- Email: [email protected]
2)0.935Ba0.065TiO3–CaYAlO4 lead–free ceramics
Accepted Manuscript Electric field–induced large strain of (Bi1/2Na1/2)0.935Ba0.065TiO3–CaYAlO4 lead–free ceramics Liangliang Li, Jigong Hao, Zhijun Xu, Wei Li, Ruiqing Chu PII: DOI: Reference:
S0167-577X(17)31189-8 http://dx.doi.org/10.1016/j.matlet.2017.08.010 MLBLUE 22987
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
25 May 2017 19 July 2017 2 August 2017
Please cite this article as: L. Li, J. Hao, Z. Xu, W. Li, R. Chu, Electric field–induced large strain of (Bi1/2Na1/2)0.935Ba0.065TiO3–CaYAlO4 lead–free ceramics, Materials Letters (2017), doi: http://dx.doi.org/10.1016/ j.matlet.2017.08.010
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electric field–induced large strain of (Bi1/2Na1/2)0.935Ba0.065TiO3–CaYAlO4 lead–free ceramics Liangliang Li1, Jigong Hao1,*, Zhijun Xu 2, Wei Li1, Ruiqing Chu2,* 1
College of Materials Science and Engineering, Liaocheng University, Liaocheng
252059, People’s Republic of China 2
School of Environmental and Materials Engineering, Yantai University, 32 Qingquan
Road, Yantai 264005, China
Abstract Lead–free piezoelectric ceramics (1-x)(Bi1/2Na1/2)0.935Ba0.065TiO3–xCaYAlO4 (BNBT6.5–xCYAO) were prepared using a conventional solid sintering technique. The addition of CYAO destroyed the ferroelectric long–range order with the shift of the ferroelectric–relaxor transition temperature TF–R down to room temperature and induced the appearance of ergodic relaxor phase. Thus, it leads to a giant field–induced strain with a peak value of 0.44% at x = 0.012. Keywords: Lead–free piezoelectric ceramics; Phase transition; Ferroelectricity, Field–induced strain 1. Introduction Over the past half century, lead–based materials with the excellent piezoelectric properties such as Pb(ZrxTi1-x)O3 (PZT) were widely utilized in piezoelectric ceramic devices [1,2]. The presence of a morphotropic phase boundary (MPB) [3] is contributed to high dielectric, piezoelectric and ferroelectric response in these systems [4, 5]. -----------------------------------* Corresponding author. Email address: [email protected] (J.Hao); [email protected] (R. Chu).
However, the strong toxicity of lead–based oxides during processing has caused critical ecological issues [6]. Therefore, great attention is drawn on lead–free ceramics with excellent properties to replace the PZT–based ceramics [7]. Recently, much research has focused on Bi–based ceramics as environmental friendly alternatives to PZT since the Bi3+, like the Pb2+ ion, is highly polarizable due to a lone electron pair [8, 9]. Among various Bi–based perovskite system, lead–free (Bi0.5Na0.5)TiO3(BNT)–based perovskite ceramics aroused great attention since Zhang et al [10] made a break–through. Since then, plenty of studies that related to the strain properties of BNT–based piezoceramics have been reported. (Bi1/2Na1/2)TiO3–BaTiO3 (BNT–BT) system, similar to the PZT ceramics [11], exhibits good piezoelectric and ferroelectric properties owing to the existence of a rhombohedral–tetragonal MPB at compositions with the BT concentration of 6–7mol% [12, 13]. However, the strain properties of BNT–BT system are not enough for practical applications. It has proved comparatively easy to form new solid solutions of BNT–BT with other perovskites compounds [14,15]. Near the MPB, these systems showed promising performances which are very attractive as lead–free replacements for PZT. Multiferroic materials A2B2O6 with the double–perovskite structure can effectively diffuse into the lattice of BNT–based ceramics to form a solid solution. Moreover, it promotes the piezoelectric properties or field–induced strain response significantly [16, 17]. eg, Bi2NiMnO6 and Sr2ZrMnO6 have been selected as the addition into BNT–based ceramics by our research group to enhance the strain response [16, 17]. Results showed that the enhanced strain response was linked with a significant disruption of the
