12023 research outputs found
Sort by
Repeatability of Joint-Dominated Deployable Masts
Get PDFDeployable masts are a class of structure that can be stowed in a small volume and expanded into long, slender, and stable booms. Their greatest benefit as space structures is their packing ratio: masts can typically be packed to a fraction of their deployed length at a diameter only modestly wider than their deployed width. This thesis is concerned with precision deployable masts, which can be stowed and deployed with repeatability of the tip position of better than 1 mm over 60 m. The methods of investigation are experimental measurements of a sample mast and numerical modeling of the mast with specially attention to hysteretic joints.
A test article of an ADAM mast was used for the experimental work. Two categories of experi- ment were pursued: measurements of mast components as inputs to the model, and measurements of full bays as validation cases for the model. Measurements of the longeron ball end joint friction, cable preload, and latch behavior are of particular note, and were evaluated for their variability. Further measurements were made of a bay in torsion and a short two-bay mast in shear, showing that there is residual displacement in this mast after shear loading is applied and released.
The modeling approach is described in detail, with attention to the treatment of the mast latches, which lock the structure in its deployed configuration. A user element subroutine was used within the framework of the Abaqus finite element analysis solver to model the behavior of the latches with high fidelity.
Validation cases for the model are presented in comparison with experimental observations of a two-bay mast. These cases show that the model captures a number of important and complex nonlinear effects of the hysteretic mast components. Parametric studies of the impacts of component behaviors and modeling practices are explored, emphasizing the impacts of part variability and the idealization of the mast latching mechanisms.</p
Surface Chemistry at the Nanometer Scale
Get PDFThis thesis describes research towards understanding surface chemical and physical processes, as well as their effects on the underlying substrate properties, at the nanometer and atomic scales. We demonstrate a method to tune the density of etch pits on Si(111) during the chlorination process so as to change the surface reactivity. Subsequent grafting of an azide group to replace chlorine demonstrates an example of non-oxidative passivation of silicon surfaces with new functionalities. Depending upon the solvent used in the azidation process, it is shown to yield different azidation kinetic rates, different final azide coverages, and different surface-area distributions. Scanning tunneling spectroscopy studies show that both chlorination and azidation processes significantly modify the surface electronic structures, with the former leading to a non-zero density of states at the Fermi level.
Our studies on a new class of corrugation, i.e., wrinkles, in exfoliated graphene on SiO2 show that a "three-for-six" triangular pattern of atoms is exclusively and consistently observed on wrinkles, suggesting the local curvature of the wrinkle is a perturbation that breaks the six-fold symmetry of the graphene lattice. Lower electrical conductance is also found on the top of wrinkles compared to other regions of graphene. The wrinkles are characterized by the presence of midgap states, which is in agreement with recent theoretical predictions. A general method is also reported for reliably fabricating ultrahigh-density graphene nanoribbon (GNR) arrays. We have clearly observed how the properties of GNRs evolve as a function of number of graphene layers. The band gap (and so the on-off ratio) decreases as the number of layers increases. These results suggest that, in addition to single layer graphene, properties of GNRs of different thicknesses can also be harnessed for engineering GNRs as different building blocks towards FET applications.
A novel imaging technique, graphene-templated scanning probe microscopy, has been developed and applied for the study on the condensation process of water and small organic molecules on mica. We found that these molecular adlayers grow epitaxially on the mica substrate in a layer-by-layer fashion. In particular, submonolayers of water form atomically flat, faceted islands of height 0.37 plus or minus 0.02 nm, in agreement with the height of a monolayer of ice. The second adlayers also appear ice-like, and thicker layers appear liquid-like. This general mechanism, however, is not universal. Exclusively three-dimensional droplets of water are observed on chemically modified (hydrophobic) mica surfaces, suggesting a 3D growth mechanism.
This thesis also includes my work on the design of a quartz-tuning-fork-based force sensor and related electronics for applications on low-temperature atomic force microscopy. Results show that the force-sensor-global-feedback circuit detector system induced lowest noise floor. The high detection sensitivity of this system demonstrates its ability to be used in frequency-modulated AFM at cryogenic temperatures. Surface topographic imaging of H-terminated Si(111) has been achieved at low temperatures.
