Varistor and dielectric properties of Cr2O3 doped SnO2–Zn2SnO4 composite ceramics

• The Schottky barriers are mainly located at SnO2–SnO2 or SnO2–Zn2SnO4 grain boundaries.
• The formation of the Schottky barriers is related to the negative trapped charges and depletion layer.
• The negative trapped charge layer is mainly composed of O′ and O″.
• Each SnO2–SnO2 or SnO2–Zn2SnO4 grain boundary is characterized by low breakdown voltage and high dielectric constant.

Cr2O3 doped SnO2–Zn2SnO4 composite ceramics were prepared by traditional ceramic processing and the varistor, dielectric properties were investigated. With increasing Cr2O3 content, the breakdown electrical field EB increases from 11 to 92 V/mm and the relative dielectric constant εr measured at 1 kHz, 50 °C decreases from 11,028 to 3412, respectively. The barrier height ϕB about 0.8–0.84 eV and the decreasing of SnO2 grain size suggest that the varistor behavior with high εr is originated from SnO2–SnO2 or SnO2–Zn2SnO4 grain boundary. In the dielectric spectra lower than 1 kHz, a dielectric peak is presented and depressed with increasing bias voltage. Similarly, at high temperature, the dielectric constant also presents a dielectric peak in the temperature spectra and the peak becomes faint with increasing frequency. The exhibition of the dielectric peak is thought to be attributed to the conduction of grain boundary since it is accompanied by the sharp increase of dielectric loss. In addition, a dielectric relaxation with the activation energy about 0.4–0.5 eV was observed in the temperature range of 20–100 °C. Based on the results, the formation mechanism of Schottky barriers at grain boundaries and the varistor behavior with high dielectric constant are well understood.