Thermal accommodation coefficients of inert gases on stainless steel and UO 2

Abstract

Energy transfer between gases and solid surfaces plays a vital role in many technologies. Thermal accommodation coefficient (TAC), which specifies the energy transferred between a surface composed of thermally-vibrating atoms and a colliding gas molecule, can be calculated by sampling the incident velocities of gas molecules by the Maxwell-Boltzmann (MB) distribution. However, the MB distribution describes gas behavior in free space rather than near a solid surface. In this work, we calculated the TAC between helium and graphite by a modified Maxwell-Boltzmann distribution accounting for the fast atom effect where gases of higher out-of-plane velocity have a larger chance of reaching the solid surface at the same period. A significant reduction in TAC is found by correcting the MB distribution with the modified-MB distribution, 32.7% for helium at 500 K and 10.74% at 300 K. Further analysis shows that the out-of-plane velocity component dominates the scattering process and energy transfer. This result underlines the importance and necessity to correct the MB distribution to incorporate the fast atom effect. Moreover, the lower and upper boundaries of the scattering events were depicted. These findings can provide a fundamental understanding of gas-surface energy transfer and guidelines for molecular dynamics simulations for TAC.