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School of Materials Science and Engineering

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College of Engineering

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Now showing 1 - 10 of 24

Tensile testing and stabilization/carbonization studies of polyacrylonitrile/carbon nanotube composite fibers

(Georgia Institute of Technology, 2012-11-14) Lyons, Kevin Mark

This study focuses on the processing, structure and properties of polyacrylonitrile (PAN)/ carbon nanotube (CNT) composite carbon fibers. Small diameter PAN/CNT based carbon fibers have been processed using sheath-core and islands-in-a-sea (INS) fiber spinning technology. These methods resulted in carbon fibers with diameters of ~3.5 μm and ~1 μm (for sheath-core and INS respectively). Poly (methyl methacrylate) has been used as the sheath or the sea component, which has been removed prior to carbonization. These fibers have been stabilized and carbonized using a batch process. The effect of stabilization has been characterized by Fourier Transform Infrared Spectroscopy (FTIR), wide-angle X-ray diffraction (WAXD), and differential scanning calorimetry (DSC). A non-isothermal extent of cyclization (Mcyc) from the DSC kinetics study was developed in order to obtain an unbiased method for determining the optimal stabilization condition. The results of Mcyc were found to be in good agreement with the experimental FTIR and WAXD observations. The carbon fiber fracture surfaces have been examined using SEM. Various test parameters that affect the tensile properties of the precursor fiber (both PAN and PAN/CNT), as well as carbon fiber have been studied. In an attempt to validate single filament tests, fiber tow testing has also been done using standard test methods. Batch processed carbon fibers obtained via sheath-core geometry exhibited tensile strengths as high as 6.5 GPa, while fibers processed by islands-in-a-sea geometry exhibited strength values as high as 7.7 GPa.
Nanocomposite glass-ceramic scintillators for radiation spectroscopy

(Georgia Institute of Technology, 2012-10-24) Barta, Meredith Brooke

In recent years, the United States Departments of Homeland Security (DHS) and Customs and Border Protection (CBP) have been charged with the task of scanning every cargo container crossing domestic borders for illicit radioactive material. This is accomplished by using gamma-ray detection systems capable of discriminating between non-threatening radioisotopes, such as Cs-137, which is often used in nuclear medicine, and fissile material, such as U-238, that can be used to make nuclear weapons or "dirty" bombs. Scintillation detector systems, specifically thallium-doped sodium iodide (NaI(Tl)) single crystals, are by far the most popular choice for this purpose because they are inexpensive relative to other types of detectors, but are still able to identify isotopes with reasonable accuracy. However, increased demand for these systems has served as a catalyst for the research and development of new scintillator materials with potential to surpass NaI(Tl). The focus of a majority of recent scintillator materials research has centered on sintered transparent ceramics, phosphor-doped organic matrices, and the development of novel single crystal compositions. Some of the most promising new materials are glass-ceramic nanocomposites. By precipitating a dense array of nano-scale scintillating crystals rather than growing a single monolith, novel compositions such as LaBr₃(Ce) may be fabricated to useful sizes, and their potential to supersede the energy resolution of NaI(Tl) can be fully explored. Also, because glass-ceramic synthesis begins by casting a homogeneous glass melt, a broad range of geometries beyond the ubiquitous cylinder can be fabricated and characterized. Finally, the glass matrix ensures environmental isolation of the hygroscopic scintillating crystals, and so glass-ceramic scintillators show potential to serve as viable detectors in alpha- and neutron-spectroscopy in addition to gamma-rays. However, for the improvements promised by glass-ceramics to become reality, several material properties must be considered. These include the degree of control over precipitated crystallite size, the solubility limit of the glass matrix with respect to the scintillating compounds, the variation in maximum achievable light yield with composition, and the peak wavelength of emitted photons. Studies will focus on three base glass systems, sodium-aluminosilicate (NAS), sodium-borosilicate (NBS), and alumino-borosilicate (ABS), into which a cerium-doped gadolinium bromide (GdBr₃(Ce)) scintillating phase will be incorporated. Scintillator volumes of 50 cubic centimeters or greater will be fabricated to facilitate comparison with NaI(Tl) crystals currently available.
Linking phase field and finite element modeling for process-structure-property relations of a Ni-base superalloy

(Georgia Institute of Technology, 2012-08-28) Fromm, Bradley S.

