Rozprawy doktorskie na temat „Bulk oxides”

This thesis work is focused on the study of oxides in form of nanostructured samples for functional applications. These applications take advantage of their high mechanical and chemical stability, high melting point, electric, optical, and magnetic properties. In particular the work is divided in three parts, which represent the three different functional applications studied: magnetic materials, protonic conductors, and lithium-ion batteries cathodes. For the magnetic materials, CoFe2O4 nanostructured sintered samples were studied in their magnetic properties, leading to interesting values. The magnetic parameters have been evaluated in sintered sample having different type of microstructure, relative density, and grain size. The protonic conductors studied are TiO2, CeO2 and ZrO2 doped with sulfur. The nanostructured sintered samples of these oxides were evaluated in their protonic conduction properties, the protonic conductivity has been evaluated in this oxide at different values of relative humidity. The results are that S-TiO2 and S-ZrO2 showed very good conductivity values, instead S-CeO2 presented values of conductivity not so good to be applied as protonic conductor. Finally new materials in form of high-entropy oxides were tried to synthesize in order to obtain new cathodes nanostructured oxides for Li-ion batteries, having the same crystal structure of LiMn2O4 but with the substitution of some cations on the MnIII and MnIV sites. Different trivalent and tetravalent cations have been used in order to enter in the spinel structure and lead to an enlargement of the spinel unit cell: Y3+, Fe3+, Al3+, Cr3+, In3+, Ti4+, Zr4+, Tb4+. Some interesting compounds, as LiFe0.6Ti0.6Mn0.8O4, LiFe0.7Mn0.6Ti0.7O4, LiFe0.5Mn0.5TiO4 and LiFe0.5Ti0.5MnO4 were synthesized.

Metal oxides of bismuth show applications from photocatalysts to dielectric materials often due to the influence of the so called “lone pair” electrons that give rise to structural distortions and modified electronic structure. These effects upon which fundamental properties are derived are still not yet fully understood. Polycrystalline Bi 2 Ti 2 O 7 , Bi 2 Zr 2 O 7 , and Bi 2 Hf 2 O 7 materials - the latter in both pyrochlore (p) and monoclinic (m) phases - were prepared via co-precipitation and, except for Bi 2 Hf 2 O 7 (m), thin films. The bulk, surface, and electronic properties were characterised using a combination of X-ray diffraction and spectroscopic techniques. The structural determination, completed in Chapter 4, confirmed the formation of a directional lone pair by O 2p-assisted Bi 6s-6p hybridisation in all except Bi 2 Zr 2 O 7 . X-ray photoelectron spectroscopy quantified a Bi surface excess that in Chapter 5 was probed further by low-energy ion spectroscopy achieving greater surface sensitivity. The top atomic layer was determined to be almost exclusively BiO x with surface relaxation of the directional lone pair suggested as the driving force behind this surface reconstruction. By in-situ diffuse reflectance infrared Fourier-transformed spectroscopy, CO 2 was found to strongly chemisorb onto the surface of Bi 2 Ti 2 O 7 , Bi 2 Zr 2 O 7 , and Bi 2 Hf 2 O 7 (p) indicating a basic surface. Both the surface reconstruction and surface basicity were not observed in Y 2 Ti 2 O 7 demonstrating the clear influence of the Bi cation. In Chapter 6, the electronic structure of the films was investigated by XPS and polarisation-dependent HAXPES and indicated a strong Bi contribution to the valence band either by O 2p-assisted Bi 6s-6p hybridisation or, as in Bi 2 Zr 2 O 7 , anti-bonding interactions between O 2p and Bi 6s. The Bi 6s was shown to shift the valence band towards the Fermi edge, in addition, a valence band with a high metal character is beneficial for the formation of holes and mobility while Bi 6p contributions to the conduction band improve the stability of excited electrons from the valence band with high O 2p character.

Nanoparticles are particles with sizes between 1 and 100 nanometers (nm). Owing to their unique chemical, electrical, mechanical, optical, and piezoelectric properties, zinc oxide nanoparticles (ZnO-NPs) are finding widespread use in numerous applications with yearly production over 550 tons per year. Increasing use of ZnO NPs, and NPs in wastewater discharges from domestic and industrial sources will have significant potential for adverse impacts on aquatic phototrophic organisms. Comparative studies on microalgae species response to ZnO NPs and variation in tolerance among species is still mostly unexplored. The proposed research aims to evaluate interspecies’ variation in tolerance to ZnO NPs among marine and freshwater microalgae. Multi-well culture plate and flask culture screening methods were utilized for assessing microalgae species’ tolerance to various levels of ZnO NPs. Microalgae cell morphology changes in response to nano ZnO exposure were explored using both the Optical Coherence Microscope (OCM) and SEM. Availability of Nano ZnO tolerant microalgae species may provide an impetus for future studies to understand the mechanism of tolerance and potential applications in NPs bioremediation in aquatic systems.

Natural manganese oxides are generally formed in surficial environments that are near ambient temperature and water-rich, and may be exposed to wet-dry cycles and a variety of adsorbate species that influence dramatically their level of hydration. Manganese oxide minerals are often poorly crystalline, nanophase, and hydrous. In the near-surface environment they are involved in processes that are important to life, such as water column oxygen cycling, biomineralization, and transport of minerals/nutrients through soils and water. These processes, often involving transformations among manganese oxide polymorphs, are governed by a complex interplay between thermodynamics and kinetics. Manganese oxides are also used in technology as catalysts, and for other applications.

The major goal of this dissertation is to examine the energetics of bulk and nanophase manganese oxide phases as a function of particle size, composition, and surface hydration. Careful synthesis and characterization of manganese oxide phases with different surface areas provided samples for the study of enthalpies of formation by high temperature oxide melt solution calorimetry and of the energetics of water adsorption on their surfaces. These data provide a quantitative picture of phase stability and how it changes at the nanoscale.

The surface energy of the hydrous surface of Mn₃O₄ is 0.96 ± 0.08 J/m², of Mn₂O₃ is 1.29 ± 0.10 J/m², and of MnO₂ is 1.64 ± 0.10 J/m². The surface energy of the anhydrous surface of Mn₃O₄ is 1.62 ± 0.08 J/m ², of Mn₂O₃ is 1.77 ± 0.10 J/m ², and of MnO₂ is 2.05 ± 0.10 J/m². Supporting preliminary findings (Navrotsky et al., 2010), the spinel phase (Mn₃O₄) has a lower surface energy (more stabilizing) than bixbyite, while the latter has a smaller surface energy than pyrolusite. These differences significantly change the positions in oxygen fugacity—temperature space of the redox couples Mn₃O₄-Mn₂O ₃ and Mn₂O₃-MnO₂ favoring the lower surface enthalpy phase (the spinel Mn₃O₄) for smaller particle size and in the presence of surface hydration.

Chemisorption of water onto anhydrous nanophase Mn₂O ₃ surfaces promotes rapidly reversible redox phase changes at room temperature as confirmed by calorimetry, X-ray diffraction, and titration for manganese average oxidation state. Water adsorption microcalorimetry (in situ) at room temperature measured the strongly exothermic integral enthalpy of water adsorption (-103.5 kJ/mol) and monitored the energetics of the redox phase transformation. Hydration-driven redox transformation of anhydrous nanophase Mn(III) ₂O₃, (high surface enthalpy of anhydrous surfaces 1.77 ± 0.10 J/m²) to Mn(II,III)₃O₄ (lower surface enthalpy 0.96 ± 0.08 J/m²) occurred during the first few doses of water vapor. Surface reduction of nanoparticle bixbyite (Mn ₂O₃) to hausmannite (Mn₃O₄) occurs under conditions where no such reactions are seen or expected on grounds of bulk thermodynamics in coarse-grained materials.

Layered structure manganese oxides contain alkali or alkaline earth cations and water, are generally fine-grained, and have considerable thermodynamic stability. The surface enthalpies (SE) of layered and tunnel structure complex manganese oxides are significantly lower than those of the binary manganese oxide phases. The SE for hydrous surfaces and overall manganese average oxidation state (AOS) (value in parentheses) are: cryptomelane 0.77 ± 0.10 J/m ² (3.78), sodium birnessite 0.69 ± 0.13 J/m² (3.56), potassium birnessite 0.55 ± 0.11 J/m² (3.52), and calcium birnessite 0.41 ± 0.11 J/m² (3.50). Surface enthalpies of hydrous surfaces of the calcium manganese oxide nanosheets are: δCa _0.39MnO_2.3nH₂O 0.75 ± 0.10 J/m² (3.89) and δCa_0.43MnO_2.3nH₂O 0.57 ± 0.12 J/m² (3.68). The surface enthalpy of the complex manganese oxides appears to decrease with decreasing manganese average oxidation state, that is, with greater mixed valence manganese (Mn ^3+/4+). Low surface energy suggests loose binding of H₂O on the internal and external surfaces and may be critical to catalysis in both natural and technological settings.

Die vorliegende Arbeit ist eine umfassende Studie über das Wachstum mittels Molekularstrahlepitaxie (MBE) und die gemessenen Seebeck Koeffizienten und Lochtransport Eigenschaften von p‑Typ Oxiden, eine Materialklasse welche die optische Transparenz und die einstellbare Leitfähigkeit verbindet. Insbesondere, Nickeloxid (NiO) und Zinnmonoxid (SnO) wurden mittels plasmaunterstützter MBE unter Einsatz von einer Metall‑Effusionszelle und einem Sauerstoffplasma gewachsen. Für das NiO Wachstum wurden vor allem die Wachstumsgrenzen bei hohen Temperaturen festgelegt, welche von der Substratstabilität im Falle von Magnesiumoxid und Galliumnitrid abhängen. Es wird die Möglichkeit der Qualitätsbewertung mittels Ramanspektroskopie für Natriumchlorid-Strukturen gezeigt. Untersuchung der NiO Dotierung durch Oberflächen-Akzeptoren und der damit verbundenen Oberflächen‑Loch‑Anreicherungsschicht offenbart eine neue Dotierungsmöglichkeit für p‑leitende Oxide im Allgemeinen. Die metastabile Phase des SnO wird mittels PAMBE unter Verwendung bekannter Wachstumskinetik von Zinndioxid und verschiedener in‑situ Methoden stabilisiert, die anwendungsrelevante thermische Stabilität wird untersucht. Anschließende ex‑situ Charakterisierungen durch XRD und Ramanspektroskopie identifizieren das kleine Wachstumsfenster für das epitaktische Wachstum von SnO. Elektrische Messungen bestätigen die p‑Typ Ladungsträger mit vielversprechenden Löcherbeweglichkeiten welche auch für Hall Messungen zugänglich sind. Temperaturabhängige Hall Messungen zeigen einen bandähnlichen Transport welcher auf eine hohe Qualität der gewachsenen Schichten hindeutet. Die Funktionalität der gewachsenen Schichten wird durch verschiedene Anwendungen nachgewiesen. Zum Beispiel werden pn‑Heteroübergänge wurden durch das heteroepitaktische Wachstum der SnO Schichten auf einem Galliumoxid-Substrat erlangt. Die ersten bisher berichteten SnO-basierten pn‑Übergänge mit einem Idealitätsfaktor unter zwei wurden erreicht.
This thesis presents a comprehensive study on the growth by molecular beam epitaxy (MBE) and the measured Seebeck coefficients and hole transport properties of p‑type oxides, a material class which combines transparency and tunable conductivity. Specifically, Nickel oxide (NiO) and tin monoxide (SnO) were grown by plasma‑assisted MBE using a metal effusion cell and an oxygen plasma. For NiO growth, the focus lies on high temperature growth limits which were determined by the substrate stability of magnesium oxide and gallium nitride. Quality evaluation by Raman spectroscopy for rock‑salt crystal structures is demonstrated. Investigations of NiO doping by surface acceptors and the related surface hole accumulation layer reveal a new doping possibility for p‑type oxides in general. The meta‑stable SnO is stabilized by PAMBE utilizing known growth kinetics of tin dioxide and various in‑situ methods, its application-relevant thermal stability is investigated. Following ex‑situ characterizations by XRD and Raman spectroscopy identify secondary phases and a small growth window for the epitaxial growth of SnO. Electrical measurements confirm the p‑type carriers with promising hole mobilities accessible to Hall measurements. Temperature dependent Hall measurements show band‑like transport indicating a high quality of the grown layers. The functionality of the grown layers is proven by various applications. For example, pn‑heterojunctions were achieved by heteroepitaxial growth of the SnO layers on gallium oxide substrates. The first reported SnO based pn‑junction with an ideality factor below two is accomplished.

