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Dielectric and piezoelecrtic properties of lead-free

(Na0.5Bi0.5)TiO3–NaNbO3 ceramics

Yueming Lia,b, Wen Chena,*, Jing Zhoua, Qing Xua, Huajun Suna, Renxin Xua

aInstitute of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, Hubei, PR ChinabJingdezhen Ceramics Institute, Jingdezhen 333001, PR China

Received 25 February 2004; accepted 26 April 2004

www.elsevier.com/locate/mseb

Materials Science & Engineering B 112 (2004) 5–9

Abstract

In this paper, lead-free [Na0.5(1+x)Bi0.5(1�x)](Ti(1�x)Nbx)O3 (x = 0–0.08) ceramics were fabricated by using conventional ceramic technique.

The phase structure of the solid solution has been determined by the X-ray diffraction. Dielectric study revealed that the dielectric relaxor

behavior becomes more obviously by doping of NaNbO3 into (Na0.5Bi0.5)TiO3. The samples in the composition range from x = 0.01 to 0.02

exhibited excellent electrical properties, piezoelectric constant d33 = 80–88 pC/N; electromechanical planar coupling coefficients kp = 17.92%.

The results show that the [Na0.5(1+x)Bi0.5(1�x)](Ti(1�x)Nbx)O3 ceramics are one of the promising lead-free materials for ultrasonic transducer

applications.

# 2004 Elsevier B.V. All rights reserved.

Keywords: Dielectric properties; Piezoelectric properties; Na0.5Bi0.5TiO3; Perovskite; Relaxor

1. Introduction

Lead oxide based ferroelectrics, 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 [1,2]. However, volatiliza-

tion of toxic PbO during high-temperature sintering not only

causes environmental pollution but also generate un-stabi-

lity of composition and electrical properties of products.

Therefore, it is necessary to develop environment-friendly

lead-free piezoelectric ceramics to replace PZT based cera-

mics, which has become one of the main trends in present

development of piezoelectric materials.

Sodium bismuth titanate, Na0.5Bi0.5TiO3 (NBT), is a

kind of perovskite (ABO3-type) ferroelectric discovered

by Smolenskii et al. in 1960 [3]. NBT is considered to be

an excellent candidate of lead-free piezoelectric ceramics

because it is rhombohedral symmetry with a = 3.891 A and

a = 898360 at room temperature. It is ferroelectric with a

relatively large remanent polarization, Pr = 38 mC/cm2, and

* Corresponding author. Tel.: +86 27 8786 4033; fax: +86 27 8764 2079.

E-mail address: chenw@public.wh.hb.cn (W. Chen).

921-5107/$ – see front matter # 2004 Elsevier B.V. All rights reserved.

oi:10.1016/j.mseb.2004.04.019

a relatively large coercive field, Ec = 7.3 kV/mm [3]. It

reveals a very interesting anomaly of dielectric properties as

a result of low temperature phase transition from ferro-

electric to anti-ferroelectric phase at 200 8C. However,

NBT has a drawback of high conductivity and high coercive

field Ec to cause problems in poling process. To improve its

properties, solid solution of NBT with BaTiO3 [4], SrTiO3

[5], K0.5Bi0.5TiO3 [6,7], NaNbO3 [8] have been investigated.

Lanthanum (La2O3) was also introduced to modify NBT’s

properties [9].

In this paper, the lead-free [Na0.5(1+x)Bi0.5(1�x)](Ti(1�x)-

Nbx)O3 [(1 � x)NBT–xNN, x = 0–0.08] solid solution was

fabricated by using conventional ceramic technique. The

dielectric and piezoelectric properties of (1 � x)NBT–xNN

were also characterized.

