<p>In the manufacture of sulphuric acid more stringent environmental standards and operation economics have forced the industry to improve product utilization, energy efficiency and reliability. A key to improving both the thermal efficiency and reliability is the use and/or development of more corrosion resistance materials including stainless steels, especially in the parts of the plant that handle the condensed acid. Application of more corrosion resistant material requires a better understanding of the corrosion mechanism involved in concentrated H2 SO4 -H2 O (>90 wt.%) solutions. While corrosion kinetics of carbon steel, the traditional material of construction, are relatively well understood, this is much less true in the case of the cyclic active-passive corrosion of stainless steels. Models proposed to explain the cyclic active-passive corrosion involve a periodic formation of either a protective metal sulphate film or an insoluble sulphur layer. To better understand the reactivity and/or passivity of stainless steel in concentrated H2 SO4 -H2 O solutions a study employing immersion and electrochemical techniques, including rotating electrodes, was conducted in order to clarify the following: (1) The state of stainless steel passivity. (2) The conditions in which passivity is stable. (3) The role played by the major alloying elements in establishing and maintaining the passive state. The study involved evaluating the corrosion behaviour of stainless steels S30403 and S43000 along with iron, chromium and nickel in 93.5 wt.% H2 SO4 at temperatures between 25-80°C. Major discoveries of the study include: (1) A content of 17-18 wt.% chromium is sufficient to anodically passivate S43000 as the potential is made more noble. Passivity is not stable and requires anodic polarization. (2) Alloyed nickel plays an active role in improving the corrosion resistance of stainless steel. A content of 8 wt.% nickel is sufficient promote a periodic passivation of the base Fe-(17-18)wt.% Cr stainless steel under open-circuit conditions which reduces the corrosion rate by at least an order of magnitude. (3) The electrolysis of concentrated H2 SO4 -H 2 O solutions involves a potential-dependent reduction of H2 SO 4 molecules to sulphur-containing species with an oxidation state lower than six (6). The various reduction products have a significant effect on the stainless steel corrosion resistance. (4) Successful modelling of the corrosion of nickel has been accomplished by using a galvanic interaction between a noncontinuous nickel sulphide (NiS) deposit, formed in situ, and the uncovered nickel metal. (5) Successful modelling of the active-passive corrosion of S30403 has been accomplished using a galvanic interaction between NiS(Ni) and S43000.</p> / Doctor of Philosophy (PhD)

<p>The evolution of molecular dynamics during the polymerization of linear-chain and network forming liquids was studied using dielectric spectroscopy and differential scanning calorimetry. Polymerization was carried out using step-addition reactions between epoxide and amine molecules, and by catalysis of epoxide molecules with tertiary amines. The former resulted in linear-chain or network structured polymers while the latter resulted in network polymers. The step-addition polymerizations resulted in linear-chain polymers by reacting stoichiometric quantities of diepoxide and monoamine molecules, and network structure polymers by reacting stoichiometric quantities of triepoxide and monoamine molecules, or diepoxide and diamine molecules. The growth and extinction of localized (or secondary) relaxation processes during the polymerization were studied by measuring the changing dielectric properties using two techniques; fixed frequency dielectric measurements during heating of the partially polymerized samples, and isothermal dielectric measurements over the frequency range of 1 MHz to 20 GHz. The number of covalent bonds formed at any instant during the polymerization was determined by isothermal calorimetric measurements. Thus, the change in the dielectric properties during polymerization was associated with the increase in the number of covalent bonds. It was found that the localized relaxations evolve in a manner that is independent of the spontaneous increase in configurational entropy. The results also tend evidence towards the concept that these relaxations occur in regions of relatively high energy, and the collapse of such regions led to the observed changes in the dielectric properties. As well, an analogy was made between the structural relaxation of a physically metastable glass and the polymerization of a chemically metastable liquid. This has lead to the concept of a chemical fictive temperature to describe the state of the polymerizing liquid. The effects of pressure on the dielectric properties of the polymerizing liquids was studied using fixed frequency dielectric measurements. Pressure was step-increased, both at the beginning of the polymerization and at a time during the polymerization, then maintained. An increase in pressure increased the rate of polymerization, and thus the dielectric properties evolved more rapidly with time. The chemical effects of pressure during polymerization were examined using transition state theory and the concept of negative feed-back. The physical effects of pressure were investigated in terms of its effects on the equilibrium dielectric properties. It was shown that the polymerization kinetics are increased with increased pressure when the kinetics are in the mass-controlled regime, and decreased with increased pressure when the kinetics are in the diffusion-controlled regime. The transition from mass- to diffusion-controlled kinetics was investigated using the rate of change of the dielectric loss. Finally, the chemical and physical effects of step-increased pressure upon the dielectric properties of polymerizing liquids in the mass-controlled regime were mathematically simulated using current concepts and formalisms. The simulated results were qualitatively similar to those obtained experimentally, demonstrating the adequacy of the understanding.</p> / Doctor of Philosophy (PhD)

