Structure and electrical properties of (Bi0.5Na0.5)0.94Ba0.06TiO3–Bi0.5(Na0.82K0.18)0.5TiO3–BiAlO3 lead free piezoelectric ceramics

Materials Chemistry and Physics 138 (2013) 140e145

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Structure and electrical properties of (Bi0.5Na0.5)0.94Ba0.06TiO3e Bi0.5(Na0.82K0.18)0.5TiO3eBiAlO3 lead free piezoelectric ceramics Peng Fu a, b, *, Zhijun Xu a, Ruiqing Chu a, Wei Li a, Xueyan Wu b, Meiju Zhao a a b

School of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, PR China School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China

h i g h l i g h t s < BNBKTexBA ceramics were prepared by solid-state reaction method. < Electrical properties of BNBKT ceramics are improved by the addition of BA. < BNBKTexBA ceramics at x ¼ 0.030 have the best electrical properties. < Td of BNBKTexBA ceramics decreases with increasing x.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 June 2012 Received in revised form 5 November 2012 Accepted 18 November 2012

0.09(Bi0.5Na0.5)0.94Ba0.06TiO3e(0.01  x) Bi0.5(Na0.82K0.18)0.5TiO3ex BiAlO3 (BNBKTexBA) lead-free piezoelectric ceramics were synthesized by conventional solid-state reaction processes. Structure, ferroelectric and piezoelectric properties of BNBKTexBA ceramics were investigated. X-ray diffraction data shows that BNBKTexBA ceramics form the pure perovskite phases and the ceramics have the morphotropic phase boundary when x  0.030. At room temperature, the BNBKTexBA ceramics at x ¼ 0.030 have better electrical properties, the piezoelectric constant d33 and planar coupling factor kp of BNBKTexBA ceramics reaches peak values at x ¼ 0.030: d33 ¼ 217 pC N1, kp ¼ 0.308. The remnant polarization Pr, mechanical quality factor Qm and relative dielectric constant 3 r of BNBKTexBA ceramics at x ¼ 0.030 attains 33.8 mC cm2, 133 and 928 (100 KHz), respectively. As BA content increase, the depolarization temperature Td shift toward lower temperature, and Td of BNBKTexBA ceramics with x ¼ 0.030 decreased to 55  C. Ó 2012 Elsevier B.V. All rights reserved.

Keywords: Ceramics Microstructure Piezoelectricity Ferroelectricity Dielectric properties

1. Introduction Bi0.5Na0.5TiO3 (BNT), one of the promising candidates to replace lead-based piezoelectric ceramics, shows a great prospect not only for environment protection but also for various applications [1]. Among the BNT systems that have been developed so far, (Bi0.5Na0.5)1xBaxTiO3 (BNBT) [2e4] and Bi0.5(Na1xKx)0.5TiO3 (BNKT) [5,6] [7], seem more interesting because they all exist the rhombohedral - tetragonal morphotropic phase boundary (MPB) near x ¼ 0.06 and 0.18, respectively. Compared with pure BNT ceramics, the (Bi0.5Na0.5)0.94Ba0.06TiO3 (BNBT6) and Bi0.5(Na0.82K0.18)0.5TiO3 (BNKT18) ceramics provide excellent piezoelectric properties due to the existence of MPB. For more extensive practical applications, other materials have been incorporated into BNBT and BNKT * Corresponding author. School of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, PR China. Tel./fax: þ86 635 8230923. E-mail addresses: [email protected], [email protected] (P. Fu). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.11.033

ceramics near MPB to form the multicomponent system solid solutions [8e14], and the electrical properties of the BNBT and BNKT ceramics all have been further enhanced, respectively. Baettig et al. [15] theoretically predicted the large ferroelectric polarization and piezoelectricity in the hypothetical perovskitestructure BiAlO3 (BA). Therefore, BA has received tremendous attention as multiferroic oxide in recent years. Belik et al. [16] prepared BA used by high-pressure high-temperature technique. BA has the hexagonal structure closely related to that of multiferroic perovskite with space group R3c, and has a noncentrosymmetric structure which can provide important properties, such as piezoelectricity and ferroelectricity [17,18]. However, its poor thermal stability and the extreme conditions used to synthesize this material limit its usability in technological applications [19,20]. Therefore, it is favorable to stabilize BA by incorporating it into other perovskite materials to form solid solutions [21e24]. Until now, there has been no report about the work on the BNBTeBNKTeBA multicomponent ceramics. In this work,

