Heat-initiated oxidation of aluminum nanoparticles

This dissertation uses multi-million atom molecular dynamics simulations to determine the key mechanisms which control the oxidation of aluminum nanoparticles (ANPs) due to uniform heating. The primary simulation studied is the oxidation of a 46 nm ANP uniformly heated to 1100 K (the melting temperature of aluminum metal is 933 K). In this system, 3 stages are identified, corresponding to simulation times of 0 to 50 ps (Stage 1), 60 to 110 ps (Stage 2), and 120 to 1000 ps (Stage 3). Radial analysis, fragment analysis, and other simulation tools are used to determine the controlling mechanisms in each stage, as well as determining the cause of each stage transition. Stage 1, initiated by the ANP temperature exceeding the melting temperature of the aluminum core, is seen to be a confined burning stage. Oxygen penetrating from the core-shell interface reacts exothermically with the core metal, releasing local heat. When this heating drives the local temperature of the shell to exceed the melting temperature of alumina (the shell), a sudden increase in the number of oxidation reactions is seen. The development of asymmetry in the shell and increased uptake of oxygen from the exterior also support the conclusion that melting of the shell initiates Stage 2. Local heating continues until the shell temperature reaches a critical temperature (T = 3000 K), at which point aluminum atom ejections from the exterior of the shell into the surrounding oxygen begin. These ejections lead to direct oxidation outside the ANP and the formation of oxygen-rich clusters. Both internal and external oxidation continue throughout the remainder of Stage 3, leading to complete oxidation of the particle.;After having determined the basic mechanisms in the 46nm simulation, the effect of initial temperature on the oxidation stages is studied. Three 26nm ANP simulation systems, heated to 1100 K, 1200 K, and 1400 K, are analyzed using the tools developed in the preceding study. By comparing these systems, it is determined that many of the features identified in the model are strongly dependant on the initial temperature of the nanoparticle (including the time delay to the onset of each stage and the rate of oxidation in stages 2 and 3), while others have a relatively small dependence on the initial temperature provided (including the initial rate of heating, the rate of aluminum ejections in stage 3, and the overall temperature of a system of a given size).;Finally, the tools and models developed in this study are applied to the case of a flash-heated ANP, providing a comparison point to aid in the understanding of this more complicated reaction. From this, it is determined that the initial heating of flash-heated ANPs is dominated by a mechanical mixing mechanism resulting from the pressure-driven volume expansion of the flash-heated core. This volume expansion significantly increases the chemical reaction zone of the core-shell oxidation, resulting in the very fast release of energy seen in the flash-heating cases.