ELECTRICAL NONUNIFORMITY OF GRAIN-BOUNDARIES WITHIN ZNO VARISTORS

ZnO varistors with and without ZnO crystalline seeds have been prepared through conventional ceramic processing. Their electrical nonuniformity has been carefully examined using microcontact measurements of single grain boundaries, current-voltage (I-V) characteristics, and dielectric temperature spectra of bulk samples. Three types of grain boundaries with barrier heights of 0.2, 0.5, and 0.6 eV are identified in a ZnO varistor with the seeds, while only one with 0.4 eV barrier height has been found in a varistor without the seeds. Using a computerized electric circuit simulation, the influence of boundary thickness and grain size on I-V characteristics of varistor is investigated extensively. The simulated results show that currents passing across various grain boundaries are quite different. For a model varistor (1 x 1 x 1 mm3) with different grain size, a good agreement between measured data and simulated curves can be achieved.