liu2014 (1)

Journal of Alloys and Compounds 613 (2014) 219–225
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Structure and electrical properties of Li-doped BaTiO3–CaTiO3–BaZrO3
lead-free ceramics prepared by citrate method
Xing Liu a, Min Zhu a, Zhihui Chen a, Bijun Fang a,⇑, Jianning Ding a,b,⇑, Xiangyong Zhao c, Haiqing Xu c,
Haosu Luo c
a
School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Changzhou University, Changzhou 213164,
Jiangsu, China
b
School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, Jiangsu, China
c
Key Laboratory of Inorganic Function Material and Device, Chinese Academy of Sciences, Shanghai 201800, China
a r t i c l e
i n f o
Article history:
Received 14 May 2014
Received in revised form 6 June 2014
Accepted 6 June 2014
Available online 16 June 2014
Keywords:
BaTiO3-based ceramics
Citrate method
Doping
Perovskite structure
Electrical properties
a b s t r a c t
1 wt% Li-doped 0.75BaTiO3–0.15CaTiO3–0.1BaZrO3 (BCZT) ceramics were prepared by the citrate method.
XRD measurement showed that pure perovskite BCZT precursor powders are obtained calcined at
600–700 °C, which exhibit rather homogeneous microstructure morphology with nm-scale grain size.
The sintered BCZT ceramics exhibit pure perovskite structure with composition locating at rhombohedral
side around the morphotropic phase boundary (MPB). Oxide doping and citrate method exert great influences on microstructure and electrical properties of the Li-doped BCZT ceramics. All the synthesized
ceramics exhibit excellent dielectric and ferroelectric properties. The Li-doped BCZT ceramics calcined
at 700 °C and sintered at 1500 °C exhibit the best dielectric, ferroelectric and piezoelectric properties.
The high piezoelectric response of the synthesized Li-doped BCZT ceramics is considered as relating to
the MPB effect, Li-doping and wet chemical synthesis method.
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
Lead-based
ferroelectric
perovskites,
represented
by
(1 x)PbZrO3–xPbTiO3 (PZT), have dominated the field of industrial piezoelectric applications, including sonars, buzzers, motors
and ultrasonic cleaners, owing to their superior piezoelectric,
dielectric, and electromechanical properties especially with the
compositions around the morphotropic phase boundary (MPB)
[1,2]. The MPB of PZT is known to locate at the composition of
x = 0.48, which is considered as a region connecting different ferroelectric phases, allowing facile domain orientation reversal during
the poling process. Further modifications using acceptor and donor
dopants provide us the wide range of piezoelectric compositions
we have today [3]. However, such lead-based materials contain
almost 60 wt% lead, which tends to release into the environment.
This occurs during calcination and sintering processes, and after
usage accompanied recycling and waste disposal, causing the
⇑ Corresponding authors. Address: School of Materials Science and Engineering,
Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering,
Changzhou University, Changzhou 213164, Jiangsu, China (B. Fang and J. Ding). Tel.:
+86 519 86330100; fax: +86 519 86330095.
E-mail addresses: [email protected], [email protected] (B. Fang), dingjn@cczu.
edu.cn (J. Ding).
http://dx.doi.org/10.1016/j.jallcom.2014.06.046
0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
environmental pollution and harming to human health. Hence,
the high toxicity of PZT and the growing demand of replacing these
lead containing piezoceramics have driven considerable research
of environment-friendly materials [4].
Over the past years, much efforts have been done to search for
lead-free piezoceramics to substitute PZT. Recent reports on
lead-free piezoelectrics are focus on three perovskite families:
K0.5Na0.5NbO3 (KNN) [4,5], Na0.5Bi0.5TiO3 (NBT) [6,7], and BaTiO3
(BT) [8,9]. In the KNN and NBT systems, the alkali components
become highly unstable and easily to evaporate during high temperature sintering, which makes it difficult to maintain chemical
stoichiometry and leads to the deterioration of electrical properties
[5]. As a classical perovskite ferroelectric, BT is another lead-free
ceramic system and possesses good composition controllability.
It is well known that BT is widely used as a commercial dielectric
material rather than piezoelectric material due to its high dielectric
property and low piezoelectric coefficient (d33 = 191 pC/N) [10].
