Thesis (Ph. D.)--University of Chicago, Dept. of Astronomy and Astrophysics, March, 2001. / Includes bibliographical references. Also available on the Internet.

Abstract This thesis will present an investigation into the variability of the maximum height of the ionospheric F2 layer, hmF2, with hour, season and latitude over the South African region. The dependence of hmF2 on solar and magnetic activity is also investigated. Data from three South African stations, namely Madimbo (22.4 S, 26.5 E), Grahamstown (33.3 S, 26.5 E) and Louisvale (28.5 S, 21.2 E) were used in this study. Initial results indicate that hmF2 shows a larger variability around midnight than during daytime for all the seasons. Monthly median values for hmF2 were used in all cases to illustrate the variability, and the International Reference Ionosphere (IRI) model has been used to investigate hmF2 predictability over South Africa. This research represents the initial steps towards a predictive model for the hmF2 parameter, with the long term aim of developing a new global hmF2 predictive model for the IRI. It is believed that this work will contribute signi cantly towards this aim through the understanding of the hmF2 parameter over a region that has not previously been investigated.

Room temperature magnetoelectric BiFeO3-BaTiO3 superlattices with strong out-of-plane magnetic anisotropy have been prepared by pulsed laser deposition. We show that the out-ofplane magnetization component increases with the increasing number of double layers. Moreover, the magnetoelectric voltage coefficient can be tuned by varying the number of interfaces, reaching a maximum value of 29 V/cmOe for the20×BiFeO3-BaTiO3 superlattice. This enhancement is accompanied by a high degree of perpendicular magnetic anisotropy, making the latter an ideal candidate for the next generation of data storage devices.

Dans ce travail, nous avons décrit mathématiquement les tissus double et triple couches en s’appuyant sur un ensemble des matrices binaires et des équations mathématiques et développé des modules mathématiques programmables, pour générer : 1- Un double tissu lié par chaînes de liage supplémentaire ; 2- Un triple tissu lié par les chaînes de la couche intermédiaires. Ces deux modules mathématiques ont a été vérifiés par un logiciel développé spécifiquement à l’aide de « Visual Basic ». Les deux tissus sont générés automatiquement avec tous les points de liage éventuels. Puis nous avons développé un troisième module mathématique programmé par « Visual Basic » pour classer les points de liage générés. Ce classement se fonde sur la définition de deux termes : l’armure de liage et le taux de liage. Ce module a été vérifié sur un tissu triple couches lié par les chaînes de la couche intermédiaire, dont tous les armures de liage ont été générésautomatiquement par le logiciel développé. Par ailleurs, nous avons développé une procédure pour regrouper les armures de liage suivant la répartition des points de liage dans l’armure du tissu. Pour ce but, nous avons appliqué deux méthodes : la méthode du moment d’inertie des points de liage autour du centre du tissu, tandis que la deuxième méthode est la méthode du plus proche voisin où nous avons calculé l’indice de dispersion des points de liage dans l’armure. L’un des objectifs de ce travail est de comprendre l’action des paramètres de l’étoffe (distribution des points de liage ou armure de liage, armures de différentes couches et le duitage) sur les propriétés mécaniques et physiques d’un tissu triple couche à double liage. Pour cela, nous avons choisi trois tissus, dont l’armure de la couche intermédiaire est variée entre toile, sergé, et cannelé. À l’aide du logiciel créé, nous avons généré les différentes possibilités de liage (armures de liage). Nous avons fixé la densité de liage à (25%), puis nous avons choisi 3 armures de liage ayant une répartition différente. Trois duitages (16, 20, et 24 duites/cm) ont été choisi pour réaliser les tissus. À l’aide de la méthode des plans d’expériences, nous avons limité le nombre de tissus à 9 tissus différents. 15 propriétés ont été évaluées à savoir : l'embuvage, le retrait, la masse surfacique, la perméabilité, la rigidité de cisaillement, la compression: (l'énergie de compression, la résilience, la compressibilité, l’épaisseur), la flexion: (la rigidité de flexion, le module d'Yong), la traction: (laforce de rupture, l'allongement maximal, l'énergie de rupture, le module d'Young). Les résultats des tests ont été analysés par analyse de variance et analyse en composantes principales (ACP) à l'aide du logiciel JMP, afin de calculer l’influence de chaque paramètre sur les propriétés physiques et mécaniques, et d'estimer les coefficients du modèle statistique qui nouspermet d’obtenir les propriétés théoriques des tissus que nous n’avons pas testé. Nous avons constaté que le liage est le paramètre qui a le moins d’influence sur les propriétés des tissus. / Ln this work, we described mathematically the double and triple layers fabrics based on a set of binary matrices and mathematical equations. Mathematical modules developed and programmed by Visual Basic to generate: 1- A double layers fabric stitched by additional warps ; 2- A triple layer fabric stitched by the intermediate layer warps. Both fabrics are automatically generated with all the stitches possible. Then we developed a third mathematical module, programmed by "Visual Basic" to classify all stitches generated. ln addition, we developed a procedure to regroup the stitching weaves according to the distribution of stitches in the weave. For this purpose, we applied two methods : the moment of inertia of stitches around the center of the fabric, and the method of the nearest neighbor. Then, using the developed softwares and the method of design of experiments, we studied the effect of the following parameters (distribution of stitches, the stitching weaves and the weft density) on the mechanical and physical properties of triple layers fabric. Finally, the test results were analyzed by using the JMP software to calculate the influence of each parameter on the physical and mechanical properties, and estimate the coefficients of the statistical model which allows us to obtain the theoretical properties of fabrics we have not tested, We found that the stitches distribution is the parameter that has the least influence on the properties of fabrics.

