Combustion of a rapidly initiated fully dense nanocomposite Al–CuO thermite powder

Abstract

Very short burn times of nanocomposite, fully dense, stoichiometric 2Al·3CuO thermite particles ignited by electro-static discharge (ESD) observed in earlier experiments are interpreted assuming that the reaction occurs heterogeneously at the Al–CuO interfaces while the initial nanostructure is preserved even after the melting points of various phases present in the particle are exceeded. The heating rate for the ESD-ignited particles is very high, reaching 109 K s−1. The reaction model assumes that the rate of reaction is limited by transport of the reacting species across the growing layer of Al2O3 separating Al and CuO. The model includes the redox reaction steps considered earlier to describe ignition of 2Al·3CuO nanocomposite thermites and adds steps expected at higher temperatures, when further polymorphic phase changes may occur in Al2O3. A realistic distribution of CuO inclusion sizes in the Al matrix is obtained from electron microscopy and used in the model. The model accounts for heat transfer of the nanocomposite particles with surrounding gas and radiative heat losses. It predicts reasonably well the burn times observed for such particles in experiments. It is also found that neglecting polymorphic phase changes in the growing Al2O3 layer and treating it as a single phase with the diffusion-limited growth rate similar to that of transition aluminas (activation energy of ca. 210 kJ mol−1) still leads to adequately predicted combustion temperatures and times for the nanocomposite particles rapidly heated by ESD. The model highlights the importance of preparing powders with fine CuO inclusion sizes in the nanocomposite particles necessary to complete the redox reaction; it is also found that the particle combustion temperatures may vary widely depending on their dimensions. Higher combustion temperatures generally lead to greater reaction rates and, respectively, to the more complete combustion.