Low- Frequency Dielectric Dispersion and Impedance Spectroscopy of
Lead-Free Na0.5 Bi0.5 Tio3 (Nbt) based Ferroelectric Ceramic
B.Tilak
Department of Materials Science, Addis Ababa University, Addis Ababa, Ethiopia
*Corresponding author:
B.Tilak
Department of Materials Science
Addis Ababa University
Addis Ababa, Ethiopia E-mail: tilakb22@rediffmail.com
Received March 28, 2012; Accepted April 18, 2012; Published April 25, 2012
Citation: Tilak B (2012) Low- Frequency Dielectric Dispersion and Impedance
Spectroscopy of Lead-Free Na0.5 Bi0.5 Tio3 (Nbt) based Ferroelectric Ceramic. J
Material Sci Eng 1:108. doi:10.4172/2169-0022.1000108
(Na0.5Bi0.5)0.89Ba0.11 Zr0.04Ti0.96O3 (0.11BNBZT) ceramics were prepared by using solid-state reaction method. A
single phase perovskite (ABO3) structure with tetragonal symmetry was confirmed by X-ray diffraction. The dielectric
behavior, impedance relaxation were investigated in a wide range of temperature (30°C-600°C) and frequencies
(45Hz-5MHz). The grain morphologies were analyzed by using a Scanning Electron Microscopy (SEM) analysis.
The temperature dependence of dielectric behavior and the frequency dependence of impedance relaxation were
investigated. A broaden dielectric constant peak were observed over a wide temperature range and also indicate a
relaxor behavior. The complex impedance plot exhibited semicircle plot, which is explained by the grain effect of the
bulk. The centers of the impedance semicircles lie below the real axis, which indicates that the impedance response
is a Cole-Cole type relaxation.
Lead–containing ferroelectric materials such as Pb(Zr, Ti)O3
(PZT) and PZT- based [1,2] multi component ceramics have been
widely used as sensors, actuators and ultrasonic transducers in various
electronic devices because of their excellent piezoelectric properties.
However, PbO vaporization problems during the sintering process
and treatment of Pb containing products cause crucial environment
pollution. Recently, much more attention has been paid to the
investigation of lead free piezoelectric materials for the replacement of
the lead containing ferroelectrics [3].
As such, lead free ceramics have attracted considerable attention.
The Na0.5Bi0.5 TiO3 (NBT) and NBT based system with ABO3
perovskite structure is one of important lead-free based materials
[4-7]. NBT has a high Curie temperature (Tc=320°C) and possesses
strong ferroelectricity, exhibiting a relatively large polarization (Pr=38
μC/cm2) and high coercive field (Ec=73 kV/cm) at room temperature.
Although, NBT exhibits a large coercive field and relatively large
conductivity, it is hard to be poled for practical applications. NBT had
two phase transitions; it has anti ferroelectric phase below 200°C and
paraelectric phase at 320°C. Recently, the effect of doping in NBT [5-
7] and several solid solutions to improve the properties of NBT have
been investigated. It has been reported that NBT –based ceramics
with their compositions modified with the addition of BaTiO3 [8],
BiFeO3[9], NaNbO3[10], CeO2[11], MnO2 [12]. Among them (Bi 0.5
Na0.5)1-xBaxTiO3 (BNBT) ceramics are more interesting because of the
existence of rhombohedral, tetragonal structure, Morphotropic Phase
Boundary (MPB), MPB for the BNBT composition lies between 6%
and 7% BaTiO3.
A necessary condition for fabrication of high performing materials
is knowledge of their physical properties, but the dielectric dispersions
and the impedance relaxation of the NBT based systems are not fully
studied. The contribution of the grains and the grain boundaries
are evaluated by using complex impedance. Complex impedance
spectroscopy is a good technique to investigate the microstructure and
the electrical properties.
In the present work, ZrO2 has been added to (Bi 0.5 Na0.5) 1-x Bax TiO3
(BNBT) i.e., (Na0.5 Bi0.5)0.89Ba0.11 Zr0.04Ti0.96O3 ceramic has been prepared and investigated in the dielectric and impedance properties over a wide
frequency range from 45 Hz to 5 MHz and temperature range from
30°C -600°C.
