Gigantic Electrostrain in Duplex Structured Alkaline Niobates

Article pubs.acs.org/cm

Gigantic Electrostrain in Duplex Structured Alkaline Niobates Si-Young Choi,*,†,# Soon-Jong Jeong,*,‡,# Dae-Su Lee,‡ Min-Soo Kim,‡ Jae-Shin Lee,§ Jeong Ho Cho,∥ Byoung Ik Kim,∥ and Yuichi Ikuhara⊥ †

Advanced Characterization & Analysis Group, Korea Institute of Materials Science, Changwon 642-831, Korea Advanced Materials & Application Research Laboratory, Korea Electrotechnology Research Institute, Changwon 642-120, Korea § School of Materials Science and Engineering, University of Ulsan, Ulsan 680-749, Korea ∥ Electronic Component Center, Korea Institute of Ceramic Engineering and Technology, Seoul 153-801, Korea ⊥ Institute of Engineering Innovation, University of Tokyo, Tokyo 113-8656, Japan ‡

S Supporting Information *

ABSTRACT: We demonstrate that an exceptionally large strain can be induced in CaZrO3-modified alkaline-niobates by electric fields. The maximum induced strain of our niobate-based ceramics could reach more than 1,000 pm/V, which is a much higher value than that of commercial soft PZT ceramics. Atomic-scale annular bright-field (ABF) and annular dark-field (ADF) scanning transmission electron microscopy (STEM) directly revealed that individual single grains were composed of an electrically duplex core−shell structure; relaxor-like cores and paraelectric shells. Based on this ABF STEM analysis along with electrical measurements, a plausible mechanism explaining the high strain effect in the present work was suggested. This new material offers an unprecedented opportunity to produce efficient Pb-free piezoelectrics for applications that require large electrostrictive motion. KEYWORDS: core/shell, perovskites, electron microscopy, electric field induced strain, actuator

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INTRODUCTION Many ABO3-type perovskite oxides have been extensively studied, as they exhibit notable ability to convert mechanical strain to electrical energy and vice versa.1,2 Pb(Zr,Ti)O3 (PZT) is a well-known piezoelectric material that has been rigorously utilized in actuators and transducers over the last several decades. In addition to Pb-based titanates, a number of other complex perovskite oxides that do not contain Pb have been suggested as alternative piezoelectrics from an environmental viewpoint. Although various reports on different compositions and synthetic approaches can be found, as recently reviewed in many papers,3−12 new developments including materials processing and design for piezoelectric perovskites are necessary to achieve better electromechanical performance. Among the parameters dictating piezoelectric properties, electric field-induced strain (EFIS) is regarded as one of the most important characteristics. As the magnitude of EFIS directly reflects how much mechanical strain can occur by an applied bias, it is usually represented in terms of the inverse piezoelectric coefficient dS/dE, induced strain at a given electric field, or normalized, large-signal strain d33* ≡ S3,max/E3,max, the ratio between the maximum induced strain and the electric field in the same direction. Considering that the values of d33* of commercial PZT range between 400 and 700 pm/V,4 recent reports of more than 500 pm/V of d33* with chemically modified (Bi,Na)TiO35 and BaTiO311 and ∼750 pm/V with textured (K,Na)NbO312 show fairly notable electrostrain © 2012 American Chemical Society

properties. Therefore, a variety of efforts to synthesize new Pb-free perovskite piezoelectrics having remarkably enhanced EFIS with temperature stability are still ongoing for applications that require both high strain effect and environmental benignity. Herein, we have successfully fabricated (K,Na)(Nb,Ta)O3based polycrystalline ceramics having a core−shell configuration in the individual grains. Such a duplex structure composed of a polar core and a nonpolar shell could be easily attained when a small amount of CaZrO3 was used as an additive. We also have directly demonstrated two different polarization behaviors in the core and shell regions at an atomic scale, using annular bright-field scanning transmission electron microscopy, which is an efficient means of visualizing atomic columns with a low average atomic number.13,14 Most significantly, the core−shell (K,Na)(Nb,Ta)O3 ceramics in the present work showed outstanding EFIS characteristics. It is noted that their d33* values exceed 1,000 pm/V at 3.5 kV/mm. This EFIS behavior is superior to that of commercial soft PZT, representing exceptionally large d33* values over those of other piezoelectric ceramics5 that have been developed thus far. Received: April 30, 2012 Revised: July 20, 2012 Published: July 20, 2012 3363