Lorsqu'un solide est en contact avec un liquide, des phénomènes physico-chimiques conduisent à polariser l'interface. Deux zones de charge, de signe opposé, apparaissent à cette interface, une dans le solide et l'autre dans le liquide, formant ainsi la double couche électrique (DCE). Par rapport à la littérature existante, l’originalité de ce travail est de s’intéresser à la DCE à l’interface acier inoxydable 304L /film passif d’oxyde/ solution de NaCl (0.01M) en couplant des caractérisations électrochimiques, électriques et physiques. Une méthodologie de caractérisation par voie électrochimique en utilisant les méthodes de spectroscopie d’impédance (SIE) et de voltammétrie cyclique (CV) a été mise au point pour accéder à deux paramètres: la capacité effective et la densité surfacique de charge. Des modifications de la concentration de l'électrolyte, du potentiel appliqué et de l'état de surface ont ensuite été réalisées : la capacité effective de la DCE dépend principalement de la concentration et du potentiel et la densité surfacique de charge croît avec la concentration. Dans la gamme étudiée, la rugosité a une faible influence sur la capacité effective mesurée. Des analyses physico-chimiques de la surface ont permis de caractériser les films passifs formés sur l'acier, sans révéler de différences significatives entre les surfaces avant et après immersion. Un autre paramètre caractéristique de la DCE, la densité volumique de charge à la paroi, a été aussi déterminé par la méthode d'électrisation par écoulement du liquide. Les 3 méthodes de caractérisation (SIE, CV et électrisation) confirment l'influence de la concentration sur les caractéristiques de la DCE. / When a metal is immersed in an electrolyte, a charge distribution is created at the interface and a potential is set up across the two phases. The separation between charges give rise to what are known as electric double layers (EDL). Compared to the usual solid/liquid interfaces investigated in the literature, this study is dedicated to the EDL at the stainless steel 304L / passive film / NaCl (0.01M) interface by combining electrochemical, electrical and physical methods. First, a methodology has been set up to characterise the EDL by the electrochemical methods: Electrochemical Impedance Spectroscopy (EIS) and cyclic voltammetry (CV). Both the effective capacity and the surface charge density were evaluated. Then, several parameters were investigated such as the electrolyte concentration, the applied potential and the influence on the EDL of the surface preparation. The results showed that the double layer capacitance depends mainly on the concentration of the electrolyte and on the applied potential. The roughness seems to have a poor influence on the measured capacitance. Various physico-chemical analysis were performed in order to characterise the passive films formed at the surface: no significant difference could have been evidence between the surfaces before and after immersion. Finally, the volume charge density was also determined by the liquid flow electrification measurement. Whatever the characterization techniques used (EIS, CV or flow electrification), the same trend was observed with regard to the influence of the concentration on the characteristics of the EDL.

Electrochemical systems, which rely on coupled phenomena of the chemical change and electricity, have been utilized for development an interface between biological systems and conventional electronics.  The development and detailed understanding of the operation mechanism of such interfaces have a great importance to many fields within life science and conventional electronics. Conducting polymer materials are extensively used as a building block in various applications due to their ability to transduce chemical signal to electrical one and vice versa. The mechanism of the coupling between the mass and charge transfer in electrochemical systems, and particularly in conductive polymer based system, is highly complex and depends on various physical and chemical properties of the materials composing the system of interest. The aims of this thesis have been to study electrochemical systems including conductive polymer based systems and provide knowledge for future development of the devices, which can operate with both chemical and electrical signals. Within the thesis, we studied the operation mechanism of ion bipolar junction transistor (IBJT), which have been previously utilized to modulate delivery of charged molecules. We analysed the different operation modes of IBJT and transition between them on the basis of detailed concentration and potential profiles provided by the model. We also performed investigation of capacitive charging in conductive PEDOT:PSS polymer electrode. We demonstrated that capacitive charging of PEDOT:PSS electrode at the cyclic voltammetry, can be understood within a modified Nernst-Planck-Poisson formalism for two phase system in terms of the coupled ion-electron diffusion and migration without invoking the assumption of any redox reactions. Further, we studied electronic structure and optical properties of a self-doped p-type conducting polymer, which can polymerize itself along the stem of the plants. We performed ab initio calculations for this system in undoped, polaron and bipolaron electronic states. Comparison with experimental data confirmed the formation of undoped or bipolaron states in polymer film depending on applied biases. Finally, we performed simulation of the reduction-oxidation reaction at microband array electrodes. We showed that faradaic current density at microband array electrodes increases due to non-linear mass transport on the microscale compared to the corresponding macroscale systems.  The studied microband array electrode was used for developing a laccase-based microband biosensor. The biosensor revealed improved analytical performance, and was utilized for in situ phenol detection.

