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Giant strain response with low hysteresis in potassium sodium niobate based lead-free ceramics
Ceramics International 45 (2019) 14675–14683
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Giant strain response with low hysteresis in potassium sodium niobate based lead-free ceramics
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Lixiang Yua,b,c, Hao Xia,b,c, Zhenglei Yua,b,c, Yunfei Liua,b,c,∗, Yinong Lyua,b,c,∗∗ a
The State Key Laboratory of Materials-Oriented Chemical Engineering, College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), China c Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, China b
ARTICLE INFO
ABSTRACT
Keywords: Potassium-sodium niobate Giant strain Low hysteresis Thermal stability
In this work, the relationships between the composition-driven phase boundary, ferroelectricity and strain properties of the (1-x)(K0.48Na0.52)(Nb1-ySby)O3-xBi0.5(Na0.82K0.18)0.5ZrO3 (abbreviated as (1-x)KNN1-ySyxBNKZ) ceramics were investigated. A giant electric field-induced strain of 0.3% (d33∗ = 750 p.m./V) and a low hysteresis (16.4%) were obtained in the 0.97KNN0.98S0.02-0.03BNKZ ceramics. The giant strain is attributed to the enhanced piezoelectricity induced by the appearance of the O-T phase boundary and the electric-fieldinduced phase transition from the relaxor phase to the ferroelectric phase. Furthermore, the 0.97KNN0.98S0.020.03BNKZ ceramics exhibit good thermal stability in the temperature range from 25 °C to 150 °C. Hence, this work can promote the practical applications of KNN-based lead-free piezoelectric ceramics in highly sensitive and precise piezoelectric actuators.
1. Introduction Piezoceramics actuators that can carry out the interconversion of electric energy and mechanical energy have been important functional materials for a wide variety of applications [1–5]. Due to the toxicity of lead, lead-free piezoelectric ceramics with high performance are currently being actively developed in order to replace lead-based ceramics. Among these lead-free ceramics, K0.5Na0.5NbO3 (KNN)-based ceramics have attracted much attention because of their excellent piezoelectricity and high Curie temperature (Tc) [6,7]. In the past several years, some significant breakthroughs in KNNbased ceramics have been reported [3,8–11]. For example, in 2004, Saito et al. achieved a high d33 value (∼416 pC/N) in Li+, Ta5+, and Sb5+ comodified KNN-based ceramics via the complicated reaction template grain growth (RTGG) method [3]. Since then, researchers have devoted increasing effort to the development of high-performance KNN ceramics and to the investigation of the relationship between the electric properties and phase boundaries in the KNN-based systems. It was found that the piezoelectricity of KNN ceramics can be enhanced by forming the rhombohedral-orthorhombic (ReO) and orthorhombictetragonal (O-T) phase boundaries [8,10,11]. Recently, Wang et al.
obtained a giant d33 (∼490 pC/N) in KNN-based ceramics via the conventional solid-state method through the doping of BNKZ and Sb, which led to the appearance of a rhombohedral-tetragonal (R-T) phase boundary by increasing the TR-O and decreasing the TO-T [9]. Previous studies of KNN-based ceramics have focused mainly on the improvement in the piezoelectricity. By contrast, the electric field-induced strain response and the hysteresis have been rarely investigated. It is well-known that pure KNN ceramics usually exhibit low strain because it is difficult to achieve dense ceramics. Therefore, different dopants were used to modify the strain, but a relatively low strain (approximately 0.2%) was usually observed [12]. Thus, the aim of the present study is to enhance the strain properties by choosing appropriate components so that the KNN-based ceramics can be used in practical applications. In this work, we developed the (1-x)(K0.48Na0.52)(Nb1-ySby)O3xBi0.5(Na0.82K0.18)0.5ZrO3 materials system for simultaneously attaining a giant strain and a low hysteresis. Compositions with x = 0.02–0.04 and y = 0.01–0.05 were used. The phase structure, ferroelectricity, the electric field-induced strain and corresponding hysteresis were investigated in detail. The enhanced strain was obtained by constructing the O-T phase boundary. Additionally, the temperature stability of the
∗ Corresponding author. The State Key Laboratory of Materials-Oriented Chemical Engineering, College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China. ∗∗ Corresponding author. The State Key Laboratory of Materials-Oriented Chemical Engineering, College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China. E-mail addresses: [email protected] (Y. Liu), [email protected] (Y. Lyu).
https://doi.org/10.1016/j.ceramint.2019.04.187 Received 9 March 2019; Received in revised form 18 April 2019; Accepted 22 April 2019 Available online 23 April 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Fig. 1. XRD patterns of (1-x)KNN1-ySy-xBNKZ ceramics as a function of (a) x (y = 0.05), (c) y (x = 0.03) measured at 2θ = 10–60 °. (b) and (d) are the magnified XRD patterns at 2θ = 44–47 °.