<p>This work has involved a study of diffusion and ion implantation enhanced diffusion in GaAs based semiconductor heterostructures. A knowledge of kinetic parameters underlying the diffusion process is of practical value in the fabrication of optoelectronic devices. Two main problems have been explored. (a) Thermal interdiffusion of (InGaAs/GaAs)quantum well structures and superlattices. Transmission electron microscopy, X-ray diffraction and photoluminescence experiments have been undertaken to follow the temporal evolution of the indium composition during annealing of the superlattice. A linear model of diffusion has been utilized to calculate the evolution of strain, indium composition and the X-ray diffraction intensity during annealing. Calculation of energy levels of the electrons and holes in the superlattice structure and their evolution with annealing have also been performed following a transfer matrix approach. This has been used to predict the experimentally observed trends in photoluminescence. Non linear aspects of diffusion including composition dependence and strain dependence (by growth on 311A substrates) have also been experimentally examined. (b) Ion implantation enhanced disordering of III-V compound semiconductor heterostructures (InGaAs/GaAs, GaAs/AlGaAs). Various parameters which influence the disordering process, namely the implant energy, ion species, implant temperature and heterostructure composition have been studied with transmission electron microscopy (conventional/high resolution) and photoluminescence techniques. Several interesting phenomena with respect to the position and crystallographic nature of implant damage have been observed. A phenomenological model to rationalize data has been developed by numerically solving the governing diffusion equations.</p> / Doctor of Philosophy (PhD)

<p>Metal-matrix composites (MMCs) have potential as load bearing components for particular functional requirements. However, MMCs are more difficult to form than conventional metals and alloys due to the tendency of suffering microstructural damage at low plastic strains. It is thus of importance to understand the forms of microstructural damage, its level and spatial distribution. In this work, three main areas of investigation related to damage accumulation in MMCs were considered: the hydrostatic extrusion of MMCs, a theoretical investigation of scale effects in MMCs and plane strain compression experiments on model composites to study load transfer and fibre distribution effects on the patterns of plastic flow in the matrix. In the experiments on the hydrostatic extrusion of particulate MMCs, it was found that damage occurred in the form of reinforcement cracking, that the amount of damage varied linearly with applied plastic strain and that the rate of damage accumulation depended on the morphology of the reinforcement. The extrusion process was examined in regard to its ability to limit or suppress damage through the action of high compressive hydrostatic pressures. A slip-line field analysis was adopted to relate the effect of process parameters such as the amount of reduction and die angle to the magnitude and distribution of hydrostatic pressure in the extrusion dies. The linear relationship between damage and applied plastic strain was explained using a micro-mechanical description of the response of MMCs to plastic flow and the effect of microstructural variables such as volume fraction, shape and size of the reinforcement on the rate of damage accumulation was discussed. In addition, the tensile properties of MMCs subsequent to hydrostatic extrusion were quantified and the effect of previous extrusion was described in terms of the strain hardening of the matrix materials and the damage accumulation in the reinforcement materials and their influence on the stability of the plastic flow process. In the work on plane strain compression of model composites, the effect of reinforcement distribution on the pattern of flow in Cu-W MMCs was described using maps of equivalent strain obtained from the analysis of deformed surface grids. The level and distribution of strain was found to be related to the spatial distribution of the reinforcements. The fiber distribution also influenced the efficiency of the fibers as obstacles to macroscopic flow. Comparison of the results with numerical analyses allowed a link to be established between the observed damage mechanism of decohesion and the state of stress and local strain level in the composite. An optical method was used to get information on the load transfer process in a Cu-sapphire MMC plastically deformed in plane strain compression. The fracture of the sapphire fiber was related to the measured stress state. In the theoretical analysis of scale effects in MMCs, scale dependent bounds on strength were developed and scale dependent composite design charts were constructed. The results highlighted the importance of the scale of the reinforcement in MMC design, as well as the importance of the processing route to fabricate MMCs on a fine scale. The conclusions of the various areas of investigation demonstrated how damage in MMCs links together issues of process design, microstructural optimization and resultant mechanical properties of MMCs.</p> / Doctor of Philosophy (PhD)

