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Enhanced electric field-induced strain and ferroelectric behaviorof (Bi0.5Na0.5)TiO3–BaTiO3–SrZrO3 lead-free ceramics

Adnan Maqboola, Ali Hussaina, Jamil Ur Rahmana, Tae Kwon Songa, Won-Jeong Kimb,Jehyun Leea, Myong-Ho Kima,n

aEngineering Research Center for Integrated Mechatronics Materials and Components, Changwon National University, Gyeongnam 641-773, Republic of KoreabDepartment of Physics, Changwon National University, Gyeongnam 641-773, Republic of Korea

Received 7 March 2014; received in revised form 3 April 2014; accepted 4 April 2014

Abstract

The crystal structure, electric field-induced strain (EFIS) and piezoelectric properties of lead-free SrZrO3-modified Bi0.5Na0.5TiO3–

0.065BaTiO3 (BNBT–SZ100x, with x¼0–10) ceramics were investigated. The X-ray diffraction analysis revealed a phase transformation fromtetragonal to pseudocubic symmetry with increasing amounts of SZ. A large EFIS of 0.39% was obtained at the critical composition of BNBT–SZ2 which corresponds to a normalized strain (Smax/Emax) of 722 pm/V at a low applied field of �5.5 kV/mm. Ferroelectric curves indicated adisruption of ferroelectric order and a decrease in the remnant polarization and coercive field. A maximum value of piezoelectric constant(197 pC/N) and electromechanical coupling coefficient (29.4%) was obtained for SZ1 ceramics. The maximum dielectric constant temperature(Tm) and depolarization temperature (Td) shifted towards lower temperatures and curves became more diffuse with increasing amounts of SZ. Theresults indicate that BNBT–SZ100x ceramics can be promising candidates for lead-free actuators.& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: C. Ferroelectric properties; Lead-free; Relaxor behavior; Electric field-induced strain

1. Introduction

Piezoelectric materials are commonly used as actuators andsensors in various micro-electromechanical devices rangingfrom mobile phones and inkjet printers to high-precisiondevices such as medical ultrasonicators and automotive fuelinjection systems. Lead-free perovskite materials such asBi0.5Na0.5TiO3 (BNT) have been suggested as key lead-freematerials for ferroelectric, piezoelectric and dielectric applica-tions. At room temperature, BNT has ABO3 perovskitestructure with a ferroelectric rhombohedral symmetry (R3C),similar to the structure of PZT [1,2]. It shows two phasetransition temperatures transforming first from rhombohedral–tetragonal phase (TR–T) around 300 1C and then it transformsform tetragonal–cubic phase (TT–C) around 540 1C [3]. Toenhance the piezoelectric properties of BNT ceramic, it has

10.1016/j.ceramint.2014.04.02614 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author. Tel.: þ82 55 213 3719; fax: þ82 55 262 6486.ss: [email protected] (M.-H. Kim).

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been modified with other perovskite materials such as BaTiO3

(BT) [4–8], SrTiO3 (ST) [9–12], BaZrO3 (BZ) [13], BaSrTiO3

(BST) [14,15], Bi0.5K0.5TiO3 (BKT) [16,17] and K0.5Na0.5NbO3 (KNN) [18].It is well known that morphotropic phase boundary (MPB)

compositions of rhombohedral BNT with tetragonal BaTiO3

(BT) [4–8] and Bi0.5K0.5TiO3 (BKT) [16,17] possess betterelectromechanical properties. A solid solution of BNT and BT(0.935BNT–0.065BT) ceramics revealed improved propertieswith a piezoelectric constant (d33) of 146 pC/N at roomtemperature [7]. Based on the MPB composition, Bi0.5Na0.5-TiO3–BaTiO3 (BNBT) has been further modified to trigger alarge electric field-induced strain (EFIS) response. Suchmodified compounds include Bi0.5K0.5TiO3 (BKT) [19,20],K0.5Na0.5NbO3 (KNN) [21,22], SrTiO3 (ST) [23], BaZrO3

(BZ) [24] and BiAlO3 (BA) [25,26]. Recently, in ST-modifiedBNBT ternary system, Wang et al. [23] reported the largenormalized strain response (Smax/Emax¼490 pm/V) at4 kV/mm. It was suggested that the large normalized strain

strain and ferroelectric behavior of (Bi0.5Na0.5)TiO3–BaTiO3–SrZrO3 lead-.2014.04.026

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response was due to the transformation of a ferroelectric long-range-ordered structure to a short-range coherence structure.Moreover, (Ba1�xSrx)TiO3-modified BNT ceramics [14,15]showed enhanced piezoelectric properties and revealed thephase transformation from rhombohedral to tetragonal. By theaddition of Sr2þ , the Curie temperature (Tm) of BNBTceramics shifted towards room temperature, with higherdielectric constant (εr) and lower dielectric loss (tan δ).

