The research presented in this thesis is a collection of many different, yet connected, parts that stemmed from the development of a new alternative material intended to be utilised as anode material in solid oxide fuel cells. The main part is the research conducted in the development and characterisation of the novel A-site deficient La₀.2₂Sr₀.₇₋ₓCaₓTiO₃. Calcium introduction resulted in reducing this perovskite unit cell volume which, at the beginning, enhanced its electrical conductivity in reducing conditions. However, the ideal cubic symmetry coud not be maintained, as in the starting material LA₀.₂Sr₀.₇TiO₃, as a result of the increased A-site ionic radius mismatch and two lower symmetries were observed at room temperature. These were the tetragonal I4/mcm for 0.1 ≤ x ≤ 0.35 and orthorhombic Pbnm for 0.4 ≤ x ≤ 0.7. Higher temperature NPD data showed that the orthorhombic samples transformed into higher symmetries with Pbnm → I4/mcm → Pm3-m phase transitions. Detailed crystallographic analysis is discussed; where the different unit cells showed changes to the tilts of the BO₆ octahedra, along with distortions to these octahedra. DC conductivity measurements showed a high electrical conductivity of 27.5 S/cm for a pre-reduced composition La₀.₂Sr₀.₂₅Ca₀.₄₅TiO₃ at 900°C and pO₂ = 10⁻¹⁹ atm. This material showed very encouraging features; which makes it a very promising anode material for SOFCs. A study was also done which explores the best renewable energy options for the United Arab Emirates given its local climate and other aspects. The reliance on seawater desalination is argued to by unsustainable for different reasons. Thus, water security should be a main element in the planning process for adopting renewable energy technologies. A system that combines different technologies; with a focus on fuel cells technology; is outlined which is thought of to be a very promising basis for a broader system that will secure power and water in a very environment friendly way. Different compositions of the system La₀.₂Sr₀.₇₋ₓCaₓTiO₃ were also studied using AC impedance spectroscopy in order to establish whether or not this system can show a ferroelectric behaviour. Results showed a variation in the dielectric constant of different samples with temperature; however, no Curie point was observed. Nonetheless, the results did show that the different compositions were very homogeneous when fully oxygenated and there were some indications of possible symmetry changes at sub-ambient temperatures. The final part of this thesis outlined the work done towards the development of a new analytical instrument. An existing TGA instrument was altered in order to provide a simultaneous thermogravimetric analysis and DC conductivity measurement for solid solutions at controlled temperature and oxygen partial pressure. Results were obtained for different samples of the system La₀.₂Sr₀.₇₋ₓCaₓTiO₃ which showed a great dependence of the electrical conductivity on the oxygen stoichiometry in these oxides. Also, a direct method is possible with this instrument to estimate the oxygen chemical diffusion coefficient using the electrical conductivity relaxation method. This new setup will be very useful for different electrochemical and thermal studies which can broaden the understanding of the different mechanisms that affect the performance of different solid state materials.

The aim of this thesis was to improve the understanding of microstructure in single crystal ferroelectrics. This was achieved through macroscopic testing of Lead Magnesium Niobate – Lead Titanate (PMN-PT) and microscopic observations of Barium Titanate (BT) single crystals. Multi-axial polarization rotation tests on PMN-PT showed a gradual increase in the change in dielectric displacement due to ferroelectric switching as the electric field is applied at increasing angles to the initial polarization direction. A relatively high remnant polarization for loading angle near to 90° suggested that PMN-PT is more polarizable in certain directions. Strains measured in two directions, parallel to the electric field and perpendicular to the electric field, showed a noticeable variation on two opposite faces of the specimen suggesting an effect of local domain configurations on macroscopic behaviour. A micromechanical model gave an insight into the switching systems operating in the crystal during the polarization rotation test. Domain structure in BT was mapped using synchrotron X-ray reflection topography. By making use of the angular separation of the diffracted reflections and specimen rocking, different domain types could be unambiguously identified, along with the relative tilts between adjacent domains. Fine needle domains (width ≈ 10μm) were successfully mapped providing a composite topograph directly comparable with optical micrograph. The domain structure was confirmed using other techniques such as piezoresponse force microscopy and atomic force microscopy/scanning electron microscopy and optical observations on the etched crystal. Results show that combined use of multiple techniques is necessary to gain a consistent interpretation of the microstructure. Finally, domain evolution in BT under compressive mechanical loading was observed in-situ using optical and X-ray diffraction techniques providing a series of images that show ferroelastic transition. The domain configurations influence the switching behaviour and constitutive models that can account for such effects need to be developed. Quantitative and qualitative data presented in this thesis can assist model development and validation.

