Fundamental studies of charge transfer in multiphase atmospheric pressure plasma reactors

Multiphase plasmas composed of ionized and radical gas-phase species and either solid or liquid aerosols are becoming increasingly important for technological applications. Plasmas enable the synthesis of crystalline nanoparticle materials with high crystallization temperatures such as silicon which have potential applications in emerging optoelectronic devices, medical imaging, and photovolatics. Aerosol particles can be electrostatically charged in a plasma followed by their removal using electrostatic precipitators which is important for clean rooms, process exhaust, and air purification. However, in many of these cases, the fundamental interactions between plasma species and the aerosol nanoparticles remains poorly understood. The goal of this dissertation is to provide relevant insight into some of these interactions. We initially focused on charge interactions between solid aerosol nanoparticles and a plasma. A two-stage, experimental system was set up consisting of one stage where nanoparticles were nucleated and grown in a plasma, followed by another stage where the nanoparticles were introduced into an identical plasma and the effect on the plasma was studied. Non-contact diagnostics based on electrical characterization and emission spectroscopy were applied to obtain key parameters, and computations were performed in support of the experiments. We found that for an atmospheric pressure plasma, nanoparticles had a minimal effect on the electron density of the plasma which is in contrast to previous reports at low pressure. Next, we introduced aerosol nanoparticles generated by atomization in a plasma and characterized the charge distribution as a function of size using a tandem differential mobility analyzer setup. The nanoparticles were found to have a bipolar charge distribution which was unexpected based on the higher mobility of electrons in the plasma and the general assumption that nanoparticles preferentially charge negatively in the plasma. These results suggest the presence of a distinct region outside of the plasma where charge density decreases and negatively-charge nanoparticles may be neutralized and charged positively, termed the spatial afterglow. This region is analogous to previous reports of a similar effect when a plasma is turned off, termed the temporal afterglow. To confirm and understand the spatial afterglow, we set up an apparatus with diagnostics aimed at characterizing the spatial afterglow. Double Langmuir probe measurements provided the ion density and electron temperature as a function of distance in the spatial afterglow. We then developed a simple decay model and identified a dimensionless parameter that describes the prevalence of three-body recombination in the spatial afterglow. We showed that the decrease in the ion as well as electron density leads to a shift in the behavior of the plasma including a possible transition from ambipolar to free diffusion. Thus, electrons are able to escape the region leaving behind positively-charged ions which are responsible for charging nanoparticles positively. Finally, we extended these studies to interactions between liquid aerosol nanoparticles and a plasma. Liquid water droplets were generated by an atomizer and the evaporation of the droplets was studied by light scattering and differential mobility analysis. We found that when the droplets contained a precursor, the presence of liquid water was critical for nanoparticle formation. These results suggest that the mechanism for particle nucleation involves the diffusion of plasma species in to the liquid droplet.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Chemical Engineering

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2022

Author dc:creator

• Abuyazid, Nabiel Hilmy

Contributors dc:contributor

• Sankaran, Ramanathan M
• Kenis, Paul J
• Ruzic, David N
• Su, Xiao

• Spatial Afterglow
• Dusty Plasma
• Plasma Charging

Statement dc:rights

• Copyright 2022 Nabiel H. Abuyazid

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/117538

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F117538