ELECTRICAL BEHAVIOR OF COMPRESSED MAGNESIA POWDER

Magnesia powder has an important application for electrical insulation . In order to study the leakage conduction mechanisms, test samples we re made by compressing 10 cm long alloy tubes filled with industrial c lass magnesia between a central electrode rod and the outer casing. Th e electrical behaviour between the central electrode rod and the outer casing was characterized up to 1200-degrees-C by measurement of the d .c. resistance and the frequency dependence of the a.c. impedance. Thr ee regimes can be identified. Below 700-degrees-C, an activation energ y of 1 eV is attributed to second phase(s) containing silicon and calc ium which acts as a bypass. Above this temperature ionic conduction in the magnesia grains becomes predominant with an activation energy clo se to 2 eV. Finally, at even higher temperature above 1000-degrees-C, the activation energy increases further, indicating mixed ionic-electr onic conduction. This last regime yields a voltage dependence to the r esistance explained by blocking of the ionic component at the electrod es.