Shielding Effectiveness Study of Barium Hexaferrite-Incorporated, Î²

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 23701−23713

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Shielding Effectiveness Study of Barium Hexaferrite-Incorporated, β‑Phase-Improved Poly(vinylidene fluoride) Composite Film: A Metamaterial Useful for the Reduction of Electromagnetic Pollution Soumyaditya Sutradhar,* Suman Saha, and Sana Javed

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Department of Physics, Amity University, Kolkata 700156, India ABSTRACT: The present work reports the high electromagnetic interference (EMI) shielding effectiveness of ∼−93.5 dB at 8.63 GHz and −97.6 dB at 8.61 GHz in the X- and Ku-bands for 10 and 20 wt % of barium hexaferrite (BaH) nanoparticle-loaded poly(vinylidene fluoride) (PVDF)-based composite films with a thickness of ∼0.210 and 0.260 mm, respectively. BaH−PVDF composite films with a layer structure have been considered in the present report in order to establish an excellent EMI shielding material for the suppression of electromagnetic pollution, with good control on flexibility, surface area, and thickness. Structural and morphological measurements reveal that the polar β-phase crystallization of the BaH−PVDF composite films has been enhanced in comparison to the pure PVDF film, and these measurements also reveal the influence of BaH nanoparticles on structural alteration from nonpolar α-phase to the polar/electroactive β-phase of the PVDF matrix. The resultant BaH− PVDF composite films produce multiple interfaces between magnetic BaH nanoparticles and β-phase-enriched electroactive PVDF, which plays the most significant role for the enhancement of the EMI shielding effectiveness (SE) in the microwave/ GHz frequency range. This high value of the EMI SE with >99.999999999% attenuation has not been found so far in the PVDF-based composite materials by anyone else. This particular feature of BaH−PVDF composite materials suggests that the BaH−PVDF composite films can be considered as the most useful ones for the fabrication of lightweight, flexible, and thicknesscontrolled EMI shielding materials for the reduction of pollution created by the electromagnetic waves in the microwave/GHz frequency region. KEYWORDS: hexaferrite nanoparticles, PVDF film, electroactive property, Maxwell−Wagner interfacial polarization, microwave absorption narrow thickness, a high degree of flexibility, and a large and controllable surface area. These altogether make the polymerbased laminated composite material an important and essential component for the technology-based research as compared to the nanocomposite powders. It is to be mentioned here that the poor flexibility, small surface area, and poor thickness control restrict the usefulness of the powdered nanocomposites in many important applications. The selection process of polymers and their copolymeric forms mostly depends on the required development or modulation of the physical properties/applications such as organic solar cells, supercapacitors, piezoelectric responses, and microwave absorbers.16−19 In this direction, the most challenging problem related to the modern technology is EMI that creates EM pollution for many signal processing system devices and high-frequency electronic switching components in frequency range of several GHz.20−24 Poly(vinylidene fluoride) (PVDF) and its electroactive βphase-improved copolymeric form is getting more and more

1. INTRODUCTION In the current century, research work is mainly focused on the improvement and also the vast use of the modern and multifunctional composite materials in various electrical and electromagnetic (EM) equipment for many important technological applications.1−4 Magnetic recording media, multiferroic application for memory devices, piezoelectric behavior in self-charging nanogenerators, and electromagnetic interference (EMI) shielding behavior in different highfrequency devices and switches are some applications where researchers have shown their highest confidence in multifunctional composite materials in the nanoscale regime.5−7 Depending on the behavior of the constituent materials, these composite materials can also be made suitable for the technological advancement, particularly in the field of defense applications, radar systems, sensor applications, wireless telecommunications, and medical as well as various technological applications in the GHz frequency range.8−15 In this regard, various polymers and laminated polymer composite materials are considered as the most effective ones in comparison to the powder nanocomposites due to their various unique characteristic properties such as light weight, © 2019 American Chemical Society

Received: March 22, 2019 Accepted: June 12, 2019 Published: June 12, 2019 23701

DOI: 10.1021/acsami.9b05122 ACS Appl. Mater. Interfaces 2019, 11, 23701−23713

Research Article

ACS Applied Materials & Interfaces

ment of the microwave absorption have not been mentioned in any of the articles reported to date. In the present article, the simultaneous contributions of structurally modified PVDF matrix as well as the magnetic nanoparticles for the improvement of the microwave absorption have been investigated with proper justification. Among all available magnetic materials, M-type barium hexaferrite (BaFe12O19) is a very important magnetic material having a magnetoplumbite crystal structure. The large intrinsic coercivity, high saturation magnetization, and magnetocrystalline anisotropy make it an important component for technological applications in the magnetic domain.37−39 Recent research on BaH nanoparticles confirms the presence of its microwave absorption due to its high permeability and moderate permittivity. Mu et al. investigated microwave absorption of BaH nanoparticles where they have observed −20 dB of reflection loss with 2 mm sample thickness. Also, the value of the reflection loss of −42.2 dB was observed by them at 8.0 GHz when BaH nanoparticles were prepared by poly(methyl methacrylate).40 The high value of permeability, permittivity, and microwave absorption capacity of BaH nanoparticles motivated us to consider this magnetic material as an essential component of microwave absorbing laminated composite material along with PVDF for further improvement of SE. In this direction, Ohlan et al. reported microwave absorption of conducting polyphenyl amine-based polymer composite with BaH nanoparticles and the composites have shown microwave SE of nearly −28.9 dB in the frequency range of 12.4−18 GHz.41 Yakuphanoglu et al. have reported EMI SE of usual rubber-loaded carbon and BaH nanoparticles at microwave frequency. In this work, the authors have shown the variation of SE caused by the change of thickness of the composite systems, and they have got maximum SE of nearly 70 dB for 20 wt % of BaH-loaded composite system in the range of 1−12 GHz.42 Therefore, a proper combination of highly magnetic BaH nanoparticles and highly electroactive or large permittivity of β-phase-enriched PVDF can build PVDF polymer-based composite systems that are very efficient to interact with microwave radiation in order to reduce EM pollution. In this present article, the solution casting method was employed for the preparation of BaH nanofillers-loaded PVDF composite films. Different investigations were carried out to establish the microwave absorber property of the resultant composite films. Also, discussions have been made in order to provide the explanations and justifications of the usefulness of BaH nanofillers-loaded PVDF composite films as improved microwave-shielding materials. The distribution of BaH nanofillers inside the PVDF matrix initiates the nucleation of PVDF in the form of all transplanar zig-zag (TTTT) conformation, which leads to an easy process for the fabrication of smart, flexible, thin microwave-absorbing composite materials.

