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Microwave absorption properties of long carbon nanotubes - epoxy composites

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MICROWAVE ABSORPTION PROPERTIES OF LONG CARBON
NANOTUBES - EPOXY COMPOSITES
Thesis
Submitted to
The College of Sciences and Engineering
Southern University and A&M College
In Partial Fulfillment of the Requirements for
The Degree of
Master of Science in Mathematics and Physics
with concentration in Physics
By
Kuo Li
Baton Rouge, Louisiana
August, 2016
ProQuest Number: 10168941
All rights reserved
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© Copyright by
Kuo Li
All rights reserved
2016
iii
MICROWAVE ABSORPTION PROPERTIES OF LONG CARBON
NANOTUBES - EPOXY COMPOSITES
In this work, we investigated the microwave absorption properties of carbon
nanotubes (CNT) - epoxy composites. We used multi-walled carbon nanotubes
(MWCNTs) as filler materials and the epoxies as matrix materials. The dielectric
constants, microwave absorption properties, and microstructure of the composites have
been studied. To develop a material with considerable microwave absorbing properties,
in the frequency ranges from 2 to 26.5 GHz, is highly challenging. We utilized several
methods to improve these properties of the composites. By controlling the weight
percentage (wt %) loadings of the CNTs in the composites from 0 wt% to 10 wt%, we
found that the microwave absorption properties strongly depend on the CNTs loading in
the composites. Epoxy 300 and epoxy 828 were studied to understand how the epoxy can
influence the microwave absorption properties of the composites. We also used the
surfactant sodium dodecyl benzene sulfonate (NaDDBs) to improve the dispersion of
MWCNTs in the epoxy matrix. Our results showed that the distribution property of the
CNTs has an influence on the microwave absorption properties of the composites. Finally,
we explored the multi-layer structures of the MWCNTs - epoxy composites and
demonstrated that the microwave absorption properties can be further improved by using
the impedance matching principle for the design of MWCNTs - epoxy layers, with
iv
different dielectric properties due to different CNTs loading in the composites. The
multi-layer structure can change the microwave absorption properties including reflection
loss, absorption ratio and transmission performance of the composites.
v
Dedicated to my parents
vi
ACKNOWLEDGMENTS
I want to express my sincere appreciation to my graduate advisor, Dr. Guang-Lin
Zhao, whose guidance, understanding, leadership, patience and support were an
important part of the completion and success of this thesis work.
I want to thank Dr. Diola Bagayoko, Southern University Distinguished Professor
of Physics, and Dean of the Honors College, for the opportunity to study at Southern
University, his patience, introduction to the program, and support for my graduate study.
I want to thank Dr. Laurence Henry, Dean of College of Science and Agriculture,
for his assistance.
I am indebted to Ms. Lashounda Franklin and Ms. Dorothy Brandon for their help
in the language editing.
I am grateful for the kind support from the professors and staff of the Department
of Mathematics and Physics: Dr. Reese, Dr. Lam, Dr. Wang, Ms. Modica, and Ms.
McKneely. Sincere thanks are extended to my research team member: Dr. Gao, Dr. Jahan,
and Mr.Inakpenu.
Last, but not least, I would like to thank my parents, family, and beloved Adela,
for their moral support and listening ears.
This research was funded in part by the Army Research Office (Award No
W911NF-15-1-0483) and the National Science Foundation (NSF) LASIGMA project
(Award No EPS-1003897, NSF92010-15-RII-SUBR).
vii
TABLE OF CONTENTS
APPROVAL PAGE……………………….………………………………………………ii
COPYRIGHT PAGE…………………………………………………………..…………iii
ABSTRACT……………………………………………………………………….……..iv
DEDICATION PAGE……………………………………………………………………vi
ACKNOWLEDGEMENTS……………………………………………………………..vii
TABLE OF CONTENTS……………………………………………………………. viii
LIST OF TABLES……………………….………………………………….…………….x
LIST OF FIGURES……………………..………………………………….…………….xi
LIST OF SYMBOLS AND ABBREVIATIONS……………………….………………xiii
1. INTRODUCTION…………………………………………………..……….…………1
1.1 MICROWAVE ABSORPTION……………………….……………………..1
1.2 EPOXY. ………………………………………………………………...…….5
1.3 CARBON NANOTUBES…………………………….……………………….5
2. EXPERIMENTS…………………………………………………………………….10
2.1 MATERIALS…………………………………………………….…………10
2.2 SAMPLE PREPARATION…………………………………………………10
2.3 INSTRUMENTS AND MEASUREMENTS………………………………..12
3. RESULTS AND DISCUSSION………………………………………………………16
3.1MICROWAVE ABSORPTION PROPERTIES OF MWCNT - EPOXY
COMPOSITES……………………………………………………..…………….16
viii
3.2 MWCNT - EPOXY COMPOSITES WITH SURFACTANT-TREATED
MWCNTS…………………………………………………………..……………24
3.3 COMPLEX DIELECTRIC PERMITTIVITY…………………………….....30
3.4 MULTI-LAYER STRUCTURE MICROWAVE ABSORBER…….……….35
4. CONCLUSION …………………………………………………………….…………46
BIBLIOGRAPHY…………………………………………………………….………….47
APPENDIX…………………………………………………………………….………...57
ix
LIST OF TABLES
Table 1
Samples’ formation of figure 3.4.2 (a)……………………………...……39
Table 2
Samples’ formation of figure 3.4.2 (b) and figure 3.4.3 (d) ……......…...39
Table 3
Samples’ formation of figure 3.4.2 (c) and figure 3.4.3 (a)…...…………40
Table 4
Samples’ formation of figure 3.4.2 (d) and figure 3.4.3 (c)……….……..41
Table 5
Samples’ formation of figure 3.4.3 (b)…………………………….…….44
Table 6
The relation between dB value and reflection ratio………………...……57
x
LIST OF FIGURES
Figure 1.1.1
Spectrum of electromagnetic waves…………………………………….3
Figure1.2.1
(a) Single-walled carbon nanotube (SWCNT) and (b) Multi-walled
carbon nanotube (MWCNT)………………………………………………7
Figure 2.3.1
Principle diagram of a two-port VNA…………………………………13
Figure 2.3.2
S-parameters of two ports network analyzer………………………….…14
Figure 3.1.1
Schematic representation of EMI shielding mechanism………………..17
Figure 3.1.2
EMI shielding effectiveness of MWCNTs - epoxy composites with (a)
epoxy 300 (b) epoxy 828, respectively. The loading of MWCNTs is (a) 17 wt% and (b) 1-10 %, and the microwave frequency range is 1-26.5
GHz……………………………………………………………………..18
Figure 3.1.3
Reflection loss of MWCNTs - epoxy composites with (a) epoxy 300 and
(b) epoxy 828, respectively. The loading of MWCNTs is (a) 1-7 wt% and
(b) 1-10 % and the microwave frequency range is 1-26.5
GHz…………………………………………………………...………….21
Figure 3.1.4
Absorption ratio of the MWCNTs - epoxy composites with (a) epoxy 300
and (b) epoxy 828. The loading of MWCNTs is (a) 1-7 wt% and (b) 110 %, and the microwave frequency range is from 1-26.5
GHz………………………………………………………………………21
xi
Figure 3.2.1
EMI shielding effectiveness of MWCNT - epoxy composites with
surfactant treated CNTs……………………………………...…………..27
Figure 3.2.2
Reflection loss of MWCNTs - epoxy composites with surfactant treated
CNTs……………………………………………………………….…….29
Figure 3.2.3
Absorption ratio of MWCNT - epoxy composites with surfactant treated
CNTs……………………………………………………………………29
Figure 3.3.1
The (a) real part and (b) imaginary part of permittivity of MWCNT epoxy samples with different CNTs loading in the frequency range from 1
to 26.5 GHz………………………………………………………………31
Figure 3.3.2
Loss tangent of MWCNTs - epoxy composites with different CNTs
loading in the frequency range from 1 to 26.5 GHz…………………...34
Figure 3.4.1
Schematic of microwave absorption performances of the Multi-layer
structure microwave absorber………………………………............….37
Figure 3.4.2
Reflection loss of (a) Aluminum and Teflon and (b) (c) (d) Multi-layer
structure microwave absorbers……………………………………….….38
Figure 3.4.3
Absorption ratio of different Multi-layer structure microwave absorbers.43
xii
LIST OF SYMBOLS AND ABBREVIATIONS
CNT
Carbon Nanotube
MWCNT
Multi-Walled Carbon Nanotube
SWCNT
Single-Walled Carbon Nanotube
EMI
Electromagnetic Interference
DUT
Device under Test
VNA
Vector Network Analyzer
Wt %
Weight percentage
NaDDBs
Sodium Dodecylbenzene Sulfonate
EM
Electromagnetic
RL
Reflection Loss
∇
Gradient
E
Electric field
Charge density
0
Permittivity
B
Magnetic field
μ0
Permeability
xiii
J
Current Density
m
Micrometer
Zin
Normalized Impedance
Z0
Characteristic Impedance of Free Space
Attenuation Constant
tan
Loss Tangent
xiv
CHAPTER 1
INTRODUCTION
1.1
Microwave Absorption
Electromagnetic (EM) waves are synchronized oscillations of magnetic and
electric fields. The magnetic and electric fields are perpendicular to each other and to the
wave spread direction. They are created by the vibration of charged particles, and travels
at the speed of light in a vacuum. An electromagnetic wave appears when the changing
magnetic field causes a changing electric field, which will subsequently lead to another
change of the magnetic field.
The electromagnetism theory was developed in the 19th century. Astronomer
William Herschel first described the infrared radiation in 1800. Infrared radiation is a
kind of electromagnetic radiation of wavelengths greater than visible lights [1]. The
electromagnetism theory was first proposed by Scottish scientist James Clerk Maxwell in
1865 in the field of mathematical physics. He developed equations for the EM fields,
which describe how electric and magnetic fields are generated by each other. He also
discovered that these fields could travel at a speed which is very close to the speed of
light [2]. These equations appeared in his publication A Dynamical Theory of the
Electromagnetic Filed.
1
Oliver Heaviside grouped the twenty equations together into a set which contains only
four equations, and these equations are now universally known as Maxwell’s Equations
[3].
