Engineering Commons^™

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 …

Synthesis And Characterization Of Doped Mayenite As A Transparent Conducting Oxide, Sabina Nwamaka Ude

Doctoral Dissertations

Base and doped mayenite (Ca₁₂Al₁₄O₃₃) were synthesized using both solid-state and citrate gel techniques; and fired in air and reducing atmospheres. The prepared compositions were studied using neutron and x-ray diffraction at room temperature, high temperature x-ray diffraction (HTXRD), differential thermal analysis (DTA), scanning electron microscopy (SEM), digital optical microscopy, and Brunauer, Emmett and Teller (BET) surface area analysis. Electrically conductivity was measured on the Fe (iron) doped samples using a 2-point, after firing in a reducing atmosphere.

The HTXRD data on pure mayenite synthesized using citrate gel method and fired in air showed …

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 = …

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 …

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, …

Laser Textured Calcium Phosphate Bio-Ceramic Coatings On Ti-6al-4v For Improved Wettability And Bone Cell Compatibility, Sameer R. Paital

Doctoral Dissertations

The interaction at the surfaces of load bearing implant biomaterials with tissues and physiological fluids is an area of crucial importance to all kinds of medical technologies. To achieve the best clinical outcome and restore the function of the diseased tissue, several surface engineering strategies have been discussed by scientific community throughout the world. In the current work, we are focusing on one such technique based on laser surface engineering to achieve the appropriate surface morphology and surface chemistry. Here by using a pulsed and continuous wave laser direct melting techniques we synthesize three dimensional textured surfaces of calcium phosphate …

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 …

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 …

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 …

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 …

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.

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 …

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 …

Studies Of Defect Structure And Oxidation-Reduction Behavior Of Undoped Lamno₃, Sr-Doped Lamno₃ And Mg-Doped Lamno₃, Jen-Ho Kuo

Doctoral Dissertations

"Undoped LaMnO₃, Sr-doped LaMnO₃ and Mg-doped LaMnO₃ showed reversible oxidation-reduction behavior. These perovskites can be excess, stoichiometric or deficient in oxygen content depending on the specific conditions. Under very reducing conditions these oxides will decompose into new phases. Phase stabilities for these oxides were determined. The results showed that both Sr and Mg doping caused the LaMnO₃ to dissociate at higher oxygen activities than for undoped LaMnO₃.

Defect models were proposed to interpret the TG results, in which metal vacancies are assumed for the oxygen excess condition and oxygen vacancies are assumed for the oxygen deficient condition. Thermodynamic properties were …

Bubble Formation, Movement And Distortion In Viscous Glass, Simon Cheng-Ping Wang

Doctoral Dissertations

"A study of bubble formation, movement and distortion in viscous glass is described. A glass rod containing an irregularly shaped hole is heated to a temperature at where the glass viscosity is low enough to let the hole form a spherical bubble. Spheration occurs as the bubble moves upward in the glass rod. At the proper time, the rising bubble is decelerated and brought to a stop by increasing the glass viscosity by slowly reducing the temperature. The entrapped bubble is then cut from the glass rod and heated again, if necessary, to a lower temperature, to reduce distortion of …

Oxidation Kinetics Of Polycrystalline Srtio₃ And Lacro₃, Chi-Kung Jerry Yu

Doctoral Dissertations

"The oxidation kinetics of polycrystalline SrTiO₃ and LaCrO₃ were determined by measuring the time and temperature dependence of the weight and conductivity change of reduced samples. A region of fast diffusion followed by a reduced diffusion tail was observed in the TGA measurements. This observation can be interpreted as the initial diffusion along the boundaries and then spreading out into the lattice. The absence of the tail in the conductivity measurements is due to the high electron and hole mobility at the boundaries. The electrical conductivity and TGA results on Nb-doped LaCrO₃ show that at low oxygen …

Dissolution Of Multi-Component Systems, Robin Ashley Murphy

Doctoral Dissertations

"The validity of ideal barrier layer diffusion kinetics was examined in the NaCl/KCl - glycerol and Al₂O₃-CaO-Al₂O₃-SiO₂/slag corrosion systems. Single crystals and dense polycrystalline plates of pure NaCl or KCl exhibit steady-state corrosion rates within 10%, of calculated theoretical values, while dense mixtures of the two salts corrode at rates higher than either pure material. The results are compared to the grooving phenomena of NaCl/KCl bicrystals.

Polycrystalline Al₂O₃ is shown to corrode at the same rate as single crystal sapphire at 1400°C in CaO-SiO₂-Al_{2 …}

Thermogravimetric And Electrical Conductivity Studies Of Mg-Doped Lacro₃ And La-Doped Srtio₃, Brian Keith Flandermeyer

Doctoral Dissertations

"Mg-doped LaCrO₃ and La-doped SrTiO₃ are perovskite compounds that display electrical conductivity which is a function of partial pressure of oxygen (po2). It has been proposed that acceptor-doped LaCrO₃ becomes highly resistive at low po2's due to ionic compensation of the acceptor-dopant present by oxygen vacancies. SrTiO₃ has been thought to absorb O₂ into a crystallographic shear structure and exsolve strontium vacancies which can then ionically compensate the La present and give a high resistance at high po2's.

