Dissertations / Theses on the topic 'Sesquioxides'

The work reported in this thesis systematically investigates the structural, electronic and/or magnetic properties of fcc AxCs3-xC60 fullerides (A = K and Rb) and of sesquioxide Cs4O6 under ambient and non-ambient temperature and pressure conditions, employing X-ray and neutron powder diffraction, muon spin relaxation, and SQUID magnetometry. In the AxCs3-xC60 alkali fullerides, unconventional superconductivity can emerge through tuning of intermolecular distances. While conventional BCS-like response is found in the underexpanded regime of the electronic phase diagram, significant deviations appear in the overexpanded region adjacent to the Mott boundary where strong electron correlations are prominent. In this work, the solid-state synthesis of fcc KxCs3−xC60 (0.12 ≤ x ≤ 2) superconducting materials, with intermolecular distances controlled via adjusting the K+/Cs+ dopant ratio, is reported. Whilst the structural, electronic and magnetic properties of fcc KxCs3−xC60 are reminiscent of those previously reported for fcc RbxCs3−xC60, i.e. the Mott boundary can be shifted to ambient pressure and the metal-insulator crossover temperature can be tuned by such chemical pressurisation of Cs3C60, cation-specific effects are of importance on the electronic properties of fcc AxCs3−xC60. This work also provides strong evidence for correlated behaviour in the overexpanded regime through examining the presence or not of the Hebel-Slichter coherence peak and extracting the superconducting gap magnitude. Crystallographically, the alkali sesquioxides A4O6 (A = Rb, Cs), at high and low temperature, had been reported to adopt a cubic structure which generates a single crystallographic position for dioxygen, implying charge disorder: A4(O2(4/3)-)3. Conversely, spectroscopy studies at 5 K found evidence for two localized valence states of dioxygen, indicating charge ordering: (A+)4(O2−)2(O22−). This issue is addressed with the first systematic investigation of the temperature- and cooling-protocol-dependent structural evolution of Cs4O6, revealing the existence of a valence-ordered state at low temperature and also that valence disorder-order transitions can be induced by temperature, pressure and X-ray illumination.

In dieser Arbeit werden die grundlegenden Wachstumsprozesse von Ga2O3 , mittels Transmissionselektronenmikroskopie analysiert. Dazu gehört die Untersuchung des heteroepitaktischen Wachstums von Galliumoxidschichten welche mittels Molekularstrahlepitaxie (molekular beam epitaxy MBE), der gepulsten Laser Abscheidung (pulsed laser deposition (PLD)) und der metallorganischen Gasphasenepitaxie (metalorganic vapor phase epitaxy (MOVPE)) auf (0001) orientierte Saphir Substraten abgeschieden wurden. Heteroepitaktisches Wachstum von Ga2O3 auf Saphir (0001) erfolgt bis zu einer Dicke von 3 Monolagen pseudomorph als α-Ga2O3 welches durch die Gitterfehlpassung zwischen Galliumoxid und dem Saphire Substrat induzierte Gitterverspannung stabilisiert wir. Weiterhin, im Fokus der Arbeit stehend, wird das homoepitaktische Wachstum von Galliumoxid auf (100) orientierten Galliumoxidsubstraten untersucht. Neben den Besonderheiten des Schichtwachstums, die sich aus den eingesetzten metallorganischen Präkursoren und Sauerstoffquellen ergeben, wird die Schichtstruktur in Abhängigkeit der typischen Wachstumsparameter (Wachstumstemperatur, Wachstumsrate, Kammerdruck und Fehlorientierung des Substrates) analysiert. Dabei wird gezeigt das homoepitaktischen Wachstum auf (100) orientiertem, β-Ga2O3, mittels MOVPE, die kristalline Perfektion der gewachsenen Schichten stark von den verwendeten Präkursoren (Trimethylgallium (TMGa) und Triethylgallium (TEGa) als metallorganische Ausgangsstoffe und H2 O oder purer Sauerstoff als Oxidant) und den chemischen Prozessen an der Oberfläche bestimmt wird. Des Weiteren wird die Entstehung von Zwillingslammelen in abhähngigkeit der Fehlorientierung untersucht. Durch die Einführung von vorbestimmten Fehlorientierungswinkeln der Substrate ist es möglich das Entstehen der Zwillingslamellen zu verhindern, und ein Stufenflusswachstum zu generieren. Durch die Anwendung eines Ratengleichungsansatzes ist es möglich die experimentell beobachteten Dichten an Zwillingslamellen zu erklären und einen Diffusionskoeffizienten zu bestimmen. (i) Heteroepitaktisches Wachstum von Ga2O3 auf Saphir (0001) erfolgt bis zu einer Dicke von 3 Monolagen pseudomorph als alpha-Ga2O3. Oberhalb dieser Schicht wächst relaxiertes ß-Ga2O3 in Form von 3 Rotationsdomänen auf. Die Stabilisation der dünnen alpha-Ga2O3 Schicht wird, durch die Gitterfehlpassung zwischen Galliumoxid und dem Saphire Substrat induzierte Gitterverspannung bewirkt. (ii) Beim homoepitaktischen Wachstum auf (100) orientiertem ß-Ga2O3 mittels MOVPE wird die kristalline Perfektion der gewachsenen Schichten stark von den verwendeten Präkursoren (Trimethylgallium (TMGa) und Triethylgallium (TEGa) als metallorganische Ausgangsstoffe und H2O oder purer Sauerstoff als Oxidant) und den chemischen Prozessen an der Oberfläche bestimmt. Während beim Wachstum mittels TMGa und O2 vorwiegend polykristalline Schichten entstehen, ergeben sich beim Wachstum mittels TMGa und H2O sowie TEGa und O2 geschlossenen epitaktische Schichten. Dieser signifikante Unterschiede lässt sich durch die unterschiedlichen Reaktionswege der Ausgangsstoffe sowie durch die katalytische Wirkung der (100) Flächen des ß-Ga2O3 erklären. (iii) Die Perfektion, mittels MOVPE gewachsener, homoepitaktischer Schichten, auf (100) orientierten Substraten, ist stark von der Fehlorientierung des Substrates bestimmt. Schichten die auf Substraten mit geringen Fehlorientierungen abgeschieden werden (< 2° bei Wachstumstemperaturen < 850°C) sind durch eine hohe Dichte an Zwillingslamellen gekennzeichnet. Die Entstehung der Zwillingslamellen ist ein Resultat eines Doppelpositionierungsprozesses der Atome auf der Oberfläche der Wachstumsebene. Durch die Einführung von vorbestimmten Fehlorientierungswinkeln der Substrate ist es möglich das Entstehen der Zwillingslamellen zu verhindern, und ein Stufenflusswachstum zu generieren. Durch die Anwendung eines Ratengleichungsansatzes, welcher die konkurrierenden Prozesse des Einbaus von Atomen in Oberflächenstufen sowie der Nukleation und des Wachstum von zweidimensionalen Inseln beschreibt, ist es möglich die experimentell beobachteten Dichten an Zwillingslamellen zu erklären und einen Diffusionskoeffizienten zu bestimmen, Dieser ist um zwei Größenordnungen geringer als bei klassischen Halbleitern, wie z. B. GaAs. In dieser Arbeit werden die grundlegenden Wachstumsprozesse von Ga2O3 , mittels Transmissionselektronenmikroskopie analysiert. Dazu gehört die Untersuchung des heteroepitaktischen Wachstums von Galliumoxidschichten welche mittels Molekularstrahlepitaxie (molekular beam epitaxy MBE), der gepulsten Laser Abscheidung (pulsed laser deposition (PLD)) und der metallorganischen Gasphasenepitaxie (metalorganic vapor phase epitaxy (MOVPE)) auf (0001) orientierte Saphir Substraten abgeschieden wurden. Heteroepitaktisches Wachstum von Ga2O3 auf Saphir (0001) erfolgt bis zu einer Dicke von 3 Monolagen pseudomorph als α-Ga2O3 welches durch die Gitterfehlpassung zwischen Galliumoxid und dem Saphire Substrat induzierte Gitterverspannung stabilisiert wir. Weiterhin, im Fokus der Arbeit stehend, wird das homoepitaktische Wachstum von Galliumoxid auf (100) orientierten Galliumoxidsubstraten untersucht. Neben den Besonderheiten des Schichtwachstums, die sich aus den eingesetzten metallorganischen Präkursoren und Sauerstoffquellen ergeben, wird die Schichtstruktur in Abhängigkeit der typischen Wachstumsparameter (Wachstumstemperatur, Wachstumsrate, Kammerdruck und Fehlorientierung des Substrates) analysiert. Dabei wird gezeigt das homoepitaktischen Wachstum auf (100) orientiertem, β-Ga2O3, mittels MOVPE, die kristalline Perfektion der gewachsenen Schichten stark von den verwendeten Präkursoren (Trimethylgallium (TMGa) und Triethylgallium (TEGa) als metallorganische Ausgangsstoffe und H2 O oder purer Sauerstoff als Oxidant) und den chemischen Prozessen an der Oberfläche bestimmt wird. Des Weiteren wird die Entstehung von Zwillingslammelen in abhähngigkeit der Fehlorientierung untersucht. Durch die Einführung von vorbestimmten Fehlorientierungswinkeln der Substrate ist es möglich das Entstehen der Zwillingslamellen zu verhindern, und ein Stufenflusswachstum zu generieren. Durch die Anwendung eines Ratengleichungsansatzes ist es möglich die experimentell beobachteten Dichten an Zwillingslamellen zu erklären und einen Diffusionskoeffizienten zu bestimmen. Des Weiteren wird das Wachstum im Mischsystem (InxGa1-x)2O3 untersucht und gezeigt das Indium als grenzflächenaktive Substanz wirken kann.
