J. Mater. Sci. Technol., Vol.25 No.6, 2009
777
Synthesis and Electrical Properties of Li-modified
Bi0.5 Na0.5 TiO3 -BaTiO3 Lead-free
Piezoelectric Ceramics
Yunwen Liao1,2)† and Dingquan Xiao2)
1) Institute of Applied Chemistry, China West Normal University, Nanchong 637002, China
2) Department of Materials Science, Sichuan University, Chengdu 610064, China
[Manuscript received March 10, 2008]
Lead-free piezoelectric ceramics (1−y)Bi0.5 (Na1−x Lix )0.5 TiO3 -yBaTiO3 with x=0−0.125 and y=0.02−0.12
were fabricated by a solid-state reaction process, and their dielectric, piezoelectric and ferroelectric properties were investigated. The results show that the addition of Li+ significantly improves the sintering performance and piezoelectric properties of the ceramics. X-ray diffraction (XRD) patterns indicate that the
ceramics possess pure perovskite structure. At room temperature, the ceramics provide high piezoelectric
charge constant d33 (up to 210 pC/N), high planar electromechanical coupling factor kp (34.5%), large remanent polarization Pr (up to 40 µC/cm2 ), and low coercive field Ec (3.0 kV/mm), which indicates that
(1−y)Bi0.5 (Na1−x Lix )0.5 TiO3 -yBaTiO3 is a good lead-free piezoelectric ceramic.
KEY WORDS: Lead-free piezoelectric ceramic; Bi0.5 Na0.5 TiO3 ; Microstructure; Piezoelectricity
1. Introduction
PbTiO3 -PbZrO3 (PZT) based materials have been
employed in many piezoelectric applications for years.
However, there is a strong push to develop alternate lead-free piezoelectric systems due to environmental concerns with lead in recent years[1,2] . Bismuth sodium titanate (Bi0.5 Na0.5 TiO3 , abbreviated
to BNT) discovered by Smolenskii et al.[3] in 1960,
has been considered to be one of the most favorable lead-free candidates due to its relatively large
remanent polarization (Pr =38 µC/cm2 ) and high
Curie temperature (Tc =320◦ C)[4] . However, BNT itself provides low piezoelectric properties because of
its high coercive field (Ec =7.3 kV/mm) and high
electrical conductivity that cause difficulties in poling process. Modifications on the electrical properties of BNT ceramics have been carried out utilizing BaTiO3 (BT), Bi0.5 K0.5 TiO3 (BKT), SrTiO3 ,
BaCu0.5 W0.5 O3 , BiFeO3 , Bi2 O3 ·Sc2 O3 , CeO2 , MnO,
etc[5–15] . Among these, BNT-BT system demonstrates relatively good piezoelectric and electromechanical properties. However, the high sintering
temperature (1180–1250◦ C) of BNT-BT system has
a negative effect on the piezoelectric property because of the volatilization of Bi and Na[16] . In this
study, to decrease the sintering temperature and improve the electrical properties of BNT-BT system, the
lithium was introduced into BNT-BT and a new system of (1−y)Bi0.5 (Na1−x Lix )0.5 TiO3 -yBaTiO3 (abbreviated to BNLT-BT(x/y)) multicomponent piezoelectric ceramics was proposed and fabricated by conventional process for electronic ceramics, and the electrical properties were investigated.
2. Experimental
2.1 Preparation of samples
A conventional ceramic process was used to
† Corresponding author. Assoc. Prof., Ph.D.; Tel: +86 817
2568067; E-mail address: liao-yw@163.com (Y.W. Liao).
prepare BNLT-BT(x/y) ceramics with x=0−0.125
and y=0.02−0.12. Industrial-grade Bi2 O3 , Na2 CO3 ,
Li2 CO3 , BaCO3 and TiO2 were used as starting materials. All the starting materials were mixed by ball
milling for 6 h and then calcined at 900◦ C for 2 h.
