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 …

Dissolution Behavior Of Phosphate Glasses, Li Na Ma

Doctoral Dissertations

"The solubility of phosphate glasses in aqueous solutions can be tailored through compositional control to obtain a wide range of ion release rates required for a variety of applications. The principal objective of this dissertation is to advance the understanding of the dissolution behavior of phosphate glasses in aqueous environments.

Two families of glasses, sodium-iron phosphate (NFP) glasses and sodium-calcium phosphate (NCP) glasses, were evaluated. The dissolution behavior depends on the phosphate anions that constitute the glass structure and the associated metal (Me) cations. The phosphate glass structure, defined by the distribution and average size of phosphate anions and depending …

First-Principles Investigations Of Iron-Based Alloys And Their Properties, Krista Renee Limmer

Doctoral Dissertations

"Fundamental understanding of the complex interactions governing structure-property relationships in iron-based alloys is necessary to advance ferrous metallurgy. Two key components of alloy design are carbide formation and stabilization and controlling the active deformation mechanism. Following a first-principles methodology, understanding on the electronic level of these components has been gained for predictive modeling of alloys.

Transition metal carbides have long played an important role in alloy design, though the complexity of their interactions with the ferrous matrix is not well understood. Bulk, surface, and interface properties of vanadium carbide, VC_x, were calculated to provide insight for the carbide …

The Structure And Properties Of Ternary Zinc Phosphate Glasses For Optical Applications, Charmayne Elizabeth Smith

Doctoral Dissertations

"This dissertation focuses on the properties and structures of ternary zinc phosphate glasses that have recently been investigated as substrates for femto-second (fs) laser writing. Although these glasses have potential for use as optical substrates, their poor chemical properties limit their applications. In this work, ternary zinc phosphate glasses were studied to find compositions with enhanced chemical durability and the properties and structures of the investigated glasses are reported.

Paper 1 and Paper 2 include the first systematic studies of the properties, structures of zinc aluminophosphate (ZAP) glasses and phase relationships in the ZnO-Al₂O₃-P₂O₅ system, respectively. Adding alumina to a …

Bioactive Glass/Polymer Composites For Bone And Nerve Repair And Regeneration, Ali Mohammadkhah

Doctoral Dissertations

"Bioactive glasses have several attractive properties in hard and soft tissue repair but their brittleness limited their use, as scaffolding materials, for applications in load- bearing hard tissue repair. At the same time, because of their bioactive properties, they are being studied more often for soft tissue repair. In the present work, a new glass/polymer composite scaffold was developed for the repair of load-bearing bones with high flexural strength and without brittle behavior. The new composites have 2.5 times higher flexural strength and ~100 times higher work of fracture (without catastrophic failure) compared to a similar bare glass scaffold.

Also …

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 …

Cerium-Based Conversion Coatings On Magnesium Alloys, Carlos E. Castaño

Doctoral Dissertations

"This research is primarily focused on gaining a better understanding of the deposition and corrosion behavior of cerium-based conversion coatings (CeCCs) on AZ31B and AZ91D Mg alloys. Deposition of homogenous and protective CeCCs was highly dependent on the surface preparation steps. The best results were obtained when Mg samples underwent grinding, acid cleaning, and alkaline cleaning processes. This reduced the number of active cathodic sites and promoted the formation of a protective Al-rich Mg oxide/hydroxide layer. Electrochemical properties of the CeCCs were also strongly correlated with morphological, microstructural, and chemical characteristics. Protective CeCCs were deposited on both AZ31 and AZ91 …

Functionally Graded Ti6sl4v And Inconel 625 By Laser Metal Deposition, Syamala Rani Pulugurtha

Doctoral Dissertations

"The objective of the current work was to fabricate a crack-free functionally graded Ti6Al4V and Inconel 625 thin wall structure by Laser Metal Deposition (LMD). One potential application for the current material system is the ability to fabricate a functionally graded alloy that can be used in a space heat exchanger. The two alloys, Inconel 625 and Ti6Al4V are currently used for aerospace applications. They were chosen as candidates for grading because functionally grading those combines the properties of high strength/weight ratio of Ti6Al4V and high temperature oxidation resistance of Inconel 625 into one multifunctional material for the end application. …

Improving The Oxidation Resistance Of Diboride-Based Ceramics, Maryam Kazemzadeh Dehdashti

Doctoral Dissertations

"Oxidation behavior has restricted the development of ZrB₂-based ceramics for aerospace and hypersonic flight vehicles applications. The research presented in this dissertation focuses on the effect of transition metal (TM) additives on oxidation behavior of ZrB₂ ceramics. In the first stage of the research, the effect of Nb additions on the morphology of the oxide particles and stability of the protective B₂O₃ glassy layer, which formed on the top surface during oxidation, was investigated. Addition of Nb increased the thickness of the glassy layer and, as a result, improved the oxidation resistance of ZrB₂ after oxidation at 1500⁰C. Next, the …

Phosphate-Doped Borosilicate Enamel Coating Used To Protect Reinforcing Steel From Corrosion, Xiaoming Cheng

Doctoral Dissertations

"Phosphate-doped sodium borosilicate glasses were developed for enamel coatings to provide corrosion protection for reinforcing steel in concrete. Phosphates have low solubilities in borosilicate glasses, generally less than about 4 mol%, and are incorporated mainly as isolated PO₄^3- and P₂O₇^4- anions, along with some borophosphate bonds. With the addition of P₂O₅, the silicate network is repolymerized and tetrahedral borates are converted to trigonal borates, because the phosphate anions scavenge sodium ions from the borosilicate network. The addition of Al₂O₃ significantly increases the solubility of P₂O …

The Effect Of Composition On The Viscosity, Crystallization And Dissolution Of Simple Borate Glasses And Compositional Design Of Borate Based Bioactive Glasses, Kathryn Lynn Goetschius

Doctoral Dissertations

"Borate glasses have recently been developed for a variety of medical applications, but much less is known about their structures and properties than more common silicate glasses. Melt properties and crystallization tendency for compositions in the Na₂O-CaO-B₂O₃ system were characterized using differential thermal analysis and viscosity measurements. Characteristic viscosity (isokom) temperatures varied with the ratio between the modifier content (Na₂O+CaO) and B₂O₃, particularly at lower temperatures, consistent with the changes in the relative concentrations of tetrahedral borons in the glass structure.

Similar glasses were used to study dissolution processes …

Thermophysical Properties Of Reaction Processed Zirconium Diboride, Jason Michael Lonergan

Doctoral Dissertations

"This dissertation focuses on the thermophysical properties of high purity zirconium diboride ceramics. These ceramics have shown promise for potential applications such as leading edge materials for next generation hypersonic vehicles. The overall goal of this work was to improve the understanding of the thermal properties and maximize the thermal conductivity of ZrB₂. Four main areas were investigated in this work. First, the sintering kinetics and the intrinsic thermal properties of reaction processed ZrB₂ were studied and compared to ZrB₂ produced by hot pressing commercial powders. The reaction process produced ceramics with higher thermal conductivity and …