Miniaturized Broadband Vibration Energy Harvesting

This thesis is a contribution to the research in piezoelectric-based energy harvesting with a focus in miniaturized devices. The aim of this project is to develop a small scale vibrational energy harvester that shows a broadband piezoelectric response by applying magnetic fields that force the devices to enter into a non-linear regime. The energy harvesters fabricated in this project use AlN as piezoelectric material and are fabricated using micro and nanotechnology processing techniques. They consist of a silicon-based beam on top of which the piezoelectric material and electrodes are implemented through an extensive process development. AlN material with fairly good c-axis orientation was obtained by reactive sputtering techniques. However, the machine was decommissioned and in the end this material together with the electrodes were deposited by PIEMACS Sarl S.A. in Switzerland. The fragility of these beams is high enough to be considered, and an effort was put on improving the harvesters robustness by a two-step lithography-free process which consisted on rounding the anchoring point of the beams. The enhanced devices showed to withstand a mean acceleration of 5.9 g without breaking, which is almost as twice as the non-enhanced ones, which withstood only an average of 3 g. Once this robustness enhancement was achieved, magnets were implemented into the system. In order for the beam to interact with the magnets, ferromagnetic foil was incorporated on either side of the beam’s tip. COMSOL multi-physics software was used for simulating the interaction between the beam and the magnets to the end of obtaining the optimal distances between both the beam and the magnets and between the magnets themselves. The results showed that small-scale dimensions were feasible for the set-up considered.
The characterization of the developed harvesters in impedance terms required a new setup to be developed by which control over the three dimensional coordinates was achieved. Both softening and hardening effects, which were preciously observed when performing the simulation studies, were also obtained in the characterization part. The devices were also characterized in electromechanical terms, for which a more robust set-up was obtained by minimizing mechanical noise both from the deflection control laser sensor and from the shaking device. A maximum output power of 0.32 µW was obtained under an acceleration of 0.8 g.