New physical insights into the electromagnetic shielding efficiency in PVDF nanocomposites containing multiwall carbon nanotubes and magnetic nanoparticles

Abstract

This work attempts to bring critical insights into the electromagnetic shielding efficiency in polymeric nanocomposites with respect to the particle size of magnetic nanoparticles added along with or without a conductive inclusion. To gain insight, various Ni–Fe (Ni_xFe_1−x; x = 10, 20, 40; Ni: nickel, Fe: iron) alloys were prepared by a vacuum arc melting process and different particle sizes were then achieved by a controlled grinding process for different time scales. Poly(vinylidene fluoride), PVDF based composites involving different particle sizes of the Ni–Fe alloy were prepared with or without multiwall carbon nanotubes (MWNTs) by a wet grinding approach. The Ni–Fe particles were thoroughly characterized with respect to their microstructure and magnetization; and the electromagnetic (EM) shielding efficiency (SE) of the resulting composites was obtained from the scattering parameters using a vector network analyzer in a broad range of frequencies. The saturation magnetization of Ni–Fe nanoparticles and the bulk electrical conductivity of PVDF/Ni–Fe composites scaled with increasing particle size of Ni–Fe. Interestingly, the PVDF/Ni–Fe/MWNT composites showed a different trend where the bulk electrical conductivity and SE scaled with decreasing particle size of the Ni–Fe alloy. A total SE of −35 dB was achieved with 50 wt% of Ni₆₀Fe₄₀ and 3 wt% MWNTs. More interestingly, the PVDF/Ni–Fe composites shielded the EM waves mostly by reflection whereas, the PVDF/Ni–Fe/MWNT shielded mostly by absorption. A minimum reflection loss of −58 dB was achieved in the PVDF/Ni–Fe/MWNT composites in the X-band (8–12 GHz) for a particular size of Ni–Fe alloy nanoparticles. This study brings new insights into the EM shielding efficiency in PVDF/magnetic nanoparticle based composites in the presence and absence of conducting inclusion.

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1.Introduction

The recent developments in wireless communication systems and other electronic circuitry has challenged the world to tackle the problems arising due to the interference of the waves emitted by them.^1–3 Therefore it is necessary to shield these devices which are susceptible to electromagnetic interference (EMI). To do so, studies have been carried out to develop a new and efficient material for shielding these EM waves. Conventionally, metallic materials are dominant over other materials however, due to the associated problems like less corrosion resistance, high density and difficulty in processing limits their use in applications where lightweight, flexibility and oxidation resistance is of prime importance.^2,3 Nowadays materials such as metallic filler, graphene, carbon nanotubes (MWNTs), ferrites etc. have been widely used for improving EMI shielding since these materials possess good electrical conductivity, magnetic and dielectric properties.^3–5 There are two major ways by which EMI shielding could be targeted i.e. either by reflection or absorption.⁵ It is well know that metallic materials and their alloys attenuates EMI radiations mostly by reflection whereas, magnetic particles by absorption.⁵ Magnetic materials have widely been adapted for EMI shielding application due to their high saturation magnetization, high magnetic permeability and low coercivity in the frequency range of 2–18 GHz.⁶ These materials bear high Snoek's limit due to their high saturation magnetization.⁷ The magnetic effect can further be enhanced by introducing easy magnetization plane.⁸ Many studies have reported that flakes of magnetic particles have higher saturation magnetization, high resonance peak and low eddy current losses than those of spherical particle due to their particle shape effect.^6,8 In recent past, ferromagnetic nanoparticles like iron, cobalt, nickel and their alloy have received great deal of attention due to their wide range of application in the field of nanoelectronics, medicine, magnetic data storage and catalysis.^7,9,10 However, it has been well-established that these materials show unusual combination of properties below a certain critical size.¹¹

