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Dielectric and piezoelectric properties of Li-substituted lead-free (Bi0.5Na0.5)TiO3–(Bi0.5K0.5)TiO3–BaTiO3 ceramics
Current Applied Physics 10 (2010) 1059–1061
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Dielectric and piezoelectric properties of Li-substituted lead-free (Bi0.5Na0.5)TiO3–(Bi0.5K0.5)TiO3–BaTiO3 ceramics L. Wang a, T.K. Song a,*, S.C. Lee a, J.H. Cho a, Y.S. Sung a, M.-H. Kim a, K.S. Choi b a b
School of Nano & Advanced Material Engineering, Changwon National Univ., Changwon, Gyeongnam 641-773, Republic of Korea Department of Physics Education, Sunchon National Univ., Chonnam 540-742, Republic of Korea
a r t i c l e
i n f o
Article history: Received 18 February 2009 Received in revised form 8 December 2009 Accepted 23 December 2009 Available online 29 December 2009 Keywords: Perovskites Ferroelectrics Piezoelectric materials Dielectrics
a b s t r a c t Lead-free 0.79(Bi0.5Na0.5)TiO3–0.14[Bi0.5(K0.5 xLix)]TiO3–0.07BaTiO3 (BNBK79 + xLi, x = 0.0, 0.1, 0.2, 0.25, 0.3, and 0.4) ceramics were prepared by conventional solid state reaction process. The crystalline structures and surface morphologies are investigated by X-ray diffraction method and scanning electron microscopy. Dielectric and piezoelectric properties were measured. With increasing of lithium substitution, the Curie temperatures of BNBK79 + xLi ceramics increase, but the maximum value of the dielectric constant decreases. And a relatively large remnant polarization of 17.6 lC/cm2 and 157 pC/N of d33 has been obtained when x = 0.3. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Lead-based perovskite ferroelectric materials exhibiting high piezoelectric coefficients have been widely used for dielectric or piezoelectric materials in applications of transducers and other electromechanical devices. However, the products containing Pb deteriorates the crucial environment and human health. Therefore many efforts have been done to find a promising way to solve this problem and develop new lead-free piezoelectric ceramics to replace Pb(Zr,Ti)O3 (PZT) ceramics to minimize lead pollution. Recently, much attention has been paid to the investigation of leadfree piezoelectric ceramics [1–12]. (Bi0.5Na0.5)TiO3 (BNT) with a rhombohedral perovskite structure has been considered to be a good candidate for lead-free piezoelectric ceramics due to its strong ferroelectric property (Pr = 38 lC/ cm2) and high Curie temperature (TC = 320 °C). However, it is hard to pole the pure BNT piezoelectric ceramics because of the relatively large coercive field (EC = 73 kV/cm) and high electrical conductivity [3,6]. To improve its properties, the solid solutions of BNT with BaTiO3 (BT), (Bi0.5K0.5)TiO3 (BKT), Ce2O3, Bi2O3, Sc2O3, etc. were investigated. Among these ceramics, xBNT–yBKT–zBT (x + y + z = 1, y:z = 2:1) demonstrates excellent piezoelectric and electromechanical properties when x = 0.79. In this work, we investigated the lithium substitution effect to substitute K ion in order to improve the hygroscopic, sintering, piezoelectric, and dielectric properties of 0.79(Bi0.5Na0.5)TiO3–0.14[Bi0.5(K0.5 xLix)]* Corresponding author. Tel.: +82 55 213 3713; fax: +82 55 262 6486. E-mail address: [email protected] (T.K. Song). 1567-1739/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2009.12.041
TiO3–0.07BaTiO3 (x = 0, 0.1, 0.2, 0.25, 0.3, and 0.4) ceramics (BNBK79 + xLi) [9–18].
