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Enhancing Electromechanical Properties of Lead-Free Ferroelectrics With Bilayer Ceramic/Ceramic Composites Azatuhi Ayrikyan, Virginia Rojas, Leopoldo Molina-Luna, Matias Acosta, Jurij Koruza, and Kyle G. Webber Abstract—The macroscopic electromechanical behavior of lead-free bilayer composites was characterized at room temperature. One layer consisted of a nonergodic relaxor, (Bi 1/2Na 1/2)TiO 3–7BaTiO 3, with an electric-field-induced longrange ferroelectric order, whereas the other is understood to be an ergodic relaxor [(Bi 1/2Na 1/2)TiO 3–25SrTiO 3 ] that undergoes a reversible electric-field-induced macroscopic nonpolar-to-polar transition. Microstructural evidence of a bilayer with low diffusion between the two components is also demonstrated. By taking advantage of the different macroscopic strain– and polarization–electric-field responses of the two constituents, internal mechanical and electrical fields can be developed that enhance the unipolar strain over that expected by a rule of mixtures approximation, thereby improving the properties needed for application of such materials to actuator systems. It is possible through further tailoring of the volume fractions and macroscopic properties of the constituents to optimize the electromechanical properties of multilayer lead-free ferroelectrics.

I. Introduction

P

erovskite ferroelectrics are materials that possess a spontaneous polarization of the unit cell, which can be switched between at least two energetically equivalent variant states through the application of an electrical field. After electrical poling, ferroelectric materials also show a piezoelectric response, whereby an applied electrical field induces mechanical strain and vice versa. As a result of these unique properties, ferroelectric materials have garnered strong interest for use in a variety of applications, including sensors, actuators, and transducers. The most widely used ferroelectric and piezoelectric material in applications for many years has been polycrystalline lead zirconate titanate [Pb(Zr,Ti)O3, PZT]. Because of the toxicity of lead, there has been a significant research effort over the past 10 years to identify a lead-free alternative [1]. Although several lead-free material classes of interest have been identified that show comparable or even improved properties in specific areas [2]–[4], there remains no single Manuscript received August 4, 2014; accepted February 3, 2015. This work was supported by the Deutsche Forschungsgemeinschaft under WE4972/2. The work of J. Koruza was supported by the Deutsche Forschungsgemeinschaft under SFB595/D6. The authors are with the Institute of Materials Science, Technische Universität Darmstadt, Darmstadt, Germany (e-mail: ayrikyan@ ceramics.tu-darmstadt.de). DOI http://dx.doi.org/10.1109/TUFFC.2014.006673

material that can replace PZT over a broad range of applications [1], [5], [6]. Specifically for actuator applications, the primary challenge has been generating a high strain response at low electrical fields, while minimizing the hysteretic behavior, i.e., reduced self-heating [7]. Recently, ferroelectric materials based on (Bi1/2Na1/2)TiO3 (BNT) have been found to display exceptionally large unipolar strains [8]–[10], which are unfortunately accompanied by large poling fields and significant electrical hysteretic behavior [11], [12]. The origin of the large strain behavior is understood to be due to an electric-field-induced transition from a macroscopic nonpolar state to a macroscopic polar state with a ferroelectric long-range order [13]. In the unpoled state, the material is a nonergodic relaxor [14], comprised of polar nanoregions (PNRs) without long-range order, where the net macroscopic crystal structure appears to be pseudocubic [15], [16]. Upon the application of an electrical field, the PNRs coalesce and form stable ferroelectric domains, with a resulting large remanent strain upon removal of the field. Through destabilization of this ferroelectric order, either compositionally or thermally modulated [17]–[24], the observed remanent strain can be lost due to a reverse macroscopic polar → nonpolar (P → NP) transition during electrical unloading as well as formation of polar nanoregions [14], thereby increasing the usable unipolar strain important to actuator applications. Materials with a reversible macroscopic NP → P transition are in an ergodic relaxor state. In situ high-resolution transmission electron microscopy (TEM) studies have revealed the development and subsequent loss of ferroelectric domains during electrical loading in ergodic relaxor ferroelectrics [25], whereas in situ neutron and synchrotron diffraction studies have shown the evolution of the observable crystal phase from pseudo-cubic to tetragonal crystal structure as a function of applied electrical field [16] and increasing ambient temperature [26], [27]. BNT-based materials can have both ergodic and nonergodic relaxor behavior depending on their composition. (1 − x)(Bi1/2Na1/2)TiO3– xBaTiO3 is a widely known lead-free nonergodic relaxor [4], [22], [28]–[31], whereas lead-free systems showing ergodic relaxor ferroelectric behavior include (1 − x − y) (Bi1/2Na1/2)TiO3–xBaTiO3–y(K0.5Na0.5)NbO3 (BNT-BTKNN), (1 − x)Bi0.5(Na0.75K0.25)0.5TiO3–xBiAlO3 (BNKTBA), and (1 − x)(Bi1/2Na1/2)TiO3–xSrTiO3 (BNT-ST) [18], [22], [32]–[35].

