Engineering Commons^™

Intragranular Tungsten-Titanium Carbide Composite Ceramics Via Gas-Solid Displacement Reactions, Ryan Daniel Dempsey

Doctoral Dissertations

"Research presented herein details the synthesis and characterization of nanocomposite ceramics featuring novel heterogeneous microstructures of potential interest in a variety of electrical and structural applications. Specifically, W-TiCy composite ceramics featuring tungsten nanoprecipitates located primarily within TiCy grains have been produced via sintering of intragranular nanocomposite powders produced via reaction processing-based techniques. This dissertation details the thermodynamic basis and applied kinetics of a processing scheme for fabricating nanocomposite ceramics whose morphological heterogeneity reflects that originally developed in the powder state. The first study in this series overviews the motivation for applying metallothermic displacement reactions, conducted through gaseous intermediates, to solid …

Joining Of Ultra-High Temperature Ceramics To The Same And Refractory Metals Via Fusion Welding, Jecee Jarman

Doctoral Dissertations

"This research focuses on the joining of ultra-high temperature ceramics to either themselves or refractory metals/alloys via fusion welding and the mechanical strength of produced joints. The ceramics studied consisted of compositions within a diboride-carbide-carbide ternary system while metals studied consisted of molybdenum and a molybdenum alloy. SiC-ZrB2-ZrC ceramics on a binary join between pure SiC and the ternary eutectic composition were found to be weldable when SiC content was below ~57.5 vol%. Compositions above 57.5 vol% SiC were unweldable based on pitting and off-gassing behavior. Welded strength of ceramics was ~180-300 MPa in the SiC-ZrB2-ZrC ternary eutectic composition, ~180-220 …

Mechanical Properties Of Boride Based Ceramics, Alec Murchie

Doctoral Dissertations

“This research focuses on the elevated mechanical properties of diboride based materials at room and elevated temperatures. These materials include a ZrB₂ ceramic, a ZrB₂-WC-SiC ceramic, and a (Hf,Mo,Nb,Ta,W,Zr)B₂ high entropy boride (HEB). The room temperature flexural strength for the ZrB₂ ceramic was 447 MPa and the fracture toughness was 2.3 MPa·m^1/2. The strength decreased to 196 MPa at 1800°C, and then increased to 360 MPa at 2300°C. The fracture toughness increased to 3.1 MPa·m^1/2 at 2200°C. The strength up to 1800°C was likely controlled by a combination of effects: surface damage …

Contributions To The Performance Of Thin Film Capacitors For High Reliability Applications, Daniel Krueger

Doctoral Dissertations

"Capacitors are critical devices in microelectronic assemblies that must be incorporated into electronic systems through a variety of ways such as integrated or discrete devices. This work has developed new thin film capacitors deposited directly onto multichip module or printed circuit board surfaces to benefit from closer integration that enhances system performance for use in high reliability applications. The capacitors serve as filters or provide tuning and energy storage functions. Unexpected performance was observed during development that included low adhesion of the films to the substrates, higher effective dielectric constants than reported in literature, and low yields. Three publications resulted …

Thermal And Mechanical Properties Of Novel Carbide Materials, Evan Charles Schwind

Doctoral Dissertations

"This research focuses on studying the thermal and mechanical properties of novel carbide materials at room and elevated temperatures. The novel carbide materials investigated include zeta phase tantalum carbide (ζ-Ta₄C_3-x) and a high entropy (Ta,Hf,Nb,Zr)C carbide. The electrical resistivity and thermal conductivity of zeta phase tantalum carbide (~96 wt.%) were measured as 160 ± 4.2 μΩ-cm and 9.6 W/m•K. These are higher and lower (respectively) than for cubic tantalum carbide, most likely due to planes of carbon vacancies present in the ζ-Ta₄C_3-x crystal structure. The thermal conductivity of (Ta,Hf,Nb,Zr)C was lower than any …

Intrinsic Mechanical Properties Of Zirconium Carbide Ceramics, Nicole Mary Korklan

Masters Theses

“This research focuses on the processing and mechanical properties of zirconium carbide ceramics (ZrC_x). The first goal of this project was to densify near stoichiometric (i.e., x as close to 1 as possible) and nominally phase pure ZrC_x. The maximum stoichiometry achieved for the ZrC_x (C/Zr ratio of 0.92) was measured using gas fusion analysis. Hot pressing was used to obtain dense ZrC_x. Archimedes was used to determine the relative density of hot pressed ZrC_x at > 95%. Scanning electron microscopy (SEM) was used to determine the overall microstructure of the hot pressed …

