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Microstructure and electrical properties of La2O3-doped Bi0.5(Na0.68K0.22Li0.1)0.5TiO3 lead-free piezoelectric ceramics
Current Applied Physics 11 (2011) 888e892
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Microstructure and electrical properties of La2O3-doped Bi0.5(Na0.68K0.22Li0.1)0.5TiO3 lead-free piezoelectric ceramics Hong Pan, Yuting Hou, Xiaolian Chao, Lingling Wei, Zupei Yang* Key Laboratory for Macromolecular Science of Shaanxi Province and School of Chemistry and Materials Science, Shaanxi Normal University, Xi’an, 710062, 199 Chang’an south road Xi’an, Shaanxi 710062, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 March 2010 Received in revised form 21 July 2010 Accepted 20 December 2010 Available online 4 January 2011
La2O3-doped Bi0.5(Na0.68K0.22Li0.1)0.5TiO3 lead-free piezoelectric ceramics have been successfully prepared by conventional solid state method and the effects of La2O3 doping on microstructure and electrical properties of the ceramics were investigated. The X-ray diffraction patterns demonstrated all ceramics possessed pure perovskite structure with rhombohedral symmetry. The temperature dependence of dielectric constant and dielectric loss revealed two abnormal dielectric peaks of Td and Tm, which implied that from room temperature to 500 C all ceramics underwent two phase transitions which were from ferroelectric to anti-ferroelectric and anti-ferroelectric to paraelectric, respectively. It was also found that the transition temperature Td decreased with the addition of La2O3. Diffuse phase transition and frequency dispersion of the dielectric constant were observed in all ceramics. Dielectric relaxor behavior obeyed the modified CurieeWeiss law. The results revealed that all samples had relaxor characteristics. In addition, for the ceramics with 0.1 wt.% La2O3, optimized electrical properties were obtained, which were as follows: 3r ¼ 1254 (1 kHz), tan d ¼ 0.0483, d33 ¼ 192 pC/N, Kp ¼ 0.27, Pr ¼ 29.1 mC/cm2, and Ec ¼ 29.0 kV/cm. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: Ceramics Doping effects Relaxation phenomena Electrical properties Piezoelectricity properties
1. Introduction Lead based ceramics have been widely used for electronic and microelectronic devices due to their excellent piezoelectric properties [1,2]. However, on account of the high volatilization of toxicant lead oxide during sintering process, a great many countries have enacted laws to restrict their applications. In recent decades, more and more researches have been focused on searching for environmentally friendly lead-free piezoelectric ceramics to replace the conventional lead based materials. Bismuth sodium titanate, (Bi0.5Na0.5TiO3, abbreviated to BNT), discovered by Smolenskii et al. in 1960 [3], has been considered as a promising candidate for lead-free piezoelectric ceramics due to its strong ferroelectricity (large remnant polarization Pr ¼ 38 mC cm2 and high Curie temperature Tc ¼ 320 C) at room temperature. However the BNT ceramic is very difficult to be polarized because of its large coercive field (Ec ¼ 7.3 kV/mm), which limits its practical application as a piezoelectric material. In order to obtain BNT-based ceramics with optimum properties, new BNT-based lead-free piezoelectric ceramic systems have been studied and reported. A
* Corresponding author. Tel.: þ86 29 8531 0352; fax: þ86 29 8530 7774. E-mail address: [email protected] (Z. Yang). 1567-1739/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.12.013
large number of multi-systems have been investigated, such as BNTeKNaNbO3 [4e10]. Meanwhile, the effects of some doping on BNT-based ceramics have already been investigated. A. Herabut et al. [11] found that introduction of a small amount of La3þ to A-site in BNT had an significant impact on the grain sizes of ceramics, and d33 increased from 58 pC/N (pure system) to 91 pC/N. Yuan et al. [12] studied the effects of La doping on BNKLT, a small amount of La (less than 2 wt.%) led to an improvement in the ferroelectric and piezoelectric properties. In our previous research work, (1xy)Bi0.5Na0.5TiO3exBi0.5 K0.5TiO3yBi0.5Li0.5TiO3 with good electrical properties had been invented at x ¼ 0.22 and y ¼ 0.10 [8]. In this paper, La2O3-doped Bi0.5(Na0.68K0.22Li0.1)0.5TiO3 were prepared by conventional solid state method to get better electrical properties. The effects of La2O3 on the microstructure and electrical properties of ceramics were studied and discussed. 2. Experimental Conventional solid state method was used to prepare Bi0.5(Na0.68K0.22Li0.10)0.5TiO3 þ x wt.% La2O3 (x ¼ 0.0, 0.1, 0.2, 0.3, 0.4, 0.6) ceramics. Reagent grade oxide or carbonate powders of Bi2O3, Na2CO3, K2CO3, TiO2, Li2CO3 and La2O3 were used as raw materials. The powers were weighted in the ratio of above formula and mixed
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H. Pan et al. / Current Applied Physics 11 (2011) 888e892
x=0.6 x=0.4 x=0.3 x=0.2 x=0.1 x=0
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2Theta(deg.) Fig. 1. XRD patterns of ceramics as a function of La2O3 content.
