Mechanism of Ga2O3 addition on the enhancement of electrical and mechanical properties of ZnO based varistor

Abstract ZnO based varistors added with minor content of Bi2O3, Sb2O3, Co2O3, SiO2, MnO2 et al. with high voltage gradient, excellent mechanical and thermal properties were fabricated by Ga2O3 doping and using raw ZnO nanoparticles. The compositions and microstructure of the varistors, as well as their electrical, mechanical, and thermal properties were investigated via XRD, XPS, SEM, E-J, C–V, mechanical, and thermal expansion measurements. Also, the mechanism of Ga2O3 addition on the electrical and mechanical properties of the varistors was discussed in detail. Results showed that the added Ga2O3 preferentially occupied the lattice position of ZnO crystal through the formation of a substitutional solid solution (Donor doping), it then occupied the void position through the formation of an interstitial solid solution (Acceptor doping), in which the residual Ga2O3 existed in the grain boundary and served as inversion boundaries. The formation of the substitutional and interstitial solid solutions helped to improve the electrical properties, when Ga2O3 content was 0.40 mol%, values of E1mA, α, and K were 1235.00 V mm−1, 46.0, and 1.37, respectively, being due to the small particle size and the related content of donor, acceptor and grain boundary in ZnO grain; The increased content of inversion boundaries stimulated the abnormal growth of ZnO grain, and the formed plate-like grain helped to improve the mechanical properties and thermal expansion coefficient of the varistors, values of σf, Ef, and KIC could reach 147.43 MPa, 213.61 GPa and 2.05 MPa m1/2, showing improvements of 25.29%, 47.67%, and 38.51%, respectively, compared with those of ZnO based varistors without Ga2O3.