Influence of an Externally-Applied Static Charge on the Oxidation Kinetics of Copper

Abstract

The kinetics of copper oxidation under theinfluence of an externally-supplied static charge ofeither kind at one of the reaction interfaces of agrowing oxide film on its subsequent thickening weredetermined in the temperature range of 523-1173 K andoxygen-pressure range of 5.06-50.66 kPa. The kineticsconformed to the parabolic rate law under all conditionsof experimentation. In the temperature range of 523-723 K, charge supply of either kind ateither of the oxide interfaces, reduced the ratescompared to normal oxidation. The reduction in rates ismore pronounced with (-)ve charge supply. In thistemperature range, Mott's in situ electrical-potentialgradient across the oxide film is identified as thepredominant driving force for migration of copper ionsduring the subsequent film-thickening process. On the other hand, in the temperature range of 873-973K, a charge supply of either kind enhanced the ratescompared to normal oxidation, where Wagner'selectrochemical-potential gradient acts as the maindriving force for ion diffusion. However, at 1073 K and1173 K, the rates were found to decrease slightlycompared to normal oxidation. The oxygen-pressuredependencies of rate constants at 623 K exhibitedrelations of the type k_P ∝P ^1/4_O2 for normal and k_p ∝P ^1/8_O2 (approximately) for oxidation witheither (+)ve or (-)ve charge supply at the oxide/oxygeninterface. However, at 873 K the oxygen-pressuredependencies of rate constants conform to k_P ∝P ^1/6_O2 for normal as well as for oxidationwith either (+)ve or (-)ve charge supply at theoxide/oxygen interface. The estimated activationenergies are 54 kJ/mol and 160 kJ/mol in Mott's and Wagner's parabolic ranges,respectively. It is established that migration of Cu+ions through the growing film is the rate-limiting stepunder all conditions of experimentation. This study has clearly demonstrated that changes inoxidation rates can be brought about by disturbing theinterfacial defect equilibria with anexternally-supplied static charge when no net currentflows through the oxide film. The estimated self-diffusivityvalues of Cu⁺ ions in the growingCu₂O at 873 K are also reported.