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Electromechanical and dielectric properties of Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3–BaTiO3 lead-free ceramics
Materials Chemistry and Physics 94 (2005) 328–332
Electromechanical and dielectric properties of Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3–BaTiO3 lead-free ceramics Yueming Li a,∗ , Wen Chen b , Qing Xu b , Jing Zhou b , Xingyong Gu a , Siqin Fang a b
a Jingdezhen Ceramics Institute, Jingdezhen 333001, PR China Institute of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, PR China
Received 2 January 2005; received in revised form 17 April 2005; accepted 10 May 2005
Abstract Electromechanical and dielectric properties of (1–3x)NBT–2x KBT–xBT lead-free ceramics were investigated. XRD results show that the crystalline structure of the ceramics was perovskite. The morphotropic phase boundary (MPB) of the ternary system between rhombohedral and tetragonal locates in the range of x = 0.025–0.035. The preferred piezoelectric constant d33 = 150 pC N−1 and planar electromechanical coupling factor kp = 0.298 are observed at x = 0.035, corresponding to a relatively large remanent polarization of Pr = 35.0 C cm−2 and a relatively low coercive field of Ec = 4.55 kV mm−1 . Temperature dependence of dielectric constant at different frequencies shows that the ferroelectric characteristic of (1–3x)NBT–2xKBT–xBT lead-free ceramics transforms from relaxor ferroelectric to normal ferroelectric with increasing of KBT and BT concentration. © 2005 Elsevier B.V. All rights reserved. Keywords: Ceramics; Dielectric properties; Ferroelectricity; Piezoelectricity
1. Introduction Lead oxide-based piezoelectric ceramics, represented by lead zirconate titanate (Pb(Zr, Ti)O3 , PZT) are widely used for piezoelectric actuators, sensors and transducers due to their excellent piezoelectric properties [1]. However, volatilization of toxic PbO during high-temperature sintering not only causes environmental pollution but also generate unstability of composition and electrical properties of products. Therefore, it is necessary to develop environment-friendly lead-free piezoelectric ceramics to replace the PZT-based ceramic, which has become one of the main trends in present development of piezoelectric materials. Sodium bismuth titanate, Na0.5 Bi0.5 TiO3 (NBT), is a kind of perovskite (ABO3 -type) ferroelectric discovered by Smolenskii et al. (1960) [2]. NBT is considered to be an excellent candidate of lead-free piezoelectric ceramics because it ∗
Corresponding author. Tel.: +86 798 8491446; fax: +86 798 8491446. E-mail address: [email protected] (Y. Li).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.05.009
is a strong ferroelectricity with a large remanent polarization, Pr = 38 C cm−2 [2]. However, NBT has a drawback of high coercive field, Ec = 7.3 kV mm−1 and high conductivity to cause problems in poling process. Therefore, NBT-based ceramics that can be poled easily have recently been investigated [3–7]. Among these investigations, barium titanate, BaTiO3 (BT) and potassium bismuth titanate, K0.5 Bi0.5 TiO3 (KBT) are well-known lead-free piezoelectric materials with a tetragonal phase. The binary systems of NBT–BT and NBT–KBT piezoelectric ceramics reported by Takenaka et al. [3] and Sasaki et al. [5], respectively. The good piezoelectric and dielectric properties of these systems have been revealed at the composition near the morphotropic phase boundary (MPB). Recent reports [8,9] have demonstrated that preferred piezoelectric properties have been obtained by adding KBT and BT into NBT to form NBT–KBT–BT ternary system. However, the piezoelectric, dielectric properties and MPB composition are still not very clear. In this work, a (1–3x)NBT–2xKBT–xBT ternary system by keeping the KBT and BT ratio constant was investigated. The MPB
Y. Li et al. / Materials Chemistry and Physics 94 (2005) 328–332
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composition range of the ternary system was confirmed. The piezoelectric and dielectric properties were also reported.
