Milliwatt order vertical vibratory energy harvesting using electret and ferroelectric — Discharge does not occur with small gap and only one wiring is required

This paper proposes a milliwatt order vertical type capacitive energy harvesting using polymer electret and ferroelectric. The ferroelectric prevents discharge between electret and facing electrode, thus the ultra small gap between mass and electret is feasible, allowing even their contact. The smaller gap leads to the enormous change in capacitance, realizing the larger output power. Note that this device is not piezoelectric but capacitive. The electret needs no comb-shaped patterning, which is effective for realizing stable and high surface potential. The alignment between mass and electret is not necessary, making fabrication process simple and economical. The output power of 1.2 mW was realized for a forced vibration at 40 Hz/250 μm amplitude. Even if the wire is not connected to the facing counter electrode, which is set over the ferroelectric, the current still flows to the ground thanks to the polarization of ferroelectric. One wiring simplifies the packaging and reduces the electric noise. The output power of one wiring type was 0.2 mW for a free vibration at 20 Hz/1 mm amplitude.