Acoustic energy harvesting with irradiated cross-linked polypropylene piezoelectret films

Piezoelectret films prepared by irradiated cross-linked polypropylene (IXPP) not only feature a large figure of merit (d₃₃ · g₃₃, FoM) and a nearly flat response of the sensitivity as a microphone (4 mV Pa⁻¹) in the audio range, but also exhibit a good impedance match to air. Therefore, this material is appropriate for air-coupled sonic and ultrasonic applications. In this work, we report acoustic energy harvesting using IXPP piezoelectret films without mass loading both in ultrasonic and low-frequency ranges. Under an input sound pressure level (SPL) of 100 dB (or 2 Pa) and a resonance frequency of 53 kHz, a maximum output power of 7.2 nW is obtained for an IXPP film harvester. Despite its high resonance frequency, the large FoM of IXPP piezoelectret films suggests itself to be a promising candidate also for low-frequency acoustic energy harvesting with the help of Helmholtz resonators. An output power of 10.3 nW is achieved for a harvester with a 16 cm² large IXPP film within a Helmholtz resonator, which features a resonance frequency of 900 Hz, with an optimized load resistance of 962 kΩ under an input SPL of 100 dB. In comparison to acoustic energy harvesters based on ferroelectric polymer polyvinylidene fluoride cantilever beams, our devices have much higher output power density under the same conditions and much broader bandwidth. Theoretical analysis and numerical simulations are performed to confirm the experimental results. Moreover, the output power of the IXPP acoustic energy harvesters can be further improved by increasing the active area of the piezoelectret films.