Degradation of ZnO varistors as estimated by aging tests

Abstract

AbstractApplication of voltage during aging tests caused interstitial zinc ions to migrate in the depletion layer and to accumulate on the Schottky barrier, while oxygen ions were evaporated from the layer. Electrons were trapped on the Schottky barrier by the evaporation of the ions, keeping the electric charge neutral in the layer. During no‐voltage periods, interstitial zinc ions that had accumulated were returned to the layer, and oxygen ions were absorbed and returned to it. Trapped electrons were removed by absorption. Thus, the Schottky barrier was changed by the application of a voltage and was restored by removal of the applied voltage, resulting in a change in the resistive leakage current. The evaporated ions were among the released ions, which included oxygen ions dissociated by the continuous operating voltage. The Arrhenius plots used to estimate varistor life crossed each other because the concentrations of interstitial zinc ions and dissociated oxygen ions were lower in the more highly degraded varistors. Thus, it was inferred that cracks occurred in the layer, because ions cannot migrate very well. As a consequence, the resistive leakage current in the more highly degraded varistors was increased because the ZnO grain interfaces were spread by cracks, thus lowering the Schottky barrier. Cracks were caused by dissociated ions which made the Schottky barrier higher and increased stresses, because in the presence of these ions, electrons trapped by evaporation produced a stronger electric field, and oxygen vacancies resulted in a greater positive charge. To prevent cracks, methods based on stress mitigation and reducing oxygen ion migration are suggested. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 170(2): 1–18, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20934