close

Вход

Забыли?

вход по аккаунту

?

Utilization of microwaves in ceramic processing

код для вставкиСкачать
INFORMATION TO USERS
This manuscript has been reproduced from the microfilm master. UMI
films the text directly from the original or copy submitted. Thus, some
thesis and dissertation copies are in typewriter face, while others may
be from any type of computer printer.
The quality of this reproduction is dependent upon the quality of the
copy submitted. Broken or indistinct print, colored or poor quality
illustrations and photographs, print bleedthrough, substandard margins,
and improper alignment can adversely affect reproduction.
In the unlikely event that the author did not send UMI a complete
manuscript and there are missing pages, these will be noted. Also, if
unauthorized copyright material had to be removed, a note will indicate
the deletion.
Oversize materials (e.g., maps, drawings, charts) are reproduced by
sectioning the original, beginning at the upper left-hand corner and
continuing from left to right in equal sections with small overlaps. Each
original is also photographed in one exposure and is included in
reduced form at the back of the book.
Photographs included in the original manuscript have been reproduced
xerographically in this copy. Higher quality 6" x 9" black and white
photographic prints are available for any photographs or illustrations
appearing in this copy for an additional charge. Contact UMI directly
to order.
University Microfilms International
A Bell & Howell Information Company
300 North Zeeb Road. Ann Arbor. Ml 48106-1346 USA
313/761-4700 800/521-0600
R ep ro d u ced with p erm ission o f th e copyright ow ner. Further reproduction prohibited w ithout p erm ission .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Order N um ber 9518741
U tilization o f microwaves in ceramic processing
Fang, Yi, P h.D .
The Pennsylvania State University, 1994
UMI
300 N. ZeebRd.
Ann Arbor, MI 48106
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
The Pennsylvania State University
The Graduate School
U T IL IZ A T IO N O F M ICROW AVES IN C E R A M IC PR O C E S S IN G
A Thesis in
Solid State Science
by
Yi Fang
Submitted in Partial Fulfillment
of the Requirements
for the Degree of
Doctor of Philosophy
December, 1994
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
We approve the thesis of Yi Fang.
Date of Signature
Della M. Roy
y
Chair of Committee
Professor o f Materials Science, Emerita
Thesis Co-Advisor
JU.
Dinesh K^Agrawal
Associate Professor o f Materials
Thesis Co-Advisor
Rustum Roy
Evan Pugh Professor of the S^lid State
"Ttcsis-Co-Advisor
Paul W. Brown
Professor of Ceramic Science
and Engineering
David J. Green
Professor of Ceramic Science
and Engineering
<— ^
d .~ r
- S’,
Robert N. Pangbom
Professor of Engineering Mechanics
In Charge o f Graduate Program in
Solid State Science
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
in
ABSTRACT
The studies described in this thesis are focused mainly on microwave processing of
various ceramic materials. The material systems selected for the studies include some
traditional ceramic materials such as alumina (AI2 Q 3) and mullite (3 A l 2 ( V 2 Si0 2 ), new
materials such as biomedical materials-hydroxyapatite [HAp, Ca 10 (PO4 )6 (OH)2 ] ceramics,
newly developed low-thermal-expansion m aterials based on NZP (NaZr 2 P 3 0 12 ), and
various zirconia-containing ceramic composites.
M icrowave processing studies were carried out on the selected m aterials in a
m ultim ode m icrowave cavity at 2.45 GHz. Conventional sintering experiments were
conducted for comparison. In the HAp system, dense, porous, and transparent HAp
ceramics were successfully fabricated by microwave processing at ambient pressure. A
kinetics study on the densification shows that the regularly dense HAp ceramics could be
fabricated by microwave processing at temperatures 100°C lower than that by conventional
method. The apparent activation energy for the densification of HAp was found to be 112
kJ/mole and 181 kJ/mol. in microwave and conventional sintering, respectively.
M icrow ave sintering studies o f m ullite ceramics were carried out using both
crystalline and amorphous (gel) precursors. In the temperature range up to 1500°C,
microwave enhancing effect on densification o f crystalline mullite and monophasic mullite
gel was not significant. The densification o f diphasic mullite xerogel was significantly
accelerated in the temperature range between 1100-1250°C. Microwave irradiation for 5-10
minutes lowered the mullitization temperature of the diphasic gel by 75-100°C. In addition,
microwave heating promotes seeding effect o f the diphasic gel. By microwave processing,
0.5% seeding (monophasic) gel significantly accelerated mullite transformation o f the
diphasic gel, while by conventional heating, seeding effect did not show up with less than
10% seeding gel. A kinetics study found that the apparent activation energy for the
densification o f the diphasic mullite xerogel in the microwave field is 396 kJ/mole, while
that under conventional conditions is 702 kJ/mole.
Using the diphasic mullite aerogel as starting material, transparent mullite ceramics
R ep ro d u ced with p erm ission o f th e copyright ow ner. Further reproduction prohibited w ithout perm ission.
were fabricated under am bient pressure by microwave and conventional sintering,
respectively.
It is suggested that the microwave enhanced densification, mullite transformation, and
epitaxy of the diphasic mullite gel are mainly attributed to the decrease in viscosity of the
amorphous silica in the gel, which effectively promotes viscous flow o f the gel and
accelerates the diffusion of the dissolved alumina. In contrast, the low dielectric loss and
low solid-state diffusivity of the ions are responsible for the sluggish sintering kinetics of
crystalline mullite and monophasic gel which converts to mullite below 1000°C.
Microwave processing enhanced the densification of the amorphous alumina by 2649% over conventional sintering, but only 2.3% for the crystalline precursor, indicating
that the amorphous alumina absorbs microwaves more efficiently than the crystalline one.
Significant microwave enhancing effect on densification was also verified with two
materials of [NZP] structure and various zirconia (PSZ)-containing composites. In the case
o f the composites, the microwave enhancement decreases with increasing the PSZ content
due to the better sinterability of PSZ than the matrix materials.
Based on the observations in the current work, it is suggested that, besides the
intrinsic dielectric properties, free energy potential, or activity, which is related to various
higher-energy defects and surface area, is an important factor responsible for microwave
absorption o f materials irradiated. Specific surface area is an indicator of free energy.
Also, a higher surface area also implies low er bulk thermal conductivity and higher
scattering effect to microwaves. Therefore, the higher the surface area, the stronger the
microwave absorption.
It is also suggested that the microwave enhancing effect on densification and related
processes is due to an athermal effect which promotes the chemical potential as the driving
force for the thermal processes. In a thermal process of a specified material, the degree of
microwave enhancing effect depends on the relative magnitude of the athermal microwave
effect and the total thermal energy supplied to the material. Therefore, the microwave
enhancing effect is shielded when the processing temperature is too high.
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
V
TABLE OF CONTENTS
Page
A BSTRA CT
.................................................................................................................
iii
L IS T O F FIG U R E S .....................................................................................................
xi
LIST OF TABLES ........................................................................................................... xviii
PRINCIPAL SYMBOLS AND ABBREVIATIONS ....................................................... xx
A C K N O W L E D G M E N T S ............................................................................................... xxi
Chapter
1. IN T R O D U C T IO N
.......................................................................................................
1
1.1.
1.2.
G eneral S ta te m e n t ..........................................................................................
S tatem ent o f P roblem ...................................................................................
1
2
1.3.
O bjectives and Scope o f Study ..................................................................
5
2. LITERATURE R E V IE W ..........................................................................................
2.1. M icro w av es ......................................................................................................
2.2. M icrowave Processing o f M aterials ............................................................
2.2.1. M icrow ave-M aterial Interactions ....................................
7
7
10
10
2.2.2. Dielectric Properties o f Ceramic M aterials ....................................
2.2.3. C haracteristics of M icrowave H eating ...........................................
12
15
2.2.4. H istorical Review o f M icrowave Sintering ..................................... 22
2.2.5. Current Status of Microwave Sintering Technology .......................... 24
2 .2.5.1.
2 .2.5.2.
M icrow ave A pplicators .................................................... 24
N ew D evelopm ents ........................................................... 25
3. EXPERIMENTAL PR O C E D U R E S
3.1.
3.2.
.......................................................................
3.2.1. M icrow ave F u rn ace ...........................................................................
3.2.2. Temperature M easurement in Microwave P ro c essin g ......................
3.2.3. C on v en tio n al F u rn ace ........................................................................
3.3. S in terin g P ro c ess ...........................................................................................
3.4.
28
S tartin g M aterials .......................................................................................... 28
Experim ental Setups for Sintering ............................................................. 28
28
31
32
33
C h a ra c te riz a tio n ............................................................................................... 33
3.4.1. P o w d er C h a ra cte riz atio n ...................................................................... 34
3.4.2. T herm ogravim etric A nalysis ............................................................... 34
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
TABLE OF CONTENTS (continued)
Chapter
Pages
3.4.3. D ifferential Therm al A nalysis ......................................................... 34
3.4.4. D ila to m e try ........................................................................................... 35
3.4.5. Pow der X -R ay D iffraction ..............................................................
3.4.6. L attice Param eter Refinem ent .........................................................
35
35
3.4.7. D ensity M easurem ent .......................................................................
3.4.8. Mercury Intrusion Porosimetry.................................................................
35
36
3.4.9. Diametral Tensile Strength M easurem ent ........................................
3.4.10. Fracture Toughness M easurem ent ..................................................
3.4.11. Scanning E lectron M icroscopy ....................................................
3.4.12. Transm ission Electron M icroscopy .............................................
36
37
37
39
3.4.13. In frared Spectrom etry
.................................................................... 39
4. MICROWAVE SINTERING OF HADROXYAPATITE
.........................................
40
4.1. In tro d u c tio n ....................................................................................................... 40
4.1.1. G e n e ral .................................................................................................. 40
4.1.2. Application o f HAp as a Biom aterial ............................................. 41
4.2. Synthesis and Thermal Stability of Hydroxyapadte (H A p )........................ 44
4.2.1. In tro d u c tio n
........................................................................................ 44
4.2.2. E xperim ental Procedure
4 .2 .2 .1 . H A p Synthesis
.................................................................. 47
........................................................... 47
4 .2 .2 .2 . C haracterization
..................................................................49
4.2 .2 .3 . T herm al Stability
............................................................. 49
4.2.3. R esults and D iscussion ................................................................... 50
4.2 .3 .1 . M orphology
.........................................................................50
4.2 .3 .2 .
4.2.3.3.
C ry sta llin ity
S to ichiom etry
....................................................................... 54
.................................................................... 54
4.2 .3 .4 . TG A R esults............... ............................................................. 57
4.2.3.5. L attice Param eters
.......................................................... 62
4 .2 .3 . 6 . T herm al Stability
............................................................. 62
4.2.4. S um m ary ............................................................................................... 65
4.3. Fabrication of Regularly Dense HAp Ceramics ......................................... 71
4.3.1. In tro d u c tio n .......................................................................................... 71
4.3.2. E xperim ental Procedure
.................................................................. 72
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
vii
TABLE OF CONTENTS (continued)
Chapter
Page.
4 .3 .2 .1 .
4 .3 .2 .2 .
M a te ria l................................................................................... 72
S in terin g Setup ................................................................. 72
4 .3 .2 .3 .
4 .3 .2 .4 .
Tem perature M easurem ent .............................................. 73
S in te rin g .............................................................................. 76
4 .3 .2 .5 .
4 .3.3. R esults
C h a ra cte riz atio n
and D iscussion
................................................................
................................................................
76
77
4 .3 .3 .1 .
4 .3 .3 .2 .
Tem perature M easurem ent .............................................. 77
S intering Process ............................................................. 77
4 .3 .3 .3 .
4 .3 .3 .4 .
U niform ness in Sintering ..............................................
81
S in tered D ensity ............................................................... 81
4 .3 .3 .5 .
4 .3 .4 .6 .
4 .3 .3 .7 .
4 .3 .3 . 8 .
G rain Size and M icrostructure .......................................
Phases
...............................................................................
M echanical Strength
......................................................
E nergy Consum ption
....................................................
85
85
89
89
............................................................................................
4.3.4. S um m ary
4.4. M icrowave Processing o f Porous HAp C eram ics........................................
92
92
....................................................................
4.4.1. In tro d u c tio n
4.4.2. E xperim ental Procedure ....................................................................
92
93
4 .4 .2 .1 .
4 .4 .2 .2 .
H A p Precursor Pow ders ...............................................
Pow der C onsolidation ....................................................
93
93
4 .4 .2 .3 .
4 .4 .2 .4 .
...........................................................................
S in te rin g
C h aracterizatio n ...............................................................
95
95
4.4.3. R esults
4 .4 .3 .1 .
4 .4 .3 .2 .
4 .4 .3 .3 .
and D iscussion ................................................................... 95
P o ro sity
............................................................................ 97
P h a se s .................................................................................
M icrostructure
.................................................................
97
99
4 .4 .3 .4 . S tren g th
.............................................................................
4.4.4. S um m ary ................................................................................................
99
99
4.5. Fabrication o f Transparent HAp Ceramics ................................................ 102
4.5.1. In tro d u c tio n .......................... .................................................................... 102
4 .5 .2 . E xperim ental Procedure
.................................................................... 103
4.5.3. R esults and D iscussion .....................................................................103
4.5.4. S um m ary
................................................................................................ 112
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout perm ission.
viii
TABLE OF CONTENTS (continued)
Chapter
4 .6 .
Page,
D ensification K inetics o f HA p ..................................................................... 112
4.6.1. In tro d u c tio n ........................................................................................... 112
4.6.2. E xperim ental Procedure
................................................................. 113
4.6.3. R esults and D iscussion ...................................................................... 113
4.6.4. S u m m ary
................................................................................................ 115
5. MICROWAVE SINTERING O F M ULLITE CERAMICS ............................... 118
5.1. In tro d u c tio n ...................................................................................................... 118
5.2.
Synthesis o f
M ullite P recursors .............................................................. 120
5.2.1. M o n o p h asic G el ................................................................................... 120
5.2.2. D ip h a sic G el ....................................................................................... 120
5.2.3. Seeded D iphasic G els ...................................................................... 121
5.2.4. D iphasic M ullite A erogels ............................................................... 122
5.3. M ullite Transform ation o f D iphasic Gels ................................................... 123
5.3.1. In tro d u c tio n ........................................................................................... 123
5.3.2. E xperim ental Procedure
.................................................................. 123
5.3.3. R esults and D iscussion .................................................................... 124
5 .3 .3 .1 . C rystallization o f Gels ................................................... 124
5.4.
5 .3 .3 .2 . Seeding Effect o f D iphasic Gels .................................... 124
Microwave Sintering of M ullite Ceramics (Regular and Transparent)
132
5.4.1. In tro d u c tio n
....................................................................................... 132
5.4.2. E xperim ental P rocedure ................................................................... 132
5.4.3. R esults and
D iscussion .................................................................. 133
5 .5. D ensification
Kinetics
o f D iphasic Gel .................................... 146
5.5.1. In tro d u c tio n ........................................................................................... 146
5.5.2. E xperim ental Procedure
....................................................................146
5.5.3. R esu lts and D iscu ssio n ...................................................................... 147
5.6. Summary on M icrowave Processing o f M ullite Materials ........................ 151
6
. MICROWAVE SINTERING OF OTHER CERAM ICS ...................................... 153
6.1.
M icrow ave Sintering o f A lum ina ............................................................ 153
6.1.1. In tro d u c tio n ........................................................................................... 153
6.1.2. E xperim ental Procedure .................................................................... 153
R ep ro d u ced with p erm ission o f th e copyright ow ner. Further reproduction prohibited w ithout perm ission.
ix
TABLE OF CONTENTS (continued)
Chapter
6.2.
Page.
6.1.3. R esults and D iscussion .................................................................... 154
M icrow ave Sintering o f [NZP] Ceramics ................................................ 157
6.2.1. In tro d u c tio n ............................................................................................ 157
6.2.2. E xperim ental P rocedure ................................................................... 157
6.2.3. R esults and D iscussion ......................................................................158
6.2.4. S um m ary ..................................................................................................164
7.
MICROWAVE SINTERING OF CERAM IC COMPOSITES .............................. 165
7 .1 . In tro d u c tio n ......................................................................................................... 165
7 .2 . M icrowave Sintering of Hydroxyapatite-Based C om posites..................... 166
7 .2 .1 . In tr o d u c tio n
................................................................................................. 166
7.2.2. E xperim ental P rocedure ..................................................................... 167
7.2.2.1.
1.2.2.2.
1.2.23.
7.2.2.4.
S tarting M aterials
......................................................... 167
C om paction
.......................................................................168
S in te rin g
.......................................................................... 168
C haracterizatio n
............................................................. 170
7.2.3. Results and D iscussion
.................................................................. 170
7 .2.3.1. H e a tin g
.............................................................................. 170
1.23.2.
1.233.
Phase C om position
....................................................... 170
S intered D ensity
........................................................... 172
7.2.3.4.
7.2.3.5.
....................................................... 174
Fracture Toughness
M ic ro stru c tu re .....................................................................175
7 .2.4. Sum m ary
..........................................................................................
7.3. Microwave Sintering o f Various Zirconia-Containing C o m p o sites
175
178
7.3.1. In tro d u c tio n
....................................................................................... 178
7.3.2. E xperim ental Procedure
................................................................. 179
7 .3.2.1.
13.2.2.
1 3 .2 3 .
Starting M aterials
......................................................... 179
S in te rin g ............................................................................. 180
C haracterizatio n
............................................................. 181
7.3.3. Results and D iscussion
.................................................................. 181
7.3.4. S um m ary
.................................................................................................. 196
R ep ro d u ced with p erm ission o f th e copyright ow ner. Further reproduction prohibited w ithout p erm ission .
x
TABLE OF CONTENTS (continued)
Chapter
8
Page
. DISCUSSION ON MICROWAVE ENHANCING EFFECTS ...............................197
8.1. M icrowave Phenom ena o f M icrowave Effects ........................................... 197
8.2. Driving Force for Densification in Solid-State S in te rin g ............................ 198
8.3. Loss M echanism s in M icrow ave H eating ................................................... 202
8.4. Interactions o f M aterial Surfaces with M icrowaves .....................................203
8.4.1. D ie le c tric L osses ................................................................................. 203
8.4.2. T herm al C onductivity .........................................................................204
8.4.3. S urface S cattering ............................................................................ 204
8.5. Atherm al E ffect in M icrowave Processing ..................................................204
8 .6 .
M icrow ave E nhanced D iffusion .................................................................. 206
8.7. M icrowave Effects on Diphasic M ullite Gels ..............................................207
8.7.1. Observed M icrowave Effects on Mullite M a te ria ls...........................207
8.7.2. C haracteristics o f M ullite G els .........................................................208
8.7.3. Microwave Effect on Densification of Diphasic Mullite X erogel
209
8.7.4. M icrowave Effect on Crystallization and E p ita x y .......................... 212
8 . 8 . M icrowave Effect on Sintering of Other Materials .................................... 213
8.9. Influence of Processing Temperature on M icrowave E f f e c t.......................... 214
8.10.G rain G row th during M icrowave Processing ............................................ 216
9. C O N C L U S IO N S ...........................................:.............................................................. 217
9.1. Sum m ary and C onclusions ......................................................................... 217
9.2. Suggestions fo r F uture S tudies................................................................... 222
9.2.1. Dielectric Properties o f Amorphous Materials .............................. 222
9.2.2. M icrowave Sintering o f HAp in Moist E nvironm ent.................... 222
9.2.3. Better Transparent Mullite Ceramics by Process M odification
R E FE R E N C E S
223
............................................................................................................... 224
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
xi
LIST OF FIGURES
Page
Chapter 2
Figure 2.1
Electromagnetic spectrum showing the band o f microwaves ...............
8
Figure 2.2.
Microwave frequencies and wavelengths used in materials processing....
9
Figure 2.3.
Classification of m icrowave-m aterial interactions...................................
11
Figure 2.4.
Relative dielectric constant (8-10 GHz) versus temperature.....................
16
Figure 2.5.
Loss tangent (8-10 GHz) versus tem perature..........................................
17
Figure 2.6.
Heating patterns in conventional and microwave furnaces......................
18
Figure 2.7.
Therm al runaway during m icrowave heating........................................
20
Schematic display of the sintering packet in the microwave processing.
1.Turning table. 2. Zirconia cylinder. 3. Samples. 4. Thermocouple.
5. Fibermax insulator. 6 . MoSi 2 rods. 7. M icrowave port.................
30
Schematic showing of the cracks in Vickers and Knoop indentations
38
Figure 4.1.
Solubility of calcium phosphates at 25°C............................................
42
Figure 4.2.
Influence of moisture on the decomposition of H A p............................. 46
Figure 4.3.
Micrographic morphology of HAp synthesized by (a) hydrolysis of
a-TCP, (b) hydrolysis of brushite (DCPD) plus ripening treatment, (c)
precipitation, and (d) hydrothermal m ethod.............................................
51
A micrograph showing that the HAp crystals were damaged by electron
beam bombardment during TEM observation at 120 kV. ...........................
52
Morphology of HAp produced by the hydration of a-C a 3P 0 4 at 25°C
fo r 7 days ..................................................................................................
53
Chapter 3
Figure 3.1.
Figure 3.2.
Chapter 4
Figure 4.4.
Figure 4.5.
Figure 4.6.
XRD patterns of the as-synthesized HAp prepared by (a) hydrolysis of
DCPD, (b) hydrolysis of DCPD plus triple ripening treatment, (c)
precipitation, (d) hydrothermal method, and (e) hydrolysis of a-T C P ....... 55
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
LIST OF FIGURES (continued)
Page
Figure 4.7.
XRD patterns showing that the crystallinity of the precipitated HAp
increased with heating temperature. All samples were fired for 1 h.
The marked (•) peaks belong to TCP; all unmarked belong to HAp
56
Figure 4.8.
IR spectra of HAp prepared by various m ethods................................
60
Figure 4.9.
TG thermograms of HAp synthesized by (a) hydrolysis of a-TCP, (b)
hydrothermal method, (c) DCPD hydrolysis, ripened twice, (d) DCPD
hydrolysis, ripened thrice, (e) DCPD hydrolysis without ripening,
(f) DCPD hydrolysis (ripened once),and (g) precipitation...................
63
Figure 4.10. XRD patterns showing that the hydrothermally (200 °C, 1.5 MPa)
synthesized HAp fired at 1370 °C in air of 50% RH (11.88 mm Hg)
rem ains undecom posed .........................................................................
66
Figure 4.11. XRD patterns o f the as-synthesized HAp by DCPD hydrolysis, heated
at various temperatures. It is seen that the decomposition started at about
700 °C. 714 °C, p-TCP + HAp; 7 3 3 ,1100°C, p-TCP. 1300°C; a-T C P
+ (3-TCP.
67
Figure 4.12. XRD patterns showing the influence of repeated ripening on the thermal
stability of HAp obtained by DCPD hydrolysis. Samples were heated 1 h
at 1100 °C in air of 50% RH. (a) Without ripening: (3-TCP. (b) Ripened
once: HAp+ P-TCP. (c) Ripened twice: HAp+P-TCP (tr.). (d) Ripened
thrice: HAp only, no deco m p o sitio n........................................................
68
Figure 4.13. XRD patterns showing the phase composition of SOR (strengthened
one-step ripening) heated 1 h at 1350 °C and 1370 °C, respectively. The
ripening after the hydrolysis of DCPD was carried out at 75 °C and
(a) 20% CCD, 70 min., initial pH 8.87, (b) 60% CCD, 24 h, initial
pH=8.87, (c) 20% CCD, 70 min., initial pH=11.0, and (d) 60% CCD,
24 h, initial pH =l 1. The marked peaks (•) belong to a-TCP. .................. 69
Figure 4.14. Schematic showing of the arrangements in the microwave sintering
cavity. The sintering package includes (1) thermocouple, (2) zirconia
cylinder, (3) pack of samples, (4) thermal insulating fibers, (5) power
level control, and (6 ) tim e control.......................................................
74
Figure 4.15. Schematic showing of relative positions of thermocouple and p-TCP
pellets during the temperature measurement calibration. Labeled are (1)
thermocouple, (2) porous zirconia, and (3) TCP
p e lle ts ................................................................................................................ 75
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
LIST OF FIGURES (continued)
Page
Figure. 4.16. XRD patterns (Cu Ka) o f /J-TCP samples used to calibrate temperature
m easurement (a) Conventionally sintered for 15 min. at temperatures
indicated, (b) Microwave sintered at 1125±10 °C for 15 min. (pellet
positions are indicated). (•): a-TCP. Unmarked peaks: /J-TCP. ............ 78
Figure 4.17. Temperature-time curves in the microwave sintering of HAp at (a)
1200 °C and (b) 1300 °C. Sintering time: (A) 5 min., (-----) 10 min.,
(••••) 2 0 m in ...............................................................................................
80
Figure 4.18. Photograph showing the uniform shrinkage of microwave sintered HAp
pellets. The dime-pattems on the green pellets (large) have been
uniformly reduced to the sintered pellets (small) by microwave sintering
at 1200 °C for 7 m in...............................................................................
82
Figure 4.19. Relative density (% theoretical) of HAp ceramics sintered by (1)
microwave/ 1300 °C/10 min., (2) microwave/1200 °C/10 min., (3)
conventional/1300 °C/2 h, and (4) conventional/1200 °C/2 h................
83
Figure 4.20. SEM micrographs showing (a) as-sintered surface and (b) fracture
surface of HAp conventionally sintered at 1200 °C for 120 m in..............
86
Figure 4.21. SEM micrographs of (a) as-sintered surface and (b) fracture surface
of HAp sintered by microwave processing at 1200 °C for 10 min........... 87
Figure 4.22. Diametral tensile strength of HAp ceramics (a) versus sintering density:
(O) microwave sintered, and (A) conventionally sintered; (b) versus green
density (1) microwave/20 min.; (2) microwave/10 min., and (3)
conventional/2 h .......................................................................................... 90
Figure 4.23. Comparison of temperature-time profiles in the sintering of HAp by (1)
microwave and (2 ) conventional m ethods.............................................. 91
Figure 4.24. Morphology of the starting HAp powders composed of (A) whiskers
synthesized by the hydrolysis of a-T C P and (B) agglomeration of fine
single crystals synthesized by the hydrolysis of brushite followed by
ripening treatm ent with calcium chloride................................................ 94
Figure 4.25. Microstructures o f the microwave-processed porous HAp ceramics
(fracture surface), (a, b) Starting with fine HAp powder: (a) 1150°C,
3 min., porosity 73.3%. (b) 1200°C, 5 min., porosity 24.5%. (c, d)
Starting with HAp whiskers, pressed to different green densities
(c: 43.58%, d: 54.29%), both fired at 1150°C for 1 min. Final porosity
(c) 47% ; (d) 2 8 % .................................................................................. 100
Figure 4.26.
Diametral tensile strength of microwave-processed HAp ceramics
101
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
xiv
LIST OF FIGURES (continued)
Page
Figure 4.27. Morphology o f fine hydroxyapatite powders synthesized by (a)
hydrothermal treatment and (b) hydrolysis of brushite followed by
"ripening" treatm ent ............................................................................... 104
Figure 4.28. Heating curves of sintering of hydroxyapatite ceramics by (a) microwave
processing and (b) conventional sintering............................................... 105
Figure 4.29. Dilatometry curve of the hydrothermally synthesized HAp heating at
5°C per m in u te .......................................................................................... 106
Figure 4.30. Transparent HAp ceramics fabricated at ambient pressure in air (a-c) by
microwave processing for 5 min. at (a) 1150°C, (b) 1125°C, (c)
1100°C, totally irradiated for 18 min., and (d) by conventional sintering
at 1150 °C for 5 min. after heating to 1150°C at 5 °C/min., totally heated
fo r 230 m in ................................................................................................. 109
Figure 4.31. Micrographs of the as-sintered surface of the transparent hydroxyapatite
ceramics sintered by (a) microwave processing and (b) conventional
m e th o d .............................................................................................................
110
Figure 4.32. Powder XRD pattern of the transparent HAp ceramics by microwave
processing at 1150 °C for 5 min., showing that the ceramics are highly
crystalline single phase H A p.................................................................
Ill
Figure 4.33. Sintered density as a function o f time of HAp sintered by (a) microwave
and (b) conventional m ethods................................................................ 114
Figure 4.34. Arrhenius plot of the densification of HAp under (a) microwave and (b)
conventional conditions...........................................................................
117
Chapter 5
Figure 5.1.
DTA curve o f monophasic aluminosilicate gel, showing that crystal­
lization of the gel takes place at around 970°C. Heating rate was
10°C per m in u te ......................................................................................... 125
Figure 5.2.
DTA curve o f diphasic gel. The exothermal peak at 1328°C indicates
the occurrence o f m ullitization............................................................... 126
Figure 5.3.
XRD patterns showing that in conventional firing, mullitization of the
diphasic aluminosilicate gel has not started at 1300°C. Mullite shows
up in the sample fired at 1325°C........................................................
127
XRD patterns showing that at 1250°C, crystallization of the diphasic
aluminosilicate gel starts when subjected to microwave irradiation
128
Figure 5.4.
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
xv
LIST OF FIGURES (continued)
Page
Figure 5.5.
Dilatometry curves of (a) monophasic and (b) diphasic mullite xerogels
heated at 3°C /m in...................................................................................... 139
Figure 5.6.
Microstructure of mullite ceramics step-sintered at 1200°C for 30 min.
followed by 1500°C for 20 min. by (a) microwave and (b) conventional
141
m ethod.
...............................................................................................
Figure 5.7.
Microstructure of (a) as-sintered surface and (b) fracture microstructure
of a diphasic mullite sample microwave sintered for 20 min. at 1300°C
to relative density o f 94% ...................................................................
142
Microstructure of conventionally sintered (1250°Cx30 min. + 1500°C
x30 min.) specimen of (a) pure and (b) seeded (5%) diphasic mullite
g e ls ...................................................................................................................
143
Figure 5.8.
Figure 5.9.
Mullite ceramics sintered by (a) microwave (1300°Cxl0 min.) and (b,
c, d) conventional (b, c: 1300°Cxl0 min.; d: 5°C/min. to 1320°C with
no holding) methods. Sample (c) was mode of the diphasic xerogel
and opaque, while the rest were made of aerogel and transparent.......... 144
Figure 5.10. Dilatometry curve o f diphasic mullite aerogel heated at 3°C/min...........
145
Figure 5.11. Densification curves o f diphasic mullite gel under (a) microwave and
(b) conventional conditions...................................................................
148
Figure 5.12. Comparison in density o f diphasic mullite gel sintered by conventional
and microwave processing at various tem peratures.............................
149
Figure 5.13. Arrehenius curve o f diphasic mullite gels sintered by (a) microwave
and (b) conventional m ethods................................................................ 150
Chapter 6
Figure 6.1.
Morphology o f the alumina sintered by microwaves at 1500°C for
20
m in .........................................................................................................
156
Figure 6.2.
A typical heating curve of CSZP ceramic in a microwave field............
159
Figure 6.3.
XRD patterns of CSZP (a) before and (b) after microwave sintering at
1300°C fo r 30 m in .................................................................................. 160
Figure 6.4.
Influence o f compaction pressure (green density), sintering temperature
and time on the sintered density o f CSZP.
............................................
161
Comparison of the sintered density of CSZP ceramics processed by
microwave and conventional methods at 1300°C................................
162
Figure 6.5.
R ep ro d u ced with p erm ission o f th e copyright ow ner. Further reproduction prohibited w ithout perm ission.
xvi
LIST OF FIGURES (continued)
Page
Figure
6 .6 .
Micrographs o f CSZP sintered at 1300°C by (a) microwave sintering
for 30 min. and (b) conventional sintering for 24 h............................
163
Figure 7.1.
The arrangem ent inside the m icrowave oven.....................................
169
Figure 7.2.
A typical heating curve for microwave processing of HAp (HAp-F.)
and HAp based composites, where porous zirconia was used as
accelerator. .......................................................................................
171
The microstructure of microwave sintered HAp/PSZ composite,
showing the simultaneous sintering o f HAp and PSZ particles
176
Chapter 7
Figure 7.3.
Figure 7.4.
The microstructures of the fracture surfaces o f the HAp/SiCw composites
sintered by (A) microwave processing and (B) hot pressing
(perpendicular to the compaction direction)......................................... 177
Figure 7.5.
Heating temperature as a function of time for the zirconia-containing
composites based on (a) alumina and (b) mullite, (c) CSZP, and
(d) B Z P S ...................................................................................................... 182
Figure 7.6.
Densities of AI2 Q 3/PSZ composites at various mixing proportions.
The alumina powder is (a) amorphous (RP-alumina) and (b) highly
crystalline, high purity, crystalline alum ina............................................... 184
Figure 7.7.
Sintered density of mullite/PSZ composites. Diphasic gel (a-c) and
monophasic gel was used as mullite precursor, respectively. ................
185
Figure 7.8.
Sintered density of [NZPJ/PSZ composites using (a) CSZP and (b)
BZPS as the [NZP] component, respectively. .......................................... 187
Figure 7.9.
Microstructure of A1203/PSZ (30%) composite sintered by microwave
processing at 1500°C for 20 m in............................................................ 189
Figure 7.10. Microstructure of mullite/PSZ (30%) composite sintered by microwave
processing at 1200°C for 30 min. followed by 1500°C for 20 m in
190
Figure 7.11. Microstructure of CSZP/PSZ (10%) composite conventionally sintered
at 1500°C fo r 20 m in ..............................................................................
191
Figure 7.12. Microstructure of BZPS/PSZ (20%) composite sintered by microwave
processing at 1500°C for 20 m in..........................................................
192
R ep ro d u ced with p erm ission o f th e copyright ow ner. Further reproduction prohibited w ithout perm ission.
xvii
LIST OF FIGURES (continued)
Chapter 8
Page
Figure 8.1. Alternate paths for matter transport in the initial stages of sintering
199
Figure 8.2. Schematic showing the energy barrier in the sintering process. As
the free energy (chemical potential) o f starting material increases from
A to A l, A 2 ,..., the driving force for the process increases from AG
to A G 1,..., and the activation energy required for the process
correspondingly decreases from Ea to E a l, E a 2 , ..................................201
Figure 8.3. Schematic showing of an amorphous structure..................................... 205
Figure 8.4. A model of the diphasic mullite gel system, showing that 8-A1203
particles are homogeneously dispersed in the silica gel........................
210
Figure 8.5. Schematic presentation of (a) ordered crystalline silica and (b) random
netw ork glassy form o f the same com position.....................................
211
Figure
8 .6 .
Arrhenius plot o f the rates of densification and grain growth................. 216
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
xviii
L IS T O F T A B L E S
Chapter 2
Page
Some key characteristics and features of microwave processing..............
21
Ca/P ratios of HAp samples determined by atomic absorption
spectroscopy. .................................................................................................
58
Table 4.2.
Spectrochemical analysis of chloride ions in HAp samples (in ppm )
58
Table 4.3.
Lattice parameters of the HAp synthesized in the current study, A
64
Table 4.4.
The main phase composition appearing in the HAp samples heated
in air of 50% relative humidity at various temperatures........................... 70
Table 4.5.
Intensity ratio of a-T C P (d = 2.909A) to p-TCP (d = 2.880A) in
Table 2.1.
Chapter 4
Table 4.1.
the p-TCP after firing under different conditions for temperature
m easurem ent c a lib ra tio n ............................................................................... 79
Table 4.6.
Grain size of HAp pellets sintered under different conditions, p m ..............
Table 4.7.
Experimental Conditions of the Microwave Processing to Fabricate
P orous HA p ceram ics................................................................................... 96
Table 4.8.
Relative Density of Microwave-Processed Porous HAp Ceramics, %....... 98
Table 4.9.
Processing conditions and results for microwave and conventional
sin terin g o f H A p ............................................................................................ 108
Table 4.10. The InK values from the densification curves for Arrhenius plot at
relative density 60% ...................................................................................
88
116
Chapter 5
Table 5.1.
Mullite contents (%) in the conventionally heated diphasic mullite gel
129
Table 5.2.
Mullite contents (%) in the microwave heated diphasic mullite gel
130
Table 5.3.
Relative density (%) of monophasic mullite gel sintered at 1200°C
x 20 m in ...........................................................................................................135
R ep ro d u ced with p erm ission o f th e copyright ow ner. Further reproduction prohibited w ithout perm ission.
xix
LIST OF TABLES (continued)
Page
Table 5.4.
Comparison of sintered density of different starting materials (microwave
sintering: 1300°C x 10 min. + 1600°C x 10 m in.).................................. 135
Table 5.5.
Density of mullite conventionally sintered (1550°C x 20 min., 3h total
h e a tin g ).............................................................................................................. 136
Table 5.6.
Relative density of the seeded diphasic mullite gels after sintering (%)..... 136
Table 5.7.
Sintered density of seeded diaphasic mullite gel (microwave sintering)...... 137
Table 5.8.
Relative density of diphasic mullite gel after microwave step sintering
(1200°C x 60 min. + 1500°C x 20 m in.)............................................... 137
Table 5.9.
Density of unseeded diphasic mullite gel under various microwave
sintering sch ed u les..........................................................................................138
Table 5.10. Kinetics values for activation energy calculation for diphasic gel
150
Chapter 6
Table 6.1.
Density data of RP-alumina before and after sintering (1500°C)............. 155
Chapter 7
Table 7.1.
Table 7.2.
Summary of the processing conditions and some results measured on
the sintered HAp and HA p-based composites........................................
173
Microwave enhancement in densities over the conventionally sintered
sam ples, %...................................................................................................... 191
Chapter 8
Table 8.1.
Alternate paths for matter transport during the initial stages of sintering.... 199
Table 8.2.
Influence of processing temperature on microwave effect...................... 222
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
xx
PRINCIPAL SYMBOLS AND ABBREVIATIONS
HAp = hydroxyapatite, Ca 10 (PO4 )6 (OH )2
CCD = calcium chloride dihydrate, CaCl2' 2H20
DCPD = dicalcium phosphate dihydrate, CaH P0 4 2H20
TCP = tricalcium phosphate, Ca 3 (P 0 4 ) 2
TeCP = tetracalcium phosphate, Ca4 (P 0 4)20
CSZP = calcium strontium zirconium phosphate
BZPS = barium zirconium phosphosilicate
PSZ = partially stabilized zirconia (tetragonal)
XRD = x-ray diffraction
SEM = scanning electronic microscopy
TGA = thermal gravimetric analysis
DTA = differential thermal analysis
£* = € - ye” = £o(s. r -
the relative complex permittivity
£q = 8 .8 6 x 10- 12 F/m, the permittivity of free space
e’r = the relative dielectric constant
€’ef f = th e effective relative dielectric loss factor
a = the total effective conductivity (S/m)
8 = dielectric loss factor
/ = frequency
P = power absorbed per unit volume
E = internal electric field
X0= firee-space wave length
D = the depth of microwave penetration
H = permeativity
jl0 = permeativity of ffeespace
KIC = fracture toughness (type I load)
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
xxi
ACKNOWLEDGMENTS
The author would like to express his sincere gratitude and respect to his advisors Dr.
Della M. Roy, Dr. Rustum Roy, and Dr. Dinesh K. Agrawal, for their encourage, patience,
guidance, suggestions, and various helps throughout the course of this work. The author
wishes to thank Dr. Paul W. Brown, and Dr. David J. Green, for their comments and
suggestions for the revision of this thesis. The author also wishes to thank Dr. Jiping
Cheng and Dr. Choon-Keun Park for their assistance in experimental work.
The author is indebted to all the M ends at the Materials Research Laboratory and at
the University who directly or indirectly helped the author with the work toward this thesis.
The financial support from the National Science Foundation under research contract
No. 8812824, which made the research work of this thesis possible, is greatly appreciated.
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
1
Chapter 1
INTRODUCTION
1.1. General Statement
Recent advances in microwave processing of materials has opened a completely new
area of research in the materials science and engineering. W hile the early study started in
the late forties by Sutherland, the eminent British spectroscopist who devoted considerable
energy to the study of drying o f large ceramic bodies using W W II radar (Roy, 1988), the
study got attracted more attention in the seventies and rapidly developed in the recent years.
The first symposium on microwave processing of materials held by the Materials Research
Society in 1988 marks the beginning of the worldwide professional study in the microwave
processing of materials as a new area. Since then, several annual symposia have been held
by the M aterials Research Society and the American Ceramic Society alternatively.
Hundreds of papers have been published or presented in this area and more and more
attention is drown.
