Nanoparticle Dynamics in the Non-Equilibrium to Equilibrium Transition Region of Plasma Reactors

Abstract

Nanocrystals are often assumed to be spherical and largely monodisperse during nonthermal plasma synthesis because of unipolar particle charging and selective particle heating originate from the non-equilibrium nature of nonthermal plasma synthesis environment. However, in sampling nanocrystals from nonthermal plasma reactor and utilizing inertial deposition to collect them, nanocrystals move from a non-equilibrium environment in the plasma to equilibrium environment beyond the plasma boundary. Nanocrystal charging and growth dynamics are altered inevitably during this transition; hence the state of aggregation needs to be addressed. However, lacking in non-thermal plasma nanocrystal synthesis and sampling has been (1) the development of online diagnostic techniques for monitoring nanocrystal growth in and beyond plasma reactors, and (2) fundamental investigations of the nanocrystal dynamics in spatial plasma afterglow environments. Therefore, this dissertation focuses on the development of cutting-edge nanometrologies for nonthermal plasma, as well as theoretical and numerical study of particle dynamics in nonthermal plasma spatial afterglow. First, a uniquely designed low pressure differential mobility analyzer (LPDMA) is applied to characterize particle size distribution from nonthermal plasma reactor for the first time. The LPDMA transfer function and complete data inversion routine to calculate particle size distribution is developed from tandem IMS-IMS calibration and Twomey-Markowski inversion algorithm. Second, the LPDMA measurement system is utilized to study Si nanocrystal charging and aggregation in the spatial afterglow of a flow through nonthermal plasma reactor. The bipolar charge state and aggregation of nanocrystals are revealed in measurements. Collectively, a specifically developed constant number Monte Carlo simulation model is implemented and compared with experimental results. The importance of transition diffusion of energetic species, electron desorption and collision model in spatial afterglow is noted through experiment-model comparison. Finally, ion mobility-mass spectrometry measurement yielding a 2-D size-mass distribution is utilized to analyze the morphology of aggregates from an atmospheric pressure DC microplasma. The fractal dimension of the aggregates is quantitatively analyzed by automated TEM image analysis. Both IM-MS and TEM image analysis illustrate the formation of highly polydispersity, branch-like structure aggregates exiting the microplasma. Comparison with Langevin dynamics simulation demonstrates that Coulomb interaction plays minimum role in particle aggregation out of plasma.