<p> The demand for energy is continually increasing overtime and the key to meeting future demand in a sustainable way is with energy storage. Li-ion batteries employing layered transition metal oxide cathodes are one of the most technologically important energy storage technologies. However, current Li-ion batteries are unable to access their full theoretical capacity and suffer from performance limiting degradation over time partially originating from the cathode and partially from the interface with the electrolyte. Understanding the fundamental limitations of layered transition metal oxide cathodes requires a complete understanding of the surface and bulk of the materials in their most delithiated state. </p><p> In this thesis, we employ LiNi_0.8Co_0.15Al_0.05O₂ (NCA) as a model system for Ni-rich layered oxide cathodes. Unlike its parent compound, LiCoO₂, NCA is capable of high states of delithiation with minimal structural transitions. Furthermore, commercially available NCA has little to no transition metals in the Li layer. X-ray spectroscopies are an ideal tool for studying cathodes at high states of delithiation due their elemental selectivity, range of probing depths, and sensitivity to both chemical and electronic state information. The oxidation state of the transition metals at the surface can be probed via X-ray photoelectron spectroscopy (XPS) while both bulk and surface oxidation states as well as changes in metal oxygen bonding can be probed using X-ray absorption spectroscopy (XAS). </p><p> Using X-ray spectroscopy in tandem with electrochemical, transport and microscopy measurements of the same materials, the impedance growth with increasing delithiation was correlated with the formation of a disordered NiO phase on the surface of NCA which was precipitated by the release of oxygen. Furthermore, the surface degradation was strongly impacted by the type of Li salt used in the electrolyte, with the standard commercial salt LiPF₆ suffering from exothermic decomposition at high voltages and temperatures. Substituting LiPF₆with LiBF4 suppressed NCA surface degradation and the dissolution of the transition metals into the electrolyte which is responsible for the impedance growth. Even in the most extreme conditions (4.75V vs Li⁺/Li⁰ at 60 &deg;C for > 100 hrs) the degradation (i.e. metal reduction) was restricted to the first 10-30 nm and no evidence of oxygen loss was observed in the bulk. </p><p> However, the transition metal ions were found to cease oxidizing above 4.25 V vs Li⁺/Li⁰ despite it being possible to extract 20% more lithium. Using a newly developed high efficiency resonant inelastic x-ray scattering (RIXS) spectrometer to probe the O K-edge of NCA electrodes at various conditions, it was concluded that oxygen participates in the charge compensation at the highest states of delithiation instead of the transition metals. These results are intrinsic to the physical and electronic structure of NCA and appear general to the other layered transition metal oxides currently under consideration for use as cathodes in Li-ion batteries. </p><p>

<p> Many ferroelectric devices benefit from the ability to deposit thin ferroelectric layers. Poly(vinylidene) fluoride (PVDF) is the prototypical ferroelectric polymer, but processing of thin film ferroelectric PVDF remains a challenge due to the formation of large voids in the film during traditional thin film processing. The research described in this dissertation starts by investigating the origin of these voids. The cause of these voids is found to be caused by vapor induced phase separation (VIPS). Guided by the thermodynamics of VIPS, a process is then designed to produce void-free ferroelectric PVDF thin films on polar and non-polar substrates. The films are shown to have a high remnant polarization (~6.5 C m^&ndash;2). The later part of this dissertation is focused on understanding the temperature and structural phase dependent kinetics of polarization switching in PVDF films. A polarization switching model is developed with considerations of Avrami nucleation and growth, local electric fields, temperature and structural phase. The kinetics of polarization switching are shown to follow a universal behavior when correctly accounting for temperature and structural phase.</p><p>

