Elsevier

Materials Research Bulletin

Volume 36, Issues 7–8, May–June 2001, Pages 1259-1267
Materials Research Bulletin

Smart technique for fabrication of zinc oxide varistor

https://doi.org/10.1016/S0025-5408(01)00611-0Get rights and content

Abstract

This paper describes a smart and industrially suitable technique for preparing a high performance ZnO varistor from doped zinc oxide powder. A homogeneous distribution of various dopants in the mixed powder has been achieved by coating ZnO powder with a mixed solution of dopants in the form of acetates, nitrates and citrates. The mixed powder on subsequent drying, calcination, and sintering, produced varistors with better nonlinear electrical properties (α = 51) compared to varistors prepared from conventional oxide mixing technique (α = 30). The better nonlinear properties of solution coating route varistors have been attributed to the presence of uniform potential barriers throughout the entire microstructure, which in turn is credited to the uniform distribution of dopant ions at each grain and grain boundary region.

Introduction

Zinc oxide varistors, well known for their highly nonlinear electrical characteristics are used in electrical and electronic industries for surge protection. Their primary function is to sense and limit voltages and to do so reproducibly without failure during their lifetime [1], [2]. The nonlinear current-voltage properties of a ZnO varistor is described by the empirical relation J = KEα, where J is the current density, E is the applied electric field and α is the coefficient of nonlinearity and K is the constant of proportionality [3]. Good varistors are defined by high α in the non-ohmic region, a sharp change in electrical behavior from linear to nonlinear regions and a high leakage resistance in the prebreakdown region. Varistors can be used in ac and dc fields over a wide range of voltages from a few volts to tens of kilovolts, and over a wide range of currents, from microamperes to kiloamperes. Varistors have the additional property of high energy absorption capability up to thousands of Joules. Their versatility has made them useful to power industry as well as semiconductor industry.

The origin of varistor action is attributed to the presence of insulating grain boundary regions, where a number of free charges are trapped and give rise to potential barrier formation [4], [5], [6]. The presence of potential barriers in turn generates critical voltages for breakdown per boundary and the total breakdown voltage of the device becomes proportional to the number of such grain boundaries in between two electrodes. This multijunction feature of a varistor lies at the heart of surge absorption capability and any transient surge energy absorbed, is distributed between the many ZnO intergranular junctions. Therefore, to obtain a good varistor (high breakdown voltage and sharp change from linear to nonlinear property), it is essential to have high performance grain boundaries with equally stable potential barriers throughout the sample.

The origin of trapping centers responsible for varistor action although not completely understood, it is well accepted that varistor phenomena can only be observed by adding large ions such as Bi, Ba, or Pr to the ZnO matrix. Unfortunately, binary varistors have very poor nonlinearity. Addition of cobalt and manganese oxide is needed to enhance nonlinerity [7], [8], [9]. Commercial varistors are prepared by adding seven or eight different additives (Bi2O3, MnO2, CoO, Cr2O3, NiO, Sb2O3, Al2O3, SiO2 etc.) in small amounts (0.5–1.0 mol%) to ZnO powder. These additives develop nonlinearity in ZnO matrix by being distributed preferentially at the grain and grain boundary regions and thereby forming elctron trapping states responsible for potential barrier formation. Therefore, uniform, stable potential barrier formation throughout the microstructure requires generation of similar trapping centers at the barrier region, which in turn is dependent on the same pattern of dopant distribution at each grain-grain boundary junction. This is only possible if the starting powder is completely homogeneous in terms of constituents and segregation occurs only during sintering or post heat treatment.

In this pretext, conventional oxide mixing route was felt inadequate and several efforts using chemical routes such as sol-gel [10], [11], urea [12], reaction spray pyrolysis [13], [14], evaporative decomposition of solutions (EDS) [15], and wet chemical methods [16] have been tried for producing homogeneous powder for varistor preparation. Although sol-gel and urea process resulted in small-grain powders, attainment of a perfect composition for optimum properties was difficult due to the complex nature of dopants and large variation of pH values associated with precipitation of various metal ions. Evaporative techniques were judged as time consuming. The microemulsion technique [17], although produced fine powder with narrow size distribution, needs further work before it can be used commercially. Among other methods, preparation of monodisperse ZnO particles from zinc acetate [18], amine processing [19] and citrate route [20] are worth mentioning. Most of these methods are still in the laboratory scale and need further verification and development for commercial viability. The major problems with these methods are irregular distribution of grain sizes, agglomeration, and addition of multiple complex steps which impose costly and large change in the infrastructure for commercial adoptation. Unfortunately, a convenient and commercially adaptable homogeneous powder preparation technique for varistor manufacturing remains missing.

The present work was done to explore the possibility of producing ZnO varistor powder on a large scale by a solution-coating method. Here we report synthesis of homogeneously doped ZnO powder by coating ZnO particles with solutions of dopant ions. The microstructure and electrical properties of the resulting varistors are compared with varistors made from a conventional oxide mixing route.

Section snippets

Powder preparation and microstructural analysis

The ZnO mixed powder prepared in the present study contained 96 mol% ZnO with Bi2O3, MnO2, CoO, Sb2O3, Cr2O3, NiO, and Al2O3 comprising the remaining 4 mol%. The detail of the composition is given in Table 1. For the solution coating route, ZnO source was pure ZnO powder, Mn, Co, Cr and Ni were added as acetates, Bi, Al were added as nitrates and antimony was added as citrate. Antimony citrate was prepared from antimony oxide by dissolving antimony oxide in excess citric acid. A mixed solution

Phase analysis and microstructure

Fig. 2 represents the XRD spectrum of varistor sample made by solution coating route. From XRD spectra, three major phases, viz., zinc oxide, Bi2O3-rich phase and spinel were identified. Fig. 3 presents the microstructures of varistors prepared from alternate routes. Fig. 3(a) shows the microstructure of a varistor from solution coating route whereas Fig. 3(b) represents typical microstructure of a varistor prepared by conventional oxide mixing route. Results of EDX spot analysis are

Conclusions

Homogeneously mixed ZnO powder can be prepared by coating ZnO powder with solutions of dopants. Varistors prepared from this powder exhibit higher non-linearity coefficient, higher low-voltage resistance and sharper change from ohmic to non-ohmic behavior than those prepared by the conventional oxide mixing route. The improved I-V properties can be credited to the homogeneous distribution of dopants in the starting powder. This in turn produced similar profile of dopant distribution at each

Acknowledgements

The authors sincerely acknowledge Indian Institute of Technology, India for providing financial support throughout this work. The author wishes to acknowledge Dr. G. Banerjee, Rodel, USA, for his active help during this investigation. The author wishes to thank Prof. N.K. Khosla & Prof. S.N. Malhotra, Dept. of Materials Science & Metallurgical Engineering. I.I.T. Bombay, India, for providing the I-V measurement system and computation facility during this work.

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