CARBON NANOTUBES

Abstract:

CARBON NANOTUBES are the most successful materials that  are now attracting a broad range of scientists and industries due to their fascinating physical and chemical properties. In this review, we enlighten you about this material. We are introducing here, the structure, synthesis and the most important applications of carbon nanotubes in different fields. The session will feature technology that exploits novel electronic, electro-mechanical, transistors, electrical circuits, optical and structural properties of a carbon nanotubes for the solution of engineering problems.

KEYWORDS:

buckyballs, fullerence, chiral nanotube, plasma, electronic, electro-mechanical, space elevator, ultra capacitors, polythene, computer circuits.

INTRODUCTION

Carbon nanotubes, long thin cylinders of carbon, were discovered in 1991 by Iijima. Carbon nanotubes (CNTs) are allotropes of carbon which  are members of the fullerene structural family, which also includes the spherical buckyballs. These are large macromolecules which are unique for there size, shape and remarkable physical properties.

The nature of the bonding of a nanotube is described by applied quantum chemistry, specifically, orbital hybridization. The chemical bonding of nanotubes is composed entirely of sp² bonds, similar to those of graphite. This bonding structure, which is stronger than the sp³ bonds found in diamond, provides the molecules with their unique strength. Nanotubes  naturally align themselves into “ropes” held together by Van der Waals forces. Under high pressure, nanotubes can merge together, trading some sp² bonds for sp³ bonds, giving the possibility of producing strong, unlimited-length wires through high-pressure nanotube linking.

HISTORY:                 The discovery that carbon could form stable, ordered structures other than graphite and diamond stimulated researchers worldwide to search for other new forms of carbon. The search was given new impetus when it was shown in 1990 that carbon-60(buckminister fullerence) could be produced in a simple arc-evaporation apparatus readily available in all laboratories. It was using such a evaporator that the Japanese scientist Sumio Iijima discovered fullerence-related carbon nanotubes in 1991. The tubes contained  atleast two layers, often many more, and ranged in outer diameter from about 3nm to 30nm.

In 1993, a new class of carbon nanotubes was discovered, with just a single layer. These single-walled nanotubes are generally narrower than the multiwalled tubes, with diameters typically in the range  1-2  nm, and tend to be curved rather than straight. It was soon established  that these new fibers had a range of exceptional properties, and this sparked  off an explosion of research  in to carbon nanotubes. It is important to note, however, that nano scale tubes of carbon produced catalytically, had been known for many years before Iijima’s discovery. The main reason why these early tubes did not excite wide interest is that they were structurally rarther imperfect, so

did not have particularly interesting properties. Recent reseach has focused on improving the quality of catalytically-produced nanotubes.

STRUCTURE:           The bonding in carbon nanotubes is sp², with each atom joined to three neighbours, as in graphite. The tubes can therefore be considered as rolled-up graphene sheets (graphene is an individual graphite layer). There are three distinct ways in which a graphene sheet can be rolled into a tube, as shown below.

The first two of these, known as “armchair” (fig.(b)) and “zig-zag” (fig.(c)) have a high degree of symmetry. The terms “armchair” and “zig-zag” refer to the arrangement of hexagons around the circumference. The third class of tube, which in practice is the most common, is known as chiral, meaning that it can exist in two mirror-related forms. An example of a chiral  nanotube is as shown in  fig.(d).

TYPES OF CARBON NANOTUBES

a) SINGLE-WALLED CNT’s

Most single-walled nanotubes(SWNT) have a diameter close to 1nm, with a tube length that can be many thousands of times longer. SWNTs are very important carbon nanotube because they exhibit important electric properties that are not shared by the multi-walled carbon nanotubes(MWNT) varients.

SWNTs can be excellent conductors and the most building block of  SWNT system is the electic wires. One useful application of SWNTs  is in the development of the first intramolecular field effect transistors(FETs).

b) MULTI-WALLED CNT’s:         Multi-walled nanotubes (MWNT) consist of multiple rolled in on themselves to form a tube shape. 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 up newspaper. The interlayer distance in multi-walled nanotubes is close to the distance between graphene layers in  graphite, approximately 0.33 nm.

c) FULLERITE:     Fullerites are the solid-state manifestation of fullerences and related compounds and materials. Being highly incompressible nanotube forms, polymerized single-walled nanotubes (P-SWNT) are a class of fullerites and are comparable to diamond in terms of hardness.

SYNTHESIS

a)  ARC DISCHARGE METHOD

Nanotubes were observed in 1991 in the carbon soot of graphite electrodes during an arc discharge, by using a current of 100 amperes, that was intended to produce fullerenes. However the first macroscopic production of carbon nanotubes was made in 1992 by two researchers at NEC’s Fundamental Research Laboratory at France. The method used was the same as in 1991. During this process, the carbon contained in the negative electrode sublimates because of the high temperatures caused by the discharge. Because nanotubes were initially discovered using this technique, it has been the most widely used method of nanotube synthesis.