In this work we study the basic growth processes of epitaxial Ga2O3 films, by means of transmission electron microscopy. We investigate the heteroepitaxial growth of thin layers Ga2O3 on the (0001) plane of sapphire grown by molecular beam epitaxy (MBE), pulsed laser deposition (PLD) and metal organic vapor phase epitaxy (MOVPE). Furthermore, we will focus on the homoepitaxial growth on the (100) plane by MOVPE. Beside the peculiarities of the layer growth dependence on different metal organic precursors and oxygen sources, we investigate the influence of typical growth parameters (i.e. growth temperature, growth-rate, chamber pressure and miscut angle of the substrate) on the layer morphology. Incase of heteroepitaxial growth of β-Ga2O3 on (0001) plane of sapphire, independent of the growth method, the formation of a 3 monolayer thick α-Ga2O3 layer is observed, which is stabilized through strain, as a result of the lattice mismatch between sapphire and α-Ga2O3. In case of homoepitaxial growth by MOVPE on (100) oriented β-Ga2O3, the crystalline quality of the grown layer strongly depends on the used precursor (tri-methyl-gallium (TMGa) or tri-ethyl-gallium (TEGa) as metal precursor and H2O or pure oxygen as oxidant) and the chemical processes on the surface, respectively. Further on is the crystalline perfection of homoepitaxial layers grown by MOVPE on (100) oriented β-Ga2O3 substrates strongly dependent on the miscut-angle of the substrates. Layer grown on substrate with a small miscut-angle (< 2°) show high amount of twin lamella. These twin lamella are a result of a possible double positioning mechanism of ad-atoms on the growth surface. By introducing appropriate miscut-angles of the substrate it is possible to suppress the formation of these twin lamellae, and enable step flow growth. By applying a rate equation approach, describing the competing processes of incorporation of ad-atoms at kink sites or nucleation and growth of two dimensional island, it is possible to quantitatively reproduce the experimentally observed twin lamella densities and to determine a surface diffusion coefficient of the ad-atoms. Furthermore, in case of the alloy system (InxGa1-x)2O3, it is shown that indium can act as an surfactant, by increasing the surface diffusion.

Yb3+-Lasermedien glänzen mit hoher Effizienz und relativ geringer thermischer Last, besonders in Laseroszillatoren und -verstärkern mittlerer bis hoher Leistung. Modenkopplung von Yb3+-Lasersystemen ermöglicht Subpikosekunden-Pulse bei hoher mittlerer Ausgangsleistung. Diese Arbeit widmet sich zwei Gruppen der vielversprechendsten neuen Yb3+-aktivierten Laserkristalle: den strukturell analogen, monoklinen Doppelwolframaten Yb:KGd(WO4)2 (Yb:KGW), Yb:KY(WO4)2 (Yb:KYW) und KYb(WO4)2 (KYbW) und den Yb3+-dotierten Sesquioxiden, vertreten durch Yb:Sc2O3 (Yb:Skandia). Die spektroskopischen Daten von KYbW, darunter eine extrem kurze 1/e-Absorptionslänge von 13 Mikrometern bei 981 nm, wurden im Rahmen der Dissertation vermessen. Die Lasereigenschaften niedrig Yb3+-dotierter Wolframate im Dauerstrich (cw)- und modengekoppelten Betrieb wurden in Lasern moderater Ausgangsleistung untersucht. Ultrakurzpuls-Erzeugung mit Yb:KYW, Yb:KGW und Yb:Glas wurde in einem passiv modengekoppelten Laser verglichen. Dabei wurde eine relativ hohe Lasereffizienz erreicht, Dank einer Trapezlaserdiode als Pumpquelle mit exzellenter Strahlqualität. Quasi-cw- und cw-Laserbetrieb von Yb3+ in hochdotierten und stöchiometrischen Wolframatkristallen wurden untersucht. Diese Materialien sind interessant für Mikrochip- und Scheibenlaser. Wichtige Fragestellungen waren die Kristallqualität und die Hitzeentwicklung bei hohen Yb3+-Konzentrationen. Erstmals wurde Lasertätigkeit von Wolframaten mit Yb3+-Konzentrationen >>20% und schließlich, mit KYbW, cw-Laserbetrieb eines stöchiometrischen Yb-Lasermaterials demonstriert. Weiterhin wurde mit KYbW der kleinste bisher für einen Laserkristall gemessene Laserquantendefekt erzielt, 1,6%. Unter Benutzung eines Yb:Sc2O3-Lasermediums wurde erstmals modengekoppelter Betrieb eines Oszillators mit Sequioxid-Laserkristall gezeigt. Betrieb mit nichtsolitonen- und solitonenartiger Pulsformung sowie mit Ti:Saphir-Laser oder Trapezlaserdiode als Pumplaser wurden untersucht. Mit einem Ti:Saphir-gepumpten Yb:Skandia-Laser wurde eine Konversionseffizienz von 47 % bezogen auf die absorbierte Pumpleistung erreicht, der bisher höchste Wert für einen modengekoppelten Yb3+-basierten Laser.
