In the last decade new materials appeared that are candidates to be used as an electrolyte in a Solid Oxide Fuel Cell, SOFC. Some materials show high ionic conductivity but lack in important properties, such as mechanical stability or chemical compatibility with other materials in the fuel cell. Scandia Stabilised Zirconia, SSZ, became a possibility when the scandia price dropped with the opening of the Chinese and Russian markets. In the starting system Ce[subscript(x)]Y[subscript(0.2-x)]Sc₀.₆Zr₃.₂O[subscript(8-δ)], (0≤x≤0.2), scandia is introduced to improve conductivity and stabilise the cubic phase; yttria is introduced to fully stabilise the cubic phase and ceria to enhance conductivity lost with the introduction of yttria. The aim of this project is to develop a reliable new method to produce quality ceramics that are not strongly composition dependent, then to prepare a range of compositions and compare intrinsic properties without having to be concerned that poor sintering dominates conduction properties. This project can be divided in two sections, the first section the powder production method, the characteristics of the powders and its final products are in focus. In the second section the relation between electric characteristics and microstructure of the material is reported. In the first section, the effect of different compositions of the system Ce[subscript(x)]Y[subscript(0.2-x)]Sc₀.₆Zr₃.₂O[subscript(8-δ)], (0≤x≤0.2) is studied, in terms of structure, phase and microstructure. The nature, size and shape of the powders are discussed, and their effect on the final product. The sol-gel and combustion method gives the formation of hard agglomerates (shells), during the combustion, a wide range of grain sizes, between less than 1µm and 200 µm, and the formation of grains with non spherical shape. In this project, the sol-gel and combustion process and solid state method are also compared. In the second section of this project, AC Impedance measurements, as a function of temperature, oxygen partial pressure and time are discussed. The Arrhenius plot for all compositions shows two regions (high and low temperature) and the change of region occurs at 580 °C. At low temperatures there is a slight difference between compositions but this difference is less at high temperatures. The obtained ionic conductivity, at 350 °C, varies from 3.84×10⁻⁶ to 5.53×10⁻⁵ S/cm; at 700 °C, ionic conductivity from 0.013 to 0.044 S/cm. At low temperatures, the activation energy associated with bulk process is generally lower than grain boundary process; for example, the composition Ce₀.₁Y₀.₁Sc₀.₆Zr₃.₂O₇.₆₅ has an activation energy, for the bulk process, of 1.05 eV and an activation energy, for the grain boundary process, of 1.17 eV. For compositions with higher ceria content, activation energy, for bulk and grain boundary, have similar values. The AC impedance as function of oxygen partial pressure show that the amount of ceria introduced as an effect on the conductivity at low oxygen partial pressure. For the sample with no ceria in its composition, Y₀.₂Sc₀.₆Zr₃.₂O₇.₆₀, the conductivity does not vary significantly as the oxygen partial pressure is decreased; for oxygen partial of 0.21 atm, conductivity is 0.018 S/cm and when oxygen partial pressure is approximately 10⁻²⁴ atm conductivity is 0.018 S/cm. For the sample with a higher content of ceria, Ce₀.₁₂Y₀.₀₈Sc₀.₆Zr₃.₂O₇.₆₆, there is a decrease in conductivity while oxygen partial pressure decreases; and there is also the appearance of a semi-circle for lower oxygen partial pressures. For oxygen partial pressure approximately 0.21 atm, conductivity is 0.019 S/cm, but when oxygen partial pressure is decreased to 10⁻²⁴ atm conductivity decreases to 0.011 S/cm. AC impedance measurements as a function of annealing time at 600 °C were performed. Total conductivity is fairly stable, for all compositions, until 1800 hours but after this time, conductivity slowly decreases. Some compositions show a second semi-circle in the AC impedance spectra, either from the beginning, time equals 0 hours, or after some working hours. Here, the changes in conduction and conduction processes with time are discussed.

