Note: Descriptions are shown in the official language in which they were submitted.
2~ ~ ~ 2 6 6
PREPARATION OF SUBSTANTIALLY CRYSTALLINE
SILICON CARBIDE FIBERS FROM BOROSILAZANES
This invention relates to the preparation of
thermally stable, substantially polycrystalline silicon
carbide ceramic fibers using borosilazane resins. The method
described herein provides a simple yet effective method of
preparing desirable fiber at a relatively low cost.
Silicon carbide ceramic fibers are known in the art
for their mechanical strength at high temperatures. Because
of this property, they have found utility in numerous areas
such as reinforcement for plastic, ceramic or metal matrices
to produce high performance composite materials or the
formation of fibrous products such as high temperature
insulation, belting, gaskets and curtains.
Several methods have been developed to manufacture
such fibers. For instance, it is known that organosilicon
polymers may be spun into fibers, infusibilized (cured) and
pyrolyzed at elevated temperatures to form a ceramified
fiber. Unfortunately, this method suffers from the
disadvantage that substantial amounts of oxygen and/or
nitrogen are often incorporated into the fibers either
directly through the polymer or indirectly by incorporation
during spinning, infusibilization or ceramification. When
these fibers are heated to temperatures above 1400~C., the
oxygen and nitrogen is lost causing weight loss, porosity and
decreased tensile strength.
Recently, polycarbosilane preceramic polymers
having a Si-C skeletal structure have been used to minimize
the incorporation of nitrogen and oxygen. Yajima et al. in
U.S. Patents 4,052,430 and 4,100,233, for example, teach a
method of producing silicon carbide fibers by spinning,
' -2- ~ ~ ~ 6 ~ 6 6
infusibilizing and pyrolyzing various polycarbosilanes.
Nippon Carbon Co., moreover, utilizes this technology to
produce a SiC ceramic fiber sold under the trade name
NICALONtm. These fibers too, however, are known to contain
about 9-15% oxygen and, thus, degrade at temperatures as low
as 1200~C. (see Mah et al., J. Mat. Sci. 19, 1191-1201 (1984)
Borosilazanes are also known in the art. For
instance, Japanese Kokai Patent No. Hei 2-84437 describes the
formation of borosilazanes by reacting a silazane with a
boron compound. The reference describes the resultant
materials as useful in the formation of ceramics. In
"International Symposium on Organosilicon Chemistry Directed
Towards Material Science", Abstracts, P. 95-96, Sendai,
Japan, 25-29 March (1990), the same inventors describe the
use of these materials in the formation of fibers. However,
since the fibers contained insufficient carbon and were only
pyrolyzed up to 1700~C., the resultant product is described
as an amorphous silicon boron nitride fiber. The fibers
claimed in the present application, on the other hand, are
predominantly crystalline, silicon carbide.
European Patent No. 364,323 describes a metl~od of
forming polymers based on boron and nitrogen comprising
reacting a silazane with a trihalogen borane. The resultant
materials are taught therein as being useful in the formation
of boron nitride ceramics, including fibers.
Takamizawa et al. in U.S. Patent 4,604,367 teach
the preparation of an organoborosilicon polymer by mixing an
organopolysilane with an organoborazine compound, spinning
fibers and then ceramifying the fibers by heating to
temperatures in the range of 900-1800~C. The organoboro-
silicon polymer therein, however, is described as having a
skeletal structure comprising Si, C, N and B compared to the
Si, B and N chains of the present invention. Moreover, this
3 ~Q~266
reference teaches that the tensile strength of the fibers
drops off dramatically when heated above 1500~C. (note the
graph on the cover of the reference).
U.S. Patent No. 4,910,173 granted to Niebylski
describes the formation of organoborosilazane polymers by the
reaction of a polysilazane with a boroxine. The reference
states that the resultant materials are useful in the
formation of ceramic fibers but fails to teach the
methodology.
Seyferth et al., J. Am. Ceram. Soc. 73, 2131-2133
(1990) likewise teach the formation of borosilazane polymers
by the reaction of silazanes with boranes. The reference
teaches that the resultant materials are useful in the
production of borosilicon nitride ceramic fibers.
The present inventors have now unexpectedly found
that thermally stable, substantially polycrystalline SiC
fibers can be formed by firing borosilazane fibers having
greater than about 0.2% boron and greater than about 0.1 %
free carbon incorporated therein to a temperature greater
than about 1700~C.
