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EMI SHIELDING BY CONDUCTING COMPOSITES
Satish Chandra gupta1, AA Athavale2, Sandhya Gupta2
1ARDE, DRDO, Pashan, Pune-411021, India
2Savitribai Phule Pune University, Pune-411007
satish_c_gupta@yahoo.com
Abstract - Microwave absorbing materials (MAM) based on
Conductive composites of Polyaniline in polyurethane (PANI-
PU) matrix have been developed to attenuate electromagnetic
radiation at microwave frequencies. Their conductivity,
capacitance and permittivity were measured. Microwave
attenuation observed in all PANI-PU coated aluminium plates
was broadband with 2 maxima of power loss measured by HP
vector network analyzer. Multi-layer coating showed total
power loss of 99.94 % in X and 99.2% in Ku band.
Key words — Microwave absorbing conductive composites
(MACC), Polyaniline in polyurethane (PANI-PU) matrix,
Dielectric properties, Power Return Loss, Reflection
coefficient
I. Introduction
Electromagnetic Interference (EMI) shielding means
reflection or absorption of Electromagnetic radiation by a
composite of materials. EMI shielding composite shields the
penetration of radiation through its conducting composite
material. As electromagnetic radiation particularly of high
frequency for instance radio waves emanating from mobile
phone towers, tends to interfere with electronics of guidance
system or computers. EMI shielding of both electronics and
radiation source is needed for safe operation of devices.
EMI shielding is required for reliability of electronics and
radiofrequency sources being used for many applications.
Magnetic field shielding means absorption of EM waves of
low frequencies up to 60 Hz. Primary mechanism of EMI
shielding is usually internal reflection of EM waves within
the materials of EMI shielding composite. For internal
reflection of EM waves the shield must have mobile
electrons or holes which interact with the electromagnetic
field in the radiation. For this purpose material should be
electrically conducting but not highly conducting like
metals. Volume resistivity of 1Ω.cm is typically sufficient.
EMI Shield can be a composite having conductive filler
which acts as dielectric materials. Microwaves only
partially reflect from and freely propagate through dielectric
materials. It is the combination of these two conditions
which allowed for the development of a reflection method
which permits measurement of dielectric properties and
physical dimensions [1]. When incident beam of
electromagnetic radiation impinges upon a “metallic”
surface, the electric field induces electrical currents within
the material, which have both capacitive and resistive
components. Capacitive currents generated are 90° out of
phase with the applied electric field (AC) voltage, and can
be related to the real permittivity (εr'). Capacitive currents
result
from polarization due to bound charges or dipoles inside the
conducting polymers and can give rise to polarization. The
resistive currents also generated within the material arise
from free electrons (or metallic states), are in phase with the
applied voltage, and give rise to dielectric loss (εr"). Useful
measure of absorption at microwave frequencies is loss
tangent (tan delta). Materials with tan δ > 1 are considered
lossy materials. tan δ values in the order of 10 are
considered to indicate strong absorption of radiation at a
given frequency. [2] In a time-varying case (i.e., a
sinusoid), electric fields and magnetic fields appear
together. This electromagnetic wave can propagate through
free space (at the speed of light, c = 3 x 108 m/s) or
through materials at slower speed. EM wave propagation
and its attenuation are dependent on the permittivity and
permeability of a material as shown in Fig. 1.
Fig. 1. Reflected and transmitted signals
A missile or aircraft can be made less visible to Radar by
coating materials that have electronic transitions of the
same energy as that of the radar electromagnetic waves.
Conducting polymers are a promising microwave
absorbing material as they are lossy, lightweight, and
corrosion resistant. The use of Conductive polymers
(Microwave absorbing materials, MAM) [3] is practical as
these materials can be applied in the form of a coating or a
tile. MAM attenuate electromagnetic radiation at
microwave frequencies on the order of GHz [4]. The
conductivity, and thus electrical resistance of conducting
polymers, provides a loss mechanism [5, 6], which
converts electromagnetic energy into energy in the form of
undetectable heat, thus attenuating the EM radiation by
absorption. Polyaniline (PANI) is a versatile conducting
polymer with a number of applications because of its
flexible chemistry, process-ability and ease of composite
paint formation. The electrical properties of PANI are
tailor able by dopants and there is scope for its solubility
in PU paint, which can improve radar absorption[7].
