they can return to their original state after being bent. Nanotubes have
also been found to be stable at very high temperatures, nearly 3000 K in a
vacuum and 700 K in air.
9.6.3 Methods for growing nanotubes
There are three primary methods for nanotube production: arc discharge,
laser ablation, and chemical vapor deposition (Baddour and Breins,
2005). Arc discharge uses a high-voltage arc to vaporize a carbon source.
This frees carbon, which then accumulates into nanotubes in the growth
chamber. Following accumulation the nanotubes are collected and
purified. Laser ablation is similar to the arc discharge method, but it
makes use of a high-energy laser instead of an electric current to vaporize
the carbon source. Chemical vapor deposition relies on the decomposition of a carbon-containing gas in the presence of a metal catalyst, such as
iron, cobalt, or nickel, which helps facilitate carbon accumulation and
nanotube growth. Each method has many variations with their own
strengths and weaknesses that should be considered when choosing
which to use.
9.6.3.1 Arc discharge
In the arc discharge method, two carbon rods are set up as an anode and
cathode and are placed in a closed growth chamber. The chamber is filled
with an inert buffer gas such as helium, argon, nitrogen, or hydrogen and
kept at relatively low pressures. Interestingly, the inert gas in the chamber
has been found to affect the diameter of grown nanotubes while the
pressure in the growth chamber does not. Keidar and Waas (2004) found
that when mixtures of helium and argon are used to fill the chamber, the
molar fraction of argon determines the nanotube diameter. Once the
chamber is filled with gas, a direct current is applied until a stable electric
arc is formed between the carbon rods. Once the anode reaches a critical
temperature, it vaporizes into a gaseous carbon source. The gas in the
chamber causes the carbon to deposit. The structures found in these
deposits are nanotubes, which need to be purified after collection. This
method has been used for the creation of both single-walled and
multiwalled nanotubes. In order to produce substantial amounts of
single-walled carbon nanotubes a metal catalyst needs to be added to the
electrodes.
Unfortunately, once collected, the nanotubes must be manipulated onto
substrates for further use, which can be difficult for large quantities of
CARBON NANOTUBES 373
also been found to be stable at very high temperatures, nearly 3000 K in a
vacuum and 700 K in air.
9.6.3 Methods for growing nanotubes
There are three primary methods for nanotube production: arc discharge,
laser ablation, and chemical vapor deposition (Baddour and Breins,
2005). Arc discharge uses a high-voltage arc to vaporize a carbon source.
This frees carbon, which then accumulates into nanotubes in the growth
chamber. Following accumulation the nanotubes are collected and
purified. Laser ablation is similar to the arc discharge method, but it
makes use of a high-energy laser instead of an electric current to vaporize
the carbon source. Chemical vapor deposition relies on the decomposition of a carbon-containing gas in the presence of a metal catalyst, such as
iron, cobalt, or nickel, which helps facilitate carbon accumulation and
nanotube growth. Each method has many variations with their own
strengths and weaknesses that should be considered when choosing
which to use.
9.6.3.1 Arc discharge
In the arc discharge method, two carbon rods are set up as an anode and
cathode and are placed in a closed growth chamber. The chamber is filled
with an inert buffer gas such as helium, argon, nitrogen, or hydrogen and
kept at relatively low pressures. Interestingly, the inert gas in the chamber
has been found to affect the diameter of grown nanotubes while the
pressure in the growth chamber does not. Keidar and Waas (2004) found
that when mixtures of helium and argon are used to fill the chamber, the
molar fraction of argon determines the nanotube diameter. Once the
chamber is filled with gas, a direct current is applied until a stable electric
arc is formed between the carbon rods. Once the anode reaches a critical
temperature, it vaporizes into a gaseous carbon source. The gas in the
chamber causes the carbon to deposit. The structures found in these
deposits are nanotubes, which need to be purified after collection. This
method has been used for the creation of both single-walled and
multiwalled nanotubes. In order to produce substantial amounts of
single-walled carbon nanotubes a metal catalyst needs to be added to the
electrodes.
Unfortunately, once collected, the nanotubes must be manipulated onto
substrates for further use, which can be difficult for large quantities of
CARBON NANOTUBES 373