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Unique Multi-Layered Assembly Consisting of ‘Flower-Like’ Ferrite Nanoclusters Conjugated with MWCNT as Millimetre Wave Absorbers Sourav Biswas, Sujit Sankar Panja, and Suryasarathi Bose J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b02668 • Publication Date (Web): 13 Jun 2017 Downloaded from http://pubs.acs.org on June 15, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Unique

Multi-Layered

Assembly

Consisting

of

‘Flower-like’

Ferrite

Nanoclusters Conjugated with MWCNT as Millimetre Wave Absorbers Sourav Biswasa, Sujit S. Panjaa*, Suryasarathi Boseb* a

Department of Chemistry, National Institute of Technology, Durgapur, WB, India-713209. Department of Materials Engineering, Indian Institute of Science, Bangalore, India 560012. * Author to whom all the correspondence should be addressed. Email: [email protected]; [email protected]

b

Abstract: Unique multi-layered assembly was designed here using polymeric blends containing ‘flowerlike’ ferrite nanoparticles conjugated with multiwall carbon nanotubes (MWCNTs) for attenuating 99.999% of the incoming electromagnetic (EM) radiation. In comparison to traditional single layered structure, this unique assembly is superior for myriad applications related to supressing the incoming EM radiation; mostly by absorption. The three key requirements; impedance wave matching, absorption and multiple scattering from the heterogeneous structures were accounted here by suitably modifying the nanomaterials. A bicomponent blend consisting of two immiscible polymers; polyvinylidine fluoride (PVDF) and polycarbonate (PC) was used here to construct the multi-layered assembly wherein selective localization of nanoparticles in one of the component, driven by thermodynamics, reduced the percolation threshold of the nanoparticles. In order to improve the impedance matching, MWCNTs were functionalized using the defect sites induced by harsh chemical treatment and incorporated in PC/PVDF blends as the inner layer of the multi-layered assembly. Conjugation of flower-like Fe3O4 nanocluster on the defect sites of the surface functionalized MWCNTs absorb the incident EM waves due to the interfacial polarization of different heterogeneous structures. The value of total loss tangent, attenuation constant and absorption coefficient supports absorption driven shielding in PC/PVDF blends. The efficient thermal dissipation together with high absorption led to fix this as the intermediate layer of the assembly. Finally, multiple scattering through the network of pristine MWCNTs was utilized as the outermost layer of the assembly, which guided the penetrated waves to interact with this layer resulting in maximum attenuation. This unique three-layered assembly, which exhibited a shielding

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effectiveness of -64 dB at 18 GHz for 0.9 mm thickness, powered by multi-functionality offer, amendable replacement of the existing solution related to EM absorption. Introduction: With the augmenting growth of electronic and smart communication devices for betterment of human life and economic development, electromagnetic (EM) radiation has become a major concern in the modern civilization.1-3 The issues related to this inevitable EM radiation, which can severely interrupt precise circuitry of the high end electronic gadgets and severe health problem to human beings, have become increasingly serious. Therefore, the concern of eliminating this unwanted EM radiation provides and ambit to develop efficient EM absorber materials.4-5 Metals, despite being widely used as a common EM shielding material, suffer from several disadvantages such as heavy weight, limitations in design flexibility, cumbersome processing and susceptible to corrosion. The quest to develop lightweight and effective EM shielding material is triggered by the myriad opportunities like design flexibility, ease of processing, corrosion resistant etc. offered by reinforced polymeric composites.6-9 Carbonaceous nanoparticles such as graphitic carbon, graphene oxide, carbon nanotubes, carbon fibres

10-19

etc.

have been used as conducting filler whereas different ferrites and alloys are usually utilized as magnetic nanoparticles20-26. A good absorbing materials requires high permittivity and permeability besides sufficient electrical conductivity12, 27-30. So hybrid system with synergetic properties are required that results in enhancement in EM shielding due to its unique combination of magnetic and electrical properties31-34. Zhang et al showed that synthesized carbon-coated Fe nanocapsules modified by arc discharge method are effective in microwave absorption due to matching microstructure, strong natural resonance and multiple polarization35. They observed -43.5 dB reflection loss at 9.6 GHz frequency with a shield of 3.1 mm thickness. Zhu et al suggested that Fe3O4/TiO2 core-shell structure was also effective in microwave shielding36. A reflection loss of -20.6 dB at 17.28 GHz frequency with 5 mm thickness was reported. Ren et al fabricated a quaternary nanocomposites consisting of graphene/Fe3O4/Fe core-shell structure for microwave absorption37. After mixing 20% of this quaternary nanocomposite with an epoxy resin, reflection loss of -20 dB at 7.3 GHz was reported. Liu et al showed that addition of 400 nm Fe3O4 core and 150 nm TiO2 shell resulted in -23.3 dB shielding effectiveness at 7 GHz