0.97KNN0.98S0.02-0.03BNKZ ceramics was studied. 2. Experimental procedure The (1-x)KNN1-ySy-xBNKZ piezoceramics were synthesized via the conventional solid-state process. The high-purity raw materials were K2CO3(99.0%), Na2CO3(99.8%), Nb2O5(99.5%), Sb2O3(99.5%), Bi2O3(99.0%), and ZrO2(99.0%). The weighted raw materials were ballmilled for 12 h using ZrO2 balls and alcohol as the ball-milling medium. The dried powers were calcined at 850 °C for 3 h in air. The calcined powders were pressed into pellets with a diameter and thickness of 13 mm and 1.5 mm, respectively, by uniaxial molding under a pressure of 10 MPa after mixing with 5 wt% polyvinyl alcohol (PVA) as the binder. The pellets were heated to 600 °C for 1 h to burn off the PVA. Then, the pellets were sintered at 1130 °C for 3 h in the calcined powder with the same composition in order to minimize the evaporation of volatile elements. Silver paste was attached to the sintered samples on both sides and then the samples were fired at 750 °C for 30 min. The samples were poled in a silicone oil bath at room temperature under a
direct current electric field of 30 kV/cm for 30 min. The phase compositions of the ceramics were measured by X-ray diffraction (XRD, Smartlab 3 kW, Rigaku, Japan) with Cu Kα radiation. The surface microstructural observations of the ceramics were performed by scanning electron microscopy (SEM, JSM-6510, JEOL, Japan). The electric-field-induced polarization (P-E) hysteresis loops, current-electric field (I-E) hysteresis loops, bipolar strain (S-E) curves and unipolar strain (S-E) curves were measured using a ferroelectric measuring instrument (Precision Premier II, Radiant Technology, USA) in a silicone oil bath. The piezoelectric constant d33 values were measured by a quasi-static d33 m (ZJ-2; Institute of acoustics, China). 3. Results and discussion Fig. 1(a) and (c) show the X-ray diffraction patterns of (1-x)KNN1ceramics measured at room temperature and 2θ = 10–60°. The results suggest that the BNKZ and Sb dopants completely diffuse into the KNN matrix, and all of the obtained ceramics show a typical perovskite structure. To characterize the evolution of the phase ySy-xBNKZ
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Fig. 2. SEM images of the (1-x)KNN1-ySy-xBNKZ ceramics with different compositions: (a)–(e) x = 0.02, 0.025, 0.03, 0.035 and 0.04 (y = 0.05); (f)–(j) y = 0.01, 0.02, 0.03, 0.04 and 0.05 (x = 0.03).
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Fig. 3. Grain size of the (1-x)KNN1-ySy-xBNKZ ceramics with different compositions: (a)–(e) x = 0.02, 0.025, 0.03, 0.035 and 0.04 (y = 0.05); (f)–(j) y = 0.01, 0.02, 0.03, 0.04 and 0.05 (x = 0.03); and composition dependence of average grain sizes and relative density as a function of (k) x (y = 0.05), (l) y (x = 0.03).
composition of the (1-x)KNN1-ySy-xBNKZ ceramics, the magnified XRD patterns with 2θ between 44° and 47° are plotted in Fig. 1(b) and (d). The (1-x)KNN0.95S0.05-xBNKZ ceramics for 0.02 ≤ x ≤ 0.04 show only a single (200) peak between 44° and 47°. This finding indicates that the (1-x)KNN0.95S0.05-xBNKZ ceramics for 0.02 ≤ x ≤ 0.04 belong to the pseudocubic phase [13]. The results show that a small BNKZ contents does not clearly change the phase composition for Sb doping in excess. The influence of Sb doping on the phase structure of the ceramics is shown in Fig. 1(c) and (d). It is observed that there is a clear split of the (002)/(200) peak between 44° and 47° in the 0.97KNN1-ySy-0.03BNKZ ceramics for 0.01 ≤ y ≤ 0.03. This result indicates that the phase structure of the 0.97KNN1-ySy-0.03BNKZ for 0.01 ≤ y < 0.02 is the O phase, and the O-T phase boundary is present in the 0.97KNN1-ySy0.03BNKZ ceramics for 0.02 ≤ y < 0.04. The O-T phase boundary appears because Sb doping decreases the TO-T to approximately room temperature [9]. As the Sb content exceeds 0.04, the split peaks change to a single (200) peak, indicating that the phase structure of the 0.97KNN1-ySy-0.03BNKZ ceramics transforms to the pseudocubic phase.
The SEM images of the (1-x)KNN1-ySy-xBNKZ ceramics are shown in Fig. 2. All of the ceramics exhibit a dense microstructure with small rectangular block grains. The grain sizes distribution and the composition dependence of the grain sizes and relative density are plotted in Fig. 3. The results show that the BNKZ content has little impact on the average grain size of (1-x)KNN0.95S0.05-xBNKZ ceramics. Additionally, the average grain size fluctuates approximately 0.4 μm with the increasing BNKZ dopant content. This observation is because the excess Sb inhibits the grain growth, and the BNKZ content will no longer clearly change the average grain size. The results show that the average grain size is strongly dependent on the Sb dopant content. The average grain size first decreases extremely from 2.26 μm to 0.89 μm and then decreases slowly from 0.89 μm to 0.52 μm as the Sb content increases from 0.01 to 0.05. The P-E curves of the (1-x)KNN1-ySy-xBNKZ ceramics measured under 40 kV/cm and a frequency of 1 Hz at room temperature are shown in Fig. 4(a) and (b) as a function of x and y. To further understand the ferroelectric properties of the ceramics, the curves of the
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Fig. 3. (continued)
maximum polarization (Pmax), the remnant polarization (Pr) and the coercive field (Ec) against x and y are plotted in Fig. 4(c) and (d). It is observed that the ceramics with x = 0.02–0.04 (y = 0.05) exhibit slim P-E loops with small Pr and Ec, while the ceramics with 0.01 ≤ y ≤ 0.04 (x = 0.03) are typical ferroelectrics with saturated P-E loops. The Pmax and Pr both first increase and then decrease as the BNKZ and Sb dopant contents increase, indicating that BNKZ and Sb enhance the ferroelectricity of (1-x)KNN1-ySy-xBNKZ for 0.02 ≤ x ≤ 0.03 (y = 0.05) and 0.01 ≤ y ≤ 0.02 (x = 0.03). In addition, Ec shows a decreasing trend overall with increasing BNKZ and Sb dopant contents. However, Ec increases slightly at x = 0.035 (y = 0.05) and y = 0.03 (x = 0.03). As a result, the ferroelectricity of the ceramics is enhanced with a small amount of BNKZ and Sb. As the BNKZ and Sb contents continue to increase, the P-E loops become slim, indicating that the ceramics transform from typical ferroelectrics to relaxor ferroelectrics. Fig. 5(a) and (b) show the bipolar strain curves of (1-x)KNN1-ySyxBNKZ ceramics with respect to the BNKZ and Sb contents, measured at room temperature and a frequency of 0.1 Hz. The ceramics with
