• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 1026
  • 309
  • 225
  • 101
  • 62
  • 26
  • 18
  • 15
  • 13
  • 12
  • 12
  • 12
  • 9
  • 9
  • 9
  • Tagged with
  • 2430
  • 320
  • 308
  • 304
  • 276
  • 208
  • 151
  • 145
  • 139
  • 136
  • 132
  • 126
  • 114
  • 102
  • 95
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
391

Alternatives to [4+1] carbocyclic annulations

Dossett, David Lawrence January 1986 (has links)
6-Carboethoxybicyclo [3.3.0] oct-6-en-2-one has been prepared by the intramolecular [4+1] cyclopentene annulation method. This compound and its precursor, 6-vinyl-6-carboethoxybicyclo[3.1.0]hexan-2-one were attained as the necessary standards for the investigation of the course of intramolecular Michael addition-alkylation sequence of l-phenylsulfonyl-6-carboethoxyocta-5,7-dien-2-one. / M.S.
392

Element substituted aluminophosphates

Correa, Maria del Consuela Montes de January 1989 (has links)
This dissertation reports the findings of an investigation aiming to the incorporation of the transition elements: cobalt, vanadium, and molybdenum into AlPO₄-5 molecular sieves, to the study of the redox properties of the resulting materials and to the potential application of these compounds in the partial oxidation of methane to methanol and formaldehyde. AlPO₄-5 molecular sieves containing Co, V, and Mo were synthesized by hydrothermal crystallization of typical aluminophosphate gels with the adequate metal substrates at temperatures of 200°C, 150°C, and 150°C respectively. Samples of each material were calcined in oxidant, reducing and inert atmospheres and the properties of the treated solids studied by different characterization techniques. The significant feature of the Co and V containing AlPO₄-5, CoAPO-5 and VAPO-5, is that they exhibit different colors depending on the treatment atmosphere. Evidence for cobalt contained in framework positions of CoAPO-5 is obtained by the change in its unit cell volume relative to AlPO₄-5, and by diffuse reflectance spectroscopy. CoAPO-5 also contains an appreciable quantity of extra-framework cobalt occluded in the pores, and/or as balancing cations. Evidence for this was obtained from ion exchange, and from oxygen and argon adsorption. Pentavalent vanadium incorporation is inferred from the change in the unit cell volume of VAPO-5 relative to AlPO₄-5, from pH measurements of the reaction vessel contents before and after heating, from oxygen and argon adsorption, chemical analysis, SEM, XPS, NMR, and diffuse reflectance spectroscopy. A scheme for the substitution of V into AlPO₄-5 is postulated based on the acidity observed in VAPO-5 by ion exchange, and potentiometrlc titration. Only a very small amount of Mo was found in the molybdenum containing AlPO₄-5. Evidence for Mo incorporation into the framework was not obtained. The use of CoAPO-5 and VAPO-5 as redox catalysts for the partial oxidation of methane to methanol and formaldehyde lead mostly to oxides of carbon. / Ph. D.
393

Toward the Synthesis of Nuclease Models.

Fomumbod, Enni Nina 03 May 2008 (has links)
Nucleases are enzymes that can specifically recognize nucleic acids and hydrolyze their phosphodiester bonds effectively. As is the case with many hydrolases, nucleases often carry one or more metal centers. Cooperation between such metal centers and other interactions involving general acid-base activities are believed to be essential in multifunctional catalyses. Combination of such interactions in model compounds often resulted in larger than additive effects. This work is aimed at synthesizing nuclease models that combine the ability to recognize phosphate groups and/or nitrogen bases of DNA together with the ability to catalyze phosphodiester hydrolysis. These models were designed to achieve optimum interaction between the recognition and the catalytic functionalities. Towards this goal, we chose phenonthiazonium ions (methylene blue analogues) and anthracene as spacers.
394

