Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Qu, Jianmin; Committee Member: Kohl,Paul A.; Committee Member: Liu, Meilin; Committee Member: McDowell, David L.; Committee Member: Zhu, Ting.

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

Thesis (Ph. D.)--West Virginia University, 2005. / Title from document title page. Document formatted into pages; contains xii, 148 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 119-124).

Thesis (M.S.)--Montana State University--Bozeman, 2005. / Typescript. Chairperson, Graduate Committee: Max Deibert. Includes bibliographical references (leaves 56-58).

Thesis (M.S.)--Montana State University--Bozeman, 2005. / Typescript. Chairperson, Graduate Committee: Vic A. Cundy. Includes bibliographical references (leaf 80).

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Thesis (M.S.)--West Virginia University, 2007. / Title from document title page. Document formatted into pages; contains vii, 92 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 89-92).

The MgO:TiO2 :ZrO2 ternary system was investigated as a possible novel anode material in a solid oxide fuel cell. Titanium-substituted yttria-stabilised zirconias have the necessary electrical conductivity properties for a ZrO2 -based fluorite electrode but problems have been encountered such as a decrease in unit-cell size upon reduction leading to mechanical failure. By incorporating magnesium into the titanium-stabilised zirconia structure, it was thought that the cubic-fluorite structure might be stabilised. A phase diagram study was made of the MgO:TiO2 : ZrO2 ternary system at 1500°C. Upon researching the literature phase diagram of the MgO:TiO2 :ZrO2 system, it was found that the authors had not studied the single-phase region in the ZrO2 -rich area extensively and did not use a consistent temperature for their analysis of samples. This has meant that the phase diagram has had to be reinvestigated. The results obtained at 1500°C are in disagreement with the previously published phase diagram. A large area bounded by single-phase cubic-fluorite was detected; however the central region of this domain contained both tetragonal and cubic-fluorite domains. This implies that for the central region of this phase area that the cubic- fluorite phase is not stable at 1500°C. Selected stabilised cubic-fluorite samples with ~ 10 atom% Mg were annealed at l000°C after preparation at 1500°C and it was found that due to the presence of other phases present at 1000°C, that the cubic-fluorite phase is thermodynamically unstable at lower temperatures. DTA analysis revealed that as the titanium content in the cubic-fluorite solid-solution increased, the phase transition from tetragonal phase (+ MgO) to cubic-fluorite phase decreased. These results were used to provide a basis for a temperature phase diagram showing the likely phase transitions that occur at a particular temperature range. The activation energy for conduction increased and ionic conductivity decreased with increasing titanium content in the solid solution, due to the effects of local distortions created by the smaller ionic radius of titanium when compared to zirconium.

Despite the abundance of renewable energy resources, a lack of economically feasible storage solutions for addressing intermittency remains a barrier to advancing their widespread adoption. Reversible solid oxide cells, which can store hydrogen during periods of renewable energy overproduction, have demonstrated potential for grid stabilization applications given their high potential efficiencies and power densities. However, to become economically competitive, improvements to reversible solid oxide cell performance stability and lifetimes are required. This research focuses on understanding the relationship between microstructure and performance in solid oxide cells and explores avenues for mitigating electrode polarization and degradation. Connections between microstructure and performance were first considered in Ni/YSZ symmetric cells, where the relationship between reaction site density and performance was quantified in nanocatalyst-infiltrated cells using EIS, SEM and FIB/SEM 3-D reconstruction. In Ni-infiltrated electrodes, results showed that both increased triple phase boundary density and decreased reaction rate constants contribute to lowering electrode polarization at intermediate temperatures. In electrodes infiltrated with GDC, a mixed ionic/electronic conducting material, reactions can take place on the GDC surface, greatly decreasing electrode polarization. Calculations considering the performance of baseline and GDC-infiltrated electrodes indicated that reactions take place up to 84nm from triple phase boundaries on nickel scaffold particle surfaces. Microstructure/performance relationships were also examined in full cells tested for 500h under electrolysis or reversible conditions; the fuel and oxygen electrodes were characterized with methods including low-voltage SEM, FIB/SEM 3-D reconstruction, and TEM. In both scenarios, the oxygen electrode was shown to contribute minimally to cell degradation. In the fuel electrode, degradation was mainly precipitated by Ni coarsening and loss of active sites; however, these were mitigated during reversible testing by 9% and 8% respectively compared to electrolysis-tested cells. Finally, strategies are discussed for mitigating long-term degradation. To further stabilize reversible full cells, GDC infiltration into the fuel electrode and adjustments to oxygen electrode phase compositions to prevent long-term decomposition are suggested. On the SOC system level, ALD spinel coatings for interconnect materials are considered. To this end, a successful ALD coating process for manganese oxide on stainless steel is discussed.

Silicon is conventionally extracted by the carbothermic reduction process, which is energetically very inefficient, besides being harmful to the environment. The proposed Solid Oxide Membrane (SOM) process to manufacture Silicon and other metals is energy-efficient and environmentally friendly. The process and its set-up are similar to those of a conventional Solid Oxide Fuel Cell (SOFC). However, the operating temperature is much higher, and therefore, the Nickel-Zirconia cermet that is used for anode in conventional SOFCs cannot be used in this process. The present work reports the development of a suitable anode cermet coating for the purpose. Based on known physical properties of various materials and the requirements of the application, it was decided to pursue the development of an anode based on Molybdenum-Yttria Stabilized Zirconia (YSZ) cermet. Research was conducted to develop with a process to make a Molybdenum-Yttria Stabilized Zirconia cermet coating that would adhere well to the SOM substrate, which is made from YSZ and would have good electronic conductivity and porosity. The molybdenum oxide and Yttria Stabilized Zirconia mixtures were milled together and slip-cast into pellets. The variation of the cermet characteristics was studied with respect to various milling times and sintering/reduction temperatures. YSZ substrate tubes were dip-coated with slurries of made from milled mixtures of Molybdenum Oxide and Yttria Stabilized Zirconia with methanol. They were sintered and reduced in an atmosphere of Argon with five percent Hydrogen, with the intention of getting Molybdenum-Yttria Stabilized Zirconia cermet coatings. These produced flaky coatings that did not adhere to the substrate. Thus, the experiments with Molybdenum Oxide and Yttria Stabilized Zirconia mixtures demonstrated the difficulty in making the cermets by that technique. Cermet coatings were made using a slurry of Molybdenum metal and Yttria Stabilized Zirconia powder. Molybdenum powder was milled with Yttria Stabilized Zirconia and the resulting powders were made into sluITY with methanol and dip-coated onto the substrate tubes. It was seen in the cermet coatings produced that the electrical conductivity and porosity increased, whereas adherence to the substrate decreased with increasing Mo-content in the cermets. In order to make a cem1et coating that had good electrical conductivity and porosity as well as adherence to the YSZ substrate, a double-layered Molybdenum-YSZ cermet coating was made with a zirconia-rich lower layer and a molybdenum-rich upper layer. This coating had good electrical conductivity and porosity, as well as adherence. This double-layer coating was recommended as the cermet coating for use as the anode for the SOM cell.