Browsing by Author "Weinberger, Chris, committee member"

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Open Access
Cationic-doping of mayenite electride: synthesis, processing, and effect on thermal stability
(Colorado State University. Libraries, 2021) DeBoer, Brodderic, author; Ma, Kaka, advisor; Weinberger, Chris, committee member; Bailey, Travis, committee member; Bandhauer, Todd, committee member
Mayenite electride is an electrically conductive ceramic developed from its parent phase, oxy-mayenite (12CaO•7Al2O3, commonly referred to as C12A7). C12A7 has a unique unit cell that consists of a positively charged [Ca24Al28O64]4+ framework containing twelve cages and two extra-framework O2- ions located inside two cages. The extra-framework O2- ions can be replaced with electrons when C12A7 is heated in a reducing environment, and those extra-framework electrons act like anions, forming the mayenite electride phase, denoted as C12A7:e- hereafter. The anionic electrons enable peculiar properties of C12A7:e- such as high electrical conductivity and low work function, making it a promising material for field emission devices, thermionic-cooling, and as a hallow cathode for electrical propulsion. Compared to other electride materials such as Ca2N, which barely sustain their electride properties even at ambient conditions, C12A7:e- has been reported to be stable up to 400 °C. This temperature is yet not high enough to enable its applications in the technologies mentioned above. Doped derivatives of C12A7:e- emerged in recent years to improve its electronic properties, mainly electron density and electrical conductivity. However, the effects of doping on the oxidation resistance and thermal stability of C12A7:e- remained unclear. Experimental effort on cationic doping of C12A7:e- was particularly lacking in the literature. Therefore, the goal of this study is two-fold: (1) to develop processing routes for successful cationic doping of C12A7:e-, and (2) to test if cationic doping can improve the thermal stability of C12A7:e-. Copper (Cu) and niobium (Nb) were selected as cationic dopants in this study to elucidate how cationic doping affects the thermal stability of the mayenite electride. First, effort was focused on developing synthesis and processing methods to effectively dope Cu and Nb into C12A7:e-. Three different methods were investigated, including diffusion doping; in conventional furnace or via spark plasma sintering (SPS), single-step in-situ formation via SPS, and a solid-state reaction (SSR) synthesis followed by reduction. The phase constitutions, lattice parameters, and microstructure of the various C12A7:e- samples fabricated via the aforementioned methods were characterized to verify if cationic doping was successfully achieved. Electrical conductivity was measured to verify the electride phase is sustained after the doping. Thermal analysis was performed to determine the thermal stability of the cation-doped C12A7:e- compared to undoped counterparts, including onset temperature and peak temperature of oxidation, oxidation rate, mass gain percentage resulted from oxidation, and any decomposition reaction. The key findings of this study include: (1) both Cu-doping and Nb-doping improved the thermal stability of the C12A7:e- by increasing the onset temperatures of oxidation; (2) Cu-doping was effectively and efficiently achieved via the novel SPS diffusion doping method. SPS diffusion doping of Cu at 800 °C gave rise to a minimum lattice parameter (a = 11.942 Å) of C12A7:e-, the lowest oxidation rate, and the smallest mass gain percent at 1050 °C; (3) Using oxy-mayenite and Nb2O5 as precursor for reaction sintering and in-situ reduction in SPS led to successful Nb-doping into the C12A7:e-. Despite the increased onset oxidation temperature resulted from Nb addition, pest oxidation occurred in Nb-doped C12A7:e- samples, leading to high oxidation rate, high total mass gain percentage, and fracture of the solid samples at temperature above 700 °C. In conclusion, Cu-doping was experimentally proved to be an effective approach to improve the thermal stability of C12A7:e- and meanwhile increase the electrical conductivity.
