ELECTRICAL-DISCHARGE REGIMES AND AEROSOL PRODUCTION IN POINT-TO-PLANEDC HIGH-PRESSURE COLD-PLASMAS - AEROSOL PRODUCTION BY ELECTRICAL DISCHARGES

The paper deals with the production of particles by electrical dischar ges in point-to-grid configurations (for centimetric gaps in controlle d air or nitrogen at atmospheric pressure). The original conection bet ween discharge regimes and aerosol characteristics enables us to defin e the electrical parameter governing the production rate of particles and the underlying physical mechanisms which can prevail in each elect rical regime. Whatever the point polarity, two different processes are proved to occur directly in the discharge gap. When the voltage is sw itched on, below the ionisation threshold or under non-propagated disc harge conditions, bursts of sub-micronic particles are resuspended fro m oxidised and/or contaminated electrodes. In propagated discharge, nu cleation arises from plasma-electrode interactions, when positive stre amers or sparks of both polarities reach the grid. Aerosol results fro m nucleation of metallic vapours, which are produced in the gap from t he cathodic spot either through thermal effects (sublimation of the gr id) or through atomisation by sputtering (ionic bombardment). Moreover , a post-discharge formation step of nucleated aerosol occurs with all types of discharges except spark discharges (i.e. non-propagated diff use discharge and crossing streamers), by maturation (confining) of th e gaseous effluents with a characteristic time of minutes. The importa nce of chemical reactions in the gaseous phase for production as well as growth by condensation and coagulation are depicted. Nuclei are sho wn to arise from nucleation of gaseous species created by slow reactio ns implying ozone. (C) 1998 Elsevier Science Ltd. All rights reserved.