ZnO-Bi2O3 Varistor Research Articles

The Effect of Sintering Temperature on the Microstructure and Electrical Properties of ZnO–Bi2O3 Varistor Ceramics

The Effect of Sintering Temperature on the Microstructure and Electrical Properties of ZnO–Bi2O3 Varistor Ceramics

Effects of SiO2 and B2O3 on electrical properties of low-temperature sintered ZnO–Bi2O3 varistors

Effects of SiO2 and B2O3 on electrical properties of low-temperature sintered ZnO–Bi2O3 varistors

Effects of Dopants and Processing Parameters on the Properties of ZnO-V2O5-Based Varistors Prepared by Powder Metallurgy: A Review.

This article reviews the progress in developing ZnO-V2O5-based metal oxide varistors (MOVs) using powder metallurgy (PM) techniques. The aim is to create new, advanced ceramic materials for MOVs with comparable or superior functional properties to ZnO-Bi2O3 varistors using fewer dopants. The survey emphasizes the importance of a homogeneous microstructure and desirable varistor properties, such as high nonlinearity (α), low leakage current density (JL), high energy absorption capability, reduced power loss, and stability for reliable MOVs. This study investigates the effect of V2O5 and MO additives on the microstructure, electrical and dielectric properties, and aging behavior of ZnO-based varistors. The findings show that MOVs with 0.25-2 mol.% V2O5 and MO additives sintered in air over 800 °C contain a primary phase of ZnO with a hexagonal wurtzite structure and several secondary phases that impact the MOV performance. The MO additives, such as Bi2O3, In2O3, Sb2O3, transition element oxides, and rare earth oxides, act as ZnO grain growth inhibitors and enhance the density, microstructure homogeneity, and nonlinearity. Refinement of the microstructure of MOVs and consolidation under appropriate PM conditions improve their electrical properties (JL ≤ 0.2 mA/cm2, α of 22-153) and stability. The review recommends further developing and investigating large-sized MOVs from the ZnO-V2O5 systems using these techniques.

SnO2 Doping Effect on the Microstructural and Electrical Behavior of ZnO nanoparticles-Bi2O3 Based Varistor Ceramics

In the current study, ZnO-Bi2O3 varistors manufactured from ZnO nanoparticle size and doped with SnO2 were processed by the ceramic industrial technique. The impact of the various SnO2 concentration upon the sintering, microstructural improvement and nonlinearity characteristic of the varistor system was investigated. Different SnO2 concentration was found to possess considerable influences on the varistors, particularly on improving ZnO grains growth at a lower SnO2 amount. But, the adding of greater contents of SnO2 leads to inhibit the ZnO grain growth via the raising percentage of Zn2SnO4 type spinel phase. In discs with a Sn/Bi less than 1.5 mol%, the pyrochlore phase forms and bounds all the SnO2, whereas excess Bi2O3 leads to the formation of a liquid phase at 740 °C which promotes sintering process. For Sn/Bi more than 1.5 mol%, all the Bi2O3 is bounded in Bi2Sn2O7 pyrochlore phase and sintering process is prevented. X-ray diffraction investigated that additive of SnO2 to the ZnO-Bi2O3 varistor sample caused the structure of the Bi2Sn2O7 type pyrochlore phase and the Zn2SnO4 type spinel phase during the sintering step. The SnO2 adding furthermore noticeably affected the electrical behaviors of the sample with a significant rise in the breakdown voltage with increasing the SnO2 doping more than 1.5 mol%. The resistivity also achieved an obvious increase with increase the SnO2 doping contents. Moderate SnO2 doping enhanced the nonlinear properties of the ZnO-Bi2O3 varistor system, while a significant amount of SnO2 causing its deterioration.

Investigation of phase evolution within ZnO–Bi2O3 varistors utilizing thin film prototypes

Investigation of phase evolution within ZnO–Bi2O3 varistors utilizing thin film prototypes

Synthesis and characterization of ZnO-based nano-powders: study of the effect of sintering temperature on the performance of ZnO–Bi2O3 varistors

Synthesis and characterization of ZnO-based nano-powders: study of the effect of sintering temperature on the performance of ZnO–Bi2O3 varistors

CaO doped ZnO–Bi2O3 varistors: Grain growth mechanism, structure and electrical properties

In this study, the impact of CaO doping on the microstructure, phase and electrical properties of the ZnO–Bi2O3 system was studied. The powder was synthesized by the solution combustion route. Reduction in the crystallite size of ZnO from 23 nm to 17 nm of the powder sample with CaO doping was noted. Conventional sintering was used to consolidate the powders. Solid state reacted compositions revealed, from X-ray diffraction, formation of two phases (a) Ca4Bi6O13 at 750 °C and (b) Ca0.89Bi3.11O5.56 at 900 °C. An increase in density from 4.21 to 5.45 gcc−1 and a decrease in grain size from 4.50 to 1.54 μm with increasing CaO doping were also observed. A grain growth mechanism in the ZnO–Bi2O3–CaO system was proposed. The breakdown field increased up to 1 wt % CaO (27 kV cm−1) and after that decreased. Impedance spectroscopic investigation, in the temperature range of 100300 °C and frequency range of 0.1 Hz -1M Hz, exhibited two relaxations. These were attributed to defects formed. The two-stage activation energy of conductivity was noticed, which was similar to that observed in complex ZnO varistor systems.

