Engineering Hysteresis and Non-diffusive Phase Transformations in Magnetocaloric (Mn,Fe)2(P,Si) Alloys for Magnetic Refrigeration Applications

Abstract

Magneto-structural transformations are non-diffusive phase transformations manifesting coupled magnetothermal properties that can enable useful applications like sensing, thermomagnetic generation, and especially, efficient solid-state refrigeration. However, implementation of magnetocaloric materials within these technologies requires a complex optimization over their underlying transformations’ critical temperatures, enthalpies, and hysteresis, with hysteresis losses a critical limiting factor. Successful engineering of transformations within this application space requires deeper understanding of (1) the trade-offs between transformation properties and their effects on macroscopic system efficiencies and (2) the underlying microscale mechanisms that control the transformation and its hysteresis. In this study, phase transformations in caloric hexagonal (Mn,Fe)v2(P,Si) alloys are investigated using a combination of modeling and experimental techniques. First, alloys’ macroscopic magnetothermal properties are coupled with a thermodynamic model incorporating hysteretic path-dependent evolution to simulate refrigeration efficiencies and cooling for relevant cycle classes. Results demonstrate extreme decreases in thermodynamic efficiencies of 10 % per 1 K thermal hysteresis, and the importance of First Law refrigeration work terms in governing cycle performance, both aspects rarely appreciated in the literature. Second, quantitative compositional analysis and calorimetry experiments are used to demonstrate mechanisms controlling the transformation, like heat-treatment induced multi-step behavior and oxygen-mediated shifts in transformation properties due to phase segregation. Finally, force microscopy techniques are employed to directly image reversible movement of the phase boundary, suggesting growth-dominated behavior from pockets of retained martensite. Together, these results both create rational efficiency-based benchmarks for engineering phase transformations in (Mn,Fe)v2(P,Si) alloys and point towards processing techniques for achieving them.