Yb3+ laser media excel with high efficiency and relatively low heat load, especially in medium to high power laser oscillators and amplifiers. Mode-locking of Yb3+ laser systems can provide subpicosecond pulse durations at high average power. This work deals with two groups of the most promising novel Yb3+-activated laser crystals: Yb3+-activated monoclinic double tungstates, namely the isostructural crystals Yb:KGd(WO4)2 (Yb:KGW), Yb:KY(WO4)2 (Yb:KYW), and KYb(WO4)2 (KYbW), and Yb3+-doped sesquioxides, represented by Yb:Sc2O3 (Yb:scandia). Spectroscopic data of KYbW were investigated as part of this thesis, finding an extremely short 1/e-absorption length of 13 micrometers at 981 nm. Continuous-wave (cw) and mode-locked laser performance of moderate-average-power lasers based on lowly Yb3+-doped tungstates were examined. Ultrashort pulse generation with Yb:KYW, Yb:KGW, and Yb:glass was compared in a passively mode-locked laser. A relatively high mode-locked laser efficiency was achieved due to a tapered diode pump laser with excellent beam quality. Quasi-cw and cw lasing of Yb3+ in highly doped and stoichiometric tungstate crystals were investigated. These materials are interesting for microchip and thin-disk lasers. Important issues were crystal quality and heat generation at high Yb3+ concentrations. For the first time, laser operation of tungstates with a Yb3+ concentration >>20% and finally, with KYbW, cw lasing of a stoichiometric Yb laser material was achieved. Furthermore, with KYbW, the smallest laser quantum defect ever for a laser crystal was demonstrated, 1.6%. Using a Yb:Sc2O3 laser medium, for the first time mode locking of an oscillator using a sesquioxide laser crystal was realized. Laser regimes with non-solitonlike and solitonlike pulse shaping were investigated, using a Ti:Sapphire laser and a tapered laser diode as pump sources. With a Ti:Sapphire-laser-pumped Yb:scandia laser the highest conversion efficiency with respect to absorbed pump power for any mode-locked Yb3+-based laser was achieved, 47%.

The use of molecular beam epitaxy as a method for producing solid state host crystals for planar waveguide lasers has been investigated. Single crystal yttrium sesquioxide with a very high degree of structural order has been grown on R-plane sapphire substrates. The (01Ῑ2) Al₂O₃ substrates were annealed in air at 1150°C to generate atomically smooth surfaces with parallel atomic steps. This process was important for maximizing structural quality and minimizing surface roughness of the grown Y₂O₃ film. A critical-thickness-like phenomenon was discovered, where the Y₂O₃ would grow in regions with near structural perfection at the beginning of growth. In thicker films, the x-ray diffraction peaks became wider, indicating less crystalline uniformity. The maximum equivalent “critical thickness” achieved was 7 nm for a film grown at 800°C with a growth rate of 20 nm/hr. The highly ordered material may be present in one uniform layer or distributed in smaller regions throughout the thin film. Y₂O₃ films on Al₂O₃ were annealed in air at temperatures up to 1400°C to study interdiffusion. By analyzing x-ray diffraction measurements, we found that Al migrated from the substrate into the Y₂O₃ film with an approximate activation energy for bulk diffusion of 3.0 eV. Diffusion on the Y₂O₃ surface was estimated to have an activation energy of (0.5 ± 0.3) eV from atomic force microscopy images. After annealing, the presence of Y₄Al₂O₉, YAlO₃, and Y₃Al₅O₁₂ phases was confirmed using x-ray diffraction and photoluminescence measurements. Attempts were made to use molecular hydrogen gas and gallium as surfactants during growth to improve film properties. No conclusive benefit was observed. Y₂O₃ film surface roughness was observed to increase roughly proportionally to the square root of film thickness. A 600 nm thick waveguide layer grown under optimal conditions had a root-mean-square roughness of 5.8 nm. This level of roughness could cause scattering loss at the waveguide core-cladding interface that is problematic for practical applications.

