Varistor Materials Research Articles

Influence of Ta2O5 doping and microwave sintering on TiO2-based varistor properties

TiO2 based varistor materials with Ta2O5 dopants are prepared by microwave sintering (MS) and conventional sintering (CS). The sintering characteristics, sintered microstructures, varistor voltage (V1mA), nonlinear coefficient (α) and leak current (IL) are studied. The influences of the doping amount of Ta2O5 on the above properties are discussed. For similar sintered densities, the microwave sintering can decrease the sintering temperature by about 250°Cwhile improving the microstructure of the samples. Microwave sintering leads to better properties than conventional sintering. The samples have the best overall properties when the doping amount of Ta2O5 is in the range from 0.075mol% to 0.125mol%.

Electron paramagnetic resonance study of ZnO varistor material

Matsuoka-type zinc oxide (ZnO) varistor material was synthesized using a conventional solid-state reaction method. X-band electron paramagnetic resonance (EPR) data revealed that Mn ions substitute in the ZnO lattice with a 2+ paramagnetic state. Co ions with either 3+ or 2+ oxidation states are only detectable at cryogenic temperatures. A Cr3+ EPR signal was strongly suppressed or masked by a Mn2+ signal. Photoluminescence and electrical results indicated that the varistor sample has fewer intrinsic defects and much higher resistivity as compared to undoped and metal-ion doped ZnO.

Effect of WO<sub>3</sub> Doping on Microstructural and Electrical Properties of ZnO-Pr<sub>6</sub>O<sub>11</sub> Based Varistor Materials

The effect of WO3doping on microstructural and electrical properties of ZnO-Pr6O11based varistor materials was investigated. The doped WO3plays a role of inhibitor in ZnO grain growth, resulting in decreased average grain size from 2.68 to 1.68 μm with increasing doping level of WO3from 0 to 0.5 mol%. When the doping level of WO3was lower than 0.05 mol%, the nonlinear current-voltage characteristics of the obtained varistors could be improved significantly with increasing amount of WO3doped. But when the doping level of WO3became higher, their nonlinear current-voltage performance would be dramatically deteriorated when more WO3was doped. The optimum nonlinear coefficient, varistor voltage, and leakage current of the samples were about 13.71, 710 V/mm and 13 μA/cm2, respectively, when the doping level of WO3was in the range from 0.03 to 0.05 mol%.

Progress on rare-earth doped ZnO-based varistor materials

Rare-earth (RE) doping can greatly enhance the voltage gradient of ZnO-based varistors, and their nonlinear coefficient, leakage current, energy absorption capability, through-current capability and residual voltage can also be improved to certain extent. In this review, the progress on RE-doped ZnO-based varistor materials in recent years was summarized. The mechanism of RE doping on the electrical performance of ZnO varistors was analyzed. The issues in exploring new ZnO-based varistor materials by RE doping were indicated, and the development trends in this area were proposed.

The Effects of B<sub>2</sub>O<sub>3</sub> Addition and Sintering Temperature on the Electrical Properties of SnO<sub>2</sub> Based Varistors

SnO2 based varistor ceramics, as new type of varistor materials, have attracted increasing interest around the world. However, the effect of B2O3 addition on novel SnO2 based varistors is still unclear. In this paper the effects of B2O3 addition on the electrical properties and sintering temperature of the SnO2-Co2O3-Ta2O5 system were investigated. The experiments revealed that addition of trivalent boron can improve the densification of SnO2 based ceramics. The samples doped with 0.025mol% B2O3 had the highest sintering density. However between 0mol% to 0.1mol% B2O3 addition region there was little effect on non-linearity and breakdown field of SnO2-Co2O3-Ta2O5 system. Obviously, the sintering temperature had a predominant influence on the SnO2-Co2O3-Ta2O5 system; the samples sintered at 1400°C were superior to those sintered at 1350°C.

Electrical properties of new tin dioxide varistor ceramics at high currents

New dense SnO 2 -based varistor ceramics with high nonlinear current–voltage characteristics (nonlinearity coefficients are of approximately 50) in a system of SnO 2 –CoO–Nb 2 O 5 –Cr 2 O 3 –Y 2 O 3 –SrO–MgO are reported. The current–voltage behaviour at high currents is studied by using exponential voltage pulses. The obtained SnO 2 varistor ceramics exhibit low grain resistivity values of 0.23–0.64 ohm cm. To date, such values are the lowest known for SnO 2 varistors, and are closely approaching the grain resistivity of the ZnO varistor. The current–voltage characteristics of the obtained SnO 2 -based varistor materials are reproducible in a wide current range from 10 −11 to approximately 10 4 A cm −2 . The minimum current density and the minimum electric field necessary to cause the irreversible electrical breakdown are measured. It is established that a decrease in the grain resistivity leads to an increase in the minimum current density necessary for irreversible electrical breakdown to occur.

