<p>Long-term stability and chromium (Cr) contamination are two major concerns for application of chromium-bearing metallic materials as interconnects of solid oxide fuel cells (SOFCs) at intermediate temperature (~800°C). Copper-manganese (Cu-Mn) and cobalt-manganese (Co-Mn) spinel can be promising coating materials for the metallic interconnects as they show high electrical conductivities. The first objective of this research is to develop an economical and convenient method through which the spinel coatings can be applied to the metallic substrates. The investigations on the crystal structure of CuᵪMn₃₋ᵪO₄ spinel, e.g., structure symmetry and cation distributions, have always been controversial, which hinders the total understanding of the detailed structure of the material. In order to resolve the inconsistency, in-situ neutron and X-ray diffraction were employed to determine the structure of the spinel.</p> <p>A novel method was developed to obtain high quality manganese coating without any additives (sulphur or selenium compounds). Cu-Mn and Co-Mn spinel coatings were applied to metallic coupons by electrodeposition and subsequent annealing. The method is convenient and easy to control. The performance testing showed that the area specific resistances (ASRs) of the coated samples (0.003 Ω•cm²) are much lower than that of the uncoated UNS 430 (0.189 Ω•cm²) after oxidation at 750°C for 1500 hours. Moreover, both spinel coatings can effectively suppress the outward diffusion of Cr, which resulted in reduction of Cr contamination significantly. The oxidation studies of Cu-Mn coating revealed the transformation mechanisms of Cu-Mn coating to the spinel. In-situ neutron and X-ray diffraction analysis clarified the crystal symmetry of CuᵪMn₃₋ᵪO₄ spinel and CuMnO₂ at high temperatures. Rietveld refinement revealed the cation distribution of Cu and Mn ions on tetrahedral and octrahedral sites of CuᵪMn₃₋ᵪO₄ spinel, which was compared to values in the literatures. / Thesis / Doctor of Philosophy (PhD)

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The application of the entropy concept to stabilize oxide systems opens the possibility of discovering new materials with unique structural and functional properties. High-entropy alloys and oxides, which are based on the entropy stabilization concept and composed of multi-principal elements, have the potential to tailor structural and functional properties to meet specific needs. The study of lanthanum-based perovskite materials that benefit from the entropy stabilization approach is a promising area of research.However, the inherent randomness of multi-principal elements presents new challenges, making it difficult to predict their behavior. To understand these difficulties, we have initiated a methodical investigation of La-based medium- and high-entropy perovskite oxides. This study focuses on the synthesis, characterization, sintering mechanism, and electrical conductivity properties of nine La1-xCax(A1/3, B1/3, C1/3)O3 medium-entropy perovskite oxide systems (A, B, and C = three combination of Cr or Co or Fe or Ni or Mn) and one La1-xCax(Cr0.2Co0.2Fe0.2Ni0.2Mn0.2)O3 high-entropy perovskite oxide system (for x = 0.1 to 0.3). This research aims to provide better understanding of: (1) synthesis process, (2) temperature of single-phase formation, (3) the impact of various combinations of multiple B-site transitional elements and Ca doping on crystal structure, and microstructure (4) sintering mechanism and (5) electrical conductivity properties.

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Detailed thermodynamic, kinetic, geometric, and cost models are developed, implemented, and validated for the synthesis/design and operational analysis of hybrid solid oxide fuel cell (SOFC) – gas turbine (GT) – steam turbine (ST) systems ranging in size from 1.5 MWe to 10 MWe. The fuel cell model used in this thesis is based on a tubular Siemens-Westinghouse-type SOFC, which is integrated with a gas turbine and a heat recovery steam generator (HRSG) integrated in turn with a steam turbine cycle. The SOFC/GT subsystem is based on previous work done by Francesco Calise during his doctoral research (Calise, 2005). In that work, a HRSG is not used. Instead, the gas turbine exhaust is used by a number of heat exchangers to preheat the air and fuel entering the fuel cell and to provide energy for district heating. The current work considers instead the possible benefits of using the exhaust gases in an HRSG in order to produce steam which drives a steam turbine for additional power output. Four different steam turbine cycles are considered in this M.S. thesis work: a single-pressure, a dual-pressure, a triple-pressure, and a triple-pressure with reheat. The models have been developed to function both at design (full load) and off-design (partial load) conditions. In addition, different solid oxide fuel cell sizes are examined to assure a proper selection of SOFC size based on efficiency or cost. The thermoeconomic analysis includes cost functions developed specifically for the different system and component sizes (capacities) analyzed. A parametric study is used to determine the most viable system/component syntheses/designs based on maximizing total system efficiency or minimizing total system life cycle cost. / Master of Science