Physical Insight Into the Mechanism of Electromagnetic Shielding in

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C: Energy Conversion and Storage; Energy and Charge Transport

Physical Insight Into the Mechanism of Electromagnetic Shielding in Polymer Nanocomposites Containing Multiwalled Carbon Nanotubes and Inverse-Spinel Ferrites Sourav Biswas, Sujit Sankar Panja, and Suryasarathi Bose J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05867 • Publication Date (Web): 10 Aug 2018 Downloaded from http://pubs.acs.org on August 11, 2018

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The Journal of Physical Chemistry

Physical Insight into the Mechanism of Electromagnetic Shielding in Polymer Nanocomposites Containing Multiwalled Carbon Nanotubes and Inverse-Spinel Ferrites Sourav Biswas a, Sujit S. Panja a*, Suryasarathi Bose b* a b

Department of Chemistry, National Institute of Technology, Durgapur 713209, India. Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India.

ABSTRACT A surge in the usage of electronic devices has led to a new kind of problem; electromagnetic interference (EMI). In a quest towards providing effective shielding; which offers design flexibility, lightweight, easy-to-integrate and embed, right combination of materials need to be synthesized and dispersed in a polymer matrix to design composites that can shield EM radiation. However, selection of nanoparticles from a vast library is quite challenging and hence, this study attempts to provide a physical insight into the mechanism of shielding in polymer nanocomposites containing a conducting phase (here multiwalled carbon nanotubes, MWCNTs) and a magnetic phase (here inverse-spinel ferrites, MFe2O4 (M= Fe, Co, Ni)). We adopted a bi-phasic co-continuous blend (consisting of polycarbonate, PC and polyvinylidene fluoride, PVDF) as the matrix to incorporate the conducting and the magnetic phases. MWCNTs, which offers inter-connected conductive fence and ferrites which provide magnetic dipoles that couple with incoming EM radiation can absorb the incoming EM radiation. The detailed mechanistic insight regarding absorption of EM radiation reveals that high saturation magnetization, high consolidated loss, better impedance matching, higher attenuation constant, high hysteresis loss and comparable eddy current loss help Fe3O4, compared to other ferrites employed here, to effectively shield EM wave in X and Ku band frequency through absorption. In addition, better impedance matching, low skin depth and enhanced dielectric and/or interfacial polarization losses due to π-electrons in MWCNTs suggest synergistic effect from both the phases. As a result, -31dB shielding effectiveness is observed in the case of Fe3O4 and MWCNTs which is 19% higher when compared with CoFe2O4 + MWCNTs and 24% higher when compared with NiFe2O4 +MWCNTs containing blends. Interestingly, when the nanoparticles are forced to localize in different components of the blends overall shielding efficiency enhances further due to their higher consolidated loss parameters. Hence, the mechanistic insight provided in this paper will help guide researchers working in this field from both academic and industry perspective. 1 ACS Paragon Plus Environment

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Introduction We live in an environment that has never before occurred in nature, one that is teeming with varying levels of electromagnetic (EM) radiation. EM radiation comprises of energy combining electrical and magnetic fields.1-3 These include the entire array of EM wave spectrum: from very long-wave radio waves towards one side to X-rays and gamma rays on the other side of the spectrum. Living and non-living entity in nature is continually being exposed to a natural background of EM radiation that originates from space.4 But a substantial extent of the enormous radiation originating from space is consumed by the atmosphere and just a little segment has reached the ground. However, there is no such filtering of radiation originating from the earth itself. Every single living life is evolutionarily adapted to such natural radiations in their common habitats.5 But, we, the most edified creatures of the earth utilize high-end technologically advanced electronic gadgets which emit such radiation as an offshoot. Despite the fact, fundamental for improvement assuming live, exposure to the overabundance of such radiation beyond the naturally evolved tolerance limits causes a catastrophe everywhere throughout the world.6-8 Thus curbing such consequences which are inferable from this radiation, shielding has turned into a prime essential for new appliances, due to different agencies fixing standards for electromagnetic compatibility.1, 9-11 Not only for its threat to the environment but also stealth technology has also ensured a high demand for such shielding materials in the current scenario of the world. Therefore burgeoning research is mainly focussed on to construct obstructive enclosures to isolate the core circuitry from the exterior environments so that the generated EM radiation cannot interfere with other equipment.12-13 This is not the first time, it has begun since the nineteenth century and extensive accomplishments have just been seen in both fundamental and materialistic research. Irrespective of all burdening, due to technological requirements, strategies have changed from metal sheets to polymer nanocomposites.14-17 Although the fundamental requirements are fixed, which are for the most part of the conductivity and lossy dielectric or magnetic permeability, yet the rapid growth in the new materials synthesis make this field predominantly more problematic?3, 18-20 The amalgamation conventions of late contrast from the single to hybrid nanomaterials delineating to amass each and every key property in a solitary composite for better shielding efficiency.21-23