Studying The Effects Of Various Process Parameters On Early Age Hydration Of Single- And Multi-Phase Cementitious Systems, Rachel Cook

Doctoral Dissertations

”The hydration of multi-phase ordinary Portland cement (OPC) and its pure phase derivatives, such as tricalcium silicate (C₃S) and belite (ß-C₂S), are studied in the context varying process parameters -- for instance, variable water content, water activity, superplasticizer structure and dose, and mineral additive type and particle size. These parameters are studied by means of physical experiments and numerical/computational techniques, such as: thermodynamic estimations; numerical kinetic-based modelling; and artificial intelligence techniques like machine learning (ML) models. In the past decade, numerical kinetic modeling has greatly improved in terms of fitting experimental, isothermal calorimetry to kinetic-based modelling …

Processing, Microstructure, And Properties Of Engineered Diboride Structures, Connor Charles Wittmaier

Masters Theses

"The mechanical properties and processing parameters of boride ceramics in foam and laminate architectures were evaluated. The ceramic reticulated foam was produced through a polymer substrate replication technique and the hardness and compressive strength were tested. The laminate structure was tested to evaluate the flexure strength and work of fracture as a function of temperature.

The foam architecture was produced using a TiB₂ slurry coating on a polyurethane reticulated foam preform. Foams sintered to 2150⁰C displayed an average grain size of 8.9 ± 7.3 µm, and a hardness of 17.3 ± 2.4 GPa. Crush testing foams were sintered at …

Fusion Welding Of Diboride-Carbide Ceramic Composites, Derek Scott King

Doctoral Dissertations

"Plasma and pulsed plasma arc welding (PAW and PPAW) processes were used to fusion weld ZrB₂ containing 20 vol% ZrC. Varying welding parameters resulted in changes in weld pool shape and size, and the size of ZrB₂ grains within the fusion zone. For PAW processes that resulted in a keyhole fusion zone (full penetration), the arc to workpiece power transfer efficiency was estimated to be 2 grain lengths were observed to decrease between binary PAW FZs (~1 mm in length), and binary PPAW FZs (~0.8 mm in length), and an increasing aspect ratio for ZrB₂ grains in …

Effect Of Solid Solutions And Second Phases On The Thermal Conductivity Of Zirconium Diboride Ceramics, Gregory John Kenneth Harrington

Doctoral Dissertations

"The research presented in this dissertation is focused on the thermal conductivity (k) of ZrB₂ ceramics. The goal was to develop a better understanding of how various solid solutions and second phases affect the thermal and electrical transport in ZrB₂, with a focus on the effect of C, W, and ZrC. The first study showed C additions improved densification and it was proposed that the reduction of boria was the impetus for this result. Boron carbide was formed by the reaction of excess C with reduced B and its formation was mitigated by the addition of ZrH …

Elevated Temperature Mechanical Properties Of Zirconium Diboride Based Ceramics, Eric W. Neuman

Doctoral Dissertations

"Research presented in this dissertation focused on the mechanical behavior of ZrB₂ based ceramic at elevated temperatures. Flexure strength, fracture toughness, and elastic modulus were measured at temperatures up to 2300ºC for three compositions: monolithic ZrB₂ (Z); ZrB₂ - 30 vol% SiC - 2 vol% B₄ C (ZS); and ZrB₂ - 10 vol% ZrC (ZC). In argon, Z, ZS, and ZC had strengths of 210 (at 2300ºC), 260 (at 2200ºC), and 295 MPa (at 2300ºC), the highest temperatures tested for each composition. Fractography was used extensively to characterize the strength limiting flaws as a function of temperature. Strength of ZS …

Bioactive Glass Scaffolds For The Regeneration Of Load-Bearing Bone, Xin Liu

Doctoral Dissertations

"Biocompatible scaffolds that replicate the structure and function of bone would be ideal bone substitutes for structural bone loss, provided they have the requisite mechanical properties for reliable long-term loading. In this dissertation, strong porous scaffolds of silicate 13-93 bioactive glass, created with two different microstructures, were evaluated to determine their mechanical properties and their capacity to regenerate bone in a rat calvarial defect model. Scaffolds with an oriented microstructure of columnar pores were prepared by unidirectional freezing of camphene-based suspensions, followed by thermal annealing and sintering. By optimizing the freezing conditions, annealing time, and sintering temperature, constructs (porosity = …

Processing And Properties Of Zrb₂-Zrsi₂ Matrix Composites Reinforced With Continuous Sic Fiber, Benjamin John Bowin Lai

Masters Theses

"The research presented in this thesis focuses on the processing, microstructure, and mechanical properties of ZrB₂-based composites. The main goal was to develop a process that incorporates high strength continuous SiC fiber in a ZrB₂-based matrix. In order to achieve this, the densification, microstructure, and mechanical properties of ZrB₂-ZrSi₂ monolithic ceramics were characterized along with various SiC fiber reinforced ceramic matrix composites (CMCs) utilizing the same matrix.

