Magnetic materials keep fridges cool

Magnets turn many refrigerators into message centers and art galleries -- holding up notes, shopping lists, and the kids' latest masterpieces for all to see. Someday, magnets may not only adorn the outside of refrigerators but also power the inner workings of the appliances.

 Magnetocaloric materials, which change temperature in response to an applied magnetic field, form the heart of a new class of refrigeration technologies. Magnetocaloric refrigerators have the potential to be more efficient than conventional devices without relying on ozone-depleting coolants.

 Materials now available perform their best in the powerful fields supplied by superconducting magnets, so the first applications will probably be in industry rather than home kitchens. Researchers presented a selection of recent findings at the American Physical Society meeting in Los Angeles last week.

 Carl Zimm and his colleagues at the Astronautics Corp. of America in Madison, Wis., have constructed a prototype magnetocaloric refrigerator in collaboration with researchers at the Department of Energy's Ames (Iowa) Laboratory. The field supplied by a superconducting magnet cools a piece of solid gadolinium, which in turn cools water flowing around it, says Zimm. Weaker, permanent magnets don't provide as much cooling power.

 As a heat-transferring fluid, water replaces the chlorofluorocarbons or hydrochlorofluorocarbons typical of ordinary refrigerators. "You can't get more environmentally safe than water," Zimm notes. The refrigerator cools a volume "about the size of a soda can," he adds. It has been running for nearly a year, which bodes well for reliability.

 Gadolinium has a magnetocaloric response twice that of iron, but Ames researchers Vitalij K. Pecharsky and Karl A. Gschneidner have found that alloys of gadolinium, silicon, and germanium show a response twice again as big.

 The magnetocaloric effect depends on the way a material's atomic spins align themselves. All materials store heat in the form of atomic vibrations. An applied magnetic field forces the atoms into alignment, reducing the system's heat capacity and causing it to expel energy, which the water carries away. When the field is removed, the atoms randomize again and can absorb energy from their surroundings, creating a cooling effect.

 By adjusting the alloy's composition, Pecharsky and Gschneidner can control the temperature at which the effect is greatest, from near room temperature down to about 30 kelvins.

 Another way to tune the magnetocaloric effect is to construct magnetic nanocomposites, says Robert D. Shull of the National Institute of Standards and Technology in Gaithersburg, Md. Scattering magnetic elements as small as 1 to 20 nanometers throughout a material changes its sensitivity to magnetic fields.

 Shull has modified a magnetocaloric material, gadolinium gallium garnet, by substituting iron for some of the gallium atoms. The addition of iron tripled the material's response to a magnetic field.

 The first application of magnetocaloric refrigeration would probably be for condensing hydrogen gas into a liquid for use as a clean-burning fuel. The liquefaction of hydrogen would require refrigerators with about 15 separate cooling stages, combining to reduce the temperature to 20 kelvins, says Pecharsky. This method's greater efficiency will lower costs significantly, compared to traditional hydrogen liquefaction. Zimm estimates that it will take at least 5 years for an industrial magnetic refrigerator to become available.

 Magnets turn many refrigerators into message centers and art galleries -- holding up notes, shopping lists, and the kids' latest masterpieces for all to see. Someday, magnets may not only adorn the outside of refrigerators but also power the inner workings of the appliances.

 Magnetocaloric materials, which change temperature in response to an applied magnetic field, form the heart of a new class of refrigeration technologies. Magnetocaloric refrigerators have the potential to be more efficient than conventional devices without relying on ozone-depleting coolants.

 Materials now available perform their best in the powerful fields supplied by superconducting magnets, so the first applications will probably be in industry rather than home kitchens. Researchers presented a selection of recent findings at the American Physical Society meeting in Los Angeles last week.

 Carl Zimm and his colleagues at the Astronautics Corp. of America in Madison, Wis., have constructed a prototype magnetocaloric refrigerator in collaboration with researchers at the Department of Energy's Ames (Iowa) Laboratory. The field supplied by a superconducting magnet cools a piece of solid gadolinium, which in turn cools water flowing around it, says Zimm. Weaker, permanent magnets don't provide as much cooling power.

 As a heat-transferring fluid, water replaces the chlorofluorocarbons or hydrochlorofluorocarbons typical of ordinary refrigerators. "You can't get more environmentally safe than water," Zimm notes. The refrigerator cools a volume "about the size of a soda can," he adds. It has been running for nearly a year, which bodes well for reliability.

 Gadolinium has a magnetocaloric response twice that of iron, but Ames researchers Vitalij K. Pecharsky and Karl A. Gschneidner have found that alloys of gadolinium, silicon, and germanium show a response twice again as big.

 The magnetocaloric effect depends on the way a material's atomic spins align themselves. All materials store heat in the form of atomic vibrations. An applied magnetic field forces the atoms into alignment, reducing the system's heat capacity and causing it to expel energy, which the water carries away. When the field is removed, the atoms randomize again and can absorb energy from their surroundings, creating a cooling effect.

 By adjusting the alloy's composition, Pecharsky and Gschneidner can control the temperature at which the effect is greatest, from near room temperature down to about 30 kelvins.

 Another way to tune the magnetocaloric effect is to construct magnetic nanocomposites, says Robert D. Shull of the National Institute of Standards and Technology in Gaithersburg, Md. Scattering magnetic elements as small as 1 to 20 nanometers throughout a material changes its sensitivity to magnetic fields.

 Shull has modified a magnetocaloric material, gadolinium gallium garnet, by substituting iron for some of the gallium atoms. The addition of iron tripled the material's response to a magnetic field.

 The first application of magnetocaloric refrigeration would probably be for condensing hydrogen gas into a liquid for use as a clean-burning fuel. The liquefaction of hydrogen would require refrigerators with about 15 separate cooling stages, combining to reduce the temperature to 20 kelvins, says Pecharsky. This method's greater efficiency will lower costs significantly, compared to traditional hydrogen liquefaction. Zimm estimates that it will take at least 5 years for an industrial magnetic refrigerator to become available.