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 A degrees 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 Zn-3(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 mu A/cm(2) 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.