Strontiumtitanat ist ein häufig untersuchtes Oxidmaterial, mit einem breiten Anwen- dungsgebiet z.B. in der Photokatalyse oder auch als Substratmaterial beim Wachstum ande- rer Oxidschichten. Dabei spielen chemische Prozesse an der Oberfläche eine herausragende Rolle, deren Kenntnis für eine tieferes Verständnis der genannten Anwendungen unentbehrlich ist. Darüberhinaus gibt es deutliche Hinweise darauf, dass diese Oberflächenprozesse sehr stark, u.a. von Wasserstoff und Wasser beeinflußt werden. Sowohl wegen der Relevanz als auch wegen der technischen Machbarkeit, stellt eine Untersuchung des Adsorptionsverhaltens von Sauerstoff und Wasserstoff mit Hilfe der Dichtefunktionaltheorie einen natürlichen Ausgangspunkt dar, um genaue Einblicke in die Prozesse auf der Oberfläche auf atomare Ebene zu gewinnen. Bei der Adsorption des Sauerstoffs und des Wassers ist gleichermaßen auffällig, dass die Bindungsenergien sehr stark durch langreichweitige Verzerrungen im Substratgitter beeinflußt werden, welche damit eine effektive repulsive Wechselwirkung der adsorbierten Spezies bewirken. Adsorbierte Sauerstoffatome bilden mit jeweils einem Sauerstoffatom des Subtrats ein „Quasi-Peroxid-anion“, wodurch das adsorbierte Sauerstoffatom einen Gitterplatz einnimmt, der nicht einem Sauerstoffplatz des Perovskitgitters entspricht. Im Gegensatz dazu wurden bei allen untersuchten atomaren Konfigurationen, die Wasser und Hydroxylgruppen enthielten, beobachtet, dass sich hier die adsorbierten Sauerstoffatome an den Plätzen des forgesetzten Perovskitgitters befinden. Bemerkenswert ist die spontane Dissoziation und Bildung eines Hydroxylpaares auf der Strontiumoxidterminierung während des Adsorption des Wassermoleküls. Auf der Titandioxidterminierung hingegen werden abhängig von der Bedeckung Wassermoleküle und Hydroxylgruppen beobachtet. Die Energetik, die diesem Verhalten zugrunde liegt, zeigt sehr gute Übereinstimmung mit den experimen- tellen Beobachtungen von Iwahori und Kollegen. / Strontium titanate is an extensively studied material with a wide range of application, for instance in photo-catalysis and most importantly, it is used as a substrate in growth of functional oxides. The surface chemistry is crucial and hence understanding the surface structure on atomic scale is essential for gaining insight into the fundamental processes in the aforementioned applications. Moreover, there exist a lot of evidence that this surface chemistry might be controlled to considerably by extrinsic species, such as residual hydrogen and water. Investigating the properties of water and oxygen on the strontium titanate surface is certainly a natural starting point for a theoretical study based on density functional theory, because these species are practically present on the surface on a wide range of experimental conditions and they are computationally feasible. For the oxygen and water adsorption the binding energy is controlled by long-range surface relaxations leading to an effective repulsion of the adsorbed specimen. The isolated oxygen ad-atom forms a covalently bonded “quasi-peroxide anion” in combination with a lattice oxygen atom. Contrariwise, in all investigated configurations containing water molecules and hydroxyl groups, the respective oxygen atoms assumed positions close to the oxygen sites of the continued perovskite lattice of the substrate. Most remarkably, on the strontium oxide termination, the water molecules adsorbs and dissociates effortlessly leading to the formation of a pair of hydroxyl groups. For the titanium dioxide termination, a coverage dependent adsorption mode is observed. Densely packings stabilize water molecules, whereas at lower coverage and finite temperatures the formation of hydroxyl groups is found. The energetics responsible for this behavior is consistent with recent experiments by Iwahori and coworkers.

