Enhanced yield of positively charged particles from a spark discharge generator via in situ corona discharges

Aerosol particles generated by spark discharge methods generally have a Gaussian charge distribution centered at zero, and these charges come from the plasma between the electrodes as well as the ions generated during the process. However, some particle manipulation applications demand control in charges of the particles, for instance, in electrostatic deposition of metal nanoparticles through ion assisted aerosol lithography (IAAL). Neutralizing and re-charging the particles to a particular charge leads to low yield, especially at particle sizes below 10 nm. Therefore, we have devised a new strategy to increase the yield of positively charged sub-10 nm particles, by continuously providing additional positive ions to charge the generated particles in situ, using the electrodes as the ion source. To this end, we have designed a new spark control circuit that rapidly restores corona discharges right after a spark discharge event. In the new circuit, an additional voltage source connected in series with the anode restores its electric potential to the corona discharge voltage. When used with pin-to-plate electrode configuration, the amount of positively charged particles increased by up to 1.8-fold while negatively charged particles were reduced by half, at an optimum voltage set point of 3400 V. Rod-to-plate and wire-in-hole configurations were also tested with the new circuit to investigate the effect of electrode geometry. Lastly, the effect of carrier gas flow rate was studied to confirm that the particles were indeed charged by positive ions from corona discharges.