2. Experimental 0.79(Bi0.5Na0.5)TiO3–0.14[Bi0.5(K0.5 xLix)]TiO3–0.07BaTiO3 (x = 0, 0.1, 0.2, 0.25, 0.3, 0.4) ceramics were prepared by a conventional solid state reaction method from the component oxides powders of BaTiO3 (Aldrich 99%), Bi2O3 (Aldrich 99%), K2CO3 (Aldrich 99.5%), Na2CO3 (Aldrich 99.5%), Li2CO3 (Aldrich 99%), and TiO2 (Aldrich 99%). These oxide and carbonate powders were weighed and mixed by ball milling in ethanol for 24 h. After being dried at 80 °C and they were calcined at about 800 °C for 2 h. The calcined powders were mixed with 5 wt.% polyvinyl alcohol as a binder. And then they were pressed into pellets with 10 mm diameter and 1 mm thick. The pellets were sintered for 2 h at 1150 °C in air. For the electric properties, electrodes were applied by silver paste on both surfaces and cured at 650 °C for 30 min. For piezoelectric measurements, samples were poled by applying a dc electric field of 40 kV/cm for 30 min in silicone oil at room temperature. When samples were aged for 24 h after poling, the piezoelectric coefficient d33 values were measured by a quasi-static d33 meter (ZJ-6B, HC Materials). X-ray diffraction (XRD, X’Pert, Philips) was used to determine the formation of the crystalline phases. Silicon powders were sprinkled on the ceramics surfaces before the XRD measurements for the reference peak positions. With different Li-substitution contents, grain morphologies and sizes were analyzed via a scanning
L. Wang et al. / Current Applied Physics 10 (2010) 1059–1061
electron microscope (SEM, S-2004, Hitachi). Resonance–antiresonance frequencies and capacitance at 1 kHz were measured, using an impedance analyzer (HP 4194A). Ferroelectric hysteresis loops were measured at 11 Hz by a Sawyer–Tower circuit with a function generator (FG300, Yokogawa), voltage amplifier (610E, Trek), and digital oscilloscope (DL7100, Yokogawa). From these results, electromechanical coupling factor kp and mechanical quality factor Qm were calculated. Dielectric constants were measured at 1 kHz in the temperature range from 30 to 650 °C and variation of TC with different Li-substitutions was investigated.
8000
Tm Dielectric Constant
6000
Td 4000
2000
3. Results and discussion
0
+0.4Li
100
500
600
30
2
Polarization ( μC/cm )
20 10 0 +0.00 Li +0.10 Li +0.20 Li +0.25 Li +0.30 Li +0.40 Li
-10 -20 -30 -50
-40
-30
-20
-10
0
10
20
30
40
50
Electric Field (kV/cm) Fig. 3. P–E hysteresis loops of BNBK79 + xLi ceramics for x = 0.0, 0.1, 0.2, 0.25, 0.3, and 0.4 substitutions.
17.6 lC/cm2 for x = 0.3 which is similar to that of 18.1 lC/cm2 for un-substituted sample. It can be easily seen from Fig. 3 that the lithium substitution in BNT–BKT–BT ceramics tends to reduce the strength of the coercive field. The coercive field decreases as increasing lithium substitution with EC = 12.4 kV/cm for x = 0.3
150
+0.2Li
90 60
+0.1Li
30
BNBK79
50
kp (%)
d33 (pC/N)
+0.25Li
40
400
Fig. 2. Temperature dependent dielectric constants of the BNBK79 + xLi ceramics at 1 kHz for different lithium substitutions. With the increasing of lithium substitution, the Curie temperature of BNBK79 ceramics increases, but the maximum value of the dielectric constant and Td decreases. Thermal hysteresis was observed at around Td.
120
30
300
o
(211)
Si
+0.3Li
20
200
Temperature ( C)
(200) Si
0
180 (201) (210)
Si
(002)
(111)
Intensity (a. u.)
