The diffusion of63Ni along grain boundaries in nickel oxide

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Abstract
The product of the grain boundary diffusion coefficient and width, D′δ, has been measured directly for the diffusion of 63Ni along grain boundaries in high-purity NiO. These measurements have been made at an oxygen pressure of 1 atm in the temperature range 500–800°C, and also at an oxygen pressure of 7 × 10−6 atm at 700°C. At po 2 = 1 atm, D′δ is given by D′δ = 3·0× 10−8 exp {−1.78(eV)/kT} cm3 S−l. The width of the boundary, δ, is estimated to be 7 × 10−8 cm and in this region near the boundary the concentration and mobility of Ni vacancies are greater than in the bulk. The measured values of D’δ confirm quantitatively that the rate of oxidation of Ni to NiO at 500–800°C is controlled by the diffusion of Ni along grain boundaries in the NiO scale.