<p>Understanding the curing kinetics of a thermoset resin has a significant importance in developing and optimizing curing cycles in various industrial manufacturing processes. This can assist in improving the quality of final product and minimizing the manufacturing-associated costs. One approach towards developing such an understanding is to formulate kinetic models that can be used to optimize curing time and temperature to reach a full cure state or to determine time to apply pressure in an autoclave process. Various phenomenological reaction models have been used in the literature to successfully predict the kinetic behavior of a thermoset system. The current research work was designed to investigate the cure kinetics of Bisphenol-A based Benzoxazine (BZ-a) and Cycloaliphatic epoxy resin (CER) system under isothermal and nonisothermal conditions by Differential Scanning Calorimetry (DSC). The cure characteristics of BZ-a/CER copolymer systems with 75/25 wt% and 50/50 wt% have been studied and compared to that of pure benzoxazine under nonisothermal conditions. The DSC thermograms exhibited by these BZ-a/CER copolymer systems showed a single exothermic peak, indicating that the reactions between benzoxazine-benzoxazine monomers and benzoxazine-cycloaliphatic epoxy resin were interactive and occurred simultaneously. The Kissinger method and isoconversional methods including Ozawa-Flynn-Wall and Freidman were employed to obtain the activation energy values and determine the nature of the reaction. The cure behavior and the kinetic parameters were determined by adopting a single step autocatalytic model based on Kamal and Sourour phenomenological reaction model. The model was found to suitably describe the cure kinetics of copolymer system prior to the diffusion-control reaction. Analyzing and understanding the thermoset resin system under isothermal conditions is also important since it is the most common practice in the industry. The BZ-a/CER copolymer system with 75/25 wt% ratio which exhibited high glass transition temperature compared to polybenzoxazine was investigated under isothermal conditions. The copolymer system exhibited the maximum reaction rate at an intermediate degree of cure (20 to 40%), indicating that the reaction was autocatalytic. Similar to the nonisothermal cure kinetics, Kamal and Sourour phenomenological reaction model was adopted to determine the kinetic behavior of the system. The theoretical values based on the developed model showed a deviation from the obtained experimental values, which indicated the change in kinetics from a reaction-controlled mechanism to a diffusion-controlled mechanism with increasing reaction conversion. To substantiate the hypothesis, Fournier et al?s diffusion factor was introduced into the model, resulting in an agreement between the theoretical and experimental values. The changes in cross-linking density and the glass transition temperature (Tg) with increasing epoxy concentration were investigated under Dynamic Mechanical Analyzer (DMA). The BZ-a/CER copolymer system with the epoxy content of less than 40 wt% exhibited the greatest Tg and cross-linking density compared to benzoxazine homopolymer and other ratios.

<p> In today&rsquo;s industry, engineering materials are continuously pushed to the limits. Often, the application only demands high-specification properties in a narrowly-defined region of the material, such as the outermost surface. This, in combination with the economic benefits, makes case hardening an attractive solution to meet industry demands. While case hardening has been in use for decades, applications demanding high hardness, deep case depth, and high corrosion resistance are often under-served by this process. Instead, new solutions are required.</p><p> The goal of this study is to develop and characterize a new borochromizing process applied to a pre-carburized AISI 8620 alloy steel. The process was successfully developed using a combination of computational simulations, calculations, and experimental testing. Process kinetics were studied by fitting case depth measurement data to Fick&rsquo;s Second Law of Diffusion and an Arrhenius equation. Results indicate that the kinetics of the co-diffusion method are unaffected by the addition of chromium to the powder pack. The results also show that significant structural degradation of the case occurs when chromizing is applied sequentially to an existing boronized case. The amount of degradation is proportional to the chromizing parameters.</p><p> Microstructural evolution was studied using metallographic methods, simulation and computational calculations, and analytical techniques. While the co-diffusion process failed to enrich the substrate with chromium, significant enrichment is obtained with the sequential diffusion process. The amount of enrichment is directly proportional to the chromizing parameters with higher parameters resulting in more enrichment. The case consists of M₇C₃ and M₂₃C₆ carbides nearest the surface, minor amounts of CrB, and a balance of M₂B.</p><p> Corrosion resistance was measured with salt spray and electrochemical methods. These methods confirm the benefit of surface enrichment by chromium in the sequential diffusion method with corrosion resistance increasing directly with chromium concentration. The results also confirm the deleterious effect of surface-breaking case defects and the need to reduce or eliminate them. </p><p> The best combination of microstructural integrity, mean surface hardness, effective case depth, and corrosion resistance is obtained in samples sequentially boronized and chromized at 870&deg;C for 6hrs. Additional work is required to further optimize process parameters and case properties.</p><p>

