β-Phase improved Mn-Zn-Cu-ferrite-PVDF nanocomposite film: A metamaterial for enhanced microwave absorption

Materials Science & Engineering B 245 (2019) 17–29

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Materials Science & Engineering B journal homepage: www.elsevier.com/locate/mseb

β-Phase improved Mn-Zn-Cu-ferrite-PVDF nanocomposite film: A metamaterial for enhanced microwave absorption Papiya Sahaa,b, Tanumoy Debnatha,b, Sukhen Dasb, Souvik Chatterjeec, Soumyaditya Sutradhara,

T ⁎

a

Department of Physics, Amity University, Kolkata 700156, West Bengal, India Department of Physics, Jadavpur University, Kolkata 700032, West Bengal, India c UGC-DAE Consortium for Scientific Research, III/LB-8, Kolkata 700098, West Bengal, India b

A R T I C LE I N FO

A B S T R A C T

Keywords: Nanoparticles β-Phase Magnetic property Dielectric property Microwave absorption

Magnetic nanoparticles embedded poly(vinylidene fluoride) (PVDF) nanocomposite films with different weight percentages of Mn-Zn-Cu-ferrite (MZCF) nano-fillers were prepared by solution casting method. The structural, morphological, magnetic, dielectric and microwave absorption properties of all the MZCF-PVDF nanocomposite films were studied. Experimental observations reveal the enhancement of the electroactive β-phase of PVDF by the modification of internal structure of the PVDF. Variation of magnetization with magnetic field reveals the presence of magnetic ordering and high magnetic moment in the MZCF-PVDF nanocomposite films. Variation of dielectric properties shows the significant modulation of dielectric response of the MZCF-PVDF nanocomposite films. High value of reflection losses of the nanocomposite films within the range of 8–18 GHz was observed and large reflection loss makes the sample most suitable one for the applications in microwave devices. The present work also suggests that these nanocomposite films would be very useful as Radar absorbing material for EMI shielding applications.

1. Introduction Recent research works on nanocomposite and laminated composite systems have attracted the attention of the research community due to their extensive uses in various fields of applications such as lithium ion battery, photo catalysis and packaging as well as biomedical field [1–5]. In this direction polymer based laminated composite systems are considered as the most effective one due to some of the unique characteristic features such as, low density, flexibility, narrow thickness, large surface area as well as easy and low cost preparation techniques. The selection of polymers and their copolymers are very important for the modulation of physical properties. Among various polymer materials poly(vinylidene fluoride) (PVDF), is a semi-crystalline, highly elastic, semi-transparent polymer material with large pyro- and piezoelectric behaviours. These properties make this PVDF the most interesting one in the field of polymer research among all the available polymers [6–8]. Also, PVDF is very well known polymer material for its various polymorphism and four different crystalline phases such as α, β, δ and γ are available [9–11]. Among these four different crystalline phases, α- and β-phase polymorphs are the most common types available therein. Here α-phase is the non-polar phase and the β-phase is the polar phase of PVDF and the later one is the most interesting one for

⁎

different technological applications as it shows effective piezoelectric, pyroelectric and ferroelectric responses [12]. The most common way to obtain the electroactive β-phase of PVDF from its non-polar α-phase is the mechanical stretching method of PVDF [13]. But, this method is not suitable for the preparation of laminated nanocomposite films because the stretching process restricts the loading of high amount nano-fillers and/or leads to the uncontrolled reconfigurations and the agglomeration of the nano-fillers [14]. All though the development of PVDF reinforced laminated nanocomposites have been considered as the subject of intensive research by various researchers but the architectural difficulties related to the formation of desired phase of PVDF makes the research work very challenging to them. In this direction, improved βphase crystallization of nano-fillers loaded PVDF nanocomposites can be obtained very easily by cost effective solution casting method [15]. In this method the production of polymer nanocomposite films with nearly uniform distribution of the nano-fillers in the matrix of polymer can be obtained and it also helps to develop/modulate many important and interesting properties. The addition of nano-fillers in the matrix of PVDF can enhance its performance or it can provide new responses, by capitalizing the size, shape and properties of the nano-fillers [16]. Also, the processability and mechanical quality of the polymer is an advantage compared to bare ferrites nanoparticles. Moreover, a

Corresponding author. E-mail address: [email protected] (S. Sutradhar).

https://doi.org/10.1016/j.mseb.2019.05.006 Received 9 April 2018; Received in revised form 22 January 2019; Accepted 6 May 2019 Available online 11 May 2019 0921-5107/ © 2019 Elsevier B.V. All rights reserved.

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composition of Mn0.8Zn0.2Cu0.2Fe1.8O4 were taken in a beaker and a complete homogeneous solution was prepared by ethyl alcohol. In this solution the stoichiometric ratio of Mn, Zn, Cu and Fe was 0.8:0.2:0.2:1.8. Further, one mole of citric acid was dissolved in ethyl alcohol and this homogeneous citric acid solution was added into the salts solution under vigorous stirring condition. This prepared solution was then sonicated for 3 h at about 60 °C. After the sonication, the solution was stirred vigorously and continuously to get heavily dense form of the solution. The solution was then put into the oven at 80 °C and the prepared solution has been dried slowly for over 2 days to get the flake form of the solution. Finally, the as-prepared MZCF sample was annealed at 400 °C for 3 h to obtain the powder Mn-Zn-Cu-ferrite nanoparticles with preferred size and magnetic moment.

