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).

4

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:

Energy Harvesting with Carbon Nanotubes

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

5

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

Energy Harvesting with Carbon Nanotubes

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.

6

"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.

7

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

8

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

9

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

10

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

11

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 2s

2 and 2p

2 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

12

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]

13

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]

14

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

18

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 10

5 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 50–500C, 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]

57

In the last decade, energy scavengers have been utilized by a vast number of

applications, including embedded and implanted sensor nodes for medical applications,

distributed wireless sensor nodes for structural health monitoring, battery recharging,

monitoring tire pressure in automobiles, powering unmanned vehicles, and running

security systems in household conditions. This trend has driven the development and

advancement in energy harvesting materials, electronics, and integration. Human

motion, low-frequency seismic vibrations, and acoustic noise are some of the sources

which can be exploited to harvest energy.

3.1.1. Vibrations Energy Harvesting

There are three major energy transduction mechanisms to harvest energy from

vibrations, namely electrostatic, electromagnetic and piezoelectric approaches:

i. Electrostatic (capacitive) energy harvesting is based on the phenomenon of

changing capacitance of vibration-dependent varactors (variable capacitors).

The initially charged varactors are separated by vibrations, thereby transducing

the mechanical energy into electrical energy.

ii. Electromagnetic induction; A magnet induces current in a coil as it moves and

magnetic field lines cross the coil.

iii. Piezoelectric energy harvesting operates using the direct piezoelectric effect that

converts mechanical strain into electrical current. [25]

Fig.3.2, Energy Harvesting estimates for different sources type [27]

58

3.1.2. Energy Harvesting Devices

In most cases, the available energy sources provide energy in very small packets

that have been previously difficult if not nearly impossible to capture for use. The

energy management must be well-defined –– and tolerate a wide range of voltage,

current and waveform inputs, including over-voltage, overcharge and other irregular

input conditions.

The classic (high-efficiency) energy-harvester system consists of an energy

generator, capture/storage/management electronics and a load designed to be powered

by the harvester, typically a wireless sensor network. In the block diagram above, a

piezoelectric crystal membrane is shown as the energy generator. The piezoelectric

generator transforms mechanical vibrations, strain or stress into electrical

voltage/current. [28]

3.1.3. Piezoelectric Energy Harvesting

Electroactive materials such as piezoelectrics and electrostrictors have been

chosen for the conversion of mechanical energy to electrical energy in several energy-

harvesting applications. Piezoelectric materials have gained popularity because of their

possible integration and their coupling coefficient. However, for applications requiring

large strains and low frequencies, typically for harvesting energy on human movement,

piezoelectric materials are not the prime candidates, since these materials tend to be stiff

and limited in mechanical strain abilities. Rather, organic materials seem interesting due

to their flexibility, low cost and the fact that they can be deposited on large surfaces.

Fig.3.3, Schematic block diagram for a classic energy-harvesting system [28]

59

3.1.4. Power Harvesting Using CNTs

The potential to use CNTs as actuators has been investigated since 1999.

However, using the CNT as a power harvesting system, an interesting related

application, has not been studied much. The CNT generates electric energy when it is

immersed in a flowing electrolyte. The generation of electric current in CNT when it is

immersed in flowing liquids has been theoretically predicted and recently validated

experimentally. Kral and Sharpiro reported that metallic CNTs immersed in flowing

liquids generate an electrical current because the ions in the liquid have a coulomb drag

effect on the free charge carriers in the CNTs.

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]

3.2. Piezoelectricity

3.2.1. History of Piezoelectricity

The first scientific publication describing the phenomenon, later termed as

piezoelectricity, appeared in 1880. It was co-authored by Pierre and Jacques Curie, who

were conducting a variety of experiments on a range of crystals at the time. In those

experiments, they catalogued a number of crystals, such as tourmaline, quartz, topaz,

cane sugar and Rochelle salt that displayed surface charges when they were

mechanically stressed.

In the scientific community of the time, this observation was considered as a

significant discovery, and the term ―piezoelectricity‖ was coined to ex-press this effect.

The word ―piezo‖ is a Greek word which means ―to press‖. Therefore, piezoelectricity

Fig.3.4, Adaptive energy harvesting piezoelectric circuit [29]

60

means electricity generated from pressure - a very logical name. This terminology

helped distinguish piezoelectricity from the other related phenomena of interest at the

time; namely, contact electricity (Static electricity generated by friction) and

pyroelectricity (Electricity generated from crystals, when heated).

The discovery of the direct piezoelectric effect is, therefore, credited to the Curie

brothers. They did not, however, discover the converse piezoelectric effect. Rather, it

was mathematically predicted from fundamental laws of thermodynamics by Lippmann

in 1881. Having said this, the Curies are recognized for experimental confirmation of

the converse effect following Lippmann‘s work.

The discovery of piezoelectricity generated significant interest within the

European scientific community. The first serious application for piezoelectric materials

appeared during World War I. This work is credited to Paul Langevin and his co-

workers in France, who built an ultrasonic submarine detector. The transducer they built

was made of a mosaic of thin quartz crystals that was glued between two steel plates in

a way that the composite system had a resonance frequency of 50 KHz. The device was

used to transmit a high-frequency chirp signal into the water and to measure the depth

by timing the return echo. Their invention, however, was not perfected until the end of

the war.

Following their successful use in sonar transducers, and between the two World

Wars, piezoelectric crystals were employed in many applications. Quartz crystals were

used in the development of frequency stabilizers for vacuum-tube oscillators. Ultrasonic

transducers manufactured from piezoelectric crystals were used for measurement of

material properties. Many of the classic piezoelectric applications that we are familiar

Fig.3.5, Marie Curie (1867

- 1934), Polish-born

physicist in her laboratory

and husband Pierre (1859 -

1906), eminent French

chemist, with the quartz

piezo-electroscope he

invented, by which rays of

radium are measured.

(Photo by Hulton

Archive/Getty Images)

61

with, applications such as microphones, accelerometers, ultrasonic transducers, etc.,

were developed and commercialized in this period.

Development of piezoceramic materials during and after World War II helped

revolutionize this field. During World War II, significant research was performed in the

United States and other countries such as Japan and the former Soviet Union which was

aimed at the development of materials with very high dielectric constants for the

construction of capacitors. Piezoceramic materials were discovered as a result of these

activities, and a number of methods for their high-volume manufacturing were devised.

The ability to build new piezoelectric devices by tailoring a material to a specific

application resulted in a number of developments, and inventions such as: powerful

sonars, piezo ignition systems, sensitive hydrophones and ceramic phono cartridges, to

name a few. [33]

3.2.2. Piezoelectric Ceramics

A piezoelectric ceramic is a mass of perovskite crystals. Each crystal is composed

of a small, tetravalent metal ion placed inside a lattice of larger divalent metal ions and

O2, as shown in Fig.3.6.

To prepare a piezoelectric ceramic, fine powders of the component metal oxides

are mixed in specific proportions. This mixture is then heated to form a uniform

powder. The powder is then mixed with an organic binder and is formed into specific

shapes, e.g. discs, rods, plates, etc. These elements are then heated for a specific time,

and under a predetermined temperature. As a result of this process the powder particles

sinter and the material forms a dense crystalline structure. The elements are then cooled

and, if needed, trimmed into specific shapes. Finally, electrodes are applied to the

appropriate surfaces of the structure.

Fig.3.6, Crystalline structure of a piezoelectric ceramic, before and after polarization [32]

62

Above a critical temperature, known as the ―Curie temperature‖, each perovskite

crystal in the heated ceramic element exhibits a simple cubic symmetry with no dipole

moment, as demonstrated in Fig.3.7. However, at temperatures below the Curie

temperature each crystal has tetragonal symmetry and, associated with that, a dipole

moment. Adjoining dipoles form regions of local alignment called ―domains‖. This

alignment gives a net dipole moment to the domain, and thus a net polarization. As

demonstrated in Fig.3.7(a), the direction of polarization among neighbouring domains is

random. Subsequently, the ceramic element has no overall polarization.

The domains in a ceramic element are aligned by exposing the element to a

strong, DC electric field, usually at a temperature slightly below the Curie temperature

Fig.3.7(b). This is referred to as the ―poling process‖. After the poling treatment,

domains most nearly aligned with the electric field expand at the expense of domains

that are not aligned with the field, and the element expands in the direction of the field.

When the electric field is removed most of the dipoles are locked into a configuration of

near alignment Fig.3.7(c). The element now has a permanent polarization, the remnant

polarization, and is permanently elongated. The increase in the length of the element,

however, is very small, usually within the micrometre range.

Fig.3.8, Reaction of a poled piezoelectric element to applied stimuli [33]

Fig.3.7, Poling process: (a) Prior to polarization polar domains are oriented randomly; (b) A very

large DC electric field is used for polarization; (c) After the DC field is removed, the remnant

polarization remains [32]

(a) (b) (c)

63

Properties of a poled piezoelectric ceramic element can be explained by the series

of images in Fig.3.8. Mechanical compression or tension on the element changes the

dipole moment associated with that element. This creates a voltage. Compression along

the direction of polarization, or tension perpendicular to the direction of polarization,

generates voltage of the same polarity as the poling voltage (Fig.3.8(b)), Tension along

the direction of polarization, or compression perpendicular to that direction, generates a

voltage with polarity opposite to that of the poling voltage (Fig.3.8(b)), When operating

in this mode, the device is being used as a sensor. That is, the ceramic element converts

the mechanical energy of compression or tension into electrical energy. Values for

compressive stress and the voltage (or field strength) generated by applying stress to a

piezoelectric ceramic element are linearly proportional, up to a specific stress, which

depends on the material properties. The same is true for applied voltage and generated

strain3. If a voltage of the same polarity as the poling voltage is applied to a ceramic

element, in the direction of the poling voltage, the element will lengthen and its

diameter will become smaller (Fig.3.8(c)). If a voltage of polarity opposite to that of the

poling voltage is applied, the element will become shorter and broader (Fig.3.8(d)). If

an alternating voltage is applied to the device, the element will expand and contract

cyclically, at the frequency of the applied voltage. When operated in this mode, the

piezoelectric ceramic is used as an actuator. That is, electrical energy is converted into

mechanical energy. [33]

3.2.3. Piezoelectric Constitutive Equations

In this section we introduce the equations which describe electromechanical

properties of piezoelectric materials. The presentation is based on the IEEE standard for

piezoelectricity which is a widely accepted standard that assumes that piezoelectric

materials are linear (Truth at low electric fields and at low mechanical stress levels.

