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https://www.nwo-i.nl/en/fom-history/annual-reports/highlights/highlights2013/magnetocaloric-materials-not-only-for-refrigeration-nr-i28/

Printed on :
April 24th 2019
05:49:04

Efficient refrigeration is important for guaranteeing the preservation of food, for example. Magnetic cooling is an alternative for the widely used gas compression refrigeration process. The most important advantages of magnetic cooling are a high energy efficiency, scalability and the fact that the cooling happens completely silently. Recycling is also very simple as the refrigeration technique makes use of a solid substance as the cooling agent. Researchers from this programme are searching for suitable magnetocaloric materials for this application.

Magnetic cooling is based on the magnetocaloric effect. If you place a magnetic field over a material then the spin direction of the electrons will adjust to that of the magnetic field. As a result of this energy is released in the form of heat. This heat can be transported away via a liquid or gas. If the magnetic field is subsequently switched off then the electron spins become randomly oriented once again. That costs energy as a result of which the temperature of the material drops.

Suitable materials
Each magnetic material exhibits the magnetocaloric effect. The effect is particularly large in the vicinity of the Curie temperature , the temperature at which spontaneous magnetic ordering disappears, and in the case of materials with a large magnetic moment. In the rare earth metal gadolinium, the effect is the greatest at about room temperature. Gadolinium is expensive, however, and at temperatures above 30 °C the magnetoclaoric effect rapidly decreases.

The material MnFePSi (manganese, iron, phosphorous and silicon) exhibits a far greater magnetocaloric effect than gadolinium. In addition, the working temperature can be set much higher than room temperature by varying the phosphorous/silicon and manganese/iron ratios. The larger magnetocaloric effect arises because the magnetic transition from order to disorder is associated with a change in the atomic lattice as result of which heat is released. The sum of these heat effects is greater than the usual magnetocaloric effect. This effect is therefore termed the giant magnetocaloric effect.

To achieve a large magnetocaloric effect for less strong magnetic fields, boron is added to MnFePSi. In MnFePSi boron was found to end up on the lattice positions of phosphorus and silicon. This was not really expected because boron is much smaller than these elements, and in the structure there is enough space to accommodate boron. The new compound MnFePSiB does, however, exhibit the intended improved characteristics.