UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
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Item THE ROLES OF MATERIAL, SURFACE, & MICROSTRUCTURAL EFFECTS IN DEVELOPING CERAMICS FOR ENERGY APPLICATIONS(2022) Ostrovskiy, Yevgeniy; Wachsman, Eric; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Ceramics have a wide variety of applications for energy conversion and other industries because of their unique properties. Conduction of multiple charged species simultaneously enables their use as membranes, electrodes, and more. Perovskites especially, have highly tunable features, and can be modified through doping, surface coating, and microstructure. In this work, each of those approaches was used to improve and/or characterize ceramic components for either proton conducting membranes or solid oxide fuel cells (SOFCs). In the case of membranes, perovskites have limited electronic conductivity, which reduces their ability to permeate hydrogen. Through changing the dopants used in existing perovskite compositions, the electronic conductivity was improved dramatically allowing its use as an n-type conductor. This was achieved by using Pr as a dopant, which introduces electronic conductivity due its multivalent nature. It also has a favorable ionic radius for proton conduction, which is required for hydrogen permeation. In the case of fuel cells, both performance and stability need to be improved for their widespread adoption. The surface chemistry and physical properties of two major cathode materials were evaluated, La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) and Sm0.5Sr0.5CoO3 (SSC). Both are susceptible to the formation of unwanted secondary phases during operation as SOFC cathodes. By studying the surface chemistry of LSCF it was possible to better understand the mechanism of cathode degradation. In the case of LSCF, it was found that electrostatic forces that result from a chemical potential difference between the bulk and surface, promote the segregation of Sr cations from the bulk which is responsible for the degradation. However, the behavior of SSC was more difficult to determine. In SSC, cation segregation was far more dependent on the grain orientation than LSCF and therefore was more difficult to quantify, and the techniques used for improving stability in LSCF were unsuccessful when applied to SSC. Additionally thin films deposited through atomic layer deposition (ALD) were tested as a means of enhancing the performance of LSCF. Due to the challenges of using ALD on porous substrates, the role of variables in the deposition process that can be widely implemented were studied, with a focus on oxygen vacancies. It was found that the choice of oxidizer and the addition of an annealing step can dramatically improve the effectiveness of thing film electrode coatings. Although ALD may not be practical for modifying SOFC electrodes, these are process steps that can be easily implemented by other researchers to improve their existing approaches. Finally, the role of microstructure was addressed as well. Tuning the porosity of SOFC anodes is essential for large scale fabrication of fuel cells and improving their performance and reliability. A microstructure featuring a hierarchal porosity was able to improve the performance of SOFC anodes, especially at lower temperatures and fuel ratios. Improvements in microstructure will allow the fabrication of larger scale SOFCs that are more reliable and mechanically stronger, with minimized performance losses associated with using a thicker anode. The primary scientific merit of this research is demonstrated in the work on cathode degradation and coatings. There the focus was on using a methodology based on a fundamental material property, oxygen vacancies, which are essential to many applications of metal oxides. With this type of approach, it is possible to apply similar techniques to other areas of research involving metal oxides or thin films. The main engineering merit of this research is evaluating the relationship between microstructure, SOFC performance, and large SOFC production. Commercialization of SOFCs requires that they are as effective as possible outside of ideal conditions (pure fuel, high temperatures). Hierarchal porosity has been shown to improve performance under both conditions and can also be applied to cathodes.Item Topological Superconductivity and Majorana Zero Modes(2017) Setiawan, FNU; Das Sarma, Sankar; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Recent years have seen a surge interest in realizing Majorana zero modes in condensed matter systems. Majorana zero modes are zero-energy quasiparticle excitations which are their own anti-particles. The topologically degenerate Hilbert space and non-Abelian statistics associated with Majorana zero modes renders them useful for realizing topological quantum