While individual nanotubes are capable of transmitting nearly 1,000 times more current than copper, the same tubes coalesced into a fibre using other technologies fail long before reaching that capacity.

But a series of tests at Rice showed the wet-spun carbon nanotube fibre still handily beat copper, carrying up to four times as much current as a copper wire of the same mass. That makes nanotube-based cables an ideal platform for lightweight power transmission in systems where weight is a significant factor, like aerospace applications, said the researchers. The analysis was led by Rice Professors Junichiro Kono and Matteo Pasquali.

Present-day transmission cables made of copper or aluminum are heavy because their low tensile strength requires steel-core reinforcement. Scientists working with nanoscale materials have long thought there’s a better way to move electricity from here to there. Certain types of carbon nanotubes can carry far more electricity than copper. The ideal cable would be made of long metallic “armchair” nanotubes that would transmit current over great distances with negligible loss, but such a cable is not feasible because it’s not yet possible to manufacture pure armchairs in bulk, said Pasquali.

In the meantime, the Pasquali lab has created a method to spin fibre from a mix of nanotube types that still outperforms copper. The cable developed by Pasquali and Teijin Aramid is strong and flexible even though at 20 microns wide, it’s thinner than a human hair.

Pasquali said there has been a disconnect between electrical engineers who study the current carrying capacity of conductors and materials scientists working on carbon nanotubes.

The researchers analyzed the fibre’s “current carrying capacity” (CCC), or ampacity, with a custom rig that allowed them to test it alongside metal cables of the same diametre. The cables were tested while they were suspended in the open air, in a vacuum and in nitrogen or argon environments.

Electric cables heat up because of resistance. When the current load exceeds the cable’s safe capacity, they get too hot and break. The researchers found nanotube fibre exposed to nitrogen performed best, followed by argon and open air, all of which were able to cool through convection. The same nanotube fibre in a vacuum could only cool by radiation and had the lowest CCC.

“The outcome is that these fibers have the highest CCC ever reported for any carbon-based fibre. Copper still has better resistivity by an order of magnitude, but we have the advantage that carbon fibre is light. So if you divide the CCC by the mass, we win,” said Kono.

Kono plans to further investigate and explore the fibre’s multifunctional aspects, including flexible optoelectronic device applications. Pasquali suggested the thread-like fibre are light enough to deliver power to aerial vehicles.