Leghe Heusler e leghe ad alta entropia per refrigerazione magnetica e recupero del calore di scarto

Energy plays a fundamental role in modern society, significantly impacting daily activities. Moreover, the increasing demand for energy, driven by new technologies, contributes to a profound impact on the environment and society. The high energy consumption becomes even more concerning considering that over 70% of the energy is lost during the transformation processes required to convert primary energy carriers into energy services. Furthermore, most energy losses occur in the form of heat emitted at temperatures below 100°C. The necessity for more efficient energy conversion solutions and the exploration of methods to recover waste energy is becoming increasingly crucial for the scientific community. Two promising fields within materials science addressing this challenge are magnetic refrigeration, which focuses on the development of more energy-efficient cooling technologies (magnetocaloric applications), and waste low-grade heat recovery (thermomagnetic applications). This thesis aims to contribute to address these issues by synthesizing NiMn-based Heusler alloys and High Entropy Alloys, composed of elements considered environmentally and economically non-critical. The magnetic transitions in the synthesized alloys are all second-order, ensuring full reversibility of the phenomena under investigation. The first part of the work focuses on the synthesis and magnetic characterization of various NiMnSn-based Heusler alloys, examining how the most relevant magnetic characteristics for magnetocaloric and thermomagnetic applications vary with changes in alloy composition. This analysis demonstrates the optimal applicability of NiMnSn-based Heusler alloys, as their Curie transitions cover a temperature range between 300 K and 350 K. Additionally, the thesis presents the development and application of a thermomagnetic device that converts controlled heat into easily measurable mechanical energy, enabling comprehensive thermomagnetic analysis of the materials under operando conditions for the first time. This device was initially employed to test the thermomagnetic properties of three representative Heusler alloys: NiMnSn, NiMnIn, and NiMnCuGa. The results demonstrated, firstly, the potential of the thermomagnetic motor prototype as an effective testing platform for thermomagnetic generation designs, as well as the promise of NiMn-based second-order Heusler compounds for low-grade heat energy harvesting applications. Notably, the NiMnIn alloy could generate an electric power of 2.6 W/kg (20.4 mW/cm³), which significantly exceeds the power generated by other devices presented in the literature. Subsequently, 3D-printed additive manufacturing rotors based on NiMnSn and NiMnIn with varying thicknesses were developed to evaluate their thermomagnetic properties. A straightforward composite filament production method enabled uniform dispersion of magnetic powders within a polymer matrix, allowing for the fabrication of rotors with different thicknesses. The findings indicated that an increased rotor surface-to-volume ratio correlated with higher power peaks, confirming previous observations. Notably, the power peak curves of the 3D-printed rotors exhibited a sharp peak near the alloy’s transition temperature, reflecting a temperature dependence that aligned with simulated trends. The study underscored a promising relationship between rotor geometry and thermomagnetic performance. Finally, two new high-entropy alloys, FeNiGaMnSi-based, were synthesized. Magnetic characterizations identified two second-order magnetic transitions near room temperature for both compositions, indicating the alloys’ suitability for potential magnetocaloric and thermomagnetic applications. The magnetic entropy changes and the magnetic work generated in an ideal thermomagnetic cycle (Wm) were calculated for both alloys. The ΔS values were comparable to the highest reported values for rare-earth-free HEAs across the Curie transition, demonstrating strong magnetocaloric application potential for these synthesized HEAs. Additionally, the calculated Wm values were similar to those obtained for NiMn-based Heuslers, further suggesting promise for TM applications. Characterizations performed with the TM device confirmed these findings, showing for the first time that the synthesized HEAs are promising candidates for waste energy recovery, which enhances interest in this novel class of functional materials. In conclusion, this work aims to present new and effective solutions for magnetic energy conversion applications, starting with the synthesis of functional materials with magnetic properties that can be easily tuned and concluding with the evaluation of their performance under operational conditions. This approach addresses the existing gap between the functional study of magnetic alloys and their practical applications. The work has been carried out primarily at the Institute of Materials for Electronics and Magnetism of the National Research Council (IMEM-CNR), where all the activity on synthesis and fundamental characterization of the thermomagnetic materials has been realized. Thanks to the strong collaboration with the Department of Physics of the University of Parma it has been possible to characterize the magnetic properties of the materials produced, and to test their functional behaviour and application potential in a thermomagnetic prototype developed there. A stage at the University of Sevilla, Spain, allowed for the realization of composite filaments containing the thermomagnetic materials and their use for 3D printing thermomagnetic rotors that were tested in the prototype.