THE CONVERSION OF CUBIC TO HEXAGONAL SILICON CARBIDE AS A FUNCTION OF TEMPERATURE AND PRESSURE
Physical sciences research papers no. 336
AIR FORCE CAMBRIDGE RESEARCH LABS HANSCOM AFB MA
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Polytypism in silicon carbide is reviewed. The geometric properties of close-packed hexagonal structures are illustrated by models of the stacking arrangements in the more common polytypes. The inadequacies of a number of theories on the origin of polytypes are briefly discussed, and it is concluded that no satisfactory theory exists. From the results of recent AFCRL experiments in the temperature range of 2100 to 2800 deg C, it is concluded that the transformation of beta to alpha silicon carbide can be suppressed by 20 atm of inert gas, and consequently that the alpha silicon carbide is a defect structure probably involving deficiencies in the carbon sublattice. It is further concluded that both polytypism and the beta-alpha transformation are impurity-dominated and that the most significant impurities are boron and nitrogen. Speculating on the origin of polytypes, it appears that any adequate theory must include the influence of chemical and physical imperfections, temperature, and ambient gas composition and pressure.
- Ceramics, Refractories and Glass
- Solid State Physics