J. Am. Ceram. Soc., 93 [9] 2452–2455 (2010) DOI: 10.1111/j.1551-2916.2010.03778.x r 2010 The American Ceramic Society
Journal In Situ Transmission Electron Microscopy of Electric Field-Triggered Reversible Domain Formation in Bi-Based Lead-Free Piezoceramics Jens Kling,w,z Xiaoli Tan,y Wook Jo,z Hans-Joachim Kleebe,z Hartmut Fuess,z and Ju¨rgen Ro¨delz z
Institute of Materials Science, Technische Universita¨t Darmstadt, 64287 Darmstadt, Germany
y
Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011, USA
proved the field-induced phase transition from pseudocubic to tetragonal BNT–BT at the MPB,16 which was further corroborated by a demonstration of an electric field-induced phase change in Mn-doped BNT–BT single crystals.17 In this communication, the microscopic mechanism for the electric field-induced strain in the (1–x–y)BNT–xBT–yKNN ternary system was investigated. Specifically, the unique electric field in situ TEM technique18–20 was applied to the composition 0.91BNT–0.06BT–0.03KNN, in order to reveal the domain morphology evolution and crystal structure change under applied electric fields. Recent TEM investigations on this composition showed a two-phase mixture of rhombohedral R3c and tetragonal P4bm in the absence of large ferroelectric domains.21
A lead-free piezoelectric 0.91(Bi1/2Na1/2)TiO3–0.06BaTiO3– 0.03(K0.5Na0.5)NbO3 ceramic with high strain was examined in situ under an applied electric field using the transmission electron microscope. No domain structure is observed without an electric field, but an alternating electric field leads to the reversible formation of a lamellar domain structure. Correlations to polarization and strain hysteresis loop measurements indicate an electric field-induced phase transition from a nonpolar to a ferroelectric state and vice versa. I. Introduction
E
NVIRONMENTAL concerns with lead in Pb(Zr1xTix)O3 ceramics have stimulated intensive search for lead-free piezoelectric materials worldwide.1–3 (Bi1/2Na1/2)TiO3 (BNT)-based ceramics are among the most studied ones due to the existence of a morphotropic phase boundary and the ease of processing.4,5 The structure of pure BNT has been characterized; it is ferroelectric with a R3c space group.6 This rhombohedral phase exhibits aaa-type oxygen octahedral tilting.7 When heated, a phase transition to a tetragonal P4bm phase with a0a0c1-type octahedra tilting occurs.8 These two space groups can be distinguished by their different superstructure reflections, namely 1/2{ooo} for R3c and 1/2{ooe} for P4bm,9 where o and e are odd and even Miller indices, respectively. A transmission electron microscopy (TEM) analysis indicated that the tetragonal phase persists down to room temperature in the form of nanometer-sized platelets embedded in the rhombohedral matrix.10 Recent studies revealed that a (Bi1/2Na1/2)TiO3-based ternary solid solution system, (1-x-y)(Bi1/2Na1/2)TiO3-xBaTiO3-y (K0.5Na0.5)NbO3 ((1-x-y)BNT-xBT-yKNN) (0.05rxr0.07, 0.01ryr0.03), looks promising for future actuator applications due to its ultrahigh electric field-induced strain.11,12 The origin of the large strain was suggested to be associated to an electric field-induced nonpolar to ferroelectric phase transition.13 The return to the unpoled state at zero electric field is consistent with X-ray diffraction analyses on these ceramics indicating a pseudocubic structure.11 In addition, TEM examination revealed no domain structure but superstructure reflections. At the same time, dielectric measurements showed a strong frequency dispersion.12 As suggested by Tan et al.14 such features resemble a relaxor-like ferroelectric behavior. The current study has relevance to the base system BNT–BT, which has been reported to exhibit a nearly cubic15 or pseudocubic11 structure at the MPB. A recent in situ synchrotron study
II. Experimental Procedure Ceramic samples whose composition is denoted as 0.91BNT– 0.06BT–0.03KNN were synthesized by the conventional solidstate fabrication method11,12 via calcination of Bi2O3, Na2CO3, K2CO3, BaCO3, TiO2, and Nb2O5 powders at 9001C for 3 h and sintering at 11501C for 3 h. The choice of this specific composition 0.91BNT–0.06BT–0.03KNN was made for the in situ experiment, because previous investigations showed a giant electric field-induced strain in spite of the disappearance of ferroelectricity on the removal of electric field, which implies a possible field-induced phase transition.12 Polarization hysteresis loops (P(E)) and strain hysteresis loops (S(E)) were obtained on disk-shaped specimens with sputtered Ag-electrodes. For P(E) characterization, a standardized ferroelectric test system (RT-66A, Radiant Technologies, Albuquerque, NM) with a frequency of 4 Hz was used. The S(E) measurements were performed at a frequency of 1 Hz using a fiber optic displacement sensor (D63, Philtec, Annapolis, MD). The electric field in situ TEM technique, as reported previously for ceramics of Pb(Zr1xTix)O3,18 single crystals of Pb(Mg1/3Nb2/3)O3–PbTiO319 and BT,20 was applied to the polycrystalline ceramic specimen. In contrast to these ferroelectric oxides studied previously with regular micron-sized domains, the composition 0.91BNT–0.06BT–0.03KNN displays only nanostructured regions in the as-processed state.21 The in situ TEM setup consists of a specially designed specimen holder and a specially prepared TEM sample. The deposited electrodes on the specimen surface are connected to two electric feed-throughs at the holder tip via thin Pt wires. Thereby, an electric field perpendicular to the electron beam can be applied. TEM specimens were prepared by grinding, polishing, and dimpling from one side, and thermally annealed at 4001C in air to minimize induced stresses during preparation. The specimens were Ar ion thinned to electron transparency. Afterwards, two semicircular-shaped Au-electrodes were vapor-deposited on the sample surface with a 0.2-mm-wide screened region in-between. One corner of the specimen was glued to the TEM holder and electrical contact was applied by Pt wires, which were glued to the surface and the feed-throughs on the holder by conductive
N. Bassiri-Gharb—contributing editor
Manuscript No. 26824. Received September 14, 2009; approved March 11, 2010. Supported by the Deutsche Forschungsgemeinschaft (DFG), Sonderforschungsbereich 595 ‘‘Electrical Fatigue in Functional Materials’’. w Author to whom correspondence should be addressed. e-mail:
[email protected]
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epoxy. A schematic diagram of the electrode geometry is given in Fig. 1. To achieve electric field strength at least as high as calculated for the simple capacitor geometry, the sample region marked in Fig. 1 (dark gray area) was chosen for the in situ experiment.18 At an applied voltage of 7500 V, a nominal electric field strength of 725 kV/cm can be reached. Considering the field concentration by the presence of the central hole in the TEM specimen,18 the actual field is up to 750 kV/cm. Because of the holder geometry, only limited tilting about one tilt axis is possible. The in situ TEM experiment was carried out on a FEI CM30 (FEI, Eindhoven, the Netherlands) operated at 300 kV and equipped with a Gatan CCD camera (Gatan, Pleasanton, CA). During the experiment, the applied voltage was increased stepwise and micrographs or electron diffraction patterns (EDP) were recorded at each constant field level.
III. Results and Discussion In Fig. 2, a representative grain configuration of 0.91BNT– 0.06BT–0.03KNN is depicted. The grains show a very weak contrast as long as they are not oriented near a zone axis. In the grain below the EDP, small square shaped nano features are visible (marked by an arrow). Detailed investigations identified these as faceted pores in the material, also found in other compositions of this system.14 The darker grain in Fig. 2 is oriented along [001]c. The contrast is mainly homogeneous and shows a ‘‘grainy’’ structure, but clearly no lamellar domain configuration. The EDP indicates 1/2{ooe}-type superstructure reflections (SR), implying tetragonal symmetry (P4bm).9 As both, rhombohedral and tetragonal, superstructure reflections are only visible in specific zone axes and due to the limitation in specimen tilting, no other superstructure reflections were observed in this grain. However, 1/2{ooo}-type SR, which are characteristic for rhombohedral symmetry (R3c),9 were also detected in neighboring grains oriented along other zone axes. Detailed TEM investigations showed that both phases are present within the same grain.21 The tetragonal phase has a higher phase fraction than the rhombohedral one, evidenced by the higher intensity of the tetragonal superlattice reflection. This phase appears in nanoscaled plate-like regions, which is supported by the apparent streaking in superlattice reflections. The observation is consistent with Dorcet and Trolliard’s results on pure BNT.10 Figure 3(a) shows the grain chosen for the in situ electric field experiment. As in Fig. 2, the contrast is homogenous but weak and shows the characteristic ‘‘grainy’’ structure. The grain is not aligned along a prominent zone axis. Some dislocations but no domain structure is visible. At a nominal field strength of 125 kV/cm (actual field about 50 kV/cm), however, it is noted that the appearance of small domains becomes visible at the lower right side in Fig. 3(b) with a bending contrast due to the inverse piezoelectric effect. The grain orientation is nearly the same as in Fig. 3(a). The electric field direction is marked by an arrow,
Fig. 1. Schematic sketch of the electrode geometry. The examined sample region is marked in dark gray.
