Photovoltaic solar cells can generate electricity directly from sunlight without emitting harmful greenhouse gases. This makes them ideal candidates as large scale future energy producers for the global energy economy. Ideally, solar cells should be efficient and inexpensive to compete in the global energy market. Unfortunately, a number of fundamental limitations exist for the efficiency due to fundamental loss mechanisms of the semiconductor materials used to make solar cells. One of the dominant loss mechanisms from a conventional silicon solar cell is the transparency of sub-bandgap near-infrared photons. Up-conversion is an optical process involving the sequential absorption of lower energy photons followed by luminescence of a higher energy photon. This mechanism could be exploited to minimise photovoltaic sub-bandgap losses. Rare-earth doped materials have ideal up-conversion luminescent properties and have been utilised for many near-infrared to visible applications. This thesis investigates the near-infrared to near-infrared up-conversion processes required for the sub-bandgap photon utilisation within a silicon photovoltaic device. Various sodium yttrium fluoride phosphors doped with rare-earths were characterised theoretically and experimentally. Erbium doped phosphors were found to be ideal for single wavelength power dependent investigations for the non-linear up-conversion processes. The radiative and non-radiative rates of various erbium doped sodium yttrium fluoride phosphors have been approximated and compared with experimental photoluminescence results. These phosphors have been applied to the rear of a bi-facial silicon solar cell and an enhancement in the near-infrared region has been demonstrated. An external quantum efficiency close to 3.4% was measured at 1523nm under 6mW laser excitation. The non-linear dependence on incident pump power has been investigated along with the dominant up-conversion mechanisms involved. It can be concluded that up-conversion phosphors can enhance the near-infrared spectral response of a silicon device. These phosphors have high luminescent efficiencies once up-conversion occurs, but suffer from poor infrared absorption and low up-conversion efficiencies. The results from this study show that relatively high doping levels of selected rare-earths into low phonon energy crystals can improve the absorption and luminescent properties of the phosphor.

When a metallic alloy is placed in an oxygen environment, oxide scale may be formed on the metal surface. The continuous growth of such oxide scale is enabled by the diffusion of various ionic species in the scale layer primarily driven by the gradient of chemical potentials of these ionic species. In addition, the molar volume of oxide is typically greater than that of the base metal. Consequently, mechanical stresses are generated in the oxide scale. Such mechanical stress, in return, may affect the diffusion of ionic species resulting in different oxide growth kinetics. Such interaction between ionic diffusion and mechanical stresses and its effect on oxide scale growth have not been studied. The goal of this thesis is to develop a systematic model for oxide scale growth that takes into account the diffusion-stress interaction. To achieve this goal, the coupled equations based on continuum formulas for diffusion and stresses are developed in first part of this study. The chemical potentials are defined as a stress dependent function. The variation of stress can therefore change the diffusion force, which is the gradient of chemical potentials, to affect the ionic species distribution and consequently have effects on the oxidation kinetics. The model is used to investigate several important aspects of oxidation including scale growth kinetics, stress distribution in the oxide scale, void formation near the metal/oxide interface, and initiation of oxide scale spallation. The reactive element effect (REE) during oxidation of reactive element doped alloy is extensively studied in this study using the developed stress-diffusion interaction model. The key information, such as the modification effects of reactive element upon the diffusion properties of ionic species in oxide scale are quantitatively accessed for yttrium doped Cr alloy. Finite element method was used through a User Element subroutine for ABAQUS to solve the fully coupled stress-diffusion equations in 2D domains with accounting for both elastic and inelastic deformations. The REEs are comprehensively investigated by studying the effects of yttrium on interfacial delamination driving force, energy release rate (G), oxide-alloy interface morphology, and defect diffusion. The outcomes of this study give (1) a deeper understanding of how stresses affect the oxidation, (2) a model to simulate oxide scale growth, and (3) design guidelines on rare earth element doping for improving oxidation resistance. The results of this work elucidate the impact and importance of stress-diffusion coupling on oxidation kinetics and mechanical reliability.

Thesis (Ph. D.)--Auburn University, 2007. / Abstract. Vita. Includes bibliographical references.