Significant enhancement in the EFIS of BNBT–KNNceramics developed by Zhang et al. [21,22] was attributed tothe phase transition from ferroelectric (FE) to anti-ferroelectric(AFE) phases. It was proposed that the structural transition andthe switching of domains resulted in the large EFIS response.Interestingly, by investigating both the longitudinal andtransverse strain response in the BNT–BT–KNN, Jo et al.[27] demonstrated that the giant EFIS was due to decrease inthe remnant strain and it was associated with the presence of anintrinsic nonpolar phase at the zero electric field. This explainsthe mechanism where a large EFIS can be achieved bystructural modification to induce a distortion in the long-range ferroelectric state while sacrificing the piezoelectricconstant. By studying the in-situ XRD and Raman scatteringof BNT–Fe system, Aksel et al. [28] associated the large EFISresponse with the nucleation of nanodomains that was the mainreason for the distortion of long-range ferroelectric order ratherthan long-range phase transition. Further, Schutz et al. [29]attributed this behavior to the disruption of hybrid Bi–O bondstudied using temperature-dependent Raman scattering. InZr-modified BNKT ceramics [30], a large EFIS response wasattributed to the coexistence of ferroelectric and nonpolarphases. More recently, Ullah et al. [31] reported the cause ofstructural distortion under high fields and strain response bythe electric field-dependent X-ray analysis in Nb-dopedBNKT–BST system. They examined the phase transformationat zero field from pseudocubic to a mix of rhombohedral–tetragonal phases under an applied electric field of 40 kV/cmand the system came back to its original state upon removal ofthe electric field.

To design new lead-free ceramics for the possible applica-tions in actuator devices, SrZrO3 (SZ) is considered to enhancethe electromechanical properties of BNT–0.065BT. SZbelongs to the perovskite family and it has an orthorhombicstructure at room temperature with a Pbnm space group [32]. Itis expected that SZ modification in BNT–0.065BT ceramicscan cause structural distortion and enhance the electromecha-nical properties. In this work, we modified BNBT with SZusing a conventional solid-state reaction method and wesystematically studied the influence of compositional modifi-cation on the crystal structure, EFIS response, ferroelectric andpiezoelectric properties.

2. Experimental

A conventional mixed oxide method was utilized to prepare(1�x)(0.935Bi0.5Na0.5TiO3–0.065BaTiO3)–xSrZrO3 (BNBT–SZ100x with x¼0–0.10) ceramics. Commercially availablereagent grade oxide and carbonate powders of Bi2O3

Please cite this article as: A. Maqbool, et al., Enhanced electric field-inducedfree ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint

(99.90%), Na2CO3 (99.95%), TiO2 (99.90%), BaCO3 (99%),SrCO3 (99.90%) and ZrO2 (99.0%) (Sigma Aldrich Co.St. Louis, MO) were used as the starting raw materials. Priorto measuring the weights, the hygroscopic Na2CO3 powderwas dried in an oven at 100 1C for 24 h. For each composition,the starting materials were weighed according to the stoichio-metric formula and ball milled for 24 h in ethanol. The driedslurries were calcined at 850 1C for 2 h and then ball milledagain. After calcinations, the mixtures were ball milled for24 h and dried. The dried powders were pulverized, mixedwith an aqueous polyvinyl alcohol (PVA) solution as a binderfor granulation and passed through a 150 mesh sieve. Thegranulated powders were subsequently pressed into green diskswith diameter 10 mm at 150 MPa. The compacts were sinteredat 1160 1C for 2 h in a covered alumina crucible. To minimizethe evaporation of the volatile elements Bi and Na, the diskswere embedded in the powder of the same composition.Silver paste was coated on both faces of the sintered samples