High Resolution Simulation of Synthetic Aperture Radar Imaging

Romero, Cindy G 01 June 2010 (has links)
The goal of this Master’s thesis is to develop a more realistic simulation of Synthetic Aperture Radar (SAR) that has the ability to image detailed targets, and that can be used for Automatic Target Recognition (ATR). This thesis project is part of ongoing SAR ATR research at California Polytechnic State University (Cal Poly) sponsored by Raytheon Space & Airborne Systems and supervised by Dr. John Saghri. SAR is a form of radar that takes advantage of the forward motion of an antenna mounted on a moving platform (such as an airplane or spacecraft) to synthetically produce the effect of a longer antenna. Since most SAR images used for military ATR are classified and not available to the general public, all academic research to date on ATR has been limited to a small data set of Moving and Stationary Target Acquisition and Recognition Radar (MSTAR) images. Due to the unavailability of radar equipment or a greater range of SAR data, it has been necessary to create a SAR image generation scheme in which the parameters of the radar platform can be directly modified and controlled to be used for ATR applications. This thesis project focuses on making several improvements to Matthew Schlutz’s ‘Synthetic Aperture Radar Imaging Simulated in Matlab’ thesis. First, the simulation is optimized by porting the antenna pattern and echo generator from Matlab to C++, and the efficiency of the code is improved to reduced processing time. A three-dimensional (3-D) graphics application called Blender is used to create and position the target models in the scene imaged by the radar platform and to give altitude, target range (range of closest approach from the platform to the center area of the target) and elevation angle information to the radar platform. Blender allows the user to take pictures of the target as seen from the radar platform, and outputs range information from the radar platform plane to each point in the image. One of the major advantages of using Blender is that it also outputs range and reflectivity information about each pixel in the image. This is a significant characteristic that was hardcoded in the previous theses, making those simulations less realistic. For this thesis project, once the target scene is created in Blender, an image is rendered and saved as an OpenEXR file. The image is rendered in orthographic mode, which is a form of projection whereby the target plane is parallel with the projection plane. This parameter means that the simulation cannot image point targets that appear and disappear during the platform motion. The echo generation program then uses the range and reflectivity obtained from the OpenEXR file, the optimized antenna pattern, and several other user defined parameters to create the echo (received signal). Once the echo is created in the echo generation program, it is then read into Matlab in order for it to go through the Range Doppler Algorithm (RDA) and then output the final SAR image.
395

Multichannel synthetic aperture radar

Rosenberg, Luke January 2007 (has links)
"In this thesis, the two problems of image formation for a Multichannel Synthetic Aperture Radar (MSAR) and suppressing interferences while forming a good quality image have been addressed. For the first problem, three wavefront reconstruction algorithms were presneted based on the multichannel Matched Filter (MF) imagining equation which demonstrated differing levels of performance and accuracy. A fourth algorithm known as multichannel backprojection was also presented to provide comparative quality with a reduced computational load. To address the second problem, a detailed jammer model was described and tested with a multichannel imaging algorithm to demonstrate the effect of hot-clutter on a SAR image. Multi-channel imaging and optimal slow-time Space Time Adaptive Processing (STAP) were shown to only partially suppress the hot-clutter interference, while optimal fast-time STAP demonstrated a much greater performance." --p. 185 of source document. / Thesis (Ph.D.)--School of Electrical and Electronic Engineering, 2007.
396

Measurements and modeling of turbulent consumption speeds of syngas fuel blends

Venkateswaran, Prabhakar 19 February 2013 (has links)
Increasingly stringent emission requirements and dwindling petroleum reserves have generated interest in expanding the role of synthesis gas (syngas) fuels in power generation applications. Syngas fuels are the product of gasifying organic-based feedstock such as coal and biomass and are composed of mainly H₂ and CO. However, the use of syngas fuels in lean premixed gas turbine systems has been limited in part because the behavior of turbulent flames in these mixtures at practical gas turbine operating conditions are not well understood. This thesis presents an investigation of the influence of fuel composition and pressure on the turbulent consumption speed, ST,GC, and the turbulent flame brush thickness, FBT, for these mixtures. ST,GC and FBT are global parameters which represent the average rate of conversion of reactants to products and the average heat release distribution of the turbulent flame respectively. A comprehensive database of turbulent consumption speed measurements obtained at pressures up to 20 atm and H₂/CO ratios of 30/70 to 90/10 by volume is presented. There are two key findings from this database. First, mixtures of different H₂/CO ratios but with the same un-stretched laminar flame speeds, SL,0, exposed to the same turbulence intensities, u'rms , have different turbulent consumption speeds. Second, higher pressures augment the turbulent consumption speed when SL,0 is held constant across pressures and H₂/CO ratios. These observations are attributed to the mixture stretch sensitivities, which are incorporated into a physics-based model for the turbulent consumption speed using quasi-steady leading points concepts. The derived scaling law closely resembles Damkhler's classical turbulent flame speed scaling, except that the maximum stretched laminar flame speed, SL,max, arises as the normalizing parameter. Scaling the ST,GC data by SL,max shows good collapse of the data at fixed pressures, but systematic differences between data taken at different pressures are observed. These differences are attributed to non-quasi-steady chemistry effects, which are quantified with a Damkhler number defined as the ratio of the chemical time scale associated with SL,max and a fluid mechanic time scale. The observed scatter in the normalized turbulent consumption speed data correlates very well with this Damkhler number, suggesting that ST,GC can be parameterized by u'rms/SL,max and the leading point Damkhler number. Finally, a systematic investigation of the influence of pressure and fuel composition on the flame brush thickness is presented. The flame brush thickness is shown to be independent of the H₂/CO ratio if SL,0 is held constant across the mixtures. However, increasing the equivalence ratio for lean mixtures at a constant H₂/CO ratio, results in a thicker flame brush. Increasing the pressure is shown to augment the flame brush thickness, a result which has not been previously reported in the literature. Classical correlations based on turbulent diffusion concepts collapse the flame brush thickness data obtained at fixed u'rms/U₀ and pressure reasonably well, but systematic differences exist between the data at different u'rms/U₀ and pressures.
397