When a poled piezoelectric ceramic is mechanically strained it becomes

electrically polarized, producing an electric charge on the surface of the material. This

property is referred to as the ―direct piezoelectric effect‖ and is the basis upon which the

piezoelectric materials are used as sensors. Furthermore, if electrodes are attached to the

surfaces of the material, the generated electric charge can be collected and used. This

property is particularly utilized in piezoelectric shunt damping applications.

64

The constitutive equations describing the piezoelectric property are based on the

assumption that the total strain in the transducer is the sum of mechanical strain induced

by the mechanical stress and the controllable actuation strain caused by the applied

electric voltage. The axes are identified by numerals rather than letters. As is shown on

Fig.3.10.

The describing electromechanical equations for a linear piezoelectric material can

be written as:

εi = SijE σj + dmiEm

Dm = dmiσi +ξikσ Ek

where the indexes i, j = 1, 2, . . . , 6 and m, k = 1, 2, 3 refer to different directions within

the material coordinate system, as shown in Fig.3.10. The above equations can be

rewritten in the following form, which is often used for applications that involve

sensing:

εi = SijD σj + gmiDm

Ei = gmiσi + βikσ Dk

Fig.3.9, Schematic diagram of piezoelectric transducer [33]

Fig.3.10, Axis nomenclature [33]

(5.1)

(5.2)

(5.3)

(5.4)

65

where σ stress vector (N/m

2)

ε strain vector (m/m)

E vector of applied electric field (V/m)

ξ permitivity (F/m)

d matrix of piezoelectric strain constants (m/V )

S matrix of compliance coefficients (m2 /N )

D vector of electric displacement (C/m2)

g matrix of piezoelectric constants (m2/C)

β impermitivity component (m/F )

Furthermore, the superscripts D, E, and σ represent measurements taken at

constant electric displacement, constant electric field and constant stress.

Equations (5.1) and (5.3) express the converse piezoelectric effect, which describe

the situation when the device is being used as an actuator. Equations (5.2) and (5.4), on

the other hand, express the direct piezoelectric effect, which deals with the case when

the transducer is being used as a sensor. The converse effect is often used to determine

the piezoelectric coefficients.

In matrix form, Equations (5.1)-(5.4) can be written as:

[ ]

[

]

[

]

[

]

[

]

[

] [

]

[

]

[

] [

]

Assuming that the device is poled along the axis 3, and viewing the piezo-electric

material as a transversely isotropic material, which is true for piezo-electric ceramics,

many of the parameters in the above matrices will be either zero, or can be expressed in

terms of other parameters. In particular, the non-zero compliance coefficients are:

S11 = S22

S13 = S31 = S23 = S32

(5.5)

(5.6)

66

S12 = S21

S44 = S55

S11 = 2 (S11 – S12)

The non-zero piezoelectric strain constants are

d31 = d32 d15 = d24

Finally, the non-zero dielectric coefficients are eσ

11 = eσ22 and e

σ33. Subsequently,

the equations (5.5) and (5.6) are simplified to:

[ ]

[

– ]

[

]

[

]

[

]

and

[

] [

]

[

]

[

] [

]

The ―piezoelectric strain constant‖ d is defined as the ratio of developed free

strain to the applied electric field. The subscript dij implies that the electric field is

applied or charge is collected in the i direction for a displacement or force in the j

direction.

3.2.4. Piezoelectric Coefficients

This section reviews the physical meaning of some of the piezoelectric

coefficients introduced in the previous section. Namely dij , gij , Sij and eij .

Fig.3.11, A piezoelectric transducer arrangement for d31 measurement [33]

(5.8)

(5.7)

67

3.2.4.a. Piezoelectric Constant dij

The piezoelectric coefficient dij is the ratio of the strain in the j-axis to the electric

field applied along the i-axis, when all external stresses are held constant. In Fig.3.11, a

voltage of V is applied to a piezoelectric transducer which is polarized in direction 3.

This voltage generates the electric field

which strains the transducer. In particular

in which

The piezoelectric constant d31 is usually a negative number. This is due to the fact

that application of a positive electric field will generate a positive strain in direction 3.

Another interpretation of dij is the ratio of short circuit charge per unit area flowing

between connected electrodes perpendicular to the j direction to the stress applied in the

i direction. As shown in Fig.3.12, once a force F is applied to the transducer, in the 3

direction, it generates the stress which results in the electric charge q flowing through

the short circuit:

q = d33F

If a stress is applied equally in 1, 2 and 3 directions, and the electrodes are

perpendicular to axis 3, the resulting short-circuit charge (per unit area), divided by the

applied stressed is denoted by dp.

Fig.3.12, Charge deposition on a piezoelectric transducer - An equal, opposite force, F , is not shown [33]

(5.11)

(5.12)

(5.10)

(5.9)

(5.13)

68

3.2.4.b. Piezoelectric Constant gij

The piezoelectric constant gij signifies the electric field developed along the i-axis

when the material is stressed along the j-axis. Therefore, in Fig.3.13 a), the applied

force F , results in the voltage

Another interpretation of gij is the ratio of strain developed along the j-axis to the

charge (per unit area) deposited on electrodes perpendicular to the i-axis. Therefore, in

Fig.53.13b), if an electric charge of Q is deposited on the surface electrodes, the

thickness of the piezoelectric element will change by

3.2.4.c. Elastic Compliance Sij

The elastic compliance constant Sij is the ratio of the strain the in i-direction to the

stress in the j-direction, given that there is no change of stress along the other two

directions. Direct strains and stresses are denoted by indices 1 to 3. Shear strains and

stresses are denoted by indices 4 to 6. Subsequently, S12 signifies the direct strain in the

1-axis when the device is stressed along the 2-axis, and stresses along directions 1 and 3

are unchanged. Similarly, S44 refers to the shear strain around the 2-axis due to the shear

stress around the same axis.

A superscript ―E‖ is used to state that the elastic compliance SijE is measured with

the electrodes short-circuited. Similarly, the superscript ―D‖ in SijD denotes that the

measurements were taken when the electrodes were left open-circuited. A mechanical

Fig.3.13, a) An open-circuited piezoelectric transducer under a force in direction 1 - An equal, but

opposite force, F , is not shown. b) A piezoelectric transducer subject to applied charge. [33]

(5.15)

(5.14)

69

stress results in an electrical response that can increase the resultant strain. Therefore, it

is natural to expect SijE to be smaller than Sij

D . That is, a short-circuited piezo has a

smaller Young‘s modulus of elasticity than when it is open-circuited. [33]

3.2.4.d. Dielectric Coefficient eij

The dielectric coefficient eij determines the charge per unit area in the i-axis due to

an electric field applied in the j-axis. In most piezoelectric materials, a field applied

along the j-axis causes electric displacement only in that direction. The relative

dielectric constant, defined as the ratio of the absolute permitivity of the material by

permitivity of free space, is denoted by K. The superscript σ in eσ11 refers to the

permitivity for a field applied in the 1 direction, when the material is not restrained. [33]

3.2.4.e. Piezoelectric Coupling Coefficient kij

The piezoelectric coefficient kij represents the ability of a piezoceramic material to

transform electrical energy to mechanical energy and vice versa. This transformation of

energy between mechanical and electrical domains is employed in both sensors and

actuators made from piezoelectric materials. The ij index indicates that the stress, or

strain is in the direction j, and the electrodes are perpendicular to the i-axis. For

example, if a piezoceramic is mechanically strained in direction 1, as a result of

electrical energy input in direction 3, while the device is under no external stress, then

the ratio of stored mechanical energy to the applied electrical energy is denoted as k312.

There are a number of ways that kij can be measured. One possibility is to apply a

force to the piezoelectric element, while leaving its terminals open-circuited. The

piezoelectric device will deflect, similar to a spring. This deflection z, can be measured

and the mechanical work WM done by the applied force F can be determined

Due to the piezoelectric effect, electric charges will be accumulated on the

transducer‘s electrodes. This amounts to the electrical energy WE

which is stored in the piezoelectric capacitor. Therefore,

√

√

(5.17)

(5.16)

(5.18)

70

When a force is applied to a piezoelectric transducer, depending on whether the

device is open-circuited or short-circuited, one should expect to observe different

stiffness. In particular, if the electrodes are short-circuited, the device will appear to be

―less stiff‖. This is due to the fact that upon the application of a force, the electric

charges of opposite polarities accumulated on the electrodes cancel each other.

Subsequently no electrical energy can be stored in the piezoelectric capacitor. [33]

3.2.5. Piezoelectric Sensor/Generator

When a piezoelectric transducer is mechanically stressed, it generates a volt-age.

This phenomenon is governed by the direct piezoelectric effect (5.2). This property

makes piezoelectric transducers suitable for sensing applications. Compared to strain

gauges, piezoelectric sensors offer superior signal to noise ratio, and better high-

frequency noise rejection. Piezoelectric sensors are, therefore, quite suitable for

applications that involve measuring low strain levels. They are compact, easy to embed

and require moderate signal conditioning circuitry. [33]

If a PZT sensor is subject to a stress field, assuming the applied electric field is

zero, the resulting electrical displacement vector is:

[

] [

]

[

]

The generated charge can be determined from

∬[ ] [

]

where dA1 , dA2 and dA3 are, respectively, the differential electrode areas in the 2-3, 1-3

and 1-2 planes. The generated voltage Vp is related to the charge via

where Cp is capacitance of the piezoelectric sensor.

Having measured the voltage, Vp, strain can be determined by solving the above

integral. If the sensor is a PZT patch with two faces coated with thin electrode layers,

e.g. the patch in Fig.3.13a, and if the stress field only exists along the 1-axis, the

capacitance can be determined from

(5.22)

(5.19)

(5.20)

(5.21)

71

Assuming the resulting strain is along the 1-axis, the sensor voltage is found to be

∫

where Ep is the Young‘s modulus of the sensor and ε1 is averaged over the sensor‘s

length. The strain can then be calculated from

In deriving the above equation, the main assumption was that the sensor was

strained only along 1-axis. If this assumption is violated, which is often the case, then

the equation (5.24) should be modified to

where is the Poisson‘s ratio.

3.2.5.a. Piezoelectric Generator Performance

Piezoelectric generators are usually specified in terms of their closed-circuit

current (or charge) and open-circuit voltage. Closed-circuit current, ICC, refers to the

total current developed, at the maximum recommended strain level and operating

frequency, when the charge is completely free to travel from one electrode to the other,

and not asked to build up voltage. Open-circuit voltage, Voc, refers to the voltage

developed at the maximum recommended strain level, when charge is prohibited from

traveling from one electrode to the other.

