A Miniaturized Autonomous Thermomagnetic Generator

Thermomagnetic generators (TMG) are promising devices for converting low-grade waste heat into electricity. Self-actuation using permanent magnets is the key to autonomy. Tailoring thermomagnetic materials in the form of flexible and freestanding films is a way towards high power-density through fast, and efficient heat transfer. Design optimization, and the use of micromagnets represent a unique opportunity towards miniaturization. Here the operation and performance of a TMG using a flexible freestanding Gd film as the active substance, and an array of NdFeB micromagnets as field source is presented. The Gd film self oscillates under the effect of the magnetic field gradient, entering in contact with the heat source, and the heat sink alternatively. At an operating frequency of $34$~Hz, a designed piezoelectric spring converts the mechanical oscillation into electrical energy, reaching up to $4.7~\mu$W power-output with $20$\textdegree C temperature difference. The magnetic field gradient acting over the thermal switch, and triggering the caloric response of this $16~\mu$m thick Gd film, is generated by an array of $50~\mu$m thick NdFeB micromagnets, which produces a $0.2$~T variation in field strength over a $45~\mu$m gap. The power density normalized to the combined volume of the Gd film and of the magnet reaches up to $1.46~$mW cm$^{-3}$.