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Ordered mesoporous materials have attracted much attention recently for use in a wide range of applications. The oxidising materials, ceria (CeO₂) and CGO (Ce₀.₉Gd₀.₁O[subscript(2-δ)]) have both been synthesised with ordered mesopores, but a method for the simple fabrication of these materials in high yields with crystalline pore walls has not yet been reported in the literature. This thesis details the development of the vacuum impregnation method for the synthesis of ordered mesoporous materials with emphasis on ceria and CGO. Using the vacuum impregnation method both materials were successfully prepared. The materials exhibited the porous single crystal morphology in high yields, with unusual crystallographic features. Nitrogen physisorption, transmission electron microscopy (TEM), TEM tomography and temperature programmed studies were employed. Temperature programmed studies showed the materials to be catalytically active at lower temperatures than traditionally-prepared ceria. Photovoltaic studies showed that the materials exhibited efficient exciton quenching. The observation of nanowire extrusion during the synthetic procedure assisted in the postulation of a mechanism for product formation in the vacuum impregnation method. The vacuum impregnation method was subsequently shown to be applicable to the synthesis of other materials, with encouraging results presented for ordered mesoporous carbon and Zr₀.₈₄Y₀.₁₆O[subscript(2-δ)]. The syntheses of ordered mesoporous La₀.₈₅Sr₀.₁₅GaO[subscript(3-δ)] and La₀.₇₆Sr₀.₁₉CoO[subscript(3-δ)] were unsuccessful.

Les oxydes mixtes de cérium-zirconium sont au cœur de nombreux sujets de recherche. En effet, ces matériaux sont utilisés dans différents domaines d'application, et tout particulièrement en catalyse, dans les catalyseurs trois voies en automobile ou pour l'oxydation de polluants organiques. L'intérêt pour ce composé réside dans ses propriétés remarquables d'oxydoréduction, sa capacité de stockage de l'oxygène et sa résistance au frittage. En conséquence, un nombre important de méthodes de synthèse ont été mises au point et soulignent la grande sensibilité de cet oxyde mixte à la voie de préparation employée. Dans le cadre de ce travail deux grandes voies de synthèses ont été utilisées, la co-précipitation (milieu aqueux) et la dégradation des -dicétonates métalliques (milieu non aqueux). Ces synthèses ont été réalisées à l'aide de plusieurs techniques que sont (i) le chauffage à pression atmosphérique, (ii) le traitement hydrothermal, (iii) la sonolyse et (iv) la combinaison de la sonolyse et du traitement hydrothermal, de façon successive ou en simultané. Afin de pouvoir réaliser cette dernière méthode de synthèse innovante, un réacteur permettant de réaliser un traitement sonochimique en température et sous pression a été développé (réacteur sonothermal). L'activité chimique des ultrasons dans ces conditions a pu être mise en évidence lors de la sonolyse de l'eau (légère ou lourde) et de celle d'un hydrocarbure. Au cours de la préparation de (Ce,Zr)O2, des paramètres autres que le dispositif de synthèse ont été étudiés, comme la présence d'un surfactant ou la longueur de chaine du solvant. Ainsi, il a pu être démontré que, dans le cas de la voie aqueuse, l'utilisation du réacteur sonothermal a une réelle incidence sur la surface spécifique du matériau final en comparaison de la réalisation successive des traitements sonochimique et hydrothermal. Les oxydes présentant les propriétés structurales les plus intéressantes ont été sélectionnés afin d'être employé en tant que support de métal noble dans des catalyseurs de type Pt/(Ce,Zr)O2. L'évaluation de l'activité catalytique de ces matériaux, lors de l'oxydation catalytique à l'air humide de l'acide formique, a montré que celle-ci dépend non seulement du mode de synthèse de l'oxyde mixte, mais également de la voie de dépôt du platine et du couple méthode de dépôt du platine/méthode de synthèse du support. / Cerium-zirconium mixed oxides are at the heart of numerous research