Journal of Alloys and Compounds 603 (2014) 149–157

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Journal of Alloys and Compounds

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Effect of SrTiO3 template on electric properties of textured BNT–BKTceramics prepared by templated grain growth process

http://dx.doi.org/10.1016/j.jallcom.2014.03.0330925-8388/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding authors. Tel.: +86 21 65980544; fax: +86 21 65985179.E-mail addresses: shenbo@tongji.edu.cn (B. Shen), apzhai@tongji.edu.cn (J. Zhai).

Wangfeng Bai a, Lingyu Li a, Wei Li a, Bo Shen a,⇑, Jiwei Zhai a,⇑, Haydn Chen b

a Functional Materials Research Laboratory, Tongji University, Siping Road, Shanghai 200092, Chinab University of Macau, Macau, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 30 December 2013Received in revised form 27 February 2014Accepted 9 March 2014Available online 20 March 2014

Keywords:Templated grain growth(Bi0.5Na0.5)TiO3 ceramicsElectrostrictionLead-free ceramics

In this work, h00li textured lead-free piezoelectric (1�x)(0.83 Bi0.5Na0.5TiO3–0.17Bi0.5K0.5TiO3)–xSrTiO3

(BNT–BKT–xST) ceramics were fabricated by template grain growth in combination with tape castingusing plate-like SrTiO3 as a template. The effect of ST template on the grain orientation, strain behavior,dielectric, ferroelectric, pyroelectric and piezoelectric properties was systematically investigated in orderto search the lead-free piezoelectric materials with an excellent actuating performance. The templatedgrowth of h00li oriented grains on the ST template resulted in textured ceramics with brick wallmicrostructures and significantly high h00li texture degree corresponding to >90% Lotgering factor at9–15 mol% ST template. Textured ceramics showed excellent strain properties as compared to therandom counterparts and encouraging results of large strain of 0.38% with normalized strainSmax/Emax = 626 pm/V for the textured critical composition with 15 mol% ST template were obtained atroom temperature. Furthermore, the textured ceramics with 27 mol% ST template exhibited largeelectrostrictive coefficient Q11 of 0.023 m4 C�2 and superior temperature stability compared to therandom counterparts, which are very promising and makes them tremendous potential for theenvironmental-friendly solid-state actuator applications.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Structural transformations in functional materials under exter-nal stimuli are known to impart exotic responses and play animportant role towards controlling material properties [1,2].Piezoelectric materials are used in a variety of applications in amultitude of devices such as actuators, sensors, and transducers,with the most common material being lead zirconatetitanate(PZT) due to the high performances. However, environmentalrestrictions in many parts of the world have prompted globalinvestigations to search for lead-free piezoelectric ceramics withelectrical properties comparable to traditional lead-based system.Among lead-free piezoelectric materials, due to the high strainresponse, BNT-based systems that exhibit an electric-field-inducedstructural transformation have gained tremendous importanceas a potential alternative to lead-based ceramics for actuatorapplication [3–6].

Recently, a giant strain response of 0.45% at 80 kV/cm has beenachieved by Zhang et al. [3], in lead-free K0.5Na0.5NbO3-modifiedBi0.5Na0.5TiO3–BaTiO3 (BNT–BT-KNN) system. Jo et al. [5] proposed

that the mechanism for the giant strain response observed in thisBNT–BT-KNN system is mostly a consequence of a reversible‘‘non-polar’’ to ferroelectric phase transformation under electricfield. The above mentioned mechanism suggests that the long-rangeferroelectric order can be disrupted with chemical modifications ina way that its reestablishment is achievable by the applicationof electric field and effective chemical modifier enables oneto come up with materials of optimized properties, e.g.,lower electric field to trigger the intended large strain [7]. In thecase of known lead-free compositions, Bi1/2Na1/2TiO3–Bi1/2K1/2TiO3

(BNT–BKT) solid solution, which exhibits a typical morphotropicphase boundary (MPB) between rhombohedral and tetragonal fer-roelectric phases at compositions with the BKT concentrationapproximately16–20 mol% [8,9], has emerged as a potential candi-date as a result of excellent piezoelectric performances and a highdepolarization temperature. As shown and proved in previousreports [10,11], the addition of SrTiO3 (ST) to BNT-based solid solu-tions could be effective to improve the strain properties, indicatingthe fact that the free energy between ferroelectric order and the‘‘non-polar’’ is so competitive that an easy transformation ofthe ‘‘non-polar’’ phase to ferroelectric is possible. Very recently,the additional work in the lead-free ST-modified BNT–BKT(BNT–BKT–ST) ternary system was demonstrated to have a giant

Fig. 1. The sketch map of templated grain growth progress.

Fig. 3. (a) XRD patterns of BNT–BKT–ST ceramics with different content of STtemplate particle and (b) Lotgering factor and relative density as a function of STtemplate content.

