Method of preparing silicon carbide ceramic materials, and start

Details Method of preparing silicon carbide ceramic materials, and start Method of preparing silicon carbide ceramic materials, and start

1997-07-08

1999-03-02

Green, Anthony

Compositions: ceramic

Ceramic compositions

Carbide or oxycarbide containing

528 10, 528 31, 528 33, C04B 35571

Patent

active

058771042

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The field of the present invention is that of the preparation of silicon carbide based ceramic materials from polymer precursors of ceramics. These ceramic materials are in the form of fibres, films, binders, construction elements, matrices or analogues, and are intended for use inter alia in electronics, electro-optics, semiconductor technology, and aeronautics.
Obtaining such SiC based ceramics is carried out by pyrolysis, e.g. under vacuum, under inert atmosphere or in the presence of a reactive gas, of polymer precursor compounds, which are carbon and silicon compounds.


PRIOR ART

Research and development efforts in the field of the synthesis of SiC ceramic materials lies first of all upon the simplification of the methodology and upon the lowering of the costs of the method and the starting materials.
The disposition is sought of easily handled and easily stored ceramic precursors or synthetic intermediates having controllable properties (viscosity, melting point . . . ) and which are therefore adaptable in various types of formings specific to the final applications sought (fibres, coatings, films, matrices or analogues).
In addition, obtaining a ceramic of high SiC purity is sought, i.e. a ceramic with a Si/C molar ratio=1 and which has minimal amounts of impurities of the oxygen or free carbon type.
Finally and above all, it is the improvement of the yields of pyrolysis which constitutes the principal object to be achieved in the synthesis of SiC.
The starting materials used in this synthetic technology and which are thin the context of the invention are in fact polysilanes, of which notably, lyalkylsilanes and, even more particularly, polymethylhydrogenosilanes MHS) and/or polyphenylhydrogenosilanes (PPHS).
PMHSs are traditionally prepared by condensation of ethyldichlorosilane (MeSiHCl.sub.2) and an alkali metal, such as sodium: ##STR2##
The analysis of these polymers by 1H NMR reveals that x ranges from 0.3 and 1,with x+y=1.
Such a preparation is notably described in the patent EP 0 152 704. This preparation suffers from being long (4 days), of low yield (lower than 60%) and from leading to polymer compounds of limited solubility. This patent thus discloses ceramic precursors obtained by allowing polyhydrogenosilanes of the polyphenylhydrogenosilane type to react with vinyl compounds, of the divinylbenzene type, in the presence of platinum. The polymer precursors are in fact solids and lead to low pyrolysis yields.
The patent application EP 0 217 539 also discloses a synthesis of polymethylhydrogenosilane (PMHS), which allows obtaining pasty liquid or solid products according to the conditions used. In particular, it is a question of ceramic precursors deriving from the cross-linking of PMHS in the presence of vinyl compounds and a hydrosilylation catalyst. This cross-linking is sensitive to increase the yields of pyrolysis.
An object of the patent application EP 0 382 651 is the preparation of polysilane ceramic precursors which are liquids at ambient temperature. Amongst the polysilanes concerned are copolymers with Si-vinyl and Si-H motifs with chain blockers derived from benzyl chloride or trimethylhydrogenosilane. These polymers may be mixed with Si-H motif homopolymers. The method of preparation considered also comprises a hydrolysis step of the polymer which notably increases the level of oxygen present in the polymer in a harmful way. It is known that high levels of O.sub.2 disturb the yield of the pyrolysis and the purity of the SiC sought. Moreover, this document itself does not in any way mention the viscosity of the polymers obtained.
It arises from the foregoing that the above-mentioned prior art references propose ceramic precursors obtained by treating PMHSs with cross-linking agents of the unsaturated type and, most particularly, of vinyl nature. This known treatment corresponds to a hydrosilylation, leading to bridges between the SiH functions and the unsaturated groups used. Up to now, this hydrosilylation was only envisaged in resorting imperatively to catal

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