(001) (100)
(110)
Densities of 0.79(Bi0.5Na0.5)TiO3–0.14[Bi0.5(K0.5 xLix)]TiO3– 0.07BaTiO3 (BNBK79 + xLi) sintered ceramics were measured using the Archimedes method. All BNBK79 + xLi ceramics had high density around 5.79–5.89 g/cm3. Fig. 1 shows XRD patterns of the BNBK79 + xLi ceramics in the 2h range from 20° to 60°. In the BNT–BKT–BT ternary system, the structure of BNBK79 is tetragonal. The crystal structure does not change significantly and still retains the tetragonal structure with Li substitution. In the SEM images of microstructures of BNBK79 + xLi ceramics, the grain size has slightly increased with increasing of lithium substitution. The average grain size of BNBK79 + xLi is about 0.2–0.25 lm from x = 0 to 0.4 samples and BNBK79 + 0.3Li has a more rectangular grain shape. It seems that lithium substitution amount does not change the grain size too much for x = 0, 0.1, and 0.2, however, the relatively larger grain size appears when x = 0.3. Fig. 2 shows the temperature dependence of dielectric constant er(T) of BNBK79 + xLi ceramics with different lithium substitution amounts. The temperature at which the transition between ferroelectric (FE) and anti-ferroelectric (AFE) took place was called as depolarization temperature (Td), and the temperature corresponding to maximum value of dielectric constant was named as maximum temperature (Tm). Thermal hysteresis were observed at around Td. Tm shifts from 277 °C for un-substituted sample to 335 °C for sample with lithium content x = 0.4, but the maximum value of dielectric constant decreased from 7100 to 4600. The decrease of Td and increase of Tm with increasing lithium content was probably due to the stability enhancement of AFE phase by Li addition. Fig. 3 shows the P–E hysteresis loops of BNBK79 + xLi ceramics at room temperature for x = 0.0, 0.1, 0.2, 0.25, 0.3, and 0.4, respectively. At room temperature, the remnant polarization Pr is about
0.00 Li 0.10 Li 0.20 Li 0.25 Li 0.30 Li 0.40 Li
18.0
250
17.5
200
17.0
150
16.5
100
16.0
60
2θ (deg.) Fig. 1. X-ray diffraction patterns of the BNBK79 + xLi ceramics in the 2h range from 20° to 60°. Although the Li addition increased from 0 to 40 mol.%, the structure does not change significantly and still retains the tetragonal structure. Si peaks come from the Si-powder strip applied on the sample surfaces to get the angle-reference of X-ray diffractions.
0.0
0.1
0.2
0.3
0.4
Qm
1060
50
Li Content, x
0
0.0
0.1
0.2
0.3
0.4
Li Content, x Fig. 4. Compositional dependence of the piezoelectric coefficients, d33, for BNBK79 + xLi ceramics. Inset shows electromechanical coupling factor (kp) and mechanical quality factor (Qm) of BNBK79 + xLi ceramics as functions of Li content.
1061
L. Wang et al. / Current Applied Physics 10 (2010) 1059–1061
which is smaller than that of EC = 19.8 kV/cm for un-substituted sample. For x = 0.4 sample, when most of K+ was replaced by Li+ in BKT, ferroelectricity disappeared and anti-ferroelectric-like double hysteresis loop was observed. But it is not so clear this loop comes from real antiferroelectricity or non-linear behavior mediated by defects. The composition dependences of the piezoelectric coefficients (d33) for BNBK + xLi are shown in Fig. 4. For this ceramic system, d33 values are higher than 135 pC/N reported by Zhang et al. in BNBK79 [9]. The increase of d33 were measured as increasing lithium substitution and the highest d33 value (157 pC/N) was obtained in x = 0.3. Therefore, d33 was enhanced in BNBK79 + xLi ceramic system as increasing lithium substitution. With higher substitution, d33 decreased may due to the secondary phase. Inset of Fig. 4 shows kp, Qm of BNBK79 + xLi ceramics as functions of Li content. Qm decreased with x = 0.1 and remains almost same value of about 80 with further substitution, but kp keeps almost constant to about x = 0.25 similarly to d33 behavior. The change of physical properties such as ferroelectric hysteresis loop and piezoelectric properties is related with the decreased of Td. The effects of AFElike phase were observed in x = 0.40 system [14,16]. 4. Conclusions Lead-free piezoelectric BNBK79 + xLi ceramics have been fabricated by conventional solid state reaction method. Their piezoelectric and dielectric properties were investigated. As increasing Li content, Tm increased but Td decreased. And d33 increased slightly to x = 0.3. A relatively large remnant polarization of 17.6 lC/cm2 and low coercive field of 12.4 kV/cm have been found for x = 0.3
lithium substitution d33 = 157 pC/N.
