Synthesis and Characterisation of Multiwalled Carbon Nanotubes Composite
based on Mill Scale and Boron Trioxide via Chemical Vapor Deposition
By
LEE SHIAN BOON
Thesis Submitted to the Department of Physics, Universiti Putra Malaysia, in
partial Fulfilment of the Requirements for the Degree of Bachelor of Science
(Hons.) Materials Science
MAY 2018
All material contained within the thesis, including without limitation text, logos, icons,
photographs and all other artwork, is copyright material of Universiti Putra Malaysia
unless otherwise stated. Use may be made of any material contained within the thesis
for non-commercial purposes from the copyright holder. Commercial use of material
may only be made with the express, prior, written permission of Universiti Putra
Malaysia.
Copyright © Universiti Putra Malaysia
I
DEDICATION
I dedicate this research to Almighty God who helps us in accomplishing this thesis and
for giving us wisdom. I also dedicate this to my parents who give financial support and to
my friends for giving us moral support.
II
ABSTRACT
Synthesis and Characterisation of Multiwalled Carbon Nanotubes Composite based on
Mill Scale and Boron Trioxide via Chemical Vapor Deposition
by
Lee Shian Boon
178654
MAY 2018
Supervisor: Dr. Raba’ah Syahidah binti Azis
Faculty: Faculty of Science
In this work, multiwalled carbon nanotubes (MWNT) composite has been synthesized by
chemical vapor deposition (CVD) technique. Mill scales and boron trioxide were used as
catalytic substrate. The CVD product was characterized by x-ray diffraction (XRD),
thermogravimetry analysis (TGA), transmission electron microscopy (TEM) and
electromagnetic absorption analysis using vector network analyser (VNA). Typical XRD
peak of (002) phase of MNNTs and distinct nanotube structures revealed by TEM
confirms MWNT presence. For microwave properties, it was observed from 8 GHz to 12
GHz and at 3 mm thickness, composite MWNT/mill scale has the maximum reflection
loss (RL = -28.26 dB) at 9.20 GHz.
III
ABSTRAK
Sintesis dan Pencirian Komposit Nanotiub Karbon Berbilang Dinding berdasarkan Sisik
Besi dan Boron Trioxide melalui Kaedah Pemendapan Wap Kimia
Oleh
Lee Shian Boon
178654
MEI 2018
Penyelia: Dr. Raba’ah Syahidah binti Azis
Fakulti: Fakulti Sains
Dalam kajian ini, pelbagai komposit nanotiub karbon berbilang dinding (MWNT) telah
disintesis melalui kaedah pemendapan wap kimia. Sisik besi dan boron trioksida
digunakan sebagai substrat pemangkin. Produck CVD ini dicirikan menggunakan difraksi
sinar-X (XRD), mikroskop transmisi electron (TEM), analisis termogravimetrik (TGA)
dan analisis penyerapan electromagnet (VNA). Puncak XRD (002) fasa MNNTs yang
biasa dijumpai dan struktur nanotube yang ditunjukkan oleh TEM mengesahkan
kehadiran MWNT. Bagi ciri-ciri gelombang mikro, ia diperhatikan dari 8 GHz hingga 12
GHz dan ketebalan pada 3 mm, komposit sisik besi/MWNT mempunyai kehilangan
pantulan maksimum (RL = -28.26 dB) pada 9.20 GHz.
IV
ACKNOWLEDGEMENT
The researcher would like to thank the following people who helped to make this research
possible. To his supervisor, Dr. Raba’ah Syahidah binti Azis and co-supervisor, Dr.
Ismayadi bin Ismail who patiently taught him everything he needs to know. Thank you I
appreciate from the bottom of my heart. Also, to Dr. Muhammad Syazwan Mustaffa for
always making sure that all questions or requests answered. The researcher appreciates
everything you have done. And now the researcher would like to thank all the people who
supported throughout this research, their families, friends, and classmates.
V
APPROVAL
This thesis entitled “Synthesis and Characterisation of Multiwalled Carbon Nanotubes
(MWNT) Composite based on Mill Scale and Boron Trioxide via Chemical Vapor
Deposition (CVD)” by Lee Shian Boon (Matric No.: 178654), was submitted to the
Department of Physics, Faculty of Science, Universiti Putra Malaysia and has been
accepted as partial fulfilment of the requirement for the degree of Bachelor of Science
(Hons.) Major in Materials Science.
Approved by,
Date: ………………………………
Dr. Raba’ah Syahidah binti Azis
Project Supervisor
Department of Physics
Faculty of Science
Universiti Putra Malaysia
Date: …………………………………
Assoc. Prof. Dr. Chen Soo Kien
Course Coordinator
Department of Physics
Faculty of Science
Universiti Putra Malaysia
Date: …………………………………….
Assoc. Prof. Dr. Zulkifly Abbas
Head of Department
Department of Physics
Faculty of Science
Universiti Putra Malaysia
VI
DECLARATION
Declaration by student
I hereby confirm that:
• this thesis is my original work;
• quotations, illustrations and citations have been duly referenced;
• this thesis has not been submitted previously or concurrently for any other degree at any
other institutions;
• intellectual property from the thesis and copyright of thesis are fully-owned by Universiti
Putra Malaysia, as according to the Universiti Putra Malaysia (Research) Rules 2012;
• written permission must be obtained from supervisor and the office of Deputy Vice-
Chancellor (Research and Innovation) before thesis is published (in the form of written,
printed or in electronic form) including books, journals, modules, proceedings, popular
writings, seminar papers, manuscripts, posters, reports, lecture notes, learning modules or
any other materials as stated in the Universiti Putra Malaysia (Research) Rules 2012;
• there is no plagiarism or data falsification/fabrication in the thesis, and scholarly integrity
is upheld as according to the Universiti Putra Malaysia (Research) Rules 2012.
