Microstructures and characteristics of deep trap levels in ZnO varistors doped with Y2O3

In this paper discussions on ZnO based varistor ceramics doped with different ratios of Y2O3 are presented. Analysis on the phase and microstructures of the samples indicates that an additional phase is detected in the samples doped with Y2O3, and the average grain size of the specimens decreases from about 9.2 mu m to 4.5 mu m, with an increase in the addition of Y2O3 from 0 mol% to 3 mol%. The corresponding varistor's voltage gradient markedly increases from 462 V/mm to 2340 V/mm, while the nonlinear coefficient decreases from 22.3 to 11.5, respectively. Furthermore, the characteristics of deep trap levels in these ZnO samples are investigated by measuring their dielectric spectroscopies. The trap energy level and capture cross section evaluated by relaxation peak of the Cole-Cole plot vary slightly as the addition of Y2O3 increases. These traps may be ascribed to the intrinsic defects of ZnO lattice.