with
best
piezoelectric
constant
Acknowledgments This work was supported by the Korean Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2007313-C00213). References [1] Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, et al., Nature 432 (2004) 84. [2] T.R. Shrout, S.J. Zhang, J. Electroceram. 19 (2007) 113. [3] T. Takenaka, H. Nagata, J. Eur. Ceram. Soc. 25 (2005) 2693. [4] X.X. Wang, X.G. Tang, H.L.W. Chan, Appl. Phys. Lett. 85 (2004) 91. [5] J. Shieh, K.C. Wu, C.S. Chen, Acta Mater. 55 (2007) 3081. [6] T. Takenaka, H. Nagata, Y. Hiruma, Y. Yoshii, K. Matumoto, J. Electroceram. 19 (2007) 259. [7] X.Y. Wang, C.L. Wang, M.L. Zhao, J.F. Wang, K. Yang, J.C. Li, Mater. Lett. 61 (2007) 3847. [8] Y. Yuan, S.R. Zhang, X.H. Zhou, J.S. Liu, J. Mater. Sci. 41 (2006) 565. [9] S.J. Zhang, T.R. Shrout, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 54 (2007) 910. [10] Y. Hiruma, K. Yoshii, R. Aoyagi, H. Nagata, T. Takenaka, Key Eng. Mater. 320 (2006) 23. [11] S. Eitssayeam, U. Intatha, K. Pengpat, G. Rujijanagul, K.J.D. MacKenzie, T. Tunkasiri, Curr. Appl. Phys. 9 (2009) 993. [12] S. Wada, J. Korean Phys. Soc. 55 (2009) 858. [13] W. Chen, Y. Li, Q. Xu, J. Zhou, J. Electroceram. 15 (2005) 229. [14] Y. Hiruma, H. Nagata, T. Takenaka, Ceram. Int. 35 (2009) 117. [15] Y. Watanabe, Y. Hiruma, H. Nagata, T. Takenaka, Ceram. Int. 34 (2007) 761. [16] D. Lin, K.W. Kwok, Curr. Appl. Phys. 9 (2009) 1369. [17] C. Zhou, X. Liu, W. Li, C. Yuan, G. Chen, Curr. Appl. Phys. 10 (2010) 93. [18] H.-G. Yeo, Y.-S. Sung, T.K. Song, J.H. Cho, M.-H. Kim, T.-G. Park, J. Korean Phys. Soc. 54 (2009) 896.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Dielectric and piezoelectric properties of Li-substituted lead-free (Bi0.5Na0.5)TiO3–(Bi0.5K0.5)TiO3–BaTiO3 ceramics L. Wang a, T.K. Song a,*, S.C. Lee a, J.H. Cho a, Y.S. Sung a, M.-H. Kim a, K.S. Choi b a b
School of Nano & Advanced Material Engineering, Changwon National Univ., Changwon, Gyeongnam 641-773, Republic of Korea Department of Physics Education, Sunchon National Univ., Chonnam 540-742, Republic of Korea
a r t i c l e
i n f o
Article history: Received 18 February 2009 Received in revised form 8 December 2009 Accepted 23 December 2009 Available online 29 December 2009 Keywords: Perovskites Ferroelectrics Piezoelectric materials Dielectrics
a b s t r a c t Lead-free 0.79(Bi0.5Na0.5)TiO3–0.14[Bi0.5(K0.5 xLix)]TiO3–0.07BaTiO3 (BNBK79 + xLi, x = 0.0, 0.1, 0.2, 0.25, 0.3, and 0.4) ceramics were prepared by conventional solid state reaction process. The crystalline structures and surface morphologies are investigated by X-ray diffraction method and scanning electron microscopy. Dielectric and piezoelectric properties were measured. With increasing of lithium substitution, the Curie temperatures of BNBK79 + xLi ceramics increase, but the maximum value of the dielectric constant decreases. And a relatively large remnant polarization of 17.6 lC/cm2 and 157 pC/N of d33 has been obtained when x = 0.3. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Lead-based perovskite ferroelectric materials exhibiting high piezoelectric coefficients have been widely used for dielectric or piezoelectric materials in applications of transducers and other electromechanical devices. However, the products containing Pb deteriorates the crucial environment and human health. Therefore many efforts have been done to find a promising way to solve this problem and develop new lead-free piezoelectric ceramics to replace Pb(Zr,Ti)O3 (PZT) ceramics to minimize lead pollution. Recently, much attention has been paid to the investigation of leadfree piezoelectric ceramics [1–12]. (Bi0.5Na0.5)TiO3 (BNT) with a