Grain boundary diffusion in CoO

Article abstract of DOI:10.1016/0022-3697(95)00115-8

The diffusion of ⁶⁰Co, ⁶³Ni, and ⁴⁵Ca at 953°C and ⁵¹Cr at 953 and 1109°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°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′δα, where D′ is the grain boundary diffusion coefficient, δ is the grain boundary thickness, and α 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′δα/D, for ⁶⁰Co and ⁶³Ni assumes similar values, which are one order of magnitude larger than those for ⁴⁵Ca and ⁵¹Cr. 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°C, exhibits the same value for specimens prepared at 1000 and 1200°C, in spite of their different porosity and different grain size.