Most people know all about refrigerator magnets. Magnetic refrigerators, though, not so much.
According to Gary Mankey, a University of Alabama professor researching thin film materials at the Spallation Neutron Source, a magnetic refrigerator operating at room temperature would use a third as much electricity as a typical home refrigerator. Since refrigerated devices, including air conditioners, use about a third of the electricity consumed in a home, switching to magnetic cooling could substantially cut a power bill.
Magnetic refrigeration relies on magnetocaloric materials that heat up in a magnetic field but rapidly cool below their original temperature when withdrawn from the field. The magnetic field causes the molecules in the material to align, releasing thermal energy within the material. When the field is withdrawn, the extra thermal energy begins to bang around the molecules and cause them to lose their magnetic orientation. The magnetic domains start rapidly absorbing energy to reorient themselves, and as they do so, the temperature of the material drops.
Scientists are familiar with magnetocaloric materials, but magnetic refrigeration requires that they operate near room temperature. Gadolinium has long been known for its magnetocaloric effect near room temperature, which is even stronger for a gadolinium alloy containing silicon and germanium. However, gadolinium and its alloys are expensive. Arsenic alloys can also be used to achieve this effect, but as everyone knows, elemental arsenic is poisonous.
Mankey noted that issues with currently tested magnetocaloric materials are a barrier to the commercialization of magnetic refrigerants. They are toxic, expensive, and/or rare, so researchers are striving to synthesize alternate materials.
In 1997, the first near-room-temperature, proof-of-concept, magnetic refrigerant was demonstrated at the U.S. Department of Energy’s Ames Laboratory, which used commercial-grade gadolinium alloys. As a result, scientists and companies worldwide started developing new types of room-temperature materials and magnetic refrigerant designs. In 2002, the University of Amsterdam demonstrated the giant magnetocaloric effect in manganese-iron alloys containing phosphorus and arsenic. These alloys are based on less expensive, more abundant elements.
The principal advantage of the NASA Ames Research Center group’s magnetic refrigerator was that it used an electric motor to rotate a permanent magnet rather than a compressor. It saves a lot of energy, because compressing a gas requires more energy than a simple rotation.
Mankey’s research group is currently searching for better alloys that meet the basic requirements. The ideal material, he said, would be inexpensive, nontoxic, easy to produce in bulk, and able to provide cooling with a modest magnetic field of 1 to 2 tesla. (A loudspeaker magnet is about 1 tesla; a refrigerator magnet is about 5 milli-tesla.) A permanent magnet can supply such a field.
Mankey and his team think that by using instruments at SNS and HFIR they can acquire the information needed for smart design of a magnetocaloric thin film to enable magnetic refrigeration, instead of using alloys containing exotic materials. The metals they are exploring include alloys of cobalt, iron, silicon, aluminum, and manganese. In pilot studies, they used SNS to probe the structure and magnetic properties of iron-rhodium-palladium thin films. To test the applicability of neutron techniques for future experiments they are employing polarized neutron reflectivity at SNS to study dysprosium-yttrium to measure changes in chirality (left- and right-handedness of atoms and molecules) as a function of temperature.
Mankey predicted that magnetic refrigeration could be available commercially in the near future, depending on the outcomes of current studies and funding for future research.
HFIR and SNS are funded by the U.S. Department of Energy (DOE) Office of Basic Energy Sciences.