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Ferrites have gained enormous attention due to its GHz matching frequency.24-27 But the electrical insulating property limits their usage.28 However, researchers find another route by adding conducting phase together with the ferrite nanomaterials which shows better efficiency. Wang et al showed the effect of graphene based Fe3O4 nanohybrid on high EM absorption efficiency because of its improved specific surface area and enhanced interfacial polarization losses.29 Further Qu et al showed the effect of coupling of hollow Fe3O4-Fe nanoparticles on graphene sheets in EM shielding application.30 They have claimed that the better impedance matched phenomena and polarization losses are the most important factor for better reflection loss value. Cao et al showed that decoration of NiFe2O4 nanoparticles on graphene oxide sheets is also effective for better attenuation.31 Carbon nanofiber and conducting polymer coated ferrite (Fe3O4, CoFe2O4 and NiFe2O4) are also studied by several groups which showed superior EM shielding properties.32-36 Pawar and co-workers extensively investigated the effect of various ferrite nanomaterials and their conjugation with graphene oxide sheets and MWCNTs in PC/SAN polymer blend system.37-39 In all cases, selective localization of nanoparticles, effective filler concentration and synergistic property obtained from different types of nanomaterials has played a significant role in EM absorption efficiency. In our earlier work, we also have studied the effect of different ferrite nanomaterials along with conducting MWCNTs in polymer blend system and we have found that saturation magnetization, different types of associated losses and the dispersibility of such materials play a major role for gaining the synergistic contribution.40-43 A mechanistic insight, in our opinion, would help guide the researchers working in this field and hence, this study aims to bring in the physical insight into the mechanism of shielding in polymer nanocomposites containing conducting phase and a magnetic phase (here inverse spinel ferrites). Most literature reports the shielding effectiveness of polymer nanocomposites containing either magnetic nanoparticles or conducting phase or a combination of both. As the particle size, shape and surface functional groups of the nanoparticles vary widely; it is difficult to compare their efficacy in shielding the incoming EM radiation. It is imperative to understand the rationale behind choosing a particular nanoparticle and get a mechanistic insight rather than synthesizing new functional nanoparticles. Herein, we have made an attempt to compare different spinel ferrites based on their saturation magnetization and coercivity as these two factors contribute towards shielding the magnetic fields of the incoming EM radiation. The conductive fence (here MWCNTs) has been fixed so that a clear mechanistic insight in the 3 ACS Paragon Plus Environment


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dielectric loss arising from interfacial polarization can be well understood. In addition, two approaches have been attempted here as to how the nanomaterials are mixed with the blend components. In the first case, all the nanoparticles are mixed along with the blend components and the second case, they were forced to localize in the different components of the blends by suitable surface functionalization. While, ferrites contribute to the tuning of magnetic permeability parameters of the blend, the preferential localization of the conductive phase controls the overall shielding performance. A facile one-pot hydrothermal synthetic protocol for synthesis of different ferrites (Fe3O4, CoFe2O4 and NiFe2O4) by using similar type of precursor can allow us to get a mechanistic insight into the mechanism of shielding as other factors like size and shape being fixed here. Experimental Materials FeCl3,

FeSO4.7H2O,

CoSO4.7H2O,

NiSO4.6H2O

and

NaOH

bead,

chloroform,

tetrahydrofuran and DMF were obtained from different viable sources. Polycarbonate (Lexan-143R) was obtained from Sabic. PVDF (Kynar-761) was acquired from Arkema. MWCNTs (d=9.5 nm, l=1.5 µm) were acquired from Nanocyl SA (Belgium). Synthesis of different ferrite nanoparticles In all cases a one pot hydrothermal reaction technique was utilized for synthesis of different ferrites nanoparticles. The typical hydrothermal reactions were carried out in a stainless-steel autoclave with a 50 ml Teflon liner under autogenous pressure at 160⁰ C for 20h. For the synthesis of Fe3O4 nanoparticles, 0.01 M FeSO4.7H2O and 0.02 M FeCl3 were dissolved in 30 ml DI water. Then 10 ml NaOH solution was added drop-wise to the mixture to maintain the pH of the solution 10. Finally, the mixture was bath sonicated for 15 min and then transferred into the autoclave. On a similar way for the synthesis of CoFe2O4 and NiFe2O4, we have used CoSO4.7H2O and NiSO4.6H2O instead of FeSO4.7H2O. After completion, the reaction mixture was cooled and washed with H2O. Finally, the product was then dried in vacuum at 80⁰ C for 24 h. we have schematically described the synthetic protocol in scheme 1.