Signature: _______________________ Date: __________________
Name and Matric No.: _________________________________________
VII
TABLE OF CONTENT
Page
ABSTRACT II
ABSTRAK III
ACKNOWLEDGEMENTS IV
APPROVAL V
DECLARATION VI
LIST OF FIGURES IX
LIST OF TABLES X
LIST OF ABBREVIATIONS XI
CHAPTER 1 INTRODUCTION
1.1 Radar Absorbing Materials 1
1.2 Historical Overview 1
1.3 Problem Statement 3
1.4 Research Overview 4
1.5 Objectives 5
CHAPTER 2 LITERATURE REVIEW
2.1 Multi-wall Carbon Nanotube (MWNT) 6
2.2 Mill Scale 7
2.3 Chemical Vapor Deposition 8
2.4 Boron Oxide 10
2.5 Electromagnetic Absorption 11
CHAPTER 3 METHODOLOGY
3.1 Overview 12
3.2 Preparation of Sample 14
3.2.1 Mill Scale 14
3.2.2 Boron Trioxide 15
3.2.3 MWNT 15
3.2.4 Composite Samples 17
3.3 Characterizations 20
3.3.1 X-ray Diffraction 20
3.3.2 TEM 21
3.3.3 TGA 23
3.3.4 VNA 24
CHAPTER 4 RESULTS AND DISCUSSIONS
4.1 Physical Analysis 28
4.1.1 X-ray Diffraction 28
4.1.2 TEM 31
4.1.3 TGA 35
4.2 Absorption Analysis 38
VIII
CHAPTER 5 CONCLUSIONS
5.1 Conclusions 42
5.2 Suggestions 43
BIBLIOGRAPHY 44
VITAE 48
IX
LIST OF FIGURES
Figure Description Page
2.1 Typical TEM images of MWNT sample (Purohit et al. 2014) 7
2.2 X-ray diffraction patterns for reduced mill scale by coke (Lo et al.,
2012)
8
2.3 Crystal structure of boron trioxide (G. E. Gurr, 1968) 10
3.1 Methodology flowchart 13
3.2 SPEX SamplePrep 8000D Dual Mixer/Mill 15
3.3 Ethanol liquid contained in a boiling flask channeled to the
furnace.
16
3.4 CVD product. 17
3.5 Aluminum mold specialized for VNA measurement. 18
3.6 X-ray Diffractometry 20
3.7 Sonicated MWNT dispersion in acetone. 22
3.8 JEM-2100F field emission electron microscope. 23
3.9 Flowchart of forming composite. 25
3.10 Vector Network Analyser. 26
4.1 XRD diagram of (a) mill scale, (b) sample A, (c) sample B and (d)
boron trioxide.
30
4.2 TEM images of sample A. 33
4.3 TEM images of sample B. 35
4.4 TGA and DTG curves of (a) mill scale, (b) boron trioxide and (c)
sample A.
37
4.5 Reflection loss versus frequency graph for (a) sample A, J and K;
(b) sample B, C and H.
41
X
LIST OF TABLES
Table Description Page
3.1 Parameters of HEBM. 14
3.2 Powder substrate weight ratio (B2O3 : Mill scale). 17
3.3 Weight ratio of sample to be incorporated into mold. 19
3.4 Composite weight ratio (resin : hardener : filler). 24
4.1 Weight change of CVD products. 27
4.2 Summary for composite mold composition. 38
4.3 Conversion of reflection loss. 39
XI
LIST OF ABBREVIATIONS
CNT Carbon nanotubes
CVD Chemical vapor deposition
MWNT Multi walled carbon nanotubes
RCS Radar cross section
RL Reflection loss
SEA Shielding effectiveness
SWNT Single walled carbon nanotubes
TEM Transmission electron microscopy
TGA Thermogravimetric analysis
VNA Vector Network Analyzer
XRD X-ray powder diffraction
1
CHAPTER 1
INTRODUCTION
1.1 Radar Absorbing Materials
Radar absorbing materials (RAM), in stealth technology or low observable technology,
are a part of electronic countermeasure. RAM absorb radar in various amounts depending
on atmospheric conditions and different frequencies. They are applied on outer surfaces
of military hardware like aircrafts, ships and missiles to deceive enemy’s radar. Together
with proper craft’s shaping design which, RAM makes military hardware less detectable
by reducing the radar cross section (RCS) and hence radar signature of a craft or a missile.
(Saville, 2005)
On the other hand, RAM is integral in measurements of electromagnetic compatibility
(EMC) and antenna radiation patterns. Test setup in such measurements unavoidably emit
unwanted signals which could cause measurement errors and ambiguities. Hence, arrays
of RAM made from lossy material will cover all internal surfaces of the anechoic chamber
to absorb incident RF radiation. Commonly, RAM in this setting comprise rubberized
foam material impregnated with controlled mixtures of carbon and iron oxide. (Gaylor,
1989)
1.2 Historical Overview
RADAR is an acronym for radio detection and ranging. It uses electromagnetic waves at
sub-optical (400 THz) frequencies to detect position or velocity of an object. Transmitted
2
from an antenna, these electromagnetic waves travelled radially outwards. Eventually,
they reflect off some distant object and return an echo to the sender, where they are
received, amplified, and processed electronically to yield an image showing the object’s
location. Object’s travelling velocity is determined by the shifted frequency due to
Doppler’s effect. (Bole et al., 2014)
RADAR started with the theoretical work of James Clerk Maxwell, followed by Heinrich
Hertz, who did all the experimental work to understand the nature of electromagnetic
waves. (Rohling, 2010). Initially developed as a mean to detect proximity object and
prevent ship collision due to poor visibility, RADAR had found itself a significant role as
navigation and tracking devices. Its application in navigation and tracking of craft at sea
and in the air, has seen rapid advancement since the World War II. It was developed
independently by researchers in the United Kingdom, France, Germany, Italy, Japan, the
Netherlands, the Soviet Union, and the United States. (Wolters et al., 2002)
In 1943, Allied RADAR was deemed to pose imminent threat to U-boats by the
Oberkommando der Marine (Nazi Germany’s Naval High Command) and a conference
was called in Berlin. The secret project to come up with a successful countermeasure was
named Schornsteinfeger (Chimney Sweep). Schornsteinfeger was an adsorbent coating
made from ferrites that, when applied to a submarine’s hull and snorkel mast, insulated
the steel and greatly reduced the reflected electronic signal in the 20-cm radar band the
Allies used. (Primus et al., 1991)
3
Commonly known types of RAM are iron ball paint, Jaumann absorber, Split-ring
resonator absorber. Research of CNT incorporation in RAM is currently undergoing. Iron
ball paint contains tiny spheres coated with carbonyl iron or ferrite. Radar waves induce
molecular oscillations from the alternating magnetic field in this paint, which leads to
conversion of the radar energy into heat. The heat is then transferred to the aircraft and
dissipated. The iron particles in the paint are obtained by decomposition of iron
pentacarbonyl. Lockheed F-117 Nighthawk utilises iron ball paint. The carbonyl iron ball
paint is most effective when the balls are evenly dispersed, electrically isolated, and
present a gradient of progressively greater density to the incoming radar waves. (Patil,
2008)
A Jaumann absorber or Jaumann layer is a radar-absorbent substance. (Gaylor, 1989)
When first introduced in 1943, the Jaumann layer consisted of two equally spaced
reflective surfaces and a conductive ground plane. One can think of it as a generalized,
multi-layered Salisbury screen, as the principles are similar. Being a resonant absorber, it
uses wave interfering to cancel the reflected wave. Because the wave can resonate at two
frequencies, the Jaumann layer produces two absorption maxima across a band of
wavelengths (if using the two layers configuration). (Primus et al., 1991)
1.3 Problem Statement
Being frequency selective, RAM never gives high absorption across whole RADAR
frequency range and can never be assumed to decrease an aircraft’s RCS values to a
considerable extent. It can absorb a portion of the incident energy, with the rest being
4
reflected. Although fighter-sized stealth aircraft could be detected by low-frequency radar,
missile lock and targeting sensors primarily operate in the X-band.
