12023 research outputs found
Sort by
Aseismic Deformation in Subduction Megathrusts: Central Andes and North-East Japan
Get PDFWe aim to characterize fault slip behavior during all stages of the seismic cycle in subduction megathrust environments with the eventual goal of understanding temporal and spatial variations of fault zone rheology, and to infer possible causal relationships between inter-, co- and post-seismic slip, as well as implications for earthquake and tsunami hazard. In particular we focus on analyzing aseismic deformation occurring during inter-seismic and post-seismic periods of the seismic cycle. We approach the problem using both Bayesian and optimization techniques. The Bayesian approach allows us to completely characterize the model parameter space by searching a posteriori estimates of the range of allowable models, to easily implement any kind of physically plausible a priori information and to perform the inversion without regularization other than that imposed by the parameterization of the model. However, the Bayesian approach computational expensive and not currently viable for quick response scenarios. Therefore, we also pursue improvements in the optimization inference scheme. We present a novel, robust and yet simple regularization technique that allows us to infer robust and somewhat more detailed models of slip on faults. We apply such methodologies, using simple quasi-static elastic models, to perform studies of inter- seismic deformation in the Central Andes subduction zone, and post-seismic deformation induced by the occurrence of the 2011 Mw 9.0 Tohoku-Oki earthquake in Japan. For the Central Andes, we present estimates of apparent coupling probability of the subduction interface and analyze its relationship to past earthquakes in the region. For Japan, we infer high spatial variability in material properties of the megathrust offshore Tohoku. We discuss the potential for a large earthquake just south of the Tohoku-Oki earthquake where our inferences suggest dominantly aseismic behavior
Oxygen Isotopes and Volatiles in Martian Meteorite
Get PDFOxygen isotopes were measured in mineral separates from martian meteorites using laser fluorination and were found to be remarkably uniform in both δ18O and Δ17O, suggesting that martian magmas did not assimilate aqueously altered crust regardless of any other geochemical variations.
Measurements of Cl, F, H, and S in apatite from martian meteorites were made using the SIMS and NanoSIMS. Martian apatites are typically higher in Cl than terrestrial apatites from mafic and ultramafic rocks, signifying that Mars is inherently higher in Cl than Earth. Apatites from basaltic and olivine-phyric shergottites are as high in water as any terrestrial apatite from mafic and utramafic rocks, implying the possibility that martian magmas may be more similar in water abundance to terrestrial magmas than previously thought. Apatites from lherzolitic shergottites, nakhlites, chassignites, and ALH 84001 (all of which are cumulate rocks) are all lower in water than the basaltic and olivine-phyric shergottites, indicating that the slow-cooling accumulation process allows escape of water from trapped melts where apatite later formed. Sulfur is only high in some apatites from basaltic and olivine-phyric shergottites and low in all other SNCs from this study, which could mean that cumulate SNCs are low in all volatiles and that there are other controlling factors in basaltic and olivine-phyric magmas dictating the inclusion of sulfur into apatite.