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unipolar strain of �0.36% (Smax/Emax of 600 pm/V) at a comparablysmaller field of 60 kV/cm [7].

In the functional ceramics community, currently there are threelines of research for addressing the fundamental problem to find areplacement for lead-based piezoelectrics used for actuators – (i)search for new systems through a combination of theory-basedprediction followed by experimental efforts (doping, solidsolutions having a MPB), (ii) stabilization of metastable phases or

Fig. 2. (a) Micrograph of the synthesized BIT template particle, and the corresponding Xtopochemical conversion from BIT, the inset shows XRD spectra recorded after conversiotemplate (c) 0 mol% and (d) 15 mol%.

finding the high temperature triclinic systems, and (iii) improvingthe properties of known compositions through microstructure

RD spectra recorded at room temperature and (b) ST templates synthesized usingn of BIT to NBT. Micrographs of BNT–BKT–ST ceramics with different content of ST

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optimization, domain engineering and multilayering [12]. Amongthese approaches, the latter line of research is more promising asit builds upon the known material systems with excellent perfor-mance that have demonstrated the potential to be practical. As aresult, what is required is a processing technique that can repro-duce the ideal texture and microstructure for a wide range of com-positions across the phase boundary. It is well known thattexturing enables the polycrystalline ceramics to resemble theirsingle crystal counterparts so that favorable domain engineeredstates can be obtained for compositions close to MPB [12], and thusdrastically enhance the piezoelectric properties of perovskites byenforcing strong crystallographic texture [12–18]. Due to thecost-effective alternative to Bridgman grown single crystals, tem-plated grain growth (TGG) technology has been extensively usedto fabricate the textured lead-free piezoelectric ceramics [16–18].To further understand the TGG progress, the sketch map of TGGprogress is presented in Fig. 1. In the TGG process, the templateswith anisotropic shape and crystallographic properties are alignedin a matrix by applying viscous forces during the shearing processthough the tape casting, subsequently, the heat treatment resultsin the nucleation and growth of the desired crystals on alignedtemplate to yield textured ceramics [19]. In recent years, withthe development of the fabrication of textured BNT-based solidsolution, the template focuses on the plate-like Bi4Ti3O12(BiT)particles in the TGG progress [17,18,20,21]. The use of BiT tem-plates results in the fact that the ceramics are difficult to be fullydensified due to the difference of the structures between thetemplate and matrix [22]. On the other hand, the addition of BiTtemplates lead to the target compound deviation induced by theinterdiffusion between template and the corresponding matrix,which gives rise to the poor dielectric, ferroelectric and piezoelec-tric properties.

Fig. 4. (a) Polarization hysteresis loops, (b) polarization current curves, (c) bipolar strainwith different ST template content.

In our present work, developing >90% textured BNT–BKT–STceramics have been fabricated by TGG method in combinationwith tape casting using SrTiO3 anisotropic particles as templatesto improve the strain response. Composition and temperaturedependence of the phase structure and the electrical propertieswere systematically studied and a schematic phase diagram wasproposed. The effect of template content on texture development,microstructure, and electric properties of the textured BNT–BKT–ST ceramics was investigated systematically.

2. Experimental details

2.1. Preparation of SrTiO3 templates

Plate-like ST template particles were prepared by double molten salt synthesis(DMSS). The Bi2O3 (99.975%), TiO2 (99.6%) and NaCl (98%) were selected as startingmaterials. First, Bi2O3 and TiO2 power were mixed with a ball milling in a ratioaccording to a predetermined number. The mixture was dry-mixed with NaCland heated at 800 �C for 1 h to melt NaCl and then at 1000 �C for 2 h to prepareplate-like Bi4Ti3O12 (BiT) particles. The resulting product was washed several timeswith hot deionized water to remove trace of NaCl. Secondly, the synthesized BiTplatelets were mixed with SrCO3 (99%)and NaCl and heated to 800 �C for 1 h tothe melt the NaCl, and then the mixture was heated to 950 �C for 2 h to completethe SrTiO3 formation topological reaction.

2.2. Fabrication of textured BNT–BKT–ST ceramics by TGG

The general formula of the target compound was 0.83BNT–0.17BKT (abbrevi-ated as BNT–BKT). The Bi2O3 (99.975%), TiO2 (99.6%), NaCO3 (99.5%) and KCO3

(99%) were selected as starting materials to synthesize BNT–BKT matrix powderby the conventional solid-state reaction rout. Random ceramics were processedusing conventional solid state sintering techniques. The plate-like ST particles syn-thesized above and pre-fabricated BNT–BKT matrix powders were mixed with thesolvent (toluene and ethanol in the weight ratio of 2:1) for 12–15 h using tumblingmill, with different amount of templates from 0 mol% to 27 mol%. Then the LSmodelbond (produced by Lingguang Electric Chemical Materials Technology

curves and (d) summary of positive strain and negative strain values of BNT–BKT–ST

