DEGRADATION DUE TO ENERGY PULSE IN HIGH-ENERGY ZnO VARISTORS

ZnOvaristorshavebeenextensivelyappliedtoelectronicandelectricsystemsasenergysurgeabsorbersincetheywereinventedin196811--4j.Ithasbeenacommerciallysuccessfultechnologytousehigh--energyZnOvaristorsforde--e-citationandovervoltageprotectionofhydroelectricgenerator.Whenappliedinthisway,thevaristorshavetobesubjecttoacontinuouselectricstressandtoadiscontinuousenergypulse,whichcauseastaticdegradationandadynamicdegradationinthehighenergyZnOvaristors,respectively['--'].ThestaticdegradationduetotheDCst…     

The Microstructure and Electrical Behavior of TiO2 Varistors Processed by Magnetized Water

A comparative study of the microstructures of two kinds of TiO₂ based varistor materials processed from the slurry using magnetized water and regular deionized water is conducted. The additives in the form of oxides of Nb, Ce, Ca, and Si were used in the recipe. The electrical behavior reveals that the water magnetized at 0.2 T yielded lower varistor voltage while the water magnetized at 0.3 T yielded higher varistor voltage compared to the regular deionized water. A second phase comprising of Ce, Si, and Ti was found in each of these samples. The distribution of second phase was found strongly dependent on the type of water used. Thus, the magnetized water shows influence on the TiO₂ varistor microstructures. It apprehended that the application of magnetized water in the general arena of advanced electronic ceramics may bring novel experimental results as demonstrated in the present work.     

Aqueous Precipitation of Spherical Zinc Oxide Powders for Varistor Applications

Aqueous precipitation has been used to prepare spherical ZnO particles. These powders have been shown to be suitable for use in the preparation of varistors. The uniform size distribution and uniform coating of these powders with the appropriate dopants allowed fabrication of varistors with improved properties over those fabricated from conventionally ball-milled and calcined powders. The improved properties were a high coefficient of nonlinearity (∼44) in the nonohmic region, a sharp change in electrical behavior from ohmic to nonohmic, and a high resistivity (5×10¹² SOM°m) at low voltages.     

Deep Levels Near the Grain Boundary in a Zinc Oxide Varistor: Energy Change Due to Electrical Degradation

Localized deep levels in a ZnO varistor were investigated by isothermal capacitance transient spectroscopy, in relation to a degradation of nonlinear electrical characteristics caused by thermal treatment in a slightly reduced atmosphere. Two discrete deep levels were observed near the grain boundary. Before the degradation, they were located at 0.36 and 0.19 eV below a conduction band. After the degradation, they shifted toward the conduction band by 0.16 and 0.09 eV, respectively: the energy shift was not identical for the two levels. It was suggested that the energy shift of the localized levels reflected lattice disorders originally existing near the grain boundary or proper changes of chemical states induced by the reduction.     

The effect of sintering temperature variations on the development of electrically active interfaces in zinc oxide based varistors

A series of zinc oxide based varistors containing 0.5 wt.% Bi₂O₃ and 0.5 wt.% Mn₂O₃ were prepared by a conventional mixed oxide route and sintered at temperatures between 950 and 1300°C. All samples showed the varistor effect, although as the sintering temperature was increased above 1000°C, the non-linear coefficient decreased from 22 to 3 at 1300°C. Local grain boundary property measurements were carried out using remote electron beam induced current (REBIC) configuration conductive mode scanning electron microscopy. The proportions of electrically active interfaces and those showing strong resistive contrast were found to increase with sintering temperature.     

Improved chemical stability of ZnO–BaO based varistors

Non-linear current–voltage response of ZnO–BaO based ceramic materials makes them valuable for varistors manufacturing. Although this behaviour is quantitatively similar to that observed in the binary ZnO–Bi₂O₃ system, their commercial use is limited because the BaO rich secondary phase is severely damaged by moisture due to its high solubility in water. In the present work, doping with P₂O₅ to form BaZn₂(PO₄)₂ and Zn₃(PO₄)₂ has been studied in order to improve the resistance of ZnO–BaO materials to degradation by moisture. The water solubility of sintered samples has been evaluated by lixiviation experiments. Sintering behaviour and microstructure development have been followed by dilatometry and microstructure was analysed by scanning electron microscopy. The electrical properties of these materials evidence varistor behaviour with non-linearity values similar to those observed for binary materials ZnO–BaO and ZnO–Bi₂O₃.     

Differences in Wetting Characteristics of Bi2O3 Polymorphs in ZnO Varistor Materials

Detailed analysis of the microstructure of grain boundaries, especially triple-grain and multiple-grain junctions, in ZnO varistor materials has been performed using transmission electron microscopy. Different polymorphs of Bi₂O₃ are shown to exhibit different wetting properties on ZnO interfaces. Recent investigations suggest that the equilibrium configuration consists of crystalline Bi₂O₃ in the triple-grain and multiple-grain junctions and an amorphous bismuth-rich film in the ZnO/ZnO grain boundaries. The present investigation supports this suggestion for δ-Bi₂O₃ and also adds to the microstructural image and wetting properties of α-Bi₂O₃.     

Inhomogeneous Conductivity in Varistor Ceramics: Methods of Investigation

The grain boundaries in zinc oxide ceramics exhibit different electrical behaviors. This results in separate paths of current flux through the microstructure and also in different breakdown voltages of each path. The paper describes some new methods for characterization of these paths. A galvanic determination is able to show the number of paths and their distribution across the varistor surface. The differences in breakdown voltage are visible using a line scan method. Current images in SEM can detect the paths of current along a varistor surface. Possible reasons for inhomogeneous current flux are inhomogeneous distribution of dopants, insufficient binder burnout, and pressing faults.     

氧化锌压敏陶瓷及其元件研究进展

介绍了氧化锌压敏陶瓷在次晶界现象，几何效应以及高温热释电等方面的应用基础研究成果，指出氧化锌压敏电阻器的发展方向为片式化和提高能量耐受能力。