What Are Carbon Nanotubes (cnts)

CNTs are long cylinders of covalently bonded carbon atoms which possess extraordinary electronic and mechanical properties. There are two basic types of CNTs: single-wall carbon nanotubes (SWCNTs) which are the fundamental cylindrical structure and multi-wall carbon nanotubes (MWCNTs) which are made of coaxial cylinders, having interlayer spacing close to that of the interlayer distance in graphite (0.34 nm). These cylindrical structures are only few nanometre in diameter, but the cylinder can be tens of microns long, with most end capped with half of a fullerene molecule. CNTs can be prepared using three methods which includes d.c arc discharge, laser ablation and chemical vapor deposition (CVD).

Types of Carbon Nanotube

SWCNTs consists of just one layer of carbon. It can have greater tendency to align into ordered bundles.MWCNTs consists of two or more layers of carbon. It tends to form unordered clumps.

Properties of Carbon Nanotubes

Carbon nanotubes are endowed with exceptionally high material properties, very close to their theoretical limits, such as electrical and thermal conductivity, strength, stiffness, toughness and low density.

Mechanical Properties

The strength of C-C bond gives a large interest in mechanical properties of nanotubes. Theoretically, these should be stiffer than any other known substance. Young's modulus of the single walled carbon nanotubes (SWCNTs) can be as high as 2.8-3.6 TPa and 1.7-2.4 TPa for multiwalled carbon nanotubes (MWCNTs) which is approximately 10 times higher than steel, the strongest metallic alloy known. Electrical PropertiesThe nanometer dimensions of CNTs, together with the unique electronic structure of a graphene sheet, make the electronic properties of these one-dimensional (1D) structures extraordinary. The one dimensional structure of CNTs helps them in making a good electric conductor. Some nanotubes have conductivities higher than that of copper, while others behave more like silicon.

Theoretically, metallic nanotubes having electrical conductivity of 105 to 106 S/m can carry an electric current density of 4 × 109 A/cm2 which is more than 1000 times greater than copper metal and hence can be used as fine electron gun for low weight displays.

Synthesis

D.c Arc DischargeThe carbon arc discharge method, initially used for producing C60 fullerenes, is the most common and perhaps easiest way to produce CNTs. But this technique produces graphitic impurities such as carbon soot containing amorphous carbon, anions and fullerens.

Laser Ablation

Laser ablation uses an intense laser pulse to vaporize a carbon target, which also contains small amount of metals such as nickel and cobalt and is placed in a tube furnace at 1200º C. As the target is ablated, inert gas is passed through the chamber carrying the grown nanotubes on a cold finger for collection. This method mainly produces SWCNT in the form of ropes.

Chemical Vapor Desposition

In this process a mixture of hydrocarbon, metal catalyst along with inert gas is introduced into the reaction chamber. This technique offers more control over the length and structure of the produced nanotubes compared to arc and laser methods. This process can also be scaled up to produce industrial quantities of CNTs.

Applications of Carbon Nanotube

With their excellent range of properties, CNTs have opened up a new age of advanced multifunctional materials. Incorporation of CNTs in polymer matrices provides materials that could be used for many high performance engineering applications. Currently, the most widespread use of CNT nanocomposites is in electronics. These nanocomposites could be used to shield electromagnetic interference and as electrostatic-discharge components. The microwave-absorbing capability of nanotubes could be exploited to heat temporary housing structures and may have applications in space exploration. Thin layers of nanotubes on plastics might also be used in transparent conducting composites.

High mechanical strength of these nanocomposites could be utilized to make some high-end sporting goods such as tennis rackets, baseball bat etc, and thus delivering superior performance. In short, the biggest market for CNT nanocomposites will undoubtedly be for high-value applications that can absorb the added costs, which includes commercial sectors such as electronics especially aerospace (which requires lightweight, high-strength, high-temperature-resistant composites) and energy (for example, in nanotube-reinforced rubber seals for large oil recovery platforms).

Once the cost of nanotubes becomes comparable to that of carbon fibre (or even to that of the much cheaper reinforcing agent, carbon black), commodities such as nanotube-filled rubber tyres could become a reality.