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Polycrystalline Inorganic FibersЧProduction Properties Applications.

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Polycrystalline Inorganic Fibers Production, Properties, Applications[**]
By Gerhard Winter[*]
This progress report ends with the sentence : In the opinion of many experts, we are entering
an “age of inorganic fibers”. An attempt is made in the article to outline the present stage of
development and the outlook for one of the most interesting fields of modem materials
science.
1. Introduction
The technology of fiber-reinforced materials is one of the
fastest-expanding fields of modern materials science. From
the number of publications and patents, it can be seen that
interest in these materials has avalanched during recent
years. This development is based on a wide range of inorganic fibrous materials with extraordinary properties,
a large part of this range being constituted by polycrystalline inorganic fibers.
2. Definitions
Certain mechanical properties will often arise in the following discussion. It is therefore convenient to recall the
meanings of the various physical properties (Fig. 1).
I
4
Hookes taw 6=E
E
moduli, such as A1,0,, Sic, or steel. The maximum tensile
stress that can be withstood by a brittle body is its tensile
strength C Y ~ .oB,unlike E, is not a material constant, but
may have lower or higher values according to the quality
of the structure of the material. The rigidity of a body
depends not only on its geometric shape but also on its
elastic modulus, and can be described by the bending
moment M required to produce a certain bending:
M - x E d 3 / 3 2 , i. e. o f two bodies with identical cross sectional areas and the same shape of cross section, the one
with the higher elastic modulus is the more rigid. As the
diameter e. g. ofa round rod is reduced, its rigidity decreases
and its flexibility increases. Inorganic fibers consisting of
materials with high elasticity moduli, e. g. Sic fibers, can
therefore have the same flexibility as e. g. man-made organic fibers if their diameter is made correspondingly smaller. Since many polycrystalline inorganic fibers are intended
for use as light structural materials, it is not only the
absolute values of crB and E that are of interest, but also
the specific values ~ ~ / the
p , specific tensile strength, and
E/p, the specific modulus (cf. Fig. 1).
The mechanical properties of some materials are given
for comparison in Table 1 - 31.
Table 1. Mechanical properties of materials
Elongation F[%I
-
Tensile strength d,[kp/mm’l
d
Specific strength S[krnl
Specific modulus
Material
oB[kp/mm21
E [kp/mm21
P [g/cm31
I
7.8
4-10
26
11
50-90
40-90
1000
0.6
0.9- 1.5
3.99
3.2
4.5
7.8
11
A1203
111
Sic
Titanium
Steel
Q-denSity
1Q [kml E
=
=modulus of elasticity
Bending moment of a round rod M-- E,$d3
d = diameter
Pine wood
Synthetic resins
300-500
36 000
49 000
11 200
21 000
rn
Fig 1 Stress-stram diagram of solids Hooke’s law, definitions
3. Classification of Inorganic Fibers
If a body is stressed by application of a tensile stress (3, an
elongation E occurs. (3 and E are proportional to each
other in the range of validity of Hooke’s law, and the slope
of the linear plot is the proportionality factor, the elastic
modulus E. E is a material constant. Materials having low
elastic moduli, such as synthetic resins, are less able to
withstand elongation than materials having high elastic
[*] Dr. G. Winter, Bayer AG
415 Krefeld-Uerdingen, Rheinuferstrasse (Germany)
[**I
Based on a lecture to the “Solid State Chemistry” group of the
GDCh on September 14, 1971, in Karlsruhe.
Angew. Chem. internat. Edir. 1 Vol. 11 11972) 1 No. 9
To obtain a general picture of the many inorganic fiber
materials and to be able to arrange the polycrystalline
fibers in a logical manner, it is convenient to divide these
materials into three groups according to their internal
structure, i. e. amorphous, monocrystalline, and polycrystalline fibers (Fig. 2)[41.
The amorphous fibers include glass, quartz, and alumina
silica fibers, rock wool and slag wool, and carbon fibers
that have not been subjected to graphitization or that can
be crystallized only with difficulty if at all (vitreous carbon).
751
The monocrystalline fibers are represented by “whiskers”,
many of which exhibit outstanding mechanical properties
because of their monocrystalline, defect-free structure.
A
glass filaments quartz filaments.
alumina silica filaments
rock wool, slag wool.
amorphous carbon fibeis
(vitreous carbon1
Amorphous fibers
/
L
-
B2&
Monocrystalllne
&Sf
/=
-
SIC. SI,N,, A@,. B,C.
BeO. and metal whiskers
whiskers
C
Polycrystalline
fibers
one phase
-
multiphase
/
metalfilamentsIFe.W.Mo.8e.etcI.
“graphite fibeis”, EN. B,C, Al,O,.
ZrO,, LiAi,0,.3Al,03
2Si0, lmuilitel
filaments. etc
B/W.B,C/W,SiC/W, TiB,/W filaments
( W = tungsten carel B/SiO,. B / C
filaments ISiO, or carbon core).
