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Polymorphic phase transition and piezoelectric properties of (Ba1−xCax)(Ti0.9Zr0.1)O3 lead-free ceramics
Physica B 405 (2010) 4513–4516
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Polymorphic phase transition and piezoelectric properties of (Ba1 xCax)(Ti0.9Zr0.1)O3 lead-free ceramics Wei Li, Zhijun Xu n, Ruiqing Chu, Peng Fu, Guozhong Zang College of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, China
a r t i c l e in fo
abstract
Article history: Received 15 June 2010 Received in revised form 4 August 2010 Accepted 6 August 2010
Lead-free (Ba1 xCax)(Ti0.9Zr0.1)O3 (x ¼ 0.12–0.18) (BCZT) ceramics were prepared successfully using a solid-state reaction technique. The polymorphic phase transitions (PPT) from rhombohedral to orthorhombic phase around room temperature were identified in the composition range 0.14 o x o 0.18. The samples at x ¼ 0.16, showing coexistence of rhombohedral and orthorhombic phase, exhibit enhanced piezoelectric and dielectric properties of d33 ¼ 328 pC/N, kp ¼ 37.6% and e0 ¼ 4800. With the increase of Ca content, the polymorphic phase transitions shift to lower temperature and cannot be observed above room temperature at x Z0.18. & 2010 Elsevier B.V. All rights reserved.
Keywords: Lead-free ceramics Ferroelectrics Dielectrics Phase transitions
1. Introduction It is well-known that lead zirconate titanate (PZT) ceramics are the most widely used piezoelectric materials due to their superior piezoelectric properties close to the morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases. Nevertheless, they are not environmental friendly for their lead oxide toxicity. With the recent growing demand of global environment protection, many researchers have greatly focused on lead-free ceramics to replace the lead-based ceramics [1–3]. Barium titanate (BaTiO3), which is one of the most widely studied lead-free piezoelectric materials [4–10], have five kinds of crystal systems: hexagonal, cubic, tetragonal, orthorhombic and rhombohedral, depending on the phase transition temperatures: 1432, 130, 5 and 90 1C, respectively [5]. It is known that the doping is an effective way to improve the material performance in electroceramics. In particular, BaTi1-xZrxO3 (BZT) ceramics were extensively investigated and show promising piezoelectric/electrostrictive properties [6–11]. The transition temperatures of BZT (rhombohedral–orthorhombic TR–O, orthorhombic–tetragonal TO– T and tetragonal–cubic TC) move closer with increase in Zr content in BZT ceramics and merge near room temperature for the composition of x ¼0.15. Further increase in Zr content, especially for x4 0.25, the samples show broad dielectric peaks with frequency dispersion, i.e., ferroelectric-relaxor behavior [6–8]. As reported in Refs. [9–11], the polymorphic phase transition plays
n
Corresponding author. Tel./fax: + 86 6358230923. E-mail address: [email protected] (Z. Xu).
0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.08.028
an important role in the dielectric and piezoelectric properties. The enhanced piezoelectric properties are observed as a consequence of compositionally shifting the polymorphic TO–T transition downwards near room temperature for BZT based ceramics. Recently, it was reported that Ba1 xCaxTiO3 ceramics showed a large piezoelectric/electrostrictive strain and multiple functions [12–14]. However, little attention has been paid to the piezoelectric properties of the (Ba1 xCax)(Ti0.9Zr0.1)O3 (BCZT) ceramics. In this paper, the effect of Ca addition on phase structure, microstructure, piezoelectric properties and dielectric properties of the lead-free (Ba1 xCax)(Ti0.9Zr0.1)O3 ceramics are studied systematically.
