Phase Transition, Electrical Properties, and Temperature-InsensitiveLarge Strain in BiAlO3-Modified Bi0.5(Na0.75K0.25)0.5TiO3 Lead-Free

Piezoelectric Ceramics

Aman Ullah,‡ Chang Won Ahn,‡ Ali Hussain,§ Sun Young Lee,‡ and Ill Won Kim†,‡

‡Department of Physics and Energy Harvest-Storage Research Center, University of Ulsan, Ulsan, 680-749, South Korea

§Faculty of Materials Science and Engineering, GIK Institute of Engineering Sciences and Technology, Topi, KP, Pakistan

Lead-free piezoelectric (1–x)(Bi0.5(Na0.75K0.25)0.5TiO3)-xBiAlO3

(BNKT25-xBA, x = 0–0.100) ceramics were synthesized using

a conventional solid-state reaction method. The effect of BA

addition into the BNKT25 ceramics was investigated by X-raydiffraction, dielectric and ferroelectric characterizations, and

electric field-induced strain. X-ray diffraction revealed a phase

transition from a tetragonal to a pseudocubic phase at

x = 0.050. As the BA content increased, the maximum dielec-tric constant as well as the depolarization temperature (Td)

decreased. The polarization and strain hysteresis loops indicate

that the addition of BA significantly disrupts the ferroelectricorder of the BNKT25 ceramics leading to a degradation of the

remanent polarization and coercive field. However, the destabi-

lization of the ferroelectric order is accompanied by a signifi-

cant enhancement in the unipolar strain which peaks atx = 0.025 with a value of ~0.29%, which corresponds to a nor-

malized strain, d*33 (=Smax/Emax) of 484 pm/V. It was

observed that the unipolar strain of 0.025xBA is fairly temper-

ature-insensitive up to 150°C, even at 130°C the d*33 is aslarge as ~415 pm/V.

I. Introduction

LEAD-BASED ferroelectric ceramics, such as Pb(Zr,Ti)O3

(PZT), are widely used as piezoelectric actuators, sen-sors, and transducers due to their excellent electromechanicalproperties.1,2 However, lead is considered to be toxic and itsuse is banned for many commercial applications. As such,lead-free piezoelectrics have become the focus of intenseinterest in both industry and academia.3,4

The ceramic Bi0.5Na0.5TiO3 (BNT) with a rhombohedralperovskite structure is considered to be a good lead-free piez-oceramic candidate due to its strong ferroelectric behavior atroom temperature.5 However, the poling of pure BNT is dif-ficult due to its large poling field and high conductivity.Furthermore, the piezoelectric properties of BNT are toopoor for practical applications. To improve the piezoelectricproperties, a number of BNT-based solid solutions such asBNT-BaTiO3 (BT),

6,7 BNT-SrTiO3 (ST),8,9 BNT-Bi0.5K0.5TiO3

(BKT),10�14 BNT-NaNbO3,15 BNT-KNbO3,

16 BiFeO3, andBiScO3

17,18 have been developed and extensively studied.Recently, a significant improvement in the electric field-

induced strain response of lead-free K0.5Na0.5NbO3-modifiedBi0.5Na0.5TiO3-BaTiO3 (BNT-BT-KNN) system has beenachieved by Zhang et al.19 even though, the emergence of

this large strain is accompanied by a significant reduction inits piezoelectric constant d33. They further showed that thematerial has good temperature stability and is highly promis-ing for actuator applications.20 Jo et al. proposed that theorigin of the giant strain response in this KNN-modifiedBNT-BT system is mostly a consequence of the disappear-ance of the remanent strain due to presence of a nonpolarphase at zero electric field.21 Daniels et al.22 recently per-formed electric field-dependent crystal structure investigationon a BNT-BT ceramic, using high energy X-ray diffraction.They reported that the large strain in BNT-BT-based systemis the consequence of a mutual contribution from an electricfield-induced phase transition from a pseudocubic to atetragonal phase as well as ferroelectric-domain contributionsfrom this field-induced ferroelectric phase. Very recently,Siefert et al.23 followed the same strategy of BNT-BT sys-tem,19,20 in which the 0.8BNT-0.2Bi0.5K0.5TiO3 system wasmodified with 0.97KNN-0.03BKT. The researcher found thatthe BNT-BKT-KNN revealed similar behavior to thatobserved in BNT-BT-KNN system, but more promising forpractical applications.