Recently, Liu and Ren [11] reported a Ca and Zr co-doped BT
system with extraordinarily high d33 (620 pC/N), which can be
comparable with PZT-5H. The high piezoactivity stems from the
tricritical triple point-like MPB composition [11]. Since then, further studies have focused on doping [12,13], sintering temperature
[14], piezoelectric [15], pyroelectric [16], optical [17] properties,
and manufacturing process [18,19].
X. Liu et al. / Journal of Alloys and Compounds 613 (2014) 219–225
The 1 wt% LiNO3-doped 0.75BaTiO3–0.15CaTiO3–0.1BaZrO3 (BCZT) ceramics
were prepared by the citrate method. Ba(NO3)2 (99.5%), Ca(NO3)24H2O (99%),
Zr(NO3)45H2O (99.5%), C16H36O4Ti (98%) and LiNO3 (99.9%) were used as starting
materials. In this method, the mole ratio of the citric acid to the total metal cations
was 1:1. The weighed amount of citric acid was dissolved into deionized water and
the pH value was adjusted to 7–9 by ammonia water. The designed amount of tetrabutyl titanate was slowly added into the citric acid solution and stirred at 70 °C to
form a transparent aqueous solution. Various nitrates were then added according
to the stoichiometric ratio and stirred at 80 °C to form a semitransparent precursor
solution. The precursor solution was heated and dried to form a gel. The gel was
pre-calcined at 300 °C for 2 h to eliminate the water, ammonia, citric acid and organics, eventually obtaining dark-brown powder. The powder was then calcined at 600–
700 °C for 2 h to form perovskite BCZT. The calcined powders were cold dry-pressed
uniaxially into pellet with 13 mm in diameter and 1.5 mm in thickness with the addition of appropriate amount of 8 wt% polyvinyl alcohol (PVA) solution as binder and
then sintered at 1490 °C and 1500 °C for 2 h in a covered zirconium dioxide crucibles.
The sintered ceramics were polished to obtain flat and parallel surfaces. Crystal
structure of the calcined Li-doped BCZT powders and sintered Li-doped BCZT
ceramics was investigated by a Rigaku D/max-2500/PC X-ray Diffraction meter
(XRD, Rigaku Corp., Japan) using the well-polished ceramics. Simultaneous thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) analysis of the
pre-calcined powder were carried out by a simultaneous thermal analyzer
(TG–DSC, Setaram Instrumentation Corp., France), which was heated from room
temperature to 1400 °C in air at a rate of 10 °C/min. Microstructure morphology of
the calcined powders were observed by a Zeiss SupraTM 55 Sapphire Field Emission
Scanning Electron Microscope (FESEM, Carl Zeiss Group, Germany). The medium
diameter of the calcined powders was measured by a BT-9300S laser scattering
particle analyzer (Dandong Bettersize Instruments Ltd., China). For electrical properties characterization, silver paste was coated on both surfaces of the well-polished
ceramics and fired at 550 °C for 30 min to provide robust electrodes. Dielectric
property was measured by a computer-controlled TH2818 Automatic Component
Analyzer under a weak oscillation level of 1 Vrms (Changzhou Tonghui Electronic
Co. Ltd., China). For piezoelectric property measurement, the ceramics were poled
at room temperature under different electric field between 0.5 and 3 kV/mm with
an interval of 0.5 kV/mm for 5 min in silicon oil to investigate the influences of
polarization electric field on piezoelectric properties. Piezoelectric properties were
measured by a ZJ-6A Berlincourt-type quasi-static d33/d31 meter (Institute of
Acoustics, Chinese Academy of Sciences, Beijing, China) and a TH2826 Wide
Frequency LCR Meter (Changzhou Tonghui Electronic Co. Ltd., China) using the
resonance-antiresonance method. Detailed electrical properties measurement
procedures were described elsewhere [22].
