Electrical characteristics and stability of low voltage ZnO varistors doped with Al

https://doi.org/10.1016/j.matchemphys.2005.12.023Get rights and content

Abstract

The nonlinear electrical characteristics and stability against AC accelerated aging stress of Al-doped low voltage ZnO varistors were investigated as a function of Al content. The varistors with Al exhibited a high nonlinear coefficient above 30, compared with that without Al. The varistors with 200 ppm Al3+ exhibited a highest nonlinearity, with a nonlinear coefficient of 42.9 and a leakage current of 9.5 μA. But the varistor without Al showed high stability, in which the variation rate of varistor voltage, of nonlinear coefficient and of leakage current are −4.1, −4.3 and +20.7%, respectively, under AC accelerated aging stress (1.0 V1 mA/125 °C/24 h).

Introduction

Zinc oxide varistors are ceramic semiconductor devices which have extreme nonlinearity in their current–voltage behavior and thus are widely used for electronic devices. The nonlinear characteristics are attributed to the formation of double Schottky barriers at the zinc oxide grain boundaries.

Zinc oxide (ZnO) varistors are formed by sintering mixture of ZnO powders with small amounts of other oxides, such as Bi2O3, Sb2O3, Co2O3, MnO2, Cr2O3, etc., the relationship between the voltage across the terminal, V, and the current in the devices, I, is typically expressed by I = kVα. The term α in the equation is a nonlinear coefficient, inherent parameter of varistors representing the degree of nonlinearity of conduction. A linear resistance has an α = 1. The higher the value of α, the better the clamp, which explains why α is sometimes used as a figure of merit, and they possess excellent surge withstanding capabilities [1], [2].

The ZnO varistors are suited for high-voltage applications. However, an increasing number of low-voltage varistors is being used for surge protection in integrated circuits and in automobiles. Since the breakdown voltage (varistor voltage) is proportional to the number of ZnO grains in series between the electrodes, low voltage varistors can be obtained by either decreasing the thickness of the specimen or by increasing the size of ZnO grains. The thin ZnO varistors are, however, difficult to prepare and apt to break. Note also that the energy absorption capability of thin ZnO varistors is very poor due to its small volume. On the other hand, low voltage ZnO varistors with grains of large size have been fabricated by using grain growth-enhancing additives. TiO2 can greatly enhance the grain growth of ZnO, thus widely used in producing low-voltage ZnO varistors, but the doping of TiO2 have restricted nonlinear current–voltage characteristics, the α value is very low. Consequently, its microstructure exhibits a nonuniform distribution of grain size, which causes an irregular current distribution and makes the varistor susceptible to hot spots.

In operation, varistor materials are often subject to various stresses (high electric fields, high temperature and aggressive ambients) that can affect their performance and useful lifetime; the continuous leakage current and resultant Joule heat may degrade the electrical properties of the devices. Therefore, in addition to nonlinearity, the electrical stability is a technologically important characteristic of ZnO varistors. It has been found that the nonlinearity and electrical stability depend critically upon the additives and the processing methods.

In this paper, the effect of a small amount of Al doping on the nonlinearty and electrical stability of low voltage ZnO varistors was investigated.

Section snippets

Experimental procedure

Reagent-grade raw materials were prepared for low voltage varistors, the base composition was: 95.95% ZnO + 0.75% Bi2O3 + 0.8% TiO2 + 0.5% NiO + 1.0% Co2O3 + 0.5% MnCO3 + 0.5% SnO2 (all in mol%). The other samples were Al-doped low voltage varistors (the base composition plus 100, 200, 300, 400, 500 ppm Al3+). The powder mixtures were wet ball-milled in a polyethylene bottle with ZrO2 balls for 24 h in deionized water. After dried and granulated, the powder was pressed into discs of 10 mm in diameter and 1.8 

Results and discussion

Fig. 1 shows the SEM micrographs of the low voltage varistors with various Al contents. The density of ceramics was decreased gradually from 5.48 to 5.08 g cm−3, corresponding to 97.6–90.5% of theoretical density (TD = 5.61 g cm−3). The average grain size was decreased in the range of 25.3–48.8 μm with increasing Al content. The decrease of grain size is attributed to the precipitation of secondary phase in the grain boundaries and nodal points. The detailed microstructural parameters are summarized

Conclusion

The electrical nonlinear characteristics and their stability against AC accelerated aging stress of Al-doped low voltage ZnO varistors were investigated with Al content. The varistors without Al content exhibited poor nonlinear characteristics as only 19.8 in nonlinear coeffcient and good stability as 0.49 μAh−1/2 in degradation rate coefficient. However, the nonlinear characteristics were greatly enhanced in nonlinear coefficient above 30 by incorportion of Al(NO3)3·9H2O, but degrades the low

References (8)

There are more references available in the full text version of this article.

Cited by (38)

  • Enhancement of electronic protection to reduce e-waste

    2015, Journal of Industrial and Engineering Chemistry
    Citation Excerpt :

    As a result, to enhance the non-ohmic property, the additives should be optimized in the starting powder of the ceramic. In the case of the optimization, the method of ‘one variable at a time’ has been widely used by varying one of the additives while other parameters are kept constant while the additives are not completely independent; it affects the electrical property of the ceramic [36–39]. Moreover, the number of experiments is quite high due to the variety of the additives which entail time consumption and possible misinterpretation of the related results.

  • Effect of Si content on electrophysical properties of Si-polymer composite varistors

    2014, Materials Chemistry and Physics
    Citation Excerpt :

    The probability of making low voltage zinc oxide-based varistors is limited by low permittivity of ZnO and the fact that energy absorption capability of thin ZnO-based varistors is very low, due to their small volume [12]. Need for low voltage varistors leads researchers to focus on new varistor materials, dopants and sintering processes [13–20]. As an example, about ZnO-based varistor case, Bi2O3 has an essential role in inducing nonlinearity whereas in new ones, conductive polymers are able to do the same task.

  • AC ageing characteristics of Y<inf>2</inf>O<inf>3</inf>-doped ZnO varistors with high voltage gradient

    2011, Materials Letters
    Citation Excerpt :

    This is the so-called ageing phenomenon and will eventually make the varistors thermally broken down or destructed [5–7]. Therefore, the ageing characteristics of ZnO varistors have attracted massive attention of researchers in this field [4,8,9]. In order to shorten the length and reduce the weight of MOA used in higher voltage level power transmission lines (e.g. 500 kV, 750 kV or 1000 kV) and gas-insulated substations (GIS), ZnO varistors with high voltage gradient have been developed [10].

  • Statistics on the AC ageing characteristics of single grain boundaries of ZnO varistor

    2011, Materials Chemistry and Physics
    Citation Excerpt :

    However, ZnO varistor subjected to long-term voltage or current stress may lead to continuous deformation of the double-Schottky barrier and eventually result in deterioration in electrical property, which is the so-called ageing phenomenon [7–10]. Therefore, the ageing characteristics of ZnO varistor have attracted more attention in this field [11–13]. Many literatures had discussed on the ageing characteristics and mechanisms of ZnO varistors, but the electrical property variation of single grain boundaries during the ageing process still has not been clearly analyzed [14–19].

View all citing articles on Scopus
View full text