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We are witnessing a great transition towards a society powered by renewable energies to meet the ever-stringent climate target. Hydrogen, as an energy carrier, will play a key role in building a climate-neutral society. Although liquid hydrogen is essential for hydrogen storage and transportation, liquefying hydrogen is costly with the conventional methods based on Joule-Thomas effect. As an emerging technology which is potentially more efficient, magnetocaloric hydrogen liquefaction can be a “game-changer”. In this work, we have investigated the rare-earth-based Laves phases RAl and RNi2 for magnetocaloric hydrogen liquefaction. We have noticed an unaddressed feature that the magnetocaloric effect of second-order magnetocaloric materials can become “giant” near the hydrogen boiling point. This feature indicates strong correlations, down to the boiling point of hydrogen, among the three important quantities of the magnetocaloric effect: the maximum magnetic entropy change ΔSmaxm, the maximum adiabatic temperature change ΔTmaxad , and the Curie temperature TC. Via a comprehensive literature review, we interpret the correlations for a rare-earth intermetallic series as two trends: (1) ΔSmaxm increases with decreasing TC; (2) ΔTmaxad decreases near room temperature with decreasing TC but increases at cryogenic temperatures. Moreover, we have developed a mean-field approach to describe these two trends theoretically. The dependence of ΔSmaxm and ΔTmaxad on TC revealed in this work helps researchers quickly anticipate the magnetocaloric performance of rare-earth-based compounds, guiding material design and accelerating the discoveries of magnetocaloric materials for hydrogen liquefaction.

The multicaloric effect is described by a temperature or entropy change of a material triggered by external stimuli applied or removed simultaneously or sequentially. The prerequisite for this is a material exhibiting multiple ferroic states. However, direct measurements of the effect are rarely reported. Now, for this reason, we built a measurement device allowing to determine the adiabatic temperature change in pulsed magnetic fields and, simultaneously, under the influence of a uniaxial load. We selected the all-𝑑-metal Heusler alloy Ni-Mn-Ti-Co for our first test because of its enhanced mechanical properties and enormous magneto- and elastocaloric effects. Ni-Mn-Ti-Co was exposed to pulsed magnetic fields up to 10 T and uniaxial stresses up to 80 MPa, and the corresponding adiabatic temperature changes were measured. With our new experimental tool, we are able to better understand multicaloric materials and determine their cross-coupling responses to different stimuli.

Natural gas is useful for the transition from traditional fossil fuels to renewable energies. The consumption of liquid natural gas has been rising, and the demand is predicted to double by 2040. In this context, magnetocaloric gas liquefaction, as an emerging and energy-saving technology, could be an alternative to the traditional gas-compression refrigeration. In this work, we report a large magnetic entropy change of 7.42 J/kg K under a magnetic field change of 2 T in Nd2In at 109 K, which is near the boiling temperature of natural gas of 112 K. The maximum adiabatic temperature change reaches 1.13 K under a magnetic field change of 1.95 T and is fully reversible. The magnetic phase transition is confirmed to be of the first-order type with the negligible thermal hysteresis. Further investigations on the thermal expansion and the magnetostriction reveal that the magnetic transition undergoes two stages with a negligible volume change. The longitudinal strain increases with magnetic fields and then decreases. These interesting properties are useful for the practical design of a magnetocaloric natural gas liquefaction system and for the fundamental understanding of the phase transitions in other RE2In intermetallics.