rhombohedral perovskite structure has been considered to be a good candidate for lead-free piezoelectric ceramics due to its strong ferroelectric property (Pr = 38 lC/ cm2) and high Curie temperature (TC = 320 °C). However, it is hard to pole the pure BNT piezoelectric ceramics because of the relatively large coercive field (EC = 73 kV/cm) and high electrical conductivity [3,6]. To improve its properties, the solid solutions of BNT with BaTiO3 (BT), (Bi0.5K0.5)TiO3 (BKT), Ce2O3, Bi2O3, Sc2O3, etc. were investigated. Among these ceramics, xBNT–yBKT–zBT (x + y + z = 1, y:z = 2:1) demonstrates excellent piezoelectric and electromechanical properties when x = 0.79. In this work, we investigated the lithium substitution effect to substitute K ion in order to improve the hygroscopic, sintering, piezoelectric, and dielectric properties of 0.79(Bi0.5Na0.5)TiO3–0.14[Bi0.5(K0.5 xLix)]* Corresponding author. Tel.: +82 55 213 3713; fax: +82 55 262 6486. E-mail address: [email protected] (T.K. Song). 1567-1739/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2009.12.041
TiO3–0.07BaTiO3 (x = 0, 0.1, 0.2, 0.25, 0.3, and 0.4) ceramics (BNBK79 + xLi) [9–18].
2. Experimental 0.79(Bi0.5Na0.5)TiO3–0.14[Bi0.5(K0.5 xLix)]TiO3–0.07BaTiO3 (x = 0, 0.1, 0.2, 0.25, 0.3, 0.4) ceramics were prepared by a conventional solid state reaction method from the component oxides powders of BaTiO3 (Aldrich 99%), Bi2O3 (Aldrich 99%), K2CO3 (Aldrich 99.5%), Na2CO3 (Aldrich 99.5%), Li2CO3 (Aldrich 99%), and TiO2 (Aldrich 99%). These oxide and carbonate powders were weighed and mixed by ball milling in ethanol for 24 h. After being dried at 80 °C and they were calcined at about 800 °C for 2 h. The calcined powders were mixed with 5 wt.% polyvinyl alcohol as a binder. And then they were pressed into pellets with 10 mm diameter and 1 mm thick. The pellets were sintered for 2 h at 1150 °C in air. For the electric properties, electrodes were applied by silver paste on both surfaces and cured at 650 °C for 30 min. For piezoelectric measurements, samples were poled by applying a dc electric field of 40 kV/cm for 30 min in silicone oil at room temperature. When samples were aged for 24 h after poling, the piezoelectric coefficient d33 values were measured by a quasi-static d33 meter (ZJ-6B, HC Materials). X-ray diffraction (XRD, X’Pert, Philips) was used to determine the formation of the crystalline phases. Silicon powders were sprinkled on the ceramics surfaces before the XRD measurements for the reference peak positions. With different Li-substitution contents, grain morphologies and sizes were analyzed via a scanning
L. Wang et al. / Current Applied Physics 10 (2010) 1059–1061
electron microscope (SEM, S-2004, Hitachi). Resonance–antiresonance frequencies and capacitance at 1 kHz were measured, using an impedance analyzer (HP 4194A). Ferroelectric hysteresis loops were measured at 11 Hz by a Sawyer–Tower circuit with a function generator (FG300, Yokogawa), voltage amplifier (610E, Trek), and digital oscilloscope (DL7100, Yokogawa). From these results, electromechanical coupling factor kp and mechanical quality factor Qm were calculated. Dielectric constants were measured at 1 kHz in the temperature range from 30 to 650 °C and variation of TC with different Li-substitutions was investigated.
8000
Tm Dielectric Constant
6000
Td 4000
2000
3. Results and discussion
0
+0.4Li
100
500
600
30
2
Polarization ( μC/cm )
20 10 0 +0.00 Li +0.10 Li +0.20 Li +0.25 Li +0.30 Li +0.40 Li
-10 -20 -30 -50
-40
-30
-20
-10
0
10
20
30
40
50
Electric Field (kV/cm) Fig. 3. P–E hysteresis loops of BNBK79 + xLi ceramics for x = 0.0, 0.1, 0.2, 0.25, 0.3, and 0.4 substitutions.