Enhancement of Microstructure Development and Current-voltage Characteristics of ZnO-Bi2O3 Varistor Ceramics Sintered with Sintering aids at the Low-Temperature

ZnO-Bi2O3 based varistor ceramics doped with SnO2 or with SnO2, B2O3 and SiO2 were prepared by traditional solid–state reaction method respectively. The microstructure and properties of samples sintered at a low temperature range from 910 to 950°C were studied. Results show that: by low-temperature sintering, the sintering aids can improve the sample’s density and homogeneity, and therefore the overall properties of the samples. It was found that the samples sintered at 910°C with SnO2 as sintering aid possess the best performance, and the breakdown voltage is 1368 V/mm, nonlinear coefficient is 12.2, and the leakage current is less than 0.1 μA.

Influence of a Zn-Bi-Sb-O Synthetic Multi-phase on Highly Nonlinear Properties of ZnO-Bi2O3 Varistor Ceramics

ZnO-Bi2O3-Sb2O3-MnO2-Co2O3-Cr2O3-based varistors were fabricated through the conventional solid state method. The influences of the synthetic multi-phase (SMP) produced by calcining the mixtures of 4ZnO·3Bi2O3·3Sb2O3 at a given temperature on the micro-structures and electrical properties were investigated. Results showed that SMP additive increased the content of the second phase and the Bi-rich phase, leading to an enhancement of the potential barrier height, which resulted in the improvement of the nonlinear properties. Sample with 0.3 at.% SMP showed the highest barrier height ϕb of 4.27 eV, and the highest nonlinear coefficient α of 91.24.

Microstructural and electrical characteristics of rare earth oxides doped ZnO varistor films

ZnO-Bi2O3 varistor films doped with two kinds of rare earth element oxides (Lu2O3 and Yb2O3) were prepared by the sol-gel method. The effects of Lu2O3/Yb2O3 doping on the microstructure and electrical characteristics of ZnO-Bi2O3 varistor films were investigated. All samples show a homogenized morphology and an improved nonlinear relationship between the electric field (E) and current density (I). Both Yb2O3 and Lu2O3 doping can decrease the grain size of ZnO-Bi2O3 varistor films and improve the electrical properties, which have a positive effect on the development of ZnO varistor ceramics. Yb2O3 doping significantly increases the dielectric constant at low frequency. 0.2 mol. % Yb2O3 doped ZnO-Bi2O3 varistor films exhibit the highest nonlinear coefficient (2.5) and the lowest leakage current (328 μA) among Lu2O3/Yb2O3 doped ZnO-Bi2O3 varistor films. Similarly, 0.1 mol. % Lu2O3 doping increases the nonlinear coefficient to 1.9 and decrease the leakage current to 462 μA.

Recent research progress of relaxation performances of defects in ZnO-Bi2O3 varistor ceamics

ZnO varistor ceramics have been widely applied to surge absorption and over-voltage protection in electronic circuit and power system because of their excellent non-ohmic characteristics.Therefore,the reaserch on ZnO varistor ceramic has long been a subject of interest for scholars and industrial circles.At present,the conductance theory of ZnO varistor ceramic has been widely studied and reviewed,and several models such as space charge limited current model,NordheimFowler tunneling current model,and Schottky barrier model have been proposed to describe the electronic transmission process and explain the non-ohmic behavior of ZnO ceramic varistor.However,the relationships of the defect structure and defect relaxation with the electrical property of ZnO varistor ceramic remain unclear,which becomes a challenge to developing new ZnO varistor ceramics.In this paper,comments on defect structures and defect types of ZnO ceramics are given,and the theortical calculation of the intrinsic point defects is discussed.Besides,the characterization technologies of the defect relaxations are introduced.The results show that the dielectric loss spectra are widely used to describe the relaxation of ZnO ceramic varistor,especially the spectra in the low frequency can provide more information about defect relaxation of ZnO ceramic varistor.It is also found that the frequency spectra of admittance in a wide temperature range and the temperature spectra of admittance in a wide frequency range play an equivalent role in characterizing the defect relaxation of ZnO ceramic varistor.The thermally stimulated current is considered to be an effective method to verify the relaxation polarization mechanism of the defects.The deep level transient spectroscopy can characterize the intrinsic and extrinsic defect relaxation processes.Moreover,several theories of relaxation mechanisms such as the Cole-Cole theory,Havriliak-Negami theory and Cole-Davidson theory are proposed to analyze the relaxation phenomena of ZnO ceramic varistors.It is suggested that the electric modulus spectrum combined with Cole-Davidson theory is more effective to characterize the defect relaxations in a wide temperature range.From the electrical degradation results, it is found that the extrinsic defect relaxation at grain boundary interface is closely related to the electrical property of ZnO ceramic varistor.A circuit model is also obtained to establish the correlation between defect relaxation and electrical performance of ZnO ceramic varistor.Therefore,the review on defect relaxations may offer some new ideas to optimize the electrical properties of ZnO ceramic varistors by modifying the defect structures.

