Purity effect on oxidation kinetics of copper at 800-1050°C

To clarify the purity effect on copper oxidation kinetics, oxidation was carried out at 800-1050degreesC using 99.99% (4 N), 99.9999% (6 N) and floating zone refined (FZR,.99.9999%) copper specimens. In the Arrhenius plots of the parabolic oxidation rate constants for the double-layer (dl) formation (Cu2O + CuO) at 800-1050degreesC in 0.1 MPa O-2 atmosphere, the points for FZR copper essentially followed a straight line. For 6 N and 4 N coppers, the oxidation kinetics at high temperatures (above 950degreesC) was almost the same as that for FZR copper, but the points at lower temperatures followed two lines with smaller slopes. This result suggests that the oxidation of FZR copper was governed by the lattice diffusion at 800-1050degreesC. Trace impurities had almost no influence on the oxidation kinetics of 6 N and 4 N coppers at temperatures above 950degreesC where the lattice diffusion predominated. However, at lower temperatures where the grain boundary diffusion also contributes to oxidation, trace impurities decreased the activation energy by impeding the growth of Cu2O grains to facilitate the grain boundary diffusion. The activation energy governed by the lattice diffusion was 173 kJ/mol for the dl formation, while it was 98 kJ/mol for the single-layer formation (Cu2O). This supports a (p(O 2))(1/4) dependence of the parabolic rate constant associated with the diffusion of copper atoms via neutral copper vacancies in the Cu2O layer. (C) 2003 The Electrochemical Society.