sufficiently high magnetic permeability can be achieved within the polymer nanocomposite films by the incorporation of the magnetic nanoparticles, and finally, the magnetic, dielectric and microwave absorption effects can be observed in such polymer composite films [17]. Due to the presence of magnetic and dielectric properties in ferrite nanoparticles, much interest has been shown by the researchers on polymer-based nanocomposites filled with nano-fillers like ferrites or hexaferrites nanoparticles, such as Co-ferrite [18], Ni-ferrite [19], Biferrite [20], etc. for their applications in various fields such as information storage, magnetostrictive effect, bio-separation, and diagnostics. The electromagnetic wave absorption properties of ferrites or hexaferrites nanoparticles also make them good choice for stealth application in defence sector [21]. Recently, many serious research works have been done on the field of military application, gigahertz (GHz) telecommunications, sensors, commercial radar systems and other high frequency technological applications where magnetic metal oxides nanoparticles-PVDF nanocomposites films are evolved as the most potential candidate [22]. Microwave absorbers are used in military applications and many other applications like EMI reduction, antenna pattern shaping and radar cross reduction applications for several decades in most of the developed countries. The most conventional microwave absorbers are all powder materials with high value of reflection loss and some of them are ferrite-CNT, hexaferrite-CNT, ferriteRGO, Fe/ZnFe2O4, Carbon Sphere/Magnetic Quantum Dots nanocomposites etc. [23–27]. However, the conventional absorption materials are quite heavy (with high molecular density) and also not at all flexible which restricts their usefulness in the applications requiring for lightweight structures like aircraft or satellite systems. One of the most effective ways to overcome these problems is the use of conventional microwave absorption materials in combination with electroactive polymers having low density. Magnetic metal oxides nanoparticles, an effective microwave absorption material, can be used as nano-fillers in the polymer matrix to produce thin sheets of laminated nanocomposites with excellent frequency response in the GHz range. This is the case where microwave absorbing materials are available with thickness in micrometer ranges and is very flexible too. Under this condition the product can be made available in rolls and any length/area can be achieved. Therefore, great research work is required on microwave absorption materials where nanoparticles of magnetic metal oxides (ferrites or hexaferrites)-PVDF nanocomposite system can exhibit significant enhancement of microwave absorption in X (8–12 GHz) and Ku (12–18 GHz) bands. Keeping this in mind, PVDF-based nanocomposites with MZCF nanoparticles as nano-fillers have been investigated in the present article.

2.3. Synthesis of MZCF-PVDF flexible film by solution casting method The Mn-Zn-Cu-ferrite-PVDF nanocomposite films were synthesized by a simple solution-casting method. In a synthesis procedure, PVDF was dissolved in DMF at 70 °C with continuous magnetic stirring and after few hours the complete dissolution of PVDF in DMF was achieved. Then, a certain mass percentage of Mn-Zn-Cu-ferrite nanoparticles (5, 10, 15, 20 and 25 wt%) were added to the solution of PVDF and the whole solution was taken for ultra-sonication. The ultra-sonication was continued for 2 h to obtain the homogeneous mixing of the MZCF nanoparticles in the solution of PVDF. Finally, MZCF nanoparticles embedded PVDF nanocomposite films were obtained by casting the whole mixture in a properly cleaned and dried glass plate and evaporating the solvent at 110 °C. The samples name are given as SP0, SP5, SP10, SP15, SP20 and SP25 for 0, 5, 10, 15, 20 and 25 wt% of MZCF nanoparticles loaded PVDF films. Pure PVDF (SP0) film was also prepared by the method as discussed above. The whole mechanism of the synthesis process has been shown in Fig. 1 with the help of flow chart. The thickness of the pure PVDF (SP0) is 0.1964 mm and that of SP5, SP10, SP15, SP20 and SP25 are 0.1750, 0.2864, 0.1956, 0.2515 and 0.2994 mm respectively. The thicknesses of the pure PVDF and MZCF nanoparticles embedded PVDF films were measured by digital micrometer. 2.4. Material characterization The XRD pattern of all the samples were recorded in powder X-ray diffractometer, Model D8, BRUKER AXS, using Cu Kα radiation (λ = 1.5405 Å) in the range of 2θ from 10 to 80°. Field emission scanning electron microscope (FESEM) was employed for morphological study using INSPECT F50 (FEI, Netherland). FTIR measurement of the samples was recorded in SHIMADZU, IR Prestige-21 (Japan). Zeta potential of MZCF nanoparticles was recorded at room temperature by Zetasizer Nano ZS (Malvern Instruments, UK). Magnetization versus applied magnetic field (M-H) data of the samples at RT was recorded by a SQUID magnetometer (MPMS XL 7, Quantum Design), where the maximum applied field was 50,000 Oe. Electrical conduction mechanism of nanocomposite films was investigated by dielectric measurement using Agilent 4294A Precision Impedance Analyzer. Microwave reflection loss of the nanocomposite samples in the microwave region of frequency (X and Ku bands) was measured by using a Agilent E8363B PNA series Network Analyzer.

2. Experimental section 2.1. Materials Mn-Zn-Cu-ferrite (MZCF) nanoparticles embedded poly(vinylidene fluoride) (PVDF) nanocomposite films were prepared by very simple solution casting method. All the precursor materials used for the preparation of MZCF and MZCF embedded PVDF nanocomposite films are manganese (II) acetate tetrahydrate Mn(CH3COO)2·4H2O (Sigma Aldrich, 99%), zinc (II) acetate dihydrate Zn(CH3COO)2·2H2O (Sigma Aldrich, 99%), copper (II) acetate monohydrate Cu(CH3COO)2·H2O (Sigma Aldrich, 99%), iron (III) nitrate nonahydrate Fe(NO3)3·9H2O (Merck Germany, 99%), citric acid, ethyl alcohol, poly(vinylidene fluoride) (PVDF) pellets (molecular weight Mw: 275,000 (hpc), Mn: 107,000, Aldrich, Germany), and N,N-dimethyl-formamide (DMF, Merck, India), respectively. All the chemicals were taken with analytical grade.

