Flat and robust out-of-plane vibrational electret energy harvester

This paper reports the design, fabrication and testing of two types of out-of-plane electret energy harvesters. Copper plates and flexible printed circuit board (FPCB) were used to fabricate the harvesters for a lower fabrication cost and a more robust structure. Several dielectric materials were investigated and SiO₂/Si3N₄ double layers were found to have better charge stability. The measured surface potential and charge density of the SiO₂/Si₃N₄ electret were -400 V and 13.5 mC m^-2, respectively. It was found that charge stability could be improved by multiple corona charging cycles. Using SiO₂/Si ₃N₄ as the electret material, the measured power output was 20.7 μW at 110 Hz for 2 G acceleration with a 50 MΩ load for the copper harvester and 0.82 μW at 172 Hz for 2 G acceleration with a 30 MΩ load for the FPCB harvester.