Preceramic materials

Object formed from a preceramic material prior to pyrolysis. 

In jiggering, machines press the preceramic material into a rotating mold of desired shape. Dinnerware products are often made using jiggering. 

Abrasives and insulators are formed from simply pressing the preceramic material into a mold of desired shape. In extrusion, the preceramic material is forced through an opening in a shaping tool. Bricks and drainpipes are formed using extrusion. 

Borazines, particularly polymeric compounds, have been extensively investigated as preceramic materials from which coatings and fibers of boron nitride can be produced upon thermolysis. B-aryl and halogeno-amino borazines are reported to have use as fire retardants in cotton and nylon textiles. Other reported uses for borazines are as epoxy resin catalysts, polymerization inhibitors of unsaturated alcohols and esters, and catalysts for polymerization of alkenes (95). 

The above thermal analysis studies demonstrated the enhanced thermal stability of POSS materials, and suggested that there is potential to improve the flammability properties of polymers when compounded with these macromers. In a typical example of their application as flame retardants, a U.S. patent39 described the use of preceramic materials, namely, polycarbosilanes (PCS), polysilanes (PS), polysilsesquioxane (PSS) resins, and POSS (structures are shown in Figure 8.6) to improve the flammability properties of thermoplastic polymers such as, polypropylene and thermoplastic elastomers such as Kraton (polystyrene-polybutadiene-polystyrene, SBS) and Pebax (polyether block-polyamide copolymer). 

The TGA curves of the graft polymer are different as well. The graft polymer showed a small weight loss between 100 and 200 °C, which began at around 100 °C. This small initial weight loss occurred only at higher temperatures (beginning at —175 °C) in the case of the in situ polymer. This difference in initial thermal stability could well have chemical consequences of importance with respect to ceramics, and both kinds of polymers may be useful as preceramic materials. 

Keywords Chlorinated Polycarbosilanes / Hydrido-Polycarbosilanes / Dehydrogenation / Preceramic Materials / Ceramic Materials... 

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. 

There are three basic steps in the manufacture of high performance, continuous ceramic fibers, all of which have important cost considerations (1) preparation of bulk, preceramic material to be spun (2) spinning the bulk material into a green fiber and (3) heat treating the spun green fiber to convert it into ceramic fiber. 

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. 

Silicone polymers can also be used in lithography, for drug delivery, in the preparation of polymer dispersed liquid crystals, and as optical and silicon o -carbide preceramic materials. 

The poly(B-vinyllborazine) polymers satisfy a number of the criteria previously identified for preceramic materials and their use as precursors to boron nitride was investigated. It was found that depending upon the polymer and pyrolysis conditions, a variety of ceramic materials may be produced, ranging from black, high carbon content materials to white h-BN. In each case, however, the polymer/ceramic conversion was found to take place with both high ceramic and chemical yields and give materials with B/N ratios of 1.0. 

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