After calcination, the ball-milled powders were mixed
with polyvinyl alcohol (PVA) as a binder for granulation. The granulated powders were pressed into
discs and then sintered at 1100−1180◦ C for 2 h in
air atmosphere. The sintered specimens coated with
silver paste to form electrodes on both sides were annealed at 700◦ C, and then poled in stirred silicone oil
bath with a dc field of 3−5 kV/mm at 40−80◦ C for
15−30 min.
2.2 Characterization of properties
The crystalline phase of the sintered samples was
examined by X-ray diffraction technique (XRD, DX1000, China). The microstructure of the sintered samples was observed by scanning electron microscopy
(SEM, JSM-5900). The dielectric properties were
determined with a capacitance-meter (HP4278A) at
1 kHz, and a quasi-static d33 analyzer (ZJ-3A, China)
was used to measure the piezoelectric constant d33
of 24 h-aged samples. The planar electromechanical
coupling factor kp was determined by the resonanceantiresonance method using an impedance analyzer
(YHP 4194A). The polarization vs electric (P -E) hysteresis loops were observed using a Radiant Precision
Workstation.
3. Results and Discussion
3.1 XRD patterns and SEM photographs of ceramics
Figure 1 shows the XRD patterns of BNLTBT(x/y) ceramics. The results of XRD indicate that
BNLT-BT(x/y) ceramics possess a single-phase perovskite structure for the compositions investigated. It
is believed that Li+ and Ba2+ partially substitute for
Na+ and diffuse into BNT lattices to form solid solutions. The result of sintering experiments shows that
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J. Mater. Sci. Technol., Vol.25 No.6, 2009
/ (pC/N)
C/2 h
160
x=0.075, y=0.08, 1130
C/2 h
x=0.075, y=0.04, 1130
C/2 h
120
d
o
30
20
33
Intensity / a.u.
o
10
T
0
1200
/
33
4.8
0
T
33
/
o
4.4
tan
800
x=0.075, y=0.06, 1130
4.0
o
C/2 h
400
x=0.025, y=0.06, 1150
m
C/2 h
30
40
50
2
60
70
C/2 h
80
160
Q
o
20
3.6
200
o
x=0, y=0.06, 1180
/ %
x=0.10, y=0.06, 1100
40
33
p
p
O
k
O
d
k
200
Perovskite
O
O
/ %
O
O
O
tan
O
O
120
80
90
/ deg.
Fig. 1 XRD patterns of BNLT-BT (x/y) ceramics
0.02
0.04
0.06
0.08
y
0.10
0.12
Fig. 3 Piezoelectric and dielectric properties of BNLTBT (0.075/y) ceramics as a function of the BT
content
20
d
T
6.5
/
1100
0
6.0
tan
5.5
/
0
33
33
p
16
1150
T
k
24
p
1050
5.0
1000
/ %
33
28
33
120
/ %
32
d
k
160
tan
/ (pC/N)
36
200
4.5
90
Q
m
100
80
0.02
0.04
0.06
0.08
x
0.10
0.12
0.14
Fig. 4 Piezoelectric and dielectric properties of BNLTBT (x/0.06) ceramics as a function of the Li+
content
Fig. 2 SEM photographs of BNLT-BT (x/y) ceramics
with x=0.075, y=0.04 (a) and x=0.025, y=0.06
(b) sintered at 1130◦ C and 1150◦ C for 2 h, respectively
the addition of Li+ evidently decreases the sintering temperature.
Figure 2 shows the SEM
photographs of BNLT-BT(0.075/0.04) ceramics and
BNLT-BT(0.025/0.06) ceramics sintered at 1130◦ C
and 1150◦ C for 2 h, respectively. It can be seen that
the sintered samples are very dense. In general, Lifree BNT-BT ceramics can be sintered to obtain dense
samples at 1180−1250◦ C for 2 h. However, the addition of Li+ significantly improves the sintering performance and decreases the sintering temperature of
BNT-BT ceramics, which mainly may be ascribed to
the low melting temperature of Li compounds that
appear to promote formation of a liquid phase during
sintering.
