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Surface tempering of dental ceramics by internal heating using microwave energy

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SURFACE TEM PERING OF DENTAL CERAM ICS BY INTERNAL HEATING
USING M ICROW AVE ENERGY
By
Karen Joan Thom pson
A Thesis
Subm itted to the Faculty o f the
U niversity o f Louisville
Speed Scientific School
as Partial Fulfillment o f the Requirements
for the Professional Degree
M ASTER OF ENGINEERING
Departm ent o f Chemical Engineering
May 2000
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
UMI Number: 1400448
UMI
UMI Microform 1400448
Copyright 2000 by Bell & Howell Information and Learning Company.
All rights reserved. This microform edition is protected against
unauthorized copying under Title 17, United States Code.
Bell & Howell Information and Learning Company
300 North Zeeb Road
P.O. Box 1346
Ann Arbor, Ml 48106-1346
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
SURFACE TEMPERING OF DENTAL CERAM ICS BY INTERNAL HEATING
USING C O NTROLLED M ICROW AVE ENERGY
Submitted by:
■ ' '/ C c s * v
Karen J. Thompson
•
A Thesis Approved on
ll» f Zooo
(Date)
by the Following Reading and Examination Committee:
%
Dr. Jam es C. Watters
Thesis Co-D irector
Dr. Lawrence Gettleman
Thesis Co-D irector
lJ
Dr. John Naber
Dr. Dean O. Harper ^
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R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
ACKNOWLEDGMENTS
I sincerely thank Dr. James C. Watters. Professor o f Chemical Engineering, and Dr.
Lawrence Gettleman. Professor of Prosthodontics and Biomaterials, School o f Dentistry, for
all o f the time, education and support that they gave me as co-directors o f this thesis.
I also acknowledge Dr. Dean O. Harper. Professor o f Chemical Engineering and Dr.
John Naber, Assistant Professor of Electrical Engineering for their lime and assistance as
members o f the evaluation committee.
I extend my gratitude to Dr. James C. W atters, Dr. Dean O. Harper and Dr. Alan
Johnson o f the Chemical Engineering Department for sharing their expertise in the area o f
ceramic processing. I also thank Dr. John Naber and Dr. Donald J. Schcer o f the Electrical
Engineering Department for their advice concerning the feasibility of this project and the
modification o f microwave ovens. Finally. I thank Dr. Lawrence Gettleman for educating
me about dental ceramics and the fabrication o f dental restorations.
I thank Ms. Rodica McCoy for her assistance in procuring the necessary chem icals
for the experimental work.
I also thank Mr. M ark Paul o f the Kersey Library for his
guidance in the preliminary literature search.
I thank Brian Knopf, Director o f Research and Allen Steinbock, President o f Whip
Mix, Corporation o f Louisville, Kentucky for their contribution to this project, including
refractory material, a dental firing oven, and advice concerning the future direction o f this
work.
Finally I thank my parents and family for their unwavering support o f all my
endeavors.
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R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
ABSTRACT
Dental ceramic restorations such as crowns are fabricated by layering and firing
ceramic powders to create esthetic reproductions o f natural teeth. The firing sequences
alternate between controlled heating and cooling phases.
The final firing step is
performed in air with heat supplied by electrical resistance coils. During the fabrication
process, the restorations are supported on refractory die materials or other refractory oven
furniture. This project proposes to modify the final firing step by heating the refractory
die materials using microwave energy while the ceramic restorations are cooling. It is
anticipated that the restorations w ill be strengthened by a thermal tempering effect,
keeping the outside cooler than the inside and controlling the rates o f cooling
simultaneously. The outer convex surface o f the restoration will be under compression as
it is rapidly cooled, while the heated inner concave surface will be under tension as it
cools more slowly and is pulled to the cooler surface.
The focus o f this work w as a review o f related literature and preliminary
experimental work, performed in a 650-watt domestic microwave oven operating at 2.45
GHz. Experiments showed that commercial refractory die materials alone do not absorb
microwave energy adequately, but the addition o f silicon carbide whiskers or copper
powder enhanced the microwave absorption o f the refractory die material and allowed it
to be heated to the required firing temperatures in less than five minutes.
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TABLE OF CONTENTS
Bags
APPROVAL PAGE............................................................................................................................ ii
ACKNOWLEDGMENTS................................................................................................................iii
ABSTRACT.........................................................................................................................................iv
NOMENCLATURE........................................................................................................................... vi
LIST OF TABLES............................................................................................................................ vii
LIST OF FIGURES.......................................................................................................................... viii
INTRODUCTION...........................................................................................................................1
BACKGROUND............................................................................................................................ 3
REVIEW OF THE LITERATURE.............................................................................................5
MICROWAVE HEATING.....................................................................................................5
SILICON CARBIDE A S A SUSCEPTOR........................................................................ 17
TEMPERATURE M EASUREM ENT IN MICROWAVE O V E N S ............................25
MICROWAVES IN THE DENTAL INDUSTRY.......................................................... 26
TEMPERING M EC H A N ISM S.......................................................................................... 28
EXPERIMENTAL A PPA R ATU S............................................................................................32
PROCEDURE................................................................................................................................ 34
RESULTS AND D ISSC U SSIO N .............................................................................................38
CONCLUSIONS........................................................................................................................... 44
RECOMMENDATIONS............................................................................................................ 45
REFERENCES..............................................................................................................................47
APPENDIX I - DENTAL REFRACTORY PRODUCT SPECIFICATIONS.....................50
VITA....................................................................................................................................................55
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R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
NOM ENCLATURE
E
=
Electric field intensity
P
=
M icrowave power absorbed
Tc
=
Critical temperature above which a material absorbs microwave energy
Tg
=
G lass transition temperature
V
=
V olum e o f sample
s'
=
Dielectric constant
e"
=
Dissipation factor
=
Dielectric loss
=
M icrowave frequency
tan
a)
6
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R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
LIST OF TABLES
TABLE I: M ICROW AV E AND CONVENTIONAL SIN TER IN G OF A12 0 3 .................11
TABLE II: PHYSICAL PROPERTIES OF SILICON C A R B ID E ....................................... 18
TABLE III: PHYSICAL PROPERTIES OF DENTAL R EFRA CTO RIES........................ 35
TABLE IV: SILICON CARBIDE WHISKERS SPEC IFIC A T IO N .................................... 36
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
LIST OF FIGURES
Figure 1 - Dental Restorations Supported by Refractory M aterial.......................................... 4
Figure 2 - Average Power Absorbed by A lum ina Samples Versus S intering
Tem perature....................................................................................................................11
Figure 3 - CEM Patented M icrowave A shing Furnace............................................................ 13
Figure 4 - CEM Patented M icrowave A shing C rucible........................................................... 14
Figure 5 - Dielectric Constant and Loss Tangent o f Silicon Carbide H exaloy SA at
Varying Frequencies and T em peratures................................................................... 18
Figure
6
- Power and Temperature D istributions for a 5 cm Slab o f S ilico n Carbide
Exposed to Microwaves from Both Sides................................................................ 19
Figure 7 - Power and Temperature D istributions for a 5 cm Slab o f S ilico n Carbide
Exposed to Microwaves from Both Sides................................................................ 19
Figure
8
- Surface Temperature Versus Tim e for SiC Irradiated with D ifferent
M icrowave Frequencies...............................................................................................21
Figure 9 - Surface Temperature o f SiC with 900 Seconds o f M icrow ave Heating at 35
W /cm 2 Followed by C ooling.......................................................................................21
Figure 10 - M icrowave Heating Rates for Silicon Carbide Dispersed in AI 2 O 3
C em ent...........................................................................................................................24
Figure 11 - Stress Distribution in Fully Tem pered G lass.........................................................28
Figure 12 - Forces Present in Tempered G lass........................................................................... 29
Figure 13 - Experimental A pparatus.............................................................................................33
Figure 14 - Silicon Carbide W hiskers M agnified 400 Tim es.................................................36
Figure 15 - Initial Stages o f Microwave Heating o f Dental Refractory Material Using
Silicon Carbide as a Susceptor................................................................................. 39
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
Figure 16 - Later Stages o f M icrowave Heating o f Dental Refractory Material Using
Silicon Carbide as a Susceptor................................................................................40
Figure 17 - Microwave Heating o f Dental Refractory Material Using Silicon Carbide as a
Susceptor...................................................................................................................... 41
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R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
INTRODUCTION
This project investigates a novel means o f strengthening dental porcelain or
ceramic restorations by m eans o f microwave heating o f the refractory die that supports a
crown in the firing furnace. The experimental procedure involves adding materials that
absorb microwave energy readily (such as silicon carbide or copper) to the refractory die
to enhance m icrow ave absorption, as well as changing the geometry o f the oven cavity to
channel the m icrow ave energy to the sample.
Proper control o f the temperature o f the
exterior and interior o f the ceramic piece during cooling should result in residual
compressive forces on the outer surface o f the restoration. It is anticipated that a thermal
tempering effect w ill occur, which should produce stronger structures.
Traditionally, ceram ic crowns have been heated using thermal heat from electrical
resistance coils. Therm al methods rely on the thermal conductivity o f ceramics, and long
processing times are required to heat up the specim ens. Slowly heating and cooling is
very time consum ing, and subjects the crowns to repeated therm al stresses, which could
weaken them.
M icrow aves heat more uniformly, o n the surface and internally, in a
shorter amount o f tim e, and are more energy efficient than traditional heating methods.
Thus microwave heating could prove to be prom ising in this application.
M icrowave energy is gaining popularity in a variety o f processing areas beyond
the traditional applications in the food and com m unications industries.
In research
laboratories, m icrow aves are used in analytical processes, ashing and in acid digestion.
Microwave energy is now being commercially used to cure polymers, sinter ceram ics,
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sterilize medical equipm ent, cure structural wood products, and to remediate nuclear and
hazardous wastes.
The ultim ate goal o f this project is to achieve a superior product through increased
strength o f the finished crow n by controlled tempering o f the ceramic.
While the
feasibility o f this idea has been confirmed by initial experim ental work, further
investigation is necessary before a commercial apparatus can be proposed.
