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Note: Theoretical predictions say that carbon nanotubes are 100 times stronger than steel but only one sixth of its weight. Therefore, they are ideal in lightweight construction, for instance in the automotive and aviation industries. Carbon nanotubes are already used in some consumer products, such as tennis rackets to add strength (without compromising weight). Note: Because of their mechanical properties, carbon nanotubes are very interesting as fillers in polymeric and inorganic composites.
Electrical and thermal properties The electrical properties of a material are based on the movement of electrons and the spaces or ‘holes’ they leave behind. These properties are based on the chemical and physical structure of the material. In nanoscale materials, some interesting electrical properties appear. Carbon nanotubes are the best example of this effect at the nanoscale. If one considers ‘building’ a nanotube by rolling up a graphene sheet, the resulting nanotube can be conductive (and, indeed, very conductive!) or semiconductive with relatively large band gaps. The electrical properties of the nanotube depend on the way it was ‘rolled up’ (technically known as chirality). If it is rolled up so that its hexagons line up straight along the tube’s axis, the nanotube acts as a metal (conductive). If it is rolled up on the diagonal, so the hexagons spiral along the axis, it acts as a semiconductor.
Why is this so? Graphene (that is, one layer of graphite) is not an insulator, but also neither a metal nor a semiconductor; it has electrical properties somewhere in between and is called a semi-metal. When rolled up, it leads to a structure that is either metallic or semiconducting. On the other hand, diamond has a tetrahedral structure (derived from the fact that carbons are hybridised sp3 rather than sp2 as in graphene) and is an insulator. One interesting property found in single-walled carbon nanotubes (SWCNT) is that electric conductance within them is ballistic (which means that all electrons that go into one end of the conductor come out the other end without scattering, regardless of how far they need to travel).
Nanotubes can be superconductors near room temperature, meaning ballistic conductors that also exhibit a resistance of zero. A superconductor can transport an enormous amount of current flow at tiny voltages. At present, known superconductors work at very low temperatures. This field of research is very important since if a material were superconductive at room temperature, it would carry current with no resistance, with no energy lost as heat. This could lead to faster, lower-power electronics and the ability to carry electricity long distances with 100 % efficiency. In terms of thermal properties, carbon nanotubes dissipate heat better than any other known material and are excellent thermal conductors.
Chemical reactivity Carbon nanotubes are very stable: they can withstand the attack of numerous chemicals and resist exposure to a large temperature range. However, their chemical structure can be changed by the addition of specific ligands with functional groups that allow interaction with different chemicals. This allows them to be used in sensors.
Synthesis of CNT • There are two main methods for preparation of CNT • Sublimation of graphite with subsequent desublimation • This method involves condensation of carbon atoms generated from evaporation of solid carbon sources of graphite. The sublimation of the solid can be done using electric arc or laser ablation where the temperature reaches to 2500 -3500°C.
Fig. 1. Schematics for CNT formation by sublimationof graphite with subsequent desublimation.
The electric arc discharge method is one of the efficient techniques for synthesis of CNT. Typically, about 60 to70 wt% of the arc-synthesized soot is CNT. The rest of the soot comprises of fullerenes, amorphous carbon and catalyst nanoparticles. In electric arc discharge production of CNT two graphite rods are used and a current is passed continuously between the electrodes. The anode is drilled and filled with catalysts. The metal oxides (Ni, Co, Fe) are used as catalyst. In some cases the catalyst/graphite composite is used as electrode. The synthesis is performed in cooled chamber in presence of helium, argon or methane environment. During the arcing, the catalyst/graphite anode is evaporated and consumed with simultaneous carbon deposition around the cathode. The quality of CNT samples depends upon arc stability, current density and cooling of cathode. In laser ablation method the graphite target is subjected to laser and sublimated carbon is recollected. Inert gas atmosphere is maintained within the chamber.
Decomposition of carbon containing compoundsThe most used method to prepare CNT is pyrolysis of hydrocarbon gases or vapors such as propane, butane, hexane, benzene, toluene etc. The method is also known as chemical vapor deposition (CVD) process. • Chemical Vapor Deposition ( CVD )By chemical vapor deposition CNTs can be produced in large quantities. The process temperature can vary from 500 – 1300°C. The hydrocarbon precursors include CH4, C2H2, C6H6, alcohols etc.
