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Mediterranean Journal of Chemistry 2019, 8(3), 245-254
Study of the dielectric, optical and microstructure properties of
CaCu3Ti4O12–PbZr0.48Ti0.52O3 ceramic system with different
compositions
Nasr Hadi 1,*, Abdi Farid 1, Taj-Edine Lamcharfi 1, Abdesselam Belaaraj2, Said Kassou 2 and
Fatimazahra Ahjyaje 1
1
2
Laboratory of Signals, Systems and Components, USMBA. FST Fez, B.P. 2202, Morocco
Laboratory Physics of Materials and Systems Modeling CNRST URAC08, Moulay Ismail University, Faculty
of Sciences Department of Physics, Morocco
Abstract: In this paper, composite ceramics (1-x)CaCu3Ti4O12–(x)PbZr0.48Ti0.52O3 (with x =0.00, 0.50 and 1.00),
denoted CCTO-PZT, were prepared by a three-stages modified method, in order to achieve high dielectric
constant and low loss. Structural investigations carried out by X-ray diffraction (XRD), and FT-IR spectroscopy
showed the formation of pure cubic and tetragonal phases for x = 0.00 and x=1.00 compositions, respectively.
XRD showed the coexistence of both phases in the CCTO-PZT composite. The morphology of the ceramics was
examined by scanning electron micrograph (SEM), results reveal a homogeneous microstructure with two types
of grains corresponding to PZT (smaller grains) and CCTO (large grains). Dielectric measurements carried out
by an impedance analyzer; show that the dielectric constant of the CCTO-PZT composite is higher than the pure
samples (CCTO and PZT) one. The temperature dependence of the ac conductivity indicated that the conduction
follows the Arrhenius law and the conduction process is due to the single and the doubly ionized. The optical
band gap of the PZT is 2.25 eV, and the band gap decreased in the CCTO/PZT composite.
Keywords: CaCu3Ti4O12-PbZr0.48Ti0.52O3; dielectric; optical; electrical; microstructure properties.
Introduction
Lead zirconate titanate PbZr1-xTixO3 (with x=0 up
to 1) (PZT) and Calcium Copper Titanate
(CaCu3Ti4O12, CCTO) are a perovskite types ABO3
(where A = Pb, B = Ti or Zr for PZT and where
A = Ca or Cu and B=Ti for CCTO) have widely used
for different applications, due to their excellent
dielectric and piezoelectric properties 1-6. However, it
is well known that PZT-based ceramic materials are
not environmentally friendly because of the PbO
evaporation during sintering. Furthermore, the PbZr1xTixO3 (PZT) is a ferroelectric perovskite and has a
tetragonal, or/and rhombohedral and orthorhombic
phases depending on the value of Zr/Ti ratio. It has
two morphotropic phase boundaries (MPB) at 53/47
and 95/5 Zr/Ti ratio7. On the other hand, CaCu3Ti4O12
(CCTO) ceramic has a high dielectric constant (104)
independent of temperature (100–400K) and
frequency (102–106 Hz) 6,7, which makes it a
promising
material
for
application
in
microelectronics, but unfortunately, CCTO ceramic
exhibits high dielectric loss that limits its practical
applications in electronic industries. Based on this,
researchers and technologists are intensively
developing lead-free and thermally stable high εr
*Corresponding author: Nasr Hadi
E-mail adresse: nassarmabbed@hotmail.com
DOI: http://dx.doi.org/10.13171/mjc8319052212nh
material as alternatives, which have a constant value
over a wide frequency region. Among those efforts,
extensive studies of the formation of perovskite
oxides of the systems Ca1- xMxTi1-xMxO3 (M = Cu, Y,
Sr, Ba, Pb, Zr, Co, Al, Fe, Li, Cr) were performed as
attempts to improve the properties of ceramic
materials 8. At present, research work has begun on
the basis of the idea of integrating ceramic materials,
among them; Almeida et al. 9, they studied the
properties of the composite (1-x)BaTiO3–xCCTO
film (x=0.00,0.50 and 1.00), N. Hadi et al. 10
investigated the dielectric properties of the (1x)CCTO-xBaTiO3 composites. In an analogous
previous work 11, we investigated the properties of the
(1-x)CCTO-xPZT composite ceramics with a ratio
Zr/Ti = 65/35, beyond the morphotropic phase and
for which PZT presents a tetragonal phase. Our
results showed that the composite CCTO-PZT with
the content near 50% of PZT has the great value of
the maximum of the dielectric permittivity.