</p
Microbial Colonization of Minerals in Marine Sediments – Method Development and Ecological Significance
Get PDFInteractions between microorganisms and minerals significantly impact microbial diversity and geochemical cycles in diverse settings. However, methodological difficulty has inhibited past study of microbe–mineral interactions in fine-grained subsurface environments. Conventional sampling poorly resolves microbial diversity at the fine scale necessary to perceive overall community differences between mineral substrates that are thoroughly mixed. In particular, the importance of microbial attachment to minerals in unconsolidated marine sediments remains poorly constrained despite extensive geobiological research in these settings. This study presents an approach for characterizing microbial colonization patterns using mineral separation techniques. Differences in density and magnetic susceptibility are used to enrich target minerals from bulk environmental samples, selecting for those minerals which may have importance as substrates for metabolic activity.
The application of this methodology to methane seep sediments of the Eel River Basin (ERB) on the California margin demonstrates that variations in microbial diversity between minerals are comparable to community differences across broad spatial scales and a range of porewater geochemistry. ERB colonization patterns determined by separation are shown to be reproducible and reflect in situ differences in the microbial community. Affinity of putative sulfide-oxidizing bacteria (primarily identified as Gammaproteobacteria) for mineral partitions enriched in authigenic sulfides suggests microbial attachment may reflect a metabolic role in sulfur cycling under reducing conditions. Mineral attachment is also shown to select between key archaeal phylotypes involved in the anaerobic oxidation of methane (AOM), providing insight into physiological differences between these uncultured groups. Preliminary results demonstrate that mineral attachment may be a significant factor in the microbial diversity of the marine subsurface, and that such community differences will be ecologically relevant.</p
Projective Dirac Operators, Twisted K-Theory, and Local Index Formula
Get PDFWe construct a canonical noncommutative spectral triple for every oriented closed Riemannian manifold, which represents the fundamental class in the twisted K-homology of the manifold. This so-called "projective spectral triple" is Morita equivalent to the well-known commutative spin spectral triple provided that the manifold is spin-c. We give an explicit local formula for the twisted Chern character for K-theories twisted with torsion classes, and with this formula we show that the twisted Chern character of the projective spectral triple is identical to the Poincare dual of the A-hat genus of the manifold
Constraints on the Global Carbon Budget from Variations in Total Column Carbon Dioxide
Get PDFDiagnosing the patterns and trends in the flux of carbon dioxide, CO₂, between the land or ocean and the atmosphere is necessary to predict the response of the carbon cycle to climate change. Atmospheric observations of the vertically averaged mixing ratio of CO₂, (CO₂), provide a new tool that complements existing observations of boundary layer CO₂ in constraining surface fluxes of CO2. My dissertation explores how variations in (CO₂) arise and how these variations can be used to estimate surface fluxes.
This thesis takes advantage of (CO₂) measurements from the Total Carbon Column Observing Network (TCCON). This global network uses ground-based Fourier transform spectrometers to obtain direct solar spectra in the near infrared, from which (CO₂) is retrieved. Because variations in atmospheric CO₂ are relatively small, it is essential that the data achieve high precision and accuracy to be useful for carbon cycle science. Using a retrieval algorithm I developed to remove transient interference from clouds and aerosols, precise measurements are achievable under a range
of meteorological conditions, allowing the inclusion of data from partially cloudy days.
At midlatitude TCCON sites, (CO₂) varies substantially on diurnal, synoptic, and seasonal timescales. A comparison of diurnal variations in (CO₂) with flux tower observations of net ecosystem exchange in northern Wisconsin suggests that local ecosystem fluxes account for only 10%–15% of variation in (CO₂) on hourly timescales. I use an atmospheric transport model with imposed surface fluxes of CO₂ to examine further the sensitivity of (CO₂) to surface fluxes and find that, as in the observations, simulated (CO₂) is relatively insensitive to local phenomena. Large variations in local fluxes and local physics (e.g., convection) produce only small changes in atmospheric (CO₂) patterns. Patterns in (CO₂) are most sensitive to the large-scale north–south flux gradient, as zonal variations in fluxes are smoothed by transport and therefore have little impact on the total column.
Rapid temporal variations in midlatitude (CO₂) arise due to transport across north-south gradients, and can be used to infer information about large-scale spatial patterns in CO₂. Here, I use the correlation between synoptic-scale variations in (CO₂) and dynamical tracers, such as potential
temperature, to infer spatial gradients in large-scale (CO₂) from sparse ground-based data. These estimated gradients, as well as the amplitude of the seasonal cycle in (CO₂), can be used as diagnostics to evaluate flux models. In simulations with one such model, the Carnegie Ames Stanford
Approach (CASA) ecosystem fluxes, gradients inferred from TCCON data are 75% larger than simulated CO₂ gradients during summer, while the seasonal cycle amplitudes in (CO₂) at midlatitude TCCON sites are between 20% and 40% larger than in simulations. Given that (CO₂) is insensitive to local fluxes, the mismatch between observations and simulations points to an underestimation of northern hemisphere ecosystem fluxes. Simulated (CO₂) diagnostics are consistent with TCCON data if boreal net ecosystem exchange is increased by 40%, a finding that suggests boreal ecosystem parameterizations must be reevaluated.
