a comparative study of dielectric properties of … comparative study of dielectric properties of...

International Journal of Engineering Technology, Management and Applied Sciences

www.ijetmas.com April 2015, Volume 3 Issue 4, ISSN 2349-4476

303 S. Moharana, M. K. Mishra, B. Behera, R.N Mahaling

A Comparative Study of Dielectric Properties of Calcined and

Un-Calcined BiFeO3-Poly (vinylidene fluoride) (PVDF)

Composite Films

S. Moharana1, M. K. Mishra

1, B. Behera

2, R.N Mahaling

1*(Corresponding Author)

1Polymeric and Materials Chemistry Laboratory, School of Chemistry, Sambalpur University,

Jyoti Vihar, Burla-768019, Odisha, India 2Materials Research Laboratory, School of Physics, Sambalpur University,

Jyoti Vihar, Burla - 768019, Odisha, India

Abstract

In the present study, the dielectric properties of calcined and uncalcined BiFeO3 with polyvinylidene

fluoride (PVDF) composite film are studied. The structure, morphology and dielectric properties are

studied by X-ray diffraction, Scanning Electron Microscope and impedance analyser. From X-ray

diffraction pattern it is found that calcined BiFeO3 with PVDF composite confirms the rhombohedral

structure but there is no clear diffraction peak in support of (structure of) uncalcined BiFeO3 with

PVDF composite film. The dielectric constant of the uncalcined BiFeO3 with PVDF composite at RT

is very high i.e., 200, which is 20 times higher than that of calcined and pure PVDF film. The

complex impedance spectroscopy shows the electrical properties of the composites which are studied

by using wide range of frequency at room temperature. The Nyquist plot suggests the contribution of

bulk and grain boundary effect with reference to the electrical impedance. The surface morphology

(SEM) reveals that the calcined PVDF/BiFeO3 composite film has an average grain size of 600 -700

nm, which is much lesser than the uncalcined composite film. The value of exponent n, pre-factor A

and ζdc are calculated by taking the reference of ζac conductivity data which obeys the Jonscher’s

universal power law.

Key words: Polymer, composite, Poly (vinylidene fluoride), X- ray diffraction, dielectric properties,

electrical conductivity.

1. Introduction

In the past few decades, the high dielectric constant polymer based composite materials have

attracted more attention because of their vast area of potential applications in electronic devices and

in electrical industry such as gate dielectrics, dynamic random access memory, miniature capacitor

for telecommunications, actuators, transducers and energy storage devices [1-4]. A ferroelectric

ceramic material such as BaTiO3 (BT), Lead Zirconate Titanate (PZT), CaCu3Ti4O12 (CCTO),

Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT) show high dielectric constant and are commonly used as

voltage capacitors due to their high breakdown voltage. Ceramic materials also exhibit poor

mechanical strength, bad energy storage capacity and are brittle in nature [5-7]. On the other hand,

polymers such as Poly(vinylidene fluoride) (PVDF), Polypropylene (PP), Polyethylene terephthalate

(PET), Polyethylene (PE) and Polystyrene (PS) are composite materials with low density, low

weight, low cost, good flexibility, elasticity, relatively low dielectric constant, high electrical

breakdown strength and high dielectric loss [5,6,8-10]. Generally, it is used in low leakage capacitors

[5, 11]. Thus, the combination of both the ceramic particles and polymers produce new composite

material with high dielectric constant and high breakdown voltage which can be efficiently used for

high energy density capacitor applications [12-14]. Recently, many researchers are keenly interested




in polymer ceramic composite materials due to their fascinating properties such as mechanical,

thermal, electrical and its fabrication as compared to their individual components and they can be

used in various commercial technological applications such as transducers, dielectric memories and

sensors etc. [15-17].