38

. Chen et al fabricated porous


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Fe3O4/carbon core-shell nanorods which shows a maximum reflection loss of -27.9dB at 14.9 GHz frequency with 5 mm thickness39. Saini et al showed that polyaniline coated Fe3O4 fabrics exhibited-19.4 dB total shielding effectiveness at 12.4-18GHz40. Hou et al showed a hydrothermal method for conjugating MWCNT and Fe3O4 nanoparticles and their effect on EM absorption41. A -18.22dB minimum reflection loss was observed at 12.05GHz. Wan et al developed a chemoselective route to develop Fe3O4@ZnO core-shell nanoparticles decorated on MWCNTs to form MWCNT/Fe3O4@ZnO42. The interfacial polarization and synergetic interaction between dielectric and magnetic absorber resulted in high shielding efficiency of ca. 40.9dB at 9.8 GHz. Liu et al developed a core-multi shell structure of MWCNT/Fe3O4/PANI/Au hybrid for very high reflection loss value ca. -60 dB43. The introduction of gold nanoparticles is beneficial for multiple reflections of EM waves within the absorber. Pawar et al conjugated MWCNT and Fe3O4to develop composites of PC/SAN blend which resulted in -32.5 dB shielding at 18 GHz44. Herein, a well-studied immiscible system: PC/PVDF polymer blend45-46 was chosen to design millimetre wave absorbers wherein selective localization of nanoparticles in one of the components reduces the percolation threshold through effective increase in the local concentration of the functional nanoparticles. In order to absorb EM radiation, two key parameters; reasonably high conductivity and lossy characteristics were achieved by incorporating MWCNTs and magnetic ‘flower like’Fe3O4 nanoclusters. The effect of surface modification of MWCNTs and conjugation of flower like Fe3O4 nanoclusters conjugated with MWCNTs. A systematic analysis of various nanocomposites in this study allowed the rational design of a unique multi-layered architecture following ‘impedance matching’, ‘electrical dipoles’ and a combination of ‘electrical and magnetic dipoles’ as the three layers of this unique multi-layered assembly. A systematic assessment of saturation magnetization, permittivity, permeability, attenuation constant and total loss tangent allowed the understanding of the shielding mechanism in the nanocomposites studied here. A very high shielding efficiency of -64 dB at 18GHz for a thickness of 0.9 mm was realized by stacking three different layers having unique advantages. Experimental section: Materials



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PVDF (Kynar-761, with Mw of 440 000 g mol-1) was kindly provided by Arkema. Polycarbonate (Lexan-143R) was obtained from Sabic (MFI 11g/10min). Pristine MWCNTs (length 1.5 µm and diameter 9.5 nm) was procured from Nanocyl SA (Belgium). Iron (III) acetylacetonate and 4dodecylbenzenesulfonic acid (DBSA) was procured from Sigma Aldrich. Analytical grades of chloroform, ethanol, N,N-Dimethylfomamide, ethylene glycol, HNO3, sodium thiosulfate and tetrahydrofuran were obtained from commercial sources. Synthesis of surface functionalized MWCNTs Carboxyl functionalized MWCNTs were synthesized by a method reported elsewhere47. Briefly, 100 mg of pristine MWCNTs were dispersed by bath sonication in 90 ml of concentrated HNO3 for 1 h. Then 10 ml of H2O was added into the dispersed solution and the total mixture was kept under reflux with vigorous stirring at 80⁰C for 24 h. Finally the mixture was poured in 2 l of DI water, collected by filtration and dried under vacuum at 80⁰C 24 h. Synthesis of Conjugated flower-like Fe3O4 nanocluster with surface functionalized MWCNTs The formation of flower-like Fe3O4 nanocluster on the defect site of MWCNTs was generally governed by the hydrothermal reduction process in polyol medium. Acid treated MWCNTs facilitate this type of conjugation because during the harsh treatment by concentrated HNO3, defects are created on the sidewall of MWCNTs, which provide specific sites for nucleation of the ferrite particles. In a typical method, 40 mg of surface functionalized MWCNTs was dispersed in 40 ml of ethylene glycol by bath sonication technique. After that, required amount of iron (III) acetylacetonate, sodium thiosulfate and DBSA were added into the dispersed solution and bath sonicated for another 30 min. The solution was then placed in Teflon lined autoclave and kept at 220⁰C 30 h. Finally the black residue was isolated and filtered by ethanol water mixture several times and vacuum dried at 80⁰C.


