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DDeakin Research Online Research Online This is the authors final peer reviewed version of the item published as: Håkansson, Eva, Amiet, Andrew, Nahavandi, Saeid and Kaynak, Akif 2007-01, Electromagnetic interference shielding and radiation absorption in thin polypyrrole films, European polymer journal, vol. 43, no. 1, pp. 205-213. Copyright : 2006, Elsevier Ltd

http://www.deakin.edu.au/dro/




Electromagnetic Interference Shielding and Radiation

Absorption in Thin Polypyrrole Films

Eva Håkanssona, Andrew Amietb, Saeid Nahavandic, and Akif Kaynakd, ∗,

a Centre for Material and Fibre Innovation, School of Engineering and Information Technology, Deakin

University, Geelong, Victoria 3217 Australia b Defence Science Technology Organisation, Maritime Platforms Division, P.O. Box 4331, Melbourne,

Victoria 3001 Australia c Intelligent Systems Research Laboratory, School of Engineering and Information Technology, Deakin

University, Geelong, Victoria 3217 Australia

d School of Engineering and Information Technology, Deakin University, Geelong, Victoria 3217

Australia

Abstract Results of permittivity measurements, electromagnetic interference shielding effectiveness, and heat

generation due to microwave absorption in conducting polymer coated textiles are reported and

discussed. Intrinsically conducting polymer, polypyrrole doped with anthraquinone-2-sulfonic acid

(AQSA) or para-toluene-2-sulfonic acid (pTSA) was applied on textile substrates and the resulting

materials were investigated in the frequency range 1-18 GHz. Conducting textile/polypyrrole

composites interacted with incident microwaves and generated absorption levels of up to 48 % in a

0.54 mm thick substrate. A thermography station was used to monitor these composites during

simultaneous subjection to microwave radiation, where absorption was confirmed via visible heat

losses. Lower conductivity samples showed larger amounts of heat loss due to microwave absorption

compared to samples with higher conductivity. A sample with an average sheet resistivity of 120 Ω/sq.

showed a maximum temperature of 27.15ºC whilst a sample with a lower resistivity (105 Ω/sq.)

reached 26.65ºC (Ambient temperature: 22.8ºC).

Keywords: Polypyrrole, electromagnetic interference shielding, thermal imaging

1. Introduction

∗ Corresponding author. Address: School of Engineering and Information Technology, Deakin

University, Geelong, Victoria 3217 Australia. Tel: +61 3 5227 2909 Fax: + 61 3 5227 2167

E-mail address: [email protected] (A. Kaynak)

Håkansson, Amiet, Nahavandi, Kaynak Page 1 of 14

Instrinsically conducting polymers (ICP) can be produced in large quantities at low cost by chemical

polymerization techniques. Polymerization takes place by the oxidation of the monomer by a metallic

salt such as ferric chloride. A wide range of conductivity values can be obtained and the conductivity

of the resulting polymer is dependent on polymerization time, temperature, concentrations of monomer

and oxidant. Electroactivity, tuneable nature of electrical properties and ease of coating on to any

substrate make conducting conductive polymers suitable for a wide range of applications including

sensors and actuators [1,2] antistatic dissipative materials [3-5], heated fabrics [6,7], photovoltaic

devices [8,9] and electromagnetic interference (EMI) shielding materials [10-12].

Although conducting polymers have interesting properties, brittleness and lack of processability inhibit

commercial applications but when coated on substrates such as textiles, flexible conductive sheets with

infinite structural variations are possible. Chemical polymerization of the monomer in the presence of a

textile substrate results in the formation of conducting polymer on the surface of the substrate as a film

and also as polymer particles in the bulk of the solution, which subsequently precipitate as insoluble

solids. The surface polymerization should be encouraged while the bulk polymerization should be

suppressed. The reactant concentrations and synthesis parameters can be optimized to achieve a thin,

homogeneous coating on the surface of textile. The thickness of the polypyrrole film formed during

chemical synthesis is in the submicron range.

EMI shielding of a device is necessary when electronic equipments interfere with one another, causing

operational malfunction. Conducting materials reflect and/or absorb electromagnetic radiation thus

enable shielding from EMI. Conducting polymer-coated textiles offer advantages including larger

flexibility, significantly lower weight compared to metallic structures, possess homogenous shielding

compared to conductive particulate filled polymeric composites and have a high absorption

contribution to shielding that is not present in traditional shielding materials. There have been a few

investigations on the EMI shielding properties of conducting polymer films or dispersions [10, 13-19]

and some work has also been reported on conducting polymer coated fabrics [20-23].

