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Lightweight Electromagnetic Interference Shielding Materials and Their Mechanisms

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Lightweight Electromagnetic
Interference Shielding Materials
and Their Mechanisms
Liying Zhang, Shuguang Bi and Ming Liu
Motivated by the increase of stress over electromagnetic pollution issues arising
from the fast-growing development and need for electronic and electrical devices,
the demand for materials with high electromagnetic interference (EMI) shielding
performance has become more urgently. Considering the energy consumption in
real applications, lightweight EMI shielding materials has been attentive in this field
of research. In this chapter, first of all, the EM theory will be briefly discussed.
Secondly, the EMI shielding performance and corresponding mechanisms of three
categories of lightweight materials, such as polymer-based composites, foams and
aerogels, are reviewed. Finally, the summary and conclusions of this field will be
Keywords: lightweight, EMI shielding, polymer-based composite, foam, aerogel
1. Introduction
Electromagnetic (EM) waves are generated when an electric field comes in
contact with a magnetic field. The oscillations of the electric field and the
magnetic field are perpendicular to each other and they are also perpendicular to
the direction of EM waves propagation. EM waves travel with a constant velocity
of 3.0 10
m/s in vacuum. Unlike mechanical waves (sound waves) which need
a medium to travel, EM waves can travel through anything, such as air, water, a
solid material or vacuum. EM radiation refers to the EM waves, propagating
through spacetime, carrying EM radiant energy [1]. It is a form of energy that is all
around us. Human activities like using global positioning system (GPS) device to
navigate precise location, heating up a food in a microwave or using X-rays detec-
tion by a doctor would be impossible without EM radiation. Figure 1 shows the EM
spectrum used to describe different types of EM energy according to their frequen-
cies (or wavelengths). The EM spectrum ranges from lower energy waves (longer
wavelength), like radio waves and microwaves, to higher energy waves (shorter
wavelength), like X-rays and gamma rays. As for the radiated emission which is
focused on in this chapter, the frequency locates in the radio frequency spectrum
(3 KHz300 GHz).
Electromagnetic interference (EMI) is a disturbance generated by conduction or
external radiation that affects an electrical circuit. The interference emission
sources are from the conducted emission (several KHz30 MHz) to the radiated
emission (30 MHz12 GHz) [2]. The conducted emission is the noise which is
internally generated from the poor designed electrical circuit such as electrical
cables and power wires. The radiated emission that is externally generated is in the
form of transmitting EM waves such as the intended EM radiation from the radio
broadcasting antenna and the unintended EM radiation from the high-speed trans-
ceivers. While detecting the EMI shielding of the device, it is usually relevant to the
radiated emission lonely. The conducted emission is another subject especially for
the noise prevention in system level.
EMI is encountered by all of us in our daily life and are expected to face
exponential rise in future due to the growing numbers of wireless devices and
standards, including cell phones, GPS, Bluetooth, Wi-Fi and near-field communi-
cation (NFC). Great effort has been dedicated for the development of EMI
shielding materials. EMI shielding can be achieved by prevention of EM waves
passing through an electric system either by reflection or by absorption of the
incident radiation power. In the past, metals were conveniently used in many
occasions. Among them, galvanized steel and aluminum are the most widely used.
Copper, nickel, pre-tin plated steel, zinc and silver are also used for some pur-
poses. When the trend in todays electronic devices become faster, smaller and
lighter, metals are disadvantageous in weight consideration. Moreover, the EM
pollution is not truly eliminated or mitigated since the EM signals are almost
completely reflected at the surface of the metal protecting the environment only
beyond the shield [3]. Hence, intensive research efforts have been focused on the
development of EMI shielding materials that work by tunable reflection and
absorption based on novel materials that possess lightness, corrosion resistance,
flexibility, easy processing, etc.
This chapter is divided into two sections. In the next section, we will describe the
EMI shielding theory in details and the parameters that influence the shielding by
reflection and absorption. After that, we introduce three categories of lightweight
EMI shielding materials, namely, polymer-based composites, foams and aerogels.
Figure 1.
A diagram of the EM spectrum showing various properties across the range of frequencies and wavelengths.
Electromagnetic Materials
2. EMI shielding theory
The EMI capability of a material is called shielding effectiveness (SE). It is
defined in terms of the ratio between the incoming power (P
) and outgoing power
) of an EM wave as [4]:
SE ¼10 log Pi
 (1)
The unit of EMI SE is given in decibels (dB). According to Eq. (1), how much
attenuation is blocked at given SE is given in Table 1.
2.1 Far field and near field
According to the distance rbetween the radiating source and the shield material,
an EM wave can be divided into near field wave and far field wave relative to the
total wavelength λof the EM wave. As shown in Figure 2, the region within the
distance r>λ/2πis the far field while the distance r<λ/2πis the near field.
In far field, the EM waves can be regarded as plane waves and EMI should
consider both electric field (E) and magnetic field (H) effects. It fulfills the condi-
tions as follows,
Hjj (2)
SE (dB) 20 30 40 50 60 70
Attenuation % 99 99.9 99.99 99.999 99.9999 99.9999
Table 1.
Shielding effectiveness and attenuation %.
Figure 2.
Wave impedance in far field and near field [5].
Lightweight Electromagnetic Interference Shielding Materials and Their Mechanisms
where Zis the intrinsic impedance or what is sometimes called wave impedance.
|E| and |H| are the electric and magnetic fieldsamplitudes, respectively. For air
), the wave impedance (Z
) is always equal to 377 Ωand can be
expressed as
where σis the electrical conductivity, μ
is the relative permeability (μ=μ
), μ
is the permeability of air (4π10
H/m), ε
is the permittivity of air
(8.85 10
In near field, the wave front is curved instead of planar, so the wave front is
not parallel to the surface of the shielding material. In this case, the wave imped-
ance (|E|/|H|) is not constant and depends on the distance and the dominant
field. For an electrical radiation source, the electrical field dominates. The wave
impedance is higher than 377 Ωand decreases as the distance rincreases. It can be
expressed as [5].
For a magnetic radiation source, the near field is mainly magnetic. The wave
impedance is lower than 377 Ωand increases as the distance rincreases, it can be
expressed as [5].
In this chapter, all the formulations and results are taken based on far field
condition because a distance of 48 cm associated with operating at a frequency of
100 MHz is already considered as far field.
2.2 EMI shielding mechanisms for homogeneous shield materials
Figure 3a illustrates the reflection and transmission of an EM wave when it
strikes on a shield material. The uniform EM wave with the electric field E
magnetic field H
is normal incident to the material. When the EM wave strikes the
left boundary of the material, portions of the EM wave are reflected in the opposite
Figure 3.