0.02 ≤ x ≤ 0.04 (y = 0.05) exhibit sprout-shaped strain curves with almost zero negative strain value (Sneg), while the ceramics with 0.01 ≤ y ≤ 0.04 (x = 0.03) exhibit butterfly shaped strain curves with large negative strain values (Sneg), indicating typical ferroelectric material behavior. This finding is consistent with the ferroelectric behavior of the ceramics (see Fig. 4(b)). Even more interestingly, Sneg decreases with the increasing Sb content and is almost equal to zero when the Sb content increases to 0.05. This finding is because the excess Sb disrupts the long-range ferroelectric order leading to reduced ferroelectricity [14]. The maximum strain S, normalized strain d33∗ and the piezoelectric constant d33 with respect to BNKZ and Sb content are shown in Fig. 5(e) and (f), respectively. It is observed that the maximum strain value first increases and then decreases as the BNKZ content increases. Additionally, the variation of the strain of the ceramics with respect to the Sb content shows a similar trend. Furthermore, Fig. 5(f) shows that both the strain and the piezoelectric constant d33 first increase and then decrease with respect to the Sb content. The maximum strain (Smax = 0.3%), the normalized strain (d33∗ = 750 p.m./V) and the
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Fig. 4. P-E curves of (1-x)KNN1-ySy-xBNKZ ceramics as a function of (a) x (y = 0.05), (b) y (x = 0.03). (c) and (d) are their corresponding Pr, Ec, and Pmax values.
piezoelectric constant (d33 = 482 pC/N) are attained in the 0.97KNN0.98S0.02-0.03BNKZ ceramics. The enhanced piezoelectricity is due to the construction of the O-T phase boundary [4] For the 0.97KNN0.98S0.02-0.03BNKZ ceramics, The well-saturated P-E hysteresis loops, the butterfly-shaped bipolar S-E curves and the high d33 indicate the dominating intrinsic contribution to the giant strain, i.e., the converse piezoelectric effect [15,16]. . To further understand the mechanism of the giant strain, the I-E loops of the ceramics measured at room temperature under 40 kV/cm and at 1 Hz are shown in Fig. 6(a) and (b). The I-E loop of 0.97KNN0.98S0.02-0.03BNKZ ceramic shows four current peaks. P2 current peaks can be ascribed to the domain switching of the long-range ferroelectric phase, while P1 current peaks are related to the electricfield-induced phase transition from relaxor phase to ferroelectric phase [16,17]. As the Sb doping content increases, the long-range ferroelectric domains are broken into the micro-domains or PNRs. The PNRs will grow up into the long-range ferroelectric order by merging small ones when the applied external electric field overcomes the effects of local random field [16]. In summary, both the enhanced piezoelectricity induced by the O-T phase boundary and the phase transition from relaxor phase to ferroelectric phase lead to the giant strain. To enable the application of the KNN-based ceramics in highly sensitive and precise actuators devices, the unipolar strain curves of 0.97KNN0.98S0.02-0.03BNKZ ceramic were studied, and the d33∗ vs. hysteresis (H = ΔS/Smax) of some reported KNN-based and BNT-based ceramics were compared, as summarized in Fig. 7 [1,2,4,7,18–36]. It is observed that a giant normalized strain (d33∗ = 1400 p.m./V) can be achieved in BNT-2.5Nb ceramics. However, a large hysteresis (H = 68.9%) was obtained at the same time [2]. Additionally, it is unexpected to find that when a low hysteresis (H = 17.2%) was achieved in textured BKT-BT-BNT ceramics, a relatively small
normalized strain (d33∗ = 400 p.m./V) was obtained. Although a low hysteresis (usually < 25%) can be obtained in some reported KNNbase ceramics [4,7,18,26–29,36], a small normalized strain (< 500 p.m./V) was usually found. In this work, a giant strain (S = 0.3%, d33∗ = 750 p.m./V) and a low hysteresis (H = 16.4%) were simultaneously obtained in the 0.97KNN0.98S0.02-0.03BNKZ ceramics. It is generally accepted that the domain wall motion accounts for the hysteresis phenomena. PNRs response to the external field much faster than macroscopic domains [37]. Therefore, the small hysteresis (H = 16.4%) of 0.97KNN0.98S0.02-0.03BNKZ ceramics is due to existence of PNRs. Fig. 8(a) shows the P-E curves of the 0.97KNN0.98S0.02-0.03BNKZ ceramics measured at a frequency of 0.1 Hz in the temperature range from room temperature to 150 °C. The Pr, Ec and Pmax and their corresponding normalized Pr, Ec and Pmax curves with respect to temperature are shown in Fig. 8(b) and (c), respectively. It is observed that both Pmax and Pr first increase and then decrease while Ec continues to decrease as the temperature ranges from 25 °C to 150 °C. The increase in Pmax and Pr is due to the occurrence of a temperature-induced phase transition from the orthorhombic phase to the tetragonal phase in the 0.97KNN0.98S0.02-0.03BNKZ ceramics. It is observed from Fig. 8(c) that the Pmax of the 0.97KNN0.98S0.02-0.03BNKZ ceramics decreases by only 0.49% when the temperature is raised from room temperature to 150 °C. This finding means that the KNNS-BNKZ ceramics are promising candidates for use in piezoelectric actuators in high temperature environment. The thermal stabilities of some of the reported KNN-based ceramics, BNT-based ceramics and BT-based ceramics were compared, as summarized in Table 1 [38–47]. In this work, the Pmax values of 0.97KNN0.98S0.02-0.03BNKZ ceramics remain relatively stable in the studied temperature range; this is mainly related to the poling state of the field-induced ferroelectric long-range phase [16].
