| dc.description |
The presence of cubic β-SiC has been identified by X-ray photographs when graphite and silicon are heated together at temperatures as low as 1 150°C, and when vitreous silica is heated with graphite the carbide is formed at 1 450-1 475°C, probably by a vapour phase reaction. No matter how the starting materials may be varied in nature and in proportion, face-centred cubic carborundum is always formed, unless the temperature is in the region of the decomposition point. Near 2 000°C face-centred cubic carborundum begins to decompose, as shown by the presence of graphite lines in an X-ray photograph, and lines corresponding to the hexagonal modification II appear. Light green commercial carborundum shows faint graphite lines when heated at 2 000°C, while at 2 050°C it turns black and a large amount of macrocrystalline graphite is formed. The long soaking periods, high vapour pressure, and high temperature conditions occurring in commercial processes favour the formation of large crystals of modification II.The evaluation of the Debye-Scherrer photographs from commercial silicon carbides shows that the elementary tetrahedra which form the basis of the structures are slightly distorted, being expanded in the c direction and compressed in the direction of the a-axis. A possible wurtzite type, not so far reported in the literature, was looked for, but without success. |