Experimental
The conventional mixed oxide technique was used to prepare
(Na0.5 Bi0.5)0.89Ba0.11 Zr0.04Ti0.96O3 (0.11BNBZT) ceramic. The commercially
available chemicals of Bi2O3, Na2 O3, Ba CO3, ZrO2 and TiO2 with purity
higher than 99.9% were used as starting materials. These oxides and
carbonates were mixed according to the desired chemical formula and
were grounded for 10 h and calcined at 850°C/2h. After calcination,
the ground powder was pressed into disks with 12 mm diameter and
about 2 mm in thickness. The disk was sintered at 1180°C/3hrs in air.
The density of the sintered sample was measured using the Archimedes
principle. The phase structure was examined using x-ray powder
diffraction analysis using a Cu Kα radiation. For the observation of the
microstructure samples were polished and thermally etched. Finally,
the microstructures were observed by scanning electron microscopy
(SEM: JEOL_JY: Model 5800F).
For electrical measurements, silver paste was applied on both
surfaces of the disk and thermally etched at 650°C. The capacitance
of the ceramic used for calculating dielectric constant was measured
as a function of temperature, complex impedance by using computer
interfaced LCR Hi-Tester (HIOKI 3532-50, japan).
Results and Discussion
Figure 1 shows an x-ray diffraction pattern of 0.11BNBZT ceramic,
exhibits pure perovskite structure with tetragonal symmetry feature
with (111) peak around 40° of 2θ. As crystal structure transformed from rhombohedral into a tetragonal symmetry there is splitting of
(200) and (002) peak around 46.5° of 2θ, which has been observed in
XRD pattern (Figure 1). Lattice parameters have been calculated by
using POWD software. Lattice parameters, cell volume, experimental
and theoretical density, porosity has been tabulated in Table 1.
Figure 1: X-ray diffraction pattern.
Table 1:
The scanning electron micrograph of the sintered ceramic
(011BNBZT) is shown in Figure 2, indicating that the ceramic is highly
dense with less porosity. Based on the measurement of Archimedes
principle, the experimental density of ceramic is 95% of theoretical
density. The average grain size has been calculated by using line
intercept method. The average grain size of the studied ceramic is1.21
μm.
Figure 2: Scanning Electron micrograph.
Figure 3; depict the temperature dependence of real part of
dielectric constant of 0.11BNBZT at various frequencies, indicating a
relaxor like behavior [13]. At low frequencies, a phase transition from
ferroelectric to anti ferroelectric is observed at around 100°C which is
referred as depolarization temperature. Such type of transition was also
reported in BNT-NNT-BT [14], BNT- PbTiO3 [15] and BNT-BKT-BLT
[16] ceramics. But, with an increase of frequency, the ceramics show
pure ferroelectric behaviors up to their transition temperatures. Such
as, phenomenon might be probably due to cation disorder by random
distribution of Na+, Bi+ and Ba2+ at the A-sites of the lattice [17-19].
There is an existence of three phases of ferroelectric, anti ferroelectric
and paraelectric in different temperature ranges.
Figure 3: Temperature dependent Dielectric constant.
The dielectric constant has frequency dependence especially, at the
low frequencies, which is called the low frequency dielectric dispersion.
A strong low-frequency dielectric dispersion has also been observed
in NBT which was previously reported [20]. At temperature above Tm
and at low frequencies, the dispersion is increased, which is caused by
an increase in the electrical conductivity. The dielectric peak appears to
broaden with respect to increase in frequency which may be attributed
to the structural disorder in the system. The real part of the dielectric
constant, describes the “elastic” loss-free reaction of the material to an
externally applied ac electric field.
Impedance spectroscopy
The electrical properties of the materials have been investigated
using Complex Impedance Spectroscopy (CIS). It is an important
tool to analyze the electrical properties of a polycrystalline material in
view of its capability of correlating the sample electrical behavior to its
microstructure [21].
Figure 4 represents the real part of impedance (Z1) as function of
frequency. Z1 has higher values at lower frequency and decreases up to
100 kHz and attains a constant value beyond that. The pattern shows
the sigmoidal variation as a function of frequency in the low frequency
region followed by a saturation region in the frequency region. This
suggests the presence of mixed nature of polarization behavior in the
material.
Figure 4: Variation of real part of impedance as function of frequency.
