Silicon carbide fibers polymer precursor processed

The first useful organosilicon preceramic polymer, a silicon carbide fiber precursor, was developed by S. Yajima and his coworkers at Tohoku University in Japan [5]. As might be expected on the basis of the 2 C/l Si ratio of the (CH3)2SiCl2 starting material used in this process, the ceramic fibers contain free carbon as well as silicon carbide. A typical analysis [5] showed a composition 1 SiC/0.78 C/0.22 Si02- (The latter is introduced in the oxidative cure step of the polycarbosilane fiber). 

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. 

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. 

The facile formation of ceramic materials from molecules has undoubtedly been one of the si ificant contributions made by chemistry to materials science (7). However, it is desirable not only to produce the ceramic per se but also to do so in a specific form, for example a fiber. Therefore, one of the key requirements for any ceramic precursor should be its processability. For this reason, there has been continued research effort aimed at the design of precursors with physical properties suitable for processing prior to pyrolysis. Two examples with sigr cant commercial application are polyacrylonitrile and polyorganosilanes, both of which may be spun into fibers, and upon pyrolysis allow for the manufacture of carbon-graphite (2) and silicon carbide (5) fibers, respectively. Despite much effort, the extension of this polymer-type precursor strategy to other ceramic systems has only met with limited success. 

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