Electrical Properties of Microwave-Sintered ZnO Varistors

Bi2O3-based ZnO varistor materials were prepared by 2.45 GHz microwave, 24 GHz millimeter-wave or conventional sintering processes. Both millimeter-wave and microwave sintering processes are superior to the conventional sintering process for densifying Bi2O3-based ZnO materials. Moreover, the millimeter-wave sintering process not only densified ZnO materials at a higher rate, but also resulted in significantly better varistor characteristics, as compared with the microwave sintering process. A large nonlinear coefficient and low leakage current density were attained by millimeter-wave sintering the samples under 1100°C for 5 min. A density as high as 95% theoretical density (T.D.) has been achieved, accompanied by a nonlinear coefficient (α) larger than 36, and a leakage current density (JL) smaller than 4.1×10-6A/cm2. The samples had a donor density (Nd) around 2.5× 1024m-3, a surface density (Ns) around 7.23×1011m-2and a potential barrier height (φb) around 2.22 eV. However, the varistor characteristics, including the nonlinear coefficient (α) and leakage current density (JL), degraded markedly for samples sintered at too high a temperature or soaked for too long. This was ascribed to the occurrence of abnormal grain growth, accompanied by the loss of Bi2O3and ZnO species.