A solid waste, known as mill scale, is generated by the oxidation of the metal surface
during the continuous casting and rolling steps in an integrated steel plant. (Cristina et al.,
2015) Being produced approximately 35–40 kg per ton of produced steel, mill scale
potentially serves as catalyst in multi walled carbon nanotube (MWNT) production. (Sun
et al., 2013)
In 2018, Kolanowska et al. stated that, in the stealth technology, composites exhibiting
appreciable electrical conductivity and high shielding effectiveness (SEA) are used. While
doped boron provides conduction carriers, it further reduces the resistivity of the MWNT.
(Ishii et al., 2011)
Hence, in this study, samples with different composition of mill scale and boron trioxide
are synthesized and their microwave absorption properties at X-band are studied.
1.4 Research Overview
The electromagnetic absorption performance of MWNT composite filler is greatly
affected by weight ratio of MWNT in MWNT composite. By altering the weight ratio, we
can produce fillers with different reflection loss across the frequency spectrum. Therefore,
investigations on the material with different MWNT samples with altered catalyst
composition will give us a valuable information in achieving low reflection loss, wide
bandwidth RAM.
5
In this research, MWNT is synthesised by CVD. After the CVD process, MWNT’s
physical properties will be studied. Then, composite material samples are prepared, and
their electromagnetic performance is probed.
1.5 Objectives
1. To synthesise MWNT via CVD method using substrate of mill scale and boron
trioxide.
2. To characterise the structural and the physical properties of MWNT samples.
3. To study the electromagnetic performance of MWNT samples.
6
CHAPTER 2
LITERATURE REVIEW
2.1 Multi-wall Carbon Nanotube (MWNT)
Carbon nanotubes are members of the carbon family with significant mechanical and
electrical properties. They can be viewed as a graphene sheet rolled up into a nanoscale
tube form to produce the single walled carbon nanotubes (SWNT). There may be
additional graphene tubes around the core of a SWNT to form multi-walled carbon
nanotubes (MWNTs). These elongated nanotubes usually have a diameter in the range of
a few angstroms to tens of nanometres and a length of several micrometres up to
millimetres with both ends of the tubes normally capped by fullerene-like structures
containing pentagons. (Schrand & Tolle, 2006)
CNT/epoxy composite coatings have been used as high radar transparency or radar
absorbing coatings to disguise or reflect objects from an enemy's night-vision equipment.
Alternatively, SWNT/epoxy mixtures can be used as sprays or sizing agents for other
composite materials to enhance the interphase/interface between reinforcements and the
matrix. (Oh et al., 2004) Composite materials reinforced by multi wall carbon nanotubes
(MWNT) as shown in Figure 2.1 are studied for their interesting microwave absorption
capability. (Micheli et al., 2014)
7
Figure 2.1: Typical TEM images of MWNT sample (Purohit et al., 2014)
2.2 Mill Scale
Mill scale is a waste from the oxidation of steel surfaces in the steelmaking process like
continuous casting and rolling steps. (Cristina et al. 2015) The separation of steel mill
scale is performed using either mechanical or chemical means. Mill scale comprises iron
oxides, such as wustite (FeO), magnetite (Fe3O4), and hematite (Fe2O3), besides traces of
non-ferrous metals, compounds of alkali metals, and oils from the rolling process. On
average, specific production of mill scale is 40 kg per ton of produced steel. (Lo et al.,
2012)
8
Figure 2.2: X-ray diffraction patterns for reduced mill scale by coke (Lo et al.,
2012)
Figure 2.2 shows the X-ray diffraction patterns for reduced mill scale similar to the mill
scale used in this research. The method of X-ray diffraction is effective in monitoring the
crystallinity of CNTs and of metallic nanoparticles. The technique has also been used for
estimating an average size of the nanoparticles and the diameter distribution of nanotubes.
(Lo et al., 2012)
2.3 Chemical Vapour Deposition
There are various techniques used for growth of CNTs. Three popular methods are arc
discharge, laser ablation, & chemical vapor deposition (CVD). The common characteristic
of these techniques is to provide energy to a carbon source for the creation of Carbon
atoms that generate CNTs. (Purohit et al., 2014)
9
In chemical vapour deposition, the decomposition of the carbon precursor and CNT
formation take place on the surface of catalyst particles. CNT size can be controlled by
varying the size of catalyst particles. Commonly used carbon sources are carbon
monoxide, methane, acetylene and ethanol. Carbon source is taken in the gas phase and
resistively heated coil imparts energy to gaseous carbon molecules. (Purohit et al., 2014)
CNT growth in CVD is affected by three main parameters which are the hydrocarbon,
catalyst and growth temperature. The yield of CNT’s synthesized by chemical vapour
deposition is greatly influenced by the growth parameters like catalyst particle size,
catalyst concentration, pressure, growth time, growth temperature and gas flow rate.