P. Fu et al. / Materials Chemistry and Physics 138 (2013) 140e145

0.09BNBT6e(0.01  x) BNKT18exBA (BNBKTexBA) ceramics were prepared by solid-state reaction method firstly, and the compositional dependence of the structure and electrical properties of the ceramics were studied. The results demonstrate that the appropriated compositional BNBKTexBA ceramics possess enhanced electrical properties.

360000

2. Experimental procedure

200000

3. Results and discussions Fig. 1(A) shows the XRD patterns of the BNBKTexBA ceramics in the 2q range of 20e70 . From Fig. 1(A), it is indicated that all samples exhibit typical ABO3 perovskite diffraction peaks and no second phases can be detected; meaning that BNBT6, BNKT18 and BA have formed a homologous solid solution with the perovsktie structure or the second phase cannot be detected because of the small content. The magnification of Fig. 1(A) in the 2q ranges of 39e 50 is shown in Fig. 1(B). In Fig. 1(B), a distinct splitting of XRD peaks is detected for specimens at 0.000  x  0.030. They can be assigned to a (003)/(021) peak splitting at about 40 and a (002)/ (200) peak splitting at about 46.5 corresponding to a rhombohedral symmetry and a tetragonal symmetry, respectively. These results show that two phases of rhombohedral and tetragonal coexist and the MPB between the rhombohedral and tetragonal phases exists in the composition range of 0.000  x  0.030 at room temperature. However, with the further increase of x, the (003)/

(A)

320000 280000

I (a.u.)

240000

160000 120000

x = 0.050 x = 0.040 x = 0.030 x = 0.020 x = 0.010 x = 0.005 x = 0.000

80000 40000 0 20

30

40

50

2θ / 180000

60

70

80

o

(111)

(200)

(B)

160000 140000 120000

x = 0.050

100000

x = 0.040 80000

x = 0.020

40000

x = 0.010 x = 0.005 x = 0.000

(003) (021)

20000 0 38

40

42

44

2θ /

46

(200)

x = 0.030

60000

(002)

I (a.u.)

The BNBKTexBA （x ¼ 0.000 to 0.050） ceramics were prepared by conventional solid-state reaction processes. The oxide or carbonate powders of high purity Bi2O3 (99.63%), Na2CO3 (99.5%), BaCO3 (99.5%), K2CO3 (99.5%), TiO2 (99.5%) and Al2O3 (99.9%) powders were used as the initial materials; the initial materials mentioned above are all produced in Sinopharm Chemical Reagent Co., Ltd, China. Firstly, the powders of these raw materials were mixed together by a planet mill in ethanol with zirconium oxide ball for 12 h. Secondly; the mixed powders were dried and calcined at 850  C for 2 h. After calcining, the powders were ball-milled again in ethanol with zirconium oxide balls for 6 h. Then the dried powders were pressed into disks (12 mm in diameter) with the polyving alcohol (PVA) as a binder. After burning off PVA at 800  C for 2 h, these disks were finally sintered at 1150  C for 2 h in air. The sintered samples were polished and pasted with silver slurry on both faces, and then fired at 740  C as electrodes. At room temperature, specimens for piezoelectric measurements were poled for 20 min by silicone oil bath with the existence of a DC electric field of 7e8 kV mm1. The density of sintered ceramics was measured by the Archimedes method. The crystal structure of the ceramics was determined by X-ray diffraction (XRD) using a Cu Ka radiation A) (Ultima IV, Rigaku co., LTD, Japan). The micro(l ¼ 1.54178  structure of the sintered ceramics was observed by scanning electron microscope (SEM; JSM-5900, Japan). The piezoelectric coefficient (d33) was measured using a quasistatic d33-m (YE2730, SINOCERA, China). The planar coupling factor (kp) and the mechanical quality factor (Qm) were determined by a resonancee antiresonance method on the basis of IEEE standards (IEEE Std 176-1987) using a precision impedance analyzer (Agilent 4294A, America). The ferroelectric polarization versus electric field (PeE) measurements was conducted at 10 Hz using a standardized ferroelectric test system (TF2000, Germany). All measurements of electrical properties above were carried out at temperatures in the range of 20e25  C. And the curve between relative dielectric constant and temperature were also measured by Agilent 4294A precision impedance analyzer.

