Temperature dependence of polarization and strainof bismuth-based ceramic composites

Soon-Jong Jeong & Dae-Su Lee & Min-Soo Kim & Seok-Myung Jang &

In-Sung Kim & Saleem Mohsin & Jae-Sung Song

Received: 10 February 2014 /Accepted: 25 June 2014# Springer Science+Business Media New York 2014

Abstract We investigated the temperature-dependent polariza-tion and strain of two Bismuth-based perovskite composites.Matrix material was chosen as 0.94Bi0.5(Na0.75K0.25)0.5TiO3-0.06BiAlO3 (BNKT-BA) which have a transition from relaxorto ferroelectric phase (TR-F). Seed materials were Bi0.5(Na0.8K0.2)0.5TiO3 (BNKT) and 0.985Bi0.5(Na0.8K0.2)0.5TiO3-0.015BiAlO3 (BNKTBA), which possessing different polariza-tion characteristics. Depending on the test temperature, thedifferent polarization and strain behaviors were observed be-tween the BNKT-BA/BNKT and BNKT-BA/BNKT BA com-posites. At T=25 °C, the two composites (BNKT-BA/BNKTand BNKT-BA/BNKTBA) exhibited double-like polari-zation loops and parabolic strain curves which involvean ergodic relaxor-to-normal ferroelectric phase transi-t ion with external electric field and a reverseferroelectric-to relaxor phase transition with removal ofthe field. At T=120 °C, two composites have slimpolarization and strain curves, which are almost the same asthose of the pure BNKT-BA. The high field-induced polari-zation and strain with respect to temperature for the compos-ites are related to the thermal stability of the ferroelectricseeds, and the nucleation and growth of ferroelectric domainin the relaxor matrix.

Keywords Lead-free piezoelectrics . Composite .

Temperature dependence . Polariation . Strain

1 Introduction

Piezoelectric/electrostrictive materials and actuators are wide-ly used in optics, astronomy, fluid control, and precisionmachining systems due to their high generative force, accuratedisplacement, and fast response to external electric fields 1.Practical materials with a large piezoelectric/electrostrictivestrain are mainly lead-based ferroelectric single crystals andceramics such as Pb(MgNb)O3–PbTiO3 and Pb(ZrNb)O3-PbTiO3 [1–3]. In Pb-based antiferroelectric ceramics such as(Pb,La,Zr)TiO3, high strain is a consequence of field-inducedtransition from an antiferroelectric to a ferroelectric phase and/or from a non-polar to a polar phase [4, 5]. The developmentof actuators based on ceramic materials that undergo such aphase transition under an electric field has attracted greatinterest in the past decade.

In view of the current demand for global environmentalprotection, lead-free materials are an important topic in studyof piezoelectric materials. Sodium-potassium-niobate((KNa)NbO3) [6, 7] and bismuth-sodium-tantalite((BiNa)TiO3) [8–14] systems are promising ceramic familiesthat might soon replace lead-based systems. One of bismuth-based system, (BiNa)TiO3-(BiK)TiO3-(KNa)NbO3 (BNT-BKT-KNN), has shown a large strain comparable to that ofPZT, which is attributed to the phase transition from therelaxor phase to the ferroelectric phase. The same strain be-haviors have been observed in many BNT-based solid solu-tions. In a real application, the phase transition materials havea drawback of requiring a high electric field to reach a largestrain (0.45 % at 80 kV/cm) [8]. To alleviate the drawback, weproposed the construction of the bismuth-based ceramic com-posites with tailored microstructures of two phase grains[14–16]. An inhomogeneous microstructure compromising

S.<J. Jeong (*) :D.<S. Lee :M.<S. Kim : S.<M. Jang : I.<S. Kim :S. Mohsin : J.<S. SongBattery Research Center, Korea Electrotechnology ResearchInstitute, 28-1 Sungju-Dong, Changwon 641-120, South Koreae-mail: sjjeong@keri.re.kr

J ElectroceramDOI 10.1007/s10832-014-9955-8

ferroelectric seeds and relaxor matrix was made. Withthis microstructure, Bi(NaK)TiO3-BiAlO3 (BNKT-BA), a Bi-based system, showed the saturated polarization and strain at30 % reduced electric field (60→40 kV/cm).

