Abstract:
Corona discharge is widely used not only to modify the physical characteristics of dielectric materials surfaces, but also as a charging technique for the granular mixtures in view of their electrostatic separation. Uniformity of the electric charge deposited at the surface of insulating materials is a major performance index for any corona electrode. The aim of the present thesis was to explore the limits of the corona charging techniques, and more specifically the charge uniformity attainable when using multi-tip-type corona electrodes, in view of their possible utilization for the surface treatment of dielectric plates or for the electrostatic separation of granular mixtures. Using a non-contact electrostatic voltmeter to measure the surface electric potential after corona discharge exposure has proven to be a convenient method to investigate the distribution and the evolution of the amount of charge deposited on insulating materials. A “static” study carried out with PVC plates at rest under the action of the corona discharge generated by such electrode arrangements enabled the evaluation of the effect of each factor: the applied voltage, the height of needles above the surface of the dielectric, and the exposure time. A “dynamic” study, in which the samples were in relative motion with respect to multi-tip-type electrodes, made possible the quantification of these effects in conditions closer to those that are expected to be encountered in industrial applications. It was found that the non-uniformity of the corona charging can be reduced to less than 5 % of the average value by choosing the appropriate inter-electrodes distance and the height of the tips of the electrodes above the surface exposed to the corona discharge.
