Thesis (Ph. D.)--University of Missouri-Columbia, 2004. / Typescript. Vita. Includes bibliographical references (leaves 178-186). Also available on the Internet.

Thesis (Ph. D.)--University of Michigan, 1938. / Caption title. Thesis note on label attached to p. 437. "Reprinted from Economic Geology, vol. XXXIV, no.4, June-July, 1939." eContent provider-neutral record in process. Description based on print version record.

Thesis--Wisconsin. / Vita. Includes bibliographical references (leaves 270-278).

Thesis (Ph. D.)--University of Michigan, 1938. / Caption title. Thesis note on label attached to p. 437. "Reprinted from Economic Geology, vol. XXXIV, no.4, June-July, 1939."

Thesis (Ph. D.)--Oregon Graduate Center, 1984.

No description available.

Pool oscillations in tungsten inert gas welding pools have been used in a closed-loop control system for orbital welding of ultra high purity tubing, determining a target level of penetration by altering the welding current in real-time. The technique is ideally suited to this application since it is does not contravene the cleanliness requirements for the inner bore and can be implemented outside the small orbital heads that are commonly used. The results presented in this thesis show how clear pool oscillation signals in extremely small molten pools can be monitored by optimising the welding conditions and signal processing of the arc voltage signal. As an indicator of the likely variation in cast behaviour present particularly in austenitic stainless steels, a 'time-to-penetrate' characterisation was made of the materials, using the time of the transition from the Mode 1 to the Mode 3 oscillation behaviour as the measured variable. By applying the test across a range of welding currents, significant insight was obtained into the cast and associated penetration behaviour. Late transitions indicated casts that exhibited significantly different responses to the more usually applied welding procedures, especially at the lower levels of welding current (highlighting their potentially more problematic penetration behaviour). It was shown that the established theoretical models were difficult to apply with certainty to moving weld pools, and consequently a fuzzy logic model was used in the control strategy. The closed-loop system comprised a user-interface PC, a control rack and commercial welding power source - control signals were applied every 2 to 3 Hz. Mode 3 pool oscillations were found to offer a more than satisfactory sensitivity to the inner bead width created for the various casts of 1.65 mm wall thickness materials studied.

No description available.

This dissertation tested the hypothesis that pulsed laser deposition (PLD) could be used to create targeted dopant profiles in few layered WS2 films based on congruent evaporation of the target. At the growth temperatures used, 3D Volmer-Weber growth was observed. Increased energy transfer from the PLD plume to the growing films degraded stoichiometry (desorption of sulfur) and mobility. Sulfur vacancies act as donors and produce intrinsic n-type conductivity. Post deposition annealing significantly improved the crystallinity, which was accompanied by a mobility increase from 6.5 to 19.5 cm2/Vs. Preparation conditions that resulted in excess sulfur, possibly in the form of interstitials, resulted in p-type conductivity. Current-voltage studies indicated that Ohmic contacts were governed by surface properties and tunneling. Extrinsic p-type doping of few layered WS2 films with Nb via pulsed laser deposition using ablation targets fabricated from WS2, S and Nb powders is demonstrated. The undoped controls were n-type, and exhibited a Hall mobility of 0.4 cm2/Vs. Films doped at 0.5 and 1.1 atomic percentages niobium were p-type, and characterized by Fermi levels at 0.31 eV and 0.18 eV from the valence band edge. That is, the Fermi level moved closer to the valence band edge with increased doping. With increased Nb doping, the hole concentrations increased from 3.9 x1012 to 8.6 x1013 cm-2, while the mobility decreased from 7.2 to 2.6 cm2/Vs, presumably due to increased ionized impurity scattering. X-ray photoelectron spectroscopy indicates that Nb substitutes on W lattice sites, and the measured peak shifts toward lower binding energy observed corresponded well with the UPS data. Throughout, a clear correlation between degraded stoichiometry and decreased mobility was observed, which indicates that point defect and ionized impurity scattering is a dominant influence on carrier transport in PLD few-layered WS2 films. The approach demonstrates the potential of PLD for targeted doping of transition metal dichalcogenides.

<p> Chan and Riley (1967) developed a colorimetric method for the determination of tungsten in natural waters and silicate rocks. Applied to silicate rocks this method contains the following stages: evaporation of silica with hydrofluoric acid, co-precipitation of tungsten and some other elements, with hydrous manganese dioxide and cation-exchange separation of vanadium, molybdenum and tungsten. When applying this method on standard solutions and the new U.S.G.S. standard rocks, the results obtained are lower than those suggested by the authors. The results obtained recently with neutron activation analysis of the same standard rocks by Johansen and Steinnes (1970), although different from the results of the present work, also disagree with Chan and Riley. The method was abandoned since a tungsten loss is believed to occur throughout the procedure.</p> / Thesis / Master of Science (MSc)