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Analysis of high temperature reduction process of Na0.5Bi0.5TiO3-based ceramics
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Original Article
Analysis of high temperature reduction process of Na0.5Bi0.5TiO3-based ceramics Wanfeng Zhoua, Haifeng Zhanga, Pan Chena, Baojin Chua,b,
⁎
a CAS Key Laboratory of Materials for Energy Conversion and Department of Materials Science and Engineering, University of Science and Technology of China, No. 96 Jinzhai Rd, Hefei, Anhui Province, 230026, China b Synergetic Innovation Center of Quantum Information Quantum Physics, University of Science and Technology of China, No. 96 Jinzhai Rd, Hefei, Anhui Province, 230026, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Lead-free piezoelectric Ferroelectric Reduction Rate-controlled Flexoelectric
Lead-free metamaterials with enormous effective apparent piezoelectric response has been fabricated by applying an asymmetric chemical reduction to Na0.5Bi0.5TiO3 (NBT)-based ceramics. To achieve high performance, optimization of the reduction conditions is required. In this study, we analyzed the effect of reduction temperature and time on the reduction thickness of NBT-based ceramics. We found that the reduction reaction between NBT-based ceramics and graphite is an interface reaction rate-controlled process. The reduction thickness has a linear relationship with the reaction time at a fixed reduction temperature. The lower activation energy of NBT-based ceramics than that of lead-based materials indicates the lead-free ceramics are easier to be reduced. The effect of the reduction on the flexoelectric-like response was further explored, and the maximum response ( > 1 mC/m) was measured in the ceramics having a reduction-thickness-to-total-thickness ratio of around 0.28. This study provides a guideline to optimize the fabrication conditions of the NBT-based metamaterials.
1. Introduction In recent years, with the increasing concern of the toxicity of lead, considerable research efforts have been made to improve the performance of lead-free piezoelectric ceramics [1–3]. Several lead-free material systems have been explored, but their piezoelectric properties are still inferior to those of lead oxide-based materials [2–5]. Designing piezoelectric metamaterials based on flexoelectric effect is a viable approach to obtain lead-free materials with high electromechanical performance and high operating temperature [6–11]. Several novel designs of lead-free metamaterials or structures, such as flexure mode metamaterials and structure with a truncated pyramid geometry, have been realized experimentally [6,12–17]. Recently, a simple asymmetric chemical reduction method applied to ferroelectric oxides is proposed to fabricate piezoelectric metamaterials [16]. NBT-based lead-free and PZT-based piezoelectric metamaterials are fabricated, and their apparent piezoelectric response are comparable to those of conventional lead oxide-based piezoelectric materials [16,18]. After chemical reduction, a composite structure with a reduced layer, an unreduced layer and an interface between the reduced and unreduced layers is produced. A unique dome-like structure
is generated because of the difference of thermal expansion between the reduced and unreduced layers, and the dome height is related to the thickness of the reduced layer of the metamaterials. It was found that the apparent piezoelectric response of the piezoelectric metamaterials is sensitive to the reduction conditions. To optimize the performance of the reduced ceramics, a suitable thickness of reduced layer has to be produced [19]. In this work, we analyzed the effect of reduction temperature and time on the reduction thickness of reduced NBT-based ceramics. We show that the chemical reduction of NBT-based ceramics is a reaction rate-controlled process. The correlation between the reduction conditions and the apparent flexoelectric response was also studied. Our results are important for the optimization of the reduction conditions of NBT-based ceramics to fabricate high performance lead-free piezoelectric metamaterials. 2. Experimental Lead-free 0.92Na0.5Bi0.5TiO3-0.08BaTiO3 (NBBT8) ferroelectric ceramics were fabricated by the conventional solid-state reaction method. Na2CO3, BaCO3, Bi2O3, and TiO2 (all 99.9% except TiO2, which is 98%, Sinopharm Chemical Reagent Co., Ltd., Shanghai,
⁎ Corresponding author at: CAS Key Laboratory of Materials for Energy Conversion and Department of Materials Science and Engineering, University of Science and Technology of China, No. 96 Jinzhai Rd, Hefei, Anhui Province, 230026, China. E-mail address: [email protected] (B. Chu).
https://doi.org/10.1016/j.jeurceramsoc.2017.12.007 Received 13 August 2017; Received in revised form 4 December 2017; Accepted 5 December 2017 0955-2219/ © 2017 Published by Elsevier Ltd.
Please cite this article as: Zhou, W., Journal of the European Ceramic Society (2017), https://doi.org/10.1016/j.jeurceramsoc.2017.12.007
