Investigation of dimensional effect in ZnO ceramics through high-temperature relaxation

The dimensional effect of ZnO-based ceramics was investigated from the aspect of high-temperature relaxation in the temperature range of 120-260 °C in this paper. A sharp decrease of breakdown field as well as nonlinear coefficient was observed under a critical thickness (~1.0 mm), when the sample was grinded gradually on both sides from 1.92 mm to 0.74 mm. Distribution of grain sizes along the thickness direction exhibited no distinct difference, indicating non-uniform characteristics of grain boundaries were likely responsible for the notable discrepancies on electrical properties. Responses of interface states, which paly a decisive role in Schottky barriers in ZnO ceramics, were employed to explain the dimensional effect. Interface states relaxation was found as a high-temperature relaxation, which was usually covered by intense dc conduction in traditional dielectric spectroscopy. By excluding the compound of dc conduction, it was successfully characterized via an improved dielectric spectroscopy. The activation energy was observed to decrease from 0.97 eV to 0.63 eV, when the sample was grinded from 1.92 mm to 0.74 mm. Significantly decreased activation energy suggested decreased density of interface states in the center due to nonuniform diffusion of oxygen during cooling process. Lowered Schottky barriers were consequently formed, which was in accordance with the degraded electrical properties. The results exhibited the relation between the dimensional effect and the high-temperature relaxation, which provides a potential approach to further understanding the dimensional effect.