In 1991, a Japanese scientist Sumio Iijima used a high-resolution transmission electron microscope to study the soot created in an electrical discharge between two carbon electrodes at the NEC Fundamental Research Laboratory in Tsukuba, Japan. He found that the soot contained structures that consisted of several concentric tubes of carbon, nested inside each like Russian dolls. These were termed as ‘Carbon Nanotubes’.
Later efficient ways of making large quantities of these multiwall nanotubes were developed. Subsequently, 1993, single-wall nanotubes were tens of nanometers across, the typical diameter of a single-wall nanotube was just one or two nanometers. The past decade has seen an explosion of research into both types of nanotube.
Today, nanotubes can be grown efficiently by the catalytic decomposition of a reaction gas that contains carbon, with iron often being used as the catalyst. This process has two main advantages. First, the nanotubes are obtained at much lower temperature, although this is at the cost of lower quality. Second, the catalyst can be grown on a substrate, which allows novel structures, such as ‘nanobrushes’, to be obtained. Currently nanotubes can be grown to lengths exceeding 100 microns, and in various shapes such as ‘nanosprings’.
A nanotube can be considered as a single sheet of graphite that has been rolled up into a tube. The electronic properties of the resulting nanotube depend on the direction in which the sheet was rolled up. Some nanotubes are metals with high electrical conductivity, while others are semiconductors with relatively large band gaps. Nanotubes also have remarkable mechanical properties that cam be exploited to strengthen materials or to act as ‘tips’ in scanning probe microscopes. And since they are composed entirely of carbon, nanotubes also have a low specific weight.
MECHANICAL PROPERTIES OF CARBON NANOTUBES
In a sheet of graphite each carbon atom is strongly bonded to three other atoms, which makes graphite very strong in certain directions. However, adjacent sheets are only weakly bound by vander waals forces, so layers of graphite can be easily pealed apart as happens when writing with a pencil. As we shall see, it is not easy to peel a carbon layer from a multiwall nanotube. Carbon fibre is already used to strengthen a wide range of materials, and the special properties of carbon nanotubes mean that they could be the ultimate high strength fibre.
Lieber and his co-workers went on to explore larger forces and deformations and compared carbon nanotubes with nanorods made from silicon carbide, another very strong material. What they found was surprising whereas the silicon-carbide nanorods eventually fractured the multiwall carbon nanotubes buckled, but did not break. This behaviour has since been confirmed in several experiments in which the nanotubes are either bent or compressed along their length.
Carbon nanotubes got a peculiar feature that they first bend over to surprisingly large angles, before they start ripple and buckle, and then finally develop kinds as well. The amazing thing about the carbon nanotubes is that these deformations are elastic- they all disappear completely when the load is removed.
To see how these properties might be useful, imagine owning a BMW car made from carbon nanotubes and being unlucky enough to crash into a wall. Due to high force of the impact, the nanotubes would bend and then buckle, squeezing your BMW into the shape of something like a Volkswagen Beetle. This would happen relatively long distance, which would provide an effective ‘crunch zone’. Moreover, after the crash all the buckles and kinks would unfold and your BMW would ‘reappear’ as if nothing had happened! To be completely safe, however, the nanotubes would have to be combined with energy-absorbing materials, otherwise the collision between the car and the wall would be completely elastic and you would rebound from the wall with the same speed as you hit it! Other, less futuristic applications might include light weight bullet proof vests and earthquake-resistant buildings, while nanotubes tips for scanning probe microscopes are already commercially available.
The high strength of carbon nanotubes makes them promising candidates in reinforcement applications but there are many outstanding problems that must be overcome. First, the properties of the individual tubes must be optimized. Second, the tubes must be efficiently bonded to the material they are reinforcing so that they actually carry the loads. Third, the load must be distributed within the nanotube itself to ensure that the outermost layer does not shear off.