∇∙
=
(1.1.1)
Equation 1.1.1 is the differential form of Gauss’s Law for Electrostatics, which
relates the distribution of the electric charge to the resulting electric field. [4] This
equation reveals that the volume left by the electric field is proportional to the charge
inside.
∇∙
=0
(1.1.2)
Equation 1.1.2 is the differential form of Gauss’s Law for Magnetism; it states
that the divergence of magnetic field B is equal to zero [5]. This implies that there is no
magnetic monopole, and the total magnetic flux through a closed surface is zero.
∇×
=−
(1.1.3)
Equation 1.1.3 is the Maxwell – Faraday Equation and also is known as Faraday’s
Law of Induction. This is a fundamental law of electromagnetism, and it describes how a
magnetic field interacts with an electric circuit to produce an electromotive force called
electromagnetic induction [6, 7].
∇×
=μ ( +ε
)
(1.1.4)
Equation 1.1.4 is Ampère's Circuital Law which was discovered by André-Marie
Ampère in 1823. This equation describes the integrated magnetic field around a closed
loop and the electric current passing through the loop [8]. It shows that electric currents
2
and changes in electric fields are proportional to the magnetic field circulating about the
area they pierce.
These equations were confirmed by Heinrich Hertz in 1887
1887, when he produced
producing EM waves. He proved the theory of electromagnetic
ectromagnetic waves by transmitting
and receiving radio pulses with experimental procedures and developed methods
method to detect
these waves [9].
EM waves are characterized by their frequency or wavelength. By increasing the
frequency or decreasing the waveleng
wavelength, EM waves can be divided into radio waves,
microwaves, infrared radiation, visible light, ultraviolet radiation, X
X-rays
rays and gamma rays.
Figure 1.1.1 shows the EM spectrum
spectrum. As illustrated in this spectrum, gamma rays have
the shortest wavelength and the highest frequency; long radio waves have the longest
wavelength and the smallest frequency.
Figure 1.1.1 Spectrum of electromagnetic waves
© ht t p s : / / c o m mo n s . w i k i me d ia . o r g / w ik i / Fi l e : EM _ s p e c t r u m. s v g
3
Microwaves are a form of EM waves which have specific frequencies ranging
from 300 MHz to 300 GHz and wavelengths ranging from one meter to one millimeter.
For this work, frequencies ranging from 2 to 26.5 GHz were studied. This range is the
most useful band in common applications [10, 11]. Microwaves have extensive
applications in modern society such as in communication, navigation, radar, spectroscopy,
etc. Radar uses microwaves to detect objects; satellites use microwaves to communicate
with each other and with ground stations; GPS system, mobile phones, wireless LAN
devices, and other applications all using microwaves technology. The proliferation of
electronics and instrumentation in commercial, industrial, healthcare and defense sectors
have led to a novel kind of pollution known as electromagnetic interference (EMI). In
recent years, the EMI pollution problem has gotten worse with the expanding usage of
electronics devices. Microwave absorption materials are drawing more attention since
they can protect the environment and sensitive circuits from EMI pollutions. Stealth
technology for defense is another application in the military in which these microwave
absorption materials can be used to resist radar technology. [12]
Microwave absorbers, also known as radiation-absorbent material (RAM), have
been critically needed for their specialty in absorbing incident radiation. Microwave
absorption properties include absorption, reflection, and transmission. Generally, the
microwave absorbers can be divided into two parts: matrixes and filling materials.
Traditional microwave absorption materials, such as ferrites, have sufficient properties in
many aspects. However, these materials are so heavy that they are not only difficult to
produce, but also have many other restrictions. In the last several years, a variety of
4
research experiments have been carried out to investigate some novel materials with
lightweight, flexible, broadband, low-cost industrial processing [13-16].
1.2
Epoxy
Matrix material provides the mechanical and structural performance in the fillers-
matrix microwave absorption system. According to general requirements for microwave
absorbers, matrix materials should have certain properties like flexibility and lightweight.
In recent years, polymer epoxies have been the subject of intense research as binder
materials. Universally, epoxies are known for their chemical and heat resistance,
excellent adhesion, and good mechanical and electrical insulating properties. Compared
with other polymer materials that have similar mechanical properties, epoxies are much
smaller in size. Usually, the epoxies are cured-end products which come from epoxy
resins and hardeners. The epoxy resin and hardener are in a liquid statue at room
temperature, which give epoxies have an unlimited flexibility property. According to
studies by other groups, many properties of epoxies can be modified [17]. Therefore,
these excellent properties make epoxies consummately fit to be matrix materials for
microwave absorbers.
1.3
Carbon Nanotubes
The discovery of carbon nanotubes by Iijima in 1991 is universally considered to
be the first report noting that the composition of multi-tubes is nested in a concentric
fashion. However, the true identity of the discovery of CNT remains a subject of some
controversy [18]. In 1952, the clear image of nanometer diameter tubes made of carbon
5
was published [19]. Morinobu Endo found hollow tubes of rolled-up graphite sheets in
1976 [20]. Many other scientists have made contributions to the exploration of carbon
nanotubes [20-23].
CNTs are allotropes of carbon with a cylindrical nanostructure. They are
constructed with various length-to-diameter ratios, known as single-wall carbon nanotube
(SWCNT) and multi-wall carbon nanotube (MWCNT), as shown in Fig 1.3.1. Graphene
is a kind of fullerene with bonded carbon atoms in sheet form one atom thick. It is an
allotrope of carbon in the form of a two-dimensional structure [25]. The SWCNTs can be
obtained by wrapping the graphene sheet into a seamless cylinder. Multi-walled carbon
nanotubes consist of multiple layers of graphene formed with a coaxial cylinder. With
this special structure, CNTs can have an extremely larger length-to-diameter aspect ratio
[24]. The diameter of most SWCNTs is nearly 1 nanometer and their lengths can be
millions of times longer. The chemical bonds of CNTs are sp 2 type, which are more
stable than those found in diamond. This particular structure makes CNTs have many
unusual properties in various areas. They are widely used in nanotechnology, electronics,
optics, and other fields of materials science and technology.
6
Figure 1.2.1 (a) Single-walled carbon nanotube (SWCNT) and (b) Multi-walled carbon nanotube
(MWCNT).
Source: www.intechopen.com
For mechanical properties, it is the strongest material in terms of tensile strength
and the stiffest material in terms of the elastic modulus. Tests in 2000 revealed that
MWCNTs have a tensile strength of 63 gigapascales in 2000 [26]. These unusual
mechanical properties are due to the sp2 bonds formed between carbon atoms. MWCNTs
have been utilized to create the world’s smallest rotational motor [27]. This motor takes
advantage of MWCNTs’ kinetic properties. MWCNTs are built with several coaxial
layers with the ability to slide along each other. CNTs also have other interesting
properties such as optical and thermal properties.
Another very important property is the electrical property. This property of CNTs
strongly depends on the structure of the nanotube. CNTs can vary from metallic to
semiconductor according to their different structures [28]. Another important factor is the
7
dimension of CNT. Its very small diameter gives it a nanoscale cross-section, which
makes the electrical property of CNTs their unique. For example, CNTs are frequently
considered as one-dimensional conductors since their electrons propagate only along their
tubes’ axes. According to calculation results, CNTs can carry an electrical current density
more than 1,000 times greater than copper [29]. Experimental and theoretical studies
have demonstrated that CNTs have outstanding electrical properties because of their
unique one-dimensional hollow tube structures [28]. As such, either by ballistic transport
or diffusive transport, electron travels along with long mean free paths [30]. The
conjugate π electrons are restricted in the one-dimensional cylinder, enabling CNTs to
show distinctive electronic responses, which giving them the ability to serve as emerging
microwave absorbing materials [31].
The unique mechanical strength, large aspect ratio, high flexibility, small
diameter, and good electric properties of CNTs make them attractive as filler materials in
microwave absorbers. Several studies have demonstrated that MWCNTs have strong
microwave absorption with a matrix polyethylene terephthalate and poly [32-34].
MWCNTs - epoxy composites are attractive for being microwave radiation
absorbing and shielding materials due to their chemical and physical properties. These
composites can offer high flexibility for design and control of microwave absorption
properties. The fundamental microwave absorption performance of MWCNTs - epoxy
composites was examined. The composites can be tailored by changing the loading
fractions, matrix materials, the number of layers, and the thickness [35-37]. These
varying factors can affect the absorption bandwidth, resonant frequency, peak value, etc.
[38]. From the experimental results and the microwave absorption theory, the surfactant
8
can improve the distribution of MWCNTs in the matrix solution. The dispersion state of
the MWCNTs in the composites was also studied in order to investigate its effect on the
microwave absorption properties of composites. The multi-layer structures of the
composites were also explored in this work due to their complex permittivity properties.
9
CHAPTER 2
EXPERIMENT
2.1
Materials
Two kinds of epoxies, epoxy 300 and epoxy 828, were utilized in this work.
Epoxy 828 and hardener were obtained from the Miller-Stephenson Chemical Company,
Inc. Epoxy 300 and two kinds of hardeners (11 and 21) were obtained from Aero Marine
Products, Inc. The MWCNTs were purchased from CheapTubes Inc., USA. The outer
diameters (OD) of the MWCNTs were in the range of 8 - 15 nm and their lengths were
greater than 100 μm. The ash content of MWCNTs is less than 1.5 wt%, while the purity
content was higher than 95 %. The surfactant agent used in this work was sodium
dodecyl benzene-sulfonate (NaDDBs), obtained from the Sigma-Aldrich Co. LLC. The
release agent used was MS-122AD from the Miller-Stephenson Chemical Company, Inc.
2.2
Sample Preparation
A mechanical mixing method was performed to fabricate the MWCNTs - epoxy
composites. The same amount of epoxy was used for all the samples with different
MWCNTs loading. The loadings of MWCNTs in the composites were controlled from 1
to 10 wt%. A suspension of a mixture of epoxy resin and MWCNTs was prepared by
stirring the solution at 80 ℃ for 2 hours at 80 rpm. To get a better dispersion of MWCNT
10
in the mixture, a high temperature of 80 ℃ was applied to reduce the viscosity of the
solution. The low stirring rate aided in the CNT distribution in the solution and dispelled
the formation of air bubbles. After transitory cooling down, the hardener was added to the
mixture and the mixture was stirred for 15 minutes. The mixture was then injected into a
hollow cylinder sample holder with a rod through the sample holder. The rod had a 1.5
mm diameter and was pretreated by the releasing agent to assist in the removal of the
sample from the sample holder after curing. The samples were then transferred to an oven
for curing at 80 ℃ for 5 hours. The sample obtained had an O-ring shape with an outer
and inner diameter of 3.5 mm and 1.5 mm, respectively. The sample was then carefully
cut into different thicknesses of 2 and 3 mm. Samples of 0.5 and 1 mm thickness samples
were also cut for the multi-layer structure measurement.