All these recent findings motivated the investigation of asystem that undergoes a similar type of field-induced phasetransition and will be attractive for actuator applicationsworking in a wide range of temperature. Among variousBNT-based solid solutions, the (1�x)BNT–xBKT (BNKT)solid solution is one of the most promising starting materialsdue to its excellent ferroelectric and piezoelectric propertiesnear the rhombohedral–tetragonal morphotropic phaseboundary (MPB) with 0.16 � x � 0.20.11�13 However, theMPB composition has a significant problem of very low depo-larization temperature (Td).

14,24,25 The piezoelectricity almostdisappears at temperatures above Td greatly limiting the pie-zoelectric working temperature range of the material.24,25 Onthe other hand, the tetragonal side Bi0.5(Na0.75K0.25)0.5TiO3

of the MPB compositions is a superior candidate material dueto its high depolarization temperature (Td) and high piezo-electric properties and thereby well-suited for the use of vari-ous electromechanical applications working in a wide rangeof temperature.14,24,25 In this study, Bi0.5(Na0.75K0.25)0.5TiO3

(BNKT25) ceramic, the composition of which lies on thetetragonal side of the MPB, was selected as a base material.

On the other hand, an increasing attention has been paid toBiMeO3 (where Me = Sc3+, In3+, Yb3+, Al3+, etc.) ceramicsdue to their relatively high remanent polarization and Curietemperature. Among them, BiAlO3 (BA) has recently garneredspecial attention due to its relatively cheap constituting ele-ments with the excellent ferroelectric properties.26 Theoreticalcalculation predicts that BA has a rhombohedral perovskitesymmetry, a large spontaneous polarization of about 76 lC/cm2, and a high Curie temperature of about ~527°C.26 Zylber-berg et al. synthesized BA and confirmed that it is, indeed,ferroelectric and has a Curie temperature (Tc) > 520°C.27

E. Suvaci—contributing editor

Manuscript No. 28710. Received October 01, 2010; approved March 29, 2011.†Author to whom correspondence should be addressed. e-mail: kimiw@mail.ulsan.

ac.kr

3915

J. Am. Ceram. Soc., 94 [11] 3915–3921 (2011)

DOI: 10.1111/j.1551-2916.2011.04595.x

© 2011 The American Ceramic Society

Journal

However, its poor thermal stability and consequent extremeconditions for synthesis has limited its use as its pure form inpractical applications.27 Therefore, attempts have been madein stabilizing BA by solid-solutioning with other stable perov-skite materials. For example, a solid solution with the ferro-electric BNT was shown to serve the purpose so effectively thata stable perovskite phase persists up to BA = ~8 mol%.28,29 Itturns out that the addition of BA into BNT decreases the tem-perature for the first dielectric anomaly, commonly referred toas Td, and thus enhances the overall electromechanical proper-ties. More importantly, it also causes Td to be so smeared anddiffused that the permittivity has a small temperature depen-dence. It follows that the addition of BA into BNKT25 whoseTd is already fairly high is expected to result in a synergic effectthat leads to a significant enhancement both in operationaltemperature range and in temperature stability. Herein wereport the dielectric, ferroelectric, and electromechanical prop-erties of BA-modified BNKT25 systems as a function of com-position and temperature.

II. Experimental Procedure

Ceramics of a stoichiometric composition, (1�x)Bi0.5(Na0.75K0.25)0.5TiO3-xBiAlO3 (x = 0–0.100), were synthesizedby a conventional solid-state reaction method using Bi2O3,TiO2, Al2O3 (99.9%, High Purity Chemicals, Saitama, Japan),Na2CO3 (99.9%, Cerac Specialty Inorganics, Milwaukee,WI, USA), and K2CO3 (� 99%, Sigma-Aldrich, St. Louis,MO, USA) as starting raw materials. Before weighing, thepowders were dried in an oven at 100°C for 12 h. Each startingmaterial was weighed according to the stoichiometric formulaand ball-milled for 24 h in ethanol with zirconia balls. Thedried slurries were calcined at 800°C for 2 h, and then ball-milled again for 24 h. The powders were pulverized, mixedwith an aqueous polyvinyl alcohol (PVA) solution, andpressed into green disks with diameters of 13 mm under a pres-sure of 100 MPa.