Variety of materials have been studied by microwave processing. The research areas
include the basic theory of the interaction between microwave and materials, microwave
sintering, joining, drying, and synthesis, microwave remediation of nuclear and hazardous
waste, m odeling o f m icrow ave processing, system design, dielectric property
measurement, and so on. The focus on the microwave processing is how to utilize the
microwave energy as an alternative in materials processing to substantially save time and
energy consumption, thus save the capital of the unit product, and to significantly improve
certain properties of the products.
M icrowave processing of materials is fundamentally different from conventional
processing in its heating mechanism. In microwave heating, heat is generated internally by
interaction of microwave and the material, while in conventional heating, the heat is
generated out of the work piece by heating elements or or by combustion of fuels then
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
2
transferred to the work piece. Thus the temperature gradients in the microwave heating is
just opposite that in conventional case. In addition, the heat generated in the microwave
heating depends on the microwave absorbing efficiency o f the material, which is then
dependent on the dielectric loss factor of the material. So that the heating efficiency in a
microwave field is m aterial-dependent. Furtherm ore, microwave can penetrate most
ceramic materials to certain depth, so that microwave heating is a volumetric heating
process.
These features m ake microwave processing to certain ceram ic materials
potentially advantageous, such as high heating rate, short processing time, thus low
processing cost; uniform volumetric heating, low grain growth to densification ratio, thus
uniform microstructure and finer grain size, resulting in improved mechanical properties.
1.2. Statement of Problem
Although microwave processing of various ceramic materials have been studied, the
unexplored important materials are still numerous, and the research on the new materials or
the improvement o f the existing materials is endless. For example, there is either no or
very little reported data on microwave processing o f NZP (NaZr2 P 3 0 12) materials, HAp
[Ca 10 (PO4 )6 (OH)2] and m ullite ceramics.
On the other hand, although microwave
enhanced sintering and densification is experimentally evident, the mechanism behind the
observation is not at all clear. Thus both intensive and extensive studies on microwave
processing o f materials are significant.
About ten years ago, Roy (1985) and coworkers observed that silica and alumina gels
can efficiently absorb m icrowave energy and get m elted even in a regular kitchen
microwave oven. That was the first discovery that these ceramic materials can be quickly
heated in microwave field. Microwave processing of numerous ceramic materials have
been studied since then, but these are mostly crystalline materials. In contrast, research on
microwave processing of noncrystalline or amorphous inorganic solids, such as gels, and
the solids o f low crystallinity is very limited. By heating the wet m ullite gels in an
insulated cavity of 2.45 GHz, 700-W ordinary microwave oven, Komarneni et al. (1988)
obtained crystalline mullite from the single phase and diphasic mullite precursor gels after
R ep ro d u ced with p erm ission o f th e copyright ow ner. Further reproduction prohibited w ithout perm ission.
3
microwave heating for 20-25 min. However, the mechanism behind the rapid heating of
the aluminosilicate gels in the microwave field as observed by Roy and coworkers is still
not clear till now.
Mullite (3 Al20 3 -2 Si0 2) is one of the most commonly used refractory materials. It is
suitable for high temperature structure applications because of its high melting temperature
(1820±10°C), low therm al expansion coefficient, low thermal conductivity, excellent
thermal shock resistance, and good creep resistance at high temperatures. Mullite-based
glass-ceram ics have now em erged as candidates for high-perform ance packaging
applications (Aksay, 1991). Low dielectric constant and optical transparency o f fine­
grained polycrystalline mullite has led to interest in potential applications as a host material
as a solid-state laser activator (Aksay, 1991). High melting point and low solid-state ion
diffusivity require the sintering of mullite to be conducted at very high temperature for a
longtim e. Practical sintering of mullite from crystalline powder needs 1600-1700°C for at
least a few hours. Reaction sintering from crystalline alumina and silica also requires
similar high temperature.
By sol-gel routes, mullite can be synthesized and sintered at
lower temperatures. Due to its low dielectric constant (e=6.7), crystalline mullite is almost
transparent to m icrowaves, so that it is difficult to sinter by m icrowave processing.
However, a diphasic mullite gel is a system totally different from crystalline mullite before
mullitization (to crystallize to mullite). Therefore, the behavior of the diphasic mullite gel
during heating, sintering, and mullitization in the microwave field is different, and thus it
seems possible to obtain desired mullite ceramics by m icrowave processing. For this
reason, mullite sintering was conducted in the microwave field mainly using mullite gels as
the precursors.
HAp is a very important biomaterial showing the best compatibility, known thus far,
with human tissues, both soft and hard. Since the 70’s, the research on the theory and
engineering design of the bioceramics based on HAp has been widely conducted. HAp
ceramics are now comm ercially available for non-load-bearing biomedical use. For
example, Interpore200™
porous HAp granules for alveolar ridge reconstruction
R ep ro d u ced with p erm ission o f th e copyright ow ner. Further reproduction prohibited w ithout perm ission.
is
4
commercially available from Interpore International in the United States, while in Europe,
OSPROVIT 0.8 is supplied in granular form for repairing bone defects, particularly in
periodontal surgery. HAp-coated metallic prosthetic implants have been used, the clinical
application of pure or composite HAp ceramics in load-bearing implantation, however, has
not yet been achieved, basically due to the intrinsic brittleness of this material in ceramic
form. Some investigators attempted the transformation toughening o f HAp ceramics by
using partially stabilized zirconia (tetragonal, PSZ) (Wu, 1988; Costa, 1993) but failed to
achieve high density by conventional sintering, simply because of the differential sintering
behavior o f HAp and zirconia. A case in which dense HAp/PSZ composite was achieved
by hot isostatic pressing (HIP) was reported by Yoshimura et al. (1989). Since microwave
processing enhanced densification of several ceramics (Janney and Kimrey, 1991), it is
worthwhile to see its feasibility for the fabrication of HAp/PSZ composite, as well as
various HAp ceramics at ambient pressure.
[NZP] is a large structural family of low thermal expansion materials named after the
parent composition of sodium zirconium phosphate, NaZr 2 P 3 0
12
(Alamo and Roy, 1984;
Roy et al., 1984; Agrawal and Stubican, 1985). These materials are expected to be used
for superionic conductors, radwaste m anagem ent, and for low therm al expansion
applications. The sintering of these materials requires very long time at high temperatures.
No study has been carried on microwave sintering o f NZP materials thus far.
Alumina is one of the most important high temperature ceramics and has been used as
model material for microwave processing by many researchers (Sutton, 1989). Usually
crystalline alumina was used for sintering study. There is no report on the microwave
sintering of alumina with amorphous precursors, which may behave quite differently from
the crystalline alumina powder in the microwave field.
As the development o f science and technology put more and higher demands to
materials science and engineering. Just like a more advanced scientific research program
usually needs the cooperation of more people, the achievement o f high performance of
materials for advanced applications often requires combination of the properties of more
than one component. And the higher the performance, the more complex the composition.
In this sense, the route of composite is a general trend in the development of new materials.
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
5
A composite is fabricated either for the desired mechanical or functional purpose. The
im proved properties can be usually achieved by tailoring the com ponents through
com prom ise.
The combination sometimes can even create new properties.
In the
composites for improved mechanical properties, “brittle A (matrix) + tough B (additive) =
moderately tough C” is an example of compromise, while “brittle A + brittle B = tough C”
is a example for the case of creation. Ceramics are intrinsically brittle, thus composite is a
new approach to make them tougher. Sintering is a necessary step in the fabrication of
ceramics and ceramic composites. Different materials often behavior differently during the
sintering. The existence of significant differentiate sinterabilities will certainly hinder the
achievement of successful sintering of the composites. This proves to be a problem in the
fabrication o f composites by conventional sintering. Fortunately, this may be the right
place for microwave processing to show its advantages, since microwave heating has a
selective heating effect to different components in micro-scale. For this reason, microwave
processing of composites was conducted in this study. Metastable zirconia in the tetragonal
phase, also known as partially stabilized zirconia (PSZ), has been investigated for
transformation toughening (Lange, 1982; Green et al, 1989) in ceramics fabrication. Since
zirconia is a good microwave absorber at elevated temperatures, the zirconia-containing
ceramic composites were chosen as a part of this microwave processing study.
1.3. Objectives and Scope of Study
The main objectives of this work are to evaluate and understand the microwave effect
on the sintering, densification and other related phenomena during microwave processing
of the selected ceramic materials, and to explore the possibility to fabricate the ceramics
with improved properties and/or energy and time saving by microwave processing.
The research involves the synthesis, processing, and characterization of various
ceramic materials of general interest in ceramic industry. Synthesis techniques include
solid state reaction, wet chemical reaction, sol-gel, hydrothermal, and ultrasonic methods.
P rocessing includes fabrication o f various ceram ics and ceram ic com posites by
conventional and microwave methods. Characterization covers various testing techniques
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
6
on composition, crystal structure, density, porosity, morphology, microstructure, thermal
properties, m echanical properties, etc.
T he m aterials studied include HAp,
[Ca 2Q(P0 4 )6 (0 H) 2 ], mullite (3 A l 20 3 .2 Si0 2), alumina (A12 0 3), zirconia (Z r0 2), and two
low-thermal-expansion [NZP] materials, i.e., calcium strontium zirconium phosphate
(CSZP) and barium zirconium phosphosilicate (BZPS).
Microwave effect on the sintering, densification, and crystallization of these materials
was evaluated. The processing parameters that influence the microwave effects, and the
mechanisms related to the microwave enhanced sintering or densification of the all selected
materials have been discussed.
The emphasis o f the study was on the synthesis and microwave effects on the
selected m aterials during sintering.
M echanical property m easurem ents were only
evaluated in a few lim ited cases. Dielectric properties are important in microwave
processing study. The measurements of dielectric properties at high temperatures constitute
another branch in the research on microwave processing of materials, thus they are not
included in the current study.
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
7
Chapter 2
LITERATURE REVIEW
2.1. Microwaves
Microwaves are electromagnetic waves ranging from 1 m to 1 mm in wavelength at
frequencies from 0.3 to 300 GHz. The most commonly used frequency for processing
foods and ceramics is 2.45 GHz with a corresponding wavelength of 122 mm. Figure 2.1
shows the microwave band in the electromagnetic spectrum.
M ost of the microwave frequency band is used for communications and radar. The
Federal Communications Commission (FCC) has allocated 0.915 MHz, 2.45, 5.85, and
20.2-21.2 GHz for industrial, scientific, and medical use. Presently, only 915 MHz and
2.45 GHz microwaves are widely used in industrial and medial application since these uses
are based on water heating. A few other frequencies such as 28, 60, 140 GHz, and 500
M Hz are available on a lim ited basis, because they are used as power sources in
accelerators plasma fusion devices, and commercial broadcasting (Katz, 1992). Figure 2.2
shows some m icrowave frequencies and wavelengths used in processing of materials
(Sutton, 1993).
Microwaves possess certain useful characteristics, one of the most important being
that microwave wavelengths are of the same size as any structure used to guide or enclose
them (Fuller, 1990). M icrowave pulses can be very short so that they can be used for
distance or tim e m easurem ent.
com puters.
Also this makes them compatible with high speed
The high frequency of m icrowaves offers very large bandwidths for
communication links. Microwave radiation penetrates fog and clouds, travels in straight
lines and gives distinct shadows and reflections enabling it to be used for distance and
direction m easurem ent and in radar systems.
Use o f m icrowaves is necessary for
communication with satellites because they can pass through the ionosphere which reflects
lower frequency radio waves. Microwave energy is absorbed very efficiently by water or
any material containing water so that microwaves can be used for heating and drying.
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
o
Microwave
■o
TJ
CO
■H
Frequencies used In ceramics processing
n
if)
(0
o
■D
CO
O
i_
(So
O-D
12
n
CO I -
T3
n
0.915 |2i45l 28 53 60 84 100 - GHz
330
a>£go,f>
CQ
122 10.7 5.7 5.0 3.6 3.0 • mm
U .0 0 -OIL
I g o z
>
ll£
>
1 1
(f) =
10
MHz
100
MHz
1
GHz
MF
HF
VHF
UHF
1000 m
100 m
m
T "
1
MHz
10 m
1
m
— i—
100
GHz
10121014
EHF
SHF
10 cm
300
GHz
1 cm
1 mm
1 p.m
Figure 2.1. Electromagnetic spectrum showing the band of microwaves. (Source: Sutton)
00
9
1000
(GHz/ mm)
(0.915/328)
A Industrial (Ukraine)
■ Industrial (U.S.) + RSD
• RSD
(2.45/122)
100
Wavelength (k.mm)
(6 /5 0 )
(28/11)
» (37/8)
0.1
0.1
0.3
1
3.0
10
30
100
< ------------------- Micro waves
300
1000
10000
W Frequency (GHz)
Figure 2.2. Microwave frequencies and wavelengths used in materials processing.
(Source: Sutton)
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
10
M icrowaves are becoming m ore and m ore widely used.
The applications include
broadcasting, communication, radar, heating, moisture measurement, power transmission.
Microwaves are also potentially hazardous due to their internal heating effect (Bruce,
1988; Fuller, 1990). The effect may not be felt until damage has already been dome
because the heating may be internal whereas our body is designed more to warn us about
externally applied heat. From heat balance consideration of standard man in standard
conditions, 100 W/m 2 (10 mW /cm2) is considered to be the safe upper limit even during
infinite exposure because thermoregulatory systems compensate for any power absorption.
A power level of 10 W /m 2 is considered to give no heating effect even under adverse
conditions of ambient temperature and humidity (Fuller, 1990). There is also some
evidence of a athermal effect through the nervous system, although the effect is hard to
prove and controversy still surrounds it. It is claimed (Fuller, 1990) that exposure over a
period of years to pow er levels greater than 2 W /m 2 can lead to nervous system
disturbances, although occupational exposure of healthy adults to this power level seems to
have no adverse effects. However the probable safe level for continuous exposure of the
general population ought to be even lower. Different countries have different recommended
limits, ranging from 100 W/m 2 to 0.01 W/m2.
2.2. Microwave Processing of Materials
2.2.1. Microwave-Material Interactions
In contrast with visible waves, except for lasers, microwaves are coherent and
polarized. Microwaves also obey the laws of optics and can be transmitted, absorbed, or
reflected , depending on the material type. As shown in Fig. 2.3. (Sutton, 1989), metals
are opaque to microwaves and thus are good reflectors. Ceramic dielectric materials (e.g.,
AI2 Q 3, MgO, S i0 2, most glasses) are transparent to microwaves at ambient temperature,
however, when heated to above a critical temperature, these materials begin to absorb
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
11
M aterial ty p e
P e n e tra tio n
TRANSPARENT
(Low lo ss
insulator)
T otal
OPAQUE
(C onductor)
None
(R eflected )
ABSORBER
(Lossy in su lato r)
P artial
to T otal
ABSORBER
(Mixed)
(a) Matrix = low lo s s in su lato r
(b) F ib e r /p a rtic le s /a d d itiv e s =
(ab so rb in g m a te ria ls)
P artial
to T otal
A/VWVW
/VA/Wwv*-
Figure 2.3. Classification of microwave-material interactions.
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
12
microwave energy more efficiently. Other ceramic materials (e.g., Co 2 0}, M n 0 2, NiO,
and CuO) absorb microwaves efficiently at room temperature.
Basic microwave-material interaction mechanisms depend strongly on the dielectric
and magnetic properties o f the material. This causes a strong dependence o f microwave
absorption on frequency, material particle size, shape, temperature, and density (Tinga,
1988). The interactions between microwaves and m atter are many. Among them, four
important polarization mechanisms are in solids, and three of them lead to losses in the
microwave region: space charges arising from localized electrical conduction, rotating
electric dipoles, and ionic polarization associated with far-infrared vibrations (Newnham,
1991).
Conduction losses from localized space charge effects are responsible for microwave
loss in transition metal oxides with mixed valence states. Alkali ions cause microwave loss
in glasses and other silicates, especially when loosely coordinated in a m anner allowing
considerable rattling room. Oscillating Na ions can be pictured either as localized
conduction or as reorientable dipoles. Organic compounds and polymers are useful in
ceramics processing. Dipole relaxation spectra are common in water, alcohol and other
polar liquids, leading to intense energy absorption in the microwave regions. Polymers
also absorb in this region of the electromagnetic spectrum with smaller segments o f the
polym er chain resonating at higher frequencies and continuing to low er temperatures.
Localized resonances caused by piezoelectric electromechanical coupling can also cause
microwave loss. And, not all microwave losses take place through the electric vector of the
wave. The magnetic field vector can initiate energy transfer through magnetic resonance
effects in which the unpaired electron spins precess about internal fields.
Kenkre (1991) developed a theory based on Debye relaxation o f interstitials,
vacancies and bivacancies to describe the absorption o f microwave energy by ceramic
materials.
2.2.2. Dielectric Properties of Ceramic Materials
The the absorption of microwaves by a dielectric material depends on the m aterial’s
R ep ro d u ced with p erm ission o f th e copyright ow ner. Further reproduction prohibited w ithout perm ission.
13
complex permittivity, £* (F/m), which is composed o f a real part ( e \ dielectric constant)
and an imaginary part (e”, dielectric loss factor) by
e* = e f-
/
e" = eo ( £ r -
//
j £e ff)
where j = (-1)1/2, £o is the permittivity o f free space (£^=8.86xl0 ' 12 F/m), e’r is the relative
dielectric constant, and e”eJyis the effective relative dielectric loss factor.
W hen microwaves penetrate and propagate through a dielectric material, the internal
electric fields generated within the affected volume induce translational motions of free or
bound charges (e.g., electrons or ions) and rotate charge complexes such as dipoles. The
resistance o f these induced motions due to inertial, elastic, and frictional forces , which are
frequency dependent, causes losses, and attenuates the electric field. As a consequence of
these losses, volum etric heating results (Sutton, 1989).
The loss tangent (tan 8 ) is
commonly used to describe these losses as
//
,an 5 = 5 # = — S _
er
2jtfeoer
where a is the total effective conductivity (S/m) caused by conduction and displacement
currents a n d /is the frequency (GHz).
The power absorbed per unit volume P (W/m3) is expressed as:
P = c r |£ |2 = 2jif so e ’rtan8l£l2
where E (V/m) is the magnitude of the internal field. This equation shows that the power
absorbed varies linearly with the frequency, the relative dielectric constant and tanS, and
varies with the square of the electric field.
When the microwaves penetrate and propagate through and absorbing material, the
electric fields are attenuated. The penetration depth, at which the incident power is reduced
by one half, is described by:
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
14
D = ---------- 3A°
8.6867rtanW e^/eo
where A0
is the incident or free-space wavelength.
It is evident that for greater
wavelengths (and lower frequencies), the penetration depth is also greater. For example,
the penetration depth of water is 7.6 cm at 0.915 GHz (X0 = 32.8 cm) but 1.3 cm at 2.45
(Aq = 12.3 cm) (Sutton, 1989).
GH z
Although low frequencies result in greater
penetration depths, the heating does not necessarily increase, since the internal field E could
be low, depending on the properties of the material.
When the wavelength (frequency) is
specified, the higher the tan 8 (the more lossy the material), the smaller the penetration
depth.
The penetrating ability of microwaves in the material is also expressed by the skin
depth (SD) which is defined as the depth at which the electric field falls to 1/e or 37% of its
initial value (Newnham, 1991; Katz, 1992):
SD = l/( 7t/|icr ) 1/2
where / is frequency, p. is magnetic permeability, and a is conductivity. For copper, a =
S .S x lO ^ ^ m -1, |X=(J.o, where |i 0 is the permeability of free space, and the skin depth is
less than 1 pm at microwave frequencies. Since field penetration is proportional to <r1/2,
microwave energy heats semimetals and semiconductors better than copper (Newnham et
al., 1991).
The relative dielectric constant (e?r) and the loss tangent (tan 8 ) are the two most
widely used and measured parameters that describe the behavior of a dielectric material
under the influence of a microwave field. They both affect the power absorbed and the
half-power depth, and thus, they influence the volumetric heating behavior of a given
material. The value of efr is a measure of the polarizability of a material in an electric field,
whereas the value o f tan 8 is a measure o f the loss (or absorption) of the microwave energy
within the material.
The values of s ’r and tan 8 increase with temperature during heating. The increase in
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
15
e?r with tem perature is due to an increase in the polarizability caused by volumetric
expansion. (Ho, 1988). For example, A12 C^ has a significantly greater coefficient of
thermal expansion, which leads to a greater increase in its e’r .
In addition, the
composition and the density have a m ajor influence on the value o f € r , higher density
tends to give a greater € r . Figure 2.4. shows the influence of temperature on dielectric
constant (Sutton, 1989).
In contrast to ef r , tan
8
is far more affected by temperature. In general, it rises
slowly with increasing temperature, until a critical point is reached, beyond which it rises
very rapidly. Compositional additives and impurities usually account for the rapid rise in
tanS. Figure 2.5. shows the influence of temperature on dielectric loss tangent. It was
reported that the rapid increase in tan5 in polycrystalline ceramics is associated with the
softening of intergranular and amorphous phases, which caused an increase in the local
conductivity, a
2.2.3. Characteristics of Microwave Heating
Microwave heating is fundamentally different from conventional heating in its heating
mechanism. Due to the penetrating nature of microwaves radiation, the microwave heating
is an internal dielectric heating. In microwave heating, heat is generated by microwavematerial interaction within the material instead o f being transferred from the surrounding
heating elements as in the conventional case. Because o f the penetrating nature o f the
microwaves, the heating is a bulk (volumetric) effect. That is, the whole volume o f the
work piece within the limit of microwave penetration is heating simultaneously. W hen the
work piece is large in size, the thermal gradients of the material being microwave heated are
reversed that of conventional heating. In other words, the center of the work piece being
microwave irradiated is hotter than the surface before thermal equilibrium is achieved.
Figure 2.6. shows the heating pattern in conventional and m icrowave heating. The
penetrating nature o f the microwave energy also implies a quick response between the input
power and the temperature of the work piece thus the thermal inertia is much lower than in
R ep ro d u ced with p erm ission o f th e copyright ow ner. Further reproduction prohibited w ithout perm ission.
16
Temperature (°C)
200
12
600
“i
i
1000
1400
r
» w 11
pure
co 10 I - AIjO,
**
m
c 9
o
C
o
U
O
©
©
©
8
7 „
^ Pyrolytic Si,N4 j~ 3-15 9/cm3
6
-
.>
5
©
GC
97% pure AljOj
•“
9606 (Glass-Ceramic)
mA>c b n 0 *-45 ^ cm i
ot-pressed
4
Fused silica *
3
1000
g/cmJ
2.06 g/cm*
1.94 a/cm*
Si*N*
i 20% ((porosity) slip cast
200 0
3000
Temperature (°F)
Figure 2.4. Relative dielectric constant (8-10 GHz) versus temperature (Source: Walton).
R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.
17
Temperature (°C)
200
^
*0
1000
1400
0 .0 1 0
97% pure
AljOj
Pyrolytic SijN«
c
£
600
0.008
SiO;
c
g) 0.006
C
Z
i/i
3
9606 (G lass-ceram ic)
99 + % pure
'A I 2O a
0.004
0 .0 0 2
Pyrolytic BN
0
500
1000
1500
200 0
2500
Temperature (°F)
Figure 2.5. Loss tangent (8-10 GHz) versus temperature (Walton, 1970).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
18
C o n v e n tio n a l
Heating element
M icrow ave
Microwave port
Sample
ulatlon
Insulation
Furnace
Metal shell
Cavity
Figure 2.6. Heating pattern under conventional and microwave heating conditions.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
19
conventional processing. In other words, there is no thermal lag during microwave
heating. This makes it possible to heat the work piece at much higher heating rates (10lOOx) than conventional process. The rapid sintering leads to limited grain growth and fine
microstructure, consequently improved mechanical properties. In contrast to the heating
process, cooling after microwave processing is basically the same as in conventional case.
Since microwave heating is a dielectric heating, microwave absorbing efficiency, and
thus the heating rate, is dependent on the dielectric losses of the material. For a dielectric
material, the dielectric constant and loss tangent are both a function of temperature. The
increase of the dielectric loss tangent is usually steeper than that of dielectric constant. The
phenomenon o f thermal runaway is a typical characteristic frequently observed in
microwave heating of materials. It is characterized by a steep temperature rise in the
materials when heated in a microwave field. As the microwave power absorption is
directly proportional to the relative dielectric constant £?n and the dielectric loss tangent
tanS, an increase in these two parameters will lead to the proportional increase in the
temperature of the work piece being processed. The higher temperature further increases
the microwave absorption and, thus the net effect is an exponential increase in temperature.
This “snow-ball” effect in temperature increase is termed as “thermal runaway”. Figure
2.7. shows a temperature-time curve with thermal runaway during microwave heating of
zirconia in this study.
The shape of the heating curves with thermal runaway may vary
with the type of material (Kenkre, et al., 1991). Thermal runaway may benefit materials
processing in some cases (Fang et al. 1991), but in some cases, it may cause thermal
gradients which may lead to cracks and inhomogeneities. Therefore, precautions should be
taken to control and monitor the thermal runaway.
Research on microwave processing of materials to date has involved drying, curing,
binder burnout, calcining, sintering, melting, joining, surface modification and waste
remediation, etc.
Some o f the key characteristics of microwave processing are listed in
Table 2.1. (Sutton, 1993).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
20
1200
900
CJ
a>"
=
4->
ro
600
c_
OJ
a.
e
CD
h—
300
30
0
Time,
min
Figure 2.7. Thermal runaway during microwave heating.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 2.1. Some key characteristics and features of microwave processing (Sutton, 1993).
Characteristic
1. Penetrating radiation,
direct bulk healing
Feature
Benefits (over conventional heating)
• Materials heat internally
• Reversed thermal gradtents (AT)
• Lower surface temperatures
•
•
•
•
•
•
Potential to heat large sections uniformly
AT favors CVI; matrix infiltration
Reduced skin effect on drying
Removal of binders a gases without cracking
Improved product qualify and yields
Materials &composite synthesis
Automation, precise temp, control
Rapid response to power level; pulsed power
Challenqes
• M/W transparent mat'ls difficult to heal
• Hot spots, cracking
• Large AT in low thermal condudivily
mal'is, and nonunilorm heating
•
•
Instantaneous power/lemp. response
Low thermal m ass
•
•
•
Applicator can be remote from power
source
• Heat in d e an environment
• Heat under conlrdled vacuum or pressure and
gas composition
• Materials synthesis
• S ee differential coupfing
• Arcing, plasmas
• Require new equipt. designs special
readion vessels
•
Controlling internal temp.
2. Field cSstributions
can be controlled
• High energy concentration
• Optimize power level vs. time
• mm-waves can b e focused or
defocused, rastered a s desired
• Precise heating ol irregular sections
• Precise healing ol selected regions (brazing,
welding, plasma generation, Ib er drawing)
• Process automation, flexbitty, energy saving
• Synthesis ol mat'ls, composites, powders, coatings
• Equpment more cosily and complex
• Requires specialized equipt.
3. Dielectric losses
accelerate rapidly
above TCI|
• Very rapid healing
•
•
•
•
• Hot spots, ardng
• Nonunilorm temperature
• Control of thermal runaway
4. Differential coupling
ol materials
• Selective heating of internal or surface
phases, additives or constituents
• Healing of MIW transparent mat'ls via additives,
fugitive phases, etc.
• Hybrid heating (adive containers)
• Materials synthesis
• Selective zone heating (joining, brazing, sealing)
• Controlled chemical readions, oxidation, redudion;
use ol M/W transparent containers
• Drying, curing, annealing; matrix inlillralion
• Readions with unwanted impurities
• Contamination with insulation or
other phases
5. Self-limiting [15]
• Selective healing ceases (self­
regulating) after certain processes
have been completed
• Below critical femp., drying & curing are sell-regulating
• Completion ol certain phase changes is self-regulating
• Undesired decoupling during healing
in certain produds
• Difficult to maintain temp.
Rapid processing (2 - 1000x lactor)
Heat mal'is > 2000°C
Density materials with min. grain growth
Capable lo heat MfW transparent materials > Ta ,
22
2.2.4. Historical Review of Microwave Sintering of Materials
Research on microwave processing of ceramics materials has been carried out for 40
years since Von Hippel (1954) began the examination o f the interaction between
microwaves and oxide materials, and how the loss characteristics of the materials vary as a
function o f microwave frequency. M uch o f his theory and some data on microwave
material interactions provided a valuable background for microwave heating. Soon after,
Feiker (1959) reported that dielectric materials could be heated rapidly by microwaves at
915 MHz. Ford (1967) conducted high-temperature chemical processing via microwave
irradiation.
Among the pioneers, Tinga (1968,1970,1973) and coworkers studied the interaction
of microwave and materials, the thermal processing o f oxides and composite materials
using m icrowaves of 915 M H z and 2.45 GHz, and carried out dielectric property
measurements in the microwave field, and developed reaction cavities for a uniform
microwave heating.
It has been reported that microwave processing of ceramic materials takes place at
accelerated rates or lower temperatures than in conventional processing. For example, the
examination of the rates o f sintering for microwave and conventional heating by Janney
(1991) and coworkers reveals that the apparent activation energy for sintering of MgOdoped alumina in a 28-GHz microwave furnace was less than one third that observed for
conventional sintering in a conventional furnace (160 vs. 575 kJ/mol.). They also found
that the grain growth in dense, hot-pressed alumina is structurally similar for microwave
and conventional annealing conditions. In both cases, normal grain growth is observed
and the m icrostructural param eters such as grain shape, anisotropy, and grain size
distribution are similar and cubic grain growth kinetics are followed. However, annealing
in a 28-GHz m icrowave furnace accelerates the growth kinetics significantly: e.g.,
m icrowave grain growth kinetics at 1500°C are the same as conventional grain growth
kinetics at 1700°C and nearly two orders of magnitude higher than conventional grain
growth kinetics at 1500°C. The activation energy for grain growth was about 20% lower
for microwave annealing as compared with conventional annealing (480 vs. 590 kJ/mol.)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
23
which probably represents an acceleration of diffusional processes. They showed that a
“microwave effect” can manifest itself in a dense ceramic body and that no free pore-solid
interface is necessary.
M eek (1987) has proposed a model in which the surfaces of the powder particles and
pores are heated preferentially in the sintering compact. And Holcombe etal. (1988) carry
the idea of local preferential heating to the extreme in suggesting that the surfaces of pores
might be heated to the melting point by microwave excitation.
Johnson (1991) carried out the heat flow calculations of microwave heated grain
boundaries in ceramics and concluded that the temperature of any given grain boundary
cannot be significantly different from the local average temperature.
So that some
mechanism other than hot grain boundaries m ust be sought to explain the observed
enhanced sintering.
In contrast, Tinga’s model (1993) indicates that very large differential heating rates at
the microstructural level are physically possible, although the implications of such rates are
not at all clear.
Booske et al. (1991,1992) have proposed that the observation of enhanced diffusion
and sintering rates in microwave (versus conventional) sinterings are due to a athermal
energy (or phonon) distribution induced by the intense microwave radiation field. This is
most likely the case if the microwave radiation resonantly couples to specific elastic waves
or normal modes of oscillation in the crystal lattice. The possibilities for coupling to
normal mode oscillations appear greater in polycrystalline media (versus single crystal) due
to additional effects associated with surfaces, grain boundaries, and concentrations of point
defects. Alternatively, microscopic spatial nonuniformity in microwave heating may yield
several populations of ions, each characterized by a different “temperature”. Averaged over
a macroscopic region, this would appear as athermal energy distribution. The exact forms
of the lattice phonon and ion energy distributions during intense microwave heating of ionic
solids, and the exact m echanism s for m icrowave-to-phonon energy transfer during
microwave sintering are unknown at this time. However, it is clear that point defect
mobility (and thus sintering rates) is m ost sensitive to the high energy tail of the
distribution, while m easurem ents with standard tem perature diagnostics (such as
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
24
thermocouples) are sensitive to “bulk” features of the distribution. It was shown that a
small, 30% difference in the mean energy between the thermal and athermal distributions
leads to orders o f magnitude difference in the probability of point defect motion. This is
due to the distribution o f an excess energy in the athermal case in the high energy tail,
where the probability is most sensitively affected.
However, refined calculations (Freeman et al., 1993) indicate that the effect of
athermal distributions of lattice ion energies caused by microwave fields is too small to
explain the observed data unless the microwave strength approaches the lattice field
strength. M ost microwave sintering experimental configurations have microwave field
strengths less than or equal to 105 to 106 V/m, whereas lattice field strengths are ~10 1 1
V/m. More recently, the same researchers (Freeman et al., 1993) proposed that a quasi­
static polarization o f the lattice near a point defect as a possible mechanism that lowers the
activation energy barrier for diffusion of that defect.
The same researchers (Freeman et al., 1993) further indicated that microwave fields
do not change microscopic activation energies for ionic mobility. Nevertheless, they have
identified the existence o f a microwave effect on ion transport in crystalline solids, which
seems to occur when there is a preexisting chemical potential gradient in the crystal.
In summary, microwave enhanced sintering of various ceram ic m aterials are
experimentally identified, but the mechanisms for explanation o f the observed microwave
effect are still at a developmental stage.
2.2.5. Current Status of Microwave Sintering Technology
2.2.5.1. Microwave Applicators
Microwave processing systems consist of a microwave source, an applicator, and the
systems to control the heating. An applicator is a device (cavity) that provides a means for
applying the microwave energy from the generator to the work piece to be processed. The
principal purpose of the applicator is to focus the microwave energy to the material load,
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
25
and to provide impedance matching in order to efficiently transfer the microwaves into the
load.
Microwave cavities can be either single-mode or multimode. A multimode applicator
is a cavity, typically a enclosed metal box as home microwave oven, in which several
fundam ental standing waves or modes are superimposed to produce a standing wave
pattern. The multi-mode applicator is m ost commonly used in microwave processing. Its
main drawback is that the non-uniform electric field distribution may cause inhomogeneous
heating. By using mode stirrer or moving the work piece through the field via a turntable,
a more uniform heating pattern can be obtained. When microwaves propagate into a cavity,
they establish a standing wave pattern. By properly choosing the dimensions of the cavity,
a fundamental standing wave pattern can be produced. Such a cavity is said to be singlemoded. The single-mode applicator is used for precise and predictable control over the
fields within the work piece. Such applicators maximize the field strength at the work piece
location so that rapid and uniform heating can be achieved. However, the heating volume
is limited, and homogeneous heating is difficult to achieve with large samples. The design
of the applicator can also be changed to provide localized heating in specific regions of the
work piece, which is desired in the case of joining o f materials, drawing glass fibers, or
creating plasmas to rapidly sinter various ceramic materials.
Single and multimode cavities may both be tuned to produce a desired mode pattern.
Tuning is usually accomplished with the aid of a sliding shorting plunger, which changes
the dimensions of the cavity.
2.2.52. New Developments
Microwave processing of ceramic materials has received considerable attention over
the past several years and its unique properties has opened up some new approaches to the
fabrication of new ceramics with improved properties and/or lower cost. On the one hand,
microwave study is being extended to more and more new material systems, on the other
hand, scale-up of the microwave sintering techniques is just starting. However, due to
some inherent limitations of the microwave process, most work has been confined only to
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
26
small laboratory samples.
M icrowave hydrothermal process has been carried out by Li and Kom am eni (Li,
1993). Various fine ceramic powders, including unary, binary, and ternary oxides have
been successfully synthesized by microwave-hydrothermal processing. The experimental
results showed that the interaction o f microwaves with materials (solids, liquids, and
interface) can lead to different morphologies and interface effect between solids and
solutions. It was also shown that microwave-hydrothermal synthesis enhances the kinetics
of crystallization of ceramic powders by one or two orders of magnitude compared to
conventional-hydrothermal process.
Rapid selective annealing of Cu thin films on silicon using microwaves was recently
reported by Brain et al. (1994). Selective annealing of metallic thin films is possible in the
lim it where the film thickness is much less than the skin depth of the m aterial at
microwaves frequencies, with the additional advantage that the relatively low absorption of
silicon deters substrate heating. It was found that as a silicon wafer is heated it becomes
less microwave absorbing, independent of the doping concentration.
M icrowave-assisted firing was reported by W roe (1993). In microwave-assisted
firing the volumetric heating provided by the microwaves heats the components, while the
mere conventional radiative heating provided by gas flame or electric resistance heating
elements minimizes heat loss from the surface of the components, by providing heat to the
surface and its surrounding. This prevents the creation of the temperature profiles/gradients
associated with both conventional and microwave-only firing. Thus microwave assisted
firing leads to producing uniform microstructures and low thermal stress.
Blake, and Katz (1993) carried out microwave sintering of large alumina bodies. By
using carbon as a setter material and a microwave susceptor, and low density aluminasilicate as insulation material, hexagonal tiles weighing 420 g were sintered to relative
density of 93% by microwave processing.
Based on the characteristics of volumetric heating of the work piece, high heating
rates, less insulation, high efficiency of the heating process, possibly as high as 80-90%
(Katz, 1992), lower processing costs are anticipated for microwave microwave sintering
besides the improved product quality. The actual data for power consumption and cost for
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
27
ceramics for microwave sintering are very limited. Reported cases are by Katz and Blake
(1991), and Patterson et al. (1991) for alumina ceramics, and Das and Curlee (1987) for
common ceramics. If using fossil fuel or natural gas for conventional sintering, microwave
processing w ould not be advantageous in cost, because o f the total efficiency of the
conversion from fossil fuel to electricity then to microwaves, and finally to heat, is lower
than that directly convert fuel to heat. This comparison seems not so valuable. A better
comparison would be that between two techniques both using electricity. Compared with
conventional electric furnace sintering, microwave sintering results in an energy saving of
as much as 90% (Petterson et al., 1991). Apparently, more work should be done for an
economic evaluation in microwave processing.
As a new direction, continuous microwave sintering o f industrial sized work pieces
has been developed by Cheng (1994). In this technique, the centrosymmetric work-piece
is continuously fed in a high-power microwave cavity to subject a dynamic sintering
process through simultaneous rotation and translation movements. M ullite and alumina
rods and rollers o f <£40x2400 mm have been fabricated by the continuous microwave
sintering. High quality, short processing time, and low rejection rate are typical features of
this process.
Although the transferring of microwave sintering technique is fraught with difficulty
and commercialization o f this technology is still far off, and yet there is no sign that
microwave sintering will replace all conventional sintering techniques, however, it is
doubtless that the unique features o f microwave processing will find its applications in the
areas where microwave processing shows definite advantages over conventional processes.
It can be expected that the ongoing extensive and intensive studies on microwave
sintering will promote both the understanding of the mechanisms behind the observed
microwave enhanced sintering effects and the development of the microwave sintering
techniques.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
28
Chapter 3
EXPERIMENTAL PROCEDURES
3.1. Starting Materials
The materials used in this study include hydroxyapatite [HAp, [C a 10 (PO 4 )6 (OH)2],
mullite (3Al 2 0 3 -2Si02), alumina (A12 0 3), calcium-strontium-zirconium phosphate (CSZP,
CaQ 5 Sr 0 5Zr4 P 6 0 24), barium-zirconium phosphosilicate (BZPS, B aj
2 5 Zr4 P 5 5 Si0 5 0 24),
and partially stabilized zirconia (PSZ, Z r0 2 -Y). Among these, HAp was synthesized by
hydrolysis, precipitation, and hydrothermal methods; mullite precursors were synthesized
by sol-gel methods; and the other materials were acquired from commercial vendors. The
precursor powders were consolidated into pellets o f 0.5 or 0.25 in. diameter by uniaxial
press for sintering. Polyvinyl alcohol (PVA) solution was used as the binder in some
cases. The details of synthesis, sources, and preparation of the test specimens of various
materials are given in the respective chapters.
3.2. Experimental Setup for Sintering
The experimental setups for sintering include both microwave and conventional
furnaces for comparison purpose.
3.2.1. Microwave Furnace
M ost microwave sintering experiments were carried out in a multimode, 900 W
(magnetron output power), 2.45 GHz microwave oven (Panasonic). The microwave
cavity is about one cubic foot (14” x 14.75” x 8.25”) in volume. There is a glass-ceramic
turntable at the bottom of the cavity. The microwave port is located on a side wall of the
cavity.
To make this kitchen microwave oven suitable for high-temperature sintering
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
29
experiments, following modifications were made:
1. To control the heating rate, a voltage regulating system consisting of two variacs
was added to the power supply to the magnetron.
2. To efficiently cool the magnetron, the power supply to the fan was separated from
that to the magnetron. This makes it possible to continuously cool the magnetron even if
the power to the magnetron is off.