The yield for this method is up to 30 percent by weight and it produces both single- and multi-walled nanotubes with lengths of up to 50 micrometres.

b)  LASER ABLATION PROCESS

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. The 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.

It was invented by Richard Smalley and co-workers at Rice University, who at the time of the discovery of carbon nanotubes, were blasting metals with the laser to produce various metal molecules. When they heard of the discovery they substituted the metals with graphite to create multi-walled carbon nanotubes. Later that year the team used a composite of graphite and metal catalyst particles to synthesise single-walled carbon nanotubes.

This method has a yield of around 70% and produces primarily single-walled carbon nanotubes with a controllable diameter determined by the reaction temperature.

c)   CHEMICAL VAPOUR DEPOSITION

The catalytic vapor phase deposition of carbon was first reported in 1959, but it was not until 1993 that carbon    nanotubes could be formed by this process. In 2007, researchers at the University of Cincinnati (UC) developed a    process to grow 18 mm long aligned carbon nanotube arrays.

uring CVD, a substrate is prepared with a layer of metal catalyst particles, 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. This can  be controlled by patterned  deposition of the metal, annealing, or by plasma etching of a metal layer. 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, hydrogen, etc.) and a carbon-containing gas (such as acetylene, ethylene, ethanol,  methane, etc.). 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  as shown in fig.(e).

CVD is a common method for the commercial production of carbon nanotubes.. For this purpose, the metal nanoparticles will be carefully mixed with a catalyst support (e.g., MgO, Al2O3, etc) to increase the specific surface area for higher yield of the catalytic reaction of the carbon feedstock with the metal particles. One issue in this synthesis route is the removal of the catalyst support via an acid treatment, which sometimes could destroy the original structure of the carbon nanotubes. However, alternative catalyst supports that are soluble in water have been shown to be effective for nanotube growth. If a plasma is generated by the application of a strong electric field during the growth process (plasma enhanced chemical vapor deposition), then the nanotube growth will follow the direction of the electric field. By properly adjusting the geometry of the reactor it is possible to synthesize vertically aligned carbon nanotubes.

In 2007, a team from Meijo University has shown a high-efficiency CVD technique for growing carbon nanotubes from camphor. A team of researchers at Rice University, until recently led by the late Dr. Richard Smalley, has concentrated upon finding methods to produce large, pure amounts of particular types of nanotubes.

CVD growth of multi-walled nanotubes is used by several companies to produce materials on the tonne scale, including NanoLab Bayer, Arkema, Nanocyl, Nanothinx, Hyperion Catalysis, Mitsui, and Showa Denko.

PROPERTIES

The most important properties of CNTs are

a) Strength:        CNTs are the strongest and stiffest materials on earth, in terms of tensile strength and elastic modulus respectively. This strength results from the covalent sp² bonds formed between individual carbon atoms.

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.

b) Thermal:        All nanotubes are expected to be very good thermal conductors along the tube, exhibiting a property known as ballistic conduction, but good insulators laterally to the tube axis. The temperature stability of carbon nanotubes is established to be up to 2800 degrees Celsius in vacuum and about 750 degrees Celsius in air.

c) Chemical Reactivity:       The chemical reactivity of a CNT is, compared with a graphitesheet, enhanced as a direct result of the curvature of the CNT surface.

d) Electrical Conductivity:       Depending on their chiral vector, carbon nanotubes with a small diameter are either semi-conducting or metallic.

APPLICATIONS

a)  STRUCTURAL

Ø  Concrete:          In concrete, they increase the tensile strength, and halt crack propagation.

Ø  Polyethylene:      Researchers have found that adding them to polyethylene increases the polymer’s elastic modulus by 30%.

Ø  Sports equipment:       CNTs are used in different sports equipments such as tennis rackets, bike parts (racing bikes), golf balls etc.

SPACE ELEVATORS:         This will be possible only if tensile strengths of more than about 70 GPa can be achieved. Monoatomic oxygen in the Earth’s upper atmosphere would erode carbon nanotubes at some altitudes, so a space elevator constructed of nanotubes would need to be protected (by some kind of coating). Carbon nanotubes in other applications would generally not need such surface protection.

Others:        Bridges, clothes, combat jackets, ultrahigh-speed flywheels etc.

b) ELECTROMAGNETIC

• Buckypaper:        It is a thin sheet made from nanotubes that are 250 times stronger than steel and 10 times lighter that could be used as a heat sink for chipboards, a backlight for LCD screens or as a faraday cage to protect electrical devices/aero planes.
• Chemical nanowires:     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.