Sintering was carried out at 1150°C for 2 h in a coveredalumina crucible. To minimize the loss of highly volatile ele-ments such as Bi, Na, and K, the disks were embedded in apowder of the same composition. The crystal structures ofthe ceramics were characterized by an X-ray diffractometer(XRD; X’pert PRO MRD; PANalytical, the Netherlands).The silicon powders were used as an internal standard. Formicrostructural analysis, the as-sintered samples were thor-oughly polished and then thermally etched at 1100°C for 1 h.Finally, scanning electron microscopy (SEM; JSM-5610LV,JEOL Ltd., Tokyo, Japan) was employed to examine themicrostructure of the polished and thermally etched samples.The sintered disks were polished to measure their electricalproperties. Silver paste was electroded on both surfaces ofthe disk samples, and fired at 700°C for 30 min. The tempera-ture dependence of the dielectric properties was measuredusing an impedance analyzer (HP4192A, Agilent, SantaClara, CA, USA). The ferroelectric hysteresis loops weremeasured using a Sawyer–Tower circuit with the maximumelectric field of 60 kV/cm as a triangular waveform. The elec-tric field-induced strain was measured using a linear variabledifferential transducer (LVDT; MCH-331 & M401, Mitutoyo,Japan). The voltage was supplied using a high voltage ampli-fier (610E, TREK Inc., Medina, NY, USA) driven by a wave-form generator (33250A, Agilent, Santa Clara, CA, USA).Normalized strains (d*33 = Smax/Emax) were calculated fromthe ratio of the maximum strain to the maximum electric fieldfrom the unipolar strain curves.

III. Results and Discussion

The XRD patterns of the (1�x)BNKT25-xBA (0 � x� 0.100) ceramics with the 2h range of 20–60o are shown inFig. 1(a). A complete solid solution with a perovskite-struc-tured single phase is evident up to x � 0.050. However, a

second phase tends to develop as x > 0.050, featured by theextra reflections around 2h = 24–31o. This second phase isidentified as Bi2Al4O9 (PDF No. 74-1097), and is marked withan asterisk in the XRD pattern. As indicated by the chemicalformula of the identified second phase, the solubility limitidentified at x = 0.05–0.075 is the tolerance of the givenperovskite structure for Al. Detailed XRD scans for theBNKT25-xBA ceramics in the 2h range from 35 to 50o areshown in Fig. 1(b). In agreement with the previously reportedstudies,11,13,14 BNKT25 ceramic without the addition of BAhad a tetragonal symmetry as evidenced by the splitting of the(002)/(200) peaks at 2h = ~46o and the existence of a single(111) peak at 2h = ~40o. However, the tetragonal distortiongradually diminished with increasing BA content, and the split(002)/(200) peaks of the tetragonal phase finally merged into asingle (200) peak at x = 0.050, suggesting that the crystal struc-ture of the BNKT25-xBA ceramics evolves from the tetragonalto a pseudocubic symmetry.

This phase transformation can be explained using thetolerance factor proposed by Goldschmidt et al.30 The toler-ance factor t was calculated using the formula,t ¼ ðrA þ rOÞ=

ffiffiffi

2p ðrB þ rOÞ; with Shannon’s ionic radii,31

where rA, rB, and rO are the radii of A, B, and O ions,respectively. The tolerance factor of BA (t = 1.010) is largerthan that of BNKT25 (t = 0.9880). Therefore, the incorpora-tion of BA into BNKT25 slightly increases the tolerance fac-tor (0.9883, 0.9885, 0.9891, 0.9896, and 0.9902 for x = 0.015,0.025, 0.050, 0.075, and 0.100, respectively) which corre-sponds to a decrease of the lattice distortion and tends tocause the structure to transition into the pseudocubic phase.

The effect of the BA content on the lattice parameters, aand c, estimated from the (002)/(200) peaks and the tetrago-nality (c/a) of the BNKT25-xBA ceramics are shown inFig. 2. It is evident that the lattice constant “c” and thetetragonality (c/a) significantly decrease with increasing BAcontent. However, the lattice constant “a” increases onlyslightly. The decreasing trend in the tetragonality clearly indi-cates that the addition of BA decreases the lattice distortionof the BNKT25 ceramic in a way that no obvious noncubicdistortion is visible when x reaches 0.050.

Figure 3 shows SEM micrographs of the BNKT25-xBAceramics with x = 0, 0.025, 0.050, and 0.075. The SEMobservations confirmed that all samples were dense withwell-developed microstructure, and cubic grain morphologies.The incorporation of BA into BNKT25 had no obviouseffect on the grain size of the BNKT25-xBA ceramics.

