Electrical Degradation Of ZnO Varistors Research Articles

Effect of ZrO2 addition on tolerance characteristics of the electrical degradation of ZnO varistors

Abstract3 ZnO varistors with excellent electrical degradation tolerance characteristics were made by addition of Bi2O3‐MnO2‐Co3O4‐Cr2O3‐SiO2‐Sb2O3‐NiO. The electrical degradation tolerance characteristics were evaluated by varying the amount of ZrO2. The evaluation methods were voltage–current (V–I) characteristics, X‐ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersion X‐ray spectroscopy (EDX). Monoclinic and tetragonal ZrO2 and compounds derived from Zr were observed at both the grain boundaries and triple points. Moreover, the compounds derived from Zr with added Sb showed improved electrical degradation tolerance characteristics. On the other hand, monoclinic ZrO2 in particular degraded the electrical degradation tolerance characteristics. One factor in the improvement of the electrical degradation tolerance characteristics is that the mobility of oxide ions or interstitial Zn2+ ions might be hindered by the formation of compounds containing Zr, Sb, Si, and Bi atoms. The electrical degradation tolerance characteristics of ZnO varistors with added ZrO2 were better than those of varistors without addition of ZrO2 but subjected to heat treatment. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 179(2): 11–19, 2012; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.21228

Effect of heat treatment on tolerance characteristic for electrical degradation of ZnO varistors with added Sb2O3

AbstractThe effects of the heat treatment of ZnO varistors with added Bi‐Mn‐Co‐Sb on their electrical degradation were investigated. Before heat treatment, the degree of the tolerance characteristics of varistors for electrical degradation was smallest for the sample with 0.12 mole‐% of added Sb2O3. After heat treatment, the electrical degradation tolerance characteristic was markedly improved. It was found that the tolerance characteristics for electrical degradation after heat treatment strongly depend on the amount of added Sb2O3. For the samples with added Sb2O3, the tolerance characteristic for electrical degradation upon heat treatment is not affected by the intensity of the X‐ray diffraction peak of the (004) plane for ZnO. For samples with 0.12 mole‐% or less of added Sb2O3, the value of the nonlinearity index α of the voltage–current (V–I) characteristic after electrical degradation was proportional to the full width at half maximum (FWHM) of the X‐ray diffraction peak at 29.4° for Zn2.33Sb0.67O4 spinel particles in samples subjected to heat treatment under various conditions. Therefore, it is speculated that the increase in the FWHM for the spinel particles is due to the existence of fine spinel particles at grain boundaries or triple points of ZnO grains. Furthermore, it is speculated that the improvement of the electrical degradation tolerance characteristics is due to a decrease in the ion mobility of O2– ions or Zn2+ ions due to their blocking by uniformly dispersed fine spinel particles at grain boundaries. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 179(2): 1–10, 2012; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.21229

Effect of ZrO2 Addition on Tolerance Characteristics of the Electrical Degradation of ZnO Varistors

ZnO varistors of the excellent tolerance characteristics of electrical degradation were made by adding Bi2O3-MnO2-Co3O4-Cr2O3-SiO2-Sb2O3-NiO in them. The tolerance characteristics of electrical degradation were evaluated by changing amount of ZrO2. The evaluation methods are voltage-current (V-I) characteristics, X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersion X-ray spectroscopy (EDX). Monoclinic, tetragonal ZrO2 and the compounds originated from Zr were observed at both the grain boundaries and triple points. Moreover, The compounds originated from Zr and added Sb improved the tolerance characteristics of electrical degradation. On the other hand, especially monoclinic ZrO2 deteriorated the tolerance characteristics of electrical degradation. It is one factor of the improvements of the tolerance characteristics of electrical degradation that the mobility of oxide ions or interstitial Zn2+ ions might be hidered by forming the compounds contained Zr, Sb, Si, and Bi atoms. The tolerance characteristics of electrical degradation for ZnO varistors added ZrO2 was improved compared with that not-added ZrO2 by heat treatment.