“Tayior wires’’
Fe. special alloys with glass sheaths.
A1,0, with SiO, sheath
Fig. 2. Classification of inorganic fibers [4]
The third group comprises all other inorganic fibrous
materials, which consist either entirely or mainly of countless small, firmly intergrown crystallites. Polycrystalline
inorganic fibers consisting of only one phase include the
metal fibers (Fe, W, Mo, Be, etc.), the “graphite fibers”,
BN and B,C fibers, and the group of polycrystalline
oxide fibers consisting of A1,0,, ZrO,, Li-A1 spinel,
mullite, etc. The multiphase fibers contain either a core
or a sheath of another material. The former include B,
B,C, Sic, or TiB, filaments deposited on a tungsten filament and boron filaments with carbon or SiO, filaments
as cores, while the second group includes “Taylor wires”
produced in a glass or quartz sheath, the crystalline inner
part consisting of iron, special metal alloys, or oxides such
as Al,O,.
Table 2. Processes for the production of polycrystalline inorganic fibers.
1. Melt-spinning process:
-amorphous glass, quartz, and alumina silica filaments,
rock and slag wool,
polycrystalline boron, AI,O,, and ZrO, filaments,
steel filaments.
2. Extrusion process:
B e 0 and A1,0, filaments.
3. Spinning process with organic supports:
TiO,, S O , , SiO,/C, and S i c filaments.
4. Spinning of sols:
A1,0,, ZrO,, LiAI,O,, and 3A1,O3.2Si0, filaments.
5. Impregnation process (“relic fibers”):
ZrO,, Ta,O,,AI,O,, TiO,, and BN filaments and webs.
6. Precursor conversion :
cellulose, PAN filaments pyro’ysls
B,O, filaments
c filaments
C filaments,
% BN filaments,
A B,C filaments
7. Chemical vapor deposition on an existing filament:
B/W, SiC/W, B,C/W, TiBJW, BISIO,, and B/C filaments
8. Wire bundle drawing process:
Superalloys.
9. Taylor process:
Fe, special alloy, and AI,O, filaments
10. Deposition from the gas phase:
Fe and Fe-containing short fibers with a shell structure
form of filaments and then consolidated by sintering. The
products obtained in this way have so far had only low
tensile strengths, are relatively thick, and will probably
be difficult to process because of the resulting low flexibility‘’].
4. Production
Many different methods are used for the production of
inorganic fibers (Table 2).
do
1. The melt-spinning method, which is extensively used
for the production of glass fibers, has so far been applied
with only slight success to polycrystalline inorganic fibers.
The main difficulties are the sharp viscosity changes that
occur on solidification of molten oxides, the high melting
points, and the problem of finding suitable containers and
spinning nozzles. The production of boron and oxide fibers
by this method is therefore still in the experimental stage.
ZrO, and A1,0, fibers have been obtained in experimental
quantities from rods rotating at a high speed in solar or arc
furnaces[5! On the other hand, a technical solution
appears recently to have been found for the spinning of
liquid steel to obtain filaments having a diameter of about
75 pm“jl.
2. In the extrusion process (Fig. 3), gel or oxide particles,
possibly with the aid of a plasticizer, are extruded in the
752
Fig. 3. Scheme of the extrusion process [7]
3. Spinning process with organic supports :Many inorganic
substances, which exhibit no spinnability, cannot themselves be spun into fibers and require a spinnable organic
support. For example, viscose rayon dope can be used
for the production of fibers containing SiO,; the viscose
rayon dope is mixed with water glass, spun, and after
coagulation of the rayon fibers in the acid precipitant bath,
the products are heated in air to obtain SiO, fibers or in
an inert atmosphere to obtain SiO, fibers containing
carbon (Fig. 5). Under certain conditions, it has been found
possible to convert the latter into p-Sic filaments without
loss of flexibility by an in-situ reaction (Fig. 6)[’’.
Angew. Chem. inrernar. Edit.
1 Vol. I I (1972) / No. 9
4. Some inorganic sols can be brought into a spinnable
state by certain procedures. These sols are then suitable
for the production of inorganic fibers by conventional
spinning methods. The spun sol fibers are converted into
gel fibers by drying in a spinning shaft, the volatile components are eliminated by heating, and the oxide skeleton
is sintered. In this way, filaments with good mechanical
properties in some cases have been obtained, examples
being A1,0,, LiAl,O,, MgAl,O,, Tho,, TiO,, ZrO,, and
3Al,O, .2SiO, (mullite) filarnent~~~l.
Fig. 4. Fracture surface of a flame-sintered AI1O3 filament produced
by the extrusion process 271.
take-up spool
Dry spinning
oxide
fi:aments
Sintering
Fig. 7. Spinning of sols (schematic).