3.2 Piezoelectric and dielectric properties of the ceramics
Figure 3 shows the piezoelectric and dielectric
properties of BNLT-BT(0.075/y) ceramics as a function of the BT content. The piezoelectric constant
d33 and planar electromechanical coupling factor kp
of BNLT-BT(0.075/y) ceramics reach the maximum
values of 190 pC/N and 33.5% at y=0.06, respectively,
but the maximum values of the relatively permittivity
(1401) and the dissipation factor tanδ (4.6%) occur at
y=0.08. The mechanical quality factor Qm decreases
with increasing BT content. However, the mechanical quality factor Qm slightly decreases with increasing BT content when y is more than 0.06. Compared
to BNT-BT system, the variation of electrical properties of BNLT-BT(x/y) system with BT content is
very similar to that of BNT-BT system[5–7] , i.e., the
ceramic shows optimum electrical properties when BT
content is about 0.06.
The piezoelectric and dielectric properties of
BNLT-BT(x/0.06) ceramics as a function of the Li+
content are shown in Fig. 4. The maximum value of
d33 (210 pC/N) of BNLT-BT(x/0.06) ceramics is observed at x=0.025, the maximum value of kp (34.5%)
and Qm (106) at x=0.05. The relatively permittivity
and the dissipation factor tanδ show the minimum
value of 984 and 4.2% at x=0.075 and 0.05, respectively. To compare with pure BNT-BT ceramics, Table 1 summarizes the electrical properties of BNLTBT(x/0.06) and 0.94BNT-0.06BT ceramics. It can be
seen that Li-modified BNT-BT ceramics shows better
piezoelectric properties than pure BNT-BT ceramics
prepared by not only the conventional ceramic process
but citrate method[5–7,17] . It can be seen that the
ceramics show excellent electrical properties. However, the piezoelectric constant d33 and planar electromechanical coupling factor kp of BNLT-BT(x/0.06)
779
J. Mater. Sci. Technol., Vol.25 No.6, 2009
Table 1 Piezoelectric properties of BNLT-BT(x/0.06) and 0.94BNT-0.06BT ceramics
Sample
0.94BNT-0.06BT[5]
0.94BNT-0.06BT[7]
0.94BNT-0.06BT[17]
(Citrate method)
BNLT-BT(0.025/0.06)
BNLT-BT(0.05/0.06)
Piezoelectric constant, Coupling factor, Dielectric constant, Dissipation factor, Mechanical quality
d33 /(pC/N)
kp /%
εT
tanδ/%
factor, Qm
33 /ε0
125
55.0(k33 )
–
–
–
122
29.0
601
1.8
180
28.0
–
–
–
210
204
31.9
34.5
1093
1008
4.5
4.2
97
106
Table 2 Ferroelectric properties of BNLT-BT(x/y)
Sample
Remanent polarization,
Pr /(µC/cm2 )
40.0
40.0
34.0
32.5
32.0
36.1
38.1
27.0
27.3
27.8
40.0
BNLT-BT(0.00/0.06)
BNLT-BT(0.025/0.06)
BNLT-BT(0.05/0.06)
BNLT-BT(0.075/0.06)
BNLT-BT(0.10/0.06)
BNLT-BT(0.075/0.02)
BNLT-BT(0.075/0.04)
BNLT-BT(0.075/0.08)
BNLT-BT(0.075/0.10)
BNLT-BT(0.075/0.12)
0.94BNT-0.06BT[7]
60
2
Polarization / ( C/cm )
40
x=0.0, y=0.06
x=0.025, y=0.06
x=0.075, y=0.06
x=0.075, y=0.08
x=0.075, y=0.10
20
0
-20
-40
-10
-8
-6
-4
-2
0
2
4
6
8
Electrical field / (kV/mm)
Fig. 5 P -E hysteresis loops of BNLT-BT(x/y) ceramics
ceramics sharply decrease when Li+ content reaches
0.125 (Fig. 4).
