actuators

Review Recent Progress on BaTiO3-Based Piezoelectric Ceramics for Actuator Applications

Jinghui Gao 1,* ID , Dezhen Xue 1 ID , Wenfeng Liu 1, Chao Zhou 1 and Xiaobing Ren 1,2,* 1 State Key Laboratory of Electrical Insulation and Power Equipment and Multi-Disciplinary Materials Research Center, Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an 710049, China; [email protected] (D.X.); [email protected] (W.L.); [email protected] (C.Z.) 2 Ferroic Physics Group, National Institute for Materials Science, Tsukuba 305-0047, Ibaraki, Japan * Correspondence: [email protected] (J.G.); [email protected] (X.R.); Tel.: +86-29-8266-4034 (J.G.); +81-29-859-2731 (X.R.)

Received: 15 June 2017; Accepted: 23 July 2017; Published: 31 July 2017

Abstract: Due to issues with Pb toxicity, there is an urgent need for high performance Pb-free alternatives to Pb-based piezoelectric ceramics. Although pure BaTiO3 material exhibits fairly low piezoelectric coefficients, further designing of such a material system greatly enhances the piezoelectric response by means of domain engineering, defects engineering, as well as phase boundary engineering. Especially after the discovery of a Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 system with extraordinarily high piezoelectric properties (d33 > 600 pC/N), BaTiO3-based piezoelectric ceramics are considered as one of the promising Pb-free substitutes. In the present contribution, we summarize the idea of designing high property BaTiO3 piezoceramic through domain engineering, defect-doping, as well as morphotropic phase boundary (MPB). In spite of its drawback of low Curie temperature, BaTiO3-based piezoelectric materials can be considered as an excellent model system for exploring the physics of highly piezoelectric materials. The relevant material design strategy in BaTiO3-based materials can provide guidelines for the next generation of Pb-free materials with even better piezoelectric properties that can be anticipated in the near future.

Keywords: piezoelectricity; Pb-free ceramics; BaTiO3; morphotropic phase boundary; strain

1. Introduction Actuators are devices that can convert input energy into mechanical energy [1]. Among the varieties of actuators with different input energy (including electromagnetic, electrostatic, and thermal energies [2–4] etc.), piezoelectric actuators feature high strain output, high response speed, and high displacement control accuracy. As a result of those advantages, piezoelectric actuators have found wide applications ranging from high-tech equipment such as scanning tunnel microscopy (STM) and atomic force microscopy (AFM), to our daily life devices such as digital cameras and cellular phone terminals [5]. Most piezoelectric actuators rely on a material with fairly large electromechanical response, known as the piezoelectric material. When applying an electric field in a certain direction, the piezoelectric material can generate a series of strain components satisfying various specific application requirements for actuators. The degree of electromechanical response for piezoelectric material, which determines the strain level with respect to external electric field, plays a crucial role on the performance of actuators. For more than 70 years, PZT (PbZr1 − xTixO3) piezoelectric ceramics have been the workhorse of piezoelectric actuator technology [6]. Later in the 1990s, relaxor type piezoelectric single crystal materials, such as PMN–PT (Pb(Mg1/3Nb2/3)O3–xPbTiO3) and PZN–PT(Pb(Zn1/3Nb2/3)O3–xPbTiO3) were discovered [6–11]. All these Pb-based piezoelectric materials dominate the actuator applications of piezoelectric materials due to their high longitudinal electromechanical coupling

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(k33) and large longitudinal piezoelectric coefficient (d33). Table1 lists the properties of these Pb-based materials [6,7,12–15]. The Pb-containing piezoelectrics have been further modified into a number of materials, and the corresponding products have been well commercialized and found mature applications. However, the family of Pb-based materials is now facing the challenge for its environmental compatibility, since it contains the toxic heavy-metal element Pb. And, there exists potential danger during the manufacture, use, and disposal of these materials. In particular, recent global restrictions are demanding the elimination of Pb from all consumer items, which makes it an urgent need to develop a Pb-free substitute that can have piezoelectric properties resembling the Pb-containing materials [16–23]. Although the majority of existing Pb-free materials have inferior piezoelectric properties compared with Pb-based ones [24–27], the solid-state physicians and material scientists have never given up on developing environmental-friendly piezoelectrics, and the publication number for Pb-free piezoelectric materials keeps growing in recent decades (Figure1).

Table 1. Piezoelectric properties of Pb-based systems and BaTiO3 based systems.

Materials Poly/Single Crystal d33 (pC/N) k33 Reference PZT–5A (soft) Polycrystalline 374 0.71 [6] PZT–8 (hard) Polycrystalline 225 0.64 [12] PMN–70PT Single crystal 1500 >0.9 [7] 92%PZN–8%PT Single crystal 2200 >0.9 [13] 0.5BZT–0.5BCT Polycrystalline 620 0.65 [14] 0.7BTS–0.3BCT Polycrystalline 530 0.57 [15]

Up to now, the widely investigated Pb-free piezoelectric materials have focused on, but are not limited to KNN ((K1/2Na1/2)NbO3)-based, BNT ((Bi1/2Na1/2)TiO3) or BKT ((Bi1/2K1/2)TiO3)-based, and BaTiO3-based ferroelectric material systems, which show large piezoelectric response [17–27]. Among these potential substitutions for PZT, the class of BaTiO3-based piezoelectric material is easy to process with better electromechanical properties. Despite its drawback of fairly low Curie temperature ◦ of approximately 100 C, the recent discoveries of large electromechanical activities in BaTiO3-based material (shown in Table1) make it a model system for understanding the underlying physics of Pb-free piezoelectrics, and thus it can provide a tutorial for the design of Pb-free piezoelectric material systems [14]. Due to these reasons, the number of research papers on the piezoelectric response in BaTiO3–based material has been increasing year by year (Figure1). This contribution will give an overview of BaTiO3-based piezoelectric materials with high piezoelectric properties and will be arranged as follows. Section2 introduces the piezoelectric response in an undoped BaTiO3 material system and its domain engineering. Section3 reviews the enhanced electromechanical effect induced by doping point defects. And, we will focus on the large piezoelectric effect caused by introducing phase boundary or interferroelectric transitions in Section4. The applications and prospective thoughts will be given in Section5. Actuators 2017, 6, 24 3 of 20 Actuators 2017, 6, 24 3 of 20

Figure 1. The publication number relates with Pb-free and BaTiO33-based piezoelectrics.

2. Undoped BaTiO 3

2.1. BaTiO 3 CeramicsCeramics and and Single Single Crystal Crystal

The BaTiO 3 waswas discovered discovered during during World World War II as a high capacitance material and was the first first discovered ferroelectric compound with a perovskite structure. Later on, the electrostriction electrostriction effect for the unpoled polycrystalline ceramics of BaTiO33 as well as the the piezoelectricity piezoelectricity for the electrically electrically poled samples was found, leading to many applications earlier than the Pb-containing material,material, PZT.PZT. Table2 2 shows shows thethe basicbasic piezoelectricpiezoelectric coefficientscoefficients forfor undopedundoped BaTiOBaTiO3 single crystal and the polycrystalline ceramic ceramic [28]. [28]. It Itcan can be beseen seen that that these these parameters parameters are fairly are fairly low lowcompared compared with withPZT ceramics.PZT ceramics. For example, For example, the piezoelectric the piezoelectric coefficient coefficient d33 cand33 onlycan reach only reachthe value the value of 191 of pC/N 191 pC/N for a BaTiOfor a BaTiO3 ceramic3 ceramic and 85.6 and pC/N 85.6pC/N for a single for asingle crystal crystal specimen, specimen, which which are only are 3–5 only fold 3–5 that fold of that PZT of ◦ materialsPZT materials [29]. [Moreover,29]. Moreover, the theCurie Curie temperature temperature for fora BaTiO a BaTiO3 system3 system Tc~130 Tc~130 °CC is is much much lower compared with PZT and the relevantrelevant Pb-containingPb-containing piezoelectric material systems. Owing to these disadvantages, although although being being earlier discovered, the BaTiO 3 hashas not been found the satisfactory application onon thethe associatedassociated piezoelectric piezoelectric devices devices such such as as actuators. actuators. Therefore, Therefore, further further treatments treatments are arenecessary necessary to enhance to enhance the the piezoelectric piezoelectric properties properties for suchfor such a material a material system. system.

Table 2. Piezoelectric coefficientscoefficients for BaTiO33 single crystal and ceramic.