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Fig. 5. (a) and (b) Bipolar strain curves, (c) and (d) unipolar strain curves, (e) and (f) the variation of the maximum strain values, normalized strain (d33∗) and d33 with respect to x and y.
Fig. 6. (a) and (b) I-E curves with respect to x and y.
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Table 1 Thermal stabilities of some of the reported BNT-based, BT-based and KNNbased ceramics.
Fig. 7. d33∗ vs. hysteresis of BNT-based and KNN-based ceramics.
4. Conclusion In summary, (1-x)KNN1-ySy-xBNKZ lead-free ceramics were synthesized via the conventional solid-state process, and it was found that the Sb content has a significant effect on the phase structure, grain size, ferroelectricity and strain properties. A giant electric field-induced strain of 0.3% (d33∗ = 750 p.m./V) was obtained in the 0.97KNN0.98S0.02-0.03BNKZ ceramics due to the enhanced piezoelectricity induced by the appearance of the O-T phase boundary and the electric-field-induced phase transition from the relaxor phase to the ferroelectric phase. Additionally, a low hysteresis (H = 16.4%) was
Compounds
Temperature Range (°C)
Pmax Fluctuation (%)
Ref.
BNT-SBT BNKL-STT BNT-100xLi BLZT BZT BCZT BT-BMZ KNNS-BNZ KNLN-BZ-BNT KNNS-CS-BKH KNNS-BNKZ
30–120 25–175 30–90 30–120 30–120 27–120 25–150 0–100 30–180 25–100 25–150
10 25 15 40 25 29 8 8 15 10 5
[41] [46] [39] [42] [43] [38] [40] [47] [45] [44] This work
simultaneously obtained at the same composition. Moreover, the 0.97KNN0.98S0.02-0.03BNKZ ceramics exhibit good temperature stability. Therefore, we believe that the KNN-based ceramics are promising candidates for the replacement of lead-based piezoceramics in the near future. Acknowledgments This work was supported by the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions, and the Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites. The authors also acknowledge the financial support from the Innovative Research Team in University (PCSIRT), IRT1146.
Fig. 8. (a) P-E curves of 0.97KNN0.98S0.02-0.03BNKZ ceramics measured from 25 °C to 150 °C; (b) Pr, Ec and Pmax vs temperature of 0.97KNN0.98S0.02-0.03BNKZ ceramics; (c) Normalized Pr, Ec and Pmax vs temperature of 0.97KNN0.98S0.02-0.03BNKZ ceramics.
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Priya, Giant strain with ultra-low hysteresis and high temperature stability in grain oriented lead-free K0.5Bi0.5TiO3-BaTiO3-Na0.5Bi0.5TiO3 piezoelectric materials, Sci. Rep. 5 (2015) 8595. A. Ullah, R.A. Malik, A. Ullah, D.S. Lee, S.J. Jeong, J.S. Lee, I.W. Kim, C.W. Ahn, Electric-field-induced phase transition and large strain in lead-free Nb-doped BNKTBST ceramics, J. Eur. Ceram. Soc. 34 (2014) 35–41. K. Wang, A. Hussain, W. Jo, J. Rodel, Temperature-dependent properties of (Bi1/ 2Na1/2)TiO3-(Bi1/2K1/2)TiO3-SrTiO3 lead-free piezoceramics, J. Am. Ceram. Soc. 95 (2012) 2241–2247. Q.R. Yao, F.F. Wang, F. Xu, C.M. Leung, T. Wang, Y.X. Tang, X. Ye, Y.Q. Xie, D. Sun, W.Z. Shi, Electric field-induced giant strain and photoluminescence-enhancement effect in rare-earth modified lead-free piezoelectric ceramics, ACS Appl. Mater. Interfaces 7 (2015) 5066–5075. H.B. Zhang, P.W. Xu, E. Patterson, J.D. Zang, S.L. Jiang, J. Rodel, Preparation and enhanced electrical properties of grain-oriented (Bi1/2Na1/2)TiO3-based lead-free incipient piezoceramics, J. Eur. Ceram. Soc. 35 (2015) 2501–2512. S.T. Zhang, A.B. Kounga, E. Aulbach, T. Granzow, W. Jo, H.J. Kleebe, J. Rodel, Lead-free piezoceramics with giant strain in the system Bi0.5Na0.5TiO3-BaTiO3K0.5Na0.5NbO3. I. Structure and room temperature properties, J. Appl. Phys. 103 (2008) 034107. Y. Zhao, J. Du, Z.J. Xu, Enhanced piezoelectric properties with a high strain in (K0.44Na0.52Li0.04)(Nb0.86Ta0.1Sb0.04)O3-x wt%Sc2O3 lead-free ceramics, Mater. Sci. Eng. B-Adv. Funct. Solid-State Mater. 