Microstructure and electrical properties of Sm2O3 doped Bi2O3-based ZnO varistor ceramics

The dependence of the bulk density, microstructure and dc electrical properties of bismuth oxide (Bi2O3)-based zinc oxide (ZnO) varistor ceramics for various samarium oxide (Sm2O3) contents was investigated. The value of bulk density was found to 5.43–5.50 g cm−3 with Sm2O3 (mol%) content. The maximum value of bulk density is observed to be 5.50 for 0.30 mol% Sm2O3 containing varistor ceramics. The grain sizes for all the samples calculated from the scanning electron micrographs were found to decrease as Sm2O3 increases. The presence of ZnO phases, Bi-rich phases, spinel phases and Sm2O3 phases were observed in the samples by the energy dispersive X-ray analysis and X-ray diffraction analysis. As the Sm2O3 amount increased in the Bi2O3-based ZnO varistor ceramics, the nonlinear coefficient, α increased up to 0.10 mol%, reaching a maximum value of 58 and then decreased. The breakdown electric field, Eb, increased with the increase of Sm2O3 content with a maximum value of 3220 V cm−1 for the 0.75 mol% Sm2O3 doped ZnO varistor ceramics. The leakage current, IL, showed a minimum value of 1.10 μA for the composition of 0.30 mol% Sm2O3 doped Bi2O3-based ZnO varistor ceramics. The 0.30 mol% Sm2O3-doped Bi2O3-based ZnO varistor ceramics sintered at 1200 °C exhibited a good stability for dc accelerated aging stress of 0.90 V1 mA/90 °C/12 h.

The EDX and XRD analyses of the samples show the presence of ZnO phases, Bi-rich phases, spinel phases and Sm2O3-based phases. For varistor ceramic containing 0.30 mol% Sm2O3 the value of α is 44, maximum Eb value is 3000 V cm−1 and minimum value of IL is 1.10 μA. It can be inferred from the above results that the 0.30 mol% Sm2O3 doped ZnO varistor ceramics is the optimum composition for best performance of these Sm2O3 doped Bi2O3-based ZnO varistor ceramics.