Master thesis: Energy harvesting using cnt. Javier Latasa

AKADEMIA GÓRNICZO-HUTNICZA
im. Stanisława Staszica w Krakowie
WYDZIAŁ INŻYNIERII
MECHANICZNEJ I ROBOTYKI
Magisterska praca dyplomowa
Javier Latasa Martínez de Irujo
Imię i nazwisko
Mechatronika (in English)
Kierunek studiów
Energy Harvesting with Carbon Nanotubes
Temat pracy dyplomowej
Prof. dr hab. inż. T. Uhl …………………..
Promotor pracy Ocena, data,
podpis Promotora
Kraków, rok 2013/2014

2
Kraków, dn……………..
Imięi nazwisko: Javier Latasa Martínez de Irujo
Nr albumu: 266083
Kierunek studiów: Mechatronics (in English)
Specjalność:
OŚWIADCZENIE
Świadomy/a odpowiedzialności karnej za poświadczanie nieprawdy oświadczam,
że niniejszą inżynierską pracę dyplomową wykonałem/łam osobiście i samodzielnie oraz
nie korzystałem/łam ze źródeł innych niżwymienione w pracy.
Jednocześnie oświadczam, że dokumentacja praca nie narusza praw autorskich
w rozumieniu ustawy z dnia 4 lutego 1994 roku o prawie autorskim i prawach pokrewnych
(Dz. U. z 2006 r. Nr 90 poz. 631 z późniejszymi zmianami) oraz dóbr osobistych
chronionych prawem cywilnym. Nie zawiera ona równieżdanych i informacji, które
uzyskałem/łam w sposób niedozwolony. Wersja dokumentacji dołączona przeze mnie na
nośniku elektronicznym jest w pełni zgodna z wydrukiem przedstawionym do recenzji.
Zaświadczam także, że niniejsza inżynierska praca dyplomowa nie była wcześniej
podstawążadnej innej urzędowej procedury związanej z nadawaniem dyplomów wyższej
uczelni lub tytułów zawodowych.
………………………………..
podpis dyplomanta

3
Kraków, ……………..
Imięi nazwisko: Javier Latasa Martínez de Irujo
Adres korespondencyjny: Francisco Aleson Kalea 4-5A, Irunea, Nafarroa (Spain)
Temat pracy dyplomowej inżynierskiej: Energy Harvesting with Carbon Nanotubes
Rok ukończenia: 2014
Nr albumu:266083
Kierunek studiów:II rok, II stopień
Profil dyplomowania:
OŚWIADCZENIE
Niniejszym oświadczam, że zachowując moje prawa autorskie , udzielam Akademii
Górniczo-Hutniczej im. S. Staszica w Krakowie nieograniczonej w czasie nieodpłatnej
licencji niewyłącznej do korzystania z przedstawionej dokumentacji inżynierskiej pracy
dyplomowej, w zakresie publicznego udostępniania i rozpowszechniania w wersji
drukowanej i elektronicznej1
.
Publikacja ta może nastąpić po ewentualnym zgłoszeniu do ochrony prawnej
wynalazków, wzorów użytkowych, wzorów przemysłowych będących wynikiem pracy
inżynierskiej2
.
Kraków, ...............… ……………………………..
data podpis dyplomanta
1 Na podstawie Ustawy z dnia 27 lipca 2005 r. Prawo o szkolnictwie wyższym (Dz.U. 2005 nr 164 poz. 1365) Art.
239. oraz Ustawy z dnia 4 lutego 1994 r. o prawie autorskim i prawach pokrewnych (Dz.U. z 2000 r. Nr 80, poz.
904, z późn. zm.) Art. 15a. "Uczelni w rozumieniu przepisów o szkolnictwie wyższym przysługuje pierwszeństwo
w opublikowaniu pracy dyplomowej studenta. Jeżeli uczelnia nie opublikowała pracy dyplomowej w ciągu 6
miesięcy od jej obrony, student, który ją przygotował, może ją opublikować, chyba że praca dyplomowa jest
częścią utworu zbiorowego."
2 Ustawa z dnia 30 czerwca 2000r. – Prawo własności przemysłowej (Dz.U. z 2003r. Nr 119, poz. 1117 z
późniejszymi zmianami) a także rozporządzenie Prezesa Rady Ministrów z dnia 17 września 2001r. w sprawie
dokonywania i rozpatrywania zgłoszeń wynalazków i wzorów użytkowych (Dz.U. nr 102 poz. 1119 oraz z 2005r.
Nr 109, poz. 910).

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Kraków, dnia
AKADEMIA GÓRNICZO-HUTNICZA
WYDZIAŁ INŻYNIERII MECHANICZNEJ I ROBOTYKI
TEMATYKA MAGISTERSKIEJ PRACY DYPLOMOWEJ
dla studenta II roku studiów stacjonarnych
Javier Latasa Martinez de Irujo
imię i nazwisko studenta
TEMAT MAGISTERSKIEJ PRACY DYPLOMOWEJ:
Promotor pracy: Prof. dr hab. inż. T. Uhl
Recenzent pracy: Podpis dziekana:
PLAN PRACY DYPLOMOWEJ
1. Omówienie tematu pracy i sposobu realizacji z promotorem.
2. Zebranie i opracowanie literatury dotyczącej tematu pracy.
3. Zebranie i opracowanie wyników badań.
4. Analiza wyników badań, ich omówienie i zatwierdzenie przez promotora.
5. Opracowanie redakcyjne.
Kraków, ....................… ……………………………..........
data podpis dyplomanta
TERMIN ODDANIA DO DZIEKANATU: 20 r.
podpis promotora

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AGH University of Science and Technology Kraków, the............
Faculty of Mechanical Engineering and Robotics
Field of Study: Mechatronics (in English)
Specialisations: Mechatronics Design
Javier Latasa Martinez de Irujo
Master DiplomaThesis
Supervisor: Prof. dr hab. inż. Tadeusz Uhl
SUMMARY
The objective of this work is to propose an energy harvesting method that takes
advantage of the outstanding properties of carbon nanotubes (CNTs).
In the first part an explanation of CNTs, their properties, applications and synthesis
technics is presented. Then, a description of energy harvesting systems and technics
completes the theoretical background. This knowledge is the base that allows us to choose
properly which alternative for harvesting energy using CNTs is to be the center of our
work.
Polymer nanocomposites with CNT as filler are chosen as the base for this thesis and
therefore a deeper study in the subject is presented. A good understanding of the
piezoelectric effect is very important for the kind of system that will be designed; therefore
this phenomenon is carefully described. The current state of the art concerning energy
harvesting with nanocomposites and using the piezoelectric effect is introduced in this test.
Additionally, a summary of the work made by several groups of scientists in the field is
also included.
After analyzing all the previous information, a flexible nanocomposite generator
(NCG) that generates electrical energy from low frequency movement is proposed. A
piezoelectric nanocomposite (p-NC) made of CNT and piezoelectric powder as fillers in an
elastomer matrix, is the main part of the proposed NCG.
A model of the NCG is prepared in order to deeply understand the working
mechanics and the role that certain parameters play in the system. The FEM software
COMSOL Multiphysics is used for model simulation. The software solves a reproduction
of a real experiment that involves the coupled effects of mechanics of materials,
piezoelectricity and electric currents that take part in the system. Results are presented and
analyzed.
Eventually, a real experiment in the laboratory is performed. P-NC samples are
prepared and a conductivity study is conducted in order to analyze the effect that CNT
concentration and preparation procedures have. Finally a variety of NCG samples are
generated, their outputs are measured and the results presented and analyzed.

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"Si no puedes tener la razón y la fuerza escoge siempre la razón y deja que el enemigo
tenga la fuerza. En muchos combates puede la fuerza obtener la victoria, pero la lucha
toda sólo la razón vence. El poderoso nunca podrá sacar razón de su fuerza, pero
nosotros siempre podremos obtener fuerza de la razón".
Sup Marcos
My sincere gratitude to those who contributed to build up a high standard public
education so that knowledge is widely accessible and people can think, decide, and
hopefully use science for other goals than just money.
Many thanks to my family because they are always ready to help.
I would like to thank for their support to:
dr inż. Michał Lubieneczki
Magdalena Młotek
Special thanks to:
mgr Krzysztof Grabowski
Prof. Tadeusza Uhla
This work would not have been possible without them.

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Table of Contents
Table of Contents ....................................................................................................... 7
1. Carbon Nanotubes................................................................................................ 11
1.1. Early History.................................................................................................. 11
1.2. Introduction to Carbon Nanotubes................................................................. 12
1.2.1. CNTs Structure.............................................................................................. 13
1.2.2. Single Wall Carbon Nanotubes, SWNT........................................................ 14
1.2.3. Multi Walled Carbon Nanotubes, MWNT .................................................... 14
1.2.4. Basic Geometry of Carbon Nanotubes. Chirality.......................................... 15
1.2.5. Chirality vs. Electrical Properties.................................................................. 17
1.2.6. Various Forms and Sizes............................................................................... 18
1.2.7. Defects in CNTs ............................................................................................ 18
1.3. CNT Properties .............................................................................................. 20
1.3.1. Mechanical Properties ................................................................................... 20
1.3.2. Thermal Properties ........................................................................................ 21
1.3.3. Electrical Properties....................................................................................... 22
1.3.4. Other properties and application ................................................................... 25
1.3.5. Defects in Carbon Nanotubes vs. Properties ................................................. 27
1.4. Production Processes ..................................................................................... 28
1.4.1. Arc-Discharge and Laser Ablation................................................................ 28
1.4.2. High Pressure Carbon Monoxide Disproportionation (HiPCO) ................... 30
1.4.3. Chemical Vapor Deposition .......................................................................... 30
1.4.4. Other Methods............................................................................................... 32
1.4.5. Major Problems ............................................................................................. 32
1.4.6. Post Synthesis Processing.............................................................................. 33
1.5. Conclusions.................................................................................................... 33
2. CNT, Present and Proposed Applications............................................................ 34
2.1. Individual Use of CNT .................................................................................. 34
2.1.1. Near-Field Microscope Probes...................................................................... 34
2.1.2. Field Emission-Based Devices...................................................................... 35
2.1.3. Chemical Sensors .......................................................................................... 36
2.1.4. Bio-Sensors.................................................................................................... 37
2.1.5. Field Effect Transistor................................................................................... 37
2.1.6. Supercapacitors.............................................................................................. 38

