Modeling contact electrification in triboelectric impact oscillators as energy harvesters

Triboelectric energy harvesters or nanogenerators exploit both contact electrification and electrostatic induction to scavenge excess energy from random motions of mechanical structures. This study focuses on the modeling of triboelectric energy harvesters in the configuration of contact-separation impact oscillators. While mechanical and electrostatic elements in such systems can be satisfactorily modeled based on existing theories, the underlying physics of contact electrification is still under debate. The aim of this work is to introduce the surface charge density of dielectric layers as a variable into the macroscopic equations of motion of triboelectric impact oscillators by experimentally investigating the relation between the impact force and the charge transfer during contact electrification. Specifically, specimens with selected pairs of materials are put under a solenoid-driven pressing tester which charges the specimens with a vertical force whose magnitude, frequency and duty cycle can be controlled. An electrometer is used to monitor the short circuit charge ow between the electrodes from which the charge accumulation on dielectric layers can be extracted. With results from parameter-sweep tests, the produced map from contact force to surface charge density can be integrated into equations of motion via curve fitting or interpolation.