A small mixing-type unipolar charger (SMUC) for nanoparticles

A small mixing-type unipolar charger for nanoparticles (SMUC) was developed to increase the charging efficiency and throughput by using a small charging chamber (0.5 cm³) to reduce particle loss caused by electrostatic dispersion. The two main components of the SMUC are a high-pressure corona ionizer (HPC Ionizer) in which high concentration unipolar ions are generated, and a charging chamber in which the unipolar ions are mixed with nanoparticles without an external electric field. The charging performance of the SMUC was evaluated experimentally by measuring the charging ratio f_c and the penetration ratio f_p. For positive charging in the particle size range of 5-40 nm, f_c⁺= 0.91-0.95 and f_p ⁺= 0.71-0.90, at an ion concentration of N_g ^+-5.25 × 10⁹ ions/cm³ and a charging time t_R = 0.025 s. For negative charging, f_c^-= 0.74-0.93 and fp^-= 0.73-0.85, at N_g^-= 6.24 × 10⁹ ions/cm³and t_R = 0.025 s. Based on these results, the extrinsic charging efficiency defined as f_e = f_c×f_p /f_d (where f_d is the dilution rate) was f_e^-= 0.46-0.66 and f_e ⁺= 0.59-0.71 for negative and positive charging. The f _c⁺ and f_c^- were estimated theoretically with and without electrostatic dispersion to clarify the effect of electrostatic dispersion on particle losses in the charging chamber. The measured f_e for d_p< 10 nm in the 0.5 cm³ chamber is higher than the theoretical results without electrostatic dispersion, indicating that particle loss caused by electrostatic dispersion is reduced by use of the small volume charging chamber.