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Scheme 1: A cartoon illustrating the conjugation of flower-like Fe3O4 nanocluster to surface functionalized MWCNTs Preparation of blends: All the blends with different nanoparticles were prepared by using a Hake minilab-II melt compounder under nitrogen atmosphere at 260°C with 60 rpm rotational speed for 20 min. We have fixed the concentration of MWCNTs as 3 wt% in the blends. This concentration is above the percolation threshold of MWCNTs. In addition, 3wt% Fe3O4 nanoparticles are added to compare the efficiency of the shielding material containing conjugated structures. Construction of multi-layered assembly: Each individual layers were prepared by compression moulding at 260°C under high pressure. After obtaining all the specific layers they were stacked again by compression moulding, at much lower presser and at 150°C. Generally the melting temperature of neat PVDF is near about 170°C, and hence by adopting this strategy the layers can be made to fuse together without melting the individual layers. Characterization: Transmission electron micrographs were acquired utilizing a FEI Technai F20 operated at accelerated voltage of 200kV. A Sirion XL30 FEG SEM with an accelerated voltage of 10 kV



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was utilized to determine the morphology of the PC/PVDF blends. The magnetic properties of the synthesized nanoparticles were assessed using a Lakeshore Vibratory Sample Magnetometer (VSM) with an applied force of -8000 to 8000 Oe at room temperature. Raman spectra were recorded using a LabRam HR (UV) system. X-ray diffraction was recorded utilizing a XPERT Pro from PANalytical. A Cu Kα radiation source (l = 1.5406 Å, 40 kV and 30 mA) was used to decipher the XRD profile of different nanoparticles. The flow characteristics of the blends were evaluated using a discovery hybrid rheometer (DHR-3) from TA-instruments under a nitrogen atmosphere to prevent any degradation of the samples. Parallel plate geometry of 25 mm diameter was used. Extruded strands were vacuum dried overnight at 80⁰ C before the rheological experiments. All the experiments were carried out under a linear viscoelastic region determined a priori. Room temperature electrical conductivity of the blend was contemplated utilizing an Alpha-N Analyser, Novocontrol (Germany) in a frequency range from 0 .1 Hz to 10 MHz. Magneto capacitance was also measured by using Alpha-N Analyser, Novocontrol (Germany) in the frequency 1000Hz by changing external magnetic field from 0 to 10000 Oe. FLIR T6000 camera was used for thermal imaging of various samples. EM interference shielding was measured by utilizing an Anritsu MS4642A VNA. A Damaskos MT-07 co-axial connector and a KEYCOM waveguide were utilized to measure the shielding efficiency. Result and discussion Synthesis and characterization of different functional nanoparticles: MWCNTs (see Figure 1a) were obtained from commercial sources with 1.5 µm average length and 10 nm diameter. The synthesis and conjugation of flower-like Fe3O4 nanoparticles on MWCNTs was confirmed by TEM. Figure 1b confirms that the average diameter of the flowerlike Fe3O4 nanoparticles is ca. 80-100 nm. HRTEM images and SAED (selected area electron diffraction) pattern further supports the polycrystallinity in the flower like nanoparticles (Figure 1c-d). The presence of surfactant usually caps the individual nanoparticles and extends the growth of flower like shape. The diffraction patterns corresponding to (220), (311), (400), (511) and (440) indicate typical face-centred cubic (fcc) crystal structure of Fe3O4with an inverse spinel structure (Figure 1d).Crystal structure of Fe3O4 nanoclusters were further analysed by Xray diffraction (Figure 2a). (220), (311), (300), (511) and (440) peaks were identified which also confirmed the fcc structure (JCPDS card 19-0629). It is observed that the strongest reflection is