in ethanol with zirconia balls by ballemilling for 12 h, then were dried at 80 C and calcined at 850 C for 2 h in air. The calcined mixtures were mixed with polyvinyl alcohol as a binder and then pressed into disks of 15 mm-in-diameter and 1 mm-thickness under pressure of 100 MPa. After a 500 C binder burnout, the compacted samples were obtained by sintering at 1120 C for 2 h in air. Silver paste was fired at 800 C on both faces of the samples as
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the electrodes. Samples for piezoelectric measurements were poled at 80 C in a silicone oil bath by applying a DC electric field of 3e4 kV/mm for 15 min. The phase purity of the sintered ceramics was detected by X-ray diffraction (XRD, Model DMX-2550/PC, Rigaku, Japan) employing Cu Ka radiation. The surface microstructures of the obtained ceramics were observed using a scanning electron microscopy (SEM, Model Quanta 200, FEI Company). Temperature dependence of dielectric properties were measured with a LCR meter (Agilent, E4980A) from room temperature to 500 C at 1, 10 and 100 kHz, respectively. The piezoelectric constant d33 was measured by a quasistatic piezoelectric d33 meter (Model ZJ-3d, Institute of Acoustics Academic Sinica, China). The electromechanical coupling factor Kp was determined by the resonance and anti-resonance technique on the basis of IEEE standards using an impedance analyzer (HP 4294A). 3. Results and discussions The X-ray diffraction patterns of ceramics are shown in Fig. 1. A pure perovskite structure is detected and no second phase can be found in all samples, suggesting La3þ diffuses into the BNKLT lattice
Fig. 2. SEM micrographs of BNT-BKT-BLT ceramics with different La2O3 content (a) 0 wt.% (b) 0.1 wt.% (c) 0.2 wt.% (d) 0.3 wt.% (e) 0.4 wt % (f) 0.6 wt.%.
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during sintering. According to the reports [13], the feature peak at about 47 does not split, which means that all ceramics have rhombohedral symmetry structure. The results reveal that a small amount of La2O3 has no significant effect on the phase structure of BNKLT ceramics. Fig. 2 shows the SEM micrographs of BNKLT ceramics doped with different La2O3 content and sintered at 1120 C, respectively. The grains of all samples are regular crystal shape with clear grain boundaries. Compared with pure BNKLT ceramics, the grain sizes decrease from 2.5 mm to 1.7 mm (with 0.3 wt.% La2O3). The smaller grain sizes of La-doped samples probably attribute to the segregation of proper quantities of La ions at boundaries, which displays a close-packing role between grains [14]. So, it is reasonable that grain sizes of BNKLT are restrained by La2O3 doping at a certain extent. With increasing x from 0 to 0.6, the density increases and reaches maximum value at 0.1 wt.%, then decreases with further addition, as the density curve of the samples shown in Fig. 3. The density results indicate that the most compacted samples can be obtained with x ¼ 0.1 wt.%. Fig. 4 shows the variations of electrical properties of ceramics as a function of x. With increasing x, dielectric constant (er) first increases, reaches maximum value and then decreases. Dielectric loss constant (tan d), piezoelectric constant (d33) and planar coupling factor (Kp) values reveal the same trends as er. The maximum values with er ¼ 1456, tan d ¼ 0.0551 are obtained at x ¼ 0.20 wt.%, d33 and Kp increase to the maximum values (192 pC/N and 0.27) at x ¼ 0.1 wt.%. It is obvious that La3þ doping can improve the dielectric properties remarkably.The ionic radius of La3þ is 1.06 Å, which is close to Naþ (1.02 Å) and Bi3þ (1.03 Å) at A-site, so it is more likely that La3þ enters into the lattice to occupy A-site. On the one hand, if La3þ substitutes Naþ at A-site, vacancies of A-site in the lattice will be formed to maintain the balance of charge; on the
other hand, La3þ will substitute Bi3þ because of the same valence and volatilization of Bi2O3 at high temperature during sintering process. The difference of ionic radius will lead to a distortion of crystal structure. The vacancies of A-site and distortion of crystal structure facilitate the movement of domain wall so as to improve the piezoelectric properties [13,15]. So it can be concluded that La3þ plays a role of “soft” doping, which leads to the increasing of d33 and Kp. The temperature dependence of er and tan d at different frequencies (1 kHz, 10 kHz, 100 kHz) of La2O3-doped BNKLT ceramics are shown in Fig. 5. From room temperature to 500 C, two abnormal dielectric peaks of Td and Tm can be observed in the curves of all