MEASUREMENT OF SPECTRAL, DIRECTIONAL REFLECTIVITIES OF SOLIDS AT HIGH-TEMPERATURES BETWEEN 9 AND 11 MU-M

A new apparatus to measure spectral, directional reflectivities of sol ids for temperatures up to 1100-degrees-C is developed. Reflectivities of two ceramic materials, silicon nitride (Si3N4) and silicon carbide alpha-SiC, are measured at wavelengths between 9 and 11 mum (the oper ating range of tunable CO2 lasers) for various temperatures, angles of incidence, and for two types of polarization, the electric vector per pendicular 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 experimen tal setup is limited to relatively specular surfaces (with a collectio n half-angle of 15-degrees). The measurements show that the reflectivi ty of alpha-SiC at room temperature rises sharply near approximately 1 0.2 mum because of the presence of a 12.6-mum reflection band (restrah len band), and the occurrence of this phenomenon gradually shifts to l onger wavelengths as the temperature is raised to 1000-degrees-C. At 1 0.6 mum, where most CO3 lasers operate, the reflectivity of SiC dimini shes rapidly as the temperature is raised. Si3N4 has two restrahlen ba nds on both sides of 9.9 mum at room temperature that gradually shift to longer wavelengths with temperature. However, the decrease in refle ctivity of Si3N4 with temperature at 10.6 mum is very small.