Carbon nanotube

A carbon nanotube is one or more concentric sheets of graphene rolled into tubes. They are amongst the stiffest and strongest materials known to mankind and their electrical properties can be varied from metallic to semiconducting.^[1]

Types

Carbon nanotubes are classified according to their concentricity, tube thickness and chirality (or zigzag/armchair status, if not chiral). There are also a number of interesting nanotube-derived materials. (Note to the reader: I haven't uploaded any images yet. The below will make a lot more sense then; so come back in a day or two if you're reading this.)

Layering

Multi-walled carbon nanotubes (MWNT) consist of more than one tube of different sizes inside one another. Single tubes are known as single-walled carbon nanotubes (SWNT).^[1]

Chirality

The shape of a carbon nanotube can be described using the value $\theta$ , the (absolute value of the) angle at which it is twisted (relative to a zigzag nanotube) when this tube is conceptually made from a sheet of graphite.

$\theta =0^{\circ }$ for zigzag nanotubes, which may be metallic conductors or semiconductors.
$\theta =30^{\circ }$ for armchair nanotubes, which are metallic conductors.
$0^{\circ }<\theta <30^{\circ }$ for all other nanotubes, which are called chiral carbon nanotubes. They may be either left-handed (following the adjacent hexagons in a clockwise direction leads you to spiral upwards along the tube) or right-handed (following the adjacednt hexagons in an anticlockwise direction leads you to spiral upwards along the tube). Chiral nanotubes may be metallic conductors or semiconductors.

Modified carbon nanotubes

It is sometimes desirable to modify carbon nanotubes to make them soluble in either water or organic solvents, bond them to another material or perform some other function.
One of the ways to achieve this is to react the carbon nanotubes with chemicals, such as fluorine, which can bond to their surface. Since each carbon atom has a double bond with one of its neigbours, it is possible to bond carbon nanotubes to other molecules or elements without disrupting the physical structure. However, this reduces electrical conductivity.^[1].

Production

Commercial production of carbon nanotubes takes place by chemical vapour deposition, laser vaporisation (of graphite) or electrical arcing between graphite electrodes^[1]. The electric arc method tends to create MWNTs. Lengths produced are currently in the micrometre range, while thicknesses vary from about 1nm for the thinnest SWNTs and 100nm for the thickest MWNTs^[1]

Possible uses in space travel

Carbon nanotubes are perhaps most notable for their extreme tensile strength, making them an ideal material for spacecraft construction.
They are also one of the only materials which have a high enough tensile strength that they might, possibly, enable a space elevator on earth.
The electrical properties of carbon nanotubes suggest that they might some day be used in computers.
Excellent resistance to thermal shock and high temperatures (which is shared by all forms of graphite, graphene, carbon nanotubes and carbon fibre), in combination with high tensile strength, raises the question of whether hypersonic parachute designs might improve the usefulness of aerobraking.