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Research Article
Metallized Skeleton of Polymer Foam Based on Metal−Organic
Decomposition for High-Performance EMI Shielding
Si-Yuan Liao,# Gang Li,# Xiao-Yun Wang,# Yan-Jun Wan,* Peng-Li Zhu,* You-Gen Hu, Tao Zhao,
Rong Sun, and Ching-Ping Wong
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sı Supporting Information
*
ABSTRACT: Highly conductive polymer foam with light weight,
flexibility, and high-performance electromagnetic interference
(EMI) shielding is highly desired in the fields of aerospace,
communication, and high-power electronic equipment, especially
in the board-level packaging. However, traditional technology for
preparing conductive polymer foam such as electroless plating and
electroplating involves serious pollution, a complex fabrication
process, and high cost. It is urgent to develop a facile method for
the fabrication of highly conductive polymer foam. Herein, we
demonstrated a lightweight and flexible silver-wrapped melamine
foam (Ag@ME) via in situ sintering of metal−organic decomposition (MOD) at a low temperature (200 °C) on the ME
skeleton modified with poly(ethylene imine). The Ag@ME with a
continuous 3D conductive network exhibits good compressibility, an excellent conductivity of 158.4 S/m, and a remarkable EMI
shielding effectiveness of 63 dB in the broad frequency of 8.2−40 GHz covering X-, Ku-, K-, and Ka-bands, while the volume content
is only 2.03 vol %. The attenuation mechanism of Ag@ME for EM waves is systematically investigated by both EM simulation and
experimental analysis. Moreover, the practical EMI shielding application of Ag@ME in board-level packaging is demonstrated and it
shows outstanding near-field shielding performance. This novel strategy for fabrication of highly conductive polymer foam with low
cost and non-pollution could potentially promote the practical applications of Ag@ME in the field of EMI shielding.
KEYWORDS: metal−organic decomposition, metallization, EMI shielding, EM simulation, near-field shielding performance
1. INTRODUCTION
With the rapid development of 5G communication technology,
electromagnetic interference (EMI) is becoming increasingly
serious, disturbing the normal operation of the electronic
device and even bringing great health risks to humans.1
Therefore, there is an urgent need to develop a novel strategy
to achieve effective and broadband shielding materials that
could be applied to electronic devices in various fields such as
aerospace and future 5G systems.2 Electrical conductivity is
one of the most important factors affecting EMI shielding
performance of shielding materials.3 Based on this factor,
metals such as Ag, Cu, and Ni were preferred as conductive
fillers to fabricate various polymer composites for EMI
shielding due to their high electrical conductivity.4−10
However, the inherent agglomeration of metallic nanofillers
in polymer and high conductive filler content are still knotty
problems.11 By contrast, the construction of a continuous 3D
conductive structure on the skeleton of polymer foam
significantly reduces the metal filler content.12,13 It shows the
advantages of lightweight, flexibility, and excellent mechanical
properties. To this end, various technics including dip coating/
spraying of nanoparticle suspension ink, electroless plating, and
© XXXX American Chemical Society
electroplating were developed. For example, Liu et al.
fabricated MXene/AgNW textile by a vacuum-assisted layerby-layer assembly method, and the conductive fabric exhibits
sheet resistance of 0.8 Ω sq−1.14 Zeng et al. developed a metalwrapped polymer nanofiber, which possesses a conductivity of
7870 S cm−1 after a long electroless plating time of 4 h.15
Accordingly, Zhang et al. prepared Ni/active carbon-filter
paper using the same method, and it shows an excellent
conductivity of 1.5 × 103 S/cm.16 Nevertheless, these
traditional preparation technologies of metallized polymer
foams inevitably involve serious problems of large pollution,
complex fabrication process, and high cost.17 Moreover, for
metal nano−/micro-particle, it usually needs high a sintering
temperature to form a conductive path, which results in the
deterioration of the mechanical properties of polymer foam.
Received: November 10, 2021
Accepted: December 28, 2021
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Figure 1. Schematic of the bonding mechanism and fabrication process of Ag@ME. (a, b) Modification process of pure ME with PEI molecules.
(b, c) Freeze-drying process of Ag+@ME. (c, d) Sintering procedure of silver nanoparticles on the skeleton of ME. (I−IV) Schematic illustration of
the bonding mechanism of Ag@ME.
attenuation of the incident EM waves is mainly based on the
reflection loss, while the porous structure is conducive to
attenuating the residual EM waves after penetrating the skin
depth of the shielding material.
Metal−organic decomposition (MOD), composed of a
metal precursor, reducing agent, complexant, and solvent, is
a novel conductive solution that can be decomposed to metal
at low temperature.17−20 For example, silver-based MOD was
fabricated using various silver salt as a metal precursor
dissolving in suitable organic solvents, including silver citrate,21
silver acetate,22 silver oxalate,23 silver tartrate,24 silver
carbonate,25 etc. The solvents depend on the property of
metal precursors and substrates. Amine, xylene, carboxylic
acids, alcohols, etc., are the typically selected solvents. No
condensation and agglomeration occur for MOD during the
dispersion process due to the absence of metal particles.26−28
More importantly, MOD can be sintered to form effective
electron conduction paths at a low temperature after the
evaporation of solvent. Therefore, MOD is an ideal candidate
for metallization of polymer foam.
