Preparation of ZnO varistors by solution nano-coating technique

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Abstract

This paper introduces a new method to produce nano-composite powder for the preparation of high performance ZnO varistors. ZnO particles were coated with Bi2O3, Sb2O3, Co2O3, Cr2O3 and other additives via liquid nano-coating technique. Then the prepared powder was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal gravity and differential scanning calorimetry (TG-DSC) and particle size distribution. The results showed that the ZnO composite powder is homogeneously coated and ultrafine. The densification, phase composition and microstructure of ZnO varistors was studied by linear shrinkage, X-ray diffraction (XRD) and SEM, respectively. The preliminary electrical parameters of ZnO varistors showed that the breakdown voltage Vb (1 mA/cm2) and nonlinear coefficient α is 5400 V/cm and 50, respectively, much higher than that of conventional ball milling method. This result shows that nano-coating is a promising route to prepare ZnO varistors.

Introduction

ZnO varistors are semiconducting ceramics made from ZnO and other metal oxides such as Bi2O3, Sb2O3, Co2O3, Cr2O3 and SiO2, etc. Due to their excellent nonlinear coefficient and low leakage current, they have been used in electrical and electronic systems as surge protection devices for many years. Homogeneous powder is necessary for the manufacture of high performance ZnO varistors, since the properties of ceramics are significantly influenced by the characteristics of preliminary powder. However, there are several drawbacks in conventional methods which were used to prepare ZnO nano-composite powder. Homogeneous composite powders can hardly be obtained by conventional ball milling method, so several chemical methods have been developed in the past, such as coprecipitation [1], [2], evaporative decomposition of solutions (EDS) [3], [4], sol–gel [5], [6], microemulsion [7], microwave technique [8] and polymerized complex method [9]. As for coprecipitation, the proportion of deposition may be changed with the pH value of the solution. During the process of evaporative decomposition of solutions, hollow shells and hollow aggregates often occur [3]. Sol–gel method is also affected by pH value greatly. Micro-emulsion and microwave methods are not suitable for manufacture in industry. The procedure of polymerized complex method is complicated and costly.

This work aims at exploring a technique suitable for industrial production. The technique is easy to control and the resulting powder is ultrafine. The property of the uniformly doped ZnO powder was studied with SEM, TEM and TG-DSC; the microstructure and electrical parameters (such as IV curve) of ZnO varistors were investigated.

Section snippets

Experimental procedure

The starting materials for the production of powder include reagent grade ZnO 96 mol%, reagent grade additives (including the salts of Bi, Sb, Co, Cr, Mn and H2SiO3) is 4 mol%. The nano-coating process was carried out following the flowchart as shown in Fig. 1. First of all, the additives were dissolved in alcohol (including ethylene glycol and ethanol, the volume ratio is 5:3) and the solution was stirred by a stirrer (the Sb2O3 was dissolved in melted citric acid [9] and then added to the

Results and discussion

The morphology of ZnO powder was observed by SEM and TEM as shown in Fig. 2, Fig. 3, respectively. Fig. 2A shows the SEM image of ZnO powder before nano-coating. As we can see, the ZnO particle size ranges from 100 to 300 nm. The surface of the ZnO particles is smooth. Contrary to Fig. 2A, Fig. 2B shows the SEM morphology after nano-coating. The larger grains are ZnO grains and the size of them keeps nearly unchanged. The surface of ZnO grains is rough because they were coated by additives. The

Conclusions

The liquid nano-coating technique for preparing ZnO varistors was investigated. This technique is a novel method to fabricate nano-composite powder for high performance ceramics. The properties of preliminary ZnO composite powder was studied by SEM, TEM, TG-DSC and particle size analysis and proved that the powder is excellent. The microstructure and densification of resulting varistors were identified by SEM, XRD and linear shrinkage. The threshold voltage of the varistors prepared by

Acknowledgement

This work was supported by plan of the Shanghai Municipal Science and Technology Commission, 04DZ11603.

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