Properties Of ZnO Varistor Ceramics Research Articles

Effect of Zn7Sb2O12 pre-synthesis on microstructure and properties of ZnO varistor ceramics

In this work, the Zn7Sb2O12 spinel phase is pre-synthesized at 980, 1000, 1020, 1040, and 1060 °C by mixing ZnO and Sb2O3 in a molar ratio of 7:1. ZnO varistors are prepared by replacing Sb2O3 with the Zn7Sb2O12 spinel phase. The effect of the Zn7Sb2O12 spinel on the electrical properties and microstructure of ZnO varistors is studied. The results show that Sb2O3 can produce free electrons and reduce the number of oxygen vacancies (VO⋅⋅) and grain size during the reaction process. The ZnO varistors prepared by replacing Sb2O3 with the Zn7Sb2O12 spinel phase have full grain growth and high density. In addition, ZnO varistors prepared by the Zn7Sb2O12 spinel phase replacing Sb2O3 can not only reduce the sintering temperature by 60 °C but also reduce the reverse voltage gradient rate of ZnO varistors from 7.85% to 1.2% on the basis of stable electrical properties and improve the impact stability of ZnO varistors. This method has the advantages of no change in formula composition and being a simple process and can provide ideas for the preparation of multilayer chip ZnO varistors.

Influence of Gd2O3 on phase, microstructure, and electrical properties of ZnO varistor ceramics

Influence of Gd2O3 on phase, microstructure, and electrical properties of ZnO varistor ceramics

An economical dopant for improving the comprehensive electrical properties of ZnO varistor ceramics

The effects of CaO (Ca) doping on the microstructure and comprehensive electrical properties of ZnO varistor ceramics were studied. A small amount of Ca doping could increase the voltage gradient and nonlinearity of ZnO varistor ceramics and reduce the leakage current. The low-cost Ca could be an excellent substitute for some expensive doping elements of ZnO varistor ceramics. The electrical parameters of the best sample were as follows: breakdown voltage E1mA = 532 V/mm, leakage current JL = 0.11μA/cm2 and non-linear coefficient α = 96.

Enhanced electrical properties of ZnO varistor ceramics by spark plasma sintering: Role of annealing

ZnO varistor ceramics are synthesized by spark plasma sintering (SPS), and then annealed in a conventional furnace in air atmosphere. Dense microstructure with more than 98% of theoretical density is achieved, and homogenous ZnO grains with typical second phases such as Zn7Sb2O12 spinel, Zn2Bi3Sb3O14 pyrochlore are observed from X-ray diffraction spectra (XRD) and scanning electron microscopy (SEM). In addition, a metallic Bi phase is also observed, which is resulted from the reduction of Bi2O3 because of inherent low oxygen partial pressure of SPS, and it can be reoxidized after a post-annealing process. The breakdown field of the optimum ZnO varistor ceramic is 725 V/mm with a nonlinear coefficient of 53.5, which stems from its high Schottky barrier height of 0.60 eV. In analysis of dielectric spectra from 173 K to 293 K, two relaxation peaks with activation energy of 0.22 eV and 0.48–0.54 eV can be observed, which are attributed to the intrinsic zinc interstitial and defect of grain boundary or intergranular phase, respectively. The grain boundary resistances are discussed according to Cole-Cole plot, from which the activation energy controlling resistance at the grain boundary is determined to be 0.87 eV.

Electrophysical characteristics of the near-surface layer of semiconductor ceramics for ZnO varistors

Electrophysical properties of near-surface layer of ZnO varistor ceramics forming during high-temperature annealing in oxidizing gaseous atmosphere are studied. Microstructure of near-surface layer and its electrical properties have features which are determined by an increased concentration of oxygen at surface of pressed ceramic billets in comparison with bulk during annealing. From the data of electron raster microscopy of cross-chipping of varistors, it is defined that the average value of thickness of near-surface layer is 40 μm. It is found that the specific resistivity value of near-surface layer is 5.3·1012 Ω·cm. The electrical capacitance of near-surface layer is calculated from capacitance measurement data for varistors containing such layer under two electrodes and without layers. It is shown that the electrical capacitance of near-surface layer exceeds that of varistor by more than an order. Conclusion is made that the near-surface layer gives a weak contribution to dielectric properties of ZnO varistor ceramics. However contribution of this layer to electrical conductivity of ceramics in pre-breakdown region is significant.

Electrical properties of ZnO varistor ceramics modified by rare earth-yttrium and gallium dopants

Scanning electron microscopy, electric field-current density measurements, capacitance-voltage testing, X-ray diffraction analysis, and energy dispersive X-ray spectroscopy were performed to study the effects of rare earth-yttrium and gallium doping on the microstructure and electrical properties of ZnO varistors. Our results indicated that the introduction of gallium and yttrium strengthened the barrier height of grain boundaries and led to a decrease in the grain resistance. Both the voltage gradient and nonlinear coefficient of the samples increased and then decreased as the gallium doping level was increased at a given rare earth-yttrium. The residual voltage ratio and the leakage current varied with the opposite relationship. The findings of this study could help to improve the protective effects of surge protection devices assembled from ZnO ceramic varistors and enhance the stability of power systems.

