Electrical conductivity of hot-pressed magnesia: Effect of heat treatment

The electrical conductivity of hot-pressed magnesia (HPM) was investigated as a function of time of heat treatment at 1350 °C. The electrical conductivity was measured in the temperature range 550-900 °C on samples having densities higher than 90% of the theoretical density. The electrical conductivity at 850 °C decreases by four orders of magnitude after heat treatment for about 50 h. Almost constant electrical conductivity at 850 °C is observed after heat treatment for 50-300 h, whereas heat treatment for more than 350 h increases the electrical conductivity at 850 °C. The grain size increases markedly from 0.4 to 8 μm during heat treatment for up to 50 h. During heat treatment for more than 50 h the grain size increases slowly, reaching 14 μm after 410 h. It is suggested that the electrical conductivity of HPM heat treated for up to 140 h is controlled by enhanced oxygen diffusion along grain boundaries. The electrical conductivity of HPM heat treated for more than 140 h is controlled by a different mechanism, the nature of which is not yet fully understood.