Surface Charge Density Measurement of Dielectric Films Using Ultrasonic Vibration

A contactless measuring method of equivalent survace-charge densities on both sides of charged films has been proposed and developed. The principle of this measurement is based on the Kelvin method (null-balance measuring method of surface potential), where a ceramic piezo-electric element is used to realize a high frequency vibration (30 to 140 kHz) for quick response and high sensitivity. The measuring device is composed of a precisely movable detection (probe) electrode and a vibrating electrode dc biased to suppress the surface potential. Surface-cahrge densities on both sides of charged films such as electrets can be calculated easily by measured surface potentials at different gaps between the sample surface and the probe. The accuracy of the sum of both surface-charge densities by this method was very high. However, the accuracy of each charge density was strongly dependent on the accuracy of the gap when the sum is large. The surface-charge densities of corona-charged PTFE Teflon®electrets were studied by this method and it was found that the polarity of the total charge depends on the surface roughness. Charge-elimination of electrets and their charge recovery by annealing were observed also.