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.