224 (2017) 110–116. L. Jin, F. Li, S.J. Zhang, Decoding the fingerprint of ferroelectric loops: comprehension of the material properties and structures, J. Am. Ceram. Soc. 97 (2014) 1–27. W.F. Bai, L.J. Wang, P. Zheng, F. Wen, L.L. Li, J.W. Zhai, Z.G. Ji, Electromechanical response and piezoelectric properties in (Ba0.85Ca0.15) (Zr0.1Ti0.9)O3 piezoceramics using nano-sized AlN modification, Ceram. Int. 44 (2018) 16040–16050. S. Gao, Z.J. Yao, L. Ning, G.Z. Dong, H.Q. Fan, Q. Li, Enhanced bipolar strain response in lithium/niobium Co-doped sodium-barium bismuth titanate lead-free ceramics, Adv. Eng. Mater. 19 (2017) 1700125. X.W. Jiang, H. Hao, S.J. Zhang, J.H. Lv, M.H. Cao, Z.H. Yao, H.X. Liu, Enhanced energy storage and fast discharge properties of BaTiO3 based ceramics modified by Bi(Mg1/2Zr1/2)O3, J. Eur. Ceram. Soc. 39 (2019) 1103–1109. L. Jin, J. Pang, R.Y. Jing, Y. Lan, L. Wang, F. Li, Q.Y. Hu, H.L. Du, D. Guo, X.Y. Wei, Z. Xu, L.Y. Zhang, G. Liu, Ultra-slim pinched polarization-electric field hysteresis loops and thermally stable electrostrains in lead-free sodium bismuth titanate-based solid solutions, J. Alloy. Comp. 788 (2019) 1182–1192. L. Jin, J. Qiao, L. Hou, L. Wang, L. Zhang, X. Lu, H.L. Du, X.Y. Wei, Y. Yan, G. 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Giant strain response with low hysteresis in potassium sodium niobate based lead-free ceramics
T
Lixiang Yua,b,c, Hao Xia,b,c, Zhenglei Yua,b,c, Yunfei Liua,b,c,∗, Yinong Lyua,b,c,∗∗ a
The State Key Laboratory of Materials-Oriented Chemical Engineering, College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), China c Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, China b
ARTICLE INFO
ABSTRACT
Keywords: Potassium-sodium niobate Giant strain Low hysteresis Thermal stability
In this work, the relationships between the composition-driven phase boundary, ferroelectricity and strain properties of the (1-x)(K0.48Na0.52)(Nb1-ySby)O3-xBi0.5(Na0.82K0.18)0.5ZrO3 (abbreviated as (1-x)KNN1-ySyxBNKZ) ceramics were investigated. A giant electric field-induced strain of 0.3% (d33∗ = 750 p.m./V) and a low hysteresis (16.4%) were obtained in the 0.97KNN0.98S0.02-0.03BNKZ ceramics. The giant strain is attributed to the enhanced piezoelectricity induced by the appearance of the O-T phase boundary and the electric-fieldinduced phase transition from the relaxor phase to the ferroelectric phase. Furthermore, the 0.97KNN0.98S0.020.03BNKZ ceramics exhibit good thermal stability in the temperature range from 25 °C to 150 °C. Hence, this work can promote the practical applications of KNN-based lead-free piezoelectric ceramics in highly sensitive and precise piezoelectric actuators.
1. Introduction Piezoceramics actuators that can carry out the interconversion of electric energy and mechanical energy have been important functional materials for a wide variety of applications [1–5]. Due to the toxicity of lead, lead-free piezoelectric ceramics with high performance are currently being actively developed in order to replace lead-based ceramics. Among these lead-free ceramics, K0.5Na0.5NbO3 (KNN)-based ceramics have attracted much attention because of their excellent piezoelectricity and high Curie temperature (Tc) [6,7]. In the past several years, some significant breakthroughs in KNNbased ceramics have been reported [3,8–11]. For example, in 2004, Saito et al. achieved a high d33 value (∼416 pC/N) in Li+, Ta5+, and Sb5+ comodified KNN-based ceramics via the complicated reaction template grain growth (RTGG) method [3]. Since then, researchers have devoted increasing effort to the development of high-performance KNN ceramics and to the investigation of the relationship between the electric properties and phase boundaries in the KNN-based systems. It was found that the piezoelectricity of KNN ceramics can be enhanced by forming the rhombohedral-orthorhombic (ReO) and orthorhombictetragonal (O-T) phase boundaries [8,10,11]. Recently, Wang et al.
