Wool-Ball Type Core-Dual-Shell FeCo@SiO2@MWCNTs Microcubes

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Wool-Ball Type Core-Dual-Shell FeCo@SiO2@MWCNTs Microcubes for Screening Electromagnetic Interference Injamamul Arief, Sourav Biswas, and Suryasarathi Bose ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00333 • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 14, 2018

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ACS Applied Nano Materials

Wool-Ball Type Core-Dual-Shell FeCo@SiO2@MWCNTs Microcubes for Screening Electromagnetic Interference Injamamul Arief a±, Sourav Biswas a,b, Suryasarathi Bose a* a

Department of Materials Engineering, Indian Institute of Science, Bangalore, India 560012.

b

Department of Chemistry, National Institute of Technology, Durgapur, WB, India 713209.

ABSTRACT: Engineered nanostructure-reinforced lightweight polymer composites with superior electromagnetic (EM) shielding effectiveness (SE) are widely employed in high-end applications such as aerospace and microelectronic devices. Recently, a carbon nanotubebased 3D nanostructure has shown enormous potential due to their unmatched processability, mechanical and electronic properties. In this study, we present for the first time, highly permeable FeCo-based core-double shell wool-ball type microcubes chemically enclosed by dielectric silica and conducting multi-walled carbon nanotubes (MWCNTs) sequentially; the resulting reinforced nanocomposites with low filler loading produced superior SET of -35 dB at 18 GHz for a specimen of 3 mm thickness. The excellent dispersion of microstructures in the soft matrix owing to the encapsulation of hard FeCo magnets by MWCNTs ensures low density and excellent flexibility for high-precision applications. The nano-engineered coredual shell strategy for fabricating magnetic-dielectric-conducting microcubes ensures strong magnetic loss, coupled with dielectric and conduction loss respectively from SiO2 and MWCNT shells. This approach, being unique in terms of nanofabrication and subsequent formulation of lightweight flexible composites demonstrates a highly efficient way towards designing advanced nanocomposites for cutting-edge shielding application. KEYWORDS: Core-dual shell, FeCo microcubes, carbon nanotubes, electromagnetic interference shielding, magnetic loss.

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1. INTRODUCTION Modern electronic and hybrid electronic-telecommunication based interfaces demand lightweight, flexible and cost-effective electromagnetic interference (EMI) shield composites containing engineered nanofillers.1-5 Although a substantial amount of researches on soft composite-based radio frequency shields have been reported since the last decade, earlier attempts were precisely concentrated on ferrite-based composites. The ferrites and their engineered nanostructures are often in spotlight because of their lightweight, high abundance, considerable chemical stability and excellent microwave absorption properties.6-8 Additionally, they have shown excellent dispersibility in soft matrices.9 Nevertheless, ferritebased soft polymer composites are losing importance nowadays, partly because of their low magnetic polarization moments compared to strong ferromagnets, poor thermal stability and low corrosion resistance.10-11 Therefore, present trend in fabrication of electromagnetic (EM)proof composites involves strong ferromagnets and their binary alloy counterparts.12-13 Additionally, to further elevate the said properties, a multistage coating strategy was developed for the nanoparticles, in which magnetic core and dielectric shell components would manifest as magnetic-dielectric absorbers.14-16 Compared to single absorber, a magneto-dielectric integrated core-shell type absorber is deemed to be more efficient for EM absorption.15, 17 For the dielectric shell, mostly oxides of Ti and Si are preferred, along with BaTiO3.13,

18

The fabrication incorporates a multistage bottom-up methodology. The

outcomes often endow excellent tunability and versatile chemical and physical properties which are subjected to changes in reaction conditions, surface electrostatic potential of the core, rate of hydrolysis etc. It has been noted previously that lightweight carbonaceous materials and their derivatives are excellent candidates for absorption of EM waves.19-23 For multi-walled carbon nanotubes (MWCNTs)-based soft composites, numerous recent advances are reported, specifically in the EM absorption phenomenon.21,

24-25

They can reproduce remarkably high electrical

conductivity and dielectric permittivity.26-27 High dielectric and conduction loss trigger substantially high EM absorption in the composites. For magnetic particles in soft polymers, MWCNTs in small weight fractions (3-5%) are critical towards designing conducting composites with high absorptions.28 However, previously reported researches exclusively employed MWCNTs externally, i.e. added prior to the melt blending in order to achieve a continuous conducting framework within matrix.8-9, 28 This strategy, albeit being fruitful in many instances, is often subjected to the quality of dispersion of the conducting carbon 2 ACS Paragon Plus Environment