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
W. Zhou et al.
disappears. The reduction thickness is about 178( ± 18) μm for the NBBT8 ceramics reduced at 750 °C for 180 min. It has also been found that after the reduced layer was completely removed, the XRD patterns of the reduced surface exhibits a perovskite structure, but it is not exactly the same as the unreduced sample. As shown in Fig. 2(c), a slight shifting of the (002) peak position and a change of (200)/(002) peak ratio can be discernible, which may be due to the slight change of composition or existence of large internal stress after the reduction [16,21]. Fig. 3(a) shows the SEM cross-section image near the reduced surface of a reduced NBBT8 ceramic wafer. Obviously, the reduced layer exhibits a more porous microstructure than the unreduced region. Fig. 3(b) shows the EDS elemental mapping images of the Bi, Ti, and Na elements in the square region shown in Fig. 3(a). Bi-rich and Ti-rich regions can be observed in the reduced layer, while the element Na is almost uniformly distributed throughout the investigated region. Therefore, Bi-rich and Ti-rich phases are produced in the reduced layer after the reduction. The backscattered electron (BSE) SEM images in Fig. 3(c) also show a complex phase compositions in the reduced layer, and primarily, there are three phases near the reduced surface. EDS analysis shown in Fig. 3(d) suggests that the bright region (region 1) in Fig. 3(c) has a high concentration of Bi. The Bi-rich phase is accumulated near the reduced surface after the reduction, and its concentration gradually decreases inside the ceramics. The region 2 with a grey color is composed of elements Na, Ba, Bi, Ti, and O, and should be the unreduced or slightly reduced NBBT8, as shown in Fig. 3(e). The region 3 with a dark color is a Ti-rich region, which contains a small amount of Na, as shown in the EDS spectra in Fig. 3(f). These regions may correspond to the perovskite, Bi or bismuth oxide, and sodium titanate phases shown in Fig. 2. By using the same method, the XRD patterns of the NBBT8 ceramics reduced at other reduction conditions (not shown here) were measured and the dependence of reduction thickness on the reduction temperatures and time was further determined. The reduction time dependence of reduction thickness at different reaction temperatures was summarized in Fig. 4(a). An approximate linear relationship exists between the reduction time and the thickness of the reduced layer at all the reduction temperatures. A similar linear relationship has been observed in the reduction of (Pb,La)(Zr,Ti)O3-based ferroelectric ceramics [22,23] and this phenomenon was attributed to the interface reactioncontrolled reduction process [24]. In some chemical reactions involving solids, the reaction kinetics is typically controlled by two processes. At the initial stage, the interface or surface reaction is the controlling process. With the reaction continuing, if the reaction products are dense, the diffusion of reactants to the reaction sites becomes difficult, and the diffusion becomes the controlling process. It is also possible that the interface reaction is always the controlling process because the loose microstructure of reaction products facilitates the diffusion process [24,25]. For NBBT8, as shown in Fig. 2(a), the reduction products seem very complex and it is difficult to determine the exact chemical reactions occurring during the reduction process. Although the reduction process is complex, the reduction can also be simplified into those two key controlling processes: one is the interface reaction process, and the other is the diffusion of various reactants to the reaction sites. If the interface reaction is the rate-controlling process in the reduction processing of NBBT ceramics, a linear relationship will exist between reduction thickness and time. In such a reaction controlled process, the relationship between the thickness of the reduction layer and time can be expressed as [25]:
Fig. 1. Schematic of the stacking of the alumina plates, graphite block and NBBT8 ceramic wafer for chemical reduction.
China), were used as raw materials. They were mixed by ball-milling machine in ethanol according to the stoichiometric ratio of 0.92Na0.5Bi0.5TiO3-0.08BaTiO3 ceramics. After calcination at 850 °C for 2 h, the calcined powders were milled for 12 h and then pressed into disks with 12.5 mm or 25.4 mm in diameter using polyvinyl alcohol (PVA) as a binder. After burning out PVA, the green compacts were sintered at 1175 °C for 2 h. During the reduction, one piece of polished NBBT8 wafer was placed on the top of a graphite block (50 mm in diameter and 10 mm in thickness) and an alumina plate was then placed on the top of the ceramic wafer, as shown in Fig. 1. The assembly was heat treated at 700–850 °C in a furnace. After holding at that temperature for 10–300 min, the assembly was quickly moved out of the furnace and cooled down to room temperature in the air. After reduction, a curvature structure was generated in the ceramic wafer. The density of NBBT8 ceramic (20 mm in diameter and 1 mm in thickness) was investigated by Archimedes method. The dome height of the curvature was characterized using a similar method that described in Ref. [18]. The X-ray diffraction (XRD) was performed on a Rigaku Smartlab diffractometer (Rigaku, Tokyo, Japan) to determine the crystal structure of the reduced and unreduced ceramics. To establish the relationship between the reduction thickness and the reduction time or temperature, the XRD patterns were measured after the reduced surface were removed layer by layer. The microstructure and the compositions of the ceramics were examined by a scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) (GeminiSEM 500, Carl Zeiss, Oberkochen, Germany). Before electrical properties were measured, the gold electrode was prepared using a DC sputtering method in a sputter coater (EMS150T, Electron Microscopy Sciences, Hatfield, PA, USA). The flexoelectric effect was characterized by measuring an apparent piezoelectric response d33 of the reduced ceramic disks, which was measured using a quasi-static d33 meter (ZJ-6A, Institute of Acoustics, CAS). The dimensions of the samples for the measurement of dome height and apparent d33 are approximately 20 mm in diameter and 0.5 mm in the thickness. The ceramics were not poled before flexoelectric measurement. 3. Results and discussion Fig. 2(a) and (b) show the XRD patterns of the unreduced NBBT8 ceramics and the ceramics reduced at 750 °C for 180 min. Unreduced NBBT8 ceramics exhibit tetragonal perovskite phase at room temperature [2,20]. After the reduction, the reduced surface exhibits mixed phases, which include NBBT8, Bi, Bi2O3, and sodium titanate. As shown in Fig. 2(b), the most evident diffraction peak of the mixed phases located at 27° to 28° is from metallic Bi [16,21]. To determine the reduction thickness of the NBBT8 ceramics, the XRD patterns were measured after the reduced surface (the surface against graphite block during the reduction) was removed layer by layer. The reduction thickness was determined when the diffraction peak from metallic Bi
dh = Kdt
(1)
where, h is the thickness of reduction layer and K is the reaction rate constant. Therefore, we can have
h= 2
∫0
t
Kdt = Kt + A
(2)
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Fig. 2. (a) X-ray diffraction patterns of the reduced and unreduced NBBT8 ceramics (10 mm in diameter and 1 mm in the thickness) reduced at 750 °C for 180 min. The reduced sample was polished from the reduced surface by the thickness indicated in the plots. (b) The diffraction peaks of metallic Bi between 27° to 28°. (c) The XRD patterns of (002) and (200) peaks.