Establishing process-structure-property relationships is an important objective in the paradigm of materials design in order to reduce the time and cost needed to develop new materials. A method to link phase field (process-structure relations) and microstructure-sensitive finite element (structure-property relations) modeling is demonstrated for subsolvus polycrystalline IN100. A three-dimensional (3D) experimental dataset obtained by orientation imaging microscopy performed on serial sections is utilized to calibrate a phase field model and to calculate inputs for a finite element analysis. Simulated annealing of the dataset realized through phase field modeling results in a range of coarsened microstructures with varying grain size distributions that are each input into the finite element model. A rate dependent crystal plasticity constitutive model that captures the first order effects of grain size, precipitate size, and precipitate volume fraction on the mechanical response of IN100 at 650°C is used to simulate stress-strain behavior of the coarsened polycrystals. Model limitations and ideas for future work are discussed.
Effect of twinning on texture evolution of depleted uranium using a viscoplastic self-consistent model

(Georgia Institute of Technology, 2012-08-20) Ho, John

Texture evolution of depleted uranium is investigated using a viscoplastic self-consistent model. Depleted uranium, which has the same structure as alpha-uranium, is difficult to model as it has an orthorhombic symmetry structure, as well as many twin systems which must be addressed in order to properly simulate the textural evolution. The VPSC method allowed for a flexible model which could not only encompass the low symmetry component but also the twinning components of depleted uranium. The model focuses on the viscoplastic regime only, neglecting the elastic regime of deformation, and uses a self-consistent method to solve the model. Different deformation processes, such as torsion, rolling, and swaging, are simulated and the theoretical textures, plotted as pole figures or inverse pole figures, are compared with previous experimental textures found for alpha uranium from previous experimental sources. A specific twin system, the (176)[512] system, is also given special consideration. This twin system is a dominant deformation mode for alpha uranium at high strain rates, but is quite elusive in general. Different deformation processes are compared where this twin system is active and not active. This allows comparison on the effect of this twin on the overall texture of depleted uranium. In addition, a sample of depleted uranium from Y12 which was analyzed for (176)[512] twins is compared to theoretical results from a VPSC simulation where the (176)[512] twin is active.
Experimental and numerical analyses of dynamic deformation and failure in marine structures subjected to underwater impulsive loads

(Georgia Institute of Technology, 2012-07-16) Avachat, Siddharth

The need to protect marine structures from the high-intensity impulsive loads created by underwater explosions has stimulated renewed interest in the mechanical response of sandwich structures. The objective of this combined numerical and experimental study is to analyze the dynamic response of composite sandwich structures and develop material-structure-property relations and design criteria for improving the blast-resistance of marine structures. Configurations analyzed include polymer foam core structures with planar geometries. A novel experimental facility to generate high-intensity underwater impulsive loads and carry out in-situ measurements of dynamic deformations in marine structures is developed. Experiments are supported by fully dynamic finite-element simulations which account for the effects of fluid-structure interaction, and the constitutive and damage response of E-glass/polyester composites and PVC foams. Results indicate that the core-density has a significant influence on dynamic deformations and failure modes. Polymeric foams experience considerable rate-effects and exhibit extensive shear cracking and collapse under high-magnitude multi-axial underwater impulsive loads. In structures with identical masses, low-density foam cores consistently outperform high-density foam cores, undergoing lesser deflections and transmitting smaller impulses. Calculations reveal a significant difference between the response of air-backed and water-backed structures. Water-backed structures undergo much greater damage and consequently need to absorb a much larger amount of energy than air-backed structures. The impulses transmitted through water-backed structures have significant implications for structural design. The thickness of the facesheets is varied under the conditions of constant material properties and core dimensions. The results reveal an optimal thickness of the facesheets which maximizes energy absorption in the core and minimizes the overall deflection of the structure.
Modulating fibrin matrix properties via fibrin knob peptide functionalized microgels