CrystalCrystal Ceramic Ceramic d15 392 270 d15 392 270 d31 −34.5 −79 d31 −34.5 −79 dd3333 85.685.6 191 191 gg1515 15.215.2 18.8 18.8 g −23.0 −4.7 g3131 −23.0 −4.7 g33 57.5 11.4 g33 57.5 11.4 k31 0.315 0.208 kk3331 0.3150.560 0.208 0.494 kk1533 0.5600.570 0.494 0.466 T ε11 k/15ε 0 0.5702920 0.466 1436 T ε33 T/ε0 168 1680 ε11S /ε0 2920 1436 ε11 /ε0 1970 1123 ε33ST/ε0 168 1680 ε33 /ε0 109 1256 11SE 0 εS11 /ε 19708.05 1123 8.55 SE εS3333 /ε0 15.7109 1256 8.93 S11E 8.05 8.55 S33E 15.7 8.93 S12E −2.35 −2.61

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Table 2. Cont. Actuators 2017, 6, 24 4 of 20 Crystal Ceramic ES13E −5.24 2.85 S12 −2.35 −2.61 E E S13 S44 −5.2418.4 2.8523.3 E S44 S66 18.48.84 23.322.3 S 8.84 22.3 66 S11D 7.25 8.18 D S11 D 7.25 8.18 DS33 10.8 6.76 S33 10.8 6.76 DS12D −3.15 −2.98 S12 −3.15 −2.98 D D S13 S13 −3.26−3.26 −1.95−1.95 D S44 S44D 12.412.4 18.318.3 Note:Note: Data Data from from [28]. [28]

2.2. Domain-Engineered BaTiO3 2.2. Domain-Engineered BaTiO3 Since the piezoelectric effect is fairly low for a BaTiO3 single crystal poled along the polar axis, a so-calledSince the domain piezoelectric wall engineering effect is fairly technique low for has a BaTiO been3 employed,single crystal making poled use along of the polarpiezoelectric axis, a so-calledanisotropy domain of single wall crystals engineering [30–32]. technique It has been has been performed employed, by poling making the use single of the crystal piezoelectric with the anisotropyelectric field of single along crystals the direction [30–32]. which It has beenhas an performed intersection by polingangle with the single respect crystal to the with crystallographic the electric fieldpolar along axis. the directionFor example, which hasFigure an intersection 2 shows anglethe schematic with respect model to the of crystallographic the engineered polar domain axis. Forconfigurations example, Figure for2 the shows tetragonal the schematic 4 mm ferroelectric model of the crystals engineered [33]. If domain the poling configurations electric field for is thealong tetragonalthe [110] 4 mmdirection, ferroelectric it will crystalsgenerate [33 two]. If equally-preferred the poling electric ferroelectric field is along domains the [110] direction,with spontaneous it will generatepolarizations two equally-preferred (PS) along [100] ferroelectric and [010], respectively. domains with In spontaneous the case of [111] polarizations poled crystal, (PS) along [100], [100] [010], andand [010], [001] respectively. are three favorable In the case PS alignments of [111] poled for crystal,ferroelectric [100], domains. [010], and Complex [001] are domain three favorable configuration PS alignmentshas been forformed, ferroelectric which enables domains. enlarged Complex piezoelect domainric configuration response of has the been domain-engineered formed, which enables crystal. enlarged piezoelectric response of the domain-engineered crystal.

Figure 2. Schematic model of the engineered domain configurations for the tetragonal 4mm ferroelectric Figure 2. Schematic model of the engineered domain configurations for the tetragonal 4mm crystals (Ps: spontaneous polar vector along the <001>c directions) [33]. ferroelectric crystals (Ps: spontaneous polar vector along the <001>c directions) [33].

TheThe intrinsic intrinsic piezoelectric piezoelectric anisotropy anisotropy can can be be considered considered as as one one of theof the reasons reasons for for the the enhanced enhanced piezoelectricpiezoelectric response response in in a BaTiOa BaTiO3 single3 single crystal crystal [32 [32].]. It It should should be be noted noted that that the the piezoelectric piezoelectric coefficientscoefficients (e.g., (e.g.,d33 d)33) usually usually show show orientation-dependentorientation-dependent ph phenomenonenomenon related related with with their their Gibbs Gibbs free freeenergy, energy, considering considering the the crystal crystal symmetry. ForFor example,example, Figure Figure3 shows3 shows the the change change of piezoelectricof piezoelectric coefficientcoefficientd33 d*33 with* with respect respect to to the the orientation orientation in ain tetragonal a tetragonal phase phase crystal crystal [30 [30].]. It canIt can be be seen seen that that the the maximummaximumd33 dvalue33 value has has not not been been achieved achieved along along the the polar polar axis axis which which is <001>is <001> for for tetragonal tetragonal crystal crystal symmetry.symmetry. Instead, Instead, the thed33 dalong33 along the the [111] [111] direction direction shows shows a larger a larger value value compared compared with with that that of <001>of <001> directions,directions, which which is mainlyis mainly due due to theto the piezoelectric piezoelectric anisotropy anisotropy in thein the crystal. crystal.

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Figure 3. (a–c) The role of an electric bias field (E3 = 0, −10 MV/m) at T = 298 K on anisotropic free Figure 3. (a–c) The role of an electric bias field (E3 = 0, −10 MV/m) at T = 298 K on anisotropic free energy flattening and piezoelectric enhancement. (d–f) The change of anisotropic free energy and energy flattening and piezoelectric enhancement. (d–f) The change of anisotropic free energy and piezoelectric coefficient around the tetragonal-orthorhombic phase transition temperature in BaTiO3 piezoelectricFigure 3. ( coefficienta–c) The role around of an electric the tetragonal-orthorhombic bias field (E3 = 0, −10 MV/m) phase at transitionT = 298 K on temperature anisotropic infree BaTiO 3 (which is adapted with permission from [30]. Copyrighted by the American Physical Society). (whichenergy is adapted flattening with and permission piezoelectric from enhancement. [30]. Copyrighted (d–f) The bychange the Americanof anisotropic Physical free energy Society). and piezoelectric coefficient around the tetragonal-orthorhombic phase transition temperature in BaTiO3 On (whichthe other is adapted hand, with the permission extrinsic from contribution [30]. Copyrighted also playsby the Americana crucial Physical role in Society). further enhancing the On the other hand, the extrinsic contribution also plays a crucial role in further enhancing the piezoelectric response for BaTiO3 single crystals. Wada et al. reported that high piezoelectric effect piezoelectriccan be Onobtained the response other by hand, forreducing BaTiO the extrinsic3 singlethe contributionsize crystals. of ferroelectric Wada also plays et al. a reported crucialdomains role that within high further a piezoelectric domain-engineeredenhancing effectthe can beconfiguration obtainedpiezoelectric by [34]. reducing response As shown thefor BaTiO size in Figure of3 ferroelectricsingle 4, crystals.as the domainsdomain Wada et size with al. decreaserepo a domain-engineeredrted thatfrom high 40 μpiezoelectricm to configuration 5.5 μm effect according [34]. As showncan be inobtained Figure4 ,by as thereducing domain the size size decrease of ferroelectric from 40 domainsµm to 5.5 withµm a according domain-engineered to their polling to their polling conditions, the piezoelectric coefficient d31 increases from 97.8 pC/N to 230 pC/N, configuration [34]. As shown in Figure 4, as the domain size decrease from 40 μm to 5.5 μm according conditions,which suggests the piezoelectricthat enhanced coefficient piezoelectric d31 increases response from can be 97.8 achieved pC/N to in 230 a fine pC/N, engineered-domain which suggests thatto enhanced their polling piezoelectric conditions, response the piezoelectric can be achieved coefficient in d a31 fineincreases engineered-domain from 97.8 pC/N configurationto 230 pC/N, [34]. configurationwhich suggests [34]. that Moreover, enhanced apiezoelectric strong piezoelectric response can response be achieved can in be a fineobtained engineered-domain by forming even Moreover, a strong piezoelectric response can be obtained by forming even smaller domains, i.e., smallerconfiguration domains, [34]. i.e., Moreover,nanodomains a strong in BaTiO piezoelectric3 ceramic, response suggested can beby obtainedTakahashi by etforming al. and even Karaki et nanodomains in BaTiO ceramic, suggested by Takahashi et al. and Karaki et al. Wada et al. even al. Wadasmaller et domains, al. even i.e.,reported 3nanodomains d33 value in of BaTiO 788 3pC/N ceramic, in asuggested template-grain-grown by Takahashi et BaTiO al. and3 ceramicKaraki et which reported d value of 788 pC/N in a template-grain-grown BaTiO ceramic which might have partial mightal. Wadahave33 partialet al. even contribution reported d33 from value orientation of 788 pC/N effects in a template-grain-grown [35–37]. Such3 a size-dependent BaTiO3 ceramic phenomenon which contributionmightmight be havedue from topartial the orientation enlargedcontribution effectscharged from [35 orientation domain–37]. Such walleffects a size-dependent density [35–37]. in Such an aengineered-domain phenomenonsize-dependent might phenomenon configuration be due to the enlarged[38].might On the charged be otherdue to domain hand, the enlarged the wall 90° densitycharged domain in domain wall an engineered-domain contri wall densitybution hasin an also configurationengineered-domain been considered [38]. configuration On as the the other reason hand, for the 90[38].◦ domain On the other wall hand, contribution the 90° domain has also wall been contri consideredbution has as also the been reason considered for enhanced as the reason piezoelectric for in enhanced piezoelectric in nanodomain tetragonal BaTiO3 single crystals [39]. nanodomainenhanced piezoelectric tetragonal BaTiO in nanodomain3 single crystals tetragonal [39 BaTiO]. 3 single crystals [39].

