High-Performance Thermomagnetic Generators Based on Heusler Alloy Films

Recent developments on Heusler alloys including Ni-Mn-X and Ni-Co-Mn-X (X = Ga, In, Sn,.) demonstrate multiferroic phase transformations with large abrupt changes in lattice parameters of several percent and corresponding abrupt changes in ferromagnetic ordering near the transition temperatures. These materials enable a new generation of thermomagnetic generators that convert heat to electricity within a small temperature difference below 5 K. While thermodynamic calculations on this energy conversion method predict a power density normalized to material volume of up to 300 mW cm(-3), experimental results have been in the range of mu W cm(-3). Challenges are related to the development of materials with bulk-like singlecrystal properties as well as geometries with large surface-to-volume ratio for rapid heat exchange. This study demonstrates efficient thermomagnetic generation via resonant actuation of freely movable thin-film devices of the Heusler alloy Ni-Mn-Ga with unprecedented power density of 118 mW cm(-3) that compares favorably with the best thermoelectric generators. Due to the large temperature-dependent change of magnetization of the films, a periodic temperature change of only 3 K is required for operation. The duration of thermomagnetic duty cycle is only about 12 ms, which matches with the period of oscillatory motion.