3. Result and discussion 3.1. Structural characterization XRD patterns of all the samples (MZCF, SP0, SP5, SP10, SP15, SP20 and SP25) are shown in Fig. 1(I). All the peaks of the pattern are duly assigned with the help of the JCPDS file no. 10-0325 for bare MZCF sample; JCPDS file no. 38-1638 for pure PVDF sample (SP0) and JCPDS files no. 10-0325 and 38-1638, respectively for MZCF nanoparticles

2.2. Synthesis of MZCF nanoparticles The stoichiometric amount of the precursor materials for the desired 18

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Mn(OC2H5)2 + CH3COOH

Mn(CH3COO)2 + C2H5OH Zn(CH3COO)2 + C2H5OH

Zn(OC2H5)2+ CH3COOH

Cu(CH3COO)2 + C2H5OH

Cu(OC2H5)2 + CH3COOH

Fe(NO3)3 + C2H5OH

Fe (OC2H5)3 + HNO3

Mn(OC2H5)2

Cu(OC2H5)2

Zn(OC2H5)2

Fe(OC2H5)3

Removal of CO2, H2O

Cu0.2O0.2

+

PVDF

+

Mn0.8Zn0.2Cu0.2Fe1.8O4

Fe1.8O2.8

+

+

Zn0.2O0.2

+

Mn0.8O0.8

DMF

PVDF-DMF solution

Ferrite (MZCF) embedded PVDF (polymer) nano-composite film Fig. 1. (I) Flowchart of MZCF embedded PVDF nano-composite synthesis by sol-gel process in alcoholic medium.

nanocomposite samples and have been given in Fig. 2(I)c–g. The average nanocrystallite diameter of MZCF nanoparticles was calculated from the broadening of 100% intense peak (3 1 1) of Fig. 2(I)a by using the Debye-Scherrer equation

loaded PVDF (MZCF-PVDF) nanocomposite samples (SP5, SP10, SP15, SP20 and SP25). All the peaks in the XRD pattern of MZCF nanoparticles have been shown in Fig. 2(I)a and the peaks are well matched with the desired phase of the mixed spinel ferrite whereas the XRD peaks of Fig. 2(I)b are well matched with the desired phase of the pure PVDF having α-, β- and γ-phase crystallizations. It is interesting to note that no extra peak due to any other impurity phase has been found in the patterns given in the Fig. 2(I)a–g. All the XRD patterns given in the Fig. 2(I)a and (I)c–g are free from any impurity phases like α-Fe2O3 due to iron oxide. In Fig. 2(I)a, only the characteristic peaks of MZCF ferrite have been observed at different 2θ positions such as, 30.67° (2 2 0), 35.57° (3 1 1), 37.08° (2 2 2), 43.85° (4 4 0), 54.47° (4 2 2), 58.18° (5 1 1), 63.81° (4 0 0). The 100% peak of MZCF ferrite has been found at 36.31° corresponding to the crystal plane (3 1 1). The XRD pattern of the pure PVDF film (SP0) has been shown in Fig. 2(I)b. According to the given Fig. 2(I)b of pure PVDF film (SP0), the XRD peaks have been found at different 2θ positions such as, 18.52° (0 2 0), 20.27° (0 2 1) (characteristic peak) and 27.65° ((2 0 1) and (3 1 0)), corresponding to the non-polar α-phase of PVDF. Also, a peak at 39.68° corresponding to the plane (2 1 1) has been observed for γ-phase of PVDF and a prominent XRD peak at 36.46° corresponding to the plane ((0 2 0) and (1 0 1)) has been found for the polar β-phase of pure PVDF film [28,29]. Now, all peaks corresponding to α-, β- and γ-phases of pure PVDF have been observed in the XRD patterns of the MZCF-PVDF nanocomposite films (SP5, SP10, SP15, SP20 and SP25) along with the characteristic peaks of MZCF nanoparticles. All the peaks are assigned for the MZCF-PVDF

〈D〉(3 1 1) =

0.9λ β1 cos θ 2

(1)

Here, D is the average nanocrystallite diameter, λ is the wavelength of the incident X-ray beam, θ is the corresponding Braggs angle, β1 is the 2 full width at half maximum (FWHM) of the (3 1 1) peak. The average nanocrystallite diameter of MZCF ferrite nanoparticles is nearly ∼15.5 nm and the corresponding lattice parameter is 0.8399 nm. It is quite clear from the XRD peak positions of different MZCF nano-filler loaded MZCF-PVDF nanocomposite samples (SP5, SP10, SP15, SP20 and SP25) that the peaks have been shifted a little bit with the increasing loading percentages of MZCF nano-filler in the PVDF matrix and the change in peak position signifies the development of the internal lattice strain due to the incorporation of the MZCF nanoparticles in the matrix of PVDF. Also, it has been observed in the XRD pattern that the peak of MZCF nanoparticles corresponding to the plane (3 1 1) at 35.57° and the peak of pure PVDF corresponding to the plane ((0 2 0) and (1 0 1)) of β-phase crystallization at 36.46° are very close to one another. Since, the average nanocrystallite diameter of MZCF nanoparticle is quite low (∼15.5 nm) and the corresponding peak (3 1 1) width is quite broad, so, in the XRD patterns of the nanocomposite samples (SP5, SP10, SP15, SP20 and SP25) the individual identification 19

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PVDF nanocomposite samples (SP10 and SP15). Fig. 2(II)c and (II)e are the clear evidences of the presence of the bubble like structures of MZCF-PVDF nanocomposite films and it is also very clear from the micrographs that the change in the surface morphology of the MZCFPVDF nanocomposite samples with respect to the pure PVDF film has been appeared due to the incorporation of the MZCF nanoparticles in the matrix of host PVDF. It has also been observed from the FESEM micrographs that with the increasing loading percentage of the MZCF nanoparticles in the matrix of pure PVDF the size of the bubble like structure have been enhanced and the surface related structural deformations get prominent. The corresponding micrographs have been shown in the Fig. 2(II)d and (II)f, respectively. The enhancement of the bubble like structures and the surface related structural deformations in the nanocomposite films to that of the pure PVDF is the confirmation of the enhancement of the β-phase of host PVDF and the enhancement of the β-phase of host PVDF has been observed due to the orientational change of the PVDF polymeric chain from non-polar to polar form due to the presence of MZCF nanoparticles with negative surface charge in the matrix of PVDF. The enhancement of the β-phase of host PVDF system due to incorporation of MZCF nanoparticles in its matrix has also been substantiated by the XRD analysis. So the improvement of the desired β-phase of the MZCF-PVDF nanocomposite films are very much clear from the FESEM micrographs and the MZCF-PVDF nanocomposite materials can be used for different magneto-dielectric device applications.