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2.1.7. Nano Electronic Interconnection................................................................... 38
2.1.8. Nano-Tools, Nano-Devices, Nano-Systems.................................................. 38
2.1.9. CNT in NEMS............................................................................................... 39
2.2. CNT Perspectives in Nano-Composites ........................................................ 41
2.2.1. Polymer Matrix Composites Perspectives..................................................... 41
2.2.2. Metal Matrix Composites Perspectives......................................................... 42
2.2.3. Ceramic Matrix Composites Perspectives..................................................... 42
2.2.4. Smart Materials ............................................................................................. 42
2.3. CNT Nano-Composites.................................................................................. 42
2.3.1. Composite fabrication techniques ................................................................. 43
2.3.2. Challenges in MWCNT Polymer Composites Fabrication ........................... 45
2.3.3. Properties of the Nanocomposites................................................................. 50
2.4. Conclusions.................................................................................................... 55
3. Energy Harvesting................................................................................................ 56
3.1. Energy Harvesting Sources and Technologies .............................................. 56
3.1.1. Vibrations Energy Harvesting ....................................................................... 57
3.1.2. Energy Harvesting Devices ........................................................................... 58
3.1.3. Piezoelectric Energy Harvesting ................................................................... 58
3.1.4. Power Harvesting Using CNTs ..................................................................... 59
3.2. Piezoelectricity............................................................................................... 59
3.2.1. History of Piezoelectricity............................................................................. 59
3.2.2. Piezoelectric Ceramics .................................................................................. 61
3.2.3. Piezoelectric Constitutive Equations............................................................. 63
3.2.4. Piezoelectric Coefficients.............................................................................. 66
3.2.5. Piezoelectric Sensor/Generator ..................................................................... 70
3.3. Piezoelectric Nano-Generators ...................................................................... 74
3.3.1. Introduction ................................................................................................... 74
3.3.2. Piezoelectric Power-Generating Devices Using ZnO Nanowires................. 74
3.3.3. Nano-Generators Using Other Piezoelectric Materials ................................. 75
3.3.4. Nano- Composite Generators (NCGs)........................................................... 77
3.4. Summary and Conclusions: ........................................................................... 80
4. Multiphysiscs Modelling of a Nanoomposite-Generator..................................... 81
4.1. Introduction.................................................................................................... 81
4.2. Model description .......................................................................................... 82

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4.2.1. Geometry implementation............................................................................. 82
4.2.2. Material properties......................................................................................... 83
4.2.3. CNT contribution........................................................................................... 84
4.3. Simulation Experiment Set-Up...................................................................... 85
4.3.1. Simulation Experiment description ............................................................... 85
4.4. COMSOL Internal Calculation Procedure..................................................... 87
4.4.1. Piezoelectric Devices Interface ..................................................................... 87
4.4.2. The Electric Currents Interface ..................................................................... 88
4.4.3. The Electrical Circuit Interface ..................................................................... 89
4.4.4. Dependent Variables ..................................................................................... 90
4.5. Model Meshing.............................................................................................. 90
4.6. Simulation and Results .................................................................................. 90
4.6.1. Introduction ................................................................................................... 90
4.6.2. Simplified Simulation: Open and Short Circuit ............................................ 91
4.6.3. NCG Model in Bending Position: Stress Distribution .................................. 91
4.6.4. NCG Model in Bending Position: ElectricPotential and Current.................. 92
4.6.5. Transient Study: NCG without PDMS Insulating Layers ............................. 93
4.6.6. Transient Study: CNT Concentration Effect Study....................................... 94
4.6.1. Transient Study: Transition Velocity Study.................................................. 96
4.7. Model Validation and Results Analysis......................................................... 97
4.7.1. Model Validation........................................................................................... 97
4.7.2. Results Analysis: CNT Concentration Effect Study ..................................... 98
4.7.3. Results Analysis: Transition Velocity Study................................................. 98
4.8. Conclusions.................................................................................................... 98
5. Fabrication of a Nanocomposite Generator (NCG) ............................................. 99
5.1. Introduction.................................................................................................... 99
5.2. Materials and Equipment............................................................................... 99
5.2.1. Materials that Compose the NCG.................................................................. 99
5.2.2. Alternative Materials for the NCG.............................................................. 100
5.2.3. Equipment for NCG Fabrication ................................................................. 102
5.2.4. Additional Equipment for NCG Fabrication ............................................... 102
5.3. Experiment: P-NC and NCG Fabrication .................................................... 103
5.3.1. Experiment 1 ............................................................................................... 103
5.3.2. Experiment 2 ............................................................................................... 104

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5.3.3. Experiment 3 ............................................................................................... 104
5.3.4. Experiment 4 ............................................................................................... 105
5.3.5. Experiment 5 ............................................................................................... 105
5.3.6. Experiment 6 ............................................................................................... 106
5.3.7. Summary of Problems Encountered During the Experimentation.............. 107
5.4. Samples Fabrication Procedures.................................................................. 107
5.4.1. Fabrication Procedures for Piezoelectric Nanocomposite (P-NC).............. 107
5.4.2. Fabrication Procedure for Nanocomposite Generator (NCG)..................... 109
5.5. P-NC and NCG Experiment Results............................................................ 109
5.5.1. Introduction ................................................................................................. 109
5.5.2. P-NC Conductivity Results ......................................................................... 109
5.5.3. P-NC Conductivity Results Analysis .......................................................... 114
5.5.4. NCG Output Results.................................................................................... 115
5.5.5. Results Analysis for NCG samples ............................................................. 119
6. Conclusions........................................................................................................ 121
6.1. Final remarks ............................................................................................... 122
6.2. Future work and applications....................................................................... 123
6.2.1. Conductive Nanocomposite perspectives.................................................... 123
6.2.2. Nanocomposite generator (NCG) perspectives........................................... 123
7. References .......................................................................................................... 124

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1. Carbon Nanotubes
1.1. Early History
Carbon is located at the 4th column of the periodic table. Each atom has six
electrons which occupy 1s2
2s2
and 2p2
atomic orbitals. When carbon is in crystalline
phase its valence electrons reach 2s and 2p orbitals playing the role of forming covalent
bonds. 2s and 2p orbitals are mixing and this effect is called hybridization. Carbon
poses unique properties. Theoretically there can be constructed infinite number of
isohedrons, one-dimensional crystalline geometries containing only carbon. One of
those geometries is the fullerene, C60, structure known at least since Leonardo da
Vinci‘s illustrations for Luca Pacioli's 1509 book, ―The Divine Proportion‖. However, it
was not until XX century that laboratory experiments confirmed the existence of
particles which consists of 12 pentagonal and 20 hexagonal faces (Tisza 1993, Osawa
1970) [2].
Fullerene has been in recent years and is still of great interest among scientists. It
is where the history of carbon nanotubes discovery started. The concept of the existence
of such structures was already reported by Radushkevich in 1952, Bacon in 1960 or
Oberlin et al. in 1976 [2]. However, the synthesis and characterization of carbon
nanotubes, also referred to as CNT, was first reported in the scientific literature by
Iijima in 1991, who found it as the product of fullerene synthesis in one of his
experiments [1]. Ever since, they have been intensively studied both theoretically and
experimentally. Great advances in fabrication techniques have been made, and
nowadays it is possible to produce high-quality carbon nanotubes in reasonable
quantities at least for research purposes [9].
Fig.1.1,[2]
a- Regular Leonardo
da Vinci’s Truncated
Isohedron
b- Fullerene,
C60 molecule

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1.2. Introduction to Carbon Nanotubes
Carbon nanotubes have been a fascinating subject of research due to their
remarkable mechanical, chemical, and electronic properties. The multiple forms and
shapes in which the carbon-based materials may appear, with varying physical and
chemical properties, are what make them so interesting for the purpose of designing and
fabricating nanoscale devices [4].
CNTs are the strongest and most flexible molecular material known due to the
unique C–C covalent bonding and seamless hexagonal network. The nanotubes also
have electrical conductivity or semiconductivity, and high thermal conductivity in the
axial direction. Structural and electrical characteristics of CNTs make them promising
for developing unique and revolutionary smart composite materials. In addition, unlike
other smart materials, CNTs have high strength as well as high thermal and electrical
conductivities, and ‗therefore‘ can provide structural and functional capabilities
simultaneously, including actuation, sensing, and generating power. These capabilities
represent the possibility for developing actuators capable of high stress and high strain
operating at low voltage, and multi-functional electrochemical and mechanical sensors.
[7]
Fig.1.2,Number of publications on
fibers in last decades [6]
Fig.1.3,Number of papers published
during the last decade (1998–2009),
including certain keywords (as indicated)
in their title [5]

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1.2.1. CNTs Structure
Carbon nanotubes are long carbon cylinders. They are constructions of rolled
graphene sheets. Graphene is a simply sp2-bonded planar sheet constructed from
carbon atoms. Graphene is the material that gives the unique properties to carbon
nanotubes. Graphene‘s in plane resistivity of 50µΩcm, which is approximately half of
the value for copper, may be reduced up to the 1/50 of this value. On the other hand
graphene has very good thermal conductivity ranging from 24 to 470 Wm-1
K-1
[8].
The ends of the tubes may be open or ―capped‖ with what is essentially a
hemisphere of fullerene. A form in which graphene is rolled up to give a spiral cross-
section is also known. [9].
There are two types of CNT:
 Single Walled Carbon Nanotube (SWNT)
 Multi Walled Carbon Nanotube (MWNT)
Fig.1.4,[Schematic illustration of CNTs: (a) carbon nano-walls (figure from); (b) arm-chair type
metallic SWNT (10, 10) (figure from); (c) structure of multi-walled nanotube (figure from]); and (d)
structure of a four-nanocone-stacked CNF [7].
Fig.1.5,[a) Graphene sheet, b) single nanotube, c) multiwalled nanotube [7]

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1.2.2. Single Wall Carbon Nanotubes, SWNT
A single wall nanotube (SWNT) has a diameter of around 1 nm, where the carbon
atoms are approximately 0.14 nm apart to each other. The typical length is about 1 µm.
However, SWCNT with lengths approaching 1 mm have been observed giving
astonishing aspect ratios (length/diameter). They are not so stiff in comparison to the
MWNT. They are harder to produce, but the structure of such a CNT is more
predictable. SWNT are also easier to model and therefore to create approximate
equations for such a properties as conductivity or strength. SWNT very often appears as
a bundle with other tubes (see Fig.1.6) [3]
1.2.3. Multi Walled Carbon Nanotubes, MWNT
Multiwalled nanotube (MWNT) may have any diameter in the range of 2–100 nm,
with 10–20 nm being typical [3]. The distance between the walls in multiwalled carbon
nanotubes is about 0.34 nm, similar to the distance between graphene layers in graphite
[9]. In comparison to the SWNT they are stiffer. They are also easier to produce,
therefore they are cheaper. Recently there were developed double-walled carbon
nanotubes which are of big interest among scientists nowadays. Other MWNT are quite
hard to obtain because of the still uncontrollable synthesis. It is hard to obtain the
desired charity (see the next section for ―chirality‖), spacing between tubes and distance
Fig.1.6,[SWNT at different length scales, a) Scanning tunneling microscope, b) HRTEM image of a
nanotube rope (Thesis, et al. Science 273, 483 (1996)), c) tanlged purified SWNT ropes and
bundles. Smalley, R.E - website

