Thermal radiative properties of magnesium oxide at high temperatures

Abstract

Results are given, obtained from measurements of the normal-hemispherical reflectivity of pure magnesium oxide at wavelengths of 0.488; 0.6328; 1.15; 3.39 and 5,4 μm during its heating in air by CO 2 laser radiation from room temperature up to intensive evaporation temperatures and during subsequent cooling. It was found that when there is a molten layer about 100 μm thick on the surface, an optically infinite layer is realized in the melt for the entire studied spectral range, and the value of the reflectivity is determined only by the refractive index. Comparison of the MgO radiation spectra in the wavelength range of 0.4–1.5 μm, measured during heating and cooling, and the radiation spectra of a blackbody model made it possible to calculate the “effective” brightness temperatures. This information enabled the nature of the spectral and temperature dependence of the absorption coefficient of MgO in both the solid and the liquid phase to be appraised. The brightness temperatures measured by a pyrometer at wavelengths of 0.55 and 0.72 ym during the course of melting indicated a substantial change in the absorption coefficient. The possible influence of radiation scattering by MgO condensate particles near the sample on the results of measuring radiative characteristics has been considered. This influence can be very appreciable in the visible and the nearest infrared region of the spectrum. Observation of the melting and solidification plateaus enabled us to determine the melting point of the magnesium oxide more precisely, viz. 3215 ± 30 K, which is much higher than the generally accepted value.