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The use of electrospun polystyrene (PS) fibers to create continuous long range ordered multi-scale porous structures in titanium diboride (TiB2) is investigated in this work. The introduction of electrospun PS fibers as a sacrificial filler into a colloidal suspension of TiB2 allows for easy control over the pore size, porosity, and long range ordering of the porous structures of the sintered ceramic. Green bodies were formed by vacuum infiltrating an electrospun-fiber-filled mold with the colloidal TiB2 suspension. The size, volume, distribution, and dispersion of the pores were optimized by carefully selecting the sacrificial polymer, the fiber diameter, the solvent, and the solid content of TiB2. The green bodies were partially sintered at 2000 C in argon to form a multiscale porous structure via the removal of the PS fibers. Aligned continuous cylindrical pores were derived from the PS fibers in a range of ~5 - 20 μm and random porosity was revealed between the ceramic particles with the size of ~0.3 - 1 μm. TiB2 near-net-shaped parts with the multi-scale porosities (~50 to 70%) were successfully cast and sintered. The multi-scale porous structure produced from electrospun fibers was characterized both thermally and mechanically, at room temperature. The conductivity ranged from 12-31 W m^(-1) K^(-1) at room temperature and the compressive strength ranged from 2-30 MPa at room temperature. Analytical thermal and mechanical models were employed to understand and verify he processing-structure-properties relationship. Finally, a method was devised for estimating the effective thermal conductivity of candidate materials for UHTC applications at relevant temperatures using a finite difference model and a controlled sample environment. This low-cost processing technique facilitates the production of thermally and mechanically anisotropic structures into near-net shape parts, for extreme environment applications, such as ultra high temperature insulation and active cooling components. / MS / Society is on the cusp of hypersonic flight which will revolutionize defense, space and transport technologies. Hypersonic flight is associated with conditions like that of atmospheric re-entry, high heat and force or specific locations of a space craft. The realization of hypersonic flight relies on innovative materials to survive the harsh conditions for repeated flight. We have created a new material with tiny holes that can help prevent heat flow from the harsh atmosphere from damaging the hypersonic craft. Thesis tiny holes are made from placing a polymer fiber in an advanced ceramic (which withstand high temperatures) and removing the fiber to leave holes. The tiny hole’s effect on strength and heat flow have been studied, to understand how the tiny holes can be made better. It is difficult to test materials in the harsh atmosphere associated with hypersonic flight, so a program has been written to estimate thermal properties of candidate materials for hypersonic flight.

The research focus was to determine diffusion mechanisms responsible for densification behavior of SHS produced Al2O3/TiB2 Ceramic Matrix Composites (CMCs). Previous research has shown SHS produced Al₂O₃/TiB₂ composites exhibited unique microstructural properties that contributed to high strength, fracture toughness, and hardness properties. Pressureless densification of SHS produced Al₂O₃/TiB₂ composites would provide a cost savings because the equipment for pressureless densification is less expensive and less complicated than equipment required for densification with pressure. Models for sintering of CMCs and calculation of Sintering Time Constants (STC) were used to predict the densification behavior of the SHS produced Al2O3/TiB2 composite. The Levin, Dirnfeld, Shwam equation was used to determine the Rate Controlling Diffusion Mechanism (RCDM) and activation energy for sintering. X-Ray Diffraction (XRD) analysis of the as-milled reaction product powder revealed the presence of an aluminum borate (Al₁₈B₄O₃₃) as a third phase, as well as, in pressureless heat treated samples. Based on experimental results and analysis, it seemed possible the Al₁₈B₄O₃₃ compound may have formed by reaction of Al₂O₃ with TiB2 along their interfaces. Aluminum borates have been observed to form Al₁₈B₄O₃₃ (s) + B₂O₃ (l) at temperatures above 1000°C. The RCDM for densification of SHS produced Al₂O₃/TiB₂ was found to be liquid phase diffusion with volume diffusion also likely being active during densification. In addition, Al₁₈B₄O₃₃ seemed to be the preferred compound formed during oxidation. Further research should be performed to control formation of Al₁₈B₄O₃₃; as well as, on the oxidation behavior of the SHS produced Al2O3/TiB2. / Master of Science