Interestingly, the shape of the nanoparticles and their interaction with other nano entities decides the properties of the material.¹¹ However, in most of the literature, the work is focused on spherical particles. In order to compare the effect of varying particle size on EMI shielding, it is very important to critically access and correlate the effect of particle size, shape and saturation magnetization on EMI shielding in a wide range of frequency (3–18 GHz).^6,12 Therefore, the present work mainly focuses on the effect of particle size and their interaction with other nanomaterial (here MWNTs) on EMI shielding in polymer based composites. In addition, due to their one dimensional structure they assist in forming conducting network in a polymer matrix at relatively lower fraction and can help in shielding EM radiation effectively.^5,13 Hence, composites with conducting filler and ferromagnetic materials would exhibit a good balance between permittivity and permeability and thereby result in good impedance match for shielding.^8,10,14,15

In the present work, we aim to develop lightweight polymeric composites containing small volume fractions of ferromagnetic material with different particle size and shape and intrinsically conducting nanomaterials like MWNTs. Therefore, Ni–Fe alloys with different compositions were prepared by vacuum arc melting process and mixed with MWNTs and PVDF using wet grinding process to design materials for shielding EM radiations. PVDF was chosen as a matrix because of its high dielectric constant, good mechanical properties, thermal stability and chemical resistance.¹³ Wei et al. reported that the reflection loss (R_L) for spherical shaped Fe₃Al particle (−30 dB) is quite higher than those of the flakes (−10 dB) in paraffin matrix.⁷ Cao et al. showed that electroconductive adhesive of silver coated carbonyl iron powder with 0.35 mm thickness could achieve a shielding effectiveness (SE) of −38 dB in the measured frequency range (1–1.6 GHz).¹⁶ Feng et al. achieved an optimal reflection loss (R_L) of −46.7 dB for 2 mm thickness layer at 3.17 GHz whereas, a R_L of −32.78 dB was observed at 13.78 GHz with thickness of 1.3 mm for FeNi@C nanocomposite.⁸ Kim et al. studied the dispersion of iron particle (flake shaped) in rubber matrix and reported a R_L of −5 dB at 1–2 GHz with 1 mm thickness.¹⁷ Zheng et al. showed a synergistic effect of super paramagnetic Fe₃O₄ nanocrystals and graphene hybrid which showed a R_L of −40 dB at 6.8 GHz with 4.5 mm matching thickness.¹⁸ Though the effect of shape and size of the magnetic nanoparticles on the overall shielding efficiency is known but the effect in presence of conducting inclusions is less well understood. Hence, in this work, PVDF based composites were developed with different particle sizes of Ni–Fe alloy and MWNTs. In addition, their structure, morphology, microwave absorption was studied systematically. Moreover, the effect of thickness on the reflection loss is discussed here in.

2. Experimental

2.1 Materials and methods

Ni–Fe alloys with different compositions (Ni_x Fe_1−x, x = 0.6, 0.8, 0.9 wt%) were prepared by vacuum arc melting technique from high purity elemental constituents. The as-cast alloys were subjected to hot rolling followed by homogenization anneal and then air-cooled. Ni–Fe alloy flakes were obtained by filing with a diamond file. Further, different particle sizes were obtained under controlled grinding using mortar grinder over a period of time (1 h, 2 h and 3 h) yielding different sizes. The average particle sizes thus obtained are 600 nm, 300 nm and 200 nm after 1 h, 2 h and 3 h of grinding respectively. Commercially available PVDF (Kynar 741), supplied by Arkema, was used in this work. The pristine MWNTs were obtained from Nanocyl (NC 7000). Different batches were prepared by mixing PVDF powder with Ni–Fe alloy particles using a mortar pestle for 30 min and the solvent (dimethylformamide, DMF) was added drop-wise to make a paste.¹⁹ This method was adopted because of the tendency of metal powder to agglomerate during processing either in solution or in melt. The resultant paste was then vacuum dried to remove the traces of solvent followed by compression moulding at 220 °C in a laboratory scale hot press. Similar process was repeated to prepare batches in presence of MWNTs in addition to Ni–Fe particles in PVDF matrix.

