Recent Submissions

  • Microstructural evolution of intercritically annealed medium-manganese steels

    De Moor, Emmanuel; Mueller, Josh J.; Speer, J. G.; Matlock, David K.; Toberer, Eric (Colorado School of Mines. Arthur Lakes Library, 2021)
    Intercritical annealing (IA) of medium-Manganese (Mn) steels is an AHSS design concept for the production of formable sheet steel. The microstructure after IA includes retained austenite (5-40 volume pct), which forms from an initial microstructure of either cold-rolled ferrite or martensite, and is stabilized to room temperature via C and Mn enrichment. Retained austenite is essential to the elevated strength-elongation, and can be engineered with equilibrium modelling and judicious selection of alloy composition and heat treatment.Long IA times (i.e. batch annealing) are expected to result in near-equilibrium microstructures with uniform Mn distributions within austenite. Relatively short IA times (<1000 s), however, may be more attractive due to energy conservation and more flexible processing routes. With shorter IA times, the phase transformation mechanism associated with austenite growth is of interest, as the sluggish diffusion of Mn in austenite is likely to result in the retention of any Mn gradients in austenite which develop during austenite growth. This thesis primarily focuses on elucidating austenite formation mechanisms during IA, and investigating the efficacy of generating Mn enrichment in austenite during relatively short IA treatments. In particular, the effect of cementite on the austenite formation mechanism, and the kinetics of Mn and carbon (C) partitioning to austenite, were investigated. Additional work focused on the effect of prior cold deformation on austenite growth and Mn redistribution during double soaking (DS) heat treatments. Scanning transmission electron microscopy with energy dispersive X-ray spectroscopy, transmission kikuchi diffraction, and field emission scanning electron microscopy were utilized for microstructural characterization, while in situ high energy X-ray diffraction, ex-situ X-ray diffraction, and dilatometry were used for bulk assessments. Phase field simulations using MICRESS®, and one dimensional diffusional simulations with the DICTRATM module of Thermo-Calc®, were also conducted for austenite formation and Mn partitioning. Austenite growth was found to be controlled predominantly by Mn diffusion. C-diffusion controlled kinetics were inhibited due to Mn enrichment in cementite, which stabilized cementite and caused dissolution to be controlled via Mn diffusion. Formation of film-like austenite from martensitic microstructures did not require the preservation of initial austenite films during heating. Phase field simulations predicted Mn-partitioning or massive transformation of austenite during DS, depending on the secondary soaking temperature. Experimental results for DS treatments were consistent with phase field simulations, which indicated that a bimodal Mn distribution can be maintained in a fully austenitic microstructure during DS treatments.
  • Design and synthesis of organic and hybrid materials for application in lighting, gas storage, and catalysis

    Sellinger, Alan; Koubek, Joshua Thomas; Lusk, Mark T.; Pylypenko, Svitlana; Vyas, Shubham (Colorado School of Mines. Arthur Lakes Library, 2021)
    The need for new materials and technological advances for an ever-changing world is paramount. Currently, one of the largest areas of change is the energy sector, from how it is produced, stored, and finally utilized; all these areas are being improved to become more environmentally friendly. The area of focus for this work is designing new organic and hybrid materials to address each of these different areas, in particular: designing and synthesizing two-dimensional metallated covalent organic frameworks (COFs) to generate energy sources and store potential sources, as well as designing novel thermally activated delayed fluorescent (TADF) emitters for applications in lighting, to reduce the overall energy demand. All these topics require a large amount of interdisciplinary collaborations and as such the focus of this work will be on the design and synthesis of these materials rather than focusing on the many details that would go into their final applications.
  • In-situ studies of strain rate effects on phase transformation and microstructural evolution in metastable β titanium alloys

    Clarke, Amy; Ellyson, Benjamin; Tucker, Garritt J.; Clarke, Kester; Findley, Kip Owen (Colorado School of Mines. Arthur Lakes Library, 2021)
    Titanium (Ti) alloys are heavily utilized in the aerospace and defense industries for their high specific strength. Ti alloys are promising candidates for lightweight crash and blast resistant structural applications. Deformation mechanisms like TRansformation Induced Plasticity (TRIP) and TWinning Induced Plasticity (TWIP) simultaneously provide increased strength and ductility by engineering the work hardening response. Although TRIP and TWIP Ti alloys promise improvements in ductility and damage tolerance, fundamental understanding is still lacking, specifically with respect to dynamic mechanical response. In this work, we use a combination of in-situ/ex-situ experimental techniques to understand strain rate effects on TRIP, TRIP/TWIP, and TWIP Ti alloys as a function of alloying, initial microstructure, and processing. A novel strengthening strategy for TRIP Ti-10V-2Fe-3Al (wt.\%) (Ti-1023) is explored by low-temperature aging to coarsen athermal ω-phase precipitates, which allows for precise tuning of strength/ductility combinations in concert with TRIP. We find that artificial aging at 423 K causes a 4x increase in yield stress with no concurrent loss in ductility. Aging up to 7200 s at 423 K was found to inhibit TRIP activity completely. Instead, heterogeneous slip dominates, resulting in the formation of concentrated slip bands and reduced uniform elongation. In-situ synchrotron x-ray diffraction was also used to study the high strain rate deformation behavior of metastable Ti alloys. Combined TRIP/TWIP Ti-12Mo (wt.%) outperforms TWIP Ti-15Mo (wt.%) at high strain rates, due to the added contribution of local strain-relaxation from TRIP. Fine scale, post-mortem microstructure characterization revealed nanometer scale transformation product in Ti-12Mo caused by TRIP within the primary twins. This contrasts strongly with Ti-15Mo, where coarser microstructures contain heavily dislocated primary twins. Ti-15Mo was also found to exhibit reduced twinning activity at high strain rates compared to microstructures produced by quasi-static and intermediate strain rate testing. Coarser microstructures and reduced twinning activity ultimately caused reduced total elongation at high strain rates in Ti-15Mo compared to TRIP enabled Ti-12Mo. The role of low temperature aging on ω-phase precipitates and TRIP and high strain rate deformation were also studied in Ti-1023. Increasing aging times were found to provide strengthening at strain rates up to 2000 /s, but also result in reduced in elongations. Post-mortem electron backscatter diffraction (EBSD) revealed increasing aging time causes a reduction in the area fraction of martensite formed, which directly correlates to reduced elongation. Lastly, the groundwork has been performed to understand the physical parameters controlling dynamic lattice stability in BCC Ti. Density Functional Theory (DFT) modelling was performed to study the effects of local structural distortions caused by solute atoms in BCC β Ti. The dispersion relation in BCC Ti was found to be highly sensitive to local distortions in the lattice. These initial results provide insights into future modelling of β Ti alloys and the design of phase stability and deformation mechanisms.
  • Exceeding the biochemical speed limit of fibrinolysis using magnetically powered microwheels