(a) Schematic illustration of EM plane wave is normal incident to a material with thickness tand
(b) schematic illustration of attenuation of an incident EM wave by a shield material (thickness of shield
material = t).
Electromagnetic Materials
direction with the electric field E
and magnetic field H
. Other portions of the EM
wave are transmitted though the material with the electric field E
and magnetic
field H
. The electric field SE can be expressed as:
SE ¼20 log Ei
The magnetic field SE can be expressed as:
SE ¼20 log Hi
Theoretically, the SE of a material is contributed from three mechanisms
including reflection, absorption and multiple-reflections., the materials with mobile
charge carriers (electrons or holes) can interact with the incoming EM wave to
facilitate reflection. Absorption depends on the thickness of the shield materials. It
increases with the increase of the thickness of the shield materials. For significant
absorption, the shield materials possess electric and/or magnetic dipoles which
could then interact with the EM fields. Multiple-reflections is the third shielding
mechanism, which operates via the internal reflections within the shield material.
Therefore, the overall SE is the sum of all the three terms:
SEoverall ¼SERþSEAþSEMR dBðÞ (9)
The EMI SE of the material depends on the distance between radiation source
and the shielding material. When the radiation source is far from the shielding
material, the SE is called as far field SE. In the case of the short distance between
radiation source and the shielding material, the SE is called as near field SE.
Figure 3b illustrates three EMI shielding mechanisms in a conductive shield
material. When an EM wave strikes the left boundary of the homogenous conduc-
tive material, a reflected wave and a transmitted wave will be created at the left
external and right external surface, respectively. As the transmitted wave propa-
gates within the shield material, the amplitude of the wave exponentially decreases
as a result from absorption, and the energy loss due to the absorption will be
dissipated as heat [6]. Once the transmitted wave reaches the internal right surface
of the shield (t), a portion of wave continues to transmit from the shield material
and a portion will be reflected into the shield material. The portion of internal
reflected wave will be re-reflected within the shield material, which represents the
multiple-reflections mechanism. The skin effect would affect the effect of multiple-
reflections to the overall shielding to a great extent. The depth at which the electric
field drops to (1/e) of the incident strength is call the skin depth (δ), which is given
as follows [7]:
where fis frequency (Hz). μand σare the magnetic permeability and the
electrical conductivity of the shield material, respectively. If the shield is thicker
than the skin depth, the multiple-reflections can be ignored. However, the effect of
multiple-reflections will be significant as the shield is thinner than the skin depth.
As shown in Figure 3b, in case the shield material is a good conductor, Z
then [8].
Lightweight Electromagnetic Interference Shielding Materials and Their Mechanisms
SER¼20 log Ei
¼20log Z0þZm
20log Z0
where Zm¼ffiffiffiffiffiffiffiffiffiffi
can be expressed as [8].
SEMR ¼20 log 1
¼168 þ10 log σr
 (12)
where ω=2πf,σ
is the relative conductivity of the material, it is related
to the electrical conductivity of the copper, the electrical conductivity of copper is
= 5.8 10
S/m. If the shield material possesses electric and/or magnetic dipoles,
the attenuation of incident EM wave happens inside the shield material due to the
absorption and multiple-reflections, the amplitude of the EM wave declines during
wave traveling, and it can be expressed as [8].
SEMR ¼20log 1Z0Zm
20log 1e2t=δe2jt=δ
where tis the thickness of the shield material, δis the skip depth under the
operation frequency, βis the propagation constant.
The mechanism of multi-reflections is complicated. For a good conductor
material, the multiple-reflection is usually insignificant because most of the inci-
dent EM waves are reflected from the external conductive surface of the shield
material, and only few penetrated EM waves can be retained for multiple-
reflections. The influence is more important for a material that has high perme-
ability and low electrical conductivity. In this case, EM waves can easily penetrate
through the external surface of the shield material and most penetrated EM waves
are reflected from the second surface of the shield material. The influence is more
important in low frequency and is reduced when the frequency gets higher
because the ratio between material thickness and skin depth (t/δ)becomelargeras
the frequency increases.
2.3 EMI shielding mechanisms for composites
Composites are made from fillers and matrices with significantly different
physical or chemical properties. Hence, EMI shielding mechanisms are more com-
plicated than those for homogeneous shield materials because of the huge surface
area available for reflection and multiple-reflections. The EMI SE of composites can
be measured experimentally, and it also can be calculated theoretically. The effec-
tive relative permittivity ε
of composites, which is one of the most important
parameters in the calculation, can be approximately calculated from the Maxwell
Garnett formula as [9]:
εeff ¼εeþ3fεe
ðÞ (15)
where ε
is the relative permittivity of the matrix, ε
is the relative permittivity of
the filler and fis the volume fraction of the filler. If the filler are electrical conduc-
tive particles, the relative permittivity ε
can be expressed as [10]:
Electromagnetic Materials
εi¼ε0jε00 ¼ε0jσ
where ε0and ε00 are the real and imaginary part of the complex relative permit-
tivity of the filler, respectively. σis the electrical conductivity of the filler. As shown
in Figure 3b, the transmission coefficient Tcan be expressed as [10]:
where T
and T
are the transmission coefficients at the boundary 0 and t,
respectively. R
and R
are the reflection coefficients at the boundary 0 and t,
respectively. γ
is the complex propagation constant. The T
, and R
further be expressed in terms of the impedance Z
and Z
where Z
and Z
are the impedance of the air and the composite material,
respectively. Z
can be expressed in Eq. (4) and Z
can further be expressed as:
The propagation constant γ
can be expressed as [10]:
eff jε00
So, the SE can be calculated in terms of T,
SE ¼20log T
ðÞ (24)
3. Lightweight EMI shielding materials
When modern electronic devices are designed, high performance EMI shielding
materials are highly demanded. In addition, lightweight is one additional important
technical requirement for potential applications especially in the areas of automo-
bile and aerospace. In the following section, we will briefly review state-of-the-art
research work regarding polymer-based composite, foams and aerogels used for
EMI shielding.
