The corona triode charging/polarization system

Abstract

The development and understanding of the corona triode charging technique, in the seventies and in the eighties is intimately related to Brazilian researchers and academic institutes. Contrarily to older two-electrode systems, the corona triode system comprises three electrodes: the discharge point electrode, the metallic grid and the plane electrode, where the charging current can be measured and controlled. It has been widely applied in the electrical charging of polymer foils for electret production, in the research of the charge stability and charging/injection processes in dielectric materials, principally in film/foil form and in electrostatic separation processes. Corona-based electrostatic precipitators have found widespread application in the treatment of contaminated gases and also in the separation of metals and non-metal components. It has, among others, the advantages not to require the deposition of an electrode on the sample surface exposed to the discharge, allowing the reversal of the discharge polarity (positive or negative), to pole/charge samples with complex non-planar geometries and it is well suited for large scale film poling/charging. A new a corona triode system developed in our facilities is presented, including some theoretical background behind this technique. This system can operate on a more traditional method and also on a more sophisticated principle based on a feedback circuit that continuously adjusts the voltage applied to the metallic grid in order to maintain the charging current flowing through the sample constant. Thus, the sample surface potential buildup can be monitored in real-time by the metallic grid voltage. Charging results on bioactive hydroxyapatite [Ca10(PO4)6(OH2) – Hap] pellets, prepared from commercial Hap powders, are presented and discussed.