Considering the temperature dependence of the ferroelec-tric properties, the thermal stability of the BNT system wouldbe more important than that of lead-based system (PZT)because KNN-doped BNT-BT has large strain behaviors overa range of temperature not at a special temperature presumablydue to the coexistence of at least two type relaxor phases [13].Because of this obscure phase identification, the ferroelectricproperties [13, 17] have been studied. Some of the Bi-basedsolid solutions like BNT-BT-KNN, BNKT, BNKT-BA et al.have been reported to have less temperature–sensitive proper-ties which may be related to the possibility of the coexistenceregion of ergodic and non-ergodic relaxor phases [17–20].The ergodic relaxor phase is transformed into a normal ferro-electric phase at field application. Then, upon the removal ofthe field the relaxor phase is fully recovered. Meanwhile, thenon-ergodic phase is transformed into a ferroelectric phasewhen a field is applied. However, even when the field isremoved, the ferroelectric phase remains stable.Consequently, the two relaxor phases reveal different polari-zation and strain behaviors. The materials having ergodicrelaxor phase show parabolic shaped-bipolar strain anddouble-like polarization. Except for the first electric fieldloading, materials with non-ergodic relaxor phase havebutterfly shaped-bipolar strain and the typical ferroelec-tric polarization hysteresis. The temperature correspond-ing to the boundary between the ergodic and non-ergodic relaxor phases is identified as Td (depolingtemperature) or TR-F. Accordingly, the Td (or TR-F) canbe an important factor in determining the ferroelectricand strain properties of the materials with such a phasetransition induced by the external electric field. Thematerials with such a relaxor phase are expected toshow the different field-induced properties at differenttemperatures. Especially, the property of the relaxor/ferroelectric composites proposed to show strain by a relativelower field would be different, depending on whether the testtemperature is above or below Td (TR-F) of the relaxor matrixand the ferroelectric seed.

In this study, the temperature dependence of the ferroelec-tric properties and strain behaviors of two relaxor/ferroelectriccomposites was described to understand the contribution ofthe relaxor matrix and the ferroelectric seeds to the properties.Two relaxor/ferroelectric composites were prepared:0.94Bi0.5(Na0.75K0.25)0.5TiO3-0.06BiAlO3 (BNKT-BA)/B i 0 . 5 ( N a 0 . 8 K 0 . 2 ) 0 . 5 T i O 3 ( B N K T ) a n d0.94Bi0.5(Na0.75K0.25)0.5TiO3-0.06BiAlO3 (BNKT-BA)/0.985Bi0.5(Na0.8K0.2)0.5TiO3-0.015BiAlO3 (BNKTBA) com-posites (designated as matrix/seed). The properties of thosecomposites were measured at the temperatures ranging from

25 to 120°C, and analyzed in terms of the modeling of twocapacitor serial connection.

2 Experimental

Two phases composites consisted of the ferroelectricseed and therelaxor matrix. Two bismuth-based mate-rials were designed as the matrix materials: 0.94Bi0.5(Na0.75K0.25)0.5TiO3-0.06BiAlO3 (BNKT-BA). Bi0.5(Na0.8K0.2)0.5TiO3 (BNKT) and 0.985Bi0.5(Na0.8K0.2)0.5TiO3-0.015BiAlO3 (BNKTBA) were prepared as the seedmaterials.

Bi2O3, Na2CO3, K2CO3, TiO2, Al2O3 and Sr2CO3 wereused as the starting raw materials. These raw powders wereball-milled altogether for 24 h in ethanol with 12-mm diam-eter zirconia balls. After drying, the mixed powders werecalcined at 800°C. Ferroelectric BNKT and BNKTBA parti-cles with a 10 μm grain size were fabricated by a molten saltmethod [21]. Then, compacts were made by pressing thepowders. The volume ratio of ferroelectric seed to relaxormatrix was 0.2. Disks of 10-mm diameter and 1-mm thicknesswere prepared. The green compacts were sintered at 1150°Cfor 4 h.

