Structure of Covalent Ceramics

The building block of silicon-based covalent ceramics, which include among others the silicates (dealt with separately in the next section) SiC and Si3N4, is in all cases the Si tetrahedron Si04 in the case of silicates, SiC4 for SiC, and SiN4 for Si3N4. The reasons Si bonds tetrahedrally were discussed in the last chapter. 

Si3N4 exists in two polymorphs a and (3. The structure of the (3 polymorph is shown in Fig. 3.11, where a fraction of the nitrogen atoms are linked to two silicons and others to three silicons. The structure of SiC also exists in many polymorphs, the simplest of which is cubic SiC, which has the zinc blende structure and is shown in Fig. 3.12 and 3.1c. 

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