Preparation and microwave shielding property of silver-coated carbonyl iron powder

Article abstract of DOI:10.1016/j.jallcom.2015.01.103

Electroless silver coating of carbonyl iron powder is demonstrated in the present investigation. The carbonyl iron powders are characterized by scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction analysis

Full text of DOI:10.1016/j.jallcom.2015.01.103

Accepted Manuscript
Preparation and Microwave Shielding Property of Silver-coated Carbonyl Iron
Powder
Xiao Guo Cao, Hao Ren, Hai Yan Zhang
PII:
S0925-8388(15)00176-0
DOI:
http://dx.doi.org/10.1016/j.jallcom.2015.01.103
JALCOM 33153
Reference:
To appear in:
Journal of Alloys and Compounds
Received Date:
Revised Date:
Accepted Date:
21 December 2014
14 January 2015
17 January 2015
Please cite this article as: X.G. Cao, H. Ren, H.Y. Zhang, Preparation and Microwave Shielding Property of Silver-
coated Carbonyl Iron Powder, Journal of Alloys and Compounds (2015), doi: http://dx.doi.org/10.1016/j.jallcom.
2
015.01.103
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Preparation and Microwave Shielding Property of Silver-coated
Carbonyl Iron Powder
a,*
b
a
Xiao Guo Cao , Hao Ren , Hai Yan Zhang
a
School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006,
Guangdong, PR China
b
Guangzhou Research Institute of O-M-E Techonlogy, Guangzhou 510006, Guangdong, PR
China
Abstract: Electroless silver coating of carbonyl iron powder is demonstrated in the present
investigation. The carbonyl iron powders are characterized by scanning electron microscope
(SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction analysis (XRD) before
and after the coating process. The relatively uniform and continuous silver coating is obtained
under the given coating conditions. In this paper, the electromagnetic interference (EMI) shielding
mechanism of the silver-coated carbonyl iron powder is suggested. The reflection of silver coating
and absorption of carbonyl iron powder dominate the shielding mechanism of the silver-coated
carbonyl iron powder. The silver-coated carbonyl iron powders are used as conductive filler in
electroconductive adhesive for electromagnetic interference shielding applications. The effect of
the thickness of electroconductive adhesive on the shielding effectiveness (SE) is investigated.
The results indicate that the SE increases obviously with the increase of the thickness of
electroconductive adhesive. The SE of the electroconductive adhesive with 0.35 mm thickness is
above 38 dB across the tested frequency range.
*
Corresponding author. Tel.: +86-20-39322425; Fax: +86-20-39322570.
E-mail address: xgcao@gdut.edu.cn (X.G. Cao).
1

Key words: Silver coating; Carbonyl iron powder; Electroless plating; Electromagnetic
interference shielding mechanism; Shelding effectiveness (SE)
1. Introduction
With the development of electronic information technology and widespread use of electronic
devices, electromagnetic interference (EMI) has become a serious problem in recent years. The
EMI may lead to malfunctioning of sensitive electronic components/appliances or may exert
harmful effect on organisms. To solve this problem, a number of electromagnetic wave shielding
materials, such as polymeric composites filled with carbon nanotubes, nickel coated carbon fibers,
silver coated cenospheres or copper powders etc, have been intensively investigated [1-4].
The primary mechanism of EMI shielding is usually reflection. The electromagnetic wave
reflecting materials must have mobile charge carriers (electrons or holes), which possessing high
conductivity. The conducting compositions based on metals (e.g. silver or copper powder),carbon
materials (e.g. graphite, carbon black, carbon fibers or carbon nanotubes) and cor-shell particles
(
e.g. silver coated cenosphere, copper or mica composites) are commonly used for EMI shielding
applications [3-16]. The secondary mechanism is absorption which requires the presence of
dielectric or magnetic dipoles (e.g. BaTiO , Fe , carbonyl iron powders, Fe-filled carbon
3
3 4
O
2
nanotubes, Co/SiO nanosphere or Ni/cenosphere cor-shell composites) [17-23].
Carbonyl iron powders are widely filled in resin or rubber as electromagnetic wave absorbers
due to their low price, high specific saturated magnetization, and Snoek’s limit [24-26]. He and
Guan [27] mixed the spherical and flaky carbonyl iron powders to adjust effectively the
electromagnetic parameters and enhance the absorption property in high frequency. Tong [28]
mixed carbon nanotubes/carbonyl iron powders (CNTs/CIPs) composite with different mixture
2

