Graduate Thesis Or Dissertation


Alloy and Defect Properties of Oxides and Nitrides for Energy Applications Public Deposited

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  • Throughout history societal advancement has been dictated largely by the development of new materials – from iron for weaponry and tools to silicon circuits for computing. Today, the physical properties of many predominant and emerging technologies rely on the departure from the ideal structure through the incorporation of defects and alloying. This thesis reports on the investigation of several systems whose properties are driven by defect formation and alloying. Spinel aluminate active materials, FeAl2O4 and CoxFe1-xAl2O4, were evaluated for solar thermochemical hydrogen (STCH) production using computational and experimental techniques Hercynite (FeAl2O4) was shown to be viable for STCH for the first time with a significantly higher H2 production than the cobalt alloy. DFT calculations demonstrated the importance of charged antisite-vacancy defect pairs in the redox behavior in these materials. A solid-state kinetic analysis was performed on hercynite and four cobalt-hercynite alloys and showed that the oxidation reaction in each of the materials was best represented by a first-order reaction model. The computed rate constants, activation energies, and pre-exponential factors were found to all increase with increasing cobalt content, indicating that cobalt may increase the reaction rate by increasing the number of active sites (vacancies) at the surface of the catalyst. Hercynite was further studied for its viability in an isothermal on-sun STCH process at the NREL High Flux Solar Furnace. The total H2 production and mass-weighted H2 production both surpassed the production of the benchmark ceria material in similar on-sun experiments, and the particles were demonstrated to be structurally and compositionally stable over multiple days of on-sun testing. Ceramic fuel cells offer an attractive method for the efficient utilization of H2 with no greenhouse gas emissions. Four complex perovskites used as cathode materials (Ba1-xSrxFe1-yZnyO3, Ba1-xSrxCo1-yFeyO3, and BaCo1-x-y-zFexZryYzO3) were studied computationally to predict their defect properties at elevated temperatures and various gas partial pressures. To do this, a new method for predicting equilibrium defect concentrations and defect formation energies beyond the dilute limit was developed and applied. Computationally-predicted and experimentally-measured oxygen nonstoichiometry were found to match within a factor of two. The Al1−xScxN system is an interesting case study in heterostructural alloying as a tool for property improvement. In this system, wurtzite-structured AlN is alloyed with rocksalt-structured ScN to produce a dramatic increase in the piezoelectric response. DFT calculations were utilized in conjunction with combinatorial thin-film synthesis to provide a unique perspective on this system. Calculations demonstrated that the combination of structural frustration and a flattened free-energy landscape lead to a substantial increase in the electromechanical response of the alloy near the heterostructural phase transition point. DFT calculations were utilized to study the bonding and alloy characteristics nine alkaline-earth chalcogenide systems (AS1-xXx: A=Mg,Ca,Sr, X=O,Se,Te). The ground-state energetics, stability, and structural and electronic properties of each alloy were assessed. The calcium- and strontium-containing alloys were all demonstrated to be isostructural systems within the rocksalt structure with the sulfur-selenium alloys having the lowest miscibility temperatures. Magnesium-containing compounds were found to have distinctly smaller polymorph energy differences and exhibited heterostructural alloy behavior within the rocksalt and wurtzite structures.
Date Issued
  • 2019
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  • 2019-11-14
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