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Polymorphic phase transition and enhanced piezoelectric properties in (Ba0.9Ca0.1)(Ti1−xSnx)O3 lead-free ceramics
Materials Letters 97 (2013) 86–89
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Polymorphic phase transition and enhanced piezoelectric properties in (Ba0.9Ca0.1)(Ti1 xSnx)O3 lead-free ceramics Mingli Chen a, Zhijun Xu a,n, Ruiqing Chu a, Yong Liu a, Lin Shao a, Wei Li a, Shuwen Gong b, Guorong Li c a
College of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, People’s Republic of China College of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng 252059, People’s Republic of China c Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, People’s Republic of China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 October 2012 Accepted 22 December 2012 Available online 31 December 2012
Lead-free (Ba0.9Ca0.1)(Ti1 xSnx)O3 (BCTS) (x ¼ 0.02–0.1) ceramics were fabricated using conventional solid state reaction method. The effects of Sn substitution on structure and electrical properties of the BCTS ceramics were researched. At room temperature, a polymorphic phase transition (PPT) from tetragonal phase to rhombohedral phase is identified from XRD patterns with increasing Sn content. High piezoelectric coefficient of d33 ¼ 405 pC/N and planar mode electromechanical coupling factor of kp ¼43.2% are obtained for the sample at x ¼ 0.06. The ceramic for x ¼ 0.06 also shows uniform microstructure that contributes to the excellent electrical properties. These results indicate that this BCTS system with optimal composition is a promising lead-free piezoelectric material. & 2012 Elsevier B.V. All rights reserved.
Keywords: PPT Electrical properties Piezoelectric materials Lead-free Diffuse phase transition
1. Introduction In recent years researchers made concerted efforts to develop lead-free piezoceramics due to the environmental concerns and restrictions [1–4]. However, lead-free ceramics generally have inferior piezoelectric coefficients (d33 o300 pC/N in most cases) compared to that of the most desired PZTs (d33 ¼300 600 pC/N) [1]. Shifting orthorhombic–tetragonal (O–T) phase transition of BaTiO3 at around 51c towards near room temperature to form orthorhombic–tetragonal phase coexistence at around room temperature and optimizing processing conditions are two important ways for enhancing piezoelectric properties of BaTiO3-based piezoceramics, like the ways in KNN-based ceramics [5]. Jaffe summarized that Sn4 þ and Zr4 þ shifted the orthorhombic–tetragonal phase transition temperature (TO-T) and rhombohedralorthorhombic phase transition temperature (TR-O) of BaTiO3 towards higher temperature and at the same time lowered its Curie temperature (TC) [6], sometimes forming the pinched phase transition [7]. And Ca2 þ could shift the TO-T of BaTiO3 to lower temperature and just slightly influence its Curie temperature [6]. These ions can diffuse into BaTiO3 lattice and change its phase transition temperature when they are doped into BaTiO3 within the solubility limit. Recently the newly discovered (Ba,Ca) (Ti,Sn)O3 system by Xue et al. [2] draws much attention from researchers [8,9]. Xue et al. [2] found that the electrical properties
n
Corresponding author. Tel./fax: þ 866358230923. E-mail addresses: [email protected], [email protected] (Z. Xu).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.12.067
of the ceramics prepared using conventional solid state reaction technique could show anomaly at around the optimal composition 30BCT (with d33 ¼530 pC/N, room temperature er ¼3800). They attributed the properties anomaly to the intrinsic polarization rotation and polarization extension and extrinsic domain wall contribution. Researchers also found the Sn4 þ substitution influenced the dielectric properties and caused diffuse phase transition in BaTiO3-based ceramics [10–12]. We designed this lead-free piezoceramic formula (Ba0.9Ca0.1)(Ti1 xSnx)O3 (x¼0.02–0.1). We hope to find the similarities and differences of the effects of Sn4þ substitution between this system and other BaTiO3-based systems. In our work, we investigate the phase structure, microstructure, piezoelectric properties, dielectric properties and diffuse phase transition of (Ba0.9Ca0.1)(Ti1 xSnx)O3 ceramics. 2. Experimental procedure (Ba0.9Ca0.1)(Ti1 xSnx)O3 (BCTS) (x ¼0.02–0.1) ceramics were prepared by conventional solid state reaction technique. Raw materials of analytical-reagent BaCO3 (99.0%), CaCO3 (99.0%), TiO2 (99.5%) and SnO2 (99.0%) were mixed with addition of distilled water. The mixtures were then ball milled and dried. The dried powders were pressed into 35 mm-diameter pellets and calcined at 11501c for 4 h. Thereafter, calcined pellets were triturated and ball milled again. The dried powders were pressed into 12 mm-diameter pellets with the addition of PVB. After burning out PVB, the pellets were sintered at 1450 1C for 4 h in air. Silver electrode was fired on both surfaces of the pellets for electrical
