Effects Of Doping CaCO₃,Fe₂O₃ And CuO On Properties And Micro-Structure Of ZnO Varistor Ceramics

In this paper, the effects of CaCO₃,Fe₂O₃ and CuO doping content on properties of ZnO-Bi₂O₃-Sb₂O₃-Co₂O₃-MnO₂-Cr₂O₃ varistor have been systematically studied; and the coincidence Doppler broadening spectrum measurements have been performed in CaCO₃, Fe₂O₃ and CuO doped ZnO-Bi₂O₃-Sb₂O₃-Co₂O₃-MnO₂-Cr₂O₃ ceramic varistors. In analyzing the experimental data, the micro-mechanism of the effects of the cationic impurities, which introduced by doping CaCO₃, Fe₂O₃ and CuO, on the varistor properties and microstructure of the ZnO ceramic varistor has been studied and discussed. The experimental results are as follows:(1) With the increase of the CaCO₃ content in ZnO varistor, the varistor voltage V_1mA increasing; leakage current decreasing, non-linear coefficient increasing and the height of the peaks in the ratio curves of Doppler broadening spectrum of ZnO varistor decreasing. This can be explained as follows: in ZnO varistor doped CaCO₃, Ca ion precipitation mainly in the grain boundaries, soluted on the ZnO grain boundary in the form of solid particles and inhibit grain growing, so that the ZnO grains decreased, thereby increasing the varistor voltage, reducing the leakage current, increasing the non-linear coefficient. At the same time, the addition of CaCO₃ will inhibit grain growing and increase the grain boundaries or the defect concentration, so that the peaks in the ratio curves of Doppler broadening spectrum of ZnO varistor Ceramics decrease.(2) With the increase of the addition of Fe₂O₃ into ZnO varistor ceramic, the varistor voltage V_1mA and the non-linear coefficient decrease firstly and then increase, the minimum values have been found at the Fe₂O₃ content of 1.00% (molar volume); while the leakage current and the height of the peaks in the ratio curves increase firstly and then decrease; the maximum values have also been found at the Fe₂O₃ content of 1.00% (molar volume).The reasons for these changes are as following: as the addition of Fe₂O₃ less than 1.00%, Fe acts as a donor impurity and mainly solutes at the ZnO lattice, which leads to the increase of carrier concentration and thus the leakage current, and the decrease of varistor voltage and nonlinear coefficient. At the same time as the Fe atom has more high-momentum electrons than that of Zn atom, when Fe₂O₃ are soluted into ZnO lattice, an increase of high-momentum electron density has been taken, therefore the height of the peaks in the ratio curves of Doppler broadening spectrum of ZnO varistor increasing. When the addition of Fe₂O₃ higher than 1.00%, a large number of solid solution are found in the grain boundary, which gives rise to the increase of the varistor voltage and the non-linear coefficient, and the decrease of leakage current and the height of the peak in the ratio curve due to the increase of the phase boundaries.(3) With the increase of the addition of CuO into ZnO varistor ceramic, the varistor voltage V_1mA, leakage current I_L and the height of the peak in the ratio curve increase; while the nonlinear coefficient decreases.This can be explained as follow: at high temperature, CuO decomposited into Cu⁺, which will substitute for the Zn²⁺ in ZnO lattice and reduce the donor carrier concentration in the ZnO lattice. This gives rise to the increase of resistivity and the varistor voltage, and the decrease of nonlinear coefficient. At the same time, because of the Cu atoms with more high-momentum electrons than Zn atom, with the increase of the addition of CuO into ZnO varistor ceramic, the probability of positron annihilation with high-momentum electrons of Cu atom will increase, therefore, the height of the peaks in the ratio curve increases. The increase of the leakage current may be due to Cu acts as a donor impurity on the grain boundary.(4) The height of the peaks in the ratio curves of ZnO varistor with doping Fe₂O₃ molar content at 1.00% are higher than that of ZnO varistor with doping CuO Molar content at 1.00%, while the height of the peaks in the Fe ratio curve is lower than that of Cu ratio curve. The main reason for this result is that when the doping molar content of Fe₂O₃ is at 1.00% into ZnO varistor, most of the Fe substitute for Zn in ZnO lattice and form solid solution, which leads to the increase of density of high-momentum electron, thus increase the height of the peaks in the ratio curve. When the doping molar content of CuO is at 1.00% into ZnO varistor, a part of the Cu substitute for Zn in ZnO lattice and form solid solution, which leads to the increase of density of high-momentum electron, thus increase the height of the peaks in the ratio curve; while another part of Cu remain on the grain boundary and form solution, which leads to the increase of defect concentration, and therefore the decrease of the height of the peaks in the ratio curves.(5) The results of the micro-structural changes due to the addition of CaCO₃, Fe₂O₃ and CuO into ZnO-Bi₂O₃-Sb₂O₃-Co₂O₃-MnO₂-Cr₂O₃ ceramic varistor obtained from the analysis on the ratio curves of Doppler broadening spectrum are in agreement with the results obtained from the analysis on the electrical properties.