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Marine structures must withstand the corrosive effects of salt water in a way that is low cost, reliable, and environmentally friendly. Aluminum satisfies these conditions, and would be a good choice for a sacrificial anode to protect steel structures if it did not passivate. However, various elements can be added to aluminum to prevent this passivation. Currently, Al-Ga alloys are used commercially as sacrificial anodes but their performance is not consistent. In this research, Thermo-Calc software was used to simulate various aspects of the Al-Ga system in an attempt to understand and potentially correct this reliability issue. Simulations showed that gallium segregates to the grain boundaries during solidification and then diffuses back into the grains during cooling to room temperature. Simulations also suggest that faster cooling rates and larger grains cause the potential segregation of gallium at the grain boundaries to remain after cooling. A set of aluminum plus 0.1% weight percent gallium alloy plates were produced with varying cooling rates, along with a control set (cooled slowly in a sand mold). Some samples were later homogenized via annealing. Samples were subjected to a 168 hour long galvanostatic test to assess voltage response. The corrosion performance of samples was found to have both consistent and optimal voltage range when subjected to quick cooling rates followed by annealing. Testing samples at near freezing temperature seems to completely remove optimal corrosion behavior, suggesting that there are multiple causes for the voltage behavior. / Master of Science / Ships must withstand the corrosive effects of salt water in a way that is low cost, reliable, and environmentally friendly. Aluminum has properties which could allow a plate of it to rust instead of a ship it is attached to, thus protecting the ships from rusting. However, because aluminum usually does not rust, gallium can be added to aluminum to allow it to rust. Currently, aluminum-gallium alloys are used commercially to protect ships, but their performance is not consistent. In this research, various aspects of the aluminum-gallium system were simulated in an attempt to understand and potentially correct this reliability issue. Simulations showed that the gallium concentration may not be uniform in the alloy, and various conditions can cause the gallium concentration to be inconsistent. A set of aluminum-gallium alloy plates were cast in molds from liquid aluminum. Some of the plates were cooled quickly, and some cooled slowly. Some samples were later heated in an oven at high temperatures in an attempt to even out the gallium concentration. Samples were subjected to tests to observe corrosion behavior. The corrosion performance of samples was found to be best when subjected to quick cooling rates followed by the oven heating. Testing the samples in cold temperatures seemed to remove the desired corrosion behavior, suggesting that there are multiple reasons for the inconsistent corrosion behavior of aluminum gallium.

