Oxidation of aluminum nanoclusters

Abstract

The dynamics of oxidation of aluminum nanoclusters ($20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ diameter) is investigated using a parallel molecular dynamics approach based on variable charge interatomic interactions due to Streitz and Mintmire that include both ionic and covalent effects. Simulations are performed for both canonical ensembles for molecular oxygen $({\mathrm{O}}_{2})$ environments and microcanonical ensembles for molecular $({\mathrm{O}}_{2})$ and atomic $({\mathrm{O}}_{1})$ oxygen environments. Structural and dynamic correlations in the oxide region are calculated, as well as the evolution of charges, surface oxide thickness, diffusivities of atoms, and local stresses. In the microcanonical ensemble, the oxidizing reaction becomes explosive in both molecular and atomic oxygen environments due to the enormous energy release associated with $\mathrm{Al}\text{\ensuremath{-}}\mathrm{O}$ bonding. Local stresses in the oxide scale cause rapid diffusion of aluminum and oxygen atoms. Analyses of the oxide scale reveal significant charge transfer and a variation of local structures from the metal-oxide interface to the oxide-environment interface. In the canonical ensemble, oxide depth grows linearly in time until $\ensuremath{\sim}30\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$, followed by saturation of oxide depth as a function of time. An amorphous oxide layer of thickness $\ensuremath{\sim}40\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ is formed after $466\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$, in good agreement with experiments. The average mass density in the oxide scale is 75% of the bulk alumina density. Evolution of structural correlation in the oxide is analyzed through radial distribution and bond angles. Through detailed analyses of the trajectories of O atoms and their formation of $\mathrm{O}{\mathrm{Al}}_{n}$ structures, we propose a three-step process of oxidative percolation that explains deceleration of oxide growth in the canonical ensemble.