The majority o f this thesis is a review o f related literature in the areas o f
microwave heating, the use o f silicon carbide as a susceptor, tem perature measurement in
microwave ovens, the role o f microwaves in the dental
industry and tempering
mechanisms. The prelim inary experimental apparatus and results are also described. O f
particular interest are the recom mendations offered as to the future goals o f this project.
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R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
BACKGROUND
Dental ceramics are com posed o f silica, alumina, kaolin, and o th er oxides. They
are used to make permanent dental restorations such as jacket crow ns, ceramic inlays,
castable ceramics and denture teeth and w hen fused to noble or base m etal alloys.
The fabrication process, which takes place at a dental laboratory, involves layering
ceramic powders and firing them in a vacuum to fuse the powders an d build up a shape
that simulates natural tooth structure.
In the deeper layers, the ceram ic is opaque,
transitioning to translucent and then alm ost transparent to mimic the shading o f natural
teeth. The final high firing is usually open to the atmosphere, which collapses remaining
pockets o f air in the ceramic structure, oxidizes metallic additions used for coloring and
staining, and produces a surface glaze. The result is a ceramic that is esthetic, minimizes
the attachment o f biofilm to the surface, and will not excessively w ear the opposing
dentition while chewing.
During the firing procedure, the restorations are supported either on fire clay
furnace furniture called sagger points, or on refractory die m aterials, as illustrated in
Figure 1. The refractory material is dim ensionally stable at high tem peratures. During
firing, the combination o f refractory die and ceramic are h eated relatively slowly
(100°C/minute) to a prescribed tem perature o f the order o f 1000°C, depending on the
m etal and/or ceramic product in use. The furnace is then cooled at a sim ilar controlled
rate to prevent unequal contraction betw een and within layers until the glass transition
(Tg) temperature is crossed. The process involves cooling from the outside to the inside.
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R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
This is due to the circumferential fum ace coil and its enclosure retaining heat longer than
the floor or ceiling o f the cham ber w hich typically do not contribute heat to the fumace.
FIGURE 1 - Dental Restorations Supported by Refractory Material
This project proposes to m odify the final air-firing step to incorporate microwave
heating.
When the restoration reaches the prescribed temperature, the resistance coils
will be shut off to begin the cooling stage, as usual. While the finished crown is cooling,
microwave energy will be used to heat the refractory die that is supporting the restoration.
The microwaves will be absorbed by the refractory die material, but not by the dental
ceramic. Thus the outer convex surface o f the restoration will be cooler than the concave
inside surface.
Thermal tempering strengthens materials by creating surface stresses as a hot
material is rapidly cooled. The surface becomes rigid, since the core o f the material tends
to shrink as it cools more slowly.
The surface is then under com pression while the
internal portion is under tension, being pulled to both surfaces.
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
REVIEW OF THE LITERATURE
MICROWAVE HEATING
When exposed to microwaves, all materials either reflect, absorb or transmit the
energy.
Those materials that absorb microwaves are termed “lossy.” As a material
absorbs microwaves, it converts the microwave energy to thermal energy, resulting in
volumetric heating. Rapid, yet controlled and selective heating can be achieved.
The use o f m icrowave energy for heating was first commercialized in 19S0 when
Raytheon’s Radarrange® developed microwave ovens for cooking in the home.
The
market for domestic microwave ovens is now saturated, as more than 90% o f homes in
the United States have a microwave oven. These ovens account for 75,000 megawatts o f
energy consumption annually, compared to 100 m egawatts for all uses o f industrial
microwave heating (Clark, 1997).
Conversion o f fo ssil energy to microwave energy is only 30 - 40
%
efficient. The
original microwave research in the 1950s and 1960s w as driven by the prediction that
fossil fuels would increase dramatically in cost, but this incentive has not materialized
(Clark 1997).
Microwaves w ere not integrated into industrial heating and drying processes until
1962 when practical choke systems were developed to prevent microwave energy from
escaping the oven w h ile specimens pass through on a conveyor belt. Chokes also prevent
leakage o f microwave energy in places where thermocouples or other monitoring
equipment enter the microwave cavity.
Currently 90% o f industrial microwave
processing is meat tempering, bacon cooking, and rubber vulcanization (Clark 1997).
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R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
In the developm ent o f new applications o f m icrowave heating, the focus should be
on tailoring parameters such as com position and additives specifically to facilitate
m icrow ave heating and to receive the full benefits o f microwave energy rather than to try
to use microwave processing with established parameters (Clark. 1997).
In any m icrowave system, safety o f the operator is an im portant issue as the polar
m olecule water is the m ajor com ponent o f the human body. In 1966. a standard o f 10
m W /cm 2 was established as the exposure limit for microwave radiation based on onetenth the am ount o f radiation required to heat human tissue one degree Celsius (Katz,
1992). This standard was revised in 1982 to give specific exposure limits for com mon
frequencies. The current exposure limit is 0.5 m W /cm 2 at 2.45 G Hz (Katz. 1992).
All materials absorb, reflect or transm it microwaves.
This interaction is
dependent on the dielectric properties or rotation o f dipoles in the material. The dielectric
loss, tan
6
, can range from
10
“* at room tem perature to values approaching one at high
tem peratures (M athis. 1993). M aterials with high dielectric loss are called "lossy" and
absorb microwaves readily. Tan 5 is the ratio o f the dissipation factor, e". to the dielectric
constant, s', as shown by the equation:
tan 8 —~ t
( 1)
£
w here e \ the real part, is in phase with the electric field and e", the imaginary part, is out
o f phase (Newnham. 1991). The dielectric constant, e', is the perm ittivity o f a material
divided by permittivity in a vacuum or free space. The dissipation factor, e", measures a
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R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
m aterial's ability to dissipate absorbed m icrow ave energy as heat.
dissipated through oscillation o f dipoles in the material.
The energy is
The dipoles align w ith the
electric field portion o f the electrom agnetic field and then relax. This oscillation causes
frictional heating in the material (M athis. 1993).
The energy absorbed by m aterials is represented by the following equation:
P=
10
e' E 2 V tan 5
(2)
w here e 1 is the dielectric constant and V is the volume of the sample (Jian. 1998). If one
holds the microwave frequency (co) and m icrow ave electric field intensity (E) constant,
the power absorbed by the samples (P) is proportional to the dielectric loss, tan 5.
Jian
(1998) states that it may therefore be necessary to adjust the microwave pow er during the
sintering process to compensate for the variations in tan 5 with temperature and density.
It is difficult to measure the lossiness o f materials because their interaction with
m icrow aves is dependent on many factors. T he dielectric constant increases linearly with
tem perature, and the loss tangent increases exponentially with temperature (K atz. 1992).
M icrow ave power absorption increases w ith increasing field intensity, frequency, loss
factor and dielectric constant (N ewnham , 1991). Frequently the dielectric properties o f a
specific material are unknown functions o f tem perature and density, and the electric field
in the m icrow ave cavity is difficult to quantify in terms o f its interaction w ith the sam ple
being heated. Tabulated values o f the loss tangent o f materials are therefore rare.
Product literature from ORPAC (O ak Ridge, TN), a chemical supplier, gives a
qualitative listing o f the microwave absorption o f some ceramics. Poor absorbers include
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R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
Y 2 O 3 , AI2 O 3 , Si0 2 , S 13 N 4 , AIN, BN, M g 0 »Al2 0 3 ,
3
AI2 O 3. 2 S 1O 2 , and CaFi.
M oderate
absorbers are Z 1O 2 (unstabilized), and T 1O 2 . The following are listed as good absorbers:
Z r 0 2 with 4% CaO, H tD 2, S n 0 2, '/2 Y 20 3 . 2 Ba0 . 3 Cu 0 , TiC, SiC (alpha and beta). TaC,
ZrB 2 . M oSi 2 . and TiN. Sturcken offers the following list o f lossy ceramics: SiC . Z 1O 2 ,
ZnO, U 0 2, U 3 0 8 and P u 0 2 (1991).
N ewnham . et al. provides another listing o f lossy materials.
The following
materials can be heated to 1000°C after a few minutes at one kW power: m ixed valent
oxides such as Fe 3 0 .|. CuO. C 0 2 O 3 , and NiO; sulfide semiconductors such as FeS 2 , PbS.
C uFeS 2 ; and carbon and graphite.
M etals reflect microwave energy because electric
fields do not penetrate beyond the surface. The skin or penetration depth o f copper is less
that one pm at microwave frequencies.
The lossiness o f materials also changes as the frequency o f the m icrow ave energy
changes. Borides and carbides are highly lossy; therefore, at 2.45 GHz, the m icrowave
heating is a surface phenom enon because the skin depth is only millimeters (K atz. 1992).
Despite the difficulty in obtaining reliable values o f the variables associated with
m icrowave interactions, microwave heating is used extensively in the ceram ic industry.
Levinson was the first to use microwaves to fire ceramics in 1969.
lie patented a
“ M icrow ave Kiln” in 1969 and wrote “ Methods o f Firing Ceramic A rticles Utilizing
M icrowave Energy” in 1971 (Sturcken, 1991).
Using conventional firing methods, the poor thermal conductivity o f ceram ics can
result in the center o f a sam ple being at significantly lower temperatures than the surface.
If the tem perature difference is too great, cracking and distortion occurs.
T he rates o f
heating and cooling m ust therefore be limited and firing is a slow process. Traditional
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tiring kilns are not energy efficient, as the specific energy consum ption o f ceramic
com ponents is often more than ten tim es the theoretical m inim um energy required to fire
the product (W roe, “ Improving E nergy...” 1993).
M icrow ave heating can provide several advantages over conventional heating,
such as faster heating rates, improved energy efficiency, and reduced thermal stresses.
Heating rates up to 1000°C per minute can be achieved in MW sintering (Rahaman,
1995). Many ceram ics do not absorb microwaves readily at room temperature, however.
The low dielectric constant o f ceramics at low tem peratures leads to low initial heating
rates and inefficiency (Hamlyn, 1997).
As the tem perature rises, heat is lost to the
surroundings by radiation to the cooler surroundings, creating an inverted parabolic
temperature distribution across the specimen.
As the critical temperature (Tc) is
approached, the ceram ic begins to absorb microwave energy readily, and thermal runaway
can occur.
The reason for the change in tan 5 above T c is unclear (Rahaman, 1995).
