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Sinds het commerciële gebruik van nucleaire energie werd het element thorium aanzien als alternatief van uranium voor brandstof applicaties. Thorium-232 zelf is niet splijtbaar maar kan gebruikt worden om uranium-233 te kweken welke splijtbaar is. Thorium is meer aanwezig dan uranium in de aardkorst. Dit maakt thorium een ideale kandidaat voor de productie van nucleaire brandstof. Tijdens de productie van brandstof wordt er thorium afval (ThO2) geproduceerd. ThO2 afval is voornamelijk afkomstig van brandstofpellets die wegens kwaliteit afgekeurd worden en ThO2 poeder afkomstig van het productieproces. De recyclage van ThO2 is echter moeilijk. Om ThO2 in vloeibare vorm te krijgen, moeten zeer ruwe methoden gebruikt worden. De meest beschreven methode in de literatuur is het THOREX proces dat een oplossing van salpeterzuur (HNO3), waterstoffluoride (HF) en aluminium nitraat (Al(NO3)3) gebruikt. Deze oplossing wordt het THOREX reagens genoemd. HF is een zeer corrosief zuur dat metalen of glazen containers aantast. Al(NO3)3 wordt toegevoegd om corrosie door HF te vermijden maar zelfs met Al(NO3)3 wordt nog steeds corrosie waargenomen. Bovendien zorgt de additie van Al(NO3)3 voor een verhoging in geproduceerd afval. Om dit te vermijden wordt er continu onderzoek gedaan naar alternatieven voor het THOREX proces. Het THOREX reagens, trifluormethaansulfonzuur en trifluorazijnzuur werden geëvalueerd in deze masterthesis als oplosmiddel voor ThO2 afval. Om het THOREX proces te evalueren, werden de hoeveelheden van de drie componenten in het reagens gevarieerd. Het werd aangetoond dat hoge concentraties HNO3 resulteerde in een zeer snel proces. Het grootste probleem tijdens dit proces was de vorming van een thorium-bevattend poeder. Dit probleem was echter eenvoudig oplosbaar door Al(NO3)3 toe te voegen aan de oplossing. Voor trifluorazijnzuur werden twee verschillende concentraties getest om te zien of het mogelijk was ThO2 in oplossing te brengen. Er werd geen methode gevonden om al het ThO2 in oplossing te brengen. Rendementen waren lager dan 5 %. Voor de evaluatie van trifluormethaansulfonzuur (CF3SO3H) werden twee verschillende concentraties getest en het was duidelijk dat geconcentreerd CF3SO3H een uitstekend oplosmiddel kan zijn voor ThO2. Een kleine hoeveelheid water bleek essentieel om ThO2 in oplossing te brengen. Om thorium te recupereren van het zure reagens, werd thorium(IV)oxalaat neergeslagen door toevoeging van oxaalzuur. Neerslag van thorium uit een CF3SO3H is in deze thesis voor het eerst beschreven. Het werd aangetoond dat het neerslagen uit zowel het THOREX reagens en CF3SO3H mogelijk is. Voor een kwantitatief proces bleek een lagere zuurconcentratie voordelig. Alle neerslagreacties uit lage concentraties zuur waren kwantitatief. Vier ThO2 poeders werden gemaakt door gebruik te maken van verschillende neerslagomstandigheden en alle poeders hadden een zeer verschillende vorm. Verdere experimenten moeten aantonen welke vorm het best geschikt is om ThO2 pellets te produceren.

