Elsevier

Ceramics International

Volume 26, Issue 7, 3 August 2000, Pages 733-737
Ceramics International

Analysis of ZnO varistors prepared by the sol–gel method

https://doi.org/10.1016/S0272-8842(00)00012-2Get rights and content

Abstract

A new sol–gel processing method to prepare ZnO-based varistor powders, using inexpensive zinc acetate dihydrate, ethylene glycol, n-propyl alcohol and glycerol as starting materials, is presented. Compared to conventional oxide-mixing techniques, this method yields powders with a more homogeneous distribution of dopants; the sintering temperature could be lowered by about 200 K to 1000°C and the grain size is much smaller which proves very helpful for achieving high breakdown strength. Microstructure and electrical characteristics of the sintered powders were investigated.

Introduction

ZnO powders are very important materials due to many interesting properties inherent in this material, such as dielectric [1], piezoelectric [2], pyroelectric [3], semiconducting [4], acousto-optic [5], optical [6], electro-optical [7], nonlinear optical [8], photoelectrochemical [9] and electrical properties [4]. Moreover, ZnO ceramics containing several metal oxides, such as Cr2O3 Bi2O3, Sb2O3, Co3O4, MnO2 etc., show highly nonlinear current-voltage characteristics which enables them to be used as protection devices against voltage surges and voltage transients [10], [11], [12], [13], [14], [15]. The varistor effects take place at the grain boundaries within the ceramics and numerous theories have been developed to explain the effect [16], [17], [18]. It is necessary to have homogeneous distribution of dopants and the correct oxygen concentration to form good varistor ceramics, since oxygen defects are presumed to be the reason for ZnO-conductivity. Commercial varistor powders are usually produced utilizing a conventional mixed oxide technique with ZnO and other metal oxide dopants as starting materials, which is recognized as being unsuitable for achieving homogeneous dopant distribution. Many papers have been published about new methods to prepare varistor powders [19], [20], [21], [22], [23]. A sol–gel method has been shown to achieve high breakdown fields by small grain size [24].

In this paper, we use a different way to process sol–gel powder for varistor application by using inexpensive source materials such as zinc acetate dihydrate, ethylene glycol, n-propyl alcohol and glycerol. The powders can be sintered at a lower temperature compared to the conventional mixed oxide method and provide smaller grain size than has been published elsewhere. Moreover, microstructure and electrical characteristics of the sintered powders were investigated.

Section snippets

Preparation of powders

The sol was prepared using zinc acetate dihydrate, ethylene glycol, n-propyl alcohol, and glycerol. Twenty five milliters of ethylene glycol was added to 100 g of zinc acetate dihydrate in a round bottomed flask fitted with a condenser and kept at 150°C for 15 min over a hot plate to obtain a uniform transparent solution. On cooling to room temperature the content of the flask solidified to a transparent brittle solid which could be dissolved in n-propanol 200 ml. The solution thus obtained was

Results and discussion

Table 1 shows the density, electrical resistance and grain size of pure ZnO powders sintered at different temperatures (termed as sample S1). The highest density is obtained about 5.4 g/cm3 which is more than 95% of the theoretical density as the sintering temperature is 1050°C (refer to Fig. 2). The maximum value of the electrical resistance is found to be about 156 MΩ as the sintering temperature is 1000°C (refer to Fig. 3). The electrical resistance of the sample could not be too small, or

Conclusions

Homogeneous pure ZnO and ZnO-based powders prepared by the sol–gel method have been successfully fabricated. Compared to conventional oxide-mixing techniques, the sintering temperature could be lowered by about 200 K to 1000°C and the grain size is much smaller (<2 μm) which is expected to show superior electrical properties.

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