Materials Research Bulletin 68 (2015) 271–275

Short communication

Structural and electrical properties of Bi2WO6 piezoceramics preparedby solid state reaction method

Tao Zeng a, Xiaoting Yu a, Shipeng Hui b, Zhiyong Zhou b,*, Xianlin Dong b

a Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power, Shanghai University of Electric Power, 2588 Changyang Road,Shanghai 200090, PR ChinabKey Laboratory of Inorganic Functional Materials and Devices, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai200050, PR China

A R T I C L E I N F O

Article history:Received 8 November 2014Received in revised form 29 March 2015Accepted 2 April 2015Available online 4 April 2015

Keywords:CeramicsDefectsFerroelectricityPiezoelectricity

A B S T R A C T

Stoichiometric Bi2WO6 (BWO) piezoelectric ceramic, with round-shaped grains and a relative density of�96%, was prepared by solid state reaction method. The dielectric, resistivity, ferroelectric andpiezoelectric properties of BWO were investigated. The dielectric constant and loss show strongfrequency dependence with increasing temperature in BWO, which can be attributed to the contributionfrom the defect dipoles. These defect dipoles have significant effects on the electrical resistivity andpolarization–electric (P–E) hysteresis loops of unpoled and poled BWO, and also thermal depolingbehavior of poled BWO.

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1. Introduction

Bismuth layer-structured ferroelectrics (BLSFs) are importantmembers of the Aurivillius phase, lead-free family of com-pounds. Some BLSFs with high Curie temperature (Tc) arepotential candidates for high-temperature piezoelectric applica-tions [1–3]. BLSFs have also attracted much attention forapplications in ferroelectric random access memories (FeRAMs)due to their excellent fatigue endurance properties in SrBi2Ta2O9

and Bi3.25La0.75Ti4O12 thin films [4,5]. BLSFs have the generalformula of (Bi2O2) (Am�1BmO3m�1), where m is an integerusually lying in the range of 1–5, built up by m(ABO3)2� layersthat alternates with (Bi2O2)2+ layers; A is a mono-, di- ortrivalent element (or a combination of them) allowing dodeca-hedral coordination (e.g., Na+, Sr2+, Ca2+, Bi3+, Ce3+), B is atransition element suited to octahedral coordination (e.g., Fe3+,Ti4+, Nb5+, Ta5+, W6+).

Bi2WO6 is the simplest member (m = 1, abbreviated as BWO) ofthe BLSFs family, as are Pr2WO6, Bi2MoO6, Bi2NbO5F, Bi2TiO4F2 [6].Recently, Bi2WO6 has been reported to undergo the followingsequence of phase transitions [7]:

* Corresponding author. Tel.: +86 21 69906095.E-mail addresses: zengtao@shiep.edu.cn (T. Zeng), zyzhou@mail.sic.ac.cn

(Z. Zhou).

http://dx.doi.org/10.1016/j.materresbull.2015.04.0040025-5408/ã 2015 Published by Elsevier Ltd.

g ðpolar; orthorhombicÞ ������!640�660 �Cg 000ðpolar; orthorhombic ���!�930�C

g00ðnonpolar; orthorhombicÞ ���!�960 �Cg 0ðnonopolar; monoclinicÞ

The g ! g 000 and g 000 ! g 00 phase transitions are ferroelectric, andg 00 ! g 0 is a high-temperature reconstructive transition. At roomtemperature, Bi2WO6 has the orthorhombic structure (g phase)with space group P21ab [8–10], and not B2cb in earlier work [6],with unit cell parameters a = 5.4559(4) Å, b = 5.4360(4) Å, andc = 16.4298(17) Å [9]. At higher temperature, the complex phasetransition of Bi2WO6 has been intensively studied [8,10–14]. Thefirst two phase transitions are in good agreement with ourexperimental results of spark plasma sintered (SPS) BWO ceramics,which showed clearly the g ! g 000 ferroelectric phase transitionaround 665 � 5 �C and the g 000 ! g 00 ferroelectric–paraelectricphase transition around 937 � 5 �C [15].