High Nonlinearity and High Voltage Gradient ZnO Varistor Ceramics Tailored by Combining Ga2O3, Al2O3, and Y2O3 Dopants

This paper deals with the electrical characteristics of rare‐earth‐doped ZnO varistor ceramics. Multiple donor dopants (Al3+, Ga3+, and Y3+) were employed to improve the comprehensive performance of ZnO varistor ceramics. The leakage current of rare‐earth‐doped ZnO varistor ceramics decreased noticeably with Ga2O3 dopants. The Ga3+ dopant occupies the defect sites of grain boundaries and increases the barrier potential of ZnO varistor ceramics, so the leakage current is effectively inhibited. Y2O3 is primarily located around the grains, which restrains ZnO grain growth, increasing the voltage gradient. The Al3+ goes into the lattices of ZnO grains, decreasing the grain resistance; thus, the residual voltage ratio can be controlled at low levels under a high impulse current. With the combined incorporation of Al3+, Ga3+, and Y3, excellent electrical properties of ZnO varistor ceramics can be acquired with a nonlinearity coefficient of 87, voltage gradient of 517 V/mm, leakage current of 0.96 μA/cm2, and residual voltage ratio of 1.60. These rare multiple donor dopants can aid in engineering high‐quality ZnO varistors.

High nonlinearity and low residual-voltage ZnO varistor ceramics by synchronously doping Ga2O3 and Al2O3

The present study examined the electrical properties of ZnO varistor ceramics co-doped with Ga2O3 and Al2O3, in particular, the current–voltage characteristics under small and intermediate currents and upturn characteristics under a large current. With increasing amounts of Ga2O3 dopant at a given Al2O3 concentration, both the threshold voltage in the small-current region and nonlinear coefficient of the varistor ceramics increased and then decreased; in contrast, the leakage current decreased and then increased. Moreover, ZnO varistor ceramics with low residual voltage ratio, high nonlinearity, and low leakage current were obtained under an optimal Ga concentration of 0.72mol% while the Al additive content was fixed at 0.1mol%. This novel finding will be helpful towards the manufacture of high-quality ZnO varistors.

Microstructure and electrical properties of ZrO2-doped ZnO varistor ceramics

The microstructure and electrical properties of ZnO varistor ceramics with different ZrO2 content prepared by a solid reaction route and sintered at 1100 °C were investigated. The microstructures of the varistor ceramics samples were characterized by X-ray diffraction and scanning electron microscopy; the electrical properties and current–voltage (V–I) characteristics of the varistor ceramics were investigated by DC parameter instrument. The microstructure of the prepared samples shows a decrease in grain size of ZnO phase with the ZrO2 content increase. ZrO2-doped ZnO varistor ceramics exhibit comparatively vastly superior comprehensive electrical properties with addition of 0.50 mol% ZrO2, such as the threshold voltage is 350 V/mm, the nonlinear coefficient is 25.4 and the leakage current is 2.96 μA.

Preparation of high-voltage ZnO varistor ceramics

We have examined the effect of dopant concentration on the properties of ZnO varistor ceramics and determined the composition and conditions of the synthesis of ceramic powders and sintering of ceramics at a temperature of 925°C. We obtained a breakdown voltage of 2.8–3.0 kV/mm and nonlinearity coefficient in the range 48–55.

Study on the Electrical Properties and the Ac Conductance Response of ZnO-Based Varistor

ZnO-based varistor ceramics with different recipes were prepared in this research. It was found that the grain growth was restrained with the addition of Ba2+. SEM results indicated that the grain size of ZnO varistor ceramics with Ba2+ was reduced to 4.7 μm. Correspondingly, the breakdown electric field of the ZnO varistor samples were extremely enhanced to 8730 V/cm. The results of ac conductance response showed that two relaxation peak 1 and 2 with activation energy at about 0.24 eV and 0.36 eV were observed in 100-250 K, which were not affected by additives. However, a new conductance relaxation peak 3 in 300-450 K for sample B and C can be found, whose activation energy were varied from 0.55 eV to 0.65 eV with the addition of Ba2+. It was put forward that the electrical properties of ZnO varistor ceramics may be associated with the conductance relaxation peak 3.

Specific features of thermal phonon scattering and ferromagnetic properties of ZnO varistor ceramics

The scattering of thermal phonons (T = 2.3–3.8 K) in ZnO single crystals and related 97.5ZnO + 0.5Bi2O3 + 0.5Co3O4 + 0.5MnO2 + 1Sb2O3 (mol %) ceramics has been studied. It is established that the transport characteristics of phonons with thermal frequencies in ceramics at liquid-helium temperatures are determined by the presence of Co2+ ions in ZnO grains. A model of phonon transport in ZnO-based ceramics is proposed that takes into account the electron states of cobalt. The energy of the corresponding level is estimated at Δ ≈ 5 K. Manifestations of the ferromagnetic effect are revealed.