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Corporation in Zhaoqing city, China) was added in the slurry and milled for another3 h. The obtained stable slurry was subjected to tape casting with the doctor bladeheight of 100 lm align the ST platelets in the powder matrix. The tapes were dried,cut, and laminated into a multilayer sheet with a size of 12 � 12 � 2 mm under apressure of 20 MPa at 50 �C for 30 min followed by a binder burnout process at500 �C for 4 h to remove organic additives with a heating rate of 0.5 �C/min. To in-crease the green density, the samples after the binder burnout process were sub-jected to the cold isostatic press (CIP) under a pressure of 200 MPa for 10 min.Then the samples were sintered at 1175 �C for10 h to promote texturing. To reducethe volatility of Bi, K and Na and maintain the chemical composition, the disks wereembedded in the powder of the same composition during high temperatureprocessing.

2.3. Characterization

The phase structure of the sintered samples was examined by X-ray diffraction(XRD, Bruker D8 Advanced, made in Germany) with Cu Ka radiation on the polishedsurface cut parallel to the tape casting plane. The degree of orientation of the tex-tured samples was evaluated from the XRD pattern in the range of 2h = 20–60�by the Lotgering factor (f) [12,21,23]. The densities of the sintered samples weremeasured by the Archimedes method. The microstructure of the thermally etchedcross-sections perpendicular to the tape casting plane and free surface parallel totape casting plane was observed by scanning electron microscopy (SEM, JSMEMP-800).

For electrical measurement, the samples were polished and painted with silverpastes on the two sides and then fired at 600 �C for 10 min. Samples were poled at adc field of 60 kV/cm for 30 min in silicone oil bath. Temperature-dependent dielec-tric constant and loss of the samples were determined using a high-precision LCRmeter (HP 4284A; Agilent, Palo Alto, CA) under different frequencies with a heatingrate of 2 �C/min. Thermally stimulated depolarization currents (TSDC) were mea-sured on poled samples using a Keithley 6517A ampere meter (Keithley Instru-ments, Cleveland, OH) at the same heating rate of 2 �C/min with the dielectricmeasurements to determine Td. The pyroelectric coefficient P and the thermalpolarization P(T) calculated from the measured pyroelectric current from the poledsamples [7,24]. The FE hysteresis loops and strain curves of the samples were mea-sured using a FE test system (Precision Premier II; Radiant Technologies Inc., Albu-querque, NM) connected with a Miniature Plane-mirror Interferometer and theaccessory Laser Interferometric Vibrometer (SP-S 120/500; SIOS Me b technikGmbH, llmenau, Germany). For the temperature-dependent measurements, thesamples were submerged in a silicon oil bath with a temperature controller andheld at each specified temperature for 20 min to minimize thermal fluctuations.After 24 h aging of the poled sample, the d33 value was measured using a quasi-sta-tic meter (Chinese Academy of Acoustics, ZJ-3, Beijing, China).

Fig. 5. (a) Unipolar strain curves and (b) unipolar strain values and hysteresis oftextured BNT–BKT–ST ceramics as a function of ST template content.

3. Results and discussion

To align plate-like ST particles into a specific orientation thusachieve the textured ceramics, the anisotropic precursors withthe specific surface must be firstly fabricated. Fig. 2(a) illustratesthe morphology and XRD pattern of the synthesized BIT particlesby the first molten salt synthesis. The synthesized BIT templateparticle with a length or width of about 5–20 lm and a thicknessof 1 lm was obtained, and thus can be regard as a suitable precur-sor with high aspect ratio and regular shape for the second step ofDMSS. The micrograph and XRD pattern of the synthesized ST tem-plate particles from the BIT particles by the topotaxial reaction areshown in Fig. 2(b). The formation of single phase ST platelets wasconfirmed by indexing the XRD pattern as shown in the inset ofFig. 2(b). Note that, ST template particles with broad faces on theorder of 10 lm, a thickness of 1 lm and a relative high aspect ratiowere obtained by DMSS, which can meet requirements for fabricat-ing textured ceramics using the TGG method. The cross-sectionalmicrograph of the textured BNT–BKT–ST ceramics with 15 mol%ST template particle displayed the brick wall microstructures withstrip-like grains representing the development of texture as shownin Fig. 2(d), while random ceramics with 0 mol% ST template par-ticle showed nearly equiaxed and random-oriented matrix grains(Fig. 2(c)). This result clearly demonstrated that textured BNT–BKT–ST ceramics with 15 mol% ST template particle was almostfully textured (f = 91%) and was in good agreement with the hightexture degree as illustrated by XRD.

