Investigation of the electrical properties of liquid-phase sintered ZnO–V₂O₅ based varistor ceramics using impedance and dielectric spectroscopy

The influence of Nb₂O₅ and a mixture of Nb₂O₅ and MnCO₃ on the electrical and dielectric properties of liquid-phase sintered ZnO–V₂O₅ ceramics were studied by impedance spectroscopy over a frequency range from 10 mHz to 1 MHz at different temperatures. The impedance data, represented by means of Nyquist diagrams, show two time constants with different activation energies, one at high frequencies and the other at low frequencies. These activation energies were associated with the adsorption and reaction of O₂ species at the grain boundary interface. The resistance and the capacitance of grain-boundary regions were determined by modeling the experimental results using equivalent circuits. Analysis of the frequency dependence of the impedance of the material shows the presence of a non-Debye type of relaxation. The Arrhenius plots show two slopes with a turnover at 150/200 °C for both the higher and lower frequency time constants. These behaviors can be related with the decrease of minor charge carrier density. Consequently, better varistor performance is achieved for 97.4 mol% ZnO + 0.5 mol% V₂O₅ + 0.10 mol% Nb₂O₅ + 2.0 mol% MnCO₃ with nonlinear coefficient α = 24.3, breakdown field E_1mA = 498.5 V/mm and leakage current density J_L = 63.114 µA/cm². X-ray diffraction and scanning electron microscopy techniques were used to characterize the crystal structure and surface morphology of the material respectively. For all the samples, other than the major ZnO phase, Zn₃(VO₄)₂ were detected as minor secondary phases. SEM morphology shows that the average grain size depends on the Nb and Mn content.