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One method for reducing the required poling field while retaining the large unipolar strain, is the formation of ceramic/ceramic composite ferroelectrics [36], [37]. The concept of enhancing the electromechanical behavior of ferroelectrics with a multilayer composite structure has been previously implemented in PZT-based systems [36]–[38], where the macroscopic hysteretic behavior was adjusted by composition, dopant concentration, and the volume fractions of each constituent. It was found that combining a ferroelectric material with either an antiferroelectric or relaxor material in a multilayer composite structure resulted in a decrease of the required poling field and an increase in the saturation polarization. This was attributed to internal electrical fields, which were simulated with a series capacitor model [37]. These concepts, however, are not limited to the PZT system and can be directly applied to other material classes to improve their macroscopic electromechanical properties. The composite approach has also been applied to leadfree ferroelectric systems to reduce the poling fields while maintaining high unipolar strain. The first successful demonstration of this approach in lead-free materials was by Lee et al. in a composite system comprised of BNKTBA matrix with BNT as a nonergodic relaxor seed material [39]. Since then, lead-free composite systems have also been reported using BNT-BT-KNN as the ergodic relaxor matrix [40], [41]. Recently, Groh and colleagues have investigated composites of BNT-BT/BNT-BT-KNN [40]–[43]. In their work, the two end members were mixed together and sintered without a predetermined structure, such as multilayer configuration. The resulting mixed composites displayed a continual variation of the electrical constitutive behavior as a function of the constituent volume fractions. Interestingly, an optimum composition was found in which the unipolar strain was enhanced, despite a monotonic decrease in the required poling field with increasing BNT-7BT seed content. This is thought to be due to a heterogeneous charge distribution between the components due to the variation of the high field permittivity and resultant different macroscopic polarizationelectric field responses between the two constituents [42]. This was, similarly to previous work by Dausch et al. [36], explained to be due to polarization coupling between the constituents. It is, however, likely that strain coupling between the relaxor and ferroelectric components plays an important role as well [41]. Despite the promising results of ceramic/ceramic composites, the origin of the macroscopic behavior is not completely understood. This is partially due to the complex microstructure that results in various electrical and mechanical fields. In addition, it is not yet understood what role diffusion plays at the interface between the relaxor and ferroelectric material. For that reason, this investigation focuses on bilayer ceramic/ceramic compositions of an ergodic relaxor as the large strain material that undergoes a reversible macroscopic NP → P transition and a nonergodic relaxor with an electric-field-induced ferroelectric order. The bilayer configuration simplifies the microstruc-

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ture over a mixed ceramic/ceramic composite structure previously used with lead-free ferroelectrics. Macroscopic constitutive behavior has been characterized for each constituent as well as the bilayer samples and contrasted with microstructural and energy dispersive X-ray spectroscopy measurements. II. Experimental A. Sample Preparation (Bi1/2Na1/2)TiO3–xBaTiO3 (BNT-7BT, x = 7 mol%) and (Bi1/2Na1/2)TiO3–xSrTiO3 (BNT-25ST, x = 25 mol%) powder were produced by a solid oxide synthesis route. BNT-7BT is a nonergodic relaxor, whereas BNT-25ST is in an ergodic relaxor state at room temperature. The oxide or carbonates of the respective elements, namely, Bi2O3 (99.975% purity), BaCO3 (99.8% purity, Na2CO3 (99.5% purity), TiO2 (99.6% purity), and Nb2O5 (99.9% purity) (all Alfa Aesar GmbH & Co. KG, Karlsruhe, Germany), were mixed according to their stoichiometric formula and ball-milled in ethanol for 24 h. The dried slurries were calcined for 2 h at 700°C, followed by 3 h at 800°C. The calcined powder was ball-milled for 24 h. Details for the production of BNT-25ST were reported previously Acosta et al. [44]. Bilayers were formed by taking equal weight percentages of each powder, pressing the first layer uniaxially, placing the second powder layer on the first pressed layer and pressing again uniaxially. The samples were then isostatically pressed at 300 MPa for 1.5 min. BNT7BT, BNT-25ST, and bilayer samples were all sintered at 1150°C for 2 h. These sintering conditions were based on the investigation of Acosta et al. [44]. Previous work has shown that minimal grain size and porosity of BNT-BT can be achieved by keeping the sintering temperature at the lower range of 1100°C to 1200°C [40], [45]. B. Microstructural Investigation Microstructural investigations were performed on bilayer samples embedded in epoxy and sectioned vertically using a diamond wire saw; the samples were then ground and polished on the vertically cut face. They were thermally etched at 1100°C for 30 min and sputtered with a thin layer of carbon for examination by scanning electron microscopy (SEM). SEM images of the bilayer cross section and compositional analysis through the thickness were performed using a scanning electron microscope (XL 30 FEG; Philips, Eindhoven, The Netherlands) and energy dispersive X-ray spectroscopy (EDAX CDU Detector, eumeX Instrumentebau GmbH, Heidenrod, Germany) and a high-resolution scanning electron microscope (JSM-7600 FEG; JEOL, Toyko, Japan) also with energy dispersive X-ray spectroscopy (EDX) [80 mm2 silicon drift detector (SSD), Oxford Instruments plc, Oxfordshire, UK]. The EDX line scan in the SEM was performed with a dwell time of 2500 ms

