Organic conducting polymer, poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS), is widely recognized for its electro-actuation mechanism and is used in flexible electronics. Its high potential as actuator is based on a strong coupling between chemical, mechanical and electrical properties which directly depends on external stimuli. There is no model today to describe the interplay between moisture absorption, mechanical expansion and electrical stimulus. Elucidating the role of each component in the effective actuation properties is needed to further optimize and tailor such materials. The objective of this thesis is to develop a macroscopic model to describe water sorption kinetics of the PEDOT:PSS film. We used gravimetric analysis of pure PEDOT:PSS film of three different thicknesses to investigate absorption kinetics over a broad range of temperatures and relative humidity. Our results revealed that the moisture uptake of PEDOT:PSS film does not follow Fickian diffusion law due to the retained amount of water after desorption process. We used an existing diffusionreaction model to describe this behavior, and COMSOL Multiphysics and MATLAB software programs to implement it. We observed that the generic model we used in our work could predict polymer behavior with 95% accuracy. However, our model was not able to properly represent the data at very high relative humidity at low temperature, which was attributed to the excessive swelling of the film. Also, we examined a relation between the moisture content of PEDOT:PSS and its mechanical strain and electrical conductivity. The results presented here are the first step towards a general multiphysics electro-thermo-mechanical description of PEDOT:PSS based actuators.

abstract: Thin-film modules of all technologies often suffer from performance degradation over time. Some of the performance changes are reversible and some are not, which makes deployment, testing, and energy-yield prediction more challenging. The most commonly alleged causes of instability in CdTe device, such as “migration of Cu,” have been investigated rigorously over the past fifteen years. As all defects, intrinsic or extrinsic, interact with the electrical potential and free carriers so that charged defects may drift in the electric field and changing ionization state with excess free carriers. Such complexity of interactions in CdTe makes understanding of temporal changes in device performance even more challenging. The goal of the work in this dissertation is, thus, to eliminate the ambiguity between the observed performance changes under stress and their physical root cause by enabling a depth of modeling that takes account of diffusion and drift at the atomistic level coupled to the electronic subsystem responsible for a PV device’s function. The 1D Unified Solver, developed as part of this effort, enables us to analyze PV devices at a greater depth. In this dissertation, the implementation of a drift-diffusion model defect migration simulator, development of an implicit reaction scheme for total mass conservation, and a couple of other numerical schemes to improve the overall flexibility and robustness of this coupled Unified Solver is discussed. Preliminary results on Cu (with or without Cl-treatment) annealing simulations in both single-crystal CdTe wafer and poly-crystalline CdTe devices show promising agreement to experimental findings, providing a new perspective in the research of improving doping concentration hence the open-circuit voltage of CdTe technology. Furthermore, on the reliability side, in agreement of previous experimental reports, simulation results suggest possibility of Cu depletion in short-circuited cells stressed at elevated temperature. The developed solver also successfully demonstrated that mobile donor migration can be used to explain solar cell performance changes under different stress conditions. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2017

This thesis covers the simulation and modelling of the transesterification of triglyceride oils to make biodiesel, using heterogeneous catalysts. Initially, data fitting was performed to fit overall kinetic rate equations to experimental data, ignoring diffusional behaviour. Additionally, experiments were undertaken to investigate the influence of feed ratio on the reaction kinetics. A single site mechanism with surface reaction as the rate limiting step was found to most closely match the experimental conversion profiles for the operating conditions studied. To incorporate diffusional behaviour into the modelling a multicomponent diffusion methodology was adapted for use within this system. To verify transport properties of the system and the suitability of this theoretical diffusion calculation, measurement of density and viscosity for a range of mixtures was undertaken, along with molecular dynamics simulation to produce diffusion coefficients. Finally, a novel algorithm was developed to simulate coupled diffusion and reaction within the pores of the catalyst and the subsequent bulk concentration changes this produced.

