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Power/Performance Bits: Oct. 27

Room-temp superconductivity; memristor inference; battery recycling with fruit.


Room-temp superconductivity
Researchers at the University of Rochester, University of Nevada Las Vegas, and Intel created a material with superconducting properties at room temperature, the first time this has been observed.

The researchers combined hydrogen with carbon and sulfur to photochemically synthesize simple organic-derived carbonaceous sulfur hydride in a diamond anvil cell, which puts materials under extraordinarily high pressure. The picoliters of carbonaceous sulfur hydride exhibited superconductivity at about 58 degrees Fahrenheit and a pressure of about 39 million psi (about 2.6 million atmospheres).

The previous high-temperature record for superconductivity was -10 to 8 degrees Fahrenheit using lanthanum superhydride, reached by the Max Planck Institute for Chemistry and University of Illinois at Chicago.

How a diamond anvil works. (Source: Courtesy of Dias lab / University of Rochester)

Superconductivity means no electrical resistance and an expulsion of magnetic fields, with potential applications including lossless electrical transmission, improved medical imaging devices, and more efficient electronics.

However, current superconducting materials require extremely cold temperatures, typically below-220 degrees Fahrenheit, limiting their application. “The cost to keep these materials at cryogenic temperatures is so high you can’t really get the full benefit of them,” said Ranga Dias, an assistant professor of physics and mechanical engineering at Rochester.

Dias explained some of the challenges involved in creating high temperature superconductors. “To have a high temperature superconductor, you want stronger bonds and light elements. Those are the two very basic criteria,” Dias said. “Hydrogen is the lightest material, and the hydrogen bond is one of the strongest. Solid metallic hydrogen is theorized to have high Debye temperature and strong electron-phonon coupling that is necessary for room temperature superconductivity.”

But extremely high pressures are required to get hydrogen into a metallic state. Instead, the team turned to hydrogen-rich materials that mimic the elusive superconducting phase of pure hydrogen, and can be metalized at much lower pressures.

In the resulting carbonaceous sulfur hydride, the “presence of carbon is of tantamount importance here,” the researchers noted, and suggested that “compositional tuning” of the combination of elements could lead to superconductivity at even higher temperatures.

The next challenge, Dias said, is finding ways to create the room temperature superconducting materials at lower pressures, so they will be economical to produce in greater volume. In comparison to the millions of pounds of pressure created in diamond anvil cells, the atmospheric pressure of Earth at sea level is about 15 PSI. Members of the team have started a new company, Unearthly Materials, to find a path to room temperature superconductors that can be scalably produced at ambient pressure.

Memristor inference
Researchers at University College London, Liverpool John Moores University, and University of Massachusetts Amherst propose a way to increase the accuracy of memristor-based artificial neural networks. Memristor-based systems have the potential to be more energy efficient than transistor-based AI hardware. However, inference accuracy has been a problem.

The team found that by arranging memristors as ‘committee machines’ in several sub-groups of neural networks and averaging their calculations, accuracy was greatly improved, cancelling out flaws in each individual network. Plus, the accuracy improvement came without increasing the total number of memristors.

“We hoped that there might be more generic approaches that improve not the device-level, but the system-level behavior, and we believe we found one,” said Dr Adnan Mehonic of UCL Electronic & Electrical Engineering. “Our approach shows that, when it comes to memristors, several heads are better than one. Arranging the neural network into several smaller networks rather than one big network led to greater accuracy overall.”

Dovydas Joksas, a PhD student at UCL, added, “We borrowed a popular technique from computer science and applied it in the context of memristors. And it worked! Using preliminary simulations, we found that even simple averaging could significantly increase the accuracy of memristive neural networks.”

The approach was tested in three different types of memristors (tantalum/hafnium oxide-based (Ta/HfO2), tantalum oxide-based (Ta2O5), and amorphous vacancy modulated conductive oxide-based (aVMCO) devices) and the researchers found that it improved the accuracy of all of them, regardless of material or particular memristor technology. It also worked for a number of different problems that may affect memristors’ accuracy, including faulty devices, device-to-device variability, random telegraph noise, and line resistance.

The team found that their approach increased the accuracy of the neural networks for typical AI tasks to a comparable level to software tools run on conventional digital hardware.

Battery recycling with fruit
Researchers at Nanyang Technological University Singapore (NTU Singapore) found a way to use fruit peels to extract metals from spent lithium-ion batteries. The team was then able to make functional batteries from the recovered metals.

Typically, recovering materials from spent batteries involves treating them with extreme heat, over 500°C, which can emit hazardous gases. Another method being explored, hydrometallurgy, uses water as a solvent for extraction. In this process, used batteries are crushed or shredded to form a material called ‘black mass.’ This black mass is then dissolved in a mix of strong acids or weak acids with chemicals like hydrogen peroxide under heat before letting the metals precipitate. However, concerns exist about the generation of secondary pollutants using this method.

“Current industrial recycling processes of e-waste are energy-intensive and emit harmful pollutants and liquid waste, pointing to an urgent need for eco-friendly methods as the amount of e-waste grows. Our team has demonstrated that it is possible to do so with biodegradable substances,” said Professor Madhavi Srinivasan, co-director of the NTU Singapore-CEA Alliance for Research in Circular Economy (NTU SCARCE) lab.

Instead, the researchers turned to orange peels. By oven-drying orange peel, grinding it to a powder, and combining it with citric acid (a weak acid found in citrus), they were able to achieve similar results as hydrometallurgy using hydrogen peroxide.

NTU Singapore’s Asst. Prof. Dalton Tay (L) and Prof. Madhavi Srinivasan (R) show orange peel and discarded Li-ion batteries, from which useful metals can be extracted. (Credit: NTU Singapore)

In lab experiments, the team found that their approach successfully extracted around 90% of cobalt, lithium, nickel, and manganese from spent lithium-ion batteries. Additionally, solid residue generated from the process was non-toxic.

“The key lies in the cellulose found in orange peel, which is converted into sugars under heat during the extraction process. These sugars enhance the recovery of metals from battery waste. Naturally-occurring antioxidants found in orange peel, such as flavonoids and phenolic acids, could have contributed to this enhancement as well,” said Dalton Tay, an assistant professor in the NTU School of Materials Science and Engineering and School of Biological Sciences.

The researchers were able to assemble new lithium-ion batteries from the recovered materials that showed similar charge capacity to commercial ones. The team is continuing research on improving charge-discharge cycling, scaling up production, and using other cellulose-rich food waste.

Tay added, “In Singapore, a resource-scarce country, this process of urban mining to extract valuable metals from all kinds of discarded electronics becomes very important. With this method, we not only tackle the problem of resource depletion by keeping these precious metals in use as much as possible, but also the problem of e-waste and food waste accumulation – both a growing global crisis.”

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