For the second part, the surfactant sodium dodecyl benzene sulfonate (NaDDBs)
was used to treat the MWCNTs. The procedures were slightly different with untreated
MWCNTs. First, the surfactant powder and the MWCNTs were placed into a container
and mixed by a mechanical stirring method. The ratio between the surfactant and
MWCNTs was controlled from 0 to 1 ~ 3 to 1. Next, ethanol was added to the mixture
and then transferred the mixture to the ultrasonic dispersion instrument. This instrument
provides a better dispersion of the surfactant in the ethanol. After 30 minutes, the epoxy
resin was added to the mixture. After 60 minutes ultrasonic, the mixture was transferred
to a hotplate stirring machine at 80 ℃ with 60 rpm of stirring to evaporate the ethanol.
For each part of the sample, the mass was measured before adding to the mixture. The
mass of the mixture was checked to ensure that the ethanol was completely evaporated.
When there is no trace of ethanol, the rest of the process was the same as part I above.
11
For the third part, the multi-layer structure measurements were taken by
combining several different samples with different MWCNTs loading together. The
samples used contained inner and outer diameters of 1.5 and 3.5 mm, respectively.
Similar to part I, these samples were cut to a thickness of 0.5 and 1 mm. The pure epoxy,
PTFE and aluminum samples were fabricated in this section.
2.3
Instruments and Measurements
A vector network analyzer (VNA) is an instrument that measures the network
parameters of electrical networks. Vector network analysis is an accurate method of
characterizing samples by measuring their effect on the amplitude and phase of sweptfrequency and swept-power test signals [39]. Network analyzers are mostly used at high
frequencies; working frequencies can range from 5 Hz to 1 THz [39]. Since VNAs can be
used to measure complex impedance of circuits, it can be found in many electronic and
radio frequency laboratories. A two-port VNA can measure both reflected and
transmitted signals.
Figure 2.3.1 is the principle diagram of a two-port VNA. The built-in source is
the one that provides the test signals. The signal separation unit can be used to split the
signals generated by the source. In the VNA, the signals were split into two parts which
lead to measurement port and reference receiver R. The receivers measure both the phase
and the magnitude of the signal. The DUT is the device under test. The signals from the
separation unit go through the transmission wire connected to port 1, and then the signals
encounter the DUT. Receiver A was used to measure the signals reflected from the DUT.
Receiver B was used to measure the transmitted signals through the DUT. The transfer
12
switch is the device utilized to switch the original signal. The transfer switch makes the
signals go through different directions from port 2 to port 1, so that receiver A can
measure the transmitted signals and receiver B can measure the reflected signals.
Figure 2.3.1 Principle Diagram of a two-port VNA
In order to complete the characterization of a DUT, measurements were made
under various conditions and a set of parameters was calculated. These parameters can be
used to describe completely the electrical properties of the DUT. There are several
different parameters that can be used to describe the behavior of the test device. These
parameters are H, Y and Z parameters. To obtain these parameters, measurements must
be made with an open circuit and short circuit conditions since the total voltage and
current must be measured. However, the total voltage and current are difficult to be
measured at higher frequency ranges. Scattering parameters (S-parameters) are generally
13
used to characterize a linear electrical network; and do not need to use open or short
circuit conditions. These parameters can be used to study many electrical properties of
the network: the gain, loss and reflection coefficient
coefficients [40]. S-parameters
parameters change with the
frequency, and these parameters are mostly used for network operations
operati
at radio
frequencies (RF) and microwave frequencies. In this work, the N5230C PNA-L
P
Microwave Network Analyzer was used as the VNA. This analyzer works at frequency
ranges from 10 MHz to 40 GHz. The frequency ranges involved in this work for the
carbon nanotube epoxy composite samples with a coaxial shape are from 2 to 26.5 GHz.
Figure 2.3.2 S
S-parameters of a two ports Network Analyzer
nalyzer [40].
Figure 2.3.2 [40] is the diagram of S
S-parameters of a two ports VNA. In this
diagram, “a” is the incident wave at each port and “b” means the outgoing wave at each
port; “1” stands for port 1 while “2” is for port 2. The relationship between the four
parameters (S11, S22, S12, and S21) and the incident and outgoing waves can be expressed
by the following linear equation [41]:
(2.3.1)
Expanding matrix 2.3.1 into equations gives
14
(2.3.2)
.
(2.3.3)
Equations 2.3.2 and 2.3.3 describe the relation between S
S-parameters
parameters and the
incident and outgoing waves. These four S
S-parameters
parameters can be explained as follows:
follow
S
= (2.3.4)
S11 is the input coefficient which is defined as the ratio of the wave quantities b1
to a1, measured at port 1.
S
= (2.3.5)
S21 is the forward transmission coefficient which is defined as the ratio of the
wave quantities b2 to a1, measured at port 1.
S
= (2.3.6)
S12 is the reverse transmission coefficient which is defined as the ratio of the
wave quantities b1 to a2, measured at port 2.
S
= (2.3.7)
S22 is the output reflection coefficient which is the ratio of the wave quantities b2
to a2, measured at port 2.
From these parameters, we can study the microwave absorption properties which
include the Reflection loss, EMI shielding effectiveness
effectiveness, and the absorption ratio.
15
CHAPTER 3
RESULTS AND DISCUSSION
3.1
Microwave Absorption Properties of MWCNTs - Epoxy Composites
Electromagnetic interference shielding prevents electromagnetic induction caused
by extensive use of alternating voltage/current. These voltages/currents can produce
corresponding induced voltages and currents in the electronic circuitry. Electromagnetic
interference may cause disturbances or a complete breakdown of the regular performance
of appliances. The shielding efficiency is usually measured in terms of the reduction in
the magnitude of the power. As shown in Figure 3.1.1 [42], shielding is a direct result of
absorption (SEA) and reflection loss, including the initial reflection (SE R) and multiple
internal reflections (SEM) that are caused by the incident EM wave/power. The shielding
effectiveness (SET) can be mathematically expressed as the following equation [42-44]:
SET= SER + SEA + SEM (dB)
(3.1.1)
Also, the transmitted wave can be measured by using VNA, and the incident wave
is known. The shielding effectiveness can be expressed as the following equation [45-48]:
SET = 10 log
= 20 log
= 20 log
16
,
(3.1.2)
where PI and PT are the power of incident and transmitted electromagnetic waves,
respectively. E and H are the electric and magnetic field intensities of EM waves,
respectively.
Figure 3.1.1 Schematic representation of EMI Shielding Mechanism (Wu, 2014 [42])
S11, S12, S22 and S21 are S-parameters; the definition of these parameters is
expressed by equations (2.3.4), (2.3.5), (2.3.6) and (2.3.7), respectively.
These
parameters can be measured via a two-port VNA system. According to the classic
transmission line theory [49], with regard to the single layer sample, the reflection and
transmission coefficients can be described as follows:
Transmittance T= |S12|2 =|S21|2
(3.1.3)
17
Reflectance
R=|S11|2 =|S22|2
(3.1.4)
Absorbance
A=1- R- T
(3.1.5)
In this case, the S-parameters can be measured by using a two port VNA and use
the S-parameters to describe the microwave absorption efficiency. For the power
attenuation, the value of the transmission/reflection coefficient was described in decibels
(dB) and the values of these numbers are negative. See Table 6 for the conversion
between decibel and percentage. Samples were fabricated using different MWCNTs
loadings and different epoxies.
(a)
(b)
Figure 3.1.2. EMI shielding effectiveness of the MWCNTs - epoxy composites with (a) epoxy 300 and (b)
epoxy 828, respectively. The loadings of the MWCNTs are (a) 1-7 wt% and (b) 1-10 %, and the microwave
frequency range is from 1 to 26.5 GHz.
Figure 3.1.2 illustrates the EMI shielding effectiveness of MWCNTs - epoxy
composites with various MWCNTs loadings in the frequency range of 2 - 26.5 GHz. All
of the samples have the same thickness of 3 mm. The results show the frequency
dependence. The EMI shielding effectiveness has a higher value at a higher frequency for
18
each individual sample. For the epoxy 828 composite sample with 8 wt% MWCNTs
loading, the EMI shielding effectiveness is -8 dB at 5 GHz, and increases to -27 dB at
25 GHz. For the epoxy 828 composite sample with 8 wt% MWCNTs loading, the EMI
shielding effectiveness is -18 dB at 5 GHz and increases to -50 dB at 25 GHz. For the
epoxy 300 composite sample with 5 wt% MWCNTs loading, the EMI shielding
effectiveness is -6 dB at 5 GHz and increases to -15 dB at -25 GHz.
The results in Figure 3.1.2 also show that the EMI shielding effectiveness
increases with increasing loading of the MWCNTs in the composites. In Figure 3.1.2 (a),
when the loading of the MWCNTs is increased from 1 to 4 wt%, the EMI shielding of the
composites increases from -1 dB to -4 at 25 GHz. Further, when the loading of the
MWCNTs was increased to 7 %, the EMI shielding effectiveness reaches to -14 dB at 25
GHz. As shown in Figure 3.1.2 (b), the EMI shielding effectiveness gradually increased
by increasing the MWCNTs loading in the composite. For example, the EMI shielding
effectiveness increased from -2 dB to -8 dB when the MWCNTs loading increased from
1 wt% to 4 wt% at 25 GHz. Further increasing the loading of the MWCNTs to 8 wt%, the
EMI shielding effectiveness reached a -25 dB.
