THERMAL INFRARED DIRECTIONAL EMISSIVITY OF POWDERED QUARTZ

Thermal infrared directional emissivity of quartz was measured for sev eral particle size ranges and packing fractions. The samples measured were (1) a polished quartz slab; (2) a 75-250 mu m powdered quartz sam ple, cleansed of clinging fines; (3) a 0-75 mu m powdered quartz sampl e, sifted into a fairy castle structure; and (4) the same sample compr essed to minimize porosity. The spectra of the particulate samples sho wed a strong dependence on exitance angle, particle size and packing f raction. In addition, thermal gradient effects significantly affected the measured emissivity of the fine, sifted sample. The measured direc tional emissivity was modeled by first using Mie theory to calculate s ingle-particle scattering properties of a quartz spheres of appropriat e size at a single wavelength and radiative transfer theory to calcula te the flux reflected from an optically thick, plane-parallel ''atmosp here'' composed of particles with these scattering properties. For the 75-250 mu m sample, close packing of individual particles was account ed for by subtracting the diffraction contribution to the scattering c ross section. These calculations are repeated wavelength-by-wavelength to determine the spectral directional hemispherical reflectance of th e quartz sample. Kirchhoff's law was then used to obtain spectral dire ctional emissivity. This model, which uses optical constants derived f rom widely used oscillator parameters for quartz, reproduces the direc tional emissivity spectrum of the large powdered quartz to better than 10%, However, model calculations for 0-75 mu m particle size quartz w ere less successful. This could be due to the several approximations u sed in the model, or to a possible error in the oscillator parameters. More successful calculations based on different optical constants sug gest that the widely used oscillator parameters may well be wrong.