In the new study led by Rice graduate students Lauren Taylor and Oliver Dewey, the researchers noted that wet-spun carbon nanotube fibres, which could lead to breakthroughs in a host of medical and materials applications, have doubled in strength and conductivity every three years, a trend that spans almost two decades.

While that may never mimic Moore’s Law, which set a benchmark for computer chip advances for decades, Pasquali and his team are doing their part to advance the method they pioneered to make carbon nanotube fibres.

The lab’s threadlike fibres, with tens of millions of nanotubes in cross-section, are being studied for use as bridges to repair damaged hearts, as electrical interfaces with the brain, for use in cochlear implants, as flexible antennas and for automotive and aerospace applications.

They are also part of the Carbon Hub, a multi-university research initiative launched in 2019 by Rice with support from Shell, Prysmian and Mitsubishi to create a zero-emissions future.

“Carbon nanotube fibres have long been touted for their potential superior properties,” Pasquali said. “Two decades of research at Rice and elsewhere have made this potential a reality. Now we need a worldwide effort to increase production efficiency so these materials could be made with zero carbon dioxide emissions and potentially with concurrent production of clean hydrogen.”

“The goal of this paper is to put forth the record properties of the fibres produced in our lab,” Taylor said. “These improvements mean we’re now surpassing Kevlar in terms of strength, which for us is a really big achievement. With just another doubling, we would surpass the strongest fibres on the market.”

The flexible Rice fibres have a tensile strength of 4.2 gigapascals (GPa), compared to 3.6 GPa for Kevlar fibres. The fibres require long nanotubes with high crystallinity; that is, regular arrays of carbon-atom rings with few defects. The acidic solution used in the Rice process also helps reduce impurities that can interfere with fibre strength and enhance the nanotubes’ metallic properties through residual doping, Dewey said.

“The length, or aspect ratio, of the nanotubes, is the defining characteristic that drives the properties in our fibres,” he said, noting the surface area of the 12-micrometre nanotubes used in Rice fibre facilitates better van der Waals bonds. “It also helps that the collaborators who grow our nanotubes optimise for solution processing by controlling the number of metallic impurities from the catalyst and what we call amorphous carbon impurities.”

The researchers said the fibres’ conductivity has improved to 10.9 megasiemens (million siemens) per meter. “This is the first time a carbon nanotube fibre has passed the 10 megasiemens threshold, so we’ve achieved a new order of magnitude for nanotube fibres,” Dewey said. Normalised for weight, he said the Rice fibres achieve about 80% of the conductivity of copper.

“But we’re surpassing platinum wire, which is a big achievement for us,” Taylor said, “and the fibre thermal conductivity is better than any metal and any synthetic fibres, except for pitch graphite fibres.”

The lab’s goal is to make the production of superior fibres efficient and inexpensive enough to be incorporated by industry on a large scale, Dewey said. Solution processing is common in the production of other kinds of fibres, including Kevlar, so factories could use familiar processes without major retooling.

“The benefit of our method is that it’s essentially plug-and-play,” he said. “It’s inherently scalable and fits in with the way synthetic fibre are already made.”

“There’s a notion that carbon nanotubes are never going to be able to obtain all the properties that people have been hyping now for decades,” Taylor said. “But we’re making good gains year over year. It’s not easy, but we still do believe this technology is going to change the world.”

Co-authors of the paper are Rice alumnus Robert Headrick; graduate students Natsumi Komatsu and Nicolas Marquez Peraca; Geoff Wehmeyer, an assistant professor of mechanical engineering; and Junichiro Kono, the Karl F. Hasselmann Professor in Engineering and a professor of electrical and computer engineering, of physics and astronomy, and of materials science and nanoengineering. Pasquali is the A.J. Hartsook Professor of Chemical and Biomolecular Engineering, of chemistry and of materials science and nanoengineering.

The U.S. Air Force Office of Scientific Research, the Robert A. Welch Foundation, the Department of Energy’s Advanced Manufacturing Office and the Advanced Research Projects Agency-Energy supported the research.

