A novel economical grain boundary engineered ultra-high performance ZnO varistor with lesser dopants

A varistor having ultra-high performance was developed from doped ZnO nanopowders using a novel composition consisting of only three (Bi, Ca and Co oxides) dopants. Improved varistor properties were obtained (breakdown field (E_b) 27.5?±?5?kVcm^?1, coefficient of nonlinearity (α) 72?±?3 and leakage current density (L_c) 1.5?±?0.06?μAcm^?2) which are attributed to the small grain size and grain boundary engineering by phases such as Ca₄Bi₆O₁₃ and Ca_0.89Bi_3.11O_5.56 along with Co⁺² doping in the ZnO lattice. Complex impedance data indicated three relaxations at 25?°C and two relaxations at high temperature (>100?°C). The complex impedance data were fitted into two parallel RC model to extract electrical properties. Two stages of activation energy for DC conductivity were observed in these varistor samples where region I (<150?°C) is found to be due to shallow traps and region II (<225?°C) is due to deep traps. The novel composition is useful for commercial exploitation in wide range of surge protection applications.