0.5
100 TG
30
o
T1=469.6 C
0.0
20
90
-0.5
-1.0
-1.5
10
o
dTG
80
T2=528.9 C
0
o
T3=599.5 C
70
-10
o
T4=612.6 C
3. Results and discussion
-2.0
60
3.1. Citrate method processing
-2.5
50
DSC
-20
Heat flow (w/g)
2. Experimental procedure
pre-calcined powder. The TG–DSC curves could be helpful to determine the temperature of the solid state reaction. The strongest
endothermic peak appears at 469.6 °C, accompanied by serious
weight loss and extremum of derivative thermogravimetry, indicating the decomposition of nitrates. Due to the dynamic measurement characteristic of DSC and the expense time for the
decomposition of nitrates, two small endothermic peaks appear
subsequently at 528.9 and 599.5 °C, which also may be correlated
with the combustion of residual organics. In addition, a minor exothermic peak appears at 612.6 °C, which can be attributed to the
solid state reaction and the initial crystallization [23]. The weight
loss reaches nearly 40% when the temperature increases to
700 °C, indicating that most organic components and nitrates have
decomposed. A wide weak exothermic peak appears around 880 °C
may be also correlated with the solid state reaction since such
reaction usually is a slow weak exothermic process depending on
diffusion and chemical reaction. Based on the TG–DSC analysis,
the calcining temperature is chosen between 600 and 700 °C considering the dynamic nature of thermal analysis.
Fig. 2 shows XRD patterns of the Li-doped BCZT powders calcined between 600 and 700 °C. All the calcined powders exhibit
typical perovskite structure, and almost no impurity phase can
be detected. Pure perovskite structure Li-doped BCZT has already
formed at calcining temperature of 600 °C, where the nitrates
and organics have decomposed and the solid state reaction begins
as shown by the TG–DSC analysis. No apparent difference in the
phase formation is observed of the powders calcined between
625 °C and 700 °C. It is clearly evident that perovskite structure
can be formed well below calcining temperature of 700 °C by the
citrate method, which is much lower than that used by the conventional solid state reaction method (1300 °C) [11,14]. The
decreased calcining temperature obtained in this study can be
attributed to the oxide doping, and small particle size, homogenous distribution and high activity of the precursor powders prepared by the citrate synthesis process.
Medium diameter and grain size of the calcined Li-doped BCZT
powders are shown in Table 1, based on laser particle size analysis
and XRD measurement of the (1 0 0), (1 1 0) and (1 1 1) diffraction
reflections. The grain size was estimated by the Scherrer formula:
d ¼ kk=b cos h, where d is the grain size; k is the shape factor, 0.89;
k is the X-ray wavelength, 0.15406 nm; b is the full-width at halfmaximum (FWHM) of the diffraction peak; h is the Bragg’s angle.
Due to the detection limitation of the laser scattering particle
analyzer, the medium diameter D50 of the particles is around
lm-scale, which is different from the nm-scale values of grain
size calculated by the Scherrer formula. Particle size tends to
increase with the increase of calcining temperature; however, no
simple regularity appears in this study. In order to detect exact
particle size of the calcined Li-doped BCZT powders, FESEM
Weight (%)
However, most Ba(ZrxTi1x)O3–(BayCa1y)TiO3 (BZT–BCT) piezoelectric ceramics reported as yet are mainly fabricated by the conventional solid-state reaction method [11–17], which requires
rather high calcination temperature (1300 °C). The solid state
sintering usually produces inhomogeneous microstructure and impurity
phases, which may deteriorate electrical properties. By contrast,
wet chemical synthesis method is beneficial to obtain high chemical
purity, rather dense microstructure, and better homogeneity [18].
As a common chemical solution synthesis method, citrate method
is essentially a polymeric precursor method, which uses citrate acid
and water as complex agent and solvent, respectively, leading to
improved homogeneity and stabilized compositions [20].
So far as we know, there are few reports on the microstructure
and piezoelectric properties of the BZT–BCT ceramics derived from
the citrate method. In this work, the composition 0.75BaTiO3–
0.15CaTiO3–0.1BaZrO3 (BCZT) was fabricated by the citrate
method, and LiNO3 was doped to adjust piezoelectric properties
of the BCZT ceramics. Such composition locates around the
reported tricritical triple point MPB composition and exhibits
excellent electrical properties [21]. The influences of ceramics processing and chemical doping on crystal structure and electrical
properties of the BCZT ceramics were studied and their possible
causations were interpreted.
dTG (% / min)
220
Exo
-30
Fig. 1 shows the simultaneous TG–DSC curves and the first
order partial differential of the TG curve of the Li-doped BCZT
0
200 400 600 800 1000 1200 1400 1600
Temperature (oC)
Fig. 1. Simultaneous TG–DSC curves of the Li-doped BCZT pre-calcined powder.
Intensity (a.u.)
