Page 47 - Carbon Nanotube Fibres and Yarns

40 Carbon Nanotube Fibers and Yarns

more amorphous carbon. The hydrogen could reduce the iron oxide, and it
also participates in the thermal decomposition of carbonaceous gas. Higher
carrier gas flow suppresses the formation of impurities and produces CNTs
with better purity; it also dilutes the catalyst, thus promoting SWNT for-
mation [12, 13, 22]. However higher carrier gas flow rate makes direct spin-
ning of the sock more difficult [14].
Growth promoters such as sulfur are regularly used in the floating
catalyst synthesis method, which can be introduced in the form of thio-
phene, CS 2 , or pure sulfur. Sulfur influences the reaction kinetics such as
the catalyst coagulation and carbon diffusion, which could increase the
yield of CNTs, and control the wall number of CNTs. It is generally ac-
cepted that stable CNT synthesis and sock formation will not be possible
without a promoter [11, 21, 29, 43], where only CNTs with substantial
impurity clusters form. However, there was one report that Paukner et al.
had successfully synthesized SWNTs, achieving CNT sock and fiber pro-
duction without any promoter [6]. From a post-synthesis characterization,
the FCC (face-centered cubic) iron catalyst core is covered with Fe 1-x S
shell [29, 44]. Based on thermodynamic calculation, it is suggested at high
temperature (1250°C), the sulfur first dissolves into Fe and later it is re-
jected upon the formation of an FCC iron, producing a liquid Fe-S shell
on FCC iron core.
Regarding the role of sulfur in carbon diffusion, Windle’s group sug-
gested that the sulfur coating on the iron catalyst encourages accelerated
carbon surface diffusion to aid the fast growth of long CNTs [6, 45, 46].
Without sulfur most catalyst particles will be encapsulated with carbon and
little CNTs will be synthesized. The sulfur may contribute to lowering the
surface tension of the molten Fe [29]. It is also suggested that the sulfur on
the surface of liquid catalyst could limit the carbon solubility and restrict
it to the surface, in other words, restricting bulk diffusion and promoting
surface diffusion. This stabilizes the edge of the nascent CNT, and later con-
tributes to extrude the graphitic layer due to its high interfacial energy with
the Fe-S-C liquid alloy [29, 44]. However, it has been proposed that a trace
amount of sulfur could reduce the surface tension of the liquid catalyst and
favor higher carbon solubility [47].
It is well accepted that higher sulfur concentration leads to the for-
mation of CNTs with a larger wall number [7, 8, 11, 48, 49]. For exam-
ple, with varying thiophene concentration of low (~0.1–0.2 wt%), medium
(~0.8 wt%), and high (~1.5 wt%), SWNT, DWNT, and MWNT can be
synthesized respectively [8, 48]. The transition from MWCT to SWNT is