obtained a giant d33 (∼490 pC/N) in KNN-based ceramics via the conventional solid-state method through the doping of BNKZ and Sb, which led to the appearance of a rhombohedral-tetragonal (R-T) phase boundary by increasing the TR-O and decreasing the TO-T [9]. Previous studies of KNN-based ceramics have focused mainly on the improvement in the piezoelectricity. By contrast, the electric field-induced strain response and the hysteresis have been rarely investigated. It is well-known that pure KNN ceramics usually exhibit low strain because it is difficult to achieve dense ceramics. Therefore, different dopants were used to modify the strain, but a relatively low strain (approximately 0.2%) was usually observed [12]. Thus, the aim of the present study is to enhance the strain properties by choosing appropriate components so that the KNN-based ceramics can be used in practical applications. In this work, we developed the (1-x)(K0.48Na0.52)(Nb1-ySby)O3xBi0.5(Na0.82K0.18)0.5ZrO3 materials system for simultaneously attaining a giant strain and a low hysteresis. Compositions with x = 0.02–0.04 and y = 0.01–0.05 were used. The phase structure, ferroelectricity, the electric field-induced strain and corresponding hysteresis were investigated in detail. The enhanced strain was obtained by constructing the O-T phase boundary. Additionally, the temperature stability of the
∗ Corresponding author. The State Key Laboratory of Materials-Oriented Chemical Engineering, College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China. ∗∗ Corresponding author. The State Key Laboratory of Materials-Oriented Chemical Engineering, College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China. E-mail addresses: [email protected] (Y. Liu), [email protected] (Y. Lyu).
https://doi.org/10.1016/j.ceramint.2019.04.187 Received 9 March 2019; Received in revised form 18 April 2019; Accepted 22 April 2019 Available online 23 April 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Fig. 1. XRD patterns of (1-x)KNN1-ySy-xBNKZ ceramics as a function of (a) x (y = 0.05), (c) y (x = 0.03) measured at 2θ = 10–60 °. (b) and (d) are the magnified XRD patterns at 2θ = 44–47 °.
0.97KNN0.98S0.02-0.03BNKZ ceramics was studied. 2. Experimental procedure The (1-x)KNN1-ySy-xBNKZ piezoceramics were synthesized via the conventional solid-state process. The high-purity raw materials were K2CO3(99.0%), Na2CO3(99.8%), Nb2O5(99.5%), Sb2O3(99.5%), Bi2O3(99.0%), and ZrO2(99.0%). The weighted raw materials were ballmilled for 12 h using ZrO2 balls and alcohol as the ball-milling medium. The dried powers were calcined at 850 °C for 3 h in air. The calcined powders were pressed into pellets with a diameter and thickness of 13 mm and 1.5 mm, respectively, by uniaxial molding under a pressure of 10 MPa after mixing with 5 wt% polyvinyl alcohol (PVA) as the binder. The pellets were heated to 600 °C for 1 h to burn off the PVA. Then, the pellets were sintered at 1130 °C for 3 h in the calcined powder with the same composition in order to minimize the evaporation of volatile elements. Silver paste was attached to the sintered samples on both sides and then the samples were fired at 750 °C for 30 min. The samples were poled in a silicone oil bath at room temperature under a
direct current electric field of 30 kV/cm for 30 min. The phase compositions of the ceramics were measured by X-ray diffraction (XRD, Smartlab 3 kW, Rigaku, Japan) with Cu Kα radiation. The surface microstructural observations of the ceramics were performed by scanning electron microscopy (SEM, JSM-6510, JEOL, Japan). The electric-field-induced polarization (P-E) hysteresis loops, current-electric field (I-E) hysteresis loops, bipolar strain (S-E) curves and unipolar strain (S-E) curves were measured using a ferroelectric measuring instrument (Precision Premier II, Radiant Technology, USA) in a silicone oil bath. The piezoelectric constant d33 values were measured by a quasi-static d33 m (ZJ-2; Institute of acoustics, China). 3. Results and discussion Fig. 1(a) and (c) show the X-ray diffraction patterns of (1-x)KNN1ceramics measured at room temperature and 2θ = 10–60°. The results suggest that the BNKZ and Sb dopants completely diffuse into the KNN matrix, and all of the obtained ceramics show a typical perovskite structure. To characterize the evolution of the phase ySy-xBNKZ
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Fig. 2. SEM images of the (1-x)KNN1-ySy-xBNKZ ceramics with different compositions: (a)–(e) x = 0.02, 0.025, 0.03, 0.035 and 0.04 (y = 0.05); (f)–(j) y = 0.01, 0.02, 0.03, 0.04 and 0.05 (x = 0.03).
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Fig. 3. Grain size of the (1-x)KNN1-ySy-xBNKZ ceramics with different compositions: (a)–(e) x = 0.02, 0.025, 0.03, 0.035 and 0.04 (y = 0.05); (f)–(j) y = 0.01, 0.02, 0.03, 0.04 and 0.05 (x = 0.03); and composition dependence of average grain sizes and relative density as a function of (k) x (y = 0.05), (l) y (x = 0.03).
composition of the (1-x)KNN1-ySy-xBNKZ ceramics, the magnified XRD patterns with 2θ between 44° and 47° are plotted in Fig. 1(b) and (d). The (1-x)KNN0.95S0.05-xBNKZ ceramics for 0.02 ≤ x ≤ 0.04 show only a single (200) peak between 44° and 47°. This finding indicates that the (1-x)KNN0.95S0.05-xBNKZ ceramics for 0.02 ≤ x ≤ 0.04 belong to the pseudocubic phase [13]. The results show that a small BNKZ contents does not clearly change the phase composition for Sb doping in excess. The influence of Sb doping on the phase structure of the ceramics is shown in Fig. 1(c) and (d). It is observed that there is a clear split of the (002)/(200) peak between 44° and 47° in the 0.97KNN1-ySy-0.03BNKZ ceramics for 0.01 ≤ y ≤ 0.03. This result indicates that the phase structure of the 0.97KNN1-ySy-0.03BNKZ for 0.01 ≤ y < 0.02 is the O phase, and the O-T phase boundary is present in the 0.97KNN1-ySy0.03BNKZ ceramics for 0.02 ≤ y < 0.04. The O-T phase boundary appears because Sb doping decreases the TO-T to approximately room temperature [9]. As the Sb content exceeds 0.04, the split peaks change to a single (200) peak, indicating that the phase structure of the 0.97KNN1-ySy-0.03BNKZ ceramics transforms to the pseudocubic phase.