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Electrodeposition is a cost-efficient and convenient method to prepare thin film materials on conductive substrates. The film morphology, composition and deposition rate can be easily controlled by changing the deposition conditions such as applied potential, current, temperature, agitation and bath composition. This PhD research investigated the electrodeposition of semiconductor materials: (1) electrodeposition of bismuth-tellurium and bismuth-antimony-tellurium thermoelectric materials from ethylene glycol solutions; and (2) electrodeposition of germanium from ionic liquids.Bismuth telluride and its derived alloys, (Bi1-xSbx)2(Se1-yTey)3 (0 < x,y<1), have the best thermoelectric properties at room temperature. In ethylene glycol, Bi(NO3)3, TeCl4 and SbCl3 have high solubilities up to 1 M which are more than 100 times higher than the solubility of Te(IV) ions in aqueous solutions. The most interesting finding is that the equilibrium alloy deposition can be achieved in chloride-free ethylene glycol solutions. This means the atomic ratio of bismuth to tellurium in the electrodeposited film is the same as the element ratio of [Bi(III)]/[Te(IV)] in the electrolyte. The film composition can be easily adjusted with the composition of the electrolyte. Removing Chloride ions also improves the film morphology and reduces the corrosiveness of the electrolyte. Mirror-like bismuth telluride films could be obtained from the chloride-free solutions. High deposition rate up to -3 A dm-2 was reached. Depending on the film composition, both p-type and n-type materials could be obtained. A maximum Seebeck coefficient of -120 µm V-1 was obtained.Preparation of multilayered Bi2Te3/(Bi1-xSbx)2Te3 thin films were studied for reducing the thermal conductivity and boosting the Seebeck coefficient. It was found that two different forms of antimony formed at different potentials. At lower overpotentials, the electrodeposited antimony is probably in a metastable ''explosive'' form that can rapidly change from amorphous into crystalline, and at higher overpotentials, crystalline antimony is electrodeposited. The deposition of bismuth-antimony and antimony-tellurium alloys followed the regular alloy deposition behavior. In a solution containing all three elements, ternary alloys with different compositions can be obtained and the composition depends on the deposition conditions. More antimony was present in the alloy if higher rotation speeds or higher overpotentials were applied. A pulsed potential was used to get multilayered structures and the layer thickness could be easily controlled by the pulse time. In theory, multilayered structures could reduce the thermal conductivity by phonon scattering at the interfaces of the layers.Electrodeposition of germanium has been performed in aqueous and organic solvents before. But due to the hydrogen evolution on germanium, only ultra thin films could be obtained from aqueous solutions and the current efficiency in organic solvent is extremely low. Ionic liquids are ideal solvents for germanium deposition because water can be easily removed and germanium tetrachloride (GeCl4) has a very high solubility in some ionicliquids. In this research, the germanium compound [GeCl4(BuIm)2], which is much less volatile than GeCl4, was synthesized and used in 1-butyl-1-methylpyrrolidinium dicyanamide ([BMP][DCA]). Black germanium films could be obtained. Only a small amount of oxygen impurity was found which could be due to the oxidation of germanium films in the air. This is a great improvement compared to the films deposited from solutionscontaining GeCl4. In those films, besides oxygen, chlorine and carbon impurities were always observed. However, electropolymerization of [DCA]- anions was observed on the cathode during deposition and the products were poorly conductive. To suppress the polymerization, [BMP]Cl was added and the deposition temperature was increased to 100 degreeC to make sure all [BMP]Cl was dissolved. The current density increase with increasing temperature. Temperatures above 100 degreeC still led to the polymerization of the [DCA]- anions.1-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [BMP][Tf2N] containing GeCl4 was used as electrolyte at elevated temperatures up to 180 degreeC. To prevent the evaporation of GeCl4, a pressure cell was designed and used. A high deposition rate of 6 µm h-1 was found at elevated temperatures due to the improved mass transport of the germanium compound and the kinetics of the reactions. The film morphology had also improved. Instead of black germanium films that always form at room temperature, metallic grey shiny germanium layers were obtained. Unfortunately, oxygen, carbon and chlorine impurities were found in the deposited films.

