The structure,perfection and annealing behaviour of SiC needles grown by a VLS mechanism

Over 50 different single crystals of SiC, grown by a vapour-liquid-solid mechanism, have been examined by optical and X-ray diffraction methods to determine their structure, perfection and annealing behaviour. All the crystals studied had either the hexagonal close-packed 2H (ABAB…) structure or the cubic close-packed 3C (ABCABC…) structure or a combination of both, and invariably contained a random distribution of stacking faults on their basal planes. The thermal stability of the structures was studied by annealing them between 1300–2200 °C in an inert argon atmosphere, and reexamining the structure of the crystals after each annealing run. Structural transformations were found to commence in different crystals at different temperatures between 1400 and 1800 °C. The transformations were found to be irreversible, not martensitic and appear to involve the nucleation of the new phase by the insertion of stacking faults followed by its growth during the annealing. The mechanism of the solid-state transformations, the influence of faults and impurities, and the thermal stability of different SiC structures are discussed.