Grain boundary diffusion in CoO

The diffusion of Co-60, Ni-63, and Ca-45 at 953 degrees C and Cr-51 at 953 and 1109 degrees C was measured in polycrystalline CoO scales by means of the radioactive tracer sectioning technique. High density scales were obtained by the oxidation of high purity Co plates at 1000 and 1200 degrees C in air. The diffusion penetration profiles exhibit two well distinguishable slopes which correspond to different diffusion regimes. The bulk diffusion coefficient, D, determined from the diffusion profile of the higher slope, is in a good agreement with the bulk diffusion data reported elsewhere. The product corresponding to the grain boundary diffusion, D'delta alpha, where D' is the grain boundary diffusion coefficient, delta is the grain boundary thickness, and alpha is the segregation enrichment factor, was determined from the diffusion profile of the lower slope. It was found that the grain boundary diffusion enhancement factor, f = D'delta alpha/D, for Co-60 and Ni-63 assumes similar values, which are one order of magnitude larger than those for Ca-45 and Cr-51. The presence of Ca within the grain boundaries of CoO was confirmed using both autoradiography and SIMS methods. It has been revealed that the diffusion coefficient for both Ni and Ca in CoO, determined at 953 degrees C, exhibits the same value for specimens prepared at 1000 and 1200 degrees C, in spite of their different porosity and different grain size.