subjects. Indeed, these materials are used in different fields of application, and particularly in catalysis in three-way catalyst for automobile or for organic pollutant oxidation. The interest for this compound resides in its remarkable oxidoreduction properties, its oxygen storage capacity and its resistance to sintering. Consequently, numerous preparation methods have been developed and underline the high sensitivity of these oxides to the synthesis way. In the present work, two main synthesis have been employed, coprecipitation (aqueous medium) and  diketonate degradation (non-aqueous medium). These syntheses have been realized with several techniques which are (i) heating at atmospheric pressure, (ii) hydrothermal treatment, (iii) sonolysis and (iv) combination of sonolysis and hydrothermal treatments, in a successive or simultaneous way. In order to realize this last and innovative method, a reactor allowing sonochemical treatment under high temperature and pressure has been developed. Under these conditions, the ultrasounds chemical activity has been proved during light or heavy water or hydrocarbon sonolysis. During the (Ce,Zr)O2 preparation, other parameters than the synthesis technique have been studied such as the surfactant presence or the solvent chain length. Thus, it has been shown that, in the case of aqueous synthesis, the use of the sonothermal reactor has a real effect on the specific surface area of the material in comparison of successive realization of sonochemical and hydrothermal treatments. Oxides with the more interesting structural properties have been selected for being used as supports of noble metal in the Pt/(Ce,Zr)O2 catalysts. The evaluation of the catalytic activity of these materials, during the catalytic wet air oxidation of formic acid, showed that it not only depends on the synthesis method, but also on the platinum deposition method and the pair platinum deposition method/support synthesis method.

Dans le cadre général des études de matériaux multifonctionnels, électrolytiques et catalytiques, susceptibles d’être utilisés au sein de dispositifs de détection de gaz, un système d’oxydes (1-x)CeO2. x/2Bi2O3 avec 0≤x≤1 a été élaboré par coprécipitation puis traitement thermique à 600°C. Le système ainsi obtenu correspondrait à un diagramme de phases original, constitué d’un domaine de solutions solides (Ce1-xBixO2-z pour x ≤ 0,20), d’un domaine multiphasé pour 0,3≤x≤0,7 comportant une phase de type quadratique b’-Bi2O3 et une phase cubique substituée limite (x=0,20), d’un autre domaine multiphasé pour les compositions 0,8≤x≤1, comportant une phase quadratique b-Bi2O3 et une phase monoclinique. Ces deux phases ont déjà été considérées dans la littérature comme phases métastables résultant de divers modes de refroidissement de la phase pure Bi2O3. Dans le cas présent, la stabilisation de ces deux phases b’ et b en présence d’une phase substituée cubique Ce1-xBixO2-z pourrait être due à la présence d’ions cérium au sein duréseau cristallin de Bi2O3. Les interactions catalytiques entre des échantillons polycristallins de ce système avec x variable et des mélanges air-CO et air CH4 ont été étudiées par spectroscopie infrarouge à transformée de Fourier dans le domaine 100 à 525°C. Il apparait que les composés riches en cérium ou riches en bismuth n'ont pas la même réactivité vis-à-vis des gaz CH4 ou CO. Cette diversité de propriétés catalytiques pourrait être utilisée au sein de systèmes multicapteurs de gaz.Une étude de la conduction électrique du système pour x variable a été effectuée par spectroscopie d’impédance électrique entre 100 et 750°C. Les représentations Nyquist des impédances électriques ont été interprétées en mettant en jeu des modèles de type élément de phase constante ou de type Warburg pour prendre en compte l’hétérogénéité des échantillons ainsi que les phénomènes de réaction-diffusion aux électrodes. La conductivité en volume (coeur de grains) augmente avec la composition, avec deux types d’évolutions distinctes : une évolution caractéristique de la phase substituée liée à l’augmentation