The Effect of Bi2O3/SiO2 Molar Ratio and Annealing on the dcDegradation of ZnO Varistors

Lei Meng,‡,§ Guorong Li,‡ Liaoying Zheng,‡,† Lihong Cheng,‡ Jiangtao Zeng,‡ and Hualing Huang‡

‡Key Laboratory of Inorganic Functional Materials and Devices, Shanghai Institute of Ceramics,Chinese Academy of Sciences, Shanghai, 200050, China

§Graduate School, Chinese Academy of Sciences, Beijing, 100039, China

The dc electrical stability of ZnO varistors was effectively

improved by controlling the composition proportion of Bi2O3

and SiO2 as well as annealing condition. The microstructure,current–voltage (I–V) property, and dc degradation character-

istics are significantly affected by the molar ratio of Bi2O3 to

SiO2 in composition. It is found that a phase transition from

b-Bi2O3 (with dissolved Si) to Bi12SiO20, with a volume con-traction of 5.88%, occurred after annealing at 850°C. The

formed Bi12SiO20 plays an important role in improving the elec-

trical stability by inhibiting the oxygen-desorption at the grainboundary.

I. Introduction

BECAUSE of their excellent nonlinear current–voltage (I–V)property, ZnO varistors have been extensively used as

protection devices against transient voltage surges in electri-cal and electronic systems. However, dc degradation of ZnOvaristors severely restricts ZnO varistors’ application in dchigh-voltage power transmission and urban rail transport.When subjected to a constant electric field, ZnO varistorsexhibit a continuous increase of the leakage current and adecrease of the nonlinearity exponent, which finally lead tothermal runaway of the varistor. Although many efforts havebeen devoted on degradation of ZnO varistors,1–4 dc degra-dation is still difficult to overcome due to the severe migra-tion of metastable ions in unidirectional electrical field.1

Regarded as the primary ingredient of ZnO varistors, isBi2O3. Depending on the processing condition, Bi2O3 mayform a-, b-, γ- or d- Bi2O3 phase.5 It has been reported thatthe degradation characteristic of ZnO varistors can beimproved by the b ? γ transition of Bi2O3 phase duringannealing at 600°–700°C.2,6 Several degradation mechanismshave been proposed to explore the relationship betweenthe various Bi2O3 phases and the dc electrical stability of Bi2O3

phase, such as “oxygen desorption”4 and the “stress effect.”7

Known as a useful additive to ZnO varistors, SiO2 exhibitsa positive effect on improving varistor voltage. Shao et al.8

reported that SiO2 addition could promote decrease of the dcdegradation level of the ZnO varistors through affectingBi2O3 phase transition during annealing. However, intensive

study on the correlation between dc electrical stability ofZnO varistors and composition proportion of Bi2O3 andSiO2 as well as annealing condition has not been disclosed.In this article, the effects of the Bi2O3 and SiO2 content, aswell as annealing on the dc degradation of ZnO varistor arestudied. The role of Bi2O3 phase and related mechanism willbe discussed.

II. Experimental Procedure

A commercial formula was adopted to fabricate ZnO varis-tor samples. The raw materials include (93.7–x) mol% ZnO,x mol% Bi2O3 (x = 0.7, 1.4, 2.1), 1.5 mol% SiO2, 1.0 mol%Sb2O3, 1.0 mol% Cr2O3, 0.5 mol% Co2O3, 0.5 mol% MnO2,and the other trace additives. The molar ratio of Bi2O3/SiO2

(R*) was kept as 0.47, 0.93, 1.4, respectively. The startingmaterials were mixed with de-ionized water and milled for8 h. The dried powder was granulated and uniaxially pressedinto disks. The disks were sintered in air at 1180°C for 2 h.The final size of the sample was about 16.8 mm in diameterand 1.0 mm in thickness. One batch of the sintered samplesis annealed in air at 850°C for 2 h and furnace cooled. Thisannealing condition was chosen based on previous study ofthe effect of annealing temperature (700°–950°C) on the dcdegradation of ZnO varistors, where it was demonstratedthat annealing at 850°C for 2 h produced the best dc stabilityof varistor sample. The samples with R*=0.47, 0.93, 1.4 weredesignated as BS1, BS2, and BS3. These samples annealedat 850°C were designated as BS1*, BS2*, and BS3*,respectively.

