(vinylidene fluoride) (PVDF) - IJETMAS

International Journal of Engineering Technology, Management and Applied Sciences

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

A Comparative Study of Dielectric Properties of Calcined and Un-Calcined BiFeO3-Poly (vinylidene fluoride) (PVDF) Composite Films 1

S. Moharana1, M. K. Mishra1, B. Behera2, R.N Mahaling1*(Corresponding Author) Polymeric and Materials Chemistry Laboratory, School of Chemistry, Sambalpur University, Jyoti Vihar, Burla-768019, Odisha, India 2 Materials 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

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

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www.ijetmas.com April 2015, Volume 3 Issue 4, ISSN 2349-4476 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) DMF BiFeO3 BiFeO3 (Polymer) (Solvent) (% PVDF) (gm) 0.5 0.5

H

F

C

C

H

F

10 10

+

10 10

BFO

0.05(calcined) 0.05 (uncalcined)

PVDF/BiFeO3 films

n PVDF 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

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www.ijetmas.com April 2015, Volume 3 Issue 4, ISSN 2349-4476 temperature according to JCPDS card No. 71-2494 with lattice parameter a=b=5.587 A0, c=13.860A0 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) 0 Where K =0.89(constant), 𝞴=1.5405A 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 nonuniformly 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.

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www.ijetmas.com April 2015, Volume 3 Issue 4, ISSN 2349-4476 4.3.

Dielectric properties of PVDF/BiFeO3 composite film 10% Calcined BFO 10 % uncalcined BFO Pure PVDF

Dielectric constant (r)

200

150

100

50

0

0.1

1

10

100

1000

Frequency (kHz)

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]. 5

10% Calcined BFO 10% uncalcined BFO Pure PVDF 4

tan 

3

2

1

0

0.1

1

10

100

1000

Frequency (kHz)

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

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www.ijetmas.com April 2015, Volume 3 Issue 4, ISSN 2349-4476 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]. 50

1 kHz 10 kHz

Dielectric constant (r)

40

30

20

10

0

10 calcination

10 uncalcination

BFO filler loading (wt.%)

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]. 2.5

1 kHz 10 kHz

2.0

tan 

1.5

1.0

0.5

0.0

10 calcination

10 uncalcination

BFO filler loading (wt.%)

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


www.ijetmas.com April 2015, Volume 3 Issue 4, ISSN 2349-4476 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. 6000000

(a)

10 % calcined BFO

5000000 (b )

200000

150000

Z' (k)

Z'(k)

4000000

1 0 % u n c a lc in e d B F O

3000000

100000

50000

2000000

0

0 .1

1

10

100

1000

F re q u e n c y(k H z )

1000000

0 100

1000

10000

100000

1000000

Frequency (kHz)

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]. -70000000

10% calcined BFO -60000000

-50000000

10% uncalcined BFO

-60

(b)

-50

(a)

Z'' (k)

-40

Z''(k)

-40000000

-30

-20

-30000000 -10

-20000000

0 0.1

1

10

100

1000

Frequency (kHz)

-10000000

0 0.1

1

10

100

1000

Frequency (kHz)

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.

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www.ijetmas.com April 2015, Volume 3 Issue 4, ISSN 2349-4476 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]. -100

10% calcined BFO

(a)

-80

Z'' (k)

Z''(k)

-60

-40

-20

-200 -190 -180 -170 -160 -150 -140 -130 -120 -110 -100 -90 -80 -70 -60 -50 -40 -30 -20 -10 0

10 % uncalcined BFO

(b)

0

10 20 30 40 50 60 70 80 90 100110120130140150160170180190200

Z' (k)

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Z' (k )

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.

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www.ijetmas.com April 2015, Volume 3 Issue 4, ISSN 2349-4476 1E-4 1E-5 1E-6

10% Calcined BFO 10% uncalcined BFO Pure PVDF

1E-7

1E-9

-1

-1

ac( m )

1E-8

1E-10 1E-11 1E-12 1E-13 1E-14 0.1

1

10

100

1000

Frequency (kHz)

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-1m-1) A n Good ness of fit (R2) Calcined BFO (10%) 6.2579×10-14 1.2451×10-17 1.1238 0.9829 -6 -8 Uncalcined BFO (10%) 5.5677×10 2.1768×10 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) Jonscher's Power law ( =0+A)

Parameter

ln0 1E-4

2.6635E-8 0.59135

A n

-1 -1

ac( m )

Value

5.0833E-6

n

BFO-10% at RT

Error

1.7654E-7 1.7409E-9 0.00476

2

Goodness of fit, R =0.99949

experimental Calculated

1E-5

0.1

1

10

100

1000

Frequency(kHz)

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

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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-1m-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. References 1. Bhadra D, Masud MDG, Sarkar S, Sannigrahi J, De SK and Chaudhuri BK. Synthesis of PVDF/BiFeO3 Nanocomposite and Observation of Enhanced Electrical Conductivity and LowLoss Dielectric Permittivity at Percolation Threshold. J Polym Sc Part b: polym Phys 2012; 50: 572-579. 2 Chen Y, Zhuang Q, Liu X, Liu J, Lin S and Han Z. Preparation of thermostable PBO/graphene nanocomposites with high dielectric constant. Nanotechnology 2013; 24: 245702-245711. 3. Kim J Y Kim, H, Kim T, Yu S, Suk JW, Jeong T, Song S, Bae MJ, Han I, Jung D and Park SH. A chlorinated barium titanate-filled polymer composite with a high dielectric constant and its application to electroluminescent devices. J Mater Chem C 2013; 1: 5078-5083. 4. Yang K, Huang X, Huang Y, Xie L and Jiang P. Fluoro-Polymer@BaTiO3 Hybrid Nanoparticles Prepared via RAFT Polymerization: Toward Ferroelectric Polymer Nanocomposites with High Dielectric Constant and Low Dielectric Loss for Energy Storage Application. Chem Mater 2013; 25(11): 2327-2338. 5. Thomas P, Satapathy S, Dwarakanath K, Varma KBR. Dielectric properties of poly (vinylidene fluoride)/ CaCu3Ti4O12 nanocrystal composite thick films. eXPRESS Polym Lett 2010; 4 (10): 632-643.

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