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Physica B 405 (2010) 2781–2784

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Physica B

0921-45

doi:10.1

n Corr

Liaoche

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Ferroelectric and dielectric properties of Sr2�x(Na, K)xBi4Ti5O18 lead-freepiezoelectric ceramics

Qian Chen a, Zhijun Xu a,c,n, Ruiqing Chu a,c, Jigong Hao a, Yanjie Zhang a, Guorong Li b, Qingrui Yin b

a College of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, People’s Republic of Chinab The State Key Lab of High Performance Ceramics and Superfinemicrostructure, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050,

People’s Republic of Chinac Liaocheng University Renewable Energy & ECO—Materials Engineering Center, People’s Republic of China

a r t i c l e i n f o

Article history:

Received 8 February 2010

Received in revised form

24 March 2010

Accepted 25 March 2010

Keywords:

Bismuth-layered structure

Dielectric properties

Piezoelectric properties

Ferroelectric properties

26/$ - see front matter & 2010 Elsevier B.V. A

016/j.physb.2010.03.072

esponding author at: College of Material

ng University, Liaocheng 252059, People’s Re

+86 635 8230923.

ail address: zhjxu@lcu.edu.cn (Z. Xu).

a b s t r a c t

(Na, K)-doped Sr2Bi4Ti5O18 (SBTi) bismuth layer structure ferroelectric ceramics were prepared by the

solid-state reaction method. Pure bismuth-layered structural Sr2�x(Na, K)xBi4Ti5O18 (x¼0.1, 0.2, 0.3,

and 0.4) ceramics with uniform grain size were obtained in this work. The effects of (Na, K)-doping on

the dielectric, ferroelectric and piezoelectric properties of SBTi ceramics were investigated. Results

showed that (Na, K)-doping caused the Curie temperature of SBTi ceramics to shift to higher

temperature and enhanced the ferroelectric and piezoelectric properties. At x¼0.2, the ceramics

exhibited optimum properties with d33¼20 pC/N, Pr¼10.3 mC/cm2, and Tc¼324 1C.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Bismuth-layered structure ferroelectric (BLSF) ceramics arecompounds of great technological interest due to their applica-tions as piezoelectric material with high Curie temperature (Tc),low temperature coefficients of dielectric, low aging rate, andstrong anisotropic characters [1,2]. These characters make BLSFceramics attractive in the field of developing piezoelectricmaterials, especially under high frequency and high temperatureconditions [3]. The BLSF ceramics belong to the Aurivilliusfamily [4,5], which have the general formula (Bi2O2)2 + (Am�1

BmO3m +1)2� . Position A is generally occupied by an alkaline,alkaline-earth, and rare-earth metal, B by a d0 transition element,and m is the number of BO6 octahedral in each pseudo-perovskiteblock (m¼1–5) [6].

Sr2Bi4Ti5O18 (SBTi) studied in this work is a well-knownmember of BLSF ceramics with m¼5, and recently the SBTicompound has been widely noticed and investigated due toits promising application for ferroelectric random access mem-ories (FeRAMs) [7–12]. However, considering the fact that thereported value of the remnant polarization Pr in SBTi is rathersmall, it limits its application for high-density FeRAMs. Some

ll rights reserved.

s Science and Engineering,

public of China.

works have been carried out by modifying SBTi ceramicsby replacing the A and/or the B site cation of SBTi to improvethe properties and thus satisfy the practical application. In recentyears, researchers [9,10] have prepared the SBTi ceramics by Asite substitution, such as La, Nd, Sm, and Dy substituting for Bi. Itwas found that the ferroelectric properties of the obtained SBTiceramics with random orientation were remarkably enhancedowing to these A site substitutions. However, there are fewreports of modified SBTi ceramics concerning substituting for Srat A site.

Recently, L. Ma et al. [13] reported the (Li, Ce)-doped NaBi5Ti4O15

ceramics, which showed relative large piezoelectric constant d33 of26.5 pC/N. Wang et al. [14] reported that (Na, Ce)-doped CaBi2Nb2O9

ceramics exhibited good performance with the enhanced d33 of16 pC/N, which was much larger than that of undoped CaBi2Nb2O9

ceramics (5 pC/N). The above results show that Na-doping caneffectively improve the piezoelectric properties of some ceramics.Based on the above results, it is expected that the doping of Nain SBTi ceramics may exhibit a similar trend in the propertiesenhancement. Furthermore, Na, K, and Sr have similar ionicradii, and (Na, K) can be replaced by Sr, which have been discoveredin KNN-based ceramics [15,16]. Hence, it is reasonable to expectthat (Na, K) dopants can completely diffuse into the SBTi latticesto form a homogeneous solid solution. Therefore, in our work,Na and K were used as the dopants in SBTi to substitute for Sr atA site.