M. Chen et al. / Materials Letters 97 (2013) 86–89
(202)
(220)
(220) (202)
(211)
(210)
(111)
(002) (200)
(110)
Fig. 1 shows the X-ray diffraction patterns of the (Ba0.9Ca0.1)(Ti1 xSnx)O3 ceramics with different Sn contents. All the compositions show a single phase perovskite structure and no impurity phases are found within the XRD detection limit. So Sn element has successfully diffused into matrix perovskite lattice. On the solubility limit of SnO2, Coffeen [13] pointed out that the solubility limit of SnO2 in BaTiO3 is above 45 mol%, and SnO2 could diffuse into the BaTiO3 lattice to form a solid solution within the solubility limit 45 mol%. The single perovskite phase here indicates our maximum doping level 10% is within the solubility limit of Sn element in (Ba0.9Ca0.1)TiO3. The enlarged patterns in the 2y ranges from 441 to 471 and from 661 to 671 give a clearer description. The ceramic at x ¼0.02 possesses tetragonal phase that is characterized by the splitting of the (2 0 0)/(0 0 2) peaks at around 2y of 45.51 [8]. With increasing Sn content, the (2 0 0)/(0 0 2) peaks at 2y of 45.51 merge into a single peak. The crystal structure of the ceramic at x¼0.1 is rhombohedral phase, featured with single (2 0 0) peak at 2y of 45.51and splitting of the (2 2 0)/(2 0 2)peaks at 2y of 661 [14]. Therefore it can be suggested that at room temperature a phase transition from tetragonal phase to rhombohedral phase occurs with increasing Sn content and it is tetragonal phase and rhombohedral phase coexistent in the composition range of 0.04ox o0.1. Fig. 2 shows the SEM micrographs of the (Ba0.9Ca0.1)(Ti1 x Snx)O3 ceramics at x¼0.02–0.10. Obvious pores exit at the grain boundary for x ¼0.02. The BCTS ceramic at x ¼0.04 forms bimodal microstructure with big grains surrounded by many small grains, which possibly results from discontinuous grain growth [15]. When x¼0.06, the microstructure becomes significantly homogeneous.
(200)
3. Results and discussion
(002)
It is shown that the ceramic has clear grain boundary and dense microstructure that is advantageous to achieve excellent electrical properties. However the microstructure of ceramics with adding larger Sn content is inhomogeneous again. The temperature dependence of dielectric constant for the BCTS ceramics at different frequencies (0.2 kHz 100 kHz) is shown in Fig. 3. The peak value of dielectric constant (abbreviated to em) corresponds to the ferroelectric–paraelectric phase transition and the corresponding temperature is Tm. The em firstly increases with the increase of Sn content, reaches the maximum at x ¼0.08, and then slightly decreases with higher Sn amount. Tm decreases obviously with Sn content increasing within our discussion range. The inset in Fig. 3 shows the change trend of Tm with the Sn content. The Tm changes almost linearly. We make the linear fitting of Tm. The linear fitting equation is Tm ¼ 962x þ135. The equation shows that Tm decreases linearly with a change rate of 201c per 2 mol %. Similar phenomenon can be found in literature [16]. It is discovered the dielectric peak broadens when x Z0.08 which is a character of diffuse phase transition behavior. The broadening of dielectric peak results from spatial composition fluctuation or polarization fluctuation in the bulk ceramic [17]. The diffuse phase transition behavior is also confirmed by frequency dispersion which is the frequency dependence of dielectric constant around and below the transition temperature [18]. From Fig. 3, the frequency dispersion is obvious for xZ0.08 that further indicates the diffuse phase transition behavior. Fig. 4 illustrates the piezoelectric coefficient d33 and planar mode electromechanical coupling coefficient kp curves of the (Ba0.9Ca0.1)(Ti1 xSnx)O3 ceramics as a function of Sn content. It can be observed that at x¼0.06 the ceramic show improved piezoelectric properties with the d33 and kp curves reaching peak values of 405 pC/N and 43.2%, respectively. XRD patterns of BCTS ceramics indicate a polymorphic phase transition (PPT) from tetragonal phase to rhombohedral phase in the composition range 0.04ox o0.10. It is believed that the observed high piezoelectric coefficient at x¼ 0.06 should be ascribed to the tetragonal– rhombohedral phase coexistence at room temperature. The phase transition induces structural instability so that the enhancement of piezoelectric property can be achieved [5]. The homogeneous microstructure also contributes to the enhanced electrical properties.
