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Applications of Polymer, Composite, and Coating Materials
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: sbose@iisc.ac.in
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
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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
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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
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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
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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
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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
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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
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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
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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)
= 10
| | (8)
|
| |
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
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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, 53The
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
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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
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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.
Materials
Filler loading
PVDF/ BaTiO3-GO/
MWCNT
Ref
Thickness
(mm)
Frequency
(GHz)
EMI SE
(dB)
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%)
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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
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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: sbose@iisc.ac.in
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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