The sesquioxides Y₂O₃, Lu₂O₃ and Sc₂O₃ have been identified as promising laser host materials due to their excellent thermo-mechanical properties and ability to be doped with rare-earth ions. However, crystals of these materials are problematic to grow due to their high melting points of ~2500°C, which means standard methods of crystal growth from a melt need to be modified to deal with such high temperatures and become more expensive. Pulsed laser deposition (PLD) has the capability to grow these materials, without requiring a melt of material to grow from. Sintered ceramic targets of the sesquioxides, often doped with rare-earth ions, are ablated with a UV laser pulse and material is ejected. This plume of material travels through a vacuum chamber to a carefully chosen substrate where material is deposited and forms a crystalline film. Planar waveguide lasers are structures of interest due to their low lasing thresholds in comparison to bulk material, owing to the greater overlap of the pump and laser modes, and their large aspect ratio that allows efficient heat removal from the laser crystal. This, combined with the high thermal conductivity of the sesquioxides, should allow for power scaling of these waveguide lasers with minimal thermal affects. The laser results in this thesis are for progressively higher powers, and no detrimental heating affects are witnessed. This thesis reports on the growth of single and multilayer sesquioxide waveguides grown by PLD, including growth optimisation, sample analysis and laser experiments. The first sample of this project to be successfully lased was a single layer Tm:Y₂O₃ waveguide on a YAG substrate, which produced a maximum output power of 35 mW with 9% slope efficiency at a wavelength of 1.95 µm, and, to the best of our knowledge, was the first Tm:Y₂O₃ planar waveguide laser. Next, Yb:Y₂O₃ was the material of choice and a multilayer sample was fabricated, where the Yb:Y₂O₃ core was sandwiched between two undoped Y₂O₃ layers, again on a YAG substrate. Laser experiments with this waveguide gave a maximum output power of 1.2 W at 1030 nm, with a slope efficiency of 20%. The highest laser output power of any of the doped sesquioxide waveguides in this project is 8.5 W, achieved using a Yb:Lu₂O₃ sample.

Thesis (MScAgric)--University of Stellenbosch, 2003.
ENGLISH ABSTRACT: Coal mining in South Africa is estimated to produce 200 Ml of acid mine drainage (AMD) per day in the Pretoria-Witwatersrand-Vereeniging (PWV) area alone, while electricity production resulted in approximately 27 Mt of ash in 2001. A large number of collieries in South Africa are tied to power stations where these two waste streams, acid mine drainage and fly ash, have the capacity to neutralize each other and provide an opportunity for co-disposal. The aim of this study was to investigate the reactions that occur during the co-disposal of fly ash leachate (FAL) and AMD and to examine the precipitates that result from the neutralisation reactions. Potentiometric titration was employed to investigate the neutralisation of Al-Fe salt solutions, simulating acid mine drainage (AMD), with alkaline solutions of Ca or Na hydroxide as well as fresh alkaline leachate from fly ash (FAL). The effectiveness of fly ash in removing metals and other salts from acid mine water was examined by analysing the neutralised water and modelling the results thermodynamically. Precipitates, prepared from large scale synthetic AMD and FAL co-disposal at various pH levels and Fe:Al ratios, were characterised according to composition, mineralogy and surface properties. The experimental neutralisation of synthetic acid mine drainage was achieved through titrating the components of SAMD (Fe and Al salt solutions) and solutions of various Fe:Al mole ratios with different bases in air and N2, and comparing the SAMD-FAL system with these simple acids and bases. The FAL used in all experiments was produced from fresh fly ash collected at Arnot power station. The SAMD was prepared as a solution with a pH of 2.5 and containing 12.7 mmol/L Al, 10.9 mmol/L Fe and 40.8 mmol/L SO4. The characterisation of reaction solids was achieved by collecting the precipitates formed from the co-disposal of FAL and SAMD with Fe:Al ratios of 7.3, 0.8 and 2.5. From the titration experiments it was found that upscale potentiometric titrations of SAMD show buffer zones at pH values of 3.5, 4, 6 and 10 corresponding to Fe(III)precipitation, Al precipitation, Fe(II) hydrolysis and oxidation, and Al redissolution, respectively, while downscale potentiometric titrations with SAMD show buffer zones at pH values 12 – 11, 9 and 4.5, which correspond to Fe oxidation and precipitation, Al precipitation and Al re-dissolution, respectively. A high concentration of Al in the simulated AMD inhibited the