Scheme 19. The alkyl chlorosilanes used as building blocks in the synthesis of organometallic precursors.

The versatility of this class of polyceramic precursors originates from the facile chemical (cross-linking, dechlorina-tion) reaction producing, sometimes by thermal treatments, tractable, soluble, and fusible precursor compounds. These stable coordinations are not always directly accessible. For example, the chlorosilanes SiHCl3, SiH2Cl2, and SiH3Cl are stable compounds and do not split off HCl, whereas the corresponding aminosilane SiCl(NH2)3 would decompose into NH4Cl and solid diimide Si(NH2)2, which seems to be an attractive precursor from its chemical composition, but is unsuitable because of its physical properties. Similarly, the tetra-amide Si(NH2)4 is an unstable compound. Although polymeric precursors for Si3N4 with the ideal atomic ratio Si:N:C = 3:4:0 cannot be directly prepared, various polysi-lazanes have been shown to convert to pure Si3N4 [505-508].

This route offers the possibility of improving the compositional homogeneity and tailoring the composition and molecular structure of the ceramic powders. Furthermore, the solubility and rheology of polymeric precursors provide potential processing routes to binders, to sintering aids, and to the formation of nanosized thin films and fibers. In addition, this methodology produces nanosized powders, which are often difficult or impossible to achieve by the more traditional ceramic processing techniques. For example, a single class of preceramic polymer, polysilazanes, has been shown to convert to either pure Si3N4 or a combination of Si3N4 and SiC, depending on the chemistry of the polymer and pyrolysis conditions [509]. Polysilazanes are silicon compounds based on an alternating Si-N backbone that may or may not contain organic substitutes (Scheme 20). The amount of carbon present in the polymer is decisive for the composition of the ceramic material. Minimizing the amount of carbon results in a ceramic with a stoichiometry closer to Si3N4, whereas the Si3N4/SiC composite is formed in the case of carbon-rich precursors [509]. In contrast to the sintering step applied for conventional ceramic powder processing, the polymeric precursors are solidified by pyrolysis, whereby the key concern is the ceramic yield determined by thermogravimetric analysis and corroborated by mass spectral studies of the pyrolysis products.

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