The effects of oxidation in the thermomechanical response of porous titanium diboride have been investigated. An in-house quasi-static material point method tool was used to perform two -dimensional plane strain simulations on unoxidized hexagonal representative volume elements (RVEs) with macroporosity volume fractions of 10%, 40% and 70% to establish a baseline for the response due to geometric effects. Compressive strains of up to 30% were applied at room temperature. The 10% and 40% RVEs showed shear banding and subsequent shear failure of the inter-pore struts, while shear banding in 70% RVE weakened the struts, which lead to buckling failure. A snapshot oxidation model was then applied to the hexagonal RVEs in place of a transient, diffusion-based oxidation solver. Compressive strain simulations were performed on RVEs with oxide layers ranging from 5 to 50 μm. In RVEs with porosity of 40% or higher, oxide percolation in the struts reduced the effective elastic modulus and compressive strength, though further oxidation beyond the percolation point did not have a significant impact. Ramped and cyclic thermal loads were applied and the damage due to thermal expansion coefficient mismatch at the oxide-substrate interface decreased as the oxide layer was increased. Finally, the snapshot oxidation modeling approach was applied to large porous RVEs derived from micro-computed tomography images of titanium diboride foam. The effective elastic modulus decreased by 47% when the 5 μm layer was applied due to many thin, flexible struts becoming fully oxidized. Subsequent oxidation did not have a significant impact on the thermomechanical response. / Master of Science / Thermal loading experienced by hypersonic flight vehicles has posed significant design challenges in the development of platforms for military and re-entry applications. The advent of hypersonic strike weapons and waveriders has led to an interest in utilizing ceramics with melting points above 3000°C, called ultra-high temperature ceramics (UHTCs), that offer improved resistance to high-temperature oxidation. Beyond load-carrying applications, UHTCs imbued with macroscale porosity have been introduced as candidates for providing thermal insulation of sensitive on-board components. This thesis presents a first pass at modeling the coupled effects of oxidation and continuum damage in the thermomechanical response of such materials. Using an in-house material point method tool, two-dimensional compressive strain simulations were performed on hexagonal representative volume elements (RVEs) of titanium diboride foam with varying levels of macroporosity, along with large porous RVEs derived from micro-computed tomography images of titanium diboride foam. A snapshot oxidation model was applied to these RVEs in place of a transient, diffusion-based oxidation solver, then simulations with applied compressive strains of up to 30% were performed on RVEs with oxide layers ranging from 5 to 50 μm. Ramped and cyclic thermal loads were applied to explore the effects of thermal expansion mismatch between the substrate and oxide phases. The oxide layers were shown to reduce the effective stiffness, compressive strength, and thermal conductivity of the RVEs, with the oxidation state of the inter-pore struts having a large impact on the overall material response.

Spark plasma sintering (SPS) is a powder consolidation technique used to rapidly densify a variety of material systems. SPS is capable of precisely controlling material microstructures and achieving non-equilibrium phases due to rapid heating and cooling rates through the simultaneous application of pressure and direct current. Due to these characteristics, SPS is an ideal processing technique for high temperature ceramics which require processing at temperatures greater than 1500°C. Due to the desirable properties obtained on small diameter materials processed by SPS, larger and more complex geometries are desired while maintaining sample microstructures. In order to accurately scale ceramics produced by SPS, a finite element model must be developed that can be used as a predictive tool. My research focuses on developing a finite element model for the spark plasma sintering furnace at the University of Arizona and validating modeled results using experimentally obtained data. Electrical and thermal conductivity as functions of temperature vary widely among different grades of commercially available electrode grade graphite at constant density. Modeled material properties are optimized in order to calibrate modeled results to experimentally obtained data (i.e. measured current, voltage, and temperature distributions). Sensitivity analysis is performed on the model to better understand model physics and predictions. A calibrated model is presented for 20mm ZrB2 and Si3N4 discs. Sample temperature gradients are experimentally confirmed using grain size and β-Si3N4 phase composition. The model is used to investigate scale up from 20mm to 30mm discs and 30mm rings as well as effects of processing conditions on β-Si3N4 content.

