Oxidation of aluminum nanoclusters is investigated with a parallel molecular-dynamics approach based on dynamic charge transfer among atoms. Structural and dynamic correlations reveal that significant charge transfer gives rise to large negative pressure in the oxide which dominates the positive pressure due to steric forces. As a result, aluminum moves outward and oxygen moves towards the interior of the cluster with the aluminum diffusivity 60% higher than that of oxygen. A stable 40 Å thick amorphous oxide is formed; this is in excellent agreement with experiments. [S0031-9007(99)09416-8]

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The dynamics of oxidation of aluminum nanoclusters ͑20 nm diameter͒ 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 ͑O 2 ͒ environments and microcanonical ensembles for molecular ͑O 2 ͒ and atomic ͑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 Al-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 ϳ30 ps, followed by saturation of oxide depth as a function of time. An amorphous oxide layer of thickness ϳ40 Å is formed after 466 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 OAl n structures, we propose a three-step process of oxidative percolation that explains deceleration of oxide growth in the canonical ensemble.

Bridge functions, the neglected terms in the hypernetted-chain (HNC) theory of classical fluids, are extracted with high precision from Monte Carlo (MC) simulation data for classical one-component plasmas. The MC bridge functions are extended by the use of the exact short-range Widom expansion and of long-range boundary conditions arising from the compressibility sum rule. An explicit analytic expression for the bridge functions is then obtained, leading to improvement on the HNC scheme. Accuracy of the improved HNC scheme is confirmed through comparison with the MC results and by examination of the thermodynamic consistency. The extracted bridge functions are compared with those of other theoretical schemes. PACS number(s): 52.25.Kn, 61.20.Gy, 61.20.Ne

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