Sulfur Kα X-rays were measured in SNC apatites using the electron probe. None of the peaks in the SNC spectra reside in the same position as anhydrite (where sulfur is 100% sulfate) or pyrite (where sulfur is 100% sulfide), but instead all SNC spectra peaks lie in between these two end member peaks, which implies that SNC apatites may be substituting some sulfide, as well as sulfate, into their structure. However, further work is needed to verify this hypothesis.</p
Interacting Single Atoms with Nanophotonics for Chip-Integrated Quantum Network
Get PDFUnderlying matter and light are their building blocks of tiny atoms and photons. The ability to control and utilize matter-light interactions down to the elementary single atom and photon level at the nano-scale opens up exciting studies at the frontiers of science with applications in medicine, energy, and information technology. Of these, an intriguing front is the development of quantum networks where N >> 1 single-atom nodes are coherently linked by single photons, forming a collective quantum entity potentially capable of performing quantum computations and simulations. Here, a promising approach is to use optical cavities within the setting of cavity quantum electrodynamics (QED). However, since its first realization in 1992 by Kimble et al., current proof-of-principle experiments have involved just one or two conventional cavities. To move beyond to N >> 1 nodes, in this thesis we investigate a platform born from the marriage of cavity QED and nanophotonics, where single atoms at ~100 nm near the surfaces of lithographically fabricated dielectric photonic devices can strongly interact with single photons, on a chip. Particularly, we experimentally investigate three main types of devices: microtoroidal optical cavities, optical nanofibers, and nanophotonic crystal based structures. With a microtoroidal cavity, we realized a robust and efficient photon router where single photons are extracted from an incident coherent state of light and redirected to a separate output with high efficiency. We achieved strong single atom-photon coupling with atoms located ~100 nm near the surface of a microtoroid, which revealed important aspects in the atom dynamics and QED of these systems including atom-surface interaction effects. We present a method to achieve state-insensitive atom trapping near optical nanofibers, critical in nanophotonic systems where electromagnetic fields are tightly confined. We developed a system that fabricates high quality nanofibers with high controllability, with which we experimentally demonstrate a state-insensitive atom trap. We present initial investigations on nanophotonic crystal based structures as a platform for strong atom-photon interactions. The experimental advances and theoretical investigations carried out in this thesis provide a framework for and open the door to strong single atom-photon interactions using nanophotonics for chip-integrated quantum networks
Advancing Electrocatalysis in Solid Acid Fuel Cells
Get PDFSolid acid fuel cells are currently performance limited by the electrochemical reaction kinetics at the electrodes. For acceptable power output, precious metal catalysts such as platinum, are required, rendering the technology too expensive for commercialization in all but niche applications. This thesis explores new approaches to solid acid fuel cell electrodes with the aim of reducing the catalyst loading or even eliminating precious metals entirely, without sacrificing performance. Two broad approaches are pursued: nanostructuring for enhanced catalyst utilization and incorporation of carbon-based materials for enhanced electrical transport and even electrocatalysis.
Electrospray deposition is shown to be a viable technique to produce nanoparticles of the solid acid fuel cell electrolyte material CsH2PO4. In situ aerosol particle size measurements using a differential mobility analyzer and a condensation particle counter allowed the characterization of the electrospray parameter space, resulting in CsH2PO4 particle size control between 10 and 50 nm. Co-deposition of the CsH2PO4 nanoparticles together with a stabilizing surfactant polyvinylpyrrolidone (PVP) and platinum catalyst nanoparticles allows the creation of highly active, porous, interconnected electrode nanostructures. These nanostructures directly deposited onto fuel cell components, either the carbon paper current collector or the thin film electrolyte layer, serve as electrodes. A 30-fold reduction of platinum loading, without sacrificing electrode performance as compared to mixed powder-electrodes, is demonstrated.
The direct deposition of CsH2PO4 nanoparticles with the stabilizing surfactant PVP onto a prefabricated CsH2PO4 electrolyte layer and subsequent magnetron sputtering of a nanometer thin platinum film lead to surprising catalyst-mass normalized electrode activities for solid acid fuel cell anodes. Specifically, a 25-fold increase in the mass normalized activity is shown as compared to the predicted values from analysis
of platinum thin films with a controlled geometry.
The second part of the thesis deals with the introduction of carbon nanotubes to the solid acid fuel cell electrodes. Three types of carbon nanotubes (CNTs) were grown directly onto the carbon paper current collector, in all cases using a chemical vapor deposition method and nickel catalyst nanoparticles.
(i) Conventional CNTs were shown to act as effective current collectors for electrosprayed composite electrode structures, containing platinum nanoparticles. Matching of scales between the current collector and the electrosprayed structure leads to improved interconnectivity of the platinum catalyst nanoparticles and a higher density of electrochemically active triple phase boundaries.
(ii) Nitrogen doped carbon nanotubes (NCNTs) were shown to actively catalyze oxygen electroreduction in solid acid fuel cells, with no platinum present.