15
between layers [11]. Because of the effectiveness in production of MWNT they seem to
be the perfect candidate in the experimental work [kg].
1.2.4. Basic Geometry of Carbon Nanotubes. Chirality
The electronic properties in particular of a carbon nanotube are dependent on the
geometry of the tube. A sheet of graphite can be wrapped in many different ways to
build the wall of a carbon nanotube.
Fig.1.7,[Electron
micrographs of microtubules
of graphitic carbon,
MWNTs. A cross section of
each tube is illustrated [1]
a) Tube consisting of five
graphitic sheets, diameter
6.7nm.
b) Two-sheet tube, diameter
5.5nm.
c) Seven-sheet tube, diameter
6.5 nm, which has the
smallest hollow diameter
(2.2 nm)
Fig.1.8,[Vectors defining the structure of CNT [8]

16
Fig.1.9, illustrates the established nomenclature of three different types of
nanotubes:
(a) The armchair.
(b) The zigzag.
(c) The chiral nanotube.
The chiral nanotube obviously incorporates, in principle, an unlimited number of
types with different wrapping angles relative to the tube axis. To define more precisely
the lattice configuration of a single shell nanotube, we take a closer look on the
honeycomb lattice of graphite in Fig.1.10. Thus, for example,
C = na1 + ma2.
where vectors C and T are the chiral and translational vectors of a nanotube,
respectively, which are defined by the unit vectors a1 and a2.
T points in the direction of the nanotube axis. To construct the nanotube, the
graphite sheet is rolled so that the beginning and the end of C coincide. Thus the
rectangle spanned by C and T is the unit cell of the nanotube lattice. The wrapping
angle of the nanotube, or chiral angle Ө, is given by the angle between a1 and C.
Because of the hexagonal symmetry of the lattice, Ө is limited to the range 0º ≤ Ө ≤ 30º.
The special cases are Ө = 0º, the zigzag nanotube, and Ө = 30º, the armchair nanotube.
The chiral vector C, or in other words, the integer pair (n, m) defines the single shell
nanotube.
Fig.1.9,[Different ways in which the graphitic wall
of SWNT can be wrapped: (a) the armchair, (b) the
zig-zag, and (c) the chiral nanotube. [3]
Fig.1.10,[The vectors C and T, shown on
the hexagonal lattice of a graphite sheet,
define the nanotube symmetry. [3]

17
The tube diameter dt and angle Ө are given as follows:
where a is the length of the unit vectors.
Armchair nanotubes have the form (n, n), while zigzag tubes are (n,0). It follows
from symmetry considerations that the restriction 0<│m│<n can be imposed.
1.2.5. Chirality vs. Electrical Properties
Through these two vectors we can figure out whether the CNT is metallic or
semiconducting. This is strongly connected to the Brillouin zone of the graphene sheet
(shown in Fig.1.12.a and Fig.1.12.b) which is calculated by the tight-binding approach.
Conduction bands, valence bands and Brillouin zone meet exactly at a point where the
Fermi energy lies, which gives unique behaviour for the graphene sheets. [12]. Cones
represent the dispersion of the energy in graphene which is close to the Fermi energy,
whereas allowed states of CNT are represented by parallel lines.
 The CNT is semiconductor when there is a gap around the Fermi level
because the lines do not intersect on the cones,
 The CNT is metallic when the lines are on the apex of the cones
Fig.1.12.a,[Different chirality CNT with
different conducting properties [3]
Fig.1.12.b, [Brillouin zones in CNT [12]
1011

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1.2.6. Various Forms and Sizes
The variety of CNT that has already been observed is impressive. The smallest
nanotube reported to date has a diameter of only 4 Å. An ordinary MWNT has 10–30
layers, each of which are SWNTs of different diameters.
Both SWNTs and MWNTs have been filled with various materials, such as
fullerenes, simple metals, and molecular compounds. Nanotubes with fullerenes inside
are called peapods, and are presently intensively investigated.
While the wall of a carbon nanotube is made up of an exclusively hexagonal
pattern, pentagons are needed to cap the tubes, as a close inspection of Fig.1.9 reveals.
More generally, pentagonal and heptagonal defects enable the graphitic sheet to take up
more complicated structures than a simple tube.
Unless special setups are used for the growth of SWNTs, they are usually
assembled into ropes by their mutual van der Waals attraction. For example, a rope of a
typical diameter of about 10 nm contains ∼100 SWNTs. Since MWNT have a much
higher bending stiffness, they do not arrange similarly. [3]
1.2.7. Defects in CNTs
Nanotubes grown under suitable conditions have a very low concentration of
defects over µm-distances, that is, over hundreds or even thousands of interatomic
spacings. However, if one was able to control the occurrence of defects, very useful and
interesting nanotube structures would emerge. Fig.1.14 shows a SWNT with a sharp
bend that is most likely caused by one or a few such simple defect structures.
Fig.1.13,[Graphene structure and the chiral axis of CNT [12]

19
Defective nanotubes are especially interesting for electronic applications where
the defect site may act as a tunnelling barrier. While high-quality MWNTs are very
straight and stiff, very defective ones have a continuous and smooth curvature as seen in
Fig.1.15. On the other hand, the curvature can be highly regular and so result in helices,
such as is shown in Fig.1.16. An SWNT, or a single shell of a MWNT, can have a
second nanotube branching out. [3]
Finally, we will mention the µm-sized rings that were observed both in SWNT-
and MWNT-based material rather early on. Fig.1.17 shows rings observed in MWNT
material by SEM. Some claims were made for the SWNT rings to be genuine toroids,
that is, seamless ring structures. Later research has, however, shown that ring structures
are readily formed by the van der Waals force–mediated attraction between the two
ends of a nanotube. The curvature is in this case therefore not caused by defects but is
Fig.1.16,[A SEM image of a coiled
MWNT produced by the CVD method [3]
Fig.1.17,[A SEM image of a carbon nanotube
ring obtained from CVD grown MWNT
material. The scale bar is 0.5 µm [3]
Fig.1.14,[An AFM image of a SWNT
with a sharp bend caused by a single
defect site [3] 141516
Fig.1.15,[AFM images of MWNTs produced by
different synthesis conditions: (a) by the arc-discharge
method and (b) by the CVD method. The curved
appearance of the latter is seen to be due to a higher
density of defects. [3]

20
determined by the competition between the strain energy of a bent nanotube and the van
der Waals attraction energy. In fact, especially in the case of SWNTs, the ring may be
composed of several turns of the nanotube (or a nanotube rope). [3]
1.3. CNT Properties
1.3.1. Mechanical Properties
Mechanical properties of carbon nanotubes are closely related to the properties of
a graphite sheet, but the tubular anisotropic form affects the mechanical behaviour. The
basis is the graphite sp2
bond, which is the strongest of chemical bonds. The overall
density of defects of carbon nanotubes can be extremely low, depending on the
synthesizing method and prevailing synthesizing parameters. This has led to predictions
of a very high axial strength [3]
Many scientists have developed simulations and experiments on single nanotubes
(see Fig.1.13). Results differ for each experiment conducted by scientists. Most of them
however, get to the conclusion that the difference is caused even by small molecular
changes which occurs during fabrication of CNT, therefore scientists still work on the
development of better synthesis processes of CNT [15].
1.3.1.a. Young‘s Modulus
Theoretical calculations of Young‘s modulus for individual SWNTs centre around
1 TPa or slightly higher [3], but values as high as 5.5 TPa have been presented. The
spread is due to different interaction models and, also to differing values of nanotube
wall thickness that is not a well-defined quantity. Most of the theoretical attention has
been on SWNTs because modelling the interlayer interaction in MWNTs is a
complicated matter. Lu presents Young‘s modulus values for multiwalled tubes as well
as SWNTs and obtains values from 0.97 TPa to 1.11 TPa with the value increasing
slightly with the number of layers. [3]
The small size of carbon nanotubes presents challenges also for experimental
characterization. Nevertheless measurements have been performed The current
agreement is that defect-free nanotubes, both SWNTs and MWNTs, have a Young‘s
modulus value around or slightly above 1 TPa, which is extremely high and sets
nanotubes as the strongest known material albeit challenged by other nano-tubular
structures such as boron nitride tubes. [3] [7]

21
1.3.1.b. Tensile Strength and Maximum Strain
Theoretically, carbon nanotube tensile strength is high, and this is supported by
calculations in which SWNTs support as high as 30% of axial strain before brittle
failure and by more recent kinetic activation based calculations that give a yield strain
of 17% with chirality and temperature-dependent defect formation activation energy
barriers [3] Other sources reported a maximum strain of SWNT >10%, which is still
greater than most structural materials‘. Compared to carbon reinforcing fibers, the
strength to weight ratio of nanotubes in the axial direction is up to four times greater [7].
Experimentally nanotube tensile strength has been measured for MWNTs by Yu
et al. Tensile strength values ranging from 11 GPa to 63 GPa were reported. For
individual SWNTs, the experimental value of tensile strength is still an open question,
but for bundles of SWNTs tensile strength values ranging between a few GPa and
several tens of GPa depending on the bundle and measurement characteristics have been
reported. [3]
1.3.2. Thermal Properties
CNTs present a very good thermal stability and thermal conductivity. They reach
values as 2000W/m-K therefore surpassing diamonds value. In the direction of the
nanotube axis there are reported values about 1750–5800 W/mK [7]. It‘s due to the
carbon bonding in CNTs. Below (Fig.1.19) is presented an experiment on MWNT
measuring its thermal conductance vs. temperature. Thermal conductivity is still of big
interest for scientists.
Fig.1.18, [Tensile test of carbon nanotube –R. Tenne et al.
1718

