discovered_01_2015

discovered 01.15 FOCUS WWW.HZDR.DE About 30 percent more efficient than traditional technologies An alternative could be magnetic cooling, which works without any traditional cooling agents or compressors. ‘If we could harness the magnetic properties of certain materials, we could build refrigerators that are 30% more efficient,’ explains HZDR doctoral candidate Mahdiyeh Ghorbani Zavareh. At the Dresden High Magnetic Field Laboratory (HLD), the 31-year-old Iranian examines materials and alloys whose magnetocaloric effect is particularly strong: ‘Magnetocaloric materials heat up when exposed to a strong magnetic field. As soon as they are removed from that field, however, they cool down again.’ Physicists use the basic thermodynamic principle that entropy – which, in very simple terms, is the degree of disorder in a system – cannot decrease in a given closed circuit. If a magnetocaloric material is exposed to a magnetic field, the magnetic moments in the material align, the magnetic entropy decreases. To compensate for it, entropy in the atomic lattice must increase: the atoms vibrate more strongly, leading to a temperature rise of the material. This heat can be released outside of the refrigerator. Once the magnetocaloric material is removed from the magnetic field, the magnetic order is lost and the atoms in the material respond accordingly: they oscillate less, the temperature drops. The now cooled material can take up heat again – in this case, from inside the device – and the cooling cycle can start anew. Search for the best material For some time, scientists have been using this interaction between magnetism and thermodynamics to achieve extremely low temperatures close to absolute zero. Besides that, some first prototypes for household use already exist: attached to the back of such a refrigerator is a permanent magnet, next to which a disk with a magnetocaloric material rotates. So far, however, a commonly used material in this process is the rare-earth metal gadolinium, which would be far too expensive for mass production. ‘Initially, I also used gadolinium in order to test my set-up. For later applications, however, it would be interesting to find alternative compounds with similar properties. With my experimental set-up, we can now study various magnetocaloric materials,’ says Mahdiyeh Ghorbani Zavareh. The scientist tests these various samples under strong magnetic fields. High currents are fed through special HZDR-produced coils at short time intervals; this generates intensive magnetic pulses. Up to a certain limit, which is different for each material, a simple rule applies: the stronger the magnetic field in one pulse, the higher the temperature Mahdiyeh Ghorbani Zavareh Mahdiyeh Ghorbani Zavareh studied physics at the University of Technology in her home town of Isfahan (Iran). She was one of the best of her graduating class and completed her Master’s degree with a theoretical thesis on the interaction of electrons in graphene- nanoribbons. Together with her husband, she moved to Dresden at the end of 2011. ‘After my husband had been accepted as a PhD candidate at the Institute of Ion Beam Physics and Materials Research at HZDR, they also advertised a position at the Dresden High Magnetic Field Laboratory.’ The young Iranian began her doctorate in February of 2012. CONTACT _Dresden High Magnetic Field Laboratory at HZDR Mahdiyeh Ghorbani Zavareh m.ghorbani@hzdr.de change. For practical applications, however, what matters is the duration of the magnetic pulses: ‘Here at the HLD, the pulses last between 10 and 100 milliseconds. That is exactly the frequency of 10 to 100 Hertz at which future magnetic refrigerators could operate in real life,’ the physicist explains. The cooling performance is measured in the lab under realistic conditions and can thus be transferred to potential applications. In her experiments so far, the temperature could be lowered by up to ten degrees centigrade per cooling cycle – more than enough to keep butter and cheese fresh in a normal household. PUBLICATION: M. Ghorbani Zavareh et al.: ‘Direct measurements of the magnetocaloric effect in pulsed magnetic fields: The example of the Heusler alloy Ni50 Mn35 In15 ’, in Applied Physics Letters 106 (2015, DOI 10.1063/1.4913446)

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