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1. INTRODUCTION One of the greatest impacts on nanoscience and nanotechnology has been made by the discovery of carbon nanotubes. Science the discovery of carbon nanotubes by Iijima in 1991 great progress has been made in many areas of sciences. Lots of research works are going on to understand their unusual behaviors and to improved it.

Carbon nanotubes (CNTs) are allotropes of Carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1, significantly larger than for any other material. Carbon nanotubes (CNTs) are made by rolling up of sheet of GRAPHENE into a cylinder. Graphene is basically a 2D single layer of graphite. In case of graphite it has a layered, planar structure. In each layer, the carbon atoms are arranged in a honeycomb lattice (fig. 1.1) with separation of 0.142 nm, and the distance between planes is 0.335 nm. Atoms in the plane are bonded covalently and this 2D honeycomb layers are bonded by weak van der waals bond which makes graphite soft materials. In ghaphene (fig. 1.2) carbon atoms are bonded covalently which makes it stronger than many strong metals. The sp2 bonds in graphene are stronger than sp3 bonds in diamond that makes grapheme the strongest material. The lattice structure of grapheme in real space consists of hexagonal arrangement of carbon atoms as shown in Fig. 1.3a. An isolated carbon atom has four valence electrons in its 2s, and 2p atomic orbitals. While forming into graphene, three atomic orbitals of the carbon atom, 2s, 2 px, and 2 p y, are hybridized into three sp2orbitals. These sp2orbitals are in the same plane while the remaining2 pz, is perpendicular to other orbitals as shown in Fig. 1.3b. The σ bonds between the adjacent carbon atoms are formed by the sp2hybridized orbitals, whereas the 2 pzorbitals form the π bonds that are out of the plane of grapheme.

Fig. 1.1 2D honeycomb lattice of Graphene Fig. 1.2 3D Graphite structure

(Carbon atoms are covalently bonded) (Each 2D layers are bonded by

Weak Van der Waals bond)

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Fig. 1.3 Basic a hexagonal and b orbital structure of graphene

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LITERATURE REVIEW

I. Pulickel M. Ajayan and Otto Z. Zhou described some of the important materials science applications of carbon nanotubes. Specifically they discuss the electronic and electrochemical applications of nanotubes, nanotubes as mechanical reinforcements in high performance composites, nanotube-based field emitters, and their use as nanoprobes in metrology and biological and chemical investigations, and as templates for the creation of other nanostructures. Electronic properties and device applications of nanotubes are treated elsewhere in the book. The challenges that ensue in realizing some of these applications are also discussed from the point of view of manufacturing, processing, and cost considerations.

II. Ray H. Baughman, Anvar A. Zakhidov, Walt A. de Heera highlights many potential applications have been proposed for carbon nanotubes, including conductive and high-strength composites; energy storage and energy conversion devices; sensors; field emission displays and radiation sources; hydrogen storage media; and nanometer-sized semiconductor devices, probes, and interconnects. Some of these applications are now realized in products. Nanotube cost, polydispersity in nanotube type, and limitations in processing and assembly methods are important barriers for some applications of single-walled nanotubes.

III. Thomas W. Ebbesen investigate on the novel mechanical and electronic properties that suggest new high-strength fibers, submicroscopic test tubes and, perhaps, new semiconductor materials.

IV. Andres Felipe Diaz Cruz presents a general overview on the main characteristics, fabrication methods current state of the art and a discussion on future potential applications in various fields such as medicine, engineering, manufacturing, military and energy storage and distribution of this material.

V. Philip G. Collins and Phaedon Avouris provides a broad overview of the electronic properties of carbon nanotubes, primarily from the point of view of the properties of individual, isolated tubes. The paper is organized according to the increasing levels of complexity found in nanotube electronics. First, the parent material graphite is briefly introduced, followed by a discussion of the electronic properties of individual, single-walled carbon nanotubes (SWNTs). Next, the experimentally observed properties of both metallic and semiconducting SWNTs are discussed in detail. An introduction to many-body effects and optoelectronic properties of SWNTs is also provided. Towards the end of the paper, more complex nanotubes and nanotube aggregates are discussed, since these types of samples derive many of their characteristics from SWNT properties. The chapter concludes with a short survey of near-term applications which take advantage of nanotube electronic properties.

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VI. Prabhakar R. Bandaru, in his paper experimentally verified electrical properties of carbon nanotube structures and manifestations in related phenomena such as thermoelectricity, superconductivity, electroluminescence, and photoconductivity are reviewed. The possibility of using naturally formed complex nanotube morphologies, such as Y-junctions, for new device architectures are then considered. Technological applications of the electrical properties of nanotube derived structures in transistor applications, high frequency nanoelectronics, field emission, and biological sensing are then outlined. The review concludes with an outlook on the technological potential of nanotubes and the need for new device architectures for nanotube systems integration.

VII. Rodney S. Ruoff , Dong Qian , Wing Kam Liu briefly reviewed the mechanical properties of CNTs and related structures. Their emphasis has been on elastic properties, deformability including buckling, twisting, and flattening, and inelastic behavior such as fracture and plastic yielding. Both experiment and theory/modeling were discussed.

VIII. Niraj Sinha, Jiazhi Ma, and John T. W. Yeow reviewed the distinct physical, electronic, and mechanical properties of CNTs. The main thrust of this paper is to highlight the present and future research and development work in the area of carbon nanotube sensors for real-world applications. The technical challenges associated with CNT-based sensors, which remain to be fully addressed, have also been outlined at the end of the manuscript. This review aims to act as a reference source for researchers to help them in developing new applications of CNT-based sensors.

IX. J. Hone, M.C. Llaguno, M.J. Biercuk, A.T. Johnson, B. Batlogg, Z. Benes, J.E. Fischer try to explore the thermal properties of nanotubes by measuring the specific heat and thermal conductivity of bulk SWNT samples. In addition, they have synthesized nanotube-based composite materials and measured their thermal conductivity.

X. Marcus Freitag in the first part of this paper, he address the electronic properties of metallic carbon nanotubes, and in the second part, he discuss the most important device made from a semiconducting nanotube, the carbon nanotube field effect transistor and its applications.

XI. Randal J. Grow In this chapter,he focused primarily on the electromechanical properties and applications of nanotubes. The dependence of the electronic properties on the structure implies that mechanical deformations can alter the band structure. This results in electromechanical effects such as piezoresistance and electrostatic actuation, which, together with the mechanical properties, may lead to nanotube-based mechanical sensors and actuators as well as more complex applications, such as oscillators or electromechanical switches.

