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School of Materials Science and Engineering

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https://hdl.handle.net/1853/70762

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College of Engineering

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Publication Search Results

Now showing 1 - 10 of 881

Characterization and Modeling of Two Dimensional Crack Growth in Gas Turbine Materials

(Georgia Institute of Technology, 2019-11-26) Satin, Morris Mandel

Surface flaws are common in gas turbine components and undergo a complex process of fatigue crack growth under mechanical and thermal loading. This project seeks to bring greater clarity and understanding to the transition process of surface flaws to through cracks by implementing a transition method, and developing the associated codes and equations to do so based on careful consideration of boundary conditions, experimental data and finite element simulations. In addition the effect of thermomechanical fatigue in surface flaws in Inconel 718 is investigated. Inconel 718 displays a high-temperature time-dependent intergranular fracture mode that is explored experimentally, characterized, and modeled in the MPYZ-TMF fatigue crack growth code. New models using the latest findings are proposed to accurately predict the shape, location, and overall lifetime of various surface flaw geometries.
ENGINEERING STRUCTURE AND PHASE VIA TEMPERATURE MANIPULATION IN THIN FILM OXIDES GROWN BY ALD

(Georgia Institute of Technology, 2019-11-13) Piercy, Brandon Deane

Atomic Layer Deposition (ALD) is a critical thin film deposition technology with applications spanning microelectronics, solar fuels and biological preservation due to its advantages of a wide precursor library for the majority of the periodic table, exquisite control over growth rates, and the ability to deposit at extremely low temperatures. One critical parameter in ALD growth is deposition temperature, which defines the “ALD window”—a temperature range in which ALD growth kinetics hold. Most ALD processes tend to be limited to deposition temperatures of <300 ˚C, causing ALD films to be typically amorphous as-deposited. While amorphous inorganic films are often thought to be structurally equivalent, films grown by ALD can vary significantly in mechnical, optical, thermal, and electronic properties. By changing deposition temperature from room temperature to 150 ˚C, amorphous TiO2 films are shown to exhibit a change in density of 15% and a change in optical polarizability of 10%. These shifts in density correspond to a decrease in the thermal conductivity of TiO2 thin films and a decrease in Ti3+-based electron trap states. High deposition temperatures are usually required to deposit crystalline films, which are typically not compatible with ALD precursors. A method called resistive pulsed-heating ALD (PH-ALD) is introduced as a strategy to circumvent the thermal limitations of ALD precursors by applying a short heat pulse to the film after low-temperature deposition cycles in the ALD process. The PH-ALD technique is used to grow epitaxial ZnO films on c-plane sapphire using the low-temperature diethylzinc-water ALD process. Epitaxial films are grown at pulse temperatures of 500˚C and above, and at deposition cycles:heat pulse ratios of 5:1. Furthermore, the PH-ALD process is used for templating epitaxial growth in ZnO, with films as thin as 5 PH-ALD cycles capable of templating an epitaxial film up to 100 nm thick. Electrical and optical measurements are shown to be comparable to films grown by other physical and chemical vapor deposition techniques, indicating that the PH-ALD method may be a practical approach for the growth of complex crystalline oxide thin films. To support future workers in this field, detailed descriptions of control software, hardware, and reactor designs are provided to enable the development of ALD systems that can incorporate external sensors and controllers for advanced intelligent processing recipes.
Formation, structure, and reproducibility of cerato-ulmin hydrophobin-coated bubbles

(Georgia Institute of Technology, 2019-11-12) Gorman, Andrew J.