attention in the modern day research work as the most essential component of multifunctional laminated composite materials due to its effective contributions in piezoelectric, pyroelectric, ferroelectric responses, and so forth.25−28 EMI shielding application is also an important issue related to the current century EM devices, where the EM pollution can be reduced by the use of high relative permittivity of polar βphase of PVDF as the component material. Further improvement of EMI shielding effectiveness (SE) of PVDF-based composite materials can be obtained by implementing an additional structural modulation over the usual host structure of PVDF, and this can be carried out successfully by incorporating different nanofillers inside the PVDF matrix. Depending on the behavior and size of the nanofillers, the PVDF-based composite materials can be made multifunctional, and many articles have already been published where the researchers have successfully shown structural modulation and physical property modulation of PVDF by the incorporation of different nanofillers.29−31 Keeping all these in mind, it is our expectation that a very well-known problem called EMI or EM pollution in many high frequency EM devices and switches can be resolved successfully by the PVDF-based laminated composite materials. Therefore, (1) proper selection of polymers as well as their copolymeric forms, (2) modulation of the structural property of host polymers by the incorporation of nanofillers, and (3) improvement of interface-induced effective interactions between the nanofillers and the host polymers in the composite materials are the key factors on which the essential modulation of the required physical property depends, and the enhancement of microwave absorption reported in this article has been successfully made by implementing all these factors.32,33 Recently, polymer-based microwave absorbers have also been developed and reported by a few research groups, but eventually, the recorded microwave absorption properties in most of the cases are only moderate. An effective microwave absorber can be made by the proper choice of both polymer matrix and nanofillers, as mentioned earlier. In this regard, magnetic nanofillers with a comparatively large magnetic moment, high relative permeability, and high magnetic loss can provide structural modification to the host PVDF for the advancement of its electroactive polar β-phase, and a better response toward the microwave absorption from this combination is expected. The report published by Bera et al. shows that the 3 wt % of multi-walled carbon nanotubes (MWCNT)-incorporated polycarbonate or poly(vinylidene) composite can absorb EM wave of −14 to −22 dB in the frequency range of 8−18 GHz, with sufficiently large thickness of 5 mm.34 Also, the microwave absorption of amorphous Fe− B/Ni−Zn ferrite nanoparticles embedded in the polymer matrix has been reported by Shimba et al., and they have observed nearly −34 dB of reflection loss at 0.83 GHz with a thickness of 3.22 mm.35 Microwave absorption has also been reported by Li et al., and they have observed −67 dB of reflection loss with a matching thickness of 2 mm.36 In their work, Li et al. used a PVDF matrix in place of epoxy resin in order to hold the nanoparticles together inside the polymer matrix. In all these works, microwave absorption is moderate in comparison to the report presented here and the role of PVDF as an important constituent material of it for the enrichment of microwave absorption has not been considered. Also, the simultaneous contributions of the structurally modified PVDF matrix along with the magnetic nanofillers for the improve-

2. EXPERIMENTAL SECTION 2.1. Materials. BaH nanoparticles-incorporated PVDF films were synthesized by the coprecipitation method followed by the solution casting method. Analytical reagent grade chemicals were used to prepare BaH−PVDF composite films, and the listed chemicals are as follows: PVDF pellets (Mw: 275 000 (hpc), Mn: 107 000, Aldrich, Germany), N,N-dimethyl formamide (DMF) (Merck, India), Ba(NO 3 )2 (Merck Germany, ∼99%), Fe(NO 3) 3 ·9H 2 O (Merck Germany, ∼99%), citric acid (C 6H 8 O7 ), and ethyl alcohol (C2H5OH). 23702