ratios to enhance the microwave absorption properties by a mechanically milling method.
Compared with CIPs, the CNTs/CIPs composites had higher electrical conductivity, permittivity,
and dielectric loss, which gradually increased with the increasing CNTs content. Silver is a typical
EMI shielding material with high SE. The core-shell structured composites with carbonyl iron
cores and silver shells may generate a high SE due to the EMW reflection of silver coating and the
absorption of carbonyl iron powder. Therefore, the development of conductive–magnetic
composite materials with composition and well-defined structures is valuable for excellent EMW
shielding capabilities. However, a few investigations have been done on the core-shell composite
fillers with magnetic cores and conductive shells. Fe
were synthesized by an easy processing route [29]. The composite nanoparticles exhibited typical
superparamagnetic behavior, and high conductivity. Gong [30] prepared the Fe /Ag
nanoparticles with both superparamagnetic and antibacterial properties by reducing silver nitrate
on the surface of Fe nanoparticles using the water-in-oil microemulsion method. And
3 4
O /Ag core-shell composite nanoparticles
3 4
O
3 4
O
Garcia-Torres [31] synthesized silver-coated cobalt (Co/Ag) nanoparticles by a micellar method.
In the present work, silver-coated carbonyl iron powders were prepared by an electroless plating
method. Electroconductive adhesive was obtained by mixing epoxy resin with silver-coated
carbonyl iron powder. The EMI shielding mechanism of the silver-coated carbonyl iron powder
was discussed. And the effect of the thickness of electroconductive adhesive on the SE was
investigated in terms of the packing structure of the silver-coated carbonyl iron powders.
2
2
. Experimental
.1. Materials and chemicals
Carbonyl iron powders used in the present investigation were supplied by Guangzhou Yuelong
3

Metal Powder Co., Ltd., China. Epoxy resin (E-44) was purchased from Guangzhou Shenghao
Chemical Industry Co., Ltd., China. The chemicals used for the electroless Ag-coating of carbonyl
3 2 5 2 8 2
iron powders included AgNO (99.9+%), HCHO (37 wt.%), C H OH (99.9%), C H N (≥98%),
and NH
Chemical Regents Company. Deionized water was used throughout the work.
.2. Electroless plating procedures
4 3
OH (28.0– 30.0% NH ), which were of analytical grade and purchased from Guangzhou
2
3
Ammonia as complexing reagent was added dropwise to the AgNO (0.05 mol/L) solution, and
+
adjusted the mixed solution pH value to 9.5. At the conditions, all Ag formed complex compound
+
[
Ag(NH
3
)
2
] with NH
3
. A total of 4 g of carbonyl iron powders were stirred in the silver ammonia
solution (300 mL), as solution A. HCHO (4 mL) was dissolved in 100 mL of ethanol, as solution
B. The solution A and B were preheated to 50•. After that, solution B was added to solution A, and
stirred for 20 min. The silver-coated particles were then filtered off, washed with deionized water,
and dried at 100• for 2 h.
2.3. Preparation of adhesives and samples
The electrically conductive adhesives were prepared by mixing epoxy resin, silver-coated
carbonyl iron powders and anhydrous ethanol used as solvent at the mixing speed 100 rpm using a
lab-scale homogenizer (FJ-200) for 15 min. The homogenizer was manufactured by Changsha
Qiulong Instrument and Equipment Co., Ltd., China and the speed range was 50～10000 r/min.
Then ethylenediamine used as hardener was mixed, and the mass ratio of ethylenediamine to
epoxy resin was 1:10. The mass ratio of silver-coated carbonyl iron powders to epoxy resin was
7:3, and the mass ratio of anhydrous ethanol to epoxy resin was 6:10. The single overlapped
adhesive joints used for studying the EMI shielding behavior were prepared with bonding of PTEF
4

