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Researchers Improve Twistron, an Energy-Harvesting Yarn

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Researchers from the University of Texas at Dallas recently improved their energy-harvester called twistrons, which are made of carbon nanotubes and generate electricity through repetitive stretching. The twistrons yarns were also sewed into a glove, enabling the wearer to produce electricity just by forming letters and phrasings with their hand in American Sign Language. This technology has potential applications for powering sensors, harvesting energy from ocean waves, powering wearables via body movements, or helping power cities in the future.

“Based on the output voltage profiles, we can easily differentiate the finger movement of different letters and phrases, and we can potentially use this glove as a self-powered sign language translator,” explains Zhong Wang, lead author of the paper.

The team initially reported the twistron technology in 2017, improving the yarn-making processes. This eventually led to more efficient fibers capable of generating more electricity per stretch than the original version.

“If you have a humanoid robot and you want to know which muscles have contracted and if they’re operating correctly, you could incorporate very fine fibers of our twistron harvesters so that when the muscle changes dimensions, it stretches the twistron, which generates electricity,” said. Dr. Ray Baughman, director of the NanoTech Institute and the Robert A. Welch Distinguished Chair in Chemistry. “That electricity can be measured, which can tell you how much that muscle has changed dimension.”

The carbon nanotubes in the twistrons are twist-spun into ultra-strong, lightweight yarns. Also, the team made the yarns extremely elastic by adding a sufficient twist, allowing the yarns to coil like an overtwisted rubber band. “The basic mechanism of these twistrons is that when you stretch them, the bundles of individual carbon nanotubes come in contact with one another, increasing the density of electrons in the material, which increases the voltage output,” Wang said. “Based on this understanding, we found that optimizing the nanotube alignment — the amount of surface area where they interact — can dramatically increase the capacitance change and dramatically increase the voltage output.”

Additionally, the researchers added graphene to the fabrication process. “We start by pulling a sheet of carbon nanotubes from a vertically aligned array of nanotubes, called a forest,” Wang said. “In these new experiments, we added a step: We deposited graphene onto that sheet and then twisted and coiled it all together into yarns. This dramatically improved the capacitance change and the amount of electricity we can harvest from the resulting twistrons.”

When stretched 30 times per second, the twistron generates 3.19 kilowatts per kilogram of peak electrical power. That’s a twelvefold increase over the highest values from other mechanical energy harvesters from frequencies ranging from 0.1 Hz to 600 Hz. The most recent twtstron achieved 7.2 times the conversion efficiency of initial twistrons.

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