computation. These Majorana zero modes can be found at the boundary of a topological superconductor. While preliminary evidence for Majorana zero modes in form of zero-bias conductance peaks have already been observed, confirmatory signatures of Majorana zero modes are still lacking. In this thesis, we theoretically investigate the robustness of several signatures of Majorana zero modes, thereby suggesting improvement and directions that can be pursued for an unambiguous identification of the Majorana zero modes. We begin by studying analytically the differential conductance of the normal-metal--topological superconductor junction across the topological transition within the Blonder-Tinkham-Klapwijk formalism. We show that despite being quantized in the topological regime, the zero-bias conductance only develops as a peak in the conductance spectra for sufficiently small junction transparencies, or for small and large spin-orbit coupling strength. We proceed to investigate the signatures of Majorana zero modes in superconductor--normal-metal--superconductor junctions and show that the conductance quantization in this junction is not robust against increasing junction transparency. Finally, we propose a dynamical scheme to study the short-lived topological phases in ultracold systems by first preparing the systems in its long-lived non-topological phases and then driving it into the topological phases and back. We find that the excitations' momentum distributions exhibit Stuckelberg oscillations and Kibble-Zurek scaling characteristic of the topological quantum phase transition, thus provides a bulk probe for the topological phase.Item Ceramic Materials Development for Intermediate Temperature Solid Oxide Fuel Cell (IT-SOFC)(2016) Pan, Ke-Ji; Wachsman, Eric D; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Solid oxide fuel cell (SOFC) is an electrochemical device that converts chemical energy into electric power with high efficiency. Traditional SOFC has its disadvantages, such as redox cycling instability and carbon deposition while using hydrocarbon fuels. It is because traditional SOFC uses Ni-cermet as anode. In order to solve these problems, ceramic anode is a good candidate to replace Ni. However, the conductivity of most ceramic anode materials are much lower than Ni metal, and it introduces high ohmic resistance. How to increase the conductivity is a hot topic in this research field. Based on our proposed mechanism, several types of ceramic materials have been developed. Vanadium doped perovskite, Sr1-x/2VxTi1-xO3 (SVT) and Sr0.2Na0.8Nb1-xVxO3 (SNNV), achieved the conductivity as high as 300 S*cm-1 in hydrogen, without any high temperature reduction. GDC electrolyte supported cell was fabricated with Sr0.2Na0.8Nb0.9V0.1O3 and the performance was measured in hydrogen and methane respectively. Due to vanadium’s intrinsic problems, the anode supported cell is not easy. Fe doped double perovskite Sr2CoMoO6 (SFCM) was also developed. By carefully doping Fe, the conductivity was improved over one magnitude, without any vigorous reducing conditions. SFCM anode supported cell was successfully fabricated with GDC as the electrolyte. By impregnating Ni-GDC nano particles into the anode, the cell can be operated at lower temperatures while having higher performance than the traditional Ni-cermet cells. Meanwhile, this SFCM anode supported SOFC has long term stability in the reformate containing methane. During the anode development, cathode improvement caused by a thin Co-GDC layer was observed. By adding this Co-GDC layer between the electrolyte and the cathode, the interfacial resistance decreases due to fast oxygen ion transport. This mechanism was confirmed via isotope exchange. This Co-GDC layer works with multiple kinds of cathodes and the modified cell’s performance is 3 times as the traditional Ni-GDC cell. With this new method, lowering the SOFC operation temperature is feasible.Item MODIFICATION OF MIXED PROTON-ELECTRON CONDUCTORS FOR HYDROGEN TRANSPORT MEMBRANES AND INVESTIGATION OF AMMONIA ION TRANSFER MEMBRANES(2015) Liang, Xuan; Wachsman, Eric; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)SrCe0.9Eu0.1O3- δ has low electron conductivity that limiting the hydrogen permeation. Effects of different dopants in SrCeO3 were studied in order to find new materials that have higher electron conductivity. Total conductivities of these materials and transference number of each species were used for calculating the electron and proton conductivities in the mixed ionic electronic conductors. SrCe0.9Pr0.1O3- δ was claimed to have higher electron conductivity than SrCe0.9Eu0.1O3- δ. The hydrogen permeability of SrCe1-xPrxO3- δ was studied as a function of temperature, hydrogen partial pressure gradient and water vapor pressure gradient. Modified Nafion® membranes can be used for electrochemical ammonia compression.