2. Experimental procedure The (Ba1 xCax)(Ti0.9Zr0.1)O3 (BCZT) ceramics at x¼0.12, 0.14, 0.16 and 0.18 were prepared by conventional solid-state reaction technique. Raw materials of BaCO3 (99.0%), CaCO3 (99.0%), ZrO2 (99.0%) and TiO2 (99.5%) were mixed with addition of anhydrous ethanol, then dried and calcined at 1200 1C for 4 h. For the purpose of refining powders and facilitating diffusion progress, the powders were placed into a nylon bottle and remixed at 140 rpm for 10 h using ZrO2 milling media. After drying, the obtained powders were pressed into 12 mm-diameter pellets and sintered at 1450 1C for 4 h in air. The sample structure or phase development was examined using an X-ray diffraction meter ˚ (D8 Advance, Bruker Inc., using a Cu Ka radiation (l ¼1.54178 A) Germany). The dielectric properties were measured by the precision impedance analyzer (4294A Agilent Inc., America)
W. Li et al. / Physica B 405 (2010) 4513–4516
controlled by a computer at 100 kHz with the testing temperature ranged from room temperature to 200 1C. Ferroelectric hysteresis loops were measured at room temperature using an aix-ACCT TF2000FE-HV ferroelectric test unit (aix-ACCT Inc., Germany). The piezoelectric constant d33 was measured using a tester quasistatic d33 meter (YE2730 SINOCERA, China). The mechanical quality factor Qm and the planar electromechanical coupling factor kp were calculated following IEEE standards [15] using the impedance analyzer, the expression is k2p 1k2p
¼
Df f ð1 þ sp Þ
h i ðsp Þ2 1 þ Z21 ,
ð1Þ
where Df is the difference between the lowest frequency of antiresonance and resonance, and Z1 is the lowest root. J1 ðZ1 Þ ¼ 1sP
ð2Þ
9000 x = 0.12 x = 0.14 x = 0.16 x = 0.18
8000 Dielectric constant
4514
7000 6000 5000 4000 3000 2000 1000 20
40
60
80
100
120
140
Temperature (°C) Fig. 2. Temperature dependence of dielectric constant for the (Ba1 xCax)(Ti0.9Zr0.1)O3 ceramics of x¼ 0.12, 0.14, 0.16 and 0.18 measured at 100 kHz.
3. Results and discussion
220 202
220 202
211 210
Intensity (a.u.)
100
111
200
110
Fig. 1 shows the XRD patterns of the BCZT ceramics with different compositions. It can be seen that all the samples show pure perovskite structure, suggesting that Ca and Zr diffuse into the BaTiO3 lattice to form a solid solution. It is also found that the positions of the diffraction peak of the BCZT ceramics shift to higher angles with increasing Ca content. It is thought that the Ca2 + substitutions induce the distortion and shrinkage of the lattice parameters, which may be attributed to the smaller ionic ˚ than that of Ba2 + (1.61 A) ˚ [12]. At room radii of Ca2 + (1.34 A) temperature, the BCZT ceramics at x ¼0.12 and x ¼0.14 possess rhombohedral phase, which is characterized by single (2 0 0) peaks at around 2y of 451 and splitting of the (2 2 0)/(2 0 2) peaks at around 2y of 661 [9,16]. The BCZT ceramics become orthorhombic phase, featured with the merging of (2 2 0)/(2 0 2) peaks at around 2y of 661, with increasing Ca content. The crystal structure of the BCZT ceramics presents only orthorhombic phase featured with single peak at 2y of 451 and 661, when xZ0.18. Therefore, it can be suggested that the PPT of the BCZT system exists in the composition range 0.14oxo0.18 at room temperature, where rhombohedral and orthorhombic phases coexist. The temperature dependence of dielectric constants for the BCZT ceramics measured at frequency of 100 kHz is shown in Fig. 2. As can be seen, the BCZT ceramics at x¼0.12 exhibit two obvious polymorphic phase transitions corresponding to the orthorhombic–tetragonal (TO–T) at 40 1C and tetragonal–cubic
x = 0.18 x = 0.16 x = 0.14 x = 0.12
20
30
40
50
60
70 65
66
67
2 Theta/(Degrees) Fig. 1. X-ray diffraction patterns of (Ba1 xCax)(Ti0.9Zr0.1)O3 ceramics for x ¼0.12, 0.14, 0.16 and 0.18.