Em Janeiro de 2001, um supercondutor totalmente novo foi apresentado por Nagamatsu, o diboreto de magnésio (MgB2), com uma temperatura crítica, Tc, surpreendentemente alta de 39 K. Atualmente, o MgB2 é considerado o condutor de alto campo do futuro. É claramente aceito que os valores excepcionais de altos campos magnético crítico superior, Hc2, (Hc2 + (0) ? 40 T para Tc ? 35 - 40 K) mostram que o MgB2 é capaz de substituir o Nb3Sn (Hc2 (0) ? 30 T para Tc ? 18 K) como a escolha para aplicações de altos campos magnéticos. Neste trabalho foram preparadas pastilhas supercondutoras de MgB2 utilizando adições de diboretos metálicos de ZrB2, TaB2, VB2 e AlB2 e adições simultâneas de diboretos metálicos e fontes diversas de carbono, como carbeto de silício, grafite e nanotubos de carbono. O objetivo da adição desses novos elementos foi criar mecanismos para melhorar a capacidade de transporte do material, tanto pela dopagem substitucional como pela geração de defeitos na matriz supercondutora, atuando como eficientes centros de aprisionamento das linhas de fluxo magnético. Para isso foram utilizados dois diferentes métodos de preparação de amostras, insitu e ex-situ. O método de preparação in-situ seguiu padrões convencionais, como mistura em moinho de bola e tratamento térmico em fluxo de argônio. Para a preparação das amostras utilizando-se o método ex-situ foram utilizadas técnicas mais sofisticadas, como moagem de alta energia e tratamento térmico em altas pressões (Hot Isostatic Press, HIP). Em geral, as adições dos diboretos metálicos melhoraram a capacidade de transporte do material em baixos campos, as fontes de carbono aumentaram os valores de densidade de corrente crítica em altos campos magnéticos, enquanto que as combinações das duas adições melhoram a capacidade de transporte, para algumas amostras, em toda a faixa de campo magnético medida. / In January 2001, a new superconductor was presented by Nagamatsu, the magnesium diboride (MgB2), with a critical temperature, Tc, extremely high of 39 K. MgB2 is considered the high field conductor of the future. The exceptional high values of upper critical magnetic field, Hc2, (Hc2 + (0) ? 40 T for Tc ? 35 - 40 K) show that the MgB2 is able to replace the Nb3Sn (Hc2 (0) ? 30 T for Tc ? 18 K) as the choice for applications in high magnetic fields. In this work, superconducting pellets of MgB2 were prepared with addition of other metal diborides of ZrB2, TaB2, VB2, and AlB2, and simultaneous additions of metal diborides and different carbon sources, such as silicon carbide, graphite and carbon nanotubes. The objective of these additions of new elements was to create mechanisms to improve the transport capacity of the material, by substitutional doping and by generation of defects in the superconducting matrix, acting as effective pinning centers of magnetic flux lines. Two different methods for sample preparation were used, the in-situ and the ex-situ method. The in-situ preparation method followed conventional standards, such as powder mixing in a ball mill and heat treatment in argon flow. The ex-situ preparation method used more sophisticated techniques, such as high energy ball milling and heat treatment under high pressures (Hot Isostatic Press, HIP). In general, the additions of metal diborides improved the transport capacity of the material at low fields, the carbon sources increased the critical current density at high magnetic fields, whereas the combination of these two additions improved the transport capacity, for some samples, in all range of applied magnetic field.

Em Janeiro de 2001, um supercondutor totalmente novo foi apresentado por Nagamatsu, o diboreto de magnésio (MgB2), com uma temperatura crítica, Tc, surpreendentemente alta de 39 K. Atualmente, o MgB2 é considerado o condutor de alto campo do futuro. É claramente aceito que os valores excepcionais de altos campos magnético crítico superior, Hc2, (Hc2 + (0) ? 40 T para Tc ? 35 - 40 K) mostram que o MgB2 é capaz de substituir o Nb3Sn (Hc2 (0) ? 30 T para Tc ? 18 K) como a escolha para aplicações de altos campos magnéticos. Neste trabalho foram preparadas pastilhas supercondutoras de MgB2 utilizando adições de diboretos metálicos de ZrB2, TaB2, VB2 e AlB2 e adições simultâneas de diboretos metálicos e fontes diversas de carbono, como carbeto de silício, grafite e nanotubos de carbono. O objetivo da adição desses novos elementos foi criar mecanismos para melhorar a capacidade de transporte do material, tanto pela dopagem substitucional como pela geração de defeitos na matriz supercondutora, atuando como eficientes centros de aprisionamento das linhas de fluxo magnético. Para isso foram utilizados dois diferentes métodos de preparação de amostras, insitu e ex-situ. O método de preparação in-situ seguiu padrões convencionais, como mistura em moinho de bola e tratamento térmico em fluxo de argônio. Para a preparação das amostras utilizando-se o método ex-situ foram utilizadas técnicas mais sofisticadas, como moagem de alta energia e tratamento térmico em altas pressões (Hot Isostatic Press, HIP). Em geral, as adições dos diboretos metálicos melhoraram a capacidade de transporte do material em baixos campos, as fontes de carbono aumentaram os valores de densidade de corrente crítica em altos campos magnéticos, enquanto que as combinações das duas adições melhoram a capacidade de transporte, para algumas amostras, em toda a faixa de campo magnético medida. / In January 2001, a new superconductor was presented by Nagamatsu, the magnesium diboride (MgB2), with a critical temperature, Tc, extremely high of 39 K. MgB2 is considered the high field conductor of the future. The exceptional high values of upper critical magnetic field, Hc2, (Hc2 + (0) ? 40 T for Tc ? 35 - 40 K) show that the MgB2 is able to replace the Nb3Sn (Hc2 (0) ? 30 T for Tc ? 18 K) as the choice for applications in high magnetic fields. In this work, superconducting pellets of MgB2 were prepared with addition of other metal diborides of ZrB2, TaB2, VB2, and AlB2, and simultaneous additions of metal diborides and different carbon sources, such as silicon carbide, graphite and carbon nanotubes. The objective of these additions of new elements was to create mechanisms to improve the transport capacity of the material, by substitutional doping and by generation of defects in the superconducting matrix, acting as effective pinning centers of magnetic flux lines. Two different methods for sample preparation were used, the in-situ and the ex-situ method. The in-situ preparation method followed conventional standards, such as powder mixing in a ball mill and heat treatment in argon flow. The ex-situ preparation method used more sophisticated techniques, such as high energy ball milling and heat treatment under high pressures (Hot Isostatic Press, HIP). In general, the additions of metal diborides improved the transport capacity of the material at low fields, the carbon sources increased the critical current density at high magnetic fields, whereas the combination of these two additions improved the transport capacity, for some samples, in all range of applied magnetic field.

