<p>The unique combination of material properties has led to the extensive use of Sn 111 a wide range of industrial applications, making it one of the most important commercial materials. In this research work, an atomic scale computational model has been developed for Sn using Molecular Dynamics (MD). The MD simulation technique has proven to be quite effective in establishing quantitative models for different materials. But accurately modeling Sn using classical interatomic potentials in MD is quite difficult due to its complex crystal structure and phase stability. The Modified Embedded Atom Method (MEAM) has been used in this study as it includes angular forces present in materials with directional bonding, which can model both the metallic and covalent phase of Sn. Using this method, a previously published pure Sn potential has been modified to improve upon the melting properties. Some predictions are presented for thermodynamic quantities, phase stability, structural properties and elastic constants. Good agreement has been found with experiments for the melting point, the phase transition temperature and the latent heats; however the predicted elastic constants are somewhat greater than those found in the literature. The crystal - melt interface and its properties are also investigated with the new potential. The (001)[100] orientation of the interface is found to be atomically rough. Capillary Fluctuation Method (CFM) is used to compute the crystal - melt interface stiffness in this orientation and the interface kinetics is investigated with CFM and Free Solidification (FS) technique. The (100)[010] and (110)[1Ī0] oriented interface is found to be flat or atomically smooth. Wulff plot is constructed to determine the equilibrium shape of a single Sn crystal and an approximate measurement of the interfacial energy for the flat interfaces is presented.</p> / Master of Applied Science (MASc)

<p>Through the study of recovery, precipitation and their effects on recrystallization at various annealing temperatures, a physically based model was developed to describe recrystallization kinetics of Mg AZ31. Based on Zener drag calculations, precipitates in AZ31 had little effect on recrystallization. Recovery activation energy and activation volume were determined from yield stress vs. time experiments. Recovery kinetics were used to determine the stored energy remaining within the material throughout the annealing process. Recrystallization experiments showed that contraction twins were preferred regions of recrystallization and in-situ recrystallization experiments showed that twin/twin and twin/GB intersections were ideal locations for nuclei growth outside of the twinned volume. Given the deformation and the annealing temperature, the proposed model is able to predict the recrystallized fraction as a function of time. Future versions of the model may be used to construct recrystallization-time-temperature (RTT) maps as well as predict recrystallized grain size after a time of anneal. The model predictions are in excellent qualitative agreement with experimental observations and can capture nucleation growth within both the twins and matrix.</p> / Master of Science (MSc)

<p>The sputtering of high velocity particles is investigated through analysis by secondary photon emission. During a sputtering event a fraction of the particles emitted from the target are in varying degrees of excitation. These excited states have a finite probability of radiatively decaying back to the ground state. The emitted photon wavelength and intensity is recorded. In addition, the monochromator can be locked onto one particular wavelength and its intensity observed as a function of distance from the target surface. Such intensity distributions are measured for several group IA and IIA metals and fluorides. Their spatial extent, for the most part, is shown to be governed by the atomic transition probability of the excited state under observation.</p> <p>A model is developed to describe the intensity distribution based on the premise that the sputtered particles are distributed in energy and we further propose that excitation in a sputtering event is a threshold process. From the experimental intensity distributions this proposed threshold energy is deduced and is found to be 10¹ - 10³ eV. These relatively high kinetic energies, along with the large sizes of excited states, indicate that their creation involves large energy transfers at or very near the surface. Such events as recoil sputtering may lead to the production of excited states.</p> <p>To this end, calculations on recoil sputtering yields and mean energies are reported which justify the observed high energies and low yields. Further, a comparison, based upon fractional yields, of recoil sputtered atoms and high energy cascade sputtered atoms show that the recoil source provides a larger number of high energy atoms available for excitation. Results relating to recoil implantation yields are also presented. In addition, recoil phenomena is used to explain some preferential effects observed in sputtering and transient effects in secondary photon emission.</p> <p>Finally, the fate of the implanted primary ion is discussed with emphasis on its diffusion behavior in both damaged and undamaged ambient surroundings.</p> / Doctor of Philosophy (PhD)

