3D-Printed High-Entropy Alloy Micro- and Nanoparticles for Magnetocaloric Energy Conversion

The HI-ENTROPY project has since summer 2019 focused on the development of novel materials for magnetocaloric energy conversion around room temperature. Two main materials classes have been intensively studied: 1.) classical high entropy alloys (HEAs), also called "multi principal element alloys" or "complex metallic alloys" with 5 or more elements at close to equiatomic composition forming randomized solid solutions (SS). 2.) MM'X-type materials (M,M'=transition metal and X=p-block element) that show both a structural and a magnetic transition upon heating/cooling. The materials that were developed are all based on cheap, abundant, and non-critical elements. The scientific achievements that were made during the last 3 ½ years have led to a better understanding of structure-property relationships and has allowed us to tailor materials to suit specific needs. HI-ENTROPY has also generated new research questions, some of which remain still unanswered, and which will occupy researchers potentially following in our footsteps for many more years to come. Some mysteries remain, such as the matter of long-term cyclability and thermal hysteresis which both still limit the practical application of these materials in future devices.

The scientific outcome is in the form of increased knowledge regarding high entropy alloys for magnetocaloric energy conversion applications around room temperature. We managed to develop novel materials and combinations of elements that were not investigated prior to the HI-ENTROPY project. Furthermore, we were able to establish some structure-property relationships for the MM'X-type compounds and gained insight into the driving force that triggers the magnetostructural transitions in those compounds. Furthermore, we developed candidate materials based on abundant and non-critical elements which could potentially find their way into future energy conversion devices.

Magnetocaloric energy conversion represents an alternative to compressor-based refrigerators, heat pumps and power generation with low enthalpy heat sources. This green technology offers an opportunity to use environmentally friendly solid refrigerants, and the potentially high energy efficiency follows the trends of future energy conversion devices. Currently employed magnetocaloric materials tend to exhibit a large magnetocaloric effect (MCE). However, they can suffer from cracking and fatigue, which severely limits their useful lifetime. The high entropy alloys (HEAs) are a class of emergent transition metal alloys that hold great potential for advanced manufacturing, and which may impact magnetocalorics. They offer supply chain and cost stability, and superior mechanical properties such as ductility, corrosion resistance, machinability, all of which ease manufacturing and bolster product longevity. HEAs have only very recently been considered for magnetocaloric applications, and are still a widely unexplored class of materials. In this project, we will study the influence of nanostructuring and chemical composition on atomic disorder, corrosion resistance, mechanical durability and thermo-magnetic properties of selected HEAs. Finally, we will create artificial structures from 3D-printed HEA micro- and nanoparticles and compare their thermo-magnetic performance to that of the neat powdered materials. A potential future application of this research lies in the area of magnetocaloric energy conversion: magnetic cooling, magnetic heat pumping and magnetic power generation. HI-ENTROPY is an interdisciplinary project that will address the key areas of materials design, synthesis, additive manufacturing, and device modelling concurrently. This interplay will facilitate the compromises necessary to reach the best combination of components and will allow implementing magnetocaloric heating and cooling as part of an efficient and environment-friendly energy system.