Page 22 - Carbon Nanotube Fibres and Yarns

Yarn production from carbon nanotube forests 15

before the catalyst deposition. A carrier gas, for example, helium, and a
carbon source, such as acetylene [7, 9–12] or ethylene [11, 13, 14], are used.
The diameter distribution and number of walls of the CNTs in a forest can
be controlled to some extent by varying the thicknesses of the catalyst layer
and the length of nanotubes by growth time [15]. Water vapor can be used
to prolong CNT growth resulting in longer CNTs [11].
Cui et al. [16] studied the effects of reaction conditions on the growth
rate of CNT forests (CNT length) in CVD using Fe catalyst on Si wafer,
C 2 H 2 as carbon source, and H 2 as inhibitor for carbonaceous species. They
found that the optimum reaction conditions were achieved at a temperature
of 750°C, acetylene flow rate of 60 sccm, and H 2 flow rate of 0.5 SLPM.
The CNT growth rate became very low after 10 min.
Huyhn et al. [18] carried out a recycling analysis of Si substrate for
spinnable CNT growth. A 100% regrowth in forest height and mass yield
of CNTs were achieved in the first four cycles, but these parameters fell to
about 20% in the fifth cycle. A decrease in nanotube diameter and increase
in areal density were also observed.
A floating catalyst CVD method was used for the production of spin-
nable CNT forests using a two-stage synthesis process. The CNT arrays
were grown on silica (quartz flake) using ferrocene as catalyst precursor and
cyclohexane as solvent and carbon source [19, 20]. The main advantages
of the floating catalyst method are that it requires simple equipment and
eliminates the procedure for the preparation of catalyst layers on substrates.

2.2 Drawing a CNT web
2.2.1 Formation of a continuous web from CNT forest
Several groups studied forest drawability using experimental and modeling
techniques and proposed a number of working mechanisms. Zhang et al.
[21] ascribed drawability to the intermittent bundling of CNTs within the
forest in which individual nanotubes migrate from one bundle of a few
nanotubes to another. Bundled nanotubes are simultaneously pulled from
different elevations in the forest sidewall, so that they join with bundled
nanotubes that have reached the top and bottom of the forest. Disordered
regions at the top and bottom of the forests, where a fraction of the nano-
tubes form loops, might help maintain continuity. They also reported that
for forests having similar topology, the highest forests were easiest to draw
into sheets, probably because increasing the nanotube length increases in-
terfibril mechanical coupling within the web.