Modified citrate gel routes to ZnO-based varistors

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Abstract

Chemical routes, based on a modified citrate gel synthesis, were developed for the production of ZnO-based varistor powders and compared with the conventional mixed-oxide route. Green bodies of powders prepared by the chemical routes were found to undergo densification in one or two discrete steps (at 770 and 910°C) compared to multiple-step densification of the conventional powder. In addition, it was found that the microstructures and electrical properties of the sintered pellets, although of nominally similar chemical composition, varied according to the synthesis procedure. Pellets from the chemical routes showed smaller grain sizes, higher densities, more evenly distributed intergranular phases and higher non-linearity coefficients than those from the conventional mixed-oxide route.

Introduction

From a technical standpoint several electrical parameters are required of commercial varistors, including particular breakdown voltages, nonlinearity coefficients and energy handling capabilities.1 In order to obtain the requisite values of these parameters much work has focused on subtle adjustments of the type and proportions of minor additives and of the sintering conditions. However, most commercial varistors are prepared by a mixed-oxide synthesis procedure, in which all of the components are mixed together as fine powders and then sintered.

The non-linear electrical properties of ZnO varistors are determined by grain boundary phenomena, whilst the low-current behaviour is a function of both the quality of the grain boundaries and the intergranular phases. The conductivity of the grains, which influences the varistor performance at high currents, is dependent on the granular chemical composition. The minor components added to commercial ZnO varistors have been found to influence not just the grain conductivity but also the grain growth, intergranular phases and grain boundaries. In accordance with common usage, the minor components shall be referred to here as dopants (grain conductivity enhancers) and additives (grain boundary formers).

One perceived problem with the mixed-oxide route is that no differentiation is made in the method of incorporation of additives and dopants. In addition impurities due to abrasion from the milling equipment may render the situation more complex. Generally high sintering temperatures and long sintering times are required to improve homogeneity and obtain satisfactory varistor pellets. However, chemical synthesis may offer routes to more homogeneous green bodies, requiring shorter sintering times at lower temperatures.

Dosch et al.2 describe a process for ZnO varistor powder synthesis in which a co-precipitation process from chlorides was used. An interesting feature of this approach is related to the sequence of precipitation of the various constituents. The strategy employed was to separate the manufacturing process into several steps and to use the constituents in the appropriate step based on their later functionality. In this respect, the dopants were co-precipitated with ZnO, since they were required within the grains to enhance the electrical conductivity. After a calcination step, additives were precipitated onto the surface of the doped ZnO grains. The additives were mostly responsible for the intergranular Schottky barrier formation and were also required to provide the flux for the liquid-phase sintering process. Related precipitation procedures are described by Haile3 and Westin4. In these cases pure ZnO was covered by a coating of dopant and additive oxides.

In this work the citrate gel process was combined with the strategies of co-precipitation employed by Dosch2 and precipitation by Westin4 to prepare ZnO-based varistors. However, the citrate-gel process commonly employed5, 6 was modified by using acetates instead of nitrates of the relevant metals (except for Bi and Al for which nitrates were used and Si for which the tetraethoxide was used). Acetates were preferred over nitrates as they produced less toxic side products during powder calcination.

Section snippets

Experimental

Varistors composed of the following were prepared: ZnO with Co, Mn, Al, Ni, and Cr dopants, and Bi, Sb, Ba, Si, and B additives. The total amount of dopants and additives equalled 7 at.%.

The conventional mixed-oxide route, used to prepare reference samples, is denoted as route 0. Powders prepared by this route were obtained from Siemens-Matsushita.

Route 1 synthesis involved a first step in which doped ZnO powder was prepared by coating a ZnO powder with a layer of citrates of the dopants using

Results and discussion

As shown in Fig. 1, Fig. 2, Fig. 3, by the differential linear expansion with temperature, the green bodies from route 1 showed a slightly earlier onset of densification than for routes 0 and 2 (680° compared with 730–750°C, respectively). However, more strikingly the densification of materials from routes 1 and 2 was largely complete (the magnitude of ΔLL0 was 95% of the maximum value) at 1100 and 980° respectively, compared with >1200°C for the route 0 material.

Local maxima were observed

Conclusions

Dense, doped ZnO ceramic pellets were produced by three different synthetic methods; conventional mixed-oxide, precipitation and co-precipitation citrate gel. The two non-conventional routes produced precursor powders which sintered more readily and in fewer distinct stages (one or two compared with many) than a powder prepared by the conventional mixed-oxide route. This was attributed to the tailored microstructural variations in minor component concentrations for the novel routes compared

Acknowledgements

We thank Siemens-Matsushita, Deutschlandsberg (now EPCOS) for providing materials. Discussions with Professor J. Schoonman are gratefully acknowledged.

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