The SEM images of the (1-x)KNN1-ySy-xBNKZ ceramics are shown in Fig. 2. All of the ceramics exhibit a dense microstructure with small rectangular block grains. The grain sizes distribution and the composition dependence of the grain sizes and relative density are plotted in Fig. 3. The results show that the BNKZ content has little impact on the average grain size of (1-x)KNN0.95S0.05-xBNKZ ceramics. Additionally, the average grain size fluctuates approximately 0.4 μm with the increasing BNKZ dopant content. This observation is because the excess Sb inhibits the grain growth, and the BNKZ content will no longer clearly change the average grain size. The results show that the average grain size is strongly dependent on the Sb dopant content. The average grain size first decreases extremely from 2.26 μm to 0.89 μm and then decreases slowly from 0.89 μm to 0.52 μm as the Sb content increases from 0.01 to 0.05. The P-E curves of the (1-x)KNN1-ySy-xBNKZ ceramics measured under 40 kV/cm and a frequency of 1 Hz at room temperature are shown in Fig. 4(a) and (b) as a function of x and y. To further understand the ferroelectric properties of the ceramics, the curves of the
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Fig. 3. (continued)
maximum polarization (Pmax), the remnant polarization (Pr) and the coercive field (Ec) against x and y are plotted in Fig. 4(c) and (d). It is observed that the ceramics with x = 0.02–0.04 (y = 0.05) exhibit slim P-E loops with small Pr and Ec, while the ceramics with 0.01 ≤ y ≤ 0.04 (x = 0.03) are typical ferroelectrics with saturated P-E loops. The Pmax and Pr both first increase and then decrease as the BNKZ and Sb dopant contents increase, indicating that BNKZ and Sb enhance the ferroelectricity of (1-x)KNN1-ySy-xBNKZ for 0.02 ≤ x ≤ 0.03 (y = 0.05) and 0.01 ≤ y ≤ 0.02 (x = 0.03). In addition, Ec shows a decreasing trend overall with increasing BNKZ and Sb dopant contents. However, Ec increases slightly at x = 0.035 (y = 0.05) and y = 0.03 (x = 0.03). As a result, the ferroelectricity of the ceramics is enhanced with a small amount of BNKZ and Sb. As the BNKZ and Sb contents continue to increase, the P-E loops become slim, indicating that the ceramics transform from typical ferroelectrics to relaxor ferroelectrics. Fig. 5(a) and (b) show the bipolar strain curves of (1-x)KNN1-ySyxBNKZ ceramics with respect to the BNKZ and Sb contents, measured at room temperature and a frequency of 0.1 Hz. The ceramics with
0.02 ≤ x ≤ 0.04 (y = 0.05) exhibit sprout-shaped strain curves with almost zero negative strain value (Sneg), while the ceramics with 0.01 ≤ y ≤ 0.04 (x = 0.03) exhibit butterfly shaped strain curves with large negative strain values (Sneg), indicating typical ferroelectric material behavior. This finding is consistent with the ferroelectric behavior of the ceramics (see Fig. 4(b)). Even more interestingly, Sneg decreases with the increasing Sb content and is almost equal to zero when the Sb content increases to 0.05. This finding is because the excess Sb disrupts the long-range ferroelectric order leading to reduced ferroelectricity [14]. The maximum strain S, normalized strain d33∗ and the piezoelectric constant d33 with respect to BNKZ and Sb content are shown in Fig. 5(e) and (f), respectively. It is observed that the maximum strain value first increases and then decreases as the BNKZ content increases. Additionally, the variation of the strain of the ceramics with respect to the Sb content shows a similar trend. Furthermore, Fig. 5(f) shows that both the strain and the piezoelectric constant d33 first increase and then decrease with respect to the Sb content. The maximum strain (Smax = 0.3%), the normalized strain (d33∗ = 750 p.m./V) and the
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Fig. 4. P-E curves of (1-x)KNN1-ySy-xBNKZ ceramics as a function of (a) x (y = 0.05), (b) y (x = 0.03). (c) and (d) are their corresponding Pr, Ec, and Pmax values.