22
1.3.3. Electrical Properties
The nanotube electronic property is a strong function of its atomic structure,
mechanical deformation and chemical doping. Changing these properties can induce
strong changes in electrical conductance of the nanotube. The electrical impedance of
CNTs was shown to be very sensitive to chemical exposure and mechanical
deformation. Temperature and magnetic fields affect the resistance of the nanotubes.
The properties depend on the type of nanotube. [7]
Electronically, the carbon nanotube can be either metallic or semiconducting,
depending on the chirality. Carbon nanotubes also have been predicted to conduct
current ballistically without dissipating heat.
Roughly it can be said that in metallic nanotubes the interesting transport
phenomena occur at low temperatures, while in semiconducting tubes much of the work
is carried out at room temperature [3]
1.3.3.a. Metallic Tubes
i. Ballistic Conduction
One of the most exciting aspects of transport in metallic carbon nanotubes is their
ability for ballistic transport over relatively large distances, exceeding 1 µm. This means
that the charge can move along the nanotube in such a way that it is not disturbed by
Fig.1.19,[Thermal conductivity of MWNT,
saturation visible at 340 K, Kim et al, Phys. Rev.
2001

23
inelastic collisions. This behavior as a quantum conductor is opposite to the classical
behavior in which the conduction takes place by diffusion of the electrons with a certain
mean free path. One of the consequences of ballistic transport is that there cannot be
dissipation of energy inside the ballistic conductor, and that the heat produced has to
appear at the leads of the ballistic element [4]
ii. Superconductivity
There have been several experiments revealing the existence of superconducting
correlations in the carbon nanotubes. These observations have taken the form of a
drastic drop in the resistance of the nanotube samples below certain temperature. In one
of the most remarkable experiments, it has been shown that a rope of carbon nanotubes
is able to carry an electric current with zero voltage drop, when embedded between
superconducting contacts. The measurement of that so called supercurrent implies
therefore a vanishing resistance of the conductor. [3] [4]
Superconducting properties have been also measured in nanotubes placed between
metallic, non-superconducting contacts.
1.3.3.b. Semiconducting Tubes
Semiconducting nanotubes are especially important for device-oriented
applications. To date, semiconducting behaviour has been observed in single SWNTs.
In MWNTs and SWNT ropes, there usually exist individual shells of both the metallic
and semiconducting kinds, as has been demonstrated by the IBM group. Therefore pure
Fig.1.20,[ (a) The I–V curves at different temperatures and (b) current modulation at
150 mK using the nanotube gate. Notice the small magnitude of the gate voltage Vg
required to produce Coulomb oscillations. [3]

24
semi-conducting behaviour in MWNTs has rarely been mentioned. Semiconducting
behaviour in carbon nanotubes is demonstrated in a FET configuration. Typically with
semiconducting SWNTs it is observed that the conducting state is attained with negative
gate voltages, implying that the carbon nanotube forms a normally-off p-type
conduction channel. Thus a semiconducting carbon nanotube is unintentionally p-
doped, with oxygen as the likely dopant [3].
Fig.1.21 shows a schematic figure of such a device and its transistor
characteristics. The IBM group has shown that a higher transconductance can be
achieved with SWNT-based FETs than with state-of-the-art silicon MOSFETs, which is
encouraging, especially considering that the fabrication technique of nanotube-FETs is
far from optimized. The resistance in the metallic state (ON state) is typically in the
range 20 k_–1 M_. With the fabrication of gate electrodes that are strongly coupled to
the nanotube, it is possible to reach ambipolar transistor action, achieving both n- and p-
type behaviour. Logic gates made from nanotube FETs have recently been
demonstrated. The nature of the Schottky barriers between bulk metal electrodes and the
SWNT, a 1D object, is still being investigated [3].
1.3.3.c. Bulk Transport
The transport physics of single SWNTs, SWNT ropes, and MWNTs is clearly
more significant than the subject of transport in macroscopic amounts of carbon
nanotubes. However, the carbon nanotube offer interesting applications as the
conductive component in composites, when mixed together with an insulating host
Fig.1.21,[ (a) Schematic picture of a FET made from an individual SWNT that is covered by a top gate.
(b) The current vs. source-drain voltage. Inset: Current vs. gate-voltage. Reprinted with permission from
[3]

25
material. In order to have a composite conducting, the volume fraction of the conductive
component has to exceed some critical value. Typically the conductive material consists
of µm-sized particles of a more or less rounded shape. For conduction to occur, the
particles have to touch each other frequently enough so that the conductive channels are
formed over macroscopic distances. As prescribed by percolation theory, this occurs at
a certain wt.% dependent on the material, whereby the conductivity of the composite
rises very sharply (as a function of filling percentage) with several orders of magnitude
[3].
1.3.4. Other properties and application
1.3.4.a. Magnetoresistance
The CNT also have spin-dependent transport properties or magnetoresistance. The
direction of magnetization of the ferromagnetic electrodes used to contact the nanotube
defines the spin direction of the charge carriers into and out of the nanotube and a
change in the resistivity of the nanotube. Spintronic nanoscale devices in theory can be
built using the superconductivity and magnetoresistance effects, where the nanotube-
metallic junction appears to have a strong effect on the spin-dependent transport. The
magnetoresistance effect is interesting, but seems difficult to use for sensing strain of
the nanotube and for use in a smart composite material. [7]
1.3.4.b. Piezoresistance
A pioneering experiment showed that the conductance of a metallic CNT could
decrease by orders of magnitude when strained by an atomic force microscope tip. It
appears that the band structure of a carbon nanotube is dramatically altered by
mechanical strain and that the conductance of the CNT can increase or decrease
depending on the chirality of the nanotube. The strain changes the structure of the
quantum states available to the electrons. Metals conduct electricity easily because their
electrons have easy access to the quantum states that carry the electrons long distances.
These states are in the conduction band of the electronic structure. In semiconducting
nanotubes, there is a band gap, which is an energy barrier that electrons must overcome
to reach the conduction band. The extra energy push to overcome the band gap can
come from heat or an electric field or strain. Actually, strain changes the band structure,
which changes the electrical properties making the nanotube more or less conductive

26
(piezoresistive) depending on the chirality of the nanotube. The piezoresistance effect is
promising for sensing. [7]
1.3.4.c. Piezoelectric Effect
In CNT, the piezoelectric effect is very small based on theory. Therefore, using
piezoelectric nanotubes/wires/ ribbons currently seems less promising than using the
electrochemical property of CNT for developing high strain smart Nanocomposite
materials. [7]
1.3.4.d. Electrochemical Effect
Introducing excess charge into CNT produces mechanical deformations that do
mechanical work. The charge injected into the valence or conduction band causes the
electronic structure to shift. The electrochemical effect should produce up to 2% strain
based on the basal plane intercalation strain of graphite. The electrochemical property
can generate large strains/forces using low voltages. Therefore, the electrochemical
property of CNTs is considered promising for actuation. [7]
1.3.4.e. Telescoping Nanotubes
The MWCNT have been proposed to be used as rotational and translational
bearings, and as a nut and screw for building nanomachines by taking advantage of the
spiral chirality of nanotubes. A screw actuator and worm gears are other ideas that come
to mind, but forming nanotubes with commensurate shells or putting defects into the
nanotubes to form the threads is difficult, particularly for large force macro-scale
actuators. Instead, a telescoping carbon nanotube actuator seems a possible device.
Electrical charge may be used to telescope the actuator and van der Waals force and
opposite electrical charge might be used to retract the actuator. The actuation forces are
being modelled but the actuation has not been verified experimentally yet. In addition,
the resistance of the nanotube depends on the telescoping length. This indicates that the
telescoping can be used as a displacement sensor that is nanoscale in size. [7]
1.3.4.f. Power Generation
This property is due to ionic flow over the nanotube surface. A coulomb drag
property causes charge to flow in the nanotubes in an electrolyte. The current flow
depends on the ionic fluid and flow velocity. The power generation is small, but is

27
promising for medical applications and flow sensing because it continuously produces
power based on flow only. [7]
1.3.5. Defects in Carbon Nanotubes vs. Properties
As in any material, defects play an important role in nano-tube properties.
Structurally, defects make the tube less strong and thus in general defects are not
desirable from the purely mechanical point of view. However, they alter the electronic
properties locally, which can be utilized in the creation of single-tube devices. Defects
are generated in the synthesizing process, and they can also be caused by mechanical
manipulation, or, for example, by ion or electron beam bombardment of the tube. The
most typical structural defects are fivefold (pentagon) and sevenfold (heptagon) rings in
the sixfold (hexagonal) lattice. Other types of typical defects are vacancies and
miscellaneous bonding configurations such as amorphous diamond. Noncarbon-based
defects include substitutional atoms or atom groups. In addition to these, MWNTs
exhibit diverse defects based on discontinuous inner layers. Defects may alter the tube
form from a straight tube to a bulging, kinked, spiral, or even more miscellaneous form.
[3]
1.3.5.a. Coulomb Bockade, CNT as Single-Electron Transistors
One of the main interests in the technological application of the carbon nanotubes
arises from the possibility of developing electronic devices made of a single molecule.
Semiconducting nanotubes have been proposed to act as field-effect transistors. In these
devices, source and drain electrodes are attached to the semiconducting nanotube, while
this is separated from the substrate (the gate electrode) by an oxide layer which acts as a
dielectric. The capacitive coupling between the nanotube and the substrate is what
makes it possible to change the density of charge carriers and the conduction properties
in the nanotube by varying the voltage of the gate.
Unlike field-effect transistors, however, single-electron devices are based on the
intrinsic quantum-mechanical character of the tunnel effect. In the case of metallic
nanotubes, the development reported is that the electrons can be confined in short
islands between two buckles of the tubule, so they can be added one by one by suitable
variations of the voltage applied to the external gate. One of the structures which have
been produced with this technique can be observed in Fig.1.22. The short nanotube
segment that appears there between the buckles has a length of the order of 25 nm. [4]

28
1.4. Production Processes
Through the last decade there was significant development in the technology of
producing the CNT. Successes in these studies and experiments are making CNT more
affordable. CNT unique properties might get altered when defects and failures appear
during the synthesis process. Therefore there were developed many different
approaches to try to obtain the best results
All growing conditions for synthesising CNTs require a catalyst to achieve high
yields, where the size of the catalyst nanoparticles will determine the diameter and
chirality of the CNT. The CNTs that are formed are generally in a mixture with other
carbonaceous products including amorphous carbon and graphitic nanoparticles [10].
Three technics are currently the most common ones to obtain CNT.
1.4.1. Arc-Discharge and Laser Ablation
Both Laser ablation and arc-discharge methods for the growth of CNTs involve the
condensation of carbon atoms generated from the evaporation of carbon sources. High
temperature is involved, ranging from 3000ºC – 4000ºC [10].
 In Arc-discharge, See fig1.23, various gases such as Helium or Hydrogen are
Fig.1.22,[Atomic force microscope image of a short nanotube island between two
buckles, formed by manipulation with the atomic force microscope tip [4]

