Despite the excellent thermal properties of individual carbon nanotubes (CNTs), the thermal characteristics of macroscopic CNT assemblies are poor. This is probably due to the presence of numerous nontrivial intertube boundaries. Currently, clarity on the inherent difference between intra- and inter-CNT thermal conductivities is not well-established. Herein, we report an in situ nanoscale observation on the anisotropic thermal transport of a single bundle of CNTs by monitoring evaporated gold nanoparticles as "thermomarkers". The experimental results indicate that even a small bundle shows colossal thermal anisotropy due to the intertube boundaries. The results are validated by finite element analysis that estimates an anisotropic thermal conductivity ratio greater than 100. The estimated value is much greater than most of the reported values measured on macroscopic specimens and matches with that of highly ordered pyrolytic graphite. Our study reveals the intrinsic thermal anisotropy of bundled CNTs and aids in visualizing nanoscale thermal transport.
Keywords: anisotropic materials; carbon nanotubes; thermal transport; van der Waals materials.