Tailoring surface conditions for enhanced reactivity of aluminum powders with solid oxidizing agents

Abstract

The effect of processing liquids on particle surface hydration and subsequent reactivity of mixtures containing aluminum (Al) with different oxidizing agents was investigated. Recently, polar processing liquids were shown to significantly increase the surface hydration layer on Al particles and effect the reactivity of Al combined with polytetrafluoroethylene (PTFE). Processing mixtures of Al and PTFE using hexane (e.g., a non-polar liquid) limited surface hydration and produced significantly lower flame speeds than the same mixture processed in isopropanol (e.g., a polar liquid). Increased surface hydroxyl concentration was linked to higher exothermic behavior within a pre-ignition reaction (PIR) which may contribute to higher overall flame speed. This study extends the previous analysis toward assessing the influence of processing liquid on reactivity of aluminum with other oxidizing agents, specifically CuO, MoO3 and I2O5. Results from DSC analysis show no PIR kinetics associated with Al and CuO or MoO3, and Al+ CuO showed no difference in reactivity as a function of processing liquid. But, MoO3 FTIR shows modified surface structures after treatment in a polar solvent. Correspondingly, Al+MoO3 processed in polar solvent exhibited increased flame speed by 19% when compared to Al+MoO3 processed in a non-polar liquid. For Al+I2O5, water in polar processing liquids produces various hydrated states of iodic acid (i.e., HIO3 and HI3O8). Changing the hydration state of I2O5 significantly impacts reactivity. Results from this study confirm that carrier fluid used to process Al with metal oxides can also alter the surface structure of the metal oxide, thereby promoting greater reactivity with Al. A polar carrier fluid not only modifies the surface of Al but also hydration sensitive metal oxides such as MoO3 and correspondingly promotes greater reactivity.