measurements. Crystal structure was examined using an X-ray diffractometer with a Cu Ka radiation (D8 Advance, Bruker Inc., Germany). Surface microstructure of the specimens was examined using scanning electron microscope (SEM) (JSM-5900, Japan). Dielectric properties were measured using the precision impedance analyzer (4294A Agilent Inc., Malaysia). The ceramics were poled under a DC field of 3–4 kV/mm in a silicon oil bath for 15 min at room temperature. Piezoelectric constant d33 of the poled ceramics was measured using a quasi-static meter d33 meter (YE2730 SINOCERA, China).
(100)
87
x=0.1
Intensity
x=0.08 x=0.06
x=0.04
x=0.02 20
30
40
50
60
70 44 45 46 47 65 2 Theta/(Degrees)
Fig. 1. X-ray diffraction patterns of the (Ba0.9Ca0.1)(Ti1 xSnx)O3 ceramics.
66
67
88
M. Chen et al. / Materials Letters 97 (2013) 86–89
Fig. 2. SEM micrographs of the (Ba0.9Ca0.1)(Ti1 xSnx)O3 ceramics sintered at 1450 1C ((A), x ¼0.02; (B), x¼ 0.04; (C), x¼ 0.06; (D), x ¼0.08; (E), x¼ 0.10).
18000 16000
x=0.1
14000
400
100 80 60
d33 kp
42
350
0.2kHz
20 x=0.06
1kHz
10000
0.02
10kHz
0.04 0.06 0.08 Sn content x=0.04
0.10
x=0.02
100kHz
8000
40 300
kp(%)
40
12000
d33(pC/N)
Dielectric constant
44
120 x=0.08
38
250
6000
2000 0
36
200
4000
150
34 0.02
20
40
60 80 Temperature (°C)
100
120
Fig. 3. Temperature dependence of dielectric constant for the (Ba0.9Ca0.1) (Ti1 xSnx)O3 ceramics at 0.2 kHz, 1 kHz, 10 kHz and 100 kHz. The inset shows Tm as a function of Sn content and the linear fitting of Tm.
0.04
0.06 0.08 Sn content x
0.10
Fig. 4. Piezoelectric coefficient d33 and planar mode electromechanical coupling coefficient kp of the (Ba0.9Ca0.1)(Ti1 xSnx)O3 ceramics as a function of Sn content x.
gradually exhibit diffuse phase transition behavior with increasing the addition of Sn.
4. Conclusion At room temperature, a polymorphic phase transition (PPT) from tetragonal phase to rhombohedral phase is identified for the (Ba0.9Ca0.1)(Ti1 xSnx)O3 ceramics with increasing Sn content. At PPT composition of x¼ 0.06, the BCTS ceramic exhibits excellent piezoelectric properties with high piezoelectric coefficient of d33 ¼ 405 pC/N and planar electromechanical coupling factor of kp¼ 43.2%. The dielectric constant data shows the BCTS ceramics
Acknowledgements This work was supported by the National High Technology Research and Development Program of China (No. 2013AA030501), the Natural Science Foundation of Shandong Province of China (No. ZR2012EMM004), the Ph.D. Programs Foundations of Shandong
M. Chen et al. / Materials Letters 97 (2013) 86–89
Province of China (No. Bs2010CL010), the Natural Science Foundation of Shandong Province of China (No. Zr2011EMQ015). References [1] Saito Y, Takao H, Tani T, Nonoyama T, Takatori K, Homma1 T, et al. Lead-free piezoceramics. Nature 2004;432:84–7. [2] Xue Dezhen, Zhou Yumei, Bao Huixin, Gao Jinghui, Zhou Chao, Ren Xiaobing. Large piezoelectric effect in Pb-free Ba(Ti,Sn)O3 x(Ba,Ca)TiO3 ceramics. Appl Phys Lett 2011;99:122901. [3] EU-Directive 2002/96/EC: waste electrical and electronic equipment (WEEE). Off J Eur Union 2003;46 [L37]:24–38. [4] EU-Directive 2002/95/EC: restriction of the use of certain hazardous substances in electrical and electronic equipment (RoHS). Off J Eur Union 2003;46 [L37]:19–23. ¨ ¨ [5] Rodel Jurgen, Jo Wook, Seifert Klaus T P, Anton Eva-Maria, Granzow Torsten, Damjanovic Dragan. Perspective on the development of lead-free piezoceramics. J Am Ceram Soc 2009;92:1153–77. [6] Jaffe B, Cook W R, Jaffe H. Piezoelectric ceramics. London: Academic; 1971. [7] Yu Zhi, Ang Chen, Guo Ruyan, Bhalla AS. Dielectric properties of Ba(Ti1 xZrx)O3 solid solutions. Mater Lett 2007;61:326–9. [8] Li Wei, Xu Zhijun, Chu Ruiqing, Fu Peng, Zang Guozhong. Enhanced ferroelectric properties in (Ba1 xCax)(Ti0.94Sn0.06)O3 lead-free ceramics. J Eur Ceram Soc 2012;32:517–20.
