U.S. patent application number 10/380605 was filed with the patent office on 2007-07-05 for silicoboroncarbonitride ceramics and precursor compounds, method for the production and use thereof.
Invention is credited to Jurgen Clade, Martin Jansen, Utz Muller, Dieter Sporn.
Application Number | 20070155611 10/380605 |
Document ID | / |
Family ID | 38294099 |
Filed Date | 2007-07-05 |
United States Patent
Application |
20070155611 |
Kind Code |
A9 |
Jansen; Martin ; et
al. |
July 5, 2007 |
Silicoboroncarbonitride ceramics and precursor compounds, method
for the production and use thereof
Abstract
The present invention relates to novel
alkylhalosilylaminoboranes, in particular
alkylchlorosilylaminoboranes, which make it possible to adjust the
viscosity of polyborosilazane compounds by varying the number of
reactive centers, to novel borosilazane compounds, to novel
oligoborosilazane or polyborosilazane compounds which have the
structural feature R.sup.1--Si--NH--B--R.sup.2, where R.sup.1 or
R.sup.2 or both is/are a hydrocarbon radical having from 1 to 20
carbon atoms, in particular an alkyl, phenyl or vinyl group, to
silicon borocarbonitride ceramic powder, to ceramic material based
on SiC, SiN and BN and to processes for producing each of these and
to the use of the polyborosilazanes and the ceramic materials.
Inventors: |
Jansen; Martin; (Leonberg,
DE) ; Muller; Utz; (Bonn, DE) ; Clade;
Jurgen; (Wurzburg, DE) ; Sporn; Dieter;
(Wurzburg, DE) |
Correspondence
Address: |
SUTHERLAND ASBILL & BRENNAN LLP
999 PEACHTREE STREET, N.E.
ATLANTA
GA
30309
US
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20050119106 A1 |
June 2, 2005 |
|
|
Family ID: |
38294099 |
Appl. No.: |
10/380605 |
Filed: |
September 14, 2001 |
PCT Filed: |
September 14, 2001 |
PCT NO: |
PCT/EP01/10667 |
371 Date: |
September 16, 2003 |
Current U.S.
Class: |
501/88; 501/92;
501/96.1 |
Current CPC
Class: |
C04B 35/589 20130101;
C04B 35/62281 20130101; C04B 35/62295 20130101; C04B 35/583
20130101; C04B 2235/3826 20130101; C07F 7/12 20130101; C04B 35/6229
20130101; C04B 2235/483 20130101; C04B 2235/386 20130101; C04B
2235/6022 20130101; C04B 2235/486 20130101; C04B 35/571 20130101;
C07F 7/10 20130101; C04B 2235/3873 20130101 |
Class at
Publication: |
501/088; 501/092;
501/096.1 |
International
Class: |
C04B 35/589 20060101
C04B035/589; C04B 35/571 20060101 C04B035/571 |
Claims
1. A compound of the formula (I)
R.sub.xHal.sub.3-xSi--NH--BR.sub.yHal.sub.2-y, where R are each,
independently of one another, a hydrocarbon radical having from 1
to 20 carbon atoms, Hal are each, independently of one another, Cl,
Br or I, x=1 or 2 and y=0 or 1.
2. A compound as claimed in claim 1, characterized in that Hal is
Cl on each occurrence.
3. A compound as claimed in claim 1, characterized in that R is,
independently on each occurrence, a hydrocarbon radical having from
1 to 3 carbon atoms, a C.sub.1-C.sub.20-alkyl group, a phenyl group
or a vinyl group.
4. A process for preparing a compound as claimed in any of claims 1
to 3, comprising reacting a compound of the formula (II)
R.sub.xHal.sub.3-xSi--NH--SiR.sub.3, where R are each,
independently of one another, a hydrocarbon radical having from 1
to 20 carbon atoms, Hal are each, independently of one another, Cl,
Br or I and x=1 or 2, with a compound of the formula (III)
BR.sub.yHal.sub.3-y, where R and Hal are as defined above and y=0
or 1, at a temperature in the range from -100.degree. C. to
+25.degree. C.
5. A process for preparing a compound as claimed in any of claims 1
to 3, comprising reacting compound of the formula (II)
R.sub.xHal.sub.3-xSi--NH--SiR.sub.3, where R are each,
independently of one another, a hydrocarbon radical having from 1
to 20 carbon atoms, Hal are each, independently of one another, Cl,
Br or I and x=1 or 2, with a compound of the formula (III)
BR.sub.yHal.sub.3-y, where R and Hal are as defined above and y=0
or 1, in a molar ratio of from 1:1 to 1:10.
6. The process as claimed in claim 4, characterized in that Hal is
Cl on each occurrence.
7. The process as claimed in claim 4, characterized in that R is,
independently on each occurrence, a hydrocarbon radical having from
1 to 3 carbon atoms, a C.sub.1-C.sub.20-alkyl group, a phenyl group
or a vinyl group.
8. A process for preparing a compound of the formula (II)
R.sub.xHal.sub.3-xSi--NH---iR.sub.3, where R are each,
independently of one another, a hydrocarbon radical having from 1
to 20 carbon atoms and Hal is Cl, Br or I and x=1 or 2, comprising
reacting R.sub.xSiHal.sub.4 and R.sub.3Si--NH--SiR.sub.3 without
the addition of a Lewis acid.
9. The process as claimed in claim 8, characterized in that
x=2.
10. The process as claimed in claim 9, characterized in that
R.sub.2SiHal.sub.2 and R.sub.3Si--NH--SiR.sub.3 are reacted in a
molar ratio of from 1:1 to 1.5:1 at a reaction temperature of from
40.degree. C. to 80.degree. C.
11. The process as claimed in claim 9, characterized in that Hal is
Cl.
12. The process as claimed in claim 9, characterized in that R is,
on each occurrence, a hydrocarbon radical having from 1 to 3 carbon
atoms, a C.sub.1-Cl.sub.2-alkyl group, a phenyl group or a vinyl
group.
13. A borosilazne compound of the formula (IV)
(R'R''N).sub.qR.sub.xHal.sub.3-x-qSi--NH--BR.sub.yHal.sub.2-y-z(NR'R'').s-
ub.z, where R' and R'' are each, independently of one another,
hydrogen or a hydrocarbon radical having from 1 to 20 carbon atoms,
R are each, independently of one another, a hydrocarbon radical
having from 1 to 20 carbon atoms, Hal are each, independently of
one another, Cl, Br or I, q=0, 1 or 2, x=1 or 2, y=0 or 1 and z=0,
1 or 2, with the proviso that q+z.gtoreq.1, x+q.ltoreq.3 and
y+z.ltoreq.2.
14. A borosilazane compound as claimed in claim 13, characterized
in that Hal is Cl on each occurrence.
15. A borosilazane compound as claimed in claim 13, characterized
in that R is, on each occurrence, a hydrocarbon radical having from
1 to 3 carbon atoms, a C.sub.1-C.sub.20=alkyl group, a phenyl group
or a vinyl group.
16. A borosilazane compound as claimed in claim 13, characterized
in that R', R'' are each selected independently from among
C.sub.1-C.sub.3-alkyl, C.sub.1-C.sub.3-alkenyl, in particular
vinyl, and phenyl.
17. A process for preparing a borosilazane compound as claimed in
any of claims 13-16 or 46-48, comprising reacting a compound of the
formula (I) as defined in any of claims 1 to 3 with at least 4 to 8
times the molar amount of a compound of the formula (V) R'R''NH at
a temperature of from -80.degree. C. to +300.degree. C., where R'
and R'' are as defined in claim 13, 16, 47 or 48.
18. An oligoborosilazane or polyborosilazane compound obtainable by
reaction of a compound of the formula (I) as defined in any of
claims 1 to 3 or/and a compound of formula (IV) as defined in any
of claims 13-16 or 46-48 with a compound of the formula (V)
R'R''NH, where R' and R'' are as defined in claim 13-16, 47 or 48,
or by polymerization of a compound of the formula (I) or the
formula (IV), characterized in that it has the structural features
C--Si--N--B, Si--N--B--C or/and C--Si--N--B--C.
19. A process for preparing an oligoborosilazane or
polyborosilazane compound, comprising polymerizing or reacting one
or more compounds of the formula (I) as defined in any of claims 1
to 3 or/and one or more compounds of the formula (IV) as defined in
any of claims 13-16 or 46-48 with one or more compounds of the
formula (V) R'R''NH, where R' and R'' are as defined in claim 13,
16, 47 or 48.
20. The process as claimed in claim 19, characterized in that the
polymerization is carried out as a polycondensation at a
temperature of .ltoreq.200.degree. C.
21. The process as claimed in claim 20, characterized in that the
polymerization is carried out under a reduced pressure of
.ltoreq.90 kPa.
22. A polyborosilazane compound obtainable by a process as claimed
in claim 19, characterized in that the polyborosilazane compound is
melted at a temperature in the range from 50 to 300.degree. C.,
preferably from 100 to 150.degree. C., and the melt reaches a
viscosity of from 40 to 200 Pas, preferably from 90 to 120 Pas, and
a loss factor of from 10 to 500, preferably from 50 to 100, within
this temperature range.
23. A process for producing a silicon carboboronitride ceramic,
comprising tempering a borosilazane compound of the formula (IV) as
claimed in claim 18 or an oligoborosilazane or polyborosilazane
compound as claimed in claim 18 at temperatures in the range from
800.degree. C. to 1700.degree. C. in an inert atmosphere or an
amine- or/and NH.sub.3-containing atmosphere.