This thesis is an investigation into the accuracy of hybrid density functional theory to predict the properties of two transition metal oxides: Ilmenite (FeTiO3) and haematite (sigma-Fe2O3). The hybrid density functional theory examined is Becke's B3LYP functional, which is an empirical mix of density functional theory and exact nonlocal exchange from Hartree-Fock theory. For bulk ilmenite, results from the B3LYP functional are compared with Hartree-Fock and pure density functional theory calculations. The computed properties are found to be very sensitive to the treatment of electronic exchange and correlation, with the best results being achieved using the hybrid functional. Calculations performed using the hybrid functional benefit from its better treatment of the electronic self interaction and its reasonable estimate of the pair correlation energy of the doubly occupied Fe-d orbital. To assess the performance of the hybrid functional in simulating Fe2O3 and FeTiO3 with different cation-anion coordination than that found in ilmenite or haematite, studies were performed on their high pressure polymorphs, for which there are a range of experimental results for comparison. This tests the transferability of the functional before examining cases, such as the surfaces of these materials, where there are little or no experimental or theoretical results. For the currently known high pressure polymorphs of ilmenite and haematite, the structural and elastic parameters computed using the hybrid functional are found to be in good agreement with those observed, as is the predicted stability of the phases. In ilmenite, the calculations predict the stability of a new high-pressure polymorph with space group Cmcm, occurring at pressures above 44 GPa. Calculations of the high pressure polymorphs of haematite involve the examination of a range of charge, spin, and magnetic states for each of the polymorphs. Magnetic ordering was found to be important for all the polymorphs, and for each polymorph an antiferromagnetic ordering was found to be lower in energy than the ferromagnetic ordering. The predicted transition pressure from the corundum structure and the magnetic collapse of the Fe3+ cations were in good agreement with experiment. At high pressures the lowest energy configuration for the orthorhombic perovskite structure was computed to occur with mixed high-spin /low-spin Fe3+ cations, in contrast to predictions in the literature of a Fe2+/Fe4+ solution. The CaIrO3-type structure was also computed to be stable with a mixed high-spin/ low-spin Fe3+ configuration at high pressures, and is computed to be the most stable polymorph at pressures above 46 GPa at 0 K. The structure of the ilmenite (0001) surface is examined using the B3LYP functional, and for this surface twelve different terminations are considered, with surface energies and relaxed geometries calculated. The Fe terminated (0001) surface was found to have the lowest cleavage energy, and also to be the most stable surface at low oxygen partial pressures suggesting it is most likely to form when ilmenite is cleaved under high vacuum.

Ce travail concerne l'étude de la texture et de la microstructure en relation avec les propriétés supraconductrices Tc et Jc de supraconducteurs haute température critique SHTc : YBCO et NBCO. Les techniques de caractérisation utilisées pour la microstructure sont la diffraction de RX et la diffraction d'électrons (Electron Back-Scatter Diffraction) "EBSD". Tc et Jc ont été déduits des mesures d'aimantation et de susceptibilité magnétique au SQUID (Superconducting Quantum Interference Device) et PPMS (Physical Properties Measument System). La première partie de l'étude traite de la caractérisation de la texture de couches minces SHTc d'YBCO, et de la possibilité de déterminer les relations d'épitaxie par EBSD entre le film déposé et la couche tampon. Ce type d'échantillon est constitué d'un substrat de saphir sur lequel 3 couches successives ont été déposées dont 300 nm de YBCO, destinées aux applications électrotechniques comme limiteurs de courant. La deuxième partie concerne l'étude de matériaux massifs supraconducteurs YBCO et NBCO fabriqués par différentes techniques. L'effet du dopage d'YBCO avec de l'argent a été étudié sur des échantillons fabriqués par la technique dite de "texturation par croissance orientée" soit MTG (Melt Textured Growth) et sous une faible pression partielle d'oxygène. Le dopage jusqu'à un certain pourcentage d'argent améliore la microstructure ainsi que la densité de courant critique. Une comparaison des caractéristiques physiques et structurales d'échantillons NBCO oxygénés ex-situ et YBCO in-situ, élaborés par MTG sous champ magnétique intense a été effectuée. De même des échantillons NBCO texturés par la technique de fusion de zone (Zone Melting) oxygénés respectivement in-situ et ex-situ ont été étudiés. Ils présentent des textures similaires avec présence de macles pour NBCO oxygéné ex-situ
The aim of this work is the study of crystallographic texture and microstructure in connection with the superconducting properties (Tc and Jc) of High Temperature Superconducting (HTS) materials: YBCO and NBCO. Microstructure is studied by X-Ray Diffraction (XRD) and Electron backscatter Diffraction (EBSD). The Determination of critical temperature (Tc) and critical current density (Jc) are made with Superconducting Quantum Interference Device (SQUID) magnetometer or Physical Properties Measurement System (PPMS). The first part of this work investigates the crystallographic textures of the YBCO film by using EBSD in order to deduce the epitaxial relationship between the superconducting layer and the buffer layer. This thin film is made up of three successive deposits (among which 300 nm of YBCO), used in fault current limiters for electrical engineering applications. The second part presents the study of NdBa2Cu3O7-d (NBCO) and YBCO bulks prepared by various techniques. The effect of silver doping of YBCO has been studied on samples prepared by the Melt Textured Growth (MTG) technique under low oxygen partial pressure. The doping up to a given amount of silver enhances the microstructure and the critical current density. A comparison of the physical and structural characteristics between NBCO oxygenated exsitu and YBCO oxygenated in-situ, prepared by MTG under high magnetic field has been made. Zone-melted NBCO samples textured by zone melting method oxygenated respectively in-situ and ex-situ have been studied. These samples exhibit the same texture with the occurrence of twins for the NBCO oxygenated ex-situ

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)
Nos solos brasileiros, a caulinita, os óxidos de ferro e os hidróxidos de alumínio são alguns dos minerais encontrados na fração argila influenciando o comportamento dos atributos físicos do solo. O objetivo do presente trabalho foi avaliar a influência dos minerais da fração argila nos atributos físicos de um Argissolo Amarelo, submetido a dois tipos de preparo e cultivado com cana-de-açúcar. As amostras deformadas e indeformadas de solo foram coletadas após oito meses do plantio da cana-de-açúcar, em quatro camadas (0,00-0,10, 0,10-0,20, 0,20-0,40 e 0,40-0,60 m) na linha e entrelinha de plantio nas áreas submetidas à escarificação em área total e escarificação na linha da cultura. Através das amostras deformadas determinou-se a textura do solo, ferro extraído por ditionito-citrato-bicarbonato (FeDCB), ferro extraído por oxalato ácido de amônio (FeOAA) e razão caulinita/(caulinita+gibbsita) [(Ct/(Ct+Gb)]. Com as amostras indeformadas determinou-se a densidade do solo (Ds) e a resistência do solo à penetração (RP). Os resultados apresentados pela estatística descritiva demostraram que houve uma separação das áreas de estudo devido à diferença mineralógica encontrada nessas áreas, sendo a primeira área caracterizada pela maior razão Ct/(Ct+Gb) e a segunda área caracterizada pelos maiores teores de ferro extraído por ditionito-citrato-bicarbonato (FeDCB). Pela análise de componentes principais foi possível observar que o FeDCB apresentou correlação inversa com a Ds e a RP, a razão Ct/(Ct+Gb) apresentou correlação direta com os atributos físicos Ds e RP. A mineralogia influenciou o comportamento da densidade do solo e resistência do solo à penetração. A área com maior razão caulinita apresentou maiores valores de densidade do solo e de resistência do solo à penetração e a área com maiores teores de óxidos de ferro apresentou menores valores de densidade do solo e de resistência do solo á penetração. O preparo do solo não influenciou o comportamento da densidade do solo e da resistência do solo à penetração.
In Brazilian soils, kaolinite, iron oxides and aluminium hydroxides are some of the minerals found in the clay fraction influencing the behavior of the soil physical attributes. The objective of the present work was to evaluate the influence of minerals of the clay fraction on the physical attributes of a yellow Ultisol under two tillage and sugar cane cultivation. The disturbed and undisturbed samples of soil were collected in the row and interrow in the areas with chiselling in the total area and chiselling in the planting row in four layers (0,00-0,10, 010-0,20, 0,20-0,40, 0,40-0,60 m) eight months later the sugar cane cultivation. The soil texture, citrate/bicarbonate/dithionite extractable Fe, oxalate/acid/ammonium extractable Fe (FeOAA) and Ct / (Ct+ Gb) ratio were evaluated from disturbed samples and bulk density (Bd) and soil penetration resistance (SPR) were evaluated from undisturbed samples. The results presented by the descriptive statistics showed that there was a separation of the study areas due to the mineralogical difference found in these areas, being the first area characterized by the higher ratio Ct / (Ct + Gb) and the second area characterized by the higher citrate/bicarbonate/dithionite extractable Fe (FeDCB). In the principal component analysis, it was observed that the FeDCB showed an inverse correlation between Bd and SPR. It was also observed that the Ct / (Ct + Gb) ratio showed a direct correlation with the physical attributes Bd and SRP. The mineralogy influenced the behavior of the Bd and SPR. The area with higher Ct/ (Ct+Gb) ratio, showed higher values of Bd and SRP and The area with higher content of iron oxides had lower values of Bd and SRP. The soil tillage didn’t influence the behavior of Bd and SPR.
FAPESP: 2014/14490-2

Zinc oxide is a II-VI semiconductor with considerable potential for optoelectronic and power-electronic applications in the UV spectrum, due to its wide direct band gap (3.35 eV at 300 K), high exciton binding energy (60 meV), high melting point, and excellent radiation hardness. A key requirement for many device applications is the consistent production of high performance Schottky contacts. Schottky contact formation to n-type ZnO was investigated via systematic studies into the relative performance of different metal and metal oxide Schottky contacts to hydrothermal and melt grown, bulk ZnO. The results of these studies can be explained by the dominating influence of two key mechanisms in the formation of high quality contacts: the removal of the natural hydroxide termination of ZnO and the associated surface accumulation layer, and the minimisation of process induced oxygen vacancies which tend to pin the barrier height of ZnO Schottky contacts in the 0.6 - 0.8 eV range. These investigations also led to the discovery of a new technique for the consistent production of high quality ZnO Schottky contacts, using the deposition of metal oxide films in reactive oxygen ambients. Specifically, silver oxide, iridium oxide, and platinum oxide films were used to consistently produce highly rectifying, very low ideality factor Schottky contacts to bulk ZnO, with figures of merit significantly better than those published in the literature. In addition, a number of previously unreported, surface polarity related effects were discovered in the electrical and optical properties of ZnO, which increase in magnitude with decreasing carrier concentration of the ZnO material. For example, metal oxide Schottky contacts fabricated on the Zn-polar surface of hydrothermal ZnO have significantly higher barrier heights than those on the O-polar surface, and low temperature (4 K) photoluminescence emission, from free excitons and excitons bound to ionised donors, is also significantly stronger from the Zn-polar face of the same material. These effects are thought to be related to the large spontaneous polarisation (-0.057 Cm-2) of ZnO, and indicate that surface polarity is an important variable when comparing experiment results with theoretical models, and in the future design of ZnO based devices.

