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Positron-annihilation spectroscopy (PAS) was used in the lifetime mode to study changes in the grain-boundary defect equilibrium associated with de-bias-induced degradation of a ZnO varistor. The PAS lifetime spectra were collected while the sample was under an applied bias ranging from 100 V (-400 V /em) to 500 V (~ 2000 V /em). The current through the sample was continuously monitored. The simple trapping model was used to interpret the lifetime PAS results and to obtain an estimate for the positron-capture rate. The experimental results show that an• increase in the bias voltage results in a decrease in the positron-trap density and an increase in the positron lifetime associated with the dominant positron trap. These results are explained on the basis of a decrease in the concentration of negatively charged zinc vacancies at the grain boundary. The PAS results support the ion-migration model for degradation, which suggests that the bias-induced migration of positively charged zinc interstitials to the grain boundary reduces the concentration of negatively charged zinc vacancies at the boundary. This results in a reduction in the barrier height (degradation) and is consistent with the PAS data.

Positron annihilation spectroscopy (PAS) was performed in a Doppler mode to characterize the negatively charged grain-boundary defect, V′Zn, in ZnO varistor. The PAS study was conducted as a function of annealing treatment. As the annealing temperature increases from 400 to 600 °C the concentration of VZn increases, at intermediate temperatures from 600 to 800 °C the V′Zn concentration decreases, and finally, beyond 800 °C it increases again. These results are explained in terms of a grain-boundary defect model presented earlier [T. K. Gupta and W. G. Carlson, J. Mater. Sci. 20, 3487 (1987)]. The effect of quenching on PAS response was also explained in terms of a defect model.

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