Concepts and microstructure design for multicaloric cooling using Ni-Mn-based Heusler compounds

The world’s rising population as well as environmental and economic changes go hand in hand with an increasing need for sustainable and energy-efficient cooling solutions. The most promising alternative to currently used vapor-compression technology is solid-state caloric cooling which utilizes the giant thermal response to an external field in vicinity of first-order phase transitions. However, there are still several limitations such as irreversibilities and energy losses during cyclic operation as well as the necessity of a rather large external field which hamper its application. The utilization of more than one external field which is known as multicaloric cooling, promises to overcome these limitations. For this purpose, materials with a pronounced cross-response to multiple stimuli are required. In this work, metamagnetic Ni-Mn-based Heusler compounds are investigated with respect to their multicaloric properties under magnetic fields and uniaxial stress. Different combinations of the two external stimuli are explored and the influence of microstructure on the caloric response and mechanical stability is investigated. It is demonstrated that a suitable combination of magnetic field and uniaxial stress can enable a significant improvement of the magnitude and reversibility of the caloric effect as compared to its single caloric counterparts. Moreover, a strong influence of microstructural features like precipitates, grain diameter and texture on the functional and mechanical performance is revealed. It is shown that a tailored microstructure design in metamagnetic Ni-Mn-based Heusler allows to simultaneously achieve excellent caloric and mechanical properties