CoreâMultishell Heterostructure with Excellent Heat Dissipation for

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Core-multishell heterostructure with excellent heat dissipation for electromagnetic interference shielding Yudhajit Bhattacharjee, Dipanwita Chatterjee, and Suryasarathi Bose ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10819 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 19, 2018

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ACS Applied Materials & Interfaces

Core-multishell heterostructure with excellent heat dissipation for electromagnetic interference shielding Yudhajit Bhattacharjee a, Dipanwita Chatterjee b and Suryasarathi Bose a* a

Department of Materials Engineering, Indian Institute of Science, Bangalore – 560012, India b

Materials Research Centre, Indian Institute of Science, Bangalore-560012, India

*Author to whom all the correspondence should be addressed: [email protected]

Abstract Herein, we report high electromagnetic interference (EMI) shielding effectiveness of -40 dB in the Ku- band (for a 600 µm thick film) through unique core-shell hetereostructure consisting of a ferritic core (Fe3O4) and a conducting shell (multiwalled carbon nanotubes, MWCNT) supported onto a dielectric spacer (here SiO2). In recent times, materials with good flexibility, heat dissipating ability and sustainability together with efficient EMI shielding at minimal thickness is highly desirable; especially if they can be easily processed into thin films. The resulting composites here shielded EM radiation mostly through absorption driven by multiple interfaces provided by the heterostructure. The shielding value obtained here is fairly superior among the different polymer nanocomposite based EMI shielding material. In addition to EMI shielding capability, this composite material exhibits outstanding heat dissipation ability (72ºC to room temperature in less than 90 s) as well as high heat sustainability. The composite material retained its EMI shielding property even after repeated heat cycles thereby opening new avenues in the design of lightweight, flexible and sustainable EMI shielding material. Keywords: core-shell, polymer nanocomposites, EMI shielding, heat dissipation, sustainability

Introduction In the recent years, unprecedented growth in digital electronics and smart communication sector enables scientist to fabricate robust, smarter, smaller and aesthetically sound devices. As electronics and their components operate at faster speeds and smaller sizes, a substantial increase in Electromagnetic interference (EMI) results, which can lead to malfunctioning and degradation of electronics.1-4 Conduction coupling of Electromagnetic (EM) signals and EM 1 ACS Paragon Plus Environment

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radiation are the predominant factors of these kind of interferences.5 The surge in numerous electronic devices in commercial, civil, and military fields generated excessive electromagnetic waves which in turn causes EM pollution in the human living space.6-8 As a result, the efficient EM screening materials, where reduction in EMI is envisaged either by reflection or by absorption from materials surfaces, through its dielectric (permittivity) or magnetic (permeability) losses9-12 are in huge demand for strategic as well as in commercial sector. Moreover, apart from this, state of the art EM screening materials should possess several qualities like light weight, low cost, flexibility and the ability to operate at wider bandwidth.13-15 Magnetic nanostructures of iron and its oxides have been of prime interest for EM wave absorption applications, as they preserve EM absorption ability in the high-frequency range due to their high saturation magnetization (Ms) and significantly large Snoek’s limit.16-17 The Snoek's limit18, provides cubic magneto crystalline anisotropy in the absence of an external EM field. However, the weak magneto crystalline anisotropy and attenuated permeability due to the eddy current phenomenon usually limit their usage as EM screener at high frequencies.19 Keeping iron as a core and coating them with an insulating material was the strategy utilized to elevate the surface anisotropy energy and reduce the eddy current effect.20 Wide-range of studies have been carried out by different groups on uniform coating of the metal oxides nanoparticles with silica shells.21-22 The silica shell not only enhances the colloidal stability but also controls the distance between the core particles within the assemblies through shell thickness in a composite.23-24 Apart from insulating shell, conducting shell coupled with magnetic core provides a significant alternative for EM screener.25 A combination of electrically conductive and magnetic materials could be more effective in enhancing screening capability synergistically.25-26 The electrical conductivity of conducting material allows the flow of eddy current induced by the magnetic field imparted by magnetic component and leads to absorption of EM radiation.27 Among the conducting carbonaceous materials, carbon nanotubes (CNTs) have clearly demonstrated better properties due to their high aspect ratio, higher strength and flexibility and lower density, making them ideal fillers.28 Further, the percolation can be achieved at minimal concentration due to high aspect ratio.29 Core-shell materials are versatile class of materials due to their enhanced properties with respect to core and shell individually.30 In the context of EMI shielding, core-shell materials with varied arrangement of either magnetic, conducting or

