Silicon carbide fiber precursor preparation

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. 

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. 

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. 

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. 

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. 

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. 

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... 

Smith and coworkers reported the preparation of both particles and fibers via RESS from a variety of polymers (57,68-72,80). For example, a solution of polystyrene (molecular weight about 300,000 and melting point about 170°C) in supercritical pentane was rapidly expanded at 350°C and 170 bar using a 25-p,m nozzle the result was spherical particles with an average diameter of 20 p,m (68). Other polymers, including polypropylene, poly(carbosilane) (an important precursor for silicon carbide), poly(phenylsulfone), poly(methyl methacrylate), and cellulose acetate, were processed into micrometer-sized particles via RESS in a similar fashion (68). However, when the preexpansion temperature of the supercritical pentane solution was lowered from 350°C to 200" C, polystyrene fibers (100-1000 txm in length and 1 ixm in diameter) were obtained. 

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