3.3 Ferroelectric properties of the ceramics
Figure 5 shows the hysteresis loops of BNLTBT(x/y) ceramics with different Li+ and BT content at room temperature, from which the remanent
polarization (Pr ) and coercive field (Ec ) values were
derived and listed in Table 2. Saturated hysteresis loops were observed over the whole investigated
composition range except BNLT-BT(0.075/0.08) ceramic. For BNT-based solid solutions, a decreasing coercive field was usually regarded as the main
strategy to modify the poling process and improve
the piezoelectric properties[18,19] . In addition, it
was found that an increase of the remanent polarization benefits an enhancement of the piezoelectric properties[20] . The remanent polarization
and coercive field of BNL-BT(x/0.06) ceramics tend
to decrease with increasing Li+ content.
However, it can be seen that the remanent polarization
(Pr =40 µC/cm2 ) and coercive field (Ec =3.0 kV/mm)
Coercive field,
Ec /(kV/mm)
3.00
3.00
3.08
2.27
2.44
5.59
4.50
1.31
2.63
3.16
2.88
of BNLT-BT(0.025/0.06) ceramic are almost identical
to those of BNLT-BT(0.00/0.06)ceramic[7] , which indicates that it cannot almost affect the ferroelectric
properties of BNLT-BT(x/0.06) ceramics when the
Li+ content is less than 0.025. The remanent polarization of BNL-BT(0.075/y) ceramic enhances with increasing BT concentration through a maximum value
of Pr =38.1 µC/cm2 at x=0.04 and then decreases,
while the coercive field sharply decreases with increasing BT concentration through a minimum value of
Ec =1.31 kV/mm at x=0.08 and then increases. This
is generally consistent with the variation trend of the
piezoelectric properties with Li and BT content. It indicates an essential relation between the ferroelectric
and piezoelectric properties of BNLT-BT(x/y) ceramics. The superior piezoelectric properties of the specimen with x=0.025 and y=0.06 have been obtained,
which could be attributed to the composition near
MPB (morphotropic phase boundary), large remanent polarization (40.0 µC/cm2 ) and a relatively low
coercive field (3.0 kV/mm). However, it was noticed
that BNLT-BT(0.075/0.06) ceramic shows lower remanent polarization (32.5 µC/cm2 ) and coercive field
(2.27 kV/mm), but it exhibits higher piezoelectric
properties compared to BNLT-BT(0.075/0.02) and
BNLT-BT(0.075/0.04) ceramics. It seems that the
influence of ferroelectric properties on piezoelectric
properties is complex. It has been proposed that it
is important to lower coercive field in order to attain
high piezoelectric properties[6] . Hence, in the present
work, the low coercive field of BNLT-BT(0.075/0.06)
ceramic is presumably mainly responsible for the relatively high piezoelectric properties.
In addition, a double-like hysteresis loop
can be observed for the composition of BNLTBT(0.075/0.08) (Fig. 5), which indicates that the
ceramic is of the characteristic of anti-ferroelectric
phase. However, this composition exhibits relatively
high remanent polarization (Pr =27.0 µC/cm2 ) and
piezoelectric properties (d33 =179 pC/N, kp =22.5%).
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J. Mater. Sci. Technol., Vol.25 No.6, 2009
It can be inferred that there exists field-induced phase
transition from anti-ferroelectric to ferroelectric phase
under the electrical field[21] .
4. Conclusion
Lead-free piezoelectric ceramics BNLT-BT (x/y)
(x=0−0.125, y=0.02−0.12) have been fabricated by
conventional ceramic process. The crystalline structure, piezoelectric, dielectric and ferroelectric properties were investigated. XRD patterns of the ceramics
show a single-phase perovskite structure. The addition of Li+ significantly improves the sintering performance and piezoelectric properties of the ceramics.
The ceramics perform satisfactorily with the optimum
properties: piezoelectric constant d33 =210 pC/N, planar electromechanical coupling factor kp =34.5%, remanent polarization Pr =40 µC/cm2 , and coercive
field Ec =3.0 kV/mm at room temperature.
Acknowledgements
This project was supported by the National Natural Science Foundation of China (No. 50572066), the
Sichuan Provincial Department of Education, China (No.
2006A075) and the Doctor Start-up Foundation of China
West Normal University (No. 06B059).
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