Hexagonal SiC

As can be seen in figure 9.1 the reaction mixture is covered up to intercept and remove the formed carbon monoxide. Carbon monoxide is combustible and consequently can be used as a fuel. The produced SiC is extremely hard and we distinguish two qualities i.e. a) hexagonal SiC, which is used as an abrasive and is a raw material in the ceramic industry and b) cubic B-SiC, which is used as a deoxidator and carbon supplier in the cast iron and steel industry. When the reaction... 

Figure 6.34. The 4-2PT structure of hexagonal SiC with an ABAC sequence of layers.

SiC(a), also called hexagonal II or 6H, is one of the more common of many hexagonal forms which arise from various possible stacking sequences of the hexagonal SiC layers (1 ). The properties of these phases are so similar that they have not been adequately differentiated thermodynamically. It has frequently been assumed that cubic SiC(B) transforms to alpha at about 2300 K, but this seems unlikely since both phases have been prepared over temperature ranges of 1700-3000 K (1 4). Heat of formation and equilibrium data Indicate that alpha is less stable up to 2000 K. The adopted functions suggest that this is the case at all temperatures however, the stability difference is small. 

Figure 9.14. A. C MAS NMR spectra of hexagonal SiC polytypes, from Hartman et al. (1987). The spectrum of the cubic (zincblende) structure was not detected by these authors under a wide range of conditions. B. Angultu" dependence of the C NMR lines of single-crystal 6H polytype of SiC. Curve (a) corresponds to the resonance at 21.9 ppm in this crystal, curve (b) corresponds to the resonance at 17.2 ppm and curve (c) to the resonance at 25.4 ppm. From Richardson etal. (1992). Both figures used by permission of the American Chemical Society.

Figure 3.12. Structure of hexagonal SiC which crystallizes in the wurtzite structure.

This Datareview describes recent progress in bipolar junction transistors, thyristors and random access memories made in SiC. The first two device types have been made in both cubic and hexagonal SiC polytypes. The first random access memory made in hexagonal SiC will be described. 

As mentioned, the sinterability of pure silicon carbide strongly depends on the properties of the powder used. Powders made by high-temperature routes are predominantly composed of different hexagonal SiC polytypes (e.g., 2H, 4H, and 6H). In contrast, SiC produced by polymeric routes almost exclusively consists of p-SiC. In principle, SiC can be formed by the pyrolysis of either polysilanes or polycarbosilanes. 

Experimentally, a phase transition to a rocksalt-type structure was observed in 3C SiC under a static pressure of 100 GPa by X-ray diffraction (XRD) studies [221]. This transformation was followed by an abrupt volume reduction of 20%. Among the hexagonal SiC polytypes, only 6H has received extensive attention in the high-pressure studies. In the same high-pressure XRD studies found in Reference [221], 6H SiC was found to be stable to 95 GPa with emergence of extra peaks attributed to the premonition of a phase transition at this pressure. Shock compression data on 6H SiC indicate a phase transition to a sixfold-coordinated phase (most likely rocksalt), which starts around 100 GPa [222,223] and completes at 137 GPa [223]. 

Ruffino Francesco, and Grimaldi Maria Gratia. Island-to-percolation transition during the room-temperature growth of sputtered nanoscale Pd films on hexagonal SiC. J. App. Phys. 107 (2010) 074301-1-074301-6. 

Zinovev, A.V. et al.. Etching of hexagonal SiC surfaces in chlorine-containing gas media at ambient pressure. Surface Science, 2006. 600(11) 2242-2251. 

Hexagonal SiC yields bond lengths of dg = 187.5 pm for the six-membered rings in chair conformation (parallel to (001)), while the bond length of the ideal boat-oriented six-membered rings comes to d = 193.0 pm. In connection with this, the fact is noteworthy that the sum of the bond orders is exactly Z n = 4 for the atoms in hexagonal SiC (3 x 1.035 + 1 x 0.895 with dj = 188.8 189 pm). 

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