(
polytetrafluoroethylene, Yangcheng Electric Model Co., Ltd., China) film with Φ115 mm and 2
mm thickness. The adhesive joints were hardened at 50℃ for 10 h.
.4. Characterization
2
The morphology of the uncoated and silver-coated carbonyl iron powders were characterized by
SEM (S-3400N(II), Hitachi). The structure analysis of the powders was carried out using XRD
(
Ultima IV, Rigaku) with Cu Ka1 radiation.
.5. Measurements of SE
The SE of the sample for plane-wave conditions/simulation was measured by means of the
2
coaxial cable method [32]. The setup consisted of a DN15115 SE tester, which was connected to
an Agilent 4396B RF network spectrum impedance analyzer. The scanning frequency ranged from
100 KHz to 1.5 GHz. The thickness of the adhesive layers were 0.2 and 0.35 mm.
3. Results and discussions
Fig.1 shows the XRD patterns of (a) carbonyl iron powders and (b) silver-coated carbonyl iron
powders. The XRD pattern in Fig.1(a) indicates that the microstructures of carbonyl iron powders
are mainly composed of Fe comparison with the standard data of JCPDS (06-0696). As shown in
Fig.1(b), the diffraction peaks at 38.12°, 44.28°, 64.43°, 77.40° and 81.54° are clearly seen,
besides the weak diffraction peaks of carbonyl iron powders. The results are matched with the
major peaks (111), (200), (220), (311) and (222) of Ag comparison with the standard data of
JCPDS (04-0783), which reveals a pure silver layer successfully coated on the surface of carbonyl
iron powders.
Fig.2 shows the SEM micrographs of (a) carbonyl iron powders and (b) silver-coated carbonyl
iron powders. As seen from Fig.2(a), the carbonyl iron powders have a regular spherical surface
5

morphology and are well dispersed from each other. As shown in Fig.2(b), the uniform and
compact coating is observed, where a few grains adhering on the surface of silver-coated carbonyl
iron powders. It appears that during the electroless silver deposition, the uniform and compact
silver coating develops through silver particles formation.
Fig.3 shows the SEM micrographs of (a) cross-section and (b) surface of the electroconductive
adhesive filled with silver-coated carbonyl iron powders. The silver-coated carbonyl iron powders
as the conductive particles are surrounded by epoxy resin and uniformly distributed.The particles
touch one another and form a more compact packing structure, which benefits for the electrical
conductivity and SE of the electroconductive adhesive.
Electromagnetic shielding means that the energy of electromagnetic radiation is attenuated by
reflection or absorption of an electromagnetic shielding material. According to the theory of
electromagnetic shielding [33, 34], the SE is described as:
SE = SE
A
+ SE
R
+ SE
M
(1)
Here SE
reflections, respectively.
Reflection loss (SE ) is the result of interaction between conducting particles in the conducting
A R M
, SE and SE are the shielding efficiency due to absorption, reflection, and multiple
R
material (free electron or vacancy) and electromagnetic field, and it has relationship with the value
of reflection coefficient (R). R was obtained by the formula as follows:
(
1- Z / Z )
R =
s w
(2)
(
1+ Z / Z )
s w
Z w =377Ω
(3)
(4)
Z = G/m
s
r
Where G and mr are relative conductivity and relative permeability of the materials,
6

r
respectively. The reflection coefficient increases with the decrease of G/μ , which lead to the
increase of reflection loss.
A
Absorption loss (SE ) is caused by the heat loss under the action between electric dipole or
magnetic dipole in the shielding material and the electromagnetic field. SE
follow:
A
was calculated as
SE =131.4t fGmr
(5)
A
Where f is the the frequency of the electromagnetic wave, t is the transmission distance of the
electromagnetic wave in the material. At the same frequency and transmission distance of the
electromagnetic wave conditions, SE
A
has relationship with the value of G· m . The absorption
r
loss increases with the increase of G· mr of the material. Multiple reflection loss SE
M
is the loss
caused by the constantly touch to the wall surface of the shielding material, but this loss is very
low and is the correction term of reflection loss exactly [35].
Silver is a kind of EMI shielding material with good prospects of application because of its
electrical conductivity. As the resistivity of the surface layer of the silver-coated carbonyl iron
powders decreasing, electromagnetic impedance of the composite surface layer becomes smaller
and smaller. Therefore, the reflection loss of the electromagnetic waves is strengthened and the SE
increased. In addition, core part (carbonyl iron powder) absorbs the electromagnetic waves
penetrated the silver coating. This suggests that the reflection and absorption dominate the
shielding mechanism of the silver-coated carbonyl iron powders. The electromagnetic interference
shielding mechanism of the silver-coated carbonyl iron powder is shown in Fig.4.
Fig.5 shows the variationin SE of electroconductive adhesive filled with silver-coated carbonyl
iron powders in the frequency range of 100 KHz～1500 MHz. As seen from Fig.5, the SE of the
7