Fig. 3(a) shows the evolution of the XRD patterns of BNT–BKT–STceramics with different amount of ST templates. All samples are

crystallized into a single-phase perovskite structure without anytrace of secondary phase. Compared to the randomly-orientedsample, the intensity of h00li peaks increase rapidly while otherpeaks show significantly reduced intensity, suggesting an increasein the strength of texture which was quantified using the Lotgeringfactor. Recently, Maurya et al. [12] investigated the synthesismechanism of grain-oriented lead-free piezoelectric though TEMmeasurement and revealed that the driving force for the growthof textured grain derives from the difference in surface energyalong with the chemical potential gradient between the stabletemplate and the metastable liquid phase. Fig. 3(b) displays thetexture degree computed by Lotgering factor method and relativedensity as a function of ST template content. With the introductionof ST template, the Lotgering factor (f) increased dramatically firstto the maximum value >90% (x = 9%–15 mol%), revealing a signifi-cant amount of template growth, and then decreased graduallywith further increasing ST template. Similar phenomenon can beobserved in other lead-free piezoelectric ceramics and are attrib-uted to the overlap and collision between each other with furtherincreasing ST template content [25,26]. In addition, the relativedensity of the samples decreased monotonously with increasingthe template contents, which is due to formation of pores formedby rearranging the template grains to have a face-to-face contactin the early stage of sintering [26]. Interestingly, an feature thatthe broaden h00li peaks displayed multiple overlapping can be ob-served in the textured ceramics as shown in Fig. 3(a), which couldbe ascribed to the lattice distortions due to the interdiffusion be-tween the template and the templated BNT–BKT–ST grains [27].Furthermore, the (002) peak splitting from (200) peak can beobserved indicating an increase in tetragonality of the textured

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ceramics, resulting from lattice distortion caused by the stress be-tween ST template and the template BNT–BKT–ST grains. However,the phenomenon of (200) peak splitting has been inconspicuous inthe textured samples with ST template content exceeded 21 mol%.This may be due to the diffuse direction affected by the change inchemical composition between the template and matrix [26]. Thediffusion of ST templates into BNT–BKT–ST matrix powders leadto a disappearance of template, and thus caused the weakeningof clamping effect and the decrease of tetragonality.

The polarization hysteresis loops and polarization currentcurves which are known to be more sensitive to polarization statewere measured at room temperature, as shown in Fig. 4(a) and (b),respectively. The well saturated hysteresis loop of the base compo-sition, BNT–BKT without addition of ST template, exhibits thebehavior of a typical ferroelectric with a large remanent polariza-tion and maximum polarization. Note that, a pinched polarization

Fig. 6. (a and d) Polarization hysteresis loops, (b and e) bipolar strain curves, and (c and f)for the addition of 9 mol% ST template and (d–f) for 15 mol% ST template, respectively.

hysteresis loop with increment of ST template content to 15 mol%was observed along with an additional current peak (assigned toP2) without affecting the maximum polarization, resulting from adrastic decrease of the remanent polarization and coercive field.These results imply the ferroelectric order is disturbed with theaddition of ST template, leaving a ‘‘weak’’ ferroelectric phase witha very small non-cubic distortion or a ‘‘nonpolar’’ phase at zeroelectric field that transforms reversibly into a ferroelectric phaseby an applied field [7,28]. Consistent with polarization hysteresisloops, the compositionally induced transformation from ferroelec-tric to weakly polar is also confirmed from bipolar strain loops, aspresented in Fig. 4(c). The positive strain and negative strain asso-ciated with domain back switching (the definition is given in theinset figure in Fig. 4) are summarized in Fig. 4(d). With increasingST template content, a pronounced reduction of the negative strainfrom 0.1% for BNT–BKT without addition of ST template to 0.005%

unipolar strain curves of the textured (-) and random (–) BNT–BKT–ST system (a–c)

Fig. 7. Temperature-dependent dielectric constant and loss for (a) unpoled and (b) poled ceramics of BNT–BKT. (c) Temperature dependence of pyroelectric coefficients as afunction of poling electric field for the random BNT–BKT ceramics. (d) Temperature-dependent dielectric constant and loss measured at 1 kHz for poled textured BNT–BKT–STceramics with different ST template content.

Fig. 8. Temperature dependence of (a) pyroelectric coefficients and (b) remanentpolarization (thermal depolarization process) for poled textured BNT–BKT–STceramics with different ST template content.

154 W. Bai et al. / Journal of Alloys and Compounds 603 (2014) 149–157

for ST template content to 15 mol% was observed, with a signifi-cant increase of strain up to 0.38%. This can clearly imply that asmall negative strain promotes the potential for a material todisplay large strain response due to the appearance of the weaklypolar phase.

To investigate the suitability of the BNT–BKT–ST ceramics foractuator applications, the strain response of the materials mea-sured under unipolar electric field cycling is shown in Fig. 5(a).The unipolar strain values as well as hysteresis (the definition is gi-ven in the inset figure in Fig. 5) values for all the compositions aresummarized in Fig. 5(b). Similar to bipolar strain, a promisingstrain of 0.38% was obtained for ST template content to 15 mol%with more hysteretic behavior, whereas other studied composi-tions with different ST template content showed a maximum strainof 0.10–0.23% and smaller hysteresis. Similar strain behavior hasbeen found in some BNT-based ceramics, such as BNT–BT-KNN[3], BNT–BKT–ST [7], BNT–BT-ST [11], and BNT–BAT [29]. This indi-cates that a unipolar strain is closely related to the hystereticbehavior and the enhanced unipolar strain is accompanied by morehysteretic behavior, as evidenced by the calculated hystereticshown in Fig. 5(b).