The present invention relates to a method for the
preparation of thermally stable, substantially polycry-
stalline silicon carbide fibers. The method comprises
forming a fiber from a preceramic borosilazane resin
characterized by containing at least about 0.2% by weight
boron and by its ceramic char containing at least about 0.1%
free carbon. The fiber is infusibilized to render it
non-melting and then pyrolyzed at a temperature greater than
about 1700~C. in a non-oxidizing environment.
The present invention also relates to silicon
carbide fibers which have at least 75% crystallinity, a
density of at least about 2.9 gm/cc, a very low residual
~a ~
--4--
oxygen content and which contain greater than 1 weight
percent nitrogen.
The present invention is based on the discovery
that borosilazane polymers which have at least about 0.2% by
weight boron and at least about 0.170 by weight free carbon
incorporated therein can be used to form substantially
polycrystalline SiC fibers which retain their strength at
high temperatures. These fibers have at least 75%
crystallinity, a density of at least about 2.9 gm/cc and a
very low residual oxygen content.
The borosilazanes used in the present invention are
especially valuable precursors for such fibers since they are
relatively simple and inexpensive to manufacture and can be
modified to produce a char with nearly any desired
stoichiometry. In addition, the use of these polymers
assures a uniform distribution of boron throughout the
fibers. Such uniformity avoids the boron agglomeration flaws
which may occur when boron is incorporated into the fibers by
other methods.
The borosilazanes useful herein are generally well
known in the art and can comprise any which provide
sufficient boron and carbon on pyrolysis. These can include,
for instance, those described in US Patent Number 4,910,173
granted to Niebylski, those described by Funayama et al.,
International Symposium on Organosilicon Chemistry Directed
Towards Material Science, Abstracts, P. 95-96, Sendai, Japan,
25-29 March (1990), those described by Seyferth et al., J.
Am. Ceram. Soc. 73, 2131-2133 (1990), those described by
Noth, B. Anorg. Chem. Org. Chem., 16(9), 618-21, (1961),
those described by Araud et al. in European Patent
No. 364,323 and those described by Funayama et al. in
Japanese Kokai Patent No. 2-84437.
- ~n~6~
The method for preparing such compounds is likewise
known in the art and described in the above references. The
preferred method, however, comprises reacting a boron
trihalide with a silazane oligomer such as (RSi(NH)l 5)x or
((CH3)3Si)2NH, wherein R is selected from the croup
consisting of hydrogen, a hydrocarbon radical and a
substituted hydrocarbon radical and x is an integer of 2-20.
Examples of the R groups include alkyls such as methyl,
ethyl, propyl, butyl, etc., alkenyls such as allyl, vinyl,
etc. and saturated or unsaturated cyclic groups such a
cyclopentane, cyclohexane, cycloheptane, phenyl, etc.
Especially preferred are hydrocarbons with 1-6 carbon atoms
with methyl being most preferred.
If boron tribromide is to be used in this process,
the amount of the silazane oligomer should be greater than
2.7 equivalents to avoid gelation. Since such gelation may
be a potential problem with the use of boron tribromidej it
is more preferred to use boron trichloride. Other equivalent
methods, however, are also contemplated herein. Specific
methods for preparation of suitable borosilazanes are also
illustrated in the examples.
The specific polymer chosen should be one which
contains at least about 0.2 weight percent boron based on the
total weight of the polymer. This amount of boron is
necessary for the fibers to undergo a densification process
which decreases porosity and strengthens the fiber. Polymers
containing at least about 0.6% by weight are more preferred
for this effect.
The specific polymer chosen should also yield a
ceramic char in which at least about 0.1% free carbon is
generated. Preferably, the polymer should yield a char
having between about 0.1 and about 5 weight percent free
carbon. What is meant by free carbon in this invention is
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the amount of free or excess carbon derived from the boro-
silazane during pyrolysis. The total amount of carbon in the
ceramic char equals the amount of free or excess carbon plus
the amount of carbon combined with silicon in the form of
silicon carbide.
