Paper PS-TuM11
A Combined Experimental and Modeling Study of Reactive Vapor-nanoparticle-plasma Interactions in a Dusty Atmospheric-pressure Plasma

Tuesday, October 22, 2019, 11:20 am, Room B130

Low-temperature (non-thermal), atmospheric pressure plasmas are characterized by several important fundamental and technological advantages for the gas-phase synthesis of nanoparticle materials. However, the effect of the particles on these plasmas remains poorly understood. It is generally accepted that nanoparticles acquire charge, typically negative, which leads to a reduction in the electron (plasma) density. The degree of reduction is not known and experimental measurements are challenged by several issues. One, plasmas operated at atmospheric pressure are small in size (~1 mm) and probes cannot be easily introduced into the plasma volume. Two, there are strong gradients in the plasma volume as the precursor vapor is dissociated and nanoparticles nucleate and grow, and the effect of particles on the plasma must be decoupled. Three, the material could have other effects on the plasma, for example by undergoing further reactions or by vaporizing after particle formation, that must also be isolated or avoided.

We present a tandem, atmospheric-pressure plasma system that separates a first “reactive” plasma, where the precursor vapor is dissociated leading to particle growth, from a second “dusty” plasma, where the effect of particles on the plasma can be studied. Two non-contact methods, an external electrical conductivity probe (Impedans Octiv Poly) and spectroscopy are applied on the dusty plasma to monitor changes in the electron density. We focused our study on carbon which has a relatively high boiling point and should be chemically stable within the plasma environment. The measurements show that electron densities decrease as expected upon the introduction of the nanoparticles into the second plasma at all powers. For example, at a power of 50 W, the electron density decreased from 4.0 x 10¹⁴ cm^-3 for a pure Ar plasma to 3.6 x 10¹⁴ cm^-3 for a dusty Ar plasma with a total particle concentration of about 4.0 x 10⁶ particles/cm³. Monte Carlo simulations were carried out in support of experiments and showed that by preferential negative charging, particles in plasmas can reduce bulk electron concentrations. We will also discuss the effect of residual hexane vapor and possible particle evaporation on plasma properties.