    Marr, David W. M.; Neeves, Keith B.; Disharoon, Dante; Trewyn, Brian; Wu, Ning; Leiderman, Karin; Petruska, Andrew J. (Colorado School of Mines. Arthur Lakes Library, 2021)
    The biochemical lysis of blood clots, or thrombolysis, using tissue plasminogen activator (tPA) is an effective treatment option for some types of clots, but its use has been limited by its rate of transport to and dissolution of thrombi. This thesis develops a technology that could broaden the indications for thrombolytic therapy using rotating magnetic fields to assemble and direct drug-bearing microwheels (μwheels) to target and lyse blood clots.μWheels driven by rotating magnetic fields translate because of friction against a surface. The translational velocity of μwheels depends on the thickness of the liquid gap separating them from the surface, which can be controlled by applying magnetic load and colloidal forces. μWheel rolling is characterized by stick-slip behavior where μwheels near the surface “stick” and translate rapidly and μwheels farther from the surface “slip” and translate slowly. μWheels functionalized with tPA (tPA-μwheels) can be targeted to plasma clots formed in vitro, demonstrating their utility as drug delivery vehicles. tPA-μwheels are five- to tenfold more effective than therapeutic concentrations of free tPA because they translate more rapidly than diffusion and localize near the clot at concentrations two orders-of-magnitude above the bulk. However, at high tPA concentrations, fibrinolysis rates are limited by the concentration of tPA’s substrate, plasminogen. To address this biochemical limitation, plasminogen-laden mesoporous silica nanoparticles are conjugated to tPA-μwheels (pgn-tPA-μwheels) to co-deliver both molecules. Pgn-tPA-μwheels match the maximum lysis rate achieved using artificially high concentrations of free plasminogen and tPA. By tuning the ratio of tPA to plasminogen and combining co-delivery with magnetically-driven mechanical action, we achieve lysis rates beyond what are possible using fibrinolytic agents alone.
  • Use of waterborne automotive paint sludge as an alternative binder for magnetite ore pellets, The

    Anderson, Corby G.; Vaccarezza, Victoria; Taylor, Patrick R.; De Moor, Emmanuel; Eggert, Roderick G.; Spiller, D. Erik (Colorado School of Mines. Arthur Lakes Library, 2021)
    Due to environmental and economic concerns, there has been a large push to investigate recycling applications for automotive paint sludge. Automotive paint sludge is considered a nuisance and hazardous waste material within the automotive industry around the world. The focus of this project was to investigate and develop a recycling application that would employ the use of the automotive paint sludge material as received. The paint sludge was used as a binder for iron ore pellets, typically used for iron- and steelmaking. The development of the type of recycling application conducted during this project was aided by a review of different binders used in iron ore pellets, as well as the various techniques used to recycle automotive paint sludge materials. Two different paint sludge samples were characterized for their organic and inorganic components. It was then determined to be a possible substitute for the standard iron ore pellet binder: bentonite clay. Unlike bentonite, paint sludge has half the concentration of silica and alumina, which are detriments in the ironmaking process. Iron ore pellets were made using magnetite ore, 1.0 – 3.0 wt.% limestone, 0.0 – 1.0 wt.% bentonite, and 0.0 – 1.0 wt.% paint sludge from two different paint shops. The pellet recipes were evaluated via a statistical analysis software where low, middle and high material weight percentages were output to ensure optimized results. The iron ore pellets with paint sludge material as a binder were then tested for their standard physical and chemical properties to make sure they were comparable to pellets made with bentonite clay. It was concluded via the physical and chemical property tests that iron ore pellet made with paint sludge material were just as capable of withstanding typical transportation and handling used in the iron- and steelmaking industries.
  • Fluid-fluid interaction in shallow hydrothermal systems: implications to silica vein textures in epithermal deposits