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2. HISTORY & DISCOVERY Until the mid-1980’s pure solid carbon was thought to exist in only two physical forms, diamond and graphite. Diamond and graphite have different physical structures and properties however their atoms are both arranged in covalently bonded networks. These two different physical forms of carbon atoms are called allotropes.

In 1985 a group of researchers led by Richard Smalley and Robert Curl of Rice University in Houston and Harry Kroto of the University of Sussex in England made an interesting discovery. They vaporized a sample of graphite with an intense pulse of laser light and used a stream of helium gas to carry the vaporized carbon into a mass spectrometer. The mass spectrum showed peaks corresponding to clusters of carbon atoms, with a particularly strong peak corresponding to molecules composed of 60 carbon atoms, C60. The fact thatC60. clusters were so easily formed led the group to propose that a new form or allotrope of carbon had been discovered. It was spherical in shape and formed a ball with 32 faces. Of the 32 faces, 12 were pentagons and 20 were hexagons exactly like a soccer ball (fig. 2.1).

Fig.2.1 C60 Fullerene.

After this discovery, other related molecules (C36, C70, C76 and C84) composed of only carbon atoms were also discovered and they and the buckyball were recognized as a new allotrope of carbon. This new class of carbon molecules is called the fullerenes. Fullerenes consist of hexagons and pentagons that form a spherical shape. The unique geometric properties of this new allotrope of carbon did not end with soccer shaped molecules, In June 1991, Japanese scientist SUMIO IIJIMA NEC Laboratory in Tsukuba found an extremely thin needle-like material when examining carbon materials under an electron microscope. Soon thereafter the material was proved to have a graphite structure basically, and its details were disclosed. He named these materials “carbon nanotubes” since they have a tubular structure of carbon atom sheets, with a thickness scaled in less than a few nanometers. The name has been widely accepted now.

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3. TYPES OF CARBON NANOTUBES

Carbon nanotubes are mainly classified into two:-

Single-walled Nanotubes (SWNTS);

Multi-walled Nanotubes (MWNTS).

Single-walled Nanotubes (SWNTS)

• A single-walled carbon nanotube (SWNT) may be thought of as a single atomic layer thick sheet of graphite (called graphene) rolled into a seamless cylinder.

• Most single-walled nanotubes (SWNT) have a diameter of close to 1 nanometer, with a tube length that can be many millions of times longer.

• Single-walled nanotubes are an important variety of carbon nanotube because they exhibit electric properties that are not shared by the multi-walled carbon nanotube (MWNT) variants

Fig. 3.1 Single-walled Carbon Nanotube (SWNT)

Commercially these single walled carbon nanotubes (SWNTs) were produced via the Catalytic Carbon Vapor Deposition (CCVD) process.

Potential applications of SWNTs are in field effect transistors (FETs), solar cells, photo detectors, high performance computation, renewable energy, optoelectronics etc. Other applications include electronics, sensors, composites, energy storage and study of life science systems.

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Multi-walled Nanotubes (MWNTS

Multi-walled nanotubes (MWNT) consist of multiple rolled layers (concentric tubes) of graphite.

There are two models which can be used to describe the structures of multi-walled nanotubes.

In the Russian Doll model, sheets of graphite are arranged in concentric cylinders.

In the Parchment model, a single sheet of graphite is rolled in around itself, resembling a scroll of parchment or a rolled newspaper.(The Russian Doll structure is observed more commonly).

The telescopic motion ability of inner shells and their unique mechanical properties will permit the use of multi-walled nanotubes as main movable arms in coming Nano mechanical devices.

MWNTs are highly conductive when properly integrated into a composite structure. One must note that the outer wall alone is conducting; the inner walls are not instrumental to conductivity.

MWNTs have a high aspect ratio with lengths typically more than 100 times the diameter, and in certain cases much higher. Their performance and application is based not just on aspect ratio, but also on the degree of entanglement and the straightness of the tubes, which in turn is a function of the both the degree and dimension of defects in the tubes.

Defect–free, individual, MWNTs have an excellent tensile strength and when integrated into a composite, such as a thermoplastic or thermoset compounds, can significantly increase its strength.

MWNTs have a thermal stability more than 600 °C, based on the level of defects and to certain extent on the purity as residual catalyst in the product can also catalyze decomposition.

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MWNTs are an allotrope of sp2 hybridized carbon similar to graphite and fullerenes and as such have high chemical stability. However, one can functionalize the nanotubes to enhance both the strength and dispersibility of composites.

Fig.3.2 Electron micrographs of nanotubes of graphitic carbon. The parallel dark lines correspond to the (002) lattice images of graphite. A cross-section of each tubule is illustrated: (a) tube consisting of five graphitic sheets, diameter 6.7 nm, (b) two-sheet tube, diameter 5.5 nm, and (c) seven-sheet tube, diameter 6.5 nm, which has the smallest hollow diameter of 2.2 nm (Iijima, 1991)

Table 1- Comparison between SWNT and MWNT

Sr. No.

SWNTs MWNTs

1 Single layer of graphene. Multiple layer of graphene.

2 Catalyst is required for synthesis. Can be produced without catalyst..

3 Bulk synthesis is difficult as it requires proper control over growth and atmospheric condition.

Bulk synthesis is easy.

4 Purity is poor. Purity is high.

5 A chance of defect is more duringfictionalization

A chance of defect is less but once occurred it’s difficult to improve.

6 Less accumulation in body..

More accumulation in body.

7 Characterization and evaluation is easy

It has very complex structure.

8 It can be easily twisted and are more pliable.

It cannot be easily twisted.

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4. STRUCTURES OF CARBON NANOTUBES To understand the crystal structure of CNTs, it is necessary to understand their atomic structure. Both CNTs and GNRs (graphene nanoribbons) can be understood as structures derived from a graphene sheet, shown in Fig. 4.1. A graphene sheet is a single layer of carbon atoms packed into 2D honeycomb lattice structure. CNT, considered as rolled-up graphene sheet, have the edges of the sheet joint together to form a seamless cylinder. The dashed arrows in Fig. 2.2a, b show the circumferential vector C, which indicates the rolling up direction for CNT. The vector is defined as C ¿ n1 a1+n2 a2where a1 and a2 are the lattice vectors of graphene and n1and n2are the chiral indices. The chiral indices (n1,n2) uniquely defines the chirality, or the rolled-up direction of graphene sheet. Depending on the chiral indices CNTs can be classified to zigzag and armchair structures as shown in Fig. 4.1 a, b, respectively. For armchair CNTs, the chiral indices n1and n2are equal while for zigzag CNTs, n1or n2= 0. For other values of indices, CNTs are known as chiral. Depending upon their different structures, CNTs can exhibit metallic or semiconducting properties. By satisfying the condition n1- n2= 3i (where i is an integer), the armchair CNTs are always metallic, whereas zigzag CNTs are either metallic or semiconducting in natural. Statistically, a mix of CNTs will have 1/3rd metallic and 2/3rd semiconducting characteristic.