This work demonstrates that variability in (CO₂) is driven by large-scale phenomena rather than local fluxes, and has important implications for interpreting (CO₂) measurements from satellites such as GOSAT and OCO-2. These results suggest that total column measurements will provide a strong constraint on large-scale flux estimates. At the same time, extracting information about regional fluxes from (CO₂) observations alone will be challenging. Coupling column and surface CO₂ observations will yield improved flux estimates as surface CO₂ observations will constrain regional flux patterns, superimposed on top of the large-scale flux distribution revealed by total column observations.</p
Measurement of the Rare Transition b→sγ Using the Complete BABAR Data Set
Get PDFWe present the results of a measurement of the total rate and photon energy spectrum in b -> sγ transitions using the entire BABAR data set, 429 fb-1. These results use a "sum of exclusives" approach in which we reconstruct a subset of the final states of the s-quark system and correct for the final states that are missing. We find B(B- -> Xsγ)=(329 ± 19 ± 48)x10-6 for Eγ>1.9 GeV. We also measure the mean and variance of the photon spectrum and find <E>=2.346 ± 0.018+0.027-0.022 and <E2> - <E>2 = 0.0211 ± 0.0057+0.0055-0.0069. Finally we fit two classes of models for the photon spectrum and extract their respective HQET parameters
Biological Activity of a Py-Im Polyamide Androgen Receptor Antagonist
Get PDFPy-Im polyamides are cell-permeable, programmable, sequence-specific, DNA minor groove-binding small molecules. When designed to bind a DNA sequence that matches the consensus DNA-binding sequence of a transcription factor, they can be used to block the binding of that transcription factor to its response element in vitro and in cell culture. We have used this approach to inhibit the genotropic activity of the endogenous transcription factors HIF1α, glucocorticoid receptor (GR), and androgen receptor (AR). In this work, we report the completion of a library of hairpin Py-Im polyamides targeted to all possible 5’-WGNNNW-3’ (W = A or T) sequences. These compounds bind their target DNA sequences with high affinity. One compound from this set targets the sequence 5’-WGWWCW-3’, which matches the DNA binding consensus sequence of GR and AR and has been shown to inhibit the gene regulatory activity of these proteins in cell culture. Herein, we show that a cyclic derivative of this compound maintains its activity against AR-driven gene expression in hormone-sensitive LNCaP prostate cancer cells. As androgen receptor signaling is crucial to prostate cancer growth and metastasis even in its recurrent form, we next examine the activity of the AR/GR antagonist in a tissue culture model of castration-resistant prostate cancer. In this model, the polyamide retains its activity against AR-driven mRNA expression, but it fails to inhibit the binding of AR to its response element. The polyamide-mediated repression is also accompanied by significant cell stress and cytotoxicity, which are explored in the final two chapters of this thesis. The former investigates a role for polyamides as inhibitors of DNA Topoisomerase II. Despite in vitro evidence indicating polyamides prevent Topoisomerase II binding, no evidence for this is found in cell culture. The final chapter reveals that polyamide-mediated cytotoxicity is likely due to inhibition of DNA synthesis. This occurs at concentrations similar to those used for transcription factor inhibition, suggesting that S-phase disturbance accompanies efforts to regulate gene expression with polyamides
Precision Measurement of Neutrino Oscillation Parameters and Investigation of Nuclear Georeactor Hypothesis with KamLAND
Get PDFA combined analysis of examining the neutrino oscillation parameters and investigation of nuclear georeactor hypothesis with the KamLAND experiement is presented. With a total exposure of 2.75 kton-years, 930 anti-electron-neutrino candidate events above 3.4 MeV neutrino energy threshold were detected, with estimated 109±13 events from backgrounds. Assuming CPT invariance by combining with solar neutrino results, the best-fit value of georeactor fission power is 4.9+3.8-4.8 TW. The 90% upper limit on the georeactor power is determined to be 11.2 TW. This result has put a significant constraint on the contribution of a possible georeactor to the total heat from the Earth. The best-fit values of the neutrino oscillation parameters, including the georeactor power as a free parameter, is consistent with KamLAND's previously published results with null-georeactor assumption.</p
Multiscale Modeling and Simulation of Damage by Void Nucleation and Growth
Get PDFVoids are observed to be generated under sufficient loading in many materials, ranging from polymers and metals to biological tissues. The presence of these voids can have drastic implications at the macroscopic level including strong material softening and more incipient fracture. Developing tools to appropriately account for these effects is therefore very desirable.
