On the Formation of the 15R-type in the β-α Transformation of SiC

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Abstract
The transformation of β-SiC in a high purity to α-SiC has been studied experimentally in the temperature range from 2100°C to 2300°C. The results obtained are summarized as follows:1) At the initial stage of the transformation only the 6H-type was found as a newly grown phase. The 15R-type appeared always after the formation of 6H. However, the 15R thus derived disappeared with the progress of the subsequent heating. The 4H-type was not recognized throughout the present experiments.2) The 6H-type has, in the range of 2100°-2300°C, a thermal stability higher than the other basic polytypes of SiC such as 3C, 4H and 15R.3) The β-α transformation is considered to take place mainly through a vapour phase, because the activation energy associated with this change was estimated to be as high as 134±4kcal/mole. However, it should also be pointed out that a premonitory twinning is required to complete this transition unobstrusively.4) The 15R-type is considered to grow, not direct from β-SiC, but through the 6H type already formed.5) The possibility is conceived that 15R is a metastable form in the whole range of temperature. On the other hand, if a proper stable range exists for this polytype, it will be very narrow indeed, probably at about 2000°C.

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