141

48

50

o

20

Fig. 1. (A) XRD patterns at 2q between and 70 of BNBKTexBA ceramics; (B) XRD patterns at 2q between 39 and 50 of BNBKTexBA ceramics.

(021) peak splitting at around 2q of 40 is combined into a single (111) peak and the (002)/(200) peak at around 2q of 46.5 is merged into a single (200) peak nearly when x > 0.030. These results indicate that the crystal structures of BNBKTexBA ceramics are transformed into pseudo cubic symmetry, this result is similar to Refs [19,25]. Fig. 2 shows the SEM micrographs of the BNBKTexBA ceramics sintered at 1150  C. It can be observed that the sizes of crystal grain are obvious big as x  0.020. Table 1 collects physical properties of the BNBKTexBA ceramics. From Table 1, compared with the pure 0.09 BNBT6e0.10 BNKT18 ceramics with an average crystal grain size of 5 mm, the average crystal grain size of the BNBKTexBA ceramics increases obviously as x  0.020 and attains the maximum value of 7 mm as x ¼ 0.010. The grain size increases only when the additive level of BA is rather low probably because BA in a small amount acts as an impurity, which significantly improves the sintering performance. However, the grain size of the ceramics decreases obviously when x > 0.020, and the crystal grain becomes homogeneous. Piezoelectric ceramics usually require a high mechanical strength. A fine grain size and uniform grain microstructure is able to enhance the density and mechanical strength of piezoelectric ceramics [26,27]. Therefore, the ceramics with a fine grain size and homogeneous microstructure are advantageous for piezoelectric ceramics

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Fig. 2. SEM micrographs of surfaces for BNBKTexBA ceramics.

applications. However, further amount of BA (x  0.040) inhibits the grain growth and leads to the decrease of crystalline grains obviously. With further increasing x to 0.050, the average crystal grain size of the ceramics decreases gradually to 1.5 mm, as shown in Table 1. Meanwhile, many distinct pores exist in the grain boundary of the ceramics and the crystal shape becomes irregular

and crystalline boundaries become smeared-out as x  0.040, which can lead to the decrease of the density and electrical properties of the samples as discussed below. The bulk densities of the sintered samples were determined by the Archimedes method, and the results are list in Table 1. The bulk density gradually increases as x increases from 0.000 to 0.030 and

P. Fu et al. / Materials Chemistry and Physics 138 (2013) 140e145 Table 1 Physical properties of BNBKTexBA ceramics.

143 9

40

x

Average crystal grain size (mm)

Density (g cm-3)

d33 (pC N-1)

kp

Qm

Td ( C)

Tm ( C)

0.000 0.005 0.010 0.020 0.030 0.040 0.050

5.0 6.0 7.0 6.0 3.0 1.5 1.5

5.64 5.73 5.78 5.84 5.87 5.60 5.53

141 149 159 168 217 46 21

0.283 0.288 0.301 0.293 0.308 e e

370 179 184 194 133 e e

119 100 97 82 55 e e

237 249 239 237 237 312 310

Pr Ec

36

6 2

5 24 4

20

3

x=0.000 x=0.005 x=0.010 x=0.020 x=0.030 x=0.040 x=0.050

30

2

P (μ C/cm )

20 10 0 -10 -20 -30 -40 -50 -10

-8

-6

-4

-2

0

2

4

6

8

10

E (kV/mm) Fig. 3. Typical room temperature ferroelectric hysteresis loops of BNBKTexBA ceramics at a frequency of 10 Hz.