Fig. 3. Cross-section SEM images of a NBBT8 ceramic wafer reduced at 825 °C for 120 min. (b) EDS elemental mapping images of the Bi, Ti, and Na elements of square region in (a). (c) BSE-SEM image of the square region shown in (a). (d), (e) and (f) are EDS spectra of the regions 1, 2, and 3 in (c).
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prepared NBBT8 ceramics have a density of 5.83( ± 0.08) g/cm3. After reduction, the density of NBBT8 ceramic becomes lower (approximately 5.68( ± 0.11) g/cm3) because of the volatilization of metallic Bi and the loss of oxygen. Fig. 3(a) also shows that the reduced layer exhibits a loose structure. In Fig. 4(a), an increase of slope K with the increase of reduction temperature indicates that the rate of reaction between NBT-based ceramics and graphite can be accelerated by increasing the reduction temperature. According to Arrhenius equation [24,26]:
K = K 0 exp(−
where A is a constant. As observed in Fig. 4(a), the reduction thickness is proportional to the reaction time at all the reduction temperatures, indicating the reduction reaction between the NBBT8 ceramics and graphite is mainly controlled by the rate of the interface chemical reactions. The solid lines give the linear fit to the data and the fitting equations of the lines are (3)
h800 = 1.02(± 0.02) t + 85.22(± 3.68)
(4)
h850 = 1.56(± 0.03) t + 122.94(± 4.53)
(5)
(6)
where Ea is activation energy, R is the gas constant, and K0 is pre-exponential factor. Fig. 4(b) shows the plot lnK versus 1/T, which can be fitted by a linear relationship. The activation energy Ea and pre-exponential factor K0 for this reaction can be calculated by Eq. (6) and the data shown in Fig. 4(b), which are approximately 55.3 kJ/mol and 9.37 × 10−6 m/s, respectively. Compared with lead oxide based ferroelectric ceramics, NBBT8 ceramics are much easier to be reduced. To achieve a reduction thickness of about 400 μm at the same reduction time (about 3 h), PLZT ceramics were reduced at 975 °C [22], PZT-5H ceramics at 1055 °C [24], and NBBT8 ceramics at a temperature about 850 °C. These observations are consistent with the results that the activation energy of reduction of NBBT8 ceramics (Ea ∼ 55.3 kJ/mol) is lower than that of lead oxide-based materials (Ea of PZT-5H ceramics ∼113 kJ/mol) [24]. Fig. 5 shows the reduction temperature dependence of reduction thickness at different reaction times. It is interesting that a linear relationship has been also found between ln (h) and 1/T. This result agrees with the relationship described in Eq. (6) and further indicates that the reduction of NBBT8 ceramics is a reaction-controlled process. The results shown in Fig. 4(a) and Fig. 5 suggest that the reduction thickness can be increased either by reducing the NBBT8 ceramics at a higher temperature or increasing reduction time. The thickness of the reduction layer is directly related to the performance of piezoelectric metamaterials. Optimization of the reduction time and temperature is required to achieve desirable properties of the materials. Fig. 6(a) shows the reduction time dependence of the dome height of the reduced NBBT8 ceramics. Because the curvature structure is caused by the different thermal expansions of the reduced and unreduced layers, a suitable thickness ratio of the reduced layer to unreduced layer can produce an optimized curvature structure. For the NBBT8 ceramics reduced at 800 °C, the maximum dome height is observed in the materials reduced for a period between 30 min and 60 min. The chemical reduction also has a significant effect on the flexoelectric response and the effective d33 of the piezoelectric
Fig. 4. (a) Variation of the reduction thickness of the reduced NBBT8 ceramics with reduction times at different reduction temperatures. (b) The linear relationship between the reaction rates constant K and 1/T in the logarithmic scale.
h750 = 0.87(± 0.09) t + 10.74(± 13.91)
Ea ) RT
where, h750, h800, and h850 are reduction thickness of ceramic reduced at 750 °C, 800 °C and 850 °C respectively, and t is reduction time. We note that the intercepts at Y-axis of the curves in Fig. 4(a) are not zero. It implies the reduction reaction has already started during the heating of the ceramics to the preset reduction temperatures (the heating rate 4 °C/min). The NBBT8 piezoelectric ceramics were prepared by conventional solid state reaction and they are not fully dense. The reason that the diffusion is not a controlling process of the reduction of NBBT8 ceramic may be due to the relative high porosity of the ceramics and the porous reduced layer, and the diffusion of reactants, such as CO and oxygen vacancies, is easier than the chemical reaction process. For lead-oxide based materials, the reduced layer is porous in (Pb,La)(Zr,Ti)O3 ceramics [22], and the thickness of the reduced layer is linear with the reduction time, similar to the observation in NBBT8 ceramics. However, a parabolic law was observed between reduced layer thickness and reduction time in PZT-5H ceramics because of its dense structure of reduction products and free of micro-cracks or micro-channels in the reduced layer [24]. For NBBT8 ceramics, we found the chemical reduction can significantly reduce the density of the materials. The as-
Fig. 5. The reduction thickness vs. reduction temperatures curves of the reduced NBBT8 ceramics.
