Large Strain Response in the Ternary Bi0.5Na0.5TiO3–BaTiO3–SrTiO3

Solid Solutions

Feifei Wang,†,‡ Min Xu,‡ Yanxue Tang,‡ Tao Wang,‡ Wangzhou Shi,‡ and Chung Ming Leung§

‡Key Laboratory of Optoelectronic Material and Device, Mathematics & Science College,Shanghai Normal University, Shanghai 200234, China

§Department of Electrical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong

A ternary solid solution (0.935-x)Bi0.5Na0.5TiO3–0.065BaTiO3–xSrTiO3 was designed and fabricated using a conventional fabri-

cation process. Temperature and composition dependence ofthe ferroelectric, dielectric, and piezoelectric properties were

systematically investigated and a schematic phase diagram was

established. The SrTiO3 substitution was found to induce atransition from ferroelectric to relaxor pseudocubic phases.

Around a critical composition x of 0.22, large strain response

of ~0.2% (under a moderate field of 4 kV/mm) with normal-

ized strain of 490 pm/V was obtained. The large unipolarstrain response would be of great interest for environmental-

friendly “on-off” actuators.

I. Introduction

LEAD zirconate titanate (PZT) ceramics have been exten-sively used for piezoelectric transducers, sensors, and

actuators due to their excellent piezoelectric propertiesaround the morphotropic phase boundary (MPB).1 Neverthe-less, because of concerns over their high lead content, envi-ronmental legislation in the European Union, parts of Asia,and the United States demands elimination of toxic lead forthese materials systems,2 which spurred a large effort in theresearch of new lead-free counterparts.3–5 Among them,sodium bismuth titanate Bi0.5Na0.5TiO3 (BNT) is consideredto be one promising candidate ascribed to the strong room-temperature ferroelectricity and high piezoelectric properties.To decrease the electrical conductivity and also furtherimprove the electromechanical responses, MPB solid solu-tions based on BNT were designed and fabricated such asthe Bi0.5Na0.5TiO3–BaTiO3 (BNT–BT),6 Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3 (BNT–BKT)7 which exhibited enhanced piezo-electric properties.

Although some improvements have been achieved and afew ultrasonic devices with acceptable performance weredemonstrated, the overall piezoelectric properties of the lead-free counterparts were still inferior to the lead-based system.8

How to further improve the piezoelectric response is not onlyscientifically interesting but also technologically importantfor piezoelectric device applications. Recently Kounga andZhang et al. reported large electric-field-induced strainresponse in Bi0.5Na0.5TiO3–K0.5Na0.5NbO3 (BNT–KNN) andBi0.5Na0.5TiO3–BaTiO3–K0.5Na0.5NbO3 (BNT–BT–KNN)solid solutions with high normalized strain (Smax/Emax) of~300 and ~560 pm/V, respectively.9,10 Through substitutingZr4+ into BNT–BKT, significantly enhanced strain up to

0.43% with Smax/Emax of 614 pm/V was obtained.11 Thelarge strain level in these quite limited studies was compara-ble to the traditional PZT system. Regarding the high strainresponse, Zhang et al. ascribed it to a field-induced antiferro-electric (AFE) to ferroelectric (FE) phase transition alongwith the domain contribution.10 However, for an AFE-FEphase transition, a volume change should be always accom-panied. Through simultaneously monitoring the longitudinaland transverse strain, Jo et al. excluded this assumption.12

Hussain et al. considered the high strain originated from thecoexistence of the ferroelectric and nonpolar phases at roomtemperature.11 In a word, up to the present the underlyingmechanism responsible for the high strain behavior is stillcontroversial.

At room temperature, SrTiO3 (ST) has a perovskite struc-ture and a cubic symmetry. In previous works the ST substi-tution of BNT was found to be an effective way to modifythe dielectric, piezoelectric, and ferroelectric properties.13–15

Hiruma et al. reported a large electric-field induced strainresponse at a critical composition of 0.28.15 Nevertheless, thestudy was mainly focused around room temperature. System-atic investigations of the temperature-dependent dielectric,ferroelectric, and piezoelectric response were quite necessaryto clarify the composition and temperature induced structureand performance revolution so as to promote practical appli-cations. Under these considerations, in this article a ternarysolid solution was designed and fabricated through substitut-ing ST into the MPB composition BNT–0.065BT. A system-atic investigation of the electrical properties was performedand a schematic phase diagram was established. A largestrain value of ~0.2% (under a moderate field of 4 kV/mm)with Smax/Emax up to 490 pm/V was found at a critical com-position. Raman spectroscopy along with the X-ray diffrac-tion (XRD) data were adopted to investigate thecomposition induced structure evolution process.

