A Study of Particle Charging for Electric Field Enhanced Deposition

Abstract

A theoretical study of charge accumulation on small particles was performed with application to the problem of electric field enhanced deposition (EFED) in flame synthesized materials manufacturing. Silica is synthesized in a flame by hydrolysis or direct oxidation of silicon tetrachloride. A silica particle exposed to flame ions in an external electric field acquires charge due to the thermal motion of ions and the motion induced on the ions by the applied electric field. Charging may also occur in a high temperature environment due to the thermionic electronic emission. Particle charging by combined field and diffusion charging in the continuum regime was reviewed, then thermionic electronic emission was incorporated into the theory. Numerical calculations were carried out for 0.2 μm particles at various flame temperatures and electric fields and assuming that hydronium ions (H₃O⁺) are the only flame ion species contributing to particle charging. When the applied electric field is less than 4–5 (kV/cm) at typical flame temperatures, particle charging is mainly due to thermal ion collison and thermionic electronic emission. Electric field effects dominate over diffusion and thermionic emission effects for fields higher than 70–100 (kV/cm). Assuming that particles are dilute with respect to the ion concentration, the number of ions attached to a particle ranges from 50 to 200 for the typical charging time, flame characteristics, and electric field conditions in the EFED process.