Advanced Materials Research Vols. 284-286 (2011) pp 1408-1411 Online available since 2011/Jul/04 at www.scientific.net © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.284-286.1408
Effect of La2O3 on the microstructure and electrical properties of 0.82Bi0.5Na0.5TiO3-0.18Bi0.5K0.5TiO3 ceramics Yan Li，Yanjie Zhang, Ruiqing Chu*, Zhijun Xu, Qian Chen, Yong Liu College of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, People’s Republic of China *
[email protected] Keywords: Microstructure; Grain sizes; La2O3-doped; Ceramics; Electrical properties.
Abstract. La2O3-doped lead-free 0.82Bi0.5Na0.5TiO3-0.18Bi0.5K0.5TiO3 (abbreviated to 0.82BNT-0.18BKT) piezoelectric ceramics were synthesized by the conventional mixed-oxide method, and the effect of La2O3 addition on the dielectric and piezoelectric properties was investigated. X-ray diffraction (XRD) patterns show that La2O3 diffuses into the lattice of the 0.82BNT-0.18BKT ceramics to form a solid solution with a pure perovskite structure. SEM images indicate that the grain size of the 0.82BNT-0.18BKT ceramics increased with the addition of La2O3 doping. The electrical properties of 0.82BNT-0.18BKT ceramics have been greatly improved by certain amount of La2O3 substitutions. At room temperature, the 0.82BNT-0.18BKT ceramics doped with 0.25 wt. % La2O3 exhibited the optimum properties with high piezoelectric constant (d33 = 142 pC/N) and high planar coupling factor (kp = 0.23). Introduction Currently, lead-free piezoelectric ceramics have attracted considerable attention from researchers as they have no lead pollution. Among them, bismuth sodium titanate, Bi0.5Na1/2TiO3 (abbreviated as BNT), which was discovered by Smolenskii et al. in 1960, is considered as an excellent lead-free piezoelectric ceramic candidate because of its large remanent polarization (Pr = 38µC/cm2) and high Curie temperature (Tc = 320 °C) [1]. However, high conductivity and high coercive field (Ec = 73 kV/cm) can cause problems in the poling process, and thus limit its practical application [1-3]. To improve its properties, several BNT-based solid solutions have been developed. BNT-BaTiO3 [4], BNT-NaNbO3 [5], BNT-Bi0.5K0.5TiO3 [6-8], or combinations of multiple additives [9-12] have been studied. Among them, the (1−x) BNT-xBKT ceramics have attracted considerable attention for the existence of a tetragonal-rhombohedra morphotropic phase boundary (MPB) located at x = 0.16-0.20 [7, 8]. Compared with pure BNT, the BNT-BKT compositions near the MPB provide excellent piezoelectric properties. However, for practical applications, the piezoelectric and dielectric properties of BNT-BKT ceramic need to be further improved. Similar to the case of PZT-based piezoelectric ceramics, rare earth elements are the often-employed additives for BNT-based compositions [13-15]. Li. et al [16] reported that CeO2 doping could reduce the coercive field Ec and improve the piezoelectric properties of Bi0.5Na0.44K0.06TiO3 ceramics. Yang et al [17] found that at a low Nd2O3 concentration, the Nd doped 0.82Bi0.5Na0.5TiO3-0.18Bi0.5K0.5TiO3 ceramics show a high piezoelectric constant. Herabut reported that electrical properties could be improved by doping appropriate amount of La in BNT-system [18]. It can be concluded that these rare earth elements were effective additives in enhancing the electrical properties of the BNT-based system. La as a kind of rare earth element is reported to improve the piezoelectric properties