X. Liu et al. / Journal of Alloys and Compounds 613 (2014) 219–225
7500
700 o C
6000
675 o C
4500
650 o C
3000
625 o C
1500
600 o C
0
0
10
20
30
40
50
60
70
80
90
221
Based on the XRD measurement results, cell parameters of the
sintered Li-doped BCZT ceramics are shown in Table 2. Synthesis
method, chemical doping, and calcining and sintering temperature
exert great effects on microstructure of the synthesized ceramics.
With the increase of calcining and sintering temperature, grains
grow up, porosity eliminates gradually, and microstructure
becomes dense. It can be deduced from the tolerance factor that
small ions (r(R3+) < 0.87 Å) will occupy the B-site, large ions
(r(R3+) > 0.94 Å) will occupy the A-site, and intermediate ions will
occupy both sides with different ratios of the perovskite structure
[12]. Taking radius of Li+ (r = 0.59 Å) into consideration, Li+ tends to
enter interstitial sites of the perovskite structure, which exerts
slight influence on crystal lattice. In general, optimum sintering
conditions for ceramics can be established by analyzing cell
parameters, perovskite phase content and densification combining
with electrical properties measurements.
3.3. Dielectric properties
2θ ( o )
Fig. 2. XRD patterns of the Li-doped BCZT powders calcined at different
temperatures.
Table 1
Laser particle size and grain size of the calcined Li-doped BCZT powders estimated by
Scherrer formula.
Calcining temperature (°C)
600
625
650
675
700
D50 (lm)
Grain size (1 0 0) (nm)
Grain size (1 1 0) (nm)
Grain size (1 1 1) (nm)
2.1
21.08
21.00
20.89
1.8
22.69
21.11
20.23
2.86
18.28
19.17
17.96
1.9
19.68
18.95
18.4
1.41
22.06
18.48
17.96
analysis was undertaken and the micrographs are shown in Fig. 3.
Due to the decomposition of nitrates and organics, particle size of
the calcined powders is rather homogenous although different
shape of particles can be observed. However, it is definitely found
that grain size of all the calcined powders is in nm-scale, which is
in agreement well with the values estimated by the Scherrer
formula. Agglomeration exists in all the calcined Li-doped BCZT
powders with different content, which is a normal phenomenon
in the wet chemical synthesis method and should be improved
further to eliminate its influence on sintering capability.
3.2. Structure characterization
XRD patterns of the Li-doped BCZT ceramics sintered at different temperatures are shown in Fig. 4. Except slight impurity phase
appearing in the Li-doped BCZT ceramics calcined at 675 °C and
sintered at 1490 °C, all the other ceramics exhibit pure perovskite
structure without detectable impurity within the detection limitation of the XRD measurement, indicating that Ca2+, Zr4+, and Li+
may diffuse into the BaTiO3 lattice to form complete solid solutions
in the studied composition range. It is well known that lithium
salts can be used as sintering acid to improve homogeneity and
microstructure morphology of ceramics. As compared to the
undoped 0.75BaTiO3–0.15CaTiO3–0.1BaZrO3 (BCZT) ceramics
reported before [21], the formation of pure perovskite Li-doped
BCZT ceramics can be attributed to the Li-doping and the citrate
synthesis process. Except slight broadening of the (3 1 0) and
(3 1 1) diffraction reflections, all the other diffraction peaks exhibit
rather symmetric, single and sharp diffraction peak with narrow
FWHM, indicating that the Li-doped BCZT ceramics locate at the
rhombohedral side around the MPB composition.
Fig. 5 shows the temperature dependence of dielectric constant
of the Li-doped BCZT ceramics calcined and sintered at different
temperatures measured at 1 kHz upon cooling. It can be seen that
the values of dielectric constant maximum (em) and Curie temperature (TC/Tm) are influenced greatly by the calcining and sintering
temperatures. For the Li-doped BCZT ceramics calcined at the same
calcining temperature, both the values of em and TC/Tm increase
with the increase of sintering temperature since the microstructure becomes denser and the grain size grows larger [14]. The
dielectric anomalies appearing at the TC/Tm temperature can be
attributed to the ferroelectric phase transition from rhombohedral
ferroelectric phase to cubic paraelectric phase, which is a common
critical phenomenon in a ferroelectric system near the phase transition point [21]. The Li-doped BCZT ceramics calcined at 700 °C
and sintered at 1500 °C exhibit the largest em, being 11,292, and
the Curie temperature is 72.9 °C.