3.1 Polymer-based composites
Polymer/conductive fillers composites was seen as a promising advanced EMI
shielding materials since the discovery that an insulating polymer would allow the
Lightweight Electromagnetic Interference Shielding Materials and Their Mechanisms
flow of current through the conductive network stablished by conductive fillers
above the percolation threshold. The conductive composite materials preserve the
advantages of lightness of polymers, low cost, design flexibility and ease of
processing, and the incorporation of conductive fillers circumvent intrinsic nature
of polymers being transparent to EM waves through interaction between EM wave
and the conductive fillers. Metallic fillers, intrinsically conductive polymers and
carbon based electrically conductive fillers are discussed in this section with specific
examples. Polymer/magnetic particles composites will also be briefly introduced as
magnetic portion is an important component in EM waves that should not be
ignored. This section aims to provide a general overview on the preparation of
polymer-based EMI shielding materials and the advantages and challenges faced by
each category and possible strategies towards enhancing the EMI shielding perfor-
3.1.1 Polymer-based composites containing metallic fillers
Metals are typical wave-reflection materials used for EMI shielding purpose owing
to their abundance in mobile charge carriers that can interact with the incident EM
radiation. Metallic fillers of various physical forms, such as fibers or nanoparticles,
were dispersed in the polymer matrix to increase the interaction with the incident EM
radiation. Injection-molding provides a direct method to disperse metallic fillers into
a polymer matrix. Stainless steel fibers (SSF) introduced into polycarbonate matrix
through injection molding shown that EMI SE is heavily dependent on the molding
parameters which would give an optimum electrical conductivity [11]. Blended tex-
tiles of polyester fibers with SSF showed that the EMI SE is more than 50 dB in the
frequencies ranging from 30 MHz to 1.5 GHz [12] (see Figure 4a). As shown in
Figure 4b and c, comparison of reflectance, absorbance and transmittance, (identi-
fied as reflectivity, absorptivity and transmissibility in Figure 4) for SSF and SSF/
polyester fiber fabrics as a function of frequency revealed absorption as the dominant
EMI shielding mechanism. In the case of SSF/polyester with 10 wt% SSF, EMI
shielding by absorption increased from 30 MHz to maximum at 500 MHz and then
decreased with the increase in frequency.
The challenges in achieving a good dispersion of metallic fillers and the weight
increase make polymer/metallic fillers composites a less popular choice. Much
attention was switched to intrinsically conductive polymers (including polyaniline,
polyacetylene, and polypyrrole), carbon-based materials (including carbon fibers,
carbon black, graphite, graphene, carbon nanotubes and mesoporous carbon), and
magnetic materials like carbonyl iron and ferrites (including Fe
and α-Fe
Figure 4.
(a) The EMI SE of the SSF/PET fabric as a function of frequency; (b) reflectivity/absorptivity/transmissibility
of SSF fabric and (c) SSF/PET fabric with 10 wt% SSF as a function of frequency [12].
Electromagnetic Materials
3.1.2 Intrinsically conductive polymers-based composites
Blends of a polymer with an intrinsically conductive polymer results in a com-
posite combining the desired properties of the two components, that is, adequate
mechanical properties of the polymer matrix for mechanical support and the elec-
trically conducting component for interaction with the EM radiation. Conducting
polymers are conjugated polymers, which on doping exhibit electronic conductiv-
ity. Distinctive to metallic fillers, the electrical conductivity of conducting polymers
arises from the polymer molecular structure. Alteration of parameters such as chain
size, doping level, dopant type and the synthesis route directly affect the molecular
structure, hence the EMI shielding properties of the material.
Among the available conducting polymers, polypyrrole (PPY) and polyaniline
(PANI) are the most widely used conductive fillers for EMI shielding purposes. PPY
is known to possess high conductivity, easy synthesis, good environmental stability
and less toxicological problem. Chemical and electrochemical polymerization of
PPY on a polyethylene terephthalate (PET) fabric is given as an example for elec-
trically conducting composite. Pyrrole was first dissolved in an aqueous solution
containing 10 wt% polyvinyl alcohol (PVA) and sprayed on the PET fabric before
subject to electrochemical polymerization at room temperature under a constant
current density. The resultant PPY coated PET fabric was shown to exhibit EMI SE
about 36 dB over a wide frequency range up to 1.5 GHz [13].
PANI was studied extensively for its various structures, unique doping mecha-
nism, excellent physical and chemical properties, stability, and the readily obtain-
able raw materials. Lakshmi et al. [14] prepared PANI-PU composite film by adding
aniline to polyurethane (PU) solution in tetrahydrofuran (THF). Doping of com-
posites was done by adding camphor sulfonic acid to the composite solution. The
EMI SE of the PU-PANI film was found to increase with thickness and the fre-
quency specific material is ideal for shielding at 2.2 and 8.8 GHz.
Other intrinsically conducting polymers, such as poly(p-phenylene-vinylene)
[15, 16] and poly(3-octylthiophene) [17], were also investigated for EMI shielding
applications, but too much lesser extent, mainly due to the unsatisfactory perfor-
mance and complex processing procedures involved.
In general, the EMI shielding performance arises by the addition of conductive
polymer consequently dominated by reflection mechanism due to the increase of
the level of impedance mismatch with air. One obvious advantage of such polymer-
polymer system is the lightweight being preserved, also there is no issue on sub-
strate flexibility as those associated with metallic or carbon-based fillers. However,
the main drawbacks of such composites include (1) poor mechanical properties of
the most of the intrinsically conducting polymers require a matrix material for
structural support; (2) the insoluble and infusible characteristics caused conducting
polymers to exhibit poor processability and (3) high filler (conducting polymer)
level is usually needed for acceptable performances.
3.1.3 Polymer-based composites containing carbon-based fillers
Similar to metallic fillers, carbon-based fillers come in various shapes and aspect
ratios. Carbon black (CB), including graphite and CB, is the generic name given to
small particle size carbon pigments which are formed in the gas phase by thermal
decomposition of hydrocarbons [18]. Carbon fibers (CFs) are 1D carbon structure
of diameter generally lies between 50 and 200 nm and aspect ratios around 250 and
2000, largely produced by chemical vaporization of hydrocarbon [19, 20]. Carbon
nanotubes (CNTs) can be considered as rolled-up hollow cylinders of graphene
sheets of very high aspect ratio due to the small diameter, constituted of a single
Lightweight Electromagnetic Interference Shielding Materials and Their Mechanisms
hollow cylinder, that is, single-walled carbon nanotubes (SWCNTs) or of a collec-
tion of graphene concentric cylinders, that is, multi-walled carbon nanotubes
(MWCNTs) [21, 22]. Graphene sheet (GS), an atomically thick two-dimensional
structure, exhibited excellent mechanical, thermal and electrical properties [23].
Both CNTs and graphene offer substantial advantages over conventional carbon
fillers and the percolation threshold can be achieved by both at very low content if
properly dispersed.
In general, carbon fillers with high aspect ratio are generally more effective in
imparting electrical conductivities to a polymer matrix, hence it is no surprise to
observe the highest SE from fillers with the highest aspect ratio, that is, SWCNTs >
MWCNTs >CNFs >CB when the volume fraction of the fillers is the same. The
different methods of fillers dispersion and various carbon filler surface modification
methods were comprehensively reviewed in the published paper and will not be
discussed in detail here [3, 24]. The EMI shielding performance of the
polymer/carbon-fillers composites can also be found in Ref. [3, 7, 24, 25].
