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Carbon Nanotubes Carbon nanotubes are fullerene -related structures. They consist of graphene cylinders closed at either end with caps containing pentagonal rings. Carbon nanotubes are considered for many potential applications. Their large length (extending up to several microns) and small diameter (few nanometers) enable them to be used in a variety of applications. Carbon nanotubes are of 2 types
• Single walled carbon nanotube- It is a long wrapped graphene sheet. It consists of 2 parts – sidewall of the tube and end cap of the tube. • Multi walled carbon nanotube- It is a collection of single walled carbon nanotubes with different diameters. Some of the properties are: • Chemical reactivity- The chemical reactivity of CNT (carbon nanotubes) is enhanced due to the curvature of its surface. • Electrical conductivity- CNT are either semi-conducting or metallic. This difference is attributed to its molecular structure. • Mechanical strength- CNT are very flexible because of their long length. They have a very large young's modulus. Because of the above-mentioned properties carbon nanotubes are used in wide variety of applications. They are used for energy storage, molecular electronics, nanoprobes and sensors. Synthesis of Carbon Nanotubes by Laser Vapourization of Graphite In this process a continuous or pulsed laser beam is used to vaporize a graphite target. The vaporization is carried in an oven whose temperature is maintained at 1200 o C. The oven is filled with helium or argon gas to maintain a pressure of about 500 torr. A very hot vapor called plume is formed. This vapor expands and cools rapidly. Small carbon molecules in the vapor are condensed to form large aggregates. The catalyst also condenses and prevents carbon molecules from forming closed cage structures.
From this aggregates tubular molecules grow into single walled carbon nanotube. The nanotube stops growing when the carbon molecule becomes too large or the temperature conditions have cooled sufficiently until no carbon molecules can diffuse over the catalyst. Wan der Waal's forces of attraction hold the single walled carbon nanotubes together. The nanotubes produced by this technique are 5-20 microns in length and 1-2 mm in diameter. The disadvantage of using this technique is that it requires expensive lasers and power requirement.
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