3. To maintain the cavity at low temperature, so that there is no excess heat
transferred from the cavity to the magnetron, a hosepipe of copper was soldered onto the
outside surface (under the outer shell) of the top of the cavity for water cooling.
4. An aluminum plate was placed on the turntable to prevent the glass-ceramic
turntable from being overheated.
5. A sintering packet was made for efficient microwave heating. The sintering packet
(Fig.3.1) consists of a crucible, zirconia cylinder (Zircar Products, Inc., Florida, NY),
m olybdenum disilicide (M oSi2) rods, and therm al insulation D uraboards™ 3000
(Carborundum Co., Niagara Falls, NY). The crucible is also made of the same board.
The Duraboard was made from a blend of Fiberfrax bulk ceramic fibers and Fibermax
polycrystalline high alumina fibers. The typical chemical composition of the board is 70%
A12 0 3 and 26% S i0 2 and 4-6% weight loss on ignition. As claimed by the manufacturer,
Duraboards™3000 offers low thermal conductivity, high temperature stability and excellent
resistance to thermal shock and chemical attack. It can withstand temperatures up to
1650°C (3000°F) for continuous use.
The zirconia cylinder vertically surrounding the samples functions both as a
microwave susceptor (microwave-absorbing material) and as a thermal insulator. As the
microwave susceptor, it absorbs and converts microwaves to heat, thus preheats the
samples if the samples do not absorb microwaves well at low temperatures. As an
insulator, it prevents heat of the samples from dissipation.
Molybdenum disilicide (M oSi2) is an excellent heating elem ent material used in
electric resistance furnace, with maximum operating temperature of about 1870°C. MoSi 2
is resistant to thermal shock and can be subjected to extremely rapid heat-cool cycling for
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
30
Ka
Figure 3.1.
Schematic display of the sintering packet in the m icrowave processing.
l.Tumtable. 2. Zirconia cylinder. 3. Samples. 4. Thermocouple. 5. Fibermax insulator.
6.
MoSi2 rods. 7. Microwave port.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
31
prolonged periods of time with no adverse effects. Also, the resistance of these high
temperature heating elements does not change with use, which allows for replacement of
individual elements in the electric furnace at any time without having to match resistance
values of new elements to the older. Although there is no previous report on the use of
MoSi2 as heating elements in microwave processing, it has been found to be an excellent
candidate materials for microwave heating. Eight molybdenum disilicide rods vertically
inserted around the zirconia cylinder can trigger microwave plasma at lower temperatures
and help homogenize temperature distribution around the samples, and lower temperature
gradient in the vertical direction inside the zirconia cylinder. The rotation of the turntable
during sintering ensures uniform heating conditions in the microwave field. The heating
rate can be controlled fairly well by an external variac voltage-regulating system (course
and fine control).
In the sintering of HAp and CSZP ceramics, a different microwave furnace (500 W,
JE43, GE) was used, in which there was not a turntable, instead, there was a mode stirrer
for homogeneous field distribution. The configuration and the principle were the same.
3.2.2. Temperature Measurement in Microwave Processing
Temperature measurement in microwave sintering heating is one area which is very
critical and matter of concern for its accuracy and reliability. This is so because microwave
enhancement in ceramics sintering is usually reported as the lowering of processing
temperatures, or at the same temperature but improved properties, while only precise
measurement can make the comparison between microwave and conventional processing
methods reliable.
Typical methods for measuring temperature in high temperature furnaces are radiation
pyrometers, thermocouples, and optical fiber probes. Fiber optics and pyrometry methods
are based on the spectra o f infrared radiation at high temperatures and are not interfered
with microwaves. However, in the use of pyrometer, the emissivity value of the material is
critical for an accurate temperature measurement, and fiber optics needs the use of a
standard material. The sources for the data of emissivity are very limited, and the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
32
emissivity of a material depends on surface roughness, density, temperature, etc., hence
pyrom etry is not a simple and direct method.
Thermocouples are widely used in
conventional high-temperature furnaces and have excellent reliability and accuracy.
However, in a microwave field, the interference of microwaves with the electromotive
force of the thermocouple makes the temperature readings random or erroneous before the
temperature rises to such a level that microwaves are efficiently absorbed by the workpiece.
When properly shielded and grounded, microwave interference can be totally prevented and
thermocouples can provide reliable temperature readings.
In this study, temperature was measured by a platinum-shielded S-type (Pt-PtlORh)
thermocouple. The shielding Pt foil on the thermocouple is properly grounded to the
m etallic wall of the microwave cavity, so that the microwave interference to the
electromotive force can be completely avoided during the sintering. Since the samples are
under consistent rotation mode, and the thermocouple is stationary, it is impractical to allow
the tip of the thermocouple in contact with the samples. To solve this problem, a small
piece of alumina or that o f the material being processed, is attached with the shielded tip of
the thermocouple, so that the temperature measured by the thermocouple is the temperature
of the material and not that of the air in the vicinity of the specimen. Furthermore, since the
tip of the thermocouple is very close to the surface of sample, the measured temperature is
identical or very close to that of the specimen.
3.2.3. Conventional Furnace
An electric furnace, exclusively devoted for this study, was built with molybdenum
disilicide as heating elements. The furnace is capable of high operating temperature (up to
1650°C), high heating rate, and low thermal inertia. The heating rates were comparable to
that in microwave sintering. The furnace has the capacity of providing heating rates up to
300°C per min. below 1000°C, and can attain a temperature of 1500°C in 13 min. The
temperature in the conventional sintering was measured with the same S-type thermocouple
as in the microwave sintering, but without platinum shielding. The samples were placed in
a platinum crucible which was in contact with the tip o f the thermocouple. The heating
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
33
rates and soaking temperatures were also controlled by regulating the power input with a
variac.
In the earlier experiments of this study on HAp, conventional sintering was carried
out in a 5 kW programmable box furnace (Model 51524, Lindberg, Watertown, WI).
Although temperature as high as 1400°C could be achieved, the heating speed in this
furnace was lower than in the microwave processing. In the case this furnace was used, it
will be specified.
3.3. Sintering Process
M ulti-pellet runs were conducted in both microwave and conventional sintering
experiments. In microwave sintering, the pellets, usually in two (0.25 inch pellets) to three
(0.5 inch pellets) layers, were placed at the center of the sintering packet. Each layer has 6 7 pellets of the 0.25 inch diameter, but only one pellet of the 0.5 diameter.
Heating rate in both microwave and conventional sintering was about 100°C per
minute. Since heating and cooling rates are comparable, for the duplicate sample of the
same material, when sintering time and temperature are identical in both case, the difference
in the sintered samples produced by two methods should be attributed to the microwave
effect.
Because the sintering characteristics of different materials are different, the sintering
temperature and heating schedule may vary from material to material. The details of the
sintering conditions for each material will be given in the respective section/chapter.
3.4. Characterization
A variety o f characterization tools were employed for the characterization o f the
starting materials and the sintered specimens. The main techniques are described as the
following. For those that are routinely used, the description and principle of the techniques
are omitted for clarity.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
34
3.4.1. Powder Characterizations
Specific surface area, particle size, and morphology o f the starting powders were
determined by standard techniques. The specific surface area of the starting materials
before com paction was m easured by BET m ethod using nitrogen absorption on an
Autosorb-1 Automatic Volumetric Sorption Analyzer (Quanta Chrome, Syosset, NY).
Particle size and m orphology o f the powders were exam ined by either scanning or
transmission electronic microscopy.
3.4.2. Thermogravimetric Analysis
Thermogravimetric analysis (TGA) was carried out on the synthetic HAp powders
and the aluminosilicate gels to study the weight loss of these materials as a function of
temperature (thermogram profile). Any reaction, decomposition, or phase transformation
involving weight change will be recorded as a function of tem perature.
The TGA
experiments were performed on a Perkin-Elmer DELTA/TGA 7 system. The system is
automatically controlled by a computer program. The heating rate for all the materials used
in this study was kept at 10°C per minute.
3.4.3. Differential Thermal Analysis
Differential thermal analysis (DTA) was carried out on the aluminosilicate (mullite)
gels to determine the mullite transformation (crystallization) temperatures. The DTA used
in this study was Perkin-Elmer DTA, which is also computer programmed. The heating
rate used was usually 10°C per minute. The maximum operation temperature is 1500°C.
The phase transformation process is accompanied by heat absorption or heat evolution,
corresponding to endothermal or exothermal peaks on the thermograms. For example, the
process of crystallization from amorphous state is an exothermal process, while the process
o f melting is an endothermal process. From the peaks on the thermograms, the phase
transformation temperatures can be identified.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
35
3.4.4. Dilatometry
The thermal mechanical analysis (TMA) was carried out on selected specimens to
study the dimensional change during heating. The TMA setup is a computer-driven system
(Innovative Thermal Systems, NY) in which desired heating and cooling rates can be
programmed. The maximum temperature is 1500°C. The heating rate of 5°C/min. was
used in this study. The dimensional change as the percentage o f the original size is
automatically recorded as a function of temperature, or alternatively, as a function of time.
The range of sintering temperature can be conveniently determined from the TM A curve.
3.4.5. Powder X-Ray Diffraction
X-ray diffraction was carried on the powdered materials for phase identification.
The equipment used in this study is a computerized Scintag Diffractometer, Model V
(Scintag Inc., Sunnyvale, CA). For qualitative analysis, the samples were ground to fine
powders and placed on glass slides with acetone. For quantitative analysis, the samples
were pressed in the cavity of the glass slide. Internal reference material such as quartz or
corundum was used in the quantitative analysis. The continuous scan at 2° (two theta)/min.
was used for both qualitative and quantitative analyses.
3.4.6. Lattice Parameter Refinement
In the study of the thermal stability of HAp, crystal lattice parameter refinement was
perform ed by XRD analysis on the Scintag D iffractom eter in conjunction with a
computerized calculation program developed at the Intercollege M aterials Research
Laboratory at The Pennsylvania State University, University Park, PA.
3.4.7. Density Measurement
Since the specimens were in simple circular shape, green and sintered density of the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
36
pellets were m easured by w eight and geometrical m easurem ents.
The weight was
measured by an electronic balance with precision of ±0.0001 gram. The dimension was
measured to an accuracy of ±0.0001 inch with a micrometer. In a few cases when the
samples are irregular in shape, the density was measured using the wax-coated samples by
Archimedes principle using kerosene or water as immersing liquid.
3.4.8. Mercury Intrusion Porosimetry
The porosities o f selected sintered porous HAp ceramic samples were determined by
a Quantachrome Scanning M ercury Porosimeter. The evacuated porous material to be
detected is immersed in mercury. The mercury is then forced into the pores of the material
under pressure. The pore size is inversely proportional to the pressure applied. Under the
operation pressures up to 386.2 M Pa (56000 psi), the corresponding pore sizes are in the
range from 7.5 (im to 1.8 nm. During scanning, the porosity, or volume of the intruded
mercury, is recorded as a function of applied pressure.
3.4.9. Diametral Tensile Strength Measurement
The mechanical strength of sintered regular and porous HAp ceramics were measured
by diam etral m ethod (Rudanick et al., 1963).
The pellets used for the strength
measurement were sintered samples with green diameter of one half inch in diameter.
Ordinary blotter paper was used as a padding material between the sample and each of the
platens. Except where otherwise indicated, the crosshead speed was set at 0.002 inch/min.
The maximum applied load at failure of the sample was recorded on a X-Y chart recorder.
The diametral tensile strength (a), in MPa, was calculated by the following formula:
o = 2 L 1%Dt
where L is the applied load (N), D is the sample diameter (m), and t is the thickness of the
pellet (m).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
37
3.4.10. Fracture Toughness Measurements
Toughness is a m easure of the resistance o f a m aterial to fracture.
Fracture
toughness, also known as critical stress intensity factor, of a brittle material was calculated
from the formula below,
K c = JcPC3/2
where % is a calibration factor, for Vickers indentation, % = 0.016CE/H)1/2, P is the
indentation load, and c is the size of the crack causing failure (Fig. 3.2.), E is Young’s
modulus, and H is the hardness.
The ratio E/H was determined on Knoop indentation using the formula (Marshall, et
al., 1982, 1988; Chantikul et a l ., 1981):
E/H = a (b/a - b’/a ) " 1
where a is a gradient (= 0.45) between indentation dimensions and the ratio E/H, a and b
are the length of the major and minor axes of the indentation diagonal, respectively, in the
loaded state, b ’ is the distortion length o f the residual impression of m inor axis after
releasing the load.
The indentation was conducted on the polished mirror planes of the selected sintered
pellets to determine K jo
3.4.11. Scanning Electron Microscopy
The morphology of the starting materials, the microstructures of the as-sintered
surfaces as well as the fracture surfaces o f the sintered samples were observed with a
scanning electron microscope (SEM, ISI-DS 130, Akashi Beam Technology Corp.). The
average grain sizes of HAp ceramics were also measured from the scanning electron
micrographs. Before the microscope observation, samples were dried and coated with gold
by sputtering method to avoid electric charging.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
38
cracks
i
2c
Vickers indentation
cracks
T
I
JL
■H
Knoop indentation
Figure 3.2. Schematic showing of Vickers and Knoop indentations.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
39
3.4.12. Transmission Electron Microscopy
D ue to the lim itation in resolution, the m orphology and particle size o f the
hydrothermally synthesized nanocrystalline HAp could not be resolved by SEM, so that
transmission electron microscopy (TEM) was performed on a Philips EM420 transmission
electron microscope at an acceleration voltage of 120 kV. The high acceleration voltage
substantially increased the resolution o f the image, which made the study o f the unique
morphology o f the hydrothermally synthesized HAp successful.
3.4.13. Infrared Spectrometry
To detect the existence o f the functional ions, especially hydroxyls, in the synthetic
HAp samples, infrared (IR) spectrometry was carried out on the samples in KBr pellets in a
Perkin-Elmer 283-B dipersive infrared spectrometer. The IR absorption o f the selected
specimens was measured in the wave-number range from 200 to 4000 cm"1.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
40
Chapter 4
MICROWAVE SINTERING OF HYDROXYAPATTTE
4.1. Introduction
4.1.1. General
Hydroxyapatite (also known as hydroxylapatite, HAp) is one of the minerals in the
apatite [Ca 5 (P0 4 ) 3 (F, Cl, OH)] family. The name “apatite” is derived from a Greek apatao,
deceive, since this mineral was long confused with other minerals (Milovsky, 1982).
Apatite represents the most abundant o f the naturally occurring phosphatic minerals
(McConnell, 1973), serving as the source for development of many phosphoms containing
compounds and the production o f phosphatic fertilizers. During the past few decades, the
use of apatite crystalline phases for fluorescent lamp phosphors (Johnson, 1962) has been
a major development, and other solid state applications have been explored, such as use for
single-crystal laser host materials. The third and perhaps the m ost important field for
human kind are the biological apatites which in polycrystalline or semicrystalline form
constitute the major inorganic matter of bones and teeth o f vertebrates.
The above define principally the family o f phosphatic minerals and related synthetic
compounds. In addition, apatite also is used to describe a structure type, the specific
crystal structure having the space group symmetry P63lm possessed by the compound
Ca 10 (PO4 )6F 2 , fluorapatite, or slight distortions thereof. Nearly half the periodic table may
be substituted into the position o f one of the four ions Ca2+, P 5+, 0 2‘, or F", singly or in
various combinations to yield m odified compounds or entirely different isostructural
chemical species (Roy, 1978).
The stoichiometric chemical formula o f HAp is C a 10 (PO4 )6 (OH)2. The crystal
structure of HAp belongs to hexagonal system; theoretical density is 3.16 g/cm 3; M ohr’s
hardness is 5. HAp crystals occur in variety o f morphologies: prismatic, tabular, granular,
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
41
reniform, stalactitic, fibrous, etc. HAp is brittle, fracture is uneven to conchoidal.
4.1.2. Application of HAp as a Biomedical Material
In all the phosphate com pounds, HAp is the only one that has excellent
biocompatibility without biodegradation. Actually, HAp shows the best biocompatibility
over any other biomaterials used for implants so far. This brings about a great potential for
HAp in the applications in dental prostheses and cements, and orthopedic surgery, and so
on. A few examples are given below.
a. Combinations of calcium phosphates as remineralizer and dental cements
The solubility study in the system C aO -H gP C ^-^O at ambient temperature (Fig.
4.1) shows that in the region which is not too acidic nor too basic, HAp is the least soluble
in all the listed phosphates. In other words, a solution which is saturated with respect to
the calcium phosphates represented by the isotherms over that of HAp is supersaturated
with respect to HAp already. Any intercept in the diagram is a singular point at which two
calcium phosphates are in equilibrium at the corresponding pH. When the solids of the two
compounds exist in the solution in excess, the pH will rem ain stable and HAp will
precipitate from the solution with continuous dissolution o f the two reacting salts. If the
pH drifts away from the value of intercept, say, the solution becomes more acidic, the salt
which is more basic will dissolve and drag the pH o f the solution back, and vice versa. So
that HAp can keep crystallizing at a stable pH regime.
Based on these principles, a series of dental remineralizers and dental cements were
invented by Brown and Chow (1986) by various combinations from the phosphate
compounds. The main advantages o f these materials include long active time, stable pH,
adjustable remineralization rate, and suitable for dental cements.
b. HAp ceramics used for implantation
Aging, cosmetic corrections, injuries due to sports, traffic and other accidents, and
medically-related surgery, etc., are the main reasons for repair and replacem ent o f body
parts. Compounded by increased life expectancy, frequent traveling and complex life
styles, demand for dental and orthopedic surgical procedures has risen steadily in the recent
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
• moles
per /iter
42
7or<3/ Calcium
T \
CaHP04 2HjO
CaHPQ,
Solubility Isotheim s at 25° C
pH
Figure 4.1. Solubility of calcium phosphates at 25‘C. (Source: Brown et al.)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
43
decades. The materials used for tissue reconstruction have been changing with the
development of science and technology. Historically, ivory, gold, brass, silver, platinum,
tantalum, stainless steel, polymethylmethacrylate, cobalt alloy, titanium, alumina ceramics,
graphite, tricalcium phosphates, and HAp ceramics, and so on, have been used for tissue
reconstruction. The use of phosphate ceramics for implants was suggested in the 1970’s.
Since then a lot of research work has been done in this area. Some cases o f using HAp
ceramics for loaded (e.g. tooth implants) and unloaded (e.g. middle ear canal wall
prostheses) implantation have been reported. In most cases, HAp was used only for some
so-called “small” surgeries, such as oral surgery, ear, nose, and throat surgeries, where the
implants are not subjected to mechanical loads other than compressive forces, because of
the brittleness o f HAp ceramics. Before the mechanical property, especially the toughness
of the HAp ceramics is solved, some alternative approaches were taken for applications.
HAp coating on certain metallic implanting materials is a way to combine the excellent
biocompatibility and the excellent mechanical properties of the metals, thus load-bearing
implants can be made. It was found that after implantation, a dense HAp surface will lose
up to 15 pm thickness due to dissolution processes, so that a coating should have a
minimal thickness sufficiently exceeding this lower limit. On the other hand, the thicker the
coating, the more it will behave like a brittle ceramic. Therefore, a coating with a thickness
of 30-50 pm is thought to be suitable. HAp coating can be obtained electrolytically, but the
coating is loose. De Groot reported the spray technique for HAp coatings for implants in
surgery. Flame spraying or plasma spraying technique has been used. The fatigue tests
and tensile bond testing as compared with uncoated or titanium coated substrates for two
different Ti alloys shows that the HAp coated surfaces are at least equal to the other
surfaces, both in fatigue and tensile bonding. It is therefore concluded that HAp coatings,
as obtained by plasma-spraying on titanium alloys as substrates, are suitable for loadbearing high strength bioactive implants (de Groot, 1987).
Hydroxyapatite, with the excellent biocompatibility, is a very new and promising
candidate for hard tissue implantation in biomedical field. Some new dental cements with
HAp as hydration product, has been patented (Brown and Chow, 1985, 1986). HAp
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
44
ceramics are expected to be used as implantation. However, a lot of problems remain to be
solved before their wide practical uses, especially, the m echanical properties and
biodegradation.
On the other hand, in order for the pure HAp ceramic implants to be practically
useful, the mechanical strengths should be comparable to those of natural bones, i.e.,
compressive strength greater than or equal to 4000 kg/cm 2 (~400 MPa), bending strength
greater than or equal to 1500 kg/cm 2 (-150 MPa). Because of the generally relation
between porosity and strength, the m ost promising approach to achieve these is to make
HAp into dense ceramics or ceramic composites.
In addition, the durability of HAp ceramics must be ensured. It is known that at high
temperature, HAp may decompose into (3-triclalcium phosphate (P-TCP) or a-tricalcium
phosphate (a-T C P ), depending on the heating temperature and duration.
W hen the
temperature is higher than 1150°C which is the transformation temperature between a - and
P-TCP, a-T C P will be obtained. This decomposition causes a few problems. First, the
decomposition may result in cracks in the sintered body during sintering process due to the
volum e change.
Second, if a -T C P which is reactive with water, is produced,
biodegradation will be induced and thus result in the damage of the ceramic body.
Also, it is noticed that the decomposition of HAp may take place at high temperatures
partially or completely, depending on the stoichiometry of the sample.
So that it is
important to investigate the factors affecting the thermal stability of HAp and offer ways to
enhance its thermal stability.
4.2.
Synthesis and Thermal Stability of HAp
4.2.1. Introduction
Synthetic hydroxy apatite (H A p), w ith a stoichiom etric com position of
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
45
Ca 10 (PO4 ) 6 (OH)2, has excellent biocompatibility with human tooth and bone, and thus
making it very attractive for biomedical applications. The studies o f HAp cements (Brown
& Chow, 1985; Brown & Chow, 1986; Brown & Fulmer 1991), ceramics (Heise, et al.,
1990; Roy, 1975; Fang et al. 1991; Fang et al. 1992; Agrawal et al. 1992), and composites
(Ioku et al. 1989; W u & Yeh, 1988; Yoshimura et al., 1989; Fang et al. 1993), for
biomedical applications, have been well documented in the recent years (Ducheyne &
Lemons, 1988). As a biomedical material, HAp is mainly used in its ceramic forms
fabricated by sintering the compacted powder of synthetic HAp at temperatures between
1000 and 1350 °C (Willmann, 1990). The standards for HAp used for bone replacement
(ASTM F 1185-88, 1988; Jarcho, 1976) specify that the material must be in the ceramic
state and exclude non-ceramic hydroxyapatite since non-ceramic HAp does not exhibit the
biological/medical properties as ceramic HAp does. Because of this, high thermal stability
o f starting HAp powder is very important for fabricating good HAp ceramics.
HAp can be prepared by various synthesis methods, including solid-state reaction
(dry process) and wet-chemical methods (Kanazawa, 1989; Jarcho et al., 1976; Monma &
Kamiya, 1987; Monma, et al. 1988; Monma et al. 1981; Hattori et al., 1988). In general,
the wet processes which have been widely used to provide finer, more homogeneous and
more reactive powders than in dry methods. Decomposition o f HAp at high temperatures
is frequently encountered during the fabrication of HAp ceramics. This inhibits the
sinterability of the apatite ceramics and transforms HAp partially or completely into a
mixture of tetracalcium phosphate (TetCP), Ca4 (P0 4 )2 0 , and tricalcium phosphate (TCP),
Ca3 (P 0 4)2. TetCP and the a-form of TCP readily react with water. If such reactions occur
in the bioceramics under the physiological conditions, the mechanical integrity of the
ceramics would be deteriorated. To obtain ceramics of phase pure HAp, the decomposition
o f HAp during ceramic fabrication should be prevented.
The dehydroxylation and
decomposition o f HAp is a process accompanied by the release of hydroxyls or molecular
water from HAp, therefore the partial pressure o f water vapor is an important factor
affecting the thermal stability o f HAp. Riboud (1968) showed the dependence of the
decomposition temperature o f HAp on the water vapor partial pressure (Fig. 4.2.). Thus it
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
46
3CaOPoOc + 4CaO'P9Oi
Temperature,
1500
1450
1400
Hydroxyapatite
1350
1300
100
P m o nimHg
Figure 4.2 Influence of moisture on the decomposition of HAp. (Source: Riboud)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Al
is possible that the dehydroxlation and decomposition o f HAp can be suppressed by
sintering it in the atmosphere o f higher partial pressure o f water vapor. This has been
demonstrated by Wang & Chaki (1994) recently. The objective here is to study various
aspects related to the thermal stability (in air) o f synthetic HAp powders prepared by wetchemical methods, and investigate the influence o f synthesis conditions on the thermal
stability of HAp, at temperatures above 700°C.
4.2.2. Experimental Procedures
4.2.2.1. HAp Synthesis
A. By hydrolysis of a-T C P [a-C a 3 (P 04)2]
First, a-T C P was synthesized by a solid-state reaction method in the following way.
Stoichiometric amounts o f fine powders o f C a C 0 3 and N H 4 H 2 PO 4 (both were reagent
grade) were hand-mixed in acetone to obtain a homogeneous mixture. This mixture was
air-dried and then heated at 300 °C for 16 hours to remove NH 3 and H 2 0 , and then at 900
°C for 24 hours to remove C 0 2- The calcined product was once again ground to improve
homogeneity, and finally fired at 1200 °C for 16 hours to form a-TCP.
50 grams o f a-T C P powder (-325 mesh) in 2000 ml deionized water was used for
hydrolysis at 60 °C to form HAp. The pH of the system was adjusted in the range of 7.68.5 with NH 4 OH and maintained relatively unchanged during the hydrolysis by using more
NH 4 OH as desired.
The suspension was subjected to continuous stirring condition
throughout the entire hydrolysis process which lasted for about two hours. The product
was then repeatedly washed three times with deionized water and dried at 90 °C in a
ventilated oven.
Synthesis o f HAp was also tried in the early stage by the hydration o f a-TCP. In this
method, pellets o f 0.5 inch diam eter were pressed with the fine a-T C P pow der as
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
48
synthesized above. The pellets were then cured in deionized water at pH
8
at 38°C for the
tim e periods up to 28 days. Phase and morphology studies were carried out on the
resulting materials.
B. By hydrolysis of brushite plus ripening
Brushite ( C a H P O ^ ^ O ) , also called dibasic calcium phosphate dihydrates (DCPD)
(J.T. Baker Chemical Co., Philipsburg, NJ) was used as a precursor to prepare HAp by
hydrolysis under the same conditions as employed for the a-T C P hydrolysis mentioned
above. The powder thus obtained was washed with deionized water and dried, and then
ripened. The ripening treatment was conducted under varying conditions as described
below:
a. Adding 20% (by weight) calcium chloride dihydrate (CCD), CaCl 2 , 2 H 2 0 , under
the same conditions as in the hydrolysis. The ripening was carried out three times
repeatedly. The product was washed with deionized water and dried at 90 °C after each
ripening.
b. Instead of the above three-step treatment, ripening was alternatively carried out at
75±5 °C using 60% CCD, with the initial pH at 8.87 and 11.00, respectively. This
treatment lasted 24 h. The ripening of 20% CCD was also carried out at the same pH for
70 min. for comparison. After the ripening treatment, the powders were washed with
deionized water and dried at 100 °C. This strengthened (high CCD content, high pH, and
long treating time) one-step ripening will be referred as “SOR” hereafter.
C. By precipitation
A precipitate was prepared by mixing stoichiometric amounts (Ca/P Mol. ratio =1.67)
of Ca(N 0 3 )2 *4 H 2 0 and NH 4 H 2 P 0 4 solutions (both o f 0.5 M) at room temperature and pH
11 (adjusted w ith NH 4 O H ).
NH 4 H 2 P 0 4
solution was slow ly added to the
C a(N 0 3 )2 ,4H 20 solution over an hour, and the mixture was subjected to constant stirring
for 24 hours. The resultant white precipitate was centrifuged to a slurry. The slurry thus
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
49
obtained was then dried at 60 °C.
D. By hydrothermal method
A portion of the precipitate obtained in (C) above, was hydrothermally treated at 200
°C in a Parr bomb under the vapor pressure of about 1.5 MPa for 24 h. The product
obtained by the hydrothermal treatment was washed with deionized water and dried at
110
°C for further experiments.
4.2.2.2. Characterization
The morphology of the HAp powders prepared by various methods was examined by
electron microscopy. The phase composition was determined by powder X-ray diffraction
(XRD) using a Scintag diffractometer (Scintag Inc., Sunnyvale, CA) and CuK a radiation.
Lattice parameter refinement was conducted by XRD in conjunction with a computerized
calculation program. Thermogravimetric analysis (TGA) was carried out on HAp powders
at a heating rate of 10 °C per min. The chemical analysis was performed on selected HAp
samples using spectrochemical methods.
4.2.2.3. Thermal Stability
The thermal stability of the HAp samples prepared in Sec. 4.2.2.1. was investigated
by XRD analysis of the air-quenched samples after isothermal heating. The samples were
heated at various temperatures in air with 50% relative humidity. The firing time for all
samples at all temperatures was 1 h. The heated samples were ground to fine powders and
spread on the slides with acetone for qualitative XRD analysis. The thermal stability was
characterized by the decomposition temperature of the specific HAp sample. Since the
decomposition of HAp is always accompanied by the appearance of TCP (either (3- or
De­
form), the minimum temperature at which the HAp sample was heated and TCP was
detected by XRD from the sample was recorded as the decomposition temperature of that
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
50
sample. A high decomposition temperature of the HAp implies good thermal stability, and
low decomposition temperature, poor thermal stability. The appearance of diffraction peak
at d=2.905A (20=30.8°) was used as the evidence of a-T C P and ^=2.88A (20=31°) for (3TCP.
4.2.3.
Results and Discussion
4.2.3.1.
Morphology
Figure 4.3 shows the morphologies of the as-synthesized HAp powders as seen
under SEM or TEM. As can be seen, hydrolysis o f a-T C P produces highly crystalline
HAp whiskers (Fig.4 3A). Typical grain size of these single crystals was about 0.2 x 5
(im. HAp obtained from the hydrolysis of DCPD was a mixture o f finer grains and some
non-uniform needle-shaped crystallites (Fig.4.3B). The morphology of the HAp from
precipitation was agglomerates of tiny particles (Fig.4.3C).
The hydrothermal method
produces very fine HAp powder. The single crystal particles are uniform in shape, with an
average particle size of 0.025 x 0.1 |J.m (Fig.4.3D). It is obvious from this study that the
method o f preparation and the nature of precursors strongly influences the morphology of
HAp powders.
It was also noticed that in vacuum, the electron bombardment (at 120 kV accelerating
voltage) made the hydrothermally synthesized HAp particles decompose when the same
observed area was irradiated continuously for more than five minutes. Figure 4.4 shows
the electron-beam-damaged HAp crystals during a few minutes delayed TEM observation.
The hydration of a-T C P at 25°C for 7 days at pH
8
produced HAp crystals o f stalactitic
morphology (Fig. 4.5.). XRD analysis indicated that most of the a-T C P had not hydrated
at this stage, so that the pure HAp was not obtained by the hydration of a-TCP.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
51
Figure 4.3. M orphology o f HAp synthesized by (a) hydrolysis o f a-TCP, (b) hydrolysis
of brushite (DCPD) plus ripening treatm ent, (c) precipitation, and (d) hydrothermal
method.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
52
! 0.1 |Jim
Figure 4.4. Micrographs showing (a) the undecomposed crystals and (b) decomposed
crystals of HAp by electron beam bombardment during TEM observation at 120 kV. The
HAp powder was hydrothermally synthesized at 200*C and 1.5 MPa.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
53
Figure 4.5. M orphology o f HAp produced by the hydration o f a-C a 3 P 0
4
at 25°C for 7
days.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
54
4.2.3.2. Crystallinity
The crystallinity of the synthetic HAp can be evaluated from either electron
micrographs or and/or X-ray diffraction patterns. An XRD pattern containing strong and
sharp peaks corresponds to high crystallinity, and that o f broad and low intensity
diffraction peaks corresponds to poor crystallinity.
Figure 4.6 indicates that the
crystallinity of the as-synthesized HAp prepared by the hydrolysis of a-T C P was very
high; that from hydrothermal method was high; that from DCPD was poor; and HAp
prepared by precipitation method was almost X-ray amorphous (Fig.4.6A). Comparison of
the crystallinity of the precipitate and the hydrothermally synthesized HAp samples
indicated that the amorphous precipitate had undergone crystallization under the
hydrothermal conditions. This made the hydrothermally synthesized HAp different from
that directly from the the precipitation. It was also noticed that ripening treatment did not
significantly change the crystallinity of the product. Heating did promote the crystallinity
of the HAp. For example, crystallinity of the precipitated HAp increased with the firing
temperature (Fig. 4.7.). Similar effect was also observed in the HAp prepared by the
hydrolysis of DCPD plus ripening.
4.2.3.3. Stoichiometry
The Ca/P m ole ratios o f the HAp samples determ ined by atomic absorption
spectroscopy technique are presented in Table 4.1. According to the chemical formula,
stoichiometric HAp should have Ca/P molar ratio of 1.67, therefore the HAp with Ca/P
lower than 1.67 is characterized as calcium deficient, or nonstoichiometric. Although
nonstoichiometry of HAp may also include hydroxyl-deficiency, the latter will not be
considered in this study. It was noticed that the HAp samples prepared by hydrolysis of a TCP and DCPD (unripened) were both calcium -deficient, while the samples from
precipitation and hydrothermal treatment were stoichiometric. The ripening treatment
increased the Ca/P ratio of the treated sample from 1.52 to 1.68, thus stoichiometric HAp
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.6. XRD patterns of the as-synthesized HAp powders prepared by (a) hydrolysis
of C aH P 0 4 *2H2 0 , (b) hydrolysis o f C aH P 0 4 2H20 plus triple ripening treatment, (c)
precipitation, (d) hydrothermal method, and (e) hydrolysis o f a-C a 3 P 0 4.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
56
a s - s y n th e s iz e d
T w o th eta , d e g re e
Figure 4.7. XRD patterns showing that the crystallinity o f the precipitated HAp increased
with heating temperature. All samples were heated for 1 h. The marked (•) peaks belong
to TCP, and all unmarked peaks belong to HAp.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
57
was obtained through ripening.
In order to examine whether the material obtained after repeated ripening with calcium
chloride was chloroapatite Ca5(P 04)3Cl or HAp, special chemical analysis of the powders
was carried out by atomic absorption spectroscopic technique on the ripened samples,
using an unripened sample as reference. The results are given in Table 4.2.
It should be noted that N 0 3' and S 0 42' sources were never used nor introduced
during the synthesis. Thus the fact that the amounts of C l' were at the same level as that of
N 0 3' and S 0 42', could be considered either as the impurities present in the material, or as
the signal of “instrument noise”. The exceptionally high count of Cl" in sample “C” might
be due to incomplete washing after ripening, otherwise the C l' content of sample “D” ,
which was ripened one more time than sample “C”, would be even higher. A simple
calculation shows that if 1% O H ' in the HAp is substituted by Cl", the content o f C l' would
be about 700 ppm. If complete substitution occurred, the content of Cl" should be
6 . 8 %,
or 68000 ppm. Furthermore, the incorporation of C l' in HAp would make O H ' stretching
band in IR absorption pattern shift from 3573 to 3495 cm '^ M o n m a et al., 1981), but the
IR spectra of the samples ripened with CaCl 2 showed O H ' stretching at 3560 cm'-*- which
was very close to 3573 cm '
1
(Fig. 4.8.). Apparently the substitution of Cl" for O H ' to
form chloroapatite did not occur in the ripening process under the current processing
conditions.
4.2.3.4. TGA Results
It was reported (Yamashita & Kanazawa, 1989) that nonstoichiom etric HAp
decomposes via dehydration according to the following reactions:
<700 °C
Caio-x(HP 0 4 )x(p 0 4 )6 _x(OH)2 .xnH 2 0 ------ >C a 10.x(P2 O7 )x(PO4 )6 .2 x(OH) 2 +nH 2 O(g)
(a)
700 - 800 °C
---------------> (l-x)C a 10 (PO4 )6 (OH)2 +3Ca 3 (PO4 )2 +xH2O (g)
(b)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
58
Table 4.1 Ca/P ratios of HAp samples determined by atomic absorption spectroscopy.
Method
Ca/P
Synthesis conditions
Hydrolysis of a-T C P
1.65
60 °C, 2 h, pH 7.6 - 8.5
Hydrolysis of DCPD
1.52
As hydrolyzed, 60°C, 2 h, pH 7.6 - 8.5
Hydrolysis of DCPD
1 .6 8
Ripened once, 60°C, 2 h, 20% CCD
Precipitation
1.67
25°C, 24 h, pH 11
Hydrothermal
1.67
200°C, 1.5 MPa, 12 h, pH 11
Table 4.2. Spectrochemical analysis of chloride ions in HAp samples (in ppm).
cr
n o 3-
S 0 42’
A
80
90
110
B
130
70
40
Ripened once
C
460
200
140
Ripened twice
D
130
70
50
Ripened thrice
Sample
Remarks
Before ripening
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
59
The resultants after therm al treatm ent are stoichiom etric HAp and TCP.
At
temperatures higher than 900 °C, further partial dehydration of HAp may take place as
follows.
>900 °C
Ca 10 (PO4 )6 (OH ) 2 ----------- > C a 10 (PO4 )6 (OH)2 _2 yOy + yH20 (g)
(c)
According to above reactions one would observe a gradual weight loss before 700 °C,
a sharp loss in 700-800 °C region, and a slight and gradual loss above 900 °C in the TGA
thermogram of a nonstoichiometric HAp sample. There should not be any sharp loss in
w eight for stoichiom etric HAp in its TG A curve. Thus stoichiom etric and
nonstoichiometric HAp samples can be easily distinguished by TGA.
The TGA thermograms of HAp prepared by different methods are shown in Fig. 4.9.
Among these only two samples, i.e., HAp prepared by the hydrolysis of a-T C P and
DCPD (without ripening) showed sharp deflection (Fig. 4.9. A, E) between 700-800 °C in
the thermograms, indicating they were both nonstoichiometric. The rest of the samples did
not show such a sharp loss, and therefore they were considered to be stoichiometric (Fig.
4.9.B, C, D, F, G). The difference in the wt. loss below 300 °C was due to the degree of
drying before the TGA experiment. The substantial wt. loss o f the precipitated HAp below
200 °C indicated that this sample contained large amount of entrapped water. The DCPDderived samples also showed moderate weight loss, at low temperature range. The HAp
from a-T C P hydrolysis showed the least weight loss in the low temperature range, because
this sample was composed of relatively large crystals and undergone near complete drying
prior to TGA experiment.
The whiskered HAp had good crystallinity but still showed apparent wt. loss between
700 and 800 °C (Fig. 4.9., A) and is believed to be nonstoichiometric. This conclusion is
in good agreement with what reported by Monma et al. (1981) These results indicate that
good crystallinity does not necessarily imply good stoichiometry, and vice versa. The
precipitated HAp had poor crystallinity (broad XRD peaks, Fig. 4.6C) but did not show
any sharp weight loss in the 700-800 °C region in the TGA curve (Fig. 4.9.F).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
60
co|-
4000
3500
3000
200 0
1800
1600
1400
Wave number, cm '
1200
1000
800
T ift
JT ft
7T5*o
1
Figure 4.8. (a) IR spectra o f HAp prepared by (1) hydrolysis of DCPD, (2-4) hydrolysis
o f DCPD followed by ripening treatment for 1,2, and 3 times with CaCl2, (5) hydrolysis
o f a-TC P, and (6 ) precipitation.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
61
48
•8
■uS
H
u
■S
I•C
.o'
•S
0)
£
c
C /3
<o
o
fi
o
<D
a*
C /3
g
00
Tf
B
s
W)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
62
4.2.3.5. Lattice Parameters
The lattice parameters of the HAp samples determined by XRD are listed in Table 4.3.
The reported values from the JCPDS file (9-432) was considered as references for
stoichiometric HAp. Compared with the as-synthesized HAp by DCPD hydrolysis (sample
A) it is seen that ripening reduced (sample C) both a and c slightly, as the Ca/P ratio was
increased (Table 4.3.). Compared with the values of the precipitate (sample D), it is
obvious that hydrothermal treatment reduced both a and c through crystallization thus
making the resultant HAp denser in crystal structure.