Fig. 6 provides the effect of strong texture (>90%) on the polar-ization, bipolar and unipolar response for the representative com-positions. As can be seen from Fig. 6(d)–(f), the shape ofpolarization hysteresis loops and bipolar strain curves betweenthe random and textured ceramics for the composition with15 mol% ST template has little change although the strain responseof textured ceramics can be enhanced. Teranishi et al. [30] reportedthat the giant strain of 0.87% in BNT–BKT–BT single crystal alongh00li c direction originates from switching of nanosized 90�domains, which was confirmed by In situ X-ray diffraction mea-surements and transmission electron microscope observations.Therefore, the enhancement of the strain response in the texturedceramics mainly results from the 90� domain contribution, becausetextured ceramics are preferentially aligned in the h00li direction

as demonstrated in Fig. 3(a), which produces a larger amount of90� domains compared to the random counterparts [13,31].

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However, the composition with 9 mol% ST template exhibits dis-tinctly different characteristics between the random and texturedceramics, as evidenced by the enhanced ferroelectricity (Fig. 6(a))and strain response (Fig. 6(b)). Note that, the strain value oftextured sample with 9 mol% ST template reaches 0.28%, a nearly50% increase compared to random counterparts (0.19%). Theenhanced strain response of the composition with 9 mol%ST template may be ascribed to polarization extension in thefield-induced tetragonal phase [32–34]. In situ diffraction ofBNT–5.6BT single crystals revealed that an induced transitionbetween pseudocubic and tetragonal structures can be observedunder electric field applied along h00li c texture direction, wherethe electric-induced phase transition depends on the field direction[33,35]. The unipolar strain response shown in Fig. 6(c) for tex-tured composition with 9 mol% ST template reveals the enhancedferroelectricity, which is consistent with the polarization hystere-sis loops shown in Fig. 6(a) and could derive from sluggish phaserelaxation kinetics. Fancher et al. [34,36] reported that h00litextured BNT–7BT-2KNN has been demonstrated to undergo apseudocubic to tetragonal field-induced phase transformation overthe electric field range of 1–3 kV/mm.

Temperature-dependent dielectric constant and loss for bothunpoled and poled ceramics of BNT–BKT in frequency range of1–200 kHz is contrasted, as shown in Fig. 7(a) and (b), which arerepresentative for all studied compositions. Two distinctive dielec-tric anomalies for both poled and unpoled samples, one at a lowertemperature with a strong frequency-dependent dispersion repre-sented as a ‘shoulder’ and the other at a higher temperature with arelatively weak frequency-dependent dispersion, are observed,which suggests that all investigated samples can be classified asrelaxor ferroelectrics [37]. According to the TEM and XRD results

Fig. 9. Temperature-dependent polarization hysteresis loops and bipolar strain curves ((aof BNT–BKT–ST system (a and b) for the addition of 27 mol% ST template and (c and d)

at ambient temperature for BNT–BT system reported by Jo et al.[38], it was proposed that the commonly observed two dielectricanomalies in BNT-based systems are correlated with thermal evo-lution of ferroelectric polar nanoregions of rhombohedral andtetragonal structure, which coexist nearly throughout the entiretemperature range and reversibly transform into each other withtemperature. The profiles of dielectric constant and loss beforeand after the poling treatment present a clear difference. An addi-tional dielectric anomaly appears when the materials are electri-cally poled and it is referred to as ferroelectric-to-relaxortransition temperature (TF–R) which is determined by the fre-quency-independent peak in the dielectric loss [7,24,38,39], as pro-vided in Fig. 7(b). It is important to note that the origin of thedielectric anomaly for poled samples in BNT-based solid solutionsis widely debated in literature and the consideration of it being anindication of FE–AFE transformation (Td) is only one of the pro-posed possibilities in literature [3,9–11,28,37,40]. However, re-cently Anton et al. [24] revealed the difference between TF-R

obtained from dielectric measurement and Td measured by TSDC.To further explore the difference between TF-R and Td, TSDC was ap-plied to determine Td for BNT–BKT as a function of poling electricfield, as provided in Fig. 7(c). It is evident that Td itself is a functionof electric field and lower than the previously determined TF-R val-ues at same poling electric field, which is similar to the observedbehavior in other BNT-based ceramics [7,24]. Fig. 7(d) shows thetemperature-dependent dielectric constant and loss measured at1 kHz for poled textured BNT–BKT–ST ceramics with different STtemplate contents. With increasing ST template content, the TF-R

varies considerably and shifts to lower temperature. It is seen thatTF-R decreases from �79 �C for BNT–BKT to �53 �C for ST templatecontent to 9 mol% and then is not detectable for ST template

and c) for textured ceramics), electrostrictive coefficient (b), and strain response (d)for 15 mol% ST template, respectively.