Herein, we developed a lightweight and flexible silver
wrapped melamine foam (Ag@ME) via in situ sintering of Agbased MOD on the ME skeleton modified with poly(ethylene
imine) molecules (PEI), which substantially improves the
interface interaction between the Ag and ME substrate. The
abundant −NH2 groups attached to the ME skeleton are
conducive to absorbing more Ag+ to construct an electrically
conductive network after evaporation of solvent and in situ
sintering at 200 °C. The Ag@ME possesses an electrical
conductivity of 158.4 S/m, leading to superb EMI shielding
effectiveness (SE) of over 63 dB in the broadband of 8.2−40
GHz covering X-band, Ku-band, K-band, and Ka-band.
Meanwhile, the Ag@ME exhibits outstanding mechanical
resilience with 80% reversible compressibility. Moreover, the
attenuation mechanism of Ag@ME for EM waves in the Xband, Ku-band, K-band, and Ka-band was simulated, which
theoretically reveals the effect of thickness or conductivity on
the propagation of EM waves in a rectangular waveguide.
Additionally, we also demonstrated the shielding mechanism of
Ag@ME with a porous structure through EM simulation. The
2. EXPERIMENTAL SECTION
2.1. Chemical Raw Materials. Silver acetate (Ag(OCOCH3), AR
99.5%), ethylenediamine ((CH2NH2)2, >99% (GC), boiling point =
118 °C), formic acid (HCOOH, AR, 88.0%, boiling point = 100.5
°C), and poly(ethyleneimine) ((CH2CH2NH)n, AR 99.0%) were
purchased from Aladdin. Ammonium hydroxide (NH4OH, 25.0−
28.0% NH3 basis, boiling point = 25 °C) and ethylene glycol
((CH2OH)2, AR 99.0%) were purchased from Sinopharm Chemical
Reagent Co., Ltd. ME foam was supplied by Sinoyqx Co., Ltd. All the
experimental raw materials are used directly without further
purification.
2.2. MOD Synthesis Process. To obtain the MOD, first, 5 grams
of silver acetate was mixed in a glass beaker with 12.5 mL of
ethylenediamine. Meanwhile, the mixed solution was stirred at 300
rpm for 5 min. After it was cooled to room temperature, the ammonia
was added in the mixed solution, and the mixed solution color was
slowly changed from gray-black to transparent with constant stirring.
Subsequently, 1 mL of formic acid solution diluted with ethylene
glycol was added dropwise into the above solution. Finally, the silver
precursor organic solution was filtered through centrifugation at 5000
rpm. The obtained MOD was used in the following experiments.
2.3. Ag@ME Preparation Process. In a typical preparation
process, the purchased ME was cleaned with ethyl alcohol; after
drying, the ME was cut into squares and then immersed in PEI
solution for 4 h. The modified ME was soaked in MOD for 0, 4, 8, 12,
and 24 h. Subsequently, the ME-coated Ag+ was freeze-dried to get rid
of excess moisture. Finally, the freeze-dried Ag+@ME was sintered at
200 °C for 10 min with a nitrogen atmosphere using a tubular
annealing furnace, and the heating rate was 10 °C/min.
2.4. Characterization and Test Methods. The morphologies of
Ag@ME were imaged by using an FEI NovaNano-450 scanning
electron microscope (SEM). The structure and crystal phase of Ag@
ME were analyzed by X-ray diffraction (XRD) using a Smartlab 9 KW
(Rigaku, Japan). Fourier transform infrared (FT-IR) spectroscopy
between 500 and 4000 cm−1 was identified by using the attenuated
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Figure 2. Morphology changes of Ag@ME with concentration of MOD. (a) Pure ME. Ag@ME with silver of (b) 0.89, (c) 2.03, (d) 2.58, and (e)
3.48 vol %.