Temperature-dependent dielectric permittivity and loss ofthe poled samples of BNKT25-xBA ceramics measured atdifferent frequencies (1, 10, 50, and 100 kHz) are displayedin Fig. 4. In agreement with the reported studies on BNT-based ceramics,5,6,32 the dielectric curves of the BNKT25-xBA ceramics exhibited two dielectric anomalies: one ataround 100°C and the other at around 300°C within themeasuring temperature range. The former is often designatedas Td, where the ferroelectricity of the given system signifi-cantly decreases, while the latter called Tm, where the dielec-tric permittivity reaches its maximum. Several features arenoted. First, high dielectric permittivity of BNKT25 ceramicsis shown to decrease with the addition of BA. It is seen thatthe dielectric maximum of ~6213 in BNKT25 decreasesalmost a half at x = 0.075. Second, the inflection point forthe temperature for the first dielectric anomaly decreasesfrom 115° to 82°C as BA increased from 0 to 0.075. Thedecrease in this temperature is considered to be closelyrelated to the depronounced noncubic distortion with theincorporation of BA into BNKT25 (Fig. 2) and a conse-quent destabilization of ferroelectric order. On the otherhand, the maximum dielectric constant temperature (Tm)increases from 256° to 278°C. The dielectric loss of theBNKT25-xBA ceramics increases to an extent with increas-ing BA content and eventually becomes excessively highwhen x � 0.050.The significant increase in the dielectric loss

3916 Journal of the American Ceramic Society—Ullah et al. Vol. 94, No. 11

at x = 0.075 is considered to be as a result of the presence ofsecond phase.

Figure 5 shows the room temperature polarization hystere-sis loops of the (1�x)BNKT25-xBA ceramics with x = 0,0.010, 0.015, 0.020, 0.025, 0.030, 0.035, and 0.075. The polar-ization hysteresis loop of the BNKT25-xBA ceramics withoutthe addition of BA exhibits typical ferroelectric behavior,having a large remanent polarization and maximum polariza-tion of 22 and 39 lC/cm2, respectively, and the coercive fieldof 20 kV/cm. As the BA content increases, the tetragonal dis-tortion gradually decreases, resulting in a slightly pinchedhysteresis loop. A significant decrease in Pr and Ec, but asmall decrease in Pm values is noted. However, at the highestBA content (x = 0.075) investigated, both Pr and Ec weredrastically decreased, indicating the material becomes electro-strictive without any apparent switching taking place. Thesignificant decrease in Pr and Ec along with the concurrentminor decrease in Pm implies that the long-range ferroelectric

order dominant in BNKT25 is disrupted with the addition ofBA possibly because of stabilization of a high temperature‘weak’ ferroelectric phase with a very small noncubic distor-tion33 or a ‘nonpolar’ phase.21,23

Similar results have been reported in (Bi0.5Na0.5)0.94Ba0.06ZryTi1�yO3,

34 Bi0.5(Na0.78K0.22)0.5(Ti1�xZrx)O3,35 (Bi0.92

Na0.92�xLix)0.5Ba0.06Sr0.02TiO3,36 (Bi0.94�xLaxNa0.94)0.5Ba0.06-

TiO3,32 and BNT–KNbO3 ceramics.37 The substitution of Zr4+

for Ti4+ in 0.94BNT–0.06BaTiO3 and Bi0.5(Na0.78K0.22)0.5-TiO3, Li+ for Na+ in BNT-BST, La3+ for Bi3+ in BNT-BT, and the introduction of KNbO3 to BNT decreases andsmears the inflection point for the first dielectric anomaly,leading to the appearance of apparently ‘nonpolar’ phase. Inaddition, Jo and his coworkers recently reported that a sub-stitution of (K0.5Na0.5)NbO3 for (Bi0.5Na0.5)TiO3 stabilizes a

Fig. 2. Lattice constant a, c, and tetragonality (c/a) as a function ofx in (1�x)BNKT25-xBA ceramics.

Fig. 3. SEM micrographs of (1�x)BNKT25-xBA ceramics sinteredat 1150 °C for 2 h (a) x = 0, (b) x = 0.025, (c) x = 0.050, and (d)x = 0.075.