transitions (TC) at 65 1C [7,8]. The third polymorphic phase transition rhombohedral–orthorhombic (TR–O) at 30 1C is not evident. The TR–O and TO–T transition peaks shift towards lower temperature with increasing Ca content [16], meanwhile the peaks of transition are broad and cannot be observed clearly. With further increase of Ca content (x Z0.18), the polymorphic TR–O and TO–T transitions cannot be observed above room temperature. On the other hand, the dielectric constants for the BCZT ceramics increase with the increase of Ca content (0.16 ZxZ0.12) and then decrease at xZ0.18. The highest dielectric constant (4800) at room temperature is obtained for the BCZT ceramics at x¼ 0.16. At a certain extent good dielectric properties are observed, resulting from the grain growth and becoming dense of the BCZT ceramics with increasing Ca content, which, as discussed below. The dielectric constant in the composite phase at x ¼0.16 is higher than that in the pure phase (rhombohedral, xr0.14 and orthorhombic, xZ0.18), unlike the simple mixture of two phases where the dielectric constant of the composite lies between those of the individual phases.[17] This fact indicates that some interaction exists between the rhombohedral and orthorhombic phases. When xZ0.18, pure orthorhombic phase is obtained and the dielectric constant decrease with the increase of Ca content. There are some possible reasons for the phenomena. It is shown in this article that the replacement of Ba2 + by Ca2 + reduces the lattice constant with increasing hydrostatic pressure and moves the PPT downwards. Moreover, a diffuse phase transition in relaxor ferroelectrics is observed with the increase of Ca content [18,19]. On the other hand, further increase of Ca content could cause the replacement of Ti4 + by Ca2 + , resulting in a compositional and structural disorder, which is brought about by the difference in valences (4+ versus 2+ ), ionic radii and oxygen vacancies [20]. Fig. 3 shows the SEM micrographs of the BCZT ceramic samples. It is clearly shown that the Ca modified BCZT ceramics exhibit the coarse grain size. The grain sizes of the BCZT ceramics are 4–10 mm at x¼ 0.12, 5–10 mm at x¼0.14, 8–12 mm at x¼0.16 and 10–15 mm at x¼0.18. It indicates that the addition of Ca enhances the grain growth of the BCZT ceramics. The hysteresis loops of polarization versus electric field of the BCZT ceramics at x ¼0.12, x¼0.14, x ¼0.16 and x¼0.18 are shown in Fig. 4. The coercive fields Ec values of BCZT ceramics at x¼ 0.12, x¼0.14, x¼0.16 and x ¼0.18 are 4.5, 4.9, 5.0 and 5.1 kV/cm, respectively, which means that Ec increases with the increase of Ca content. The remnant polarization Pr values of the BCZT ceramics at x¼ 0.12, x ¼0.14, x ¼0.16 and x ¼0.18 are 6.4, 7.0, 9.0
W. Li et al. / Physica B 405 (2010) 4513–4516
4515
Fig. 3. The SEM micrographs of the (Ba1 xCax)(Ti0.9Zr0.1)O3 ceramic of x¼ 0.12, 0.14, 0.16 and 0.18 sintered at 1450 1C.
340
20
320
15 d33 (pC/N)
Pr (µC/cm2)
x = 0.12 x = 0.14 x = 0.16 x = 0.18
10 5
300 280 260
0 -40
-30
-20
0
-10 -5
10
20
30
240
40
Ec (kV/cm)
38 36
-10
-20
34 kp (%)
-15
32 30 28
Fig. 4. Polarization versus electric field for the (Ba1 xCax)(Ti0.9Zr0.1)O3 ceramics of x¼ 0.12, 0.14, 0.16 and 0.18 at room temperature.
26 280
2
260 Qm
and 7.0 mC/cm . It can be seen clearly that with increasing Ca content, Pr values increase to a maximum value of 9.0 mC/cm2 at x ¼0.16 and then decrease. Fig. 5 shows the piezoelectric coefficient d33, planar mode electromechanical coupling coefficient kp and mechanical quality factor Qm of the BCZT ceramics as a function of Ca content. It can be observed that both of the d33 and kp curves possess a peak with increasing Ca content. At x ¼0.16, the d33 and kp of the BCZT ceramics reach their maximum values of 328 pC/N and 37.6%, respectively. The highest d33 of BCZT ceramics at x ¼0.16 should be attributed to the highest Pr of 9.0 mC/cm2 and relative low Ec of 5.0 kV/cm, which indicate the improvement of poling process. Moreover, the enhanced piezoelectric properties are considered to be reasonably consistent
240 220 200 180 0.12
0.14
0.16
0.18
Ca content x Fig. 5. Piezoelectric constant d33, planar electromechanical coefficient kp and mechanical quality factor Qm of the (Ba1 xCax)(Ti0.9Zr0.1)O3 ceramics as a function of x.
4516
W. Li et al. / Physica B 405 (2010) 4513–4516
with coexistence of rhombohedral phase and orthorhombic phase occurring near room temperature, as is similar to other BT and KNN systems [2,9,21,22]. The Qm value of the sample increases with increasing Ca content. The partial substitution of Ba2 + by small Ca2 + will decrease the lattice parameter and make the BCZT ceramics ‘‘harder’’ [12,23].