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nanofillers and their compatibility within the matrix. For better dispersion, MWCNTs are often undergone chemical modification, thus compromising the electrical conductivity and EM absorption. Although core-shell type structures with magnetic core-reduced graphene oxide shell has been reported recently,14, 29-31 an integrated nanostructure containing highly permeable magnetic core with impedance matching binary shell containing carbon nanotubes has not been attempted so far. Recently binary ferromagnetic alloys are proved to be widespread choice for EM attenuation. A handful of recent advances were reported with that of CoNi and FeNi alloy-based soft composites for significantly high reflection loss and shielding effectiveness.13, 32-33 Out of all, FeCo is very promising because of its remarkably high saturation (240 emu/g).34 Excellent tunability of FeCo nanostructures in wet chemical approach makes them suitable for various other applications as well. The anisotropy in shape and magnetization can further induce surface scattering microwave absorption. However, bare FeCo nanoparticle offers several shortcomings, which include magnetic aggregation, high density and prone to oxidation. A multistage core-shell morphology can efficiently address these problems by effective ‘wrapping’ of the magnetic core. Among the wave-transparent dielectric materials known, SiO2 is one of the most popular materials with excellent dielectric and optical properties employed in bottom up methodologies.13-14 The ease of functionalization of the SiO2 further qualifies for the additional docking of functional sites for multistage assemblies. SiO2 acts as a bridge or buffer layer between magnetic core and conducting shell, thereby accounts for excellent synergy between magnetic and dielectric components within the nanostructures.14 A few recent advances were reported on core-shell morphologies for EMI shielding application. The most notable results include NiCo-SiO2-TiO2 core-dual shell, FeCo/C/BatiO3 coredouble shell, and ZnFe2O4@SiO2@RGO core-shell with magnetic-dielectric-conducting architectures (Table S1).13-14,

35

However, the reported findings often involve high particle

weight fractions in composites and thereby, offer little flexibility and mechanical strength. Under

this

framework,

we

present

a

facile

bottom-up

route

comprising

of

solvothermal/hydrothermal/hydrolysis reaction for the fabrication of hierarchical FeCo microcubes, FeCo@SiO2, and core-dual shell Fe50Co50@SiO2@MWCNTs microcubes, respectively. The sequential core-dual shell cubic microstructures with FeCo at core and Silica/MWCNTs as shells have not been reported so far in the literature. The continuous bottom-up strategy to develop functional core-shell type structures were designed to produce high absorption owing to the presence of high permeable FeCo at core and conducing 3 ACS Paragon Plus Environment


MWCNTs wrap at the outer shell, whereas, the intermediate dielectric SiO2 layer was accounted for dielectric loss. The encapsulation of MWCNTs has also been critical in reducing the density of microstructures by nearly 40% from that of bare FeCo microcubes. In a nutshell, strategy for making bilayer or multilayer porous particles stems from the fact that magnetic core bound by dielectric/conducting shell (s) can give rise to synergistic contribution to ultrahigh absorption. The intermediate silica layer acts as impedance matching layer between the magnetic core and outer shell, respectively. To anchor the acidfunctionalized carbon nanotubes, we have functionalized the existing silica layer with amineterminated functional groups. The core-dual shell particles wrapped with MWCNTs in small weight fraction (10%) in soft PVDF composites were observed to be excellent microwave shielding materials with substantially high shielding effectiveness value (SET ~35 dB at 18 GHz for 3 mm thickness) than that of previously studied cases.35-36 In most reported instances, undesirably high weight fractions of magnetic phase in the composites made them unsuitable for many applications.13-14, 36 With the proposed material and composite, we are able to reproduce high SET with better dispersion in matrix and higher synergy with very small particle load. This design can also display improved mechanical properties and tremendous flexibility owing to conducting wrapping. The flexible nature of composites can be useful for custom EM applications, ranging from screening of mobile phones to aerospace industries. 2. EXPERIMENTAL SECTION Materials PVDF (Kynar-761, with Mw of 4, 40,000 g mol-1) was provided by Arkema. Pristine MWCNTs (Average diameter 9.5 nm and average length 1.5 µm) were acquired from Nanocyl SA (Belgium). Iron (III) acetylacetonate and Co (III) acetylacetonate, Tetraethylorthosilicate (TEOS), (3-aminopropyl)triethoxysilane (APTES) and Jeffamine® ED 2003 were bought from Sigma Aldrich. N,N-dicyclohexylcarbodiimide (DCC), 4dimethylaminopyridine (DMAP), chloroform, ethanol, ethylene glycol, HNO3 were procured from other notable commercial sources. Synthesis of FeCo magnetic microcubes The cuboidal FeCo microparticles were synthesized in a typical one-pot chemical coreduction of Fe (III) acetylacetonate and Co (III) acetylacetonate in ethylene glycol (EG). 4 ACS Paragon Plus Environment