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dielectric system as the core or the shell plays an important role not only in providing better shielding values but also some fundamental insights to the composite systems.25, 31-32 Polymer nanocomposites and bi-phasic polymer blend nanocomposites evolved rapidly as one of the promising EM absorbers. Heterogeneous dispersion of conducting and magnetic nanoparticles in polymer matrix leads to synergistic absorption of microwave radiation in these systems.33-34 But materials derived out of these nanocomposites are often encountered with few drawbacks such as high filler loading and large thickness making them unsuitable for sophisticated applications. Among the various nanocomposites, polyvinylidene difluoride (PVDF) based nanocomposites have been studied for EMI shielding applications involving various kind of fillers. 2D dielectric materials such as graphene and others like barium titanate(BaTiO3) were used.35 Ferromagnetic inclusion such as alloy magnetic microcluster and paramagnetic inclusion such as cobalt (Co) nanowire were been also tested along with MWCNTs.35 Ceramics inclusion such as hollow glass microspheres along with MWCNT also provides satisfactory shielding values.35-37 However, inclusion of only single component (like conducting, magnetic or dielectric) failed to provide significant shielding values at smaller thicknesses. However, though satisfactory shielding may be achieved by increasing the filler loading, but may result in processing difficulties and poor structural properties in case of composites. In context to shielding at GHz frequencies, as discussed earlier, magnetic nanoparticle alone fail to provide effective shileding due to Snoek’s limit. However, decorating them onto 2D materials (like graphene oxide sheets) have proved to extend their capability to shield even at high frequencies38 besides improving their dispersion quality in the host. Although this strategy showed promising results, but the amount of filler loading required for effective shielding is usually high. In this context, Areif et. al39 suggested the use of FeCo ferromagnetic alloy heterostructure to shield GHz frequencies however, the thickness of the shield still remains a key challenge. Under this framework, we designed multi-component core-shell heterostructure which has the potential to deliver better shielding values at relatively lower filler loading and low thickness; extremely desired under the current trend. A novel heterostrucure comprising of Fe3O4 as a core and MWCNT wrapped SiO2 as shell was designed to shield microwave frequency. Incorporation of this hetereostrucure in polymer matrix (here PVDF) may create multiple interfaces due its unique morphology leading to enhanced shielding. We have



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designed various core-shell structures with magnetic core (Fe3O4) and dielectric (SiO2) shell (Fe3O4@SiO2); magnetic core and a conducting (polyaniline, PANI) shell (Fe3O4@PANI) and core-multishell structure (Fe3O4@SiO2@MWCNT) for effective shielding through multiple interfaces in the microwave frequency domain. The resulting core-shell and coremultishell structures were thoroughly characterized and composited with PVDF to design nanocomposites for effective shileidng of EM radiation. The EMI shielding effectiveness of these composites were evaluated systematically and their heat dissipating ability and their fate under repreated heat-cycles were assessed.

Experimental Section Materials Pristine MWCNTs (length 1.5 µm and diameter 9.5 nm) NC7000 was procured from Nanocyl SA (Belgium). Polyvinylidine fluoride (KYNAR-761) was purchased from Arkema (MW 4,40 ,000 g mol-1). Ferric chloride hexahydrate (FeCl3,6H2O), Polyethylene glycol (PEG3000),

Sodium

acetate

(NaAc),

dimethylaminopyridine(DMAP),

N,

N-dicyclohexylcarbodiimide

3-aminopropyl

triethoxysilane

(DCC),

(APTES),

4-

Tetraethyl

orthosilicate (TEOS), Aniline, Ammonium persulfate (APS) and were procured from Sigma Aldrich. Analy tical grades of chloroform, 28% ammonia solution, ethanol, N,Ndimethylformamide and tetrahydrofuran were obtained from commercial sources.

Synthesis of nanomaterials and nanocomposites Synthesis of Fe3O4 nanoparticles 6mmol of FeCl3 was dissolved in a beaker with 40 mL of glycol. To this solution, 43mmol of NaAc and 1g of PEG were added and the whole mixture was stirred for 30 min at room temperature. This mixture was then poured into a Teflon autoclave and heated at 2000C for 24 h. The autoclave was then allowed to cool to room temperature and the precipitate was collected via magnetic separation using lanthanide magnet. It was then repeatedly washed with DI water and ethanol and dried overnight at 1000 C. Synthesis of Fe3O4 @ PANI nanoparticles For the synthesis of Fe3O4@PANI nanoparticles, 0.1 g of Fe3O4 pre-synthesized nanoparticles were added into a two-neck flask (100 ml, capacity) containing 40 ml of 0.1 M 4 ACS Paragon Plus Environment

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HCl aqueous solution. The suspension was ultrasonically treated for 10 min, and then kept at 5ºC for 10 h under mechanical stirring. The HCl aqueous solution was decanted by applying an external magnet. Then, 10 ml of HPLC grade ethanol and 0.1 mL of redistilled Aniline were added into the flask. The mixture was kept at 5ºC using an ice water mixture bath in a nitrogen atmosphere ultrasonically treated for 12 h with intermittent stirring in between, followed by addition of 0.17 mL of 12 M HCl and dropwise addition of precooled APS aqueous solution (0.04 M, 30 ml). The mixture was treated with constant ultrasonication in a nitrogen atmosphere for further 3 h. The resulting product was magnetically collected, washed with distilled water and ethanol several times, and finally dried in a vacuum oven. Synthesis of Fe3O4 @ SiO2 nanoparticles Fe3O4 @ SiO2 were synthesised by treating pre synthesized Fe3O4 with TEOS. 100 mg of Fe3O4 was dissolved in 200 ml of ethanol and sonicated for 10 min. On to it 0.68 ml of TEOS was added (Fe3O4 and TEOS in 1:6 ratio) and further sonicated for 5 mins. On to it 2 ml of liquid ammonia was added and the solution was further sonicated for 6 h. It was then Centrifuged, collected and dried over vacuum at 100ºC.

