Note on Diffusion of Cobalt Oxide and Ferrous Oxide in Polycrystalline Magnesium Oxide

Polycrystalline MgO of 95% of theoretical density, 50 mu in grain size, and 99.99% pure was cut into bars 0.3 by 0.3 by 2.0 cm. FeO in fine-powder form was tightly packed around each MgO bar. The specimens were placed in an electrical resistance furnace at 1100 deg C for 25 hours in a controlled atmosphere of hydrogen and water vapor, ensuring the stability of FeO. A sharp, straight boundary was observed between the diffused and nondiffused part of the MgO bar. There was no evidence of grain-boundary diffusion but only of bulk diffusion similar to the diffusion observed with single crystals and at a rate comparable with single crystals. The diffusion process appeared to take place by a counterdiffusion of the cations through a relatively rigid oxygen lattice. Because of the nonstoichiometric property of FeO, cation vacancies were present. The diffusion process appeared to prefer bulk movement by means of vacancies over grain-boundary diffusion. Cobalt oxide was also packed around the polycrystalline MgO in the same manner as with FeO. The specimens were placed in a gas furnace open to the atmosphere for 25 hours at 1600 deg C. Results showed a straight boundary indicating only bulk diffusion and no grain-boundary diffusion. (N.W.R.)