    Monecke, Thomas; Ranville, James; Taksavasu, Tadsuda; Pfaff, Katharina; Kuiper, Yvette; Mauk, Jeffrey; Ranville, James F. (Colorado School of Mines. Arthur Lakes Library, 2021)
    Epithermal deposits are an important source of precious metals that form at shallow depth by subaerial hydrothermal systems. This study aimed to unravel the processes that result in the formation of high grades in these deposits through textural investigations on epithermal veins.High-grade vein ores in low-, intermediate-, and high-sulfidation epithermal deposits are typically hosted in specific colloform bands. The ore minerals form dendritic aggregates that are hosted by a matrix that originally consisted of opal-A showing a microspherical texture. The opal-A was originally gel-like and could be shaped by the hydraulic action of the hydrothermal fluids. The opal-A in the veins was deposited broadly contemporaneously with the ore minerals although textural evidence suggests that delicate dendrites could also have grown within the silica gel. Experimental investigations confirm that the growth of mineral dendrites in silica gels is possible at far-from-equilibrium conditions. The microspherical opal-A hosting the ore mineral dendrites is thermodynamically unstable and in most deposits investigated has matured and recrystallized to mosaic quartz characterized by highly irregular and interpenetrating grain boundaries. In most deposits, ore minerals are associated with this mosaic quartz and relic microspheres may or may not be preserved in the quartz matrix. The mosaic quartz present in mineralized colloform bands is texturally distinct from quartz occurring in barren bands in epithermal veins, which includes comb quartz and quartz pseudomorphs formed after bladed calcite. It is proposed here that ore mineral formation and deposition of opal-A within the veins occurred as a result of metal and silica supersaturation achieved during short-lived events of vigorous boiling of flashing that may have been triggered by seismic events. Ore deposition occurred in the area of two-phase liquid and vapor flow whereby the degree of vapor production varied along the vein and over time. In contrast, barren bands in epithermal veins formed during periods of gentle boiling or nonboiling. The observation that flashing is the principal mechanism resulting in high-grade ore formation in the epithermal environment has significant implications to exploration as it predicts that the boiling zone and mineralization can occur at variable depths below the paleowater table.
  • Sedimentology of bottom current processes and their bedforms, The

    Wood, Lesli J.; Beelen, Daan; Sarg, J. F. (J. Frederick); Carr, Mary; Plink-Björklund, Piret; Snieder, Roel, 1958- (Colorado School of Mines. Arthur Lakes Library, 2021)
    This doctoral thesis (this work) is aimed at better understanding ocean bottom current processes and their bedforms in the contexts of 1) ancient outcrops, 2) modern oceans and 3) geophysical bedform processes. First, Miocene Rifian Corridor deposits in Northern Morocco are described, which have been interpreted to have formed under the action of ocean bottom currents. The independent paleontological, sedimentary, and stratigraphic analysis presented in this work shows that these deposits have likely been misinterpreted and actually represent shallow marine tide-dominated delta deposits. The implications of these findings to our understanding of ancient bottom current deposits in an outcrop are discussed and independent interpretations for these sediments are outlined. Second, the abyssal plain geomorphologies are compiled and analyzed to illustrate the bedform types and the diversity of ocean bottom current-controlled landscapes. This work shows that ocean bottom current deposits comprise mostly of migrating dunes and have morphological, sedimentological, and process similarities to aeolian deposits. Compiled abyssal dune data is then used to develop a global map with regions of ocean bottom deposition, erosion, and stasis. A separate chapter of this work discusses the use of ocean floor sediment core to quantify the origin of oceanic sediments, and documents that on average ± 8% of the sediments on the ocean floor are derived from authigenic suspension fallout, as compared to ± 92% that is detrital sediment derived from continental sources. The final two chapters of this work examine active aeolian systems to define physical laws of dune formation and migration, showing that the shapes and migration rates of dunes are controlled by surface-to-volume ratios. These laws are then applied to better understand the role of topography on dune morphology transitions.
  • Controls on the formation of disseminated- and vein-style low-sulfidation epithermal precious metal deposits

    Monecke, Thomas; Tharalson, Erik R.; Holley, Elizabeth A.; Pfaff, Katharina; Goldfarb, R. J.; Heffernan, Scott (Colorado School of Mines. Arthur Lakes Library, 2021)
    Low-sulfidation epithermal deposits are major sources of Au and Ag. They form in the shallowsubsurface (<1.5 km) from near-neutral chloride waters at <300°C. The ore-forming waters are rock- buffered and have a low salinity (<3‒4 wt.% NaCl equiv.). Many low-sulfidation epithermal deposits are characterized by bonanza-type ore zones confined to banded quartz veins and breccia zones and are mined as high-grade, small-tonnage deposits. However, the ore zones in some of these deposits consist of disseminated hypogene sulfides and may be extracted by low-grade, large-tonnage operations. Research at the Castle Mountain low-sulfidation epithermal deposit in California highlighted the importance of lithological controls on the nature of the deposit style. Castle Mountain represents a low- grade, large-tonnage deposit hosted in a Miocene volcanic succession that is dominated by volcaniclastic rocks. The highest gold grades occur where breccia deposits associated with rhyolite flows and domes and vertical breccia pipes interpreted to represent diatreme breccias coincide spatially with extensional faults. These host rocks provided cross-stratal permeability for thermal liquids that precipitated metals primarily through cooling during their upflow. In contrast, bonanza-type precious metal enrichment apparently occurs primarily in competent rocks of flow-dominated volcanic successions. Detailed textural studies on samples collected from bonanza-type ore zones in low-sulfidation epithermal deposits in Nevada, California, and Japan suggest that high-grade precious metals are deposited as a result of flashing of the thermal liquids. This process leads to an efficient precipitation of metals, typically forming ore mineral dendrites, which are hosted by noncrystalline silica formed by homogeneous nucleation in the liquid. The textural observations suggests that the noncrystalline silica that originally makes up the bulk of the mineralized veins recrystallizes to thermodynamically more stable quartz during and after the ore deposition. The combination of field and microanalytical research provided new insights into the mechanisms by which low-sulfidation epithermal deposits are formed. It highlights volcanological and rheological controls on the nature of these deposits as high-grade deposits can only develop in competent host rocks allowing flashing of the thermal liquids to depth. The improved understanding of ore-forming processes has implications to the design of exploration strategies for this deposit type.
  • Development of novel six-dimensional anisotropic and asymmetric yield approaches applied to the study of dwell fatigue using various x-ray techniques, The

