Carbon nanotubes are a type of fullerene, an allotrope of carbon. They have some fascinating properties and a lot of reasearch is currently being undertaken in their electrical properties.
What is a nanotube?
Nanotubes are a type of fullerene, a class of carbon allotrope which contains tubes or spheres of carbon atoms. By definition, fullerenes contain at least 60 carbon atoms but nanotubes often contain many more. They have a very similar structure to graphite and are made up of rings of carbon atoms.
Graphite is made up of sheets of carbon and the layers can easily slide over one another. Therefore graphite is slippery, often used as a lubricant due to this property. Nanotubes are formed by rolling the sheets of graphite into cylinders, which changes the physical properties of the material and provides a range of useful applications.
As mentioned above, nanotubes are produced from sheets of graphite. The way they are then rolled into cylinders has a huge influence on their physical properties and multiple forms of carbon nanotubes can be made.
Zig-zag, armchair and chiral are the most common forms of nanotube:
- Zig-zag nanotubes, with hexagons arranged in a circle around the nanotube, act more like semiconductors
- Armchair nanotubes, with hexagons parallel to long axis, are very good electrical conductors, with higher conductivity than copper. They are also very light so could have potential applications in aircraft where weight is an issue.
- Chiral nanotubes have a twist in them meaning that the hexagons will not line up at all. This structure will only conduct electricity in certain situations, if light or an electric field is applied, for example.
Properties and Applications
The carbon-carbon covalent bonds across a sheet of the nanotube are extremely strong, resulting in some very interesting properties.
Nanotubes usually have a Young Modulus (stiffness) of around 5x that of steel and a breaking strength of about 63GPa,making them 50x stronger than steel. They are also very lightweight. These properties alone give them useful applications in a range of areas such as sports equipment.
Their electrical properties are also very interesting, with some structure having better conductivity than copper but others act as semiconductors. There is currently a lot of development and research into using nanotubes in electronics and they are already being used in some cases.
Nanotubes have a very large surface area for their size, as they can be a small as one atom thick. A catalyst can therefore be spread across the nanotube and will be extremely effective due to the high surface area greatly increasing the number of collisions.
Also certain types of nanotube will release electrons when a current passes through. This can be used to donate electrons to molecules which collide with the nanotube, increasing the rate at which a reaction occurs.
Image: By James Hedberg, licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.