Growth mechanisms of nanotube are different for different methods. Researchers have
reported various mechanisms, such as vapor-solid-solid (VSS), vapor-liquid-solid (VLS).
(Purohit et al., 2014)
Chemical vapour deposition (CVD) is a unit equipped with a horizontal tube furnace; a
quartz tube was used as a reactor and installed in the furnace. Using a crucible boat to hold
the metal catalyst, CVD process sources its mixture of hydrocarbon and argon gas from a
duct. Ethanol is heated up to its boiling temperature and its vapor is channeled into the
furnace when furnace temperature is at its pre-set temperature.
After carbon deposition of 20 minutes, reactor is cooled to room temperature under argon
gas. After that, the products can be collected. The morphology, length and diameter of the
10
CNTs produced were determined by transmission electron microscopy (TEM). (Iyuke &
Danna, 2005)
2.4 Boron Oxide
Boron trioxide is an odourless stable white powder. Being in vitreous form usually, boron
trioxide crystallizes under extensive annealing. Kracek et al. in 1938 found B2O3 to form
as a fine powder during dehydration of meta-boric acid under carefully controlled
conditions. Each boron is triangularly surrounded by 3 oxygen atoms with a B-O distance
of 1.37 Å as shown in Figure 2.3. It melts at 450 ⁰C.
Figure 2.3: Crystal structure of boron trioxide (G. E. Gurr, 1968)
11
2.5 Electromagnetic Absorption
Upon leaving a transmitting antenna, a radio wave propagates in a widening beam at the
speed of light (> 186,000 miles per hour or 3 × 108 m/sec). It continues propagating until
it encounters an obstacle, a medium whose characteristic impedance differs from that of
air and vacuum (> 377 Ω), it splits into two parts. One part passes into the obstacle and is
generally absorbed, and the other is reflected. Shape of the obstacle dictates the
propagation direction of reflected wave. Roundish or irregular obstacles tend to scatter
energy through a wide angle, while flat or facet-like surfaces tend to send it off in a single
direction, just as a flat mirror reflects light. If any part of the outgoing wave is reflected at
180° (which is not guaranteed) it will return to the transmitter. This returned or
backscattered signal is usually detected by the same antenna that sent the outgoing pulse;
this antenna alternates rapidly between transmitting pulses and listening for echoes, thus
building a real-time picture of the reflecting targets in range of its beam. Electromagnetic
radiation power is attenuated due to reflection, absorption and/or multiple reflection.
(Patil, 2008)
12
CHAPTER 3
METHODOLOGY
3.1 Overview
This chapter describes the mechanism used to grow MWNT via chemical vapor deposition
(CVD) procedures and the preparation of composite samples. The study will comprise
characterization techniques like X-ray diffraction (XRD), Thermogravimetric analysis
(TGA), transmission electron microscopy (TEM) and PNA Vector Network Analyzer
(VNA).
For CVD process, 5 powder substrates with different weight ratio of boron trioxide to mill
scale are prepared by weighing powders and mixing them accordingly. MWNT synthesis
for each sample runs separately. Before introducing hydrocarbon into the horizontal
furnace, the crucible containing 0.5 g of catalytic substrate is heated gradually to 750 ⁰C
under steady flow of argon gas. Upon furnace temperature reaching the 750 ⁰C mark,
ethanol vapor heated in a boiling flask is channeled into the furnace alongside with argon
gas for 20 minutes. Finally, the ethanol vapor valve is closed, and argon continues flowing
until furnace temperature drops to room temperature. The physical properties and
electromagnetic absorptive ability of samples were examined under several
characterization processes. The methodology flowchart is briefly illustrated in Figure 3.1.
13
Figure 3.1: Methodology flowchart
XRDA
BMill scae
Boron trioxide
TGA
A
Mill scale
Boron trioxide TEM
A B
Resin Composite
(VNA)A
B
C H
J
K
A
B
C
D
E
Weighing
Mill Scale
Boron Trioxide
CVD process
Synthesis process
Characterization
14
3.2 Preparation of Sample
3.2.1 Mill Scale
High energy ball milling (HEBM) prepares mill scale using parameters as stated in Table
3.1. Milling machine is shown in Figure 3.2 and it is used for substrate powder mixing as
well.
Table 3.1: Parameters of HEBM.
Parameter Values
Rotation per minutes 1760
Ball to powder ratio 10:1
Ball milling time (hrs) 7
The average particle size of iron oxide milled at 6 hours obtained is 53.76 nm. High energy
imparted from the collision of milling media to the iron oxide particles causes the size of
iron oxide nanoparticles decreased. (Suhada et al., 2017)
The decomposition of the carbon precursor & CNT formation take place on the surface of
catalyst particles. Hence, manipulation of catalyst particle size affects the morphology of
MWNT. (Purohit et al., 2014)
15
Figure 3.2: SPEX SamplePrep 8000D Dual Mixer/Mill
3.2.2 Boron Trioxide
Boron trioxide in this study passes through a 60-mesh sieve (Alfa Aesar, 97.5%).
3.2.3 Multi-walled Carbon Nanotuubes
Horizontal quartz furnace is cleaned thoroughly and heated up to 900 ⁰C to remove any
moisture as well as carbonaceous particles. Prior to CVD process, catalytic substrate
powder is weighted and distributed evenly on a boat-shaped crucible. The wall edge of
crucible facing the hydrocarbon vapor feed is removed to facilitate the decomposition of
the carbon precursor & CNT formation on substrate. The furnace temperature raises as
resistively heated coil imparts energy to it. It is sealed and argon gas flows steadily through
a stainless steel pipe into the furnace. Argon is an inert gas which acts as carrier gas.
(Adnan et al., 2015) Liquid ethanol is held in a boiling flask and it is boiled to supply
16
carbon source in the gas phase. The setup of apparatus is shown in Figure 3.3. (Purohit et
al., 2014)
Figure 3.3: Ethanol liquid contained in a boiling flask channeled to the furnace.
When furnace reached 750 ⁰C, the carbon source valve is opened, and carbonaceous vapor
is formed in the furnace. At pressures below 10 atm, the decomposition of CH3CH2OH
occurs primarily by the dehydration reaction producing C2H4 + H2O. (Park et al., 2012)
Steady flow of ethanol is maintained for 20 minutes. Finally, the valve is closed, and the
furnace is left to cool down under continuous steady flow of argon gas. The CVD products
are shown in Figure 3.4. Powder substrate weight ratio for respective samples is tabulated
in Table 3.2.