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Scheme 1: Synthetic protocol of different ferrite synthesis Blends preparation method and compositions All the polymer blends 50/50 (w/w) PC/PVDF with filler nanomaterials were processed by utilizing a melt compounder (Hake minilab-II) with a rotational speed of 60 rpm at 260°C for 20 min under N2 atm. All blends contain 3 wt% MWCNTs where ever it has used. Reasons behind the choosing of such concentration of MWCNTs are mainly percolation limit, cost effective and relatively mechanical properties which we have explored in our previous reports.21,39 Additionally, we also fixed the amount of ferrite nanoparticles which is 3 wt% in all blend composition, although the composition of ferrites changes from Fe3O4, CoFe2O4 and NiFe2O4. Characterization TEM (FEI Technai T20) worked at 200 kV was used to assess the microstructure of synthesized nanoparticles. Sirion XL30 FEG SEM was used to evaluate the microstructure of synthesized nanoparticles and morphology of blends. It is worth to mention that to improve the contrast between the phases of blends we have removed PC phase from the cryofracture blend samples prior to acquire SEM micrographs. XPERT Pro (PANalytical) XRD was utilized to record the crystal data (with Cu Kα radiation source, λ = 1.5406 Å, 40 kV and 30 mA). Room temperature magnetism of ferrite nanoparticles was studied by a VSM (Lakeshore). Alpha-N Analyser, (Novocontrol) was utilized for electrical conductivity measurements. Electromagnetic shielding efficiency was assessed by VNA (Anritsu MS4642A), where a co-axial adapter (Damaskos MT-07) was utilized to measure the scattering parameters of toroidal samples.



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Result and discussion Characterization of different synthesized nanomaterials Herein, we adopted hydrothermal synthetic protocol to synthesize various ferrite nanoparticles form their salt precursor. Figure 1 depicts the TEM micrographs of synthesized Fe3O4, CoFe2O4, and NiFe2O4 nanoparticles. It is observed that the average size of cube shaped Fe3O4 nanoparticles is ca.61 nm (Figure 1a). In case of distorted cube shaped CoFe2O4 nanoparticles the average size varies between 55-62 nm (Figure 1b). However, by following similar reaction conditions we observe that synthesized NiFe2O4 nanoparticles are spherical in nature and the average diameter is in the range of 59-65 nm (Figure 1c). HRTEM images and SAED patterns additionally confirm the nanocrystalline nature of each of the synthesized nanoparticles (Figure 1d-f). In all cases, lattice fringe spacing showed in Figure 1d-f, are consistent with the lattice spacing of cubic magnetite in (311) planes. The diffraction patterns confirm that all synthesized nanoparticles are typical face-centered cubic inverse spinel structure with two different lattice sites. Tetrahedral sites which are filled by the Fe (III) and octahedral sites are filled by M (II) and Fe (III). Each primitive cell overlaid with two Fe (III) in tetrahedral site and four in octahedral sites in which two are M (II) and two are Fe (III).44 Corresponding SEM micrographs and EDS confirm the structure, shape and exact compositions of the synthesized ferrite nanoparticles (Figure 2). EDS also confirms the atomic ratios of Co and Fe in CoFe2O4 and Ni and Fe in NiFe2O4 are 1:2. The XRD patters of various synthesized nanoparticles are presented in Figure 3a. The peaks were identified and indexed for matching with JCPDS (Joint Committee on Powder Diffraction Standards) file numbers. The obtained XRD patterns of all nanoparticles are fully consistent with the previously shown SAED pattern of cubic inverse spinel structure. The size of the nanocrystallites was calculated by Scherer’s equation (Eq. 1), based on the intense (311) peak, Dxrd = 0.9 λ/β cosθ

(1)

where, average crystallite size is denoted by D, wave length of X-ray beam is λ, β is the full width of diffraction line at half of the maximum intensity and θ is the Bragg’s angle.45 The Dxrd of Fe3O4 is 59 nm, for CoFe2O4 is 58 nm and for NiFe2O4 is 63 nm which echoes with the SEM and TEM images. Figure 3b depicted the room temperature magnetic hysteresis loops of all the synthesized ferrite nanoparticles. While looking at the impact of the 6 ACS Paragon Plus Environment

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compositions of synthesized nanoparticles, we discovered intriguing varieties regarding saturation magnetization, coercivity and remanence. It is well observed form the literature that magnetic moment of the individual sub-lattices in the inverse-spinel structure is the key property for determining the overall magnetic properties.46 Exchange interaction between electrons and ions in these sub-lattices of each inverse-spinel structure has different value due to the presence of different number of unpaired electrons in each individual central metal ions.47