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obtained from (311) plane. The d-spacing corresponding to (311) plane is around 0.26nm, which corroborates with lattice fringe in the HRTEM images of flower-like Fe3O4 nanocluster (see Figure 1c). The attachment of flower-like Fe3O4 nanocluster with the MWCNTs was also confirmed by XRD. Thermo-gravimetric analysis is an ideal tool to determine the exact composition of MWCNTs and Fe3O4 nanocluster in the conjugated structure. The weight loss up to 200oCis due to the removal of surface absorbed water and residual surfactant molecule. The oxidation of MWCNTs was observed which is similar in both the cases. Finally, we observed that the conjugated product contained 47% of MWCNTs and 45% of Fe3O4 (Figure 2b). Raman spectra or more specifically, the intensity ratio of ID/IG (the ratio of disorder to graphitic structures) can shed some light on the defect chemistry of MWCNTs. It has been observed from Figure 2c that the intensity ratio increases from 1.25 to 1.32 during harsh acid treatment manifesting the fact that defect sites can provide nucleation sites for ferrite clusters. As the Fe3O4 nanoparticles exhibit inverse spinel structure, Fe3+ and Fe2+ atoms are in the opposing position with different magnitudes providing ferrimagnetic property. VSM study showed that the saturation magnetization of conjugated flower-like Fe3O4 nanocluster is 45.45 emu g-1, remnant magnetization is 21 emu g-1 and coercivity is 352 Oe (Figure 2d). The high initial permeability that usually predicts strong magnetic loss ability can be expressed by, µi = Ms2/(aKHcMs + bλξ )

(1)

where, material compositions are determined by the constant a and b, magneto restriction constant is λ, elastic strain parameter of the crystal is ξ and K is the proportionality coefficient MS is the saturation magnetization and HC is the coercivity 48-49. It can be inferred that higher Ms and lower Hc values are favourable for improving the initial permeability. Interestingly, conjugation of 45% Fe3O4 nanocluster on surface functionalized MWCNTs readily enhances the saturation magnetization. It is noteworthy to mention here that obtained saturation magnetization is quite impressive for only 45% Fe3O4 particles that are conjugated with MWCNTs. Heterostructure, dispersion state and microstructure The lack of specific interactions between the immiscible polymer components; generally result in heterogeneous morphology.46 A co-continuous type of morphology is observed here when these two immiscible polymers (PC and PVDF) were mixed in 50/50 weight ratios (Figure 3a).



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The SEM micrographs suggest that co-continuous morphology is retained even after the addition of different nanoparticles (Figure 3b-c). It is indeed an interesting finding that all the nanoparticles are selectively localized in PVDF, driven by its high polarity, despite the fact that PC component is favoured when thermodynamic considerations are taken into account.45 Higher magnification SEM micrograph also confirms the selective localization of nanoparticles in PVDF while PC component is etched out for better resolution (Figure 3d). The localization of nanoparticles was further analysed using EDAX. The presence of iron in the EDAX spectrum confirms the localization of flower-like Fe3O4 nanoclusters in the PVDF component after removal of PC phase (Figure 4a). SEM morphology of the blends structure without etching of any components also confirms the selective localization of nanoparticles in the PVDF component of the blend (Figure S1). EDAX spectra clearly indicates that PVDF component contains all the nanoparticles and appears as rough while the smooth PC component indicates the absence of nanoparticles in this particular phase (Figure S1). Solution dissolution test further confirms the selective localization of nanoparticles in the PVDF phase. The clear solution, after dissolving PC component suggests no particles in PC whereas dark solution obtained after dissolving PVDF indicate the presence of functional nanoparticles in the PVDF component (Figure 4b). To gain more insight, DSC studies were performed wherein change in the crystallization behavior of PVDF was correlated with the presence of nucleating agents, here the different functional nanoparticles. As PVDF is semi crystalline in nature, the presence of nanoparticles induces the crystallization temperature50-51. It is observed that addition of MWCNTs or surface functionalized MWCNTs readily enhances the crystallization temperature of PVDF which clearly suggest that the nanoparticles are selectively localized in the PVDF component wherein MWCNTs promotes heteronucleation of PVDF crystals (Figure 4c). Further addition ofFe3O4 nanocluster along with MWCNTs also enhances the crystallization phenomena.


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Figure 1: TEM image of (a) MWCNT, (b) MWCNT-Fe3O4. (c) HRTEM of MWCNT-Fe3O4. (d) diffraction pattern of Fe3O4 nanocluster



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Figure 2: (a) XRD, (b) TGA of MWCNT, MWCNT-COOH and MWCNT-Fe3O4 nanoparticles respectively (c) Raman of MWCNT-COOH and MWCNT-Fe3O4 nanoparticles, (d) VSM of MWCNT-Fe3O4 nanoparticles


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Figure 3: SEM micrograph of 50/50 PC/PVDF blends where PC phase was etchedout previously (a) neat (b) with MWCNT, (c) with MWCNT-Fe3O4, (d) High magnification SEM micrograph of MWCNT-Fe3O4 contain blend after removal of PC phase where arrows indicates MWCNTs



