Silicon carbide precursor preparation

The potential of silicon carbide pre-ceramic compounds was recognized especially after Yajima et al. had prepared silicon carbide-based ceramic fibers [1]. The development of tailor-made silicon carbide precursor molecules has to be seen in close relation to the tremendously fast growth of the field of... 

In the field of thermostmctural composites, silicon carbide occupies a privileged place, whether it is for the production of ceramic matrices or fibers. Silicon carbide fibers prepared from organometallic precursors are the most stable at high temperature in oxidizing atmosphere. 

Soluble polydiorganosilane homo and copolymers have recently shown great potential in such areas as precursors for the preparation of silicon carbide fibers (1), as photoinitiators in alkene polymerization (2), as photoconductors (3), and as positive or negative self-developing photoresists for photolithographic applications (4). A number of copolydiorganosilane copolymers have been reported recently (5) in which the copolymer contained equal amounts of both monomers in the feed. 

Other organosilicon polymer precursors for ceramics have either been prepared or improved by means of transition metal complex-catalyzed chemistry. For instance, the Nicalon silicon carbide-based ceramic fibers are fabricated from a polycarbosilane that is produced by thermal rearrangement of poly(dimethylsilylene) [18]. The CH3(H)SiCH2 group is the major constituent of this polycarbosilane. 

Twenty-five years later, Burhard reported the preparation of permethylated. polysilane (2). These materials were, however, highly crystalline, insoluble white solids which evoked little scientific interest until recently when it was discovered that silane polymers could be used as thermal precursors to / -silicon carbide fibers (3-5). In this regard, Yajima and co-workers reported that poly (dimethyl) silane could be converted by the two-step process shown below to / -silicon carbide, a structural material of considerable industrial importance. 

As observed by D. Johnson and J. Stiegler, "Polymer-precursor routes lor fabricating ceramics offer one potential means or producing reliable, cost-effective ceramics. Pyrolysis of polymeric metalloorganic compounds can be used to produce a wide variety of ceramic materials." Silicon carbide and silicon oxycarbide fibers have been produced and sol gel methods have been used In prepare line oxide ceramic powders, such as spherical alumina, as well as porous and fully dense monolithic forms. 

Research and development in the field are still continuing at a fast pace, particulady in the area of absorption and emission characteristics of the polymers. Several reasons account for this interest. First, the intractable poly dime thylsilane [30107-43-8] was found to be a precursor to the important ceramic, silicon carbide (86—89). Secondly, a number of soluble polysilanes were prepared, which allowed these polymers to be studied in detail (90—93). As a result of studies with soluble polymers it became clear that polysilanes are unusual in their backbone CT-conjugation, which leads to some very interesting electronic properties. 

Silicon carbides are generally synthesized by the pyrolysis of precursors, prepared by liquid phase methods. One possible way for precursor synthesis is the addition of carbon black or sucrose, to a gelling silica.8 In this method, the carbon is introduced from an external source. A more intimate contact between the carbon and silicon in the precursor is assured with the use of organometallic polymer precursors. The use of silane polymers for silicon carbide production was initiated by Yajima.9,10 Polymers having a -[Si-C]- backbone are crosslinked and pyrolysed to yield SiC." In the initial work, dimethyldichlorosilane was used as a starting monomer, which was subjected to a sodium catalyzed polymerization (reaction (C)). 

Summary Polysilacarbosilanes and polysilasilazanes prepared according to a copolymer strategy offer an easy, coherent approach to polycarbosilanes and silazanes, precursors of SiC and SiCN-based materials with variable C/Si and C/Si/N ratios. In contrast with the polysilazane route which leads, upon pyrolysis, to carbon-containing silicon nitride, the synthesized polycarbosilazanes are finally converted into nitrogen-containing silicon carbide. 

For the preparation of technically important metal carbide and metal nitride materials the application of organosilicon compounds as preceramic precursors is advantageous under certain conditions [1-5]. Compared with the conventional metallurgical powder process, one benefit is the utilization of very low process temperatures for the preparation of individual ceramic materials. Another improvement is the high purity of the ceramics obtained from tailor-made preceramic precursors. Usually, after pyrolysis organosilicon compounds afford silicon-containing ceramic powders Likewise, they can also be used under certain conditions for the production of silicon carbide or silicon nitride fibers. 

This route has been used to prepare, e.g., (PhMeSi) . Crosslinked polysilane polymers useful as precursors for silicon carbide are obtained by redistribution from chlorodisilanes ... 

Polysilanes have been the first class of precursors used to prepare silicon carbide ceramics. In all cases, on pyrolysis, polysilanes are converted into polycarbosilanes by a Kumada rearrangement prior to the formation of an amorphous silicon carbide network. Several synthetic routes including dehydro-polymerization, ring-opening polymerization of strained cyclosilanes, polymerization of masked disilenes, or base catalyzed disproportionation reactions of disilanes have been described to prepare linear or branched polysilanes but despite its drawbacks the Wurtz-coupling route remains the method applied most, especially to prepare linear polysilanes. 

Summary Novel poly(silylenemethylene)s have been prepared by ring-opening polymerization of 1,3-disilacyclobutanes followed by a protodesilylation reaction with triflic acid. Reactions of the triflate derivatives with organomagnesium compounds, LiAlH4, amines or alcohols gave functional substituted and branched poly(silylene-methylene)s, which may serve as suitable precursors for silicon carbide and Si/C/N-based materials. 

In some instances materials with potentially useful properties have not been exploited until prepared as pure crystals and films. Examples of this include doped poly thi-azyl, (SN), and polyacetylene, (CH) c, which have metallic properties, including electrical conductivity. The use of polymer precursors for ceramics (e.g., silicon carbide) is another interesting solid-state preparation technique. 

Examples of polymers of the type -(M-M-) containing a bond in the main chain are known with metals or semimetals like B, Si, Ge, Sn, As, Sb, Th and Po [1]. Examples are shown in 66. Well-described are polydiorganosilicones (polysilanes) which are investigated for use as precursors of B-silicon carbide, as photoconductors, in nonlinear optics and microlithography. Preparations and properties are reviewed in [1,42,271,272]. 

H.-J. Wu and L. V Interrante (1989), Preparation of a Polymeric Precursor to Silicon Carbide via Ring-Opening Polymerization Syntheses Poly[(methylchlorosilylene)-methylene] and Poly(sila-propylene), Chem. Mater. 1, 564. 

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