In questa tesi presento il lavoro da me svolto durante il mio dottorato su film vetro-ceramici nanostrutturati contenenti ossido di gallio riguardo alla possibile uso di questa classe di materiali in geometria planare all’interno di dispositivi optoelettronici. Negli ultimi tre anni, le mie ricerche sono state rivolte alla comprensione della relazione fra la presenza delle nanostrutture – analizzate attraverso uno studio strutturale dettagliato grazie a diverse tecniche di microscopia e diffrazione in funzione delle condizioni di deposizioni e trattamenti successivi – e i meccanismi fisici di trasporto di carica e polarizzazione che accadono all’interno di film di ossidi-in-ossidi. Il materiale studiato è stato preparato tramite sputtering a radiofrequenze a partire da un precursore vetroso contenente ossidi di Li, Na, Si, Ga e Ge. Come conseguenza del processo di deposizione abbiamo ottenuto un materiale parzialmente cristallino con nanocristalli ricchi in Ga incorporati nel resto della matrice amorfa. Attraverso la microscopia a forza atomica, la riflessione dei raggi x, la diffrazione 2D dei raggi x, la dispersione 2D dei raggi x a piccoli angoli – anche usando luce di sincrotrone – abbiamo completamente caratterizzato i film nanostutturati a diverse scale di grandezza, da pochi nm a qualche micron. L’analisi dettagliata dei risultati mostra la formazione di molteplici nanocristalli anisotropi di spinello di ossido di gallio con dimensioni di 3 nm, che formano aggregati lenticolari di dimensioni maggiori con differenze significative nelle dimensioni nel piano e fuori dal piano. Come risultato dell’indagine di campioni trattati differentemente, lo studio ha inoltre mostrato che la distribuzione e la morfologia di nanostrutture può essere controllata dalle condizioni della deposizione, dalla durata e dalla temperatura del trattamento termico seguente. Partendo dalla conoscenza delle caratteristiche strutturali, l’attività di ricerca è stata indirizzata verso l’approfondita comprensione delle proprietà di trasporto di carica risultanti dalla natura nanostrutturata del materiale, inclusa la nanofase in Ga2O3 semiconduttore e la matrice ospite dielettrica. La risposta elettrica – analizzata con l’aiuto della spettroscopia di impedenza complessa – si è rivelata essere la risultante di molteplici meccanismi di trasporto e di carica dovuti a matrice e nanofase, incluse le interfacce interne. I dati suggeriscono che il trasporto di carica è supportato da una combinazione di meccanismi attivati termicamente per hopping e per effetto tunnel mediati da un percorso di percolazione costruito dalle caratteristiche nanostrutturali del materiale. Curiosamente, la risposta nel suo complesso ha fornito indizi preliminari di plasticità elettrica, rendendo i film sottili nano-vetroceramici dei promettenti candidati come sistemi chiave in dispositivi avanzati per memorie completamente inorganiche ossido-in-ossido ispirate al cervello umano.
In this thesis I present the work I have carried out during my PhD on Ga-oxide containing nanostructured glass-ceramic films as a potential breakthrough for the implementation of this class of material in planar geometry for applications in the field of optoelectronics. In the last three years, my investigations have been aimed at understanding the relationship between the occurrence of nanostructuring – analysed through a detailed structural studies by means of different microscopy and diffractometric techniques as a function of deposition conditions and post-deposition treatments – and the physical mechanisms of charge transport and polarization taking place in oxide-in-oxide films. The studied material was produced by RF sputtering deposition starting from a glass target of Li, Na, Si, Ga and Ge mixed oxide. As a consequence of the deposition process we have obtained partially crystallized material with Ga-rich nanocrystals incorporated in the remaining amorphous matrix. Through atomic force microscopy, x-ray reflectivity, 2D-x-ray diffraction, 2D-small angle x-ray scattering - also employing synchrotron radiation facilities - we have fully characterized the nanostructured films at different length scales, from few nm to few microns. The detailed analysis of the results shows the formation of multiple anisotropic spinel Ga-oxide nanocrystals with size of about 3 nm, forming larger lenticular aggregates with significant differences between the in-plane and the out-of-plane dimensions. As a result of the investigation of differently treated samples, the study also shows that size distribution and morphology of the nanostructures can be controlled by deposition conditions, duration and temperature of post-deposition thermal treatments. Starting from the knowledge of the structural features, the research activity has been directed to the deep understanding of the charge transport properties resulting from the nanostructured nature of the material, including the Ga2O3 semiconductor nanophase and the dielectric host matrix. The electrical response – analysed with the aid of complex impedance spectroscopy – turns out to be the results of multiple contributions to transport and charging mechanisms by the matrix and the nanophase, including the effects of the internal interfaces. The data suggest that the charge transport is sustained by a combination of thermally activated hopping and tunnelling mechanisms mediated by the percolation path built up by the nanostructured features of the material. Interestingly, the overall response gives preliminary evidences of electrical plasticity, making nano-glassceramic thin films potential candidates as key systems in advanced devices for brain-inspired oxide-in-oxide fully inorganic memories.

The optical properties of zinc oxide crystals and thin films prepared by different methods are investigated. Single crystal zinc oxide samples prepared by melt and hydrothermal growth techniques were obtained. The influence of polarity and growth method on the optical properties were studied and correlated with their electronic properties. Thin films prepared by molecular beam epitaxy (MBE) and sputtering were studied and the influence of growth conditions and post growth treatment on the optical properties of the films was investigated. The photo-luminescence (PL) of bulk zinc oxide was examined at high resolution. Line widths of less than 0.1 meV were observed. More than a dozen different transitions in the near band edge region (NBE 360-380 nm) were noted, several of which displayed a separation of <0.5 meV which goes some way to illustrating the complexity of the system. Attempts were made, with some success, to reconcile the two main competing identification systems of the NBE transitions and explanations for some of the discrepancies are provided. The controversial deep level transitions in the visible part of the spectrum are fit with 3 Gaussians and their identities discussed with relation to the available literature. The presence of copper impurities was detected in annealed films and a model to explain their behaviour under annealing conditions is hypothesised. Films grown by MBE here at the University of Canterbury are shown to have PL line widths of as little as 2.2 meV, the ratio of active oxygen species in the growth chamber during deposition is shown to effect the optical quality of the films. It is shown that annealing can improve the optical quality of the films and various other methods of influencing the films properties are discussed. Reactive, magnetron, direct current sputtering is shown to be the optimal method of growth for maximising both optical and piezo-electric properties. Optimum annealing temperatures were found at 900 and 1100 ℃ with a local minimum at 1000 ℃. X-ray diffraction, atomic force and scanning electron microscopy measurements in addition to optical PL measurements show the influence of annealing on the polycrystalline sputtered ZnO films. Films grown on glass, silicon, sapphire and quartz were shown to display similar behaviour under annealing conditions. It was found that zinc oxide based devices were liable to be chemically unstable at temperatures above 1100 ℃. The piezo electric properties of the films were examined and attempts were made to prepare a zinc oxide film optimised for both optical quality and piezoelectric properties for possible future applications of a hybrid opto-mechanical coupled devices.

This thesis reports the results of investigations into efficient, miniature solid state lasers and optical amplifiers in rare earth doped oxide crystals. Waveguides doped with the trivalent thulium ion were grown in YAG and YSO host crystals and lased, when optically pumped, at 2.012µm and 1.884µm, respectively. The Tm:YAG laser had absorbed power laser thresholds as low as 7mW and slope efficiencies as high as 68%. The planar devices were fabricated using the technique of liquid-phase epitaxy which produces waveguides of excellent optical quality and with very low propagation losses. High gain optical amplifiers have been demonstrated in Nd:YAG planar waveguides pumped by a Ti: sapphire laser and a diode laser. A small-signal gain of 28.4dB was obtained in a waveguide fabricated by liquid-phase epitaxy; in the same waveguide 290 mW of power was extracted for only ~550mW of absorbed pump power. A small-signal gain of 23.5dB was achieved in a Nd:YAG waveguide, fabricated by thermal bonding, for 250mW of absorbed pump power. Pumping directly into the upper laser manifold of a bulk Nd:YAG rod, at 869nm has produced a highly efficient laser on the quasi-three-level 946nm transition. The lower energy defect for a laser pumped in this manner relative to conventionally pumped Nd:YAG lasers should result in higher slope efficiencies and fewer thermal problems when pumped at high powers. Using such a pumping scheme a slope efficiency of 75% with respect to absorbed power was obtained.

Oxide semiconductors have been shown to exhibit rich physics related to their bulk, defect and interface properties. First-principles calculations have and will continue to play a major role in developing an understanding of the microscopic origins of these phenomena. In this thesis, first-principles studies are presented for several oxide semiconductors, with a view to understand how their microscopic properties ultimately determine device functionality. In Chapter 3, a detailed study of bulk SrZrO3 and Sr(Ti,Zr)O3 alloys is performed. For Sr(Ti,Zr)O3 alloys with 50% Ti concentration, we find that arranging the Ti and Zr atoms into a 1×1 SrZrO3/SrTiO3 superlattice along the [001] direction leads to breaking of the conduction band t2g orbital degeneracy, which could suppress scattering due to electron-phonon interactions. In Chapter 4, we present an investigation into the properties of native defects and hydrogen in SrZrO3. It is found that oxygen and strontium vacancies are the dominant defects in the absence of impurity doping, and will form deep donor and deep acceptor states, respectively. Hydrogen is found to be amphoteric in this material at different lattice sites; additionally, this impurity forms a stable complex with oxygen vacancies. In Chapter 5, the tendency for ABO3 perovskite oxides with 3dn B-cations to exhibit ferroelectricity and multiferroicity is investigated. Using the LaBO3 series as a model, we find that initially, as electrons are added to the B-cation d orbital, the tendency for the system to exhibit a ferroelectric distortion disappears - however, for high spin d5 - d7 and d8 cations a strong ferroelectric instability is recovered, and this effect is explained within the pseudo Jahn-Teller theory for ferroelectricity. This finding provides a new route for the design of strongly coupled magnetoelectric materials. In Chapters 6 and 7 the fundamental properties of the technologically important oxide heterostructure systems ZnO/MgZnO and SrTiO3/LaAlO3 are characterized. For the latter, we identify a previously unreported mechanism for interface induced magnetism based on surface aluminium vacancies, which will aid in interpreting experimental results for this system and other polar/non-polar oxide heterostructures.

