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A nanotube has a structure similar to a fullerene, but where a fullerene's carbon atoms form a sphere, a nanotube is cylindrical and each end is capped with half a fullerene molecule. Their name derives from their size; nanotubes are on the order of only a few nanometres wide (on the order of one ten-thousandth the width of a human hair), and their length can be millions of times greater than their width.
While it has long been known that carbon fibres can be produced with a carbon arc, and patents were issued for the process, it was not until 1991 that Sumio Iijima, a researcher with the NEC Laboratory in Tsukuba, Japan, observed that these fibres were hollow. This feature of nanotubes is of great interest to physicists because it permits experiments in one-dimensional quantum physics. Techniques have been developed to produce nanotubes in sizeable quantities, but their cost still prohibits any large scale use of them.
Fullerenes and carbon nanotubes are not necessarily products of high-tech laboratories, and are also formed in such mundane places as candle flames. However, these naturally occurring varieties are highly irregular in size and quality, and attempting to ensure the high degree of uniformity necessary to meet the needs of research and industry is impossible in such an uncontrolled environment.
Nanotubes can be opened and filled with materials such as biological molecules, raising the possibility of applications in biotechnology. They can be used to dissipate heat from tiny computer chips. The strength and flexibility of carbon nanotubes makes them of potential use in controlling other nanoscale structures, which suggests they will have an important role in nanotechnology engineering. Though it is debatable if nanotube materials can ever be made with a tensile strength approaching that of individual tubes, composites may still yield incredible strengths potentially sufficient to allow the building of such things as space elevators, artificial muscles, ultrahigh-speed flywheels, and more. MIT is working on combat jackets utilizing carbon nanotubes for ultrastrong fibers and for monitoring its wearer's condition.
One use for nanotubes that has already been developed is as extremely fine electron guns, which could be used as miniature cathode ray tubes in thin high-brightness low-energy low-weight displays. In this type of display, a group of many tiny CRTs would provide the electrons to hit the phosphors of one pixel, instead of having one giant CRT whose electrons are aimed using electric and magnetic fields. These diplays are known as Field Emission Displays (FEDs) A nanotube formed by joining nanotubes of two different diameters end to end can act as a diode, suggesting the possibility of constructing electronic computer circuits entirely out of nanotubes. Nanotubes have been shown to be superconducting at low temperatures.
In April of 2001, IBM announced it had developed a technique for automatically developing pure semiconductor surfaces from nanotubes.
Other applications for nanotubes that are currently being researched include high tensile strength fibers. Two methods are currently being tested for the manufacture of such fibers. A French team has developed a liquid spun system that involves pulling a fiber of nanotubes from a bath which yields a product that is approximately 60% nanotubes. The other method, which is simpler but produces weaker fibers uses traditional melt-drawn polymer fiber techniques with nanotubes mixed in the polymer. After drawing, the fibers can have the polymer burned out of them to make them purely nanotube or they can be left as they are.
High purity (80%) nanotubes with metallic properties can be extracted with electrophoretic techniques. See: Krupke and Hennrich
These take advantage of the incredible aspect ratio and strength of nanotubes. Computer storage devices using nanotubes are currently in the prototype stages. Both high speed non-volatile memory which can be used to replace nearly all solid state memory in computers today, and high density storage that may replace hard drives, are being developed. Major limiting factors in development include orienting the nanotubes, which tend to tangle because of their length, and their price.
On September 19, 2003, NEC Corporation, Japan, announced stable fabrication technology of carbon nanotube transistors.Current Progress