Initial stages of oxidation of aluminum powder in oxygen

Abstract

Oxidation of aluminum powders is known to include several stages, corresponding to growth of different polymorphs of alumina. The initial oxide layer is amorphous, which transfers to γ-Al₂O₃ at elevated temperatures and greater oxide thicknesses. This work focuses on quantitative characterization of oxidation for thin initial oxide layers occurring at relatively low temperatures. The experiments include different types of thermo-gravimetric (TG) measurements with increased amounts of powder loaded for greater sensitivity. Modulated and isothermal TG measurements were found to be less useful than constant heating rate measurements. Results were processed using both, an explicit oxidation model and a model-free isoconversion method. The latter approach was more productive in identifying the activation energy of oxidation. Using the activation energy as a function of reaction progress, the pre-exponent was also determined as a function of reaction progress assuming a diffusion-limited reaction mechanism. The reaction kinetics was validated by comparison between predicted and measured oxidation rates for nanoaluminum powders reported in the literature. Finally, the oxidation model was combined with a heat transfer model to describe ignition of aluminum particles exposed to a heated oxidizing environment. A sharp increase in the ignition temperature from 850 to 2260 K is predicted as the particle size increases from 0.3 to 1.2 µm. The results are found to be sensitive to the assumed initial oxide thickness (2.5 nm); they are also somewhat affected by the value of thermal accommodation coefficient used in the heat transfer model.