Microstructure and electrical properties of Ho2O3 doped Bi2O3-based ZnO varistor ceramics

The microstructure and electrical properties of Ho2O3 doped Bi2O3-based ZnO varistor ceramics were investigated. The bulk density varies between 5.41 and 5.47 g cm−3 with the maximum value of 5.47 g cm−3 for 0.50 mol% Ho2O3 content. The average grain size for all the samples was calculated from the scanning electron micrographs and were found between 5.1 and 7.1 μm. The microstructure of the prepared samples shows a decrease in grain size of ZnO phase with Ho2O3 doping. The energy dispersive X-ray analysis and X-ray diffraction analysis of the samples show the presence of ZnO, Bi-rich, spinel Zn7Sb2O12 and Ho2O3-based phases. The nonlinear coefficient, α, obtained from electric field–current density plots has a maximum value of 78 for the ceramics with 0.50 mol% Ho2O3 content. The leakage current, IL, has a minimum value of 1.30 μA for the 0.50 mol% Ho2O3 doped ZnO varistor ceramics. The breakdown field, Eb, was found to increase with Ho2O3 content.