Scientists are using a very small visible beam to burn laser-induced graphene, a foamy form of carbon, into microscopic patterns.

The labs of Rice University chemist James Tour, which discovered the original method to turn a common polymer into graphene in 2014, and Tennessee/Oak Ridge National Laboratory materials scientist Philip Rack revealed they can now watch the conductive material form as it makes small traces of LIG in a scanning electron microscope (SEM).

The altered process, which appears in ACS Applied Materials & Interfaces, creates LIG with features more than 60% smaller than the macro version and almost 10 times smaller than typically achieved with the former infrared laser.

Lower-powered lasers also make the process less expensive, Tour says. That could lead to wider commercial production of flexible electronics and sensors.

“A key for electronics applications is to make smaller structures so that one could have a higher density, or more devices per unit area,” Tour says. “This method allows us to make structures that are 10 times denser than we formerly made.”

To prove the concept, the lab made flexible humidity sensors that are invisible to the naked eye and directly fabricated on polyimide, a commercial polymer. The devices were able to sense human breath with a response time of 250 milliseconds.

“This is much faster than the sampling rate for most commercial humidity sensors and enables the monitoring of rapid local humidity changes that can be caused by breathing,” says lead author Michael Stanford, a postdoctoral researcher at Rice.

The smaller lasers pump light at a wavelength of 405 nanometers, in the blue-violet part of the spectrum. These are less powerful than the industrial lasers the Tour Group and others around the world are using to burn graphene into plastic, paper, wood, and even food.

The SEM-mounted laser burns only the top five microns of the polymer, writing graphene features as small as 12 microns. (A human hair, by comparison, is 30 to 100 microns wide.)

“The LIG features we were creating were so small that they would have been next to impossible to find if we were to lase the patterns and then search for them in the microscope later,” says Stanford, who used advanced equipment at Oak Ridge National Laboratory.

Tour, whose group recently introduced flash graphene to instantly turn trash and food waste into the valuable material, says the new LIG process offers a new path toward writing electronic circuits into flexible substrates like clothing.

“While the flash process will produce tons of graphene, the LIG process will allow graphene to be directly synthesized for precise electronics applications on surfaces,” says Tour, chair in chemistry and professor of computer science and of materials science and nanoengineering.

Coauthors of the paper are from UT Knoxville and Oak Ridge National Laboratory.

The Air Force Office of Scientific Research and the US Department of Energy supported the research.

Source: Rice University

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By infusing laser-induced graphene, a flaky foam of the atom-thick carbon, with plastic, rubber, cement, wax, or other materials, researchers have created composites with lots of possible uses.

The composites could be useful for wearable electronics, in heat therapy, in water treatment, in anti-icing and deicing work, in creating antimicrobial surfaces, and even in making resistive random-access memory devices.

Here, a hydrophilic composite of laser-induced graphene and other materials readily soaks up water. (Credit: the Tour Group):

In 2014, the lab of James Tour, chair in chemistry as well as a professor of computer science and of materials science and nanoengineering at Rice University, first made LIG when it used a commercial laser to burn the surface of a thin sheet of common plastic, polyimide. The laser’s heat turned a sliver of the material into flakes of interconnected graphene. The one-step process made much more of the material, and at far less expense, than through traditional chemical vapor deposition.

Since then, the lab and others have expanded their investigation of LIG, even dropping the plastic to make it with wood and food. Last year, the researchers created graphene foam for sculpting 3D objects.

“LIG is a great material, but it’s not mechanically robust,” says Tour, coauthor of an overview of laser-induced graphene developments in the Accounts of Chemical Research journal last year.

“You can bend it and flex it, but you can’t rub your hand across it. It’ll shear off. If you do what’s called a Scotch tape test on it, lots of it gets removed. But when you put it into a composite structure, it really toughens up.”

To make the composites, the researchers poured or hot-pressed a thin layer of the second material over LIG attached to polyimide. When the liquid hardened, they pulled the polyimide away from the back for reuse, leaving the embedded, connected graphene flakes behind.

Soft composites can be used for active electronics in flexible clothing, Tour says, while harder composites make excellent superhydrophobic (water-avoiding) materials. When a voltage is applied, the 20-micron-thick layer of LIG kills bacteria on the surface, making toughened versions of the material suitable for antibacterial applications.