24. A silicon carboboronitride ceramic obtainable by a process as
claimed in claim 23, characterized in that C--Si--N--B, Si--N--B--C
or/and C--Si--N--B--C structural units are present in the ceramic
and the elements Si,N,B and C are present in an amount of more than
93% by mass.
25. A silicon carboboronitride ceramic obtainable by a process as
claimed in claim 23, characterized in that Si--N--B structural
units are present in the ceramic and the elements Si,N,B and C are
present in an amount of more than 93% by mass, in particular more
than 97% by mass, with the proviso that C.gtoreq.3% by mass.
26. A silicon carboboronitride ceramic as claimed in claim 24,
characterized in that it is an amorphous silicon carboboronitride
ceramic powder.
27. A process for producing a composite ceramic comprising
SiC,Si.sub.3N.sub.4 and BN, comprising ageing a silicon
carboboronitride ceramic as claimed in claim 24 at temperatures of
>1700.degree. C.
28. The process as claimed in claim 27, characterized in that an at
least part crystalline composite ceramic is produced.
29. A composite ceramic obtainable by a process as claimed in claim
27 by crystallization of a silicon carboboronitride ceramic as
claimed in claim 27, characterized in that SiC,Si.sub.3N.sub.4 and
BN are present in molecularly dispersed form.
30. The use of a borosilazane compound of formula IV of claim 18 or
an oligoborosilazane or polyborosilazane compound as claimed in
claim 18 comprising producing therewith ceramic fibers, ceramic
coatings, shaped ceramic bodies, ceramic sheets or/and ceramic
microstructures.
31. The use of ceramic materials as claimed in claim 24 comprising
producing therewith shaped ceramic bodies, ceramic fibers, ceramic
coatings or ceramic microstructures.
32. A ceramic fiber obtainable as claimed in claim 30,
characterized in that a polyborosilazane compound as claimed in
claim 30 is melt-spun in the monofilament or multifilament mode
under an inert atmosphere, the spun green fiber is made infusible
in situ in the spinning shaft and/or in a subsequent process step
by treatment with a reactive gas selected from among NH.sub.3,
ethylenediamine, trichlorosilane, dichlorosilane, boranedimethyl
sulfide adduct, borane-triethylamine adduct, B.sub.2H.sub.6 or by
means of electromagnetic radiation or particle radiation and the
cured green fiber is ceramicized at temperatures in the range from
800 to 1600.degree. C., preferably 1200.degree. C.
33. A ceramic fiber as claimed in claim 32, characterized in that
the melt temperature of the spinning composition is from 50 to
300.degree. C., preferably from 100 to 150.degree. C., the
capillary diameter of the spinneret is from 50 to 500 .mu.m,
preferably 300 .mu.m, at a capillary length of from 1 to 30 mm,
preferably from 5 to 10 mm, and the take-off velocity is from 150
to 1000 m/min, preferably from 300 to 600 m/min.
34. A ceramic fiber as claimed in claim 32, characterized in that
the fiber has a tensile strength of from 0.5 to 2 Gpa, preferably
1.5 Gpa, a modulus of elasticity of from 50 to 200 Gpa, preferably
150 Gpa, and an oxygen content of .ltoreq.3% by weight, preferably
.ltoreq.1% by weight.
35. The use as claimed in claim 30, characterized in that
microstructures are produced by injection molding or lithographic
processes.
36. The use as claimed in claim 30, characterized in that woven or
braided fabrics are manufactured from the ceramic fibers.
37. The use of a borosilazane compound as claimed in any of claims
13 to 16 in chemical vapor deposition (CVD) or physical vapor
deposition (PVD).
38. The process as claimed in claim 5, characterized in that Hal is
Cl on each occurrence.
39. The process as claimed in claim 5, characterized in that R is,
independently on each occurrence, a hydrocarbon radical having from
1 to 3 carbon atoms, a C.sub.1-C.sub.20-alkyl group, a phenyl group
or a vinyl group.
40. The process as claimed in claim 6, characterized in that R is,
independently on each occurrence, a hydrocarbon radical having from
1 to 3 carbon atoms, a C.sub.1-C.sub.20-alkyl group, a phenyl group
or a vinyl group.
41. The process as claimed in claim 38, characterized in that R is,
independently on each occurrence, a hydrocarbon radical having from
1 to 3 carbon atoms, a C.sub.1-C.sub.20-alkyl group, a phenyl group
or a vinyl group.
42. The process as claimed in claim 10, characterized in that Hal
is Cl.
43. The process as claimed in claim 10, characterized in that R is,
on each occurrence, a hydrocarbon radical having from 1 to 3 carbon
atoms, a C.sub.1-Cl.sub.2-alkyl group, a phenyl group or a vinyl
group.
44. The process as claimed in claim 11, characterized in that R is,
on each occurrence, a hydrocarbon radical having from 1 to 3 carbon
atoms, a C.sub.1-Cl.sub.2-alkyl group, a phenyl group or a vinyl
group.
45. The process as claimed in claim 42, characterized in that R is,
on each occurrence, a hydrocarbon radical having from 1 to 3 carbon
atoms, a Cl-Cl.sub.2-alkyl group, a phenyl group or a vinyl
group.
46. A borosilazane compound as claimed in claim 14, characterized
in that R is, on each occurrence, a hydrocarbon radical having from
1 to 3 carbon atoms, a C.sub.1-C.sub.20=alkyl group, a phenyl group
or a vinyl group.
47. A borosilazane compound as claimed in claim 14, characterized
in that R', R'' are each selected independently from among
C.sub.1-C.sub.3-alkyl, C.sub.1-C.sub.3-alkenyl, in particular
vinyl, and phenyl.
48. A borosilazane compound as claimed in claim 15, characterized
in that R', R'' are each selected independently from among
C.sub.1-C.sub.3-alkyl, C.sub.1-C.sub.3-alkenyl, in particular
vinyl, and phenyl.
49. A process for producing a silicon carboboronitride ceramic,
comprising tempering a borosilazane compound of the formula (IV) as
claimed in claim 22 or an oligoborosilazane or polyborosilazane
compound as claimed in claim 22 at temperatures in the range from
800.degree. C. to 1700.degree. C. in an inert atmosphere or an
amine- or/and NH.sub.3-containing atmosphere.
50. A silicon carboboronitride ceramic as claimed in claim 25,
characterized in that it is an amorphous silicon carboboronitride
ceramic powder.
51. A process for producing a composite ceramic comprising
SiC,Si.sub.3N.sub.4 and BN, comprising ageing a silicon
carboboronitride ceramic as claimed in claim 25 at temperatures of
>1700.degree. C.
52. A composite ceramic obtainable by a process as claimed in claim
51 by crystallization of a silicon carboboronitride ceramic as
claimed in claim 51, characterized in that SiC,Si.sub.3N.sub.4 and
BN are present in molecularly dispersed form.
53. The use of ceramic materials as claimed in claim 25 comprising
producing shaped ceramic bodies, ceramic fibers, ceramic coatings
or ceramic microstructures.
Description
[0001] The present invention relates to novel
alkylhalosilylaminoboranes, in particular
alkylchlorosilylaminoboranes, which make it possible to adjust the
viscosity of polyborosilazane compounds by varying the number of
reactive centers, to novel borosilazane compounds, to novel
oligoborosilazane or polyborosilazane compounds which comprise the
structural feature R.sup.1--Si--NH--B--R.sup.2, where R.sup.1 or
R.sup.2 or both is/are a hydrocarbon radical having from 1 to 20
carbon atoms, in particular alkyl, phenyl or vinyl groups, to
silicon carbonitride ceramic powder, to ceramic material based on
SiC, Si.sub.3N.sub.4 and BN, and to processes for the preparation
of each and to the use of the polyborosilazanes and the ceramic
materials, in particular for the production of fibers.
[0002] The production of multinary, nonoxidic ceramics via
molecular single component precursors has achieved great
importance. It makes it possible to obtain nitridic, carbidic and
carbonitridic material systems which are not obtainable via
conventional solid state reactions. The products have a high
purity, homogeneous element distribution and uniform particle
size.
[0003] Materials comprising silicon (Si), boron (B) and nitrogen
(N) and possibly also carbon (C) and at the same time contain no or
very little oxygen display particular properties in respect of the
thermal stability and the oxidation resistance. They can be used
industrially as bulk materials, in composites, for coatings or as
ceramic fibers. The boron-containing materials generally display
increased crystallization inhibition, while carbon-containing
materials additionally have a higher decomposition temperature than
do carbon-free ceramics. Owing to the high mechanical strength, the
corrosion resistance at high temperatures, the thermal shock
resistance and the high-temperature strength of such materials,
they can be used, for example, as reinforcing materials for
high-temperature composites and are employed in the automobile
industry and in the aircraft industry, for example in
turbochargers, turbines of jet engines and also for the lining of
rocket nozzles and combustion chambers.
[0004] The production of ceramics via inorganic polymers is very
promising. Crosslinking of molecular structural units gives
polymers which can be converted into ceramics by pyrolysis. This
route, which has already been followed by Chantrell and Popper,
Special Ceramics (editor: E. P. Popper), Academic Press, New York
(1964), 87-103, offers new opportunities, in particular for
carbidic and nitridic ceramics of main groups 3 and 4.
[0005] Winter, Verbeek and Mansmann (Bayer AG) (1975), U.S. Pat.