BWO has high ionic conductivity owing to its fast oxygen iontransport [16]. Therefore, it has been studied for catalyticapplication [17] and as negative electrode for super capacitorapplication [18]. BWO has also been considered as a hightemperature piezoelectric material because of its high Curietemperature and high piezoelectric coefficients (d33 �40 pC/N)[15,19,20]. Up to now, almost all the investigations related with thecrystal structure and phase transitions mentioned above werebased on BWO single crystals, and electrical properties studiedwere of BWO single crystals or textured ceramics by tape casting[21] and SPS [15,22] methods.

Fig. 1. XRD patterns of as-prepared BWO ceramics. Inset: the density of BWOceramics sintered at different temperatures.

272 T. Zeng et al. / Materials Research Bulletin 68 (2015) 271–275

From the compositional point of view, BWO contains equalmolar ratio of Bi2O3 and WO3, and the weight fraction of Bi2O3 isvery high up to 66.78%. The melting point of Bi2O3 (815.8 �C) ismuch lower than that of WO3 (1470 �C) [23]. In general, theoptimum sintering temperature for BWO ceramics is over 850 �C.Due to the large difference of melting points of the two oxides, it issupposed that liquid and solid phases coexist and solid WO3

particles are wetted by liquid phase of Bi2O3 generated during thesintering. Liquid bridge can be formed between solid particlesbecause of capillary pressure, accompanying with the generationof voids because of the migration of low-melting-point phase [24],which may result in composition segregation. Compared withpressureless sintering, press sintering is beneficial to speed upmass transfer and particle rearrangement, and hence improves thedensity and uniform composition distribution in case of coexistingof liquid and solid phases. This can explain we achieved easilydense BWO ceramics by SPS in our previous study [15].

In this work, we explored the approach of how to get denseBWO ceramics by conventional solid state reaction method, andthen investigated the structural and electrical properties system-atically.

2. Experimental procedure

BWO ceramic samples were prepared by conventional solidstate reaction method using powders of Bi2O3 (99.78% purity) andWO3 (99.00% purity). The raw materials were ball-milled withagate balls and water-free ethanol for 24 h. After drying, thestoichiometric BWO was obtained using two-step synthesismethod. First, the mixed powder crushed and calcined at 600 �Cfor 4 h followed by recrushing, then ball milling and recalcinationat 700 �C for 4 h. BWO powders were then pressed into disc-shapedsamples with a diameter of 15 mm and a thickness of about 1.5 mm.The samples were heated to 750 �C to remove the binder and thenplaced in a sealed Al2O3 crucible and finally sintered from 850 �C to900 �C for 2 h at an interval of 10 �C in air. The ramp-down speed isset as 1 �C/min instead of natural cooling to avoid glass–ceramicsformation because of fast cooling. The density of BWO ceramicswere measured with Archimedes method. Electrodes for dielectric,electrical conductivity, and piezoelectric measurements weremade of fired-on platinum paste.

X-ray diffraction (XRD) patterns of the sintered ceramicpowders were performed with a diffractometer (D/max 2550 V,Rigaku, Japan) using CuKa radiation with a step scan of 0.02� from20�–80� 2u. The morphology and composition of as-prepared BWOceramics were characterized by field-emission scanning electronmicroscopy (FESEM, JEOL, Tokyo, Japan) equipped with an energy-dispersive X-ray spectroscopy (EDS). The dielectric properties werecharacterized by an LCR meter (Model HP4284A) at 1 MHz.Samples for piezoelectric measurement were poled in a siliconeoil bath at 80 �C by applying a DC electric field of 12 kV/mm for20 min. The piezoelectric constants, d33, were measured using a d33meter (Model ZJ-3, Institute of Acoustics, Chinese Academy ofScience). Thermal depoling experiments were conducted byholding the poled samples for 4 h at various higher temperatures,cooling to room temperature, measuring d33, and repeating theprocedure up to the temperature above Tc. Temperature depen-dence of the electrical resistivity was measured over thetemperature range from 100 �C to 550 �C with a high-resistancemeter (Model HP4339B). The two-terminal method was used inmeasuring the resistances.

