Microstructural and electrical characteristics of Y2O3-doped ZnO–Bi2O3-based varistor ceramics

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Abstract

The microstructural and electrical characteristics of ZnO–Bi2O3-based varistor ceramics doped with Y2O3 in the range from 0 to 0.9 mol% have been investigated. The addition of Y2O3 resulted in the formation of a fine-grained Bi–Zn–Sb–Y–O phase along the grain boundaries of the ZnO grains which inhibits the grain growth. The mean ZnO grain size decreased from 11.3 to 5.4 μm with increasing amounts of Y2O3. The threshold voltage (VT) of the ceramics increased from 150 to 274 V/mm, the non-linear coefficient α was not influenced and remained at approximately 40, and the leakage current also increased with the amount of Y2O3 added. On the basis of the Mukae et al. (Mukae, K., Tsuda, K. and Nagasawa, I., Capacitance-vs-voltage characteristics of ZnO varistors. J. Appl. Phys., 1979, 50, 4475–4476) Schottky barrier model of ZnO varistors, the addition of Y2O3 resulted in a slight increase in the density of interface states (NS) and a more pronounced increase in the donor density (ND), causing a decrease of the barrier height (ΦB) and the depletion layer width (t). The increase of the leakage current (IL) with higher amounts of Y2O3 added can be ascribed to the increase in donor density (ND) as well as to the increased amount of Y2O3-containing phase at the grain boundaries of ZnO.

Introduction

ZnO-based varistors are characterised by highly non-linear current-voltage characteristics and a high energy-absorption capability. As a result they are widely used as surge absorbers in electronic circuits, devices and electrical power systems to protect against dangerous over-voltage surges. In the classical ZnO-based varistor, Bi2O3 is used as the varistor-former, while other oxides such as Sb2O3, Co3O4, Mn3O4, NiO and others are added in small amounts to further enhance the non-linearity of the varistor's behaviour. The non-linear current-voltage characteristics of ZnO varistor ceramics results from the formation of double Schottky barriers at the grain boundaries. These non-ohmic ZnO–ZnO grain boundaries each have a break-down voltage of 3V and so the overall break-down voltage of the varistor builds up from the non-ohmic grain boundaries between the electrodes of the varistor and can be controlled either by the varistor thickness or the ZnO grain size.1 High-voltage varistor ceramics require a fine-grained microstructure and Sb2O3 is usually added to inhibit the ZnO grain growth.2, 3 Recently it has been reported that the breakdown voltage and energy characteristics of varistor elements can be significantly increased by the introduction of various rare-earth oxides (REO) and Y2O3 to the varistor ceramics.4

In the present work, the influence of the amount of added Y2O3 on the microstructure, current–voltage (I–V) and capacitance–voltage (C–V) characteristics of ZnO–Bi2O3-based varistor ceramics has been investigated.

Section snippets

Experimental

ZnO–Bi2O3-based varistor samples with the nominal composition (96.2–x) mol% ZnO+0.9 mol% Bi2O3+2.9 mol% (Sb2O3+Co3O4+Mn3O4+NiO+Cr2O3)+xY2O3 for x=0, 0.1, 0.3, 0.45 and 0.9 (sample labeled Y0, Y1, Y3, Y5 and Y9, respectively) were prepared by the classical ceramic procedure. Reagent-grade oxides were mixed in proper ratios and homogenized in absolute ethanol using a planetary mill. The powders were dried at 70°C and pressed with 200 MPa into discs 10 mm in diameter and 2 mm thick. The pellets

Results and discussion

XRD patterns of the investigated samples are given in Fig. 1. In the sample without Y2O3, three phases were identified: the ZnO phase, γ-Bi2O3 phase and the Zn7Sb2O12-type spinel phase. However, in samples doped with Y2O3, additional peaks are evident and their intensity increases with increasing amounts of Y2O3 in the starting composition. Fig. 2 shows microstructures of the investigated samples. As can be seen from these back-scattered SEM micrographs, the three phases already identified by

Conclusions

Doping of ZnO–Bi2O3-based varistor ceramics with Y2O3 results in the formation of a Bi–Zn–Sb–Y–O phase with a cation ratio close to 0.4:1:1:1. Y2O3 does not enter the ZnO grains and was not detected in either the Bi2O3-rich phase or the Zn7Sb2O12-type spinel phase. In samples with higher amounts of Y2O3 in the starting composition and hence a higher amount of Y2O3-containing phase which bounds the Bi2O3, the onset of sintering is shifted to a higher temperature. The fine-grained Y2O3-containing

Acknowledgements

The work has been carried out as part of the PROTEUS project for Slovene–French scientific and technical collaboration. The financial support of the Ministry of Science and Technology of Slovenia and EDIGE is gratefully acknowledged.

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