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John O | September 2018

Researchers develop a cooling cycle using magnetic memory of special alloys


By Josh Perry, Editor
[email protected]

 

Researchers at the Technische Universität Darmstadt (TU Darmstadt) and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in Germany have developed a novel idea for a cooling cycle based on magnetic materials in magnetic fields, according to an article from TU Darmstadt

 


Scientists at the HZDR are exploring the potential and limits of magnetizable materials. The capacitor bank enables ultrashort magnetic pulses. (André Wirsig/TU Darmstadt)

 

Magnetic properties of metals change when heated or cooled. Some alloys become ferromagnetic when heated and when these alloys are put into external magnetic fields just below the transition temperature they will spontaneously become magnetic and simultaneously cool down.

 

Researchers also said that magnetic field can change the density of the alloys, increasing their volume. The scientists used an alloy of nickel, manganese, and indium in experiments because its transition can occur at room temperature.

 

“The researchers generated the magnetic field using the strongest permanent magnets known to date – containing the rare-earth metal neodymium in addition to iron and boron,” the article explained. “They can generate magnetic fields up to a flux density of 2 tesla – that is 40,000 times stronger than the Earth's magnetic field.”

 

Under this magnetic field, the alloy cooled several degrees and it took only a millisecond in the field to cause the transformation.

 

“In the next step of the six-step cycle, the researchers removed the cooling element from the magnetic field, which retained its magnetization,” the article continued. “In step three, the heat sink comes into contact with goods to be cooled down and absorbs its heat. The alloy even remains magnetic if the material returns to its original temperature. This can be remedied by mechanical pressure: in step four, a roller compresses the shape memory alloy.

 

“Under pressure, it switches to its denser, non-magnetic form and heats up in the process. When the pressure is removed in step five, the material retains its state and remains demagnetized. In the final step, the alloy releases heat into the environment until it has returned to its initial temperature and the cooling cycle can recommence.”

 

Because of the environmental impacts of typical coolants used in refrigeration cycles, or the potential for explosion of using propane or butane, the research team is convinced of a future for solid coolants.

 

Currently, manufacturing issues, the need for powerful magnets, and the limited supply of metals for this type of cycle have limited its potential, but researchers at TU Darmstadt hope to have a prototype by 2022 to demonstrate this cooling cycle under real-world conditions.

 

The research was recently published in Nature Materials. The abstract stated:

 

“The giant magnetocaloric effect, in which large thermal changes are induced in a material on the application of a magnetic field, can be used for refrigeration applications, such as the cooling of systems from a small to a relatively large scale. However, commercial uptake is limited.

 

“We propose an approach to magnetic cooling that rejects the conventional idea that the hysteresis inherent in magnetostructural phase-change materials must be minimized to maximize the reversible magnetocaloric effect.

 

“Instead, we introduce a second stimulus, uniaxial stress, so that we can exploit the hysteresis. This allows us to lock-in the ferromagnetic phase as the magnetizing field is removed, which drastically removes the volume of the magnetic field source and so reduces the amount of expensive Nd–Fe–B permanent magnets needed for a magnetic refrigerator.

 

“In addition, the mass ratio between the magnetocaloric material and the permanent magnet can be increased, which allows scaling of the cooling power of a device simply by increasing the refrigerant body. The technical feasibility of this hysteresis-positive approach is demonstrated using Ni–Mn–In Heusler alloys.

 

“Our study could lead to an enhanced usage of the giant magnetocaloric effect in commercial applications.”

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