Microstructural and electrical characteristics of Y2O3-doped ZnO–Bi2O3-based varistor ceramics

The microstructural and electrical characteristics of ZnO–Bi2O3-based varistor ceramics doped with Y2O3 in the range from 0 to 0.9 mol% have been investigated. The addition of Y2O3 resulted in the formation of a fine-grained Bi–Zn–Sb–Y–O phase along the grain boundaries of the ZnO grains which inhibits the grain growth. The mean ZnO grain size decreased from 11.3 to 5.4 μm with increasing amounts of Y2O3. The threshold voltage (VT) of the ceramics increased from 150 to 274 V/mm, the non-linear coefficient α was not influenced and remained at approximately 40, and the leakage current also increased with the amount of Y2O3 added. On the basis of the Mukae et al. (Mukae, K., Tsuda, K. and Nagasawa, I., Capacitance-vs-voltage characteristics of ZnO varistors. J. Appl. Phys., 1979, 50, 4475–4476) Schottky barrier model of ZnO varistors, the addition of Y2O3 resulted in a slight increase in the density of interface states (NS) and a more pronounced increase in the donor density (ND), causing a decrease of the barrier height (ΦB) and the depletion layer width (t). The increase of the leakage current (IL) with higher amounts of Y2O3 added can be ascribed to the increase in donor density (ND) as well as to the increased amount of Y2O3-containing phase at the grain boundaries of ZnO.