FigureFigure 4. ( a4.– d(a)– Domaind) Domain configurations configurations of of 31 31 resonators resonators withwith different domain domain sizes sizes ranging ranging from from 5.5 5.5 µm μm to 40 μm. (e) Domain size dependence of d31 and k31 for <111>c-poled BaTiO3 single crystals [34]. toFigure 40 µm. 4. ( ae–)d Domain) Domain size configurations dependence ofof d3131 resonatorsand k31 for wi <111>th differentc-poled BaTiOdomain3 single sizes ranging crystals from [34]. 5.5 μm to 40 μm. (e) Domain size dependence of d31 and k31 for <111>c-poled BaTiO3 single crystals [34]. 3. Large Non-Linear Electrostrain in Aging Point-Defect-Doped BaTiO3

3. Large Non-Linear Electrostrain in Aging Point-Defect-Doped BaTiO3

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3. Large Non-Linear Electrostrain in Aging Point-Defect-Doped BaTiO3 Actuators 2017,, 6,, 2424 6 of 20 The non-180◦ domain switching is always accompanied with a large electrostrain due to the exchangeThe non-180° of non-equal domain crystallographic switching is axes. always The acco levelmpanied of strain duewith to a suchlarge a electrostrain process can reach due oneto the or twoexchange orders of of non-equal magnitude crystallographic larger than the axes. linear The strain level of of poled strain piezoelectric due to such materials.a process can However, reach one on removalor two orders of the of electric magnitude field, itlarger cannot than return the linear to the st originalrain of poled unpoled piezoelectric domain configuration, materials. However, leading toon aremoval remnant of polarizationthe electric field, when it cannot the electric return field to the is zero.original Hence, unpoled it makes domain the configuration, domain switching leading a one-timeto a remnant or irrecoverable polarizationeffect when that the restrictselectric field the application is zero. Hence, of non-180 it makes◦ domain the domain switching switching for actuator a one- applications.time or irrecoverable In 2004, effect our groupthat restricts reported the a application giant recoverable of non-180° electrostrain domain inswitching a ferroelectric-aged for actuator BaTiOapplications.3 single In crystal 2004, dopedour group by acceptor reported ions a giant [40]. recoverable And, the strain electrostrain level can in reach a ferroelectric-aged up to 0.75% at a 3 lowBaTiO field3 single of 200 crystal V/mm, doped which by acceptor largely exceeds ions [40]. the And, strain the ofstrain soft level PZT can ceramics reach andup to PZN–PT 0.75% at single a low crystalsfield of 200 under V/mm, the which sameelectric largely fieldexceeds conditions. the strain The of soft similar PZT effectceramics was and later PZN–PT reported single in the crystals aged acceptor-dopedunder the same BaTiOelectric3-based field conditions. ceramic (BaSr The0.05 similaTiO3–0.01Mn)r effect was with later a large reported recoverable in thenonlinear aged acceptor- strain 3 0.05 3 ofdoped about BaTiO 0.12–0.15%3-based ceramic at a field (BaSr of 30.05 kV/mm,TiO3–0.01Mn) which with is higher a large thanrecoverable that of nonlinear conventional strain hard of about PZT piezoelectric0.12%–0.15% ceramicsat a field of (as 3 shownkV/mm, in which Figure is5 )[higher41]. Itthan should that beof conventional noted that the hard electric PZT fatigue piezoelectric study showsceramics that [41]. the It electrostrain should be noted has a that good the recoverability electric fatigue even study after shows 10,000 that cycles the at electrostrain 3 kV/mm electric has a good field asrecoverability shown in Figure even6 after. 10,000 cycles at 3 kV/mm electric field as shown in Figure 5.

3 Figure 5.5. TheThe non-linear non-linear electrostrain electrostrain in in aged point-defect-doped BaTiO BaTiO3 singlesingle crystalcrystal ( (left) andand ceramics (right)[) [40,41].40,41].

Figure 6. Electric-fatigue in Mn-doped BaTiO3 ceramic [41]. Figure 6. Electric-fatigue in Mn-doped BaTiO33 ceramic [41]. [41].

In order to understand the large recoverable electrostrain in such a material system, in-situ In order to understand the large recoverable electrostrain in such a material system, in-situ 3 optical microscopic observation has been conducted for BaTiO3 single crystals upon electric field optical microscopic observation has been conducted for BaTiO3 single crystals upon electric field loading [42]. The mesoscopic evidence suggests that using in-situ domain observation shows that the 3 aged Mn-doped BaTiO3 single crystal exhibits a remarkable reversible domain switching during electric field cycling. The giant recoverable electrostrain effect switching a maximum strain of 0.4% has also been observed simultaneously.

Actuators 2017, 6, 24 7 of 20 loading [42]. The mesoscopic evidence suggests that using in-situ domain observation shows that Actuators 2017, 6, 24 7 of 20 the aged Mn-doped BaTiO3 single crystal exhibits a remarkable reversible domain switching during electric field cycling. The giant recoverable electrostrain effect switching a maximum strain of 0.4% The ferroelectric aging effect is quite difficult to understand by the classical ferroelectric theories. has also been observed simultaneously. It is now generally considered that ferroelectric aging effect originates from the migration of oxygen The ferroelectric aging effect is quite difficult to understand by the classical ferroelectric theories. vacancies inside the materials with time. There are several models to understand the exotic It is now generally considered that ferroelectric aging effect originates from the migration of oxygen ferroelectric aging effect, including the grain boundary effect, domain wall effect, and the volume vacancies inside the materials with time. There are several models to understand the exotic ferroelectric effect. (1) The grain boundary effect refers to the oxygen migration into the secondary phase in the aging effect, including the grain boundary effect, domain wall effect, and the volume effect. (1) The grain boundary to form the space charge layer, driven by the depolarization field, and stabilizes the grain boundary effect refers to the oxygen migration into the secondary phase in the grain boundary ferroelectric domains [43,44]. (2) The domain wall effect involves the oxygen vacancy migration to to form the space charge layer, driven by the depolarization field, and stabilizes the ferroelectric the domain walls and pinning of the domain walls electrically or elastically [45,46]. (3) The volume domains [43,44]. (2) The domain wall effect involves the oxygen vacancy migration to the domain effect refers to the reorientation of point defects with respect to the PS direction with the whole walls and pinning of the domain walls electrically or elastically [45,46]. (3) The volume effect refers volume, and thus stabilizes the ferroelectric domain states [47–53]. Among them, the volume effect to the reorientation of point defects with respect to the P direction with the whole volume, and can explain all ferroelectric aging effects in different systems,S and is thus the intrinsic effect for thus stabilizes the ferroelectric domain states [47–53]. Among them, the volume effect can explain all ferroelectric aging. ferroelectric aging effects in different systems, and is thus the intrinsic effect for ferroelectric aging. Based on the volume effect explanation for ferroelectric aging, a more detailed microscopic Based on the volume effect explanation for ferroelectric aging, a more detailed microscopic model model has been proposed to consider the interaction between the spontaneous polarization (PS) and has been proposed to consider the interaction between the spontaneous polarization (PS) and the defect the defect dipolar polarization (PD) [40]. The idea of such a principle is that the distribution of oxygen dipolar polarization (P )[40]. The idea of such a principle is that the distribution of oxygen vacancies vacancies around the acceptorD dopant shows statistic symmetry, known as “defect symmetry”, which around the acceptor dopant shows statistic symmetry, known as “defect symmetry”, which has the has the tendency to conform to the crystal symmetry after aging. As shown in Figure 7, the cubic tendency to conform to the crystal symmetry after aging. As shown in Figure7, the cubic crystal crystal symmetry has the cubic defect symmetry with symmetric distribution of oxygen vacancies symmetry has the cubic defect symmetry with symmetric distribution of oxygen vacancies around the around the acceptor dopants. However, on cooling the sample below TC, the ferroelectric transition acceptor dopants. However, on cooling the sample below T , the ferroelectric transition will occur, but will occur, but such a diffusionless transition does not allowC the defect symmetry to change abruptly. such a diffusionless transition does not allow the defect symmetry to change abruptly. During aging, During aging, the oxygen vacancies will migrate into a non-symmetric defect configuration, which is the oxygen vacancies will migrate into a non-symmetric defect configuration, which is suitable for suitable for the tetragonal crystal symmetry and causes gradual stabilization with time. The above- the tetragonal crystal symmetry and causes gradual stabilization with time. The above-mentioned mentioned degradation of small-signal properties and change of large-signal properties can be degradation of small-signal properties and change of large-signal properties can be explained by such explained by such a principle. Besides, a recent study shows that the stabilization effect can be a principle. Besides, a recent study shows that the stabilization effect can be manifested as the Curie manifested as the Curie temperature gradually increases with aging time [54]. temperature gradually increases with aging time [54].