4.2.3.6. Thermal Stability
The thermal stability is discussed with respect to the decomposition o f the specific HAp
sample. The precipitated HAp, which was chemically stoichiometric as observed in the
previous section, was found to be stable up to about 900 °C. The highest thermal stability
up to 1370 °C was found in the hydrothermally synthesized HAp (Fig. 4.10.). It is evident
that the crystallization of the precipitate under the hydrothermal conditions is a very
effective way to drive the amorphous precipitate of exact stoichiometry towards the highly
thermally stable crystalline HAp. Although crystallization also occurred when heating the
precipitate in air, the pressure of water vapor in air was low. Under the hydrothermal
condition of high pH region, the atmosphere is saturated and the pressure is high, thus
favoring the formation of fine, uniform, stoichiometric, and structurally dense crystalline
HAp.
For the DCPD-derived HAp samples, ripening treatment really made difference. The
as-synthesized HAp by the hydrolysis of DCPD was nonstoichiom etric which had
completely transformed to P-TCP at 733 °C (Fig. 4.11.), but the sample ripened once with
20% CCD remained stable up to 1000 °C. Repeated ripening treatment (three times) further
enhanced the thermal stability up to about 1200 °C. Figure 4.12. illustrates the effect of
ripening on the thermal stability of HAp. The increase of Ca/P ratio of the HAp from 1.52
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
63
o
5
oo
10
15
0
500
1000
T e m p e ra tu re , *C
Figure 4.9.
TG thermograms o f HAp synthesized by (a) hydrolysis o f a-TCP, (b)
hydrothermal method, (c) DCPD hydrolysis, ripened twice, (d) DCPD hydrolysis, ripened
thrice, (e) DCPD hydrolysis without ripening, (f) DCPD hydrolysis (ripened once), and (g)
precipitation.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
64
Table 4.3. Lattice parameters o f the HAp samples synthesized in the current study, A.
Sample No.
a
c
Synthesis method
Reference
9.418
6.884
JCPDS file (No.9-432)
A
9.432
6.890
Hydrolysis of DCPD
B
9.430
6.878
Hydrolysis of a-T C P
C
9.428
6.875
“A” plus ripening
D
9.427
6.893
Precipitation
E
9.418
6.884
Hydrothermal
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
65
to 1.67 or 1.68 (Table 4.3.) clearly proves that the stoichiometry is one of the key factors
that control the thermal stability of HAp. However, since both temperature and pressure in
the ripening process were much lower than in the hydrothermal process, the repeated
ripening did not produce results as good as in the hydrothermal method. Also, increasing
treating time, increasing amount of CCD, and higher pH, all favored the transformation of
nonstoichiometric HAp into stoichiometry. This is quite evident in the SOR samples. For
example, the HAp ripened with 60% CCD at initial pH of 11 and treated for 24 h was
stable up to 1370 °C. The samples used for comparison, i.e., those treated at lower pH,
less CCD, or shorter period of soaking time, were stable up to 1300-1350 °C. Fig. 4.13
shows the XRD patterns of the SOR treated HAp sample, heating at 1350 °C and 1370 °C
for
1
h.
Table 4.4 summarizes the phase composition results of the HAp samples heated at
various temperatures. It can be seen that the thermal stability of the synthetic HAp depends
upon both the nature of the precursor and the synthesis method. The hydrothermal
treatment substantially enhanced the thermal stability. The hydrolysis of DCPD followed
by the strengthened one-step ripening (SOR) offered good thermal stability.
The as-
synthesized HAp by hydrolysis of a-T C P began to decompose at 800 °C and remained as a
mixture of HAp and TCP over a large temperature range. For the SOR samples, only
thermal stability was studied. Comprehensive explanation on the behavior o f these samples
needs further study. It should be indicated that during heating in air, HAp might have at
least partially transformed to the so-called oxy-hydroxyapatite (OHA) by dehydroxylation.
If this is the case, then the HAp detected in the samples heated at high temperatures should
be more accurately called OHA. In order to completely prevent the appearance of OHA,
sintering of HAp should be conducted in moisture (Wang & Chaki, 1993).
4.2.4. Summary
Thermal stability of several synthetic hydroxyapatite samples prepared by wetchemical methods was studied. The HAp powder synthesized by the hydrothermal method
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
66
.
. A_A
I
80
as
90
99
CALCIUM PHOSPHATE HYDROXIDE / HYOROXYLAPATITE
.....................
1
,
.
l.l I................. .
40
1
*9
,J.
CAS ( P 0 4 ) 3
90
( 0 H I
Two theta, degree
Figure 4.10. XRD patterns of the hydrothermally (200 °C, 1.5 MPa) synthesized HAp
fired at 1370 °C in air of 50% RH (11.88 mm Hg) remains undecomposed.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
67
693 C , 1 h r
733 C , I h r
UOO C, 1 h r
. i
r -■»*
20
Ji
M i
^
T
30
i^ i
I
~~*1
~T
40
1100c> 1hr
~*'V ** r‘ ~i
^ tf “ * |‘
50
60
26, degrees
Figure 4.11, XRD patterns of the as-synthesized HAp by DCPD hydrolysis, heated
at various temperatures. It is seen that the decomposition started at about 700 °C. 714 °C,
p-TCP + HAp; 7 3 3 ,1 100°C, p-TCP. 1300°C; a-T C P + P-TCP.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
68
I
20
T'A' A"1 ■ !
25
I"
J
L
m
I ' ’ H * ^ ^ 1 '1*1
30
35
. . a . *8
w .
AO
*4^
A5
T w o th eta , d e g re e
Figure 4.12. XRD patterns showing the influence of repeated ripening on the thermal
stability of HAp obtained by DCPD hydrolysis. Samples were heated 1 h at 1100 °C in air
o f 50% RH. (a) W ithout ripening: p-TCP.
(b) Ripened once: HAp+ P-TCP. (c) Ripened
twice: HAp+P-TCP (tr.). (d) Ripened thrice: HAp only, no decomposition.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
69
1 3 5 0 * C
» : a
. 1
i .
a . L
1
i
i
a
U
. 1
r
t
20
r
l
i
i i l
: .
A :
.
J
l
^
. 1 . ; .
a
'
j A
i»
1
A
l " i
2j
30
J
-
■
^
■ * i
-1 - r
1 1‘
35
1
1
1*
40
3 t
1*1
l
i ^ i "
45
50
1370 *C
. J L !_
i
a
-1
. . a Av.I .
.1.
aA
1.
a
I
Ai i
: .l . A »
A: :
*A_ ...
.A . A a ^ l\
A ..
. B . I\.
J
■A -+ T -,4 --r-A -i 1 . i
l. .
f„,». ,
■y.L-.-.A-.
T\vo theta, degree
Figure 4.13. XRD patterns showing the phase composition of SOR (strengthened one-step
ripening) heated 1 h at 1350 °C and 1370 °C, respectively. The ripening after the
hydrolysis o f DCPD was carried out at 75 °C and (A) 20% CCD, 70 min., initial pH 8.87,
(B) 60% CCD, 24 h, initial pH=8.87, (C) 20% CCD, 70 min., initial pH=11.0, and (D)
60% CCD, 24 h, initial pH = l 1. The marked peaks (•) belong to a-TCP.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
70
Table 4.4. The main phase composition appearing in the HAp samples heated in air of 50%
relative humidity at various temperatures.
T
CC)
Hvdrolvsis of DCPD
Hydrolysis of
a-TCP
PPT.
Hydrothermal
Ripened
As is
i
n
m
SOR
700
H
H
H
H
H
H
H
H
800
H + p(tr.)
H
H
H
H
H
900
H+P
H
H
H
H
H+a(tr.)
H
1000
H+P
H+p
H+P(tr.) H
H
H+a(tr.)
H
1100
H+P
P
P
P
P
H
H+P
H+P(tr.) H
H
H+a
H
1200
H+a
a
-
H+P
H+a(tr.) H
H+a
H
1300
H+a
a
H+a
H +a
H+a
H
H+a
H
a ,p —a - or P-TCP. H—HAp. PPT.—precipitated. I, II, and III indicate the number of ripening time.
SOR—strengthened one-step ripening, (tr.)— trace amount, defined arbitrarily by relative intensity of XRD
patterns.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
71
was uniformly nanocrystalline. This material showed very high thermal stability up to
1370 °C in air of 50% relative humidity. The as-synthesized HAp by the hydrolysis of
brushite showed poor thermal stability, starting decomposing at about 700 °C. Repeated
ripening with addition o f calcium chloride enhanced the thermal stability up to about
1200
°C. The strengthened one-step ripening treatment of this DCPD-derived HAp had increased
the decomposition temperature o f the HAp up to 1370 °C. The HAp obtained by the
hydrolysis o f a -T C P was highly crystalline but nonstoichiom etric and partially
decomposes at about 800 °C. The as-synthesized HAp obtained directly by precipitation
was almost X-ray amorphous. The crystallinity of this HAp increased with temperature till
about 900 °C at which it decomposes.
The results in this study indicate that the thermal stability of the synthetic HAp does
not depend upon the crystallinity but the stoichiometry and method of synthesis. In air of
50% relative humidity, the chemically stoichiometric and structurally dense HAp showed
the best thermal stability. To avoid dehydroxylation and decomposition of HAp during
ceramics preparation, either humid atmosphere or HAp powder of high thermal stability
should be used. However, the combination of both should enable to completely prevent
the decomposition of HAp in the general sintering temperature range.
4.3. Fabrication of Regularly Dense HAp Ceramics
4.3.1. Introduction
Microwave processing technique is still in an exploratory stage of development.
Although it has been investigated for many ceramic materials, no microwave sintering of
HAp prior to this study was reported. In the current study, regular dense HAp ceramics
have been fabricated by microwave sintering in a 500 W microwave furnace. Circular-plate
specimens of various green densities were sintered in the furnace at 1200 and 1300 °C, for
5, 10, and 20 minutes, respectively. Ceramics with density up to 97% of the theoretical
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
72
were obtained. The ceramics sintered by microwave and by conventional methods were
characterized for density, grain size, microstructure, and strength, and the results were
compared.
4.3.2. Experimental Procedure
4.3.2.1. Material
HAp powder prepared by the hydrolysis of brushite (C aH P 0 4 -2H 2 0 ) , followed by
ripening with 60% wt. CaCl2 2H20 was used in this study. The ripened HAp was dried at
90 °C in a ventilated oven and then ground to -325 mesh followed by heating at 500 °C for
24 h. The BET surface area of this powder was found to be 47.4 m 2 /g. Powder XRD
confirmed that the m aterial was pure HAp with low crystallinity.
W ithout using any
binder, pellets of 12.7 mm (0.5 inch) diameter were made at uniaxial pressures from 35 to
351 MPa to get various green densities. Two 19.05 mm (3/4 in.) diameter pellets, on which
the patterns of a dime were pressed, were used to test the uniformity of shrinkage during
microwave sintering.
4.3.2.2. Sintering Setup
A 500 W, 2.45 GHz commercial microwave oven (Model JE43, GE Co.) was
modified for the sintering experiments. The arrangement inside the oven is schematically
shown in Fig. 4.14. In this case there was no turntable, but there was a mode stirrer above
the cavity. Samples were stacked in layers at the center o f a vertically placed porous
zirconia cylinder (Zircar Products, Inc., Florida, NY) (28 mm ID, 51 mm OD and 30 mm
in height), which was placed on a thick Fiberfrax (Fiberfrax, Harbinson-Carborundum
Corp., Niagara Falls, NY) cushion, and then covered all around with a layer of Fiberfrax
thermal insulation fibers.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
73
4.3.2.3. Temperature Measurement
An alumina-sheathed and Pt foil-shielded S-type thermocouple was inserted from the
left wall o f the oven to m onitor the temperature of the samples during sintering. The
thermocouple was so arranged that the tip was only about 2 mm away from the surface of a
sample (Fig. 4.15).
The microwave interference with the tem perature m easurem ent is a common
observation at low temperatures, but once coupling between microwaves and the material
is initiated (i.e. when zirconia and specimens begin to absorb microwave effectively) the
tem perature readings get stabilized. The lower temperature readings are recorded by
m omentarily switching off the power supply to the m icrowave oven. The response time
was less than 3 seconds.
Tricalcium phosphate (TCP), C a 3 (P04)2, was used to calibrate and determine the
accuracy o f the therm ocouple-temperature-measurem ent m ethod used in the current
m icrowave experiments. TCP exists in two crystalline forms: a-TC P (high-temperature
form) and j3-TCP (low-temperature form). The transition of /i-TCP to a-T C P starts at
about 1125 °C and the process is sluggish. P-TCP will transform completely to a-T C P if
temperature is sufficiently greater than 1125°C or if kept above the transition temperature
for a sufficiently long time. Although a-T C P is a high-temperature form, it can also
m etastably exist at room temperature even in regularly cooled samples. Therefore by
comparing the relative amounts of two TCP phases in the sample of /J-TCP heat-treated by
conventional and microwave methods at temperatures around the transition temperature, an
accurate temperature of microwave heated sample can be determined indirectly.
For this purpose, the following experiments were conducted. Three pellets of labsynthesized P-TCP were stacked one over another inside the porous zirconia cylinder in the
m icrowave set-up, with the thermocouple located at a distance o f about
2
mm from the
pellet in the middle (Fig. 4.15). The rest of the experimental setup was identical to that
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
74
Figure 4.14. Schematic showing of the arrangements in the microwave sintering cavity.
The sintering package includes (1) thermocouple, (2) zirconia cylinder, (3) pack of
samples, (4) thermal insulating fibers, (5) power level control, and (6 ) time control.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
75
Figure 4.15. Schematic showing of relative positions of thermocouple and (3-TCP pellets
during the temperature measurement calibration. Labeled are (1) thermocouple, (2) porous
zirconia, and (3) TCP pellets.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
76
used for HAp sintering (Fig. 4.14). The samples were heated in the microwave field at
1125±10 °C for 15 min., then cooled to room temperature. Another set of three pellets was
sintered (one pellet per run) in a conventional furnace for 15 min. at 1100,1125, and 1150
°C, respectively, then air-quenched to room temperature. XRD was carried out on all
samples sintered for phase identification.
4.3.2.4. Sintering
Six multi-sample runs (six HAp pellets with green densities ranging from 32.6 to
51.4%, stacked in two layers in each run) of microwave processing were carried out for 5,
10 and 20 minutes at 1200 and 1300 °C, respectively. The two pellets with the dimepatterns were microwave sintered at 1200 °C for 7 min. to check the uniformness in
shrinkage. During the sintering, the microwave oven was set at the full power level (500
W output), and the temperature was controlled manually by on-and-off operations of the
pow er supply. After each run, the system was allowed to cool naturally to room
temperature. For comparison, conventional sintering of the same material was also carried
out at the same temperatures
(2
h soaking at each sintering temperature, with heating and
cooling rates o f 5 °C per min.) in a 5 kW programmable box furnace (Model 51524,
Lindberg, Watertown, W I ).
4.3.2.5. Characterization
Density, grain size, microstructure, and tensile strength of the sintered pellets were
measured. Density was determined by direct measurement of the geometric size and weight
of the specimens. Grain size was measured from the electron micrographs o f the assintered surfaces. M icrostructure was examined on the fracture surfaces with the SEM.
Phase compositions were identified by XRD. The tensile strength was m easured by
diametral-compression test (Rudanick et al.) in a MTS810 (Material Test System, MTS
System Corp., Minneapolis, MN) with a crosshead speed of 0.051 mm (0.002 in) per
minute.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
77
4.3.3. Results and Discussion
4.3.3.1. Temperature Measurement
Figure 4.16. shows the XRD patterns of six TCP samples processed in this study.
As can be seen, conventionally, the sintered sample at 1100 °C remained pure P-TCP [Fig.
16, (a) 1]; a-T C P appeared as a minor phase at 1125 °C [(a) 2] and as a major phase at
1150 °C [(a) 3]. These results indicate that in conventional heating, the phase transition
from p- to a-T C P started at about 1125 °C when soaked for 15 min., and substantially
transformed at 1150 °C. In contrast, in case of microwave processing at 1125+10 °C, the
pellet located at the bottom of the stack remained p-TCP [(b) 1], the middle pellet contained
only trace amount o f a-TCP, and p-TCP was still the major phase in the top pellet [(b)3j.
Table 4.5. lists the alp intensity ratio (using intensity of d = 2.909A for a-TCP, and d =
2.880A for p-TCP) of the characteristic reflections in the XRD patterns. As can be seen,
the middle pellet sintered by microwave at 1125110 °C is comparable in the a//3 intensity
ratio to that of the conventionally sintered sample at 1125 °C, while the top pellet shows
higher a/p ratio than the one conventionally sintered at 1125 °C, but obviously much lower
than that sintered at 1150 °C. The bottom pellet was the same as conventionally sintered at
1100 °C (no trace o f a-TCP). Considering the temperature deviation of +10 °C in the
microwave sintering, the measured temperature (on the middle pellet) was in excellent
agreement with the expected value. Thus the validity and accuracy of the temperature
measurement with the thermocouple in the current microwave processing is confirmed.
4.3.3.2. Sintering Process
Figure 4.17. shows the temperature-time curves of the six sets of HAp specimens
sintered by microwave processing. The run at 1200 °C for 5 min. was irregular, therefore
the results of this run are not included here. As can be seen, each curve has three parts,
corresponding to three stages: heating, soaking, and cooling. The sharp temperature
increase in the heating curve is characterized by the phenomenon of thermal runaway. It is
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
78
3. top
3. 1150°C
1
A—
2 . middle
2.1125 °C
J aJLa^ l-J
1.1100 °C
20
1.
30
29, degrees
40
20
30
bottom
40
20, degrees
Figure. 4.16. XRD patterns (Cu K a ) o f p-TCP samples used to calibrate temperature
measurement, (a) Conventionally sintered for 15 min. at temperatures indicated, (b)
Microwave sintered at 1125±10 °C for 15 min. (pellet positions are indicated). (•): a-TCP.
Unmarked peaks: /J-TCP.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
79
Table 4.5. Intensity ratio of a-T C P id = 2.909A) to /J-TCP (d = 2.880A) in the /3-TCP
after firing under different conditions for temperature measurement calibration.
Conventional, 15 min.
1150°C
1125°C
1.769
0.172
Microwave, 1125±10 °C, 15 min.
1100°C
0
top
middle
bottom
0.282
0.111
0
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
80
1400
1400
1200’C
1200
1300‘C
1200
1000
1000
800
BOO
O
o
I
I*
£
8,
600
400
400
200
200
0
0
0
10 20 30 40
60
20 30 40 50 60
Time, min
Time, min
(a)
(b)
Figure 4.17. Temperature-time curves in the microwave sintering o f HAp at (a) 1200 °C
and (b) 1300 °C. Sintering time: (A) 5 min., (-----) 10 min., (• • •) 20 min.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
81
well known that zirconia is a microwave absorber, so that the zirconia cylinder plays a dual
role during the m icrowave heating:
(i) a thermal insulator, and (ii) a preheater.
Furthermore, since the amount o f HAp (less than
6
g per run) used in the sintering was
much less than the amount of zirconia (about 2 0 g), it is proposed that the zirconia cylinder
triggered thermal runaway.
To confirm this, and to determ ine the role of Fiberfrax
insulation, two blank tests were carried out: (a) with Fiberfrax only (no sample and no
zirconia cylinder), temperature rose only to 369 °C after a 60-min irradiation; (b) with
Fiberfrax and the zirconia cylinder, thermal runaway was noticed after 21 min. This
confirms that the zirconia cylinder played a key role in triggering thermal runaway in the
current microwave sintering. At low temperature, HAp absorbed little microwave and was
heated up by the zirconia cylinder which got heated first by absorbing microwave energy.
When temperature was sufficiently high, both zirconia and HAp would absorb microwave
more efficiently, thus the sintering o f the HAp samples was a complex result of both direct
and indirect microwave heating.
4.3.3.3. Uniformness in Sintering
Although having experienced high-rate heating (thermal runaway) and cooling (in the
first few min. of cooling the rate was high), the HAp pellets did not show any cracks after
microwave sintering. As shown in Fig. 4.18., all the details o f the coin-pattems have been
uniformly reduced and preserved on the sintered HAp pellets after 7 min. microwave
sintering at 1200 °C. This indicates that under proper conditions, microwave sintering is
quite uniform and homogenous. The uniform shrinkage should be attributed to the
volumetric heating, good thermal insulation, neglected thermal gradients, and the small size
of the specimens.
4.3.3.4. Sintered Density
Figure 4.19. shows the relative densities of the microwave sintered pellets versus the
corresponding green densities. The values of the conventionally sintered pellets were also
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
82
Figure 4.18. Photograph showing the uniform shrinkage o f microwave sintered HAp
pellets. The dim e-pattem s on the green pellets (large) have been uniformly reduced and
preserved on the sintered pellets (small) by microwave sintering at 1200 °C for 7 m in ..
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
83
100
30
35
40
45
50
55
Green density, %
Figure 4.19.
Relative density (% theoretical) o f HAp ceram ics sintered by (1)
microwave/1300 °C/10 min., (2) microwave/1200 °C/10 min., (3) conventional/1300 °C/2
h, and (4) conventional/1200 °C/2 h.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
84
plotted in the same graph for comparison. It can be seen that at the same temperature,
higher densities were achieved in the microwave sintered pellets, and this is more obvious
when green density was high (typically over 45%). Generally, the densification of
ceramics in a solid-state sintering is a process of shrinkage of the compact or removal of
porosity. Although quite a few mass transfer processes take place simultaneously during
sintering, only transfer o f matter from the particle volume or from the grain boundary
between particles causes shrinkage and pore elimination (Kingery et al., 1976, p.475).
Therefore, it can be suggested that grain boundary diffusion between the HAp particles was
substantially enhanced during the microwave sintering. The grain boundary diffusion takes
place only after the particles are necked (in contact) and at a suitable temperature (to
overcome the energy barrier). In the pellets of low green density, the number of contact
points is limited. As the green density increases, the particles-in-contact correspondingly
increases, leading to increased sintered density in both microwave and conventional
sintering (Fig. 4.19.).
However, the increase in the microwave sintered samples was
more than that in the conventionally sintered samples. Also, the microwave sintering took
much shorter time (< 1 0 % of the time of the conventional sintering), it can be further
suggested that the densification rate (through boundary diffusion) in the microwave
sintering process was much higher than in the conventional sintering. The volumetric
heating in the microwave sintering is a consequence of a series of energy losses. The
principle of microwave sintering has been reviewed by Sutton (1989). When microwaves
penetrate and propagate through a dielectric material, the internal fields generated within the
affected volume induce translational motions of free and bound charges and rotate charge
complexes such as dipoles. The resistance of these induced motions due to inertial, elastic,
and frictional forces, causes losses and attenuate the electric field. As a consequence of
these losses, volumetric heating results.
This principle should be applicable to the
microwave sintering of HAp also. It is also suggested that the hydroxyl groups in the HAp
structure might have played an important role in causing losses and enhancing densification
in the microwave sintering of HAp.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
85
4.3.3.5. Grain Size and Microstructure
Figures 4.20. and 4.21 show the SEM photographs of the HAp specimens sintered at
1200°C by microwave and conventional processing, respectively. The differences in
morphology o f these samples can be clearly seen. Compared with the conventionally
sintered sample (Fig. 4.20.a) in which the entire as-sintered surface is rough and uneven,
and the grains are angular with boundaries shown by the straight lines, the microwave
sintered sample (Fig. 4.21.a) shows unique features:
the whole area looks flat and
smooth, and the grains are tightly bounded. Since the driving force for the densification of
solid-phase sintering is the decrease in surface area and the lowering of the surface free
energy by the elimination of solid-vapor interfaces (Kingery, et al., 1976, p.469), the
rough and uneven as-sintered surfaces correspond to a higher residual surface area and
imply a lesser degree of sintering. By comparing the SEM photographs of the fracture
surfaces, it can be seen that the microwave sintered HAp (Fig. 4.21.b) has a lower total
porosity and finer pore size than the conventionally sintered specimen (Fig. 4.20.b).
Table 4.6. shows a comparison between the average grain size of the microwave and
the conventionally sintered HAp samples. The grain size of the conventionally sintered
HAp (1200°C) is close to that of microwave sintered one at the same temperature. For the
pellets sintered at 1300 °C, the grain size of both microwave and conventionally sintered
samples had increased substantially, but the grains in conventionally sintered sample were
much larger. In addition, the abnormal grain growth was pronounced in the conventionally
sintered pellet, but was less obvious in the microwave sintered one. The relatively lower
grain growth during microwave sintering is attributed to the shorter sintering time and a
more effective densification process.
4.3.3. 6 . Phases
XRD analysis on the sintered HAp samples detected a-T C P in trace, but still, HAp
was the major phase. This indicates that a partial decomposition of HAp took place during
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
86
8 . 02KX
5U 0 0 1 7
MW11 8F
Figure 4.20. SEM micrographs showing (a) as-sintered surface and (b) fracture surface of
HAp sintered by conventional processing at 1200 °C for 120 min.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
87
20KU
3 . 8 7 KX
Figure 4.21. SEM micrographs o f (a) as-sintered surface and (b) fracture surface of HAp
sintered by microwave processing at 1200 °C for 10 min.sintering.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
88
sintering. To prevent the decomposition, either lower processing temperatures should be
used, or some special precautions, such as special atmosphere or using HAp of high
thermal stability, should be taken.
Table 4.6. Grain size of HAp pellets sintered under different conditions, |im.
Microwave
5 min.
10
min.
Conventional
20
min.
120
min.
1200 °C
...
1.05
1.48
1 .1 2
1300°C
2.06
3.16
3.39
6.52
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
89
4.3.3.7. Mechanical Strength
Figure 4.22 (a) shows the diametral tensile strength as a function of the sintered
density of the HAp ceramics, regardless of the sintering time and temperature. The data for
both the microwave and conventionally sintered HAp seem to follow the same densitystrength relation. The diametral tensile strength of the pellets sintered at 1200 °C has been
plotted in Fig. 4.22 (b).
In this figure, it can be seen that starting with the same green
density, a microwave sintered pellet exhibited higher strength than the conventionally
sintered HAp: 10 min. microwave sintering achieved higher strength [Fig.4.22 (b), 2] than
the 2 h conventional sintering [Fig.4.22 (b), 3], and the pellets of 20 min. microwave
sintering demonstrated even higher strength [Fig.4.22 (b), 1]. Comparing with Fig.4.22
(a), it is clear that the enhanced mechanical strength in the microwave sintered HAp
ceramics should be attributed to their higher densities.
4.3.3. 8 . Energy Consumption
Besides the denser microstructure, finer microstructure, smaller grain size, and the
improved mechanical strength, the economic issues of microwave sintering in terms of both
time and energy are very important. Fig. 4.23 compares the temperature-time profiles of
microwave and conventional sintering processes. Taking a typical run at 1200 °C for 10
min. with a 500 W microwave oven for example, the microwave sintering process
consumed energy only in the heating and soaking periods, which was about 0.5 h in total,
while the conventional sintering process required
10
h (it consumed energy in the cooling
cycle, also) and a furnace with a minimum of 1 kW (although a 5 kW furnace was used in
the current study). Thus microwave sintering for the laboratory scale specimens requires
only about 5% of the time, and even a smaller percentage of energy, of the conventional
process.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
90
60
•B
60
S
40
£ia
JD
"S
c
3
e
aj
20
0
BO
70
80
90
100
Sintered density, %
(a)
30
35
40
45
50
Green density, %
(b)
Figure 4.22. Diametral tensile strength of HAp ceramics (a) versus sintered density: (o)
microwave sintered, (A) conventionally sintered; (b) versus initial green density: (1)
microwave/20 min., (2) microwave/10 min., and (3) conventionally/2 h.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
91
1400
1200
U 1000
800
600
.<o
400
200
0
100
200
300
400
500
600
Time, min
Figure 4.23. Com parison o f tem perature-time profiles in the sintering o f HAp by (1)
microwave and ( 2 ) conventional methods.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
92
4.3.4. Summary
HAp ceramics have been sintered by microwave heating. The results show that HAp
samples with varying green density can be sintered by microwave irradiation to high
densities within a short period o f time. In contrast to conventional method, microwave
sintering of hydroxyapatite consum ed m uch less energy, offered apparently denser
m icrostructure, finer grain size, and consequently, im proved m echanical strength.
Technically, if a microwave oven is adjustable in both power and frequency, with a
combined feedback temperature controlling system, the efficiency o f the process could be
even higher.
4.4. Microwave Processing of Porous Hydroxyapatite Ceramics
4.4.1. Introduction
Porous hydroxyapatite ceramics may be used for highly effectively restoring bony
mass. For example, porous HAp granules can be used to restore the deficient alveolar
ridge. The pioneering work to develop porous HAp ceram ics by the route of the
replamineform technique was done by Weber etal. (Weber etal., 1969; R.A. W hite etal.,
1972;
E.W. W hite et al., 1975; D.M. Roy, 1975).
Based on the study o f the
microstructure of sea-urchins and corals by Weber et al. (1969), the first and possibly only
commercially available biomimetic material even twenty years later was developed by
W hite, W eber and W hite (R.A. W hite, 1972; E.W. W hite, 1975) who devised the
replamine process, and D. Roy (Roy, 1975; Roy and Linnehan, 1974; Eysel and Roy,
1975) who learned to convert the C aC 0 3 to HAP. These porous HAp ceramics are
available commercially from Interpore International for dental and bone transplants. HAp is
the obvious material expected to be widely used for biomedical applications in humans. For
implants, HAp ceramics, either dense or porous, can be used for different purposes.
As shown in previous section, regularly dense HAp ceramics have been successfully
fabricated by m icrowave processing. The results show that, in addition to lowering
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
93
sintering temperature and enhancing mechanical strength, one can sinter HAp ceramics in a
few minutes, consuming less than one tenth o f the energy or time than required for the
conventional process. Because o f the usefulness o f porous HAp ceramics in biomedical
applications, fabrication o f porous HAp ceramics by m icrow ave processing was
investigated in the current study.
4.4.2. Experimental Procedure
4.4.2.1. HAp Precursor Powders
Two kinds of HAp-precursor powders were used.
A. W hiskers o f pure HAp, with typical size of 0.2 x 5 (J.m, BET surface area 10.8
m 2/g, synthesized by the hydrolysis o f tricalcium phosphate [a-C a 3 (P0 4 )2 , a-TCP)] at
60°C, pH 8±0.5, dried at 165°C (Fang, et al. 1994).
B. Pure HAp, agglomerations of very fine needle-like crystals, with typical size of
single crystal size at 0.05 x 0.5 pm (estimated from microgragh), BET surface area 47.4
m 2/g, prepared by the hydrolysis of brushite (C aH P0 4 .2H 2 0 ) followed by "ripening"
treatment with CaCl2 (Fang et al., 1994; M onma and Kamiya, 1987), sieved to -325
mesh, and dried at 500°C. Figure 4.24. shows the morphologies of these powders.
4.4.2.2. Pow der Consolidation
Three groups of circular pellets of 12.7 mm (0.5 in) diameter were pressed with (1)
powder A and (2) powder B, both at 35 - 140 MPa obtain various green densities, and (3)
mixture (will be designated as C) o f powder B with various amounts of ammonium
carbonate [(NH 4 )2 C 0 3 'H 2 0 ], pressed at 140 MPa. Ammonium carbonate was used to
introduce additional porosity in the specimens.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
94
Figure 4.24.
M orphology o f the starting HAp pow ders composed of (a) whiskers
synthesized by the hydrolysis o f a-T C P and (b) agglomeration of fine single crystals
synthesized by the hydrolysis o f brushite followed by ripening treatment with calcium
chloride.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
95
4A.2.3. Sintering
Sintering was carried out in air in a 2.45 GHz, 500 W commercial microwave oven.
The experimental setup was described earlier in Sec. 4.3. The processing conditions are
listed in Table 4.7. For pure HAp (A and B), the specimens (batches I to HI) were heated
directly from room temperature to the final sintering temperature, and soaked at the peak
tem perature for only 1 min., then cooled to room temperature. In the case of HAp
containing ammonium carbonate (C), one batch (TV) was heated at 120°C for 12 h and then
400°C for 20 min. in an electric furnace to remove ammonium carbonate (decomposition
temperature 58°C), and then transferred to the microwave oven, processed at 1150°C for 3
min. Another batch (V) was first heated in the microwave oven at the lower power level to
remove ammonium carbonate, then heated at the full power input of the oven to 1200°C,
and allowed to cool after soaking at 1200°C for 5 min.
4.4.2.4. Characterization
Porosity and density of the sintered specimens were determined by geometrical
measurements. M ercury intrusion porosim etry experiments were carried out on three
representative specimens to determine open porosity. The maximum pressure for intrusion
was 362 MPa (52,500 p.s.i.). Microstructures of the processed specimens were examined
by SEM. XRD studies were carried out to identify the phase composition of the sintered
samples. Tensile strength was determined by means of diametral-compression (Brazilian)
tests (Rudanick etal.), with a crosshead speed of 0.051 mm (0.002 in)/min.
4.4.3. Results and Discussion
The products were porous HAp ceramics with porosities from 10 to 75%. All the
processed specimens were perfect in shape without any deformation or visual microcracks.
The details are given as the following.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
96
Table 4.7. Microwave processing conditions in fabricating porous HAp ceramics.
Batch
I
II
in
IV
V
Starting material
A
B
B
c*
c*
Wt. of HAp, g
1.5
2 .8
3.0
2 .8
2 .8
1200
1150
1200
1
3
5
Sintering temp., °C 1150
Sintering time, min.
1
1150
1
* Powder B plus ammonium carbonate.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
97
4.4.3.1. Porosity
Apparent densities of the sintered ceramics vs. the green densities are listed in Table
4.8. Within the same batch, a linear relation between final and green densities is observed.
Porosity can be controlled by adjusting the starting material, green density, or sintering time
and/or temperature. The starting materials influence the final porosity because of the
difference in sinterability and microstructure. The higher the specific surface area, the
higher the sinterability. The specific surface area of powder B (finer powder) is much
higher than that of A (whiskers), so that at the same green density, under the same sintering
conditions, the pellets made o f powder B (batch n , Tables 4.7 and 4.8) achieved higher
density than those of powder A (batch I). In addition, increasing processing temperature
results in higher density, which can be seen by comparing batch II with III. Increasing
sintering duration also has a similar effect (II and IV).
The mercury intrusion porosimetry detects the open porosity of a sample. The
porosimetry experiment was carried out on three representative samples (apparent density
around 50%). Among them, the specimen made o f whiskers showed total open porosity,
and those o f powders B and C had 82% and 84% open porosity, respectively.
The
difference is again due to the morphology and sinterability of the starting materials.
4.4.3.2. Phases
XRD analysis made on the specimens after microwave processing shows that the
samples of powder B (the ripened HAp) contain pure HAp, but those made o f the whiskers
(powder A) show small amount of a-TCP, which is attributed to partial decomposition of
the starting HAp during sintering. The use of ammonium carbonate, during compaction,
effectively increased porosity (batch IV and V, Table II) without introducing any apparent
contamination. W hen sugar was tried for the same purpose, however, some problems
were encountered. First, sugar fuses during heating, thus making the samples deform or
collapse, especially when the sugar/powder ratio is high. Secondly, because of the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
98
Table 4.8. Relative density (%) of microwave-processed porous HAp ceramics.
Density
n
I
Batch
D j#
d 2#
III
IV
V
Dl
°2
Dl
d2
Dl
d2
Dl
d2
43.6
52.9
34.2
51.4
34.1
58.0
16.3
26.7
15.5
30.3
49.3
55.8
39.0
65.8
39.0
77.1
25.5
43.3
23.2
44.5
51.8
68.8
44.5
68.7
44.5
81.9
27.3
51.2
27.3
52.5
72.0
48.3
82.4
48.7
91.2
32.2
56.6
30.4
58.4
37.6
65.3
36.3
69.6
40.4
69.0
38.8
75.5
54.3
# D j, D2, relative density before and after microwave processing. For IV and V, D j's are
the values after removal of ammonium carbonate.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
99
insufficient oxygen and very rapid sintering, some carbon is trapped in the ceramics before
it is com pletely oxidized, resulting in C-contam ination.
Both the collapse and C-
contamination (darkening) of the specimens were observed in the samples containing sugar.
Obviously, ammonium carbonate is much more suitable for the purpose of introducing high
porosity in the rapid sintering of HAp by microwave processing.
4.4.3.3. M icrostructure
The m icrostructures of a few representative porous HAp ceramics are shown in
Figure 4.25. It can be seen that the microstructures o f these ceramics are statistically
homogeneous. In the whisker-made samples, some whiskers are still visible inside the
cavity. Such a microstructure is expected to help the ingrowth of the tissue when used in
biomedical applications. However, compared to the size (>100 p.m) required for ingrowth
of bone tissue (Lavernia and Schoenung, 1991), the pore size should be much larger.
Nevertheless, this should not to be difficult to achieve by m odifying the processing
conditions described above.
4.4.3.4. Strength
The tensile strength measured by the diametral-compression method is plotted as a
function of density in Fig. 4.26. It can be seen that most porous HAp ceramics fabricated
in the current research have shown "reasonable" strength values, while, of course, there is
a trade-off between the porosity and the strength o f the HAp ceramics.
4.4.4. Summary on Microwave Processing of Porous HAp Ceramics
Porous HAp ceramics with porosity up to 73% have been fabricated by microwave
processing at 1150 to 1200°C for 1 to 5 min. The sintering can be accomplished within a
few minutes under optimum conditions. For the specimens of about 50% porosity, mercury
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
100
1 9KU
19KU
803X
2 . 96KX
10U
Figure 4.25. Microstructures o f the microwave-processed porous HAp ceramics (fracture
surface), (a, b) Starting with fine HAp powder: (a) 1150°C, 3 min., porosity 73.3%. (b)
1200°C, 5 m in., porosity 24.5%. (c, d) Starting with HAp whiskers, pressed to different
green densities (c: 43.58%, d: 54.29%), both fired at 1150°C for 1 min. Final porosity (c)
47%; (d) 28%.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
101
porosimetry shows that 82-100% of the pores are connected (open). The porosity of the
ceramics can be controlled by adjusting the starting material, green density, sintering time
or temperature. Pore size can also be adjusted. The addition o f ammonium carbonate in
the green specimens helps in adjusting porosity. The microstructure o f the microwave
sintered porous ceramics is homogeneous and the tensile strengths are reasonably good.
50
40
cS
g
30
to
G
£
GO
20
’S3
G
£
10
0
40
60
80
100
Density, %
Figure 4.26.
D iam etral tensile strength vs. density o f m icrow ave-processed HAp
ceramics.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
102
4.5. Fabrication of Transparent HAp Ceramics
For the first time, transparent hydroxyapatite ceramics have been fabricated at ambient
pressure in this study by microwave and conventional sintering m ethods. This was
achieved essentially using a fine crystalline hydroxyapatite pow der synthesized
hydrothermally as a starting material. The sintered hydroxyapatite ceramic was phase pure
and the average grain size was around 0 .2 (am.
4.5.1. Introduction
Aoki et al. (1987) used a HAp percutaneous device for continuous and long-term
blood pressure and deep body temperature measurement without introducing any infection.
In fact, transparent HAp can be a better candidate for such an application, since it can work
as a window to observe the changes inside.
The com bination o f the excellent
biocompatability and transparency makes the transparent HAp ceramics unique.
It is
expected that such ceramics will find good application in the biomedical area, and probably
in some other areas as well.
Some work on the fabrication of transparent HAp ceramics by hot isostatic pressing
(HIP) has been reported (Yoshimura et al., 1989; Uem atru et al., 1989; Li and
Hermansson, 1990). In these methods, either filter cake, hydrothermally synthesized
ultrafine powder, or commercial HAp powder was used as starting material and the HIP
processing conditions were 800 - 1275°C at the pressures o f 100 - 200 M Pa for 1 - 2 h.
U nder the HIP conditions nearly complete densification with limited grain growth can be
achieved at relatively low processing temperatures, so that transparent HAp ceramics could
be obtained. However, the procedure of the HIP process and conditions are very complex
and the experiment takes almost a whole day for completion.
In the current study, a
different approach, i.e., the ambient-pressure sintering, or, in other words, pressureless
sintering, was adopted to fabricate transparent H A p ceramics. The experiments were
carried out in air following conventional and microwave heating.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
103
4.5.2. Experimental Procedure
The HAp powders synthesized in (Sec. 4.2.2.1.) by hydrothermal method (Powder
I) and by the hydrolysis of brushite (C aH P0 4 -2H 20 ) followed by a "ripening" treatment
(Powder II) were used in this study. Figure 4.27. shows the morphology of these starting
powders . Before compaction, the powder was sieved to -325 mesh and heated at 500 °C.
