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Carbon nanotube ‘twistrons’ harvest mechanical energy to generate electricity

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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

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.

Nanotube artificial muscles pick up the pace

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electrochemical yarn muscles

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.

UT Dallas researchers create fridges with a twist

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By twisting small fibers, a new device produces changes in temperature. Years from now, it could change the way we cool things down.

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From left to right, research scientist Jiuke Mu, graduate student Zhong Wang and research professor Ali Aliev, in the Nano Tech Institute lab on the campus of UT-Dallas in Richardson, Oct. 24, 2019.(Ben Torres / Special Contributor)

Dallas scientists have developed a fundamentally new approach to cooling things down — by understanding that twisting and untwisting fibers can result in temperature changes. For example, as a thin rubber strand is twisted tightly, the strand gets quite hot. As the rubber untwists, it cools. Researchers at the University of Texas at Dallas affixed fibers such as rubber to a motorized twister that applied a torsional force. The scientists demonstrated their twist on rubber, fishing lines and nickel titanium wire — the same kind of wire used in dental braces. The cooling effect was observed in all of them. The UTD team collaborated with scientists at Nankai University in China to produce the prototype of the cooling device, which they call a “twist fridge.” The findings were recently published in the journal Science. It’s the first time scientists have shown that twisting and untwisting fibers can cause cooling. In the research paper, the scientists demonstrated the effects of twist, also referred to as torsion, on a very small scale. The cooling device is a thin tube as long as a ballpoint pen. Just a few nickel titanium fibers are contained within it.

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The device can cool about one gram of water by almost 14 degrees Farenheit after one twist-untwist cycle. By comparison, today’s fridges keep their contents around 40 degrees cooler than their surroundings. The research team is working on improvements to lower temperatures even further. And in theory, scaling up the cooler could be as simple as just linking several devices together.

“We’ve already had inquiries from commercial manufacturers for refrigerators interested in our work,” says Ray Baughman, a chemistry professor at UTD and an author of the paper.

Baughman estimates that in just a few years, twist-untwist cooling may be used in special cases when conventional fridges simply can’t be made small enough to work. Their applications could include portable coolers for camping and the cooling of electronic devices such as computers and cellphones. However, it’ll likely be at least another decade before the twist fridge starts seriously competing with conventional refrigerators like those found in kitchens.

From left: Research professor Ali Aliev, research scientist Jiuke Mu and graduate student Zhong Wang operate equipment and devices that are used to transfer heat at the Nano Tech Institute lab on the campus of UTD in Richardson on Oct. 24, 2019.(Ben Torres / Special Contributor)

The UTD research finding is “definitely high-quality, that’s for sure,” says Jun Cui, a materials science professor at Iowa State University. Cui, who was not involved in the study, says previous studies had explored stretching and unstretching metallic fibers to get a similar effect. But stretching fibers poses a number of problems. First, stretching takes much more effort than twisting to produce the same cooling effect. Second, to get a reasonable cooling effect from stretching alone, there must be room to stretch rubber by four to six times its length. The UTD researchers still stretched the rubber a bit after twisting. But Baughman says that twisting the fiber reduces the extent to which it must be stretched by about a sixth. The twisting of fibers is just the latest approach to replace the current cooling method in refrigerators and air-conditioning units.

Typical refrigerators cool by evaporating a liquid into a gas to dissipate heat from inside the fridge. The gas is compressed then condensed into liquid through coils outside the fridge, causing the heat siphoned from the refrigerator to be expelled outside. Refrigeration and air conditioning, which rely primarily on this vapor-compression cooling, use a fifth of the world’s electricity. That usage is expected to grow as developing countries prosper and as the planet continues to warm, according to the International Institute of Refrigeration, or IIR. “Conventional vapor-compression, I think, has reached the end of its potential,” says Cui. “If we want anything better, more efficient or less environmentally unfriendly, we really need a new platform.”

