Two-step microwave sintering of nanostructured ZnO-based varistors

Abstract ZnO is a versatile semiconductor material and is suitable for application in UV and gas sensors, photocatalysts, dye-sensitized solar cells, and in electronic devices such as LEDs and varistors. In this work, samples of nanostructured ZnO-based varistor compositions doped with 1 mol% of Sb 2 O 3 and 0.5 mol% of bismuth, cobalt, manganese and chromium oxides were processed by the two-step microwave sintering technique. The samples were heat-treated at 1050, 1100 and 1150 °C for 1 min, applying heating rates of 100 °C/min, then cooled at 50 °C/min to 850 and 900 °C, at which temperature they were held for 10, 30 and 60 min. The microstructure, density and electrical properties were analyzed. Samples with 96% of theoretical density and an average grain size smaller than 2.1 µm were obtained. The samples exhibited enhanced electrical properties, such as coefficient of nonlinearity and breakdown electric field higher than 30 and 9 kV/cm, respectively, and leakage current lower than 70 µA.