Circular pellets o f 12.7 mm diam eter and about 1 mm thick were uniaxially
com pacted at pressure up to 350 MPa. As-pressed thin pellets from pow der I were
translucent, and those of powder II were all opaque. For the conventional sintering, thin
pellets of 6.35 mm diameter were pressed at 350 MPa.
The microwave sintering was carried out in a modified 500-W microwave oven. The
experimental details and temperature measurement have been described in Section 4.4. The
samples were heated in the microwave oven in the air of 50% relative humidity directly
from room temperature and sintered for 5 min. at 1150 °C, then allowed to air cool
naturally. The conventional sintering was carried out in a dilatometer furnace in which
heating rates could be well controlled through a computer program. This sample was heated
at 5 °C per minute to 1150 °C and held at this temperature for 5 min., then allowed to cool
down to room temperature, by turning off the power.
4.5.3. Results and Discussion
Figure 4.28. shows the heating curves of the ambient-pressure sintering o f HAp
ceramics. In the microwave sintering, the sintering temperature of 1150 °C had reached in
13 min. In the conventional sintering, it took 225 min. to reach the same temperature. A
dilatometry study (Fig. 4.29.) showed that the sintering of HAp begins at around 670 °C.
At the heating rate of 5 °C/min., it took 96 min. from 670 °C to 1150 °C. Thus there were
actually 101 min. over 670 °C for sintering under the conventional condition. This was
over ten times longer than that in the microwave processing (in which the sintering above
670 °C was only
8
min.).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
104
Figure 4.27. Morphology o f the HAp powders synthesized by (a) hydrothermal method,
and (b) hydrolysis of C aH P04-2H20 plus ripening treatment.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
105
1200
000
u
u
<u
3
nia>
a
E
<u
H
800
600
400
Microwave
Conventional
200
0
50
150
1 00
200
25 0
3 00
Tim e, m in.
Figure 4.28. Heating curves o f sintering of hydroxyapatite ceramics by (a) microwave
processing and (b) conventional sintering.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
106
0
300
- 2 .5
250
Percent Expansion
6 S 8 .1 - C
-5 .0
200 i
-7 .5
-
150
10.0
-12.5
8 8 3 .7 -C
j
*
00
-15.0
50
-17.5
-
20.0
250
500
I
_L
750
1000
1250
^tem perature ,°C
Figure 4.29. Dilatometry curve of the hydrothermally synthesized HAp heating at 5 °C per
min.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
107
The processing conditions are listed in Table 4.9. All the pellets sintered by both the
m icrowave and the conventional processes were perfect in shape and pure in phase, but
only the pellets prepared from Powder I turned transparent (Fig. 4.30). The specimens
made from Powder II were all opaque. This indicates that the nature of starting material is
crucial for achieving transparency in the HAp ceramics.
Density measurement showed that the sintered specimens made from Powder I were
nearly 100% dense (+99%), while those from Pow der II were only 93-95% of the
theoretical density. The SEM micrographs of the as-sintered surface of the transparent
HAp ceramics (Fig. 4.31) show that the specimens were well sintered, and the grain
growth was very limited, with average grain size of 0.2 - 0.3 |xm. XRD pattern (Fig.
4.32.) indicates that the transparent HAp ceramics are highly crystalline, pure HAp.
Transparency is an optical property of materials. For a specific m aterial, to be
transparent it must not absorb light. The nature o f material is of course the most important
factor that affects transparency. For example, it is impossible to achieve transparency in
metals under normal conditions, since the numerous free electrons in metals will absorb the
photons when visible light strike them. A glass is transparent because it has short-range
ordered structure only, and also it is optically isotropic. There is no grain boundary in a
glass, and hence very little scattering or absorption of light. Ceramics are generally
polycrystalline. The grain boundaries in ceramics strongly scatter light. However, if the
grain size is smaller than the wavelength of the visible light (0.4 - 0.7 p.m), light can
transmit through the ceramic just like it travels through a grating. Owing to the difference
in light absorption, the impurity phases in a ceramic will certainly affect transparency,
usually decreasing transparency by scattering or absorption.
Porosity also influences
transparency in the same manner. In short, density, purity, and grain size are the key
factors that influence the transparency of a ceramic. To achieve transparency in a ceramic,
efforts should be made to eliminate or minimize scattering or absorption of light.
In this Study the transparency was achieved by using the hydrothermally synthesized
HAp powder (Powder I) which has the following characteristics: high purity, high thermal
stability, fine and uniform particles, good crystallinity, excellent sinterablity, etc. These
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
108
Table 4.9. Processing conditions and results for microwave and conventional sintering of
HAp.
Powder Compactioni Process
MPa
Sintering
Hold
Total Processing
temp., °C
min.
min.
Sintered pellets
I
350
microwave
1150
5
20
transparent
n
350
microwave
1150
5
20
opaque
i
350
conventional
1150
5
230
transparent
n
350
conventional
1150
5
230
opaque
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.30. Transparent HAp ceramics fabricated at ambient pressure in air (a-c) by
microwave processing for 5 min. at (a) 1150°C, (b) 1125'C, (c) 1100°C, totally irradiated
for 18 min., and (d) by conventional sintering at 1150 "C for 5 min. after heating to
1150°C at 5 °C/min., totally heated for 230 min.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
110
111 0 0 1 9
MW2.4SB
Figure 4.31. M icrographs o f the as-sintered surface o f the transparent hydroxyapatite
ceramics sintered by (a) microwave processing and (b) conventional method.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Ill
■tftf
20
30
40
50
Two theta, degree
Figure 4.32.
Pow der XRD pattern of the transparent HAp ceramics by microwave
processing at 1150 °C for 5 min., showing that the ceramics are highly crystalline single
phase HAp.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
112
unique properties, as well as the relatively high green density (60%), and sintering without
substantial grain growth, made it possible to realize transparency in the resultant HAp
ceramics.
By contrast, Powder II was composed of nonuniform and relatively large particles,
with some large needle-like single crystals of aspect ratio o f
showed lower packing efficiency and poorer sinterability.
10,
so that this powder
It was noticed that, at the
compaction pressure of 350 MPa, the green density of the pellets made from Powder I
reached 59.9% but that of Powder II was 54.9%. It is more difficult to fully densify the
pellets made of the powder with the large needle-shaped particles, there were always some
residual porosity in the pellets of Powder II after sintering. Besides, the hydrolysisderived HAp was probably less pure than the hydrothermally synthesized one, thus making
it impossible to achieve transparency in the specimens made of Powder II.
4.5.4. Summary
Transparent HAp ceramics were successfully fabricated at ambient pressure in air by
m icrow ave as well as by conventional sintering o f the pow der-com pacts o f the
hydrothermally synthesized HAp. The microwave sintering was accomplished in 5 min. at
1150 °C, and the total processing time was only about 20 min., while the conventional
sintering took about 4 h. High quality of the starting HAp powder is a key factor to
achieve transparency.
Specifically, the fine nanocrystalline HAp prepared by the
hydrothermal method has high purity, high thermal stability, high sinterability, which are
essential to fabricate transparent HAp ceramics.
4.6. Densification Kinetics of HAp
4.6.1. Introduction
It has been observed in the previous sections that by microwave processing, one can
achieve significant enhancement in sintering or densification of HAp. In the case o f the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
113
sintering of regularly dense HAp, the microwave effect was observed as the enhanced
densification; in the fabrication o f porous HAp ceramics, the microwave effect was mainly
noticed as enhancement in sintering, since in the latter case, connection between particles
to produce certain mechanical strength instead of densification was the main purpose of the
processing. To evaluate the microwave effect on the densification of HAp, kinetics study
was carried out.
4.6.2. Experimental Procedures
Densification kinetics of HAp was studied under both microwave and conventional
conditions.
U sing the DCPD hydrolysis derived HAp (triply ripened with calcium
chloride) as the starting material, pellets of 0.25 inch diameter were uniaxially pressed at
350 MPa, with appropriate PVA (2% solution) as the binder. The samples were sintered
by microwave and conventional methods in a temperature range of 950 to 1200°C for 5 to
60 min. The microwave sintering setup was the modified 900-W furnace with a turntable
(Sec. 3.2.). The conventional sintering setup was the one with comparable heating rates as
the microwave furnace. The heating rate in both microwave and conventional cases were
about 100°C per minute. The sintering time and temperature were the same in both cases.
The relative densities after sintering were m easured by weight and dim ensional
measurements to determine the kinetics of densification. Activation energy of densification
under both microwave and conventional conditions were derived from Arrehnius formula.
4.6.3. Results and Discussion
Densities of HAp sintered under microwave and conventional conditions in the
temperature range of 950 to 1200°C and soaking time for up to 60 min. show that density
increases as increasing sintering temperature or time. Figure 4.33 shows the sintered
densities as a function of sintering time at various temperatures.
In general, the influence of temperature on densification rate can be expressed by the
Arrhenius Equation (Ye et al., 1986):
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
114
%
1 0 0 -i
Relative
den sity,
90 80 -
mw1200°C
m w 1100°C
mw1000°C
mw 950°C
60 -
◦
0
10
20
30
40
50
60
Sintering time, min
d en sity,
%
100
80 cv1100°C
cv1000°C
cv 950°C
Relative
70 -
b
50
0
10
20
30
40
50
60
Sintering time, min.
Figure 4.33. Sintered density as a function o f sintering time o f HAp sintered by (a)
microwave and (b) conventional methods.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
115
K = A expC-Eg/RT)
where K is the densification rate, A is a constant with the same dimension as K, E a is the
apparent activation energy of densification, R the gas constant, and T the absolute
temperature in Kelvin. Taking logarithm gives
lnK = ln A - E a(RT )-1
Plotting InK versus 1/T gives Fig. 4.34. The tangent of the line in the plot is -Eg/R, from
which the apparent activation energy Ea can be calculated.
Using densification rates (Table 4.10) at 60% relative density, plot o f logarithm of
densification rate (InK) versus reciprocal absolute temperature (1/T) was made (Fig. 4.34)
It is noticed that the apparent activation energy for the densification o f HAp under the
microwave processing condition is about 40% lower than that for conventional sintering.
Since microwave sintering effect o f any material directly depend on the dielectric properties
of the material. The lower activation energy of HAp may indicate a relatively high dielectric
loss factor of this material, which might be attributed to the dipoles of hydroxyl ions in the
crystal structure of HAp. The tangent values for the lines for conventional and microwave
sintering are -2.1737 and -1.3452, respectively. Accordingly, the apparent activation
energies for conventional and m icrowave sintering are 180.7 and 111.8 kJ/m ole,
respectively.
4.6.4. Summary on the Densification Kinetics of HAp
Densification kinetics study of HAp in temperature range from 950 to 1200°C under
microwave and conventional conditions shows that microwave processing apparently
enhances densification of HAp. A t relative density of 60%, the apparent activation energy
for microwave sintering o f HAp is 112 kJ/mole, which is about 40% lower than the
conventional process (181 kJ/mole). Since the sintering temperature, time, and heating rate
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
116
in microwave and conventional processes were comparable, the lower activation energy
indirectly indicates that HAp has fair dielectric loss properties to effectively couple with
microwaves to enhance its densification.
Table 4.10. The InA values from the densification curves for Arrhenius plot at relative
density 60%.
Microwave
Conventional
1/T
K (l/m in.)
InK
K (l/m in.)
950°C (1223K)
8 .1 7 7 x l0 '4
0.0241
-3.7232
0.0160
-4.1382
1000°C (1273K)
7 .85 5 x l0 ’4
0.0588
-2.8336
0.0417
-3.1780
1100°C (1373K)
7.283x1 O' 4
0.1163
-2.1514
1200°C (1473K)
6.789xl0-4
Temperature
0.1795
InK
-1.7176
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
117
y = 13.737-2.1737x RA2
10000(1 fT), 1/K'
Figure 4.34.
Arrhenius plot o f the densification o f HAp under (a) microwave and (b)
conventional conditions.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
118
Chapter 5
MICROWAVE PROCESSING OF MULLITE CERAMICS
5.1. Introduction
Mullite, 3 A l 20 3 -2 Si0 2 , is one of the m ost commonly used ceramic refractory
materials. It has been considered an important material for high temperature structure
applications. The favored role of mullite for this is mainly due to its high melting point
(1828±10°C, M ah et
al., 1983), low thermal expansion coefficient, low therm al
conductivity, excellent resistance against therm al-shock, and good high-tem perature
mechanical properties (e.g., high strength and good creep resistance). Mullite is also
characterized by its low dielectric constant (e = 6.7, compared to 9.8 for AI2 Q 3) and thus it
is a promising candidate for substrates in electronic packaging (Aksay et al., 1991).
Moreover, mullite has a good transmission at high temperature in middle IR band, so that it
can be used as windows in middle IR spectrum, especially at high temperatures. Low
dielectric constant and optical transmittance of fine-grained polycrystalline mullite has also
triggered interest in its potential applications as a solid-state laser activator (Aksay et al.,
1991). However, it is not easy to obtain dense mullite because the temperature required for
the sintering is very high, usually in the range o f 1600 - 1700 °C under conventional
conditions.
High purity mullite powders are not easily found in the market compared to silicon
oxide and aluminum oxide powders. The use of reaction sintering of mullite is an attractive
approach (Rodrigo and Boch, 1985) to prepare dense mullite ceramics. Reaction sintering
of mullite may be carried out with fine crystalline alumina and silica powders as starting
material, but the sintering temperatures in such a case are very high, because both the solidstate reaction to form mullite and the subsequent sintering are sluggish at relatively low
tem peratures.
The sol-gel m ethod is a popular approach to obtain fine, pure,
homogeneous, and reactive mullite precursor powders. Both monophasic (polymeric) and
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
119
diphasic (colloidal) mullite gels are used in ceramic fabrication. Monophasic is a mullite
precursor mixed at molecular scale. Complete conversion of this kind of gel to crystalline
mullite can be achieved below 1000 °C (at about 970 °C). Correspondingly, the activation
energy of nucleation for this conversion was found to be 293-362 kJ/m ol (Li and
Thomson, 1991), but its sintering after crystallization requires temperatures around 16001700 °C or hot pressing (Huling and Messing, 1989). Diphasic mullite gel is a mullite
precursor m ixed at nanometer scale.
The crystallization temperature o f a colloidal
boehmite-silica diphasic gel was observed as high as 1364 °C (Huling and Messing, 1989).
Wei and Halloran (1988) calculated the activation energy of nucleation in this case to be
987±63 kJ/mol. The higher activation energy slows the kinetics of diphasic mullite gel and
allows it to be densified before crystallization to high density by viscous deformation of
amorphous silica (Wei and Halloran, 1988). In this way, it is possible to obtain sintered
mullite at lower temperatures with diphasic gel.
In order to fabricate high-performance mullite ceramics, it is necessary to tailor their
microstructures. Interlocked acicular grain structure, for example, is necessary when
creep resistance is desired, while fine and equiaxed grained mullite is suitable for IR
transparence or room temperature strength (Mroz andLaughner, 1989).
It is well known that by adding seeds to a precursor, crystallization direction can be
preferentially controlled, or crystallization temperatures can be lowered, since the energy
barrier for nucleation can be overcome in this way.
The crystallization temperature of monophasic mullite gel is significantly lower than
that of diphasic mullite gel. When appropriate amount of momophasic mullite gel is mixed
with diphasic mullite gel and heated, the crystallization o f monophasic gel at lower
temperature will provide diphasic mullite gel with many mullite crystallites which would
then work as in-situ seeds and promote the crystallization of diphasic mullite gel. Since the
mixing is conducted in the wet state, obviously it is easier to obtain best homogeneity and
finest seeds. Huling and Messing (1989) have reported their results on nucleation of such a
gel mixture knows as hybrid gel. They found that seeding diphasic gel with monophasic
gel can effectively decreased the grain size of the sintered mullite and remove intragranular
pores usually trapped in the sintered body, although the decrease in the m ullite
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
120
transformation temperature of diphasic mullite is limited (about 30 °C).
Microwave sintering usually offers a controllable grain-growth and more uniform and
finer grain sizes compared to the conventional sintering. The objective of this investigation
was to study the sintering behavior of m ullite in m icrowave processing, including
densification and mullite transformation of seeded and unseeded mullite gels.
5.2. Synthesis of Mullite Precursors
Sol-gel derived precursors are characterized by high purity, high surface area, high
activity, and high homogeneity. Mullite precursors used in this study were synthesized by
sol-gel methods. The mullite gels are the aluminosilicate gels with mullite stoichiometry
(3Al2 0 3 -2Si02), including monophasic, diphasic, and seeded gels. By different drying
methods, xerogels and aerogels were obtained. Xerogels were obtained by drying the gels
in air, while aerogels were obtained by supercritical drying. The details will be described
in the following sections.
5.2.1. Monophasic Gel
Monophasic mullite gel was prepared by mixing stoichiometric amounts (mullite,
3 Al20 3 -2 Si0 2
) of tetraethyl orthosilicate (TEOS) and Al ( N 0 3 )2 -9H20 dissolved in
absolute ethanol at room temperature, gelling at 60 °C, drying at 120 °C, and denitrating for
4 h at 465 °C. The xerogel thus obtained was ground with an agate mortar and sieved to 325 mesh (45 pm opening). The bulk density of the dried powder is 2.03 g/cm3.
5.2.2. Diphasic Gel
W ith addition of concentrated nitrate acid (H N 0 3) to get pH 2.0 (at room
temperature) in water, boehmite (AlOOH) was allowed to hydrolyze in the acidic water
overnight at 95 °C under reflux condition.
The solid/water ratio was about 1:50.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
121
Translucent boehmite sol was obtained at the end of the hydrolysis. The boehmite sol was
mixed by vigorous stirring with an ammonium-stabilized colloidal silica sol (Ludox,
DuPont Chemicals, Wilmington, DE) at room temperature. The A l 2 0^/SiO 2 molar ratio
was 3:2. The mixed sol was heated at 60 °C with continuous magnetic stirring to remove
excessive water, and this finally gelled at the same temperature by prolonged sitting.
The gel was dried at 110 °C then denitrated at 550 °C, and then ground and sieved to
fine powder o f -325 mesh. The bulk density of the powder was 2.94 g/cm3. The heating
process at 550°C was necessary to transform boehmite to y-Al2 0} and prevent cracking of
the pellets. In the earlier trials, it was found that all the pellets pressed from the gel powder
dehydrated at 400 °C cracked after microwave sintering, but those made the powder
calcined at 700 °C were intact after the same process. It is believed that the cracking of the
pellets of the 400 °C-heated powder occurred due to the transition of boehmite into yA12 Q 3. Further experiment showed that the minimum temperature for the denitration was
500°C. Therefore, in all the subsequent experiments, the powders were calcined at 500 550 °C. The heating time and temperature were limited to a minimum value to preserve the
highest possible specific surface area, ensuring the best reactivity of the resulting powder.
5.2.3. Seeded Diphasic Gels
In order to study the influence of seeding on the crystallization and densification of
diphasic m ullite gel in m icrowave processing, diphasic gels were seeded with (a)
monophasic gel and (b) crystalline mullite as seeds.
For gel seeding, the seeding (monophasic) gel as prepared in Sec. 5.2.1. (0.5 to 30%
by weight of dry material) was homogeneously mixed with the diphasic gel matrix by
magnet stirring when the gel was wet. The seeded gel was dried at 60°C followed by
120°C to get xerogel. The xerogel was further dehydrated and denitrated at 550°C after
grinding into fine powder.
For crystalline mullite seeding, submicron mullite seeds were prepared by grinding
crystalline mullite and separating appropriate size by decantation in deionized water. The
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
122
fine mullite particles in the supernatant were collected by drying as seeds. The seeds of 2
or 5% by weight were added to the diphasic sols as prepared in Sec. 5.2.2. before gelation.
The drying and powder-making processes were identical to that in monophasic gel seeding.
5.2.4. Diphasic Mullite Aerogel
Aerogels are porous materials produced by a sol-gel process followed by supercritical
drying o f the wet gel in an autoclave (Fricke & Emmerling). Aerogels have a spongelike,
open-pore structure with a large inner surface. The pore size o f aerogels is in the range
between 1-100 nm. Monolithic aerogels are used in Cerenkov detectors in high-energy
physics; translucent granular aerogels can be used as insulator for the purpose of passive
solar energy usage;
opacified aerogel powders are being tested as substitutes for
chlorofluorocarbon-blown polyurethane foams and as thermal super insulations in heatstorage systems; experiments have been performed with aerogels as catalytic substrates,
acoustic impedance matching layers, containment for fusion fuels, and gas filters; and
aerogels are also used in radioluminescent light and energy sources.
Since aerogels free from agglomeration, and still possess high surface area which
means higher activity, the sinterability of mullite aerogel is very high. Rahaman et al.
(1988) sintered the mechanically compacted aerogels prepared by supercritical extraction
with C 0 2 to nearly theoretical density below 1200°C. In this study, mullite aerogel was
prepared by supercritical extraction with methanol (CH 3 OH). In the preparation, the same
wet diphasic gel as prepared for the diphasic xerogel (Sec. 5.2.2.) was introduced in glass
cells and immersed in a methanol (CH3 OH) bath at room temperature to exchange the water
trapped in the gels. After 24 h, the immersing liquid was replaced by fresh absolute
methanol to allow further exchange to take place. The exchange was repeatedly carried out
three times. After the exchange, the cells were placed in an autoclave with 17-20 mL
methanol in it fo r supercritical drying.
The supercritical drying was conducted at
260°C,1200 p.s.i., which was slightly above the supercritical conditions o f methanol,
240°C, 1155 p.s.i.. The system was heated from room temperature to 260°C in 3 h, and
soaked at 260°C for 30 min. Then the methanol was extracted under the supercritical
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
123
conditions under flowing argon. Since the methanol is removed under the supercritical
conditions, at which there is no distinction between liquid and gaseous phases, and absence
of surface tension, practically no shrinkage, thus highly porous and translucent aerogels are
resulted.
5.3. Mullite Transformation o f Diphasic Gels
5.3.1. Introduction
Since the particle size of the diphasic mullite gels is in nanometer scale, so that the
mullitization temperature of diphasic gel is significantly higher than that of monophasic gel.
Now that microwave processing can enhance sintering of many ceramic materials, it may
also affect the mullitization of diphasic mullite gel. In the current study, microwave effect
on the crystallization of seeded and unseeded diphasic gels was investigated.
5.3.2. Experimental Procedure
The microwave effect on the crystallization of diphasic aluminosilicate gel was
studied using the diphasic xerogel. Influence of seeding on the crystallization was also
studied. The hybrid gels as described in Sec. 5.2.3. containing up to 30% monophasic gel
were used for this study. For convenience, the starting powders were pressed into pellets
of 0.25 in. diameter.
Samples were heated in a microwave field as well as in the conventional furnace at
various temperatures between 1250 to 1400°C for 10 min. Both qualitative and quantitative
XRD analyses were performed on the samples heated under various conditions. For the
quantitative analysis, a-alum ina was used as the internal standard. The XRD integrated
intensities o f mullite at (210) and that of corundum at (113) were used to evaluate the
degree o f conversion of the diphasic gel to the crystalline mullite.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
124
5. 3.3. Results and Discussion
5.3.3.1. Crystallization of the Gels
Crystallization o f an amorphous phase is a process toward a state of low er free
energy, so that it is an exothermic process. Therefore, the exothermic peaks on the DTA
curves correspond to the crystallization temperature. DTA study shows that monophasic
mullite gel crystallizes at 970°C (Fig. 5.1.), and diphasic gel crystallizes at 1328°C (Fig.
5.2.). The difference in the crystallization temperature can be explained by the degree of
mixing scale of the compositions. Monophasic gel is an amorphous precursor with atomicscale arrangement of -Al-O-Si- groupings very similar to that of mullite (Sunderesan &
Aksay, 1991), so that it readily crystallizes to m ullite at low er tem perature. The
homogeneity between alumina and silica sources in diphasic gel is at nanometer scale, thus
the crystallization is controlled by the reaction between the two phases and requires a higher
temperature.
Qualitative XRD studies show that under conventional conditions, diphasic mullite
gel transforms to crystalline mullite at around 1325°C by conventional heating (Fig. 5.3),
whereas by microwave heating, crystalline mullite was detected in the sample heated at
1250°C (Fig. 5.4.).
This indicates that m icrow ave heating accelerates m ullite
transformation o f diphasic gel. The appearance o f phases as sequence of temperature,
however, was the same in both conventional and microwave processing. Silica remains
amorphous before mullitization, while alumina exists as 8-Al2 0 3 before mullitization.
Therefore mullite transformation of this diphasic gel is the reaction between 5-Al2 0 3 and
amorphous silica. This result is in agreement with Wei and Halloran (1988).
5.3.3.2. Seeding Effect of Diphasic Gels
The results of quantitative XRD o f the conventionally and microwave fired diphasic
mullite gels are listed in Tables 5.1. and 5.2., respectively. Seeding with 5% or less mullite
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
125
a. qo
a
a
z
DEGREES
UJ
V
4.00'
a oo
60.00
190.00
320.00
450.00
5B0.00
710.00
TE M PER A TU R E
F igure 5,1.
970.00
CC)
DTA therm ogram o f m onophasic alum inosilicate gel, show ing that
crystallization of the gel takes place at around 970°C. Heating rate was 10°C per minute.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
126
100
1328*C
LU
UJ
oz
(J
UJ
□
acoi5un
22aao
34aoo
uaoo
aaoo
m oo
oka
loeam
ukoo
wdaoo
TEMPERATURE <C)
Figure 5.2. DTA thermogram o f diphasic gel. The exothermal peak at 1328'C indicates
the occurrence of mullitization.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
127
soo c
1300X5
****"*
»",
1325X 5
... .
T w o theta, d e g re e
Figure 5.3. XRD patterns o f diphasic mullite gels heated conventionally, indicating that
mullitization did not occur below about 1325*C. Phases are (a) amorphous, (b) 8-Al2 0 3,
(c) mullite + 8 -AI2 O3 , and (d) mullite.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
128
500 DRIED
1250X5
MW 1300C
20
10
30
40
00
00
70
Two theta, degree
Figure 5.4. XRD patterns showing that crystallization o f the diphasic mullite gel starts at
1250°C when subjected to microwave irradiation. Detected phases are (a) amorphous, (b)
5-Al2 0 3,
(c)
mullite + 5-Al2 0 3, and (d) mullite.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
129
Table 5.1. Mullite contents (%) in the conventionally heated diphasic mullite gel.
seeds%
0
0.5
1
5
10
20
1250°C
0
0
0
0
0
0
—
1275°C
0
0
0
0
0
0
14.33
1300°C
0
0
0
0
25.73
32.23
2 1 .0 0
1325°C
75.1
81.85
67.81
54.12
74.01
56.54
31.85
1350°C
77.24
83.66
66.36
55.72
55.26
26.54
1400°C
91.74
83.98
65.78
53.53
27.07
85.79
30
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
130
Table 5.2. Mullite contents (%) in the microwave heated diphasic mullite gel.
seeds%
0
0.5
1
5
10
20
30
1250°C
0
34.11
31.81
37.16
52.00
19.11
1275°C
55.93
79.94
74.50
40.51
71.27
45.96
21.57
1300°C
79.90
77.66
69.58
48.69
82.29
49.94
25.41
1325°C
84.75
72.67
67.85
54.17
85.05
52.15
29.11
1350°C
87.41
90.77
71.41
55.95
98.06
57.28
25.48
1400°C
91.86
84.25
93.36
69.42
90.53
64.87
34.48
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
131
seeds did not show any improvement in crystallization. W eak XRD intensities of mullite
show up in the samples with 10% or more seeds, when fired at 1300°C.
At or above
1325°C, the XRD intensities become stronger. At the same processing temperature, the
rate of mullite conversion decreases as the content of seeds increases. For the same amount
of seeds, conversion rate increases with increasing temperature.
Under microwave conditions, crystalline mullite phase was detected on the sample
heated at 1250°C (Fig. 5.3). W ithout seeds, the conversion rate was about 60% at
1275°C, and about 92% at 1400°C. In the seeded samples, even with only 0.5% seeds,
34% of the gel got converted to mullite at 1250°C. At the same temperature, for the sample
containing same amount of seeds, microwave processed samples always show higher
conversion rate. Therefore, microwave irradiation promotes seeding effect on diphasic gel.
The conversion rate increases as increasing amount o f seeds up to 10%, and then
decreases. This can be explained from the point o f view of nutrient (gel) available to the
seeds for the growth of the crystallites. The process o f mullitization of the gel includes
nucleation followed by crystal growth. In the seeded gel, mullite crystallites converted
from monophasic gel at lower temperatures function as seeds in the diphasic gel, so that at
higher temperatures, the diphasic gel around each seed will supply crystal nutrient to the
seed with A12 0 3 to S i0 2 ratio of mullite stoichiometry to form crystalline mullite. This is
an epitaxy process. Within the limitation o f the crystal nutrient available to each seed to
grow to a mature crystal, increase seeds will promote mullite transformation, when the
numerical concentration o f seeds is too high, the nutrient available to each seed decreases to
such a level that no well defined crystals will be obtained. This explains why in microwave
processing o f seeded diphasic mullite gel at 1250°C, XRD intensity increases with seeds to
10%, then decreases with further increase in seeds. With 30% seeds, mullite could not
even be detected by XRD.
Compared with the case of microwave processing in which mullite was detected at
1250°C, it is seen that the m icrowave enhancem ent in crystallization o f diphasic
aluminosilicate gels o f mullite stoichiometry is about 75-100°C.
Although it is well known that the detecting limitation of XRD for a crystalline phase
is about 5%, XRD did not reveal the existence of the seeds converted from the seeding
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
132
(monophasic) gel in the diphasic matrix, even in amount up to 30%, when conventionally
fired at temperatures below 1325°C. Since the monophasic gel crystallizes at 970°C, there
is no reason to doubt about the existence of crystalline mullite from the monophasic phase
after firing at such high temperatures. There are at least two reasons to explain this
phenomenon. First, the crystallites were too small to induce X-ray diffraction but variable
amount of line broadening would occur. Second, the microabsorption effect (Cullity,
1978) o f the untransformed diphasic gels would further weaken the diffraction. The final
result is that the XRD intensities totally disappeared.
5.4. Microwave Sintering o f Mullite Ceramics (Regular and Transparent)
5.4.1. Introduction
Due to the low diffusion rate of the ions in mullite ions, the sintering of mullite
ceramics generally requires very high temperature (1600-1700°C) and long sintering times
(at least a few hours). Reaction sintering may apparently lower the sintering temperature
for mullite. Crystalline alumina and silica powders may be used for the reaction sintering
to obtain mullite, but high temperature is still required in this case since both components
are in crystalline state and the homogeneity between the reactants is very limited due to the
existence o f high activation energy barrier.
Adopting an approach of diphasic gel,
however, the mullite precursor can be prepared with high reactivity and high homogeneity
to favor reaction sintering. In this study, microwave sintering o f mullite ceramics was
conducted using diphasic gels as the starting material, and also using monophasic gel and
crystalline materials for comparison. Transparent mullite ceramics were sintered using the
mechanically compacted diphasic aerogel, with the xerogel o f the same composition as
comparison.
5.4.2. Experimental Procedure
The monophasic and diphasic gels as well as the seeded diphasic gel as described in
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
133
Sec. 5.2. were used for sintering experiments. For comparison, fine powder o f crystalline
mullite (from Chichibu Cement Co., Ltd., Japan) was also used in one experiment. The
m icrowave sintering was carried out in the m odified 900-W, 2.45 GHz m ultim ode
microwave furnace. Conventional sintering was conducted in a high temperature, high
heating rate electric furnace. Both sintering setups have been described in detail in Sec. 3.2
and 3.3. For the best densification effect, step sintering was adopted for samples of
diphasic gels, in which samples were processed below m ullitization tem perature for
densification by viscous flow and then crystallized at elevated temperatures. Various
sintering schedules were used. Appropriate amount of PVA (2% solution) was used as a
binder during compaction.
For transparent mullite ceramics, the diphasic aerogel as described in Sec. 5.2.5 was
used as starting material. Thin circular pellets of 0.25 or 0.5 inch diameter were compacted
by uniaxial pressing at 280-350 MPa. To avoid contamination, no binder was used. The
as-compacted green pellets were translucent. Microwave sintering o f transparent mullite
was carried out at 1300°C for 10 min.
Conventional sintering was carried out in a
dilatometer furnace heating at 5°C/min. to 1320°C. Once the specimen temperature reached
1320°C, the power was shut o ff to let the furnace cool down naturally.
The peak
temperature was so chosen to provide best densification with least crystallization, because
the diphasic gel transforms to mullite at 1330°C.
Thermal mechanical behavior of the monophasic gel, diphasic gel, and diphasic
aerogel, was studied by dilatometry. Fine powders of the above three gels were compacted
into pellets of 0.25 in. diameter and then subjected to heating in a dilatometry furnace (Sec.
3.4.4.) at 3°C/min. from room temperature to 1500°C. During heating, the change in
thickness of the specimen as a percentage of the original dimension was continuously
monitored and recorded by the computer of the system. The thermal mechanical analysis
(TMA) curve was then plotted.
5.4.3. Results and Discussion
Tables 5.3. through 5.9 list the density results of the mullite samples sintered under
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
134
various conditions, including influence of starting material, green density, and sintering
schedule on densification o f mullite. It is seen that the crystalline mullite and monophasic
mullite gel are difficult to sinter at temperatures up to 1600°C, while diphasic mullite gel
can be sintered to around 95% relative density. The sinterability of crystalline mullite
precursor is the lowest; that o f m onophasic gel is also poor, because it converts to
crystalline form below the sintering temperature; the sinterability of diphasic gel is quite
high, since it densities before crystallization; the sinterability of the mixture of diphasic gel
and monophasic gel of equal amounts is in between that of diphasic and monophasic gels.
The sinterability difference between monophasic and diphasic gels is more clear from
the TMA curves (Fig. 5.5.). The first sharp shrinkage of monophasic gel takes place at
970°C, corresponding to the crystallization of the gel (Fig. 5.1.). After this, the shrinkage
with temperature slows down. The initial shrinkage of diphasic xerogel starts at about
700°C through the viscous flow of silica gel. Then obvious sintering takes place at about
1050°C when the viscosity of silica gel further lowers at elevated temperatures. This
process continues till around 1300°C, presumably as a result of nucleation of mullite
crystals. As mullite transformation takes place at 1328°C (Fig. 5.2.), the viscous flow
mechanism slows down, and so does the sintering. The shrinkage of diphasic gel is much
greater and takes place at higher temperature than that of monophasic gel, indicating that the
diphasic gel favors sintering of mullite ceramics.
It is also found that seeding does not enhance densification of the diphasic mullite gel.
This is due to the fact that seeding lowers m ullitization tem perature and enhances
conversion of diphasic gel to crystalline mullite which has low diffusion rates than the gel
itself. As expected, increasing compaction pressure or green density increases sintered
density.
Step sintering works well for diphasic mullite gel, since it allows the diphasic gel to
densify before crystallization and the densification of the gel by viscous deformation is
more efficient than the solid state diffusion of the crystalline mullite.
Figures 5.6-8 show the microstructures of sintered mullite samples of diphasic gel.
The samples in Fig. 5.6 were polished and thermally etched at 1400°C for 30 min. The
microstructure of the microwave sintered specimen (Fig. 5.6.a) is more dense than the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
135
Table 5.3. Relative density (%) of monophasic mullite gel sintered at 1200°C x 20 min.
compaction, M Pa
microwave
conventional
40
140
280
420
560
700
840
50.04
49.52
51.72
54.07
52.78
56.76
53.44
57.05
56.16
46.16
46.01
46.75
Table 5.4. Comparison of sintered density of different starting materials (microwave
sintering: 1300°C x 10 min. + 1600°Cx 10 min.).
green density, %
sintered density, %
crystalline
monophasic
59.58
40.15
66.99
67.80
diphasic
44.66
91.40
starting material
Table 5.5. Density of mullite conventionally sintered (1550°C x 20 min., 3h total heating).
starting materials
green density, %
sintered density, %
diphasic gels (D)
monophasic (M)
47.14
40.15
90.60
67.48
D : M = 50 : 50
47.64
74.93
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
136
Table 5.6. Relative density (%) of the seeded diphasic mullite gels after sintering.
________ 1200°C___________________
1200x20* 1200x40
10 min.
+1400x5 +1400x5 +1400x5 +1500x20 +1500x20
20 min.
40 min.
60 min.
1200x60 1200x20 1200x40
m icrow ave
no seed 76.94
79.05
81.14
86.09
89.64
90.58
88.91
89.41
90.72
2.5%
74.61
78.93
82.40
83.32
84.76
84.79
85.22
85.73
85.36
5.0%
73.76
81.21
81.35
83.24
84.85
85.59
85.82
85.27
85.79
D:M=1 63.73
65.25
66.52
67.64
67.21
67.17
67.79
71.57
72.49
87.41
92.04
91.22
conventional
no seed 61.79
66.53
70.09
70.35
90.51
2.5%
55.98
62.39
65.43
68.34
84.97
—
85.68
87.61
87.88
5.0%
56.67
64.74
65.45
71.34
86.13
—
83.29
87.02
86.49
D:M=1
52.65
55.25
57.46
60.27
67.71
—
67.38
70.78
70.76
* Temperature (°C) x time (min.).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
137
Table 5.7. Sintered density of seeded diphasic mullite gel (microwave sintering).
densitv. %
Seed%
green
no seeds
2 .0
%
5.0%
sintered
green
sintered
1300°C x 20 min.
90.06
45.53
1450°C x 2 0 min.
48.20
94.10
44.87
44.89
47.07
47.22
87.91
87.84
92.00
91.00
Table 5.8. Relative density of diphasic mullite gel after microwave step sintering (1200°C
x 60 min. + 1500°C x 20 min.).
comp, pressure (MPa)
sintered density (%)
140
210
280
420
560
82.93
86.46
89.08
89.93
91.15
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
138
Table 5.9. Density of unseeded diphasic mullite gel under various microwave sintering
schedules.
Sintering schedule
(°C x min. + °C x min.)
green density, %
sintered density, %
1300 x 15 + 1400 x 5
46.24
87.22
1300 x 15 + 1500 x 20
44.98
87.56
1300 x 30 + 1475 x 20
44.48
86.73
1300 x 30 + 1550 x 20
47.47
94.63
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
139
10
5
200
Tine
Percent
(nin)
Expansion
400
-1 5
0
250
500
750
1000
1250
1500
Tenperature (°C)
o
500
400
Percent
Tine
1 2 9 2 . »C
Cnin)
Expansion
-20
B
Tenperature (°C)
Fig. 5.5. Dilatometry curves of (a) monophasic and (b) diphasic mullite xerogels heated at
3°C/min.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
140
conventional sample (Fig.5.6.b). A lamellar structure in the microwave sintered sample is
obvious, while the sample conventionally sintered does not show the same ordered
structure. Figure 5.7. shows the as-sintered surface (a) and fracture microstructure (b) of a
diphasic mullite sample sintered by microwave processing at 1300°C for 20 min. The
relative density of this sample was 94%. The grain size ranges between 1-5 |im. The two
specimens shown in Fig. 5.8. were step-sintered conventionally (1250°C x 30 min.
followed by 1500°C x 30 min.). Sample (a) was pure diphasic gel, while (b) was seeded
by 5% mullite seeds. It can be seen that the average grain size of the seeded sample was
significantly smaller.
This is attributed to the increased nucleation frequency.
The
observation is in good agreement with what reported by Huling and Messing (1989). The
residual porosity of about 5% may be attributed to the intragranular pores.
Figure 5.9. shows the mullite ceramic pellets sintered by (a) microwave and (b-d)
conventional methods. Among them, only the samples made from aerogel (a, b, and d) are
transparent. Sample (c), made from diphasic xerogel, is opaque, although it was sintered
under identical conditions (conventional, 5°C/min. to 1320°C, no holding) as (b) and (d).
Dilatometry study (Fig. 5.10.) shows that except for an additional shrinkage below
479°C, the thermal mechanical behavior of the diphasic aerogel is basically the same as the
xerogel (Fig. 5.5.b).
The agglom eration in the powders decreases hom ogeneity and hinders the
achievement of full densification. It introduces additional intergranular porosity which may
partially rem ain in the sintered body as residual porosity.
The large porosity
and
inhomogeneity of microstructure strongly scatter visible light, so that transparent mullite
was not achieved with the diphasic xerogel. The aerogel preparation is a key for the
fabrication of the transparent mullite ceramics, since it prevents the gel from agglomeration.