Fired-on silver paste at 700°C was used as the electrode forthe measurement of electrical properties. These specimenswere poled in silicone oil by applying a dc field of over5 kV/mm at 60°C.

P-E hysteresis loops were measured using a modified Saw-Tawyer circuit at 60 Hz. Field-induced strains were measuredusing a contact-type displacement sensor at 0.1 Hz.Piezoelectric properties were measured by a resonance-antiresonance method with an impedance analyzer(HP4294A). The electromechanical coupling factor was mea-sured using an IEEE standard. A small-size chamber wasconstructed to control the test temperature ranging from −20to 120°C.

3 Results & discussion

3.1 XRD and microstructure

The X-ray diffraction (XRD) patterns of BNKT-BA withoutand with the ferroelectric BNKT and BNKTBA seeds areshown in Fig. 1. The XRD patterns of all the ceramics indi-cated pure perovskite structure without secondary phases.Figure 2 shows the scanning electron micrographs ofBNKT-BA without and with the ferroelectric BNKT andBNKTBA seeds. All specimens exhibited a mixture of smallrelaxor and large ferroelectric phase. The grain size of therelaxor matrix ranged 0.5~2 μm, and that of the ferroelectricseeds 5~20 μm.

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3.2 Seed and matrix materials

The thermal stability and transition temperatures of the seedand the matrix materials were investigated by measuring thetemperature dependence of the dielectric constant andpolarization-electric field curves, respectively.

Recently, bismuth-based perovskite oxides have been foundto possess more than two relaxor phases in crystallograpicaland dielectric aspect. According to previous literatures [17, 22],BNT-based ceramics show the large strain under an electricfield at or above the depolarization temperature (Td) or relaxor-to-ferroelectric transition point (TR-F), where long range ferro-electric domain breaks down into polar nanoregions (PNR). (Toavoid any confusion, hereafter, this point is named as relaxor-to-ferroelectric transition point TR-F.)

In temperatures near the TR-F, the 0.94BNT-0.06BT (BNT-BT) [22] exhibited the phase stability where a relaxor hasPNRs with different symmetries but competitive free energies.It was suggested that the relaxor-to- ferroelectric phase tran-sition takes place reversibly upon application and removal ofan electric field, resulting in large strain behavior which isvery temperature-sensitive. However, the KNN doped-BTshows a large strain behavior in the temperature range of aninterest not at a specific temperature. This presumably indi-cates that the ergodic and non-ergodic relaxors coexist in thesystem at the temperature range with an interest. Similarphenomena were also observed in the Bi(NaK)TiO3 systemwith some dopings [13, 17–20, 23–26]. Especially, in Sn-doped Bi(NaK)TiO3, Bi(NaK)TiO3-BiAlO3 and so on [17,19] the relatively stable temperature dependence of electricfield-induced strain is related to the nonergodicity in theergodic relaxor phase. The pure BA-doped BNKT, investigat-ed in this study, also showed similar temperature-dependentferroelectric properties.

Figure 3 shows the temperature dependence of the dielec-tric constant for the seed and the matrix specimens in the poledand the un-poled states. All specimens showed the shoulder inthe temperature range of 50~150°C.

The poled-BNKT has the sharp transition at 120°C, and theun-poled BNKT a broaden rise around this temperature. TheBNKTBA with the poling treatment has a small edge shapedcurve at 80°C, indicating that the BNKT and BNKTBA seedmaterials have a TR-F of 120 and 80°C, respectively.

Meanwhile, BNKT-BA matrix specimen has broad dielec-tric constant rises near 120°C and in poled and un-poled states,respectively, indicating presumably that there were some tem-perature ranges which supported the coexistence region of thenon-ergodic (or ferroelectric) and ergodic relaxor phases.