du taux de lacunes, une évolution dans le système biphasé avec une forte augmentation de conductivité au-dessus de x=0,3 et un maximum atteint pour x=0,7. La phase quadratique de type b’-Bi2O3 connue comme phase métastable est ainsi stabilisée au sein de ce système mixte, au moins à 600°C: elle serait à l’origine de la forte conductivité ionique observée pour la composition proche de x = 0,7. / To develop multifunctional and sensitive materials for gas sensor, catalytic microsystems and electrolytic applications, the multiphase system (1-x).CeO2 + ½ x.Bi2O3 has been investigated in the bismuth composition range 0≤x≤1. A series of ceramics samples has been prepared via a coprecipitation route followed by a thermal treatment at 600°C. X-ray diffraction analyses showed that, for x ≤ 0.20, a solid solution Ce1- xBixO2-x/2 with fluorine structure was formed. For x ranging between 0.25 and 0.7, a tetragonal b’-Bi2O3 phase coexisting with the solid solution was observed. For x ranging between 0.8 and 0.9, a new tetragonal b'-Bi2O3 phase, closely related to the b' phase was evidenced. Finally, close to x=1, the classical monoclinic a-Bi2O3 structure crystallized. The formation of such intermediate tetragonal b and b’ phases could be due of theprobable presence of cerium cations in the Bi2O3 lattice. The solid–gas interactions between these polycristalline materials and air–CH4 and air–CO flows have been studied as a function of time, temperature and composition x, using Fourier transform infrared (FTIR) analyses of the conversions of CH4 and CO gases into the CO2 gas. For all compositions, a low catalytic reactivity was observed with air–CH4 gas flows, while, for the highest bismuth compositions, a high catalytic reactivity was observed with air–CO gas flows. The electrical conduction of this series of polycrystalline samples has been analyzed using electrical impedance spectroscopy, in the temperature range 25 to 750°C. To interpret the Nyquist representations of electrical analyses, various impedance models including constant phase elements and Warburg impedances have been used. The optimal conduction observed close to composition x=0.7 should be due to the stabilization of the tetragonal b’ Bi2O3 phase. This specific multiphase system could present a high interest in catalytic and electrolytic applications.

O uso de formulações fotoprotetoras tem aumentando muito nas últimas décadas devido ao maior conhecimento dos danos causados pela radiação UV e maior preocupação com o fotoenvelhecimento. Desta maneira observa-se um grande esforço por parte dos pesquisadores e indústrias para o desenvolvimento de novos filtros solares estáveis, eficazes e que agradem ao consumidor. Neste contexto destaca-se o fosfato de cério (CePO4), substância proposta para ser utilizada como filtro solar inorgânico devido a sua alta absortividade no UV, além de apresentar baixa atividade fotocatalítica e relativa transparência na luz visível, diferente dos filtros inorgânicos comumente utilizados em fotoprotetores. Assim o objetivo do presente trabalho foi o desenvolvimento e avaliação da estabilidade e das propriedades fotoprotetoras de formulações cosméticas contendo CePO4. Inicialmente realizou-se a caracterização dos filtros solares inorgânicos em estudo para um melhor conhecimento das características do material. Posteriormente foi desenvolvida uma formulação base (F8) na qual se associou uma combinação de filtros orgânicos (F8A) a qual foi acrescida de filtros inorgânicos ZnO, TiO2 e CePO4 (F8B, F8C e F8D, respectivamente). Após a realização de testes preliminares de estabilidade, as formulações foram submetidas à avaliação da estabilidade física (reologia) e química (CLAE). Ainda, a atividade fotocatalítica destas formulações foi avaliada para verificar a influência dos filtros inorgânicos objeto de estudo na estabilidade da formulação. Observou-se que a formulação contendo CePO4 além de apresentar superior estabilidade, quando comparada com os tradicionais filtros inorgânicos, ainda demonstrou menor atividade fotocatalítica no veículo a qual foi acrescida. Em