Ce travail s’inscrit dans le programme de recherche sur les réacteurs nucléaires de « génération IV » à caloporteur sodium. Un des verrous technologiques à lever concerne le contrôle de la corrosion des matériaux (matériaux de structure, gaines des combustibles, etc.) par le sodium liquide. Un des paramètres clé est la teneur en oxygène dissous dans le caloporteur. Cette thèse porte sur l’élaboration et la caractérisation de matériaux céramiques de type ThO2 dopé avec Y2O3 pour la réalisation de sondes potentiométriques à oxygène utilisables dans le sodium liquide. Des travaux ont été effectués et des sondes ont été testées par le passé, cependant les sondes présentaient généralement, à l’époque, une absence de fiabilité dans le temps. L’objectif de cette thèse est d’élaborer et de caractériser des électrolytes à base d’oxyde de thorium dopé à l’oxyde d’ytrium à l’aide de techniques de synthèse permettant de contrôler la pureté, la taille des grains, la compacité, etc. Pour mettre en place les protocoles expérimentaux une céramique modèle a été choisie, la cérine dopée à l’oxyde d’yttrium. Les processus de transport ont été étudiés à l’aide de la spectrocopie d’impédance. Une interprétation des phénomènes de blocage de la conduction ionique dans les deux céramiques étudiées en fonction de la pression partielle d’oxygène a pu être proposée. / This PhD thesis was prepared within the framework of the research program on "Generation IV" nuclear reactors with sodium as coolant. One of the main technological problem concerns the control of the corrosion processes of the materials (structural materials, fuel claddings, etc.) by liquid sodium. A key parameter is the dissolved oxygen content in the coolant. This thesis focuses on the development and characterization of ceramic materials based on ThO2 doped with Y2O3 for making potentiometric oxygen sensor used in liquid sodium. Work has been carried out and probes were tested in the past, however, the probes had at the time, a lack of reliability. The objective of this thesis is to develop and characterize electrolytes based on thorium oxide doped with yttrium oxide using specific synthesis techniques to control purity, grain size, compactness, etc. To develop experimental protocols a ceramic model has been chosen, i.e., yttria-doped ceria. Transport processes were studied using the impedance spectroscopy technique. An interpretation of the blocking phenomena of the ionic conduction in both ceramics as a function of the oxygen partial pressure has been given.

Titanium dioxide or titania is applied in paints, in the paper industry, fibbers, cosmetics, sunscreen products, toothpaste, foodstuffs, optical coatings, beam splitters and anti-reflection coatings. It is also used as support catalyst and its use as humidity and high-temperature oxygen sensor is under consideration. These applications are related to its high refractive index, oil absorption, tinting strength and inert chemical properties. Commonly, titania is recovered either by leaching ilmenite with sulphuric acid and subsequently hydrolysing the resulting sulfate solution by boiling. In another process, titanium feedstock is converted into titanium tetrachloride and further oxidised to titanium dioxide. These methods are reportedly time-consuming and environmentally unfriendly. They are also unable to use all existing types of titanium minerals. In this study, a novel process for the extraction of titanium valuables from its minerals is presented. The process entails the roasting of titanium ore with alkaline metal salt. The roasted product is hydrolysed with water and acid, and subsequently reacted with sulphuric acid. Alternatively, the hydrolysed product can be used as feedstock in the chloride process. Roasting at 900°C and using a 2:1 (NaOH:ilmenite) mole ratio proved to be the most efficient in releasing titanium units from its ore. Ternary phases dominate under these conditions. Na0.75Fe0.75Ti0.25O2 was the dominant titanium-bearing phase. NaFeTiO4a ndNa2Fe2Ti3O10 were also present. Whenever the Ti:Fe atom ratio was different from one, the surplus titanium was accommodated in single titanates, mainly Na2TiO3, while iron was accommodated in NaFeO2. In many cases Na8Ti5O14 was also present as a result of Na2TiO3 polymerisation. This is consistent with a fusion period of one hour or more. Shorter fusion periods tended to produce binary phases. Similar results were obtained when lower fusion temperatures were employed, i.e. below 550°C. When anatase reactant was used to resemble an anatase ore, Na2Ti6O3,Na2TiO3, Na8Ti5O14 and Na16Ti10O28 were identified in the products. Optimum recoveries were obtained using a 1:1 NaOH:TiO2 mole ratio, and fusing at 800°C for 2 h. Close to 100% of the titanium was recovered. A one-step leaching process was found to be effective compared with multi-step leaching. The leaching step was found to be dependent on time, solid:liquid ratio and temperature. The optimum conditions for solid:liquid ratio, time and temperature were found to be 0.20, and 15 min at 75°C, respectively. Acidic hydrolysis was controlled by the relative amount of iron and titanium in solution. It was found that less than 1% was dissolved between 3 and 7 in pH units. Higher pH values are recommended, since less acid will be used. Any excess of sulphuric acid in the sulfation step proved to be unnecessary. No significant changes were observed in the amount of dissolved iron and titanium. Therefore the stoichiometric amount can be used in the sulfation process. / Thesis (PhD)--University of Pretoria, 2012. / Chemistry / unrestricted

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Bare, non-pretreated platinum wires and screen-printed platinum electrodes were used as both working and counter electrodes in the measurement of dissolved oxygen using a chronoamperometric method. The oxygen reduction current response in the diffusion state was used for a linear determination of air saturation. We evaluated the two different materials in general for their sensing performance such as conditioning time, accuracy, resolution and stability over 13 h of continuous mid-term measurement. A good performance was found for the wire electrodes in terms of accuracy with a current slope of 1.0–1.6 μA (% as)-1 and a resolution of 10–15 nA (Lowest Level of Detection = 0.1% as), but with an unstable current response result over the course of the measurement. The screen-printed electrodes have a resolution of 10–18 nA (Lowest Level of Detection = 0.6–0.8% as) and an accuracy of 620–660 nA (% as)-1 but they showed promising reproducibility and stability. Both materials require several hours of conditioning in the chronoamperometric method before a stable current response is achieved. For biotechnological applications, the platinum screen printed electrodes were evaluated in typical parameter settings (pH 4.0 and 7.4, salinity 0.1 to 10x phosphate buffered saline and temperature 12 to 42 °C) and showed correlations between the response time and stability and the temperature. No correlations were found between salinity, pH and the current response. In this paper, we present inexpensive electrode materials and a simple to implement chronoamperometric method for reliable direct measurement of dissolved oxygen in aqueous media.

Hydrogen sulfide mediates various physiological functions and along with carbon monoxide and nitric oxide it is an important gaseous signaling molecule. Cellular targets for H2S are proteins, enzymes, transcriptional factors or ion channels. In many cases, the effect of H2S on the regulatory protein is mediated by modifications of its cystein residues. In hemeproteins, the regulation of catalytic activity is induced by formation of the Fe(III)-SH complex or by reduction of the heme iron with subsequent formation of Fe(II)-O2 complex. The effect of Na2S on model sensor heme-containing proteins is presented in this thesis. Protein, isolated from bacterium Anaeromyxobacter sp. strain FW109-5, containing a globine coupled sensor domain and a histidine kinase domain is one of the studied proteins, the second one is protein isolated from bacterium Escherichie coli, containing a globine coupled sensor domain and a diguanylate cyclase domain. The effect of Na2S on both model proteins and their mutants was studied by UV-Vis spectral analysis. Spectra of YddV-HD Y43A were very unique, because thery confirmed formation of a homogenous complex Fe(III)-SH in this protein, whereas only mixtures of varous heme complexes were detected in other tested proteins. Additionally the effect of Na2S on functional domain...