Re: Room temperature superconduction in carbon fibers?

Brian D Williams (
Thu, 10 Sep 1998 07:48:32 -0700 (PDT)

4. ON THE FUTURE OF CARBON NANOTUBE CHEMISTRY Fullerenes are large molecules composed entirely of carbon, with the chemical formula C(sub n), where n is any even number from 32 to over 100. They apparently have the structure of a hollow spheroidal cage with a surface network of carbon atoms connected in hexagonal and pentagonal rings. Carbon nanotubes are similar to fullerenes, except their shape is tubular. They were first discovered by Sumio Iijima (NEC Laboratories, JP) in 1991, they come in both multi-walled and single-walled versions, and they have diameters of the order of 10 to 30 nanometers. .... ... Robert F. Service (*Science*, US) reviews the reasons for current excitement concerning nanotubes and recent research in the field, and makes the following points: 1) Carbon nanotubes are stronger than steel; lightweight; able to withstand repeated bending, buckling, and twisting; can conduct electricity as well as copper, or semiconduct like silicon; and they transport heat better than any other known material. 2) The possible important applications of carbon nanotubes include superstrong cables, wires for microscale electronic devices, charge storage devices in batteries, and microscale electron guns for flat-screen television. 3) The key to the potential applications of carbon nanotubes lies in the unique structure of carbon nanotubes and in the possible defects in that structure that confer special properties. The structure itself depends on the unique properties of carbon. Under intense pressure, carbon atoms form bonds with 4 neighboring atoms to produce diamond. Under special conditions, however, sheet-like carbon structures involving 3-bonded carbon can be formed (graphite), and with still further specialization of conditions, these sheets organize into spherical (fullerene) or tubular (nanotube) arrays. One critical aspect of the 3-bonded carbon array is that the nature of the bonding produces a cloud of unpaired electrons floating above and below the sheet, and these electrons are mobile enough to make the material a good electrical conductor. The importance of defects in this context is that specifically designed defects in the carbon arrays can alter the physical properties of these arrays, including the electrical properties -- all in a controlled manner. It has recently been reported, for example, that single-walled carbon nanotubes can function not only as conductors but also as semiconductors, depending on the conditions, which is of considerable significance for the possible use of carbon nanotubes as semiconductor switches in computer devices. It has also been possible to form hybrid carbon nanotubes such that one end of the tube behaves as a metallic conductor while the other end of the tube behaves as a semiconductor, and such tubes have the potential to act as molecular diodes, devices that allow electric current to flow in one direction, from a semiconductor to a metal but not in reverse. 4) Perhaps of greatest interest is the recent demonstration by Heer et al (Georgia Institute of Technology, US), confirming theoretical predictions, that carbon nanotubes can carry current at room temperature with essentially no resistance. The mechanism for this involves so-called
"ballistic transport", which refers to the passage of electrons
through a semiconductor whose length is less than the mean free path of electrons in the system, so that most of the electrons pass through the semiconductor without scattering. 5) At the present time, one critical aspect of carbon nanotube research is that before the mentioned potential applications can be achieved, the technology of carbon nanotube production must be improved so that carbon nanotubes are available in bulk quantities for materials research and development. At the present time, singlewalled carbon nanotubes are commercially available for approximately US$200 per gram. This price needs to be severely reduced by technological advances before these new structures can be fully developed for practical use.
QY: Robert F. Service <> (Science 14 Aug 98 281:940) (Science-Week 11 Sep 98)

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