Magnetic Deep Freeze

LOS ANGELES--Refrigerator magnets are best known for holding shopping lists and old postcards onto refrigerator doors. But in a few years, much more powerful magnets could be the key to keeping food cold in so-called magnetocaloric refrigerators, which would be much more energy efficient and less polluting than standard models. Now a new class of magnetocaloric materials, announced yesterday here at a meeting of the American Physical Society, could make these magnetic refrigerators more practical and versatile.

The magnetocaloric effect works when strong magnetic fields align quantum-mechanical "spins" of atoms. This transition reduces one aspect of the randomness, or entropy, of the atoms. But according to laws of thermodynamics, some other aspect of randomness has to increase in compensation, so the atoms increase the randomness of their velocities--vibrating and heating up. Once this heat is carried away by a coolant such as water, the field is removed and the effect works in reverse, chilling the material and cooling a refrigerator. To date, the peak performance has been with the element gadolinium.

By adding various amounts of silicon and germanium to the gadolinium lattice, Vitalij Pecharsky and Karl Gschneidner of the Ames Laboratory at Iowa State University discovered a new class of materials that can chill two to six times further in a single magnetic cycle, meaning that refrigerators could operate with weaker magnetic fields or less material. The new materials also operate from about room temperature all the way down to minus 253 degrees Celsius, which could allow magnetocaloric freezers to liquefy hydrogen or natural gas for use in clean-burning power plants or future automobiles.

"These new materials give you a lot more flexibility in designing magnetocaloric [refrigerators]," says Carl Zimm, a senior scientist in magnetic refrigeration at Astronautics Corporation of America in Madison, Wisconsin. The team hopes to try its new material in Zimm's prototype gadolinium-based refrigerator, which has been running for more than a year.