Browsing by Subject "Diffusion"

Now showing 1 - 7 of 7
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    Buffer and Barrier Layers for CIGS Based Tandem Photovoltaics
    (2019-02) Bontrager, Timothy
    As the world’s energy consumption increases, a technology for energy production that does not release greenhouse gases has become desirable. Of these technologies, solar devices with an absorber composed of copper, indium, gallium, and selenium (CIGS) have become industrially viable. It is possible to increase the efficiency of a CIGS based device by implementing a so-called “tandem” configuration consisting of a wide bandgap device stacked on top of a narrow bandgap device, but actual demonstration of these devices remains elusive. This work seeks to address two concerns that arise during fabrication of this device. The first of these is the diffusion of cadmium from the buffer layer into the narrow bandgap absorber during top cell deposition. In this work, a diffusion barrier has been shown to be a partially effective mechanism for limiting the damages caused by this diffusion. The procedure for depositing the diffusion barrier and the electrical and chemical effects of the diffusion barrier are discussed. The second limitation to a tandem configuration discussed in this work is the optimization of the band structure between the top-cell wide bandgap absorber and the top-cell buffer layer. This interaction is measured directly, and the effect of varying the buffer layer on device efficiency is examined.
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    Calibration of New Concrete Shrinkage and Creep Model for ACI Committee 209
    (2022-04) Clement, Timothy
    A new semi-empirical concrete shrinkage and creep model is proposed and calibrated in which creep ages according to solidification theory. The impetus for this CRC-funded research is to propose new creep and shrinkage design guidelines for ACI Committee 209; previous guidelines were published in 1982 and are no longer up to date with modern understanding of time-dependent behavior. The new model proposes a coupling between autogenous and drying shrinkage using a volume-average pore relative humidity and treats drying creep as an additional stress-dependent shrinkage, linking together all these phenomena. The proposed expressions are designed to facilitate traditional integral-type analysis, but also uniquely support rate-type calculations that can be leveraged by analysis software. Model calibration uses the Northwestern University (NU) database of creep and shrinkage tests to determine new model parameters. Profile likelihood curves for each individual mix in the database are computed to quantify the sensitivity of the fitting parameters to the available data and to mix design inputs.
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    Feeding the future: the global emergence of school lunch programs.
    (2009-06) Rutledge, Jennifer Geist
    My dissertation is motivated by a puzzle of international social policy and norm emergence and diffusion. Today, children in one hundred and forty-one countries receive free or subsidized school lunches. Yet less than a century ago, no state had a national child nutrition policy. Feeding children was clearly not considered a state responsibility a century ago, why is it considered one today? In addition to analyzing this policy emergence and diffusion, I argue that this policy emergence represents an emergent international norm - a norm that there is public responsibility beyond the family to feed children. Scholars tend to explain policy and norm emergence and diffusion as due to the work of activists, diffusion effects or with world polity theory. However, these explanations tend to focus on either the national or international level as the causal source. Instead, I look at how the national and international levels interact in the creation of policy. In addition, my argument incorporates ideational factors into the field of social policy, which has long focused on material factors to explain policy emergence. I do this by utilizing insights from constructivist international relations theory. Specifically, my argument focuses on the ability of policy entrepreneurs to manipulate certain, internationalized, frames, or ideational cultural structures, within their domestic context in order to produce school lunch programs. The dissertation is structured around the historical development of school lunch programs and traces their progress from their inception as food surplus disposal and military readiness programs, to their current use as a development tool by the international community. After using my global dataset of school lunch programs to assess conventional social policy theories, I develop my argument through in-depth case studies of the US, UK, Canada, India, the UN's World Food Programme, Catholic Relief Services and the New Partnership for African Development. In each case study I focus on the interaction between the different ways the problem of child malnutrition was framed in each context and the political consequences of the emergence of structural agricultural surpluses at the global level.