5. Relic fibers can be produced by a process developed
by Union Carbide, in which single rayon filaments or entire
webs are impregnated for a long time with inorganic salt
solutions and then subjected to controlled pyrolysis and
sintering, in the course of which the organic components
are burnt off.Unlike in the similar process for the production of Welsbach gas mantles, where relatively brittle webs
are produced, it was possible by this method to obtain
Fig. 5. SiO, and SiO,/C filaments [8].
Fig. 6 . SIC fiber web [8].
Anqew. Chem. inrernar. Edit. I
Fig. 8. ZrO, filaments, produced by the sol spinning process [S].
Vof. I 1 (1972) 1 No. 9
753
fibers and entire webs having useful tensile strengths.
Single fibers of ZrO, also have useful properties for reinforcing purposes. BN fabrics afe also reported to be
obtainable in this way["].
6. By precursor conversion, existing non-fusible organic
filaments, which are usually obtained by conventional
methods of the fiber-spinning industry, are transformed
into carbon or graphite fibers, which are particularly
interesting for reinforcing purposes. The source of carbon
in this method is cellulose or polyacrylonitrile (PAN)
fibers. The American process starts with cellulose fibers,
which must be carbonized and stretched by up to 50% of
their length-at temperatures above 2400°C in order to
orient the graphite crystallites in the fiber direction, to
give the fibers the high elastic modulus required for reinforcement. The British process (Fig. s),which is superior,
avoids this difficult step and begins with PAN fibers, which
are first subjected to an oxidizing pretreatment below
300"C, the shrinkage of the fibers (and this is the decisive
step) being prevented by firm clamping so that preorientation in the direction of the fiber axis is provided for the
graphite crystallites to be formed later. This is followed by
a carbonization step at 1ooO"C. For the production of
graphite fibers with particularly high tensile strengths
(type HT = high tensile) the fibers are then heated at
1600-2000"C, while for the production of fibers with high
moduli (type HM = high modulus) they are heated at
2600-2800"C~1'1.
d i s s o l v e d PAN
1st furnace (oxidation]
.air
I
PAN,fiber
3rd fuinace (heating in inert atmosphere1
Ip90041
'
Fig. 10. BN fibers [5]
7. A number of filaments can be produced by chemical
vapor deposition on heated tungsten wires approximately
15 pm in diameter at temperatures around 1200°C. For
the deposition of boron or of Sic, mixtures of BCIJH,
or CH3SiC13are passed over the hot wire. Because of the
high specific gravity of the tungsten core, the filament
thickness must be built up to about 100 pm to obtain a
filament having a low density. At this diameter, however,
the filaments are very rigid and difficult to process. Similarly,
this method (Fig. 11)can also be used for the production of
B,C and TiB, filaments, starting with BCl,/H,/CH, or
BCl,/H,/TiCl, mixtures respectively. Attempts are also
being made to replace the heavy tungsten core by a cheaper
and lighter filamentary substrate. One step in this direction
is the deposition of boron from B,H, on carbon-coated,
heated SiO, filaments or the deposition of boron on carbon
filament ~ [ ' ~ ] .
g_;
-
s
e
inlet
\
Fig. 9. Process for the production of carbon fibers (schematic) [11]
Numerous efforts have been made to replace the relatively
expensive starting fibers rayon and PAN by cheaper materials. However, attempts to spin tar, pitch, lignin, etc.
into fibers and to carbonize the fibers gave amorphous
carbon fibers with low elastic moduli. Crystalline fibers
with high elastic moduli were obtained only by the difficult
and once again expensive step of graphitization under
stress at 2600--2850°C just below the breaking strength
of the fibers['21.
BN filaments (Fig. 10) are produced by reaction of B 2 0 3
filaments spun from the melt with NH,. On reaction of
B,O, fibers having a diameter of about 10 pm at temperatures below the melting point of B,O,, complete conversion
into BN can be achieved in this way. The fibers are stabilized by high-temperature treatment",]. Carbon filaments can
be converted into B,C in a similar manner by reaction with
a BCl,/H, mixture.
754
Djjjg
heating current
Fig.11. Scheme of the chemical vapor deposition method [ S ]
8. For the production of metal filaments, methods have
been developed that enable complete bundles of metal
wires, which are either coated or embedded in a soft matrix, to be drawn through series of dies with decreasing
diameters. A bundle of up to 100 wires made of superalloys,
such as Chrome1 R or Rent 41, can be drawn in this way
to a diameter of 12.7 pml*].
Angew. Chem. internat. Edit.
Vol. I 1 (1'972) / N o . 9
9. Another process for the production of metallic microwires is the Taylor process, which was developed further
inter alia by Nixdorf at the Battelle Institute. In this
process, metals or alloys are melted in glass or quartz
tubes and drawn out to extremely fine capillaries. The glass
The majority of the processes described can be carried
out continuously and proceed with relatively high yields
per unit time and volume, so that reasonable prices may
be expected with mass production. Moreover, most of the
filaments can be obtained in any desired lengths, so that the
production of wound components or the parallel alignment
of the fibers for components with anisotropic properties
becomes much simpler than when whiskers are used.
