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Publication Defects, scattering, and mobility in complex thermoelectric materials(Colorado School of Mines. Arthur Lakes Library, 2024)Growing numbers of newly discovered materials hold the possibility of high thermoelectric performance. However, the requirement of the material to conduct electricity while maintaining a high Seebeck coefficient can be limited by native defects or poor electronic mobility which are difficult to predict \textit{a priori}. The research goal of this dissertation is to investigate how the thermoelectric community can understand defects and scattering in complex thermoelectric materials. Using recently discovered compound Hg$_2$GeTe$_4$ as a case study, we demonstrate manipulation of native and extrinsic defects to optimize thermoelectric performance. Finally, we explore the impact of carrier scattering on mobility in classic material SnTe using a custom-designed Nernst effect instrument we built. To understand the impact of defects in complex thermoelectric materials, we adopt a joint computational-experimental approach focusing on Hg$_2$GeTe$_4$ as a case study material. We perform phase boundary mapping, defect calculations, and synthesis of native \& extrinsic doped samples to identify the highest performance in Hg$_2$GeTe$_4$. We succeed in manipulating the carrier concentration of Hg$_2$GeTe$_4$ by half an order of magnitude \textit{via} manipulating concentrations of native defects. We follow our native doping study with an extrinsic doping study which examines the impact of 15 different dopants on the thermoelectric performance in Hg$_2$GeTe$_4$. Ultimately, we achieve the highest figure of merit to date in Hg$_2$GeTe$_4$ by doping with silver. Our curiosity about the dominant source of scattering in Hg$_2$GeTe$_4$ leads us to construct an instrument to measure the Nernst effect, which can shed light on the dominant scattering mechanism in a material. The Nernst coefficient, when combined with three other measured experimental parameters (Hall coefficient, electrical resistivity, and Seebeck coefficient) can be used to solve for material parameters that elucidate scattering mechanisms. Nernst measurements are scant across the thermoelectric literature in comparison with Hall and Seebeck instruments. We describe our design and build a room temperature apparatus that automatically measures and calculates the Nernst coefficient in a material. Finally, we apply our Nernst instrument to explore carrier scattering in SnTe under various doping regimes. Undoped SnTe possesses high mobility, and by synthesizing samples doped with indium, iodine, or natively doped, we explore how mobility evolves in response to two orders of magnitude change in carrier concentration. We demonstrate that the traditional single parabolic band model is insufficient to describe the scattering in SnTe, but applying the more appropriate two band Kane/parabolic model is a complex optimization problem. We provide an interpretation of the raw Nernst data, along with standard thermoelectric measurements to offer insight into the chemistry dependence of scattering in SnTe.Publication Advancing biomass to biofuel conversion via multiple process technologies(Colorado School of Mines. Arthur Lakes Library, 2024)Lignocellulosic biomass is a renewable feedstock for fuels and other value-added products. The advancement of biofuel production requires a multifaceted approach. There are numerous ways to produce biofuels from biomass, many of which are not discussed in this work. Presented here are methods for improving three aspects of improving biofuel production: rheology of enzymatically hydrolyzed biomass, temperature modeling in biomass feeding systems, and cell free synthesis of fuel products. Chapter 3 is a discussion of the influence of enzymatic hydrolysis on the rheological properties of biomass, such as yield stress and plastic viscosity. These properties change during enzymatic hydrolysis and alter the energy requirements for pumping and mixing, an important consideration for the design of processing equipment. The dynamic changes that occur in a corn stover feedstock undergoing enzymatic hydrolysis are characterized, and the influence on pressure losses in piping systems are estimated. Chapter 4 is an exploration of temperature profiles in auger feeders for pyrolysis reactors. Auger feeders experience particle agglomeration and plugging of the auger screw. One-dimensional and two-dimensional simulations of heat transfer throughout the auger feeder are developed to investigate whether heat transfer from the reactor could be responsible for plugging of the screw auger. The one-dimensional model was used to determine the affect of changing operating parameters. Chapter 5 includes a discussion of a Python model for a twenty-three step enzymatic pathway to convert glucose to monoterpenes. The Python model was developed by myself and coworkers and is validated with experimental results from Korman, et al. Key enzymes and co-factors are identified as potential bottlenecks for the production of monoterpenes. In summary, this dissertation provides insights into key fundamental aspects of biofuel production that can be used to optimize biofuel production design in the future.Publication Bayesian approach to alloy