Exceptionally High Piezoelectric Coefficient and ... - ACS Publications

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Exceptionally High Piezoelectric Coefficient and Low Strain Hysteresis in Grain-Oriented (Ba, Ca)(Ti, Zr)O3 through Integrating Crystallographic Texture and Domain Engineering Yingchun Liu, Yunfei Chang, Fei Li, Bin Yang, Yuan Sun, Jie Wu, Shantao Zhang, Ruixue Wang, and Wenwu Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08160 • Publication Date (Web): 11 Aug 2017 Downloaded from http://pubs.acs.org on August 13, 2017

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ACS Applied Materials & Interfaces

Exceptionally High Piezoelectric Coefficient and Low Strain Hysteresis in Grain-Oriented (Ba, Ca)(Ti, Zr)O3 through Integrating Crystallographic Texture and Domain Engineering Yingchun Liu,† Yunfei Chang, *, †, ┴ Fei Li,‡, ┴ Bin Yang,† Yuan Sun,† Jie Wu,† Shantao Zhang,§ Ruixue Wang,† Wenwu Cao*, †, ǁ † Condensed Matter Science and Technology Institute, School of Science, Harbin Institute of Technology, Harbin 150080, China ‡

Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education, Xi’an Jiaotong University, Xi’an 710049, China

§

National Laboratory of Solid State Microstructures and Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, China

┴

Department of Materials Science and Engineering and Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA

ǁ

Department of Mathematics and Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA

KEYWORDS Piezoelectric properties, strain hysteresis, lead-free, textured ceramics, domain engineering, templated grain growth

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ABSTRACT Both low strain hysteresis and high piezoelectric performance are required for practical applications in precisely controlled piezoelectric devices and systems. Unfortunately, enhanced piezoelectric properties were usually obtained with the presence of a large strain hysteresis in BaTiO3 (BT)-based piezoceramics. In this work, we propose to integrate crystallographic texturing and domain engineering strategies into BT-based ceramics to resolve this challenge. [001]c grain-oriented (Ba0.94Ca0.06)(Ti0.95Zr0.05)O3 (BCTZ) ceramics with a texture degree as high as 98.6% were synthesized by templated grain growth. A very high piezoelectric coefficient (d33) of 755 pC/N, and an extremely large piezoelectric strain coefficient (d33*=2027 pm/V) along with an ultra-low strain hysteresis (Hs) of 4.1% were simultaneously achieved in BT-based systems for the first time, which are among the best values ever reported on both lead-free and lead-based piezoceramics. The exceptionally high piezoelectric response is mainly from the reversible contribution, and can be ascribed to the piezoelectric anisotropy, the favorable domain configuration and the formation of smaller sized domains in the BCTZ textured ceramics. This study paves a new pathway to develop lead-free piezoelectrics with both low strain hysteresis and high piezoelectric coefficient. More importantly, it represents a very exciting discovery with potential application of BT-based ceramics in high-precision piezoelectric actuators.

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INTRODUCTION High-performance piezoelectric materials with the energy conversion capability between electrical and mechanical energies are widely utilized in a lot of electronic devices, including actuators, smart sensors, ultrasonic transducers, and energy harvesters. Pb(Zr, Ti)O3 (PZT)-based materials, which possess superior properties and structural flexibility, have dominated the global piezoelectric market in the past decades.1,2 Relaxor PbTiO3 based materials,

such

as

Pb(Mg1/3Nb2/3)O3-PbTiO3

(PMN-PT)

and

Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT), whose piezoelectric responses are even higher than PZT, are also commercially essential.2-4 However, due to growing environmental and human health concerns on lead toxicity, extensive attention has been moved to sustainable lead-free piezoelectric materials.5-10 BaTiO3 (BT)-based ceramics are among the most promising lead-free replacements owing to the relatively good piezoelectric properties, low losses and high tunability. A peak piezoelectric coefficient d33~560-620 pC/N and a large piezoelectric strain coefficient d33*~1140 pm/V were reported in A- and B-sites co-substituted (Ba0.85Ca0.15)(Ti0.90Zr0.10)O3 piezoceramics (BZT-50BCT) by Liu et.al6 in 2009, with the composition designed at a phase boundary starting from a rhombohedral-tetragonal-cubic tricritical point near room temperature. Both the structural instabilities and facilitated polarization rotation are believed to contribute to the high d values.6,11 Since then, in order to further improve piezoelectric performance in BT-based systems, numerous efforts have been made on the construction of phase boundaries through compositional modifications12-33 in ceramics synthesized by conventional solid-state approach. However, the obtained comprehensive piezoelectric properties are still inferior or only comparable to those of the BZT-50BCT ceramics. Moreover, modified BT-based ceramics usually have large hysteresis (Hs) in strain-electric field curves,6,12-20,22,24-36 which seriously degrades their actuating performance and restricts their applications in precisely controlled devices and systems.23,37,38 In particular, besides maximizing the d33 value, a large strain (d33*) 3