Entrapment of RADAR microwave in a multilayer
dielectric of graphite doped PANI conducting polymer
composites is the best way to reduce RADAR cross section
RCS. In case of oblique incidences scattering from a
multilayer dielectric [8] causes Total Internal Reflection
(TIR) thereby inducing microwave power loss (Fig. 2).
Single layer dielectrics are ineffective to produce TIR.
Fig. 2. EM Wave Propagation and reflection
in multilayer dielectrics D1, D2, D3
II. EXPERIMENTAL
A. Materials
The Synthesis and doping of polyaniline (PANI) was
done by different methods based on process ability to
form a blend with PU (Polyurethane). The composites of
PANI with varying quantities of dopant like DBSA
(dodecyl benzene sulphonic acid), nano graphite have been
prepared on the requirement of conductivity. In the
present work, PANI-PU blends were prepared in the paint
form using polyurethane as the matrix. The PU Paint
comprises of two components, one is Polyurethane resin
and the other is Hardener. For our experiment, the PU
Resin prepared by condensation polymerization of Polyol
i.e. poly ethylene glycol and acrylic acid and the hardener
is HMDI i.e. Hexa methylene diisocynate. The thinner
used was a mixture of butyl acetate and xylene.
B. Characterization
B.1 FT Infrared Spectral Analysis
Perkin Elmer Spectrum1 Fourier Transform Infrared
spectrophotometer was used for Infrared spectral analysis
of polyaniline (PANI) samples with KBr pellets. Specific
functional groups were observed for PANI samples in their
FTIR spectra.
B.2 Dielectric Permittivity Measurements
Aplab 4910 LCR-Q meter was used to measure
capacitance and permittivity of the different PANI
samples. A sheet or plate of dielectric material is
sandwiched between two electrodes to form a capacitor.
The PANI samples were pressed into circular pellets with
diameter 13 mm and thickness 2 mm and 3 mm for
consistent reading under a pressure of 400 M Pa in a
hydraulic press machine. After coating with silver paint on
both the surfaces as electrodes, the values of capacitance
measured for all samples at frequency of 100 Hz and 1
KHz. The calculation process is given below.
Cp= ε0εr’ A/t, Cp = Equivalent parallel capacitance of
material under test (MUT)
ε0 = 8.854× 10 -12 F/m = Free space permittivity;
Electrode surface ‘A’ = πd2/4, where diameter d = 13
mm, thickness t = 3 mm
From the measured capacitance Cp values, the dielectric
constant (real permittivity εr’ ) and Dielectric loss
factor ( εr” ) can be calculated by the equation 1 and 2.
εr’ = Cp * t / ε0A = Cp* 2.55×1012 Eqn. 1
Eqn. 2
ω = 2 π f = operating frequency , Rp = equivalent
parallel resistance of the MUT.
B.3 Conductivity Measurements
Conductivity measurement was done on PANI samples
using Vander Paaw method (4 probe DC method) using a
HAMEB current source and Keithley digital multimeter.
In this method we take a flat lamella, completely free of
holes, and provide it with four small contacts, M, N, O and
P, at arbitrary places on the periphery. We apply a current
I M N to contact M and take it off the contact N. We
measure the potential difference Vp- VO and define
resistance as follows between the current in and out points.
Eqn. 3(a)
(b)
In that case, M and O are placed on the line of symmetry,
while N and P are disposed symmetrically with respect to
this line. From the reciprocity theorem for passive four
poles, we have R NO, PM = R PM, NO (interchange of current
and voltage contacts) and it follows from the symmetry
that R PM, NO = R MN, OP. Resistivity ρ can then easily be
found from Equation 4.