simulation, A(Colorado School of Mines. Arthur Lakes Library, 2024)Alloy simulation is—seemingly—rife with intractable mathematical problems: the combinatorial explosion of atomic decorations, the irreducible global nature of convex hulls, and the curse of dimensionality in configuration space. Due to the importance of alloys across materials science, considerable attention has been given to surmounting these computational obstacles. The typical angle is to tackle daunting mathematical problems with increasingly complex model-Hamiltonians, ranging from lattice models to machine-learned interatomic potentials. Even in the fortunate cases where these models provide accurate predictions, extracting insight from large-scale simulation is challenging, limiting our theoretical understanding. Herein, I take a Bayesian approach to alloy simulation. All predictions provided from calculations are thus represented as probabilistic distributions over possible outcomes rather than isolated, singular values. The classic materials science questions are used to ground this thesis: i) can a given chemical composition be made as a single-phase alloy; and ii) what will be its local and long-range atomic structure? Equipped with Bayesian modeling, I show that relatively few calculations are necessary to provide sufficient estimations for the stated questions. As such, advanced alloy simulation is returned to the domain of first-principles calculations, enabling accuracy and functionality. While I focus solely on first-principles simulation, model-Hamiltonian research also stands to benefit from the developed Bayesian approach, which could accelerate exploration across time scales, length scales, and composition spaces. If there is an overarching thesis to this amalgamation of work, it is the following. Bloated mathematical constructs conceal the underlying simplicity of alloys—through the success of relatively simple simulation, I highlight the elegance within these disordered materials.Publication Utilizing spectroscopy and electrochemistry to understand europium, terbium, and uranium redox chemistry in novel media(Colorado School of Mines. Arthur Lakes Library, 2024)The projects in this thesis span three different solution matrices—highly basic media (potassium carbonate) in Chapter 2, aqueous media (water) in Chapter 3, and highly corrosive and ionic media at high temperatures (lithium and potassium chloride molten salt) in Chapter 4. In Chapter 2, the redox properties of europium and terbium are investigated through X-ray absorption spectroscopy, UV-visible spectroscopy, near-infrared spectroscopy, and electron paramagnetic spectroscopy showing the prevalence of trivalent oxidations states and a unique, red-hued Tb(III) complex. Chapter 3 sets the stage for Chapter 4 in which it investigates the well-known reversible redox couple of ferrocyanide in aqueous media using less common spectroelectrochemical approaches to support diffusion coefficient assessment. In Chapter 4, a multiscale electrochemical approach was used to study redox chemistry in a high temperature molten salt. In part one of this chapter, the concentration of uranium tetrachloride in the salt mixture was increased far beyond what has ever been reported and diffusion coefficients were measured through cyclic voltammetry. The second part of this chapter focused on developing a hybrid technology capable furnace that was able to monitor the oxidation of uranium in molten salts through UV-visible spectroelectrochemistry to calculate a conditional diffusion coefficient value. Overall, this thesis provides fundamental understandings on select redox elements across a wide range of solution environments using various electrochemical and spectroscopy methods.Publication Synthesis and characterization of polymer charge transport materials for applications in high efficiency and stability perovskite solar cell technologies(Colorado School of Mines. Arthur Lakes Library, 2024)Perovskite solar cells (PSCs) have shown great promise as a low-cost and efficient renewable energy source since their conception in the late 2000s, recently achieving over 26% power conversion efficiencies (PCE) for single junction devices. The hole transport material (HTM) plays an integral part in the performance of PSCs; however, previous studies have shown that the HTM interface with adjacent perovskite/ITO layers can often be a bottleneck for device efficiency and long-term stability. Avenues for improving HTMs include addressing issues with energetic alignment, physical contact with adjacent layers, and lowering manufacturing costs1. Previous attempts to address these issues have included the synthesis of different carbazole and fluorene-based polymers via the Buchwald-Hartwig amination, whose modified backbone allowed for varying thermal and optical properties. This existing inexpensive polymer HTM family was modified with polar side-chain moieties containing methoxyethoxyethyl (O), alkyl oxetane chain (Ox) N,N-dimethyl aminopropyl (N), butanoate (COOCH3), and butanenitrile (CN) in order to improve the interaction with adjacent layers. The synthesis and characterization of monomers and polymers are reported and the effects of these wettable monomer additions on optical, thermal, molecular weight properties, and increased device performance were measured.