EFFECTS OF In_2O_3 DOPING AND SINTERING TEMPERATURE ON THE ELECTRICAL PROPERTIES OF ZnO VARISTORS

ZnO varistors are prepared using the 0.1-0.3mm ZnO powders. The effects of the sintering temperature, contents of In2O3 doping on the non-linear properties of ZnO varistors have been investigated. Theresults show that this kind of ZnO powder has a high sintering activity. It is suitable for making the low voltage varistors. The Vc decreases with the increase of sintered temperature, when the In2O3 content is fixed(0. 98 %, mass fraction), and increases with the increase of In2O3 contents when the temperature is steady.

Fabrication of ZnO-based thick film varistors with high potential gradient

ZnO-based thick film varistors have been fabricated by Y203 doping and low-temperature sintering, of which the sample with the best electrical properties has a high potential gradient value of 3159.4 V/mm. The effects of Y2O3 doping concentration and sintering temperature on the potential gradient of the samples were systematically investigated. The results show that the sample with the best electrical properties can be obtained by doping 0.08 mol% Y2O3 and sintering at 725℃. Under these optimum preparation conditions, the leakage current and the nonlinear coefficient are found to be 36.4 gA and 13.1. The sample with the best electrical properties has a grain size of 1.290um, a single grain boundary voltage of 4.08 V, a barrier height of 0.81 eV, and a depletion layer width of 10.2 nm, which are determined by thermionic emission. Small grain size with good grain boundary characteristics is beneficial to improve the electrical properties of varistors and promote the potential gradient.

DEGRADATION DUE TO ENERGY PULSE IN HIGH-ENERGY ZnO VARISTORS

The degradation phenomena due to the energy pulse in the high-energy ZnO varistors used for deexitation and overvoltage protection of hydroelectric generator are investigated. The energy pulse, obtained by releasing the energy stored in an inductor, can be equivalent to the combination of the DC field components and the energy component. The variations of the characterized voltages, nonlinear coefficients and pre-breakdown V-A characteristics, increase with the number of the applied energy pulse. The asymmetrical variations of the electric properties of the high-energy ZnO varistors after the energy pulse arise from the deformation of the double Schottky barriers due to the ion migration occuring in the depletion layer and in the grain boundary.

Improvement of sintering,nonlinear electrical,and dielectric properties of ZnO-based varistors doped with TiO2

The effects of TiO2 on sintering and nonlinear electrical properties of（98.5-x）ZnO–0.5MnO2–0.5Co2O3-0.5Bi2O3–xTiO2（x = 0.3,0.5,0.7,0.9 mol%） ceramic varistors prepared by the ceramic technique are investigated in this work.The optimum sintering temperature of the prepared samples is deduced by determining the firing shrinkage and water absorption percentages.The optimum sintering temperature is found to be 1200℃,at which each of the samples shows a maximum firing shrinkage and minimum water absorption.Also minimum water absorption appears in a sample of x = 0.9 mol%.Higher sintering temperature and longer sintering time give rise to a reduction in bulk density due to the increased amount of porosity between the large grains of ZnO resulting from the rapid grain growth induced by the liquid phase sintering.The crystal size of ZnO decreases with increasing TiO2 doping.The addition of TiO2 improves the nonlinear coefficient and attains its maximum value at x = 0.7 mol% of TiO2,further addition negatively affects it.A decrease in capacitance consequently in the dielectric constant is recorded with increasing the frequency in a range of 30 kHz–200 kHz.The temperature and composition dependences of the dielectric constant and AC conductivity are also studied.The increase of temperature raises the dielectric constant because it increases ionic response to the field at any particular frequency.

Effects of Rare Earth Oxide on Microstructure of Zinc Varistors

The influence of Pr6O11 on the microstructure of ZnO varistors was researched. The results of experiment indicate that addition of Pr6O11 leads to the change of the formation process of the spinel phase. A lot of pyrochlore phases （ Bi3Zn2Sb3O14 ） produce at about 700℃, and decompose to fine spinel phase （Zn7Sb2O12） when the temperature reaches to 900℃.This type of spinel phase makes ZnO crystal size minor. Also, the phase contained the Pr and different Pr oxides, which makes the whole material crystal size more uniform and compact. The fine structure improves the threshold voltage by about 60%, and modifies nonlinear coefficient of the ZnO material.

