Carbon nanotube yarn harvests mechanical energy ...

Researchers at The University of Texas at Dallas say they have made novel carbon nanotube yarns that convert mechanical movement into electricity more effectively than other material-based energy harvesters. Constructed much like traditional wool or cotton yarns, the new spun carbon nanotube (CNT) yarns – called "twistrons" – generate electricity when stretched or twisted.

Sewn into textiles, twistrons can sense and harvest human motion. When deployed in salt water, twistrons can harvest energy from the movement of ocean waves. And, say the researchers, twistrons can even charge supercapacitors.

"Our materials do something very unusual," says Dr. Ray Baughman, director of the Alan G. MacDiarmid NanoTech Institute at UT Dallas and the corresponding author of a study on the research. "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."

Twistrons are constructed from CNTs that 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 coil like an overtwisted rubber band. Electricity is generated by the coiled yarns by repeatedly stretching and releasing them, or by twisting and untwisting them.

With the new twistron version, say the researchers, they didn't 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," says Baughman. "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, the plied CNT yarns demonstrated an energy conversion efficiency of 17.4% for tensile (stretching) energy harvesting and 22.4% for torsional (twisting) energy harvesting. Previous versions of the researchers’ coiled twistrons reached a peak energy conversion efficiency of 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 2 hertz and 120 hertz — than previously reported for any non-twistron, material-based mechanical energy harvester," says Baughman.

The improved performance of the plied twistrons, say the researchers, results from 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.

The researchers found that constructing the yarn from three plies provided the optimal performance. Several proof-of-concept experiments were conducted using three-ply twistrons: In one demonstration the researchers 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 3.2 milligrams 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 researchers also 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, demonstrating the potential use of the fibers for sensing and harvesting human motion.

The researchers say they have applied for a patent based on the technology. For more, see "Mechanical energy harvesters with tensile efficiency of 17.4% and torsional efficiency of 22.4% based on homochirally plied carbon nanotube yarns."