Change the World? An Amazing Material Carbon Nanotube
Can you believe that there is a material that is stronger than steel, unbreakably elastic, resistant to chemicals and high temperature, a better conductor of electricity than silver, and a better heat conductor than diamond? The material could improve the performance of computers, and itfs even a potential solution to the energy problem. The dream material was discovered in the city of Tsukuba in Japan, which coincides with where I live.@
The Birth of the Novel Material
Following the introduction of large scale synthesis of C60 (figure 1), the early 1990's for scientific community was the time of fullerene boom. As I wrote in the previous fullerene column, the synthetic process involves the vaporization of carbon electrode by arc discharge, which results in the formation of C60 in the gsooth collected on the anode (the positive side of the electrode). When laboratories around the world were busy focusing on making fullerenes, there was one person who was studying the soot on the cathode (the negative side). It was Sumio Iijima, a senior scientist at NEC Corporationfs Fundamental Research Laboratory (who is also a professor at Meijo University now).
Fig 1 C60 (fullerene)
When Dr. Iijima was analyzing the soot on the cathode under an electron microscope, what he observed was a large amount of thin tube-like material which was completely different from spherical fullerene. This was the moment when the amazing material carbon nanotube showed itself before mankind.
Carbon nanotube (figure 2) was initially regarded as mere gstrange cousinh of fullerene. However, the discovery of large scale synthesis in 1990 by Smalley (one of the discoverers of fullerene, a Nobel Laureate) accelerated the progress of research and today it has become even more popular than fullerene.
Fig 2 carbon nanotube
The Secret of Carbon Nanotube
The most common form of carbon is graphite (contained in pencil lead), which consists of the layers of flat sheets of honeycomb structure. On the other hand, carbon nanotube is the graphite sheet rolled into a tube (Figure 3). The first nanotubes discovered were multi-walled with differently sized tubes, but soon it became possible to make single-walled ones. The diameter of a carbon nanotube is literally in the order of nanometers (one billionth of a meter), while its length reaches several thousand times longer.
Fig 3 graphite
The chemical bond between the carbon atoms of a benzene ring (known as sp2 carbon atoms) is considered the strongest among all atomic bonds. Carbon nanotube is made entirely of this very stable bond, so itfs strong against bending or stretching and unreactive towards most chemicals. Thanks to the sp2 carbons, nanotubes also possess good property as the material needed for electronic devices. Itfs been shown that they can be either good conductors of electricity or semiconductors.
I said that nanotube is a rolled-up sheet of graphite, so you can imagine that it can be rolled either parallel or with a little twist. The degree of the twist and the tube diameter determines whether it is a conductor or semiconductor
Semiconducting nanotubes are promising as the component of computer circuits, and conducting ones are expected to have superior property to metal electrical wires. For example, while an ordinary copper wire burns out with the electricity of one million ampere per square centimeters, strong and stable nanotube can withstand one billion ampere. For these reasons, some people even say that humans wonft be able to create better material than carbon nanotube in next several thousand years!
The Strongest Fiber Ever
The excellent properties of nanotubes have led to the development of impressively many applications, some of which are already moving towards practical use.
Since nanotube is an extremely strong material, weaving it would make a superbly strong fiber. By calculation, a rope made of defect-free nanotube with the diameter of one centimeter would be able to lift 1,200 tons, which is by far the best tensile strength among all other known materials. Nanotubes are already used in bulk products like tennis rackets, golf clubs, and in car fenders. You can expect more products in near future in the areas using special material like architecture.
One of the ideas proposed for future is the application of nanotube in outer space development. Today, you have to launch a rocket every time when flying into space, spending huge amount of energy with fuels releasing toxic substances into the environment. The concept is to extend a cable into space to construct gspace elevatorh which would transport cargos and people up and down. It might sound too unrealistic, but can be the most economical and environmentally friendly way of space travel if realized.
The idea is interesting but has been thought impossible to achieve because the material used for the cable must be of extreme strength. The advent of strong, light-weight carbon nanotube, however, is changing that notion. The synthesis of nanotubes with necessary length and purity hasnft been successful yet and so many problems must be solved. Still, this is a fascinating dream.
