Development of Magnetic Refrigeration Materials for Cryogenic Applications

Hydrogen will play a major role in the establishment of a renewable-energy-based society and realization of carbon neutrality. However, one of the major challenges is the storage and transportation of hydrogen: the H₂ gas has large volume and its liquefaction is necessary. The current available technology for hydrogen liquefaction is conventional gas compression systems based on the Joule-Thompson effect, which is costly and inefficient at low temperatures. Cryogenic magnetic refrigeration (CMR) is a prospective environmentally friendly technology for temperature below 120 K, and it can theoretically exhibit a higher efficiency as compared to those of conventional gas compression systems. In the CMR, the gaseous hydrogen is pre-cooled down to ~77 K by using liquid nitrogen, and, at the second stage, the magnetic liquefier provides a further decrease in temperature down to 20 K (H₂ liquefaction temperature). However, the lack of magnetic refrigerant materials with high magnetic entropy change in a wide temperature range of 77-20 K required for the hydrogen liquefaction is a bottle-neck for practical applications of MR cooling systems<br/>In this talk, we will first demonstrate how data science can be used to develop magnetocaloric materials with giant and reversible magnetocaloric effect at the cryogenic temperatures. We conducted machine learning on a dataset extracted from literature on Fe₂P based alloys to predict optimum alloy composition which results in a decrease of transition temperature below 100 K. Combination of machine learning and experimental validations resulted in realization of rare-earth free (Mn,Fe,Co)₂(P,Si) based compounds which exhibit a large magnetocaloric performance in isothermal magnetic entropy change (Δ<i>S_m</i>) of 7.5–11.5 J/kgK at the temperatures below 100 K [1]. This study demonstrates that data-driven development of magnetocaloric materials can efficiently boost the optimization of their properties, thus aiding the practical applicability of magnetic refrigeration technology. In the second part of the talk, we will show a series of materials with a giant magnetocaloric effect (MCE) in magnetic entropy change (-Δ<i>S_m</i> &gt; 0.2 Jcm^-3K^-1) in the Er(Ho)Co₂-based compounds, suitable for operation in the full temperature range required for hydrogen liquefaction (20-77 K) [2]. We found that the giant MCE becomes reversible, enabling sustainable use of the MR materials, by eliminating the magneto-structural phase transition revealed by detailed cryogenic and in-situ microstructure characterizations. We will discuss how this discovery can lead to the application of Er(Ho)Co₂-based alloys for hydrogen liquefaction using MR cooling technology for the future green fuel society.<br/>[1] J. Lai et al. “Machine learning assisted development of Fe₂P-type magnetocaloric compounds for cryogenic applications” Acta Mater. 232 (2022) 117942.<br/>[2] Xin Tang, et al. “Magnetic refrigeration material operating at a full temperature range required for hydrogen liquefaction” Nature Comm. 13 (2022) 1.