Scheme 1: Schematic illustration of synthesis of various core-shell particles. Synthesis of amine terminated Fe3O4@ SiO2 nanoparticles



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Amine terminated Fe3O4 nanoparticles were synthesized by treating the pre-synthesized Fe3O4@SiO2 with APTES. 100 mg Fe3O4 nanoparticles were first dispersed in 40 ml of H2O2 by sonication for 2 h and then hydroxylated by keeping it for reflux at 105°C for 4 h. The particles were dried overnight at 100°C and dispersed in 100 ml of dry DMF by sonicating it for an hour. APTES was added dropwise into the mixture during sonication and subsequently, the mixture was kept for reflux in an inert atmosphere for 24 h at 105°C with constant stirring. The mixture was allowed to cool to room temperature and the particles were washed repeatedly with DI water and ethanol. The collected particles were then dried overnight at 100°C. Synthesis of acid functionalized MWCNT Acid functionalized MWCNTs were obtained by treating pristine MWCNTs with conc. HNO3. 100 mg of MWCNTs were taken in a beaker along with 90 ml of HNO3 and 10 ml of water sonicated for about an hour. The mixture was then kept under reflux at 80ºC for 24 h while stirring constantly. It was then cooled to room temperature and diluted with DI water. Solution was filtered and washed several times with water until pH = 6. The particles were then collected by filtration and dried at 80°C. Synthesis of fragmented acid functionalized MWCNT Acid functionalized MWCNTs were dispersed in DMF and probe sonicated for 1 h. The solution was centrifuged twice and dried under vacuum. Synthesis of heterostructure nanoparticles Amine terminated Fe3O4@SiO2 and acid functionalized MWCNTs were conjugated via condensation reaction. 100 mg of Fe3O4@SiO2-NH2 and 30mg of MWCNT were dispersed in DMF by bath sonication for half an hour. 10mg of DCC and 20mg of DMAP were added into the mixture and further sonicated for 15 min. The mixture was then refluxed under inert atmosphere at 105º C for 24 h with constant stirring.


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Scheme 2: Schematic illustration of Synthesis of heterostructure

Synthesis of Various nanocomposites PVDF/ MWCNT nanocomposite: PVDF was dissolved in dimethyl formamide (DMF) using bath sonication for 45 min. In a separate beaker MWCNTS dispersed in DMF using probe sonicator for 15 mins to remove primary agglomeration and then bath sonicated for one hour. Resulting MWCNT/DMF solution was poured into PVDF/DMF solution prepaered earlier and final solution was bath sonicated for an hour. Resulting solution was casted in Teflon mould and subsequently compression moulded film was made at 220ºC.

PVDF/ Nanoparticle/ MWCNT nanocomposites: Pre-synthesized nanoparticles and MWCNTs were probe sonicated in DMF for 20 min and bath sonicated for another 20 minutes in a different beaker. In a separate container, PVDF was taken in DMF and sonicated until it dissolves all the PVDF (approx. 45 mins). In to that, pre sonicated DMF solution containing nanoparticles and MWCNTs were mixed and further bath sonicated for 1 h. Resulting solutions were poured into a Teflon mould and kept for 7 ACS Paragon Plus Environment


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solvent evaporation. Finally solution casted films were compression moulded at 220ºC to obtain polymer thin film.

Characterization Magnetic property of the synthesized nanoparticles was assessed by Lakeshore Vibratory Sample Magnetometer (VSM) with an applied magntic field of −20000 to 20000 Oe at room temperature. X-ray diffraction was recorded using a XPERT Pro from PAN analytical. A Cu Kα radiation source (λ = 1.5406 A, 40 kV and 30 mA) was used to determine the XRD profile. Room temperature electrical conductivity of the blends was studied using an Alpha-N Analyser, Novocontrol (Germany) in a frequency range from 0.1 Hz to 10 MHz. EM shielding interference was studied by Anritsu MS4642A vector network analyzer (VNA). KEYCOM waveguide is used to measure the s parameters of the thin layered samples in the 12-18GHz frequency region. Raman spectra were recorded using a Horiba LabRam HR Raman spectrometer. Transmission electron micrographs and HAADF (high angle annular dark field) images were acquired using a FEI Technai F30 instrument operated at accelerating voltage of 300 kV. A Sirion XL30 FEG SEM with an acceleration voltage of 10 kV was utilized to determine the morphologies various nano-strucutre. FTIR spectra on the films were recorded on a PerkinElmer frontier by accumulating 16 scans over a range of 4000-600 cm−1 in ATR mode to obtain information about the covalent conjugation. UV-Visible spectroscopy measurements were carried out in diffused reflectance mode (DRS) using Perkin Elmer Lambda 750 spectrophotometer fitted with integrating sphere (60mm, Labsphere). Labsphere certified reflectance standards are used for baseline correction. The powder sample was tightly packed in a sample holder having quartz window. For heat dissipation measurements, sample was heated with laser (Synrad 48, tuneable CO2 laser of beam diameter of 3.5 mm and wavelength of 10.1 µm. Laser Power Used: 80 kW/m2). IR imaging was done by infrared camera: FLIR SC5200, operated at 25 fps. For heat stability experiment kaleidoscope programmable environmental chamber (KEW/PEC-70) were used.