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together with a low stain hysteresis are very important for high-precision piezoelectric actuator applications. Hence, the question arises: can the two critical challenges (i.e. large hysteresis and the bottleneck for piezoelectric performance) be overcome simultaneously in modified BT-based ceramics to expand their application areas? Crystallographic texturing of piezoelectric ceramics, which can be achieved by templated grain growth (TGG) process, allows the utilization of inherent anisotropy in material properties and the application of domain engineering,5,38-44 potentially offering a solution to the aforementioned challenges in modified BT-based ceramics. Based on the concept of domain engineering, significantly enhanced piezoelectric response could be expected in highly textured rhombohedral or orthorhombic ceramics poled along [001]c, as a consequence of facilitated polarization rotation due to the formation of “4R” or “4O” domain configuration.2,4,39,45 Another important characteristic of domain engineered piezoelectrics is a low strain hysteresis, which is related to the coexistence of four equally favored domain states, reducing the driving force for extensive domain wall motion. Recently, the well-known BZT-50BCT composition has been textured along [001]c crystallographic orientation,36,46-48 but the piezoelectric properties (e.g. d33 ~350-470 pC/N) did not show obvious improvement, which partially resulted from the low texture degree (56%-82%) caused by the lack of densification and thus limited TGG in the ceramics. Most importantly, those textured ceramics were dominated by tetragonal phase, so the formation of “1T” domain configuration after poling could not benefit from polarization rotation effect.2,4,39,45 In this work, we proposed a new strategy to develop BT-based piezoceramics with both high piezoelectric response and low hysteresis by considering anisotropy/composition/phase structure selection, crystallographic texturing, and domain characteristics. Orthorhombic phase dominated (Ba0.94Ca0.06)(Ti0.95Zr0.05)O3 (BCTZ) was chosen as the texturing composition. [001]c grain-oriented BCTZ ceramics with texture degree as high as ~98.6% were developed by templated grain growth. Indeed, a very high d33 ~755 pC/N, and an extremely large d33*~2027 pm/V along with an ultra-low Hs ~4.1% were simultaneously achieved, which are superior to 4


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those of BT-based ceramics developed thus far.6,12-22,24-36,46-49 The underlying mechanisms for the exceptionally high piezoelectric performance were also addressed. This work not only paves a new path to produce high d33/d33*-low Hs piezoelectrics for precisely controlled device applications, but also can provide guidelines for further development of novel functional materials with superior properties. EXPERIMENTAL SECTION Platelet BaTiO3 templates with lengths/widths around 5-12 µm and thicknesses about 0.5-1.2 µm (Figure S1) were produced by topochemical microcrystal conversion with the detailed processing procedure described in the literature50. The large faces of these templates are perpendicular to the pseudocubic perovskite [001]c direction. (Ba0.937Ca0.063)(Ti0.947Zr0.053)O3 equiaxed matrix powders of ~250 nm in average size were synthesized by conventional solid-state approach (See supporting information for a detailed procedure). For TGG process, (Ba0.937Ca0.063)(Ti0.947Zr0.053)O3 matrix powders and BT templates were mixed according to a molar ratio of 95:5 to achieve the final (Ba0.94Ca0.06)(Ti0.95Zr0.05)O3 (BCTZ) composition of the textured ceramics. The mixture slurry was tape casted according to the procedure described in the supporting information. After drying, the tapes were cut, stacked and laminated at 75 ºC and 20 MPa for 1 h. The organics were removed by heating the specimens at 600 ºC for 2 h, and then they were isostatically pressed at 200 MPa for 3 min. After that, some specimens were sintered at 4 ºC/min to temperatures between 1300 ºC and 1575 ºC without holding. Besides, some specimens were held at 1575 ºC for 0.25-9 h. For comparison, randomly oriented (Ba0.94Ca0.06)(Ti0.95Zr0.05)O3 ceramics were also prepared without adding BT templates. Phase structure and texture quality quantified by the pseudocubic Lotgering factor51 F00l were evaluated using X-ray diffraction (XRD, D/max 2400, Rigaku, Tokyo, Japan). Microstructural and compositional features were examined by field-emission scanning electron microscopy (FE-SEM, Helios NanoLab 600i, FEI, OR, USA) and energy dispersive spectroscopy (EDS). Specimen density was measured by the Archimedes method. Temperature dependences of dielectric properties were measured with an LCR meter (Agilent E4980A, 5



