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60 • Chapter 3 / Structures of Metals and CeramicsMATERIALS OF IMPORTANCE
Carbon Nanotubes
A
nother molecular form of carbon has recently
been discovered that has some unique and
technologically promising properties. Its structure
consists of a single sheet of graphite, rolled into
a tube, both ends of which are capped with C 60
fullerene hemispheres. Thiscarbon nanotubeis
represented schematically in Figure 3.19. Thenano
prefix denotes that tube diameters are on the or-
der of a nanometer (i.e., 100 nm or less). Each nan-
otube is a single molecule composed of millions of
atoms; the length of this molecule is much greater
(on the order of thousands of times greater) than its
diameter. Multiple-walled carbon nanotubes con-
sisting of concentric cylinders have also been found
to exist.
These nanotubes are extremely strong and
stiff, and relatively ductile. For single-walled nan-
otubes, tensile strengths range between 50 and 200
GPa (approximately an order of magnitude greater
than for carbon fibers); this is the strongest known
material. Elastic modulus values are on the order
of one tetrapascal [TPa (1 TPa= 103 GPa)], with
fracture strains between about 5% and 20%. Fur-
thermore, nanotubes have relatively low densities.
On the basis of these properties, the carbon nan-
otube has been termed the “ultimate fiber” and is
extremely promising as a reinforcement in com-
posite materials.
Carbon nanotubes also have unique and
structure-sensitive electrical characteristics. De-
pending on the orientation of the hexagonal units
in the graphene plane (i.e., tube wall) with the tubeFigure 3.19 The structure of a carbon nanotube. (Reprinted by permission fromAmerican
Scientist, magazine of Sigma Xi, The Scientific Research Society. Illustration by Aaron Cox/
American Scientist.)axis, the nanotube may behave electrically as ei-
ther a metal or a semiconductor. It has been re-
ported that flat-panel and full-color displays (i.e.,
TV and computer monitors) have been fabricated
using carbon nanotubes as field emitters; these dis-
plays should be cheaper to produce and have lower
power requirements than CRT and liquid crystal
displays. Furthermore, it is anticipated that future
electronic applications of carbon nanotubes will in-
clude diodes and transistors.An atomically resolved image of a carbon nanotube
that was generated using a scanning tunneling
microscope (a type of scanning probe microscope,
Section 5.12). Note the dimensional scales (in the
nanometer range) along the sides of the micrograph.
(Micrograph courtesy of Vladimir K. Nevolin, Moscow
Institute of Electronic Engineering.)