89
[9] Li Wei, Xu Zhijun, Chu Ruiqing, Fu Peng, Zang Guozhong. Large piezoelectric coefficient in (Ba1 xCax)(Ti0.96Sn0.04)O3 lead-free ceramics. J Am Ceram Soc 2011;94:4131–3. [10] Wei XY, Yao X. Preparation, structure and dielectric property of barium stannate titanate ceramics. Mater Sci Eng, B 2007;137:184–8. [11] Shvartsman VV, Kleemann W, Dec J, Xu Z K, Lu S G. Diffuse phase transition in BaTi1 xSnxO3 ceramics: an intermediate state between ferroelectric and relaxor behavior. J Appl Phys 2006;99:124111. [12] Cai Wei, Fan Yize, Gao Jiacheng, Fu Chunlin, Deng Xiaoling. Microstructure, dielectric properties and diffuse phase transition of barium stannate titanate ceramics. J Mater Sci: Mater Electron 2011;22:265–72. [13] Coffeen William W. Dielectric bodies in the quaternary system BaTiO3– BaSnO3–SrSnO3–CaSnO3. J Am Ceram Soc 1953;36:215–21. [14] Liu Wenfeng, Ren Xiaobing. Large piezoelectric effect in Pb-free ceramics. Phys Rev Lett 2009;103:257602. [15] Frey M H, Payne D A. Grain-size effect on structure and phase transformations for barium titanate. Phys Rev B 1996;54:3158–68. [16] Gu H H, Chen X M, Qin N. Tunable dielectric characteristics of (Ba0.95Ca0.05)(Ti1 ySny)O3 ferroelectric ceramics. J Electroceram 2008;21:495–8. [17] Hegenbartha. E. The influence of hydrostatic pressure on the phase transitions of some ferroelectrics. Ferroelectrics 1978;20:79–86. [18] Tang XG, Chew K H, Chan HLW. Diffuse phase transition and dielectric tunability of Ba(ZryTi1 y)O3 relaxor ferroelectric ceramics. Acta Mater 2004;52:5177–83.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Polymorphic phase transition and enhanced piezoelectric properties in (Ba0.9Ca0.1)(Ti1 xSnx)O3 lead-free ceramics Mingli Chen a, Zhijun Xu a,n, Ruiqing Chu a, Yong Liu a, Lin Shao a, Wei Li a, Shuwen Gong b, Guorong Li c a
College of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, People’s Republic of China College of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng 252059, People’s Republic of China c Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, People’s Republic of China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 October 2012 Accepted 22 December 2012 Available online 31 December 2012
Lead-free (Ba0.9Ca0.1)(Ti1 xSnx)O3 (BCTS) (x ¼ 0.02–0.1) ceramics were fabricated using conventional solid state reaction method. The effects of Sn substitution on structure and electrical properties of the BCTS ceramics were researched. At room temperature, a polymorphic phase transition (PPT) from tetragonal phase to rhombohedral phase is identified from XRD patterns with increasing Sn content. High piezoelectric coefficient of d33 ¼ 405 pC/N and planar mode electromechanical coupling factor of kp ¼43.2% are obtained for the sample at x ¼ 0.06. The ceramic for x ¼ 0.06 also shows uniform microstructure that contributes to the excellent electrical properties. These results indicate that this BCTS system with optimal composition is a promising lead-free piezoelectric material. & 2012 Elsevier B.V. All rights reserved.