An initial-value problem consists of an ordinary differential equation subject to an initial condition. The right-hand side of the differential equation can be interpreted as additively split when it is comprised of the sum of two or more contributing factors. For instance, the right-hand sides of initial-value problems derived from advection-diffusion-reaction equations are comprised of the sum of terms emanating from three distinct physical processes: advection, diffusion, and reaction. In some cases, solutions to initial-value problems can be calculated analytically, but when an analytic solution is unknown or nonexistent, methods of numerical integration are used to calculate solutions. The runtime performance of numerical methods is problem dependent; therefore, one must choose an appropriate numerical method to achieve favourable performance, according to characteristics of the problem. Additive methods of numerical integration apply distinct methods to the distinct contributing factors of an additively split problem. Treating the contributing factors with methods that are known to perform well on them individually has the potential to yield an additive method that outperforms single methods applied to the entire (unsplit) problem. Splittings of the right-hand side can be physics-based, i.e., based on physical characteristics of the problem, such as advection, diffusion, or reaction terms. Splittings can also be based on linearization, called Jacobian splitting in this thesis, where the linearized part of the problem is treated with one method and the rest of the problem is treated with another. A comparison of these splitting techniques is performed by applying a set of additive methods to a test suite of problems. Many common non-additive methods are also included to serve as a performance baseline. To perform this numerical study, a problem-solving environment was developed to evaluate permutations of problems, methods, and their associated parameters. The test suite is comprised of several distinct advection-diffusion-reaction equations that have been chosen to represent a wide range of common problem characteristics. When solving split problems in the test suite, it is found that additive Runge–Kutta methods of orders three, four, and five using Jacobian splitting generally outperform those same methods using physics-based splitting. These results provide evidence that Jacobian splitting is an effective approach when solving such initial-value problems in practice.

In this dissertation, we examine an advection diffusion model for insects inhabiting a spatially heterogeneous environment and moving toward a more favorable environment. We first study the effects of adding a term describing drift or advection toward a favorable environment to diffusion models for population dynamics. The diffusion model is a basic linear two-dimensional diffusion equation describing local dispersal of species. The mathematical advection terms are taken to be Fickian and describe directed movement of the population toward the favorable environment. For this model, the landscape is composed of one homogeneous habitat patch embedded in a spatially heterogeneous environment and the boundary of the habitat inhabited by the population acts as a lethal edge. We also derived the mean occupancy time and the boundary flux of the habitat patch. The diffusion rate and advection parameters of the advection diffusion model are estimated based on mean occupancy time and boundary flux. We then introduce two methods for the identification of these coefficients in the model as well as the capture rate. These two new methods have some advantages over other methods of estimating those parameters, including reduced computational cost and ease of use in the field. We further examine the statistical properties of new methods through simulation, and discuss how mean occupancy time and boundary flux could be estimated in field experiments.

abstract: Semiconductor devices often face reliability issues due to their operational con- ditions causing performance degradation over time. One of the root causes of such degradation is due to point defect dynamics and time dependent changes in their chemical nature. Previously developed Unified Solver was successful in explaining the copper (Cu) metastability issues in cadmium telluride (CdTe) solar cells. The point defect formalism employed there could not be extended to chlorine or arsenic due to numerical instabilities with the dopant chemical reactions. To overcome these shortcomings, an advanced version of the Unified Solver called PVRD-FASP tool was developed. This dissertation presents details about PVRD-FASP tool, the theoretical framework for point defect chemical formalism, challenges faced with numerical al- gorithms, improvements for the user interface, application and/or validation of the tool with carefully chosen simulations, and open source availability of the tool for the scientific community. Treating point defects and charge carriers on an equal footing in the new formalism allows to incorporate chemical reaction rate term as generation-recombination(G-R) term in continuity equation. Due to the stiff differential equations involved, a reaction solver based on forward Euler method with Newton step is proposed in this work. The Jacobian required for Newton step is analytically calculated in an elegant way improving speed, stability and accuracy of the tool. A novel non-linear correction scheme is proposed and implemented to resolve charge conservation issue. The proposed formalism is validated in 0-D with time evolution of free carriers simulation and with doping limits of Cu in CdTe simulation. Excellent agreement of light JV curves calculated with PVRD-FASP and Silvaco Atlas tool for a 1-D CdTe solar cell validates reaction formalism and tool accuracy. A closer match with the Cu SIMS profiles of Cu activated CdTe samples at four different anneal recipes to the simulation results show practical applicability. A 1D simulation of full stack CdTe device with Cu activation at 350C 3min anneal recipe and light JV curve simulation demonstrates the tool capabilities in performing process and device simulations. CdTe device simulation for understanding differences between traps and recombination centers in grain boundaries demonstrate 2D capabilities. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2019

