Synergistic effect of co-doping of nano-sized ZnO and Nb2O5 on the enhanced nonlinear coefficient of TiO2 varistor with low breakdown voltage

•Nano-sized ZnO was added to TiO2 varistor and good electrical properties were obtained.•The effects of nano-sized ZnO on the microstructure were studied and some defects (Ti3+, O vacancies) were generated.•A doping mechanism was put forward to account for the low breakdown voltage and high nonlinear coefficient. In this study, the enhanced nonlinear coefficient (8.6) of TiO2 varistor with a low breakdown voltage (3.58 V/mm) was achieved by co-doping of nano-sized ZnO and Nb2O5. Meanwhile, the TiO2 varistor exhibited a low leakage current (0.096 mA) and high relative density (98.73%). Further analysis indicated that such improvement could be attributed to the changes in the microstructure including the grain size, pores, defects distribution and grain boundary characteristics in particular. Separately, SEM results demonstrated that the dopant of nano-sized ZnO could promote the sintering process of TiO2 varistor and increase the density of the sample. The XPS and EPR analysis showed that Nb2O5 could help to produce free electrons and Ti3+ ions through donor doping, while ZnO would be beneficial to generate oxygen vacancies and other defect ions through acceptor doping. On the one hand, the defects generated in the grain could result in the grain semiconducting; on the other hand, some defects diffusing to the grain boundary would participate in the formation of grain boundary potential barrier, thus the height of grain boundary barrier was improved. Consequently, the breakdown voltage descended while the nonlinear coefficient ascended.