The characterization of the interaction of microwaves with ICP coated fabrics is important because

accurate determination and evaluation of the shielding effectiveness is an incentive for further

investigations and subsequent implementation of the conducting textiles in suitable applications, which

include space materials, protective clothing for employees in electronically polluted environments, and

frequency selective protection from electronic espionage. In this paper, we investigate the heat

generated in polypyrrole coated nylon fabrics due to microwave absorption. The techniques employed

include a non-contact, non-destructive free space method for determining broadband dielectric

properties and thermal mapping with a thermography station. The free-space measurements facilitate

calculation of the absorption/reflection/transmission behaviour of the conducting polymer textiles and

the thermal imaging is used to confirm and map the presence of absorption in selected conductive

fabrics. The visual confirmation of heat generated from absorption of microwave radiation, including

consequent patterns of heating and rates of heat dissipation, improves our understanding of the

absorption that occurs within conducting polymers.


2. Experimental

2.1 Reagents

Fabric samples (0.54 mm thick Nylon-Lycra®) were scoured in Imerol (0.5 g/l) and sodium carbonate

(2.0 g/l) at 80°C prior to polymerization in order to remove any waxes, residual chemicals, oils and

particulates, providing a good basis for application of the conducting polymer coating. The samples

were dried in a FED 115 Lab oven (Binder) at 105°C prior to polymerization and allowed to cool to

room temperature before being introduced to the reaction solution.

Polypyrrole coatings were applied via direct chemical polymerization onto textile substrates using

different concentrations of anthraquinone-2-sulfonic acid (AQSA) sodium salt monohydrate (Aldrich)

or para-toluene-2-sulfonic acid (pTSA) sodium salt (Aldrich) as dopants and ferric chloride

hexahydrate (Fluka) as oxidant. An optimized oxidant to monomer ratio of 1:2.23 was used [24,25].

The polymerization time was kept to 180 minutes and occasional stirring was carried out.

2.2 Instrumentation

The surface resistivity of the conducting fabrics was measured in a standard atmosphere (20°C at 65%

RH) to AATCC 76-1995 standard. The thicknesses of the conducting polymer composites were

determined utilizing a fabric thickness tester (DGTW01B, Mitutoyo, Japan) to ISO 9073-2 standard

(0.5 kPa). An average of 20 measurements was used in the permittivity calculations.

The interaction of the conducting polymer samples with microwaves were investigated using a non-

destructive, non-contact method, where the sample is placed between two horn antennas [26]. An

8510C Vector Network Analyzer (Agilent Technologies), an 8517A S-Parameter Test Set (Agilent

Technologies) and an 83651B Synthesized Frequency Source (Agilent Technologies) were used to

produce the output radiation and collect the data. The frequency range of the mentioned system is able

to cover measurements from 45 MHz to 50 GHz. An IBM compatible computer controlled the system

and recorded the data.

The swept frequency radiation for detection of microwave absorption in the conducting polymer coated

fabrics was transmitted by a high performance dish antenna VHP1-130-111 (Andrew Australia) using a

PA750-8 probe antenna (Continental Microwave and Tool) as a feed. The antenna was connected to a

system consisting of an 8510B Network Analyser (Hewlett Packard), an 8350B Sweep Oscillator with

83592A RF Plug in (Hewlett Packard), and a 20T4G18 microwave amplifier (Amplifier Research).

Thermal mapping of the heat generated by absorption was performed utilizing a JADE-MW Advanced

Thermography Station (CEDIP Infrared Systems, France) in conjunction with Altaïr Software. This

system operates in the medium wavelength IR (3-5 μm) and has a thermal resolution as low as 20 mK.

2.3 Calculations

The behaviour of a material under the influence of an electromagnetic field can be determined using

complex permittivity ε* (due to electrical component of field) and complex permeability μ* (due to

magnetic component of field). The complex permittivity of any material consists of a real part

associated with the capacitive or "energy storage" component, and an imaginary part related to the

electric dissipation occurring within the material. Further, permittivity is often expressed in relative

units compared to that of free space and hence the relative complex permittivity is equal to


rrr iεεεεε ′′+′==

0

* (1)

Where, ε is the permittivity of the material and ε0 is the permittivity of free space (ε0 = 8.854 × 10-12

F/m). All permittivity values presented in this work are relative permittivities (i.e. εr*) but the suffixes

will not be used from here on. The conducting polymers under investigation have permeability equal to

that of free space.

