CARBON NANO-TUBES

AN ADVANCED APPLICATION OF NANO-TECHNOLOGY

ABSTRACT

In the present world of development, technology is growing at a tremendous pace making works

easier, faster, efficient, & compact. The introduction of nano-technology has revolutionized the

world of science influencing each & every field. This paper aims at highlighting the technology

that is emerging fast and is in latest use…nano carbon tubes ,a latest advent of nano technology.

Carbon Nano-tubes are an allotrope of carbon. These are a tubular material with a hexagonal

honeycomb structure of a carbon atom connected to other carbon atoms. Carbon nano-tubes are

known to have excellent mechanical, electrically selective, high efficient hydrogen storage

properties and be new and almost defect-free of all the existing materials. Carbon nano-tubes are

called a new dream material in the 21st century and broadening their applications to almost all

the scientific areas, such as aerospace science, bio-engineering, environmental energy, materials

industry, medical and medicine science, electronic computer, security and safety, and science

education with the development of science.

This paper deals with the basics of nano carbon tubes, history, their structures, different

properties, types, various processing techniques and an analysis on these along with the

perspective applications.

INTRODUCTIONAs numerous researches has been reported on new physical phenomena and advanced properties

of materials in the extremely infinitesimal areas of nanoscale size in recent years, a new area

called nanotechnologies came into being. These nanotechnologies have emerged as a future

leader in areas, such as electronic information communications, medicine, material, production

process, environment, and energy.

As new material properties of carbon nano-tubes, in particular, can be realized among other nano

technologies, both the importance in basic research and industrial applicability are being in the

great limelight

HISTORYFullerene, one of carbon allotropes (a cluster of 60 carbon atoms: C60) was discovered by

KROTO and SMALLEY for the first time in 1985. Dr Sumio Iijima, a researcher of the NEC

laboratories in JAPAN, in 1991 discovered thin and long straw shaped carbon nano-tubes during

a TEM analysis of carbon clusters synthesized by arc-discharge method. He found that the

central core of the cathodic deposit contained a variety of closed graphitic structures including

nano-particles and nano-tubes, of a type which had never previously been observed. The nano-

tubes range in length from a few tens of nanometers to several micrometers, and in outer

diameter from about 2.5 nm to 30 nm. A carbon atom in nano-tubes forms a hexagonal

honeycomb lattice of sp2 bond with 3 other carbon atoms. In 1992, Ebbesen and Ajayan reported

that increasing the pressure in the chamber of an arc-evaporation had greatly improved the nano-

tube yield on the cathode of graphite. In the year 1993 Bethune of IBM and Iijima of NEC

independently synthesized carbon nano-tubes, using arc-discharge methods. In the Year 1998

using the plasma-enhanced chemical vapor deposition, a major breakthrough was made in

synthesis and application of carbon nano-tubes by growing highly pure carbon nano-tubes

vertically aligned on a glass substrate. Since then, researches into synthesis and application have

been vigorously conducted around the world.

STRUCTURE

A free carbon atom has the electronic structure 1s22s22p2. In order to form covalent bonds, one of

the 2s electrons is promoted to 2p. In graphite, one carbon atom forms a strong ¥ò bond called as

sp2 with three other adjacent atoms in a plane. The others in the p orbital have a weak ¥ð bond at

90o to this plane which gives the semi-metal characteristics.

There are two possible high symmetry structures for carbon nano-tubes, known as 'zig-zag' and

'armchair'. In practice, it is believed that most carbon nano-tubes do not have these highly

symmetric forms but have structures called 'chiral' in which the honeycomb-shaped hexagons are

arranged helically around the tube axis. The simplest way of specifying the structure of an

individual tube is in terms of a vector, which is labeled as C, joining two equivalent points on the

original graphene lattice. The cylinder is produced by rolling up the sheet such that the two end-

points of the vector are superimposed. Fig 2. Shows the graphene sheet labeled. Each pair of

integers (n, m) represents a possible tube structure in this image. Thus the vector C can be

expressed as

C = na1 + ma2

Where a1 and a2 are the unit cell base vectors of the graphene sheet. It can be seen that m = 0 for

all zig-zag tubes, while n = m for all armchair tubes. All other tubes are 'chiral'. In the case of the

two 'archetypal' nano-tubes which can be capped by one half of a C60 molecule, the zig-zag tube

is represented by the integers (9, 0) while the armchair tube is denoted by (5, 5). The chiral

angle, ¥è, is given by

¥è = sin-1{31/2m/2(n2+nm+m2)1/2}

If nano-tubes are considered as a 'one-dimensional crystal', we can define a translational unit cell

along the tube axis. Unit cells for 'armchair' and 'zigzag' nano-tubes are shown in Fig. 3. For the

armchair tube, the width of the cell is equal to the magnitude of a, while for the zig-zag tube the

width of the cell is 31/2a. Larger diameter armchair and zig-zag nano-tubes have unit cells which

are simply longer versions of these. For chiral nanotubes, the lower symmetry results in larger

unit cells.