In this study, the ceramic material like BiFeO3 (BFO) belongs to the novel multiferroic

family shows various special properties. It is a single phase material having both ferroelectric and

anti-ferromagnetic natures simultaneously so it is known as multiferroics [18-19]. Usually, properties

of multiferroics include ferroelectricity, ferromagnetism and ferrotoroidicity [20-21]. This type of

multiferroic material has wide range of applications such as in spintronics, smart sensor devices,

multistate memory devices, data storage media, memory elements, tunable microwave devices etc

[22-23]. Generally, very few materials are ferroelectric and ferromagnetic, but in room temperature

the magnetoelectric couplings are very weak [24-25]. Recently, D. Bhadra and S. Sarkar research

groups [1] have extensively studied the dielectric properties of BiFeO3-PVDF composite film which

reveals high dielectric constant and low loss tangent.

In our work, the polymeric materials we have used to prepare the composite film are poly

(vinylidene fluoride) and the multiferroic material BiFeO3. Here PVDF is chosen as the polymer host

because it is a well-known material having high dielectric constant which can be easily processed.

They have numerous applications in pyroelectric, piezoelectric, transducers and actuators [26-28, 1].

Generally, PVDF is a semicrystaline polymer with complicated structure and also forms different

crystalline phases (α, β, γ and δ). The most common polymorph i.e. α phase is an electrically inactive

non-polar phase, while the other phases are electrically active polar [29-30]. Normally, they have a

chain like linear structure i.e. [-CH2-CF2-] and are formed by free radical polymerization. It has low

piezo, pyro-electric coefficient and dielectric constant while the ceramics have high piezo, pyro-

electric coefficient and dielectric constants [1]. Due to this reason, the polymer composites have

become an ultimate substitute for both classes as they possess useful properties of both materials.

The general objective of this study is to compare the efficacy of calcined and uncalcined BiFeO3

with PVDF composite to investigate the effects of the quantity of BiFeO3 loaded. In recent years,

several reports show the dielectric properties of calcined BiFeO3 with PVDF composite. However, to

the best of our knowledge, there is no report till now on dielectric properties of uncalcined BiFeO3

with PVDF composite.

Therefore in this paper we have examined the effect of dielectric behaviour of calcined and

uncalcined BiFeO3 with PVDF composite film.

2. Experimental details

2.1. Materials Semi-crystalline polymer Poly (vinylidene fluoride) (PVDF) was purchased from Alfa Aesar.

Bismuth Oxide (Bi2O3) 99.5% purity and Iron Oxide (Fe2O3) 99% purity, were obtained from Merck,

India. The solvent N, N-Dimethylformamide (DMF,>99.0%) was supplied from Loba Chemie PVT,

Ltd., India. All these chemicals were used as received without further purification. Deionised water

was used throughout the experiment.

2.2. Preparation of Bismuth Ferrite Powder

BiFeO3 ceramic was prepared by the conventional solid state reaction method. The equi-molar

mixture of Bi2O3 and Fe2O3 were mixed thoroughly in agate mortar in an air atmosphere for 2 h and

then in alcohol for another 2 hour. The mixed powders were calcined in a high purity alumina

crucible at an optimized temperature of 800oC for 4h in a high-purity alumina crucible.




2.3. Synthesis of PVDF/ BiFeO3 composite film The PVDF/ BiFeO3 composite films were prepared by simple solution casting method. The desired

amount of PVDF was dissolved initially in a flask containing N, N-Dimethylformamide (DMF) by

stirring at room temperature to obtain transparent solution. At the same time, previously prepared

BiFeO3 particles were dispersed in DMF under sonication. Then the mixed (BiFeO3 + DMF) solution

was transferred in to the PVDF solution. The obtained mixture was stirred at room temperature for

30 minutes for fabrication of PVDF/BiFeO3 composites. The resulting composite was casted in to

polypropylene container and placed in an oven at 80oC for 4 hours. The resulted film (thickness

around 55 ± 5 µm) was used for measurement. The composition and scheme of prepared composite

film was shown in Table 1 and Scheme 1, respectively.