Study on Recovery of Electrical Degradation of ZnO Varistors

The mechanism of the recovery of the nonlinearity voltage-current (V-I) characteristics after electrical degradation was investigated from various viewpoints. The recovery processes of ZnO varistors in various gas atmospheres, and at various pressures, temperatures, and absolute humidity levels were evaluated. The evaluation involved the analysis of V-I characteristics, capacitance-voltage (C-V) characteristics, and X-ray diffraction (XRD). The height of the Schottky barrier and the thickness of the depletion layer were almost constant for sintering times between 3 and 5 h. On the other hand, the rate of recovery exhibited a change with sintering time. The recovery speed of the nonlinearity was also temperature dependent. It is suggested that the recovery process is caused by the transport of oxide ions through the grain boundaries between ZnO grains. It was found that the recovery process is affected by the crystal structure of Bi2O3 particles, which exist at the boundaries between ZnO grains.

Study on Recovery of Electrical Degradation of ZnO Varistors

The relationship between the crystal structure of Bi2O3 and the recovery of the electrical degradation of ZnO varistors was investigated. The evaluation methods used were analysis of the voltage-current (V-I) characteristics, scanning electron microscopy (SEM), energy-dispersion X-ray spectroscopy (EDX), and X-ray diffraction (XRD) measurements. The electrical degradation was estimated by the nonlinearity of the V-I characteristics. A marked recovery of the nonlinearity of the V-I characteristics was observed for a sample having the crystal structure of δ-type Bi2O3. It is speculated that the electrical degradation is markedly recovered for optimum conditions of both the amount and the crystal structure of the added Bi2O3, because, for samples sintered for a long time, the recovery speed is shorter than that for samples sintered for a shorter time. During long-time sintering, the amount of Bi2O3 decreased because of evaporation. No change of the crystal structure of Bi2O3 was observed upon repeated the electrical degradation and the recovery. It is suggested that another mechanism apart from the diffusion of oxygen ions through the grain boundaries of ZnO particles contributes to the electrical degradation and recovery of the nonlinearity of V-I characteristics.

Relationship between grain boundaries and electrical degradation of ZnO varistors

AbstractThe effects of the electrical degradation characteristics and microstructure of Sb2O3‐added ZnO varistors were investigated by field emission scanning electron microscopy (FE‐SEM), energy dispersion X‐ray spectroscopy (EDX), optical microscopy, X‐ray diffraction (XRD), and voltage– current (V−I) characteristic measurement. The nonlinearity index α of the V−I characteristic for Bi‐Mn‐Co‐Sb2O3‐added ZnO varistors decreased with increasing amount of Sb2O3 after electrical degradation. Twin crystals of ZnO were formed by the addition of Sb2O3. The number of twin crystals with two c‐axes perpendicular to the twin plane increased and the number of twin crystals with two c‐axes parallel to the twin plane decreased with increasing addition of Sb2O3. It is suggested that electrical degradation is affected by a combination of orientations of ZnO grains containing twin planes and that a double Schottky barrier may not be formed in the twin plane. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 159(3): 1– 9, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20493

Effect of Mn and Co Addition on Electrical Degradation of ZnO Varistors

The effects of Mn and Co addition on the electrical degradation of ZnO varistors were investigated on the basis of voltage-current (V-I) characteristics, X-ray diffraction (XRD), capacitance-voltage (C-V) characteristics, and isothermal capacitance transient spectroscopy (ICTS). The optimum Co content for preventing electrical degradation was determined to be approximately 0.5mol% in Bi-Mn(0.5mol%)-Co(0~1.0mol%)-added ZnO varistors. Correlations between the improvement of electrical degradation and the structural changes of additives were investigated by XRD. It is suggested that the crystal structure of a-Bi2O3 at the grain boundary changes to a different structure, such as d-Bi2O3 or Bi7.65Zn0.35O11.38 with a fluorite-type structure, at the optimum Co content. It is speculated that the structural change of Bi2O3 at the grain boundary contributes to the improvement of electrical degradation characteristics. Three trap levels were detected in all Bi-Mn-Co-added samples by ICTS. These trap levels showed a local maximum at the optimum Co content.