Effect of a Bi–Cr–O synthetic multi-phase on the microstructure and electrical properties of ZnO–Bi2O3 varistor ceramics

ZnO–Bi2O3–MnO2 (ZBM)-based varistors were fabricated via doping a novel synthetic multi-phase (SMP) additive produced by calcining the mixture of 18Bi2O3·Cr2O3 at a given temperature. The effects of the SMP on the microstructural and electrical properties of ZBM varistors were investigated. It was found that the SMP dopant was a compound crystalline phases including Bi–Cr–O phases (Bi7.38Cr0.62O12+x and CrBi18O30) and small amounts of Bi2O3 rather than a synthesized polycrystal. The Bi–Cr–O phases were not emerged for samples with x=1, indicating that the amount of it is tiny and the small Bi2O3 may accelerate ZnO grain growth. With more SMP doping (x>1) in the ZBM ceramics, it acted as a barrier inhibiting grain growth. For samples with x=5, excellent electrical properties were obtained: the nonlinear coefficient α increased up to 50.19 corresponding to the highly barrier height of 2.62eV; the leakage current IL reduced to 0.3μA. The dielectric constant εa is proportional to the ratio of the grain size d to the thickness of the depletion layer width t, which explained the εa increased at f=1kHz for the samples with x=1 and 5. The improvement of the electrical properties can be explained by the oxygen absorption mechanism.

Effects of BN on phase compositions and electrical properties of ZnO–Bi2O3 varistors prepared by the reduction–reoxidation method

A reduction–reoxidation method is frequently used for preparing multilayer ceramic components with base metal inner electrodes (BME). However, ZnO varistors has been hardly prepared by the reduction–reoxidation method because that ZnO is easy to be reduced. We proposed that Boron nitride (BN) as an anti-reduction additive in this paper, which obviously prevented ZnO–Bi2O3 from reducing when sintering in a reducing atmosphere. For the BN-added samples, the B-containing phase formed during the sintering process promoted the liquid-phase sintering process of ZnO and prevented the composition loss of them. BN-added samples sintered in a reducing atmosphere (N2(99.5%)–H2(0.5%)) at 1000°C for 2h and re-oxidized in air at 850°C for 2h exhibited excellent nonlinear properties with nonlinear coefficient of 43.3 and leakage current of 0.12μA/cm2. This is promising for producing BME ZnO-based multilayer varistors (MLVs).

Effect of Cr2O3 on the property and microstructure of ZnO–Bi2O3 varistor ceramics in different sintering temperature

The electrical property, phase transformation, and microstructure of Cr2O3 doped ZnO–Bi2O3–Sb2O3–Co2O3–MnO2 (ZBSCM)-based varistors were investigated for different sintering temperature. The structure and morphological modifications were analyzed by X-ray diffraction and scanning electron microscopy (SEM), respectively. Doping Cr2O3 could lower the decomposition temperature of the pyrochlore phase Zn2Bi3Sb3O14, resulting in more Bi2O3 rich liquid phase and spinel phase Zn7Sb2O2. The results indicated that Cr2O3 played multiple roles in the grain growth and microstructure. Cr2O3 which acted as an indirect sintering aid to promote the grain growth was an effective additive which contributed to lower sintering temperature of ZnO varistors.

Microstructure and Electrical Properties of La2O3 and Sm2O3 Co-doped ZnO-Based Varistor Ceramics Prepared From Nanosize ZnO Powder

High-performance ZnO–Bi2O3 varistors co-doped with a fixed amount of La2O3 and different amounts of Sm2O3 have been fabricated from nanosize zinc oxide powders by solid-state reaction. Phase composition, microstructure, and electrical properties were studied by x-ray diffractometry (XRD), scanning electron microscopy, and direct current electrical measurement. XRD analysis of the samples showed that a ZnO phase, a Bi2O3-rich phase, a spinel phase, and a pyrochlore-type phase were present; an Sm-rich phase was found in varistor ceramics containing large amounts of Sm2O3. The varistor ceramics had fine microstructures, and the corresponding grain size varied from 2.30 to 3.10 μm. Varistor ceramics doped with 0.1 mol% La2O3 and 0.5 mol% Sm2O3 then sintered at 1100°C for 2 h had good electrical properties (a breakdown field of 1130 V/mm, a nonlinear coefficient of 74, and a leakage current of 4.2 μA).