The twist fridge already shows signs of being moderately more efficient than vapor-compression systems, says Baughman. Given the cooling industry’s immense electricity consumption, “if you can make a fridge that’s 7% more efficient than conventional fridges, what an impact it’d have,” he says. The twist fridge also cuts out conventional fridges’ most direct environmental impact: their cooling fluid. Coolants most commonly used in American refrigerators and air-conditioning units are hydrofluorocarbons — greenhouse gases that are often thousands of times more potent than carbon dioxide. Theoretically, these gases should remain sealed in the fridges’ coils, but “you have paths of the system that can leak,” says the IIR’s general director Didier Coulomb. For big systems with long paths along which the coolant must flow — such as those found in supermarkets — leaks here and there can amount to large emissions, says Didier.

Household refrigerators do tend to be leak-tight. However, these fridges may be discarded without regard for the coolants inside, and as a result, more refrigerants escape into our atmosphere. Because twist fridges work with materials like rubber, instead of coolants, their environmental impact is enticingly lower. The twist fridge is off to a promising start, but there’s a difference between proving that a twist fridge cools and proving that it’s commercially viable. “The main problem is capacity,” says Didier. “Currently, most of these applications are for very small [pieces of] equipment. The capacity is very low. You have some prototypes, but not for big systems.”

Graduate student Zhong Wang inspects equipment and devices that are used to transfer heat at the Nano Tech Institute lab on the campus of UTD in Richardson on Oct. 24, 2019.(Ben Torres / Special Contributor)

There’s also a question about how long fibers can last when they are constantly being twisted. Baughman and his team have shown only that nickel titanium fibers can survive a thousand twists. For a typical refrigerator’s lifetime of a decade or so, however, it’s likely that the fibers must be twisted and untwisted much more. Even so, Cui points out that the damage to these fibers isn’t necessarily a dealbreaker. “For my refrigerator, I change the water filter every half year. … Material damaged? So what? It’s not that expensive to just change it.” At this point, no one knows how long a twist fridge could keep running before the fibers must be switched out.

Baughman says that will be the next problem they seek to address. After all, theirs is only the first demonstration of cooling technology like this. “There are many possibilities for materials that provide even higher performance than we’ve already seen,” he explains. “This is the beginning of the story, not the end.”

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Article by Jordan Wilkerson

Dallas Morning News

January 23, 2020

Scientists Make Strong, Super-tough Carbon Sheets at Low Temperature

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Material’s Properties Exceed Those of Carbon Fiber Composites Used in Aircraft Bodies, Sports Equipment

An international research team led by scientists at Beihang University in China and The University of Texas at Dallas has developed high-strength, super-tough sheets of carbon that can be inexpensively fabricated at low temperatures.

The team made the sheets by chemically stitching together platelets of graphitic carbon, which is similar to the graphite found in the soft lead of an ordinary pencil. The fabrication process resulted in a material whose mechanical properties exceed those of carbon fiber composites currently used in commercial products.

Dr. Ray Baughman

“These sheets might eventually replace the expensive carbon fiber composites that are used for everything from aircraft andautomobile bodies to windmill blades and sports equipment,” said Dr. Ray Baughman, the Robert A. Welch Distinguished Chair in Chemistry at UT Dallas and director of the Alan G. MacDiarmid NanoTech Institute. Baughman is a corresponding author of an article describing the material published online this week in the Proceedings of the National Academy of Sciences.

Today’s carbon fiber composites are expensive in part because the carbon fibers are produced at extremely high temperatures, which can exceed 2,500 degrees Celsius (about 4,500 degrees Fahrenheit).

“In contrast, our process can use graphite that is cheaply dug from the ground and processed at temperatures below 45 degrees Celsius (113 degrees Fahrenheit),” said Dr. Qunfeng Cheng, professor of chemistry at Beihang University and a corresponding author. “The strengths of these sheets in all in-plane directions match that of plied carbon fiber composites, and they can absorb much higher mechanical energy before failing than carbon fiber composites.”

‘Mother-of-Pearl’

Graphite consists of platelets made up of stacked layers of graphene. Graphene is simply a single layer of carbon atoms, arranged in a pattern that looks like a chicken wire mesh fence, where each hexagon in the mesh is formed by six carbon atoms.

“While scientists can continuously make large sheets of graphene by high-temperature processing, and have shown these sheets to have remarkable strength, it is impractical to make thick plates of graphite by merely stacking large-area graphene sheets,” Cheng said. “One would need to stack about 150,000 graphene sheets to make a graphite sheet having about the thickness of a human hair.”

