Charging efficiency of nanoparticles in needle-to-plate chargers with micro discharge gaps

Simple chargers, based on needle-to-plate configuration with micro gap distance of several hundred micrometers, for the direct charging of aerosol particles, have been designed and constructed. Charging efficiency and particle deposition loss (electrostatic and diffusional) of this type of chargers have been evaluated systematically for particles (polystyrene) with diameters ranging from 28.9 to 287 nm, at different applied voltages, different aerosol flow rates, and different gap distances. The charging efficiency increases with particle size, applied voltage, and mean residence time of aerosol particles in the charger, while decreases with gap distance. As a consequence of the small effective volume of the chargers, a negligible diffusional loss of particles was obtained. The electrostatic loss of charged particles within the chargers was relatively large for smaller particles at higher applied voltage, but a charging efficiency of 82% was achieved for 28.9-nm particles using the charger with a gap distance of 150 μm. The chargers can charge nanoparticles effectively and are easy to be integrated into minimized devices such as sensor chips.