This work is an extension of Yin's work studying the diffusion of Na ion in the PbCl2-NaCl system, but at more dilute compositions of NaCl.

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The specific goal of the investigation was the measurement, as a function of temperature, of the self-diffusion coefficients of Pb210 and Cl36 in PbCl2-KCl compositions in the region of the first compound, and to calculate from these data the activation energy necessary for the diffusion of these ions.

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A new sensor technology based on optical diffraction of visible light shows promise for sensing metal ions and other species that employ chemically-responsive metal oxide and conducting polymer grating elements. These materials undergo reversible redox processes upon interaction with a chemical analyte that subsequently induces changes in the materials refractive index. The two key design parameters of this sensing technique involve preparation of micropatterned sensor elements and the evaluation of appropriate wavelengths for detection of diffracted light. Much of the ability to “tune” a desired sensing response is dictated by the understanding of how factors of size, dimension, crystallinity, morphology, porosity, and heterogeneity influence analyte/sensor interactions (i.e., adsorption, binding, and transport). The effect of composition, structure, and morphology of MoO₃, WO₃, Moₓ W₁₋ₓO₃, IrOₓ and polyaniline grating materials on chemical, electrochemical and optical properties of these systems will be examined by a range of spectroscopic and electrochemical techniques. Comprehensive evaluation and correlation of materials’ optical properties to diffraction-based detection will advance understanding of the capabilities and limitations for the diffraction-based sensing methodology. This information can then used to determine optimal sensing parameters to improve detection limits, enhance sensitivity and increase the dynamic range for detection of model analytes. / text

Reservoir fluids are rarely found in homogeneous structures having homogeneous properties. The various elements and processes of the petroleum system result in complex fluid distributions and compositions. A sound understanding of these complexities can avoid disappointing results and costly mistakes when designing the completion and production of the reservoir. The earlier these complexities are understood in the exploration phase, the better are the chances of a successful decision making process in the design phase of the project. Assessing reservoir compartment connectivity is of paramount importance for a optimal field development. Recent technological advances in wireline formation testing and sampling provide asset teams with a new methodology to evaluate in situ fluid properties and reservoir connectivity. After a review of the technology of downhole fluid analysis (DFA), the currently available methods of modeling equilibrated fluid gradients are presented. Fluid composition equilibrium is a stationary state where all components have reached zero mass flux. A reservoir model is designed to simulate numerically equilibration processes over geologic timescales at isothermal conditions where diffusion and gravity are the active mechanisms. A variety of initial conditions and reservoir fluid types is considered. Non-equilibrium fluid gradients and their transient behavior as they evolve towards fluid composition equilibrium are the main interest of this study. The results are compared in case studies, that are available in published literature. The modeling methods allow modeling of vertical and lateral fluid gradients. After a discussion of the cases, this thesis gives recommendations on 1) what fluid properties should be assessed and 2) how many data points are needed to reduce the chance of misinterpretation of non-equilibrium gradients in the presence of faults. To make best use DFA data, the property that exhibits the largest gradient needs to be investigated, as it yields the greatest potential to assess connectivity. The shape of the distribution of fluid composition within a compartment is found to be an important part in investigating reservoir connectivity. During data acquisition efforts should be made to acquire enough data points to reveal this shape. In combination with the presented techniques to identify non-equilibrium conditions, this will optimize DFA data acquisition and maximize the value of the data.

In the beginning of this study, Molecular dynamics simulation is utilized to investigate the behavior of water molecules confined between two Au plates of (001) planes separated by gaps of 24.48, 16.32, 12.24, 11.22, and 10.20 . The simulation results indicate that the arrangements of the water molecules are dependent on the gap size. An inspection of the variation of the self-diffusion coefficients with the gap size suggests that the difference between the dynamic properties of the water molecules in the z-direction and the x-y plane decreases as the distance between the two Au plates increases. Moreover, we discuss the effects of different lattice structures, (100), (110) and (111)¡Aon the water molecules. The simulation results indicate that the arrangements of the water molecules are dependent on Au plate surface structures. The adsorption of the plate creates flat water layers in the proximity of each plate surface for (100) and (111) cases, but wave-like water layer for Au (110) plate. The absorbed water layer is the most close to plate surface for (110) lattice structure. Moreover, the self-diffusion coefficient in the z-direction for (110) case is the largest, meanwhile, the water molecules have a greater ability to diffuse in the x-y plane for (100) case. Finally¡Athe density distribution, velocity profile, and diffusion coefficients of the water film in a Couette flow are studied. Shear viscosity and its dependence on the shear rate of the water film are also examined in the present research. The diffusion of the whole film increases dramatically as the shear rate greater than a critical value. The shear viscosity decreases as the shear rate increases, especially for the water film with a small thickness, which implies the shear-thinning behavior for viscosity of the nanoconfined film. Moreover, increase in shear viscosity with a decrease in the film thickness can also be found in the present study.

<p>Technological advancements in electronics industry are driven by innovations in device fabrication techniques and development of novel materials. Halide perovskites are one of the latest additions to the semiconductor family. The performance of solid-state devices based on halide perovskites is now competing with other well-established semiconductors like silicon and gallium arsenide. However, the intrinsic instability of three-dimensional (3D) perovskites poses a great challenge in their widespread commercialization. The soft crystal lattice of hybrid halide perovskites facilitates anionic diffusion which impacts material stability, optoelectronic properties, and solid-state device performance.</p> <p>Two-dimensional (2D) halide perovskites with organic capping layers have been used for improving the extrinsic stability as well as suppressing intrinsic anionic diffusion. Nevertheless, a fundamental understanding of the role of compositional tuning, especially the impact of organic cations, in inhibiting anionic diffusion across the perovskite-ligand interface is missing. In our research, we first developed a library of atomically sharp and flat 2D heterostructures between two arbitrarily determined phase-pure halide perovskite single crystals. This platform was then used to perform a systematic investigation of anionic diffusion mechanism and quantify the impact of structural components on anionic inter-diffusion in halide perovskites. </p> <p>Stark differences were observed in anionic diffusion across 2D halide perovskite lateral and vertical heterostructures. Halide inter-diffusion in lateral heterostructures was found to be similar to the classical Fickian diffusion featuring continuous concentration profile evolution. However, vertical heterostructures show a “quantized” layer-by-layer diffusion behavior governed by a local free energy minimum and ion-blocking effects of the organic cations. For both lateral and vertical migrations, halide diffusion was found to be faster in perovskites with larger inorganic layer thickness. The increment becomes less apparent as the inorganic layer thickness increases, akin to the quantum confinement effect observed for band gaps. Furthermore, we found that bulkier and more rigid π-conjugated organic cations inhibit halide inter-diffusion much more effectively compared to short chain aliphatic cations. These results offer significant insights into the mechanism of anionic diffusion in 2D perovskites and provide a new materials platform for heterostructure assembly and device integration.</p>

Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2009. / Please refer to full text for abstract. Abstract contains special characters.

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