Figure 7. Cont.

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FigureFigure 7. (a )7.Direct (a) Direct evidence evidence for for reversible reversible domain-switching domain-switching process process and and its itsrelation relation to a to double a double

hysteresishysteresis loop loop in aged in aged Mn-BaTiO Mn-BaTiO3 single-crystal3 single-crystal sample.sample. ( (bb) )Its Its comparison comparison to an to anunaged unaged specimen. specimen. (c) Based(c) Based on the on observation,the observation, the the mechanism mechanism is is given given regarding defect defect symmetry symmetry and and related related crystal crystal symmetry in a perovskite BaTiO3 structure doped with Mn3+3+ ions at Ti4+ sites4+ [42]. symmetry in a perovskite BaTiO3 structure doped with Mn ions at Ti sites [42].

4. Large Piezoelectric Response Caused by Morphotropic Phase Boundary in BaTiO3 4. Large Piezoelectric Response Caused by Morphotropic Phase Boundary in BaTiO3

4.1. Ba(Zr0.2Ti0.8)O3–(Ba0.7Ca0.3)TiO3 4.1. Ba(Zr0.2Ti0.8)O3–(Ba0.7Ca0.3)TiO3 The common approach to achieving high piezoelectricity is to place the composition of the Thematerial common in the approachproximity toof achievinga phase transition high piezoelectricity regime between istwo to ferroelectric place the composition phases. Such ofa the materialcomposition-induced in the proximity phase of a transition phase transition region in regimethe pseudo-binary between twocomposition-temperature ferroelectric phases. phase Such a composition-induceddiagram has been known phase transitionas the “morphotropic region in thephas pseudo-binarye boundary (MPB)”. composition-temperature Figure 8 shows the typical phase diagramphase has diagram been known of PZT as[55,56], the “morphotropic which is characteri phasezed boundary by a vertical (MPB)”. MPB separating Figure8 shows a ferroelectric the typical phaserhombohedral diagram of PZT(R) phase [55,56 and], which a ferroelectric is characterized (T) tetragonal by a verticalphase. At MPB the separatingMPB, there are a ferroelectric strong rhombohedralpolarization (R) instabilities phase and and avanishing ferroelectric polarization (T) tetragonal anisotropy, phase. which Atfacilitate the MPB, ease of there polarization are strong rotation by external stress or electric field. It thus results in a high piezoelectricity and permittivity. polarization instabilities and vanishing polarization anisotropy, which facilitate ease of polarization It should be noted that although MPB and the relevant mechanisms for high piezoelectricity have rotation by external stress or electric field. It thus results in a high piezoelectricity and permittivity. It mainly been discussed for Pb-based systems, it seems to be applicable to Pb-free systems as well. should beIn noted our previous that although work, we MPB proposed and the a design relevant stra mechanismstegy that combines for high two piezoelectricity Pb-free end compositions have mainly been discussedto establish fora pseudo-binary Pb-based systems, system with it seems MPB. to We be selected applicable a certain to Pb-free composit systemsion that asundergoes well. only Incubic our (C) previous to tetragonal work, (T) we phase proposed transition a design at the T- strategyend and thata certain combines composition two Pb-free with only end cubic compositions (C) to to establishrhombohedral a pseudo-binary (R) phase transition system withat the MPB. R-end. We Then selected the combination a certain of composition the selected thatT-end undergoes and R-end only cubicwill (C) toresult tetragonal in a pseudo-binary (T) phase transitionphase diagram, at the posse T-endssing and a MPB a certain separating composition R and T phases. with only The cubicfirst (C) to rhombohedralsystem we designed (R) phase was the transition (1 − x)Ba(Zr at0.2 theTi0.8 R-end.)O3–x(Ba Then0.7Ca0.3)TiO the combination3 ((1 − x)BZT–xBCT) of the [14], selected where x T-end is the and 2+ R-endmolar will resultpercent in of a pseudo-binaryBCT, as shown in phase Figure diagram, 8. The Ca possessing concentration a MPB was separating set to be 30 R andat.% Tto phases. ensure The the pure C-T phase transition, and the Zr4+ concentration was fixed to be 20 at.% to let the pure C-R first system we designed was the (1 − x)Ba(Zr Ti )O –x(Ba Ca )TiO ((1 − x)BZT–xBCT) [14], phase transition occur. The resulting pseudo-binary0.2 0.8 phase3 diagram0.7 of 0.3BZT–BCT3 is thus characterized where x is the molar percent of BCT, as shown in Figure8. The Ca 2+ concentration was set to be 30 at.% by a MPB separating ferroelectric R (BZT side) and T (BCT side) phases. It should be noted that this 4+ to ensurematerial the system pure C-T was phase abbreviated transition, as BZCT and or theBCZT Zr in someconcentration literatures. wasHowever, fixed it to should be 20 be at.% pointed to let the pure C-Rout that phase it is transitionphysically occur.clearer to The use resulting BZT–BCT pseudo-binary to demonstrate the phase morphotropic diagram ofphase BZT–BCT boundary is thus characterizedorigin for bysuch a MPBa material separating system. ferroelectric Such a physical R (BZT idea side)has also and been T (BCT adopted side) inphases. the designing It should of be notedKNN–based, that this material lead-free system piezoelectri was abbreviatedc materials, and as BZCTa promisingly or BCZT large in some piezoelectric literatures. response However, has it shouldbeen be achieved pointed on out the that phase it is boundary physically of some clearer material to use systems BZT–BCT [18,22,57,58]. to demonstrate the morphotropic phase boundary origin for such a material system. Such a physical idea has also been adopted in the designing of KNN–based, lead-free piezoelectric materials, and a promisingly large piezoelectric response has been achieved on the phase boundary of some material systems [18,22,57,58]. Actuators 2017, 6, 24 9 of 20

Actuators 2017, 6, 24 9 of 20 The terminology “morphotropic phase boundary (MPB)” has been employed for BZT–BCT The terminology “morphotropic phase boundary (MPB)” has been employed for BZT–BCT piezoelectric ceramics. Admittedly, for most of the BaTiO3-based systems such a boundary is named piezoelectric ceramics. Admittedly, for most of the BaTiO3-based systems such a boundary is named “polymorphic phase boundary (PPB)”, because it is not vertical to the composition axis and thus “polymorphic phase boundary (PPB)”, because it is not vertical to the composition axis and thus dependentdependent on temperature. on temperature. The reasonThe reason why why MPB MPB is isstill still used for for BZT–BCT BZT–BCT hereafter hereafter is to illustrate is to illustrate its similarityits with similarity PZT inwith the PZT phase in the diagram. phase diagram.

Figure 8. TheFigure comparison 8. The comparison of phase of phase diagram diagram for PbZrfor PbZr1 −1 −x xTixOO3 (3left(left) and) and Ba(Zr Ba(Zr0.2Ti0.80.2)OTi3–(Ba0.8)O0.7Ca3–(Ba0.3)TiO0.73 Ca0.3)TiO3 (right)[14(].right) [14].

The piezoelectric response and the associated physical properties have been largely enhanced The piezoelectricon MPB composition response at room and temperature, the associated i.e., physical0.5BZT–0.5BCT. properties As shown have in beenFigure largely 9c, the enhanced on MPBcomposition-dependence composition at room of the temperature, piezoelectric coefficient i.e., 0.5BZT–0.5BCT. exhibits a peak with As a maximum shown d in33 value Figure of 9c, the 620 pC/N for 0.5BZT–0.5BCT. Such a d33 level is superior than most of the Pb-free piezoelectric d composition-dependenceceramics, and is even of comparable the piezoelectric with commerciali coefficientzed high-end exhibits PZT a ceramics peak with [13]. The a maximum strain for a 33 value of 620 pC/N0.5BZT–0.5BCT for 0.5BZT–0.5BCT. ceramic can reach Such 0.05% a d 33in anlevel electric is superior field of 500 than kV/mm most [14], of which the is Pb-free even larger piezoelectric ceramics,than and PZT–5H is even at comparable the same electric with field commercialized condition. Moreover, high-end Bowman’s PZT group ceramics and other [groups13]. The even strain for a 0.5BZT–0.5BCTreported ceramic a larger strain canreach level at 0.05% a near-MPB in an composition electric field [59–61], of500 which kV/mm makes BZT–BCT [14], which ceramic is evena larger promising material for actuator application [62–65]. Besides piezoelectric coefficients, other than PZT–5Hparameters, at the such same as electricmaximum field polarization, condition. remnant Moreover, polarization, Bowman’s dielectric permittivity group and etc. other are also groups even reported aenhanced larger strainon the MPB level composition at a near-MPB 0.5BZT–0.5BCT. composition The detailed [59– physical61], which parameters makes for BZT–BCT the optimal ceramic a promisingcomposition material for of 0.5BZT–0.5BCT actuator application are shown [62 in– Table65]. Besides3 [66]. It should piezoelectric be emphasized coefficients, that the values other of parameters, such as maximumthe listed polarization,parameters are remnantlargely determined polarization, by the dielectric material processing permittivity conditions etc. are and also poling enhanced on conditions for BZT–BCT ceramics [58,64,65], and the appropriate condition should be selected in the MPB compositionorder to achieve 0.5BZT–0.5BCT. best properties for the The piezoceramic. detailed physical parameters for the optimal composition of 0.5BZT–0.5BCT are shown in Table3[ 66]. It should be emphasized that the values of the listed parameters are largely determinedTable 3. The by piezoelectric the material coefficients processing for 0.5BZT–0.5BCT conditions [66]. and poling conditions for