total reflection (ATR) model of a Nicolet iSTM10 FTIR
spectrometer. The elemental compositions and chemical bond of
samples were recorded by X-ray photoelectron spectroscopy (XPS,
Escalab 250xi). Thermogravimetric analysis (TGA) was performed on
An SDT Q600 (TA instruments, UK) with a heating rate of 10 °C/
min under an argon atmosphere (50 mL/min). The mechanical
properties of Ag@ME were tested by using a mechanical test machine
(Instrumentation System Co., LTD, Japan) with a strain ramp of 5
mm/min, and all samples were cut into 20 mm × 20 mm. The
electrical resistivity of sintered Ag@ME was measured using a twopoint method; each sample was cut into regular shape (20 mm × 20
mm), and each end of the sample was adhered to copper foil with
conductive silver paste to decrease contact resistance. EMI SE of Ag@
ME in a broad frequency range of 8.2−40 GHz, including X-band
(8.2−12.4 GHz), Ku-band (12−18 GHz), K-band (18−27.6 GHz),
and Ka-band (28−40 GHz), was calculated from the S parameters
that were obtained by a vector network analyzer VNA, KEYSIGHT,
N5227B with a rectangular waveguide. Test samples were cut into
22.86 × 10.16, 15.80 × 7.90, 10.67 × 4.32, and 7.12 × 3.56 mm2
(length × width) corresponding to X-, Ku-, K-, and Ka-bands with
thicknesses of 1, 3, and 5 mm, respectively. The EMI SE, SEA and SER
can be obtained by the following equations:
R = |S11|2
(1)
T = |s21|2
(2)
A=1−R−T
(3)
SE R = − 10 log(1 − R )
(4)
i T yz
zz
SEA (dB) = − 10logjjj
k1 − R {
EMI SE(dB) = SE R + SEA
3. RESULTS AND DISCUSSION
The preparation of Ag@ME is schematically shown in Figure
1. The fabrication process can be divided into two parts: the in
situ absorption of Ag+ on the ME skeleton and the sintering of
MOD. Pristine ME shows white color (inset of Figure 1a) and
lacks a functional group.29 To improve the interaction between
MOD and ME, the ME was treated with oxygen plasma and
PEI molecules to introduce the hydrophilic group (−OH,
−COOH) and more −NH2 groups on the skeleton surface of
ME, as shown in Figure 1a,b and Figure 1I,II. Without oxygen
plasma and PEI treatment, the ME only contains a small
amount of −NH2 groups, leading to only a little of MOD being
absorbed on the ME skeleton (see Figure S1, Supporting
Information). As a result, the shielding efficiency of Ag@ME
treated without PEI and plasma is obviously worse than that of
Ag@ME treated with PEI and plasma (Figure S1, Supporting
Information). Whereafter, the treated ME was immersed in
MOD solution (inset of Figure 1b), in which the silver amine
complex was absorbed by the −NH2 groups on the skeleton
surface (Figure 1b). The sample was freeze-dried, as shown in
Figure 1c and Figure 1II,III, to get rid of the solvent. In this
step, large amounts of free Ag+ in the ME cavity were further
accumulated and attached to the ME skeleton, and the color of
Ag+@ME changed to black (see inset of Figure 1c).
It is well known that the sintering temperature plays a vital
role in the conductivity of MOD.24,30 Nevertheless, for the
present study, the high-temperature treatment will inevitably
deteriorate the mechanical properties of ME.31 To confirm the
optimized sintering temperature, we conducted thermal
thermogravimetric analysis (TGA) for each group of samples,
as shown in Figure S2 (Supporting Information). According to
the TGA results, the MOD is almost decomposed when the
temperature reaches 200 °C, while the main loss for ME (less
than 5 wt %) is the evaporation of the absorbed water and
moisture contained in the sponge, which is consistent with the
reported results.31 In addition, the products after sintering are
volatile, leaving without any residues that impede the
conductivity of the Ag@ME, where eqs 7 and 8 show the
(5)
(6)
where T is transmission coefficient, R is reflection coefficient, and A is
the absorption coefficient. The near-field EMI SE of Ag@ME was
tested by using a Smart Scan-350/550 EMI (API, UK). The EMI
shielding mechanism simulation of Ag@ME was verified. The detailed
simulation parameter and corresponding model are listed in Table S1
(Supporting Information).
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Figure 3. Various characterizations of Ag@ME. (a) FTIR spectra, exhibiting the evolution of functional groups of samples with different
treatments. (b) XRD patterns and (c) XPS analysis of Ag@ME. (d) High-resolution XPS spectra of N 1s and Ag 3d, verifying the elements of C, N,
O, and Ag in Ag@ME.
shielding material (inset of Figure 1d). The microstructure of
Ag@ME with different Ag vol % values was observed by SEM,
as shown in Figure 2. Original ME possesses a continuous
three-dimensional network that is composed of staggered
skeletons (Figure 2a). It can be seen from the SEM image of
magnification that the surface of pure ME foam is smooth,
which is not conducive to the precipitation and adsorption of
free Ag+ in MOD on ME foam. As Figure 2b−e shows, the
SEM images and EDS mapping of the Ag@ME demonstrate
the change of metalized skeleton with the increase of the
concentration of Ag+ in MOD, and the wrapping of silver
particles on the ME foam becomes denser gradually after
sintering.
As shown in Figure 2b, only a tiny amount of silver
nanoparticles are located on the ME skeleton after sintering at
200 °C when the concentration of MOD solution is low
(0.0003 mol/mL), which cannot form an effective conductive
network. A thin and continuous silver layer is formed when the
concentration of MOD reaches 0.0006 mol/mL. Interestingly,
the thickness of silver layers coated on the ME rapidly
increases with the concentration of MOD increase to 0.0009
mol/mL. When the concentration of MOD is further increased
to 0.0012 mol/mL, the metallized skeleton of Ag@ME is
coated with a heavy silver layer after sintering. However, the
excess silver cannot attach to the ME skeleton due to the
limited binding sites, which results in the aggregation of silver
particles in the cavity of ME. Therefore, the silver layer made
by in situ wetting and sintering with an MOD solution is tightly
coated on the ME skeleton (see Figure S4, Supporting
Information), forming whole electron transmission paths that
can provide an essential guarantee for improving the overall
EMI shielding ability of Ag@ME. The ME foam serves as a
three-dimensional skeleton, and PEI is deposited on the
surface of ME skeleton to enhance its surface adhesion. The
chemical reaction of MOD formation and its decomposition, in
which the silver acetate particles are first complexed with
ethylenediamine and ammonia and thus forming a clear
solution (inset of Figure 1a).