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Fig. 6. The dome height (a), the effective d33 (b), and the apparent flexoelectric coefficient (c) of the reduced NBBT8 ceramics reduced at 800 °C for a period between 10 and 180 min. (d) The apparent flexoelectric coefficient of the NBBT8 ceramics reduced at a temperature between 750 and 850 °C for 30 min. Each data point is an average of three samples and the error bar shows the standard deviation.
thickness ratio is about 0.28 according to Fig. 5. The results in NBBT8 ceramics are consistent with those in (Pb,La)(Zr,Ti)O3–based piezoelectric Rainbow actuators, in which the maximum electromechanical response was obtained in the reduced samples having the thickness ratio of around 0.3 [28]. The enhancement of the apparent piezoelectric response in the reduced (Pb,La)(Zr,Ti)O3-based ceramics was attributed to the internal stress caused by the reduction in the materials. Consequently, although the mechanisms for the enhancement of the apparent μρ in NBBT8 ceramics are unknown, the finding that the maximum μρ of reduced NBBT8 ceramics and the maximum of d33 of the reduced (Pb,La)(Zr,Ti)O3–based ceramics were observed at a similar ratio of the thickness of the reduced layer to the total thickness indicates that the chemical reduction induced internal stress may play an important role in the enhancement of the apparent μρ of the reduced NBBT8 ceramics.
metamaterials. The apparent piezoelectric response of the piezoelectric metamaterials was measured using the point-ring test by a quasi-static d33 meter [16], and the effective d33 first increases and then decreases with the reduction time, as shown in Fig. 6(b). A maximum d33 higher than 3000 pC/N is measured in the materials reduced for 30 min. The apparent flexoelectric coefficient μρ was calculated according to the following equation [17]:
d33 =
μρ (1 − σ ) R2 8(1 + σ ) D
(7)
In the equation above, d33 is the apparent piezoelectric response, σ is poisson’s ratio, R is the radius of the ceramic wafer, and D is a coefficient controlling the flexural rigidity of the wafer. Fig. 6(c) shows the reduction time dependence of the apparent flexoelectric coefficient μρ of NBBT8 ceramics reduced at 800 °C. The μρ can be significantly enhanced by the reduction and the maximum μρ is around 700 μC/m for the ceramics reduced for 30 min. In Fig. 6(d), the apparent μρ of reduced NBBT8 ceramic is plotted against the reduction temperatures. The μρ is also strongly dependent on the reduction temperatures. The μρ reaches a maximum value of nearly 1.1 mC/m for the ceramics reduced at 825 °C for 30 min, and the value is comparable to that reduced BaTiO3 crystals [27]. These results indicate that reduction conditions have a significant effect on the μρ of the NBBT8 ceramics. Fig. 6(c) indicates that the maximum μρ was obtained in the ceramics reduced at 800 °C for 30 ∼ 60 min. According to Eq. (4), the reduction thickness of the reduced ceramic is estimated to be about 115 ∼ 147 μm, and the thickness ratio of the reduced layer to the total thickness is approximately 0.23 ∼ 0.29. Likewise, the reduction thickness of the materials reduced at 825 °C for 30 min, the conditions at which a maximum μρ is measured, is approximately 139 μm and the
4. Conclusions In this study, the effect of reduction conditions on the reduction thickness of the NBT-based ceramics was investigated. We found the chemical reduction of NBT-based ceramic is an interface reaction ratecontrolled process, and a linear relationship between reduction thickness and time was obtained. The activation energy of the reduction of the NBT-based ceramics is lower than that of lead oxide-based ferroelectric ceramics and the lead free materials can be reduced at a much lower temperature. We found the apparent flexoelectric coefficient of the ceramics can be enhanced either by reducing the NBBT8 ceramics at a higher temperature or increasing reduction time and the maximum flexoelectric coefficient was measured in reduced ceramics with a reduced layer of approximately 1/4–1/3 of the total thickness of the ceramics. Our results provide a guideline to optimize the reduction 5
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[12] B.J. Chu, W. Zhu, N. Li, L.E. Cross, Flexure mode flexoelectric piezoelectric composites, J. Appl. Phys. 106 (2009) 104109. [13] W. Zhu, J.Y. Fu, N. Li, L.E. Cross, Piezoelectric composite based on the enhanced flexoelectric effects, Appl. Phys. Lett. 89 (2006) 192904. [14] J.Y. Fu, W. Zhu, N. Li, N.B. Smith, L.E. Cross, Gradient scaling phenomenon in microsize flexoelectric piezoelectric composites, Appl. Phys. Lett. 91 (2007) 182910. [15] W. Ma, L.E. Cross, Flexoelectric polarization of barium strontium titanate in the paraelectric state, Appl. Phys. Lett. 81 (2002) 3440–3442. [16] W.F. Zhou, P. Chen, Q. Pan, X.T. Zhang, B.J. Chu, Lead-free metamaterials with enormous apparent piezoelectric response, Adv. Mater. 27 (2015) 6349–6355. [17] X.T. Zhang, J.L. Liu, M.J. Chu, B.J. Chu, Flexoelectric piezoelectric metamaterials based on the bending of ferroelectric ceramic wafers, Appl. Phys. Lett. 109 (2016) 072903. [18] W.F. Zhou, B.J. Chu, Strong electromechanical response in lead zirconate titanate metamaterials, J. Am. Ceram. Soc. 99 (2016) 3317–3324. [19] M.W. Hooker, Correlation of the coercive field and reduced layer thickness in piezoelectric RAINBOW ceramics, J. Mater. Sci. 34 (1999) 2407–2410. [20] B.J. Chu, D.R. Chen, G.R. Li, Q.R. Yin, Electrical properties of Na1/2Bi1/2TiO3BaTiO3 ceramics, J. Eur. Ceram. Soc. 22 (2002) 2115–2121. [21] W.F. Zhou, B.J. Chu, Sodium bismuth titanate-based lead-free RAINBOW piezoelectric devices, J. Eur. Ceram. Soc. 37 (2017) 2373–2377. [22] G.H. Haertling, Chemically reduced PLZT ceramics for ultra-high displacement actuators, Ferroelectrics 154 (1994) 101–106. [23] C. Elissalde, L.E. Cross, C.A. Randall, Structural–property relations in a reduced and internally biased oxide wafer (RAINBOW) actuator material, J. Am. Ceram Soc. 79 (1996) 2041–2048. [24] Q.M. Wang, L.E. Cross, Analysis of high temperature reduction processing of RAINBOW actuator, Mater. Chem. Phys. 58 (1999) 20–25. [25] V.I. Dybkov, Reaction Diffusion and Solid State Chemical Kinetics, The IPMS Publications, Kyiv, 2002. [26] A. Hiroki, Jay A. LaVerne, Decomposition of hydrogen peroxide at water- ceramic oxide interfaces, J. Phys. Chem. B 109 (2005) 3364–3370. [27] J. Narvaez, F. Vasquez-Sancho, G. Catalan, Enhanced flexoelectric-like response in oxide semiconductors, Nature 538 (2016) 219–221. [28] G. Li, E. Furman, G.H. Haertling, Stress-enhanced displacements in PLZT rainbow actuators, J. Am. Ceram. Soc. 80 (1997) 1382–1388.