For the samples containing the same MWCNTs loading with different epoxies,
the EMI shielding effectiveness performance is similar. In Figure 3.1.2 (a), for samples
with 6 wt% MWCNTs loading the EMI shielding effectiveness goes from -6 dB to -10
dB in the frequency range of 5-25 GHz. In Figure 3.1.2 (b), the sample has the same
MWCNTs loading with a different epoxy and the EMI shielding effectiveness goes from
-6 dB to -10.5 dB in the frequency range 5-25 GHz. Based on the results mentioned
19
above MWCNT are the main reason for the EMI shielding effectiveness and the epoxy
can greatly influence the performance.
Figure 3.1.3 illustrates the experimental data of reflection loss versus frequencies
for the MWCNTs - epoxy composites with various loadings from (a) 1 wt% to 7 wt% and
(b) 1 wt% to 10 wt% in the frequency range of 2 - 26.5 GHz. The epoxies used for these
two series of the samples are (a) epoxy 300 and (b) epoxy 828. The reflection loss peak
position is related to the MWCNTs loadings. The reflection loss peak shifts to a lower
frequency with the increasing of the MWCNTs loading increases. In Figure 3.1.3 (b), the
reflection loss peak of the sample with the 1 wt% MWCNTs loading is around 26 GHz,
and it reduces to 18 GHz, 15 GHz, 12 GHz and 5 GHz for the sample with the 3 wt%, 6
wt%, 8 wt% and 10 wt% MWCNTs loadings, respectively. Remarkably, the desired
reflection loss frequency can be achieved by controlling the MWCNTs loading of the
composites. It is also observed that the reflection loss is around -20dB (about 1% incident
wave is reflected) at 25 GHz with 1 wt% MWCNTs loading. Further, the reflection loss
value decreases with increasing the MWCNTs loading. For the samples with MWCNTs
loadings below 8 wt%, increasing MWCNTs loading the reflection loss performance
decreased, in the high frequency range (20-26.5 GHz) and in the low frequency range (110 GHz). Considering the middle frequency range from 10 to 20 GHz in Figure 3.1.1, the
results show opposite trends, i.e., the reflection loss increases with increasing the
MWCNTs loadings. The reflection loss performance is low for the MWCNTs loadings of
more than 8 wt%. Presumably, this is due to the network between the CNTs caused by
the high CNTs contents [32].
20
(b)
(a)
Figure 3.1.3 Reflection loss of the MWCNTs - epoxy composites with (a) epoxy 300 and (b) epoxy 828,
respectively. The loadings of the MWCNTs are (a) 1-7 wt% and (b) 1-10 %, and the microwave frequency
range is from 1-26.5 GHz.
(a)
(b)
Figure 3.1.4 Absorption ratio of the MWCNTs - epoxy composites with (a) epoxy 300 and (b) epoxy 828,
respectively. The loadings of the MWCNTs are (a) 1-7 wt% and (b) 1-10 %, and the microwave frequency
range is from 1-26.5 GHz.
Figure 3.1.4 shows the microwave absorption ratio of the MWCNTs - epoxy
composites with epoxy 300 and 828. The thickness for all the samples is 3 mm, and the
MWCNTs loadings in the composites are (a) 1 wt% to 7 wt% and (b) 1 wt% to 10 wt%.
21
The results show that the microwave absorption ratio of the composites has frequency
dependence in the range of 1-26.5 GHz. The behavior of the absorption ratio is to
increase with increasing the frequency. Higher frequency ranges have a higher absorption
for each composite sample. Hence, the absorption ratio is 10% at 5 GHz for the sample
with 1 wt% MWCNTs loading, which is shown in Figure 3.1.4 (a), and it increases up to
20% and 30% at 20 GHz and 26GHz, respectively.
For the samples with MWCNTs loadings of 1-7 wt%, the results indicate that the
absorption ratio increases along with the increasing of the MWCNTs loading. In Figure
3.1.4 (a), at 20 GHz frequency, the absorption ratio for the sample with 1 wt% MWCNTs
loading is 20%. The absorption ratio increases to 30%, 50%, 56% and 58% for the
samples with 3 wt%, 4 wt%, 6 wt% and 7 wt% MWCNTs loadings, respectively.
Especially for the samples with 6 wt% and 7 wt% CNT loadings, the absorption ratio is
similar in the frequency range of 15 - 26.5 GHz. This may indicate that the microwave
absorption ratio of the composites reaches its maximum value with the 6 wt% MWCNTs
loading [35]. In Figure 3.1.4 (b), the absorption ratio of samples with the MWCNTs
loadings from 1 wt% to 6 wt% gradually increases with the increasing of the MWCNTs
loadings. However, the microwave absorption ratio of the samples with the MWCNTs
loadings of 8 wt% and 10 wt% does not show much improvement in the frequency range
of 12 to 26 GHz. The absorption ratio may reach its maximum with certain MWCNTs
loadings (e.g. 6 wt% MWCNTs). Another reason may be that the samples with a high
MWCNTs loading (e.g., 8 wt% MWCNTs) have low reflection loss property. In Figure
3.1.3 (b), the samples with 8 wt% and 10 % CNT loadings have very low reflection loss,
meaning most of the incident waves have been reflected. Consequently, the absorption
22
ratio is affected because the amount of the wave energy that entered the sample was
reduced.
When exposed to an electromagnetic field, electrons inside the material will
generate inductive currents. The inductive currents give rise to the attenuation of
electromagnetic waves by converting the currents to heat. The electrons inside the
MWCNTs can freely move along the tube, making it possible for MWCNTs to have good
conductivity [16]. Therefore, the individual MWCNTs and MWCNT bundles can absorb
the electromagnetic energy and dissipate the radiation via the interactions between the
external radiation and electrons inside the MWCNTs.
For the MWCNTs - epoxy
composites with low MWCNTs loading, the MWCNTs in the composites exist as
individual MWCNT particles and MWCNT bundles. The microwave absorption capacity
of composites with low MWCNTs loadings mainly depend on the interaction between
interior electrons and exterior microwave radiation. Due to van der Waals forces,
MWCNTs will attract each other and tend to agglomerate [35]. With the increasing of the
MWCNTs loadings in the composites, larger aggregates and agglomerates form in the
composites. In other words, an electrically conductive network is formed by MWCNT
bundles and MWCNT fibers. This can be described by the percolation theory for charge
transport and effectiveness of electron tunneling or hopping [51, 52]. In particular, the
MWCNTs used in this work are relatively long, i.e., their lengths are about 100
m,
which can enhance the constitution of the conductive network. Once the network is
formed, the induction current induced by the external electromagnetic radiation will be
able to transfer in the insulating polymer matrix throughout the conductive network. This
will reinforce the interaction between the internal electrons and external electromagnetic
23
radiation. In fact, it will strengthen the dielectric loss and raise the microwave absorption
ability of the MWCNTs - epoxy composites [16]. Other research groups stated that the
defects in the MWCNTs will act as polarization centers and generate the polarization
phenomena [35, 50]. This will also contribute to the electromagnetic wave absorption of
the MWCNTs [16, 35, 50].
3.2
MWCNT - Epoxy Composites with Surfactant-Treated MWCNT
The MWCNTs supply a good conductive network in the MWCNTs - epoxy
composites as filler material. Their microwave absorption originates from polarization,
Ohmic losses and multiple scattering [53]. As mentioned above, MWCNTs provide the
composites with most of its microwave absorption ability due to the MWCNT’s dielectric
relaxation. The microwave absorption performance of MWCNT-filled polymer
composites is affected by multiple factors, such as the MWCNTs loading, geometry of
the particles, and dispersion of the particles and interfacial properties of MWCNTs, etc
[50].
The loadings of MWCNTs in the MWCNTs - epoxy composites can efficiently
change the microwave absorption properties of composites. It can affect the peak value,
the absorption bandwidth, and the response frequency, as discussed in Section 3.1.
The geometry of the particles will dramatically influence the microwave
absorption performance of the MWCNT-filler composites. There are several studies on
the microwave absorption performance with different kinds of CNTs which have various
lengths, diameters, number of layers, etc. [54, 55, 56].
24
It has been reported that in the polymer and CNT composite system, CNT’s
aspect ratio and its dispersion state in the matrix are critical parameters for the composite
conductivity [57]. These parameters will influence the microwave absorption
performance of the composites. It has been found that dispersion is a major parameter of
influence on the electrical percolation when the CNT’s aspect ratio is more than 100 [57].
As previously mentioned, the MWCNT’s used in this work have an outer diameter of 815 nm with the length greater than 100 μm, which makes the aspect ratio much greater
than 100. In this case, the dispersion state of the CNTs in the composite should be studied
further. Due to van der Waals forces, CNTs in the polymer matrix tend to attract each
other. The separation of the CNT aggregates and dispersion of the CNTs uniformly in the
matrix should be investigated. The dispersion of conductive fillers in the matrix has great
influence on the electrical performance of the composites; however, some research
publications state that CNT agglomeration could enhance the constitution of the
percolating network [57 - 59]. It was reported that MWCNT polymer composites with
CNT agglomerations have a better electrical conductivity performance than those with
well-dispersed CNTs [60], but a good dispersion is required to enhance the polymer
matrices effectively [61]. To form the conductive network, the distribution of the disentangled CNTs and CNT agglomerates is significantly important [62]. The effects of
dispersion on the conductivities and dielectric properties have been reported in some
research studies [63-65]. Among different dispersion methods (such as centrifugation,
ultrasonication and chemical medication etc.) it was found that the stirring rate and pretreatment are important factors [65, 66].
25
How the pre-treatment of CNTs influences the microwave absorption properties
of the MWCNT - epoxy composites is a factor which remains to be explored. In this
work, sodium dodecyl benzene sulfonate (NaDDBs) was used to treat the MWCNTs [6770]. The ratio between NaDDBs and MWCNTs were modified. The motivation was to
study the relationship between the dispersion state of the MWCNTs and the microwave
absorption performance of the composites.
In Figures 3.2.1, 3.2.2 and 3.2.3, the ratios of the samples are 1:0.5, 1:1, 1:2, 1:3
and 1:4, respectively, which means that the weight ratios between the MWCNTs and
Surfactant are 1:0.5, 1:1, 1:2, 1:3 and 1:4, respectively. The sample with the ratio 1:0 has
no surfactant added to the composites, meaning that the MWCNTs are untreated. All the
samples have the same thickness of 3 mm and the measured frequency range is from 2 to
26.5 GHz. The epoxy 300 was used to fabricate MWCNTs - epoxy composite samples in
this section. The loading of the MWCNTs in the samples was fixed at 3 wt% for
comparisons.