2. Experimental A conventional ceramic fabrication technique was used to prepare (1–3x)NBT–2x KBT–xBT (x = 0, 0.010, 0.020, 0.025, 0.030, 0.035 and 0.040) ceramics. Reagent grade oxide or carbonate powders of Bi2 O3 , Na2 CO3 , K2 CO3 , BaCO3 and TiO2 were used as starting raw materials. The oxides and carbonates were mixed in ethanol with agate balls by ball milling for 6 h. After being mixed, the dried powder was calcined at 900 ◦ C for 2 h. The calcined powder was reground by ball milling for 10 h. The dried powder was mixed with PVA and pressed at 150 MPa into pellets 20 mm in diameter and about 1.5 mm in thickness. The green compacts were sintered at 1160 ◦ C for 2 h in air atmosphere. Silver paste was fired on the surfaces of the disc as electrodes. The specimens for measurement of piezoelectric properties were poled in a silicon oil bath at 80 ◦ C under 3.5 kV mm−1 for 15 min. X-ray powder diffraction (XRD) patterns for the unpoled ceramics were taken on a D/MAX-III X-ray diffractome˚ and graphite ter with Cu K␣ radiation (λ = 1.5418 A) monochrometer. The piezoelectric constant d33 of the samples were measured by means of quasistatic d33 meter (ZJ-3A) at 110 Hz based on Berlincourt method. The piezoelectric properties were measured by means of the resonance–antiresonance method on the basis of IEEE standards using a precision impedance analyzer (HP4294A). The electromechanical coupling factor kp were calculated from the resonance and antiresonance frequencies based on the Onoe and Jumonji’s formulas [10]. The relative dielectric constant εr at room and elevated temperature was taken at 1, 10 and 100 kHz using TH2816 LRC meter with 2 ◦ C min−1 heating rate. The remanent polarization, Pr , and coercive field, Ec , were determined from P–E hysteresis loops measured by a radiant precision workstation.
Fig. 1. XRD patterns of (1–3x)NBT–2xKBT–xBT lead-free ceramics.
symmetry. Therefore, it can be suggested that the MPB of (1–3x)NBT–2xKBT–xBT system ceramics lies in the composition range of 0.025 ≤ x ≤ 0.035 at room temperature, where rhombohedral and tetragonal phases coexist. 3.2. Electromechanical properties of the (1–3x)NBT–2xKBT–xBT ceramics Fig. 3 presents the dielectric constant εT33 /ε0 and dielectric loss tan δ as a function of the substituting concentration x of KBT and BT. It can be seen the dielectric constant εT33 /ε0 increases monotony with increasing x. The dielectric loss tan δ of specimens decreases after substituted KBT and BT, but with increasing x, the dielectric loss tan δ increases with increasing of x, reaches the maximum value at x = 0.035 and then decreases with more concentration. The mechanical quality factor Qm shown in Fig. 4 reveals a reversed tendency compared to the dielectric loss tan δ. The same trend of frequency constant N with Qm is also shown in Fig. 4, which only the maximum value locates at different concentration x. Fig. 5 presents the piezoelectric constant d33 and electromechanical coupling factor kp as a function of the substituting
3. Results and discussion 3.1. Crystal structure of the (1–3x)NBT–2xKBT–xBT ceramics Fig. 1 shows the XRD patterns of (1–3x)NBT– 2xKBT–xBT ceramics. A pure perovskite structure without any secondary impurity phases could be certified. Fig. 2 shows the XRD patterns of (1–3x)NBT–2xKBT–xBT ceramics in the 2θ ranges of 38–50◦ . The rhombohedral symmetry for the system in the range of x = 0–0.020 is characterized by a (0 0 3)/(0 2 1) peak splitting between 39 and 41◦ and a single peak of (2 0 2) between 45 and 48◦ . The (0 0 3)/(0 2 1) peak splitting is obvious until x = 0.035. A distinct (0 0 2)/(2 0 0) peak splitting between 45 and 48◦ can be seen when x ≥ 0.025, corresponding to a tetragonal
Fig. 2. XRD patterns of (1–3x)NBT–2xKBT–xBT lead-free ceramics in 2θ range of 38–50◦ .