Taking 3.56, 2.2, and 3.17 as the densities of 8-Al2 0 3, amorphous silica, and crystalline
mullite, respectively, calculation shows that the mullite transformation will leave about 3%
porosity in the mullite ceramics made from pure diphasic mullite gel. These residual pores
will decrease transparency in the case of transparent mullite ceramics. This problem may be
solved by seeding. Huling and Messing (1989) reported that sintering the unmodified
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
141
Figures 5.6 M icrostructures of diphasic m ullite gel step-sintered (1200°C/30 min. +
1500'Cx20 min.) by (a) microwave and (b) conventional method. The samples were
polished and thermally etched at 1400*C for 30 min.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
142
Figure 5.7. Microstrucure of (a) as-sintered surface and (b) fracture surface o f a diphasic
mullite sample microwave sintered for 20 min. at 1300*C to relative density o f 94%.
i
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
143
2 5 KU
1 0 . 5KX
1U 00 0 5
YF230D
Fig. 5.8. Microstructure of conventionally sintered (1250°Cx30 min. + 1500°Cx30 min.)
specimen of (a) pure and (b) seeded (5%) diphasic mullite gels.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
144
Transparent Mullite Ceramics
Ceramics
4 .
Figure 5.9. M ullite ceramics sintered by (a) microwave (1300*Cxl0 min.) and (b, c, d)
conventional (b, c: 1300’CxlO min.; d: 5'C/m in. to 1320*C without holding) methods.
Sample (c), made from diphasic xerogel is opaque, while the rest made from aerogel show
transparency.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
145
o
Tine
(n ln )
sag
a
258
508
250
to w
1258
1588
Tenperature (°C)
Figure 5.10. Dilatometry curve o f diphasic mullite aerogel heated at 3 °C/min.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
146
colloidal (diphasic) gel for 2 h at 1550°C leaves many faceted intragranular pores that are
removed only by increasing the sintering time or temperature. In contrast, they found that
the increased nucleation frequency and resulting increased grain-boundary area in the
hybrid gels (diphasic gel seeded by monophasic gel) allows short-range vacancy diffusion
to promote pore removal at lower temperatures or shorter times. Thus by using hybrid gel,
the transparency of transparent mullite ceramics could be further improved by effectively
remove the intragranular pores.
XRD study indicated that the transparent sample sintered by conventional method was
not mullitized, while that sintered by the microwave method was crystalline. This is in
good agreement with the results on the crystallization of diphasic gels, i.e., mullitization
takes place at low er temperatures in the m icrowave processing than in conventional
sintering. However, it is interesting that mullitization in the microwave processed sample
did not reduce the transparency, showing that transparent mullite ceramics can be fabricated
directly by sintering.
5.5. Densification Kinetics of Diphasic Mullite Gel
5.5.1. Introduction
It has been seen that the densification o f diphasic alum inosilicate gels under
m icrowave conditions is substantially enhanced. To better understand and evaluate the
microwave effect, a kinetic study was carried out. Since the densification of the diphasic
gels is mainly attributed to the viscous sintering of the gels, the firing process in the
kinetics study was limited below the mullitization temperature. This means that there is no
crystallization occurring during the firing so that the kinetics applies only to a single
viscous densification process.
5.5.2. Experimental Procedure
Pellets of quarter inch diameter were pressed at 350 M Pa unaxially, then sintered at
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
147
1100, 1150, 1200, 1250, and 1300°C for various durations, under microwave and
conventional conditions, respectively. Same sintering setups as described in Sec. 5.3. and
5.4. were used. Densities after sintering were measured and densification (density versus
time) curves were plotted. By plotting the natural logarithm of densification rate versus the
inverse absolute temperature (1/T), the apparent activation energies for densification were
derived from Arrhenius equation.
5.5.3. Results and Discussion
The densification curves o f diphasic m ullite gel are shown in Fig. 5.11. The
densities achieved by microwave processing are always higher than that by conventional
sintering (Fig.5.12). For example, at 1100°C, the density (48.54%) after 5 min. of
microwave processing is equal to that by conventional sintering for 60 min.; at 1200°C, 60
min. of conventional sintering gave 67.54% relative density, while microwave processing
for 10 min. achieved 68.52%, and 78.16% at 60 min. Similarly, the density achieved at
1250°C by 60 min. of conventional sintering is 81.74%, which could be obtained within
10 min. by microwave processing. At 1300°C, the difference is less obvious.
In microwave processing, the densification in the temperature range from 1150°C to
1250°C is very significant. Beyond this range, the density increase with temperature is
much smaller because of mullite transformation. Under conventional conditions, however,
the density change between 1250°C and 1300°C is still large, since mullitization has not
started in this case.
By the same treatment as in Sec. 4.6.3., the densification rates K as the tangents of
the densification curves (Fig.5.11.) at 50% relative density at various temperatures. The
values are listed in Table 5.10. Plotting InK versus 1/T, two lines are obtained (Fig.
5.13.). For microwave processing,
InK = 29.635 - 4.7675 T 'l
Accordingly, the apparent activation energy
Ea = 4.7675 x 104 x 8.314 = 396.4 (kJ/mole)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
148
«niiM‘,*l,,,1Bt
IIIIIIIIIH I« ',,,1 m ,,,I ,,,,,I I ,I I A
o ,,IIMI
— O —
(A
C
mnHij^iiiini
•a
—O "
a>
0)
>
ro
a)
DC
•IIIW#""'*1
mw1300°C
mw1250°C
mw1200°C
mw1150°C
mw1100°C
Sintering time, min.
Q
uihh111111"
■' 0 ■
■mitiQiiiiii
(0
c
a>
•a
>
*->
cv1300°C
cv1250°C
■"A cv1200°C
IIIIWI0 IIIIIH cv1100°C
llllllllllll■••|■■l■■•■ln" ," ,l,,,,,,,l,,ll^ ll," , .................
•
<u
DC
20
40
Sintering time, min.
Figure 5.11. Densification curves of diphasic m ullite gel under (a) microwave and (b)
conventional conditions.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
149
54
80
52
jfi
50
70
5k
«C
*ao
©
*0
46
60
o
44
1 100°C
40
20
Sintering
30
40
tim e,
80-
SO
1200°C
DC
microwave
•hmohm> conventional
"m ^ ^ m
42
«*•*#«*«
40
60
0
10
m in .
20
S in te rin g
30
microwave
conventional
40
tim e,
50
60
min.
"
^
5k
C
SO
1300°C
1250°C
microwave
conventional
o
10
20
S i n t e ri n g
30
tim e,
40
50
60
m in .
0
10
20
S in te rin g
30
tim e,
40
50
60
min.
Figure 5.12. Comparison in densities of diphasic mullite xerogel sintered by conventional
and microwave processing at various temperatures.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
150
Table 5.10. Kinetics values for activation energy calculation of diphasic mullite gel.
microwave
m
conventional
K (l/m in.)
InK
1150°C (1423K) 7.027xl0-4
1.9385
-3.9433
1200°C (1473K) 6.789xl0-4
7.5927
1250°C (1523K) 6.566xl0-4
17.400
temperature
K (l/m in.)
InK
-2.5780
I.5466
-4.1691
-1.7487
II.0 0 4
-2.2069
59.395
-0.5210
1300°C (1573K) 6.357x10-4
y = 29.635 - 4.7675X R A2 = 0.985
-
2-
c
•
O
-3 -
microwave
conventional
-4 -
y = 53.21 4 - 8.4486X RA2 = 0.999
6.2
6 .4
6.6
6.8
7 .0
7 .2
10000(1/T), 1/K°
Figure 5.13. Arrhenius curve of diphasic mullite gels sintered by (a) microwave and (b)
conventional methods.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
151
Similarly, for conventional sintering,
InK = 53.214 - 8.449 T ' 1
and
Ea = 8.449 x 8.314 x 104 = 702.4 (kJ/mole).
It is seen that under the microwave processing conditions, the apparent activation
energy for the densification of diphasic mullite gel is significantly (about 44% ) smaller
than that under conventional sintering conditions.
The smaller activation energy in the microwave processing indicates that the diphasic
gel couples well with microwaves, which in turn implies that the dielectric loss of the
diaphasic gel at high temperature is fairly high. This should be attributed to the special
microstructure and the particle size o f the diphasic gel. In the diphasic gel, the alumina
particles exist in 8-Al2 0 3 of low crystallinity, and the silica exists in amorphous state.
Additionally, both phases are highly divided (high specific surface area, or very fine
particle size). The combination of these factors makes the diphasic gel system different
from the crystalline materials of the same components (crystalline alumina and silica), as
well as the crystalline mullite, creating unique conditions that may favor the microwave
coupling.
5.6. Summary on Microwave Processing of Mullite Materials
Monophasic, diphasic, hybrid, and seeded aluminosilicate gels were synthesized by
sol-gel methods. Xerogel was obtained by drying these materials in air. Aerogel was
prepared by drying the diphasic gel under supercritical conditions.
M ullite ceramics were sintered using the above materials. It was found that in the
temperature range between 1200 to 1500°C, for the time period of an hour, diphasic gels
sinters well. The sinterability of the crystalline mullite is poor, the monophasic gel
crystallizes before temperature reaching this range thus behaves like the crystalline mullite,
the sinterability of the seeded gels is in between the diphasic gel and the crystalline mullite.
M icrowave processing accelerates the mullite transformation o f the diphasic gel,
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
152
lowering transformation temperature by about 75-100°C. Microwave processing also
enhances densification of the diphasic aluminosilicate gels. The study on the densification
kinetics of the diphasic gel shows that the apparent activation energy of this material under
microwave processing conditions is 396 kJ/mole, while that in the conventional case is 702
kJ/mol. The significantly lower activation energy indicates that the diphasic aluminosilicate
gels couple well with the microwaves. It also implies that the dielectric loss factor of the
diphasic gels at high temperatures is fairly high. The observed high microwave coupling
phenomenon of the diphasic gel is attributed to the unique characteristics of this gel,
including the high free energy state of the two components, particle size, high porosity,
high surface area, and high homogeneity, etc. The interaction of microwaves with mullite
gels will be discussed further in Chapter 8 .
Regular m ullite ceramics with relative density up to 95% were fabricated by
microwave sintering using the diphasic gel as starting material. Step sintering works well
for the diphasic gels in both microwave and conventional processes.
Transparent mullite ceramics were fabricated at ambient pressure by microwave at
1300°C and conventional processes at 1320°C, respectively, using the diphasic aerogel.
The transparency of the obtained samples is good, but might be further improved by using
hybrid gel and/or optimizing sintering schedule.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
153
Chapter 6
MICROWAVE SINTERING OF OTHER CERAMICS
Besides HAp and mullite, alum ina and [NZP] ceramics were also sintered by
microwave processing. The precursors of these two materials were acquired directly from
commercial sources.
6.1. Microwave Sintering of Alumina
6.1.1. Introduction
Alumina is a very important material and exists in various polymorphs. a-A l 2 0 3 ,
corundum, a crystal form of alumina, is widely used in ceramic industry for numerous
applications. The melting temperature of a-Al 2 0 3 is over 2000°C, so that the sintering of
alumina requires very high temperatures. Microwave sintering of alumina has been carried
out (Cheng et al., Patil et al.) by many investigators who used crystalline alumina powder
as starting material. In this study, an amorphous alumina was used as starting material to
investigate the sintering of A12 0 3 in a microwave field.
6.1.2. Experimental Procedure
Two starting materials were used in this study: one is crystalline and the other is
amorphous. The crystalline powder is a fine a-alum ina generally used in polishing
(Buehler). The average particle size is 0.05 |0.m. The amorphous powder is a commercial
material (Rhone-Poulenc, France). The powder was compacted into circular pellets of a
half inch diameter. No sintering aids were used. Microwave sintering was carried out at
1500°C for 20 min. in the modified 900-W m icrowave furnace with the turntable.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
154
Conventional sintering was carried out in the high heating rate electric furnace. Heating
rates in both cases were about 100°C/min. Green and sintered densities were measured.
Microstructure of the sintered sample o f amorphous alumina was examined by SEM.
6.1.3. Results and Discussion
Crystalline alum ina powder with the green density of 52%, when sintered by
microwaves at 1500°C for 20 min., produced a relative density of 78%, compared with
76% achieved by conventional sintering.
In this case, microwave enhancem ent in
densification is very little.
The density data for the amorphous starting material are listed in Table 6.1. Although
the samples were not sintered to high densities, microwave effect is clearly seen. At
1500°C, the microwave sintered density is 26% higher when sintered for 20 min., and
31% higher when sintered for 60 min., than the conventionally sintered samples.
SEM micrograph in Fig. 6.1. shows that the porosity of the sintered alum ina is
consisted o f two types o f pores: intergranular and intragranular. The primary particles are
only about
1 -2
jxm, while the agglomerates of tens or hundreds of micro are composed of
numerous primary particles. Under the experimental conditions, sintering mainly occurred
between the tiny primary particles. Further densification could be achieved by increasing
temperature, time, or improving the morphology of starting material.
The microwave effect on the densification of amorphous alumina was found to be one
order of magnitude higher than that on crystalline alumina, although the starting material of
the latter was very fine, and the green density o f the compacts was high. This again
indicates that the amorphous alumina couples with microwaves more efficiently than the
crystalline alumina. It is believed that the high free energy state of the amorphous alumina
and the scattering effect to the microwaves made the difference. The related mechanisms
will be discussed in Chapter 8 .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
155
Table 6.1. Density data of RP-alumina before and after sintering (1500°C).
microwave
20
min,
conventional
60 min.
20
min.
60 min.
Green, %
38.70
39.54
39.70
39.76
Sintered, %
59.97
64.34
47.60
48.97
microwave effect
20
min.
26%
60 min.
31.4%
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 6 . 1 . Microstructure of the alumina sintered by microwaves at 1500°C for 20
min.using from amorphous as starting material.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
157
6.2. Microwave Sintering of [NZP] Ceramics
6.2.1. Introduction
C a 1.xSrxZr4 P 6 0
24 ,
(0<x<1.0), or CSZP, is an important member of a large structural
family o f low thermal expansion materials known as [NZP] system after the parent
composition of sodium zirconium phosphate, NaZr2 P 3 0
12
(Alamo and Roy, Roy et al.,
Agrawal and Stubican). The potential applications in superionic conductors, radwaste
management and low thermal expansion ceramics, have made the study o f these materials
very attractive. Synthesis and conventional sintering, as well as characterization of CSZP,
have been carried out recently by Limaye et al. Microwave sintering o f these materials,
however, has not yet been reported. It is the objective o f the present study to show the
possibility of sintering CSZP ceramics by microwave processing, and the influence of
microwave interactions on the resultant ceramics.
6.2.2. Experimental Procedure
The precursor powder of C a 0 -5 Sr0 5 Zr4 P 6 0 24 was synthesized by a solution sol-gel
method, as previously described in the literature (Limaye et al.). Using fine powders of
-325 mesh, cylindrical pellets of 12.7 mm (0.5 in) in diameter were uniaxially pressed at
various pressures up to 840 MPa. An appropriate amount of 2% aqueous solution of
polyvinyl alcohol (PVA) was used as a binder during compaction.
Microwave sintering was carried out in a 500 W, 2.45 GHz commercial microwave
oven (GE, model J43) with suitable modifications as described in Sec. 3.2. Multipellet
sintering was conducted. The pellets were placed on a Fiberfrax™ insulation layer and
then shielded with a porous zirconia cylinder which was then surrounded by Fiberfrax
insulating material. The specimens were directly heated from room temperature to the
sintering temperatures for various time periods, then allowed to cool naturally by turning
off the power.
Temperature was controlled by manually pulsing the microwave power
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
158
input. For comparison, conventional sintering was also carried out at 1300°C for 30 min.
and 24 h, respectively, in a programmable furnace.
6.2.3. Results and Discussion
A typical heating curve o f the microwave processed CSZP ceramics is shown in Fig.
6.2.
Thermal runaw ay was noticed in the current sintering after about 40 min. of
microwave irradiation. XRD study (Fig. 6.3.) showed that the sintered CSZP ceramic was
highly crystalline compared with the starting material. Figure 6.4. shows the sintered
densities of the microwave processed samples as a function of compaction pressure or
green density. The sintered density was obviously dependent on sintering temperature,
time, and green density.
As can be expected, with increasing green density, sintering
tem perature, or sintering time, sintered density increased.
Figure 6.5 provides a
comparison o f sintered densities o f CSZP at 1300°C by microwave processing and the
conventional method. As can be seen from the figure, at the same sintering temperature
(1300°C), a 30-min. microwave processing provided the sintered densities of the CSZP
ceramics 13-17% higher than the conventional sintering of 30 min., and 5-8% higher than
those for 24 h .
The fact that the 24 h conventionally sintered CSZP was still less than 90% dense
indicates that the diffusion o f the CSZP material under the conventional processing
conditions was low. The significantly higher sintered densities achieved after a short period
of microwave processing indicates that the densification rate o f CSZP in the microwave
field was much greater than in the conventional oven. Since boundary diffusion is the
primary mechanism for densification of ceramics during sintering, the boundary diffusion
of the CSZP material must have been enhanced during the microwave processing.
The m icrostructures of the as-sintered surfaces o f both m icrowave and the
conventionally sintered CSZP are shown in Fig. 6 .6 . The grain size of the CSZP ceramics
sintered both by microwave and by conventional heating did not show much difference.
Compared with the higher sintered density achieved by microwave processing within a
short processing time, the rate o f grain growth in the microwave processed sample was
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
159
Temperature,
2000
1000
0
20
40
60
80
100
Time, min.
Figure 6.2. A typical heating curve o f CSZP ceramic in a microwave field.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
160
AS
R E C E IV E D
CS2P2
10
20
30
40
50
TWo theta, degree
Figure 6.3. XRD patterns of CSZP (a) before and (b) after microwave sintering at
1300°C for 30 min.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
161
100
>*
"<7>
c
o
■o
73
1450°C, 45 min.
1450°C, 30 min.
1300°C, 30 min.
1450°C, 20 min.
0)
»_
0)
c
W
20 0
400
600
800
1000
Compaction pressure, MPa
Figure 6.4. Influence of compaction pressure (green density), sintering temperature and
time on the sintered density of CSZP.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
162
80
Sintered
density, %
100
M/W 30 min.
CN 24 h
C/V 30 min.
200
400
600
800
1000
Compaction pressure, MPa
Figure 6.5. Comparison of the sintered density of CSZP ceramics processed
by microwave and conventional methods at 1300*C.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
163
A' ■
4 . 0 7 KX
Figure
6 .6 .
5U
0007
YF i 8 2 E 5
M icrographs o f CSZP sintered at 1300°C by (a) microwave method for 30
min. and (b) conventional method for 24 h.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
16 4
relatively suppressed. Higher sintered density could be achieved by increasing sintering
temperatures and/or time.
6.2.4. Summary on Microwave Sintering of CSZP
Microwave sintering o f CSZP ceramics was conducted at 2.45 GHz at 1300 and
1450°C, respectively. The results show that the sintered density of CSZP increases with
increasing green density, sintering temperature and sintering time. Compared with the
samples conventionally sintered at 1300°C for 30 min. and 24 h, a higher sintered density
was achieved in microwave sintering for 30 min. This indicates that the densification rate
of CSZP was substantially enhanced during the microwave processing.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
165
Chapter 7
MICROWAVE SINTERING OF VARIOUS CERAMIC COMPOSITES
7.1. Introduction
A composite consists o f two or more physically distinct and mechanically separable
phases. The importance o f a composite lies in the fact by modifying the amounts of
components one can control and tailor the properties to desired values. To meet the
increasing demands in high-performance applications, often a composite approach is
adopted to improve the functional and mechanical properties.
In a composite, one can not only tailor properties o f the components through
compromise (e.g. brittle + tough —> moderately tough), but also “create” new properties.
Two different brittle com ponents can lead to a tough com posite.
For exam ple,
mullite/SiC(whiskers) is such a brittle-brittle composite in which mullite is the matrix and
silicon carbide whiskers work as a reinforcing or toughening component.
M any
composites have been developed using whiskers or particulate materials as reinforcing
ag e n t.
As composite m aterials become a trend in material development, processing of
composites becomes more and more important. In the preparation o f ceramic composites,
sintering is an important step. Different components in a composite may have different
sinterability, which makes sintering of the composite difficult in conventional process.
Since microwave processing is fundamentally different from conventional processing in
heating mechanism, it may be unique, or an alternative to conventional method. In this
study, various zirconia-containing ceramic composites based on HAp, mullite, alumina,
CSZP, and BZPS, were sintered by microwave processing.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
166
7.2. Microwave Sintering of HAp-Based Composites
7.2.1. Introduction
In the last two decades, m uch work has been done to develop a fundamental
understanding of various aspects of HAp. HAp ceramics have now been used clinically for
nonload-bearing applications. However, pure HAp ceramic is intrinsically brittle. The
fracture toughness o f dense HAp ceramics is about 1 MPaVm which is only a quarter to a
half that of bones. Because of this, HAp ceramics could not be used for any load-bearing
implantation unless its toughness is substantially improved. Therefore, toughening o f HAp
is an important subject. Unfortunately, research work carried out in this area is very
limited.
The reported cases include HAp dispersed with Z r 0 2 particles and a-S i 3 N4
whiskers.
W hen preparing H A p/Z r0 2 composites, sintering has to be carried out at high
temperatures which could lead to decomposition of HAp (Ioku et al., W u and Yeh). Wu
and Yeh studied conventional sintering of H A p/Z r0 2 composites containing 20% by weight
of Z r0 2 (about 11.5% by volume). They found that a 3-h sintering at 1100-1400°C led to
relative densities o f about 50 to 60% in one case, and 75 to 80% in another. The
decomposition o f HAp and formation of C aZ r0 3 were observed in their study. Fang et al.
(1989) also tried to sinter the HAp/PSZ composite by conventional sintering alone, or
conventional sintering follow ed by hot isostatic pressing (HIP).
H ow ever, a 2-h
conventional sintering at 1250°C led to relative densities of only 64-80%, while the post
sintering HIP at 100 MPa improved the density only by 1-2%. Compared to the pure HAp
which can be easily sintered to around or over 95% by conventional sintering, it is obvious
that the addition o f Z r 0 2 to HAp has a deteriorating effect on the sinterability of the
mixture.
It is impossible to achieve a dense H A p/Z r0 2 composite by conventional
sintering at the ambient pressure. By hot pressing at 1050°C and 30 M Pa for 1 h, then
post-sintered at the same tem perature at 200 M Pa in Ar for 1 h, Yoshimura et al.
(Yoshimura et al., 1989) could make a dense H A p/Zr0 2 composite without decomposition
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
167
of HAp. They also prepared H A p/a-Si 3 N 4 by hot pressing at 30 MPa and obtained a
dense composite, but they observed a reaction between HAp and a-S i 3 N4. Through the
dispersion of Z i0 2 particles or a -S i 3 N 4 whiskers in HAp, the fracture toughness, K IC,
had been improved by these investigators to around 3 MPaVm.
As described in Chapter 4, fabrication of regular, porous, and transparent HAp
ceramics has been achieved. Microwave sintering has a heating mechanism totally different
from that of the conventional method. The sintering efficiency in microwave processing is
material dependent. Materials that have a high dielectric loss absorb microwave energy
efficiently and are easy to heat in a microwave field, while those of low dielectric loss
absorb microwave energy less efficiently and are hard to heat. Because of this, selective
sintering/heating is possible when processing a multi-phase system. The success in
microwave sintering of HAp indicates that the microwave absorption of HAp is good
enough at least at elevated temperatures, zirconia also absorbs microwaves very efficiently
One can expect good sinterability o f HAp/PSZ composites by microwave processing.
Silicon carbide is also a good m icrowave absorber, therefore one can expect good
sinterability on HAp/SiC composites in a microwave field. In the current study, microwave
sintering of HAp-based composites, i.e., HAp/PSZ and HAp/SiC, was attempted.
7.2.2. Experimental Procedure
7.2.2.1. Starting Materials
Two HAp powders were used in the current study. HAp-A was synthesized by
hydrotherm al m ethod, and H A p-B w as prepared by hydrolysis o f brushite
(CaHP0 4 -2H 2 0 ) followed by ripening treatment (Section 4.2.).
For the HAp/SiC composite, the volume proportion of HAp-A:SiC was 90:10. The
SiC whiskers were o f an average thickness 0.5 pm with varying lengths up to 25 pm. The
whiskers were added to the precipitate slurry of HAp-A by magnetic stirring. The mixture
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
168
of precipitate and whiskers was then subjected to the same hydrothermal, drying, and
disagglomeration processes as HAp-A.
For the HAp/PSZ composite, the tetragonal phase zirconia stabilized by 3 mol% yttria
[Zirconia Sales (America), Inc., Atlanta, GA] of sub-micrometer particle size was used.
The volume proportion of HAp-B:PSZ was 90:10. The two powders were dry mixed in a
plastic jar mill using zirconia balls as mixing media. The mixture was then sieved to -325
mesh to further improve homogeneity.
1.2.22. Compaction
Pellets (12.7 mm diameter x 5 mm thick) of HAp, HAp/PSZ and HAp/SiC powders
w ere com pacted uniaxially for both m icrowave and conventional sintering.
The
compaction pressure for all pellets was 350 MPa, except for the case of HAp/PSZ in which
420 M Pa was used.
1.2.23. Sintering
A 500 W, 2.45 GHz commercial microwave oven (GE, Model JE 43) was used for
sintering. Only one pellet was used in each run. The arrangement inside the microwave
oven is schematically shown in Fig.7.1. The pellet was placed on a board of Fibermax
insulation (Carborundum Co., Niagara Falls, NY) at the center o f the cavity o f the
microwave oven, then vertically surrounded by two coaxial porous zirconia insulating
cylinders o f 30 mm in height, and then further insulated with the Fibermax boards. One
end-open Pt-PtlORh thermocouple, sheathed with alumina, was inserted from the bottom
of the microwave oven, with the tip touching the sample, to monitor the temperature during
sintering. The processing temperature was controlled by pulsing the microwave power.
Heating started from the room temperature. After sintering, the system was allowed to cool
to room temperature.
For comparison, conventional sintering of some pellets with the same green densities
was carried out at 1200°C in a 5 kW programmable box furnace, with both heating and
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
169
Mlcrowava ow n
/
/
/
/
Zirconia cylinder
/
/
/
/
/
/
/
/
/
/
/V /V ,V ,V /V /V .
\
\
\
Kv a
VA'
\
N
\
V xV
\
\
\
\
\
\
\
\
\
v a v a v
*
/
\
\
\
\
\
\
\
\
\
S
\
\
\
\
\
\
\
/ / /
S *
/
4
/ / / / / ,
\ S\ \ \
A WQ
(Sample
1
|
|
S
S
S
5
/ / / / / ./ ./ /. /.
vW W W V o
/ V/ \ / \/ <
»////
/ / / / \/ <\ \/ 4 > x/V/ v/ W
/ /s /W/ V
/
■
/
/
/
/
/
✓
/
/
/
✓
/
Rofractory wools /
. Thermocouple
Figure 7.1. The arrangement inside the microwave oven.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
170
cooling rates at 5°C/min. A HAp/SiC sample was uniaxially hot pressed in vacuum at
1200°C and 36.6 M Pa for 15 Min. in a graphite m old of 12.7 mm diameter.
7.2.2.4. Characterization
The sintered samples were characterized to determine density, phase composition,
microstructure, and fracture toughness. Density was determined by weight and dimension
measurements. Phase composition was analyzed by powder X-ray diffraction (XRD)
using a Scintag diffractom eter (Scintag Inc., Sunnyvale, CA). M icrostructures were
exam ined with a scanning electron microscope (SEM, ISI-DS 130, Akashi Beam
Technology Corp., Japan). Fracture toughness was measured by diamond indentation
using a Vickers indenter as described in Sec. 3.4.10.
7.2.3. Results and Discussion
7.2.3.1. Heating
A typical heating curve for microwave processing in the current study is shown in
Fig. 7.2. It usually took 20 to 50 min. to trigger thermal runaway. Because of the high
m icrowave absorption and relatively large mass o f the Z r0 2 cylinders over that of the
sample, the therm al runaway behavior here had much to do with the porous zirconia
cylinders used to vertically shield the sample.
1.23.2. Phase Composition
By XRD analysis, it was found that both the microwave and conventionally sintered
HAp/PSZ composites were composed of HAp and tetragonal zirconia with trace amounts
of a- and /J-Ca3 (P0 4 )2 . No C aZ r0 3 was detected. Trace a- and /?-Ca3 (P 0 4 ) 2 were also
detected in the pure HAp pellet after 1 h conventional sintering at 1200°C. This indicates
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
171
1400
1200
1000
*«
800
600
a.
400
200
0
10
20
30
40
50
Time, Min.
Figure 7.2. A typical heating curve for microwave processing o f HAp-based composites,
where porous zirconia was used as accelerator.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1 72
that a partial decomposition o f HAp took place during sintering, but the decomposition was
due to the thermal stability o f the starting HAp rather than the microwave processing or any
reaction between HAp and Z r0 2.
In the case of the HAp/SiC system, the conventional sintering at 1200°C led to a
slight oxidation on the surface o f the pellet, while the microwave processing caused very
extensive oxidation. A thick layer o f oxidation product was observed on the surface o f the
HAp/SiC pellets after microwave sintering even without additional holding.
A few
minutes of microwave processing at 1200°C would lead to complete oxidation of the SiC in
the HAp/SiC mixture. Microwave sintering o f HAp/SiC in Ar gas was also attempted, but
it did not help. It is believed that the lack o f moisture in the sintering atmosphere caused
HAp to decompose at lower temperatures, while the OH- ion decomposition of HAp made
the oxidation of SiC even worse. A complex mixture o f S i0 2 phases was detected in the
oxidation product. The hot-pressed HAp/SiC sample was composed of HAp and a large
amount of a-C a 3 (P 04)2. The decomposition o f HAp in this case was very serious because
the sample was hot-pressed in vacuum. The hot-pressed HAp/SiC composite turned black
due to the diffusion of graphite from the mold.
7.2.3.3. Sintered Density
For HAp/PSZ, conventional sintering at 1200°C led to a sintered density o f only 75%
of the theoretical (Table 7.1), and prolonged sintering tim e from ~30 min. to 4 h did not
bring about any significant increase in sintered density. Under microwave processing
conditions, however, a 20 min. sintering at 1200°C led to a sintered density o f 92.7% in
the pellet o f 59.8% green density, which was 24% m ore dense with respect to the
conventionally sintered sample. This substantial enhancement in density is attributed to
selective heating in the microwave processing. Zirconia absorbs microwave energy much
more effectively than HAp, so that it could promote densification o f a HAp/PSZ composite
in microwave field when the amount of Z r 0 2 in the composite is appropriate. This is just
the reverse of the case in conventional sintering where the existence of Z r0 2 makes the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
173
Table 7.1. Summary o f the processing conditions and some results m easured on the
sintered HAp and HAp-based composites.
No.
Processing
Composition
p*g, %
p*s, %
K ic
1
HAp
m/w
1200°C x 10 min.
52.82
93.3
1.56
2
HAp/10%PSZ
m/w 1200°C x 20 min.
54.33
87.0
1.18
3
HAp/10%PSZ
m/w 1100°C x 30 min.
54.02
89.1
1 .2 0
4
HAp/10%PSZ
m/w
1200°C x 20 min.
59.79
92.7
3.88
5
HAp/10%PSZ
c/v
1200°C x 24 min.
60.42
74.81
—
6
HAp/10%PSZ
c/v
1200°C x 60 min.
59.86
75.08
—
7
HAp/10%PSZ
c/v
1200°Cxl20 min.
60.42
75.43
—
8
HAp/10%PSZ
c/v
1200°Cx240 min.
60.06
75.2
—
9
HAp/10%SiC
HP
1200°Cx 15 min.
—
72.5
2.85
10
HAp/10%SiC
c/v
1200°Cxl20 min.
60.0
66.83
—
11
HAp/10%SiC
m/w 1200°C no holding
58.5
—
—
*Pg, p s — green density and sintered density, respectively.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
174
sinterability o f HAp/PSZ very poor. This demonstrates the advantage o f microwave
processing for such a special system. It should be mentioned that the low er sintered
densities of other two microwave sintered HAp/PSZ samples were due to their lower green
densities.
In the case of HAp/SiC, the conventional sintering at 1200°C for 2 h led to a density
of
6 6 . 8 %,
and hot pressing, 72.5%. The density of the microwave sintered sample was
not measured due to the serious oxidation on the surface after processing, but from SEM
micrographs, its density looks comparable to that o f the hot pressed one. The low sintered
density o f HA p/SiC was due to the m orphology o f the silicon carbide whiskers.
Obviously, it is more difficult to achieve a high sintered density in a whisker dispersed
system than in the particle dispersed one.
7.2.3.4. Fracture Toughness
K IC of 3.88 M Pa'®- has been achieved in the HAp/10%PSZ composite o f 92.7%
density sintered by microwave processing. Compared with 1.5 MPaVm" from the pure
HAp o f 93.3% density, a 150% net increase in K IC was realized. The significant
im provem ent in KIC was attributed to the transformation toughening m echanism of
tetragonal Z r0 2 dispersion. The toughening effect was not observed in the two other
HAp/PSZ samples with density less than 90%. This seems to show that the transformation
toughening could be realized only when density is over a certain value, most likely greater
than 90%. Apparently, fracture toughness, K IC, is a function o f density. It increases with
increasing density, but the relation is not linear. Since the densities of the HAp/PSZ
samples sintered by the conventional method were well below 90% o f the theoretical, no
toughening was expected in these samples, and thus KIC was not measured for them.
In the case of the SiC-containing composite, although the density of the hot-pressed
sample was only 72.5%, an increase in fracture toughness was still observed. K IC of 2.85
MPaVm was achieved in this sample, compared with the zirconia dispersed sample, where
no toughening effect was achieved below 90% density. This is not surprising since the
mechanism o f whisker toughening is different from that of transformation toughening.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
175
Another factor which should be considered is that the K IC o f this HAp/SiC sample was
measured on the plane perpendicular to the compaction direction and thus more whiskers
had contributed to the toughening because o f the preferred orientation o f the whiskers due
to the uniaxial compaction.
7.2.3.5. Microstructure
Figure 7.3. shows the m icrostructures o f the HAp/PSZ composite sintered by
m icrow ave at 1200°C.
It can be seen that the distribution o f PSZ particles is
homogeneous, and the microstructure is quite dense. The sintering o f some PSZ particles
is also evident. The microstructures of HAp/SiC composites, sintered by microwave as
well as by hot pressing are shown in Fig. 7.4.
Obviously, both samples show very
porous microstructure. The bonding between SiC whiskers and HAp matrix, however,
looks good. If the oxidation o f SiC could be avoided, yet high densification could be
achieved, then improved toughening could be expected. In both the cases of PSZ and SiC
dispersed HAp ceramics, no microcracks between the dispersed phase and the matrix were
observed, indicating that there was no significant mismatch in the thermal expansion
coefficients between the toughening agents and the HAp matrix.
7.2.4. Summary
Com posites o f hydroxyapatite/partially stabilized zirconia (HAp/PSZ) and
hydroxyapatite/silicon carbide whiskers (HAp/SiC) were sintered at 1100-1200°C by
microwave at 2.45 GHz. Characterization o f the sintered composites was carried out by
density, microstructure, phase composition, and fracture toughness measurements. The
results show that although not yet fully densified, a much higher sintered density in the
HAp/PSZ composite was achieved by microwave sintering than by conventional sintering
at the same temperature. A relative density o f 93% was achieved by 20 min. microwave
processing at 1200°C. Comparatively, 2 h conventional sintering of the same material at
1200°C led to only 75.5% relative density. KIC o f this microwave sintered HAp/PSZ of
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
176
Figure 7.3. The microstructure o f microwave sintered HAp/PSZ composite, showing the
simultaneous sintering o f HAp and PSZ particles.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
177
Figure 7.4. The m icrostructures o f the fracture surfaces o f the HAp/SiCw composites
sintered by (a) microwave processing and (b) hot pressing (perpendicular to the compaction
direction).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
178
93% density was found to be 3.88 MPaVm, which is 2.5 times of the value for pure HAp
o f the same density. A further increase in KI(-< could be expected if full or nearly full
densification was achieved. Sintering of PSZ particles in the HAp/PSZ composite was also
observed in the microwave processed sample. Microwave sintering of HAp/SiC was not
successful in the current study due to the oxidation of SiC in air at high temperature.
7.3. Microwave Sintering o f Various Zirconia-Containing Composites
7.3.1. Introduction
Ceramics are usually brittle and transformation toughening is adopted to improve
fracture toughness o f various ceramics. Partially stabilized tetragonal zirconia is a
toughening agent popularly used. Since any toughening effect in such a composite through
transformation o f zirconia directly depends on the density of the composite, a high density
is critically important to achieve the desired transformation toughening. The addition of
zirconia, however, may reduce sinterability of the ceramic and thus high densities become
difficult to achieve under conventional sintering conditions. For example, about 95%
relative density is easy to achieve in conventional sintering o f HAp, once zirconia is added
to HAp, the sinterability o f the composite becomes very poor. In conventional sintering of
HAp/Zi0 2 >earlier researchers achieved a relative density of only about 50-78% at 11001400°C for 3 hours (Wu and Yeh, 1988). W hile by microwave processing, a relative
density of 93% in HAp/10%ZrO 2 composites (Sec. 7.2.) at 1200°C for only 30 min.
Since microwaves have the selective heating effect to the materials of different
dielectric properties, microwave processing may be advantageous for sintering some
ceramic composites. Stabilized tetragonal zirconia is a transformation toughening agent
and also an good microwave absorber. It can be preferentially heated in a microwave
field. The objective o f this study was to see how well microwave processing works with
zirconia-containing composites.
For this purpose, microwave sintering of ceramic composites o f zirconia and four
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
179
matrices, including alumina, mullite, calcium strontium zirconium phosphate (CSZP),
Cao.5Sr0-5Zr4P6024, and barium zirconium phosphosilicate (BZPS or B S25),
Baj
25 Zr4 P 5 5 Si0.5 Q 24 ,
was carried out in the current study. Among them, alumina and
mullite are important ceramic materials commonly used, CSZP and BS25 belong to a very
large new family known as [NZP] and characterized for its low thermal expansion behavior
(Limaye et al., 1991; Roy et al., 1989 ).
Conventional sintering of the duplicate samples was also carried out in a high
capacity, low therm al inertia furnace at the heating rates comparable to those in the
m icrowave processing. In this way, any differences in the sintering behavior would be
attributable to the effect of microwave irradiation.
7.3.2. Experimental Procedure
7.3.2.I. Starting Materials
The starting m aterial o f alum ina was an amorphous alum ina pow der from a
commercial source (Rhone-Poulenc, France). The powder was sieved to -325 mesh (45
pm opening) before compaction. For comparison, a crystalline alumina powder (Linda B,
high purity a-alum ina) of 0.05 pm average particle size was also used for composites
fabrication. The mullite precursor was prepared by a diphasic xerogel method (Sec. 5.2.)
by mixing sols o f boehmite (AlOOH) and silica (Ludox) at a m olar ratio of 3:2. A
monophasic gel powder was also used for mullite/PSZ composites for comparison. The
CSZP powder was also prepared by a sol-gel method (Limaye et al., 1991). The BS25
pow der was synthesized by a solid-state reaction method using dry oxide precursors
(Huang et al., 1994). The zirconia powder (HSY-3.0, Zirconia Sales, Inc., Atlanta, GA)
used in this study was tetragonal phase partially stabilized by Y 2 Q 3 (5.4 wt.%), with a
minor monoclinic phase. The average particle size of this zirconia powder was about 0.1
pm .
Each matrix powder was mixed with zirconia in various proportions. An appropriate
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1 80
amount o f 2% PVA solution was used as a binder during mixing. The m ixture was
hom ogenized by hand m ixing with an agate mortar.
The alum ina/zirconia and
BS25/zirconia mixtures were compacted into pellets of 0.5 inch diameter, and the mixtures
o f mullite/zirconia and CSZP were compacted into pellets o f 0.25 inch diameter. The
compaction was uniaxially carried out at 350 MPa. The relative green densities of the ascompacted pellets of various systems were in the range of 39-53% for alumina/zirconia,
46-51% for mullite/zirconia, 53-58% for CSZP/zirconia, and 60-67% for BS25/zirconia,
respectively. No sintering additive was used in this study.
13.2.2. Sintering
M icrowave sintering was carried out in a 900 W , 2.45 GHz m icrowave oven
(Panasonic) with a turntable. The sintering packet has been described and schematically
illustrated in Section 3.2.