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Briefly, 0.1M of Co (III) and 0.1M Fe (III) complexes were sonicated and subsequently stirred mechanically to dissolve in 50 mL of EG, followed by the addition of 0.1 g of Jeffamine® ED 2003 solution in EG. The precursor was then agitated under argon blanket for 30 min. Afterwards, freshly prepared 10 mL 0.5M NaOH in EG was added onto the precursor solution under constant mechanical stirring. Subsequently, the opaque solution was transferred to a Teflon-lined steel autoclave and maintained at 215°C for 8 h. After the completion, the suspension was centrifuged and washed using copious amount of water, acetone and ethanol for at least twice. Finally, the product was kept for drying in vacuum oven at 65°C. Synthesis of Core-double shell FeCo@SiO2@MWCNTs Fabrication of dielectric-conducting dual shell on magnetic core is a three step procedure and is shown schematically in scheme 1. Prior to amine modification for anchoring MWCNTs, a thin coating of SiO2 was performed on FeCo microcubes. Typically 100 mg of FeCo microparticles were dispersed in a mixture of 100 ml ethanol and 5 ml ammonia solution by probe and bath sonication. Then, 2 ml TEOS was added drop-wise under vigorous stirring at room temperature. After continuous stirring for 12 h, silica coated FeCo microparticles were centrifuged and dried under vacuum. For amine modification, 100 mg FeCo@SiO2 microparticles were treated in 30 ml toluene with 5 mmol APTES for 24h at room temperature. Functionalized particles were then separated and dried under vacuum. Finally, Acid treated MWCNTs were covalently attached on amine-modified FeCo@SiO2 microparticles. Acid functionalization of MWCNTs was performed following a previously reported method.37 Covalent conjugation between –COOH and -NH2 was performed in 1:1 mixture of both functionalized nanoparticles in presence of DCC and DMAP at 80C for 24h. Finally, the particles were centrifuged and collected after drying. To the best of our knowledge, this is the first ever reported fabrication of core-dual shell microcubes containing highly permeable FeCo at core and functionalized MWCNTs at the outer shell, separated by amine-modified silica layer in between. Preparation of PVDF nanocomposites PVDF nanocomposites were fabricated by typical solvent mediated mixing-cum-casting approach. The appropriate amounts of nanoparticles (10% by weight) were dispersed in DMF and then PVDF was added onto it. The mixture was further undergone 2h bath sonication and then casted onto a Teflon flatbed tray. Following the slow evaporation of the solvent at 5 ACS Paragon Plus Environment


ambient temperature, the film was collected and subsequently vacuum dried at 80C for overnight.

Scheme 1: Bottom-up synthetic route for FeCo@SiO2@MWCNTs core-dual shell microstructures Characterization Particle morphologies and composite morphologies were investigated by field emission scanning electron microscope (ESEM Quanta, FEI and Carl Zeiss Ultra 55) with energy dispersive X-ray (EDAX) attachment. TEM images were acquired utilizing FEI Technai T20 (200kV operating voltage). The magnetic properties were assessed using a Quantum Design MPMSXL-5 superconducting quantum interference device (SQUID) magnetometer with an applied field of -20000 to 20000 Oe at room temperature. A PerkinElmer GX FTIR instrument was utilized to obtain FTIR spectra. Raman spectra were recorded using a LabRam HR (UV) system. X-ray diffraction was recorded utilizing a PANalytical X’PERT PRO diffractometer. For AC electrical conductivity measurement, we used Alpha-N Analyser, Novocontrol (Germany) in the frequency range from 0.1 Hz to 10 MHz at room temperature. Prior to the measurement, 1 mm thick circular disc samples with 12 mm diameter were prepared by compression molding of extruded batch. A room temperature UTM was employed for evaluating the mechanical properties of the various composite structures. A 5 mm min-1 cross head speed was used for all the experiments. Prior to EMI shielding measurement, toroidal samples (5 mm thickness) were prepared by compression 6 ACS Paragon Plus Environment

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molding. Anritsu MS4642A Vector Network Analyzer (VNA) was utilized to determine the shielding effectiveness (SE) of the composites. A Damaskos MT-07 coaxial connector was used to quantify the shielding coefficients of the samples. Prior to measurement, the coaxial setup was calibrated by SOLT (short-open-load-through). The S parameters (S11, S12, S22 and S21) and associated permeability and permittivity parameters were measured in 2-18GHz frequency region for all the measurements. The total shielding efficiency data were acquired in dB units. 3. RESULT AND DISCUSSION FeCo nanoparticles are obviously the best choice for the core magnetic materials for EMI applications, with their very high permeability and saturation magnetization. Since frequency of magnetic resonance and permeability are both functions of magnetic saturation (Ms), higher Ms ensures better EMI shielding effectiveness. However, due to high conductive nature of core-shell ferromagnetic binary alloy systems, the permeability drops at much lower frequency. Therefore, functionalization of the magnetic phase poses additional challenges leading to enhanced shielding. The synthesis of FeCo follows standard polyol reduction. We have reported previously the highly efficient hydrothermal synthesis of FeCo nanocubes with an average size of 100 nm.38 However, in this presentation, the synthesized FeCo microcubes are fairly larger, with average diameter of 1.1 µm, and with high degree of polydispersity. The cubes were shown to have high surface roughness, which was attributed to the hierarchical growth in glycol-mediated reduction of metal precursor. Scheme 1 illustrates the nanofabrication of FeCo microcubes. The synthesis was performed in a typical one-pot bottom-up polyol method, followed by stepwise addition of dielectric shell and MWCNTs (for a detailed schematic representation of mechanism, see Figure S1). This ‘click’ reaction ensures the ‘wrapping’ of CNTs onto the amine-treated FeCo@SiO2, resulting in core-dual shell FeCo@SiO2@MWCNTs. The morphology of the bare FeCo microcubes is shown in Figure 1A. The FeCo microcubes, following SiO2 coating, appeared to have porous surface with thickness of the coating varying widely (Figure S2). The average size of the FeCo@SiO2@MWCNTs is 1.3 µm, indicating a roughly 150±20 nm MWCNT shell around the silica-coated FeCo microcubes (Figure S3). The average thickness of silica coating was measured from TEM and was found to be approximately 50 nm (Figure S4). The flaky porous surface of FeCo@SiO2 demonstrated the surface coating of silica onto FeCo microcubes (Figure 1B). Following the incorporation of amine terminated functional group on FeCo@SiO2, the porous surface remained largely unaltered with the development of 7 ACS Paragon Plus Environment