II. Experimental Procedure

Using a conventional solid-state reaction method using Bi2O3

(99.0%), Na2CO3 (99.8%), BaCO3 (99.0%), TiO2 (98.0%),and SrCO3 (99%) as starting raw materials (0.935-x)Bi0.5Na0.5TiO3–0.065BaTiO3–xSrTiO3 (BNBST or BNBSTx)ceramics with x from 0.02 to 0.26 were prepared. For eachcomposition, the starting materials were weighed accordingto the stoichiometric formula and ball-milled for 6 h in etha-nol. The dried slurries were calcined at 850°C for 2 h andthen ball-milled again. The powders were subsequentlypressed into green disks and sintered at 1 200°C for 2 h. Tominimize the evaporation of the volatile elements Bi and Na,the disks were embedded in the powder with the same com-position. Silver paste was coated on both sides of the sinteredsamples and fired at 650°C for 0.5 h to form electrodes. Thespecimens for measurement of electrical properties werepoled in silicone oil bath with a dc field of 3–4 kV/mm for

J. Roedel—contributing editor

Manuscript No. 30139. Received August 09, 2011; approved January 17, 2012.†Author to whom correspondence should be addressed. email: f_f_w@sohu.com

1

J. Am. Ceram. Soc., 1–5 (2012)

DOI: 10.1111/j.1551-2916.2012.05119.x

© 2012 The American Ceramic Society

Journal

15 min. All the electrical measurements were performed afteraging for at least 24 h.

The crystal structures were characterized using X-ray dif-fractometry (D8 Focus; Bruker AXS, Karlsruhe, Germany)using unpoled crushed sintered samples. Dielectric constantand loss of the ceramics were measured using an automaticacquisition system using an impedance analyzer (AgilentHP4294A, Santa Clara, CA) in the range of 25°C–400°Cunder different frequencies. The piezoelectric constant d33was measured using a Berlincourt d33 meter at 55 Hz. Theferroelectric hysteresis loops and bipolar strain curves weremeasured in silicon oil with the aid of a Sawyer-Tower cir-cuit (TF1000 analyzer, Aixacct, Aachen, Germany).

III. Results and Discussion

Figure 1 illustrates three representative room-temperatureferroelectric P–E loops, polarization current density J–Ecurves, and electric-field-induced S–E curves of BNBST0.02,BNBST0.18, and BNBST0.22. The BNBST0.02 exhibitedtypical rectangular loop with the remnant polarization Pr

and coercive field Ec of 28 lC/cm2 and 2.5 kV/mm, respec-tively. One sharp polarization current peak (denoted as P1)ascribed to the domain switching could be observed whenthe applied field reached Ec. The bipolar strain curve[Fig. 1(b)] exhibited typical butterfly shape ascribed to theferroelectric domain and domain wall motions. In addition,the strain under unipolar electric field exhibited quasi-linearresponse with little hysteresis and the Smax/Emax (calculatedfrom the unipolar strain response under 0.5 Hz) was about303 pm/V. With x increased to 0.18, slightly pinched P-Eloop appeared along with an additional current peak[denoted as P2 shown in Fig. 1(d)]. The Ec corresponding toP1 shifted to lower field direction (~0.9 kV/mm fromFig. 1(d)) and the bipolar strain curve exhibited deformedbutterfly shape with the negative strain value decreased from

~0.15% [Fig. 1(b)] to ~0.1% [Fig. 1(e)]. With x furtherincreased to 0.22, obvious pinched behavior appeared. Inter-estingly the current peak P1 appeared in the electric fielddecreasing process and the P2 exhibited a composition-inde-pendent behavior, suggesting a different nature from that ofP1. From Figs. 1(h) and (i) the strain curve exhibited someelectrostrictive behavior characterized by the zero negativestrain with the Smax/Emax up to 490 pm/V. Nevertheless, thecorresponding quasi-static d33 decreased to only ~9 pC/N.