156 W. Bai et al. / Journal of Alloys and Compounds 603 (2014) 149–157

content to 15 mol%. The absence of apparent TF-R in ST15 in thecurrent measurement is indicative of the lowering of TF-R belowthe room temperature by the addition of ST template [7,28]. Thedownward shift of TF-R with increasing ST template content dem-onstrates a compositionally induced ferroelectric-to-relaxor phasetransition, which is consistent with the corresponding straincurves and polarization hysteresis loops shown in Fig. 4.

It is well known that thermally stimulated depolarization cur-rents (TSDC) can be applied to determine the depolarization tem-perature Td. Fig. 8(a) shows the pyroelectric coefficient Pcalculated from the thermal depolarization current for the texturedBNT–BKT–ST with different ST template content. With increasingST template content, the Td shifts to low temperature and lies be-low the room temperature. This result is good agreement with thedielectric measurements (Fig. 7(d)) and demonstrates the strongcomposition-dependent behavior. It is interesting to see that thepyroelectric coefficient P curve for the composition with 15 mol%ST template is distinctively diffusive compared with other investi-gated compositions. As the P value demonstrates the amount of in-duced FE phase that can exist stably after removal of electric field[41], the stability of the induced FE phase is on the decline withincreasing ST template content and the critical composition with15 mol% ST template contains coexisting FE and relaxor phases.The calculated temperature-dependent polarization P(T) [7], whichequals Pr for poled FE, is provided in Fig. 8(b). Obviously, the com-positions with ST template content <15 mol% are denoted by therather sharp decay of Pr near corresponding Td, whereas the electri-cal polarization is highly smeared and persists to temperatureswell above Td for composition with 15 mol% ST template, whichis similar to the obtained results in BNT–BT-KNN and BNT–BKT–ST systems [7]. Jo et al. [5,38] demonstrated that the above diffused

Fig. 10. The schematic phase diagram for textured BNT–BKT–ST system and ST templ

depolarization was rationalized by assuming the coexistence of arelaxor phase and polar phases over a wide temperature range inthe base compound (Bi0.5Na0.5)TiO3.

To improve the sensitivity of solid-state actuator, the electricfield-induced strain was generally required to be accompaniedwith low hysteresis. In the present study, with higher ST templatecontent, the textured BNT–BKT–ST system could substantially de-crease the hysteresis behavior. Fig. 9(b) demonstrates the straincurve follows a quasi-linear relation with the square of the polar-ization derived from the temperature dependence of P–E and S–Eloops as shown in Fig. 9(a). This behavior can also be observed inother ST-modified BNT-based systems and indicates the observedstrain in textured BNT–BKT–ST as a phenomenologically electro-strictive one [10,11,42,43]. The averaged electrostrictive coefficientQ11 of the textured ceramics was calculated to be of 0.023 m4 C�2

at room temperature, which is no smaller than that of the repre-sentative electrostrictive materials PMN of 0.017 m4 C�2 and therandom oriented ones and also comparable to the recently re-ported BNT-based ceramics [40,42,43]. As commonly expected,both the electrostrictive coefficient and electrostrictive loops ofthe textured ceramics are more stable and much higher than thoseof the random oriented counterparts with measured temperaturerange. Fig. 9(c) shows the temperature-dependent polarizationhysteresis loops and bipolar strain curves of the textured ceramicswith 15 mol% ST template. Note that, the rate of strain degradationof textured ceramics is slightly less sensitive to temperaturechanges than that of random counterparts, evidenced by 44% deg-radation from �0.38% to �0.21% compared to 51% from �0.34% to�0.16% over the temperature range RT and 120 �C as shown inFig. 9(d). All these results demonstrated that textured BNT–BKT–ST ceramics show high electromechanical response and improved

ate dependence of piezoelectric and ferroelectric properties at room temperature.

W. Bai et al. / Journal of Alloys and Compounds 603 (2014) 149–157 157

temperature stability, and thus are suitable for high-precision posi-tioning devices and solid-state actuators.

