We attempt to understand the fundamental factors that determine the growth rate
of carbon nanotubes. In a series of experiments on growing multiwall carbon nanotubes
(MWNTs) by thermal chemical vapor deposition, we found that the addition of carrier
gas and the type of carrier gas can change the growth rate, growth density, and structures
of MWNTs. We explain these results based on the dissociative adsorption of C2H2 on Fe
nanoparticles and the vapor-liquid-solid (VLS) growth model. Finally, high-density,
vertically aligned MWNTs were grown when decomposition and segregation rates of
carbon were balanced.