Biomedical Engineering Reference
In-Depth Information
hydrocarbons
[53]
. With a similar growth mechanism, CNFs also can be synthesized by the above
methods
[54]
. But with the fast development of electrospinning, CNFs fabricated from polymer
solution electrospinning and carbonization has been preferred
[55]
. To date, therefore, CVD and
electrospinning are the most promising ones among many techniques used for the synthesis of
CNTs and CNFs, respectively.
18.4.1
Carbon nanotubes (CNTs)
CNTs were discovered as a microscopic miracle in the cathode deposits obtained in the arc evapora-
tion of graphite
[56]
. Since then, arc discharge, laser ablation, and CVD have been well developed
as the three main and modified methods to obtain good yields of both MWCNTs and SWCNTs. The
arc discharge and laser ablation employ solid-state carbon precursors to provide carbon sources
needed for nanotube growth and require high temperatures (thousands of degrees Celsius) to vapor-
ize carbon. And in the procedure, large amounts of by-products are associated
[5]
. CVD technique
utilizes hydrocarbon gases as sources for carbon atoms and metal catalyst particles as “seeds” for
nanotube growth, which can take place at relatively lower temperatures (500
1000
o
C)
[53]
. The
synthesis of CNTs by CVD generally involves heating a catalyst material in a furnace and passing a
hydrocarbon gas through the tube reactor for a period of time. The catalytic species are transition-
metal nanoparticles that serve as seeds to nucleate the growth of nanotubes. Briefly, supersaturation
occurs when carbon is dissolved in a transition metal that melts to form a carbon
iron solid-state
solution, and carbon atoms will precipitate out from the nanoparticle, leading to the growth of a
nanotube to a maximum length (typically 50
m)
[2]
. Over patterned catalyst arrays, organized
nanotube structures can be synthesized for nanotubes growing from specific sites on surfaces. The
most effective metals have been shown to be iron, nickel, and cobalt. Typically, the as-prepared
CNTs contain metal particles, metal clusters coated with carbon, amorphous carbon, and in some
cases fullerenes, with a 30 wt % abundance of CNT ropes. Thus, the pristine CNTs cannot be used
directly in biomedical applications without purification and functionalization.
μ
18.4.2
Carbon nanofibers (CNFs)
With a similar growth mechanism to CNTs, CNFs also can be synthesized by methods like vapor
growth, arc discharge, laser ablation, and CVD
[53]
. Among them, CVD is commonly used for its
lower process temperature. For example, Agiral et al.
[57]
developed nickel thin-film catalyst
coated inside a closed channel fused silica microreactor in order to grow CNFs on Ni/alumina. By
directly flowing reactant gases over a catalytic coating inside the microchannels, a mechanically
stable and porous CNF
alumina composite was formed with high surface area (160 m
2
/g). Mori
et al.
[58]
reported a catalyst-free low-temperature growth of CNFs by microwave plasma-
enhanced CVD, whose diameter was about 50
100 nm and growth rate about 5 nm/s. The maxi-
mum length of CVD produced CNFs was around tens of microns. However, these are very expen-
sive processes due to low product yield and the expensive equipment required. To produce
relatively long and continuous CNFs at low cost, the rapidly developing technology of electrospin-
ning has provided a unique opportunity
[55]
. In this technique, a polymer precursor for producing
CNFs is dissolved in organic solvent and electrospun into fibers of several hundreds of nanometers
under the application of an electrostatic force. The applied electric field and solution conductivity
