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Copyright © 2011 by ASME
Proceedings of 2011 International Conference on Mechanical Engineering and Technology
ICMET 2011 November 24-25, 2011, London, UK
PYROELECTRIC PROPERTIES OF NANOCOMPOSITE OF POLYVINYLIDENE FLUORIDE AND BATIO3
Sh. Ebrahim/ Department of Materials Science, Institute of Graduate Studies and Research, Alexandria
University
I. Morsi /Arab Academy for Science and Technology , Electronics and Communications
Department
M. Soliman/ Department of Materials Science, Institute of Graduate Studies and Research, Alexandria
University
S. Ibrahim /Arab Academy for Science and Technology , Electronics and Communications
Department
ABSTRACT In recent years, polymer-ceramic nanocomposite materials
have been given great attention due to the possibility of their
use in piezoelectric and pyroelectric transducers.
Nanocomposite of polyvinylidene fluoride (PVDF) and barium
titnate (BaTiO3) is prepared using cast technique. When
infrared spectra were used, it is concluded that pure PVDF and
their composite with BaTiO3 exist in the unpoled state (α-
phase). It is found that incorporation of BaTiO3 into PVDF has
destroyd the spherulite structure and has dispersed in the PVDF
matrix with nanosize particles. It is observed that
nanocomposite of 30 wt. % of PVDF has the highest
pyroelectric coefficient of 1.00 nC/cm2/oC.
Keywords: Polyvinylidene fluoride, BaTiO3, Thermal
stimulated depolarization current , Nanocomposite
INTRODUCTION Ferroelectric ceramics, such as lead zirconium titanate
(PZT) and barium titanate (BaTiO3), with very high
pyroelectric, piezoelectric coefficients and dielectric constants,
are used in various applications. However, inflexibility and
poor processibility, inherent in ceramics, can limit these
applications. Ferroelectric polymers, such as poly (vinylidene
fluoride) (PVDF) and its copolymers have been mainly used in
transducers, since they are flexible, easy to process and present
low mechanical impedance. However, PVDF has low
pyroelectric, piezoelectric coefficients and dielectric constants.
Therefore, a polymer/ceramic composite would be an ideal
replacement for both classes and would have the desirable
properties of both materials.
Heterostructural materials, such as polymer ceramic
composites, have received a lot of attention recently, since these
materials can combine the excellent pyroelectric and
piezoelectric properties of ceramics with the flexibility,
processing facility, levity and strength of polymers. Amongst,
the most widely studied composites are those consisting of
PVDF or its copolymers and PZT or BaTiO3 .
Polyvinylidene fluoride (PVDF) and its copolymers have
been extensively studied due to their excellent pyroelectric and
piezoelectric properties over the last three decades [1–4]. PVDF
is famous for its multiple characters with four different
crystalline forms, i.e., α, β, γ, and δ. These crystalline phases
could be transformed into each other under specific conditions,
such as the application of mechanical milling or high
temperature electrostatic field. In all phases, the β-PVDF
exhibits very good piezoelectric, pyroelectric, and dielectric
properties, so it is utilized to fabricate high β-phase PVDF for
its use in sensors and actuators. In general, β-PVDF is obtained
by uniaxial stretching, elevated pressure crystallization, high
electric field polarization, and solution crystallization.
Composites can be prepared by various methods, e.g.
embedding poled piezoelectric fibers, drilling holes in blocks of
poled ceramic and then filling them with polymer, and mixing
powdered ceramic with polymer and then poling it [5-9]. The
last method is a convenient way to prepare a composite of a
required size and composition. However, the sample must be
poled after its preparation in order to exhibit the pyroelectric
effects.
The aim of this work is to prepare and measure thermally
stimulated depolarization current (TSDC) of the composite of
PVDF and BaTiO3. The effect of composite composition on the
pyroelectric coefficient will be investigated.
NOMENCLATURE PVDF: polyvinylidene fluoride
BaTiO3: barium titanate
PZT : lead zirconium titanate
TSDC: thermally stimulated depolarization current
DMF: dimethyl formamide
FTIR: Fourier transform infrared spectroscopy
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Copyright © 2011 by ASME
XRD: X-ray diffraction
SEM: scanning electron microscope
EXPERIMENTAL WORK
Sample preparation
The composite samples is prepared by a solvent-
cast technique by PVDF and fine powder of BaTiO3
(particle size 0.2 µ m) in different weight proportions as
shown in Table (1). Since there is no common solvent for
both materials, so first dissolving PVDF in dimethyl
formamide (DMF) and then dispersing BaTiO3 powder in
the solution of PVDF. The solvent-cast films are
prepared by pouring the solution on a glass substrate and
heating it in an oven at about 80oC for 2-3 h till the film
is created and the traces of solvent are removed.