Orientador: Sidney Jose Lima Ribeiro / Banca: Younes Messaddeq / Banca: Denis Boudreau / Banca: Cid Bartolomeu de Araujo / Banca: Anna Marie Ritcey / Banca: Mauricio da Silva Baptista / Banca: Rogéria Rocha Gonçalves / Resumo: Na área biomédica, existe uma crescente demanda por nanossistemas multifuncionais para realização de imageamento e terapia simultaneamente, visando um diagnóstico precoce e máximo benefício terapêutico. Nanopartículas para conversão ascendente de energia (UCNPs) vêm sendo propostas como a sonda biológica ideal devido às suas vantagens únicas relacionadas ao fenômeno de upconversion apresentado por materiais contendo íons lantanídeos, isto é, emissão no visível obtida sob excitação no infravermelho, tais como penetração profunda nos tecidos, uma baixa taxa de autofluorescência e um fotodano mínimo. Além disso, as propriedades luminescentes dos íons lantanídeos podem ser usadas para termometria por serem fortemente dependentes da temperatura. A termometria luminescente é uma técnica de não-contato e alta resolução que vem ganhando atenção na nanomedicina uma vez que a temperatura é um parâmetro fundamental para o funcionamento das células. Danos térmicos às células podem ser localmente fotoinduzidos pelo uso de nanoestruturas metálicas iluminadas em sua banda de ressonância plasmônica por causa da sua elevada absortividade. A primeira parte deste trabalho consiste no desenvolvimento de um sistema multifuncional baseado em nanocascas de ouro (AuNSs) decoradas com UCNPs podendo ser utilizadas para aumentar e medir a temperatura em escala nanométrica. Este sistema foi desenvolvido com a finalidade de uma eventual utilização como agente em terapia fototérmica (PTT), na qual a capac... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: In the biomedical field, there is an increasing demand for multifunctional nanosystems to perform imaging and therapy simultaneously, aiming at early diagnosis and maximum therapeutic benefit. Upconversion nanoparticles (UCNPs) have been proposed as an ideal bio-probe because of their unique advantages related to the upconversion phenomenon presented by materials containing lanthanide ions, e.g. visible emission obtained under near-infrared (NIR) excitation, such as deep tissue penetration, low autofluorescence background and low photo-damage. Moreover, the luminescent properties of lanthanide ions may be used for thermometry because of a strongly temperature-dependent effect. Luminescence nanothermometry is a noncontact and high-resolution technique that has been gaining attention in nanomedicine since temperature is a fundamental parameter in events that occur in cells. The thermal damage of cells may be locally photoinduced by using metal nanostructures illuminated at their localized surface plasmon resonance (LSPR) band because of the enhancement of light absorption. In this work, a multifunctional system was designed combining gold nanoshells (AuNSs) and UCNPs intended as an optical heater and temperature probe at the nanoscale. This system was studied aiming its application as an agent for photothermal therapy (PTT), guided by the thermometer capacity of UCNPs, which allows to optimize the therapeutic benefits. The synthesis of NaGdF4 UCNPs doped with ions Yb3+:Er3+ was... (Complete abstract click electronic access below) / Résumé: Dans le domaine biomédical, il y a une demande croissante pour les nanosystèmes multifonctionnels pour effectuer simultanément l'imagerie et la thérapie, en visant le diagnostic précoce et apporter du bénéfice thérapeutique maximal. Les nanoparticules à conversion ascendante d'energie (UCNPs) ont été proposés comme une bio-sonde idéale en raison de leurs avantages uniques liés au phénomène d'upconversion présenté par les matériaux contenant des ions lanthanides, c'est-à-dire l'émission visible obtenue sous excitation dans le proche infrarouge (NIR), tels qu'une meilleure pénétration dans les tissus, une bas taux d'autofluorescence et un photo-dommage minimal. De plus, les propriétés luminescentes des ions lanthanides peuvent être utilisées pour la thermométrie en raison de leur forte dépendance sur la température. La thermométrie par luminescence est une technique sans contact et à haute résolution qui a attiré l'attention en nanomédecine puisque la température est un paramètre clé dans le fonctionnement des cellules. Des dommages thermiques aux cellules peuvent être localement photoinduits par l'utilisation de nanostructures métalliques illuminées dans leur bande de résonance plasmon en raison de leur absorptivité élevée. La prémière partie de ce travail implique le développement d'un système multifonctionnel, basé sur des nanocoquilles d'or (AuNSs) décorées avec des UCNPs, pouvant être utilisé pour augmenter et mesurer la température à l'échelle nanométrique. Ce système a é... (Résumé complet accès életronique ci-dessous) / Doutor

