Structure, microstructure and dielectric properties of lead-free BCT-xBZT ceramics near the morphotropic phase boundary

Abstract

There is an urgent demand for high performance Pb-free piezoelectrics to substitute for the current workhorse, the lead zirconate titanate (PZT) family. In the present study, the high performance lead-free BCT-xBZT ceramics near the morphotropic phase boundary (MPB) were prepared by traditional method. The structure, microstructure of the ceramics samples were analyzed. Results show that MPB for these ceramics is achieved with BZT component of x = 0.48. Dielectric properties show ferroelectric-relaxor behaviour of this system. Furthermore, the combination of ImageJ and Lince software allows analyzing SEM images of the ceramics for estimating average particle size and particle size distribution.

Full text

Indian Journal of Pure & Applied Physics
Vol. 53, June 2015, pp. 409-415
Structure, microstructure and dielectric properties of lead-free BCT-xBZT
ceramics near the morphotropic phase boundary
Dang Anh Tuan1,*, VoThanh Tung1, Truong Van Chuong1, Nguyen Trong Tinh2 & Nguyen Thi Mai Huong2
College of Sciences, Hue University, Vietnam, Institute of Applied Physics and Scientific Instrument of
Vietnamese Academy of Science and Technology, Vietnam
*E-mail:datuan1984gmail.com
Received 16 September 2014; revised 20 January 2015; accepted 25 February 2015
There is an urgent demand for high performance Pb-free piezoelectrics to substitute for the current workhorse, the lead
zirconate titanate (PZT) family. In the present study, the high performance lead-free BCT-xBZT ceramics near the
morphotropic phase boundary (MPB) were prepared by traditional method. The structure, microstructure of the ceramics
samples were analyzed. Results show that MPB for these ceramics is achieved with BZT component of x = 0.48. Dielectric
properties show ferroelectric-relaxor behaviour of this system. Furthermore, the combination of ImageJ and Lince software
allows analyzing SEM images of the ceramics for estimating average particle size and particle size distribution.
Keywords: BCT-xBZT ceramics, ImageJ, PZT family, Piezoelectrics
1 Introduction
During the past 50 years, Pb-based ceramics such
as PZT and modified PZT have exhibited an
outstanding piezoelectric performance (d33 = 200-600
pC/N, k33 = 0.6-0.8). They became promptly main
materials in piezodevices1. Unfortunately, Pb
component in these materials are now facing global
restrictions in its usage because of Pb toxicity to the
environment and human body. Therefore there is an
urgent need to study Pb-free piezoelectric materials
with good properties that can compete with PZT family2,3.
BaTiO3 (BTO) is one of the most basic and widely
applied ferroelectric oxide materials with a
perovskite-type crystalline structure. It is also
dielectric material to fabricate dielectric devices like
ceramic capacitors and multilayered ceramic
capacitors. It is well known that physical properties of
BaTiO3 can be altered by doping with either A or/and
B site substitutions4-6. For example, the addition of
calcium (Ca) into barium (Ba) site and zirconium (Zr)
into titanium (Ti) site in barium titanate established
compositionally modified BZT and BCT that received
much attention due to the tunable structure and the
electrical properties to the specific applications7,8.
What happens when BZT and BCT combine to make
new material systems? Wenfeng Liu and Xiaobing
Ren9 designed Pb-free ferroelectric Ba(Zr0.2Ti0.8)O3-
x(Ba0.7Ca0.3)TiO3 systems (BZT-BCT) that have
exhibited an equally excellent piezoelectricity as in
Pb-based materials9. In Liu and Ren’s report9, a very
high value of d33 is 620 pC/Nwhich can be compared
to the PZT material (d33 = 500-600 pC/N). They
predicted that the single-crystal form of the MPB
composition of the Pb-free ferroelectric system may
reach enormous d33 = 1500-2000 pC/N. The colossal
piezo-parameters were received by other researchers
for BZT-BCT and modified BZT-BCT materials. For
0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 composition,
the values of d33 and kp are 630 pC/N and 0.56,
respectively, when the poling conditions were
optimized10. By modifying 0.06 mol % ZnO in the
Ba0.85Ca0.15Ti0.9Zr0.1O3 system, the values11 of d33and
kp are 521 pC/N and 47.8%, respectively. For
0.2 mol. % BiFeO3 doped Ba0.85Ca0.15Ti0.90Zr0.10O3
material12, the values of d33 and kp possess 405 pC/N
and 0.44, respectively. Fairly high d33 ~ 423 pC/N
and kp ~ 51.2% values are also obtained for
(Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 compound13. Yerang Cui
et al14. found that CeO2 used as a sintering aid
could be enhanced the piezo-parameters for the
(Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 system which owned d33 of
600 pC/N and kp of 0.51. Because of the achieved
piezoelectric properties, the BZT-BCT ceramics are
suitable for the passive applications such as diagnostic
instruments in health, sonar devices in military field,
ambient energy harvesting, etc15-17.
The BCT-BZT systems show a phase diagram with
a morphotropic phase boundary (MPB) starting from
a triple point of a paraelectric cubic phase (C),
ferroelectric rhombohedral (R), and tetragonal (T)