Keywords: PPT Electrical properties Piezoelectric materials Lead-free Diffuse phase transition
1. Introduction In recent years researchers made concerted efforts to develop lead-free piezoceramics due to the environmental concerns and restrictions [1–4]. However, lead-free ceramics generally have inferior piezoelectric coefficients (d33 o300 pC/N in most cases) compared to that of the most desired PZTs (d33 ¼300 600 pC/N) [1]. Shifting orthorhombic–tetragonal (O–T) phase transition of BaTiO3 at around 51c towards near room temperature to form orthorhombic–tetragonal phase coexistence at around room temperature and optimizing processing conditions are two important ways for enhancing piezoelectric properties of BaTiO3-based piezoceramics, like the ways in KNN-based ceramics [5]. Jaffe summarized that Sn4 þ and Zr4 þ shifted the orthorhombic–tetragonal phase transition temperature (TO-T) and rhombohedralorthorhombic phase transition temperature (TR-O) of BaTiO3 towards higher temperature and at the same time lowered its Curie temperature (TC) [6], sometimes forming the pinched phase transition [7]. And Ca2 þ could shift the TO-T of BaTiO3 to lower temperature and just slightly influence its Curie temperature [6]. These ions can diffuse into BaTiO3 lattice and change its phase transition temperature when they are doped into BaTiO3 within the solubility limit. Recently the newly discovered (Ba,Ca) (Ti,Sn)O3 system by Xue et al. [2] draws much attention from researchers [8,9]. Xue et al. [2] found that the electrical properties
n
Corresponding author. Tel./fax: þ 866358230923. E-mail addresses: [email protected], [email protected] (Z. Xu).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.12.067
of the ceramics prepared using conventional solid state reaction technique could show anomaly at around the optimal composition 30BCT (with d33 ¼530 pC/N, room temperature er ¼3800). They attributed the properties anomaly to the intrinsic polarization rotation and polarization extension and extrinsic domain wall contribution. Researchers also found the Sn4 þ substitution influenced the dielectric properties and caused diffuse phase transition in BaTiO3-based ceramics [10–12]. We designed this lead-free piezoceramic formula (Ba0.9Ca0.1)(Ti1 xSnx)O3 (x¼0.02–0.1). We hope to find the similarities and differences of the effects of Sn4þ substitution between this system and other BaTiO3-based systems. In our work, we investigate the phase structure, microstructure, piezoelectric properties, dielectric properties and diffuse phase transition of (Ba0.9Ca0.1)(Ti1 xSnx)O3 ceramics. 2. Experimental procedure (Ba0.9Ca0.1)(Ti1 xSnx)O3 (BCTS) (x ¼0.02–0.1) ceramics were prepared by conventional solid state reaction technique. Raw materials of analytical-reagent BaCO3 (99.0%), CaCO3 (99.0%), TiO2 (99.5%) and SnO2 (99.0%) were mixed with addition of distilled water. The mixtures were then ball milled and dried. The dried powders were pressed into 35 mm-diameter pellets and calcined at 11501c for 4 h. Thereafter, calcined pellets were triturated and ball milled again. The dried powders were pressed into 12 mm-diameter pellets with the addition of PVB. After burning out PVB, the pellets were sintered at 1450 1C for 4 h in air. Silver electrode was fired on both surfaces of the pellets for electrical
M. Chen et al. / Materials Letters 97 (2013) 86–89
(202)
(220)
(220) (202)
(211)
(210)
(111)
(002) (200)
(110)
Fig. 1 shows the X-ray diffraction patterns of the (Ba0.9Ca0.1)(Ti1 xSnx)O3 ceramics with different Sn contents. All the compositions show a single phase perovskite structure and no impurity phases are found within the XRD detection limit. So Sn element has successfully diffused into matrix perovskite lattice. On the solubility limit of SnO2, Coffeen [13] pointed out that the solubility limit of SnO2 in BaTiO3 is above 45 mol%, and SnO2 could diffuse into the BaTiO3 lattice to form a solid solution within the solubility limit 45 mol%. The single perovskite phase here indicates our maximum doping level 10% is within the solubility limit of Sn element in (Ba0.9Ca0.1)TiO3. The enlarged patterns in the 2y ranges from 441 to 471 and from 661 to 671 give a clearer