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Fig. 1. (a) A scanning electron micrograph of the bilayer composite, where the white line indicates the EDX scannning axis and the arrow indicates the BNT-25ST/BNT-7BT interface. (b) The EDX scanning results for strontium content as a function of distance through the thickness.

per point, over 275 points across the sample. The EDX line scan in the HRSEM was performed with a dwell time of 3000 ms, over 10 points across the interface region. Both line scans were acquired using a beam energy of 30 kV. Raman spectra were measured using a LabRAM HR high-resolution microscope with a 633 nm wavelength laser and a 50× objective (model HR 800, Horiba Jobin Yvon, Villeneuve-d’Ascq, France). C. Electrical Measurements For measurement of electrical properties, the sintered samples were ground to a final surface finish of 15 μm and silver electrodes were burnt in at 400°C for 2 h. The polarization P(E) and strain S(E) hysteresis were measured by means of a Sawyer–Tower setup with an optical sensor (Philtec Inc., Annapolis, MD, USA) at 50 mHz with a maximum applied electrical field of 6 kV/mm orthogonal to the interface between the two layers. Care was taken to ensure that only thickness-direction strain was measured in the bilayer composites. Because of the asymmetric layup, a bending mode is present that may give incorrect displacement values. To avoid this error, the displacement of the sample was measured using only a point contact. Poling was performed by applying a sinusoidal electrical field at 50 mHz and a maximum electrical field of 6 kV/mm at room temperature. Temperature- and frequency-dependent relative permittivity ε r′ values were obtained on unpoled and poled (after 5 bipolar cycles at 50 mHz) samples at 10 kHz, 100 kHz, and 1 MHz with a measurement voltage of 1 V from room temperature up to 500°C during heating with a heating rate of 2°C/min. A precision LCR meter (4192A, Hewlett Packard Corp., Palo Alto, CA, USA) was used to perform the measurements.

III. Results A. Microstructure Microstructural analysis of the samples provides evidence of the formation of a bilayer structure. In Fig. 1(a), the SEM image of the bilayer used for EDX analysis is shown, where the white line shows the scan line and the arrow indicates the location of the BNT-7BT/BNT-25ST interface. The EDX line scan across the sample reveals a sharp drop in strontium content at approximately 1200 μm, indicating that half of the sample is composed of BNT-25ST, while the other half, lacking strontium, is BNT-7BT. To analyze the interface region, a similar scan was performed with a high-resolution scanning electron microscope. Here, 10 points were measured along a 22.5 μm distance (2.5 μm step size) across the interface. The intensity of the strontium peaks were monitored at each point and are plotted as a function of distance in Fig. 2(b) together with the electron backscatter (BSE) image [Fig. 2(a)], showing compositional contrast. Inset figures are also included for both materials showing the grain structure. It can be clearly observed from the measurements that the strontium content changes continually across the interface, indicating an interdiffusion layer on the order of approximately 20 μm. The overall impact of this layer on the macroscopic electromechanical behavior is, however, not currently understood. Also of note here is the presence of considerable porosity in the BNT-25ST layer, whereas the BNT-7BT layer showed little porosity development. This is likely due to the in-plane biaxial stresses developed during sintering by the variation in the sintering rates between the two layers, which can influence the sintering potential of each constituent [46]. Investigations by Jamin

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Fig. 2. (a) Electron backscatter image of the interface region in the BNT-7BT/BNT-25ST bilayer. (b) At ten points along the scanning line (white line), the intensities of strontium peaks were determined using EDX, showing the compositional variation.

