Effect of impulse current degradation on the electrical properties and dielectric relaxations of ZnO-based ceramic varistors

The effects of impulse current degradation on the electrical properties and dielectric relaxations of zinc oxide (ZnO) ceramic varistors are systematically investigated in this work. The J-E characteristics show that the breakdown field E1mA (electric field at 1mA/cm2) increases, stabilizes, and then decreases with increasing impulse degradation, while the nonlinear coefficient α drops significantly over the entire degradation process. The DC conduction mechanism is consistent with a Schottky thermionic emission and the impulse degradation decreases the barrier height from 1.05 to 0.48 eV. It is also found that the electrical properties of the degraded samples can be partly recovered after removing the impulse degradation. SEM, XPS observations of the microstructure show increased voids at the grain boundaries and the content of chemisorbed oxygen is greatly reduced for degraded samples. Dielectric relaxations are characterized over a wide frequency and temperature range. Two dielectric relaxation peaks with activation energies of 0.25 and 0.37 eV are observed in 253 K, which do not change with impulse degradation. Another two dielectric relaxation with activation energies of 0.72 and 0.83 eV are characterized by δϵ'/δω spectra over 423-473 K. The activation energy of one of the latter peaks decreases from 0.83 to 0.76 eV with impulse current degradation and may be related to interfacial states at grain boundaries. A schematic energy band diagram is proposed to interpret the degraded process of the ceramic varistors.