Figure 1: TEM of (a) Fe3O4, (b) CoFe2O4 and (c) NiFe2O4, HRTEM of (d) Fe3O4, (e) CoFe2O4 and (f) NiFe2O4 on inset corresponding SAED patterns. As a result of this, highest saturation magnetization is observed in case of Fe3O4 nanoparticles, which is 65 emu.g-1. However, the saturation magnetization of CoFe2O4 and NiFe2O4 are quite similar which are 51emu.g-1 and 46 emu.g-1 respectively. Coercivity and remanence of all nanoparticles are listed in Table S1, which clearly depicts the ferromagnetism characteristics of all synthesized nanoparticles. The depletion of magnetic property of different ferrites nanoparticles may also be attributed to the presence of disordered spins of surface atom that prevent the core spins to align the field direction. This surface layers usually called magnetic dead layer (MDL), which can be estimated by, Ms=Msbulk (1-6t / d)

(2) 7 ACS Paragon Plus Environment


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where, the bulk saturation magnetization is denoted as Msbulk, t is the MDL thickness and d is the average size of the particles.43 Several factors including reaction conditions and growth mechanism of nanoparticles along with the surface spin canting are involved in enhancing the thickness of such dead layers which mainly reduces the saturation magnetization. We have calculated MDL thickness of each ferrite nanoparticles which are being synthesized by similar protocol. Interestingly, we have found that MDL thickness varies in this order; Fe3O4> CoFe2O4> NiFe2O4 which are recorded in Table S1. So, it is obvious from the above discussion that synthesized Fe3O4 nanoparticles have higher magnetic property than other two ferrites.

Figure 2: EDS spectra of (a) Fe3O4, (b) CoFe2O4 and (c) NiFe2O4 on inset corresponding SEM micrographs 8 ACS Paragon Plus Environment

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Figure 3: (a) XRD and (b) VSM data of synthesize ferrite nanoparticles Characterization of blend structures with fillers localization Polymer blends, where two or more polymers are integrated to produce novel material with arrays of optimized properties focused towards specific applications. But the final properties of the blend are strongly reliant on the interaction of ingredients and developed morphology during processing. Literature suggests that absence of any particular interaction between PC and PVDF drives a heterogeneous morphology.40 Here, we observe a co-continuous type of morphology in 50/50 (w/w) PC/PVDF blend composition which is delineated from Figure 4a, where PC phase was selectively etched out by dissolving in chloroform to improve contrast. However, after incorporation of different filler nanomaterials, the co-continuity is not altered although more coarse morphologies are obtained (Figure 4b-d). Presently it is established that limitation of nanofillers in one particular phase of the blend structure is an ingenious route to improve the optimum properties at lower filler concentration. The reason to choose PC/PVDF blend is to restrict the nanofillers localization in a specified phase (here PVDF).3 SEM micrographs of different filler containing blends efficiently show the network of MWCNTs in the PVDF phase as PC was etched out (Figure 4b-d). However, ferrite nanomaterials are not clearly visible here due to their lower content in the blends. So, for more clear understanding we have performed selective dissolution test for ferrite containing blends and observed that all nanofillers are confined in PVDF only without any migration to PC (Figure 4b-d inset).



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Figure 4: SEM of (a) 50/50 (neat) PC/PVDF blend, (b) Fe3O4 and MWCNT contain 50/50 PC/PVDF blend, (b) CoFe2O4 and MWCNT contain 50/50 PC/PVDF blend and (c) NiFe2O4 and MWCNT contain 50/50 PC/PVDF blend. On inset solution dissolution test where (1) clear solution of chloroform dipped samples suggests no migration of filler nanomaterials at PC whereas (2) black color DMF solution suggests all filler nanomaterials are at PVDF only. Comparative EM shielding efficiency of the synthesized nanomaterials Shielding efficiency of EM waves is the logarithmic proportion of incident and transmitted power and is expressed in dB.40, 48 As showed by Schelkunoff’s theory, the incident power can be isolated in three segments; reflected, absorbed and transmitted power, when the approaching EM waves encounter any shield.18 Therefore, an advanced incentive in shielding 10 ACS Paragon Plus Environment

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efficiency infers less measure of energy which is experiencing by the shield, where an extensive portion of the radiation is either reflected or consumed.49-50 Total shielding effectiveness can be presented as follows,51

SET = SER + SEMR + SEA = 10 log (Pt/Pi)

(3)