2
long-range ferroelectric order induced by the A2B2O6, where the phase structure transition caused by ion substitution effect is the original reason. Unlike A2B2O6 with the double–perovskite structure, the calcium yttrium aluminate (CaYAlO4, i.e., CYAO) has a two–dimensional perovskite with a tetragonal structure [18, 19]. It belongs to the series of A3+B2+AlO4 aluminates (A3+ = Y, Nd, La; B2+ = Ca, Sr), which crystallize in the K2NiF4 type structure consisting of an ABO3 perovskite and AO rock–salt (NaCl) type layers. Moreover, A and B elements present ninefold coordination whereas Al is octahedrally coordinated [20,21]. Owing to the close ionic radius, Ca2+ (1.18Ǻ, CN=9) and Y3+ (1.075Ǻ, CN=9) will enter into the A–site of perovskite while Al3+ (0.535Ǻ, CN=6) will enter into the B–site of perovskite. Ca2+, Y3+, and Al3+ were proved to be effective modifier to promote the strain response of BNT–based ceramics [22–24]. Based on the above, we expect CYAO plays a similar role to produce large strain response in BNT–based ceramics. In the present work, we selected (Bi1/2Na1/2)0.935Ba0.065TiO3 (BNBT6.5), which is known to lie at the MPB between rhombohedral BNT and tetragonal BT as a base material with small amounts of CYAO being introduced as doping species. The phase structure and electric properties of the new BNBT6.5–CYAO lead–free ceramics were studied systematically. 2. Experimental procedure The ceramics (1-x)(Bi1/2Na1/2)0.935Ba0.065TiO3–xCaYAlO4 (BNBT6.5–xCYAO, x = 0–0.015) were fabricated with metal oxides or carbonate powders: Na2CO3 (99.8%), BaCO3 (99%), CaCO3 (99%), TiO2 (99.5%), Y2O3 (99.99%), Al2O3 (99.5%) and Bi2O3 (99.9%) by the conventional solid–state reaction method [25]. The dried powders were
3
ball–milled for 15h in ethanol. After drying, the mixed powders were calcined at 850 oC and ball–milled again. The mixtures were mixed with an appropriate amount of polyvinyl alcohol (PVA) solution as a binder for granulation. Samples were finally sintered at 1150 oC in covered alumina crucible for 2h in air. The crystal structures of the sintered ceramics were characterized by X-ray diffraction analysis (XRD) using a Cu Kα radiation (λ=1.54178Å) (D8 Advance, Bruker Inc., Karlsruhe, Germany). The microstructure of the ceramics was examined by a scanning electron microscope (SEM, JSM–6380, Japan). The electric field-induced polarization (P–E) and strain (S–E) were measured at room temperature at 10Hz by using an aix–ACCT TF2000FE-HV ferroelectric test unit (aix ACCT Systems GmbH, Aachen, Germany). The temperature dependence of dielectric properties was measured at temperatures ranging from room temperature to 500oC with a heating rate of 3 oC /min (Novocontrol, Germany). Impedances spectroscopy of the samples was performed using a Broadband Dielectric Spectrometer (Novocontrol Germany) in the 0.01 to 20 MHz frequency range at various temperatures. 3. Results and discussion Fig. 1(a) presents the X–ray diffraction patterns of the BNBT6.5–xCYAO samples in the 2θ ranges from 20° to 70°. All ceramics exhibit a pure perovskite structure, indicating that CYAO has diffused into BNBT6.5 lattices to form a new solid solution in the composition range studied. According to the reports, there exists a splitting peak near 46.5° for pure BNBT6.5 composition in the poled state [26, 27]. However, all compositions studied here are characterized by the absence of a noncubic distortion.