Rajabtabar et al. 12 have investigated the dielectric
properties of CaCu3Ti4O12/Pb(Zr0.52Ti0.48)O3, the
considered ratio Zr/Ti of PZT corresponds to the
morphotropic phase with the coexistence of both
rhombohedral and tetragonal phases of PZT, their
Received February 7, 2019
Accepted March 23, 2019
Published May 22, 2019
Mediterr.J.Chem., 2019, Special Issue 8(3)
N. Hadi et al.
results showed that the composite ceramics with the
Pb(Zr0.52Ti0.48)O3 content of 20% has the highest
dielectric constant and the lowest dielectric loss. In
the present research, a new CCTO-PZT with a ratio
Zr/Ti=48/52 which is close to the morphotropic phase
boundary with a single tetragonal phase was
synthesized from PZT and CCTO which were
prepared by sol-gel and solid state route respectively.
We investigate the dielectric, electrical, structural and
optical properties of the synthetized samples. Also,
the relationships between the microstructure and
dielectric properties of the PZT, CCTO and CCTOPZT ceramics were discussed.
246
PZT gel formed. The dry gel was calcined at 700 °C
for 4 h in the atmosphere. Finally, the (1-x)CCTOxPZT prepared of CCTO and PZT were carefully
weighed in stoichiometric proportion and mixed
thoroughly in an agate mortar for 1h then stirred in
acetone for 2h. The powder was then dried at 400 °C
for 2h. The dried powder was then pressed into disks
of 12mm as diameter and 1mm of thickness under
pressure around 430 MPa and using the polyvinyl
alcohol (PVA) as a binder. The final sintering of the
pellets was done at 1000 oC for 8h with a heating rate
of 3 oC/min
Results and discussion
Experimental
The (1-x)CCTO-xPZT (where x = 0.00, 0.50 and
1.00) ceramic composites were synthesized by a
modified route via three steps. Firstly, CCTO was
synthesized by a solid-state reaction using CaCO3,
CuO and TiO2 were used as raw materials. In this
route, stoichiometric ratios of the reagents were
mixed in an agate mortar for 1h, after that stirred in
the medium of acetone for 3h, then ground again. The
mixed powder calcined in air at 1050 0C for 4h.
Secondly, PbZr0.48Ti0.52O3 (PZT) ceramic powder was
synthesized by a Sol-Gel process using zirconium
acetate, Zr(CH3COO)4, Lead (II) acetate trihydrate,
Pb(CH3COO)2.3H2O and titanium isopropoxide,
Ti(OCH(CH3)2)4. The metal acetates were dissolved
in distilled water to obtain standard aqueous solutions
of Pb2+ and Zr4+ and mixed in separate beakers along
with stoichiometric amounts of Ti(OCH(CH3)2)4.
X-ray diffraction patterns of the CCTO, PZT and
CCTO-PZT ceramic are shown in Figures 1a, 1b,
and Figure 2, respectively. The PZT and CCTO
ceramics show a single phase, which is highly
crystalline in nature where the main peaks of the
ceramic powders are comparable to those of the
standard ceramic XRD patterns of PZT (JCPDS 330784) and CCTO (JCPDS 75-2188), which have
been indexed to Im-3 and P4mm space groups with
cubic and tetragonal symmetries respectively. For
the CCTO-PZT composite, each XRD pattern can be
disassembled into two evident sets of well-defined
peaks that belong to the PZT and CCTO phases
without a secondary one. A similar result has
observed in (Nylon11 + μCCTO) composites 13
where both the Nylon11 and μCCTO peaks were
observed
to
be
unchanged.
