Made available in DSpace on 2014-12-17T15:41:54Z (GMT). No. of bitstreams: 1 ElisamaVS_DISSERT.pdf: 3256609 bytes, checksum: 7bb56ee7f0f5f70ae8afd8e24e21825f (MD5) Previous issue date: 2011-07-05 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / In this work, chitosan was used as a coating of pure perlite in order to increase the accessibility of the groups OH- e NH2+the adsorptionof ions Mn2+ e Zn2+.The characterization results of the expanded perlite classified as microporous and whose surface area 3,176 m2 g-1after the change resulted in 4,664 m2g-1.From the thermogravimetry(TG) it was found that the percentage of coating was34,3%.The infrared analysis can prove the presence of groups Si-OH, Si-O e Al-O-Siresulting from the perlite and C=O, NH2and OH characterization of chitosan. The experiments on experiments on the adsorption of Mn and Zn were performed in the concentration range of10 a 50 mgL-1and the adsorption capacity inpH 5,8 e 5,2 was 19,49 and 23,09 mgg-1to 25 oC,respectively.The adsorption data were best fitted to Langmuir adsorption model to Langmuir adsorption model for both metalionsisindicative of monolayer adsorption. The kinetics of adsorption were calculated from the equation of Lagergren fitting the model pseudo-second-order for all initial concentrations, suggesting that adsorption of ions Mn2+ and Zn2+ follows the kinetics of pseudo-second-order and whose constant Speedk2(g/mg.min) are 0,105 e 3,98 and capacity and maximum removal qe 4,326 e 3,348,respectively.In this study we used a square wave voltammetry cathodic stripping voltammetry to quantify the adsorbed ions, and the working electrode glassy carbon, reference electrode silver / silver chloride and a platinum auxiliary electrode. The attainment of the peaks corresponding to ions Mn2+ and Zn2+ was evaluated in and electrochemical cell with a capacity of 30 mL using a buffer system (Na2HPO4/NaH2PO4)at pH 4 and was adjusted with solutionsH3PO4 0,1molL-1and NaOH 0,1 molL-1and addition of the analyte has been a cathodic peak in- 0,873 Vand detection limit of2,55x10-6molL-1para Zn.The dough used for obtaining the adsorption isotherm was 150 mg and reached in 120 min time of equilibrium for both metal ions.The maximum adsorption for 120 min with Mn concentration 20 mgL-1 and Zn 10 mgL-1,was91, 09 e 94, 34%, respectively / Nesse trabalho a quitosana foi utilizada como revestimento da perlita expandida pura com o objetivo de aumentar a acessibilidade dos grupos OH- e NH2+ na adsor??o dos ?ons Mn2+ e Zn2+. A perlita expandida foi classificada como microporoso com uma ?rea superficial de 3,176m2 g-1 e ap?s modifica??o resultou em 4,664m2 g-1. A partir da Termogravimetria (TG) verificou-se que o percentual de revestimento foi de 37,5%. A an?lise de infravermelho comprovou a presen?a dos grupos Si-OH, Si-O e Al-O-Si decorrentes da perlita e C=O, NH2 e OH caracter?sticos da quitosana. Os experimentos na adsor??o de Mn2+ e Zn2+ foram realizados no intervalo de concentra??o de 10 a 50 mg L-1 e acapacidade de adsor??o em pH 5,2 e 5,8 foi de 23,09 e 19,49 mg g-1 a 25 oC, respectivamente. Os dados de adsor??o se ajustaram melhor ao modelo de adsor??o de Langmuir para ambos os c?tions met?licos sendo indicativo de adsor??o f?sica em monocamada.As constantes de velocidade de adsor??o foram calculadas a partir da equa??o Lagergren se ajustando ao modelo de pseudo-segunda-ordem para todas as concentra??es iniciais, sugerindo que a adsor??o dos ?ons Mn2+ e Zn2+ segue a cin?tica de pseudo-segunda-ordem e cujas constantes de velocidade k2(g/mg.min) s?o 0,105 e 3,98 e capacidade m?xima de remo??o qe 4,326 e 3,348 mg g-1, respectivamente. Utilizou-se a voltametria de onda quadrada de redissolu??o cat?dica para a quantifica??o dos ?ons adsorvidos, com o eletrodo de trabalho de carbono v?treo, eletrodo de refer?ncia prata/cloreto de prata e um eletrodo auxiliar de platina. A obten??o dos picos correspondente aos ?ons Mn2+ e Zn2+ foi avaliado em uma c?lula eletroqu?mica com capacidade para 30 mL utilizando um sistema tamp?o (Na2HPO4/NaH2PO4) cuja concentra??o 0,1 mol L-1em pH 4, sendo ajustado com solu??es H3PO4 0,1 mol L-1e NaOH 0,1 mol L-1e adi??o do analito, tendo verificado um pico cat?dico em 0,873 V com limite de detec??o de Mn 7,76x10-7 mol L-1 e um pico an?dico de -1,1 V e limite de detec??o de 2,55x10-6mol L-1 para Zn2+.A massa utilizada para a obten??o das isotermas de adsor??o foi de 150 mg, tendo atingido o tempo de equil?brio de 120 min para ambos ?ons met?licos. A m?xima adsor??o em 120 min para Mn2+ com concentra??o de 20 mg L-1 e Zn 10 mg L-1, foi de 91,09 e 94,34% ,respectivamente

