Cool Magnet: A little bit of iron gives magnetic refrigeration a boost

June 23, 2004 at 5:58 pm

Magnets are big-time materials, finding roles in products ranging from motors to medical-imaging systems. Now, a team of engineers’ improvement of a custom-made magnetic material increases the odds that refrigeration will soon join the roster of magnet-based technologies.

With the goal of making refrigerators and air conditioners more efficient, several groups around the world are developing magnetic-refrigerant materials. A magnetic-cooling system could also be less polluting than current systems because it wouldn’t use environmentally harmful chemicals, such as ammonia or chlorofluorocarbons.

What’s more, the technology requires few moving parts, so it can be simple, silent, and reliable. When a magnetic-refrigerant material is exposed to a magnetic field, the field forces the spins of electrons in the material to align. As a result, the material heats up. Removing the field permits the electrons to relax into less-ordered states, and the material cools down. By cycling the material through these hot and cold states and venting away the heat, the system can generate an overall cooling effect.

The field of magnetic refrigeration has progressed rapidly in recent years. Researchers at the Department of Energy’s Ames (Iowa) Laboratory and the Astronautics Corporation of America in Madison, Wis., have created a prototype magnetic refrigerator that operates at room temperature (SN: 1/5/02, p. 4: Magnetic refrigerator gets down and homey). The Ames group also developed a new magnetic-refrigerant material—a combination of gadolinium, germanium, and silicon—that produces the largest cooling effect of any material to date.

Robert Shull at the National Institute for Standards and Technology in Gaithersburg, Md., points out that researchers can’t yet take full advantage of this new material. Each time a magnetic field is applied to the material, it shifts the arrangement of the atoms, changing the material’s crystal structure and releasing energy. This shift would reduce the cooling efficiency of any system made with the material, explains Shull.

To eliminate these losses, Shull and his colleagues added iron to the gadolinium-germanium-silicon compound. With just 1 percent of all the atoms in the material consisting of iron, the material no longer changed its crystal structure when exposed to a magnetic field. However, it retained its magnetic-cooling properties. In the June 24 Nature, the researchers report that their subtle modification reduced the material’s energy losses by almost 95 percent.

“The results are pretty amazing,” says Michael DiPirro, a cryogenic engineer at the NASA Goddard Space Flight Center in Greenbelt, Md. Previously, researchers had overlooked the importance of energy losses, he notes.

Shull says magnetic-cooling systems could have uses beyond refrigerators and other household appliances. “One of the things that is really limiting the development of all-electric cars is the fact that they don’t have an air conditioner,” he says. “They can’t generate enough power to run it.” An efficient magnetic-cooling system could solve that problem, he predicts.