electroconductive adhesive with 0.20 mm thickness is above 32 dB across the tested frequency
range. The value of SE peaks at about 825 and 1470 MHz are 37.5 dB and 40.0 dB, respectively.
The SE of the electroconductive adhesive with 0.35 mm thickness is above 38 dB across the tested
frequency range. The value of SE peaks at about 825 and 1470 MHz are 42.5 dB and 49.3 dB,
respectively. The results indicate that the electroconductive adhesive filled with silver-coated
carbonyl iron powders is positively correlated to the thickness. The SE increases obviously with
the increase of the thickness of electroconductive adhesive.
4. Conclusions
Successful silver coating has been achieved on carbonyl iron powder by the electroless plating
process. The SEM results show that the uniform and compact silver layer has been successfully
coated onto the surface of carbonyl iron powders under the given coating conditions. The
silver-coated carbonyl iron powder is one of the most promising alternative conductive fillers for
electromagnetic interference shielding applications. The electromagnetic waves are firstly
reflected by silver coating, then absorbed by carbonyl iron powders. The reflection and absorption
dominate the shielding mechanism of the silver-coated carbonyl iron powder. The SE of the
electroconductive adhesive filled with silver-coated carbonyl iron powders is positively correlated
to the thickness. With the increase of the thickness of electroconductive adhesive, the SE increases
obviously.
Acknowledgement
This work is supported by the National Natural Science Foundation of China(Grant No.
51302042), and the Pearl River New Star Plan of Science and Technology of Guangzhou City of
China (Grant No. 2013J2200038).
8

References:
1] R. Wang, H. Yang, J. Wang, F. Li, The electromagnetic interference shielding of silicone
rubber filled with nickel coated carbon fiber, Polymer Testing, 38 (2014) 53-56.
2] H. Nishikawa, S. Mikami, K. Miyake, A. Aoki, T. Takemoto, Effects of Silver Coating
[
[
Covered with Copper Filler on Electrical Resistivity of Electrically Conductive Adhesives,
Materials Transactions, 51 (2010) 1785-1789.
[
3] B.-S. Yim, J.-M. Kim, Characteristics of Isotropically Conductive Adhesive (ICA) Filled with
Carbon Nanotubes (CNTs) and Low-Melting-Point Alloy Fillers, Materials Transactions, 51 (2010)
329-2331.
4] W. Wang, Q. Li, Y. Li, H. Xu, J. Zhai, Electroless Ag coating of fly ash cenospheres using
polyaniline activator, Journal of Physics D-Applied Physics, 42 (2009).
5] D.I. Petukhov, M.N. Kirikova, A.A. Bessonov, M.J.A. Bailey, Nickel and copper conductive
patterns fabricated by reactive inkjet printing combined with electroless plating, Materials Letters,
32 (2014) 302-306.
6] X.G. Cao, H.Y. Zhang, Preparation of silver-coated copper powder and its oxidation resistance
research, Powder Technology, 226 (2012) 53-56.
7] J.-T. Tsai, S.-T. Lin, Silver powder effectiveness and mechanism of silver paste on silicon solar
cells, Journal of Alloys and Compounds, 548 (2013) 105-109.
8] S.R. Mahapatra, V. Sridhar, D.K. Tripathy, Impedance analysis and electromagnetic
2
[
[
1
[
[
[
interference shielding effectiveness of conductive carbon black reinforced microcellular EPDM
rubber vulcanizates, Polymer Composites, 29 (2008) 465-472.
[9] X.G. Cao, H.Y. Zhang, Investigation into conductivity of silver-coated cenosphere composites
9