BZT–BCT ceramics [58,64,65], and the appropriatePiezoelectric condition Coefficients should be selected in order to achieve best properties for the piezoceramic. dij (10−12 N/C) eij (C/m2) gij (10−3 Vm/N) hij (108 V/m)

Material d33 Tabled31 3. Thed15 piezoelectric e33 e coefficients31 e15 for 0.5BZT–0.5BCTg33 g31 g15 [66 ]. h33 h31 h15 BZT–50BCT 546 −231 453 22.4 −5.7 12.1 15.3 −6.5 31.0 8.6 −2.1 8.3 Piezoelectric Coefficients BaTiO3 191 −79 270 11.6 −4.4 18.6 11.4 −4.7 18.8 9.2 −3.5 16.6 −12 2 −3 8 dij (10 N/C) eij (C/m ) gij (10 Vm/N) hij (10 V/m) PZT5A 374 −171 584 15.8 −5.4 12.3 24.9 −11.4 38.0 21.4 −7.3 15.0 Material d33 d31 d15 e33 e31 e15 g33 g31 g15 h33 h31 h15 Dielectric Constants BZT–50BCT 546 −231 453 22.4 −5.7 12.1 15.3 −6.5 31.0Electromechanical8.6 Coupling−2.1 8.3

− − −4 − Factors − BaTiO3 191 79 ࢿ࢏࢐270(0) 11.6 4.4 18.6ࢼ࢏࢐(10 /0) 11.4 4.7 18.8 9.2 3.5 16.6 PZT5A 374 −171 584 15.8 −5.4 12.3 24.9 −11.4 38.0 21.4 −7.3 15.0 Dielectric Constants Electromechanical Coupling

−4 Factors εij (ε0) βij (10 /ε0) T a T a S a S a T T S S ε33 ε11 ε33 ε11 β33 β11 β33 β11 k33 k31 k15 kt kp BZT-50BCT 4050 2732 2930 1652 2.47 3.66 3.41 6.05 0.65 0.31 0.48 0.42 0.53 BaTiO3 1898 1622 1419 1269 5.3 6.2 7.0 7.9 0.49 0.21 0.48 ... 0.35 PZT5A 1700 1730 830 916 5.9 5.8 12.0 10.9 0.70 0.34 0.68 0.49 0.60 a Directly measured properties. Actuators 2017, 6, 24 10 of 20

ࢀ a ࢀ a ࡿ a ࡿ a ࢀ ࢀ ࡿ ࡿ ࢿ૜૜ ࢿ૚૚ ࢿ૜૜ ࢿ૚૚ ࢼ૜૜ ࢼ૚૚ ࢼ૜૜ ࢼ૚૚ k33 k31 k15 kt kp

BZT-50BCT 4050 2732 2930 1652 2.47 3.66 3.41 6.05 0.65 0.31 0.48 0.42 0.53

BaTiO3 1898 1622 1419 1269 5.3 6.2 7.0 7.9 0.49 0.21 0.48 … 0.35

PZT5A 1700 1730 830 916 5.9 5.8 12.0 10.9 0.70 0.34 0.68 0.49 0.60 Actuators 2017, 6, 24 10 of 20 a Directly measured properties

Figure 9. The composition dependence of the piezoelectric,piezoelectric, dielectric, and ferroelectricferroelectric properties for BZT–BCT piezoelectricpiezoelectric system [[14].14]. (a) P-E hysteresis loop.loop. (b1–b6) Maximum polarization, remnant

polarization, coercivecoercive field, field, dielectric dielectric permittivity, permittivity,d31, dS /dd31E, .(dcS,d/d)E The. (c comparison,d) The comparison for the piezoelectric for the parameterspiezoelectric between parameters BZT–BCT between and BZT–BCT other systems. and other (e1,e2 systems.) The temperature (e1,e2) The stability temperature for BZT–BCT. stability for BZT–BCT. 4.2. Crystal Structure 4.2. Crystal Structure The original idea for the material design for BZT–BCT system was to mix two binaries with tetragonalThe original and rhombohedral idea for the symmetries,material design respectively. for BZT–BCT Therefore system it is was natural to mix to expect two binaries that on MPBwith thetetragonal crystal and structure rhombohedral is a combination symmetries, of tetragonal respectively and. Therefore rhombohedral. it is natural The to lab expect X-ray that diffraction on MPB studiesthe crystal on 0.5BZT–0.5BCTstructure is a combin suggestsation that of the tetragonal result can beand interpreted rhombohedral. as a coexisting The lab ofX-ray tetragonal diffraction and rhombohedralstudies on 0.5BZT–0.5BCT crystal symmetries suggests [14 that,67]. the Haugen result ca etn al. be further interpreted employed as a coexisting the high-resolution of tetragonal XRD and to studyrhombohedral the crystal crystal structure symmetries of 0.5BZT–0.5BCT [14,67]. Haugen followed et byal. Rietveldfurther employed analysis. The the measuredhigh-resolution data can XRD be fittedto study by usingthe crystal the T-R structure coexisting of 0.5BZT–0.5BCT model [68]. We fo alsollowed used by the Rietveld convergent analysis. beam The electron measured diffraction data methodcan be fitted to prove by using that diffraction the T-R coexisting symmetry model satisfies [68]. the We tetragonal also used and the rhombohedral convergent modelbeam electron [69,70]. In contrast, Keeble et al. pointed out that there exists an intermediate phase interleafing the tetragonal and rhombohedral models by investigating the temperature-evolution of high-resolution XRD reflections, and the result is shown in Figure 10[ 71]. It can be seen that the measured temperature window can be divided into 4 regions for 0.5BZT–0.5BCT, which is the indication that there exists another phase besides the already-known C, T, and R phases. The Rietveld analysis further suggests an orthorhombic crystal symmetry for such an intermediate phase. And similar conclusions have been drawn from the temperature-dependence of elastic anomalies as well as the Rama spectrum [72,73]. Actuators 20172017,, 66,, 2424 1111 ofof 2020 diffraction method to prove that diffraction symmetry satisfies the tetragonal and rhombohedral model [69,70]. InIn contrast,contrast, KeebleKeeble etet al.al. pointedpointed outout thatthat thertheree existsexists anan intermediateintermediate phasephase interleafinginterleafing thethe tetragonaltetragonal andand rhombohedralrhombohedral modelsmodels byby investigatinvestigatinging thethe temperature-evoluttemperature-evolutionion ofof high-resolutionhigh-resolution XRD reflections, and the result is shown in Figure 1010 [71].[71]. ItIt cancan bebe seenseen thatthat thethe measuredmeasured temperaturetemperature windowwindow cancan bebe divideddivided intointo 44 regionsregions forfor 0.5BZT–0.5BCT,0.5BZT–0.5BCT, whichwhich isis thethe indicationindication thatthat therethere existsexists anotheranother phasephase besidesbesides thethe already-knalready-known C, T, and R phases. The Rietveld analysis furtherActuatorsfurther 2017suggestssuggests, 6, 24 anan orthorhombicorthorhombic crystalcrystal symmetrsymmetry for such an intermediate phase. And similar11 of 20 conclusionsconclusions havehave beenbeen drawndrawn fromfrom thethe temperature-temperature-dependence of elastic anomalies as well as the Rama spectrum [72,73]. The phase diagram has been modified accordingly as shown in Figure 11. It The phase diagram has been modified accordingly as shown in Figure 11. It should be noticed that shouldshould bebe noticednoticed thatthat eveneven ifif ththee orthorhombicorthorhombic phasephase interleafsinterleafs thethe TT andand RR phases,phases, itit appearsappears inin aa even if the orthorhombic phase interleafs the T and R phases, it appears in a very narrow temperature very narrow temperature and composition region, which blurs the measurements results. and composition region, which blurs the measurements results.

FigureFigure 10. TheTheThe high-resolutionhigh-resolution high-resolution X-rayX-ray X-ray diffractiondiffraction diffraction (XRD)(XRD) (XRD) (( (leftleftleft))) andand and RamanRaman Raman spectrumspectrum forfor BZT–BCTBZT–BCT piezoceramics ( rightright))[) [71,72].[71,72].71,72].