26
Figure 3.2.1.
EMI shielding effectiveness of MWCNTs - epoxy composites with surfactant treated
MWCNTs.
Figure 3.2.1 shows the EMI shielding effectiveness (transmission loss) in dB of
the MWCNTs - epoxy composites with surfactant treated MWCNTs. The EMI shielding
effectiveness performance of the sample with the ratio 1:1 is lower than other samples in
the measured frequency range of 2- 26.5 GHz. The EMI shielding effectiveness of other
samples show a slight increase with increasing the surfactant ratio in the frequency range
of 5 to 22 GHz. The samples with surfactant treated MWCNTs do not show much more
differences with each other, except for the sample with the ratio 1:1. In the frequency
range of 22 to 26.5, the sample with untreated MWCNTs shows an even better EMI
shielding effectiveness than the other samples. The different ratio between the MWCNTs
and the surfactant offer different dispersion conditions of the MWCNTs. Overall, the
27
results show that the surfactant could influence the EMI shielding effectiveness. The
dispersion states of the MWCNTs in the composites can either increase or decrease the
EMI shielding effectiveness of the composites, and it depends on the dispersion level and
frequency range.
Figure 3.2.2 shows the reflection loss performance of MWCNTs - epoxy
composites with NaDDBs treated MWCNTs. In the frequency range of 1 to 12 GHz, the
sample with the ratio 1:1 shows a slight increase in the reflection loss performance, while
the other samples do not show much difference when compared to the one without
surfactant. In the frequency range of 12 to 22 GHz, the samples with surfactant treated
MWCNTs show a decrease in the reflection loss performance. In the frequency range of
22 to 26.5 GHz, the samples with surfactant treated MWCNTs show an increase in the
reflection loss performance. Specifically, for the sample with the ratio 1:1, the reflection
loss performance was dramatically reinforced; it has more than a 5 dB increase in the
frequency range of 22 to 26.5 GHz. The reflection peak position of the sample with ratio
1:1 shifts from 20 GHz to 23GHz and the peak value rises up from -17 dB to -24 dB. The
reflection loss peak position of the samples with surfactant treated MWCNTs moved to a
higher frequency. All the results show that surfactant influenced the reflection loss
performance of the MWCNTs - epoxy composites; the dispersion states of the MWCNTs
in the composites can either increase or decrease the reflection loss performance of the
composites, depending on the dispersion level and the frequency range. The dispersion
states can change the reflection peak position, bandwidth and peak value.
28
Figure 3.2.2. Reflection loss of MWCNTs - epoxy composites with surfactant treated MWCNTs.
Figure 3.2.3. Absorption ratio of MWCNTs - epoxy composites with surfactant treated CNTs.
29
Figure 3.2.3 is the absorption ratio of MWCNTs - epoxy composites with
surfactant treated MWCNTs. The results show that in the frequency range of 1 to 15 GHz,
the samples with surfactant treated MWCNTs have similar absorption performances
when compared to the samples with untreated MWCNTs. In the frequency range of 15 to
23 GHz, the microwave absorption performance of the samples with surfactant treated
MWCNTs decreased. The sample with untreated CNTs showed the best absorption
ability in this frequency range. In the frequency range of 23 to 26.5 GHz, the absorption
performance of the samples with surfactant treated MWCNTs was reinforced with the
exception of the sample with the 1:1 ratio. Because the sample with the 1:1 ratio shows a
better reflection loss performance (Figure 3.2.2), the amount of the incident wave energy
entered into the sample was less than others; this may have caused the decrease of the
absorption performance.
3.3
Complex Dielectric Permittivity
Complex permittivity ε (ε = ε' - ε") and complex permeability
( = ' - ") are
the two fundamental physical quantities related to the microwave absorption properties of
the materials.
It has been reported that the complex permeability of the MWCNTs - epoxy
composites system has a very low value [16, 35]. The reason is that both the epoxy and
the MWCNTs have weak magnetism performances which mean the permeability does
not contribute much to the MWCNTs - epoxy microwave absorption composites. In this
work, we mainly studied the permittivity properties of the MWCNTs - epoxy composites.
30
(a)
(b)
Figure 3.3.1. The (a) real part and (b) imaginary part of the complex permittivity of the MWCNTs - epoxy
samples with different MWCNTs loading in the frequency range from 2 to 26.5 GHz.
The real part of the complex permittivity of MWCNTs - epoxy composites, ε', is
shown in Figure 3.3.1 (a). Here, the loadings of the MWCNTs were controlled from 1 to
7 wt%. The real part of the complex permittivity is related to the energy storage
capability from the external electromagnetic field in the materials. For the samples with
lower MWCNTs loadings, the ε' are almost independent of the frequencies in the range
from 2 to 26.5 GHz. For example, the value of ε' for the 1 wt% sample is around 4.5 in
the frequency range from 2 to 26.5 GHz. However, for the sample with higher MWCNTs
loadings, the values of ε' showed a frequency dependence in which ε' decreased with
increasing the frequency. In the frequency range from 1 to 26.5 GHz, the sample with 6
wt% CNTs has a value of 12 for ε' at 2 GHz and gradually deceases to 10.5 at 26 GHz.
The sample with 7 wt% CNTs has a value of 12 for ε' at 2 GHz and decease to 11 at 18
GHz, following that a slightly increasing of ε' as the frequency increases.
31
The values of ε' also show the MWCNTs loadings dependence in that ε' increases
with increasing the MWCNTs loading. For example, the value of ε' slightly rises from 4.5
to 5.8 when the MWCNTs loading increasing from 1 wt% to 3 wt% at 5 GHz. Further ε'
increased to 12 and 15 for the samples with MWCNTs loadings of 6 wt% and 7 wt% at 5
GHz, respectively.
Figure 3.3.1 (b) is the imaginary part of the complex permittivity of the
MWCNTs - epoxy composites with MWCNTs loadings from 1 to 7 wt%. The
measurement frequency range is from 1 to 26.5 GHz. The imaginary part of the complex
permittivity, ε", is related to the electromagnetic energy dissipation capability of the
materials. The ε" values of the samples with MWCNTs loadings 1-4 wt% are very small
(between 0-1) in the frequency range of 2 - 26.5 GHz. These ε" values for lower
MWCNTs loading composites do not show a strong frequency dependence, i.e., the ε"
slightly increased along with the increasing of the frequency. However, when the
MWCNTs loadings are more than 4 wt%, ε" shows an apparent frequency dependence.
Thus, for the sample with the 6 wt% MWCNTs loading ε" increased from 1.5 to 3.5 as
the frequency increased from 2 to 26.5 GHz.
The ε" values for the samples with
MWCNTs loadings of 1-4 wt% slightly increased with the increased MWCNTs loading.
But when the MWCNTs loadings increases from 4 wt% to 7 wt%, the ε" gradually
increase from 1 to 3 and 6 at 20 GHz, when the MWCNTs loadings increased from 4 wt%
to 6 wt% and 7 wt%, respectively.
In Figure 3.3.1 the real and imaginary parts of the complex permittivity of
MWCNTs - epoxy composites show a slight increase with increasing the MWCNTs
loadings in the frequency range of 2 to 26.5 GHz for the samples with MWCNTs
32
loadings of 1-4 wt%. When a critical MWCNTs loading (e.g. 5 wt%) is reached, the
permittivity shows a drastic increase. This phenomenon also has been confirmed by other
research groups [72 - 75]. This can be explained by the percolation theory for charge
transport and effective electron tunneling or hopping [76, 77]. One step further, when the
MWCNTs loading is low, the MWCNT aggregates and MWCNT bundles are in
separated states, which means the whole composite may be in an insulating state. With
the increase of the MWCNTs loading, once it reaches the percolation threshold, the
continuous conductive network will be formed, and the composites will transfer to a more
conductive state [16]. The percolation thresholds may depend on many parameters such
as the materials and the sample processing methods. For the MWCNTs - epoxy
composites system, it is believed that the percolation threshold is between MWCNTs
loadings of 4 wt% and 6 wt%.
The loss tangent of a dielectric material is defined as the ratio of ε" to ε'. It is
related to the attenuating factor of the material, which is the ability to convert stored
energy to heat. The large attenuating factor means that the material has a better
microwave absorption performance. Figure 3.3.2 shows the loss tangent of MWCNTs epoxy composites with MWCNTs loadings from 1 to 7 wt%. The result shows that the
loss tangent is a function of frequency in the frequency range of 2 to 26.5 GHz. For all of
the samples, the loss tangents increased along with the increasing of the frequency in the
frequency range of 2 to 16 GHz. Then, the samples began to decrease in the frequency
range of 16 to 26.5 GHz, with the exception of the sample with the 6 wt% MWCNTs
loading. For example, the sample with the MWCNTs loading of 7 wt%, the loss tangent
increased from 0.2 to 0.55 as the frequency increased from 2 GHz to 20 GHz. The loss
33
tangent decreased to 0.45 when the frequency increased to 26.5 GHz. The results also
showed that the loss tangent increases with increasing the MWCNTs loading. For the
samples with MWCNTs loadings of 1 to 4 wt%, the loss tangents showed a slight
increase along with the increasing of MWCNTs loadings. Further, when we increased the
MWCNTs loading to 7 wt%, the loss tangents showed a drastic increase. As mentioned
earlier, this may be due to the percolation threshold in which the conductive network
formed with a high MWCNTs loading.
Figure 3.3.2 Loss tangent of MWCNTs - epoxy composites with different MWCNTs loadings in the
frequency range from 1 to 26.5 GHz.
34
3.4
Multi-layer Structure Microwave Absorber
As mentioned in the previous sections, the microwave absorption properties of
MWCNTs - epoxy composites are mainly due to the dielectric loss in the materials. The
conductivity and dielectric permittivity of the fillers are definitely important for the
MWCNTs - epoxy system microwave absorber. For a better microwave absorption
performance, the material should have both low-reflection and high-absorption properties
[78]. From Section 3.1, the sample with lower MWCNTs loadings shows a better
reflection loss performance. But in materials, if the amount of conductive fillers
(MWCNTs) is too low, the absorption performance is restricted, because only a few filler
particles can dissipate the microwave energy. However, if the amount of the MWCNTs in
the material is too high, the reflection performance will be drastically decreased [63].