Semi-synthetic proteins for catalytic and analytical applications

Huettinger, Karl 06 April 2009 (has links)
Proteins have evolved over millions of years to serve a plethora of highly specialized functions in biological systems. Given the enormous diversity in structure and function, it is truly surprising that only 20 different amino acids are utilized as the building blocks of proteins. Furthermore, only a small set of metal cations that are biologically available are used as structural or catalytically active cofactors in proteins, whereas rare metal cations such as platinum, ruthenium or rhodium remain absent. In the 20th century myriad catalysts, based on non-biological transition metals, emerged that can facilitate numerous organic transformations. The goal of the thesis was to introduce new functions into proteins by attaching platinum metals and fluorescent metal sensors. Thus, semi-synthetic proteins for catalytic and analytical applications were generated. The replacement of organic solvents by environmentally benign solvents such as water is an imperative step towards achieving "green chemistry". The combination of small molecule catalysts with proteins may introduce new functions and take advantage of the benefits of "both worlds" while avoiding their potential drawbacks. Therefore semi-synthetic catalysts were developed for enantioselective organic reactions in aqueous medium. A suitable reaction, reaction conditions and catalytic system for later utilization in a semi-synthetic protein were designed, developed and characterized. Ruthenium porphyrins catalyzed cyclopropanation reactions with fair yields and high stereoselectivity in aqueous medium. The successful reaction in water was a crucial requirement for a catalytically active semi-synthetic protein. Mechanistic studies did not elucidate the actual catalytic species for the formation of the cyclopropanation product and the side-product diethyl maleate; however, new insights were gained from the analysis of potential reaction pathways. Moreover, studies of the influence of axial ligands, resembling likely residues coordinating to the ruthenium metal center in the active site of a semi-synthetic protein, on the carbene formation of ruthenium porphyrins illustrated that coordination of axial ligands may inhibit the catalytic activity. The generation of ruthenium porphyrin based semi-synthetic proteins and their subsequent catalysis of cyclopropanation reactions was carried out. Myoglobin and myoglobin mutants were successfully reconstituted with a heme-like ruthenium carbonyl porphyrin; however, none of the formed semi-synthetic proteins catalyzed the enantioselective cyclopropanation of styrene. Efforts to determine the reconstitution efficiency of the generated semi-synthetic were hampered by problems to purify the generated semi-synthetic proteins that are probably due to non-specific binding of the ruthenium porphyrin to the protein surface. The exploration of labile metal pools of the biologically relevant transition metals copper, iron and zinc in cells was the goal of developing semi-synthetic proteins for analytical applications. Combining fluorescent proteins with colored or fluorescent metal chelators by forming semi-synthetic proteins allows taking advantage of their beneficial properties while avoiding their downsides. This design offers an attractive platform for in vivo metal sensing. Plasmids encoding fluorescent proteins, targeting sequences and AGT or intein fusion domains (necessary for labeling) for eukaryotic and prokaryotic expression were generated. The targeting of intracellular compartments (mitochondria, nucleus and TGN) was successful (confirmed by light microscopy experiments with transfected mammalian cells). In vitro labeling experiments of expressed and purified fusion proteins with rhodamine derivatives succeeded with AGT based fusion proteins; however, labeling of fusion proteins by trans-splicing with split-inteins failed. A new Zinc(II)-chelator was attached to an AGT based protein and the resulting semi-synthetic protein exhibited strong changes of fluorescence in the presence of zinc(II). This represents an important step towards the goal of in vivo cell imaging of labile zinc(II) pools. Despite extensive efforts, all attempts failed to generate a chelator that forms Cu(I)-complexes with the 1:1 stochiometry (ligand:metal) that is necessary for metal sensing with semi-synthetic proteins.
398