Ec (kV/mm)

Pr (μ C/cm )

28

reaches a maximum value of 5.87 g cm3 at x ¼ 0.030. Then, the bulk density decreases with further increasing x to 0.050, as shown in Table 1. These results indicate that the optimum BA addition can promote sintering and thus improve the density of the ceramics. However, excess BA addition can also result in the decrease of the bulk density. This result agrees with the result discussed in Fig. 2. The polarization versus electric field hysteresis loops of all samples were measured at room temperature and a frequency of 10 Hz, and the results are presented in Fig. 3. The variations of the remnant polarization Pr and coercive field Ec with x are shown in Fig. 4. It is evident that the ferroelectric properties of BNBKTexBA ceramics have been changed significantly with the variations of x. Compared with the 0.09 BNBT6e0.01 BNKT18 ceramics, the remnant polarization Pr of BNBKTexBA ceramics increases with increasing x and then decreases, giving a maximum value of 38.6 mC cm2 at x ¼ 0.005. The remnant polarization Pr of BNBKTe xBA ceramics as x  0.030 all has the higher value. Higher remnant polarization also shows that the BNBKTexBA ceramics have better ferroelectric properties. Moreover, the coercive field Ec of the sample decreases gradually with x further increasing to 0.050, as shown in Fig. 4. Low coercive field is beneficial to the polarization of the samples. Table 1 collects various room-temperature piezoelectric properties of BNBKTexBA ceramics sintered at 1150  C. From Table 1, the piezoelectric coefficient d33 of 0.09 BNBT6e0.01 BNKT18 ceramics attains 141 pC N1, which is higher than that of previous prepared pure BNKT18 ceramics [13,28,29] but lower than that of pure BNBT6 ceramics [10,30,31]. With the increasing value of x, the piezoelectric

40

7

32

16

50

8

2

12

1

8

0 0.00

0.01

0.02

0.03

0.04

0.05

x Fig. 4. The remnant polarization Pr and coercive field Ec of BNBKTexBA ceramics as a function of x.

coefficient d33 and planar coupling factor kp increase firstly and reach peak values at x ¼ 0.030: d33 ¼ 217 pC N1, kp ¼ 0.308, and then the two values decrease. Compared with the pure 0.09 BNBT6e 0.01 BNKT18 ceramics, piezoelectric properties of BNBKTexBA ceramics at x ¼ 0.030 have been improved obviously, as shown in Table 1. Meanwhile, the piezoelectric properties of BNBKTexBA ceramics at x ¼ 0.030 are significantly larger than the reported values of other modified BNBT6 and BNKT18 system ceramics [8e 13,28e31]. The results show that the addition of appropriate BA improves the piezoelectric properties of the BNBT6eBNKT18 system ceramics significantly. It is well known that the MPB plays a very important role in piezoelectric ceramics. When tetragonal and rhombohedral system exists simultaneously, and electric domain wall turns easily owing to the small difference of free energy between two crystalline phases. So the spontaneous polarization intensity and remnant polarization increases greatly, which can provide excellent piezoelectric properties [32e34]. Higher piezoelectric properties of BNBKTexBA ceramics at x  0.030 may be attributed to possess of MPB, just as discussed in Fig. 1(B). However, Excessive BA (x > 0.030) makes piezoelectric properties of BNBKTe xBA ceramics decreased obviously. According to point group theory and ferroelectric, the perovskite-type material with cubic symmetry has no spontaneous polarization vector in crystal cell, and no piezoelectric properties [33]. Pseudo cubic possesses high structure symmetry and approaches the cubic structure, so the perovskitetype material with pseudo cubic symmetry has no or very small piezoelectric properties. The BNBKTexBA ceramics possess pseudo cubic structures when x > 0.030, as discussed in Fig. 1(B), so the samples have not piezoelectric properties nearly when x > 0.030. Piezoelectric coefficient d33 of the BNBKTexBA ceramics become so little that the planar coupling factor kp and the mechanical quality factor Qm cannot be measured as x > 0.030, as shown in Table 1. Typically, large remnant polarization usually facilitates the increase of piezoelectric properties. At the range of the experiments, the BNBKTexBA ceramics as x  0.030 all have the higher remnant polarization (Pr) as discussed in Fig. 4, so it has the higher piezoelectric coefficient d33 as x  0.030, as discussed in Table 1. Compared with Pr of BNBKTexBA ceramics as x < 0.030, Pr of BNBKTexBA ceramics at x ¼ 0.030 decreases slightly, as shown in Fig. 4, but d33 of BNBKTexBA ceramics at x ¼ 0.030 attains the maximum value. It’s probably due to the lower Ec of BNBKTexBA ceramics at x ¼ 0.030 as shown in Fig. 4, which makes the