Figure 1. XRD patterns and their Rietveld refinements for a) CCTO and b) PZT (48/52) samples.
The XRD patterns have been analyzed by
employing the Rietveld method using Fullprof
Software program 14 using the Im-3 and P4mm space
groups for CCTO and PZT, respectively. The X-ray
diffraction patterns along with Rietveld refined data
hare shown in the Figures 1a, 1b and Figure 2. In
these figures, the black points represent our
experimental results, and the solid line (red)
represents Rietveld refined data. The bottom lines
show the difference between the experimental and
refined data. The small vertical lines (blue) represent
Bragg allowed positions. From Rietveld analysis, the
crystalline structure was confirmed, and the unit cell
parameters calculated. The lattice constants, we
obtain for pure CCTO, and PZT (Table 1) are in good
agreement with those reported in literature 15-17. In the
case of the (CCTO -PZT) mixture, Rietveld analysis
shows that the crystal structure is a composition of
Mediterr.J.Chem., 2019, Special Issue 8(3)
247
N. Hadi et al.
both CCTO and PZT structures as separate ones, the
same phenomenon has been observed for The
BaM/CCTO composites which exhibit a single
crystalline phase of both BaM and CCTO18.
However, we remark a slight change in the lattice
parameters. The lattice constant decreased in CCTO
while it increased in PZT accompanied by a decrease
in c.
The fitting quality of the experimental results has
been assessed by computing the parameters such as
the “fit goodness” X2, RB (Bragg factor) and RF
(crystallographic factor) 14 obtained from Rietveld
refinement. They are given in Table 1 for all the
samples.
The Crystallite size (D) of the compounds was
determined by the Scherrer’s formula 19. We can
notice that the crystallite size of CCTO was found to
decrease on adding PZT while the PZT crystallite size
increases (Table 1).
Figure 2. XRD pattern and Rietveld refinement for the CCTO-PZT sample
Table 1. Reliability factors (RBragg, RF and X2), crystalline size (D), lattice parameters and cell volume V for
CCTO, PZT and CCTO and PZT in the mixture.
Parameters
CCTO
PZT
CCTO-PZT
RBragg
6.31
6.69
CCTO
9.01
RF
X2
7.98
2.19
9.26
2.93
7.76
1.13
2.79
1.13
597. 4
126.5
556.2
130.6
a=b=c=7.3
905
1
a=b=4.0449 and
c=4.11118
1.06386
403.665
67.264
D (Å)
a. b and c
(Å)
c/a
V (Å3)
The formation of the phases and purity of CCTO
and PZT were confirmed from the FT-IR spectrum
shown in Figure 3. The IR spectra of the CCTO
perovskite in the region of 400–1200 cm−1 is
dominated by three broad absorptions centred at 566,
516 and 447 cm−1 9,11. A similar spectrum for the
ceramic CCTO we prepared in our study is observed,
a=b
=c=7.38876
1
403.381
PZT
4.42
a=b=4.04603 and
c=4.10945
1.0567
67.273
with absorptions at 573, 523 and 451 cm−1 (Figure 3).
These absorptions are related to Ca-O, Cu-O and
Ti-O-Ti, respectively.
For the PZT (48/52)
perovskite, two mains absorptions were observed at
576 and 400 cm−1, which is consonant for the PZT
perovskite 11. For both the samples, a broad
absorption band is observed at 1000 – 600 cm-1 that
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N. Hadi et al.
suggests a structural rearrangement of the BO6 unit
248
resulting in perovskite phase formation.
Figure 3. FT-IR spectrum of pure CCTO and PZT.
Figure 4 shows scanning electron microscopy
(SEM) images of the PZT, CCTO and CCTO-PZT
pellets sintered at 1000 °C for 8 h. The average grain
sizes of the CCTO, PZT and CCTO-PZT pellets were
found to be 8.75, 6.01 and 1.75 μm, respectively.