2. Experimental

The conventional solid state reaction method was used

to prepare [Na0.5(1+x)Bi0.5(1�x)](Ti(1�x)Nbx)O3 (x = 0, 0.01,

0.02, 0.03, 0.04, 0.05, 0.06, 0.08) ceramics. Reagent grade

oxide or carbonate powders of Bi2O3, Na2CO3, TiO2 and

Y. Li et al. / Materials Science & Engineering B 112 (2004) 5–96

Nb2O5 are used as starting raw materials. The oxides and

carbonates were mixed in ethanol with agate balls by ball

milling for 4 h. After being mixed, the dried powder was

calcined at 850–900 8C for 2 h. The calcined powder was

reground by ball milling for 6 h. The dried powder was mixed

with polyyinyl alcohol and pressed at 150 MPa into pellets

20 mm in diameter and about 1.5 mm in thickness. The green

compacts were sintered at various temperatures (1150–

1200 8C) 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 silicon

oil at 80 8C under 3–4 kV/mm for 15 min.

X-ray powder diffraction (XRD) patterns were taken on a

D/MAX-III X-ray diffractometer with Cu Ka radiation (l =

1.5418 A) and graphite monochrometer. The relative dielec-

tric constant er and dissipation factor (tan d) at room and

elevated temperature were measured at 1, 10 and 100 kHz

using TH2816 LRC meter. Piezoelectric constant d33 of the

samples were measured by means of quasistatic d33 meter

(ZJ-3A) based on Berlincourt method. Dielectric and piezo-

electric properties were measured by means of the reso-

nance-antiresonance method using a precision impedance

analyzer (HP4294A). The electromechanical coupling fac-

tor kp was calculated from the resonance and antiresonance

frequencies based on the Onoe’s formulas [10].

3. Results and discussions

3.1. The X-ray diffraction patterns of (1 � x)NBT–xNN

ceramics

The XRD analysis of the ceramics powder shows that

(1 � x)NBT–xNN is pure of single phase with a perovskite

structure and forms a solid solution (Fig. 1). At room

temperature, the NBT is rhombohedral ferroelectric,

whereas NaNbO3 is known to be an orthorhombic structure

anti-ferroelectric [11]. XRD pattern of NBT, all of the peaks

could be indexed on the basis of the rhombohedral unit cell

Fig. 1. Powder XRD patterns of the (1 � x)NBT–xNN ceramics.

with a = 0.389 nm and a = 89.68. The result reveals that solid

solution samples without any secondary impurity phases can

be prepared in the NBT–NN system. With increasing of

NaNbO3 content, the diffraction peaks shift toward a lower

angle since it is expected the Ti4+ (0.61 A, ionic diameter)

could substitute by Nb5+ (0.64 A). The substitution seems to

have caused the enlargement of the NBT unit cell.

3.2. Dielectric properties of (1�x)NBT–xNN

3.2.1. Temperature dependence of the dielectric constant

The temperature dependence of dielectric constant (er) at

10 kHz for (1 � x)NBT–xNN samples with 0 � x � 0.08 is

shown in Fig. 2. For undoped NBT, two sharp phase

transition are observed at 180 and 305 8C, corresponding

to the phase transitions of ferroelectric (rhombohedral)-

anti-ferroelectric (tetragonal) (at Tf) and anti-ferroelectric

(tetragonal)-paraelectric (tetragonal) (at Tc), respectively

[4,12]. For the samples with x = 0.01, 0.02 and 0.03, the two

phase transitions are also observed. However, their phase

transition temperatures shift to lower temperatures and the

peaks become much broader than that of pure NBT. When

the NaNbO3 content is higher than 0.04 (x � 0.04), only

one rounder e peak is observed in the examined tempera-

ture range. At room temperature, the dielectric constant

er (x = 0–0.08) varies from 467 to 889, the dielectric loss

tangent, tan d is 4.11–6.26 % (Table 1), indicating that

(1 � x)NBT–xNN ceramics should be suitable for practical

application.

The temperature dependence of dielectric constant er and

dielectric loss tan d of (1 � x)NBT–xNN samples with x =

0.02, 0.04, 0.06, 0.08 under various frequencies is shown in

Fig. 3. A strong frequency dispersion of the dielectric

permittivity is clearly seen for all samples. The temperature

(Tm) of the dielectric constant maximum increases and the

em value decreases with increasing frequency. It is proved

that NBT is a relaxor ferroelectric and the Na+ and Nb5+ co-

doped NBT make the ceramics become more relaxor-like

ferroelectric.

