Abstract

<p>The absorption coefficient of single-crystal Al<sub>2</sub>O<sub>3</sub> (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 Al<sub>2</sub>O<sub>3</sub> 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.</p><p>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.</p>

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