Effect of two-step sintering on rare earth (RE = Y2O3, Pr6O11) doped ZnO–Bi2O3 varistors processed from ‘nano-precursor’ powders

Nano size ZnO–Bi2O3 varistor precursor powders containing Y2O3 and Pr6O11 rare earth dopants were prepared by low temperature refluxing at 80 °C. Effect of rare earth dopants, densification by two-step sintering, evolution of microstructures and their influence on varistor properties were investigated. Chemically synthesized nano- precursor varistor powders produced controlled grain size in two-step sintering in which the average sintered ZnO grain size was reduced to at least half compared to the conventionally processed ZnO–Bi2O3 varistors. The study revealed that such grain size reduction is highly beneficial to attain enhanced varistor properties.

Sintering of surfactant modified ZnO–Bi2O3 based varistor nanopowders

Abstract Surfactant modified nano-origin ZnO–Bi2O3 varistor powder was prepared in presence of cetyltrimethyl ammonium bromide (CTAB) surfactant through an aqueous reflux reaction at 100 °C. The compacted varistor discs made from the nano-origin powders were subjected to step-sintering, microwave sintering and solid-state sintering. The influences of CTAB in different sintering methods were analyzed from the densification characteristics, evolution of sintered microstructures and associated varistor properties (I–V). The conventional solid-state sintering produced 96% theoretical sintered dense samples at 1100 °C. The step and microwave sintered samples showed 93% and 99% sintered densities, respectively, with controlled microstructures having grain sizes in the range of 2–6 μm at the given conditions. The CTAB advantages were clearly seen in grain structuring and grain boundary properties, in addition to the enhanced densification and homogenous microstructures for obtaining high breakdown voltage and non-linearity coefficient.

Effects of alumina surrounding in sintering process on ZnO–Bi 2O 3varistors doped with CoO

Nonlinear current–voltage properties of zinc oxide-bismuth oxide varistors doped with cobalt oxide were investigated. The composition of ZnO doped with 0.5 mol% Bi 2O 3 and 0.75 mol%CoO was prepared and sintered in two different surroundings, zinc oxide–bismuth oxide–cobalt oxide powder and alumina powder. Microstructures and element tracing were studied using scanning electron microscopy with X-ray microanalysis, electron probe microanalysis and X-ray diffraction. Results clearly showed that the alumina surrounding led to bismuth loss from the green body and degraded the nonlinear property of varistors. At sintering temperatures higher than 1150 °C, the alumina-surrounding sintered specimens became ohmic and the electrical resistivity of 200 Ω cm was found in the specimen sintered at 1200 °C.

Microstructural Development in SnO2‐Doped ZnO–Bi2O3 Ceramics

The effects of adding small quantities of SnO2 to the basic ZnO–Bi2O3 varistor composition were studied in terms of phase reactions, microstructural development, and the formation of inversion boundaries. Scanning and transmission electron microscopy studies showed that the inversion boundaries, triggered by the addition of SnO2, cause anisotropic grain growth in the early stages of sintering. ZnO grains that include inversion boundaries grow exaggeratedly, at the expense of normal grains, until they dominate the microstructure. Higher additions of SnO2 lead to an increase in number of grains with inversion boundaries and to a more fine‐grained microstructure. The increasing amount of secondary phases is also related to a higher level of SnO2 addition; however, the influence of these phases on ZnO grain growth is subordinate to the role of inversion boundaries.

Microstructural and electrical characteristics of SiO2 doped ZnO-Bi2O3 varistors

Varistors in the new system ZnO-Bi2O3-SiO2 were prepared through conventional ceramic processing route. The effect of sintering temperature and time (0·5 h to 2 h between 1000° and 1250°C) on the microstructure and current/voltage characteristics of the varistors of the new system were investigated and the results were compared with those of ZnO-Bi2O3 system varistors prepared. An increase in nonlinear coefficient (α) value was observed in the SiO2 added varistors. The microstructure and the phase of the varistors were examined by means of SEM and XRD. The Zn2SiO4 spinel phase was found to be present in the intergranular region. The grain growth exponent was determined to be 2·5±0·2 and the activation energy for the ZnO grain growth was estimated to be 251±11 kJ/mol. These values were compared with those estimated for ZnO-Bi2O3 system varistors.