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Flash graphene is made in 10 milliseconds by heating carbon-containing materials to 3,000 Kelvin (about 5,000 degrees Fahrenheit). The source material can be nearly anything with carbon content. Food waste, plastic waste, petroleum coke, coal, wood clippings and biochar are prime candidates.

A concentration of as little as 0.1% of flash graphene in the cement used to bind concrete could lessen its massive environmental impact by a third. Production of cement reportedly emits as much as 8% of human-made carbon dioxide every year.

This is a big deal, the world throws out 30% to 40% of all food, because it goes bad, and plastic waste is of worldwide concern. We’ve already proven that any solid carbon-based matter, including mixed plastic waste and rubber tires, can be turned into graphene.

By strengthening concrete with graphene, less concrete needs to be used for building, and it would cost less to manufacture and less to transport by trapping greenhouse gases like carbon dioxide and methane that waste food would have emitted in landfills. Those carbons are being converted into graphene and added to concrete, thereby lowering the amount of carbon dioxide generated in concrete manufacture.

Turning trash to treasure is key to the circular economy, graphene acts both as a 2D template and a reinforcing agent that controls cement hydration and subsequent strength development.

In the past graphene has been too expensive to use in these applications however, manufacturing graphene using the flash process will greatly lessen the price while it helps improve waste management.

The process aligns with Rice University’s recently announced Carbon Hub initiative to create a zero-emissions future that repurposes hydrocarbons from oil and gas to generate hydrogen gas and solid carbon with zero emission of carbon dioxide. The flash graphene process can convert that solid carbon into graphene for concrete, asphalt, buildings, cars, clothing and more.

Flash Joule heating for bulk graphene, developed in the Tour lab by Rice graduate student and lead author Duy Luong, improves upon techniques like exfoliation from graphite and chemical vapour deposition on a metal foil that require much more effort and cost to produce just a little graphene.

The process produces turbostratic graphene, with misaligned layers that are easy to separate. A-B stacked graphene from other processes, like exfoliation of graphite, is very hard to pull apart. The layers adhere strongly together but turbostratic graphene is much easier to work with because the adhesion between layers is much lower. They just come apart in solution or upon blending in composites.

The lab noted that used coffee grounds transformed into pristine single-layer sheets of graphene. Bulk composites of graphene with plastic, metals, plywood, concrete and other building materials would be a major market for flash graphene, according to the researchers, who are already testing graphene-enhanced concrete and plastic.

The flash process happens in a custom-designed reactor that heats material quickly and emits all noncarbon elements as gas. When this process is industrialised, elements like oxygen and nitrogen that exit the flash reactor can all be trapped as small molecules because they have value.

The flash process produces very little excess heat, channelling almost all of its energy into the target. You can put your finger on the container a few seconds afterwards because with the flash process, the heat is concentrated in the carbon material and not in a surrounding reactor.

All the excess energy comes out as light, in a very bright flash, and because there aren’t any solvents, it’s a super clean process

The researchers hope to produce a kilogram (2.2 pounds) a day of flash graphene within two years, starting with a project recently funded by the Department of Energy to convert U.S.-sourced coal. This could provide an outlet for coal in large scale by converting it inexpensively into a much-higher-value building material.

A grant from the Department of Energy has been awarded to scale up the flash graphene process which will be co-funded by the start-up company, Universal Matter Ltd.

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New experiments show that the biocompatible fibres are ideal candidates for small, safe electrodes that interact with the brain’s neuronal system, and could replace much larger electrodes currently used in devices for deep brain stimulation therapies in Parkinson’s disease patients.

They could also advance technologies to restore sensory or motor functions and brain-machine interfaces as well as deep brain stimulation therapies for other neurological disorders, including dystonia and depression, the researchers wrote.

The fibres are made from bundles of long nanotubes originally intended for aerospace applications where strength, weight and conductivity are paramount. The individual nanotubes measure only a few nanometers across, but when millions are bundled in a process called wet spinning, they become thread-like fibres about a quarter the width of a human hair.