Tab.1 The different samples proportions of PVDF and BaTiO3
Sample No. PVDF
(by weight %)
BaTiO3
(by weight %)
A 100 0.0
B 90 10
C 80 20
D 50 50
E 30 70
Characterization and measurements
IR spectra are taken by Perkin Elmer FTIR Spectroscopy
BX instrument. The crystalline structure is identified by X-ray
diffraction (XRD) using X-ray 7000 Schimadzu diffractometer.
It is employed to characterize the phase and structure of the
absorber samples operating with Cu Kα radiation (λ=0.154060
nm). It is generated at 30 kV and 30 mA with scanning rate of
4º min-1 for 2θ values between 10 and 80 degrees.
The microstructure characterization of the samples are
observed by a scanning electron microscope (SEM) (JEOL
JSM-6360LA).
The dielectric measurements are carried out by a Hewlett
Packard (HP 4277A) LCZ meter. The samples are sandwiched
between pressure-contact electrodes. The whole assembly is
kept in a controlled heating rate maintained at 4oC/min. The
electrodes are connected to an electrometer (Keithley 616) in
order to measure the pyroelectric current.
Thermal stimulated depolarization current (TSDC)
technique is based on depolarization of sample by thermal
activation. Before TSDC measurements, samples are subjected
to an electric poling. At a given temperature Tp (called poling
or polarization temperature), a static electric field is applied to
the investigated sample for a time tp that is long enough to
permit the different mobile entities in the material to orient
themselves within the field. This configuration is then frozen by
a rapid decrease in temperature keeping the electric field
applied to it in order to avoid any relaxation of dipoles and/or
charges. The field is then removed and the sample is short
circuited for a certain time to eliminate the eventual surface
charges and stabilize the sample at this temperature.
The poled sample is then short-circuited through a high
sensitive digital electrometer in an oven, which is programmed
to rise temperature linearly with time (4oC/min). This rate
ensures a good resolution of the TSDC spectrum and gives
measurable current value sufficiently high to make the
background current negligible. The samples are coated with
carbon paste. Two runs are carried out to obtain the pyroelectric
current (reversible) because some charges is released during the
first and second run. The presence of space charges in the first
run gives an unreal pyroelectric current. Pyroelectric coefficient
(p) can be calculated from the following equation (1)[10]:
(1)
where I is the pyroelectric current, A is the electrodes area and
dt/dT is heating rate.
RESULTS AND DISCUSSION
FTIR spectra
Normally, IR spectroscopy of the composite is carried out
to explore the possible interactions between the blend
components. Figure (1) shows FTIR spectra of the pristine
PVDF and composite of PVDF/BaTiO3 with 90 wt% of PVDF
films. A typical vibration band is observed at about 1400 cm-1,
and it corresponds to the deformed vibration of the CH2 group
[11]. The bands observed at 850 cm-1 are assigned to the
characteristic frequency of the vinylidene compound. The
absorption band seen at 490 cm-1 can be attributed to the
wagging vibrations of CF2. The characteristic absorption
bands observed at 491, 600, 666 and 740 cm-1 are assigned to
the α- phase of PVDF, and the band 840 cm-1 is assigned to the
β-phase of PVDF. In addition, the band observed at 3020 cm-1
corresponds to the β- phase of PVDF. The pure PVDF and its
composite have the same IR spectra as shown in Figure (1a and
b). From FTIR spectra we can conclude that pure PDVF and its
composite with BaTiO3 exist in the α- phase [11, 12].
X-ray diffraction
PVDF is a semicrystalline polymer that consists of
amorphous phase and crystalline (α, β and γ) phases. The amorphous and α-phases are the nonpolar phases, the
crystalline β and γ-phases are polar phases. XRD patterns of pure PVDF and composites of PVDF/BaTiO3 films with
different compositions are indicated in Figure (2). XRD pattern
of pure PVDF shows a semicrystalline behavior. PVDF has
major peak at the 2θ value of 19.6o of the plane (020). The main
peaks of BaTiO3 appear for (200)/(002) (2θ =45o), (210)/(201)
(2θ =51o), (112)/(211) (2θ =56
o), (202)/(220) (2θ =66
o),
(202)/(220) (2θ =66 o) and (103)/(301)/(310) (2θ =75
o) imply
that the primary phase is tetragonal perovskite structure
[13,14].
)/( dtdTA
IP =
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Copyright © 2011 by ASME
Fig.1 FTIR spectra of the pristine PVDF and composite of PVDF/BaTiO3 with
90 wt% of PVDF.
