Magnetic cooling alloys
Many magnetic materials react with a change in temperature, when placed in a magnetic field or away from it. Researchers at the Leibniz Institute for Solid State and Materials Research Dresden and TU Darmstadt have Heuslerlegierung of nickel, manganese, cobalt, indium, and examined more closely, in the magnetocaloric effect is particularly large. The researchers propose to use the material for the development of highly efficient refrigerators. This could achieve a higher efficiency than the established compression chillers.
This phenomenon has been known since 1881 found only a few technical applications. Physicists use it for example in order to produce extremely low temperatures near absolute zero. For a broader application, it is still a lack of suitable materials. The main reason is that the Curie temperature above which no longer occurs, the effect is very low for many substances. Several research groups are working on materials exhibiting magnetocaloric behavior at higher temperatures up to room temperature, require only small magnetic field strengths, while inexpensive, nontoxic and readily available are.
Physics of the cooling process
If a conventional magnetocaloric material placed in a magnetic field, its disordered magnetic moments align parallel from the magnetic field. The higher magnetic ordering state corresponds to a lower internal energy than before. The released energy is emitted as heat under adiabatic conditions. If you cool down the heated substance in a magnetic field again to the starting temperature, and then turns off the magnetic field, the reverse process takes place and the material cools further. The material now reaches a few degrees Celsius lower temperature than at the beginning of the cycle. In this state it can absorb heat and thus serve as a coolant.
Larger effects of structural entropy
Even with the examined nickel-manganese alloy occurs on the described conventional magnetocaloric effect. The magnetic orientation of the magnetic field is thus free energy in the form of a moderate increase in temperature. At the same time forces the magnetic field also has a structural transformation of the crystal lattice. Here, a far greater degree of energy is stored. This inverse caloric effect thus leads to a marked cooling of the material. The height of the cooling effect resulting from the sum of these oppositely acting effects when applying the magnetic field.
Material improvements
With the optimal choice of the chemical composition of the researchers achieve a change in temperature of up to 6 degrees at moderate magnetic fields of 2 Tesla. From theoretical considerations and model, they infer that the following conditions for high temperature changes in magnetocaloric materials are low: a complete phase change in a narrow temperature interval and the optimal field dependence of the transition temperature.
In addition, the scientists another problem of nickel-manganese alloys, advanced on the body: the high temperature changes in these alloys can be achieved so far only in the first cycle and decrease drastically in the following cycles. They found that external pressure, the cyclic behavior improved significantly. The possible temperature ranges can be influenced by the precise adjustment of the crystallographic lattice parameters and the stacking of layers of certain magnetocaloric alloys.
Trade publication
A publication in the journal "Nature Materials" presents the research results in detail.
"Giant magnetocaloric effect structural transitions driven by Jian Liu, Tino Gottschall, Constantine P. Skokov, James D. Moore, Oliver Gutfleisch Nature Materials, Advance Online Publication (AOP), DOI 27/05/2012: 10.1038/NMAT3334