of (Bi0.5Na0.5)0.94Ba0.06TiO3 ceramics significantly [19]. However, few researchers had reported the effects of La2O3 on electrical properties of BNT-BKT system. Due to the good piezoelectric and ferroelectric properties [8], we choose 0.82BNT-0.18BKT ceramics to examine the effect of La2O3 on BNT-based ceramics. In this paper, the La2O3-doped 0.82BNT-0.18BKT ceramics were synthesized by the conventional solid-state reaction method. The effects of La2O3 addition on the phase composition, microstructure, dielectric and piezoelectric properties were investigated. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 221.2.209.101-06/07/11,11:51:56)
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Experimental Powders with a nominal composition of 0.82Bi0.5Na0.5TiO3-0.18Bi0.5K0.5TiO3+ x wt. % La2O3 (abbreviated to 0.82BNT-0.18BKT-La-x; x = 0.00, 0.25, 0.50, 0.75) were synthesized by a conventional solid state reaction method. Reagent grade oxide or carbonate powders of Na2CO3 (99.8%), K2CO3 (99%), TiO2 (99.5%), Bi2O3 (99.64%) and La2O3 (99.9%) (All raw materials were made by Sinopharm Chemical Reagent Co., LtdS) were used as starting materials. All the starting materials were mixed by ball milling for 8h and then were calcined at 850 °C for 2 h. After calcination, the mixture was ball milled again and mixed thoroughly with a poly vinylbutyral (PVB) binder solution and then pressed into 12 mm diameter and 1.5 mm thickness disks. After burning off PVB, the pellets were embedded into pre-prepared powder with similar composition and sintered in air in the temperature range of 1140-1160°C for 2h. The bulk density of the sintered samples was determined by the Archimedes method. The phase structure was examined by X-ray diffraction (XRD) analysis using a Cu Kα radiation (λ = 1.54178Å) (D8 Advance, Bruker Inc., Germany). The surface morphology of the ceramics was studied by scanning electron microscope (SEM) (JSM-5900, Japan). The average grain size in a 0.82BNT-0.18BKT-La-x ceramics is given by n
∑L
i
(2 2 0)
(2 1 1)
(2 0 0)
(1 1 1)
(1 0 0)
(1 1 0)
(1) NB In Eq.(1), G, n, Li, and NB each represent the average grain size in the disk, the total number of straight lines drawn on the SEM image(Fig. 4), the length of the ith straight line drawn on the SEM image, and the total number of grain boundaries, x = 0.75 which are intersected by all of the straight lines drawn on the SEM image, respectively[20]. x = 0.50 For the electrical measurements, silver paste was x = 0.25 coated on both sides of the sintered samples and fired at 740°C for 20 min to form electrodes. Dielectric x = 0.00 properties were measured using an Agilent 4294A precision impedance analyzer (Agilent Inc., 20 30 40 50 60 70 2θ(°) America) in the temperature range from room Fig. 1. XRD pattern of the temperature to 400°C. P-E hysteresis loops were 0.82BNT-0.18BKT-La-x ceramics recorded using an aix-ACCT TF2000FE-HV (b) ferroelectric test unit (aix-ACCT (a) Inc., Germany). For the measurement of piezoelectric properties, samples were poled in silicon oil at room temperature under 70-100 kV/cm for 15 min. The 5µm 5µm piezoelectric constant d33 was measured using a quasi-static d33 (c) (d) meter (YE2730 SINOCERA, China). The planar electromechanical coupling factor kp was obtained by a resonance-antiresonance method 5µm 5µm through an impedance analyzer (HP 4294A) on the basis of IEEE Fig. 2. SEM images of the 0.82BNT-0.18BKT-La-x ceramics standards. intensity(a.u.)