To determine the coexistence range of the two phases,polarization-electric field curves were measured at several tem-peratures as shown in Fig. 4. The pure BNKT showed thetypical ferroelectric polarization reversal up to 100°C.However, a pinch-type polarization was observed at 120°C,suggesting a phase transition. Therefore, the transition point(TR-F) of the BNKT can be defined as 120°C. For theBNKTBA specimen, the transition point lies between 80 and100°C.

Fig. 1 X-ray diffraction patterns of BNKT-BA, BNKT-BA/BNKT andBNKT-BA/BNKTBA

BNKT-BA/BNKTBABNKT-BA/BNKTBNKT-BA

50 m

Fig. 2 Scanning electron microscope images showing the microstructures of BNKT-BA, BNKT-BA/BNKT and BNKT-BA/BNKTBA

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On the other hand, in the BNKT-BA pinch-type polarizationcurves are present at all temperatures ranging up to 120°C,revealing that the ergodic and nonergodic relaxor phases

co-existed in the temperature from 25 to 120°C. However, thisspecimen showed a normal ferroelectric polarization behavior at−20°C Accordingly, BNKT-BA has a TR-F range of 20~100°C

Fig. 3 Plots of dielectric constant vs. temperature for un-poled and poled BNKT, BNKTBA and BNKT-BA

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lying at the boundary between the ergodic and non-ergodicrelaxor phases.

3.3 Small signal ferroelectric properties

Table 1 shows the electrical properties of the BNKT-BAceramics with the ferroelectric BNKT and BNKTBA seeds.The pure BNKT-BA has the low Pr (7.12 μC/cm2) and d33(17 pC/N). revealing the relaxor feature. Meanwhile, BNKT-BA/BNKT and BNKT-BA/BNKTBA have the moderatePr (15 and 13 μC/cm2) and d33 (13 and 12.1 pC/N), respec-tively, which indicate very weak ferroelectric and relaxorcharacteristics.

3.4 P-E & S-E curves

Figure 5 shows the polarization-electric field (P-E) loops of theBNKT-BA/BNKT and BNKT-BA/BNKTBA at 25, 100, 120and 140°C. Both specimens exhibited slightly pinched-typehysteresis loops with significant increases in remnant polariza-tion Pr and coercive field E, when comparedwith BNKT-BA. Ecand Pr were reduced with increasing temperature up to 140°C.

At 25°C, the BNKT-BA/BNKT and BNKT-BA/BNKTBAcomposites have larger polarization than the pure BNKT-BA.With respect to the un-saturated polarization curves, the po-larization values of the three specimens are the same untilE=20 kV/cm. At E>30 kV/cm, polarization increases in the

Fig. 4 Plots of polarization vs. electric field for BNKT, BNKTBA, BNKT-BAwith respect to temperature

Table 1 Ferroelectric and piezoelectric characteristics at room temperature

er Pr (μC/cm2) Ec (kV/cm) Pmax (μC/cm

2) d33 (pC/N) kp (%)

BNKT seed 1143 37.32 29.1 43.49 151 19.02

BNKTBA seed 1459 22.76 14.1 32.99 56 16.19

BNKT-BA 1534 7.12 8.4 31.17 17 13.63

BNKT-BA/BNKT 1483 14.97 13.0 39.89 58 15.11

BNKT-BA/BNKTBA 1530 13.00 12.1 38.90 30 14.89

er: Relative dielectric constant, Pr: Remnant polarization, Ec: Coercive field, Pmax: Maximum polarization (at 5 kV/mm, BNT:7 kV/mm), d33:Piezoelectric constant, kp: Electromechanical coupling factor

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order of BNKT-BA/BNKT>BNKT-BA/BNKTBA>BNKT-BA. This indicates that the ferroelectric seeds increase thepolarization of the composite, revealing the acceleration ofrelaxor-to ferroelectric phase transition with field >30 kV/cm.No contribution of the ferroelectric seeds to phase transitionwas observed at E<20 kV/cm.