Matteo Pasquali said on the creation of the fibres;

We developed these fibres as high-strength, high-conductivity materials. Yet, once we had them in our hand, we realised that they had an unexpected property: They are really soft, much like a thread of silk. Their unique combination of strength, conductivity and softness makes them ideal for interfacing with the electrical function of the human body.

Intensive testing on cells and then in rats with Parkinson’s symptoms proved the fibres are stable and as efficient as commercial platinum electrodes at only a fraction of the size. The soft fibres caused little inflammation, which helped maintain strong electrical connections to neurons by preventing the body’s defences from scarring and encapsulating the site of the injury.

Doctors who implant deep brain stimulation devices start with a recording probe able to “listen” to neurons that emit characteristic signals depending on their functions, once a surgeon finds the right spot, the probe is removed and the stimulating electrode gently inserted. Rice carbon nanotube fibres that send and receive signals would simplify implantation.

The fibres could one day lead to self-regulating therapeutic devices for Parkinson’s and other patients. Current devices include an implant that sends electrical signals to the brain to calm the tremors that afflict Parkinson’s patients.

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These computer-drawn maps are “dimensionless” and their findings will work as well for materials built with nanoscale blocks as they would for a brick wall, or bigger.

[padding type=”small_left”][quote_colored name=”” icon_quote=”no”]That’s the beauty of this approach: It can scale to something very large or very small.[/quote_colored][/padding]

The research which appeared this week in Nature Communications relies on four characteristics of the individual materials under consideration for a composite: their length, a ratio based on their respective stiffness, their plasticity and how they overlap.

If you know these characteristics, you can predict the stiffness, strength and toughness of the final composite. They call this a universal map because all of those input parameters are relevant to all composites and their structural properties.

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To materials scientists and engineers, stiffness, toughness and strength are distinct, important mechanical properties. Strength is the ability of a material to stay together when stretched or compressed, stiffness is how well a material resists deformation and toughness is the ability of a material to absorb energy before failure.

The team designed maps to show how materials rate in all three categories and where they overlap. Their goal is to help engineers calculate the ultimate qualities of a material and cut down on trial and error.

The study began when Shahsavari took a close look at the architecture of mother of pearl (nacre), which maximises both strength and toughness. Under a microscope, it looks like a well-built brick wall with overlapping platelets of different lengths held together by thin layers of an elastic biopolymer.

[padding type=”small_left”][quote_colored name=”” icon_quote=”no”]It has a very particular structure and property: It optimises different mechanical properties at the same time.[/quote_colored][/padding]

However, engineering nacre-like composites has been difficult so far, mainly because of the lack of a design map that can reveal the various links between the structure, materials and properties of nacre-like materials. The work is an important milestone toward a better ability to decode and replicate nacre’s architecture for lightweight, high-performance composites which could benefit the aerospace, auto and construction industries in the future.

The work has spanned three years of calculation and experimentation that involved mapping the properties of natural composites like collagen and spider silk as well as synthetic stacks like hexagonal boron-nitride/graphene and silumin/alumina. They also tested their theory on macro-scale, 3-D printed composites of hard plastic and soft rubber that mimicked the properties they observed in nacre.

A map of 15 of the materials they tested shows natural ones like nacre tend to be strong and tough while synthetics lean toward strong and stiff. Shahsavari said he hopes materials scientists will use the design maps to give their composites the best possible combination of all three properties.

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The lab of chemist James Tour at Rice University has enhanced a polymer material to make it far more impermeable to pressurised gas and far lighter than the metal in tanks now used to contain the gas. The combination could be a boon for an auto industry under pressure to market consumer cars that use cheaper natural gas. It could also find a market in food and beverage packaging.

Tour and his colleagues at Rice and in Hungary, Slovenia and India reported their results in the online edition of the American Chemistry Society journal ACS Nano.

By adding modified, single-atom-thick graphene nanoribbons (GNRs) to thermoplastic polyurethane (TPU), the Rice lab made it 1,000 times harder for gas molecules to escape, Tour said. That’s due to the ribbons’ even dispersion through the material. Because gas molecules cannot penetrate GNRs, they are faced with a “tortuous path” to freedom, he said. The researchers acknowledged that a solid, two-dimensional sheet of graphene might be the perfect barrier to gas, but the production of graphene in such bulk quantities is not yet practical, Tour said.