The aim of this study was to produce zirconium diboride (ZrB2) and other metal borides such as lanthanum hexaboride (LaB6) and cerium hexaboride (CeB6) by magnesiothermic reduction (reaction of metal oxide, boron oxide and magnesium) using volume combustion synthesis (VCS) and mechanochemical process (MCP). Production of ZrB2 by VCS in air occurred with the formation of side products, Zr2ON2 and Mg3B2O6 in addition to MgO. Formation of Zr2ON2 was prevented by conducting VCS experiments under argon atmosphere. Wet ball milling was applied before leaching for easier removal of Mg3B2O6. Leaching in 5 M HCl for 2.5 hours was found to be sufficient for removal of MgO and Mg3B2O6. By MCP, 30 hours of ball milling was enough to produce ZrB2 where 10% of excess Mg and B2O3 were used. MgO was easily removed when MCP products were leached in 1 M HCl for 30 minutes. Complete reduction of ZrO2 could not be achieved in either production method because of the stability of ZrO2. Hence, after leaching VCS or MCP products, final product was composed of ZrB2 and ZrO2. Formation of LaB6 and CeB6 were very similar to each other via both methods. Mg3B2O6 appeared as a side product in the formation of both borides by VCS. After wet ball milling, products were leached in 1 M HCl for 15 hours and pure LaB6 or CeB6 was obtained. As in ZrB2 production, 30 hours of ball milling was sufficient to form these hexaborides by MCP. MgO was removed after leaching in 1 M HCl for 30 minutes and the desired hexaboride was obtained in pure form.

Nuclear magnetic resonance force microscopy (NMRFM) is a technique that combines aspects of scanning probe microscopy (SPM) and nuclear magnetic resonance (NMR) to obtain 3 dimensional nanoscale spatial resolution and perform spectroscopy. We describe the components of a helium-3 NMRFM probe and studies of ammonium sulfate ((NH₄)₂SO₄) and magnesium diboride (MgB₂). For our room temperature (NH₄)₂SO₄ studies we were able to perform a 1-D scan and perform nutation and spin echo experiments. In our 77 K MgB₂ we demonstrate a 1-D scan of a 30 micron powder sample. In addition, we describe magnetic measurements of the possible dilute semiconductors Mn[subscript x]Sc[subscript 1-x]N and Fe₀.₁Sc₀.₉N. / text

Die vorliegende Arbeit zielte auf die Erarbeitung der verfahrens- und legierungstechnischen Grundlagen zur Randschichtbehandlung von TiAl6V4 durch Dispergieren/Legieren mit Diboriden ab. Für die Untersuchungen zum Dispergieren/Legieren wurden sowohl unterschiedliche Lasersysteme (CO2, CO2-Slab, diodengepumpter Nd:YAG, Diodenlaser) als auch der Elektronenstrahl gewählt. Das Konzept für die Vorbehandlung und die Prozessgestaltung wurde entwickelt und erfolgreich getestet. Anhand von Untersuchungen zum Dispergieren/Legieren wurde das Prozessparameterfeld erarbeitet, wobei sich die nachfolgend aufgeführten Einflussgrößen als wesentlich erwiesen: • Pulverkorngröße (Kornfraktion 5 - 125 µm), • Pulvereintragsmenge (Förderparameter, Förderrate, Pulvervor-/-nachlauf, Pastenschichtdicke), • Prozessparameter (Strahlleistung, Defokussierung, Streckenenergie, Ablenkfigur). Zusammenfassend wurde die gute Eignung des Dispergierens/Legierens mittels Hochleistungsstrahlquellen für die Erzeugung boridverstärkter Randschichten an Bauteilen aus TiAl6V4 nachgewiesen.