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Y. Li et al. / Materials Chemistry and Physics 94 (2005) 328–332 Table 1 Ferroelectric properties of (1–3x)NBT–2xKBT–xBT lead-free ceramics
Fig. 3. Dielectric constant εT33 /ε0 and dielectric loss tan δ as a function of KBT and BT concentration x for (1–3x)NBT–2xKBT–xBT ceramics.
amount x. A full same tendency of them can be seen that the d33 and kp increase with increasing x, reach the maximum value of d33 = 150 pC N−1 and kp = 0.298 at x = 0.035 then decrease. These properties demonstrate that the compositions near the MPB have relatively high piezoelectric and electromechanical activities. It is attributed to an increase in the number of possible spontaneous polarization direction for the compositions near the MPB due to the coexistence of rhombohedral and tetragonal phases [11].
x
Pr (C cm−2 )
Ec (kV mm−1 )
0.000 0.010 0.020 0.025 0.030 0.035 0.040
29.0 33.5 31.0 32.1 33.6 35.0 33.3
6.11 7.32 7.72 6.41 5.74 4.55 4.82
The measurement of P–E hysteresis loops was conducted to examine the ferroelectric properties of (1–3x)NBT–2xKBT–xBT ceramics. Table 1 shows the remanent polarization (Pr ) and coercive field (Ec ) values derived from the P–E hysteresis loops of (1–3x)NBT–2xKBT–xBT ceramics. It can be seen that in the composition range of x = 0–0.040, the coercive field decreases with the increase of KBT and BT concentration through a minimum value at x = 0.035 and then tends to increase, which is generally consistent with the variation trend of piezoelectric constant with KBT and BT concentration. Fig. 6 shows the P–E hysteresis loops of (1–3x)NBT–2xKBT–xBT ceramics with x = 0 and 0.035. It can be seen evidently that the ceramics of x = 0.035 has a remanent polarization Pr of 35.0 C cm−2 ,
Fig. 4. Mechanical quality factor Qm and frequency constant N as a function of KBT and BT concentration x for (1–3x)NBT–2xKBT–xBT ceramics.
Fig. 5. Piezoelectric constant d33 and electromechanical coupling factor kp as a function of KBT and BT concentration x for (1–3x)NBT–2xKBT–xBT ceramics.
Fig. 6. P–E hysteresis loops for the specimens of x = 0 and 0.035.
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Fig. 7. Temperature dependence of dielectric constant (εr ) for (1–3x)NBT–2xKBT–xBT ceramics at 1, 10 and 100 kHz with x = 0, 0.010, 0.025, 0.035 and 0.040.
which is significantly larger than 29.0 C cm−2 of pure NBT ceramic. In addition, it can also be seen that the coercive field Ec of 4.55 kV mm−1 for x = 0.035 composition is distinct smaller than that of 6.11 kV mm−1 for x = 0. In generally, high polarization remanent Pr and low coercive field Ec are presumably responsible for their large piezoelectric properties [12]. In present work, good electromechanical properties of NBT–KBT–BT ceramics have been obtained, which could be attributed the composition near MPB and the good ferroelectric properties. 3.3. Dielectric properties of the (1–3x)NBT–2xKBT–xBT ceramics Fig. 7 shows the temperature dependence of dielectric constant εr of x = 0.010, 0.025, 0.035 and 0.040 for (1–3x)NBT–2xKBT–xBT ceramics during heating process taken at measurement frequencies of 1, 10 and 100 kHz. It can evidently be seen that two dielectric abnormal peaks exist for all detected specimens between room temperature and 500 ◦ C. Here, the temperature of lower dielectric peak is called as Td and the temperature corresponding to maximum value of dielectric constant named as maximum temperature Tm . For x = 0.010, a strong frequency dispersive especially at Td was observed between the tested temperature range
