Magnetocaloric effect
The magnetocaloric effect describes the phenomenon of a temperature increase in a magnetizable material when it is exposed to a stronger magnetic field . The effect is reversible and manifests itself as a falling temperature as a result of a decreasing magnetic field.
Effect and application
The effect arises when the magnetic moments of the material are aligned by the magnetic field; this orientation also decreases again as the magnetic field decreases. The alignment speed of the magnetic moments usually shows a clear hysteresis behavior, which depends on the respective material. The search for suitable alloys with low hysteresis should result in materials that are suitable as heat pumps : through periodic magnetization and simultaneous dissipation of the resulting heat , they can be used to achieve a cooling effect . The test of corresponding prototypes serves to test the effect for applications in household appliances. Many of the prototypes use gadolinium , which has a Curie temperature of 19 ° C, or alloys containing this element .
The magnetocaloric effect can also be used for heating, e.g. B. via underfloor heating (magnetic heat pump). The Coefficent of Performance (COP) is higher than that of a conventional heat pump.
At the electronics trade fair CES in 2015, BASF and Dutch scientists presented the first prototype of a wine cooler in which a magnetocaloric heat pump generates the cold. The device consumes 35 percent less electricity than comparable models and works almost silently. The new material Quice is used, which is made from the inexpensive and easy-to-obtain elements iron and manganese. When this material comes into a magnetic field, it heats up by 20 degrees and more. This thermal energy is absorbed by a cooling circuit in which water circulates. It transports the heat into the environment. At the same time, the inside of the cooling device cools down.
entropy
The total entropy of a system is constant or increasing. In the magnetocaloric effect, the total entropy consists of the thermal and the magnetic entropy. By aligning the magnetic moments in a material, the magnetic entropy decreases. Thus, the thermal entropy must rise to compensate, and thus the temperature.
See also
literature
- Horst Stöcker: Pocket book of physics . 4th edition. Verlag Harry Deutsch, Frankfurt am Main 2000, ISBN 3-8171-1628-4 .