M ulti-pellet run was adopted in the sintering. The pellets,
usually in two (0.25 inch pellets) to three (0.5 inch pellets) layers, were placed in the center
o f the packet. Each layer containing 6-7 pellets of the 0.25 inch samples, but only one
pellet o f the 0.5 inch samples. Conventional sintering was carried out in the high capacity,
low thermal inertia, high temperature furnace (see Section 3.2.).
The sintering of the composites based on alumina, mullite, and CSZP, was conducted
at 1500°C. Composites of BZPS (BS25) were sintered at 1550°C. All samples were
soaked at the peak temperature for 20 min. The sintering of the diphasic mullite gel is a
reaction sintering process. The earlier results (Sec. 5.3.) showed that the diphasic mullite
gel used in this study starts crystallizing (mullitization) at about 1320°C. Since the
densification o f gels through viscous flow is far more efficient than that o f the related
crystalline materials through ordinary solid diffusion, step sintering was adopted for the
composites based on the mullite gel, in which the samples were first heated to 1200°C,
then soaked 30 min. for densification by viscous flow, and finally heated to 1500°C for
mullitization. Sintering of the composites based on crystalline mullite (Chichibu Cement
Co.) was 20 min. at 1500°C.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
181
13 .23 . Characterization
Sintered density was m easured by weighing and dimensional measurement of the
pellets. Powder x-ray diffraction (CuAT^) was carried out with a Scintag diffractometer on
the starting powders and on some selected sintered samples for phase identification. The
microstructure of the sintered specimens was examined by SEM.
7.3.3. Results and Discussion
The heating curves are plotted in Fig. 7.5. It is seen from the figures that while the
soaking times in both microwave and conventional processes are identical, heating time to
the desired sintering temperatures for microwave processing was never longer than in
conventional process. Actually, conventional heating took a few minutes longer in the
cases of alumina, CSZP and BZPS (BS25) composites.
Sintered densities as a function of zirconia content are shown in Figures 7.6-8. From
Fig. 7.6., it is clearly seen that the microwave enhancing effect on the composites based on
the amorphous alumina (a) is much stronger than that based on the crystalline alumina (b).
W ith the amorphous alumina as the matrix, microwave effect is stronger when zirconia
content is lower, and weaker as zirconia content increases. W hen crystalline alumina
(Linda B) was used as the matrix, microwave effect is not as obvious, and not dependent
on the zirconia content (b). This clearly indicates that the amorphous alumina precursor
couples better with microwaves than the crystalline powder.
Figure 7.7. shows sintered density o f mullite/zirconia composites. When using the
diphasic mullite gel as the matrix, no significant microwave effect was observed on the
samples step-sintered at 1200°C for 30 min. followed by 1500°C for 20 min. (Fig. 7.7.a).
Keeping the low er tem perature sintering conditions unchanged but reducing higher
temperature (1500°C) sintering time from 30 to 4 min. did not show much microwave
effect either (Fig. 7.7.b). W hile when sintered only at 1200°C for 30 min., the microwave
effect was very clear (Fig. 1.1.c). Since at 1200°C, the diphasic gel has not transformed to
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1 82
1600
• 0 8o0© *00 0000003
1400 -
O
o
1200-
O
l_
1000
3
(0
0)
O.
V.
E
a
l-
cP
-
◦
800 -
e
600 400 -
200
alumina/zirconia
•
microwave
O conventional
•
0
20
10
T
-r
T
30
40
50
60
Time, min.
1600
O
o
0)
L-
3
••
1400 -
1200-
•0*0 * 0 * 0 oOO
•O
1000
+*
800-
k
0)
CL
600-
• o
400
K>
(0
E
0)
H
CTOOOOO
• O
mullite/zirconia
200 H
O
i
0
i__
microwave
conventional
—r20
40
60
80
Time, min.
Figure 7.5.
H eating tem perature as a function of tim e for the zirconia-containing
composites based on (a) alumina, (b) mullite.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
183
1600
^ ••O * 0 * 0 * 0 * 0 0 0 0 0
O
•
1200
o
o
• O
Temperature,
o
1400
:°°
o
c
o
o
o *
o
••
CSZP/zirconia
•
microwave
O conventional
•
10
20
30
40
50
Time, min.
1600
• o*o*oc*ooo
Temperature,
°C
1400
•
1200
o_o
p
• o
o
o
&
o
o
o
1000
800 600400
•IT
BS25/zirconia
•
microwave
O conventional
O*
0
200 H
■>.
0
10
T
T
20
30
40
50
Time, min.
Figure 7.5. (Continued) Heating tem perature as a function o f time for the zirconiacontaining composites based on (c) CSZP, and (d) BZPS.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
184
100
■
90 - □
m/w 1500°C x 20 min
c/v 1500°Cx20min
80 7 0 -,
60 50 -
20
0
40
60
80
100
Zirconia Content, vol.%
100
•
O
m/w 1500°C x 20 min
c/v 1500°Cx20m in
90 -
o
<0
o
c
O
a
o
>
O
80
0)
cc
ooo
70
l b
T
T
T
T
20
40
60
80
100
Zirconia Content, vol.%
Figure 7.6. Sintered densities of AI2 O3/PSZ composites at various mixing proportions.
The alumina powder is (a) amorphous and (b) highly crystalline, high purity alumina.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
185
100
95 -
90 c/v 1200°Cx30' + 1500°Cx30'
m/w 1200°Cx30' +1500°Cx30'
85
20
0
40
60
80
100
Zirconia Content, vol.%
100
-I
sS
~
w
c
0)
95 H
■o
0>
>
90-
M
■
O
O
0)
m/w 30 min.
m/w 4 min.
DC
85 £
—T“
—T“
—r -
—r~
20
40
60
80
—i
100
Zirconia content, vol.%
Figure 7.7. Sintered densities of mullite/PSZ composites. Diphasic xerogel (a-c) and
monophasic xerogel (d) was used as mullite precursor, respectively.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
186
100 -1
c
90 -
>•
«
c
<u
■o
Q)
>
re
"aj
DC
•
O
m/w 1200Cx30 min
C/v 1200 0x30 min
•
80-
•
•
70
bO '
o
o
o
O O O o
60 50
I
■>
20
i
r~
40
1
60
i—
80
100
Zirconia content, vol%
100
-i
90 80 -
•
60
O
m/w 1500°C x 20 min
c/v 1500°C x 20 min
50
0
10
20
30
40
50
60
Zirconia Content, vol.%
Figure 7.7. (continued) Sintered densities o f mullite/PSZ composites using (c) dipasic
mullite gel and (d) monophasic mullite gel as the matrix, respectively.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
187
mullite, while at 1500°C, the mullite transformation is over. It is clear that the diphasic
mullite gel strongly absorbs microwaves before mullitization, while the crystalline mullite
does not in the temperature range up to 1500°C. This is evidenced in the result of the
composites using monophasic gel as the matrix (Fig.7.7.-d), in which the microwave
effect on densification is negligible. Since monophasic gel crystallizes to mullite at 970°C,
at higher temperature it behaves just as crystalline mullite.
The sintered densities of the composites based on [NZP] materials (CSZP and BZPS)
were plotted in Fig. 7.8. Both systems show the same trend of density dependence on the
zirconia content. Generally, as zirconia content increases, the relative density increases,
then decrease after reaching the maximum. The decrease stops at about 80% PSZ content,
then increases again to a higher steeply to the maximum density o f pure PSZ. The
microwave enhancing effect on densification in both cases is obvious, but that in the
BZPS-based system is more significant when PSZ content is low. The peak density on the
density curve indicates the optimum mixing ratio of the two components in the systems,
while the “valley” density indicates inefficient sintering at the related proportion range of
the two components. The density o f pure PSZ is much higher than that o f the [NZP]
materials, indicating that PSZ has better sinterability. Addition of PSZ to the two [NZP]
materials promotes sinterability, while the addition of the [NZP] materials to PSZ matrix
deteriorates sinterability, but the relationship is not linear.
In all the materials studied, density increases as increasing zirconia content in both
microwave and conventional sintering, with the mullite system before crystallization as an
exception. Higher densities have been achieved in microwave sintering in all cases.
Significantly higher sintered density was produced in the microwave sintered
specim ens of alum ina/zirconia system.
The difference betw een m icrow ave and
conventional sintering in the low zirconia content side is more significant. As zirconia
content increased, the difference diminished, indicating that the microwave enhancement in
sintering o f alum ina is more effective than o f zirconia, which is due to the better
sinterability of zirconia over alumina. The relatively low
density was due to the
coarseness of the starting powder, low green density, low processing temperature, and
short processing time. Furthermore, no sintering additive was used. Microwave
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
188
100
•
m/w 1500°C x 20 min
c/v 1500°Cx20m in
O
'«
c
0>
Q
0>
90 -
A
>
JO
0>
DC
20
0
40
60
80
100
Zirconia Content, vol.%
100
•
<
•
O O o
o
•
•
<n
c
0)
o
d>
>
90-
•
o
>•
0
•
O
O
•
9
o
80
B
0°
•
<D
O
DC
70
m/w 1500°C x 20 min
c/v 1500°C x 20 min
I
T
i
20
40
60
80
100
Zirconia Content, vol.%
Figure 7.8. Sintered density o f [NZP]/PSZ composites using (a) CSZP and (b) BZPS as
the [NZP] component, respectively.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
189
frequency might also partly responsible for the relatively low density.
K im rey and
cow orkers (1991) have shown that alum ina/zirconia system couples better with
microwaves at 28 GHz than at 2.45 GHz.
In the mullite/zirconia system, no significant difference in sintered density was
observed after sintering at 1500°C. A comparison of the density after the first-stage
sintering (1200°C x 30 min.), however, clearly showed the difference (Fig. 7.7.). The
densities after the 30 min. microwave heating at 1200°C were much higher than those
conventionally heated samples. Obviously, microwave irradiation is more efficient in the
densification of the diphasic mullite gels before mullitization. This is in good agreement
with the earlier observation o f Roy et al. (1984, 1985) that the alumina and silica gels
absorb microwave energy and get heated very quickly. Once the crystalline mullite is
formed, the sintering mechanism changes and there is no more viscous flow. Further
densification can only rely on the boundary diffusion and related factors as in ordinary
solid state sintering.
Since the solid state diffusion of ions in m ullite is very low,
conventional sintering of m ullite is usually carried out at 1600-1700°C (Huling and
Messing, 1993) for many hours. Due to the limit in processing temperature and time, and
more importantly, the transparency of the crystalline mullite to microwaves, the microwave
sintering of the m ullite system after m ullitization, under the current experimental
conditions, did not show much advantage over conventional method.
In the B S25/zirconia system containing up to 30% zirconia, the microwave
enhancement was also obvious. The increase in density o f the CSZP/zirconia system,
although obvious, was not as significant. This is probably due to the high processing
temperature. When processing temperature is sufficiently high, the solid state diffusion of
ions under the conventional conditions might also have been substantially accelerated to
allow the microwaves to lose their effectiveness. The results of the previous study (Sec.
6.2.) showed that pure CSZP could be sintered to a relative density of 94% by microwave
processing at 1300°C for 30 min. which was about 24% higher than that achieved in
conventional sintering under the same temperature and same duration. This may explain
why the CSZP/zirconia system sintered at 1500°C did not show as obvious microwave
enhancement.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
190
Table 7.2. summarizes the enhancement of sintered density of the composites through
microwave sintering. The values are obtained from the difference of sintered densities
(microwave sintered density minus conventionally sintered density) divided by the
conventionally sintered density, and they can be considered as the microwave effect in
densification. It can be seen that with increasing zirconia content up to 40 vol.%,
microwave enhancement in densification decreases in alumina and mullite systems, slighdy
increases in the CSZP system, but the trend is not clear in the BS25 system. Actually, an
optimum zirconia content may exist in certain systems for the best densification. The
microwave enhancement in the composites o f alumina, mullite, CSZP, and BS25 are 2046%, 24-34%, 1-2%, and 4-13%, respectively. The ceramics without addition o f zirconia
showed even higher enhancement, with CSZP as an exception.
In both microwave and conventional sintering, there is a general trend of increasing
sintered density with zirconia content, in all the systems studied, with the mullite/zirconia
before mullitization as an exception. This is due to the fact that all the matrix materials used
in this study are hard to sinter, while the sinterability of zirconia is significantly better, so
that the addition of zirconia improves the sinterability of the resultant composites. This
was also observed by other investigators under both m icrow ave and conventional
conditions (Kimrey et al., 1991; M oya and Osendi, 1984). In the mullite/zirconia system,
the sintering before mullitization is caused by the viscous deformation o f silica gel, which
starts at temperature as low as 700°C (Wei and Halloran, 1988). The addition of zirconia
powder separates the gel and thus reduces the viscous deform ation, so that before
mullitization, the density of the composites decreases as increasing zirconia content. The
trend is reversed after mullitization since at higher temperatures, the sintering mechanism
changes to solid state diffusion, so that the presence of the easier-to-sinter zirconia will
improve the sinterability of the crystalline mullite/zirconia composites.
Powder XRD study showed that the sintered alumina was pure a-A l 2 0 3 phase when
there was no zirconia addition, and was a mixture of the alumina (matrix) and tetragonal
zirconia in the composites, with monoclinic zirconia in trace amount. Same is true for the
mullite and BS25 systems. In the CSZP system, zirconia had completely transformed to
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
191
Table 7.2. M icrowave enhancement in densities (%) over the conventionally sintered
samples.
N o.
Z r 0 2, vol.%
A lu m in a
M ullite (gel)
CSZP
1500°C/20 min
BS25
1500°C/20 min
1200°C/30 min
48.69
40.29
1 .0 0
13.36
1550°C/20 min
1
0
2
2
34.46
1 .1 2
7.68
3
5
32.45
0.64
11.16
4
10
46.40
30.52
1.97
13.40
5
20
38.55
26.85
1 .0 0
9.91
6
30
32.41
25.61
1.23
3.86
7
40
19.82
23.75
2.04
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
192
monoclinic phase. Further study is needed to explain this phenomenon. The pure mullite
gel remained amorphous after sintering at 1200°C for 30 min., while after the processing at
1500°C, the m aterial was found to be highly crystalline single phase m ullite.
No
decomposition was found in any of these systems.
SEM observations (Figs. 7.9.-12.) indicated that the m icrostructures of the
composites were fairly homogeneous. Since the powders were m ixed in dry condition,
agglomerates of zirconia could be seen. The homogeneity could be substantially improved
by mixing in the wet state, especially during the synthesis of the matrix materials. As
expected, the microwave sintered samples showed denser m icrostructure in general
because of the increased density.
Because duplicate green samples were used, the heating rates in the microwave and
conventional sintering were comparable, temperature measurement was the same, and
sintering time and temperatures were identical, the enhanced densification in the microwave
processed samples could be certainly attributed to the microwave effect. Although some of
the composites were not very dense, we believe that it is only a problem of optimization of
processing parameters.
7.3.4. Summary on Microwave Sintering of Zirconia-Containing Composites
Microwave sintering studies of various ceramic composites containing zirconia were
carried out along with parallel conventional sintering studies. The results show that
microwave processing significantly enhanced the densification in most cases, but the
enhancement decreased with increasing zirconia content. This is due to the relatively
higher sinterability o f zirconia over the matrix materials. The maximum enhancement was
up to 46%, varying with the system and zirconia content. The influence of zirconia
addition on the densification of the composites is determined by the sinterability difference
of the components.
The addition of zirconia to a harder-to-sinter matrix enhances
densification, under both microwave and conventional conditions.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
193
Figure 7.9.
M icrostructure o f A12 O j/PSZ (30%) com posite sintered by m icrowave
processing at 1500'C for 20 min.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
194
Figure 7.10. Microstructure of mullite/PSZ (30%) composite sintered by microwave
processing at 1200*C for 30 min. followed by 1500*C for 20 min.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
195
Figure 7.11. As-sintered surface o f CSZP/PSZ (10%) composite sintered at 1500°C for
20
min.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
196
Figure 7.12. M icrostructure o f BZPS/PSZ (20%) composite sintered by microwave
processing at 1500°C for 20 min.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
197
Chapter 8
DISCUSSION ON MICROWAVE ENHANCING EFFECTS
8.1. Observed Phenomena of Microwave Enhancing Effects
It has been observed in the experiments described in Chapters 4 to 7 that microwave
processing generally enhanced densification of the ceramic materials selected for the current
studies, including HAp, diphasic mullite gel, alumina, [NZP] materials, and zirconiacontaining composites. By microwave enhancing effect, or microwave enhancement, it is
meant that a process is accelerated by microwave heating under the same heating schedule
as in conventional processing, or lower temperature is required to achieve the same results.
The process is thermal, including sintering, densification, and cheminal reaction, etc.
The observed microwave enhancing phenomena in the current research are basically
in two categories: microwave enhanced densification and microwave accelerated mullite
transformation. The densification includes sintering of crystalline, semicrystalline, and
amorphous ceramic materials. There are two phenomena in this category. First, for the
same composition, the microwave enhancement was more significant for the amorphous
and semicrystalline forms than for the crystalline m aterial. Second, the microwave
enhancement effect diminished at higher processing temperatures. The examples for the
first phenomenon are the sintering of mullite and alumina. Under the current experimental
conditions, at 1500°C, the microwave enhancement in densification of amorphous alumina
was found to be one order of magnitude higher than that of the crystalline alumina.
Similarly, the microwave enhancing effect on densification of crystalline mullite was
negligible at 1500°C, while 40% higher density was achieved with diphasic mullite gel by
sintering at 1200°C for 20 minutes. The apparent activation energies for densification of
HAp and diphasic mullite gel in the microwave heating were found to be substantially
lower than that in conventional processing. A typical example for the second phenomenon
in the first category is the sintering of CSZP, in which the microwave enhancing effect was
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
198
very significant at 1300°C, but negligible at 1500°C.
The microwave enhancement of the mullite transformation of the diphasic mullite gel
was manifested as the decrease in mullitization temperature and the promotion of seeding
effect.
In order to prom ote our understanding o f these m icrowave effects, we need to
discuss the driving force for densification in general solid-state sintering and the
mechanisms of the interactions between microwaves and the specific materials.
8.2. Driving Force for Densification in Solid-State Sintering
Solid-state diffusion was the fundam ental m echanism responsible for the
densification of all the materials in the current study except the diphasic mullite gel and the
composites related to it. It is well known that in solid-state sintering the mechanisms
related to material transfer include evaporation and condensation, viscous flow, surface
diffusion, grain-boundary or lattice diffusion, and plastic deformation (Kingery et al., p.
470). During the initial stages of sintering, the difference in free energy or chemical
potential between the neck area and the surface of the particle provides a driving force
which causes the transfer of material by the fastest means available. If the vapor pressure
is low, material transfer may occur more readily by solid-state processes, several of which
can be imagined. As shown in Figure 8.1. and Table 8.1, in addition to vapor transport
(process 3), matter can move from the particle surface, from the particle bulk, or from the
grain boundary between particles by surface, lattice, or grain-boundary diffusion. Which
one or more of these processes actually contributes significantly to the sintering process in
a particular system depends on the relative magnitude of their rate, since each is a parallel
method of lowering the free energy of the system. However, there is a most significant
difference between these paths for matter transport: the transfer of material from the surface
to the neck by surface or lattice diffusion, like vapor transport, does not lead to any
decrease in distance between particle centers. In other words, these processes do not result
in shrinkage o f the compact and a decrease in porosity. Only transfer of matter from the
particle volume or from the grain boundary between particles causes shrinkage and pore
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
199
Grain boundary
Figure 8.1. Alternate paths for matter transport in the initial stages of sintering. (Ashby)
Table 8.1. Alternate Paths for Matter Transport During the Initial Stages o f Sintering.
Mechanism
(Fig. 8.1.)
Transport Path
Source of Matter
Sink of Matter
1
Surface diffusion
Surface
Neck
2
3
Lattice diffusion
Vapor transport
Surface
Surface
Neck
Neck
4
5
Boundary diffusion
Lattice diffusion
Grain boundary
Grain boundary
Neck
Neck
6
Lattice diffusion
Dislocations
Neck
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
200
elim ination (Kingery et al., p. 474).
From Fig. 8.1. and Table 8.1., we can see
thatprocesses 4-6 are mechanisms possibly responsible for shrinkage. W hether all or
which of these mechanisms are enhanced by microwave irradiation needs a more detailed
analysis.
Sintering is a necessary thermal process of ceramics fabrication in which bonding and
densification take place in a particulate material, leading to a solid body with mechanical
strength. The driving force for sintering process is the difference in Gibbs free energy of
the material before and after sintering (AG). This is schematically shown in Fig. 8.2. The
fine ceramic powder before sintering is in a higher free energy state (A) than the sintered
body with free energy (B). However, the change from state A to B is not a spontaneous
process because there is an energy barrier (Ea) to overcome. For the process from state A
to B to occur, a certain amount o f energy must be supplied to the material. The minimum
energy required for per m ole o f material to overcome the energy barrier is known as
activation energy (Ea) for the process. Heating is a means to supply activation energy to
the material to overcome this energy barrier.
In a simple sintering case of a crystalline ceramic powder compact during which there
is no phase transformation, the free energy change that gives rise to densification is the
decrease in surface area and lowering o f the surface free energy by the elimination of solidvapor interfaces. This takes place with the coincidental formation of new but lower-energy
solid-solid interfaces (Kingery et al., p.469). Obviously, increasing the free energy of the
starting powder would lower the activation energy for sintering. It is seen in Fig. 8.2. that
when free energy increases from A to A j, A2, A 3 ..., the activation energy correspondingly
decreases from E a to Eal, E ^ , E ^ ... Inversely, the lowering of the apparent activation
energy implies an increase in free energy o f the material before sintering. Notice that
surface area is a storage of free energy, because on the surfaces there are various kinds of
crystal defect, vacancies, lattice distortion, dangling bonds, etc., which are at higher energy
state than the bulk o f regular lattice. The higher the surface area (the finer the powder), the
higher the free energy, and the more reactive the powder, so that very fine powders may
sinter at lower temperatures.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
201
E
A3
i
Eal
i
Ea
Process coordinate
Figure 8.2. Schematic showing the energy barrier in the sintering process. As the free
energy (chemical potential) of starting material increases from A to A j, A 2, ..., the driving
force for the process increases from AG to AGj, AG2, ..., and the activation energy
required for the process correspondingly decreases from Ea to ^ a l’ ^a 2 ’
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
202
8.3. Loss Mechanisms in Microwave Heating
From the point of view of microwave-material interaction, a higher surface area will
also benefit microwave absorption. Most ceramic materials are ionic solids and can be
considered as dielectrics. The dielectric losses are related to electric polarization. There are
various possible mechanisms for polarization in a dielectric m aterial, such as electric
polarization, atom ic (ionic) polarization, orientation polarization, and space charge
polarization. There is a frequency dependence o f the dielectric constant (Kingery et al., p.
923). Tinga (1988) suggests that ionic conduction and dipole orientation are the major
mechanisms causing microwave heating at the lower ISM (Industrial, Science and Medical)
frequencies for which commercial power sources are available. Similarly, Katz et al.
(1991, 1992) indicate that electromagnetic energy can be dissipated in materials through
electronic polarization, ionic polarization, conduction, and interfacial polarization. Among
these, electronic polarization and ionic vibration are resonance phenomena and, if operable
during microwave heating, have the potential to directly change the ion jum p frequency.
However, due to the big difference between the microwave frequencies used in microwave
heating (109 to 1010 Hz) and the ion jump frequency (about 1013 Hz), ion jump mechanism
is not possible. Electronic polarization occurs at even higher frequencies than ionic
vibration, thus it is not thought to be operable at microwave frequencies either. Instead, ion
jump relaxation, interfacial polarization, and conduction are possible in microwave heating.
Ion jump relaxation in a crystalline ceramic occurs when an aliovalent ion (impurity
cation or anion with a valence different from that of its host sublattice) and vacancy form an
associated pair. An aliovalent ion-vacancy pair has a dipole moment associated with it that
responds to the applied electric field. The vacancy is thought to jum p around the aliovalent
ion to align its dipole moment with the electric field.
Interfacial polarization occurs at a structural inhomogeneity such as a grain boundary,
dislocation, or vacancy cluster. In an ionic lattice there will be a localized disruption in
electroneutrality at such a structural inhomogeneity with a net dipole moment that will align
itself with the applied field.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
203
Conduction occurs when vacancies are not associated with other defects and hence
are not localized. Unassociated vacancies are much more mobile than associated pairs and
migrate in response to the electric field. This mechanism is mostly of interest at low
frequencies.
In addition, as indicated by Meng et al. (1994), photon-phonon interactions are
another mechanism for absorption of microwave energy, especially in the high frequency
range (millimeter-wave and far-infrared). However, its contribution to the frequency range
o f 2-20 GHz cannot be ignored, particularly in relatively pure crystals where it could even
be the dominant process. The interaction between phonons and photons resulting from
quantization o f the electrom agnetic fields and lattice vibrational dynamics leads to
microwave absorption in crystals (Gurevich & Tagantsev, 1991).
In summary, dipole orientation through ion jum p relaxation and interfacial
polarization, ionic conduction, and possibly also photon-phonon interactions, are some
mechanisms responsible for microwave absorption in microwave heating.
8.4. Interactions of Material Surfaces with Microwaves
8.4.1. Dielectric Losses
Powders are collection of finely dispersed particles. It is well known that the finer
the particles, the more active the powder. The origin of the activity o f the powders is the
surface area, lattice distortions, and defects. To certain extent, the surface structure of the
fine powder is similar to the that of the amorphous. On the surfaces o f the fine particles,
there are numerous defects and lattice distortions, which readily couple with microwaves
through the mechanisms described above, thus the microwave absorption of finer powders
should be stronger. Inversely, surface area is a measure of defect concentration, activity,
free energy state, and thus microwave absorption. The higher the free energy state, the
stronger the microwave absorption. Accordingly, grain boundaries will couple with
microwave more strongly than the bulk of the grain, and amorphous materials will couple
much more strongly with microwaves than the crystalline form of the same composition.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
204
8.4.2. Thermal Conductivity
Structural imperfections also affect thermal conductivity.
Am ong insulators,
amorphous solids have smaller thermal conductivities than crystals because the phonon
waves are scattered more often in an aperiodic structure (Newnham, 1975). Chemical
impurities also promote phonon scattering. Similarly, the thermal conductivities of grain
boundaries should be smaller than that of the bulk due to the concentrated structural defects
and impurities along the grain boundaries. This will lead to localized heat dissipation and
consequently more microwave absorption during irradiation.
8.4.3. Surface Scattering
The scattering of microwaves by the material being processed is another possible
m echanism to prom ote m icrowave absorption.
A green ceramic com pact contains
numerous particles and is porous. The irregular shapes, surface roughness, and various
defects on and near the surfaces of the particles will scatter microwaves. Such a scattering
effect will increase with increasing specific surface area and the degree of imperfection of
the material. The scattering effect will increase microwave absorption of the material. As a
consequence, powders o f amorphous m aterials such as gels, should have a strong
microwave absorption. Figure 8.3. schematically shows the structure of the amorphous
material.
8.5. Athermal Effect in Microwave Processing
In Sec. 4.6.3., the apparent activation energy for the densification of HAp was found
to be 112 kJ/mole in microwave processing, compared with 181 kJ/mole in conventional
processing. In the case of diphasic mullite gel (Sec. 5.5.3.), the apparent activation energy
for m icrowave and conventional processing was found to be 396 and 702 kJ/mole,
respectively.
Similar phenomena were observed by other researchers. For example,
Janney and Kimrey (1991) reported that the apparent activation energy for microwave
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
205
Figure 8.3. Schematic showing o f an amorphous structure.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
206
sintering o f alumina at 28 GHz was 160 kJ/m ole, compared with 575 kJ/m ole for
conventional sintering. W hat does the lower apparent activation energy mean? From Fig.
8 .2 .,
we can see that a lower apparent activation energy implies an increase in free energy
o f the material. In other words, microwave irradiation seems to promote the chemical
potential of the material besides thermal heating. Since the accelerated densification was
observed at the same temperatures, the microwave effect must be an athermal effect, which
is an additional energy gain beside the volumetric thermal heating during microwave
irradiation.
8 .6 .
Microwave Enhanced Diffusion
Although no satisfactory model has yet been established, microwave enhanced
diffusion is evident in experiments. Freeman (1993, 1994) et al., for example, observed
an increase of ionic current in an NaCl crystal due to the application of microwave field.
Their experiment was conducted to make the with/without microwave comparison in-situ
and simultaneously, so that there were no questions of tem perature m easurem ent
inaccuracies. These researchers even measured ionic current without supplying external
voltage to the crystal. If this result is confirmed, then the microwave fields m ust be
supplying the driving force (chemical potential) for the ionic conduction. W hether the
driving force effect observed with the single crystal cancels out in polycrystals is still
unknown.
Rothman (1994) confirms the the microwave enhanced diffusion in his critical review
and indicates that the enhanced diffusion observed by Janney and Kimrey (1991) with
sapphire single crystal and Fathi et al. (1992) with silicate glass was volume diffusion,
since there was no grain boundary in these cases. For polycrystalline m aterials, the
microwave enhancement could not be so simply attributed to volume diffusion because
numerous grain boundaries with various defects exist. Ionic mobility along a grain
boundary can be 3 to 4 orders of magnitude greater than in the bulk (Freeman et al., 1994),
so that grain boundary diffusion should be mainly responsible for the densification in the
sintering of ceramics. Also, it has been indicated (Sec. 8.4.1.) that grain boundaries
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
207
couple with microwaves more strongly than the bulk, thus the enhancement in sintering
should be attributed to the enhancement in grain boundary diffusion. M eek (1987) and
Brooske et al. (1991, 1992) have proposed some models to interpret the microwave
enhanced densification, but no satisfactory theory has yet been developed. Obviously,
more efforts are needed in this aspect.
In summary, the above discussion suggests that free energy, or, indirectly, surface
area, is an important factor that influences microwave absorption of the material, beside
other loss mechanisms. Generally, a higher free energy state corresponds to higher
dielectric loss, lower thermal conductivity, stronger scattering effect, thus better microwave
absorption. For the same composition, the microwave effect on sintering should increase
in order of single crystals, polycrystals, and the amorphous. It is also suggested that
microwave enhanced sintering of polycrystalline ceramic materials is mainly resulted from
the enhancement in grain boundary diffusion. Now that the microwave effect exists, then
the more microwaves absorbed by the material irradiated, the stronger the microwave
effect. This is confirmed by the observation o f Freeman et al. (1994) that the ionic current
in the NaCl crystal in the microwave field increases with increasing microwave power.
8.7. Microwave Effects on Diphasic Mullite Gels
8.7.1. Observed Microwave Effects on Mullite Materials
It has been shown in Chapter 5 that microwave processing significantly accelerated
crystallization and densification of diphasic mullite xerogels. By microwave processing,
the mullite transformation temperature o f the diphasic gel was lowered by about 75-100°C.
The sintering of mullite and m ullite-based composites was much more efficient than
conventional processing when using the diphasic xerogel as the mullite precursor.
By
utilizing microwave processing, the apparent activation energy for the densification o f the
diphasic xerogel was lowered from 702 to 396 kJ/mole. By contrast, in the temperature
range up to 1500°C, the microwave effect on the densification of monophasic mullite gel
and crystalline mullite was not significant. This clearly indicates that the microwave
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
208
enhancing effect on the densification and mullite transformation is directly related to the
unique properties of the diphasic gel.
8.7.2. Characteristics of Mullite Gels
The scale of homogeneity of the alumina and silica species in the monophasic mullite
gel is at the atomic level, mullite forms in the monophasic gel at temperature as low as
970°C. Below this temperature, the sintering o f the monophasic gel is negligible, while
over this temperature, crystalline mullite has formed thus sintering and densification rely on
the solid-state diffusion mechanism. Since the crystalline mullite is a low-loss material, it
does not couple so efficiently with microwaves in the tem perature range of current
experiments.
Also, due to the low diffusivity of the crystalline m ullite, the rate of
densification o f mullite below 1500°C is limited.
In contrast, the two phases, alumina and silica sources, in the diphasic gel are mixed
at the scale of the nanometer range (~ 1 -1 0 0 nm), thus mullite formation can be delayed to
significantly higher temperatures around 1350°C, depending upon the fineness of the
alum ina particles, or the degree of hydrolysis o f alum ina source during the sol-gel
preparation. Huling and M essing (1989) observed m ullitization at 1364°C with the
colloidal boehmite-silia gel.
During mullitization, the formation and growth of mullite from the diphasic gels
occurs via a nucleation-and-growth mechanism by the reaction between S-A12 0 3 and
amorphous S i0 2. It has been established (Sundaresan and Aksay, 1991) that this reaction
takes place within the amorphous silicate-rich matrix rather than at the alumina/silica
interfaces. Now that microwave processing substantially enhanced mullite transformation
and densification of the diphasic gel, the microwave effect must have functioned through
the controlling mechanism(s) of these processes. Then, what is the mechanism in each of
these two processes? We will discuss them separately.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
209
8.7.3. Microwave Effect on Densification of Diphasic Mullite Xerogel
In the diphasic mullite gel, the alumina particles of nanometer size are mixed with
silica gel. It has been identified previously that the mullite transformation o f the current
diphasic gel takes place at about 1328°C under conventional condition, and 1250°C in the
microwave processing (Sec. 5.3.3.2.). The densification o f the diphasic gel by viscous
flow of the silica occurs before the mullite transformation. Figure 8.4. shows the diphasic
gel system, in which 8-Al2 0 3 particles are homogeneously dispersed in the silica gel
matrix.
Taking 3.65 and 2.2 g/cm 3 as the densities o f
8 -AI2 O 3
and the silica gel,
respectively, calculation shows that in such a gel system o f m ullite stoichiometry
(3Ai2 0 3 -2Si02), the volume percentage of 8-Al2 0 3 and the amorphous silica is 60.6% and
39.4%, respectively. If we suppose that all the alumina particles are uniformly covered by
a thin layer of silica gel, then the thickness of the silica layer on an alumina particle of 30
nm diameter, for example, would be 3.52 nm, roughly one tenth of the particle size. Silica
gel shows viscous flow starting about 700°C and remains amorphous before the
mullitization of the diphasic gel. The viscosity of silica decreases as temperature increases,
so that the densification of the diphasic mullite gel is a result of viscous flow of the silica
gel. Any factors that lower the viscosity of the silica gel will enhance the densification of
the diphasic gel. Thus it is obvious that the observed microwave enhanced densification of
the diphasic mullite gel was a result of the decrease in the viscosity o f silica gel in the
microwave field.
As described previously, the amorphous materials should absorb microwave more
efficiently than crystals o f the same composition. For example, it was found that the low
density polyethylene has twice the microwave loss level of high density polyethylene, and
the higher losses o f low density polyethylene have been attributed to the amorphous
regions where potions of polyethylene chains are able to oscillate in response to the
microwave field (Newnham, 1991). Similarly, the silica gel is amorphous and has a
distorted structure (Fig. 8.5.), thus it should have higher dielectric losses and absorb
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
alumina
silica
Figure 8.4. Model o f the diphasic mullite gel system, showing that
8 -AI2 O3
particles
are homogeneously dispersed in the silica gel.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 8.5. Schematic presentation of (a) ordered crystalline silica and (b) random network
glassy form o f the same composition. (Kingery e t al., p. 97)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission
212
microwave energy more efficiently. It can be suggested that the microwave absorption by
the gel brings about the volumetric heating as well as athermal effects. The microwave
enhancing effect on sintering and densification is a result o f the athermal effect, which
might be attributed to the increased kinetic energy o f the silica segments locally without
creating thermal effect in the bulk.
8.7.4. Microwave Effect on Crystallization and Epitaxy
In the mullite transformation of the diphasic gel, either nucleation or growth controls
the total process. If nucleation was the process controlling mechanism, then seeding would
significantly accelerate the transformation. However, this was not the case. Both the
current study (Sec. 5.3.3.2.) and previous work (Huling and M essing, 1989) show that
the m ullite transformation temperature o f the diphasic gel lowered under conventional
heating is very limited, only about 30° C. This clearly indicates that it is the growth rather
than nucleation that controls the mullite transformation in the diphasic gel.
The growth rate of m ullite grains can be controlled either by the dissolution of
alumina particles into the amorphous phase, or by the diffusion of the dissolved Al species
to the mullite grains. W ei and Holloran (1988) suggested that the mullite forms on the
interface and grows toward the diphasic region, and the growth mechanism is controlled by
a near-interface diffusion process (about 30 nm scale) or by the interfacial reaction itself.
On the other hand, Sundaresan and Aksay (1991) established a model to support the
concept that the growth rate of mullite grains in the diphasic gel is controlled by the
dissolution of the alumina particles into the amorphous phase.
The dissolution of alumina particles in the silica gel can be accelerated by increasing
the kinetic energy o f the Al species on the surfaces of
5 -Al2 Q 3
particles, whereas the
diffusion rate of the Al species in the silica gel may be accelerated by decreasing the
viscosity of the gel.
W hen subjected to m icrowave irradiation, the absorption of
microwaves by the alumina particles might excite the lattice of the alumina and allow more
Al species to be released to the region near the surfaces of alumina particles, leading to a
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
213
greater concentration gradient of the dissolved alumina between the surfaces of alumina
particles and the matrix. This would increase the driving force for the diffusion of alumina
species. On the other hand, the microwave irradiation also increases the kinetic energy of
silica segments of the gel, thus increases the motion of the silica particles and decreases the
viscosity of the silica gel. As a result, the mobility of the alumina species in the silica gel is
promoted. Although both mechanisms are possible, the latter appears to be dominating.
This is so because that it has been concluded that the microwave enhanced densification of
the diphasic mullite gel is a result o f lowering viscosity of silica gel, while the enhanced
densification was observed at about 1200°C which was below the temperature of mullite
transformation of the diphasic gel in the microwave field. In summary, the accelerated
mullite transformation of the diphasic gel is mainly attributed to the lowering of the
viscosity of the silica gel.
Seeding lowers the energy barrier in nucleation and may preferentially guide the
direction of the crystallization. Seeding in the diphasic mullite gel increases the nucleation
frequency and, at the same time, decreases the diffusion distance of the ions. Increasing
nucleation num ber increases the demand of ions to the crystal growing sites, while
decreasing diffusion distance shortens the time of ion transport. In conventional heating, if
the process is not nucleation-controlled, then the enhancement in grain growth is attributed
to the decrease in diffusion distance. In microwave processing, however, the microwave
effect lowers the viscosity of the amorphous silica and accelerates the ion transport. This
additional microwave effect significantly accelerates the crystallization process. Thus an
enhanced epitaxy o f the diphasic mullite gel is a natural result of lowering viscosity of the
silica gel. This result, in turn, suggests that the crystallization of the diphasic mullite gel is
a diffusion-controlled process.
8 .8 .
Microwave Effect on Sintering of Other Materials
The microwave enhancement in the sintering o f alumina is very significant when
using amorphous alum ina as starting material. For example, 20 min. o f microwave
sintering o f amorphous alumina at 1500°C achieved 49% higher densification than that
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
214
achieved by conventional sintering at the same temperature for same duration. When using
crystalline alumina as starting material, the microwave effect is neglected at the same
temperature. Apparently, the amorphous alumina had a higher microwave absorbing
efficiency thus showed stronger microwave effect.
The densification of HAp was significantly enhanced by microwave processing. It
has been found (Sec. 4.6.) that the apparent activation energy for the densification of HAp
in microwave sintering is substantially lower than that in conventional sintering
181 kJ/m ole).
(1 1 2
versus
In this case, the hydroxyls in the crystal structure of HAp,
Ca 10(PO4 )6 (OH) 2 , are thought to play an important role in microwave absorption. The
hydroxyls are polar function groups which may orient themselves in response to the
microwave fields thus make HAp absorb microwaves efficiently. In addition, high surface
area of the HAp must also have made contribution to the the microwave absorption.
To the two [NZP] materials, CSZP and BZPS, surface effect may be the main
interaction mechanism with microwaves contributing to microwave enhanced densification.
8.9. Influence of Processing Temperature on Microwave Effect
As shown in Fig. 8.2., for the process to occur from state A to B, energy greater than
Ea has to be supplied to the system for activation. E a is the activation energy for the system
with respect to this process. The energy supplied to the system can be either thermal or
athermal. The thermal energy increases kinetics, and athermal energy increases the driving
force (chemical potential). The relative magnitude of the activation energy and the supplied
energy determines the occurrence of the process.
