Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

12j 1 Introduction
nanoparticles on the substrate. Hydrocarbon gas is then introduced into the quartz
tube reactor [48, 53].
According to the VLS model, an initial step in the CVD process involves catalytic
decomposition of hydrocarbon molecules on metal nanoparticles. Carbon atoms
then diffuse through the metal particles, forming a solid solution. When the solution
becomes supersaturated, carbon precipitates on the surface of particles and grows
into CNTs. Growth can occur either below or above the metal catalyst as carbon is
precipitated from supersaturated solid solution. Both base and tip growth mechanisms
have been suggested. For a strong catalyst–substrate interaction, a CNT grows
up with the catalyst particle pinned at its base, favoring a base-growth phenomenon
[56, 57]. In the case of a tip growth mechanism, the catalyst–substrate interaction is
weak, hence the catalyst particle is lifted up by the growing nanotube such that the
particle is eventually encapsulated at the tip of a nanotube [58–60]. Figure 1.9(a) and (b)
Figure 1.9 TEM images of CNTs synthesized using the catalyst
with 10% nickel at 450 C [(a) and (b)], and at 650 C [(c) and (d)].
Reproduced with permission from [59]. Copyright Ó (2008)
Elsevier.