Ag(OOCCH3) + (CH 2NH 2)2 + NH4OH + HCOOH
→ NH3(HCOO)C2H8N2Ag + CH3COOH + H 2O
(7)
NH3(HCOO)C2H8N2Ag + CH3COOH
→ Ag ↓ + CO2 ↑ +H 2O ↑ +NH3↑
(8)
To investigate the effect of soaking time on microtopography of Ag@ME, different immersion times with 4, 8,
12, and 24 h were comparably explored. As clearly shown in
Figure S3a−e, with the immersion time increased from 4 to 12
h, the content of silver layer coated on the ME increases from
0.17 to 2.03 vol %. No significant difference can be found when
soaking time exceeds 12 h. Therefore, 12 h is an optimized
soaking time, and the Ag vol % of Ag@ME can be regulated by
altering soaking time. Interestingly, it seems that there is more
silver attached on the joints than those of the skeleton bar.
After the low-temperature decomposition with 200 °C under a
nitrogen atmosphere, the Ag@ME shows gray color (inset of
Figure 1d). The ME skeleton was wrapped with silver particles
tightly to form a conductive network (Figure 1IV), providing a
continuous electron transport channel and bringing excellent
EMI shielding performance. The incident EM waves are
attenuated when they encounter the Ag@ME, and some EM
waves are first reflected back due to impedance mismatch
between the air and the surface of Ag@ME. Another part of
EM waves that penetrated the surface is attenuated due to the
multiple reflection losses inside the Ag@ME. Eventually, only a
very small amount of transmitted EM waves penetrated the
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Figure 4. Mechanical properties of the Ag@ME. (a) Schematic diagram of a set of cyclic compression tests. (b) Typical stress−strain curves of
Ag@ME-2.03 vol % during stress−strain cycles in compressing amplitudes of 30, 50, and 80%. (c) Stress−strain curves of Ag@ME during loading−
unloading cycles (80% strain) for different cycles. (d) Large version of maximum compression strength, and the inset is images of the initial
position compression and recovery. (e) Statistical graph of compression strength and the reduced ratio with different numbers loading−unloading
cycles (80% strain).
The XRD pattern of Ag@ME is shown in Figure 3b; no
crystallization peaks are observed for the raw ME foam, while
the sharp crystal peaks of Ag@ME after low-temperature
decomposition have appeared. The peaks located at 37.98,
44.17, 64.32, 77.27, and 81.44° correspond to the (111),
(200), (220), (311), and (222) planes of silver,34,35 and the
intensity of these peaks increase with the increase of silver
contention (from 0.89 to 3.48 vol %). In addition, no other
characteristic peaks can be found, indicating that no impurities
were generated during the low-temperature decomposition
process. The XRD results imply that all the MOD inside ME
foam have been completely decomposed into silver at 200 °C
in the atmosphere of N2 for 10 min. XPS results further prove
elementary compositions of raw ME foam and Ag@ME, as
shown in Figure 3c,d. The N 1s, C 1s and O 1s peaks originate
from the ME foam.36−38 The new peaks of (Ag 2p and Ag 3d)
Ag@ME are observed after coating MOD (Figure 3c). The
coordination bonding of Ag@ME can be further certified by
high-resolution XPS spectra (Figure 3d). The peaks at 368.12
and 374.2 eV correspond to Ag 3d5/2 and Ag 3d3/2,
respectively.39,40 Therefore, the silver layer is attached on the
ME skeleton successfully, which is consistent with the XRD
results.
sintered silver particles stick to the foam surface by physical
adsorption.
To further explore the evolution of functional groups during
the preparation process and the structure of Ag@ME, Fourier
transform infrared (FI-IR), X-ray diffraction (XRD), and X-ray
photoelectron spectroscopy (XPS) analysis were carried out.
Figure 3a shows ATR-FTIR spectra of pure ME foam and
Ag@ME with different reaction steps. For the raw ME foam,
the peaks at ∼3375, ∼2939, and ∼1568 cm−1 are the stretching
vibration peaks of N−H, C−H, and CN,32,33 respectively.
After being modified by PEI molecules, the intensity of peaks
for N−H and CN is increased, suggesting that the −NH2
group is successfully attached on the ME skeleton. Compared
to the raw ME foam, the ME foam deposited by PEI molecules
possesses more −NH2 groups, which is conducive to adsorbing
MOD. For Ag+@ME, the intensity of characteristic peaks
corresponding to CN, C−H, and N−H groups is enhanced,
suggesting that the quantity of these functional groups is
further increased. After sintering process, the intensity of C
N and N−H groups becomes weak, indicating that most
functional groups of Ag@ME are decomposed; meanwhile, the
detected C−N and N−H may be originated from the ME
foam, which is verified by the TGA results.