From the XRD measurements all the compositions exhib-ited pure perovskite structure and three representative pat-terns of BNBST0.02, BNBST0.18, and BNBST0.22 areshown herein in Fig. 2(a). Close inspection of the slow-scan-ning XRD patterns indicated that no characteristic peakssplitting appeared, indicating pseudocubic structure for allthe composition range studied. To give an insight into thestructure reason for the strong composition-dependentbehavior, Raman spectroscopy, which could probe a localionic configuration with short length scale usually less than10 nm,16 was utilized to complement the XRD results (whichreflected the large-length-scale average structures) as shownin Fig. 2(b). From the Raman data the high-frequency over-lapping bands in the range of 450–650 cm�1 changed little.Nevertheless, the mode corresponding to Ti–O vibration cen-tered around 260 cm�1 of BNBST0.22 (denoted by thearrow) became broader and started splitting into two bandsthat shifted apart from each other, indicating particular pho-ton behavior in the structure evolution caused by the substi-tution of Sr2+ into (Bi0.5Na0.5)

2+. Recent in situ synchrotronstudy on BNT–BT indicated that unpoled BNT–0.065BTceramic was ferroelectric rhombohedral at room tempera-ture.17 Poling of BNT–0.065BT under a high electric field of6 kV/mm could induce the phase coexistence of rhombohe-dral and tetragonal at ambient temperature, however, thephase fraction of tetragonal is much smaller compared tothat of the rhombohedral phase.17 Combined with present

Fig. 1. The room-temperature ferroelectric P–E loops, polarization current density J–E curves, and electric-field-induced S–E curves ofBNBST0.02, BNBST0.18, and BNBST0.22.

2 Journal of the American Ceramic Society—Wang et al.

measured electrical properties, here we suggested that forBNBST with x < 0.22, the ferroelectric rhombohedral phaseshould be the room-temperature dominant phase. Forx > 0.22, the sharp decrease in the remnant polarization andweak quasi-static d33 indicated that ferroelectric long-range-order structure was disrupted and pseudocubic phase becamethe dominant one.

In addition, the temperature dependence of the dielectric,ferroelectric, and piezoelectric behavior was also investigatedin detail and a phase diagram was established. Fig 3 showsthe temperature-dependent P–E loops of BNBSTx with x of0.02, 0.18, and 0.22 at 10 Hz. For the BNBST0.02[Fig. 3(a)], similar pinched loops can also be observed at

elevated temperature of ~80°C. With the temperature furtherincreasing, Pr and Ec decreased substantially and the loopbecame much slimmer, indicating the shape of a dielectricmaterial. The temperature and composition dependence ofPr, Ec, and maximum polarization Pm under 4 kV/mm wassummarized in Fig. 3(d). From the inset of Fig. 3(d) the dif-ference between the Pm and Pr (Pm�Pr) was found toincrease gradually with x increasing, indicating that the Sr2+

substitution caused the lattice distortion and disrupted theferroelectric long-range-order leading to a decrease in thepolarization states (Pm and Pr). The corresponding bipolarstrain curves are illustrated in Figs. 4(a)–(c) and the temper-ature-dependent maximum strain data (the amplitude

Fig. 3. The composition and temperature dependence of the ferroelectric properties: (a) BNBST0.02, (b) BNBST0.18, (c) BNBST0.22, and (d)temperature dependence of remnant polarization Pr and coercive field Ec and the inset shows the composition dependence of the maximumpolarization Pm, Pr, and Ec.

Fig. 2. The XRD patterns and Raman data for BNBST0.02, BNBST0.18, and BNBST0.22.