Table-1

Composition Used for the preparation of calcined and uncalcined PVDF/BiFeO3composite films

PVDF (gm)

(Polymer)

DMF

(Solvent)

BiFeO3

(% PVDF)

BiFeO3

(gm)

0.5 10 10 0.05(calcined)

0.5 10 10 0.05 (uncalcined)

Scheme 1 Schematic Illustration of PVDF/BiFeO3 composite films

3. Characterization

X-ray diffraction (XRD) technique at room temperature with a powder diffractometer (D8 advanced,

Bruker, Karmsruhe, Germany) using CuKα radiation (λ=1.5405 Å) in a wide range of Bragg’s angles

2θ (20o

< 2θ < 80o) were carried out to get an idea of crystalline structure, phase, composition and

crystallite size of composite. The morphology and microstructure were analysed by scanning

electron microscope (HITACHI COM-S-4200) operated at 300 kV. The dielectric measurement of

composite films were analysed by an impedance analyzer (HIOKI 3532 LCR HiTESTER) in a

frequency range (100 Hz–1 MHz) at room temperature.

4. RESULTS AND DISCUSSION

4.1. Structural Characterization

The X-ray diffraction (XRD) patterns of PVDF/BiFeO3 composite with 10 wt. % of calcined and 10

wt. % of uncalcined BiFeO3 at room temperature are shown in Figure 1a and 1b. Figure 1a,

(uncalcined BiFeO3 with PVDF composite) shows that the first six diffraction peaks are similar to

that of calcined BiFeO3 with PVDF composite film. After that there is no additional peaks are

obtained. On the other hand, in Figure 1b ( calcined BiFeO3 with PVDF composite) all the

diffraction peaks are identified which supports rhombohedral structure with R3c space group at room

C C

FH

H F

n

PVDF

PVDF/BiFeO3 filmsBFO+




temperature according to JCPDS card No. 71-2494 with lattice parameter a=b=5.587 A0, c=13.860A

0

this is in agreement with the reported literature [31-32,18]. The crystallite size of composite films are

calculated using Scherrer’s equation [33-34]

P= K𝞴/ (β1/2cosθhkl) (1)

Where K =0.89(constant), 𝞴=1.5405A0

and β1/2 = peak width of the reflection at half intensity. The

average crystallite sizes of uncalcined and calcined BiFeO3 with PVDF composite film are about 4

and 54 nm respectively.

Fig.1. X-ray diffraction patterns of (a) PVDF/BiFeO3 composite with 10 wt. % of uncalcined

BiFeO3. (b) 10 wt. % of calcined BiFeO3 with PVDF composite.

4.2. Surface Morphology study Figure 2a and 2b shows the freeze-fractured cross section of PVDF/BiFeO3 composite films with 10

wt. % of calcined and uncalcined BiFeO3. In the Figure 2a, it is found that the 10 wt.% of calcined

BiFeO3 powders are well dispersed homogeneously in the PVDF matrix and the BiFeO3 particles are

uniformly distributed in this composite. The average particle size is calculated from the surface

micrograph to be about 600-700nm. Figure 2b shows the uncalcined BiFeO3 particles are non-

uniformly distributed in the polymer matrix and it is very difficult to calculate the average particle

size. The chemical composition of BiFeO3-PVDF composite film is confirmed by EDS analysis.

Figure 2c shows the chemical composition is uniformly present in the composite i.e., carbon (C),

fluorine (F), Iron (Fe) and Bismuth (Bi) with an atomic ratio of 45.30, 54.54, 0.11 and 0.05

respectively.

Fig.2 Cross sectional SEM images for PVDF/BiFeO3 with calcined and uncalcined composite films:

(a) 10% (b) 10% (c) EDS study of PVDF/BiFeO3 composite films.




4.3. Dielectric properties of PVDF/BiFeO3 composite film

0.1 1 10 100 1000

0

50

100

150

200

Frequency (kHz)

Diel

ectri

c co

nsta

nt (

r)

10% Calcined BFO

10 % uncalcined BFO

Pure PVDF

Fig.3 shows frequency dependent dielectric constant of calcined and uncalcined PVDF/BiFeO3

composites at RT.

