Study on the electrical properties and defect structures of a high voltage gradient ZnO varistor

In this research, ZnO-based varistor ceramics with different recipes were obtained. It was found that the grain growth can be effectively restrained with the addition of Ba
²⁺
and the decrease of sintering temperature. SEM results indicate that the grain size of ZnO varistor ceramics sintering at 950 °C was reduced to 1.28 μm, which is about 8 times smaller than that of ordinary ZnO varistor ceramics. Correspondingly, the voltage gradient of the ZnO varistor samples were also extremely enhanced to 3845 V/mm. The dielectric properties of the ZnO varistor ceramics were measured by Broadband Dielectric Spectroscopy. The results showed that two dielectric loss peaks with activation energy at about 0.22 eV and 0.36 eV observed in 163-203 K are common for all the ZnO varistor samples, which were considered to be consistent with the electronic levels of intrinsic defect interstitial zinc and vacancy oxygen in the depletion layer. However, for sample C, the dielectric loss was reduced greatly in low frequency range with the addition of Ba
²⁺
and the resistance of grain boundary increased to 128 MΩ at 473 K. Therefore, it was proposed that the addition of Ba
²⁺
and the sintering procedure may play a great role in improving the electrical properties of ZnO varistors.