Test pieces of GaAs were cut from Czochralski grown <100> wafers. Prior to deformation the dislocation configuration was established by cathodoluminescence (CL). Etch pits produced by molten KOH on examined crystal surfaces coincided with the CL images. The test pieces were capped with Si₃N₄, heated to between 950 and 1050°C, and plastically deformed by bending. The dislocation configuration after bending was then compared to that of the undeformed crystal. It was observed that heating to 1050°C did not significantly change the as grown cellular dislocation arrays in the crystal. With strain the dislocation configuration changed appreciably. New bands of dislocations were formed, parallel to the bend axis with dislocation free regions between them. Increasing the strain increased the number of bands. Observations were made on undoped crystals with high and low dislocation densities, and Si doped crystals. The luminescent properties of the dislocations were observed to change with heating and strain. As grown, a dislocation imaged as a dark spot surrounded by a bright halo, giving bright dislocation networks. After heating to 950°C samples showed only the dark spots without halos. After deformation, all the new dislocations appeared as dark spots or lines without halos. At very low strains, the original dislocations were still evident but were distinct from the new arrays. In an attempt to correlate the dislocation images with impurity segregation some observations of the samples were made using secondary ion mass spectroscopy (SIMS). The results suggest the possibility of the dark areas in the CL images being associated with the presence of carbon. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The temperature fields and resulting stress fields have been calculated for a growing GaAs crystal produced by the LEC process. The calculations are based in a finite element numerical thermoelastic stress analysis. The calculated temperature fields have been compared to reported experimental measurements with good agreement. The stress fields have been used to calculate the resolved shear stresses, in the growing crystal, from which the dislocation density and distribution were determined. Using the model the effects of a range of growth and environmental parameters on the dislocation density and distribution were determined. Theses parameters include crystal length, crystal diameter, cone taper, boron oxide thickness, gas pressure, solid/liquid interface shape, vertical temperature gradients and others. The results show that the temperature distribution in the gas surrounding the crystal, and the boron oxide thickness, were critical factors in determining the dislocation density and distribution in the crystal. The crystal radius, crystal length and interface curvature also strongly influenced the dislocation configuration. After crystal growth, the dislocation density at the end of the crystal was strongly influenced by the cooling procedure adopted. The dislocation distribution on cross-sections of the crystal exhibited two-fold, four-fold and eight-fold symmetry depending on growth and cooling conditions and position in the crystal. / Applied Science, Faculty of / Mining Engineering, Keevil Institute of / Graduate

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

Aluminum is an attractive metal for use as an anode in the cathodic protection of steels in seawater due to its low cost and high current capacity. Zinc is often used for its ability to readily corrode, but it has a low current capacity and it operates at very negative voltages, leading to hydrogen generation at the steel cathode, which may cause hydrogen embrittlement. Aluminum can operate at less-negative voltages, therefore reducing hydrogen generation, but it forms a passive oxide film, preventing the anode from corroding. Ga is added to aluminum in small amounts (0.1 wt%) to destabilize this oxide film and allow for active corrosion. The mechanism of how Ga activates Al is still not well-known, though there are prevailing proposals. A previous study noted a difference in behavior between Al-Ga master heats and the alloys that were later produced by re-melting them. This study is focused on characterizing the corrosion behavior of Al-0.1 wt% Ga in synthetic seawater, with samples from a master heat and two subsequent remelts. Galvanostatic, potentiostatic, and open-circuit tests were run, as well as galvanic coupling with 1123 steel. It was found that the remelted anodes behaved more consistently and maintained stable corrosion behavior for longer times than the master heat. X-ray Photoelectron Spectroscopy analysis showed elevated concentrations of Ga inside the oxide layer. The findings support the mechanism in the literature of discrete particles of Ga forming under the oxide film but do not support the mechanism of an amalgam layer formation. This project was funded by NACE International, Virginia Tech project number 457789. / Master of Science

Thesis (Ph.D.)--University of Western Australia, 2006.