No. 3,892,583, have developed the first spinnable inorganic
polymers which have been prepared by aminolysis or ammonolysis of
methylchlorosilanes. These were able to be converted into Si/C/N
fibers by pyrolysis. The first fibers of commercial significance go
back to Yajima who converted polycarbosilanes into carbon-rich SiC
fibers (trade name: NICALON), S. Yajima, J. Hayashi, M. Omori
(1978), U.S. Pat. No. 4,100,233.
[0006] The first homogeneous ceramic in the system Si/B/N/C of the
approximate composition SiBN.sub.3C was produced by Wagner, Jansen
and Baldus, O. Wagner (1992), EP 502399. The fibers of this
material, which is prepared by aminolysis of the single-component
precursor trichloro-silylaminodichloroborane (TADB)
Cl.sub.3Si--(NH)--BCl.sub.2, have an excellent property profile, H.
P. Baldus, M. Jansen, Dr. Sporn, Science (1999) 285, p. 699.
Further improved high-temperature properties are displayed by
ceramics from the precursor TSDE
(trichlorosilyldichloroborylethane,
Cl.sub.3Si--[CH--CH.sub.3]--BCl.sub.2), M. Jansen, H. Jungermann
(Bayer AG) (1997), WO98/45302 A1.
[0007] Thus, the high-temperature stability of the ceramics appears
to improve with increasing carbon content. This is shown, for
example, by the progression of the decomposition temperatures
(under inert gas conditions) of the ceramics
Si.sub.3B.sub.3N.sub.7, SiBN.sub.3C
(=Si.sub.3B.sub.3N.sub.7C.sub.2.4) and SiBN.sub.2.5C.sub.2
(=Si.sub.3B.sub.3N.sub.7C.sub.6) which differ essentially only in
their carbon content. The thermal stability increases from
1750.degree. C. through 1900.degree. C. up to >2000.degree.
C.
[0008] The carbon and/or nitrogen content of the ceramics can be
varied by the choice of the crosslinking reagents, M. Jansen, H.
Jungermann (1997), U.S. Pat. No. 5,866,705. Thus, TADB can be
reacted not only with methylamine or ammonia but also with further
amines such as guanidine to form polymers.
[0009] In the ceramics mentioned, silicon and boron are coordinated
exclusively by nitrogen. The realization of new structural features
such as Si--C or B--C bonds in ceramics leads to improved
mechanical strength and thermal stability of a ceramic.
[0010] The Theological properties, e.g. the viscosity of a polymer
for an appropriate processing method, can be adjusted by, for
example, thermal pretreatment. However, the known polymers in the
system Si/B/N/C have the disadvantage that thermal crosslinking
increases continuously in the liquid, i.e. molten, state and the
rheological properties such as the viscosity thus do not remain
constant during processing. This causes considerable problems such
as blocking of the nozzles in the drawing of fibers. Linearly
crosslinked molecules are advantageous for achieving a very high
elongation in the drawing of fibers. The single-component
precursors mentioned have too many reactive centers which lead to
multidimensional crosslinking. A polymer consisting predominantly
of chains would be advantageous.
[0011] It is therefore an object of the invention to provide novel
organometallic precursor compounds which can be prepared in high
yields and a process for converting these precursor compounds into
nitridic ceramics which comprise Si, N, B and C and overcome the
abovementioned disadvantages. Adjustment of the number of reactive
centers, i.e. the halogen atoms, in the monomeric precursors should
make it possible to adjust the rheological properties, in
particular to adjust the viscosity, in the reaction to form
polyborosilazanes. In particular, the constancy of the rheological
properties such as the viscosity in liquid form should be improved
thereby. Furthermore, this process should make it possible to
prepare ceramics having a high proportion of carbon. In the
ceramic, the silicon should be partly coordinated by carbon.
[0012] This object is achieved according to the invention by
amorphous ceramics or nanocomposites, by their precursor compounds,
by the respective processes for producing them and by the use of
the polyborosilazanes and the ceramic materials, as disclosed in
the claims.
[0013] A first aspect of the invention is a compound of the formula
(I) R.sub.xHal.sub.3-xSi--NH--BR.sub.yHal.sub.2-y (I) where R are
each, independently of one another, a hydrocarbon radical having
from 1 to 20 carbon atoms, preferably from 1 to 10 carbon atoms,
[0014] Hal are each, independently of one another, Cl, Br or I,
[0015] x=1 or x=2 and [0016] y=0 or y=1.
[0017] The compounds of the invention are
alkylhalosilylaminoboranes which have at least one hydrocarbon
radical bound to the silicon atom. Such compounds have the
structural feature C--Si--N--B, C--Si--N--B--C or/and Si--N--B--C
and thus have carbon present in the basic skeleton. Such compounds
make it possible to produce ceramics which, owing to the increased
carbon content and the realization of new structural features such
as Si--C or B--C bonds, display improved mechanical strength and
thermal stability. The replacement of halogen radicals in the
alkylhalosilylaminoboranes of the invention by hydrocarbon
radicals, both on the Si and on the B, leads not only to the
advantageous introduction of carbon but also to a targeted
reduction in the number of reactive halogen atoms. The rheological
properties, in particular the viscosity, of the oligomers or
polymers formed from the compounds of the invention can be varied
or/and set by this means. Particularly advantageous compounds are
ones which have three hydrocarbon radicals (x+y=3) containing two
halogen atoms capable of crosslinking, which limits
multidimensional crosslinking.
[0018] In the formula I, the radicals R are each, independently of
one another, a hydrocarbon radical having from 1 to 20 carbon
atoms, preferably from 1 to 10 carbon atoms. A hydrocarbon radical
is a radical which is formed by the elements carbon and hydrogen.
According to the invention, the hydrocarbon radical can be branched
or unbranched, saturated or unsaturated. The hydrocarbon radical
can also contain aromatic groups which may in turn be substituted
by hydrocarbon radicals. Examples of preferred hydrocarbon radicals
are unbranched saturated hydrocarbon radicals such as
C.sub.1-C.sub.20-alkyl, in particular methyl, ethyl, n-propyl,
n-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl or n-decyl.
However, the radicals R can also be branched saturated hydrocarbon
radicals, in particular branched C.sub.3-C.sub.20-alkyls such as
i-propyl, i-butyl, t-butyl and further branched alkyl radicals. In
a further preferred embodiment, the radical R comprises one or more
olefinically unsaturated groups. Examples of such radicals are
vinyl, allyl, butenyl, pentenyl, hexenyl, heptenyl, octenyl,
nonenyl, decenyl, butadienyl, pentadienyl, hexadienyl, heptadienyl,
octadienyl, nonadienyl and decadienyl. The radical R can also be an
alkyne group, i.e. contain a C.ident.C bond. In a further preferred
embodiment, at least one radical R and preferably all radicals R
contains/contain an aromatic group, in particular an aromatic group
having from 5 to 10 carbon atoms, in particular 5 or 6 carbon
atoms, for instance a phenyl group or an aromatic group, in
particular a phenyl group, substituted by a hydro-carbon, in
particular a C.sub.1-C.sub.10-hydrocarbon, for instance
methylphenyl, dimethylphenyl, trimethylphenyl, ethylphenyl or
propylphenyl. Including the substituents, the aromatic radical
preferably has from 5 to 20 carbon atoms, in particular up to 10
carbon atoms. The hydrocarbon radicals R can be varied
independently of one another.
[0019] Particular preference is given to at least one radical R and
in particular all radicals R being a C.sub.1-C.sub.20-alkyl group,
a phenyl group, a vinyl group or a hydrocarbon radical having from
1 to 3 carbon atoms, in particular methyl, ethyl or propyl and most
preferably methyl.
[0020] The radical Hal is a halogen atom and is particularly
preferably Cl, Br or I, with preference being given to at least one
radical Hal and preferably all radicals Hal being Cl. Such
compounds are alkylchlorosilylaminochloroboranes.
[0021] Particularly preferred embodiments of the invention are
compounds of the formula RHal.sub.2Si--NH--BHal.sub.2, where the
radicals R and Hal are as defined above and in particular have the
meanings indicated above as preferred. These compounds contain the
structural feature C--Si--N--B and have four halogen atoms which
are reactive in oligomerization or polymerization. A particularly
preferred example of such a compound is
(methyldichlorosilylamino)dichloroborane (MADB). Preference is also
given to compounds of the formula R.sub.2HalSi--NH--BHal.sub.2.
Such compounds contain two hydrocarbon radicals on the Si atom, as
a result of which the carbon content of a ceramic produced from
such compounds can be increased further. Furthermore, such
compounds have only three halogen atoms which are reactive in
oligomerization or polymerization, which makes it possible to
achieve further variations in the rheological properties, e.g. the
viscosity, of oligomers or polymers formed therefrom. A
particularly preferred example of such compounds containing two
alkyl radicals and three halogen radicals is
(dimethylchlorosilylamino)dichloroborane (DADB). In general,
compounds of the formula (I) having the structural element
BHal.sub.2 are very preferred.
[0022] Further preferred compounds include
(vinyldichlorosilylamino)dichloroborane,
(divinylchlorosilylamino)dichloroborane,
phenyldichlorosilylamino)dichloroborane,
(diphenylchlorosilylamino)dichloroborane,
(ethyldichlorosilylamino)dichloroborane,
(diethylchlorosilylamino)dichloroborane and
(methylvinylchlorosilylamino)dichloroborane.