Results and Discussion (a) Structure and properties of different nanostructures The morphology of various synthesized nanoparticles were characterized by SEM (Scanning electron microscopy) and bright field transmission electron microscopy (TEM) and is presented in Fig.1. The SEM and TEM images [Fig. 1a-b] of Fe3O4 nanoparticles confirm 8 ACS Paragon Plus Environment

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spherical morphology with size varying between 400-600 nm and indicating each sphere is polycrystalline and contains several finer grains of Fe3O4 [Fig.1c]. Coating of Fe3O4 particles with PANI retains the spherical morphology though makes the surface rough, as can be seen in SEM image [Fig. 1d]. A uniform shell of PANI forms around Fe3O4 core, observed from the bright field TEM images [Fig.1 e and 1f] and diffuse dark field image of Fe3O4@PANI is shown as an inset [Fig.1e]. Coating of Fe3O4 particles with SiO2 also retains the spherical morphology as can be observed in the SEM and bright field TEM images Fig. 1(g) and (h) respectively. High magnification bright field TEM image in Fig. 1(i) shows the presence of SiO2 shell. Fe3O4@SiO2 wrapped with MWCNT is shown in [Fig. 1(j) and bright field TEM image is represented [Fig.1 k-l]. The presence of MWCNT around the surface of Fe3O4@SiO2 particle can be clearly observed. EDS mapping (see SI, Fig. S2) further confirms the different element in the heterosturcture. X-Ray diffraction pattern of heterostructure confirm the spinel structure of Fe3O4 with peaks (220), (311), (400), (422), (511), (440) and (533) match as well with the JCPDS no. 88-0866 (see Fig.2a). It is observed that after modification there is no peak shift for the core Fe3O4 which confirms that the crystal structure of the core was unaffected despite various functionalization processes. An additional characteristics peak for (002) of MWCNTs indicates the presence of MWCNT in the heterostructure. The crystallite size of Fe3O4 as estimated using Debye-Scherrer method (see SI: Fig. S1and Table T1) is found to be 24 nm. It is observed to be 32 nm for Fe3O4@PANI, 31 nm for Fe3O4@SiO2 and 39 nm for Fe3O4@SiO2 wrapped with MWCNT (heterosturcutre). This additionally confirms that Fe3O4 actually contains finer crystallites or grains of Fe3O4. The size of the Fe3O4 crystallites increases with increasing time of treatment of Fe3O4 nanoparticles to form the other variants. In order to determine the nature of interaction between the interfaces.



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Figure 1. SEM image, bright field TEM image and HR-TEM images of Fe3O4 particles are shown in (a), (b) and (c) respectively. The HR-TEM image shows that each spherical Fe3O4 particle is polycrystalline and contains finer grains. Low magnification SEM image, bright field TEM image and high magnification bright field TEM images of Fe3O4@PANI particles are shown in (d), (e) and (f) respectively. The inset in (e) shows a diffuse dark field image of PANI coated Fe3O4 particle where it clearly shows an 10 ACS Paragon Plus Environment

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amorphous coating on Fe3O4 particle. (g) and (h) show low magnification SEM and bright field TEM images of Fe3O4@SiO2 particles respectively. (i) shows a high magnification image of the particle where the SiO2 shell is marked. (j) and (k) show low magnification SEM and bright field TEM images of Heterostructure respectively. High magnification image of the particle in (l) shows the SiO2 shell which is marked and a MWCNT wrapping its surface. Fourier-transform infrared (FTIR) spectroscopy of the samples were performed (see Fig 2(b)). In Fig.2b (bottom pane), peak at 2982 cm-1 confirms the OH stretching and peak at 1682 cm-1 indicating the presence of C=O stretch in acid functionalized MWCNT. In Fig.2b (middle pane) the peak at 1639 cm-1 represents the NH- bending and peak at 3737 cm-1 represents NH- stretching of amine terminated Fe3O4@SiO2 nanoparticles. The acid groups of MWCNT and surface functionalized amine group of Fe3O4@SiO2 react covalently to form the amide linkage, the signature of which is observed in the Fig. 2b. Peak at 1633 cm-1 depicts the C=O stretching for amide linkage and the doublet at 3777 cm-1 and 3707 confirms the NH- stretching in Fig.2b (upper pane). These finding provides a strong indication towards the formation of final heterostructure which is also confirmed by SEM and TEM micrograph as discussed earlier. Fig. (S4) confirms the formation of polyaniline coating over Fe3O4.