Although solar energy is the most abundant energy resource available, photovoltaic solar cells must consist of sufficiently abundant and environmentally friendly elements, for scalable low-cost production to provide a major amount of the world’s energy supply. However, scalability is limited in current thin-film solar cell technologies based on Cu(In,Ga)(S,Se)2 and CdTe due to scarce, expensive, and toxic elements. Thin-film solar cells consisting of earth-abundant and non-toxic materials were made from pulsed chemical vapor deposition (pulsed-CVD) of SnS as the p-type absorber layer and atomic layer deposition (ALD) of Zn(O,S) as the n-type buffer layer. Solar cells with a structure of Mo/SnS/Zn(O,S)/ZnO/ITO were studied by varying the synthesis conditions of the SnS and Zn(O,S) layers. Annealing SnS in hydrogen sulfide increased the mobility by more than one order of magnitude, and improved the power conversion efficiency of the solar cell devices. Solar cell performance can be further optimized by adjusting the stoichiometry of Zn(O,S), and by tuning the electrical properties of Zn(O,S) through various in situ or post-annealing treatments. Zn(O,S) can be post-annealed in oxygen atmosphere or doped with nitrogen, by ammonium hydroxide or ammonia gas, during the ALD growth to reduce the carrier concentration, which can be critical for reducing interface recombination at the p-n junction. High carrier concentration buffer layers can be critical for reducing contact resistance with the ITO layer. Zn(O,S) can also be incorporated with aluminum by trimethylaluminum (TMA) doses to either increase or decrease the carrier concentration based on the stoichiometry of Zn(O,S).

The performance of ultra-thin film solid oxide fuel cells (μ-SOFC) is highly dependent on the structural, microstructural and transport properties of the electrolyte. The focus of this thesis is on understanding the effect of synthesis and processing parameters of BaY0.2Zr0.8O3 (BYZ), a complex oxide proton-conducting electrolyte, on thin-film solid oxide fuel cell (SOFC) performance. The properties of BYZ thin films are highly dependent on film growth techniques and parameters. The relationship between electrolyte thickness and fuel cell performance is investigated in the ultra-thin film thickness range of ~ 70 nm to ~ 200 nm for BYZ films grown by RF sputtering. The microstructure, crystal structure, and electrical behavior of BYZ films were examined as a function of thickness to attain high power density in SOFCs. The optimal thickness that allows for a balance between the leakage current and Ohmic resistance for these devices was determined to be t0 ~150 nm. XRD examination showed a thickness dependent stress behavior in BYZ thin films, with the most compressive state occurring for films of thickness t0. A Volmer-Weber thin film growth mode is proposed for the observed thickness dependent evolution in film properties. The findings of this examination can allow for an increase in the limits of SOFC power density in the ultra-thin regime for proton conducting electrolytes. The presence of a large number of grain boundaries in BYZ films processed at intermediate temperatures leads to diminished conductivity. To mitigate this reduced conductivity while maintaining reasonable processing temperatures, it is essential to increase the effective surface area or TPB of the device. A study of the insertion of ion-selective interfacial layers between the electrode-electrolyte interfaces in μ-SOFCs performance is presented. A nearly two-fold increase in power density of μ-SOFCs in the intermediate temperature range is demonstrated by the addition of ultra-thin palladium interlayers. In addition to enhancing performance, this approach may yield important insight into the proton conduction behavior of BYZ and other proton conducting materials. Finally, to address some of the shortcomings in the current synthesis techniques for BYZ, a novel intermediate temperature thin film synthesis route is demonstrated. This new technique (SP-GNP) is a combination of a thin film deposition technique, Spray Pyrolysis (SP), with a low temperature oxide powder synthesis technique, Glycine Nitrate Process (GNP). A proposed working mechanism and a discussion of the principal parameters that dictate film properties is presented. By using this technique, single-phase perovskite BYZ films were successfully grown at a temperature of 200 °C followed by annealing at 750 °C. The compositional and microstructural evolution of BYZ thin films obtained by SP-GNP is investigated as a function of several technique parameters such as precursor concentration, solvent properties and substrate properties. A microstructural evolution from porous to dense in BYZ thin films by changing precursor composition is demonstrated. This intermediate temperature technique may allow for a deeper insight into the properties of refractory complex oxides through incorporation of novel dopants and may lead to the emergence of new applications for these materials. / Engineering and Applied Sciences - Applied Physics