seguida a segurança das formulações foi avaliada por meio de ensaios de citotoxicidade em culturas de fibroblastos 3t3 e teste de irritabilidade cutânea in vivo. Nenhuma das formulações apresentou irritabilidade cutânea e o ainda o CePO4 apresentou ótima compatibilidade no estudo de citotoxicidade in vitro. As formulações foram então submetidas à avaliação da eficácia por determinação do FPS in vivo e apresentaram valores entre 19,9 e 22,9, não havendo diferença significativa entre elas. A avaliação do residual branco das formulações também foi estudada em placas de silicone e observou-se que quando acrescida de CePO4 a formulação apresenta menor efeito inestético, demonstrando mais um diferencial deste material, o que possibilitará a utilização em maiores concentrações dos já usuais filtros UV inorgânicos. Assim, podemos concluir que o material proposto como novo filtro inorgânico UV (CePO4), possui potencial para ser utilizado em fotoprotetores uma vez que foram demonstradas todas as características necessárias para tal, ou seja, estabilidade, segurança e eficácia comprovados. Ainda, o CePO4 permite o desenvolvimento de formulações com sensorial adequado para aplicação, proporcionando um menor residual branco na pele, fato que interfere diretamente na aplicação desta classe de produtos. / The use of sunscreens has increased in recent years due to increase of knowledge about the damage caused by UV radiation and a greater concern with the photoaging. Because of that, it have been observed a effort by the research community and industry to develop new stable and effective UV filters, and formulations with appropriate sensorial during application. In this context it could be emphasize the cerium phosphate (CePO4), material proposed to be used as inorganic UV filter due to its high absorptivity in the UV range, besides its low photocatalytic activity and relative transparency in visible light, unlike inorganic filters commonly used in sunscreens. Thus the aim of this study was the development and evaluation of stability and photoprotective efficacy of cosmetic formulations containing CePO4. Initially the CePO4 was evaluated for a better understanding of the characteristics of this material. Subsequently a base formulation (F8) containing a combination of organic UV filters (F8A) was increased by the inorganic filters ZnO, TiO2 and CePO4 (F8B , F8C and F8d , respectively). After conducting preliminary stability tests, the formulations were evaluated for their physical (rheology) and chemical (HPLC) stability. Then, the photocatalytic activity of these formulations was evaluated to verify the influence of the addition of inorganic UV filters in formulations stability. It was observed that the formulation containing CePO4 besides having superior stability compared with conventional inorganic filters, also showed a lower photocatalytic activity. After, the safety of the formulations was evaluated by means of cytotoxicity assays on 3T3 fibroblast cultures and in vivo irritability skin test. None of the formulations presented cutaneous irritability and, besides of that the CePO4 showed excellent compatibility in the in vitro cytotoxicity study. The efficacy of the studied formulations were then evaluated by determining the in vivo SPF and it was obtained values between 19.9 and 22.9, with no significant difference between the different formulations studied. Also, the evaluation of the white residual after application was assessed in silicon plates and it was observed that the formulation containing CePO4 shows less unaesthetic effect, demonstrating a differential of this material , which will allow the use in higher concentrations them the usual inorganic UV filters. Thus, we conclude that the material proposed as new inorganic UV filter (CePO4) has potential to be used in sunscreens with stability, safety and efficacy demonstrated. Further, the CePO4 allows the development of formulations with suitable sensory for application providing a smaller white residual on skin, which interferes directly in the application of these kin of products.

