Carbon nanotubes are tubular carbon molecules provided with very particular properties. Their structure is similar to fullerene; but while the fullerene's molecules form a spherical shape, nanotubes are cylindrical structures with the ends covered by half a fullerene molecule. Nanotube diameter is of the order of a few nanometers, while their length can be of the order of several millimeters.
The Physical Structure of Carbon Nanotubes
The physical properties of nanotubes make them potentially useful in nanometric scale electronic and mechanical applications, since they show unusual strength, unique electrical properties and extremely high thermal conductivity. The chemical bonding between carbon atoms inside nanotubes is always of sp2 type, the same that we find in graphite, and provides them their unique strength. Moreover, they align themselves into ropes held together by the Van der Waals force and can merge together under high pressure, trading some sp2 bonds to sp3 and producing very strong wires of nanometric lateral dimension.
The Two Types of Carbon Nanotubes: Single-Walled (SWNT) and Multi-Walled (MWNT)
Several types of nanotubes exist; but they can be divided in two main categories: single-walled (SWNT) and multi-walled (MWNT). The form of nanotubes is identified by a sequence of two numbers, the first one of which represents the number of carbon atoms around the tube, while the second identifies an offset of where the nanotube wraps around to.
The Properties of Carbon Nanotubes and Potential Industry Applications
The electrical properties of nanotubes are very different: they can be excellent conductors (with conductivity 1000 times the one of copper) as well as insulators or semiconductors, depending of their structure. Thermal conductivity is expected to be very high in the axial direction, but very low in the lateral direction. The particular properties of carbon nanotubes make them of great interest for potential use in biotechnology since they could be opened and filled with other molecules.
Fabrication of Carbon Nanotubes
The fabrication of nanotubes is not a difficult task, since they can be found also in common environments such as the flame of a candle. But is very difficult to control their size, orientation and structure, in order to be able to use them for technological tasks. They were synthesized for the first time in 1991 by Sumio Iijima, a researcher working in the NEC laboratories in Tsukuba, Japan, while he was studying the material deposited on the cathode during the arc-evaporation synthesis of fullerenes. He found that the central core of the cathodic deposit contained a variety of closed graphitic structures including nanoparticles and nanotubes.
Producing Bulk Quantities of Carbon Nanotubes by Varying the Arc-Evaporation Conditions
A short time later, Thomas Ebbesen and Pulickel Ajayan, from Iijima's lab, showed how nanotubes could be produced in bulk quantities by varying the arc-evaporation conditions. This paved the way to an explosion of research into the physical and chemical properties of carbon nanotubes in laboratories all over the world.
Synthesis of Carbon Nanotubes
The synthesis of nanotubes can be performed by different kind of techniques: among them we can cite arc evaporation (the one used by Iijima), sputtering, Chemical Vapor Deposition (CVD) and Plasma Enhanced Chemical Vapor Deposition (PECVD).
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