Making Magnets Cool

Think refrigerator magnets are for shopping lists and works of youthful art? Think again. Researchers around the world are trying to revolutionize refrigeration by bringing magnets into the cold.

A group at the Ames Laboratory, an Energy Department lab at Iowa State University, has built a prototype refrigeration unit that turns materials with certain magnetic properties into natural coolants. Specifically, the prototype contains the rare earth element gadolinium, which they claim has cooled contents to 42 degrees Fahrenheit.

Magnetic cooling could result in more energy efficient and smaller cooling systems, whether for consumer refrigerators and air conditioners or for industrial uses like manufacturing liquid hydrogen to run fuel cells. But finding materials that cool at room temperatures, for a price that doesn’t require government subsidy, is not easy.

Scientists have been searching for improved materials for some time. The heart of the research is a phenomenon called the magnetocaloric effect, in which some materials cool off when subjected to a changing magnetic field. There are two indicators of the magnetocaloric effect: an alignment of electron spins in the material-magnetization-and an actual drop in temperature. In January, researchers from the University of Amsterdam in the Netherlands that a compound of iron, manganese, and arsenic displayed a strong electron spin alignment, which suggested that a cheap material with the magnetocaloric effect might just be possible.

Yet numbers can be misleading, says Ames researcher Karl Gschneidner. There’s a problem, he says, with relying on magnetization. “You only get the [change in the material’s] entropy, and that’s only one measure of the magnetocaloric effect.” The change in temperature, he says, is the more important factor to researchers interested in building a magnetic cooling device.

There are other problems, says Lawrence Bennett, an engineering professor at George Washington University’s Institute for Magnetics Research. Refrigerators and air conditioners transfer heat from one location to another. A pump compresses a gas like freon, making it dense and hot. The gas runs through a set of tubes; as the tubes radiate heat, the gas gets colder and turns to a liquid. The liquid is forced through a special valve, expands into an even colder gas, absorbs heat from the inside of the cooling system and then repeats the cycle.

For a magnetic refrigerator to work, the magnetocaloric material must absorb heat on one side and expel it on the other. But most of the materials that have been tested are poor conductors of heat. Furthermore, the best magnetocaloric materials are metals, but the changing magnetic fields create small electrical currents that use energy and can make cooling far less effective.

Such drawbacks are why some experts like Bennett are investigating whether nanocomposites could be created with the necessary characteristics. “In principle, the nanocomposites might be much better,” he says. For example, the composite is actually a mass of tiny particles that do not touch each other, stopping the electrical currents almost as soon as they started. Magnetocaloric material could be matched with some other substance that could more easily dissipate heat.

So far, though, magnetocaloric nanocomposites are only a concept. “No one is able to produce what we need yet,” Bennett says, although George Washington’s chemistry department has received an energy department grant to try.
 
Even if such development is successful, Barrett warns, it is unclear whether engineers can design commercially-viable products using the technology. So scientists will continue searching for a material that is suitably attractive and the magnets will remain, for now, on the refrigerator door.