144

P. Fu et al. / Materials Chemistry and Physics 138 (2013) 140e145

ceramics be prone to be polarized and have better piezoelectric properties. By and large, compared with the pure 0.09 BNBT6e0.01 BNKT18 ceramics, the mechanical quality factor Qm of the BNBKTexBA ceramics as x > 0.000 decreases totally, and Qm attains 133 at x ¼ 0.030, as shown in Table 1. Generally, BA acts as an additive and promotes the movement of the domains, which lead to an increase of inner attrition together with a decrease of Qm of BNBKTexBA ceramics compared with the pure 0.09 BNBT6e0.01 BNKT18 ceramics. Fig. 5 shows temperature dependence of the relative dielectric constant and loss tangent of BNBKTexBA ceramics at a frequency of 100 kHz. In Fig. 5, Td is the depolarization temperature which corresponds to the transition from a ferroelectric state to so-called “anti-ferroelectric” state which is defined as one in which lines of ions in the crystal are spontaneously polarized, but with neighboring lines polarized in antiparallel directions [35], while Tm is the maximum temperature at which relative dielectric constant 3 r reaches a maximum value and corresponds to a transition from an “anti-ferroelectric” state to a paraelectric state [36]. The inflexion of the curves was called “a shoulder” on the curve in some reports and indicated an intermediate phase transition. An opinion about this shoulder is that the temperature region between the shoulder and Tm is an anti-ferroelectric phase [37]. The Curie point (Tc) can be approximately determined by using the maximum temperature (Tm) [38]. From Fig. 5, at room temperature, the relative dielectric constants of the BNBKTexBA ceramics are lower than that of the 0.09BNBT6e0.01 BNKT18 ceramics. But BNBKTexBA ceramics at x ¼ 0.030 have still the higher relative dielectric constant, and 3 r of the BNBKTexBA ceramics at x ¼ 0.030 attains 928. Furthermore, compared with pure 0.09BNBT6e0.01 BNKT18 ceramics, the dielectric maxima also increase significantly as x  0.030 and it attains 5681 and 3951 for the BNBKTexBA ceramics at x ¼ 0.020 and 0.030, respectively. From Table 1, the BA addition induces a decrease in Td of the BNBKTexBA ceramics, and the obtained Td of the ceramics decreases from 119 to 55  C as x increases from 0.000 to 0.030. However, when x > 0.030, Td of the BNBKTexBA ceramics is not observed in Fig. 5, implying that Td has moved to lower temperature than the measurement temperature that results in the weak ferroelectric and piezoelectric properties of the ceramics. The results are consistent with Table 1 and Fig. 3. The shift of the Td peak toward lower temperatures ascribed to the decreased polarization in the ceramics. Because BA incorporation reduced the tetragonality of the crystal structure of the BNBKTexBA ceramic system as shown in Fig. 2. This phenomenon is similar to the reports of

4000

3



-8

0.30

-10

¼

ðT  Tm Þg C

x=0.000,γ x=0.005,γ x=0.010,γ x=0.020,γ x=0.030,γ x=0.040,γ x=0.050,γ

-12

tanδ

0.15

1000

3m

0.35

0.20

2000

1

(1)