Titanatos nanoestruturados (TNS) obtidos pelo tratamento hidrotérmico de TiO2 são semicondutores muito estudados por suas propriedades de alta área superficial e capacidade de troca iônica. No entanto, sua estrutura cristalina e a influência das condições de síntese e pós-tratamento ainda são motivos de controvérsia. Neste estudo, TNSs foram produzidos em diversas condições e submetidos a diferentes tipos de tratamento ácido e térmico. Os materiais foram caracterizados por difratometria de raios X (DRX), espectroscopias vibracionais (Raman e FTIR), espectroscopia de refletância difusa (DRS), análise térmica (TG, DTG e DSC), análise textural por adsorção de N2 a 77 K, análise química por ICP-OES, e microscopia eletrônica de transmissão. Acompanhando a transformação hidrotérmica de TiO2 (anatase) nanocristalino obtido em laboratório com o tempo, observou-se que os nanocristais de TiO2 se transformam em estruturas lamelares com formato de folhas entre 3h e 12h. As nanofolhas se enrolam parcial ou totalmente formando nanotubos. A transformação da morfologia é acompanhada por uma transformação de fase de anatase para uma fase titanato lamelar, que se completa entre 12h e 24h. Utilizando TiO2 P25 como precursor, observou-se que as amostras obtidas apresentam alto teor de Na+, que é progressivamente eliminado por lavagens do sólido com H2O ou solução ácida. Quanto menor o pH de equilíbrio da suspensão, menor foi o teor de Na+ até o limite de pH 2, em que esse cátion foi praticamente eliminado. A diminuição do teor de sódio foi acompanhada de aumento da área superficial (BET, 155 e 205 m2.g-1 para pH 9 e 2, respectivamente); aumento do espaço interlamelar; diminuição da cristalinidade; e diminuição do bandgap (3,60 e 3,45 para pH 9 e 2, respectivamente). Em pH 1,5, ocorreu ainda maior aumento do espaço interlamelar e da área superficial (368 m2.g-1) indicando que a troca iônica de Na+ por H3O+ não é unicamente responsável pelas transformações estruturais que ocorrem durante a neutralização de TNSs. A desidratação em baixa temperatura (até 150 °C) sofrida por TNSs acidificados é irreversível, levando à diminuição do espaço interlamelar, e formação de vacâncias de oxigênio responsáveis pela absorção de radiação acima de 420 nm (visível). A transformação de fase de titanato para anatase ocorreu à temperatura ambiente quando a neutralização foi feita com HF; entre 300 e 400 °C quando feita com HCl, HNO3, H2SO4 ou ácido acético; e acima de 600 °C quando usado H3PO4. Foi possível inserir diferentes quantidades de prata em TNS através da suspensão dos sólidos em solução de AgNO3. A reação levou à formação de nanopartículas cristalinas de 3 a 5 nm, contendo prata, na superfície das partículas de TNS. Ag+ foi reduzido a Ag0 pelo tratamento térmico das amostras a 250 °C em presença de H2(g). Essas amostras apresentaram absorção de radiação em todo o espectro visível e menor bandgap (3,06 em amostra contendo 3% de Ag, em massa). Em amostras com pouca quantidade de prata (menos de 0,05% em massa), foram observadas bandas largas de absorção (DRS) de ressonância de plasmon de superfície quando calcinadas a 250 °C em H2(g). / Titanate nanostructures (TNS) obtained by the hydrothermal treatment of TiO2 are extensively studied due to their high surface area and ion-exchange ability. However, their crystal structure and influence of synthesis and post-treatment conditions are still under debate. In this study, TNSs were produced under different synthetic conditions and underwent different types of acid and thermal treatments. The materials were characterized by means of X-ray diffractometry (XRD), vibrational spectroscopy (Raman and FTIR), diffuse reflectance spectroscopy (DRS), thermal analysis (TG, DTG and DSC), textural analysis by N2 adsorption at 77 K, chemical analysis by ICP-OES, and transmission electron microscopy (TEM). Following the hydrothermal transformation of homemade crystalline TiO2 (anatase) with time, we observed that TiO2 nanocrystals change into lamellar sheet-like structures between 3h and 12h. The nanosheets roll up partial or totally, thus forming nanotubes. The morphological transformation is accompanied by a phase transformation from anatase to lamellar titanate, which is completed between 12h and 24h. Using TiO2 P25 as precursor, we observed that as-obtained samples have a high Na+ content, which is progressively removed by washing the solids with H2O or acidic solution. The smaller the suspensions equilibrium pH, the smaller the Na+ content to the limit of pH 2, when the removal of this cation was complete. The decrease in sodium content was followed by an increase in the surface area (BET, 155 and 205 m2.g-1 at pH 9 and 2, respectively); an increase in the interlamellar distance; a decrease in crystallinity; and a decrease in bandgap energy (from 3.60 eV at pH 9 to 3.45 eV at pH 7). After treating at pH 1.5, the interlamellar distance and the surface area (368 m2.g-1) increased further, suggesting that the Na+ to H3O+ ion-exchange is not the only factor in the structural transformations that take place during the acid treatment of TNSs. Acidic TNSs undergo an irreversible dehydration process at low temperature (150 °C). It leads to the decrease of the interlamellar distance and to the formation of oxygen vacancies responsible for the absorption of radiation in the visible range (> 420 nm). The phase transformation of the titanate phase to TiO2 anatase took place at room temperature when the TNS was treated with HF; between 300 and 400 °C for samples neutralized with HCl, HNO3, H2SO4 or acetic acid; and over 600 °C when H3PO4 was used. It was possible to insert different amounts of silver by the immersion of the solids in AgNO3 solution, leading to the formation of nanocrystalline-silver-containing nanoparticles (3 to 5 nm) on the surface of the TNS particles. Ag+ was reduced to Ag0 by heat treating the samples at 250 °C in presence of H2(g). These materials showed absorption of radiation in entire visible spectrum and narrowed bandgap energy (3,06 eV for sample with 3wt% of Ag). Samples containing low amounts of Ag (less than 0,05 wt%) showed a wide surface plasmon resonance band (DRS) when calcined at 250 °C under H2(g).