This thesis provides a mathematical framework for the development of efficient control strategies that satisfy the charters of Integrated Pest Management (IPM) which aims to maintain pest population at a low impact level. This mathematical framework is based on a dynamical system approach and comprises the construction of mathematical models, their theoretical study, the development of adequate schemes for numerical solutions and reliable procedures for parameter identification. The first output of this thesis is the construction of trap-insect spatio-temporal models formulated via advection-diffusion-reaction processes. These models were used to simulate numerically trapping to compare with field data. As a result, practical protocols were identified to estimate pest-population size and distribution as well as its dispersal capacity and parameter values related to the attractiveness of the traps. The second major output of this thesis is the prediction of the impact of a specific control method: mating disruption using a female pheromone and trapping. A compartmental model, formulated via a system of ordinary differential equations, was built based on biological and mating behaviour knowledge of the pest. The theoretical analysis of the model yields threshold values for the dosage of the pheromone above which extinction of the population is ensured. The practical relevance of the results obtained in this thesis shows that mathematical modelling is an essential supplement to experiments in optimizing control strategies. / Thesis (PhD)--University of Pretoria, 2016. / Mathematics and Applied Mathematics / PhD / Unrestricted

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Obtaining high magnetic susceptibility phases from Hekimhan&amp / #8211 / Deveci siderite orevia preliminary thermal treatment has been the basic target of the thesis study.Thermal decomposition characteristics of samples, determined bythermogravimetric analysis (TGA), differential thermal analysis (DTA), and differential scanning calorimetry (DSC), were referenced in advancement of thestudy. Heat treatment experiments, particularly roasting, were carried out byconventional heating and microwave heating. Results showed that roasting of Hekimhan&amp / #8211 / Deveci siderite samples could not be achieved by microwave energywhilst conventional heating experiments recorded success. Subsequentlow&amp / #8211 / intensity magnetic separation of roasted samples gave recovery above 90%, where low&amp / #8211 / intensity magnetic separation of run&amp / #8211 / of&amp / #8211 / mine sample had failed. Formation of high magnetic susceptibility phases was verified by magneticsusceptibility balance and x&amp / #8211 / ray diffraction analysis (XRD), on roasted samples. Statistical modeling was applied to determine the optimum conditions of roastingin conventional heating system / based on heating temperature, time of heating, particle size as factors.It was concluded that roasting at T= 560 &ordm / C, for t= 45 minutes was adequate toobtain desired results. Particle size was noted to be not much effective on the process as other factors at the studied size range. Kinetics (E, n) and reaction mechanism for the thermal decomposition in conventional heating system were evaluated with different solid&amp / #8211 / state reaction models by interpretation of the model graphs.Three&amp / #8211 / dimensional diffusion reaction models reported to characterize the thermal decomposition well, with values of activation energy (E), E= 85.53 kJ/mol (Jander) / E= 85.49 kJ/mol, (Ginstling&amp / #8211 / Brounshtein).

Disertacijoje nagrinėjami modeliai, aprašantys dviejų ir trijų reagentų chemines sintezes aukštose temperatūrose. Darbe pristatomos reagentų koncentracijų dinamiką aprašančios diferencialinių lygčių sistemos, sintezės parametrų skaičiavimo metodai bei parametrų paieškos lygiagretinimo algoritmas. Konkretūs skaičiavimai atlikti ir rezultatai pateikti remiantis realių laboratorinių eksperimentų duomenimis itrio aliuminio granato (du reagentai) bei sintetinio kalcio hydroksiapatito (trys reagentai) sintezių atvejais. / The dissertation deals with models describing two- and three-reactant chemical syntheses at high temperatures. The work presents the differential equation systems describing the dynamics of concentrations of reactants, methods for calculation of synthesis parameters, and a parallelization algorithm for faster search of parameters. Concrete calculations were performed and the results presented on the basis of data from real laboratory experiments of syntheses of yttrium aluminum garnet (two reactants) and synthetic calcium hydroxyapatite (three reactants).