This thesis is concerned with both, the appearance of voids (nucleation process) and the modeling and simulation of materials in the presence of voids. A particular nucleation mechanism based on vacancy aggregation in high purity metallic single crystals is analyzed. A multiscale model is developed in order to obtain an approximate value of the time required for vacancies to form sufficiently large clusters for further growth by plastic deformation. It is based on quantum mechanical results, kinetic Monte Carlo methods and continuum mechanics estimates calibrated with quasi-continuum results. The ultimate goal of these simulations is to determine the feasibility of this nucleation mechanism under shock loading conditions, where the temperature and tensions are high and vacancy diffusion is promoted.
On the other hand, the effective behavior of materials with pre-existent voids is analyzed within the general framework of continuum mechanics and is therefore applicable to any material. The overall properties of the heterogeneous material are obtained through a two-level characterization: a representative volume element consisting of a hollow sphere is used to describe the "microscopic" fields, and an equivalent homogeneous material is used for the "macroscopic" behavior. A variational formulation of this two-scale model is presented. It provides a consistent definition of the macro-variables under general loading conditions, extending the well-known static averaging results so as to include microdynamic effects under finite deformations. This variational framework also provides a suitable starting point for time discretization and consistent definitions within discrete time. The spatial boundary value problem resulting from this multiscale model is solved with a particular spherical shell element specially developed for this problem. The approximation space is based on spherical harmonics, which respects the symmetries of the porous material and allows the representation of the fields on the sphere with very few degrees of freedom. Numerical tools, such as the exact representation of the boundary conditions and an exact quadrature rule, are also provided. The resulting numerical model is verified extensively, demonstrating good convergence results, and its applicability is shown through several material point calculations and a full two-scale finite element implementation.</p
Electrochemical and Thermochemical Behavior of CeO₂-δ
Get PDFThe mixed-valent nature of nonstoichiometric ceria (CeO2-δ) gives rise to a wide range of intriguing properties, such as mixed ionic and electronic conduction and oxygen storage. Surface and transport behavior in rare-earth (samaria) doped and undoped ceria were investigated, with particular emphasis on applications in electrochemical and thermochemical energy conversion processes such as fuel cells and solar fuel production. The electrochemical responses of bulk- processed ceria with porous Pt and Au electrodes were analyzed using 1-D and 2-D transport models to decouple surface reactions, near-surface transport and bulk transport. Combined experimental and numerical results indicate that hydrogen electro-oxidation and hydrolysis near open-circuit conditions occur preferentially over the ceria | gas interface rather than over the ceria | gas | metal interface, with the rate-limiting step likely to be either surface reaction or transport through the surface oxygen vacancy depletion layer. In addition, epitaxial thin films of ceria were grown on zirconia substrates using pulsed-laser deposition to examine electrocatalysis over well-defined microstructures. Physical models were derived to analyze the electrochemical impedance response. By varying the film thickness, interfacial and chemical capacitance were decoupled, with the latter shown to be proportional to the small polaron densities. The geometry of microfabricated metal current collectors (metal = Pt, Ni) was also systematically varied to investigate the relative activity of the ceria | gas and the ceria | metal | gas interfaces. The data suggests that the electrochemical activity of the metal-ceria composite is only weakly dependent on the metal due to the relatively high activity of the ceria | gas interface. In addition to electrochemical experiments, thermochemical reduction-oxidation studies were performed on ceria. It was shown that thermally-reduced ceria, upon exposure to H2O and/or CO2, can be reoxidized to form H2, CO, and/or CH4. Analysis of gas evolution rates confirms that the kinetics of ceria oxidation by H2O and CO2 are dominated by surface reactions, rather than by ambipolar oxygen diffusion. Temperature-programmed oxidation experiments revealed that, even under thermodynamically favored conditions, carbonaceous species do not form on the surface of neat ceria, thereby giving a high CO selectivity when dissociating CO2. A scaled-up ceria-based solar reactor was designed and tested to demonstrate the feasibility of solar fuel production via thermochemical cycling