17
Figure 3.4: CVD product.
Table 3.2: Powder substrate weight ratio (B2O3 : Mill scale).
Sample Powder substrate weight ratio (B2O3 : Mill scale)
A 0 : 1.0
B 0.5 : 1.0
C 1.0 : 1.0
D 1.5 : 1.0
E 2.0 : 1.0
3.2.4 Composite Samples
As a second phase, the inorganic fillers can also affect cure exotherms, shrinkage, thermal
and electrical conductivity, machinability, hardness, compressive, flexural, and impact
strength. The extent to which the fillers modify polymer properties is closely associated
with the size, shape and dispersion uniformity of the filler as well as the degree of
interaction between the inorganic filler and the organic matrix. An ideal performance is
18
achieved with inorganic fillers consisting of small particles that are uniformly dispersed
throughout the polymer and interact strongly with the organic matrix.
Substrates were made by proportionating samples according to Table 3.3 using weight
balance. Then, the mixed powder samples are dispersed briefly in the resin system using
magnetic stirrer. Epoxy resins are easily moulded using sturdy aluminium mould
exclusively made for X-band VNA measurement as shown in Figure 3.5. Epoxy resins
have moderate strength, and low hardness. Resin system used consist of two parts, a resin
and a hardener, which are mixed and cured at elevated temperatures of 60 ⁰C. They are
QUICKMOUNT 2 Fast Epoxy Resin Model - ERF-3000-32 and Hardener Model - EHF-
3000-08. The ratio of epoxy to hardener is 17:1 while the filler loading level is carefully
maintained at 10 %. The loading level influence the material’s intensive properties such
as permeability and permittivity.
Figure 3.5: Aluminum mold specialized for VNA measurement.
19
Table 3.3: Weight ratio of sample to be incorporated into mold.
Composite Sample Composition
Weight ratio of
powder
Remark
A
As synthesized
CVD product
Mill scale
Comprises
significant amount
of carbon product
B
0.5 B2O3 : 1 Mill
scale
Comprises
moderate amount of
carbon product
C 1 B2O3 : 1 Mill scale
Comprises
insignificant
amount of carbon
product
H Raw mill scale Mill scale
Magnetic dark
brownish metal
oxide without any
carbon product
J Mixing sample A
and raw boron
trioxide
0.5 B2O3 : 1 sample
A
Comprises certain
amount of carbon
product from
sample A K 1 B2O3 : 1 sample A
20
3.3 Characterisations
3.3.1 X-ray Diffraction Analysis
X-ray powder diffraction (XRD) was performed by employing Cu Kα radiation (λ= 1.54
A ˚, 40 kV, 40.0 mA). All samples were scanned in 2θ = 20⁰ to 80⁰ at a rate of 2⁰ per
minute. The bulk chemical compositional analysis of mill scale is normally carried out
using X-ray. The structural and phase analysis of the samples were performed using X-
ray diffractometer (Philips PW 3040/60 X'pert Pro) as shown in Figure 3.6 with CuKα
radiation (wavelength of 1.5405 Å). Phase identification of the samples was performed
using X’Pert Highscore software with the support of ICDD-PDF-2 database.
Figure 3.6: X-ray Diffractometry
Diffraction occurs as waves interact with a regular structure whose repeating distance is
about the same as the wavelength. X-Rays have wavelengths on the order of a few
angstroms, the same as typical interatomic distances in crystalline solids, and can be
diffracted from crystalline solids that have regular repeating atomic structures. When
21
certain geometric conditions like X-Ray constructive interference occurrence are met, X-
rays diffract into beams.
XRD provides effective, fast and non-destructive technique to study the average structural
properties of MWNTs. All possible orientations are hit by the X-ray incident beam when
nanotubes are probed. This leads to powder-like diffraction profiles which give statistical
characterization of MWNTs. Thus, this shows that XRD is not adopted to study isolated
nanotubes.
The XRD pattern of a MWNT exhibits graphite-like peak (002) and a family of smaller
peaks also attributable to graphene crystals ((100), (101), (102), (004), (110), and (112)),
closely resembling that of a well-crystallized graphitic material. However, the XRD
profile is incapable of differentiating the microstructural details in the nanotubes samples.
The presence of impurities such as catalyst particles will be evidenced by additional peaks
that can be assigned to respective planes.
3.3.2 TEM
Transmission electron microscopy (TEM) is a very useful technique for studying in detail
the structure MWNTs, including defects present in nanotubes. TEM provides information
on the structure of internal and external closures, on the nature of defects, and on how the
tubes are stacked. TEM samples are simply dispersed in acetone and a copper grid will be
dipped into the resulting dispersion. The dilute nanotube slurry was subjected to ultrasonic
dispersion for approximately 5 minutes to reduce agglomeration as shown in Figure 3.7.
22
Figure 3.7: Sonicated MWNT dispersion in acetone.
A JEM-2100F field emission electron microscope (Figure 3.8) was operated at 200kV
accelerating voltage. An advantage of this microscope is its potential for low current
density with a highly coherent beam. Low accelerating voltage avoid damage of the
MWNTs. The FE electron gun (FEG) produces highly stable and bright electron probe
that is never achieved with conventional thermionic electron gun. This feature is essential
for ultrahigh resolution in scanning transmission microscopy and in an analysis of a nano-
scaled sample. The diameter of nanotubes can be obtained directly from the electron
microscopy images in real space.
23
Figure 3.8: JEM-2100F field emission electron microscope.
3.3.3 TGA
Thermal properties of CNT are investigated using thermogravimetric analysis (TGA).
TGA was performed to determine CNTs yield. Mettler Toledo TGA/DSC 1 is used.
Thermogravimetric analysis is a thermal analysis technique in which the percent weight
loss of a sample is recorded while the sample is being heated at a uniform rate in an
appropriate environment. The weight loss over specific temperature range reveals
information of the sample composition (including volatiles and inert fillers) and it thermal
stability. Conventionally, TGA for MWNTs is done under air (thermal decomposition).
24
Oxidation temperature is indicated by a peak of the temperature derivative of the weight
𝑑𝑚/𝑑𝑇. During the thermal oxidation of the samples in ambient air, the sample was
heated from 50 to 1000 °C at 10 °C/min in order to determine the weight loss.