and fired at 650 1C for 0.5 h to form electrodes. The specimensused to measure the piezoelectric properties were poled in asilicone oil bath with a dc field of 4 kV/mm for 15 min. All theelectrical measurements were performed after aging for at least24 h. The crystal structure of the sintered samples wascharacterized using X-ray diffractometry (XRD, X’pert MPD3040, Philips, Netherlands). For phase solubility and structuralevolution, a Micro Raman spectrophotometer (JASCO, JapanNRS-3300) was used. Surface morphology was checkedthrough scanning electron microscopy (SEM, JP/JSM 5200,Japan). The dielectric constant and loss of the specimens weremeasured using an automatic acquisition system with animpedance analyzer (Agilent HP4292A, USA) in the25–500 1C temperature range at different frequencies. Thepiezoelectric properties were measured using a Berlincourt d33meter (IACAS, ZJ-6B). The electromechanical coupling factor(kp) was measured using IEEE standard with an impedanceanalyzer (HP4194A). The dependence of the electric polariza-tion under an external electric field was measured in a siliconoil bath by using a precision material analyzer (RadiantTechnologies, Inc. Albuquerque, NM). EFIS was measuredusing a contact-type displacement sensor (Millitron,Model 140).

3. Results and discussion

3.1. Phase and microstructural analysis

The XRD patterns of sintered SZ-modified BNBT ceramicsamples are shown in Fig. 1. Within the detection limit of theXRD, patterns of all samples revealed a pure perovskite phaseindicating that SZ diffused into the lattice structure of theBNBT ceramics to form complete solid solutions. ExpandedXRD patterns in 2θ range of �39–481 revealed that pureBNBT composition exhibited typical characteristics of tetra-gonal symmetry evident by the splitting of (002)/(200) peaks at2θ of near 46.51, as shown in Fig. 1(a). However, increasingthe SZ content induced a phase transition from tetragonal topseudocubic symmetry and it became more prominent with the





Fig. 1. X-ray diffraction patterns of the BNBT–SZ100x ceramics in the 2θ range of (a) 20–701 and (b) 39–481.

Fig. 2. Raman spectrum of BNBT–SZ ceramics with different SZ contents.

A. Maqbool et al. / Ceramics International ] (]]]]) ]]]–]]] 3

increase in the SZ concentration i.e., above SZ5. As a result,peak splitting of (002)/(200) merged into a single peak (200)showing pseudocubic symmetry at SZ7. In addition, theoverall effect of SZ substitution on the XRD patterns of theBNBT ceramics was the slight shift of intensity peaks towardslower angles, and this peak shifting behavior increased withincreasing SZ concentration. This may have been due to thereplacement of small ions with the larger ionic radii Sr2þ

(1.44 Å) on A-site (i.e., Bi3þ¼1.36 Å and Naþ¼1.39 Å) andZr4þ (0.72 Å) on B-site (i.e., Ti3þ¼0.605 Å) in the BNBTceramics. Thus, a critical amount of Sr2þ and Zr4þ ionsinduced a phase transformation from tetragonal to pseudocubicsymmetry. Similar peak shifting behavior has also beenobserved in Zr [33] and BZ-modified [24] BNBT and BST-modified [14,15] BNT ceramics.

Raman scattering spectroscopy was used to further explorethe phase transition behavior on a short-range scale that couldreveal the local ionic configuration. As shown in Fig. 2, fourmain active modes were perceived in the BNBT–SZ ceramicsfrom 100–900 cm�1, which is consistent with other BNT-based ceramics [12,28,29]. First, a Raman-active mode (A1)observed at 146 cm�1 is related to the vibrations of an A-siteperovskite structure, which could be due to cations distortionor clusters of octahedral [BiO6] and [NaO6]. It was observedthat the plateau of the mode position shifted slightly towardsthe higher point due to A-site distortion by the [SrO12] clustersand the curves became steeper with higher SZ substitution[23,34]. The peak at 278 cm�1 is associated with the Ti–Ovibrations, the wavenumber range 450–700 cm�1 host modesare related to the TiO6 octahedra vibrations and the high-frequency range above 700 cm�1 is due to overlapping of A1