INDIAN J PURE & APPL PHYS, VOL 53, JUNE 2015
410
phase that likes Pb contained systems. MPB separates
phase diagram into 2 regions with different symmetry:
R-phase (BZT side) and T-phase (BCT side). The
change of composition ratio R/T strongly influences
the electric properties of the BZT-BCT. The similar
results were observed for Ba(Sn0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3
and Ba(Hf0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 systems18,19.
In the present work, the lead-free system
(Ba0.7Ca0.3)TiO3-xBa(Zr0.2Ti0.8)O3, or BCT-xBZT has
been designed, where x is a molar per cent of BZT,
x = 0.42 – 0.56. The effect of BZT concentration on
structure and microstructure and some dielectric
properties of BCT-xBZT system near MPB have also
been studied. The optimal properties of these systems
around room temperature (just in the MPB
temperature regime) are comparable with the MPB
composition BCT-48BZT.
2 Experimental Details
A conventional ceramics fabrication technique was
used to prepare Pb-free ceramics (Ba0.7Ca0.3)TiO3-
xBa(Zr0.2Ti0.8)O3 (x = 0.42 – 0.56) (abbreviated as
xBZT, with x being the molar fraction of BZT). The
raw materials with high purity (> 99%) of BaCO3,
CaCO3, ZrO2, TiO2 were weighed and mixed in a
planetary milling machine (PM400/2-MA-Type)
using ethanol as a medium for 20 h. The obtained
powders were dried in an oven and then calcined at
1250°C for 3 h. After the second milling in ethanol
for 20 h, the obtained powders were dried and pressed
into disks specimens with a diameter of 12 mm and a
thickness of 1.2 mm only by uniaxial pressing with a
pressure of 100 MPa. Sintering was carried out at
1450°C for 4 h. The crystalline structure of sintered
ceramics was examined by X-ray diffraction (XRD,
D8-Advanced, BRUKER AXS). The sintered samples
after chemically-etching were cleaned by ultrasonic
cleaner. The surface microstructure was observed by
scanning electron microscopy (SEM, Nova NanoSEM
450-FEI).The silver pastes were fired at 450°C for
30 min on both sides of these sintered bulks as
electrodes for electrical measurements. Dielectric
properties of the materials were obtained together
using an impedance analyzer (Agilent 4396B, Agilent
Technologies, America, HIOKI3532) by measuring
the capacitance and phase angle of the specimens
from room temperature to 120°C.
3 Results and Discussion
The TGA-DSC curves recorded at a heating rate of
10°C/min in air for an equimolar mixture in
stoichiometric proportion of 0.48BZT composition is
shown in Fig. 1. There are two distinct weight losses
in the TG curve corresponding to two endothermic
peaks in the TGA curve. The first weight loss occurs
around 709°C, and the second one locates at 927°C
with weight losses in percentage of 6.974 and 17.643,
respectively. In principle, a solid reaction occurs
completely to form BZT-BCT solid solution at the
second endothermic peak (corresponding to the
highest weight loss in the investigated temperature
region). It means that the temperature for calcination
was chosen around 927oC. However, the initial mass
of mixture in stoichiometric proportion was used for
measuringTGA-DSC curves, was very small as
compared to the amount of the raw materials in our
work, thus calcining temperature was selected over
250-300°C than endothermic peak, i.e. 1200-1250°C.
These temperatures were also selected by other
researchers20-23.
Figure 2(a) shows the XRD patterns of xBZT
ceramics, measured at room temperature. All
ceramics exhibit a pure perovskite phase and no
secondary phase was observed in the range
investigated. The stable solid solutions were well
established. Figure 2(b) shows the expanded XRD
patterns in the range 44°-46° of xBZT ceramics. The
BZT contents of 0.42-0.44, xBZT ceramics have
tetragonal symmetry characterized by splitting
(002)/(200) peaks at around 2Theta of 45°. The
intensity of (002)T peak decreases with increasing
BZT content. The (200) and (002) diffraction peaks of
xBZT ceramics gradually merge with the BZT content
beyond 0.48, confirming an involvement of a phase
transition. More clearly, we fitted XRD data of the
Fig. 1 — TGA-DSC curves for the mixture in stoichiometric
proportion of 0.48BZT