Thermal runaw ay is a direct result o f non-uniformity o f the M W field and o f the
properties o f the sample (Raham an, 1995). The surfaces lose heat by radiation, and the
center o f the sam ple (which is at a higher temperature) absorbs m icrowave in preference
to the surfaces, causing the thermal runaway to be self-propagating (W roe, "Improving
E nergy...” 1993).
Therm al runaway and cracking can be avoided by changing the properties o f the
sam ple to im prove the therm al conductivity and by varying the input pow er to control the
heating rate. A dding SiC or ZrCh o r another highly therm al conductive ceramic not only
im proves the microwave absorption o f the sample, but also elim inates thermal runaway
(Tian, 1991). A ny com pound that is added to im prove the absorption o f the sam ple is
9
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
called a “ susceptor.”
Boron carbide (B4C) has been used successfully as a susceptor.
Silicon carbide w hiskers, 10 vol% have been used as susceptors to heat alumina at 60
G H z (Katz, 1992). The use o f susceptors, however, may ch an g e the properties o f the
m aterial being heated.
It is also difficult to predict and control the temperature that
results from the heating o f the susceptor and, ultimately, th e entire sample (Hamlyn,
1997V
The advantages o f both conventional and microwave heating can be exploited in a
hybrid oven, which com bines radiant heating and m icrow ave heating. Gas burners or
electric radiant elem ents provide the radiant heat at ceramic firing temperatures. At lower
tem peratures, hot air or infra-red can be used as the radiant h eat source (Hamlyn, 1997).
Low-loss insulation should be used, in quantities com parable to conventional ovens.
Sam ples are usually conveyed through the oven autom atically for operator safety so that
m inim al handling o f hot sam ples occurs. The main advantage o f hybrid heating is that
therm al gradients can be m inim ized. The radiant heating also helps to bring the ceramic
sam ples up to their critical tem peratures so that microwave heatin g occurs more readily.
At the U niversity o f W uhan, China, researchers com pared m icrowave sintering o f
AI2O3 w ith conventional sintering (Jian, 1998). The m icrow aved sam ples have superior
strength and density to those sintered conventionally, as show n in Table I.
In this
experim ent, the average pow er absorbed by the alum ina sam ples w as also measured, and
is graphed in Figure 2 as a function o f time. The pow er absorbed increases rapidly at
900°C until the m axim um value o f the loss tangent o f the alu m in a sam ples is reached at
about 1050°C.
10
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
TABLE 1
M ICRO W AVE AND CONVENTIONAL SIN T E R IN G OF Al20 3
Jian, et a/., University o f W uhan, C hin a, 1998
Microwave Sinterinu
Conventional Sintering
Rate o f tem perature
increase (°C/m in)
Sintering tem perature (°C)
Sintering time (min)
Relative density (%)
Bending strength (M Pa)
15 to 20
1450
60
94
300
150 to 250
1250
15
98
380
200
150
100
S.
Il
800
900
1000
1100
1200
1300
Temperature (°C)
FIGURE 2 - Average M icrowave Power A bsorbed by Alum ina Samples versus
Sintering Tem perature
EA Technology, Inc. also combined conventional gas firing and electric resistance
elem ents with m icrow ave heating to develop a hybrid heating system. The energy costs
for conventional electric tiring are seven times m ore than conventional gas firing. The
study concluded that energy costs for m icrowave-assisted electric firing were 3.5 times
less than conventional electric firing and one-fifth o f m icrow ave firing alone. The energy
11
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
costs for microwave-assisted gas firing were h alf o f the cost o f conventional gas firing
(W roe, “ Scaling u p ...” 1993).
A nother type o f hybrid heating system involves the placement o f lossy furniture
inside the microwave cavity to induce convective heating o f a non-lossy sample. In one
such exam ple, samples o f Ti+C pow der m ixtures are placed in a fused quartz crucible and
this crucible is placed inside a larger crucible containing silicon carbide granules. Both
crucibles are then placed inside non-lossy insulation in the cavity o f a simple domestic
700-W m icrowave oven. Ignition o f the samples occurs within several minutes. A study
o f this system concluded that the greater the mass for a given density, the shorter time for
the sample to ignite. Similarly, an increase in packing density led to a shorter time for
ignition.
Also, increasing the am ount o f SiC in the crucible increased the rate o f
com bustion (Ahmad. 1991).
A patent assigned to CEM Corporation, M atthews. N C, describes a microwave
ashing apparatus that incorporates both microwave and radiative heating in a hybrid
arrangem ent.
O f primary interest to the current application is the description o f the
furniture used in the microwave oven cavity, shown in figure 3.
The patent (Collins et
al.. 1986) describes a chair-shaped insulating block made o f a refractory material such as
firebrick. This insulation stands on legs to m inim ize contact with the metal floor o f the
oven cavity. Sitting on the “chair” is a block o f silicon carbide in non-particulate form.
The silicon carbide is used as a susceptor to absorb m icrowave energy and to heat the
sam ple by radiative heating. According to the authors, silicon carbide is the “most useful
and most preferred” o f the variety o f possible susceptor materials.
Atop the silicon
carbide block sits a thin (0.08 - 0.2 m m ) support plate with a surface area o f 20 - 25 cm 2
12
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
made o f non-woven fibers o f fused quartz. If a circular plate is used, a 5.4-cm diam eter is
recommended. This plate h o ld s the sample, and an identical plate covers the sample.
The quartz is transparent to m icrow ave, and serves only to contain the sam ple.
quart/ supports
/
silicon carbide block
FIGURE 3 - C E M Patented Microwave Ashing Furniture
An alternative arrangem ent involves a thin-walled quartz crucible, such as Vycor
crucibles 2-mm thick, surrounded by tightly packed blocks o f silicon carbide, which are
contained in a box made o f refractory material, as shown in figure 4. T he authors state
that the slabs o f silicon carb id e may be in any non-particulate shape, but that particles,
powders or granules o f silico n carbide do not provide sufficient heating as susceptors.
Ideal blocks o f silicon carb id e are 0.5 - 1.5 cm thick with sides 6.5 - 8.5 cm in length,
such as “finishing sticks” so ld by N orton Company. It is expected th at the silicon carbide
blocks will deteriorate and w ill need to be replaced after every 1,000 to 5,000 uses.
13
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
quartz crucible
refractory material
silicon carbide
\
blocks
FIGURE 4 - CEM Patented M icrowave Ashing Crucible
Using the furniture described above, samples tested by the authors heated to
glowing red in three to four m inutes (600 - 800 °C). The microwave applicator was a
600-watt oven capable o f being adjusted from 0 to 100% power in one-percent
increments. The authors state that increasing the am ount o f insulation or increasing the
pow er to the m icrowave will allow the sample to reach higher temperatures.
In the
experiments cited, the turntable w as removed from the oven, but a fan (rotating baffle or
paddle) was utilized to evenly distribute the microwave energy.
The design o f the m icrowave applicator should be considered in microwave
heating systems.
processing.
An applicator is a device into which a material is inserted for
The mode defines the way in which the electric and magnetic fields are
distributed within the cavity. Transverse electric (TE) describes the situation in which the
electric field is transverse to the w ave propagation. Transverse magnetic (TM ) refers to
situations in which the magnetic Held is transverse to the wave propagation. In a single
m ode cavity, superposition o f the incident and reflected waves gives a standing wave
14
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
pattern that is w ell-defined in space.
In a m ultim ode cavity, the microwave signal is
coupled through a slot or launcher and suffers m ultiple reflections, giving rise to a
standing w ave pattern so that the applicator w ill support a number o f resonant m odes
(M etaxas, 1993).
Either single mode applicators o r m ultim ode applicators may be used in the
m icrow ave sintering o f ceramics (Tian, 1991). T he single mode applicator is used m ore
often because it offers several benefits. It yields higher electric field strength, couples
m ore energy into ceram ic samples, and allow s heating rates o f 100-1000 °C per m inute.
A single m ode applicator also allows for on-line electric feedback to precisely control the
tem perature and heating rate.
Single m ode applicators are often used to m easure the
changes in m aterial properties with changing electrom agnetic field (Tian, 1991).
The use o f m icrowave energy for heating m ay also provide econom ic benefits in
ceram ic processing. The faster heating rates ach iev ed through microwaves should lead to
m ore efficient use o f capital equipment and larger throughput rates (Katz, 1992).
The
overall conversion o f a fossil fuel to m icrow aves is only about 15% efficient, as
conversion o f fossil fuel to electricity is 30% efficient and conversion o f electricity to
m icrow aves is 50% efficient. Conversion o f m icrow aves to heat is 80% efficient, so
conversion o f a fossil fuel to heat during a typical sintering process is 40% efficient
(K atz, 1992). H am lyn adds that the conversion o f electric energy to microwave energy is
50% efficient at 2.45 G Hz, but is increased to 80-90% efficiency at 915 M Hz (1997).
The cost o f microwave ovens using the standard 2.45 G H z is becom ing
increasingly m ore attractive. At 2.45 G H z, a 2 0 kW experimental apparatus co sts less
than $100,000 (in 1992 dollars), while at h ig h er frequencies (28 - 60 G H z), a 2 0 kW
15
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
apparatus costs $1,000,000 (Katz, 1992).
For small batch processes, simple domestic
microwave ovens can be purchased for a few hundred dollars.
In the design o f new
microwave heating processes, it is therefore not advisable (from an economic standpoint)
to deviate far from the dim ensions and operating parameters o f simple domestic
microwave ovens.
16
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
SILICON C A R B ID E AS A SUSCEPTOR
Silicon carbide has been used extensively in research as a susceptor to promote
the microwave heating o f non-lossy materials. Some ceram ics such as Fe^Ch, Cr 2 0 3 and
SiC absorb m icrow ave radiation efficiently at room tem perature. Others must be heated
to a critical tem perature, above which they will absorb M W radiation (Rahaman, 1995).
Silicon carbide is a covalent compound with a decom position temperature o f
2500°C at atm ospheric pressure (Swain, 1994). It can ex ist as a (hexagonal) or p (cubic)
crystal structures.