Fig.3.14, Simplified representation of piezoelectric coupling. [48]

(5.23)

(5.25)

(5.24)

72

The amount of energy extracted from a piezoelectric transducer is greatly

dependent on its load. Optimum load must be connected to the transducer for optimum

energy transfer. In practice, it is nearly impossible to always match the circuit load with

the optimum load required. Hence, interfacing circuit is essential. This interfacing

circuit must be able to provide fix optimum impedance to the transducer regardless the

actual circuit load connected. [34]

Fig.3.16. shows the characteristic output curve for a piezoelectric generator. We

can clearly observe here, the importance of matching the load to maximize the power

generated. At low values of resistance no voltage is produced (short circuit) and no

power is generated. At high resistances (open circuit) no current flows and no power is

generated.

Fig.3.16, Power as a function of resistive load: 1-Hz 800-N sine wave. [30]

Fig.3.15, Power as a function of resistive load: 1-Hz 800-N sine wave. [29]

73

3.2.5.b. Output Power Optimization

Regardless of energy source, the key to effective energy harvesting from

biological sources remains one of maximizing the transducer's output. Piezoelectric

transducers are constant impedance devices, reflecting a characteristic current-voltage

profile (Fig.3.17).

For these devices, maximizing power output means maintaining the load on the

transducer at its maximum power point (MPP). Operation at loads significantly away

from MPP result in a significant reduction of power from the transducer, and power

available to charge local energy devices or the load directly. For stable environments,

MPP is typically found at one-half the transducer's open-circuit voltage value.

Fig.3.18, Resistive impedance matching circuit [35]

Fig.3.17, Efficiency as a function of frequency and resistive load. [30]

74

3.3. Piezoelectric Nano-Generators

3.3.1. Introduction

Energy harvesting using mechanical energy sources is a promising candidate for

energy generation with improved accessibility and eco-compatibility. To harvest

electrical energy from ambient mechanical energies created by natural sources or from

human movements, piezoelectric energy harvesting devices called a nanogenerator

(NG) have been proposed and developed by many researchers [36-42]. The

development of milliwatt (mW)-scale piezoelectric nanogenerator has been attempted to

establish wireless communication-sensor networks and self-powered flexible electronics

in biological and environmental monitoring systems [36].

3.3.2. Piezoelectric Power-Generating Devices Using ZnO Nanowires

A single nanowire can act as a diode and when under mechanical strain can pass

the charges generated to an external circuit under favourable conditions. It conducts in

one direction only, as metal/ZnO contacts mostly result in metal-semiconductor

Schottky contacts, giving a unidirectional flow of current in the external circuit. The

current and power produced by a single nanowire are not enough for real devices. The

integration of a large number of nanowires has been done in an effort to increase the

output power. Key issues for harvesting piezoelectric energy included the simultaneous

generation of piezoelectric potential in a large number of nanowires, the extraction of

that energy and the application of the same to an external load. [38]

Fig.3.20, (A) Schematic diagram showing the design

and structure of the nano-generator built using ZnO

nanowires and platinum-coated Si zigzag electrode.

(B) Cross-sectional SEM image of the nano-

generator made of aligned ZnO nanowires and a

platinum-coated Si zigzag electrode [38].

Fig.3.19, Principle of power generation in a ZnO

NW: (A) Schematic of the NW. (B) Longitudinal

strain εz distribution in the NW after deflection

by an AFM tip. Image shows a FEMLAB

simulation for a ZnO NW of length 1 μm and an

aspect ratio of 10. (C) Electric field distribution

in the NW induced by the piezoelectric effect. (D)

Potential distribution in the NW [38].

75

Many researching groups such as I. Dakua and N. Afzulpurkar based their work

on ZnO properties to develop a Nanocomposite generator There have been reported

output values up to 1.26 V, with a maximum current of 28.8n.A and output power

density values up to 2.7 mW/cm3. [38]

3.3.3. Nano-Generators Using Other Piezoelectric Materials

While ZnO nanowires had been the principal material for the exploitation of

piezoelectric energy generation, some other ceramic crystals and polymer materials

have been used for the same purpose at the bulk level and have been demonstrated to

give good power generation at the atomic-scale as well. Nano-generators built using

nanofibres of PZT and PVDF have been reported and have attracted attention in

research [36-40].

The most common ceramic material used for piezoelectric energy harvesting is

lead zirconium titanate (or PZT). It has exceptional piezoelectric properties. Another

material used for nano-scale energy harvesting is a polymer, polyvinylidene fluoride (or

PVDF). Organic nanofibres of PVDF are lightweight, flexible, bio-compatible and can

be produced in different sizes and shapes. PZT comes with an inherent polar crystalline

structure, like ZnO, while PVDF needs to be converted from a non-polar α-phase to a

polar β-phase by applying an electric field through a process called ‗poling‘.

3.3.3.a. Nano-Generators Using PZT Nanowires

Lead zirconate titanate (PZT) has been used for piezoelectric energy generation at

the macro-scale. PZT nanofibres are found to have a higher piezoelectric voltage

constant than semiconducting nanowires due to their inherent polar crystal structure and

high dielectric value, and they can be synthesized with a very high aspect ratio.

However, bulk PZT and its thin films are extremely fragile, and are not useful for

energy generation under alternating loads. They have been found to be very sensitive to

Fig.3.21, (Left) Schematic of the polar crystal structure of PZT and (Right) the schematics

of the non-polar and polar structures of PVDF [38].

76

high frequency. The problem of fragility, however, disappears for high aspect ratio

nanostructures. Several reserchin groups with different approaches have reported an

output voltage of 1.63 V and a power of 0.03 µW at a load resistance of 6 MΩ (Chen et

al.), an output voltage of ~0.7 V and a current density of 4 µAcm-2

, with an average

power density of 2.8 mWcm-3

(Xu et al.) and an output voltage of 6 V and produce a

current of 45 nA. (Wu et al.). [38]

.

3.3.3.b. Nano-Generators using PVDF Nanofibres

For the first time, polyvinylidene fluoride (or PVDF) nanofibres were directly

written using a near-field electrospinning process by Chang et al. [94] Piezoelectric

properties were produced and tested by electrical poling and in situ mechanical

stretching. Repeatable and consistent electrical output voltages up to 8.5 mV with an

output power of 7.2 pW were achieved under the mechanical stretching of a single

electrospun PVDF nanofibre. The energy conversion efficiency was found to be much

higher than PVDF thin films [94, 95].

3.3.3.c. Nano-Generators Using Barium Titanate

Recently piezoelectric power generation is perovskite BaTiO3, which is not only

piezoelectric but also ferroelectric. Park et al. [42] have demonstrated the use of

BaTiO3 thin films on a flexible substrate for the conversion of mechanical energy into

electrical energy for the first time. By applying a periodic bending force, the nano-

Fig.3.22, Schematic arrangement of a PZT nanofibre-based nano-generator, the distribution of forces

for piezoelectric voltage generation and voltage generated as a result of applied force (clockwise). [38]

77

generator produced an output current density of 0.19 μA/cm2 and a power density of ~7

mW/cm3. Fig.3.23 gives the schematic of the fabrication procedure of the nano-

generator.

The analysis of piezoelectric potential distribution was done for the thin film. The

results obtained are shown in Fig.3.24. It was found that when stretched from both ends,

the potential increased from the bottom of the thin field (at 0 V), which is connected to

the substrate, to a maximum of 0.529 V at the topmost layer.

3.3.4. Nano- Composite Generators (NCGs)

Nanocomposites (NCs) are formed by a polymer matrix and some micro and nano

particles as fillers. Combining the properties of all those elements we can obtain

outstanding materials. The wide range of available polymers allows us to create NCs

with different mechanical properties depending on the applications. In the case of the

Nanocomposite generators (NCGs) piezoelectric particles are introduced as filler inside

a polymer matrix. Other nanomaterials, such as CNTs, might be added when the

mechanical or electrical properties of the resultant NC are to be modified.

In recent years some researchers are investigating in this direction. Several

nanocomposite generators (NCG), using different materials have been reported. C. K.

Jeong et al. (Fig.3.25) [36] presented a Large-Area and Flexible Lead-Free

Nanocomposite Generator Using Alkaline Niobate Particles and Metal Nanorod Filler

reporting an output voltage up to 12 V and current of 1.2 µA. Other study by Kwi-Il

Park et al. (fig.3.26) [37] reported a Highly-Efficient, Flexible Piezoelectric PZT Thin

Fig.3.24, Piezoelectric potential distribution

inside the BaTiO3 thin film. Pure tensile

strain is assumed, bent over 90°, radius

1.0cm. [42]

Fig.3.24, Schematic illustration of the process

for fabricating a flexible BaTiO3 nano-

generator on plastic substrates [42].

78

Film Nanogenerator on Plastic Substrates (1.0 V and 26 nA output with 7 mW·cm -3

achieved).

The same researching group Kwi-Il Park et al. (Fig.3.27) [40] prepared a NCG in

a PDMS matrix using BaTiO3 as piezoelectric element, and CNT for mechanical

reinforcement, dispersing agent and conductivity improver.

Under the continual bending and unbending cycles, the NCG device repeatedly

generates an open-circuit voltage (V) of ~ 3.2 V and a short-circuit current signal of 250

to 350 nA; these output values are produced for a maximum horizontal displacement of

Fig.3.26, a) Schematic of an NCG device using

KNLN particles and Cu NRs. b) Photograph of

the flexible p-NC layer attached to a rolled

paper. The inset shows the final NCG device

bent by fingers. c) Cross-sectional SEM image

of a bent KNLN-based NCG [36]

Fig.3.26, (a) Schematic diagram of the

fabrication process for a high-efficient, flexible,

and large-area PZT thin fi lm-based NG using

the LLO method. (b) and a PET substrate (c)

The insets show the top surfaces of PZT thin

film on sapphire and PET substrates. [37]

Fig.3.27, Schematics of cross-sectional structure of NCG devices and calculated piezopotential

distributions. The CNTs act as dispersing (ii) and stress reinforcing agents (v) which are well

supported by the calculated piezopotential difference (iii and vi). [40]

79

5 mm from an original 4 cm long sample at a deformation rate of 0.2 m ·s - 1 (see Video

S1 for real-time live views of the energy harvesting). The amplitude of the output

voltage generated from. The NCG device increases from 0.2 V to 3.2 V After the poling

process and also depends on the composition of nanomaterials, the angular bending

strain, and strain rate.

Another experiment reported by the same group Kwi-Il Park et al. [39] only last

year shows a thin elastic NCG with PZT micro-particles and CNT in a PDMS matrix

with ITO coated PET on top and bottom as electrodes. They describe a generated open-

circuit voltage and short-circuit current signal of the NCG device with a working area of

3cm × 3cm of up to 10 V and 1.3µA. (See Fig.3.29)

Fig.3.28, (On top) The picture shows the periodic bending experiment as performed. (Bottom)

The output voltages generated from a device containing only BaTiO3 NPs and a NCG device.