Fig. 2. (I) XRD patterns of the samples (a) MZCF, (b) SP0, (c) SP5, (d) SP10, (e) SP15, (f) SP20 and (g) SP25 and (III) FESEM images of (a) SP0, (b) MZCF, (c), (d) SP10 and (e), (f) SP15.

3.3. FTIR analysis of both (3 1 1) plane of MZCF sample and ((0 2 0) and (1 0 1)) of βphase crystallization of host PVDF is very much difficult. But the presence of β-phase crystallization even in the nanocomposite samples can be realized by considering the variations of the relative intensity of the peak at around 36° for all the MZCF-PVDF nanocomposite samples (SP5, SP10, SP15, SP20 and SP25). It has been observed that the relative intensities of the peak (3 1 1) at 36° for all the nanocomposite samples show a clear variation with the increase of the MZCF loading percentage in the matrix of PVDF. This variation of peak intensity also appears due to the variation of β-phase crystallization of MZCF-PVDF nanocomposite samples due to the successful incorporation of the MZCF nanoparticles in the matrix of PVDF which leads to the electrostatic interaction (discussed later) between the MZCF nanoparticles (having negative surface charge) and the hydrogen ligand (having positive charge) of PVDF host matrix.

Fig. 3(I) shows the Fourier transform infrared (FTIR) absorption spectra of pure PVDF (SP0) and MZCF modified PVDF (MZCF-PVDF) nanocomposite films (SP5, SP10, SP15, SP20 and SP25). The FTIR spectrum of the pure PVDF shows characteristic peaks positioned at 495 cm−1 (CF2 waging), 540 cm−1 (CF2 bending), 620 cm−1 (CF2 bending), 770 cm−1 (CF2 skeletal bending), 817 cm−1 and 982 cm−1 (CH2 rocking) assigned to the IR bands of non-polar α phase of PVDF and also three peaks positioned at 524 cm−1 (CF2 stretching), 847 cm−1 (CH2 rocking, CF2 stretching and skeletal C–C stretching) and 898 cm−1 (CH2 and CF2 groups generated from the CH2 rocking and CF2 stretching) conforming the presence of β-phase of PVDF [30]. Fig. 3(I) shows that the relative intensity of the characteristic absorption bands corresponding to non-polar α-phase of PVDF are gradually decreased with increasing MZCF nanoparticles in the host PVDF matrix and the relative intensity of the characteristic absorption bands corresponding to polar β-phase of host PVDF has been enhanced with reference to the non-polar α-phase for nanocomposite sample. Hence, the FTIR results indicate the sustainability of β-phase of host PVDF even in presence of MZCF nanoparticles and the transformation of α- to β-phase of host PVDF due to the loading of the MZCF nanoparticles in the PVDF matrix. The β phase fraction (F(β)) has been calculated for the pure PVDF film and the value is ∼25%. The same has been estimated for MZCF-PVDF nanocomposite films and it varies from 27.7% to 31%. The calculation of β-phase fraction (F(β)) has been done from the FTIR spectra using Lambert-Beer law as given in Eq. (2) to quantitatively measure the phase transformation of host PVDF film due to the incorporation of MZCF nanoparticles in the matrix of PVDF films [31,32].

3.2. Morphological characterization FESEM micrographs of some selected nanocomposite samples have been observed and the corresponding images are depicted in Fig. 2(II). It is to be mentioned here that the FESEM measurement is a very important evidence to comprehend about the enhancement of the β-phase crystallization of host PVDF from the observed variation of the structural morphology of MZCF-PVDF nanocomposite samples. The surface morphologies of pure PVDF and MZCF-PVDF nanocomposite samples have been observed in FESEM measurement of Fig. 2(II) and with the help of some representative micrographs the unambiguous differences in the surface morphologies of pure PVDF and MZCF-PVDF nanocomposites have been portrait in this section. It is clear from the given micrograph in Fig. 2(II)a that the surface morphology of the pure PVDF sample is uniform spherullite in nature, highly compact and the typical bubble like structures are also formed at the surface, which is basically the signature pattern of the PVDF film with β -phase crystallization. Fig. 2(II)b shows the micrograph of MZCF nanoparticles with spherical or semi-spherical nature. The average particle size of MZCF nanoparticles was calculated from the FESEM micrographs and the particles have been found in the range of 17–20 nm. Fig. 2(II)c–f illustrate the distinguishable change in surface morphologies of the resultant MZCF-

F (β ) =

Aβ

( )A Kβ

Kα

α

+ Aβ

(2) −1

−1

and 847 cm , rewhere Aα and A β are the absorbance at 770 cm spectively, and Kα (6.1 × 104 cm2 mol−1) and Kβ (7.7 × 104 cm2 mol−1) are the absorption coefficients at their respective wavenumbers. Fig. 2(II) shows the variation of (F(β)) with the incorporation of MZCF nanoparticles in the PVDF matrix. From the Fig. 3(II) it can be seen that with the variation of loading fraction of MZCF nanoparticles (F(β)) varies and the maximum value of (F(β)) of 20

