Interface stability during displacement reactions between Cu₂O and Co_1-XFe_X alloys at 1223 K

The stability of the reactive interface during the solid-state displacement reaction, Cu₂O + Co_1-XFe_X = 2Cu + (Co_1-XFe_X)O, is studied as a function of Co-Fe alloy composition at 1223 K. For X ≤ 0.03, the reaction zone has a layered structure, and the cation diffusion in (Co, Fe)O is the rate-limiting step. The interface is unstable in the early stages of the reaction; the instability decreases with time as the oxide thickness increases, and the interface becomes planar at long times. The time required for the attainment of interface planarity increases with the value of X. The reaction kinetics are consistent with the available cation-diffusion data in (Co, Fe)O. For X ≥ 0.045, the product zone is a composite of Cu + (Co, Fe)O, and the rate is limited by the oxygen transport in copper. The transition to interface instability occurs when the oxide can support a cation flux that exceeds the maximum possible oxygen flux in copper. During the reaction, composition gradients develop in (Co, Fe)O because of higher diffusion rates for iron than for cobalt.