Fig. 6 shows the temperature dependence of dielectric constant
and loss of the Li-doped BCZT ceramics calcined at 700 °C and sintered at 1500 °C measured at several frequencies. Although the
dielectric peaks are broad measured at different frequencies, dielectric frequency dispersion is not apparent, i.e., the values of em and
TC/Tm maintain almost unchangeable with the variation of frequency as shown in the inset, which indicates that the Li-doped
BCZT ceramics synthesized by the citrate method approach normal
ferroelectrics. A dielectric shoulder appearing around 20–30 °C on
the dielectric constant and loss peaks may be related to the polymorphic phase transition, where the ferroelectric phase transition
from rhombohedral ferroelectric phase to tetragonal one takes
place, accompanied by the improvement of electrical properties
[11]. Fig. 7 shows the linear fitting of its dielectric behavior above
TC/Tm at 1 kHz according to the Curie–Weiss law: e ¼ C=ðT T 0 Þ,
where C is the Curie–Weiss constant, T0 is the Curie–Weiss temperature. The fitting equation is determined as e ¼ 1:208 105 =
ðT 94:7Þ, in which the Curie–Weiss law takes effect between
120 °C and 183.9 °C. The Curie–Weiss constant C is around 105
magnitude, indicating further the normal ferroelectric characteristic of the Li-doped BCZT ceramics prepared by the citrate method.
3.4. Ferroelectric properties
The ferroelectric properties of ferroelectric ceramics are strongly
influenced by calcining and sintering temperature. Figs. 8 and 9
show the room-temperature P–E hysteresis loops of the Li-doped
BCZT ceramics sintered at 1490 and 1500 °C, respectively. At 2 kV/mm
and 1 Hz, the hysteresis loops are rather saturated and fully
developed, confirming partly excellent ferroelectric property of
the Li-doped BCZT ceramics synthesized by the citrate method. It
222
X. Liu et al. / Journal of Alloys and Compounds 613 (2014) 219–225
(110)
Fig. 3. SEM micrographs of the Li-doped BCZT powders calcined at (a) 600 °C, (b) 650 °C and (c) 700 °C.
8000
Intensity (a.u.)
Dielectric constant
(220)
(300)
(310)
(311)
(211)
(210)
(111)
(100)
(200)
12000
o
o
o
o
o
o
o
o
o
o
o
o
700 C-1500 C
700 C-1490 C
6000
o
o
650 C-1490 C
o
o
650 C-1500 C
o
o
675 C-1490 C
o
o
675 C-1500 C
o
o
700 C-1490 C
o
o
700 C-1500 C
10000
8000
6000
4000
2000
675 C-1500 C
4000
675 C-1490 C
2000
0
0
0
30
60
90
120
150
180
Temperature (oC)
650 C-1500 C
650 C-1490 C
0
10
20
30
40
50
60
70
80
90
2θ ( o )
Fig. 4. XRD patterns of the Li-doped BCZT ceramics calcined and sintered at
different temperatures.
is found that the value of remnant polarization Pr shows great
dependency on the calcining and sintering temperatures, whereas
their coercive field Ec changes little. All the ceramics exhibit low
Ec value (<0.5 kV/mm), indicating that the Li-doped BCZT ceramics
Fig. 5. Temperature dependence of dielectric constant of the Li-doped BCZT
ceramics calcined and sintered at different temperatures measured at 1 kHz upon
cooling.
prepared by the citrate method are rather ‘‘soft’’ and may be feasible
to tailor the poling condition. Generally, the Li-doped BCZT ceramics
calcined at 650 °C and sintered at 1500 °C exhibit the optimum
ferroelectric properties, whose values of Pr = 7.96 lC/cm2 and
Ec = 0.20 kV/mm. The slight asymmetry of the hysteresis loops can
be attributed to the existence of space-charge field, which is
constructed by point-charge defects induced by oxide doping and
Table 2
Cell parameters of the Li-doped BCZT ceramics calcined and sintered at different temperatures.
