Materials Letters 65 (2011) 259 5 – 2597

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Materials Letters

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AC ageing characteristics of Y2O3-doped ZnO varistors with high voltage gradientJinliang He ⁎, Jun Liu, Jun Hu, Wangcheng LongState Key Laboratory of Power Systems, Department of Electrical Engineering, Tsinghua University, Beijing 100084, China

a r t i c l e i n f o

Article history:Received 3 May 2011Accepted 4 June 2011

a b s t r a c t

In this paper, AC ageing characteristics of high voltage gradient ZnO varistors doped with different Y2O3

concentration were investigated. The voltage gradient of these samples is markedly improved by increasing Y2O3 content, however, the nonlinear coefficient decreases and the leakage current increases at the same time.

Available online 13 June 2011 Y2O3-doped ZnO samples exhibit lower stability under accelerated AC ageing stress of 0.85 V

1mA/135 °C/168 h,

Keywords: ZnO VaristorsElectrical propertiesDefects

compared with samples without Y2O3. Double-Schottky barrier(DSB) parameters before and after ageing tests indicate that the decrease of barrier height for traditional sample is less than that for high voltage gradient sample, which should be ascribed to its slight variation in the interface state density.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

ZnO varistor behaves high nonlinear current–voltage characteris- tic and has been extensively used in metal oxide arresters (MOA) to protect electrical device against overvoltage surges in power systems [1–4]. However, ZnO varistor in-use is usually subjected to a long- term AC or DC voltage stress which may lead to the increase of leakage current. 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]. In general, the voltage gradient of ZnO varistor is proportional to the number of ZnO grains in series per unit thickness. As a result, high voltage gradient ZnO varistor could be obtained by decreasing the grain sizes. Recently, it is reported that several rare earth oxides, such as Y2O3 and Dy2O3, could be considered as ZnO grain growth inhibitors during the sintering process. Although the microstructures and compositional phases of ZnO varistors doped with Y2O3 have been proposed inseveral literatures [11–13], investigation on the influence of Y2O3 onthe AC ageing characteristics of ZnO varistors has not been carried out yet. Since high voltage gradient ZnO varistors are subjected to higher operation voltage and overvoltage surge stress, the ageing character- istics should be paid more attention to.

In present work, ZnO varistor samples doped with different content of Y2O3 were prepared and degraded under accelerated AC ageing stress

⁎ Corresponding author. Tel.: + 86 10 62775585; fax: + 86 10 62784709.E-mail address: hejl@tsinghua.edu.cn (J.L. He).

of 0.85 V1mA/135 °C/168 h. The electrical properties and double- Schottky barrier (DSB) parameters before and after the ageing tests were measured and compared. And then, the AC ageing mechanism was discussed based on the experimental results, as well.

2. Experimental procedure

The compositions of ZnO varistors were (95.05 − x) mol% ZnO,0.70 mol% Bi2O3, 0.50 mol% MnO2, 1.00 mol% Co2O3, 0.50 mol% Cr2O3,1.00 mol% Sb2O3, 1.25 mol% SiO2 and x mol% Y2O3, where x = 0, 0.50,0.75 and 1.00, respectively. These analytical-grade raw reagents were homogenously mixed, dried and pressed into round discs. And then, the pellets were sintered at 1200 °C for 3 h. Finally, both sides of the samples were painted with silver paste. The obtained samples were labeled as ZY000, ZY050, ZY075 and ZY100, respectively. The surface microstruc- ture graphs of obtained samples were obtained by SEM (JSM-6460LV, JEOL Inc., USA).

Accelerated AC (50 Hz) ageing tests were carried out under continuous voltage stress of 0.85 V1mA at 135 °C for 168 h. The current density-electric field (J–E) characteristics were measured by a digitalsource meter (Model 2410, Keithley Instruments Inc., USA). The voltage gradient E1mA is defined as the electric field where the current is 1 mA, the nonlinear coefficient α is calculated by equation α = 1/log (E1mA/E0.1mA) and the leakage current IL is the current corresponding to 0.75 E1mA. And then, the ageing rate coefficient KT was calculated by equation IL = IL0 + KTt

1/2, where IL0 and IL is the leakage current before and after stress. The capacitance–voltage (C–V) characteristics were measured by a dielectric spectrometer (Concept 80, NovocontrolTechnologies GmbH & Co. KG., Germany). The DSB parameters, including donor density Nd, interface state density Ni and barrierheight ϕb, were determined by linearly fitting (1/C − 1/2C0)

2 − Ugb

curves [14].

0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.06.022

2596 J.L. He et al. / Materials Letters 65 (2011) 2595–2597

Fig. 1. The surface microstructure graphs of ZnO varistors with high voltage gradient.

3. Results and discussions

SEM micrographs of these samples are illustrated in Fig. 1. The mean grain size of these samples are measured by linear interception method. With the Y2O3-doping concentration increasing, samples’ grain size is 10.13 μm, 7.22 μm, 6.85 μm and 6.03 μm, respectively, which indicates the grain growth inhibition with the introduction of Y2O3.

Fig. 2 shows the J–E characteristic curves of ZnO varistor samples

before and after the AC ageing test. Table 1 lists the electrical properties of these samples before and after AC ageing test derived from Fig. 2. E1mA0, α0 and IL0 represent their initial values before ageing, while E1mA, α and IL represent their final values after ageing.