The amount of free carbon derived from the
borosilazane is determined by pyrolysis of the borosilazane
to an elevated temperature under an inert atmosphere until a
stable ceramic char is obtained. For purposes of this
invention, a "stable ceramic char" is defined as the ceramic
char produced under an inert atmosphere at an elevated
temperature which will not significantly decrease in weight
upon further exposure at the elevated temperature. Normally,
a stable ceramic char is produced upon pyrolysis at 1800~C.
for about 30 minutes under argon. Other elevated
temperatures can be used to form the stable ceramic char but
the length of exposure to the elevated temperature will need
to be increased for temperatures less than 1800~C. Both the
ceramic yield and the silicon and carbon content of the
stable ceramic char are then determined. Using a rule of
mixtures, the amount of SiC and free carbon of the stable
ceramic char can be calculated.
To be useful herein, the borosilazane should also
preferably be 1) solid at room temperature, 2) readily
spinable into small diameter fibers, and 3) infusible such
that the polymer will remain in fiber form during pyrolysis.
More preferably, I) the solid polymers have softening points
less than about 100~C. so that they are readily extrudable
for conventional fiber spinning techniques and II) the
polymers have Si-H functional groups for faster cure rates.
Alternatively, a liquid borosilazane may be
utilized to spin the fibers. However, when fibers are spun
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in this manner, they are generally solidified by rapid curing
immediately after exiting the spinning apparatus.
The borosilazanes may, however, be formed into
fibers by any conventional spinning technique. For instance,
techniques such as melt spinning, dry spinning or wet
spinning may all be used in the practice of this invention.
The spun fibers formed in this manner are generally
drawn to diameters of less than about 100 micrometers. More
preferably, the fibers are drawn to diameters of about 10-50
micrometers. Fibers of this size are generally more flexible
than larger fibers and, thus, can be more readily woven into
reinforcing matrices for composite materials.
The fibers formed above are then infusibilized to
prevent melting during pyrolysis. The fibers may be
infusibilized, for example, by exposure to various gases
such as HCl, HCl/moist air, HCl/ammonia, boron
trichloride/ammonia, borane or chlorine/ammonia. The
concentration of HCl used is generally in the range of about
0.1 to about 50 weight percent, the concentration of water is
generally in the range of about 0.1 to about 20 weight
percent, the concentration of ammonia gas is in the range of
0.1 to 50 weight percent, the concentration of boron
trichloride is in the range of 0.1 to 20 weight percent, the
concentration of borane is in the range of 0.01 to 20 weight
percent and the concentration of chlorine gas is in the range
of 0.1 to 50 weight percent. Generally, the fibers are cured
at temperatures in the range of about room temperature to
about 400~C. in a time of from about less than a second to
several hours. For example, times of from about 0.1 second
to about 6 hours may be used.
If enough boron is not incorporated into the
initial polymer, additional boron may be incorporated into
the fibers during infusibilization or the early stages of
-' - 8 ~ fi ~
pyrolysis. This can be accomplished, for instance, by
exposing the fiber to a boron containing gas such as
diborane, a boron halide, diborane, boron hydrides, borazine
andtor trichloroborazine.
After infusibilization, the fibers are pyrolyzed by
heating to temperatures greater than about 1700~C. and
preferably at temperatures of about 1800-1900~C. in a
non-oxidizing environment (eg., argon, vacuum, etc.). The
present inventors have found that most of the oxygen and
nitrogen are eliminated from the fibers at temperatures above
about 1400~C. which is believed to result in an initial
weakening of the fiber. However, when an appropriate amount
of boron is incorporated into fibers and said fibers are
pyrolyzed above about 1700~C., it is believed the fibers
undergo a densification process which decreases porosity and
strengthens the fiber. Temperatures in excess of about
2000~C. are not preferred as there is undesirable grain size
growth of the silicon carbide ceramic which adversely affects
fiber strength.
The fibers are heated at the desired temperature
for a time sufficient to reduce the oxygen content of the
fibers to below about 0.5% by weight. For example, if the
fibers are heated to about 1800~C., it has been found -that
temperature should be maintained for about 1 hour.
During this pyrolysis step, the nitrogen content of
the fiber is also lowered. However, since some residual
amounts of nitrogen (i.e., up to about 3 weight percent)
typically remain in the fibers, it is postulated that a
portion of the boron is present as boron nitride.
The ceramic fibers which result from the process of
this invention have at least 75% crystallinity and have a
density of at least about 2.9 gm/cc, which represents about
90-95% of the theoretical density of SiC. The fibers also
-9- ~ ~ ~
have a smooth surface structure and a grain size less than
0.5 micrometers, typically less than 0.2 micrometers.
