Introduction to Nanotechnology

5.5. APPLICATIONS OF CARBON NANOTUBES 125
counterparts. For example, multi-walled nanotubes of 200 nm diameter have a tensile
strength of 0.007 TPa (i.e., 7 GPa) and a modulus of 0.6 TPa.
5.5. APPLICATIONS OF CARBON NANOTUBES
The unusual properties of carbon nanotubes make possible many applications
ranging from battery electrodes, to electronic devices, to reinforcing fibers, which
make stronger composites. In this section we describe some of the potential ap-
plications that researchers are now working on. However, for the application poten-
tial to be realized, methods for large-scale production of single-walled carbon
nanotubes will have to be developed. The present synthesis methods provide only
small yields, and make the cost of the tubes about $1500 per gram ($680,000 per
pound). On the other hand, large-scale production methods based on chemical
deposition have been developed for multiwalled tubes, which are presently available
for $60 per pound, and as demand increases, this price is expected to drop sig-
nificantly. The methods used to scale up the multiwalled tubes should provide the
basis for scaling up synthesis of single-walled nanotubes. Because of the enormous
application potential, it might be reasonable to hope that large-scale synthesis
methods will be developed, resulting in a decrease in the cost to the order of $10
per pound.
5.5.1. Field Emission and Shielding
When a small electric field is applied parallel to the axis of a nanotube, electrons are
emitted at a very high rate from the ends of the tube. This is calledjeld emission.
This effect can easily be observed by applying a small voltage between two parallel
metal electrodes, and spreading a composite paste of nanotubes on one electrode. A
sufficient number of tubes will be perpendicular to the electrode so that electron
emission can be observed. One application of this effect is the development of flat
panel displays. Television and computer monitors use a controlled electron gun to
impinge electrons on the phosphors of the screen, which then emit light of the
appropriate colors. Samsung in Korea is developing a flat-panel display using the
electron emission of carbon nanotubes. A thin film of nanotubes is placed over
control electronics with a phosphor-coated glass plate on top. A Japanese company
is using this electron emission effect to make vacuum tube lamps that are as bright as
conventional light bulbs, and longer-lived and more efficient. Other researchers are
using the effect to develop a way to generate microwaves.
The high electrical conductivity of carbon nanotubes means that they will be poor
transmitters of electromagnetic energy. A plastic composite of carbon nanotubes
could provide lightweight shielding material for electromagnetic radiation. This is a
matter of much concern to the military, which is developing a highly digitized
battlefield for command, control, and communication. The computers and electronic
devices that are a part of this system need to be protected from weapons that emit
electromagnetic pulses.