PZT (48/52) shows a dense microstructure and the
presence of many clusters. The CCTO ceramics
exhibited a completely homogeneous morphology
with abnormally large grains with a small grain
segregated at the grain boundaries. The grains of
CCTO have smooth surfaces associated with a
spherical appearance, while the morphology of the
CCTO-PZT ceramic shows the unique features with a
large conglomerate formed by a cluster of small
granules with the presence of large pores between
these conglomerates.
Figure 4. SEM micrographs for CCTO (ref 11), PZT and (1-x)CCTO-xPZT for x = 0.00, 0.50 and 1.00
Since the modification of the grain size of the
ceramics affects the dielectric responses, we
investigate the dielectric properties of our
synthetized samples. The temperature dependence of
the dielectric properties of the CCTO, PZT and
CCTO-PZT ceramics sintered at 1000 °C for 8 h at
the frequency of 10 kHz is shown in Figure 5a. We
can see that ε’r increases up to maximum values
27620 at 400 0C for PZT and 18205 for CCTO-PZT
at 3630C, and then ε’r decreased with increasing
temperature. The temperature at which the
transitions occur is termed the Curie temperature,
(Tc), where PZT undergoes a transition from a
ferroelectric to a paraelectric. The figure shows a
shift of Tc to low temperatures for CCTO-PZT
composite, while for CCTO sample an abroad
dielectric peak appeared between 150 and 400 °C.
Figure 5b shows the frequency variation of the
dielectric constant (102 up to 2.106 Hz) at room
temperature for all composites. The values of ε’r for
the CCTO, PZT and CCTO-PZT samples at 1 kHz
are found 1662, 957and 4923, respectively. It can be
Mediterr.J.Chem., 2019, Special Issue 8(3)
N. Hadi et al.
seen that the value of the dielectric constant (ε’r) for
the CCTO-PZT ceramics is higher than the values of
the CCTO and PZT ceramics. The ε’r decreased with
249
increasing frequency in CCTO and CCTO-PZT
samples, but it remains independent of frequency in
pure PZT.
Figure 5. Real part of the relative dielectric constant of CCTO, PZT and CCTO-PZT as a function of (a)
temperature at 10 kHz (b) frequency at R.T
Many researchers interpreted the relaxation
behavior in the CCTO ceramic samples through the
complex impedance spectrum by three semicircles in
Nyquist plot or three peaks in electric modulus plot
(M'' versus frequency plot), and modeled the
dielectric response into an equivalent circuit which
consists of three parallel RC elements connected in
series 16. For the same purpose, in our study, the
frequency ranged from 100 Hz to 2 MHz. The
impedance spectrum study of all samples with
increasing temperature showed that at lower
temperatures, only one semicircular arc (data not
shown). This suggests the presence of grain bulk
properties (capacitance and resistance) of the
materials. However, at higher temperatures, the
semicircle arc is distorted, and another arc appears,
and the spectrum includes two semicircular arcs with
their centers lying out of the real axis for all samples
(Figure 6). This means that the relaxation process
becomes non-Debye. This behavior can originate
from several factors such as grain boundary, grain
size distribution or orientation, defects distribution,
and so on. Thus the presence of two semicircle arcs
indicates the presence of both bulk and boundary
contributions and the electrode interface effects 20 to
the electrical properties of the samples 16. The
equivalent resistance is connected to the diameter of
the semicircle arcs, so we can observe that in the
studied frequency range the PZT resistance was
larger that CCTO and CCTO-PZT resistance as is
shown in the Figure 6b.