Fig. 4.1 Schematic view of CNT made from graphene sheet a zigzag and b armchair CNT

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Fig. 4.2a.Armchair arrangement b. Zig-zag arrangement of c. Chiral arrangement of carbon atoms of carbon atoms of carbon atoms

5. SYNTHESIS OF CARBON NANOTUBES

Techniques have been developed to produce nanotubes, including arc discharge, laser ablation and chemical vapor deposition (CVD). Most of these processes take place in vacuum or with process gases. CVD growth of CNTs can take place in vacuum or at atmospheric pressure. Large quantities of nanotubes can be synthesized by these methods; advances in catalysis and continuous growth processes are making CNTs more commercially viable.

SWNTs and MWNTs are usually made by carbon-arc discharge, laser ablation of carbon, or chemical vapor deposition (typically on catalytic particle). Nanotube diameters range from 0.4 to 3 nm for SWNTs and from 1.4 to at least 100 nm for MWNTs. Nanotube properties can thus be tuned by changing the diameter. Unfortunately, SWNTs are presently produced only on a small scale and are extremely expensive. All currently known synthesis methods for SWNTs result in major concentrations of impurities. These impurities are typically removed by acid treatment, which introduces other impurities, can degrade nanotube length and perfection, and adds to nanotube cost.

MWNTs produced catalytically by gas-phase pyrolysis, like the Hyperion nanotubes, have high defect densities compared to those produced by the more expensive carbon- arc process.

5.1 Arc Discharge Method

CNT production requires 3 elements,

1. Carbon feed.

2. Metal catalyst.

3. Heat the nanotubes were initially discovered using this technique; it has been the most widely-used method of nanotube synthesis.

Two Graphite electrodes placed in an inert Helium atmosphere. When DC current is passed anode is consumed and material forms on cathode. For SWNT mixed metal catalyst is inserted into anode

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Pure iron catalyst + Hydrogen-inert gas mixture gives 20 to 30cm long tube

Fig. 5.1 Arc discharge method.

5.2 Laser Ablation

In the laser ablation process, a pulsed laser vaporizes a graphite target in a high-temperature reactor while an inert gas is bled into the chamber.

Nanotubes develop on the cooler surfaces of the reactor as the vaporized carbon condenses.

A water-cooled surface may be included in the system to collect the nanotubes.

The laser ablation method yields around 70% and produces primarily single-walled carbon nanotubes with a controllable diameter determined by the reaction temperature.

It is more expensive than either arc discharge or chemical vapor deposition.

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Fig. 5.2 Laser ablation method

5.3 Chemical Vapor Deposition (Cvd)

During CVD, a substrate is prepared with a layer of metal catalyst articles, most commonly nickel, cobalt, iron, or a combination.

The diameters of the nanotubes that are to be grown are related to the size of the metal particles.

The substrate is heated to approximately 700°c. To initiate the growth of nanotubes, two gases are bled into the reactor: a process gas

(such as ammonia, nitrogen or hydrogen) and a carbon-containing gas (such as acetylene, ethylene, ethanol or methane).

Nanotubes grow at the sites of the metal catalyst; The carbon-containing gas is broken apart at the surface of the catalyst particle, and

the carbon is transported to the edges of the particle, where it forms the nanotubes.

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fig. 5.3 Chemical vapor deposition (CVD) metod.

6. PROPERTIES OF CARBON NANOTUBES

6.1 Mechanical & Physical Properties Carbon nanotubes are the strongest and stiffest materials yet discovered in terms of tensile strength and elastic modulus respectively. This strength results from the covalent sp2 bonds formed between the individual carbon atoms which are stronger than 3D diamond bonds. Because of their exceptional properties Steady progress has been made in exploring the mechanical properties and potential applications of two types of CNTs namely single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT). The measured specific tensile strength of a single layer of a multi-walled carbon nanotube can be as high as 100 times that of steel, and the graphene sheet (in-plane) is as stiff as diamond at low strain. These mechanical properties motivate further study of possible applications for lightweight and high strength materials. Composite materials reinforced by either SWCNT or MWCNT have been fabricated and significant enhancement in mechanical properties has been recently reported. In 2000, a multi-walled carbon nanotube was tested to have a tensile strength of 63 GPa(9,100,000 psi). (For illustration, this translates into the ability to endure tension of a weight equivalent to 6,422 kilograms-force (62,980 N; 14,160 lbf) on a cable with cross-section of 1 square millimeter (0.0016 sq in).) Further studies, such as one conducted in 2008, revealed that individual CNT shells have strengths of up to ~100 GPa(15,000,000 psi), which is in agreement with quantum/atomistic models. Since carbon nanotubes have a low density for a solid of 1.3 to 1.4 g/cm3, its specific strength of up to 48,000 kN·m/kg is the best of known materials, compared to high-carbon steel's 154 kN·m/kg..

The reasons for such exceptional properties are explained below-

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6.1.1 Molecular Structure

Structure of the bond CNT is a cylindrical molecule composed of carbon atoms. A typical SWCNT structure is illustrated in Fig. 6.1. A major feature of the structure is the hexagon pattern that repeats itself periodically in space. As a result of the periodicity, each atom is bonded to three neighboring atoms. Such structure is mainly due to the process of sp2 hybridization during which one

S-orbital and two p-orbitals combine to form three hybrids sp2-orbital at 120ᵒto each other within a plane (shown in Fig. 6.2 for part of a graphene sheet). This covalent bond (referred to as the σ -bond) is a strong chemical bond and plays an important role in the impressive mechanical properties of CNTs. In addition, the out-of-plane bond (the π-bond) that is relatively weak contributes to the interaction between the layers in MWCNTs, and between SWCNT’s in SWCNT bundles. Of course, the bonding is not purely sp2 in nanotubes, as curving the graphene sheet into a tube re-hybridizes the σ and π orbitals, yielding an admixture.