Silicon carbide has high hardness, excellent high temperature creep
resistance, high therm al conductivity, good sem iconducting properties and excellent
oxidation/corrosion resistance. It has applications at high tem peratures under corrosive
conditions and in areas where wear must be prevented. O ne o f the critical problems with
SiC is low fracture toughness (3-4 MPa m l/2). Table II lists som e physical properties o f
SiC. The dielectric constant and loss tangent are representative values. The actual values
depend on the tem perature and density o f the sample, and on the measurement technique
used in the experim ental set up. Iskander reports a value o f e' = 29.36 at 2.45 GHz for
silicon carbide (1991).
The dielectric constant and loss tangent for silicon carbide are plotted as a
function o f frequency from 8 to 40 GHz from 25 to 150°C in Figure 5. Both parameters
decrease with increasing frequency and increase with increasing temperature.
17
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
T A B L E II
PHYSICAL PROPERTIES O F SILIC O N CARBIDE
Rahaman, 1995 a n d S w ain, 1994
Crystal Structure
a (hexagonal)
or p (cubic)
Density
P
3100 kg/m3
Heat Capacity
Cp
3300 J/kg-K
Therm al Conductivity
k
40 W/m-K
Dielectric Constant
s’ (at 2,450 M Hz)
26.66
Loss tangent
tan 5 (at 2,450 MHz)
27.99
17*
Silicon
100
Carbide
1!rjioli>v
S A
a
&
25
140 . 4 ------
0 .3 ------
I oo
3
0 . 2 ------
0.1
70
75
t)
I ’r e ^ i * e i » 4 . ' y ( C i l ! / . )
FIGURE 5 - Dielectric Constant an d L oss Tangent of Silicon C arbide H exaloy SA
at Varying Frequencies and T em peratures (from Hollinger, 1991).
18
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
Using a m athem atical m odel, Chatteijee graphed the pow er and tem perature
distributions in SiC slabs for a 5-cm slab (Figure 6) and a 2-cm slab (Figure 7).
1300
7 456 mm
x
m
d
3
(X
-i
i
I
1200
1100
i
3 728 mm
1000
tu
Q.
5 900
UJ
1.863 m!
O 20
H
I
800
700
0.5
0
1
FRACTIONAL DISTANCE
FRACTIONAL DISTANCE
FIGURE 6 - Power and Tem perature Distributions for a 5 cm Slab o f Silicon
Carbide Exposed to M icrowaves from Both Sides (Chaterjee, 1998).
1800
125
1
SiC
x
7.456 min
1600
7 100
i
UJ
£T
3.728 min
3 1400
HI
r
__
75
i
£
u 1200 -
£
1.863 min
50
/
- \
-
UJ
•“
1000
800
E 25
-
i
0
0.5
--------------------------- _
0
1
FRACTIONAL DISTANCE
.................. -
0
-
■
0.5
1
FRACTIONAL DISTANCE
FIGURE 7 - Pow er and Tem perature Distributions for a 2 cm Slab o f Silicon
Carbide Exposed to M icrowaves from Both Sides (Chaterjee, 1998).
19
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
A mathematical m odel w as developed by Singh et al. to describe the microwave
heating o f slabs o f highly lossy ceram ics (1993). The model incorporates the temperature
dependence o f the dielectric properties o f the material, th e non-linear radiative and
convective losses from th e slab, and the geometric considerations o f the sample. The
model shows that m icrow ave energy penetrates four centim eters into silicon carbide slabs
at room temperature. At elevated temperatures, however, this penetration depth decreases
to 0.35 cm at 700° C due to the changes in the dielectric properties o f silicon carbide as
the temperature is increased. A s the penetration depth decreases, the heating becomes a
surface phenomenon, an d large temperature gradients develop in the slab. This model
also shows that the tem perature profile as samples are heated is initially linear with time.
Once the sample reaches high tem peratures o f approximately 730°C to 1230°C, however,
the heat losses from radiation becom e significant, and the surface tem perature reaches a
constant value, know n a s th e steady state temperature. For a given material, the steady
state temperature can be increased by increasing the m icrow ave pow er input, as shown in
Figure 8.
W hen the m icrow ave pow er is turned off, the surface o f the sample cools
rapidly by radiation d u e to the fourth power dependence o f the radiative losses on
temperature. For exam ple, a slab o f silicon carbide heated to 1180°C cools to less than
900°C in the first 2.5 m inutes o f cooling as shown in Figure 9.
Joining o f ceram ic com ponents is another area in w hich m icrowave energy has
been used to supply the required heating duty. Sintered silicon carbide (SSiC) is used as
the material o f construction for ceram ic heat exchangers because it is able to withstand
high temperatures and corrosive environments. Solid SSiC is difficult to manufacture,
20
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
1500
/
/
Temperature (K)
1200
/
/
/
000
/
/
35 W/cm2
/
- - - 20 W/cm2
/ /
/ /
600
~“
-
10 W/cm2
V
/,
300
o
300
600
400
1200
1500
Time (s)
FIGURE 8 - Surface Tem perature Versus Tim e for SiC Irradiated with
Different M icrowave Intensities
I 500
1200
Healing
600
C oolin g
300
300
400
600
1200
1500
Time (s)
FIGURE 9 - Surface T em perature o f SiC with 900 Seconds o f M icrowave
Heating at 35 W /cmx Followed by C ooling
21
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
however, using conventional means such as slip casting, pressure casting and injection
molding. Reaction bonded silicon carbide (R B S C ) is much easier to fabricate, but it is
not an acceptable alternative to SSiC due to its low melting temperature, 1410°C (the
m elting point o f silicon) (Ifikhar, 1993).
Research in this application involved tu b es o f SSiC (sintered silicon carbide) and
sockets o f RBSC (reaction bonded silicon carbide), heated to facilitate bonding along the
joint.
The sam ples were placed inside a hybrid heating apparatus o f refractory brick
surrounded by alum ina insulation with a th in layer o f SiC on the inside.
The hybrid
heating apparatus was wrapped in alum ina insulation and heated inside a 900-watt multim ode m icrow ave oven.
The tubes and sockets reached a maximum temperature o f
1530°C, and the joints were leak-tight, indicating that microwave energy was a good
source o f heat for this application (Ahmad. 1993).
Experim ental work by Binner (1995) and (1998) in the microwave joining o f
ceram ics revealed an interesting phenom enon concerning the microwave absorption
characteristics o f silicon carbide.
Despite all attempts, hot pressed silicon carbide
(H PSC) could not be heated to temperatures g reater than 1312°C using microwave pow er
up to 600 W (0.6 °C per second) for 30 m inutes. Binner uses the fact that silicon carbide
is a good electrical conductor to explain th is behavior.
It is believed that, as the
tem perature o f HPSC is raised, the electrical conductivity o f the material increases,
resulting in a decrease in the penetration dep th o f the microwave energy.
Thus the
ceram ic begins to reflect microwaves rather than absorb them at elevated tem peratures.
The result is that when the ceramic reaches 1 3 10°C, increases in power do not result in an
increase in temperature.
This tem perature plateau may be advantageous in some
22
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
applications, but can be overcome using a hybrid heating system to contribute conductive
heat to further increase the temperature.
In contrast, reaction bonded silicon carbide (RBSC) heated easily to 1376 °C at
300 W pow er (2.0 °C per second) in 15 m inutes total time. Heating was done in a TE
102 rectangular tunable cavity. Control o f the R B SC heating was difficult, how ever, as a
rapid increase in the com plex permittivity occurred at 1150°C which resulted in a large
shift in the tuning o f the cavity (Binner, 1998).
During the period o f 1991 to 1997, the U niversity o f Florida and Atomic Energy
o f Canada formed a collaboration to measure the dielectric properties o f various m aterials
(Clark, 1997). Silicon carbide was identified as a good microwave susceptor.
In one
exam ple, varying am ounts o f SiC particles w ere added to alumina/calcia cem ent
(A liO j/C aO ) which has good strength and form ability. With the addition o f 40 w t.% fine
SiC particles, the sam ples heated to nearly 1200°C in 270 seconds. Figure 10 gives the
m icrowave heating rates for this system using varying weight percent o f coarse (1000 pm
diam eter) silicon carbide and fine (85 pm diam eter) silicon carbide (Clark. 1997).
Various experim ents have shown that the am ount and arrangement o f silicon
carbide added as a susceptor is critical. Silicon carbide has high electrical conductivity;
thus excessive use o f silicon carbide may hinder the penetration o f the m icrowave energy
to the ceramic sam ples.
In this case, the SiC absorbs all the energy and heating is by
radiation only (from the SiC to the sam ple), negating any benefits o f m icrow ave
processing. The am ount o f susceptor should therefore be kept to a minimum, depending
on the size o f the sam ple to be heated.
23
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
HHnr
•nm%
1
•m i
m
m
m
m
iiiiimiim!!
MOmwactiMiaMMmi)
ai
FIGURE 10 - Microwave Heating Rate* for Silicon Carbide Dispersed in
AljOj/CaO Cement
24
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
TEMPERATURE M EA SU R EM EN T IN M ICRO W AVE O V EN S
Measurement o f the temperature o f specim ens that are heated in a microwave
oven leads to unique problem s.
The temperature-measuring device m ust be properly
grounded to the oven cavity to prevent arcing.
The most accurate tem perature o f the
sample is that m easured directly on the surface.
According to G rellinger, the tem perature on the surface o f the sample can be
measured using a therm ocouple, radiation pyrometer, or optical fiber probe (1993).
Proper
shielding
with
a
platinum/rhodium (Raham an,
metallic
protective
tube,
such
as
m olybdenum
or
1995) must be provided to avoid arcing with the
thermocouple and “ shine through” with the optical fiber probe.
The measured tem perature will be significantly lower if the m easurem ents are not
taken directly on the surface or the inside o f the samples due to the fact that microwave
heating begins internally at the m olecular level (Grellinger, 1993 and Raham an, 1995).
Hamlyn
suggests
that temperature
measurements
be
taken
with
K-type
thermocouples or w ith optical fibre probes, such as Accufiber M l00 with a black body
sensor (1997). Raham an states that optical pyrometers are more practical, and must be
focused directly on the sam ple and calibrated using a heated blackbody source (1995).
25
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
M IC RO W AV ES IN THE DENTAL INDUSTRY
The use o f microwave energy in the dental industry is not a novel concept.
M icrow aves are used for curing o f denture polymers and drying o f gypsum products. T he
m icrow ave sterilization o f dental implements has also been attempted, with lim ited
success.