(Georgia Institute of Technology, 2012-07-10) Sathananthan, Saranya

Fibrin is the body's natural provisional matrix activated in response to vascular injury in which noncovalent knob:hole interactions between fibrin monomers lead to the assembly of fibrin for clot formation. In this study we aimed to exploit fibrin knob:hole affinity interactions with swelling, space filling microgels for the development of a potential bio-synthetic hybrid polymer system with hemostatic properties. Previous work has explored the inherent binding interactions of various fibrin knobs and their complementary polymerization holes, which have led to the development of fibrin knob peptide mimic (GPRPFPAC) with enhanced binding affinity for fibrin(ogen) holes. By coupling this enhanced fibrinogen binding peptide with a pNIPAm microgel system capable of being dynamically tuned and self-assembled, we hypothesized the specific and rapidly triggered formation of a bulk hydrogel in a wound environment (i.e. in the presence of fibrinogen). We found that at the peptide ligand density and concentrations of microgels used, that a rapid formation of a gel did not occur in the presence of fibrinogen alone. However with fibrinogen and thrombin, we found that fibrin network polymerization, structure, and viscoelastic properties were greatly altered in the presence of knob peptide-conjugated microgels.
Electrical properties of BaZr0.1Ce0.7Y0.1Yb0.1O3-δ and its application in intermediate temperature solid oxide fuel cells

(Georgia Institute of Technology, 2012-07-06) Rainwater, Benjamin H.

Conventional oxygen anion conducting yttria-stabilized zirconia (YSZ) based solid oxide fuel cells (SOFCs) operate at high temperatures (800oC-1000oC). SOFCs based on proton conducting ceramics, however, can operate at intermediate temperatures (450oC-750oC) due to low activation energy for protonic defect transport when compared to oxygen vacancy transport. Fuel cells that operate at intermediate temperatures ease the critical materials requirements of cell components and reduce system costs, which is necessary for large scale commercialization. BaCeO3-based perovskite materials are candidates for use as ion conductors in intermediate temperature SOFCs (IT-SOFCs) when doped with trivalent cations in the B-site. B-site doping forms oxygen vacancies which greatly increases the electrical conductivity of the material. The oxygen vacancies are consumed during the creation of protonic defects or electronic defects, depending on the atmosphere and temperature range. High performance IT-SOFCs based on the Y3+ and Yb3+ doped BaCeO3-based system, BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) have been recently reported. High conductivity in O2/H2O atmosphere was reported, however, a more basic understanding of the BZCYYb structure, electrical conductivity, and the portion of the charge carried by each charge carrier under fuel cell conditions is lacking. In this work, the BZCYYb material is fabricated by the solid state reaction method and the crystal structure at intermediate temperatures is studied using HT-XRD. The total conductivity of BZCYYb in H2/H2O, O2/H2O, and air atmospheres in the IT-SOFC temperature range is reported. The activation energy for transport at these conditions is determined from the conductivity data and the transference numbers of protonic defects, oxygen anion defects and electronic defects in the BZCYYb material are determined by the concentration cell - OCV method. BZCYYb is a mixed proton, oxygen anion, and electronic conductor at IT-SOFC temperature ranges (450oC - 750oC), in H2, O2, and H2O containing atmospheres. Ni-BZCYYb/BZCYYb/BZCYYb-LSCF fuel cells were constructed and peak power densities of ~1.2 W/cm2 were reported at 750oC after optimization of the Ni-BZCYYb anode porosity. Decreasing the Ni-BZCYYb anode porosity did not significantly affect the electrical conductivity of the anode, however the peak power densities of the IT-SOFCs based on the anode with less porosity, calculated from I-V curve data, showed dramatic improvement. The fuel cell with the lowest anode porosity demonstrated the highest performance. This finding is in stark contrast to the optimal anode porosity needed for high performance in YSZ-based, oxygen anion conducting SOFCs. Because of significant proton conduction in the BZCYYb material, fuel cell reaction products (water) form at the cathode side and less porosity is required on the anode side. The improvement in performance in the BZCYYb based IT-SOFC is attributed to the unique microstructure formed in the Ni-BZCYYb anode when no pore forming additives are used which may contribute to high electrocatalytic behavior for anode reactions. This work provides a basic understanding of the electrical properties of BZCYYb and clarifies the feasibility of using BZCYYb in each component of the IT-SOFC system as well as in other electrochemical devices. The high performance of the Ni-BZCYYb/BZCYYb/BZCYYb-LSCF IT-SOFC, due to low anode porosity, provides a new understanding for the rational development of high performance IT-SOFCs based on electrolytes with significant protonic conduction.
Stress corrosion cracking of 316L austenitic stainless steel in high temperature ethanol/water environments