The bottom-right insets show the magnified output voltage generated. The bottom-left inset

shows the equivalent circuit of NCG devices. [40]

Fig.3.29, a) After poling process, the dipole moments are well arrayed. The electrons generated by

mechanical stress flow to the top electrode through the circuit, due to the negative piezopotential on the

bottom side. In the unbending state, the accumulated electrons flow back to the bottom electrode. [39]

80

It is important to outstand that all this NCGs created with randomly distributed

piezoelectric particles must be polarized in order to realign the piezoelectric dipoles,

otherwise its performance would be dramatically lower.

3.4. Summary and Conclusions:

Piezoelectricity is naturally available in certain ceramics and crystals. Some

polymers have also been used for piezo-energy harvesting. The various materials in use

to date for energy harvesting at the micro- and nano-scales have been explored.

Although ZnO is the most widely used crystal material for this purpose, some ceramics

and polymers such as PZT, PVDF and more recently BaTiO3 have also been used for

the same purpose.

Ceramic materials like PZT and BaTiO3 have high piezoelectric sensitivity and

coupling coefficients and are easily available. However, they suffer from the loss of

polarization and properties are also dependent on the operating temperatures.

Piezoelectric single crystal materials (ZnO) are easily and economically

synthesized in the required sizes and shapes. They have a high piezoelectric coefficient

and electromechanical coupling, is highly tensile and its properties are not temperature-

dependent [38].

Polymers are not inherently polarized and undergo polarization with the

application of an external field in a special environment. However, because of their

properties of being light-weight, flexible and biocompatible, they are increasingly seen

as potential wearable and foldable energy harvesters for various biomedical applications

and are attracting research [38].

On the other hand, concerning the development of nanocomposite generators,

there are two important challenges to overcome; to generate power and to extract it. For

power generation, piezoelectric elements like the ones described above will be used. To

optimize the energy extraction, CNTs properties are to be exploited.

Additionally, as we could see in the piezoelectricity section of this chapter, for a

successful implementation of any kind of piezoelectric generator it is essential to count

on an adequate conversion circuit to maximize the power extracted after the generation

phase. In other case, the generated energy will not be extracted efficiently.

In the next chapter we analyse and study the Nanocomposite-generator trough

simulation in order to optimize energy that it provides.

81

4. Multiphysiscs Modelling of a Nanoomposite-Generator

4.1. Introduction

A model for the Nanocomposite Generator was created to analyse the system

behaviour before the real experiment is set. The model analysis should allow us to

deeply understand the system working mechanism and the influence that certain

parameters have.

To create such model we must compromise simplicity and accuracy in order to

produce appropriate results with an affordable computation demand and not losing

important information on the way.

COMSOL Multiphysics V4.3 is used for modeling and simulation purposes.

COMSOL is a finite element analysis software package for various engineering and

physics applications, with special emphasis on coupled phenomena (multiphysics).

The model validation will be based on the values from real measurements of

similar Nanocomposite Generator systems that are described on the literature [39][40].

Those systems are also the base for the experiments that are described in the next

chapter.

1µm 24µm

400µm

? µm

? µm

1µm

174µm

PDMS

PET

PET

PDMS

+

CNT

+

PZT

*ITO

layer

*ITO

layer

PDMS

Fig.4.1, (a) Schematic draw of the representative volume, model of the NCG used for simulation.

(b) Detail of the model cross section with thickness values and material distribution.

500µm 1000µm

600µm +

2x PDMS

(insulation)

(a) (b)

82

(a) NCE-55 piezoelectric particles (b) PDMS with MWNTs

(c) PDMS (high voltage isolator) (d) Conductive ITO layer (electrodes) (e) PET

Fig.4.2, Detailed distribution of the different materials in the NCG model

4.2. Model description

4.2.1. Geometry implementation

The model represents a volume, part of the whole NCG. It has the same layer

distribution as the real system but a reduced surface.

A plane PDMS layer is introduced because it has been reported in the literature

that it is needed to avoid extremely high voltage picks in the output [36-40].

The real ITO* layer thickness has of around 100-300 nm [57]. However, to bring

simplicity to the model, a thickness of 1µm and equivalent conductive properties to the

real one has been given. Moreover in the model ITO will have the same mechanical

properties as the PET layer (see fig.4.1).

The piezoelectric particles are represented by cubes uniformly distributed within

the central PDMS+CNT matrix layer. Their size was properly calculated to obtain an

equivalent volume to the 12wt% that has been reported as optimal weight fraction [36-

40].

83

4.2.2. Material properties

The material properties values used were taken from the manufacturers‘ datasheet

or other technical sources when the first ones were not clear or accessible. The table

below shows the material data used for the model

The piezoelectric powder NEC-55 is classified as 600 in European standard EN

50324-1 and Navy type VI. PZT-H5 is the common name for an analogue widely

commercialized. Since the PZT-H5 properties are already implemented in the

COMSOL library, we checked that they were correct and used them in our model.

PDMS+CNT

[58,59]

PDMS

[58,59]

ITO

[52-57]

PET

[60-63]

Young's modulus E [Pa] 1.32E+06 1.32E+06 1.16E+11 3.45E+09

Poisson's ratio nu [1] 0.49 0.49 3.5 0.37

Density rho [kg/m^3] 970 970 6800 1400

Relative permittivity epsilonr [1] 2.72 2.72 1.00E+03 3.2

Electrical conductivity sigma [S/m] c1 4.20E-15 5.70E+06 1.00E-15

Fig.4.3, Table with material properties used in the model

Fig.4.4, Properties for piezoelectric material NCE 55 [50]

84

4.2.3. CNT contribution

When CNTs are added to a PDMS matrix, the electrical and mechanical

properties of the matrix change. Because of the impossibility of modelling the CNTs in

COMSOL, the influence of CNT is introduced to the model by modifying the material

properties of the matrix as the nanotubes would do it. The way their effect influence the

mechanical an electrical properties is described below:

4.2.3.a. CNTs Effect in Electrical conductivity

As it was previously explained in this work in the section 2.3.3.a, the presence of

CNT increases the electrical conductivity following a known relation that is described

in the literature [58-59]. The percolation theory can be applied to explain the electrically

conducting behaviour. According to percolation theory the conductivity follow the

expression below, close to threshold percolation zone:

where σ is the composite conductivity, σ o is a constant , p the weight fraction of

nanotubes, po is the percolation threshold and t the critical exponent [17].

The electrical conductivity of CNT/polymer composites is also affected by

dispersion and aspect ratio of CNTs. Threshold concentration values for CNT in PDMS

from 0.2 to 4wt% have been described in the literature depending on the preparation

procedure and aspect ratio, therefore in order to have a good knowledge of the

resistivity – CNT wt.% relation in our experiment, a conductivity study is performed.

4.2.3.b. CNTs Effect in Young Modulus

In the case of the Young modulus, the relation between CNT wt% and its

increase is not very well known jet, and contradictory results are found depending on

the source [17]. Therefore, no effect of the CNT in the mechanical properties of the

nanocomposite has been introduced in this study.

85

4.3. Simulation Experiment Set-Up

The main characteristics that help us to describe this kind of electrical generator

are the Open Circuit Voltage and the Short Circuit current, as it was explained in the

previous chapter. In the simulation those values and also the output with an output load

connected are measured.

For model validation we will use the values described for a similar experiment

that was performed in the literature (see fig.4.5) [39]

4.3.1. Simulation Experiment description

The Bending unbending movement exerted to the NCG can be characterized as

composed of two positions, bended position, relaxed position, and a transition between

them.

The positions can be described by the curvature radios for each state:

R0 (relaxed) =

R1 (bended) = 5 mm

Fig.4.5, Bending-unbending experiment for measuring flexible NCG output values [39]

Fig.4.6, Output measurement plots from a bending-unbending experiment with a flexible NCG [39]

86

The transition must be smooth (derivable) and can be described by the time taken

to pass from one state to the other one. As it can be observed in the simulation, this

transition speed is a variable that influences the output.

Below, the values to introduce in COMSOL for a boundary condition that

provides a position to the model equivalent to the real bending position are calculated.

The expressions below describe the relationship within the circle geometry:

The chord length is

The angle is

The height is

Where c is the cord, Ө is the central angle and h is the heath as we can see on the figure.

Assuming a curvature radius R = 5mm and after using the formulas above the values to

simulate the bended position are obtained.

Radius of Arc

(mm)

Length of Arc

(mm)

Width of Arc

(mm)

Height of Arc

(mm)

Apothem

(mm)

Angle Subtended by Arc

(rad)

5 1 0.998 0.0250 4.975 0.2

A cyclic bending-unbending movement is simulated by as the product of the

―bending position boundary condition‖ and a periodic function that changes from 0 to 1

with a derivable transition zone that can be changed.

Fig.4.7, Circle representation

Fig.4.8, View of the bend model during Bending-unbending experiment

87

4.4. COMSOL Internal Calculation Procedure

The modelled system combines Solid-Mechanics, Piezoelectricity and Electrical

currents all coupled in one. To try to solve such simulation problem, COMSOL

Multiphysics utilizes the formulas that describe those phenomena using a Finite

Elements Analysis (FEA) approach. The procedure and equations used by COMSOL

are explained below.

4.4.1. Piezoelectric Devices Interface

The Piezoelectric Devices interface ( ), combines Solid Mechanics and

Electrostatics for modeling of piezoelectric devices, for which all or some of the

domains contain a piezoelectric material. The interface has the equations and features

for modeling piezoelectric devices, solving for the displacements and the electric

potential.

The piezoelectric coupling can be presented in stress-charge or strain-charge

form. All solid mechanics and electrostatics functionality for modeling is also

accessible to include surrounding linear elastic solids or air domains.

Linear Elastic Materials

The total strain tensor is written in terms of the displacement gradient

where the strain is named ε, and the displacement is u.

Piezoelectric Constitutive Relation

The constitutive relation using COMSOL Multiphysics symbols for the different

constitutive forms are thus:

Stress-charge

Strain-charge

where the strain is named ε, the stress is named σ, E is the elastic matrix, D electrical

displacement, cE the elastic matrix, sE the compliance matrix, d the coupling matrix.

88

Relative permittivity is (ɛrS or ɛrT) This makes the names consistent with those used in

the other structural mechanics interfaces.

