Progressive degradation of high voltage ZnO commercial varistors upon Fe2O3 doping

In the complex multicomponent formulation of standard commercial varistors used in high tension devices, those based in the ZnO-Bi2O3-Sb2O3 (ZBS) ternary system, the presence of iron is ordinarily prevented in any of its plausible sources: dopant, impurity and/or unwanted contamination. Otherwise the material undergoes a systematic degradation in its electrical performance which rapidly disallows it for practical uses. In order to understand the origin of such degradation phenomena, we have deliberately doped the ZBS varistor composition with different amounts of Fe2O3. The research here presented evidences that the prime reason for the varistor electrical ruination is the progressive incorporation of iron inside the spinel secondary grains, which eventually leads to the formation of a highly conductive skeleton of secondary phases with an unbearable level of leakage currents below the varistor breakdown voltage.