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The apparent inconsistency illustrates the BNBT6.5–xCYAO system may underwent to a phase transition during poling as reported in BNT–BT–based systems [10, 28]. Moreover, it is noted that there is a very slight shift in the XRD peaks towards lower angles in composition of x < 0.006, and then obviously to higher angles when x ≥ 0.009, indicating the change of cell volumes induced by the addition of CYAO. In the present work, owing to the close ionic radius, Ca2+ (1.18Ǻ, CN=9) and Y3+ (1.075Ǻ, CN=9) will enter into the A–site of perovskite and replace the larger (Bi0.5Na0.5)2+ (1.21Ǻ). While Al3+ (0.535Ǻ, CN=6) will enter into the B–site of perovskite and replace the larger Ti4+ (0.605Ǻ, CN=6), leading to the shrinkage of cell volumes. This can be explain why there is an obvious shift in the XRD peaks towards higher angles in composition of x ≥ 0.009. For the lower shift of XRD peaks in the composition of x < 0.006, it cannot be linked with the expansion of cell volumes due to the relative smaller ionic radius in Ca2+, Y3+ and Al3+ dopants. The very slight shift in the XRD peaks towards lower angles in composition of x < 0.006 may be caused by the XRD measurement errors. Fig. 1(b) displays the SEM micrographs of the BNBT6.5–xCYAO ceramics with x = 0.009, 0.012. The specimens were almost dense and have an obvious grain boundary. The results suggests the ceramics have been well sintered, which is also reflected by the large relative density >96% measured by the Archimedes drainage method. Fig. 2 shows (a) ferroelectric hysteresis (P–E) loops, (b) bipolar strain curves , (c) unipolar strain curves and (d) the strain and large signal d 33* of BNBT6.5–xCYAO ceramics measured at room temperature (RT), respectively. The P–E loops of ceramics
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change from well saturated to slim with the increase of CYAO. It is obvious that the remnant polarization Pr and coercive field Ec decrease gradually. The results indicate that the chemical modification destroyed the ferroelectric order and produced the ergodic relaxor phase [29]. The compositionally induced ferroelectric–relaxor transition is also verified by bipolar strain measurements. The BNBT6.5 ceramic shows a typical ferroelectric behavior with butterfly shaped strain and visible negative strain S neg related to the domain back switching during bipolar cycles [28]. With the doping of CYAO, Sneg gradually disappears accompanied by the increase in the positive strain Spos and the butterfly–like loop drastically changes into the sprout–shaped loop. Moreover, the changes contributed to the large unipolar strain: at x = 0.012, the value of unipolar strain reaches a maximum value of 0.44% at 75kV/cm, equivalently a large signal d 33* (Smax/Emax) of 470 pm/V, as shown in Fig. 2(d). The stability of the relaxor phase depends not only on composition but also on temperature. The P–E hysteresis loops, bipolar/unipolar S–E curves and variation of strain of BNBT6.5–xCYAO ceramics with x = 0 and 0.012 at different temperatures (20–120 oC) are shown in Fig. 3. The P–E loop of BNBT6.5 at RT shows a saturated shape which is emblematic for ferroelectrics. When the temperature is above TF–R (the ferroelectric–relaxor transition temperature), the loops become pinched and the bipolar strain curves change from butterfly to the sprout. Moreover, the strain increase before the temperature reaches 100oC and then decrease. By contrast, for BNBT6.5–0.012CYAO ceramics, since the TF–R is near RT, the RT P–E curve already exhibits a relaxor characteristic. The strain continuous decreases and the strain loops
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become slimmer as the temperature increases. Fig. 4 displays the temperature dependence of dielectric permittivity (εr) and dielectric loss (tanδ) for unpoled BNBT6.5–xCYAO ceramics, from room temperature to 500oC at 1MHz, and inset shows the temperature dependence of the dielectric constant of poled and unpoled BNBT6.5 samples. For unpoled samples, compared with pure BNBT6.5 ceramics, the dielectric maxima decrease significantly, suggesting that the dielectric properties are weakened by the addition of CYAO. After poling treatment, the dielectric response of BNBT6.5 ceramic exhibits two anomalies that are related to two–phase transitions are observed in the inset of Fig. 4. TF–R is