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Figure 4: (a) EDAX spectra of MWCNT-Fe3O4 contain blend when PC phase was etched out, (b) solution dissolution test where (i) samples were dissolved in chloroform for extracting PC component, colour less solution indicated absence of nanoparticles at that component, (ii) samples were dissolved in DMF for extracting PVDF component, black colour solution indicated presence of nanoparticles in that component, (c) DSC crystallization temperature of various blends Interconnected network-like structures of MWCNT conjugated with flower-like Fe3O4


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To perceive the state of dispersion of different nanoparticles in the blend structure, viscoelastic properties were evaluated here by melt rheological studies52. In general melt-rheological study in polymer blends also shed light on the evolved structure during processing especially in blends containing nanoscale materials53. It is observed here that all the blend structures exhibited yield stress at lower frequency region (Figure 5a). The addition of nanoparticles in the bend structures results an observable change in the complex viscosity especially in lower frequency region where sufficient time is available for chain relaxation. It is interesting to note that surface functionalization of conducting MWCNTs is not effective to change the complex viscosity when compared with blends filled with MWCNTs. But after conjugation of flower like Fe3O4 nanocluster with MWCNTs, a dramatic increase in complex viscosity is observed manifesting ‘rigidity percolation’. This is due to the presence of two different rigid nanoparticles in one component of the blend which restricts the macromolecular motion reflecting in enhanced viscosity. In general, preferential localization of conducting nanomaterials in one of the components in biphasic polymer blend reduces the percolation threshold significantly54. After addition of 3wt% MWCNTs, a considerable increase in AC electrical conductivity is observed. After harsh acid treatment of MWCNTs, the structural defects in MWCNTs reduce the bulk electrical conductivity (Figure 5b). The conjugation of flower likeFe3O4 nanocluster on to surface functionalized MWCNTs further reduces the overall conductivity due to an increase in defect concentration. This was also supported by Raman spectroscopy wherein enhanced structural defects led to an increase in the intensity ratio of ID/IG.



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Figure 5: (a) complex viscosity and (b) AC electrical conductivity of various blends

Absorption of millimetre wave: effect of surface functionalization of MWCNTs and conjugation with flower-like Fe3O4 nanoparticles The ability to shield the incident EM radiation is expressed in terms of total shielding effectiveness (SET) in dB. Three different losses; reflection, absorption and multiple reflection constitute the total SE (see figure 6a).2 The surface reflection (ER) is mainly observed due to impedance mismatch and the presence of electrical/magnetic dipoles result in absorption. The amplitude of these two opposing waves depends on the intrinsic impedance and incident wave propagation domain of the shielding material. During the propagation of the wave from t=0 to t=d (d is a critical thickness of the shield), the strength of the amplitude of the transmitted waves decreases due to absorption, and finally the absorbed energies are dissipated as heat. Once the wave reaches t=d, then again it divides in two opposing direction where one is reflected back into the material and other transmitted. Now if the thickness of the material is greater than the skin depth the internal reflection of the material can be absorbed which otherwise is lost. So if the thickness is greater than skin depth, the internal reflection can be ignored. So the total


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shielding efficiency for thicker shields is mainly from reflection and absorption. SET can be estimated by the ratio of incident to transmitted electric field intensity. Here we have estimated SET by using the scattering parameters obtained directly from the vector network analyzer (VNA). A considerable shielding efficiency is detected in the blends with the addition of only MWCNTs while neat blends are transparent to EM radiation (Figure 6b). Preferential localization of MWCNTs into the energetically favoured component of an immiscible polymer blend leads to significant enhancement in AC electrical conductivity as well as EM shielding efficiency. The perceived changes in AC conductivities can be attributed to the formation of interconnected conducting networks within the polymer component. The insulating polymer matrix enhances the energy barrier between MWCNTs, so charge transport will be facilitated by the electron hopping mechanism. Power law fitting45,

55-56

of AC electrical conductivity data also provide insights

which clearly suggest that the electron hopping is the main mechanism of charge transportation in this case (see Table S1). The micro current which is generating during the hopping process is also effective for EM shielding when the concentration of the conducting particles is enough in the blend structure57. Therefore, materials should possess adequate electrical conductivity and good network connectivity in order to interact with the EM radiation58. However incorporation of non-conducting nanoparticles may impede this network-like structure of MWCNTs thereby lowering the blend conductivity. It is observed that blends containing surface functionalized MWCNTs showed lower electrical conductivity and lower EM shielding property (Figure 6b, Table1). It is interesting to note that by conjugating surface functionalized MWCNTs with flower-like Fe3O4nanocluster, the EM shielding effectiveness has increased significantly although the bulk conductivity has decreased. It is noteworthy to mention here that our previous study showed that the conjugation of both conducting and magnetic nanoparticles was superior to direct physical mixing of these nanoparticles.47 Now if we take a closer look at the absorption and reflection components of the total shielding efficiency, it is well evident that the reduction in bulk electrical conductivity facilitated in more EM radiation to penetrate thereby enhancing the absorption of these millimetre waves (Figure 6c-d).