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Ionic liquids possess some interesting properties for solvent extraction experiments such as a negligible volatility, a low flammability and high structure tuneability. Moreover, their ionic structure and metal complex solvation power is totally different from apolar aliphatic or aromatic solvents. Even though ionic liquids are considered as safer and more environmentally friendly alternatives for traditional organic diluents, they have one main disadvantage which is their slower extraction kinetics due to the higher viscosity and mass transport of this kind of solvents.In the last years, the supply of rare earths and NdFeB magnets has been under a constant pressure due to a cheaper production process of China resulting in a quasi-monopoly and its strong export. Therefore, the recovery of rare-earths from end-of-life materials such as NdFeB magnets becomes strategically very interesting as it reduces the rare-earth supply dependency on China.In the first results part of this PhD dissertation, the basic extractant trihexyl(tetradecyl)phosphonium in combination with chloride and nitrate anions is used to separate some main transition metals from the rare earths present in NdFeB or SmCo magnets. The process is based on a salting-out procedure by using high concentrations of salt or acid in the aqueous phase. The most promising process was tested on a real NdFeB magnet, which was first roasted and leached selectivily to remove the iron. Than, the remaining transition metals were removed by solvent extraction with trihexyl(tetradecyl)phosphonium chloride in the presence of 3.5 M of NH4Cl in the aqueous phase. Afterwards, the rare earths were precipitated by the addition of oxalic acid and calcinated. In this way, a highly pure mixture of the rare-earth oxides was produced which can be used directly as starting material for the production of NdFeB magnets. The processes are operated in that way that they minimize the amount of waste streams and the amount of chemicals consumption. Moreover, the ionic liquid or even aqueous phases are reused to obtain a closed and environmentally friendly process.The second part of this PhD dissertation focuses on the use of the ionic liquid betainium bis(trifluoromethylsulfonyl)imide for the extraction of metals. An innovative process, increasing the reaction and extraction rate by reducing the ionic liquid phase viscosity during the extraction process, is worked out. In this method, called homogeneous liquid-liquid extraction, the aqueous/ionic liquid mixture is heated above its critical temperature, at which one homogeneous phase is formed. Afterwards, the mixture is cooled and two phases are reformed. In this way, mixing and reaction between the metal and the extractants occurs at molecular scale in the homogeneous state, whereas phase and metal separation can be achieved by cooling down and obtaining twonbsp;The ionic liquid betainium bis(trifluoromethylsulfonyl)imide, in combination with trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide and water was used as well for a triphasic extraction system. In this triphasic system, three different metals (Sc(III), Y(III) and Sn(II)) can be separated in one single step, a separation that cannot be achieved when working with the conventional two phases.

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In the nuclear field, research must advance fast enough to keep pace with the industry’s growing demands and new applications. Safety and high performance needs to be maintained for both fuel and reactor. Major progresses have been made in this matter thanks to the development of a new generation of nuclear fuel. A wide variety of fission products (FP) are formed during the fission chain reaction.Therefore, it is important to delve the understanding of the local phenomena at atomic scale, the crystal chemistry and the type of defect that is formed when a foreign element is accommodated in the UO2 matrix. The First perpose of this research is practical, such as, the improvement of the understanding of the UO2 crystal stability. Secondly, there are structural and fundamental purposes, for instance the research on the coordination and valence state of the foreign elements on the UO2 matrix, as well as the modellingof f-electron interactions. The study of the highly similar ThO2 structure and the behaviour of foreign elements in their latter structure, allow us to use more exact characterization techniques. The complex part is that Thorium remains in its 4+valence state, while Uranium canassume a 4+, 5+ and even a 6+ valence state. In order to reach these goals, UO2 and ThO2 will be synthesized so that they contain non-radioactive FP such as Cerium, Europium, Gadolinium, Zirconium.Two synthesis methodes has been choosen solid state that consistin on the mechanical mixing of UO2 and the dopant oxides and sol gel process in solution,followed by a thermal decomposition of the molecular prcursors. The materials will be sintered in various atmospheres (from highly reducing to moderately reducing) at high temperatures (from 1500 °C to 1750 °C). Under these conditions, solid solutions of the impurity element and the UO2 matrix are obtained (either over the entire mixing domain or up to a certain solubility limit). Our primary interest is the study of the distorted host matrix structure near to the dopant. The resulting compounds will be investigated by X-ray diffraction to determine the influence of the dopant on the crystal lattice parameter. The experimental lattice parameters results will be used for the validation of molecular dynamics (MD) calculations. The homogeneity and microstructure of the doped UO2 or ThO2 pellets will be determined by scanning electron microscopy (SEM) and electron microprobe analysis (EPMA). The doped ThO2 will be characterised by optical absorption and luminescence spectroscopy in order to determine the local structure of the dopants ions in the host matrix. The results from the characterization would depend of the two selected synthesis methods: sol-gel and solid-state.