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Figure 2. (a) XRD patterns of the Fe3O4, Fe3O4@PANI, Fe3O4@SiO2 and heterostructure. (b) FTIR spectra of acid functionalised MWCNT, amine functionalised Fe3O4@SiO2 and heterostructure respectively. Fig.(S5) shows the TGA profile of acid functionalized MWCNTs, amine terminated Fe3O4@SiO2 and heterostrucutre. From the profile it is observed that the peak degradation temperature of acid functionalised MWCNTs, amine terminated Fe3O4@SiO2 and final heterostructure is ca. 160ºC, 235ºC and 280ºC respectively. Enhancement in the degradation temperature for heterostrucuture from its parent component indicates the strong covalent linkage between the amine terminated Fe3O4@SiO2 and acid functionalised MWCNT. The organic content in the heterostructure is ca. 9% and is found to increase with respect to amine terminated Fe3O4@SiO2 (ca.7%). The EMI shielding by absorption is the preferred way of screening the unwanted interference. EM radiation is the proliferation of both electric and magnetic components. Thus significant absorption necessitate electric and/or magnetic dipoles that can interact with the incoming electromagnetic field. The room temperature magnetometric studies (M−H hysteresis loops) of various synthesized particles are shown in Figure 3a. The saturation magnetizations of the Fe3O4, Fe3O4@PANI, Fe3O4@SiO2, and heterostructure were estimated to be 61.7, 48.6, 31.4 and 9.41 emu/g, respectively. The coercivity (Hc) and remanent (Mr) magnetization values were also obtained. A noticeable decrease in saturation magnetization of Fe3O4 was observed after coating with PANI, SiO2, and further decrease was observed in case of the final heterostructure where MWCNTs were wrapped on Fe3O4@SiO2. The spread in Ms values can be correlated with the existence of magnetic dead layer on the nanostructure surface. Table 1 shows different values of magnetic parameters. The higher coercivity value implies larger magnetic anisotropy leading to high-frequency resonance in terms of anisotropy constant (K), anisotropy energy (Ha), and resonance frequency (fr) as evaluated on the basis of the following equations40

= µ0MsHc / 2

(1)

Ha = 4 |K| / 3 µ0Ms

(2)

2πfr = rHa

(3)

K


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where µ0, r, and Ha stand for the universal value of permeability in free space (4π × 10−7 H m−1), gyromagnetic ratio, and anisotropy energy, respectively. The interrelationships of these equations suggests higher values of Hc lead to larger magnitudes of K (anisotropy constant), (Ha), and (fr).

Table 1: Room temperature magnetic parameters of different particles.

Sample

Saturation

Coercivity (HC)

Remanent

magnetization (MS)

(Oe)

magnetization

(emu/g)

(Mr) (emu/g)

Fe3O4

61.7

154

11.4

Fe3O4@PANI

48.6

162

9.3

Fe3O4@SiO2

31.4

158

5.5

Fe3O4@SiO2@MWCNT

9.4

160

1.8

(Heterostructure)

Fig.3b, depicts the Raman spectra of MWCNTs and acid functionalised MWCNTS. The spectrum clearly delineates two signature bands; G-band which corresponds to the stretching of sp2 hybridized carbon atoms and D-band which corresponds to the disordered carbon atoms and defects. The intensity ratio of the bands (ID/IG) indicates the amount of disorder and defects in the structure. It is observed that the acid functionalization of MWCNTs introduced a greater number of defect sites when compared with its pristine counterpart. Fig. 3c, depicts solid state UV-vis spectra of different synthesized nanostructures observed earlier. As the crystallite size and also the bulk size of the system increases from Fe3O4, Fe3O4@SiO2, and MWCNT wrapped heterostructure, the corresponding red shifts were observed from parent nanostructure (Fe3O4) and thus confirms the surface modification in various steps.



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Figure 3: (a) Hysteresis loops of various component at 300 K. Inset shows the enlarged areas of the M−H curves to depict the room temperature coercivity and remanant magnetization. (b) Raman spectra of MWCNT and acid functionalised MWCNT. (c) UV-vis spectra of various component. (b) Charge transport: Reflection of incident EM waves is the primary mechanism of shielding. The shield must have sufficient electrical conductivity and mobile charge carriers to achieve this phenomenon.41 However, high electrical conductivity is not the main criterion for attenuating EM radiation as conduction requires connectivity of the conduction pathways, which is mainly obtained above the percolation threshold in case of high aspect ratio materials like carbon nanotubes. It is known that the interconnected network of conducting nanoparticles is 14 ACS Paragon Plus Environment

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the prime requisite for charge transportation through an insulating polymer matrix.42 In this case, the percolation threshold for MWCNT in PVDF was estimated [Fig.S6(a)]. PVDF being insulating in nature, its conductivity scales linearly with frequency.43 After incorporation of MWCNT in the PVDF matrix conductivity goes up for a concentration region 0-1 wt %. [Fig. S6(a)]. Fig.S6(c) shows a growth in conductivity is obtained after incorporation of 3 wt % MWCNTs into the composites. Interestingly, after incorporation of Fe3O4 and core shell nanoparticles into the composite, the composite conductivity is decreased [Fig. S6 b-c]. This may be due to impediment of the interconnected network of MWCNTs, although the effects are not significant. The charge transport phenomena of the conducting network was further analysed by fitting the AC electrical conductivity data with a power law44 (eq.4)

σ/ (ω) = σ (0) + σAC (ω) = σDC + Αωn

(4)

where ω is the angular frequency, σDC is the direct electrical conductivity, A is the temperature dependent constant, and n is the exponent. The exponent is the measure of the three-dimensional (3D) network of capacitor or resistor and depends on both frequency and temperature, and the value is in the range of 0−1.

