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 degrees C for 5 min. A density as high as 95% theoretical density (T.D.) has been achieved, accompanied by a nonlinear coefficient (alpha) larger than 36, and a leakage current density (J(L)) smaller than 4.1 x 10(-6) A/cm(2). The samples had a donor density (N-d) around 2.5 x 10(24) m(-3), a surface density (N-s) around 7.23 x 10(11) m(-2) and a potential barrier height (phi(b)) around 2.22 eV. However, the varistor characteristics, including the nonlinear coefficient (a) and leakage current density (J(L)), degraded markedly fur 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 Bi2O3 and ZnO species.