Characterization of ZnO-degraded varistors used in high-tension devices

The effects of the degradation process on the structural, microstructural and electrical properties of ZnO-based varistors were analyzed. Rietveld refinement showed that the BiO_2−x phase is affected by the degradation process. Besides the changes in the spinel phase, the degradation process also affects the lattice microstrain in the ZnO phase. Scanning electron microscopy analysis showed electrode-melting failure, while wavelength dispersive X-ray spectroscopy qualitative analysis showed deficiency of oxygen species at the grain boundaries in the degraded samples. Atomic force microscopy using electrostatic mode force illustrated a decrease in the charge density at the grain boundaries of the degraded sample. Transmission electron microscopy showed submicrometric spinel grains embedded in a ZnO matrix, but their average grain size is smaller in the degraded sample than in the standard one. Long pulses appeared to be more harmful for the varistors’ properties than short ones, causing higher leakage current values. The electrical characteristics of the degraded sample are partially restored after heat treatment in an oxygen-rich atmosphere.     

高电位梯度ZnO压敏电阻的高压稳压性能研究

采用稀土掺杂和低温烧结的方法制备出高电位梯度的ZnO压敏电阻,并研究了Y2O3掺杂浓度对试样电位梯度和稳压性能的影响.结果表明,在800℃的烧结条件下,掺杂0.08%(摩尔分数)Y2O3后试样的电位梯度增加了49%,稳压系数达到1.99×10-2.从微观角度对ZnO压敏电阻烧结过程中的物化反应分析发现,较低的烧结温度能够抑制Y2O3的受主固溶和Bi2O3的挥发,减缓晶粒的生长速度,改善晶界质量,从而提高试样的电位梯度,实现高压稳压.     

高能ZnO压敏电阻在发电机灭磁中的应用

论述了发电机灭磁原理及要求.高能ZnO压敏电阻作为大容量吸能元件,由于其优异的非线性特性和浪涌吸收特性,在发电机灭磁中具有优越性,得到很好的应用.介绍了高能ZnO压敏电阻的制造方法,在此基础上论述了改善高能ZnO压敏电阻能量耐量和通流性能的方法,最后提出了要规范高能ZnO压敏电阻性能参数和检测标准.     

Cooling rate effects in ZnO varistors

Cooling rate effects on ZnO-Bi₂O₃ varistors are presented. In particular, the oxygen vacancies (V _o ⁺ ) concentration was determined by electron paramagnetic resonance (EPR) spectroscopy and related to the conductivity. It was found that a higher cooling rate provokes an increase of the V _o ⁺ concentration and a higher conductivity. Current density versus temperature curves are fitted with a modified Schottky barrier model which reflects a higher active donor concentration near the grain interface.     

叠层片式ZnO压敏电阻的微观结构研究

通过光学显微镜、扫描电镜观察及能谱分析等方法，分析研究了叠层片式压敏电阻器（MLV,mutilayer varistor）的微观结构．实验结果表明，在半成品叠层片式压敏电阻器的微观结构中ZnO晶粒较小且不均匀，结构较疏松，强度差．而成品由于进行了低温液相烧结，其微观结构则为较规则的ZnO晶粒，晶粒变大尺寸较为均匀，致密度明显提高．由于在陶瓷叠片上内电极浆料分布不均匀或电极印刷的原因，经过烧结后个别地方可能出现内电极层的缺损，以至观察到的内电极是断续的。同时观察到空洞可影响到内电极扣外电极的连接，以及电极变形等现象。这些缺陷结构的形成与叠层片式压敏电阻器的制作工艺有关．     

A model of non-Ohmic conduction in ZnO varistors

A simple phenomenological approach to non-Ohmic conduction in zinc oxide based varistors is suggested. The decrease of the barrier height on voltage is considered as a reason of varistor effect. This model gives the relationship between the current density j and the average electric field E in the form , where the nonlinearity factor α is proportional to the rate of change of the barrier height on voltage . The nonlinearity factor α or the normalized nonlinearity coefficient (E ₁ is the electric field at fixed current density) can be used instead of the traditional but empirical nonlinearity coefficient . Fairly reasonable agreement between suggested model and experimental results is found. On leave from Dniepropetrovsk National University, Dniepropetrovsk, Ukraine; Fax: +52-953-5320214, ext. 106     

ZnO compact layers used in third-generation photovoltaic devices: a review

Journal of Materials Science - ZnO is a well-known semitransparent semiconductor with wide applicability in semiconducting devices such as solar cells, LEDs, MOSFETs, gas sensor devices, or...     

