title: On the rare-earth-based intermetallic compounds for magnetocaloric hydrogen liquefaction: a matter of performance and criticality
Creators: name: Liu, Wei
Publisher: name: UNSPECIFIED
Year: 2024

description: Hydrogen energy plays an essential role in transitioning to a climate-neutral society. Although important for the efficient use of hydrogen energy, liquid hydrogen is not economical enough for large-scale energy-related applications such as energy storage and hydrogen-powered vehicles. The reason for this is the low efficiency of the conventional liquefaction technologies that are based on Joule-Thomson expansion. As an emerging liquefaction technology based on the magnetocaloric effect (MCE), magnetocaloric (MC) hydrogen could be a "game changer" for the liquid hydrogen industry for its great potential to achieve higher efficiency. This thesis focuses on the magnetocaloric effects of rare-earth-based MC materials, as they are promising candidates for MC hydrogen liquefaction. The first part of this work reveals a feature by a study on the heavy rare-earth Laves phases: Second-order MC materials can also achieve a giant MCE at low temperatures. Two trends have been summarized from an extensive literature review: (1) the maximum magnetic entropy change increases as the Curie temperature decreases; (2) the maximum adiabatic temperature change decreases near room temperature as the Curie temperature decreases, but increases in cryogenic temperature range. Although heavy rare-earth MC materials possess excellent magnetic entropy change and adiabatic temperature change, heavy rare earths are highly critical, questioning the feasibility of using heavy rare-earth alloys for large-scale MC hydrogen liquefaction applications. In contrast, the lower criticality of light rare-earth elements makes their alloys appealing. Based on the discovery that the maximum MCE becomes more pronounced toward lower Curie temperature in cryogenic temperature range, a method of designing a light rare-earth Laves phase series for hydrogen liquefaction is developed: mixing different light rare-earth elements with different de Gennes factors at the rare-earth sublattice to adjust Curie temperature. Successfully, light rare-earth-based (Pr,Ce)Al₂ and (Nd,Pr)Al₂ materials are developed, showing a competitive MCE that covers the temperature range of 77 ~ 20 K. Recognizing that second-order MC materials do not exhibit the same excellent magnetic entropy change near 77 K as they do near 20 K, studies are carried out on light rare-earth-based intermetallic compounds with a first-order phase transition. A large magnetic entropy change is discovered in Nd₂In. In particular, this compound demonstrates a first-order phase transition at approximately 108 K, with negligible thermal hysteresis. Further studies on Pr₂In reveal that the Debye temperature plays an important role in achieving a large adiabatic temperature change: materials with a higher Debye temperature tend to exhibit a larger adiabatic temperature change, especially in the cryogenic temperature range. This thesis studies the MCEs of the heavy rare-earth Laves phases, the light rare-earth Laves phases with a second-order phase transition, and the light rare-earth-based first-order magnetocaloric materials---Nd₂In and Pr₂In. This work shows that maximum MCE becomes more pronounced as the Curie temperature decreases in the cryogenic temperature range. The aim of this work is to reduce the criticality of the raw materials while maximizing the magnetocaloric performance.