dx.doi.org/10.1021/cm301324h | Chem. Mater. 2012, 24, 3363−3369

Chemistry of Materials

Article

Figure 1. Core−shell structure in CZ-modified KNLNT. DF-STEM and EDS mapping images show that Ta and Ca inhomogeneously exist in the shell region: (a) bright-field and (b) dark-field STEM images and (c), (d), (e), and (f) are EDS mapping images for Ta, Nb, Ca, and Zr atoms, respectively. SADP obtained from both core and shell is inserted in (b), indicative of the crystallographic coherency between the core and shell.

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performed using an electron microscope (JEM-2100F, JEOL, Japan) at 200 kV. The electron microscope was equipped with a spherical aberration corrector (CEOS GmbH, Germany) for the probe forming, and thus the size of the electron probe in STEM mode was ∼0.9 Å at 200 kV. The inner and outer detection angles for ABF imaging were adjusted to ∼17 and ∼40 mrad, respectively, to visualize both the light and heavy atoms. Electrical measurements were carried out after screen-printing Ag paste on both sides of a disk-shaped specimen and subsequent firing at 700 °C for 30 min. P-E hysteresis loops were measured using a modified Saw-Tawyer circuit at 60 Hz. The electric field-induced strain (S-E) measurements were carried out using a linear variable differential transducer at 1 Hz after poling under a dc field of 3 kV/mm at 80 °C for 30 min. Piezoelectric properties were measured by the resonance-antiresonance method with an impedance analyzer (HP4294A). The electromechanical coupling factor was measured using an IEEE standard (ANSI/IEEE std. 176-1987).

EXPERIMENTAL SECTION

A solid state reaction route was applied to prepare powder with a composition of 0.96 (K0.51Na0.47Li0.02)(Nb0.8Ta0.2)O3−0.04CaZrO3. Reagent grade powders of K2CO3 (99.0%), Na2CO3 (99.9%), Nb2O5 (99.95%), Li2CO3 (99.9%), Ta2O5, CaCO3, and ZrO2 (99.9%) were used as raw materials. The powders were weighed according to the chemical formula and then ball-milled for 24 h in anhydrous ethanol with zirconia balls. The slurry was dried and calcined at 900 °C for 2 h, after which the calcination step was repeated. The calcined powder was mixed with polyvinyl alcohol as a binder and pressed into circular disks with a diameter of 12 mm at 300 MPa. The green compacts were sintered in a covered alumina crucible at 1100 °C for 2 h in air. To promote densification during sintering, excess 1 mol% Li2CO3 and 2 mol% MnO2 were added as a sintering aid. Electron microscopic analyses, including selected area diffraction pattern (SADP), energy dispersive X-ray spectroscopy (EDS) mapping, and atomic-resolution ABF and ADF imaging, were 3364



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Figure 2. (a) HAADF-STEM image of the core−shell structured grains in the whole sample. (b) Atomic scale HAADF STEM image from the core/ shell interface area, where the brighter, upper shell is distinguished from the less bright, bottom core. The red arrow and line indicate the interface between the core and shell. (c) and (d) are ABF STEM images from the shell and core region before poling. (e) and (f) are ABF STEM images from the shell and core regions after poling under a dc field of 3 kV/mm at 80 °C for 30 min. The brighter spots denoted by the red arrow indicate the heavy B-site atomic column of Nb in the core and Nb and Ta in the shell; the less bright spots, denoted by the yellow arrow, indicate light A-site atomic columns of K, Na, and Li.