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Figure 5. EMI shielding performance, S parameters, and power coefficients of Ag@ME in the frequency of 8.2−40 GHz covering X-band, Ku-band,
K-band, and Ku-band. (a) EMI SE of raw ME and Ag@ME with an average thickness of 1 mm. (b) EMI SE of Ag@ME-2.03 vol % with different
thicknesses. (c) Comparison of electrical conductivity of Ag@ME. (d) SET, SER, and SEA and (e) average power coefficients A, R, and T of Ag@
ME-2.03 vol % with different thicknesses. (f) Changes of EMI SE of Ag@ME-2.03 vol % in the X-band as the number of cycles of compression
cycles at about 80% strain.
Mechanical properties, including compression strength and
recovery ability, are important indicators that need to be
considered for conductive foam used in the EMI shielding of
electronic devices.41 The compressive elasticity of Ag@ME is
demonstrated by the uniaxial compression test, as schematically illustrated in Figure 4a. The compressive stress of Ag@
ME as a function of strain with different stepped strain
amplitudes (30, 50, and 80% in sequence) was measured.
When the compressive strain is lower than 50%, the stress−
strain curves show an approximate linear region, and a sharply
increasing slope occurs when strain is over 70%, as displayed in
Figure 4b. Apparently, the hysteresis loop can be observed
during stress−strain cycles even at compressive strain as low as
30%, indicating that the Ag@ME possesses sensitive
compression resilience and broad prospects for the precise
electronic equipment that need to have both the shock
absorption and EMI shielding. When the compression strain is
increased to 50%, the compressive stress is increased slightly
from 1.12 to 1.86 kPa. When compression strain is further
increased to 80%, the compressive stress is increased
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Table 1. Summary of Volume Fraction, Density, Conductivity, and EMI SE of Ag@ME
sample
Ag@ME-0.89
Ag@ME-2.03
Ag@ME-2.58
Ag@ME-3.48
vol
vol
vol
vol
%
%
%
%
Ag content (vol %)
density (g/cm−3)
conductivity (S/m)
average EMI SE (8.2−40 GHz, dB)
SSE/t (dB/cm2/g−1)
0.89
2.03
2.58
3.48
0.1442
0.2291
0.2983
0.3802
1.2
158.4
198.3
279.2
5.9
21.1
22.8
25.2
409.2
921.3
764.2
625.9
GHz, which indicates that a continuous conductive network is
not formed in the ME. Further, by increasing the loading of
silver to 2.03 vol %, the EMI SE is sharply promoted to ∼21
dB with the same thickness of 1 mm, corresponding to specific
SE (SSE/t) of 921.3 dB/cm2/g−1 (see Table 1), indicating that
the conductive path in the Ag@ME has been constructed
(Figure 2c). Nevertheless, when the loading of silver in
composites reaches 3.48 vol %, there is a slight decrease of
SSE/t, which indicates that the silver absorption has almost
reached saturation. Furthermore, the EMI SE of the Ag@ME is
increased with the frequency of EM waves, which is consistent
with the reported literature.15,44
Shielding thickness is a critical factor for its EMI shielding
performance.45 We measured the EMI SE of Ag@ME-2.03 vol
% with different thicknesses in the frequency of 8.2 to 40 GHz,
as displayed in Figure 5b. The average EMI SE is calculated to
be ∼65 dB for the sample with a thickness of 5 mm, which is
enough to block over 99.9999% incident EM radiation.
Generally, the total EMI SE (SET) is mainly divided into
absorption loss (SEA), reflection (SER), and multiple reflection
loss (SEM). The SET, SEA, and SER can be deduced from the
following equations (eqs 9−11):
dramatically to 21.81 kPa. With the compressive strain
gradually stepped strain amplitudes of 30, 50, and 80%, the
compression curves still can overlap.
Figure 4c and Figure S5 show stress−strain curves (80%
strain) of Ag@ME after 10 loading−unloading cycles.
Compared to the original ME foam, Ag@ME owns a higher
maximum stress. With the increase in the silver particles
loading of samples from 0.89 to 3.48 vol %, the maximum
stress is increased from ∼18 to ∼28 kPa, which is attributed to
the compact binding the between ME foam and silver particles
coating. In addition, the stress−strain curve of all Ag@ME
could recover to the initial position forming a closed curve and
almost overlap with each other for different loading−unloading
cycles, showing that Ag@ME can recover its original size
without plastic deformation. We further studied and amplified
the maximum compression strength position (Figure 4d) and
the initial position (the illustration of Figure 4d) of the stress−
strain curves of Ag@ME-2.03 vol %. The original position for
each loading−unloading cycle slippage has changed, and all the
starting position is moving in a range controlled at about 1%.
The maximum compressive strength is slightly reduced after
suffering 10 times compressive cycles at 80% strain, from 21.81
to 17.86 kPa. Meanwhile, no appearance of a twine
phenomenon for each compressive curve is observed. The
compressive strength reduction ratio and maximum compressive strength for different compression times at 80% strain are
counted in Figure 4e and Figure S5. Even after suffering from
10 times compression of 80% ratio, the mechanical properties
of the Ag@ME-2.03 vol % can still maintain at the initial of
about 80%. To further explore the effect of mechanical
compression on EMI shielding performance stability, the EMI
shielding performances of Ag@ME-2.03 vol % under different
compression strains was measured, as shown in Figure S6. The
EMI SE of Ag@ME-2.03 vol % shows almost no change under
∼30, ∼50, and ∼80% compression strains, respectively, which
is attributed to the tight bond between the silver particles and
ME (see Figure S4, Supporting Information), and it is also
consistent with the test results of mechanical properties.
