M. Ing. (Electronic and Electrical) / Porous silicon has been obtained by anodizing monocrystalline silicon in hydrofluoric acid solutions. The experimental conditions needed to obtain this material are described. A scanning electron microscopy study revealed the morphology of the pores. The diffusion-limited model is the current accepted model explaining the formation of porous silicon. The structure of porous silicon formed on lightly doped n-type silicon does, however, not agree with the morphologies predicted by the basic model. The model was·therefore extended to include the n-type observations. The observed straight pores, the fluctuation of pore length and the double layered structure were succesfully simulated. Visible photoluminescence were obtained from p-type porous silicon. The properties of the emitted light were investigated together with the quantum confinement and siloxene models describing this behaviour of porous silicon. The effect of the anodizing parameters on the electroluminescence visible during the anodic oxidation of porous silicon were investigated leading to additional proof of the quantum confinement model. A model describing the band structure of the silicon-electrolyte system during electroluminescence is proposed.

Magister Scientiae - MSc / South Africa

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Spatial heterogeneity of geologic media leads to uncertainty in predicting both flow and transport in the vadose zone. In this work an efficient and flexible, combined analyticalnumerical Monte Carlo approach is developed for the analysis of steady-state flow and transient transport processes in highly heterogeneous, variably saturated porous media. The approach is also used for the investigation of the validity of linear, first order analytical stochastic models. With the Monte Carlo analysis accurate estimates of the ensemble conductivity, head, velocity, and concentration mean and covariance are obtained; the statistical moments describing displacement of solute plumes, solute breakthrough at a compliance surface, and time of first exceedance of a given solute flux level are analyzed; and the cumulative probability density functions for solute flux across a compliance surface are investigated. The results of the Monte Carlo analysis show that for very heterogeneous flow fields, and particularly in anisotropie soils, The linearized, analytical predictions of soil water tension and soil moisture flux become erroneous. Analytical, linearized Lagrangian transport models also overestimate both the longitudinal and the transverse spreading of the mean solute plume in very heterogeneous soils and in dry soils. A combined analytical-numerical conditional simulation algorithm is developed to estimate the impact of in-situ soil hydraulic measurements on reducing the uncertainty of concentration and solute flux predictions. In soils with large spatial variability and in dry soils, soil water tension measurements significantly reduce the uncertainty in the predicted solute concentration. Saturated hydraulic conductivity data are valuable in relatively wet soils. A combination of tension and saturated hydraulic conductivity data gives the best results, especially if some data are available on the unsaturated hydraulic conductivity function. It is also found that if soil heterogeneity is large, the conditional spatial moments of inertia of the mean concentration plume and the conditional mean breakthrough curves are poor means of depicting the actual solute plume distribution and the actual solute flux. Nevertheless, conditional simulation is one of the most rational approaches for modeling unsaturated flow and transport, if in-situ data are available.

Thesis (M.S.)--University of Nevada, Reno, 2006. / "December, 2006." Includes bibliographical references (leaves 92-94). Online version available on the World Wide Web.