In this paper, SBTi ceramic samples with Sr2 + substituted byNa+ and K+ ions have been prepared by the conventional

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Q. Chen et al. / Physica B 405 (2010) 2781–27842782

solid-state reaction method. The effects of (Na, K) substitution onthe structure, microstructures, and electrical properties of SBTiceramics have been studied.

2. Experimental

The Sr2�x(Na, K)xBi4Ti5O18 [abbreviated to SBTi-(Na, K)x]ceramics with x¼0.1, 0.2, 0.3, and 0.4 were prepared byconventional solid-state reaction process. Reagent grade SrCO3

(99%), K2CO3 (99%), Na2CO3 (99.8%), Bi2O3 (99.64%), and TiO2

(99.5%) were used as the starting materials. The raw materialspowders were accurately weighed according to chemical compo-sition and fully mixed through planetary ball grinding mill withagate ball media in ethanol for 8 h. Then the slurry was dried andcalcined at 870 1C for 2 h. After calcination, the powders wereball-milled again for 8 h, then dried and burned at 550 1C for30 min to exclude the impurity. Then these powders were mixedthoroughly with a polyvinylbutyral (PVB) binder solution and

20 30 40 50

(0 0

16)

(1 0

23)

(2 0

0)

(1 1

12)

(1 1

0)

(1 0

11)

(1 0

9)

(1 0

1)

(0 0

12)

x = 0.2

x = 0.1

x = 0.3

Inte

nsity

(a.u

)

2θ (degree)

x = 0.4

Fig. 1. XRD patterns of the SBTi-(Na, K)x powders.

Fig. 2. SEM micrographs of SBTi-(Na, K)x ceramics sintered at 11

pressed into disks 12 mm in diameter and about 0.3–0.5 mm inthickness. After burning off PVB, these disks were finally sinteredat 1130–1150 1C for 2 h in air, followed by furnace cooling. Thesintered ceramics were polished and pasted with silver slurry onboth sides, and then fired at 740 1C for 20 min to form theelectrode.

The X-ray diffraction (XRD) patterns of the ceramics weredetermined using X-ray powder diffraction analysis (XRD) (D8Advance, Bruker Inc., Germany). The microstructure and surfacemorphology of the ceramics were characterized a using scanningelectron microscope (SEM) (JSM-5900, Japan). The ferroelectrichysteresis loops were measured at 10 Hz using an aix-ACCTTF2000FE-HV ferroelectric test unit (aix-ACCT Inc., Germany). Thetemperature dependences of the dielectric properties weremeasured using HP4294A precision impedance analyzer (AgilentInc., USA). For the measurement of piezoelectric properties,samples were poled in silicone oil at room temperature under adc electric field from 5.0 to 6.0 kV/mm for about 30 min. Thepiezoelectric constants d33 were measured using a quasi-static d33

meter (YE2730 SINOCERA, China).

3. Results and discussion

Fig. 1 shows the XRD patterns of the SBTi-(Na, K)x powders inthe 2y range of 20–501. The peaks of SBTi-(Na, K)x samples areindexed according to the diffraction data of SBTi (PDF#140276). Itis found that all samples are single phase of BLSF crystal structure,indicating that Na, K substitution does not change the basiccrystal structure of SBTi ceramics. The highest intensity ofdiffraction peak is (1 0 1 1) reflection in all XRD patterns, whichis the characteristic peak of the bismuth layer-structuredceramics with m¼5.

Fig. 2 shows the SEM micrographs of SBTi-(Na, K)x ceramicssintered at 1140 1C for 2 h. 1140 1C is the optimum sinteringtemperature of the SBTi-(Na, K)x ceramics, which can bedetermined by the measured densities. It is found that themicrostructures of all sintered samples are typical of the ceramicmaterials based on Aurivillius bismuth-layered compounds withthe strongly anisotropic and plate-like grains [17,18]. Dense

40 1C for 2 h: (a) x¼0.1, (b) x¼0.2, (c) x¼0.3, and (d) x¼0.4.

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microstructures with uniform grain size are obtained in allsamples and the distinct pores decrease obviously withincreasing (Na, K) content. This result suggests that (Na, K)-doping can effectively promote sintering of the SBTi ceramics andobviously suppress the occurrence of defects. Moreover, it isobvious that the grain size increases gradually with the increasing(Na, K) content, which reveals that (Na, K) dopant acts as a graingrowth accelerant and has an evident effect on grain sizeaccretion of SBTi ceramics.