PROPERTIES The strongest and most flexible molecular material because of C-C covalent bonding and

seamless hexagonal network architecture.

Young’s modulus of over 1TPa vs. 70GPa for aluminum & 700GPa for C-fiber.

Maximum strain ~ 10% much higher than any material.

Thermal conductivity ~3000 W/mK in axial direction with small values in the radial

direction.

Electrical conductivity six orders higher than copper.

Can be metallic or chirality’s based on 1. The tunable band gap

2.electrical properties can be tailored through application of external magnetic field,

application of mechanical deformation…..

Very high current carrying capacity

Excellent field emitter; higher aspect ratio and a smaller tip radius of curvature which are

ideal for field emission.

TYPES

Single-walled carbon nanotubes (SWNTs)

Single-walled carbon nanotubes (SWNTs) are nanometer-diameter cylinders consisting of a

single graphene sheet wrapped up to form a tube. Since their discovery, there has been intense

activity exploring the electrical properties of these systems and their potential applications in

electronics. These tubes can be either metals or semiconductors, and their electrical properties

can rival, or even exceed, the best metals or semiconductors known. The remarkable electrical

properties of SWNTs stem from the unusual electronic structure of the two-dimensional material,

graphene, from which they are constructed [6, 7]. Graphene - a single atomic layer of graphite -

consists of a 2D honeycomb structure of sp2 bonded carbon atoms. In an SWNT, the momentum

of the electrons moving around the circumference of the tube is quantized, reducing the available

states to slices through the 2-D band structure, this quantization results in tubes that are either

one-dimensional metals or semiconductors, depending on how the allowed momentum states

compare to the preferred directions for conduction.

SINGLE WALL NANO-TUBE

Multiwalled nanotubes (MWNT)

Carbon multiwall nanotubes were first synthesized in 1983 by scientists at Hyperion Catalysis

International. Carbon multiwall nanotubes (CMWNT) have been in commercial use as a

conductive additive for plastics only since the early 1990’s. These nano-tubes are approximately

10 nanometers in diameter and 10 microns long. They are made by a continuous, catalyzed, high

temperature gas phase reaction of low molecular weight hydrocarbons. Current production using

this process is multiple tons, with the capability to readily expand to meet demand. Carbon

nanotubes have proven to be an excellent additive to impart electrical conductivity in plastics.

Their high aspect ratio (length divided by diameter) of 1000 means that a very low loading is

needed to form a percolating mixture in a polymer compared to materials with lower aspect

ratios, such as carbon black, chopped carbon fiber, or stainless steel fiber. Because of their small

size and low loading, nanotubes have less of an effect on part surface quality. Carbon nano-tube-

filled plastics are being used in nylon 12 automotive fuel. Because moving fuel can build up a

static charge, the fuel line needs to be conductive enough to bleed off the charge. The low

loading of CMWNT preserves more of the tensile elongation of the resin and reduces the chance

of a fuel line rupture in a low temperature accident.

Carbon multiwall nanotubes are a relatively new additive for plastics. They are being

successfully used commercially as a conductive additive and may be a new, non-halogenated

flame retardant that is effective in low loadings.

FULLERITES

A new super hard material composed of polymerized single wall carbon nanotubes (P-SWNT)

has been synthesized which exhibits a bulk modulus exceeding or comparable with diamond.

Polymerized SWNT (P-SWNT) via sp3 inter-tube bonding are expected to have highly

anisotropic mechanical and electrical (metallic and semiconductor) properties while exhibiting

super hard material characteristics. Polymerization of nanotubes is possible by analogy with ultra

hard fullerite, since the nanotube is a graphite sheet curved into a cylinder, while fullerene is that

curved to sphere.

