13 - Covalent Nitrides: Properties and General Characteristics

Abstract

This chapter reveals the properties of covalent nitrides—boron nitride, aluminum nitrides, and silicon nitride—and provides a summary of the fabrication processes and applications of these compounds. The refractory covalent nitrides have remarkable properties and are industrial materials of major importance, produced on a large scale in the form of powders, monolithic shapes, and coatings. The three covalent nitrides are low-density materials with melting points, which are higher than those of their parent elements—boron, aluminum, and silicon. Of the three, boron nitride has the highest melting point and is more refractory than boron carbide. On the other hand, silicon nitride is not as refractory as silicon carbide. The thermal conductivity of the covalent nitrides decreases with increasing temperature. Thermal expansion of the covalent nitrides is low and, like that of the covalent carbides, increases with increasing temperature. This increase is not entirely linear and is slightly more rapid at high temperature. The covalent nitrides are excellent electrical insulators. Their electrons are strongly and covalently bonded to the nucleus and are not available for metallic bonding.