Electrical discharge regimes and aerosol production in point-to-plane dc high-pressure cold plasmas: Aerosol production by electrical discharges

The paper deals with the production of particles by electrical discharges in point-to-grid configurations (for centimetric gaps in controlled air or nitrogen at atmospheric pressure). The original conection between discharge regimes and aerosol characteristics enables us to define the electrical parameter governing the production rate of particles and the underlying physical mechanisms which can prevail in each electrical regime. Whatever the point polarity, two different processes are proved to occur directly in the discharge gap. When the voltage is switched on, below the ionisation threshold or under non-propagated discharge conditions, bursts of sub-micronic particles are resuspended from oxidised and/or contaminated electrodes. In propagated discharge, nucleation arises from plasma-electrode interactions, when positive streamers or sparks of both polarities reach the grid. Aerosol results from nucleation of metallic vapours, which are produced in the gap from the cathodic spot either through thermal effects (sublimation of the grid) 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 diffuse discharge and crossing streamers), by maturation (confining) of the gaseous effluents with a characteristic time of minutes. The importance of chemical reactions in the gaseous phase for production as well as growth by condensation and coagulation are depicted. Nuclei are shown to arise from nucleation of gaseous species created by slow reactions implying ozone. (C) 1998 Elsevier Science Ltd. All rights reserved.