The crystal phases of samples were studied by X-raydiffraction (Bruker AXS D8 Advance, Frankfurt, Germany).Electron probe micro analyzer (EPMA, JEOL-JXA8100,Tokyo, Japan) was used to observe the microstructure. Theaverage grain size (d) was determined by the linear interceptmethod.9 The density of the sintered samples was measuredby the Archimedes method. Silver paste was applied to thesample surface as electrodes.

The I–V characteristics of the samples were measured byusing a digital electrometer (Advantest R8240, Suzhou,China) with a dc source. The breakdown voltage (V1mA) wasmeasured as the voltage at a current of 1.0 mA and the leak-age current (IL) was measured under 0.75 V1mA (75% of thebreakdown voltage) at room temperature. The nonlinearexponent (a) is defined by the following equation:

a ¼ log I2 � log I1logV2 � logV1

ð1Þ

where V1 and V2 are the voltage corresponding toI1 = 0.1 mA and I2 = 1 mA, respectively.

S. Bernik––contributing editor

Manuscript No. 29343. Received February 19, 2011; approved April 26, 2011.This work was supported by the NSAF of China (No.10876041), the Innovation

Project of Shanghai Municipality (No.07XI-023), the 973 Project of China(No.2009CB623305), and the Innovation Project of Shanghai Institute of Ceramics,Chinese Academy of Sciences (No.Y11ZC1110G).

†Author to whom correspondence should be addressed. e-mail: zhengly@mail.sic.ac.cn

2300

J. Am. Ceram. Soc., 94 [8] 2300–2303 (2011)

DOI: 10.1111/j.1551-2916.2011.04654.x

© 2011 The American Ceramic Society

Journal

The dc accelerated degradation tests were performed byapplying a dc voltage of 85% V1mA at 130 ± 1°C for 14 h tothe samples. The variation of the leakage current with timeunder degradation condition was measured at an interval of1 min. The degradation rate coefficient KT is calculatedaccording to the equation below10:

IL ¼ IL0þ KTt

1=2 ð2Þ

where IL is the leakage current of the sample at time, t andIL0 is the initial leakage current at time, t = 0.

III. Results

(1) MicrostructureFigure1 shows the cross section EPMA micrographs ofvaristor samples with different R* in the back-scatteredmode. The BS1 sample shows four typical phases of ZnOvaristors: ZnO, Zn7Sb2O12, Zn2SiO4, and Bi-rich intergranu-lar phase. The dark Zn2SiO4 scatters sporadically amongZnO grains. With increasing R*, the white Bi-rich intergran-ular phase obviously increases and shows a better wettingto ZnO grains. Furthermore, several independent darkaggregations of ZnO grain surrounded by a layer ofZn2SiO4 phase appear in BS2 and BS3 sample. In eachaggregation, Zn2SiO4 phase shows an interconnected net-work distribution, replacing the blocky distribution in BS1sample. The average ZnO grain size and the density of thesamples are summarized in Table I. All the samplesshow high densification (� 98%). On increasing Bi2O3 addi-tion, the density and ZnO grain size increase somewhat.Annealing has little influence on the density and ZnO grainsize.

(2) Phase AnalysisFour different phases were found in XRD pattern of all BSsamples, as shown in Fig. 2(a), which is in agreement with

the EPMA analysis. But after annealing, a new phaseBi12SiO20 (JCPDS Card No. 37-0485) appears and b-Bi2O3

phase (JCPDS Card No. 27-0050) disappears in all BS* sam-ples. The intensity of diffraction peaks was divided by that of(101) peak of ZnO for reasons of comparison.

(3) Electrical characteristicsThe I–V characteristic parameters (a, V1mA, and IL) of thevaristor samples are listed in Table I. As increasing R*,V1mA of samples apparently decreases, a slightly increases,and IL shows little change. After annealing, V1mA and a geta little lower than the ones before annealing.

Figure3 exhibits the dc degradation curves of differentsamples. The calculated KT is listed in Table I. All of the BSsamples show a rapidly increased leakage current withincreasing degradation time. The BS1 and BS2 present thelowest and highest uptrend of leakage current. However, theBS2* and BS3* samples show a slowly reduced leakage cur-rent, i.e. a minus KT. The BS1* sample shows a sloweruptrend of leakage current than BS samples.

