Grain-boundary characterization of ZnO varistors by positron annihilation spectroscopy

Abstract

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.