Fig. 3 shows the temperature dependence of dielectricconstant of SBTi-(Na, K)x ceramics at 100 kHz. Sharp dielectricpeaks appear when the temperature is higher than 310 1C, whichcorresponds to the Curie temperature. As the (Na, K)-doping levelincreases, the obtained Curie temperature (Tc) increases linearly,which is clearly illustrated in Fig. 4. For the pure SBTi ceramics, Tc

is measured to be 286.7 1C [10], while the Curie temperaturesobtained in (Na, K)-doped ceramics in the present work are allhigher than that of pure SBTi ceramics. At x¼0.4, the value of theCurie temperature reaches up to 351 1C, which is much higherthan that of SBTi ceramics. Two possible reasons for the enhancedCurie temperature of (Na, K)-doped SBTi ceramics are as follows:on the one hand, due to the lower valence of (Na+, K+) than that ofSr2 + , the substitution of (Na+, K+) for Sr2 + introduced anionvacancy in A site, which could lead to an enhancement of

0 100 200 300 400

400

600

800

1000

1200

351

340

324

314

ε r

Temperature (°C)

x = 0.1x = 0.2x = 0.3x = 0.4

Fig. 3. Temperature dependence of dielectric constant of the SBTi-(Na, K)x

ceramics at 100 kHz.

0.1 0.2 0.3 0.4310

320

330

340

350

T c (°

C)

Doping content x

Fig. 4. Dependence of the Curie temperature for SBTi-(Na, K)x ceramics on doping

content.

ferroelectric structure distortion and thus result in a higher Tc

[19,20]; on the other hand, (Na, K)-doping decreases the tolerancefactor t [t¼ ðrAþrOÞ=

ffiffiffi

2pðrBþrOÞ where rA, rB, and rO are the ionic

radii of an A site cation, a B-site cation, and an oxygen ion,respectively] of SBTi-(Na, K)x, and according to the theory ofDonaji, the decreased tolerance factor t caused by (Na, K)-dopingwill increase the Curie temperature [21].

Fig. 5 shows temperature dependence of dielectric loss of SBTi-(Na, K)x ceramics at 100 kHz. It is found that dielectric losstangent (tan d) of all samples is lower than 3% from roomtemperature to 300 1C and reaches a peak at the Curietemperature, after which it increases rapidly owing toconductive losses. Furthermore, it is clearly shown that theroom temperature dielectric loss decreases significantly with theincreasing (Na, K) content as illustrated in the inset of Fig. 5. Atx¼0.3, the room temperature dielectric loss factor (tan d) reachesa minimum value of 1.38%, which is much lower than that ofundoped SBTi sample reported in the literature [22]. Thedecreased tan d confirms that (Na, K)-doping can obviouslysuppress the occurrence of defects of the ceramics.

Fig. 6 shows the P–E hysteresis loops of the SBTi-(Na, K)x

ceramics measured at room temperature and 10 Hz. The insetshows the remnant polarization (Pr) and coercive field (Ec) ofSBTi-(Na, K)x ceramics as a function of x. The shape of P–E loopsvaries greatly in different (Na, K)-doped compositions, and boththe values of Pr and Ec increase and then decrease with theincrease of doping content, giving the maximum values of10.3 mC/cm2 and 49 kV/cm at x¼0.2. The obtained Pr of SBTi-(Na, K)x samples with x¼0.2 is larger than that of SBTi ceramics(8.15 mC/cm2) [10]. The change of remnant polarization atdifferent compositions is considered to be dominated by thecompetition of the decrease of oxygen vacancy concentration andthe relief of structural distortion [10]. These results indicate that(Na, K)-doping with appropriate content can improve theferroelectric properties of the SBTi ceramics.

Fig. 7 shows the piezoelectric constant d33 of the SBTi-(Na, K)x

ceramics as a function of x. Similar to the variation of the remnantpolarization, the piezoelectric constant d33 of SBTi-(Na, K)x

ceramics increases with a small amount of (Na, K)-doping, andthen decreases with further increase in the value of x. At x¼0.2,the piezoelectric constant d33 reaches the maximum value of20 pC/N.

0 100 200 300 4000

1

2

3

4

5

6

1.5

1.8

2.1

2.4

tanδ

(%

)

Doping content x

tanδ

(%)

Temperature (°C)

x = 0.1x = 0.2x = 0.3x = 0.4

0.1 0.2 0.3 0.4

Fig. 5. Temperature dependence of dielectric loss of SBTi-(Na, K)x ceramics at

100 kHz (the inset shows room temperature dielectric loss of SBTi-(Na, K)x

ceramics as a function of x).