Fig. 1. X-ray diffraction (XRD) patterns of (1�x)BNKT25-xBA ceramics (x = 0-0.100) in the 2h ranges from (a) 20–60o and (b) 35–50o, (*)denotes the Bi2Al4O9 second phase.

November 2011 Temperature‐Insensitive Large Strain in BNKT-BA 3917

‘nonpolar’ phase in (Bi0.5Na0.5)TiO3-BaTiO3-(K0.5Na0.5)NbO3,

21 and (Bi1/2Na1/2)TiO3-(Bi1/2K1/2)TiO3-(K0.5Na0.5)NbO3 ceramics.23

Figure 6 shows the bipolar electric field-induced straincurves of BNKT25-xBA ceramics measured at room temper-ature. BNKT25 without the addition of BA shows a typicalbutterfly-shaped strain hysteresis loop with the maximumstrain of 0.17% and a negative strain (which denotes thedifference between the zero field strain and the lowest strain

Fig. 4. Dielectric constants and loss of (1�x)BNKT25-xBA ceramics as a function of temperature and frequency: (a) x = 0, (b) x = 0.010, (c)x = 0.015, (d) x = 0.025, (e) x = 0.030, (f) x = 0.035, (g) x = 0.050, and (h) x = 0.075.

Fig. 5. P-E hysteresis loops of (1�x)BNKT25-xBA (x = 0, 0.010,0.015, 0.020, 0.025, 0.030, 0.035, and 0.075) ceramics.

Fig. 6. Bipolar S-E loops of (1�x)BNKT25-xBA (x = 0, 0.010,0.015, 0.020, 0.025, 0.030, 0.035, and 0.075) ceramics.

3918 Journal of the American Ceramic Society—Ullah et al. Vol. 94, No. 11

and is only visible in the bipolar cycle)38 of ~0.09%. On theother hand, as BA content is increased, the strain loops showa drastic deviation from typical ferroelectric behavior. This isevident from the absence of the negative strain that is closelyrelated to domain back-switching during bipolar cycles.Instead, a pronounced enhancement in the maximum strainresponse is observed. Maximum strains of 0.22%, 0.28%,0.28%, 0.29%, 0.27%, and 0.23% are observed forx = 0.010, 0.015, 0.020, 0.025, 0.030, and 0.035, respectively.Beyond this region, the maximum strain drastically decreases,whereas the negative strain nearly vanished.

Figure 7 shows the unipolar field-induced strain curves ofBNKT25-xBA ceramics measured at room temperature. Thestrain increased significantly with increasing BA content up

to x = 0.025 but decreased thereafter. The normalized strainsd*33 of BNKT25-xBA ceramics as a function of BA contentare depicted in Fig. 8. A large d*33 of 484 pm/V wasobtained for x = 0.025 at an applied electric field of 60 kV/cm. The values of d*33 for the compositions x = 0.015, 0.020,0.025, 0.030, and 0.035 were 439, 454, 484, 454, and 425 pm/V, respectively. These values were higher (>400 pm/V) in thestudied composition range, implying that a strain derivedunder an electric field arises from a common origin in thiscomposition range (0.015 � x � 0.035). The field-inducedstrain caused by the ferroelectric-‘nonpolar’ phase transitionwas larger (S = 0.29%, at x = 0.025) than that caused by fer-roelectric-domain switching alone (S = 0.17%, at x = 0).These results indicate that the enhanced unipolar strain withthe BA addition should be a consequence of a reversiblephase transition between a ‘nonpolar’ phase at zero field anda field-induced ferroelectric phase.21�23,33 In the meantime, itis interesting to note that, the large unipolar strain has afairly large compositional tolerance, which is advantageousfor industrial applications.

Figures 9(a) and (b) display the P-E hysteresis loops ofBNKT25-xBA ceramics with x = 0, and 0.025 measuredunder an electric field of 50 kV/cm at different temperatures.BNKT25-xBA ceramics, without the addition of BA, exhibita typical ferroelectric hysteresis loop at room temperature,

Fig. 7. Unipolar S-E loops of (1�x)BNKT25-xBA (x = 0, 0.010,0.015, 0.020, 0.025, 0.030, 0.035, and 0.075) ceramics.

Fig. 8. Normalized strain (d*33) as functions of x in (1�x)BNKT25-xBA ceramics.

Fig. 9. Temperature-dependent polarization P-E loops [(a) and (b)]and unipolar strain S-E loops [(c) and (d)]. (a) and (c) for BNKT25-xBA with x = 0 and (b) and (d) for composition x = 0.025.

