Individual single-walled carbon nanotubes (SWCNTs) have thermal conductivity that is several times higher than copper. However, once researchers start to assemble SWCNTs into macroscale composites, fibers or films, they notice that thermal conduction drops significantly.

"One of the most critical causes may be the thermal transport at boundaries between SWCNTs," says Dr. Hiromu Hamasaki, a researcher in the Department of Mechanical Engineering at Osaka University. "Nontrivial intertube boundaries – e.g., spacing, surface contaminations, and contact angles – occur in macroscopic assemblies, and therefore, the inherent difference between intra- and inter-SWCNT thermal conductivities remains unclear."

Additionally, the effects of boundaries are not simple because the structure of the boundaries are not simple and uniform.

A recent paper in Nano Letters, ("Visualization of Thermal Transport within and between Carbon Nanotubes"), first-authored by Hamasaki, reports the experimental investigation of the anisotropic thermal transport in a bundle of SWCNTs via in situ transmission electron microscopy (TEM).

In their work,  the team, led by Prof. Kaori Hirahara, visualized a colossal anisotropy of thermal conductivity of nanoscale materials. In their experiments, they focused on structured bundles of carbon nanotubes that they decorated with gold nanoparticles. These nanoparticles on the bundle surface serve as temperature markers.

In their experiments, they focused on structured bundles of carbon nanotubes that they decorated with gold nanoparticles. These nanoparticles on the bundle surface serve as temperature markers.

Experimental setup for the in situ observations of thermal transport on a bundle of SWCNTs. (a) Schematic of the experimental setup. An electrode probe tip is brought in contact with the middle-side of the bundle projecting from the edge of conductive substrate by controlling the manipulator. Joule heat is generated by electric current flowing through the circuit, which is monitored by a digital multimeter. Gold nanoparticles are deposited on the bundle for visualizing the thermal transport by monitoring the evaporation of individual nanoparticles when the bundle is partially heated. (b) Transmission electron microscopy images of a typical bundle when in contact with the electrode probe tip. Small black particles on the bundle represent the gold nanoparticles. The average diameter of an individual SWCNT is 1.4 nm.

Reprinted with permission by American Chemical Society

By increasing the heat flow through the nanotube bundle, the scientists observed the evaporation of individual gold nanoparticles, thereby visualizing the temperature distribution on the bundle.

"Our experiments revealed that even a simple structured bundle shows colossal thermal anisotropy due to the internanotube boundaries," Hamasaki points out. "We are now working to understand the properties of more general boundaries of carbon nanotubes, not only the ideal boundaries of the bundles."

The team says that their methodology to visualize nanoscale thermal transport is not limited to carbon nanotubes.

By Michael Berger – Michael is author of three books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology, Nanotechnology: The Future is Tiny, and Nanoengineering: The Skills and Tools Making Technology Invisible.

NW. Posted: April 06, 2021.

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