Two-step microwave sintering of nanostructured ZnO-based varistors

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 Sb2O3 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 degrees C for 1 min, applying heating rates of 100 degrees C/min, then cooled at 50 degrees C/min to 850 and 900 degrees 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 mu m 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 mu A.