Thesis (Ph. D.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2004.
Hess, Dennis, Committee Member; Meindl, James, Committee Chair; Allen, Phillip, Committee Member; Cressler, John, Committee Member; Davis, Jeffrey, Committee Member. Vita. Includes bibliographical references (leaves 108-119).

Fibrous aluminum oxide γ-Al2O3 and lanthanum sesquioxide A-type La2O3 were synthesized by the thermal decomposition (700 °C, air) of cotton fibers impregnated by aqueous solutions of corresponding nitratesalts. The fibers of alumina consist of 20 nm sized particles, while the particle size of the lanthana ranges within 100–150 nm. The formation of chemisorption products on the lanthana surface is alsodiscussed. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/35041

Organic photovoltaics have shown much promise as an alternative photovoltaic technology for application in low-cost, large-scale and flexible solar cells. The application of metal oxides in organic solar cells (OSCs) and the impact of the properties of metal oxide/organic hetero-interfaces on cell performance have attracted a lot of attention. The metal oxide/organic interfaces have a crucial impact on interfacial charge transfer, charge collection and the overall device performance. This thesis is aimed at clarifying the principal interfacial phenomena occurring at the metal oxide/organic hetero-interfaces as well as effective engineering of those interfacial properties in OSCs. Photo-generated electrons and holes undergo different recombination processes, e.g., bimolecular recombination and trap-assisted recombination, before being collected by the electrodes in OSCs. Light intensity-dependent current densityvoltage (JV) characteristics of OSCs were analyzed to study the effect of recombination on charge collection efficiency. Effect of metal oxide/organic hetero-interfaces on charge transfers at organic/electrode interface was analyzed using transient photocurrent (TPC) measurements. Light intensity-dependent JV characteristics and TPC characteristics were applied to explore the charge recombination dynamics in OSCs with a metal oxide interlayer. This project concentrated on an in-depth investigation of the physics and the interface phenomena such as interfacial exciton dissociation, charge recombination processes, charge collection and interface engineering for high performing OSCs. The fundamentals about light intensity-dependent J-V characteristics for OSCs were summarized. The relationship between the charge recombination dynamics and light intensity-dependent J-V characteristics in OSCs were developed. Light intensity-dependent JSC, VOC and FF in OSCs made with different bulk-heterojunction (BHJ) systems of PTB7:PC70BM, PTB7-Th:PC70BM and PNB4:PC70BM were investigated. It is found that bimolecular recombination is the most prominent factor limiting the performance of OSCs. For freshly made OSCs fabricated based on the commercial polymers, e.g. PTB7 & PTB7-Th, and the new polymer PNB4 synthesized in-house, the trap-assisted charge recombination process in the BHJ active layer plays a relatively small role. This suggests that reducing the bimolecular recombination in OSCs through selecting proper materials and device structures is crucial for enhancing the power conversion efficiency (PCE) of OCSs. In this work, device structures which enable reducing bimolecular recombination in OSCs were investigated. The effect of ZnO interlayer at the interface between BHJ and Al cathode on the performance of PTB7:PC71BM based OSCs was studied by a combination of theoretical simulation and experimental characterization techniques, e.g., using light intensity-dependent JV characteristic and TPC measurements etc. It shows that ZnO interlayer has a profound effect on the performance of the PTB7:PC70BM-based OSCs, although it does not have a significant influence on the maximum absorptance in the active layer. The origin of the improvement in the cell performance is associated with the efficient charge collection due to the favorable exciton dissociation at the electrode/active layer interface. It is shown that the presence of the ZnO interlayer allows using a thinner active layer without moderating the absorption in the optically optimized control OSCs without the ZnO interlayer. OSCs with a ~10 nm thick ZnO interlayer are found to be favorable for the efficient charge collection, and thereby improving the cell performance. The TPC measurements also reveal that the dissociation of excitons at the metal/organic interface of regular OSCs hinders the electron collection. The unfavorable interfacial exciton dissociation can be removed by interposing a ZnO interlayer at the Al/organic interface, thus bimolecular recombination at the electrode/active layer interface can be reduced for improving the charge collection efficiency. PCE of the OSCs using ZnO interlayer was 6.5%, which is about 20% higher than a control cell (5.4%), having an identical device configuration without a ZnO interlayer. Solution-processed anode interlayer, a mixture of solution-processed MoOX and PEDOT:PSS, was adopted for application in inverted PTB7:PC71BM-based OSCs. The ratio of MoOX to PEDOT:PSS in the mixed solution was optimized for achieving the best cell performance. A PCE of 7.4% was obtained for OSCs with an optimal MoOX-PEDOT:PSS based interlayer, interposed between the BHJ active layer and Ag anode, which means 10% enhancement over the PCE of control cell made with an evaporated MoOX interlayer. Light intensity-dependent JV characteristics implied that the bimolecular recombination in OSCs with a MoOX-PEDOT:PSS interlayer was reduced. TPC measurements showed that the favorable exciton dissociation occurs at the organic/MoOX interface for the inverted OSCs. The favorable interfacial exciton dissociation generates an electrical field within a very small space near the interface, contributing significant additional photocurrent when the effective bias across the active layer in the OSCs is low, and thereby assisting in an efficient charge collection at the organic/electrode interface. In addition to the improvement in the cell performance, the solution-processed MoOX-PEDOT:PSS interlayer does not require a post-annealing treatment, which is beneficial for application in solution-processed tandem and flexible OSCs.

The growing need for high-performance electrode materials in electrochemical conversion and storage applications requires further fundamental investigation on the working and degradation mechanisms of these materials. Among various functional materials, transition metal oxides are still one of the main choices due to their tunable chemical compositions and diverse crystal structures in most aqueous and organic electrolytes. The charge transfer process mainly occurs at the electrode-electrolyte interface, and controlling the electrochemical interfacial stability represents a key challenge in developing sustainable and cost-effective electrochromic materials. The present thesis focuses on classical tungsten trioxide (WO3) materials as the platform to uncover the previously unknown interrelationship between phase transformation, morphological evolution, nanoscale color heterogeneity, and performance degradation in these materials during 3,000 cyclic voltammetry cycles. Through the application of novel cell design, synchrotron/electron spectroscopic, and imaging analyses, we observe that the interface between the WO3 electrode and 0.5 M sulfuric acid electrolyte undergoes constant changes due to the tungsten oxide dissolution and redeposition. The redeposition of dissolved tungsten species provokes in situ crystal growth, which ultimately leads to phase transformation from the semicrystalline WO3 to a nanoflake-shaped, proton-trapped tungsten trioxide dihydrate (HxWO3·2H2O). The multidimensional (surface and bulk) quantification of the electronic structure with X-ray measurements reveals that the tungsten reduction caused by proton trapping is heterogeneous at the nanometric scale and is responsible for the nanoscale color heterogeneity. The Coulombic efficiency, optical modulation, apparent diffusion coefficients, and switching kinetics are gradually diminished during 3,000 cyclic voltammetry cycles, resulting from the structural and chemical changes of the WO3 electrode. We hypothesize that the high interfacial reactivity in the electrode-electrolyte interfacial region could be the universal underlying mechanism leading to undesired bulk structural changes of inorganic electrochromic materials.
M.S.
With the rapid development of human society, the research of new energy-saving materials has become a focus of attention. Among them, electrochromic devices can effectively adjust their color through a controllable electrochemical reaction and have a wide range of uses in our daily life. For example, smart windows can reduce glare and heat without blocking the natural light, thereby providing buildings and vehicles with better thermal and visual comfort. Electrochromic optical displays can lower energy consumption. Variable reflectance mirrors such as anti-glare car rear-view mirrors can ensure the safety of driving. Lastly, wearable apparel such as electrochromic lenses for spectacles and sunglasses can protect users from ultraviolet radiation. Although electrochromic materials and devices have not expanded from the niche market, the enormous potential that they hold cannot be ignored and wide-scale commercialization should be sought after. Tungsten oxides electrochromic devices have proved to utilize the full spectrum of the incident light through structure design. These devices can also be configured with solar cells as a state-of-art integrated self-powered system with satisfactory optical modulation that can be obtained without any external electrical energy input. Moreover, WO3-based devices have also been combined with electrodeposition technology to achieve fast color-switching kinetics. However, the long-term durability in the acidic electrolyte under electrochemical cycling conditions needs to be further improved, and the road of full commercialization is still unpaved. To design high-performance electrochromic materials, it is imperative to study the degradation mechanism under long-term electrochemical cycling conditions. In the present thesis, the performance degradation of the WO3 electrode in acid electrolytes involves chemical changes. Through a better understanding of the fundamental degradation process, the design of high- performance electrochromic metal oxides can be developed.

Many new applications are being proposed and developed for use in the terahertz (THz) frequency region. Similarly, many new materials are being characterized for possible use in this area. Nanostructured forms are of particular interest since they may yield desirable properties, but they remain especially challenging to characterize. This work focuses on the characterization of zinc oxide (ZnO) in bulk and nanowire form. A method for characterizing nanostructures at THz by use of a parallel-plate waveguide (PPWG) is presented. This method is novel in that it is simple, both in theory and practice, and does not require the use of complex measurement techniques such as differential and double modulated terahertz time-domain spectroscopy (THz-TDS). To enable easy evaluation of the quality of the result the maximum deviation in the material response measurement is presented. The dielectric properties of bulk and nanowire ZnO as determined by THz-TDS measurements are reported, and the electrical conductivity extracted from both are presented for comparison. Experimental results are compared to the well established pseudo-harmonic phonon dielectric model. Shortcomings in the pseudo-harmonic phonon model are resolved when coupled with a modified Drude model. This work will enable the determination of THz material properties from nano-scale and very-thin film materials with better reliability and practicality than what has been possible until now.

Thesis (Ph. D.)--Materials Science and Engineering, Georgia Institute of Technology, 2007.
Wang, Zhong Lin, Committee Chair ; Wong, C.P., Committee Member ; Summers, Christopher J., Committee Member ; Degertekin, F. Levent, Committee Member ; Bottomley, Lawrence A., Committee Member.