(iii) Undoped but defective carbon nanotubes (dCNTs) were shown to be highly efficient catalysts of the oxygen electroreduction reaction, surpassing the activity of the state of the art, platinum containing electrodes. This is the first time undoped carbon nanotubes have been reported to be catalytically active for electroreduction of oxygen.
In addition, catalytically active carbon nanotubes show excellent catalysis of the water splitting reaction, creating the opportunity for new applications of these solid state electrochemical devices.</p
Molecular Design of Side-Group Liquid Crystalline Polymers: Understanding Their Interactions with Small Molecule Liquid Crystal Solvent
Get PDFLiquid crystal (LC) gels – the combination of macromolecules with small molecule LCs – couple the
elasticity and mechanical strength of polymers to the order inherent to LCs and are attractive to many
researchers hoping to marry liquid crystals' optical and electro-optical responsiveness with polymers'
mechanical strength and ease of processing. In particular, side-group liquid crystal polymers (SGLCPs)
are flexible-chain polymers that are functionalized with LC side-groups. Here we introduce the concept
of polymer dopants: homogenously dissolved LC-containing SGLCP homopolymers that are molecularly
designed for solubility in and coupling to small molecule LC solvents. Using polymer analogous
chemistry (changing the molecular makeup of the side groups and their linkers, while keeping backbone
molecular weight, polydispersity index, and degree of polymerization constant), we’ve targeted the effect
of side-group orientation, dipole position and strength, spacer length and linking-group type on
polymer solubility and bulk material properties. We've shown that, at low concentration, these dopants
can have significant effects on the bulk material properties of two types of LCs: ferroelectric and vertically
aligned nematic LCs
X-ray Ionization and the Influence of Magnetic Fields on the Growth of Gas Giant Planets
No full textThe past two decades have witnessed an explosion in the number of known planets outside our Solar System. The exoplanets' number and diversity are surprising, with many planetary systems quite unlike our own. Naturally, the question of their origins arise. Yet our knowledge of how planetary systems form and evolve are far from complete, and even for our own Solar System, many fundamental questions remain. One puzzle concerns the formation of gas giants with masses intermediate between Jupiter (MJ = 320M⊕, where M⊕ = 6 x 1027 g equals the mass of the Earth) and Neptune (MN = 17M⊕ ), which our Solar System provides a prime laboratory for study: Saturn (MS = 95M⊕)· According the popular core accretion scenario, gas giants form through the growth of an ice and rock core that, upon becoming sufficiently massive, accretes a gaseous envelope from the surrounding protoplanetary disk. If the mass of the envelope reaches the mass of the core, runaway gas accretion commences, with terminal masses typically a Jupiter mass or more. The combined mass of the envelope and core at the critical point when runaway accretion begins is about a Neptune mass. Saturn, being a factor of a few times more massive than Neptune, achieved this critical mass, yet appears not to have undergone runaway accretion to an extent similar to Jupiter, posing a problem for the core accretion hypothesis.
In this thesis, we investigate one possible explanation for Saturn's low mass: magnetic fields. If the gas surrounding a protoplanet is sufficiently ionized, it can be well coupled to the ambient magnetic field; as magnetized gas accretes onto the protoplanet, magnetic pressure and tension forces would grow, prematurely halting the flow of gas. To investigate this possibility, a preexisting Monte Carlo transport code was extended to calculate x-ray transfer. The code was applied to calculate ionization rates for a proto-Saturn still embedded in the protosolar disk. The results were inputted into a simplified chemical reaction network, which takes into account recombination and charge transfer and returns the ionization fraction of the disk. Magnetic diffusivities in proto-Saturn's vicinity can then be calculated, allowing us to infer whether the magnetic field was coupled well enough to the gas surrounding proto-Saturn to affect gas dynamics.