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ACS Applied Materials & Interfaces 2.2. Synthesis of BaH Nanoparticles. The M-type BaH (BaFe12O19) nanoparticles were synthesized by a simple sol−gel method. The precursor salts with proper stoichiometry were taken with ethyl alcohol to form a homogeneous solution. The solution of the precursor salts was stirred for 1 h at 60 °C. Thereafter, 1 M C6H8O7 was also added in ethyl alcohol using a magnetic stirrer for 1 h at 60 °C, and the homogeneous C6H8O7 solution was added dropwise to the homogeneous salts solution under rapid stirring condition. After the complete addition of the C6H8O7 solution, the resultant solution was again stirred for 3 h at 60 °C and as a result, a largely thick solution was obtained. The thick solution was then placed inside an oven at 80 °C and in time, a gel was formed, which was then dried slowly for 2 days. After 2 days, dried-up flakes were obtained from the prepared gel. Finally, the BaH solid powder was ground and annealed at 900 °C for 6 h in a tube furnace in order to obtain the BaH nanoparticles with the desired phase and size. 2.3. Synthesis of BaH−PVDF Composite Film. The BaH− PVDF composite films were formed by the method of solution casting. At first, PVDF pellets were taken with DMF. They were then placed on a magnetic stirrer at 50 °C for the preparation of PVDF gel. After few hours, the solution of PVDF was converted into a thick and clear gel. Thereafter, the BaH nanoparticles were added into the thick and clear gel. The whole system was placed in an ultrabath sonicator for few hours to get the homogeneous mixing of BaH nanoparticles inside the PVDF gel. Thereafter, the homogeneously mixed BaH− PVDF gel was poured over the clean and dry glass substrate, and the substrate was placed inside the hot air oven at 75 °C. The boiling point of the DMF solution is 153 °C. Hence, the given temperature inside the hot air oven initiates the slow evaporation of the solvent under normal pressure. This slow evaporation also helps to get the development of a uniform interfacial area between the BaH nanoparticles and PVDF throughout the resultant film structure. The substrate was kept inside the hot air oven for 24 h and then the whole system was gradually cooled down to room temperature (RT). Finally, the dried BaH−PVDF composite film with large surface area, small thickness, and high flexibility was collected. This process was conducted for the formation of two resultant composite films of BaH nanoparticles with PVDF. Also, the film of bare PVDF was prepared by following the same process, where the thick and clear gel of PVDF was poured on a cleaned, dry, and warm glass substrate. Recently, Ribeiro et al. reported the formation of electroactive PVDF with different structures and advanced applications.43 They have shown various methods for the synthesis of modified and improved β-phase PVDF films. It is to be mentioned here that the synthesis process we have adopted in this article is similar to their method called films by a doctor blade. This method is a very easy and cost effective process out of many others for the synthesis of modified and improved β-phase PVDF film. The pure PVDF film and two resultant BaH−PVDF composite films were designated as BaF-0 (0 wt % BaH nanofillers in the PVDF matrix, pure/bare PVDF), BaF-10 (10 wt % BaH nanofillers in the PVDF matrix), and BaF-20 (20 wt % BaH nanofillers in the PVDF matrix), respectively. Surely, it is the simplest and most cost-effective method by which small thickness, large surface area, lightweight, and highly flexible resultant composite films of BaH−PVDF can be fabricated. These composite films of BaH−PVDF were considered for different measurements for the determination of various physical properties, which were required to explain the improvement of microwave SE of BaH−PVDF composite films. 2.4. Characterization Techniques. The X-ray diffraction (XRD) pattern of bare and BaH nanofiller-loaded PVDF films was obtained by using a powder X-ray diffractometer (Bruker AXS, model D8) having Cu Kα radiation of wavelength λ = 1.5405 Å, within the 2θ range of 10 to 80°. The surface morphologies of all samples were obtained by field emission scanning electron microscopy (FESEM, INSPECT F50 of FEI, Netherland). Fourier transform infrared (FTIR) study of all samples was conducted in a Fourier transform infrared spectrophotometer (IRPrestige-21, Shimadzu, Japan). Zeta potential measurement of the BaH nanoparticles was conducted at RT by using a Zetasizer Nano ZS (Malvern Instruments, UK). In this direction, the BaH nanoparticles were dispersed in the polar

dispersant water. Then, the solution was sonicated for several hours. After getting the solution of nearly monodispersed aqueous solution of BaH nanoparticles, the solution was taken for zeta potential measurement. For this measurement, a laser light source of 633 nm was used and from the scattering phenomenon, the zeta potential data was extracted. The dielectric responses of the film samples were investigated by using a precision impedance analyzer (Agilent 4294A). Microwave SE of all composite films in the 8−18 GHz frequency region (X-band and Ku-bands) was investigated by using the PNA microwave network analyzer (Agilent E8363B). Figure 5 shows that BaF-10 and BaF-20 resultant composite film samples are free to stand and the thickness lies in the range of ∼0.210−0.260 mm. So, these films can be placed in between the waveguide flanges and no materials like, wax, resin, or paraffin are required to hold the film samples for this purpose. BaF-10 and BaF-20 were cut into the shape of the flange dimensions of the X- and Ku-band waveguides and the composite film samples were placed between the X- and Ku-band waveguide flanges of the PNA microwave network analyzer (Agilent E8363B) for microwave SE measurement.

