Preparation And Research Of High Potential Gradient Chip/Thick-Film ZnO Varistors

In this thesis, high potential gradient chip/thick-film ZnO varistors were prepared by rare-earth doping and low-temperature sintering. With the comparison of electrical properties, phase composition and microstructure, the preparation process was optimized. The effect of the process on the potential gradient was also investigated. In addition, recombination effect was taken into consideration to explain the conductive mechanism in the prebreakdown region of high potential gradient ZnO varistors. The experiment results were in agreement with computer simulation data and validated the high value of potential gradient.It was found that the promotion of potential gradient was very obvious by Y2O3-doping in both samples. The samples with maximum values of potential gradient could be obtained by doping 0.08 mol% Y2O3. It achieved 2460.5 V/mm in chip samples, and 3159.4 V/mm in thick-film samples. The increase amplitudes were 49% and 19%, respectively. The improvement of potential gradient is mainly resulted from the decrease of ZnO grains. In general, the potential gradient is closely related to the number of grain boundaries. The more the number of grain boundaries per unit thickness, the higher the potential gradient. In our experiment, the average grain size decreased by 21% in chip samples and 13% in thick-film samples, which was the origin for the increase of potential gradient.Additive Y2O3 existed in the form of Y2O3 phase in the ZnO varistors. Deliquescent Y2O3 particles between ZnO grains restrained the grain growth. In the low doping concentration areas, the grain size sharply decreased and it promoted the potential gradient significantly. With the increasing Y2O3 content, the tensile stress resulted from the lattice distortion influenced the grain boundary characteristics and reduced the potential gradient. In the high doping concentration areas, the effect of doping concentration on residual stress was much more remarkable than that on grain size, so the potential gradient began to decrease. The optimal Y2O3 doping concentration was 0.08 mol% and the value of potential gradient reached the maximum under this doping condition.In the sintering process of ZnO varistors, the sintering temperature had an important effect on the potential gradient. High-energy ball milling in the early period of experiment induced the grain refinement and made the grain size down to quasi-nanometer level, which could realize the wetting of ZnO grains at the lower temperature. The volatilization of Bi2O3 and the solid solution of Y3+ions were serious at the conventional sintering temperature of 1200℃. And exaggerated grain growth occurs at this sintering temperature. Due to the exaggerated grain growth and the volatilization of Bi2O3, Y2O3 doping nearly made no improvement on the microstructure of ZnO varistors during the high temperature sintering. In our experiment, for the chip samples, the best sintering temperature was 800℃, and it was 725℃for the thick-film samples.It is well known that the I-V characteristics of ZnO varistors are based on the model of double Schottky barriers. High and narrow barrier is helpful to improve the electrical properties of ZnO varistors, especially in potential gradient and nonlinear coefficient. The barrier of chip samples formed at the sintering temperature of 800℃had a height of 0.89eV and a width of 15 nm. The barrier of thick-film samples formed at the sintering temperature of 725℃had a height of 0.81eV and a width of 10 nm. These showed that the grain boundaries formed at the given sintering temperatures were good enough to promote the nonlinear properties of the samples.ZnO varistors begin to degrade because of gradually increasing leakage current with the stress time. The electrical stability is a technologically important characteristic of varistors. All the chip samples had an increase in leakage current with the increasing stress time and this indicated that the varistors were degraded under the DC bias. The chip samples sintering at high temperature of 1200℃had a larger variation in leakage current during the DC stress. In the light of the results, it could be concluded that the resistance against DC aging stress was greatly affected by the sintering temperature. Higher sintering temperature of 1200℃directly made the electrical stability become worse. After being stressed for 20 h, the leakage current of chip samples sintering at 800℃increased only from 4.45 to 57.96μA. Less change in leakage current showed that the chip samples sintering at 800℃had the better electrical stability.The dielectric parameters of thick-film ZnO varistors, including the permittivity, the dissipation factor and the resistivity, were measured in the frequency range of 102-107 Hz. The results showed that the thick-film samples sintered at 725℃possessed the highest permittivity and lowest dissipation factor in the lower measuring frequency range. In the low frequency range, the effective capacitance is originated from grain boundaries, while the dielectric loss mainly depends on the electric conductance and is determined by the grain boundary resistance. High permittivity and low dissipation factor indicated that the grain boundaries formed at 725℃were better. At the same time, the difference of the resistivity between low and high frequency range actually represents the resistivity difference of grain boundaries and ZnO grains. Therefore, due to the higher resistivity difference, the thick-film samples sintered at 725℃possessed the better grain boundary characteristics with lower leakage current and higher nonlinear coefficient.The electrical properties of ZnO varistors under high voltage condition will determine whether they can be applied to high voltage field. All the samples appeared high voltage stabilization and the stabilization voltage showed good agreement with the potential gradient. The stabilization coefficient of chip samples and thick-film samples were of 10-2 grade and 10-1 grade. The voltage stabilization properties of chip samples were much better with the best sample being obtained by sintering at the temperature of 800℃.The electrical properties of chip samples under high current surge were investigated. After doping 0.08 mol% Y2O3, the residual voltage ratio was 1.48, the maximum discharge current capacity was 1263 A and the energy absorption capacity was 404.7 J/cm3. High potential gradient samples had good performance in withstanding high current surge. The chip samples had a uniform size distribution after doping 0.08 mol% Y2O3. Smaller grain size with uniform size distribution was the origin for the promotion of the electrical properties under high current surge.The chip samples with different Y2O3 contents were given a series of current impulse (3kA,8/20μs) for a consideration of degradation in potential gradient. All the chip samples had a decrease in potential gradient after the current impulse. With the increase of impulse times, the decrease in potential gradient increased. The potential gradient of chip samples decreased in 9.8% after the second current impulse. And even after the sixth current impulse, the decrease remained in 13.8%. These indicated our chip samples had good performance under high current impulse and could be applied to high energy field.Lastly, the conductive behavior in the prebreakdown region of high potential gradient ZnO varistors was analysed. Recombination effect made theⅠ-Ⅴcharacteristics in the prebreakdown region depart from the ideal curve determined by thermionic emission effect. The simulation experiment results indicated that both thermionic emission and recombination effect played obvious roles in conductive mechanism of high potential gradient ZnO varistors, while the latter accounted for about 40% of the total amount.