    Stebner, Aaron P.; Brice, Craig Alan, 1975-; Brunson, Zachary D.; Findley, Kip Owen; Berger, John R.; Kappes, Branden Bernard; Pilchak, Adam L. (Colorado School of Mines. Arthur Lakes Library, 2021)
    In an ever increasingly technological world, it is frequently the case that as industries search for solutions to problems of light-weighting, high-temperature applications, biocompatibility, and cost, they turn towards anisotropic and asymmetric materials such as magnesium (Mg) alloy plate, titanium (Ti) alloy forgings, 3D-printed nickel (Ni) based superalloys, super-elastic Ni-Ti binary alloy extrusions, and a whole host of composites to name but a few. Anisotropy and asymmetry are not only interesting or potentially useful in niche cases, but increasingly vital to understanding and successfully using advanced materials. And yet, the paucity of clear and in-depth understanding and capable mathematical descriptions of the elastic limit for these materials marks a gap in engineering knowledge, a gap worth considering and studying. Galileo once wrote about the importance of comprehending the mathematics of a particular scientific endeavor as a prerequisite to understanding the science itself. Quoting from Sillman Drake’s 1957 translation of Galileo Galilei’s 1623 book Il Saggiatore (page 25 of the original work): [The philosophy of the universe] cannot be understood unless one first learns to comprehend the language and read the letters in which it is composed. It is written in the language of mathematics and its characters are triangles, circles, and other geometric figures without which it is humanly impossible to understand a single word of it; without these, one wanders about in a dark labyrinth. This is no less relevant a sentiment in the niche field of solid mechanics or the even more specialized domain of inelastic anisotropy and asymmetry. As such, any endeavor to understand these aspects of material behavior must begin by comprehending the mathematics which underlie and describe them. The geometry of the elastic limit for the most anisotropic of materials requires the actions on each component of stress, or strain, to be considered as independent – unique from other combinations of actions. This demands a six-dimensional perspective to adequately describe anisotropic inelasticity. By describing techniques for visualizing and comprehending such a geometry, new approaches are proposed for mathematically capturing a greater range of behaviors for the elastic limit and for calibrating such theories experimentally. These novel approaches are first tested on the macro-scale inelastic character of a 3D-printed Ni-based superalloy, Inconel 718, to demonstrate their efficacy. The concepts are subsequently studied through applications to the micro-scale phenomena in Ti which give rise to bulk yield behaviors and to dwell-fatigue failures: grain-scale anisotropic elastic limits. Specifically, multiple in-situ X-ray diffraction experiments are proposed and carried out to monitor the states of stress and strain at the grain-scale in order to allow for grain-scale calibrations of anisotropy and asymmetry and for mapping the anisotropic interactions of neighboring grains during dwell-fatigue loading.
  • Comprehensive modeling of process-molten pool condition-property correlations for wire-feed laser additive manufacturing

    Zhang, Xiaoli; Jamnikar, Noopur; King, Jeffrey C.; Brice, Craig Alan, 1975-; Wakin, Michael B. (Colorado School of Mines. Arthur Lakes Library, 2021)
    Wire-feed laser additive manufacturing (WLAM) is gaining wide interest due to its high level of automation, high deposition rates, and good quality of printed parts. The complexity of modeling the directed energy deposition (DED) process, high characterization and printing cost, and the destructive testing of the final build part for quality testing motivates the need for developing in situ quality assurance and control techniques. In-process monitoring and feedback controls that would reduce the uncertainty in the quality of the fabricated parts are in the early stages of development. Machine learning (ML) promises the ability to accelerate the adoption of in-process monitoring and control in additive manufacturing (AM) by modeling and predicting process-sensing-property connections between process setting inputs and material quality outcomes. However, the lack of sufficient sensing and characterization data for training ML models is a significant challenge in the field of AM industry due to associated high costs. This thesis explores the in situ quality assurance and control methods by studying the process-molten pool condition-property relation for the robotic laser wire-feed DED process. Analysis and characterization are performed on the experimentally collected in situ sensing data for the molten pool under a set of controlled process parameters for a WLAM system. The real-time molten pool dimensional information and temperature data are the indicators for achieving good quality of the build, which can be directly controlled by processing parameters. Thus, the process-molten pool condition-property relations are of preliminary importance for developing a quality control and assurance framework. The results highlight collaborative and quantitative multi-modality models for controlling and estimating the process and quality parameters using real-time sensing data.
  • Role of nitrogen in the structure and properties of proton irradiated 12Cr1MoWV ferritic/martensitic steel for advanced nuclear reactor cores, The

