(544d) Modeling of Reduction-Oxidation Reactions in Al-MoO3 Nanocomposite Powders

Recently, reactive Al-MoO₃ nanocomposites were prepared by arrested reactive milling for potential applications in various energetic formulations, including pyrotechnics, explosives, and propellants. Other manufacturing approaches, including nanopowders mixing and sol-gel processing have also been recently explored to produce similar nanocomposite thermite compositions. The reactions in nanocomposite powders were investigated by differential scanning calorimetry and correlated with their ignition kinetics quantified from heated filament ignition experiments. It was observed that many overlapping processes control the reaction rate and ignition so that a simplified model based on several independent processes is inadequate for practically useful predictions. The objective of this work is to develop a model of oxidation processes in the Al-MoO₃ nanocomposites that enables one to describe the experimental data and account for differences in ignition kinetics as a function of the scale and morphology (e.g., spherical vs. flat) of nano-domains. The model describes the reduction of two oxide phases, MoO₃ and MoO₂. Oxygen ions produced as a result of molybdenum oxide reduction diffuse to aluminum through a growing aluminum oxide layer. The model accounts for simultaneous growth of different aluminum oxide polymorphs and for polymorphic phase transformations occurring within the aluminum oxide layer. This modeling approach was previously successfully used to describe oxidation and ignition of aluminum particles in air. Compared to the model of Al oxidation in air, the Al-MoO₃ nanocomposite reaction model includes new assumptions about diffusion characteristics of growing alumina polymorphs films sandwiched between molybdenum oxides and aluminum. Most of the kinetic parameters describing specific reactions are determined from processing the scanning calorimetry and ignition experiments. The model formulation and results will be presented and compared to experimental data. Phase analysis of quenched samples is planned to verify the model predictions about the phases formed at different stages of the reduction-oxidation reactions.