Electrical Material Model

The Electrical Material Model adds an electric field to domains in a piezoelectric

device model that only includes the electric field. We select a constitutive relation—

relative permittivity, polarization, or remanent displacement.

where E is the electric field and V the voltage

Piezoelectric Material Properties

Once we select a Constitutive relation—Stress-charge form or Strain-charge form.

For each of the following, COMSOL needs the properties below solve the system:

For Stress-charge form, select an Elasticity matrix (ordering: xx, yy, zz, yz, xz,

xy) (cE) (SI unit: 1/Pa).

For a Strain-charge form, select a Compliance matrix (ordering: xx, yy, zz, yz, xz,

xy) (sE) (SI unit: 1/Pa).

Coupling matrix (ordering: xx, yy, zz, yz, xz, xy) (d) (SI unit: C/m2 or C/N).

Relative permittivity (ɛrS or ɛrT) (unitless).

Density (p) (SI unit: kg/m3).

4.4.2. The Electric Currents Interface

The Electric Currents interface ( ), has the equations, boundary conditions, and

current sources for modeling electric currents in conductive media, solving for the

electric potential.

Current Conservation is the main feature, which adds the equation for the electric

potential and provides a settings window for defining the electrical conductivity as well

as the constitutive relation for the electric displacement field and its associated material

properties such as the relative permittivity.

Electric Currents Interface Equations

89

The electric currents interface solves the system for the following equations:

where J density is the current, E is the electric field, V the electric potential, σ is

the electrical conductivity (SI unit: S/m), Je is an externally generated current density

(SI unit: A/m2). and Qj is an external current source (SI unit: A/m3).

4.4.3. The Electrical Circuit Interface

The Electrical Circuit interface ( ), has the equations for modeling electrical

circuits with or without connections to a distributed fields model, solving for the

voltages, currents and charges associated with the circuit elements.

The Electrical Circuit interface makes it possible to add nodes representing circuit

elements directly to the model tree in a COMSOL Multiphysics model. The circuit

variables can then be connected to a physical device model to perform co-simulations of

circuits and multiphysics. The model acts as a device connected to the circuit so that its

behaviour is analysed in larger systems.

Electric Circuit Interface Equations

The fundamental equations solved by the electrical circuit interface are

Kirchhoff‘s circuit laws, which in turn can be deduced from Maxwell‘s equations.

External Circuit Connection to the Model

The external circuit implemented in the electrical circuit interface is connected to

the model through the ITO electrodes outside boundary surfaces (see fig.4.9). An

external resistive load is used to analyse de model output behaviour.

Fig.4.9, Schematic representation of the external circuit connected to our

model for output characteristics analyses and measurements

R (external load)

90

4.4.4. Dependent Variables

In this model, the dependent variable (field variable) is the electric potential V.

The name can be changed but the names of fields and dependent variables must be

unique within a model.

4.5. Model Meshing

A mesh study was carried out to ensure that the mesh size was adequate for

obtaining a correct solution from the solver in COMSOL. In a stationary study, the size

of the mesh was gradually reduced, starting from a coarse mesh, until it was observed

that the solver results remained the same. Especially fine mesh was locally in the

piezoelectric elements to have the model properly defined.

4.6. Simulation and Results

4.6.1. Introduction

Several simplified simulations where carried out in order to get familiar with the

software and to ensure that every aspect is understood and working properly. Then

simulations for the whole model were run in different conditions.

Fig.4.10, Meshed model of the NCG used for the simulations in COMSOL

91

4.6.2. Simplified Simulation: Open and Short Circuit

Simulation of a single piezoelectric particle within a PDMS with carbon

nanotubes Matrix (Matrix conductivity = 0.01 S/m). This simulation was performed to

observe and understand the way the charges move within the composite when the

matrix surrounding the piezoelectric particle is conducting.

4.6.3. NCG Model in Bending Position: Stress Distribution

In this simulation it is proved that the stress distribution within the piezoelectric-

nanocomposite have the same sign for the whole cross section. This is important

because this way, all the piezoelectric elements produce a voltage with the same

polarity. This is true in case of nanocomposites that when through a poling process.

Fig.4.12, View of the NCG model in bending position. On the left side a displacement graph is

presented. On the right side the figure shows the stress distribution in a cross section that passes

through the piezoelectric particles

Fig.4.11, View of the current density (red lines) for a single piezoelectric particle in a PDMS matrix

with CNTs (Matrix electrical conductivity = 0.001 S/m); (a) Short circuit configuration, (b) Open

circuit configuration

(a) (b)

92

4.6.4. NCG Model in Bending Position: ElectricPotential and Current

In this simulation, it can be observed the electric potential and the current density

due to the charge generated by the piezoelectric particles in a bending position.

Fig.4.14, View of the current density (red lines) and the electric potential, for a single piezoelectric

particle in a PDMS matrix with CNTs (Matrix electrical conductivity = 0.001 S/m).

Fig.4.13, View of the current density (red lines) and the electric potential, for a single piezoelectric

particle in a PDMS matrix with CNTs (Matrix electrical conductivity = 0.001 S/m).

93

4.6.5. Transient Study: NCG without PDMS Insulating Layers

An important issue that is observed in the simulations is the fact that, as it can be

observed (see fig.4.15.), an extremely high voltage pick is produced when bending and

unbending the NCG. This problem has been described in the literature before [36]. To

solve this issue, an additional pure PDMS layer between the piezoelectric nano-

composite layers (p-NC) is introduced as described in the literature.

In the graphics below, we can observe the high voltage and current picks that

occurred in a NCG output that already had two pure PDMS layers for protection of

100µm each, against high voltage picks (see fig.7.15).

Fig.4.15,View of the current density (red lines) and a central slide cross section of the electric

potential, for a single piezoelectric particle in a PDMS matrix with CNTs (Matrix electrical

conductivity = 0.001 S/m).

Fig.4.16, The graph shows the output of the NCG model in a transient study with a bending movement.

The left plot shows the voltage [V] in the electrodes. The right plot shows the output power [w].

(Transition time=100ms; p-NC conductivity=0.001S/m)

[s] [s]

94

The same NCG model that had given a high voltage pick in the output where

tasted with an insulating PDMS layer. It was observed that in some cases the same the

high pick was reduced or disappeared. This issue is problematic, because depending on

the thickness of this isolating-protecting PDMS layer, which would be difficult to create

with precision in a real NCG, the results of the simulation change substantially.

4.6.6. Transient Study: CNT Concentration Effect Study

In this study all the parameters are kept the same and only the piezoelectric

nanocomposite conductivity is changed. Only one bending movement is applied to the

model. The graphs below are present models with p-NC conductivity values going from

lower to higher. Some simulations did not finish due to convergence errors.

Fig.4.18, Output values for the NCG model after transient simulation of bend-unbend movement.

(Simulation type = Bend once; Movement transition time = 50ms; p-NC conductivity = 1e-4; PDMS

insolate thickness = 0.1 mm; simulation time = 4 sec; COMSOL file M7)

Fig.4.17, Output values for the NCG model after transient simulation of bend-unbend movement.

(Simulation type = Bend once; Movement transition time = 50ms; p-NC conductivity = 1e-14;

PDMS insolate thickness = 0.1 mm; simulation time = 4 sec; COMSOL file M1)

[s] [s] [s]

[s] [s]

95

Fig.4.19, Output values for the NCG model after transient simulation of bend-unbend movement.

(Simulation type = Bend once; Movement transition time = 50ms; p-NC conductivity = 1e-3; PDMs

insolate thickness = 0.1 mm; simulation time = 4 sec; COMSOL file M2)

Fig.4.21, Output values for the NCG model after transient simulation of bend-unbend movement.

(Simulation type = Bend once; Movement transition time = 50ms; p-NC conductivity = 5e-3; PDMS

insolation thickness = 0.1 mm; simulation time = 1 sec; COMSOL file M3)

Fig.4.20, Output values for the NCG model after transient simulation of bend-unbend movement.

(Simulation type = Bend once; Movement transition time = 50ms; p-NC conductivity = 5e-3; PDMs

insolate thickness = 0.1 mm; simulation time = 4 sec; COMSOL file M8)

[s] [s] [s]

[s] [s] [s]

[s] [s] [s]

96

4.6.1. Transient Study: Transition Velocity Study

In this study all the parameters are kept the same and only the transition velocity

between bended and relaxed state is changed. Both bending and unbending movement

are applied to the model.

Fig.4.23, Output values for the NCG model after transient simulation of bend-unbend movement.

On the top: general view of the graph. At the bottom: a detail of the transition between bending and

relaxed states is shown. (Simulation type = Bend + Unbend once; Movement transition time = 50ms; p-

NC conductivity = 1e-3; PDMs insolate thickness = 0.1 mm; simulation time = 2 sec; COMSOL file M9)

Fig.4.22, Output values from the NCG model after transient simulation with bend-unbend movement.

(Simulation type = Bend once; Movement transition time = 50ms; p-NC conductivity = 1e-2; PDMS

insolation thickness = 0.1 mm; simulation time = 1.1 sec; COMSOL file M4)

[s] [s] [s]

[s] [s] [s]

[s] [s] [s]

97

4.7. Model Validation and Results Analysis

4.7.1. Model Validation

The NCG was simulated for a cyclic bending unbending movement. Simplified

simulation for many aspects where performed successfully which gives confidence

about the final simulation results. Moreover, the obtained results for the NCG transient

simulation agree with the data found in the literature and with the data obtained in the

experimental section. However, the simulation was very problematic. Complete

simulations could not be performed as it would have been desired. All the solvers

provided by COMSOL with different parameter configurations where tried. Most

simulations with more than one movement did not finish and gave a convergence error.

Due to the nature of the model, it could be observed that some variables suffer of abrupt

Fig.4.25, Output values for the NCG model after transient simulation of bend-unbend movement.

(Simulation type = Bend + Unbend once; Movement transition time = 300ms; p-NC conductivity =

1e-3; PDMs insolate thickness = 0.2 mm; simulation time = 2 sec; COMSOL file M5)

Fig.4.24, The graph shows the output of the NCG model in a transient study with a bending movement.

(Simulation type = Bend + Unbend once; transition time=100ms; p-NC conductivity=0.001S/m) The left

plot shows the voltage [V] in the electrodes. The right plot shows the output power [w]

[s] [s] [s]

[s] [s] [s]

98

changes that, together with the multiple physics that are involved at once, make the

solver eventually encounter a situation that is not able to overcome. Further work with

COMSOL technical service should be done to solve this issue.

4.7.2. Results Analysis: CNT Concentration Effect Study

It can be observed in the plots that the conductivity affects the output in the NCG

model. For very low and very high conductivity values, the output from the NCG is a

big voltage pick. For values in between, it is observed that the wave generated by the

movement changes gradually in clear relation with the conductivity.

