Much of the recent nanotechnology/nanomaterial hype has centred around carbon nanotubes, sometimes also referred to as buckytubes. This name is derived from the spherically structured carbon allotropes known as Buckminster fullerene. Their amazing properties have been linked to possible applications as far fetched as the space elevator, a carbon nanotube based cable that would allow people to travel into space.
So what is a carbon nanotube?
A carbon nanotube is essentially a graphite sheet rolled up to form a hollow tube. The graphite sheets consist of carbon atoms arranged in hexagons, with each atom being bound to three others to form a flat sheet, which can best be visualized by thinking of chicken wire.
When were they discovered?
Carbon nanotubes were first observed by Roger Bacon, then an employee at Union Carbide. When studying carbon near its triple point he observed straight, hollow tubes of carbon. Similar structures were observed in the 70’s by Morinobu Endo. However, it was not until 1991 when Sumio Ijima characterized them and named them carbon nanotubes that their potential began to be understood.
Do they come in different forms?
There are two basic types of carbon nanotubes single and multiwalled carbon nanotubes:
• Single walled carbon nanotubes, as the name suggest consist of a single layer of carbon atoms in the form of a graphite sheet (graphene) that wraps around to form a seamless tube. These tubes may be open at both ends or capped at one or both ends.
• Multiwalled carbon nanotubes are of two types, singe walled carbon nanotubes arranged concentrically or a grapheme sheet that is wrapped around onto itself, like a scroll.
While related to the more common carbon fibre used commonly to reinforce plastics in yacht sails, etc, nanotubes have the advantage that they are molecularly perfect, which leads to their extraordinary properties, in particular strength. Unlike nanotubes, carbon fibres can contain surface defects that can lead to catastrophic failures.
What amazing properties do carbon nanotubes possess?
Probably the biggest claim to fame for carbon nanotubes is their strength. They are the strongest material known to man. To give you an idea, they are about 100 time stronger than high strength steel, at a sixth of the weight.
They also claim the mantle of highest Young’s modulus and possess unprecedented toughness. While exhibiting such strength, they are also amazingly flexible.
They also trump diamond, the long term title holder in the thermal conductivity stakes. Although diamond doesn’t discriminate, carbon nanotubes conduct much more efficiently in the direction parallel to the axis of the tube.
While graphene behaves like a semimetal in terms of electrical conductivity, the tube structure of carbon nanotubes enables them to conduct electricity like a metal.
How are carbon nanotubes made?
There are several methods that have been used to synthesise carbon nanotubes. Some of the more common techniques include:
• Chemical Vapour Deposition (CVD)
• Arc discharge
• Laser ablation
• High pressure carbon monoxide
While many other processes have been reported, many of these are only suited to laboratory scale production. There are however, numerous companies around the world who can produce carbon nanotubes in commercial quantities.
What can carbon nanotubes be used for?
To date there are only a few reports of carbon nanotubes having been used in commercial applications, although there is no shortage of research being carried out to integrate them into both consumer and high-tech applications as either modifications to existing technologies or completely original technologies.
Potential areas where carbon nanotubes can be applied include, but are certainly not restricted to:
• Flat panel displays
• Conductive plastics
• Energy storage
• Conductive adhesives and connectors
• Molecular electronics
• Thermal materials
• Structural composites
• Fibres and fabrics
What does the future hold for carbon nanotubes?
There is no doubt that nanotechnology, and carbon nanotubes will continue to infiltrate our lives. As with most advanced materials they will initially find uses in somewhat more exotic applications. With increasing commercial production and more efficient production techniques, prices will come down and these amazing materials will quietly find their way into our homes with the aid of recent developments in metrology and microscopy.
Will carbon nanotubes be used to build a space elevator? Unlikely to happen in this lifetime, but beyond that, only time will tell.