which means that this material is a relaxor ferroelectric. It can be interpreted by the compositional fluctuation model put forward by Smolenskii and Isupov [13]. It is believed that a little amount of KBT and BT substituted to NBT could not change the relaxor ferroelectric characteristic because of the NBT itself is a relaxor ferroelectric [14,15]. However, when x = 0.025, 0.035 and 0.040, these curves of temperature dependence of dielectric constant are obviously different from that of x = 0.010. From these curves, it can be seen that dielectric constant increases slowly as the increasing of temperature initially. However, the dielectric constant increases dramatically near the Td , which is often called the characteristics of normal ferroelectric. After Td , the dielectric dispersion gradually disappears and the dielectric dispersion disappears as the concentration x increasing. These results clearly show that ferroelectric characteristic of (1–3x)NBT–2xKBT–xBT ceramics transform from relaxor ferroelectric to normal ferroelectric. The relaxor ferroelectric to normal ferroelectric transformation phenomenon has been found in many compounds such as La-modified tetragonal structure PLZT [16], Tidoped PMN [17] and Pb-doped NBT [18]. It is believed that relaxor to normal transformation is the reason of ferroelectric order–disorder type phase transitions of spontaneous micro–macrodomain switching. For those of some
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B-site co-occupied A(B1/2 B1/2 )O3 compounds, the larger the difference of ion radius between B and B , the easier forming ordered structure, which is also applicable for the (A1/2 A1/2 )BO3 -type compounds [19,20]. Due to the larger ion radius of Ba2+ (rBa2+ = 0.140 nm) and K+ (rK+ = 0.133 nm) were doped to A-site replacing the small ion radius of Na+ (rNa+ = 0.097 nm) and Bi3+ (rBi3+ = 0.096 nm), it would increase the ordered degree of A lattice site of ABO3 and decrease the compositional fluctuation. 4. Conclusions The ternary (1–3x)NBT–2xKBT–xBT lead-free ceramics were prepared by conventional ceramic technique. The crystalline phases, electromechanical, ferroelectric and dielectric properties of the ceramics were investigated. The results of XRD show the MPB of the ceramics exists in the composition range of 0.025 ≤ x ≤ 0.035 at room temperature. The preferred piezoelectric constant d33 (150 pC N−1 ) and electromechanical coupling factor kp (0.298) are obtained at x = 0.035. P–E hysteresis loops shows that it has a relatively large polarization remanent Pr and a relatively low coercive field Ec than pure NBT ceramic. The ferroelectric characteristic of (1–3x)NBT–2xKBT–xBT ceramics would transform from relaxor ferroelectric to normal ferroelectric with increasing of KBT and BT concentration. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 50272044),
Natural Science Foundation of Hubei province, China (Grant No.2002AB076) and Nippon Sheet Glass Foundation for Materials Science and Engineering (Japan).