Giant dielectric and low voltage varistor behaviors of Ba-doped Bi1/2Na1/2Cu3Ti4O12 ceramics

Bi1/2−xNa1/2−xBa2xCu3Ti4O12 (x = 0, 0.025, 0.05, 0.075) polycrystalline ceramics are prepared by a modified Pechini method. Effect of Ba doping on the dielectric and nonlinear electrical behaviors are investigated in detail. All the samples exhibit giant dielectric effect and nonlinear electrical behavior. Compared with Bi1/2Na1/2Cu3Ti4O12 ceramics, the permittivity drops down and the nonlinear electrical behavior shifts to low voltage area by Ba doping. The permittivity reduces from 20,004 to 7,006 at 1 kHz. The breakdown voltage and nonlinear coefficient values decrease to 10.8 V/mm and 1.5, respectively. Additionally, the electrostatic barrier height drops down to 0.66 eV. Giant dielectric response and nonlinear electrical behavior can be interpreted in terms of internal barrier layer capacitors model. These results imply that Ba substitution can adjust the dielectric and nonlinear electrical behaviors of Bi1/2Na1/2Cu3Ti4O12 ceramics, which is a promising candidate for low voltage varistor materials.

Influence of Chromium on Microstructure and Electrical Properties of ZnO-Based Varistor Materials

Abstract The addition of Cr 2 O 3 into a ZnO-based varistor composition was systematically investigated. All samples had the general chemical formula was Zn 0.96 Bi 0.02 Co 0.02-x Cr x O β (x = 0, 0.005, 0.01 and 0.02) and prepared by solid-state processing. Addition of chromium had a great influence on several microstructural characteristics which, in turn, resulted in different varistor behaviors. All samples could be sintered up to 90% at 900 °C which could be attributed to liquid-phase sintering; though densification rate of Cr-added samples was greatly reduced especially below 900 °C. Such retardation in sintering might be possibly related to differences in phase formation among different compositions. X-ray diffraction (XRD) of the samples sintered at 1000 °C revealed a higher degree of multi-phasic nature upon increasing amount of chromium. Secondary phases include ZnCr 2 O 4 , Bi 2 O 3 and Zn-Bi-O compounds. Scanning electron microscopy (SEM) showed that the first phase was distributed all over the microstructure. The latter two phases, however, were found mostly in the grain boundary region and are likely associated with the mechanism of liquid-phase sintering. Surprisingly, these two liquid phases could not be detected in the non-Cr sample by XRD in spite of the similar initial amounts of Bi 2 O 3 in all samples. This finding indicates possible alterations in both crystallization and phase formability in this varistor system. Besides the phasic unconformity, grain growth was also affected. Increasing amount of chromium significantly reduced the averaged grain size of the ZnO main phase (from 6.3 μm at x = 0.005 to 4.5 μm at x = 0.02). The smaller grains among these samples could be related to the presence of chromium itself which somehow possibly plays a role in atomic diffusion during sintering. In addition, the induced crystallization of the liquid phases, as confirmed by XRD, might also be responsible for grain boundary pinning, inhibiting grain growth during heat treatment. Samples with smaller grain sizes yielded a better varistor property; the breakdown voltages were improved (up to 740 V/cm) when the amount of chromium was increased from 0 to 0.02. This improvement was likely related to a higher volume of grain boundary phases which were widely proposed to facilitate the non-linear behavior in ZnO-based varistor materials.

Influence of Fe 2O 3 doping on microstructural and electrical properties of ZnO–Pr 6O 11 based varistor ceramic materials

The doping effect of Fe 2O 3 on the microstructural and electrical properties of ZnO–Pr 6O 11 based varistor ceramic materials was investigated. Fe 2O 3 doping would inhibit the growth of ZnO grains, whose average sizes were found to decrease from 3.0 to 2.7 μm with the doping level of Fe 2O 3 increased from 0 to 1 mol%. When the doping level of Fe 2O 3 was 0.005 mol%, the varistors exhibited the optimum nonlinear electrical characteristics with nonlinear coefficient of about 26, breakdown voltage of approximately 571 V/mm and leakage current of less than 65 μA. With higher doping level of Fe 2O 3, more Fe atoms would segregate at grain boundaries, providing more extra electrical carriers, decreasing the resistances of the grain boundaries, and PrFeO 3 would be formed, destroying the construction of grain boundaries. Therefore, the nonlinear electrical properties of the resultant varistor materials were deteriorated.