Microwave Sintering and Electric Properties of ZnO Varistor Ceramics

The ZnO varistor ceramics were fabricated through microwave sintering at 800~1150°C using ZnO powder doped and undoped TiO2 prepared by planetary ball mill. And the effects of milling time, sintering temperature and doping on microwave sintering technique have been studied. The result shows that the powder is more homogeneously and with smaller particle size with the increasing of milling time, therefore the electric properties is improved. However the particle size and the electric properties tend towards stability after 20h milling time. The density and electric properties increase with the increasing of sintering temperature, but decrease sharply at 1100°C, so the best sintering temperature is 1100°C. Doping TiO2 improves the electric properties of ZnO, however the electric properties decrease with more than 2.5% doping.

The effects of precipitant on the defect structures and properties of ZnO varistor ceramics

The effects of different precipitants (NH4HCO3 and NaOH) on the performances of ZnO varistor ceramics which were prepared by coprecipitation method were analyzed. There are noticeable influences for different precipitants to the microstructure and electrical performances of ZnO varistor ceramics. The change of microstructure is caused by the properties of precipitants themselves while the change of electrical performance is caused by the influence of precipitants to the grain boundary potential barrier properties. Besides, the donor impurity ion Na+ introduced by NaOH in the ZnO varistor ceramic will increase the free electron concentration in the grain. So the densities of intrinsic defects (zinc interstitial and oxygen vacancy) are restrained. The density of zinc interstitial is more sensitive to donor impurity ions compared with oxygen vacancy. Therefore the choice of precipitant is important when using coprecipitation method to prepare the powder of ZnO varistor ceramics. Even little impurity ions will result in the obvious change of the performance of varistor ceramics. The impurity ions should be avoided in the powder of ZnO varistor ceramics.

Effect of Sintering Temperature on Electrical Properties of ZnO Varistor Ceramics

To obtain ZnO varistors with high voltage gradient, ZnO varistors were fabricated by traditional ceramic sintering technique, the effect of different sintering temperature (1135～1155 °C) on electrical properties of ZnO varistors were investigated. The experimental results show that with increasement of sintering temperature, the grain size of ZnO varistor ceramic becomes bigger, the voltage gradient of varistor decreases and the density is improved. When the sintering temperature is at 1135 °C, the voltage gradient of varistor is up to 329V/mm, the leakage current is 8μA and the density is 96.4%. When the sintering temperature is at 1140 °C, the voltage gradient of varistor is 301V/mm, the leakage current is 4μA and the density is 96.6%. Compared the results at 1135 °C with 1140 °C , it is found that the comprehensive electrical properties of ZnO varistors reach maximum at 1140 °C.

Study on Microstructure and Electrical Properties of ZnO Varistor Ceramics

To obtain ZnO varistors with high voltage gradient, ZnO varistors were fabricated by traditional ceramic sintering technique, which were sintered from 1135 to 1155 °C, the microstructure and properties of varistors were characterized by SEM, X-rays diffraction and DC parameter instrument for varistors. The experimental results show that the spinel phase Zn7Sb2O12 generated during sintering process, very thin amorphous Bi rich films are formed between the ZnO/ZnO grain boundaries, with increasement of sintering temperature, the grain size of ZnO varistor ceramic becomes bigger, the voltage gradient of varistor decreases and the density is improved. The optimized parameters are that when the sintering temperature is at 1140 °C, the voltage gradient of varistor is 301V/mm, the leakage current is 4μA and the density is 96.6%, the comprehensive electrical properties of ZnO varistors reach maximum.

Research on Mechanism of Improving Electrical Properties of ZnO Varistor Ceramics by Laser Shock Processing

In order to improve electrical properties, annealed ZnO varistor ceramics have been treated by laser shock processing (LSP). Electrical parameters of the annealed ZnO varistor ceramics were measured by CJ1001 Varistor Tester. Morphology and microstructure of the annealed ZnO varistor ceramics were gained by XRD and SEM. The results indicate that compared with the electrical properties of the annealed ZnO varistor ceramics, the voltage gradient of the ZnO varistor ceramics treated by LSP remains the same, the nonlinear coefficient substantially increases by 31.6%, and the leakage current reduces by 33.3%. Meanwhile, the phase composition of the annealed ZnO varistor ceramics is mainly composed of ZnO main phase, Zn2.33Sb0.67O4, β-Bi2O3, and δ-Bi2O3 phase; while the phase composition of the ZnO varistor ceramics remains the above four phases in the compound state (anneal and then LSP), but the content of δ-Bi2O3 phase increases. The increase of δ-Bi2O3 phase content is the main reason for improving the electrical properties of the annealed ZnO varistor ceramics.