Figure 3 shows the variation of relative dielectric constant (εr) with frequency of PVDF/BiFeO3

composite film of 10 wt. % of calcined and uncalcined BiFeO3 at RT. It is observed that the

uncalcined PVDF/ BiFeO3 composite, shows very high dielectric constant i.e., εr 200 at low

frequency range, which is 20 times higher than calcined composite and pure PVDF and also have

dielectric constant decreases with increase in frequency at low frequency range and then merge at

high frequency region. Further, the calcined BiFeO3 with PVDF composite, the dielectric constant at

low frequency range is very low. This result shows that the dielectric constant decreases with

increase in frequency and it is due to the presence of interfacial polarization, as some impurities

accumulate and migrate between polymer matrix and fillers. These accumulation and migration

processes of charge carrier develop a high performance of polarization and produce high dielectric

constant. Similarly in calcined PVDF/BiFeO3 composite, we clearly observe that the dielectric

constant increases with BiFeO3 loading over the whole range of frequency as shown in Figure 5. At

low and high frequency range (Figure 3) we find that with increase in frequency the dielectric

constant (εr) sharply increases for the same wt. % of BiFeO3 filler loading. This may be due to the

presence of space charge polarization. The higher dielectric constant of polymer ceramic composite

on BiFeO3 particle in the PVDF matrix improves the polarization of dipole-dipole interaction in

PVDF/BiFeO3 composites [35-36, 5].

0.1 1 10 100 1000

0

1

2

3

4

5

tan

Frequency (kHz)

10% Calcined BFO

10% uncalcined BFO

Pure PVDF

Fig. 4 shows frequency dependent loss tangent with calcined and uncalcined PVDF/BiFeO3

composite at RT.




Figure 4 shows frequency dependence of the loss tangent at room temperature for calcined and

uncalcined BiFeO3 particle in PVDF/BiFeO3 composite. It is found that, the uncalcined BiFeO3 with

PVDF composite, dielectric loss increases in comparison to calcined BiFeO3 with PVDF composite

film. However, all the composites show the decrease in dielectric loss with increase in frequency and

also similar observations are found in the pure PVDF. This is mainly due to the presence of

interfacial polarization effect [4,5,37].

10 calcination 10 uncalcination

0

10

20

30

40

50

Diele

ctric

cons

tant (

r)

BFO filler loading (wt.%)

1 kHz

10 kHz

Fig.5 shows the Dielectric constant values of calcined and uncalcined PVDF/BiFeO3 composites at 1

kHz and 10 kHz at room temperature.

Figure 5 displays the comparison of calcined and uncalcined BiFeO3 with PVDF composite at RT. It

is observed that, at low frequency region the calcined BiFeO3 with PVDF composite the dielectric

constant decreases with BiFeO3 filler loading at 1 kHz and 10 kHz, respectively. This may be due to

the interfacial and space charge polarization [38]. As a result, the dielectric constant of the composite

is linearly dependent [39- 40]. On the other hand, same proportion of BiFeO3 (Uncalcined) with

PVDF composite film is prepared w.r.t. BiFeO3 (calcined) with PVDF composite (table-1) and the

dielectric properties are carried out within a range of 1 kHz and 10 kHz respectively. However, it is

found that at high frequency region the dielectric constant increases as compared to 1 kHz and then

decreases at 10 kHz, which may be due to the space charge polarization [36]. It also indicates that

this type of behaviour is observed in some polymer ceramic composite and this may be due to the

formation of voids and porosity in the composite [36].

10 calcination 10 uncalcination

0.0

0.5

1.0

1.5

2.0

2.5

tan

BFO filler loading (wt.%)

1 kHz

10 kHz

Fig.6 shows the Dielectric loss value of calcined and uncalcined PVDF/BiFeO3 composites at 1 kHz

and 10 kHz at room temperature.

Figure 6 shows the loss tangent of calcined and uncalcined BiFeO3 with PVDF composite as a

function of same wt. % of BiFeO3 loading at 1 kHz and 10 kHz. It is found that, at low frequency

region, the dielectric loss tangent decreases with the same BiFeO3 filler loading at 1 and 10 kHz.

Further in the uncalcined BiFeO3 with PVDF composite of same BiFeO3 filler loading i.e. 10 wt. %,




the dielectric loss tangent increases at 1 kHz and then decreases at 10 kHz which may be due to the

presence of phase inversion [36].