Mechanism and Development of TiO_2-Doped ZnO-Bi_2O_3-Based Varistors

This paper reviews the history of ZnO varistor,discribes its properties and recenttechnological status and forecasts its evolution.The future development trend is to produce the low-voltage high-energy multi-layer ZnO varistors.After the two additives are classified by their functions,the effect mechanism of Bi_2O_3 and TiO_2 additives are researched theoretically.TiO_2 will make ZnO graingrow bigger and V_ImA/mm be depressed down.Especially the colloid TiO_2 additive in the scale ofnanometer brings about a new method to realize the low voltage of ZnO varistor,which resolves theproblem of how to disturb nanometer powder evenly.Moreover the sintering temperature has prominenteffect on the electrical properties of ZnO varistors.Generally,the appropriate sintering temperature forlow-voltage ZnO varistor ceramics should not be more than 1 250℃.These provide an effective methodand rationale for studying low-voltage ZnO varistors.

Effect of SiO_2 addition on the microstructure and electrical properties of ZnO-based varistors

The microstructure and electrical properties of ZnO-based varistors with the SiO2 content in the range of 0-1.00mol% were prepared by a solid reaction route. The varistors were characterized by scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray spectrometry, inductively coupled plasma-atomic emission spectrometry, and X-ray photoelectron spectroscopy. The results indicate that the average grain size of ZnO decreases with the SiO2 content increasing. A new second phase （Zn2SiO4） and a glass phase （Bi2SiO5） are found. Element Si mainly exists in the grain boundary and plays an important role in controlling the Bi2O3 vaporization. The electric measurement shows that the incorporation of SiO2 can significantly improve the nonlinear properties of ZnO-based varistors, and the nonlinear coefficients of the varistors with SiO2 are in the range of 36.8-69.5. The varistor voltage reaches the maximum value of 463 V/mm and the leakage current reaches the minimum value of 0.11 μA at the SiO2 content of 0.75mol%.

ZnO varistors with high voltage gradient and low leakage current by doping rare-earth oxide

The surge arrester of 1000 kV gas-insulated substation (GIS) needs ZnO varistor with high voltage gradient to effectively improve the potential distribution along ZnO varistor column inside the metal-oxide surge arresters. In this paper, the elec-trical and structural parameters of ZnO varistors are changed by doping with some rare-earth oxides, and the mechanism which leads these changes is discussed. When rare-earth oxide additives are added into ZnO varistors, the growing speed is slowed down due to the stabilization of the new spinel phases formed in the grain-boundary by rare-earth oxide additives, then the size of ZnO grains is smaller, and the voltage gradient of varistor increases obviously. By adding suitable amount of oxides of metal Co and Mn, the leakage current can be effectively decreased and the nonlinearity coefficient increased. The novel ZnO varistor samples sintered with the optimal additives have a voltage gradient of 492 V/mm, and the nonlinearity coefficient of 76, but their leakage currents are only 1 μA.

Microstructures and characteristics of deep trap levels in ZnO varistors doped with Y_2O_3

In this paper discussions on ZnO based varistor ceramics doped with different ratios of Y2O3 are presented.Analysis on the phase and microstructures of the samples indicates that an additional phase is detected in the samples doped with Y2O3,and the average grain size of the specimens decreases from about 9.2μm to 4.5μm,with an increase in the addition of Y2O3 from 0 mol%to 3 mol%.The corresponding varistor’s voltage gradient markedly increases from 462 V/mm to 2340 V/mm,while the nonlinear coefficient decreases from 22.3 to 11.5,respectively.Furthermore,the characteristics of deep trap levels in these ZnO samples are investigated by measuring their dielectric spectroscopies.The trap energy level and capture cross section evaluated by relaxation peak of the Cole-Cole plot vary slightly as the addition of Y2O3 increases.These traps may be ascribed to the intrinsic defects of ZnO lattice.