The fraction of incident radiation that is reflected from the front surface of a material is referred to as

the reflection coefficient Γ, whilst the ratio of transmitted electric field strength to the incident electric

field strength is called transmission coefficient Τ. Both reflection and transmission coefficients can be

expressed in terms of relative permittivity and permeability as per

1

1

+

−=Γ

εμεμ

(2)

and

εμω dc

ie

⎟⎠⎞

⎜⎝⎛−

=Τ (3)

Where, ω is the angular frequency and d is the thickness of the material.

The output from the vector network analyser is in terms of scattering parameters, Sxy. The first number

in the suffix refers to what port the output is measured at and the second number refers to where the

signal originates from. Hence S11 is the reflected signal and S21 is the transmitted signal. For

electrically thin materials, Nicolson and Ross [68] showed that the scattering parameters S11 and S21

can be described as

( ) ( )22

2

11 1 1Γ−Γ−

=TTS ω (4)

( ) ( )22

2

21 1 1Γ−

Γ−=

TTS ω (5)

Using the free space technique, it is possible to extract both complex permittivity and permeability of

the sample by measuring both S11 and S21. However, when only the relative permittivity is unknown

and the samples under test are non-magnetic, it is possible to approximate the relative complex

permeability to unity. As a result of this, permittivity can be calculated using only the scattering

parameter S21. This can be done by substitution of the values of Γ and T (from Eq. 2 & 3) in Eq. 5,

resulting in the transmission S21 expressed in terms of μ and ε.


Since it is not possible to explicitly express the permittivity in terms of S21, the Newton method has

been used to solve the implicit equation for permittivity [26] by rearranging the equation to the form

( ) 0=εf as:

( ) ( ) 0sin14

cos2111 =−⎪⎭

⎪⎬⎫

⎪⎩

⎪⎨⎧

⎥⎥⎦

⎤

⎢⎢⎣

⎡++

⎥⎥⎦

⎤

⎢⎢⎣

⎡≡ r

rr

rrr c

di

cd

Sf εεω

εεεωεε (6)

It is then possible to iteratively find the roots using

( ))( 1

11

−

−− ′

=n

nnn xf

xfxx (7)

The iteration was stopped when the difference between the roots xn and xn-1 was less than 10-7 [26]. The

same calculation was carried out at 401 frequencies across the frequency span. Based on the calculated

permittivity from the transmission measurements, the reflection and transmission coefficients were

calculated [27] for normal electromagnetic incidence on single dielectric slab. These magnitudes can

be used to calculate the percentages of reflected, transmitted and absorbed radiation.

2.4 Procedures

Free space transmission measurements were used to determine the dielectric properties of the

conducting polymer composites. The samples, measuring 305 mm square, were placed in a horizontal

manner symmetrically in the path of radiation between the send and the receive horn as shown in

Figure 1. The distance between the transmission horn and the sample under test was 0.310 m, and the

distance from the sample to the receive horn was 0.165 m.

Figure 1. Free space set-up for determination of dielectric properties of conducting polymer samples.


Calibration of the system was carried out every 5 minutes in order to adjust for changes in ambient

conditions and movements in the equipment. Diffraction around the sample under test can lead to

errors in the transmission measurements. The diffracted signal around the sample can be estimated by

measuring the signal reached from port 1 to port 2 with a metal sheet that has exactly the same size as

the test specimen placed in the sample plane [28-29], thus allowing only the diffraction signal to reach

port 2. The diffracted signal can then be mathematically removed from the transmission measurements

[26, 30]. The differences in conductivity between the samples tested and the metal used for calibration

may lead to a change in the diffraction signal, however the technique has been shown to perform

satisfactorily for similar samples [30, 31].

Thermal mapping was carried out by fixing the conductive fabric on a vertical polystyrene foam stand

and pointing both the antenna and the thermal camera directly at the sample as indicated in Figure 2.

The distance between the measurement devices and the sample under test was set to 3.0 m and the total

fabric area mapped was 305 mm by 305 mm.

Figure 2. Schematic microwave heat detection set-up.