However, generally, it is demonstrated that when the SET is equivalent or higher than 15 dB, the fundamental assumption is adequately high to avoid the multiple reflection parts.52 In this study, we assessed that total shielding efficiency varies with the frequency by utilizing scattering parameters which are specifically accessible from the VNA. In all cases, the thickness of the samples is 5 mm. Figure 5a exhibits total shielding efficiency of various blend structure at X (8-12 GHz) and Ku (12-18 GHz) frequency band region. As the polymer matrix is inherently insulating in nature, neat 50/50 (w/w) blend is transparent to EM wave while infiltration of 3 wt% MWCNTs resulted in a reasonably high shielding efficiency (21dB). On a comparable note, the addition of just 3wt% insulating ferrite nanoparticles into the blend system was not effective in hoisting the shielding effectiveness (Table S2). However, the addition of MWCNTs together with the ferrite nanoparticles displayed better outcome, -31 dB (Figure 5a). The perceived alteration in the shielding efficiency can be attributed towards the development of conducting network inside the matrix which is an essential necessity of any EM shield. From our previous study, it is understood that selective localization of MWCNTs in the preferred phase of the PC/PVDF blend system improves the electrical conductivity and it can be additionally tuned up by the double percolation effect.40 Figure 5b displays that the dc conductivity drops on addition of ferrite nanomaterials together with the MWCNTs while raising the shielding efficiency from -21 dB to -31 dB. This observation is more interesting when we have differentiated the total shielding efficiency into reflection and absorption part at 18GHz frequency which are presented in Figure 5c. So, it is clearly observed that shielding by reflection is the major mechanism of shielding when only MWCNTs are present in the blend. However, addition of ferrites along with MWCNTs resulted in shielding by absorption rather than reflection. This drives us to study the mechanism of shielding in more detail especially in presence of hybrid nanomaterials.



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Figure 5: (a) SET, (b) DC electrical conductivity, (c) absorption and reflection part at 18 GHz of various blends Mechanism of EM shielding It is well understood that reflection is the main mechanism of shielding in highly conducting materials like metals. Theoretically, the magnitude of shielding by reflection can be assessed by this equation, SER = -10

(4)


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where, conductivity of the shield is denoted by σT and relative permeability of the shield is µr.53 However, mechanism of shielding is more complex for filled polymer blends as opposed to homogeneous conductive hindrance.54 Incorporation of conducting nanomaterials inside the blend exhibits 29% of absorption although the overall mechanism is reflection dominated. Here the absorption mostly emerges due to two distinct variables. Conducting MWCNTs in the preferred phase generally improves the charge transport inside the blend. But the presence of insulating polymer matrix between the conducting networks of MWCNTs hinders the charge transport and electron-hopping dominates. By fitting power law to the obtained ac electrical conductivity data also conveys electron hopping mechanism as exponent n is greater than 0.7 (Table S3). Now, this electron-hopping reduces the bulk electrical conductivity due to the formation of micro-current network, which for the most part encourage the absorption driven shielding due to dissipation of charge through heating. Besides, multiple internal reflections from the network of MWCNTs additionally contribute to the absorption efficiency. However, shielding by reflection in the case of only MWCNTs begins to suggest high impedance miss match. Such uncontrolled surface reflection may appear as a secondary source of EM radiation. However, addition of ferrite nanoparticles together with MWCNTs improves the impedance matching and limits the surface reflection (Figure S1) and enhances the absorption from 29% to 65%. The theoretical assumption additionally strengthened our understanding that absorption emerges as an effect of energy losses and successive dissipation as heat when a harmonic electromagnetic wave encounters at an adequate angular frequency with the surface of the shield material. Presently, the dielectric permittivity and magnetic permeability parameters are mainly assessed in order to understand the microwave-matter interaction. These two parameters comprise of two different parts of the permittivity and permeability.55 Real parts mainly represent the storage ability and the imaginary part corresponds to the losses which are depicted in Figure 6.56 So for the realization of actual EM absorption mechanism, the imaginary parts of both parameters are discussed here. Permittivity loss originates due to the consolidation of conductivity loss and polarization losses. As we have discussed earlier, that a drop in the conductivity due to impeded charge transport, mainly generates conductivity losses. The incorporation of ferrite nanomaterials together with MWCNTs mainly supplements the barriers in charge transport, which enhances the exponent value (Table S3) as well as conduction loss. Now, the resistance concerning the conduction loss clearly imposes obstruction to the mobile charge carrier which causes heating by the impact of charged molecules with adjacent insulators. In addition, polarization loss is mainly attributed here due 13 ACS Paragon Plus Environment


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to the interfacial polarization of different dielectric materials. Mainly, dipoles of different dielectric materials are sensitive to incoming electric fields of EM radiation and will have a tendency to align itself with the field by oscillation.57 However in the GHz frequency region, the dipoles do not have adequate time to react to the oscillating field, because of this phase lag, they collide with each other and the power is dissipated to produce heat in the material.58 More importantly here we have used MWCNTs as conducting fillers which has its own π electron cloud, can also be induced through the incident electric field of the EM wave. However in the other hand insulating ferrite nanomaterials cannot couple with such charges during their orientation induced by the EM field and dissipates more energies through heating which is mainly called Maxwell-Wagner polarization effect.58 Now, the consolidated dissipated power through dielectric heating can be calculated per unit volume power loss factor by, P = ω ‫׳׳‬

(5)

now ω=2πf, where f is the frequency of EM wave, represent the value of electric field strength at a particular point, permittivity of the free space corresponds to and effective ‫׳׳‬

dielectric loss factor is . The effective dielectric loss factor can also be expressed as, ‫׳׳‬