Virtually all of the oxygen originally present in, or
introduced into, the fibers is removed by the high
temperature pyrolysis step and the nitrogen, other than that
present as boron nitride, is also lost. Less than about
0.5% and preferably less than about 0.2%, by weight oxygen
remains and generally less than about 3% nitrogen remains.
The following non-limiting examples are included in
order that one skilled in the art may more readily understand
the invention.
In the following examples, Ph = phenyl, Me = Methyl
and HMDZ = hexamethyldisilazane.
Carbon and nitrogen analyses were carried out on a
CEC 240-XA elemental analyzer. Silicon and boron were
determined by a fusion technique which consisted of
converting the material to soluble forms of silicon and boron
and analyzing the solute for total silicon or boron by atomic
absorption spectrometry.
Gel permeation chromatography (GPC) data were
obtained on a Waters GPC equipped with a model 600E systems
controller, a model 490 W and model 410 Differential
Defractometer detectors; all values are relative to
polystyrene.
All furnace firings were done in a 2 inch Lindberg
tube furnace, a two inch Astro furnace or a six inch Vacuum
Industries hot press on graphite foil fiber holders.
Example 1
To prepare a polymer having the formula
(phsi(NH)l 5)0 3s(MeSi(NH)1.5)0~55(B(NH)l~5)0 1
HMDZ, 362.25 g (2.25 moles) and xylene, 100 g, were placed in
a 1 L three necked flask under an argon atmosphere. The
- 10-
flask was fitted with an addition funnel, an overhead stirrer
and the argon inlet. The addition funnel was charged with
74 g (0.35 mole) of PhSiC13 and 82.2 g (0.55 mole) of MeSiC13
under argon. The chlorosilane mixture was then added to the
HMDZ solution dropwise over a 30 minute period with stirring.
The addition funnel was replaced with a water cooled
condenser and the reaction refluxed at 80~C. for 20 hours.
The mixture was then cooled to ambient temperature and the
condenser replaced with a rubber septum. BBr3 was added
(24.8 g, 0.1 mole) via a syringe through the septum over a 10
minute period. This addition resulted in an exotherm and the
formation of a milky white suspension. After stirring for 2
hours at ambient temperature, the septum was replaced with a
distillation head and the mixture heated to 170~C. over 145
minutes with overhead volatiles beginning at 70~C. The warm
resin was then quenched with xylene (100 g) and the resulting
solution was heated to 215~C. and held for 45 minutes. The
warm resin was again quenched with 100 g xylene, allowed to
cool and filtered through a medium glass frit. The filtrate
was placed in a 500 mL three necked flask fitted with an
argon inlet, overhead stirrer and a distillation head
connected to a receiver which was cooled in dry ice. The
filtrate was stripped for 90 minutes at 235~C. at 10 mm Hg.
The above synthesis yielded 103 g of a brittle resin which is
characterized in Table 2.
The above resin (5.9 g) was powdered, using a
mortar and pestle and pressed into a spinning evaluation rod
using 300 psi in a stainless steel rod mold. The rod sample
was loaded into an Ultraspin apparatus. This apparatus
heated the polymer and mechanically fed controlled, small
amounts of molten polymer through a small diameter orifice.
Once extruded, the filament fell through a draw down zone in
which the atmosphere was controlled to allow the option of
6 ~ -
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introducing and removing cure gases. The filaments were
collected on a drum rotated at a controlled speed located in
an inert take-up chamber.
Fibers were spun at varying temperatures, extrusion
rates and take-up speeds. Spinning conditions of 142~C.,
extrusion rate of 0.0136 g/min and take-up speed of 40 m/min
resulted in spinning 19 micron diameter fibers. Spinning
conditions of 142~C., extrusion rate of 0.0136 g/min and
take-up speed of 70 m/min resulted in spinning 14 micron
diameter fibers. Spinning conditions of 141~C., extrusion
rate of 0.0208 g/min, take-up speed of 40 m/min and 10% flow
of HCl gas into and out of the draw down zone resulted in
spinning 24 micron diameter fibers.