It was noted that when sintered at 1300°C for 30 min., the sintered density of CSZP
was significantly higher than that achieved by conventional sintering for 24 h (Sec. 6.2.),
while when sintered at 1500°C for 20 min., microwave effect was almost negligible (1%).
Why the microwave enhancing effect in densification diminishes at elevated temperatures?
It can be explained as the following.
In m icrowave processing, the sintering or
densification is a combined effect of thermal heating plus microwave effect (presumably
athermal), while under conventional conditions, the same process simply relies on thermal
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
215
heating. Since microwave effect accelerates the process of sintering or densification, it is
an additional driving force. W hen the material is processed at such a temperature that the
thermal energy alone is not enough to trigger off sintering, sintering may be already in
process in the microwave process due to the additional microwave effect. W hen the
material is processed at a temperature that thermal energy alone is just enough to trigger
sintering, the microwave process will certainly result in a promoted sintering. In these
cases, the net promotion is totally attributed to the microwave effect. When temperature is
sufficiently high, the conventional heating alone supplies excess therm al energy to
substantially enhance the sintering or densification, then the microwave effect would be
negligible.
Let E t be the thermal energy, and Emw be the athermal microwave effect, T be
temperature (T 0 <T 1<T 2 <T 3 ), and suppose Emw<Ea, then the above discussion can be
summarized as Table 8.2. Thus it is clear that the sintering of CSZP at 1300°C is case 3,
while that at 1500°C is case 4 in Table 8.2.
Table 8.2. Influence of processing temperature on microwave effect.
Process
Microwave Effect
1. T = T 0 , E l0 < E a, a n d E l0 + E mw < Ea
Does not occur
Invisible
2. T = T j, E t l < E a, but E tl + Emw > Ea
Occurs
Yes
3. T = T2, E g > Ea, so E g + Emw » Ea
Enhanced
Yes
4. T = T 3, E t3 » Ea, then E g + Emw ~ E g
Enhanced
Negligible
Conditions
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
216
8.10. Grain Growth during Microwave Processing
Grain growth is a process taking place in the late stage of the sintering process of
ceramics. Generally, it is thought that at lower tem peratures, grain growth is more
significant but densification is not as significant, while at elevated tem peratures,
densification process dominates (Fig. 8 .6 .). When the densification processes to such a
stage that the removal of porosity is difficult, the recrystallization takes place and the
irregular grain growth will occur. In microwave processing, the heating rate is generally
higher, so that the time expended on heating stage is shorter, and grain growth in the low
temperature range can be relatively suppressed. Although microwave processing enhances
both densification and grain growth, the ratio of grain growth rate to the densification rate
is smaller than in conventional sintering. This was identified in microwave sintering of
HAp ceramics (Sec. 4.3).
sintering
0)
ca
cn
o
grain g ro w th
_i
high T
low T
1 /R T
Figure 8 .6 . Arrhenius plot of the rates of densification and grain growth. (Katz, 1992).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
217
Chapter 9
CONCLUSIONS
9.1. Summary and Conclusions
This thesis has focused on microwave processing of various ceramic materials,
including hydroxyapatite, mullite, alumina, NZP, and composites o f these materials and
partially stabilized zirconia. M icrowave processing experiments were carried out in a
mulitmode microwave cavity of 2.45 GHz. Conventional sintering experiments of the
duplicate samples were carried out in a special electric furnace with heating rates
comparable to those in microwave processing. Microwave effect on densification of these
materials has been evaluated. The mechanisms related to the microwave enhancement in
densification o f the selected materials and the crystallization of diphasic mullite gel have
been discussed.
In the synthesis of HAp, emphasis was put on the achievement o f pure, active HAp
powder with high thermal stability. The studies o f thermal stability were based on the
consideration that HAp o f poor thermal stability will decompose during high-temperature
sintering; only highly thermally stable HAp starting material can ensure the achievement of
pure HAp ceramics after sintering in air at high temperatures.
HAp powders were synthesized by various w et chemical m ethods, including
hydrolysis o f brushite (C aH P 04-2H 2 0 ) in a basic region (~pH 8 ) at temperatures around
70°C; hydrolysis of tricalcium phosphate [a-C a 3 (P 04)2] under the save conditions;
precipitation method from solutions; and hydrothermal synthesis. It was found that the
activity o f HAp products from these methods are comparable, with that by hydrolysis of
tracalcium phosphate as exception (lower specific surface area), while the thermal stability
varies one from the other.
Hydrolysis of DCPD produces fine HAp powder of poor crystallinity. This HAp is
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
218
nonstoichiometric (calcium deficient) and shows poor thermal stability. It decomposes at
about 800°C. Ripening treatment of this HAp with calcium chloride in basic region
significantly increased the Ca/P ratio and thus improves the thermal stability. The ripened
sample was found to be stable to 1350°C.
Hydrolysis o f tricalcium phosphate produced highly crystalline HAp whiskers.
However, this HAp is also nonstoichiometric and could not be ripened with calcium
chloride as m entioned above. Partial decomposition, or dehydroxylation, was observed
when heated to 900°C.
Precipitation from solutions of C a(N 0 3) 2 and NH 4 H 2 P 0 4 is amorphous and fine. It
becomes crystalline when heated to above 700°C.
The crystallinity increases with
temperature. However, partial decomposition of this HAp was observed at 900° C.
The HAp powder synthesized by hydrothermally treating the precipitate from the
above solutions at 200°C and 1.5 MPa for 10 to 24 h results in well defined nanocrystalline
HAp. This product is stoichiometric and show excellent thermal stability. It is stable to
1370°C in air of 50% relative humidity.
Lattice param eter refinem ent of the above HAp samples reveals that chemical
stoichiometry and dense crystal structure are the keys to obtain high thermal stability of
synthetic HAp.
Microwave sintering of various HAp ceramics has been carried out. Regularly dense
HAp ceramics were achieved by microwave sintering of compacts of ripened HAp powder.
Relative density up to 97% has been achieved in about 10 min. at 1200°C. Porous HAp
ceramics with up to 74% porosity were produced with the ripened HAp powder, as well as
the HAp whiskers. In the former case, ammonium carbonate was used to create porosity,
while in the latter case, the porosity is created naturally by the entanglement of HAp
whiskers. Diametral tensile strength measurement o f the above ceramics show that they
were fairly strong. Highly transparent HAp ceramics were achieved by both microwave
and conventional sintering in air at ambient pressure. Hydrothermally synthesized HAp
was used for the fabrication of the transparent HAp ceramics. The microwave processing
of such ceramics takes only 5 min. at 1150°C.
A kinetics study shows that the densification of HAp by microwave processing has
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
219
lower apparent activation energy than in conventional process,
112
kJ/mol. vs. 181 kJ/mol.
It is believed that the hydroxyl ions in the HAp crystal structure is responsible for the
efficient sintering of HAp ceramics in the microwave field.
M icrowave sintering o f mullite ceramics was carried out with crystalline mullite,
monophasic, diphasic, as well as hybrid aluminosilicate gels as starting materials. Owing
to the limitation in the working temperature range o f the refractory materials used for
thermal insulation, most sintering experiments were carried out only up to 1500°C.
It was observed that diphasic gels can be sintered to relative density near 95% at
1300-1500°C within half an hour. Step sintering process works well for the diphasic gels.
In this process, the diphasic gel was first densified at 1200-1300°C, then crystallized to
mullite by soaking at elevated temperatures for minutes. The crystalline mullite is difficult
to densify under the sam e processing conditions, because m ullite does not absorb
microwaves efficiently, and at the processing temperatures, the solid state diffusion rate of
m ullite is very low. The monophasic gel transforms to mullite at 970°C. Before the
transformation, sintering effect is negligible, and after the transformation, it becomes
crystalline and behaves in the same way as the crystalline mullite. The densification
behavior o f the seeded gels is in between diphasic and monophasic gels. Seeding the
diphasic gel low ers m ullitization tem perature by about 30°C, but also lowers the
densification process, because of the slow densification kinetics o f the crystalline mullite.
The viscous flow o f amorphous silica in the diphasic gels allows the gels densify much
more efficiently than crystalline mullite by solid state diffusion.
Transparent mullite ceramics were fabricated in this study by both microwave and
conventional sintering in air at ambient pressure. Aerogels m ade from the diphasic
aluminosilicate gels by supercritical drying were used as starting material for the transparent
mullite. High purity, excellent sinterability, and most importantly, the agglomeration-free
microstructure, are the key points leading to the transparency.
A kinetics study shows that the diphasic mullite gel densifies efficiently at 11501200°C. The densification rate of the diphasic gel in the microwave field is significantly
higher than in conventional case. The apparent activation energy for the densification of the
diphasic gel in microwave processing found in this study is 396 kJ/mole, compared with
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
220
702 kJ/mole for conventional sintering.
It was found in this study that microwave irradiation not only significantly enhances
the densification of the diphasic mullite gel, but also apparently accelerates its mullite
transform ation.
W ithout seeding, m icrowave processing lowers the transform ation
temperature by 75-100°C. In conventional processing, the effect o f seeding on mullite
transformation was not detected when the content of seeds (monophasic gel) was less than
10%, but 0.5% seeds shows apparent acceleration in m ullite transform ation in teh
microwave irradiated samples. This clearly indicates that microwave processing promotes
seeding effect on the diaphasic mullite gel.
It is suggested, from the above observations, that the microwave enhancement in the
densification, and the mullite transformation, and the effect of epitaxy of the diphasic gels
are mainly attributed to the decrease in the viscosity o f the amorphous silica in the diphasic
gels by microwave irradiation. It is supposed that the loosely bonded fragments of the
distorted silica netw ork orient themselves in response to the m icrowave field thus
efficiently obsorb microwaves, lowering the viscosity. The attribution of the microwave
enhenced densfication and mullite transformation to the decrease in viscosity implies that
the mullite transformation in the diaphsic gel is controlled by the diffusion instead of
dissolution of the 8 -alumina in the gel.
M icrowave sintering of alum ina was conducted at 1500°C for 20 min. using
amorphous and crystalline starting materials, respectively. It was found that under the
current experimental conditions, when using the amorphous alumina as starting material,
the microwave enhancing effect on densification is very significant, while when using
crystalline starting m aterial, the microwave enhancing effect was negligible. This
indicates that the amorphous alumina precursor absorbs microwaves more efficiently than
the crystalline alumina.
M icrowave sintering experiments of CSZP ceramics were conducted at 1300 and
1450°C, respectively. The samples microwave sintered at 1300°C for 30 min. achieved
higher density than those conventionally sintered at the same temperature for 24 h,
indicating that the densification rate of CSZP was substantially enhanced by microwave
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
221
processing. The average grain size of the microwave sintered sample is about the same as
that conventionally sintered for 24h. This implies that microwave irradiation enhances
densification as well as grain growth, but the extent of enhancement in grain growth is less
than that in the densification. In other words, the grain growth process o f CSZP in the
microwave processing process was relatively suppressed.
Various ceramic composites containing partcially stabilized zirconia were sintered by
microwave processing. The microwave enhancement in densification was also identified
with these material systems. For the HAp/PSZ system, the typical density of microwave
sintered sam ple was 24% higher than the conventionally sintered sam ples.
Correspondingly, significant toughening effect has been achieved. The fracture toughness
(KIC) o f 3.88 MPaVm has been m easured in the m icrow ave sintered sample of
HAp/10%PSZ, compared with 1.56 MPaVm of the pure HAp ceramic of the same relative
density.
The microwave enhancing effect on densification of the composites based on the
amorphous alumina and the diphasic mullite gel was substantial. That with the composites
based on the NZP materials (CSZP and BZPS) was not as significant due to the high
processing temperatures. The microwave enhancing effect on densification decreases with
increasing zirconia content due to the relatively higher sinterability of PSZ over the matrix
materials.
From the data obtained in the current studies, it is proposed that the free energy state,
or chemical potential, of the powder materials is an important extrinsic cause of microwave
absorption due to the defects, lattice distortions, and surface scattering which couple with
microwave well.
The higher the free energy state, the stronger the microwave enhancing
effect. Thus the microwave effect on the materials of the same composition increases in the
order of single crystals, polycrystals, semicrystal, and the amorphous.
It was also proposed that the microwave enhancement in densification and related
phenomena is an athermal effect, although the mechanisms are not clear at present. The
obtained data indicates that the influence of the microwave enhancement on a desired
process o f the materials also depends on the relative magnitude of the athermal effect and
thermal effect, thus on the processing temperatures. When processing temperature is
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
222
substantially higher than that necessary for regular conventional heating, the microwave
effect will be shielded.
9.2. Suggestions for Future Studies
9.2.1. Dielectric Properties of Amorphous Materials
From the studies carried out in this thesis, microwave enhancement in densification of
the selected ceramic materials is evident. The most significant microwave effect was
observed on the materials of higher free energy state, such as the diphasic mullite gel,
amorphous alumina, and HAp of low crystallinity or high surface area. The higher the free
energy, the stronger the microwave enhancing effect.
Although the mechanisms of
microwave enhancement in the densification of these materials, as well as the acceleration
in m ullite transformation of diphasic aluminosilicate gels, has been proposed, a more
detailed analysis is definitely necessary to promote the understanding of the microwave
effect. Based on the experimental observations and the proposed mechanisms, a model
could be developed to describe the relationship of the dielectric properties and of the free
energy status of the ceramic materials. This will lead to a better control and process design
o f the microwave processing.
9.2.2. Microwave Sintering of HAp in Moist Environment
In addition, decomposition temperature of HAp depends on the moisture content, or
partial pressure of water vapor, in the sintering atmosphere. All sintering experiments in
this thesis were carried out in air, necessitating the use of highly thermally stable HAp to
ensure that pure HAp was obtained in the resultant ceramics. As indicated in the previous
chapter, only the HAp synthesized by hydrothermal method and by ripening of hydrolysis
product o f DCPD meet the requirement in thermal stability for high temperature sintering.
However, if sintered in a moist environment, the decomposition of HAp would be largely
suppressed. It is suggested that study of microwave sintering is carried out in moisture
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
223
using unripened as-synthesized HAp, or HAp prepared by precipitation.
9.2.3. Better Transparent Mullite Ceramics by Process Modification
Transparent HAp and m ullite ceram ic disks were fabricated in this thesis by
m icrowave and conventional sintering, respetively.
Such transparent ceramics were
achieved for the first time by ambient pressure sintering. Although the transparency of
HAp is high, there is some room for improvement o f the transparency of the mullite
ceramics. Realizing the important potential of transparent mullite ceramics in special
applications, efforts should be made to effectively rem ove the residual porosity in the
ceramics. This may be accomplished by seeding the diphasic mullite gel, or lowering
heating rate to completely rem ove the chem ical w ater (silonal groups). These two
approaches may be adopted independendy, or in combination.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
224
REFERENCES
Agrawal, D.K., Fang, Y., Roy, D.M ., and Roy, R., Fabrication o f hydroxyapatite
ceramics by microwave processing, Mat. Res. Soc. Symp. Proc., Vol. 269, 231-236,
MRS, Pittsburgh, PA, 1992.
ASTM F I 185-88, Standard specification for composition of ceramic hydroxyapatite for
surgical implant, American Society of Testing and Materials, 1988.
Blake, R.D., and J.D. Katz, “Microwave sintering of large alumina bodies”, in Ceramic
Transactions, Vol. 36, M icrowaves: Theory and Application in M aterials Processing”,
edited by D.E. Clark, W.R. Tinga, and J.R. Laia, The Am. Ceram. Soc., Westerville, OH,
pp. 459-465, (1993).
Brain, R.A., H.A. Atwater, “Rapid selective annealing of Cu thin films on silicon using
microwaves”, Mat. Res. Soc. 1994 Spring M eeting Abstracts, pp. 311, 1994.
Brooske, J.H., R.F. Cooper, and I. Dobson, and L. McCaughan, “Models o f nonthrmal
effects on ionic mobility during microwave processing of crystalline solids”, in Ceramic
Transactions, Vol. 21, M icrowaves: Theory and Application in M aterials Processing”,
edited by D.E. Clark, F.D. Gac, and W.H. Sutton, Am. Ceram. Soc., W esterville, OH,
pp. 185-191, (1991).
Brooske, J.H., R.F. Cooper, and I. Dobson, “Mechanisms for nonthermal effects on ionic
mobility during microwave processing of crystalline solids”, J. Mater. Res., Vol. 7., No.
2, 495-501, (1992).
Brooske, J.H., R.F. Cooper, and I. Dobson, and L. McCaughan, “Models o f nonthrmal
effects on ionic mobility during microwave processing of crystalline solids”, in Ceramic
Transactions, Vol. 21, Microwaves: Theory and Application in M aterials Processing”,
edited by D.E. Clark, F.D. Gac, and W.H. Sutton, Am. Ceram. Soc., W esterville, OH,
pp. 185-191, (1991).
Brow n, P.W., and M. Fulm er, “K inetics o f hydroxyapatite form ation at low
temperature”, / . Am. Ceram. Soc., 74(5), 934-940, 1991.
Brown, W .E., L.C. Chow, “Dental Restorative Cement Pastes”, US Pat. No. 4518430,
May 21, 1985.
Brown, W.E., L.C. Chow, “A new calcium phosphate, water-setting cement”, in Cements
Research Progress, edited by P.W. Brown, Cements Division, Am. Ceram. Soc., 1986.
Brown, W.E., Chow, L .C ., Combinations o f sparingly soluble calcium phosphates, in
slurries and pastes as mineralizers and cements, US Pat. 4612053, Sep. 16,1986.
Bruce, R.W., “New frontiers in the use of microwave energy: power and m etrology”,
Materials Research Society Symposium Proceedings, Vol. 124, Microwave Processing of
Materials, edited by W.H. Sutton, M.H. Brooks, and I J . Chabinsky, Materials Research
Society, Pittsburgh, PA., 3-15, 1988.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
225
Cheng, J., Y. Fang, D.K. Agrawal, Z. W an, L. Chen, Y. Zhang, J. Zhou, and X. Dong,
“Continuous m icrowave sintering o f ceram ics” , Ceramic Transactions, Microwaves:
Theory and Applications in M aterials Processing, Am. Ceram. Soc., Westerville, OH
1994.
Cheng, J., J. Qiu, J. Zhou, and N. Ye, “Densification kinetics o f alumina during
m icrowave sintering” , in M icrowave Processing of Materials III, M aterials Research
Society Symposium Proceedings Vol. 269, Edited by R.L. Beatty, W.H. Sutton, and M.F.
Iskander, pp. 323-328, Materials Research Society, Pittsburgh, Pennsyvania (1992).
Cheng, J., Y. Fang, D.K. Agrawal, Z. W an, L. Chen, Y. Zhang, J. Zhou, and X. Dong,
“ Continuous microwave sintering o f ceram ics”, Ceramic Transactions, Microwaves:
Theory and Applications in M aterials Processing, Am. Ceram. Soc., Westerville, OH
1994.
Costa, B.J., and J.O. Herrm an, “Preparation and sintering o f an hydroxylapatite
composite”, in Sintering and microstructure development symposium (paper No. SXVP-993), the 95th Annual Meeting of Am. Ceram. Soc. Abstracts, pp. 377, (1993).
Costa, B.J., and J.O. Herrm an, “Preparation and sintering o f an hydroxylapatite
composite”, in Sintering and microstructure development symposium (paper No. SXVP-993), the 95th Annual Meeting of Am. Ceram. Soc. Abstracts, pp. 377, (1993).
Brown, W .E., and L.C. Chow, “Dental restorative cement pastes”, U.S. Pat. 4518430,
M ay 21, 1985.
Cullity, B.D., Elements o f X-Ray Diffraction, 2nd edition, pp.418, Addison-Wesley
Publishing Compary, Inc., 1978.
Ducheyne, P., and J.E. Lemons, Eds. Bioceramics: Material characteristics versus in vivo
behavior, Annals of the New York Academy of Sciences, Vol. 523, The New York
Academy of sciences, New York, NY., 1988.
Eysel, W., D. M. Roy, "Topotactic Reaction o f Aragonite to Hydroxyapatite", Zeits. Kirst.
141, 11-24(1975).
Fang, Y., D.K. Agrawal, D.M. Roy, and R. Roy, Rapid sintering o f hydroxyapatite
ceramics by microwave processing, Ceramic Transactions, Vol. 21, edited by D.E. Clark,
F.D. Gac, and W.H. Sutton, American Ceramic Society, Westerville, Ohio, 1991.
Fang, Y., D.K. Agrawal, D.M. Roy, and R. Roy, “Fabrication of porous hydroxyapatite
ceramics by microwave processing”, J. Mater. Res., \b l. 7(2), 490-494, 1992.
Fang, Y., D.K. Agrawal, D. M. Roy, and R. Roy, "Rapid Sintering of Hydroxyapatite
Ceramics by Microwave Processing", Microwaves: Theory and Application in Microwave
Processing, Ceramic Transactions, Vol. 21, pp. 349-356 (1991).
Fang, Y, D.K. Agrawal, D.M . Roy, and R. Roy, “Microwave sintering of calcium
strontium zirconium phosphate ceramics”, Ceramic Transactions, vol. 36, Microwaves:
Theory and application in materials processing n, edited by D.E. Clark, W.R. Tinga, and
F.R. Laia, pp. 109-114, Am. Ceram. Soc., Westerville, OH, 1993.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
226
Fang, Y., D.K. Agrawal, R. Roy, and D.M. Roy, "Synthesis, dehydration, and ceramics
of hydroxylapatite", 34-T-89, 91st Anuual M eeting o f Am. Ceram. Soc., Indianapolis,
April 23-27,1989.
Fang, Y, D.M . Roy, J. Cheng, R. Roy, and D.K. Agrawal, “M icrowave sintering of
hydroxyapatite-based composites”, Ceramic Transactions, vol. 36, Microwaves: Theory
and application in materials processing n, edited by D.E. Clark, W .R. Tinga, and F.R.
Laia, pp. 397-407, Am. Ceram. Soc., Westerville, OH, 1993.
Fang, Y., D.M. Roy, J. Cheng, R. Roy, and D.K. Agrawal, Microwave sintering of
hydroxyapatite-based composites”, in Ceramic Transactions, Microwaves: Theory and
Application in Materials Processing II, Am. Ceram. Soc., Inc., Westerville, OH (1993).
Fathi, Z., D.E. Clark, and R. Hutcheon, “ Surface m odification o f ceram ics using
microwave energy”, in Microwave Processing of M aterials III, M at. Res. Soc. Symp.
Proc., Vol. 269, edited by R.L. Beatty, W.H. Sutton, and M.F. Iskander, pp. 347-351,
Pittsburgh, PA 1992.
Feiker, G.E., and N.C. Gittinger, “Rapid Heating of Dielectric M aterials at 915 Me”,
Electrical Engineering, Vol. 78, No. 11, pp. 1089, (1959).
Ford, J.D ., “H igh Temperature Chemical Processing via M icrowave Absorption”, J.
Microwave Power, Vol. 2-2, pp. 61-64, (1967).
Freem an, S.A., J.H. Booske, R.F. Cooper, B. Meng, J. Kieffer, and B J . Reardon,
“ Studies o f microwave field effects on ionic transport in ionic crystalline solids”, in
Ceramic Transactions, Vol. 36, M icrowaves: Theory and Application in M aterials
Processing”, edited by D.E. Clark, W.R. Tinga, and J.R. Laia, The Am. Ceram. Soc.,
Westerville, OH, pp. 213-220, (1993).
Freem an, S.A., J.H. Booske, R.F. Cooper, B. Meng, J. Kieffer, and B J . Reardon,
“Effects of high power microwave fields on ionic transport in ceramics and ionic crystalline
solids”, Proc. M icrowave Absorbing Materials for Accelerators Conf., Newport News,
VA, 1993, to be published.
Freem an, S.A., J.H. Booske, R.F. Cooper, B. Meng, “M icrowave radiation effects on
ionic current in ionic crystalline solids”, in Microwave Processing of Materials IV, Mat.
Res. Soc. Symp. Proc., Vol. 347, edited by M.F. Iskander, R J . Lauf, and W.H. Sutton,
pp. 479-485, Pittsburgh, PA, 1994.
Fricke, J. and A. Emmerling, “Aerogels”, J. Am. Ceram. Soc., 75 [8 ] 2027-36 (1992).
Fuller, A J . Baden, M icrowaves, An introduction to microwave theory and techniques,
third edition, Pergamon Press, p p .1-3, 1990.
Green, D .J., R .H J.H annink, and M.V.Swain, Transformation Toughening of Ceramics,
CRC Press, Boca Raton, Florida, 1989.
Gurevich, V.L., A.K. Tagantsev, Adv. in Phys. 40, 719-767 (1991), quoted by Meng et
al. (1994).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
227
De Groot, K., “Hydroylapatite coating for implants in surgery”, in Ceramics in Clinical
Applications, edited by P. Vincenzini, Elsevier (1987), pp381-386.
Hanker, J.S., B.L. Giammara, Biomedical Mat. and Devices, MRS Symp. Proc., Vol.
110, MRS, Pittsburgh, PA, 1989.
Hattori, T., Y. Iwadate, T. Kato, Hydrothermal synthesis of hydroxyapatite from calcium
acetate and triethyl phosphate, Advanced Cer. Mat. 3 (4), 426-428, 1988.
H eise, U., J.F. Osborn, and F. Duwe, Hdroxyapatite ceramic as a bone substitute,
International Orthopedics (SICOT) 14:329-338,1990.
Hench, L.L., Bioceramics: from concept to clinic, J. Am. Ceram. Soc., 1A (7), 14841510, 1991.
Ho, W.W., “Hign-temperature Dielectric Properties of Ploycrystalline Ceramics”, pp. 137148 in M icrowave Processing of M aterials, Vol. 124. Edited by W.H. Sutton, MH.
Brooks, and I.J. Chabinsky. Materials Research Society, Pittsburgh, PA, 1988.
Holcombe, C.E., T.T. Meek, and N.L. Dykes, “Enhanced Thermal Shock Properties of
Y2 Q 3 -2 wt% Zr 0 2 Heated Using 2.45 GHz R adiation”, pp. 227-234 in M icrowave
Processing o f M aterials, Vol. 124, Edited by W .H. Sutton, M.H. Brooks, and I.J.
Chabinsky. Materials Research Society, Pittsburgh, PA, 1988.
Huling, J.C., G.L. Messing, “Hybrid gels for homoepitactic nucleation of mullite”, J.
Am. Ceram. Soc., 72(9) 1925-29 (1989).
Ioku, I., M. Yoshimura, S. Somiya, Hydrothermal synthesis of hydroxyapatite powders
with zirconia dispersion, in Biomedical materials and devices, MRS Sympo. Proc., Vol.
110, 445-450, 1989.
Janney, M ark A., Hal D. Kimrey, M onica A.Schmidt, and James O. Kiggans, “Grain
Growth in M icrowave-Annealed Alumina”, J. Am. Ceram. Soc., 74 [7] 1675-1681
(1991).
Janney, M ark A., and Hal D. Kimrey, “Diffusion-controlled processes in microwave-fired
oxide ceramics”, Mat. Res. Soc. Symp. Proc. Vol. 189, 215-227 (1991).
Jarcho, M ., Bolen, C.H., Thomas, M .B., Bobick, J., Kay, J.F., D orem us, R.H.,
Hydroxyapatite synthesis and characterization in dense polycrystalline form, J. Mat. Sci.
11, 2027-35, 1976.
Johnson, D. Lynn, “Microwave Heating o f Grain Boundaries in Ceramics”, J. Am.
Ceram. Soc., 74 [4] 849-850 (1991).
Kanazawa, T ., Ed., Inorganic Phosphate Materials, Elsevier, 1989.
Katz, J.D., “Microwave sintering o f ceramics”, Annu. Rev. Mater. Sci. 1992. 22:153-70.
Kenkre, V.M., “Theory of microwave interactions with ceramic materials”, in Ceramic
Transactions, Vol. 21, Microwaves: Theory and Application in Materials Processing”, Am.
Ceram. Soc., Westerville, OH, pp. 69-80, (1991).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
228
Kenkre, V.M, L.Skala, and M.W. Weiser, “Theory of microwave interactions in ceramic
materials: the phenomenon of thermal runaway”, J. Mater. Sci. 26 (1991) 2483-2489.
Kimrey, H.D., J.O. Kiggans, M.A. Janney, and R.L. Beatty, “Microwave sintering of
zirconia-toughened alumina composites”, Materials Research Society Symposium
Proceedings, vol. 189, pp.243-255, MRS, 1991.
Kingery, W.D., H.K. Bowen, D.R. Uhlm ann, Introduction to Ceramics, 2nd edition,
John W iley & Sons, 1976.
Kom am eni, S.,
and R. Roy, "Anomalous Microwave M elting of Zeolites", Materials
Letters, 4 [2] 107-110 (1986).
Lange, F.F., “Transformation toughening”, J. Mater. Sci., 17 (1982) 225-262.
Lavernia, C., and J. M. Schoenung, "Calcium Phosphate Ceramics as Bone Substitutes",
Am. Ceram. Soc. Bull., 70[1], 95-100 (1991).
Li, Qing Hua, “Synthesis of ceramic powders by m icrowave-hydrothermal process”,
Master thesis, The Pennsylvania State University, 1993.
Limaye, S.Y., D.K. Agrawal, R. Roy, and Y. Mehrotro, “Synthesis, sintering and thermal
expansion o f C a 1.xSrxZr4 P 6 0 24 -an ultra-low thermal expansion ceramic system”, J.
Mater. Sci., Vol. 26, pp. 93-98,(1991).
Mah, T.-I., K.S. M azdiyasni, “M echanical properties of m ullite”, J. Am. Ceram. Soc.,
\b l. 6 6 , No. 10, 699-703 (1983).
M ason, T.J., and J.P. Lorim er, Sonochemistry: Theory, A pplicatins and Uses of
Ultrasound in Chemistry (Ellis H orw oodLtd., Chichester, U.K., 1988).
McConnell, D., Apatite: Its Crystal Chemistry, Mineralogy, Utilization, and Geologic and
Biologic Occurrences, Springer-Verlag, New York, Wien, 1973.
Meek, T.T. “Proposed Model for the Sintering of a Dielectric in a M icrowave Field”, J.
Mater. Sci. Lett., 6 , 638-640 (1987).
Meng, B., J.Booke, R.Cooper, andS. Freeman, “Microwave absorption in NaCl crystals
with various controlled defect conditions”, in Microwave Processing o f Materials IV, Mat.
Res. Soc. Symp. Proc., Vol. 347, edited by M.F. Iskander, R.J. Lauf, and W.H. Sutton,
pp. 467-472, Pittsburgh, PA, 1994.
Milovsky, A.V., Mineralogy & Petrography, pp.235, Mir Publishers, Moscow, 1982.
Monma, H., and T. Kamiya, Preparation of hydroxyapatite by the hydrolysis of brushite,
J. Mat. Sci. 22, 4247-50, 1987.
M onma, H., Takahashi, T., Ushio, H ., Soeda, S., and Kiyosawa, T., Preparation and
properties of porous apatite by the hydration and hardening method, Gypsum & Lime,
212, 25-28, 1988.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
229
M onma, H., S. Ueno, and T. Kanazawa, Properties of hydroxyapatite prepared by the
hydrolysis of tricalcium phosphate, J. Chem. Tech. Biotechnol., 31,15-24, 1981.
Moya, J.S., M.I. Osendi, “Microstructure and mechanical properties of mullite/Zr0 2
composites”, J. Mat. Sci., 19(1984) 2909-2914.
Newnham, R.E., S.J. Jang, M. Xu, and F. Jones, “Fundamental interaction mechanisms
between microwaves and matter”, in Ceramic Transactions, Vol. 21, Microwaves: Theory
and Application in Materials Processing”, edited by D.E. Clark, F.D. Gac, and W .H.
Sutton, Am. Ceram. Soc., Westerville, OH, pp. 51-67, (1991).
Newnham, R.E., Structure-Property Relations, Springer-Verlag, pp. 57, 1975.
Patil, D.S., B.C. Mutsuddy, J. Gavulic, and M. Dachimene, “M icrowave sintering of
alumina ceramics in a single mode applicator”, in Ceramic Transactions, Microwaves:
Theory and Application in Materials Procession, edited by D.E. Clark, F.D. Gac, and
W.H. Sutton, Am. Ceram. Soc., Westerville, Ohio, pp.301-309 (1991).
Raham an, M .N., L.C. De Jonghe, S.L. Shinde, and P.H. Tewari, “Sintering and
microstructure of mullite aerogels” , J. Am. Ceram. Soc., 71 [7] C-338-C-341 (1988).
Riboud, P.V., Bull. Soc. Chim. Fr., Spec. No. Cpmparaison de la stabilite de I’apatite
d’oxydedeferet del’hydroxyapatite a haute temperature, 1701-1703, 1968.
Rothman, S.J., “Critical assessment of microwave-enhanced diffusion” , in Microwave
Processing of Materials IV, M at. Res. Soc. Symp. Proc., Vol. 347, edited by M.F.
Iskander, R.J. Lauf, and W.H. Sutton, pp. 9-18, Pittsburgh, PA, 1994.
Roy, R., D.K. Agrawal, and V. Srikanth, “Acoustic wave stimulation of low temperature
ceramic reactions”, / . Mater. Res. 6, 2412-2416 (1991).
Roy, D.M ., Porous Biomaterials and M ethod o f Making Same, US Pat. No. 3929971,
1975.
Roy, D. M., and S. K. Linnehan, "Hydroxyapatite formed from Coral Skeletal Carbonate
by Hydrothermal Exchange", Nature, 247, 220-22 (1974).
Roy, D.M ., L.E. Drafall, land R. Roy, Crystal Chemistry, Crystal Growth, and Phase
Equilibria of Apatites, Academic Press, Inc. (1978) pp.186.
Roy, R., L.J. Yang, and S. Komameni, Am. Ceram. Soc. Bull., 63 (1984) 459.
Roy, R., S. Komarneni, and L.J. Yang, “Controlled microwave heating and melting of
gels”, J. Am. Ceram. Soc., 6 8 (7) 392-395 (1985).
R udanick, A., A. R. H unter, and F. C. Holden, "An Analysis of the DiametralCompression Test", Materials Research Standards, pp. 283, April, 1963.
Stein, D.F., “Microwave processing — An emerging industrial technology?”, Mat. Res.
Soc. 1994 Spring Meeting Abstracts, pp. 299,1994.
Suslick, K.S., “Sonochemistry”, Science, Vol. 247, 1439-1445 (1990).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
230
Sundaresan, Sankaran and Ilhan A. Aksay, “Mullitization of Diphasic Alumino-silicate
Gels”, J. Am. Ceram. Soc., 74[10] 2388-92 (1991).
Sutton, W.H., "Microwave Processing of Ceramic Materials", Am. Ceram. Soc. Bull.,
[2] 376-86 (1989).
68
Tinga, W.R., and E.M . Edwards, “Dielectric M easurement Using Sw ept Frequency
Techniques”, J. Microwave Power, vol. 3, No. 3, pp. 144-75 (1968).
Tinga, W.R., and W.A.G. Voss, M icrowave Power Engineering,Vol. 2, pp.189-194,
Academic Press, New York (1968).
Tinga, W.R., “Interaction of Microwaves with Materials”, in Proceedings of IMPI, Short
Course for Users o f Microwave Power (1970).
Tinga, W .R., “Fundamentals of microwave-material interactions and sintering”, Mat. Res.
Soc. Symp. Proc. Vol. 124, edited by W.H. Sutton, M.H. Brooks, and I J . Chabinsky,
Mat. Res. Soc., pp. 33-43, 1988.
Tinga, W .R., “Microwave material interactions and process design modeling”, in Ceramic
Transactions, Vol. 36, Microwaves: Theory and Application in Materials Processing n,
edited by D.E. Clark, J.R. Laia, Jr., pp. 29-43, Am. Ceram. Soc., Westerville, OH, 1993.
Vbn Hipel, A.R., Dielectric Materials and Applications, Technology Press o f M IT and
Wiley, New York, (1954).
W alton, J.D ., Jr., “Inorganic random es” ; pp. 229-344 in Random e Engineering
Handbook. Edited by J.D. Walton, Jr., Marcel Dekker, Inc., New York, 1970.
Wang, P.E., T.K. Chaki, Sintering behaviour and mechanical properties o f hydroxyapatite
and dicalcium phosphate, J. Mat. Sci.: Mat. in Medicine, 4, 150-158, 1993.
Weber, J. N., R. T. Greer, B. T. Voight, E. W. White, and R. Roy, "Unusual Strength of
Echinoderm Calcite Related Structure", J. Untrastructure Research, 26, 355-66 (1969).
Wei, W en-Cheng, and John W. H alloran, “Phase T ransform ation o f D iphasic
Aluminosilicate Gels”, J. Am. Ceram, soc., 71 [3] 166-72 (1988).
Wei, W en-Cheng, and John W. H alloran, “Transform ation Kinetics o f Diphasic
Aluminosilicate Gels”, / . Am. Ceram, soc., 71 [7] 581-87 (1988).
W hite, E. W., J. N. Weber, D. M. Roy, E. L. Owen, R. T. Chiroff and R. A. White,
"Replamineform Porous Biomaterials for Hard Tissue Implant Applications", J. Biomed.
Mat. Res. Symp. No. 6 , 23-27 (1975).
W hite, R. A., J. N. Weber, and E. W. W hite, "Replamineform: A New Process for
Preparing Ceramics, Metal, and Polymer Prosthetic M aterials", Science, 176, 922-24
(1972).
W illmann, G., Normbestimmungen fur hydrxyapatit als Knochenersatz (Standards for
Hydroxyapatite used for Bone Replacement), Sonderdruck aus Biomedizinische Tecknik,
35(9), 205-207, 1990.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
231
W roe, F.C.R., “Scaling up the microwave firing o f ceramics”, in Ceramic Transactions,
\fol. 36, Microwaves: Theory and Application in Materials Processing II, edited by D.E.
Clark, W.R. Tinga, and J.R. Laia, Am. Ceram. Soc., Wsterville, OH, 449-458, 1993.
W u, J.-M ., T.-S. Yeh, Sintering o f Hydroxyapatite-zirconia composite materials, J. Mat.
Sci., 23, 3771-3777, 1988.
Yamamuro, T., L.L. Hench, J. Wilson, Eds. Handbook o f Bioactive Ceramics, Vol. II,
Calcium Phosphate and Hydroxylapatite Ceramics, CRC Press, Boca Raton, FL 1990.
Yamashita, K., Kanazawa, T., “Hydroxyapatite”, in Inorganic Phosphate Materials, edited
by Kanazawa, T., Elsevier, 1989, p l8 .
Ye, Ruilun, Y.H. Fang, and P.W. Lu, Physical Chemistry of Inorganic Materials, China
Building Industry Press, 1986, 337-338.
Yoshimura, M., K. Ioku, andS. Somiya, “Apatite ceramics with tailored microstructures
from hydrothermally-prepared ultra-fine hydroxyapatite crystals”, Euro-Ceramics, Vol. 3,
edited by G. de W ith, R.A.Terpstra, and R. Metselaar, Elsevier, 3.17-3.20, 1989.
Zhong, J.P., Z. F athi, G.P. LaTorre, D.C. Folz, and D.E. Clark, “ M icrowave
densification of porous silica gel”, in Ceramic Transactions, Vol. 36, Microwaves: Theory
and Application in Materials Processing II, edited by D.E. Clark, W.R. Tinga, and J.R.
Laia, Am. Ceram. Soc., Wsterville, OH, 157-164, 1993.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
VITA
Yi Fang was born in Buchu Milage, Zhaotong, Yunnan Province, China.
He
attended The First Middle School of Zhaotong Prefecture. He studied at Wuhan Institute of
Building Materials (presently Wuhan University o f Technology) and graduated with a B.S.
in materials engineering in 1982. He worked at Guangxi University until he joined the
Intercollege Materials Research Laboratory, The Pennsylvania State University, University
Park, PA, as an exchange visitor in 1987. H e worked at the Intercollege M aterials
Research Laboratory as a visiting research assistant untill 1989. In the spring of 1989, he
joined the graduate program in Solid State Science at The Pennsylvania State University for
his Ph.D. During these years as a research assistant, he worked m ainly in materials
synthesis and characterization. He authored and coauthored around 20 technical papers on
cementitious materials, bioceramic materials, ultrasonic chemical synthesis, ultrasonic
sieving, and microwave sintering of various ceramic materials. He is a member o f The
American Ceramic Society and Materials Research Society.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Документ
Категория
Без категории
Просмотров
0
Размер файла
9 847 Кб
Теги
sdewsdweddes
1/--страниц
Пожаловаться на содержимое документа