et al. [47], for example, show the sintering behavior of alumina patterns deposited on sapphire substrates, where the biaxial tensile lateral sintering strain reduced the relative density and resulted in a local density gradient. In this configuration, densification occurs primarily through the thickness of the film. This indicates that the BNT-25ST is in a state of biaxial tension during the densification process, while the BNT-7BT is in biaxial compression. Additional studies are required, however, to elucidate the role of these internal stresses and the resulting porosity on the macroscopic behavior. The EDX analysis is corroborated by Raman spectroscopy, which was used to compare individual end members sintered separately with the two end members of the bilayer, shown in Fig. 3. The spectrum for BNT-7BT corresponds very well with the spectrum from the strontiumpoor region of the bilayer composite, shown in Fig. 3(a). This is particularly apparent by the splitting of the A1 mode involving the Ti–O bond at approximately 270 cm−1 and the BO6 modes centered at ~550 cm−1, which has

been shown by Foronda et al. to increase with the concentration of BT [48]. Similarly, the spectrum for as-sintered BNT-25ST corresponds well in terms of the positions of the peaks with the portion of the bilayer with higher strontium content. These results also agree with previous investigations on the room-temperature Raman spectra for various compositions of BNT-ST [49]. It was shown that with increasing ST content a peak broadening that eventually appears as overlapping peaks occurs at ~256 cm−1, and a shoulder occurred at ~560 cm−1, consistent with the findings of this work. B. Dielectric Properties The frequency-dependent permittivity was characterized on poled (Fig. 4, top row) and unpoled (Fig. 4, bottom row) samples from 25°C to 500°C, which revealed interesting changes in electrical properties with the bilayer structure (Fig. 2). The effect of poling on the permittivitytemperature behavior of BNT-BT has been investigated

Fig. 3. Raman spectra for BNT-7BT and BNT-25ST in the as-sintered state and in a bilayer composite structure.

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Fig. 4. Permittivity as a function of temperature and frequency for BNT-7BT, BNT-7BT/BNT-25ST bilayer composites, and BNT-25ST.

by numerous authors [30], [50], [51] and is understood to be due to an induced long-range ferroelectric order in nonergodic relaxors [26], [52]. Upon the increase of temperature there is a transformation from the induced macroscopic polar state into an ergodic relaxor state at the depolarization temperature (Td), which is close in temperature to a sharp increase in permittivity [50], [53]. This was found to be approximately 112°C for BNT-7BT. The BNT-25ST composition, however, does not show this trend, because the long-range order is already unstable near room temperature because of the ergodic relaxor state [44]. This results in only minor effects of poling on the permittivity–temperature response, i.e., the permittivity–temperature behavior of poled and unpoled samples is similar. The bilayer composite, however, shows the formation of a sharp increase of permittivity at approximately 112°C, consistent with the results of BNT-7BT, and the development of a macroscopic polar order. In addition, the transition is not as sharp in the bilayer as that observed in the BNT-7BT material, likely because of the influence of the BNT-25ST layer. It is apparent that the permittivity–temperature behavior of the bilayer is a composite response of the two constituents. The maximum relative permittivity of the composite ranged between 5300 and 5600 depending on frequency. In comparison, the maximum permittivity of pure BNT25ST was between 4800 and 5200, depending on the frequency. At the same frequency, the bilayer permittivity was approximately 10% higher than the pure BNT-25ST. The peak permittivity in pure BNT-25ST was observed at the temperature range of 188°C and 194°C. In the bilayer, this peak occurs between 218°C and 224°C. In BNT7BT, the maximum permittivity occurs at approximately 276°C, and has a value of approximately 6600. Therefore, the bilayer permittivity seems to fall between the two end members both in maximum permittivity and depolarization temperature

C. Large Signal Electrical Properties The polarization- and strain-electric field hysteretic behavior of each end member, i.e., BNT-25ST and BNT7BT, was characterized and compared with the bilayer composite under unipolar and bipolar electrical fields (Fig. 5). The characteristic parameters of each end member and the bilayer composite are presented in Table I. In the current measurements the unpoled curves were not recorded, meaning that the remanent strain is not available. For this reason, the reference point, i.e., the remanent strain, is taken as zero. The strain below this reference point is often referred to as negative strain, although in the current work, this region is labeled the minimum strain to avoid confusion. The poling field is defined as the electrical field at the inflection point in the S-E loop dur∗ ing poling from the unpoled state. d 33 is defined as the ratio of the maximum strain to the maximum applied electrical field, whereas the total strain is the difference between the maximum and minimum strain. It can be clearly observed that BNT-7BT displays typical ferroelectric constitutive behavior, with a corresponding small usable strain (0.16%) and a large minimum strain (−0.13%). This behavior has been previously observed in various BNT-BT compositions and has been explained by the formation of a metastable long-range order during poling from the unpoled state [31]. TEM investigations have shown the presence of a grainy contrast, consistent with the presence of polar nanoregions in the as-sintered state of BNT-BT [25], [30]. Under an applied electrical field, ferroelectric domains are formed because of an electricfield-induced phase transition from an initially macroscopic nonpolar phase into a macroscopically polar one [30]. This is also supported by in situ synchrotron diffractions studies that reveal a pseudo-cubic structure in the unpoled state and a transition to the tetragonal phase under the application of electrical field [54]. In contrast, BNT-

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Fig. 5. Bipolar (thin line) and unipolar (thick line) strain– and polarization–electric field behavior of BNT-7BT, BNT-7BT/BNT-25ST bilayer composites, and BNT-25ST.