Portions of fiber spun by the above technique
(20-80mg; 142~C., extrusion rate of 0.0136 g/min and take-up
speed of 40 m/min) were cured in a sealed chamber designed to
allow inert transfer and mounting of the fiber samples. The
fiber sample was mounted in the chamber and the desired cure
gas(s) admitted through a stopcock. The cured fibers were
then pyrolyzed at the desired temperature. The following
cure/pyrolysis conditions were used:
i) 60 mg of the fiber was exposed to 20% HCl for 10
minutes and then pyrolyzed at 3~C./min to 1200~C. under
argon. The fiber retained its shape.
ii) 108 mg of the fiber was exposed to 20% HCl for 10
minutes and then 10% moist air for 5 minutes. The sample was
then pyrolyzed at 3~C./min to 1200~C. under argon (46.7%
yield). The fiber retained its shape and had a tensile
strength of 38 ksi.
iii) 30 mg of the fiber was exposed to 20% C12 for 18
hours and then pyrolyzed at 0.75~C./min to 200~C. under
50%ammonia/50%nitrogen, 1.7~C./min to 800~C. under argon and
-12- ~ ~ 8 ~ 2 6 ~ -
3~C./min to 1200~C. under argon. The fiber retained its
shape and had a tensile strength of 31 ksi.
iv) 26 mg of the fiber was exposed to 0.2% borane and
pyrolyzed at 1~C./min to 200~C. (held at 75~C. for 45 min)
and then at 3~C./min to 1200~C. under argon (60.4% yield).
The fiber retained its shape and had a tensile strength of 65
ksi.
v) 30 mg of the fiber was exposed to 5% BC13 for 10
minutes, exposed to 50%ammonia/50%nitrogen while pyrolyzed at
0.75~C./min to 150~C., pyrolyzed at 0.75~C./min under
nitrogen to 200~C., pyrolyzed at 1.7~C./min to 800~C. under
argon and 3~C./min to 1200~C. under argon. The fiber
retained its shape and had a tensile strength of 44 ksi.
Portions of the on line HCl exposed fibers (141~C.,
extrusion rate of 0.0208 g/min, take-up speed of 40 m/min and
10% flow of HCl gas into and out of the draw down zone) were
batch exposed to 10% moist air for 5 minutes and pyrolyzed at
3~C./min to 1200, 1800, 1900 and 2000~C. under argon. The
results are summarized in the following table.
Table 1
Temp(~C) Char Yield Carbon Nitrogen Result
1200 NA NA NA Shape retention
123 ksi
1800 36.1 wt% 36.7 wt% 2.2 wt% beta-SiC
1900 36.8 wt% 36.4 wt% 1.1 wt% beta-SiC
2000 36.4 wt% 45.3 wt% 0.1 wt% beta-SiC
2000 36.4 wt% 35.6 wt% 1.4 wt% beta-SiC ****
**** uncured borosilazane polymer
NA = Not Available
-13- ~Q ~ ~ 2~ ~
Example 2
A polymer having the formula
(PhSi(NH)l 5)0 2s(MeSi(NH)1.5)0.65(B(NH)1.5)0.1
was formed in the same manner as example 1 except the
reactants comprised 435 g (2.7 mole) of HMDZ, 53 g (0.25
mole) PhSiC13, 97 g (0.65 mole) MeSiC13 and 24.8 g (0.1 mole)
BBr3. The resultant product is characterized in Table 2.
The above resin (5.7 g) was powdered, using a
mortar and pestle and pressed into a spinning evaluation rod
using 300 psi in a stainless steel rod mold. The rod sample
was loaded into the Ultraspin apparatus of Example 1. Fibers
were spun at varying temperatures, extrusion rates and
take-up speeds. Spinning conditions of 143~C., extrusion
rate of 0.0208 g/min and take-up speed of 40 m/min resulted
in spinning 0.1 g 24 micron diameter fibers. Spinning
conditions of 143~C., extrusion rate of 0.0154 g/min, take-up
speed of 40 m/min and 10% flow of HCl gas into and out of the
draw down zone resulted in spinning 0.4 g of 22 micron
diameter fibers.
Portions of the on line HCl exposed fibers (143~C.,
extrusion rate of 0.0154 g/min, take-up speed of 40 m/min and
10% flow of HCl gas into and out of the draw down zone) were
batch exposed to 10% moist air for 8 minutes and pyrolyzed at
3~C./min to 1200~C. under nitrogen to yield a fiber that
retained its shape and had 58.3 wt % yield.
Portions of the on line HCl exposed fibers (143~C.,
extrusion rate of 0.0154 ~/min, take-up speed of 40 m/min and
10% flow of HCl gas into and out of the draw down zone) were
batch exposed to 10% moist air for 8 minutes and pyrolyzed at
3~C./min to 1200~C. and 10~C./min to 1800~C. under argon to
yield a fiber that retained its shape and had 34.2 wt %
yield.