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RESULTS AND DISCUSSION Li-added (K,Na)(Nb,Ta)O3 polycrystals that are chemically modified with Ca and Zr were fabricated through a solid-state route and subsequent sintering in air. The precise composition of the solid solution is (1-x)(K0.51Na0.47Li0.02) (Nb0.8Ta0.2)O3− xCaZrO3. As significant changes of physical properties were found in the x = 0.04 (KNLNT-CZ) compound over the x = 0.00 (KNLNT) case, atomic-scale analyses and electrical measurements have been intensively performed on the KNLNT-CZ samples. Figure 1 shows typical bright-field and

dark-field STEM images of a single grain in the KNLNT-CZ specimen along with corresponding compositional maps obtained by EDS. Both STEM images (Figures 1 (a) and (b)) clearly indicate the presence of a core−shell structure. Since the contrast in dark-field mode is proportional to the atomic number, the brighter shell region in the dark-field image implies a much higher concentration of heavier atoms, compared with that in the core region. The EDS maps in Figures 1 (c) and (f) directly demonstrate that most Ta ions are present in the shell, consistent with the STEM images, 3365



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Figure 3. (a) Unipolar S-E loops of KNLNT and KNLNT-CZ in comparison with those of soft PZT and textured KNN ceramics.12 (b) Temperature-dependence of EFIS of KNLNT-CZ as a function of a maximum electric field. (c) Dielectric constant and loss as a function of temperature for KNLNT and KNLNT-CZ. (d) Polarization (P)-electric field (E) hysteresis loops of KNLNT-CZ as a function of temperature.

As aforementioned, the atom columns containing such light atoms as Na and K are difficult to visualize in the HAADF mode, because the contrast in dark-field STEM is proportional to the average atomic number of each column.15 Although the HAADF STEM image shown in Figure 2 (b) indicates that there is no significant lattice mismatch between the core and shell regions, relative displacement between cations and resulting polarization of the two regions could not be identified by HAADF STEM due to the invisible A-site columns in our sample. Therefore, ABF STEM was carried out to precisely probe the atomic columns having K and Na in the lattice. Figures 2 (c) and (d) are ABF STEM images in the [011] projection taken from the shell and core regions of a KNLNTCZ sample, respectively. It should be noted that the contrast of ABF STEM images was reversed for better understanding, and thus the strong contrast in the images is shown as black contrast in the original ABF STEM images. The B-site columns showing stronger contrast in the image for the shell region appear to be centered in the unit cell, denoted by broken lines in Figure 2 (c). By contrast, substantial atomic displacement of the B-site columns can be observed in the core region of Figure 2 (d). Such atomic displacement behavior was intensified after a KNLNT-CZ sample was poled under a high electric field. Figures 2 (e) and (f) compare ABF STEM images in the [011] projection for the shell (Figure 2(e)) and core (Figure 2(f)) regions after poling under 3 kV/mm. It is noted that remarkable polarization has been induced in the core region

while more Nb ions are detected in the core. A slightly higher concentration of Ca in the shell region can also be found (Figure 1 (e)), while the local distribution of Zr in the shell is not detected (Figure 1 (f)). During the STEM and EDS analyses, most of the individual grains in the KNLNT-CZ sample were confirmed to have this chemically distinct duplex structure. It should be mentioned that the A site composition does not exhibit the inhomogeneous location (not in the present view), and thus the chemical inhomogeneity is mainly attributed to the B site composition and the added CaZrO3. The core and shell have the same crystallographic orientation in spite of the difference in chemical composition, as revealed in the electron diffraction pattern of Figure 1 (b). To examine the variation in atomic arrangements and resulting polarizations in both the core and the shell regions at an atomic scale, spherical aberration-corrected STEM was utilized. Figures 2 (a) and 2 (b) show high-angle annular dark-field (HAADF) STEM images of KNLNT-CZ. The image in Figure 2 (a) at a low magnification consistently demonstrates the presence of a core−shell structure in each grain. When the interface area between the core and shell regions was observed at an atomic resolution, much brighter contrast in atomic columns of the shell region is readily recognized, as shown in Figure 2 (b). Thus, it is understood that this different column contrast results from a much higher concentration of Ta in the shell, indicating good agreement with the EDS result in Figure 1. 3366