G=
i=1
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Materials and Design
2
0
P(µC/mm )
Results and discussion 90
Fig. 1 shows the XRD patterns of 0.25 0.5 60 0.82BNT-0.18BKT-La-x ceramics. All ceramics exhibit a 0.75 30 pure perovskite structure and no second phases can be 0 detected, which implies that the La3+ has diffused into the -75 -50 -25 0 25 50 75 E(kV/cm) -30 0.82BNT-0.18BKT lattices to form a solid solution. -60 Fig. 2 shows the micrographs of the sintered -90 0.82BNT-0.18BKT-La-x ceramics. It can be clearly seen that Fig. 3. P-E hysteresis loops of all the ceramics are very dense, with a relative density (measured by the Archimedesmethod) larger than 95%. 0.82BNT-0.18BKT-La-x ceramics 150 0.24 However, the addition of La2O3 leads to no obvious change -ο-d in the feature of grain shape. The clear and neat rectangular 0.22 --k 140 shapes can be obtained in all samples. 0.20 130 Fig. 3 depicts the P-E hysteresis loops of all specimens at 0.18 room temperature. The optimum values of the remanent 0.16 120 polarization (Pr) and the coercive field (Ec) are 75µC/cm2 and 0.14 110 45kV/cm, respectively, at 0.25 wt. % La2O3 addition. Compared 0.00 0.25 0.75 x 0.50 to pure BNT ceramics, the observed Ec of BNKT ceramics with Fig. 4. Variations of 0.25 wt. % La2O3 addition is decreased distinctly (from 73 d33, kp with different x kV/cm to 45kV/cm). As is well known, the low Ec will 3000 facilitate the poling process, making the x = 0.00 Tm x = 0.25 0.82BNT-0.18BKT ceramics doped with 0.25 wt. % La2O3 x = 0.50 x = 0.75 exhibit better piezoelectric properties. It is hence Td 2000 anticipated that the ceramics obtained at 0.25 wt. % La2O3 additions are advantageous for piezoelectric ceramics applications. The piezoelectric constant d33 and electromechanical 1000 coupling factor kp as a function of La2O3 content are 100 200 300 400 o illustrated in Fig. 4. The observed d33 and kp first increase T/ C Fig. 5. Temperature dependence and then decrease with the increasing La2O3 content. The of dielectric constant εr of maximums of d33 and kp are obtained at 0.25% La2O3 0.82BNT-0.18BKT-La-x system addition, which are 142pC/N and 0.23, respectively. This phenomenon is in accordance with the P-E loop results in Fig. 3. According to Shannon’s effective ionic radius [21], La is expected to substitute in the A-site. The substitution of La3+ for Bi3+ causes a great distortion in the crystal lattice, which will enhance the motion of the ferroelectric domains and result in the improvement of piezoelectric properties. Additionally, the substitution of La3+ for Na+ can lead to some A-site vacancies in the lattice, which facilitate the movement of the domains and thus improve the piezoelectric properties significantly [22-24]. Fig. 5 shows the temperature dependence of dielectric constant εr of 0.82BNT-0.18BKT-La-x ceramics at 100 kHz, while Td and Tm of 0.82BNT-0.18BKT-La-x ceramics as a function of x is similar to the BNT ceramic [2, 4], the 0.82BNT-0.18BKT ceramics exhibited two abnormal dielectric peaks at Td and Tm. After adding La2O3, both Td and Tm are strongly composition dependence. The maximums of Td and Tm are obtained at 0.25% La2O3 addition, which are 114 and 311 ºC, respectively. Besides, the highest εr values at Tm was obtained at x = 0.25 (εr =2614). It is much higher than that of pure 0.82BNT-0.18BKT ceramics (εr =1978), suggesting that the dielectric properties were enhanced by doping a small amount of La2O3. 33