References [1] B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic, New York, 1971. [2] G.A. Smolenskii, V.A. Isupv, A.I. Afranovskaya, N.N. Krainik, J. Sov. Phys. Solid State 2 (1961) 2651. [3] T. Takenaka, K.-I. Mareyama, K. Sakata, Jpn. J. Appl. Phys. 30 (9B) (1991) 2236. [4] S.-E. Park, K.-S. Hong, J. Mater. Res. 12 (1997) 2152. [5] A. Sasaki, T. Chiba, Y. Mamiya, Y. Mamiya, E. Otsuki, Jpn. J. Appl. Phys. 38 (1999) 5564. [6] Y.M. Li, W. Chen, J. Zhou, Q. Xu, H.J. Sun, R.X. Xu, Mater. Sci. Eng. B 112 (2004) 5. [7] X.X. Wang, H.L.W. Chan, C.L. Choy, Solid State Commun. 125 (2003) 395. [8] H. Nagata, M. Yoshida, Y. Makiuchi, T. Tanenaka, Jpn. J. Appl. Phys. 42 (2003) 7401. [9] X.X. Wang, X.G. Tang, H.L.W. Chan, Appl. Phys. Lett. 85 (2004) 91. [10] M. Onoe, H. Jumonji, J. Acoust. Soc. Am. 41 (1967) 974. [11] Y. Hosono, K. Harada, Y. Yamashita, Jpn. J. Appl. Phys. 40 (2001) 5722. [12] T. Takenaka, Ferroelectrics 230 (1997) 87. [13] G.A. Smolenskii, V.A. Isupov, Dokl Akad Nank, USSR 96 (1954) 53. [14] I.G. Siny, E. Husson, J.M. Beny, S.G. Lushnikov, E.A. Ragocheva, Physica B 293 (2001) 382. [15] J. Kreisel, A.M. Glazer, Phys. Rev. B 63 (2001) 174106-1. [16] X. Dai, Z. Xu, D. Viehland, J. Appl. Phys. 79 (1996) 1021. [17] H.Q. Fan, L.B. Kong, L.Y. Zhang, X. Yao, J. Mater. Sci. 34 (1999) 6143. [18] S. Kunharuangrong, J. Mater. Sci. Lett. 18 (1999) 1155. [19] N. Setter, L.E. Cross, J. Mater. Sci. 15 (1980) 2478. [20] N. Setter, L.E. Cross, J. Appl. Phys. 51 (1980) 4356.
Electromechanical and dielectric properties of Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3–BaTiO3 lead-free ceramics Yueming Li a,∗ , Wen Chen b , Qing Xu b , Jing Zhou b , Xingyong Gu a , Siqin Fang a b
a Jingdezhen Ceramics Institute, Jingdezhen 333001, PR China Institute of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, PR China
Received 2 January 2005; received in revised form 17 April 2005; accepted 10 May 2005
Abstract Electromechanical and dielectric properties of (1–3x)NBT–2x KBT–xBT lead-free ceramics were investigated. XRD results show that the crystalline structure of the ceramics was perovskite. The morphotropic phase boundary (MPB) of the ternary system between rhombohedral and tetragonal locates in the range of x = 0.025–0.035. The preferred piezoelectric constant d33 = 150 pC N−1 and planar electromechanical coupling factor kp = 0.298 are observed at x = 0.035, corresponding to a relatively large remanent polarization of Pr = 35.0 C cm−2 and a relatively low coercive field of Ec = 4.55 kV mm−1 . Temperature dependence of dielectric constant at different frequencies shows that the ferroelectric characteristic of (1–3x)NBT–2xKBT–xBT lead-free ceramics transforms from relaxor ferroelectric to normal ferroelectric with increasing of KBT and BT concentration. © 2005 Elsevier B.V. All rights reserved. Keywords: Ceramics; Dielectric properties; Ferroelectricity; Piezoelectricity
1. Introduction Lead oxide-based piezoelectric ceramics, represented by lead zirconate titanate (Pb(Zr, Ti)O3 , PZT) are widely used for piezoelectric actuators, sensors and transducers due to their excellent piezoelectric properties [1]. However, volatilization of toxic PbO during high-temperature sintering not only causes environmental pollution but also generate unstability of composition and electrical properties of products. Therefore, it is necessary to develop environment-friendly lead-free piezoelectric ceramics to replace the PZT-based ceramic, which has become one of the main trends in present development of piezoelectric materials. Sodium bismuth titanate, Na0.5 Bi0.5 TiO3 (NBT), is a kind of perovskite (ABO3 -type) ferroelectric discovered by Smolenskii et al. (1960) [2]. NBT is considered to be an excellent candidate of lead-free piezoelectric ceramics because it ∗
Corresponding author. Tel.: +86 798 8491446; fax: +86 798 8491446. E-mail address: [email protected] (Y. Li).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.05.009
is a strong ferroelectricity with a large remanent polarization, Pr = 38 C cm−2 [2]. However, NBT has a drawback of high coercive field, Ec = 7.3 kV mm−1 and high conductivity to cause problems in poling process. Therefore, NBT-based ceramics that can be poled easily have recently been investigated [3–7]. Among these investigations, barium titanate, BaTiO3 (BT) and potassium bismuth titanate, K0.5 Bi0.5 TiO3 (KBT) are well-known lead-free piezoelectric materials with a tetragonal phase. The binary systems of NBT–BT and NBT–KBT piezoelectric ceramics reported by Takenaka et al. [3] and Sasaki et al. [5], respectively. The good piezoelectric and dielectric properties of these systems have been revealed at the composition near the morphotropic phase boundary (MPB). Recent reports [8,9] have demonstrated that preferred piezoelectric properties have been obtained by adding KBT and BT into NBT to form NBT–KBT–BT ternary system. However, the piezoelectric, dielectric properties and MPB composition are still not very clear. In this work, a (1–3x)NBT–2xKBT–xBT ternary system by keeping the KBT and BT ratio constant was investigated. The MPB
Y. Li et al. / Materials Chemistry and Physics 94 (2005) 328–332
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composition range of the ternary system was confirmed. The piezoelectric and dielectric properties were also reported.