3.3.4 VNA
The manufacturing of composite material samples enables computing of the relative
permittivity as function of the frequency of the applied EM field. Electromagnetic and
microwave absorbing properties of the prepared samples were performed using N5227A
PNA Network Analyser (Figure 3.10). The Agilent Technologies 85,071 Materials
Measurement software streamlined the process of measuring complex permittivity and
permeability with VNA in the frequency range of 8-12 GHz.
Table 3.4: Composite weight ratio (resin : hardener : filler).
Composite
weight, g
Composite weight ratio
(resin : hardener : filler)
Resin
weight, g
Hardener
weight, g
Filler
weight, g
1.200 17 : 1 : 2 1.0200 0.0600 0.1200
25
Figure 3.9: Flowchart of forming composite.
Two port calibration is done using ECal calibration which uses four impedance states to
compute the VNA’s systematic error terms to reduce calibration errors. Calibration is
essential to make accurate measurements. The reflection loss of electromagnetic wave
comprises frequency, intensity, and bandwidth of absorption peaks. By measuring the
Resin and hardener is mixed
Filler is added to resin system.
Then, they are mixed homogenously.
After that, it is placed into different sample holders.
The mixture is dried in an oven at 70⁰C for 24 hours for the solidification of the
epoxy.
VNA: Line transmission theory
to characterize microwave absorbing properties.
26
reflection coefficient of the composite backed with a metal plate, the data of reflection
loss peak is measured.
Retrieval of the dielectric parameters was obtained by measuring the microwave scattering
parameters Sij (S11, S21, S12, S22) by means of a vector network analyser (AGILENT, PNA-
L N5235) and a coaxial transmission line. The composite specimens for the measurement
of the microwave absorber properties were prepared by mixing according to Table 3.4.
The filler and the epoxy resin were mixed homogenously using magnetic stirrer, and the
mixture were placed into different sample holders. The mixture was dried in an oven at
70oC for 24 h for the solidification of the epoxy. The flowchart is shown in Figure 3.9.
Figure 3.10: Vector Network Analyser.
27
CHAPTER 4
RESULTS AND DISCUSSION
In the CVD process, CNT growth depends on the carbon source, synthesis temperature
and catalyst. In most cases, the catalyst chosen is of iron, cobalt or nickel based. Generally,
low-temperature CVD (600-900 °C) yields MWNTs. (Purohit et al. 2014) Experiments
were carried out at different weight ratio ranging from 0:1 to 2:1. MWNT growth is
monitored by the weight change of the sample after ethanol decomposition expressed by
the equation
𝑤 (%) = [𝑚𝑓 − 𝑚𝑜
𝑚𝑜] ∗ 100,
where 𝑚𝑜 is the initial weight of substrate before introducing the ethanol into the reactor
and 𝑚𝑓 is the weight of final CVD product.
Table 4.1: Weight change of CVD products.
Sample
Powder substrate
weight, 𝑚𝑜 (g)
CVD product
weight, 𝑚𝑓 (g) Weight change, w
(g)
Weight change,
w (%)
A 0.5074 1.7142 1.2068 237.8
B 0.5023 0.5415 0.0392 7.804
C 0.5016 0.3577 -0.1439 -28.69
D 0.4987 0.2321 -0.2666 -53.46
E 0.5089 0.2038 -0.3051 -59.95
For sample A, the powder substrate is basically mill scale, the weight change is 237.8 %.
This indicates good MWNT growth. However, the yield decreases for sample B (7.804
%), in which boron trioxide has been introduced one third in weight of the powder
28
substrate. Subsequent increase of (B2O3 : mill scale) ratio (1.0 : 1.0, 1.5 :1.0 and 2.0 : 1.0)
leads to negative weight growth (-28.69 %, -53.46 % and -59.95 %) as tabulated in Table
4.1.
This might be explained by the low melting temperature of boron trioxide(450°C). At 750
°C, the boron trioxide constituent has certainly melted and might be engulfing other solid
mill scale substrate, inhibiting decomposition of vapor hydrocarbon on the catalyst.
Catalyst acts as the nucleation site for the nanotubes to grow and further introduction of
boron trioxide has overwhelmed any possible MWNT synthesis.
4.1 Physical Analysis
4.1.1 X-ray Diffraction
Figure 4.1 shows the X-ray diffractograms (XRD) for 4 samples (A, B, H and Boron
Trioxide). For raw mill scale Figure 4.1 (a), the major diffraction peaks of magnetite
(Fe3O4), hematite (Fe2O3) and wüstite (FeO) are observed and can be indexed. Peaks at
30.1⁰ and 35.5⁰ can be assigned to (022) and (113) crystalline plane diffraction peaks
respectively which coincides with cubic magnetite. Peaks at 33.2⁰ is assigned to (104)
crystalline plane diffraction peak of hexagonal hematite. Peaks at 36.5⁰, 42.1⁰ and 61.1⁰
can be assigned to (111), (002) and (022) crystalline plane diffraction peaks respectively
which belong to wüstite.
For sample A shown in Figure 4.1 (b), the MWNTs showed a typical peak of (002) phase
of MWNTs or graphite at 26.4⁰. Peaks at 42.8⁰ and 44.6⁰ can be assigned to (100) and
29
(101) crystalline plane diffraction peaks, respectively and show the presence of MWNTs.
XRD studies demonstrated well-crystallized structure of the MWNT composite.
The XRD pattern of sample B is shown in Figure 4.1 (c). However, the graphite-like (002)
peak is somewhat weakened with a downward shift (2θ = 26.2⁰). The peaks at 42.9⁰ and
43.7⁰ can be assigned to (100) and (101) planes of MWNTs.
The XRD spectrum of boron trioxide is shown in Figure 4.1 (d). The two peaks (2θ =
28.0⁰ and 2θ = 40.3⁰) in the spectrum can be indexed as (3 1 0) and (4 2 0) crystal planes
of the cubic structure B2O3 respectively, not only in the peaks’ positions, but also in their
relative intensity. (Yang et al., 2005)
30
Figure 4.1: XRD diagram of (a) mill scale, (b) sample A, (c) sample B and (d)
boron trioxide.