Extracellular neuroelectronic interfacing has important applications in the fields of neural prosthetics, biological computation and whole-cell biosensing for drug screening and toxin detection. While the field of neuroelectronic interfacing holds great promise, the recording of high-fidelity signals from extracellular devices has long suffered from the problem of low signal-to-noise ratios and changes in signal shapes due to the presence of highly dispersive dielectric medium in the neuron-microelectrode cleft. This has made it difficult to correlate the extracellularly recorded signals with the intracellular signals recorded using conventional patch-clamp electrophysiology. For bringing about an improvement in the signalto-noise ratio of the signals recorded on the extracellular microelectrodes and to explore strategies for engineering the neuron-electrode interface there exists a need to model, simulate and characterize the cell-sensor interface to better understand the mechanism of signal transduction across the interface. Efforts to date for modeling the neuron-electrode interface have primarily focused on the use of point or area contact linear equivalent circuit models for a description of the interface with an assumption of passive linearity for the dynamics of the interfacial medium in the cell-electrode cleft. In this dissertation, results are presented from a nonlinear dynamic characterization of the neuroelectronic junction based on Volterra-Wiener modeling which showed that the process of signal transduction at the interface may have nonlinear contributions from the interfacial medium. An optimization based study of linear equivalent circuit models for representing signals recorded at the neuron-electrode interface subsequently iv proved conclusively that the process of signal transduction across the interface is indeed nonlinear. Following this a theoretical framework for the extraction of the complex nonlinear material parameters of the interfacial medium like the dielectric permittivity, conductivity and diffusivity tensors based on dynamic nonlinear Volterra-Wiener modeling was developed. Within this framework, the use of Gaussian bandlimited white noise for nonlinear impedance spectroscopy was shown to offer considerable advantages over the use of sinusoidal inputs for nonlinear harmonic analysis currently employed in impedance characterization of nonlinear electrochemical systems. Signal transduction at the neuron-microelectrode interface is mediated by the interfacial medium confined to a thin cleft with thickness on the scale of 20-110 nm giving rise to Knudsen numbers (ratio of mean free path to characteristic system length) in the range of 0.015 and 0.003 for ionic electrodiffusion. At these Knudsen numbers, the continuum assumptions made in the use of Poisson-Nernst-Planck system of equations for modeling ionic electrodiffusion are not valid. Therefore, a lattice Boltzmann method (LBM) based multiphysics solver suitable for modeling ionic electrodiffusion at the mesoscale neuron-microelectrode interface was developed. Additionally, a molecular speed dependent relaxation time was proposed for use in the lattice Boltzmann equation. Such a relaxation time holds promise for enhancing the numerical stability of lattice Boltzmann algorithms as it helped recover a physically correct description of microscopic phenomena related to particle collisions governed by their local density on the lattice. Next, using this multiphysics solver simulations were carried out for the charge relaxation dynamics of an electrolytic nanocapacitor with the intention of ultimately employing it for a simulation of the capacitive coupling between the neuron and the v planar microelectrode on a microelectrode array (MEA). Simulations of the charge relaxation dynamics for a step potential applied at t = 0 to the capacitor electrodes were carried out for varying conditions of electric double layer (EDL) overlap, solvent viscosity, electrode spacing and ratio of cation to anion diffusivity. For a large EDL overlap, an anomalous plasma-like collective behavior of oscillating ions at a frequency much lower than the plasma frequency of the electrolyte was observed and as such it appears to be purely an effect of nanoscale confinement. Results from these simulations are then discussed in the context of the dynamics of the interfacial medium in the neuron-microelectrode cleft. In conclusion, a synergistic approach to engineering the neuron-microelectrode interface is outlined through a use of the nonlinear dynamic modeling, simulation and characterization tools developed as part of this dissertation research.