25ST displays little minimum strain (−0.03%) and a large usable strain (0.29%), corresponding with the strain values observed by Hiruma et al. in the same material [55]. A blurred contrast corresponding to polar nanoregions was also observed by TEM on the BNT-ST system [56]. Combining BNT-7BT and BNT-25ST into a bilayer composite resulted in a significant change of the largefield hysteretic behavior that corresponded to an increase of the usable strain over BNT-7BT (0.21%) and pinching of the polarization–electric field hysteresis, which has been previously explained as a reverse macroscopic P → NP transition [57]. In addition, the remanent polarization of the bilayer composite was found to be approximately 26 μC/cm2, lying between that observed for BNT-7BT (29 μC/cm2) and BNT-25ST (9 μC/cm2). The remanent polarization is related to the remanent strain through electrostriction. Assuming that the electrostrictive properties of each component remain the same in the bilayer, the rule of mixtures should hold for both the polarization and strain. It is important to note that the resulting remanent polarization of the bilayer composite is not a simple rule of mixtures combination of the constituents, which would give a value of 19.2 μC/cm2. This indicates the presence of additional influences on the macroscopic behavior. In addition, the minimum strain of the bilayer (−0.18%) was found to be larger than that of the BNT7BT or BNT-25ST materials, which resulted in an overall

increase in hysteresis. This result is not expected because the bilayer is comprised of 50% ergodic relaxor, and thus a lower minimum strain than the nonergodic material with an induced ferroelectric order was expected [43]. The origin of the increased minimum strain and the observed remanent polarization is not currently understood. One important factor is the internal stress state generated by the mismatch of thermal expansion coefficients and different remanent states upon poling. Moreover, internal electrical fields are expected to be formed because of polarization coupling between the end members, discussed in further detail by Okatan et al. with respect to compositionally graded and spatially localized charges at the interlayer interface of compositionally graded ferroelectrics [58]. In poled ferroelectric materials, a sub-coercive electrical field applied antiparallel to the remanent polarization direction can lead to strain lower than the remanent strain. A detailed explanation of how minimum strain can be induced by the application of a small (E < Ec) field is given by Gerber et al. [59]. Non-180° domain switching is the primary mechanism for inducing the strain response due to applied electrical field. An important factor in switching polarization states is the contribution of nearest neighbors in terms of both electrical and mechanical fields. Previous modeling work has shown that these interactions can greatly limit the amount of domains switching

TABLE I. Characteristic Material Parameters for BNT-7BT, BNT-25ST, and the Bilayer Composites.

BNT-7BT Bilayer composite BNT-25ST

Remanent polarization (μC/m2)

Unipolar strain (%)

Negative strain (%)

Total strain (%)

∗ d 33 (pm/V)

29.4 26.0 9.0

0.16 0.21 0.29

−0.13 −0.18 −0.03

0.29 0.39 0.33

274 350 487

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by non-180° [60]. Such an analysis on bilayer structures could provide further detail on interactions at the interface boundary between the two components of our system. Minimum strain is known to occur in poled materials when the non-180° domains switch polarization direction. The field at which domain switching occurs indicates the internal field that must be overcome to switch the polarization direction of a poled material [61], [62]. Fedosov and von Seggern discuss a comparable system in their work on ferroelectric polarization of PVDF composites [63]. They suggest that in a two-component system, in which one component is ferroelectric, an applied field results in two distinct internal electrical fields on each component. When the internal field on the ferroelectric material exceeds the coercive field, back-switching is expected until the internal field is equal to the coercive field. Cross and Takuda discussed the effect of heterogeneities in a ferroelectric layer on switching and the preference of a material to have one polarization state over the other [64]. Similar factors may be involved in this work. It is expected that the ergodic and non-ergodic relaxor layers interact with each other through strain- and polarization-coupling. In the case of strain coupling, the large ∗ signal d 31 coefficients and the remanent strain values are responsible for the changing internal stresses during electric field loading. After the initial electric poling cycle, this difference in remanent strains is expected to result in inplane biaxial tensile and compressive stresses in the BNT7BT and BNT-25ST layers, respectively. Previous investigations on the influence of biaxial stress on the macroscopic constitutive behavior have shown an increase in the remanent polarization and a decrease in the electrical poling field [65]. These internal stresses are expected, however, to change during electrical loading because of ∗ the different d 31 coefficients of each end member. Jo and Rödel [66] have reported the longitudinal and radial strain behavior of BNT-BT-KNN during unipolar electric field ∗ loading, showing that the effective macroscopic d 31 value for materials undergoing a reversible NP → P phase transition is larger than those with a metastable long-range ferroelectric order. The radial strain, in addition to the longitudinal strain, was characterized for BNT-25ST up to ∗ 4 kV/mm, providing a d 31 coefficient of approximately –220 pm/V. This value is larger than that of BNT-7BT, which was determined by Jo and Rödel [24] to be –135 pm/V with an applied electrical field of 6 kV/mm. It is, therefore, expected that during electrical loading in the bilayer composite, the internal compressive stress is reduced because of the larger biaxial in-plane contraction of BNT-25ST compared with BNT-7BT. It is important to note, however, that other internal stresses can be present, such as mismatches in coefficients of thermal expansion and elastic compliance, as well as shrinkage strains developed during sintering. The multilayer composite approach offers a possible technique for tailoring the macroscopic electrical properties of lead-free ferroelectrics. These systems, however, are a complex combination of each nonlinear, hysteretic con-