Large Strain Response in Ternary Lead-Free Ceramics 3

between the maximum and the minimum strain under bipo-lar electric field) are summarized in Fig. 4(d). ForBNBST0.02 and BNBST0.18, with temperature increasingthe total strain (under 4 kV/mm) both first increased to amaximum and then followed a decrease instead. The temper-ature corresponding to the maximum strain was 80°C and40°C, respectively, close to the temperature of pinching. ForBNBST0.22 the maximum total strain was obtained at roomtemperature (also close to the temperature of pinching). Allthese indicated that around this temperature large strainlevel can generally be obtained due to the phase coexistence.A comparison of the quasi-static d33 and Smax/Emax is alsosummarized as shown in the inset of Fig. 4(d). With xincreasing, the d33 decreased slightly first then followed asharp reduction from ~150 to ~9 pC/N. Meanwhile, the uni-polar strain increased to a maximum value up to ~0.2% atx of 0.22 and normalized strain Smax/Emax reached as highas 490 pm/V. Such Smax/Emax value was entirely comparableto the PZT,1 BNT–KNN,9 and BNT–BT–KNN solid solu-tions.10

Figure 5 shows the temperature dependence of the dielec-tric constant (eT33=e0) and loss (tand) for the poled BNBSTxwith x of 0.02, 0.18, and 0.22 under 100 Hz–100 kHz. Twodielectric anomalies, corresponding to the depolarizationtemperature Td and permittivity-maximum temperature Tm,can be determined as indicated by arrows. Apart from Tm

and Td, another characteristic temperature where the fre-quency dispersion vanished (denoted as TDV) was also mani-fested. From Fig. 5, at room temperature the frequencydispersion decreased greatly in poled ceramics with x < 0.18due to the formation of the macrodomains. However, forBNBST0.22, external poling field could not set up the long-

range-order at room temperature and obvious frequencydispersion remained. The Tm was shifted down to lowertemperature with x increasing and finally almost disap-peared at x of 0.22. Such phenomena was quite interestingand similar response was also recently reported in the(Sr1�1.5aBia)TiO3 system, which may be correlated withformation of the sublattice structure in the ternary solidsolutions.18

Based on the above electrical data and structure analysis,a schematic low-temperature phase diagram was constructedfor poled BNBST system shown in Fig. 5(d). A critical com-position between the ferroelectric long-range-order and relax-or pseudocubic phases was proposed at x around 0.22. Forx < 0.22 below Td, ferroelectric rhombohedral phase shouldbe the dominant one and was responsible for the typical elec-tromechanical behavior. The temperature and compositionincreasing would disrupt the long-range-order structure.Combined with Eerd et al.’s results on BNT–BT16 and ourRaman data, here we suggested the pinched polarizationloops, large reversible strain behavior with zero negativestrain, and the relaxor characteristics between Td and TDV

originated from the composition and temperature inducedshort-range ionic displacement order. The coexistence of theferroelectric long-range-order and short-range coherencestructure crossing Td also gave a trace of residual macro-scopic piezoelectric properties in poled samples.

IV. Conclusion

In summary, composition and temperature dependence ofthe dielectric, ferroelectric, and piezoelectric properties wereinvestigated and a schematic diagram was constructed for

Fig. 4. The composition and temperature dependence of the piezoelectric properties: (a) S–E curves for BNBST0.02, (b) S–E curves forBNBST0.18, (c) S–E curves for BNBST0.22, and (d) temperature dependence of the maximum bipolar strain and the inset shows thecomposition dependence of the quasi-static d33 and normalized Smax/Emax.

4 Journal of the American Ceramic Society—Wang et al.

BNBST system. Around the critical composition x of 0.22,high strain level of ~0.2% with normalized strain Smax/Emax

up to 490 pm/V was obtained which was attributed to therevolution of ferroelectric long-range-order to short-range-coherence structure. The high strain has great applicationpotential in lead-free solid-state actuators.

Acknowledgments

This work was supported by the Science and Technology Commission ofShanghai Municipality (Grant No. 10ZR1422300 and 09520501000), the“Chenguang” Program of Shanghai Educational Development Foundation ofChina (Grant No. 11CG49), the Innovation Program of Shanghai MunicipalEducation Commission (Grant No. 11YZ82, 11YZ83, and 11ZZ117), NationalNatural Science Foundation of China (Grant Nos. 60807036 and 51071105).

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Fig. 5. Frequency, composition and temperature dependence of the dielectric properties in poled ceramics from 25°C to 400°C: (a) BNBST0.02,(b) BNBST0.18, (c) BNBST0.22, and (d) a schematic low-temperature phase diagram.

Large Strain Response in Ternary Lead-Free Ceramics 5