3. RESULT AND DISCUSSIONS 3.1. XRD Analysis. The XRD patterns of all samples (pure PVDF, BaH nanoparticles, BaF-10, and BaF-20 composite films) have been depicted in Figure 1. All observed peaks in Figure 1(I,II)(a−c) of the XRD pattern are matched properly with the preferred phases of bare PVDF [JCPDS (file no. 381638)], bare BaH nanoparticles [JCPDS (file no. 043-0002)], and the coexisting phases of both BaH and the PVDF polymer in both the BaH−PVDF resultant composite films (BaF-10 and BaF-20). It is interesting to mention here that no additional peak for any other undesired phases has been observed in any of the XRD images. The average crystallite size of BaH nanoparticles was estimated by using the Scherrer equation given in Figure 1(II)(a) for the intense XRD peak (114) of the BaH nanoparticles ⟨D⟩(114) =

0.9λ β1/2 cos θ

(1)

Here, D signifies the average crystallite size, λ signifies the wavelength of incident X-ray, θ signifies the Braggs angle corresponding to the central position of the XRD peak (114), and the corresponding full width at half maximum is marked by β1/2. The D value of ∼59.27 nm has been observed for the BaH nanoparticles from the XRD study of Figure 1(II)(a). The uncertainty estimated in the measurement of D value of the peak (114) lies in the range ±1. The XRD pattern given in Figure 1(I) confirms the simultaneous existence of α-, β-, and γ-phase for the bare PVDF (BaF-0) film. Figure 1(II)(b,c) exhibits the XRD of BaF-10 and BaF-20, confirming the presence of the β-phase at 20.27° (characteristic peak) corresponding to the plane ((110) (200)) and γ-phase at 38.68° corresponding to the plane (211), respectively, along with the distinctive peaks of BaH nanoparticles.44,45 From the given Figure 1(II)(b,c) it is fairly clear that the relative intensity corresponding to the β-phase at 20.27° has been reduced for BaF-20 as compared to BaF-10. This has happened because higher percentage of BaH nanoparticles are loaded in the PVDF matrix, which restricts further elongation of the polymer chains corresponding to all transplanar zig-zag (TTTT) conformation over the BaF-10 composite film and, thereby, reduces the intensity of the β-phase in comparison to the BaF-10 composite film. Figure 1(III) shows the XRD peak positions of different BaH−PVDF resultant composite films in comparison with the bare BaH nanoparticles. From the Figure 23703


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Figure 2. Zeta potential distributions of BaH nanoparticles.

with the CH2 groups inside the PVDF matrix with a positive surface charge density. This electrostatic interaction has been developed by the successful inclusion of the BaH nanofillers inside the structure of the PVDF matrix. 3.2. Morphology Study. The surface morphology of BaH nanoparticles, BaF-0, BaF-10, and BaF-20 composite films has been observed by using FESEM micrographs. The FESEM micrographs of the respective samples have been depicted in Figure 3. Figure 3a displays the microstructure of BaH

Figure 3. FESEM images of (a) BaH nanoparticles, (b) BaF-0, (c) BaF-10, (d) BaF-20, and (e,f) dispersivity of BaH nanoparticles in the matrix of PVDF.

Figure 1. (I) XRD pattern of BaF-0 film (II) XRD patterns of the samples (a) BaH nanoparticles (b) BaF-10 and (c) BaF-20 composite films and (III) relative shift of peaks of the samples (a) BaH nanoparticles and (b) BaF-10 and (c) BaF-20 composite films.

nanoparticles that are more or less spherical in nature. The average size of BaH nanoparticles has been calculated from the observed FESEM micrographs, and it has been observed within the range of 65−70 nm. This observation is slightly different from the D value obtained in XRD, but in well agreement with the D value extracted from the XRD analysis.46 Figure 3b displays the surface morphology of BaF-0. The surface morphology of the BaF-0 film shows that the surface of the BaF-0 film contains both spherulite and radial lamellar structures. The spherulite shape on the BaF-0 surface confirms the presence of the polar β-phase, but the lamella shape on the BaF-0 surface confirms the presence of a nonpolar α-phase of the BaF-0. The observed surface morphology of the BaF-0 film is the clear evidence of having large fractional of the spherulite structure along with a small fractional of radial lamella structure, which signifies the presence of the β-phase at the cost of the α-phase in the BaF-0 film. Also, the distinguishable change in the surface morphology of BaF-10 and BaF-20 has been observed in Figure 3c,d. In different studies, it has been reported that the microstructures of PVDF films change due to the change in their crystalline phases, and one such modulation of this crystalline phase from α-phase to the electroactive β-

1(III), it is very much evident that the peaks of the planes (114), (108), and (203) are shifted toward the higher 2θ value as the BaH nanoparticles loading percentage was enhanced in the PVDF matrix in comparison to BaF-0, and these shifts of the given peaks toward the higher 2θ value have been observed because a large internal structural deformation has been formed inside the PVDF matrix. The presence of more BaH nanoparticles inside the PVDF matrix induces large internal structural deformation at the cost of the α-phase. The observed shifting of the peaks toward a higher 2θ value is more for BaF20 and BaF-10 as compared to BaF-0. This large change in position of the XRD peaks indicates that the internal structural deformation is more for BaF-20 than that of BaF-10. This may be ascribed by the existence of a large amount of BaH nanofillers in the PVDF matrix for BaF-20 for which the internal structural deformation has been improved. Also, in the given XRD patterns, the variation of peak intensities of the BaH−PVDF composite films has appeared due to the variation of β-phase crystallization. This has happened due to the electrostatic interactions between the BaH nanoparticles having zeta potential of ∼−18.5 mV, shown in Figure 2, 23704