Made available in DSpace on 2016-12-23T14:41:53Z (GMT). No. of bitstreams: 1 Fernanda Dalto.pdf: 2110489 bytes, checksum: 772ad5a67cbe75024d598e2f0d60fb91 (MD5) Previous issue date: 2011-02-15 / The cerium oxide is a very useful material in a series of processes among which we highlight the automotive catalysts, where it acts as a regulator of oxygen. We studied the synthesis of complex diphenyl-4-amine sulfonate cerium (III) pentahydrate, Ce (C12H10NSO3) 3.5H2O, and its application as a precursor to the oxide CeO2. The complex was synthesized in aqueous solution under argon atmosphere and protected from light. The solid obtained was characterized by elemental analysis of carbon, hydrogen and nitrogen and also by complexometry EDTA, for determining of Ce3+ in the precursor. From an infrared analysis it was possible to verify the presence of functional groups which are characteristic of the compound, and also the TGA in an atmosphere of oxygen and nitrogen allowed to observe the thermal behavior of the complex. These techniques combined, led to the determination of the molecular compound such as Ce (C12H10NSO3)3.5H2O. Considering this compound as a reaction product synthesis proposal we achieve an efficiency of 75% of a product sensitive to air. The TG in oxygen atmosphere at 1000 º C showed the formation of a thermally stable product, identified as cerium oxide (IV), CeO2. Therefore, this complex was subjected to heat treatment under various conditions. The oxides obtained at 800 ° C were characterized by X-ray diffraction, scanning electron microscopy and evaluated on heating under an atmosphere of CO and new heating under an atmosphere of compressed air. The results show that CeO2 is best done under an atmosphere of oxygen. The CeO2-O2 and CeO2-air sample studied presented crystallite sizes of 6 and 7 nm, respectively. Although the sample CeO2-air is not pure, the samples were stable when exposed to reducing atmospheres of CO and oxidizing air compressed under heating / O Óxido de cério é um material muito útil em uma série de processos dentre os quais destacamos os catalisadores automotivos, onde atua como regulador de oxigênio. Neste trabalho, estudamos a síntese do complexo difenil-4-amina sulfonato de cério (III) penta-hidratado, Ce(C12H10NSO3)3.5H2O, e sua aplicação como precursor para o óxido CeO2. O complexo foi sintetizado em meio aquoso, sob atmosfera de argônio e protegido da luz. O sólido obtido foi caracterizado por análise elementar de carbono, hidrogênio e nitrogênio e ainda por complexometria com EDTA, para determinação do conteúdo de Ce3+ no precursor. A partir da análise por infravermelho foi possível verificar a presença de grupos funcionais característicos do composto bem como, a análise termogravimétrica em atmosfera de oxigênio e argônio permitiu observar o comportamento térmico do complexo. Essas técnicas somadas, levaram a determinação da fórmula molecular do composto como Ce(C12H10NSO3)3.5H2O. Considerando este composto como produto da reação de síntese proposta obteve-se um rendimento de 75%, de um produto sensível ao ar. A análise termogravimétrica em atmosfera de oxigênio até 1000ºC, mostrou a formação de um produto termicamente estável e identificado como o óxido de cério (IV), o CeO2. Portanto, tal complexo foi submetido a tratamento térmico sob condições variadas. Os óxidos obtidos a 800ºC foram caracterizados por difratometria de raio X, microscopia eletrônica de varredura e avaliados no aquecimento sob atmosfera de CO e reaquecimento sob atmosfera de ar comprimido. Os resultados mostram que o CeO2 é obtido mais eficientemente sob atmosfera de oxigênio. As amostras estudadas CeO2-O2 e CeO2-ar, apresentaram tamanho médio de cristalito de 6 e 7nm, respectivamente. Embora a amostra CeO2-ar não esteja pura, essas amostras se mostraram estáveis quando submetidas a atmosferas redutora de CO e oxidante de ar comprimido, sob aquecimento

