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

Rare earth oxides: Pr6O11, Y2O3, La2O3, Ce2O3 and Nd2O3 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/cm3. It is practically constant whatever the REO percentage. Results from standard composition are exploited for the second composition by optimization of Sb2O3 and REO concentrations in order to control the growth of the ZnO grains while maintaining the energy absorption capability above 100 J/cm3. 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/cm3 ensured by the REO, whereas it had fallen to 52 J/cm3 because of the great quantity of Sb2O3. 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.