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Quantum materials show promise for greener, better refrigerators

by Lindsay Jolivet Nov 6 / 14

Refrigerators are wasteful heat removers. They rely on refrigerant flowing through coils, absorbing and expelling heat — a very inefficient process that makes them one of the biggest energy-sucking appliances in most homes. But scientists are exploring ways to eliminate the need for refrigerant, using quantum materials to pump away heat with magnetism.

A magnetocaloric material heats up when magnetized (b). If cooled and then demagnetized (c), its temperature drops dramatically (d). 
Photo by Talbott, NIST

CIFAR Fellow Patrick Fournier (Université de Sherbrooke) says that physicists have understood for about a century that applying a magnetic field to certain materials heats them up, and removing the magnetic field cools them down. Moving these materials in and out of the magnetic field can pull heat away like a pump, and some low-temperature laboratory equipment already uses this process, called a magnetocaloric effect, or more specifically, adiabatic demagnetization.

GE and others are working toward using the effect to make commercial refrigerators. However, existing machines use metal substances such as gadolinium that degrade quickly and only work in a narrow range of temperatures close to room temperature, making them costly and clunky. A time-varying magnetic field induces currents in metals that warm them up, which is a disadvantage if you try to cool them down using the magnetocaloric effect. Researchers are trying to figure out how to use the effect more efficiently.

“The key word is ‘efficiently,’” says Fournier, a fellow of CIFAR’s program in Quantum Materials.

At his lab, Fournier and his colleagues are investigating the special properties of materials that don’t have the same faults as metals, including oxides such as HoMn2O5 that are magnetic insulators. Oxides don’t rust and insulators resist electricity, which means they won’t generate excess heat as they are cycled in the magnetic field.

In a recent experiment published in Applied Physics Letters, the researchers discovered that applying a magnetic field to HoMn2O5 single crystals produces a large magnetocaloric effect at a temperature of -263.15 degrees Celsius only when the field is aligned along a specific direction of the crystals. In other directions, it is smaller. As a consequence, it isn’t necessary to move this material in and out of the magnetic field to cool it down. Simply rotating a crystal of HoMn2O5 in a constant magnetic field produces the same cooling effect.

“You can get exactly the same effect as increasing the magnetic field and decreasing the magnetic field by rotating the crystal instead. So that’s the trick,” Fournier says. “Doing it this way is actually very efficient because then you can rotate the crystal at very high speed with less friction and get a large cooling efficiency. And that, without the induced currents in a metal.”

The rotating method also makes this process adaptable to existing technology, because all motors use circular motion. A rotating crystal requires few mechanical parts to change its orientation in the magnetic field compared to a piston-like motion of current prototypes, and it allows for smaller appliances . “That’s why people are getting a bit excited about it,” Fournier says.

Fournier and his colleagues are planning a prototype using this new material and other oxides with similar properties.

While a HoMn2O5 refrigerator won’t turn up in kitchens, Fournier says refrigeration using this material could vastly improve laboratory technology for cooling substances such as nitrogen, hydrogen or helium from a gas at room temperature to a liquid at extremely low temperatures.

“That would be a big improvement compared to what we have now,” he says.

Discoveries of other quantum materials with these valuable properties could eventually revolutionize our kitchens as well as our labs.

Fournier says the next challenge is finding ways to produce magnetocaloric effects at close to room temperature, by broadening the temperature range at which HoMn2O5 continues to cool as it rotates, or finding cheaper, similar materials that perform even better at room temperature.