Therefore, the Ag@ME exhibits an outstanding mechanical
property since the MOD can be decomposed to form a
continuous conductive path at a fairly low temperature that
does not damage the mechanical property of ME, which is
superior to other types of conductive foam such as GO
composites foam prepared by high-temperature carbonization.42,43
The EMI shielding performance of Ag@ME was measured
by a vector network analyzer in the X-band, Ku-band, K-band,
and Ka-band. As shown in Figure 5a, the EMI SE of Ag@ME
increases with silver content. The raw ME foam has almost no
attenuation ability for incident EM waves. The decomposition
of MOD attached to the ME skeleton constructs a conductive
network, which provides a conductive path for shielding
incident EM waves. The ME foam with 0.89 vol % silver
content shows the electrical conductivity of only 1.2 S/m and
an average EMI SE of ∼6 dB in the frequency range of 8.2−40
SE R = 168 − 101log(fur /σr )
(9)
SEA = 1.31d fur σr
(10)
SE T = SE R + SEA + SEM
(11)
where d (cm), f (Hz), ur, and σr are the sample thickness,
frequency, magnetic permeability, and electrical conductivity,
respectively (ur = 1 in this study). For any shielding materials,
the reflection loss is determined by the significant impedance
mismatch at the contact surface between the air and shield, and
the level of impedance mismatch at the interface depends on
the intrinsic conductivity.46−49 The absorption loss is related
to the thickness, conductivity, and structure of shielding
materials.41,50 From eq 10, it can be clearly seen that increase
thickness enhances the SEA of the shield with constant
conductivity and permeability and thus further improves the
overall EMI SE (Figure 5d). Moreover, the porous structure is
conducive to enhancing the SEM of the shielding material.51,52
The porous structure within Ag@ME foam provides a vast
point to reflect the EM waves. To further investigate the
shielding mechanism, the power coefficients of Ag@ME with
different thicknesses, including A, R, and T were calculated
from S parameters, as displayed in Figure 5e. The SEA is much
greater than SER in the wide frequency band from 8.2 GHz to
40 GHz, indicating that SEA is dominated in EMI SE.
However, power coefficient R is much larger than A. From the
perspective of energy, it can be concluded that the attenuation
mechanism of Ag@ME for incident EM waves is mainly
reflected loss.53 When masses of EM waves are incident on
Ag@ME, the majority of EM waves are reflected because of
vast free electrons existing on the surface of Ag@ME. The
residual EM waves penetrate Ag@ME and interact with the
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Figure 6. EMI simulation results of Ag@ME in different frequency bands including X-band, Ku-band, K-band, and Ka-band. Electric field
distribution of waveguide (a−d) without and (a1−d1) with Ag@ME-2.03 vol %. Magnetic field distribution of waveguide (e−h) without and (e1−
h1) with Ag@ME-2.03 vol %.
shown in Figure 6). The red area in the center of the
waveguide is the EM radiation with high energy, and the blue
area at the edge of the waveguide is the weak EM radiation.
The wavelength of the transmitted EM wave in the rectangular
waveguide cavity decreases as its frequency increases. In the
absence of a shielding material (Figure 6a−d), the EM waves
are directly transmitted from port S1 to port S2, where S1 and
S2 are defined as the excitation port and receiving port,
respectively. When Ag@ME is placed in the middle of the
rectangular waveguide cavity, the energy transmission path of
the EM waves are blocked, resulting in the weakening of signal
strength in the side of S2.
It is worth noting that the intensity of monitored EM waves
attenuated by Ag@ME on the port S2 slightly is weaken with
the increase of frequency from the X-band to Ka-band (Figure
6a1−d1, Figure 6e1−h1), which is consistent with the
experimental results (Figure 5a). The effect of conductivity
of Ag@ME on EMI shielding performance is further explored
by the simulation, as shown in Figure S7. For Ag@ME-0.89 vol
%, a constant alternating electric field signal on the side of port
S2 is detected, suggesting that the Ag@ME-0.89 vol % shows
high-density electron of cavities inside Ag@ME, which induces
currents to produce ohmic losses. Specially, these EM waves
can be reflected back and forth in the porous structure until
they are completely absorbed in the cavity of Ag@ME and
eventually dissipated as heat. Moreover, there is a vast
conductivity difference between the silver and ME skeleton,
which generally induces dielectric relaxation, inherent electric
dipole, and interface polarization to improve the overall
shielding efficiency synergistically. Finally, the Ag@ME
exhibits outstanding cycling stability that EMI SE almost no
longer decreases after several compressions at about 80% strain
(see Figure 5f), which is attributed to the tight binding
between ME and silver coating.