temperature and time of NBT-based ceramics and achieve high performance materials. Acknowledgements This research was supported by the National Natural Science Foundation of China (No. 51672261 and 51373161), 1000 Young Talents Program. References [1] G.H. Haertling, Ferroelectric ceramics: history and technology, J. Am. Ceram. Soc. 82 (1999) 797–818. [2] J. Rödel, W. Jo, K.T.P. Seifert, E. Anton, T. Granzow, D. Damjanovic, Perspective on the development of lead-free piezoceramics, J. Am. Ceram. Soc. 92 (2009) 1153–1177. [3] Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T. Nagaya, M. Nakamura, High performance lead-free piezoelectric material, Nature 432 (2004) 84–87. [4] D. Damjanovic, N. Klein, J. Li, V. Porokhonskyy, What can be expected from leadfree piezoelectric materials? Funct. Mater. Lett. 3 (2010) 5–13. [5] T.R. Shrout, S.J. Zhang, Lead-free piezoelectric ceramics: Alternatives for PZT? J.Electroceram 19 (2007) 113–126. [6] L.E. Cross, Flexoelectric effects: charge separation in insulating solids subjected to elastic strain gradients, J. Mater. Sci. 41 (2006) 53–63. [7] P. Zubko, G. Catalan, A.K. Tagantsev, Flexoelectric effect in solids, Annu. Rev. Mater. Res. 43 (2013) 387–421. [8] S.M. Kogan, Piezoelectric effect during inhomogeneous deformation and acoustic scattering of carriers in crystals, Sov. Phys. Solid State 5 (1964) 2069–2070. [9] A.K. Tagantsev, Piezoelectricity and flexoelectricity in crystalline dielectrics, Phys. Rev. B 34 (1986) 5883–5889. [10] B.J. Chu, W. Zhu, N. Li, L.E. Cross, Flexoelectric composite – a new prospect for lead-free piezoelectrics, Funct. Mater. Lett. 3 (2010) 79–81. [11] T.D. Nguyen, S. Mao, Y.W. Yeh, P.K. Purohit, M.C. McAlpine, Nanoscale flexoelectricity, Adv. Mater. 25 (2013) 946–974.
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Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Original Article
Analysis of high temperature reduction process of Na0.5Bi0.5TiO3-based ceramics Wanfeng Zhoua, Haifeng Zhanga, Pan Chena, Baojin Chua,b,
⁎
a CAS Key Laboratory of Materials for Energy Conversion and Department of Materials Science and Engineering, University of Science and Technology of China, No. 96 Jinzhai Rd, Hefei, Anhui Province, 230026, China b Synergetic Innovation Center of Quantum Information Quantum Physics, University of Science and Technology of China, No. 96 Jinzhai Rd, Hefei, Anhui Province, 230026, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Lead-free piezoelectric Ferroelectric Reduction Rate-controlled Flexoelectric
Lead-free metamaterials with enormous effective apparent piezoelectric response has been fabricated by applying an asymmetric chemical reduction to Na0.5Bi0.5TiO3 (NBT)-based ceramics. To achieve high performance, optimization of the reduction conditions is required. In this study, we analyzed the effect of reduction temperature and time on the reduction thickness of NBT-based ceramics. We found that the reduction reaction between NBT-based ceramics and graphite is an interface reaction rate-controlled process. The reduction thickness has a linear relationship with the reaction time at a fixed reduction temperature. The lower activation energy of NBT-based ceramics than that of lead-based materials indicates the lead-free ceramics are easier to be reduced. The effect of the reduction on the flexoelectric-like response was further explored, and the maximum response ( > 1 mC/m) was measured in the ceramics having a reduction-thickness-to-total-thickness ratio of around 0.28. This study provides a guideline to optimize the fabrication conditions of the NBT-based metamaterials.
1. Introduction In recent years, with the increasing concern of the toxicity of lead, considerable research efforts have been made to improve the performance of lead-free piezoelectric ceramics [1–3]. Several lead-free material systems have been explored, but their piezoelectric properties are still inferior to those of lead oxide-based materials [2–5]. Designing piezoelectric metamaterials based on flexoelectric effect is a viable approach to obtain lead-free materials with high electromechanical performance and high operating temperature [6–11]. Several novel designs of lead-free metamaterials or structures, such as flexure mode metamaterials and structure with a truncated pyramid geometry, have been realized experimentally [6,12–17]. Recently, a simple asymmetric chemical reduction method applied to ferroelectric oxides is proposed to fabricate piezoelectric metamaterials [16]. NBT-based lead-free and PZT-based piezoelectric metamaterials are fabricated, and their apparent piezoelectric response are comparable to those of conventional lead oxide-based piezoelectric materials [16,18]. After chemical reduction, a composite structure with a reduced layer, an unreduced layer and an interface between the reduced and unreduced layers is produced. A unique dome-like structure
is generated because of the difference of thermal expansion between the reduced and unreduced layers, and the dome height is related to the thickness of the reduced layer of the metamaterials. It was found that the apparent piezoelectric response of the piezoelectric metamaterials is sensitive to the reduction conditions. To optimize the performance of the reduced ceramics, a suitable thickness of reduced layer has to be produced [19]. In this work, we analyzed the effect of reduction temperature and time on the reduction thickness of reduced NBT-based ceramics. We show that the chemical reduction of NBT-based ceramics is a reaction rate-controlled process. The correlation between the reduction conditions and the apparent flexoelectric response was also studied. Our results are important for the optimization of the reduction conditions of NBT-based ceramics to fabricate high performance lead-free piezoelectric metamaterials. 2. Experimental Lead-free 0.92Na0.5Bi0.5TiO3-0.08BaTiO3 (NBBT8) ferroelectric ceramics were fabricated by the conventional solid-state reaction method. Na2CO3, BaCO3, Bi2O3, and TiO2 (all 99.9% except TiO2, which is 98%, Sinopharm Chemical Reagent Co., Ltd., Shanghai,
⁎ Corresponding author at: CAS Key Laboratory of Materials for Energy Conversion and Department of Materials Science and Engineering, University of Science and Technology of China, No. 96 Jinzhai Rd, Hefei, Anhui Province, 230026, China. E-mail address: [email protected] (B. Chu).