The network analyser system generated an average output power of 30 to 35 W, with a total sweep time

of 0.5 seconds across two frequency bands 8 – 9 GHz and 15 – 16 GHz. The thermal camera was set to

record at a frame rate of 50 frames per second, a total recording time of 120 seconds and a sub

sampling rate of 2. The thermal maps obtained with the thermal camera were evaluated using the

functions readily available in the Altaïr software that is incorporated into the operating system of the

thermal camera. Emissivity is the fraction of incoming radiation that a surface absorbs; hence a perfect

absorber has emissivity of 1. As the emissivity of a surface decreases, irradiation causes a greater

temperature rise. A high emissivity value of 0.98 for all conducting polymer samples was used in the

thermal camera software.

3. Results and discussion

3.1 Morphology of conducting polymer film

The conducting polymer adheres to the textile and forms a continuous black film on the surface of each

fibre, as can be seen in the micrograph in Figure 3. The thickness of the coating can be determined

using optical microscopy and optical fibre diameter analysis and is in the order 0.5 - 1 μm.

Figure 3. Scanning electron microscope image of conducting polymer coating on nylon fibre.

Since PPy coatings are conductive, neither gold-sputtering nor carbon coating was applied before SEM

imaging. The inner pristine fibre is non-conducting therefore uncoated regions of the surface appear

white in the scanning electron microscopy images due to charging.

3.2 Permittivity measurements and shielding effectiveness

When electromagnetic radiation encounters a material interface, some of the radiation is reflected and

some is transmitted. Radiation entering the material may be absorbed if the material properties allow,


or it can pass through virtually unattenuated. An insulating material will generally transmit almost all

the radiation not reflected from the surface, and a very highly conductive material (eg metal) reflects

almost all radiation.

Since it is difficult to accurately determine the bulk resistivity of the coated fabrics, surface resistivity

in ohms per square is preferable to use as the material parameter rather than the volume resistivity.

Materials with low surface resistivity value will have high microwave reflection coefficients and vice

versa.

The permittivity of three conductive textiles with varying dopant concentrations has been evaluated.

The negative values refer to the imaginary component (ε´´) of the permittivity, which is directly

proportional to the total conductivity. This results in the characteristic shape of the imaginary

permittivity as seen in Figure 4, which decreases inversely with frequency.

2 4 6 8 10 12 14 16 18

-250

-200

-150

-100

-50

0

50

100

ε'

ε'' 0.018 mol/l AQSA Re 0.018 mol/l AQSA Im 0.027 mol/l AQSA Re 0.027 mol/l AQSA Im 0.027 mol/l pTSA Re 0.027 mol/l pTSA ImP

erm

ittiv

ity [R

elat

ive

Uni

ts]

Frequency [GHz]

Figure 4. Relative permittivity vs. frequency of Nylon-Polypyrrole composites prepared at

polymerization time of 180 minutes. Dopant concentration as indicated.

The positive values refer to the real part of permittivity ε´, which displays a smaller variation with

frequency than the imaginary permittivity. This implies that the capacitive or “energy storage”

component remains more or less constant throughout the frequency range tested.

The permittivity values show a slight oscillation in values caused by incomplete removal of the

diffraction signal. The shielding effectiveness (sum of the reflected and absorbed radiation) of the

conductive fabrics reaches 89.9 % at 18 GHz for the sample with an AQSA concentration of 0.027

mol/l. Although the total shielding effectiveness is highest for the most conducting sample (0.027 mol/l

AQSA), the absorption levels are higher in samples with lower conductivity. The variation of


absorption with frequency and differences in the absorption levels among the three samples are seen in

Figure 5. The solid curves correspond to a Sigmoidal (Boltzmann) fit.

2 4 6 8 10 12 14 16 1840

4142

4344

45

4647

4849

50

0.027 mol pTSA/l 0.018 mol AQSA/l 0.027 mol AQSA/l

Abs

orpt

ion

[%]

Frequency [GHz]

Figure 5. Absorption contribution to shielding for two concentrations of anthraquinone-2-sulfonic Acid

(AQSA) and one concentration of p-Toluene-2-sulfonic acid (pTSA).

The highest level of absorption was recorded in the sample with 0.027 mol/l pTSA, followed by

samples with 0.018 mol/l AQSA and 0.027 mol/l AQSA respectively. However, the reflection level is

highest in the sample with 0.027 mol/l AQSA, and decreases in the samples with lower concentrations

of AQSA and pTSA as can be seen in figure 6.


2 4 6 8 10 12 14 16 1825

30

35

40

45

50

AQSA 0.027 mol/l AQSA 0.018 mol/l pTSA 0.027 mol/l

Ref

lect

ion

[%]

Frequency [GHz]

Figure 6. Reflection contribution to shielding for two concentrations of anthraquinone-2-sulfonic Acid

(AQSA) and one concentration of p-Toluene-2-sulfonic acid (pTSA).

