Air Breakdown in Contact Electrification
Contact electrification of solids in a gas medium involves two stages, i.e., surface charge deposition immediately at separation, and dissipation due to dielectric breakdown of the medium as the gap increases. The presumption that such gas breakdown obeys Paschen's law, which is conventionally determined for gas between electrodes with constant charge supply, is widely accepted yet unverified. The present work experimentally validates such dependence of the breakdown voltage of air between charged dielectric surfaces on both its pressure and the gap distance. Sample surfaces are brought to cycles of contact electrification in a vacuum chamber and charge relaxation due to air breakdown is monitored with measurements of the Coulomb attraction by fixing either the air pressure or gap distance and varying the other. The results indicate thresholds of pressure and distance to facilitate investigations of the raw amount of charge transfer prior to any breakdown discharge, which is adopted to examine the saturation trend of surface charge density in the contact electrification of multiple material combinations using the same test apparatus. Comparatively consistent results are obtained in repeated tests for a variety of contact pairs, while a reduction of saturated surface charge density is observed for PTFE against PDMS after breakdown discharge in low-pressure air, which is preliminarily attributed to alternations of PTFE surfaces caused by accelerated cation strikes during air breakdown, based on SEM images and estimations of particle energy in Townsend avalanches. Conclusions on both the general raw level of surface charge density and the air breakdown during separation in contact electrification are applied to complement models of vibro-impact triboelectric energy harvesters for predicting their performance under various air pressures and physical dimensions in order to either prevent or exploit air breakdown to enhance the power output.