Given these models, relationships between weight loss and conductivity vs. po₂ are developed for both these …

Chemical Corrosion Of Ag Films On Glass, Hyun Soo Park

Doctoral Dissertations

"The chemical corrosion of chemically, vacuum and sputter deposited Ag films on an aluminoborosilicate and a soda lime glass was investigated in wet HCl and saturated water vapor as a function of concentration, temperature and time. The higher corrosion resistance of chemically deposited Ag films in comparison to evaporated or sputtered Ag films is attributed to their lower microvoid density and higher adhesion strength. Ag films on the more chemically durable aluminoborosilicate glass had a higher corrosion resistance. Furthermore, films which received a moderate thermal annealing (80 to 140° C) or were deposited on a glass surface washed with HN0 …

Thermally Stimulated Polarization And Depolarization Current (Tspc/Tsdc) In Glass, Chi-Ming Hong

Doctoral Dissertations

"The feasibility of studying ionic motion in glass using thermally stimulated polarization (TSPC)/depolarization current (TSDC) techniques was investigated with 4Na₂O·96SiO₂ and 30PbO·70SiO₂ (mol%) glasses. The TSDC peaks in these glasses were dependent on glass composition and attributed to bulk polarization. The high temperature background TSPC is shown to be due to dc conductivity, whereas the TSPC/TSDC peaks in the 4Na₂O glass are attributed to shorter range Na⁺ motion.

The electrical conductivity of glasses determined by TSPC agrees well with that measured by conventional ac or dc techniques. The TSPC technique is a rapid and easy method of measuring the …

Phase Equilibria In Aqueous Magnesium Chloride, Sulfate, Chromate And Phosphate Systems, Ladawan Urwongse

Doctoral Dissertations

"In the first part of the present study, which is presented in Paper I, solubility of MgO in aqueous HCl solutions at 23° ± 3°C was measured and combined with analyses of neat magnesium oxychloride cements to construct an equilibrium phase diagram for the system MgO-MgCl₂-H₂O. Invariant liquids in equilibrium with Mg(OH)₂ and 5Mg(OH)₂·MgCl₂·8H₂O (5.1.8), with 5·1·8 and 3Mg(OH)₂·MgCl₂·8H₂O (3·1·8), and with 3·1·8 and MgCl₂·6H₂O were located at 0.8 ± 0.2 MgO, 17.0 ±0.5 MgCl₂; at 1.0 …

Electrical Degradation In High Purity Barium Titanate, Jeffrey Duncan Keck

Doctoral Dissertations

"The stability of BaTiO₃-type insulators to DC fields has long been recognized as one of the important factors in determining the quality of a capacitor which uses such a material as its dielectric. The slow increase of the electrical conductivity of an insulator which is stressed by DC fields at levels below the breakdown strength is normally termed degradation. Since degradation is a very common malady to the ceramic capacitor manufacturers, many studies have been conducted over the past two decades in efforts to learn how to remedy the problem.

The results of previous studies have shown that …

Solid Transformations And Premelting Phenomena In Fluorite Structures And The System Srcl₂Bacl₂, Damri Sukhotanang

Doctoral Dissertations

"Thermal expansion and phase transformations in CaF₂, BaF₂, SrCl₂, and BaCl₂ were studied by high temperature x-ray diffractometry, differential thermal analysis, and thermodilatometric analysis. Thermal expansion characteristics of CaF₂, BaF₂ and SrCl₂ are very similar. The structures expand normally at lower temperatures and depart substantially from linearity at higher temperatures, because of formation of defects and disordering of anions. At high temperatures CaF₂ and BaF₂ react with water vapor, even at low humidities, forming oxides and emitting HF, leading to difficulty in observation of phase transformations. SrCl …

Phase Equilibria And Transformations In Potassium Sulfate, Chromate, Molybdate, And Tungstate, Roy Richard Ramey

Doctoral Dissertations

"Thermal expansion and transformations of K₂SO₄, K₂CrO₄, K₂MoO₄, and K₂WO₄ were studied by high temperature diffractometry, differential thermal analysis, and dilatometry. K₂SO₄ and K₂CrO₄ transform from an orthorhombic to a hexagonal structure at 580°C and 670°C respectively, with initiation of the transformation probably related to increased amplitude of S-O tetrahedral vibrations. K₂MoO₄ and K₂WO₄ transform from a monoclinic to an orthorhombic structure at 300° and 370°C respectively. A minor K₂MoO_{4 …}

Initial Surface Reactions And Nucleation Kinetics Of Boron On Atomically Clean Molybdenum Substrates, Richard Eugene Moore

Doctoral Dissertations

"A study of the chemical vapor deposition (CVD) of boron from boron triiodide (BI₃) on molybdenum has been done with field emission microscopy (FEM) and low energy electron diffraction (LEED).