piezoelectric constant (d33 = 482 pC/N) are attained in the 0.97KNN0.98S0.02-0.03BNKZ ceramics. The enhanced piezoelectricity is due to the construction of the O-T phase boundary [4] For the 0.97KNN0.98S0.02-0.03BNKZ ceramics, The well-saturated P-E hysteresis loops, the butterfly-shaped bipolar S-E curves and the high d33 indicate the dominating intrinsic contribution to the giant strain, i.e., the converse piezoelectric effect [15,16]. . To further understand the mechanism of the giant strain, the I-E loops of the ceramics measured at room temperature under 40 kV/cm and at 1 Hz are shown in Fig. 6(a) and (b). The I-E loop of 0.97KNN0.98S0.02-0.03BNKZ ceramic shows four current peaks. P2 current peaks can be ascribed to the domain switching of the long-range ferroelectric phase, while P1 current peaks are related to the electricfield-induced phase transition from relaxor phase to ferroelectric phase [16,17]. As the Sb doping content increases, the long-range ferroelectric domains are broken into the micro-domains or PNRs. The PNRs will grow up into the long-range ferroelectric order by merging small ones when the applied external electric field overcomes the effects of local random field [16]. In summary, both the enhanced piezoelectricity induced by the O-T phase boundary and the phase transition from relaxor phase to ferroelectric phase lead to the giant strain. To enable the application of the KNN-based ceramics in highly sensitive and precise actuators devices, the unipolar strain curves of 0.97KNN0.98S0.02-0.03BNKZ ceramic were studied, and the d33∗ vs. hysteresis (H = ΔS/Smax) of some reported KNN-based and BNT-based ceramics were compared, as summarized in Fig. 7 [1,2,4,7,18–36]. It is observed that a giant normalized strain (d33∗ = 1400 p.m./V) can be achieved in BNT-2.5Nb ceramics. However, a large hysteresis (H = 68.9%) was obtained at the same time [2]. Additionally, it is unexpected to find that when a low hysteresis (H = 17.2%) was achieved in textured BKT-BT-BNT ceramics, a relatively small
normalized strain (d33∗ = 400 p.m./V) was obtained. Although a low hysteresis (usually < 25%) can be obtained in some reported KNNbase ceramics [4,7,18,26–29,36], a small normalized strain (< 500 p.m./V) was usually found. In this work, a giant strain (S = 0.3%, d33∗ = 750 p.m./V) and a low hysteresis (H = 16.4%) were simultaneously obtained in the 0.97KNN0.98S0.02-0.03BNKZ ceramics. It is generally accepted that the domain wall motion accounts for the hysteresis phenomena. PNRs response to the external field much faster than macroscopic domains [37]. Therefore, the small hysteresis (H = 16.4%) of 0.97KNN0.98S0.02-0.03BNKZ ceramics is due to existence of PNRs. Fig. 8(a) shows the P-E curves of the 0.97KNN0.98S0.02-0.03BNKZ ceramics measured at a frequency of 0.1 Hz in the temperature range from room temperature to 150 °C. The Pr, Ec and Pmax and their corresponding normalized Pr, Ec and Pmax curves with respect to temperature are shown in Fig. 8(b) and (c), respectively. It is observed that both Pmax and Pr first increase and then decrease while Ec continues to decrease as the temperature ranges from 25 °C to 150 °C. The increase in Pmax and Pr is due to the occurrence of a temperature-induced phase transition from the orthorhombic phase to the tetragonal phase in the 0.97KNN0.98S0.02-0.03BNKZ ceramics. It is observed from Fig. 8(c) that the Pmax of the 0.97KNN0.98S0.02-0.03BNKZ ceramics decreases by only 0.49% when the temperature is raised from room temperature to 150 °C. This finding means that the KNNS-BNKZ ceramics are promising candidates for use in piezoelectric actuators in high temperature environment. The thermal stabilities of some of the reported KNN-based ceramics, BNT-based ceramics and BT-based ceramics were compared, as summarized in Table 1 [38–47]. In this work, the Pmax values of 0.97KNN0.98S0.02-0.03BNKZ ceramics remain relatively stable in the studied temperature range; this is mainly related to the poling state of the field-induced ferroelectric long-range phase [16].
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Fig. 5. (a) and (b) Bipolar strain curves, (c) and (d) unipolar strain curves, (e) and (f) the variation of the maximum strain values, normalized strain (d33∗) and d33 with respect to x and y.
Fig. 6. (a) and (b) I-E curves with respect to x and y.
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Table 1 Thermal stabilities of some of the reported BNT-based, BT-based and KNNbased ceramics.
Fig. 7. d33∗ vs. hysteresis of BNT-based and KNN-based ceramics.
4. Conclusion In summary, (1-x)KNN1-ySy-xBNKZ lead-free ceramics were synthesized via the conventional solid-state process, and it was found that the Sb content has a significant effect on the phase structure, grain size, ferroelectricity and strain properties. A giant electric field-induced strain of 0.3% (d33∗ = 750 p.m./V) was obtained in the 0.97KNN0.98S0.02-0.03BNKZ ceramics due to the enhanced piezoelectricity induced by the appearance of the O-T phase boundary and the electric-field-induced phase transition from the relaxor phase to the ferroelectric phase. Additionally, a low hysteresis (H = 16.4%) was
Compounds
Temperature Range (°C)
Pmax Fluctuation (%)
Ref.
BNT-SBT BNKL-STT BNT-100xLi BLZT BZT BCZT BT-BMZ KNNS-BNZ KNLN-BZ-BNT KNNS-CS-BKH KNNS-BNKZ
30–120 25–175 30–90 30–120 30–120 27–120 25–150 0–100 30–180 25–100 25–150
10 25 15 40 25 29 8 8 15 10 5
[41] [46] [39] [42] [43] [38] [40] [47] [45] [44] This work
simultaneously obtained at the same composition. Moreover, the 0.97KNN0.98S0.02-0.03BNKZ ceramics exhibit good temperature stability. Therefore, we believe that the KNN-based ceramics are promising candidates for the replacement of lead-based piezoceramics in the near future. Acknowledgments This work was supported by the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions, and the Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites. The authors also acknowledge the financial support from the Innovative Research Team in University (PCSIRT), IRT1146.
Fig. 8. (a) P-E curves of 0.97KNN0.98S0.02-0.03BNKZ ceramics measured from 25 °C to 150 °C; (b) Pr, Ec and Pmax vs temperature of 0.97KNN0.98S0.02-0.03BNKZ ceramics; (c) Normalized Pr, Ec and Pmax vs temperature of 0.97KNN0.98S0.02-0.03BNKZ ceramics.
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