O objetivo desta tese de doutorado foi estudar a influência da adição do íon estanho (Sn4+) à rede do composto SrTiO3 em substituição ao íon de titânio visando otimizar as propriedades elétricas desse composto e, como consequência, obter uma melhora de seu desempenho como um sensor de gás na forma de filmes finos. Para realizar a deposição destes filmes finos através dos métodos de Deposição por Feixe de Elétrons (EBD), alvos cerâmicos de composição SrTi1-xSnxO3 (STSO) com x = 0; 0,20; 0,40; 0,60; 0,80; 0,85; 0,90; 0,95; 1 foram obtidos através do método dos precursores poliméricos modificado. A sequência de formação de soluções sólidas foi determinada através do refinamento Rietveld das amostras STSO na forma de pó sinterizadas, obtidas através dos métodos dos precursores poliméricos e de reação de estado sólido, mostrando que a transição da fase cúbica Pm3̄m do composto SrTiO3 até a fase ortorrômbica Pnma do composto SrSnO3 não depende do método de síntese. As medidas de espectroscopia Raman e absorção de raios-X (XANES, na borda K do Ti) das amostras tanto na forma de pó, obtidas através do método dos precursores poliméricos e de reação de estado sólido, quanto na forma de filme fino obtidas por EBD revelaram a existência de uma desordem local na rede do composto SrTiO3 que diminui com o aumento da temperatura e com a diminuição da concentração de Sn. Os filmes finos STSO obtidos por EBD foram avaliados como sensores utilizando-se os gases O3 e NH3. Em medidas realizadas com o gás ozônio (O3), os resultados mostraram que os filmes finos de 100 nm de espessura apresentaram uma maior sensibilidade tendo a amostra com 60% de Sn com o melhor desempenho a 350°C para 0,15 ppm do gás. As análises de performance dos filmes STSO quanto a seletividade indicaram que não foram seletivos e que apresentaram uma maior resposta ao gás ozônio quando comparados ao gás NH3. / The objective of this work was to study the influence of the addition of tin ion (Sn4+) into the SrTiO3 compound lattice, to replace the titanium ion (Ti4+). The aim was to optimize the electrical properties of SrTiO3 compound and, as a consequence, to obtain an improvement of its performance as a gas sensor in the thin films samples. To perform the deposition of these thin films through Electron Beam Deposition (EBD), ceramic targets of composition SrTi1-xSnxO3 (STSO) with x = 0; 0.20; 0.40; 0.60; 0.80; 0.85; 0.90; 0.95; and 1 were obtained by the modified polymer precursor method. The solid solution formation sequence was determined by the Rietveld refinement of the STSO sintered powdered samples, obtained by both polymeric precursor and solid-state reaction methods, showing that the transition from the cubic Pm3̄m phase of the SrTiO3 compound to the orthorhombic Pnma phase of the SrSnO3 compound does not depend on the synthesis method. The measurements of Raman spectroscopy and absorption of X-rays (XANES, at Ti K-edge), of the powdered samples obtained by both synthesis methods and of the thin films obtained by EBD, revealed the existence of a local disorder in the SrTiO3 compound lattice which decreases with increasing of temperature and with decreasing of Sn concentration. The STSO thin films were evaluated as sensors using the O3 and NH3 gases. In measurements accomplished with the ozone gas (O3), the results showed that thin films of 100 nm thickness had a higher sensitivity. The sample having 60% of tin showed the best performance at 350°C for 0.15 ppm of ozone gas. The performance analysis related to the selectivity of the STSO films indicated they were not selective and that presented a higher response to the ozone gas when compared to the NH3 gas.