The semiconductor industry scaling has mainly been driven by Mooreâ s law, which states that the number of transistors on a single chip should double every year and a half to two years. Beyond 2011, when the channel length of the Metal Oxide Field effect transistor (MOSFET) approaches 16 nm, the scaling of the planar MOSFET is predicted to reach its limit. Consequently, a departure from the current planar MOSFET on bulk silicon substrate is required to push the scaling limit further while maintaining electrostatic control of the gate over the channel. Alternative device structures that allow better control of the gate over the channel such as reducing short channel effects, and minimizing second order effects are currently being investigated. Such novel device architectures such as Fully-Depleted (FD) planar Silicon On Insulator (SOI) MOSFETS, Triple gate SOI MOSFET and Gate-All-Around Nanowire (NW) MOSFET utilize Silicon on Insulator (SOI) substrates to benefit from the bulk isolation and reduce second order effects due to parasitic effects from the bulk. The doping of the source and drain regions and the redistribution of the dopants in the channel greatly impact the electrical characteristics of the fabricated device. Thus, in nano-scale and reduced dimension transistors, a tight control of doping levels and formation of pn junctions is required. Therefore, deeper understanding of the lateral component of the diffusion mechanisms and interface effects in these lower dimensional structures compared to the bulk is necessary. This work focuses on studying the dopant diffusion mechanisms in Silicon nanomembranes (2D), nanoribbons (â 1.Xâ D), and nanowires (1D). This study also attempts to benchmark the 1D and 2D diffusion against the well-known bulk (3D) diffusion mechanisms.
Master of Science

We study the structure and energetics of water molecules adsorbed at ceria (111) surfaces below one monolayer coverage using density-functional theory. The results of this study provide a theoretical framework for interpreting recent experimental results on the redox properties of water at ceria (111) surfaces. In particular, we have computed the structure and energetics of various absorption geometries at stoichiometric ceria (111) surface. In contrast to experiment results, we do not find a strong coverage dependence of the adsorption energy. For the case of reduced surface, our results show that it may not be energetically favorable for water to oxidize oxygen vacancy site at the surface. Instead, oxygen vacancies tend to result in water more strongly binding to the surface. The result of this attractive water-vacancy interaction is that the apparent concentration of oxygen vacancies at the surface is enhanced in the presence of water. Finally, we discuss this problem with reference to recent experimental and theoretical studies of vacancy clustering at the (111) ceria surface. We also describe the simulation results for the structure and dynamics of liquid water using the SIESTA electronic structure approach. We find that the structure of water depends strongly on the particular basis set used. Applying a systematic approach to varying the basis set, we find that the basis set which results in good agreement with experimental binding energies for isolated water dimers also provides a reasonable description of the radial distribution functions of liquid water. We show that the structure of liquid water varies in a systematic fashion with the choice of basis set. Comparable to many other first-principle studies of liquid water using gradient-corrected density functionals, the liquid is found to be somewhat overstructured. The possibility of further improvements through a better choice of the basis set is discussed. We find that while improvements are likely to be possible, application to large-scale systems will require use of a computational algorithm whose computational cost scales linearly with system size. Finally, we study the molecular and atomic adsorption of oxygen on the gold nano-clusters. We show multiple stable and metastable structures for atomically and molecularly adsorbed oxygen to the gold cluster. We plan to predict the reaction pathway and calculate activation energy barrier for desorption of molecular oxygen from the atomically adsorbed gold cluster which is very important for any catalytic reaction occurring using gold nanoparticles.
M.S.
Department of Mechanical, Materials and Aerospace Engineering;
Engineering and Computer Science
Materials Science and Engineering

Three types of Calcium Copper Titanium oxide (CCTO), i.e. pure CCTO, niobium doped CCTO, and tin doped CCTO were obtained by mixed oxide route. The niobium and tin as a dopant were selected with the variation of doping concentration to create the stoichiometric formula CaCu3Ti4-xMxO12 (x = 0.05, 0.10, 0.15, 0.20, and 0.25; M = Nb, and Sn). Three different sintering times were applied for all the samples. X-ray diffraction, scanning electron microscopy,impedance spectroscopy at high and low temperature were used to determine the phases, dielectric constant, dielectric loss, resistivity and capacitance of bulk and grain-boundary, nonlinear coefficient of conduction, breakdown voltage-field, barrier height, and microstructural changes of CCTO. The purpose of this research is to investigate and provide the transition of dielectric properties from pure CCTO to non-linear behaviour in niobium and tin doped CCTO. The dielectric properties is represented by grain boundary thickness (tgb) and activation energy for electrical conductivity of bulk (Eab) and grain boundary (Eagb). The non-linear behaviours is represented by non-linear coefficient of conduction () and barrier height (B). The relative dielectric constant tends to decrease with increasing the amount of niobium and tin dopant. The activation energy of niobium and tin doped CCTO are in line with barrier height which are associated with conduction processes; the dopants are shown to reduce the conductivity of samples. The barrier height and grain boundary thickness tend to decrease with increasing the amount of niobium and tin dopant. The narrowest grain boundary thickness will increase rapidly the effective dielectric constant which is attributed to the formation of thin insulating boundaries with the conducting grains. The non-linear coefficient of conduction () of niobium and tin doped CCTO depend on the sintering hold time and the amount of dopant. The maximum value of alpha of niobium doped CCTO is ~600 at CCT10N4, and the maximum value of alpha of tin doped CCTO is ~830 at CCT15S4. Identically to the  value trend, the barrier height (B) also varies according to sintering hold time and the amount of dopant, and reaches maximum ~0.180 eV at CT05N4 in the range of ~0.173 to ~0.180 eV. There is no direct correlation between non-linear coefficient of conduction and barrier height in terms of sintering hold time for niobium and tin doped CCTO. The bulk activation energy of CCTO is in the range of 0.044 and 0.118 eV and the grain boundary activation energy is in the range of 0.389 and 0.706 eV. The large difference between bulk and grain boundary Ea is a strong indicator that different charge transport mechanisms exist for niobium and tin doped CCTO.

LiMn2O4 is ideal as a high-capacity Li-ion battery cathode material by virtue of its low toxicity, low cost, and the high natural abundance of Mn. Surface related reactions and bulk kinetics have been the major focus of this work. The main techniques exploited have been: electrochemical cycling, X-ray diffraction, X-ray photoelectron spectroscopy, infrared spectroscopy and thermal analysis. Interface formation between the LiMn2O4 cathode and carbonate-based electrolytes has been followed under different pre-treatment conditions. The variables have been: number of charge/discharge cycles, storage time, potential, electrolyte salt and temperature. The formation of the surface layer was found not to be governed by electrochemical cycling. The species precipitating on the surface of the cathodes at ambient temperature have been determined to comprise a mixture of organic and inorganic compounds: LiF, LixPFy (or LixBFy, depending on the electrolyte salt used), LixPOyFz (or LixBOyFz) and poly(oxyethylene). Additional compounds were found at elevated temperatures: phosphorous oxides (or boron oxides) and polycarbonates. A model has been presented for the formation of these surface species at elevated temperatures. The cathode surface structure was found to change towards a lithium-rich and Mn3+-rich compound under self-discharge. The reduction of LiMn2O4, in addition to the high operating potential, induces oxidation of the electrolyte at the cathode surface. A novel in situ electrochemical/structural set-up has facilitated a study of the kinetics in the LiMn2O4 electrode. The results eliminate solid-phase diffusion as the rate-limiting factor in electrochemical cycling. The electrode preparation method used results in good utilisation of the electrode, even at high discharge rates.

With increasing environmental awareness, the desire to protect human beings and the environment from adverse effects induced by dispersed metals has become an issue of great concern and interest. New policies, such as REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) within the European Community, have been implemented to reduce hazards posed by the use of chemicals on producers and downstream users. The generation of exposure assessment data and relevant test procedures able to simulate realistic scenarios are essential in such legislative actions. This doctoral study was initiated to fill knowledge gaps related to the metal release process of stainless steels. A wide range of stainless steel grades, fourteen in total, were investigated. They cover a very broad range of applications, and the focus in the thesis was to simulate a few selected exposure scenarios: precipitation, the human body and food intake. Comparisons were made between metal release from stainless steel alloys and the pure metals that constitute each stainless steel in order to explore the differences between alloys and pure metals, and to provide quantitative data on metal release rates of different alloy constituents. Because of similar surface properties between stainless steel and pure chromium, this metal exhibits similar release rates, whereas iron and nickel exhibit significantly lower release rates as alloy components than as pure metals. Detailed studies were also performed to elucidate possible relations between metal release and steel surface properties. Key parameters turned out to be chromium enrichment of the self-passivating surface film, surface roughness, the electrochemically active surface area and the microstructure of the steel substrate. The degree of metal release increased with decreasing chromium content in the surface oxide, increasing surface roughness, and increasing presence of inhomogeneities in the bulk matrix. More detailed studies were initiated to possibly correlate the nucleation of metastable pits and the extent of metal release. Evidence was given that metastable pits exist even when the stainless steel is passive, and may cause extremely short-lived bursts of released metal before the surface film repassivates again.
QC 20100810