G ettlem an el al. (1977) compared the
m icrow ave curing o f polym ethyl
m ethacrylate (PM M A) with other curing methods. PM M A is used in a variety o f dental
im plants because o f its rapid curing time and m echanical strength. A microwave curing
tim e o f six minutes resulted in PMMA with a tensile strength o f 8.610 psi. but the
m echanical properties decreased at a curing time o f seven minutes.
Steam autoclave
curing resulted in better tensile strength (9,130 psi), but a curing time o f 30 m inutes w as
required.
The authors noted that the exact curing tim e m ust be determined for each
polym er to avoid over-curing or under-curing in the m icrow ave oven.
A related study found that PMMA cured using microwaves resulted in superior
bond strength to denture teeth compared to PM M A cured by conventional m ethods
(G eerts, 1993).
A study at the University o f Istanbul com pared the properties o f m icrow ave-dried
gypsum products (used for refractory casts in the dental industry) with air-dried products.
T he three gypsum products tested were M oldano dental stone, Glastone dental stone and
M ulti-vest partial denture investment. The study concluded that microwave drying o f
M ulti-vest at low power resulted in a significant tim e savings over the air-dried m ethod,
w ithout a loss o f com pressive strength (Tuncer, 1993). T he surfaces o f the specim ens
26
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
should appear dry before they are microwaved, however, because water may escape
violently from very w et specim ens as they are heated in the m icrowave oven. The other
microwaved gypsum products in the study had lower com pressive strength than air-dried
specimens, regardless o f the pow er level o f the microwave energy.
Researchers at the National Institute o f Standards and Technology (NIST)
developed a m icrowave-sterilization method for dental and medical instruments.
Sterilization is traditionally done w ith dry-heat or steam in an autoclave, a process which
can take up to two hours to com plete. Repeated heating can dull the cutting edges and
damage the rubber seals and gaskets o f metal instruments. In the microwave method, the
instruments were placed in a ja r inside the microwave and a vacuum is drawn. W hen the
ja r is irradiated with m icrowaves, a gas plasm a forms as the atmosphere ignites.
The
plasm a destroys the bacteria and prevents the metal instrum ents from arcing (Hemenway
1986).
A similar study conducted at the University o f Adelaide used a simpler
experimental set up without the vacuum conditions.
The researchers concluded that
microwave sterilization under the tested conditions was not effective, as microorganisms
survived irradiation for up to 64 m inutes (Hume, 1975).
27
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
TEM PERING M EC H A N ISM S
Thermal tempering strengthens materials by creating surface stresses as a hot
material is rapidly cooled.
T he tem pering o f all-ceramic m aterials is sim ilar to the
tempering o f glass, which is discussed in detail. All information contained in this section
regarding tempering o f glass is attributed to “Tempered Glass: M ore Than You Want to
Know” by C. Bay in http://newsuroup.sci.aqm iria.rcc.aQ uaria.alt.aquaria.htm .
In glass tempering processes, a sheet o f glass is heated to a m alleable state (red
hot) in a furnace. The surfaces are then quickly cooled by blasting cold air on both sides
(Bay, 1995). The cooled surfaces rem ain rigid as the core o f the m aterial shrinks as it
cools more slowly. The surfaces are then under very high com pression while the internal
portion is under tension, being pulled by both surfaces.
This is advantageous as the
surfaces o f glass objects, w hen exposed to external forces, are susceptible to surface flaws
which lead to cracks, crack propagation and failure. The opposing forces strengthen the
material (Bay, 1995).
D iagram s o f the stress distribution and the forces present in
tempered glass are given in Figures 11 and 12.
--------- 10.000 psi (mm)
02 Compression Zone
Neutral Zone
— ►
1-02
Tension Zone
02 Neutral Zone
H m H l
o 2 Compression Zone
FIGURE 11 - Stress Distribution in Fully Tem pered G lass
(from Saflex Technical Information)
28
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
/
surfaces in
compression
compression
neutral
center in
tension
tension
neutral
compression
FIGURE 12 - Forces Present in Tempered Glass (from Saflex Technical Info.)
Tempered glass is resistant to cracking because cracks will only propagate under
tensile stress. Any cracks or flaws on the surface o f tempered glass are trapped by the
compressive forces and can not extend into the internal regions o f the sheet o f glass.
The surfaces o f fully tempered glass are under compressive forces o f 15,000
pounds o f force per square inch (psi).
In order to shatter the glass, one must exceed
15,000 pounds o f force per square inch at one point on the surface. Thus a hit from an
object with a small surface area such as a needle or ice pick is more likely to shatter the
glass than a hammer or a baseball, which has a larger surface area.
The pieces o f
shattered fully tempered glass ate one-eighth to one-sixteenth o f an inch long.
Curved glass, such as automobile windows, can not be fully tempered because
curved glass will not cool uniformly. Automobile glass is tempered to 10,000 psi. The
shattered pieces measure approximately one-fourth o f an inch. The degree o f tempering
29
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
is m easured by shattering a sheet o f tem pered glass and counting the num ber an d sizes o f
the pieces.
A non-destructive method o f m easuring the degree o f tempering is to shine light
through the surface and measure the polarization.
This is an effective m ethod o f
m easurem ent because the process o f tem pering also polarizes the glass.
O nce a sheet o f glass has been tem pered, it can not be cut or shaped. The only
way to remove the tempering is to reh eat the glass to molten tem peratures.
Tem pered
glass is non-uniform, and has ripples, warps and twists formed by the opposing
com pressive and tensile forces.
Non-tempered (annealed) glass is under lower com pressive surface
force,
approxim ately 400 psi. Annealed glass will not shatter, but it will crack o r break more
easily. Tem pering glass does not change its density, but tempered glass is m ore than ten
tim es stronger than annealed glass d u e to the opposing com pressive and tensile forces
present.
Tem pering can also be accom plished using ionic forces. A patent issued in 1989
to LaCourse describes a process for strengthening glass through ion-exchange using
m icrow ave radiation at frequencies from 0.9 to 22.1 GHz. This strengthening process
involves exchanging smaller ions in th e glass with larger ions contained in a coating on
the surface o f the glass. Thus the su rface o f the glass is placed in com pression (w ith the
larger ions occupying space meant for sm aller ions) while the internal portion o f the glass
is under tension.
The microwave radiation heats th e glass to a temperature below its strain point to
facilitate the ion exchange. The auth o rs state that conventional ion exchange results in
30
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
shallow penetration o f the larger ions, which causes the glass to lose its strength over
time. Longer processing tim es can compensate for this, but are impractical and subject
the internal portion o f the glass to extreme tension, often resulting in violent breakage o f
the glass.
This patent claim s that glass strengthened using microwave radiation produces
deeper penetration depth o f the larger ions in a shorter am ount o f time than conventional
m ethods o f ion exchange (LaCourse, 1989).
31
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
EXPERIM ENTAL APPARATUS
The experimental apparatus is a standard 650-watt domestic microwave oven.
The turntable has been removed, and the oven is inverted so that the microwaves enter
through the floor and com e in direct contact with the sample, rather than first dissipating
into the oven cavity.
To further direct the microwaves tow ards the sample, a copper cham ber has been
fabricated over the trapezoidal opening o f the waveguide as shown in Figure 13.
A
copper baffle, grounded with five machine screws, couples the cham ber to the floor o f the
m icrow ave oven. The cham ber consists o f a soldered copper structure, which channels
the microwave energy upward into a trapezoidal copper tower (low er chamber). A fire
clay table was cut to shape and placed in the lower cham ber a few centimeters above the
baffle to hold the furnace furniture and the ceramic restorations to be fired. Above this a
second copper cham ber was constructed to simulate the enclosure that a resistance
furnace firing muffle would occupy in the com pleted device.
The upper cham ber hinges open and latches shut in order to place and remove
objects on the table. A copper apron was attached at the junction where the upper and
low er cham bers jo in, to prevent arcing between the metallic components.
C opper screening was inserted in the front wall o f the upper cham ber to view the
firing as it proceeds. A second copper screen was placed at the top o f the upper cham ber
to vent hot gases during firing.
32
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
FIGURE 13 - E sp ertaeatal Apparatus
33
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
PROCEDURE
The preliminary experim ental
procedure
involves
testing
the
microwave-
absorbing capabilities o f the dental refractor)' materials with and w ithout lossy additives.
The first step prepared sm all sam ples o f dental refractory casting investm ent material.
Five refractory die materials, provided by Whip M ix Corporation o f Louisville, KY,
include Polyvest and V H T (phosphate-bonded refractory die m aterials). Hi-Temp
(carbon-free refractory m aterial). Beauty Cast (gypsum-bonded refractory material), and
Ceramigold (carbon-bonded refractory material).
The physical properties o f these
materials are summarized in T ab le III, and additional information from the Whip Mix
Product Director)’ is included in A ppendix I.
The refractory m aterial is m ixed with water to form a paste.
The paste hardens
chemically and dries into a so lid in about 30 minutes. The samples are then placed in the
microwave oven. The refractory m aterials heated up only enough to drive o ff the water
they contained.
One way to improve the ability o f a material to absorb m icrow aves is to use lossy
additives as susceptors. Based o n a thorough survey o f the literature, different forms of
silicon carbide and copper p o w d er were used to try to improve the heating.
Powdered
silicon carbide was mixed w ith the refractor)' casting material in varied proportions. The
susceptors that were tested include tine (320 grit) SiC gray p o w d er from Fischer
Scientific Company ( a o r (3, h o t pressed or reaction-bonded SiSiC ), coarse SiC from a
grinding wheel, green SiC w h isk ers and 99.7% copper pow der (3 m icron) from Aldrich
34
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
Chemical Com pany. C om bination o f the above were also tested. The physical properties
o f silicon carbide w hiskers are tabulated in Table IV.
Figure 14 shows a 400%
magnification
by
of
silicon
carbide
whiskers,
provided
Advanced
Refractory
Technologies.
TA BLE III
PH Y SIC A L PR O PERTIES O F DENTAL REFR AC TO R IES
f
'’
Color
Thermal
Expansion
Compressive
Strength
Compressive
Strength After
Firing
Maximum
Furnace
Temperature
Compatibility
t
(W HIP MIX CORPORATION)
v i .i t
: r , , . . . ..
