The absorption coefficient of single-crystal Al2O3 (sapphire) has been found to increase at most wavelengths from 0.56 to 6 μ as the temperature is increased up to 2020°C. As the temperature is increased further until the material melts, the absorption coefficient appears to increase discontinuously by a factor of about 30 or 40. The emissivity of some ceramic material which consists of more than 99% polycrystalline Al2O3 is greater than that of sapphire at the same temperature, but shows the same general dependence on temperature as it is heated up through the melting point.
The index of refraction of sapphire at wavelengths less than 4 μ was found to increase 0.05 (+0.01, −0.03) as the temperature increases from ambient to 1700°C.
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