ZnO varistor manufactured by composite nano-additives

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Abstract

Based on the analysis of the effects of ZnO grain boundaries, the new method of manufacturing ZnO varistors prepared by composite nano-additives is put forward in this paper. The emphasis of this technique is mainly on improving the microstructural uniformity and the effects of ZnO varistors grain boundaries so as to obtain the high quality components. As a result of the experiment, fine and active multiplex composite nano-additives are prepared by chemical coprecipitation, and highly uniform microstructure and larger surge absorption capability compared with the conventional oxide mechanical mixed technique are achieved. Moreover, this method is easy to perform and the shape of the obtained powders is apt to control. Also this new technique of manufacturing ZnO varistors is more feasible and of great prevalence in spawning ceramic materials among the existing equipments.

Introduction

ZnO varistors is a typical sort of semiconductor ceramics. Their excellent nonlinearly and electrical characteristics arising out of the grain boundaries are mostly dependent on the micro structural uniformity[1] (especially on the shape and the size of all the ZnO grains). According to the theory of Ce-Wen Nan and David R. Clark [2], the more narrow the three dimensional distribution of barrier height and the width of the depletion layer, the more excellent the macroelectrical property of ZnO varistors. And on the basis of the thought experiment introduced by Pike [3], the formation of grain boundary may be considered to consist of two identical semiconducting grains with an intervening layer of grain-boundary material. The grain boundary material is assumed to consist of the same semiconducting material but contains defects and dopants. As a result, its Fermi level is different from that of the two grains, and it also has electronic states because of the defects and dopants within the bandgap energy. The grain boundary is thus formed by joining together the three pieces of material. The relations of two important parameters: the barrier height, Φ0, and the width, d, of the depletion layer are given as follows:Φ0=e2nt28εε0n0d=εΦ0n01/2where nt is the trapped charge of area density, and n0 is the carrier concentration in the grains. Obviously, the expressions above are closely related to the types and centralizing state of the adsorbed dopants. Accordingly, how to improve the topsy–turvydom of the adsorbing dopants and attaining the microstructural uniformity is the very core of the subject of enhancing the macroelectrical property of ZnO [4]. For achieving this purpose, the much smaller scale of the preparatory powders, and the more narrow of the three dimensional distribution are absolutely essential qualifications.

At present time, the new techniques of manufacturing high quality Zinc oxide varistors have been reported on many occasions [5], [6], [7], [8]. Through the analysis of microstructure and disabled mechanism of ZnO varistors, we think the microstructure uniformity of grain boundary abounded in defects and dopants is the basis of improving the macroelectrical property of ZnO varistors [9]. Consequently, a new method of manufacturing ZnO varistors prepared by composite nano-additives is put forward. The emphasis is mainly on meliorating the micro structural uniformity and the effect of ZnO varistors grain boundary, especially the accumulative state of adsorptive dopants in obtaining the high quality components. Our research is based upon the thought stated above, and the perfect results achieved by this means.

Section snippets

Experimentals

All the dopants such as Mn, Co, Bi, Ba, Al, Cr, Zr, Sb, etc. are divided into two groups: Labeled A and B. The mixed metal–salt solutions are confected by the volumetric molar concentration of 1 mol·l−1. After mixing at the temperature of 50 °C, under a certain pH value, the chemical coprecipitation is carried out by adding the composite precipitators of NH4HCO3–NH3·H2O–NH4Ac to the mixed solutions. In succession, the proper dispersant is been added before filtration, and after sufficiently

Characteristic of the powders

The TEM photos of the multiplex composite nano-additives prepared by the chemical coprecipitation are shown as Fig. 1, Fig. 2, and the curves of the granular distributing measured by the WQL-11 are shown in Fig. 3, Fig. 4. From these figures, we can see that the granularmetric is much less than 100 nm, and the range of the granular distributions is much narrower than usual, with approximately 85% smaller than 100 nm. Due to the high superficial activity, the composite powders reunite to a

Conclusions

In this paper, a new technique of manufacturing ZnO varistors prepared by multiplex composite nano-additives is put forward on the basis of analyzing the effects of grain boundary. As a result of the experiment, the much uniform microstructure and larger surge absorption capability compared with those of conventional mixed oxide technique are achieved by this means. So do the very large nonlinearity. Moreover, the method is easy to perform and the shape of the attained powders by the chemical

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