the ferroelectric–relaxor transition temperature while Tm is the maximum temperature. The behavior is similar to those previously observed in BNT–BT ceramics [30, 31]. While for poled CYAO-modified ceramics, the TF–R peak disappears while Tm shifts slightly. It is likely that TF–R has moved to below RT [32]. The downward shift of TF–R with increasing CYAO content indicates a compositionally induced ferroelectric–relaxor transition, which is in good agreement with the results from strain as well as P–E hysteresis measurement. In our work, we have also study the influence of CYAO content on the impedance spectra of the BNBT6.5 ceramics, the results and discussion have been summarized in Fig. S1 (Supplementary data). 4. Conclusions The effect of CYAO substitution on BNBT6.5 ceramics was systematically investigated. With increasing CYAO content, the ceramics underwent a phase transition
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from ferroelectric to relaxor phase with the shift of TF–R down to room termperature. Encouraging results of field–induced unipolar strain (0.44%) is realized for BNBT6.5–0.012CYAO ceramics. It is expected that the piezoceramic is promising candidate that can be used in practical applications. Acknowledgments This work was supported by the National Key R&D Program of China (NO.2016YFB0402701)，National Natural Science Foundation of China (No. 51372110, 51402144, and 51502127 ), the Natural Science Foundation of Shandong Province of China (ZR2016EMM02), Independent innovation and achievement transformation in Shandong Province special, China (No. 2014CGZH0904), the Natural Science Foundation of Shandong Province of China (ZR2014JL030), The Project of Shandong Province Higher Educational Science and Technology Program(No. J14LA11, No.J14LA10). References [1] L. Zheng, X. Lu, H. Shang, et al. Phys. Rev. B. 91 (2015) 184105. [2] L. Zheng, et al. J. Appl. Phys., 114 (2013) 104105. [3] M. Zhang, K. Wang, Y. Du, et al. J. Am. Chem. Soc. 139 (2017) 3889–3895. [4] B. Noheda, D.E. Cox, G. Shirane, J. Gao, Z.G. Ye, Phys. Rev. B. 66 (2002) 054104. [5] A. Hussain, A. Zaman, Y. Iqbal, M.H. Kim, J. Alloys. Compd. 574 (2013) 320–324. [6] Y. Li, K.-S. Moon, C.P. Wong, Science. 308 (2005) 1419–1420. [7] L.E. Cross, Ferroelectrics. 151 (1994) 305–320. [8] T.R. Shrout, S.J. Zhang, J. Electroceram. 19 (2007) 111–124. [9] M.R. Suchomel, A.M. Fogg, M. et al. Chem. Mater. 38 (2006) 4987–4989. [10] S.T. Zhang, A.B. Kounga, E. Aulbach, H. Ehrenberg, J. Rödel, Appl. Phys. Lett. 91 (2007) 112906
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[11] Z. Surowiak, J.Dudek, Y.I. Goltzov, I.A Bugajan, V.E. Yurkevich, J. Mater. Sci. 26 (1991) 4407–4410. [12] T. Takenaka, H. Nagata, J. Eur. Ceram. Soc. 25 (2005) 2693–2700. [13] W. Zhao, H. Zhou, Y. Yan, D.Liu, Key Eng. Mater. 368 (2008) 1908–1910. [14] F. Wang, M. Xu, Y. Tang, T. Wang, W. Shi, C. Leung, J. Am. Ceram. Soc. 95 (2012) 1955–1959. [15] D.S. Yin, Z.H. Zhao, Y.J. Dai, Z. Zhao. X.W. Zhang, S.H. Wang, J. Am. Ceram. Soc. 99 (2016) 2354–2360. [16] R. Cheng, Z. Xu, R. Chu, J. Hao, J. Du, G. Li, J. Alloys. Compd. 632 (2015) 580–584. [17] R. Cheng, L. Zhu, Y. Zhu, et al. Mater. Lett. 165 (2015)143–146. [18] S. Lv, Y. Wang, Z. Zhu, et al. Appl. Phys. B 116 (2014) 83–89. [19] R.D. Shannon, R.A. Oswald, J.B. Parise, et al. J. Solid State Chem. 98 (1992) 90–98. [20] A.E. Lavat, E. J. Baran. J. Alloys. Comp. 368 (2004) 130–134. [21] Y. Matsushima, N. Ishizawa, N. Kodama, Physica C. 338 (2000) 166–169. [22] J.S. Lee, D.N. Binh, A. Hussain, et al. J. Korean. Phys. Soc. 57 (2010), 892–896. [23] I.K. Hong, H.S.Han, C.H. Yoon, et al. J. Intell. Mater. Syst. Struct. 24 (2013) 1343–1349. [24] A. Ullah, W.A. Chang, A. Hussain, et al. J. Am. Ceram. Soc. 94 (2011) 3915–3921. [25] L. Li, J. Hao, Z. Xu, et al. Mater. Lett. 184 (2016) 152–156. [26] J.E. Daniels, W. Jo, J.L. Jones, Appl. Phys. Lett. 95 (2009) 032904. [27] C. Xu, D. Lin, K.W. Kwok, Solid State Sci. 10 (2008) 934–940. [28] K.T.P. Seifert, W. Jo, R. Rödel, J. Am. Ceram. Soc. 93 (2010) 1392–1396. [29] F.Z. Yao, K. Wang, W, Jo, et al. Adv. Funct. Mater. 26 (2016) 1217–1224. [30] A. Maqbool, A. Hussain, J. Ur Rahman, T. Kwon Song, W.–J. Kim, J. Lee, M.–H. Kim, Ceram. Int. 40 (2014) 11905–11914. [31] J. Hao, B. Shen, J. Zhai, H. Chen, J. Appl. Phys. 115 (2014) 034101. [32] J. Hao, Z. Xu, R. Chu, W. Li, P. Fu, J. Du, J. Alloys. Compd. 677 (2016) 96–104.