Open Access
Computational feasibility of simultaneous analysis and design in interior point topology optimization
(Colorado State University. Libraries, 2023) O'Connor, Justin, author; Bangerth, Wolfgang, advisor; Weinberger, Chris, committee member; Shipman, Patrick, committee member; Liu, James, committee member; Weinberger, Chris, committee member
Topology optimization is a class of algorithms designed to optimize a design or structure to accomplish some goal. It is part of a process of computer generated design that allows engineers to design better products faster. One such algorithm that has piqued the imagination of developers is called Simultaneous Analysis and Design (SAND), especially in the context of Interior Point Optimization (IPO). This method is known to generate extremely optimal designs, and is good at avoiding local minima. However, this method is not used in practice, due to its computational cost. This thesis examines the SAND IPO method, and develops an effective algorithm to generate a design using it. I begin by discussing nonlinear optimization algorithms, selecting pieces that work together for this problem, to generate a cohesive algorithm for the whole process. Inside this developed algorithm, as with most nonlinear optimization algorithms, the most ex- pensive part is a linear solve. In my case, it is a linear solve of a block system. I develop and implement a multi-tier preconditioning approach to solve this system in a reasonable amount of time. Finally, I present a large topology optimization problem presented in three dimensions that has been solved using IPO and SAND, demonstrating the usability of the implemented algorithm.
Open Access
From recycled machining waste to useful powders: sustainable fabrication and utilization of feedstock powder for metal additive manufacturing
(Colorado State University. Libraries, 2018) Fullenwider, Blake Patrick, author; Ma, Kaka, advisor; Weinberger, Chris, committee member; Neilson, James, committee member
Gas atomized (GA) powders are the most common feedstock for state-of-the-art metal additive manufacturing (AM) technologies because of their spherical morphology and controllable particle size distribution. However, significant resource consumption, e.g., energy and inert gas, are required to produce GA powders, leading to high costs and limited availability in alloy compositions. To fulfill the growing demand for alternative and sustainable feedstock production for metal AM, my research aimed to explore a mechanical milling strategy to fabricate 304L stainless steel powders from recycled machining waste chips. A theoretical analysis was performed to evaluate the impact force on powder and the consequent maximum deformation depth per impact during ball milling with different ball diameters. The modeling results suggest that 20-mm-diameter balls effectively reduce the powder particle size while 6-mm-diameter balls are favorable in terms of forming spherical morphology of the powder. Various ball milling procedures were implemented to experimentally investigate the effect of ball diameter on the powder morphology evolution and particle size refinement. It is found that a novel dual-stage ball milling strategy effectively converts machining chips to powder with desirable characteristics (near spherical morphology with particle sizes of 38-150 μm) for metal additive manufacturing. The ball milled powders created from the machining chips also exhibit a higher hardness than GA powder, based on nanoindentation testing. To verify the viability of using the ball milled powder created from machining chips in metal AM, single tracks (ST) have been successfully deposited via laser engineered net shaping (LENS®) and compared to the single tracks made from GA powder (ST-GA) using identical deposition conditions. The microstructures of these single tracks exhibited adequate adhesion to the substrate, a uniform melt pool geometry, continuity, and minimal splatter. Minimal differences in grain structure were observed between the single tracks made from ball milled powder (ST-BM) and ST-GA. However, the average nanoindentation hardness of ST-BM is approximately 21% higher than that of ST-GA. Although the chemical compositions of both types of single tracks are within the compositional range of a 304L stainless steel, the increase in hardness of ST-BM is attributed to a 1.7 wt.% decrease in Ni content, potentially leading to an increase in the amount of martensite. Therefore, my research has discovered a sustainable approach to fabricate powders from recycled machining chips and has proved it is feasible to utilize these powders as feedstock in metal AM. Future work on depositing bulk samples with more complex geometry using the ball milled powder is proposed.
