Rare earth oxides effects on both the threshold voltage and energy absorption capability of ZnO varistors

Abstract

Rare earth oxides: Pr 6O 11, Y 2O 3, La 2O 3, Ce 2O 3 and Nd 2O 3 are added with contents of 0.01–0.5 wt.% to ZnO standard and antimony-rich varistor compositions. It is found that the rare earth oxide: REO, allows reaching large energy absorption capability value for the high threshold voltage ZnO-based varistors. A 30% maximum increase in threshold voltage is observed with the addition of 0.1 wt.% REO. However, degradation is accentuated with REO addition. Between 0 and 0.1 wt.% of REO the degradation remain still acceptable, but beyond 0.1% it becomes strong, up to 20% and energy absorption capability remains more than 100 J/cm 3. It is practically constant whatever the REO percentage. Results from standard composition are exploited for the second composition by optimization of Sb 2O 3 and REO concentrations in order to control the growth of the ZnO grains while maintaining the energy absorption capability above 100 J/cm 3. On the one hand, a high threshold voltage 300 V/mm ensured by antimony oxide, and on the other hand, a good capacity for absorption in energy 107 J/cm 3 ensured by the REO, whereas it had fallen to 52 J/cm 3 because of the great quantity of Sb 2O 3. Satisfactory values of non-linearity coefficient ( α) between 40 and 52 are obtained. These results are explained by the presence of extra-pyrochlore phase suggesting less bismuth oxide in the ceramics, especially at grain boundaries. Varistors present more active grains and hence a larger conduction section which account for large absorption capability.