20 25 30 35 40 45 50 55 60 65 70 75 80
Inte
nsi
ty (
a.u
.)
2θ (°)
(d)
(c)
(b)
(a)
(002)
(002)
(100)
(100)
(101)
(101)
(310)
(420)
MWNT
Magnetite (Fe3O4)
Hematite (Fe2O3)
Wüstite (FeO)
Boron trioxide
(022)
(104)
(113)
(111)
(002)
(022)
31
4.1.2 TEM
Figure 4.2 displays representative TEM images of the fabricated MWNTs. Figure 4.2 (a)
contains a micrograph of sample A, showing MWNTs with diameters as small as 55.76
nm and as big as 64.63 nm. Most of MWNTs exhibited bamboo-type morphology and
rough nanotube surfaces. In Figure 4.2 (b), the inner diameter is 9.21 nm and outer
diameter is 55.58 nm. The outer region of multiple essentially continuous layers of ordered
atoms is observed and a distinct inner core is visible, almost coaxial yet tampered with
kinks at several sites. Besides, multiple nanoparticles with different shapes and sizes are
isolated and contained as shown in the circled region. In Figure 4.2 (c), it is clearly seen a
MWNT with an inner diameter of 10.83 nm and an outer diameter of 25.48 nm. The
bamboo morphology is easily observed along the inner core. There is a nanoparticle-sized
lump believed to be mill scale encapsulated along a distinct nanotube below. It is probable
that the different MWNTs morphologies found in samples could be attributed to the
catalytic metallic nanoparticles. The final shape of these nanoparticles could give us
information about the process involved in the growth of different MWNTs observed.
Encapsulated nanoparticles with irregular morphologies distort the MWNTs structure and
form junctions (circled parts).
32
(a)
(b)
33
(c)
Figure 4.2: TEM images of sample A.
In Figure 4.3 (a), the outer tube diameters depicted are 170.36 nm, 211.49 nm and
160.43 nm which are larger than that of sample A. Sample B is synthesized by
substrate comprising mill scale and boron trioxide via CVD method. The lump seen
in Figure 4.3 (b) is excessively larger than 300 nm which might explain the lack of
distinct nanotubes. The catalyst particle size has been found to dictate the nanotube
diameter. Figure 4.3 (c) exhibits twisted nanotubes with multi collapsed sites and
lumps of varied sizes.
34
(a)
(b)
35
(c)
Figure 4.3: TEM images of sample B.
4.1.3 TGA
To analyse the thermal stability and reactivity of samples, thermo gravimetric analysis
(TGA) is carried out under oxygen atmosphere. The samples were analysed using a rate
heating of 10 ⁰C/min. Figure 4.4 shows the degradation curve for each sample, plotted as
loss mass (%) versus temperature.
No observable weight loss for mill scale as depicted in Figure 4.4 (a). Residual mass is
same with initial mass. This is anticipated as mill scale has been sintered at elevated
temperature during powder preparation. Any water or gases has been liberated
beforehand, rendering it stability throughout the thermal analysis.
36
Boron trioxide curve in Figure 4.4 (b) has the maximum value of the first loss mass
derivative 132.23 ⁰C. The degradation temperature is obtained from the maximum value
of the value of the first loss mass derivative. The mass loss halted at 447.63 ⁰C. By the
end of the thermal analysis, 68.08 % of initial sample is left. The residue is amorphous
boron trioxide. The weight liberated is believed to be water and impurities.
For sample A curve in Figure 4.4 (c), the degradation temperature is found to be 872.55
⁰C, where residual mass is 64.42 %. The mass starts to drop at 809.91 ⁰C and continues to
drop beyond 1000 ⁰C. The residual ash content observed at the final degradation
temperature can be attributed to the mill scale. The derivation curve shows a shoulder at
849.42 ⁰C before starting to plummet to the head at -0.00087 s-1 and eventually levelling
to -0.00021 s-1 like that of during at the shoulder earlier.
(a)
37
(b)
(c)
Figure 4.4: TGA and DTG curves of (a) mill scale, (b) boron trioxide and (c)
sample A.
38
4.2 Absorption Analysis
All composite samples are successfully prepared using resin system according to Table
4.2. Electromagnetic and microwave properties of the prepared samples were performed
using Agilent Technologies N5227A PNA Network Analyzer. 85071 Material
Measurement Software E07.02.29 automates complex permittivity and permeability
measurements. Measurements are taken within X-band (8-12 GHz).
Table 4.2: Summary for composite mold composition.
Sample Composition Remark
Weight ratio (B2O3 : Mill scale)
CVD product A 0 : 1.0
B 0.5 : 1.0
C 1.0 : 1.0 Weight ratio (Mill scale)
Raw mill scale without
undergoing CVD H 1.0
Weight ratio (B2O3 : A)
Mixing CVD product A
with unreacted raw B2O3
J 0.5 : 1.0
K 1.0 : 1.0
The log of the ratio of two powers P1 and P0, is measured in Bels:
1 𝐵𝑒𝑙𝑠 = 10 𝑑𝐵
and
𝑟𝑎𝑡𝑖𝑜 (𝑑𝐵) = 10 × 𝑙𝑜𝑔𝑃1
𝑃0= 20 × log Γ
where Γ is the reflection coefficient and
39
Γ = 𝑉𝑜𝑙𝑡𝑎𝑔𝑒1
𝑉𝑜𝑙𝑡𝑎𝑔𝑒0
Radar cross section (RCS) reduction is given by:
RCS = 1 − Γ2
The conversion of several reflection loss value is tabulated in Table 4.3 for convenient
reference.
Table 4.3: Conversion of reflection loss.