[0023] In addition, the invention provides compounds having the
structural feature --BRHal, so that the boron atom is bound to a
hydrocarbon radical and to a halogen. Such compounds preferably
have the formula RHal.sub.2Si--NH--BRHal or
R.sub.2HalSi--NH--BRHal. Replacement of a halogen on the boron by a
hydrocarbon radical makes it possible to form precursors having a
further-increased carbon content and a further-reduced
functionality in respect of crosslinking. Thus, compounds
RHal.sub.2Si--NH--BRHal have only three halogen atoms which are
reactive in oligomerization or polymerization and compounds
R.sub.2HalSi--NH--BRHal have only two such halogen atoms. Preferred
examples of compounds of this type are
(methyldichlorosilylamino)methylchloroborane,
(dimethylchlorosilylamino)methylchloroborane,
(phenyldichlorosilylamino)phenylchloroborane,
(diphenylchlorosilylamino)phenylchloroborane,
(vinyldichlorosilylamino)vinylchloroborane,
(divinylchlorosilylamino)vinylchloroborane and
(methylvinylchlorosilylamino)phenylchloroborane.
[0024] The novel compounds of the formula (I) can be obtained by
reacting a compound of the formula (II) R.sub.xHal.sub.3-xSi
--NH--SiR.sub.3 (II) with a compound of the formula (III)
BR.sub.yHal.sub.3-y (III) at a temperature in the range from
-100.degree. C. to +25.degree. C.
[0025] Suitable and preferred meanings of R, Hal, x and y are as
indicated above. The reaction can be carried out in an organic
solvent such as n-hexane or toluene, with, for example, the
compound of the formula (II) being added dropwise to a compound of
the formula (III) dissolved in an organic solvent. However,
preference is given to carrying out the reaction in the absence of
a solvent. The reaction temperatures are preferably at least
-90.degree. C. and very particularly preferably at least
-80.degree. C. and preferably not more than 0.degree. C.,
particularly preferably not more than -50.degree. C. Particularly
advantageous results are obtained when the reaction is carried out
at a temperature of about -78.degree. C.
[0026] The novel compounds of the formula (I) can also be obtained
by reacting a compound of the formula (II)
R.sub.xHal.sub.3-xSi--NH--SiR.sub.3 (II), where R are each,
independently of one another, a hydrocarbon radical having from 1
to 20 carbon atoms, Hal are each, independently of one another, Cl,
Br or I and
[0027] x is 1 or 2,
[0028] with a compound of the formula (III) BR.sub.yHal.sub.3-y
(III) where R and Hal are as defined above and [0029] y is 0 or 1,
in a molar ratio of from 1:1 to 1:10. The compound of the formula
(II) and the compound of the formula (III) are preferably reacted
in a molar ratio in the range from 1:1 to 1:5. Particularly
advantageous results are obtained in a process in which both the
temperature limits indicated here and the molar ratio are adhered
to.
[0030] The starting compound of the formula (II)
R.sub.xHal.sub.3-xSi--NH--SiR.sub.3 used in the preparation of the
novel compound of the formula (I) can be prepared by reacting
R.sub.xSiHal.sub.4-x and R.sub.3Si--NH--SiR.sub.3. In a preferred
embodiment, a compound of the formula (IIa)
R.sub.2HalSi--NH--SiR.sub.3 as starting compound is prepared by
reacting R.sub.2SiHal.sub.2 and R.sub.3Si--NH--SiR.sub.3 in a molar
ratio of from 1:1 to 1.5:1, preferably from 1.1:1 to 1.4:1 and
particularly preferably from 1.2:1 to 1.3:1. Particularly good
results are obtained when the two compounds R.sub.2SiHal.sub.2 and
R.sub.3Si--NH--SiR.sub.3 are used in a molar ratio of about
5:4.
[0031] The starting compound of the formula (IIa)
R.sub.2HalSi--NH--SiR.sub.3 and in particular the silane in which
R=CH.sub.3 can also be prepared by reacting R.sub.2SiHal.sub.2 and
R.sub.3Si--NH--SiR.sub.3 at a reaction temperature of from 40 to
80.degree. C., in particular up to 60.degree. C. Particularly high
yields of the starting compound of >70%, preferably >80%, can
be obtained when the reaction is carried out both in the reaction
temperature range indicated and in the molar ratio range
indicated.
[0032] The preparation of the starting compounds and the compounds
of the formula (I) will be described in detail once again below
using the preparation of the two particularly preferred compounds
MADB and DADB as examples.
[0033] Surprisingly, the reaction of
(1,1-dichlorotetramethyl)disilazane or
(chloropentamethyl)disilazane with boron trichloride made it
possible to prepare the two new compounds
(methyldichlorosilylamino)dichloroborane (MADB) and
(dimethylchlorosilylamino)dichloroborane (DADB). These compounds
both contain the structural feature C--Si--N--B. MADB has four
halogen atoms which are reactive in oligomerization or
polymerization, while DADB has only three. The choice of one of the
molecules or mixing of the two molecules in any desired ratio thus
enables the rheological properties, in particular the viscosity, of
the oligomers or polymers to be prepared to be varied. Both
molecules are subject matter of the invention.
[0034] The starting material (1,1-dichlorotetramethyl)-disilazane
can be prepared in a yield of over 80% from hexamethyldisilazane
and methyltrichlorosilane by stirring at room temperature. The
starting material (chloropentamethyl)disilazane can be prepared by
reaction of hexamethyldisilazane and dimethyldichlorosilane.
[0035] According to the invention, the preparation of
(chloropentamethyl)disilazane gives a yield of over 70% when the
ratio of the reactants Me.sub.2SiCl.sub.2 and hexamethyldisilazane
is 5:4 and the reaction temperature is from 40 to 60.degree. C.
[0036] According to the invention, the compounds MADB and DADB are
formed in yields of 80% and 70% of theory by dropwise addition of
the starting materials to boron trichloride, which may be dissolved
in an organic solvent (e.g. n-hexane, toluene). The molar ratios of
boron trichloride to the starting materials are in the range from
5:1 to 1:1. The reaction temperatures can vary from -100.degree. C.
to room temperature, and the preferred value is -78.degree. C.
[0037] Monomeric, oligomeric or polymeric borosilazane compounds
can be prepared from the novel compounds of the formula (I) by
reaction with primary or secondary amines. In such borosilazane
compounds, all or some of the halogen atoms of the compound of the
formula (I) are replaced by amino groups. Accordingly, the
invention further provides borosilazane compounds of the formula
(IV):
(R'R''N).sub.qR.sub.xHal.sub.3-x-qSi--NH--BR.sub.yHal.sub.2-y-z(NR'R'').s-
ub.z' (IV) where R' and R'' are each, independently of one another,
hydrogen or a hydrocarbon radical having from 1 to 20 carbon atoms,
[0038] R are each, independently of one another, a hydrocarbon
radical having from 1 to 20 carbon atoms, [0039] Hal are each,
independently of one another, Cl, Br or I, [0040] q=0, 1 or 2,
[0041] x=1 or 2, [0042] y=0 or 1 and [0043] z=0, 1 or 2, with the
proviso that q+z.gtoreq.1, x+q.ltoreq.3 and z.ltoreq.2.
[0044] The novel borosilazane compounds of the formula (IV) contain
at least one hydrocarbon radical which is bound to the Si atom, so
that they have the structural feature C--Si--N--B. Compounds in
which some of the halogen atoms are replaced by the amino groups
R'R''N thus contain hydrocarbon radicals, halogen and amine
radicals as substituents on the Si or B. However, preference is
given to borosilazane compounds of the formula (IV) in which all of
the halogen atoms are replaced by amino groups. Such compounds have
the formula (IVa)
(R'R''N).sub.qR.sub.xSi--NH--BR.sub.y(NR'R'').sub.z, where q+x=3
and y+z=2.
[0045] If one or more halogen atoms is/are present in the
borosilazane compound, it is preferred that Hal is Cl on at least
one occurrence and preferably on each occurrence.
[0046] Preference is also given to borosilazane compounds of the
formula [0047] (R'R''N) R Hal Si--NH--B Hal.sub.2, [0048] (R'R''N)
R.sub.2 Si--NH--B Hal.sub.2, [0049] (R'R''N).sub.2 R Si--NH--B
Hal.sub.2, [0050] R.sub.2 Hal Si--NH--B Hal (NR'R''), [0051]
R.sub.2 Hal Si--NH--B R (NR'R''), [0052] R.sub.2 Hal Si--NH--B
(NR'R'').sub.2, [0053] (R'R''N) R.sub.2 Si--NH--B Hal (NR'R''),
[0054] (R'R''N) R Hal Si--NH--B Hal (NR'R''), [0055] (R'R''N) R Hal
Si--NH--B R (NR'R''), [0056] (R'R''N) R Hal Si--NH--B
(NR'R'').sub.2, [0057] (R'R''N).sub.2 R Si--NH--B Hal (NR'R''),
[0058] (R'R''N) R Hal Si--NH--B R Hal, [0059] (R'R''N) R.sub.2
Si--NH--B R Hal or [0060] (R'R''N).sub.2 R Si--NH--B R Hal.
[0061] Particular preference is also given to borosilazane
compounds in which all halogen atoms have been replaced by amino
groups, so that they bear only hydrocarbon radicals or amino
groups. Such compounds have the formulae [0062] (R'R''N) R.sub.2
Si--NH--B (NR'R'').sub.2, [0063] (R'R''N) R.sub.2 Si--NH--B R
(NR'R''), [0064] (R'R''N).sub.2 R Si--NH--B R (NR'R'') or [0065]
(R'R''N).sub.2 R Si--NH--B (NR'R'').sub.2.
[0066] In the abovementioned formulae, the radical R has, on each
occurrence, the meanings indicated above for the compound (I) and
in particular the meanings which are indicated there as preferred.
