Investigating the growth cycles of titania and carbonaceous nano dusty plasmas

Abstract

Understanding and controlling dust formation in plasmas — especially from gaseous chemical precursors is a key technological goal, and the overarching theme of this dissertation. Solid nanoparticles (i.e. nano dust) which range in size from ∼ 1 to 500 nm can spontaneously grow from reactive gaseous precursors in non-thermal plasmas. This dissertation studies the particle size, and growth time with and without a background magnetic field in the plasma. Traditionally, studies have focused on the growth of either carbonaceous or silicate dust from either acetylene or silane, respectively. However, recently, there have been a shift towards studying new kinds of dust. For example, recent studies have grown polymers and metallic dust in nonthermal plasmas. This dissertation first introduces and studies the growth of titanium dioxide (a.k.a. titania) dust from the metal-organic vapor precursor of titanium tetraisopropoxide (TTIP). The as-grown materials are of amorphous structure but high temperature annealing crystallizes the samples into anatase and subsequently rutile. Then, the growth of titania dusty plasma is and compared to the growth of carbonaceous dust from acetylene, in argon plasma. They are grown during the presence and absence of weak magnetic fields of ∼ 500 Gauss. Both kinds of dust growth exhibit a growth cycle, which had already been shown for various nano dusty plasma. This occurs because once the dust accumulates a critical radius and mass, they move away from the central region of the plasma, allowing a new generation of growth begins. Ultimately, the new generation of growth also moves away leading to a continuous cycle of particle formation and transport as long as the plasma is on. However, with the presence of the magnetic field, the cycle time decreases, and the spatial distribution of the dust cloud appears differently. For example, titania dust are more concentrated in the middle of the plasma where the field strength is higher but carbonaceous dust appear to move away from the high magnetic field region. Finally, we focus on the growth cycle time of carbonaceous dust and how it decreases with a gradual increase in magnetic field, varying from ∼ 20- 1000 Gauss. We particularly noticed a minimum at ∼ 330 Gauss, which coincides with electron magnetization in the plasma. To understand the physical factors contributing to observed changes in the growth cycle, we study the growth rate of carbonaceous dust, and plasma potential of the background plasma, as a function of magnetic field. The former is done via scanning electron microscope measurement of the dust size distribution throughout the first growth cycle, and the latter is done using emissive probe measurements in the plasma. Our results suggest that both the growth rate and the plasma potential of the background plasma decreases during weak magnetic fields. Therefore, we conclude that the physics of the plasma changes during the presence of the weak magnetic fields; specifically, the magnitude of the electric field decreases causing smaller dust particles to be levitated in the plasma. However, we also have initial results from optical emission spectroscopy (OES) of the dusty plasma which suggest that the chemistry governing dust formation is changing. For example, we see that the amount of time needed to reach the first maximum peak in intensity of OES increases with an increasing magnetic fields. It is possible that the nucleation and growth rate of the particles are also changing as seen by OES.