Orientadores: Sergio Gama, Pedro Jorge von Ranke / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-08T01:09:57Z (GMT). No. of bitstreams: 1 Carvalho_AlexandreMagnusGomes_D.pdf: 6808779 bytes, checksum: b779b54728ac489a69ae449f1e33d093 (MD5) Previous issue date: 2006 / Resumo: Este trabalho apresenta resultados de estudos estruturais, magnéticos e magnetocalóricos de alguns compostos à base de gadolínio, germânio e silício. Os estudos estruturais incluem análises por microscopia ótica e eletrônica, além de difração de raios-X. As análises magnéticas restringem-se a medidas de magnetização em função da temperatura e do campo magnético. Utilizando essas técnicas, são estudados compostos sob diferentes condições de processamento, tais como: amostras como fundidas; tratadas termicamente; pulverizadas e sinterizadas. Os compostos Gd5Ge4 e Gd5Ge2Si2 são também analisados magneticamente sob pressão hidrostática. Adicionamos hidrogênio aos compostos Gd5Si4, Gd5G e2Si2 e Gd5Ge2,1S i1,9 e substituímos Ge e Si por Sn no composto Gd5Ge2Si2, criando novas famílias de materiais, as quais também foram analisadas pelas técnicas supracitadas. Além do trabalho experimental, são apresentados resultados teóricos para o efeito magnetocalórico, utilizando o modelo de Landau-Devonshire. Apresentamos também resultados fenomenológicos para o composto Gd5Ge2Si2 sob pressão hidrostática, utilizando o Modelo de Acoplamento Spin-Rede (MASR) / Abstract: This work presents the results from structural, magnetic and magnetocaloric studies about some compounds based on Gd, Ge and Si. Structural studies include optical and electronic microscopy analyses, besides X-ray diffraction. Magnetic analyses are limited to measurements of magnetization as a function of temperature and magnetic field. Using these techniques, the compounds are investigated under different processing conditions, such as: as-cast, heat-treated, powdered and sintered samples. Gd5Ge4 and Gd5Ge2Si2 compounds are also analyzed through magnetic measurements performed under hydrostatic pressure. We have inserted hydrogen atoms into Gd5Si4, Gd5G e2Si2 and Gd5Ge2,1S i1,9 compounds and substituted Sn for Ge and Si in Gd5Ge2Si2 compound. Thus, new families of materials were developed, which are also investigated in this work using the techniques mentioned above. Besides the experimental work, theoretical results are presented for the magnetocaloric effect using the Landau-Devonshire model. We also present phenomenological results for Gd5Ge2Si2 compound under hydrostatic pressure using the Coupling Magnetic-Lattice Model / Doutorado / Física da Matéria Condensada / Doutor em Ciências

Density functional theory is an efficient and useful method of solving single-reference computational chemistry problems, however it struggles with multi-reference systems. Modifications have been developed in order to improve the capabilities of density functional theory. In this work, density functional theory has been successfully applied to solve multi-reference systems with large amounts of non-dynamical correlation by use of modifications. It has also been successfully applied for geometry optimizations for lanthanide trifluorides.

Two adjacent caldera systems, the Mahogany Mountain and the Three Fingers caldera constitute voluminous rhyolitic volcanic deposits on the eastern margin of the Oregon-Idaho graben during the middle-Miocene. Both calderas are part of the Lake Owyhee volcanic field that in turn is part of widespread rhyolite deposits associated with the Columbia River Basalt province. We focus on establishing relationships between intracaldera units of Three Fingers caldera and caldera-forming tuff of Spring Creek and surveying the distribution of entrained mafic clasts which often display anomalous concentrations of rare earth elements. Previous mapping identified two intra-caldera facies and one outflow facies of the tuff of Spring Creek, in addition to a younger rhyolite within the caldera (Trp). New 40Ar/39Ar dates show these units are nearly time equivalent at 15.64 ± 0.08 Ma for Trp and 15.64 ± 0.09 Ma for tuff of Spring Creek. Field evidence shows extensive coverage of Trp and associated facies emplaced after a period of sedimentation within the caldera. The main reinterpretations are: i) the mostly devitrified units of Trp are time equivalent to flows and domes of glassy, vesicular, or brecciated rhyolite previously mapped as intra-caldera tuff of Spring Creek; and ii) mafic clasts present in dense glass and porous rhyolite are fragments of mafic lava flows entrained by the subsequent eruptions. New geochemical and mineralogical evidence clearly distinguish the outflow tuff of Spring Creek and intracaldera rhyolites. Compared to the outflow tuff, intracaldera rhyolite flows are less Fe-rich, (2 vs. 3 wt.% FeO), and higher silica (77 vs. 74 wt.% SiO2) rhyolites that lack vitrophyric texture. I interpret the investigated area as a rhyolite dome field, erupted subsequent to caldera collapse. The proximity of vents resulted in a complex stratigraphic overlap of rhyolite flows and clastic debris issued from coalescing domes. The predominance of high-standing dome interiors reflects the more resistant nature of dense devitrified rhyolite as compared to pumiceous, glassy, or brecciated facies of intra-caldera rhyolite. Enrichment of REE in mafic clasts is highly variable, and does not correlate with their entrainment in a specific facies of intra-caldera rhyolite. Individual clasts contain up to 2400 ppm Nd, 1800 ppm Ce, and 1400 ppm La in the most enriched samples. Linear regression shows these highly anomalous concentrations are not correlated with variations in major element chemistry between enriched and un-enriched clasts. The geographic extent of mafic clast-bearing units is limited to less than 5 percent of the area mapped, and their distribution within these units is typically volumetrically insignificant, limiting their economic potential. Mechanisms for enrichment of REE within these rocks is however significant to our understanding of a yet unexplained phenomenon and may lead to further discoveries with greater economic potential.