Nesse trabalho foram estudados os compostos intermetalicos do tipo RAg (R = Nd, Gd e Ho) por calculos teoricos desenvolvidos dentro do formalismo da Teoria do Funcional Densidade (DFT). O metodo APW+lo (Augmented Planes Waves + lo- cal orbitals) foi aplicado para resolver a equacao de Kohn-Sham e a Aproximacao do Gradiente Generalizado (GGA) usada para tratar de forma aproximada o potencial de troca-correlacao. O codigo computacional utilizado foi o WIEN2k. As atividades foram focalizadas em duas frentes. Numa delas, determinou-se a fase magnetica do estado fundamental dos compos- tos HoAg e NdAg. Para isso, simulou-se as 4 estruturas magneticas possiveis (para as celulas cristalinas cubicas) desses dois sistemas (uma ferromagnetica (0,0,0) e as antifer-romagneticas: (0,0,¼), (¼,¼,0) e (¼,¼,¼)) e com a confeccao de gra¯cos de energia das celulas cristalinas dessas estruturas magneticas pela variacao dos volumes de tais celulas, chegou-se na estrutura magnetica (¼,¼,0) como a mais provavel para o estado fundamental magnetico de ambos os compostos. Tambem observou-se que o sistema de NdAg apre- senta uma pequena diferenca entre as energias das estruturas antiferromagnetica (¼,¼,0) e ferromagnetica. Creditou-se a esse efeito a explicacao de encontrar-se na literatura re- sultados experimentais diferentes para a estrutura magnetica do composto de NdAg puro e dopado com o atomo de 140Ce (entrando no sitio do Nd). Acredita-se que a dopagem do NdAg com o atomo de 140Ce (em uma porcao reduzida), gera alteracao no sinal da integral de troca (acoplamento RKKY), mudando-o de positivo para negativo, e isso implica na modficacao da fase magnetica do estado fundamental, passando da estrutura (¼,¼,0) µa 0,0,0). Assim, com a informacao anterior da estrutura magnetica do composto de NdAg quando dopado com o atomo de 140Ce, entrou-se na segunda etapa do estudo. Nela, fez-se os calculos das estruturas eletronicas usando a aproximacao de supercelulas nos compostos de GdAg e NdAg dopados com o atomo de Ce, garantindo que esse substituisse um atomo de terra rara nas supercelulas cristalinas montadas para determinar no Ce o campo hiperfino magnetico e suas componentes geradoras. Os compostos de GdAg e NdAg (dopados com Ce), respectivamente, tiveram suas celulas cristalinas montadas ferromagnetica e antiferromagneticamente. Utilizou-se a polarizacao orbital (DFT+U) nos eletrons da camada 4f das terras raras (excluindo-se o atomo de Gd que nao apresenta momento angular). Dessa forma, em ambos os sistemas foi possivel fazer varias simulacoes, nas quais a camada 4f do atomo de Ce foi populada de maneiras diferentes, afim de se obter varios valores de momento angular e com isso diversos resultados de campo hiperfino orbital foram conseguidos; assim, automaticamente o campo magnetico hiperfino tambem assumiu diferentes valores. Escolheu-se os sub-estados da camada 4f do Ce para serem simulados por meio da regra de Hund. Como esperado o campo hiperfino magnetico orbital gerado na camada 4f e a principal componente do campo hiper¯no total no atomo de Ce e ele apresenta sinal contrario µa componente de contato. Apesar dos resultados teoricos do campo hiperfino magnetico terem consideravel discrepancia dos resultados experimentais, pode-se verificar que o unico eletron da camada 4f do atomo de Ce (nos dois compostos) possivelmente esta em um dos seguintes sub-estados da camada 4f: ml = ¡2, ml = ¡1 ou ainda uma combinacao dos dois. / In this work the magnetic hyper¯ne ¯eld acting on Ce atoms substituting the rare-earths in RAg compounds (R = Gd e Nd) was studied by means of ¯rst-principles electronic structure calculations. The employed method was the Augmented Plane Waves plus local orbitals (APW+lo), embodied in the WIEN2k program, within the framework of the Density Functional Theory (DFT) and with the Generalized Gradient Approximation (GGA) for the exchange and correlation potential. The super-cell approach was utilized in order to simulate for the Ce atoms acting as impurities in the RAg matrix. In order to improve for correlation e®ects within the 4f shells, a Hubbard term was added to the DFT hamiltonian, within a procedure called GGA+U. It was found that the magnetic hyper¯ne ¯eld (MHF) generated by the Ce 4f electron is the main component of the total MHF and that the Ce 4f ground state level is probably a combination of the ml = ¡2 and ml = ¡1 sub-levels. In addition, the ground-state magnetic structure was determined for HoAg and NdAg by observing the behavior of the total energy as a function of the lattice volume v for several possible magnetic ordering in these compounds, namelly, ferromagnetic, and the (0,0,¼), (¼,¼,0) and (¼,¼,¼) types of anti-ferromagnetic ordering of rare-earth atoms. It was found that the ground-state magnetic structure is anti-ferromagnetic of type (¼,¼,0) for both, the HoAg and NdAg compounds. The energy di®erence of the ferromag-netic and antiferromagnetic ordering is very small in the case of the NdAg compound.