‫׳׳‬

‫׳׳‬

= + "#$ %$

(6)

So, as a combination of these two loss factors, permittivity loss of the blend structure or the effective loss rises sharply as depicted in Figure 6b. Now, the elevation in effective dielectric loss ensures the amplified generation of dissipated heat than only MWCNTs containing blend. On the other hand, magnetic ferrite nanomaterials are incorporated in a nonmagnetic matrix and the demagnetizing field is caused by the magnetic poles on the surface of the magnetic fillers, as a result both permeability parameters are improved (Figure 6c-d). In the subsequent section we have discussed the effect of permeability parameters in dissipation of incoming EM waves, elaborately. So, it is now understood that after incorporation of ferrites in the blend structure, both dielectric and magnetic loss parameters are enhanced which result in better absorption efficiency rather than reflection due to higher amount of heat dissipation of incident energies.


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Figure 6: (a) real and (b) imaginary part of complex permittivity and (c) real, (d) imaginary part of complex permeability Mechanistic comparison of shielding efficiency in different ferrites So it is well proven from the above discussion that inclusion of ferrite nanomaterials induces both the loss parameters which enhance the absorption ability of the shield material and it is well consistent with literatures.59-60 However, in the event that we nearly investigate the total shielding effectiveness and absorption reflection part of it, are not similar in all ferrite nanomaterials. DC electrical conductivity data exhibits comparable conductivity in all cases resembles quite similar permittivity loss parameter. So it is clear that elevation of total



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shielding efficiency as well as absorption ability mainly attributes only to the permeability parameters. Theoretically shielding by absorption is assessed by this equation,61

1/2 ) .

SEA = -8.68t(

(7)

It is also interesting to note that initial permeability (µi) of any materials mostly relies upon their Ms values followed by this equation, µi = Ms2/(aKHcMs + bλξ )

(8)

where ξ is elastic strain, K is proportionality coefficient, magneto restrictive constant is λ, and compositions of materials are represented by the constant a and b.62 From Figure 3b, we observed a trend in saturation magnetization which is Fe3O4> CoFe2O4> NiFe2O4. So it is clear that Fe3O4 possess a very high magnetic permeability which makes it superior for EM shielding application. Now the current research suggests us that high initial permeability indicates more magnetically lossy materials that can dissipate the incident EM energies through heat.60 It is well established in the literature that dissipated heat energy is mainly originated due to induction heating in an alternating magnetic field. Associated power loss fundamentally initiates from hysteresis, eddy current loss and residual loss.61,

63-65

The

hysteresis loss originates due to irreversible magnetization in the applied alternating magnetic field. Generally, the area under the hysteresis loop (Wh) is the measure of total hysteresis losses. Now the generated power loss can be evaluated as, Ph=Wh f

(9)

where, f is frequency. In the alternating magnetic field, hysteresis loop also varies with the amplitude of the frequency parameters. So, in a more simplified way when ferrite materials are encountered with an incoming alternating magnetic field, the magnetic dipoles will oscillate as magnetic poles change their alignment in each cycle.57 Now, because of this rapid flipping of magnetic domain in ferrite nanomaterials considerable frictions and heating is generated resulting in power loss. However, VSM data which is listed in Table S1, clearly shows that the coercivity is in the order Fe3O4> CoFe2O4> NiFe2O4. So, it is expected that the permeability loss and heat dissipation would also be in that order. Now, the eddy current loss is the other contributor in permeability loss parameter as well as joule heating due to eddy current induced by the altering magnetic field.66 Figure 7a, depicts the calculated eddy current loss of ferrite containing blends in respect of frequency. All the ferrite nanoparticles


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containing blends exhibit similar kind of eddy current loss which suggests similar amount of space charge accumulation at the outer surfaces because of their saturation magnetization value which partially alters the incoming electromagnetic field density. So due to the eddy current loss, ferrite nanomaterials actually widens up the interaction with incoming electromagnetic field and maximizes the dissipation through heat. On the other hand, the complex identification technique of residual loss makes this field more complicated. However, it can be calculated from the various relaxation effect of magnetization in low frequency. So, taken together we now understand that eddy current loss, and hysteresis loss in the case of Fe3O4 dissipates maximum portion of interacted EM energies through heat. Now for estimation of associated dielectric and magnetic loss parameters of each blend with various ferrite nanomaterials we have calculated consolidated tangent loss parameters (tanδε + tanδµ). Figure 7b, exhibits a significant enhancement in loss parameters after incorporation of ferrite nanoparticles into the blend system together with MWCNTs which clearly suggests amplified total shielding efficiency. Now in comparison to ferrite nanoparticles, a trend in consolidated loss parameters is observed which is Fe3O4> CoFe2O4> NiFe2O4. As a result similar type of trend is observed in total shielding effectiveness and absorption measurement. It is not the fact that only consolidated loss parameter play a key role in elevating the shielding efficiency but attenuation constant additionally which is displayed in Figure 7c also explains the differences. The attenuation constant is calculated theoretically in combination with different complex parameters as defined in our earlier study and displayed with frequency variation.43 Besides, SEMR originating from the inner surface of the shield material also contribute here as, absorption. Mainly SEMR decrease the SET when the shield is thinner than the skin depth.40 Hypothetically, when the electric field of the incident plane wave penetrates into the shield material, the intensity of the field drops exponentially with the depth of the shield and at the point when the depth of the field drops to 1/e of the incident value; it is called skin depth of that material. Now literature shows that, skin depth (δ) can be evaluated theoretically by this equation,16,67 δ = (&'()* )-1.