These advantages have gained a lead in development for
polycrystalline inorganic fibers over whiskers, with which
they cannot compete in many of their properties.
5. Properties
The properties of particular interest in polycrystalline
inorganic fibers are their tensile strength, their elastic
modulus, and the dependence of these properties on temperature, as well as their density and their thermal and
chemical stability.
Fig. 12. Surface of a boron filament
sheath can be removed by ultrasonic radiation, chemical
etching, or mechanical crushing, and very fine metal
filaments with high tensile strengths are obtained. Under
certain conditions, filamentary single crystals can also be
produced in this way. It is even possible to melt A1,0, in
a quartz tube, which is then drawn out to form a filament
with an S O , skin.
glass-sheathed metal
or oxide rod
\\\\\\\\\\
If one considers the tensile strength of compact ceramic
bodies and compares this with the values calculated from
the bond strengths, one finds that the strengths attained
in practice are often two to three orders of magnitude lower
than the theoretical values. To this fact, which has been
known for a long time as the “crystal paradox”, is added
another remarkable observation, i. e. that macro single
crystals of many substances have a lower mechanical
strength than polycrystalline bodies of the same material.
In the course of time, the search for an explanation of
these phenomena produced many findings that have been
of the utmost importance to the production of high-tensile
polycrystalline inorganic fibers with high elastic moduli,
and that have contributed in general to our understanding
of the mechanical properties of solids.
One of the first attempts to calculate the theoretical
strength of ionic crystals was made by Polanyi in 1921[”].
His formula for the caIculation of the maximum tensile
strength omax
:
/ spool
take-up
11900131
Fig. 13. Scheme of the Taylor process [15]
10. According to Schladitz, polycrystalline iron fibers
having surprisingly high tensile strengths can be produced
in the form of a felt by decomposition of pentacarbonyliron
in a magnetic field. These short, polycrystalline metal
fibers can be produced in many modifications, e. g. with a
shell structure of various metals and non-metals. The process is reminiscent of the production of whiskers from the
gas phase, but has the advantage of a much higher yields
per unit time and
Anyew. Chem. internat. Edit.
1 Vol. I 1
11972) 1 N o . 9
y
=
E
Lo
=
=
surface energy,
elastic modulus,
elongation necessary to cause fracture
was later modified by various workers, and can be expressed
in its simplest form by eq. (2).
The strengths found from this formula (e.g. a value of
approximately 4000 kp/mmz as the tensile strength of
cc-Al,O,) are not even approached in practice (cf. Table 1).
An explanation for this is provided by Griffith’s fracture
theory[’*! When a tensile stress is applied to a body, the
externally acting force is not uniformly distributed in the
interior, but gives stress concentrations at flaws in the
755
crystal structure, e.g. at a notch. At the tip of a notch, a
stress crk of
The combined influence of crystallite size and porosity on
the strength of a polycrystalline inorganic body can be
expressed, according to Knudsen, by eq. (6)"91, and is
represented graphically in Figure 14.
(3)
a = overall applied stress,
a, = stress at the notch tip,
C = half the length of an internal crack or depth of a surface crack,
r
=
radius of curvature at the notch tip
is reached. The greater C and the smaller r, the more easily
will the tensile stress crk reach or exceed the theoretical
strength value, whereupon breakage will occur in the case
of a single crystal. For a polycrystalline body, however,
the fracture of one crystallite does not necessarily lead to
breakage of the entire body. The break will certainly
extend over the entire crystallite, but it is fairly difficult for
a growing crack to pass over into the differently oriented
neighboring crystallites, and the fracture process consequently stops here. It is thus clear that the strength of
polycrystalline bodies is partly dependent on the crystallite
size; this is expressed in eq. (4).
= k x K-"
K = crystallite size,
k, n = empirical constants
However, this by no means completes the description of
the factors affecting the strength of polycrystalline inorganic
fibers. In the discussion of the Griffith's fracture theory,
mention was made of flaws in the crystal structure. According to Seeger, these defects can be divided into three
groups :
1. Zerodimensional defects e. g. lattice vacancies, interstitial atoms, atoms in incorrect lattice sites, foreign atoms.
2. One-dimensional defects (flaws that extend along open
or closed lines in the crystal), e. g. dislocations (Figure 15)
or acicular precipitations.
a
(4)
High tensile strengths are therefore obtainable in polycrystalline bodies only if the crystallites are very small and
if the surface is practically perfect and free from notches.
The crystallite sizes of polycrystalline inorganic fibers are
mostly less than 300 A, and often even less that 100 A.
The pores in the structure of a polycrystalline body have
an effect similar to that of notches and cracks. They considerably reduce the tensile strength; this is expressed quantitatively by
o
=
o,,xe-"
a. = tensile strength at P = 0,
= porosity,
= empirical constant.
