Cation Self-Diffusion and Electrical Conductivity in Polycrystalline Beryllium Oxide

Beryllium self-diffusion coefficients were determined in high-density polycrystalline BeO in the temperature range 800° to 1795°C. A precision lapping method using Be as a radiotracer was used in the interval above 1180°C, and electrical conductivity between 800° and 1250°C. Volume diffusion coefficients determined by both methods correlate well between 1100° and 1250°C. The results can be expressed over the temperature range 1100° to 1800°C by the relation D=3.2×10−3 exp(−63.0×103/RT). Above 1100°C, the agreement between the results obtained by the two techniques supports an ionic drift mechanism for the electrical conductivity of BeO. Below this temperature, however, the transport behavior appears to be considerably influenced by the history of the sample as well as the nature and concentrations of impurities. Classical volume diffusion was observed in a region near the surface of the specimens; at greater depths, deviations occurred and the magnitude of the deviations increased with decreasing grain size of the BeO. These discrepancies have been explained on the basis of a simultaneous grain-boundary diffusion mechanism.