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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.
511

Synthesis, Modification, and Analysis of Silicate Cosmic Dust Analogues Using Ion-Beam Techniques

Young, Joshua Michael 08 1900 (has links)
Silicates analogous to cosmic dust were synthesized, modified, and analyzed utilizing ion-beam techniques with Rutherford backscattering spectrometry (RBS) and x-ray diffraction (XRD). Silicate dust is a common constituent in interstellar space, with an estimated 50% of dust produced in the stellar winds of M class Asymptotic Giant Branch (AGB) stars. Silicate dust acts as a surface upon which other chemicals may form (water ice for example), increasing significance in the cosmochemistry field, as well as laboratory astrophysics. Silicate formation in the stellar winds of AGB stars was simulated in the laboratory environment. Three sequential ion implantations of Fe-, MgH2-, and O- with thermal annealing were used to synthesize a mixture appropriate to silicate dust in the surface layers of a p-type Si substrate. Post implantation He+ irradiation was shown to preferentially induce crystalline formation in the analogue prior to thermal annealing. This effect is believed to originate in the ion-electron interaction in the Si substrate. The effects of ionization and ion energy loss due to electronic stopping forces is believed to precipitate nucleation in the amorphous media. For annealing temperatures of 1273 K, predominant quartz formation was found in the substrate, whereas lower annealing temperatures of 1000 K formed enstatite without post-implantation He+ irradiation, and olivine with He+ irradiation. Post annealed crystalline phase modification was investigated via x-ray diffraction and elemental compositions were investigated utilizing RBS. Finally, the interdiffusion of Fe and Mg at temperatures of 900-1100 K was investigated with RBS, and activation energies for interdiffusion were extracted for the transition from amorphous to crystalline phase in the silicate analogues. Fe had an interdiffusion energy of 1.8 eV and Mg 1.5eV. The produced analogues have similar properties to those inferred from infrared spectroscopy of the stellar winds of M-class AGB stars with an oxygen-rich outflow. This work established a method of silicate production using ion beam modifications, explored He+ irradiation effects in the annealed structures, and derived interdiffusion activation energies for Fe and Mg in the amorphous structure. Grain sizes were <100 nm with the observed formation of quartz, enstatite, and olivine.
512

Determining Oxygen Abundance vs. Age and Iron Abundance in Halo Field Dwarfs

Steele, Matthew 06 July 2005 (has links)
No description available.
513

Analysis of the Halo Globular Cluster M30 and its Variable Stars

Smitka, Michael Thomas 05 July 2007 (has links)
No description available.
514

Astrophysical Constraints on Fundamental Parameters in Physics and Astronomy

Davis, Adam N. 29 October 2004 (has links)
No description available.
515

TOPICS IN COSMOLOGY: ISLAND UNIVERSES, COSMOLOGICAL PERTURBATIONS AND DARK ENERGY

Dutta, Sourish 18 July 2007 (has links)
No description available.
516

A PHYSICAL DISCUSSION OF THE SINGLE PARABOLIC RADIO TELESCOPE AND THE TWO-TELESCOPE INTERFEROMETER

Hott, Douglas Allen 11 October 2001 (has links)
No description available.
517

Self-Gravitating Eccentric Disk Models for the Double Nucleus of Μ31

Salow, Robert M. 30 June 2004 (has links)
No description available.
518

Nebular Emission in X-ray Selected Brightest Cluster Galaxies

Samuele, Rocco January 2004 (has links)
No description available.
519

Spatial distribution and function of ion channels on neural axon

Zeng, Shangyou 21 April 2005 (has links)
No description available.
520

Studies of the Nuclear Three-Body System with Three Dimensional Faddeev Calculations

Liu, Hang January 2005 (has links)
No description available.

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