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Fig. 2. (continued)

nearly ∼ 31% is achieved for 20 wt% MZCF loaded PVDF nanocomposite (SP20). The interaction between the MZCF nano-filler and the polymer matrix leads to this enhancement of (F(β)) upto 20% MZCF loaded PVDF nanocomposite. When the filler content is low the effective interfacial area between the polymer and the MZCF nanoparticles is less and with the increment of the homogeneously dispersed MZCF nanoparticles in the polymer matrix the interfacial area increases. Hence, the amount of aligned chains having ‘all-trans’ (TTTT) conformation has been enhanced and leads to the successful increment of the fraction of β-phase for the MZCF-PVDF nanocomposites. However, the β-phase is reduced further for the higher percentages of the MZCF nano-fillers more than 20 wt% of MZCF nanoparticles in the matrix of PVDF. These phenomena may be occurred due to the movement restriction of elongation of the polymer chains in ‘all-trans’ (TTTT) conformation for further loading of the nanofillers, which in turns reduces the β-phase content. Thus, Fig. 3(II) shows that (F(β)) has been improved from SP0 to SP10 vary largely, but the variation is less for SP10 and SP15 nanocomposite. Again it shows prominent variation for SP20 nanocomposite film to that of other films and attains the maximum value for SP20 nanocomposite film. Also, for large fraction of MZCF

nanoparticles in the matrix of PVDF above 20 wt% of MZCF nanoparticles loaded PVDF matrix the value of (F(β)) has been decreased. The overall observation shows that the electroactivity of the host PVDF is improved initially due to the incorporation of MZCF nanoparticles in the matrix of PVDF for which the number of aligned chains having ‘alltrans’ (TTTT) conformation gets improved. This observation is very important to get idea about the required percentage of MZCF nanoparticles for which the host PVDF shows highest electroactivity. In the present report the maximum value of (F(β)) of ∼31% has been achieved for 20 wt% of MZCF loaded PVDF nanocomposite (SP20). So, among all the different loading percentages of MZCF nanoparticles in the matrix of PVDF the 20 wt% of MZCF nanoparticles loading can be considered as the most significant percentage of loading of MZCF nanoparticles in the matrix of PVDF to get maximum number of aligned chains having ‘all-trans’ (TTTT) conformation and the corresponding maximum electroactivity. However, in nano-scale region the production of nano-fillers with exactly same particle size and distribution is very difficult to control and also in the nanocomposite state of Mn-ZnCu-ferrite-PVDF nanocomposite films, it is very difficult to control the distribution of the nano-fillers in the matrix of PVDF. Due to these 21

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negative zeta potential is responsible for the enhancement of the electroactive β-phase of MZCF-PVDF nanocomposite systems as compared to the pure PVDF. The negative zeta potential of MZCF nanoparticles in DLS measurement signifies the presence of negative charges all around the surface of the spherically shaped MZCF nanoparticles. It has been observed from FTIR spectra and calculated from the Lambert-Beer law that the pure PVDF (SP0) has β-phase fraction (F(β)) of only 25% and this value enhances upto 31% for 20% MZCF loaded PVDF nanocomposite (SP20). This enhancement of (F(β)) has been appeared due to the formation of large area of interfaces between MZCF and PVDF as compared to the pure PVDF. Now, the interfaces in between MZCF ferrite nanoparticles and the polymer matrix are formed due to the electrostatic interaction between negatively charged MZCF ferrite nanoparticles and positively charged hydrogen atom of the PVDF polymer matrix. This electrostatic interaction leaves the negatively charged fluorine atoms uncovered and thereby it enhances the polarizability of MZCF modified PVDF (MZCF-PVDF) nanocomposite system. The detail of the electroactive β-phase formation mechanism in PVDF due to the presence of MZCF ferrite nanoparticles in the matrix of PVDF has been shown in the schematic diagram of Fig. 4. Now, the development of this additional polarizability due to the presence of negatively charged fluorine atoms under the influence of MZCF-PVDF electrostatic interaction in the MZCF-PVDF nanocomposite samples (SP5, SP10, SP15, SP20 and SP25) makes the nanocomposite samples more and more polarized than that of the pure PVDF sample (SP0).

3.4. Static magnetic study Static hysteresis loops of SP10, SP15 and SP20 were recorded at room temperature and the maximum applied magnetic field was ∼50,000 Oe. The systematic developments of the loops with the increasing percentages of the MZCF nanoparticles in the matrix of PVDF have also been observed in Fig. 5. Fig. 5a–c represents the loops of SP10, SP15 and SP20, respectively recorded at 300 K. It is clear from the loops shown in Fig. 5a–c that the magnetization increases with the increase of magnetic field and the respective curves are more or less saturated at around ∼50,000 Oe which indicates the soft magnetic nature of the sample. Close inspection of these loops revealed that the value of magnetization increases very slowly even when the applied field is 50,000 Oe. This is mainly due to the presence of a small fraction of SPM particles in the samples at 300 K. Different magnetic quantities like saturation magnetization (Ms), remanent magnetization (Mr) and coercive field (Hc) etc. of all the samples extracted from the different loops recorded at 300 K are 4.16, 5.84 and 9.91 emu/g, 0.02, 0.06 and 0.18 emu/g and 2.9, 4.2 and 10.4 Oe respectively. From the values of saturation magnetization, remanent magnetization and coercive field it is seen that the overall magnetic response increases with the increasing percentages of the MZCF loading inside the matrix of PVDF film, which is normally happened in case of nanocrystalline ferrite materials. The high values of saturation magnetization of the coated samples even in presence of the PVDF matrix make them most suitable for the applications in the magnetic and magneto-electric domain. The differences observed in the coercive field for SP10, SP15 and SP20 samples suggest that all the nanoparticles are successfully encapsulated inside the matrix of PVDF film due to which dipolar and/or exchange interaction comes into play among the magnetic nanoparticles. Thus the dipolar and/or exchange interaction is strongly modulated by the presence of PVDF matrix. Thus, from the magnetic measurement it is clear that the MZCF-PVDF nanocomposite films are quite able to modify the structure as well as the orientation of the PVDF matrix when the material is subjected to the external magnetic field and this property of the material is mostly helpful for the consideration of these types of materials as the potential candidate for microwave absorption.