Open Access
Geometry considerations for high-order finite-volume methods on structured grids with adaptive mesh refinement
(Colorado State University. Libraries, 2022) Overton-Katz, Nathaniel D., author; Guzik, Stephen, advisor; Gao, Xinfeng, advisor; Weinberger, Chris, committee member; Bangerth, Wolfgang, committee member
Computational fluid dynamics (CFD) is an invaluable tool for engineering design. Meshing complex geometries with accuracy and efficiency is vital to a CFD simulation. In particular, using structured grids with adaptive mesh refinement (AMR) will be invaluable to engineering optimization where automation is critical. For high-order (fourth-order and above) finite volume methods (FVMs), discrete representation of complex geometries adds extra challenges. High-order methods are not trivially extended to complex geometries of engineering interest. To accommodate geometric complexity with structured AMR in the context of high-order FVMs, this work aims to develop three new methods. First, a robust method is developed for bounding high-order interpolations between grid levels when using AMR. High-order interpolation is prone to numerical oscillations which can result in unphysical solutions. To overcome this, localized interpolation bounds are enforced while maintaining solution conservation. This method provides great flexibility in how refinement may be used in engineering applications. Second, a mapped multi-block technique is developed, capable of representing moderately complex geometries with structured grids. This method works with high-order FVMs while still enabling AMR and retaining strict solution conservation. This method interfaces with well-established engineering work flows for grid generation and interpolates generalized curvilinear coordinate transformations for each block. Solutions between blocks are then communicated by a generalized interpolation strategy while maintaining a single-valued flux. Finally, an embedded-boundary technique is developed for high-order FVMs. This method is particularly attractive since it automates mesh generation of any complex geometry. However, the algorithms on the resulting meshes require extra attention to achieve both stable and accurate results near boundaries. This is achieved by performing solution reconstructions using a weighted form of high-order interpolation that accounts for boundary geometry. These methods are verified, validated, and tested by complex configurations such as reacting flows in a bluff-body combustor and Stokes flows with complicated geometries. Results demonstrate the new algorithms are effective for solving complex geometries at high-order accuracy with AMR. This study contributes to advance the geometric capability in CFD for efficient and effective engineering applications.
Open Access
Implementation and evaluation of backward facing fuel consumption simulation and testing methods
(Colorado State University. Libraries, 2019) Johnson, Troy, author; Bradley, Thomas, advisor; Pasricha, Sudeep, committee member; Weinberger, Chris, committee member
The Colorado State University Vehicle Innovations Team (VIT) participates in numerous Advanced Vehicle Technology Competitions (AVTC's) as well as several hybrid-electric vehicle projects with outside sponsors. This study seeks to develop and quantify the accuracy of simulation and testing methods that will be used in the VIT's predictive optimal energy management strategy research that is to be used in these projects. First, a backward facing vehicle simulation model is built and populated with real-world OBD-II drive data collected from a 2019 Toyota Tacoma. This includes the creation of both an engine speed vs accelerator position vs engine load map as well as an engine speed vs engine load vs engine fuel rate map. Acceleration events (AE's) are performed with a baseline shift schedule and vehicle performance is recorded. The backward facing vehicle simulation model is used to predict how a modified shift schedule will affect the vehicle's fuel consumption. Further AE's are performed with the modified shift schedule and the performance data is compared to the vehicle simulation. The backward facing simulation model was capable of predicting average engine speed within 0.3 RPM, average engine load within 5.2%, and average total fuel consumption within 0.2 grams of the actual testing data. This study concludes that the vehicle simulation methods are capable of predicting fuel consumption changes within 1.4% of what is actual measured during real-world testing with a 95% confidence.
Open Access
Molecular design of a fatigue-resistant and energy-dissipative hydrogel
(Colorado State University. Libraries, 2022) Klug, Allee Shiryce, author; Bailey, Travis, advisor; Reynolds, Melissa, committee member; Chen, Eugene, committee member; Weinberger, Chris, committee member