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Figure captions: Fig.1. (a) X–ray diffraction patterns of BNBT6.5–xCYAO ceramics in the 2θ range of 20°–70°; (b) the SEM photographs of BNBT6.5–xCYAO (x = 0.009,0.012) ceramics. Fig.2. (a) P–E hysteresis loops, (b) bipolar strain curves, (c) unipolar strain curves and (d) the large signal d33*and strain of BNBT6.5–xCYAO measured at RT. Fig.3.The P–E hysteresis loops and bipolar and unipolar S–E curves of BNBT6.5–xCYAO ceramics with (a), (c), (e) x=0 and (b), (d), (f) x=0.012 at different temperatures (20–120 oC). The strain values as a function of temperature of BNBT6.5–xCYAO ceramics with (g) x = 0 and (h) x = 0.012. Fig.4. Temperature dependence of the dielectric constant and loss of unpoled BNBT6.5–xCYAO ceramics at various frequencies, inset figure shows the temperature dependence of the dielectric constant of poled and unpoled BNBT6.5 samples.
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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• The coexistence of ferroelectric phase and relaxor phase promotes the electric field induced strain response. • BNBT6.5–xCYAO ceramics show large electric field-induced bipolar strain of 0.44% at 75kV/cm. • The relationship between strain and temperature is interesting.
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S0167-577X(17)31189-8 http://dx.doi.org/10.1016/j.matlet.2017.08.010 MLBLUE 22987
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
25 May 2017 19 July 2017 2 August 2017
Please cite this article as: L. Li, J. Hao, Z. Xu, W. Li, R. Chu, Electric field–induced large strain of (Bi1/2Na1/2)0.935Ba0.065TiO3–CaYAlO4 lead–free ceramics, Materials Letters (2017), doi: http://dx.doi.org/10.1016/ j.matlet.2017.08.010
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electric field–induced large strain of (Bi1/2Na1/2)0.935Ba0.065TiO3–CaYAlO4 lead–free ceramics Liangliang Li1, Jigong Hao1,*, Zhijun Xu 2, Wei Li1, Ruiqing Chu2,* 1
College of Materials Science and Engineering, Liaocheng University, Liaocheng
252059, People’s Republic of China 2
School of Environmental and Materials Engineering, Yantai University, 32 Qingquan
Road, Yantai 264005, China
Abstract Lead–free piezoelectric ceramics (1-x)(Bi1/2Na1/2)0.935Ba0.065TiO3–xCaYAlO4 (BNBT6.5–xCYAO) were prepared using a conventional solid sintering technique. The addition of CYAO destroyed the ferroelectric long–range order with the shift of the ferroelectric–relaxor transition temperature TF–R down to room temperature and induced the appearance of ergodic relaxor phase. Thus, it leads to a giant field–induced strain with a peak value of 0.44% at x = 0.012. Keywords: Lead–free piezoelectric ceramics; Phase transition; Ferroelectricity, Field–induced strain 1. Introduction Over the past half century, lead–based materials with the excellent piezoelectric properties such as Pb(ZrxTi1-x)O3 (PZT) were widely utilized in piezoelectric ceramic devices [1,2]. The presence of a morphotropic phase boundary (MPB) [3] is contributed to high dielectric, piezoelectric and ferroelectric response in these systems [4, 5]. -----------------------------------* Corresponding author. Email address: [email protected] (J.Hao); [email protected] (R. Chu).
