Browsing by Subject "Alloys"
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Item The effects of ternary alloying additions on solute-drag creep in Al-Mg alloys(2003-05) Qiao, Jun; Taleff, Eric M.Item Evaluation of Biomimetic and Alloy-based Materials for Orthopedic Applications(2013-07-17) Guiza-Arguello, Viviana R.The basic principle of tissue engineering is the combination of appropriate cells with biomaterials under conditions that promote and lead to tissue formation. A tissue engineering scaffold is a material that supports cells for their growth, proliferation, and differentiation in the absence of native extracellular matrix (ECM). The ECM was originally thought to provide primarily a mechanical support for the cells, but through receptors on the surface of cells, the ECM takes part in promoting cell adhesion, migration, growth, differentiation, and apoptosis. Ideally, a tissue engineered scaffold should mimic both the form and function of native ECM. Additionally, like any other biomaterial for implantation, a tissue engineered scaffold should be biocompatible and not initiate tissue reactions or immune responses. This work focuses on the evaluation of the biocompatibility of novel alloy-based materials for orthopedic applications. In addition, in the context of bone regeneration, it examines the influence of select native ECM constituents on mesenchymal stem cell (MSC) osteogenic differentiation in 3-D contexts. On the other hand, given the crucial role of vasculogenesis in cell nutrition in the scaffolds, ECM mimics found to support osteogenesis were further evaluated for endothelial cell adhesion and migration. For the hydrogel systems presented in this manuscript, poly(ethylene glycol) diacrylate (PEGDA) networks were selected as the base scaffold due to the broad tunability of their mechanical properties and their previous use in bone regeneration applications. In addition, pure PEGDA hydrogels do not intrinsically promote cell adhesion. Thus, cell interactions with PEGDA gels are initially isolated to the interactions supported by the proteins tethered to the scaffold. This work attempts to contribute to the development of novel materials that provide biocompatibility and enhanced versatility in orthopedic applications. Moreover, in the context of bone regeneration, the use of selective ECM biomolecules in hybrid hydrogel scaffolds will aid in the understanding of MSC osteogenic responses to specific ECM constituents. Additionally, incorporation of ECM mimics that support both osteogenesis and vasculogenesis will provide a more controlled platform which will serve as a foundation for the fabrication of more efficient vascularized bone constructs.Item First-principles investigation of the surface reactivity of Pd-based alloys for fuel cell catalyst applications(2011-12) Ham, Hyung Chul; Hwang, Gyeong S.; Ekerdt, John G.; Mullins, Buddie C.; Arumugam, Manthiram; Ferreira, Paulo J.In recent years, palladium (Pd) has been extensively studied for a possible alternative for Pt that has been most commonly used as a catalyst in fuel cells. However, Pd shows lower activity than Pt towards the cathodic oxygen reduction reaction (ORR) and also exhibits poor tolerance toward carbon monoxide (CO) poisoning occurring in the anode process. To improve its performance, alloying Pd with other transition metals has been suggested as one of promising solutions as the Pd-based alloys have been found to boost the ORR activity and yield significant improvement in the CO tolerance. However, a detailed understanding of the alloying effects is still lacking, despite its importance in designing and developing new and more cost effective fuel cell catalysts. This is in large part due to the difficulty of direct characterization. Alternatively, computational approaches based on quantum mechanics have emerged as a powerful and flexible means to unravel the complex alloying effects in multimetallic catalysts; such first principles-based computational studies have provided many invaluable insights into the mechanisms of catalytic reactions occurring on the alloy surfaces. Using first-principles density-functional theory calculations, we have examined the surface reactivity of Pd-based bimetallic catalysts with the aim of better understanding the alloying effects in association with atomic arrangement, facet, local strain, ligand interaction, and effective atomic coordination number at the surface. More specifically, this thesis work has focused on examining the following topics: Role of Pd ensembles in