Multi-stimuli cooling cycle

Magnetic refrigeration is probably the most developed of the solid state cooling technologies and has been in focus of intense research since the discovery of giant magnetocaloric effects in the 1990’s. It is based on the magnetocaloric effect — a useful adiabatic temperature change in a substance exposed to a changing magnetic field [1]. A typical problem of the solid state refrigerants is that the largest effects are shown by the materials with first-order phase transitions, which have the hysteresis that hinders the efficiency of the material. A multicaloric cooling cycle rethinks the whole concept of the conventional caloric cooling. Instead of trying to squeeze the best out of magnetostructural phase-change materials in relatively low magnetic fields by attempts of the reduction of the thermal hysteresis, we intend to make use of it by introducing a second stimulus in the form of mechanical load.

Exploiting the thermal hysteresis

We exploit, rather than avoid, the thermal hysteresis of magnetocaloric materials which undergo a first-order transition [2]. By applying magnetic field at a temperature near the transition, the material transforms to the high-magnetization phase. When the magnetic field is removed, the material remains “locked” in the phase with high magnetization because of the hysteresis. Now another stimulus, such as mechanical stress, can be applied to transform the material back to the original state. In this way, we turn the thermal hysteresis from being a problem for magnetocalorics into an advantage for multi-stimuli caloric materials. The working principle of such multicaloric cycle is shown below on the left, while the design of a possible future cooling device is illustrated on the right.

[1] I. Takeuchi, K. Sandeman, Solid-state cooling with caloric materials, Physics Today (2015)
DOI: 10.1063/PT.3.3022

[2] T. Gottschall, A. Gràcia-Condal, M. Fries, A. Taubel, L. Pfeuffer, L. Mañosa, A. Planes, K.P. Skokov and O. Gutfleisch. A multicaloric cooling cycle that exploits thermal hysteresis, Nature Materials (2018)
DOI: 10.1038/s41563-018-0166-6

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