Thermal air oxidation of copper in an applied electric field

Two copper disks connected as DC capacitor electrodes were oxidized by annealing in air for 1 h at a temperature T of 100-500°C. An electric field of 0-5000 V/cm was applied between the electrodes during oxidation. A dense and homogeneous layer of CuO with a characteristic grain size of 2-4 μm formed on the anode. The oxide on the cathode and on a sample annealed without applied voltage consisted of separated grains of CuO on a background of smaller Cu₂O grains. The grain sizes were 6-10 and 1.5 μm, respectively, on the cathode and 5-7 and 1 μm, respectively, on the sample without applied voltage. The oxide thickness and mass were greater on the anode than on the cathode. For applied fields greater than 1000 V/cm, spallation of the oxide layers and a resultant decrease in the electrode mass were observed. The cathode mass decreased slightly, while the anode mass decreased by a factor of 13 at 5000 V/cm in comparison with that at 1000 V/cm. A leakage current between the electrodes was measured during annealing. The current observed only for T≥400°C increased with time during annealing up to maximum, and then decreased.