3.1.4 Polymer-based composites containing magnetic particles composites
A binary or even ternary component consists of two or more types of the fillers
provide an effective way to bypass the inherent shortcomings of a single-filler
composite. The incorporation of magnetic components will supplement the attenu-
ation properties of a carbon-based EMI shielding material.
Physical blending or deposition of metallic particles within a polymer blend or
structure is the most direct way of incorporation a third element, however, such
method faces the problem of uniform dispersion and deposition at the bottom layers
due to the higher density of metallic particles. Electroless plating of metals on
carbon substrates provides a neat way of incorporating metal components uni-
formly into a system without excessive weight addition. Works by Kim et al. [26]
and Yim et al. [27] dispersed nickel coated MWCNT through electroless plating in
epoxy and high-density polyethylene, respectively. Figure 5a gives an illustration of
Figure 5.
(a) Schematic diagram of the electroless Ni-plating process; (b) SEM images of (1) pristine MWCNTs and
(2) Ni-coated MWCNTs, respectively; (c) comparison of the EMI SE of MWCNTs/HDPE and Ni-MWCNTs/
HDPE and (d) the proposed shielding mechanism of Ni-MWCNTs/HDPE [27].
Electromagnetic Materials
the nickel coated MWCNTs. It is apparent that the nickel coated MWCNTs
appeared rougher comparing to the pristine ones due to the presence of nickel
particles as shown Figure 5b. Yim achieved 140% (at 1 GHz, Figure 5c)in
enhancement of the EMI SE compared to the pristine MWCNT/polymer compos-
ites. The enhancement was attributed to the increased surface conductivity.
Figure 5d shows the proposed shielding mechanism of Ni-MWCNTs/HDPE. EM
wave was firstly reflected at the composite surfaces upon reaching the surface of the
composite. When the penetrated EM wave meets the nickel layer on the MWCNTs,
the metallic layer functioned as EM absorbable or reflective fillers. It is evident that
the EMI absorbing nature of the metallic layer can be used as an effective additional
shielding material despite the small amount present in the systems.
3.2 Foams and aerogels used in EMI shielding
In view of the rigid index of fuel-economy in the applications of automobile and
aerospace, lightweight EMI shielding materials with the combination of reduced
density and high EMI SE are much preferred. In this section, we aim to provide a
general overview on the preparation of foam and aerogel materials used in EMI
shielding and the advantages and challenges faced by each category and possible
strategies towards enhancing their EMI shielding performances. The specific EMI
SE, defined as the ratio of the EMI SE to the density (SSE) or both density and
thickness (SSE/t), is a more appropriate criterion to compare the EMI shielding
performance with those of other typical materials for the applications where light-
weight is required.
3.2.1 Polymer-based composite foams
Conductive polymer-based composites foams offer significant reduction in
weight, while the pores decrease the real part of the permittivity, accordingly
reducing the reflection at the material surface. The porous structure enhances the
energy absorption through wave scattering in the walls of the pores. Electrically
conductive fillers, including CNFs, CNTs and graphene sheets, are commonly used
to form a desirable conducting network within the inherently insulating polymer
foam matrix. Yang et al. [28] first reported CNFs reinforced polystyrene (PS)
composite foam as a conductive foam for EMI shielding application. The EMI SE of
PS/CNFs foam containing 1 wt% CNFs was less than 1 dB, upon increasing CNFs
content to 15 wt%, EMI SE increased to 19 dB. Following this work, the authors
reported PS/CNTs composite foam with varying CNTs contents from 0 to 7 wt%
[4]. The PS/CNTs composite foam achieved a higher EMI SE of above 10 dB com-
pared to 3 dB for the PS/CNFs composite foam at the same filler content of 3 wt%.
The difference in the results originated from the remarkable electrical and struc-
tural properties of CNTs, such as larger aspect ratio, smaller diameter, higher
electrical conductivity and strength, compared to CNFs.
3.2.2 Syntactic foams
Syntactic foam, filling hollow spheres in a matrix, is a kind of lightweight
composite materials. The approaches to enhance the EMI SE of syntactic foams
include (i) hollow particles made of a conductive material; (ii) coating a conductive
layer onto the surface of hollow particles and (iii) adding a second conductive filler
in syntactic foam matrix.
Zhang et al. [29] added a second conductive filler, (CNFs, chopped carbon fiber
(CCF), and long carbon fiber (LCF)), into syntactic foams containing conductive
Lightweight Electromagnetic Interference Shielding Materials and Their Mechanisms
hollow carbon microspheres (HCMs). The EMI SE values of used syntactic foams at
the same filler content were compared, as shown as Table 2. The results showed
that CNFs is more effective in providing EMI shielding compared to CCF and LCF
due to the larger aspect ratio of CNFs.
Zhang et al. [30] also demonstrated the effect of functionalization of HCMs on
the EMI SE of the epoxy-HCMs syntactic foam. HCMs were coated with
polydopamine (PDA) via the self-polymerization of dopamine. The PDA coating
promotes dispersion and served as a reducing agent to deposit silver (Ag) particles
on the surface of HCMs as illustrated in Figure 6a. The average EMI SE of the
epoxy-HCMs syntactic foam containing Ag-PDA-HCMs with 28.5 and 30.5 wt% of
silver in the X-band achieved 49.5 and 60.2 dB, respectively as shown in Figure 6b.
The SSE reached up to 46.3 dB cm
/g, demonstrating the prospect of epoxy/Ag-
PDA-HCMs syntactic foam as a lightweight high-performance EMI shielding mate-
rial. The corresponding EMI shielding mechanism of this syntactic foam was ana-
lyzed by comparing the values of reflectance (R), absorptance (A), and
Filler content (vol%) CNF
Aspect ratio: 5001700
Aspect ratio: 650
Aspect ratio: 150750
0.5 5.2 2.2 2.8
1.0 11.3 3.4 4.4
1.5 16.4 3.7 6.5
2.0 24.9 4.3 7.5
Table 2.
Comparison of the EMI SE (dB) of CNF, CCF, and LCF reinforced syntactic foam.
Figure 6.
(a) Schematic illustration of the procedure for preparation of PDA-HCMs and Ag-PDA-HCMs; (b) EMI SE
in the frequency range from 8 to 12 GHz for syntactic foam containing pristine HCMs and Ag-PDA-HCMs
with different silver contents; and (c) reflectance (R), absorbance (A), and transmittance (T) of EM radiation
over syntactic foams containing Ag-PDA-HCMs with different silver content at 10 GHz [30].
Electromagnetic Materials
transmittance (T)inFigure 6c. The specimens were both reflective and absorptive
towards EM radiation at silver content less than 17.8 wt%. The contribution of
reflection (0.83) towards EMI SE surpassed that from absorption (0.16) when silver
content increased to 28.5%. The dense and thick electrically conductive silver
formed due to further increasing the silver content to 30.5 wt% increased the Rto
0.97 and resultant in reflection as the dominant shielding mechanism.