Fabrication and electrical characteristics of flash-sintered SiO_(2)-doped ZnO-Bi_(2)O_(3)-MnO_(2) varistors

The dense ZnO-Bi_(2)O_(3)-MnO_(2)(ZBMS)varistors for x=0,1,2,3 wt% were fabricated by flash sintering method under the low temperature of 850℃ within 2 min.The sample temperature was estimated by a black body radiation model in the flash sintering process.The crystalline phase assemblage,density,microstructure,and electrical characteristics of the flash-sintered ZBMS varistors with different SiO_(2)-doped content were investigated.According to the XRD analysis,many secondary phases were detected due to the SiO_(2) doping.Meanwhile,the average grain size decrease with increasing SiO_(2)-doped content.The improved nonlinear characteristics were obtained in SiO_(2)-doped samples,which can be attributed to the ion migration and oxygen absorption induced by the doped SiO_(2).The flash-sintered ZBMS varistor ceramics for x=2 wt% exhibited excellent comprehensive electrical properties,with the nonlinear coefficient of 24.5,the threshold voltage and leakage current of 385 V·mm^(-1 )and 11.8μA,respectively.

Influence of processing on microstructure and electrical characteristics of multilayer varistors

The paper reports on the influence of processing on microstructure and electrical properties of multilayer varistors based on zinc oxide doped with Bi2O3,Sb2O3,Co2O3,MnO,Cr2O3,B2O3,and SiO2.0.5–1 wt%of AlF3–CaB4O7 was used as a new effective sintering aid.The behavior of green laminates during heating was characterized using differential thermal analysis and a heating microscope.As revealed by XRD,SEM,and EDS methods,the varistor layers are composed of ZnO grains of 1–5μm size,submicrometer spinel and pyrochlore grains situated at the ZnO grain boundaries,and nanometric Bi2O3-rich films surrounding ZnO grains.Complex impedance studies carried out in the frequency range of 0.01 Hz–2 MHz at temperatures changing from –30 to 150℃ imply the formation of semiconducting grains and insulating grain boundaries.Frequency dependence of dielectric permittivity shows a high plateau at lower frequencies,typical for barrier layer capacitance effect.The fabricated multilayer varistors show nonlinear current–voltage characteristics with a high nonlinear coefficient of 26–38.The breakdown voltage was found to decrease within the range of 66–130 V with sintering temperature increasing from 1000 to 1100℃.Good surge current capability of the varistors was confirmed by the tests using 8/20μs pulses.

A grain boundary defect model for ZnO ceramic varistors by deep heat treatment

Studies on ZnO ceramic varistors by deep heat treatment at 650\900 ℃ are reported. The current creep time curve exhibits a peak during the continuous action of a dc biasing voltage; the forward V I characteristic is improved rather than degraded after the action of the biasing voltage. We assume that the zinc interstitial cations Zn · i are out diffused rapidly and the concentration of Zn · i in the depletion layer is decreased rapidly during deep heat treatment; the oxygen anions O′ O could be accumulated at the grain interface if the out diffusion quantity of Zn · i is not enough to react with the O′ O ; the current creep phenomenon above results from the migration of the interface O′ O by the biasing voltage. We suggest an improved grain boundary defect model for the ZnO varistors by deep heat treatment, and examine the model using the experimental data of lifetime positron annihilation spectroscopy.

Percolation effects in the capacitive properties of metal-oxide varistors in the range of high voltage

C-V characteristics of ZnO-based ceramic structures used in manufacturing high-voltage and low-voltage varistors of different chemical compositions and manufacturing techniques have been investigated.A correlation between the intensity of electric field corresponding to transition of the C-V characteristics to the negative capacitances and average sizes of grains of a varistor structure has been established.Obtained data have been interpreted with the use of notions of the percolation theory of electric conductivity.The Shklovskii-De Gennes model has been used.It has been shown that on the highly nonlinear segment of C-V characteristics of a varistor structure,the size of an infinite cluster are limited to several intercrystallite potential barriers.This result is observed in all kinds of investigated varistor ceramics.

Preparation and characterization of layered low-voltage ZnO varistors

Double-layered, low-voltage ZnO varistors have been fabricated by feeding two kinds of ZnO powders into a die using dry extrusion molding. Compared with ZnO varistors fabricated by the conventional route, the layered ZnO varistors have larger non-linear coefficients, lower breakdown electric fields, and lower leakage current densities. The improvement in electrical performance of the layered low-voltage ZnO varistors is attributed to the asymmetric band structure at grain boundary between the two layers.