The management and disposal of huge volumes of coal combustion by products such as fly ash has constituted a major challenge to the environment. In most cases due to the inadequate alternative use of coal fly ash, the discarded waste is stored in holding ponds, slag heaps, or stock piled in ash dumps. This practice has raised concerns on the prospect of inorganic metals release to the surface and groundwater in the vicinity of the ash dump. Acceptable scientific studies are lacking to determine the best ash disposal practices. Moreover, knowledge about the mobility patterns of inorganic species as a function of mineralogical association or pH susceptibility of the dry disposed ash dump under natural weathering conditions are scarce in the literature. Fundamental understanding of chemical interactions of dry disposed ash with ingressed CO2 from atmosphere, percolating rain water and brine irrigation within ash disposal sites were seen as key areas requiring investigation. The mineralogical association of inorganic species in the dry disposed ash cores can be identified and quantified. This would provide a basis for understanding of chemical weathering, mineralogical transformations or mobility patterns of these inorganic species in the dry ash disposal scenario. The current study therefore aims to provide a comprehensive characterisation of weathered dry disposed ash cores, to reveal mobility patterns of chemical species as a function of depth and age of ash, with a view to assessing the potential environmental impacts. Fifty-nine samples were taken from 3 drilled cores obtained respectively from the 1 year, 8 year and 20-year-old sections of sequentially dumped, 
weathered, dry disposed ash in an ash dump site at Tutuka - a South African coal burning power station. The core samples were characterized using standard analytical procedures viz: X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier transforms infrared (FTIR) techniques, Scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) and Acid neutralisation capacity (ANC) test. A modified sequential extraction (SE) method was used in this study. The chemical partitioning, mobility and weathering patterns in 1 year, 8 year and 20-year-old sections of the ash dump were respectively investigated using this modified sequential extraction scheme. The sequence of the extractions was as follows: (1) water soluble, (2) exchangeable, (3) carbonate, (4) iron and manganese and (5) residual. The results obtained from the 5 steps sequential extraction scheme were validated with the total metal content of the original sample using mass balance method. The distribution of major and trace elements in the different liquid fractions obtained after each step of sequential extraction of the 59 drilled core samples was determined by inductively coupled plasma mass spectrometry (ICPMS). The data generated for various ash core samples were explored for the systematic analysis of mineralogical transformation and change in ash chemistry with ageing of the ash. Furthermore, the data was analyzed to reveal the impact of ingressed CO2 from atmosphere, infiltrating rain water and brine irrigation on the chemistry of ash core samples. Major mineral phases in original ash core samples prior to extraction are quartz (SiO2) and mullite (3Al2O3·
2SiO2). Other minor mineral phases identified were hematite (Fe2O3), calcite (CaCO3), lime (CaO), anorthite (CaAl2Si2O8), mica (Ca (Mg, Al)3 (Al3Si) O10 (OH)2), and enstatite (Mg2Si2O6). X-ray diffraction results show significant loss of crystallinity in the older ash cores. The presence of minor phases of calcite and mica in dry disposed ash cores are attributed to reduction in the pore water pH due to hydration, carbonation and pozzolanic reactions. The X-ray diffraction technique was unable to detect Fe-oxyhydroxide phase and morealuminosilicate phases in ash core samples due to their low abundance and amorphous character. X-ray fluorescence results of the original ash core samples showed the presence of major oxides, such as SiO2, Al2O3, Fe2O3, while CaO, K2O, TiO2, Na2O, MnO, MgO, P2O5, and SO3 occur in minor concentrations. The ratio of SiO2/Al2O3 classified the original core samples prior to extraction as a silico-aluminate class F fly ash. The ternary plot of major elements in 1-year-old ash core samples was both sialic and ferrocalsialic but 8 year and 20-year-old ash core samples were sialic in chemical composition. It is noteworthy that the mass % of SiO2 varies through the depth of the core with an increase of nearly 3 %, to 58 mass % of SiO2 at a depth of 6 m in the 1-year-old core whereas in the case of the 8-year-old core a 2 % increase of SiO2 to a level of 57.5 mass % can be observed at levels between 4-8 m, showing dissolution of major components in the matrix of older ash cores.. The Na2O content of the Tutuka ash cores was low and varied between 0.6-1.1 mass % for 1-year-old ash cores to around 0.6-0.8 mass % for 8-year-old ash cores. Sodium levels were higher in 1-year-old ash cores compared to 8 year and 20-year-old ashcores. Observed trends indicate that quick weathering of the ash (within a year) leached out Na+ from the ash dump. No evidence of Na+ encapsulation even though the ash dump was brine irrigated. Thus the dry disposal ash placement method does not result in a sustainable salt sink for Na-containing species over time. The total content of each of the elements in 1 year and 20-year-old ash cores was normalised with their total content in fresh ash from same power station to show enrichment and depletion factor. Major elements such as K+, Mn showed enrichment in 1-year-old ash cores whereas Al, Si, Na+, Ti, Ca, Mg, S and Fe showed depletion due to over time erosion. Trace elements such as Cr, Sr, P, Ba, Pb, V and Zn showed enrichment but Ni, Y, Zr showed depletion attributed to over time erosion. In 20-year-old ash cores, major elements such as Al, Na+ and Mn showed enrichment while Si, K+, Fe, Mg and Ca showed depletion highlighting their mobility. Trends indicated intensive flushing of major soluble components such as buffering constituents (CaO) by percolating rain water. The 1-year-old and 20-year-old coal ash cores showed a lower pH and greater loss/depletion of the soluble buffering constituents than the 2-week-old placed ash, indicating significant chemical weathering within a year. Based 
on ANC results the leaching behaviours of Ca, Mg, Na+, K+, Se, Cr, and Sr were found to be controlled by the pH of the leachant indicating high mobility of major soluble species in the ash cores when in contact with slightly acid rain water. Other investigated toxic metals such as As, Mo and Pb showed amphoteric behaviour with respect to the pH of the leachant. Chemical alterations and formation of transient minor secondary mineral phases was found to have a significant effect on the acid susceptibility and depletion pattern of chemical species in the core ash samples when compared to fresh ash. These ANC results correlated well with the data generated from the sequential extraction scheme. Based on sequential extraction results elements, showed noticeable mobility in the water soluble, exchangeable and carbonate fractions due to adsorption and desorption caused by variations in the pore water pH. In contrast, slight mobility of elements in the Fe and Mn, and residual fractions of dry disposed fly ashes are attributed to the co-precipitation and dissolution of minor amount of less soluble secondary phase overtime. The 1-year-old dry disposed ash cores were the least weathered among the 3 drilled ash cores. Therefore low concentration of toxic metals in older ash cores were ascribed to extensive weathering with slower release from residual mineral phases over time. Elements were found to associate with different mineral phases depending on the age or depth of the core samples showing greater heterogeneity in dispersion. For instance the average amount of total calcium in different mineral associations of 1-year-old ash cores is as follows
water soluble (10.2 %), exchangeable (37.04 %), carbonate (37.9 %), Fe and Mn (7.1 %) and residual (2.97 %). The amount of total Na+ in different mineral phases of 1-year-old ash cores followed this trend: water soluble (21 %), exchangeable (11.26 %), carbonate (2.6 %), Fe and Mn (4.7 %) and residual (53.9 %). The non-leachable portion of the total Na+ content (namely that contained in the residual fraction) in the 1-year-old ash core samples under conditions found in nature ranged between 5-91 %. This non-leachable portion of the Na+ showed the metastability of the mineral phases with which residual Na+ associates. Results showed older ash cores are enriched in toxic elements. Toxic elements such as As, B, Cr, Mo and Pb are enriched in the residual fraction of older ash cores. For instance As concentration in the residual fraction varied between 0.0003- 0.00043 mg kg-1 for 1-year-old ash cores to around 0.0003-0.0015 mg kg-1 for 20-year-old ash cores. This suggests that the older ash is enriched in toxic elements hence dust from the ash dump would be toxic to human health. The knowledge of mobility and ecotoxicological significance of coal fly ash is needed when considering its disposal or reuse in the environment. The mobility and ecotoxicology of inorganic metals in coal fly ash are determined by (i) mineralogical associations of inorganic species (ii) in-homogeneity in the ash dumps (iii) long and short term exposure to ingress CO2 and percolating rain water. Management issues such as inconsistent placement of ash in the dumps, poor choice of ash dump site, in-homogeneity in brine irrigation, no record of salt load put on the ash dumps and lack of proper monitoring requires improvement. The thesis provides justification for the use of the modified sequential extraction scheme as a predictive tool and could be employed in a similar research work. This thesis also proved that the dry ash disposal method was not environmental friendly in terms of overall leaching potential after significant chemical weathering. Moreover the study proved that the practice of brine co-disposal or irrigation on ash dumps is not sustainable as the ash dump did not act as a salt sink.

Organic photovoltaics (OPVs) have received a great deal of focus in recent years as a possible alternative to expensive silicon based solar technology. Current challenges for organic photovoltaics are centered around improving their lifetimes and increasing their power conversion efficiencies. One approach to improving the lifetime of such devices has been the inclusion of inorganic metal oxide layers, but interaction between the metal oxides and common conjugated polymers is not favorable. Here we present two methods by which the interactions between polythiophenes and nanostructured ZnO can be made to be more favorable. Using the first method, direct side on attachment of polythiophene to ZnO nanowires via chemical grafting, we demonstrate chemical linkage between the polymer and ZnO phases. The attachment was confirmed to affect the morphological properties of the polymer layer as well, inducing highly ordered regions of the polymer at the ZnO surface via chemical attachment and physical adsorption. Using the second method to improve polythiophene ZnO interactions, we have functionalized ZnO nanowires with organic molecules that favorably interact with conjugated polymer and organic solvents. Photovoltaic devices were made using a blended active layer of functionalized ZnO nanowires and P3HT. Electrical analysis of the resultant devices concluded that the devices were functional photovoltaic cells and isolated the dominant loss mechanisms for further device improvement.

Abstract The qualitative and quantitative characteristics of each component layer constituting the structure of organic bulk heterojunction solar cells (OSC-BHJ) contribute significantly towards its overall performance. One of the prevalent issues resulting in reduced device efficiency is due to the conformational inhomogeneities in the active and buffer layers. The mechanical stress, extended thermal exposure and presence of mutually reactive component layers etc., affects negatively on the device stability. Effective methods to address these issues will be extensively benefited by the industry since the current commercialisation of the technology is hindered owing to the lower efficiency and stability of these devices. This dissertation focuses on methods to coherently enhance the performance and longevity of the OSC-BHJ devices. The efficiency enhancements of the devices in this work were achieved through two main routes. The first route was through morphological improvement of the active layer. The second route was through boosting the electrical characteristics of hole transporting conducting polymer layer (HTL) by controlled annealing conditions. The introduction of a suitable additive in the active layer was found to reduce unfavourable phase segregation thus resulting in enhanced morphology. Further, the annealing conditions in different atmospheres (air, nitrogen and vacuum) were found to have a clear influence on the optimum functioning of the HTL in the device. Regarding the stability improvement study done in this work, a method of employing suitable interlayer was developed to effectively abate the internal degradation occurring in the device due to etching reaction on the indium tin oxide (ITO) anode by the HTL. Moreover, experimental investigations were carried out for drawing fundamental understanding of stability degenerating issues such as the influence of mechanical defects on transparent conducting metal oxide (ITO) anode on the performance of the device and heat induced degradations in the low band gap polymer-fullerene active layer. The highlight of this research is that the discovered methods are inexpensive, efficient, and easy to adopt. The results of the study could help the technology to overcome some of its limitations and accelerate its progress towards commercialisation
Tiivistelmä Orgaanisten heteroliitosaurinkokennojen kerrosrakenteen ominaisuudet ja laatu vaikuttavat merkittävästi aurinkokennojen toiminnallisuuteen. Erityisesti rakenteelliset epähomogeenisuudet aktiivi- ja puskurikerroksissa heikentävät kennon hyötysuhdetta. Kennojen stabiilisuutta tarkasteltaessa myös mekaanisella rasituksella, pitkittyneellä lämpöaltistuksella ja materiaalien reagoinneilla keskenään kerrosten välillä, on selkeä negatiivinen vaikutus kennojen stabiilisuuteen. Orgaanisen aurinkokennoteknologian kaupallistamisen rajoitteina ovat kennojen heikko hyötysuhde ja stabiilisuus, joten menetelmät jotka tarjoavat ratkaisuja edellä mainittuihin ongelmiin, ovat erittäin tärkeitä teknologiaa kaupallistavalle teollisuudelle. Tämä väitöskirja keskittyy johdonmukaisesti selvittämään tapoja, joilla voidaan parantaa heteroliitosaurinkokennojen hyötysuhdetta ja elinikää. Hyötysuhteen tehostamiseksi valittiin kaksi eri lähestymistapaa, joista ensimmäisessä keskityttiin aktiivikerroksen morfologian parantamiseen ja toisessa aukkoja kuljettavan kerroksen sähköisten ominaisuuksien parantamiseen lämpökäsittelyprosessin avulla. Sopivan lisäaineen avulla aktiivikerroksen ei-toivottua kiteytymistä voidaan pienentää ja parantaa näin kerroksen morfologiaa. Lisäksi työssä todettiin, että lämpökäsittelyn aikaisella ympäristöolosuhteella (ilma, typpi, tyhjiö) on merkittävä vaikutus puskurikerroksen optimaaliseen toimintaan aurinkokennossa. Stabiilisuuden parantamiseksi kehitettiin välikerroksen hyödyntämiseen perustuva menetelmä, jolla voidaan tehokkaasti vähentää kennojen sisäisessä rakenteessa tapahtuvaa toiminnallisuuden heikkenemistä, joka aiheutuu aukkoja kuljettavan kerroksen syövyttävästä vaikutuksesta indiumtinaoksidi (ITO) pohjaiseen anodiin. Tämän lisäksi työssä tutkittiin kokeellisesti stabiilisuuteen heikentävästi vaikuttavia tekijöitä, kuten mekaanisen rasituksen aiheuttamia vaurioita metallioksidi (ITO) anodissa ja lämpöaltistuksesta aiheutuvia vikoja polymeeri-fullereeni rakenteeseen perustuvassa aktiivikerroksessa. Tutkimuksen keskeisin tulos on, että esitellyt keinot aurinkokennojen hyötysuhteen ja stabiilisuuden parantamiseen ovat edullisia, tehokkaita ja helppoja hyödyntää. Tulokset voivat merkittävästi edistää orgaanisten aurinkokennojen teknistä kehitystä ja kiihdyttää niiden tuloa kaupallisiksi tuotteiksi

