THERMAL RADIATION CHARACTERISTICS OF TRANSPARENT, SEMI-TRANSPARENT AND TRANSLUCENT MATERIALS UNDER NON-ISOTHERMAL CONDITIONS
Abstract
Thermal radiation from refractory oxide materials is a volume, rather than a surface, phenomenon. General equations describing emissivity, transmissivity, and reflectivity for scattering and absorbing systems have been derived in a form that allows determination of the necessary material constants and calculation of emissivity under isothermal and nonisothermal conditions. Methods of evaluating conditions under which these equations are applicable have been derived, and a simple expression developed for radiant energy transfer when conduction in the solid is negligible relative to the radiant energy transfer in the solid. Experimental apparatus has been developed and measurements of the isothermal emissivity of several refractory oxides and the transmissivity of single crystals have been made over the wavelength range 1-15 microns at temperatures up to 1350 C. Emissivity was found to depend strongly on composition, microstructure and wavelength. Different grades of alumina ceramics range from about 0.2 to above 0.9 in the near infrared. Diffuse transmissivity measurements provided experimental values from which scattering was calculated for each sample. Large scattering values were obtained, however, emissivities calculated from these measurements agreed well with experimentally measured data. Results show that the normal emissivity in these systems is substantially larger than the hemispherical emissivity in contrast to opaque solids where they are nearly the same.
Document Details
- Document Type
- Technical Report
- Publication Date
- Apr 01, 1964
- Accession Number
- AD0600370
Entities
People
- Robert C. Folweiler