A new apparatus to measure spectral directional reflectivities of solids for temperatures up to 1100 °C is developed. Reflectivities of two ceramic materials, silicon nitride (Si3N4) and silicon carbide α-SiC, are measured at wavelengths between 9 and 11 μm (the operating range of tunable CO2 lasers) for various temperatures, angles of incidence, and for two types of polarization, the electric vector perpendicular and parallel to the plane of incidence. Reflectivities are measured by comparing the power of the beam reflected from the sample (heated in the furnace) with that of the incident beam. This experimental setup is limited to relatively specular surfaces (with a collection half-angle of 15°). The measurements show that the reflectivity of α-SiC at room temperature rises sharply near ∼10.2 μm because of the presence of a 12.6-μm reflection band (restrahlen band), and the occurrence of this phenomenon gradually shifts to longer wavelengths as the temperature is raised to 1000 °C. At 10.6 μm, where most CO3 lasers operate, the reflectivity of SiC diminishes rapidly as the temperature is raised. Si3N4 has two restrahlen bands on both sides of 9.9 μm at room temperature that gradually shift to longer wavelengths with temperature. However, the decrease in reflectivity of Si3N4 with temperature at 10.6 μm is very small.
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