Therefore, it is difficult to find the best microwave absorption performance in a single
system.
To reach the best microwave absorption performance, there are two fundamental
requirements. The first one is a low-reflection, meaning that the incident microwave
energies can enter the materials as much as possible. This is decided by the impedance
matching condition, see equations 3.4.1 to 3.4.4 [50, 79]. The second one is highabsorption, meaning that the microwave entering the materials can be attenuated as much
as possible. This is related to the attenuation factor; see equation 3.4.5 [78]. To fit these
two requirements, the microwave absorbers with multi-layer structure have been
designed.
= 20
=
(3.4.1)
ℎ
(3.4.2)
35
When you combine equation 3.4.1 and 3.4.2 you get
√
= 20
,
(3.4.3)
√
where RL is reflection loss (as mentioned in Section 3.1);
j is the imaginary unit; c is the speed of light;
f is the frequency;
d is the sample thickness;
is the complex permeability;
is the complex permittivity of the sample;
Zin is the normalized impedance of the material;
Z0 is the characteristic impedance of the free space.
If electromagnetic waves traveling through one material meet another material,
which has a very different impedance from the first material, the waves will reflect or
scatter from the boundary between the materials.
Regarding the MWCNTs - epoxy composites, since they are a dielectric absorber,
'=1 and "=0 are assumed. Then equation 3.4.3 can be rewritten as
√
= 20
(3.4.4)
√
This equation describes the impedance matching condition and reflection loss
performance. For the best possible absorption condition, the attenuation constant is given
as (3.4.5), where
=
√
"
is attenuation constant and tan is the loss tangent [50].
1+
−
,
(3.4.5)
36
Figure 3.4.1 Schematic representation of the microwave absorption performances of the multi-layer
structure microwave absorber..
Based on the
he discussion above, the multi
multi-layer structure MWCNTs - epoxy
composites microwave absorbers have been designed. The impedance matching theory
was used to design the low reflection samples. According to equation 3.4.4,
3.4.4 the complex
permittivity is important for the fit impedance matching condition.
condition The complex
permittivity values of the MWCNTs - epoxy composites can be found from Section 3.3.
The material on the first layer should have a very small permittivity since the
th
impedance of the free space or air is around 1. The materials on the N and N+1 layers
layer
should have similar permittivity values so that they can have similar impedance for the
low reflection. The hollow arrows in Figure
igure 3.4.1 indicate the weak reflection under the
impedance matching condition.
The material on the last layer should be the conducting metal substrate.
substrate In this
work an aluminum sheet was used as the metal back material. The reason for using metal
as the last layer is because of its high reflec
reflection.
tion. Before the incident waves the reach
metal layer, a portion of them have been reflected at each boundary interface and s
37
portion of them have been absorbed by each layer. The rest of the incident wave will
almost be reflected at the metal layer boundary surface. These waves then have to go
through the entire absorber layers again, which will enhance the absorption performance.
(a)
(b)
(c)
(d)
Figure 3.4.2 Reflection loss of (a) Aluminum and Teflon and (b) (c) (d) Multi-layer structure microwave
absorbers.
38
The following tables indicate the construction formation of each sample. Table 1,
Table 2, Table 3 and Table 4 are formations for Figure 3.4.2 (a), (b), (c) and (d),
respectively. In Figure 3.4.2 (b), sample “3 mm” consists of 5 layers, as shown in Table 2.
The first layer is pure epoxy with thickness of 0.5 mm, and the second layer is 1 wt%
MWCNTs - epoxy composites with thickness of 0.5 mm. The third, fourth and fifth
layers are 3 wt%, 5 wt% and 7 wt% MWCNTs - epoxy composites with thickness of 0.5
mm, 0.5 mm and 1mm, respectively.
TABLE 1. Samples’ construction formation of Figure 3.4.2 (a).
Sample No.
1 mm Al
1 mm T
1st layer
1 mm Aluminum
1 mm Teflon
TABLE 2. Samples’ construction formation of Figure 3.4.2 (b) and Figure 3.4.3 (d).
Sample No.
st
3 mm
3.5 mm
4 mm
4.5 mm
5 mm
0.5mm
0.5mm
0.5mm
0.5mm
0.5mm
Pure Epoxy
Pure Epoxy
Teflon
Pure Epoxy
Teflon
0.5mm
0.5mm
0.5mm
0.5mm
0.5mm
1 wt%
1 wt%
0.1wt%
1 wt%
Pure Epoxy
0.5mm
0.5mm
0.5mm
0.5mm
0.5mm
3 wt%
3 wt%
1 wt%
2 wt%
1 wt%
0.5mm
1 mm
0.5mm
0.5mm
0.5mm
5 wt%
5 wt%
3 wt%
3 wt%
2 wt%
1mm
1mm
1 mm
0.5mm
0.5mm
7 wt%
7 wt%
5 wt%
4 wt%
3 wt%
1 layer
nd
2 layer
rd
3 layer
4th layer
5th layer
39
6
7
8
9
th
th
th
th
layer
-
layer
-
layer
-
1mm
0.5mm
0.5mm
7 wt%
5 wt%
4 wt%
0.5mm
0.5mm
6 wt%
5 wt%
1mm
0.5mm
7wt%
6 wt%
-
-
-
-
1mm
layer
-
-
-
7wt%
TABLE 3. Samples’ construction formation of Figure 3.4.2 (c) and Figure 3.4.3 (a).
Sample No.
1st layer
2nd layer
3rd layer
4th layer
5th layer
6
th
3.5 mm
4.5 mm
0.5mm
0.5mm
Pure Epoxy
Pure Epoxy
0.5mm
0.5mm
1 wt%
1 wt%
0.5mm
0.5mm
3 wt%
3 wt%
1 mm
1 mm
5 wt%
5 wt%
1mm
1mm
7 wt%
7 wt%
1mm
layer
Aluminum
40
TABLE 4. Samples’ formation of Figure 3.4.2 (d) and Figure 3.4.3 (c).
Sample No.
1st layer
2nd layer
3rd layer
4th layer
5th layer
6th layer
7th layer
8
9
th
th
th
5 mm
6 mm
0.5mm
0.5mm
Teflon
Teflon
0.5mm
0.5mm
0.1 wt%
0.1 wt%
0.5mm
0.5mm
1 wt%
1 wt%
0.5mm
0.5mm
2 wt%
2 wt%
0.5mm
0.5mm
3 wt%
3 wt%
0.5mm
0.5mm
4 wt%
4 wt%
0.5mm
0.5mm
5 wt%
5 wt%
0.5mm
0.5mm
6 wt%
6 wt%
1mm
1mm
7wt%
7wt%
layer
layer
10 layer
1mm
Aluminum
41
Figure 3.4.2 shows the reflection loss performance for the different samples.
Figure (a) illustrates the reflection loss for Teflon and the aluminum sheet. Since Teflon
has a very small permittivity value, the reflection loss in frequency range of 2-26.5 GHz
is below -13 dB, which means that less than 5 % of the microwave energies have been
reflected. This makes Teflon very suitable as the first layer. For the aluminum sheet, the
reflection loss in the frequency range of 5 to 25 GHz is above -5 dB, which means most
of the microwave energies were reflected. This makes aluminum suitable as the last layer.
Depending on this phenomenon, several different experiments were performed.
Figure 3.4.2 (b) is the reflection loss of the different samples. As the results
showed, the reflection loss peak goes to a lower frequency with the increasing of the
sample thickness. For example, the 3 mm sample has a reflection loss peak around 24
GHz, and the reflection loss peak goes to 18.5 GHz, 17.5 GHz, 15 GHz and 13 GHz for
the 3.5 mm, 4 mm, 4.5 mm and 5 mm samples, respectively. The reflection loss can reach
-37 dB at 18.5 GHz for the 3.5 mm sample.
The only difference between the 3.5 mm sample and the 4 mm sample is the first
two layers. The 4 mm sample has 0.5 mm Teflon and 0.5 mm 0.1 wt% composites at the
first two layers, and the 3.5 mm sample has 0.5 mm pure epoxy on the first layer.
However, these two samples show similar reflection loss performances. This may
indicate that the combination of Teflon and the 0.1 wt% composite has a similar
reflection performance with pure epoxy when they are the first layer of the absorber.
This can be verified by the 4.5 mm and 5 mm samples, since they show similar reflection
loss performances. The differences between these two are also in the first two layers. The
4.5 mm sample has pure epoxy as the first layer and the 5 mm sample has Teflon and
42
pure epoxy as the first two layers. The 3 mm sample shows a better reflection loss
performance in the frequency range. The reflection peak value or bandwidth is better than
the single layer sample in figure 3.1.3.
Figure 3.4.2 (c) gives the reflection loss for the 3.5 mm and 4.5 mm samples. As
listed in Table 3, the only difference between these two samples is that the 4.5 mm
sample has the 1 mm aluminum metal back. The results showed that the reflection loss
peak goes to a lower frequency range when the thickness is increased. Figure 3.4.2 (d)
shows the same performance.
(a)
(b)
(c)
(d)
Figure 3.4.3. Absorption ratio Multi-layer structure MWCNTs-epoxy composites.
43
TABLE 5. Samples’ construction formation of figure 3.4.3 (b).
Sample No.
1st layer
nd
4 mm
5 mm
0.5mm
0.5mm
Teflon
Teflon
0.5mm
0.5mm
0.1wt%
0.1wt%
0.5mm
0.5mm
1 wt%
1 wt%
0.5mm
0.5mm
3 wt%
3 wt%
1 mm
1 mm
5 wt%
5 wt%
1mm
1mm
7 wt%
7 wt%
2 layer
rd
3 layer
th
4 layer
th
5 layer
th
6 layer
7
th
1mm
layer
Aluminum
Figure 3.4.3 illustrates the absorption ratio of the different samples. Table 3,
Table 5, Table 4 and Table 2 are the construction formations for Figures 3.4.3 (a), (b), (c)
and (d), respectively. Figure 3.4.3 (d) is the absorption ratio for the different samples.