4.4. Impedance analysis

The complex impedance spectroscopy (CIS) [41] is an important technique which is used to

determine the electrical characteristics i.e. transport properties of bulk, grain boundary and electrode

effect of the composite material as a function of frequency. The composite material also depends on

the frequency dependent electrical parameters, which can be obtained in terms of complex dielectric

constant (ε*), complex impedance (Z*), complex modulus (M*) and loss tangent (tan δ). These

factors are co-related to each other as follows:

Complex dielectric constant (ε*) = ε'-j ε" (2)

Complex impedance (Z*) = Z'- jZ"=1/jωC0 ε* (3)

complex modulus (M*) =M'-jM"=1/ε* and tan δ= ε'/ ε" (4)

where ε' and ε" are real and imaginary parts of complex dielectric constant, Z' and Z" are real and

imaginary parts of complex impedance, M' and M" are real and imaginary parts of complex modulus,

ω=2πf is the angular frequency, C0 is the free space capacitance and j= is the imaginary part

respectively.

100 1000 10000 100000 1000000

0

1000000

2000000

3000000

4000000

5000000

6000000

0.1 1 10 100 1000

0

50000

100000

150000

200000(b)

Z' (k)

Frequency(kHz)

10% uncalcined BFO

(a)

Z'(k

)

Frequency (kHz)

10 % calcined BFO

Fig. 7 (a) and 7 (b) Variation of real parts of Z' as a function of frequency with calcined and

uncalcined PVDF/BiFeO3 composite at RT.

Figure 7a and 7b (inset) shows the variation of real (Z') with calcined and uncalcined PVDF/BiFeO3

composite as a function of frequency at RT. It is observed that both the value of Z' decreases with

rise in frequency in the low frequency region. At higher frequency region all the curves appear to

merge. It indicates the presence of space charge polarization at low frequency region but disappear in

high frequency region [42].

0.1 1 10 100 1000

0

-10000000

-20000000

-30000000

-40000000

-50000000

-60000000

-70000000

0.1 1 10 100 1000

0

-10

-20

-30

-40

-50

-60(b)

Z'' (k

)

Frequency (kHz)

10% uncalcined BFO

(a)

Z''(k)

Frequency (kHz)

10% calcined BFO

Fig. 8 (a, b) shows variation of imaginary part of impedance (Z") as a function of frequency for

calcined and uncalcined PVDF/BiFeO3 composite at RT.




Figure 8 (a) shows the variation of imaginary part of impedance (Z') as a function of frequency at

RT. It is found that the magnitude of Z" i.e. Resistance decreases with increase in frequency at low

frequency region, indicating the presence of negative frequency coefficient resistance like that of

semiconductor and at higher frequency region it remains constant which may be due to the

possibility of space charge effect [43].

Figure 8 (b) (inset) shows the variation of imaginary part of impedance (Z") as a function of

frequency. It is observed that the value of Z" first increases towards low frequency and then

decreases with increase in frequency at high frequency region. The curve exhibits that conduction is

increasing with increasing frequency. This behaviour again indicates the presence of space charge

polarization [43].

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0

-20

-40

-60

-80

-100

0 10 20 30 40 50 60 70 80 90 100110120130140150160170180190200

0

-10

-20

-30

-40

-50

-60

-70

-80

-90

-100

-110

-120

-130

-140

-150

-160

-170

-180

-190

-200

(b)

Z''

(k

)

Z' (k)

10 % uncalcined BFO(a)

Z''(

k)

Z' (k)

10% calcined BFO

Fig.9. complex impedance spectrum of PVDF/BiFeO3 composite

The variation of Z' with Z" (Nyquist plot) at calcined and uncalcined PVDF/BiFeO3

composite at RT is shown in Figure 9 (a, b). We obtain the nature of the above curve by Cole-Cole

formalism [44]. Generally, this type of plot demonstrates the transport properties of the composite

material. Consecutively, two semi circles are formed, due to the lack of data to complete the

semicircle in the lower frequency region. Clearly, it also indicates that there are second semicircles

which are formed in higher frequency region for 10 wt. % of uncalcined BiFeO3 with PVDF

composite. In Figure 9 the lower frequency semicircle region can be attributed to grain boundary

(Rgb) while higher frequency semicircle region can be regarded as bulk properties of the composites.

The bulk property arises due to the parallel arrangement of bulk resistance (Rb) and bulk capacitance

(Cb) of PVDF/BiFeO3 composites.

