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    A Carbon Nanotube is a tube-shaped material, made of carbon, having a diameter measuring on the nanometer scale. A nanometer is one-billionth of a meter, or about one ten-thousandth of the thickness of a human hair. The graphite layer appears somewhat like a rolled-up chicken wire with a continuous unbroken hexagonal mesh and carbon molecules at the apexes of the hexagons.

    Carbon Nanotubes have many structures, differing in length, thickness, and in the type of helicity and number of layers. Although they are formed from essentially the same graphite sheet, their electrical characteristics differ depending on these variations, acting either as metals or as semiconductors.

   As a group, Carbon Nanotubes typically have diameters ranging from <1 nm up to 50 nm. Their lengths are typically several microns, but recent advancements have made the nanotubes much longer, and measured in centimeters.

 Carbon Nanotubes can be categorized by their structures:

·                          Single-wall Nanotubes (SWNT)

·                          Multi-wall Nanotubes (MWNT)

·                          Double-wall Nanotubes (DWNT)

 Single-wall Nanotubes (SWNT)

 Single-wall nanotubes (SWNT) are tubes of graphite that are normally capped at the ends. They have a single cylindrical wall. The structure of a SWNT can be visualized as a layer of graphite, a single atom thick, called graphene, which is rolled into a seamless cylinder.

 Most SWNT typically have a diameter of close to 1 nm. The tube length, however, can be many thousands of times longer.

SWNT are more pliable yet harder to make than MWNT. They can be twisted, flattened, and bent into small circles or around sharp bends without breaking.

 SWNT have unique electronic and mechanical properties which can be used in numerous applications, such as field-emission displays, nanocomposite materials, nanosensors, and logic elements. These materials are on the leading-edge of electronic fabrication, and are expected to play a major role in the next generation of miniaturized electronics.

Multi-wall Nanotubes (MWNT)

 Multi-wall nanotubes can appear either in the form of a coaxial assembly of SWNT similar to a coaxial cable, or as a single sheet of graphite rolled into the shape of a scroll.

 The diameters of MWNT are typically in the range of 5 nm to 50 nm. The interlayer distance in MWNT is close to the distance between graphene layers in graphite.

 MWNT are easier to produce in high volume quantities than SWNT. However, the structure of MWNT is less well understood because of its greater complexity and variety. Regions of structural imperfection may diminish its desirable material properties.

 The challenge in producing SWNT on a large scale as compared to MWNT is reflected in the prices of SWNT, which currently remain higher than MWNT.

 SWNT, however, have a performance of up to ten times better, and are outstanding for very specific applications.

Double-wall Nanotubes (DWNT)

 Double-wall nanotubes (DWNT) are an important sub-segment of MWNT.

 These materials combine similar morphology and other properties of SWNT, while significantly improving their resistance to chemicals. This property is especially important when functionality is required to add new properties to the nanotube.

 Since DWNT are a synthetic blend of both SWNT and MWNT, they exhibit the electrical and thermal stability of the latter and the flexibility of the former.

 Because they are developed for highly specific applications, SWNT that have been functionalized are more susceptible to breakage. Creating any structural imperfections can modify their mechanical and electrical properties.

 However, with DWNT, only the outer wall is modified, thereby preserving the intrinsic properties.

 Also, research has shown that DWNT have better thermal and chemical stability than SWNT. DWNT can be applied to gas sensors and dielectrics, and to technically-demanding applications like field-emission displays, nanocomposite materials, and nanosensors.

 What are the Properties of a Carbon Nanotube?

 The intrinsic mechanical and transport properties of Carbon Nanotubes make them the ultimate carbon fibers. The following tables (Table 1 and Table 2) compare these properties to other engineering materials.

 Overall, Carbon Nanotubes show a unique combination of stiffness, strength, and tenacity compared to other fiber materials which usually lack one or more of these properties. Thermal and electrical conductivity are also very high, and comparable to other conductive materials.

 Table 1. Mechanical Properties of Engineering Fibers

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Table 2. Transport Properties of Conductive Materials

What are the Potential Applications for Carbon Nanotubes?

Carbon Nanotube Technology can be used for a wide range of new and existing applications:

·                          Conductive plastics

·                          Structural composite materials

·                          Flat-panel displays

·                          Gas storage

·                          Antifouling paint

·                          Micro- and nano-electronics

·                          Radar-absorbing coating

·                          Technical textiles

·                          Ultra-capacitors

·                          Atomic Force Microscope (AFM) tips

·                          Batteries with improved lifetime

·                          Biosensors for harmful gases

·                          Extra strong fibers