ZnOバリスタの粒界と課電劣化の関係

ZnOバリスタの粒界と課電劣化の関係

ZnOバリスタのMn，Co両添加による課電劣化への影響

The effect of both Mn and Co addition on the electrical degradation of ZnO varistors were investigated based on voltage-current (V-I) characteristics, capacitance-voltage (C-V) characteristics, isothermal capacitance spectroscopy (ICTS), and X-ray diffraction (XRD). The optimum Co content to prevent the electrical degradation is approximately 0.5mol% for Bi-Mn(0.5)-Co-added ZnO varistors. Correlation of the improvement of the electrical degradation with the structure changes of ZnO varistor by Mn and Co addition was investigated by XRD. It is found that the crystal structure of α-Bi2O3 in the grain boundary changed to Bi7.65Zn0.35O11.38 for Bi-Mn-Co(0.5mol%) added samples. It is suggested that these structure changes of varistors with Mn and Co addition contribute to the improvement of the electrical degradation characteristics of Bi-Mn-Co-added ZnO varistors.

The Effect of Both Mn and Co Addition on Electrical Degradation of ZnO varistors

The Effect of Both Mn and Co Addition on Electrical Degradation of ZnO varistors

The Effects of Si Addition on Electrical Degradation of ZnO Varistors

The Effects of Si Addition on Electrical Degradation of ZnO Varistors

Effects of Addition of Al on Electrical Degradation of ZnO Varistors

Effects of Addition of Al on Electrical Degradation of ZnO Varistors

The Effect of Trasition of Valence State of Additives Electrical Degradation of ZnO Varistors

The correlation between electrical degradation and valence states of additives in the Bi2O3 and MnO2(Bi-MnO2) or Mn2O3(Bi-Mn2O3) added ZnO varistors and Bi2O3 and CoO(Bi-CoO) or Co3O4(Bi-Co3O4) added varistors was investigated by ICTS and XPS measurements. The results of ICTS and XPS measurements showed that Mn as the additive contributes differently to the factors of degradation in the cases of Bi-MnO2 and Bi-Mn2O3 added ZnO and Co as the additive contributes similarly to the factors of degradation in the cases of Bi-CoO and Bi-Co3O4 added ZnO. The recovery of varistor characteristics of Bi-Mn-Co added ZnO added Co3O4 after degradation was more remarkable than that of the sample added CoO by the thermal annealing. The results of ICTS measurements showed that the interface states changed for Bi-Mn-Co added ZnO after the thermal annealing. XPS spectra of the Co-2p3/2 orbit and Mn-2p3/2 orbit for Bi-Mn-Co added ZnO showed different binding energies after degradation.

The Effects of Addition of Al on Electrical Degradation of ZnO Varistors.

The effects of addition of Al on the electrical degradation of the Bi, Mn, and Al added ZnO varistors and the Bi, Co, and Al added ZnO varistors were investigated by V-I, C-V, ICTS, and XPS methods. The optimum quantity of Al additive to prevent the electrical degradation is approximately 5ppm in the case of Bi-Mn-Al added ZnO varistor and approximately 10ppm in the case of Bi-Co-Al added ZnO varistor. For each optimum quantity of Al content for Bi-Mn-Al or Bi-Co-Al added varistor, the distribution of values of the energies of the trap levels became narrowest. On the other hand, values of the full width at half maximum of Mn-2p and Co-2p orbits became largest.

The Effect of Transition of Valence State of Additives on Electrical Degradation of ZnO Varistors

The Effect of Transition of Valence State of Additives on Electrical Degradation of ZnO Varistors