A novel twist on carbon-nanotube yarns

These scanning electron microscope images show how UT Dallas researchers made a new kind of twistron by intertwining three individual strands of spun carbon-nanotube fibers to make a single yarn. A previous version of a harvester (right) was made by coiling the CNT fibers. The scale bars indicate 100µm. Image: UT Dallas.

Nanotechnology researchers at the University of Texas at Dallas (UT Dallas) have made novel carbon nanotube yarns that convert mechanical movement into electricity more effectively than other material-based energy harvesters.

In a paper in Nature Energy, the UT Dallas researchers and their collaborators describe improvements to high-tech yarns they invented called ‘twistrons’, which generate electricity when stretched or twisted. Their new version is constructed much like traditional wool or cotton yarns.

Twistrons sewn into textiles can sense and harvest human motion. When deployed in salt water, they can harvest energy from the movement of ocean waves. They can even charge supercapacitors.

First described in a paper in Science in 2017, twistrons are constructed from carbon nanotubes (CNTs), which are hollow cylinders of carbon 10,000 times smaller in diameter than a human hair. To make twistrons, the nanotubes are twist-spun into high-strength, lightweight fibers, or yarns, into which electrolytes can also be incorporated.

Previous versions of twistrons were highly elastic, which the researchers accomplished by introducing so much twist that the yarns coiled like an overtwisted rubber band. Electricity is generated by repeatedly stretching and releasing the coiled yarns, or by twisting and untwisting them.

In the new study, the research team did not twist the fibers to the point of coiling. Instead, they intertwined three individual strands of spun carbon-nanotube fibers to make a single yarn, similar to the way conventional yarns used in textiles are constructed – but with a different twist.

“Plied yarns used in textiles typically are made with individual strands that are twisted in one direction and then are plied together in the opposite direction to make the final yarn,” explained Ray Baughman, director of the Alan G. MacDiarmid NanoTech Institute at UT Dallas and corresponding author of the paper. “This heterochiral construction provides stability against untwisting.

“In contrast, our highest-performance carbon-nanotube-plied twistrons have the same-handedness of twist and plying – they are homochiral rather than heterochiral.”

In experiments with the plied CNT yarns, the researchers demonstrated an energy conversion efficiency of 17.4% for tensile (stretching) energy harvesting and 22.4% for torsional (twisting) energy harvesting. Previous versions of their coiled twistrons reached a peak energy conversion efficiency of just 7.6% for both tensile and torsional energy harvesting.

“These twistrons have a higher power output per harvester weight over a wide frequency range – between 2Hz and 120Hz – than previously reported for any non-twistron, material-based mechanical energy harvester,” Baughman said.

He added that the improved performance of the plied twistrons is due to the lateral compression of the yarn upon stretching or twisting. This process brings the plies in contact with one another in a way that affects the electrical properties of the yarn.

“Our materials do something very unusual,” Baughman said. “When you stretch them, instead of becoming less dense, they become more dense. This densification pushes the carbon nanotubes closer together and contributes to their energy-harvesting ability. We have a large team of theorists and experimentalists trying to understand more completely why we get such good results.”

The researchers found that constructing the yarn from three plies provided the optimal performance, conducting several proof-of-concept experiments using these three-ply twistrons. In one demonstration, they simulated the generation of electricity from ocean waves by attaching a three-ply twistron between a balloon and the bottom of an aquarium filled with salt water. They also arranged multiple-plied twistrons in an array weighing only 3.2mg and repeatedly stretched them to charge a supercapacitor, which then had enough energy to power five small light-emitting diodes, a digital watch and a digital humidity/temperature sensor.

The team also sewed the CNT yarns into a cotton fabric patch that was wrapped around a person’s elbow. Electrical signals were generated as the person repeatedly bent their elbow, demonstrating the potential use of the fibers for sensing and harvesting human motion.

This story is adapted from material from the University of Texas at Dallas, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.