However, the strong toxicity of lead–based oxides during processing has caused critical ecological issues [6]. Therefore, great attention is drawn on lead–free ceramics with excellent properties to replace the PZT–based ceramics [7]. Recently, much research has focused on Bi–based ceramics as environmental friendly alternatives to PZT since the Bi3+, like the Pb2+ ion, is highly polarizable due to a lone electron pair [8, 9]. Among various Bi–based perovskite system, lead–free (Bi0.5Na0.5)TiO3(BNT)–based perovskite ceramics aroused great attention since Zhang et al [10] made a break–through. Since then, plenty of studies that related to the strain properties of BNT–based piezoceramics have been reported. (Bi1/2Na1/2)TiO3–BaTiO3 (BNT–BT) system, similar to the PZT ceramics [11], exhibits good piezoelectric and ferroelectric properties owing to the existence of a rhombohedral–tetragonal MPB at compositions with the BT concentration of 6–7mol% [12, 13]. However, the strain properties of BNT–BT system are not enough for practical applications. It has proved comparatively easy to form new solid solutions of BNT–BT with other perovskites compounds [14,15]. Near the MPB, these systems showed promising performances which are very attractive as lead–free replacements for PZT. Multiferroic materials A2B2O6 with the double–perovskite structure can effectively diffuse into the lattice of BNT–based ceramics to form a solid solution. Moreover, it promotes the piezoelectric properties or field–induced strain response significantly [16, 17]. eg, Bi2NiMnO6 and Sr2ZrMnO6 have been selected as the addition into BNT–based ceramics by our research group to enhance the strain response [16, 17]. Results showed that the enhanced strain response was linked with a significant disruption of the
2
long-range ferroelectric order induced by the A2B2O6, where the phase structure transition caused by ion substitution effect is the original reason. Unlike A2B2O6 with the double–perovskite structure, the calcium yttrium aluminate (CaYAlO4, i.e., CYAO) has a two–dimensional perovskite with a tetragonal structure [18, 19]. It belongs to the series of A3+B2+AlO4 aluminates (A3+ = Y, Nd, La; B2+ = Ca, Sr), which crystallize in the K2NiF4 type structure consisting of an ABO3 perovskite and AO rock–salt (NaCl) type layers. Moreover, A and B elements present ninefold coordination whereas Al is octahedrally coordinated [20,21]. Owing to the close ionic radius, Ca2+ (1.18Ǻ, CN=9) and Y3+ (1.075Ǻ, CN=9) will enter into the A–site of perovskite while Al3+ (0.535Ǻ, CN=6) will enter into the B–site of perovskite. Ca2+, Y3+, and Al3+ were proved to be effective modifier to promote the strain response of BNT–based ceramics [22–24]. Based on the above, we expect CYAO plays a similar role to produce large strain response in BNT–based ceramics. In the present work, we selected (Bi1/2Na1/2)0.935Ba0.065TiO3 (BNBT6.5), which is known to lie at the MPB between rhombohedral BNT and tetragonal BT as a base material with small amounts of CYAO being introduced as doping species. The phase structure and electric properties of the new BNBT6.5–CYAO lead–free ceramics were studied systematically. 2. Experimental procedure The ceramics (1-x)(Bi1/2Na1/2)0.935Ba0.065TiO3–xCaYAlO4 (BNBT6.5–xCYAO, x = 0–0.015) were fabricated with metal oxides or carbonate powders: Na2CO3 (99.8%), BaCO3 (99%), CaCO3 (99%), TiO2 (99.5%), Y2O3 (99.99%), Al2O3 (99.5%) and Bi2O3 (99.9%) by the conventional solid–state reaction method [25]. The dried powders were
3
ball–milled for 15h in ethanol. After drying, the mixed powders were calcined at 850 oC and ball–milled again. The mixtures were mixed with an appropriate amount of polyvinyl alcohol (PVA) solution as a binder for granulation. Samples were finally sintered at 1150 oC in covered alumina crucible for 2h in air. The crystal structures of the sintered ceramics were characterized by X-ray diffraction analysis (XRD) using a Cu Kα radiation (λ=1.54178Å) (D8 Advance, Bruker Inc., Karlsruhe, Germany). The microstructure of the ceramics was examined by a scanning electron microscope (SEM, JSM–6380, Japan). The electric field-induced polarization (P–E) and strain (S–E) were measured at room temperature at 10Hz by using an aix–ACCT TF2000FE-HV ferroelectric test unit (aix ACCT Systems GmbH, Aachen, Germany). The temperature dependence of dielectric properties was measured at temperatures ranging from room temperature to 500oC with a heating rate of 3 oC /min (Novocontrol, Germany). Impedances spectroscopy of the samples was performed using a Broadband Dielectric Spectrometer (Novocontrol Germany) in the 0.01 to 20 MHz frequency range at various temperatures. 3. Results and discussion Fig. 1(a) presents the X–ray diffraction patterns of the BNBT6.5–xCYAO samples in the 2θ ranges from 20° to 70°. All ceramics exhibit a pure perovskite structure, indicating that CYAO has diffused into BNBT6.5 lattices to form a new solid solution in the composition range studied. According to the reports, there exists a splitting peak near 46.5° for pure BNBT6.5 composition in the poled state [26, 27]. However, all compositions studied here are characterized by the absence of a noncubic distortion.