selective H₂O₂ formation on AuPd alloys; Effect of local strain and low-coordination number at the surface on the performance of Pd monomer in selective H₂O₂ formation; Different facet effects on the activity of Pd ensembles towards ORR; Structure of ternary Pd-Ir-Co alloys and its reactivity towards ORR; Pd ensembles effects on CO oxidation on CO-precovered Pd ensembles; Role of ligand and ensembles in determining CO chemisorptions on AuPd and AuPt. Our first principles-based theoretical investigation of bimetallic alloys offers some insights into the rational design and development of alloyed catalysts.Item Linear and nonlinear optical spectroscopies of SiGe interfaces and Si nanocrystals(2002) Jiang, Yingying; Downer, Michael CoffinLinear and nonlinear optical spectroscopies are used to study SiGe alloy films and Si nanocrystals (NCs). With spectroscopic ellipsometry (SE), a bulk-sensitive linear optical probe, we demonstrate in-situ monitoring and control of compositionally graded SiGe films grown on Si(001) by chemical vapor deposition. Feedback control is achieved by comparing the Ge composition of the most recently deposited layer determined from SE to the set values, then adjusting the flow of disilane gas accordingly. Second harmonic generation (SHG), a surface/interfacesensitive nonlinear optical probe, complements SE greatly in monitoring film growth. We develop a real-time SHG technique by tracking surface Ge composition with the peak of the SHG spectrum (E1 resonance) using a 15 femtosecond broad bandwidth laser. Data acquisition is much faster than traditional SHG spectroscopy, in which a 100 femtosecond narrow bandwidth laser must be tuned. Using broadband SHG and SE, we also explore the strain effect caused by adding a small amount of C into SiGe alloys. SHG studies are extended from the planar surface/interface such as SiGe/Si to the sharply curved Si/SiO2 interfaces of Si NCs embedded in SiO2. We observe SHG from 3-dimensional distributions of spherical Si NCs prepared by ionimplantation into glass, which have applications in photonic and light-emitting devices. The results suggest that SHG originates microscopically from Si/SiO2 interfaces states, which are passivated by hydrogen annealing of NC samples, and macroscopically in part from fluctuations in NC size, shape and density. We also study SHG from dense (1010 or 6×1011 cm−2 ) 2-dimensional layers of Si NC (5 or 8 nm average diameter) prepared by chemical vapor deposition of Si precursor gases onto an oxidized Si wafer, and subsequently embedded in SiO2. Such Si NC layers act as a controllable planar charge storage layer in flash-memory devices. Time-dependent SHG measures the electrostatic charging and discharging of the NC layer in real-time. By polarization-dependent and frequency-domain interferometric SHG (FDISH) spectroscopy, SHG intensity and phase spectra of Si NCs are distinguished from contributions of the Si substrate, and reveal a NC-size-dependent blue-shift of the E1 resonance, consistent with quantum confinement, that can be used as an in-situ size diagnostic. Although these results were obtained ex-situ, they show that SHG can probe key material and electrical properties of Si NCs sensitively without contacting the sample, and thus can be transferred readily to in-situ, real-time monitoring of the deposition of Si NCs.Item Low-temperature solution synthesis of alloys and intermetallic compounds as nanocrystals(2009-05-15) Vasquez, YolandaThe synthesis of solid state materials has traditionally been accomplished using rigorous heating treatments at high temperatures (1,000?C) to overcome the slow rate of diffusion between two reactants. Re-grinding and re-heating treatments improve the rate of reaction between two solids; however, the high temperatures required to overcome the diffusion barrier limit the products accessible to the most thermodynamically stable phases. In this work, nano-scale solids such as alloys and intermetallics were synthesized via solution techniques where metal compounds are reduced by NaBH4 or n-butyllithium at temperatures below 300?C. To form hollow particles, metal nanoparticles of Co, Ni, Pb were synthesized via reduction by NaBH4 in water and reacted with K2PtCl6, which resulted in the formation of alloys in the case of Co-Pt and Ni-Pt. PbPt intermetallic hollow particles were synthesized by heating a composite of PbO and hollow Pt nanoparticles in tetraethylene glycol (TEG) at 140 ?C. With n-butyllithium as a reducing agent, Au3M (M= Fe, Co, Ni) nanoparticles could be synthesized as