Where 3 is dielectric constant at a temperature T and 3 m is its maximum value at Tm, C is a constant and g is called the degree of relaxation which is used to express the diffuseness exponent of the phase transition. By fitting the experimental data based on Eq. (1), we can obtain the value of parameter g. The value of g can vary from 1, for normal ferroelectrics with a normal Curie-Weiss behavior, to 2, for completely disordered relaxor ferroelectrics [41,42]. Fig. 6 shows the plot of ln(1/3  1/3 m) as a function of ln(T  Tm) at 100 kHz for BNBKTexBA ceramics and the g values of each specimen. It is obvious that the value of g for all BNBKTexBA ceramics is close to 2, suggesting the appearance of the typical relaxor behavior. Moreover, the dielectric properties sensitively depend on the BA content; the value of g increases with the increasing BA content and then decreases, the value of g reaches peak values 1.95 as x ¼ 0.010. This result indicates that the degree of the relaxor behavior for BNBKTexBA ceramics increases gradually as x  0.010, but the degree of the relaxor behavior decreases as the further BA content increases. From Fig. 5, it is also found that the loss tangent tand gradually increases with the increase of temperature and then decreases because of the less distortion in the crystalline structure after depolarization until Tm [43]. At higher temperature, increased conductivity of samples causes the increase of tand dramatically, so the curve starts to rise again. Moreover, lower dielectric loss at

0.25

Td

3000

1

ln(1/ε -1/ε m)

5000

εr

Tm

x=0.000 x=0.005 x=0.010 x=0.020 x=0.030 x=0.040 x=0.050

6000

Ref. [19]. However, Tc of the BNBKTexBA ceramics as x  0.030 is barely changed by the addition of BA. Tc of the BNBKTexBA ceramics at x > 0.030 increases greatly, but the relative dielectric constant 3 r at x > 0.030, on the whole, becomes very small, suggesting that the dielectric properties are weakened obviously when addition of BA is superabundant. From Fig. 5, it is also found that the temperature-dependent dielectric curves of BNBKTexBA ceramics all show broad peaks and the broadening degree is changed with addition of BA. This implies that the BNBKTexBA ceramics display a diffusing phase transition character and the degree of diffusion is changed with the addition of BA. In order to characterize the dielectric dispersion and diffuseness of the phase transition, the modified CurieeWeiss law is proposed by many researchers [39,40]:

=1.51 =1.58 =1.95 =1.70 =1.64 =1.52 =1.50

-14

-16

0.10

-18 0

0.05

-20

-1000

0.00 0

100

200

300

400

500

o

T ( C) Fig. 5. The relative dielectric constants and loss tangent of BNBKTexBA ceramics as a function of temperature (100 kHz).

-3

-2

-1

0

1

2

3

4

5

6

7

ln(T-Tm) Fig. 6. Plots of ln(1/3  1/3 m) as a function of ln(T  Tm) at 100 kHz for BNBKTexBA ceramics.