The relative stability of a and V1mA of samples in the dcdegradation test is presented in Fig. 4. From Fig. 4(a), the aof BS samples and BS1* drop greatly after the degradationtest, while BS2*-3* samples show a more stable a than them.An obvious downtrend of V1mA with increasing R* is found,for both BS and BS* samples, in Fig. 4(b).

IV. Discussions

When R*� 1, a network distributed Zn2SiO4 phase, abun-dant Bi-rich intergranular phase, and an enhanced ZnO grainsize are found in the EPMA analysis. These should resultfrom the increased Bi2O3 addition with a fixed SiO2 additionin starting materials. With increasing Bi2O3 addition, moreBi2O3 liquid formed in the sintering period. Such increasedBi2O3 liquid enhanced the mass transportation during sinter-ing. It prompted grain growth of ZnO, formation of Zn2SiO4

network layer, and abundant Bi-rich intergranular phase with

Fig. 1. The back-scattered EPMA micrographs of polished cross-section of (a) BS1, (b) BS2, and (c) BS3 (etched in a 10-M NaOH solution).The phases marked by A, B, C, and D in Fig. 1(a) refers to ZnO, Zn7Sb2O12, Zn2SiO4, and Bi-rich intergranular phase, respectively.

Table I. The Densification, Grain Size, and Electrical Properties of Sintered Varistor Samples

Samples R* ρ(g/cm3) D (%) d(lm) V1mA(V/mm) a IL(lA/cm2) KT

BS1 0.47 5.52 98.4 9.2 367 37.6 1.5 60BS2 0.93 5.57 99.3 10.3 332 43.7 0.7 130BS3 1.4 5.58 99.5 11.7 306 41.3 0.3 100BS1* 0.47 5.54 98.8 9.9 366 38.0 4.4 23.3BS2* 0.93 5.58 99.5 10.4 341 36.7 4.0 �2.25BS3* 1.4 5.59 99.6 11.6 298 34.9 4.8 �2.22

ρ is the sintered density, D is the relative proportion of the theoretical density ZnO (5.61 g/cm3), KT is the degradation rate coefficient.

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a better wetting to ZnO grains. Contrarily, with a low Bi2O3

addition (R*<1), Zn2SiO4 phase was formed as small blocksamong the ZnO grains due to finite mass transportation.

According to the Phase analysis, it is reasonable in thisarticle to conclude that a phase transition from b-Bi2O3 withsome dissolved Si at intergranular region to Bi12SiO20

occurred during the annealing. Bi12SiO20, i.e. 6Bi2O3∙SiO2,refers to a γ-Bi2O3 (bcc) matrix with finite solid solution ofSiO2. Fei et al.

11 reported that in the Bi2O3-SiO2 system oneeutectic reaction Liquid ? d-Bi2O3 + Bi12SiO20 occurred at825°C. Takamori12 also confirmed the formation of d-Bi2O3

and γ-Bi2O3 (Bi12SiO20) phases through cooling of theBi2O3-rich liquid phase (<15 mol% SiO2) at eutectic tempera-ture of 825°C. Here, after sintering b-Bi2O3 was formed withmain existence of SiO2 as Zn2SiO4 at the intergranularregion. Subsequent heating up in annealing melted the b-Bi2O3 phase and caused a reconstruction of intergranularlayer. Finally, Bi12SiO20 was formed at the intergranularregion from molten Bi2O3 by the eutectic reaction at 825°C.But, no d-Bi2O3 was identified in this article. This is probably

because of its low content beyond the detection limit ofXRD measurement.

Such unique microstructure from increased Bi2O3 additiondoes a little harm to I–V properties: V1mA drops obviouslywith increasing R*. This is attributed to the increased ZnOgrains. It is well-known that the V1mA of ZnO varistors wasinversely proportional to the ZnO grain size.13 In addition,annealing at 850°C does not show a significant effect on I–Vcharacteristic of varistor samples.