Hydrogels at the most basic iteration are cross-linked polymer networks swollen in water. They show promise in biomedical applications due to their high water content and flexibility. However, intentional design of new hydrogel networks by modification of the choice of polymer, the fabrication of the polymer network, and the choice of cross-link have resulted in hydrogels which have useful properties ranging from fatigue resistance to elasticity to bulk toughness. Of particular interest is a hydrogel which can dissipate energy as a way to resist failure of the polymer network. Unfortunately, many of the design strategies previously used to insert an mechanism for energy dissipation into the hydrogel result in hydrogels which are not elastic or their mechanical properties fatigue throughout multiple cycles of use. Therefore, our goal was to design a hydrogel network that is able to both dissipate energy and be resistant to fatigue of mechanical properties. This design strategy is based on the self-assembly of blends of ABC and ABCBA block polymers, specifically polystyrene-b-polyisoprene-b-poly(ethylene oxide) (PS-PI-PEO, SIO) and polystyrene-b-polyisoprene-b-poly(ethylene oxide)-b-polyisoprene-b-polystyrene (PS-PI-PEO-PI-PS, SIOIS) into a sphere morphology where the A block is spheres of glassy, hydrophobic polystyrene surrounded by the B block of rubbery, hydrophobic polyisoprene as the surface of the sphere. These AB spherical domains sit in a matrix of the C block, poly(ethylene oxide). The spherical domains are tethered together by the SIOIS polymer so that the glassy spheres are evenly-spaced physical crosslinks in the polymer network. The tethered spheres provide the network with elasticity and fatigue resistance while the hydrophobic PI block is accessible to forcibly mix with water as a way to dissipate energy when the hydrogel is strained. This dissertation describes the design, testing, and optimization of a hydrogel where an energy dissipation mechanism was placed directly onto every crosslink of a known elastic and fatigue-resistant network. The possibility of even designing such a network was tested by studying the self-assembly of the SIO polymers into the ABC block polymer sphere morphology. Once, the formation of the sphere morphology in the tethered micelle network was confirmed, the effectiveness of the design strategy of a fatigue-resistant network with an intrinsic energy dissipation mechanism was studied by comparison of the mechanical properties of the SIOIS hydrogel to a similar hydrogel that is fatigue-resistant but does not contain an energy dissipation mechanism. Finally, the design of the SIOIS hydrogel is optimized by studying the effect of changes to the hydrogel processing method and changes to the PS molecular weight on the self-assembly of the energy dissipation PI block and the formation of the tethered micelle network.
Open Access
One-dimensional effective continuum mechanics models of braided and trapezoidal wires
(Colorado State University. Libraries, 2017) Alkharisi, Mohammed K., author; Heyliger, Paul, advisor; Chen, Suren, committee member; Weinberger, Chris, committee member
As the use of wires in different engineering applications increases, investigation into and better understanding of the wire's behavior become more important. Over the past years, heavy work has been done to study the mechanical and dynamical behavior of wires using analytical, experimental, and finite element models. This attention explains the importance of such a structure. However, studying such a structure is more challenging than with other ordinary structures, due to the nonlinearity of the geometry. In this work, the axial elastic behavior was studied using linear three-dimensional finite element Fortran 77 code. The wire was discretized, element matrices were built, and varying boundary conditions were applied to find the four elastic coefficients of the global matrix: pure tensile stiffness, two coupling terms between the tensile and torsional stiffness. Couple action appears when there is a twist in the wire, for that varying twist angles (0°, 5°, 10°, 15°, 20°, 25°, and 30°) were used to check their effect on the stiffness. To validate the model used, a simple straight wire rope (1+6) of known behavior was tested using same approach and twist angles, and then compared with 7 existing analytical models available in literature. Results showed a good agreement with the finite element model, which indicates that the approach used to solve for the trapezoidal wire was reliable and valid. The results showed that the trapezoidal wire is stiffer than the simple straight wire rope and exhibited extensional and torsional coupling behavior values, which can be critical in the design process of these structures. This model can also be used to decrease the high costs associated with experimental tests needed to determine its behavior. The method was extended, as, to evaluate the integrity of such a structure, it was essential to conduct a free vibration analysis using a one-dimensional finite element approximation for the trapezoidal wire as well as for the simple straight wire rope, which had not been done before, to investigate the extensional and torsional behavior of the motion of these wires. First, an aluminum straight bar was tested by solving the mass and stiffness matrices using 2-, 4-, 8-, and 16-element approximations, and the convergence was checked against the known exact axial and torsional frequency solutions. The 16-element approximation was applied to both the trapezoidal and the simple straight wire rope with all the lay angles considered. The coupled extensional and torsional vibration for these wires was solved using closed-form equations for the mass matrices; with these and the stiffness matrices constructed, the eigenproblem was solved to find the frequencies and the corresponding mode shapes. The two types of displacement, axial and torsional, were found in each frequency while having coupled stiffness. The simple straight wire rope behaved similarly to the trapezoidal wire, but with relatively lower frequencies. Which conclude that it is important to the design, safety, and monitoring, depending on the application for which these wires are used, that the coupled frequencies suggested be considered and studied carefully.
