Dynamics of Energy Transfer between Nanoscale Aluminum/Aluminum Oxide Particles and Nitrogen Gas in the Noncontinuum Regime

The dynamics of energy transfer between aluminum/aluminum oxide nanoparticles and nitrogen gas in the noncontinuum regime is investigated using a multiscale modeling framework. Density functional theory (DFT) computations are first performed to compute adsorption energies of nitrogen gas for aluminum and alpha-alumina slabs. The DFT adsorption energy data set is used to develop pair potentials for gas-surface interactions. Using the developed pair potentials, classical molecular dynamics (MD) simulations are conducted to compute the energy accommodation coefficients (EACs) for a broad range of slab temperatures, covering both thermodynamic phases of the slab. All EAC components jumped abruptly upon melting of the slab, except for the rotational EAC component of the aluminum-nitrogen system. This is attributed to the activation of low frequency lattice vibration modes and roughening of the surface of the slab upon melting. Several numerical experiments are conducted to explore the effects of the mass ratio, gas-surface well depth, and temperature on rotational EAC. A 1D collision model is also developed to explain the rotational EAC trends. It is discovered that the magnitude of rotational EAC is a function of the extent of asynchrony between the reversal of directions of rotational and translational motions of the gas molecule and phase lag between translational motions of solid atoms and gas molecules during scattering. Larger asynchrony and phase lag resulted in higher rotational EAC values. Finally, the MD-derived EACs are used as inputs to the transition regime heat transfer model to simulate the cooling of nanoalumina powder in a nitrogen gas environment. Favorable agreement with the experimental data is achieved.