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Novel carbon nanotube yarns that convert mechanical movement into electricity more effectively than other material-based energy harvesters have been developed at The University of Texas at Dallas.

Twistrons, made from spun CNTs, convert mechanical movement into electricity. Scanning electron microscope images show how UT Dallas researchers made a new kind of twistron by intertwining three individual strands of spun carbon nanotube fibres to make a single yarn, similar to the way conventional yarns used in textiles are constructed. A previous version of a harvester (right) was made by coiling the CNT fibres. The scale bars indicate 100 micrometres – UT Dallas

In a study published in Nature Energy, UT Dallas researchers and their collaborators describe improvements to high-tech yarns dubbed ‘twistrons,’ which generate electricity when stretched or twisted.

According to UT Dallas, twistrons sewn into textiles can sense and harvest human motion; when deployed in salt water, twistrons harvest energy from the movement of ocean waves; and twistrons can even charge supercapacitors.

First described by UTD researchers in a study published in 2017 in Science, twistrons are constructed from carbon nanotubes (CNTs) that are twist-spun into high-strength, lightweight fibres, or yarns, into which electrolytes can 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.

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In the new study, the research team intertwined three individual strands of spun carbon nanotube fibres to make a single yarn, similar to the way conventional yarns used in textiles are constructed.

“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,” said Dr. Ray Baughman, director of the Alan G. MacDiarmid NanoTech Institute at UT Dallas and the corresponding author of the study.

“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, the Robert A. Welch Distinguished Chair in Chemistry in the School of Natural Sciences and Mathematics.

In experiments with the plied CNT yarns, the researchers demonstrated an energy conversion efficiency of 17.4 per cent for tensile energy harvesting and 22.4 per cent for torsional energy harvesting. Previous versions of their coiled twistrons are said to have reached a peak energy conversion efficiency of 7.6 per cent for 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 in a statement.

According to Baughman, the improved performance of the plied twistrons 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.

“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.

The team conducted several proof-of-concept experiments using 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 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 then wrapped around a person’s elbow. Electrical signals were generated as the person repeatedly bent their elbow, demonstrating the potential use of the fibres for sensing and harvesting human motion.

The team, assisted by researchers from Xi’an Jiaotong University and Wuhan University in China, Hanyang University in South Korea, and Lintec of America Inc.’s Nano-Science & Technology Center – have applied for a patent based on the technology.

An electrochemically powered artificial muscle made from twisted carbon nanotubes contracts more when driven faster thanks to a novel conductive polymer coating. The device, which was developed by Ray Baughman of the University of Texas at Dallas in the US and an international team of collaborators, overcomes some limitations of previous artificial muscles, and could have applications in robotics, “smart” textiles and heart pumps.

Carbon nanotubes (CNTs) are rolled-up sheets of carbon with walls as thin as a single atom. When twisted together to form a yarn and placed in an electrolyte bath, these hollow carbon cylinders can be made to expand and contract in response to electrochemical inputs, much as a human or animal muscle does. In a typical setup, a voltage difference, or potential, between the yarn and a counter electrode drives ions from the electrolyte into the yarn, causing the “muscle” to actuate.

While these electrochemically driven CNT muscles are highly energy efficient and extremely strong – they can lift loads up to 100,000 times their own weight – they do have limitations. The main one is that they are bipolar, meaning that the direction of their movement switches whenever the potential drops to zero. This effect reduces the overall stroke of the actuator. Another drawback is that the muscle’s capacitance – that is, its ability to store the charge it needs to expand or contract – decreases when the potential is scanned more quickly, which also causes the stroke to decrease.

Polymer “guest”

In this study, as in their previous work, Baughman and colleagues created their artificial muscle from a “forest” of CNTs all vertically aligned in the same direction. Next, they drew a thin sheet of nanotubes from the forest and twisted it to make a yarn containing helices of intertwined CNTs. In the final step, which was unique to this series of experiments, they coated the interior surfaces of the CNTs with an ionically conducting polymer that contains either positively or negatively charged chemical groups.

The first polymer “guest” material the group studied was poly(sodium 4-styrenesulphonate), PSS. The resulting structure is known as a PSS@CNT yarn and contains around 30 percent PSS by weight. To determine the zero-charge potential of this yarn – that is, the potential at which the stroke switches direction – the researchers used a technique called piezoelectrochemical spectroscopy, which they developed themselves. They then tested their yarns in baths of either aqueous or organic electrolytes.

Bipolar to unipolar

Baughman and colleagues, who report their work in Science, found that the polymer coating converts the normally bipolar actuation of CNT yarns into unipolar actuation. In other words, the coated muscle actuates in only one direction over the entire potential range at which the electrolyte remains stable.

The team’s explanation for this unusual behaviour is that the dipolar field of the polymer shifts the yarn’s zero-charge potential to a value that lies outside the electrolyte’s stability range. This means that ions of only one polarity (positive or negative) are driven into the yarn, explains team member Zhong Wang. Hence, the muscle’s stroke changes in only one direction before the direction of voltage change reverses. Team member Jiuke Mu adds that the number of electrolyte molecules that are electro-osmotically pumped into the muscle also increases the faster the potential is changed, or scanned, across its range.READ MOREArtificial muscles go with the twist

As for the new unipolar muscles’ performance, the researchers found that the maximum average output mechanical power they generate is 2.9 W per gram of muscle. This is about 10 times the typical capability of human muscle, Mu says, and about 2.2 times the weight-normalized power capability of a turbocharged V-8 diesel engine.

Dual-electrode, all-solid-state yarn muscle

In the final stage of their research, the scientists demonstrated that they could combine two different types of unipolar yarn muscles to make a dual-electrode, all-solid-state yarn muscle, thereby dispensing with the need for a liquid electrolyte bath. Here, Wang explains that a solid-state electrolyte laterally interconnects two coiled CNT yarns containing different polymer guests – one with negatively-charged substituents, and the other with positively charged ones. The injection of positive and negative ions means that both yarns contribute to actuation during charging, Wang says. He suggests that such dual electrode unipolar muscles could, in the future, be woven together to make actuating textiles that “morph” in response to electrical stimuli.

Members of the team, which includes scientists from the University of Illinois at Urbana-Champaign, Changzhou University, Jiangsu University, the Harbin Institute of Technology, Hanyang University, Seoul National University, Deakin University, the University of Wollongong, Opus 12 and MilliporeSigma, now plan to exploit these muscles in robots and artificial limbs as well as textiles.