Open Access
Processing of mayenite electride and its composites in spark plasma sintering
(Colorado State University. Libraries, 2019) Kuehster, Adam Edward, author; Ma, Kaka, advisor; Weinberger, Chris, committee member; Williams, John, committee member
Mayenite electride, as the first inorganic room temperature stable electride, has attracted intensive research interests since the early 2000s due to its great potential in various applications such as catalysts, conductive oxides and thermionic emission materials. Mayenite electride is developed from mayenite, a stoichiometric compound of CaO and Al2O3 (12CaOˑ7Al2O3, referred to as C12A7 hereafter) that has a cubic unit cell with a positively charged lattice framework [Ca24Al28O64]4+ of twelve crystallographic subnano-cages per unit and O2- anions clathrated in the cages to maintain charge neutrality. When mayenite is heat treated in a reducing environment, electrons replace O2- ions clathrated in the cages. The electrons can migrate through the inter-cage framework, leading to the formation of electride (C12A7:e-), an electrically conductive form of C12A7. A variety of methods to make C12A7:e- powder and bulk materials have been investigated in the literature, all of which involve multiple steps and long-time (days to weeks) of heat treatment at high temperatures (>1100 ˚C). Although fundamental knowledge of the structure and functionality of C12A7:e- is advancing in the field, the formation of other calcium aluminate phases during the synthesis of mayenite or its electride has been overlooked. Most of the previous studies also lack detailed microstructure characterization. In addition, monolithic C12A7:e- does not provide continuous ohmic contact due to the destruction of the surface cages during processing, which limits its direct use in thermionic emission devices. To address the aforementioned practical issues and to fill in the fundamental knowledge gap, we investigated the effect of adding different reinforcing particles, including carbon black (CB), Ti, and TiB2, on the formation of C12A7:e- via spark plasma sintering (SPS), with attention particularly paid to address phase formation during the processing. Specifically, preformed C12A7 powder was synthesized via a solid-state reaction and used as the precursor base in SPS to study the effect of additives. In addition, a novel approach using in-situ reaction in SPS was proposed in the present work to significantly reduce the processing time. My research revealed that both Ti and TiB2 effectively reduced C12A7 to its electride phase, C12A7:e-. However, addition of Ti and TiB2 also led to partial decomposition of C12A7 into secondary calcium aluminate phases, primarily Al2O3-rich calcium monoaluminate (CA) and CaO-rich tricalcium aluminate (C3A). Although CB did not effectively reduce C12A7 to C12A7:e-. it did not result in the formation of any secondary calcium aluminate phases. Using Ti foils on the top and bottom of the preformed C12A7 powder in SPS created C12A7:e- with a near-theoretical maximum electron concentration ~ 10^21/cm^3. For the in-situ reaction approach, the chemical homogeneity and size distribution of precursor powders are critical to forming C12A7:e- in the typical processing time frame of SPS (5-15 minutes). The fast heating rate and C-rich environment in SPS increased the CaCO3 decomposition temperature to above 930°C, which is consequential to the calcium aluminate formation reaction. Adding Ti powder lowered the CaCO3 decomposition temperature in SPS and allowed for the formation of C12A7:e- via in-situ reaction sintering. The work function of a 50-50wt% C12A7:e- -Ti composite in this study is ~ 2.6 eV.