4
The apparent inconsistency illustrates the BNBT6.5–xCYAO system may underwent to a phase transition during poling as reported in BNT–BT–based systems [10, 28]. Moreover, it is noted that there is a very slight shift in the XRD peaks towards lower angles in composition of x < 0.006, and then obviously to higher angles when x ≥ 0.009, indicating the change of cell volumes induced by the addition of CYAO. In the present work, owing to the close ionic radius, Ca2+ (1.18Ǻ, CN=9) and Y3+ (1.075Ǻ, CN=9) will enter into the A–site of perovskite and replace the larger (Bi0.5Na0.5)2+ (1.21Ǻ). While Al3+ (0.535Ǻ, CN=6) will enter into the B–site of perovskite and replace the larger Ti4+ (0.605Ǻ, CN=6), leading to the shrinkage of cell volumes. This can be explain why there is an obvious shift in the XRD peaks towards higher angles in composition of x ≥ 0.009. For the lower shift of XRD peaks in the composition of x < 0.006, it cannot be linked with the expansion of cell volumes due to the relative smaller ionic radius in Ca2+, Y3+ and Al3+ dopants. The very slight shift in the XRD peaks towards lower angles in composition of x < 0.006 may be caused by the XRD measurement errors. Fig. 1(b) displays the SEM micrographs of the BNBT6.5–xCYAO ceramics with x = 0.009, 0.012. The specimens were almost dense and have an obvious grain boundary. The results suggests the ceramics have been well sintered, which is also reflected by the large relative density >96% measured by the Archimedes drainage method. Fig. 2 shows (a) ferroelectric hysteresis (P–E) loops, (b) bipolar strain curves , (c) unipolar strain curves and (d) the strain and large signal d 33* of BNBT6.5–xCYAO ceramics measured at room temperature (RT), respectively. The P–E loops of ceramics
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change from well saturated to slim with the increase of CYAO. It is obvious that the remnant polarization Pr and coercive field Ec decrease gradually. The results indicate that the chemical modification destroyed the ferroelectric order and produced the ergodic relaxor phase [29]. The compositionally induced ferroelectric–relaxor transition is also verified by bipolar strain measurements. The BNBT6.5 ceramic shows a typical ferroelectric behavior with butterfly shaped strain and visible negative strain S neg related to the domain back switching during bipolar cycles [28]. With the doping of CYAO, Sneg gradually disappears accompanied by the increase in the positive strain Spos and the butterfly–like loop drastically changes into the sprout–shaped loop. Moreover, the changes contributed to the large unipolar strain: at x = 0.012, the value of unipolar strain reaches a maximum value of 0.44% at 75kV/cm, equivalently a large signal d 33* (Smax/Emax) of 470 pm/V, as shown in Fig. 2(d). The stability of the relaxor phase depends not only on composition but also on temperature. The P–E hysteresis loops, bipolar/unipolar S–E curves and variation of strain of BNBT6.5–xCYAO ceramics with x = 0 and 0.012 at different temperatures (20–120 oC) are shown in Fig. 3. The P–E loop of BNBT6.5 at RT shows a saturated shape which is emblematic for ferroelectrics. When the temperature is above TF–R (the ferroelectric–relaxor transition temperature), the loops become pinched and the bipolar strain curves change from butterfly to the sprout. Moreover, the strain increase before the temperature reaches 100oC and then decrease. By contrast, for BNBT6.5–0.012CYAO ceramics, since the TF–R is near RT, the RT P–E curve already exhibits a relaxor characteristic. The strain continuous decreases and the strain loops