2. Experimental A conventional ceramic fabrication technique was used to prepare (1–3x)NBT–2x KBT–xBT (x = 0, 0.010, 0.020, 0.025, 0.030, 0.035 and 0.040) ceramics. Reagent grade oxide or carbonate powders of Bi2 O3 , Na2 CO3 , K2 CO3 , BaCO3 and TiO2 were used as starting raw materials. The oxides and carbonates were mixed in ethanol with agate balls by ball milling for 6 h. After being mixed, the dried powder was calcined at 900 ◦ C for 2 h. The calcined powder was reground by ball milling for 10 h. The dried powder was mixed with PVA and pressed at 150 MPa into pellets 20 mm in diameter and about 1.5 mm in thickness. The green compacts were sintered at 1160 ◦ C for 2 h in air atmosphere. Silver paste was fired on the surfaces of the disc as electrodes. The specimens for measurement of piezoelectric properties were poled in a silicon oil bath at 80 ◦ C under 3.5 kV mm−1 for 15 min. X-ray powder diffraction (XRD) patterns for the unpoled ceramics were taken on a D/MAX-III X-ray diffractome˚ and graphite ter with Cu K␣ radiation (λ = 1.5418 A) monochrometer. The piezoelectric constant d33 of the samples were measured by means of quasistatic d33 meter (ZJ-3A) at 110 Hz based on Berlincourt method. The piezoelectric properties were measured by means of the resonance–antiresonance method on the basis of IEEE standards using a precision impedance analyzer (HP4294A). The electromechanical coupling factor kp were calculated from the resonance and antiresonance frequencies based on the Onoe and Jumonji’s formulas [10]. The relative dielectric constant εr at room and elevated temperature was taken at 1, 10 and 100 kHz using TH2816 LRC meter with 2 ◦ C min−1 heating rate. The remanent polarization, Pr , and coercive field, Ec , were determined from P–E hysteresis loops measured by a radiant precision workstation.
Fig. 1. XRD patterns of (1–3x)NBT–2xKBT–xBT lead-free ceramics.