TUAN et al.: STRUCTURE, MICROSTRUCTURE AND DIELECTRIC PROPERTIES OF CERAMICS
411
samples around 0.48BZT composition with Gauss
function [Fig. 2(c)]. As shown in Fig. 2(c), the sample
0.48BZT contains two phases simultaneously:
tetragonal phase [(002)T, (200)T peaks positioned at
45.11°, 45.36°, respectively] and rhombohedral phase
((200)R located at 45.21°). Moreover, the diffraction
peak positions shift to the lower angles as increasing
BZT content. The BZT concentration can lead to
crystal lattice distortion and change in crystalline
structure. On the other hand, the coexistence of
tetragonal-rhombohedral phases was observed at
x = 0.48, implying that MPB is located at 0.48BZT
composition. This result is different from previous
report9, where MPB composition is x = 0.50.
Figure 3 shows the surface morphologies of the
polished and chemically-etched xBZT ceramics. No
secondary phase or phase segregation at grain
boundary was detected. In order to estimate grain
size, the microstructural photos of the ceramics were
analyzed using ImageJ software24,25. Figure 4 shows
the particle size distribution with data obtained from
ImageJ software. The grain sizes are located between
5 and 95 µm and gathered round the top of Gauss
fitting plot. As a comparison, a linear cutting method
using Lince software26 was performed to determine
average grain size (Table 1).
Figure 5(a) shows the BZT concentration
dependence of permittivity and dielectric loss of the
xBZT ceramics at 1 kHz and room temperature. It can
be seen that BZT content has strong effect on
dielectric properties. As amount of BZT increases, the
values of permittivity and dielectric loss vary
simultaneously and reach the maximum of 3321 and
the minimum of 1.3% at 0.48BZT composition,

Fig. 2 — XRD partten of xBZT ceramics
Fig. 3 — Surface morphologies of xBZT ceramics: (a) 0.42BZT, (b) 0.44BZT, (c) 0.46BZT (d) 0.48BZT, (e) 0.50BZT,
(f) 0.52BZT, (g) 0.54BZT, (h) 0.56BZT

