spotlight: carbon nanotubes

Carbon nanotubes were discovered by Sumio Iijima in 199. Carbon nanotubes are even newer to the scientific world than buckyballs, so they are also in no way widely used, however, they continue to be researched today to determine their useful qualities. Each individual nanotube’s properties greatly depend on its length, diameter, and character of spiral.

There are several ways to synthesis carbon nanotubes, but the most widely used methods are arc discharge and laser ablation. Arc discharge happens when a direct current arc is applied across two electrodes, in this case graphite, which are immersed in an inert gas, usually helium. A soot containing fullerenes is then deposited in the chamber, to be separated out using chromatography. Arc discharge is generally favored because it is easy and does not produce produce any toxic byproducts. Laser ablation is the practice of removing a material from a solid by irradiating it with a laser beam. The absorption of the material being ablated is very important. If we were to ablate diamond rather than graphite it would not only very very expen$$$ive, but it would be very difficult. The diamond does not absorb light very well, so a layer of graphite would need to be added in order to “force” the diamond to irradiate. Lucky for us carbon nanotube-lovers, they are synthesized using graphite, so we don’t have any problem$$$.


Laser Ablation

As mentioned in my blog down to the basics, there are three types of carbon nanotubes: the armchair, the zigzag, and the chiral. The structure is important because the armchair form is a metal conductor, while the zigzag and the chiral are just semiconductors. If the structure of a nanotube decreases the number of collisions between conductive electrons and atoms, it will be very conductive. In addition, due to the sp^2 hybridization, the immense strength of the bonds allow high electric currents to flow — even higher than copper. Again, because of the strength created by the sp^2 hybridization, the carbon nanotubes are able to withstand very high temperatures, making them good thermal conductors.

Though there is nothing in practice today that is built around carbon nanotubes, there is a whole world of speculation surrounding their potential. Nanotubes have been considered to replace some parts in the traditional lithium battery. In the lithium battery, there is a graphite electrode and a metal oxide electrode, and lithium atoms are transferred between them to convert chemical energy into electrical energy. It has been theorized that replacing the graphite with carbon nanotubes could greatly increase the efficiency and desirability of the battery. The carbon nanotubes would present a lighter, thinner electrode, making the system easier to transport, and a significant amount of battery power would be saved in having to transport less weight. In addition, they have a much high conductivity than amorphous carbon.

However, on the other hand, there are issues to be overcame regarding the production and mass use of carbon nanotubes. There is currently no way to synthesize nanotubes homogeneously. According to the collision theory, only a fraction of all the collisions between atoms produce the desired molecules, because a significant amount of energy is needed to overcome the activation energy, and, more importantly for our purposes, the atoms must collide in the correct orientation. Unfortunately for the proprietors of carbon nanotubes, there are three different ways for carbons to collide to form the tubes, and there is no way to monitor them. This wouldn’t pose too large of a problem, except that some nanotubes are conductors and some are only semiconductors. Because all the molecules have the same number of carbon atoms, there is no way to use a mass spectrometer to separate out the conductive from the semi-conductive.

Information about carbon nanotubes is still surfacing today, and we still do not have the whole picture. Though there are difficulties involved in using them on a mass scale, scientists have hope that they may soon contribute positively to our society.




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