Multiscale defect responses in understanding degradation in zinc oxide varistor ceramics

Dielectric responses of multiscale defects, including electron trapping behaviours of intrinsic point defects in grains, heterogeneous interface of depletion/intergranular layers, and dynamic responses of interface states at grain boundaries, were investigated in ZnO varistor ceramics via the DC ageing process. The dielectric relaxation peaks corresponding to zinc interstitials and oxygen vacancies increased, revealing that the donor densities gradually increase with ageing time. Dynamic response of the interface states, which were observed to undergo significant dc conductance, using traditional dielectric spectroscopy, is unveiled based on an optimised (∂ε'/∂lnω)/ε′ spectroscopy. The energy level of interface states monotonously decreased while the relaxation peak value increased, corresponding to decreased interface states with degradation time. Collectively, the degradation of Schottky barrier during DC ageing process is discussed based on multiscale defect responses with the help of the (∂ε'/∂lnω)/ε′ spectroscopy.