Analysis of High-Voltage ZnO Varistor Prepared from a Novel Chemically Aided Method

High-voltage varistor materials were prepared using an ultrafine additives precursor obtained from the precipitation method. Microstructural and electrical studies showed that the sample exhibited a smaller average grain size (d=4.5 μm) and a considerably higher breakdown voltage (Eb=1610±30 V/mm) as compared with the conventional one (d=6.5 μm, Eb=400±30 V/mm). The high voltage is attributed to the improvement in microstructure homogeneity and the formation of more active grain boundaries per unit volume. This novel route may explore the commercial possibility of device miniaturization.

ZnO-Pr6O11-Co3O4-TiO2-Based Ceramic Varistor Materials

The preparation and characterization of ZnO-Pr6O11-Co3O4-TiO2 (ZPCT) based varistor materials with different doping levels of TiO2 and Pr6O11 were investigated. The results reveal that: (1) TiO2 is an important additive, acting as an inhibitor of ZnO grain growth. The doping of appropriate amount of TiO2 can significantly improve the nonlinear properties and decreases the leakage current of the varistors, achieving a relatively high nonlinear exponent and low leakage current with 1.0 mol% TiO2 doped. (2) The oxide of Pr6O11 microstructurally plays the role of inhibition in grain growth. The doping of appropriate amount of Pr6O11 can improve the nonlinear property, and decrease the leakage currents of the varistors, acquiring the optimum results with 1.5 mol% Pr6O11 doped.

Electrical properties of Pr 6O 11-doped WO 3 capacitor–varistor ceramics

The microstructure and electrical properties of Pr 6O 11-doped WO 3 ceramics were investigated. Results showed that the breakdown voltage of doped samples was lower than that of the undoped. The dielectric constant of doped samples was higher than that of the undoped, and the high dielectric constant made Pr 6O 11-doped WO 3 ceramics to be applicable as a kind of capacitor–varistor materials. A small content of Pr 6O 11 could significantly improve nonlinear properties of the samples. The WO 3–0.03 mol% Pr 6O 11 obtained a large nonlinear coefficient of 3.8, a low breakdown voltage of 8.8 V/mm, and a high dielectric constant of 7.69 × 10 4 at 1 kHz. The defects theory was introduced to explain the nonlinear electrical behavior of Pr 6O 11-doped WO 3 ceramics.

X-ray powder diffraction and electron diffraction studies of the thortveitite-related L phase, (Zn,Mn)2V2O7

The phase designated gamma-Zn3(VO4)2 reported as a minor second phase in zinc oxide-based varistor materials doped with vanadium oxide and manganese oxide is shown to be the L phase, (Zn(1-x)Mn(x))2V2O7 (0.188 < x < 0.538), in the pseudo-binary Mn2V2O7-Zn2V2O7 system. Analysis of X-ray powder diffraction patterns and electron diffraction patterns of this phase shows that the previously published a, c and beta values for this thortveitite-related phase are incorrect. Instead, Rietveld refinement of the X-ray powder pattern of the L phase shows that it has a monoclinic C lattice with Z = 6, with a = 10.3791 (1), b = 8.5557 (1), c = 9.3539 (1) A and beta = 98.467 (1) degrees. Although prior convergent-beam electron diffraction work of 'gamma-Zn3(VO4)2' confirmed the C Bravais lattice, the space group was found to be Cm rather than C2/m, the difference perhaps arising from the inability of the X-rays to detect small displacements of oxygen. Attempts to refine the structure in Cm did not produce improved R factors. The relationship between the crystal structure of the L phase and the high-temperature C2/m beta'-Zn2V2O7 thortveitite-type solid solution is discussed.

SnO2-based varistors capable of withstanding surge current

SnO2-based varistor material was fabricated by doping of CoO, Nb2O5, Cr2O3, and Y2O3 or Ho2O3. The microstructure, nonlinear I-V characteristic and surge current performances have been investigated. The average grain boundary voltage of the obtained materials is close to that of ZnO varistors. Similar to Cr3+, Y3+ or Ho3+ acts as the acceptor in the grain boundaries and the depletion layers. Moderate co-doping Y2O3 or Ho2O3 with Cr2O3 is helpful to improve the electrical performances and to prevent poor densification. The obtained optimal samples with a diameter of about 14 mm have a nonlinear coefficient (α) of about 60, threshold electric field (Eb) of about 380 V/mm, and the withstanding peak current of 8/20 μs wave is about 2400 A. Higher surge current will cause failure but no visual damage happen. This kind of failure mode is helpful for safety use of varistors.

Study of Ag/ZnO composite material by universal power law

The dielectric properties of Ag/ZnO composite material are investigated and a strong low-frequency dispersion phenomenon is found. The experimental results are systematically studied in the frequency range of 0.1—107 Hz and in the temperature range of 173—473 K. By means of normalization of conductivity, it is found that samples with different Ag contents show similar mechanism of dielectric loss, which is suggested as electron jumping between deep levels. In addition, it is also found that the dependence of E1mA on Ag content is similar to that of αlf on Ag content, which is a novel result in the research of varistor materials. The results can be fully understood by means of universal power law.

