Influence of Fe2O3 doping on microstructural and electrical properties of ZnO–Pr6O11 based varistor ceramic materials

doi:10.1016/j.jallcom.2010.08.100

Journal of Alloys and Compounds

Volume 508, Issue 2, 22 October 2010, Pages 494-499

https://doi.org/10.1016/j.jallcom.2010.08.100 Get rights and content

Abstract

The doping effect of Fe₂O₃ on the microstructural and electrical properties of ZnO–Pr₆O₁₁ based varistor ceramic materials was investigated. Fe₂O₃ 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 Fe₂O₃ increased from 0 to 1 mol%. When the doping level of Fe₂O₃ 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 Fe₂O₃, more Fe atoms would segregate at grain boundaries, providing more extra electrical carriers, decreasing the resistances of the grain boundaries, and PrFeO₃ would be formed, destroying the construction of grain boundaries. Therefore, the nonlinear electrical properties of the resultant varistor materials were deteriorated.

Research highlights

▶ ZnO–Pr₆O₁₁ based varistor ceramic materials. ▶ Eliminating the few drawbacks due to the high volatility and reactivity of Bi₂O₃ during liquid sintering by Pr₆O₁₁ substituting Bi₂O₃. ▶ Optimizing doping of Fe₂O₃ in ZnO–Pr₆O₁₁ based varistor ceramic materials.

Introduction

ZnO-based varistors are polycrystalline electronic ceramic devices, which are obtained by sintering ZnO powder with small amounts of other metal oxides. The most excellent property of ZnO varistor materials is their nonlinear voltage–current characteristics due to the potential barriers formed at grain boundaries. Their primary function is to protect electronics and electrical circuits from being destroyed by transient voltage surges, and this protection function can be performed promptly and repeatedly. So they are very much useful both in power industry as well as in electronic industry [1], [2], [3], [4], [5].

The conducting mechanism of ZnO-based varistors is a double Schottky phenomenon. The key that a ZnO-based varistor functions is the formation of grain boundary layer of high electrical resistivity. According to the different varistor forming oxides in the grain boundary layer, ZnO-based varistor materials are classified into Bi-doped, Pr-doped, V-doped and Ca-doped ZnO varistor materials, and so on [5], [6]. Other oxides are doped into varistor ceramics to achieve, hopefully, a larger nonlinear coefficient and higher resistivity [7], [8], [9], [10], [11], [12].

Up to now, the majority of commercial ZnO varistor materials are Bi-doped ZnO-based semiconductors [5], [6]. However, the most commercially applied Bi-doped ZnO varistor materials have a few drawbacks due to the high volatility and reactivity of Bi₂O₃ during liquid sintering. The former changes varistor characteristics with the deviation of inter-composition ratio of additives, and the latter destroys the multi-layer structure of chip varistors, which generates an insulating spinel phase deteriorating surge-absorption capabilities [5], [13], [14]. To overcome these problems, various fabrication techniques to reduce the volatility have been continuously developed for Bi-doped ZnO varistor materials [2], [13], [14], [15], [16], [17], [18], and different varistor forming oxides have been tentatively investigated into ZnO varistor materials [19], [20], [21], [22], [23], [24], in which Pr-doped ZnO varistor materials are most promising in application and have been intensively studied [19], [20], [21], [22], due to their advantages over Bi-doped ZnO varistor materials in relatively simple two-phase microstructure of ZnO grains and praseodymium oxide intergranular phases, reducing quantity of composition materials and improving the electrical properties of the ceramics, and stable composition at high temperature sintering stemming from the high melting point of Pr₆O₁₁ [5], [21].

It is a pity, however, that Pr-doped ZnO varistor materials have not been studied as extensively as done for the Bi-doped ZnO varistor materials. As main dopants, to our knowledge, only the effects of Co-doping and rare-earth doping on the microstructural and electrical properties of Pr-doped ZnO varistor materials have been reported in literature. And several other kinds of metal oxides such as MnO₂, Sb₂O₃, Al₂O₃, NiO, Cr₂O₃, TiO₂ and K₂O which are commonly used in the ZnO–Bi₂O₃ system were tried into Pr-doped ZnO varistor materials [5], [19], [20], [21], [22]. Their mutual interactions, especially the one between the varistor forming oxide, Pr₆O₁₁, and other electrical property improvers, have been under dimness.