Composition (°C)
a = b = c (Å)
a (°)
b (°)
c (°)
Cell volume (Å3)
650–1490
650–1500
675–1490
675–1500
700–1490
700–1500
3.9997(43)
4.0011(14)
4.0026(25)
4.0066(13)
3.9996(26)
3.9966(21)
90.349(578)
90.18(183)
90.375(326)
90.164(172)
90.249(340)
90.232(279)
89.873(382)
89.878(121)
89.784(215)
89.954(114)
89.831(224)
89.850(184)
90.088(95)
89.906(30)
90.007(53)
90.066(28)
89.930(55)
89.930(45)
63.985
64.052
64.123
64.317
63.979
63.835
223
80
18000
1kHz
10kHz
50kHz
100kHz
300kHz
6000
C
15000
72
o
9000
76
εm
12000
TC / Tm
68
12000
64
9000
60
6000
0
60
3000
120
180
240
300
Frequency ( kHz )
0
0.04
0.03
0.02
0.01
0
30
60
90
120
150
Dielectric loss
Dielectric constant
X. Liu et al. / Journal of Alloys and Compounds 613 (2014) 219–225
0.00
180
Temperature (oC)
Fig. 6. Temperature dependence of dielectric constant and loss tangent of the Li-doped BCZT ceramics calcined at 700 °C and sintered at 1500 °C measured at several
frequencies upon cooling.
0.0008
Measured data
Linear fitting
0.0007
5
ε=1.208x10 /(T-94.7)
o
o
120 C-183.9 C
0.0005
0.0004
0.0003
0.0002
110
120
130
140
150
160
170
180
Polarization (μC/cm2)
5
0
-5
-10
-15
-20
-10
0
-5
-10
-20
-10
0
10
20
Electric field (kV/cm)
Fig. 9. Room-temperature P–E hysteresis loops of the Li-doped BCZT ceramics
sintered at 1500 °C measured at 2 kV/mm and 1 Hz.
3.5. Piezoelectric properties
o
650 C-1490 C
o
o
675 C-1490 C
o
o
700 C-1490 C
10
5
-15
Fig. 7. Linear fitting of the dielectric behavior above TC/Tm of the Li-doped BCZT
ceramics calcined at 700 °C and sintered at 1500 °C at 1 kHz according to the Curie–
Weiss law. The fitting parameters and the temperature range where the Curie–
Weiss law takes effect are shown in the plot.
o
o
190
Temperature (oC)
15
o
650 C-1500 C
o
o
675 C-1500 C
o
o
700 C-1500 C
10
Polarization (μC/cm2)
0.0006
1/ ε
15
0
10
20
Electric field (kV/cm)
Fig. 8. Room-temperature P–E hysteresis loops of the Li-doped BCZT ceramics
sintered at 1490 °C measured at 2 kV/mm and 1 Hz.
tends to exhibit pinning-down phenomenon [20,21]. The breach
appearing in the hysteresis loops provides additional evidence of
the existence of crystal defects.
Piezoelectric constant and electromechanical coupling coefficient are important parameters for piezoelectric applications.
Fig. 10 shows the frequency dependence of impedance and phase
degree of the Li-doped BCZT ceramics calcined at 700 °C and
sintered at 1500 °C as an example. The Li-doped BCZT ceramics
synthesized by the citrate method exhibit excellent resonance
response peaks, which provides efficient energy interconversion
between electrical energy and mechanical energy.
Poling conditions exert great influences on piezoelectric properties. The low poling electric field makes the ferroelectric domains
switch and reverse incompletely, whereas excessively large poling
electric field leads to high leakage current, increase of conductivity
and finally dielectric breakdown of the piezoceramics [24]. The
poling electric field dependence of piezoelectric constant d33 and
electromechanical coupling factor Kp are given in Fig. 11 and
Table 3, respectively. The value of Kp is calculated by the equation:
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
K p 2:51 ðfp fs Þ=fs . The gradual increase of d33 and Kp results
from the easy switching of 180° domains under lower poling electric field, whereas non-180° domains switching is responsible for
the improvement of d33 and Kp during higher poling electric field
[25]. It can be seen that most Li-doped BCZT ceramics exhibit the
224
X. Liu et al. / Journal of Alloys and Compounds 613 (2014) 219–225
Table 3
Poling electric field dependence of electromechanical coupling factor Kp of the Li-doped BCZT ceramics calcined and sintered at different temperatures.
