Ames Laboratory, U.S. Department of Energy, Ames, Iowa
Ames Laboratory, U.S. Department of Energy, Ames, Iowa
Ames, Iowa -- Scientists at the Department of Energy's (DOE) Ames Laboratory have discovered a new class of materials that represents a significant advance in the cooling power of materials currently used for magnetic refrigeration.
The new materials are made of a gadolinium-silicon-germanium (Gd5Si2Ge2) alloy.Scientists discovered the new class of materials when they lowered the magnetic ordering temperature of the compound, gadolinium-silicon (Gd5Si4), by substituting germanium for silicon. The new compound, Gd5Si2Ge2, exhibits a magnetocaloric effect about twice as large as that exhibited by gadolinium, the best known magnetic refrigerant material for near room temperature applications.
The new material, which is described in articles to be published in Physical Review Letters on June 9, 1997, and Applied Physics Letter on June 16, 1997, has two advantages over existing magnetic coolants: it exhibits a giant magnetocaloric effect, the ability of certain materials to heat up when placed in a magnetic field and cool when taken back out again; and its operating temperature can be tuned from about 30K (-400 degrees F) to about 290K (65 degrees F) by adjusting the ratio of silicon to germanium -- the more germanium, the lower the temperature.
"There are very few systems that will give you that temperature span -- it's unheard of," says Karl Gschneidner, senior scientist at the Laboratory and Anson Marston distinguished professor in materials science and engineering at Iowa State University (ISU), who has been characterizing the materials with Vitalij Pecharsky, associate scientist at Ames Lab.
The efficiency of the new materials make magnetic refrigeration even more competitive with conventional gas-compression technology by replacing complex and costly superconducting magnets with permanent magnets in refrigerator designs. The elimination of superconducting magnets may also open the way for small-scale applications of this technology, such as climate control in cars and homes, and in home refrigerators and freezers. In addition, Gschneidner says the discovery may also launch totally new applications for efficient refrigerators at very low refrigeration powers since gas compression technology cannot be scaled down to such low cooling powers and since thermoelectric cooling is very inefficient (30 times less than magnetic refrigerants).
Gschneidner teamed with Astronautics Corporation of America earlier this year to unveil the first gadolinium-based magnetic refrigerator. The refrigerator has been operating for over six months, which far exceeds the few hours or days of operation recorded by similar units. In addition, the unit has achieved cooling power 20 to 1,000 times greater than previous units.
Currently, Gschneidner and Pecharsky are working with another Ames Laboratory group to find practical means of processing the new materials and with Astronautics to design, construct and test a variety of magnetic refrigerators, which span temperatures from 20K (-425 degrees F) to 300K (80 degrees F) and have cooling powers ranging from one watt to 50,000 watts.
"We're the only team in the world that is looking at the whole picture, from the design of the coolant to the operation of the refrigerator," says Gschneidner.
Gschneidner's research is funded by DOE's Advanced Energy Projects and Technology Research Division and the Materials Sciences Division.
Ames Laboratory is operated for the DOE by ISU. The Lab conducts research into various areas of national concern, including energy resources, high-performance computing, environmental cleanup and restoration, and synthesis and study of new materials.
Contact: Karl Gschneidner
ph: 515-294-7931
Steve Karsjen
ph: 515-294-5643
Last revision: 4/17/98 sd
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