There were two main areas of study in this thesis. The initial study demonstrated that additions of ZrSi₂ into a ZrB₂ ceramic increases densification at significantly reduced processing temperatures ≤1600ºC). This study revealed …

Liquid Phase Sintering Of 20bi(Zn₀.₅Ti₀.₅)O₃-80batio₃ Dielectrics With Bismuth-Zinc-Borate And Bismuth Borosilicate Glasses, David I. Shahin

Masters Theses

"Dielectrics in the Bi(Zn₀.₅Ti₀.₅O₃-BaTiO₃ system (specifically 20BZT-80BT, in mol%) are promising candidates for high energy density capacitor applications due to broad temperature-dependent dielectric constant maxima and a relatively field-independent permittivity. Bulk samples require sintering temperatures of greater than 1180ºC to reach useful densities. Due to incompatibility of Bi with low-pO₂ processing, BZT-BT-based multilayer capacitors must utilize noble metal electrodes that resist oxidation during sintering. Sintering temperatures must be reduced to allow use of less expensive electrode materials (Cu, etc.). This work studies the reduced temperature sintering behavior and dielectric properties of BZT-BT sintered with 30 Bi₂O₃-30ZnO-40B₂O₃ and 50Bi₂O₃-25B₂O₃-25SiO₂ (mol%) liquid …

Thermal Properties Of Spinel Based Solid Solutions, Kelley R. Wilkerson

Doctoral Dissertations

"Solid solution formation in spinel based systems proved to be a viable approach to decreasing thermal conductivity. Samples with systematically varied additions of MgGa₂O₄ to MgAl₂O₄ were prepared and thermal diffusivity was measured using the laser ash technique. Additionally, heat capacity was measured using differential scanning calorimetry and modeled for the MgAl₂O₄-MgGa₂O₄ system. At 200⁰C thermal conductivity decreased 24% with a 5 mol% addition of MgGa₂O₄ to the system. The solid solution continued to decrease the thermal conductivity by 13% up to 1000⁰C with 5 mol% addition. The decrease in thermal conductivity ultimately resulted in a decrease in heat flux …

Microstructure And Mechanical Properties Of Silicon Carbide-Titanium Diboride Ceramic Composites, Derek Scott King

Masters Theses

"The microstructure, hardness, fracture toughness, Young's modulus, strength and Weibull modulus of silicon carbide-titanium diboride (SiC-TiB₂) ceramics were studied. First, SiC-TiB₂ ceramics with 15 vol.% TiB₂ particles were processed using two green processing methods, spray drying (ST) and ball milling (SiC-15TiB₂). In addition, SiC-TiB2 ceramics with TiB₂ contents ranging from 0 to 100 vol.% were produced to determine a TiB₂ content that produced the best combination of mechanical properties. From spray drying, segregation of the TiB₂ particles in ST led to a granule-like microstructure and spontaneous microcracking in the final ceramic. In ceramics containing 20 and 40 vol.% TiB₂, the …

Sputter Deposition Of Thin-Film Capacitors Onto Low Temperature Co-Fired Ceramic Substrates, Jack Murray

Masters Theses

"Single layer thin film capacitor structures were fabricated on the surface of low temperature co-fired ceramic (LTCC) substrates using sputter deposition. The capacitor structures had areas between ~10⁴ µm² to ~10⁶ µm² and featured ~200 nm Al or Pt electrodes with 150-1500 nm Al₂O₃ dielectric layers. Impedance analysis and current-voltage testing were carried out to determine the effect of electrode material and dielectric thickness upon capacitor performance. Capacitance values for devices with Al electrodes ranged from ~10 to ~700 pF, depending on capacitor area and dielectric thickness, and the fit to expected values was better than devices with Pt or …

Processing And Characterization Of Boron Carbide-Hafnium Diboride Ceramics, Harlan J. Brown-Shaklee