P
b
8
Fig. 15. Scheme of a) edge and b) screw dislocations [20]
3. Two-dimensional defects, e. g. grain boundaries or twin
boundaries.
The structure of a real crystal, with its vacancies, foreign
atoms, dislocations, etc., is reproduced very vividly by a
two-dimensional soap bubble model (Fig. 16)120'.
Whereas no clear dependence exists between the crystallite
size and the elastic modulus, pores also considerably
decrease the elastic modulus.
a = k x K - a x e-hP
m0.m
Fig. 14. Tensilestrength asa function ofcrystaIlitesizeand porosity [19].
756
Fig. 16. Soap-bubbie model of the defects in a crystal [20].
Angew. Chem. internat. Edit. 1 Vof. I 1 (1972) 1 No. 9
Table 3. Comparison of mechanical properties of compact and fibrous
materials [2,3].
The dislocations are defects in the crystal lattice that begin
to migrate under a tensile stress, and that can thus lead to
slip processes and hence also to fracture. There are two
fundamental ways of preventing this. Either one tries to
produce bodies that are totally free from dislocations, or
one tries to inhibit the movement of the dislocations.
Compact material
Steel
AI,03
ZrO,
Sic
Glass
The first method is used in the production of whiskers,
where an effort is made to produce practically dislocationfree single crystals. The second method is the only one that
can be used in the case of polycrystalline inorganic fibers.
Several alternatives are available. Dislocations may be
anchored by strong lattice distortions such as those
resulting from vacancies or foreign atoms in lattice or
interstitial sites. The number of dislocations also influences
their mobility. Thus the dislocation density can be in-
Fiber
u [kp/mm2]
E [kp/mm2]
u [kp/mm2]
40-90
26
14
11
3-9
21 000
36 OOO
18900
48 000
7 400
420
210
21 0
320
z 200
E [kp/mm2]
20600
(17 500)
17500
45 000
7 400
in strength. An impressive practical example for which
many of these theoretical considerations have been confirmed is provided by the polycrystalline Fe fibers produced
by Schladitz by decomposition of pentacarbonyliron in a
Table 4. Mechanical properties of various fibrous materials [4].
Fiber
M.P. ["CI P Ig/cm31
oB[kp/mm2]
E [kp/mm2]
o B / p [kml
E/P[kml
~-
Amorphous
E glass
SiO,
Polycrystalline
single phase
C-HT
C--H M
A1203
ZrO,
BN
3A1,03.ZSi0,
W
steel
Be
multiphase
SjW
BISiO,
SiCIW
700
1660
2,55
2,19
200
150
7 300
7 300
80
70
2850
3 350
3650
3650
2 040
2715
2980
1920
3 400
1 400
1280
1.76
1.95
3.1 5
4.84
1.90
2.9
19.4
7.74
1.83
280
200
200
200
140
245
400
400
130
26 500
38 000
17 500
35000
9 000
17500
41 000
20 500
24 500
160
105
65
40
75
85
20
50
70
1 5000
19500
5 550
7 200
4750
6050
2100
2650
13400
2 300
2 300
2690
2.7
2.36
3.5
350
230
320
40 000
37 000
45 000
130
95
90
14 800
15 700
12850
2 040
2 690
1540
3.96
3.21
7.83
2 000
2 000
1300
42 000
50000
20 500
500
620
165
10600
15600
2650
10-125
200-1
Monocrystalline
(whiskers)
Sic
Fe
For comparison
organic synthetic fibers
creased to such a degree by quenching heated bodies
(freezing the flaws) or by cold deformation of metals that
the dislocations anchor one another and their mobility is
thus reduced,
It has already been mentioned that crystallite boundaries
present obstacles to the propagation of cracks and to the
migration of dislocations. The smaller the crystallite, the
smaller is the length of the dislocation source to be activated, and the greater is the work required to set the dislocation in motion. Finely dispersed precipitates of a foreign
substance, which can prevent the passage of a dislocation
line when separated by certain distances, also present
obstacles to dislocationsf2'!
Attempts are being made, with greater or lesser success,
to bring these mechanisms into play in polycrystalline
inorganic fibers. As can be seen in Table 3, the transition
from the compact material to the fiber with high surface
and internal perfection, taking into account the relationships
discussed above, is accompanied by a considerable increase
Angew. Chem. internat. Edit.
Vol. I 1 (1972) No. 9
1.1-1.5
500
magnetic field. These fibers contain crystallites having a
size of 80 A, and have a dislocation density of 1.5 x 10"
dislocation per cm', compared with about 106-108 in
pure iron. The formation of the ideal lattice is suppressed
by precipitation of Fe,C crystallites at short distances
from one another, and the dislocations are anchored by
precipitation of 1.2% of carbon in interstitial sites. The
fibers have tensile strengths of up to 700 to 800 kp/mm2,
i.e. approximately 60% of the tensile strength of good iron
whiskers ( z1300 kp/mm2)['6].
