Silicon nitride polymorphs

This chapter describes the preparation and examination of ceramic matrix composites realized by the addition of different carbon polymorphs (carbon black nanograins, graphite micrograins, carbon fibers and carbon nanotubes) to silicon nitride matrices. In the following sections, structural, morphological and mechanical characteristics of carbon-containing silicon nitride ceramics are presented. 

Figure 4.31. A, B. The Si spectra of the two polymorphs of silicon nitride containing tetrahedral Si-N units. C. The Si MAS NMR spectrum of cubic SisN4 showing both tetrahedral and octahedral Si-N resonances in the expected tetrahedral octahedral ratio of 1 2. From Sekine et al. (2001) by permission of the copyright owner.

Silicon nitride has two major polymorphs a and P for engineering application, in addition to the amorphous and the cubic polymorph first reported in 1999. Both silicon nitride (Si3N4) and its form in solid solution (SiAlON) have been reviewed in detail " ". 

Table 7.1 Crystallographic characteristics of polymorphic modifications of silicon nitride.

Si3N4 (silicon nitride) is also produced synthetically. It exhibits allotropy. The lower-temperature a form and the higher-temperature P form are the two polymorphic forms of Si3N4. When the a form transforms to the P form, the crystals are elongated. The following routes are used for the production of silicon nitride ... 

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