positive charge around the silica layer. The MWCNTs-wrapped FeCo@SiO2 showed distinct wool-ball type of structure (Figure 1D) with acid-functionalized MWCNTs encapsulating the FeCo@SiO2 microcubes. The TEM micrographs of the FeCo@SiO2 and FeCo@SiO2 @MWCNTs are shown in Figure 2D and 2E. The typical triple-layer morphology is also evident from TEM micrographs. The high magnification TEM image (Figure 2F) of the shell segment further reflects the attachment of carbon nanotubes. The morphology of the outer shell changed with the concentration of the modified MWCNTs used and are shown in Figure 2A, 2B and 2C. With the varying molar ratios of FeCo@SiO2 to MWCNTs from 1:1 to 1:2, one can clearly observe the surface morphological changes in the ‘wrapped’ microstructures. The 1:1 composition and homogeneous distribution of Fe and Co in the FeCo microcubes and the presence of Silica on the shell was confirmed by EDAX mapping (Figure S5-S6). The presence of nanotube shell was confirmed by EDAX and the corresponding line spectra of the FeCo@SiO2 @MWCNTs microcubes (Figure S7-S8). We have performed thermogravimetric analyses (TGA) of the as-prepared microstructures for further enunciating the composition (Figure S9). As shown in the TGA data, the core-shell particle displayed significant weight loss owing to the presence of MWCNTs (both as shell and free CNTs). The weight loss studies were performed in TGA under air and it corresponds to approximately the amount of MWCNTs present in the microstructures. However, significant loss in weight was reported only beyond 350⁰C, which is well above the processing temperature for the PVDF-based composites (260⁰C). Assuming the polymer composites may undergo heat dissipation during operation, the rise in temperature can no way go beyond the specified temperature. The observation coincided with the initial weight% of the Silica and MWCNTs in the precursor. The phase information of the as-prepared FeCo microstructures were acquired by X-ray diffraction studies (Figure 3).


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Figure 1. SEM micrographs of the as-synthesized (A) FeCo cubes, (B) FeCo@SiO2, (C) amine-functionalized FeCo@SiO2 and (D) FeCo@SiO2@MWCNTs. Scale bar measures 1 µm.



Figure 2. SEM micrographs of samples containing varying amount of MWCNTs used during the synthesis. (A) FeCo@SiO2: MWCNTs = 1:2, (B) FeCo@SiO2: MWCNTs = 1:1 and (C) FeCo@SiO2: MWCNTs = 2:1. (D) TEM image of the (D) FeCo@SiO2, (E, F) FeCo@SiO2@MWCNTs with different magnifications.


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Figure 3. X-ray diffraction patterns of as-synthesized FeCo, FeCo-coated with SiO2 and MWCNTs-wrapped FeCo@SiO2 nanostructures, respectively. The peaks corresponding to (110), (200) and (220) are associated to body centred cubic (bcc) phase of α-FeCo (JCPDS card no. 49-1568). The reduction in peak intensities of FeCo@SiO2 indicates reduced crystallinity owing to the coating of SiO2. In addition, the diffraction peaks at 2θ = 25.3o and 44.2o are ascribed to the (002) and (100) reflection of MWCNTs, thereby confirming the nanotube-encapsulated structure. Selected area diffraction patterns (SAED) for the bare FeCo microcubes and FeCo@SiO2@MWCNTs core-shell microstructures further echoed the XRD findings (Supporting information, Figure S10). The Raman spectra of MWCNTs,

acid-functionalized

MWCNTs

and

wrapped

microstructures

of

FeCo@SiO2@MWCNTs are demonstrated in Figure 4. The Raman spectra of MWCNTs, acid-functionalized MWCNTs and wrapped microstructures of FeCo@SiO2@MWCNTs are demonstrated in Figure 4. The presence of D and G bands of MWCNTs and MWCNTCOOH were observed in final MWCNT-wrapped microstructures approximately at 1330 and 1590 cm-1, corresponding to disordered phase and graphitic carbon in the nanotube, respectively.