Titanatos nanoestruturados (TNS) obtidos pelo tratamento hidrotérmico de TiO2 são semicondutores muito estudados por suas propriedades de alta área superficial e capacidade de troca iônica. No entanto, sua estrutura cristalina e a influência das condições de síntese e pós-tratamento ainda são motivos de controvérsia. Neste estudo, TNSs foram produzidos em diversas condições e submetidos a diferentes tipos de tratamento ácido e térmico. Os materiais foram caracterizados por difratometria de raios X (DRX), espectroscopias vibracionais (Raman e FTIR), espectroscopia de refletância difusa (DRS), análise térmica (TG, DTG e DSC), análise textural por adsorção de N2 a 77 K, análise química por ICP-OES, e microscopia eletrônica de transmissão. Acompanhando a transformação hidrotérmica de TiO2 (anatase) nanocristalino obtido em laboratório com o tempo, observou-se que os nanocristais de TiO2 se transformam em estruturas lamelares com formato de folhas entre 3h e 12h. As nanofolhas se enrolam parcial ou totalmente formando nanotubos. A transformação da morfologia é acompanhada por uma transformação de fase de anatase para uma fase titanato lamelar, que se completa entre 12h e 24h. Utilizando TiO2 P25 como precursor, observou-se que as amostras obtidas apresentam alto teor de Na+, que é progressivamente eliminado por lavagens do sólido com H2O ou solução ácida. Quanto menor o pH de equilíbrio da suspensão, menor foi o teor de Na+ até o limite de pH 2, em que esse cátion foi praticamente eliminado. A diminuição do teor de sódio foi acompanhada de aumento da área superficial (BET, 155 e 205 m2.g-1 para pH 9 e 2, respectivamente); aumento do espaço interlamelar; diminuição da cristalinidade; e diminuição do bandgap (3,60 e 3,45 para pH 9 e 2, respectivamente). Em pH 1,5, ocorreu ainda maior aumento do espaço interlamelar e da área superficial (368 m2.g-1) indicando que a troca iônica de Na+ por H3O+ não é unicamente responsável pelas transformações estruturais que ocorrem durante a neutralização de TNSs. A desidratação em baixa temperatura (até 150 °C) sofrida por TNSs acidificados é irreversível, levando à diminuição do espaço interlamelar, e formação de vacâncias de oxigênio responsáveis pela absorção de radiação acima de 420 nm (visível). A transformação de fase de titanato para anatase ocorreu à temperatura ambiente quando a neutralização foi feita com HF; entre 300 e 400 °C quando feita com HCl, HNO3, H2SO4 ou ácido acético; e acima de 600 °C quando usado H3PO4. Foi possível inserir diferentes quantidades de prata em TNS através da suspensão dos sólidos em solução de AgNO3. A reação levou à formação de nanopartículas cristalinas de 3 a 5 nm, contendo prata, na superfície das partículas de TNS. Ag+ foi reduzido a Ag0 pelo tratamento térmico das amostras a 250 °C em presença de H2(g). Essas amostras apresentaram absorção de radiação em todo o espectro visível e menor bandgap (3,06 em amostra contendo 3% de Ag, em massa). Em amostras com pouca quantidade de prata (menos de 0,05% em massa), foram observadas bandas largas de absorção (DRS) de ressonância de plasmon de superfície quando calcinadas a 250 °C em H2(g). / Titanate nanostructures (TNS) obtained by the hydrothermal treatment of TiO2 are extensively studied due to their high surface area and ion-exchange ability. However, their crystal structure and influence of synthesis and post-treatment conditions are still under debate. In this study, TNSs were produced under different synthetic conditions and underwent different types of acid and thermal treatments. The materials were characterized by means of X-ray diffractometry (XRD), vibrational spectroscopy (Raman and FTIR), diffuse reflectance spectroscopy (DRS), thermal analysis (TG, DTG and DSC), textural analysis by N2 adsorption at 77 K, chemical analysis by ICP-OES, and transmission electron microscopy (TEM). Following the hydrothermal transformation of homemade crystalline TiO2 (anatase) with time, we observed that TiO2 nanocrystals change into lamellar sheet-like structures between 3h and 12h. The nanosheets roll up partial or totally, thus forming nanotubes. The morphological transformation is accompanied by a phase transformation from anatase to lamellar titanate, which is completed between 12h and 24h. Using TiO2 P25 as precursor, we observed that as-obtained samples have a high Na+ content, which is progressively removed by washing the solids with H2O or acidic solution. The smaller the suspensions equilibrium pH, the smaller the Na+ content to the limit of pH 2, when the removal of this cation was complete. The decrease in sodium content was followed by an increase in the surface area (BET, 155 and 205 m2.g-1 at pH 9 and 2, respectively); an increase in the interlamellar distance; a decrease in crystallinity; and a decrease in bandgap energy (from 3.60 eV at pH 9 to 3.45 eV at pH 7). After treating at pH 1.5, the interlamellar distance and the surface area (368 m2.g-1) increased further, suggesting that the Na+ to H3O+ ion-exchange is not the only factor in the structural transformations that take place during the acid treatment of TNSs. Acidic TNSs