Effect of process variables on the grain growth and microstructure of ZnO–Bi2O3 varistors and their nanosize ZnO precursors

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Abstract
The basic building block of the ZnO varistor is the ZnO grain formed as a result of sintering. Nanosized ZnO particles are prepared by carrying out the reaction in the controlled size nanoreactors—the droplets of microemulsions. Chemical doping of the ZnO nanoparticles provides ZnO-based ceramic varistors displaying superior varistor properties. These varistors show a higher value of the nonlinear coefficient, lower leakage current, and higher critical electric field value as compared to those for conventional samples in their log E versus log J curve. The present work has also been aimed at studying the effect of processing variables such as sintering temperature and duration on the microstructure and grain growth of ZnO nanoparticles and ZnO-Bi2O3 ceramics. The activation energy calculated from this data is found to be 175 kJ/mol for pure ZnO. For Bi2O3-doped ZnO, the activation energy is found to decrease considerably (∼148 kJ/mol). All these advantages are due to greater structural homogeneity, smaller particle size, higher surface area, and higher density of the ZnO nanoparticles which are precursors for ZnO varistors, as compared to coarser particles for making varistors.

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