Xu et al. [31] fabricated syntactic foams (hybridized epoxy composite foams
according to authors) through impregnating expandable epoxy/MWCNT/micro-
sphere blends into a preformed, highly porous, and 3D silver-coated melamine
foam (SF) sponge. The highly conductive SF resolved the problem of the foam
reduction of high filled epoxy blends and provided channels for rapid electron
transport. MWCNTs were used to offset the loss of conductive pathways due to the
crystal defects in the silver layer and the insulating epoxy resin. As a result, the EMI
SE of 68.1 dB was achieved with only 2 wt% of MWCNTs and 3.7 wt% of silver due
to the synergy of the MWCNT and SF.
3.2.3 Carbon foams
Carbon foam is a class of three-dimensional (3D) architecture consisting of a
sponge-like interconnected network of porous carbon. Carbon foams have been
wildly used as candidates for realistic EMI shielding applications due to their excel-
lent properties, such as low density, resistance to chemical corrosion, high thermal
and electrical conductivity, and high temperature resistance.
Zhang et al. [32] prepared a novel ultralight (0.15 g/cm
) carbon foam by direct
carbonization of phthalonitrile (PN)-based polymer foam, as shown in Figure 7a.
High EMI SE of 51.2 dB (see Figure 7b, C1000 was labeled as the carbonization of
1000°C) was contributed by the high graphitic carbonaceous species and the
intrinsic nitrogen-containing structure. The carbon foams showed the best SSE of
341.1 dB cm
/g so far when mechanical property was considered. The carbon foam
developed by Zhang provides an excellent low-density and high-performance EMI
shielding material for use in areas where mechanical integrity is desired.
3.2.4 CNTs/graphene foams
The EMI SE of carbon foams was closely related to the char yield of polymer
precursors and the demanding carbonization conditions. Therefore, a new kind of
filler-free lightweight EMI shielding material, is in demand, which can be prepared
without the stringent processing conditions. In view of the lightweight require-
ment, assembling one dimensional (1D) CNTs and two-dimensional (2D) graphene
sheets into three dimensional (3D) macroscopic porous structures (e.g., sponges,
foams and aerogels) emerged as an efficient approach.
Figure 7.
(a) Schematic representation of the preparation of PN-based carbon foams and (b) EMI SE of carbon foams [32].
Lightweight Electromagnetic Interference Shielding Materials and Their Mechanisms
Lu et al. [33] synthesized a flexible CNTs sponge with a density of 10.0 mg/cm
via chemical vapor deposition (CVD) process, composed of self-assembled and
interconnected CNT skeletons. The freestanding CNTs sponge showed the high
EMI SE and SSE of 54.8 dB and 5480 dB cm
/g in X-band, respectively. After
composited with polydimethylsiloxane (PDMS) by directly infiltrating method, the
CNT/PDMS composites still exhibited excellent EMI SE (46.3 dB) at the thickness
of 2.0 mm, while the CNT loading content was less than 1.0 wt%.
Surface modification is employed to increase the EMI shielding ability of
graphene foams. Zhang et al. [34] prepared surfaced modified 3D graphene foams
via self-polymerization of dopamine with a subsequent foaming process, as shown
in Figure 8a. The polydopamine (PDA) served as a nitrogen doping source and an
enhancement tool to achieve higher extent of reduction of the graphene through
providing wider pathways and larger accessible surface areas. The enhanced reduc-
tion of graphene sheets and the polarization effects introduced by PDA decoration
compensated the negative effect of the barrier posed by PDA. As a result, the
resultant EMI SE showed 15% improvement compared to PDA-free graphene foam
as shown in Figure 8b. Wu et al. [35] also fabricated an ultralight, high performance
EMI shielding graphene foam (GF)/poly(3,4-ethylenedioxythiophene):poly(sty-
rene sulfonate) (PEDOT:PSS) composites by drop coating of PEDOT:PSS on the
freestanding cellular-structured GFs, as illustrated in Figure 8c. The GF/PEDOT:
PSS composites possess an enhanced electrical conductivity from 11.8 to 43.2 S/cm
after the incorporation of PEDOT:PSS. The modified grapheme foam with a density
of 18.2 10
provide a remarkable EMI SE of 91.9 dB (identified as SE
Figure 8d).
3.2.5 Graphene aerogels
Aerogel is a synthetic porous ultralight material derived from a gel, in which the
liquid component used in gel are replaced by air. In recent years, the great potential
of graphene aerogel (GAs) in EMI shielding applications has been confirmed by
several researchers. Song et al. [36] reported that the EMI SE of GA-carbon textile
hybrid with a thickness of 2 mm was 27 dB. The 3D scaffold GA greatly enhances
Figure 8.
(a) Schematic representation of the preparation of PDA-GO and PDA-rGO; (b) EMI SE of rGO foam and
PDA-rGO foam [34]; (c) schematic procedure of the preparation of GF/PEDOT:PSS composites; (d) EMI SE
of GF/PEDOT:PSS composites as a function frequency [35].
Electromagnetic Materials
the conductive network while maintaining the advantage of light carbon textile.
Singh et al. [37] studied the EMI SE of pure GA, which was 20 dB, with a density
75 mg /cm
and a thickness of 2 mm. They discussed the EMI shielding mechanism
by correlating the EM wave interaction with the 3D porous structure. Zeng et al.
[38] fabricated an ultralight and highly elastic rGO/lignin-derived carbon (LDC)
composite aerogel with aligned microspores and cell walls by directional freeze-
drying and carbonization method. The EMI SE of rGO/LDC composite aerogels
with a thickness of 2 mm could reach up to 49.2 and 21.3 dB under ultralow
densities of 8.0 and 2.0 mg/cm
, respectively.
The graphitization of GAs facilitates to improve its electrical conductivity, thus
improving the EMI SE. Liu et al. [39] reported an effective method of manufacturing
an integrated graphene aerogel (IGA) using a complete bridge between rGO sheets
and polyimide macromolecules via graphitization at 2800°C, as shown in Figure 9a.
The rGO sheets were efficiently reduced to graphene during graphitization, while the
polyimide component was graphitized to turbostratic carbon to connect the graphene
sheets, resulting in a high EMI SE of 83 dB in X-band at a low density of 18 mg/cm
as shown in Figure 9b. The EMI shielding mechanism analysis for the porous IGA
revealed that most of the incident EM wave was dissipated through absorption, thus
forming an absorption-dominant EMI shielding mechanism.
Different reduction process of graphene oxide (GO), including chemical reduc-
tion and thermal reduction would affect the EMI shielding performance of GAs.