<p>The typical microstructure of silicon nitride consists of rigid Si3 N4 grains and intergranular amorphous films associated with the liquid phase sintering process involved during densification. The presence of the amorphous films may affect the creep behaviour of silicon nitride ceramics at elevated temperatures. The advent of high resolution transmission electron microscopy (HRTEM) coupled with fine-probe chemistry analysis enables us to investigate the structure and chemical composition of the nano-scale grain boundary amorphous films and the role they play in creep deformation of silicon nitride. The materials investigated consist of β-Si3 N4 grains with and without secondary crystalline phases. All grains were covered with a thin intergranular amorphous film at both homophase and heterophase boundaries. It was found that these amorphous films have a characteristic value of thickness, independent of grain misorientation, but dependent on the chemical composition of the film and the grains on either side of the film. The creep behaviour of the materials were evaluated by compressive and tensile testing. The grain-boundary film thickness distribution was measured before and after creep using both high-resolution lattice imaging technique and Fresnel fringe imaging technique. The results show a narrow range of film widths in the uncrept material but a bimodal distribution after creep. This provides, for the first time, direct evidence for the occurrence of viscous flow of intergranular amorphous films during creep deformation of silicon nitride. Finally, a model is developed to describe the viscous flow process in multi-phase Si3 N4 materials in contrast to prior models which are only applicable to "pure" Si3 N 4 materials. The creep response predicted by the model is consistent with the experiment.</p> / Doctor of Philosophy (PhD)

<p>Alumina composites with 10, 20 and 30 volume % SiC whiskers were fabricated using colloidal processing methods followed by uniaxial hot pressing. The tensile creep properties of these materials have been studied between 1200°C and 1400°C. The composite slurries showed the best stability at pH = 2, which led to uniform distribution of whiskers in the final products. However, at pH ≥ 6 flocculation occurred between whiskers, resulting in whisker agglomerates in the matrix. Distribution of whiskers was characterised using neutron diffraction methods, which indicated that the whisker orientation could not be altered significantly by adjusting pH. All the composites showed much superior tensile creep resistance compared to pure alumina and the effect of increasing whisker volume fraction was significant up to 30%. Relatively high stress exponents were found, which is most probably associated with much enhanced cavitational creep in tension. The activation energy varied with whisker volume fraction, temperature and applied stress in a complex manner. This combined with the temperature-dependent stress exponents makes the identification of creep mechanisms difficult. Nevertheless, it appears that at moderate stress level grain boundary diffusion and grain boundary sliding (GBS) become more significant as whisker volume fraction increases. The composites containing 20 and 30% whiskers showed significant anelastic strain recovery (∼0.001) following tensile creep, which is consistent with earlier reports that involved bending creep tests. The whisker bending effect was studied by measuring the peak width of (111) SiC planes (perpendicular to the whisker axis) at various conditions. The difference in the peak width at room temperature was found to be insignificant before and after creep. Moreover, during in-situ neutron diffraction measurement at 1400°C, no measurable variation in the peak width was recorded from the crept samples that were cooled under load. It may be that the neutron diffraction technique used in this study is not sufficiently sensitive to measure the small bending strains developed. However, These results along with other evidence in the literature suggest that Herztian contact deformation of networked whiskers, rather than bending deformation of whiskers, may be the dominant mechanism to explain the observed anelastic strain recovery. This mechanism predicts similar strain recovery in any composite that contains constrained 'hard' inclusions with sufficient contact numbers. Models based on this mechanism have been developed, which seem to predict the magnitude of the recoverable strain reasonably well.</p> / Doctor of Philosophy (PhD)

<p>BAZ microstructure stress and hydrogen level are the fundamental factors influencing the cold cracking susceptibility of the HAZ. Implant testing at various hydrogen levels over a range of steel compositions and heat inputs shows how the microstructure and hydrogen level influence the critical stress necessary for cold-cracking.</p> <p>Based on implant test data, a correlation formula predicting the critical stress necessary for cold cracking for given HAZ hardness martensite in the HAZ and hydrogen level, is proposed. Employing this correlation, together with prediction of martensite in the HAZ and HAZ hardness, based on heat transfer caculations, martensite-composition-cooling rate relations and hardness-composition-cooling rate relations, an algorithms which can predict the critical stress necessary for cracking for given implant composition, cooling rate and hydrogen level, is constructed.</p> <p>This method of predicting the critical stress necessary for cold cracking is an improvement over the existing regression formulas for estimating cold-cracking susceptibility.</p> <p>This formula has been successfully adapted to predict cold-cracking susceptibility data as obtained through other tests such as rigid restraint, and to recommend prehear levels necessary to avoid cold-cracking. Development and use of implant testing machine with an automatic welding and loading facility is also reported.</p> / Master of Engineering (ME)