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Figure 6: (a) EM shielding mechanism, (b) total shielding effectiveness, (c) shielding by absorption, (d) shielding by reflection of various blends Absorption driven shielding: effect of MWCNTs conjugated with flower-like Fe3O4 nanoparticles flower-like The primary mechanism of shielding is usually regarded as reflection where high conductivity and dielectric constant are the prime concern1. Moreover reflection of incident EM waves originates from the surface of the material due to impedance mismatching. So


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incorporation of only MWCNTs into the blend readily enhances the conductivity which may enhance the surface reflection phenomena. On the other hand, absorption mainly arises due to energy losses and subsequent dissipation into heat49. The losses during attenuation originate mainly due to the dielectric and magnetic losses that can be evaluated from the scattering parameters as relative complex permittivity and permeability parameters where, µr = µ‫ – ׳‬jµ‫ ׳׳‬and εr = ε‫ – ׳‬jε‫׳׳‬. These complex permittivity and permeability parameters are frequency dependent and the real parts mainly represent the storage ability whereas the imaginary parts correspond to the loss parameters (Figure S2)49. Polarization loss and conductivity loss are primarily attributed towards the dielectric loss. Magnetic permeability, on the other hand, is attributed to the incorporation of Fe3O4 nanocluster while high saturation magnetization is the main source of enhancement of initial permeability which is discussed earlier. The effect of magnetic nanoparticles depends largely on the dispersion state in the polymer matrix, which by conjugating flower-like Fe3O4 nanocluster with surface functionalized MWCNTs results in uniform dispersion of these clusters in the composites. It is well envisaged that for better absorption, the surface reflection should be minimum and this can be achieved by proper impedance wave matching. To achieve this, the characteristic impedance of the shield should be equal/close to that of the free space59. The characteristic impedance is determined by the ratio of real part of complex permittivity to permeability. If the real part of complex permittivity is much higher than the real part of complex permeability, most of the incident electromagnetic waves will be reflected off the surface due to low surface resistance rather than penetrating into the shield59. From Figure 7a, it is observed that both surface functionalized MWCNTs and flowerlike Fe3O4 nanocluster conjugated with surface functionalized MWCNTs show impedance wave matching closer to 1 when compared with only MWCNTs. The surface functionalization of MWCNTs reduces the bulk electrical conductivity of the composites as we discussed earlier and has a direct consequence on the real part of the permittivity. On the other hand conjugating flower-like Fe3O4 nanocluster also helps in impedance wave matching besides effective dispersion of the magnetic nanomaterials, which enhances magnetic permeability. The consolidated loss parameters, by evaluating dielectric tangent loss (tanδε = ε‫׳׳‬/ε‫ )׳‬and magnetic tangent loss (tanδµ = µ‫׳׳‬/µ‫)׳‬, are assessed here60. It is observed that after addition of surface functionalized MWCNTs conjugated with flower-like Fe3O4nanocluster, the total losses increase (Figure 7b). As discussed earlier, the total dielectric loss is mainly contributed by



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interfacial polarization and conductivity losses. As observed earlier, the charge transport was through hopping in this case; which results in conductivity losses. However, further oxidation of MWCNTs leads to enhancement in conductivity losses due to the presence of defect sites. Besides magnetic permeability loss, the enhanced consolidated loss is due to high saturation magnetization of flower-like Fe3O4 nanoclusterwhich is conjugated with MWCNTs. Hence, enhancement in consolidated loss clearly exhibits the enhancement of absorption efficiency in the blend. The attenuation constant (α)49 which is the ability of EM absorption can also be evaluated by the corresponding permittivity and permeability parameters. The attenuation constant can express as follows; α = √2