INDIAN J PURE & APPL PHYS, VOL 53, JUNE 2015
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Table 1 — Grain size for xBZT ceramics
Grain size (µm) BZT
content,x Lince software Image J software
Density
(kg/m3)
0.42 22.6 22.5 5351
0.44 24.1 22.0 5482
0.46 28.4 25.2 5534
0.48 32.4 29.2 5624
0.50 30.0 25.5 5602
0.52 26.4 23.5 5531
0.54 27.9 27.0 5493
0.56 24.8 23.2 5452
respectively. This result could be due to the variation
in grain size of the (Ba, Ca)(Zr, Ti)O3 ceramics27.
Measurements on the dielectric permittivity as a
function of the temperature reveal anisotropic
behaviour. The highest permittivity of 12514 is
observed for the 0.48BZT composition (as shown in
Fig. 6). The temperature, Tm, corresponding to the
maximum permittivity, İm, reduces with the increase
of BZT concentration due to the change in crystal
structure as shown in XRD pattern. The top of
permittivity-temperature curves is broadened,
indicating of a ferroelectric relaxor behaviour in the
literature28-30.
A modified empirical expression was proposed by
Uchino and Nomura31 to describe the diffuseness of
the ferroelectric phase transition as:
m
m
( )
1 1
, (1 2)
T T
C
− γ
− = ≤ γ ≤
ε ε …(1)
\where Ȗ and C are assumed to be constant. The
parameter Ȗ, called degree of diffuseness, gives
information on the character of phase transition. As Ȗ
= 1, expression given in Eq. (1) describes a normal
Curie-Weiss law and Ȗ = 1 that presents a complete
diffuse phase transition. The plots of ln(1/İ – 1/İm)
Fig. 4 — Grain size distribution for xBZT ceramics: (a) 0.42BZT, (b) 0.44BZT, (c) 0.46BZT (d) 0.48BZT, (e) 0.50BZT,
(f) 0.52BZT, (g) 0.54BZT, (h) 0.56BZT
Fig. 5 — (a) Permittivity and dielectric loss as a function of BZT content at 1 kHz, room temperature, (b) The plot of Tm versus BZT
content, and (c) Temperature dependence of permittivity and dielectric loss of xBZT ceramics

TUAN et al.: STRUCTURE, MICROSTRUCTURE AND DIELECTRIC PROPERTIES OF CERAMICS
413
versus ln(T – Tm) of xBZT ceramics at 1 kHz are
shown in Fig. 7(a). The parameter Ȗ varies as
increasing BZT and reaches the highest value of 1.825
at 0.48 BZT composition. It is said that the transitions
are of diffuse type, and the ceramics are highly
disordered. By fitting Eq. (1) to the data, the Ȗ
constant is obtained from the s loop of the fitting
curves. Figure 7(b) shows the dependence of Ȗ versus
BZT concentration.
To study the frequency dependence of Tm, it is
necessary to apply Vogel-Fulcher relationship32,33
o
o
m f
exp T
f f
T T
ª º
= −
« »
−
¬ ¼
…(2)
Fig. 6 — Temperature dependence of permittivity and dielectric loss of xBZT ceramics at various frequencies
Fig. 7 — (a) ln(1/İ – 1/İm) as a function of ln(T – Tm) at 1 kHz for the xBZT ceramics, (b) BZT content dependence of diffuseness degree, γ

INDIAN J PURE & APPL PHYS, VOL 53, JUNE 2015
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Fig. 8 — Plot of ln(f) versus Tm for the xBZT ceramics. The
symbols are the experimental points, and the fitted lines
correspond to Vogel-Fulcher law
Table 2 — Values of To , Tf (°C), with various BZT content, x
x 0.42
0.44
0.46
0.48
0.50
0.52
0.54
0.56
To 68 157 66 150 191 87 340 147
Tf 92 85 89 78 75 80 62 59
where, is the attempt frequency, and is a constant. The
parameters To, Tf, were obtained by fitting experiment
data in Eq. (2) and listed in Table 2. A good fit of the
data with Vogel-Fulcher equation (Fig. 8) confirms a
static freezing temperature of thermally activated
polarization fluctuations in xBZT ceramics.
4 Conclusions
Complete perovskite lead-free xBZT ceramics were
prepared by using the solid state reaction method. As
varying BZT content, the structure of the material
changes from tetragonal phase to rhombohedral
phase. At 0.48BZT composition, the tetragonal and
rhombohedral phases coexist which demonstrates that
MPB is located at this composition. Our work
indicates that with x = 0.48, the characteristic
parameters such as density, dielectric constant,
average grain size reach maximum values3, which are
5624 kg/m12514, 32.4 µm, respectively. The results
also show the diffuse transitions in the material. The
experimental Tm data points are found to be in good
agreement with Vogel-Fulcher relationship which
constitutes strong evidence for a static freezing
temperature of thermally activated polarization
fluctuations in lead-free ceramics BZT-BCT.
Acknowledgement
The work was carried out in the frame of the “Basic
Project of Hue University 2013-2015”.
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