Open Access
Systems and operational modeling and simulation to address research gaps in transportation electrification
(Colorado State University. Libraries, 2023) Rabinowitz, Aaron I., author; Bradley, Thomas, advisor; Daily, Jeremy, committee member; Pasricha, Sudeep, committee member; Weinberger, Chris, committee member
Transportation electrification is increasingly thought of as a necessity in order to mitigate the negative effects of climate change and this has recently resulted in large investments, within the US and globally, into green transportation technology. In order to ensure that the electrification transition of the transportation sector is carried out in an efficient and effective manner, it is important to address key research gaps. The proposed research involves addressing 4 important research gaps related to electrification in the transportation sector. The four research gaps addressed are quantifying the energetic benefits which may be achieved via the use of Connected Autonomous Vehicle (CAV) technology to enable optimal operational and dynamic control in Electric Vehicles (EVs), the quantification of the operational inconvenience experienced by Battery Electric Vehicle (BEV) users compared to Internal Combustion Vehicle (ICV) users for given infrastructural parameters, and quantification of the potential economic competitiveness of BEVs for Heavy Duty (HD) Less Than Truckload (LTL) fleets. The identified research gaps are addressed via quantitative, data-based, and transparent modeling and simulation. In the first two cases, comprehensive simulation experiments are conducted which show both the potential energetic improvements available as well as the best methods to achieve these improvements. In the second case, a novel method is developed for the quantification of operational inconvenience due to energizing a vehicle and an empirical equation is derived for estimating said inconvenience based on vehicular and infrastructural parameters. The empirical equation can be deployed on a geo- spatial basis in order to provide quantitative measures of BEV inequity of experience. In the last case a novel, data-driven simulation based Total Cost of Ownership (TCO) model for class 8 BEV tractors is developed and used to project economic competitiveness in the near and medium term future. Findings from the proposed research will provide critical information for industry and policy-makers in their mission to enable an efficient and equitable transportation future.
Open Access
The impact of non-local elasticity factors on natural frequencies of a rectangular cantilever beam
(Colorado State University. Libraries, 2020) Bouzaid, Ibrahim F., author; Heyliger, Paul, advisor; Chen, Suren, committee member; Weinberger, Chris, committee member
The natural frequencies of a structural element are important factors in attaining a safe design. Natural frequency is the frequency at which an element tends to vibrate in the absence of any driving or damping force. When an object vibrates at a frequency equivalent to its natural frequency, its vibration amplitude increases significantly, which could lead to severe damage. A safe design would thus require having a different natural frequency compared to the frequency of the vibrating element. In some cases, obtaining accurate natural frequencies is challenging. In cases in which non-local elasticity, where the stress at a point is a function of the strain at the close region around that point, provides a better solution to the mechanical problems compared to other theories, natural frequencies should be studied. The non-local elastic solution to the non-local elastic natural frequencies of a rectangular cantilever beam problem was developed using a Fortran code, and the finite elements of non-local mesh were generated using a MATLAB code. The eigenvalue problem was solved, and the mode shapes were plotted using another MATLAB code. The results indicate that the natural frequencies for the non-local solution have dropped 25–30 percent. The non-local factors, mesh size, and slenderness influenced the drop in the natural frequencies. The non-local natural frequencies tended to match the local natural frequencies up to the third frequency, then start diverged. The mode shapes are similar to the local elastic mode shapes in all cases.
Open Access
Understanding and utilization of thermal gradients in spark plasma sintering for graded microstructure and mechanical properties
(Colorado State University. Libraries, 2022) Preston, Alexander David, author; Ma, Kaka, advisor; Weinberger, Chris, committee member; Neilson, Jamie, committee member; Heyliger, Paul, committee member
Spark plasma sintering (SPS), also commonly known as electric field assisted sintering, utilizes high density electric currents and pressure to achieve rapid heating and significantly shorter sintering times for consolidating metal and ceramic powders, which could otherwise be difficult, time consuming, and energy intensive. SPS has attracted extensive research interests since the early 1990's, with the promise of efficient manufacturing of refractory materials, ultrahigh temperature ceramics, nanostructured materials, functionally graded materials, and non-equilibrium materials. Thermal gradients occur in SPS tooling and the samples during sintering, which can be a drawback if homogeneous properties are desirable, as the temperature inhomogeneity can lead to large gradients in microstructure such as porosity, grain size, and phase distribution. Many researchers have looked to mitigate or control these gradients by design and use of specialized tooling. However, the effect of the starting powder