Advances in the design of adaptive matter capable of programmable, environmentally-responsive changes in shape would enable myriad applications including smart textiles, scaffolds for tissue engineering, and smart machines. 4D printing is an emerging approach in which 3D objects are produced whose shape changes over time. Initial demonstrations have relied on commercial 3D printers and proprietary materials, which limits both the tunability and mechanisms that can be incorporated into the printed architectures. My Ph.D. thesis focuses on a new 4D printing method, which is inspired by the movements or natural plants. Specifically, we encode swelling and elastic anisotropy in printed hydrogel composites through the alignment of stiff cellulose fibrils on-the-fly during printing. Filler alignment parallel to the print path leads to enhanced stiffness in that direction; hence, upon immersion in water, the printed filaments expand preferentially in the direction orthogonal to the printing path. When structures are patterned with broken-symmetry, i.e., as bilayers, their anisotropic swelling leads to programmable out-of-plane deformation, determined by the orientation of printed filaments. We have demonstrated complex changes in curvature including bending, twisting, ruffling, conical defects, and more, all using a single hydrogel-based ink printed in a single step. We have demonstrated the ability to precisely control curvature by varying the actual and the effective thickness, the latter of which is governed by the interfilament spacing within the printed architectures. With collaborators, a model has been developed for solving both the forward and inverse design problems, based on an adaptation of the classic Timoshenko bending theory, allowing us to create nearly arbitrary structures. Our filled hydrogel ink is modular, allowing a broad range of hydrogel chemistries and anisotropic filler compositions to be explored. For example, both reversible and non-reversible hydrogels were explored; namely poly(N-isopropyl acrylamide) (PNIPAm) and poly(N,N-dimethylacrylamide) (PDMAm), respectively. Additionally, light-absorbing carbon microfibers were incorporated to demonstrate reversible, multi-stimuli responsive 4D printing. In this case, reversible shape changes were encoded via 4D printing and then triggered either by heating PNIPAm or illuminating the printed architectures with a near IR laser. In summary, this biomimetic 4D printing platform enables the design and fabrication of complex, reversible shape changing architectures printed with one composite hydrogel ink in a single step. These biocompatible shape-shifting architectures with interesting mechanical and photothermal properties may find applications in smart textiles, tissue microgrippers or scaffolds, or as actuators and sensors in soft machines. / Engineering and Applied Sciences - Engineering Sciences

<p> Electron Beam Additive Manufacturing (EBAM) is a relatively new additive manufacturing (AM) technology that uses a high-energy electron beam to melt and fuse powders to build full-density parts in a layer by layer fashion. EBAM can fabricate metallic components, particularly, of complex shapes, in an efficient and cost-effective manner compared to conventional manufacturing means. EBAM is an enabling technology for rapid manufacturing (RM) of metallic components, and thus, can efficiently integrate the design and manufacturing of aerospace components. However, EBAM for aerospace-related applications remain limited because the effect of the EBAM process on part characteristics is not fully understood. In this study, various techniques including microhardness, optical microscopy (OM), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and electron backscatter diffraction (EBSD) were used to characterize Ti-6Al-4V components processed using EBAM. The results were compared to Ti-6Al-4V components processed using conventional techniques. In this study it is shown that EBAM built Ti-64 components have increased hardness, elastic modulus, and yield strength compared to wrought Ti-6Al-4V. Further, it is also shown in this study that the horizontal build EBAM Ti-6Al-4V has increased hardness, elastic modulus, and yield strength compared to vertical build EBAM due to a preferential growth of the &beta; phase.</p><p>

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