DIFFUSION OF CO-57 IN SURFACE-LAYERS OF NICKEL-OXIDE AND ALUMINA

Diffusion of Co-57 isotope on NiO (110) and Al2O3 (0001) surfaces was investigated by using the edge source method. The surface diffusion pa rameter alphaD(s)delta, where alpha is the segregation factor, D(s) th e surface diffusion coefficient, and delta the thickness of the high-d iffusivity layer, was determined for the temperature range 750-degrees -1200-degrees-C. For calculation of experimental results, the Whipple solution was used. The Arrhenius plot shows a break at almost-equal-to 900-degrees-C for the surface diffusion of Co-57 isotope on Al2O3 (00 01) plane. Above this temperature, vapor transport seems to be the ove rriding diffusion mechanism. Below this temperature, ionic transport p redominates. The apparent activation energy for the ionic transport wa s calculated to be 120 +/- 12 kJ/mol. Ionic transport predominated in the surface diffusion of Co-57 on NiO over the entire investigated tem perature range. This can be explained by the weak bond between Co-vapo r species and the NiO surface. The results obtained suggest that the s urface diffusion of Co2+ ion on NiO at 750-degrees-C is approximately 7 orders and at 1200-degrees-C approximately 5 orders of magnitude fas ter than volume diffusion. Activation energies are 139 and 227 kJ/mol, for surface and volume diffusion, respectively.