Doctoral Dissertations

"Hafnium diboride based ceramics are promising candidate materials for advanced aerospace and nuclear reactor components. The effectiveness of boron carbide and carbon as HfB₂ sintering additives was systematically evaluated. In the first stage of the research, boron carbide and carbon additives were found to improve the densification behavior of milled HfB₂ powder in part by removing oxides at the HfB₂ surface during processing. Boron carbide additives reduced the hot pressing temperature of HfB₂ by 150⁰C compared to carbon, which reduced the hot pressing temperature by ~50⁰C. Reduction of oxide impurities alone could not explain the difference in sintering enhancement, however, …

The Properties And Structure Of Tin Phosphate Glasses Modified With Other Oxides, Jong Wook (Austin) Lim

Masters Theses

"The main objective of this research was to evaluate the properties and structures of glasses from the stannous (SnO) phosphate system for possible use in low temperature optical applications. Glasses in the binary xSnO-(100-x)P₂O₅ system (50 ≤ x ≤ 70) were prepared and their thermal and optical properties were evaluated. Differences in glass transition temperatures (T_g) measured here and reported in the literature were explained by differences in the residual water content in the glasses, as determined by infrared spectroscopy. In general, increasing the SnO content of the binary glasses increased the refractive index and the glass transition …

Ceramic Dielectrics For High Energy Density Capacity Application, Sheng Chao

Doctoral Dissertations

"The objective of this dissertation is to investigate the relationship between the processing parameters, microstructural development, defect chemistry and electrical properties of titanium oxide (TiO₂) dielectrics for high energy density capacitor applications. The effects of aliovalent dopants on the dielectric properties of TiO₂ ceramics were investigated, aiming to further improve the desired dielectric properties especially at elevated temperatures (up to 200°C). Due to the segregation of acceptor type impurities in the starting powders, space charge polarization took place in TiO₂ ceramics with relative large grain size (>̲500nm), leading to high dielectric loss and low energy storage efficiency. Increased ratio …

Sintering And Microstructural Effects On Mechanical Properties Of Zirconium Carbide-Tungsten Cermets, Melissa M. Giles Craft

Masters Theses

"Zirconium carbide-tungsten cermets were prepared by in situ reaction sintering a mixture of zirconia and tungsten carbide. Tungsten is a refractory metal with a melting point of 3422⁰C; however, its mechanical strength decreases significantly with increasing temperature. One approach to improve the elevated temperature strength is to add ceramic particulates to the matrix. Zirconia and tungsten carbide were compacted and then reacted and sintered in a single step by heating to 2050⁰C. The resulting stoichiometric zirconium carbide-tungsten cermet had a relative density greater than 96%. Cermets with two different grain sizes were produced by altering the final sintering temperature. Sintering …

Phase Equilibria In Iron Phosphate System, Liying Zhang

Doctoral Dissertations

"The main objectives of this research were to synthesize iron phosphate compounds, study phase equilibria in the iron phosphate system, focusing on the glass forming area, and investigate glass formability and properties based on the liquidus regions. Twelve iron phosphate compounds were successfully prepared and studied in terms of liquidus temperature or decomposition behavior. The liquidus surface of the Fe₃PO₇- Fe(PO₃)₃ system was re-determined and found to be significantly different from that originally presented by Wentrup in 1935. Eutectic points exist at 58.0 mole% Fe₂O₃ (1070⁰C), 42.7 mole% Fe₂O₃ (925°C), and 37.0 mole% Fe₂O₃ (907°C). The latter two eutectic points …

Anti Wetting Additives For Aluminosilicate Refractories In Molten Aluminum Contact Applications, Devdutt Pramod Shukla

Doctoral Dissertations

"Aluminosilicate based refractories are widely used in furnace installations for melting aluminum because they are inexpensive, readily available and generally exhibit the properties desired from a refractory material. However, they face severe corrosion and degradation issues due to the extremely reducing nature of molten aluminum alloys. Isothermal static cup testing is widely used as a tool to evaluate the performance of refractories against penetration by molten aluminum alloys. Various testing methods were reviewed and an upgraded static cup test was recommended. Commercially available aluminosilicate refractories were tested using this method and their results were studied in order to understand the …

Freeze Casting Of Bioactive Glass And Ceramic Scaffolds For Bone Tissue Engineering, Qiang Fu

Doctoral Dissertations

"The main objectives of this dissertation were to explore the production of bioactive ceramic and glass scaffolds with oriented pore architectures by unidirectional freezing of suspensions, and to characterize the mechanical and biological performance of the scaffolds. Freezing of aqueous suspensions of hydroxyapatite (HA) or bioactive 13-93 glass particles resulted in the formation of scaffolds with a lamellar-type microstructure (pore width = 5-30 µm). The addition of polar organic solvents (such as 60 wt% dioxane) to the aqueous suspensions markedly changed the morphology and size of the oriented pores, giving scaffolds with a columnar-type microstructure and larger pre width (90-110 …