RL dB = 20 log Γ Γ = 𝑉𝑜𝑙𝑡𝑎𝑔𝑒1
𝑉𝑜𝑙𝑡𝑎𝑔𝑒0
𝑃𝑜𝑤𝑒𝑟1
𝑃𝑜𝑤𝑒𝑟0 RCS reduction RCS reduction (%)
-1.0000 0.8913 0.7943 0.2057 20.5672
-2.0000 0.7943 0.6310 0.3690 36.9043
-3.0000 0.7079 0.5012 0.4988 49.8813
-4.0000 0.6310 0.3981 0.6019 60.1893
-5.0000 0.5623 0.3162 0.6838 68.3772
-6.0000 0.5012 0.2512 0.7488 74.8811
-7.0000 0.4467 0.1995 0.8005 80.0474
-8.0000 0.3981 0.1585 0.8415 84.1511
-9.0000 0.3548 0.1259 0.8741 87.4107
-10.0000 0.3162 0.1000 0.9000 90.0000
-15.0000 0.1778 0.0316 0.9684 96.8377
-20.0000 0.1000 0.0100 0.9900 99.0000
-25.0000 0.0562 0.0032 0.9968 99.6838
-30.0000 0.0316 0.0010 0.9990 99.9000
-40.0000 0.0100 0.0001 0.9999 99.9900
Figure 4.5 shows the frequency dependence of reflection loss of six composite samples
which describes the electrical behaviour of linear electrical networks when undergoing
various steady state stimuli by electrical signals. Composite samples backed with metal
plate will exhibits quarter-wavelength cancellation through destructive superposition.
40
With similar composite thickness (t = 3mm), the reflection loss (RL) of each composite is
plotted in Figure 4.5 across the X band frequency (8-12 GHz). The RL value of composite
in Figure 4.5 (a) starts small in the low frequency region. It rapidly increases at the high
frequency region before gradually decreases in magnitude. Composite A has the
maximum reflection loss (RL = -28.26 dB) at 9.20 GHz. This translates to 99.85 % of
radar cross section reduction. The maximum reflection loss of composite J (RL = -13.50
dB) occurs at 9.76 GHz. Whereas the RL of composite K does not cross -5 dB mark before
10.46 GHz and attains its maximum reflection loss (RL = -6.74 dB) at 11.22 GHz.
For Figure 4.5 (b), none of the composites attains RL across the -5 dB mark. Composite
H has three RL peaks (RL = -2.18 dB, -4.14 dB and -3.30 dB) at 8.88 GHz, 11.12 GHz
and 11.80 GHz respectively. It is noted that no RL peak is found in composite B curve
across X-band although showing an increasing trend towards higher frequency. Maximum
reflection loss recorded at 12.00 GHz is -2.64 dB. Composite C has a RL peak (RL = -
0.90 dB) at 8.74 GHz and starts levelling off beneath RL = -1.00 dB border.
41
(a)
(b)
Figure 4.5: Reflection loss versus frequency graph for (a) sample A, J and K;
(b) sample B, C and H.
-30.0
-25.0
-20.0
-15.0
-10.0
-5.0
0.08.00E+09 9.00E+09 1.00E+10 1.10E+10 1.20E+10
Ref
lect
ion
loss
(d
B)
Frequency (Hz)
-5
-4
-3
-2
-1
08.00E+09 9.00E+09 1.00E+10 1.10E+10 1.20E+10
Ref
lect
ion
loss
(d
B)
Frequency (Hz)
H
B
C
A
J
K
42
CHAPTER 5
CONCLUSION
5.1 Conclusions
In this study, MWNT composites are synthesized via chemical vapor deposition method
using mill scale and boron trioxide as catalytic substrate. The variation of powder substrate
weight ratio (B2O3 : Mill scale) sees the introduction of boron trioxide annihilating the
MWNT growth due to its unsuitability as a hydrocarbon deposition catalyst. The
mechanism of growth involves hydrocarbon in gas phase dissociation on metal surfaces,
followed by diffusion to the particle surface of the particle where MWNT is precipitated.
Boron trioxide has a lower melting temperature (T = 450 ⁰C) than the CVD reactor
temperature (T = 750 ⁰C). Nevertheless, only sample A obtains a good yield of 237.8 %
of weight change after CVD process. Thus, they are studied using X-ray diffractometry,
transmission electron microscopy, thermogravimetric analysis and electromagnetic
absorption analysis.
All major XRD peaks is assigned to boron trioxide and raw mill scale powder which
contains magnetite, wüstite and hematite. CVD products, Sample A and Sample B, show
distinct (002) graphitic peak which indicates presence of MWNTs. Despite this, the TEM
images reveal that MWNT present contains multiple defects and some encapsulated
nanoparticles are observed along the nanotubes. Composite A starts to degrade at 872.55
⁰C and continues to lose weight beyond the terminal test temperature. While, mill scale
43
has no weight loss in thermal decomposition that runs up to 1000 ⁰C and boron trioxide
suffer a 32 % final weight loss long after its degradation temperature at 132.23 ⁰C.
From 8 GHz to 12 GHz, the microwave absorption studies reveal that composite A has
the maximum reflection loss (RL = -28.26 dB) at 9.20 GHz. It denotes composite A being
the more promising electromagnetic absorptive composite with the highest RL that
translates to 99.85 % of radar cross section reduction. Composite A is produced from CVD
process using only mill scale as its catalytic substrate and vaporized ethanol as its
hydrocarbon source. With the introduction of boron trioxide in CVD process, composite
B and C have insignificant electromagnetic absorption performance in which both
recorded maximum reflection loss not more than -4.5 dB across X-band.
5.2 Suggestions
Across 8-12 GHz, MWNT presence in sample A has distinctly improves the
electromagnetic absorption ability (RLmax = -28.26 dB) as compared to sample H, a raw
mill scale where RLmax does not cross -5.00 dB. Despite that, the introduction of boron
trioxide into the CVD substrate has retarded MWNT growth. Hence, further study could
be directed towards a binary system comprising both MWNT synthesis catalyst. Further
investigation on the Raman spectroscopy of MWNT synthesized in this project could be
done to better understand carbon nanotubes’ structures.
44
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48
VITAE
Lee Shian Boon
Home Address: 1, Jln Bunga Tanjung Satu, 12300 Butterworth, Penang, Malaysia.
Current address: No 4111, Tingkat 1, Blok 30, Kolej Sultan Alaeddin Suleiman
Shah, Universiti Putra Malaysia, 43400 Serdang.
Email: [email protected]
Phone No.: 012-4411962
Education:
University Putra Malaysia
Bachelor Science of Materials Science
CGPA : 3.75
Graduation Year : June 2018
SMJK (C) Chung Ling Butterworth, Penang
STPM
CGPA : 3.25
Graduation Year : December 2013
Personal Particulars:
Age: 24 Years Old Date of Birth: 28-03-1994
Nationality: Malaysian Marital Status: Bachelor
49