R is particularly preferably a hydrocarbon radical having from 1 to
3 carbon atoms, in particular a methyl, ethyl or propyl radical, or
a phenyl radical or a vinyl radical.
[0067] The radicals R' and R'' are each, independently of one
another, hydrogen or a hydrocarbon radical having from 1 to 20
carbon atoms, preferably from 1 to 10 carbon atoms. Preference is
given to compounds in which at least one of the radicals R' and R''
is a hydrocarbon radical having from 1 to 20 carbon atoms. R' and
R' are particularly preferably selected from among
C.sub.1-C.sub.20-alkyl groups, in particular C.sub.1-C.sub.3-alkyl
groups such as methyl, ethyl and propyl, and phenyl and vinyl
groups.
[0068] Particular preference is given to compounds in which both R
and R' and R'' are methyl on each occurrence. The borosilazane
compounds of the invention can be prepared by reacting a compound
of the formula (I) with at least, depending on the molecule, from
four to eight times the molar amount (number of halogen atoms
multiplied by two), preferably at least ten times the molar amount,
of a compound of the formula (V) R'R''NH at a temperature of from
-80.degree. C. to +300.degree. C. This makes it possible to prepare
monomeric or oligomeric or polymeric compounds of the preferred
formulae [0069] (NR'R'').sub.2 R Si--NH--B (NR'R'').sub.2 [0070]
(NR'R'') R.sub.2 Si--NH--B (NR'R'').sub.2 or [0071] [--(NR'R'') R
Si--NH--B (NR'R'')--].sub.a, where a indicates the degree of
polymerization,
[0072] from the compounds of the formula (I), which can be used
either individually or in any mixing ratios, by reaction with
primary or/and secondary amines. Other monomeric borosilazanes and
polyborosilazanes are also possible, in particular those which have
transversely crosslinked structures. Use of the compounds MADB and
TADB described above by way of example makes it possible to obtain,
in particular, compounds of the formula
(NRR').sub.2(CH.sub.3)Si--NH--B(NRR').sub.2 or
(NRR')(CH.sub.3).sub.2Si--NH--B(NRR').sub.2 or [(NRR')
(CH.sub.3)Si--NH--B(NRR')].sub.a. In the monomeric or oligomeric
units, the first coordination sphere of each silicon atom consists
of carbon and nitrogen atoms. The novel borosilazane compounds of
the formula (IV) are preferably prepared by reacting a compound of
the formula (I) with at least, depending on the molecule, four to
eight times the molar amount (number of halogen atoms multiplied by
two), preferably at least ten times the molar amount, more
preferably at least twenty times the molar amount, of a compound of
the formula (V) R'R''NH (V) at a temperature of from -80.degree. C.
to +300.degree. C.
[0073] The monomeric or oligomeric units can be converted into
polymers by thermal treatment and/or by crosslinking using ammonia
or an amine.
[0074] The thermal treatment is preferably carried out at
temperatures of from -80.degree. C. to +500.degree. C., more
preferably up to +300.degree. C. and most preferably up to
+200.degree. C. The treatment is preferably carried out under
atmospheric pressure or under reduced pressure. However, in some
cases it can also advantageously be carried out under
superatmospheric pressure.
[0075] The invention therefore further provides an
oligo-borosilazane or polyborosilazane compound which is obtainable
by reaction of a compound of the formula (I) or/and a compound of
the formula (IV) with a compound of the formula (V) or by
polymerization of a compound of the formula (I) or the formula
(IV). Such oligo-borosilazane or polyborosilazane compounds have
the structural feature C--Si--N--B or/and Si--N--B--C. The first
coordination sphere of the silicon atoms of the oligoborosilazane
or polyborosilazane compounds of the invention preferably consists
of both carbon and nitrogen, with the silicon atoms and/or the
boron atoms bearing a radical R and the nitrogen atoms bearing a
radical R' or R''.
[0076] In particular, the oligoborosilazane or polyboro-silazane
compounds have the structural features C--Si--N--B--N--B--N--Si--C,
C--Si--N--B--N--Si--N--B or/and Si--N--B--N--Si--N--B--C. The
structural features are, in the interests of clarity, represented
as linear sequences in which Si is of course always bound to four
adjacent atoms, B and N is always bound to three adjacent atoms and
C is in each case bound to three or four adjacent atoms. The
corresponding bonds have been left out for reasons of clarity but
can readily be visualized by a person skilled in the art. Branches
can occur at any atom.
[0077] B and Si are preferably surrounded by only N or/and C.
Particular preference is given to at least one C being bound to
each B or/and Si or at least more than 50%, in particular more than
80%, of all B and/or Si. N and C can be surrounded by any atoms,
with N--N bonds preferably not being present.
[0078] The rheological properties, in particular the viscosity, of
the oligoborosilazane or polyborosilazane compounds of the
invention can be varied by the choice of the radicals R in the
compound of the formula (I) used, by the choice of the radicals R'
and R'' of the amines used and/or by the type of thermal
treatment.
[0079] According to the invention, the rheological properties, in
particular the viscosity, can be varied for the same thermal
treatment and use of the same amines by, for example, the choice of
the monomer MADB or DADB or a mixture of these molecules. The
methyl group(s) of these single-component precursors can also be
entirely or partly replaced by, for example, alkyl, phenyl or vinyl
groups so as to give a higher carbon content of the polymers.
[0080] The reaction of the monomers with the amines mentioned can
be carried out both in open systems and in closed systems. The
reaction temperatures are in the range from -78.degree. C. to
+500.degree. C., and the reaction time is from 5 minutes to 20
days. The pressure is preferably from 0.001 kPa to 5.times.10.sup.5
Pa, more preferably in the range from 0.001 kPa to atmospheric
pressure.
[0081] Amines suitable for the reaction include, for example,
methylamine, ethylamine, dimethylamine, aniline and ammonia. The
reaction can be carried out either in the pure components or in an
aprotic solvent such as hexane, toluene, THF or methylene chloride.
The reaction temperature is preferably at least -78.degree. C.,
more preferably at least -50.degree. C. and most preferably at
least -30.degree. C., and is preferably up to not more than
100.degree. C., more preferably up to not more than 5.degree.
C.
[0082] The consistency of the polyborosilazanes of the invention
thus extends, depending on the radicals R, R' and R'' and on the
degree of polymerization, from slightly viscous via resinous or
waxlike to a solid amorphous or crystalline state. Thermal
crosslinking occurs by elimination of an amine radical and
formation of new Si--N or B--N bonds. Crosslinking by means of
ammonia occurs by replacement of an NR'R'' group by an NH.sub.2
group which then crosslinks further.
[0083] The degree of crosslinking of the polyborosilazanes can thus
be set in a targeted manner via the type of polymerization, for
example polycondensation by means of thermal treatment or
crosslinking using ammonia or an amine. It has been found that, in
particular, the processing properties of the polyborosilazanes of
the invention to produce fibers can be improved further by setting
appropriate reaction parameters in the polycondensation. The
process of the invention for preparing an oligoborosilazane or
polyborosilazane compound therefore preferably encompasses at least
one process step in which a polycondensation is carried out at
temperatures of .ltoreq.200.degree. C. or/and under reduced
pressure, preferably from 0.01 kPa to 10 kPa. Under these reaction
conditions, largely "linearized" oligomers or prepolymers having
rheological properties which are particularly favorable for the
melt spinning process are formed. The curability of the green
fibers by means of reactive gases is not impaired. In addition, the
homogeneity of the polymer in respect of the element distribution
is improved further under these preferred reaction conditions.
[0084] Polyborosilazanes which are to be processed further to
produce fibers can be obtained from chlorinated precursors such as
TADB by reacting the precursors in an inert solvent such as hexane
with an amine such as methylamine. This gives, apart from insoluble
methylammonium chloride which can be filtered off, a soluble
borosilazane oligomer mixture. After distilling off the solvent,
the still liquid material can then be polycondensed thermally with
elimination of methylamine to give a product which is solid at room
temperature and is suitable for the melt spinning process. In the
case of the borosilazane oligomer mixture prepared from TADB,
temperatures of about 250.degree. C. are advantageously used for
the thermal polycondensation. In the case of borosilazane oligomer
mixtures prepared from MADB or DADB, higher temperatures of up to
500.degree. C. are often advantageous because of the smaller number
of crosslinking sites per monomer unit.
[0085] Moreover, compounds having the structural unit
.ident.Si--N(R)--B= are thermally sensitive and can easily
decompose to form monosilane and borazine derivatives. Such a
decomposition reaction with elimination of monosilazanes and
oligosilazanes can also take place during the polycondensation of
borosilazane oligomer mixtures at elevated temperatures. This
decomposition reaction often results in the occurrence of
inhomogeneities in the element distribution. The advantage of a
single-component precursor, namely, in particular, a homogeneous
element distribution in the ceramic end product, can be lost by
separation into a two-component system which can impair the
high-temperature properties of the material. For this reason,
process temperatures for the polycondensation of an oligomeric
mixture of .ltoreq.200.degree. C., preferably .ltoreq.180.degree.
C., more preferably .ltoreq.150.degree. C., and at least 50.degree.
C., more preferably .gtoreq.100.degree. C., have been found to be
advantageous. If this temperature is, in the case of the starting
materials selected, not sufficient to give a product which is solid
at room temperature, as is required, in particular, for future use
in the melt spinning process, the reaction is advantageously
carried out under reduced pressure. The pressure is then preferably
.ltoreq.90 kPa, more preferably .ltoreq.10 kPa, particularly
preferably .ltoreq.1 kPa and even more preferably .ltoreq.0.1
kPa.
