Temperature induced structural changes in sapphire whiskers
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The effect of high temperature annealing (800 to 1500° C) on the structure of individual sapphire whiskers has been determined with an electron microscope technique. Two types of whiskers, grown by a similar process, but containing different levels of silicon impurity (6 and 0.2% respectively) were studied. Discrete second phase particles were observed within and at the surface of many of the whiskers with 6% silicon. After heat treatments at 1000 to 1300° C in high purity argon, these particles coarsened and coalesced in the larger whiskers and spheroidised on the surface of the smaller whiskers. In addition, a dispersion of fine particles was formed in some whiskers free from “grown in” particles.
Some melting of the second phase occurred between 1000 and 1400° C, with an attendant disintegration of the whiskers. Although the sapphire whiskers with 0.2 silicon % did not contain second particles, some breakdown of the whiskers also occurred at about 1300° C, a process which is attributed to the presence, and melting, of a surface coating.
Surface pits were formed at temperatures above 1000° C, and became extensive at 1400 to 1500° C, particularly in the 6% silicon whiskers. It is considered that the surface pitting is a consequence of impurity diffusion and internal stress in the sapphire whiskers.
KeywordsSilicon Heat Treatment Sapphire Fine Particle Surface Coating
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