O uso de dióxidos de metais de transição em aplicações tecnológicas é bastante amplo, pois esses compostos possuem características importantes de semicondutores. Apesar de existir um grande número de estudos experimentais e teóricos, o entendimento das propriedades estruturais e eletrônicas desses compostos ainda não é satisfatória, principalmente quando se envolve o estudo de clusters. Clusters podem ser definidos como uma fase embrionária da matéria, pois são partículas contendo um número muito reduzido de átomos em comparação com partículas macroscópicas. Logo, suas propriedades estruturais e eletrônicas são totalmente distantes da fase cristalina do material, o que permite o desenvolvimento de novos materiais para aplicações tecnológicas. Portanto, existe um grande interesse em compreender as propriedades estruturais e eletrônicas dos clusters. Neste projeto de mestrado temos como objetivo estudar as propriedades estruturais e eletrônicas de clusters de TiO2 e CeO2 , usando para isso cálculos de primeiros princípios com base na teoria do funcional da densidade (DFT). Um dos principais problemas no estudo de clusters é a determinação da estrutura atômica, devido as dificuldades experimentais envolvidas em se trabalhar com estruturas tão pequenas. Dessa forma, um dos nossos maiores desafios foi a determinação das estruturas atômicas dos clusters desses dois tipos de dióxidos de metais de transição, uma vez que, o nosso grupo (QTnano) já conta com grande experiência, principalmente no desenvolvimento e implementação de algoritimos de otimização global. Devido as dificuldades envolvidas no estudo de partículas tão pequenas, ficamos restritos ao estudo de clusters com composição (MO2)n , com n = 1-15, ou seja, os nossos maiores clusters tem um total de 45 átomos, formando estruturas com diâmetro de inferior a 3 nm. Além da obtenção das estruturas, foi realizado o estudo das propriedades eletrônicas, energéticas e vibracionais para cada uma das composições geradas, o que propiciou a melhor compreensão sobre os efeitos eletrônicos nas estrutura atômica dos clusters. / The use of transition metal dioxides in technological applications is wide spread, because these compounds have important characteristics of semiconductors. Although there a large number of theoretical and experimental studies, the acknowledgement about the structural and electronic properties these compounds are not yet satisfactory, especially when studying clusters. Clusters can be defined as an embryonic phase of matter, because they are particles that contain a very small number of atoms in comparison with macroscopic particles. So, the structural and electronic properties are very distinct of the material crystalline phase, when allow the development of new materials in technological applications. Therefore a large interest exist in understanding the structural and electronic properties of clusters. In this master\'s degree project we have as objective to study the electronic and structural properties of TiO2 and CeO2 clusters, using first principle calculations based on the density functional theory (DFT). One of the main problems in the study of clusters is to determine the atomic structure, due the experimental difficult of work with so small particles. Thus, a great challenge was to determine the atomic structures of these two different transition metal dioxides, considering that our group (QTnano), has a large knowledge in the development and implementation of global optimization algorithms. Due the difficulty involved in studying small particles, we were restricted to the clusters of composition (MO2)n, with n = 1-15. In other words, our largest clusters have 45 atoms and a diameter smaller than 3 nm. Along with securing the structures, we realize the study of electronic, energetic and vibrational properties to each generated composition, providing an understanding of electronic effects in the atomic structure of clusters.

