new energy-harvesting nanotube yarn generates electricity when stretched or twisted

New high-tech yarn harvests mechanical energy

A team of researchers at the University of Texas at Dallas has invented nanotube yarns that turn mechanical motion into electricity when they are stretched or twisted. As first described in a study published in 2017 in the journal Science, the yarns are constructed from carbon nanotubes, which are hollow cylinders of carbon 10,000 times smaller in diameter than a human hair. A study published on Jan. 26 in Nature Energy reported that these prior versions of yarns dubbed twistrons were highly elastic and could generate electricity by being repeatedly stretched and released or twisted and untwisted.

Since then, the team has refined the twistrons process, resulting in fibers that are more efficient and produce more electricity than the older models. In their new study, the researchers did not twist the fibers to the point of coiling. Instead, they intertwined three individual strands of spun fibers to make a single yarn, much like how traditional wool or cotton yarn is constructed but with a different twist.

Dr. Ray Baughman, director of the Alan G. MacDiarmid NanoTech Institute at UT Dallas and the corresponding author of the study explains: ‘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. This heterochiral construction provides stability against untwisting’. Baughman, the Robert A. Welch Distinguished Chair in Chemistry in the School of Natural Sciences and Mathematics adds: ‘In contrast, our highest-performance carbon-nanotube-plied twistrons have the same-handedness of twist and plying — they are homochiral rather than heterochiral,’ said Baughman.

all images by the University of Texas at Dallas

new yarns for sensing & harvesting human motion

The researchers could show through experiments that the new yarns demonstrated an energy conversion efficiency of 17.4 % for tensile (stretching) energy harvesting and 22.4 % for torsional (twisting) energy harvesting. This achieved a peak energy conversion efficiency of 7.6%. ‘These twistrons have a higher power output per harvester weight over a wide frequency range — between 2 hertz and 120 hertz — than previously reported for any non-twistron, material-based mechanical energy harvester,’ Baughman said.

Baughman mentioned that the team was able to enhance the performance of plied twistrons by introducing a lateral compression of the yarn upon stretching or twisting. This updated process enables the plies to come in contact with one another in a way that affects the electrical properties of the yarn.

‘Our materials do something very unusual,’ Baughman explained. ‘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 discovered that forming the yarn from three plies provided optimal performance. After several tests, they found out that the new yarns could serve for sensing and harvesting human motion. To show practicality, the team sewed the CNT yarns into a cotton fabric patch that was then wrapped around a person’s elbow. Electrical signals were generated as the person repeatedly bent their elbow.

the nanotubes are hollow cylinders of carbon 10,000 times smaller in diameter than human hair