prepared by a modified electroless process, Applied Surface Science, 264 (2013) 756-760.
10] L.-N. Ho, H. Nishikawa, N. Natsume, T. Takemoto, K. Miyake, M. Fujita, K. Ota, Effects of
[
Trace Elements in Copper Fillers on the Electrical Properties of Conductive Adhesives, Journal of
Electronic Materials, 39 (2010) 115-123.
[11] X. Luo, G.A. Gelves, U. Sundararaj, J.-L. Luo, Silver-coated copper nanowires with
improved anti-oxidation property as conductive fillers in low-density polyethylene, Canadian
Journal of Chemical Engineering, 91 (2013) 630-637.
[12] N. Yousefi, X. Sun, X. Lin, X. Shen, J. Jia, B. Zhang, B. Tang, M. Chan, J.-K. Kim, Highly
Aligned Graphene/Polymer Nanocomposites with Excellent Dielectric Properties for
High-Performance Electromagnetic Interference Shielding, Advanced Materials, 26 (2014)
5480-5487.
[13] I. Tavman, I. Krupa, M. Omastova, M. Sarikanat, I. Novak, K. Sever, I. Ozdemir, Y. Seki, S.
Podhradska, D. Moskova, E. Erbay, F. Guner, Effects of conductive graphite filler loading on
physical properties of high-density polyethylene composite, Polymer Composites, 33 (2012)
1
071-1076.
14] X.G. Cao, H.Y. Zhang, Study of Pretreatment Techniques and Characterization of Electroless
Silver on Cenospheres, Electronic Materials Letters, 8 (2012) 519-522.
15] Y.N. Ko, H.Y. Koo, J.H. Kim, Y.C. Kang, Characteristics of silver-glass composite powders
as the silver electrode for Si solar cells, Journal of Alloys and Compounds, 491 (2010) 584-588.
16] H.T. Hai, H. Takamura, J. Koike, Oxidation behavior of Cu-Ag core-shell particles for solar
cell applications, Journal of Alloys and Compounds, 564 (2013) 71-77.
17] X.J. Yin, K. Peng, A.P. Hu, L.P. Zhou, J.H. Chen, Y.W. Du, Preparation and characterization
[
[
[
[
1
0

of core-shell structured Co/SiO2 nanosphere, Journal of Alloys and Compounds, 479 (2009)
72-375.
18] D.-L. Zhao, X. Li, Z.-M. Shen, Preparation and electromagnetic and microwave absorbing
properties of Fe-filled carbon nanotubes, Journal of Alloys and Compounds, 471 (2009) 457-460.
19] Z. Song, J. Xie, P. Zhou, X. Wang, T. Liu, L. Deng, Toughened polymer composites with
3
[
[
flake carbonyl iron powders and their electromagnetic/absorption properties, Journal of Alloys and
Compounds, 551 (2013) 677-681.
[20] L. Liu, Y. Duan, S. Liu, L. Chen, J. Guo, Microwave absorption properties of one thin sheet
employing carbonyl-iron powder and chlorinated polyethylene, Journal of Magnetism and
Magnetic Materials, 322 (2010) 1736-1740.
[21] P. Saini, M. Arora, G. Gupta, B.K. Gupta, V.N. Singh, V. Choudhary, High permittivity
polyaniline-barium titanate nanocomposites with excellent electromagnetic interference shielding
response, Nanoscale, 5 (2013) 4330-4336.
[
22] X. Yu, Z. Shen, The electromagnetic shielding of Ni films deposited on cenosphere particles
by magnetron sputtering method, Journal of Magnetism and Magnetic Materials, 321 (2009)
890-2895.
23] W. Zhou, X. Hu, X. Bai, S. Zhou, C. Sun, J. Yan, P. Chen, Synthesis and Electromagnetic,
2
[
Microwave Absorbing Properties of Core-Shell Fe3O4-Poly(3, 4-ethylenedioxythiophene)
Microspheres, Acs Applied Materials & Interfaces, 3 (2011) 3839-3845.
[
24] W. Zhang, S. Bie, H. Chen, Y. Lu, J. Jiang, Electromagnetic and microwave absorption
properties of carbonyl iron/MnO2 composite, Journal of Magnetism and Magnetic Materials, 358
2014) 1-4.
(
1
1