Compared

to

the

pristine

MWCNTs,

the

MWCNT-COOH

and

FeCo@SiO2@MWCNTs showed significant variation in intensities and shift in the D and G 11 ACS Paragon Plus Environment


bands, which suggested that the charge-transfer and dipolar interaction might have occurred between the functionalized SiO2 layer and outer MWCNT shell. The slightly increased ID/IG ratio in the MWCNT-COOH (1.16), as compared to the pristine CNT (1.12) additionally confirmed the presence of disordered structures of carbon.39-41 However, the decreased ID/IG ratio (0.99) for the FeCo@SiO2@MWCNTs indicates that the atomic ordering or crystalloid of the MWCNTs was enhanced and structure defect was reduced somewhat. It also suggests that the precipitation of FeCo@SiO2@MWCNTs involved the formation of strong chemical interaction between the acid groups on CNTs surfaces and amine-silica on the intermediate layer rather than being adsorbed physically.24,

42

The FT-IR spectra of FeCo, FeCo@SiO2,

amine-treated FeCo@SiO2 and FeCo@SiO2@MWCNTs are shown in Figure S11. Peak at 1066 cm-1 corresponds to the Si-O bond confirming the coating of SiO2 at FeCo core nanoparticles. A broad absorption peak at 3400 cm-1 and 1606 cm-1 corresponding to the N-H stretching and bending confirmed the amine treatment on FeCo@SiO2 surface. Evidently, after wrapping of acid functionalized MWCNTs shell on the amine treated FeCo@SiO2, a shift in N-H bending peak is observed owing to the formation of amide linkage. In order to further ascertain the mechanism of formation, we performed XPS (Figure S12). The peaks of C1s and O1s were observed in the XPS survey spectrum of acid-functionalized MWCNTs, whereas, pristine MWCNTs showed no O1s peak. Furthermore, the survey scan of FeCo@SiO2@MWCNTs demonstrated all the relevant peaks correspond to Co2p, Fe2p, O1s, N1s (owing to the presence of amide linkage), C1s and Si2p respectively, clearly accounting for a core-shell type morphology. Following the Gaussian fits, the deconvoluted C1s spectra of FeCo@SiO2@MWCNTs additionally confirmed the C-N (286 eV) and C(O)-N linkages (288 eV) in the CNT-wrapped core-bilayer shell structures. The presence of both Fe2p and Co2p account for the FeCo core.


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MWCNTs (ID/IG = 1.12)

D band (1342 cm-1)

MWCNT-COOH (ID/IG = 1.16) FeCo@SiO2@MWCNTs (ID/IG = 0.99)

Raman Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1000

G band (1578 cm-1)

1330

1200

1400

1590

1600

1800

2000

-1

Wavenumber (cm )

Figure 4. Raman spectra of the FeCo microcubes, core-shell FeCo@SiO2 microcubes and core-double shell FeCo@SiO2@MWCNTs. Since frequency of magnetic resonance and permeability are both functions of magnetic saturation (Ms), higher Ms ensures better EMI shielding effectiveness. Figure 5 shows the MH hysteresis loops of all the microcube samples (FeCo, FeCo@SiO2 and FeCo@SiO2 @MWCNTs) recorded by SQUID at room temperature. The saturation magnetization (Ms) of the uncoated FeCo microcube was found out to be 176 emu/g, comparable to our previous results for FeCo microspheres for 1:1 compositions. The Ms for subsequently wrapped microcubes were shown to have slightly lower values, 162 emu/g and 136 emu/g for FeCo@SiO2 and FeCo@SiO2@MWCNTs, respectively.



Figure 5. M-H hysteresis loops of different samples measured at 300K. Saturation magnetization (Ms) for the core-double shell was found out to be 136 emu/g, whereas the uncoated pure FeCo possessed to have Ms of 175 emu/g.

The decline in Ms was apparently due to non-magnetic silica and MWCNT shells onto the magnetic core. This observation was similar to other core-shell ternary nanostructures containing magnetic core and non-magnetic shells. However, the Ms for FeCo microcubes was significantly lower than the bulk Ms for the 1:1 alloy, and it can addressed by the existence of magnetic dead layers on the nanoparticle surface and surface spin disorders.43-45 The coercivity (Hc) values for the samples indicate a clear trend, being higher in FeCo@SiO2 and FeCo@SiO2 @MWCNTs (175 Oe and 181 Oe, respectively), compared to the FeCo microcubes (141 Oe). The increased anisotropy due to step-wise coating and existence of lamellar porous structures following the silica coating can be attributed to an increase in Hc. However, the spread in Hc was not too high; owing to the fact that same magnetic FeCo core was involved in all the samples. The electromagnetic shielding mechanism is explicitly governed by fabrication and design aspects of polymeric nanocomposites.23 Composite nanostructures are widely revered for their lightweight, easy-to-process in large-scale production and chemical stability. However, 14 ACS Paragon Plus Environment