Bi et al. [40, 41] carried out a comprehensive study of EMI shielding mechanisms of
GAs solely consisted of graphene sheets to determine the main parameters of high
EMI SE. As shown in Figure 10a, two types of ultralight (4.55.5 mg/cm
were prepared by chemical reduction and thermal reduction of GO aerogels. The EMI
SE reached 27.6 and 40.2 dB for chemically reduced graphene aerogel (GAC) and
thermally reduced graphene aerogel (GAT), respectively. The distinct graphene
surface resulted from different processing pathway led to different EM wave
Figure 9.
(a) Schematic illustration for fabricating IGA and (b) effect of annealing temperature on EMI shielding
performance of IGAs [39].
Figure 10.
(a) Schematic representation of the preparation process of GAC and GAT [42] and (b) R&Aof GA9 and
GA9F [41].
Lightweight Electromagnetic Interference Shielding Materials and Their Mechanisms
response upon striking the graphene/air interface. Nitrogen-doping and side polar
groups induced strong polarization effects in GAC. Higher extent of reduction of the
grapheme sheets in GAT left a smaller amount of side polar groups and formed more
graphitic lattice, both favored π-πstacking between the adjacent graphene sheets.
The enhanced polarization effects and the increased electrical conductivity of GAT
contributed to better EMI shielding performance. Bi further investigated the effect of
porosity on EMI shielding mechanisms compressing the aerogel (GA9) into thin film
(GA9F), as shown in Figure 10b. The highly connected conducting network resulted
in a significant increase in the electrical conductivity of GA9F, while the EMI SE
remained unchanged at constant rGO content. The observation was contradictory to
the previous outcomes that higher electrical conductivity or better-connected
network contributed to higher EMI SE. Hence, the fact can be believed that the EMI
SE is highly dependent on the effective amounts of materials response to the EM
waves. Despite the similar intrinsic properties of rGO, the amount of absorption of
EM waves in GA9 was much higher than that in GA9F when the EM waves
penetrated through the porous structure. The cavities within the highly porous GA
absorbed the EM waves through multiple internal reflections and eventually depleted
the energy. Hence, the tightly connected conducting network within GA9F changed
the EMI shielding mechanism from absorption to reflection.
4. Conclusions
Generally, EMI shielding is defined as the prevention of the propagation of EM
waves from one region to another by using shield materials. With the development of
electronic industry, weight reduction is an additional technical requirement besides the
good EMI shielding performance. Metal as a traditional EMI shielding material has been
replacing with lighter materials, such as polymer-based composites, foams and
aerogels. This chapter reviewed various types of lightweight materials with their EMI
SEs corresponding to their EMI shielding mechanisms. To verify the benefits of using
lightweight materials for EMI shielding applications, a comprehensive comparison was
performed as shown in Figure 11.AllthedatainFigure 11 were collected from the
Figure 11.
Comparison of EMI SEs of lightweight materials as a function of density of materials.
Electromagnetic Materials
reference papers listed in this chapter. Although the data are not involved all the
published results, they are representative to the library of lightweight EMI shielding
materials. The reported EMI SEs of polymer-based composites containing conductive
fillers varied in the range of 2060 dB corresponding to the densities higher than 0.8 g/
. Polymer-based foams reinforced with additional conductive fillers and carbon
foams outperform polymer-based composites in terms of EMI SE. They possessed
comparable EMI SE of 2080 dB with the lower density (<0.8 g/cm
). Aerogels with
ultralow densities (<100 mg/cm
) exhibited high EMI SEs in the same range of poly-
mer- and carbon-based foams, indicating they can be used as an ideal potential light-
weight EMI shielding materials though the mechanical properties of aerogels still
remain a big issue.
Liying Zhang would like to acknowledge the support by the initial research funds
for young teachers of Donghua University. Shuguang Bi would like to acknowledge the
financial support of Wuhan Engineering Center for Ecological Dyeing & Finishing and
Functional Textiles, Key Laboratory of Textile Fiber & Product (Wuhan Textile Uni-
versity), Ministry of Education, Hubei Biomass Fibers and Eco-dyeing & Finishing
Key Laboratory. Zhang and Bi would also thank the funding support by State Key
Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua
University (KF1827). Ming Liu would like to acknowledge the support from School of
Materials Science and Engineering at Nanyang Technological University for this work.
Conflict of interest
No conflict of interest.
Author details
Liying Zhang
, Shuguang Bi
and Ming Liu
1 Center for Civil Aviation Composites, Donghua University, Shanghai, China
2 Chemistry and Chemical Engineering College, Wuhan Textile University, Wuhan,
3 Temasek Laboratories, Nanyang Technological University, Singapore
*Address all correspondence to:
These authors contributed equally to this work.
© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms
of the Creative Commons Attribution License (
by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,
provided the original work is properly cited.
Lightweight Electromagnetic Interference Shielding Materials and Their Mechanisms
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Electromagnetic Materials
... Hence, the benefits and the potential risks need to be accessed judiciously. The problem of protection against high energy microwaves EM radiation is typical due to high intensity reciprocal interference of electronic instruments working in close proximity [1][2][3][4][5][6]. Furthermore, uncontrolled long-term exposure to these radiations imposes hazardous consequences on living organism creating potential health risk even jeopardizing their life [7]. ...
... A number of studies related to the effect of number of layers on EMSE conducted by researchers confirm that the EMSE increases with an increase in the number of conductive fabric layers [1][2][3][4][5][6][7][8][9]. Prerumalraj et al. 2009 [13] produced copper core yarns to make twill and plain-woven fabrics, it was observed that, the EMSE of double layered fabrics in both weave configurations increases in comparison with their single layer variants. ...
... Hence, the benefits and the potential risks need to be accessed judiciously. The problem of protection against high energy microwaves EM radiation is typical due to high intensity reciprocal interference of electronic instruments working in close proximity [1][2][3][4][5][6]. Furthermore, uncontrolled long-term exposure to these radiations imposes hazardous consequences on living organism creating potential health risk even jeopardizing their life [7]. ...
... A number of studies related to the effect of number of layers on EMSE conducted by researchers confirm that the EMSE increases with an increase in the number of conductive fabric layers [1][2][3][4][5][6][7][8][9]. Prerumalraj et al. 2009 [13] produced copper core yarns to make twill and plain-woven fabrics, it was observed that, the EMSE of double layered fabrics in both weave configurations increases in comparison with their single layer variants. ...