It would have been desired to perform a whole parametric study of the

conductivity for a complete bending unbending cycle or even for a few cycles.

However, unfortunately it was not possible to perform such simulation because of

problems of convergence with the solver.

4.7.3. Results Analysis: Transition Velocity Study

It can be observed on the plots that for higher movement velocity, the pick

obtained is also higher.

4.8. Conclusions

A model of a NCG was created and the internal working mechanism of the system

and the stress distribution were observed and understood. The transient studies showed

that a PDMS insulation layer is desirable to avoid high output picks. It was also seen

that certain resistivity in the matrix is needed don‘t to get too high pick in the output.

The CNT conductivity was observed to influence the NCG output. Finally the results

showed that the velocity of the movements affect the output amplitude by increasing it

for higher speeds.

A deeper parametric research could not be made due to problems with the

software to solve such complex system.

99

5. Fabrication of a Nanocomposite Generator (NCG)

5.1. Introduction

In the first part of the chapter a range of samples of a piezoelectric nanocomposite

(p-NC) are prepared. The objective is to observe the material behaviour, to check the

conductivity of the obtained nanocomposite, and to establish an adequate fabrication

procedure. A study of p-NC samples conductivity is carried out, in order to observe the

influence that CNTs concentration and fabrication procedure have in this parameter.

The p-NC is only the main part of the whole nanocomposite generator (NCG). In

the second part of this chapter a range of samples of NCGs are created, their output is

measured and the results analysed.

The insufficient equipment and media has restricted our possibilities; therefore

some adapted procedures had to be used instead of the described in the literature. The

used material and procedures are described but also alternatives solution and procedures

that could improve several aspects of the experiment have been included in this text.

5.2. Materials and Equipment

5.2.1. Materials that Compose the NCG

The material used to create the Nanocomposite Generator (NCG) is listed below:

i. Matrix:

- PDMS, polydimethylsiloxane elastomer. Sylgard 184, Dow Corning, Base +

Curing Agent. [51]

ii. Filler:

- Carbon Nanotubes: MWCNT

- Piezoelectric powder : Lead Zirconate Titanate (PZT), NCE-55 Noliac [50]

iii. Electrodes:

- Bottom electrode: ITO (Indium Tin Oxide) coated PET film. (175 µm in

thickness)

- Top electrode: Aluminium foil

iv. Solvent, dispersing agent:

- Ethanol

100

5.2.2. Alternative Materials for the NCG

5.2.2.a. Alternatives for Flexible Electrodes

The material needed for the electrodes should be conducting and flexible enough to

bend periodically without damage. Moreover, the top electrode must be thinner or lower

in strength than the bottom one, because it is desired to shift the mechanically neutral

region, from the stress distribution diagram, to a position outside the p-NC layer (See

fig.5.1). If the stress along the whole p-NC cross section has the same sign, then all the

piezoelectric particles generate voltage with the same polarity. Obviously this is true if

the material has been previously poled. Alternative materials for electrode are presented

below:

i. ITO coated PET (25 µm thickness)

The lower electrode is 175 µm thicknesses. This is a simple and practical

alternative that has been reported to work in the literature [38-40]. In our case it I was

not used, because it was not found in the market in and affordable manner.

ii. Conductive Nanocomposite: PDMS+CNT high concentration

This option is a proposed new application for the conductive nanocomposite that

is obtained in this work.

It can be applied either only for the top electrode or for both, but taking into

account the issue about the mechanically neutral region that is explained above.

To make the final connexion with the electrodes it could be done in an area were

the NCG does not move. A simple alternative would be to introduce copper wires in the

nanocomposite electrode perpendicularly to the movement. The extremes of the wires

would be used for the final connexions.

PDMS

Aluminium foil

PDMS + CNT + PZT

(p-NC)

PDMS

ITO coated PET

Electrode 1

Electrode 2 Mechanically

Neutral Region

Fig.5.1, Schematic draw with layer distribution of materials that compose the NCG. A diagram

shows the stress distribution along the cross-section when bending the NCG. The mechanically

neutral region is not in the centre due to different strength for each material [36]

Stress distribution

101

5.2.2.b. Alternatives for Piezoelectric Materials

For the piezoelectric material to be used, high d33 is desired. Electro mechanical

coupling coefficient (dij), represents how much electric charge is generated in material

for force applied. ―i‖ represent poling direction and ―j‖ represent the direction of

applied stress on crystal (see section 3.2. for detailed information) [53].

NCE55 rom Noliac is the material used in this work. It is equivalent to the well-

known PZT 5H (Lead Zirconium Titanate), a very high sensitivity material featuring

extremely high permittivity, large coupling factor and piezoelectric constant. It has have

a relatively low Curie temperature which makes poling possible without damaging the

rest of the elements. This material is suitable for a wide range of high sensitivity

applications with limited temperature range of operation [50].

i. Lead Based Piezoelectric Materials PZT

Lead Zirconium Titanate (Pb (Zr1-xTix) O3 or PZT) ceramics, has been market-

dominating due to its excellent properties. However, the large amount of lead contained

in PZT materials has drawn much attention during the past decade, due to the

environmental concern as well as government regulations against hazardous

substances.[39][64-65]

ii. Lead-Free Piezoelectric Materials

Advantage: Environmental and safety concerns with respect to the utilization of

lead-based piezoelectric ceramics encourage the induction of lead-free piezoelectric

ceramics. Below, some examples of lead-free materials are listed:

iii. KNLN, Alkaline Niobate Particles

They have been reported as the most attractive lead-free piezoelectric material due

to remarkable piezoelectric properties, biocompatibility, high Curie temperature, and

large electromechanical coupling factor. [36]

Nominal composition 0.942(K0.480 Na0.535) NbO3-0.058LiNbO3

Piezoelectric coefficient d33 ≈ 310 pC N-1

Synthetized Using solid state method (Detailed description below)

iv. PVDF (Polyvinylidene Fluoride) [64-65] d33 = -33 pC/N-1

v. BaTiO3 [40] d31 = 78 pC/N-1

102

5.2.3. Equipment for NCG Fabrication

- Digital balance (0.1mg).

- Ultrasonication device: CNT dispersion.

- Moulds: Samples preparation.

- Oven (80-100 ºC): Nanocomposite Curing.

- Vacuum chamber, for material degasification before curing.

5.2.4. Additional Equipment for NCG Fabrication

5.2.4.a. Poling Equipment

Poling usually involves subsequent heating above the Curie point (159ºC for

NCE-55 piezoelectric powder [51]), application of an electric field (2 kV), cooling

below the Curie point, and finally removal of the electric field.

- Advantage: It has been reported that the generated output voltage and current

signals are dramatically increased by poling process. The insets below (see fig.5.3)

show the magnified output signals [36-40]

5.2.4.b. Spin or Bar Coating Equipment: For thin layer deposition (see fig.5.4).

- Advantage: A thin homogeneous material layer is easily obtained [36-40].

Fig.5.2, a ) CAD design of a mould for NCG in using inventor. b) Mould printed using a 3D printer

a) b)

Fig.5.3, Measurements plot for a NCG similar to the one in our work output. a) Open circuit Voltage

before and after poling. b) a) Short circuit Current before and after poling [39]

103

5.2.4.c. Vacuum-Dry-Oven: For degasification and curing the composite.

- Advantage: Removes undesired bubbles from the Nanocomposite due to air and

rests of solvent [36-40].

5.2.4.d. Oven Calcination: Calcination of piezoelectric powder (750 - 1050 ºC)

- Advantage: Eliminates impurities [36-40]

5.2.4.e. Ball-Milling and Sieving: Crushing and sieving by #100 standard mesh.

- Advantage: Small homogenous powder size is achieved [36-40]

5.2.4.f. Magnetic stirring (650 rpm): CNTs dispersion.

- Advantage: Alternative additional way of dispersing the CNTs [36-40]

5.3. Experiment: P-NC and NCG Fabrication

5.3.1. Experiment 1

Material Used:

Description:

MWNT, Piezoelectric powder and PDMS base were mixed together and sonicated

for 30 minutes. After the hardener was added, the mixture was stirred and sonicated for

2 minutes. Then it was poured into moulds and surfaces and put into the oven to cure,

85C for 25 min.

Fig.5.4, a ) Schematic view of spin coating process. b) Schematic view of bar coating process.

b) a)

104

Observations:

In the obtained samples it could observed that most of the piezo-powder was

deposited at the bottom. The experiment was a good first contact with the materials in

which the way they behave and react could be seen.

5.3.2. Experiment 2

Material Used:

Description:

Two different composites with different CNT concentration were prepared. In

both cases, MWNT and Piezoelectric powder was mixed with Dimethylformamide

solvent and ultrasonicated for 15 minutes. After the PDMS base was added to the

mixture it could be observed that the mixture became very thick. It was stirred and

ultrasonicated for 15 minutes. After the hardener was added, the mixture was deposited

into moulds and surfaces and finally in the oven, 85C for 30 min. The resultant

composite did not harden after the curing process.

Observations:

This practice helped us to become more familiar with the materials and

equipment. The obtained samples were useless. It was noticed that Dimethylformamide,

the solvent used, is not an appropriate solvent for PDMS based composites.

5.3.3. Experiment 3

Material Used:

Description:

Two nanocomposite samples were prepared using solvent for dispersion of CNT

in one of them. The general procedures ―A‖ and ―B‖ presented later were followed,

with the exception that the solvent was not evaporated before curing in the oven.

105

Observations:

Most samples with solvent came out full of bubbles and gaps due to the presence

of solvent while curing in the oven. Only some samples of this composite with solvent

where fine, because they were so thin that the solvent had been able to escape during the

curing process.

5.3.4. Experiment 4

Material Used:

Description:

Two different composites were prepared with the only difference that one had

solvent while the other one did not. The general procedures ―A‖ and ―B‖ presented later

were followed.

Observations:

The resultant samples where irregular and rough on their top layer. This was

caused by the high thickness of the mixture before curing due to a high CNT

concentration (4wt.%). Generally the results were satisfactory.

5.3.5. Experiment 5

Material Used:

Fig.5.5,Top view of the NCG mixed solution during sonication process

106

Description:

In this case the p-NC was prepared to later build the complete NCG. Procedure

detailed in 5.4.1.a without degasing was used for the p-NC and procedure described in

5.4.2 to create the NCG.

Observations:

Some piezoelectric particles were deposited at the bottom of the samples. It has

been observed that the CNTs act as a dispersing agent for the piezoelectric powder. This

problem might be reduced if the piezoelectric powder is further milled to reduce the

size, as it is suggested in the additional equipment section.