Structures of single-walled carbon nanotube As described above, the bonding in CNTs is similar, but not identical, to the graphene sheet. A widely used approach to identify the types of SWCNT is by reference to rolling up the graphene sheet. The key geometric parameter associated with this process is the roll-up vector r, which can be expressed as the linear combination of the lattice basis (a1 anda2).

C ¿ n1 a1+n2 a2It is then possible to associate a particular integer pair (n1 , n2) with each SWCNT. The relation between n and m also defines three categories of CNT:n1 = 0, ‘Zigzag’,

n1n = n2, ‘Armchair’,

Other, ‘Chiral’. (Fig. 4.1 & 4.2)

Mechanical as well as electrical properties vary depending upon the configuration of SWCNT.

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Fig. 6.1. Molecular structure of a section of (10, 10) CNT. Each node shown is a carbon atom and lines are the chemical bonds. Fig. 6.2 Basic hexagonal bonding

structure for one graphite layer (the grapheme sheet’). Carbon nuclei shown as filled circle, out-of-plane π-bonds, and σ-bonds connect the C nuclei in-plane.

Structures of multi-walled carbon nanotubes:

In MWCNT there is a between two layers in the ranges from 0.342 to 0.375 nm and this out-of-plane bond (the π-bond) that is relatively weak contributes to the interaction between the layers in MWCNTs. That’s why the properties of MWCNTs are different from SWCNTs. The experimental values of MWCNTs and SWCNTs are mentions letter.

6.1.2 Bonding Models:

Several empirical potentials are available that model the covalent bond in CNT. These atomistic models can be used to predict the mechanical properties. The existing empirical models can be categorized as follows:

Force field model Bond order model Semi-empirical model Interlayer potentials

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6.1.3 Elastic Properties

By assuming CNT as a structural member, the elastic properties of CNT can be obtained from experimental observations. Typical examples of structural member include bar, beam, and shell models. The bar model has been used in the experiment by Lourie and Wagner, in which the compressive response was measured using micro-Raman spectroscopy. They reported Young’s modulus of 2.8–3.6 TPa, for SWCNT and 1.7–2.4 TPa for MWCNT. Direct tensile loading tests of SWCNT and MWCNT have been performed by Yu et al. The Young’s modulus obtained ranges from 320 to 1470 GPa (mean: 1002 GPa) for SWCNT and from 270 to 950 GPa for MWCNT. The stress–strain curves obtained for these two types of CNT are shown below (Figs. 6.3 and 6.4). Note that the stress and strain correspond to the definition of engineering stress and strain, and the stress is evaluated by assuming an equivalent thickness of 0.34 nm for each layer of loaded CNT.

A cantilevered beam model has been used in the experiment by Wong et al. in which individual MWCNT were bent using an atomic force microscope tip. By fitting the measured static response to the analytical solution for a cantilevered beam, a Young’s modulus of 1.28 ± 0.59 TPa was obtained. The simply-supported beam model was used by Salvetat et al. to model the deflections of individual MWCNTs and of different-sized SWCNT ropes; a Young’s modulus of ~1 TPa for MWCNTs grown by arc discharge was reported, whereas CNT grown by the catalytic decomposition of hydrocarbons, however, had a modulus 1–2 order of magnitude smaller.

Fig. 6.3 Eight stress versus strain curves obtained from the tensile-loading experiments on individual SWCNT bundles. The values of the nominal stress are calculated using the cross-sectional area of the perimeter SWCNTs assuming a thickness of 0.34 nm. The strain is the engineering strain.

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Fig. 6.4 Plot of stress versus strain curves for 5 individual MWCNTs.

A theoretical evaluation of Young’s modulus can be obtained either by directly computing the mechanical response or by deriving it analytically. Overney et al. calculated Young’s modulus using an empirical Keating Hamiltonian with parameters determined from first principles in the computation. Their calculation, as pointed out by Treacy et al., implies a Young’s modulus ranging from 1.5 to 5.0 TPa.

The Young’s modulus can also be estimated by evaluating the energy in the CNT system. The relation that the strain energy of the tube is proportional to 1/R2 (where R is the radius of the CNT) was reported in the energetic analysis of Tibbetts, Robertson et al., and Gao et al. Gao et al. obtained values of Young’s modulus from 640.30 GPa to 673.49 GPa by computing the second derivative of the potential energy.

By using the molecular dynamics (MD) Lu has reported a Young’s modulus of ~1 TPa, a shear modulus of ~0.5 TPa, and also that chirality, radius and the number of walls have little effect on the value of Young’s modulus.

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6.1.4 Strength of CNTs

Measuring the tensile strength of CNTs is an extremely challenging task. Under excessive tensile strain, the tubes will undergo plastic deformation, which means the deformation is permanent. This deformation begins at strains of approximately 5% and can increase the maximum strain the tubes undergo before fracture by releasing strain energy.

Tensile load testing was performed by Yu et al. on SWCNT bundles and tensile strength values ranging from 13 to 52 GPa were reported; shown in Fig. 6.5 is an example of a SWCNT bundle that is being loaded and the maximum tensile strain obtained was 5.3%, which is close to the prediction by Nardelli et al. Yu et al. have also conducted tensile testing of MWCNTs. It was found that only the outermost layer breaks during the loading process (Fig. 6.6). The tensile strength corresponding to this layer of CNT ranges from 11 to 63 GPa.

Fig.6.5. SEM image of a tensile loaded SWCNT bundle between an AFM tip and a SWCNT “buckytube paper” sample

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Fig.6.6. Tensile loading of individual MWCNTs. (A) An SEM image of a MWCNT attached between two AFM tips; (B) higher magnification image of the indicated region in (A) showing the MWCNT between the AFM tips

Although the strength of individual CNT shells is extremely high, weak shear interactions between adjacent shells and tubes lead to significant reduction in the effective strength of multi-walled carbon nanotubes and carbon nanotube bundles down to only a few GPa. This limitation has been recently addressed by applying high-energy electron irradiation, which crosslinks inner shells and tubes, and effectively increases the strength of these materials to ~60 GPa for multi-walled carbon nanotubes and ~17 GPa for double-walled carbon nanotube bundles.

CNTs are not nearly as strong under compression. Because of their hollow structure and high aspect ratio, they tend to undergo buckling when placed under compressive, torsional, or bending stress.

