View Generic Document: Magnetic Refrigeration: Perfecting the Process
Citation:
Baumgold, Ben (2005). Magnetic Refrigeration: Perfecting the Process. National Institute of Standards and Technology, Technology Administration, U.S. Department of Commerce..
Currently, conventional refrigeration technology cyclically pressurizes and depressurizes Chlorofluorocarbons (commonly known as Freon) to provide cooling in modern air conditioners
and kitchen refrigerators. One of the main problems with this process is the unavoidable leakage of Freon into the atmosphere, causing ozone depletion and pollution. Additionally, current
refrigeration cannot easily reach temperatures below 30 K (-405° F), which are necessary for satellite sensor cooling and hydrogen liquification (for the future fleet of hydrogen-powered cars)
without multi-stage cooling that can be grossly inefficient. An exciting and revolutionary alternative to the presently inefficient and environmentally harmful conventional refrigeration is
magnetic refrigeration. This technology is based on the magnetocaloric effect of a material (the temperature change of that material due to the alignment of its magnetic spins when subjected to a
magnetic field). Prototype magnetic refrigerators use metal alloys and permanent magnets to exploit this effect. A metal alloy is cyclically magnetized and demagnetized to provide powerful and
efficient cooling without damage to the earth’s atmosphere. The alloy is placed in the cooling chamber and a strong magnetic field is applied to magnetize the substance, thereby increasing its
temperature but cooling the surrounding air in the refrigeration compartment. Then the material is removed from both the cooling chamber and the magnetic field. This allows the atomic spins to
return to a disordered state and the stored heat to be released, preparing the material for another cycle of cooling. Magnetic refrigeration is both nondestructive and theoretically more efficient
than either vapor expansion or radiant cooling. In order to optimize the cooling effects of a magnetic refrigerator, a metal alloy must be found that exhibits optimal magnetocaloric effects at
temperature ranges of interest. A recent publication in Nature (“Hysteresis losses in Gd5Ge2Si2 by addition of iron”, Nature, Volume 429, 24 June 2004, pages 853-857) shows that certain compounds,
doped with iron, possess greatly enhanced effects. This project was focused on examining Holmium-Titanium-Germanium (HoTiGe) alloys doped with iron under the prediction they would also possess
similar enhancements. Improved cooling potential around 90 K (-298° F) was previously documented by a group of Dutch scientists (“Magnetic-phase transitions and magnetocaloric effects”, Physica B,
Volume 319, 15 February 2002, pages 174-192). This summer, we revisited the Dutch research and found large magnetocaloric effects on this particular system at 15 K (-433° F) and 2 K (-456° F) as
well. Further, we have identified the specific phase of the alloy that is responsible for these low-temperature magnetocaloric effects, Ho58Ti5Ge36Fe. These results could have a major impact on
low-temperature cooling techniques in the future by greatly improving the efficiency of magnetic cooling at low temperatures.
Publisher
National Institute of Standards and Technology, Technology Administration, U.S. Department of Commerce.
Guest - January 07, 2009
View Generic Document: Magnetic Refrigeration: Perfecting the Process 