is relatively less investigated or overlooked. Feedstock powders can come in various shapes, particle size distributions, and surface chemistry. Effects of these powder characteristics on the SPS process and the consequent microstructure of the sintered parts remain as a gap in the fundamental knowledge of SPS. To fill in this gap, my research investigated the role of thermal gradients during SPS, and how the thermal gradients subsequently affect the location-specific pore distribution, and the consequent mechanical properties of the materials. From a practical point of view, design and fabrication of a bulk sample with a fully dense surface and an engineered pore architecture in the sample interior via one-step SPS will enable mechanical properties unattainable via conventional processing of fully dense bulk materials, such as alike combination of lightweight, high surface hardness, and wear resistance, and high toughness. Therefore, the overarching goal of my research was to provide fundamental insights into the material processing - microstructure - properties correlation so that the field assisted sintering technology can be advanced to control location-specific microstructure. To fulfill this goal, two metallic materials were selected in my study, austenitic stainless steel and commercially pure titanium, representing inherently heavy but widely used alloys, and a pure metal that is inherently lightweight, these materials were used to investigate the effects of powder morphology on the sintering behavior. The pure Ti was selected specifically to gain fundamental insight into the effect of powder shape on sintering, while mitigating the concern of alloying/precipitation events and integrating FEM with my experimental work. This work identified a relationship between decreasing pore size and increasing yield strength in stainless steel, which was attributed to fine precipitate formation surrounding submicron pores inducing local stiffening. Whereas larger pores where precipitates were not found are concluded to not have the necessary driving force for the precipitation event to occur. Ball milled stainless steel powders with higher aspect ratios were also shown to have smaller porosity gradients in comparison to their spherical gas atomized counterparts. A thermal electric finite element model is also proposed which incorporates the master sintering curve to simulate densification as an alternative to the more computationally costly and difficult to parametrize fully coupled thermal-electric-mechanical finite element model. Results from the combined model indicate strong agreement with experimental results within 2% accuracy of measured densification. Additionally, the model predicts higher porosity gradients for gas atomized powders in comparison to ball milled powders which is experimentally verified.
Embargo
Understanding structure property relationships in niobium–based oxides for high-rate anodes
(Colorado State University. Libraries, 2024) Salzer, Luke David, author; Sambur, Justin, advisor; Dong, Yuyang, committee member; Henry, Chuck, committee member; Weinberger, Chris, committee member
With the growing usage of portable electronic devices, electric vehicles, and grid level storage, a diverse set of energy storage devices is required for each application. Current commercial level lithium-ion batteries commonly utilize graphite as the anode material. While graphite possesses impressive energy storage, graphite struggles with high (dis)charge applications. One class of materials of interest to replace graphite are niobium-based oxides, some of which fall into a group of materials called Wadsley-Roth crystallographic shear compounds. Wadsley-Roth (W-R) compounds possess unit cells with nxm blocks of edge-shared octahedra, which boast high-rate capabilities, having higher volumetric capacities than graphite at various (dis)charge rates. While various (W-R) compositions of have been synthesized and their electrochemical properties explored, the origin of the excellent rate capabilities and capacities is unclear. Herein, niobium-based anodes for high-rate lithium-ion batteries are investigated to understand the structure-property relationships in W-R materials with different block sizes, levels of disorder, and composition. Additionally, a niobium oxide polymorph falls into a unique class of energy storage materials called pseudocapacitors, which possess high energy density while the charge storage mechanism mimics that of a capacitor. Sections of this work describe current and future investigations of pseudocapacitive niobium oxide to better understand the origin of this interesting material. Chapter 1 begins with a brief introduction, background, and motivation on the need for high-rate, high-capacity anode materials as an alternative for graphite, to address the growing need for high-power, high-energy density materials. Chapter II describes the synthesis of three structurally similar W-R compounds with different block sizes and investigates the electrochemical performance of each material. Chapters III and IV investigate methods to improve the electrochemical performances of W-R compositions through defects and dopants. Chapter V investigates the pseudocapacitive niobium oxide that also exhibits high-rate capabilities through a process called pseudocapacitance, in which the material possesses electrochemical characteristic similar to both batteries and capacitors. In the final chapter, Chapter VI, concludes the dissertation by describing further directions necessary to better understand the structure property relationships resulting in high-rate, high-capacity niobium-based oxide anodes.