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Figure 4. Proposed mechanism of large electric field-induced strain behavior in a core−shell structured ferroelectric ceramic. (a) Unipolar S-E curve for normal ferroelectric ceramics. Polarization reorientation and induced strain in traditional ferroelectric ceramics, in which randomly oriented polarization directions in unpoled state (state I) are largely aligned parallel to the applied electric field through poling (state II), resulting in a large remnant strain Sr. Electrostrain Se is observed under external fields (state III). (b) Unipolar S-E curve for our KNLNT-CZ ceramic, which shows unpoled core−shell structures (state IV), generation of polarization in the cores by poling (state V), and polarization propagation to the nonpolar shell region (state VI) under applied fields. The dashed line in the S-E curve represents a poling process by electrical as well as thermal activation.

became remarkably diffuse with CZ addition. For KNLNT-CZ, a broad maximum of the permittivity could be found at Tm = 200 °C, while dielectric losses of both KNLNT-CZ and KNLNT are flat up to 300 °C. The polarization-electric field hysteresis behavior (Figure 3(d)) was also observed in KNLNT-CZ up to Tm, showing a remnant polarization Pr = ∼6.0 μC/cm2 and a coercive field Ec = ∼0.7 kV/mm at up to 50 °C and furthermore a finite remnant and saturated polarization with rather a small coercive electric field up to 200 °C. Therefore, based on these macroscopic electrical properties and the atomic-scale ABF STEM analysis shown in Figure 2, it can be inferred that the core regions of KNLNT-CZ have ferroelectric characteristics, while the shell regions remain paraelectric at room temperature, although the KNLNT-CZ outwardly appears to be a relaxor-like ferroelectric material, exhibiting a small piezoelectric coefficient d33, a significant Pr, and a diffuse dielectric permittivity. Variations of structural and electrical properties as a function of CZ addition to KNLNT are provided in Supporting Information S2 and are also summarized in Table S1. Therefore, the mechanism for generating the giant strain in our core−shell structured ceramics can now be explained. Figure 4 compares unipolar EFIS behaviors between normal ferroelectric ceramics and our core−shell duplex material. Ferroelectric ceramics consist of randomly oriented domains in an unpoled state (state I). A poling process, which is generally referred to as domain switching, causes the polarization axis of each domain to reorient parallel to the direction of poling field as much as possible (state II). A large remnant strain (Sr) is accompanied by domain switching, as depicted in Figure 4 (a), due to the exchange of nonequal crystallographic axes.16 Once such Sr occurs, it is usually irrecoverable unless sufficient temperature is provided to restore each polarization to the initial state. As a result, inducing large electrostrain after poling

by the poling process, while no distinguishable atomic displacement has occurred in the shell region. Therefore, the series of ABF STEM images shown in Figures 2 directly demonstrates that the cores are in a ferroelectric polar state, whereas the shells are in a paraelectric nonpolar state. When the EFIS behavior of KNLNT-CZ samples was measured, an exceptionally large electrostrain could be obtained. Figure 3 (a) plots the unipolar strain characteristics as a function of applied electric field. For comparison, the strain loops of other ceramic materials including soft PZT (Product ID: S-55, Sunnytec Co, Taiwan; d33 = 650 pC/N and d31 = −200 pC/N) are presented together. It can be readily recognized from this plot that the KNLNT-CZ has superior EFIS. The normalized strain d33* of KNLNT-CZ reaches 1030 pm/V (Emax = 3.5 kV/mm), which is remarkably higher than those reported on PZT ceramics (d33* = 400−700 pm/V)4 as well as textured KNN ceramics (d33* = 750 pm/V)12 and polycrystalline KNLNT ceramics without CZ (d33* = 440 pm/ V, see Table S1 in the Supporting Information). It should be noted that the giant strain could be properly observed only after poling treatment in our specimens, which is similar to normal piezoelectric ceramics that have randomly oriented ferroelectric domains. Moreover, Figure 3 (b) confirms that such a large strain was not deteriorated even above 40 °C, showing fairly stable characteristics with temperature. Upon measuring the strain up to 50 °C, the strain value of 0.2% was maintained under E = 1.0 kV/mm, although we could not reach higher temperature yet due to an instrumentation limit. Even if the strain measured at higher E = 2.0 and 3.0 kV/mm show a slightly higher decreasing rate upon increasing temperature, their values are always higher than 0.2% up to 50 °C. The dielectric permittivity of KNLNT as a function of temperature was observed to show a sharp peak at TC = 360 °C, as in other typical ferroelectrics (Figure 3(c)). In contrast, this sharp peak 3367