isolatable solids in the L12 structure. PtSn and AuCu3 intermetallics were synthesized using NaBH4 and TEG. The PtSn and AuCu3 nanoparticles were characterized by transmission electron microscopy in attempts to learn about the phase diagrams of nanoscale solids. The purpose of this work was to synthesize nanoparticles via solution-mediated routes at low temperatures in compositions and morphologies not observed in the bulk, and learn about the phase diagrams of nanoparticles to understand why it is possible to access solids at temperatures significantly below those used in traditional solid state chemistry.Item Nanoparticles as Reactive Precursors: Synthesis of Alloys, Intermetallic Compounds, and Multi-Metal Oxides Through Low-Temperature Annealing and Conversion Chemistry(2010-07-14) Bauer, John C.Alloys, intermetallic compounds and multi-metal oxides are generally made by traditional solid-state methods that often require melting or grinding/pressing powders followed by high temperature annealing (> 1000 degrees C) for days or weeks. The research presented here takes advantage of the fact that nanoparticles have a large fraction of their atoms on the surface making them highly reactive and their small size virtually eliminates the solid-solid diffusion process as the rate limiting step. Materials that normally require high temperatures and long annealing times become more accessible at relatively low-temperatures because of the increased interfacial contact between the nanoparticle reactants. Metal nanoparticles, formed via reduction of metal salts in an aqueous solution and stabilized by PVP (polyvinylpyrrolidone), were mixed into nanoparticle composites in stoichometric proportions. The composite mixtures were then annealed at relatively low temperatures to form alloy and intermetallic compounds at or below 600 degrees C. This method was further extended to synthesizing multi-metal oxide systems by annealing metal oxide nanoparticle composites hundreds of degrees lower than more traditional methods. Nanoparticles of Pt (supported or unsupported) were added to a metal salt solution of tetraethylene glycol and heated to obtain alloy and intermetallic nanoparticles. The supported intermetallic nanoparticles were tested as catalysts and PtPb/Vulcan XC-72 showed enhanced catalytic activity for formic acid oxidation while Pt3Sn/Vulcan XC-72 and Cu3Pt/y-Al2O3 catalyzed CO oxidiation at lower temperatures than supported Pt. Intermetallic nanoparticles of Pd were synthesized by conversion chemistry methods previously mentioned and were supported on carbon and alumina. These nanoparticles were tested for Suzuki cross-coupling reactions. However; the homocoupled product was generally favored. The catalytic activity of Pd3Pb/y-Al2O3 was tested for the Heck reaction and gave results comparable to Pd/y-Al2O3 with a slightly better selectivity. Conversion chemistry techniques were used to convert Pt nanocubes into Ptbased intermetallic nanocrystals in solution. It was discovered that aggregated clusters of Pt nanoparticles were capable of converting to FePt3; however, when Pt nanocubes were used the intermetallic phase did not form. Alternatively, it was possible to form PtSn nanocubes by a conversion reaction with SnCl2.Item Theory of alloy broadening of deep electronic energy levels in ternary semiconductor alloys(Texas Tech University, 1986-05) Ford, William CliffordThe effects of alloy disorder on deep electronic levels produced by point defects in some of the technologically important ternary semiconductor alloys are investigated using the embedded cluster method and the theory of deep levels. Deep levels produced by defects both with nearest-neighbor and with second-neighbor alloy disorder are studied. The alloy host is treated by embedding an ensemble of clusters each of which contains five atoms (for nearest-neighbor disorder) or seventeen atoms (for second-neighbor disorder) in a virtual crystal approximation effective medium. The theory of deep levels is then used to find the deep levels in the bandgap of the host material for each cluster configuration when the central atom is replaced by a point defect. Results are presented for the inhomogenously broadened deep level spectra due to substitutional impurities on the anion site in AlxGa-x.As, Hg1-xCdxTe, and GaAS1-x, Px as well as due to the ideal As vacancy in AlxGa1-xAs. Specifically, the alloy composition dependences of the first moment, the component deep levels, and the linewidths of the spectra associated with these defects are presented.