29
induced into plasma by large currents generated at a carbon anode and
cathode. This process leads to the evaporation of carbon atoms which produces
very high quality MWNTs and SWNTs [10].
 Laser ablation, See fig.1.24, also produces very high quality CNTs with a high
degree of graphitisation by focusing a CO laser (in pulsed or in continuous wave
mode) for a period of time onto a rotating carbon target [10].
Diameters accomplished through arc-discharge are approximately 5-30 nm and
the length is in the order of microns. SWNTs are harder to produce using this method
since metal catalyst is needed. The CNT produced are among the ones with better
crystalline structure quality (due to the high temperature of the process) [13] [14].
Fig.1.23,[Laser ablation schematic, ―Carbon nanotubes from basic research to nanotechnology‖ 2006
Fig.1.24,[Arc discharge schematic, from ―Carbon nanotubes from basic research to nanotechnology‖ 2006

30
1.4.2. High Pressure Carbon Monoxide Disproportionation (HiPCO)
The HiPCO process utilises clusters of Fe particles as catalysts to create very high
quality SWNTs. Catalyst is formed in situ by thermal decomposition of iron
pentacarbonyl, which is delivered intact within a cold CO flow and then rapidly mixed
with hot CO in the reaction zone. Upon heating, the Fe(CO)2 decomposes into atoms
that condense into larger clusters. SWNTs nucleate and grow on these particles in the
gas phase [10].
1.4.3. Chemical Vapor Deposition
The CVD method usually consists of a furnace, catalyst material, carbon source, a
carrier gas, a conditioning gas, and a collection device (usually a substrate). The carrier
gas is responsible for taking the reacting material onto the substrate where CNT growth
occurs at catalyst sites. The components mentioned are essential; however, different
groups and researchers have alternative experimental conditions which can contain
multiple types of furnaces, and a variety of catalyst and carbon sources. The key
advantage of this technique is its capability to directly deposit the CNTs onto the
substrate, unlike arc discharge and laser ablation that produces a soot / powder [10].
The growth may be specifically controlled due to the size of the particle on which
nanotube is formed. Due to the lower temperature for the CVD it is believed that the
CNT has lower quality (low energy form). However, in comparison to two other
methods CVD does not produce unwanted graphite material [15].
Fig.1.25,[CVD growth using as growing base different materials. A) picture
of pattern, b) CNT forests, c) CNT forests, c) schematic, Hongije Dai

31
Fig.1.27,[A forest of carbon nanotubes produced by Plasma Enhanced Chemical Vapor Deposition
(PECVD). The substratum must first be covered with metal (e.g., Fe or Ni) catalyst islands.
Hydrocarbon feedstock (acetylene) is then passed over the substratum heated to several hundred C. The
acetylene decomposes at the surface of the catalyst and the carbon nanotubes grow up from the catalyst
particle, or grow up beneath it (pushing it up). [9]
Fig.1.26,[Scanning electron micrographs of carbon nanotubes grown on the surface of a carbon fiber
using thermal chemical vapor deposition. The right-hand image is an enlargement of the surface of the
fiber, showing the nanotubes in more detail. Reproduced with permission from [9]

32
1.4.4. Other Methods
Recent developments by Harris et. al.. has led to the development of a large scale
batch process for fabricating MWNTs. Here, a furnace like system called a fluidised bed
reactor continuously flows a carrier gas over a porous alumina powder that is
impregnated with the catalyst material, leading to a continuous creation of MWNTs
where tens of grams can be synthesised in one run. [10].
On the fig.1.28, Catalytic method (CoMoCAT®) that produces SWNT of high
quality at a very high selectivity, and a remarkably narrow distribution of tube
diameters (OU Nanotube Research Group, http://www.ou.edu)
1.4.5. Major Problems
Major problems remain with the large-scale utilization of carbon nanotubes. The
most severe are [9]:
 making pure preparations
 dispersing them in solvent (since they can scarcely be solvated (cf. Section
3.2) they tend to be strongly aggregated into bundles)
 reducing their length (a 20 nm diameter tube may be 20 m long as fabricated,
unnecessary for many applications)
 manipulating them into a desired position
Fig.1.28, Catalytic method (CoMoCAT®
)

33
1.4.6. Post Synthesis Processing
Post synthesis processing of nanotube material therefore typically requires [9]:
 Purification—methods include thermal annealing in air or oxygen; acid
treatment, microfiltration; typically 50% of mass reduction
 De-agglomeration to separate the tubes. Methods include ultrasonication (but
this can damage the tubes), electrostatic plasma treatment, electric field
manipulation and polymer wrapping, ball milling (can damage the tubes);
these methods can also reduce their length
 Chemical functionalization (with electron-donating or electron-accepting
groups) to improve interactions with a solid or liquid matrix
1.5. Conclusions
In this section CNT, their properties and production procedures where introduced.
It can be observed that CNT properties are still not entirely known. Sometimes data
mismatch might appear when searching through different sources, therefore the more
often and recently reported data where chosen after analysis and deeper research. It was
noticed the great potential that CNTs have for developing revolutionary technologies.
One of the main troubles that scientists have to face, is the difficulty of
synthetizing good quality CNT in an affordable manner. Great affords are been made to
improve production procedures, thus new methods constantly appear. Better and more
affordable technologies that allow for efficiently work in the nanoscale are also needed
for properly testing and manipulating CNT. This would permit to accurately define and
exploit their outstanding properties.
Presented properties are just highlights of the researches going on concerning
CNT. The scientific community is still investigating in multiple directions and the
perspectives are great. Carbon nanotubes offer exciting possibilities. Understanding
their properties is essential to design new smart composite materials and develop
revolutionary technologies in nanotechnology. Applications for individual CNT are
presented in the next chapter.
The main characteristics and properties for individual CNTs have been
introduced. However a large number of them can form secondary structures, such as
ropes or fibers, and take part in nanocomposites as fillers. The new specific properties
that arise from those forms are explained in the second part of next chapter.

34
2. CNT, Present and Proposed Applications
Carbon nanotubes can be inert and can have a high aspect ratio, high tensile
strength, low mass density, high heat conductivity, large surface area, and versatile
electronic behaviour including high electron conductivity. While these are the main
characteristics of the properties for individual nanotubes, a large number of them can
form secondary structures such as ropes, fibers, papers, thin films with aligned tubes,
etc., or take part as fillers in nanocomposites; arising for each case specific properties
[44]. The wide range of properties makes them ideal candidates for a large number of
applications that will get bigger once their cost is sufficiently low. CNTs applications
can be divided in following way:
-
Individual CNTs
-
Bulk CNT (Nanocomposites)
The form is choses depending on the application needs. For example, for MEMS
and NEMS devices, CNTs are used, while if we want to work in the macro-scale CNT
as filler of a nanocomposite will be selected.
2.1. Individual Use of CNT
2.1.1. Near-Field Microscope Probes
Carbon nanotubes can be used as tips in scanning probe microscopes, which
provides several advantages over usual silicon tips. The ability that the nanotube tips
have to buckle elastically reduces the damage that can be produced when crashing into
the sample. [4] Such nanotube-based SPM tips also offer the perspective of being
functionalized, in the prospect of making selective images based on chemical
discrimination by ―chemical force microscopy‖ (CFM).
Fig.2.1,[Scanning
electron microscopy
image of carbon
nanotube (MWNT)
mounted onto a regular
ceramic tip as probe for
atomic force
microscopy. [44]

35
Chemical function imaging using functionalized nanotubes represents a huge step
forward in CFM because the tip can be functionalized very accurately (ideally at the
very nanotube tip only, where the reactivity is the highest), increasing the spatial
resolution. The interaction between chemical species present at the end of the nanotube
tip and a surface containing chemical functions can be recorded with great sensitivity,
allowing the chemical mapping of molecules [44].
2.1.2. Field Emission-Based Devices
Based on a pioneering work by de Heer et al., carbon nanotubes have been
demonstrated to be efficient field emitters and are currently being incorporated in
several applications, including flat panel display for television sets or computers (whose
a first prototype was exhibited by Samsung in 1999) or any devices requiring an
electron producing cathode, such as X-ray sources [44].
The principle of a field-emission-based screen is demonstrated in Fig.2.2,a). The
emission is produced by applying a voltage between a surface with nanotube fibers,
acting as a cathode, and a substrate with phosphor arrays. The high local fields created
in the nanotube geometry make the electrons jump toward the anode, where the contact
with the phosphor produces the spots of light in the display. The flat panel nanotube
displays turn out to save more energy and to have higher brightness than liquid crystal
displays. A similar field-emission effect can be applied to the generation of X-rays,
when the anode is replaced by a metal surface, which can lead to interesting
Fig.2.2, a) Principle of field-emitter-based screen. b)
Scanning electron microscope image of a nanotube-based
emitter system (top view). Round dots are MWNT seen
through the wholes corresponding to de extraction grid.
Legagneux (Thales research and technology, Orsay) [44]
Fig.2.3, Prototype of using CNT layer
as FED, Dr. W. Choi, Samsung
Advanced Institute of Technologies. [3]

36
applications for medical purposes. [4], As opposed to regular (metallic) electron
emitting tips, the structural perfection of carbon nanotubes allows higher electron
emission stability, higher mechanical resistance, and longer life time. First of all, it
allows energy savings since it needs lower (or no) heating temperature of the tips and
requires much lower threshold voltage. The market associated with this application is
huge. With such major companies involved as Motorola, NEC, NKK, Samsung, Thales,
Toshiba, etc. Samsung has produced several generations of prototype FED ranging from
4.5 inch (Fig.2.3) with red-green-blue phosphor columns, while companies such as
Oxford Instruments and Medirad work on miniature X-ray generators for medical
applications on the basis of nanotube-based cold cathodes developed by Applied
Nanotech Inc. [44]
2.1.3. Chemical Sensors
The electrical conductance of semiconductor SWNTs was recently demonstrated
to be highly sensitive to the change in the chemical composition of the surrounding
atmosphere at room temperature, due to the charges transfer between the nanotubes and
the molecules from the gases adsorbed onto the SWNT surface. It has also been shown
that there is a linear dependence between the concentration of the adsorbed gas and the
difference in electrical properties, and that the adsorption is reversible. Sensors are
characterized by extremely short response time (Fig.2.4), thus being different from
conventionally used sensors. High sensitivity toward water or ammonia vapors has been
measured on SWNT-SiO composite. The determination of CO concentrations on
SWNT-SiO composite has also been reported. By doping nanotubes, detection of other
gases has been reported.
Generally speaking, the sensitivity of the new nanotube-based sensors is three
Fig.2.4, Demonstration of the ability of SWNT sin detecting molecule traces in inert gases. (a)
Increase in a single SWNT conductance when 20 ppm of NO are added to an argon gas ﬂow.
(b) Same with 1% NH3 2 added to the argon gas ﬂow [44]