Made available in DSpace on 2015-03-26T13:35:20Z (GMT). No. of bitstreams: 1 texto completo.pdf: 1927502 bytes, checksum: 3b216b20861fe585d8cac540c9638d0b (MD5) Previous issue date: 2006-12-21 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In the present work, we study films CdTe electrodeposited galvanostatically from aqueous acidic solution (pH<1) onto n-type monocrystalline silicon wafer (111). The optimum current density to obtain good crystalline quality CdTe films was found to be 0,3 mA/cm2. The films were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and photoacoustic spectroscopy (PAS). The CdTe films were deposited at room temperature or at 85 ºC and covered evenly the substrate surface. The as-prepared films were non-reflecting and presented a grey color which showed dependence with current density and temperature The SEM pictures revealed a granular structure, without indications of faces that evidenced some crystalline arrangement. The XRD patterns of the films deposited at 85 ºC show peaks associated to the CdTe cubic phase. Additionally, peaks corresponding to a hexagonal phase of the elemental cadmium and tellurium were also detected. However, in the XRD patterns of the films which were grown at low temperature, no peaks were associated to CdTe. This film was passed by chemical treatment in saturated solution of CdCl2 followed by thermal annealing at 420 and 450 °C in nitrogen atmosphere. The XRD patterns of the films after annealing showed only signals associated to the cubic phase of CdTe. These results suggest improvement in the crystallinity of the films grown at room temperature and reduction of the amount of present elemental Te e Cd. Change in microstructure due to the annealing with CdCl2 was observed in the SEM pictures, which show completely recrystallized grains. The measurements of optical absorption in the films grown after annealing by PAS, showed an absorption band around the expected band gap energy value of CdTe. Our experimental results also showed that the CdTe films adhered better to unpolished surfaces. / No presente trabalho, estudamos filmes finos de CdTe eletrodepositados galvanostaticamente em meio aquoso ácido (pH<1) sobre silício monocristalino, tipo-n, (111). A densidade de corrente de 0,3 mA/cm2 permitiu a obtenção de filmes de melhor qualidade. Os filmes foram caracterizados por microscopia eletrônica de varredura (SEM), difração de raios-X (XRD) e espectroscopia fotoacústica (PAS). Obtivemos depósitos que cobriram efetivamente o substrato tanto a temperatura ambiente quanto a 85 ºC. Os filmes depositados são cinza escuros e a tonalidade varia com a temperatura de deposição e a densidade de corrente. A SEM revelou nos filmes uma estrutura granular, sem indicações de faces que pudessem evidenciar algum arranjo cristalino. A XRD dos filmes depositados a 85 ºC mostrou picos associados à fase cúbica do CdTe e também picos correspondendo à fase hexagonal do telúrio e do cádmio livres. Porém a XRD dos filmes depositados a temperatura ambiente não mostrou picos que pudessem ser associados ao CdTe. Os filmes foram submetidos a tratamento térmico a 420 e 450 ºC, em atmosfera de nitrogênio, depois de umedecidos com solução saturada de CdCl2. A XRD dos filmes depois do tratamento térmico mostrou picos intensos associados à fase cúbica do CdTe, com orientação preferencial dos grãos na direção (111), tanto nos filmes depositados a temperatura ambiente quanto a 85 ºC. Os resultados mostraram também uma redução na concentração do Te e Cd livres. Micrografias SEM mostraram recristalização da superfície do filme devido ao tratamento térmico. As medidas de absorção óptica por PAS, efetuadas em filmes tratados termicamente, revelaram uma banda de absorção em torno do valor esperado do gap óptico do CdTe. Nossos experimentos mostraram também que os filmes de CdTe aderem melhor em substratos de silício não polidos.

Chromium passivation treatment on tinplate is important in stabilising the active tin surface in terms of oxidation resistance, sulphide stain resistance and ensuring good lacquer adhesion properties. Through this research, the reason was explored why dip passivation treatment, in sodium dichromate, provides superior lacquer adhesion properties on tinplate compared to electrolytic sodium dichromate (CDC) treatments. A critical assessment of the current knowledge of chromium passivation treatments on tinplate formed the basis of the experimental work. Through electrochemical and surface analytical techniques (X-Ray Photoelectron Spectroscopy and Auger Electron Spectroscopy) both dip and CDC treated tinplate surfaces were characterised in terms of surface species. It was shown through XPS, using angle resolved techniques, that no metallic chromium forms on the tinplate surface during CDC treatments. Furthermore, a difference in chromium surface species was established for the dip and CDC processes showing additional SnO and Cr(OH)3.nH2O species for the latter. Complementary to these findings, it was verified that the formation of additional chromium hydroxide species is a function of pH, governed by the applied cathodic current during the CDC process itself. The inferior adhesion properties of CDC treated tinplate were shown to be related to the difference in the respective surface species formed during dip and CDC applications. Copyright 2004, University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria. Please cite as follows: Biermann, MC 2004, A critical assessment of the current understanding of chromium passivation treatments in tinplate, MSc dissertation, University of Pretoria, Pretoria, viewed yymmdd < http://upetd.up.ac.za/thesis/available/etd-04172007-123305 / > / Dissertation (MSc(Applied Sciences))--University of Pretoria, 2007. / Materials Science and Metallurgical Engineering / unrestricted