The construction formation is described in Table 2. The results show that the 3 mm
sample has the worst absorption ratio among these samples because it has the smallest
thickness. The absorption performance for the 4.5 mm and 5 mm samples is almost
44
similar in the frequency range of 2- 26.5 GHz. The only difference between these two
samples is the first layer. As mentioned earlier, the combination of Teflon and 0.1 wt%
has similar microwave absorption properties with pure epoxy. The 4 mm and 3.5 mm
samples show the best absorption performances among these samples in the frequency
range of 2 to 26.5 GHz. The common point of these two samples is that they both have 1
mm thickness and 5 wt% and 1 mm thickness and 7 wt%, respectively, on the last two
layers. The higher microwave absorption may due to the larger thicknesses of the 5 wt%
and 7 wt% layers. The 5 wt% and 7 wt% MWCNTs - epoxy composites have better
microwave absorption performances than the sample with lower MWCNTs loading.
Figure 3.4.3 (a), (b) and (c) are the absorption ratio of multi-layer samples with 1
mm aluminum as the last layer. In the Figure 3.4.3 (a), after adding the aluminum sheet,
the microwave absorption performance was drastically enhanced in the frequency range
of 1 to 7 GHz. For the samples with different thicknesses, the absorption performances
were enhanced in different frequency ranges. For instance, the absorption performance of
the 4 mm sample was enhanced in the frequency range of 5 to 10 GHz, and the
absorption performance of the 5 mm sample was enhanced in the frequency range of 7 to
12 GHz. Compared to the multi-layer samples without aluminum or the single layer
sample in Figure 3.1.4, the results showed that the microwave absorption performance
was obviously enhanced in the frequency range of 2 to 26.5 GHz when the 1 mm
aluminum sheet was placed as the last layer. In Figure 3.4.3 (b), the absorption peak
value reached 97 % for the 5 mm sample at 7 GHz.
45
CHAPTER 4
CONCLUSION
MWCNTs - epoxy composite microwave absorbers were fabricated. The
MWCNTs loading was controlled from 1 to 10wt% and the measurement frequency
range was from 1 to 26.5 GHz. The single layer MWCNTs - epoxy composite microwave
absorber, the MWCNTs - epoxy composite with surfactant treated MWCNTs and multilayer structure microwave absorbers were fabricated. The reflection loss, EMI shielding
effectiveness, absorption ratio performance and complex permittivity of these samples
were studied. The results showed that the microwave absorption performances strongly
depend on the MWCNTs loading of the single layer samples. The dispersion states of the
MWCNTs in the composites could influence the microwave absorption performance of
the sample. The multi-layer structure MWCNTs - epoxy composites showed an enhanced
microwave absorption performance compared to the single layer MWCNTs - epoxy
composites.
46
BIBLIOGRAPHY
1.
William Herschel, Phil. Trans. R. Soc. Lond., vol. 90, pp. 284-292, 1800.
2.
Maxwell, J.C., Phil. Trans. R. Soc. Lond., vol. 155, pp. 459-512, 1865.
3.
P. Nahin, Oliver Heaviside. Baltimore, Md.: Johns Hopkins University Press, pp.
108-112, 2002.
4.
E. Bellone, A world on paper. Cambridge, Mass.: MIT Press, 1980.
5.
T. Chow, Introduction to electromagnetic theory. Boston: Jones and Bartlett
Publishers, 2006.
6.
M. Sadiku, Elements of electromagnetics. New York: Oxford University Press,
2001.
7.
Applications of electromagnetic induction. Boston University. 1999-07-22.
8.
P. Lorrain, D. Corson and F. Lorrain, Fundamentals of electromagnetic
phenomena. New York: W.H. Freeman, 2000.
9.
J. Jeans, The growth of physical science. Cambridge [England]: University Press,
1948.
10.
D. Pozar, Microwave engineering. Reading, Mass.: Addison-Wesley, 1990.
11.
R. Sorrentino and G. Bianchi, Microwave and RF engineering. Chichester, West
Sussex, U.K.: Wiley, 2010.
12.
John D, Washington M. USAF unveils stealth Figurehter. Aviat Week Space
Technol, 129 (November 14): 28–9, 1988.
47
13.
X. Liu, D. Geng, H. Meng, P. Shang and Z. Zhang, "Microwave-absorption
properties of ZnO-coated iron nanocapsules", Appl. Phys. Lett., vol. 92, no. 17, p.
173117, 2008.
14.
H. Nalwa, Handbook of organic conductive molecules and polymers. Chichester:
Wiley, pp. 367, 1997.
15.
M. Cao, W. Song, Z. Hou, B. Wen and J. Yuan, "The effects of temperature and
frequency on the dielectric properties, electromagnetic interference shielding and
microwave-absorption of short carbon fiber/silica composites", Carbon, vol. 48,
no. 3, pp. 788-796, 2010.
16.
Z. Wang and G. Zhao, "Electromagnetic wave absorption of multi-walled carbon
nanotube–epoxy composites in the R band", J. Mater. Chem. C, vol. 2, no. 44, pp.
9406-9411, 2014.
17.
C. May, Epoxy resins. New York: M. Dekker, pp. 194, 1988.
18.
M. Pacios Pujadó, "Carbon Nanotubes as Platforms for Biosensors with
Electrochemical and Electronic Transduction", Springer Theses, p. 208, 2012.
19.
L. V. Radushkevich and V. M. Lukyanovich, Soviet Journal of Physical
Chemistry, vol 26, pp. 88–95, 1952.
20.
Nesterenko, A.M.,
Prilutskii, O.V.,
Kolesnik, N.F., Akhmatov, Yu.S., Suhomlin, V.I., and
“Characteristics of the phase composition and structure of
products of the intera ction of nickel (II)and iron (III)oxide with carbo n
monoxide. Izvestia Akadem ii Nauk SSSR, Metal ly, 3, pp. 12–17, 1982.
21.
S. Iijima, "Helical microtubules of graphitic carbon", Nature, vol. 354, no. 6348,
pp. 56-58, 1991.
48
22.
D. Bethune, C. Klang, M. de Vries, G. Gorman, R. Savoy, J. Vazquez and R.
Beyers, "Cobalt-catalysed growth of carbon nanotubes with single-atomic-layer
walls", Nature, vol. 363, no. 6430, pp. 605-607, 1993.
23.
J. Mintmire, B. Dunlap and C. White, "Are fullerene tubules metallic?", Phys. Rev.
Lett., vol. 68, no. 5, pp. 631-634, 1992.
24.
X. Wang, Q. Li, J. Xie, Z. Jin, J. Wang, Y. Li, K. Jiang and S. Fan, "Fabrication
of Ultralong and Electrically Uniform Single-Walled Carbon Nanotubes on Clean
Substrates", Nano Letters, vol. 9, no. 9, pp. 3137-3141, 2009.
25.
"The 2010 Nobel Prize in Physics - Press Release". Nobelprize.org. Nobel Media
AB 2014.
26.
M. Yu, "Strength and Breaking Mechanism of Multiwalled Carbon Nanotubes
Under Tensile Load", Science, vol. 287, no. 5453, pp. 637-640, 2000.
27.
A. Fennimore, T. Yuzvinsky, W. Han, M. Fuhrer, J. Cumings and A. Zettl,
"Rotational actuators based on carbon nanotubes", Nature, vol. 424, no. 6947, pp.
408-410, 2003.
28.
X. Lu and Z. Chen, "Curved Pi-Conjugation, Aromaticity, and the Related
Chemistry of Small Fullerenes and Single-Walled Carbon Nanotubes", Chemical
Reviews, vol. 105, no. 10, pp. 3643-3696, 2005.
29.
S. Hong and S. Myung, "Nanotube Electronics: A flexible approach to mobility",
Nature Nanotech, vol. 2, no. 4, pp. 207-208, 2007.
30.
P. Avouris, "Carbon nanotube electronics and photonics", Phys. Today, vol. 62,
no. 1, pp. 34-40, 2009.
49
31.
C. Wang, X. Han, P. Xu, X. Zhang, Y. Du, S. Hu, J. Wang and X. Wang, "The
electromagnetic property of chemically reduced graphene oxide and its
application as microwave absorbing material", Appl. Phys. Lett., vol. 98, no. 7, p.
072906, 2011.
32.
Z. Liu, G. Bai, Y. Huang, F. Li, Y. Ma, T. Guo, X. He, X. Lin, H. Gao and Y.
Chen, "Microwave Absorption of Single-Walled Carbon Nanotubes/Soluble
Cross-Linked Polyurethane Composites", J. Phys. Chem. C, vol. 111, no. 37, pp.
13696-13700, 2007.
33.
Z. Fan, G. Luo, Z. Zhang, L. Zhou and F. Wei, "Electromagnetic and microwave
absorbing properties of multi-walled carbon nanotubes/polymer composites",
Materials Science and Engineering: B, vol. 132, no. 1-2, pp. 85-89, 2006.
34.
C. Ma, Y. Huang, H. Kuan and Y. Chiu, "Preparation and electromagnetic
interference shielding characteristics of novel carbon-nanotube/siloxane/poly(urea urethane) nanocomposites", J. Polym. Sci. B Polym. Phys., vol. 43, no. 4, pp.
345-358, 2005.
35.
Z. Wang and G. Zhao, "Microwave Absorption Properties of Carbon NanotubesEpoxy Composites in a Frequency Range of 2 - 20 GHz", Open Journal of
Composite Materials, vol. 03, no. 02, pp. 17-23, 2013.
36.
D. Zhao, J. Zhang, X. Li and Z. Shen, "Electromagnetic and microwave absorbing
properties of Co-filled carbon nanotubes", Journal of Alloys and Compounds, vol.
505, no. 2, pp. 712-716, 2010.
50
37.
T. Ting, Y. Jau and R. Yu, "Microwave absorbing properties of polyaniline/multiwalled carbon nanotube composites with various polyaniline contents", Applied
Surface Science, vol. 258, no. 7, pp. 3184-3190, 2012.
38.
J. Wu and L. Kong, "High microwave permittivity of multiwalled carbon
nanotube composites", Appl. Phys. Lett., vol. 84, no. 24, p. 4956, 2004.
39.
Agilent - Network Analyzer products, Agilent Technologies, Inc. Published in
USA, February 27, 2014 5989-7603EN
40.