[0086] Under these preferred process conditions, a polyborosilazane
having a very homogeneous element distribution is obtained.
Furthermore, it has good processability to produce green fibers and
can be cured chemically and ceramicized. A further advantage of
carrying out the process under reduced pressure is that any
monosilazanes or oligosilazanes formed can be distilled off under
reduced pressure and thus do not contaminate the product.
[0087] The desired silicon borocarbonitride ceramics can be
produced from the polyborosilazanes of the invention. Accordingly,
the invention further provides a process for producing a silicon
carboboronitride ceramic which is characterized in that a
monomeric, oligomeric or polymeric borosilazane compound as
described herein is heated in an inert or amine-containing and, if
carbon-free ceramics are desired, ammonia-containing atmosphere at
temperatures of from 800.degree. C. to 2000.degree. C., preferably
from 1000.degree. C. to 1800.degree. C. and most preferably from
1350.degree. C. to 1750.degree. C. Aminolysis or ammonolysis
reactions and subsequent pyrolysis convert the borosilazanes into a
silicon carboboronitride ceramic powder. C--Si--N--B structural
units are preferably present in the ceramic and the elements Si, N,
B and C are preferably present in an amount of more than 93% by
mass, more preferably more than 97% by mass. The silicon
carboboronitride ceramic of the invention has, in particular, a low
oxygen content of preferably <7% by mass, more preferably <1%
by mass and most preferably <0.5% by mass. The process of the
invention for producing a silicon carboboronitride ceramic makes it
possible to produce ceramics which are virtually free of oxygen.
For the reaction of the borosilazane compounds with ammonia, it is
possible to utilize all aminolysis or ammonolysis processes known
from the literature for tetrachlorosilane, for example reaction
with solid or liquid ammonia at low temperatures (U.S. Pat. No.
4,196,178), reaction with gaseous ammonia in an organic solvent
(U.S. Pat. No. 3,959,446) or reaction with ammonia in a
high-temperature reaction with elimination of hydrogen chloride
(U.S. Pat. No. 4,145,224).
[0088] The inert atmosphere can be selected from among a noble gas
atmosphere, for example an argon or helium atmosphere, a nitrogen
atmosphere and an atmosphere comprising another inert gas which
does not react with the reactants under the reaction conditions of
from 800.degree. C. to 2000.degree. C.
[0089] The ceramic yields in the pyrolysis are generally in the
range from 65% to 80%. The pyrolysis product is a ceramic material
which comprises more than 93% by mass, preferably more than 97% by
mass, of the elements Si, N, B and C and contains the structural
units C--Si--N--B, Si--N--B--C or/and Si--N--B. The ceramic
material preferably contains the structural unit C--Si--N--B--C and
in particular the structural units C--Si--N--B--N--B--N--Si--C,
C--Si--N--B--N--Si--N--B or/and Si--N--B--N--Si--N--B--C. The
structural features indicated are linear sequences in which Si is
of course always bound to four adjacent atoms, B and N are always
bound to three adjacent atoms and C is in each case bound to three
or four adjacent atoms. The corresponding bonds have been left out
in the interests of clarity but can readily be visualized by a
person skilled in the art. Branches can occur on any atom.
[0090] B and Si are preferably surrounded by only N or/and C.
Particular preference is given to at least one C being bound to
each B or/and Si or at least to more than 50%, in particular more
than 80%, of all B or/and Si. N and C can be surrounded by any
atoms, with N--N bonds preferably not being present.
[0091] In the pyrolysis, the silicon borocarbonitride ceramic of
the invention can be obtained in amorphous or at least partially
crystalline form. It is preferably an amorphous silicon
borocarbonitride. The silicon borocarbonitride ceramic of the
invention has, in particular, a high thermal stability and is inert
toward oxygen. The elements present are distributed virtually
completely homogeneously within the ceramic. Crystallization of the
amorphous material to form a composite ceramic comprising SiC,
Si.sub.3N.sub.3 and BN can be achieved by aging at a temperature of
>1700.degree. C. In such a crystalline composite ceramic, SiC,
Si.sub.3N.sub.3 and BN crystallites, preferably on a nanometer
scale, are distributed essentially fully homogeneously, i.e. are
molecularly dispersed. The ceramics of the invention display, in
particular, a high thermal stability. Apart from the amorphous
ceramics, crystalline ceramics and processes for producing them,
the present invention also provides for the use of the monomeric,
oligomeric or polymeric borosilazane compounds and the amorphous
and at least partly crystalline ceramic materials for producing
ceramic fibers, ceramic coatings, shaped ceramic bodies, ceramic
sheets or/and ceramic microstructures.
[0092] The polyborosilazanes can be processed directly or as
solutions in organic solvents to produce shaped bodies, fibers,
sheets or coatings. The Theological properties and in particular
the viscosity of the polymers can, according to the invention, be
matched to requirements by the choice of the compounds of the
formula (I) used, e.g. the ratio of MADB and DADB, and also by
appropriate choice of the parameters for crosslinking.
[0093] The shaped polyborosiloxanes can be subjected to pyrolysis
and/or physical or chemical pretreatment, e.g. curing or
crosslinking, to make the polymer infusible.
[0094] An appropriate treatment for preparing infusible
polyborosilanes is described, for example, in DE 195 30 390 A1,
where infusible compounds are obtained by reaction with
borane-amine adducts. As further reagents for making shaped
polyborosilazane bodies, preferably green polyborosilazane fibers,
infusible, it is possible to use, in particular, reactive gases
such as ammonia, gaseous ethylenediamine, trichlorosilane or
dichlorosilane and also boranes (e.g. B.sub.2H.sub.6). Hydrogen
compounds such as HSiCl.sub.3, H.sub.2SiCl.sub.2 or B.sub.2H.sub.6
are particularly suitable for making polymers containing
unsaturated side groups such as vinyl or allyl infusible by means
of hydroboration or hydrosilylation reactions.
[0095] Microstructures can be produced, for example, by injection
molding or lithographic processes. The ceramics are particularly
preferably produced in the form of fibers from which, for example,
woven or braided fabrics are manufactured. These fabrics can be
used as fillers for increasing the strength or toughness of other
ceramics.
[0096] Furthermore, the borosilazane compounds of the invention can
also be used in chemical vapor deposition (CVD) or physical vapor
deposition (PVD) . Coating of substrates by means of CVD or PVD
makes it possible to produce ceramic coatings. Vapor deposition can
be carried out as described in the prior art (cf., for example, DE
196 35 848 C1).
[0097] The invention is illustrated below by means of some
examples, without this implying any restriction:
EXAMPLE 1
Synthesis of (1,1-dichlorotetramethyl)disilazane
[0098]
Me.sub.3Si(NH)SiMe.sub.3+MeSiCl.sub.3.fwdarw.MeCl.sub.2Si(NH)SiMe.-
sub.3+Me.sub.3SiCl
[0099] In a 250 ml three-neck flask provided with an overpressure
valve and a magnetic stirrer, 178.8 g (1.20 mol, 141.2 ml) of
MeSiCl.sub.3 together with 64.4 g of hexamethyldisilazane (0.40
mol, 50.0 ml) are stirred at room temperature for two days.
[0100] The excess methyldichlorosilane and the
trimethyl-chlorosilane formed are slowly distilled off at room
temperature under a continuously decreasing pressure. Purification
is carried out by fractional distillation via a Vigreux column.
[0101] The boiling point is 39.degree. C. at p =13 mbar, and the
yield is 85% of theory.
[0102] Mass spectroscopy on (1,1-dichlorotetramethyl)-disilazane:
m/e=201 (M.sup.+), 186 (M.sup.+-CH.sub.3), 171 (M.sup.+-2
CH.sub.3), 151 (M.sup.+--Cl--CH.sub.3).
[0103] NMR spectroscopy on (1,1-dichlorotetramethyl)-disilazane:
.sup.1H NMR (C.sub.6D.sub.6): .delta.=0.03 ppm (s, 9 H); 0.47 ppm
(s, 3 H).
EXAMPLE 2
Synthesis of (chloropentamethyl)disilazane
[0104]
Me.sub.3Si(NH)SiMe.sub.3+Me.sub.2SiCl.sub.2.fwdarw.Me.sub.2ClSi(NH-
)SiMe.sub.3+Me.sub.3SiCl
[0105] In a 250 ml three-neck flask provided with an over-pressure
valve and a magnetic stirrer, 51.6 g of Me.sub.2SiCl.sub.2 (0.50
mol, 50.0 ml) together with 64.4 g (0.40 mol, 66.8 ml) of
hexamethyldisilazane are stirred at 50.degree. C. for two days. The
excess methylchlorosilane and the trimethylchlorosilane formed are
slowly distilled off at room temperature under a continuously
decreasing pressure (diaphragm pump). Purification is carried out
by distillation via a Vigreux column. The boiling point is
34.degree. C. at p=10 mbar, and the yield is 70% of theory.
[0106] MS (EI): m/e=181 (M.sup.+) , 166 (M.sup.+-CH.sub.3) , 151
(M.sup.+-2 CH.sub.3), 146 (m.sup.+-Cl). .sup.1H-NMR
(C.sub.6D.sub.6): .delta.=0.07 (s, 9H) ; 0.29 (s, 6H).