A correlation of the efficiency by which BI₃ is effectively dissociated to form adsorbed boron, and the temperature of the substrate has been shown through work function measurements on the boron covered molybdenum field emitter surfaces. The boron-induced work function change of molybdenum has been shown to be in agreement with a theoretical model which predicts positive, as well as negative work functions for adsorbate-substrate systems. It …

Synthesis And Properties Of Cadmium Manganese Ferrites, B. W. Jong

Doctoral Dissertations

"The formation sequence of (Cd,Mn,Fe) (Mn,Fe)₂O₄ from the individual oxides,x CdO + (1-x) MnCO₃ + α-Fe₂O₃,was found by X-Ray diffraction. Cd(Mn,Fe)₂O₄ was formed completely from CdO·Fe₂O₃ and CdO·Fe₂O₄ at 900°C and reacted with MnFe₂O₄ for 0 < x < 0.5 and with α-Fe₂O₃ for 0.5 < x < 1.0 to form a single phase of (Cd,Mn,Fe) (Mn,Fe)₂O₄ at 1150°C. This is the first time that the details involved in the formation sequence of CdMn-ferrite have been shown. A mixture of 0.5 CdO + 0.5 MnCO₃ + α-Fe₂O₃ was calcined at 900°C for 2 hrs forming Cd(Mn,Fe)₂O₄ with no Cd present in the other phases. Sintering was accomplished at 1150°C for 30 minutes forming a single phase of Cd₀.₅Mn₀.₅Fe₂O₄ with a bulk sintered density of 98-99% of the X-Ray density. The rate of changing density followed the relationship, p = K ln t + c, at temperatures between 1050°C and 1300°C. Magnetic properties of CdₓMn₁₋ₓFe₂O₄ were affected by composition x. A relationship, B = gx + h, was found between maximum magnetic flux density B and composition x at room temperature for 0 <̲ x <̲ 0.3. This was rationalized by a modification of Néel's theory. The maximum magnetic flux density decreased with increasing cadmium concentration for 0.3 < x <̲ 0.5. This decrease was attributed to the changing direction of the magnetic moment in the octahedral site sub-lattice according to the superexchange theory. No magnetic properties were observed due to the formation of a compensated antiferromagnetic phase for 0.5 < x <̲ 1.0. Maximum magnetic flux density increased with bulk density, while the coercive force decreased with the grain growth and the initial permeability increased with the bulk density and grain size. A stator core used for a-c electric motor application was formed by pre-sintering at 900⁰C for 2 hrs. Slots were then sawed with a tungsten carbide blade and final sintering was accomplished at 1150⁰C for 1 hr"--Abstract, pages iii-iv.

A Study Of The Solid Regions In The System Zr-Te, Frances Dolores Jenkins

Doctoral Dissertations

"Relative intensity measurements of X-ray powder diffraction lines were made of compositions in the region ZrTe₀.₅ to ZrTe₂.₀, fired between 600º and 817ºC in evacuated sealed tubes. The experimental relative intensities of samples between ZrTe and ZrTe₂ were compared with values calculated for several theoretical models with different Zr vacancy ordering schemes and Te vacancy concentrations. The most probable structural model for compositions in the range ZrTe to ZrTe₁.₅ is one in which the number of Zr vacancies located at z = 0 ranges from 0% to 30%, with the concentration of Te vacancies ranging from 0 to 35%. A …

Structure And Properties Of Silver Borate Glasses, Edward Nashed Boulos

Doctoral Dissertations

"Clear glasses form in the system Ag₂O-B₂O₃ up to about 35 mol.% (65 wt.%) Ag₂O. Infrared absorption, thermal expansion and density data indicated an analogy to the Na₂O-B₂O₃ system. Pentaborate-triborate group pairs appear to be formed upon the addition of Ag₂O to B₂O₃ up to 20 mol.% Ag₂O and diborate groups from 20 to 33 mol.% Ag₂O. This interpretation is supported by the comparison of the infrared absorption spectra of quenched and crystallized glasses. One crystallization product ,Ag₂O.4B₂O₃, has been identified previously. A new compound starts to appear at 28 mol.% Ag₂O. Silver is generally present as a network modifier …

Internal Friction Of Ion Exchanged Li₂O•Al₂O₃•2sio₂ Glasses, A. Ismail A. Abdel-Latif

Doctoral Dissertations

"Li₂O•Al₂O₂•2SiO₂ glass fibers were ion-exchanged from 1 to 300 minutes in a sodium nitrate bath at 366ºC. The internal friction was measured along with the lithium and sodium concentration profiles. As sodium progressively replaced lithium, the alkali internal friction peak became smaller while a new peak (mixed alkali peak) appeared and increased in magnitude. These changes in the internal friction are similar to those that occur upon the addition of a second alkali to glasses prepared by conventional melting. The magnitude of both internal friction peaks in ion exchanged glasses was dependent on the overall glass composition. The magnitude of …