In the present study, a strategic designing of multilayer shield was planned to enhance the multiple reflection phenomenon to achieve maximum absorption properties in microwave frequency (C & X band) range. Multi-layer EMR shields were developed using pure cotton fabric and conductive woven fabrics, incorporated with copper-based & silver-plated hybrid yarn. First of all, single layer fabrics were produced in five variants, nomenclature as L1A (pure cotton) L1B, L1C (copper-based hybrid yarn), LS1B and LS1C (silver plated hybrid yarn). These five variants were used to prepare four sets of double & triple layer fabric. In both double and triple layer composition , L1A fabric (pure cotton) was used as top layer followed by B and C series fabrics, containing copper and silver-plated hybrid yarn. The EMSE performance in C and X band frequency range of single layer, double layer and triple layers in terms of scattering parameters S11(reflectance) & S21 (transmittance) in vertical and horizontal wave polarization was studied. It was found that number of layers, layer composition, orientation of metallic yarn, frequency and EM wave polarization have significant influence on overall electromagnetic shielding effectiveness.
... However, EM waves from some nature sources and emitted by human devices can interrupt the working of electronic equipment to some extent, i.e., the electromagnetic interference (EMI) [5,6]. The EMI can be generated by an conducted emission or an external radiated emission, from several kilohertz to gigahertz frequencies, significantly affecting an electrical circuit [7]. The emitted EM radiations can interfere with sophisticated electronic devices by affecting their signal transmission, energy consumption, and working efficiency. ...
... The effective relative permittivity ε eff of composites is a key parameter for EMI shielding, it can be calculated from by the Eq. (6) which is derived from the Maxwell Garnett formula [7,71]: ...
Electromagnetic interference has become a serious pollution concern in modern society, which has significantly driven the development of lightweight electromagnetic interference shielding materials based on porous carbon/polymer nanocomposites. This work discusses the state-of-the-art methods for fabricating polymer foams and carbon foams, aerogels, and sponges for electromagnetic interference shielding. In order to obtain an ideal electromagnetic interference shielding effectiveness, it is crucial to create carbon/polymer nanocomposites with effective conductive networks at low filler loadings. To this end, there have been three design strategies, including the use of carbon foams with polymer backfilling, the use of carbon-based hybrid fillers, and the formation of segregated structures in conductive polymer composite. This review also discusses electrical conductivity and electromagnetic interference shielding performances as well as associated mechanisms behind of lightweight carbon-polymer nanocomposites, and their potential applications are summarized. Some key challenges on lightweight polymer-carbon nanocomposites as electromagnetic interference shielding materials are presented followed by some future perspectives.
... Additionally, the shielding mechanism of metals is dominantly based on the reflection of electromagnetic radiations from their surfaces which limit their utilization where absorption is primarily essential for blocking the radiated electromagnetic energy e.g., in stealth technology. Consequently, development of lighter, thinner and flexible EMI shielding materials which can effectively mitigate electromagnetic interference predominantly by absorption in compliance with EMC standards are in big demand which are being investigated for fastest growing smart electronics industry [16,17]. Hence, to meet such demands of the industry various formulations of polymer nanocomposites are being explored because of their tunable electrical conductivity that make them suitable for a wide range of applications like charge storage devices, electrostatic discharge (EDS) safety, antistatic dissipation, EMI shielding, etc. Polymer nanocomposites comprising of blended matrix, types of conductive fillers and their dominant attenuation mechanisms against electromagnetic interference have attracted considerable attention to employ them as novel functional EMI shielding materials in megahertz (MHz) to terahertz (THz) frequency range [18]. ...
Full-text available
Electromagnetic (EM) wave absorbing materials have attracted remarkable attention for their use against mitigation of EM interference in smart electronics. Flexible polymer nanocomposites thin films containing homogeneously embedded carbon nanofibers (CNFs) in poly(methyl methacrylate) (PMMA) matrix have been synthesized for investigation of their attenuation characteristics against EM noise in THz frequency band. These PMMA-CNF nanocomposite films comprising of CNFs loadings varying from 0.00–12.50wt% were synthesized using solvent casting method. The samples were characterized for evaluation of their structure, morphology, electrical and EM wave absorption properties in 0.2–1.2 THz range by X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy, Field emission electron microscopy, IV measurements and Terahertz time domain spectroscopy. The electrical conductivity enhancement in the samples has been attributed to the formation of conducting network by uniform distribution of CNFs in the insulating PMMA matrix. The percolation threshold for electrical conductivity was found at 5.0wt% of CNFs loading in these samples. As a result, the electrical conductivity and shielding effectiveness (SE) have been observed to improve with the increase in CNFs loading in the polymer matrix. The SE is also a function of frequency of incident EM waves, which is attributed to the increase in the skin depth. A systematic increasing trend in the SE has been observed by the THz-TDS but a maximum SE of 56.8 dB has been determined in the samples containing CNFs loading of 12.50wt%. These polymer nanocomposite film samples are of significant importance for practical applications as light weight and flexible shielding materials in the terahertz frequencies.
... SE overall consists of reflection (SE R ), absorption (SE A ), and multiple reflections (SE MR ), which can be expressed as [22]: ...
Carbon fiber reinforced composites (CFRC) are in huge demand in aviation industry for reducing the fuel consumption, despite the unfavorable electromagnetic interference (EMI) shielding property. In this work, carbon fiber fabrics (CF) were coated by a thin layer of nickel (Ni) using electroless plating to increase the electrical conductivity of the composites. Dopamine was then self-polymerized on Ni coated CF (CF-Ni) surfaces to enhance the interfacial interactions between fibers and epoxy matrix. The results showed that the introduction of 0.39 wt.% of polydopamine (PDA) content leads to a significant increase of interlaminar shear strength (ILSS), tensile strength and modulus by 70.7%, 22.7% and 15.3%, respectively, compared with CF-Ni/epoxy composites with free of PDA. The dominant facture mechanisms changed from fiber pulling-out and/or debonding to fiber breakage after the introduction of PDA. Compared with CF/epoxy composites, the EMI shielding effectiveness (SE) of CF-Ni/epoxy composites increased by 77.2% and slightly decreased with the increase of the PDA content. In order to further optimize the overall performances of the composites, the laminates structures were specially designed by replacing 2 plies of CF-Ni-PDA with CF-PDA in the middle according to EMI shielding mechanisms. The composites with special laminate stacking exhibited outstanding ILSS (61.2 MPa) and EMI SE (31.0 dB), which are dominant over most reported structural composites. The effects of humidity on the mechanical and EMI shielding properties were evaluated as well, indicating that the composites played a huge application potential in aircraft.
