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/Si3N4 as the electret material, the measured power output was 20.7 mu W at 110 Hz for 2 G acceleration with a 50 M Omega load for the copper harvester and 0.82 mu W at 172 Hz for 2 G acceleration with a 30 M Omega load for the FPCB harvester.