The researchers found inspiration in natural nacre, also known as mother-of-pearl, which gives some seashells their strength and toughness. Nacre is composed of parallel platelets that are bound together by thin layers of organic material, similar to the way bricks in a wall are held together by mortar.

“Instead of mechanically stacking large-area graphene sheets, we oxidize micron-size graphite platelets so that they can be dispersed in water, and then filter this dispersion to inexpensively make sheets of oriented graphene oxide,” Baughman said. “This process is akin to handmaking sheets of paper by filtering a slurry of fibers.

This false-color, scanning electron microscope image shows the fractured surface of a sequentially bonded graphene sheet developed by scientists at UT Dallas and Beihang University. (Image courtesy of Beihang University)

“At this stage, the sheets are neither strong nor tough, meaning they cannot absorb much energy before rupturing,” he said. “The trick we use is to stitch together the platelets in these sheets using sequentially infiltrated bridging agents that interconnect overlapping neighboring platelets, and convert the oxidized graphene oxide to graphene. The key to this advance is that our bridging agents separately act via formation of covalent chemical bonds and van der Waals bonds.”

Sheets that incorporated the bridging agents were 4.5 times stronger and 7.9 times tougher than agent-free sheets, said Beihang University PhD student Sijie Wan, who is a lead author of the journal article. “Unlike carbon fiber composites, no polymer matrix is needed,” he said.

“While sheets of expensive carbon fiber composites can provide a similar strength in all sheet-plane directions, the energy that they can absorb before fracture is about one-third that of our sequentially bridged graphene sheets,” Wan said. “Because our sheets are fabricated at low temperatures, they are low cost. In addition to exhibiting high sheet strength, toughness and fatigue resistance, they have high electrical conductivity and are able to shield against electromagnetic radiation. These properties make these sequentially bridged graphene sheets quite attractive for possible future applications.”

Other team members from the NanoTech Institute at UT Dallas are Dr. Ali Aliev and Dr. Shaoli Fang, both research professors, and Dr. Jiuke Mu, a co-author of the study and a postdoctoral researcher.

Additional Beihang University members are Dr. Lei Jiang, an academician of the Chinese Academy of Sciences and a foreign member of the U.S. National Academy of Engineering; and Yuchen Li, an undergraduate student who is also a co-author of the study. Dr. Nicholas Kotov, the Joseph B. and Florence V. Cejka Professor of Chemical Engineering at the University of Michigan in Ann Arbor, also contributed.

Support for the U.S. researchers was from the Air Force Office of Scientific Research and the National Science Foundation.

NanoExplorers Program for High-Schoolers Lauded with Award

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A UT Dallas program aimed at igniting interest in the sciences among high school students earned a Tech Titan award from the Metroplex Technology Business Council (MTBC).

The George A. Jeffrey NanoExplorers program at UT Dallas was awarded the annual Tech Titan of the Future award from the council.

Dr. Ray Baughman, director of the Alan G. MacDiarmid NanoTechnology Institute at the University, founded the NanoExplorers program in 2002 as a way to bring in promising pre-collegiate students to conduct lab work during the summer.

The program inspires high school students to become productive scientists and engineers by helping them to do original research work.

Once accepted into the program, students join faculty members’ research labs based on their interests. They are taught the skills needed to test their ideas and work with others, and are encouraged to play with new concepts and invent. More than 160 students have gone through the program.

“This award is a meaningful and profound recognition of our efforts to interest young people in science.”
Dr. Ray Baughman

“Many individuals at UT Dallas have helped make NanoExplorers successful over the years, and we are very grateful for their support,” Baughman said.

The NanoExplorers’ namesake, Dr. George A. Jeffrey, was a professor at the University of Pittsburgh, who gave a high school-aged Baughman the chance to study in a lab. The UT Dallas program promotes nanotechnology-based education for the next generation of scientists and is funded by the Robert A. Welch Chair grant that Dr. Baughman received in chemistry.

The Tech Titan Award-University Level recognizes higher education institutions that encourage students to choose engineering and technology-related disciplines. The Metroplex Technology Business Council is the largest technology trade non-profit organization in Texas.

The University of Texas at Dallas