Electrical behavior of compressed magnesia powder

Magnesia is an important insulation material for high-temperature electrical applications. In order to study the leakage conduction mechanisms, test samples were made by compressing 10 cm long alloy tubes filled with industrial class MgO powder between a central electrode rod and the outer casing. The electrical behaviour was characterised up to 1200 C by measuring the d.c. resistance and the frequency dependence of the a.c. impedance. Three regimes were identified. Below 700 C, an activation energy of 1 eV is attributed to second phase(s) containing silicon and calcium which acts as a bypass. Above this temperature, ionic conduction in the MgO grains becomes predominant with an activation energy close to 2 eV. Finally, at even higher temperatures, above 1000 C, the activation energy increases further, indicating mixed ionic-electronic conduction. This last regime yields a voltage dependence to the resistance explained by blocking of the ionic component at the electrodes. 10 refs.