Building upon the above dielectric results, pyroelectric mea-surements and the structure analysis, a schematic phase diagramfor the poled textured BNT–BKT–ST system was constructed andprovided in Fig. 10(a) and (d) along with the ST template depen-dence of ferroelectric and piezoelectric properties in Fig. 10(b)and (c). With increasing ST template content, the depolarizationtemperature Td can be adjusted to RT at the composition with15 mol% ST template. Therefore, a critical composition betweenferroelectric rhombohedra and relaxor pseudocubic phases wasproposed at ST template content around 15 mol%. There was anapparent trade-off (marked by bar-type shown in Fig. 10(c)) be-tween the large strain Smax/Emax values and achievable d33. Thed33 values decreased gradually first then diminished quite remark-ably at the critical composition. Meanwhile, a sudden jump in thenormalized strain Smax/Emax values up to 626 pm/V could be ob-served at the critical composition. Fig. 10(b) shows the correspond-ing ST template dependence of maximum polarization Pm, remnantpolarization Pr, and coercive field Ec. At around the critical compo-sition with 15 mol% ST template, a sharp decrease in the remnantpolarization Pr and Ec was also observed, resulting in pinchedpolarization hysteresis loops shown in Fig. 4. In the present study,combined with recently reported TEM and PFM [43,44], Jo et al.analysis on BNT–BT and BNT–BT-KNN [5,38], the uniaxial stresson BNT-based ceramics [45], the phase transition theory in BNT-based solid solutions proposed by Hiruma et al. [9,10], XRD mea-surements and TEM microscope observations in the BNT–BKT–BTsingle crystals [30], and our XRD and electric measurements, wepropose that two factors contribute to the large strain responsearound the critical composition with 15 mol% ST template in tex-tured BNT–BKT–ST system – (i) electric field induced phase transitionfrom the relaxor pseudocubic phase to ferroelectric rhombohedraphase and (ii) the contribution of 90� domain wall motion becauseh00li – textured ceramics produce 90� domain wall.

4. Conclusions

In summary, lead-free piezoelectric (1�x)(0.83Bi0.5Na0.5TiO3

�0.17Bi0.5K0.5TiO3)–xSrTiO3 ceramics with a high h00li texturewere prepared successfully by template grain growth methodusing anisotropically SrTiO3 templates. In this work, the effect ofST template content on the electric properties and the relationshipbetween the phase diagram and the electrical properties wereinvestigated. The results show that the ST template contentstrongly affects the grain orientation, strain behavior and temper-ature stability of BNT-based textured ceramics. A large strain of0.38% with normalized strain Smax/Emax = 626 pm/V was obtainedat critical composition containing 15 mol% ST template witha > 90% Lotgering factor. For the textured ceramics with 27 mol%ST template, the electrostrictive coefficient Q11 can reach as highas 0.023 m4 C�2 at room temperature and is less sensitive to tem-perature than the random counterparts. The introduction of STtemplate was found to induce a phase transition from the relaxorpseudocubic phase to ferroelectric rhombohedra phase and pro-mote high grain orientation, which result in the enhanced strainresponse and large electrostrictive strain along with good temper-ature stability.

Acknowledgements

The authors would like to acknowledge the support from theShanghai Municipal Natural Science Foundation under Grant No.

12ZR1434600 and the National Natural Science Foundation ofChina under Grant Nos. 51372171, 51332003. This work wassupported in part by Research Committee of the Universityof Macau under Research & Development Grant for ChairProfessor.

References

[1] D. Maurya, A. Pramanick, K. An, S. Priya, Appl. Phys. Lett. 100 (2012).[2] X. Tan, J. Frederick, C. Ma, W. Jo, J. Rodel, Phys. Rev. Lett. 105 (2010).[3] S.T. Zhang, A.B. Kounga, E. Aulbach, H. Ehrenberg, J. Rodel, Appl. Phys. Lett. 91

(2007).[4] W. Jo, R. Dittmer, M. Acosta, J.D. Zang, C. Groh, E. Sapper, K. Wang, J. Rodel, J.

Electroceram. 29 (2012) 71–93.[5] W. Jo, T. Granzow, E. Aulbach, J. Rodel, D. Damjanovic, J. Appl. Phys. 105 (2009).[6] K.T.P. Seifert, W. Jo, J. Rodel, J. Am. Ceram. Soc. 93 (2010) 1392–1396.[7] K. Wang, A. Hussain, W. Jo, J. Rodel, J. Am. Ceram. Soc. 95 (2012) 2241–

2247.[8] A. Sasaki, T. Chiba, Y. Mamiya, E. Otsuki, Jpn. J. Appl. Phys. 38 (1999) 5564–

5567.[9] Y. Hiruma, K. Yoshii, H. Nagata, T. Takenaka, J. Appl. Phys. 103 (2008).

[10] Y. Hiruma, Y. Imai, Y. Watanabe, H. Nagata, T. Takenaka, Appl. Phys. Lett. 92(2008).

[11] F.F. Wang, M. Xu, Y.X. Tang, T. Wang, W.Z. Shi, C.M. Leung, J. Am. Ceram. Soc.95 (2012) 1955–1959.

[12] D. Maurya, Y. Zhou, Y.K. Yan, S. Priya, J. Mater. Chem. C 1 (2013) 2102–2111.[13] W.F. Bai, J.G. Hao, F. Fu, W. Li, B. Shen, J.W. Zhai, Mater. Lett. 97 (2013) 137–

140.[14] Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T. Nagaya, M.