37
orders of magnitude higher than that of standard solid state devices. In addition, the
interest in using nanotubes as opposed to current sensors is the simplicity and the very
small size of the system in which they can be placed, and their selectivity, which allows
a limited number of sensor device architectures to be built for a variety of industrial
purposes. Nanotube-based sensors are currently developed in both large and small
companies, such as Nanomix (USA), for example. [44]
2.1.4. Bio-Sensors
Attaching molecules of biological interest to carbon nanotubes is an ideal way to
realize nanometer-sized biosensors. Indeed, the electrical conductivity of such
functionalized nanotubes would depend on modiﬁcations of the interaction of the probe
with the studied media, because of chemical changes or as result of their interaction
with target species. The science of attaching biomolecules to nanotubes is rather recent
and was inspired by similar research in the fullerene area. Some results have already
been patented, and what was looking like a dream a couple of years ago may become
reality in the near future. The use of the internal cavity of nanotubes for drug delivery
would be another amazing application, but little work has been carried out so far to
investigate the harmfulness of nanotubes in the human body. [44]
2.1.5. Field Effect Transistor
An interesting finding has been that the field-effect transistors made of single
nanotubes can have better performance than the leading silicon transistor prototypes. [4]
Fig.2.5, Cross sections of different geometries of carbon nanotube field-effect
transistors: (a) back-gated CNTFETs, (b) top-gated CNTFETs, (c) wrap-around
gate CNTFETs, and (d) suspended CNTFETs. [47]

38
2.1.6. Supercapacitors
They have been proposed for the construction of supercapacitors, which may take
advantage of the large surface area accessible in nanotube arrays. These can give rise to
capacitors with high power and storage capabilities. [4]
Supercapacitors include two electrodes immersed in an electrolyte (e.g., 6 M
KOH), separated by an insulating ion-permeable membrane. Charging the capacitors is
achieved by applying a potential between the two electrodes, making the cations and the
anions moving toward the electrode oppositely charged. Suitable electrodes should
exhibit a high electrical conductivity and a high surface area since the capacitance is
proportional to it. [44]
2.1.7. Nano Electronic Interconnection
The use of carbon nanotubes as wiring for interconnection of nanoscale circuit
elements is being explored primarily because SWNTs can carry a current density of up
to 109
Acm−2
, compared to 105
Acm−2
for normal metals [3].
2.1.8. Nano-Tools, Nano-Devices, Nano-Systems
Due to the ability of graphene to expand slightly when electrically charged,
nanotubes have been found to act conveniently as actuators. Kim et al. demonstrated it
by designing ―nano‖-tweezers able to grab, manipulate, release nano-objects (the
―nano‖-bead having been handled for the demonstration was actually closer to
micrometer than nanometer), and measure their electrical properties [44] [3]. This was
made possible quite simply by depositing two non-interconnected gold coatings onto a
pulled glass micropipette (Fig.2.6), then attaching two MWNTs (or two SWNT-
bundles) ~ 20–50nm in diameter to each of the gold electrodes.
Fig.2.6, Sketch
explaining how the first
nano-tweezers were
designed. First is a glass
micropipete (dark cone
top). Then two Au
coating (in grey middle)
are deposited so that they
are not in contact. Then a
voltage is applied to the
electrodes. [44]

39
Applying a voltage (0–8.5 V) between the two electrodes then makes the tube tips
to open and close reversibly in a controlled manner. A similar experiment, again rather
simple, was proposed by Baughman et al. the same year (1999), consisting in mounting
two SWNT-based paper strips (―bucky-paper‖) on both sides of an insulating
doubleside tape. The two bucky-paper strips were previously loaded with Na + and Cl -
, respectively. When 1 V was applied between the two paper strips, both expand, but the
strip loaded with Na + expands a bit more, forcing the whole system to bend. Though
performed in a liquid environment, such a behaviour has inspired the authors to predict
a future for their system as ―artificial muscles.‖ [44]
Another example of amazing nano-tools is the nano-thermometer proposed by
Gao et al.. A single MWNT was used, in that case, partially filled with liquid gallium.
Upon the effect of temperature variations in the range 50–500◦C, the gallium goes up
and down reversibly within the nanotube cavity at reproducible level with respect to the
values of the temperature applied. Of course, nano-tools such as nano-tweezers or nano-
thermometers will hardly reach a commercial development so to justify industrial
investments. But such experiments are more than amazing laboratory curiosities. They
definitely demonstrate the ability of carbon nanotubes as parts for future nano-devices,
including nano-mechanics-based systems. [44]
2.1.9. CNT in NEMS
The impact of Nano-Electro-Mechanical Systems (NEMS) is likely to be as
significant as microelectromechanical systems. Carbon nanotubes are promising for the
design and development of NEMS, not only because of the excellent mechanical and
electrical properties, but also because the significant progress in the fabrication of
carbon nanostructures of the last few years points to possible implementation of
recently proposed carbon nanotube-based NEMS devices such as a non-volatile random
access memory for molecular computing. [44]
Fig.2.7,
MEMS
multiaxis
force sensor
with CNT,
Cullinan et
al. [48]

40
The predicted behavior of carbon nanotube nanoelectromechanical switches,
which is the basis of many NEMS devices, is favorable, and electronic properties have
been shown to be reversible with mechanical deformation by a local probe [44]. Fig.2.7-
2.9 show some examples of CNT in already constructed NEMS devices.
Fig.2.8, (a) CNT ﬁlm strain gauge, (b) single suspended CNT displacement
sensor, and (c) pressure sensor with CNT piezoresistors. [44]
Fig.2.9, Rotational actuator using MWNT as the axle for the rotor. Top a)
represents concept, b) picture from SEM, bottom: pictures during the
performance. [48]

41
2.2. CNT Perspectives in Nano-Composites
Because of their exceptional morphological, electrical, thermal, and mechanical
characteristics, carbon nanotubes are particularly promising materials as reinforcement
in composite materials with metal, ceramic, or polymer matrix. Key basic issues include
the good dispersion of the nanotubes, the control of the nanotube/ matrix bonding, the
densification of bulk composites and thin films, and the possibility of aligning the
nanotubes. In addition, the nanotube type (SWNT, c-MWNT, h-MWNT, etc.) and
origin (arc, laser, CCVD, etc.) is also an important variable since determining the
structural perfection, surface reactivity, and aspect ratio of the reinforcement.
Considering the major breakthrough that carbon nanotubes are expected to make in the
field, the following will give an overview of the current work on metal-, ceramic- and
polymer-matrix composites reinforced with nanotubes. [44]
2.2.1. Polymer Matrix Composites Perspectives
Dispersion of carbon nanotubes in polymer composites may improve their
strength, stiffness and thermal and electrical conductivities. The strength improvement
depends on the degree of load transfer and on the level of dispersion achieved in the
matrix.
Improvements in electrical properties are dramatic even at very low volume
Fig.2.10, Publications on CNT composites divided by material type [37]

42
fractions. The percolation threshold is reached at very low load with nanotubes.
Tailoring the electrical conductivity of a bulk material is then achievable by adjusting
the nanotube volume fraction in the formerly insulating material while not making this
fraction too large anyway. [44]
Typical current applications for these materials include electrically conducting
paint, conducting polymer structures, lighter and stiffer structures, heat sinks for
electronics, motor components, and smart polymer coatings. [4]
2.2.2. Metal Matrix Composites Perspectives
Nanotube-metal matrix composites are still rarely studied. The materials are
generally prepared by standard powder metallurgy techniques, but the dispersion of the
nanotubes is not optimal. Thermal stability and electrical and mechanical properties of
the composites are investigated. [44]
2.2.3. Ceramic Matrix Composites Perspectives
Carbon nanotube-containing ceramic-matrix composites are a bit more frequently
studied, most efforts made to obtain tougher ceramics. [44]
2.2.4. Smart Materials
Smart materials are solid-state transducers that have piezoelectric, pyroelectric,
electrostrictive, magnetostrictive, piezoresistive, electroactive, or other sensing and
actuating properties. Existing smart materials such as piezoelectric ceramics,
electroactive polymers, and shape memory alloys have various limitations holding them
back from practical applications. The limitations centre on the requirement for high
voltage or high current, or the material is brittle, heavy, or has a small range of strain or
force actuation. Smart nanoscale materials may reduce these limitations and represent a
new way to generate and measure motion in devices and structures. Among the various
nanoscale materials, carbon nanotubes (CNTs) exhibit extraordinary mechanical and
electric properties. [7]
2.3. CNT Nano-Composites
As described previously, CNTs are amongst the strongest and stiffest ﬁbers ever
known. These excellent mechanical properties combined with other physical properties
of CNTs exemplify huge potential applications of CNT/polymer nanocomposites. For

43
example, they may be used as reinforcements in high strength, low weight and high
performance composites. Presently there is a great interest in exploiting the exciting
properties of these CNTs by incorporating them into some form of polymer matrix. [17]
Unlike traditional polymer composites containing micron-scale fillers, the
incorporation of nanoscale CNTs into a polymer system results in very short distance
between the fillers, thus the properties of composites can be largely modified even at an
extremely low content of filler. For example, the electrical conductivity of CNT/epoxy
nanocomposites can be enhanced several orders of magnitude with less than 0.5 wt.% of
CNTs. We can observe below how the distribution of the filler within the matrix
changes for different types of fillers with good dispersion. [19]
2.3.1. Composite fabrication techniques
A large number of techniques have been used for the fabrication of CNT-polymer
nanocomposites based on the type of polymer used. The most popular ones are
explained below.
2.3.1.a. Solution Casting / Blending
The solution casting is the most valuable technique to form CNTs/polymer
nanocomposites. However, its use is restricted to polymers that are soluble. One of the
benefits of this method is that agitation of the nanotubes powder in a solvent facilitates
nanotubes‘ disaggregation and dispersion. Almost all solution processing methods are
based on a general theme which can be summarised as [18]:
1) Dispersion of nanotubes in either a solvent or polymer solution by
energetic agitation.
2) Mixing of nanotubes and polymer in solution by energetic agitation.
3) Controlled evaporation of solvent leaving a composite film.
Fig.2.11, Distribution of micro- and nano-scale fillers of the same 0.1 vol.% in a reference volume of
1 mm3
: A) Al2O3 particle; B) carbon fiber; C) GNP, graphite nanoplatelets; D) CNT. [19]

