Paper-thin nanogenerator also works as both mic, loudspeaker

May 18, 2017 // By Julien Happich
Led by Nelson Sepúlveda, Associate Professor at the Michigan State University (Lansing, MI), a team of researchers who recently reported a breakthrough in ferroelectret nanogenerators (FENGs) was able to further characterize the behavior of their thin-film FENG as a flexible acoustic transducer capable of operating both as a loudspeaker and a microphone.

Their paper "Nanogenerator-based dual-functional and self-powered thin patch loudspeaker or microphone for flexible electronics" published in Nature Communications, examines the FENG's energy conversion mechanism under various sound pressure levels (SPL).

Because the 0.1mm thin polypropylene ferroelectret (PPFE) includes compressible charged voids behaving as deformable dipoles in a metal-insulator-metal structure (with conductive silver layers for electrodes on the faces of the film), the alternating waves of sound pressure are faithfully transduced into alternating electrical signals. Here, rather than focus on energy generation, the study focused on reading out the analogue signal produced by the polymer-based FENG and the results are compelling.

Energy conversion mechanisms of a FENG. (a) The charge distribution and giant dipoles with upper and lower surfaces oppositely charged. (b) Electromechanical effect under direct pressure. (c) When the pressure is released, the giant dipoles restore their original sizes and a reverse current is generated. (d) Reverse electromechanical interaction: applying a positive potential further expands the giant dipoles. (e) The giant dipoles shrink as a negative potential is applied.

Performing measurements using three different configurations (the FENG as a freestanding film, affixed to a soft substrate, and attached to a soft substrate and rolled up into a cylinder), the researchers were able not only to accurately record sound in all three configurations, but also reversibly to play sounds by applying electric signals to the transducer.

The study shows the FENG-based microphone to be highly sensitive to a broad range of frequency including the full 20Hz to 20kHz human hearing range and extrapolates that such polypropylene ferroelectrets could be used as a thin, wearable and self-powered loudspeaker or microphone patch.