Power/Performance Bits: Aug. 20

Six-angstrom waveguide; rare earth recycling; cooling buildings.


Six-angstrom waveguide
Engineers at the University of California San Diego, City University of New York, and Johns Hopkins University created the thinnest optical waveguide yet. At only three atoms thick, the team says the waveguide serves as a proof of concept for scaling down optical devices.

The waveguide consists of a tungsten disulfide monolayer (made up of one layer of tungsten atoms sandwiched between two layers of sulfur atoms) suspended on a silicon frame. The monolayer is also patterned with an array of nanosized holes forming a photonic crystal.

“Fundamentally, we demonstrate the ultimate limit for how thin an optical waveguide can be built,” said senior author Ertugrul Cubukcu, a professor of nanoengineering and electrical engineering at UC San Diego.

The waveguide measures six angstroms, making it more than 10,000 times thinner than a typical optical fiber and about 500 times thinner than on-chip optical waveguides in integrated photonic circuits.

Additionally, the waveguide channels light in the visible spectrum. “This is challenging to do in a material this thin,” Cubukcu said. “Waveguiding has previously been demonstrated with graphene, which is also atomically thin, but at infrared wavelengths. We’ve demonstrated for the first time waveguiding in the visible region.”

Nanosized holes etched into the crystal allow some light to scatter perpendicular to the plane so that it can be observed and probed. This array of holes produces a periodic structure that makes the crystal double as a resonator as well.

“This also makes it the thinnest optical resonator for visible light ever to be demonstrated experimentally,” said Xingwang Zhang, who worked on the project as a postdoctoral researcher at UC San Diego. “This system does not only resonantly enhance the light-matter interaction, but also serves as a second-order grating coupler to couple the light into the optical waveguide.”

Creating the waveguide was a challenge. “The material is atomically thin, so we had to devise a process to suspend it on a silicon frame and pattern it precisely without breaking it,” said Chawina De-Eknamkul, a nanoengineering PhD student at UC San Diego.

The team plans to continue exploring the fundamental properties and physics of the waveguide.

Rare earth recycling
Researchers at Oak Ridge National Laboratory and collaborators are commercializing a process to extract rare earth minerals from scrapped magnets of used hard drives and other sources.

“We have developed an energy-efficient, cost-effective, environmentally friendly process to recover high-value critical materials,” said Ramesh Bhave, who leads the membrane technologies team in ORNL’s Chemical Sciences Division. “It’s an improvement over traditional processes, which require facilities with a large footprint, high capital and operating costs and a large amount of waste generated.”

The magnets are dissolved in nitric acid, and the solution is continuously fed through a module supporting polymer membranes. The membranes contain porous hollow fibers with an extractant that creates a selective barrier and lets only rare earth elements pass through. The rare-earth-rich solution collected on the other side is further processed to yield rare earth oxides at purities exceeding 99.5%.

Typically, 70% of a permanent magnet is iron. “We are essentially able to eliminate iron completely and recover only rare earths,” Bhave said. Currently, no commercialized process recycles pure rare earth elements from electronic-waste magnets.

The team used magnets of varied composition from a range of sources, including hard drives, MRI machines, cell phones, and cars. After two years of work, the tailored membrane was able to recover more than 97% of the rare earth elements.

Three rare earth elements, neodymium, praseodymium, and dysprosium, are extractable as a mixture of oxides. A mixture of the three oxides sells for $50 a kilogram. Current efforts work on separating dysprosium from neodymium and praseodymium, which could be sold for five times as much.

Another focus is separating other in-demand elements from lithium ion batteries. “The expected high growth of electric vehicles is going to require a tremendous amount of lithium and cobalt,” Bhave said.

The process has been patented, and ORNL is working with licensee Momentum Technologies of Dallas to scale the process further to produce commercial batches of rare earth oxides. The goal is to recover hundreds of kilograms of rare earth oxides each month and validate, verify and certify that manufacturers could use the recycled materials to make magnets equivalent to those made with virgin materials.

Cooling buildings
Researchers at the State University of New York at Buffalo, King Abdullah University of Science and Technology, and University of Wisconsin-Madison developed a system that can help cool buildings in crowded metropolitan areas without consuming electricity.

Individual units are made up of an inexpensive polymer/aluminum film installed inside a foam box at the bottom of a specially designed solar “shelter.” The film absorbs heat from the air inside the box and the shelter helps to block incoming sunlight and beams the thermal radiation emitted from the film into the sky.

“The polymer stays cool as it dissipates heat through thermal radiation, and can then cool down the environment,” said Lyu Zhou, a PhD candidate in electrical engineering in the University at Buffalo School of Engineering and Applied Sciences. “This is called radiative or passive cooling, and it’s very interesting because it does not consume electricity — it won’t need a battery or other electricity source to realize cooling.”

The film itself is made from a sheet of aluminum coated with the clear polymer polydimethylsiloxane. The aluminum reflects sunlight, while the polymer absorbs and dissipates heat from the surrounding air.

The shelter-and-box system measures about 18 inches tall (45.72 centimeters), 10 inches wide and 10 inches long (25.4 centimeters). To cool a building, numerous units of the system would need to be installed to cover a roof.

“During the night, radiative cooling is easy because we don’t have solar input, so thermal emissions just go out and we realize radiative cooling easily,” said Haomin Song, UB assistant professor of research in electrical engineering. “But daytime cooling is a challenge because the sun is shining. In this situation, you need to find strategies to prevent rooftops from heating up. You also need to find emissive materials that don’t absorb solar energy. Our system address these challenges.”

When placed outside during the day, the heat-emanating film and solar shelter helped reduce the temperature of a small, enclosed space by a maximum of about 6 degrees Celsius (11 degrees Fahrenheit). At night, that figure rose to about 11 degrees Celsius (about 20 degrees Fahrenheit).

“If you look at the headlight of your car, it has a certain structure that allows it to direct the light in a certain direction,” added Qiaoqiang Gan, UB associate professor of electrical engineering. “We follow this kind of a design. The structure of our beam-shaping system increases our access to the sky. The ability to direct the emissions improves the performance of the system in crowded areas.”

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