Fig. 3. (I) FTIR spectra of (a) SP0, (b) SP5, (c) SP10, (d) SP15, (e) SP20 and (g) SP25, (II) Variation of β-phase content with increasing MZCF nanoparticles in MZCF-PVDF nanocomposites obtain form FTIR spectra, (III) Zeta potential distributions of MZCF nanoparticles.

issues, (F(β)) with exactly same amount is difficult to expect. But, from the present study we can say that if the preparation condition and the other parameters remains nearly same for Mn-Zn-Cu-ferrite-PVDF nanocomposite films, then the highest (F(β)) can be expected near the 20 wt% of MZCF loaded PVDF nanocomposite (SP20) system. In the present report we have tried to give an idea about the variation of (F(β)) of host PVDF due to the incorporation of nano-fillers and the expected improvement of (F(β)) with the increase of nano-fillers percentage upto a certain limit after which the effect will start decreasing [33]. The ‘alltrans’ (TTTT) conformation as well as the enhancement of the electroactive β-phase of MZCF-PVDF nanocomposites can be explained with the help of the Fig. 3(III) and Fig. 4. Fig. 4 shows the graph of negative zeta potential of MZCF nanoparticles in aqueous medium and this 22

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E-phase PVDF structure

MZCF nanoparticles

Carbon atom Fluorine atom Hydrogen atom

All trans (TTTT) conformation on MZCF ferrite nanospheres MZCF ferrite nanospheres PVDF polymer layer MZCF ferrite-PVDF polymer layer structure

MZCF ferrite-PVDF polymer flexible thin film image Fig. 4. Schematic diagram of the proposed polar β-phase formation mechanism of PVDF polymer on MZCF nanoparticles.

3.5. Dielectric analysis The variations of dielectric responses as a function of frequency and temperature of all the samples have been measured and the observed responses are shown in Figs. 6 and 7 respectively. Frequency dependent dielectric response is one of the most important measurements to understand the behaviour of the MZCF-PVDF nanocomposites as a potential candidate for microwave absorption. A high quality microwave absorber contains magnetic dipoles, electric dipoles as well as favourable coupling in between them and in this section we have discussed the modulation of the dielectric response of MZCF-PVDF nanocomposite films due to the incorporation of MZCF nanoparticles inside the matrix of PVDF. Dielectric response of the nanocomposite materials are characterized by complex permittivity of the material which is represented by

ε = ε′ + jε″

(3)

Here in Eq. (3) the symbols have their usual meanings. Here the values of ε′ and ε″ have been determined to understand the behaviour of some specific informations such as, the effect of electrostatic energy storing ability, the quantitative response of the electric dipoles, energy dissipation etc. by the MZCF-PVDF nanocomposite samples in presence of externally applied alternating electric field. ε′ and ε′′ of pure PVDF and PVDF based nanocomposite films have been calculated by using the

Fig. 5. Static hysteresis loops of (a) SP10, (b) SP15 and (c) SP20 at 300 K.

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Fig. 6. (I) Variation of dielectric constant with frequency of (a) SP0, (b) SP5, (c) SP10, (d) SP15, (e) SP20 and (g) SP25 at different temperature from RT (30 °C) to 140 °C and (II) Variation of dielectric constant with MZCF concentration at different frequency range of (a) SP0, (b) SP5, (c) SP10, (d) SP15, (e) SP20 and (g) SP25.

nanoparticles inside the matrix of PVDF film, SP15) loading percentages of MZCF nanoparticles has been decreased further. Thus, the frequency dependent dielectric response among the set of present samples is maximum for SP15. The variation of dielectric response with frequency can be explained most conveniently by two major phenomena. The first one is variation of intrinsic β-phase crystallization of the host PVDF film due to the incorporation of the MZCF nanoparticles in the matrix of PVDF. It has already been observed in the FTIR spectra that the β-phase fraction (F(β)) of SP5 nanocomposite sample increases with respect to SP0 and this (F(β)) becomes maximum at the loading percentage of 20% of MZCF nanoparticles in the matrix of PVDF (SP20) and the maximum (F(β)) contribution has been obtained for SP20 is nearly 31%. It has also been observed that (F(β)) decreases with further increase of the MZCF nanoparticles in the matrix of PVDF and the value of (F(β)) for SP25 is lower than that of SP20. Following the variation of the β-phase crystallization of the host PVDF film due to the incorporation of the MZCF nanoparticles the dielectric constant varies accordingly. The second one is the Maxwell-Wagner-Sillars interfacial polarization effect which appears for heterogeneous medium consisting of different phases having dissimilar permittivity and conductivity; and

formula

Cd ε0 A

(4)

ε″ = ε′ × tan δ

(5)

ε′ = And

where C is the capacitance of the sample, d and A are thickness and area, respectively of the nanocomposite film and ε0 is the free space permittivity. Fig. 6(I) shows the variation of ε′ as a function of frequency for the pure PVDF and MZCF-PVDF nanocomposites, ranging from 103 to 105 Hz at different temperatures (RT to 140 °C) and Fig. 7(I) represents ε′ as a function of temperature for same set of samples with temperature ranges from RT to 140 °C and at different frequency values (1–50 kHz). It is quite clear from Fig. 6(I) and (II) that 5% MZCF loaded PVDF (SP5) nanocomposite sample shows initial decrement of the dielectric constant to that of pure PVDF (SP0) and the different values of the dielectric constant for other nanocomposite films have shown gradual increment from SP10 to SP15. The dielectric constant of the MZCFPVDF nanocomposite samples with higher (higher than 15% of MZCF 24