Novel varistor material based on terbium oxide

A new varistor system based on terbium oxide is introduced in this paper. The ceramic samples were prepared using the standard ceramic method and sintered in air at 1300 °C for 2 h. From the I–V measurements, the samples exhibit varistor action with the nonlinear coefficient α = 3.9 and the breakdown voltage Eb = 40 V mm−1 and possess excellent electrical stability up to 70 °C. X-ray diffraction reveals that the ceramic is a single phase structure. It is proposed that the varistor action can be attributed to the adsorbed oxygen on the grain surfaces. This is on the basis of the barrier formation model for metal oxide varistor systems and the intrinsic characteristics of terbium oxide ceramic.

An investigation of co-fired varistor-NiZn ferrite multilayers

The co-firing characteristics of ZnO varistor and NiZn ferrite thick films were investigated, the objective being to create an integrated passive device. Bismuth oxide additions were used to increase the shrinkage of the ferrite during sintering. Dilatometry analysis proved that well-matched shrinkage characteristics could be obtained for bismuth oxide-modified ferrite compositions and the starting varistor material. Cross-boundary diffusion across the co-fired varistor–ferrite interface occurred during sintering, but application of a barrier layer of pure zinc oxide reduced this phenomenon considerably. By tailoring the shrinkage, strong, crack-free co-fired layers with no evidence of cracking or delamination were obtained. No deleterious effects were observed due to electroding of the varistor and ferrite materials.

Investigation of Electron Traps in SnO&lt;sub&gt;2&lt;/sub&gt; Based Varistor Ceramics

Dense tin oxide based ceramics are a new type of varistor materials. To further understand the electrical properties of SnO2 varistors doped with CoO, Nb2O5, and Cr2O3, the techniques of capacitancevoltage (C-V) measurement and deep level transient spectroscopy (DLTS) were used to investigate the electron traps in the SCN samples (doped with 1.0 mol% CoO and 0.05mol% Nb2O5) and SCNCr samples (doped with 1.0 mol% CoO, 0.05mol% Nb2O5 and 0.05mol% Cr2O3). Two electron traps were detected: trap T1 is located at Ec - 0.30 ± 0.01eV and trap T2 is located at Ec – 0.69 ± 0.03eV for both SCN and SCNCr samples. The variations in the donor density and trap density could be related to the addition of chromium oxide. The features of these traps are discussed based on the defect theory related to the SnO2 varistors.

Soft core behavior in ZnO–Bi2O3‐based varistors containing oxides of Ce and Gd

AbstractThe effects of sample lengthd0on the average breakdown electric fieldEBof ZnO–Bi2O3‐based varistor materials containing oxides of Ce and Gd sintered in the temperature range from 1150 to 1200 °C were investigated. It is observed thatEBdecreases with increasingd0. The breakdown electric fieldE′Bat each location along the axial direction was measured by gradually grinding and lapping both sides of the sample. The experimental results indicate thatE′Bin the core of the varistor material is as low as about 35% of that in the surface layers. This behavior of varistor materials is termed the soft core phenomenon. Further experimental results reveal that the Schottky barrier heightΦBin the core of the material is much lower than in the surface layers. This is indicative of non‐uniform distribution of breakdown electric field. The reasons for the formation of the soft core were explored. The experimental results of X‐ray photoelectron spectroscopy analysis indicate that the binding energy of Zn in the core is lower than that in the surface layers implying variation in the chemical environment of the Zn atoms along the axial direction. The analysis of the phases obtained by X‐ray diffraction showed no evidence of Bi2O3phase in the core while Bi2O3phase appears in the surface layers. Consequently, it is proposed that the soft core behavior originates from the decrease of Schottky barrier height along the axial direction of the varistor material doped with rare earth oxides, which may be caused by the non‐uniform distribution of sintering atmosphere and phase morphology at the grain boundaries. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Voltage Response of ZnO Varistors to 8/20 µs Surge Current

The voltage response of ZnO varistors to 8/20 µs surge current was investigated. The observed frontal spikes on the residual voltage waveforms are caused by the ignition gap, and no frontal spike was observed when a thyristor was used as the discharge trigger. The rear part of the waveform is determined by the damping coefficient of the RLC-circuit. Near the critical point, the residual voltage waveform changes from non-oscillating attenuation modes to distinct across zero oscillating modes along with the increase of the peak current, but there will be no oscillation happen when a thyristor is used as the discharge trigger. The residual voltage peak is not synchronized with the current peak, and the voltage peak is leading, implying that the ZnO varistor appears to be inductive. According to the experiment results, it can be reasonably explained that the voltage peak leading phenomenon is attributed to the transient skin effect of the varistor materials.