Polyvcsti
Beauty
VHT
Hi-Temp
Cast
while
while
while
blue
1.2%
0.65 %
0.80 %
0.55 % to
1.20%
6.000 psi
1.500 psi
700 psi
2.500 psi
10 M Pa
5 MPa
42 M Pa
17 M Pa
6,500 psi
4,800 psi
N/A
N/A
46 M Pa
34 M Pa
Ceramigold
while
1.2 %
1.500 psi
10 MPa
N/A
1.200 °C
1.200 °C
N/A
N/A
N/A
use with
m edium expanding
porcelains
use with
highexpanding
porcelains
use with
nonprecious
metal
alloys
use with
low
fusing
alloys
use with
ceramic
gold alloys
In the proposed com mercial apparatus, heating will occur not only by microwave
energy, but also by resistance coils. This was tested by first heating the samples in simple
convective ovens, then quickly placing the hot sam ples into the microwave oven. Most
materials absorb m icrow aves more readily at elevated temperatures, so this should allow
the samples to reach a much higher temperature as they absorb microwaves.
35
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
TABLE IV
SILICON CARBIDE WHISKERS SPECIFICATION
# ART-PS-2003-04
Advanced Refractory Technologies, Inc., Buffalo, NY
M ean W hisker Length
M ean W hisker Diam eter
Mean W hisker Aspect Ratio
Particulate Content
Particulate Size
Surface Area
Free Carbon
Free Silica
M inim um S.00 pm
M inim um 0.60 pm
6-15
< 30 vol. %
< 50 pm
2 - 5 m2/g
< 0.2 wt. %
< 6.0 w t %
FIGURE 14 - Silicon Carbide Whiskers Magnified 400 Times
(Advanced Refractory Technologies)
36
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
The 650-watt microwave oven is not equipped with a temperature-measuring
device. The temperature has been estim ated by observing the color o f the fired samples
and by feeling the radiation from the surfaces. An optical pyrom eter was also used to
m easure the temperature, but the 400 °C m axim um range o f this device was not sufficient
to accurately gauge the tem peratures o f the hotter samples.
The temperature o f the
sam ples was therefore measured only qualitatively in the preliminary experiments.
Control o f the microwave energy was achieved by cycling the power on and o ff
manually when the samples reached the desired temperature range.
37
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
RESULTS AND DISSCUSSION
it was determined that the dental refractory materials alone do not absorb
appreciable am ounts o f microwave energy at room temperature. All sam ples heated only
enough to drive o ff the water that they contained; some exploded from steam pressure if
heated before their green strength w as established. It is apparently necessary to com bine
the refractory materials with lossy susceptors before microwave heating can occur.
The first success encountered in this experiment involved silicon carbide from
intact pieces o f a grinding wheel as a susceptor. In less than three m inutes, these samples
heat to a glowing orange, as can be seen in the photographs in figures 15, 16 and 17. In
figure 17. the temperature was so high that the solder in the upper cham ber o f the copper
tow er melted. The upper cham ber w as rem oved for the photograph in figure 17. The
particles o f silicon carbide from the grinding wheel are not uniform, and som e arcing was
observed between the components after heating began. The arcing is possibly due to the
electrical conductivity o f organic com pounds such as the matrix binder, w hich became
carbonized after burning. Arcing is a form o f heating that is unpredictable and difficult to
control, so it is undesired in this project.
Silicon carbide whiskers have also been successful as an additive.
uniform heating was observed.
Rapid and
T h e use o f silicon carbide w hiskers may not be
econom ical, however, as they cost approxim ately $900 per kg, but their density is low and
surface area is high.
38
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
FIGURE 15 - laitial Stages of Microwave Heattag of Deatal Refractory Material
asiag Silieoa Carbide as a Sasceptor
39
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
FIGURE 16 - L ater Stages of Microwave Heating of Dental Refractory Material
Using Silicon Carbide aa a Snaceptor
40
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
FIGURE 17 - Microwave Heatiag of Deotal Refractory Material Uaiag Silkoa
Carbide aa a Saaceptor
41
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
Copper pow der is the m ost successful susceptor tested in term s o f rapid heating.
These samples consistently heat to a glow ing orange in less than two minutes. Copper
may not be the optim al choice, how ever, because the copper could fuse with metal
structures in the restoration and/or cause greening o f the ceramic layers in the restoration.
The preliminary experim ental investigation and literature search revealed several
advantages and possible disadvantages o f the proposed hybrid heating system. The
heating provided by the resistance coils m ight bring the refractory die material to a
tem perature exceeding the critical tem perature (Tc) for microwave absorption.
This
should improve the efficiency o f the hybrid heating.
It w as determ ined that tuning the w avelengths o f the m icrowave energy may not
be necessary, as successful heating was achieved using 2.45 GHz only. Custom-made
m icrowave applicators utilizing tuned cavities are considerably more expensive than the
sim pler domestic m icrowave units w hich use 2.45 GHz.
In general, increasing the w avelength o f the microwave energy increases the
pow er absorbed by the material.
Since silicon carbide is such a strong absorber o f
m icrowave energy, how ever, higher wavelengths reflect from the surface o f silicon
carbide sam ples rather than penetrate into the samples.
This is another reason that a
sim ple 2.45 G Hz oven seem s to be preferred over a more com plex tuned cavity for this
application.
Control o f the m icrow ave energy entering the hybrid oven could be accomplished
using a) tem poral control (off/on cycles), b) variation o f the am perage to the magnetron
tube, or c) a hot pressed SiC iris heat sink choke driven by a linear motor. Temporal
control was found to be a sufficient controlling m ethod by switching the magnetron on
42
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
and off. More investigation o f the later two possibilities is necessary, as the prelim inary
experimental work used only temporal control.
Another challenge presented by the proposed apparatus is that the tem pering
effect could lead to undesirable mechanical properties in the finished all-ceram ic
restoration.
Thermal tem pering not only leads to com pressive forces on the external
convex surface, but may also generate tensile forces on the internal concave surface. This
could cause the interior o f the ceramic to be w eak and brittle, trading high external
strength for low internal strength.
The proposed com m ercial apparatus will not achieve faster processing o f dental
restorations despite the m ore rapid heating. The production o f crowns and bridges is a
multi-step batch process, so the time saved by m icrow aves at one step may not save time
in the overall process because material-in-progress ju st backs up at the next processing
step. To avoid excessive therm al stresses, the heating and cooling stages should n ot be
rushed.
Therefore, the success o f this project relies on the improved properties that
should be achieved through thermal tempering.
43
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
CONCLUSIONS
1. Dental refractory materials do not absorb microwave energy appreciably.
The
addition o f lossy susceptors allow s m icrowaves to influence the rate and direction o f
heating o f the refractory materials.
2. Silicon carbide whiskers are successful susceptors for dental refractory materials.
3. A sim ple 2.45 GHz microwave applicator with temporal control is sufficient for the
firing o f dental refractory materials using susceptors.
It is not necessary to use a
tunable cavity nor is it advisable to use higher wavelengths, as silicon carbide is no
longer a good m icrowave absorber at higher wavelengths.
4. The use o f furniture in the m icrowave oven to channel the energy to the sample is
critical to ensure sufficient heating for firing the dental restorations.
5. The use o f m icrowave energy resulted in faster heating tim es o f dental refractory
materials (w ith proper susceptors) than are possible through conventional heating
m ethods using resistance coils.
44
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
RECOMMENDATIONS
Before this idea can be converted into a commercially viable unit, more
experim entation is needed.
A ceramic oven m uffle w ith resistance coils should be obtained from a dental
equipm ent manufacturer.
This oven should be disassembled and the resistance coils
fitted inside the upper portion o f the experim ental chamber. Some m odification o f the
existing cham ber design may be required.
The experim ental oven should be further modified to incorporate a device to
evenly distribute the microwaves w ithin the cham ber. An alum inum paddle with four
vanes should be installed in the lower chamber. The paddle will be held in place by a
non-m etallic support, and the paddle will rotate on plastic needle bearings. T he incoming
m icrowave energy will rotate the vanes o f the paddle, resulting in m ore even attenuation
o f the microwave energy into the chamber.
Tem perature-m easuring devices and feedback control systems sh o u ld be designed
and incorporated into the experim ental apparatus to quantify the heating effects and
control the heating and cooling o f the exterior and interior o f th e ceram ic dental
restoration.
This new experim ental apparatus will allow for the o p tim ization o f the
geom etry o f the oven furniture and susceptor m aterials to get the best p o ssib le heating.
A m ethod m ust be developed to m easure and quantify the strength gained through
therm al tem pering o f the dental restorations to show that the hybrid m icrow ave heating
45
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
method is an im provem ent over current heating methods. A dditional sam ples should be
prepared to match exactly the crow ns and that will be heated in the com m ercial device.
Additional experim entation may be warranted to test other possible susceptors. In
cooperation with a dental laboratory, the exact chemical com position o f the refractory die
materials should be determ ined to ensure that the susceptors are com patible with dental
ceramics.
46
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
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application in materials processing IV. Ed. Clark, D. E. e t a l . Cincinnati: The
American Ceramic Society. 555-562.
Hemenway, C. G. Microwave S terilizes in Seconds. Dentistry Today. 5,1:12.
Hollinger, R. D., Varadan, V. V ., Varadan, V. K., and Ghodgaonkar, D . K. 1991. “Freespace Measurements o f High-temperature, Complex Dielectric Properties at
Microwave Frequencies." M icrowaves: theory and application in m aterials
processing. Ed. Clark, D . E. e t a l . Cincinnati: The American C eram ic Society.
243-250.
Hume, W. R. and Makinson, O. F. 1975. Microwave Radiation in Dental Sterilization.
Journal o f Dental Research. 54,3:40.
Iskander, M. F., Andrade, O., Virkar, A ., Kimrey, H., Smith, R., Lamoreaux, S., Cheng,
C., Tanner, C ., Knowiton, R ., and Mehta, K. 1991 “M icrowave Processing o f
Ceramics at the U niversity o f Utah - Description o f A ctivities and Summary o f
Progress." Microwaves: theory and application in materials processing. Ed.