Table 2. Comparison of mechanical properties

Materials Young's modulus (TPa)

Tensile strength (GPa

Elongation at break (%)

SWCNTs ~1 (from 1 to 5) 13–53 16

Armchair SWNT 0.94 126.2 23.1Zigzag SWCNT 0.94 94.5 15.6–17.5Chiral SWCNT .92MWNT 0.2 - 0.9 63 - 150Stainless Steel 0.183 – 0.264 0.38 – 1.55 15 - 50Kevlar 0.06 – 0.18 3.6 – 3.8 ~2

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6.1.5 Hardness

Standard single-walled carbon nanotubes can withstand a pressure up to 25 GPa without (plastic/permanent) deformation. They then undergo a transformation to superhard phase nanotubes. Maximum pressures measured using current experimental techniques are around 55 GPa. However, these new superhard phase nanotubes collapse at an even higher, albeit unknown, pressure.

The bulk modulus of superhard phase nanotubes is 462 to 546 GPa, even higher than that of diamond (420 GPa for single diamond crystal).

6.1.6 Wettability

The surface wettability of CNT is of importance for its applications in various settings. Although the intrinsic contact angle of graphite is around 90°, the contact angles of most as-synthesized CNT arrays are over 160°, exhibiting a superhydrophobic property. By applying a low voltage as low as 1.3V, the extreme water repellant surface can be switched into super hydrophilic.

6.1.7 Aspect Ratio

It was found that the MWNT aspect ratio has a significant effect on both electrical and mechanical properties of nanocomposites with significantly better properties for MWNTs of smaller diameter. The high aspect ratio in the CNT structure means they are susceptible to structural instability. It also play an important role in CNT composite making.

6.2 ELECTRICAL AND ELECTRONICS PROPERTIES

6.2.1. Behaviors According to Structure

The electronic band structure of a nanotube can be described by considering the bonding of the carbon atoms arranged in a hexagonal lattice. Each carbon atom (Z = 6) is covalently bonded to three neighbor carbons via s p2 molecular orbital. The fourth valence electron, in the P z orbital, hybridizes with all the other P z orbital to form a delocalized Π-band. As the unit cell of graphene has two carbon atoms an even number of electrons are contained in the basic nanotube structure, which consequently can be metallic or semiconducting predicted through tight-binding electronic structure calculations that the relationship between the coefficients (n1∧n2) of the translational vector ch = n1 a1+n2 a2, which connects two crystallographically equivalent sites, determines the conducting properties.

If n1=n2 the nanotube is metallic; if (n-m) is a multiple of 3, then the nanotube is semiconducting with a very small band gap, otherwise the nanotube is a moderate semiconductor. Thus all armchair (n1=n2) nanotubes are metallic, and nanotubes (6,4), (9,1), etc. are semiconducting.

If all values of the chiral vectors were equally probable, it would be expected that 1/3 of the total SWNTs would be metallic while the remaining 2/3 would be semiconducting, which is indeed what is found in synthesis.

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6.2.2 Semiconducting and Doping For semiconducting SWNTs, in the absence of impurities or defects (doping), the Fermi energy (EF) is taken to be at a reference value of zero. However, for a realistic grapheme based nanotube a finite doping is inevitable due to the presence of adsorbents, from the ambient, which would cause charge transfer. In that case, the EF is either <0 (for hole doping, electron transfer from the NT—p type) or >0 (for electron doping, electron transfer to the NT— n type).The effects of temperature also have to be taken into consideration i.e., (i) k BT >EF or (ii) k BT <EF

Case (i), suitable for high temperatures, corresponds to low doping while low temperatures (case (ii)) are typify strong doping conditions. In metallic nanotubes the results of the shifts in the Fermi level are masked by a higher density of states and doping effects are less marked.

6.2.3 Piezoresistance

It’s a electromechanical properties of CNT. Piezoresistance is the tendency of a material to change its resistivity under strain. As a force deforms a crystal and changes the lattice spacing, the electronic band structure changes, which changes the resistivity. This effect is well understood in bulk semiconductors like silicon and germanium. The resistivity of a semiconductor is given by-

ρ= 1qn μn+qpμ p

Where q is the carrier charge, n and p are the carrier densities, and μn and μpare the carrier mobilities. The mobility is given by μ = 1/mτ, where m is the effective mass of the carrier and τ is the scattering time. In silicon, piezoresistance is explained primarily by strain in a particular direction breaking the symmetry and splitting degenerate bands, which causes a shift in the population of carriers between subbands with different mobilities. In addition, the splitting suppresses band–band scattering because there are no longer phonons available with the correct energy and momentum. Another, smaller effect is a change in the overall band gap, which changes the number of carriers in the entire conduction band. The overall change in resistivity tends to be linear with strain. In nanotubes, on the other hand, the second-lowest subband of the conduction band is typically ~1 eV above the lowest sub-band, so it is too high in energy to have any substantial carrier population. Only the lowest sub-band plays a role, allowing a change in the band gap to be the dominant reason for a change in resistance (Figure 6.7). If the band gap changes, that changes the overall number of carriers in the nanotube, changing the resistance.

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Tube axis.

Fig. 6.7 A zigzag nanotube being stretched along its length and the resultant change in the dispersion relation (and therefore the band gap) near EF.

In nanotubes, piezoresistance may be useful for mechanical sensors as well as electromechanical switches. It may also be possible to use this effect to tune the electronic and optical properties of nanotube devices; much like piezoresistance in silicon is used to increase the carrier mobility in transistors.

6.2.4 Photoconductivity of Carbon Nanotubes

Fig.6.8 Schematic diagram of photoconductive mechanism.

It has been observed that infrared laser excited photoconductivity from a single carbon nanotube incorporated as the channel of an ambipolar field-effect transistor (FET). Electron−hole pairs are generated within the nanotube molecule, and the carriers are separated by an applied electric field between the source and drain contacts. The photocurrent shows resonances whose energies are in agreement with the energies of exciton states of

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semiconducting nanotubes of the appropriate diameter. The photocurrent is maximized for photons polarized along the direction of the carbon nanotube. Thus, the nanotube FET acts as a polarized photodetector with a diameter 1000 times smaller than the wavelength of the light it detects and has an estimated quantum efficiency of >10%. A photovoltage is observed when an asymmetric band lineup due to two nonequivalent Schottky barriers or an asymmetric coupling of the gate to the nanotube is present.