Composition (°C)
Kp (%)
650–1490
650–1500
675–1490
675–1500
700–1490
700–1500
500 V/mm
1000 V/mm
1500 V/mm
2000 V/mm
2500 V/mm
3000 V/mm
28.52
28.19
37.53
29.51
27.73
34.75
31.20
29.57
35.60
31.12
30.35
36.58
32.19
30.58
34.90
32.77
30.37
35.95
32.68
31.38
39.44
32.75
34.03
38.39
34.21
33.27
40.94
33.60
35.72
39.23
33.18
32.78
41.31
34.77
36.72
40.10
90
4.2
60
2.8
0
logZ
o
30
θ
logZ
3.5
-30
2.1
-60
θ
1.4
-90
240000
252000
264000
276000
4. Conclusions
288000
Frequency (Hz)
Fig. 10. Frequency dependence of impedance and phase degree of the Li-doped
BCZT ceramics calcined at 700 °C and sintered at 1500 °C in radial–extension–
vibration-mode.
o
o
650 C-1490 C
o
o
650 C-1500 C
o
o
675 C-1490 C
o
o
675 C-1500 C
330
d33 (pC/N)
growth of grains and further improve electrical properties. Lithium
has a very small ionic radius, which tends to enter interstitial sites
of the perovskite structure. Due to the low local energy barrier of
the lithium ions, the rotation of ferroelectric domains becomes
easier, leading to the improvement of piezoelectric properties. In
addition, the wet chemical synthesis process, i.e., citrate method,
is feasible to obtain homogenous, small size and high activity precursor powders, which are helpful to improve sintering capability
of ceramics. Thus, enhanced ceramics with rather uniform and
densified microstructure can be synthesized, leading to improved
electrical properties.
300
270
240
o
o
700 C-1490 C
o
o
700 C-1500 C
210
0.5
1.0
1.5
2.0
2.5
3.0
1 wt% Li-doped 0.75BaTiO3–0.15CaTiO3–0.1BaZrO3 (BCZT)
ceramics were synthesized successfully by the citrate method.
The Li-doped BCZT precursor powders calcined between 600 and
700 °C exhibit rather homogeneous microstructure with nm-scale
grain size. The Li-doped BCZT ceramics sintered at 1490 and
1500 °C exhibit pure rhombohedral perovskite structure. Oxide
doping and citrate method exert great influences on microstructure and electrical properties of the synthesized Li-doped BCZT
ceramics. Dielectric behavior of the Li-doped BCZT ceramics prepared by the citrate method approaches normal ferroelectrics. All
the sintered ceramics exhibit rather saturated and fully developed
P–E hysteresis loops, whose values of Pr are influenced greatly by
calcining and sintering temperatures, whereas Ec is influenced little. The Li-doped BCZT ceramics calcined at 700 °C and sintered at
1500 °C exhibit the best dielectric and piezoelectric properties, in
which em = 11,292, TC/Tm = 72.9 °C, d33 = 336 pC/N and Kp = 40.1%
poled at 3 kV/mm. The high piezoactivity of the synthesized
ceramics can be attributed to the low energy barrier for the
polarization rotation and the densified microstructure induced by
Li-doping and the wet chemical synthesis method.
Poling electric field (kV/mm )
Fig. 11. Poling electric field dependence of piezoelectric constant d33 of the Lidoped BCZT ceramics calcined and sintered at different temperatures.
maximum values of d33 and Kp under the poling electric field of
2.5–3 kV/mm. The Li-doped BCZT ceramics calcined at 700 °C and
sintered at 1500 °C exhibit the optimum piezoelectric properties,
where the largest value of d33 is 336 pC/N, poled at 3 kV/mm,
and its Kp reaches 40.1%.
The improvement of the piezoelectric properties is related to
the MPB effect, oxide doping and preparation method. The composition of the selected BCZT ceramics locates around the reported
MPB composition, where polymorphic ferroelectric phases coexist
at room temperature. The free energy of these ferroelectric phases
around the MPB composition is nearly equivalent, which causes a
low energy barrier for polarization rotation and lattice distortion,
thus gives rise to high piezoelectric properties [11,26,27]. The
Li-doping also takes effect. Lithium salts are usually used as
sintering acid to promote ceramic densification, inhibit abnormal
Acknowledgements
The authors thank the National Natural Science Foundation of
China (Grant No. 51342002) and the Priority Academic Program
Development of Jiangsu Higher Education Institutions for financial
support.
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