Thermal accommodation coefficients by high speed vibration of solid samples. IV. Determination of translational and internal energy accommodation coefficients

Abstract

A method has been developed to measure both translational and internal energy accommodation in gas–surface collisions from the surface temperature rise which occurs upon ultrasonic oscillation of a solid sample in a rarefied test gas. Thin, flat samples are vibrated normal to their exposed faces at rms velocities of ∼3×104 mm sec−1. About 35% more gas–surface collisions occur when the sample moves into the gas than when it recedes, so that, relative to the surface, incident gas molecules have more translational energy than do surface-equilibrated molecules. Gas–surface energy transfer causes the surface temperature to rise. A steady-state surface temperature is attained due to a balance between the translational energy transferred to the surface by the gas with thermal emission, conduction, and energy transfer involving gas internal modes. Incident gas molecules are cooler internally than the heated surface because gas internal energies are independent of the surface motion. Analysis of the energy balance enables a determination of both translational γt and internal γi energy accommodation coefficients from measured steady-state temperature rises as a function of gas pressure. For the diatomics N2 and O2, γi=γrot. Values of γrot for N2 on Ni, Cu, and Pt are, respectively: 0.09, 0.04, and 0.05. The values for O2 on the same surfaces are: 0.06, 0.05, and 0.08. For the polyatomics—CO2, C2H2, and C6H6−on Cu, the values of γi are 0.45, 0.54, and 0.07. The implications of the results and the experimental method are discussed.