Luminescent lanthanide containing coordination polymers and metal-organic frameworks hold great potential in many applications due to their distinctive spectroscopic properties. While the ability to design coordination polymers for specific functions is often mentioned as a major benefit bestowed upon these compounds, the lack of a meaningful understanding of the crystal engineering and luminescence in lanthanide coordination polymers remains a significant challenge toward functional design. Currently, the study of luminescence attributed to these compounds is based on the antenna effect as derived from molecular systems, where organic antennae are used to facilitate lanthanide-centered luminescence. This molecular based approach does not take into account the unique features of extended network solids, particularly the formation of band structure. By comparing molecular and band-based approaches, it was determined that the band structure of the organic sensitizing linker needs to be considered when evaluating the luminescence of lanthanide coordination polymers. This new model, as well as work on the crystal engineering and sensor applications of these materials will be presented. / Includes bibliography. / Dissertation (Ph.D.)--Florida Atlantic University, 2017. / FAU Electronic Theses and Dissertations Collection

In der vorliegenden Arbeit wurden Verbindungen untersucht, die reguläre oder verzerrte planar-quadratische Chalkogenschichten als ein wesentliches Strukturelement enthalten. Neben der Darstellung neuer Verbindungen und der Identifizierung des Phasenbestandes in den binären Zustandsdiagrammen Ln – Se lag das Hauptaugenmerk der Untersuchungen auf der Aufklärung der z. T. komplexen Überstrukturen und deren kristallchemischen Einordnung sowie auf den elektronischen Eigenschaften der Verbindungen. Methodisch kamen zur Strukturlösung und -beschreibung dabei sowohl Röntgen- und Elektronenbeugung, als auch hochauflösende Transmissionselektronenmikroskopie und Elektronenholographie zum Einsatz. Bei Strukturverfeinerungen der kommensurabel und inkommensurabel modulierten Strukturen wurde in vielen Fällen das Superraumkonzept angewandt, das eine einheitlichere Beschreibung verwandter Strukturen in höherdimensionalen Superraumgruppen ermöglicht. Im einzelnen wurden die Polyselenide PrSe2 und NdSe2 (beide kristallisieren im CeSe2-Typ) und SmSe1.9 (CeSe1.9-Typ) sowie die selenärmeren Verbindungen Ln8Se15 mit Ln = Y, Gd, Tb, Dy, Ho und Er (alle: Gd8Se15-Typ) hergestellt und erstmals strukturell charakterisiert. Diese Verbindungen kristallisieren als kommensurable Überstrukturen eines hochsymmetrischen Aristotyps, des ZrSSi-Typs. Die ebenfalls neu aufgefundenen Selenide Nd0.6Gd0.4Se1.85 und PrSe1.85 bilden dagegen inkommensurabel modulierte Strukturen aus und werden mit Hilfe des Superraumformalismus beschrieben. Die untersuchten Polyselenide weisen halbleitendes Verhalten auf und enthalten dreiwertige Lanthanoidmetalle. Die ebenfalls neu aufgefundenen Substanzklasse der Lanthanoidselenidditelluride LnSeTe2 (Ln = La, Ce, Pr, Nd, Sm) sind als ternäre Ordnungsvarianten des NdTe3-Typs zu beschreiben. Die Verbindungen LaSeTe2, CeSeTe2, PrSeTe2 und NdSeTe2 durchlaufen reversible, temperaturabhängige Phasentransformationen von einer nicht modulierten Hochtemperaturphase in eine inkommensurabel modulierte Tieftemperaturphase, die mit einem Metall-Halbmetall-Übergang korreliert.