Interdiffusion and Association Phenomena in the System NiO-Al2O3

The rate of formation of NiAl2O4 by reaction between single crystals of NiO and Al2O3 can be described by k= 1.1 × 104 exp (−108,000 ± 5,000/RT) cm2/s. In NiO the behavior of D as a function of concentration supports the Lidiard theory of diffusion by impurity‐vacancy pairs. A good fit of the theory to the experimental results was obtained by assuming that Al3+ ions diffuse as [AlNi· VNi]’pairs. The diffusion coefficient of pairs, Dp, obeys the equation 6.6 × 10−2 exp (−54,000 ± 3,000/RT) cm2/s. The free energy of association for pairs was calculated to range from 6.5 kcal/mol at 1789°C to 9.0 kcal/mol at 1540°C. The interdiffusion coefficients in the spinel showed a constant small increase with increasing concentration of Al3+ dissolved in the spinel.