Resistivity Л d R MN, OP
ρ =
ln 2
Conductivity σ = 1 / ρ S/m. …Eqn. 4
B.4 Reflection Coefficient Measurement
These measurements were carried out using HP vector
network analyzer using an APC 7 coaxial cable and a
rectangular waveguide. Power ratio of the reflected to the
incident micro wave is called return loss (RL). The RL
gives how many dB the power of the reflected wave is
below the incident wave [7]. The reflection coefficient (Г)
in dB is given as
.. Eqn. 5
Reflection coefficient, power reflected and loss are related
as per Table 1.
TABLE I POWER LOSS VALUES FOR DIFFERENT
REFLECTION COEFFICIENTS
S11 (dB) Reflected
Power (%)
Power
loss (%)
-∞ 0 100
-41.362 0.0074 99.9926
-30.737 0.084 99.9156
-5.451 28.504 71.496
-34.912 0.0234 99.9677
Direct measurement of power loss can be given as
… Eqn. 6
III. RESULTS AND DISCUSSION
A. FTIR Spectral Analysis
The vibration bands observed in FTIR spectra (Table 2)
for Polyaniline and doped composite are reasonably
explained on basis of normal modes of Polyaniline.
B. Capacitance, Permittivity & Conductivity
Capacitance observed, real permittivity values and DC
conductivity for the Graphite blended PANI-A, B, C, D
and E samples at 100 Hz and 1 KHz frequency are given
in Table 3. Larger doping agent DBSA reduces
conductivity of PANI-9 due to increased inter-atomic
distance. With increasing quantity of graphite in 1:2:4:8:16
ratio conductivity increases as observed for PANI-E to
PANI-A. It has effect on permittivity values also. The
frequency dependent dielectric permittivity observed in
PANI conducting polymers is dominated by reorientation
of molecular dipoles. Electronic, atomic, and orientation
polarization inside the semi-conducting polymers occur as
charges are locally bound in atoms, molecules, or
structures of solids. Charge carriers also exist that can
migrate over a distance through the material when a low
frequency electric field is applied. Interfacial or space
charge polarization occurs when the motion of these
migrating charges is impeded. Charges become trapped
within the interfaces of a material and are not freely
discharged or replaced at the electrodes. Field distortion
caused by accumulation of these charges increases the
overall capacitance which appears as an increase in є'r.
Real part of permittivity (є'r) measures the energy from an
external electric field that is stored in the conducting
polymer composite, a dielectric material. Measured
variation of real permittivity of differently doped (HCl,
H2SO4, DBSA, p-TSA dopants) conducting PANI and
PANI-PVAcetate hybrid composites with frequency
alteration (Fig. 3) is as expected (Equation 2).
TABLE II FTIR - SPECTRAL DATA
DBSA DOPED PANI COMPOSITE
Wave number
(cm-1)
Band assignment
3439 Asymmetric –NH + stretching
3202, 2924 N-H2 stretching
3071 Aromatic –CH stretching
2854 –CH2- stretching (DBSA)
1606, 1577 N=Q=N ring stretching
1589 Stretching of N=Q=N
1552 Aromatic C-C stretching Benzoid
segment
1465 N-B-N ring stretching
1460 Aromatic C-C stretching Quinoid
segment
1387 Out-of-plane –CH bending of –
1301 Q=N-B stretching
1296 Out -of-plane –NH bending
1156 N=Q vibration
1015-1175 CH in-plane bending
(1,4- disubstituted ring)
1006-1030 S=O stretching (DBSA)
C. Microwave Power Attenuation
For microwave power attenuation studies PANI-PU
coatings of 50-micron thickness were made on test
aluminium plates. Microwave power attenuation observed
in all PANI-PU samples was broadband and generally
showed two maxima between 8-11 GHz measured by HP
vector network analyzer using an APC-7 coaxial cable and
a rectangular waveguide. The electromagnetic waves
absorption of RADAR waves in the 9-11 GHz frequency
region was studied using the multilayer of STEALTH
Paints comprising graphite doped PANI-PU composites.
