High nonlinearity and low residual-voltage ZnO varistor ceramics by synchronously doping Ga2O3 and Al2O3

•ZnO varistor ceramics co-doped with Al2O3 and Ga2O3.•Al additive decreases the residual voltage under high currents.•Ga-doping increases the barrier potential and further reduces the grain resistance.•Optimal Ga additive concentration suggested as 0.72 mol%.

The present study examined the electrical properties of ZnO varistor ceramics co-doped with Ga2O3 and Al2O3, in particular, the current–voltage characteristics under small and intermediate currents and upturn characteristics under a large current. With increasing amounts of Ga2O3 dopant at a given Al2O3 concentration, both the threshold voltage in the small-current region and nonlinear coefficient of the varistor ceramics increased and then decreased; in contrast, the leakage current decreased and then increased. Moreover, ZnO varistor ceramics with low residual voltage ratio, high nonlinearity, and low leakage current were obtained under an optimal Ga concentration of 0.72 mol% while the Al additive content was fixed at 0.1 mol%. This novel finding will be helpful towards the manufacture of high-quality ZnO varistors.