FigureFigure 11. 11. NewNewNew phasephase phase diagramdiagram diagram forfor BZT–BCTBZT–BCT basedbased onon the the crystalcrystal structure structure studies studies [72]. [[72].72].

4.3.4.3. Tricritical Tricritical Phenomenon ItIt shouldshould be be noted noted thatthat anan importantimportant featurefeaturefeature of ofof this thisthis phase phasephase diagram diagramdiagram (Figure (Figure(Figure 11 11)11)) is isis the thethe existence existenceexistence of ofof a a C-R-T triple point or a C-T-O-R quadruple point in the phase diagram located at x of 32% and at Tc aC-R-T C-R-T triple triple point point or or a C-T-O-Ra C-T-O-R quadruple quadruple point point in in the the phase phase diagram diagram located located at xatof x 32%of 32% and and at TatC Tofc of57 57◦C. °°C. Now, Now, the the phase phase transition transition behavior behavior for such for such a point a point remains remain controversial,ss controversial,controversial, and a central andand aa central issuecentral is issuewhetherissue isis whetherwhether or not the oror multi-phase-coexisting notnot thethe multi-phase-coexistingmulti-phase-coexisting point is a thermodynamicpointpoint isis aa thermodynamicthermodynamic tricritical point, tricriticaltricritical and whether point,point, andand the high piezoelectricity at MPB is affected by the tricritical phenomenon. Liu and Ren proposed that large the piezoelectric response for BZT–BCT is associated with the starting point of MPB, i.e., the C-T-R triple point, which shows a tricritical behavior with the crossover between first order and second order transition (as shown in Figure 12)[14]. On the other hand, it has also been proposed that there is no need for the coincidence between the tricritical point and the triple point, and Acosta et al. pointed out that the maximum coefficient d33 value (best piezoelectric property) does not occur at the triple point [74–76]. This indicates the irrelevance between the tricritical point and the high piezoelectric Actuators 2017, 6, 24 12 of 20

whether the high piezoelectricity at MPB is affected by the tricritical phenomenon. Liu and Ren proposed that large the piezoelectric response for BZT–BCT is associated with the starting point of MPB, i.e., the C-T-R triple point, which shows a tricritical behavior with the crossover between first order and second order transition (as shown in Figure 12) [14]. On the other hand, it has also been proposed that there is no need for the coincidence between the tricritical point and the triple point, Actuators 2017, 6, 24 12 of 20 and Acosta et al. pointed out that the maximum coefficient d33 value (best piezoelectric property) does not occur at the triple point [74–76]. This indicates the irrelevance between the tricritical point and response.the high piezoelectric It should be response. noted that It the should reason be fornoted such that a disputethe reason stems for fromsuch thea dispute challenge stems in accuratefrom the determinationchallenge in accurate of the positiondetermination of the of triple the position point and of the tricriticaltriple point point. and the Further tricritical investigations point. Further are requiredinvestigations in order are to required solve the in discrepancyorder to solve [77 the–80 discrepancy]. [77–80].

FigureFigure 12.12. Thermal hysteresis studies (left)) and permittivity maximum measurement ((rightright)) indicatesindicates aa tricriticaltricritical pointpoint forfor thethe tripletriple pointpoint ofof BZT–BCTBZT–BCT [[14].14].

4.4.4.4. Microstructure ofof FerroelectricFerroelectric DomainsDomains ItIt isis well-knownwell-known thatthat thethe ferroelectricferroelectric domaindomain configurationconfiguration asas wellwell asas itsits domaindomain dynamics playplay veryvery importantimportant rolesroles onon thethe piezoelectricpiezoelectric response.response. ThereThere areare intensiveintensive studiesstudies onon thethe microstructuremicrostructure forfor thethe Pb-containingPb-containing piezoelectricpiezoelectric materials.materials. For For example, example, for for PZT, PZT, PMN–PT, and PZT–PT, thethe ferroelectricferroelectric domaindomain structuresstructures manifestmanifest themselvesthemselves asas thethe nano-sizednano-sized domainsdomains insideinside aa domaindomain hierarchy.hierarchy. TheThe domain domain dynamics dynamics or domainor domain wall wall motion, motion, on the on other the hand,other hashand, invoked has invoked great interest great forinterest Pb-containing for Pb-containing piezoelectric piezoelectric materials. materials. Hence, the Hence, microstructure the microstructure study of Pb-free study piezoelectric of Pb-free materialspiezoelectric is also materials crucial is for also understanding crucial for underst the mechanismanding the underneath mechanism the underneath piezoelectric the response piezoelectric and furtherresponse utilization and further of the utiliz materials.ation of the materials. ForFor domaindomain structurestructure observation,observation, transmissiontransmission electronelectron microscopymicroscopy (TEM)(TEM) waswas used.used. BrightBright fieldfield (BF)(BF) TEMTEM imagesimages inin FigureFigure 1313 showshow thethe microstructuremicrostructure evolutionevolution withwith compositioncomposition atat roomroom temperaturetemperature [ 69[69].]. The The 40 BCT40 BCT specimen specimen shows shows a zigzag a domainzigzag pattern,domain which pattern, is a typicalwhich characteristicis a typical forcharacteristic rhombohedral for rhombohedral phase (Figure phase 13a). (Figure The 40 13a BCT). specimen,The 40 BCT on specimen, the other on hand, the other shows hand, a lamellar shows domaina lamellar structure domain with structure 90◦ domain with walls, 90° domain which iswall a typicals, which tetragonal is a typical microstructure tetragonal (Figure microstructure 13c). The above(Figure two 13c). microstructures The above two are microstructures plain structures thatare canplain be structures observed in that ordinary can be ferroelectrics. observed in However,ordinary atferroelectrics. 50 BCT in the However, MPB regime, at 50 the BCT sample in the exhibits MPB regi as ame, totally the differentsample exhibits microstructure: as a totally fine domainsdifferent withmicrostructure: an average fine domain domains size ofwith 20 nman areaverage embedded domain in size the frameof 20 nm ofmicron-scale are embedded domains in the frame forming of amicron-scale kind of “domain domains hierarchy”. forming Thea kind study of “domain of microstructure hierarchy”. evolution The study with of microstructure composition uncovers evolution a micron-nano-micronwith composition uncovers microdomain-nanodomain-microdomain a micron-nano-micron microdomain-nanodomain-microdomain nature in domain structure across nature MPB. Thein domain miniaturized structure domain across structure MPB. The corresponds miniaturized to the domain MPB state structure with the corresponds highest d33. Ato similar the MPB domain state patternwith the has highest also been d33. A observed similar indomain KNN-based, patternlead-free has also piezoelectricbeen observed materials in KNN-based, [81,82]. Guo lead-free et al. furtherpiezoelectric reported materials the observation [81,82]. Guo of et nanodomain al. further reported structure the changing observation to a of single-domain nanodomain statestructure in a 3 BaTiOchanging3-based to a polycrystallinesingle-domain ceramicstate in ata BaTiO intermediate-based polingpolycrystalline electric fields ceramic with at in intermediate situ TEM as shownpoling inelectric Figure fields 14[ 83 with,84]. in This situ has TEM been as shown taken as in theFigure evidence 14 [83,84]. for the This existence has been of taken orthorhombic as the evidence symmetry. for Moreover,the existence by usingof orthorhombic the same method, symmetry. Zakhozheva Moreover et, al.by reported using the that same the method, domain configurationZakhozheva et after al. electricreported field that loading the domain can reappear, configuration which after raises electric interest field to studyloading the can domain reappear, switching which or raises domain interest wall motionto study for the the domain BZT–BCT switching material or systemdomain [ 85wall]. motion for the BZT–BCT material system [85]. The domain wall displacement for BZT–BCT studies can be divided into two categories. One is the long-range domain wall displacement under a large electric field, which leads to large strain that can be utilized for actuators. Tutuncu et al. pointed out that the domain switching process accompanied with domain wall displacement contributes the majority of the effect under a large electric field, and it thus can be considered as the main effect for the large strain of BZT–BCT for actuator applications [86]. On the other hand, the piezoelectric effect at the subswitching condition is determined by Rayleigh Actuators 2017, 6, 24 13 of 20 Actuators 2017, 6, 24 13 of 20