4. Conclusions The (Ba1 xCax)(Ti0.9Zr0.1)O3 ceramics were prepared by solidstate reaction technique. XRD study indicates that the rhombohedral and orthorhombic phases coexist at room temperature in the range 0.14oxo0.18. Enhanced piezoelectric and dielectric properties of d33 ¼328 pC/N, kp ¼37.6% and e’¼ 4800 are obtained for the sample of x¼0.16. These properties indicate that this system is a potential lead-free candidate material to be further studied.
Acknowledgment This work was supported by the program for the National Natural Science Foundation of China (Grant nos. 50602021 and 50802038).
References [1] Y. Saito, H. Takao, T. Tani, T. Nonoyama, Nature 432 (2004) 84. [2] E. Hollenstein, M. Davis, D. Damjanovic, N. Setter, Appl. Phys. Lett. 87 (2005) 182905. [3] P. Kantha, K. Pengpat, P. Jarupoom, Curr. Appl. Phys. 9 (2009) 460. [4] Z. Yu, R.Y. Guo, A.S. Bhalla, J. Appl. Phys. 88 (2000) 410. [5] X.S. Wang, L.L. Zhang, H. Liu, J.W. Zhai, X. Yao, Mater. Chem. Phys. 112 (2008) 675. [6] Z. Yu, C. Ang, R.Y. Guo, A.S. Bhalla, J. Appl. Phys. 92 (2002) 148. [7] P.S. Dobal, A. Dixit, R.S. Katiyar, J. Appl. Phys. 89 (2006) 8085. [8] Z. Yu, C. Ang, R.Y. Guo, A.S. Bhalla, J. Appl. Phys. 92 (2002) 2656. [9] W.F. Liu, X.B. Ren, Phys. Rev. Lett. 103 (2009) 257602. [10] J.L. Zhang, X.J. Zong, L. Wu, Appl. Phys. Lett. 95 (2009) 022909. [11] S.J. Zhang, R. Xia, T.R. Shrout, J. Appl. Phys. 100 (2006) 104108. [12] L.L. Zhang, X.S. Wang, H. Liu, X. Yao, J. Am. Ceram. Soc. 93 (2010) 1049. [13] X. Wang, C.N. Xu, H. Yamada, K. Nishikubo, X.G. Zheng, Adv. Mater. 17 (2005) 1254. [14] S.W. Zhang, H.L. Zhang, B.P. Zhang, G.L. Zhao, J. Euro. Ceram. Soc. 29 (2009) 3235. [15] IEEE Standard on Piezoelectricity, ANSI/IEE Standard 176, 1987. [16] P.S. Dobal, A. Dixit, R.S. Katiyar, J. Appl. Phys. 86 (2001) 8085. [17] X.S. Wang, H. Yamada, C.N. Xu, Appl. Phys. Lett. 86 (2005) 022905. [18] L.L. Zhang, X.S. Wang, W. Yang, H. Liu, X. Yao, J. Appl. Phys. 104 (2008) 014104. [19] P. Victor, R. Ranjith, S.B. Krupanidhi, J. Appl. Phys. 94 (2003) 7702. [20] P.S.R. Krishna, D. Pandey, V.S. Tiwari, R. Chakravarthy, B.A. Dasannacharya, Appl. Phys. Lett. 62 (1993) 231. [21] B. Jaffe, W.R. Cook, H. Jaffe, in: Piezoelectric Ceramics, Academic Press, London, 1971. [22] S.J. Zhang, R. Xia, T.R. Shrout, Appl. Phys. Lett. 91 (2007) 32913. [23] P.Z. Zhang, M.R. Shen, L. Fang, F.G. Zheng, X.L. Wu, J.C. Shen, H.T. Chen, Appl. Phys. Lett. 92 (2008) 222908.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Polymorphic phase transition and piezoelectric properties of (Ba1 xCax)(Ti0.9Zr0.1)O3 lead-free ceramics Wei Li, Zhijun Xu n, Ruiqing Chu, Peng Fu, Guozhong Zang College of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, China
a r t i c l e in fo
abstract
Article history: Received 15 June 2010 Received in revised form 4 August 2010 Accepted 6 August 2010
Lead-free (Ba1 xCax)(Ti0.9Zr0.1)O3 (x ¼ 0.12–0.18) (BCZT) ceramics were prepared successfully using a solid-state reaction technique. The polymorphic phase transitions (PPT) from rhombohedral to orthorhombic phase around room temperature were identified in the composition range 0.14 o x o 0.18. The samples at x ¼ 0.16, showing coexistence of rhombohedral and orthorhombic phase, exhibit enhanced piezoelectric and dielectric properties of d33 ¼ 328 pC/N, kp ¼ 37.6% and e0 ¼ 4800. With the increase of Ca content, the polymorphic phase transitions shift to lower temperature and cannot be observed above room temperature at x Z0.18. & 2010 Elsevier B.V. All rights reserved.