At the test temperature 100°C (<TR-F (BNKT) and >TR-F(BNKTBA)), different polarization reversals were observed.The polarization curve of the BNKT-BA/BNKTBA compositeis very similar to that of pure BNKT-BA. The polarization ofthe BNKT-BA/BNKT composite is slightly larger than that ofpure BNKT-BA at E>20 kV/cm. These similar behaviors werealso observed at 120°C (~TR-F (BNKT) and >TR-F

(BNKTBA)) and 140°C (>TR-F (BNKT) and TR-F

(BNKTBA)), although the polarization difference betweenBNKT-BA/BNKT and pure BNKT-BA is reduced.

Because the TR-F of BNKTBA seed is 100°C (ergodicrelaxor phase becomes stable with disappearance of longrange ordering ferroelectric characteristics), the ferroelec-tric instability of the BNKTBA seed at 100°C impedes theseed from activating the relaxor-to-ferroelectric phasetransition. As a result, the polarization curves of BNKT-BA and BNKT-BA/BNKTBA a temperatures over 100°Cbecome almost identical.

Accordingly, it can be suggested that the nucleation of theferroelectric phase from the relaxor phase in the matrix was acontrolling factor for the abrupt change in the polarization andstrain behaviors of the BNKT-BA/BNKT and BNKT-BA/BNKT composites.

These were ascertained by measuring the maximum polar-ization value of each specimen with 50 kV/cm. Figure 6shows the polarization values of the specimens attained with50 kV/cm as a function of temperature. The polarizationvalues of pure matrix and seeds were also included. The

BNKT-BA/BNKT and BNKT seeds show the decreasing po-larization with increasing temperature, while the pure BNKT-BA has, to a measureable extent, constant polarization. TheBNKT-BA/BNKTBA and BNKTBA show similar relationsof polarization to temperature.

This difference of temperature-dependent polarizationbetween BNKT-BA/BNKT may be related to the degreesof contribution of the matrix and the seed materials in therelaxor-to-ferroelectric phase transition. In the case ofBNKT-BA/BNKT, the role of the BNKT seed to the phasetransition formed in the matrix seems to be dominantunder an electric field.

The strain versus electric field (S-E) curves were similar tothe P-E curves. Figure 7 shows the bipolar strain as a functionof the electric field for the composites. Analogous to thebehavior of polarization with respect to electric field, the strainloop changes to a thin shape with increasing temperature. PureBNKT-BA has, to a measureable extent, almost the samemaximum strain with 50 kV/cm in the temperature range of25~120°C. Meanwhile, the BNKT-BA/BNKT and BNKT-BA/BNKTBA composites exhibited the decreasing strainwith increasing temperature.

3.5 Modeling and P-S relation

The temperature-dependence of the polarization and strainbehaviors of the relaxor/ferroelectric composite varies withthe properties of the seed and matrix materials. The degree ofcontribution of each phase to phase transition, resulting insharp change in the polarization and strain behaviors, wasaccessed by comparing the measurements with the calcula-tions based on a model of two capacitors connected in seriesbased on a polarization model proposed in previous studies[27–29]. Using the model proposed by Miller and et al., the

Fig. 5 Plots of polarization vs. electric field ld for BNKT-BA/BNKT and BNKT-BA/BNKTBA at various temperatures

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polarization-electric field loop for a composite was calculatedand then plotted in Fig. 8.

For the BNKT-BA/BNKT and BNKT-BA/BNKTBA com-posites tested at 25°C (Fig. 8a and b), at E<20 kV/cm thecalculated polarization values (broken blue lines) are in fairagreement with the measured values (red lines). At E>30 kV/cm, the measured polarization values deviate from the calculatedones. This polarization difference meant that other factors mayhave been involved in high field-induced polarization, alongwithpolarization due to the serial connection of two different dielec-tric materials. That may be related to the nucleation of ferroelec-tric domains by the ferroelectric seed and the growth of thesedomains in the relaxor matrix. The existing ferroelectric seed ishighly polar, so it can quickly form a ferroelectric embryo inthe relaxor matrix during the relaxor-to-ferroelectricphase transition under a high electric field, as mentionedin our previous reports [13, 14]. The measured and calculatedpolarization behaviors at 80°C showed similarity to those at25°C, although the degree of polarization was less.