https://doi.org/10.1016/j.jeurceramsoc.2017.12.007 Received 13 August 2017; Received in revised form 4 December 2017; Accepted 5 December 2017 0955-2219/ © 2017 Published by Elsevier Ltd.
Please cite this article as: Zhou, W., Journal of the European Ceramic Society (2017), https://doi.org/10.1016/j.jeurceramsoc.2017.12.007
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
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disappears. The reduction thickness is about 178( ± 18) μm for the NBBT8 ceramics reduced at 750 °C for 180 min. It has also been found that after the reduced layer was completely removed, the XRD patterns of the reduced surface exhibits a perovskite structure, but it is not exactly the same as the unreduced sample. As shown in Fig. 2(c), a slight shifting of the (002) peak position and a change of (200)/(002) peak ratio can be discernible, which may be due to the slight change of composition or existence of large internal stress after the reduction [16,21]. Fig. 3(a) shows the SEM cross-section image near the reduced surface of a reduced NBBT8 ceramic wafer. Obviously, the reduced layer exhibits a more porous microstructure than the unreduced region. Fig. 3(b) shows the EDS elemental mapping images of the Bi, Ti, and Na elements in the square region shown in Fig. 3(a). Bi-rich and Ti-rich regions can be observed in the reduced layer, while the element Na is almost uniformly distributed throughout the investigated region. Therefore, Bi-rich and Ti-rich phases are produced in the reduced layer after the reduction. The backscattered electron (BSE) SEM images in Fig. 3(c) also show a complex phase compositions in the reduced layer, and primarily, there are three phases near the reduced surface. EDS analysis shown in Fig. 3(d) suggests that the bright region (region 1) in Fig. 3(c) has a high concentration of Bi. The Bi-rich phase is accumulated near the reduced surface after the reduction, and its concentration gradually decreases inside the ceramics. The region 2 with a grey color is composed of elements Na, Ba, Bi, Ti, and O, and should be the unreduced or slightly reduced NBBT8, as shown in Fig. 3(e). The region 3 with a dark color is a Ti-rich region, which contains a small amount of Na, as shown in the EDS spectra in Fig. 3(f). These regions may correspond to the perovskite, Bi or bismuth oxide, and sodium titanate phases shown in Fig. 2. By using the same method, the XRD patterns of the NBBT8 ceramics reduced at other reduction conditions (not shown here) were measured and the dependence of reduction thickness on the reduction temperatures and time was further determined. The reduction time dependence of reduction thickness at different reaction temperatures was summarized in Fig. 4(a). An approximate linear relationship exists between the reduction time and the thickness of the reduced layer at all the reduction temperatures. A similar linear relationship has been observed in the reduction of (Pb,La)(Zr,Ti)O3-based ferroelectric ceramics [22,23] and this phenomenon was attributed to the interface reactioncontrolled reduction process [24]. In some chemical reactions involving solids, the reaction kinetics is typically controlled by two processes. At the initial stage, the interface or surface reaction is the controlling process. With the reaction continuing, if the reaction products are dense, the diffusion of reactants to the reaction sites becomes difficult, and the diffusion becomes the controlling process. It is also possible that the interface reaction is always the controlling process because the loose microstructure of reaction products facilitates the diffusion process [24,25]. For NBBT8, as shown in Fig. 2(a), the reduction products seem very complex and it is difficult to determine the exact chemical reactions occurring during the reduction process. Although the reduction process is complex, the reduction can also be simplified into those two key controlling processes: one is the interface reaction process, and the other is the diffusion of various reactants to the reaction sites. If the interface reaction is the rate-controlling process in the reduction processing of NBBT ceramics, a linear relationship will exist between reduction thickness and time. In such a reaction controlled process, the relationship between the thickness of the reduction layer and time can be expressed as [25]:
Fig. 1. Schematic of the stacking of the alumina plates, graphite block and NBBT8 ceramic wafer for chemical reduction.