17.6 lC/cm2 for x = 0.3 which is similar to that of 18.1 lC/cm2 for un-substituted sample. It can be easily seen from Fig. 3 that the lithium substitution in BNT–BKT–BT ceramics tends to reduce the strength of the coercive field. The coercive field decreases as increasing lithium substitution with EC = 12.4 kV/cm for x = 0.3
150
+0.2Li
90 60
+0.1Li
30
BNBK79
50
kp (%)
d33 (pC/N)
+0.25Li
40
400
Fig. 2. Temperature dependent dielectric constants of the BNBK79 + xLi ceramics at 1 kHz for different lithium substitutions. With the increasing of lithium substitution, the Curie temperature of BNBK79 ceramics increases, but the maximum value of the dielectric constant and Td decreases. Thermal hysteresis was observed at around Td.
120
30
300
o
(211)
Si
+0.3Li
20
200
Temperature ( C)
(200) Si
0
180 (201) (210)
Si
(002)
(111)
Intensity (a. u.)
(001) (100)
(110)
Densities of 0.79(Bi0.5Na0.5)TiO3–0.14[Bi0.5(K0.5 xLix)]TiO3– 0.07BaTiO3 (BNBK79 + xLi) sintered ceramics were measured using the Archimedes method. All BNBK79 + xLi ceramics had high density around 5.79–5.89 g/cm3. Fig. 1 shows XRD patterns of the BNBK79 + xLi ceramics in the 2h range from 20° to 60°. In the BNT–BKT–BT ternary system, the structure of BNBK79 is tetragonal. The crystal structure does not change significantly and still retains the tetragonal structure with Li substitution. In the SEM images of microstructures of BNBK79 + xLi ceramics, the grain size has slightly increased with increasing of lithium substitution. The average grain size of BNBK79 + xLi is about 0.2–0.25 lm from x = 0 to 0.4 samples and BNBK79 + 0.3Li has a more rectangular grain shape. It seems that lithium substitution amount does not change the grain size too much for x = 0, 0.1, and 0.2, however, the relatively larger grain size appears when x = 0.3. Fig. 2 shows the temperature dependence of dielectric constant er(T) of BNBK79 + xLi ceramics with different lithium substitution amounts. The temperature at which the transition between ferroelectric (FE) and anti-ferroelectric (AFE) took place was called as depolarization temperature (Td), and the temperature corresponding to maximum value of dielectric constant was named as maximum temperature (Tm). Thermal hysteresis were observed at around Td. Tm shifts from 277 °C for un-substituted sample to 335 °C for sample with lithium content x = 0.4, but the maximum value of dielectric constant decreased from 7100 to 4600. The decrease of Td and increase of Tm with increasing lithium content was probably due to the stability enhancement of AFE phase by Li addition. Fig. 3 shows the P–E hysteresis loops of BNBK79 + xLi ceramics at room temperature for x = 0.0, 0.1, 0.2, 0.25, 0.3, and 0.4, respectively. At room temperature, the remnant polarization Pr is about
0.00 Li 0.10 Li 0.20 Li 0.25 Li 0.30 Li 0.40 Li
18.0
250
17.5
200
17.0
150
16.5
100
16.0
60
2θ (deg.) Fig. 1. X-ray diffraction patterns of the BNBK79 + xLi ceramics in the 2h range from 20° to 60°. Although the Li addition increased from 0 to 40 mol.%, the structure does not change significantly and still retains the tetragonal structure. Si peaks come from the Si-powder strip applied on the sample surfaces to get the angle-reference of X-ray diffractions.
0.0
0.1
0.2
0.3
0.4
Qm
1060
50
Li Content, x
0
0.0
0.1
0.2
0.3
0.4
Li Content, x Fig. 4. Compositional dependence of the piezoelectric coefficients, d33, for BNBK79 + xLi ceramics. Inset shows electromechanical coupling factor (kp) and mechanical quality factor (Qm) of BNBK79 + xLi ceramics as functions of Li content.
