Transport Processes in Ceramic Oxides

Progress is described in a program intended to clarify the nature of mass transport in MgO through (1) growth of crystals of improved perfection and purity, (2) measurement of cation self-diffusion rates over a wide range of temperatures, and (3) extension of diffusion measurements as close as possible to the melting point of the material. Crystals of MgO2 cm in diameter and 2 mm in thickness are routinely and reproducibly grown epitaxially on MgO substrates by means of chemical vapor transport with HCl at 1000 C. Measurements of Ni(2+) diffusion in MgO have been extended to 2460C (0.88 T(M)) in an attempt to reveal intrinsic transport. The measurements extend by 70% the temperature range over which transport data for MgO are now available, but fail to reveal a change in transport mechanism. Mg26 self-diffusion in MgO in the temperature range 1600- 2250C was studied. The results are interpreted as extrinsic diffusion and are not in accord with limited data in the literature for diffusion of the radioisotope Mg28. Neither of the present diffusion studies has revealed a significant difference between transport in moderate purity crystals and crystals of the best quality which are commercially available.