On the oxygen pressure dependence of high temperature oxidation of copper

High purity copper has been oxidized at 900-1050 degrees C and oxygen pressures from 5.10(-4) to 0.21 atm. The oxidation as measured by thermogravimetry was parabolic. In the stability range of CuO, so that the scale consists of Cu2O and a thin outer layer of CuO, the parabolic rate constant was independent of the ambient oxygen pressure. But at oxygen pressures where only Cu2O is formed, the oxygen pressure dependence gradually changes from being proportional to p(O2)(1/7) at low to p(O2)(1/4) at near-atmospheric oxygen pressure. The reaction kinetics and oxygen pressure dependence is interpreted to reflect that the oxidation is governed by diffusion of copper predominantly via singly charged vacancies at the lower oxygen pressures and neutral vacancies at the near-atmospheric oxygen pressures. The activation energy when only Cu2O is formed ranges from 110 to 120 kJ/mol depending on the oxygen pressure.