Solar energy conversion and storage into hydrogen is a valuable approach to capture the energy that is freely available from sunlight and to turn it into a clean fuel. Photoelectrochemical (PEC) water splitting through the dual-absorber tandem cell technology has emerged as a promising strategy to this aim. The work conducted in the frame of this PhD thesis aimed at playing a part in the development and optimization of efficient oxide-based semiconductor photoanodes for water oxidation, which is the kinetic bottleneck of the overall PEC water splitting process. Photoanodes based on films of absorbing materials were successfully synthesized with a high optical transparency as important requirement for maximizing the solar energy conversion efficiency of the final tandem cell device. Subsequently, their intrinsic properties as single photoabsorber photoanodes were largely improved, on the basis of the results obtained through comprehensive PEC studies in parallel with thorough structural, morphological, and spectroscopic investigations. The attention was focused on three different classes of promising ternary metal oxides, able to absorb a large portion of the solar spectrum, namely i) BiVO4 (bandgap Eg = 2.4 eV), known for its excellent solar light to hydrogen conversion efficiency, ii) the copper tungstate-based materials CuWO4 (Eg = 2.3 eV) and CuW(1-x)Mo(x)O4 (Eg = 2.0 eV), ideal to be employed as visible-light active alternative to WO3, and iii) ZnFe2O4 (Eg = 2.0 eV) belonging to the spinel ferrites class, possessing excellent photothermal and chemical stability. Specifically, BiVO4 was studied either as a visible light sensitizer towards TiO2 or as a single photoanode material to focus on the identification and improvement of its intrinsically poor electron transport and interfacial transfer properties. In the first case, the TiO2/BiVO4 heterojunction system was proved to be effective in producing highly reductive electrons, suitable for overall water splitting, through TiO2 sensitization towards visible light. This, together with the counterintuitive mechanism at the basis of the observed impressive functionality, was effectively disclosed through combined PEC and photocatalytic reduction test studies. The multifaceted role of Mo6+ doping onto both the bulk and surface properties of BiVO4 films was also revealed through an in-depth PEC and impedance spectroscopy study. By improving either the bulk conductivity or the interfacial charge transfer of optimized Mo6+ doped BiVO4 photoanodes a conspicuous enhancement was attained of their photoactivity towards water oxidation with respect to the pure material. The presence of intra-gap states in CuWO4, acting as electron traps and thus being responsible for a severe internal charge recombination, was verified by means of the first ultrafast transient absorption study performed with this material, in combination with both an electrochemical and a photochromic characterization. This issue, which strongly limits the PEC performance of CuWO4 photoanodes, was addressed by adopting a 50% Mo for W substitution resulting in CuW0.5Mo0.5O4 photoanodes, exhibiting not only a greatly extended visible light-induced photoactivity compared to the pure material, as a result of enhanced absorption, but also a considerably improved charge separation. All these factors contributed to the much better PEC performance attained with respect to CuWO4 electrodes. This study was finalized by the identification of a suitable hole scavenger species for copper tungstate-based materials, able to ensure enhanced photocurrent generation compared to pure water oxidation while minimizing dark currents. Finally, in the frame of my seven months stage in Prof. Sivula’s group at the EPFL in Lausanne, a thorough study was performed on the impact that several parameters, such as the annealing temperature, the film thickness and the creation of oxygen vacancies through a reductive treatment in hydrogen atmosphere, have on the PEC performance of ZnFe2O4 photoanodes. The verified synergism between the higher crystallinity of the films subjected to a high-temperature annealing treatment and the hydrogenation efficiency, which proved effective in optimizing charge separation in the thicker photoactive layers, allowed one to maximize the performance of ZnFe2O4 electrodes for water oxidation. This study also shed light onto the strict correlation occurring between structural parameters, i.e. the film crystallinity and the spinel inversion degree, and the resulting PEC performance, which proved to be in turn controlled by the film morphology.

[Some symbols cannot be rendered in the following metadata – please see the PDF file for an accurate version of the Abstract] ¶ Recombination within the bulk and at the surfaces of crystalline silicon has been investigated in this thesis. Special attention has been paid to the surface passivation achievable with plasma enhanced chemical vapour deposited (PECVD) silicon nitride (SiN) films due to their potential for widespread use in silicon solar cells. The passivation obtained with thermally grown silicon oxide (SiO2) layers has also been extensively investigated for comparison. ¶ Injection-level dependent lifetime measurements have been used throughout this thesis to quantify the different recombination rates in silicon. New techniques for interpreting the effective lifetime in terms of device characteristics have been introduced, based on the physical concept of a net photogeneration rate. The converse relationships for determining the effective lifetime from measurements of the open-circuit voltage (Voc) under arbitrary illumination have also been introduced, thus establishing the equivalency of the photoconductance and voltage techniques, both quasi-static and transient, by allowing similar possibilities for all of them. ¶ The rate of intrinsic recombination in silicon is of fundamental importance. It has been investigated as a function of injection level for both n-type and p-type silicon, for dopant densities up to ~5x1016cm-3. Record high effective lifetimes, up to 32ms for high resistivity silicon, have been measured. Importantly, the wafers where commercially sourced and had undergone significant high temperature processing. A new, general parameterisation has been proposed for the rate of band-to-band Auger recombination in crystalline silicon, which accurately fits the experimental lifetime data for arbitrary injection level and arbitrary dopant density. The limiting efficiency of crystalline silicon solar cells has been re-evaluated using this new parameterisation, with the effects of photon recycling included. ¶ Surface recombination processes in silicon solar cells are becoming progressively more important as industry drives towards thinner substrates and higher cell efficiencies. The surface recombination properties of well-passivating SiN films on p-type and n-type silicon have been comprehensively studied, with Seff values as low as 1cm/s being unambiguously determined. The well-passivating SiN films optimised in this thesis are unique in that they are stoichiometric in composition, rather than being silicon rich, a property which is attributed to the use of dilute silane as a process gas. A simple physical model, based on recombination at the Si/SiN interface being determined by a high fixed charge density within the SiN film (even under illumination), has been proposed to explain the injection-level dependent Seff for a variety of differently doped wafers. The passivation obtained with the optimised SiN films has been compared to that obtained with high temperature thermal oxides (FGA and alnealed) and the limits imposed by surface recombination on the efficiency of SiN passivated solar cells investigated. It is shown that the optimised SiN films show little absorption of UV photons from the solar spectrum and can be easily patterned by photolithography and wet chemical etching. ¶ The recombination properties of n+ and p+ emitters passivated with optimised SiN films and thermal SiO2 have been extensively studied over a large range of emitter sheet resistances. Both planar and random pyramid textured surfaces were studied for n+ emitters, where the optimised SiN films were again found to be stoichiometric in composition. The optimised SiN films provided good passivation of the heavily doped n+-Si/SiN interface, with the surface recombination velocity increasing from 1400cm/s to 25000cm/s as the surface concentration of electrically active phosphorus atoms increased from 7.5x1018cm-3 to 1.8x1020cm-3. The optimised SiN films also provided reasonable passivation of industrial n+ emitters formed in a belt-line furnace. It was found that the surface recombination properties of SiN passivated p+ emitters was poor and was worst for sheet resistances of ~150./ . The hypothesis that recombination at the Si/SiN interface is determined by a high fixed charge density within the SiN films was extended to explain this dependence on sheet resistance. The efficiency potential of SiN passivated n+p cells has been investigated, with a sheet resistance of 80-100./ and a base resistivity of 1-2.cm found to be optimal. Open-circuit voltages of 670-680mV and efficiencies up to ~20% and ~23% appear possible for SiN passivated planar and textured cells respectively. The recombination properties measured for emitters passivated with SiO2, both n+ and p+, were consistent with other studies and found to be superior to those obtained with SiN passivation. ¶ Stoichiometric SiN films were used to passivate the front and rear surfaces of various solar cell structures. Simplified PERC cells fabricated on 0.3.cm p-type silicon, with either a planar or random pyramid textured front surface, produced high Voc’s of 665-670mV and conversion efficiencies up to 19.7%, which are amongst the highest obtained for SiN passivated solar cells. Bifacial solar cells fabricated on planar, high resistivity n-type substrates (20.cm) demonstrated Voc’s up to 675mV, the highest ever reported for an all-SiN passivated cell, and excellent bifaciality factors. Planar PERC cells fabricated on gettered 0.2.cm multicrystalline silicon have also demonstrated very high Voc’s of 655-659mV and conversion efficiencies up to 17.3% using a single layer anti-reflection coating. Short-wavelength internal quantum efficiency measurements confirmed the excellent passivation achieved with the optimised stoichiometric SiN films on n+ emitters, while long-wavelength measurements show that there is a loss of short-circuit current at the rear surface of SiN passivated p-type cells. The latter loss is attributed to parasitic shunting, which arises from an inversion layer at the rear surface due to the high fixed charge (positive) density in the SiN layers. It has been demonstrated that that a simple way to reduce the impact of the parasitic shunt is to etch away some of the silicon from the rear contact dots. An alternative is to have locally diffused p+ regions under the rear contacts, and a novel method to form a rear structure consisting of a local Al-BSF with SiN passivation elsewhere, without using photolithography, has been demonstrated.

Orientador: Julio Ricardo Sambrano
Banca: Miguel Angel San-Miguel Barrera
Banca: Rogério Custódio
Banca: Ieda Maria Garcia dos Santos
Banca: Luis Vicente de Andrade Scalvi
Resumo: A química computacional tem se mostrado uma ferramenta muito útil no meio científico e tem sido cada vez mais utilizada na pesquisa de novos materiais. Dentre os muitos sistemas estudados com o auxílio da química computacional, destaca-se o óxido de zinco (ZnO), muito utilizado em diversos dispositivos eletrônicos tais como, sensores, células solares, diodos de emissão de luz UV e diodos a laser. À temperatura e pressão ambientes, a estrutura cristalina mais estável do ZnO é hexagonal do tipo wurtzita, na qual os átomos de zinco estão coordenados a quatro átomos de oxigênio. Devido a coordenação tetraédrica e falta de centro de simetria dessa estrutura, o ZnO apresenta propriedades piezoelétricas podendo ser aplicado em sensores piezoelétricos, por exemplo. Atualmente, existem muitos trabalhos científicos relacionados com o ZnO, porém o número de trabalhos teóricos em relação aos trabalhos experimentais ainda é pequeno. Neste sentido, este projeto teve como objetivo a análise das propriedades do ZnO em três morfologias diferentes, bulk, superfícies e nanotubos, aplicando as principais técnicas de modelagem computacional aplicada ao estado sólido tais como escolha do funcional de densidade e funções de base, otimização da geometria, dopagem por substituição de átomos, cálculo de constantes elásticas e piezoelétricas, simulação de pressão hidrostática aplicada a célula unitária, secção do bulk para gerar superfícies, substituição de átomos para formar interfaces, nanotubos e ad... (Resumo completo, clicar acesso eletrônico abaixo)
Abstract: Computacional chemistry has proved to be a very useful tool in the scientific field and has been incrasingly used in the research of new materials. Among the many systems studied with the aid of computacional chemistry, we highlight zinc oxide (ZnO), widely used in many electronic devices such as sensors, solar cells, UV light emitting diodes and laser diodes. At room temperature and pressure, the most stable crystalline structure of ZnO is hexagonal of the wurtzite type, in the zinc atoms are coordinated to four oxygen atoms. Due to the tetrahedral coordination and lack of center of symmetry of this structure, the ZnO presents piezoelectric properties and can be applied in piezoelectric sensors, for example. Currently, there are many papers related to ZnO, however the number of theoretical articles in relation to the experimental works are still small. In this sense, this project aimed the analysis of ZnO properties in three different morphologies, bulk, surfaces and nanotubes, applying the main techniques of computational modeling to solid state such as the choise of density functional and basic functions, optimization of geometry, doping by atom replacement, calculation of elastic and piezoelectric constants, hydrostatic pressure simulation applied to unit cell, bulk section to generate surfaces, replacement of atoms to form interfaces, nanotubes and adsorption of molecules in nanotubes. The calculations were performed applying th Density Functional Theory, with the help of the CRYSTAL14 program, using the hybrid function B3LYP, with the ste of all-electron base functions. The applied methodology preserves the periodicity of the crystalline system (1D for nanotubes, 2D for surfaces or 3D for bulk), in which the building blocks are composed of unit cells and can be replicanted by the symmetry operator. The topological reviews were performed applying the Quantum Theory... (Complete abstract electronic access below)
Doutor