symmetry. Therefore, it can be suggested that the MPB of (1–3x)NBT–2xKBT–xBT system ceramics lies in the composition range of 0.025 ≤ x ≤ 0.035 at room temperature, where rhombohedral and tetragonal phases coexist. 3.2. Electromechanical properties of the (1–3x)NBT–2xKBT–xBT ceramics Fig. 3 presents the dielectric constant εT33 /ε0 and dielectric loss tan δ as a function of the substituting concentration x of KBT and BT. It can be seen the dielectric constant εT33 /ε0 increases monotony with increasing x. The dielectric loss tan δ of specimens decreases after substituted KBT and BT, but with increasing x, the dielectric loss tan δ increases with increasing of x, reaches the maximum value at x = 0.035 and then decreases with more concentration. The mechanical quality factor Qm shown in Fig. 4 reveals a reversed tendency compared to the dielectric loss tan δ. The same trend of frequency constant N with Qm is also shown in Fig. 4, which only the maximum value locates at different concentration x. Fig. 5 presents the piezoelectric constant d33 and electromechanical coupling factor kp as a function of the substituting
3. Results and discussion 3.1. Crystal structure of the (1–3x)NBT–2xKBT–xBT ceramics Fig. 1 shows the XRD patterns of (1–3x)NBT– 2xKBT–xBT ceramics. A pure perovskite structure without any secondary impurity phases could be certified. Fig. 2 shows the XRD patterns of (1–3x)NBT–2xKBT–xBT ceramics in the 2θ ranges of 38–50◦ . The rhombohedral symmetry for the system in the range of x = 0–0.020 is characterized by a (0 0 3)/(0 2 1) peak splitting between 39 and 41◦ and a single peak of (2 0 2) between 45 and 48◦ . The (0 0 3)/(0 2 1) peak splitting is obvious until x = 0.035. A distinct (0 0 2)/(2 0 0) peak splitting between 45 and 48◦ can be seen when x ≥ 0.025, corresponding to a tetragonal
Fig. 2. XRD patterns of (1–3x)NBT–2xKBT–xBT lead-free ceramics in 2θ range of 38–50◦ .
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Y. Li et al. / Materials Chemistry and Physics 94 (2005) 328–332 Table 1 Ferroelectric properties of (1–3x)NBT–2xKBT–xBT lead-free ceramics
Fig. 3. Dielectric constant εT33 /ε0 and dielectric loss tan δ as a function of KBT and BT concentration x for (1–3x)NBT–2xKBT–xBT ceramics.
amount x. A full same tendency of them can be seen that the d33 and kp increase with increasing x, reach the maximum value of d33 = 150 pC N−1 and kp = 0.298 at x = 0.035 then decrease. These properties demonstrate that the compositions near the MPB have relatively high piezoelectric and electromechanical activities. It is attributed to an increase in the number of possible spontaneous polarization direction for the compositions near the MPB due to the coexistence of rhombohedral and tetragonal phases [11].
x
Pr (C cm−2 )
Ec (kV mm−1 )
0.000 0.010 0.020 0.025 0.030 0.035 0.040
29.0 33.5 31.0 32.1 33.6 35.0 33.3
6.11 7.32 7.72 6.41 5.74 4.55 4.82
The measurement of P–E hysteresis loops was conducted to examine the ferroelectric properties of (1–3x)NBT–2xKBT–xBT ceramics. Table 1 shows the remanent polarization (Pr ) and coercive field (Ec ) values derived from the P–E hysteresis loops of (1–3x)NBT–2xKBT–xBT ceramics. It can be seen that in the composition range of x = 0–0.040, the coercive field decreases with the increase of KBT and BT concentration through a minimum value at x = 0.035 and then tends to increase, which is generally consistent with the variation trend of piezoelectric constant with KBT and BT concentration. Fig. 6 shows the P–E hysteresis loops of (1–3x)NBT–2xKBT–xBT ceramics with x = 0 and 0.035. It can be seen evidently that the ceramics of x = 0.035 has a remanent polarization Pr of 35.0 C cm−2 ,
Fig. 4. Mechanical quality factor Qm and frequency constant N as a function of KBT and BT concentration x for (1–3x)NBT–2xKBT–xBT ceramics.
Fig. 5. Piezoelectric constant d33 and electromechanical coupling factor kp as a function of KBT and BT concentration x for (1–3x)NBT–2xKBT–xBT ceramics.
Fig. 6. P–E hysteresis loops for the specimens of x = 0 and 0.035.