Lithium-ion batteries are the most are widely used as electrical storage device in various applications such as portable electronics, electric vehicles, Photovoltaic application, telecommunication etc due to the characteristics of the batterie such as high-power density, long cycling and high-power efficiency. Extensive condition monitoring of the battery should be implemented due to the usage of the battery so that there will be an increase in all the overall performance and expectancy. This research is focused on implementing an online condition monitoring on the Li-ion battery using a signal injection through a power converter. The implemented technique in this research is known as the Electrochemical Impedance Spectroscopy (EIS). The EIS is a widely known technique used in determining the internal impedance of a battery cell. The estimated impedance can be used to determine the state of charge (Soc) and State of health (SoH) of a battery. The EIS is used to characterize the electrochemical behaviour thereby monitoring the change in the impedance of the cell of the battery. The EIS technique is accomplished by sinusoidally injecting current at different frequencies and measuring the voltage response. A standard Frequency Response Analyser (FRA) is used as an offline test while the battery is disconnected from the Load. The limitation of this standard FRA analyser is that it is bulky and Expensive. Attempts have been made to migrate the techniques to online operations, each having their own challenges. For an online Implementation, the interfacing power converter is used for Signal injection to measure the impedance of the battery. This work explores the low current ripple advantage of a threelevel boost converter to implement EIS on lithium ion battery.

Capacitive deionization based on electrosorption has become a viable process for brackish water desalination. In this study, activated biochar was employed as low-cost and alternative carbon-based electrodes substituting activated carbon with comparable adsorption capacity. Effects of different activation temperatures of the biochar were studied by physical characterization (i.e. SEM, TEM, elemental analysis, and Raman spectroscopy) and electrochemical characterization (i.e. cyclic voltammetry and galvanostatic charge/discharge measurement) based on the electrical double layer theory. The highest specific capacitance obtained (118.50 F g-1) was from activated biochar electrode treated at 800°C. The removal capacity was investigated by AAS and conductivity measurements. Several limitations associated with them were identified to improve the measurements. The removal capacity of biochar electrode is ~ 2 mg g-1 with significant results for both one-sided and two-sided t-test. In summary, activated biochar can be used as a cheap-alternative electrode material for desalination based on capacitive deionization.

This thesis describes the metallurgical and environmental factors that influence hydrogen embrittlement of super duplex stainless steels and presents a model to predict the rate at which embrittlement occurs. Super duplex stainless steel has an austenite and ferrite microstructure with an average fraction of each phase of approximately 50%. An investigation was carried out on the metallurgical and environmental factors that influence hydrogen embrittlement of super duplex stainless steels. Tensile specimens of super duplex stainless steel were pre-charged with hydrogen for two weeks in 3.5% NaCl solution at 50º C at a range of applied potentials to simulate the conditions that exist when subsea oilfield components are cathodically protected in seawater. The pre-charged specimens were then tested in a slow strain rate tensile test and their susceptibility to hydrogen embrittlement was assessed by the failure time, reduction in cross-sectional area and examination of the fracture surface. The ferrite and austenite in the duplex microstructures were identified by analysing their Cr, Ni, Mo and N contents in an electron microscope, as these elements partition in different concentrations in the two phases. It was shown that hydrogen embrittlement occurred in the ferrite phase, whereas the austenite failed in a ductile manner. An embrittled region existed around the circumference of each fracture surface and the depth of this embrittlement depended on the hydrogen charging time and the potential at which the charging had been carried out. The depth of embrittlement was shown to correlate with the rate of hydrogen diffusion in the alloy, which was measured electrochemically using hydrogen permeation and galvanostatic methods. A two-dimensional diffusion model was used to calculate the hydrogen distribution profiles for each experimental condition and the model could be employed to provide predictions of expected failure times in stressed engineering components.

The thesis deals with a study of the various carbon materials for negative electrode of lithium ion accumulators. The object of the work is to gain the most knowledge about these materials and to project their possible improvements. In the first part of the work is described the latest knowledge about carbon materials and their application. In the following parts of the work is handled about manufacture,optimization of composition of electrode and the choice of carbon material for liquid and gel electrolyte.

<p>In this research project, nanostructured composites based on Tin dioxide (SnO2) and Titanium dioxide (TiO2) with poly-4-styrene sulfonic acid (PSSA) doped polyaniline (PANI) conducting polymer has been investigated based on their structural, electrical and electrochemical properties. The synthesis of conducting polymers and their metal oxide or composites have been carried out chemically or electrochemically according to methods modified from the literature. Layer-by-layer construction of nano-Metal Oxide/PSSA doped polyaniline composites were successfully constructed by electroanalytical methods on the surface of a glassy carbon working electrode (GCE).</p>