P. Fu et al. / Materials Chemistry and Physics 138 (2013) 140e145

room temperature is obtained in all samples, and the BNBKTexBA ceramics at x ¼ 0.030 have a lower dissipation factor (tand ¼ 0.061) at a frequency of 100 kHz. 4. Conclusions The BNBKTexBA ceramics were designed and prepared by a conventional solid-state process. Effect of the variations of x on the structure and electrical properties of BNBKTexBA ceramics has been investigated. XRD data shows that BNBKTexBA ceramics have formed the pure perovskite phase, and the ceramics have the MPB between the rhombohedral and tetragonal phases when x  0.030. SEM images indicate that the grain sizes of BNBKTexBA ceramics are obvious big as x  0.020, but the grain size of the ceramics decreases obviously as x > 0.020. At room temperature, the ferroelectric and piezoelectric properties of the BNBT6eBNKT18 ceramics have been improved with the addition of appropriate BA. The remnant polarization Pr of BNBKTexBA ceramics increases with the increase of x and reaches peak value at x ¼ 0.005: Pr ¼ 38.6 mC cm2. Moreover, BNBKTexBA ceramics at x ¼ 0.030 have the highest piezoelectric coefficient (d33 ¼ 217 pC N1), the highest planar coupling factor (kp ¼ 0.308), lowest mechanical quality factor (Qm ¼ 133), higher dielectric constant (3 r ¼ 928) and lower dielectric loss (tand ¼ 0.061) at a frequency of 100 kHz. Electrical properties are obviously higher than that of the previous prepared modified BNBT6 and BNKT18 system ceramics. However, as BA content increase, Td shift toward lower temperature. Moreover, the BNBKTexBA ceramics have the ferroelectric relaxor behavior and the degree of the relaxor behavior attains peak value (g ¼ 1.95) as x ¼ 0.010. Acknowledgments This work was supported by the Natural Science Foundation of Shandong Province of China (No. ZR2012EMM004). References [1] J. Rödel, W. Jo, K. Seifert, E.M. Anton, T. Granzow, D. Damjanovic, Perspective on the development of lead-free piezoceramics, J. Am. Ceram. Soc. 92 (2009) 1153e1177. [2] T. Takenaka, K. Maruyama, K. Sakata, (Bi1/2Na1/2)TiO3eBaTiO3 system for leadfree piezoelectric ceramics, Jpn. J. Appl. Phys. Part. 1 30 (9B) (1991) 2236e2239. [3] Q. Xu, S.T. Chen, W. Chen, S.J. Wu, J. Zhou, H.J. Sun, Y.M. Li, Synthesis and piezoelectric and ferroelectric properties of (Na0.5Bi0.5)1xeBaxTiO3 ceramics, Mater. Chem. Phys. 90 (2005) 111e115. [4] H. Simons, J. Daniels, W. Jo, R. Dittmer, A. Studer, M. Avdeev, J. Rödel, Mark Hoffman, Electric-field-induced strain mechanisms in lead-free 94%(Bi1/ 2Na1/2)TiO3e6%BaTiO3, Appl. Phys. Lett. 98 (2011) 082901. [5] O. Elkechai, M. Manier, J.P. Mercurio, Na0.5Bi0.5TiO3eK0.5Bi0.5TiO3 (NBT-KBT) system: a structural and electrical study, Phys. Stat. Sol.(a) 157 (1996) 499e506. [6] A. Sasaki, T. Chiba, Y. Mamiya, E. Otsuki, Dielectric and piezoelectric properties of (Bi0.5Na0.5)TiO3e(Bi0.5K0.5)TiO3 systems, Jpn. J. Appl. Phys. 38 (1999) 5564e5567. [7] T. Takenaka, H. Nagata, Current status and prospects of lead-free piezoelectric ceramics, J. Eur. Ceram. Soc. 25 (2005) 2693e2700. [8] G.F. Fan, W.Z. Lu, X.H. Wang, F. Liang, Effects of manganese additive on piezoelectric properties of (Bi1/2Na1/2)TiO3eBaTiO3 ferroelectric ceramics, J. Mater. Sci. 42 (2007) 472e476. [9] Y.M. Li, W. Chen, Q. Xu, J. Zhou, X.Y. Gu, S.Q. Fang, Electromechanical and dielectric properties of Na0.5Bi0.5TiO3eK0.5Bi0.5TiO3 - BaTiO3 lead-free ceramics, Mater. Chem. Phys. 94 (2005) 328e332. [10] P. Fu, Z.J. Xu, R.Q. Chu, W. Li, G.Z. Zang, J.G. Hao, Piezoelectric, ferroelectric and dielectric properties of Sm2O3-doped (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics, Mater. Chem. Phys. 124 (2010) 1065e1070. [11] Y. Hiruma, H. Nagata, T. Takenaka, Phase transition temperatures and piezoelectric properties of (Bi1/2Na1/2)TiO3e(Bi1/2K1/2)TiO3eBaTiO3 lead-free piezoelectric ceramics, Jpn. J. Appl. Phys. 45 (9B) (2006) 7409e7412. [12] Y. Makiruchi, R. Aoyagi, Y. Hiruma, H. Nagata, T. Takenaka, (Bi1/2Na1/2)TiO3e (Bi1/2K1/2)TiO3eBaTiO3-based lead-free piezoelectric ceramics, Jpn. J. Appl. Phys. 44 (6B) (2005) 4350e4353. [13] P. Fu, Z.J. Xu, R.Q. Chu, W. Li, Q. Xie, G.Z. Zang, Effects of Eu2O3 on the structure and electrical properties of 0.82Bi0.5Na0.5TiO3e0.18Bi0.5K0.5TiO3 lead-free piezoelectric ceramics, Curr. Appl. Phys. 11 (2011) 822e826.

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