(longitudinal optical) and E (longitudinal optical) bands.Furthermore, the peaks around 540 cm�1 mode started broad-ening and shifted slightly to lower wavenumber at higherconcentration of SZ which may be due to photon behavior


owing to the structural evolution by the Zr4þ substitution intoTi4þ anion [35,36]. Increase in the peak intensity around610 cm�1 is also evident at SZ concentration from SZ1–SZ3,indicating the structural distortion induced by Zr4þ ion intoBNBT ceramics. These results are consistent with the XRDresults and are also reflected in the electrical properties ofBNBT–SZ ceramics.Fig. 3 presents the SEM micrographs of the sintered surfaces

of SZ-modified BNBT ceramics. All ceramics are wellsintered, grains are tightly bound and have uniform micro-structures. Pure BNBT ceramic has a characteristic rectangulargrain morphology with homogenous grain size distributionwhich is similar to the result of previous studies [7,24]. Theaddition of SZ had a small influence on the grain size of theBNBT ceramics. Overall, the grain size was increased withincreasing SZ concentration. An obvious change in grainmorphology from rectangular to round shape can be observed.





Fig. 3. SEM micrographs of BNBT–SZ ceramics with different SZ contents.

Fig. 4. (a) Effect of SZ-modification on the unipolar S–E loops of BNBT–SZceramics. (b) Characteristic values of maximum strain (Smax) and normalizedstrain (Smax/Emax) as a function of SZ content.


Careful analysis of the SEM images revealed that the sharpcorners of rectangular grains in SZ0 sample were transformedinto round edges by SZ substitution and they were finallyconverted into round shaped grains in the SZ4 sample. Similarchanges in grain profile have been observed in the Zr [33] andBiAlO3-modified [25,26] BNBT ceramics. Using a linearintercept method, the average grain size was found to increasefrom 2.52 mm for SZ0 to 4.3 mm for SZ4. In SZ-modifiedBNBT ceramics, ionic radii mismatch might have createddefects which increased the transport of the mass and energybetween the reactants [7]. In addition, melted compounds ofcarbonate and zirconate could also assist the grain growth toproduce round type grains in SZ4 and high concentrationceramics. Similar grain growth behavior has also beenobserved in BNBT-based studies [24,33].

3.2. Electrical properties

3.2.1. Electric field-induced strain responseEFIS response of piezoelectric ceramics is considered an

important property of the materials used in actuator applica-tions. To analyze the performance of the BNBT–SZ ceramics,the EFIS response under unipolar electric field loadingmeasured at an electric field of 6 kV/mm is depicted inFig. 4(a). The EFIS level significantly increased with increas-ing SZ content up to SZ2 and then it decreased. For pureBNBT ceramics, a maximum observed strain was 0.15%. Thehighest strain (Smax¼0.405%) was attained in the SZ2ceramics along with increased hysteresis. Further increase inSZ concentration reduced the strain level to 0.37% for SZ2.5with lesser hysteresis. However, the rest of the BNBT–SZcompositions displayed lower hysteresis behavior but a max-imum unipolar strain of less than 0.3%. The characteristicvalues of EFIS Smax and a normalized strain (Smax/Emax) ofBNBT ceramics as a function of SZ content are presented inFig. 4(b). An enhanced EFIS more than three times that of pure


BNBT and Smax/Emax of 676 pm/V was obtained for SZ2 thatis significantly higher value than those obtained in previousstudies. Table 1 compares the maximum achievable strain andcorresponding normalized strain values of BNBT–SZ2 with





Table 1Comparison of normalized strain (Smax/Emax) of various BNT-based ceramics.

Material E(kV/mm)

Smax

(%)Smax/Emax

(pm/V)Year References

BNBT–SZ 5.4 0.39 722 2014 Current work4 0.235 590

BNBT–ST 4 0.20 490 2012 [23]BNBT–AN 5 0.29 575 2014 [37]BNKT–SZ 6 0.37 617 2014 [38]BNKTN–BST 6 0.38 634 2014 [31]BNKT–ST 6 0.36 600 2012 [39]BNKT–Li,Ta 6 0.435 727 2012 [40]BNKT–BA 6 0.35 592 2010 [41]BNBT–BZ 7 0.38 542 2014 [24]BNKTN–LS 7 0.43 614 2013 [42]BNKT–Nb 7 0.45 641 2010 [43]BNKT–Zr 7 0.43 614 2010 [30]BNKT–KNN 8 0.46 575 2013 [44]BNBT–KNN 8 0.45 550 2008 [22]

Fig. 5. Unipolar strain of BNBT–SZ2 ceramics as a function of the appliedfield. The inset shows the corresponding values of Smax and Smax/Emax.