[25] X.J. Wei, J.T. Jiang, L. Zhen, Y.X. Gong, W.Z. Shao, C.Y. Xu, Synthesis of Fe/SiO2
composite particles and their superior electromagnetic properties in microwave band, Materials
Letters, 64 (2010) 57-60.
[
26] L. Liu, Y. Duan, L. Ma, S. Liu, Z. Yu, Microwave absorption properties of a wave-absorbing
coating employing carbonyl-iron powder and carbon black, Applied Surface Science, 257 (2010)
42-846.
27] J. He, W. Wang, A. Wang, J. Guan, Electromagnetic parameters and microwave-absorption
8
[
properties for the mixtures of mechanically milled spherical and flaky carbonyl iron powders, Acta
Metallurgica Sinica-English Letters, 25 (2012) 201-206.
[28] G. Tong, W. Wu, Q. Hua, Y. Miao, J. Guan, H. Qian, Enhanced electromagnetic
characteristics of carbon nanotubes/carbonyl iron powders complex absorbers in 2-18 GHz ranges,
Journal of Alloys and Compounds, 509 (2011) 451-456.
[29] Y. Sun, Y. Tian, M. He, Q. Zhao, C. Chen, C. Hu, Y. Liu, Controlled Synthesis of Fe3O4/Ag
Core-Shell Composite Nanoparticles with High Electrical Conductivity, Journal of Electronic
Materials, 41 (2012) 519-523.
[
30] P. Gong, H. Li, X. He, K. Wang, J. Hu, W. Tan, S. Zhang, X. Yang, Preparation and
antibacterial activity of Fe3O4@Ag nanoparticles, Nanotechnology, 18 (2007).
31] J. Garcia-Torres, E. Valles, E. Gomez, Synthesis and characterization of Co@Ag core-shell
nanoparticles, Journal of Nanoparticle Research, 12 (2010) 2189-2199.
32] Y.P. Duan, S.H. Liu, H.T. Guan, Investigation of electrical conductivity and electromagnetic
shielding effectiveness of polyaniline composite, Science and Technology of Advanced Materials,
(2005) 513-518.
[
[
6
1
2

[
33] T. Araki, Electromagnetic interference and Shielding measures, China Zhijian Publishing
House, (1989) 151-153.
34] J. Joo, A.J. Epstein, Electromagnetic radiation shielding by intrinsically conducting polymers,
Applied Physics Letters, 65 (1994) 2278-2280.
35] A. Kaynak, Electromagnetic Shielding Effectiveness of Conducting PPy Films in the
00-2000 MHz Frequency Region Materials Research Bulletin 31 (1996) 845-860.
[
[
3
1
3

Figure captions:
1
. XRD patterns of (a) carbonyl iron powders and (b) silver-coated carbonyl iron
powders.
2
. SEM micrographs of (a) carbonyl iron powders and (b) silver-coated carbonyl
iron powders prepared at 50°C for 20min. The dosages of AgNO solution was 300
3
mL. And the concentration of AgNO
. SEM micrographs of (a) cross-section and (b) surface of the
electroconductive adhesive filled with silver-coated carbonyl iron powders.
. Mechanism of shielding electromagnetic of silver-coated carbonyl iron
powders.
3
solution was 0.05 mol/L.
3
4
5
. Shielding effectiveness of the electroconductive adhesive filled with
silver-coated carbonyl iron powders. The thickness of the coating is 0.20 and 0.35
mm.
1
4

Fig.1
1
5

Fig.2
1
6

Fig.3
1
7

Fig.4
1
8

Fig.5
1
9

Highlights
1. The silver-coated carbonyl iron powder is prepared by the electroless plating
process.
2
3
. The silver-coated carbonyl iron powder is a new kind of conductive filler.
. The reflection and absorption dominate the shielding mechanism of the prepared
powder.
4. Increasing the thickness of electroconductive adhesive will increase the SE.
2
0