undergo an irreversible dehydration process at low temperature (150 °C). It leads to the decrease of the interlamellar distance and to the formation of oxygen vacancies responsible for the absorption of radiation in the visible range (> 420 nm). The phase transformation of the titanate phase to TiO2 anatase took place at room temperature when the TNS was treated with HF; between 300 and 400 °C for samples neutralized with HCl, HNO3, H2SO4 or acetic acid; and over 600 °C when H3PO4 was used. It was possible to insert different amounts of silver by the immersion of the solids in AgNO3 solution, leading to the formation of nanocrystalline-silver-containing nanoparticles (3 to 5 nm) on the surface of the TNS particles. Ag+ was reduced to Ag0 by heat treating the samples at 250 °C in presence of H2(g). These materials showed absorption of radiation in entire visible spectrum and narrowed bandgap energy (3,06 eV for sample with 3wt% of Ag). Samples containing low amounts of Ag (less than 0,05 wt%) showed a wide surface plasmon resonance band (DRS) when calcined at 250 °C under H2(g).

O objetivo desta dissertação foi estudar a produção de pós de titanato de alumínio - tialita - pelo processo sol-gel associado à aspersão térmica por plasma. Foram investigados 3 tipos de solventes (etilenoglicol, álcool etílico e água) para a obtenção de um pó com características adequadas para a aplicação por aspersão ténnica a plasma. Os pós foram preparados por co-precipitação de sais inorgânicos de titânio e alumínio através da desestabilização das soluções com uma base forte. Os produtos obtidos foram então secos, calcinados em diferentes temperaturas, observando a evolução das fases. Alternativamente, pós pré-calcinados foram submetidos à aspersão térmica a plasma contra um recipiente com água, e contra um substrato de aço baixo carbono, proporcionando um resfriamento rápido. Os pós obtidos foram caracterizados por análises terrnodiferenciais (ATD) e termogravimétricas (ATG), análise granulométrica por difração de laser, análise da evolução de fases em função da temperatura de calcinação (e após aspersão térmica) por difração de raios X, análise da homogeneidade química e morfologia das partículas utilizando microscopia eletrônica de varredura (MEV) com o auxílio de espectrômetro de energia dispersiva (EDS). Os resultados mostraram que é possível obter, via sol-gel, matérias-primas adequadas para a formação da fase tialita após calcinação a temperaturas acima de 1300°C ou calcinando-se a cerca de 600°C e aspergindo em chama plásmica. Os diferentes solventes utilizados proporcionaram a produção de pós com diferentes morfologias e graus de homogeneidade química. Os pós apresentaram uma distribuição de titânio e alumínio na mesma partícula, sendo que a utilização de etilenoglicol corno solvente levou à maior homogeneidade. Com menores taxas de aquecimento nas calcinações (150Kfh), em temperaturas entre 800 e 1200°C, ocorre a segregação desses elementos com formação e crescimento de grãos de rutila e coríndon. Já com o rápido aquecimento e resfriamento, característicos da aspersão térmica por chama plásmica, a fase final preponderante é a tialita, tanto com resfriamento em água quanto no resfriamento sobre um substrato. / This work has the purpose of studying the production of a ceramic powder of aluminurn titanate - tialita - by sol-gel process associated with plasma spraying. Three kinds of solvents (ethyleneglicol, ethyl alcohol, and distillated water) were investigated in order to obtain a powder with adequate characteristics to be used as feedstock in plasma spray technique. The powders were made by co-prectpttation of inorganic salts of titanium and aluminurn through destabilization of the solutions using a strong base. The products obtained were dried, calcinated at different temperatures with attention to the phase evolution. Alternately, powders pré-calcinated were submitted to plasma thermal spraying in water and in a low-carbon steel substrate, providing an intense cooling rate. The powders obtained were characterized by differential thermal analysis (DT A) and thermogravimetric analysis (TGA), particle size analysis by laser diffraction, phase evaluation in function of calcinations temperature by X ray diffraction, chemical homogeneity and particle morphology using scanning electron microscopy (SEM) with energy dispersive spectrometer (EDS). The results showed that is possible obtain, by sol-gel, adequate feedstock to formation of tialita phase after calcinations at temperatures above 1300°C, or calcining at temperatures near 600°C followed by thermal spraying with plasma flame. The different kinds of solvents used, propitiated the production of powders with different morphologies and degrees of chemical homogeneity. With low heating rates in calcinations (150Kih), at temperatures between 800°C and 1200°C, occurs the segregation of aluminum and titaniurn with formation and growing of corundum and rutile phases. Since with fast heating and cooling propitiate by plasma spraying, the preponderant final phase is tialite in the substrate (water or metallic) aspersion.