Carbon nano-tube synthesis 1. Arc-discharge

Arc-discharge is a method which was usually used in synthesis of carbon nanotubes in the early

stage. Two graphite rods are used as the cathode and anode, between which arcing occurs when

DC voltage power is supplied. Large quantities of electrons from the arc-discharge move to the

anode and collide into the anodic rod. Carbon clusters from the anodic graphite rod caused by the

collision are cooled to low temperature and condensed on the surface of the cathodic graphite

rod. The graphite deposits condensed on the cathode contain both carbon nanotubes,

nanoparticles, and clusters. The graphite clusters synthesized in these initial experiments

contained a very small amount of carbon nanotubes, but modifications to the procedure later

have enabled greatly improved the yield.

2. Laser vaporization

A laser is used to vaporize a graphite target in an oven at 1200 oC. Then helium or argon gas is

filled to keep the pressure in the oven at 500 Torr. Carbon clusters from the graphite target are

cooled, adsorbed, and condensed on the Cu collector at a low temperature. The condensates

obtained this way are mixed with carbon nanotubes and nanoparticles. MWNT would be

synthesized in the case of a pure graphite, but uniform SWNT could be synthesized if a graphite

of a mixture of Co, Ni, Fe, Y were used instead of a pure graphite. SWNTs synthesized this way

exist as 'ropes'.

3. Thermal Chemical Vapor Deposition

Thermal CVD has advantages of variety in products and hydrocarbon sources, adequacy for

synthesis of high quality materials, and controllability of microscopic structures. The synthesis

method of carbon nanotubes using thermal chemical vapor deposition is as follows. Fe, Ni, Co,

or alloy of the tree catalytic metals is initially deposited on a substrate. After the substrate is

etched in diluted HF solution with distilled water, the specimen is placed in a quartz boat. The

boat is positioned in a CVD reaction furnace, and nano-size fine catalytic metal particles are

formed after an additional etching of the catalytic metal film using NH3 gas at a temperature in

750 to 1050oC. As carbon nanotubes are grown on these fine catalytic metal particles in CVD

synthesis, forming these fine catalytic metal particles is the most important process.

4. Vapor Phase Growth

Most synthesis methods of carbon nanotubes are carried out by deposition of catalytic metals on

a substrate using conventional gas, such as C2H2, CH4, C2H4, and C2H6. However, vapor phase

growth is a synthesis method of carbon nanotubes, directly supplying reaction gas and catalytic

metal in the chamber without a substrate. It has been suggested as a good method for mass

production. A mass flow controller is placed in one corner and a boat for catalytic metal powder

in the chamber. The chamber is composed of two stage furnaces. Relatively low temperature is

kept in the first furnace while higher temperature is maintained in the hot second furnace where

the synthesis occurs. When fine catalytic particles vaporized from metal powder in the low

temperature area reach the second furnace, decomposed carbons in the hot second area are

adsorbed, diffused to catalytic metal particles, and synthesized as carbon nano-tubes.

5. Electrolysis

MWNTs are synthesized using this method which involves the electrolysis of molten lithium

chloride using a graphite cell in which the anode was a graphite crucible. The temperature of the

graphite crucible is approximately 600 oC in the Ar atmosphere. MWNTs with 2-10 nm in

diameter and 0.5 § or more in length are synthesized when DC power at less than 3-20A and 20

V is applied. Amorphous carbons and encapsulated CNTs are synthesized as by-products.

6. Flame synthesis

In this method, combustion heat is a heating source of carbon nanotubes produced from

combustion of CH4 in the small amount of oxygen atmosphere. MWNTs and SWNTs are

synthesized by flowing hydrocarbon source like C2H2 and catalytic precursors in the diffusion

flame atmosphere. As the temperature range of 600-1300 oC is not uniform in the Flame

atmosphere, the tubes contain a great amount of amorphous carbons and show poor crystallinity.

It facilitates mass production and is promising in the application to electrolytic materials.

APPLICATIONS AND USESSince discovering them more than a decade ago, scientists have been exploring possible uses for

carbon nanotubes, which exhibit electrical conductivity as high as copper, thermal conductivity

as high as diamond, and as much as 100 times the strength of steel at one-sixth the weight. In

order to capitalize on these properties, researchers and engineers need a set of tools -- in this

case, chemical processes like pyrolytic fluorination -- that will allow them to cut, sort, dissolve

and otherwise manipulate nanotubes.

Molecular and Nanotubes Memories

Nanotubes hold promise for non-volatile memory; with a commercial prototype nanotubes-

based RAM predicted in 1-2 years, and terabit capacity memories ultimately possible. Similar

promises have been made of molecular memory from several companies, with one projecting a

low-cost memory based on molecule-sized cylinders by end 2004 that will have capacities

appropriate for the flash memory market. These approaches offer non-volatile memory and if the

predicted capacities of up to 1Tb can be achieved at appropriate cost then hard drives may no

longer be necessary in PCs.