<p>Using a newly developed method of co-electrodeposition, superparamagnetic films containing iron oxide nano-particles in a polymer matrix have been fabricationed. The method is based upon the electro synthesis of inorganic particles dispersed within a polymer matrix obtained by electrophoretic deposition. The results showed the prepared materials to exhibit superparmagnetic properties at room temperature. The influence of the deposition conditions and the electrolyte composition on the magnetic properties of the film have been studied. Adjusting the polymer content within the electrolyte has been shown to control the iron oxide to polymer composition ratio in the deposited film. This significantly affects the particle size, inter-particle distance and magnetic properties of the material. TGA analysis was used to determine the iron oxide to polymer composition ratio. The morphology of the films was studied by SEM and AFM techniques. Two theoretical approaches have been used to calculate the particle size in the developed films and the results are in good agreement with TEM observations. Saturation magnetization ranged up to 19.71 emu/g at room temperature in the films obtained. The blocking temperatures have been determined from the results of DC and AC measurements, the latter performed from 10 to 10,000 Hz. Blocking temperatures varied with film composition but remained below 111ºK. These results set the framework for discussion of the structure and magnetic properties of nano-materials synthesized using this fabrication method.</p> / Master of Applied Science (MASc)

<p>This research has been carried out to understand the relation between work hardening, deformation behaviour, dislocation substructure and crystallographic texture developed during plastic flow in pure Al and continuous cast and direct chill cast AA5754 AI-Mg alloys. Tensile and strain rate sensitivity tests are performed in the temperature range 4.2K-295K; details of the process of dislocation accumulation, the dislocation substructure development and texture evolution are followed using a range of techniques such as TEM, in-situ resistivity measurements and X-ray diffraction. The tensile deformation results show an unprecedented level of strength and unusual work hardening behaviour of AI-alloys at 4.2K. The electrical resistivity data suggest that fracture is initiated by the collapse of the dislocation network at places where dislocations develop a critical spacing for spontaneous annihilation. This spacing is estimated at approximately 8nm at 4.2K for both pure Al and Al alloys and rv12nm at 78K for pure AI. Strain rate sensitivity measurements suggest that deformation of high purity Al is governed by dislocation-dislocation interactions in a broad range of temperatures, whereas Al alloys exhibit a larger thermal component of flow stress due to the presence of solute atoms in the matrix. The texture studies show that, independently of initial texture, the tensile axis of deformed samples rotates to (111) stable end orientation, and the intensity of the (111) peak increases parabolically with the flow stress, also strongly affected by the temperature. The experiments carried out on samples oriented at different angles to the rolling direction indicate that the crystallographic texture is the major factor responsible for the anisotropy of the alloys' mechanical properties. This is attributed to the effect of the operating slip systems activated during tensile deformation. The Visco-Plastic Self Consistent modeling has been carried out to predict mechanical behaviours of AA5754 alloys and to provide insight into operating mechanisms of plastic flow as well as the origin of the anisotropy of mechanical properties. These results indicate that the more effective dislocation storage in the substructure at a low temperature significantly increases the work hardening rate at the later stage of deformation, and the crystallographic texture is the major cause of the anisotropy in the flow stress behaviour.</p> / Doctor of Philosophy (PhD)