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the concentration of nanofillers within soft matrix plays pivotal role in different applications. Higher loadings generally deteriorate the mechanical properties while connectivity between the nanofillers is the primary factor for EM shielding application. To incur considerable absorption loss, recent advancements have reported a higher loading fraction of magnetic or dielectric nanostructures. Contrary to the trend, we have reported a unique approach to maximize the dispersibility of rigid magnetic nanostructures by forming a shell of conducting MWCNTs in a model PVDF matrix. SEM micrographs clearly showed that bare magnetic FeCo were aggregated due to their van der Walls’ forces and high permeability in the PVDF matrix, while in comparison with bare nanomaterials, FeCo@SiO2@MWCNTs, dispersion is slightly improved which facilitates the interconnected networks formation inside the PVDF matrix which is evident from Figure 6A and 6B. As we described earlier that few MWCNTs were unbound while forming the shell on the magnetic core, it is evident from Figure 6C that the free MWCNTs indeed helped in the bridging the core-shell nanostructures in the matrix. The AC electrical conductivity plot as a function of frequency in Figure S13 clearly reiterated the

formation

of

inter-connected

network

following

the

incorporation

of

FeCo@SiO2@MWCNTs whereas both bare FeCo and FeCo@SiO2-based composites were insulator in nature. This may be due to the presence of conducting MWCNTs. However, presence of conducting MWCNT shell serves two purposes: 1) better dispersion of highly permeable FeCo-based core-shell micro-assembly in the PVDF matrix, and 2) formation of effective bridging by free MWCNTs between the fillers and thereby enhancing the overall connectivity.



Figure 6. SEM micrograph of (A) PVDF with FeCo, (B) PVDF with FeCo@SiO2 @MWCNTs and (C) High magnification SEM micrograph of PVDF with FeCo@SiO2 @MWCNTs where arrows indicated the interconnected network formation by free MWCNTs. Mechanical properties of the composites can be altered owing to the incorporation of MWCNTs into the matrix. The behavior is controlled by a few microstructural parameters, e.g., network property, exfoliation of fillers inside the matrix, permeation limit, and interfacial bonding. The reinforcing effect of FeCo and FeCo@SiO2@MWCNTs in PVDF matrix was assessed by performing the room temperature tensile test (Figure S14). A general decline in the mechanical properties is seen after incorporation of FeCo, might be accredited to the agglomeration of the nanostructured materials. However, following the addition of 16 ACS Paragon Plus Environment

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conducting MWCNT shell, the resulting composite undergoes enhanced exfoliation that largely affects the elasticity and Young’s modulus of the composites. For instance, a remarkable 63% increment in Young’s modulus and ca. 17% enhancement in tensile strength are observed in comparison with only FeCo contain nanocomposites. The elevation of Young’s modulus begins to suggest that MWCNTs act as an efficient reinforcing agent although; FeCo alone severely affected the mechanical properties. As discussed above, free MWCNT played a pivotal role in connecting these core-shell particles which reflected in the mechanical properties. Besides reinforcing, the presence of rigid nanostructures and their exfoliation inside the matrix can affect the crystallization behavior. It has been also observed that the crystallization temperature follows a similar trend on incorporation of FeCo nanoparticles in the matrix. However, following the addition of FeCo@SiO2@MWCNTs, crystallization temperature has shifted (Figure S15). This is essentially in light of the fact that MWCNTs possibly act as hetero-nucleating agent. Nevertheless, elongation at break is significantly compromised due to premature failure of MWCNTs agglomerates that mainly contributes as stress concentrators. However, the flexibility of the nanocomposites is still observed up to 10 wt% addition of FeCo@SiO2@MWCNTs in PVDF matrix (Figure S16). The shielding efficiency is a measure of the ability to attenuate the incident electromagnetic radiation. The total shielding efficiency (SET = − 10 = 10 × log |

=10 × log |

|

|

can be measured by the scattering parameters ( ) which we

)

have obtained from the VNA by the ratio of transmitted power and incident power of the EM wave. Now the total shielding efficiency is the contribution of three different parameters which are mainly shielding by reflection (SER), shielding by absorption (SEA) and multiple reflection.23,

28, 46

However multiple reflection can be ignored if the shield has sufficient

thickness which is higher than the skin depth or efficiency is greater than 10 dB. So SET can be expressed as, SET = SEA + SER. By evaluating the scattering parameters we can also assessed the total reflection and absorption contribution of the total shielding effectiveness as,

SER = 10 × log

and SEA = 10 × log

. Figure 7A and S16 exhibits SET of

PVDF composites with varying loadings of different microstructures over the frequency range 2-18 GHz. The result reveals that bare magnetic FeCo nanoparticles are insufficient to shield EM radiation in low concentration (-7 dB for 5 mm, 18 GHz). However, presence of conducting nanotube shell on magnetic core shows a massive improvement in shielding efficiency. A staggering jump in SET for the core-dual shell composites (-38 dB, 5 mm at 18 17 ACS Paragon Plus Environment


