Effect of Melting on Energy Accommodation Coefficients of Aluminum–Noble Gas Systems

Molecular dynamics simulations are conducted to investigate the effect of melting on the energy accommodation coefficient (EAC) of Al–noble gas systems. The accommodation coefficients are computed for a gas temperature of 3000 K and slab temperatures in the range of 600–1500 K. Three different noble gases, helium, argon, and xenon, are considered. Density functional theory (DFT)-derived gas–metal interatomic potentials are used to obtain accurate predictions of accommodation coefficients. An abrupt jump in the accommodation coefficient upon melting is observed for argon and xenon, whereas the accommodation coefficient is negligibly affected for helium gas. The effects of gas–metal potential and gas atom mass are probed separately, and it is found that the gas–metal potential has a negligible effect on the magnitude of EAC jumps. The gas atom mass, on the other hand, exerted a strong effect; heavier gases exhibited greater EAC jumps than lighter gases. The underlying physics is then unraveled by studying the effects of surface roughness and lattice dynamics on the accommodation coefficient. Surface roughness increases the tangential EACs significantly for all gases, but the normal EACs are not as strongly amplified. Analysis of the vibrational density of states of solid and liquid slabs suggests the activation of low-frequency vibrational modes upon melting. This coupled with the roughening of surface upon melting results in an abrupt jump in the accommodation coefficient, especially for heavier gases.