A sharp increase in σDC after incorporation of MWCNTs indicates dynamic percolation of the nanotubes. Conductivity further decreased with respect to bare Fe3O4 after incorporation of Fe3O4@SiO2 in finely percolated MWCNT. This happens due to the polarization loss because of incorporation of dielectric shell around Fe3O4. Moreover as the size of the core-shell structure has increased with respect to core so the impediment is more prominent which leads to decrease in conductivity. We expect a jump in the conductivity with respect to Fe3O4@SiO2 when Fe3O4@PANI was incorporated [Fig. S6(c)] in the percolated network of MWCNT. Due to its intrinsic conducting nature, PANI imparts an order of magnitude higher conductivity in the system though it also hinders the interconnected network of MWCNT. In this study, various core-shell and core-multshell structures were designed to systematically understand their effects on shileidng EM radation. As conducting MWCNT shell on ferritic core is difficult to design hence, we used a dielelctric (here SiO2) spacer to wrap MWCNT on the ferritic core. We compared the EM shielding capability of the resulting heterostructure with MWCNT wrapped ferritic core to gain insight on the effect of multiple interfaces on the 15 ACS Paragon Plus Environment


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EM shielding ability of the material. For the heterostructure coupled with MWCNT a significant increase in conductivity for Fe3O4@PANI, Fe3O4@SiO2 and with respect to only Fe3O4 was obtained which implies fragmented acid MWCNT which were wrapped around the heterostructure helped in effective charge transport. Heterostructure alone in PVDF showed a significant

decrease in the conductivity as the fragmented acid MWCNTs which were

wrapped around the Fe3O4@SiO2 failed to provide strongly interconnected percolated pathway in the host matrix.

(c) EMI shielding Property of composite material The EMI shielding effectiveness of various composited studied here is shown in Fig. 4a. EM shield effectiveness (SET) is a combination of shielding by absorption (SEA), reflection (SER) and multiple reflection (SEMR).45 But due to the increasing absorption of reflected waves from the internal surface, the multiple reflections can be ignored when the shield thickness is greater than the skin depth. In this context, SET can be expressed as, (5)

SET = SEA + SER

From the vector network analyser (VNA), the total shielding effectiveness (SETotal) can be estimated by the following relation through scattering parameters where the scattering parameters are the representatives of reflected, absorbed and transmitted power,

= 10

| |

= 10

| |

(6)

Where S12 is a reverse transmission coefficient whereas S21 is a forward transmission coefficient. With the help of these coefficients we can express the total reflection and absorption by the following equations,

= 10 |

(7)

(8)

|

= 10

| | | |

where S11 is a forward reflection coefficient. All scattering parameters were measured here in the frequency region 12-18 GHz (Ku-band). The shielding effectiveness by absorbtion, as derived from above equations, is illustrated in Fig. 4b. From the EMI theory it is understood that in order to screen EM radiation, materials should have sufficient electrical conductivity. In case of polymer nanocomposites, the connectivity of the conducting nanoparticles is essential for achieving bulk electrical conductivity. In the case of MWCNT filled polymer, the dielectric losses are higher and the attenuation is due to wave reflection rather than 16 ACS Paragon Plus Environment

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absorption.46 The shielding mechanism is dominated by reflection when MWCNTs alone are considered due to their lower dielectric loss at below percolation and poor magnetic properties.46 But heterogeneous inclusion can alter the shielding mechanism from reflection to absorption through various loss parameters.47-48 Generally, when an EM wave interacts with such heterogeneous component, the resulting local field variation can have a strong effect on the energy of absorption.

To begin with, when MWCNTs were incorporated in PVDF matrix, a gradual increase in shielding efficiency with increase in concentration was observed (Fig.S7), which also correlates with conductivity of the system as increase in concentration give rise to enhanced electrical conductivity and enhanced electrical conductivity would substantially contribute to the EMI shielding performance.49 PVDF/MWCNT

composites showed predominant

reflection.46 As for reflection, there is slight increment with the increased concentration of MWCNTs, as amount of mobile charge carriers plays a decisive role for SER. But absorption increases significantly due to increasing MWCNT content as gradual increase in the MWCNT concentration would result in higher complex permittivity and more conductive networks serve as dissipating mobile charge carriers. 50-51



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Figure 4: EMI shielding properties of various PVDF nanocomposites (a) Total shielding effectiveness with respect to frequency (b) total absorption ability with respect to frequency, (c) % absorption and % reflection .

It is envisaged that polarization loss and conductivity loss constitutes to overall dielectric loss. Magnetic permeability, on the other hand, is attributed to the incorporation of ferrimagnetic nanoclusters while high saturation magnetization is the main source of enhancement of initial permeability and thereby leading to increase in absorption.