samples, which indicates that all samples undergo two phase transitions from ferroelectric to anti-ferroelectric phase (corresponding to Td) and anti-ferroelectric to paraelectric phase (corresponding to Tm) [16]. er exhibits an obvious dependence on frequency in Fig. 5, while this dependence becomes weaker between Td and Tm (the temperature at which er reached the maximum), and appears clearly again above Tm. With increasing frequency, Td shifts toward higher temperature and er becomes lower; under the same situation, tan d increases when the temperature is lower than Td, and decreases when the temperature is above Td. Furthermore, all samples have expanded dielectric peaks, showing frequency dispersion of er and diffusive phase transition characteristics. The results imply that all samples with 0.0 wt.%e0.6 wt.% La2O3 are relaxor ferroelectrics. One abnormal dielectric loss peak corresponding to Td can be observed in curves of dielectric loss factor vs. temperature. It can be seen that tan d increases sharply above 300 C. With frequency increasing, tan d becomes larger, which also displays relaxor characteristics. The variation of tan d can be explained by macro-micro domain switching model of relaxor ferroelectrics proposed by Yao xi et al. [17]. The dielectric loss of BNT-based ferroelectric ceramics mainly be attributed to the vibration of domain walls. When the temperature is low, the ceramics form a stable macro-ferroelectric domain structure; as the temperature rises to near Td, thermally driven makes the dipole overcome the directional arrangement along the direction of electric field, macrodomains gradually transform into microdomains; increased domain walls cause the increasing of tan d. With further increasing temperature, more and more microdomains loss stability and convert into the anti-ferroelectric polar microregions. In this case, domain walls disappear and dielectric loss decreases. When the temperature is over Tm, the loss generated from electrical conduction becomes higher, which plays a leading role in rapidly increasing of tan d. To further explain the diffuse characteristic of the ceramics with different La2O3 content, the modified CurieeWeiss law was used [18,19].
Fig. 4. Variations of er , tan d, Kp and d33 of ceramics as a function of La2O3 content.
H. Pan et al. / Current Applied Physics 11 (2011) 888e892
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Fig. 5. er and tan d as a function of temperature for La2O3 added ceramics (a) 0.0 wt.% (b) 0.1 wt.% (c) 0.2 wt.% (d) 0.3 wt.% (e) 0.4 wt.% (f) 0.6 wt.%.
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Fig. 7. Td and Tm as function of La2O3 content.
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H. Pan et al. / Current Applied Physics 11 (2011) 888e892
piezoelectric properties of BNKLT ceramics, while excessive amount (more than 0.1 wt%) makes the ferroelectrics and piezoelectric properties weaken [22]. Generally the result is in accordance with the characteristic of “soft” doped ferroelectrics in Fig. 4. 4. Conclusions
Fig. 8. PeE hysteresis loops of BNKLT and La2O3-doped BNKLT ceramics at room temperature.
em ðT Tm Þg ¼ 1þ e 2D2 The plot of Ln[(em/e)1] as a function of Ln(TTc) of all samples at 1 kHz is shown in Fig. 6. A linear relationship is observed in all samples by linear fitting the experimental date. The diffuseness exponent g of all ceramics is between 1 and 2. According to the relaxor concept proposed by Smolenskii et al. [3], the phase transition of all ceramics shows the diffuse characteristic. This result is in accordance with that of Fig. 5. Td and Tm as a function of the La2O3 content are indicated in Fig. 7. With increasing x, Td decreases while Tm changes slightly. Following the theory of stability of ferroelectric domains of ferroelectric material with perovskite structure proposed by Thomas [20], when the stability of ferroelectric domains decreases, the transition temperature from ferroelectric to anti-ferroelectric phase (Td) is likely to reduce. Ferroelectric domains can be seen as the coupling function between A-site cations and BO6 octahedron in the ABO3 perovskite structure, and the stability of ferroelectric domains is determined by the intensity of dipole moment of BO6 octahedron and coupling degree between A-site cations and BO6 octahedron [21]. For the La2O3-doped BNKLT ceramics, from the view of ionic radius, La3þ will enter A-site rather than B-site. The introduction of La3þ to A-site will lead to charge imbalance because of the different ionic radius between La3þ and A-site cations, and to compensate the charge