Deposition of piezoelectric films for energy harvesting
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Coatings were done on silicon cantilever
Thus, the process of FEP has the potential for highly productive manufacturing of Sc-doped AlN films. Working in collaboration with the Technical University of Dresden and Oulu University in Finland, tests on energy harvesting with AlN coatings of up to 50 µm were performed. The coatings were done on silicon cantilever of the size approx. 70 mm x 5 mm. These served as standard measurement samples, to characterize the film material. The measurement setup consists of an electromagnetic shaker system for the generation of defined mechanical vibrations regarding frequency and displacement amplitude and an electrical measurement circuit (Image 2).
The shaker system simulates the vibration environment of the harvester. The coated silicon cantilever is stimulated to oscillate and is electrically connected with the measurement circuit. In the most basic form, an AC measurement circuit with purely resistive load is used. With that, the measured powers at resonance of the cantilever for pure AlN with thickness 10 µm reached 70 µW at vibration amplitudes at the basis of ±2.5 µm. The usage of Sc-doped AlN resulted in a significant increase of generated power (5 times higher for Al60Sc40N compared to AlN), confirming the expectations from piezoelectric film characterization (Table).
Supply autonomous sensors with energy
However, to supply autonomous sensors with energy, the harvesters need to be able to charge a battery or capacitor: a DC output is required. To realize this, an AC/DC- or a DC/DC-converter has to be included. In the most basic form, a diode rectifier bridge can be used. However, due to the inevitable electric losses, the power output at the end is lower than for the pure AC circuit. An improved circuit based on non-linear processing of the voltage according to the SSHI-principle (Synchronised Switch Harvesting on Inductor) can remedy this (Image 3). In the SSHI-circuit, an inductor and an electronic switch are connected in series with the piezoelectric element and the rectifier input (Series-SSHI).
The switch is triggered at the maximum value of displacement of the Si cantilever. In that moment, the switch closes and a part of the energy stored in the capacitor of the piezoelement is transferred to the filter capacitor. A voltage inversion is occurring. This setup significantly improves the energy conversion efficiency, thereby increasing the harvested power.
Support several industrial partners
A DC power of 380 µW was measured for a 60 mm x 10 mm silicon cantilever coated with 10 µm Al60Sc40N at resonance (1.17 kHz, base vibration amplitude ±2.5 µm). This is an almost two-times increase compared to the basic diode rectifier, which resulted at same conditions in a generated power slightly above 200 µW (Image 4). The scientists now are prepared to support industrial partners across the entire scale of product development, starting from feasibility studies under laboratory conditions up to integrated packages for the deposition of AlN and AlScN films, consisting of key components, fully automatic process and control systems as well as technology.