Densification, Microstructure, And Mechanical Properties Of Zirconium Diboride Based Ultra-High Temperature Ceramics, Sumin Zhu

Doctoral Dissertations

"The first part of this dissertation was aimed at studying the densification of ZrB₂ ceramics by pressureless sintering techniques. Various processes have been applied to coat ZrB₂ powders with polymer precursors, which were used to produce C after charring. After sintering at 1900⁰C, relative density increased of ~70% for uncoated ZrB₂ to >99% for ZrB₂ coated with at least 1.0 wt% C. Thermodynamic analysis suggested that C reacted with and removed oxide impurities (ZrO₂ and B₂O₃) that were present on the ZrB₂ particle surfaces, which promoted densification by minimizing grain coarsening"--Abstract, page iv.

Properties Of Alkaline-Resistant Calcium-Iron-Phosphate Glasses, Jiawanjun Shi

Masters Theses

"The physical properties and alkaline corrosion resistant properties of calcium-iron-phosphate glasses were studied. The addition of Fe₂O₃ decreases the thermal expansion coefficient and increases the glass fiber's Young's modulus in comparison with the addition of CaO. Bulk glass dissolution behavior in alkaline Lawrence solution was studied by weight loss experiments, Raman spectroscopy, and analytical scanning electron microscopy. The corrosion of glass powder in NaOH solution was studied by X-ray diffraction, infrared spectroscopy and high pressure liquid chromatography. The results show that the dissolution rate decreases when the phosphate chain length decreases. Calcium oxide can improve the glass durability in high …

Iron Phosphate Glass For The Vitrification Of Ineel Sodium Bearing Waste And Hanford Low Activity Waste, Robert Douglas Leerssen

Masters Theses

"The current study investigates the feasibility of using iron phosphate glass for the immobilization of nuclear wastes that have limited solubility in the current borosilicate glass used for vitrification. These wastes contain elements such as sulfur and phosphorus, which have proven problematic (causing phase separation at >1 wt%) for vitrification in borosilicate glasses, as well as having a high soda content (50-75 wt%) which also limits wasteloading"--Abstract, page iv.

Immobilization Of Radioactive Materials In Iron Phosphate Glass, Melissa G. Mesko

Doctoral Dissertations

"The current study investigates the feasibility of using iron phosphate glass for the immobilization of radioactive wastes considered incompatible with borosilicate glass. Wastes may be incompatible with borosilicate glass due to high phosphorus content (Hanford tank sludges), the presence of halides (CsCl and SrF₂ capsules), or large amounts of uranium (SNF).

Between 20 and 60 wt% Hanford tank sludge was vitrified with an iron phosphate glass at ≤ 1200°C in 1 hour. The chemical durability of iron phosphate glassy wasteforms was as good as, or better than a reference borosilicate glass (10^-9 g/cm²/min) in distilled water …

Iron Phosphate Glasses As A Potential Nuclear Waste Disposal Medium, Melissa G. Mesko

Masters Theses

”Phosphate glasses containing large amounts of Fe₂O₃ (up to 43wt%) were studied to determine their potential suitability as nuclear waste disposal glasses for high level radioactive wastes containing large concentrations of iron oxide, phosphate, and fluoride compounds. Glass formation and chemical durability was the prime focus of this study, while properties such as density, thermal expansion, dilatometric softening temperature, and devitrification temperature were examined as well. It was found that large amounts of simulated nuclear waste material (up to 44.4wt% Cs₂O or 37.5wt% SrO) could be incorporated into the base iron phosphate glass while retaining …

Toughening Mechanisms In Weak Matrix Ceramic Composites, Jowoong Ha

Doctoral Dissertations

"Tough behavior of ceramic matrix composites is closely related to the weak interface between fiber and matrix. The weak interfaces in composites have been provided by reducing the chemical bonding and the residual thermal stresses between constituents. A new way to introduce the weak interface by induced porosity or damaged zones, so called porosity toughening or damage toughening respectively, was proposed in this study to provide the rationale for guiding development of oxide/oxide composites. The existence of porosity toughening was examined by qualitative theoretical consideration and experiments. Porous oxide/oxide composites were fabricated using alumino-silicate fibers (Nextel 440) and magnesium alumino-silicate …