O desenvolvimento de novos revestimentos ou métodos de proteção considerados ambientalmente amigáveis é extremamente importante para a substituição das eficientes camadas de conversão do cromo, as quais são amplamente utilizadas para proteção anticorrosiva do alumínio e suas ligas. Entre as alternativas relatadas na literatura, podemos destacar a aplicação de filmes de polianilina (PAni) e o uso de camadas de conversão de cério. Neste trabalho é proposta a utilização de revestimentos do tipo bicamadas formada pela combinação de: 1) duas camadas de polianilina ou 2) camadas de conversão de cério (depositadas pelo método electroless ou catodicamente) e filmes de PAni para proteção contra a corrosão das ligas de alumínio AA7075-T6 e AA8006-F em meio aquoso contendo íons cloreto. Nos ensaios de corrosão foram utilizados o monitoramento do potencial de circuito aberto e a polarização potenciodinâmica em meio de NaCl 0,6 mol L-1. A superfície das ligas com e sem os recobrimentos foram analisadas por microscopia óptica e microscopia eletrônica de varredura. Também foi realizado o teste de aderência dos filmes de PAni presentes nas bicamadas sobre as ligas estudadas. O revestimento bicamada composto por uma camada de filme de PAni quimicamente depositada e um filme casting de PAni apresenta excelente aderência sobre as ligas AA7075-T6 e AA8006-F e combina a proteção oferecida pelos filmes casting com a boa aderência dos filmes de PAni depositados quimicamente. O uso da bicamada de cerato electroless e PAni apresenta propriedades anticorrosivas inferiores quando comparadas ao filme de PAni aplicado diretamente sobre o substrato metálico, entretanto, o uso de cerato electroless melhora a aderência dos filmes casting de PAni sobre as ligas estudas. O uso de bicamadas formadas por cerato depositado catodicamente e PAni sobre a liga AA8006-F apresenta valores de potencial de corrosão e potencial de pite inferiores ao do filme casting de PAni aplicado diretamente sobre o substrato metálico, enquanto que para a liga AA7075-T6 foram obtidas melhores propriedades anticorrosivas quando utilizada uma camada de cerato espessa (obtida utilizando uma solução de cério contendo 8 ou 25 µL mL-1 de H2O2). / The development of new coatings or protection methods considered environmentally friendly is extremely important to replace the efficient conversion layers of chromium, which are widely used for corrosion protection of aluminum and its alloys. Among the alternative methods reported in the literature, it can be highlighted the application of polyaniline (PAni) films and the use of cerium conversion layers. This work proposes the use of bi-layers based on: 1) two layers of polyanilines or 2) combination cerium conversion layers (deposited by electroless or cathodic electrodeposition) and PAni films for corrosion protection of AA7075-T6 and AA8006-F alloys in aqueous solutions containing chloride ions. Open circuit potential and potentiodynamic polarization curves were used for the corrosion tests, which were performed in 0.6 mol L-1 NaCl solution. The surface of the alloys with and without the coatings were characterized by optical microscopy and scanning electronic microscopy. It was also performed the adhesion test of the PAni films present in the bi-layers on the studied alloys. The bi-layers formed by a layer of PAni chemically deposited and a layer of PAni film exhibited excellent adhesion to the AA7075-T6 and AA8006-F alloys and combined the protection offered by the cast film of PAni with the good adhesion of PAni chemically deposited. The use of a bi-layer formed by cerium oxides (deposited by electroless) with PAni film as coating presented lower anticorrosive properties compared to the PAni films applied directly on the substrate, however, the use of the cerium layer improves the adhesion of the PAni films on the alloys. The use of bi-layers formed by electrodeposited cerium oxides and PAni films on AA8006-F showed corrosion potential and pitting potential lower when compared with the PAni film directly applied on the substrate, whereas for AA7075-T6, the best anticorrosive properties was obtained using a thick cerium layer (cerium solution containing 8 or 25 µL mL-1 of H2O2).