The domain wall displacement for BZT–BCT studies can be divided into two categories. One is The domain wall displacement for BZT–BCT studies can be divided into two categories. One is the long-range domain wall displacement under a large electric field, which leads to large strain that the long-range domain wall displacement under a large electric field, which leads to large strain that can be utilized for actuators. Tutuncu et al. pointed out that the domain switching process can be utilized for actuators. Tutuncu et al. pointed out that the domain switching process accompanied with domain wall displacement contributes the majority of the effect under a large accompaniedActuators 2017, 6 ,with 24 domain wall displacement contributes the majority of the effect under a 13large of 20 electric field, and it thus can be considered as the main effect for the large strain of BZT–BCT for electric field, and it thus can be considered as the main effect for the large strain of BZT–BCT for actuator applications [86]. On the other hand, the piezoelectric effect at the subswitching condition is actuator applications [86]. On the other hand, the piezoelectric effect at the subswitching condition is determined by Rayleigh relation under a small electric field [87]. The result shows that the intrinsic determinedrelation under by Rayleigh a small electric relation field under [87 ].a small The result electr showsic field that[87].the The intrinsic result shows piezoelectric that the responseintrinsic piezoelectric response exhibits a peak close to the composition-induced R–MPB and MPB–T phase piezoelectricexhibits a peak response close to exhibits the composition-induced a peak close to the R–MPB composition-induced and MPB–T phase R–MPB transitions, and MPB–T but the phase value transitions,is much less but than the total valued isvalue. much In less contrast, than total the extrinsicd33 value. piezoelectric In contrast, response,the extrinsic inparticular piezoelectric the transitions, but the value33 is much less than total d33 value. In contrast, the extrinsic piezoelectric response, in particular the one corresponding with reversible domain wall motion, has been greatly response,one corresponding in particular with the reversible one corresponding domain wall with motion, reversible has domain been greatly wall motion, enhanced has close been togreatly MPB. enhanced close to MPB. Therefore, it is concluded that the extrinsic piezoelectric activity is the major enhancedTherefore, close it is concludedto MPB. Therefore, that the extrinsicit is concluded piezoelectric that the activity extrinsic is piezoelectric the major contributor activity is to the the major high contributor to the high piezoelectricity in BZT–BCT ceramics under subswitching conditions. contributorpiezoelectricity to the in high BZT–BCT piezoelectricity ceramics under in BZT–BCT subswitching ceramics conditions. under subswitching conditions.

Figure 13. Transmission electron microscopy (TEM) images of different compositions of (1 − x)BZT– FigureFigure 13. 13. TransmissionTransmission electron electron microscopy microscopy (TEM) images (TEM) of images different of compositions different compositions of (1 − x)BZT– of xBCT. (a) x = 0.4; (b) x =0.5; (c) x = 0.6; (a1,b1,b2,c1) the convergent beam electron diffraction pattern x(1BCT.− x ()BZT–a) x = x0.4;BCT. (b) ( ax) =0.5;x = 0.4;(c) x (=b )0.6;x = (a1 0.5;,b1 (,b2c) ,xc1=) the 0.6; convergent (a1,b1,b2,c1 beam) the elec convergenttron diffraction beam electronpattern obtained from one grain [69]. obtaineddiffraction from pattern one grain obtained [69]. from one grain [69].

Figure 14. In situ TEM observations of a series of grains in the same specimen of the 0.5Ba(Zr0.2Ti0.8)O3– FigureFigure 14. 14. In situIn TEM situ observations TEM observations of a series of ofa grains series in the of grainssame specimen in the of same the 0.5Ba(Zr specimen0.2Ti of0.8)O the3– 0.5(Ba0.7Ca0.3)TiO3 ceramic. (a,c,e) show the virgin state of domain patterns; (b,d,f) bright-field 0.5(Ba0.5Ba(Zr0.7Ca0.3Ti)TiO)O3 ceramic.–0.5(Ba (Caa,c,e))TiO showceramic. the virgin (a, cstate,e) show of domain the virgin patterns; state of(b domain,d,f) bright-field patterns; micrographs0.2 0.8are compared3 0.7 with0.3 differe3 nt electric field conditions [83]. micrographs(b,d,f) bright-field are compared micrographs with arediffere comparednt electric with field different conditions electric [83]. field conditions [83].

4.5. General Systems BZT–BCT has been proved to be a promising Pb-free piezoelectric system with a large electromechanical response. Later, following the same strategy as BZT–BCT, we have developed Actuators 2017, 6, 24 14 of 20

4.5. General Systems Actuators 2017, 6, 24 14 of 20 BZT–BCT has been proved to be a promising Pb-free piezoelectric system with a large electromechanical response. Later, following the same strategy as BZT–BCT, we have developed

Ba(Sn0.12TiTi0.880.88)O)O3–3x–(Bax(Ba0.7Ca0.7Ca0.3)TiO0.3)TiO3 (BTS–BCT)3 (BTS–BCT) and and Ba(Hf Ba(Hf0.2Ti0.20.8Ti)O0.83–)Ox(Ba3–x0.7(BaCa0.70.3)TiOCa0.33)TiO (BHT–BCT),3 (BHT–BCT), and andlarge large piezoelectric piezoelectric activity activity has also has alsobe reported be reported in a inBaTiO a BaTiO3–BaSnO3–BaSnO3 (BT–BS)3 (BT–BS) [15,88]. [15 ,88]. The phase phase diagram diagram of of the the Ba(Sn Ba(Sn0.120.12Ti0.88Ti)O0.883–)Ox(Ba3–x0.7(BaCa0.70.3)OCa30.3 (BTS–)O3 (BTS–xBCT)x materialBCT) material system systemis shown is shownin Figure in 15. Figure It is characterized15. It is characterized by its phase by bounda its phasery boundarystarting from starting the triple from point the tripleof a paraelectric point of a paraelectriccubic phase, cubic ferroelectric phase, ferroelectric rhombohedral, rhombohedral, and tetragonal and tetragonal phases. phases.The room The temperature room temperature MPB MPBcomposition composition BTS–30BCT BTS–30BCT exhibits exhibits a high a high piezoelectric piezoelectric coefficient coefficient d33d 33= =530 530 pC/N. pC/N. It Itcan can thus thus be considered asas aa promising promising high-performance high-performance Pb-free Pb-fre piezoceramic.e piezoceramic. Another Another high-property, high-property, triple-point triple- typepoint Pb-freetype Pb-free material material system, system, Ba(Ti Ba(Ti0.8Hf0.80.2Hf)O0.23)O–(Ba3–(Ba0.7Ca0.7Ca0.3)TiO0.3)TiO3 was3 was also also designed designed accordingaccording toto a similar idea. The system shows anomalies of both small field field (dielectric and piezoelectric) and large fieldfield (P-E hysteresis) propertiesproperties atat thethe MPB,MPB, andand especiallyespecially thethed d3333 coefficientcoefficient cancan reachreach 550550 pC/NpC/N at room temperature, whichwhich furtherfurther verifiesverifies thethe generalitygenerality forfor suchsuch aa designdesign idea.idea.

Figure 15. (a) Phase diagram for for the the BST–BCT BST–BCT material system. system. ( (bb––ee)) Dielectric Dielectric permttivity permttivity for for 0BCT, 0BCT, 5BCT, 90BCT. ((ff)) ChangeChange ofof thermalthermal hysteresishysteresis withwith compositioncomposition [[15].15].

Further large piezoelectric properties with d33 of 697 pC/N have been found in BaTiO3–xBaSnO3 Further large piezoelectric properties with d33 of 697 pC/N have been found in BaTiO3–xBaSnO3 Pb-free ferroelectric system (Figure 16) [89]. By re-examining the Sn-doped BaTiO3, we found it also Pb-free ferroelectric system (Figure 16)[89]. By re-examining the Sn-doped BaTiO3, we found it alsoexhibits exhibits a T-O a phase T-O phase boundary. boundary. Its quasi-quadruple Its quasi-quadruple point, point, which which is a point is a pointwhere where four phases four phases (C-T- (C-T-O-R)O-R) nearly nearly coexist coexist together together in the intemperature-composition the temperature-composition phase diagram, phase diagram, the piezoelectric the piezoelectric as well as welldielectric as dielectric coefficient coefficient values valuesreach their reach optimal their optimal values. values. Therefore, Therefore, it is highly it is highly possible possible that thatthe themulti-phase multi-phase coexisting coexisting point point (i.e., (i.e., quadruple quadruple point point or ortriple triple point) point) is isof of great great significance significance for for the piezoelectric responseresponse ofof ferroelectric ferroelectric system, system, which which might might provide provide a guidelinea guideline for for developing developing Pb-free Pb- piezoelectricfree piezoelectric materials materials with with large large electrometrical electrometrical responses. responses.

Actuators 2017, 6, 24 15 of 20 Actuators 2017, 6, 24 15 of 20

FigureFigure 16.16. Contour map of thethe dd3333 coefficientcoefficient in temperature-compositiontemperature-composition dimensions of BT–BSBT–BS piezoelectricpiezoelectric materialsmaterials [[89].89].