It is interesting to note that when R*� 1, once annealedat 850°C, the samples show a reduced leakage current, stableV1mA and a in the dc degradation test. It means that in suchcases the dc electrical stability of varistor samples wasremarkably improved. This improvement must be closelyrelated to the newly formed Bi12SiO20 during annealing.Takemura et al.7 pointed out that the b-Bi2O3 to γ-Bi2O3

transition is accompanied by a volume contraction of 3.5%.Shao et al.8 stated that a phase transition from d-Bi2O3 (withdissolved Si) to Bi24Si2O40 took place during annealing witha volume contraction of 6.02%. It was reported that this vol-ume contraction of Bi2O3 phase led to internal stress at inter-granular regions, caused band gap changes at surfaces ofZnO grains and consequently affected the electrical charac-teristics of ZnO varistors. Based on the oxygen-desorptiontheory,4 the degradation of ZnO varistors results from themigration of oxygen vacancies toward grain-boundariesunder external electrical field and the subsequent reaction[as shown by Eq. (3)] with the adsorbed oxygen locating atthe grain interface. The formed oxygen could diffuse outfrom the intergranular layer.

V�O þO0

ad ¼ V�O þ 1=2O2 " ðgÞ ð3Þ

In this article, it could be inferred that a volume contrac-tion of intergranular phase must occur during the phase tran-sition of b-Bi2O3 (with dissolved Si) to Bi12SiO20. In terms ofthe same crystallographic calculation method as the Takem-ura7 and Shao,8 the volume contraction of the b-Bi2O3

(P421c space group, tetragonal, a = 7.742 A, c = 5.631 A)with dissolved Si to Bi12SiO20 (I23 space group, cubic,a = 10.107 A) phase transition is about 5.88%. It is higherthan that of b-Bi2O3 to γ-Bi2O3 transition, implying uniquecontribution of Si participation. Thus, the role of Bi12SiO20

phase in improving the dc electrical stability of ZnO varistorsis quite possibly as follows: Due to the great volume contrac-

(a)

(b)

Fig. 2. XRD patterns of sintered samples after treatment (a) before(b) annealing at 850°C for 2 h.

Fig. 3. The dc degradation I–t curves of samples (with different R*)annealed (a) before and at (b) 850°C for 2 h.

(a)

(b)

Fig. 4. The variation of (a) a and (b) V1mA before and after dcdegradation test for the varistor samples with different R*.

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tion, the tensile-stressed Bi12SiO20 has smaller interstitialspace and more compact structure than the b-Bi2O3. Accord-ingly, it is more difficult for oxygen to migrate in Bi12SiO20

than in b-Bi2O3. The Bi12SiO20 phase at the intergranularlayer effectively prevents the oxygen from diffusing out andindirectly inhibits oxygen-desorption [Eq. (3)].

The improved level of dc electrical stability is also associ-ated with the R*. It was found that only BS2* and BS3*samples, i.e. samples with R*� 1, show an improved dc sta-bility. This should be related to microstructure change. WhenR*� 1, a network distributed Zn2SiO4 replaced its blockydistribution (R*<1) in microstructure. Therefore, the possibil-ity of Bi12SiO20 formation increases greatly due to morechance of participation of dissolved Si from Zn2SiO4 networkin the Bi2O3 phase transition. Consequently, more abundantBi12SiO20 will be formed after annealing at 850°C, whichleads to more effective inhibiting function for chemical reac-tion of Eq. (3).

V. Conclusions

A network distributed Zn2SiO4 phase, abundant whiteBi-rich intergranular phase and an enhanced ZnO grain sizeare formed in the samples with increased R* in the raw mate-rials. A high R* would have a detrimental effect on V1mA

due to the increased ZnO grain size. A new Bi12SiO20 phasewas formed from b-Bi2O3 (with dissolved Si) after annealingat 850°C with a volume contraction of 5.88%. For the sam-ple with R*� 1, abundant Bi12SiO20 phase at intergranularlayer effectively inhibits the oxygen-desorption at grainboundary and consequently forms a stable electrical barrier.Such a process effectively improves the dc degradation ofvaristor samples. Overall, considering the negative effect on

I–V characteristics, R*�1 and annealing at 850°C for 2 hwould become one promising process to improve the dc elec-trical stability of ZnO varistors.

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