Two Types of Nanotubes

There are two main types of nanotubes: single-walled nanotubes (SWNTs) and multiwalled nanotubes (MWNTs).

Most SWNTs have a diameter of close to 1 nanometer, with a tube length that can be many thousands of times longer. In fact, SWNTs can reach the length of centimeters. The structure of a SWNT can be conceptualized by wrapping a one-atom-thick layer of graphite called graphene into a cylinder. A graphene is a two-dimensional single sheet of carbon-bonded atoms.

A single-walled nanotube is commonly called a buckytube and consists of one single shell. The nanotube is cylindrical, with at least one end typically capped with a hemisphere of the buckyball structure. The diameter of a nanotube is only a few nanometers wide and can extend up to 50 microns in length.

Nanotubes have the following physical, chemical, and mechanical properties that make them such an outstanding material:

• Electrical conductivity: Depending on their precise structure, carbon nanotubes can be either metallic conductors or semiconductors. The electrons in nanotubes can travel much faster than in metals such as copper, and they do not dissipate or scatter. The electrical conductivity of the nanotubes could be useful in absorbing static noise, storing energy, or in replacing silicon circuits in computer chips.

• Thermal conductivity: The thermal conductivity of nanotubes is superior to that of a diamond. In some tests, nanotubes have been shown to have a thermal conductivity at least twice that of diamond. The nanotubes could be potentially handy for cooling off confined spaces inside computers and other nanoelectronics.

• Mechanical: Nanotubes are the stiffest, strongest, and toughest fiber known. For their small size, nanotubes are six times lighter than steel but more than 500 times stronger. They could be used to replace copper wires or to create superstrong plastics.

MWNTs are multiwalled nanotubes which may have 7-20 concentric grapheme cylinders. Double-walled carbon nanotubes have higher thermal and chemical stability than single-walled carbon nanotubes. MWNT can be applied to gas sensors, nanoelectronic devices, and nanocompos-ites.

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