The influence of dislocations on the nonlinearity of ZnO:Cu varistors

Zinc Oxide doped only with Cu shows highly nonlinear I–V characteristics. Microstructural observations of these ceramics reveal the presence of extensive dislocation network. The transmission electron microscopy (TEM) indicates that the dislocations are impurity decorated which arise as a result of limited solubility of CuO in ZnO. It is envisaged that the depletion region is generated in the region containing the dislocations because of the presence of acceptor type traps.     

AC ageing characteristics of Y2O3-doped ZnO varistors with high voltage gradient

In this paper, AC ageing characteristics of high voltage gradient ZnO varistors doped with different Y₂O₃ concentration were investigated. The voltage gradient of these samples is markedly improved by increasing Y₂O₃ content, however, the nonlinear coefficient decreases and the leakage current increases at the same time. Y₂O₃-doped ZnO samples exhibit lower stability under accelerated AC ageing stress of 0.85 V_1mA/135 °C/168 h, compared with samples without Y₂O₃. Double-Schottky barrier(DSB) parameters before and after ageing tests indicate that the decrease of barrier height for traditional sample is less than that for high voltage gradient sample, which should be ascribed to its slight variation in the interface state density.     

Electrically active grain boundaries in ZnO varistors by liquid-infiltration method

The varistors were fabricated by spreading a thin layer of Pr₆O₁₁ powder paste on the surface of ZnO pellets and heating to various temperatures (1200–1400 °C) and times (0–60 min). Higher heat-treatment temperatures and/or times resulted in progressively higher breakdown voltages. Eventually the devices became varistor, which was attributed to the formation of a liquid (ZnO-PrO_x) layer between the grains. Microstructures of cross-sections of wetting pellets have shown that the infiltration rate was increased with the amount of Co₃O₄ and heat-treatment temperature. In addition, on the basis of the small variations of the varistor properties per grain boundary (e.g., threshold voltage, donor concentration, and barrier height), the number of active grain boundaries are believed to be increased when the samples were heat-treated above the liquid-phase temperature.     

The relation between residual voltage ratio and microstructural parameters of ZnO varistors

The effect of the sample thickness d on the residual voltage ratio K_r indicates the existence of the dimensional effect in the commercial ZnO varistors. The empirical relations derived are based on experimental findings for K_r and breakdown electric field strength E_{1 mA cm⁻²} and the microstructural parameter average grain size μ. It is observed that K_r decreases with increasing E_{1 mA cm⁻²}, while it increases with increasing μ. An integrated hybrid parameter σ²μ, obtained as the product of average grain size μ and grain size variance σ², is found to be a representative relation between electrical properties and microstructure of the ZnO varistors. A microstructural model is used for computer simulation that also verified the dimensional effect in these varistors. The simulated results are consistent with the experimental results.     

提高ZnO变阻器电性能的研究

ZnO变阻器电气性能的提高对避雷器的运行可靠性起着主要作用.本文首先采用两种新型球磨工艺制备了细化的ZnO陶瓷变阻器的添加剂.并且通过改进粘结剂和无机/有机复合侧面绝缘层配方,使ZnO变阻器的残压比、2ms方波通流、4/10μs大电流冲击等电性能有了大幅度的改善.     

钛的复合掺杂对ZnO压敏电阻器性能的影响

本文研究了钛以不同的复合物加入时，对ZnO压敏电阻器电性能的影响。实验表明，不同的复合物掺杂对ZnO压敏电阻器电性能影响规律不同，当钛的复合物在压敏电阻器烧结过程中有利于钛溶于晶界处的富铋液相中时，则有利于晶粒的生长。     

Effects of cobalt doping on the electrical characteristics of Al-doped ZnO varistors

The effects of cobalt additive on the electrical characteristics of the Al-doped ZnO varistors are studied in this paper. The current-voltage characteristics of the varistor samples have been investigated in a range from small to large current. With the amount of the doped cobalt increased, the leakage currents of the Al-doped ZnO varistors are inhibited, and their nonlinear coefficients increase remarkably as well. In addition, their breakdown electric fields increase in a small extent and their residual voltage ratios change slightly. Furthermore, the donor densities and the barrier heights of samples decrease with the cobalt content increased. The XRD patterns show that the Bi-rich phase and the willemite phase vary obviously in the varistor samples with various content of cobalt additive.