O objetivo desta tese de doutorado foi estudar a influência da adição do íon estanho (Sn4+) à rede do composto SrTiO3 em substituição ao íon de titânio visando otimizar as propriedades elétricas desse composto e, como consequência, obter uma melhora de seu desempenho como um sensor de gás na forma de filmes finos. Para realizar a deposição destes filmes finos através dos métodos de Deposição por Feixe de Elétrons (EBD), alvos cerâmicos de composição SrTi1-xSnxO3 (STSO) com x = 0; 0,20; 0,40; 0,60; 0,80; 0,85; 0,90; 0,95; 1 foram obtidos através do método dos precursores poliméricos modificado. A sequência de formação de soluções sólidas foi determinada através do refinamento Rietveld das amostras STSO na forma de pó sinterizadas, obtidas através dos métodos dos precursores poliméricos e de reação de estado sólido, mostrando que a transição da fase cúbica Pm3̄m do composto SrTiO3 até a fase ortorrômbica Pnma do composto SrSnO3 não depende do método de síntese. As medidas de espectroscopia Raman e absorção de raios-X (XANES, na borda K do Ti) das amostras tanto na forma de pó, obtidas através do método dos precursores poliméricos e de reação de estado sólido, quanto na forma de filme fino obtidas por EBD revelaram a existência de uma desordem local na rede do composto SrTiO3 que diminui com o aumento da temperatura e com a diminuição da concentração de Sn. Os filmes finos STSO obtidos por EBD foram avaliados como sensores utilizando-se os gases O3 e NH3. Em medidas realizadas com o gás ozônio (O3), os resultados mostraram que os filmes finos de 100 nm de espessura apresentaram uma maior sensibilidade tendo a amostra com 60% de Sn com o melhor desempenho a 350°C para 0,15 ppm do gás. As análises de performance dos filmes STSO quanto a seletividade indicaram que não foram seletivos e que apresentaram uma maior resposta ao gás ozônio quando comparados ao gás NH3. / The objective of this work was to study the influence of the addition of tin ion (Sn4+) into the SrTiO3 compound lattice, to replace the titanium ion (Ti4+). The aim was to optimize the electrical properties of SrTiO3 compound and, as a consequence, to obtain an improvement of its performance as a gas sensor in the thin films samples. To perform the deposition of these thin films through Electron Beam Deposition (EBD), ceramic targets of composition SrTi1-xSnxO3 (STSO) with x = 0; 0.20; 0.40; 0.60; 0.80; 0.85; 0.90; 0.95; and 1 were obtained by the modified polymer precursor method. The solid solution formation sequence was determined by the Rietveld refinement of the STSO sintered powdered samples, obtained by both polymeric precursor and solid-state reaction methods, showing that the transition from the cubic Pm3̄m phase of the SrTiO3 compound to the orthorhombic Pnma phase of the SrSnO3 compound does not depend on the synthesis method. The measurements of Raman spectroscopy and absorption of X-rays (XANES, at Ti K-edge), of the powdered samples obtained by both synthesis methods and of the thin films obtained by EBD, revealed the existence of a local disorder in the SrTiO3 compound lattice which decreases with increasing of temperature and with decreasing of Sn concentration. The STSO thin films were evaluated as sensors using the O3 and NH3 gases. In measurements accomplished with the ozone gas (O3), the results showed that thin films of 100 nm thickness had a higher sensitivity. The sample having 60% of tin showed the best performance at 350°C for 0.15 ppm of ozone gas. The performance analysis related to the selectivity of the STSO films indicated they were not selective and that presented a higher response to the ozone gas when compared to the NH3 gas.

