  Individual carbon nanotubes (CNTs) possess extremely high thermal
conductivities. However, the thermal conductivities and their anisotropy of
macroscopic assemblies of CNTs have so far remained small. Here, we report
results of directional thermal transport measurements on a nearly-perfectly
aligned CNT film fabricated via controlled vacuum filtration. We found the
thermal conductivity to be 43 +- 2.2 W m^-1 K^-1 with a record-high thermal
anisotropy of 500. From the temperature dependence of the thermal conductivity
and its agreement with the atomistic phonon transport calculation, we conclude
that the effect of intertube thermal resistance on heat conduction in the
alignment direction is negligible because of the large contact area between
CNTs. These observations thus represent ideal unidirectional thermal transport,
i.e., the thermal conductivity of the film is determined solely by that of
individual CNTs.
