No description available.

No description available.

Sustainable fusion power is within reach; however, more research is needed in the field of material science and engineering. One critical component of a fusion reactor is the plasma facing material. Very little literature exists on the sustainability of tungsten as a plasma facing material (PFM). During operation, PFM must withstand harsh conditions with combined effects from high temperature, mechanical stress, irradiation, transmutation, and the production of hydrogen (H) and helium (He) from nuclear reactions. Therefore, this thesis will focus on co-implantation of H and He into tungsten to investigate the mechanical and microstructural material response. For the first part of this study, Molecular Dynamics (MD) was used to qualitatively understand defect migration and mechanical property changes in tungsten. A Brinell hardness test was simulated using MD in tungsten to study the dependence on void size and void density hardness. It was found that hardness changes vary as the square root of the void size and void density. Also the movement of dislocations and its interaction with voids were investigated. For the second part of the study, H and He were co-implanted into tungsten to look at the mechanical and microstructural changes. Hardness changes were measured using a nano-indenter ex-situ on post-irradiated specimen. Results show that the hardness of tungsten after co-implantation is proportional to the square root of the fluence. Additionally, the microstructure of irradiated tungsten samples was investigated by using a Transmission Electron Microscope (TEM). It was observed that the defect microstructure in tungsten, after co-implantation, is quite complex, with a number of intriguing features, such as the presence of the nano-bubbles and dislocation loops. Also it was observed that there was an effect that H has on the nucleation of He nano-bubbles. The results from this work suggest that the effect of co-implanting H and He into tungsten is crucial to fully understand its viability as a PFM.

Research Doctorate - Doctor of Philosophy (PhD) / Helium has the highest ionization energy of any species and is as a consequence difficult to detect by conventional means. On the other hand, it is the ideal surface probe, having no net charge or spin, a low mass and a short de Broglie wavelength. Therefore, there exists a strong incentive to develop a microscopy technique based on helium atom scattering. The purpose of this thesis is to investigate in detail how an efficient helium detector might be developed using the phenomenon of field ionization, an ionization method that relies on quantum mechanical tunneling rather than the more conventional electron impact ionization techniques. In particular, the work focusses on the potential use of a novel nanomaterial, carbon nanotubes, as the source of the high electric fields required for field ionization detection. In Chapter 1 we review the history of field ionization research and the properties and synthesis methods for carbon nanotubes. Chapter 2 describes the experimental apparatus and procedures used for the present research, and Chapter 3 introduces the theoretical framework and background for field ionization. In Chapter 4, the prototypical field ionization system is considered from a detector viewpoint. The work demonstrates that existing theory is not sufficiently quantitative for describing a field ionization detector and therefore a semi-empirical theory is advanced for that purpose. Chapter 5 considers the problem of nanotube field enhancement in detail using computational methods, leading to a complete description of the maximum field enhancement of a nanotube array based on the four fundamental array parameters. Efforts to synthesize carbon nanotubes in the Newcastle plasma-enhanced chemical vapor deposition system are described in Chapter 6. Several procedures are developed for reproducible growth of nanotube films and the chemical vapor deposition system is characterized with single parameter studies. Chapter 7 presents the results of electron field emission and helium field ionization experiments carried out using the grown nanotube films. We demonstrate for the first time the field ionization of helium using a planar film of carbon nanotubes. Finally, we conclude the investigation of field ionization detection in Chapter 8 with a discussion on how such a detection method integrates into a helium microscope and in particular we detail the design and initial calculations for the planned Newcastle helium microscope.

Zugl.: Rostock, Univ., Diss., 2007

Thesis (Ph. D.)--University of Hong Kong, 2007. / Title proper from title frame. Also available in printed format.

Thesis (Ph. D.)--University of Michigan, 1933. / Cover title. Reprinted from the Physical review, vol. 47, no. 6, March 15, 1935."

Thesis (Ph. D.)--Florida State University, 2004. / Advisor: Dr. Steven W. Van Sciver, Florida State University, College of Engineering, Dept. of Mechanical Engineering. Title and description from dissertation home page (viewed May 25, 2005). Includes bibliographical references.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1962. / Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 60-62).

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Science, 1982. / Supervised by William J. Jenkins. Includes bibliographical references (leaves 265-290).