Keywords: Lead-free ceramics Ferroelectrics Dielectrics Phase transitions
1. Introduction It is well-known that lead zirconate titanate (PZT) ceramics are the most widely used piezoelectric materials due to their superior piezoelectric properties close to the morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases. Nevertheless, they are not environmental friendly for their lead oxide toxicity. With the recent growing demand of global environment protection, many researchers have greatly focused on lead-free ceramics to replace the lead-based ceramics [1–3]. Barium titanate (BaTiO3), which is one of the most widely studied lead-free piezoelectric materials [4–10], have five kinds of crystal systems: hexagonal, cubic, tetragonal, orthorhombic and rhombohedral, depending on the phase transition temperatures: 1432, 130, 5 and 90 1C, respectively [5]. It is known that the doping is an effective way to improve the material performance in electroceramics. In particular, BaTi1-xZrxO3 (BZT) ceramics were extensively investigated and show promising piezoelectric/electrostrictive properties [6–11]. The transition temperatures of BZT (rhombohedral–orthorhombic TR–O, orthorhombic–tetragonal TO– T and tetragonal–cubic TC) move closer with increase in Zr content in BZT ceramics and merge near room temperature for the composition of x ¼0.15. Further increase in Zr content, especially for x4 0.25, the samples show broad dielectric peaks with frequency dispersion, i.e., ferroelectric-relaxor behavior [6–8]. As reported in Refs. [9–11], the polymorphic phase transition plays
n
Corresponding author. Tel./fax: + 86 6358230923. E-mail address: [email protected] (Z. Xu).
0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.08.028
an important role in the dielectric and piezoelectric properties. The enhanced piezoelectric properties are observed as a consequence of compositionally shifting the polymorphic TO–T transition downwards near room temperature for BZT based ceramics. Recently, it was reported that Ba1 xCaxTiO3 ceramics showed a large piezoelectric/electrostrictive strain and multiple functions [12–14]. However, little attention has been paid to the piezoelectric properties of the (Ba1 xCax)(Ti0.9Zr0.1)O3 (BCZT) ceramics. In this paper, the effect of Ca addition on phase structure, microstructure, piezoelectric properties and dielectric properties of the lead-free (Ba1 xCax)(Ti0.9Zr0.1)O3 ceramics are studied systematically.
2. Experimental procedure The (Ba1 xCax)(Ti0.9Zr0.1)O3 (BCZT) ceramics at x¼0.12, 0.14, 0.16 and 0.18 were prepared by conventional solid-state reaction technique. Raw materials of BaCO3 (99.0%), CaCO3 (99.0%), ZrO2 (99.0%) and TiO2 (99.5%) were mixed with addition of anhydrous ethanol, then dried and calcined at 1200 1C for 4 h. For the purpose of refining powders and facilitating diffusion progress, the powders were placed into a nylon bottle and remixed at 140 rpm for 10 h using ZrO2 milling media. After drying, the obtained powders were pressed into 12 mm-diameter pellets and sintered at 1450 1C for 4 h in air. The sample structure or phase development was examined using an X-ray diffraction meter ˚ (D8 Advance, Bruker Inc., using a Cu Ka radiation (l ¼1.54178 A) Germany). The dielectric properties were measured by the precision impedance analyzer (4294A Agilent Inc., America)
W. Li et al. / Physica B 405 (2010) 4513–4516
controlled by a computer at 100 kHz with the testing temperature ranged from room temperature to 200 1C. Ferroelectric hysteresis loops were measured at room temperature using an aix-ACCT TF2000FE-HV ferroelectric test unit (aix-ACCT Inc., Germany). The piezoelectric constant d33 was measured using a tester quasistatic d33 meter (YE2730 SINOCERA, China). The mechanical quality factor Qm and the planar electromechanical coupling factor kp were calculated following IEEE standards [15] using the impedance analyzer, the expression is k2p 1k2p
¼
Df f ð1 þ sp Þ
h i ðsp Þ2 1 þ Z21 ,
ð1Þ
where Df is the difference between the lowest frequency of antiresonance and resonance, and Z1 is the lowest root. J1 ðZ1 Þ ¼ 1sP
ð2Þ
9000 x = 0.12 x = 0.14 x = 0.16 x = 0.18
8000 Dielectric constant
4514
7000 6000 5000 4000 3000 2000 1000 20
40
60
80
100
120
140
Temperature (°C) Fig. 2. Temperature dependence of dielectric constant for the (Ba1 xCax)(Ti0.9Zr0.1)O3 ceramics of x¼ 0.12, 0.14, 0.16 and 0.18 measured at 100 kHz.