Fig. 2. Temperature dependence of the dielectric constant (er) of (1� x)NBT–

xNN ceramics at 10 kHz.

Y. Li et al. / Materials Science & Engineering B 112 (2004) 5–9 7

Table 1

The dielectric and piezoelectric properties of (1 � x)NBT–xNN ceramics

NaNbO3 content (x) eT33=e0 (10 kHz) Tan d (%) (10 kHz) d33 (pC/N) kp (%) Qm N’

0 467 4.11 61 17.95 140 3143

0.01 637 5.60 80 17.48 90 3178

0.02 624 5.90 88 17.92 87 3173

0.03 754 5.96 60 14.90 85 3188

0.04 753 6.26 50 12.50 92 3187

0.05 801 5.40 32 12.38 69 3197

0.06 811 6.01 31 12.50 52 3220

0.08 889 4.53 – – – –

This relaxor phenomenon has been found in many

compounds such as Pb(Mg1/3Nb2/3)O3, Pb(Zn1/3Nb2/3)O3,

(Pb, La)(Zr, Ti)O3 and doped BaTiO3 with perovskite

structure [13–16]. The relaxor behavior can be induced

by many reasons such as the merging of micropolar regions

into macropolar regions [16], local compositional fluctua-

tion [17], superparaelectric[18] and dipolar glass model

[19]. In the solid solution of (1 � x)NBT–xNN, Na+ and

Bi3+ ions co-occupy the A-site of ABO3 perovskite struc-

ture, Ti4+ and Nb5+ ions co-occupy the B-site, therefore

the cations disorder in perovskite unit cell should be

one of the reason for the appearance of relaxor state.

On the other hand, it is known that the NaNbO3 shows

Fig. 3. Temperature dependence of dielectric constant (er) and dielectric loss (tan d) f

(c) x = 0.06, and (d) x = 0.08.

anti-ferroelectric at room temperature. In this case, the

macrodomain in pure NBT should be divided into micro-

domains with increasing Na+ and Nb5+ ion doping, which

also may result in the appearance of the more relaxor-like

behavior.

3.2.2. Diffusion phase transition

For a normal ferroelectric, the dielectric constant above

the Curie temperature follows the Curie–Weiss law

described by:

e ¼ C

T � T0(1)

or (1� x)NBT–xNN ceramics at 1, 10, 100 kHz with (a) x = 0.02, (b) x = 0.04,

Y. Li et al. / Materials Science & Engineering B 112 (2004) 5–98

Fig. 4. The inverse dielectric constant (1/er) as a function of temperature at

10 kHz for (1 � x)NBT–xNN ceramics with x = 0.02, 0.04, 0.06, and 0.08

(the symbols: experimental data, the solid line: fitting to Curie–Weiss law).

Table 2

The Curie–Weiss temperature (T0), the Curie–Weiss constant (C), the

temperature above the dielectric constant follows the Curie–Weiss law

(Tcw), and the diffuseness coefficient (g) for (1 � x)NBT–xNN ceramics at

10 kHz

Composition x = 0.02 x = 0.04 x = 0.06 x = 0.08

T0 (8C) 300 285 275 305

C � 107 (8C) 2.579 1.325 0.711 0.612

Tcw (8C) 340 340 340 360

DTm = Tcw � Tm (8C) 40 55 65 55

g 1.295 1.42 1.525 1.56

where C is the Curie–Weiss constant, and T0 is the Curie–

Weiss temperature. Fig. 4 shows the inverse e as a function of

temperature at 10 kHz, the fitting results obtained by Eq. (1)

are listed in Table 2. Deviation from the Curie–Weiss law

can be defined by DTm as the following:

DTm ¼ Tcw � Tem (2)

where Tcw is the temperature at which e starts to follow the

Curie–Weiss law, and Tem is the temperature at which e value

reaches the maximum.