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我們主要研究了多孔體系在乾燥過程中的兩個現象: 一是裂縫行為，二是污染相在孔隙間的擴散特點、機理以及收集污染相、解決污染問题的方法。 / 在本文的第一部分中，我們用共聚焦顯微鏡觀察到了膠體懸浮體系在溶劑揮發時產生裂縫。我們尋找到一個可令裂縫完全消除的辦法：加入乳液到膠體體系後，裂縫的數量減少；當乳液達到一定濃度時，裂縫完全被消除。同時我們也發現加入乳液帶給體系的另一效應：空氣進入體系的速度也發生改變，從而實現了乾燥速度可控。通過用流變儀研究樣品的基本力學性質，我們找出了裂縫數量減少的根本原因。我們的方法可同時有效地控制裂縫數量和體系乾燥速度，因而在許多涉及到多孔體系乾燥或裂縫的工業過程中有潛在的應用。 / 在本文的第二部分中，我們受到石油洩漏污染海灘的啟發，研究了污染相在多孔體系里的擴散特點及機理，並在此基礎上設計了有效的處理方案。我們搭建了一個小尺度的系統來模擬石油滲入沙灘的過程，記錄了隨著海水沖洗次數的增加污染相在多孔體系里的分佈變化。我們發現不論是親水還是疏水的多孔體系，在幾次海水沖刷週期后，污染相都有了明顯的擴散；但是對於不同的體系，污染相呈現出不同的分佈和擴散特點。通過分析小孔尺度上體系內部的壓強分佈，我們很好地解釋了造成這種不同的基本原因。最後我們尋找到兩種可令污染相集中的方法，這對解決實際污染問题有一定的幫助。 / We study two interesting phenomena occurred during the evaporation of solvent in porous medium: first, the cracking behavior; and second, the expanding mechanism and the collecting methods of the non-evaporative phase. / In the first part of this thesis, we visualize the cracking behavior of colloidal suspensions during drying by a confocal microscope. We develop an effective method which can completely eliminate cracking during drying: by adding emulsion droplets into colloidal suspensions, we can systematically decrease the amount of cracking, and eliminate it completely above a critical droplet concentration. We also find another effect that the emulsion droplets can bring: it varies the speed of air invasion and provides a powerful method to adjust drying rate. Besides, we investigate the samples’ fundamental mechanical properties with a rheometer and clarify the underlying physical mechanism for the decreasing of crack amounts. With the effective control over cracking and drying rate, our study may find important applications in many drying and cracking related industrial processes. / In the second part of the thesis, we conduct a study on the expanding mechanism and collecting methods of the non-evaporative phase in porous medium, which is inspired by a practical pollution problem that occurs when oil spills to the sandy beach. We build a system in a smaller scale to mimic the practical pollution and investigate the distribution change of the polluting phase as the flushing cycle increases. We find an obvious expansion of the polluting phase after several flushing cycles in both hydrophilic and hydrophobic porous media, but with different distributions and expanding behaviors. We explained this difference by analyzing the pressure distribution in the system at the pore level. Finally, we develop two methods to concentrate the polluting phase in some particular regions, which is beneficial to collect and solve the practical pollution problem. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Jin, Qiu = Duo kong ti xi lie feng he you ru qin de yan jiu / Jin Qiu. / Thesis (Ph.D.) Chinese University of Hong Kong, 2014. / Includes bibliographical references (leaves 62-67). / Abstracts also in Chinese.

Although extensive literature exists on NMR of water in MCM-41, the origin of a number of NMR spectral features in this material had not been understood. Specifically, the OH proton resonance observed in the dry material disappears completely as it is hydrated to 0.2 mono-layer hydration level. The purpose of this study was to gain insight into the physical basics for these spectral features and in the process broaden our understanding of behaviour/interactions of water molecules in porous material. First, measurements of MAS spectra as a function of temperature and hydration, at very low hydrations, made possible a definitive spectral peak assignment. Second, using 1D and 2D selective inversion recovery and magnetization exchange experiments, as well as MAS and non-MAS techniques, magnetization exchange between the water protons and surface OH group protons was quantified. The present results lead to the conclusion that chemical exchange is not responsible for producing the observed changes in proton spectra in MCM-41 as this material is hydrated up to the 0.2 mono-layer hydration level. This represents an important result as it is at odds with what is assumed in the literature in this connection and means that previous conclusions about hydration dynamics in this material need to be revisited. A dynamics model of water interaction with the surface OH hydration sites was introduced to explain the observed proton spectra. The model can successfully predict the observed chemical shifts and temperature dependent changes of proton spectra in the very low hydration MCM-41.

Thesis (M.S.)--Worcester Polytechnic Institute. / Keywords: electrospinning. Includes bibliographical references (p. 95-96).