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    Innovation Adoption: Adopting change management techniques to positively impact new product diffusion
    (2018-05) Kennedy, Christine
    Several decades of innovation diffusion research has utilized varying lenses – from forecasting innovation diffusion to controllable variables such as cost, quality, and marketing. Yet with so much research into the affecting factors, innovation adoption continues to fail at an alarmingly high rate. One factor that could be influencing this failure rate is adopting an innovation requires a behavior change on the part of the consumer and the resistance to change may lead to the resistance to adopt an innovation. In this thesis, the effectiveness of applying change management techniques to promote innovation diffusion is investigated. It is shown that change management techniques used proactively in marketing can positively influence the overall diffusion of a working innovation. This unique research utilizes organizational change management techniques to promote diffusion and provides a pathway for doing so when dealing with diffusion in the market. Seven propositions were derived from combing consumer personality types and organizational change management. The propositions were challenged against successful and failed innovation adoption case studies for verification. Findings show that each of the seven propositions can positively influence innovation diffusion and as such, seven steps for innovation change management were identified as a pathway for managerial use.
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    Nonlinearity and Complexity in Gravel Bed Dynamics
    (University of Minnesota. Institute for Mathematics and Its Applications, 2009-02) Singh, Arvind; Lanzoni, Stefano; Foufoula-Georgiou, Efi
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    Optimization and Sampling using Iterative Algorithms
    (2022-07) Bhattacharya, Riddhiman
    Sampling and optimization are considered as the two key pillars in statistics with applications in many other fields like machine learning, physics, chemistry just to name a few. Although there has been a recent boom in literature for both optimization and sampling algorithms, some key questions on the properties of the aforementioned topics still remain unanswered. In this thesis we study some of these algorithms in detail and address some of the key questions. The common aspect that connects all the algorithms considered by us is that they are iterative in nature. We study the Langevin Monte Carlo (LMC) algorithm with incorrect gradient and establish asymptotic results. We also consider the Multiplicative Stochastic Gradient Descent Algorithm (M-SGD) and establish that the error term is asymptotically Gaussian. Also, we exhibit that the M-SGD algorithm is close to a certain diffusion, irrespective of the weights used, in order of the step size under suitable assumptions. Next, we establish the convergence of the algorithm and a CLT around the optimum in the regime of strong convexity. Lastly, we consider the preconditioned LMC algorithm and exhibit non-asymptotic bounds in the regime of strong convexity.
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    Temperature dependent sulfate transport in aquatic sediments
    (2014-11) DeRocher, Will D.
    Sulfate, released to overlying waters from natural sources and human activity, has the potential to be reduced to sulfide within the anoxic environments of aquatic sediments and negatively impact the growth of aquatic vegetation. Wild Rice is of particular concern within Minnesota as it is both an economic and cultural resource within the state. This study was conducted to characterize the temperature dependence of sulfate transport, via diffusion, between overlying waters and sediment porewaters through the use of laboratory experimentation and mathematical analysis to study the transient response to changes in the overlying water concentration. Two riverine sediments with contrasting organic carbon content from the St. Louis River watershed in northern Minnesota were characterized for their bulk geochemistry and incubated under laboratory conditions to observe the temperature dependence of ion transport between overlying water and sediment porewaters. Two identical sets of laboratory microcosms, incubated under warm and cold conditions, were subjected to a sulfate loading phase in which the overlying water was spiked with sodium sulfate to induce a concentration gradient between the sediment porewaters and overlying water. At the end of the sulfate loading phase, the sulfate gradient was reversed by replacing the overlying water with fresh water, causing sulfate to diffuse out of the sediment, back into the overlying water. During the sulfate recovery phase, sodium bromide was spiked into the overlying water. Bromide, acting as an inert chemical tracer, provided a diffusion-only baseline with which to compare to reactive sulfate. The anion concentrations in the overlying waters were closely monitored to quantify changes in the concentration through the sulfate loading and recovery phases. Non-destructive porewater samples were collected using Rhizon® soil moisture samplers to measure concentrations of sulfate, bromide, ferrous iron, pH, and sulfide at discrete depths in the sediment during key times after changes in surface boundary conditions.Averaged results from both the high and low organic sediments showed sulfate transport occurred 49% faster out of the overlying waters into the sediments at 23°C when compared to 4.5°C. Estimated rates of sulfate reduction at 4.5°C were on average, 40% of those estimated at 23° C. After seven weeks of recovery from the sulfate loading, porewater sulfate concentrations in the warm microcosms had dropped back to ambient levels while slightly elevated sulfate levels were still noticed within the cold microcosm porewater. Even though more sulfate diffused into the warm sediments, the cold sediments retained the sulfate for a significantly longer period of time after the change in boundary layer conditions due to the retarded rates of diffusion and reaction. The longer the sediment is exposed to elevated sulfate levels a greater potential exists for the wild rice seed within the sediment to be exposed to sulfide.