Graphite doping in PANI has provided effective RADAR
signal absorption up to 41 dB with > 99% microwave
power reflection loss.
Fig. 3.Real permittivity with frequency for PANI polymer
composites
TABLE III CAPACITANCE AND REAL
PERMITTIVITY OF PANI SAMPLES
IV Conclusion
Microwave absorption measurements on 100 mm x100
mm aluminium plates coated with PANI-PU paint
composites in Figs. 4 display the S11 microwave
absorption curves and reflection coefficients by PANI
B+C+E and PANI E respective coatings with PU paint of
varying thickness from vector network analyzer. The
attenuation observed in all the PANI-PU composites is
broadband and generally shows two or more maxima
between 8-12 GHz. Table 4 reflects the maximum power
loss in case of each sample. Polyurethane (PU) resin
provides thermal stability to the PANI-PU composite. The
coatings with conducting polymer PANI with PU paint can
be utilized for Radar absorption to reduce the signature of
the airborne objects and EMI shielding of electronic
appliances.
ACKNOWLEDGEMENT
Authors express their thanks to Director, ARDE, and
DRDO for support and encouragement to publish the
work.
TABLE IV POWER LOSS BY PU-PANI COATINGS
Fig. 4. Microwave Reflection Loss by PU- PANI coatings
REFERENCES
1., P. G. Lederer, “An Introduction to Radar Absorbent
Materials (RAM)” Royal Signals and Radar Establishment,
Malvern, 1986.
2. K. Gaylor, “Radar Absorbing Materials - Mechanisms and
Material,” DSTO Materials Research Laboratory, 1989.
3. K.J. Vinoy, and R.M. Jha, Radar Absorbing Materials: from
theory to design and characterization. Boston: Kluwer
Academic Publishers 1996.
4. A.J. Heeger, Semiconducting and Metallic Polymers: The
Fourth Generation of Polymeric Materials, Angew. Chem. Int.
Ed. Engl., 2001, 40, p.2591- 2611.
5. H.M. Altschuler, Dielectric Constant, Chapter IX of
Handbook of Microwave Measurements, Sucher, M. and Fox, J.
ed., Wiley 1963.
6. Arthur von Hippel (ed.), Dielectric Materials and
Applications, Cambridge, Massachusetts: MIT Press, 3rd
printing, January 1961.
7. Aditya Panwar, and S.C. Gupta, Polymer Based Microwave
Absorbing Material , Dissertation of the degree in Master of
Engineering (Mechanical) in Air Armament, Defence Institute of
Advanced Technology, Pune 2009.
Sample Capacita
nce
( n f )
Real
permittivity
( є'r )
Conductivity
(S/m)
At frequency
100Hz
PANI-9
PANI-B
PANI -C
PANI- D
PANI- E
15
50
10.2
2.50
2.30
4.28x105
127.66x105
26.044x105
8.51x105
6.8515 x105
0.07
126.298
51.138
26.675
5.121
At frequency
1kHz
PANI-9
PANI-A
PANI-B
PANI-C
PANI-D
PANI-E
16
9
6
6
2.80
2.20
0.404x105
17.95x105
15.32x105
15.32x105
0.9525x105
0.655 x105
0.1
161.55
129.18
68.5
29.78
7.41
Sample Thickness
(Micron)
S11
(dB)
Max
Peak
Power
Loss
(%)
Frequenc
y (GHz)
1. PANI B+C+ E 350 -41.36 99.99 9.98
2. PANI B+C+E 250 -30.73 99.91 9.30
3. PANI B+C+E 150 -5.451 71.49 10.2
4. PANI E 200 -34.91 99.96 11.4
8. Knott, F. Eugene, J.F. Schaeffer, and M.T. Tuley, RCS its
prediction Measurement and reduction, Artec House Inc. 1985.
9. M.I. Skolnick, Introduction to Radar Systems, McGraw-Hill,
1980.
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