2.2 Characterization

The magnetization measurement of various Ni–Fe flakes, Ni–Fe nanoparticles and the composite were obtain using vibrating sample magnetometer (VSM). The morphology of the product and the state of dispersion of the flakes and MWNTs in PVDF were investigated using field emission scanning electron microscope (Sirion XL 30). The composite samples were cryo-fractured prior to SEM. The crystal structures were carried out using PANalytical X'pert Pro (Cu Kα, λ = 0.154 nm). Co-axial method was used for evaluating the scattering parameters of toroidal samples in the frequency range of 2–18 GHz. Calibrated Vector Network Analyzer (VNA, Anritsu MS4642A) with full two-port measurement was employed to measure the reflection and transmission. The relative permittivity and permeability were calculated from VNA using the well-established Nicolson–Ross model.²⁰ The AC electrical conductivity of hot pressed discs (10 mm) was measured by using an Alpha-N analyser (Novocontrol) with a broad frequency range of 10⁻¹ to 10⁷ Hz.

3. Results and discussion

3.1 Synthesis and characterization of Ni–Fe alloy

The preparation of Ni–Fe alloy using vacuum arc melting process is schematically illustrated in Fig. 1a. The microstructure of the hot rolled and homogenized annealed alloy is shown in Fig. 1b. The obtained ingot was filed and subsequently milled to obtain nanoparticles of specific size. The SEM images of the various Ni–Fe alloy particles are shown in Fig. 1c. After 1 h of grinding the average particle size was ca. 600 nm as can be seen from Fig. 1c1. By extending the grinding time further to 2 h, the average particle size was ca. 300 nm as can be seen from Fig. 1c2. After a long period (3 h) of grinding the particle size was ca. 200 nm (see Fig. 1c3). Fig. 1d shows the X-ray diffraction (XRD) pattern for as cast Ni–Fe alloy. Three distinctive diffraction peaks at 43.86°, 50.75° and 74.86°, corresponding to (111), (200) and (220) plane of Ni–Fe alloy are indexed as face-centered cubic (FCC) γ phase of Ni–Fe alloy.²¹ Also, no impurity or any second phase was found in the XRD pattern for Ni–Fe alloy. Fig. 1e shows the hysteresis loop for Ni–Fe alloy flakes at room temperature with varying composition of iron. It has been observed that the saturation magnetization scales with concentration of iron in Ni–Fe alloy. Ni₆₀Fe₄₀ showed the highest saturation magnetization (126.59 emu g⁻¹) than the remaining two. The saturation magnetization of Ni₉₀Fe₁₀ and Ni₈₀Fe₂₀ are 58.68 emu g⁻¹ and 105.39 emu g⁻¹ respectively. Therefore, further study was carried out on Ni₆₀Fe₄₀ powder sample and the magnetic properties of Ni₆₀Fe₄₀ powder samples have been explained by using hysteresis loop between −20 and 20 kOe as shown in Fig. 1f. From Fig. 1f it is clearly visible that the saturation magnetization has decreased with decreasing particle size as compared to flakes. It is well known that ferromagnetic materials get spontaneously magnetized in the bulk. Thus, in order to minimize the magnetostatic energy, the formation of domains is favored in bulk ferromagnetic materials. However, such domain formation is not energetically favored below some critical size and the material prefers to be in a single domain.¹¹ In this case also all the atomic spins are oriented along one direction.¹¹ A decrease in coercivity from 154.94 Oe (flakes) to 69.60 (3 h) Oe is also observed after reduction of particles size.

Fig. 1 (a) Processing of Ni–Fe alloys (b) microstructure of Ni–Fe alloys (c1) SEM micrograph of Ni–Fe alloys ground after 1 h (scale 1 μm) (c2) 2 h (scale 500 nm) and (c3) 3 h (scale 200 nm) (d) XRD pattern for Ni–Fe alloys (e) hysteresis loop for Ni–Fe alloys with varying concentration of iron (f) hysteresis loop for Ni–Fe alloys with different particle size.