<p>The influence of dynamic behaviour of materials (i.e. the response of materials to large strain, high strain rate, deformation under large hydrostatic pressure occurring during metal cutting) on machinability (i.e., chip morphology, tool wear and surface finish) has been investigated in ferrous alloys with a wide range of matrix and volume fraction of second phase particles. With the increase of cutting speed, there is change in the tribological phenomenon at the tool-chip interface from sliding to seizure. A physical model for seizure is proposed based on atomic contact at the tool-chip interface. The model predicts the critical cutting speed for onset of seizure from force measurements. Seizure is said to occur when the normal stress exceeds yield strength of asperities such that the true area of contact approaches the apparent area of contact. The tribological phenomenon of seizure is shown to cause thermoplastic shear localisation. In consequences, the temperature at the tool-chip interface rises, leading to dissolution wear of the tool into the chip by a diffusion mechanism which causes chemical wear of the tool. The technique of ICP-MS has been developed and used to separate the physical and chemical wear aspects of the tool by measurement of minute concentrations of tungsten present in the chips as WC as distinct from tungsten atomically dissolved in the chips. The results have confirmed that chemical crater wear dominates at high cutting speeds. The temperature distribution at the tool-chip interface has been predicted by finite element analysis and used to compute diffusion wear. A comparison of theoretical and experimental values of diffusion wear suggests that high diffusivity paths operate at the tool-chip interface to enhance the diffusivity by more than two orders of magnitude. The maximum depth of the measured crater depth profile has been found to coincide with the phase transformation temperature of the workpiece material rather than the maximum predicted temperature at the tool-chip interface. The amount of dissolution wear, as measured by the amount of tungsten transferred into the chips is attributed to dislocation pipe diffusion. It is further suggested that dislocations generated by deformation concomitant with phase transformation provide high diffusivity paths that contribute to enhanced diffusion wear. The implication is that dissolution crater wear of the tool is phase transformation coupled. Dissolution crater wear can be suppressed if the tribological phenomenon of seizure can be prevented. This can be achieved by in-situ lubrication at the tool-chip interface through inclusion engineering of the workpiece. Alternatively, the diffusion wear can be minimized by coating the tool with a compound which hasa the least solubility in the workpiece. The microstructural response to changes of metal cutting variables during the machining of a range of iron alloys with varying heat treatment condition and microstructural constituents of the matrix has been investigated to establish the inter-relationship among chip morphology, tool wear and surface finish. The microstructural changes in the chips have been analysed by optical microscopy, scanning electron microscopy, transmission electron microscopy and x-ray diffractiion techniques. The results from the chips formed during machining of martenistic Fe-28.9%Ni-0.1%C alloy confirmed the presence of austenite, exhibiting grains as fine as 40-100 nm in the white shear bands. This structure is attributed to a sequence of events: the reverse phase transformation of martensite to austenite, shear localisation, formation of the transformation shear bands, and probably dynamic recrystallisation. This it is demonstrated that thermal softening due to phase transformation causes shear localisation leading to chip segmentation in the primary shear zone. Phenomenological observations are presented to confirm that shear localisation is caused by (i) geometrical softening due to second phase particles (graphite inclusions in cast iron, inclusions in free cutting steels), (ii) thermal softening of the matrix due to phase transformation or recrystallisation and (iii) a combination of the above. Shear localisation causes step temperature rise in a narrow band, referred to as shear band. The interaction of the priary shear band with cutting edge of the tool is found to cause dissolution wear of the cutting edge of the tool. The loss of the cutting edge in turn is shown to impair surface finish. The effect of metal cutting variables, i.e., speed, feed, depth of cut, external lubricants on shear localisation, chip morphology and tool wear is investigated. It is shown that chip segmentation can be suppressed by decreasing the feed. This is analyzed in terms of damage concepts underlying chip fracture behaviour.</p> / Doctor of Philosophy (PhD)

<p>A number of perovskite oxides, typically, heavily doped SrTiO₃ samples, were synthesized and characterized with a view to establishing their potential as anode materials for solid oxide fuel cells (SOFCs). The structure, microstructure, electrical conductivity, reduction-oxidation behavior, phase stability, compatibility with electrolytes, and performance in SOFC operation were assessed.</p> <p>Ceramic samples were prepared with the formula (Sr₁ᵪRᵪ)(Ti₁_yTy)O₃ (R = rare earth elements, T = transition metals) and with charge balance achieved by A-site deficiency. Electrical conductivities were examined by the do four-probe method and impedance spectroscopy. It was found that yttrium is soluble in SrTiO₃ (SYT) up to 8 mol% and has marked effects on conductivity. Electrical conductivities were observed to increase with increasing donor-doping level, on reduction in low oxygen partial pressures. Electrical conductivity with values as high as 82 S/cm was achieved at 800°C and P(O₂) = 10ˉ¹⁹ atm. Electrical conductivities were reversible upon reduction and oxidation. The thermal expansion coefficient is compatible with electrolyte materials such as yttria-stabilized ZrO₂ and doped LaGaO₃. Cobalt-doped SYT, which showed a relatively high resistance to oxidation, was tested as the anode material in a fuel cell. Yttrium-doped SrTiO₃ meets the requirements for the anode in SOFCs to a substantial degree, and is a promising alternative anode material.</p> / Doctor of Philosophy (PhD)