(10)

So it is clear that both conductivity and magnetic permeability are governed the δ of that material. Figure 7d, clarifies the effect of ferrite nanoparticles along with conducting MWCNTs on alteration of skin depth value at 18 GHz. Such result further specifies the boost in absorption of incident EM waves as, 17 ACS Paragon Plus Environment


δ = -8.68

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(11)

+,-

where t is the thickness of the material.60 So, lowering the skin depth value simultaneously lifts its practical applicability in lowering thickness and enhancing the absorption driven shielding efficiency. In the case of different ferrites, previously obtained trend is again valid in lowering the skin depth of the shield materials from 3.4 mm for NiFe2O4 to 2.1 mm for Fe3O4. In the current scenario researchers are more focused to meet the modern technological requirements which are dominated by lightweight, corrosion free and easy to integrate systems. By considering this we have strengthened our discussion by evaluating the thickness. It is obvious that below the skin depth of any shield material, shielding efficiency will decrease drastically.43 But interestingly by lowering the skin depth here we have managed to limit a considerable high shielding efficiency up to 2 mm thickness irrespective of the nature of ferrite nanomaterials. The result is more pronounced in the case of Fe3O4 nanoparticles in particular. However, a decrease in total shielding efficiency is observed in all cases from 5 mm to 1 mm thickness, mainly due to the lessening of inter-connected conducting mesh which is arbitrarily located in each shield (Figure S2). So as a whole, the above discussion clearly suggests the superiority of Fe3O4 nanoparticles over CoFe2O4 and NiFe2O4 in better EM wave absorption mechanism.


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Figure 7: (a) Eddy current loss, (b) total loss parameters, (c) attenuation coefficienct and (d) skin depth values of different blends Synergistic contribution from hybrid nanomaterials towards high EM shielding efficiency As we stated earlier that the exact shielding mechanism is difficult to predict in polymer nanocomposites containing various functional nanomaterials as compared to homogeneous materials. As we have showed in the earlier section that MWCNTs alone are incapable for shielding through absorption. On a similar note inclusion of only ferrites nanomaterials in the blend below the percolation limit also showed very low EM shielding effectiveness. In addition by following the Maxwell-Wagner model for a two layer system consisting of high



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permittivity materials such as ferrites and low permittivity materials such as polymers, the relaxation frequency at which absorption occurs vary according to this equation, ω=σ/ε

(12)

Many literatures also suggest that ferrite nanomaterials are only active in lower frequency shielding application due to their Snoek’s limit.60,

68

In a perfect situation least surface

reflection is required because of its high impedance matching between the incident EM wave and external surface of the shield material for amplifying the interaction of EM waves with the inward surface of the shield material. So the surface conductivity ought to be as low as conceivable besides connectivity being the fundamental requirement. In the present study combination of ferrite nanomaterials together with MWCNTs into the blend decrease the overall conductivity by imposing more resistance in the conducting network. This phenomenon actually widens up the charge dissipation at the inner surface of the material by reducing the prompt surface reflection during interaction with any incident EM wave at the shield surface. Now the interacted EM waves encounter with the assortment of perceptible heterogeneous structure, absorption and scattering of energies because of the local field variety and charge aggregation by the Maxwell-Wagner-Siller polarization at the interface position of the heterogeneous dielectrics and different types of the conduction losses.3 The complex parameters and associated conductivity likewise disperse the energies because of polarization, hysteresis loss and eddy current losses which finally dissipate maximum portion of interactive energies through heat. The multiple reflections from the internal surface of the materials additionally improve the absorption phenomena. So it is clear that a synergistic contribution from both conducting and magnetic nanoparticles is essential for enhancing the total shielding efficiency through absorption which is schematically represented in Figure 8a. Finally, it is clearly observed form the above mechanistic study that absorption is governed by synergistic effect from both conducting and magnetic nanomaterials. However the choice of ferrite nanomaterials controls the shielding effectiveness due to its interactions with the incident EM waves. High saturation magnetization, high consolidated loss parameters, high attenuation constant, high impedance matching and low eddy current losses are the prime factors for the elevation of shielding effectiveness which is portrayed in Figure 8b, by considering their individual values at 18GHz for better understanding.


