Page 230 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 8 6 Graphite 20

Carbon and Graphite Foams. These foams have high service temperatures, chemi-
cal inertness, low thermal expansion, and thermal and electrical properties that can
be tailored to specific applications. Carbon foams are available in either graphitic or
nongraphitic structures. Graphitic foams (typically produced from petroleum, coal
tar, and synthetic pitches) have low density, high thermal and electrical conductivity
(but lower mechanical strength), and are much more expensive than nongraphitic
foams (produced from coal or organic resins), which are highly amorphous.
These foams have a cellular microstructure with interconnected pores; thus,
their mechanical properties depend on density (see also Section 8.3). They easily can
be machined into various complex shapes with appropriate tooling. Applications of
carbon foams include their use as core materials for aircraft and ship interior panels,
structural insulation, sound-absorption panels, substrates for spaceborne mirrors,
lithium-ion batteries, and fire and thermal protection.

8.6.l Fullerenes

A more recent development is the production of carbon molecules (usually C60) in
the shape of a soccer balls, called fullerenes or buckyballs, after Buckminster Fuller
(1895-1983), the inventor of the geodesic dome. These chemically inert spherical
molecules are produced from soot and act much like solid lubricant particles.
Fullerenes can become low-temperature superconductors when mixed with metals.
Despite their promise and significant research investment, no commercial applica-
tions of buckyballs currently exist.

8.6.2 Nanotubes
Carbon nanotubes can be thought of as tubular forms of graphite and are of interest
for the development of nanoscale devices. (See also nanomaterials, Section 8.8.)
Nanotubes are produced by laser ablation of graphite, carbon-arc discharge, and,
most often, by chemical vapor deposition. They can be single-walled (SWNTS) or
multiwalled (MWNTs) nanotubes and can be doped with various species.
Carbon nanotubes have exceptional strength, which makes them attractive as
a reinforcing fiber for composite materials. However, they have very low adhesion
with most materials, so that delamination with a matrix limits their reinforcing
effectiveness. Also, it is difficult to disperse the nanotubes properly, and their effec-
tiveness as a reinforcement is limited if the nanotubes are clumped. However, a few
products have used carbon nanotubes, such as a bicycle frame used in the 2006 Tour
de France and specialty baseball bats and tennis racquets. Note that the nanotubes
provide only a fraction of the reinforcement by volume or effectiveness in these
products, with graphite filling the major role.
The other material characteristic of carbon nanotubes is their very high electri-
cal current carrying capability. They can be made as semiconductors or conductors,
depending on the orientation of the graphite in the nanotube (see Fig. 8.4).
Armchair nanotubes are theoretically able to carry a current density more than 1,000
times larger than silver or copper, which makes them attractive for electrical connec-
tions in nanodevices (see Section 29.5). Carbon nanotubes have been incorporated
into polymers to improve their static electricity discharge capability, especially in
fuel lines for automotive and aerospace applications. Other proposed uses for
carbon nanotubes include storage of hydrogen for use in hydrogen-powered vehi-
cles, flat-panel displays, catalysts, and X-ray and microwave generators. Highly sen-
sitive sensors using aligned carbon nanotubes are now being developed for detecting
deadly gases, such as sarin.