Essa tese de doutorado apresenta o estudo eletroquímico da oxidação do inseticida fipronil bem como o desenvolvimento de metodologias analíticas para a determinação deste composto em amostras de águas naturais. Os experimentos foram realizados sobre os eletrodos de compósito grafite-poliuretana (GPU) e carbono vítreo modificado com nanotubos de carbono de paredes múltiplas (GC-MWCNTs) utilizando a Voltametria de Onda Quadrada (SWV). A Voltametria Cíclica (CV) foi utilizada para diagnosticar o grau de reversibilidade da reação de oxidação do inseticida assim como a natureza do transporte do material eletroativo para a superfície dos eletrodos. Os resultados dos estudos da oxidação eletroquímica do fipronil utilizando o eletrodo de GPU mostraram que a oxidação do fipronil apresenta um em 0,70 V (vs. EAg/AgCl) e ocorre de forma totalmente irreversível e controlada por adsorção das espécies na superfície do eletrodo. Foram obtidas curvas analíticas para o fipronil no intervalo de 2,0 a 14,0 x 10-5 mol L-1, resultando um limite de detecção (LD) de 139 µg L-1 e Limite de Quantificação (LQ) de 480 µg L-1. Para a oxidação do fipronil no eletrodo GC-MWCNTs os resultados mostraram que a oxidação do fipronil apresenta apenas um pico em 0,50 V (vs. EAg/AgCl) e ocorre de forma totalmente irreversível e controlado por difusão a adsorção das espécies na superfície do eletrodo. Os LD e LQ obtidos foram de 26 µg L-1 e 147 µg L-1, respectivamente. O estudo por eletrólise a potencial controlado revelou que a oxidação do inseticida fipronil envolve a participação de um elétron. A metodologia desenvolvida para ambos os eletrodos foi aplicada em amostras de águas naturais, testes de recuperação foram realizados mostrando eficiência de recuperação 96,0% para o eletrodo de GPU e de 94,6% para o eletrodo de GC-MWCNTs. / In this thesis a study of the electrochemical oxidation of the insecticide fipronil and the development of analytical methodologies for the determination of this compound in natural water samples is presented. The experiments were performed at graphite-polyurethane composite electrodes (GPU) and a glassy carbon electrode modified with multi-walled carbon nanotubes (MWCNTs-GC). The electrodes were caracterized using square wave voltammetry (SWV). Cyclic voltammetry (CV) was used to diagnose the degree of reversibility of the oxidation reaction of the insecticide as well as the nature of the transport of the electroactive material to the surface of the electrodes. The electrochemical oxidation of fipronil studies using the GPU electrode showed that fipronil oxidation presents a peak at 0.70 V (vs. EAg/AgCl) which is totally irreversible and controlled by adsorption of species on the electrode surface. Analytical curves were obtained for fipronil in the range 2.0 to 14 x 10-5 mol L-1, with a detection limit (LD) of 139 µg L-1 and a quantification limit (LQ) of 480 µg L-1. For the oxidation of fipronil in the GC-MWCNTs electrode the results showed that fipronil oxidation presents a peak at 0.50 V (vs. EAg/AgCl) which is totally irreversible and diffusion controlled the adsorption of species on the electrode surface. The LD and LQ were obtained 26 µg L-1 and 147 µg L-1, respectively. The study by controlled potential electrolysis showed that oxidation of the insecticide fipronil involves the participation of one electron. The methodology developed for both electrodes was applied to natural water samples, recovery tests were performed showing recovery efficiency of 96.0% for the GPU electrode and 94.6% for MWCNTs-GC electrode.