    Clarke, Kester; Clarke, Amy; Rietema, Connor J.; Bourne, Gerald; Findley, Kip Owen; Saleh, Tarik A.; Eberhart, Mark E. (Colorado School of Mines. Arthur Lakes Library, 2021)
    The 12Cr1MoWV (wt%) ferritic/martensitic steel HT9 is a candidate material for fuel cladding in advanced nuclear reactors, such as the Versatile Test Reactor, currently under development. As such, understanding the relationship between microstructure and mechanical properties in the context of irradiation environments for these steels is critical. Nitrogen content has been hypothesized to have a significant effect on the irradiated properties of alloy HT9. In this work, three otherwise similar alloys of HT9 with varying N contents (10 ppm, 190 ppm, 440 ppm N) are thoroughly characterized prior to irradiation with 1.5 MeV protons to 1 dpa of dose at 300 ̊C. In the unirradiated condition, the presence of ultrafine, intralath vanadium carbonitride (V(C,N)) precipitates are revealed for the first time in the 190 ppm and 440 ppm N alloys via centered dark field transmission electron microscopy (TEM). Lower N content result in finer intralath precipitates, whereas higher N content results in larger, elongated disks or needles. In addition to the quantitative assessment of interlath and intralath V(C,N) by TEM, thermodynamic simulations with ThermoCalc, and, for the first time, time-of-flight secondary ion mass spectrometry (ToF-SIMS) are utilized as complementary techniques, providing a high- throughput method for assessing trends in precipitate volume fractions. ToF-SIMS, combined with internal friction measurements, also provides the relative amount of interstitial N present across the three alloys. Characterization of the alloys shows that N content has a profound effect on the irradiated defect structures. On-zone scanning TEM (STEM) is used to determine that, as N content increases, the average dislocation loop diameter decreases, while the number density of loops increases, a behavior consistent with a reduction in self-interstitial atom (SIA) cluster mobility. Additionally, STEM energy dispersive spectroscopy finds extensive Ni clustering on dislocations and V(C,N) interfaces. The Mid and High N specimens exhibit significantly less hardening relative to the Low N sample. The decrease in hardening is attributed to the presence of V(C,N), which provides an alternative short-range site beyond dislocation loops and line dislocations for the formation of Ni clusters, resulting in fewer Ni clusters on dislocations in the Mid and High N alloys. The data indicates increasing the N content in HT9 may have a desirable effect on the irradiated structure and properties at the dose studied, as well as the swelling resistance at higher doses. In other words, N content appears to be a powerful tool for tailoring the SIA cluster mobility in F/M steels for different temperature and dose applications.
  • Design, implementation and interpretation of algorithms to predict the progression of Alzheimer's disease

    Wang, Hua; Brand, Lodewijk Willem Cornelis; Zhang, Hao; Mehta, Dinesh P.; Klein-Seetharaman, Judith; Crosby, Ralph (Colorado School of Mines. Arthur Lakes Library, 2021)
    Alzheimer's Disease (AD) is a serious public health issue that results in significant social and financial burdens on the individuals and communities impacted. In order to tackle this public health crisis it is critical that the clinical and computational research communities collaborate to identify possible causes of this progressive memory disease. Close collaboration between these two communities has the potential to result in promising therapeutic treatments for AD and other health conditions. This dissertation presents a collection of algorithms and associated derivations designed to predict the progression of AD using multi-task and structured regularization techniques, clustering membership by way of nonnegative matrix factorization, and COVID-19 clinical outcome prediction using multi-instance learning methods. This work presents novel algorithms for handling multimodal and longitudinal data and details approaches for multitask and multi-instance learning techniques that can be applied in other fields. Extensive discussions on algorithm predictive performance, interpretability, and implementation are provided for each method and are designed to serve as a framework for future research.
  • Morphological and microstructural evolution in BZY/Ni catalyst materials

    Reimanis, Ivar E. (Ivar Edmund); Jennings, Dylan M.; Diercks, David R.; Pylypenko, Svitlana; Ricote, Sandrine; O'Hayre, Ryan P. (Colorado School of Mines. Arthur Lakes Library, 2021)
    Yttria doped barium zirconate (BZY, BaZr 1-xYxO3-δ) is of interest for its potential uses as a catalyst and in protonic ceramic fuel cells. BZY is often doped with transition metals, such as Ni, which can form into metallic nanoparticles and greatly increase the catalytic performance of the material. The process of precipitating Ni nanoparticles during a reduction treatment, termed 'exsolution', is utilized to produce stable catalytic nanoparticles. Studies are presented here which aim to further the understanding of morphological and microstructural evolution in BZY/Ni, focusing on how that evolution will affect catalytic performance. To begin, BZY/Ni is analyzed as a catalyst in the water-gas-shift (WGS) reaction, demonstrating bi-functionality of BZY as a WGS support for the first time. The loss of catalytic surface area through the coarsening of metallic nanoparticles is a major degradation mechanism for supported metal catalysts, and has not been examined in BZY/Ni. To study Ni coarsening in BZY/Ni, the kinetics of Ni nanoparticle growth are analyzed, allowing for a determination of the dominant coarsening mechanisms. In addition, the morphology in Ni particles produced through exsolution and those produced through metal deposition and dewetting are compared; it is demonstrated that Ni particle morphology is controlled by the thermodynamics of surfaces and interfaces. Finally, the potential of in situ HRTEM as a technique for studying exsolution in BZY/Ni is demonstrated in preliminary experiments. Epitaxial BZY thin films are used to provide more control for fundamental studies into the relationships between the BZY support and Ni nanoparticles. BZY thin films have been studied in the literature for their excellent protonic conductivity when compared to bulk BZY, but the morphological and microstructural evolution of these films at high temperatures has not been examined thoroughly. Here, two studies are presented that describe the decomposition of BZY thin films, beginning with the formation of crystallographically oriented barium carbonate grains from the BZY film. Subsequently, the addition of Fe and Ni are observed to have different effects on the decomposition of BZY thin films, and analysis is provided to explain the effects of the dopants.
  • From tree to tap: the impacts of climate change on biogeochemical processes during conifer needle decomposition and broader implications for water quality in Colorado

