A new way to capture heat and turn it into electricity

Researchers have made an important discovery that could make it easier to collect energy from heat.

Scientists find new way to capture heat that otherwise would have been lost

An international team of scientists has figured out how to capture heat and turn it into electricity.

The discovery, published last week in the journal Science Advances, could create more efficient energy generation from heat in things like car exhaust, interplanetary space probes and industrial processes.

“Because of this discovery, we should be able to make more electrical energy out of heat than we do today,” said study co-author Joseph Heremans, professor of mechanical and aerospace engineering and Ohio Eminent Scholar in Nanotechnology at The Ohio State University. “It’s something that, until now, nobody thought was possible.”

The discovery is based on tiny particles called paramagnons—bits that are not quite magnets, but that carry some magnetic flux. This is important, because magnets, when heated, lose their magnetic force and become what is called paramagnetic. A flux of magnetism—what scientists call “spins”—creates a type of energy called magnon-drag thermoelectricity, something that, until this discovery, could not be used to collect energy at room temperature.

“The conventional wisdom was once that, if you have a paramagnet and you heat it up, nothing happens,” Heremans said. “And we found that that is not true. What we found is a new way of designing thermoelectric semiconductors—materials that convert heat to electricity. Conventional thermoelectrics that we’ve had over the last 20 years or so are too inefficient and give us too little energy, so they are not really in widespread use. This changes that understanding.”

Magnets are a crucial part of collecting energy from heat: When one side of a magnet is heated, the other side—the cold side—gets more magnetic, producing spin, which pushes the electrons in the magnet and creates electricity.

The paradox, though, is that when magnets get heated up, they lose most of their magnetic properties, turning them into paramagnets—“almost-but-not-quite magnets,” Heremans calls them. That means that, until this discovery, nobody thought of using paramagnets to harvest heat because scientists thought paramagnets weren’t capable of collecting energy.

What the research team found, though, is that the paramagnons push the electrons only for a billionth of a millionth of a second—long enough to make paramagnets viable energy-harvesters.

The research team—an international group of scientists from Ohio State, North Carolina State University and the Chinese Academy of Sciences (all are equal authors on this journal article)—started testing paramagnons to see if they could, under the right circumstances, produce the necessary spin.

What they found, Heremans said, is that paramagnons do, in fact, produce the kind of spin that pushes electrons.

And that, he said, could make it possible to collect energy.

Learn more: A new way to turn heat into energy

 

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New alternatives could reduce demand for rare-earth permanent magnets

Single crystals of CeCo3 synthesized by solution growth method.
Credit: Andriy Palaysuik

From computer hard discs and smart phones to earbuds and electric motors, magnets are at the forefront of today’s technology. Magnets containing rare-earth elements are among the most powerful available, allowing many everyday objects to be ever smaller. But rare-earth elements can be difficult to obtain, given either their scarcity or the challenging geopolitical climates of some of the nations where they are mined. Now, scientists have identified magnets based on more readily obtainable rare earths, as well as some promising magnets that don’t contain these materials at all.

The researchers will present their findings today at the American Chemical Society (ACS) Spring 2019 National Meeting & Exposition. ACS, the world’s largest scientific society, is holding the meeting here through Thursday. It features nearly 13,000 presentations on a wide range of science topics.

“We have developed new ways to better predict which materials make good magnets,” says Thomas Lograsso, Ph.D., who led the team. “Experimentally, we can ‘rehabilitate’ near-magnet systems, called paramagnets. We start with alloys or compounds that have all the right properties to be ferromagnetic at room temperature. Many times, these materials have high proportions of iron or cobalt.”

Paramagnets are materials that are weakly attracted to a magnetic field and are not permanently magnetized. But by adding alloys, paramagnets have been transformed into ferromagnets, or regular permanent magnets, like the metal surface of a refrigerator. Lograsso’s team at the Critical Materials Institute at Ames Laboratory has identified two promising candidates thus far using this “rehabilitative” approach, and both are forms of cerium cobalt: CeCoand CeCo5. Although cerium is called a rare-earth element, it is very abundant and easy to obtain.

Previous work on CeCo3 showed that it exhibited classic paramagnetic behavior. Calculations predicted that by adding magnesium, paramagnetic CeCo3  could be transformed into a ferromagnet. These predictions have been experimentally validated, Lograsso says, and this property has been observed in measurements of single crystals of the compound.

CeCois a strong ferromagnet. The researchers combined theoretical calculations with high-throughput experiments to zero in on the exact amount of copper and iron to add that would optimize the compound’s ferromagnetism. With these additives, the team anticipates that CeCo5 could someday be used in place of the strongest rare-earth magnets that contain neodymium (Nd) and dysprosium (Dy), thus easing demand for those critical elements. Lograsso and colleagues continue to investigate other similar metals that can be added to CeCo5 to further improve its suitability as a viable substitute for Nd and Dy magnets.

“Replacing rare-earth magnets, which are in high demand, would be ideal, both economically and environmentally,” Lograsso says. “Although our modified cerium-cobalt compounds are not as powerful as rare-earth magnets, they could still be highly valuable for certain commercial applications. So, our goal is to match the right magnet material to a specific application — a so-called ‘Goldilocks’ non-rare-earth magnet.”

To that end, the group continues to use their strategy to optimize the key characteristics of poor magnets or non-magnets to transform them into alternatives that are completely free of rare-earth elements. For example, they are now using cobalt to optimize the performance of iron germanium, Fe3Ge. The resulting compound’s high magnetization is comparable with the best Nd-based magnets. This strategy is not just limited to Fe3Ge and is being applied to other promising rare-earth-free compounds to selectively improve magnet properties.

Learn more: New alternatives may ease demand for scarce rare-earth permanent magnets

 

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