description. The ceramic at x ¼0.02 possesses tetragonal phase that is characterized by the splitting of the (2 0 0)/(0 0 2) peaks at around 2y of 45.51 [8]. With increasing Sn content, the (2 0 0)/(0 0 2) peaks at 2y of 45.51 merge into a single peak. The crystal structure of the ceramic at x¼0.1 is rhombohedral phase, featured with single (2 0 0) peak at 2y of 45.51and splitting of the (2 2 0)/(2 0 2)peaks at 2y of 661 [14]. Therefore it can be suggested that at room temperature a phase transition from tetragonal phase to rhombohedral phase occurs with increasing Sn content and it is tetragonal phase and rhombohedral phase coexistent in the composition range of 0.04ox o0.1. Fig. 2 shows the SEM micrographs of the (Ba0.9Ca0.1)(Ti1 x Snx)O3 ceramics at x¼0.02–0.10. Obvious pores exit at the grain boundary for x ¼0.02. The BCTS ceramic at x ¼0.04 forms bimodal microstructure with big grains surrounded by many small grains, which possibly results from discontinuous grain growth [15]. When x¼0.06, the microstructure becomes significantly homogeneous.
(200)
3. Results and discussion
(002)
It is shown that the ceramic has clear grain boundary and dense microstructure that is advantageous to achieve excellent electrical properties. However the microstructure of ceramics with adding larger Sn content is inhomogeneous again. The temperature dependence of dielectric constant for the BCTS ceramics at different frequencies (0.2 kHz 100 kHz) is shown in Fig. 3. The peak value of dielectric constant (abbreviated to em) corresponds to the ferroelectric–paraelectric phase transition and the corresponding temperature is Tm. The em firstly increases with the increase of Sn content, reaches the maximum at x ¼0.08, and then slightly decreases with higher Sn amount. Tm decreases obviously with Sn content increasing within our discussion range. The inset in Fig. 3 shows the change trend of Tm with the Sn content. The Tm changes almost linearly. We make the linear fitting of Tm. The linear fitting equation is Tm ¼ 962x þ135. The equation shows that Tm decreases linearly with a change rate of 201c per 2 mol %. Similar phenomenon can be found in literature [16]. It is discovered the dielectric peak broadens when x Z0.08 which is a character of diffuse phase transition behavior. The broadening of dielectric peak results from spatial composition fluctuation or polarization fluctuation in the bulk ceramic [17]. The diffuse phase transition behavior is also confirmed by frequency dispersion which is the frequency dependence of dielectric constant around and below the transition temperature [18]. From Fig. 3, the frequency dispersion is obvious for xZ0.08 that further indicates the diffuse phase transition behavior. Fig. 4 illustrates the piezoelectric coefficient d33 and planar mode electromechanical coupling coefficient kp curves of the (Ba0.9Ca0.1)(Ti1 xSnx)O3 ceramics as a function of Sn content. It can be observed that at x¼0.06 the ceramic show improved piezoelectric properties with the d33 and kp curves reaching peak values of 405 pC/N and 43.2%, respectively. XRD patterns of BCTS ceramics indicate a polymorphic phase transition (PPT) from tetragonal phase to rhombohedral phase in the composition range 0.04ox o0.10. It is believed that the observed high piezoelectric coefficient at x¼ 0.06 should be ascribed to the tetragonal– rhombohedral phase coexistence at room temperature. The phase transition induces structural instability so that the enhancement of piezoelectric property can be achieved [5]. The homogeneous microstructure also contributes to the enhanced electrical properties.
measurements. Crystal structure was examined using an X-ray diffractometer with a Cu Ka radiation (D8 Advance, Bruker Inc., Germany). Surface microstructure of the specimens was examined using scanning electron microscope (SEM) (JSM-5900, Japan). Dielectric properties were measured using the precision impedance analyzer (4294A Agilent Inc., Malaysia). The ceramics were poled under a DC field of 3–4 kV/mm in a silicon oil bath for 15 min at room temperature. Piezoelectric constant d33 of the poled ceramics was measured using a quasi-static meter d33 meter (YE2730 SINOCERA, China).