China), were used as raw materials. They were mixed by ball-milling machine in ethanol according to the stoichiometric ratio of 0.92Na0.5Bi0.5TiO3-0.08BaTiO3 ceramics. After calcination at 850 °C for 2 h, the calcined powders were milled for 12 h and then pressed into disks with 12.5 mm or 25.4 mm in diameter using polyvinyl alcohol (PVA) as a binder. After burning out PVA, the green compacts were sintered at 1175 °C for 2 h. During the reduction, one piece of polished NBBT8 wafer was placed on the top of a graphite block (50 mm in diameter and 10 mm in thickness) and an alumina plate was then placed on the top of the ceramic wafer, as shown in Fig. 1. The assembly was heat treated at 700–850 °C in a furnace. After holding at that temperature for 10–300 min, the assembly was quickly moved out of the furnace and cooled down to room temperature in the air. After reduction, a curvature structure was generated in the ceramic wafer. The density of NBBT8 ceramic (20 mm in diameter and 1 mm in thickness) was investigated by Archimedes method. The dome height of the curvature was characterized using a similar method that described in Ref. [18]. The X-ray diffraction (XRD) was performed on a Rigaku Smartlab diffractometer (Rigaku, Tokyo, Japan) to determine the crystal structure of the reduced and unreduced ceramics. To establish the relationship between the reduction thickness and the reduction time or temperature, the XRD patterns were measured after the reduced surface were removed layer by layer. The microstructure and the compositions of the ceramics were examined by a scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) (GeminiSEM 500, Carl Zeiss, Oberkochen, Germany). Before electrical properties were measured, the gold electrode was prepared using a DC sputtering method in a sputter coater (EMS150T, Electron Microscopy Sciences, Hatfield, PA, USA). The flexoelectric effect was characterized by measuring an apparent piezoelectric response d33 of the reduced ceramic disks, which was measured using a quasi-static d33 meter (ZJ-6A, Institute of Acoustics, CAS). The dimensions of the samples for the measurement of dome height and apparent d33 are approximately 20 mm in diameter and 0.5 mm in the thickness. The ceramics were not poled before flexoelectric measurement. 3. Results and discussion Fig. 2(a) and (b) show the XRD patterns of the unreduced NBBT8 ceramics and the ceramics reduced at 750 °C for 180 min. Unreduced NBBT8 ceramics exhibit tetragonal perovskite phase at room temperature [2,20]. After the reduction, the reduced surface exhibits mixed phases, which include NBBT8, Bi, Bi2O3, and sodium titanate. As shown in Fig. 2(b), the most evident diffraction peak of the mixed phases located at 27° to 28° is from metallic Bi [16,21]. To determine the reduction thickness of the NBBT8 ceramics, the XRD patterns were measured after the reduced surface (the surface against graphite block during the reduction) was removed layer by layer. The reduction thickness was determined when the diffraction peak from metallic Bi
dh = Kdt
(1)
where, h is the thickness of reduction layer and K is the reaction rate constant. Therefore, we can have
h= 2
∫0
t
Kdt = Kt + A
(2)
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Fig. 2. (a) X-ray diffraction patterns of the reduced and unreduced NBBT8 ceramics (10 mm in diameter and 1 mm in the thickness) reduced at 750 °C for 180 min. The reduced sample was polished from the reduced surface by the thickness indicated in the plots. (b) The diffraction peaks of metallic Bi between 27° to 28°. (c) The XRD patterns of (002) and (200) peaks.
Fig. 3. Cross-section SEM images of a NBBT8 ceramic wafer reduced at 825 °C for 120 min. (b) EDS elemental mapping images of the Bi, Ti, and Na elements of square region in (a). (c) BSE-SEM image of the square region shown in (a). (d), (e) and (f) are EDS spectra of the regions 1, 2, and 3 in (c).
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prepared NBBT8 ceramics have a density of 5.83( ± 0.08) g/cm3. After reduction, the density of NBBT8 ceramic becomes lower (approximately 5.68( ± 0.11) g/cm3) because of the volatilization of metallic Bi and the loss of oxygen. Fig. 3(a) also shows that the reduced layer exhibits a loose structure. In Fig. 4(a), an increase of slope K with the increase of reduction temperature indicates that the rate of reaction between NBT-based ceramics and graphite can be accelerated by increasing the reduction temperature. According to Arrhenius equation [24,26]:
K = K 0 exp(−
where A is a constant. As observed in Fig. 4(a), the reduction thickness is proportional to the reaction time at all the reduction temperatures, indicating the reduction reaction between the NBBT8 ceramics and graphite is mainly controlled by the rate of the interface chemical reactions. The solid lines give the linear fit to the data and the fitting equations of the lines are (3)
h800 = 1.02(± 0.02) t + 85.22(± 3.68)
(4)
h850 = 1.56(± 0.03) t + 122.94(± 4.53)
(5)
(6)
where Ea is activation energy, R is the gas constant, and K0 is pre-exponential factor. Fig. 4(b) shows the plot lnK versus 1/T, which can be fitted by a linear relationship. The activation energy Ea and pre-exponential factor K0 for this reaction can be calculated by Eq. (6) and the data shown in Fig. 4(b), which are approximately 55.3 kJ/mol and 9.37 × 10−6 m/s, respectively. Compared with lead oxide based ferroelectric ceramics, NBBT8 ceramics are much easier to be reduced. To achieve a reduction thickness of about 400 μm at the same reduction time (about 3 h), PLZT ceramics were reduced at 975 °C [22], PZT-5H ceramics at 1055 °C [24], and NBBT8 ceramics at a temperature about 850 °C. These observations are consistent with the results that the activation energy of reduction of NBBT8 ceramics (Ea ∼ 55.3 kJ/mol) is lower than that of lead oxide-based materials (Ea of PZT-5H ceramics ∼113 kJ/mol) [24]. Fig. 5 shows the reduction temperature dependence of reduction thickness at different reaction times. It is interesting that a linear relationship has been also found between ln (h) and 1/T. This result agrees with the relationship described in Eq. (6) and further indicates that the reduction of NBBT8 ceramics is a reaction-controlled process. The results shown in Fig. 4(a) and Fig. 5 suggest that the reduction thickness can be increased either by reducing the NBBT8 ceramics at a higher temperature or increasing reduction time. The thickness of the reduction layer is directly related to the performance of piezoelectric metamaterials. Optimization of the reduction time and temperature is required to achieve desirable properties of the materials. Fig. 6(a) shows the reduction time dependence of the dome height of the reduced NBBT8 ceramics. Because the curvature structure is caused by the different thermal expansions of the reduced and unreduced layers, a suitable thickness ratio of the reduced layer to unreduced layer can produce an optimized curvature structure. For the NBBT8 ceramics reduced at 800 °C, the maximum dome height is observed in the materials reduced for a period between 30 min and 60 min. The chemical reduction also has a significant effect on the flexoelectric response and the effective d33 of the piezoelectric
Fig. 4. (a) Variation of the reduction thickness of the reduced NBBT8 ceramics with reduction times at different reduction temperatures. (b) The linear relationship between the reaction rates constant K and 1/T in the logarithmic scale.