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Agilent technologies, CA, USA) combined with a temperature regulated oven. Piezoelectric coefficient d33 was recorded by a piezo d33 meter (ZJ-4A, Institute of Acoustics, Beijing, China). Polarization against electric field (P-E) loops and strain against electric field (S-E) curves were characterized using a modified Sawyer-Tower circuit. Based on the S-E curve, piezoelectric strain coefficient d33* was calculated from the slope of the increasing field (E95%, suggesting that TGG is closely related to microstructure connectivity. A very high texture fraction of 98.6% was achieved at RD~99% for the templated BCTZ ceramics sintered at 1575 °C for 9 h.

Figure 1. (a) XRD patterns of randomly oriented counterpart and BT templated BCTZ ceramics with different texture fractions F00l during the TGG process. The peaks are indexed using pseudocubic Miller indices. (b) Textured fraction of templated BCTZ as a function of relative density. Cross-section SEM images of templated BCTZ ceramics sintered at different sintering temperatures/holding times: (c) 1300 ºC, 0 h, (d) 1400 ºC, 0 h, (e) 1500 ºC, 0 h, (f) 1575 ºC, 0 h, and (g) 1575 ºC, 6 h, and (h) the corresponding randomly oriented ceramics. Images e, f and g were taken in backscattered electron (BSE) mode. Figure 1c-g demonstrates microstructure evolution of BCTZ ceramics during the texture 7



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development. BT templates were extremely well aligned in the submicrometer-sized matrix (Figure S1). Heteroepitaxial nucleation of oriented crystallites has occurred on the templates at 1300 ºC, which gradually lead to visible thickening of the templated grains at 1400 ºC. With further increasing sintering temperature, templated grains rapidly grow in a homoepitaxy way, even though the matrix grains become coarser. Impingement of large templated grains appears at 1575 °C without holding. By holding the samples for 6 h at 1575 °C, almost all matrix grains disappear leaving highly [001]c oriented grains. Here BCTZ texture develops directly from the epitaxial growth of the orthorhombic regions on aligned tetragonal BT templates during sintering. Compared to the non-textured ceramics with randomly oriented grains (Figure 1h), textured samples show a microstructure with well-aligned BT templates (brighter lines) inside the oriented grains of ~10-18 µm in size. In addition, a tiny amount of voids can be detected at three-grain junctions from the SEM image (Figure 1g) of highly textured BCTZ after polishing and thermal etching. Compared with the fresh fracture micrograph (Figure S3), the observed voids could either be closed pores formed during sintering or be caused by pullout of oriented grains with irregular shapes during grinding. According to the EDS analysis shown in Figure S3, all elements are homogenously distributed in the regions near both grain boundaries and triple points within the EDS detection limit. There is no evidence suggesting the presence of amorphous phases in the textured ceramics. To understand interface evolution during TGG, a templated BCZT ceramic at the early stage of texture development (F00l=22.4%) was selected for TEM observation. Figure 2a shows obvious growth of a textured grain on a BT template at the expense of surrounded matrix. Both the crystallization interface (textured grain boundary) and the interface between BT template and BCTZ oriented region are clearly observed. According to the HR-TEM images provided in Figure 2b and 2c, the crystallization interfaces between matrix grains and the textured grain are coherent and defect-free. There is no clear evidence suggesting the presence of a liquid phase film at such interface, which indicates that the TGG here may not be dominated by liquid phase texturing mechanism. SAED patterns (Figure 2b-d) demonstrate the different crystallographic 8


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orientations between the textured grain and two matrix grains. The HR-TEM image and SAED patterns in Figure 2d-f confirmed that the BT platelets successfully templated the [001]c oriented grain growth of BCTZ grains. According to the EDS analysis, there is very little diffusion of Ca2+ and Zr4+ through the interface between BT templates and BCTZ oriented regions, suggesting that such composition difference cannot be homogenized during TGG. In addition, this interface is also coherent without defects, which may be attributed to the small lattice difference between the templates and oriented grains, and is very important towards obtaining superior functionality.42

Figure 2. (a) Cross-section TEM image of a textured BCTZ grain growing by consuming surrounded matrix grains. (b, c) HR-TEM images of the interfaces between the textured grain and two matrix grains with different crystallographic orientations. The insets are SAED patterns of matrix grains. (d) SAED pattern of the textured grain. (e) HR-TEM image of the interface between the textured grain and BT template. (f) SAED pattern of the BT template. Here the SAED patterns of the textured grain and BT template were indexed according to pseudocubic 9



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and tetragonal structures, respectively.

