Design and characterization of rare-earth-free high entropy alloys for thermomagnetic heat recovery

Thermomagnetic technologies provide an innovative pathway for recovering low-grade waste heat, leveraging the temperature dependence of magnetic properties for energy conversion. In this study, FeNiMnGaSi-based High Entropy Alloys (HEAs) are synthesized and characterized to evaluate their potential for thermomagnetic applications. The alloys were designed according to high-entropy criteria to obtain Rare-Earths-free compositions, mainly based on sustainable and high-availability elements, exhibiting second-order Curie-type transitions within the temperature range suitable for low-grade heat recovery. Structural analyses confirm a body-centred cubic structure with a high degree of compositional uniformity, while magnetic measurements demonstrate single-phase Curie transitions near room temperature, making these materials ideal candidates for thermomagnetic conversion. To complement the experimental measurements, calculations based on magnetization data are performed to estimate the magnetic work produced during ideal thermomagnetic cycles. The results confirm the suitability of both alloys for thermomagnetic applications, demonstrating significant energy conversion potential. In-operando tests in a bespoke thermomagnetic motor prototype confirm their ability to deliver high mechanical and electrical power outputs, 4 mW cm⁻³ and 2.8 mW cm⁻³ respectively, surpassing benchmark values reported in the literature. These findings establish the potential of HEAs for advancing thermomagnetic technologies and enabling efficient and sustainable energy applications.