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become slimmer as the temperature increases. Fig. 4 displays the temperature dependence of dielectric permittivity (εr) and dielectric loss (tanδ) for unpoled BNBT6.5–xCYAO ceramics, from room temperature to 500oC at 1MHz, and inset shows the temperature dependence of the dielectric constant of poled and unpoled BNBT6.5 samples. For unpoled samples, compared with pure BNBT6.5 ceramics, the dielectric maxima decrease significantly, suggesting that the dielectric properties are weakened by the addition of CYAO. After poling treatment, the dielectric response of BNBT6.5 ceramic exhibits two anomalies that are related to two–phase transitions are observed in the inset of Fig. 4. TF–R is the ferroelectric–relaxor transition temperature while Tm is the maximum temperature. The behavior is similar to those previously observed in BNT–BT ceramics [30, 31]. While for poled CYAO-modified ceramics, the TF–R peak disappears while Tm shifts slightly. It is likely that TF–R has moved to below RT [32]. The downward shift of TF–R with increasing CYAO content indicates a compositionally induced ferroelectric–relaxor transition, which is in good agreement with the results from strain as well as P–E hysteresis measurement. In our work, we have also study the influence of CYAO content on the impedance spectra of the BNBT6.5 ceramics, the results and discussion have been summarized in Fig. S1 (Supplementary data). 4. Conclusions The effect of CYAO substitution on BNBT6.5 ceramics was systematically investigated. With increasing CYAO content, the ceramics underwent a phase transition
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from ferroelectric to relaxor phase with the shift of TF–R down to room termperature. Encouraging results of field–induced unipolar strain (0.44%) is realized for BNBT6.5–0.012CYAO ceramics. It is expected that the piezoceramic is promising candidate that can be used in practical applications. Acknowledgments This work was supported by the National Key R&D Program of China (NO.2016YFB0402701)，National Natural Science Foundation of China (No. 51372110, 51402144, and 51502127 ), the Natural Science Foundation of Shandong Province of China (ZR2016EMM02), Independent innovation and achievement transformation in Shandong Province special, China (No. 2014CGZH0904), the Natural Science Foundation of Shandong Province of China (ZR2014JL030), The Project of Shandong Province Higher Educational Science and Technology Program(No. J14LA11, No.J14LA10). References [1] L. Zheng, X. Lu, H. Shang, et al. Phys. Rev. B. 91 (2015) 184105. [2] L. Zheng, et al. J. Appl. Phys., 114 (2013) 104105. [3] M. Zhang, K. Wang, Y. Du, et al. J. Am. Chem. Soc. 139 (2017) 3889–3895. [4] B. Noheda, D.E. Cox, G. Shirane, J. Gao, Z.G. Ye, Phys. Rev. B. 66 (2002) 054104. [5] A. Hussain, A. Zaman, Y. Iqbal, M.H. Kim, J. Alloys. Compd. 574 (2013) 320–324. [6] Y. Li, K.-S. Moon, C.P. Wong, Science. 308 (2005) 1419–1420. [7] L.E. Cross, Ferroelectrics. 151 (1994) 305–320. [8] T.R. Shrout, S.J. Zhang, J. Electroceram. 19 (2007) 111–124. [9] M.R. Suchomel, A.M. Fogg, M. et al. Chem. Mater. 38 (2006) 4987–4989. [10] S.T. Zhang, A.B. Kounga, E. Aulbach, H. Ehrenberg, J. Rödel, Appl. Phys. Lett. 91 (2007) 112906
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Figure captions: Fig.1. (a) X–ray diffraction patterns of BNBT6.5–xCYAO ceramics in the 2θ range of 20°–70°; (b) the SEM photographs of BNBT6.5–xCYAO (x = 0.009,0.012) ceramics. Fig.2. (a) P–E hysteresis loops, (b) bipolar strain curves, (c) unipolar strain curves and (d) the large signal d33*and strain of BNBT6.5–xCYAO measured at RT. Fig.3.The P–E hysteresis loops and bipolar and unipolar S–E curves of BNBT6.5–xCYAO ceramics with (a), (c), (e) x=0 and (b), (d), (f) x=0.012 at different temperatures (20–120 oC). The strain values as a function of temperature of BNBT6.5–xCYAO ceramics with (g) x = 0 and (h) x = 0.012. Fig.4. Temperature dependence of the dielectric constant and loss of unpoled BNBT6.5–xCYAO ceramics at various frequencies, inset figure shows the temperature dependence of the dielectric constant of poled and unpoled BNBT6.5 samples.
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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• The coexistence of ferroelectric phase and relaxor phase promotes the electric field induced strain response. • BNBT6.5–xCYAO ceramics show large electric field-induced bipolar strain of 0.44% at 75kV/cm. • The relationship between strain and temperature is interesting.
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