We find that for a disk depleted in small grains and possessing magnetic fields of about 0.1 G, magnetic fields were likely well-coupled to the gas near proto-Saturn and may have influenced its gas accretion rate.</p
Improving the Efficiency of Ruthenium-Catalyzed Olefin Metathesis with Solid-Supported Catalysts, Microfluidic Reactors, and Novel X-Type Ligands
Get PDFOlefin metathesis has become an important tool in modern organic chemistry. Key to the development of olefin metathesis as a methodology has been the discovery of the highly active, selective, and tolerant ruthenium-based Grubbs catalysts. The overall efficiency and utility of these catalysts are determined by a complex set of parameters including catalyst design, reaction conditions, reactor design, and purification strategy. These parameters can be varied to achieve higher catalyst turnovers, better product selectivity and reduced product contamination. This research seeks to improve the efficiency and utility of olefin metathesis using three strategies; the covalent attachment of catalysts to silica supports, the development of biphasic microfluidic reactors, and the synthesis of novel catalyst architectures.
Solid-supported catalysts present an effective strategy to eliminate metal contamination in metathesis products. These catalysts, however, are generally ill defined and their active species and decomposition pathways are poorly understood. In order to further study both the activity and decomposition of silica-supported catalysts, both a brominated alkylidene ligand and a cleavable linker were prepared. The brominated ligand was designed to bind only active catalyst, but was found to indiscriminately bind all ruthenium species. The cleavable linker was synthesized with an ortho-benzyl nitro ether moiety, rendering it cleavable by UV light. Future studies will use this UV-triggered lability to study the solid-supported catalysts with solution phase techniques.
Biphasic microfluidic reactors were developed to address the generation or consumption of ethylene gas in metathesis. By using either alternating flow or parallel flow gas-liquid reactors, the mass transfer of ethylene was facilitated. The enhanced mass transfer gave higher yields and catalyst turnovers in ethenolysis and ring-closing metathesis reactions.
Novel catalyst architectures were designed and synthesized to increase catalyst activity. While chloride-based catalysts have generally been used because of their higher activity, the activity of fluoride and hydroxide catalysts remains under explored, due mainly to practical challenges associated with their synthesis. A set of fluoride catalysts based on the Piers-type catalyst and a hydroxide catalyst based on the recently developed Z-selective catalysts were synthesized and characterized. The hydroxide catalyst showed promising activity while the fluoride catalyst was found to be inactive under all but the most forcing of conditions.
In general, the utility of ruthenium-based catalysts has caused rapid growth in the field of olefin metathesis. The work presented herein covers a variety of strategies to improve the overall utility and efficiency of these catalysts, including insights into decomposition pathways, controlling phase interactions, and synthesizing novel catalysts. Further pursuits of these strategies will prove valuable to the advancement of olefin metathesis.</p
Simulation of Richtmyer-Meshkov Flows for Elastic-Plastic Solids in Planar and Converging Geometries Using an Eulerian Framework
Get PDFThis thesis presents a numerical and analytical study of two problems of interest involving shock waves propagating through elastic-plastic media: the motion of converging (imploding) shocks and the Richtmyer-Meshkov (RM) instability. Since the stress conditions encountered in these cases normally produce large deformations in the materials, an Eulerian description, in which the spatial coordinates are fixed, is employed. This formulation enables a direct comparison of similarities and differences between the present study of phenomena driven by shock-loading in elastic-plastic solids, and in fluids, where they have been studied extensively. In the first application, Whitham's shock dynamics (WSD) theory is employed for obtaining an approximate description of the motion of an elastic-plastic material processed by a cylindrically/spherically converging shock. Comparison with numerical simulations of the full set of equations of motion reveal that WSD is an accurate tool for characterizing the evolution of converging shocks at all stages. The study of the Richtmyer-Meshkov flow (i.e., interaction between the interface separating two materials of different density and a shock wave incoming at an angle) in solids is performed by means of analytical models for purely elastic solids and numerical simulations when plasticity is included in the material model. To this effect, an updated version of a