EXAMPLE 3
Synthesis of 1,1-dichloro-1-vinyltrimethyldisilazane
[0107] In a 250 ml three-neck flask, 50 ml of hexamethyl-disilazane
(38.7 g, 0.24 mol) are admixed with 70 ml of vinyltrichlorosilane
(88.9 g, 0.55 mol) and stirred overnight at room temperature. The
mixture is fractionated under reduced pressure. After
trimethyl-chlorosilane and excess vinyltrichlorosilane have been
distilled off, (1,1-dichloro-1-vinyl)trimethyl-disilazane goes over
as a clear, colorless liquid. The boiling point is 40.degree. C. at
11 mbar, and the yield is 80%.
[0108] .sup.1H-NMR spectrum: .delta.=0.18 ppm: Si(CH.sub.3);
.delta.=1. 40 ppm (broad): NH; .delta.=6.17 ppm (multiplet): vinyl.
.sup.13C-NMR spectrum: .delta.=1.8 ppm: Si(CH.sub.3).sub.3;
.delta.=134.0 ppm: CH=CH.sub.2; .delta.=137.3 ppm: CH=CH.sub.2.
[0109] The vinyl carbon signals have been assigned on the basis of
a DEPT spectrum.
[0110] IR spectrum [cm.sup.-1]: 3380 vs, 3198 w, 3145 w, 3070 m,
3022 w, 2960 vs, 2900 m, 1598 s, 1402 vs, 1251 vs, 1180 vs, 1000
vs, 960 vs, 850 vs, 770 vs, 725 vs, 690 vs, 605 sh, 575 vs.
[0111] MS: m/e=198 (2 Cl): M.sup.+-CH.sub.3; m/e=170 (2 Cl):
M.sup.+-CH.sub.3-C.sub.2H.sub.4; m/e=162: M.sup.+-CH.sub.3-HCl. The
molecular peak is not visible since very rapid elimination of a
methyl group evidently takes place. The number of Cl atoms in the
fragments was determined by means of the isotope pattern.
EXAMPLE 4
Synthesis of (methyldichlorosilylamino)dichloroborane (MADB)
[0112]
MeCl.sub.2Si(NH)SiMe.sub.3+BCl.sub.3.fwdarw.MeCl.sub.2Si(NH)BCl.su-
b.2+Me.sub.3SiCl
[0113] 23.4 g (0.02 mol, 16.36 ml) of BCl.sub.3 are placed in a 250
ml three-neck flask provided with a dropping funnel, an
overpressure valve and a magnetic stirrer at -78.degree. C. 21.2 g
(0.10 mol) of methylchlorodisilazane are added dropwise over a
period of 1 hour while stirring. The mixture is allowed to warm up
while stirring overnight.
[0114] Excess BCl.sub.3 and the trimethylchlorosilane formed are
slowly distilled off at room temperature under a continuously
decreasing pressure (diaphragm pump). The product is a colorless
oil. The crude product is purified by distillation. The boiling
point is 38.degree. C. at p=13 mbar, and the yield is about 80% of
theory.
[0115] MS (EI): m/e=194 (M.sup.+- CH.sub.3), 173 (M.sup.+- HCl) ,
158 (M.sup.+-HCl - CH.sub.3), 137 (M.sup.+-2 HCl). .sup.1H-NMR
(C.sub.6D.sub.6): 0.47 (monomer), 0.49 (dimer).
EXAMPLE 5
Synthesis of (dimethylchlorosilylamino)dichloroborane (DADB)
[0116] Me.sub.2ClSi(NH)SiMe.sub.3+BCl.sub.3
.fwdarw.Me.sub.2ClSi(NH)BCl.sub.2+Me.sub.3SiCl
[0117] 29.8 ml (0.36 mol) of BCl.sub.3 are placed in a 250 ml
three-neck flask provided with a dropping funnel, an overpressure
valve and a magnetic stirrer at -78.degree. C. 32.9 g (0.18 mol) of
methylchlorodisilazane are added dropwise over a period of 1 hour
while stirring. The mixture is allowed to warm up overnight while
stirring. Excess BCl.sub.3 and the trimethylchlorosilane formed are
slowly distilled off at room temperature under a continuously
decreasing pressure (diaphragm pump). The product is a colorless
oil. The crude product is purified by distillation. The boiling
point is 28.degree. C. at p=0.1 mbar, and the yield is about 70% of
theory.
[0118] MS (EI): m/e=174 (M.sup.+-CH.sub.3) , 154 (M.sup.+-Cl), 139
(M.sup.+-Cl-CH.sub.3). .sup.1H-NMR (C.sub.6D.sub.6): 0.16
(monomer), 0.32 (dimer).
EXAMPLE 6
Synthesis of (vinyldichlorosilylamino)dichloroborane
[0119] 80 ml of BCl.sub.3 (114.4 g, 0.98 mol) are placed in a 500
ml three-neck flask at -78.degree. C. While stirring vigorously, a
total of 80 ml (81.6 g, 0.38 mol) of
vinyldichlorosilylaminotrimethylsilane are added dropwise over a
period of 1.5 hours. The mixture is allowed to warm to room
temperature overnight and is subsequently fractionated under
reduced pressure by means of a 20 cm Vigreux column. After
trimethyl-chlorosilane and excess BCl.sub.3 have been separated
off, VADB is obtained as main fraction. The boiling point is
33.degree. C. at 12 mbar, and the yield is 65%.
[0120] The pure compound polymerizes within a few hours at room
temperature to give a white, infusible mass. It is therefore
diluted with an inert solvent (e.g. pentane) and stored at low
temperature (-20.degree. C.).
[0121] .sup.1H-NMR spectrum: .delta.=4.9 ppm (broad): NH;
.delta.=6.3 ppm (multiplet): vinyl. .sup.13C-NMR spectrum:
.delta.=130.7 ppm: CH=CH.sub.2; .delta.=139.8 ppm: CH=CH.sub.2.
[0122] The vinyl carbon signals were assigned on the basis of a
DEPT spectrum.
[0123] IR spectrum [cm.sup.-1]: 3450 w, 3360 sst, 3075 w, 3040 vw,
2995 vw, 2970 w, 1600 s, 1447 s, 1370 vs, br, 1225 s, 1118 w, 1010
s, 998 s, 990 s, 949 vs, 920 vs, 849 s, 724 vs, 622 vs, 600 vs, 551
vs.
EXAMPLE 7
Synthesis of (dimethylchlorosilylamino)phenylchloroborane
[0124] In a 250 ml three-neck flask, a solution of 20 g of
phenyldichloroborane (24.5 g, 154 mmol) in 100 ml of absolute
pentane is cooled to -78.degree. C. 20 ml of
chloropentanemethyldisilazane (18.4 g, 102 mmol) are subsequently
added dropwise. The mixture is slowly brought to room temperature
and stirred at room temperature for 12 hours. The solvent, the
trimethyl-chlorosilane formed as by-product and excess
phenyldichloroborane are subsequently taken off under reduced
pressure. This leaves (dimethylchloro-silylamino)phenylchloroborane
as a clear, yellowish liquid. Yield: 78%. The substance can also be
distilled undecomposed under reduced pressure; an increase in
temperature to .gtoreq.50.degree. C. leads to decomposition into
dimethyldichlorosilane and B-triphenylborazine. The compound is
significantly more stable toward air than are the previously known
aliphatically or olefinically substituted borosilazanes.
[0125] .sup.1H-NMR spectrum: .delta.=0.69 ppm: CH.sub.3;
.delta.=4.77 ppm (broad); NH; .delta.=7.3-7.8 ppm (AA'BB'C spin
system): phenyl. .sup.13C-NMR spectrum: .delta.=4.7 ppm: CH.sub.3;
.delta.=128.5, 132.3, 133.9 and 135.0 ppm: phenyl.
[0126] IR spectrum [cm.sup.-1]: 3370 s, 3078 m, 3052 m, 3018 m,
2965 m, 2905 w, 1600 s, 1500 s, 1440 vs, 1405 sh, 1380 vs, 1295 s,
1260 vs, 1230 s, 1180 s, 1150 sh, 1100 vw, 1074 m, 1043 m, 1005 w,
945 sh, 900 vs, 851 vs, 826 vs, 801 vs, 753 s, 700 vs, 680 sh, 635
s, 550 nm.
EXAMPLE 8
Ammonolysis of MADB
[0127] 80 ml of ammonia which has been dried over sodium are placed
in a 500 ml three-neck flask provided with a dropping funnel, an
overpressure valve and a magnetic stirrer at -78.degree. C. 15.6 g
of MADB dissolved in 100 ml of pentane are added dropwise over a
period of 2 hours while stirring. A white precipitate is formed at
the point at which the drops enter the ammonia, but this is quickly
redissolved in the large excess of ammonia. The reaction mixture is
allowed to warm to room temperature overnight while stirring.
Colorless hydrochlorides precipitate during this time. The polymer
formed is insoluble in organic solvents. The ammonium chloride
obtained as by-product is extracted with liquid ammonia. After the
extraction, the polymer remains as a colorless powder.
[0128] IR (KBr) : 3435: .nu.(N-H) , 2964: .nu..sub.as (C-H), 2903:
.nu..sub.s (C-H), 1406: .nu. (B-N), 1265: .delta..sub.s (CH.sub.3),
994: .nu. (Si-N), 779: .nu. (Si-C).
EXAMPLE 9
Ammonolysis of DADB
[0129] 80 ml of ammonia which has been dried over sodium are placed
in a 500 ml three-neck flask provided with a dropping funnel, an
overpressure valve and a magnetic stirrer at -78.degree. C. 12.6 g
of DADB dissolved in 100 ml of pentane are added dropwise over a
period of 2 hours while stirring. A white precipitate is formed at
the point at which the drops enter the ammonia, but this is quickly
redissolved in the large excess of ammonia. The reaction mixture is
allowed to warm to room temperature overnight while. stirring.