<p>Five aspects of disordered solids and polymers have been studied, as follows. (1) The temperature and pressure modulation effects on structural relaxation have been formulated and simulated. The calculated heat capacity for the modulated conditions shows extra features over that for unmodulated conditions, which may cause misinterpretation of a disordered solid's characteristics. (2) A new mean field approximation in the lattice-hole model is developed for monodispersed polymer chains. Calculations of the configurational entropy, Sconf for polydispersed chains, has led to two predictions, (i) a maximum in the plot of configurational heat capacity against the extent of polymerization, and (ii) S conf remaining positive at 0 K. Both predictions have been verified by others. The lattice occupancy density contribution to S conf has been related to a liquid's viscosity and divergence of the viscosity-pressure plots explained. (3) From calorimetry and x-ray diffraction studies, a new phase of CuCN, which remains metastable on cooling to 77 K, has been discovered. It shows features characteristic of glasses. Sconf of this phase has been, calculated by using a flexible chain model. (4) A calorimetric method for determining the transition from mass-controlled to diffusion-controlled reaction kinetics during polymerization has been developed, and verified by experiments. In this transition range, the plot of the reaction rate against the reciprocal temperature at fixed value of the extent of polymerization deviates from the Arrhenius behavior. (5) Dielectric studies of linear chain polymerization of a melt in real time, and the polymers ultimately formed have shown that their properties depend upon the thermal history. This is attributed to different molecular level structures, e.g., chains and loops formed under different polymerization conditions. Altogether these theoretical and experimental studies have a broader consequence for our current understanding of the nature of disordered solids and of their formation from liquids, both molecular and polymeric.</p> / Doctor of Philosophy (PhD)

<p>The investigation concentrates on the morphology of the oxidation products produced in Ag-1.8, 7.5, 10.9, 12.9 and 18.7 wt % Zn alloys at 550°C under an oxygen pressure of 1 atm. Particular attention is given to the structure of the internal ZnO produced and the morphological development of the internal precipitation zone in the alloys containing Ag-1.8, 7.5 and 10.9 wt % Zn. Penetration depth of the internal ZnO was measured using a light microscope, and was observed to obey the parabolic law. A model based on volume diffusion control of oxygen has been proposed for the velocity of the internal ZnO precipitates.</p> <p>The Ag-12.9 wt % Zn alloy was observed to form an internal ZnO scale in combination with an external ZnO scale. An exclusive protective external scale of ZnO is formed on the Ag-18.7 wt % Zn alloy. The growth of the scale was found to obey parabolic kinetics. The morphological development of the external ZnO on the above alloys has also been studied.</p> <p>The theoretical basis for the transition from internal to external scale formation has been given according to the Wagner(²⁶) criteria for binary alloys.</p> <p>It was necessary to extend the Wagner binary treatment for the transition into a ternary thermodynamic and diffusion analysis. A portion of the Ag-Zn-O ternary isotherm has been proposed, thus making possible the placement of representative diffusion paths.</p> / Master of Engineering (ME)

<p>This thesis consists of two parts. The first describes work on swelling - the second, on creep damage accumulation.</p> <p>The first part develops a model for pressure-driven diffusional dedensification (or swelling) in a polycrystal containing isolated pores. The model is sufficiently general to be directly applicable also to pressure sintering. It is tested against experimental data for hot-pressed alumina (HPA) which swells during high-temperature, pressureless anneals in air.</p> <p>The model includes two independent, coupled, pore growth rate components: (1) diffusional (de)densification, driven by gas pressures in closed pores, and (2) coalescence of pores attached to grain boundaries, driven by grain growth against pore drag. Internal pore pressures may be due to inert trapped gas or to gas generated by chemical reactions.</p> <p>In a poly crystal with inert trapped gas, the trapped gas drives early swelling, but is self-dissipating. Pore coalescence, by progressively releasing capillarity constraint, becomes the dominant cause of swelling at longer times.</p> <p>The model predicts what magnitudes of chemically-generated pore pressure will increase swelling rates beyond those arising from inert trapped gas alone. And, for research on creep damage accumulation, it predicts what precreep anneals are required to dissipate an initial trapped-gas pore pressure, and the range of distinct, predamaged porous microstructures available.</p> <p>The second part of the thesis presents experimental data from flexural and uniaxial tensile creep tests. The data demonstrated that HPA's cavitation damage tolerance and failure strains are high enough that creep fracture tests should be done in uniaxial tension rather than in bending. A high temperature tensile test system, with optical extensometry, was designed, built and tested. The uniaxial tensile data suggest design improvements for testing ductile ceramics.</p> <p>The combined mechanical and microstructural data indicate that, at 1250 C, failures at high stress are controlled by slow crack growth from microstructural heterogeneities. An abrupt transistion at about 200 MPa seperates rapid failures from failures at strains on the order of 0.1. At about 50 MPa, a more gradual transition occurs, to fracture at higher strains, controlled by strain-driven damage. The internal microstructural data show internal creep damage at levels at which cavity coalescence generates macrocracks in HPA.</p> / Doctor of Philosophy (PhD)