h750 = 0.87(± 0.09) t + 10.74(± 13.91)
Ea ) RT
where, h750, h800, and h850 are reduction thickness of ceramic reduced at 750 °C, 800 °C and 850 °C respectively, and t is reduction time. We note that the intercepts at Y-axis of the curves in Fig. 4(a) are not zero. It implies the reduction reaction has already started during the heating of the ceramics to the preset reduction temperatures (the heating rate 4 °C/min). The NBBT8 piezoelectric ceramics were prepared by conventional solid state reaction and they are not fully dense. The reason that the diffusion is not a controlling process of the reduction of NBBT8 ceramic may be due to the relative high porosity of the ceramics and the porous reduced layer, and the diffusion of reactants, such as CO and oxygen vacancies, is easier than the chemical reaction process. For lead-oxide based materials, the reduced layer is porous in (Pb,La)(Zr,Ti)O3 ceramics [22], and the thickness of the reduced layer is linear with the reduction time, similar to the observation in NBBT8 ceramics. However, a parabolic law was observed between reduced layer thickness and reduction time in PZT-5H ceramics because of its dense structure of reduction products and free of micro-cracks or micro-channels in the reduced layer [24]. For NBBT8 ceramics, we found the chemical reduction can significantly reduce the density of the materials. The as-
Fig. 5. The reduction thickness vs. reduction temperatures curves of the reduced NBBT8 ceramics.
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Fig. 6. The dome height (a), the effective d33 (b), and the apparent flexoelectric coefficient (c) of the reduced NBBT8 ceramics reduced at 800 °C for a period between 10 and 180 min. (d) The apparent flexoelectric coefficient of the NBBT8 ceramics reduced at a temperature between 750 and 850 °C for 30 min. Each data point is an average of three samples and the error bar shows the standard deviation.
thickness ratio is about 0.28 according to Fig. 5. The results in NBBT8 ceramics are consistent with those in (Pb,La)(Zr,Ti)O3–based piezoelectric Rainbow actuators, in which the maximum electromechanical response was obtained in the reduced samples having the thickness ratio of around 0.3 [28]. The enhancement of the apparent piezoelectric response in the reduced (Pb,La)(Zr,Ti)O3-based ceramics was attributed to the internal stress caused by the reduction in the materials. Consequently, although the mechanisms for the enhancement of the apparent μρ in NBBT8 ceramics are unknown, the finding that the maximum μρ of reduced NBBT8 ceramics and the maximum of d33 of the reduced (Pb,La)(Zr,Ti)O3–based ceramics were observed at a similar ratio of the thickness of the reduced layer to the total thickness indicates that the chemical reduction induced internal stress may play an important role in the enhancement of the apparent μρ of the reduced NBBT8 ceramics.
metamaterials. The apparent piezoelectric response of the piezoelectric metamaterials was measured using the point-ring test by a quasi-static d33 meter [16], and the effective d33 first increases and then decreases with the reduction time, as shown in Fig. 6(b). A maximum d33 higher than 3000 pC/N is measured in the materials reduced for 30 min. The apparent flexoelectric coefficient μρ was calculated according to the following equation [17]:
d33 =
μρ (1 − σ ) R2 8(1 + σ ) D
(7)
In the equation above, d33 is the apparent piezoelectric response, σ is poisson’s ratio, R is the radius of the ceramic wafer, and D is a coefficient controlling the flexural rigidity of the wafer. Fig. 6(c) shows the reduction time dependence of the apparent flexoelectric coefficient μρ of NBBT8 ceramics reduced at 800 °C. The μρ can be significantly enhanced by the reduction and the maximum μρ is around 700 μC/m for the ceramics reduced for 30 min. In Fig. 6(d), the apparent μρ of reduced NBBT8 ceramic is plotted against the reduction temperatures. The μρ is also strongly dependent on the reduction temperatures. The μρ reaches a maximum value of nearly 1.1 mC/m for the ceramics reduced at 825 °C for 30 min, and the value is comparable to that reduced BaTiO3 crystals [27]. These results indicate that reduction conditions have a significant effect on the μρ of the NBBT8 ceramics. Fig. 6(c) indicates that the maximum μρ was obtained in the ceramics reduced at 800 °C for 30 ∼ 60 min. According to Eq. (4), the reduction thickness of the reduced ceramic is estimated to be about 115 ∼ 147 μm, and the thickness ratio of the reduced layer to the total thickness is approximately 0.23 ∼ 0.29. Likewise, the reduction thickness of the materials reduced at 825 °C for 30 min, the conditions at which a maximum μρ is measured, is approximately 139 μm and the
4. Conclusions In this study, the effect of reduction conditions on the reduction thickness of the NBT-based ceramics was investigated. We found the chemical reduction of NBT-based ceramic is an interface reaction ratecontrolled process, and a linear relationship between reduction thickness and time was obtained. The activation energy of the reduction of the NBT-based ceramics is lower than that of lead oxide-based ferroelectric ceramics and the lead free materials can be reduced at a much lower temperature. We found the apparent flexoelectric coefficient of the ceramics can be enhanced either by reducing the NBBT8 ceramics at a higher temperature or increasing reduction time and the maximum flexoelectric coefficient was measured in reduced ceramics with a reduced layer of approximately 1/4–1/3 of the total thickness of the ceramics. Our results provide a guideline to optimize the reduction 5
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