Colorless hydrochlorides precipitate during this time. The
pentane-soluble polymer is separated off from the precipitated
ammonium chloride by filtration. The precipitate is washed three
times with 20 ml each time of pentane. The pentane is distilled off
from the filtrate at room temperature under reduced pressure (10
mbar) . This leaves a highly viscous colorless liquid.
[0130] IR (KBr): 3451: .nu.(N-H), 2960: .nu..sub.as (C-H), 2900:
.nu..sub.s (C-H), 1382: .nu.(B-N), 1254: .delta..sub.s (CH.sub.3),
939: .nu. (Si-N), 783: (Si-C).
EXAMPLE 10
Aminolysis of MADB
[0131] 130 ml of methylamine which has been dried over molecular
sieves (4 .ANG.) are placed in a 500 ml three-neck flask provided
with a dropping funnel, an overpressure valve and a magnetic
stirrer at -78.degree. C. 16.9 g of MADB dissolved in 120 ml of
pentane are added dropwise over a period of 2 hours while stirring.
A white precipitate is formed at the point at which the drops enter
the methylamine, but this is quickly redissolved in the large
excess of methylamine. The reaction mixture is allowed to warm to
room temperature overnight while stirring. Colorless hydrochlorides
precipitate during this time. The pentane-soluble polymer is
separated off from the precipitated methylammonium chloride by
filtration. The precipitate is washed three times with 20 ml each
time of pentane. The pentane is distilled off from the filtrate at
room temperature under reduced pressure (10 mbar). This leaves a
highly viscous colorless liquid.
[0132] IR (KBr) : 3475: .nu.(N-H), 2955: .nu..sub.as (C-H), 2890:
.nu..sub.s (C-H), 1381: .nu. (B-N), 929: .nu. (Si-N), 755:
(Si-C).
EXAMPLE 11
Aminolysis of DADB
[0133] 130 ml of methylamine which has been dried over molecular
sieves (4 .ANG.) are placed in a 500 ml three-neck flask provided
with a dropping funnel, an overpressure valve and a magnetic
stirrer at -78.degree. C. 16.1 g of MADB dissolved in 120 ml of
pentane are added dropwise over a period of 2 hours while stirring.
A white precipitate is formed at the point at which the drops enter
methylamine, but this is quickly redissolved in the large excess of
methylamine. The reaction mixture is allowed to warm to room
temperature overnight while stirring. Colorless hydrochlorides
precipitate during this time. The pentane-soluble polymer is
separated off from the precipitated methylammonium chloride by
filtration. The precipitate is washed three times with 20 ml each
time of pentane. The pentane is distilled off from the filtrate at
room temperature under reduced pressure (10 mbar). This leaves a
highly viscous colorless liquid.
[0134] IR (KBr) : 3455: .nu.(N-H), 2961: .nu..sub.as (C-H), 2918:
.nu..sub.s (C-H), 1383: .nu. (B-N), 1261: .delta..sub.s (CH.sub.3),
825: .nu. (Si-N), 778: .nu. (Si-C).
EXAMPLE 12
Pyrolysis of a Polymer from Examples 5 to 8 to Form an Amorphous
Ceramic Powder
[0135] The polymer is placed in a boron nitride boat. It is firstly
heated at 150 K/h to 700.degree. C. in a stream of argon,
maintained at 700.degree. C. for 2 hours and cooled to room
temperature at 150 K/h. It is then heated at 300 K/h to
1500.degree. C. in a stream of nitrogen, maintained at 1400.degree.
C. for 2 hours and cooled to room temperature at 150 K/h.
[0136] The resulting ceramic consists of coarse- to fine-pored
black fragments. The X-ray powder diffraction patterns of the four
novel networks demonstrate that all samples are X-ray-amorphous
after pyrolysis.
[0137] Elemental analysis from Example 7 (percent by mass) Si:
26.3, B: 8.9, N: 36.3, C: 18.7, O: 3.9.
EXAMPLE 13
Preparation of a Borosilazane Oligomer Mixture
[0138] 500 ml of absolute pentane are cooled to -78.degree. C. and
200 ml of methylamine are subsequently condensed into the cooled
solvent. While stirring vigorously, 50 ml of MADB or DADB diluted
with 100 ml of absolute pentane are then added dropwise over a
period of one hour. After removal of the cold bath, two phases are
observed; the upper phase comprises the oligomer mixture dissolved
in pentane while the lower phase comprises methylammonium chloride
dissolved in liquid methylamine. The mixture is warmed to room
temperature overnight, resulting in evaporation of the excess
methylamine. The methylammonium chloride which has now crystallized
is filtered off under protective gas and washed with 3.times.50 ml
of absolute pentane. The solvent is taken off from the combined
filtrates under reduced pressure. This gives an oligomer mixture
which is a viscous liquid at room temperature.
EXAMPLE 14
Preparation of a Polyborosilazane
[0139] An oligomer mixture obtained by aminolysis of MADB or DADB
as described in Example 10 is heated at 50.degree./h to 200.degree.
C. under a protective gas atmosphere. After a hold time of 3 hours,
the material is allowed to cool to room temperature and the
reaction flask is connected to a distillation apparatus. The
contents of the flask are then heated at 50.degree./h to
150.degree. C. under reduced pressure and this temperature is
maintained until no more by-products go over. The temperature is
then increased to 200.degree. C. over the course of 1 hour. After a
hold time of 3 hours, the contents of the flask are again allowed
to cool to room temperature, giving a solid, brittle, colorless,
clear product.
EXAMPLE 15
[0140] Rheological Characterization of a Polyborosilazane Prepared
from MADB
[0141] On a rotation rheometer, the viscosity is firstly determined
in a temperature range from 120 to 145.degree. C. An Arrhenius plot
gives a viscosity of 100 Pas at 131.degree. C. Oscillation tests
are carried out at this temperature. The region of linear
viscoelasticity is firstly determined by variation of the
deformation amplitude .gamma. from 0.01 to 10 (this range of linear
viscoelasticity extends to .gamma.=1) and the variation of the
storage modulus G' and the loss modulus G'' on varying the
oscillation frequency from 6 to 600 Hz at .gamma.=0.1 is then
determined. During the test, G' rises from 9.6 Pa to 2480 Pa; G''
rises from 724 Pa to 60600 Pa. This gives a loss factor (tan
.delta.) of 61.2 at .omega.=10 Hz. According to R. Beyreuther and
R. Vogel, Intern. Polymer Processing XI, Hanser Publishers, Munich
1996, a loss factor of at least 10 is necessary for a readily
spinnable polymer.
EXAMPLE 16
Rheological Characterization of a Polyborosilazane Prepared from
DADB
[0142] On a rotation rheometer, the viscosity is firstly determined
in a temperature range from 70 to 100.degree. C. An Arrhenius plot
gives a viscosity of 100 Pas at 88.6.degree. C. Oscillation tests
are carried out at this temperature. The region of linear
viscoelasticity is firstly determined by variation of the
deformation amplitude .gamma. from 0.01 to 10 (this range of linear
viscoelasticity extends to .gamma.=0.7) and the variation of the
storage modulus G' and the loss modulus G'' on varying the
oscillation frequency from 0.1 to 100 Hz at .gamma.=0.1 is then
determined. During the test, G' rises from 55 Pa to 54000 Pa; G''
rises from 0.3 Pa to 150 Pa. This gives a loss factor (tan .delta.)
of 200 at .omega.=1 Hz.
EXAMPLE 17
Production of Green SiBNC Fibers from a polyborosilazane Prepared
and Characterized as Described in the above Examples
[0143] The polyborosilazane is placed in a heatable pressure
vessel, melted at 90.degree. C. and extruded through a capillary
nozzle (O300 .mu.m) under a pressure of pure nitrogen (p=4-8 bar) .
The polymer thread leaving the nozzle is wound up on a godet
(takeoff velocity=300 m/min) and plaited.
EXAMPLE 18
Curing of Green Fibers Produced as Described in Example 13 by a
Batch Method using Ammonia or Amines
[0144] The green fibers are introduced into a batch reactor in
which an atmosphere of pure NH.sub.3 or ethylenediamine-containing
N.sub.2 (EDA content in the range from 0.5 to 1.5%) is provided.
After aging for 24 hours at room temperature, the temperature of
the reactor is increased to 60.degree. C. and aging is continued
for a further 24 hours. The cured green fibers can subsequently be
pyrolyzed at 1200.degree. C. without defects (fused regions,
conglutination) occurring. The ceramic fibers typically have a
tensile strength of from 1 to 1.5 GPa, a modulus of elasticity of
from 100 to 150 GPa and an oxygen content of .ltoreq.1% by
weight.
EXAMPLE 19
In-situ Shaft Curing of Green Fibers During the Spinning
Process
[0145] The polyborosilazane is spun as described in Example 13 to
give green fibers. However, a mixture of N.sub.2 with about 1-2% of
trichlorosilane is passed into the spinning shaft during the
process. It has to be ensured by means of an extraction device that
no trichloro-silane reaches the spinning nozzle. The spun and cured
green fibers can subsequently be pyrolyzed at 1200.degree. C. The
ceramic fibers typically have a tensile strength of from 1 to 1.5
GPa, a modulus of elasticity of from 100 to 150 GPa and an oxygen
content of .ltoreq.1% by weight.
* * * * *