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ACS Applied Materials & Interfaces phase can be performed successfully by the proper choice of the synthesis procedure and also by the existence of the required amount of nanofillers.47−49 In the present study, the solution casting process has been used for the preparation of all composite film samples, and in each composite film sample, different amount of BaH nanofillers are incorporated. By this process, the presence of the polar β-phase of the composite film samples in the form of spherulite shapes can be improved at the cost of the nonpolar α-phase of the lamellar shape. Now, in case of the BaF-0 sample, the BaF-0 sample has been prepared by the solution casting method, which has improved the polar β-phase crystallization of the film sample and the spherulite structure of the polar β-phase dominates over the nonpolar α-phase in the form of the lamellar shape. In case of the BaF-10 and BaF-20 composite films, the polar β-phase has enhanced even higher than that of the BaF-0 film because of the presence of BaH nanofillers in the PVDF matrix. The presence of BaH nanoparticles act as the center of nucleation inside the PVDF matrix and change the individual dipoles of the PVDF structure from antiparallel to parallel alignment because of which the polar β-phase of the composite films gets improved. From Figure 3, it is quite clear that the spherulite structure is present in all film samples, although the presence of more bubble-like structures on the surface of the BaF-10 and BaF-20 is the clear evidence of having larger polar β-phase crystallization in the BaF-10 and BaF-20 in comparison to the BaF-0 film. Here, the change has appeared with the variation of the BaH nanofillers content in the PVDF matrix. In the composite films, a large area is covered up by the bubble-like structures, which is the clear evidence of the improvement of polar β-phase crystallization of the BaH−PVDF composite films. The discussion has already been made in the previous section that the presence of bubble-like spherulite structures over the composite film surface signifies the improvement of the polar β-phase of the BaH−PVDF composite films. The bubble-like spherulite structures all over the surface of the composite films signify that the dipoles corresponding to each molecule of the PVDF chains inside the unit cell are parallel to each other. In the present case, the BaH nanofillers act as the center of nucleation inside the PVDF matrix. Each of the nucleation centers helps to get parallel configuration of the molecular dipoles present inside the PVDF matrix. Here, in the present case, the presence of BaH nanofillers inside the PVDF matrix for both BaF-10 and BaF-20 help to get bubble-like spherulite structures all over the surface of the composite films, which also signifies the presence of improved β-phase crystallization of the BaH−PVDF composite films. The detail of the variation of the β-phase crystallization of the BaF-10 and BaF-20 has also been discussed in the FTIR section. 3.3. FTIR Spectroscopy Study. Figure 4a−c(I) shows the FTIR absorption spectra of BaF-0, BaF-10, and BaF-20, respectively. Table 1 provides the list of peaks associated with the α- and β-phase of BaF-0, BaF-10, and BaF-20, respectively. The observed FTIR absorption spectrum of the BaF-0 shows all distinctive peaks located at 525, 602, 847, and 1082 cm−1, confirming the existence of the polar β-phase of BaF-0 and the distinctive peaks located at 493, 538, 618, 773, 814, 898, and 981 cm−1, confirming the existence of the nonpolar α-phase of BaF-0.50 Also, the FTIR absorption spectra of BaF-10 and BaF20 show the relative change in intensity corresponding to the characteristic absorption bands of the BaH−PVDF composite films. The relative increase in intensity of the polar β-phase located at 847 cm−1 at the cost of the α-phase located at 773

Figure 4. (I) FTIR spectra of the samples (a) BaF-0, (b) BaF-10, and (c) BaF-20 composite films and (II) variation of β-phase content with increasing BaH nanoparticles in BaH−PVDF composite films obtained from FTIR spectra.

Table 1. FTIR Peaks Corresponding to the α-Phase and βPhase of the Samples BaF-0, BaF-10, and BaF-20 peak position corresponding to sample name BaF-0

BaF-10 BaF-20

α-phase (cm−1) 493 (CF2 waging) 538 (CF2 bending) 618 (CF2 bending) 773 (CF2 skeletal bending) 814 (CH2 rocking) 898 (CH2 rocking) 981 (CH2 rocking) same as above same as above

β-phase (cm−1) 525 (CF2 stretching)

(F(β) %) 26.1

602 (CF2 waging) 847 (CH2 rocking, CF2 stretching, and skeletal C−C stretching)

1082 (CH2 and CF2 groups generated from the CH2 rocking and CF2 stretching) same as above

32.4

same as above

31.3

cm−1 has also been observed in the FTIR absorption spectra because of the loading of BaH nanofillers in the PVDF matrix in comparison to BaF-0. Hence, the FTIR results confirmed the sustainability as well as the enhancement of the polar βphase of PVDF in the BaH−PVDF system due to the presence of BaH nanoparticles. The transformation of α- to β-phase of BaF-0 occurred due to the solution casting synthesis process, but in case of composite systems, the further enhancement of β-phase at the cost of α-phase has appeared due to both 23705


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Figure 5. Schematic diagram of the proposed electroactive β-phase formation mechanism of BaH−PVDF composite film.

particles inside the PVDF matrix. Because of this large magnetic moment, the BaH nanoparticles interact strongly with themselves and produce agglomeration even in the presence of the PVDF matrix. This agglomeration effect of BaH nanoparticles inside the matrix of PVDF gets reduced when the loading percentage of BaH is less and the agglomeration effect increases gradually with the rising loading percentage of BaH nanoparticles inside the PVDF matrix even after having an adequate number of sonication processes. Here, in the present article, it is quite clear that the agglomeration effect between the magnetic BaH nanofillers increases when the percentage of BaH nanofillers increases from 10 to 20 wt % inside the PVDF matrix, and as a consequence, it reduces F(β) % of the corresponding composite film. Hence, in the 10 wt % of BaH-loaded PVDF composite film (BaF-10), the effective number of aligned chains with an all-transplanar zig-zag (TTTT) conformation has been enhanced and it leads to the successful increment of F(β) % to 32.4% in comparison to 26.1% for bare PVDF. The advancement of the all-trans (TTTT) conformation mechanism of BaH−PVDF composite systems has been displayed in Figure 5. However, F(β) % is reduced for further enhancement of the BaH nanofillers inside the PVDF matrix. This has been observed particularly in BaF20, where F(β) % has been reduced to 31.3%. This phenomenon may be appearing because of the restriction of the movement of elongation of the polymer chains in all transplanar zig-zag (TTTT) conformation for higher loading percentage of nanofillers because of which the β-phase crystallization of the composite system get reduced. The all transplanar zig-zag (TTTT) conformation, as well as the improvement of the polar β-phase of all BaH−PVDF composite films in comparison to BaF-0, can be explained using Figure 2. Figure 2 represents the negative zeta potential of ∼−18.5 mV for BaH nanoparticles. The negative zeta potential of BaH nanoparticles signifies the presence of a negative charge effect all around the surface of the spherically shaped BaH nanoparticles when the nanoparticles are immersed in the PVDF gel. It has been obtained from FTIR