As Electronic Component
There has been a great expectation for carbon nanotube as the component in electronic devices. As mentioned earlier, carbon nanotubes can either be good conductor of electricity or semiconductor, and the semiconducting ones are drawing attentions in this area.
As you know, todayfs computers are built with chips made of silicon crystal and tiny circuits on the silicon. It is known, however, that within next few years the capacity of the circuit wiring will reach its physical limit above which you canft put any more in. If the progress of computer technology hits the ceiling, the damage it will cause to the current economy is not difficult to imagine.
How can carbon nanotube help? The theoretical limit for the wire diameter in silicon chips is 50 to 100 nanometers, whereas nanotubes are about one nanometer thick, which allows the production of much higher density circuits. Computers built with nanotube-based circuits are expected to consume less electricity and at the same work accurately more than a thousand times faster (at greater than one terahertz). Despite many technical hurdles that need to be overcome, the practical use of nanotube transmitter is expected to arrive around 2010.
Inside the Tube
It is possible to put things into a cylindrical nanotube. The usually closed ends of a nanotube can be opened by gburningh them under appropriately chosen conditions, and things can be pulled in by capillary action.
For example, nanotube is known to take in fullerene, which is its close carbon relative (Figure X). The fabulous view of a nanotube packed with multiple fullerene balls can be seen under electron microscope. It was then found that when this gpeapodh was heated, the neighboring fullerenes fused with one another to become a nanotube themselves. This was the first controlled synthesis of double walled nanotube, which may lead to another interesting application.
Fig 4 Bucky peapod
The interesting behaviors of other molecules when trapped inside a nanotube have been reported. In 2007, Professor Eiichi Nakamura of Tokyo University succeeded in the first-ever direct observation of the movement of a single organic molecule by studying it in a nanotube under electron microscope. Nanotube therefore has had tremendous impact on academic researches as well as industrial applications.
Energy Problem and Nanotube
Energy development is one of the biggest challenges human society is facing today. Fossil fuels like petroleum will be exhausted eventually, and their combustion releases environmental pollutants like sulfur and nitrogen oxides and greenhouse gas like carbon dioxide. We also know that other energy sources such as nuclear and hydroelectric energies have their own problems.
Then what can we turn to? One of the possible solutions under serious consideration today is methane (CH4). Methane releases only half as much carbon dioxide as petroleum by combustion, and a huge oceanic deposit as methane hydrate has been confirmed to exist. A reason it hasnft been utilized on practical scale so far is because of the difficulty of storing the gas. It can be stored compressed in tanks, but the risk of explosion is too problematic to be used as common energy source.
So thatfs why the research is ongoing to use the ability of nanotubes to take in other molecules as a way to store methane gas. Of particular interest is carbon nanohorn , a relative of nanotube, which adsorbs methane gas with high efficiency.
Carbon nanohorn is made of benzene rings just like nanotube, but is corn-shaped instead of a cylinder. Under electron microscope, nanohorn molecules appear as a sea-urchin like aggregate with each nanohorn pointing the center. Already the prototype of nanohorn-based battery has been made, so itfs possible that it will evolve into something that supports future computer or automobile. Since the mass-production and purification processes for nanohorns are easier than those for nanotubes, the practical use of the former will probably be ready sooner.
Despite the dream possibilities of carbon nanotubes I have talked about, the biggest obstacle to their realization is the cost of the material, which is roughly 500 to 1,000 dollars per gram. Even pure gold costs about 10 dollars per gram, so you can tell how expensive it is. The cost should drop if the current production process is improved, considering that nanotubes are made of abundant element, carbon after all. Scientists and engineers will continue to strive for the invention of a better process, which will unquestionably bring them an enormous fortune.
Professor Sumio Iijima, the discoverer of carbon nanotube, and Professor Morinobu Endo of Shinshu University, who is known for his pioneering works in the field, are both predicted to win the Nobel Prize in near future. The Japanese science has always been more famous for applications than for fundamentals, but I want to emphasize that there are in fact great fundamental researches in Japan.
Japan is an important contributor to gnanotechnologyh, a science that deals with substances on nanometer scale. It will be exciting to follow the field and see how carbon nanotubes will be leading the way to change our world.