Understanding the fundamental principles of vector network analysis. © Agilent
Technologies, Inc 2012. Published in USA. December 12,2012 5965-7707E
41.
J. Choma and W. Chen, Feedback networks. Hackensack, NJ: World Scientific,
2007.
42.
F. Wu, Z. Xu, Y. Wang and M. Wang, "Electromagnetic interference shielding
properties of solid-state polymerization conducting polymer", RSC Advances, vol.
4, no. 73, p. 38797, 2014.
43.
N. Colaneri and L. Schacklette, "EMI shielding measurements of conductive
polymer blends", IEEE Trans. Instrum. Meas., vol. 41, no. 2, pp. 291-297, 1992.
44.
J. Joo and A. Epstein, "Electromagnetic radiation shielding by intrinsically
conducting polymers", Appl. Phys. Lett., vol. 65, no. 18, p. 2278, 1994.
45.
C. Lee, H. Song, K. Jang, E. Oh, A. Epstein and J. Joo, "Electromagnetic
interference shielding efficiency of polyaniline mixtures and multilayer films",
Synthetic Metals, vol. 102, no. 1-3, pp. 1346-1349, 1999.
46.
Y. Hong, C. Lee, C. Jeong, D. Lee, K. Kim and J. Joo, "Method and apparatus to
measure electromagnetic interference shielding efficiency and its shielding
51
characteristics in broadband frequency ranges", Rev. Sci. Instrum., vol. 74, no. 2,
p. 1098, 2003.
47.
Y. Wang and X. Jing, "Intrinsically conducting polymers for electromagnetic
interference shielding", Polym. Adv. Technol., vol. 16, no. 4, pp. 344-351, 2005.
48.
K.
Singh,
A.
Ohlan,
P.
ethylenedioxythiophene)γ-Fe2O3
Saini
and
polymer
S.
Dhawan,
composite–super
"Poly
(3,4-
paramagnetic
behavior and variable range hopping 1D conduction mechanism–synthesis and
characterization", Polym. Adv. Technol., vol. 19, no. 3, pp. 229-236, 2008
49.
RWP King, Transmission-Line Theory, McGraw-Hill press, 1995.
50.
F. Qin and C. Brosseau, "A review and analysis of microwave absorption in
polymer composites filled with carbonaceous particles", J. Appl. Phys., vol. 111,
no. 6, p. 061301, 2012.
51.
W. Bauhofer and J. Kovacs, "A review and analysis of electrical percolation in
carbon nanotube polymer composites", Composites Science and Technology, vol.
69, no. 10, pp. 1486-1498, 2009.
52.
F. Du, R. Scogna, W. Zhou, S. Brand, J. Fischer and K. Winey, "Nanotube
Networks in Polymer Nanocomposites: Rheology and Electrical Conductivity",
Macromolecules, vol. 37, no. 24, pp. 9048-9055, 2004.
53.
X. Gui, K. Wang, J. Wei, R. Lü, Q. Shu, Y. Jia, C. Wang, H. Zhu and D. Wu,
"Microwave absorbing properties and magnetic properties of different carbon
nanotubes", Sci. China Ser. E-Technol. Sci., vol. 52, no. 1, pp. 227-231, 2009.
54.
B. Che, B. Nguyen, L. Nguyen, H. Nguyen, V. Nguyen, T. Van Le and N.
Nguyen, "The impact of different multi-walled carbon nanotubes on the X-band
52
microwave absorption of their epoxy nanocomposites", Chemistry Central
Journal, vol. 9, no. 1, p. 10, 2015.
55.
X. Gui, K. Wang, J. Wei, R. Lü, Q. Shu, Y. Jia, C. Wang, H. Zhu and D. Wu,
"Microwave absorbing properties and magnetic properties of different carbon
nanotubes", Sci. China Ser. E-Technol. Sci., vol. 52, no. 1, pp. 227-231, 2009.
56.
F. Castillo, R. Socher, B. Krause, R. Headrick, B. Grady, R. Prada-Silvy and P.
Pötschke, "Electrical, mechanical, and glass transition behavior of polycarbonatebased nanocomposites with different multi-walled carbon nanotubes", Polymer,
vol. 52, no. 17, pp. 3835-3845, 2011.
57.
J. Li, P. Ma, W. Chow, C. To, B. Tang and J. Kim, "Correlations between
Percolation Threshold, Dispersion State, and Aspect Ratio of Carbon Nanotubes",
Adv. Funct. Mater., vol. 17, no. 16, pp. 3207-3215, 2007.
58.
C. Martin, J. Sandler, M. Shaffer, M. Schwarz, W. Bauhofer, K. Schulte and A.
Windle, "Formation of percolating networks in multi-wall carbon-nanotube–
epoxy composites", Composites Science and Technology, vol. 64, no. 15, pp.
2309-2316, 2004.
59.
G. Seidel and A. Puydupin-Jamin, "Analysis of clustering, interphase region, and
orientation effects on the electrical conductivity of carbon nanotube–polymer
nanocomposites via computational micromechanics", Mechanics of Materials, vol.
43, no. 12, pp. 755-774, 2011.
60.
J. Aguilar, "Influence of carbon nanotube clustering on the electrical conductivity
of polymer composite films", expresspolymlett, vol. 4, no. 5, pp. 292-299, 2010.
53
61.
M. Morcom, K. Atkinson and G. Simon, "The effect of carbon nanotube
properties on the degree of dispersion and reinforcement of high density
polyethylene", Polymer, vol. 51, no. 15, pp. 3540-3550, 2010.
62.
F. Gojny, M. Wichmann, B. Fiedler, I. Kinloch, W. Bauhofer, A. Windle and K.
Schulte, "Evaluation and identification of electrical and thermal conduction
mechanisms in carbon nanotube/epoxy composites", Polymer, vol. 47, no. 6, pp.
2036-2045, 2006.
63.
F. Nanni, P. Travaglia and M. Valentini, "Effect of carbon nanofibres dispersion
on the microwave absorbing properties of CNF/epoxy composites", Composites
Science and Technology, vol. 69, no. 3-4, pp. 485-490, 2009.
64.
A. Saib, L. Bednarz, R. Daussin, C. Bailly, Xudong Lou, J. Thomassin, C.
Pagnoulle, C. Detrembleur, R. Jerome and I. Huynen, "Carbon nanotube
composites for broadband microwave absorbing materials", IEEE Transactions on
Microwave Theory and Techniques, vol. 54, no. 6, pp. 2745-2754, 2006.
65.
J. Kovacs, B. Velagala, K. Schulte and W. Bauhofer, "Two percolation thresholds
in carbon nanotube epoxy composites", Composites Science and Technology, vol.
67, no. 5, pp. 922-928, 2007.
66.
S. Bose, R. Khare and P. Moldenaers, "Assessing the strengths and weaknesses of
various types of pre-treatments of carbon nanotubes on the properties of
polymer/carbon nanotubes composites: A critical review", Polymer, vol. 51, no. 5,
pp. 975-993, 2010.
54
67.
M. Islam, E. Rojas, D. Bergey, A. Johnson and A. Yodh, "High Weight Fraction
Surfactant Solubilization of Single-Wall Carbon Nanotubes in Water", Nano
Letters, vol. 3, no. 2, pp. 269-273, 2003.
68.
E. Camponeschi, B. Florkowski, R. Vance, G. Garrett, H. Garmestani and R.
Tannenbaum, "Uniform Directional Alignment of Single-Walled Carbon
Nanotubes in Viscous Polymer Flow", Langmuir, vol. 22, no. 4, pp. 1858-1862,
2006.
69.
O. Matarredona, H. Rhoads, Z. Li, J. Harwell, L. Balzano and D. Resasco,
"Dispersion of Single-Walled Carbon Nanotubes in Aqueous Solutions of the
Anionic Surfactant NaDDBS", The Journal of Physical Chemistry B, vol. 107, no.
48, pp. 13357-13367, 2003.
70.
L. Vaisman, H. Wagner and G. Marom, "The role of surfactants in dispersion of
carbon nanotubes", Advances in Colloid and Interface Science, vol. 128-130, pp.
37-46, 2006.
71.
R. Che, L. Peng, X. Duan, Q. Chen and X. Liang, "Microwave Absorption
Enhancement and Complex Permittivity and Permeability of Fe Encapsulated
within Carbon Nanotubes", Adv. Mater., vol. 16, no. 5, pp. 401-405, 2004.
72.
Y. Chen, M. Cao, T. Wang and Q. Wan, "Microwave absorption properties of the
ZnO nanowire-polyester composites", Appl. Phys. Lett., vol. 84, no. 17, p. 3367,
2004.
73.
W. Bauhofer and J. Kovacs, "A review and analysis of electrical percolation in
carbon nanotube polymer composites", Composites Science and Technology, vol.
69, no. 10, pp. 1486-1498, 2009.
55
74.
F. Du, R. Scogna, W. Zhou, S. Brand, J. Fischer and K. Winey, "Nanotube
Networks in Polymer Nanocomposites: Rheology and Electrical Conductivity",
Macromolecules, vol. 37, no. 24, pp. 9048-9055, 2004.
75.
Y. Feng, T. Qiu and C. Shen, "Absorbing properties and structural design of
microwave absorbers based on carbonyl iron and barium ferrite", Journal of
Magnetism and Magnetic Materials, vol. 318, no. 1-2, pp. 8-13, 2007.
76.
E. Michielssen, J. Sajer, S. Ranjithan and R. Mittra, "Design of lightweight,
broad-band microwave absorbers using genetic algorithms", IEEE Transactions
on Microwave Theory and Techniques, vol. 41, no. 6, pp. 1024-1031, 1993.
56
APPENDICES
TABLE 6. The relation between dB value and reflection ratio
dB value
Reflection ratio
-1 dB
79.4%
-2 dB
63.1%
-3 dB
50.1%
-4 dB
39.8%
-5 dB
31.6%
-6 dB
25.1%
-7 dB
20%
-8 dB
15.8%
-9 dB
12.6%
-10 dB
10%
-11 dB
7.9%
-12 dB
6.3%
-13 dB
5%
-14 dB
3.9%
-15 dB
3.1%
-16 dB
2.5%
-17 dB
2%
-18 dB
1.5%
-19 dB
1.25%
-20 dB
1%
57
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