For example, it was reported that Fe₂O₃ would be a fatal additive for ZnO–Bi₂O₃ system [25], but Song and Liu [26] claimed that when the content of Fe₂O₃ was less than 0.1 mol% in ZnO–Bi₂O₃ system, it could improve the electrical properties of ZnO–Bi₂O₃ based varistor materials. However, the influence of Fe₂O₃ doping on ZnO–Pr₆O₁₁ based varistor materials has not been reported. So, the aim of this work is to investigate the effect of Fe₂O₃ doping (which is hardly to be avoided during the industrial production of ZnO varistors due to the usage of iron-made equipments) on the microstructural and electrical properties of ZnO–Pr₆O₁₁ based varistor materials, and to try to understand the mutual interaction and reaction mechanism between the oxides in the material system.

Section snippets

Sample preparation

The samples were fabricated using a conventional ceramic process [27], [28] with a nominal composition of (98.0 − x) mol% ZnO + 0.5 mol% Pr₆O₁₁ + 1.0 mol% Co₃O₄ + 0.5 mol% Cr₂O₃ + xmol% Fe₂O₃ (x = 0.0, 0.001, 0.003, 0.005, 0.01, 0.025, 0.1, 0.5, 1.0). The powder mixtures were ball-milled in water for 24 h with highly wear-resistant ZrO₂ as media, in which the weight ratio of ZrO₂ balls to powder mixture was 2:1. The milled slurries were dried in air at 120 °C for at least 24 h. The dried powder mixtures were then

Composition and microstructure

Fig. 1a shows the XRD patterns with normalized peak intensities of the as-prepared ZnO–Pr₆O₁₁ based varistor ceramic samples doped with different amounts of Fe₂O₃. The experimental results indicate that, when the doping amounts of Fe₂O₃ into the samples are less than 0.10 mol%, the identifiable phases in the samples are almost the same as that in the sample without Fe₂O₃ doped, where excepting the major phase of ZnO, only could praseodymium oxide (Pr₆O₁₁) be detected, as can be seen in Fig. 1a

Conclusions

In ZnO–Pr₆O₁₁–Co₃O₄–Cr₂O₃–Fe₂O₃ based varistor ceramic materials, Fe₂O₃ was an important additive, acting as an inhibitor of ZnO grain growth. With tricky and appropriate amount of Fe₂O₃ doped, the electrical property of ZnO–Pr₆O₁₁ based varistor ceramics would be significantly improved. When the doping level of Fe₂O₃ was no more than 0.005 mol%, more addition of Fe₂O₃ could improve the nonlinear coefficient and varistor voltage of the materials. The optimum nonlinear coefficient, 26, and

Acknowledgments

This work was supported by “Scientific Research Foundation for Returned Overseas Chinese Scholars, State Education Ministry”, the National Natural Science Foundation of China (grant no. 60806005), “Transfer and Industrialization Project of Sci-Tech Achievement, Cooperation Project between University and Factory, Beijing Municipal Commission of Education”, and “State Key Laboratory of New Ceramic and Fine Processing, Tsinghua University (grant no. KF0903)”.

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Influence of Fe2O3 doping on microstructural and electrical properties of ZnO–Pr6O11 based varistor ceramic materials

Abstract

Research highlights

Introduction

Section snippets

Sample preparation

Composition and microstructure

Conclusions

Acknowledgments

J. Eur. Ceram. Soc.

Ceram. Int.

J. Alloys Compd.

Ceram. Int.

Mater. Lett.

J. Eur. Ceram. Soc.

Met. Soc. China

Mater. Sci. Eng.

Mater. Chem. Phys.

J. Alloys Compd.

Mater. Sci. Eng.

Ceram. Int.

Physica B

Mater. Lett.

J. Alloys Compd.

Ceram. Int.

J. Alloys Compd.

J. Hazard. Mater.

Solid State Commun.

Influence of Fe₂O₃ doping on microstructural and electrical properties of ZnO–Pr₆O₁₁ based varistor ceramic materials