Table 4 shows a survey of the tensile strengths and elastic
moduli obtained with polycrystalline inorganic fibers in
comparison with those of other fibers. It can be seen that
polycrystalline inorganic fibers have extraordinarily high
tensile strengths, which are superior even to those of organic fibers and glass fibers, and are exceeded only by
those of whiskers. They normally also exhibit the high
elastic moduli found for the corresponding compact ceramic
materials. In this respect, they are only slightly inferior to
757
whiskers. Fine metal filaments also stand comparison in
this respect ; however, with the exception of Be, when one
considers the specific tensile strengths and specific moduli,
they immediately lose ground because of their high specific
gravity. These specific values, however, are decisive for use
in light structural components, e. g. for aircraft and space
applications.
+----
8001
I:
I!
600
600
,
"
,
"
6 E -giass
. -Q-400
E
a
1
2
4
200-
1
6
5
Fig. 17. Temperature dependence of the tensile strength of inorganic
fibers [4]. AI,O, as whiskers, all others as endless fibers.
disadvantage that, with few exceptions, they are too
heavy.
The ideal situation would be achieved if the positive
properties of groups I and II could be combined, This
can be achieved in a composite material, in which fibers
of a material from group If are embedded in a matrix
of a material from group I ; it is necessary to ensure good
adhesion between the fiber and the matrix and good
transfer of force. As will be shown later, however, other
combinations are also possible.
The strengthening and stiffening mechanism that can act
in such fiber-reinforced materials can be seen from the
stress-strain diagram (Fig. 18). If a fiber-reinforced material with good adhesion between the fiber and the
matrix is subjected to an elongation a by application
of a tensile stress, the fiber with the high elastic modulus
will bear most of the load (of),
while the matrix, because
of its low elastic modulus, will bear the smaller share
(om);
the function of the latter is to hold the fibers together
and to transfer the tensile stress to the fibers. This combination principle has been followed in nature since the
earliest times, examples being the strength and rigidity of
bone material or of bamboo cane.
liber
Polycrystalline inorganic fibers naturally have extremely
high melting points. However, this does not mean that
they can be used up to these temperatures. Recrystallization
at temperatures far below the melting point may lead to
structural changes that drastically impair the mechanical
properties. Moreover, the strength values decrease at higher
temperatures because of the greater ease with which creep
and slip processes occur.
Nevertheless, polycrystalline inorganic fibers are the
strongest materials known at these temperatures, apart
from whiskers. As can be seen from Figure 17, for example,
graphite fibers still have a greater strength at 1500°C
than steel at room temperature.
6. Applications
The materials in common use can be roughly divided into
three groups according to their properties (cf. Table I).
Materials of group I, which include, e.g. synthetic resins,
are thermally unstable, easy to process, light, tough,
impact-resistant materials having low strengths. Because
of their low elastic modulus, they have little rigidity, and
consequently cannot be used for the production of large
load-bearing structural components. The ceramic materials of group 11, e.g. A1,0, or Sic, are relatively light and
thermally stable, have very high elastic moduli and are
therefore very rigid, but they are also very brittle and
sensitive to impact, and have low strengths. They are also
unsuitable for use alone as construction materials. The
third group of materials, which within certain limits
combines in itself the desirable properties of the other
two groups (rigidity, strength, toughness, thermal stability),
are the solid metals ; however, these have the important
758
d,,, =d,V,+d,V,
Em, = 6 V i + E m V m
FRM = fiber reinforced material
l=fiber
m=matrix
V = voiume fraction
E = moduius of elasticity
D
matrix
v)
c
a,
a
Elongation
E
------)
Fig. 18. Stress distribution in a fiber-reinforced material
In the ideal case, the tensile strength of a fiber-reinforced
material (FRM) oFRMis made up additively from the
products of the tensile strength of the fiber ofand its
volume fraction V, on the one hand and the tensile strength
of the matrix omand its volume fraction V, on the other.
The same is true of the elastic modulus, and hence of the
rigidity of the reinforced material.
Fig. 19. Cross section through a boron-fiber reinforced material.
Angew. Chem. Infernat.Edif. / Vol. 11 (1972) 1 No. 9
The most common case is the reinforcement and stiffening
of synthetic resins. As can be seen from Table 5, it is
possible in this way to obtain materials whose strength
and rigidity is similar to that of steel, but which are lighter
by a factor of four to five.