Percolation effects in dc degradation of ZnO varistors

For quantitative estimation of the degree of electrical disorder(electrical inhomogeneity)in ZnO varistor ceramics caused by a variation in the barrier height at different grain boundaries in a sample,the comparison of threshold electric fields(onsets of highly nonlinear current-voltage characteristics)in ceramics and single grain boundary(GB)is suggested and approved.At dc degradation similar behavior of the current-voltage characteristics of ZnO varistor ceramics and single GB is observed.The percolation model of Shklovskii-De Gennes is applicable for the description of a disorder in ZnO varistor ceramics.The degree of the disorder in ZnO varistor ceramics is not dependent on the duration of dc degradation at least at degradation time below 60 h.At voltages close to the onset of a highly nonlinear region of current-voltage characteristic the correlation radius of infinite cluster is~5 times greater than the average grain size.

高温超导储能磁体MOV保护方法的研究

超导磁储能系统(SMES)电感大,在与电力系统进行快速的能量交换时超导磁体两端将产生较大的感应电压,并受到内部饼间环流的影响,而磁体绝缘水平有限,其过电压保护一直是人们关注的热点。本文从电力系统对入网设备的要求出发,提出并联金属氧化锌阀片(MOV)的高温超导磁体保护方法,并与常规技术进行了比较,不同标准雷电波和周期振荡波冲击以及自放电条件下的仿真和试验。结果表明:该方法有效地限制磁体两端过电压,并且能作为超导磁体的泄能电阻,快速释放能量。

Nonlinear Electrical Properties of SnO_2-Li_2O-Nb_2O_5 Varistor System

The electrical properties of (Nb, Li)-doped SnO2 ceramics as a new varistor material were investigated. The sample 97.95%SnO2·0.50%Li2O·0.05%Nb2O5 (mol fraction) sintered at 1450= possess the highest density (ρ=6.77 g/cm3) and nonlinear electrical coefficient (α=11.6). The substitution of Sn4+ with Li+ increases the concentration of oxygen vacancies, together with the formation of solid solution, which will increase the sintering rate greatly and decrease the optimized sintering temperature. The substitution of Sn4+ with Li+ and the variation of temperature play very important effects on the densities, dielectric constant, nonlinear electrical properties and other characteristics of the samples. The properties of the grain boundary barrier and the microstructural characteristics were investigated to ensure the effect of the dopants and the temperature. A grain boundary defect barrier model was used to illustrate the grain boundary barriers formation in SnO2-Li2O-Nb2O5 varistors.

Effects of Aluminum Doping on the Microstructure and Electrical Properties of ZnO- Pr_6O_(11)-Co_3O_4-MnCO_3-Y_2O_3 Varistor Ceramics

Abstract： The effect of Al_2O_3 doping on the microstructure and electrical properties of the ZnO- Pr_6O11-CO_3O_4-MnCO_3-Y_2O_3 system was investigated in the range of 0.0-0. lmol%. The results reveal that Al_2O_3 doping has slight influence on the densification process. The microstructure of the ceramics comprises of ZnO phase, ZnAl_2O_4 spine phase and Pr-rich phases. The addition of Al_2O_3 greatly affects the electrical properties. The varistor voltage （E_1mA/cm^2） of ZPCMYAl samples decreases over a wide range from 5 530 V/cm to 1 844 V/cm with the increasing Al_2O_3 content. The nonlinear exponent（a） increases with the increasing Al_2O_3 content to 0.01mol%, whereas it is decreased by the further doping. The ZPCMYAI-based varistor ceramics with 0.01mol% Al_2O_3 exhibit the best electrical properties, with the nonlinear exponent （ct） attaining the highest value of 33.4 and the lowest leakage current of 2.7 μA. The capacitance-voltage （C-V） measurement shows that the donor density （Nd） at the grain boundaries increase from 1.58×10^18 to 3.15×10^18 cm^-3, the barrier height （Чb） increases from 1.60 to 2.36 eV, and the depletion layer width （t） decreases from 24.9 to 21.6 nm.

Microstructure and electrical properties of Y_2O_3-doped ZnO-based varistor ceramics prepared by high-energy ball milling

Y2O3-doped ZnO-based varistor ceramics were prepared using high-energy ball milling （HEBM） and low-temperature sin- tering technique, with voltage-gradient of 1934-2197 V/mm, non-linear coefficients of 20.8-21.8, leakage currents of 0.59-1.04 μA, and densities of 5.46-5.57 g/cm3. With increasing Y2O3 content, the voltage-gradient increases because of the decrease of ZnO grain size; the non-linear coefficient and the leakage current improve but the density decreases because of more porosity; the donor con- centration and density of interface states decrease, whereas the barrier height and width increase because of the acceptor effect of Y2O3 in varistor ceramics.