Silicon carbide overview

To set the stage for a discussion of our own efforts, it is educational to briefly discuss several examples of next generation precursors and to provide an overview of silicon carbide preceramics. 

Boron-containing nonoxide amorphous or crystalline advanced ceramics, including boron nitride (BN), boron carbide (B4C), boron carbonitride (B/C/N), and boron silicon carbonitride Si/B/C/N, can be prepared via the preceramic polymers route called the polymer-derived ceramics (PDCs) route, using convenient thermal and chemical processes. Because the preparation of BN has been the most in demand and widespread boron-based material during the past two decades, this chapter provides an overview of the conversion of boron- and nitrogen-containing polymers into advanced BN materials. 

This section presents a brief overview of a few other compounds that have not been described in previous sections. Because it can function as a nonmetal, silicon forms sihcides with several metals. These materials are often considered as alloys in which the metal and silicon atoms surround each other in a pattern that may lead to unusual stoichiometry. Examples of this type are Mo3Si and TiSi2. In some sihcides, the Si-Si distance is about 235 pm, a distance that is quite close to the value of 234 pm found in the diamond-type structure of elemental silicon. This indicates that the structure contains Si22-, and CaSi2 is a compound of this type. This compound is analogous to calcium carbide, CaC2 (actually an acetylide that contains C22- ions (see Chapter 10)). 

What type of compounds will be covered in this overview. Strictly, only compounds which have at least one metal-carbon bond (with the exception of metal carbides) are called organometaiiic. However, this definition was not adopted in this chapter because many families of compounds that are relevant in organometaiiic chemistry would not be included (e.g., alkoxides). By metal we mean any element from groups 1 (except hydrogen), 2, 3 (including lanthanides and actinides), 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 (except boron), 14 (except carbon and silicon), and 15 (antimony and bismuth only). Most of the available thermochemical data for all these species are freely available in a single on-line database the NIST Chemistry WebBook Unless stated otherwise, all the data included in this chapter were quoted from that reference. 

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