4.5. Electrical conductivity

The ac conductivity (ac) was determined using the experimental dielectric data and an empirical

equation:

ζac= ωεrε0 tan δ (5)

where o=8.85 × 10-12

F/m is the permittivity in free space, εr is the relative dielectric constant, ω=2πf

is the angular frequency and tan δ is the loss tangent.




0.1 1 10 100 1000

1E-14

1E-13

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

ac(

-1m

-1)

Frequency (kHz)

10% Calcined BFO

10% uncalcined BFO

Pure PVDF

Fig.10 shows frequency dependence of ac conductivity for calcined and uncalcined PVDF/BiFeO3

composite at RT.

Table-2

Fitting parameters obtained from Jonscher’s power law at calcined and uncalcined PVDF/ BiFeO3

composite.

Composition (%) σdc(ῼ-1

m-1

) A n Good ness of fit

(R2)

Calcined BFO (10%) 6.2579×10-14

1.2451×10-17

1.1238 0.9829

Uncalcined BFO (10%) 5.5677×10-6

2.1768×10-8

0.60667 0.9996

Pure PVDF 3.867×10-14

4.0682×10-20

1.49742 0.99735

Figure 10 shows the variation of ac conductivity as a function of frequency for the calcined and

uncalcined PVDF/BiFeO3 composite. It is observed that in the calcined and uncalcined BiFeO3 with

PVDF composite the ac conductivity decreases on decreasing frequency and at high frequency ζac

ωn .This increase in value of ac conductivity is due to the presence of space charge polarization [43].

On the other hand, the uncalcined BiFeO3 with PVDF composite have much higher conductivity in

comparison to the calcined PVDF/BiFeO3 composite film when the total BiFeO3 filler loading is the

same The composites obey Jonscher’s power law [45].

ζac =ζ0+Aωn (6)

0.1 1 10 100 1000

1E-5

1E-4

BFO-10% at RT

Calculated

experimental

Goodness of fit, R2=0.99949

0.00476

1.7409E-9

1.7654E-7

0.59135

2.6635E-8

5.0833E-6

n

A

ln0

Parameter Value Error

Jonscher's Power law ( =0+A)

n

ac(

-1 m-1 )

Frequency(kHz)

Fig. 11 shows Non-linear fitting of ac Conductivity obeying Jonscher’s universal power law.




Where ζ0 is the dc conductivity, A is the frequency dependent parameter and the value of n around 0

and 1 for the electrolyte. The detail study of ζac of PVDF/BiFeO3 composite proposed by universal

power law is obeyed.

This is confirmed by the typical fit (Table-2) of the above equation to the experimental data

shown in Figure 10 and Figure 11 respectively. According to Jonscher’s, low frequency dispersion

can be attributed to the ac conductivity whereas the frequency independent plateau regions

correspond to the dc conductivity of the composite material.

5. Conclusions

In this paper, we have synthesized two types of composite films i.e. uncalcined and calcined BiFeO3

with PVDF, which are prepared by solution casting method at RT. From X-ray diffraction pattern it

is found that calcined BiFeO3 with PVDF composite confirms the rhombohedral structure but there is

no clear diffraction peak in support of (structure of) uncalcined BiFeO3 with PVDF composite film.

In comparision, there is remarkable differences have been noticed in both the composites as per as

dielectric behaviour is concerned. For uncalcined PVDF/BiFeO3 composite the dielectric constant

(εr ) is noticed which is 20 times higher than the uncalcined one and pure PVDF. The variation

at low and high frequency region as per as the dielectric loss tangent is concerned, which is clearly

revealed from the graph this may be due to the presence of phase inversion / voids / porosity [36].

The universal Jonscher’s power law is well fit to the conductivity spectrum of both the composite

film that means conductivity can be well described by Arrhenius equation. The ac conductivity of the

composite film is in the order of 10-5

Ω-1

m-1

which is much greater than the calcined composite film

and pure PVDF.

Acknowledgement: This work is fully funded by the University Grant Commission (UGC), New

Delhi, Govt. of India, under the grant head F. No. 42 – 277/2013 (SR), UGC – MRP.

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