0

10

20

30

40

50

0.0270.0180.0090.004OxidantPristine

Abs 8-9 GHz Abs 15-16 GHz

Abs

orpt

ion

[%]

Dopant concentration [mol/l pTSA]

Figure 7. Absorption level as function of dopant concentration for pTSA, including pristine fabric.

Polymerisation time 180 min.


The level of absorption in the conducting samples as a function of pTSA concentration is presented in

Figure 7. In the uncoated fabric, very little of the signal is reflected and most is transmitted, due to

factors such as the areal density of the material, the thickness and the permittivity of the thread. As the

concentration of dopant increases in the PPy coated fabrics, the absorption level increases to reach a

maximum, after which the absorption level decreases slightly. The decrease in absorption is more

pronounced at the higher of the two frequencies. The sample that is the most absorptive, i.e. the one

doped with 0.027 mol/l pTSA, should generate the most heat of the investigated samples. As the

conductivity increases, it is expected that the temperature rise upon irradiation will decrease, since a

larger amount of the incident microwave radiation is reflected and thus does not contribute to heating

of the samples.

3.2 Thermal mapping

Thermal mapping of the pristine nylon fabric while irradiated shows that the uncoated material does

not absorb any of the microwave energy, which is in accordance with the result presented in figure 7.

As can be seen in figure 8, no detectable rise in temperature is observed in the pristine sample upon

irradiation, only residual heating effects in the corners caused by sticking the sample onto the stand.

Figure 8. Non-conducting uncoated nylon fabric subjected to electromagnetic radiation in the

frequency range 15-16 GHz. Analysis from Altaïr software for three lines shown in thermal image.

The fact that no temperature increase is observed proves that no energy absorption is due to residual

moisture present in the fabrics; hence all heat originates in absorption of microwave energy in the

conductive coatings. The temperature profiles along the selected lines in Figure 8 display waviness due

to slight changes in ambient conditions during the duration of the test.


Pristine

Fabric

AQSA

0.018 mol/l

AQSA

0.027 mol/l

pTSA

0.027 mol/l

Rs [ohms/sq.] Several Mega ohm 120 105 155

Average absorption [%] 0.28 46.05 43.24 47.76

In contrast to this result, the thermal maps show a distinguishable heat development in all of the

conducting polymer samples. An example is given in Figure 9, where a temperature of up to 27.15ºC is

achieved upon irradiation of the sample which had been exposed to a solution of 0.018 mol/l AQSA as

the dopant.

Figure 9. Thermal image of conducting polymer sample irradiated with electromagnetic radiation in the

frequency range 15-16 GHz. Dopant: 0.018 mol/l AQSA. Analysis from Altaïr software.

The sheet resistivity, average absorption and reflection values and temperature increases upon

irradiation for all samples are listed in Table 1.

Table 1. Sheet resistances, average absorption and reflection values and temperature increases upon

irradiation for three conducting polymer samples.


8-9 GHz

Average Reflection [%]

8-9 GHz

0.05 38.77 45.10 32.70

Temperature increase upon

irradiation [°C], 8-9 GHz

Negligible 3.05 2.75 3.54

Average Absorption [%]

15-16 GHz

0.16 44.97 42.59 46.86

Average Reflection [%]

15-16 GHz

0.17 41.92 47.37 36.40

Temperature increase upon

irradiation [°C], 15-16 GHz

Negligible 4.35 3.85 5.27

The sample with the largest microwave absorption levels also shows the highest temperature increase,

and the lowest temperature rise is generated in the most reflective sample. The most reflective sample

is the sample with the lowest surface resistivity, i.e. the highest conductivity.

The temperature rise in the lower frequency band (8 – 9 GHz) is lower than the higher band since the

microwave beam is less focussed at lower frequencies.

4. Conclusions

Conducting polymer coated textiles have been characterized in regards to permittivity, reflection,

absorption and heat generation during microwave irradiation. A maximum shielding effectiveness of

89.9 % at 18 GHz was displayed by the sample with an AQSA concentration of 0.027 mol/l. Thermal

mapping with an infrared camera whilst being irradiated with microwaves showed that the most

absorptive sample had the highest temperature increase. The most conductive, hence most reflective

sample showed the lowest amount of absorption. These fabrics offer an alternative to the traditional

materials normally used to shield EM radiation as they have reasonable SE values and ability to both

reflect and absorb the incoming signal.

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