Electron transport in E x B devices

A Hall thruster, an E × B device used for in-space propulsion, utilizes an axial electric field to electrostatically accelerate plasma propellant from the spacecraft. The axial electric field is created by positively biasing the anode so that the positivelycharged ions may be accelerated (repelled) from the thruster, which produces thrust. However, plasma electrons are much smaller than ions and may be accelerated much more quickly toward the anode; if electrons were not impeded, a "short circuit" due to the electron flow would eliminate the thrust mechanism. Therefore, a magnetic field serves to "magnetize" plasma electrons internal to the thruster and confines them in gyro-orbits within the discharge channel. Without outside factors electrons would be confined indefinitely; however, electron-neutral collisions provide a mechanism to free electrons from their orbits allowing electrons to cross the magnetic field toward the anode, where this process is described by classical transport theory. To make matters worse, cross-field electron transport has been observed to be 100-1000 times that predicted by classical collisional theory, providing an efficiency loss mechanism and an obstacle for modeling and simulations in Hall thrusters. The main difficulty in studying electron transport in Hall thrusters is the coupling that exists between the plasma and the fields, where the plasma creates and yet is influenced by the electric field. A device has been constructed at MTU’s Isp Lab, the Hall Electron Mobility Gage, which was designed specifically to study electron transport in E × B devices, where the coupling between the plasma and electric field was virtually eliminated. In this device the two most cited contributors to electron transport in Hall thrusters, fluctuation-induced transport, and wall effects, were absent. Removing the dielectric walls and plasma fluctuations, while maintaining the field environment in vacuum, has allowed the study of electron dynamics in Hall thruster fields where the electrons behave as test particles in prescribed fields, greatly simplifying the environment. Therefore, it was possible to observe any effects on transport not linked to the cited mechanisms, and it was possible to observe trends of the enhanced mobility with control parameters of electric and magnetic fields and neutral density– parameters that are not independently variable in a Hall thruster. The result of the investigation was the observation of electron transport that was ~ 20-100 times the classical prediction. The cross-field electron transport in the Mobility Gage was generally lower than that found in a Hall thruster so these findings do not negate the possibility of fluctuations and/or wall collisions contributing to transport in a Hall thruster. However, this research led to the observation of enhanced cross-field transport that had not been previously isolated in Hall thruster fields, which is not reliant on momentum-transfer collisions, wall collisions or fluctuations.

Biological materials : Part A. tuning LCST of raft copolymers and gold/copolymer hybrid nanoparticles and Part B. biobased nanomaterials

The research described in this dissertation is comprised of two major parts. The first part studied the effects of asymmetric amphiphilic end groups on the thermo-response of diblock copolymers of (oligo/di(ethylene glycol) methyl ether (meth)acrylates, OEGA/DEGMA) and the hybrid nanoparticles of these copolymers with a gold nanoparticle core. Placing the more hydrophilic end group on the more hydrophilic block significantly increased the cloud point compared to a similar copolymer composition with the end group placement reversed. For a given composition, the cloud point was shifted by as much as 28 °C depending on the placement of end groups. This is a much stronger effect than either changing the hydrophilic/hydrophobic block ratio or replacing the hydrophilic acrylate monomer with the equivalent methacrylate monomer. The temperature range of the coil-globule transition was also altered. Binding these diblock copolymers to a gold core decreased the cloud point by 5-15 °C and narrowed the temperature range of the coil-globule transition. The effects were more pronounced when the gold core was bound to the less hydrophilic block. Given the limited numbers of monomers that are approved safe for in vivo use, employing amphiphilic end group placement is a useful tool to tune a thermo-response without otherwise changing the copolymer composition. The second part of the dissertation investigated the production of value-added nanomaterials from two biorefinery “wastes”: lignin and peptidoglycan. Different solvents and spinning methods (melt-, wet-, and electro-spinning) were tested to make lignin/cellulose blended and carbonized fibers. Only electro-spinning yielded fibers having a small enough diameter for efficient carbonization ( Peptidoglycan (a bacterial cell wall material) was copolymerized with poly-(3-hydroxybutyrate), a common polyhydroxyalkanoate produced by bacteria with the objective of determining if a useful material could be obtained with a less rigorous work-up on harvesting polyhydroxyalkanoates. The copolyesteramide product having 25 wt.% peptidoglycan from a highly purified peptidoglycan increased thermal stability by 100-200 °C compared to the poly-(3-hydroxybutyrate) control, while a less pure peptidoglycan, harvested from B. megaterium (ATCC 11561), gave a 25-50 °C increase in thermal stability. Both copolymers absorbed more moisture than pure poly-(3-hydroxybutyrate). The results suggest that a less rigorously harvested and purified polyhydroxyalkanoate might be useful for some applications.