Fundação de Amparo a Pesquisa do Estado de Minas Gerais
PbS quantum dots has been grown in oxide glasses by two distinct methodologies according to the dopants, using the fusing method at 1200ºC. The host glass matrices used were SNAB (SiO2.Na2CO3.Al2O3.B2O3) and SNABP (SiO2.Na2CO3.Al2O3.B2O3.PbO2), with PbS bulk and S (sulphur) added as dopants, respectively. Thermal treatments were applied in a systematic way in order to allow a controlled growth of quantum dots. Different experimental techniques were realized in order to study the growth kinetic of dots as a function of thermal treatment times and dopant concentrations. It was analyzed thermal properties of the glass matrices by Calorimetry Scanning Diferencial (DSC). Many techniques have allowed discussion of the optical properties by Optical Absorption (OA) and Photoluminescence (PL), morphologic properties by Atomic Force Microscopy (AFM), and structural properties by X-Ray Diffractometry (DRX) of the nanocrystals. Qualitative and quantitative analyse of the chemical composition of the samples has been possible by spectrometry of Fluorescence of Raios-X (FRX). Using the ] 4 4 [ . × p k r r method we were able to estimate the avarage diameter of the PbS quantum dots. Calculations of dispersion sizes of PbS quantum dots, from the AO bands, has been presented. We have found the dependence between the quantum dot gap and the dopant concentrations. The relation between the Stokes Shift and the average size of the PbS quantum dots was also presented. Through the previous results, we have defined the methodology referring the host glass matrix SNABP with S bulk added as the most favorable to the growth PbS quantum dots with low size dispersion.
Pontos Quânticos de PbS foram sintetizados em vidros à base de óxidos, a partir duas metodologias distintas quanto a dopagens, pelo método da fusão a 1200ºC. As matrizes vítreas hospedeiras utilizadas foram: SNAB (SiO2.Na2CO3.Al2O3.B2O3) e SNABP (SiO2.Na2CO3.Al2O3.B2O3.PbO2), e os dopantes adicionados à elas foram PbS bulk e S (enxôfre), respectivamente. Tratamentos Térmicos variados e adequados foram efetuados nestas amostras para permitir o crescimento controlado dos pontos quânticos de PbS. Com o objetivo de estudar e acompanhar a cinética de crescimento destes pontos em função dos tempos de tratamentos térmicos e das variadas concentrações de dopantes, foram efetuadas várias técnicas de caracterizações. Estas permitiram análises das propriedades térmicas dos vidros por Calorimetria Exploratória Diferencial (DSC); ópticas, por Absorção Óptica (AO) e Fotoluminescência (PL); morfológicas, por Imagens de Microscopia de Força Atômica (AFM); e estruturais, por Difratometria de Raios-X (DRX) dos referidos nanocristais. Análises qualitativas e quantitativas das composições químicas das amostras sintetizadas neste trabalho foram possíveis através da técnica de espectrometria de Fluorescência de Raios-X (FRX). Com o auxilio do método ] 4 4 [ . × p k r r , foi possível estimar o diâmetro médio dos pontos quânticos de PbS. Cálculos de dispersão de tamanhos ξ a partir da largura a meia altura (W) das bandas de AO e discussões sobre a dependência de energia e o tamanho médio dos pontos quânticos foram apresentados e discutidos. Mostrouse graficamente a relação de dependência entre o Gap dos pontos quânticos e as concentrações de dopantes adicionados às matrizes vítreas. Apresentou-se de forma gráfica a relação entre o deslocamento Stokes e o tamanho médio dos pontos quânticos de PbS. A partir de análises dos resultados obtidos, definiu-se então a metodologia correspondente à matriz SNABP dopada com S bulk como sendo a mais favorável para a formação e crescimento dos pontos quânticos de PbS com menor dispersão de tamanhos entre si.
Mestre em Física

La manutention et la mise en œuvre des matériaux granulaires libèrent de fines particules de poussière qui, dans un contexte professionnel, peuvent gravement affecter la santé et la sécurité des travailleurs, ainsi que le fonctionnement global de l'installation. L’émission de poussières et la capacité d'un matériau à libérer des particules de poussière, dépendent de plusieurs paramètres relatifs au matériau mais aussi au procédé. Ces émissions sont généralement mesurées par des tests d'empoussièrement à l'échelle du laboratoire. Ces tests reposent principalement sur des études expérimentales et manquent de capacité prédictive fiable en raison d'une compréhension limitée des mécanismes mis en jeu et des interactions complexes entre particules, paroi et fluide, survenant simultanément pendant la génération de poussières. Dans le cadre du projet EU ITN T-MAPPP, cette thèse utilise des approches expérimentales et statistiques pour comprendre les mécanismes de génération de poussières en étudiant: a) les effets des caractéristiques des particules et poudres en vrac sur l’émission de poussières; b) la nature et l'ampleur des interactions entre particules, entre particules et parois, et entre particules et fluides; c) l'évolution de l'empoussièrement et des mécanismes de génération pour des applications de poudre de longue durée. Les résultats indiquent que les mécanismes de génération de poussière diffèrent en fonction de la taille des particules et de la distribution de taille de la poudre. Pour les échantillons d'essai et les conditions expérimentales donnés, les différences dans les modèles initiaux de libération de poussière peuvent être caractérisées par trois groupes différents de poudres : - des poudres contenant des particules cohésives fines, - des poudres bimodales (constituées de fines et de grosses particules), - et enfin des poudres constituées de grosses particules. Tandis que la cohésion globale, surtout celle due aux forces de van der Waals (mesurée à l'aide de testeurs de cisaillement) détermine le niveau de poussières pour les poudres fines, de telle sorte qu'une cohésion globale plus élevée conduit à moins de poussière, la fraction de particules fines et la cohésion déterminent toutes deux l'empoussièrement provenant des poudres bi-modales. Les grosses particules peuvent émettre de la poussière uniquement par usure des particules primaires en particules fines aérosolisables plus petites. L'analyse d'un mouvement de particules tracées à l'intérieur d'un tube cylindrique agité par un testeur d'empoussiérage à vortex montre une nature cyclique du mouvement des particules. Le mouvement des particules (position et vitesse) est symétrique et isotrope dans le plan horizontal, les vitesses radiales les plus basses et les plus élevées étant proches du centre du tube et de la paroi, respectivement. Les particules ont tendance à s'élever lentement au milieu du tube tout en descendant rapidement près de la paroi. Les valeurs les plus élevées de la vitesse se trouvent aux hauteurs les plus élevées et près de la paroi interne du tube à essai, où les densités de population sont les plus faibles. Les valeurs plus élevées de la vitesse pourraient provenir d’une diminution du nombre de chocs due à des densités de population plus faibles. L'augmentation de la taille des particules et des vitesses de rotation des tourbillons tend à augmenter la vitesse des particules tandis que l'augmentation de la masse de poudre conduit à une diminution de la vitesse des particules pour des vitesses de rotation allant jusqu'à 1500 tr / min. Pour les échantillons donnés (carbure de silicium, alumine et coke d'acétylène) et les conditions expérimentales, l'empoussièrement initial est déterminé par la fraction de fines particules respirables présentes dans la poudre, mais les modèles et les niveaux de génération de poussière à long terme sont influencés par le comportement d’attrition matérielle
Handling and processing of granular material release fine solid dust particles, which in an occupational setting, can severely affect worker health & safety and the overall plant operation. Dustiness or the ability of a material to release dust particles depends on several material and process parameters and is usually measured by lab-scale dustiness testers. Dustiness tests remain mostly experimental studies and lack reliable predictive ability due to limited understanding of the dust generation mechanisms and the complex interactions between the particles, wall and fluid, occurring simultaneously during dust generation. In the framework of EU ITN project T-MAPPP, this thesis uses an experimental approach to understand the dust generation mechanisms by studying: a) the effects of key bulk and particle properties on powder dustiness; b) the nature and magnitude of inter-particle, particle-wall and particle-fluid interactions; c) the evolution of dustiness and generation mechanisms for long duration powder applications. The results indicate that the dust generation mechanisms differ based on particle size and size distribution of the powder. For the given test samples and experimental conditions, the differences in powder dustiness and dust emission patterns can be characterized by three different groups of powders; powders containing fine cohesive particles, bi-modal (consisting of fine and large particles) powders and lastly, powders consisting of only large particles. While bulk cohesion, especially that stemming from van der Waals forces (measured using shear testers) determines the level of dustiness for the fine powders (in such a way that higher bulk cohesion leads to lower dustiness), both the fraction of fine particles and cohesion determine the dustiness of bi-modal powders. The large particles can emit dust only through attrition of the primary particles into smaller aerosolizable fine particles. Analysis of a traced particle motion inside a cylindrical tube agitated by a vortex shaker dustiness tester shows the cyclic nature of the particle motion. The motion (position and velocity) is symmetric and isotropic in the horizontal plane with lowest radial velocities close to the tube centre and highest at the boundary wall of the test tube. The particles tend to rise up slowly in the middle of the tube while descending rapidly close to the wall. The highest values of the velocity are found at the highest heights and close to the wall of the test tube, where the population densities are lowest. Increasing particle size and vortex rotation speeds tends to increase particle velocity whereas increase in powder mass leads to a decrease in particle velocity for rotation speeds up to 1500 rpm. For the given samples (silicon carbide, alumina and acetylene coke) and the experimental conditions, the initial dustiness is determined by the fraction of fine respirable particles present in the powder but the long-term dust generation patterns and levels are influenced by the material attrition behaviour. Dust is generated by the fragmentation and/or abrasion of primary particles, which may lead to the production and emission of fine daughter particles as dust. The samples with large irregularly shaped particles are likely to show high dustiness by shedding angular corners through inter-particle and particle-wall collisions, thus becoming more spherical in shape. On the contrary, the smaller particles are more resistant to abrasion and generate relatively less dust. While the vortex shaker dustiness tests show similar trends as an attrition tester, our study using alumina and acetylene coke indicate that the results are not interchangeable. Results from this thesis help understand the influence of powder and process parameters which may be manipulated to reduce dust generation. Furthermore, experimental results can be used to develop and validate numerical models to predict dustiness