Despite the steady improvement of the heat resistance of
synthetic resins, full use cannot be made of the outstanding
high-temperature properties of polycrystalline inorganic
fibers with synthetic resins as the matrix materials. As is
Table 5. Mechanical properties of fiber-reinforced materials [4]
Composite
Fiber
Density oB
content
[voI.-%] (g/cm3] [kplmm’]
59
C/epoxy
Bfepoxy
55
B/polylmide 69
1.5
2.1
2.1
140
145
200
B/Al
2.1
125
1.2
2.1
7.8
10
10
50
50
For comparison:
Epoxy resin 0
Aluminum
0
Building
0
steel
E
Remarks
[kp/mmz]
14000
21 000
24000 - tensile strength
at 315°C
130 kp/rnm2
24000 - tensile strength
at 500°C
I 0 kp/mm2
300
7000
21 000
shown by the example B/polyimide, the medium-temperature range is attainable, but metallic matrices will play
a part in future for the high-temperature range. The metals
offer higher heat resistance, higher elastic moduli, and in
some cases toughness and hardness, as valuable properties
in fiber-reinforced materials.
Polycrystalline inorganic fibers above all improve the
tensile strength at higher temperatures and reduce the
density of the metals, with the result that the specific
properties are improved. It is clear that such materials will
find use wherever great strength is required at high temperatures, as in pressure vessels, in reactor engineering, and
above all where weight saving is important, i.e. in aerospace
technology where resistance to high temperatures is
necessary. As can be seen from Figure 20, e. g. aluminum
reinforced with boron fibers has a strength at 500°C
similar to that of steel at room temperature.
whisker composite
B~C’A’
0 continuous filament composite
Ib
2b
steel
’Oi
m@j
1 i
8
o;
Elp [1O3krn1-
Fig. 20. Specific properties of various composite materials [4].
Anqew. Chem. inrernar. Edrt I Vol. 11 11972) f No. 9
It is mentioned only to complete the picture that fiber reinforcement of ceramic materials is also possible, and can
lead to improved properties. This field of application
is still in its infancy, but amazing properties have already
been obtained, e . g . in the reinforcement of glass with
carbon fibers.
In addition to mechanical reinforcement, another important application of inorganic fibers is thermal insulation at high temperatures. However, since extreme
mechanical properties are less important here than
resistance to high temperatures, low thermal conductivity,
resistance to thermal shock, and low embrittlement,
this field is dominated not by polycrystalline inorganic
fibers but by the amorphous SiO, and alumina silica
fibers.
Certain polycrystalline inorganic fibers have special fields
of application. The steel filaments produced by Monsanto
by melt spinning are intended for tire cord. Fabrics made
from polycrystalline oxide filaments can be used as filters
for hot corrosive gases, liquids, or molten metals. BN
fibers, for example, are suitable for electrical insulation
at high temperatures; these fibers are also unaffected by
boiling alkalis and acids or by molten Fe or Al, and even
withstand an atmosphere of chlorine at 700°C.
Synthetic resins reinforced with polycrystalline inorganic
fibers may be expected in the future to find a spectrum of
application similar in breadth to that existing at present
for glass-fiber reinforced plastics. The former class of
composite materials currently seems particularly attractive
in aircraft and space applications, since they allow a
reduction of weight without loss of strength and rigidity,
and hence greater ranges and lower fuel consumptions.
However, the weight saving is economically justifiable
only if the cost does not exceed a certain limit. This limit is
around $60 per kg of weight saved for conventional
Iight aircraft, and around $600 for supersonic aircraft.
The cost of saving 1 kg of structural weight by the use of e.g.
carbon-fiber reinforced materials is at present between
$350 and $1900. By 1975, these costs should have fallen to
approximately $ 90 to $500. A considerable saving
would thus be possible in particular for supersonic
aircraftrzzJ.For the near future, the addition of smalI
proportions of polycrystalline inorganic fibers to glassfiber reinforced plastics may be expected ; in this way, their
rigidity and their fracture energy can be increased, so that
they are better suited to the construction of tanks, silos,
pipelines, vehicles, containers, etc. Other applications
of polycrystalline inorganic fibers will become interesting
as prices fall. Conceivable applications include lighter
bridges with higher carrying capacities and load-bearing
structural components in tall buildings in construction
engineering, underwater buildings in the exploitation
of the sea, rigid but lighter, corrosion-resistant structural
components in shipbuilding and boatbuilding, and corrosion-resistant pressure vessels, tanks, and reactors in
chemistry and in reactor engineering. The applications
that have already been successful in sports (fiber reinforced
components in racing cars, racing canoes, yacht masts,
etc.) are likely to be extended to skis, boat hulls, golf
759
clubs, etc. In the opinion of many experts we are entering
an “age of inorganic fibers”.
Received : October 8, 1971 [A 900 IE]
German version: Angew. Chem. 84, 866 (1972)
Translated by Express Translation Service, London
[l] P. H. Selden I Glasfaserverstarkte Kunststoffe. Springer-Verlag,
Berlin 1967, p. 181.
[2] I. E. Campbell and E. M . Sherwood: High-Temperatur Materials
and Technology. Wiley, New York 1967, pp. 256ff.
[3] L. Piatri: Werkstoffe der chemischen Technik, Vol. 3. Verlag
Sauerlander, Aarau 1955, p. 76.