solution casting synthesis process and the inclusion of BaH nanoparticles inside the PVDF matrix. In Figure 4(II), the modulations of the β-phase fraction (F(β) %) of the PVDF matrix in the BaH−PVDF system with the growing percentage of BaH nanoparticles have been projected. From Figure 4(II), the F(β)% has been measured for BaF-0, BaF-10, and BaF-20 films, and the corresponding results have been provided in Table 1. The estimation of F(β) % has been carried out by using the Beer−Lambert law given below.47,49 F (β ) =

Aβ (Kβ /Kα)Aα + Aβ

(2)

Here Aα and Aβ are the absorbance values corresponding to the peaks located at 770 and 847 cm−1, respectively. Kα (6.1 × 104 cm2 mol−1) and Kβ (7.7 × 104 cm2.mol−1) are the corresponding absorption coefficients. The electrostatic interaction present in between BaH and PVDF is mainly responsible for this enhancement of F(β) % of BaF-10. The calculated value of F(β) % also shows that the value has been reduced for BaF-20. It shows that the effective interfacial area between BaH and PVDF in the composite film is large for low filler content and with the enhancement of the nanofillers content inside PVDF, the agglomeration between the magnetically strong BaH nanoparticles ruled over the effectiveness of the interfacial area and F(β) % decreased due to this agglomeration effect. Now, this agglomeration effect can be minimized by the choice of suitable percentage of BaH nanofillers in the PVDF matrix. Also, this agglomeration effect can be reduced by the choice of a suitable synthesis method. The sonication process can also play an important role to minimize the agglomeration effect between BaH nanofillers inside the PVDF matrix. Different processes can be opted to minimize the agglomeration effect between BaH nanofillers inside the PVDF matrix, but the agglomeration effect between BaH nanofillers is not possible to rule out completely for higher loading percentage of BaH nanoparticles and the corresponding high magnetic moment of the BaH nano23706


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ACS Applied Materials & Interfaces absorption spectra that pure PVDF (BaF-0) has F(β) % of nearly ∼26.1% and this value enhances up to 32.4% for BaF10. This improvement of F(β) % has appeared due to the formation of a large area of interfaces between BaH and PVDF in comparison to the BaF-0. Now, the interfaces in between BaH and PVDF are formed due to the electrostatic interaction in between the negative surface charge of BaH and the CH2 groups of PVDF matrix with a positive surface charge density.51 This electrostatic interaction improves the polarization effect and the electroactive behavior of BaH-modified PVDF composite systems.52 Now, the development of this additional polarization effect and the electroactive behavior of BaH−PVDF composite films under the influence of BaH− PVDF electrostatic interactions (BaF-10 and BaF-20) make the resultant composite films more and more polarized/ electroactive than the pure PVDF film (BaF-0). 3.4. Dielectric Spectroscopy Study. In the present report, the frequency-dependent dielectric study has been conducted for these polymer-based composite systems to realize the character of the pure PVDF (BaF-0) and BaH− PVDF composite films as a prospective candidate for EMI shielding applications.53 Figure 6(I) shows the variation of the