crystallinity of the precipitates and resulted in a large quantity of SO4 being removed from solution, which suggests that an aluminium sulphate phase is precipitating, but it is not crystalline and cannot be identified by XRD. Titrations performed up-scale by adding FAL to AMD showed near-complete metal and substantial SO4 removal from solution. The characterisation of reaction solids by x-ray diffraction, infrared spectroscopy, thermal gravimetric and differential thermal analysis revealed that the precipitates consist of poorly crystalline, highly Al-substituted goethite and ferrihydrite with large amounts of SO4 included in the structure. Poorly crystalline bayerite appears at a high pH and high Al concentration, and calcite is present in precipitates made by adding SAMD to FAL. High surface charges of between 330 cmolc/kg positive and 550 cmolc/kg negative charge and potentially large specific surface areas between 7 and 236 m2/g suggest a strong potential for the precipitates to function as low-grade adsorbents in wastewater treatment. The similarity of these ochre precipitates to soil minerals implies that land disposal of the neutralised solids is also viable.
AFRIKAANSE OPSOMMING: Steenkool mynbou in SA produseer na benaming 200 ML suur mynwater per dag in die PWV area alleenlik, terwyl opwekking van elektrisiteit naastenby 27 Mt vliegas geproduseer het in 2001. ’n Groot aantal steenkoolmyne in SA word verbind met kragsentrales, waar hierdie twee strome afval, suur mynwater en vliegas, die kapasiteit het om mekaar te neutraliseer en die weg te baan vir gesamentlike wegdoening. Die doel van hierdie studie was om die reaksies wat plaasvind gedurende gesamentlike wegdoening van vliegas loog (VAL) en suur mynwater (SMW) te ondersoek, asook die neerslae wat mag vorm as gevolg van neutralisasie reaksies. Potensiometriese titrasies was gebruik om die neutralisering tussen Al:Fe-sout oplossings te ondersoek as nabootsing van SMW met gebruikmaking van alkaliese oplossings van Ca of Na hidroksied asook vars loog van VA. Die effektiwiteit van VA om metale en soute uit SMW te verwyder was getoets deur outleding van die geneutraliseerde water en modellering van die termodinamika. Neerslae berei uit groot-skaal sintetiese SMW en VAL en met gelyktydige storting by verskeie pH vlakke en Fe:Al verhoudings, was gekarakteriseer volgens samestelling, mineralogie en oppervlak eienskappe. Die eksperimentele neutralisering van sintetiese suur mynwater (SSMW) was gedoen deur titrering van die SSMW komponente en oplossings van verskeie Fe:Al molêre verhoudings met verskillende basisse in lug en N2, en vergelyking van SSMW-VAL sisteem met hierdie eenvoudige sure en basisse. Die VAL gebruik in alle eksperimente was geproduseer van vliegas verkry van die Arnot kragsentrale. Die SSMW was berei as ’n oplossing met ’n pH van 2.5 en bevat 12.7 mmol/L Al, 10.9 mmol/L Fe en 40.8 mmol/L SO4. Die karakterisering van vastestowwe uit die reaksie was gedoen deur die bemonstering van neerslae gevorm as gevolg van die gelyktydige wegdoening van VAL en SSMW met Al:Fe verhoudings van 7.3, 0.8 en 2.5.Die was waargeneem in die titrasie eksperimente dat hoër-skaal potensiometriese titrasie van SSMW buffersones, by pH waardes 3.5, 4, 6 en 10, ooreenstem met Fe(III) presipitasie, Al presipitasie, Fe(II) hidrolise en oksidasie, en Al her oplossing, terwyl laer skaal potensiometriese titrasie met SSMW buffer sones by pH waardes 12 - 11, 9 en 4.5 ooreenstem met Fe(III) presipitasie en oksidasie, Al presipitasie en heroplossing respektiewelik. ’n Hoë konsentrasie Al in die sintetiese SMW het kristalliniteit van die neerslae geïnhibeer en veroorsaak dat ’n hoeveelheid SO4 uit oplossing verwyder is, wat suggereer dat die AlSO4 fase neerslaan maar nie kristallyn is en gevolglik nie opgetel word met x-straal diffraksie nie. Titrasies gedoen by hoërskaal deur byvoeging van VAL tot SSMW, het feitlik volledige metaal en SO4 verwydering uit oplossing getoon. Die karakterisering deur x-straal diffraksie, infrarooispektroskopie, termies gravimetries en differentiële termiese analise, het getoon dat die presipitate bestaan uit swak kristallyne, hoë Al-gesubstitueerde goethiet en ferrihidriet met groot hoeveelhede SO4 vasgevang in die struktuur. Swak kristallyne bayeriet verskyn by hoë pH en hoë Al-konsentrasies en kalsiet is teenwoordig in neerslae gevorm deur byvoeging van SSMW tot VAL. Hoë oppervlakladings van tussen 330 cmolc/kg positief en 550 cmolc/kg negatiewe lading en ook potensieel groot spesifieke oppervlak van tussen 7 en 236 m2/g, dui op ’n sterk potensiaal vir neerslae om as laegraadse adsorbeermiddels in afvalwaterbehandeling gebruik te word. Die ooreenstemming in hierdie geelbruin neerslae met grond minerale, impliseer dat die land storting van geneutraliseerde vastestowwe ook lewensvatbaar is.