3. Results and discussions

XRD patterns of as-prepared BWO ceramic are shown in Fig. 1.All the diffraction peaks can be clearly indexed as an orthorhombic

phase of BWO and match very well with the standard data (JCPDFcard No. 01-073-1126, a = 5.4570 Å, b = 5.4360 Å, and c = 16.4270 Å),except for the little peak at 2u of 27.2� which might belong to Bi2O3.The strongest diffraction peaks for BWO ceramics are (113), whichis consistent with the (112m + 1) highest diffraction peak inAurivillius phases [25]. The inset of Fig. 1 is the density of BWOceramics sintered at different temperature. As it can be seen thatall the samples have a relative high density over 94% and up to amaximum of 96% with sintering temperature of 880 �C whichwould be the optimum sintering temperature in this work. Thehigh dense BWO ceramic obtained by conventional sintering can beattributed to the two step synthesis method, as well as theoptimum sintering temperature. If the mixture of Bi2O3 and WO3 iscalcined directly at 700 �C, it may cause volatilization of Bi2O3 sincethe calcination temperature is close to the melting point of Bi2O3

(815.8 �C). Synthesis of Bi2O3 and WO3 mixed powder at a lowertemperature of 600 �C for 4 h in the first step can form relativelystable intermediate phases to avoid volatilization of Bi2O3 [15]. Inthe second step, after crushing, then ball milling, these intermedi-ate phases were recalcined at 700 �C for 4 h to form astoichiometric BWO, which is important for sintering denseBWO ceramics.

Fig. 2 shows the scanning electron microscopy (SEM) images ofthe natural and fractured surfaces and EDS analysis of BWOceramics. BWO shows round-shaped grains as shown in Fig. 2(a)and (b), not the normal plate-like grains in Aurivillius compoundswith m � 2, which can be attributed to that BWO has an m value of 1(the parameter c is about half of that for m = 2) and the solidparticles are wetted by massive liquid phase thus results inspherical grains during sintering. Consistent with the XRD result,the EDS analysis further confirms only the elements of Bi, W and Oare contained in the BWO sample and the composition isstoichiometric with atomic ratio of Bi:W:O equal to 2:1:6 asshown in Fig. 2(c).

The frequency dependence of dielectric constant and loss ofBWO from 25 �C to 200 �C is shown in Fig. 3. As it can be seen, thedielectric properties of BWO are strongly frequency dependent,which increases with the increasing temperature. It is reportedthat some defect dipoles are able to follow the field at lowfrequency, thereby contributing to high dielectric loss at lowerfrequency [26]. When the temperature increases, the response ofthe defect dipoles increases so these phenomena are amplified.Therefore, the observed strong frequency dependence of dielec-tric constant and loss with increasing temperature in BWO can be

Fig. 2. Scanning electron microscopy (SEM) images of the natural surface (a) and fractured surface (b) and EDS analysis (c) of BWO ceramics.

T. Zeng et al. / Materials Research Bulletin 68 (2015) 271–275 273

attributed to the contribution from the defect dipoles, which isconsistent with reports that oxygen vacancies exist in BWO[22,27].

Fig. 4 shows the temperature dependence of the electricalresistivity of both unpoled and poled BWO ceramic samplesmeasured in the temperature range of 100–550 �C. As it can be seenthat the resistivity of both samples decreases with the increasingtemperature and it is interesting to note that the resistivity of poledBWO is a little higher than that of unpoled one below 200 �C. As wepreviously reported in SPS BWO ceramics [22], there are defectdipoles which can be decoupled and randomized by increasingtemperature up to 200 �C or applying high voltage such as polingvoltage. A possible reason for this is that some of the defect dipoles

Fig. 3. Frequency dependence of dielectric constant and loss of BWO from 25 �C to200 �C.

are thermally decoupled and randomized with increasing temper-ature. When temperature is below about 200 �C, these decoupledand randomized defects move and act as additional mobile carrierswhich provide extra contribution to the electrical conductivity ofunpoled BWO. However, for poled BWO, the movement of defectsis inhibited by internal electric field created by oriented domainafter poling, resulting in higher resistivity of the poled BWO thanthat of unpoled BWO below about 200 �C. On the other hand, thegap between the resistivity of poled and unpoled BWO samples isnarrowing gradually with the increasing temperature up to about200 �C, which might be due to the effect of inhibition by internalelectric field on the movement of defects is weakening. Therefore,both unpoled and poled BWO ceramics have the same conduction

Fig. 4. Temperature dependence of the electrical resistivity of unpoled and poledBWO ceramics from 100 �C to 550 �C.