Nakamura, Nature 432 (2004) 84–87.[15] S. Kwon, E.M. Sabolsky, G.L. Messing, S. Trolier-McKinstry, J. Am. Ceram. Soc.

88 (2005) 312–317.[16] W. Zhao, J. Ya, Y. Xin, L.E.D. Zhao, H.P. Zhou, J. Am. Ceram. Soc. 92 (2009) 1607–

1609.[17] F. Gao, C.S. Zhang, X.C. Liu, L.H. Cheng, C.S. Tian, J. Eur. Ceram. Soc. 27 (2007)

3453–3458.[18] C.G. Ye, J.G. Hao, B. Shen, J.W. Zhai, J. Am. Ceram. Soc. 95 (2012) 3577–3581.[19] M.M. Seabaugh, I.H. Kerscht, G.L. Messing, J. Am. Ceram. Soc. 80 (1997) 1181–

1188.[20] D.L. West, D.A. Payne, J. Am. Ceram. Soc. 86 (2003) 769–774.[21] D.L. West, D.A. Payne, J. Am. Ceram. Soc. 86 (2003) 1132–1137.[22] J.T. Zeng, K.W. Kwok, H.L.W. Chan, J. Am. Ceram. Soc. 89 (2006) 2828–2832.[23] Y.M. Kan, P.L. Wang, Y.X. Li, Y.B. Cheng, D.S. Yan, J. Eur. Ceram. Soc. 23 (2003)

2163–2169.[24] E.M. Anton, W. Jo, D. Damjanovic, J. Rodel, J. Appl. Phys. 110 (2011).[25] W.F. Bai, J.G. Hao, B. Shen, F. Fu, J.W. Zhai, J. Alloys Comp. 536 (2012) 189–

197.[26] S.K. Ye, J.Y.H. Fuh, L. Lu, Appl. Phys. Lett. 100 (2012).[27] E.M. Sabolsky, S. Trolier-McKinstry, G.L. Messing, J. Appl. Phys. 93 (2003)

4072–4080.[28] A. Ullah, C.W. Ahn, A. Ullah, I.W. Kim, Appl. Phys. Lett. 103 (2013).[29] W.F. Bai, Y.L. Bian, J.G. Hao, B. Shen, J.W. Zhai, J. Am. Ceram. Soc. 96 (2013)

246–252.[30] S. Teranishi, M. Suzuki, Y. Noguchi, M. Miyayama, C. Moriyoshi, Y. Kuroiwa, K.

Tawa, S. Mori, Appl. Phys. Lett. 92 (2008).[31] M. Nemoto, Y. Hiruma, H. Nagata, T. Takenaka, Jpn. J. Appl. Phys. 47 (2008)

3829–3832.[32] D. Damjanovic, Appl. Phys. Lett. 97 (2010).[33] W.W. Ge, C.T. Luo, Q.H. Zhang, C.P. Devreugd, Y. Ren, J.F. Li, H.S. Luo, D.

Viehland, J. Appl. Phys. 111 (2012).[34] C.M. Fancher, T. Iamsasri, J.E. Blendell, K.J. Bowman, Mater. Res. Lett. 1 (2013)

156–160.[35] C.T. Luo, W.W. Ge, Q.H. Zhang, J.F. Li, H.S. Luo, D. Viehland, Appl. Phys. Lett. 101

(2012).[36] C.M. Fancher, J.E. Blendell, K.J. Bowman, Scr. Mater. 68 (2013) 443–446.[37] H.S. Han, W. Jo, J. Rodel, I.K. Hong, W.P. Tai, J.S. Lee, J. Phys. Condens. Mater. 24

(2012).[38] W. Jo, S. Schaab, E. Sapper, L.A. Schmitt, H.J. Kleebe, A.J. Bell, J. Rodel, J. Appl.

Phys. 110 (2011).[39] E. Sapper, S. Schaab, W. Jo, T. Granzow, J. Rodel, J. Appl. Phys. 111 (2012).[40] Y.P. Guo, M.Y. Gu, H.S. Luo, Y. Liu, R.L. Withers, Phys. Rev. B. 83 (2011).[41] Y.P. Guo, M.Y. Gu, H.S. Luo, J. Am. Ceram. Soc. 94 (2011) 1350–1353.[42] H.S. Han, W. Jo, J.K. Kang, C.W. Ahn, I.W. Kim, K.K. Ahn, J.S. Lee, J. Appl. Phys.

113 (2013).[43] F.F. Wang, C.C. Jin, Q.R. Yao, W.Z. Shi, J. Appl. Phys. 114 (2013).[44] J. Kling, X.L. Tan, W. Jo, H.J. Kleebe, H. Fuess, J. Rodel, J. Am. Ceram. Soc. 93

(2010) 2452–2455.[45] X. Tan, E. Aulbach, W. Jo, T. Granzow, J. Kling, M. Marsilius, H.J. Kleebe, J. Rodel,

J. Appl. Phys. 106 (2009).