• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 5829
  • 1902
  • 687
  • 680
  • 245
  • 225
  • 146
  • 128
  • 103
  • 75
  • 74
  • 74
  • 74
  • 74
  • 74
  • Tagged with
  • 12574
  • 2223
  • 1695
  • 1572
  • 1312
  • 980
  • 830
  • 829
  • 799
  • 686
  • 683
  • 657
  • 656
  • 622
  • 610
  • 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.
11

The electrochemical oxidation and combustion of carbon.

Turnbull, John Douglas Shand January 1957 (has links)
The oxidation and combustion of graphite in a lead-borosilicate slag of the composition PbO.SiO₂.0.1Na₂B₄O₇ were studied with and without an applied potential. The reaction was followed by the analysis of the desorbed anode gas. The CO/CO₂ ratio of the anode gas in any one experiment was found to increase linearly with time in all cases. This increase was thought to be related to the decreasing oxygen-ion concentration. The CO/CO₂ ratio was found to increase with temperature and to decrease with increasing current density. The apparent difference in activation energies for the desorption of CO and CO₂ without an applied potential was found to be 32 ± 6 kilocalories. This is considerably higher than the values (8 to 17 kilocalories) reported for the gaseous reaction. A theoretical explanation for the increased production of CO₂ in chemical and electrochemical combustion over that observed in gaseous combustion was advanced. This explanation extended the desorption model of Long and Sykes to the slag-graphite reaction. This extended model explains why the CO/CO₂ ratio increases with time in chemical combustion, but does not predict this observed effect for the electrochemical reaction. The rate of oxygen removal from the slag during the chemical reaction was calculated from the rate of evolution and the composition of the desorbed gas. The activation energy is 26 ± 5 kilocalories. Absolute reaction rate calculations were made for rate-controlling steps of immobile adsorption, mobile adsorption, chemical reaction, and desorption. The calculated rates were at least a factor of 10⁵ different from the observed rate. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate
12

Gel spun PAN and PAN/CNT based carbon fibers: From viscoelastic solution to elastic fiber

Newcomb, Bradley Allen 27 May 2016 (has links)
This study focuses on the processing, structure, and properties of gel spun polyacrylonitrile (PAN) and polyacrylonitrile/carbon nanotube (PAN/CNT) carbon fibers. Gel spun PAN based carbon fibers are manufactured beginning with a study of PAN dissolution in an organic solvent (dimethylformamide, DMF). Homogeneity of the PAN/DMF solution is determined through dynamic shear rheology, and the slope of the Han Plot (log G’ vs log G’’). Solutions were then extruded into gel spun fibers using a 100 filament fiber spinning apparatus in a class 1000 cleanroom. Fibers were then subjected to fiber drawing, stabilization, and carbonization, to convert the PAN precursor fiber into carbon fiber. Carbon fiber tensile strength was shown to scale with the homogeneity of the PAN/DMF solution, as determined by the slope of the log G’ vs log G’’ plot. After the development of the understanding between the homogeneity of the PAN/DMF solutions on the gel spun PAN based carbon fiber tensile properties, the effect of altering the fiber spinning processing conditions on the gel spun PAN based carbon fiber structure and properties is pursued. Cross-sectional shape of the gel spun PAN precursor fiber, characterized with a stereomicroscope, was found to become more circular in cross-section as the gelation bath temperature was increased, the amount of solvent in the gelation bath was increased, and when the solvent was switched from DMF to dimethylacetamide (DMAc). Gel spun fibers were then subjected to fiber drawing, stabilization, and carbonization to manufacture the carbon fiber. Carbon fibers were characterized to determine single filament tensile properties and fiber structure using wide-angle x-ray diffraction (WAXD) and high resolution transmission electron microscopy (HRTEM). It was found that the carbon fiber tensile properties decreased as the carbon fiber circularity increased, as a result of the differences in microstructure of the carbon fiber that result from differences in fiber spinning conditions. In the second half of this study, the addition of CNT into the PAN precursor and carbon fiber is investigated. CNT addition occurs during the solution processing phase, prior to gel spinning. As a first study, Raman spectroscopy is employed to investigate the bundling behavior of the CNT after gel spinning and drawing of the PAN/CNT fibers. By monitoring the peak intensity of the (12,1) chirality in the as-received CNT powder, and in differently processed PAN/CNT fibers, the quality of CNT dispersion can be quickly monitored. PAN/CNT fibers were then subject to single filament straining, with Raman spectra collected as a function of PAN/CNT filament strain. As a result of the PAN/CNT strain, stress induced G’ Raman band shifts were observed in the CNT, indicating successful stress transfer from the surrounding PAN matrix to the dispersed CNT. Utilization of the shear lag theory allows for the calculation of the interfacial shear strength between the PAN and incorporated CNT, which is found to increase as the quality of CNT (higher aspect ratio, increased graphitic perfection, and reduced impurity content), quality of CNT dispersion, and fiber drawing increase. PAN/CNT fibers were then subjected to stabilization and carbonization for the manufacture of gel spun PAN/CNT based carbon fibers. These fibers were then characterized to investigate the effect of CNT incorporation on the structure and properties of the carbonized fibers. The gel spun PAN/CNT based carbon fibers were compared to commercially produced T300 (Toray) and IM7 (Hexcel) carbon fibers, and gel spun PAN based carbon fiber. Fiber structure was determined from WAXD and HRTEM. Carbon fibers properties investigated include tensile properties, and electrical and thermal conductivity. PAN/CNT based carbon fibers exhibited a 103% increase in room temperature thermal conductivity as compared to commercially available IM7, and a 24% increase in electrical conductivity as compared to IM7. These studies provide a further understanding of the processing, structure, property relationships in PAN and PAN/CNT based carbon fibers, beginning at the solution processing phase. Through the manufacture of more homogeneous PAN/DMF solutions and investigations of the fiber spinning process, gel spun PAN based carbon fibers with a tensile strength and modulus of 5.8 GPa and 375 GPa, respectively, were successfully manufactured in a continuous carbonization facility. Gel spun PAN/CNT based carbon fibers exhibit room temperature electrical and thermal conductivities as high as 74.2 kS/m and 33.5 W/m-K.
13