3. Results and discussion
220 202
220 202
211 210
Intensity (a.u.)
100
111
200
110
Fig. 1 shows the XRD patterns of the BCZT ceramics with different compositions. It can be seen that all the samples show pure perovskite structure, suggesting that Ca and Zr diffuse into the BaTiO3 lattice to form a solid solution. It is also found that the positions of the diffraction peak of the BCZT ceramics shift to higher angles with increasing Ca content. It is thought that the Ca2 + substitutions induce the distortion and shrinkage of the lattice parameters, which may be attributed to the smaller ionic ˚ than that of Ba2 + (1.61 A) ˚ [12]. At room radii of Ca2 + (1.34 A) temperature, the BCZT ceramics at x ¼0.12 and x ¼0.14 possess rhombohedral phase, which is characterized by single (2 0 0) peaks at around 2y of 451 and splitting of the (2 2 0)/(2 0 2) peaks at around 2y of 661 [9,16]. The BCZT ceramics become orthorhombic phase, featured with the merging of (2 2 0)/(2 0 2) peaks at around 2y of 661, with increasing Ca content. The crystal structure of the BCZT ceramics presents only orthorhombic phase featured with single peak at 2y of 451 and 661, when xZ0.18. Therefore, it can be suggested that the PPT of the BCZT system exists in the composition range 0.14oxo0.18 at room temperature, where rhombohedral and orthorhombic phases coexist. The temperature dependence of dielectric constants for the BCZT ceramics measured at frequency of 100 kHz is shown in Fig. 2. As can be seen, the BCZT ceramics at x¼0.12 exhibit two obvious polymorphic phase transitions corresponding to the orthorhombic–tetragonal (TO–T) at 40 1C and tetragonal–cubic
x = 0.18 x = 0.16 x = 0.14 x = 0.12
20
30
40
50
60
70 65
66
67
2 Theta/(Degrees) Fig. 1. X-ray diffraction patterns of (Ba1 xCax)(Ti0.9Zr0.1)O3 ceramics for x ¼0.12, 0.14, 0.16 and 0.18.
transitions (TC) at 65 1C [7,8]. The third polymorphic phase transition rhombohedral–orthorhombic (TR–O) at 30 1C is not evident. The TR–O and TO–T transition peaks shift towards lower temperature with increasing Ca content [16], meanwhile the peaks of transition are broad and cannot be observed clearly. With further increase of Ca content (x Z0.18), the polymorphic TR–O and TO–T transitions cannot be observed above room temperature. On the other hand, the dielectric constants for the BCZT ceramics increase with the increase of Ca content (0.16 ZxZ0.12) and then decrease at xZ0.18. The highest dielectric constant (4800) at room temperature is obtained for the BCZT ceramics at x¼ 0.16. At a certain extent good dielectric properties are observed, resulting from the grain growth and becoming dense of the BCZT ceramics with increasing Ca content, which, as discussed below. The dielectric constant in the composite phase at x ¼0.16 is higher than that in the pure phase (rhombohedral, xr0.14 and orthorhombic, xZ0.18), unlike the simple mixture of two phases where the dielectric constant of the composite lies between those of the individual phases.[17] This fact indicates that some interaction exists between the rhombohedral and orthorhombic phases. When xZ0.18, pure orthorhombic phase is obtained and the dielectric constant decrease with the increase of Ca content. There are some possible reasons for the phenomena. It is shown in this article that the replacement of Ba2 + by Ca2 + reduces the lattice constant with increasing hydrostatic pressure and moves the PPT downwards. Moreover, a diffuse phase transition in relaxor ferroelectrics is observed with the increase of Ca content [18,19]. On the other hand, further increase of Ca content could