Influence of Cr₂O₃ on electrical and microstructural characteristics of ZnO–V₂O₅–MnCO₃–Nb₂O₅ varistor ceramics

In order to clarify the controversy on whether Cr₂O₃ can enhance the nonlinearity of ZnO based varistor ceramics, ZnO–V₂O₅–MnCO₃–Nb₂O₅ (ZVMN) ceramics with 0-0.5 mol% Cr₂O₃ were synthesized by traditional mixing oxide procedure ended by sintering at 900 °C for 3 h. Then XRD, FESEM, EDS and V–I tests were used to investigate the influence of Cr₂O₃ on electrical and microstructural characteristics of the obtained ceramics. The result shows ZVMN ceramics with Cr₂O₃ addition consisted of Zn₃(VO₄)₂, ZnV₂O₄ and ZnCr₂O₄ as the minor secondary crystalline phases and ZnO as the main phase. The average size of ZnO grains decreased monotonously with the increase of Cr₂O₃ concentration from 0 to 0.5 mol%, which in turn led to a monotonous elevation of the breakdown field from 354 to 1032 V/mm. The nonlinearity coefficient of ZVMN ceramics increased to its maximum value of 41.0 at 0.1 mol% Cr₂O₃, while the breakdown field and leak current density were 415 V/mm and 95.0 μA/cm² respectively. The comparison of this study with those of peers suggests Cr₂O₃ is more likely to enhance the nonlinearity of ZnO–V₂O₅ based varistor ceramics than it does in the case in ZnO–Bi₂O₃ system, wherein co-doping Sb₂O₃ may be a necessary step to secure an overall enhancing effect of Cr₂O₃ on the nonlinearity.