6.3 THERMAL PROPERTIES

6.3.1 Specific Heat

The thermal properties of carbon nanotubes are directly related to their unique structure and small size. Because of these properties, nanotubes may prove to be an ideal material for the study of low-dimensional phonon physics, and for thermal management, both on the macro and the micro-scale. Fig.6.9 shows the calculated low-temperature specific heat of an isolated nanotube. Because the PDOS (Phonon density of state) is constant at low energy, the specific heat displays linear temperature dependence at low temperature

Fig. 6.9 Calculated single-nanotube specific heat

6.3.2 Thermal conductivity As diamond and graphite display the highest known thermal conductivity at moderate temperatures, it is likely that nanotubes should be outstanding in this regard as well. Indeed,

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recent theoretical work has predicted that the room temperature thermal conductivity of nanotubes is as high as 6600W/m K. Figure 6.10 shows the measured temperature-dependent thermal conductivity of bulk samples of SWNTs that have been aligned by filtration in a high magnetic field. In the alignment direction, the room-temperature thermal conductivity is greater than 200W/m K, which is comparable to a good metal and within an order of magnitude of that of highly crystalline graphite or diamond. The thermal conductivity of unaligned samples is about one order of magnitude smaller. The temperature dependence of the thermal conductivity is shown in Fig. 6.11.

Fig.6.10 Thermal conductivity of a bulk sample of SWNTs. The measurement is taken in the direction parallel to the tubes.

Fig.6.11 Thermal conductivity divided by temperature, K/T, of SWNT samples with different average diameter.

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7. ADVANTAGES AND DISADVANTAGES Carbon nanotubes are a great piece of technology, and new uses for them are being found every day. However they do have their draw backs.

Most of the unique characteristics are already discussed in ‘properties’ section. Some of the advantages and disadvantages of CNTs with respect to other materials can be list out as-

Advantages Extremely small and lightweight, making them excellent replacements for metallic wire. Resources required to produce them are plentiful, and many can be made with only a

small amount of material. Are resistant to temperature changes, meaning they function almost just as well in

extreme cold as they do in extreme heat. Have been in the R&D phase for a long time now, meaning most of the kinks have been

worked out. As a new technology, investors have been piling into these R&D companies, which will

boost the economy.

Disadvantages Despite all the research, scientists still don't fully understand exactly how they work. Extremely small, so are difficult to work with. Currently, the process is relatively expensive to produce the nanotubes. Would be expensive to implement this new technology in and replace the older

technology in all the places that we could. At the rate our technology has been becoming competitive; it may be a gamble to bet on

this technology.

8. APPLICATION OF CARBON NANOTUBES

8.1 Electronic Applications of Carbon Nanotubes

Because nanotubes exhibit many interesting electronic properties, a wide range of electronic applications have been proposed and investigated. Various prototype devices have been demonstrated with excellent, commercially-competitive properties.

8.1.1 Conductive Composites

Because of their extreme aspect ratios, nanotubes can form conductive percolation networks at very small volume fractions. By embedding nanotubes into a polymer matrix, antistatic and electrically conductive plastics are readily produced. In fact, such products are the earliest known commercial applications of carbon nanotubes. The volume fraction of nanotubes can be as small as 0.1% allowing the intrinsic properties of the matrix, which might include transparency, flexibility, and strength, to be retained. At still higher densities, the nanotubes can also begin to tailor the matrix’s mechanical properties.

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When not confined within a matrix, conductive nanotube films exhibit large surface areas and significant mechanical flexibility. These properties enable electrochemical and electromechanical applications. For example, nanotube “supercapacitors” exhibit very large capacitances for their size, because even a compressed nanotube film remains porous to most electrolytes. A nanotube electrode with a macroscopic area A has an effective surface area hundreds of times larger than A and exhibits capacitances exceeding 100 Farads per gram of nanotube material. Furthermore, nanotube supercapacitors can charge and discharge at power rates of 20 –30 W per gram, sufficient for high power applications such as hybrid-electric vehicles. The chemical inertness of nanotubes is critical to this type of application: nanotubes do not readily degrade with repeated cycling the way that other high-surfacearea materials do. Of course, some electrolytes are more chemically active than others, and the application of nanotubes to lithium ion storage has been hampered by large, irreversible capacitances. Nevertheless, nearly all lithium ion batteries manufactured today include a small percentage of lifetime-enhancing carbon nanotubes. Both capacitative and chemical energy storage using nanotube electrodes remain active research topics, with products successfully deployed in both areas. However, these applications are quite sensitive to the high costs of nanotube materials and will benefit from ongoing research in bulk nanotube synthesis.

8.1.2 Electron Emitters As chemically inert wires with high aspect ratios, carbon nanotubes exhibit all of the properties desired in a good field emitter. Both SWNTs and MWNTs exhibit emission at low threshold electric fields of 1 to 2 V/μm, indicating that their sharp tips geometrically enhance local electric fields by factors of 1000 or more. The tolerance of nanotubes for extraordinary current densities further allows these emitters to operate at high power densities.

Individual field emitters constitute a small commercial market, but arrays of emitters can be used for general lighting, flat panel displays, and high power electronics. In each of these applications, the nanotube emission exceeds the minimum performance specifications to be competitive; the primary barriers to commercialization involve other issues such as fabrication, compatibility, and product complexity. In lighting, these barriers are relatively small and high-brightness, high-efficiency lighting products exist. Alternately, flat panel displays are highly complex products with interrelated technical, economic, and practical specifications. Display technologies demand thousands of identical pixels, a stringent requirement on emitter variability. In this field, multiple demonstration prototypes by companies including Samsung and Motorola have not yet resulted in consumer products.

Field emission is intrinsically a nondissipative, energy-efficient tunneling process. Emission devices are therefore useful in a variety of high power density applications, such as the frequency modulation of very large currents or surge protection for power and telecommunications substations. In the past, potential power devices such as these have been severely limited by the available field emission materials, and nanotubes have made new performance levels possible.

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Additionally, a field emission beam is very energetic, and when directed at a metal target it can generate x-rays. Before nanotubes, field emission sources lacked the current densities necessary to produce high flux x-ray sources for medical imaging or scientific equipment. As a result, x-ray equipment has depended on power-hungry, water-cooled, thermal electron sources. The advent of nanotube-based x-ray sources allows low weight, low power, portable devices, with the added advantage of higher spatial resolution and longer lifetimes compared to thermal sources. These devices are currently being developed for multiple applications including biomedical scanning, as depicted in Fig. 8.1 airport security, and space exploration,

Fig. 8.1 A SWNT cathode is a very effective field emitter, with turn-on fields as low as 1V/μm. The emitted electron beam can be used to light a phosphor or, as shown here, to generate X-rays medical imaging.

