Self-propagating reaction induced by ball milling in a mixture of Cu2O and Al powders

The mechanochemical reduction of Cu2O with Al was induced via ball milling. A self-propagating reaction took place in two distinctly different ways. If the mass of the reactant powder mixture was large enough, ignition happened within a few seconds after starting the mill. Neither the activation of the reactants nor the formation of a powder layer covering the surfaces of the milling tools were necessary. If the mass of the charge is smaller than a critical value, the self-propagating reaction is ignited only after several minutes of mechanical activation. A slight excess of Al promoted an immediate reaction. The mechanism of the process is determined by the balance of the reaction heat, the heat transfer within the powder, and the heat loss to the environment. Local melting plays an important role, especially by promoting the formation of partially molten aggregates from free particles.