<p>The cathodoluminescence of various thin films deposited on (111)Si substrates was systematically studied for the first time. The best dopants for red, green and blue (RGB) phosphor films are Eu, Tb and Ce, respectively. A program was developed for the calculation of chromaticity coordinates from the luminescent spectra. High luminescence can only be achieved by post-annealing thin films above 900°C for yttrium silicates or above 850°C for zinc silicates. The band-gaps of the thin films were measured by UV absorption spectroscopy.</p> <p>Rare earth (Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, E³⁺) doped Y₂SiO₅, Y₂Si₂O₇, Y₄.₆₇(SiO₄)₃O and Mn²⁺ doped Zn₂SiO₄ thin films were prepared by magnetron sputtering. Their compositions were studied by Rutherford backscattering and X-ray diffraction. Sputtering parameters such as magnetic field, pressure, target-substrate distance and substrate temperature were studied. The chromaticity coordinates and brightness of thin film phosphors were compared with their powder counterparts. These films show potential for practical applications.</p> <p>Electroluminescence was found only in Zn₂SiO₄:Mn²⁺ and Y₂SiO₅:Ce³⁺ thin film phosphors. The devices of ITO/Zn₂SiO₄:Mn²⁺ (or Y₂SiO₅:Ce³⁺)/BaTiO₃/Al and multi-insulated phosphor thin films were prepared and their electroluminescent (EL) response under different voltages, frequencies and pulse widths, and their decay curves and transferred charges were studied. With the same chromaticity coordinates as Sylvania powder CL samples, Zn₂SiO₄:Mn²⁺ (2% mole) thin films are shown to have very good EL performance with a brightness as high as 80 foot lambents (fL) excited by 400 Hz a.c. pulses, with a short decay time of 0.6 ms and with an efficiency of 0.78 lumin/watt.</p> / Doctor of Philosophy (PhD)

<p>A systematic study of the influence of oxygen partial pressure, temperature and time on 110K phase formation in the BiPbSrCaCuO system has been undertaken. The optimum nominal composition to promote the Bi₂Sr₂Ca₂Cu₃Ow (2223, 110K) phase, ie; Bi₁.₈₄Pb₀.₃₄Sr₁.₉₁Ca₂.₀₀Cu₃.₀₄Ow was studied. The relationships between 110K phase fraction, oxygen partial pressure, sintering temperature and time has been mapped for this composition via experimental data. The preferred conditions for 110K phase formation were identified as 0.01 atm≤Po₂≤0.35 atm, at 820≤T≤880°C for ≤36 hours with an intermediate grinding of the material every 6 hours. The optimum conditions for pure 110K phase were Po₂=0.08 atm at 845-865°C for 12 hours with grinding every 6 hours. The oxygen partial pressure range for single 110K phase stability was determined as 0.001 atm</p> / Doctor of Philosophy (PhD)

<p>Heavily deformed, two-phase materials have been shown to exhibit strengths well in excess of the rule-of-mixtures prediction using the bulk properties of the constituents. The work described in this thesis investigates a number of aspects regarding high-strength, high-conductivity, two-phase materials. Microstructural observations and mechanical testing were used to examine the process of co-deformation, the evolution of mechanical properties, and the thermal stability of these heavily deformed structures. Materials selection procedures for the design of high-field magnet coils were developed.</p> <p>The sustained co-deformation of the two-phases leads to the conclusion that the second phase behaves as a shearable obstacle to dislocation motion. The increase in strength displayed by the two-phase material over the bulk constituent strengths may be explained by existing models of particle shearing if increased interfacial energy due to residual stress is accounted for. The increased work hardening displayed by copper-silver alloys subjected to intermediate annealing treatments is explained by a continuum description of the evolution of slip line length.</p> <p>The combination of materials selection procedures and investigations into the strengthening and resistive mechanisms have led to a new approach to the design of non-uniform composite materials to optimize strength and resistivity in two-phase materials.</p> / Doctor of Philosophy (PhD)