Fig. 6. (a) Field induced bipolar S–E loops of BNBT ceramics with differentSZ contents and (b) negative strain (Sneg), piezoelectric constant (d33) andelectromechanical coupling coefficient (kp) of BNBT ceramics as a function ofSZ content.


previously reported results for BNT based perovskite ceramics[22–24,30,31,37–44].

To evaluate the optimized EFIS response of the SZ2ceramics, it was examined under different electric fields. Alarge EFIS of 0.39% was perceived at a low applied field of5.4 kV/mm which corresponds to Smax/Emax of 722 pm/V.Even at low applied field of 4 kV/mm, SZ2 displayed a highstrain of 0.235% corresponding to Smax/Emax of 590 pm/Vwhich is higher than other BNT-based ceramics, as shown inFig. 5. Most of the BNT-based systems required a high drivingfield in order to achieve a high EFIS and Smax/Emax4600 pm/V. This large EFIS at low applied field indicates thatthe BNBT–SZ2 system is a potential candidate material forenvironmentally friendly electromechanical devices and actua-tor applications. The right combination of BNBT solid solutionas a base composition with a chemical modifier such as SZ isfavorable for the enhancement of actuating behavior aspresented by the Smax/Emax of 722 pm/V.

This enhancement in the EFIS and corresponding normal-ized strain is due to partial replacement of small ions with thelarger ionic radii Sr2þ on the A-site (for Bi3þ and Naþ ) andZr4þ on B-site (for Ti3þ ) in BNBT ceramics. The introductionof SZ in the host perovskite unit cell of BNBT ceramicsinduced the structural distortion by the formation of sufficientdefect dipoles in the initial ferroelectric (FE) phase. This led tothe phase transition of BNBT ceramics from FE (polar) torelaxor ferroelectric (RFE) phase. Furthermore, the RFE phasewas dominated at higher concentrations, SZ2 or above, whichdelayed the transformation from the RFE phase to the FEphase as evidenced by the substantial decrease in the polariza-tion response. The free energy of the FE phase was comparableto that of the RFE phase under zero field, such that it can beeasily induced by an external electric field and becomessaturated, as shown in Fig. 4(b). Similar to previous works[21–23,30], the results of this study suggest that high unipolarstrain is located only in a narrow region near SZ2 in whichboth FE and RFE phases coexist in the BNBT ceramic system.


Beyond this narrow region, either the FE or RFE phasedominates. Neither of the phases can solely deliver a strainas large as that measured from compositions (SZ2–SZ3) nearthe boundary between polar and RFE phases as suggested bythe S–E curves.For the analysis of the enhanced unipolar strain, the bipolar

EFIS response of BNBT–SZ ceramics was measured under anapplied electric field of 6 kV/mm. Fig. 6(a) shows theexemplary bipolar EFIS curves of SZ0, SZ1, SZ2, SZ2.5 andSZ4. The pure BNBT ceramic exhibited a butterfly shapedcurve typical of ferroelectric materials with a large negative






strain (Sneg) of 0.27% and Smax of 0.08%. However, the SZ-modified samples showed disrupted curves, i.e., a deviationfrom the butterfly-shape. By the introduction of a smallamount of SZ, such as SZ1, the curve changed shape withthe increase in Smax value. Furthermore, above this criticalcomposition, a significant drop in Sneg was observed with thechange in typical FE order and significant enhancement inSmax. The decreasing trend for Sneg of SZ-modified BNTceramics is presented in Fig. 5(b). However, the large Smax of0.38% was observed for SZ2 during bipolar loading with asmall Sneg of 0.02%. The large unipolar strain in SZ2 indicatesthat the reduction in Sneg is interrelated with domain back-switching during bipolar cycling with the appearance of theweak relaxor phase [6,18]. Hence, the significant enhancementin bipolar strain at SZ2 can be attributed to the FE to RFEphase transition, which is supported by the polarization versuselectric field (P–E) hysteresis loops as shown in Fig. 7, wherethe relatively slim loops indicate the coexistence of FE andRFE phase [38]. Fig. 6(b) shows the piezoelectric constant(d33) and electromechanical coupling coefficient (kp) of theBNBT ceramics as a function of SZ content. The d33 parameterfirst increased up to SZ1 ceramics with a maximum value of197 pC/N. Further increase in SZ concentration resulted in asignificant reduction in d33. Similar trends in kp were observedwhich increased up to 29.4% for SZ1 ceramic and thendecreases. The observed trends of d33 and kp are in goodagreement with P–E hysteresis loops as shown in Fig. 6(a);similar behavior is also observed in other studies [25,26,38].The substantial increase in d33 at SZ1 is attributed to a largeremnant polarization (Pr), maximum polarization (Pm) and alower coercive field (Ec). This is because a lower Ec enablesthe ceramics to be more easily poled, whereas a large Pr andPm favors piezoelectricity.