52

As a

result, incorporation of Fe3O4 nanoparticles coupled with MWCNT shows a significant increase in the absorption with respect to only MWCNT in PVDF matrix. The interfacial dielectric polarization of MWCNTs results in the absorption of the electric component at the surface, whereas magnetic hysteresis losses through incorporation of ferrimagnetic nanoparticles leads to absorption of magnetic vector component of the incident electromagnetic wave. Now functionalisation of Fe3O4 nanopartices with polyaniline increases the reflection. This is due to the fact that the permittivity values increases after incorporation of conducting component in the system25 which manifests in high storage capability and dielectric loss due to higher electrical conductivity of carbon.25,

53

The

nonmagnetic silica coating on the magnetic nanoparticles enhances the effective reluctance of the polymer nanocomposites and thus results in the relatively weak frequency dispersion phenomena32 compared to conducting and magnetic coating. As a result, slight increase in the 18 ACS Paragon Plus Environment

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absorption [Fig. 4(c)] in the case of Fe3O4@SiO2 was observed as compared to Fe3O4 nanoparticles coupled with 3 wt % MWCNT. As it is known that the magnetic loss of originates mainly from hysteresis, domain wall resonance, natural ferromagnetic resonance and the eddy current effect, the observed changes in Ms doesn’t scale with SET. The hysteresis loss comes from irreversible magnetization and is negligible in weak applied field. The domain wall resonance occurs only in multi-domain materials and usually in the 1–100 MHz range.54-55 In this study all the EM parameters were measured at 12-18 GHz frequency range, so domain wall resonance is the main contributor to magnetic loss of the nanoparticles. In case of heterostructure, creating multiple interfaces of various properities (magnetic, dielectric, conducting) was lead to increase in shielding effectiveness. This is due to the synergestic influence of individual interface which lead to various losses in the system in which eddy, conduction and dielectric loss plays a pivotal part.

56-57

Furthermore, it is

observed that the coercivity of the hetereostructure has increased from its parent material (Fe3O4), indicating higher anisotropy. Now, we know that for high anisotropy material, the high activation energy barrier for the spin reversal takes place at high magnetic field and for this reason magnetic anisotropic energy (Ha) remains the main factor for determining the magnetic loss of any core shell composite which is expressed in Eq(2). As the saturation magnetization (Ms) value changes with the shell structures it actually enhances the anisotropic contribution which results in absorption of EM wave particularly at higher frequency region. Moreover, it can be also inferred that, the interfacial polarization of MWCNTs coupled with the heterostructure leads to higher shielding values of the final composite materials. Heterostructure alone in PVDF fails to show a significant SE values as very little amount of fragmented acid MWCNT which was wrapped around the Fe3O4@SiO2 nanoparticles were not sufficient enough to produce sufficient conductive paths through PVDF matrix. In recent times, multi-layered architecture has offered some promise in desiging effective EMI shielding materials as they facilitate in multiple air-dielectric interfaces and improved impedence mismatch in the system.14 This drive us to study the potential of stacked multilayers containing heterostrucure-MWCNT (each having thickness of 100 microns) for effective shielding. The 3D surface plot Fig.5(a) showing thickness versus shielding efficiency in the frequency range 12-18 GHz. It is observed that at very low thickness (100 µm), our materials show remarkable shielding efficiency -23 dB (> 99 % attenuation) and when we increase the thickness of this multi-layered stack to 600 µm, the shielding efficiency 19 ACS Paragon Plus Environment


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increased from -23 dB to -40dB (> 99.999% attenuation). Fig 5b. showing the mechaniscm of shielding. Table 2 lists the competitiveness of our designed material with the recent state of the art.

Figure 5: (a) 3D representation of total shielding effectiveness for PVDFheterostructure –MWCNT nanocomposite as a function of thickness in the frequency range of 12-18 GHz. The color bar represents SET (dB) values. (b) Schematic illustration of EMI shielding mechanism in PVDF- heterostructure-MWCNT composites.


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Table 2: Comparison Chart of various PVDF based composites for EMI shielding. Thickness (mm)

Frequency (GHz)

EMI SE (dB)

Ref

Materials

Filler loading

PVDF/ BaTiO3-GO/ MWCNT

BaTiO3-GO (5 Vol %) MWCNT (3 wt%)

5

18

- 27

35

PVDF/ CoNw’s/ MWCNT

CoNW’s (2.2 Vol %) MWCNT (3 wt%)

5

18

-33

35

PVDF+ Carbonyl Iron powder (CIP)

CIP (50 vol%)

1.2

12

-20

58

chlorinated polyethylene (CPE)/carbon nanofiber (CNF) PVDF/ FeCo nanocubes/ MWCNT

CNF (25 Wt %)

2

12

-42

59

FeCo nanocubes (10 wt%) MWCNT (5 Wt%)

5

18

-39

36

PVDF/ FeCo nanorod/ MWCNT

FeCo nanorod (10 wt%) MWCNT (5 Wt%)

5

18

-44

36

PVDF/rGO-MDAFeCo/MWCNT

rGO-MDA-FeCo (10 wt%) MWCNT (5 wt%)

5

18

-41

43

PVDF/Ag-np/graphite

Ag-np (5 wt%) Graphite (10 wt%)

1

12.4

-29

60

PVDF/Hollow glass microspheres (HGMs)/MWCNT

HGN (2 wt %) MWCNT (10 wt%)

2

12.4

- 43

37

PVDF/(Fe3O4)decorated polyaniline/single wall carbon nanohorn (SWCNH))

Fe3O4- decorated polyaniline (10 wt%) SWCHN (1wt %)

2

>18

-30

61

PVDF/Wool-ball heterostructure

10 wt% wool-ball heterostructures

3

18

35

39

PVDF/Heterostructur e/ MWCNT

Heterostructure (10wt%) MWCNT (3 wt%)