Figure 6. Impedance spectrum of samples for; (a) CCTO-PZT for different temperatures and (b) CCTO, PZT
and CCTO-PZT at 220 0C
According to impedance spectrum data obtained
for pure CCTO and PZT samples, each sample can be
represented by several elements. For the pure PZT,
two parallel elements, R|C and R|CPE connected in
series (Figure 7a), are used to fit the impedance data
sample, while three parallel elements represent the
pure CCTO sample (Figure 7b) (R|C-R|C-R|CPE). On
the other hand, the CCTO-PZT composite is
represented by four parallel elements, three of them
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N. Hadi et al.
R|C and other R|CPE as shown in Figure 7c. These
250
circuits gave the best fittings.
Figure 7. The equivalent electric circuits for the samples: a) PZT, b) CCTO and c) CCTO-PZT.
Figure 8a shows the variation of real (Z’) and
(Z") part of complexes impedance as a function of
frequency (102–2.106Hz) at different temperatures for
CCTO-PZT. We observe that Z' decreases with
increasing temperature or frequency and attains a
constant value at higher frequencies irrespective of
temperatures. This may be due to an increase in the
ac conductivity with temperature increasing. The
merger of the real part of impedance Z' for all
temperatures at the higher frequencies is due to the
RC network where the current passes through
conducting regions in the material at lower
frequencies, but at higher frequencies, the current
passes through the insulating (capacitive) regions 9.
The temperature dependence of resistivity showed the
typical behavior found for semiconductors. The
changes of imaginary part Z’’ of impedance for
CCTO-PZT ceramic composite are shown in Figure
8b. The typical variation indicates that Z" attains a
maximum value at a particular frequency. The
maximum value and the frequency position of this
maximum depend on temperature. The behavior of Z"
shows a considerable decrease in the magnitude with
a shift in the peak frequency position towards the
higher side when the temperature increases. This
feature becomes notable at a higher temperature. The
trend of variation of Z" with a shift in the peak
frequency suggests the presence of electrical
relaxation phenomenon in the material. A relative
lowering in the magnitude of Z" accompanied by a
shift in the peak frequency position towards the
higher side with the rise in temperature originates
from the presence of space charges in the material.
This result is in good agreement with the observation
of complex impedance spectrum results in the
literature 6,16.
Figure 8. Frequency variations of a) the real part Z´ and b) the imaginary part of CCTO-PZT (48/52) ceramic at
different temperatures.
The maximum values Z′′max of Z” follow the
temperature dependent Arrhenius law, where the
frequency position fmax associated with Z′′max can be
expressed as:
(
)
Where Ea is the activation energy in the
relaxation process, fo is the pre-exponential factor, K
is the Boltzmann constant and T is the absolute
temperature. Figure 9 shows a plot of log(fmax) vs
1000/T for CCTO-PZT composite with the
theoretical fit along to the above equation. The slop
of the curve gives the value
, term enabling us
to estimate the activation energy of the samples. The
Ea values for all samples are given in Table 2. The
activation energy obtained for CCTO-PZT suggests
that the conduction process is mainly due a todoubly
21.
ionized
phenomenon
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N. Hadi et al.
0.6 eV
Figure 9. log(fmax) vs.1000/T plot for CCTO-PZT composite
Table 2. Activation energy of CCTO, PZT and CCTO-PZT composite.
Composite
CCTO
CCTO-PZT
PZT
In order to understand the mechanism of
conduction and relaxation in CCTO, PZT and
CCTO-PZT materials, we use the AC conductivity
measured as a function of temperature at 10 kHz. As
shown in Figure 10, the conductivity of the PZT
ceramics remained relatively constant between room
temperature up to 50°C. However, as the temperature
Activation Energy (eV)
0.8
0.6
0.23
was further increased, the conductivity increases
significantly. The conductivity of the CCTO-PZT
pellets is higher than that of the CCTO and PZT
ceramics. The high conductivity of the CCTO-PZT
ceramic is responsible for the high dielectric constant
of this ceramic, which also supports the presence of
the IBLC structures 3,17.
Figure 10. Variation of ac conductivity of CCTO–PZT ceramics with the temperature at 10 kHz.