imbalance, the vacancies at A-site will be formed. The presence of A-site vacancies would reduce the degree of the coupling function between A-site cations and BO6 octahedron, consequently causing a depressed effect on the stability of ferroelectric domains. This is probably responsible for the decrease in depolarization temperature of the samples with the addition of La2O3. To evaluate the effect of La2O3 on the ferroelectric properties, the PeE hysteresis loops of the samples were measured at room temperature, as shown in Fig. 8. The PeE hysteresis loops of pure BNKLT and 0.1 wt% La2O3-doped BNKLT exhibit typical ferroelectric behavior, with a large remnant polarization (Pr) and coercive field (Ec). The samples with x ¼ 0.0, 0.1 and 0.2 possessed Pr of 29.1, 29.4 and 21.1 mC/cm2 with Ec of 29.0, 24.2 and 19.2 kV/cm, respectively. With the addition of La2O3, Ec decreases and Pr slightly increases and then decreases sharply. It indicates that proper amount of La2O3 (about 0.1 wt%) doping enhances the ferroelectrics and
In this work, the effects of La2O3 doping on the microstructure and electrical properties of BNKLT lead-free ceramics have been investigated. The X-ray diffraction patterns showed that all ceramics were pure rhombohedral perovskite and La3þ diffused into the lattice of BNKLT and formed a solid solution. SEM images indicated that the grain sizes of BNKLT were restrained by closepacking effect of a certain extent of La doping. All ceramics had two abnormal dielectric peaks of Td and Tm, corresponding to the phase transition from ferroelectric to anti-ferroelectric and anti-ferroelectric to paraelectric, respectively. The variation of tan d can be explained by macro-micro domains switching model. Td and Tm exhibited an obvious dependency on frequency and Td shifted to higher temperature with increasing frequency. The results proved that all samples had relaxor character. The relaxor characteristics were further proved by modified Curie-Weiss law. With the addition of La2O3, d33 and Kp of the samples increase and Ec decreases, which indicate that La3þ plays a role of “soft” doping. Acknowledgments This work was supported by National Science Foundation of China (NSFC) (Grant No. 20771070), Foundation of Doctorial Program in China (No. 20070718004), Natural Science Research Program of Shaanxi Province (Grant No. 2009JZ003). Fundamental Research Funds for the Central Universities (Program No. GK200901003). References [1] B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics. Academic Press, New York, 1971. [2] Z. Yang, X. Chao, L. Yang, Jpn. J. Appl. Phys. 46 (2007) 6746e6750. [3] G.A. Smolenskii, V.A. Isupov, A.I. Agranovskaya, N.N. Krainik, Sov. Phys. Solid State 2 (11) (1961) 2651e2654. [4] A.B. Kounga, S.T. Zhang, W. Jo, T. Granzow, J. Rödel, Appl. Phys. Lett. 92 (2008) 222902. [5] Y. Li, W. Chen, J. Zhou, Q. Xu, H. Sun, M. Liao, Ceram. Int. 31 (2005) 139e142. [6] M. Chen, Q. Xu, B.H. Kim, B.K. Ahn, J.H. Ko, W.J. Kang, O.J. Nam, J. Eur. Ceram. Soc. 28 (2008) 843e849. [7] S.T. Zhang, A.B. Kounga, E. Aulbach, T. Granzow, W. Jo, H.J. Kleebe, J. Rödel, J. Appl. Phys. 103 (2008) 034107. [8] Z.P. Yang, Y.T. Hou, H. Pan, Y.F. Chang, J. Alloys Compd. 480 (2009) 246e253. [9] C.R. Zhou, X.Y. Liu, W.Z. Li, C.L. Yuan, G.H. Chen, Curr. Appl. Phys. 10 (2010) 93e98. [10] H. Yu, Z.G. Ye, Appl. Phys. Lett. 93 (2008) 112902. [11] A. Herabut, A. Safari, J. Am. Ceram. Soc. 80 (1997) 2954e2958. [12] Y. Yuan, S.R. Zhang, X.H. Zhou, J.S. Liu, J. Mater. Sci. 41 (2006) 565e567. [13] J.H. Shi, W.M. Yang, J. Alloys Compd 472 (2009) 267e270. [14] J.Y. Yoo, J. Hong, H. Lee, Y. Jeong, B. Lee, H. Song, J. Kwon, Sens. Actuators. A 126 (2006) 41e47. [15] Z.Y. Yang, Y.T. Hou, B. Liu, L.L. Wei, Ceram. Int. 43 (1) (2008) 81e89. [16] X.X. Wang, H.L.W. Chan, C.L. Choy, Solid State Commun. 125 (2003) 395e399. [17] Y. Xi, C.Z. Li, L.E. Cross, J. Appl. Phys. 54 (6) (1983) 3399e3403. [18] A.E. Glazounov, A.K. Taganstev, A.J. Bell, Phys. Rev. B. 53 (1996) 11281e11284. [19] X.P. Jiang, L.Z. Li, M. Zeng, H.L.M. Chan, Mater. Lett. 60 (2006) 1786e1790. [20] N.W. Thomas, J. Phys, Chem. Solids 51 (1990) 1419e1431. [21] X. Dai, A. Digiovanni, D. Viehland, J. Appl. Phys. 74 (5) (1993) 3399e3405. [22] A. Ullah, C.W. Ahn, A. Hussain, I.W. Kim, H. Hwang, N.K. Cho, Solid State Commun. 150 (2010) 1145e1149.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Microstructure and electrical properties of La2O3-doped Bi0.5(Na0.68K0.22Li0.1)0.5TiO3 lead-free piezoelectric ceramics Hong Pan, Yuting Hou, Xiaolian Chao, Lingling Wei, Zupei Yang* Key Laboratory for Macromolecular Science of Shaanxi Province and School of Chemistry and Materials Science, Shaanxi Normal University, Xi’an, 710062, 199 Chang’an south road Xi’an, Shaanxi 710062, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 March 2010 Received in revised form 21 July 2010 Accepted 20 December 2010 Available online 4 January 2011