Fig. 2. A grain in 0.91BNT–0.06BT–0.03KNN, oriented along the [001]c zone axis; the diffraction pattern shows 1/2{ooe} superstructure reflections. No lamellar domains are visible. The arrow marks faceted pores, commonly observed in this sample.
whose tip points approximately 101 away from the paper normal. The lamellar domain configuration is clearly visible in the [110]c zone axis as shown in Fig. 3(c). The electric field direction is oriented close to the /001Sc direction. The EDP clearly reveals 1/2{ooo} superstructure reflections, indicating the presence of the rhombohedral distortion. On removal of the field, however, it is noted that the domain configuration disappears, leaving only the contrasts from the strain-induced bending and dislocations, as depicted in Fig. 3(d). Little change in the EDP in this zone axis is noticeable in the observed voltage range. The superstructure reflections, visible in the inset in Figs. 3(c) and (d), have been retained and unchanged. In order to correlate the observed domain formation under electric fields with macroscopic properties, electrical polarization P and longitudinal strain S were measured in bulk disk samples ( 7 mm diameter and 0.4–0.8 mm thickness) of 0.91BNT– 0.06BT–0.03KNN as a function of applied field. Figure 4 depicts loops under fields of up to 45 and 70 kV/cm. With the peak field of 45 kV/cm, both P(E) (dark gray) and S(E) (light gray) show a slim loop with a very small remanent polarization and strain. Considering the fact that this composition displays a nanosized microstructure and frequency dispersion in dielectric behavior, the P(E ) and S(E ) relations shown in Fig. 4 suggest an electrostrictive behavior like in usual relaxor materials. Applying higher electric fields, the loop shapes change considerably. At about 50 kV/cm, there is a steep increase in both polarization and strain. With decreasing field, a significant hysteretic behavior is apparent, as revealed by both P(E) and S(E) curves. In addition, a significantly suppressed remanent polarization and strain are observed at zero electric field, which indicate the disappearance of the induced ferroelectric order. At the current stage, the origin for the constricted polarization curve is not clear yet, as the constriction in the polarization loop can be caused, for example, by relaxor-to-ferroelectric transition,22 antiferroelectric-to-ferroelectric transition,23 or a significant ageing as in Fe-doped PZT.24 For a similar composition of (1–x–y)BNT–xBT–yKNN, the phase was described as nonpolar.13 In connection with the in situ TEM results shown in Fig. 3, we propose that there is a nonpolar (possibly relaxor or relaxor-like) to ferroelectric phase transition under the application of electric fields in this lead-free piezoceramic. At zero field, the polarization is close to zero and the microstructure is characterized with nanoscale domains, similar to that in Lamodified Pb(Zr1xTix)O3 relaxors.25 With increasing fields up to 45 kV/cm, electrostrictive P(E ) and S(E ) behaviors are observed. At about 50 kV/cm, a phase transition to a ferroelectric phase occurs, as manifested by the sudden increase in both polarization and strain. As the actual field strength during the TEM experiment is intensified to roughly twice the nominal field
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Fig. 3. The grain chosen for the in situ TEM experiment (a) The grain before applying electric field; no domains are visible. (b) The grain with the same orientation as in (a) at nominal 25 kV/cm (actual field about 50 kV/cm); lamellar domains become visible. The black arrow indicates the electric field direction (the tip points out of the paper plane approximately by 101). (c) The same grain oriented in [110]c direction at nominal 25 kV/cm (actual field about 50 kV/cm); lamellar domains are still visible. The diffraction pattern shows 1/2{ooo} superstructure reflections. The electric field direction is close to /100Sc. (d) The same grain at 0 kV/cm with the same orientation as in (c); lamellar domains have disappeared. The 1/2{ooo} superstructure reflections are still visible.
strength,18 the appearance of large ferroelectric domains occurs at the actual field of 50 kV/cm. Therefore, the domain formation observed in the in situ TEM experiment is the microstructural evidence for the phase transition to the ferroelectric state. When the electric field is released, the induced large ferroelectric domains disappear. Correspondingly, both macroscale polarization and strain return to a minimum value. However, EDP analyses did not confirm this electric field-induced phase transition because only 1/2{ooo} rhombohedral SR were visible in this specific grain orientation.
In summary, it was shown that large ferroelectric domains form under applied electric fields in the BNT-based lead-free piezoelectric material. In the observed voltage range, these domains disappear with diminishing electric fields. The observations are consistent with an abrupt increase in polarization and strain as revealed in the P(E) and S(E) hysteresis measurements. A reversible electric field-induced nonpolar (possibly relaxor or relaxor-like) to ferroelectric phase transition is proposed to rationalize the experimental observations.
Acknowledgments The in situ TEM experiment was carried out at Ames Laboratory, which is operated for the U.S. Department of Energy by Iowa State University. The authors wish to thank Dr Gerhard Miehe for the analysis of the pores, Julia Glaum for strain measurements, Josh Frederick for polarization measurements, and Dr. Ljubomira Schmitt and Manuel Hinterstein for helpful discussions.
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Fig. 4. P(E) hysteresis loops (dark gray) and S(E) hysteresis loops (light gray); the loops up to 45 kV/cm show a nearly linear behavior with low remanent polarization and strain. The loops up to 70 kV/cm show a steep increase in polarization and strain above 50 kV/cm. Close to 0 kV/ cm, both exhibit low remanence.
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