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 parameter alphaD(s)delta, where alpha is the segregation factor, D(s) the surface diffusion coefficient, and delta the thickness of the high-diffusivity 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 (0001) plane. Above this temperature, vapor transport seems to be the overriding diffusion mechanism. Below this temperature, ionic transport predominates. The apparent activation energy for the ionic transport was calculated to be 120 +/- 12 kJ/mol. Ionic transport predominated in the surface diffusion of Co-57 on NiO over the entire investigated temperature range. This can be explained by the weak bond between Co-vapor species and the NiO surface. The results obtained suggest that the surface diffusion of Co2+ ion on NiO at 750-degrees-C is approximately 7 orders and at 1200-degrees-C approximately 5 orders of magnitude faster than volume diffusion. Activation energies are 139 and 227 kJ/mol, for surface and volume diffusion, respectively.