Fig. 10. Summary of the normalized strain (d*33) of (1�x)BNKT25-xBA ceramics with x = 0, and 0.025 as function oftemperature.

November 2011 Temperature‐Insensitive Large Strain in BNKT-BA 3919

25°C [Figs. 5, and 9(a)]. As the temperature is increased to100°C, which roughly corresponds to the first inflection point(or peak) in the dielectric curve [Fig. 4(a)], a sharp reductionin remanent polarization Pr and coercive field Ec areobserved. As a result of which, the hysteresis loop becomespinched, suggesting the coexistence of ferroelectric and non-polar phases.21,23,32,35 Further increment in temperature to150°C results in further reduction in Pr and Ec, causing theloop to become slimmer. On the other hand, the compositionx = 0.025 shows the coexistence of ferroelectric and nonpolarphases at room temperature. This is evident from Figs. 5 and9(b). Moreover, it can also be seen from Fig. 9(b), thatabove 100°C, the material becomes purely electrostrictive,which again supports the idea that the origin of the largestrain is because of an electric field-induced phase transitionfrom a ‘nonpolar’ electrostrictive phase to a ferroelectric one.

To examine the temperature dependence of the normalizedstrain, the unipolar strains measured under an electric fieldof 60 kV/cm for the compositions x = 0, and 0.025 in thetemperature range from 25° to 150°C are shown in Figs. 9(c)and (d). In agreement with the reported studies of BNT-BT-KNN,20,21 and BNT-BKT-KNN ceramics,23 the strainresponse of BNKT25-xBA without the addition of BA showsa drastic change with temperature. As depicted in Fig. 9(c), asudden increase in the unipolar strain is observed at 100°C,whereas a sharp drop is noticed when the temperature wasfurther increased to 150°C. On the other hand, BNKT25-xBA with x = 0.025 shows distinctly different behavior asdepicted in Fig. 9(d). It is seen that up to ~90°C, littlechange is noted in the maximum unipolar strain response,whereas the hysteresis decreases significantly. Once thehysteresis vanishes, the strain level continuously decreases.Considering the fact that the degree of hysteresis reflects thatof the amount of induced ferroelectric order, it can be saidthat an electric field required to induce a ferroelectric phaseincreases with temperature, and the domain switching inthe induced ferroelectric phase has little contribution to theobserved large strain. This leads to a conclusion that thelarge strain in the studied materials comes from a mutualcontribution from the inherently large electrostrictive strainof the ‘nonpolar phase’ and an additional strain arising dur-ing the electric field-induced phase transition. The calculatednormalized strain d*33 for the composition x = 0, and 0.025at various temperatures is presented in Fig. 10. The d*33value of the composition x = 0.025 stays nearly constant at470 pm/V up to 90°C, and then decreases linearly.

IV. Conclusion

Lead-free BiAlO3-modified Bi0.5(Na0.75K0.25)0.5TiO3 piezo-electric ceramics were successfully fabricated by the conven-tional solid-state reaction method. Dielectric, ferroelectric,and piezoelectric properties were characterized in terms ofboth composition and temperature. It was found that theaddition of BA disrupts the ferroelectric order in theBNKT25 ceramics, inducing a ‘nonpolar’ phase that can beconverted into a ferroelectric one under the electric field. Theelectric field-induced large unipolar strain was realized over awide range of composition. Temperature dependence ofpolarization and strain hysteresis loops suggested that theorigin of the large strain is an inherently large electrostrictivestrain combined with an additional strain introduced duringelectric field-induced phase transition. As both the electro-strictive strain and phase-transition-induced strain have littletemperature dependence, the currently developed materialsshowed a highly temperature-insensitive large strain, which ishighly promising for actuator applications.

Acknowledgments

This work was financially supported by the Ministry of Education, Scienceand Technology (MEST), and the Korea Industrial Technology Foundation

(KOTEF) through the Human Resource Training Project for Regional Inno-vation. The authors also acknowledge the Priority Research Centers Programthrough the National Research Foundation of Korea (NRF) funded by theMinistry of Education, Science and Technology (2009-0093818). Aman Ullahis indebted to Prof. Jurgen Rodel (Technische Universitat Darmstadt,Germany) for the temperature-dependent strain measurements.

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November 2011 Temperature‐Insensitive Large Strain in BNKT-BA 3921