44
Solvent casting facilitates nanotube dispersion and involves preparing a
suspension of CNTs in the desirable polymer solution via energetic agitation (magnetic
stirring or sonication) and then allowing the solvent to evaporate to produce CNT-
polymer nanocomposites. A lot of study is available in open literature for the formation
of CNT nanocomposites by this method. The choice of solvent is generally made based
on the solubility of the polymer. The solvent selection for nanotube dispersion also had
a significant influence on the properties of the nanocomposites. It is reasonable that,
easier the solvent can evaporate, the less solvent will remain to affect the curing
reaction. The presence of residual solvent may alter the reaction mechanism by
restricting the nucleophile-electrophile interaction between the hardener and epoxy,
henceforth, affect the cross-linking density and thus degrade the transport properties and
mechanical properties of the cured structures. Nanocomposites with other thermoplastic
materials with enhanced properties have been fabricated by solvent casting.
The limitation of this method is that during slow process of solvent evaporation,
nanotubes may tend to agglomerate, that leads to inhomogeneous nanotube distribution
in polymer matrix. The evaporation time can be decreased by dropping the
nanotube/polymer suspension on a hot substrate (drop casting) or by putting suspension
on a rotating substrate (spin-casting). [17]
2.3.1.b. Melt Mixing Method
The alternative and second most commonly used method is melt mixing, which is
Fig.2.12, Schematic representation of different steps of polymer/CNTs composite processing:
solution mixing (a); melt mixing (b); in situ polymerisation (c). [18]

45
mostly used for thermoplastics and most compatible with current industrial practices.
This technique makes use of the fact that thermoplastic polymers soften when heated.
Melt mixing uses elevated temperatures to make substrate less viscous and high shear
forces to disrupt the nanotubes bundle. Samples of different shapes can then be
fabricated by techniques such as compression molding, injection molding or extrusion.
Although melt-processing technique has advantages of speed and simplicity, it is
not much effective in breaking of agglomeration of CNTs and their dispersion. [17]
2.3.1.c. In-situ Polymerization
In addition to solvent casting and melt mixing the other method which combines
nanotubes with high molecular weight polymers is in-situ polymerization starting with
CNTs and monomers. It is particularly important for the preparation of insoluble and
thermally unstable polymers, which cannot be processed by solution or melt processing.
In-situ polymerization has advantages over other composite fabrication methods. A
stronger interface can be obtained because it is easier to get intimate interactions
between the polymer and nanotube during the growth stage than afterwards. The most
common in situ polymerization methods involve epoxy in which the monomer resins
and hardeners are combined with CNTs prior to polymerizing. Generally, in situ
polymerization can be used for the fabrication of almost any polymer composites
containing CNT that can be non-covalently or covalently bound to polymer matrix. This
technique enables the grafting of large polymer molecules onto the walls of CNT. [17]
2.3.1.d. Other Technics
Some studies have been also carried out using combined methods, such as solvent
casting in conjunction with sonication, followed by melt mixing and compression
moulding.
The other less commonly known methods for CNT-polymer nanocomposites
formation are twin screw pulverization, latex fabrication, coagulation spinning and
electrophoretic deposition. [17]
2.3.2. Challenges in MWCNT Polymer Composites Fabrication
Although these fabrication methods helped to enhance the properties of CNT
reinforced composites over neat polymer but there are several key challenges that
hinder the excellent CNT properties to be fruitful in polymer composite formation.

46
2.3.2.a. Dispersion
Dispersion of nanoscale filler in a matrix is the key challenge for the formation of
nanocomposites. Dispersion involves separation and then stabilization of CNTs in a
medium. The methods described above for the nanocomposites fabrication require
CNTs to be well dispersed either in solvent or in polymer for maximizing their contact
surface area with polymer matrix. As CNTs have diameters on nanoscale the
entanglement during growth and the substantial van der Waals interaction between them
forces to agglomerate into bundles. The ability of bundle formation of CNTs with its
inert chemical structure makes these high aspect ratio fibers dissolving in common
solvents to form solution quite impossible. The SEM of MWCNTs synthesized by CVD
technique seems to be highly entangled and the dimensions of nanotube bundles is
hundreds of micrometres. This shows several thousands of MWCNTs in one bundle as
shown in Fig.2.13.a).
These bundles exhibits inferior mechanical and electrical properties as compared
to individual nanotube because of slippage of nanotubes inside bundles and lower
aspect ratio as compared to individual nanotube. The aggregated bundles tend to act as
defect sites which adversely affect mechanical and electrical properties of
nanocomposites. Effective separation requires the overcoming of the inter-tube van der
Waal attraction, which is anomalously strong in CNT case. To achieve large fractions of
individual CNT several methods have been employed. The most effective methods are
by attaching several functional sites on the surface of CNTs through some chemical
treatment or by surrounding the nanotubes with dispersing agents such as surfactant.
Fig.2.13, (a) SEM image of aligned CNT bundle synthesized by CVD technique. The inset
figure shows the very good quality of uniform CNTs (b) TEM image of as grown MWCNT
and inset image shows the MWCNTs with encapsulated metallic impurities. [17]

47
Thereafter the difficulty of dispersion can be overcome by mechanical/physical means
such as ultrasonication, high shear mixing or melt blending. Another obstacle in
dispersing the CNTs is the presence of various impurities including amorphous carbon,
spherical fullerenes and other metal catalyst particles. These impurities are responsible
for the poor properties of CNTs reinforced composites. [17]
i. Chemical Functionalization of CNTs
The best route to achieve individual CNT to ensure better dispersion is chemical
modifications of CNT surface. The chemical functionalization involves the attachment
of chemical bonds to CNT surface or on end caps. The addition of these functional
groups on CNTs possesses intermolecular repulsion between functional groups on
surface that overcomes the otherwise weak van der Waal attraction between CNTs.
Chemical functionalization can prevent reagglomeration of CNTs also. Studies found
that the composites filled with functionalized CNTs had better dispersion.
Covalent functionalization of CNTs can be achieved by introducing some
functional groups on defect sites of CNTs (see Fig.2.14) by using oxidizing agents such
as strong acids, which results in the formation of carboxylic or hydroxyl groups (-
COOH, -OH) on the surface of nanotubes (Coleman, 2000, 2006, Singh, 2009). This
type of functionalization is known as defect group functionalization. Such
Fig.2.14, Possibilities for the functionalization of SWCNTs a) л-л interaction; b) defect
group functionalization ; c) non-covalent functionalization with polymers [20]

48
functionalization improves nanotube dispersion in solvents and polymers and imparts
high stability in polar solvents.
To ensure the adhesion between polymer and nanotubes various surfactant and
chemical modification procedures have been adopted to modify the surface of otherwise
inert surface of CNTs that provides bonding sites to the polymer matrix. So the surface
modification of CNTs is the crucial factor that decides the effective dispersion and
improves the interactions between CNTs and matrix.
However there are certain drawbacks of using chemically functionalized CNTs.
Chemical functionalization normally employs harsh techniques resulting in tube
fragmentation and also disrupts the bonding between graphene sheets and thereby
reduces the properties of CNTs. Also the chemical functionalized CNTs significantly
decrease the electrical conductivity of CNTs nanocomposites. [17]
i. Dispersion of high loading of CNTs in polymer matrix
Dispersion of high loading of CNTs in any polymer is very difficult due to the
formation of agglomerates by the conventional techniques. To maximize the
improvement in properties, higher loading of CNTs is preferred. However, polymer
composites synthesized by using the conventional methods generally have low CNT
contents. It has been observed that beyond 1 wt.-% of loading, CNTs tend to
agglomerate resulting in poor mechanical properties of the composites. It is therefore
important to develop a technique to incorporate higher CNT loading in the polymer
matrices without sacrificing their mechanical properties. Recently, several methods
have been developed for fabricating CNT/polymer composites with high CNT loadings
[17].
Fig.2.15, Covalent functionalization of carbon nanotubes on defects sites [20]

49
2.3.2.b. Adhesion between CNTs and Polymer
The second key challenge is in creating a good interface between nanotubes and
the polymer matrix. From the research on microfiber based polymer composites over
the past few decades, it is well established that the structure and properties of filler-
matrix interface plays a major role in determining the structural integrity and
mechanical performance of composite materials. CNTs have atomically smooth non-
reactive surfaces and as such there is a lack of interfacial bonding between the CNT and
the polymer chains that limits load transfer. Hence the benefits of high mechanical
properties of CNTs are not utilized properly. There are three main mechanisms for load
transfer from matrix to filler:
i. The first is weak van der Waal interaction between filler and polymer. Using
small size filler and close contact at the interface can increase it. The large specific
surface area of CNTs is advantageous for bonding with matrix in a composite, but is a
major cause for agglomeration of CNTs. Therefore, uniformly dispersed individual
nanotubes in matrix is helpful.
ii. The second mechanism of load transfer is micromechanical interlocking
which is difficult in CNTs nanocomposites due to their atomically smooth surface.
However, local non uniformity along length of CNTs i.e. varying diameter and bends
due to non-hexagonal defects contributes to this micromechanical interlocking. This
interlocking can increase by using long CNTs to block the movement of polymer
chains. The contribution of this mechanism may reach saturation at low CNT content.
iii. The third and best mechanism for better adhesion and hence load transfer
between CNTs and polymer is covalent or ionic bonding between them. The chemical
bonding between CNTs and polymer can be created and enhanced by the surface
treatment such as oxidation of CNTs with acids or other chemicals. This mode of
mechanism have much importance as it provides strong interaction between polymer
and CNT and hence efficiently transfers the load from polymer matrix to nanotubes
necessary for enhanced mechanical response in high-performance polymers. [17]
1.1.1.b. Alignment of CNTs in Polymer Matrix
Another key challenge is to understand the effect of nanotube alignment on
nanocomposites properties because the nanotubes have asymmetric structure and
properties. Like other one-dimensional fiber fillers CNTs displays highest properties in