In fact, the EMI shielding mechanism is abstract54 and it is
difficult to intuitively investigate by experimental results, and
most of the reported literatures for shielding mechanism
analysis are based on speculation. Therefore, a simulation was
conducted to study the electric field (E-field) and magnetic
field (H-field) distributions in the rectangular waveguide when
Ag@ME encountered with incident EM waves in the
frequency of X-band, Ku-band, K-band, and Ka-band (as
H
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Figure 7. Near-field shielding performance of Ag@ME in the frequency range of 1−9 GHz. (a) Application scenarios of near-field shielding (i)
without and (ii) with shielding materials, (iii) schematic for measurement of near-field shielding. (b) Near-field SE of Ag@ME with different silver
loadings. (c) Near-field SE of Ag@ME-2.03 vol % with different thicknesses and (d) the corresponding near-field SE mapping.
weak shielding ability. When the silver content increased from
0.89 to 2.03 vol % (corresponding to the electrical conductivity
increased from 1.2 S/m to 158.4 S/m), it is obviously observed
that the intensity of alternating electric field received on port
S2 is rapidly decreased. Eventually, with the conductivity
increased to 279.2 S/m, the intensity of the E-field signal
received by port S2 is further weakened. By contrast, no
significant change of H-field distribution is found (Figure S8,
Supporting Information), which may be ascribed to the nonmagnetic characteristic of Ag@ME. In addition, the effect of
sample thickness on EMI shielding performance has also been
studied by simulation. The shielding performance of Ag@ME-
2.03 vol % with different layers of one layer, two layers, and
three layers are simulated in the X-band (Figures S9 and S10,
Supporting Information). The intensity of E-field signal on the
side of port S2 is weakened gradually as the layer number of
Ag@ME-2.03 vol % increases (Figure S9, Supporting
Information). Similarly, the strength of the H-field monitored
in port S2 is also reduced with the increase of thickness of the
placed Ag@ME-2.03 vol % (Figure S10, Supporting
Information). The simulation results regarding the Ag@ME
with different thicknesses and electrical conductivity are
consistent with the experimental results.
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To further analyze and verify the interaction between Ag@
ME skeleton and EM waves, one of the structural units of Ag@
ME-2.03 vol % is selected for simulation by the periodic
structure solution model, as displayed in Figure S11
(Supporting Information). When the EM waves emanate
from an infinite distance and interact with the conductive
skeleton, the energy of incident EM waves is decreased
correspondingly (Figure S11c, Supporting Information),
producing a drastic reaction on both sides of the cavity.
Therefore, each unit forms a capacitor-like structure. The
induced surface current is mainly concentrated on the surface
of the Ag@ME, and only a small amount of induced surface
current is generated inside the cavity. (Figure S11e, Supporting
Information). Combining with simulation results of periodic
units (Figure 6 and Figure S11e), it can clearly reveal the
interaction mechanism of incident EM radiation waves and
shielding material. When EM waves reach the surface of Ag@
ME, most of them are immediately reflected due to the
interaction with free electrons in silver. The residual EM waves
penetrating the surface of the shielding material are reflected
many times inside the Ag@ME. In the process of multiple
reflections, alternating EM fields are continued to be cut by the
conductive network and produce induced currents at the
surface. These simulation results illustrate that incident EM
waves are first scattered on the surface due to the impedance
mismatch between air and Ag@ME. The residual EM waves
penetrating the surface are reflected many times inside the
Ag@ME. Therefore, the attenuation mechanism of Ag@ME
for EM waves is mainly based on reflection loss from the
simulation results, which is in agreement with the experimental
results. The EMI shielding performance is strongly dependent
on not only the shielding material but also the type of EM
radiation including near-field radiation and far-field radiation.55
Generally, the EMI SE measured by the rectangular waveguide
test method is identified as far-field EMI shielding performance, and it is excited by the far-field radiation source. For the
far-field zone (KR ≫ 1, where K and R are the wavenumber
and distance from detector to radiation source, respectively),
the field is separated from the emitting source and propagation
of EM waves.56 The near-field radiation is typically dominated
in the region of KR ≪ 1. The delay between phase and energy
propagation of the EM waves can be ignored for this case, and
the near-field radiation can be served as a quasi-static
condition. Therefore, for electronic devices in practical
application, the leakage of EM waves is usually detected by
the near-field scanning method.44,57
The schematics of the corresponding shielding model and
equipment set are shown in Figure 7a. The sensitive electron
component could be disturbed by the massive leakage of EM
waves from the analog chip if without a shielding material
(Figure 7ai). By comparison, the leakage of EM waves emitted
from chip will be decreased if an enclosed Faraday cage is
formed by a shielding material and lid, as shown in Figure 7aii.
To accurately measure the near-field SE of Ag@ME, a microstrip antenna embedded in a printed circuit board is served as
an analog chip and used as a near-field EM radiation source.58
The scanning probe connected to VNA via a coaxial cable with
an SMA connector is employed as the signal collector, as
shown in Figure 7aiii and Figure S12. In our work, the nearfield SE of Ag@ME in frequency range of 1−9 GHz involved
the operating frequency range 1 (FR1, 410−7125 MHz) of 5G
communication was measured. As shown in Figure 7b, the
near-field shielding performance of Ag@ME is improved with
the increase of silver loading, and the near-field SE of Ag@ME3.48 vol % with an average thickness of ∼1 mm is about −30
dB, which is much higher than the baseline without a shield
(approximately −18 dB) and meets the requirement of
commercial standards. In addition, thickness also plays an
important role in near-field shielding, and the effect of
thickness on near-field SE was investigated, as displayed in
Figure 7dc,. The signal intensity of leaked EM waves from the
analog chip with shield can be visualized. As we can see, the
corresponding near-field SE mapping is gradually weakened
from the red switch to blue. High-intensity EM waves will be
leaked in the absence of a shielding material. After shielding
with the Ag@ME-2.03 vol % with different thicknesses (from
1.2 to 6.2 mm), the signal intensity of the detected EM waves
is decreased gradually. Surprisingly, the average near-field SE of
Ag@ME-2.03 vol % is as high as about approximately −65 dB
with the thickness of ∼6 mm at the frequency of 1−9 GHz,
covering the primary frequency range of FR1. Therefore, Ag@
ME with outstanding near-field shielding performance shows
potential application in chip packaging and 5G wireless
communication (FR1).
