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-wa ve and microwave sintering processes are superior to the conventional sinte ring process for densifying Bi2O3-based ZnO materials. Moreover, the millim eter-wave sintering process not only densified ZnO materials at a higher ra te, but also resulted in significantly better varistor characteristics, as compared with the microwave sintering process. A large nonlinear coefficien t and low leakage current density were attained by millimeter-wave sinterin g the samples under 1100 degrees C for 5 min. A density as high as 95% theo retical density (T.D.) has been achieved, accompanied by a nonlinear coeffi cient (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 p otential barrier height (phi(b)) around 2.22 eV. However, the varistor char acteristics, including the nonlinear coefficient (a) and leakage current de nsity (J(L)), degraded markedly fur samples sintered at too high a temperat ure or soaked for too long. This was ascribed to the occurrence of abnormal grain growth, accompanied by the loss of Bi2O3 and ZnO species.