εr
kp
d33
p
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Conclusions La2O3 doped 0.82BNT-0.18BKT ceramics have been prepared by conventional mixed-oxide method. The XRD patterns indicate that the 0.82BNT-0.18BKT ceramics doped with 0-0.75 wt. % La2O3 still show a pure perovskite structure. The doping of a small amount of La2O3 was effective in improving the densification. The piezoelectric properties of 0.82BNT-0.18BKT ceramics have been promoted by La2O3 doping. High magnitude of piezoelectric properties is obtained at 0.25wt. % La2O3, i. e. d33 =142 pC/N, kp = 0.23. Acknowledgements This work was supported by Ph. D. Programs Foundation of Shandong Province of China (No. BS2010CL010) and the National Natural Science Foundation of China (No. 50702068). References [1] G. A. Smolenskii, V. A. Isupov, A. I. Agranovskaya and N. N. Krainik, Sov. Phys. Solid State Vol. 2, (1961), p. 2651 [2] T. Takennaka, K. Maruyama and K. Sakata, Jpn. J. Appl. Phys. Vol. 30 (9B), (1991), p. 2230 [3] A. Herabut and A. Safari, J. Am. Ceram. Soc. Vol. 80, (1997), p. 2954 [4] C. G. Xu, D. M. Lin and K. W. Kwok, Solid State Sci. Vol. 10, (2008), p. 934 [5] Y. M. Li, W. Chen, J. Zhou, Q. Xu, H. J. Sun and R.X. Xu, Mater. Sci. Eng. B. Vol. 112, (2004), p. 395 [6] A. Sasaki, T. Chiba, Y. Mamiya and E. Otsuki, Jpn. J. Appl. Phys. Vol. 38, (1999), p. 5564 [7] W. Zhao, H.P. Zhou, Y.K. Yan and D. Liu, Key Eng. Mater. Vol. 368, (2008), p. 1908 [8] Z.Y. Yang, B. Liu, L.L. Wei and Y.T. Hou, Mater. Res. Bull. Vol. 43 (1), (2008), p. 81 [9] J. Shieh, K.C. Wu and C.S. Chen, Acta Mater. Vol. 55, (2007), p. 3081 [10] Y. Hiruma, H. Nagata and T. Takenaka, Ceram. Int. Vol. 35, (2009), p. 117 [11] D. M. Lin, D. Q. Xiao, J. G. Zhu and P. Yu, Mater. Lett. Vol. 58, (2004), p. 615 [12] D. M. Lin, D. Q. Xiao, J. G. Zhu and P. Yu, Appl. Phys. Lett. Vol. 88, (2006), p. 062901 [13] X. X. Wang, L. -W.C. Helen and C. -L. Choy, Solid State Commun. Vol. 125, (2003), p. 395 [14] D. W. Wang, D. Q. Zhang, J. Yuan, Q. L. Zhao, H. M. Liu, Z. Y. Wang and M. S. Cao, Chin. Phys. B Vol. 18, (2009), p. 2596 [15] Y. J. Zhang, R. Q. Chu, Z. J. Xu, J. G. Hao, Q. Chen, F. Peng, W. Li, G. R. Li and Q. R. Yin, J. Alloys Compd. Vol. 502, (2010), p. 341 [16] Y. M. Li, W. Chen, Q. Xu, J. Zhou, Y. Wang and H. J. Sun, Ceram. Int. Vol. 33, (2007), p. 95 [17] Z. P. Yang , Y. T. Hou, B. Liu and L. L. Wei, Ceram. Int. Vol. 35, (2009), p. 1423 [18] A. Herabut and A. Safari, J. Am. Ceram. Soc. Vol. 80, (1997), p. 2954 [19] P. Fu, Z. J. Xu, R. Q. Chu, W. Li, G. Z. Zang, J. G. Hao, Mater. Sci. Eng. B. Vol. 167, (2010), p. 161 [20] C. S. Chou, J. H. Chen, R. Y. Yang and S. W. Chou, Powder Technol. Vol. 202, 39 (2010), p. [21] R. D. Shannon, Acta. Cryst. A Vol. 32, (1976), p. 751 [22] P. Roy-Chowdhury and S. B. Deshpande, J. Mater. Sci. (USA) Vol. 22, (1987), p. 2209 [23] J. H. Shi and W. M. Yang, J. Alloys Compd. Vol. 472, (2009), p. 267 [24] Y. W. Liao, D. Q. Xiao, D. M. Lin, J. G. Zhu, P. Yu, L. Wu and X. P. Wang, Mater. Sci. Eng. B. Vol. 133, (2006), p. 172
Materials and Design doi:10.4028/www.scientific.net/AMR.284-286 Effect of La2O3 on the Microstructure and Electrical Properties of 0.82Bi0.5Na0.5TiO30.18Bi0.5K0.5TiO3 Ceramics doi:10.4028/www.scientific.net/AMR.284-286.1408
