Characteristics of ZnO-based varistor ceramics doped with Al2O3

The influence of Al2O3 doping in the range 0.00–0.83 mol% on the microstructure and current–voltage characteristics of ZnO-based varistor ceramics sintered at 1200 °C for 2 h was studied. The threshold voltage VT (V/mm) increased up to a dopant level of about 0.08 mol% Al2O3; the nonlinear coefficient α was significantly increased by additions of up to 0.04 mol% Al2O3, although larger additions of Al2O3 caused it to decrease; and the leakage current increased sharply with increasing amounts of Al2O3. Doping with Al2O3 up to about 0.12 mol% Al2O3 resulted in a significantly decreased ZnO grain size, which is mainly responsible for the significantly increased threshold voltage, VT. No ZnAl2O4 spinel phase was detected in any of the samples, and EDXS and WDXS analyses showed that most of the added Al2O3 distributed between the Zn7Sb2O12 spinel phase and the ZnO phase, while only trace amounts were detected in the Bi2O3-rich phase. The spinel phase incorporates an appropriate amount of Al2O3; however, with an increasing amount of added Al2O3, more of it remains outside the spinel phase in the Bi2O3-rich liquid, where it can incorporate into the growing ZnO grains at the sintering temperature. The amount of Al in the ZnO grains was determined. A mechanism for the grain growth inhibition resulting from the small amounts of Al2O3 in the Bi2O3-rich liquid phase is also proposed.