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��������� WO3-based capacitor-varistor ceramics doped with Er2O3 were prepared and the

microstructures and nonlinear electrical properties were investigated. The results show that there exist

second phase Er10W2O21 on the surface of WO3 grains. Doping Er2O3 in WO3 ceramic can inhibit the

grain growth. A small quantity of Er2O3 can significantly improve nonlinear properties of the

samples. The permittivity of doped samples was higher than that of the undoped, and the high

permittivity makes Er2O3-doped WO3 ceramics be applicable as a kind of capacitor-varistor

materials.

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Varistors exhibit highly nonlinear voltage-current characteristics and are used as surge protection

devices in power systems and in electronic circuits [1]. Up to now, ZnO-based varistors have been

extensively studied. However, the low permittivity of ZnO varistor weakens its ability to absorb the

sparks, which restricts its application in the low voltage circuit [2]. Actually, the recent trend in

electrical appliance design requires varistors that contain more functions and have a relatively low

breakdown voltage [3].

Tungsten trioxide has received much attention because of its potential for many technological

applications, such as electrochromic devices and gas sensors [4, 5]. In 1994, Makarov and Trontelj [6]

first reported that Na2CO3 and MnO2-doped WO3 ceramics have varistor behavior with a nonlinear

coefficient of ɑ =7 and a relatively low breakdown voltage of 6-10 V mm-1

. In addition, the

permittivity of WO3 is high [7]. Both imply that WO3 is a good candidate for capacitor-varistor with

the low breakdown voltage.

Rare earth oxides used as a dopant have been applied for fabricating varistors such as ZnO-Pr6O11

system [8]. Nahm reported the addition of Er2O3 to ZnO-Pr6O11-Co- based varistor greatly improved

the nonlinear properties [9]. Recently, Yang et al [10]reported the breakdown voltage of the WO3

ceramic containing 2 mol% CeO2 is 3.1V/mm, which is very low for varistor materials indicating that

WO3 is more suitable for low voltage varistor. Therefore, it is possible to improve the electrical

properties of sintered WO3 ceramics by doping a suitable rare earth oxide [11-13]. In this paper, the

effect of Er2O3 on the microstructure and electrical properties of WO3 ceramics were investigated for

the first time.

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The raw materials were WO3 (analytical grades, 99.0 wt. %) and Er2O3 (analytical grades, 99.99

wt.%). The compositions were (100 - x) mol% WO3 + x mol% Er2O3, where x = 0.1, 0.3, 0.5, 1.0 and

2.0. The composite powder was obtained by a conventional mixing method which used an agate ball

mill. The milled powder was pressed into pellets of 12 mm in diameter and 2 mm in thickness at the

Advanced Materials Research Vols. 233-235 (2011) pp 2503-2506Online available since 2011/May/12 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.233-235.2503

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 134.148.10.13, University of Newcastle, Callaghan, Australia-18/08/13,13:40:56)

pressure of 40 MPa, which were sintered in air for 2 h at the temperature of 1150 ℃. Then they were

furnace- cooled to room temperature.

The microstructure was examined by scanning electron microscope (SEM, JSM-5610LV, Japan).

The grain sizes were calculated by microstructure linear analysis. For electrical properties

measurement, silver electrodes were made on both sides of samples. The permittivities of the samples

were investigated in the frequency of 1 KHz by using an Impedance Analyzer (HP4294A USA). The

current-voltage characteristics were measured by using a stabilized DC power supply (RXN-605D

China) and a varistor parametric tester (CJ1001 China). The breakdown voltage (Eb) is the field

density when current density reaches as high as 1 mA/cm2. The nonlinear coefficient α was obtained

from the equation (1)[14] :

( )( )12

12

VVlog

IIlog=α (1)

Where 2� is the voltage value corresponding to 2� = 10 mA/cm2, and 1� to 1� = 1 mA/cm

2,

respectively.