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may be limited at even higher electric fields in general ferroelectrics (state III). By contrast, as revealed in the ABF STEM images of Figure 2, polarization is generated only in the core regions in our KNLNT-CZ material, as depicted in Figure 4 (b). Consequently, Sr is not as considerable as in general ferroelectrics (state V). If a higher electric field is applied, however, the generated polarization can be extended to the paraelectric nonpolar shell regions (state VI),17−19 giving rise to a gigantic electrostrain (Se). When the high field is removed, the induced polarization in the shell regions should be readily relaxed, and thus the initial state with merely small Sr is recovered from the highly strained state (state V). The nonpolar shells appear to play a significant role in the high strain effect in our KNLNT-CZ in terms of efficient polarization extension from the cores and its easy release.17 To straightforwardly demonstrate the possibility of utilizing this KNLNT-CZ ceramic in actuator applications, a cantilevertype actuator consisting of the ceramic and a copper plate was fabricated. For direct comparison, a cantilever sample of a soft PZT specimen in the same geometry was also fabricated, and then both KNLNT-CZ and soft PZT cantilevers were evaluated under electric fields of 1, 1.5, and 2 kV/mm. The reflection of the laser beam to the silicon mirror attached to the front edge of the cantilever was used in order to track the deflection motion of the cantilevers. For manageable detection of the movement of the reflected beam, an electric field was oscillated from 0 to the respective target electric field with a frequency of 8 Hz. The overall configuration of the cantilevers and a schematic illustration are shown in Figures 5 (a) and (b). When an electric field was applied to the cantilever ceramics (right, Figure 5(a)), the polarization in the ceramics occurs along the external electric field, resulting in bending motion of the cantilever and accordingly a shift of the reflected beam on the screen. Therefore, when an electric field is applied to the cantilever samples in a sinusoidal waveform, the movement of the reflected beam can be expressed as a line on the screen, as shown in Figure 5 (c). The moving distance of the reflected beam obtained from the KNLNT-CZ cantilever under 1 kV/ mm is a few millimeters longer than that from the PZT cantilever due to the similar strain rate under 1 kV/mm in both KNLNT-CZ and PZT ceramics, as predicted in Figure 3 (a). On the other hand, as the difference in strain becomes larger under a higher electric field (Figure 3 (a)), a further increase of the electric field up to 1.5 and 2 kV/mm significantly enhances the movements of reflected beams in the KNLNT-CZ cantilever, more than 10 mm and 20 mm, demonstrating that the EFIS characteristics of KNLNT-CZ are of great interest for efficient actuators.

Figure 5. (a) Schematic illustration showing the operation system of laser beam reflection using the cantilever-type actuator. (b) Photograph of two cantilever-type actuators using KNLNT-CZ and soft PZT ceramics. (c) Beam trajectory under electric fields of 1.0, 1.5, and 2.0 kV/mm reflected by the soft PZT and KNLNT-CZ cantilevers. Projection distance between the cantilevers and the screen was fixed to ∼2.0 m. The white dotted lines in (e) indicate the initial beam position under no electric field.

observations are suggested to directly visualize the plausible polarization extension17−19 and release motion between the core and shell as well as electric-field-induced phase transitions,20,21 we believe that our KNLNT-CZ ceramics with a duplex structure can be effectively applied to real electromechanical devices based on Pb-free perovskite oxides.