3.2 Synthesis and characterization of PVDF based composites

Fig. 2 schematically depicts the preparation of PVDF/Ni₆₀Fe₄₀MWNTs composite. The obtained constituents were physically mixed by dropwise addition of solvent (dimethylformamide) to get thick slurry. The prepared slurry was then vacuum dried and hot pressed subsequently to obtain composite as shown in figure. Since metal possess high density, it is very difficult to prepare composites, as they settle down during solution mixing or may get re-agglomerated during melt blending. Therefore, the unique method we adopted can give better dispersion of metal flakes/nanoparticles together with MWNTs in PVDF matrix. Fig. 2b shows the XRD patterns of as prepared PVDF/Ni₆₀Fe₄₀-MWNTs composite. It shows three additional peaks at 2θ of 18.38°, 20.13° and 26.76° in addition to FCC peaks of Ni–Fe alloy which are associated with (020), (110) and (021) planes of PVDF respectively. Fig. 2c and d shows the hysteresis loop for PVDF/Ni–Fe composites in the absence and presence of MWNT respectively. The non-magnetic PVDF causes a decrease in the saturation magnetization of magnetic particles. Interestingly it is important to note that substantial increase in saturation magnetization has been observed after the addition of MWNTs in PVDF/Ni–Fe composites. In this case the magnetic behaviour of MWNTs is governed by the presence of residual magnetic particles in MWNTs.²² The saturation magnetization for PVDF/Ni–Fe composites are 45.32, 36.38, 30.22 and 23.92 emu g⁻¹ respectively for the flakes, after 1 h, of grinding, after 2 h of grinding and after 3 h of grinding as shown in Fig. 2c. Whereas slight drift in saturation magnetization has been observed in the presence of MWNTs, the values are ca. 65.03, 41.47, 34.86, 28.66 emu g⁻¹ for the flakes, and after grinding for 1 h, 2 h and 3 h respectively as shown in Fig. 2d.

Fig. 2 (a) Preparation of PVDF/Ni–Fe/MWNTs composite (b) XRD pattern for PVDF/Ni–Fe/MWNTs composite (c) and (d) hysteresis loop for PVDF/Ni–Fe composite in the absence and presence of MWNTs.

The SEM morphology of neat PVDF, PVDF/Ni–Fe and PVDF/Ni–Fe/MWNTs composite are shown in Fig. 3 and 4 respectively. The corresponding EDAX mapping of Fe and Ni is also shown alongside. The surface roughness of the fractured surfaces increases with the addition of Ni–Fe particles/flakes (see Fig. 3b–e). In addition, the quality of dispersion is relatively good in the composites with no visual agglomeration. In the SEM morphologies of PVDF/Ni–Fe/MWNT composites, bright dots corresponding to conducting MWNTs are well evident (see Fig. 4a–d). More interestingly, an interconnected network of MWNTs and Ni–Fe particles are evident from Fig. 4d suggesting that a 3D network has been developed consisting of MWNTs and Ni–Fe. This has also resulted in enhanced electrical conductivity and will be discussed later on. From the SEM images and the elemental mapping (Fig. 5a and b), it is well evident that the Ni–Fe nanoparticles are well dispersed in PVDF. In addition, the inset of Fig. 5c illustrates the magnified image of PVDF/Ni–Fe/MWNTs composite wherein it is clearly visible that MWNTs and the Ni–Fe nanoparticles are interconnected.

Fig. 3 SEM morphology of (a) neat PVDF (b) PVDF/Ni–Fe flakes; (c–e) PVDF/Ni–Fe composites containing Ni–Fe particles subjected to grinding for 1, 2 and 3 h respectively.

Fig. 4 SEM morphology of PVDF/Ni–Fe/MWNT composites containing (a) flakes; Ni–Fe particles subjected to grinding for 1, 2 and 3 h respectively (b–d).

Fig. 5 SEM morphology and the corresponding EDAX elemental mapping of (a) PVDF/Ni–Fe flakes (b) PVDF/Ni–Fe flakes/MWNTs composite (the inset is magnified image of PVDF/Ni–Fe/MWNTs composite at 500 nm).