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Fig. 1 shows the X-ray diffraction pattern for the pure WO3 ceramics and the doped samples. There

are only monoclinic and triclinic phases of WO3 in the doped samples, if the dopant concentration is

not more than 0.1 mol%. A few additional peaks corresponding to Er10W2O21 emerge for the sample

containing 1% and 2% Er2O3. The ratio of the second phase increases with the dopant concentration.�

�Fig. 1. XRD patterns of different samples

The SEM micrographs of different samples are shown in Fig. 2. It is clearly shown that the grain

size decreases with the increase of the concentration of Er2O3, meaning the dopant could prevent

grains from growing. But the shape of the grains is almost the same.

Fig. 2. SEM micrographs of WO3 ceramics doped with different quantity of Er2O3

Fig. 3 presents the current-voltage characteristics of the WO3-Er2O3 ceramics. All the samples

show different nonlinear electrical properties. The Eb and α curves for samples doped with different

concentrations of Er2O3 are given in Fig. 4. Breakdown voltage Eb of doped samples decreases with

2504 Fundamental of Chemical Engineering

increase in concentration of Er2O3 up to 0.5 mol%, but further addition of Er2O3 increases breakdown

voltage. The value of Eb of the sample doped with 0.5 mol% Er2O3 is only 3.5 V/mm, which is very

low for low-voltage varistor materials. As a dopant, Er2O3 also influences the non-linear coefficient of

WO3 varistors in some extent. The sample doped 0.3 mol% Er2O3 has the highest non-linear

coefficient (α = 3.2), which is higher than the undoped sample.

Fig. 3. Current-Voltage characteristics of WO3 ceramics doped with different amounts of Er2O3.

Fig. 4. Eb and α vs. the content of Er2O3 dopant.

Fig. 5. shows the relative dielectric constant (εr) with different concentrations of Er2O3. The other

obvious effect is that the permittivity of doped samples is higher than that of pure WO3. The

permittivity was found to increase with increase with increase in concentration of Er2O3, reach a

maximum at 1.0 mol%, and decrease with further increase in concentration of Er2O3. It is shown that

the 0.5 mol% Er2O3 specimen possesses an ultrahigh electrical permittivity of 4.65× 104. The high

permittivity of the ceramic comes form the fact that the resistivity of WO3 grains is much lower than

that of the grain boundary layers [15]. High permittivity makes it more suitable as a capacitor-varistor

material.

Fig. 5. Effective relative dielectric constant vs. the content of Er2O3 dopant.

Advanced Materials Research Vols. 233-235 2505

���� �����

A low-voltage WO3 capacitor-varistor ceramics doped with Er2O3 was systematically researched. The

XRD analysis of sintered tungsten oxide doped with not more than 0.1 mol% Er2O3 revealed three

peaks of strongest intensity corresponding to the triclinic phase, and some peaks belonging to

monoclinic phase. There is a second phase Er10W2O21 exist in the samples containing 1% and 2%

Er2O3. The dopant Er2O3 plays an important role on the characteristics and dielectric properties of the

ceramics. Breakdown voltage of the sample containing 0.5 mol% Er2O3 is 3.5 V/mm, which is very

low for varistor materials indicating that WO3 is more suitable for low voltage varistor. The value of

the nonlinear coefficients of α is in the range of 2.0-4.0. It was found that the permittivity of the

samples doped with Er2O3 was larger than that of pure WO3.

���������������

This work was financially supported by Dr start-up fund research of Hubei University of Automotive

Technology, China (BK200926).

����������

[1] T.K. Gupta: J. Am. Ceram. Soc. Vol. 73 (1990), p. 1817

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[3] C.P. Li, J.F. Wang, W.B. Su, H.C. Chen and Y.J. Wang: Mater. Lett. Vol. 57 (2003), p. 1400

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[5] M. Penza, M. A. Taglienta and L. Mirenghi: Sens. Actuators B. Vol. 50 (1998), p. 9

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[9] C. W. Nahm: Solid State Commun. Vol. 126 (2003), p. 281

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[11] Y. Wang, X.S. Yang, Z.L. Liu and K.L. Yao: Mater. Lett. Vol. 58 (2004), p. 1017

[12] X.S. Yang, Y. Wang, L. Dong: Mater. Chem. Phys. Vol. 98 (2006), p. 225

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2506 Fundamental of Chemical Engineering

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