previously developed multi-material, level-set-based, Eulerian framework for solid mechanics is employed. The revised code includes the use of a multi-material HLLD Riemann problem for imposing material boundary conditions, and a new formulation of the equations of motion that makes use of the stretch tensor while avoiding the degeneracy of the stress tensor under rotation. Results reveal that the interface separating two elastic solids always behaves in a stable oscillatory or decaying oscillatory manner due to the existence of shear waves, which are able to transport the initial vorticity away from the interface. In the case of elastic-plastic materials, the interface behaves at first in an unstable manner similar to a fluid. Ejecta formation is appreciated under certain initial conditions while in other conditions, after an initial period of growth, the interface displays a quasi-stationary long-term behavior due to stress relaxation. The effect of secondary shock-interface interactions (re-shocks) in converging geometries is also studied. A turbulent mixing zone, similar to what is observed in gas--gas interfaces, is created, especially when materials with low strength driven by moderate to strong shocks are considered
Synthesis and Biological Studies of DNA-binding Cyclic Py-Im Polyamides
Get PDFPyrrole-imidazole (Py-Im) polyamides are programmable oligomers that bind to the minor groove of DNA in a sequence-specific manner at affinities comparable to natural DNA-binding proteins. Hairpin polyamides have been shown to localize within the nucleus of live cells, disrupt protein-DNA interactions, and modulate endogenous gene expression. Cyclic polyamides display further enhanced DNA binding affinities and exhibit similar gene regulatory effects, but investigations into their biological activity have been limited by the lack of effective synthetic methods. Herein, we demonstrate the efficient synthesis of a focused library of cyclic polyamide utilizing a novel microwave-assisted solid-phase technique. The orthogonal protection strategy allowed for selective turn modifications, and the mild cleavage conditions gave access to polyamide cores beginning with a C-terminal imidazole. In addition to expanding our synthetic repertoire, we further examined the cytotoxicity and cell uptake profiles of the cyclic polyamide variants, which highlighted the significant changes in biological activity resulting from minor structural modifications. Molecular recognition of the polyamide turn unit was also explored by installing heteroatom substituents at the α-position. Interestingly, while none of the fluoro, hydroxyl, or amino derivatives increased turn specificity, the (S)-fluoro turn exhibited better tolerance for binding a C•G pair. Finally, we optimized the synthesis of several biologically active hairpin polyamides on a 50-mg scale and examined their antitumor activity in mice xenograft models
Dynamics of Earth's Hadley Circulation
Get PDFThis thesis advances our understanding of the mechanisms controlling the Hadley circulation, and its interaction with eddies on planetary scales in particular. On Earth, and more generally in a rapidly rotating and differentially heated planet, planetary scale eddies in the extratropics interact with the mean flow in the tropics, contributing to the driving of the Hadley circulation. A hierarchy of numerical models is used to simulate and understand the relative importance of eddies in the driving of the Hadley circulation. In a global warming experiment, the Hadley circulation is found to strengthen in colder climates and weaken in warmer climates, with a maximum strength in a climate close to present-day Earth’s. This nonmonotonicity is shown to be consistent with variations in the eddy activity in the midlatitudes. The cells are also found to widen over the entire range of this climate change. A criterion quantifying the importance of baroclinic waves in setting the depth of the troposphere, which is modified to account for the effect of convective adjustment on planetary Rossby waves activity, is used to explain the shifts in the terminus of the Hadley circulation for a wide range of climate scenarios. Additionally, by comparing simulations with and without ocean heat transport, it is shown that accounting for low-latitude ocean heat transport and its coupling to wind stress is essential to obtain Hadley circulations in a dynamical regime resembling Earth’s. These changes in the strength and extent are found to be captured in a simple one-dimensional model that relies on standard assumptions about the thermodynamic properties of the atmosphere in the low-latitude regions and with a simple representation of eddy fluxes. Further work with this model, which may be amenable to analytical progress, could provide a quantitative understanding for the sensitivity of the Hadley circulation in comprehensive GCM simulations of 21st century global warming scenarios.</p