Scanning Electron Microscope
Figure (3) shows SEM micrographs of pure PVDF (a)
and composites of PVDF/BaTiO3 films with different
compositions. The characteristic spherulitic crystallite of PVDF
in the range of 20 µm diameter is shown in Figure (3a). Each
spherulitic crystallite has fiber structure. The incorporation of
BaTiO3 into PVDF leads to the growth of the spherulitic
crystallites and BaTiO3 particles inserted in these crystals with
nanosize of average 25 nm for composite of PVDF/BaTiO3
with 90 wt. % of PVDF as shown in Figure (3b). Increasing
weight content of BaTiO3 in the composite, decreases size of
particles to about 15 nm as shown in Figure (3 c and d).
Fig.2 XRD of pure PVDF and composites of PVDF/BaTiO3 with different
compositions
a) Pure PVDF
b) Composite of PVDF/BaTiO3 with 90 wt. % of PVDF
c) Composite of PVDF/BaTiO3 with 80 wt. % of PVDF
d) Composite of PVDF/BaTiO3 with 30 wt. % of PVDF
pure PVDF
400900140019002400290034003900
Wavenumber (cm-1
)
Tra
nsm
itta
nce
(a.u
.)
Composite with 90 wt % PVDF
400900140019002400290034003900
Wavenumber (cm-1
)
Tra
nsm
itta
nce
(a.u
.)
pure PVDF
10 30 50 70
2 θ (0)
Inte
nsi
ty (a.u
.)
Composite with 90 wt % PVDF
10 20 30 40 50 60 70 80
2 θ (o)
Inte
nsi
ty (a.u
.)
Composite with 80 wt % PVDF
10 30 50 70
2 θ (o)
Inte
nsi
ty (a.u
.)
Composite with 30 wt % PVDF
10 30 50 70
2 θ (o)
Inte
nsi
ty (a.u
.)
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Copyright © 2011 by ASME
Fig.3 SEM micrographs of pure PVDF and composites of PVDF/BaTiO3 films
with different compositions
Dielectric constant
The most important feature of dielectrics is the ability to
store electric charge which is determined by the homo or
heterogeneity of their structure. Figure (4) shows the dielectric
constant of pure PVDF and composites of PVDF/BaTiO3 films
with different compositions in the frequency range from 10 to
1000 kHz at the room temperature. The dielectric constant
increases by increasing of BaTiO3 content in the composite.
The dielectric constant of pure PVDF is equal to 19.47 at 10
kHz and rise to 146.6 for PVDF/ BaTiO3 composite at 30 wt %
of PVDF. This is may be due to higher conductivity and
dielectric constant of BaTiO3 than that of PVDF. The dielectric
constant falls in the small range of low frequency to reach a
plateau region at high frequency [15].
Fig.4 Frequency dependence of εr for the unpoled PVDF and their different composite with BaTiO3 at room temperature.
Thermal stimulated depolarization current (TSDC)
TSDC technique has shown that the total charge stored in
polymer electrets and different mechanisms, which contribute
to the storage of charges, are very sensitive to the structure of
the electrets material itself, because of the presence of different
groups in the main molecular chain. Thus, TSDC technique is
proved to be a basic tool to identify and evaluate the dipole re-
orientation processes, trapping and recombination levels in
electrets. TSDC method is known to be a powerful tool for
studying relaxation processes in polymer [16].
PVDF and their nanocomposite samples are poled in order
to investigate the pyroelectric behavior. The poling conditions
are [Ep= 1MV/m, tp= 30 min, Tp=70 oC].
The pyroelectric measurements are carried out after 24h of
poling in order to stabilize the current in the samples. It can be
observed from Figure (5) that the pyroelectric coefficient rises
sharply at temperatures >70 oC. This is because of a structural
relaxation peak at 70 oC due to a molecular motion in the
crystalline region [17]. Table (2) shows the dielectric constant
and pyroelectric coefficient for PVDF and their
nanocomposites. It is observed that nanocomposite of 30 wt. %
of PVDF has a high pyroelectric coefficient of 1.00 nC/cm2/ oC.
Fig.5 Pyroelectric coefficients versus temperature for PVDF and their
nanocomposite poled at (Ep = 1MV/m, tp = 30 min, Tp = 70o C)
Tab.2 The dielectric constant and pyroelectric coefficient for
PVDF and BaTiO3 nancomposites.
Sample
No.
Dielectric constant
at 10KHz
Pyroelectric coefficient
(nC/cm2/ oC)
A 19.47 0.018
B 26.14 0.02
C 28.03 0.02
D 49.9 0.1718
E 146.6 1.00
CONCLUSION
Nanocomposites of PVDF and BaTiO3 with different weights
are successfully prepared using cast technique from DMF
solvent. It concluded that pure PVDF and its composite with
BaTiO3 exist in the unpoled state. It is found that incorporation
of BaTiO3 into PVDF destroys the spherulite structure and
disperses in the PVDF matrix with nanosize particles. It is
observed that nanocomposite of 30 wt. % of PVDF has the
highest pyroelectric coefficient of 1.00 nC/cm2/ oC.
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Copyright © 2011 by ASME
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