Silicon Oxide Nitride and Carbide

Because of the covalent character of the chemical bonds, the nonoxide amorphous and nanocrystalline refractory ceramics exhibit a very high (~3000 °C) melting point, low mobility of atoms, low plasticity, and high hardness at temperatures up to 2000 °C. Therefore these materials can substitute for metals, alloys, and intermetallics in many high-temperature engineering, chemical, and electronic applications. The synthesis of silicon-containing ceramics (oxide, carbide, nitride, etc.) is an active research field in its own right, and, since the early investigations of organometallic oilgomers and polymers in the 1970s [501], numerous new organometallic compounds have been developed as precursors to all kinds of nonoxide ceramics like SiC, Si3N4, AlN, BN, B4C, TiC, and TiN.

The low-temperature synthesis (without sintering aid) of two-and three-component silicon-containing ceramics that are homogeneous on an atomic scale is possible only through organometallic precursors [502]. The most salient application of organometallic precursors is in the liquid phase synthesis of silicon nitride and carbide ceramics, which are of significant technological interest because of their high strength and thermal and chemical stability. The use of molecular precursors for refractory materials affords potential advantages in terms of controlling the composition, deposition temperature, and microstructure of the deposited product [30]. The need to control these factors is critical to achieve the extraordinary properties of the above materials, which are sensitive to local variations in composition and/or microstructure. The choice of organometallic molecules for use as SSPs has been largely an empirical process, with few guidelines available relating to the molecular structure of the precursor and the chemical composition of the final material. In the preparation of silicon preceramic polymers, compounds containing Si-Si, Si-H, Si-N, Si-C, and Si-Cl bonds or molecular units based on these linkages are used as precursor species [503] that are derived from different alkyl chlorosilanes (Scheme 19). Since most of the precursor compounds are viscous or oily, their structural characterization is generally based on NMR and mass spectral data [504].

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