3.3 Synergistic improvement in electrical conductivity: effect of Ni–Fe alloy and MWNTs

Fig. 6 shows the variation of AC electrical conductivity of composite samples as a function of frequency. From Fig. 6a it can be seen that addition of 50 wt% of Ni–Fe flakes, significantly improved the electrical conductivity (3 × 10⁻⁷ S cm⁻¹) of PVDF. The value of electrical conductivity has marginally decreased from 10⁻⁶ to 10⁻¹⁰ and 10⁻¹¹ as the particle size decreased from 600 nm to 300 nm respectively. The sudden drop in electrical conductivity may be attributed due to increase in scattering of electrons at the grain boundaries.¹¹ As the particle size decreases, they have large number of grain boundaries as compared to polycrystalline material.¹¹ Therefore the resistivity of such materials is larger compared to polycrystalline materials.¹¹ From Fig. 6b it can be seen that a considerable increase in electrical conductivity (6 × 10⁻⁶ S cm⁻¹) is observed after addition of 50 wt% of Ni–Fe flakes and 3 wt% MWNTs as compared to only Ni–Fe flakes in PVDF. The effect of addition of MWNTs has played an important role in improving the bulk conductivity. From Fig. 6b it is also evident that with addition of 50 wt% Ni–Fe nanoparticles (of ca. 600 nm particle size) in addition to 3 wt% of MWNTs has further enhanced the electrical conductivity (5.39 × 10⁻⁵). After addition of ca. 300 nm sized particles, the electrical conductivity increased by an order of magnitude. Further reduction in particle size did not result in any significant improvement in the bulk conductivity. This observation is in contrast to the composites with only Fe–Ni nanoparticles where the bulk conductivity decreased dramatically on reducing the particle size to 200 nm.

Fig. 6 Dependence of electrical conductivity on frequency for (a) PVDF composites with different particle sizes of Ni₆₀Fe₄₀ alloy (b) PVDF composites with different particle sizes of Ni₆₀Fe₄₀ alloy and MWNTs.

3.4 Attenuating electromagnetic waves: effect of Ni–Fe alloy and MWNT

Shielding effectiveness is dominated by three important mechanisms viz. reflection from the shield (SE_R), absorption within the shield (SE_A) and internal multiple reflections (SE_MR). Therefore, the total shielding effectiveness of a material is the sum of reflection, absorption and multiple reflections from the shield:

SE_T = SE_R + SE_A + SE_MR.

In practical situations the multiple reflection can be neglected if the absorption loss is greater than 10 dB.²³ The above mentioned parameters are calculated with the help of scattering parameters (S₁₁ and S₁₂) using the following equations.

SE_R = 10 log₁₀(1/(1 − [S₁₁]²))

SE_A=10 log₁₀((1 − [S₁₁]²)/[S₁₂]²)

SE_T = SE_R + SE_A = 10 log₁₀(1/[S₁₂]²)

Fig. 7 shows the total shielding effectiveness of PVDF/Ni₆₀Fe₄₀ composite as a function of frequency. As one can clearly see from this figure that the SE value for PVDF/Ni₆₀Fe₄₀ composite filled with flaky Ni₆₀Fe₄₀ is greater than that of nanoparticle filled composites. The SE observed for flakes filled PVDF/Ni₆₀Fe₄₀ is ca. −20.04 dB where as for the nanoparticle filled composite the shielding effectiveness are almost similar; −17.0 dB and −17.9 dB respectively for 600 nm and 300 nm particle size. The decrease in SE may be due to the lack of connectivity between the nanoparticles as compared to flakes in the PVDF matrix. Fig. 7b shows the shielding effectiveness of PVDF/Ni₆₀Fe₄₀ composite with 3 wt% MWNTs as a function of frequency. From this figure it is noticeable that SE total has scaled up to −21.7 dB after addition of 3 wt% MWNTs in PVDF matrix. A similar trend has been observed in the electrical conductivity measurements. The SE value has drastically increased to −28.9 dB after introduction of 50 wt% of flaky Ni₆₀Fe₄₀ powder in PVDF/MWNTs composite. The SE values of PVDF/Ni₆₀Fe₄₀-MWNTs composites are −31 dB, −34 dB and −33 dB for nanoparticles filled composite for 600 nm, 300 nm and 200 nm particle sizes respectively.

Fig. 7 Shielding effectiveness (a) PVDF composites with different particle sizes of Ni₆₀Fe₄₀ alloy (b) PVDF composites with different particle sizes of Ni₆₀Fe₄₀ alloy and MWNTs.