L’oxyde de cérium suscite un fort intérêt en catalyse hétérogène, en particulier dans le domaine de la semi-hydrogénation des alcynes. En effet, il a été montré que CeO2 est capable de dissocier la molécule de H2 en l’absence de métaux nobles en faisant intervenir un intermédiaire hydrure, espèce catalytiquement active. Cependant, sur la surface stoechiométrique (111)-CeO2, cet intermédiaire n’est pas stable et reste systématiquement défavorisé par rapport au produit de dissociation homolytique. Bien que la plupart des applications de CeO2 soient reliées à ses propriétés redox, il y a un manque de connaissance de l’effet de la réduction sur la dissociation de H2. Dans ce contexte, nous proposons une étude complète de la dissociation de H2 par la modélisation à l’échelle atomique basée sur une approche de type DFT+U appliquée en conditions périodiques. L’effet de la réduction a été pris en compte en considérant des surfaces : stoechiométriques, localement réduites via la formation de lacunes d’oxygène, et totalement réduites. L’étude systématique de la dissociation de H2 sur différentes surfaces a permis d’identifier les paramètres clés impliqués et d’en extraire des relations structure-propriété-réactivité. En particulier, nos résultats indiquent une forte dépendance entre la topologie des surfaces et leurs réactivités. De plus, nous avons mis en évidence le rôle clé de la réductibilité : l’activité catalytique augmente graduellement avec le taux de réduction. Nos travaux permettent donc d’interpréter les observations expérimentales en proposant un mécanisme réactionnel, et d’explorer des voies pour orienter la réactivité des matériaux à base d’oxydes de cérium
These last years, cerium oxide has received a great interest in heterogenous catalysis, in particular in the alkyne semi-hydrogenation field. Indeed, it has been shown that CeO2 is able to dissociate H2 molecule in absence of noble metals. Interestingly, the hydrogenation of ceria is found to take place through an hydride intermediate which is a catalytically active species. However, on the stoichiometric (111)-CeO2 surface, this intermediate is not stable and remains systematically unfavored compared to the homolytic product. Although most of the CeO2 applications are related to its redox properties, there is a lack of knowledge regarding the reduction effect on the H2 dissociation. In this context, we propose a full study of H2 dissociation by the modelling of surface reactivity at the atomic scale based on the DFT+U approach applied in periodic conditions. The reduction effect was studied by considering: (i) stoichiometric surfaces (CeO2), (ii) partially reduced surfaces with the formation of oxygen vacancies (CeO2-x), and (iii) fully reduced surfaces (Ce2O3). The systematic study of H2 dissociation on different surfaces allows us to identify the key parameters and to extract structure-property-reactivity relationships. In particular, our results show a strong relationship between the topology of the surfaces and their reactivities. Moreover, we have highlighted the key role of the reducibility: the catalytic activity increases gradually with the reduction rate. Our works allow to interpret the experimental observations by proposing a reaction mechanism, as well as to explore new ways of rational design of cerium oxides based materials

Mise au point d'un procede necessitant la construction d'un four basculant de grande capacite. Role cle joue par l'anhydride borique. Le monoferrite de ba prepare par coprecipitation reagit avec b::(2)o::(3) en donnant d'abord une phase amorphe

Titanium is one of the earth-abundant elements, and its oxides including titanium dioxide (TiO2) and strontium titanium oxide (SrTiO3) are widely used in technologies of electronics, energy conversion, catalysis, sensing, and so on. Generally, the Ti ions in these compounds have a valence of 4+ with the outer shell electron configuration of 3d0. In this thesis, we explore interface and titanite containing Ti3+ ions with 3d1 itinerate electrons, which we believe open new doors towards some new titanite-based technologies. In the first part of this thesis (Chapter 3), we will discuss the nanoscale chemical and valence evolution at a metal/oxide interface: Ti/SrTiO3. In many devices, metal-oxide interfaces are ubiquitous and play important roles in the performance of a wide range of electronic and optoelectronic devices. This motivated us to examine the microscopic structure of the interfaces