-14-
Portions of the on line HCl exposed fibers (143~C.,
extrusion rate of 0.0154 g/min, take-up speed of 40 m/min and
10% flow of HCl gas into and out of the draw down zone) were
pyrolyzed at 3~C./min to 1200~C. under nitrogen to yield a
fiber that retained its shape, had 61.7 wt % yield and
tensile strength of 81 ksi.
Table 2 Characterization of Borosilazanes
Ex Tg Mn Mw Char Wt % Wt % Wt % Wt %
No ~C Yield C N B Si
1 77 1468 8590 36.4 35.6 1.37 2.3 57.9
2 79 1772 10974 37.1 32.7 0.8
Example 3
To prepare a polymer having the formula
(PhSi(NH)l 5)0 30(Hsi(NH)l 5)o 6o(B(NH)l 5)o l
HMDZ, 3864.0 g (24.0 moles) was placed in a 12 L four necked
flask under an argon atmosphere. The flask was fitted with
an addition funnel, an overhead stirrer and the argon inlet.
The addition funnel was charged with 634.1 g (3.0 moles) of
PhSiC13. The chlorosilane was then added to the HMDZ
dropwise over a 30 minute period with stirring. The addition
funnel was replaced with a water cooled condenser and the
reaction refluxed at 90~C. for 60 hours. The mixture was
then cooled to ambient temperature and the condenser replaced
with an addition funnel and a solution of 813.2 g (6.0 moles)
HSiC13 in 501 g xylene was added over a 30 minute period
. BC13 was added (117.4 g, 1.0 mole) then added to the
mixture over a 45 minute period. This addition resulted in
an exotherm and the formation of a milky white suspension.
After stirring for 20 hours at ambient temperature, the
addition funnel was replaced with a distillation head and the
reaction heated to 190~C. over 7 hours with overhead
volatiles beginning at 70~C. The warm resin was then
2 ~ ~
-15-
quenched with xylene (1000 g) and the resulting solution was
filtered through a medium glass frit. The filtrate was
filtered, bodied and stripped to yield the final product.
This product had a Tg of 82~C., Mn of 1822, Mw of 6260, char
yield (at 2000~C.) of 45.3 weight percent and the following
chemical composition: C = 34 weight percent, N = 0.21 weight
percent and SiH = 0.123 weight percent.
The above resin was melted and mechanically fed
through a multifilament spinning apparatus having 200 14 mM
diameter orifices. The molten polymer was fed at 2.5 g/min
and pulled down approximately 12 feet to a take up spool
affording a 200 filament tow. Over the course of the 12
feet, some of the fibers were exposed to HCl followed by
moist inert gas. 35 micron cured and uncured fibers were
thus produced.
Portions of the uncured fiber spun by the above
technique were cured in a sealed chamber by exposure to HCl,
BC13 or C12 followed by exposure to ammonia. The cured
fibers were then pyrolyzed in 2 steps - first to 1200~C. and
then to 1800~C. The following table provides the results:
Table 3
Cure Post Treat* Char YieldTensile Strength
1%HCl/NH3 yes 47.4 99ksi
1%HCl/NH3 no 33.4 21ksi
2%Bcl3lNH3yes 49.7 lOOksi
2%C12/NH3 yes 22.3
J'-heated to 200~C. at 0.75~C./min under argon
Portions of the on line HCl exposed fibers were
post cured at 1.1~C./min to 200~C. under vacuum (A) or at
0.75~C./min to 200~C. under vacuum. The post cured samples
were pyrolyzed under argon in either a one step or 2-step
process. All of the fibers retained their shape. The
results are summarized in the following table:
-16~ fi ~
Table 4
Post Pyrolysis Char
70HCl 7OH2O Cure type/temp Yield %C / %N Tensile Strength
1.8 1.8 A 1 step/1800 45.628.9/1.25
3.0 1.8 A 1 step/1800 48.129.7/4.12
4.5 1.8 B 1 step/1200 72.7 NA 270 ksi
4.5 1.8 B 2 step/1800 48.533.8/0.36 75 ksi*
4.5 1.8 B 2 step/1800 43.734.4/1.61
4.5 1.8 A 1 step/1800 40.927.5/0.17 118 ksi
* 98 weight percent beta SiC by x-ray diffraction
