Enhancing charging and capture efficiency of aerosol nanoparticles using an atmospheric-pressure, flow-through RF plasma with a downstream DC bias

Charge of clustered microparticles measured in spatial plasma afterglows follows the smallest enclosing sphere model

“…It is likely that in our experiments, the externally applied electric field influenced the latter transition. Positively charged particles in afterglows were also reported in several other studies 42 , 53 – 57 . Since the photon energy of the laser system was below the work function of silver 58 , photodetachment was assumed to be negligible.…”

Modeling nanoparticle charge distribution in the afterglow of non-thermal plasmas and comparison with measurements

“…Another assumption in the modeling is that the charging reactions attain a steady state within the afterglow (based on the residence times of the particles in the plasma). While this assumption certainly becomes better for particles >100 nm, sub-80 nm particles considered here have characteristic particle-ion collision times (the slower among particle-ion and particle-electron collisions) and for ∼10 nm particles, it is possible that the particles took much longer than the 2 cm region nominally taken to be the afterglow (in our modeling as well as in the experiments [23,26]). Inclusion of an unsteady term in equation ( 2) thus becomes necessary for small particles and needs to be explored in future work.…”

“…Hence, for simplicity, we take n e = 0.5 × 10 15 m −3 , k B T e = 0.51 eV, T i = T g = 300 K in the plasma. The estimate of n e is based on the remarks of Dhawan et al [26], a follow-up study to the observations reported by Sharma et al [23], that a choice of 2.5 × 10 15 m −3 leads to reasonable agreement between the characteristic charging time analysis and their experimental results. Although our choice of electron concentration is five times smaller, we also explore the variation in the predicted particle charges with variability in electron concentration.…”

“…Thus, the reduced ion and electron concentrations at the exit of the plasma producing good agreement with measured charge fractions is not entirely fortuitous. A properly measured axial ion and electron concentration profile will allow the prediction and design of plasma process conditions to tune the particle charge distribution in the afterglow [25,26]. Another assumption in the modeling is that the charging reactions attain a steady state within the afterglow (based on the residence times of the particles in the plasma).…”

“…The particle charge is modified both in the active plasma region by the nearly neutral space charge environment (n i ∼ = n e ) wherein ion-electron pair production rate by ionization balances the losses due to recombination, diffusional deposition on surfaces and collisional charging of particles, and, by the non-neutral, positive space charge in the spatial afterglow region (n i > n e ) where electrons are lost at much higher rates than ions without replenishment by ionization; n i is the ion number concentration and n e is the electron number concentration. Motivated by recent experimental investigations of charging in the spatial afterglow downstream of non-thermal plasmas to infer the particle charge distribution [23,24] as well as to propose charge control technologies [25,26], we describe a modeling approach to calculate the charge distribution f p of aerosol particles flown through a non-thermal plasma and the afterglow; f p is the fraction of particles carrying p charges in a population of monodisperse aerosol particles. Modeling and tuning f p as a function of particle size and other plasma attributes accurately is needed for the selection of materials synthesis plasma process parameters in order to obtain tight control over the particle size distribution via collisional growth [27][28][29][30][31][32][33][34], prevent undesirable agglomeration by unipolar charging [35][36][37] as well as the promote desirable production of fractal aggregates by scavenging of nonnegatively charged monomers by negatively charged larger particles [11,27].…”