cause the replacement of Ti4 + by Ca2 + , resulting in a compositional and structural disorder, which is brought about by the difference in valences (4+ versus 2+ ), ionic radii and oxygen vacancies [20]. Fig. 3 shows the SEM micrographs of the BCZT ceramic samples. It is clearly shown that the Ca modified BCZT ceramics exhibit the coarse grain size. The grain sizes of the BCZT ceramics are 4–10 mm at x¼ 0.12, 5–10 mm at x¼0.14, 8–12 mm at x¼0.16 and 10–15 mm at x¼0.18. It indicates that the addition of Ca enhances the grain growth of the BCZT ceramics. The hysteresis loops of polarization versus electric field of the BCZT ceramics at x ¼0.12, x¼0.14, x ¼0.16 and x¼0.18 are shown in Fig. 4. The coercive fields Ec values of BCZT ceramics at x¼ 0.12, x¼0.14, x¼0.16 and x ¼0.18 are 4.5, 4.9, 5.0 and 5.1 kV/cm, respectively, which means that Ec increases with the increase of Ca content. The remnant polarization Pr values of the BCZT ceramics at x¼ 0.12, x ¼0.14, x ¼0.16 and x ¼0.18 are 6.4, 7.0, 9.0
W. Li et al. / Physica B 405 (2010) 4513–4516
4515
Fig. 3. The SEM micrographs of the (Ba1 xCax)(Ti0.9Zr0.1)O3 ceramic of x¼ 0.12, 0.14, 0.16 and 0.18 sintered at 1450 1C.
340
20
320
15 d33 (pC/N)
Pr (µC/cm2)
x = 0.12 x = 0.14 x = 0.16 x = 0.18
10 5
300 280 260
0 -40
-30
-20
0
-10 -5
10
20
30
240
40
Ec (kV/cm)
38 36
-10
-20
34 kp (%)
-15
32 30 28
Fig. 4. Polarization versus electric field for the (Ba1 xCax)(Ti0.9Zr0.1)O3 ceramics of x¼ 0.12, 0.14, 0.16 and 0.18 at room temperature.
26 280
2
260 Qm
and 7.0 mC/cm . It can be seen clearly that with increasing Ca content, Pr values increase to a maximum value of 9.0 mC/cm2 at x ¼0.16 and then decrease. Fig. 5 shows the piezoelectric coefficient d33, planar mode electromechanical coupling coefficient kp and mechanical quality factor Qm of the BCZT ceramics as a function of Ca content. It can be observed that both of the d33 and kp curves possess a peak with increasing Ca content. At x ¼0.16, the d33 and kp of the BCZT ceramics reach their maximum values of 328 pC/N and 37.6%, respectively. The highest d33 of BCZT ceramics at x ¼0.16 should be attributed to the highest Pr of 9.0 mC/cm2 and relative low Ec of 5.0 kV/cm, which indicate the improvement of poling process. Moreover, the enhanced piezoelectric properties are considered to be reasonably consistent
240 220 200 180 0.12
0.14
0.16
0.18
Ca content x Fig. 5. Piezoelectric constant d33, planar electromechanical coefficient kp and mechanical quality factor Qm of the (Ba1 xCax)(Ti0.9Zr0.1)O3 ceramics as a function of x.
4516
W. Li et al. / Physica B 405 (2010) 4513–4516
with coexistence of rhombohedral phase and orthorhombic phase occurring near room temperature, as is similar to other BT and KNN systems [2,9,21,22]. The Qm value of the sample increases with increasing Ca content. The partial substitution of Ba2 + by small Ca2 + will decrease the lattice parameter and make the BCZT ceramics ‘‘harder’’ [12,23].
4. Conclusions The (Ba1 xCax)(Ti0.9Zr0.1)O3 ceramics were prepared by solidstate reaction technique. XRD study indicates that the rhombohedral and orthorhombic phases coexist at room temperature in the range 0.14oxo0.18. Enhanced piezoelectric and dielectric properties of d33 ¼328 pC/N, kp ¼37.6% and e’¼ 4800 are obtained for the sample of x¼0.16. These properties indicate that this system is a potential lead-free candidate material to be further studied.
Acknowledgment This work was supported by the program for the National Natural Science Foundation of China (Grant nos. 50602021 and 50802038).
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