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stituent part, in addition to the internal mechanical and electrical fields that form during processing and electrical loading. One possible method for investigating the influence of the local fields on the macroscopic behavior could be by varying the relative thicknesses of the constituent components, thereby adjusting the volume fractions and the resulting internal fields. Further investigations are required to determine the responsible microstructural mechanisms. IV. Discussion and Conclusions The small signal dielectric response and macroscopic electromechanical properties of lead-free relaxor ferroelectrics in a bilayer composite structure were characterized and contrasted to the behavior of the constituent components. It was found that the bilayer composite structure resulted in a macroscopic constitutive behavior that was a nonlinear combination of both end members, e.g., the pinched polarization–electric field hysteresis loop and an increase in the unipolar strain over the ferroelectric end member. The enhancement in the unipolar strain and reduction in poling field was rationalized to be due to polarization coupling, although it is expected that internal microscopic and macroscopic mechanical fields also play an important role. This technique provides a viable method for reducing the required poling fields of large-strain leadfree ferroelectric materials. References [1] J. Rödel, W. Jo, K. T. P. Seifert, E.-M. Anton, T. Granzow, and D. Damjanovic, “Perspective on the development of lead-free piezoceramics,” J. Am. Ceram. Soc., vol. 92, no. 6, pp. 1153–1177, 2009. [2] Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T. Nagaya, and M. Nakamura, “Lead-free piezoceramics,” Nature, vol. 432, no. 7013, pp. 84–87, 2004. [3] S.-T. Zhang, A. B. Kounga, E. Aulbach, H. Ehrenberg, and J. Rödel, “Giant strain in lead-free piezoceramics Bi0.5Na0.5TiO3–BaTiO3–K0.5Na0.5NbO3 system,” Appl. Phys. Lett., vol. 91, no. 11, art. no. 112906, 2007. [4] W. Liu and X. Ren, “Large piezoelectric effect in Pb-free ceramics,” Phys. Rev. Lett., vol. 103, no. 25, art. no. 257602, 2009. [5] E. Aksel and J. L. Jones, “Advances in lead-free piezoelectric materials for sensors and actuators,” Sensors, vol. 10, no. 3, pp. 1935– 1954, 2010. [6] I. Coondoo, N. Panwar, and A. Kholkin, “Lead-free piezoelectrics: Current status and perspectives,” J. Adv. Dielectr., vol. 3, no. 2, art. no. 1330002, 2013. [7] M. S. Senousy, R. K. N. D. Rajapakse, D. Mumford, and M. S. Gadala, “Self-heat generation in piezoelectric stack actuators used in fuel injectors,” Smart Mater. Struct., vol. 18, no. 4, art. no. 045008, 2009. [8] S. Teranishi, M. Suzuki, Y. Noguchi, M. Miyayama, C. Moriyoshi, Y. Kuroiwa, K. Tawa, and S. Mori, “Giant strain in lead-free (Bi0.5Na0.5)TiO3-based single crystals,” Appl. Phys. Lett., vol. 92, no. 18, art. no. 182905, 2008. [9] Y. Guo, Y. Liu, R. L. Withers, F. Brink, and H. Chen, “Large electric field-induced strain and antiferroelectric behavior in (1 − x) (Na0.5Bi0.5)TiO3–xBaTiO3 ceramics,” Chem. Mater., vol. 23, no. 2, pp. 219–228, 2011. [10] Y. Hiruma, H. Nagata, and T. Takenaka, “Phase diagrams and electrical properties of (Bi1/2Na1/2)TiO3-based solid solutions,” J. Appl. Phys., vol. 104, no. 12, art. no. 124106, 2008.