Figure 3. (a) Temperature dependence of dielectric constant εr and loss tan δ for randomly oriented and textured BCTZ ceramics (F00l=98.6%) measured at 1 kHz, respectively; (b) In(1/εr-1/εm) as a function of In(T-Tm); (c) Piezoelectric coefficient d33 and piezoelectric voltage coefficient g33; (d) Unipolar strain vs. electric field (S-E) curves; (e) Strain hysteresis Hs as a function of maximum field induced strain Smax for modified BT based ceramics.6,12-19,24-36 Please note large strain with low strain hysteresis in the textured BCTZ ceramics (This work). (f) Polarization vs. electric field (P-E) hysteresis loops for both randomly oriented and textured BCTZ ceramics. Figure 3a compares the temperature dependences of dielectric constant εr and loss tan δ of non-textured and textured BCTZ ceramics measured at 1 kHz. For the non-textured ceramics, three dielectric anomalies were detected at ~ –18 ºC, 33 ºC and 119 ºC, corresponding to rhombohedral-orthorhombic-tetragonal-cubic phase transition temperatures (TR-O, TO-T and Tc), respectively. For the textured BCTZ, both TR-O and TO-T increase respectively to ~ –2 ºC and 42 ºC, suggesting that these samples are mainly of orthorhombic phase at room temperature. Meanwhile, Tc decreases to ~ 95 ºC, but it is still comparable to or higher than most of substituted BT based ceramics reported in the literature.6,12-25,28,29,32,34,36,46-48 Here the shifts of 10


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phase transition temperatures are probably related to the existence of heterogeneous BT templates inside the BCTZ textured grains. Because of lattice mismatch between the oriented grains and templates, interfacial stress should exist in the textured ceramics, which could cause distortion of crystal lattice and thus the phase transition temperature variations. For the textured ceramics, broader peak of εr can be observed near Tc, indicating that they may become more relaxorlike. Lower loss tan δ ~0.014 can also be observed at room temperature, and it keeps relatively stable with increasing temperature and remains below 0.021 even at the temperature as high as 190 ºC. A modified Curie-Weiss law53 was utilized to determine the dielectric relaxor behavior in the textured ceramics: 1/εr − 1/εm = (T − Tm)r/C; where εm corresponds to the maximum value of εr achieved at Tm temperature, and r and C are constants. The exponents r=1 and r=2 represent normal ferroelectric and ideal relaxor ferroelectric, respectively. The relationship between In(1/εr − 1/εm) and In(T − Tm) was plotted, and the curves are shown in Figure 3b. Both non-textured and textured ceramics exhibit a linear relationship, and the latter shows a much higher r value of 1.83 than the former (r=1.29), indicating an increase of relaxor nature. As we know, polar nanoregions (PNRs) or polar clusters form as a result of local structural distortion in ferroelectrics.42 In our textured ceramics, PNRs may exist near the interfaces between the oriented BCTZ regions and BT templates due to the possible lattice distortions, which could also modify Tm and lead to a diffused phase transition. Figure 3c presents piezoelectric properties of non-textured and textured BCTZ based ceramics. An exceptionally high piezoelectric coefficient d33 of 755 pC/N and a very large piezoelectric voltage coefficient (induced voltage under applied stress) g33 of 34.3×10-3 Vm/N were achieved in textured BCTZ ceramics, which are 2.2 and 2.8 times respectively higher than those of non-textured counterparts. We believe that both the piezoelectric anisotropy resulted from the high [001]c texture quality and the favorable “4O” engineered domain configuration (Figure S4) which facilitates polarization rotation should contribute to such significant improvements in the piezoelectric properties. Besides, to the best of our knowledge, the 11



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obtained values here are superior to what are currently available for modified BT based piezoceramics.6,12-18,20-36,46-49 To understand the texturing effects on domain switching and domain wall mobility, unipolar S-E curves of non-textured and textured BCTZ ceramics are plotted in Figure 3d. Our highly textured BCTZ ceramics exhibit a very large electrostrain Smax (~0.24% at 20 kV/cm), which is more than 2 times of that of non-textured counterparts measured at the same electric field. It is noteworthy to mention that compared to the d33* value of 501 pm/V in non-textured ones at E

Exceptionally High Piezoelectric Coefficient and ... - ACS Publications

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