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Fig. 6. (continued)

more but due to the presence of excess amount of MZCF nanoparticles the agglomeration effect occurs inside the PVDF matrix. This agglomeration of MZCF nanoparticles inside the matrix of PVDF film reduces the effective area of interfaces or electrostatic interaction in between negatively charged MZCF nanoparticles and the positively charged hydrogen atom of PVDF film and the dielectric constant decreases accordingly. Therefore, in the sample SP15 the optimization of both βphase crystallization and interfacial polarization gives maximum effect and thus, it shows highest dielectric constant among all other nanocomposite samples. Also it is quite clear from Fig. 6(I) that the dielectric response of all the samples increases with the rise in temperature. Basically, with the increasing temperature the dipoles inside the samples get sufficient energy to rotate them towards the direction of the external electric field following the frequency with proper manner. Thus,

this effect causes accumulation of the charges at the interfaces of PVDF and MZCF nanoparticles [34]. The area of interfaces between PVDF and MZCF nanoparticles increases with the increasing loading percentages of MZCF nanoparticles in the matrix of PVDF. But it has also been observed that for a very large amount of MZCF nanoparticles in the matrix of PVDF the number of effective interfaces between PVDF and MZCF nanoparticles get reduced. With the increasing amount of MZCF nanoparticles the amount of interfaces increases i.e., the electrostatic interaction in between negatively charged MZCF nanoparticles and the positively charged hydrogen atom of PVDF film becomes more and the optimization condition is achieved with 15% MZCF nanoparticles in the matrix of PVDF (SP15). Now, beyond the limit of 15% (SP15) of the MZCF nanoparticles in the matrix of PVDF film the β-phase crystallizations of MZCF-PVDF nanocomposites (SP20 and SP25) are even 25

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Fig. 7. (I) Variation of dielectric constant with temperature of (a) SP0, (b) SP5, (c) SP10, (d) SP15, (e) SP20 and (g) SP25 at different frequency of applied field and (II) Variation of ac conductivity with frequency at different temperature of (a) SP0, (b) SP5, (c) SP10, (d) SP15, (e) SP20 and (g) SP25.

migration of the induced free charge carriers through grain boundaries. From Fig. 7(II), ac conductivity for all the nanocomposite samples is completely independent of the frequency of the applied electric field.

more and more dipoles are found with favourable orientation towards the external electric field with the increasing temperature and as a result the dielectric constant increases. Fig. 6(II) shows the decrement of the dielectric response of the samples with the increase in frequency and at a particular temperature and it happens because with the increasing frequency the polarization decreases and it approaches to frequency independent behaviour and this type of behaviour is observed due to the fact that beyond a certain frequency range of the externally applied alternating electric field the hopping of the electrons cannot follow the frequency and hence lagging behind the frequency. Also, in Fig. 7(I) the above mentioned dielectric response of the samples under external electric field and at different temperatures has been substantiated. The electrical conduction mechanism of all the samples was determined using ac conductivity measurement and the curves have been shown in Fig. 7(II). It has been vastly mentioned in one of the earlier reports that ac conductivity of nano-fillers loaded PVDF nanocomposites increases with the increase in frequency of applied alternating electric field. This is mainly attributed as the enhanced

3.6. Microwave absorption study The chosen magnetic nanoparticle of MZCF is an important soft magnetic material in the family of mixed spinel nanocrystalline system [35–38]. Many important applications can be made in the field of science and technology with this soft magnetic material due to its high value of permeability, high magnetic anisotropy, high resonance frequency, electrical resistivity, chemical stability and corrosion resistance properties. Also, the frequency dispersion of complex permeability of these soft magnetic nanoparticles plays the most important role in different application areas such as inductors, converters, or electromagnetic wave absorbers. The complex permeability of ferrite nanoparticles is associated with the magnetic energy storing ability due to domain-wall motion, magnetization rotation, gyromagnetic spin 26

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Fig. 7. (continued)

smart Radar absorbing material for improved microwave applications to date by anyone else. Now, to investigate of the enhancement of microwave absorption property of the MZCF-PVDF nanocomposite films in comparison to bare ferrite nanomaterials, the reflection loss of the MZCF-PVDF nanocomposite films was measured in the frequency range of 8–18 GHz (X (8–12 GHz) and Ku (12–18 GHz)) band) [29]. The measured values of reflection loss of all the MZCF-PVDF nanocomposite films are shown in Fig. 8(I) for X band (8–12 GHz) and Fig. 8(II) for Ku (12–18 GHz) band, respectively of microwave frequency. It has been observed from the Fig. 8(I) and (II) that the reflection loss in the microwave range of frequency has been highly enhanced due to the presence of MZCF ferrite nanoparticles in the matrix of PVDF. From the measured values of reflection losses, the maximum reflection loss of –33.5 dB at 11.5 GHz (X band) in case of SP25 sample and the maximum reflection loss of 32.2 dB at 32.2 GHz (Ku band) in case of SP5 sample have been observed and all these values are quite high as compared to the other works. Also it has been found that the band

rotation of magnetic dipoles under the influence of the applied magnetic field and the magnetic energy losses, which can be ascribed to magnetic hysteresis, domain-wall resonance, eddy-current loss, exchange resonance etc. [39,40]. On the other hand, high-frequency complex permeability of polycrystalline ferrite with nonmagnetic grain boundary can be explained with the help of natural resonance and the magnetization rotation. These particular features of soft magnetic nanoparticles help to get enhancement of their physical property in the microwave/GHz frequency range. But, as per as the applications in the microwave region of frequency is concerned this soft magnetic material itself cannot fulfill the application requirements because of its low value of permittivity, poor flexibility and high density. In this regards, the magnetic nanoparticles embedded PVDF matrix may extend the area of applications of the MZCF-PVDF nanocomposites in the microwave region of frequency. For the first time here we have considered the incorporation of such important soft magnetic material in the matrix of PVDF and this nanocomposite system has not yet been considered as 27

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explained in the following discussion. The reflection loss (R(dB)) of a microwave absorbing system (having normalized input impedance (Zin)) is defined as: [43,44]

R (dB ) = 20log |(Zin − 1)/(Zin + 1)|

(6)

Again Zin is given by

Zin =

μr / εr tanh [j (2π /c ) μr ∊r fd]

