Magnetic refrigerators offer significant advantages when compared with current vapor-compression ones, such as gains in energy efficiency, lower cost of operation or elimination of environmentally damaging coolants. Unfortunately, all the materials which have been tested in the last fifty years suffer from hysteresis losses, lowering the energy available for cooling. But now, National Institute of Standards and Technology (NIST) researchers have found a solution, reported in "Nanomaterial Yields Cool Results." By adding a small amount of iron to a gadolinium-germanium-silicon alloy, they enhanced the cooling capacity by 30 percent. This very significant step may help move the promising technology of magnetically generated refrigeration closer to market.
Here is how this works.
By adding a small amount of iron (about 1 percent by volume), the NIST team enhanced the effective cooling capacity of the so-called "giant magnetocaloric effect" material by 15 to 30 percent. The result, writes materials scientist Virgil Provenzano and his NIST colleagues, "is a much-improved magnetic refrigerant for near-room-temperature applications."
The original material -- a gadolinium-germanium-silicon alloy -- already is considered an attractive candidate for a room-temperature magnetic refrigerant. However, its cooling potential is undercut by significant energy costs exacted during the on-and-off cycling of an applied magnetic field, the process that drives the refrigeration device. These costs -- called hysteresis losses -- translate into commensurate losses of energy available for cooling.
The iron supplement overcomes this disadvantage. It nearly eliminates hysteresis and the associated energy cost, permitting the material to perform near the peak of its potential.
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NIST materials scientist Robert Shull loads an alloy sample into a chamber surrounded by a superconducting-magnet as he prepares to make measurements of the magnetocaloric effect -- the property that causes certain materials to heat up when exposed to a magnetic field and to cool down when the field is removed. (Credits: legend by NIST, photo by Kathie Koenig) |
Here are more details provided by EurekAlert! in "Tiny iron supplement has chilling effect."
When exposed to a magnetic field, the gadolinium alloy and other magnetocaloric-effect materials heat up as their spinning electrons align with the field, thereby magnetizing the materials and raising their temperature. When the external field is removed, the materials demagnetize -- the electrons revert to a disordered magnetic-spin state -- and their temperature drops. The two-stage process forms the magnetic refrigeration cycle.
Adding the iron supplement largely suppresses a rearrangement of atoms that occurs as the applied magnetic field increases (or temperature decreases) in the original gadolinium alloy. In turn, stifling the shift in atomic structure all but eliminates the hysteresis loss, the NIST team found.
Yet, the NIST team is not sure of what is happening.
NIST magnetics researcher Robert Shull, one of the NIST inventors of the new nanocomposite material, notes that the modest addition of iron results in the formation of nanometer-sized magnetic clusters in the gadolinium alloy.
"How such a nanomagnetic structure developed is unknown," Shull explains. "It's existence is certain, but its cause is very subtle."
This research work has been published in the June 24 issue of Nature. Here is a link to the abstract of the paper, called "Reduction of hysteresis losses in the magnetic refrigerant Gd5Ge2Si2 by the addition of iron."
Sources: National Institute of Standards and Technology (NIST), via ScienceDaily, July 5, 2004; and various websites
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