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The presentation included 30 slides describing following geopolymer applications developed from 1972 to 2002 in France, Europe and USA. The Geopolymer chemistry concept was invented in 1979 with the creation of a non-for profit scientific organization, the Institut de Recherche sur les Géopolymères (Geopolymer Institute). 1. Fire resistant wood panels 2. Insulated panels and walls, 3. Decorative stone artifacts, 4. Foamed (expanded) geopolymer panels for thermal insulation, 5. Low-tech building materials, 6. Energy low ceramic tiles, 7. Refractory items, 8. Thermal shock refractory, 9. Aluminum foundry application, 10. Geopolymer cement and concrete, 11. Fire resistant and fire proof composite for infrastructures repair and strengthening, 12. Fireproof high-tech applications, aircraft interior, automobile, 13. High-tech resin systems. The applications are based on 30 patents filed and issued in several countries. Several patents are now in the public domain, but others are still valid. The applications show genuine geopolymer products having brilliantly withstood 25 years of use and that are continuously commercialized.
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With the development in the modern technologies such as telecommunication instruments and scientific electronic devices, large amount of the electromagnetic radiations are produced, which lead to harmful effect on the highly sensitive electronic devices as well as on the health of human beings. To minimize the effect of electromagnetic radiations produced by different technologies, more efficient shielding materials are required which must be cost-effective, lightweight and good corrosion resistive. In this review, we focused on the shielding materials based on composites of carbon nanotubes and graphene. The typical surface modification of carbon nanotubes and graphene to optimize their interactions with polymers matrix has also summarized. It was found that the composites based on these carbon fillers were more efficient for electromagnetic interference shielding due to their unique properties (i.e., superior electrical, mechanical and thermal) together with lightweight, easy processing. Hence, the carbon nanotubes and graphene-based composites are excellent shielding materials against the electromagnetic radiations.
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The presentation included 30 slides describing following geopolymer applications developedsince 1972 in France, Europe and USA. The Geopolymer chemistry concept was invented in1979 with the creation of a non-for profit scientific organization, the Institut de Recherche sur lesGéopolymères (Geopolymer Institute).1. Fire resistant wood panels2. Insulated panels and walls,3. Decorative stone artifacts,4. Foamed (expanded) geopolymer panels for thermal insulation,5. Low-tech building materials,6. Energy low ceramic tiles,7. Refractory items,8. Thermal shock refractory,9. Aluminum foundry application,10. Geopolymer cement and concrete,11. Fire resistant and fire proof composite for infrastructures repair and strengthening,12. Fireproof high-tech applications, aircraft interior, automobile,13. High-tech resin systems.The applications are based on 30 patents filed and issued in several countries. Several patents arenow in the public domain, but others are still valid. The applications show genuine geopolymer products having brilliantly withstood 25 years of use and that are continuously commercialized.
Ultralight and highly elastic reduced graphene oxide (RGO)/lignin-derived carbon (LDC) composite aerogels with aligned micron-sized pores and cell walls are prepared using a facile freeze-drying method. The presence of a small fraction of LDC in the cell walls enhances the interfacial polarization effect while almost maintaining the amount of charge carriers and conductivity of the cell walls, greatly boosting wave absorption capability of the cell walls. RGO/LDC aerogels also show a greater number of large cell walls with better integrity than RGO aerogels, further improving the multiple reflection ability of the aligned cell walls. Synergistic effects of the multi-phase cell walls and the preferred microstructures of the RGO/LDC aerogels lead to their high electromagnetic interference (EMI) shielding effectiveness of 21.3 to 49.2 dB at the ultralow density of 2.0 to 8.0 mg/cm3. This corresponds to surface specific SE (SE divided by density and thickness) up to 53250 dB·cm2/g, which is higher than reported values for other carbon- and metal-based shields. Furthermore, the critical roles that microstructures play in determining EMI shielding performance are directly revealed through comparing shielding performance in directions parallel and normal to cell walls, as well as in an in-situ compression process.
Although lightweight and three-dimensional graphene aerogels and foams combining ultrahigh electrical conductivity, superelasticity and fatigue resistance are highly desirable for widespread applications, it remains a large challenge to construct a multifunctional framework affording the rapid electron transport and efficient load transfer due to the weak interfaces between highly reduced graphene oxide sheets. Herein, we report an efficient approach for fabricating an integrated graphene aerogel by bridging its reduced graphene oxide sheets with polyimide macromolecules followed by graphitization at 2800 °C. During the graphitization process, the reduced graphene oxide sheets are thermally reduced to graphene efficiently by removing their residual oxygen-containing groups and healing their defects, while the polyimide component is graphitized to turbostratic carbon to bridge the graphene sheets, resulting in an integrated graphene aerogel with satisfactory mechanical and functional performances, including ultrahigh electrical conductivity (>1000 S m⁻¹) at a low density, unprecedented high electromagnetic interference shielding effectiveness of ∼83 dB in X-band, 90% reversible compressibility, and reliable resistance to fatigue for 1000 compressive cycles at 50% strain. The integrated graphene aerogels with such multifunctional performances hold a great promise for applications as electromagnetic interference shielding materials, oil adsorbents, and conductive scaffolds for polymer nanocomposites.
Graphene was recently demonstrated to exhibit excellent electromagnetic interference (EMI) shielding performance. In this work, ultralight (∼5.5 mg/cm³) graphene aerogels (GAs) were fabricated through assembling graphene oxide (GO) using freeze-drying followed by a chemical reduction method. The EMI shielding properties and mechanisms of GAs were systematically studied with respect to the intrinsic properties of the reduced graphene oxide (rGO) sheets and the unique porous network. The EMI shielding effectiveness (SE) of GAs was increased from 20.4 to 27.6 dB when the GO was reduced by high concentration of hydrazine vapor. The presence of more sp² graphitic lattice and free electrons from nitrogen atoms resulted in the enhanced EMI SE. Absorption was the dominant shielding mechanism of GAs. Compressing the highly porous GAs into compact thin films did not change the EMI SE, but shifted the dominant shielding mechanism from absorption to reflection.
Ultralight, high-performance electromagnetic interference (EMI) shielding graphene foam (GF)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) composites are developed by drop coating of PEDOT:PSS on cellular-structured, freestanding GFs. To enhance the wettability and the interfacial bonds with PEDOT:PSS, GFs are functionalized with 4-dodecylbenzenesulfonic acid. The GF/PEDOT:PSS composites possess an ultralow density of 18.2×10(-3) g/cm(3) and a high porosity of 98.8%, as well as an enhanced electrical conductivity by almost four folds from 11.8 to 43.2 S/cm after the incorporation of the conductive PEDOT:PSS. Benefiting from the excellent electrical conductivity, ultralight porous structure and effective charge delocalization, the composites deliver remarkable EMI shielding performance with a shielding effectiveness (SE) of 91.9 dB and a specific SE (SSE) of 3124 dB∙cm(3)/g, both of which are the highest among those reported in the literature for carbon-based polymer composites. The excellent electrical conductivities of composites arising from both the GFs with three-dimensionally interconnected conductive networks and the conductive polymer coating, as well as the left-handed composites with absolute permittivity and/or permeability larger than one give rise to significant microwave attenuation by absorption.