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[32] J. Zang, M. Li, D. C. Sinclair, W. Jo, and J. Rödel, “Impedance spectroscopy of (Bi1/2Na1/2)TiO3–BaTiO3 ceramics modified with (K0.5Na0.5)NbO3,” J. Am. Ceram. Soc., vol. 97, no. 5, pp. 1523–1529, May 2014. [33] C. M. Fancher, T. Iamsasri, J. E. Blendell, and K. J. Bowman, “Effect of crystallographic texture on the field-induced-phase transformation behavior of Bi0.5Na0.5TiO3−7BaTiO3−2K0.5Na0.5NbO3,” Mater. Res. Lett., vol. 1, no. 3, pp. 156–160, 2013. [34] S.-T. Zhang, A. B. Kounga, W. Jo, C. Jamin, K. Seifert, T. Granzow, J. Rödel, and D. Damjanovic, “High-strain lead-free antiferroelectric electrostrictors,” Adv. Mater., vol. 21, no. 46, pp. 4716–4720, 2009. [35] M. Kosec, V. Bobnar, M. Hrovat, B. Janez, B. Malic, and J. Holc “New lead-free relaxors based on the K0.5Na0.5NbO3–SrTiO3 solid solution,” J. Mater. Sci., vol. 19, no. 6, pp. 1849–1854, 2011. [36] D. E. Dausch, E. Furman, F. Wang, and G. H. Haertling, “PLZTbased multilayer composite thin films, Part I: Experimental investigation of composite film structures,” Ferroelectrics, vol. 177, no. 1, pp. 221–236, 1996. [37] D. E. Dausch, E. Furman, F. Wang, and G. H. Haertling, “PLZTbased multilayer composite thin films, Part II: Modeling of the dielectric and hysteresis properties,” Ferroelectrics, vol. 177, no. 1, pp. 237–253, 1996. [38] T. Shrout, W. a. Schulze, and J. V. Biggers, “Electromechanical behavior of antiferroelectric-ferroelectric multilayer PZT based composites,” Ferroelectrics, vol. 29, no. 1, pp. 129–134, Aug. 1980. [39] D. S. Lee, D. H. Lim, M. S. Kim, K. H. Kim, and S. J. Jeong, “Electric field-induced deformation behavior in mixed Bi0.5Na0.5TiO3 and Bi0.5(Na0.75K0.25)0.5TiO3-BiAlO3,” Appl. Phys. Lett., vol. 99, no. 6, art. no. 062906, 2011. [40] C. Groh, W. Jo, and J. Rödel, “Tailoring strain properties of (0.94− x)Bi0.5Na0.5TiO3−0.06BaTiO3–xK0.5Na0.5NbO3 ferroelectric/relaxor composites,” J. Am. Ceram. Soc., vol. 97, no. 5, pp. 1465–1470, 2014. [41] N. H. Khansur, C. Groh, W. Jo, C. Reinhard, J. A. Kimpton, K. G. Webber, and J. E. Daniels, “Tailoring of unipolar strain in lead-free piezoelectrics using the ceramic/ceramic composite approach,” J. Appl. Phys., vol. 115, no. 12, art. no. 124108, 2014. [42] C. Groh, D. J. Franzbach, W. Jo, K. G. Webber, J. Kling, L. A. Schmitt, H.-J. Kleebe, S.-J. Jeong, J.-S. Lee, and J. Rödel, “Relaxor/ferroelectric composites: A solution in the quest for practically viable lead-free incipient piezoceramics,” Adv. Funct. Mater., vol. 24, no. 3, pp. 356–362, 2013. [43] C. Groh, W. Jo, and J. Rödel, “Frequency and temperature dependence of actuating performance of Bi0.5Na0.5TiO3 -BaTiO3 based relaxor/ferroelectric composites,” J. Appl. Phys., vol. 115, no. 23, art. no. 234107, 2014. [44] M. Acosta, W. Jo, and J. Rödel, “Temperature- and frequencydependent properties of the 0.75Bi1/2 Na1/2TiO3–0.25SrTiO3 leadfree incipient piezoceramic,” J. Am. Ceram. Soc., vol. 97, no. 6, pp. 1937–1943, 2014. [45] H. Lidjici, M. Rguiti, F. Hobar, J.-F. Trelcat, C. Courtois, and A. Leriche, “The effects of sintering temperature and poling condition on the piezoelectric properties of 0.935(Bi0.5Na0.5)TiO3– 0.065BaTiO3 ceramics,” Mater. Sci., vol. 29, no. 1, pp. 9–14, 2011. [46] D. J. Green, O. Guillon, and J. Rödel, “Constrained sintering: A delicate balance of scales,” J. Eur. Ceram. Soc., vol. 28, no. 7, pp. 1451–1466, 2008. [47] C. Jamin, T. Rasp, T. Kraft, and O. Guillon, “Constrained sintering of alumina stripe patterns on rigid substrates: Effect of stripe geometry,” J. Eur. Ceram. Soc., vol. 33, no. 15–16, pp. 3221–3230, 2013. [48] H. Foronda, M. Deluca, E. Aksel, J. S. Forrester, and J. L. Jones, “Thermally-induced loss of piezoelectricity in ferroelectric Na0.5Bi0.5TiO3–BaTiO3,” Mater. Lett., vol. 115, pp. 132–135, 2014. [49] D. Rout, K.-S. Moon, S.-J. L. Kang, and I. W. Kim, “Dielectric and Raman scattering studies of phase transitions in the (100−x) Na0.5Bi0.5TiO3–xSrTiO3 system,” J. Appl. Phys., vol. 108, no. 8, art. no. 084102, 2010. [50] W. Jo, J. Daniels, D. Damjanovic, W. Kleemann, and J. Rödel, “Two-stage processes of electrically induced-ferroelectric to relaxor transition in 0.94(Bi1/2Na1/2)TiO3–0.06BaTiO3,” Appl. Phys. Lett., vol. 102, no. 19, art. no. 192903, 2013. [51] C. Xu, D. Lin, and K. W. Kwok, “Structure, electrical properties and depolarization temperature of (Bi1/2Na1/2)TiO3–BaTiO3 lead-