(‫ ׳׳ ׳׳‬− ‫ )׳ ׳‬+ ((‫ ׳׳ ׳׳‬− ‫ )׳ ׳‬+ (‫ ׳׳ ׳‬− ‫) )׳ ׳׳‬

(2)

where, c is the speed of light. So it is important for satisfying higher attenuation ability of materials, magnetic and dielectric losses should be as high as possible. Figure 7c, depicts that attenuation constant is increased significantly after addition of magnetic nanoparticles into the blend. Therefore, when external EM radiation encounters the designed shielding material, it guides the EM waves to come across a variety of microscopic boundary owing to the inclusions of hetero structure. In this case the local field variations can have considerable effect on absorption of energies at such heterogeneous boundaries due to the Maxwell Wagener polarization. Macroscopic inhomogeneities can have surprising impact on magnetotransport of the material. The magneto-dielectric behavior of the hybrid nanoparticles contain blend was analysed by magneto-capacitance (MC) measurements, by varying the magnetic field from 0 to 10000 Oe at 1000Hz. MC can be defined as,61 MC =

() () ()

100

(3)

Where ε (H) and ε (0) are the dielectric constant in the presence and absence of magnetic field respectively. The magneto capacitance study also favoured such polarization process in heterogeneous medium where dielectric constant changes by 5% as the magnetic field increased to 10000 Oe (Figure 7d). The change in the dielectric constant on varying the magnetic field is due to magnetostriction effect, which occurs due to change in lattice parameters62. So, if we apply magnetic field to a magnetoelelctric material, the material gets strained, and this strain


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further induces stress which gives rise to an electric field. This generated field is able to orient the ferroelectric domains, and hence dielectric behaviour is altered. So here magneto capacitance suggests that our synthesized materials have magento-dielectric property. Nevertheless the magneto capacitance study also shows that magneto-dielectric effect can be achieved in material system without any true magneto coupling63. The synergetic effect in screening EM wave in case of conjugated structures motivated us to take a closer look at the permittivity and permeability values. The surface functionalized MWCNTs ensure the maximum penetration of EM waves due to lower surface reflection phenomena by impedance matching while only MWCNTs containing blend shows reflection driven shielding. Further, based on the obtained parameters, a chart describing the best material to absorb the EM radiation is constructed in figure S3. The unique properties from the individual nanocomposites further led us to design a unique multi-layered architecture as illustrated in scheme 2.



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Figure 7: (a) Impedance matching, (b) consolidated loss part, (c) attenuation constant of various blends and (d) magneto capacitance of conjugated MWCNT-Fe3O4 nanoparticles


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Scheme 2: Shielding mechanism of different samples (i) MWCNT contain blend, (ii) MWCNTCOOH contain blend and (iii) MWCNT-Fe3O4 contain blend. Absorption losses and efficient heat dissipation The efficient heat dissipation is also an important parameter for designing modern age EM shielding materialas they undergo numerous heating-cooling cycles. The thermal images of various specimens were captured after irradiating them in a microwave at a constant frequency of 2.45 GHz for 4 s (Figure 8). The time- temperature response (Figure S4) of the specimen during cooling was evaluated directly from the infrared (IR) images (taken using an infrared camera FLIR T600). It is observed that the specimens follow an exponential decay with time. However, the cooling is remarkably faster for the blends containing MWCNTs conjugated with flower like Fe3O4 nanocluster when compared with the other specimens. This is an interesting observation. As these conjugated structures also showed higher attenuation constant, efficient heat dissipation together with higher absorption losses makes them potential candidates for new-age millimetre wave absorbers.



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Figure 8: Infrared images of the samples at different time interval during cooling following microwave irradiation for 4 s. (i, ii, iii, iv) MWCNT contain blend, (v, vi, vii, viii) MWCNTCOOH contain blend and (ix, x, xi, xii) MWCNT-Fe3O4 contain blend. Shield thickness and rational stacking of different nanocomposites The thickness of the shield material is also an important parameter. EM attenuation has direct correlation with the skin depth and is scales up with the thickness. Generally EM attenuation is governed by inter connected conducting and magnetic networks. Therefore attenuation of incident radiation is improved by increasing sample thickness as, the number of conductive and magnetic mesh increases and they are randomly placed into the matrix one after another. It is observed from figure 9a that in all cases total shield effectiveness is increased with increasing shield thickness. But enhancement of shielding efficiency with thickness is higher below the skin depth of each different sample. Skin depth (δ), intensity of penetration can be calculated by this equation;2