3.2.2. Ferroelectric hysteresis behaviorTo further analyze the origin of large EFIS, the

compositional-dependent ferroelectric behavior of BNBT–SZceramics measured at 50 Hz is shown in Fig. 7. The pureBNBT ceramic showed a typical saturated FE behavior in theP–E loops with Pr of 28 mC/cm2 and Ec of 30 kV/mm. It can

Fig. 7. (a) Effect of SZ-modification on the P–E hysteresis loops and the insetshows the characteristic values of Pmax and Pr, their difference (Pmax�Pr), andEc as a function of SZ content.


be clearly seen in Fig. 7 that SZ exerts a significant influenceon the loop shape and polarization; similar behavior is alsoperceived in ST and Zr-modified BNBT ceramics [23,33]. AtSZ1, Ec radically decreased from 30 to 17.5 kV/cm whereas Pr

increased substantially from 28 to 30 mC/cm2. Further, at SZ2,Pr decreased from 30 to 13.5 mC/cm2. At higher concentra-tions, i.e., above SZ4, both Pr and Ec continuously decreased,which indicates that the material became electrostructive withno apparent switching. The characteristic values of Ec, Pr, andPm and their difference (Pm�Pr) can be seen in the inset ofFig. 7. At SZ1, Ec significantly decreased with increasing SZcontent which is characterized by a pinched-type P–E hyster-esis loop. At SZ2, a large difference between Pm and Pr

indicates the reversible non-1801 domain-switching responseleading to a large EFIS response which is similar to previousreports on BNT-based ceramics [24,38]. This indicates that theFE order is destabilized with the addition of SZ, leaving anonpolar phase at zero electric field that transforms reversiblyinto a FE phase by an external electric field [27]. In otherwords, the significant decrease in Pr and Ec along withconcurrent minor decreases in Pm suggests that the long-range FE order dominant in BNBT ceramics is disrupted andRFE type behavior becomes dominant with the addition of SZ[39,42]. Moreover, in this study, the observed compositionallyinduced FE to a RFE transformation from SZ0 to SZ4 werealso consistent with bipolar strain measurements.

3.2.3. Temperature-dependent ferroelectric behaviorTo better understand the temperature stability and ferro-

electric behavior of BNBT–SZ ceramics, the temperature-dependent polarization hysteresis loops are illustrated in detailin Fig. 8(a–e). For pure BNBT ceramics, Pr decreased slightly.However, a sharp reduction in Ec is observed with increasingtemperature. For SZ-modified ceramics, a sharp reduction in Pr

and Ec is observed with increasing temperature indicating thedisappearance of the FE order and appearance of a RFE phase.A detail response of the Pr in ferroelectric P–E loops ispresented in Fig. 8(f). In pure BNBT, a small pinched-typeloop is revealed around 150 1C indicating stable ferroelectricbehavior at elevated temperatures. The arrow represents thedepolarization temperature (Td) determined from thetemperature-dependent dielectric curves shown in Fig. 9. Incase of SZ1, a pinched-type character was also observed atelevated temperatures. However, the pinched-type character-istic can be easily seen around 55 1C for SZ1 samples. Withfurther increase in temperature around 100 1C, P–E loopsbecame very slim; still the pinched-type character was notcompletely lost. However, at elevated temperatures SZ2, SZ2.5and SZ4 samples behaved like linear dielectric materials. Thischange of the P–E loops is well consistent with thetemperature-dependent dielectric properties shown in Fig. 9.Similar results of temperature-dependent P–E loops have beenreported for other BNT-based ceramics [6,45]. Similarly, forBNBT–BLT and BNST–BLT, Lin et al. [46,47] suggested thecoexistence of polar and nonpolar phases between Td and Tm.Jo et al. [27] monitored the volume change in KNN-modifiedBNBT ceramics and proposed that the pinch-type character in





Fig. 8. (a–e) P–E hysteresis loops of BNBT–SZ ceramics with different SZ contents and (f) characteristic values of Pr as a function of temperature.


the P–E loops originated from the existence of a nonpolarphase. Therefore, it can be concluded that pinched-typeanomalies in P–E loops result from the electromechanicalinteraction between the polar and nonpolar regions that coexistin the ceramics at high temperatures.