However, when the temperature is 120°C, which is TR-F ofBNKT, the calculated results are almost the same as the mea-sured results in whole range of electric field. This indicates that

the contribution of the ferroelectric BNKTseedwas trivial to thephase transition of matrix (BNKT-BA) due to the instability offerroelectric phase in the BNKT seed. This has been possiblebecause the ferroelectricity of the BNKT seed was no longerstable since its TR-F was about 120°C. A similar result was alsoobserved in the polarization behaviors of the BNKT-BA/BNKTBA composite tested at 25°C and 80°C. At120°C>TR-F of BKNTBA (100°C), however, the polarizationbehaviors of BNKT-BA/BNKTBA and pure BNKT-BA be-come identical.

In addition, to understand the contribution of each phase toits polarization and strain, the relation of strain S vs. polariza-tion P was investigated. The results at 25 and 120°C areshown in Fig. 9. Regardless of the test temperature, the strainshows a parabolic relation to the polarization, implying thatthe electrostriction coefficient is constant from the followingelectrostriction equation.

S ¼ Q33P2; ð1Þ

where Q33 is the electrostriction coefficient.The P-S relation of pure BNKT-BA is identical to those of

BNKT-BA/BNKT and BNKT-BA/BNKTBA composites.

Fig. 6 Plots of saturated polarization attained at 50 kV/cm with respect to temperature for BNKTand BNKTBA seeds, BNKT-BA matrix, and BNKT-BA/BNKT and BNKT-BA/BNKTBA composites

Fig. 7 Plots of strain vs. electric field for BNKT-BA matrix, BNKT-BA/BNKT, BNKT-BA/BNKTBA composites at various temperatures

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That is, the Q33 of the composites are the same as the Q33 ofpure BNKT-BA, revealing that the strain behaviors of thecomposite are solely related to the phase transition and itsresultant polarization in the BNKT-BA matrix.

Based on the comparison of models which was created withmeasurement and the polarization-strain relations of the com-posites described above, the role of the ferroelectric seeds wasfound to be related to a transition from the short range orderingto long range ferroelectric ordering in the matrix. The existingferroelectric seed is highly polar, so it would be served as apreferential nucleation site for the ferroelectric domain in therelaxor matrix during relaxor-to-ferroelectric phase transition

under electric field. Ferroelectric domains (phase) are expectedto nucleate and grow in a region of the matrix, near largeferroelectric seeds.

4 Conclusion

The temperature-dependent polarization and strain propertiesof two Bismuth-based perovskite composites were studied. InBNKT-BA/BNKTand BNKT-BA/BNKTBA composites, po-larization and strain decreased with increasing temperature upto 120°C due to ferroelectric BNKT and BNKTBA seeds.

Fig. 8 Measured and calculated polarization(P)-electric field(E) curves for (a) BNKT-BA/BNKT and (b) BNKT-BA/BNKTBA. For comparison, thedata of BNKT-BA are also included

Fig. 9 Plots of strain vs.polarization for BNKT-BAmatrix, BNKT-BA/BNKT,BNKT-BA/BNKTBAcomposites at 25 and120°C

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BNKT-BA matrix has temperature-insensitive saturated po-larization and strain. The thermal stability of properties of theferroelectric seeds appeared to affect the temperature depen-dence of phase transition in the matrix. Especially, at temper-atures above TR-F of the ferroelectric seeds, the compositesexhibited the polarization and strain similar to those of thepure BNKT-BA matrix. The composites were affected by thethermal stability of the ferroelectric seeds when phase transi-tion occurred over the test temperature range.

References

1. K. Uchino, Ferroelectric devices, 2nd edn. (CRC Press, BocaRaton, 2009)

2. S.E. Park, T.R. Shrout, J. Appl. Phys. 82, 1804 (1997)3. T.R. Shrout, S.J. Zhang, J. Electroceram. 19, 113 (2007)4. S.E. Park, M.J. Pan, K. Markowski, S. Yoshikawa, L.E. Cross, J.