In order to investigate the possible mechanism of attenuation, the relative permittivity and permeability of the composites were calculated using well established Nicolson and Ross²⁰ calculations. Thus, as stated the relative permittivity and permeability were determined from the reflection (S₁₁) and transmission scattering coefficients (S₁₂) in an air-filled coaxial line mode. Fig. 8 shows the relative permittivity and permeability of the PVDF/Ni₆₀Fe₄₀ composites with and without addition of 3 wt% MWNTs in the measured frequency range. Fig. 8a shows the variation of relative permittivity as a function of frequency for PVDF/Ni₆₀Fe₄₀ composites without addition of MWNTs. A substantial improvement in relative permittivity has been noted after addition of MWNTs as shown in Fig. 8b. It is well known that due to electric dipole associated with MWNTs, the permittivity of composites have been significantly enhanced after addition of 3 wt% MWNTs in PVDF/Ni₆₀Fe₄₀ composite. However, the presence of magnetic dipoles of Ni₆₀Fe₄₀ particles couldn't improve the relative permeability enough. Thus, from Fig. 8c it can be inferred that the magnetic nanoparticle alone are not effective at higher frequency range to attain enough relative permeability. However, the value of relative permeability has significantly increased after addition of 50 wt% Ni₆₀Fe₄₀ particles along with MWNTs in PVDF. This mechanism provides the support to the synergistic effect of MWNTs and Ni₆₀Fe₄₀ particles in attenuating EM radiation by absorption where the EM radiations are arrested by the interaction with electric and magnetic dipoles. Also, it is well understood that high conducting materials have low impedance hence, the effective mechanism of shielding is mostly by reflection whereas, due to higher impedance, magnetic materials attenuates EM radiation by absorption. The ideal condition for an effective absorber is ε_r/μ_r = 1. The presence of MWNTs helped in attenuating EM radiation by absorption.

Fig. 8 Relative permittivity for (a) PVDF/Ni₆₀Fe₄₀ composite without MWNTs (b) PVDF/Ni₆₀Fe₄₀ composite with MWNTs, relative permeability for (c) PVDF/Ni₆₀Fe₄₀ composite without MWNTs (d) PVDF/Ni₆₀Fe₄₀ composite with MWNTs.

Reflection loss (R_L) is another important parameter in EM radiation attenuation. R_L can be a measure of absorbing properties of material and can be calculated using relative permittivity and permeability from the well-established transmission line theory^6,24 as follow:

Z_in = Z₀(μ_r/ε_r)^1/2tanh[j(2πft/c)(μ_rε_r)^1/2]

R_L = 20 log|(Z_in − Z₀)/(Z_in + Z₀)|

where t is the thickness of the absorber, c is the velocity of the light, f is the frequency of the EM radiation, Z₀ is the impedance of vacuum and Z_in is the input impedance of the absorber.^8,14

Fig. 9 shows the variation of R_L for both with and without addition of MWNTs as a function of frequency. As shown in Fig. 9, a minimum R_L value of −55 dB is observed at 8 GHz for PVDF/Ni₆₀Fe₄₀ (ca. 600 nm)/MWNTs composite. It is worth noting that the R_L value for PVDF/Ni₆₀Fe₄₀ (flakes)/MWNTs composite is −50 dB and −45 dB at higher frequencies, 11 GHz and 13 GHz respectively. The observed R_L for PVDF/Ni₆₀Fe₄₀(2 h)/MWNTs composite at 12 GHz is −42 dB whereas for PVDF/Ni₆₀Fe₄₀(3 h)/MWNTs composite it is maximum (−42 dB) at 18 GHz. On the other hand without addition of MWNTs hardly any difference is observed in R_L values. The maximum reported R_L values for PVDF/Ni₆₀Fe₄₀(flakes)/MWNTs PVDF/Ni₆₀Fe₄₀(1 h)/MWNTs and PVDF/Ni₆₀Fe₄₀(2 h)/MWNTs composites are −42 dB, −45 dB and −46 dB at 8 GHz, 5 GHz and 12 GHz respectively.

Fig. 9 Dependence of R_L on frequency for PVDF/Ni₆₀Fe₄₀ composite (a) with MWNTs (b) without MWNTs.