8.1.3 SensorsCNTs can be used as the sensing material in pressure, flow, thermal, gas, optical, mass, position, stress, strain, chemical, and biological sensors.

Following are the promising area where CNTs can be used as sensing material.

Biomedical Industry: It is believed that CNTs incorporated sensors can bring dramatic changes to the biomedical industry. There are certain cases such as diabetes, where regular tests by patients themselves are required to measure and control the

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sugar level in the body. Children and elderly patients may not be able to perform this test properly. Another similar example is regular tests of persons exposed to hazardous radiations or chemicals. The objective in these cases is to detect the disease in its early stage so that appropriate clinical action can be taken. Nanosensors have the advantages that they are thousands of times smaller than even MEMS sensors and consume less power. Also, they are less sensitive to variations in temperature (compared to silicon piezoresistors). This enables them to perform better in many of the biomedical sensing applications. Therefore, CNT-based nanosensors are highly suitable as implantable sensors. Implanted sensors can be used for monitoring pulse, temperature, blood glucose, and diagnosing diseases. Besides, CNTs can be used for repairing damaged cells or killing them by targeting tumors by chemical reactions. Implantable nanosensors can also monitor heart’s activity level and regulate heartbeats by working with an implantable regulator. CNTbased nanobiosensors may also be used to detect DNA sequences in the body. These instruments detect a very specific piece of DNA that may be related to a particular disease. Therefore, these sensors can possibly diagnose patients as having specific sequences related to a cancer gene.

Automotive Industry: Because of the quest to improve performance, reduce cost, and enhance the reliability, sensors have a substantial utility in the automotive industry and their influence is expected to increase while designing the vehicles of the future. They are used to acquire information about vehicle parameters such as pressure, vehicle altitudes, flow, temperature, heat, humidity, speed and acceleration, exhaust gas, and engine knock and torque. Apart from enabling new desirable features, CNT-based sensors are simply replacing old technologies with cheaper and more reliable devices. To prevent stealing of cars, antitheft systems are provided by the automakers. Conventional antitheft systems incorporate tilt detection systems. The drawback with this system is that it is not possible to park the car at higher inclinations. Also, conventional sensors don’t perform well in the widely varying automotive temperature environment. CNT-based sensors can find promising applications in this case. Other areas, where CNT-based sensors can find application are ignition control, headway control, transmission control, vehicle navigation, tyre condition, and cabin air quality monitoring.

Food Industry: Sensors and biosensors have been widely used in food industry to provide safety and quality control of food products as the contamination of foods caused by bacterial pathogens may result in numerous diseases. CNT-based sensors and biosensors show great potential for applications in the food industry. CNT-based biosensors can be used in meat freshness evaluation. Luong et al. reported a MWNT-based biosensor for non-mediator detection of putrescine. 3-Aminopropyltriethoxysilane (APTES) solubilized MWNTs were coated on glassy carbon electrodes to impose the interaction and electron exchange between redox-enzymes and electrode interface. The APTES modified CNTs served as an immobilization matrix for putrescine oxidase (POx).

Environmental Monitoring: CNT-based gas sensors can offer improved performance in real-time monitoring of combustible gas alarms, gas leak detection/alarms,

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biowarfare (e.g., monitoring explosives such as TNT or RDX and nerve agents such as GB or VX), environmental pollution monitoring, and cooking controls, etc.Unlike conventional solid-state gas sensors, which require relatively high temperatures to achieve significant. CNT-based carbon monoxide (CO) sensor can be used to control ventilation system in car parks.

Nanoprobes and Sensors: The small and uniform dimensions of the nanotubes produce some interesting applications. With extremely small sizes, high conductivity, high mechanical strength and flexibility (ability to easily bend elastically), nanotubes may ultimately become indispensable in their use as nanoprobes. One could think of such probes as being used in a variety of applications, such as high resolution imaging, nano-lithography, nanoelectrodes, drug delivery, sensors and field emitters. A single MWNT can be attached to the end of a scanning probe microscope tip for imaging (Fig. 8.2) . Since MWNT tips are conducting, they can be used in STM, AFM instruments as well as other scanning probe instruments, such as an electrostatic force microscope.

Fig. 8.2 Use of a MWNT as an AFM tip

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8.2 CNTs in Mechanical Field8.2.1 CNT Based Actuator

At macro scale level, great progresses have been made in CNT actuations by coupling CNT’s excellent mechanical properties and electrical properties. Large strain and high stress actuators have been studied based upon CNT electrostatic actuation, CNT electrochemical actuation, and CNT-polymer actuation. Although practical actuators are still under developing, the success of direct conversion of electrical energy into mechanical energy from these actuations offers the opportunities for a number of high technology applications, including humanoid robots, artificial and damaged hearts, artificial limbs, medical prosthetic devices etc.

8.2.2 CNT Based Composites

The high modulus and the low weight of carbon fibres make them ideal reinforcing agents in a variety of composite materials used in the aircraft and sports industries. However, to take real advantage of the high Young’s modulus and high strength of CNTs, a number of conditions have to be fulfilled and among them is the load transfer efficiency. Indeed, load transfer from matrix to CNTs plays a key role in the mechanical properties of composites. If the adhesion between the matrix and the CNTs is not strong enough to sustain high loads, the benefits of the high tensile strength of CNTs are lost. The two principal ways of improving adhesion and shear strength are to increase the surface roughness or the surface reactivity.

Here are the some examples of commercial products based on CNT.

Easton-Bell Sports, Inc. have been in partnership with Zyvex Performance Materials, using CNT technology in a number of their bicycle components—including flat and riser handlebars, cranks, forks, seatposts, stems and aero bars.

Zyvex Technologies has also built a 54' maritime vessel, the Piranha Unmanned Surface Vessel, as a technology demonstrator for what is possible using CNT technology. CNTs help improve the structural performance of the vessel, resulting in a lightweight 8,000 lb boat that can carry a payload of 15,000 lb over a range of 2,500 miles.

Amroy Europe Oy manufactures Hybtonite carbon nanoepoxy resins where carbon nanotubes have been chemically activated to bond to epoxy, resulting in a composite material that is 20% to 30% stronger than other composite materials. It has been used for wind turbines, marine paints and variety of sports gear such as skis, ice hockey sticks, baseball bats, hunting arrows, and surfboards.

