BASIC CONCEPTS AND AN OVERVIEW - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 12

of one pixel, instead of having one giant CRT whose electrons are aimed using
electric and magnetic fields. These displays are known as Field Emission Dis-
plays (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 [55].

1.1.5.1.2 MECHANICAL PROPERTIES

The carbon nanotubes are expected to have high stiffness and axial strength as a
result of the carbon-carbon sp 2 bonding. The practical application of the nano-
tubes requires the study of the elastic response, the inelastic behavior and buck-
ling, yield strength and fracture. Efforts have been applied to the experimental
and theoretical investigation of these properties. Nanotubes are the stiffest known
fiber, with a measured Young's modulus of 1.4 TPa. They have an expected elon-
gation to failure of 20-30%, which combined with the stiffness, projects to a
tensile strength well above 100 GPa (possibly higher), by far the highest known.
For comparison, the Young's modulus of high-strength steel is around 200 GPa,
and its tensile strength is 1-2 GPa [56].

1.1.5.1.3 THERMAL PROPERTIES
Prior to CNT, diamond was the best thermal conductor. CNT have now been
shown to have a thermal conductivity at least twice that of diamond. CNT 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. The specific heat and
thermal conductivity of carbon nanotube systems are determined primarily by
phonons. The measurements yield linear specific heat and thermal conductivity
above1 K and below room temperature while a 0.62 behavior of the specific heat
was observed below1 K. The linear temperature dependence can be explained
with the linear k-vector dependence of the frequency of the longitudinal and twist
acoustic phonons. The specific heat below1 K can be attributed to the transverse
acoustic phonons with quadratic k dependence. The measurements of the thermo-
electric power (TEP) of nanotube systems give direct information for the type of
carriers and conductivity mechanisms [57].

1.1.5.2 STRUCTURE OF CARBON NANOTUBES
The multilayered nanotubes were found in the cathode tip deposits that form when
a DC arc is sustained between the graphite electrodes of a fullerene generator.
They are typically composed of 2 to 5 concentric cylindrical shells, with outer
diameter typically a few tens of nanometer and lengths of the order of micrometer.
Each shell has the structure of a rolled up graphene sheet with the sp 2 carbons
forming a hexagonal lattice. The discovery of nanotubes has revolutionized re-
searches in different directions. A light and high strength nanotube would be an