10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Fine Particle Formation in Corona Discharge
VALERY ZAGAYNOV, National Research Nuclear University MEPhI
Abstract Number: 1374 Working Group: Aerosol Physics
Abstract The corona discharge is a gas discharge where geometry confines the gas ionizing process to high field ionization region around active electrode. The corona discharge can be used as a source of positive or negative charges based on the polarity of active electrode (Goldman et.al., 1985). It ought to be taken into account that some plasma is formed across the ionizing region causing ions and electrons colliding onto metal electrodes’ surfaces. The energy of charged particles is sufficiently high capable of ionizing the gas molecules resulting from collision. Their energy could be at the level of tens of electron-volts corresponding to tens thousands degrees. It could be supposed that these impacts of electrons onto surfaces could cause some electrode erosion resulting from charged particles and electrons knocking atoms and molecules out of electrode’s surface. Then, these displaced atoms and molecules turn into vapor molecules and contribute to the surrounding gas phase. Upon moving away from the ionizing region, the temperature of such vapor molecules rapidly decreases and, any unavoidable collision between each other could potentially move them to the bond states, subject to their ability to release excessive kinetic energy. Thus, fine particles composed of electrode material could be formed (Smirnov, 1999). To investigate this process the experimental set up has been constructed. The set up consists of silver particle generator and diffusion aerosol spectrometer (DAS) capable of measuring concentration and size distribution of produced silver particles. The particle generator consisted of thin silver needle (active electrode) and ring silver electrode. Particles produced in the active electrode region were measured by the DAS at one minute interval providing information about particle size distribution and concentration. Typical size distribution of silver particles produced by the above generator is shown in Figure 1. As is seen from this graph some part of generated particles is not presented in the figure, as it is laying beyond the bottom detection limit of DAS, which is blind for particles smaller than 3 nm. Analyzing the obtained experimental data enabled to estimate an amount of silver atoms (which took part in particle formation) generated by one electron impacting onto the active electrode surface (generation efficiency). To achieve this goal, the electric current of corona discharge, total particle concentration and particle size distribution were accurately measured and the results were used in calculations. Dividing measured electric current by electron charge the electron flow on electrode surface was calculated. Then number of silver atoms which took part in particle formation was estimated by measured particle size distribution and its concentration. On the ground of these estimations it was obtained that efficiency of particle generation is between 10-3 – 10-2. At the same time it has to be taken into account that this is low boundary estimation, because part of particles (most fine) was not recorded due to their location beyond the DAS detection limit. Size distribution of the particles, formatted in corona discharge.
The proposed approach to model nanoparticle generation in corona discharge by the vapor of silver or other metals is supposed to consider coagulation equation for corresponding conditions. This method is promoted by the fact that the vapor transition from hot ionizing region to the region with room conditions is very abrupt, and any collisions between particles could result in bond state. At the same time, this method of particle generation can be useful if very fine particles with narrow size distribution are required.
Acknowledgments. The work was supported by RSCF (project # 18-05-00289)
[1] Goldman M., Goldman A., Sigmond R.S. (1985) Pure and Appl. Chem. 57, 9, 1353-1362. [2] Smirnov B M. (1999) Clusters and Small Particles in Gases and Plasmas New York: Springer.