High performance varistors prepared from doped ZnO nanopowders made by pilot-scale flame spray pyrolyzer: Sintering, microstructure and properties

Doped ZnO nanopowders (<25 nm) with production rate of 2–3 kg h−1 was synthesized by a single step solution feeding flame spray pyrolysis and were characterized by XRD, TEM, SAED, ICP-OES and BET surface area measurements. The powders were consolidated into greater than 98% dense pellets by two step conventional sintering. The sintered samples showed the formation of spinel, δ-Bi2O3, γ-Bi2O3 and pyrochlore phases along with the matrix ZnO. The grain size and density of the pellets were varied by changing the sintering temperatures. The grain growth exponent “n” was found to be in the range 3.5–4.0 and the activation energy for grain growth was found to be 304 ± 10 kJ mol−1 indicating that the grain growth to be dominated by volume and surface diffusion mechanisms. Electrical properties of samples prepared at different sintering conditions were studied using two different techniques. Breakdown voltage of 14.38 kV cm−1, coefficient of non-linearity of 171.69 and leakage current density of 1.10 μA cm−2 were obtained for pellets made under different sintering conditions. It was observed that pellets sintered at higher temperatures yielded lower impedance values. The high temperature (>175 °C) impedance data was fitted into two parallel RC circuits to calculate the electrical properties of the varistors.