5.3.6. Experiment 6

Material Used:

Description:

In this experiment a range of piezoelectric nanocomposite samples and NCG

where prepared with different CNTs concentrations. In some cases solvent was used and

a degasification process was also introduced for some samples. Procedure detailed in

5.4.1.a with and without degasing was used for the p-NC and no plane PDMS was used

for preparing the NCG samples.

Observations:

Samples were prepared by pouring the mixture between to glass layers and then

putting it to the oven for curing. The first samples obtained after curing had many

undesired air bubbles. Therefore a second round of samples was first degasificated and

107

then it was introduced to the oven. Those samples came out with flat smooth surface

and without air bubbles in them.

5.3.7. Summary of Problems Encountered During the Experimentation

The issues mentioned here are more explained in more detail in the observation

section for each experiment:

- Dimethylformamide solvent is inappropriate for PDMS composites.

- Ethanol solvent is appropriate but must be removed before the curing process,

unless the NCG layers are thin enough for it to completely evaporate while

curing.

- Piezoelectric powder precipitates. Higher CNT concentrations and the use od

solvent help for a better dispersion. Probably further milling would be also

advantageous.

- Top layer rough and with non-homogenous thickness especially for high CNTs

concentrations. Proposed ―sandwich‖ method for obtaining flat smooth surfaces.

- Air bubbles are found after curing. Degasification is highly recommended and

soles this issue.

5.4. Samples Fabrication Procedures

5.4.1. Fabrication Procedures for Piezoelectric Nanocomposite (P-NC)

The step-by-step ―general procedures‖ detailed in this section are recommended basing

on the experience gained during the six experiments performed.

Fig.5.6, The draw illustrates the NCG fabrication Process ―A‖. a) The materials are initially

mixed and ultrasonicated; b) The mixture is poured into the moulds or surfaces; c) the moulds are

introduced in the oven for curing; d) representation of a final NCG.

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5.4.1.a. P-NC Fabrication Procedure ―A‖ (Without Solvent)

1. The nanotubes and piezoelectric powder quantities are weighted via digital

balance and preliminary mixed with a spatula.

2. PDMS base product is weighted via digital balance and added to the previous

mixture. The mixture is stirred and ultrasonicated for 15 minutes.

3. The curing agent is added in a 10:1 ratio to the mixture, followed by long hand

stirring with a pestle.

4. Everything is transferred into the final moulds or surfaces we that will be used

for curing. For low concentration samples, up to 2 wt.% CNT, the consistency of

the mixture is similar to pure PDMS‘s, thence casting into moulds is performed

by simple dripping.

5. It is cured in the oven 85-90 °C for 15 minutes and put the moulds inside.

5.4.1.b. P-NC Fabrication Procedure ―B‖ (With Solvent)

1. The nanotubes are weighted and mixed with ethanol in an open container; the

amount of solvent employed rendered the mixture fluid; it is ultrasonicated for

15 minutes.

2. The piezoelectric powder is weighted and added to the solvent mixture. It is

stirred.

3. PDMS base product is weighted via digital balance and added to the previous

mixture. The mixture is stirred and ultrasonicated for 15 minutes.

4. The mixture is heated up on a hub while stirring until the solvent evaporates.

5. The procedure follows as from point 3 in preparation ―A―.

Fig.5.7,View of the NCG mixed solution during ultrasonication phase

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5.4.2. Fabrication Procedure for Nanocomposite Generator (NCG)

This procedure was follower to fabricate NCG samples. First we prepare a thin

layer of piezoelectric nanocomposite that will be the core of the NCG. Then we use the

mould for building up the NCG. In the mould, a kind of sandwich is made as explained

in the steps below:

1. ITO-PET is placed at the bottom.

2. A pure PDMS thin layer is poured on it.

3. The p-NC already cured is put on top of it.

4. Again, pure PDMS is poured over it.

5. Finally the other ITO-PET electrode is placed on the top.

Once all samples are ready, a flat ―weight‖ covering the samples is left on top of

them to keep the shape, in order to obtain a flat homogeneous surface after curing.

Finally it is put into the oven for curing.

5.5. P-NC and NCG Experiment Results

5.5.1. Introduction

Two interesting feature of the generated samples are analysed in this section. In

first place, since a piezoelectric nanocomposite (p-NC) with CNT is being created to

add conductivity to an otherwise insulating material, a conductivity versus CNTs

concentration study is performed.

On the other hand, the main characteristic concerning nanocomposites is

obviously the output power that it can provide. In the second part of this section the

nanocomposite generator (NCG) output is measured and the results are presented and

analysed.

5.5.2. P-NC Conductivity Results

The electrical resistivity of a material is a number describing how much that

material resists the flow of electricity. Resistivity will not depend on the size and shape

of the piece of material. Resistivity is measured in units of ohm·meters (Ωm). If

electricity can flow easily through a material, that material has low resistivity. Electrical

resistivity is represented by the Greek letter ρ. Electrical conductivity is represented by

the Greek letter σ, and is defined as the inverse of the resistivity.

One of the most popular ways to measure the resistivity of high resistance

110

materials is the four points probe method described below.

5.5.2.a. Four-Point Resistivity Method

The resistivity of thin films materials is often measured using commercial four-

point probes. These probes generally have four equally spaced, collinear metal points

that are pressed against the surface of the film. A current is applied between the outer

two points, while the voltage is measured across the inner two points. These probes can

also be used to measure the resistivity of bulk samples [58]

where t is the layer thickness in cm and resistivity is given in [Ω cm].

The simple formula above works for when the wafer thickness is less than half the

probe spacing (t < s/2). For thicker samples the formula becomes:

where s is the probe spacing and resistivity is given in [Ω cm]. [58]

Fig.5.8,Schematic representation of the Four Points Probe Method

(8.2)

(8.1)

111

5.5.2.b. P-NC Samples

A range of p-NC samples with various CNTs concentrations was prepared for

measurement. Silver electrodes were deposited in stripes (see fig.5.9). The distance

between electrodes was 10 mm.

5.5.2.c. Equipment to Measure Conductivity

- DC power supply, JINGCE JC2733S

- Oscilloscope, Agilant DSO-X 3024A

- Digital Multimeter, Agilant 34401A

- Digital Multimiter BM811a

5.5.2.d. Expected Conductivity Values for P-NC

A reported experiment for PDMS-CNT nanocomposite conductivity for different

filler concentrations is presented. Experimental data were fitted with the scaling law

from percolation theory (red line). The fitting curve can be expressed as:

σ=1.1·(p−0.85)2.67

mS/cm [58].

Fig.5.10,Measurment equipment for Four Points Probe method

Fig.5.9, a) and b) View of Piezoelectric Nanocomposite (p-NC) samples with deposited silver

electrode for resistivity measurement. c) Draw of schematic diagram that illustrates the premises

of the four-terminal sheet resistance measurement of PDMS/MWCNT composites [58].

a) b)

c)

112

where σ is the conductivity and p is the CNTs wt.% concentration.

The data were fitted by the well-known percolation scaling law:

σ=σ0(p−pc)t

where σ0 is a proportionality constant, pc is the percolation threshold, and t is the

critical exponent that characterise the percolative network.

As visually represented in Fig.5.11, the conductivity jumps up by several order of

magnitudes passing from low loadings around the percolation threshold (0.7 and 1.0

wt.%) to those above that critical value. The conductivity continues to rise but

seemingly its increasing trend gets sluggish for higher concentrations (5.0 and 6.0 wt).

[58]

Fig.5.12, Resistivity measurement

process using four probes method.

Fig.5.11,Graph reported for PDMS-CNT resistivity vs. CNT concentration [58]

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5.5.2.e. P-NC Conductivity Results

In the table and graphs below, the values for conductivity of the P-NC samples

obtained using the four probe method are presented:

Fig.5.13, The Graphs present the results for Conductivity vs. CNTs concentration for the

piezoelectric nanocomposite prepared. Data are divided in two groups depending on the preparation

method. Trend line for nanocomposite without solvent could not be obtained due to insufficient data.

*Could not be measured because the resultant values were lower than the equipment scale.

114

5.5.3. P-NC Conductivity Results Analysis

Piezoelectric nanocomposite samples for a range of CNT wt.% between 0 and 4

wt.% were prepared. Basically two different preparation methods were used providing

two differentiated groups of samples and results. Those methods are ―Fabrication

Procedure A‖, not using solvent (see point 5.4.1), and ―Fabrication Procedure B‖, using

solvent for CNT dispersion (see point 5.4.2).

5.5.3.a. Conductivity Results ―Procedure A‖ (No Solvent)

Some increase in the composite conductivity with respect to the plane PDMS

conductivity can be observed. However, the conductivity of this samples is considerably

lower the one obtained for samples with the same CNTs concentration using the

―fabrication procedure B‖ (with solvent). From what can be seen on the graphs 5.13, the

threshold region is expected to be for higher CNT concentrations. Therefore, the trend

equation for conductivity versus CNTs wt.% concentration can not be found; data are

insufficient.

5.5.3.b. Conductivity Results, ―Procedure B‖ (With Solvent)

The conductivity is lower in the samples where solvent for CNTs better dispersion

was used (―fabrication procedure B‖). The percolation region is found between 1 and 2

CNT wt.%, with conductivity values as high as in case of

samples with 4wt.% CNTs. The trend equation for conductivity versus CNTs wt.%

concentration was calculated and is presented below [58]

were σ is the conductivity and p is the CNTs wt.% concentration.

5.5.3.c. Conclusions of P-NC Conductivity Results

The conductivity results for the p-NC fabricated using solvent are definitely

promising. Moreover, it is likely that it will be higher if the CNTs dispersion is

improved.

Having a flexible material with such high conductivity can open a way for

multiple applications. Although this characteristic had been already described in the

literature, after this experiment confirmed it and prove that it can be easily prepared.

115

5.5.4. NCG Output Results

5.5.4.a. NCG Samples

A hole range of NCG samples where prepared during experiment 5 and 6 as

detailed in section 5.3.5 and 5.3.6 (see the fig.2):

5.5.4.b. Expected Output Values for NCG

The graphics below correspond to measurements reported by scientists from

piezoelectric nanogenerator composites with similar characteristics to the ones that are

subject of our study [40]

It can be observed that the voltage picks that appear have different polarity when

bending and unbending. Moreover, for faster movements bigger picks are obtained.

