Upper bound to the thermal conductivity of carbon nanotube pellets

Using atomistic Green's function calculations, we find that the phonon thermal conductivity of pellets composed of similar to mu m long carbon nanotubes has an upper bound of a few W/m K. This is in striking contrast with the extremely high thermal conductivity of individual nanotubes (similar to 3000 W/m K). We show that, at room temperature, this upper bound does not depend on the nanotube diameter. Conversely, for low temperatures, an inverse proportionality with nanotube diameter is predicted. We present concrete results as a function of nanotube length and chirality, pellet density, and temperature. These results imply that carbon nanotube pellets belong to the category of thermal insulators, contrasting with the good conducting properties of parallel nanotube arrays, or individual nanotubes.