La2O3-doped Bi0.5(Na0.68K0.22Li0.1)0.5TiO3 lead-free piezoelectric ceramics have been successfully prepared by conventional solid state method and the effects of La2O3 doping on microstructure and electrical properties of the ceramics were investigated. The X-ray diffraction patterns demonstrated all ceramics possessed pure perovskite structure with rhombohedral symmetry. The temperature dependence of dielectric constant and dielectric loss revealed two abnormal dielectric peaks of Td and Tm, which implied that from room temperature to 500 C all ceramics underwent two phase transitions which were from ferroelectric to anti-ferroelectric and anti-ferroelectric to paraelectric, respectively. It was also found that the transition temperature Td decreased with the addition of La2O3. Diffuse phase transition and frequency dispersion of the dielectric constant were observed in all ceramics. Dielectric relaxor behavior obeyed the modified CurieeWeiss law. The results revealed that all samples had relaxor characteristics. In addition, for the ceramics with 0.1 wt.% La2O3, optimized electrical properties were obtained, which were as follows: 3r ¼ 1254 (1 kHz), tan d ¼ 0.0483, d33 ¼ 192 pC/N, Kp ¼ 0.27, Pr ¼ 29.1 mC/cm2, and Ec ¼ 29.0 kV/cm. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: Ceramics Doping effects Relaxation phenomena Electrical properties Piezoelectricity properties
1. Introduction Lead based ceramics have been widely used for electronic and microelectronic devices due to their excellent piezoelectric properties [1,2]. However, on account of the high volatilization of toxicant lead oxide during sintering process, a great many countries have enacted laws to restrict their applications. In recent decades, more and more researches have been focused on searching for environmentally friendly lead-free piezoelectric ceramics to replace the conventional lead based materials. Bismuth sodium titanate, (Bi0.5Na0.5TiO3, abbreviated to BNT), discovered by Smolenskii et al. in 1960 [3], has been considered as a promising candidate for lead-free piezoelectric ceramics due to its strong ferroelectricity (large remnant polarization Pr ¼ 38 mC cm2 and high Curie temperature Tc ¼ 320 C) at room temperature. However the BNT ceramic is very difficult to be polarized because of its large coercive field (Ec ¼ 7.3 kV/mm), which limits its practical application as a piezoelectric material. In order to obtain BNT-based ceramics with optimum properties, new BNT-based lead-free piezoelectric ceramic systems have been studied and reported. A
* Corresponding author. Tel.: þ86 29 8531 0352; fax: þ86 29 8530 7774. E-mail address: [email protected] (Z. Yang). 1567-1739/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.12.013
large number of multi-systems have been investigated, such as BNTeKNaNbO3 [4e10]. Meanwhile, the effects of some doping on BNT-based ceramics have already been investigated. A. Herabut et al. [11] found that introduction of a small amount of La3þ to A-site in BNT had an significant impact on the grain sizes of ceramics, and d33 increased from 58 pC/N (pure system) to 91 pC/N. Yuan et al. [12] studied the effects of La doping on BNKLT, a small amount of La (less than 2 wt.%) led to an improvement in the ferroelectric and piezoelectric properties. In our previous research work, (1xy)Bi0.5Na0.5TiO3exBi0.5 K0.5TiO3yBi0.5Li0.5TiO3 with good electrical properties had been invented at x ¼ 0.22 and y ¼ 0.10 [8]. In this paper, La2O3-doped Bi0.5(Na0.68K0.22Li0.1)0.5TiO3 were prepared by conventional solid state method to get better electrical properties. The effects of La2O3 on the microstructure and electrical properties of ceramics were studied and discussed. 2. Experimental Conventional solid state method was used to prepare Bi0.5(Na0.68K0.22Li0.10)0.5TiO3 þ x wt.% La2O3 (x ¼ 0.0, 0.1, 0.2, 0.3, 0.4, 0.6) ceramics. Reagent grade oxide or carbonate powders of Bi2O3, Na2CO3, K2CO3, TiO2, Li2CO3 and La2O3 were used as raw materials. The powers were weighted in the ratio of above formula and mixed
(202) (220)
(112) (211)
(202)
(110)
(111)
Intensity(cps)
(100)
H. Pan et al. / Current Applied Physics 11 (2011) 888e892
x=0.6 x=0.4 x=0.3 x=0.2 x=0.1 x=0
20
30
40
50
60
70
80
2Theta(deg.) Fig. 1. XRD patterns of ceramics as a function of La2O3 content.