Composites made with liquid additives are best at preserving LIG flakes’ connectivity. In the lab, they heated quickly and reliably when researchers applied voltage. That should give the material potential use as a deicing or anti-icing coating, as a flexible heating pad for treating injuries, or in garments that heat up on demand.

“You just pour it in, and now you transfer all the beautiful aspects of LIG into a material that’s highly robust,” Tour says.

The research appears in ACS Nano. Additional coauthors are from Rice, the Korea Basic Science Institute, the Indian Institute of Technology Bombay, and Ben-Gurion University of the Negev in Israel. The Air Force Office of Scientific Research and the United States-Israel Binational Science Foundation supported the research.

Source: Rice University

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A new “structural battery” prototype incorporates a cartilage-like material to make the batteries highly durable and easy to shape, researchers report.

The idea behind structural batteries is to store energy in structural components—the wing of a drone or the bumper of an electric vehicle, for example. They’ve been a long-term goal for researchers and industry because they could reduce weight and extend range. But so far, structural batteries have been heavy, short-lived, or unsafe.

In a study in ACS Nano, the researchers describe how they made a damage-resistant rechargeable zinc battery with a cartilage-like solid electrolyte.

They showed that the batteries can replace the top casings of several commercial drones. The prototype cells can run for more than 100 cycles at 90 percent capacity, and withstand hard impacts and even stabbing without losing voltage or starting a fire.

“A battery that is also a structural component has to be light, strong, safe, and have high capacity. Unfortunately, these requirements are often mutually exclusive,” says research leader Nicholas Kotov, a professor of engineering, materials science and engineering, and macromolecular science and engineering at the University of Michigan.

Stronger stuff

To sidestep these trade-offs, the researchers used zinc—a legitimate structural material—and branched nanofibers that resemble the collagen fibers of cartilage.

“Nature does not have zinc batteries, but it had to solve a similar problem,” Kotov says. “Cartilage turned out to be a perfect prototype for an ion-transporting material in batteries. It has amazing mechanics, and it serves us for a very long time compared to how thin it is. The same qualities are needed from solid electrolytes separating cathodes and anodes in batteries.”

In our bodies, cartilage combines mechanical strength and durability with the ability to let water, nutrients, and other materials move through it. These qualities are nearly identical to those of a good solid electrolyte, which has to resist damage from dendrites while also letting ions flow from one electrode to the other.

In the batteries, aramid nanofibers—the stuff in bulletproof vests—stand in for collagen.

Dendrites are tendrils of metal that pierce the separator between the electrodes and create a fast lane for electrons, shorting the circuit and potentially causing a fire. Researchers previously overlooked zinc for rechargeable batteries because it tends to short out after just a few charge/discharge cycles.

Not only can the membranes ferry zinc ions between the electrodes, they can also stop zinc’s piercing dendrites. Like cartilage, the membranes are composed of ultra-strong nanofibers interwoven with a softer ion-friendly material.

In the batteries, aramid nanofibers—the stuff in bulletproof vests—stand in for collagen, with polyethylene oxide (a chain-like, carbon-based molecule) and a zinc salt replacing soft components of cartilage.

Safety first

To make working cells, the team paired the zinc electrodes with manganese oxide—the combination found in standard alkaline batteries. But in the rechargeable batteries, the cartilage-like membrane replaces the standard separator and alkaline electrolyte. As secondary batteries on drones, the zinc cells can extend the flight time by 5 to 25 percent—depending on the battery size, mass of the drone, and flight conditions.

Safety is critical to structural batteries, so the team deliberately damaged their cells by stabbing them with a knife. In spite of multiple “wounds,” the battery continued to discharge close to its design voltage. This is possible because there is no liquid to leak out.

For now, the zinc batteries are best as secondary power sources because they can’t charge and discharge as quickly as their lithium ion brethren. But Kotov’s team intends to explore whether there is a better partner electrode that could improve the speed and longevity of zinc rechargeable batteries.

The Air Force Office of Scientific Research and National Science Foundation funded the research.

Source: University of Michigan

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