been found for BaF-10 and BaF-20, respectively. The second phenomenon in the direction of frequency-dependent dielectric response can be justified by the very well-known Maxwell−Wagner interfacial polarization as well as other polarization effects, which are very much effective for the heterogeneous medium that contains charge carriers of limited mobility. In the present study, the effective dielectric constant is more than 10, and the material is not supposed to be a ferroelectric one, so there is a very good reason to believe that the effective dielectric constant appears due to the contributions from induced charge carriers of limited mobility and this effect causes growth of the induced charge carriers at the interfaces of BaH−PVDF composite films.54−57 Now, the accumulation of the induced charge carriers at the interfaces will increase with the increase of the interfacial area between PVDF matrix and BaH nanofillers. On the other hand, the interfacial area between PVDF matrix and BaH nanofillers can be improved within the permissible range by increasing the loading percentages of BaH nanofillers inside the PVDF matrix. Thus, with the rising amount of BaH nanofillers inside PVDF, the total amount of charge accumulation increases and it improves the overall polarization effects of the composite films. As a consequence, comparatively high dielectric constant of 12.3 and 19.2 at 100 Hz has been achieved for BaF-10 and BaF-20, respectively, in comparison to BaF-0 with the dielectric constant of around 5. Therefore, the presence of both electroactive β-phase crystallization and polarization effects in BaF-10 and BaF-20 composite films shows a higher dielectric constant of BaF-10 and BaF-20 in comparison to BaF-0. Figure 6(II) shows the imaginary parts of the dielectric constant (ε″) of the BaF-0, BaF-10, and BaF-20 composite films at RT. A significant dispersion in dielectric energy loss has been observed in the entire frequency range for both BaF10 and BaF-20 in comparison to BaF-0, which appears due to the change of a part of electrical energy into heat energy. The generation of high dielectric energy loss in the form of heat energy appears due to the presence of large polarization effect in BaF-10 and BaF-20 in comparison to BaF-0. Under external alternating electric field, the large space charge polarization and other polarization effects present in BaF-10 and BaF-20 in comparison to BaF-0 produce more heat energy when the direction of rotation is changed followed by the frequency of the electric field. Also, the presence of electron hopping between Ba2+ and Fe3+ ions in the BaH nanostructure produces an additional amount of heat energy from applied electrical energy due to the movement of carrier electrons confined inside the grains. The generation of dielectric energy loss due to this conduction loss also improves ε″ of BaF-10 and BaF-20 in comparison to BaF-0. From Figure 6(II), the dielectric energy loss has been estimated for BaF-10 and BaF-20 composite films and the measured values are 1.1 and 2.3, respectively at 100 Hz frequency, which is comparatively high in comparison to BaF-0 with the value of 0.32 at 100 Hz. Thus, from Figures6(I,II) it is quite clear that more polarization effects are present in both BaF-10 and BaF-20 in comparison to the BaF-0 film, and this particular feature of BaF-10 and BaF-20 composite films also plays the significant role for high SE of EM wave at GHz frequency ranges. Figure 7(I) shows the dielectric tangent loss (tan δ) of BaF0, BaF-10, and BaF-20 composite films with the variation of frequency at RT. It has been observed in Figure 7(I) that tan δ of BaF-0, BaF-10, and BaF-20 composite films lowers with the increase in frequency at RT. The observed variation of

Figure 6. (I) Variation of real part of dielectric constant with frequency of the samples (a) BaF-0, (b) BaF-10, and (c) BaF-20 films at RT, (II) variation of imaginary part of dielectric constant with frequency of the samples (a) BaF-0, (b) BaF-10, and (c) BaF-20 films at RT.

real part of the dielectric constant (ε′) with the frequency of BaF-0, BaF-10, and BaF-20 composite films at RT, and the observed dielectric response has been explained in this section by two major phenomena. The first one explains the reason behind the improvement of electroactive β-phase crystallization of BaF-10 and BaF-20 composite films in comparison to that of BaF-0. The crystallization of the host PVDF corresponding to the β-phase has been achieved successfully by inserting the BaH nanofillers in the PVDF matrix and the cause behind this improvement has already been explained in the FTIR analysis. The measured value of (F(β) %) of BaF-10 and BaF-20 has been increased in comparison to BaF-0 and sufficiently high values of (F(β) %) of 32.4 and 31.3% have 23707


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been observed for BaF-10 and BaF-20 as compared to BaF-0. The short range conductivity appears due to the streaming of induced electric charges under external applied alternating electric field in several kinds of electric dipoles. It has already been mentioned in the earlier part of this dielectric discussion that the presence of BaH nanofillers in the matrix of PVDF can accumulate charge carriers at the interfaces and improves various dipolar effects in comparison to BaF-0. The existence of large dipolar polarization effects of BaF-10 and BaF-20 causes short-range conductivity at the frequency range of 40 Hz to 10 kHz and due to this reason, the σac has been improved largely for BaF-10 and BaF-20 in comparison to BaF0. It should be mentioned here that this short range σac appears for various reasons. In the low-frequency region, this short range σac appears due to the growth of induced charge carriers at the interface of the composite films, and at high-frequency region, this short range σac appears due to the presence of various others dipolar effects such as ionic polarization and the polarization due to the electron transportation between Ba2+ and Fe3+ ions in the BaH nanostructure. Thus, σac of short range type in the low-frequency region appears due to the Maxwell−Wagner interfacial polarization effect and in the high-frequency region, the ac conductivity of short range does not depend on this Maxwell−Wagner interfacial polarization effect, rather it depends on various others dipolar effects, as mentioned above. However, as a whole, this σac of short range type in the complete frequency range depends on both accumulations of charge carriers at the interfaces as well as various other high frequency dipolar responses. Thus, from the dielectric response study, it is quite clear that the loading of BaH nanofillers inside the PVDF matrix generates various dipolar effects due to which the EMI shielding ability of BaF10 and BaF-20 has been improved largely in comparison to the BaF-0 film in the GHz frequency range. 3.5. EM and Microwave Absorption Spectroscopy Study. In this article, both εr = ε′ + jε″ and μr = μ′ + jμ″ have been investigated in the frequency range of 8−12 GHz for the X-band and 8−12 GHz for the Ku-band, and 12−18 GHz of BaF-10 and BaF-20 composite films and the corresponding graphs have been depicted in Figures 8 and 9 respectively. Here, εr represents the relative complex permittivity and μr represents the relative complex permeability of the composite systems. The values of εr and μr have been estimated in the Xband and Ku-band from the observed values of two scattering (S) parameters, that is, S11 reflection coefficient and S21

Figure 7. (I) Variation of dielectric loss tangent with frequency of the samples (a) BaF-0, (b) BaF-10, and (c) BaF-20 films at RT, (II) variation of ac conductivity with frequency of the samples (a) BaF-0, (b) BaF-10, and (c) BaF-20 films at RT.