    Sharp, Jonathan O.; Leonard, Laura T.; Munakata Marr, Junko; Spear, John R.; Voelker, Bettina M. (Colorado School of Mines. Arthur Lakes Library, 2021)
    Recent climate change has contributed to large-scale tree mortality across forested regions in Colorado. As forest health declines, concern for associated terrestrial biogeochemical and hydrologic shifts is mounting. These shifts are related to reduced tree canopy cover, cessation of belowground rhizospheric processes, and increased organic inputs to the system. This can in turn affect terrestrial carbon and nitrogen cycling at the tree scale and downstream water quality. These processes within the forest are complex, however, and the observed changes in soil and stream chemistry can vary across watersheds depending on the land cover, local climate, and location. This dissertation explores biogeochemical processes associated with conifer litter decomposition, a source of terrestrial organic matter contributions, and more broadly the associations between forest health and water quality in Colorado. This work includes field-based experiments beginning at the tree-scale and concludes more broadly with watershed-scale evaluations of water quality. Results revealed the inherent chemistry associated with tree species has a significant influence on soil biogeochemistry during isolated needle decomposition. Further, biogeochemical shifts observed with bark beetle impact are likely driven by other changes (e.g., the cessation of rhizospheric processes and tree canopy loss). Comparisons of the roles of elevation, soil type, seasonal shifts in soil moisture, and snowmelt timing on litter decomposition processes revealed needle presence and seasonal variability of soil moisture are influential in soil carbon release and magnitude. In addition, soil carbon fluxes returned the greatest release associated with more bioavailable litter. Microbial community structure was a key variable that demonstrated sensitivity but also resilience to climate shifts. Extended work beyond the field using historical drinking water reports revealed increased organic disinfection byproducts (DBPs) across certain municipalities of Colorado. The trends correlated with climate variables of reduced total annual days of frost and ice conditions. Closer inspection of these trends in a town of interest revealed long-term decadal and seasonal trends of DBPs associated with spring snowmelt events. This work reveals that as terrestrial-sourced organic matter increases under a changing climate, drinking water facilities should monitor the watershed landscape and source water chemistry to proactively respond to heightened DBP production.
  • Advances in spatial frequency modulation imaging: spatio-spectral encoding

    Squier, Jeff A.; Czerski, John; Brice, Craig Alan, 1975-; Adams, Daniel; Durfee, Charles G. (Colorado School of Mines. Arthur Lakes Library, 2021)
    In the following thesis I present my work advancing the state of the art in spatial frequency modulation imaging (SPIFI). There are many imaging scenarios in which traditional segmented detectors are impractical. Segmented detectors become prohibitively expensive outside of commonly used wavelength ranges and are susceptible to scattering. In such cases, single element detectors provide access to a larger range of wavelengths and may remove scattering ambiguity; however, they require a method for coupling spatial information into temporal signals. SPIFI couples spatial information into temporal signals from a single element detector by modulating the spatial intensity distribution of light illuminating the sample. This work extends the utility of SPIFI by developing simple optimization algorithms for the modulation pattern, developing a fiber deliverable SPIFI system, two multi-dimensional SPIFI systems, and a single shot SPIFI system. The thesis is organized into six chapters. I begin with an introduction to single element imaging and SPIFI. In chapter two I describe how to optimize the SPIFI modulation pattern for various constraints such as the manufacturing resolution or the numeric aperture of an optical system. Chapter 3 describes the technique I developed for fiber deliverable SPIFI imaging: wavelength domain SPIFI. By modulating the spectrum of the illumination beam, wavelength domain SPIFI facilitates remote delivery via optical fiber or free space transmission. I provide theoretical analysis of wavelength domain SPIFI along with experimental validation of the technique and its compatibility with fiber delivery. Wavelength domain SPIFI can be combined with SPIFI along the perpendicular transverse dimension. In chapter 4 I present experimental realization of a two dimensional SPIFI system with spatial and wavelength modulation. I also demonstrate a scan free SPIFI system where the wavelength encoded axis is sampled with a linear CMOS detector. Chapter 5 presents initial results from wavelength multiplexed single shot SPIFI. This technique provides a valuable illumination scheme for phenomena that occur faster than the scan time of traditional SPIFI. I end the thesis by describing future work related to these techniques and presenting some concluding statements.
  • Optimal design and control of cool thermal energy storage as a distributed energy resource

    Tabares-Velasco, Paulo Cesar; Deru, Michael; Heine, Karl W.; Braun, Robert J.; Newman, Alexandra M.; Vincent, Tyrone (Colorado School of Mines. Arthur Lakes Library, 2021)
    The electric grid of the future is envisioned to be a smart grid, where electricity ismanaged in coordinated manner between suppliers and users. To achieve this, a high level of electric demand flexibility must be integrated into our building infrastructure. In the U.S., 9% of all electricity generated is used to cool buildings, making this end-use an ideal target for active management through cool thermal energy storage (CTES) technologies. Historic uses for CTES are designed around central chilled water plants, but these systems cool less than 25% of U.S. commercial floorspace. Emerging technologies are under development to serve the many smaller distributed cooling systems, such as rooftop units (RTUs), and have the potential to add CTES to an additional 66% of cooled commercial floorspace. However, these unitary thermal storage systems (UTSS) lack the modeling and analysis tools to evaluate them in the future interactive grid context. The purpose of this research is to develop the modeling and optimization tools necessary to not only examine UTSS within specific building applications, but also within the multibuilding, connected community context. To do so, new simulation tools for the U.S. Department of Energy’s OpenStudio energy modeling platform are developed, and a novel mixed-integer linear program is devised to optimize the design and dispatch of multiple CTES technologies, both UTSS and central systems. An integrated simulation-optimization workflow is created to allow for rapid customized analysis. Several case studies are presented illustrating the benefits of community-scale optimization and the impacts of storage costs and utility rate structure on CTES design and dispatch. CTES modeling results, demonstrating the energy and flexibility tradeoffs of various implementations are presented. Optimization results are described in terms of minimum cost, optimal design, and optimal dispatch, with analysis on the CTES impacts on the aggregated community electricity use. It is further demonstrated that community-scale optimization yields greater cost savings potential than an individual-building approach and that certain CTES technologies are more appropriate for different demand-response cost signals.
  • Semi-analytic time-domain solutions to wave-scattering problems