Direct biocatalytic asymmetric aldol reactions

Maggiotti, Virginie January 2003 (has links)
No description available.
14

Enhanced abatement of aqueous organic compounds using stratified activated carbon adsorption columns /

Sze, Fan Fu. January 2009 (has links)
Includes bibliographical references (p. 245-271).
15

Carbon dioxide and energy exchanges in the coastal zone of Hudson Bay

Scott, Glenn 18 January 2011 (has links)
An eddy covariance system and micrometeorological station was deployed at two locations along the coastline of Hudson Bay during the summers of 2005 and 2006 to document and to understand mass and energy fluxes in high-latitude intertidal and near-shore environments. Despite the proximity of these two zones, it was found that they exhibited distinctly different characteristics. The near-shore zone was a sink for CO2 with an average uptake of -0.11 μmol·m-2·s-1 and the intertidal zone tended to be a source of CO2 with an average efflux of 0.04 μmol·m-2·s-1 with considerable variability due to the action of the tides. Sensible heat fluxes in the near-shore zone tended to be small and negative and both latent and sensible heat fluxes were significantly enhanced in the intertidal zone. Significantly, increasing wind velocities did not appear to play a role in the enhancement of these fluxes and onshore winds were observed to be unusually dry. As such, key differences were observed that stood in contrast to the results and the conclusions of other flux studies conducted in similar high-latitude coastal-marine environments. It is suggested that these differences could only be understood in the context of the proximity of these areas of living and dead kelp, their respective differences in water depth and the occasional occurrence of a sea-breeze effect that may have implications for the observed fluxes in these areas.
16

Mathematical model of solid state thermo-oxidative stabilization of acrylic precursors for carbon fibers : evaluation of the properties of carbon fibers produced from melt-spun pan-based fibers

Grove, Dale A., III 08 1900 (has links)
No description available.
17

Carbon dioxide and energy exchanges in the coastal zone of Hudson Bay

Scott, Glenn 18 January 2011 (has links)
An eddy covariance system and micrometeorological station was deployed at two locations along the coastline of Hudson Bay during the summers of 2005 and 2006 to document and to understand mass and energy fluxes in high-latitude intertidal and near-shore environments. Despite the proximity of these two zones, it was found that they exhibited distinctly different characteristics. The near-shore zone was a sink for CO2 with an average uptake of -0.11 μmol·m-2·s-1 and the intertidal zone tended to be a source of CO2 with an average efflux of 0.04 μmol·m-2·s-1 with considerable variability due to the action of the tides. Sensible heat fluxes in the near-shore zone tended to be small and negative and both latent and sensible heat fluxes were significantly enhanced in the intertidal zone. Significantly, increasing wind velocities did not appear to play a role in the enhancement of these fluxes and onshore winds were observed to be unusually dry. As such, key differences were observed that stood in contrast to the results and the conclusions of other flux studies conducted in similar high-latitude coastal-marine environments. It is suggested that these differences could only be understood in the context of the proximity of these areas of living and dead kelp, their respective differences in water depth and the occasional occurrence of a sea-breeze effect that may have implications for the observed fluxes in these areas.
18

Molecular analysis of genes involved in carbon catabolite repression in Aspergillus nidulans /

O'Connor, Susan, January 1900 (has links) (PDF)
Thesis (Ph.D.)--University of Adelaide, Dept. of Genetics, 1999? / Erratum pasted onto front end-paper. Copies of author's previously published article inserted. Bibliography: leaves 167-180.
19

Soil hydroclimate, vegetation, and substrate controls on carbon flux in an Alaskan fen

Conlin, Molly R. January 2008 (has links)
Thesis (M.S.)--Michigan State University. Dept. of Plant Biology Ecology, Evolutionary Biology, and Behavior, 2008. / The direct goal of this thesis is determine the effect of expermental soil climate manipulatoins on carbon fluxes in an Alaskan rich fen and to assess the indirect influence of substrate quality on carbon mineralizaton rates in peat--From abstract. Title from PDF t.p. (viewed on July 29, 2009) Includes bibliographical references. Also issued in print.
20

Isotopic constraints on sources and cycling of riverine dissolved inorganic carbon in the Amazon Basin /

Mayorga, Emilio, January 2004 (has links)
Thesis (Ph. D.)--University of Washington, 2004. / Vita. Includes bibliographical references (leaves 184-226).

Page generated in 0.067 seconds