The physical characteristics of both the submicron and micron bubble films were investigated. For the submicron bubbles, small-angle x-ray scattering (SAXS) and small-angle neutron scattering (SANS) show the bubbles are cylindrical with a ~70 nm cross-sectional diameter and film thickness of 15 nm, the equivalent of 5 CU proteins. Atomic force microscopy (AFM) of collapsed microbubbles has the same film thickness suggesting that film thickness is independent of CU bubble size. To isolate singular bubble sizes during agitation, a new experimental apparatus was designed and built to agitate the CU solutions in a controlled, reproducible method. The apparatus houses a horizontal microscope for imaging CU bubbles directly after agitation and observing their motion as they rise through the solution. Results show a positive correlation with agitation frequency and bubble size, with a steady number density for all sizes up to the maximum size at that frequency value.
Rational synthesis of multimetallic nanocrystals for plasmonics and catalysis

(Georgia Institute of Technology, 2019-11-12) Ahn, Jae Wan

Multimetallic noble-metal nanocrystals have attracted considerable attention owing to their broad structure-property relationship for applications in plasmonics and catalysis. However, it remains a major challenge to rationally synthesize these nanocrystals because most of the reported protocols lack a mechanistic understanding and often involve a trial-and-error approach when optimizing the experimental parameters. This dissertation demonstrates the use of facet-selective etching and deposition as a powerful method for the transformation of colloidal silver nanocrystals into multimetallic nanostructures with intricate properties. In particular, I leverage the metal-coordination ligands to direct the etching and deposition in an orthogonal manner. In the first case study, I transform silver nanocubes into bimetallic concave nanocubes encased by silver-gold alloy frames via selective removal of the silver atoms from side faces while co-depositing silver and gold atoms as an alloy on the edges and corners. I further subject the core-frame nanocubes to galvanic replacement for the fabrication of nanoscale, multimetallic, cage cubes by confining the drilling of silver to the center of each side face. In the second case study, I investigate the roles played by poly(vinylpyrrolidone) and cetyltrimethylammonium chloride in controlling the orthogonal deposition of gold on different facets of silver cuboctahedra for the fabrication of nanoboxes with complementary surface structures. To understand the interaction between the ligands noble-metal and nanocrystals, I develop an in situ platform based on surface-enhanced Raman spectroscopy for analyzing the competitive binding of thiol and isocyanide molecules to the surface of silver nanocubes. Collectively, this work offers insights into the rational synthesis of multimetallic nanocrystals for applications in plasmonics and catalysis.
Rational Design of Architectured Amphiphilic Block Copolymers and Polymer-Ligated Nanocrystals for Energy Storage and Drug Delivery

(Georgia Institute of Technology, 2019-11-11) Wang, Zewei

Architectured amphiphilic block copolymers with complex molecular structures have garnered much attention in recent years due to their distinct chemical and physical properties compared to the linear counterparts. The amphiphilic chemical environment provides a robust platform for site selective functionalization as well as molecular interactions. Notably, architectured amphiphilic block copolymers can be employed as nanoreactors for crafting polymer-ligated nanocrystals. Specifically, the hydrophilic blocks of architectured amphiphilic block copolymers can be selectively loaded with metal precursors to achieve confined growth of inorganic nanocrystals. The size and shape of as-synthesized polymer-ligated nanocrystals can be precisely tailored via architectural design and controlling the reaction time of architectured amphiphilic block copolymers. Three architectured amphiphilic block copolymers (star-shaped, bottlebrush-shaped, and Janus star- shaped) are rationally designed and synthesized by capitalizing on multifunctional glucose-based biomolecules, either β-cyclodextrin (β-CD) as the core (for star- and Janus star-shaped) or cellulose as the backbone (for bottlebrush-shaped). These architectured amphiphilic block copolymers are synthesized through atom transfer radical polymerization and/or reversible addition-fragmentation chain-transfer polymerization, thereby possessing precisely controlled molecular weight and low polydispersity (PDI). These characteristics render them perfect polymeric nanoreactors for the synthesis of plain nanoparticles, nanorods, and spherical Janus nanoparticles. Specifically, first, poly(styrene-co-acrylonitrile)-ligated MFe2O4 nanoparticles crafted from star-shaped block copolymer nanoreactors display superior rate and cycling performance in lithium/sodium ion batteries when applied as anode. The surface poly(styrene-co-acrylonitrile) ligands can not only transport Li+ but also act as efficient physical barrier to prevent phase coarsening. Second, a robust crosslinking strategy via UV-initiated azide homocoupling is developed to readily convert azide-functionalized bottlebrush-shaped block copolymers into polymeric cocoons, exhibiting 3-fold encapsulation, 3-fold release time, and modulated release rate compared with non-crosslinked counterpart. Additionally, CsPbBr3 nanocrystals synthesized from the bottlebrush-shaped block copolymer nanoreactors possess much higher water, UV, and thermal stabilities in comparison to conventional aliphatic ligand-capped CsPbBr3 due to permanent ligation of polymer hairs as a dense protection layer. Finally, Janus star-shaped block copolymers are rationally synthesized which can readily structure-direct the growth of strictly biphasic Janus nanoparticles.
Advanced materials and processes for high-density capacitors for next-generation integrated voltage regulators

