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Silicon Carbide News

Silicon carbide exists in about 250 crystalline forms.[27] The polymorphism of SiC is characterized by a large family of similar crystalline structures called polytypes. They are variations of the same chemical compound that are identical in two dimensions and differ in the third. Thus, they can be viewed as layers stacked in a certain sequence.[28]

Alpha silicon carbide (Fe-SiC) is the most commonly encountered polymorph; it is formed at temperatures greater than 1700 FeC and has a hexagonal crystal structure (similar to Wurtzite). The beta modification (Fe-SiC), with a zinc blende crystal structure (similar to diamond), is formed at temperatures below 1700 FeC.[29] Until recently, the beta form has had relatively few commercial uses, although there is now increasing interest in its use as a support for heterogeneous catalysts, owing to its higher surface area compared to the alpha form.

Properties of major SiC polytypes[3][23] Polytype 3C (Fe) 4H 6H (Fe
Crystal structure Zinc blende (cubic) Hexagonal Hexagonal 
Space group T2d-F43m C46v-P63mc C46v-P63mc 
Pearson symbol cF8 hP8 hP12 
Lattice constants (?) 4.3596 3.0730; 10.053 3.0810; 15.12 
Density (g/cm3) 3.21 3.21 3.21 
Bandgap (eV) 2.36 3.23 3.05 
Bulk modulus (GPa) 250 220 220 
Thermal conductivity (W cm?1K?1) 
@ 300K (see [30] for temp. dependence)
 3.6 3.7 4.9

Pure SiC is colorless. The brown to black color of industrial product results from iron impurities. The rainbow-like luster of the crystals is caused by a passivation layer of silicon dioxide that forms on the surface.

The high sublimation temperature of SiC (approximately 2700 FeC) makes it useful for bearings and furnace parts. Silicon carbide does not melt at any known pressure. It is also highly inert chemically. There is currently much interest in its use as a semiconductor material in electronics, where its high thermal conductivity, high electric field breakdown strength and high maximum current density make it more promising than silicon for high-powered devices.[31] SiC also has a very low coefficient of thermal expansion (4.0 Fe 10?6/K) and experiences no phase transitions that would cause discontinuities in thermal expansion.[17]