• Computer circuits:        A nanotube formed by joining nanotubes of two different diameters end to end can act as a diode, suggesting the possibility of constructing electronic computer circuits entirely out of nanotubes. Because of their good thermal properties, CNTs can also be used to dissipate heat from tiny computer chips.

• Conductive films:     CNTs are also introduced in developing transparent, electrically conductive films to replace indium tin oxide(ITO).CNT films are sustaintially more mechanically robust then ITO films ,making them ideal for more reliability touch screens and flexible displays. Printable water based inks of carbon nanotubes are desired to enable the production of these films to replace the ITO. Nanotube films show promise for use in displays for cell phones, computers,  PDAs, and  ATMs.
• Electric motor brushes:            Conductive carbon nanotubes have been used for several years in brushes for commercial electric motors.. The nanotubes improve electrical and thermal conductivity because they stretch through the plastic matrix of the brush. This permits the carbon filler to be reduced from 30% down to 3.6%, so that more matrix is present in the brush. Nanotube composite motor brushes are better-lubricated (from the matrix), cooler-running (both from better lubrication and superior thermal conductivity), less brittle (more matrix, and fiber reinforcement), stronger and more accurately moldable (more matrix). Since brushes are a critical failure point in electric motors, and also don’t need much material, they became economical before almost any other application.
• Light bulb filament:        Alternative to tungsten filaments in incandescent lamps.
• Solar cells:         GE’s carbon nanotube diode has a photovoltaic effect. Nanotubes can replace ITO in some solar cells to act as a transparent conductive film in solar cells to allow light to pass to the active layers and generate photocurrent.
• Superconductor:        Nanotubes have been shown to be superconducting at low temperatures.
• Ultracapacitors:         Nanotubes, when bound to  plates of capacitors increase the surface area and thus increase energy storage ability.
• Displays:       One use for nanotubes that has already been developed is as extremely fine electron guns, which could be used as miniature cathode ray tubes in thin high-brightness low-energy low-weight displays. This type of display would consist of a group of many tiny CRTs, each providing the electrons to hit the phosphor of one pixel, instead of having one giant CRT whose electrons are aimed using electric and magnetic fields. These displays are known as field emission displays (FEDs).
• Others:         Artificial muscles, magnets, optical ignition etc.

c) CHEMICAL

d) MECHANICAL

e) CARBON NANOTUBE INTERCONNECTS

Metallic CNTs have aroused a lot of research interest in their applicability as Very-large-scale integration (VLSI) interconnects of the future because of their desirable properties of high thermal stability, high thermal conductivity and large current carrying capacity. An isolated CNT can carry current densities in excess of 1000 MA/sq-cm without any signs of damage even at an elevated temperature of 250 degrees C, thereby eliminating electromigration reliability concerns that plague Cu interconnects. Recent modeling work comparing the performance, power dissipation and thermal/reliability aspects of CNT interconnect to scaled copper interconnects have shown that CNT bundle interconnects can potentially offer more advantages over copper.

f) TRANSISTORS

Smaller silicon based integrated circuits result in both a higher speed and device density. As a result, downscaling of these devices has been very important since their first implementation. However, at the moment it is generally accepted that silicon devices will reach fundamental scaling  limits within a decade or so. This limit is caused by the minimum wavelength of light used in lithographic techniques used for integrated circuit production nowadays. For this reason a quest for alternative, integrated circuits with smaller dimensions has started. A major step in downscaling would be the application of single molecules in electronic devices. Carbon nanotubes have already shown promising results in single molecular transistors. For successful implementation of molecular transistors in large and complex logic systems, they must show signal amplification. Signal amplification makes it possible to reference separate signals along a chain of logical operations. In addition, noise caused by thermal fluctuations and environmental disturbances is also reduced. Three terminal nanotransistors, in special, field-effect-transistors show amplifying behavior and have recently been investigated for this reason.

g) OTHER APPLICATIONS

CONCLUSION

Rise in demand and production, and ease of accessibility  of carbon nanotubes would lead to the extensive use of carbon nanotubes in a wide variety of applications. The use of nanotechnology for human will become common need in 21st century. As world is suffering from serious pollution problems, hydrogen will becoming need of 21st century & carbon nanotubes provide better solution for hydrogen storage.

Nanotubes  market, which was growing at a moderate rate till 2006-2007, is expected to rise at a skyrocketing pace  in the coming years. Hence we can conclude  that  most of the demands of human, in this and fore coming  generation  will be fulfilled by carbon nanotubes.

REFERENCES

• www.nanotechnology.com
• www.multidisciplinair projects.com
• www.nanotechnology .com
• www.cnt.com