Evaluation of ZnO varistors by photoacoustic spectroscopy

ZnO varistors degraded under various conditions were evaluated by photoacou stic spectroscopy (PAS). The degradation where the grain boundary is damage d by DC bias stress is more than that by AC bias stress. PAS, however, reve als that the interior of the grains of the sample degraded by AC bias stres s is much more damaged than that by the DC bias stress. The PA signal inten sity at a wavelength of more than 500 nm increases and the dispersion of th e spectrum decreases throughout the wide wavelength range considered with t he degradation time. In particular, the decrease of the spectral dispersion below 500 nm is caused by the change of the electronic states at the inter face. that is, the increase of the recombination center of the space charge . The annealing effect on the degradation of ZnO varistors was also studied . The PA spectrum of the sample annealed in N-2 gas corresponds to those of the sample degraded by DC and AC bias stress for a long time. This suggest s that the degradation of ZnO varistors is closely related to the release o f oxygen from both ZnO grain interior and grain boundaries.