This paper presents the design, fabrication and characterization of a thermo-magnetic film generator for efficient conversion of low-grade thermal energy into mechanical energy and an additional piezoelectric layer to convert the mechanical into electrical energy. Harnessing the abundant waste heat available at low temperatures holds immense potential to power off-grid electronics, such as IoT systems and wearable medical devices. In previous work, we introduced a new class of thermo-magnetic film-based generators (TMGs) for conversion of low-grade thermal energy consisting of movable cantilevers oscillating in resonant self-actuation mode, which boosts power output up to 50 µW/cm² while relative efficiencies can reach up to 5% even at temperature difference below 10 K [1]. Based on the concept of resonant self-actuation, we explore the option of converting the mechanical energy of oscillation into electrical energy using an additional piezoelectric layer. Compared to inductive energy conversion, this approach enables an increased voltage output, which can be rectified and stored by standard electronic components.

As shown in Figure 1 (a), the initial part of the cantilever is designed as a three-layer system consisting of brass, epoxy and a piezoelectric layer. The piezoelectric layer is fabricated from 100 µm thick bulk PZT and thinned down to 30 µm by mechanical grinding. The cantilever onset is rigidly clamped to a substrate. Two films made from the magnetic shape-memory-alloy Ni-Mn-Ga of 10 µm thickness are stacked and bonded, then attached to the end part of the cantilever and positioned beneath a magnet, which also serves as a heat source. A finite element model and an analytic model based on Bernoulli’s theory are used to design the piezoelectric layer and optimize its power density.

In ferromagnetic state below the Curie temperature (T_C), the Ni-Mn-Ga film is attracted by the magnet. While in solid-solid contact with the heated magnet, it heats above T_C and becomes paramagnetic. Therefore, the magnetic attraction force strongly decreases allowing the elastic forces of the cantilever to reset the initial deflection. As the film cools down below T_C, it transforms back to its ferromagnetic state and the cycle begins anew.

In this work, we demonstrate resonant self-actuation of the hybrid piezoelectric thermo-magnetic film generator by fine-tuning the oscillation frequency of the cantilever to achieve a sufficient temperature change during solid-solid contact with the heated magnet. Due to the epoxy and PZT layers, the thermal behavior of the system is altered, changing the conditions needed for resonant self-actuation. These changes are compensated by reducing the length of the piezoelectric layer and increasing the surface of the brass layer for enhanced convection. Characterization of a cantilever reveals peak output values of 1.5 V and 17 µA at a stroke of 1 mm and a frequency of 100 Hz. With a compact footprint of 0.2 cm², this results in a peak power density of 38 µW/cm². As shown in Figure 1 (b), the peak power output of a demonstrator device is 0.9 µW.

These preliminary performance results underline the potential of the concept of resonance self-actuation. In particular, the achieved electrical output facilitates the development of a power management system for energy use.

Figure 1: (a) Schematic of the multilayer piezoelectric thermo-magnetic film generator. (b) Power output of the generator device operating in resonant self-actuation mode.

References
[1] Joseph, J., Ohtsuka, M., Miki, H., & Kohl, M. (2020). Upscaling of Thermomagnetic Generators Based on Heusler Alloy Films. Joule, 4(12), 2718-2732. Elsevier. https://doi.org/10.1016/j.joule.2020.10.019