Clark, D. E. e t a l . Cincinnati: The American Ceramic Society. 35 -48.
Iskander, M. F., Smith, R. L., Andrade, A . O. M ., Kimrey, H. and W alsh, L. M. 1994.
“FDTD Sim ulation o f M icrow ave Sintering o f Ceramics in M ultim ode C avities.”
IEEE Transactions on M icrow ave Theory and Techniques. 42, 5: 793-799.
Jian, Z., Jiping, C., Bingchu, M ., W enbing, F. 1998. “Research on M icrow ave Sintering
Alumina Ceramics.” Journal o f Wuhan University o f Technology. 1 3 ,3 :3 4 -3 8 .
K atz,J. D. 1992. M icrowave Sintering o f Ceramics. Annual R eview o f M aterial
Science. 22:153-170.
LaCourse, W. C., Akhtar, M aysood. 1989. U .S. Patent 4,872,896.
M athis, M. D., Dewan, H. S., A graw al, D . K., Roy, R. and Plovnick, R. H . 1993.
“Microwave Processng o f N i-A hC b Composites.” M icrowaves: theory and
application in materials processing II. Ed. Clark, D. E. e t a l . C incinnati: The
American Ceramic Society. 431-438.
48
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
Metaxas, A. C. 1993. “A pplicators for Industrial M icrowave Processing.” Microwaves:
theory and application in materials processing II. Ed. Clark, D. E. etal.
Cincinnati: The A m erican Ceramic Society. 549-562.
Newnham, R. E., Jang, S. J., X u, M. and Jones, F. 1991. “Fundam ental interaction
mechanisms between m icrow aves and matter.” M icrowaves: theory and
application in m aterials processing. Ed. Clark, D. E. et al. Cincinnati: The
American Ceram ic Society. 51-67.
Rahaman, M. N. 1995. C eram ic Processing and Sintering. N ew York: Marcel Dekker.
Singh, R. K., Viatel la, J., Fathi, Z. and Clark, D. E. 1993. “Therm al analysis o f
microwave processing o f ceram ics.” Microwaves: theory and application in
materials processing II. Ed. Clark, D. E. et al. Cincinnati: The American
Ceramic Society. 247-255.
Sturchen, E. F. 1991. “The U se o f ‘S elf Heating’ Ceram ics as C rucibles o f Microwave
Melting M etals and N uclear Waste Glass.” M icrowaves: theory and application
in materials processing. Ed. Clark, D. E. et al. Cincinnati: The American
Ceramic Society. 433-440.
Swain, M. V. vol. ed. 1994. Structure and Properties o f Ceram ics. M aterials Science
and Technology. Eds. C ahn, R. W., Haasen, P. and Kram er, E. J. New York:
VCH. Vol. 11.
Tian, Yong-Lai. 1991. “ Practices o f Ultra-Rapid Sintering o f C eram ics Using Single
Mode Applicators.” M icrow aves: theory and application in m aterials processing.
Ed. Clark. D. E. et al. Cincinnati: The American Ceram ic Society. 283-300.
Tuncer, N., Tufek<;ioglu, H. B. and Qalikkocaoglu, S. 1993. “ Investigation on the
compressive strength o f several gypsum products dried by m icrowave oven with
different program s.” Journal o f Prosthetic Dentistry. 6 9 ,3:333-339.
Wroe, F. C. R. 1993. “Scaling up the microwave firing o f ceram ics.” Microwaves:
theory and application in materials processing II. Ed. Clark, D. E. et al.
Cincinnati: The A m erican Ceramic Society. 449-458.
Wroe, F. C. R. 1993. “Im proving Energy Efficiency in Firing o f C eram ics.” Materials
World. Aug. 1993: 446-448.
http://newseroup.sci.aquaria.rec.aquaria.alt.aquaria.htm. 9/2/99, 12:35 p.m.
http://www.saflex.com/Technicah'Structure/ StrCh3b.htm. 9/2/99, 2:00 p.m.
49
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
APPENDIX I
DENTAL REFRACTORY PRODUCT SPECIFICATIONS
This section contains product inform ation from the W hip Mix Products Directory,
distributed by W hip M ix Corporation, Louisville, Kentucky. The product specifications
are current as o f May 2000.
50
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
R e fr a c to r y I n v e st m e n t s
F o r D irect-F iring C e r a m i c s
Polyvcst Refractory
rO L 'iU M K tlia .'.fn P it M .iltr.il i- I rr
..lalec w nt' i i . f l l i i tin
u i , I ti N 'i v
rr.ilct;
I i h I. i i
nc:rv..il tx p .iiw .'r.
newer p o r e .ur-
W ith lb n 'n tr i’lli\l i-\p:iibum :v i ) \ ) - i
.u .m .iif i t u i i i i f :r>i, i.in..r.r r
.
•;
• 'llti'. in.in> .itita ri.i^ * iim w i- -lit :i .i• A m ple n o ik in g u n it V v t i r niivlc •
I'.r.e
t ; r . i . . ; f t l m . r . t i ; . i v . i : i : l l u i i i . i t i.-i
t'-Ct- ■ 1“
11■i>’ ; ' . .
‘'-H i
'i.-i:
• H :i'.:ii 'i ' i-.:i..;l' i t rc -M : r u
ire
ir.J .I'M - fii
• i ui'i'. Hr.; . \:'.;r.i., :i v in.i:.:i
•:..-i 'i ii.fl.iri?
V.H.T. Industrial Investment
lit-
-i
h
- .
'i
i
w
u
i
l
\
Jf.h .jh lc
' v rf" i i
'
i .i'i til i
e - e l i - i i >: ; l ,
r c 'r.u to r.
H I
.11
. v l u ' h .i c b A i M p f T . i L i n r . - ' i . . , i. r . i n i .. r . .
ri.ii
f i.i.i- 'iii
m m
, ' p f . i l f . . 1: r - ■ • . i r i . i t
. . ;■ .■
L i'v u
■ .umLm.;';iiif !-• " nii;iu:iv
. I'.t- t k ;
m
1
V •
. ^ M." 11
i.;:i . 1,'i.iJui; .••• i .in-
:. " . . . , . i
■■■ m :
-.ft: i» i!l in i t u m
.mi. u'.ii'i1.' -ir... :::■
■
,t‘ ii v l '
Prm nighed Envelope*
. I
mi
-i N«-i
Item No.
D escription
18473
P m * * r p / i * d E nvefopn
24 60 gram Box 11-340 ml)
.■ , : n p ' . . L i : ,
Item No. Description
24929
25062
24-60 giam Box 1 1 340 mi)
340 mi Pnlyves! Liquid
IK ee p Irorn Iroa/mg)
•• i- '.
Physical Properties
Color
Liquid/Powder Ratio
Working Time
Setting Time
Thermal Expansion
500-C. 2nd firing
Compressive Strength (1 hr.)
Compressive Strength (after firing)
Maximum Furnace Temperature
V.H.T.
Blue
19 ml’100 g
4-5 minutes
0 30°.
P o ly v M t
White
0 80“.
2.500 psi (17 MPa)
4.800 psi(34 MPa)
2,200'F(1.200'C)
0 6 5 °.
6.000 psi (42 MPa)
6.500 psi (46 MPa)
2.200 F t 1.200 C)
22 mo 1 0 0 g
\
2 minutes
0 8 0 °.
Compatibility Is The Key
l<ac V.HJ. with bl^i npseil^ p iH ttiH i
Ducerapild
Degussa Denial. Inc
Optec-HSP
Jenenc/Pcntmn !nc
Oprcc-VP
tenenc/Pentron Inc
Wii-Centn
ivudtr ISA/Williams Denial Co
fortune
Iiociat USA/Wllluim Denul Co.
Execko
Ney Denul [nttmanonal. Inc
Excrko inlay/Onlay Ney Dental International. Inc
Microbond
Auaenal Dental Inc7
Nobdpharma
Creations
Jensen Indusnts. Inc.
fortress
Myron IntcmanunaL Inc.
UcroUfVESTwtth
Finesse
Ceramco O'lorlogtc
Cetamco II
Ceramco Veneer
Spectrum
Blobond
Ttu-es
Ducetam
Certotn-PVS
ChameleonAtira#
S'ltaWIK 68
DentsplyOnnkci Inc
Der.isply/Ceramco. Inc
DrnupKyCrtamco. Inc
DenuplyvCeratnco. Int.
Dciusply International. Inc.
Demsplv Intetnauonal. Inc.
Cenpac Corporation
Degussa Dental. Inc.
fetenko Dental Health Products
Mvton International, Inc.
Vidtnt/Ylu ZahnFahnk
Vita Omcya
Ssr.-Spar
Pencralt
Ftancet
Vintage
Crystat
Silhouette
G-Cera
VidentAita Zahniabnk
JenetK/Ptntron Inc.
JenenoTemrcn Inc.
Eno/Suisut SA
3M/Tab Products
3M/tjb Products
Leach & Dillon
G-C International Corp
All ol the above poirelam brand names are
trademarks oi their respective manufacturers
51
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
H i-T emp
Carbon-Free Investment for High Fusing Alloys
\ v>'. . i.i, i . v H ' . u - u
; : ; u i i ’Y .ii'.l
iy :i
-(.m i p r t i h Y j u i v t - >
. . . n t.ii'b r i ''. i r 'n ’i'
V r . p r c u j'.b T iit.i. .illov- -i ijiun- .u:
.rvi-'fn-t i'-'v. i.i;.'. i. ihjm.ru:<-i: in
• , fn i|v risiti- :«>• jlinv -i.cckjqc
• Wound
:..r-n4-.;Tip-:ar..':->
• Pn»v iL‘ .i :K f.-.i> i'.ij:r..n jt:n c a tm . s p K r t
•
\ l i l ' V . . . M i l l ) . I ., I l j l l r ' - l i l ' l ’ - l l .
II
1
- ’ I k ' • v . l- i n i n :
a
'lid
;- i ' K ' v . - . . r . k
- .if I. in it !' .'.i'.."- j
-m bix
Mf a>t:r.>:.’! "Af rejJih-.niti.-vcr.i*. I.t
■ni- itit.a l n :
ir i r / n - .u .i. ! ,1-. lll.ji
- - if i .v l - v
i:.i u :r.L .e ;
’he
-I.i
!T. v .n n u > -im rli 4 ''rru.ii L...ui.l ;•
.to .;
.1 ' . u v r
i ' 1' i . t i r . ' . l
fk
a r . k v . j y : i I tr.s.
jg'.i.d
:ri
■ r.c iilutt.-n "h
I'.--- lit. i - v p . i r K i ' l ' .
