Influence of the Magnetic State in the Thermo-Structural Properties of Commercially Available La(Fe,Si,Mn) ₁₃ H _z Microparticles Applied in Magnetic Refrigeration Prototypes

La(Fe,Si,Mn)<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sub>H<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</sub> are among the most promising magnetocaloric materials (MCMs) used in room-temperature magnetic refrigeration. They exhibit an intense magnetocaloric effect (MCE), a low content of rare-earth elements and a tunable Curie Temperature (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_C</i>), the temperature at which the MCE reaches its apex. MCMs are assembled in a porous-layered component called active magnetic regenerator (AMR), where the materials are arranged to achieve a gradient of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_C</i> values. Since the material constantly changes its magnetic state to trigger the giant MCE, it is important to analyze how its properties behave in both ferro- and paramagnetic states. Besides that, although commercially available La(Fe,Si,Mn)<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sub>H<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</sub> compounds have been used in several studies in recent years, they cannot achieve the expected performance based on lab-made materials. This work investigates how the thermo-structural properties of several batches of commercially available La(Fe,Si,Mn)<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sub>H<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</sub> microspheres, with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_C</i>s ranging from 282 K to 307 K, vary according to the magnetic state of these materials at its application temperature, the room temperature. X-ray diffraction (XRD) analysis revealed that the magnetic state has a significant influence on the microstructural properties of the MCM. The crystallographic density of the magnetocaloric phase of the materials that were in the ferromagnetic state was about 1% higher than those in the paramagnetic state. Furthermore, the crystallographic results showed that the magnetic state of the magnetocaloric phase also affects the structure of secondary phases present in the material, such as the α-Fe, which suffers a 1% density reduction during the magnetic transition of the magnetocaloric phase. Similarly, photoacoustic absorption spectroscopy (PAS) results unveiled that the samples in the ferromagnetic state have higher values of thermal diffusivity, reaching up to 7.9 mm²/s, while materials in the paramagnetic state range from 4.5 to 5.5 mm²/s. Also, the PAS analysis also highlighted a key difference between commercially available materials and lab-scale compounds, the latter of which have lower values of thermal diffusivity (up to 1.5 mm²/s) because of the lack of a conductive secondary phase. The results presented in this paper highlight the importance of considering the differences between the properties of commercially available materials and laboratory-made ones and also the fluctuation of thermal and structural properties depending on the material’s magnetic state, which are key factors when designing magnetic refrigeration prototypes and simulating their performance.