(100)
87
x=0.1
Intensity
x=0.08 x=0.06
x=0.04
x=0.02 20
30
40
50
60
70 44 45 46 47 65 2 Theta/(Degrees)
Fig. 1. X-ray diffraction patterns of the (Ba0.9Ca0.1)(Ti1 xSnx)O3 ceramics.
66
67
88
M. Chen et al. / Materials Letters 97 (2013) 86–89
Fig. 2. SEM micrographs of the (Ba0.9Ca0.1)(Ti1 xSnx)O3 ceramics sintered at 1450 1C ((A), x ¼0.02; (B), x¼ 0.04; (C), x¼ 0.06; (D), x ¼0.08; (E), x¼ 0.10).
18000 16000
x=0.1
14000
400
100 80 60
d33 kp
42
350
0.2kHz
20 x=0.06
1kHz
10000
0.02
10kHz
0.04 0.06 0.08 Sn content x=0.04
0.10
x=0.02
100kHz
8000
40 300
kp(%)
40
12000
d33(pC/N)
Dielectric constant
44
120 x=0.08
38
250
6000
2000 0
36
200
4000
150
34 0.02
20
40
60 80 Temperature (°C)
100
120
Fig. 3. Temperature dependence of dielectric constant for the (Ba0.9Ca0.1) (Ti1 xSnx)O3 ceramics at 0.2 kHz, 1 kHz, 10 kHz and 100 kHz. The inset shows Tm as a function of Sn content and the linear fitting of Tm.
0.04
0.06 0.08 Sn content x
0.10
Fig. 4. Piezoelectric coefficient d33 and planar mode electromechanical coupling coefficient kp of the (Ba0.9Ca0.1)(Ti1 xSnx)O3 ceramics as a function of Sn content x.
gradually exhibit diffuse phase transition behavior with increasing the addition of Sn.
4. Conclusion At room temperature, a polymorphic phase transition (PPT) from tetragonal phase to rhombohedral phase is identified for the (Ba0.9Ca0.1)(Ti1 xSnx)O3 ceramics with increasing Sn content. At PPT composition of x¼ 0.06, the BCTS ceramic exhibits excellent piezoelectric properties with high piezoelectric coefficient of d33 ¼ 405 pC/N and planar electromechanical coupling factor of kp¼ 43.2%. The dielectric constant data shows the BCTS ceramics
Acknowledgements This work was supported by the National High Technology Research and Development Program of China (No. 2013AA030501), the Natural Science Foundation of Shandong Province of China (No. ZR2012EMM004), the Ph.D. Programs Foundations of Shandong
M. Chen et al. / Materials Letters 97 (2013) 86–89
Province of China (No. Bs2010CL010), the Natural Science Foundation of Shandong Province of China (No. Zr2011EMQ015). References [1] Saito Y, Takao H, Tani T, Nonoyama T, Takatori K, Homma1 T, et al. Lead-free piezoceramics. Nature 2004;432:84–7. [2] Xue Dezhen, Zhou Yumei, Bao Huixin, Gao Jinghui, Zhou Chao, Ren Xiaobing. Large piezoelectric effect in Pb-free Ba(Ti,Sn)O3 x(Ba,Ca)TiO3 ceramics. Appl Phys Lett 2011;99:122901. [3] EU-Directive 2002/96/EC: waste electrical and electronic equipment (WEEE). Off J Eur Union 2003;46 [L37]:24–38. [4] EU-Directive 2002/95/EC: restriction of the use of certain hazardous substances in electrical and electronic equipment (RoHS). Off J Eur Union 2003;46 [L37]:19–23. ¨ ¨ [5] Rodel Jurgen, Jo Wook, Seifert Klaus T P, Anton Eva-Maria, Granzow Torsten, Damjanovic Dragan. Perspective on the development of lead-free piezoceramics. J Am Ceram Soc 2009;92:1153–77. [6] Jaffe B, Cook W R, Jaffe H. Piezoelectric ceramics. London: Academic; 1971. [7] Yu Zhi, Ang Chen, Guo Ruyan, Bhalla AS. Dielectric properties of Ba(Ti1 xZrx)O3 solid solutions. Mater Lett 2007;61:326–9. [8] Li Wei, Xu Zhijun, Chu Ruiqing, Fu Peng, Zang Guozhong. Enhanced ferroelectric properties in (Ba1 xCax)(Ti0.94Sn0.06)O3 lead-free ceramics. J Eur Ceram Soc 2012;32:517–20.
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