Fig. 5. P–E hysteresis loops of unpoled and poled BWO ceramics at 25 �C and 10 Hz.

274 T. Zeng et al. / Materials Research Bulletin 68 (2015) 271–275

mechanism and show same level of electrical conductivity over200 �C.

Fig. 5 shows the P–E hysteresis loops of both unpoled and poledBWO ceramic samples measured at 25 �C and 10 Hz. Unpoled BWOsample shows constricted P–E loops. This is caused by defects insamples, which stabilizes the domain structure. The defects pin thedomain walls and restrict their movement, which subsequentlyresult in the reorientation of the polarization of domain walls byexternal electric fields very difficult. This happens because anychange in the polarization by an external electric field will nowrequire reorientation of aligned defects, which is manifested byshifts of ferroelectric hysteresis loops along the field axis, orconstriction of the loop. A shift of the loop occurs if defects dipolesare preferentially oriented in approximately the same direction inthe whole sample. It is observed that, in Fig. 5, a saturated P–Ehysteresis loop with remnant polarization (Pr) of 9.4 mC/cm2 andcoercive field (Ec) of 4.7 kV/mm exists in poled BWO where thepoling direction is the preferential direction of defect alignment.

The room temperature d33 values of the BLSFs are relatively lowcompared with those of lead zirconate titanate (PZT,200–600 pC/N). This is because of the two-dimensional orientationrestriction of the rotation of their spontaneous polarization Ps andtheir high coercive fields Ec [28]. BWO ceramic samples in thiswork are the same case, showing an average piezoelectric constantd33 value of 9.2 pC/N. Fig. 6 shows the effect of thermal annealingon the piezoelectric property of BWO. The relative d33 is defined as

Fig. 6. Effect of thermal annealing on the piezoelectric property of BWO ceramics.

the ratio of d33 value after thermal annealing to 9.2 pC/N. It can beobserved that there is a rapid decrease in d33 after post-annealingfrom room temperature to around 200 �C, and then becomes stableup to 900 �C. When the annealing temperature increase, the d33values drop sharply and tend to zero when the annealingtemperature is 950 �C. The reason for this can be attributed tointeraction of the defect dipole with domain walls in BWO. Thesedefect dipoles interact with domain walls and inhibit theirmovement at low temperature, and then cause a drop in d33.When the defects were completely randomized on heating up to200 �C, the d33 becomes stable even with a further increase intemperature up to the Curie point (phase transition temperaturefrom g 000 to g 00).

4. Summary

Stoichiometric Bi2WO6 (BWO) piezoelectric ceramic, withround-shaped grains and a relative density of �96%, was preparedby solid state reaction method. The dielectric constant and lossshow strong frequency dependence with increasing temperaturein BWO, which can be attributed to the contribution from thedefect dipoles. The movement of decoupled and randomizeddefects by increasing temperature or applying poling voltagesinhibited by internal electric field created by oriented domain afterpoling, resulting in higher resistivity of the poled BWO than that ofunpoled BWO below about 200 �C. Unpoled BWO shows con-stricted P–E loops because of the defects pinning the domain wallsand restricting their movement, while poled BWO has a saturatedP–E hysteresis loop, resulting from defects dipoles are preferen-tially oriented in approximately the same direction in the wholesample after poling. Meanwhile, these defect dipoles interact withdomain walls and inhibit their movement at low temperature, andthen cause a drop in d33 below 200 �C.

Acknowledgements

The authors acknowledge the financial supports by the NationalNatural Science Foundation of China (Grant No. 11304334) andScience and Technology Commission of Shanghai Municipality(Grant No. 14DZ2261000).

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