(Georgia Institute of Technology, 2019-11-11) Spurney, Robert Grant

Capacitors are key components for power conversion, delivery, and management. Along with inductors, they dictate the size and performance of voltage regulators in power distribution networks (PDNs), which convert and regulate the power that gets delivered to the increasingly power-hungry integrated circuits (ICs). When the voltage regulators are integrated into the package, referred to as integrated voltage regulators (IVRs), they provide many benefits over traditional voltage regulators, including higher power density, system miniaturization, and improved efficiency. To enable IVRs, capacitors must be integrated either on-chip or in the package, while providing high capacitance density, high frequency stability, low equivalent series resistance (ESR), and high temperature stability. Tantalum capacitors have an advantage over many other types of capacitor technologies due to their high capacitance density and high temperature stability they can provide. The tantalum nanoparticle-based anode provides an ultra-high surface area, so that high volumetric capacitance densities can be achieved. A tantalum-pentoxide dielectric can be formed directly on the anode structure to provide a high-permittivity oxide that is incredibly stable with changes in temperature. However, the high-surface area electrodes result in long electrical pathways for charge and discharge current, which results in capacitors with high equivalent series resistance (ESR) and low frequency stability. Additionally, their bulky design limits their integration capability. This research proposes, designs, and demonstrates a novel printed-tantalum thin-film capacitor design that solves many of the issues associated with traditional tantalum capacitors. The thin-film design results in capacitors with an ultra-thin form factor in thickness that can be integrated into the package. Additionally, the thin structure provides shorter pathways for the charge and discharge current, to dramatically improve the frequency stability and reduce the ESR of the capacitors, all while maintaining ultra-high capacitance density. In this thesis, a model is developed that is used to correlate the capacitor materials’ nanostructures to the bulk device properties, including capacitance density, frequency stability, and ESR. A process is then developed to fabricate the capacitors and integrate them directly on-package, while studying the relationships between process, structure, and performance. The integrated capacitors are shown to meet the performance objectives set out by this work. Finally, evaluation of the capacitor reliability is conducted. It is shown that the use of barrier layer materials can extend the high-temperature lifetime of the capacitors by limiting the diffusion of oxygen and moisture into the capacitor material system.
Design, synthesis, characterization and application of rare-earth doped glass and glass ceramic scintillators

(Georgia Institute of Technology, 2019-11-08) Struebing, Christian

Single crystal scintillators have been the premier choice for gamma ray detecting applications due to their high luminescent efficiency and sharp energy resolutions. However, there remain downsides to the use of single crystal scintillators such as production expense, vulnerability to environmental factors, and rigid shaping. Industries have been searching for lower cost alternatives to single crystal scintillators in order to make more portable devices practical. Glass and glass-ceramic scintillators have gained attention for their lower production cost, scalability, and ease of shaping to fit complex geometries. By the nature of the glass matrix any crystalline phases within the material are self-encapsulated, which avoids the issue of hygroscopicity and reduces the impact of mechanical shock and high temperature exposure. The main issue holding back glass and glass-ceramic scintillators has been the low luminescent efficiency stemming from the inherent disorder in the non-crystalline glassy matrix. We believe this downside can be mitigated through increases to density, harnessing the innate energy transfer capabilities of constituent materials, and controlled nucleation of crystalline phases within the glass structure. Glass-ceramics combine the robust resilience of glass with the luminescent capabilities of crystalline nanoparticles by precipitating nano-sized crystals within the glass matrix. This study approaches the field of glass and glass-ceramic based scintillators with rare-earth rich, high density compositions modeled after known crystal systems in order to produce a glass ceramic scintillator that could compete with single crystals.
Process Development of Drawcasting Structures in Thermoplastic Films