But graphene nanoribbons are already there. Tour’s breakthrough “unzipping” technique for turning multiwalled carbon nanotubes into GNRs, first revealed in Nature in 2009, has been licensed for industrial production. “These are being produced in bulk, which should also make containers cheaper,” he said.

The researchers led by Rice graduate student Changsheng Xiang produced thin films of the composite material by solution casting GNRs treated with hexadecane and TPU, a block copolymer of polyurethane that combines hard and soft materials. The tiny amount of treated GNRs accounted for no more than 0.5 percent of the composite’s weight. But the overlapping 200- to 300-nanometer-wide ribbons dispersed so well that they were nearly as effective as large-sheet graphene in containing gas molecules. The GNRs’ geometry makes them far better than graphene sheets for processing into composites, Tour said.

They tested GNR/TPU films by putting pressurised nitrogen on one side and a vacuum on the other side. For films with no GNRs, the pressure dropped to zero in about 100 seconds as nitrogen escaped into the vacuum chamber. With GNRs at 0.5 percent, the pressure didn’t budge over 1,000 seconds, and it dropped only slightly over more than 18 hours.

Stress and strain tests also found that the 0.5 percent ratio was optimal for enhancing the polymer’s strength.

The idea is to increase the toughness of the tank and make it impermeable to gas his becomes increasingly important as automakers think about powering cars with natural gas. Metal tanks that can handle natural gas under pressure are often much heavier than the automakers would like.

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The fibre spun at Rice is unique for the strength of its knots. Most fibres are most likely to snap under tension at the knot, but Rice’s fibre demonstrates what the researchers refer to as “100 percent knot efficiency,” where the fibre is as likely to break anywhere along its length as at the knot.

The material could be used to increase the strength of many products that use carbon fibre, like composites for strong, light aircraft or fabrics for bulletproof apparel, according to the researchers.

Credit goes to the unique properties of graphene oxide flakes created in an environmentally friendly process patented by Rice a few years ago. The flakes that are chemically extracted from graphite seem small. They have an average diameter of 22 microns, a quarter the width of an average human hair. But they’re massive compared with the petroleum-based pitch used in current carbon fibre.

Like with pitch, the weak van der Waals force holds the graphene flakes together. Unlike pitch, the atom-thick flakes have an enormous surface area and cling to each other like the scales on a fish when pulled into a fibre. The wet-spinning process is similar to one recently used to create highly conductive fibres made of nanotubes, but in this case Xiang just used water as the solvent rather than a super acid.

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Bend ability at the knot is due to the fibres bending modulus, which is a measure of its flexibility, Xiang said. “Because graphene oxide has very low bending modulus, it thinks there’s no knot there,” he said.

Tour said industrial carbon fibres — a source of steel-like strength in ultralight materials ranging from baseball bats to bicycles to bombers — haven’t improved much in decades because the chemistry involved is approaching its limits. But the new carbon fibres spun at room temperature at Rice already show impressive tensile strength and modulus and have the potential to be even stronger when annealed at higher temperatures.

Heating the fibres to about 2,100 degrees Celsius, the industry standard for making carbon fibre, will likely eliminate the knotting strength, Xiang said, but should greatly improve the material’s tensile strength, which will be good for making novel composite materials.

The Rice researchers also created a second type of fibre using smaller 9-micron flakes of graphene oxide. The small-flake fibres, unlike the large, were pulled from the wet-spinning process under tension, which brought the flakes into even better alignment and resulted in fibres with strength approaching that of commercial products, even at room temperature.

The new work from the Rice lab of chemist James Tour appears online today in the journal of Advanced Materials.

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A new type of paint made with carbon nanotubes at Rice University can help detect strain in buildings, bridges and airplanes.