Ionospheric effects on synthetic aperture radar imaging /

Liu, Jun, January 2003 (has links)
Thesis (Ph. D.)--University of Washington, 2003. / Vita. Includes bibliographical references (leaves 100-105).
399

A Path to the Formulation of New Generations of Synthetic Jet Fuel Derived from Natural Gas

Al-Nuaimi, Ibrahim Awni Omar Hassan 16 December 2013 (has links)
Characterization of jet fuels obtained from sources other than crude oil is a modern area of research that is developing continuously to replace available petroleum-based fuels with ‘drop-in’ alternative fuels. Therefore, reliable composition-property relations are developed to correlate the hydrocarbon compositions of formulated synthetic fuels with their properties to be certified for aviation commercial use. Intensive studies have been initiated at Texas A&M University Qatar in collaboration with industry and academia to study synthetic jet fuels derived from natural gas. These studies are being implemented at its Fuel Characterization Lab where the most advanced testing equipment is used and strict Quality Management and safety systems are followed. This study is divided into two tracks. The first track is focused on conducting experimental investigations using in-house formulated synthetic jet fuels derived from natural gas via Gas-to-Liquid technology and Fischer-Tropsch chemistry. Throughout this research work, these fuels will be referred to as Synthetic Paraffinic Kerosene (SPK). These experimental investigations activities are composed of three phases: the first phase focuses on the influence of SPK building blocks (paraffinic hydrocarbons) on fuels’ properties, the second phase concerns evaluating the role of aromatics and cyclo-paraffins on properties, and the third phase studies the influence of mixing SPK with conventional Jet A-1 derived from crude oil. All of the aforementioned experimental investigations are aimed at building an experimental data bank to assist the efforts of the formulation of new generations of SPKs that meet aviation industry standards. On the other hand, the second track is directed towards the development of mathematical correlations for four properties of high importance to SPK certification. These correlations aim at optimizing fuel composition whereby major physical/chemical properties of ASTM D1655 are met at the lowest cost of composed fuel. The primary findings of this study showed that GTL derived SPK paraffinic constituents can improve certain properties while affecting others negatively, and emphasizing the necessity of aromatics in improving specific properties. Further studies compensating the absence of aromatics and sulfur through blended Jet A-1 revealed a practical solution through jet fuels optimization based on cost and technical effective manners.
400

Fast circular aperture synthesis in sar all-aspect target imaging

Burki, Jehanzeb 14 October 2008 (has links)
The objective of this research is a fast circular synthetic aperture radar (F-CSAR) algorithm. Slow-time imaging distinguishes synthetic aperture radar (SAR) from its predecessor imaging radars. SAR slow-time imaging is strongly rooted in Huygens-Fresnel principle and Kirchhoff's approximation based scalar diffraction theory. Slant-plane SAR Green's function and resultant Fourier integral, unlike some Fourier integrals, cannot be analyzed using residue theory from complex analysis and Cauchy-Riemann equations yield analyticity. The asymptotic expansion of 1D and 2D Fourier integrals renders a decomposition of the Green's function leading to SAR data focusing. The research unveils Fraunhofer diffraction patterns in 2D aperture synthesis formulation corresponding to various aperture shapes including circular aperture that appears to be an optimum aperture shape from the mathematical condition in the asymptotic expansion. It is shown that these diffraction patterns may be used for refocusing of defocused images. F-CSAR algorithm is demonstrated using Householder transform recently shown to have improved error bounds and stability. Research is also carried out into various interpolation schemes. Backprojection implementation of CSAR is compared to F-CSAR and elevation coverage renders 3D reconstruction. F-CSAR is also demonstrated using GTRI T-72 tank turntable data.

Page generated in 0.0318 seconds