    Martin, P. A.; Yoder, Todd J.; Sava, Paul C.; Fasshauer, Gregory; Ryan, Jennifer K. (Colorado School of Mines. Arthur Lakes Library, 2021)
    Scattered waves are constructed in the time domain by explicitly evaluating the inverse Laplace transform integral as a summation of complex residues. The method is applied to two problems: a bead on a semi-infinite string and acoustic scattering by a sphere. In the first problem, the end of an infinitely long string is oscillated in a specified way. The incident wave is scattered by a bead of negligible width and specified mass, some given distance from the end of the string. The initial boundary value problem for the total wave is solved in the frequency domain. The corresponding time-domain solution is obtained with complex calculus; the Laplace transform is inverted by closing the Bromwich contour and applying a rotated Jordan’s lemma, allowing the time-domain solution to be written as a sum of complex frequency-domain residues. In the second problem, a rotationally-symmetric acoustic wave is scattered by a soft sphere. The same method of closing the Bromwich contour and applying Jordan’s lemma is used to write the time-domain solution as a sum of residues. Mathematically equivalent to the spherical scattering problem, the residue solution is also constructed for a point source. The closed-form solution for the point source problem is known, and evaluation on the surface of a sphere gives the appropriate boundary conditions for the residue solution. Application of Jordan’s lemma is considered in three space-time regions, corresponding to times where the incident wave is contained in the sphere, times where the incident wave has begun to exit the sphere, and times where the incident wavefront has completely exited the sphere. Solving in the frequency domain and writing the inverse Laplace transform as a summation of residues avoids the accumulation of error associated with time-stepping methods, making the method especially useful in evaluating late-time solutions. The method is entirely mesh-free, allowing evaluation of the solution anywhere in the space-time domain.
  • Optimization and data-driven methods for signal processing

    Tang, Gongguo; Xie, Youye; Wakin, Michael B.; Vincent, Tyrone; Zhang, Xiaoli; Yue, Chuan (Colorado School of Mines. Arthur Lakes Library, 2021)
    By exploiting and leveraging the intrinsic properties of the observed signal, many signal processing and machine learning problems can be effectively solved by transforming them into optimization problems, which constitutes the first part of the thesis. The theoretical sample complexity for exact signal recovery and the recovery error bound with noisy observation can be derived for the optimization methods. However, it is not efficient for optimization methods to deal with high-dimensional signals and observation with the complex noise and non-stationary sensing process. Thus, in the second part of the thesis, we focus on applying data-driven methods using deep learning techniques to high-dimensional problems in order to verify and examine their efficiency and capability of handling the complex noise and complicated sensing process in real data. Finally, in the third part, we develop optimization-inspired data-driven methods for several inverse problems in signal processing and machine learning. Experiments show that the proposed optimization-inspired data-driven methods can achieve a comparable performance of the optimization methods, are extremely efficient in handling high-dimensional signals, and are very robust against the noise and complicated sensing process. This reveals the potential to design data-driven methods, following traditional optimization approaches, to robustly address challenging problems in signal processing and machine learning. \textit{Part 1: Optimization Methods}. In this part, we apply optimization methods to several inverse problems in signal processing and machine learning, including the signal and support recovery problems for the sparse signal with non-stationary modulation and parameter estimation of damped exponentials. For the inverse problems of sparse signal with non-stationary modulation, we derive the theoretical sufficient sample complexity for exact recovery and bound the signal recovery error in the noisy case. \textit{Part 2: Data-driven Methods}. In this part, we apply data-driven methods to several machine learning problems, which include recognizing the 3-dimensional (3D) chess pieces and classifying and clustering inlier correspondences of multiple objects in computer vision. The experiment results demonstrate the efficiency and robustness of data-driven methods against complex noise in the high-dimensional real data. \textit{Part 3: Optimization-inspired Data-driven Methods}. In this part, we develop data-driven methods based on the optimization techniques. By unfolding the optimization methods and making the parameters trainable, we obtain deep architectures that can achieve a fast approximation of the original optimization approaches and deal with signal models with the complicated sensing process that can not be modeled properly by optimization methods. We also design deep networks following the atomic norm optimization process for multiband signal identification and parameter estimation of contaminated damped exponentials.
  • Control of optoelectronic properties via aliovalent disorder in ternary nitrides and oxynitrides