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0.1 0.2 0.3 0.4

14

16

18

20

d 33 (p

C/N

)

Doping content x

Fig. 7. Piezoelectric constant d33 of the SBTi-(Na, K)x ceramics as a function of x.

-100 -80 -60 -40 -20 0 20 40 60 80 100

-15

-10

-5

0

5

10

15

0.1 0.2 0.3 0.44

6

8

10

P r (μ

C/c

m2 )

Doping content x

42

44

46

48

50

Ec (

kV/c

m)

P ( μ

C/c

m2 )

E (kV/cm)

x = 0.1x = 0.2x = 0.3

x = 0.4

Fig. 6. P–E hysteresis loops of the SBTi-(Na, K)x ceramics measured at room

temperature and 10 Hz [the inset shows the remnant polarization Pr and the

coercive field Ec of SBTi-(Na, K)x ceramics as a function of x].

Q. Chen et al. / Physica B 405 (2010) 2781–27842784

4. Conclusions

SBTi ceramics were modified by A site substitution with (Na, K)dopants. Pure single phase layer structure Sr2�x(Na, K)xBi4Ti5O18

ceramics were formed in this work as (Na, K) content was from0.1 to 0.4. The Curie temperature (Tc) of the ceramics shifted tohigher temperature monotonously with the increasing (Na, K)-doping level. The ferroelectric and piezoelectric properties of SBTiceramics were obviously improved by (Na, K)-doping withappropriate content. At x¼0.2, the SBTi-(Na, K)x ceramicsexhibited the best properties with a relatively high piezoelectricconstant d33 of 20 pC/N and a large remnant polarization Pr of10.3 mC/cm2.

Acknowledgment

This work was supported by the National Natural ScienceFoundation of China (Nos. 50602021 and 50702068).

References

[1] T. Takeuchi, T. Tani, Y. Saito, Jpn. J. Appl. Phys. 39 (2000) 5577.[2] J.P. Mercurio, A. Souirti, M. Manier, Mater. Res. Bull. 27 (1992) 123.[3] C.M. Wang, J.F. Wang, Z.G. Gai, Scr. Mater. 57 (2007) 789.[4] B. Aurivillius, Ark. Kemi. 1 (1949) 463.[5] B. Aurivillius, Ark. Kemi. 2 (1950) 512.[6] H. Irie, M. Miyayama, T. Kudo, J. Appl. Phys. 90 (2001) 4089.[7] S.T. Zhang, C.S. Xiao, A.A. Fang, B. Yang, B. Sun, Y.F. Chen, Z.G. Liu, Appl. Phys.

Lett. 76 (2000) 3112.[8] Y. Noguchi, M. Miyayama, T. Kudo, Appl. Phys. Lett. 77 (2000) 3639.[9] L. Cui, Y.J. Hu, Physica B 404 (2009) 150.

[10] F. Qiang, J.H. He, J. Zhu, X.B. Chen, J. Solid State Chem. 179 (2006) 1768.[11] T. Ogawa, J. Eur. Ceram. Soc. 24 (2004) 1517.[12] J. Wang, G.X. Cheng, S.T. Zhang, H.W. Cheng, Y.F. Chen, Physica B 344 (2004)

368.[13] L. Ma, K. Zhao, J.X. Li, Q. Wu, M.L. Zhao, C.L. Wang, J. Rare Earths 27 (2009)

496.[14] C.M. Wang, S.J. Zhang, J.F. Wang, M.L. Wang, Mater. Chem. Phys. 118 (2009)

21.[15] B. Malic, J. Bernard, J. Holc, D. Jenko, M. Kosec, J. Eur. Ceram. Soc. 25 (2005)

2707.[16] Y.F. Chang, Z.P. Yang, L.L. Wei, B. Liu, Mater. Sci. Eng. A 437 (2006) 301.[17] M. Villegas, A.C. Caballero, C. Moure, P. Duran, J.F. Fernandez, J. Am. Ceram.

Soc. 82 (1999) 2411.[18] K. Shoji, M. Aikawa, Y. Uehara, K. Sakata, Jpn. J. Appl. Phys. 37 (1998)

5273.[19] Y. Noguchi, M. Miyayama, T. Kudo, Phys. Rev. B 63 (2001) 214102.[20] Y. Shimakawa, Y. Kubo, Y. Nakagawa, T. Kamiyama, H. Asano, F. Izumi, Appl.

Phys. Lett. 74 (1999) 1904.[21] D.Y. Suarez, I.M. Reaney, W.E. Lee, J. Mater. Res. 16 (2001) 3139.[22] K. Srinivas, A.R. James, J. Appl. Phys. 86 (1999) 3885.