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didate in the Institute of Materials Science of the Technische Universität Darmstadt, where she is investigating the electromechanical properties of ferroelectric composites. Her research interests include piezoelectric and ferroelectric materials and biomedical applications thereof, micromechanical systems, and surface chemistry.

Azatuhi Ayrikyan received the B.A. degree in biological sciences from Barnard College, Columbia University, New York, NY, USA, in 2006 and the M.S degree in materials science and engineering from Boston University, Boston, MA, USA, in 2011. She is currently a doctoral candidate at the Technische Universität, Darmstadt, Germany. From 2011 to 2013, she was a Research Assistant with the Laboratory for Engineering, Education and Development in the Biomedical Engineering Department of Boston University, working on the development of piezoelectric biosensors for global health applications using zinc oxide thin films. Since 2013, she has been a doctoral can-

Jurij Koruza received the diploma degree in metallurgy and material sciences from the University of Ljubljana, Slovenia, in 2008 and the Ph.D. degree in nanoscience and nanotechnology from the Jožef Stefan International Postgraduate School, Ljubljana, Slovenia, in 2013. He is currently a Postdoctoral Researcher in the Institute of Materials Science at the Technische Universität Darmstadt, Germany. His main research interests include processing of complex oxides, phase transition behavior, and processing–structure–properties relationships in lead-based and lead-free ferroelectric ceramics.

Virginia Rojas received the diploma degree in materials science and engineering in 2008 from the Simón Bolívar University, Caracas, Venezuela, and the M.Sc. degree in materials science in 2014 from the Technische Universität, Darmstadt, Germany, where she is currently a doctoral candidate investigating process optimization for ferroelectric material fabrication. Her research interests include mechanical characterization of materials, lead-free ferroelectrics, and ceramics powder processing.

Leopoldo Molina-Luna received the B.Sc. degree in physics from the Universidad de Los Andes, Mérida, Venezuela, in 1999 and the M.Sc. degree in physics from the Universität Stuttgart and the Max Planck Institute for Solid-State Research, Stuttgart, Germany, in 2003. In 2010, he obtained the Ph.D. degree in physics from the Eberhard Karls Universität Tübingen, Germany. From 2010 to 2012, he was a Postdoctoral Researcher in the Electron Microscopy for Materials Science (EMAT) group of the University of Antwerp, Belgium. In 2013, he joined the Department of Materials- and Geosciences of the Technische Universität Darmstadt as a Research Associate. His current research interests include structure–property correlations on the nanoscale of lead-free ferroelectrics, oxide nanodevices, semiconducting oxides, magnetic materials, and thin-film thermoelectrics.

Matias Acosta received the diploma degree in materials engineering from the Professor Jorge A. Sabato Institute, Buenos Aires, Argentina, in 2001. He is currently a doctoral candidate in materials science at the Technische Universität Darmstadt, Germany. His research interest includes synthesis, engineering, and characterization of functional ceramics such as lead-free piezoelectrics and high-temperature dielectrics. Mr. Acosta was awarded a full scholarship during the period of time of his studies at the Professor Jorge A. Sabato Institute. He was honored with the best poster award at SAM-CONAMET (2009), at Symposium 2 ICAE (2013), and at the Joint IEEE International Symposium ISAF/IWATMD/PFM (2014).

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Kyle G. Webber received the B.S. degree in marine systems engineering from the Maine Maritime Academy, Castine, ME, USA, in 2003, and the M.S. and Ph.D. degrees in mechanical engineering from the Georgia Institute of Technology, Atlanta, GA, USA, in 2005 and 2008. In 2008, he joined the Institute of Materials Science of the Technische Universität Darmstadt, Germany, as a Postdoctoral Researcher, where he worked on the mechanical properties of ferroelec-

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trics. In 2013, he was awarded the Emmy Noether Research Fellowship by the Deutsche Forschungsgemeinschaft. Since 2014, he has been an Assistant Professor at the Technische Universität Darmstadt, Germany. His primary research interests include temperature-dependent ferroelasticity, phase transformations, and fracture of single crystal and polycrystalline ferroelectrics.

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