5.5.4.c. Equipment to Measure NCG Output

- Oscilloscope, Agilant DSO-X 3024A

- Digital multimeter, Agilant 34401A

- Digital multimeter BM811a

Fig.5.15, Output voltage generated from the NCG device when subject to periodical cycles of

slow bending/unbending motions (a), fast bending/unbending motions (b), and slow bending/fast

unbending motions (c). [40]

Fig.5.14, View of the Nanocomposite Generator samples (NCGs) prepared for measuring

procedure. Top part view on the left picture and bottom view on the right picture.

116

Measuring procedure: the NCGs samples are held in a device that allows for a

bending-unbending movement while they stay insulated from any external contact.

5.5.4.d. NCG Output Results

Screen–shots from the oscilloscope when measuring the NCG output are

presented below. The experiment consist on a periodic bending–unbending movement.

The graphs show the open circuit voltage measured in the NCG electrodes.

Fig.5.16, a) Detail of one of the NCG samples Nanocomposite Generator samples (NCGs), b) Sample

attached to measuring device, c) NCG Measuring process and equipment used

a

b c

Fig.5.17, Sample 6.5). NCG output voltage for bending-unbending experiment.

PZT=NO; CNT=2wt.%; PDMS insulation=No; Solvent; No.

.

117

Fig.5.18, Sample 5.1). NCG output voltage for bending-unbending experiment.

PZT=12wt.%; CNT=2wt.%; PDMS insulation=Yes; Solvent=No.

Fig.5.19, Sample 5.2). NCG output voltage for bending-unbending experiment.

PZT=12wt.%; CNT=2wt.%; PDMS insulation=Yes; Solvent=No.

.

Fig.5.20, Sample 6.2). NCG output voltage for bending-unbending experiment.

PZT=12wt.%; CNT=3wt.%; PDMS insulation=No; Solvent=No.

.

118

Fig.5.22, Sample 6.1). NCG output voltage for bending-unbending experiment.

PZT=12wt.%; CNT=3wt.%; PDMS insulation=No; Solvent=Yes.

.

Fig.5.21, Sample 6.2). NCG output voltage for bending-unbending experiment.

PZT=12wt.%; CNT=3wt.%; PDMS insulation=No; Solvent=No.

(Probes polarity inverted with respect to case Fig.5.20).

.

Fig.5.23, Sample 6.3). NCG output voltage for bending-unbending experiment.

PZT=12wt.%; CNT=2.5wt.%; PDMS insulation=No; Solvent; Yes.

.

119

5.5.5. Results Analysis for NCG samples

In first place, it can be observed an obvious measurement noise in all plots,. It has

a sinusoidal shape and higher frequency than our working frequency. This noise is due

to electromagnetic waves in the surrounding environment. An electromagnetic shielding

consisting of a conductive enclosure should be implemented in order to block

electrostatic fields. This is also known as a Faraday cage. Despite this noise, the signals

generated by the NCG samples are easily observables. On the graphs, picks

synchronized with the bending–unbending movement of the samples can be observed. It

is important to highlight that the picks are alternating between positive and negative

polarity. Bending movement produces a positive pick while unbending movement gives

Fig.5.25, Sample 6.4). NCG output voltage for bending-unbending experiment.

PZT=12wt.%; CNT=No; PDMS insulation=No; Solvent; No.

.

Fig.5.24, Sample 6.4). NCG output voltage for bending-unbending experiment.

PZT=12wt.%; CNT=No; PDMS insulation=No; Solvent; No.

.

120

a negative one. As we can see on fig.5.20 and fig.5.21, this relation is inverted when the

probes polarity is changed.

The best results are observed in the samples where the p-NC conductivity was

relatively low. This could occur because in the samples were the conductivity was

higher, pure PDMS layer for insulation was not used. Hence the resultant internal

resistivity of the NCG (from top to low electrode) is comparatively low.

In the samples were better results are obtained, from fig.5.18. to fig.5.21, the

amplitude of the generated voltage is remarkable, with an average pick to pick voltage

between 2 and 3 volts. This data is really promising taking into account that the NCG

samples were not poled. According to the literature [39][40], after the poling process,

the generated voltage should increase by one order of magnitude.

Some special samples only for testing purposes were also prepared:

1- NCG sample only with CNT in a PDMS matrix (see fig.5.17)

It can be seen on the plot that this sample does not generate any voltage when a

bending-unbending movement is applied.

2- NCG sample only with Piezoelectric NCE-55 in a PDMS matrix (see fig.5.17)

The graphs show that when a bending-unbending movement is applied to this

sample it gives extremely high voltage picks with not defined polarity.

3- NCG sample only with PDMS matrix

For this sample it could not be appreciated any change in the output when bending

unbending movement was applied.

5.5.5.a. Conclusions of Nanocomposite Generator Results

In this experimental work, nanocomposite generators with output voltage of

several volts (open circuit) have been prepared. Those voltage values have been

achieved even before the poling process that, according to the literature, should increase

those values in one other of magnitude.

The short circuit current was not measured due to insufficient equipment but it

should be measured in the next step to better characterize the nanocomposite generator.

The results are promising and leave an open space for further investigation. This

further research should focus on the improvement of the fabrication process and on the

efficient use and storage of the generated power.

121

6. Conclusions

The principal goal of the thesis was to find a way to harvest energy taking

advantage of the remarkable properties that carbon nanotubes offer. In this work a

flexible nanocomposite generator with piezoelectric micro-powder and MWNTs as

fillers is presented. The functionality of this device consist on generating energy from

mechanical low frequency movements, which can be easily found in many forms in the

real life. Hence, it can be said that the main goal has been achieved.

The first part of this thesis presents carbon nanotubes characteristics, properties

and synthesis processes. Then, carbon nanotubes applications and the particular case of

nanocomposites with CNTs are explained. This theoretical background was essential to

properly orientate the rest of the work that had to be done. The idea of focusing on

CNTs nanocomposites was reached at this stage. The particular mechanical and

electrical properties of CNTs nanocomposites are promising and still relatively

unknown. Additionally, when working with CNT nanocomposites, the expensive high-

technology equipment that is still needed when working with individual nanotubes is

not required. In the case of nanocomposites, most of the process can be made from the

macro-scale. For all this reasons, it is an exciting field that can be exploited while

accurate nanoscale technology became widely available.

After CNTs nanocomposites were chosen as the center of our research, an

investigation about energy harvesting sources, technologies and devices was needed to

fulfil our main goal. In the third chapter, this information is included together with a

deep presentation about the piezoelectric effect and energy harvesting from

piezoelectricity. After this research was made, it was concluded that the biggest

challenges for using energy from piezoelectric sources, is the high voltage values that

this sources usually generate and their problematic high internal capacitance. As it is

explained in the related chapter, this capacitance makes complicated finding appropriate

devices for storing the generated energy. After a deep research on the subject and basing

on experiments reported in the literature [36-40] a flexible nanocomposite generator

was proposed.

A model of the system was made in COMSOL Multiphysics software to observe

the internal working mechanism and to try to identify the role that certain parameters

play in the system. It was of special interest to identify the effect of the conductivity in

the p-NC, because it would be directly related with the concentration of CNT. This

122

FEM software solves the model for the coupled effects that take part; mechanics of

materials, piezoelectric effect and electric currents. In the simulations, it could be

observed the stress distribution within the piezoelectric particles and the way the energy

is generated.

The simulation consists of a transient study that reproduces the real bending-

unbending experiment with the NCG model. The results tell us that some conductivity

is needed in the piezoelectric nanocomposite and also that certain insolating layer

between the p-NC and the electrodes is desired, otherwise extremely high voltage picks

are obtained in the output. Nevertheless, the simulation was very problematic and no

good simulation of a few cycles of work in a row could be completed. Moreover, for

this reason parametric study of any kind could be completed. Consequently one of the

most important objectives of the simulation could not be fulfilled. Although all the

solvers available in the software and many parameter configurations were tested the

extremely abrupt changes in the variables values due to the nature of the system

provoked discontinuities that the solver was not able to overcome.

In the final part of this work, a real nanocomposite generator was prepared in the

laboratory. In first place, a study of the conductivity of the piezoelectric-nanocomposite

was conducted. It was found that the nanocomposite that had been prepared using

ethanol solvent, for better dispersion of the CNT, had very interesting conductivity

values, (up to 1.38e-2 S/cm in the case of 4wt.% CNT concentration). This conductivity

is related to the CNTs wt.% concentration. The samples prepared without solvent

offered lower conductivity values.

Eventually, some nanocomposite generator (NCG) samples were prepared, their

output measured and the results discussed. The voltages values measured in their

electrodes for the bending-unbending experiment where remarkable. Average values of

1-2 volts pick-to-pick were obtained for samples that had not been poled jet. According

to the literature the poling process should increase those values by one order of

magnitude. Despite the shortage of equipment, in the experimental section all goals

were successfully fulfilled.

6.1. Final remarks

Promising results were achieved after the experiments. On one hand, a flexible

conductive material with a conductivity related to the CNTs concentration was proved.

123

On the other hand, a nanocomposite generator was found to generate 1-2 pick to pick

even before poling process.

6.2. Future work and applications

6.2.1. Conductive Nanocomposite perspectives

The flexible conductive material that was found can have multiple applications.

To start with, it can be used as a flexible electrode in a future flexible NCGs or in other

devices where a flexible electrode is needed. Other application could be as flexible

connections in an electric circuit, although taking into account that resistivity is not

extremely low.

6.2.2. Nanocomposite generator (NCG) perspectives

Further work is needed for the present energy harvesting device to be functional.

First of all, a definitive fabrication procedure should be stabilized. Then, the

development of an appropriate signal treatment for an effective use of the generated

energy is essential. Finally, a specific energy storage device must be selected and

proved.

124

7. References

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[2] Dresselhaus M.S, Dresselhaus G., Eklund P.C, Science of Fullerenes and Carbon

Nanotubes, Kentucky, Elsevier Science, 1996

[3] M. Ahlskog, Ch. Laurent, M. Baxendale, M. Huhtala. ―Electronic Properties and

Applications of Carbon Nanotubes‖. ISBN: 1-58883-059-4 Encyclopedia of Nanoscience and

Nanotechnology

[4] J. González, ―Electronic Properties of Carbon Nanotubes‖. Consejo Superior de

Investigaciones Científicas, Madrid, Spain. ISBN: 1-58883-059-4 Encyclopedia of Nanoscience

and Nanotechnology

[5] Sajanlal, P., Sreeprasad, T., Samal, A., & Pradeep, T. (2011). Anisotropic nanomaterials:

structure, growth, assembly, and functions. Nano Reviews, 2. doi:10.3402/nr.v2i0.5883

[6] Agarwal A., Bakashi S. R., Lahiri D., Carbon Nanotubes Reinforced Metal Matrix

Composites, New York, CRC Press, 2011

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