1061
L. Wang et al. / Current Applied Physics 10 (2010) 1059–1061
which is smaller than that of EC = 19.8 kV/cm for un-substituted sample. For x = 0.4 sample, when most of K+ was replaced by Li+ in BKT, ferroelectricity disappeared and anti-ferroelectric-like double hysteresis loop was observed. But it is not so clear this loop comes from real antiferroelectricity or non-linear behavior mediated by defects. The composition dependences of the piezoelectric coefficients (d33) for BNBK + xLi are shown in Fig. 4. For this ceramic system, d33 values are higher than 135 pC/N reported by Zhang et al. in BNBK79 [9]. The increase of d33 were measured as increasing lithium substitution and the highest d33 value (157 pC/N) was obtained in x = 0.3. Therefore, d33 was enhanced in BNBK79 + xLi ceramic system as increasing lithium substitution. With higher substitution, d33 decreased may due to the secondary phase. Inset of Fig. 4 shows kp, Qm of BNBK79 + xLi ceramics as functions of Li content. Qm decreased with x = 0.1 and remains almost same value of about 80 with further substitution, but kp keeps almost constant to about x = 0.25 similarly to d33 behavior. The change of physical properties such as ferroelectric hysteresis loop and piezoelectric properties is related with the decreased of Td. The effects of AFElike phase were observed in x = 0.40 system [14,16]. 4. Conclusions Lead-free piezoelectric BNBK79 + xLi ceramics have been fabricated by conventional solid state reaction method. Their piezoelectric and dielectric properties were investigated. As increasing Li content, Tm increased but Td decreased. And d33 increased slightly to x = 0.3. A relatively large remnant polarization of 17.6 lC/cm2 and low coercive field of 12.4 kV/cm have been found for x = 0.3
lithium substitution d33 = 157 pC/N.
with
best
piezoelectric
constant
Acknowledgments This work was supported by the Korean Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2007313-C00213). References [1] Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, et al., Nature 432 (2004) 84. [2] T.R. Shrout, S.J. Zhang, J. Electroceram. 19 (2007) 113. [3] T. Takenaka, H. Nagata, J. Eur. Ceram. Soc. 25 (2005) 2693. [4] X.X. Wang, X.G. Tang, H.L.W. Chan, Appl. Phys. Lett. 85 (2004) 91. [5] J. Shieh, K.C. Wu, C.S. Chen, Acta Mater. 55 (2007) 3081. [6] T. Takenaka, H. Nagata, Y. Hiruma, Y. Yoshii, K. Matumoto, J. Electroceram. 19 (2007) 259. [7] X.Y. Wang, C.L. Wang, M.L. Zhao, J.F. Wang, K. Yang, J.C. Li, Mater. Lett. 61 (2007) 3847. [8] Y. Yuan, S.R. Zhang, X.H. Zhou, J.S. Liu, J. Mater. Sci. 41 (2006) 565. [9] S.J. Zhang, T.R. Shrout, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 54 (2007) 910. [10] Y. Hiruma, K. Yoshii, R. Aoyagi, H. Nagata, T. Takenaka, Key Eng. Mater. 320 (2006) 23. [11] S. Eitssayeam, U. Intatha, K. Pengpat, G. Rujijanagul, K.J.D. MacKenzie, T. Tunkasiri, Curr. Appl. Phys. 9 (2009) 993. [12] S. Wada, J. Korean Phys. Soc. 55 (2009) 858. [13] W. Chen, Y. Li, Q. Xu, J. Zhou, J. Electroceram. 15 (2005) 229. [14] Y. Hiruma, H. Nagata, T. Takenaka, Ceram. Int. 35 (2009) 117. [15] Y. Watanabe, Y. Hiruma, H. Nagata, T. Takenaka, Ceram. Int. 34 (2007) 761. [16] D. Lin, K.W. Kwok, Curr. Appl. Phys. 9 (2009) 1369. [17] C. Zhou, X. Liu, W. Li, C. Yuan, G. Chen, Curr. Appl. Phys. 10 (2010) 93. [18] H.-G. Yeo, Y.-S. Sung, T.K. Song, J.H. Cho, M.-H. Kim, T.-G. Park, J. Korean Phys. Soc. 54 (2009) 896.