Environmental Engineering Reference
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ene-acetylene mixture. The advantage of using a nebulized spray is the ease of
scaling into an industrial scale process, as the reactants are fed into the furnace
continuously [55, 56].
During nanotube synthesis, impurities in the form of catalyst particles, amor-
phous carbon and non-tubular fullerenes are also produced. Thus, subsequent pu-
rification steps are needed to separate the carbon nanotubes. The gas phase pro-
cesses tend to produce nanotubes with fewer impurities and are more amenable to
large scale processing. It is believed that the gas phase techniques, such as CVD,
for nanotube growth offer greater potential for the scaling up of nanotubes pro-
duction for applications. Initially, electric-arc discharge technique was the most
popular technique to prepare the SWNTs as well as MWNTs. In this technique,
the carbon arc provides a simple and traditional tool for generating the high tem-
peratures needed for the vaporization of carbon atoms into a plasma (>3000°C).
The gas phase growth of single walled nanotubes by using carbon monoxide as
the carbon source has already been reported. They found the highest yields of sin-
gle walled nanotubes occurred at the highest accessible temperature and pressure
(1200°C, 10 atm). They have modified this process to produce large quantities of
single walled nanotubes with remarkable purity. The lower processing tempera-
tures also enable the growth of carbon nanotubes on a wide variety of substrates.
CVD method has been successful to produce the CNTs in large quantity, and also
to obtain the vertically aligned CNTs at relatively low temperature. In particular,
growth of vertically aligned CNTs on large substrate area at low temperature, for
instance, softening temperature of the glass is an important factor for the practical
application of the electron emitters to the field emission displays. A lot of reports
on the synthesis of single walled as well as multi-walled carbon nanotubes using
the plasma enhanced chemical vapor deposition and microwave plasma enhanced
chemical vapor deposition techniques, are available in the literature. Others suc-
cessfully synthesized vertically aligned carbon nanotubes (CNTs) at 550°C on
Ni-coated Si substrate placed parallel to Pd plate as a dual catalyst and a tungsten
wire filament. The bamboo shaped carbon nanotubes can be obtained only if the
reaction temperature is higher than 1000 K, and carbon fibers can be obtained at
lower temperatures. They have also discussed the role and state of the catalyst
particles. They have found that a plug-shaped Ni particle always plunged at the
top end of a nanotube. Experimental results indicated that the catalytic particles
exist in a liquid state during the synthesis procedure. After having crystallized,
the orientations of plunged Ni particles randomly distributed around the axis. All
three metals deposited on the quartz plates are found to be efficient catalysts for
the growth of CNTs in good alignment by CVD using ethylene diamine [56-57].
The CNTs are multi-walled with a bamboo-like graphitic structure. The hol-
low compartments of the tubes are separated by the graphitic interlink layers. This
is a simple and efficient way for the production of carbon nanotubes with good
order using ethylene diamine by CVD without plasma aid. Others produced CNTs
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