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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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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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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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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90

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0 200 400 600 800 1000

Frequency (KHz)

die

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Pure PVDF

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30 wt% PVDF

Copyright © 2011 by ASME

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