Grain Boundary Diffusion and Grain Boundary Chemistry of Cr-Doped Magnesium Oxide. Technical Progress Report

摘要

The grain boundary product D' delta (where D' is the grain boundary diffusion coefficient and delta is the grain boundary width) of exp 51 Cr in MgO and Cr-doped MgO was determined at 1200 to 1450 exp 0 C for Cr exp 3+ concentrations of 0 to 0.56 atom %. By assuming a grain boundary width to be between 100 and 1000 A the grain boundary diffusion coefficients are 10 exp 5 -10 exp 6 times larger than the volume diffusion coefficients. D' delta increased with increasing Cr exp 3+ concentration, but the activation energy for diffusion of 180 +- 8 kJ/mole was the same for all specimens. Surface diffusion on MgO and Cr-doped MgO was examined using grain boundary grooving at 1100 to 1400 exp 0 C for a concentration range of 0 to 0.211 atom % chromium. The activation energy for surface diffusion on MgO was found to be 239 +- 5 kJ/mole. The surface diffusivity did not change markedly with chromium concentration; however, an abrupt change in activation energy from 180 kJ/mole occurred between 0.038 and 0.254 atom % chromium. Effect of association of defects on the thermodynamical factor (Darken's equation) for diffusion was studied theoretically. It was shown that in the limit of dilute solution the thermodynamic factor (1 + par. delta 1n gamma /delta 1n N) was equal to 3/2 if monomers are the diffusing species, 2 if dimers are the diffusion species and 3 if trimers are the diffusion species. These results are consistent with the atomistic model. (ERA citation 06:030979)