frequency-dependent tan δ depends on the induced space charge polarization present in both BaF-10 and BaF-20 composite films. Also, the observed tan δ has been enhanced with the rising loading percentage of BaH nanofillers inside the PVDF matrix. This type of dielectric response can be explained partially by the high value of tan δ of the BaH nanofiller in comparison to that of the β-phase of host PVDF due to the presence of a resistive factor called equivalent resistance in series with the capacitive effect of the composite films and partially by the heterogeneous structure of the BaH nanofillers with the PVDF polymer. The structural mismatching is also responsible to induce space charges at the interfaces, which improves the conductivity inside the BaH grains and contributes the higher value of tan δ for both BaF-10 and BaF-20 in comparison to the BaF-0 film. Also, the existence of heterostructure effect in BaH−PVDF composite films enhances various other polarization effects, and as a consequence, it leads to an additional contribution to tan δ of the composite films. Also, from Figure 7(I), it can be observed that when the value of the applied electric field frequency is more than 100 kHz, the dielectric relaxation related to the β-phase crystallization of PVDF controls the overall dielectric response. The observed tan δ at and above 100 kHz of BaF-10 and BaF-20 [Figure 7(I)] appears due to the inherent dielectric response of the BaH nanofillers and host PVDF matrix, and it suggests that the Maxwell−Wagner interfacial polarization is not prominent there because the Maxwell−Wagner interfacial polarization effect is prominent in the frequency range which is 99.999999999%) among all other published research work.

previous work, also shows that the selection of component materials for the fabrication of the composite material is very important for technological applications, and this is probably the main essence of materials science research.63 The improvement of SE (total) of BaF-10 and BaF-20 composite films may be appearing due to the high magnetic losses of BaH nanoparticles and high dielectric losses of electroactive βphase-enriched PVDF matrix and the perfect balance between these two loss factors in the GHz frequency range. Also, the interfaces between BaH nanofillers and PVDF appearing in the composite systems improve the SE (total) of BaF-10 and BaF20 composite films as shown in the Figure 11. The SE (total) as a function of εr and μr can be represented as69 ij σ yz μωσAC = 10 logjjjj AC zzzz + 20d r log e 2 k 16ωε0μr {

SE (dB) = SE R + SEA

(7)

where

ij σ yz SE R (dB) = 10 logjjjj AC zzzz k 16ωε0μr {

(8)

and SEA (dB) = 20d

μωσ AC r 2

log e

4. CONCLUSIONS In this article, we have successfully synthesized composite films of BaH nanoparticles and the PVDF matrix by the solution casting method. The qualitative as well as quantitative enrichment of the polar β-phase in the resultant BaH−PVDF composite systems has been observed in comparison to the pure PVDF film. The existence of large interfacial area between the highly magnetic BaH nanofillers and the polar PVDF matrix plays the most significant role during wave−matter interaction and the resultant composite films have come out with an excellent EMI shielding effect. These unique and effective RAMs show SE (total) of −93.5 dB at 8.63 GHz and −97.6 dB at 8.61 GHz for 10 and 20 wt % of the BaH nanoparticles-embedded PVDF composite films. The observed SE (total) corresponding to the present composite systems shows the highest ever value of SE (total) measured to date. High value of μr and high magnetic loss of BaH nanoparticles and the high value of εr of the β-phase-improved PVDF build up these composite films as thinner, lighter, and flexible RAMs and the most suitable component for EM device applications for the high frequency switching devices, high frequency metal−oxide−semiconductor field-effect transistors, and high frequency switching mode power supplies as well as suitable for the reduction of pollution created by the EM waves in the GHz frequency region.

(9) 66

Here, the symbols have their usual meaning. The variations of SER and SEA of the composite films with frequency have been shown in Figure 10. For BaF-0, the dielectric loss corresponding to the electroactive β-phase, mainly contributes to the energy loss corresponding to the incident EM waves, whereas for BaH nanoparticles, the loss factor appears due to the existence of eddy current and it produces the magnetic loss which is much larger than the dielectric loss factor of the magnetic nanoparticles. Thus, in the case of individual materials, the magnetic and dielectric losses cannot produce a proper balance between one another, which induces weak EM wave absorption property. Mu et al. have investigated the property of microwave absorption of bare BaH nanoparticles, and according to their report, the microwave absorption capacity of bare BaH nanoparticles is very weak and also not useful for any technological applications in the GHz frequency range.40 However, in the case of BaF-10 and BaF-20, the structure modification of host PVDF by the BaH nanofillers produces an enhanced electroactive β-phase. Now, this enhanced electroactive β-phase improves the SE (total) of BaF-10 and BaF-20 to the maximum of −97.6 dB at 8.61 GHz (>99.999999999% attenuation). Therefore, these inexpensive BaH−PVDF composite films with low mass, large and controllable surface area, sufficiently small thickness, and greater flexibility can be used effectively in EMI shielding applications connected to microwave device components for the reduction of EM pollution in different high frequency switching devices. In this report, the thicknesses of BaF-10 and BaF-20 are lying within the range of ∼0.210−0.260 mm. Thus, the magnetic BaH nanofillers inside the PVDF matrix produces a large interfacial area between the BaH nanofillers and PVDF matrix and these interfaces enhanced the effective interactions between the EM waves with the composite films, which leads to the improvement of microwave absorption of the bariumhexagonal ferrite-functionalized PVDF composite films. Also, it

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Soumyaditya Sutradhar: 0000-0003-3679-2583 Notes

The authors declare no competing financial interest.

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