Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

show TEM images of MWNTs synthesized from thermal CVD over Ni/Al catalyst
at 450 C [59]. Long nanotubes having a diameter of about 15 nm exhibit typical
coiled, spaghetti-like morphology. Some metal catalysts appear to locate at the tip of
nanotubes. TEM image of MWNTs produced at 650 C clearly shows an interlayer
spacing between graphitic sheets of 0.34 nm (Figure 1.9(c)). A metal nanoparticle can
be readily seen at the tip of the tube formed at 650 C, thus indicating that a tip growth
mechanism prevails in this case (Figure 1.9(d)).
Despite the fact that the thermal CVD method produces tangled and coiled
nanotubes, this technique is capable of forming aligned arrays of CNTs by precisely
adjusting the reaction parameters [53, 56–58, 61–63]. For practical engineering
applications in the areas of field emission display and nanofillers for composites,
formation of vertically aligned nanotubes is highly desirable. To achieve this, a
gaseous precursor is normally diluted with hydrogen-rich gas, that is, NH3 or H2.
Lee et al. reported that NH3 is essential for the formation of aligned nanotubes [61].
Choi et al. [62] demonstrated that the ammonia gas plays the role of etching
amorphous carbon during the earlier stage of nucleation of nanotubes. Both
synthesis by C2H2 after ammonia pretreatment, and synthesis by NH3/C2H2 gas
mixture without ammonia treatment favor alignment of nanotubes. Figure 1.10(a)
shows the morphology of nanotubes synthesized by flowing C2H2 on a Ni film
at 800 C without ammonia pretreatment. Apparently, coiled nanotubes are produced
without NH3 treatment. However, well-aligned nanotubes can be formed
on the Ni film with ammonia pretreatment prior to the introduction of C2H2 gas
(Figure 1.10(b)).
Figure 1.10 Surface morphology of CNTs synthesized by C2H2
gas mixture on 20 nm thick Ni films: (a) without and (b) with
ammonia treatments. Reproduced with permission from [62].
Copyright Ó (2001) Elsevier.
1.3 Synthesis of Carbon Nanotubesj13