5.5. Applications and Outline

TheThe BaTiOBaTiO33-based materials, such asas BZT–BCTBZT–BCT piezoceramic,piezoceramic, exhibitexhibit superiorsuperior piezoelectricitypiezoelectricity comparedcompared withwith otherother Pb-freePb-free systems,systems, whichwhich isis comparablecomparable withwith softsoft PZT. ItIt thusthus bringsbrings about strong hopehope forfor Pb-freePb-free materialsmaterials thatthat cancan eventuallyeventually be substitutedsubstituted for PZT in thethe future.future. Admittedly, wewe havehave toto pointpoint outout thatthat BaTiOBaTiO33-based-based materialmaterial systemssystems havehave thethe drawbackdrawback ofof lowlow CurieCurie temperaturetemperature (Tc(Tc < < 130 130◦C), °C), which which restricts restricts their application,their application, especially especially due to their due temperature to their temperature stability performance. stability However,performance. such However, a system such can a still system be considered can still be as considered an excellent as an model excellent for exploring model for the exploring physics the of highlyphysics piezoelectric of highly piezoelectric materials (a materials teacher for (a highly teache piezoelectricr for highly Pb-freepiezoelectric materials). Pb-free Even materials). better Pb-free Even piezoelectricbetter Pb-free systems piezoelectric can be systems anticipated can be based anticipa on theted new based insights on the obtainednew insights from obtained BZT–BCT. from BZT– BCT.Here, we would like to give an example of the application of BZT–BCT piezoceramic as the transducerHere, ofwe medical would ultrasoundlike to give imaging an example systems. of Asthe shownapplication above, of the BZT–BCT 0.5BZT–0.5BCT piezoceramic ceramics as have the atransducer high dielectric of medical constant ultrasound of 2800 and imaging superior systems. piezoelectric As shown performance above, thed33 0.5BZT–0.5BCTof 600 pC/N. Although ceramics therehave area high not realdielectric applications constant of such of 280 materials0 and insuperior actuators, piezoe it haslectric been performance shown that the d33 0.5BZT–0.5BCT of 600 pC/N. ceramicAlthough is there a promising are not lead-freereal applications piezoelectric of such material materials for high-frequencyin actuators, it has transducer been shown applications. that the Yan0.5BZT–0.5BCT et al. has used ceramic the 0.5BZT–0.5BCT is a promising ceramicslead-free topiezoelectric fabricate a high-frequencymaterial for high-frequency (~30 MHz) needle-type transducer ultrasonicapplications. transducer Yan et al. forhas intravascular used the 0.5BZT–0.5BCT imaging application ceramics to [90 fabricate]. Such a a high-frequency lead-free transducer (~30 MHz) was foundneedle-type to exhibit ultrasonic a −6 dB bandwidthtransducer offor 53% intravas with ancular insertion imaging loss ofapplicatio 18.7 dB.n As [90]. shown Such in Figure a lead-free 17, an intransducer vitro intravascular was found ultrasound to exhibit a (IVUS)−6 dB bandwidth image of a of human 53% with cadaver an inse coronaryrtion loss artery of 18.7 was dB. obtained As shown by suchin Figure a lead-free 17, an transducerin vitro intravascular with adequate ultrasound resolution (IVUS) and contrastimage of to a differentiatehuman cadaver the coronary vessel wall artery and fibrouswas obtained plaque. by Various such a studies lead-free have transducer also shown with potential adequate applications resolution of and such co BaTiOntrast3-based to differentiate lead-free ceramics,the vessel ranging wall and from fibrous energy plaque. harvesting Various to energy studies storage, have also which shown can providepotential guidelines applications for theof such next generationBaTiO3-based ofPb-free lead-free materials ceramics, with ranging even better from piezoelectric energy harvesting properties, to forenergy example storage, in high which TC KNN can basedprovide piezoceramics guidelines for [18 ,91the,92 next]. generation of Pb-free materials with even better piezoelectric properties, for example in high TC KNN based piezoceramics [18,91,92].

Actuators 2017, 6, 24 16 of 20 Actuators 2017, 6, 24 16 of 20

Figure 17. An intravascular ultrasound image acquired from a 0.5BZT–0.5BCT transducer [90]. Figure 17. An intravascular ultrasound image acquired from a 0.5BZT–0.5BCT transducer [90].

6. Summary 6. Summary BaTiO3-based piezoelectric ceramics are interesting Pb-free materials for actuator applications BaTiO -based piezoelectric ceramics are interesting Pb-free materials for actuator applications as as they exhibit3 high strain under fairly low electric fields. The piezoelectric properties have been they exhibit high strain under fairly low electric fields. The piezoelectric properties have been greatly greatly enhanced via domain engineering, defect engineering, and phase boundary engineering enhanced via domain engineering, defect engineering, and phase boundary engineering processes. processes. After the report of the BZT–BCT piezoelectric ceramic systems, the research interest in this After the report of the BZT–BCT piezoelectric ceramic systems, the research interest in this family of family of materials for electromechanical applications has increased. Although the low Tc of such a materials for electromechanical applications has increased. Although the low Tc of such a system may system may hinder its application due to issues of its temperature stability, the BaTiO3-based hinder its application due to issues of its temperature stability, the BaTiO -based piezoelectrics can still piezoelectrics can still be considered as a model system for Pb-free piezoe3 lectric material design. be considered as a model system for Pb-free piezoelectric material design. Acknowledgments: We thank Yongbin Liu, Yan Wang and Zhixin He for helpful discussion. The authors Acknowledgments: We thank Yongbin Liu, Yan Wang and Zhixin He for helpful discussion. The authors gratefully acknowledge the support of National Basic Research ProgramProgram of ChinaChina (Grant(Grant No.No. 2012CB619401), thethe NationalNational NaturalNatural ScienceScience FoundationFoundation ofof ChinaChina (Grant(Grant Nos.Nos. 51571156,51571156, 51321003,51321003, 51302209,51302209, 51431007,51431007, and 51320105014), and Program for Changing Scholars and InnovativeInnovative Research Team inin UniversityUniversity (IRT13034).(IRT13034). J.G acknowledges thethe Fundamental Fundamental Research Research Funds Funds for thefor Centralthe Central Universities Universities and State and Key State Laboratory Key Laboratory of Electrical of Insulation and Power Equipment (EIPE16311) for financial support. Electrical Insulation and Power Equipment (EIPE16311) for financial support. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. References References 1. Uchino, K. Ferroelectric Devices; Marcel Dekker: New York, NY, USA, 2000. 1. Uchino, K. Ferroelectric Devices; Marcel Dekker: New York, NY, USA, 2000. 2. Malek, C.K.; Saile, V. Applications of LIGA technology to precision manufacturing of high-aspect-ratio 2. Malek, C.K.; Saile, V. Applications of LIGA technology to precision manufacturing of high-aspect-ratio micro-components and -systems: A review. Microelectron. J. 2004, 35, 131–143. [CrossRef] micro-components and-systems: A review. Microelectron. J. 2004, 35, 131–143. 3. Fennimore, A.M.; Yuzvinsky, T.D.; Han, W.Q.; Fuhrer, M.S.; Cumings, J.; Zettl, A. Rotational actuators based 3. Fennimore, A.M.; Yuzvinsky, T.D.; Han, W.Q.; Fuhrer, M.S.; Cumings, J.; Zettl, A. Rotational actuators on carbon nanotubes. Nature 2003, 424, 408–410. [CrossRef][PubMed] based on carbon nanotubes. Nature 2003, 424, 408–410. 4. Tadaki, T.; Otsuka, K.; Shimizu, K. Shape memory alloys. Annu. Rev. Mater. Sci. 1988, 18, 25–45. [CrossRef] 4. Tadaki, T.; Otsuka, K.; Shimizu, K. Shape memory alloys. Annu. Rev. Mater. Sci. 1988, 18, 25–45. 5. Setter, N.; Damjanovic, D.; Eng, L.; Fox, G.; Gevorgian, S.; Hong, S.; Kingon, A.; Kohlstedt, H.; Park, N.Y.; 5. Setter, N.; Damjanovic, D.; Eng, L.; Fox, G.; Gevorgian, S.; Hong, S.; Kingon, A.; Kohlstedt, H.; Park, N.Y.; Stephenson, G.B.; et al. Ferroelectric thin films: Review of materials, properties, and applications. J. Appl. Stephenson, G.B.; et al. Ferroelectric thin films: Review of materials, properties, and applications. J. Appl. Phys. 2006, 100, 051606. [CrossRef] Phys. 2006, 100, 051606. 6. Haertling, G.H. Ferroelectric ceramics: History and technology. J. Am. Ceram. Soc. 1999, 82, 797–818. 6. Haertling, G.H. Ferroelectric ceramics: History and technology. J. Am. Ceram. Soc. 1999, 82, 797–818. [CrossRef] 7. Park, S.-E.; Shrout, T.R. Ultrahigh strain and piezoelectric behavior in relaxor based ferroelectric single 7. Park, S.-E.; Shrout, T.R. Ultrahigh strain and piezoelectric behavior in relaxor based ferroelectric single crystals. J. Appl. Phys. 1997, 82, 1804–1811. crystals. J. Appl. Phys. 1997, 82, 1804–1811. [CrossRef] 8. Zhang, S.; Li, F. High performance ferroelectric relaxor-PbTiO3 single crystals: Status and perspective. J. Appl. Phys. 2012, 111, 031301.

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