(a)

It is noticed in Table 1 that the voltage gradient and the nonlinear coefficient of these samples change slightly after the AC degradation test. Sample ZY100 exhibits most obvious variation in E1mA. The leakage currents of these samples increase markedly, especially for sample ZY050, whose leakage current increases more than 5 times compared with its initial value. The lowest KT for high voltage gradientsamples, 0.081 μA ⋅ h− 1/2, is obtained by doping 0.75 mol% Y2O3,which is higher than that of conventional sample ZY000.

The capacitance–voltage relationships of the varistor samples before and after AC ageing test are represented as (1/C − 1/2C0)

2 − Ugb curvesin Fig. 3, where Ugb is the applied voltage per grain boundary. The DSB parameters are derived by linearly fitting these curves and listed in Table 1.

After the AC ageing test, Nd and Ni for most samples decrease compared with their initial values, while sample ZY000's Nd is the exception. According to the double-Schottky barrier model, the barrier height is expressed by the following equation:

e2 N2

ϕb = i

2ε0 εr Ndð1Þ

(b)

where e is the electron charge, ε0 is the permittivity of vacuum, εr is the relative permittivity of ZnO. It is clear that the decrease of Ni or the increase of Nd will lead to the decrease of ϕb. According to Eq. (1) and the results listed in Table 1, the barrier height largely depends on Ni. There are obvious decrease in ϕb for all of these ZnO samples, which leads to the increase of leakage current. Meanwhile, the variation of ϕb for traditional ZnO varistor is less than that for high voltage gradient samples.

It is known that ZnO is non-stoichiometric, which contains numerous intrinsic point defects, such as zinc interstitials and oxygen vacancies. During the sintering process, those defects along with doped extrinsic impurities diffused and distributed along the grain boundary. With the introduction of Y2O3, there will be a substitutional defect reaction on ZnO grain surface, as given below:

ZnO • ″ X 1Fig. 2. The J–E characteristics of ZnO varistors before and after AC ageing test.

Y2 O3 → 2YZn + VZn + 2OO + 2

O2 : ð2Þ

Sample ZY000 ZY050 ZY075 ZY100

E1mA0 (V/mm) 315.6 402.2 505.5 737.4E1mA (V/mm) 316.9 400.5 505.0 719.8α0 35.1 29.8 21.6 15.4α 34.8 29.1 20.6 15.3IL0(μA) 0.14 0.54 2.67 9.51IL(μA) 0.35 3.36 3.72 11.1KT(μA ⋅ h− 1/2) 0.016 0.218 0.081 0.120Nd0 (1017cm- 3) 1.88 0.44 0.46 0.34Nd (1017cm- 3) 2.13 0.33 0.42 0.30Ni0(1012cm- 2) 1.49 0.74 0.68 0.56Ni(1012cm-2) 1.48 0.45 0.58 0.48ϕb0 (eV) 1.25 1.30 1.09 0.99ϕb (eV) 1.10 0.66 0.87 0.81

Z

J.L. He et al. / Materials Letters 65 (2011) 2595–25972597

Table 1Comparison on electrical properties and DSB parameters of ZnO varistors before and after AC ageing test.

(a)

(b)

Y• might play a role of donor at the depletion layer. However, fewer Y3+ ions could enter into ZnO lattice, since the ion radius of Y3+

ions (0.093 nm) is larger than that of Zn2+ ions (0.074 nm). It means that the reaction must overcome large lattice stress. As a result, the improvement of donor density is limited. Zinc interstitial in ZnO lattice could diffuse to the grain boundary for its low migration activation energy. The diffused zinc atoms could react with dopants and form new phase as observed in Ref. [11,15], which might give rise to the reduction of interface states. This process could be considered as the main reason that leads to the reduction of donor density and surface sates density, as shown in Table 1. Under the AC voltage ageing stress, zinc interstitials could migrate towards the grain boundary and react with the negatively charged ions at the boundary surface, expressed as follows:

Fig. 3. The (1/C − 1/2C0)2 − Ugb characteristics of ZnO varistors before and after AC

ageing test.

reduction of Ni after ageing. Meanwhile, the variation of ϕb for ZnOsample doped without Y2O3 is much less than those doped with Y2O3.

Acknowledgements

This work was supported by the National Natural Science Foundations

Zn• + V ′ = Znx + V x : ð3Þi Zn i Zn of China under Grants 50425721 and 50737001.

As a result of the formation of neutral defects at the interface, the barrier height of the grain boundary is reduced. Although the initial donor density and interface states density for Y2O3-doped samples are less than those of un-doped sample, more negatively charged ions are neutralized at the interface. This deduction could be supported by the decrement of interface states density after ageing test, which is much higher for high voltage gradient samples than that of traditional sample.

4. Conclusions

ZnO varistors with high voltage gradient were prepared by doping with Y2O3 and aged by accelerated AC voltage stress. The results indicate that the ageing rates of high voltage gradient ZnO varistors are higher than that of traditional varistor. The lowest ageing rate for high voltage gradient varistor sample is obtained when doped with 0.75 mol% Y2O3. The decrease of barrier height ϕb

should be mostly attributed to the

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