    Toberer, Eric; Tamboli, Adele C.; Melamed, Celeste L.; Pylypenko, Svitlana; Zimmerman, Jeramy D.; Brennecka, Geoffrey (Colorado School of Mines. Arthur Lakes Library, 2021)
    The III-V family of semiconductor compounds have revolutionized optoelectronics. From record-efficiency GaAs solar cells to the GaN-based light-emitting diodes that won the 2014 Nobel Prize, these compounds and alloys have enabled contemporary optoelectronic technology. However, there are challenges that cannot be solved with the III-Vs; there is still an outstanding need for Earth-abundant and non-toxic optoelectronics, especially in the green-emitting regime. The underexplored II-IV-V2 family of materials offers the possibility of groundbreaking optoelectronic properties similar to the III-Vs yet with much broader chemical flexibility. In particular, the II-IV-Vs enable the use of aliovalent site ordering to tune materials properties, an entirely new design space which is predicted to control the optical band gap by up to 1 eV with very little corresponding change in bond length. The chemical and structural diversity of the II-IV-Vs massively opens up the property space of the achievable materials and alloys, enabling control and tunability far beyond the III-Vs. However, with this diversity comes complexity. Cation site ordering in the II-IV-Vs is challenging to control and to characterize in thin film form due to the very small structural changes involved. Consequently, the impact of cation site ordering on optoelectronic properties has yet to be understood. Additionally, the anion sublattice can also exhibit disorder which interplays with the cation sublattice; this is just as difficult to control and characterize. Finally, aliovalent disorder raises questions of the local coordination environments that enable this disorder, which have implications for extended defects and carrier localization but have never been investigated. Due to the complexity of this structural space, in this dissertation we develop directed synthesis and characterization routes to map the II-IV-Vs and concurrently leverage computational techniques to guide and frame experiments. Using the suite of high-throughput combinatorial synthesis techniques at the National Renewable Energy Laboratory (NREL), we orthogonalize the impact of structure and composition in order to elucidate fundamental structure-property relations, building a deeper understanding of knobs such as long-range and local structure as well as non-idealities such as oxygen contamination and off-stoichiometry. We first investigate the fundamental properties of ZnGeN2 in thin film form. To begin, we find that at high temperatures, cation- and anion-disordered ZnGeN2xOx grows epitaxially on c-Al2O3 despite significant incorporation of oxygen. Optical characterization, particularly room-temperature photoluminescence, reveals bandgap tuning consistent with a disordered and oxygen-containing film. This study, reported in Chapter 2, demonstrates that optically active epitaxial ZnGeN2xOx can form with a significant degree of disorder, indicating a path toward commercialization. Armed with the knowledge that unintentional oxygen has historically convoluted its reported properties, we then carefully map the phase space of cation-disordered ZnGeN2 to minimize oxygen contamination and characterize the fundamental properties of this material system. We find that disordered ZnGeN2 exhibits a decreased absorption threshold energy with cation disorder. An additional discovery enabled by combinatorial techniques is that ZnGeN2 stabilizes with significant cation off-stoichiometry in both the Zn- and Ge-rich directions, corresponding to an alloy-like structural shift and consequent tuning of the optical absorption. Off-stoichiometry is also framed from a theoretical perspective using defect complex calculations. These results, reported in Chapter 3, provide a thorough fundamental understanding of oxygen-free, cation-disordered ZnGeN2 and offer an entirely new knob for property tuning. Finally, we report in Chapter 4 a high-throughput study of the cation-disordered ZnSnN2-ZnO alloy system to understand the interactions between long-range disorder and local coordination environment. With the support of DFT calculations and FEFF simulations, we characterize octet-rule-violating bonding to both N and O anions with X-ray absorption near-edge structure (XANES). Upon annealing, we demonstrate a shift toward local ordering while long-range cation disorder is maintained, which correspondingly tunes the optical absorption onset. This study adds XANES to the characterization toolkit for II-IV-V2 materials for understanding local coordination environment, and begins to unravel the complex interactions between bonding environment and long-range disorder. Ultimately, this thesis re-affirms the potential of the II-IV-V2 semiconductor family and paves the way for the use of local and long-range aliovalent ordering and cation off-stoichiometry as novel knobs to tune materials properties.
  • Influence of reaction synthesis feedstocks on solidification defect formation and microstructure-property relationships in electron beam freeform fabrication of aluminum metal matrix composites

    Liu, Stephen; Sullivan, Ethan M.; Yu, Zhenzhen; Clarke, Amy; Brice, Craig Alan, 1975- (Colorado School of Mines. Arthur Lakes Library, 2021)
    High-strength wrought aluminum alloys are extensively used in aerospace and automotive applications for their high strength-to-weight ratios. Further improvements to properties, including specific modulus, can be achieved by the introduction of discontinuous particulates into the Al matrix, creating a metal matrix composite (MMC). The introduction of particulates can also minimize the propensity for solidification cracking that certain Al alloys possess as a result of large solidification temperature ranges and high coefficients of thermal expansion. To overcome the limitations of conventional manufacturing processes, additive manufacturing (AM) of these alloys and MMCs is emerging as a promising solution to meet the demands of the aerospace and automotive industries. In this work, Al MMCs were created in situ via exothermic reaction synthesis powders that produce ceramic and intermetallic inoculant particulates. Powder-cored tubular wires (PCTWs) were made using the reaction synthesis powders to produce varying amounts of inoculant content during electron beam freeform fabrication (EBF3) AM deposition. The EBF3 materials were compared with builds produced by laser-powder bed fusion (L-PBF). The inoculant particulates significantly refined the microstructure of the AM materials, achieving mean grain diameters as small as 9 and 2 µm in EBF3 and L-PBF, respectively. Additionally, for both AM processes, the solidification morphology was shifted to an equiaxed grain structure, which was found to be more resistant to solidification cracking. XRD and TEM were used to identify the most potent inoculants present as TiB2, TiC, and Al3Ti. Percent density in the EBF3 materials was found to decrease with increasing inoculant content, which was attributed to nucleation of vaporized Mg on poorly wetted particles. Mechanical testing showed optimal impact toughness to occur at 2 vol.% inoculant, while the highest ultimate tensile strengths of 308±6 MPa and 368±2 MPa and Young's moduli of 86±0.2 GPa and 92.8±1.6 GPa were achieved at 10 vol.% inoculant for T6 heat treated solid wire EBF3 and L-PBF builds, respectively. These materials showed a marked improvement over the UTS of uninoculated EBF3 and L-PBF builds, which were 186±6 MPa and 35±3 MPa.

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