Interactions between the Corona Discharge and a Zinc Oxide Electrode

Abstract

In a negative point-to-plane corona discharge in air, mutual interactions are observed between the electrode and the discharge when the anode is a film composed of zinc oxide incorporated in a resin binder. The current carried by the corona discharge increases by more than fivefold as the zinc oxide becomes negatively charged. When the electrostatic charge on the zinc oxide surface is dissipated by light the current falls to the value which is observed when a metal conductor replaces the zinc oxide resin film electrode in an identical geometry. Another feature of the discharge interaction is that the zinc oxide resin films are able to accept and retain over 109 electronic charges per square centimeter of excess negative electrostatic charge despite the fact that the bulk conductivity of the films measured electrodynamically either before or after corona charging is often greater than 10−11 Ω−1 cm−1. Selected single crystals of zinc oxide have also been observed to accept and retain electrostatic charge for several seconds after exposure to the negative corona discharge, while concurrent measurements show that the conductivity of the crystals remains unchanged at 10−2 Ω−1 cm−1 throughout the experimental treatment. In both cases the charge can be dissipated by less than 1014 photons/cm2 of actinic light. It was thus possible to make Xerographic prints on single zinc oxide crystals. No effects comparable to these are observed with a positive corona discharge. The paper is primarily experimental in scope. Support is introduced for the hypothesis that the effect of negative ions of the corona discharge on the zinc oxide resin film surface is the creation of small isolated charged regions, presumably the result of the ``blocking contact'' made with the zinc oxide surface by individual negative ions from the discharge. These regions behave as islands of high resistivity which can accept and retain electrostatic charge even when immersed in a relatively conducting sea of unaltered film material. The anomalous, or unexpectedly high, corona current is then attributed to an increase in conductivity of the corona gap resulting from additional ionization of the air near the charged zinc oxide resin film surface. This ionization is caused by localized high potential gradients in the vicinity of excess negative charges isolated on these islands, and the dissipation of these charges by light thus produces the observed photosensitive decrease in corona current. A comparison is made between the resistive island model and the model which constrains the pigment resin film to behave as a homogeneous insulator after electrostatic charging.