The Rice scientists call their mixture “strain paint” and are hopeful it can help detect deformations in structures like airplane wings. Their study, published online this month by the American Chemical Society journal Nano Letters details a composite coating they invented that could be read by a handheld infrared spectrometer.

This method could tell where a material is showing signs of deformation well before the effects become visible to the naked eye, and without touching the structure. The researchers said this provides a big advantage over conventional strain gauges, which must be physically connected to their read-out devices. In addition, the nanotube-based system could measure strain at any location and along any direction.

Rice chemistry professor Bruce Weisman led the discovery and interpretation of near-infrared fluorescence from semiconducting carbon nanotubes in 2002, and he has since developed and used novel optical instrumentation to explore nanotubes’ physical and chemical properties.

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The Rice lab of materials scientist Pulickel Ajayan, in collaboration with colleagues in China, India, Japan and the Texas Medical Centre, discovered a one-step chemical process that is markedly simpler than established techniques for making graphene quantum dots. The results were published online this month in the American Chemical Society’s journal Nano Letters.

“There have been several attempts to make graphene-based quantum dots with specific electronic and luminescent properties using chemical breakdown or e-beam lithography of graphene layers,” said Ajayan, Rice’s Benjamin M. and Mary Greenwood Anderson Professor of Mechanical Engineering and Materials Science and of Chemistry. “We thought that as these nano domains of graphitised carbons already exist in carbon fibres, which are cheap and plenty, why not use them as the precursor?”

Quantum dots, discovered in the 1980s, are semiconductors that contain a size and shape dependent band gap. These have been promising structures for applications that range from computers, LEDs, solar cells and lasers to medical imaging devices. The sub-5 nanometer carbon-based quantum dots produced in bulk through the wet chemical process discovered at Rice are highly soluble, and their size can be controlled via the temperature at which they’re created.

The Rice researchers were attempting another experiment when they came across the technique. “We tried to selectively oxidize carbon fiber, and we found that was really hard,” said Wei Gao, a Rice graduate student who worked on the project with lead author Juan Peng, a visiting student from Nanjing University who studied in Ajayan’s lab last year. “We ended up with a solution and decided to look at a few drops with a transmission electron microscope.”

The specks they saw were bits of graphene or, more precisely, oxidised nanodomains of graphene extracted via chemical treatment of carbon fibre. “That was a complete surprise,” Gao said. “We call them quantum dots, but they’re two-dimensional, so what we really have here are graphene quantum discs.” Gao said other techniques are expensive and take weeks to make small batches of graphene quantum dots. “Our starting material is cheap, commercially available carbon fibre. In a one-step treatment, we get a large amount of quantum dots. I think that’s the biggest advantage of our work,” she said.

Further experimentation revealed interesting bits of information: The size of the dots, and thus their photoluminescence properties, could be controlled through processing at relatively low temperatures, from 80 to 120 degrees Celsius. “At 120, 100 and 80 degrees, we got blue, green and yellow luminescing dots,” she said.
They also found the dots’ edges tended to prefer the form known as zigzag. The edge of a sheet of graphene — the single-atom-thick form of carbon — determines its electrical characteristics, and zigzags are semiconducting.

Their luminescent properties give graphene quantum dots potential for imaging, protein analysis, cell tracking and other biomedical applications, Gao said. Tests at Houston’s MD Anderson Cancer Centre and Baylor College of Medicine on two human breast cancer lines showed the dots easily found their way into the cells’ cytoplasm and did not interfere with their proliferation.

“The green quantum dots yielded a very good image,” said co-author Rebeca Romero Aburto, a graduate student in the Ajayan Lab who also studies at MD Anderson. “The advantage of graphene dots over fluorophores is that their fluorescence is more stable and they don’t photobleach. They don’t lose their fluorescence as easily. They have a depth limit, so they may be good for in vitro and in vivo (small animal) studies, but perhaps not optimal for deep tissues in humans.

“But everything has to start in the lab, and these could be an interesting approach to further explore for bioimaging,” Romero Alburto said. “In the future, these graphene quantum dots could have high impact because they can be conjugated with other entities for sensing applications, too.”

You can read the abstract here

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