Page 46 - Carbon Nanotube Fibres and Yarns

Carbon nanotube fibers spun directly from furnace 39

that the iron concentration affects the type and diameter of CNTs, where
as a lower concentration leads to dominant SWNT, and a higher concentra-
tion to MWNTs [15, 22, 23]. However in one study, Reguero et al. found
that varying Fe concentration in the feedstock did not significantly affect
the type of CNTs [8]. Any extra iron stays as redundant catalyst attached on
the CNT bundles.
Alcohols are widely used as carbon source in floating catalyst method.
Ethanol [12, 13, 24–27] is the most popular reactant although other alco-
hols have also been used, such as methanol [20, 28] and butanol [8, 29, 30].
The small amount of oxygen from OH group in alcohols could remove the
amorphous carbon and activate the catalyst by preventing carbon encapsu-
lation, thus improving the product purity [12, 13]. Different alcohols behave
differently in the reaction, possibly because of the different bonding energy
which provides a varying amount of carbon [13]. It was found that ethanol
produces high-quality SWNTs. High alkyl alcohols like isopropanol and
hexanol produce more amorphous carbon, whereas methanol alone could
not produce any carbon. The combination of alcohol and sulfur leads to
dominant collapsed DWNTs [21], which may be due to the oxygen pro-
hibiting the formation of small diameter tubes. Mas et al. used butanol as
the carbon source, which generated more uniform precursor injection into
reactor owing to their hardware setup [29].
Non-alcohol carbon sources are also used, including methane [11, 31–35],
carbon monoxide [36], acetone [37], toluene [38], and decahydronaphtha-
lene [39]. Methane gas is widely used because it is quite stable and de-
composes at relatively high temperature. In an early study, benzene, hexane,
and mesitylene were found not to be able to synthesize enough CNTs to
form a sock, instead thicker fibers and impurity particles emerged [12]. These
hydrocarbons have to be mixed with another oxygen-containing molecule
such as alcohol to form a CNT sock. Later studies showed that toluene could
be used successfully [6, 38, 40, 41]. Toluene decomposes at a lower tempera-
ture than methane. Thus the feedstock injection rate had to be reduced by a
factor of 20 to allow the formation of a sock [11]. Toluene was also observed
to produce longer CNTs due to a more efficient thermal decomposition
than butanol [29].
Typical carrier gases in floating catalyst method include hydrogen, he-
lium, argon, and nitrogen. Occasionally a combination of two gases is used
[11]. Hydrogen is generally used in gas phase pyrolysis method [13, 24, 26,
30, 37], which increases the yield significantly compared with nitrogen or
argon [42]. It is also observed that replacing hydrogen with argon produces