8.3 CNT in Medicine 8.3.1 CNTs in Drug Delivery and Cancer Therapy

Drug delivery is a rapidly growing area that is now taking advantage of nanotube technology. Systems being used currently for drug delivery include dendrimers, polymers, and liposomes, but carbon nanotubes present the opportunity to work with effective structures that have high drug loading capacities and good cell penetration qualities. These nanotubes function with a larger inner volume to be used as the drug container, large

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aspect ratios for numerous functionalization attachments, and the ability to be readily taken up by the cell. Because of their tube structure, carbon nanotubes can be made with or without end caps, meaning that without end caps the inside where the drug is held would be more accessible. Right now with carbon nanotube drug delivery systems, problems arise like the lack of solubility, clumping occurrences, toxicity, cell penetration. Further research and understanding could improve upon numerous other advancements, like increased water solubility, decreased toxicity, increased cell penetration and uptake, all of which are currently novel but undeveloped ideas.

Carbon nanotubes can be used as multifunctional biological transporters and near-infrared agents for selective cancer cell destruction. Biological systems are known to be highly transparent to 700- to 1,100-nm near-infrared (NIR) light. Researchers showed that the strong optical absorbance of single-walled carbon nanotubes (SWNTs) in this special spectral window, an intrinsic property of SWNTs, can be used for optical stimulation of nanotubes inside living cells to afford multifunctional nanotube biological transporters.

8.3.2 CNTs as Biosensors

CNT Network Bio-Stress Sensors: A single nanotube experiences a change in electrical resistance when experiencing stress or strain. This piezoresistive effect changes the current flow through the nanotube, which can be measured in order to accurately quantify the applied stress. The tube network is embedded within orthopedic plates, clamps, and screws and in bone grafts in order to determine the state of bone healing by measuring the effect of a load on the plate, clamp, screw, or other fixation device attached to the bone. A healed bone will bear most of the load while a yet unhealed bone will defer the load to the fixation device wherein the nanotube network may measure the change in resistivity. Measurement is done wirelessly by electrical induction. This allows the doctor to accurately assess patient healing and also allows the patient to know how much stress the affected area may safely tolerate.

Glucose detection biosensors: Carbon nanotube–plasma polymer-based amperometric biosensors for ultrasensitive glucose detection have been fabricated.

DNA detection biosensors: An aligned carbon nanotube ultrasensitive biosensor for DNA detection was developed. The design and fabrication of the biosensor was based on aligned single wall carbon nanotubes (SWCNTs) with integrated single-strand DNAs (ssDNA). The fabricated ultra-sensitive biosensor provided label-free real-time electronic detection of DNA hybridization between surface immobilized ssDNA and target ssDNA.

8.4 Other applications of CNTs CNTs Thermal Materials: The record-setting anisotropic thermal conductivity of

CNTs is enabling many applications where heat needs to move from one place to another. Such an application is found in electronics, particularly advanced computing, where uncooled chips now routinely reach over 100 ͦC. The technology for creating aligned structures and ribbons of CNTs is a step toward realizing incredibly efficient

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heat conduits. In addition, composites with CNTs have been shown to dramatically increase their bulk thermal conductivity, even at very small loadings.

CNTs Air and Water Filtration: Many researchers and corporations have already developed CNT based air and water filtration devices. It has been reported that these filters can not only block the smallest particles but also kill most bacteria. This is another area where CNTs have already been commercialized and products are on the market now.

CNTs Ceramic Applications: A ceramic material reinforced with carbon nanotubes has been made by materials scientists at UC Davis. The new material is far tougher than conventional ceramics, conducts electricity and can both conduct heat and act as a thermal barrier, depending on the orientation of the nanotubes. Ceramic materials are very hard and resistant to heat and chemical attack, making them useful for applications such as coating turbine blades, but they are also very brittle. The researchers mixed powdered alumina (aluminum oxide) with 5 to 10 percent carbon nanotubes and a further 5 percent finely milled niobium. The researchers treated the mixture with an electrical pulse in a process called spark-plasma sintering. This process consolidates ceramic powders more quickly and at lower temperatures than conventional processes. The new material has up to five times the fracture toughness — resistance to cracking under stress — of conventional alumina. The material shows electrical conductivity seven times that of previous ceramics made with nanotubes. It also has interesting thermal properties, conducting heat in one direction, along the alignment of the nanotubes, but reflecting heat at right angles to the nanotubes, making it an attractive material for thermal barrier coatings.

9. CHALLENGES There are also general challenges that face the development of nanotubes into functional devices and structures. First of all, the growth mechanism of nanotubes, similar to that of fullerenes, has remained a mystery .With this handicap; it is not really possible yet to grow these structures in a controlled way. Especially for electronic applications, which rely on the electronic structure of nanotubes, this inability to select the size and helicity of nanotubes during growth remains a drawback. More so, many predictions of device applicability are based on joining Nano-tubes via the incorporation of topological defects in their lattices. There is no controllable way, as of yet, of making connections between nanotubes. Some recent reports, however, suggest the possibility of constructing these interconnected Structures by electron irradiation and by template mediated growth and manipulation.

For bulk applications, such as fillers in composites, where the atomic structure (helicity) has a much smaller impact on the resulting properties, the quantities of nanotubes that can be manufactured still falls far short of what industry would need. There are no available techniques that can produce nanotubes of reasonable purity and quality in kilogram quantities. The industry would need tonnage quantities of nanotubes for such applications.

Another challenge is in the manipulation of nanotubes. Nano-technology is in its infancy and the revolution that is unfolding in this .eld relies strongly on the ability to manipulate structures at the atomic scale. This will remain a major challenge in this field, among several others.

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10. CONCLUSION There is much about carbon nanotubes that is still unknown. There is also much work to be done towards cheaper mass-production and incorporation with other materials before many of the current applications being researched can be commercialized.

Nanotechnology is predicted to spark a series of industrial revolutions in the next two decades that will transform our lives to a far greater extent than silicon microelectronics did in the 20th century. Carbon nanotubes could play a pivotal role in this upcoming revolution if their remarkable structural, electrical and mechanical properties can be exploited. The remarkable properties of carbon nanotubes may allow them to play a crucial role in the relentless drive towards miniaturization scale. Lack of commercially feasible synthesis and purification methods is the main reason that carbon nanotubes are still not widely used nowadays. At the moment, nanotubes are too expensive and cannot be produced selectively. Some of the already known and upcoming techniques look promising for economically feasible production of purified carbon nanotubes. Some future applications of carbon nanotubes look very promising. All we need are better production technique for large amounts of purified nanotubes that have to be found in the near future. Nanotube promises to open up a way to new applications that might be cheaper, lower in weight and have a better efficiency.

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