Research Article
4. CONCLUSIONS
In summary, we developed a facile and environmentally
friendly method to prepare silver coated melamine foam (Ag@
ME) via in situ sintering of metal−organic decomposition
(MOD) on the ME skeleton modified with a poly(ethylene
imine) molecule (PEI). The PEI substantially improves the
interaction between silver and ME. The low-sintering temperature characteristic but high electrical conductivity of MOD
leads to the excellent mechanical property and EMI shielding
performance of Ag@ME. The Ag@ME possesses an EMI SE of
∼63 dB in the wide frequency band from 8.2 to 40 GHz (Xband, Ku-band, K-band, and Ka-band), while the volume
content of silver is as low as 2.03 vol %. The shielding
mechanism of Ag@ME is further verified by electromagnetic
simulation. Moreover, the Ag@ME shows an outstanding
average near-field SE of over approximately −65 dB. This work
provides a new strategy to fabricate metallized skeleton of
polymer foam for application in the field of EMI shielding.
■
ASSOCIATED CONTENT
* Supporting Information
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acsami.1c21836.
Morphology of ME skeleton deposited silver and EMI
shielding performance of Ag@ME treated with and
without PEI and plasma; thermogravimetric analysis
curves of Ag@ME; SEM images of the sintered
morphologies of Ag@ME with different soaking times;
SEM image of Ag@ME, showing interfacial bonding of
silver particles to ME; compression stress−strain curves
of Ag@ME with different Ag loadings; EMI SE of Ag@
ME-2.03 vol % under different compressive strains;
detailed simulation parameters of Ag@ME for EMI
shielding; electromagnetic simulation results of Ag@ME
obtained from finite element analysis; and measurement
setup of near-field shielding performance (PDF)
J
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www.acsami.org
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AUTHOR INFORMATION
Corresponding Authors
Yan-Jun Wan − Shenzhen Institute of Advanced Electronic
Materials, Shenzhen Institute of Advanced Technology,
Chinese Academy of Sciences, Shenzhen 518055, China;
orcid.org/0000-0002-8033-3328; Email: yj.wan@
siat.ac.cn
Peng-Li Zhu − Shenzhen Institute of Advanced Electronic
Materials, Shenzhen Institute of Advanced Technology,
Chinese Academy of Sciences, Shenzhen 518055, China;
orcid.org/0000-0002-4888-2685; Email: pl.zhu@
siat.ac.cn
Authors
Si-Yuan Liao − Shenzhen Institute of Advanced Electronic
Materials, Shenzhen Institute of Advanced Technology,
Chinese Academy of Sciences, Shenzhen 518055, China;
University of Chinese Academy of Sciences, Beijing 100049,
China
Gang Li − Shenzhen Institute of Advanced Electronic
Materials, Shenzhen Institute of Advanced Technology,
Chinese Academy of Sciences, Shenzhen 518055, China
Xiao-Yun Wang − Shenzhen Institute of Advanced Electronic
Materials, Shenzhen Institute of Advanced Technology,
Chinese Academy of Sciences, Shenzhen 518055, China;
University of Chinese Academy of Sciences, Beijing 100049,
China
You-Gen Hu − Shenzhen Institute of Advanced Electronic
Materials, Shenzhen Institute of Advanced Technology,
Chinese Academy of Sciences, Shenzhen 518055, China;
orcid.org/0000-0002-5258-199X
Tao Zhao − Shenzhen Institute of Advanced Electronic
Materials, Shenzhen Institute of Advanced Technology,
Chinese Academy of Sciences, Shenzhen 518055, China
Rong Sun − Shenzhen Institute of Advanced Electronic
Materials, Shenzhen Institute of Advanced Technology,
Chinese Academy of Sciences, Shenzhen 518055, China;
orcid.org/0000-0001-9719-3563
Ching-Ping Wong − School of Materials Science and
Engineering, Georgia Institute of Technology, Atlanta 30332,
United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acsami.1c21836
Author Contributions
#
S.-Y.L., G.L., and X.-Y.W. contributed to the work equally.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was supported by the National Natural Science
Foundation of China (no. 52103090), Foundation and Applied
Basic Research Fund project of Guangdong Province
(2019A1515111034), Guangdong Provincial Key Laboratory
(2014B030301014), Shenzhen Basic Research Plan
(JCYJ20190807154409372), and the Shenzhen Post-doctoral
Funding (E19106).
■
Research Article
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