The variations of reflection and absorption with frequency are seen in Fig. 10. As seen from Fig. 10a, reflection is more dominating over absorption. It is found that in conducting composite the shielding effectiveness is mainly dominated by reflection term whereas magnetic particles contribute in absorption. The fact that as the packing efficiency increases flaky particles start connecting to each other results in the magnetic network formation and this causes improvement in absorption value as shown in Fig. 10a for PVDF/Ni₆₀Fe₄₀(flakes) composite. With further reduction in particle size, the magnetic network starts becoming weaker due to increase in surface to volume ratio of nanoparticles. Below some critical size magnetic nanoparticles start becoming anisotropic with all their atomic spins oriented in one direction which leads to decrease in magnetization hence absorption contribution scaled down as shown in Fig. 10a.

Fig. 10 Absorption and reflection contribution to total shielding effectiveness for PVDF/Ni₆₀Fe₄₀ composite (a) without MWNTs (b) with MWNTs.

After addition of MWNTs to PVDF/Ni₆₀Fe₄₀ composite, interesting results have been observed as shown in Fig. 10b. Here, the absorption factor starts dominating over reflection as the particle size decreases from flaky shape particle to nanoparticle. Addition of MWNTs has enhanced the connecting network between filler particles and MWNTs, resulted in better connectivity and dispersion in PVDF matrix. Though the flaky particle showed highest absorption value compared to nanoparticles in absence of MWNTs, reverse trend is observed in the presence of MWNTs. It is interesting to note that MWNTs are playing vital role in improving absorption properties in conjunction with magnetic nanoparticles. The magnetic nanoparticles associated with MWNTs during their synthesis²² start trying to connect with Ni–Fe nanoparticles within the PVDF matrix resulting in the formation of magnetic domain hence the substantial increment in absorption value been observed as compared with p-MWNTs. The maximum value of absorption reported here is around −27 dB with total shielding effectiveness of −33 dB as shown in Fig. 10b. Interestingly, at particular critical size with further reduction in diameter from 300 nm to 200 nm the absorption value scaled down. This decrement in absorption can be due to the loss of connectivity between MWNTs and magnetic nanoparticles since as the particle size decreases surface to volume ratio increases therefore less number of particles are available to occupy the total volume of the composite and ultimately they starts losing their connectivity with other conducting material (MWNTs).¹¹ Therefore the optimized condition for attenuating EM radiation mainly by absorption could be proposed by addition of magnetic nanoparticles of average 300 nm (2 h) particle size in PVDF/MWNTs composite. Reflection loss tells us about the matching frequency where the absorption value is maximum whereas, SE_A is the total amount of radiation absorbed by the shield. Hence, reflection loss and absorption loss are two different quantities and give information about the applicability of the shield under different frequencies. More interestingly, Fig. 11 illustrates the effect of thickness on R_L. It is intriguing to note that shield thickness of 2.5 mm can absorb almost 99.99% of the incoming radiation as manifested from their minimum R_L of >50 dB. Hence, such materials can further be explored for lightweight EMI shielding materials for a wide range of application.

Fig. 11 Reflection loss as a function of frequency and thickness for various shielding material.

4. Conclusions

This study provides new physical insights into the EM shielding efficiency in PVDF/magnetic nanoparticle based composites in presence and absence of conducting inclusion. Various Ni–Fe (Ni_xFe_1−x; x = 0.6, 0.8, 0.9; Ni: nickel, Fe: iron) alloys were prepared by vacuum arc melting process and different particle sizes were designed by controlled grinding process. As the saturation magnetization scaled with the concentration of Fe in the alloy, different PVDF based composites were hence prepared, with Ni–Fe alloys with the highest percentage of Fe, with or without multiwall carbon nanotubes (MWNTs). Contrasting effects were noted in PVDF/Ni–Fe and PVDF/Ni–Fe/MWNT composites in terms of bulk electrical conductivity and the SE. The composites without MWNTs shielded mostly by reflection whereas, absorption dominated in presence of MWNTs. A minimum reflection loss of −58 dB was achieved in PVDF/Ni–Fe/MWNT composites in the X-band (8–12 GHz) for a particular size of Ni–Fe alloy nanoparticles. This study can help in designing lightweight polymeric nanocomposites for EM shielding.

Acknowledgements

The authors like to acknowledge Department of Science and Technology DST, India for financial support. Authors also like to acknowledge Prof. S. Subramanian for extending grinding machine facility.

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