(7)

where μr and εr are the relative permeability and permittivity of the composite medium, c is the velocity of electromagnetic waves in free space, f is the frequency of microwaves, and d is the thickness of the absorber. Thus the microwave absorption can be enhanced by matching magnetic contributions from MZCF nanoparticles and dielectric contributions from the electroactive β -phase of PVDF, which depends on tanh [j (2π / c ) μr ∊r fd]. Generally excellent electromagnetic wave absorption is resulted from the efficient complement between the relative complex permittivity and the relative complex permeability of the component materials. The relative complex permittivity (εr) and the relative complex permeability (μr), the electromagnetic impedance match and the microstructure of the resultant composite absorber play the most significant role for the enhanced performance of these composite materials as Rader absorbing material. When a microwave radiation falls on the surface of an absorber, a good matching condition of the EM impedance can produce almost zero reflectivity of the incident microwave. And therefore, the transmitted microwave can be dissipated in form of dielectric loss and magnetic loss in order to enhance microwave absorption or reflection loss property. Either only the magnetic loss or only the dielectric loss may induce a weak electromagnetic wave absorption property due to the imbalance of the required electromagnetic match. Actually, for pure PVDF, only dielectric loss contributes to the energy loss of electromagnetic wave, while for bare MZCF nanoparticles, the magnetic loss due to eddy current is more prominent than the dielectric loss. Thus the magnetic and dielectric losses are out of balance in case of individual component which induces poor electromagnetic wave absorption. However, in the MZCF-PVDF nanocomposite state the microwave reflection loss is enhanced because of the well matching of the magnetic and dielectric losses originated from MZCF as well as from PVDF film.

Fig. 8. Microwave absorption characteristics at (I) 8–12 GHz and (II) 12–18 GHz range of the samples (a) SP0, (b) SP5, (c) SP10, (d) SP15, (e) SP20 and (g) SP25.

widths of MZCF-PVDF nanocomposites are large which make them potential candidate for microwave absorption. Hence, the nanoparticles of MZCF embedded PVDF films can be considered as the better candidate for microwave absorption. In recent time, many works have been done for the development of Radar absorbing smart materials in the field of microwave absorption due to its upgrading requirements in the field of microwave devices. Many of them have measured the values of reflection loss of the soft magnetic materials loaded CNT nanocomposite samples in the frequency range of 2–18 GHz. In this direction, Ghasemi et al. has reported the measured value of reflection loss of substituted Sr-ferrite-functionalized MWCNT nanocomposites [41]. In that work they reported a maximum of −39 dB reflection loss at a frequency of 15.2 GHz. Che et al. measured the reflection loss of Coferrite loaded CNT nanocomposites in the frequency range of 2–18 GHz and they have got a maximum value of 18 dB reflection loss at a matching frequency of 9 GHz [42]. In all these works the nanocomposites are in powder form and thus they are very poorly flexible. Also, these nanocomposite materials are quite heavy (with high molecular density), which restricts their usefulness in the applications requiring for lightweight masses like aircraft or satellite systems. Enhancement of EMI shielding efficiency of microwave absorbents with high absorption rate, thin coating thickness, wide absorption band width, light weight, thermal stability, good flexibility and low cost is the most desirable things to make the nanocomposite materials useful in the microwave frequency range. In this study the thickness of all the nanocomposites films lie in the range of only ∼200–300 μm and it is capable to block EM radiation prominently in X-band and Ku-band of microwave radiations. Well distributed MZCF nanoparticles in the polymer matrix cause the increased interfacial area between the nano-fillers and polymer matrix which leads to the improvement of reflection loss value. In our present work on MZCF-PVDF nanocomposites, the microwave absorption is significantly improved. The cause of enhancement of reflection loss normally depends on the matching of relative permeability (μr) and relative permittivity (εr) of the nanocomposite medium and this is

4. Conclusion The nanoparticles of MZCF were successfully prepared by sol-gel method and further the nanoparticles of MZCF were dispersed and incorporated in the matrix of PVDF by very simple and low cost solution casting method. The presence of desired phases, the encapsulation of the nanoparticles of MZCF in the matrix of PVDF and the enhancement of the electroactive β -phase of the MZCF-PVDF nanocomposite samples were confirmed by XRD, various micrographs observed in FESEM and FTIR spectroscopy. The observed saturation magnetizations of MZCFPVDF nanocomposite films are considerably high and the coercive field is low. The modulation of magnetic behaviour of MZCF nanoparticles by PVDF films may be suitable for the applications in electromagnetic devices. On the other hand the magnetic property of MZCF-PVDF has been greatly enhanced compared to that of pure PVDF and this enhanced magnetic property may find many useful applications in polymer based devices. The dielectric response of MZCF-PVDF nanocomposite as the function of frequency and temperature confirms the presence of electric dipoles in the nanocomposite samples as well as the modulation of dielectric constants due to the electrostatic interaction between the negatively charged ferrite nanoparticles and the positively charged hydrogen ligand of the PVDF matrix. The most interesting application of this nanocomposite system that makes it unique and effective one in the field of science and technology is the microwave absorption. From the variation of reflection loss versus frequency it is evident that the reflection loss observed in X and Ku bands of 28

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microwave frequency have been substantially enhanced in the nanocomposite state which would be extremely fruitful for microwave devices. It is to be mentioned here that the ferrites are considered to be the most favourable microwave absorption materials for technological operations in the field of electromagnetic device components due to their high magnetic anisotropy, high resonance frequency, high permeability, electrical resistivity, chemical stability and corrosion resistance properties. However, the conventional absorptive materials such as MZCF nanoparticles is quite heavy (with high molecular density), which restricts their usefulness in the applications requiring for lightweight masses like aircraft or satellite systems. In this direction, PVDF can be applied along with the MZCF nanoparticles as an emerging microwave absorbing material because of its desirable physical and chemical properties. PVDF is a non-magnetic polymer and its microwave absorption properties mostly owe to the dielectric loss which on the other hand depends on its electroactive β-phase. Also, the molecular density of the MZCF-PVDF nanocomposite films is less as compared to the MZCF itself and so these light weight nanocomposite films are suitable for the many applications related to aircraft or satellite systems. Thus, the nanocomposite of MZCF-PVDF films will be effective to absorb electromagnetic energy at GHz frequency range which will be very much suitable for the components in electromagnetic devices at microwave/GHz frequency range.

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