Effects of Sr(Co, Nb, Ta)O₃ addition on the defect structures and electrical properties of ZnO-based varistors

The present study reports on the effects of the addition of Sr(Co, Nb, Ta)O₃ and transition metal cations (Co and Mn) on the microstructures, varistor properties, and point defect structures of ZnO varistors. Addition of Sr(Co, Nb, Ta)O₃ and transition metal cations as donor dopants for the ZnO-matrix phase facilitates oxygen ionic conduction, promoting oxygen chemisorption at grain boundaries. The phase Sr(Co, Nb, Ta)O₃ is located in the intergranular regions and promotes oxygen diffusion in the grain boundaries. The transition metal cations were oxidized and lost electrons during sintering, enhancing the chemical absorption of oxygen molecules at the grain boundaries and resulting in the formation of additional interface traps. ZnO varistors with a breakdown voltage as high as 4000 V mm⁻¹ and a large nonlinearity coefficient reaching a value of 65 were obtained by co-doping with the intergranular phase Sr(Co, Nb, Ta)O₃ and transition metal ions of cobalt and manganese substituting zinc.