(Georgia Institute of Technology, 2012-06) Gulbrandsen, Stephani

There has been an increase in the production of bio-fuels. Organosolv delignification, high temperature ethanol/water environments, can be used to separate lignin, cellulose, and hemicelluloses in the bio-mass for bio-fuel production. These environments have been shown to induce stress corrosion cracking (SCC) in 316L stainless steel. Previous research has been done in mixed solvent environments at room temperature to understand SCC for stainless steels, but little is known about the behavior in high temperature environments. Simulated organosolv delignification environments were studied, varying water content, temperature, pHe, and Cl- content to understand how these constituents impact SCC. In order for SCC to occur in 316L, there needs to be between 10 and 90 volume % water and the environment needs to be at a temperature around 200°C. Once these two conditions are met, the environment needs to either have pHe < 4 or have more than 10 ppm Cl-. These threshold conditions are based on the organosolv delignification simulated environments tested. SCC severity was seen to increase as water content, temperature, and Cl- content increased and as pHe decreased. To prevent failure of industrial vessels encountering organosolv delignification environments, care needs to be taken to monitor and adjust the constituents to prevent SCC.
Analysis of fatigue behavior, fatigue damage and fatigue fracture surfaces of two high strength steels

(Georgia Institute of Technology, 2011-11-18) Lester, Charles Gilbert, IV

Building fuel efficient automobiles is increasingly important due to the rising cost of energy. One way to improve fuel efficiency is to reduce the overall automobile weight. Weight reductions using steel components are desirable because of easy integration into existing manufacturing systems. Designing components with Advanced High Strength Steels (AHSS) has allowed for material reductions, while maintaining strength requirements. Two Advanced High Strength steel microstructures investigated in this research utilize different strengthening mechanisms to obtain a desired tensile strength grade of 590MPa. One steel, HR590, utilizes precipitation strengthening to refine the grain size and harden the steel. The other steel, HR590DP, utilizes a dual phase microstructure consisting of hardened martensite constituents in a ferrite matrix. The steels are processed to have the same tensile strength grade, but exhibit different fatigue behavior. The central objective of this research is to characterize and compare the fatigue behavior of these two steels. The results show the dual phase steel work hardens at a low fatigue life. The precipitation strengthened microstructure shows hardening at low strain amplitudes, softening at intermediate strain amplitudes and little to no effect at high strain amplitudes. These different fatigue responses are characterized and quantified in this research. Additionally, observations showing the fracture surfaces and the bulk microstructure are analyzed.
Thin-film trench capacitors for silicon and organic packages

(Georgia Institute of Technology, 2011-08-29) Wang, Yushu

The continuous trend towards mega-functional, high-performance and ultra-miniaturized system has been driving the need for advances in novel materials with superior properties leading to thin components, high-density interconnect substrates and interconnections. Power supply and management is becoming a critical bottleneck for the advances in such mega-functional systems because power components do not scale down with the rest of the system resulting in bulky and stand-alone power modules. Amongst the power components, thin film capacitors are considered the most challenging to integrate because of several manufacturability concerns. The challenges are related to process compatibility of high permittivity dielectrics with substrates and high surface area electrodes, yield, leakage and losses. This thesis focuses on novel thin film capacitor technologies that address some of these critical challenges.