Biomedical Engineering Reference
In-Depth Information
metal catalysts into the electrodes, such as Fe, Co, Ni, Cu, Ag, Al, Pd, Pt or
a combination thereof, produces SWCNTs (Daenen et al., 2003), which
deposit on the walls of the reaction chamber. SWCNTs of diameter 1-5 nm
and length of up to 1
m are produced as well as MWCNTs with large
variation in their length, and outer and inner diameter (Dresselhaus, 2001).
These CNTs are highly graphitized with few structural defects, and possess
good electrical, thermal and mechanical properties. The yield, however, is
low due to the presence of other graphitic products, particularly
nanoparticles (Ajayan et al., 1997, Gamaly and Ebbesen, 1995).
μ
Laser ablation or evaporation
The laser ablation or evaporation process to produce CNTs was first
introduced in 1995 (Guo et al., 1995). This method is quite similar to electric
arc discharge except using a different heating mechanism, i.e. a high power
laser to evaporate graphite (Terrones, 2003) and, usually, a more controlled
heating regime. The flow of an inert gas such as argon or helium drives the
vaporized carbon atoms away from the high-temperature zone on a cold
copper collector, where CNTs deposit (Poole and Owens, 2003). MWCNTs
can be synthesized by this technique (Meyyappan, 2005), but it is more often
applied for the synthesis of SWCNTs after impregnating the graphite with
transition metal catalysts (1-2%) (Terrones, 2003). The processing
parameters, such as the type of inert gas, hydrocarbon source, flow of
inert gas, intensity of laser and the furnace temperature, influence the
morphology and properties of the CNTs produced. The use of a high-power
laser and high-purity graphite makes this technique very expensive
(Terrones, 2003) but it produces good-quality SWCNTs at higher yields
(
>
70%), with good diameter control.
Chemical vapor deposition or catalytic growth process
Chemical vapor deposition has been known for a long time as a means to
deposit carbon nanofibrils (Radushkevich and Lukyanovich, 1952); how-
ever, from 1998, the method began to be optimized for the synthesis of
CNTs (Kong et al., 1998, Ren et al., 1998). In this technique, a source of
carbon (usually a hydrocarbon or CO) is heated inside a quartz tube at an
intermediate temperature range (500-1100
C) in the presence of a catalyst
under an inert atmosphere of argon or helium gas. Hydrocarbon molecules
catalytically decompose into hydrogen and carbon; subsequently, carbon
atoms rearrange themselves into hexagonal networks on the metal catalyst
to grow CNTs (Dresselhaus, 2001). The hydrocarbon source may be a solid
(camphor, naphthalene), a liquid (benzene, alcohol, hexane) or a gas
(acetylene, methane, ethylene); metal catalysts including Fe, Co, Ni, Mo and
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