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Fig. 7. Temperature dependence of dielectric constant (εr ) for (1–3x)NBT–2xKBT–xBT ceramics at 1, 10 and 100 kHz with x = 0, 0.010, 0.025, 0.035 and 0.040.
which is significantly larger than 29.0 C cm−2 of pure NBT ceramic. In addition, it can also be seen that the coercive field Ec of 4.55 kV mm−1 for x = 0.035 composition is distinct smaller than that of 6.11 kV mm−1 for x = 0. In generally, high polarization remanent Pr and low coercive field Ec are presumably responsible for their large piezoelectric properties [12]. In present work, good electromechanical properties of NBT–KBT–BT ceramics have been obtained, which could be attributed the composition near MPB and the good ferroelectric properties. 3.3. Dielectric properties of the (1–3x)NBT–2xKBT–xBT ceramics Fig. 7 shows the temperature dependence of dielectric constant εr of x = 0.010, 0.025, 0.035 and 0.040 for (1–3x)NBT–2xKBT–xBT ceramics during heating process taken at measurement frequencies of 1, 10 and 100 kHz. It can evidently be seen that two dielectric abnormal peaks exist for all detected specimens between room temperature and 500 ◦ C. Here, the temperature of lower dielectric peak is called as Td and the temperature corresponding to maximum value of dielectric constant named as maximum temperature Tm . For x = 0.010, a strong frequency dispersive especially at Td was observed between the tested temperature range
which means that this material is a relaxor ferroelectric. It can be interpreted by the compositional fluctuation model put forward by Smolenskii and Isupov [13]. It is believed that a little amount of KBT and BT substituted to NBT could not change the relaxor ferroelectric characteristic because of the NBT itself is a relaxor ferroelectric [14,15]. However, when x = 0.025, 0.035 and 0.040, these curves of temperature dependence of dielectric constant are obviously different from that of x = 0.010. From these curves, it can be seen that dielectric constant increases slowly as the increasing of temperature initially. However, the dielectric constant increases dramatically near the Td , which is often called the characteristics of normal ferroelectric. After Td , the dielectric dispersion gradually disappears and the dielectric dispersion disappears as the concentration x increasing. These results clearly show that ferroelectric characteristic of (1–3x)NBT–2xKBT–xBT ceramics transform from relaxor ferroelectric to normal ferroelectric. The relaxor ferroelectric to normal ferroelectric transformation phenomenon has been found in many compounds such as La-modified tetragonal structure PLZT [16], Tidoped PMN [17] and Pb-doped NBT [18]. It is believed that relaxor to normal transformation is the reason of ferroelectric order–disorder type phase transitions of spontaneous micro–macrodomain switching. For those of some
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B-site co-occupied A(B1/2 B1/2 )O3 compounds, the larger the difference of ion radius between B and B , the easier forming ordered structure, which is also applicable for the (A1/2 A1/2 )BO3 -type compounds [19,20]. Due to the larger ion radius of Ba2+ (rBa2+ = 0.140 nm) and K+ (rK+ = 0.133 nm) were doped to A-site replacing the small ion radius of Na+ (rNa+ = 0.097 nm) and Bi3+ (rBi3+ = 0.096 nm), it would increase the ordered degree of A lattice site of ABO3 and decrease the compositional fluctuation. 4. Conclusions The ternary (1–3x)NBT–2xKBT–xBT lead-free ceramics were prepared by conventional ceramic technique. The crystalline phases, electromechanical, ferroelectric and dielectric properties of the ceramics were investigated. The results of XRD show the MPB of the ceramics exists in the composition range of 0.025 ≤ x ≤ 0.035 at room temperature. The preferred piezoelectric constant d33 (150 pC N−1 ) and electromechanical coupling factor kp (0.298) are obtained at x = 0.035. P–E hysteresis loops shows that it has a relatively large polarization remanent Pr and a relatively low coercive field Ec than pure NBT ceramic. The ferroelectric characteristic of (1–3x)NBT–2xKBT–xBT ceramics would transform from relaxor ferroelectric to normal ferroelectric with increasing of KBT and BT concentration. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 50272044),
Natural Science Foundation of Hubei province, China (Grant No.2002AB076) and Nippon Sheet Glass Foundation for Materials Science and Engineering (Japan).
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