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ASSOCIATED CONTENT

* Supporting Information S

Additional STEM results are summarized. Variations of structural and electrical properties as a function of CZ addition to KNLNT are also provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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CONCLUSION Our results will shed light on avenues to achieve large electromechanical coupling in non-Pb based ceramics via a new microstructural design without resorting to a complex domain texturing process.12 The present study has shown that remarkably large electrostrain can be achieved in (K,Na)(Nb,Ta)O3 polycrystals by simple chemical modification. The polar core and the nonpolar shell in each individual grain are notable features in the overall microstructure. However, since the formation of the core−shell structure is deeply related with thermodynamic phase stability and thus is very sensitive to the sintering process, more intensive studies to understand the mechanism of core−shell formation are currently underway. Moreover, although further investigations including in situ

AUTHOR INFORMATION

Corresponding Author

*Phone: +82 (55) 280 3514. Fax: +82 (55) 280 3699. E-mail: [email protected] (S.-Y.C.), E-mail: [email protected] (J.Y.J.). Author Contributions #

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. S.-Y. Chung (Korea Advanced Institute of Science and Technology) for helpful discussion on the 3368



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STEM analysis and Prof. K. H. Kim (Seoul National University) and Dr. J. Ryu (Korea Institute of Materials Science) for contribution to the property analysis. This study was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy (MKE), Republic of Korea.

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REFERENCES

(1) Uchino, K. Ferroelectric devices, 2nd ed.; CRC Press: Boca Raton, 2009. (2) Schwartz, M. Smart Materials; CRC Press: Boca Raton, 2009. (3) Takenaka, T.; Nagata, H. J. Eur. Ceram. Soc. 2005, 25, 2693− 2700. (4) Shrout, T. R.; Zhang, S. J. J. Electroceram. 2007, 19, 113−126. (5) Rödel, J.; Jo, W.; Seifert, K.; Anton, E.-M; Granzow, T; Damjanovic, D. J. Am. Ceram. Soc. 2009, 92, 1153−1177. (6) Zhang, S.-T.; Kounga, A. B.; Jo, W.; Jamin, C.; Seifert, K.; Granzow, T.; Rödel, J.; Damjanovic, D. Adv. Mater. 2009, 21, 4716− 4720. (7) Daniels, J. E.; Jo, W.; Rödel, J.; Jones, J. L. Appl. Phys. Lett. 2009, 95, 032904. (8) Aksel, E.; Jones, J. L. Sensors 2010, 10, 1935−1954. (9) Leontsev, S. O.; Eitel, R. E. Adv. Mater. 2009, 11, 044302. (10) Safari, A.; Abazari, M. IEEE Trans. Ultrasonics, Ferroelectrics Frequency Control 2010, 57, 2165−2176. (11) Liu, W.; Ren, X. Phys. Rev. Lett. 2009, 103, 257602. (12) Saito, Y.; Takao, H.; Tani, T.; Nonoyama, K.; Takatori, K.; Homma, T.; Nagaya, T.; Nakamura, M. Nature 2004, 432, 84−87. (13) Findlay, S. D.; Shibata, N.; Sawada, H.; Okunishi, E.; Kondo, Y.; Ikuhara, Y. Ultramicroscopy 2010, 110, 903−923. (14) Ishikawa, R.; Okunishi, E.; Sawada, H.; Kondo, Y.; Hosokawa, F.; Abe, E. Nat. Mater. 2011, 10, 278−281. (15) Jesson, D. E.; Pennycook, S. J. Proc. R. Soc. London, Ser. A 1993, 441, 261−281. (16) Park, S. E.; Wada, S.; Cross, L. E.; Shrout, T. R. J. Appl. Phys. 1999, 86, 2746−2750. (17) Damjanovic, D. A. Appl. Phys. Lett. 2010, 97, 062906. (18) Kling, J; Tan, X.; Jo., W.; Kleebe, H.-J.; Fuess, H.; Rödel, J. J. Am. Ceram. Soc. 2010, 93, 2452−2455. (19) Hinterstein, M.; Knapp, M.; Hölzel, M.; Jo, W.; Cervellino, A.; Ehrenberg, H.; Fuess, H. J. Appl. Crystallogr. 2010, 43, 1314−1321. (20) Park, S. E.; Pan, M.-J.; Markowski, K.; Yoshikawa, S.; Cross, L. E. J. Appl. Phys. 1997, 82, 1798−1803. (21) Davis, M.; Damjanovic, D.; Setter, N. M. Phys. Rev. B 2006, 73, 014115.

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Gigantic Electrostrain in Duplex Structured Alkaline Niobates

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