in ethanol with zirconia balls by ballemilling for 12 h, then were dried at 80 C and calcined at 850 C for 2 h in air. The calcined mixtures were mixed with polyvinyl alcohol as a binder and then pressed into disks of 15 mm-in-diameter and 1 mm-thickness under pressure of 100 MPa. After a 500 C binder burnout, the compacted samples were obtained by sintering at 1120 C for 2 h in air. Silver paste was fired at 800 C on both faces of the samples as
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the electrodes. Samples for piezoelectric measurements were poled at 80 C in a silicone oil bath by applying a DC electric field of 3e4 kV/mm for 15 min. The phase purity of the sintered ceramics was detected by X-ray diffraction (XRD, Model DMX-2550/PC, Rigaku, Japan) employing Cu Ka radiation. The surface microstructures of the obtained ceramics were observed using a scanning electron microscopy (SEM, Model Quanta 200, FEI Company). Temperature dependence of dielectric properties were measured with a LCR meter (Agilent, E4980A) from room temperature to 500 C at 1, 10 and 100 kHz, respectively. The piezoelectric constant d33 was measured by a quasistatic piezoelectric d33 meter (Model ZJ-3d, Institute of Acoustics Academic Sinica, China). The electromechanical coupling factor Kp was determined by the resonance and anti-resonance technique on the basis of IEEE standards using an impedance analyzer (HP 4294A). 3. Results and discussions The X-ray diffraction patterns of ceramics are shown in Fig. 1. A pure perovskite structure is detected and no second phase can be found in all samples, suggesting La3þ diffuses into the BNKLT lattice
Fig. 2. SEM micrographs of BNT-BKT-BLT ceramics with different La2O3 content (a) 0 wt.% (b) 0.1 wt.% (c) 0.2 wt.% (d) 0.3 wt.% (e) 0.4 wt % (f) 0.6 wt.%.
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La2O3 content(wt.%) Fig. 3. Density of ceramics as a function of La2O3 content.
during sintering. According to the reports [13], the feature peak at about 47 does not split, which means that all ceramics have rhombohedral symmetry structure. The results reveal that a small amount of La2O3 has no significant effect on the phase structure of BNKLT ceramics. Fig. 2 shows the SEM micrographs of BNKLT ceramics doped with different La2O3 content and sintered at 1120 C, respectively. The grains of all samples are regular crystal shape with clear grain boundaries. Compared with pure BNKLT ceramics, the grain sizes decrease from 2.5 mm to 1.7 mm (with 0.3 wt.% La2O3). The smaller grain sizes of La-doped samples probably attribute to the segregation of proper quantities of La ions at boundaries, which displays a close-packing role between grains [14]. So, it is reasonable that grain sizes of BNKLT are restrained by La2O3 doping at a certain extent. With increasing x from 0 to 0.6, the density increases and reaches maximum value at 0.1 wt.%, then decreases with further addition, as the density curve of the samples shown in Fig. 3. The density results indicate that the most compacted samples can be obtained with x ¼ 0.1 wt.%. Fig. 4 shows the variations of electrical properties of ceramics as a function of x. With increasing x, dielectric constant (er) first increases, reaches maximum value and then decreases. Dielectric loss constant (tan d), piezoelectric constant (d33) and planar coupling factor (Kp) values reveal the same trends as er. The maximum values with er ¼ 1456, tan d ¼ 0.0551 are obtained at x ¼ 0.20 wt.%, d33 and Kp increase to the maximum values (192 pC/N and 0.27) at x ¼ 0.1 wt.%. It is obvious that La3þ doping can improve the dielectric properties remarkably.The ionic radius of La3þ is 1.06 Å, which is close to Naþ (1.02 Å) and Bi3þ (1.03 Å) at A-site, so it is more likely that La3þ enters into the lattice to occupy A-site. On the one hand, if La3þ substitutes Naþ at A-site, vacancies of A-site in the lattice will be formed to maintain the balance of charge; on the
