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  • 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.
111

Ion exchange behavior among metal trisilicates: probing selectivity, structures, and mechanism

Fewox, Christopher Sean 10 October 2008 (has links)
One model system for the investigation of selectivity in inorganic ion exchangers is a group of synthetic analogues of the mineral umbite. Hydrothermally synthesized trisilicates with the general form A2BSi3O9.H2O, where A is a monovalent cation, and B = Ti4+, Zr4+, and Sn4+ have been shown to have ion exchange properties. The extended three dimensional framework structure offers the ability to tune the selectivity based on the size of the cavities and channels. The unit cell volume, and therefore the pore size, can be altered by changing the size of the octahedral metal. The substitution of Ge for Si can also increase the pore size. A variety of cations have been exchanged into the trisilicates including alkali and alkaline earths, lanthanides, and actinides. The reason for the selectivity rests in the pocket of framework oxygens which make up the exchange sites. Close examination of the cation environments shows that the ions with the greatest affinity are those that have the closest contacts to the framework oxygens. For example, among alkali cations, zirconium trisilicate demonstrates the greatest affinity for Rb+ and has the most A-O contact distances approaching the sum of their ionic radii. The origins of selectivity also rely upon the valence of the incoming cation. When cations are of similar ionic radius, a cation of higher charge is always preferred over the lower valence. Ion exchange studies in binary solutions of cations of different valence, but similar size (1.0Å ) have proven the selectivity series to be Th4+ > Gd3+ > Ca2+ > Na+. Through structural characterization, kinetic studies, and use of in situ x-ray diffraction techniques the origins of selectivity in these inorganic ion exchangers has been further elucidated. The principles gleaned from these studies can be applied to other inorganic framework materials. The umbite system has the potential to be altered and tailored for specific separation needs. The trisilicate materials presented in this work are representative of the types of advances in inorganic materials research and prove their potential as applicable compounds useful for solving real world problems.
112

A Study of Gallium Nitride Light Emitting Diode Optical Output Power Enhancement Based on Focused Ion Beam Technology

Kuo, Kwei-Kuan 29 January 2008 (has links)
The application of focused ion beam (FIB) technology in microfabrication has become increasingly popular. Its use in microfabrication has advantages over contemporary photolithography or other micromachining technologies, such as the ability to process without masks and being accommodating for a variety of materials and geometries. With the surface modification of the LED/air interface, like microlens array, the light emitting at large angle can be extracted because the incident angle at the interface will be less than the critical angle without total internal reflection. A microlens feature has been fabricated on GaN LED top surface (p-GaN layer) and back side (sapphire substrate) by scanning a focused Ga ion beam. The lens shape can be modulated by using computer-controlled beam direct writing and dwell time during milling process. We have used this technique even to create a sophisticated lens surface of Fresnel microlens array which can't be created with the conventional etching methods. In addition, the resistivity of p-GaN layer is highly sensitive to the process-induced damages during surface texturing, it is difficult to apply dry etching to p-GaN layer. Our method of using gas-assisted focused ion beam etching (GAFIBE) can enhance the etching rate by the assistance of chemical reaction with minimized ion dose density to provide nearly damage-free etching by varying the beam current, pixel dwell time and refresh time. Our study emphasis on direct milling and maskless techniques which can distinguish the FIB technology from the contemporary photolithography process and provide a vital alternative to it.
113

On the feasibility of dose quantification with in-beam PET data in radiotherapy with 12C and proton beams

Parodi, Katia 31 March 2010 (has links) (PDF)
This thesis has contributed to the achievement of in-beam PET as a promising clinical monitoring technique. In response to a pressing medical demand, this work has provided a tool for quantification of local dose deviations in case of observed discrepancies between the measured and expected PET images. The implemented interactive approach described in chapter 3 is in clinical use since 2001. It provides the radio-oncologist with a valuable feedback which may allow a prompt reaction in the strategy of the therapy prior to the delivery of the successive treatment fraction in case of significant deviations between planned and actually applied dose. ...
114

Rumänien und die Großmächte in der Ära Antonescu (1940-1944) /

Balta, Sebastian. January 2005 (has links)
Texte remanié de: Dissertation--Philosophische Fakultät--Bonn--Rheinische-Friedrich-Wilhelms-Universität, 2004. / Bibliogr. p. [506]-531.
115

A kinetic treatment of a perpendicular gradient in field-aligned flow in a thermally anisotropic plasma

Spangler, Robert S. January 2001 (has links)
Thesis (M.S.)--West Virginia University, 2001. / Title from document title page. Document formatted into pages; contains iv, 76 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 47-51).
116

Ion cyclotron resonance studies of vibrationally excited ions I. Low intensity multiphoton dissociation of gas phase ions using CW CO2 laser radiation. II. Infrared radiative stabilization of energized species in the gas phase /

Woodin, Richard Lawrence. Goddard, William A., Beauchamp, Jesse L. January 1979 (has links)
Thesis (Ph. D.)--California Institute of Technology, 1979. UM #79-11,639. / Advisor names found in the Acknowledgments pages of the thesis. Title from home page. Viewed 01/19/2010. Includes bibliographical references.
117

Studies of acid-base ion-molecule chemistry using ion cyclotron resonance techniques and photoelectron spectroscopy

Staley, Ralph Horton. Beauchamp, Jesse L. January 1976 (has links)
Thesis (Ph. D.). UM #76-06,542. Thesis (Ph. D.)--California Institute of Technology, 1976. UM #76-06,542. / Advisor names found in the Acknowledgments pages of the thesis. Title from home page. Viewed 01/19/2010. Includes bibliographical references.
118

A universal species ion implantation model for implants into topographically complex structures with multiple materials

Chen, Yang. January 2001 (has links)
Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references. Available also from UMI/Dissertation Abstracts International.
119

The investigation of gas-phase ion-molecule reactions with fourier transform ion cyclotron resonance mass spectrometry

Crellin, Kevin Christopher. Goddard, William A., Beauchamp, Jesse L. January 1900 (has links)
Thesis (Ph. D.)--California Institute of Technology, 1997. UM #9800349. / Advisor names found in the Acknowledgments pages of the thesis. Title from home page. Viewed 01/21/2010. Includes bibliographical references.
120

Ionic conductivity of alkali oxide glasses at microwave frequencies /

Krishnaswami, Sumithra, January 2001 (has links)
Thesis (Ph. D.)--Lehigh University, 2001. / Includes vita. Includes bibliographical references (leaves 154-165).

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