Reactions leading to ignition in fully dense nanocomposite Al-oxide systems

Aluminum-metal oxide energetic compositions with components mixed on the nano-scale are substantially more reactive than conventional thermites and are of interest as potential additives to propellants, explosives, and pyrotechnics. For such nanocomposite materials prepared by Arrested Reactive Milling (ARM), the exothermic reactions leading to ignition were detected to begin at relatively low temperatures. These materials are prepared by mechanical processing at room temperature, and the nature of the interface present between aluminum and the oxidizer (metal oxide, e.g., CuO, MoO₃, Bi₂O₃, etc.) is unknown. Experiments using a Thermal Activity Monitor (TAM III) quantify the reaction rates between aluminum and CuO at temperatures between 303 and 373K. Results of the present TAM III measurements and results of earlier measurements using differential scanning calorimetry for the same 2Al·3CuO nanocomposite are interpreted considering two different reaction models. The rate-limiting step is described either as a conventional thermally activated diffusion, or using the Cabrera-Mott model developed originally for oxidation of fresh metal surfaces. It is shown that the thermally activated diffusion model is inadequate for description of the low-temperature reactions observed in nanocomposite thermites prepared by ARM. The Cabrera-Mott model provides a description qualitatively matching the experimental results; achieving the quantitative match is expected to be possible by adjusting the model parameters.