other hand, La3þ will substitute Bi3þ because of the same valence and volatilization of Bi2O3 at high temperature during sintering process. The difference of ionic radius will lead to a distortion of crystal structure. The vacancies of A-site and distortion of crystal structure facilitate the movement of domain wall so as to improve the piezoelectric properties [13,15]. So it can be concluded that La3þ plays a role of “soft” doping, which leads to the increasing of d33 and Kp. The temperature dependence of er and tan d at different frequencies (1 kHz, 10 kHz, 100 kHz) of La2O3-doped BNKLT ceramics are shown in Fig. 5. From room temperature to 500 C, two abnormal dielectric peaks of Td and Tm can be observed in the curves of all samples, which indicates that all samples undergo two phase transitions from ferroelectric to anti-ferroelectric phase (corresponding to Td) and anti-ferroelectric to paraelectric phase (corresponding to Tm) [16]. er exhibits an obvious dependence on frequency in Fig. 5, while this dependence becomes weaker between Td and Tm (the temperature at which er reached the maximum), and appears clearly again above Tm. With increasing frequency, Td shifts toward higher temperature and er becomes lower; under the same situation, tan d increases when the temperature is lower than Td, and decreases when the temperature is above Td. Furthermore, all samples have expanded dielectric peaks, showing frequency dispersion of er and diffusive phase transition characteristics. The results imply that all samples with 0.0 wt.%e0.6 wt.% La2O3 are relaxor ferroelectrics. One abnormal dielectric loss peak corresponding to Td can be observed in curves of dielectric loss factor vs. temperature. It can be seen that tan d increases sharply above 300 C. With frequency increasing, tan d becomes larger, which also displays relaxor characteristics. The variation of tan d can be explained by macro-micro domain switching model of relaxor ferroelectrics proposed by Yao xi et al. [17]. The dielectric loss of BNT-based ferroelectric ceramics mainly be attributed to the vibration of domain walls. When the temperature is low, the ceramics form a stable macro-ferroelectric domain structure; as the temperature rises to near Td, thermally driven makes the dipole overcome the directional arrangement along the direction of electric field, macrodomains gradually transform into microdomains; increased domain walls cause the increasing of tan d. With further increasing temperature, more and more microdomains loss stability and convert into the anti-ferroelectric polar microregions. In this case, domain walls disappear and dielectric loss decreases. When the temperature is over Tm, the loss generated from electrical conduction becomes higher, which plays a leading role in rapidly increasing of tan d. To further explain the diffuse characteristic of the ceramics with different La2O3 content, the modified CurieeWeiss law was used [18,19].
Fig. 4. Variations of er , tan d, Kp and d33 of ceramics as a function of La2O3 content.
H. Pan et al. / Current Applied Physics 11 (2011) 888e892
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Fig. 5. er and tan d as a function of temperature for La2O3 added ceramics (a) 0.0 wt.% (b) 0.1 wt.% (c) 0.2 wt.% (d) 0.3 wt.% (e) 0.4 wt.% (f) 0.6 wt.%.
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La2O3 content(wt.%) Fig. 6. Ln[(em-e)/e] as a function of Ln(T-Tc) for La2O3 added ceramics measured at 1 kHz.
Fig. 7. Td and Tm as function of La2O3 content.
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piezoelectric properties of BNKLT ceramics, while excessive amount (more than 0.1 wt%) makes the ferroelectrics and piezoelectric properties weaken [22]. Generally the result is in accordance with the characteristic of “soft” doped ferroelectrics in Fig. 4. 4. Conclusions
Fig. 8. PeE hysteresis loops of BNKLT and La2O3-doped BNKLT ceramics at room temperature.
em ðT Tm Þg ¼ 1þ e 2D2 The plot of Ln[(em/e)1] as a function of Ln(TTc) of all samples at 1 kHz is shown in Fig. 6. A linear relationship is observed in all samples by linear fitting the experimental date. The diffuseness exponent g of all ceramics is between 1 and 2. According to the relaxor concept proposed by Smolenskii et al. [3], the phase transition of all ceramics shows the diffuse characteristic. This result is in accordance with that of Fig. 5. Td and Tm as a function of the La2O3 content are indicated in Fig. 7. With increasing x, Td decreases while Tm changes slightly. Following the theory of stability of ferroelectric domains of ferroelectric material with perovskite structure proposed by Thomas [20], when the stability of ferroelectric domains decreases, the transition temperature from ferroelectric to anti-ferroelectric phase (Td) is likely to reduce. Ferroelectric domains can be seen as the coupling function between A-site cations and BO6 octahedron in the ABO3 perovskite structure, and the stability of ferroelectric domains is determined by the intensity of dipole moment of BO6 octahedron and coupling degree between A-site cations and BO6 octahedron [21]. For the La2O3-doped BNKLT ceramics, from the