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The special nature of carbon combines with the molecular perfection of Buckytubes (single-wall carbon nanotubes) to endow them with exceptionally high material properties including electrical and thermal conductivity, strength, stiffness and toughness. No other element in the periodic table bonds to itself in an extended network with the strength of the carbon-carbon bond.
The de-localized pi-electron donated by each atom is free to move about the entire structure, rather than remain with its donor atom, giving rise to the first molecule with metallic-type electrical conductivity. The high-frequency carbon-carbon bond vibrations provide an intrinsic thermal conductivity which is even higher than diamond.
In most materials, however, the actual observed material properties, including strength and electrical conductivity, are degraded significantly by the occurrence of defects in their structure. For example, high-strength steel typically fails at about 1% of its theoretical breaking strength. Buckytubes, however, are unique and achieve values very close to their theoretical limits because of their perfect molecular structure. Buckytubes are an example of true nanotechnology - only 1 nm in diameter with molecules that can be manipulated chemically and physically. They open incredible applications in materials, electronics, chemical processing and energy management.
Buckytubes have the following physical, chemical and mechanical properties that make them such an outstanding material:
- Electrical conductivity- the first polymer with truly metallic conductivity
- Thermal conductivity - higher than diamond along the tube axis
- Mechanical - the stiffest, strongest and toughest fiber known to man
- Chemistry of carbon - can be reacted and manipulated with the richness and flexibility of other carbon molecules (carbon is the basis of most materials we use every day)
- Molecular perfection - essentially free of defects
- Self-assembly – as noted above, strong van der Waals attraction leads to spontaneous roping of many nanotubes, which is important in certain applications
Depending on their precise structure, Buckytubes can be either metallic conductors or semiconductors. The semiconducting tubes are of two types: one has a bandgap of hundredths of an electron volt, while others have about a 1-eV bandgap.
Ropes have been measured with a resistivity of 10-4 ohm-cm at 300 K [A. Thess et al., Science 273, 483 (1996)], making them the most conductive fibers known. Individual tubes have been observed to conduct electrons ballistically - that is with no scattering, with coherence lengths of several microns [Tans, et al., Nature 393, 49 (1998)]. In addition, they can carry the highest current density of any known material, measured as high as 109 A/cm2 [B.Q. Wei, et al., Appl. Phys. Lett. 79 1172 (2001)], and Phaedon Avouris, the Director of IBM's Center for Nanoscale Science and Technology, has suggested that it could be as high as 1013 A/cm2.
Before the discovery of Buckytubes, diamond was the best thermal conductor, however, Buckytubes have since been shown to have a thermal conductivity at least twice that of diamond [Hone J, Carbon Nanotube Topics in Applied Physics, 273 (2001)]. Since this property, like electrical conductivity, is highly anisotropic in Buckytubes, a macroscopic crystal of Buckytubes (close-packed and aligned) would have the unique property of feeling cold to the touch like metal on the sides with the tube ends exposed, but similar to wood on the other sides.
Buckytubes are the stiffest known fiber with a measured Young's modulus of 1.4 TPa [M.-F. Yu et al., Phys. Rev. Lett. 84, 5552 (2000)]. They have an expected elongation failure of 20-30% which, combined with the stiffness, projects to a tensile strength well above 100 GPa which is the highest known. In comparison, Young's modulus of high-strength steel is around 200 GPa and its tensile strength is 1-2 GPa. The toughness of Buckytubes has been demonstrated in simulations and experiments and is remarkable for a material so stiff, as they can be doubled over, snapping back with no resulting defect [Yakobson].
Chemistry and Manipulation
Buckytubes are polymers of pure carbon and possess all the richness of carbon chemistry. Carbon is by far the most chemically versatile of elements, permitting an infinite number of combinations and derivatives. Closed ends of Buckytubes are like fullerenes, whose chemistry is well understood. Open ends have the chemistry of the edges of graphite, which is also well understood. The possibility of attaching any chemical, structure or surface to either end of a Buckytube creates an entirely new situation whereby these end-entities can communicate with one another via true metallic transport of electrons. This is unprecedented as previously no molecules existed with metallic conductivity, which opens up a number of possibilities for technological applications.
Technology also exists for chemically modifying the sidewalls of Buckytubes, both covalently (by attaching chemical groups to the sides with chemical bonds) and non-covalently. Examples of the latter are the use of either simple or chemical surfactants to modify the surface energy of the Buckytubes. In addition, the ability to manipulate Buckytubes with chemical precision enables, for example, their dispersion in solvents, making them compatible with plastic resins for composites.
This article was updated on the 11th September, 2019.