PREPARATION AND CHARACTERIZATION OF ZNO BASED MATERIALS FOR VARISTOR APPLICATIONS

Zinc oxide based non-ohmic resistors in disc form have been fabricated for varistor applications. Two series of samples Z1: ZnO (99.5 at %) + Bi2O3 (0.5 at %) and Z2: ZnO (99 at %) + Bi2O3 (0.5 at %) + SnO2 (0. 5 at %) have been prepared for the present study. The sintering studie s on the disc specimens between 800 and 1300 degrees C show that the p ercentage of linear shrinkage, electrical resistivity and nonlinear ex ponent are higher In Z2 series than in Z1 series. These measured param eters and the exhibited breakdown voltage per individual ZnO/ZnO grain boundary regions varies with sintering. The XRD and SEM results confi rm that Bi-rich oxide layers are segregated at the ZnO grain boundary regions. The size of the segregated Bi-rich layers at the grain bounda ry regions are of the order of nanometer. The resistivity data shows t hat the segregated layers are insulating in nature which develops nonl inear current voltage characteristics. These grain boundary layers are evaporated from the system with increased sintering temperature and t ime. The increased grain growth, (sizes higher than 15 mu) developed o n sintering, degrade the varistor characteristics.