It is found that the dielectric constant of (1 � x)NBT–

xNN ceramics obeys the Curie–Weiss law at temperature

Fig. 5. The value (1/e� 1/em) as a function of (T � Tm) at 10 kHz for (1 � x)NBT–

data; the solid line: fitting to Eq. (3)].

increasing with the increase x (Table 2), implying that the

diffusion phase transition behavior have been enhanced with

increasing doping content.

A modified empirical expression was proposed by

Uchino et al. to describe the diffusion of the ferroelectric

phase transition [20]:

1

e� 1

em¼ CðT � TemÞg (3)

where g and C are assumed to be constant, the g value is

between 1 and 2. The limiting values g = 1 and g = 2 obey the

equation to Curie–Weiss law which are the character for the

case of normal ferroelectric and for an ideal relaxor ferro-

electric, respectively [21,22].

The value (1/e � 1/em) was plotted against the (T � Tm)

and the curves are shown in Fig. 5. A linear relationship is

observed for all samples. The slope of the fitting curves is

used to determine the g value. The g value varies from 1.295

to 1.56 (Table 2), indicating that the (1 � x)NBT–xNN solid

xNN ceramics with x = 0.02, 0.04, 0.06, and 0.08 [the symbols: experimental

Y. Li et al. / Materials Science & Engineering B 112 (2004) 5–9 9

solutions are more relaxor ferroelectric characteristic with

the increase doping of Na+ and Nb5+ ions.

3.3. Piezoelectric properties (1 � x)NBT–xNN ceramics

The piezoelectric properties of (1 � x)NBT–xNN cera-

mics are also listed in Table 1. The pure NBT sample shows

good piezoelectric properties: d33 = 61 pC/N, kp = 17.95%.

The relative large piezoelectric constants d33 = 31–88 pC/N

and electromechanical planar coefficients kp = 12.5–17.92%

were also observed for the doped NBT ceramics in the

composition range of x = 0.01–0.08. This should attribute

co-effect of the soft additive Nb5+ ion doping at B-site and

hard additive Na+ ion doping at A-site. When x = 0.01–0.02,

relative good piezoelectric properties such as the piezo-

electric constant d33 = 80–88 pC/N, kp = 17.92% were

obtained, which should be attribute the dominant of doping

Nb5+ ion. However, with further increasing the doping

content of NaNbO3 (x = 0.03–0.08), the piezoelectric prop-

erties decreased, which may be the reason for the dominant

of doping Na+ ion. Although NaNbO3 is anti-ferroelectric,

the morphotropic phase boundary does not appear between

NBT and NaNbO3 in this composition range. Therefore, it is

presumed that the piezoelectricity of NBT loses gradually by

adding NaNbO3 more than threshold content. Similar phe-

nomenon was also found in KNbO3–LaFeO3 system [23].

4. Conclusions

The [Na0.5(1+x)Bi0.5(1�x)](Ti(1�x)Nbx)O3 solid solution

has been successfully synthesized by using conventional

ceramics technique. Dielectric study revealed that (1 � x)-

NBT–xNN solid solutions become more relaxor ferro-

electric characteristic with the increase the content NaNbO3.

Excellent electrical properties, piezoelectric constant

d33 = 80–88 pC/N, electromechanical planar coupling coef-

ficients kp = 17.92% can be observed in the composition

range of x = 0.01–0.02. It is obvious that the [Na0.5(1+x)-

Bi0.5(1�x)](Ti(1�x)Nbx)O3 solid solution ceramics are one

of the promising lead-free ceramics for high frequency

ultrasonic transducer applications.

Acknowledgements

This work is supported by the National Natural Science

Foundation of China (Grant no. 50272044), Natural Science

Foundation of Hubei, China (Grant no. 2002AB076), and

Nippon Sheet Glass Foundation for Materials Science and

Engineering (Japan).

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