GHz) can be associated with the formation of inter-connected network of MWCNTs shell acting as a ‘bridge’ between magnetic nanoparticles as well as the free MWCNTs in the matrix. Generally an incident EM wave while penetrating through the shield, decays inside and it does so by conductive fencing. The induced current created by the conducting network during the interaction with EM waves can too affect the attenuation process. Evidently, the unique approach of fabricating conducting shell on a non-conducting magnetic core is effective to form an interconnected conducting network within the matrix. Despite the fact that there exists a linear relationship between conductivity and shielding by reflection (SER), we observe a decline in SER value, which we depicted in Figure 7B in terms of %-of shielding by absorption (SEA) and shielding by reflection (SER) of the total shielding effectiveness. As indicated by EM absorption theory, it has to be a perfect situation when the vast majority of the approaching electromagnetic waves encounter with inward surface of the shield material. The incident EM waves experience with assortment of minute limits identified with the consolidation of various nanomaterials that constitute the microstructures within the matrix. Homogeneous and heterogeneous microstructures are analysed within an effective medium approach and are described by two material parameters: the complex (relative) permittivity and the magnetic permeability (relative).47 The real terms of both complex parameters are associated with energy storage and imaginary terms are associated with loss or energy dissipation within a material resulting from conduction, resonance, and relaxation mechanisms. Figure S17A-B shows the complex permittivity of the FeCo, FeCo@SiO2 and FeCo@SiO2@MWCNTs containing composites. It is evident that FeCo@SiO2@MWCNTs possess much higher real and imaginary parts of the complex permittivity than bare magnetic core. The dielectric loss tangents are frequently used to characterize dielectric loss ability, additionally affirm that FeCo@SiO2@MWCNTs has higher dielectric loss. Dielectric losses can be attributed to a number of possible sources, including dipolar polarization, interfacial or Maxwell-Wagner-Sillars polarization effect, defect-induced polarization or loss and presence of oxygen functional groups on the surface of conducting nano-materials. As per Debye theory and free electron theory, interconnected network of conductive inclusion beneficially conducts charge through hoping inside the polymer matrix which generates very high conductivity losses.28, 48 However, due to the absence of conducting phase, bare FeCo and FeCo@SiO2 nanoparticles demonstrate very less conduction loss which actually decreases the overall dielectric loss of the composite. On the other hand, uneven distribution of the 18 ACS Paragon Plus Environment

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space charge between the non-conducting FeCo core and conducting MWCNTs shell and associated void spaces introduce considerable interfacial polarization due to presence of different dielectric, giving rise to polarization loss. Many researchers have reported that high defect density of conducting nano-materials may significantly improve the EM absorption phenomena due to defect induced polarization losses. For our case, the defects sites which are created during the acid treatment of MWCNTs for attaching with the core particles are helpful for accumulating the residual abundant charges and enhances the defect induced polarization losses. Figure S18A-B depict the real and imaginary permeability parameters of FeCo, FeCo@SiO2 and FeCo@SiO2@MWCNTs containing composites respectively. A slight variation in both complex permeability parameters is observed following the addition of conducting shell. The nonmagnetic coating on magnetic core enhances the effective reluctance and thus, results in the relatively weak frequency dispersion phenomena. Generally, high saturation magnetization is preferred for higher initial permeability.49 It is therefore suggested that bare magnetic nanoparticles show higher permeability parameters than core-shell nano-materials due to the slight decrease in the saturation magnetization. However, we observe a reverse trend that can be explained by magnetic loss tangent values. The magnetic loss tangent is calculated by determining the tanδµ value (tanδµ = µ‫׳׳‬/µ‫)׳‬, which is associated with hysteresis, domain wall resonance and eddy current loss in GHz frequency. The hysteresis loss in negligible in the weak field and the domain wall resonance loss actually occurs at much lower frequency region. Therefore, eddy current loss effect (eddy current loss = µ‫( ׳׳‬µ‫)׳‬-2 (f)-1) is taken as a main loss mechanism in higher frequency range which we have calculated from the permeability parameters. It is observed that conducting coating on magnetic FeCo microcubes generally reduces the eddy current throughout the frequency region, accounting for the better magnetic loss behavior. The metallic FeCo is good magnetic conductor, when an alternating magnetic field is applied the agglomerated network of bare core nanoparticles in PVDF matrix account for eddy current and skin effect resulting in partial cancellation of internal magnetic field and consequent degradation of complex permeability. But interconnected network formed after forming of conducting shell through MWCNTs enhances the overall dispersion in the PVDF matrix, which actually cuts off the eddy current and thus, the skin effect is suppressed completely (Figure 7C). To elucidate the mechanism of shielding in the hybrid system the consolidated loss parameters (tanδε + tanδµ) were plotted in Figure 7D. The result clearly shows a significant enhancement in total loss parameters after forming the conducting shell on FeCo core.



In addition to the said factors, better EM shielding property in the FeCo@SiO2@MWCNTs containing composite can further be ascribed to the enhanced anisotropic energy (Ha), which can be expressed as; = 4| | / 3

!

where, | | is the anisotropic constant.50 As the Ms

decreases with the shell structures on the magnetic core, therefore, it actually enhances the anisotropic contribution as well as absorption of EM wave particularly at higher frequency region. Figure 7E shows an incremental result of attenuation constant following the formation of conducting shell on magnetic core, suggesting the absorption ability of such core-shell nanoparticles. Attenuation constant (α) was assessed using complex permittivity and permeability parameters as follows;

α =√2

$% &

'(‫ ׳׳)׳׳‬− ‫ )׳)׳‬+ ,((‫ ׳׳)׳׳‬− ‫ )׳)׳‬+ (‫ ׳׳)׳‬− ‫) )׳)׳׳‬, where c is the speed of light.