[4] H. W Rauch, W H. Sutton, and L. R. McGreight: Ceramic Fibers
and Fibrous Composite Materials. Academic Press, New York 1968
[5] C. Z. Carroll-Porczynski: Advanced Materials. Chemical
Publishing, New York 1969.
[6] Chem. Week 108 (Feb. 24). 51 (1971).
[7] H. P. Blakelock, N . A. Hill, N . Aubrin. S. A . Lee, and C. Goatcher,
Proc. Brit. Ceram. SOC.1970, No. 15.6Y
[8] A. H. Frazer: High Temperature Resistant Fibers. Wiley, New York
1967, pp. lOSff, 183ff, 267ff.
[9] W: L. Lachmann and J . P. Srerry, Chem. Eng. Progr. 58 (lo), 37
(1962).
[lo] B. H . Hamling, A. W Naumann, and W H . Dreshrr, Polymer
Reprints, Atlantic City Meeting 9 (2), 1449 (1968).
[ll} W I: Gunston, Sci. 5, 39 (1969).
[I21 H. M . Hawthorne, Int. Conf. Carbon Fibres, Composites and
Applications, London 1971, Paper No. 13.
[I31 J . Economy, R. V Anderson, and K I . Matkooich, Polymer Reprints,
Atlantic City Meeting 9 (2), 1392 (1968).
[14] H . W Rauch, Mater. Eng. 66, 74 (1967).
[15] J . Nixdorf; Metal1 23, 887 (1969).
[16] H. J . Schladitz, 2. Metallk. 59, 18 (1968).
[I71 M . Polanyi, Z. Phys. 7, 323 (1921).
[IS] E. Orowan, 2. Phys. 86,195 (1933).
[I91 F . P. Knudsen, J. Amer. Ceram. SOC.42, 376 (1959).
[20] W Kleber: Einfiihrung in die Kristallographie. VEB Verlag
Technik, Berlin 1961, pp. 114fT.
[21] H. Griinewald, DEW (Deut. Edelstahlwerke) Techn. Ber. 8 (4),
271 (1968).
[22] J . Fray, Int. Conf Carbon Fibres, Composites and Applications,
London 1971, Paper No. 46.
Cathodic Dimerization[**]
By Fritz Beck“’
The technical, electrochemical, and preparative aspects of cathodic dimerization, which
leads to bifunctional compounds. are reviewed. With the hydrodimerization of acrvlonitrile
as an example, the effects of the reaction parameters and the mechanism are discussed in
detail. In addition to the hydrodimerization of activated compounds, the coupling can
also proceed via elimination of halide or via the discharge of cations. Processes of special
preparative interest are those in which two different molecules are coupled, which can yield e.g.
esters, alcohols, or ketones with cyano groups, as well as asymmetric diols. The reductive
dimerizations of acrylonitrile, p-chloropropionitrile, acetone, acetylpyridine, nitrobenzene
(+ benzidine), and pyridinium salts have already found industrial use.
1. Introduction
The field of cathodic dimerization has developed particularly rapidly in recent years. This is no accident, since this
area of organic electrosynthesis involves aspects of great
preparative, technical, and mechanistic interest. The object of this progress report is to present a comprehensive
survey, with special emphasis on industrial and electrochemical possibilities. Only recently, Baizer (together with
Petrowitsch), who played an extremely important part in
the development of the field, published a brilliant survey
of the synthetic and mechanical aspects[’’. This topic is
also discussed at length in a recent review on organic
eIectrosynthesisr2! Overlapping with these publications
cannot be entirely avoided, but will be kept to a minimum.
[*] Dr. F. Beck
Badische Anilin- & Soda-Fabrik AG, Hauptlaboratorium
67 Ludwigshafen (Germany)
[**I Based, in part, on a lecture at the 1st EUCHEM Conference
on Organic Electrochemistry at Ronneby Brunn, Sweden, on June 9,
1971.
760
In cathodic dimerization, the organic molecule takes up
an electron directly at the electrode. It thus passes through
the intermediate stage of a radical-ion. The negative charge
is compensated in accordance with Scheme 1 by uptake of
protons (“hydrodimerization”), by elimination of halide
ions, or by the positive charge on the substrate when the
latter is a cation. Each of these cases can in principle be
further subdivided into the dimerization of two identical
molecules, which yields a symmetrical product, the
coupling of two different molecules, and intramolecular
reactions leading to cyclic products.
If the monomer contains one or more functional groups,
bifunctional or polyfunctional dimers are formed. Cathodic
dimerization is therefore a valuable method for the synthesis of such compounds. Since oniy one electron is taken
up per monomer in every case, the energy consumption is
comparatively low. This is extremely favorable in industrial uses.
The formation of a symmetrical dimer is only one of several
possible reactions. In principle, the molecule could also be
Angew. Chem. internot. Edit. Voi. I 1 (19721 I No. 9
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