Evaluation of a soft X-ray unipolar charger for charging nanoparticles

Three types of unipolar chargers (parallel multi-electrodes, single electrode, and single electrode with compact size) using the soft X-ray were constructed and their charging performance was evaluated by measuring positive, negative, and neutral fractions of size-resolved ultrafine particles (20–100 nm) with the Tandem Differential Mobility Analyzer (TDMA) technique. The unipolar charger with a single electrode and compact size showed the highest charge fraction with least particle loss probably due to lower electrostatic loss of ions among tested chargers. With positive voltage applied to electrode to remove negative ions, we found that the positively charged particles were 43, 52, 62, 69, and 75% for 20, 30, 50, 70, and 100-nm particles, respectively, and a few particles were negatively charged although their fraction increased with size (1, 2, 4, 5, and 6% for 20, 30, 50, 70, and 100-nm particles, respectively). The positive charge fractions were about three times higher than the values estimated theoretically from a bipolar charger. Also, based on comparison of current data with previously reported values using corona discharge unipolar charger, the soft X-ray charger showed better performance in terms of charging efficiency and penetration for particles (NaCl) currently tested in the particle size range of 20–100 nm.