50
the oriented reinforced direction and the mechanical, electrical, magnetic and optical
performance of its composites are linked directly to their alignment in the matrix. So to
take the full advantage of excellent properties of CNTs these should be aligned in a
particular direction. For example, the alignment of CNT increases the elastic modulus
and electrical conductivity of nanocomposites along the nanotube alignment direction.
Several methods like application of electric field during composite formation and
carbon arc discharge, composite slicing, film rubbing, chemical vapour deposition,
mechanical stretching of CNT-polymer composites and magnetic orientation have been
reported for aligning nanotubes in composites. Electrospinning is also an effective
method for the alignment of CNTs in polymer matrix. [17]
2.3.3. Properties of the Nanocomposites
1.1.1.c. Mechanical Properties of MWCNTs Polymer Nanocomposites
Different thermoplastic and thermoset polymer matrices have been tried to realize
the superior mechanical properties of CNTs for development of light weight strong
material. NASA scientists are considering CNT-polymer composite for space elevator.
Du et al. studied the experimental results for mechanical performance of CNTs
nanocomposites carried out by different research groups and observed that the gains are
modest and far below the simplest theoretical estimates. Haggenmueller applied the
Halpen Tsai composite theory to CNT nanocomposites and observed that the
experimental elastic modulus is smaller than predicted by more than one order. It is
attributed to the lack of perfect load transfer from nanotubes to matrix due to non-
uniform dispersion and small interfacial interaction. Although chemical
functionalization of CNTs has sorted out those problems to an extent yet the best results
have to be achieved. Also aspect ratio is other source of uncertainty in mechanical
properties. Defects on the CNT surface also expected to influence the mechanical
properties significantly. The methods of handling nanotubes, including acid treatments
and sonication for long time are known to shorten nanotubes results in decreasing aspect
ratio and are detrimental to mechanical properties. The mechanical properties of CNT
based composites increased up to certain loading of CNTs and beyond it starts
decreasing. This may be because of increase in viscosity of polymers at higher CNTs
loading and also cause some surface of CNTs not to be completely covered by polymers
matrix due to the large specific surface area of CNTs. [17]

51
Researchers have observed that that the mechanical properties are always higher
for aligned CNTs composites with higher loading while the case is different for
isotropic CNT polymer composites. [17]
2.3.3.a. Electrical properties of MWCNTs Polymer Nanocomposites
i. Conductivity
CNTs because of their extraordinary electrical conductivity are also excellent
additive to impart electrical conductivity to polymers. The percolation theory can be
applied to explain the electrically conducting behaviour of composites consisting of
conducting fillers and insulating matrices. When the conducting filler content is
gradually increased, the composite undergoes an insulator-to-conductor transition. The
critical filler content is referred to as the percolation threshold where the measured
electrical conductivity of the composite sharply jumps up several orders of magnitude
due to the formation of continuous electron paths or conducting networks. Below the
percolation transition range, electron paths do not exist and the electrical properties are
dominated by the matrix material. Above the percolation transition range, multiple
electron paths exist in the matrix so that the electrical conductivity of the composite
often shows a saturation plateau. This behaviour is graphically shown in Fig.2.16B [19].
Fig.2.16 and Fig.2.17 show the general trend of electrical conductivity of CNT-
polymer nanocomposites. It can be found from almost all the experimental results and
also obvious from the figure that CNT nanocomposites exhibit a typical percolation
behaviour and CNT reinforcement to polymers can increase the conductivity of
Fig.2.16, Typical applications of conducting composites (A) and a schematic of percolation
phenomenon and conducting network in conducting composites (B). [19]

52
resulting composites to several order of magnitude or even some times higher than ten
orders of magnitude.
According to percolation theory the conductivity follow the following power law
close to threshold percolation.
where σ is the composite conductivity, σ o is a constant , p the weight fraction of
nanotubes, p o is the percolation threshold and t the critical exponent.
Many experimental results shows that the conductive CNT composites can be
constructed at low loading of CNTs due to low percolation threshold originated from
the high aspect ratio and conductivity of CNTs [17] [19].
Fig.2.17 shows the percolation threshold of nanocomposites ﬁlled with CNTs and
for different polymers [9].
The current-voltage measurements exhibited non-ohmic behaviour, which is most
likely due to tunnelling conduction mechanism. The main mechanism of conduction
between adjacent nanotubes is probably electron hopping when their separation distance
is small. At concentration greater than percolation threshold, conducting paths are
formed through the whole nanocomposites, because the distance between the
conductive CNT filler (individual or bundles) is small enough to allow efficient electron
hopping.
Fig.2.17, a) General trend of electrical conductivity of CNT polymer composites [17]. b) Percolation
threshold of CNT/polymer nanocomposites. (PA (Nylon): polyamide; PB: polybutylene; PE:
polyethylene; PI: polyimide; PP: polypropylene; PS: polystyrene; PVA: poly (vinyl acetate); PMMA:
poly(methyl ethacrylate). EP: epoxy; PU: polyurethane; VR: vulcanized rubber.) [19]
a) b)

53
The electrical conductivity of CNT/polymer composites is also affected by
dispersion and aspect ratio of CNTs. In most of the cases the CNT nanocomposites with
isotropic nanotubes orientation have greater electrical conductivity than the
nanocomposites with highly aligned CNTs especially at lower CNT loadings. By
alignment of CNTs in polymers, the percolation pathway is destroyed as aligned CNTs
seldomly intersects each other. At higher CNTs loading the conductivity is more in case
of aligned CNTs as compared to randomly oriented CNTs. [17]
i. Resistivity
For a polymer to be electrically conductive, nano-scaled fillers must either
physically touch to form electron conducting path, or be sufficiently close to each other
to allow electron transfer via tunneling effect‖ [22]. The CNTs create a electrical
network where with a resistance that depends on the number of interconnection nodes
and distances between neighbouring carbon nanotubes. For tunneling of electrons
between CNTs to occur, the distance between neighbouring CNTs needs to be on the
scale of nanometers. When a uniaxial tensile strain is applied to the nanocomposite, as
shown in Fig.2.18, carbon nanotubes are separated apart, leading to loss of contact
points and widening of inter-tubular distances. This impedes the electron transferring
ability of the conductive network and causes overall resistance to rise. Similarly, when
the nanocomposite relaxes electron conduction paths are restored, therefore resistance
drops along with decreasing strain. [22]
ii. EMI Shielding Properties of MWCNTs Polymer Nanocomposites
The electrical conductivity of CNT reinforced polymer composites makes them a
very suitable candidate to be employed for electromagnetic interference (EMI)
shielding. EMI is the process by which disruptive electromagnetic energy is transmitted
Fig.2.18, Schematic diagram showing the interconnection and spacing change of carbon
nanotubes when a PDMS-CNTs nanocomposite is exposed to tensile strain [21]

54
from one electronic device to another via radiation or conduction. As we all know that
the electromagnetic waves produced from some electronic instrument have an adverse
effect on the performance of the other equipment present nearby causing data loss,
introduction of noise, degradation of picture quality etc. So it a strong desire to shield
electronics equipment from the undesired signals. Problems with EMI can be minimized
or sometime eliminated by ensuring that all electronic equipment are operated with a
good housing to keep away unwanted radio frequency from entering or leaving. The
shielding effectiveness (SE) of the shielding material is its ability to attenuate the
propagation of electromagnetic waves through it and measured in decibels (dB) given
by the following equation:
SE (dB) = − 10 log ( P t / P 0 ) ,
where P t and P 0 are, respectively, the transmitted and incident electromagnetic
power. A SE of 10 dB means 90% of signal is blocked and 20 dB means 99% of signal
is blocked.
One of the important criterion for a material to be used for EMI shielding material
is that it should be electrically conducting. Because of their high electrical conductivity
metals have been used for past several years as EMI shielding materials. But the
shortcomings of metals like heavy weight, physical rigidity and corrosion restricts their
use. The most notable substance that could overcome these shortcomings is the CNT-
polymer composites. As discussed in previous sections these are electrically conductive,
having low density, corrosion resistant and can be molded in any form. Due to easy
processing and good flexibility, CNT-polymer composites have been employed for
application as promising EMI shielding materials.
There are few additional advantages of using MWCNTs as EMI shielding
material. The EMI SE also depends on the source of origin of electromagnetic waves.
Electrically conducting material can effectively shield the electromagnetic waves
generated from an electric source, whereas magnetic materials effectively shield the
electromagnetic waves generated from a magnetic source. The MWCNTs exhibits
electrical properties because of presence of pi electrons and magnetic properties because
of the presence of catalytic iron particles in tubes. Also one common problem
experienced with commonly used composite materials for EMI shielding is build-up of
heat in the substance being shielded. The possible solution for this is to add thermal
conducting material. Composites with MWCNTs can easily overcome this problem as it
has high thermal conductivity. [17]

55
2.3.3.b. Thermal Properties of MWCNTs Polymer Nanocomposites
As discussed above that the CNTs have thermal conductivity as high as
6600W/mK predicted for SWCNTs at room temperature and have experimental value
3000W/mK for isolated MWCNT. So it is quite expected that the reinforcement of
CNTs can significantly enhance the thermal properties of CNT-polymer
nanocomposites. The improvement in thermal transport properties of CNT polymer
composites leads their applications for usage as printed circuit boards, connectors,
thermal interface materials, heat sinks. [17]
2.4. Conclusions
In the first part of the chapter, a variety of present and proposed applications for
individual CNT were presented. Then, the properties, problems and possibilities of CNT
nanocomposites are explained in detail.
The large variety of applications exposed is noticeable. The insufficient
availability of technology, both to obtain affordable good quality CNT and to precisely
manipulate the CNTs, is the main obstacle that blocks the development of new
applications. Once this impediment is resolved only the scientists‘ imagination will
slow down the apparition of new revolutionary technologies.
Until a technology to manipulate individual nanotubes is widely extended, plenty
of work can still be done in the field of CNTs nanocomposites, where the use of such
expensive equipment is not essential. For this reason, it was decided that our work had
to focus on using the properties of CNT nanocomposites that could help us for energy
harvesting.
In the next chapter, the physical phenomena that combined with the CNT
nanocomposites properties will complete the proposed energy harvesting device are
introduced.

56
3. Energy Harvesting
3.1. Energy Harvesting Sources and Technologies
Energy harvesting, also referred to as ―energy scavenging‖ or ―energy
extraction‖, can be deﬁned as ―converting ambient energies such as vibration,
temperature, light, RF energy, etc. to usable electrical energy thanks to energy
conversion materials or structures, and subsequently storing the electrical energy in
order to power electric devices. In other words, the general concept of energy
harvesting is to convert unusable or wasted energy from the environment into a more
useful form. [25]
The form of energy that is most useful in modern applications is often electrical
energy, since it can be stored in a battery or used to power electrical circuits. The
harvesting of energy from ambient environments is an emerging technology with
promise for numerous applications such as low-power electronic devices or renewable
energy. Technological advances and scientific research trend is heading towards the
development of smaller and more energy efficient devices where MEMs and NEMs
play a decisive role. This opens an exciting field for a new power supplying philosophy,
where smaller delocalized power supplies are a promising alternative to the traditional
wired networks. The increasing number of independent miniature electronic devices and
their need for sufficient, reliable power supply make micro energy harvesting more
appealing.[24]
Fig.3.1, Possible energy sources and applications for energy harvesting devices [26]

Master thesis: Energy harvesting using cnt. Javier Latasa

Master thesis: Energy harvesting using cnt. Javier Latasa

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