Laser applications heat up for carbon nano-tubes:-Carbon nanotubes---tiny cylinders made

of carbon atoms---conduct heat hundreds of times better than today's detector coating materials.

Nanotubes are also resistant to laser damage and, because of their texture and crystal properties,

absorb light efficiently.

Nanoelectronics:-Nanotubes are either conducting or semi-conducting depending upon their

structure (or their 'twist') so they could be very useful in electronic circuitry. Nanotubes

Ropes/Fibers: These have great potential if the SWNT's can be made slightly longer they have

the potential to become the next generation of carbon fibers. Carbon nanotubes additionally can

also be used to produce nanowires of other chemicals, such as gold or zinc oxide. These

nanowires in turn can be used to cast nanotubes of other chemicals, such as gallium nitride.

These can have very different properties from CNTs - for example, gallium nitride nanotubes are

hydrophilic, while CNTs are hydrophobic, giving them possible uses in organic chemistry that

CNTs could not be used for.

Display Technologies:- Nanomaterials will help extend the range of ways in which we display

information. Several groups are promising consumer flat screens based on nanotubes by the end

of 2003 or shortly after (Carbon nanotubes are excellent field emitters). E-paper is another much

heralded application and nanoparticles figure in several approaches being investigated, some of

which promise limited commercialization in the next year or two. Soft lithography is another

technology being applied in this area.

Light Emitting Polymer Technology

Light Emitting Polymer technology is leading to a new class of flat panel displays.

Researchers have discovered that Light Emitting Diodes (LEDs) could be made from polymers

as well as from traditional semiconductors. It was found that the polymer poly p-

phenylenevinylene (PPV) emitted yellow-green light when sandwiched between a pair of

electrodes. Initially this proved to be of little practical value as it produced an efficiency of less

than 0.01%. However, by changing the chemical composition of the polymer and the structure of

the device, an efficiency of 5% was achieved, bringing it well into the range of conventional

LEDs.

Some Amazing facts and Applications

• Carbon Nanotubes possess many unique and remarkable properties (chemical, physical,

and mechanical), which make them desirable for many applications. The slender

proportions of carbon nanotubes hide a staggering strength: it is estimated that they are

100 times stronger than steel at only one sixth of the weight - almost certainly the

strongest fibres that will ever be made out of anything - strong enough even to build an

elevator to space. In addition they conduct electricity better than copper and transmit heat

better than diamond.

• Enhancements in miniaturization, speed and power consumption, size reduction of

information processing devices, memory storage devices and flat displays for

visualization are currently being developed

• The most immediate application for nanotubes is in making strong, lightweight materials.

It will be possible to build a car that is lighter than its human driver, yet strong enough to

survive a collision with a tank

• Aircraft built with stronger and lighter materials will have longer life spans and will fly at

higher temperatures, faster and more efficiently.

• Nanotubes are being explored as receptacles - storage tanks - for hydrogen molecules to

be used in the fuel cell that could power automobiles of the future. Hydrogen does not

produce pollution or greenhouse emissions when burned and is considered to be the clean

energy of the future.

FUTURE APPLICATIONSApart from the above mentioned fields the carbon nano-tubes can also be used in other

various fields in future such as...

• Nanotube actuator

• Molecular Quantum wires

• Hydrogen Storage

• Noble radioactive gas storage

• Solar storage

• Waste recycling

• Electromagnetic shielding

• Dialysis Filters

• Thermal protection

• Nanotube reinforced composites

• Reinforcement of armour and other materials

• Reinforcement of polymer

• Avionics

• Collision-protection materials

• Fly wheels Artificial muscles

• Super capacitors

• Batteries

• Field emission flat panel displays

• Field Effect transistors and Single electron

CONCLUSION As carbon nanotubes have various physical properties and are an important material which can

be broadly used for various advanced industries such as electronic and information technology in

the 21st century, they are globally in the limelight. We have seen the different aspects of carbon nano-tubes ranging from their structure, types,

manufacturing techniques & the different applications. The range of applications the carbon

nano-tubes are used in, itself depicts their effect in revolutionizing the field of science and

technology. They have paved the way for miniaturizing the present technological systems

providing greater strength, flexibility, agility, conductivity [electric & thermal] in a wide range

of applications in different fields. They hold the future of science enhancing the lives of people

in every possible manner.

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