Author: axg162431

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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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.

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.”

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

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.

“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.

“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.

UT Dallas nanotechnologists have invented a groundbreaking new technology for producing weavable, knittable, sewable and knottable yarns containing giant amounts of otherwise unspinnable powders.

A tiny amount of host carbon nanotube web holds guest powders in the corridors of highly conducting scrolls, without altering their performance for high-tech applications such as energy storage, energy conversion and energy harvesting.

With conventional technology, powders are either held together in a yarn using a polymer binder or incorporated on fiber surfaces. Both approaches can restrict powder concentration, powder accessibility for providing yarn functionality, or the strength needed for yarn processing into textiles and subsequent applications.

In the Jan. 7 issue of the journal Science, co-authors working in the Alan G. MacDiarmid NanoTech Institute of UT Dallas describe the use of “bi-scrolling” to solve these problems.

“In this study, we demonstrated the feasibility of using our bi-scrolled yarns for applications ranging from superconducting cables to electronic textiles, batteries and fuel cells,” said Dr. Ray H. Baughman, Robert A. Welch Professor of Chemistry and director of the UT Dallas NanoTech Institute.

Bi-scrolled yarns get their name from the way they are produced: A uniform layer of guest powder is placed on the surface of a carbon nanotube web. This two-layer stack is then twisted into a yarn.

The carbon nanotube webs used for bi-scrolling are not ordinary— they can be lighter than air and stronger pound-per-pound than steel. Four ounces of these webs would cover an acre of land and are about a thousand times thinner than a human hair.

These strong carbon nanotube webs hold together yarns that are mostly powders and can even be machine-washed. The web’s thinness means that hundreds of scroll layers can be included in a bi-scrolled yarn no thicker than a human hair.

The choice of imbedded powder determines yarn function. For instance:

• UT Dallas researchers used yarns imbedded with metal oxide powder to make high-performance lithium ion batteries that can be sewn into fabrics.
• Bi-scrolled yarns for self-cleaning fabrics were obtained using photocatalytic powder.
• A powder of nitrogen-containing carbon nanotubes provided highly catalytic yarns for chemical generation of electricity, avoiding the need for expensive platinum catalyst.
• Using other types of powders, the team made superconducting yarns for potential use in applications ranging from powerful magnets to underground electrical transmission lines.

“UT Dallas’s bi-scrolling technology is rich in application possibilities that go far beyond those we described in Science,” Baughman said. “For instance, our collaborator, professor Seon Jeong Kim of Hanyang University in Korea has already used bi-scrolled yarn to make improved biofuel cells that might eventually be used to power medical implants.”

“I am especially proud of two of our former NanoExplorer high school students, Carter Haines and Stephanie Stoughton, who are undergraduate co-authors of both our article in Science magazine and our internationally filed patent application on bi-scrolling,” Baughman added.

Other co-authors of this article are postdoctoral fellows Dr. Márcio Lima, who is lead author; Dr. Elizabeth Castillo-Martínez,  Dr. Javier Carretero-Gonzáles,  Dr. Raquel Ovalle-Robles and Dr. Jiyoung Oh; graduate students Xavier Lepró, Mohammad Haque, Neema Rawat, and Vaishnavi Aare;  laboratory associate Chihye Lewis; research professors Dr. Shaoli Fang and Dr. Mikhail Kozlov; and Dr. Anvar Zakhidov, professor of physics and associate director of the NanoTech Institute.

Funding for this research was provided by grants from the Air Force, the Air Force Office of Scientific Research, the Office of Naval Research, the National Science Foundation, and the Robert A. Welch Foundation.