Formation of Al-Cu Alloy during Al/CuO Thermite Combustion: Investigation on Off-Stoichiometry for Maximum Pressure Change

It is commonly observed that the highest pressurization rate is achieved where Al/CuO composites have slightly Al-rich condition compared to stoichiometric ratio. Although there were some effort for interpreting this phenomenon, proposed hypotheses cannot clearly explain the reason of phenomenon. Here, we examined off-stoichiometry of Al/CuO composites analyzing combustion product of the composites with various composition. X-ray diffraction analysis shows that there are several compounds including remained reactants, predicted products, especially, Al_xCu_y alloy. Electron microscopy analysis revealed that unreacted Al nanoparticles (NPs) are covered with Cu layer. The Cu layer on the surface of Al decreases reactivity of Al NPs and hinders combustion propagation. Analysis on heat and mass transfer shows produced Cu is gasified with generated heat during the reaction and gas-phase Cu travels from reaction sites to unreacted Al NPs before combustion wave reaches. From these analysis, we proposed combustion mechanism including formation of Cu layer on the Al NPs and Al_xCu_y alloy from intermetallic diffusion process.