Energy-efficient cooling technology is extremely important in today’s society, considering the need for energy conservation and the urgent need to mitigate global warming. Near-room-temperature magnetic refrigeration is an emerging cooling technology that has several advantages compared to conventional gas-compression technology. It utilizes the magnetocaloric effect (MCE) in which heating and cooling of a magnetocaloric material (MCM) is induced by a varying external magnetic field. The magnetocaloric effect (the temperature change of a magnetic material due to the application of an external magnetic field) is the cornerstone of magnetic cooling.

MCE is the vital element of near-room-temperature magnetic cooling, which is poised for commercialization in the future. In addition, MCE is intrinsic to every magnetic solid and is of fundamental importance; it is related to diverse topics such as spin dynamics and physical properties such as thermal conductivity.

Schematic representation of a Magnetic Refrigeration Cycle in which heat is transported from the heat load to its surroundings.
In a magnetic material, entropy is a combination of contributions from the lattice as well as from electronic and magnetic spins. Upon an adiabatic application of an external magnetic field, the magnetic spins align parallel to the field, which causes the magnetic part of the entropy to decrease. The lattice entropy consequently increases, leading to a temperature rise in the material. This heat is removed from the material to its surroundings by a heat-transfer medium (depending on the operating temperature, the heat-transfer medium may be water or air, and for very low temperatures, helium). When the field is removed under adiabatical conditions, the magnetic material cools down to below ambient temperature due to an increase in disorder of the magnetic spins; this results in lower lattice entropy.

Fe80-xGdxCr8B12 alloys exhibit good magnetocaloric properties near room temperature. Gd, which is well-known to exhibit good magnetocaloric effects in room temperature, was alloyed in Fe-Cr-B base alloys. The increase of Gd additions displaces the Curie temperature of the alloy to higher temperatures. Because of this, the experimental investigation of magnetocaloric effects and structural properties of Fe80-xGdxCr8B12 (x = 0, 5, 8, 15) alloys has been studied.

Gd addition decreases the peak entropy change slightly and displaces TC to higher temperatures. The RC values of Fe80-xGdxCr8B12 alloys compare favorably to those in GMCE materials, and the alloys are much cheaper than

the rare-earth-based MCM. The universal curve models are applicable to the studied alloys. The exponent, n, controlling the field dependence of the magnetic entropy change enables easier MCE extrapolations of the alloys at different magnetic fields.

This work was done by Raju Ramanujan of Nanyang Technological University, Singapore, for the Asian Office of Aerospace Research and Development. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Physical Sciences category. AFRL-0168


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This article first appeared in the December, 2010 issue of Defense Tech Briefs Magazine.

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