0.1

18

-23

0.6

18

-40

This work

(d) Heat dissipation and sustainability The conventional EM screening devices are frequently subjected to overheating owing to prolonged exposure to various radiation.62 With the miniaturization of various electronic interfaces, an effective thermal management is desired for high-end sophisticated product design. Heterostrucutre composites containing a small weight fraction of MWCNTs (3%) 21 ACS Paragon Plus Environment


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offer remarkable heat dissipation ability, largely due to its high carbonaceous content with respect to other variants. It is well known that CNTs display excellent thermal conductivity in the range of 2000-6000 W/(m K).63 The high thermal conductivity in carbon derivatives is related to low thermal conduction by phonons.64 For the polymer-based composites, however, the lesser polymer-filler contact area often results in reduced thermal conductivity.65 Conversely, it is assumed that large surface area of hetereostructure and MWCNTs offer improved interfaces within the polymer matrix, resulting in minimum thermal resistance in the composites.

66

As per the heat dissipation is concerned, we know that all objects emit

radiation at all temperatures. The spectrum of this radiation depends on the temperature of the body, emissivity etc.67 This emitted radiation can be analysed to determine the temperature of the object. We have obtained the emissivity value of our composite materials which were close to 0.92. As per the literature, the emissivity of neat PVDF film is 0.8. 67 The composite of thickness 5 mm was heated with a laser for 10 s and once the sample reached the desired temperature, laser was switched off and the thermal map of heat dissipation was recorded using infrared camera. This IR camera processed the received spectrum and determined the temperature of the surface. The transient temperature responses of the heterostructure coupled with very small amount of MWCNT (3 wt %) in PVDF matrix are illustrated in Figure 6a. Time-temperature response (Fig.6b) of the composite during cooling was evaluated directly from the infrared (IR) images. It is observed that the cooling was exceptionally fast and the curve follows an exponential decay with time for the composite system. Thermal images of other nanocomposites (Fig. S9) also follow the same exponential decay slightly at a slower rate due to the variation in the thermal conductivity ( Fig S11) . In case of heterostructure/ MWCNT composites , MWCNT in the matrix and wrapped MWCNT in the heterostrucutre helps in effective transport and as a result interfacial resistance to phonon propagation, which is accountable for thermal transport, is reduced.68-69

PVDF is an insulator and its degradation temperature is around 350 ºC making them capable of being heat stable at least for the application considered here.70 In order to study the fate of the material under different heat-cycles, an experiment was conducted wherein thin films of the composite samples were put in a programmable environmental chamber. Each films were kept at four different temperatures (50ºC, 60ºC, 70ºC and 80ºC) in the chamber with a hold time of 10 min at a particular temperature. The rationale behind choosing such temperature is that the material under consideration is frequently subjected to different heat-cycles. EMI shielding values were measured after each temperature ramp (see Fig.6c and Fig.S10) and no 22 ACS Paragon Plus Environment

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observable change in the shielding values were recorded thus confirming that these materials can sustain its EMI shielding capability at high temperature.



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Fig 6: (a) Infrared images of the Heterostrucutre-MWCNT- PVDF nanocomposite at different time interval during cooling following laser irradiation for 10 s (b) transient temperature responses of Heterostructure -MWCNT- PVDF nanocomposites. (C) Shielding effectiveness vs Temperature Ramp profile for Heterostrucutre - MWCNTPVDF nanocomposites thin film. Conclusion In summary, we synthesized and fabricated a unique core-shell and core-multishell heterostructure and composited with PVDF to design effective EMI shielding material. Our composite films of 600 µm exhibited high EMI shielding effectiveness of 40 dB (>22 dB for a 100 µm film) in the Ku- band, mostly through absorption (77%). The obtained shielding values are fairly competitive when compared with the available lietarture on PVDF based EMI shielding materials. Furthermore, this composite exhibit outstanding heat dissipation ability (72ºC to room temperature in less than 90 seconds). The composite material retained its shielding property even after subjecting to various heat-cycles suggesting good heat sustainability. We believe that the versatility of this strategy will open up new avenues not only in the product design but also in deciphering some fundamental insights on coremultishell based EM shielding material. Acknowledgement The authors would like to thank DST (India) and INSA for the financial support. They sincerely acknowledge Mr. Rudra Narayan Samajdar (SSCU, IISc) for conductivity measurements , Mr. Lijun T. Raju (Department of Mechanical Engineering, IISc) for heat dessipition measurements and Ms. Deepali Sonawane (Department of Materials Engineering , IISc) for heat stability experiment. Authors also acknowledge Dr. Injamamul Areif (Université Claude Bernard, Lyon) and Mr. Sourav Biswas (NIT, Durgapur) for their valuable inputs. Supporting Information: Calculation of crystallite size of the sample from XRD by Debye-Scherrer equation, SEMEDS mapping, SEM image of fragmented MWCNT, IR plot of Fe3O4@PANI, TGA profile of various component, Conductivity Plot of various composites, Shielding effectiveness plot with respect to frequency for different MWCNT content in PVDF, EMI SE for different


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arrangement of multi-layered stack, Transient temperature responses of various PVDF based naocomposites , Heat sustainability for various composites. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] Notes: The authors declare no competing financial interest. ORCID Yudhajit Bhattacharjee: 0000-0001-9079-7516 Dipanwita Chatterjee: 0000-0002-4783-5078

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