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δ = -8.68

(4)

where t is the thickness of the material. Skin depth is the intensity of penetration in to the conductive material which is inversely related to the absorption efficiency of the screening material2. The calculated skin depth is readily decreased from 4.9 mm (when only MWCNTs are present) to 1.3 mm after conjugating with flower-like Fe3O4 nanocluster, indicating a boost in EM attenuation property. The reduction of skin depth clearly suggests absorption driven shielding efficiency. For myriad application in modern technology, ultra-thin shield materials are desirable. However, reducing the shield thickness below the skin depth readily decreases the shielding effectiveness. As we discussed earlier, the advantages of using functionalized MWCNTs and conjugating it with ferrite nanoclusters; rationally stacking them in a specific sequence can further enhance the overall shielding efficiency. Hence, unique multi-layered stack is designed here following the advantages offered by individual functional nanocomposites. It is well established that the characteristic impedance should be equal/close to 1 for zero reflection from the surface of the shield. As observed earlier, good impedance wave matching was observed for functionalized MWCNTs. This ensures maximum penetration of incident EM waves inside the material. Hence, this particular nanocomposite was fixed as the outer layer of the multi-layered assembly. The inclusion of MWCNTs conjugated with flower like Fe3O4 nanoparticles in the blends readily enhances the shielding by absorption and hence, this particular layer was chosen as the next layer in the multi-layered assembly. This will help guide the incoming waves to interact with various microscopic boundary, where local field variations can have considerable effect on absorption of energies at such heterogeneous boundaries due to the Maxwell Wagener polarization64. In addition, the absorbed energy is dissipated through heat through the conducting network by MWCNTs as observed from the thermal images. The blend containing only MWCNTs showed reflection driven shielding and hence, this motivated us to use this layer as the third layer of the stack. This particular layer can scavenge the transmitted EM waves which pass through the middle absorber layer. By this rational arrangement, we aim to maximize the penetration and further accumulate the virtual charges at the interface of two layers having different dielectric constants and conductivities thereby resulting in interfacial polarization. This will significantly enhance the total EM shielding efficiency. Figure 9b clearly demonstrates the advantage of using



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multi-layered stack of different nanocomposites where in the total shielding efficiency is significantly higher than the individual material. The SEM micrograph of this layered structure is shown in the inset displaying sharp interfaces between the layers. Figure 9c illustrates the rational design of this multi-layered assembly towards effective shielding. Taking together, our study clearly demonstrates that by rational construction of multi-layered assembly, the overall shielding can be improved by many folds at much lower shield thickness (0.9 mm in this case). These materials and hence be explored as potential EM shielding materials.


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Figure 9: (a) Thickness dependent total shielding effectiveness of various blends, (b) total shielding effectiveness of multi-layer assembly, on inset SEM micrograph of multi-layer assembly, (c) schematic representation of multi-layer assembly and shielding mechanism Summary: Herein, we have demonstrated a strategic approach to enhance the shielding effectiveness by many folds by rationally stacking individual functional nanocomposites consisting of PC/PVDF blends with MWCNTs, functionalized MWCNTs and MWCNTs conjugated with flower-like Fe3O4 nanoclusters. The individual composites are systematically studied which allowed the understanding of the mechanism of shielding. While blends with only MWCNTs exhibited reflection dominated shielding, harsh chemical treatment to MWCNTs resulted though in lower bulk electrical conductivity but improved impedance wave matching. These functionalized MWCNTs when conjugated with flower-like Fe3O4 nanoclusters, illustrated in absorption driven shielding due to the presence of both electrical and magnetic dipoles together with multiple interfaces. In addition, the blends containing this conjugated structure dissipated heat effectively leading to potential EM wave absorber. The advantages of the individual layer further led to rationally stacking them in a specific sequence which resulted in a very high shielding efficiency of -64 dB at 18 GHz for a shield thickness of 0.9 mm. Table 1: Total shielding effectiveness of various samples with different thickness Compositions

Thickness (mm)

SET (dB)

PC/PVDF with MWCNT

0.3

-10

0.9

-16

0.3

-6

0.9

-10

0.3

-24

0.9

-32

PC/PVDF with MWCNT-COOH

PC/PVDF with MWCNT-Fe3O4



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PC/PVDF with Fe3O4

0.9

-4

Three-layered assembly

0.9

-64

Supporting Information: EDAX spectra of MWCNT-Fe3O4 contain blend when both phases are present, Complex permittivity and permeability parameters, Chart plot for material selection with permittivity and permeability parameters, Time temperature response of heat dissipation power, Parameters obtained from power law fitting on AC electrical conductivity. Notes The authors declare no competing financial interest.

Acknowledgement The authors gratefully acknowledge the financial support from DST-SERB (EMR/2016/001230), India.

References: (1)

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