The UV-visible reflectance spectra of the
samples CCTO, PZT and CCTO-PZT, were recorded
at room temperature using a Jasco V-570
spectrophotometer, in the wavelength range [2002000 nm]. A Barium Sulphate plate (BaSO4) is used
as the standard (100% reflectance) on which the
Mediterr.J.Chem., 2019, Special Issue 8(3)
N. Hadi et al.
crushed sample of the crystal to be analyzed is
placed.
The optical transmittance spectrum for the
materials CCTO, PZT and CCTO-PZT is shown in
Figure 11. The analysis of the spectrums reveals that
the pure PZT present a high optical transmittance
252
than the CCTO-PZT and CCTO (Table 3). The high
optical transmittance may be due to lesser crystallite
size in PZT while the irregular behavior for CCTO
and CCTO-PZT could be due to defects mainly
originated from CCTO. The scattering is strong,
resulting in low transmittance.
Figure 11. Optical Transmittance spectrum of CCTO, PZT and CCTO-PZT
The dependence of the optical absorption
coefficient on the photon energy helps to study the
band structure using the following expression.
( )
Where T is the transmittance (%), d is the thickness
of the sample (0.76 mm). The optical band gaps
energies of the compounds were evaluated using the
following expression.
Where A is a constant, Eg is the optical energy
band gap, υ is the frequency of the incident beam, and
h is the Planck’s constant. The Eg direct optical band
gap energy value is obtained by extrapolating the
linear portion of the plot of (αhν)2 versus (hν) to
(αhν)2 = 0 as shown in Figure 12. The optical band
gap energies (Eg) were found to be 2.25 eV, 3.92 eV,
3.50 eV for CCTO, CCTO-PZT and PZT respectively
(Figure 12 and Table 3) the band gap calculated in
our study is consistent with the range gap values
which mentioned in the literature 22.
Figure 12. The optical band gap energy of CCTO and CCTO-PZT
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N. Hadi et al.
Table 3. The optical transmittance and band gap
energy of CCTO, CCTO-PZT and PZT composites.
Composite
CCTO
CCTO-PZT
PZT
transmittance
46.43
69.24
99.22
Energy gab (ev)
2.25
3.92
3.50
Conclusion
The present work reports the results of our study
on the dielectric and electrical properties of CCTOPZT composite ceramics. (1-x)CaCu3Ti4O12 –xPZT
samples were synthetized with compositions x = 0.00,
0.50 and 1.00. X-ray diffraction pattern shows pure
cubic and tetragonal phase for x = 0.00 and x= 1.00
compositions respectively, while the x = 0.50,
composition presents a composite phase cubic and
tetragonal. The diffraction peaks in the pattern of the
composite sample do not reveal any change in the
structures of both CCTO and PZT in the mixture.
From Rietveld refinements, the obtained values of the
lattice parameters are close to the reported values in
the literature. Scanning electron micrographs show a
uniform grain distribution and the grain sizes and
shapes depending on the PZT amount in the
composite. The dielectric measurements show that
the CCTO- PZT presents higher values of the
dielectric constant. The Curie temperature was found
to be 400 oC for pure PZT, and it is shifted toward a
lower temperature (Tc~340 oC) in CCTO-PZT. The
electrical property indicates that the material exhibits
a single semi circular arc at lower temperatures
attributed to grain bulk conduction, and for higher
temperatures, the semicircle arc is distorted by the
appearance of another arc attributed to the grain
boundary conduction. The temperature evolution of
the diameters of the arcs suggests that the materials
present a negative temperature coefficient resistance
(NTCR). A temperature dependent relaxation
phenomenon is observed for the CCTO-PZT sample;
the ac conductivity increases with increasing the
temperature and obey the Arrhenius law. Optical
transmittance spectra show that PZT is more
transparent than CCTO-PZT and PZT, the optical
band gap energies (Eg) were found to be 2.25 eV,
3,51 eV, 3.92 eV for CCTO, PZT and CCTO-PZT
respectively.
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