between strontium titanium oxide and metals. In this work, one unit cell of cubic perovskite Ti2O3 was observed at the Ti/SrTiO3 interface, and oxygen diffusion depth of ~3.2 nm was observed in the sample fabricated at room temperature. Meanwhile, oxygen vacancy domains in the SrTiO3 substrates was observed and characterized by low angle annular dark field (LAADF) imaging and electron energy loss spectra (EELS). In the main part of this thesis, we will focus on the structure and physical properties of Ti2O3, a titanite which has received less attention so far in the research community. Different from TiO2 and SrTiO3, Ti2O3 has a much narrower band-gap (~0.1 eV), and we will discuss some preliminary results of its physical properties and potential applications. In Chapter 4, we will discuss the photothermal application and mid-infrared photodetectors using Ti2O3 nanoparticles based on its ultra-narrow bandgap. Photo-thermal effect via a Ti2O3/membrane structure is further applied to seawater desalination. A high temperature of 70 °C was achieved when this Ti2O3/membrane double layer structure floating on top surface of water subjected to the white light illumination of 7 kW/cm2. Furthermore, room temperature mid-infrared (10 μm) photodetectors based on Ti2O3/graphene hybrid structure was fabricated and studied. The photoresponsivity of this hybrid device, operated from 4.5 to 10 μm, is above ~ 100 A/W, which, to our knowledge, is the highest value for the mid-infrared photodetectors operating in the photocurrent (PC) mode. In chapter 5, structure, optical, transport properties of Ti2O3 epitaxial thin films on sapphire fabricated by pulsed laser deposition (PLD) will be discussed. By tailoring growth conditions, two different: trigonal and orthorhombic, of Ti2O3 were stabilized on Al2O3 substrates. More interestingly, the orthorhombic Ti2O3 has never been reported, and, moreover, superconductivity (~8 K) and high temperature ferromagnetism (up to 700 K) was discovered in this new stabilized phase. More details of the physical properties of Ti2O3 will be discussed in the following chapters of this dissertation.

The scientific relevance and potential for technological applications of complex materials have made them the focus of active investigation in order to fully charac- terize the competition and interactions between their electronic, structural, orbital, and spin degrees of freedom. Optical and terahertz (THz) spectroscopy provide ac- cess to electronic and low frequency quasiparticle responses, and therefore play a key role in understanding the fundamental mechanisms which dictate the macroscopic properties of complex materials. Time-resolved experiments, in turn, have the po- tential to disentangle the various coexisting energy scales through a careful selection of the pump and probe characteristics. This work investigates the role played by the electronic, structural and magnetic excitations in the insulator-to-metal transi- tions (IMT) of VO2, V2O3 and NdNiO3, through studies under different conditions of temperature, strain, doping and photoexcitation. Our work shows that a complete understanding of the IMT in VO2 requires sev- eral length scales and time scales to be considered. Indeed, epitaxial strain leads to anisotropy in the IMT characteristics of thin films of (100) and (110) VO2/TiO2, measured using THz spectroscopy, which can be explained by strain induced modi- fications both in the (microscopic) V3d orbitals and in the geometry of mesoscopic metallic domains. On the other hand, ultrafast studies which track, with femtosecond resolution, the electronic and structural dynamics of VO2 thin films following THz excitation reveal a delay in the onset of the structural response with respect to the electronic one, lending support to the correlation rather than Peierls driven picture of the IMT in this material. As for V2O3, the IMT is seen to occur via nucleation and growth of metallic domains, as previously reported in VO2. However, a scaling of the photoinduced conductivity dynamics rise time is further identified, which reveals the temperature and fluence dependence of the nucleation and growth process. Finally, strained NdNiO3 films exhibit a two step dynamical conductivity response following optical excitation, different from that of the vanadates with which they share a complex, albeit more tunable, phase diagram. This hints at a significant role being played by the magnetic structure during the IMT in NdNiO3.