Open Access
Using antimony as a model anode to study the chemical and mechanical stability of electrodes in Li-ion and next generation batteries
(Colorado State University. Libraries, 2019) Schulze, Maxwell Connor, author; Prieto, Amy, advisor; Shores, Matthew, committee member; Neilson, James, committee member; Weinberger, Chris, committee member
As humanity grapples with the ever-increasing global demand for electrical energy, we are concurrently trying to curb global greenhouse gas emissions on massive scales to avoid potentially catastrophic changes in the global climate. Strategies to address these problems include transitioning away from a fossil fuel powered society where electrical grid energy is instead generated from renewable sources and internal combustion engine vehicles are replaced with electrified ones. Both of these transitions require energy storage technologies that can deliver high efficiencies, large energy densities, large power outputs, long lifetimes, and good safety factors all while remaining affordable and sustainable to produce. Li-ion batteries have already proven their merit as an effective energy storage technology with high enough energy densities, low enough costs, and long enough lifetimes to be ubiquitous in powering portable electronic devices. While the performance metrics of Li-ion batteries have also started to allow all-electric vehicles and grid-level energy storage to become commercially feasible, limitations in their cycle lifetimes and safety concerns arising from their flammable nature still limit their widespread implementation for these application. Ultimately, the interactions between constituent materials of a battery and the modes of their degradation limit a battery's performance. As such, research to understand and mitigate the degradation of battery materials, including those that move beyond Li-ion battery chemistry, is necessary to promote the widespread, tunable, and diverse use of batteries in overcoming the challenges discussed. Herein, I present a study that uses antimony as a model anode material to develop an understanding of the critical limiting factors of next-generation battery materials. Antimony-based anodes exhibit degradation and concomitant short cycle-lifetimes that are typical of many promising next-generation battery materials, including those that move beyond Li-ion chemistries. Thus, antimony-based model anodes can be used to study such degradation, which is primarily due to chemical and mechanical instability of the electrode and its interfaces with other battery cell components. In the following chapters, strategies to improve the chemical or mechanical stability of the antimony-anode and its interfaces are developed and can be more generally applied to other promising next-generation electrode materials. The following is a journal format dissertation, with each chapter being a document that is published, submitted, or in preparation to a peer-reviewed journal. The first chapter reviews the basic operating principles of rechargeable batteries as well as critically discusses the electrochemical experiments that are common in battery materials research. In particular, the first chapter emphasizes the limits of testing half-cell configurations in representing the cycle lifetimes of full-cell batteries, the key metric needed for long cycle lifetimes in full-cells being extremely high coulombic efficiencies. Chapter two explores and develops mitigation strategies for detrimental mechano-chemical interactions at the interface between the active Cu-Sb anode and the current collector that arise from the existence of a ternary Li-Cu-Sb phase with structural similarity to both Cu2Sb and Li3Sb. While the existence of the ternary phase results in good reversibility of Cu-Sb electrodes when cycled in Li-ion batteries, it also results in the formation of voids at Cu-Sb interfaces that exacerbates delamination during cycling to result in short cycle lifetimes. Chapter three develops a procedure for the electrodeposition of antimony carbon nanotube composites as a strategy to address the bulk mechanical instability of the anode during cycling in Li- and Na-ion batteries. Results of chapter three reveal significant chemical instability at the anode-electrolyte interface and motivate much of the work performed in chapter four, which departs from focusing on antimony as an anode material and instead uses antimony to explore the properties of anode coatings. Chapter four is a systematic study that explores how annealing conditions affect properties of polyacrylonitrile coatings relevant to the chemical stabilization of the electrode-electrolyte interface. This study reveals that ion diffusion in annealed polyacrylonitrile films is correlated to the delocalization of electrons in conjugated domains within the polyacrylonitrile films. Finally, chapter five reviews the materials properties that have made the Li-ion battery so successful, such as the mechanically and chemically stable interfacial layers that form at the electrode-electrolyte interfaces. The chapter additionally highlights some recent progress in the battery materials field and suggests that electrolyte additives, interfacial coatings, and solid-state electrolytes as the most impactful types of materials to continue researching and developing for the future.
Open Access
Using electrochemical methods to synthesize and understand energy dense anodes for lithium-ion and "beyond" battery technologies
(Colorado State University. Libraries, 2021) Ma, Jeffrey, author; Prieto, Amy L., advisor; Shores, Matthew, committee member; Finke, Richard, committee member; Weinberger, Chris, committee member
To view the abstract, please see the full text of the document.