The effect of the shield thickness on EM shielding efficiency has been evaluated at 18 GHz frequency for all the samples (Figure 7F). Shielding effectiveness reduces gradually with lowering of thickness due to lacking of interconnected conducting mesh which is randomly placed within the composite matrix. However, it is observed that the SE of FeCo@SiO2@MWCNTs sustains a moderate value although thickness is reduced up to 1 mm. This may be attributed to the reduction of the skin depth of the composite, which is 1.4 mm for FeCo@SiO2@MWCNTs containing PVDF composite.


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Figure 7. (A) Total shielding effectiveness, (B) % of absorption and reflection contribution, (C) eddy current loss, (D) consolidated loss parameters, (E) attenuation constant and (F) thickness dependent shielding effectiveness in various composites. Therefore, FeCo@SiO2@MWCNTs is so far, the most preferred material in term of EM absorption compared to that of bare FeCo core microcubes. In any case, the general shielding mechanism is considerably more intricate when these microstructures are incorporated in the PVDF matrix along with some free MWCNTs. As the AC electrical conductivity is not so high and the ratio of permeability and permittivity parameters is closer to 1, the shielding material maximizes the interaction of incident EM radiation with the inner surface of the material while minimizing the surface reflection due to impedance matching. The incident EM waves interact with a variety of microscopic boundaries associated to the inclusions of both core-shell and free MWCNTs that constitute the heterostructure in the PVDF matrix. The local field variation at the heterostructure junction is associated with the absorption of incident energies as it depends quadratically with the electric field intensity. Associated void spaces of the hetero-junction core-shell nanofillers additionally contribute in the polarization 21 ACS Paragon Plus Environment


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losses. Maxwell-Wagner-Siller polarization theory also suggests the charge accumulation at the interface of two different dielectric materials and the dissipation of energies into heat synergistically. Besides, the multiple reflections at the inner surface of the shield materials also provide a part of the absorption. The mechanism of shielding is schematically illustrated in Figure 8. So in a nutshell, the core-dual shell microstructures with magnetic core and dielectric-conducting shell illustrates a very high impact in terms of designing advanced EM absorption material for high-end applications.

Figure 8. Schematic representation of the shielding contributions of PVDF nanocomposite with FeCo@SiO2@MWCNTs core-dual shell microcubes. 4. CONCLUSION In summary, we have shown for the first time that highly efficient, magnetic-dielectricconducting core-bilayer microstructure-based PVDF composites can be utilized for their excellent

electromagnetic

shielding

effectiveness.

The

core-dual

shell

FeCo@SiO2@MWCNTs-based nanocomposites demonstrated excellent EM attenuation with SET of -35 dB for the thickness of 3 mm at 18 GHz with improved Young’s modulus and tensile strength. This new type of nano-engineered architecture with distinct wool-ball shape has not been explored so far for electromagnetic screening and essentially eliminates the addition of conducting fillers (MWCNTs) externally into the blend. The low filler concentration and presence of conducting outer shell encourage flexibility and lightweight properties. The integrated magnetic-dielectric FeCo@SiO2 microcubes undergo an additional shell of conducting MWCNTs, thereby enhancing the EM absorption property owing to 22 ACS Paragon Plus Environment

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higher consolidated loss parameters, eddy current and attenuation constant. Henceforth, this novel nanocomposite offers excellent EM attenuation ability and improved mechanical properties as compared to existing solution.

ASSOCIATED CONTENT Supporting Information Schematic mechanism of core-shell microstructure growth, high resolution SEM images of the

various microstructures, low magnification SEM images and size distribution curves, high resolution TEM images of the various core-shell microstructures, elemental EDAX color mapping of FeCo cubes, elemental EDAX color mapping of FeCo@SiO2 microcubes, EDAX spectra and quantification table of FeCo@SiO2@MWCNTs, line EDAX spectra of the FeCo@SiO2@MWCNTs, TGA curves of various core-shell microstructures, SAED pattern for the FeCo microcubes and core-shell components, FTIR spectra of various synthesized microstructures, XPS spectra, AC electrical conductivities of the composites, mechanical properties of the nanocomposites, DSC crystallization curves, image of twisted PVDF film, total shielding effectiveness, complex permittivity as a function of frequency for nanocomposites, complex permeability as a function of frequency for nanocomposites, table for core-shell based materials for EMI literature, table listing all other relevant compositions and their EMI SE.

AUTHOR INFORMATION Corresponding Author * e-mail [email protected] ±

Present address: Université Lyon 1, CNRS, Ingénierie des Matériaux Polymères, UMR 5223, F-69003 Lyon, France. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support from INSA India. One of the authors, IA acknowledges DST SERB-National Post-Doctoral Fellowship (N-PDF) program (Grant PDF/2016/ 000048) for financial assistance.



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TABLE OF CONTENT Herein, we present for the first time, highly permeable FeCo-based core-double shell woolball type microcubes chemically enclosed by dielectric silica and conducting multi-walled carbon nanotubes sequentially; the resulting reinforced PVDF nanocomposites with low filler loading produced superior SET of -35 dB at 18 GHz for a specimen of 3 mm thickness.


Wool-Ball Type Core-Dual-Shell FeCo@SiO2@MWCNTs Microcubes

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