Atomic Insight into the Oxidation Mechanism of a Core–Shell Aluminum Nanoparticle: Atomic Diffusion or Micro-Explosion?

Aluminum nanoparticles (ANPs) are an effective and economical additive in various energy conversion applications. In the present work, ReaxFF molecular dynamics simulations are performed to reveal the underlying oxidation mechanism of the natural core–shell ANPs with various oxygen concentrations, particle sizes, and shell thicknesses. The oxidation of ANPs is initiated by the atomic diffusion of Al and oxygen through the oxide shell, and the subsequent oxidation can be divided into three distinct modes dependent on the initial conditions. For a small size parameter M (ratio of core radius to shell thickness), the particles remain intact during the whole oxidation process via atomic diffusion. For a large size parameter M, cracks may appear on the oxide shell as the melted Al expands, which provides pathways for Al–oxygen diffusions. Under high oxygen concentrations, the extreme self-heating rate caused by quickly exothermic reaction leads to the violent evaporation of Al and subsequent micro-explosion of ANPs, which first confirms the experimental observations from the atomic perspective. This work reveals a fundamental mechanism for describing the oxidation of ANPs and provides a guideline for improving the combustion efficiency of ANPs, that is, the micro-explosions can be promoted by increasing the initial particle size parameters and oxygen concentrations.