The self-diffusion of Ni in NiO and its relevance to the oxidation of Ni

Abstract

The tracer self-diffusion coefficient of Ni in single crystals of NiO, at atmospheric pressure of oxygen, has been measured by sectioning in the temperature range 700 to 1400° C and is given by

$$D_{Ni}^* = (1.5{\text{ }} \pm {\text{ }}0.8) \times 10^{ - 2} \exp {\text{ }}\{ - 58{\text{ }} \pm {\text{ }}1{\text{ }}(kcal)/RT\} {\text{ }}cm^2 \sec ^{ - 1} .$$

Hence it is argued that the breakdown in the application of Wagner's theory to the oxidation of Ni at intermediate temperatures is not an impurity effect, but a result of short-circuit diffusion. Diffusion of Ni along grain boundaries in NiO at 600° C has been observed using autoradiography and the productD ^*_GB δ estimated to be 3×10⁻¹⁹ cm³ sec⁻¹. This value is reasonably consistent with estimates from oxidation kinetics, but it is not yet established that grain boundary diffusion of Ni will account completely for the kinetics of nickel oxidation at this temperature.