Appl. Phys. 82, 1798 (1997)5. C. Heremans, H.L. Tuller, J. Appl. Phys. 87, 1458 (2000)6. Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T.

Nagaya, M. Nakamura, Nature 432, 84 (2004)7. K. Wang, F.-Z. Yao, W. Jo, D. Gobeljic, V.V. Shvartsman, D.C.

Lupascu, J.-F. Li, J. Rödel, Adv. Funct. Mater. 23, 4079 (2013)8. T. Takenaka, H. Nagata, J. Eur. Ceram. Soc. 25, 2693 (2005)9. S.T. Zhang, A.B.. Kounga, E. Aulbach, H. Ehrenberg, J. Rödel, Appl.

Phys. Lett. 91, 112906 (2007)10. W. Jo, S. Schaab, E. Sapper, L.A. Schmitt, H.J. Kleebe, J. Appl. Phys.

110, 074106 (2011)11. W. Jo, T. Granzow, E. Aulbach, J. Rödel, D. Damjanovic, J. Appl.

Phys. 105, 094102 (2009)

12. Y. Isikawa, Y. Akiyama, T. Hayashi, Jpn. J. Appl. Phys. 48, 09KD03(2009)

13. S.T. Zhang, A.B.. Kounga, E. Aulbach, W. Jo, T. Granzow, H.Ehrenberg, J. Rödel, J. Appl. Phys. 103, 034108 (2008)

14. D.S. Lee, D.H. Lim, M.S. Kim, K.H. Kim, S.J. Jeong, Appl. Phys.Lett. 99, 062906 (2011)

15. D.S. Lee, S.J. Jeong,M.S. Kim, J.H. Koh, J. Appl. Phys. 112, 124109(2012)

16. D.S. Lee, S.J. Jeong, M.S. Kim, K.H. Kim, Jpn. J. Appl. Phys. 52,021801 (2013)

17. B. Wylie-van Eerd, D. Damjanovic, N. Klein, N. Setter, J. Trodahl,Phys. Rev. B 82, 104112 (2010)

18. H.S. Han, W. Jo, J. Rodel, I.K. Hong, W.P. Tai, J.S. Lee, J. Phys.Condens. Matter 24, 265901 (2012)

19. H.S. Han, W. Jo, J.K. Kang, C.W. Ahn, I.W. Kim, K.K. Ahn, J.S.Lee, J. Appl. Phys. 113, 154102 (2012)

20. A. Ullah, C.W. Ahn, A. Hussain, S.Y. Lee, I.W. Kim, J. Am. Ceram.Soc. 94(11), 3915 (2011)

21. K.Watari, B. Brahmaroutu, G.L. Messing, S. Trolier-Mckinstry, S.C.Cheng, J. Mater. Res. 15, 846 (2000)

22. W. Jo, S. Schaab, E. Sapper, L.A. Schmitt, H.J. Kleebe, A.J. Bell, J.Rodel, J. Appl. Phys. 98, 152901 (2011)

23. A. Hussain, C.W. Ahn, U. Aman, J.S. Lee, I.W. Kim, Jpn. J. Appl.Phys. 49, 041504 (2010)

24. U. Aman, S.Y. Lee, H.J. Lee, I.W. Kim, J. Korean Phys. Soc. 57(4),1102 (2010)

25. A. Hussain, C.W. Ahn, J.S. Lee, U. Aman, I.W. Kim, SensorsActuators A 158, 84 (2010)

26. K. Pham, A. Hussain, C.W. Ahn, I.W. Kim, S.J. Jeong, J.S. Lee,Mater. Lett. 64, 2219 (2010)

27. S.L. Miller, R.D. Nasby, J.R. Schwank, M.S. Rodgers, P.V.Dressendorfer, J. Appl. Phys. 68, 6463 (1990)

28. D.E. Dausch, E. Fuman, F. Wang, G.H. Haertling, Ferroelectrics 177,221 (1996)

29. Z.G. Ban, S.P. Alpay, J.V. Mantese, Phys. Rev. B 67, 184104(2003)

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