This thesis describes the theoretical modeling of a response of an electrochemical BioMEMS sensor for detecting small amounts of cortisol hormone. The electrochemical sensor utilizes a catalyst enzyme (3a-HSD) to convert cortisone to cortisol and the Square Wave Voltammetry (SWV) as a preferred method to measure the forward and reverse current of the system. The parameters and equations necessary to estimate the Square Wave Voltammetry (SWV) theoretical response are determined and outlined. The response is modeled and the results are compared to the experimental data. Further, the design of the sensor is analyzed and suggestions are made on how to improve the repeatability of the sensor's response. The diffusion coefficients for cortisone and cortisol hormone are calculated to be 2.87*10-10 and 2.84*10-10 square meters per second respectively with 10 percent tolerance. The dimensionless peak current (ψ) for the system is approximately 10 percent lower than the one theoretically postulated by Bard et al. [3]. The surface area of the working electrode of the sensor varies with and is directly proportional to the concentration of the analyte. Theoretical current peaks are hypothesized to be within 10 percent tolerance limits (mainly due to the reason that the surface area of the working electrode is itself a variable).

Le travail présenté est le fruit de 15 ans de recherche, à compter de l'obtention de ma thèse, sur l'étude et la mise en œuvre de méthodes électrochimiques à échelons de potentiel destinées à l'électroanalyse et à la détermination de mécanismes réactionnels. <br />Dans le manuscrit, sont développés principalement, mes travaux relatifs aux réactions électrochimiques simples en régime transitoire ou stationnaire. Les points originaux et les applications de ces travaux se résument par catégorie comme suit.<br /><br />CONTROLE DU COURANT PAR LA DIFFUSION<br /><br />Electrode plane<br /><br /> Traitement unifié des méthodes mono et multi-impulsionnelles dans le cas des réactions électrochimiques réversibles. La recherche d'équivalents mathémati-ques à des solutions sous forme de séries conduit à des formules d'analyse simples. Ceci est particulièrement important pour la voltamétrie à signaux carrés (SWV) et la voltamétrie impulsionnelle différentielle (DPV) qui sont des techniques analytiques performantes.<br /> Etude de la voltamétrie à escalier de potentiel en mode différentiel. L'expression du courant s'adapte à la voltamétrie à balayage linéaire. <br /> Introduction de la voltamétrie impulsionnelle normale (NPV) en mode différentiel qui supprime l'effet de déplétion sur l'électrode non-renouvelée (goutte de mercure ou électrode solide) et permet l'emploi d'impulsions de courte durée. <br /> Présentation dans un but pédagogique d'un traitement simplifié de la polarographie impulsionnelle différentielle (DPP) basé sur le concept de la couche de diffusion.<br />Electrode sphérique<br /><br /> Traitement des méthodes mono et multi-impulsionnelles sur l'électrode à goutte de mercure pour des réactions réversibles avec ou sans formation d'amalgame. <br /> Etablissement d'une solution pour toute voltamétrie en l'absence d'amalgamation. <br /> Présentation d'une solution avec une correction de sphéricité au deuxième ordre qui s'applique sans restriction à toutes les polarographies mono-impulsionnelles. <br /> Etude de la SWV et de la NPV en cas d'amalgamation.<br /><br />Applications :<br /><br /> Caractérisation de la réversibilité et du nombre d'électrons de la réaction électrochimique.<br /> Détermination des coefficients de diffusion.<br /> Optimisation des paramètres expérimentaux en électroanalyse. <br /> Séparation des réponses interférentes par le choix du motif de polarisation et utilisation des expressions littérales pour la déconvolution des réponses.<br /> Prise en compte de la sphéricité de l'électrode à goutte de mercure en électroanalyse.<br /> Description du courant stationnaire sur microélectrode.<br /><br />CONTROLE DU COURANT PAR LA DIFFUSION ET PAR LE TRANSFERT DE CHARGE<br /><br /> Analyse des courbes irréversibles en DPP, SWV et DPV. L'expression du courant en DPP a été établie dans le cadre du modèle de l'électrode plane. L'effet de la sphéricité de la goutte de mercure est donné par une formule empirique.<br /> Introduction des balayages inverses en SWV.<br /> Analyse de la SWV effectuée avec des balayages dans le sens négatif et dans le sens positif. Pour le courant de pic obtenu avec un balayage négatif (Ox initialement présent en solution) des formules empiriques sont proposées à la fois pour l'électrode plane et sphérique.<br /><br />Applications<br /><br /> Détermination des paramètres cinétiques d'une réaction électrochimique irréversible à partir de la largeur et de la position des pics cathodique et anodique obtenus en SWV lors des balayages négatif et positif.<br /> Optimisation des paramètres expérimentaux en électroanalyse (amplitude des impulsions en DPP et en SWV, rapport des temps de polarisation en DPP et en DPV).<br /><br />COURANT STATIONNAIRE SOUS CONVECTION FORCEE <br /><br /> Analyse des voltammogrammes en SWV ou en DPV sur électrode à disque tournant quelle que soit la vitesse de transfert de charge.<br /><br />Applications<br /><br /> Détermination aisée du coefficient de transfert de charge à partir de la hauteur ou de la largeur des courbes différentielles irréversibles.<br /> Détermination de faibles concentrations (de l'ordre de 10-7 M) avec des électrodes solides. <br /><br /> Mes autres travaux, concernant des réactions électrochimiques avec des complications chimiques et de surface sont brièvement commentés dans le manuscrit.