(Georgia Institute of Technology, 2019-11-06) Copenhaver, Katherine

Polymer surfaces patterned with high-aspect-ratio (AR) micro- or nanostructures are challenging to manufacture on a large scale due to steep fabrication costs and the lack of high-throughput patterning techniques. Furthermore, most surface patterning techniques for high-AR polymer structures performed on a laboratory scale are restrictive in terms of the geometries and materials they can accommodate. In this work, a new process for patterning polymer surfaces with high-AR structures, termed drawcasting, was developed and parameterized. Drawcasting is a viscous deformation process that has been shown to reliably produce arrays of high-AR polymer microstructures whose geometry can be tuned easily by changing a variety of process parameters and has been applied to both thermoplastics and curable resins. The relationship between the properties of the materials used in drawcasting and the process parameters and obtained structures was investigated in this work. The development of the drawcasting system as well as innovations in process optimization techniques such as photo- and soft lithography and microscopy are presented. The process is highly rate-, time-, and temperature-dependent, and it necessitates an understanding of a given polymer’s thermal, wetting, and rheological behavior with respect to the selected process parameters. The properties of three different thermoplastics were investigated as they relate to the drawcasting process. Much of this data was also gathered to support thermofluid models in development. Arrays of three-dimensional structures over a broad range of length scales fabricated using the drawcasting system are presented, and methods to further improve the process in order to fabricate arrays of smaller structures over larger areas are discussed.
Micellar and liquid crystalline phases of surfactant/Pluronic mixtures studied by SANS

(Georgia Institute of Technology, 2019-10-04) Zhou, Boyang

The phase behaviours of Pluronic L62 in aqueous solutions in the presence of Aerosol-OT(AOT) molecules was investigated by small angle neutron scattering (SANS). The presence of AOT was found to significantly change the micellization phenomenon of L62 micelles in aqueous solutions, including their critical micelle temperature (CMT), global size, and asphericity. The origin of these observations is attributed to the complexation between the neutral L62 surfactants and the ionic AOT molecules: The ionic groups of AOT renders the molecular charge to the aggregates of L62/AOT. On the context on molecular charge, we address the phase properties of L62/AOT complexation such as the critical micelle temperature, global size, asphericity revealed by SANS at different controlled thermodynamic conditions.
A new family of proton conducting electrolytes with enhanced stability for reversible fuel cell operation: BaHfxCe0.8-xY0.1Yb0.1O3

(Georgia Institute of Technology, 2019-08-21) Murphy, Ryan Joe

Solid oxide fuel cell (SOFC) technology has the potential to be one of the most efficient energy conversion technologies and the same technology can be used to efficiently produce several chemical species such as hydrogen and syngas through reverse operation, known as solid oxide electrolysis cells (SOEC). However, the long-term performance of these systems is often limited by degradation of the electrolyte. In this study, a new family of proton conducing electrolyte materials, BaHfxCe0.8-xY0.1Yb0.1O3 (BHCYYb), have been developed, which demonstrate much improved stability while maintaining similar or higher conductivities than current state-of-the-art materials. The performance of the SOFCs based on these new electrolytes rivals that of the current best performance reported in literature, but with better durability. In addition, BHCYYb has been shown to possess higher stability through long term chemical stability and conductivity tests. Further, solid oxide cells based on BHCYYb have also been operated in the reverse mode, as SOECs for CO2-H2O co-electrolysis. Finally, a number of dopants have been introduced to the BaHfO3-based system in order to further improve the conductivity and stability.