Nanotubes: A Carbon-based Nanoparticle

A significant nanotechnology discovery that came to light in 1991 was carbon nanotubes. Carbon nanotubes are cylindrical structures that have diameters as small as 1 nm and lengths up to several centimeters. Carbon nanotubes have the highest strength-to-weight ratio of any known material. Although carbon nanotubes are strong, they are not brittle. They can be bent, and when released, they will spring back to their original shape.

One type of carbon nanotube has a cylindrical shape with open ends.

A carbon nanotube.
A carbon nanotube.

Another type of nanotube has closed ends, formed by some of the carbon atoms combining into pentagons on the end of the nanotube.

A carbon nanotube with closed ends.
A carbon nanotube with closed ends.

Carbon nanotubes can occur as multiple concentric cylinders of carbon atoms, called multi-walled carbon nanotubes (MWCTs). Logically enough, carbon nanotubes that have only one cylinder are called single-walled carbon nanotubes (SWCTs). Both MWCT and SWCT are used to strengthen composite materials.

A multi-walled carbon nanotube.
A multi-walled carbon nanotube.

Electrical properties of nanotubes

The electrical properties of carbon nanotubes depend on how the hexagons are orientated along the axis of the tube. Three orientations are possible: armchair, zigzag, and chiral.

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Armchair carbon nanotubes have electrical properties similar to metals. When you apply a voltage between two ends of an armchair nanotube, a current will flow. An armchair carbon nanotube is a better conductor than copper or any other metal.

Researchers are developing methods to spin carbon nanotubes together to make low-resistance electrical wires that could transform the electrical power grid, reduce the power consumed, and reduce the weight of wiring in weight-sensitive uses like spacecraft and airplanes. Researchers are considering using armchair carbon nanotubes to replace the metal lines in integrated circuits.

The zigzag and chiral carbon nanotubes share electrical properties similar to semiconductors. These two configurations of nanotubes will only conduct an electric current when extra energy in the form of light or an electric field is applied to free electrons from the carbon atoms. Semiconducting nanotubes could be useful in building ever smaller transistors used in integrated circuits for all kinds of electronic devices.

Another interesting property of carbon nanotubes is that their electrical resistance changes significantly when other molecules attach themselves to their carbon atoms. Companies are using this property to develop sensors that can detect chemical vapors such as carbon monoxide or biological molecules.

Nanotubes bonded with other materials form super strong composites

The carbon atoms in nanotubes are great at forming covalent bonds with many other types of atoms for several reasons:

  • Carbon atoms have a natural capacity to form covalent bonds with many other elements because of a property called electronegativity. Electronegativity is a measure of how strongly an atom holds onto electrons orbiting about it. Because carbon has an electronegativity rating in the middle of the range, it can form stable covalent bonds with a large number of elements.

  • All the carbon atoms in nanotubes are on the surface of the nanotube and therefore accessible to other atoms.

  • The carbon atoms in nanotubes are bonded to only three other atoms, so they have the capability to bond to a fourth atom.

These factors make it relatively easy to covalently bond a variety of atoms or molecules to nanotubes, which changes the chemical properties of the nanotube. (This method is called functionalization.)

Taking this bonding thing further, if the molecules attached to the carbon nanotubes also attach to carbon fibers, the functionalized carbon nanotubes can bond to the fibers in a composite, producing a stronger material.

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