>Magister Scientiae - MSc / The main aim of this study is to investigate theelectrochemical determination of two beneficial (selenium and zinc) and two toxic (chromium and vanadium) metals in the potable water within the City of Cape Town's distribution area. The Water Laboratory of the City's Scientific Services Branch analyses for these metals in their elemental state, using the Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). This is a standard method used for the detection of trace metals. The most sensitive voltammetric method for determining these metals is by adsorptive stripping voltammetry, using a thin mercury film electrode with a glassy carbon support. This voltammetric method is used for quantitative determination of specific ionic species. Because of mercury's toxicity it is not really favoured for trace metals anymore. Many other possibilities are under investigation, e.g. bismuth-film, modified glassy carbon and antimony electrodes. The boron-doped diamond (BDD) electrode has distinct advantages when used to determine metal concentrations. Advantages of BDD electrodes include lower detection limit, speciation and wider potential window. In this study cyclic voltammetry (CV) was used to determine the copper, cadmium and lead concentrations in potable water by means of square wave voltammetry (SWV) and a bare glassy carbon electrode (GCE). Furthermore, a boron-doped diamond electrode (BDDE) was used to investigate the possibilities of determining selenium, chromium and vanadium by SWV. Real samples (potable water samples) were analysed electrochemically to qualify and quantify these metals and determine whether they comply with the SANS 241:2006 drinking water guidelines.The copper, cadmium, lead and selenium peaks appear very close to the theoretical values, which indicate that these metals can be detected by SWV method, but further analysis with more samples is advised. Even though responses were observed for vanadium and chromium, it was not reliable and requires further investigation. Further studies into the analyses of zinc are also advised.

Future aircraft are likely to employ electrically powered actuators for adjusting flight control surfaces, and other high power transient loads. To meet the peak power demands of aircraft electric loads and to absorb regenerated power, an ultracapacitor-based energy storage system is examined in which a bidirectional dual active bridge DC-DC converter is used. This Thesis deals with the analysis, design, development and performance evaluation of the dual active bridge (DAB) converter, which can act as an interface between the ultracapacitor energy storage bank and the aircraft electrical power network. A steady-state analysis is performed for the DAB converter producing equations for the device RMS and average currents and the peak and RMS currents in the coupling inductor. This analysis focuses on understanding converter current shapes and identifying the zero-voltage switching (ZVS) boundary condition. A converter prototype was designed and built and its operation verified through SABER simulations confirming the accuracy of the analysis. Experimental results are included to support the analysis for 7kW, 20 kHz operating conditions giving a measured efficiency of 90%. To enhance the performance of the converter under light-loads, a quasi-square-wave mode of operation is proposed in which a dead-time is introduced either on the transformer primary voltage, or on the transformer secondary voltage, or simultaneously on both transformer primary and secondary. A similar detailed analysis as that for square-wave operation has been undertaken for all three cases and the converter performance was analysed focusing on ZVS operating range, impact of the RMS/peak inductor currents and converter efficiency. The theoretical work was validated through SABER simulations and proof of concept experimental measurements at 1kW, 20 kHz, which resulted in converter efficiency well above 91%. A 9%-17% improvement in efficiency and a 12%-17% improvement in ZVS operating range over square-wave operation are observed for similar operating conditions. Furthermore, a novel bidirectional current control technique for the DAB converter is presented. A SABER simulation has been performed and the converter operation is validated for square-wave and quasi-square-wave modes under steady-state and transient conditions.