3.2.4. Dielectric propertiesThe temperature-dependent dielectric constant (εr) and loss

(tan δ) of the poled BNBT–SZ ceramics was measured at 1, 10and 100 kHz, as presented in Fig. 9. Pure BNBT ceramicsrevealed two distinguishing dielectric anomalies. The firstinflection point is referred as the depolarization temperature(Td) which characterizes the reduction of ferroelectricity andthe latter peak is referred as the maximum dielectric constanttemperature (Tm), as specified in dielectric curves. For pureBNBT, Tm peak was observed around 260 1C with highest εrvalue of �7900 at 10 kHz and a slightly sharp Td curveappeared around 145 1C. However, with the substitution of SZ,


the transitions at both inflection points became increasinglydiffuse. First, Tm slightly increased and then decreased,indicating a compositionally induced FE to diffused phasetransition. For SZ1 ceramics, the highest εr value was obtainedaround �8100 at 10 kHz. Moreover, Td curves disappearedwith the addition of SZ. However, Td curves can be clearlyobserved in the loss curves. Similar to Tm curves, Td curvesalso shifted towards lower temperature with rise in SZ content.By the temperature variation in BNBT ceramics, it is expectedthat the substitution of SZ to BNBT would decrease thetemperature for FE to RFE phase (TF–R) transition whichwould make the FE phase unstable at room temperature. Bythe addition of 1% SZ, TF–R decreased significantly around55 1C as compared with pure BNBT. However, further SZsubstitution decreased TF–R to room temperature which isevidenced from the disappearance of the distinctive peak in theloss curves of SZ2 and above samples. The decline in TF–Rwith increasing SZ content confirms the fact that





Fig. 9. Temperature dependence of the dielectric constant and loss of BNBT–SZ ceramics with different SZ contents.


compositionally induced FE state becomes unstable near roomtemperature which is in agreement with the results of ST-modified BNKT [39] and BNBT [23] ceramics. Wang et al.[39] attributed TF–R as a critical temperature for the develop-ment of FE short-range order from long-range order. Similartransitional features can also be observed in the EFIS and P–Ehysteresis loops results as discussed in the Figs. 4 and 7,respectively. The dielectric constant perceived in BNBT–SZceramics decreased with increase in measuring frequencysimilar to other BNT-based RFE materials that show thebroadness and frequency dependence of dielectric peaks near100 1C [39,48,49]. Moreover, as suggested for the BZ-modified BNBT [24] and SZ-modified BNKT [38] ceramics,the peak broadening of εr at Tm is mainly due to compositionalalteration arising from the distortion of A- and B-sites of theperovskite unit cell. Similar behavior was observed in Zr-modified BNKT [30] and BNBT [33,50] ceramics, where Tddecreased with increasing Zr resulting in the stabilization of anonpolar phase.

4. Conclusions

The influence of the addition of SZ on the crystal structure,microstructure, electric field-induced strain, piezoelectric,ferroelectric and dielectric properties of BNBT ceramics wasinvestigated. The crystal structure of BNBT–xSZ showed aphase transformation from tetragonal to pseudocubic symme-try. SEM-analysis revealed an increase in the grain size with


increasing SZ content with an obvious change in grainmorphology from rectangular to round shaped grains. A largeEFIS of 0.39% was observed at the critical composition of SZ2which corresponds to the Smax/Emax of 722 pm/V at a lowapplied field of �5.5 kV/mm. The P–E hysteresis loopsindicated a disruption of FE order with the addition of SZ toBNBT ceramics with a reduction in the Pr and Ec at roomtemperature. A maximum value of Pr of 30 mC/cm2, d33 of197 pC/N and kp of 29.4% was obtained for SZ1 ceramics. Thetemperature-dependence of dielectric properties showed that Tdshifted towards lower temperatures and the degree of diffuse-ness of the phase transition around Td and Tm became moreobvious with the addition of SZ.

Acknowledgments

This work is supported by the Basic Research Programthrough the National Research Foundation of Korea (NRF)funded by the Ministry, Science and Technology (MEST)(2011-0030058).

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