Physical P roperties
Liquid"Powder Ratio
Working Time
Setting Expansion
Thermal Expansion
Compressive Strength, wet
C ir -
16 ml 100 g
7-8 minutes
I ' l . ' l ' i HI
0 7*0
12*o
Ik' ' . ' , i - ' m i ' '
"i.i
'1 ' . l ^ . i - k
[ j . " - v j : . r
J
v ’
Tm rr
.*■ : i i c
-• ' " ' l I r k l l " ' ,il
I'ri.'-'.i' it i ti 'tlx t'A tx ' '.:k- tc .I '.r .i..;
I 500 psi 1.10 MPa I
. - - .n t iu l: \ :h c M r *
D escription
.b
f u n it. r a u r : v .. ir 'i r it
in 1, i - i—; r t T v . l u r
•
00450
00477
..iii.l:
rc ’-.'ii'i,r: ' r lull i n n > ..r » .• :-i..|v n s;
'Suggested concentration ol Special Liquid is 75% i,3 parts liquid to I part water)
Item No.
WithLiquid
i
I '■ \ ’m : - . 'i
'
n
i • p.i
-tu-vi-itu-
' . i r . • '! i i l - . L m i
..!
. 'n n r i M t . n - ■ its- '|Y < .il I t.u .l
2 kg (4 12 lb) Jar (1-340 mi)
11 kg (25 lb. I Carton ( 1 1,ter & 1-340 ml |
PrmnigtMdemWoptt
00493
00507
00515
24-60 gram Box 11-340 ml)
24-90 gram Box 11-340 ml)
144-60 gram Package (1 Liter & 1-340 ml)
00523
144-90 gram Package (2 Liters)
!•
.i:
i:
«i
-j
ij
KKEkT UOUO COKBfltUlIM
52
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
i.
>.
'.c*
C eram igold *
Investment with Carbon fo r Crown and Bridge and Ceramic Gold Alloys
T his outstanding phosphate investm ent has
th ese advantages
C o m a iM u r k s tor bnght. gold-colored
castings
F ip a a a l a a c a n b e c o a ln U c d to ht your
n eed s - yet technic is easy to follow
E v c a u l w t ly th in scctfcm s w ill be cast
w ith sharp, d ea n m argins |ust like lowerfu sin g golds
A c c u r a te ly f h tt a g c a s t t a f i w ith sm ooth
an d clean surfaces - without gn n d in g
U niform results tim e after tim e.
The chart below show s the effective expansion
figures obtainable by varying the con cen t­
ration ol special liq u id A 7 V k concentranor..
i parts special Liquid and 1 p an Water, is
generally prelerreu (set en d ed expansion
tigures on chan I Thermal and also setting
Physical Properties
e x p n s io n will increase by greater Special
16 m l/1 0 0 g
U q w d'/P ow dw Ratio
6 - 7 m in u tes
Working Tima
0.7%
Sotting E xpantion
12%
Thermal Expansion
1 ,5 0 0 p si (1 0 M Pa)
C om p reM M Strangto, w et
’S u gg stM d concentration al S p e d e i Liquid is 75% (3 parts k qu d (o 1 pert w eler).
Liquid concentration, these expansions
w ill decrease by dilunon with m ore water
C onsequently, expansion can be controlled
to lit individual needs
There is no additional inermal expansion
item No.
b etw een TiXR! and IcXX'^C 113 00s)-' and
Description
1 8 0 0 °h l The curve tn the ch a n show s d if-
H W lU j u t f
lerent figures for the thermal e x p n s io n m erely
00 3 4 5
00361
2 kg (4 1/2 to.) Jar (1 -3 4 0 m l)
11 kg (2 5 to ) C arton (1 U tar & 1-340 ml)
as a result from higher or low er concentration
A O M lf c M t f f lM t o p M
o l Special Liquid. The setting e x p n s io n (or a
00396
00418
00 4 2 6
00434
2 4 -4 0 gram Bern (1 -3 4 0 ml)
2 4 -9 0 gram B o a (1 -3 4 0 m l)
1 4 4 -6 0 gram P K * a g a ( 1 L M r t 1 -3 4 0 ml)
1 4 4 -9 0 gram P a ck a g a (2 U la n )
b ench -set m old is show n in the low est curve
■- i
. » X ’tfl f
IC 20 *
53
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
•• .
m
B ea u ty -C a s t
For Low Fusing Crown a n d Bridge Alloys
\ n u r u
.1 s 11.1d i i i v *
1 1 \ I i i •i-siim-i'.i
• I he pruun standard :or the Hvcrosiopt.
It-ihnic
• Al'O e.uelletil lur ihe hij^h Heat Relink
rite
lechnc » a ta iiLU l)-iA > ;
.5 as simple and a iu a as the I lid'. Heat tuhnu hat '.lie n-s-.dl' ate ttuun mere ur.ili'mi
Hie Jse ol a water bath itluKi'K.UM' at
1' 3l . 100T ’ auti'matk.iilv I'rinqs all
parents to atiaotiri teni|vral'u:e. sutten.nu
ihe '.vax etuui^h let even expansion ol die
imesineiit l orovt’e l die wax cxp.ir:->..'ii
.a.U the laeii hm u'viipi. expansion „.iii.
pensate lor most ol the eek: snrnkaee an:
ihib n e low lenqvrjiurc I'tiituuit at 4H0 t
W 'h i proMdes ihe habnec ol die required
mold expansivm
Hp 1 shout; hi .qroseopie expansion ol
?FAl~'t a.s7 .ii | iV jin iF '£ d slums
Thetrna! Expansion at 0 is Jt a a o x i - W
xie unhesiutinid) reeemmend me iIv^n*
-eopn Teelmie with Hr U l i t A 'I > liest
suitniit :odi\ s needs lor a ptceise m simple
rotiune le-.nnx
Ihe mnercni htdl I nermal I xpar.sion i.i
Hl-.x. P . as : at hsp'V U lX 'T 'r e
ituhes n ixAsihie to use die standard I lid:
Iteal lee m u xttinqexpan- einar. iv oat
roiled nv .aniiiq die uumnei n nnc liners
in me Inlai Kme ■l ij’ I resuams in varvnc
■leqn'es ol i.na! expatis on xu'tinga'. t-1(1 i
IJi'd 'h '
nueisap.'lor the acoitioral : J'.
Ihemul I xp.itiHon to ..unpensjie to' o ld
'liriulsaei' In the IPA '•pet .nulior 'so . :«>t
Investments the term. Ivpe I. tnlav I dermal
re!ors to inxesiinents used
die I lid* I le.;l
Vtnn.i, Kpell s.alVdlnbx Hsqrosvopn
With I'Hl IVi .‘.-I either tee r,nu - tie
llvevseopiv o r t h e llipl Ilea! -svill priellue
.■'Uenien 'iiuoil' vusimo Ma u.h voider.
I’h v s u u i r ‘t t ' | H ' l t l v ' s
Water. Powder Ratio
Working Time
Setting Time (ADA Method)
Reedy (or Burnout (minimum)
Setting Expansion
Hygroscopic Expansion
Thermal Expansion 480'C
Thermal Expansion 650JC
Compressive Strength, wet
..nor that seldom require fUKiine
30 ml/100 g
3 minutes
16 minutes
30 minutes
0.35%
1 50%
0.55%
1.20%
700 psi (5 MPa)
Ill'll! \t>
00019
00035
2 kg (4 1/2 lb.) Jar
11 kg (25 lb) Canon
00078
00094
00108
00124
24-50 gram Box
24-75 gram Box
144-50 gram Package
144-75 gram Package
Ir r w c sr a w : OT*#SJ w
i*TW»-
j I
i
-
/
------ 1'
iS *
j j y
I J l ' S l T I p I It 111
/ /
s
Pisae|p(iid Ertvafopee
1
!.J
-ja r tr w is c o p c E X M r c w i _
I
1
i
i
*
,ta II
i ^
n
XPAftSX N
Ml
TMINMNUtU
KS
1
II.*.
U'll
FI,. I
4
!
I;
f» rTTTTl TYPE 1fc II
i
;x
is
re IK) «: toco itod t«k>
m m m m m n rc
Fl«. 1
54
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
VITA
T he author, Karen Joan Thom pson, is the daughter o f Kenneth an d Kathleen
Thompson o f Louisville, KY. She was bom in Louisville on February 7, 1976 and has
two younger sisters, Katie and Kelly. Karen is a graduate o f Assumption H igh School,
where she w as the secretary o f the N ational H onor Society and a m em ber o f the French
Honor Society, Beta Club, Ensemble Choral Group, Red Cross Club, A cadem ic Team
and yearbook staff.
She received a Commonwealth Scholarship from the University o f Louisville as
well as the L & N Federal Credit Union Scholarship. Kentucky Grocers A ssociation
Scholarship, Asparagus Club Scholarship and ValuM arket Scholarship.
She began
attending classes at the University o f Louisville in 1994 as a biology m ajor, and
transferred to the engineering school in 1995.
While a student at U o f L, Karen
participated in the Collegiate Chorale, the Speed School Student Council, and the Society
o f W omen Engineers.
Her senior year, she was the president o f A lpha O m icron Pi
sorority and as a graduate student, was the secretary o f the student ch ap ter o f the
American Institute o f Chemical Engineers.
K aren com pleted three co-op term s w ith E.I. DuPont de N em ours, Inc. in
Louisville, KY.
She received a B achelor o f Science in Chemical E ngineering in
December o f 1999. graduating w ith honors.
She was awarded second place for her
Chemical Engineering Exhibit in Engineer’s W eek 2000, for this thesis w ork. She will
receive a M aster o f Engineering in Chem ical Engineering in May o f 2000.
55
R e p r o d u c e d w ith p e r m i s s i o n o f t h e c o p y r i g h t o w n e r . F u r t h e r r e p r o d u c t i o n p r o h i b i t e d w i t h o u t p e r m i s s i o n .
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