No description available.

By 2030, it is expected that 80% of all electric power will flow through power electronics systems. Wide bandgap power modules that can tolerate higher voltages and currents than silicon-based modules are the most promising solution to reducing the size and weight of power electronics systems. These wide-bandgap power modules constitute powerful building blocks for power electronics systems, and wide bandgap-based converter/power electronics building blocks are envisaged to be widely used in power grids in low- and medium-voltage applications and possibly in high-voltage applications for high-voltage direct current and flexible alternating current transmission systems. One of the merits of wide bandgap devices is that their slew rates and switching frequencies are much higher than silicon-based devices. However, from the insulation side, frequency and slew rate are two of the most critical factors of a voltage pulse, influencing the level of degradation of the insulation systems that are exposed to such voltage pulses. The shorter the rise time, the shorter the lifetime. Furthermore, lifetime dramatically decreases with increasing frequency. Thus, although wide bandgap devices are revolutionizing power electronics, electrical insulating systems are not prepared for such a revolution; without addressing insulation issues, the electronic power revolution will fail due to dramatically increased failure rates of electrification components. In this regard, internal partial discharges (PDs) have the most effect on insulation degradation. Internal PDs which occur in air-filled cavities or voids are localized electrical discharges that only partially bridge the insulation between conductors. Voids in solid or gel dielectrics are challenging to eliminate entirely and may result simply during manufacturing process. The objective of this study is to develop a Finite-Element Analysis (FEA) PD model under fast, repetitive voltage pulses, which has been done for the first time. The model is coded and implemented in COMSOL Multiphysics linked with MATLAB, and its simulation results are validated with experimental tests. Using the model, the influence of different parameters including void shape, void size, and void air pressure on PD parameters are studied. / M.S. / To decarbonize and reduce energy consumption for commercial aviation, the development of lightweight and ultra-efficient all-electric powertrain including electric motors, drives, and associated thermal management systems has been targeted. Using wide bandgap (WBG) power modules that can tolerate high voltages and currents can reduce the size and weight of the drive. However, the operation of WBG-based power converter can endanger the reliability of the electrified systems, most importantly, the insulation system. In this study, it is attempted to model the impact of such threats to the insulation system using numerical models.

No / Electroretinograms (ERGs) elicited by transient, square-wave L- and M-cone isolating stimuli were recorded from human trichromatic (n=19) and dichromatic (n=4) observers. The stimuli were generated on a 4 primary LED stimulator and were equated in terms of cone modulation (cone contrast = 0.11) and retinal illuminance (12,000 trolands). L- and M-cone isolated ERGs had waveforms similar to those observed for luminance responses. However, M-cone ERGs exhibited a phase reversal in their responses to onset and offset stimuli relative to the L-cone responses. This on-off response reversal was observed in trichromats but not dichromats. Simultaneous counter-phase and in-phase combinations of L- and M-cone isolating stimuli generated responses that reflected chromatic and luminance processing, respectively. We conclude that L- and M-cone specific ERGs provide a measure of how photoreceptors contribute to post-receptoral mechanisms.

This thesis deals with electrochemical behaviour of tauroursodeoxycholic acid (TUDCA) at silver solid amalgam electrode modified by mercury meniscus (m-AgSAE), polished silver solid amalgam electrode (p-AgSAE) and hanging mercury dropping electrode (HMDE). This thesis is a part of a bigger scientific research that deals with synthesis and characterization of supramolecular systems based on natural steroid compounds and its conjugates. TUDCA offers one reduction peak at m-AgSAE in the environment of Britton - Robinson buffer in range of pH 6.0 - 13.0. The potential of this peak is around −1200 mV. Using cyclic voltammetry was determined that the process on the electrode surface is quasireversible, the reduction is controlled by diffusion and the anodic process is controlled by adsorption. Concentration dependence measured at HMDE by direct current voltammetry in 0.04 mol∙l-1 borat buffer (pH 9.1) is linear in two concentration intervals - 1∙10-3 - 2∙10-4 mol∙l-1 and 1∙10-4 - 8∙10-6 mol∙l-1 of TUDCA. There was no linear dependence between the increase of concentration of TUDCA and the height of the peak obtained on amalgam electrodes by methods DC, DP, cyclic and "square-wave" voltammetry. On HMDE was in several short concentration intervals measured by a CV method a linear dependence of cathodic and...