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(b)

Figure 8: (a) Schematic representation of shielding mechanism through synergistic contribution of both filler nanomaterials (b) Characteristics parameters for effective shielding of different blends containing different types of ferrite particles along with the MWCNTs



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Does shielding effectiveness depend on the localization of nanoparticles in the blend? So it is well understood from the above discussion that Fe3O4 is the best ferrite among the three in imparting effective shielding in the blends. The synergistic effect from both conducting MWCNTs and ferrites facilitate absorption driven shielding. However, this section discusses as to how the localization of nanoparticles affect the overall shielding performance. In addition to the various composites that have been discussed earlier, additional blend composites have been prepared wherein Fe3O4 is made to localize in the PC component and the MWCNTs in the PVDF component. This pre-made PC/Fe3O4 (see Figure S3) nanocomposite when blended with PVDF and MWCNTs, result in selective localization of ferrite and MWCNTs in different components in the blends. This resulted in further improvement in shielding efficiency (SET = -36dB, see Figure 9a). SEM micrographs, EDS and solution dissolution tests clearly show that covalently attached Fe3O4 nanoparticles through nucleophilic substitution reaction in PC restrict their migration to the PVDF component during blending (Figure S4). This presumably aids in effective charge transport through the network of MWCNTs in PVDF which possibly can explain higher conductivity in this particular blend (Figure 9b). When the consolidated loss parameter is evaluated, this particular blend exhibited highest loss as compared to other sets of blend (Figure 9c). It is envisaged that this strategy enhances the power loss due to Maxwell-Wagner polarization effect not only due to charge accumulation at the interfaces but each phase of the blend also contributes due to differences in the dielectric properties. So, higher the accumulation of induced charges greater will be the dissipation energies through dielectric heating mechanism. Besides, imaginary part of the magnetic permeability enhances due to the enhancement of anisotropic constant. When the ferrite nanoparticles are selectively localized in PVDF, due to the presence of MWCNTs network, they are not well dispersed or rather distributed. However, restricting them in PC via suitable chemistry allows them to be well dispersed which improves the anisotropic constant significantly. This anisotropic constant can contribute in the residual magnetic loss factor which enhances the ultimate magnetic loss permeability part. So, by the combination of amplified interfacial polarization loss and higher anisotropy leads to the higher consolidated loss parameters which eventually enhance the dissipation of energies through heat. Taken together, when the incoming radiations interact with various heterogeneous materials which are already present in the matrix the interfacial polarization contributes towards dielectric loss permittivity. Simultaneously, due to enhanced anisotropic effect, improvement 22 ACS Paragon Plus Environment

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in the residual loss factor aids in magnetic loss permeability. So, the incident EM waves experience more obstructive networks which efficiently enhance the dissipation of energies through heat. More importantly, enhancement in the number of interacting components in the ‘mesh’ due to selective localization of nanoparticles plays a significant role in attenuating the EM waves through a reflection-absorption-reflection mechanism. As understood from the above discussions that the interactive loss parameters result in the dissipation of energies through Joule heating however, in the case of polymeric materials generation of any local heating will be detrimental if such heat is not distributed throughout the matrix. Thermal conductivity measurements clearly show an enhanced thermal transport due to this selective localization of particles (Figure 9d).



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Figure 9: (a) Total shielding effectiveness of compartmentalized localized blend, (b) DC electrical conductivity comparison, (c) consolidated loss of compartmentalized localized blend and (d) thermal conductivity plot Conclusions A mechanistic insight is provided herein based on the various losses (both dielectric and magnetic) in presence of a conductive fence (MWCNTs) and three different ferrites. Among the different ferrites studied here, Fe3O4 exhibited the best shielding effectiveness in the blends. This phenomenon was explained in terms of high saturation magnetization, high consolidated loss parameters, and high attenuation constant, better impedance matching and low eddy current losses fixing the synthetic protocol to be same for all the ferrites designed here. Interestingly, by portioning the ferrites in PC and localizing the MWCNTs in PVDF, the overall shielding efficiency was enhanced due to higher consolidated loss parameters. This study will help guide researchers working in this field from both academia and industry as shielding EM radiation has become quite imperative given the surge in the usage of electronic devices. ASSOCIATED CONTENT Supporting Information Impedance matching phenomena of various blends with different frequency, total shielding effectiveness

of

various

blends

in

respect of

thickness,

SEM

micrograph

of

compartmentalized blend with EDAX and solution dissolution test, table for magnetization data, table for total shielding effectiveness data of various blend composition, table for power law fitting exponent n. AUTHOR INFORMATION Corresponding Authors *Email: [email protected] *Email: [email protected] Notes No competing financial interest. ACKNOWLEDGEMENTS


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