Preceramic polymers, requirements

A variety of other ceramics are prepared by pyrolysis of preceramic polymers.32,38 Some examples are silicon carbide, SC, tungsten carbide, WC, aluminum nitride, AIN, and titanium nitride, TiN. In some cases, these materials are obtained by simple pyrolysis in an inert atmosphere or under vacuum. In other cases a reactive atmosphere such as ammonia is needed to introduce some of the atoms required in the final product. Additional details are given in Chapter 9. 

Thus the chemistry leading to the desired ceramic product is quite satisfactory most of the requirements mentioned earlier are met. Initial evaluation of the polysilazane shows that it has promise in three of the main potential applications of preceramic polymers in the preparation of ceramic fibers and ceramic coatings and as a binder for ceramic powders. 

Due to their three-dimensional architecture, additional cross-linking of poly(silsesquiazane)s prior to thermolysis is not required. Nevertheless, there are two drawbacks that limit their applicability as preceramic polymers. First, the difficult workup, that is, removal of the couple product NH4CI from the polymer is complicated and time intensive. Second, polysilsesquiazanes are difficult to... 

By preceramic polymers, several joints were obtained through PIP, RS, and new NITE processes The NITE SiC/SiC composites were produced to demonstrate high-performance hot-pressed joining and its tensile strengths is > 200 MPa however, the joining process requires... 

The following discussion covers the chemical synthesis of ceramics derived from organometallic polymers BN, AIN, TiN, TiC, and TiBa. It should be emphasized here that some of the syntheses involve starting materials, monomers, and intermediates, as well as polymers, that are oxidatively unstable and/or susceptible to hydrolysis. These syntheses therefore generally require inert atmospheres and the extensive use of vacuum (Schlenk type) line or dry-box techniques. This makes it obvious that collaborations between synthetic chemists and materials and ceramic scientists and engineers is important. Here we outline a selected number of synthetic routes to preceramic polymers. 

The preparation of preceramic material involves processing a high purity, easily spinnable material, usually a preceramic polymer or sol. In either case, specialized equipment is required to produce the material because there is no other market for these exaet materials (although there are some close relatives). The high level of purity, the delicate balance of material properties, and the dedicated, customized equipment make preparation an expensive process. 

For the production of preceramic slurries, fillers in the submicron range have to be used to achieve homogeneous infiltration of a fiber bundle with several thousands of filaments. This requires detailed knowledge of the rheological behavior of the powder-filled dissolved polymers. Additives are necessary to achieve high filler contents and good rheological behavior. 

The condensation of secondary silanes requires more rigorous conditions and no catalyst has yet been reported to couple tertiary silanes efficiently [140]. The great interest in polysilane polymers and their potential application come from their unusual electronic, optical, and chemical properties [142], particularly as preceramic materials [143]. The reaction constitutes the only alternative to Wurtz coupling for the formation of silicon-silicon bonds. 

Another reason preceramics do not meet the above criteria is that inexpensive syndietic routes to the most desirable precursors, especially for the production of kilogram quantities, are not readily available. Thus, polymer architectural demands, curing requirements, and/or synthetic difficulties often result in preceramics with good processing characteristics but less than optimal elemental stoichiometries. 

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