The effect of bismuth oxide polymorph forms on degradation processes in ZnO varistors

The behavior of a ZnO–Bi2O3 binary system and ZnO-based varistors under a biasing electric field and temperature was investigated. Both types of samples were exposed to aging with 50 μA DC at 115 °C for 22 h. The degradation effect of DC biasing was manifested by a drop of varistor voltage as well as the presence of peaks in the thermally stimulated discharge current (TSDC) spectra. It was found that the TSDC generation is related to a particular form of the Bi2O3 in ceramics. Activation energies of the thermally stimulated responses ranged from 0.01 eV to 0.17 eV. In the high temperature range, the peaks attributable to degradation were found in the TSDC current when the Bi-rich inter-granular phase formed agglomerates and the Bi2O3 was in its β- or defective γ- variety. When Bi2O3 was in an α-form, well-crystallized γ-form, or an amorphous form, none of the high temperature TSDC maximums were identified. Under a DC bias, the varistor voltage returned to its starting value when the samples were cooled to room temperature. However, passing the current in the opposite direction by reversing the bias from positive to negative exhibited a persistent drop of the varistor voltage.