Figure 5 presents the variation of imaginary part of impedance (Z11)
as a function of frequency at different set of temperatures. With the
increase of frequency, the real part of impedance (Z1) and imaginary
part of impedance (Z11) decreases with increase in frequency. At higher
frequency side all the curves merge. As the temperature increases the
peak has been observed in (Z11) vs frequency curve [22]. The peak
shifts towards higher frequency side with increasing temperature
showing that the resistance of the bulk material is decreasing. Also,
the magnitude of Z11 decreases with increasing frequency. This
would imply that dielectric relaxation is temperature dependent and there is apparently not a single relaxation time. It is evident that with
increasing temperature, there is broadening of the peaks and at higher
temperatures, the curves appear almost flat [23].
Figure 5: Variation of imaginary part of impedance as function of frequency.
The diffusiveness of Z11 peaks indicate the distribution of relaxation
times and the increase in full width at half of the Z11 maxima with
the increase of temperature indicate the increase in distribution of
relaxation frequency. This distribution of relaxation frequency is
attributed to the cationic disorder due to the random occupations of
A and B-sites of cations with different ionic radius and valence states.
The random distribution causes multiple relaxations that are shown by
broadening of Z11 peaks. The appearance of peak in Z11 vs frequency
plots indicate some sort of relaxation related to oxygen vacancies
present in the studied sample. It has been known that the oxygen
vacancies are the main cause of domain wall clamping. This clamping will restrain the macro-micro domain switching in some degree, which
in turn will affect the polarization in the material. Increase in oxygen
vacancies will increase the domain wall clamping and decreases the
polarization state.
Figure 6 shows the frequency dependences of the real part (Z1) and
the imaginary part (Z11) of the complex impedance at a temperature
530°C. With increasing frequency, the real part Z1 decreases while the
imaginary parts Z11 shows a peak. Then, the complex impedance plot
of studied material exhibits one impedance semicircle arc. Thus, one
impedance arc of the studied material is representative of the bulk
properties of the grains (dc resistivity). The centers of the impedance
semicircular arcs lie below the real axis. Thus, the impedance relaxation
can be explained by using Cole-Cole response [24, 25]. The electrical
contribution of the grains was introduced by using equivalent circuit
as shown in Figure 7 and Figure 8 shows the temperature dependent
spectra (Nyquist plot) of 0.11BNBZT material. The impedance spectrum
is featured by semicircle arcs. The nature of variation of the arcs with
temperature and frequency provides various clues of the materials.
The impedance spectra are characterized by the appearance of a two
semicircle arcs. The presence of single semi circular arc indicated that
the electrical process contribution as form a bulk materials (grain
interior), which can be modeled as an equivalent circuit comprising
of a parallel combination of bulk resistance (Rb) bulk capacitance (Cb)
and leaky capacitance (Qb). Appearance of another semicircular arc
near above 450°C indicates the beginning of the intergranular activities
(grain boundary effect) within the sample with definite contribution
from both bulk and grain boundary effects (Figure 8). The intercept of the semicircular arc with real axis (Z1) gives us an estimate of the bulk
resistance (Rb) and grain boundary resistance (Rgb) of the material. It has
been observed that the bulk resistance of the materials decreases with
increase in temperature showing a typical semiconducting property,
i.e., Negative Temperature Coefficient of Resistance (NTCR) behavior
[26]. It can be noticed that the center of the semicircle arc lies below
the real axis suggesting the relaxation to be of poly dispersive Non-
Debye type in samples. This may be due to the presence of distributed
elements in the materials electrode system.
Figure 6: Impedance Cole-Cole plot at 5300C as function of frequency.
Figure 7: Cole-Cole Plot.
Figure 8: Variation of real and imaginary part of impedance with temperature.
Conclusion
The solid solution of (Na0.5 Bi0.5)0.89Ba0.11 Zr0.04Ti0.96O3 (0.11BNBZT)
was prepared using high temperature solid state reaction technique.
X-ray diffraction studies reveal perovskite structure with tetragonal
symmetry. SEM analysis explained the grain morphology on the surface
of the material. The temperature dependence of dielectric constant
shows relaxor behavior, leading to conclusions that three phases of
ferroelectric, anti ferroelectric, and paraelectric states exist in different
temperature ranges. The grain and grain boundary contribution have
been separated using impedance spectroscopy analysis equivalent
circuit to explain electrical phenomena occurring inside the material
which revealed the Non-Debye type relaxation present in the material.
Acknowledgements
The authors thank to Naval Science and Technology Laboratory (NSTL), Govt
of India, Visakhapatnam, for funding the research project and University Grants
Commission, Govt of India for Research fellowship.
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