view of ionic radius, La3þ will enter A-site rather than B-site. The introduction of La3þ to A-site will lead to charge imbalance because of the different ionic radius between La3þ and A-site cations, and to compensate the charge imbalance, the vacancies at A-site will be formed. The presence of A-site vacancies would reduce the degree of the coupling function between A-site cations and BO6 octahedron, consequently causing a depressed effect on the stability of ferroelectric domains. This is probably responsible for the decrease in depolarization temperature of the samples with the addition of La2O3. To evaluate the effect of La2O3 on the ferroelectric properties, the PeE hysteresis loops of the samples were measured at room temperature, as shown in Fig. 8. The PeE hysteresis loops of pure BNKLT and 0.1 wt% La2O3-doped BNKLT exhibit typical ferroelectric behavior, with a large remnant polarization (Pr) and coercive field (Ec). The samples with x ¼ 0.0, 0.1 and 0.2 possessed Pr of 29.1, 29.4 and 21.1 mC/cm2 with Ec of 29.0, 24.2 and 19.2 kV/cm, respectively. With the addition of La2O3, Ec decreases and Pr slightly increases and then decreases sharply. It indicates that proper amount of La2O3 (about 0.1 wt%) doping enhances the ferroelectrics and
In this work, the effects of La2O3 doping on the microstructure and electrical properties of BNKLT lead-free ceramics have been investigated. The X-ray diffraction patterns showed that all ceramics were pure rhombohedral perovskite and La3þ diffused into the lattice of BNKLT and formed a solid solution. SEM images indicated that the grain sizes of BNKLT were restrained by closepacking effect of a certain extent of La doping. All ceramics had two abnormal dielectric peaks of Td and Tm, corresponding to the phase transition from ferroelectric to anti-ferroelectric and anti-ferroelectric to paraelectric, respectively. The variation of tan d can be explained by macro-micro domains switching model. Td and Tm exhibited an obvious dependency on frequency and Td shifted to higher temperature with increasing frequency. The results proved that all samples had relaxor character. The relaxor characteristics were further proved by modified Curie-Weiss law. With the addition of La2O3, d33 and Kp of the samples increase and Ec decreases, which indicate that La3þ plays a role of “soft” doping. Acknowledgments This work was supported by National Science Foundation of China (NSFC) (Grant No. 20771070), Foundation of Doctorial Program in China (No. 20070718004), Natural Science Research Program of Shaanxi Province (Grant No. 2009JZ003). Fundamental Research Funds for the Central Universities (Program No. GK200901003). References [1] B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics. Academic Press, New York, 1971. [2] Z. Yang, X. Chao, L. Yang, Jpn. J. Appl. Phys. 46 (2007) 6746e6750. [3] G.A. Smolenskii, V.A. Isupov, A.I. Agranovskaya, N.N. Krainik, Sov. Phys. Solid State 2 (11) (1961) 2651e2654. [4] A.B. Kounga, S.T. Zhang, W. Jo, T. Granzow, J. Rödel, Appl. Phys. Lett. 92 (2008) 222902. [5] Y. Li, W. Chen, J. Zhou, Q. Xu, H. Sun, M. Liao, Ceram. Int. 31 (2005) 139e142. [6] M. Chen, Q. Xu, B.H. Kim, B.K. Ahn, J.H. Ko, W.J. Kang, O.J. Nam, J. Eur. Ceram. Soc. 28 (2008) 843e849. [7] S.T. Zhang, A.B. Kounga, E. Aulbach, T. Granzow, W. Jo, H.J. Kleebe, J. Rödel, J. Appl. Phys. 103 (2008) 034107. [8] Z.P. Yang, Y.T. Hou, H. Pan, Y.F. Chang, J. Alloys Compd. 480 (2009) 246e253. [9] C.R. Zhou, X.Y. Liu, W.Z. Li, C.L. Yuan, G.H. Chen, Curr. Appl. Phys. 10 (2010) 93e98. [10] H. Yu, Z.G. Ye, Appl. Phys. Lett. 93 (2008) 112902. [11] A. Herabut, A. Safari, J. Am. Ceram. Soc. 80 (1997) 2954e2958. [12] Y. Yuan, S.R. Zhang, X.H. Zhou, J.S. Liu, J. Mater. Sci. 41 (2006) 565e567. [13] J.H. Shi, W.M. Yang, J. Alloys Compd 472 (2009) 267e270. [14] J.Y. Yoo, J. Hong, H. Lee, Y. Jeong, B. Lee, H. Song, J. Kwon, Sens. Actuators. A 126 (2006) 41e47. [15] Z.Y. Yang, Y.T. Hou, B. Liu, L.L. Wei, Ceram. Int. 43 (1) (2008) 81e89. [16] X.X. Wang, H.L.W. Chan, C.L. Choy, Solid State Commun. 125 (2003) 395e399. [17] Y. Xi, C.Z. Li, L.E. Cross, J. Appl. Phys. 54 (6) (1983) 3399e3403. [18] A.E. Glazounov, A.K. Taganstev, A.J. Bell, Phys. Rev. B. 53 (1996) 11281e11284. [19] X.P. Jiang, L.Z. Li, M. Zeng, H.L.M. Chan, Mater. Lett. 60 (2006) 1786e1790. [20] N.W. Thomas, J. Phys, Chem. Solids 51 (1990) 1419e1431. [21] X. Dai, A. Digiovanni, D. Viehland, J. Appl. Phys. 74 (5) (1993) 3399e3405. [22] A. Ullah, C.W. Ahn, A. Hussain, I.W. Kim, H. Hwang, N.K. Cho, Solid State Commun. 150 (2010) 1145e1149.