A defect model for the grain-boundary barrier was proposed to explain the phenomena of voltage instability/stability of the ZnO varistor. The key element of the proposed model was the zinc interstitials which were present in the depletion layer as excess zinc, arising from the non-stoichiometric nature of ZnO. Both instability and stability were described in terms of diffusion of these interstitials in the depletion layer, followed by chemical interactions with defects at the grain-boundary interface. Finally, a large body of experimental data was presented to indirectly validate the proposed defect model.

A Grain-Boundary Defect Model for Instability/Stability of a ZnO Varistor. Gupta, T.K., Carlson, W.G.: Journal of Materials Science, 1985, 20[10], 3487-500