Óxidos à base de titânio são estudados por suas diversas aplicações. Dentre eles, compostos com estrutura cristalina de perovskita apresentam propriedades elétricas diferenciadas. Ademais, os recém-descobertos nanotubos derivados de TiO2 por método hidrotérmico têm chamado a atenção dos pesquisadores por sua alta área superficial e fácil obtenção, embora ainda haja controvérsias sobre sua estrutura cristalina e a influência das condições de síntese. Com o desenvolvimento tecnológico, tornou-se fundamental a obtenção de materiais mais puros e com propriedades controladas, o que vem sendo alcançado através de rotas brandas, relacionadas à Química Doce. Simultaneamente, a preocupação ambiental vem influenciando as metodologias sintéticas através das diretrizes da Química Verde. Esta pesquisa visa: 1. caracterização de SrTiO3 obtido por rotas brandas - sal fundido, hidrotérmica, precipitação do sol e combustão. Os produtos foram caracterizados por análise de sorção de N2, DRX e termogravimetria, para a comparação de área superficial (BET), perfil de porosidade (DFT), composição cristalina e cristalinidade, tamanho de partículas e composição química. 2. exploração do sistema de nanotubos derivados de TiO2 pelo método hidrotérmico em NaOH 10 mol.L-1. Foram comparados quatro precursores (TiO2 amorfo, anatase e rutilo obtidos via precipitação de alcóxido, e o comercial P25), dois tempos de reação (24h e 72h), e o efeito de lavagem posterior com HCl. As amostras foram caracterizadas por DRX, área superficial (BET), perfil de porosidade (BJH), MEV e MET, além de espectroscopia Raman e TG e o efeito da temperatura na composição cristalina das amostras. A rota de precipitação do sol produziu SrTiO3 com maior grau de cristalinidade e de impurezas, sendo que sua área superficial diminuiu após tratamento a 400°C. Todos os produtos continham SrCO3, eliminado após 900°C. O método da combustão não formou o titanato. Nanotubos de comprimentos variados foram obtidos de todos os precursores cristalinos, sendo que rutilo não gerou outros tipos de nanoestruturas e os nanotubos mais longos. Além disso, foi o polimorfo que reagiu mais lentamente. O precursor amorfo gerou nanoestruturas semelhantes a nanofios. Foi observado que os nanotubos são compostos de titanatos de sódio lamelares, sendo esses cátions trocados por H+ pela lavagem dos pós com HCl. Essa troca iônica promove a diminuição da região interlamelar e o aumento da área superficial das amostras. / Titanium based oxides are studied for their wide range of applications. Among them, perovskite-like oxides show especial electrical properties. Furthermore, the recently discovered TiO2-derived nanotubes through hydrothermal process have been drawing researchers\' attention for their high surface area and because they can be easily obtained, although their crystal structure and the influence of condition synthesis still being point of controversy. Due to technological advances, the production of property-controlled, composition and phase pure materials has become crucial. These aims have been achieved by the employment of soft routes, related to Soft Chemistry synthesis. Simultaneously, environmental concerns have influenced synthetic methodologies through the Green Chemistry guidelines. This research aims: 1. characterizing of SrTiO3 synthesized by soft routes, namely molten salt synthesis, hydrothermal method, sol-precipitation process and combustion synthesis. The products were characterized by N2 sorption analysis, XRD and thermogravimetry in order to compare their surface area (BET), porous structure (DTF), chemical and crystal phase composition, crystallinity and particle size. 2. exploring the system of nanotubes hydrothermally derived from TiO2 in NaOH 10 mol.L-1. The effects of precursor type (amorphous TiO2, anatase and rutile obtained by alcoxide hydrolysis, and commercial P25), reaction time (24h and 72h), and acid washing treatment were compared. The samples were characterized by XRD, surface area (BET), porous structure (BJH), SEM and TEM, besides Raman spectroscopy, TG and temperature effect on the crystal composition. The sol-precipitation route produced the SrTiO3 sample with higher crystallinity and amount of impurities, while its surface area decreased on 400°C heat-treatment. All products showed SrCO3 presence up to 900°C. Combustion synthesis did not lead to the strontium titanate. Varied-length nanotubes and other nanostructures were obtained from all crystalline precursors, although rutile-derived samples showed only nanotubes, and the longest ones. On the other hand, this precursor reacted more slowly than the others. Amorphous TiO2 produced wire-like nanostructures. It has been observed that nanotubes are composed of lamellar sodium titanate, and that HCl washing promotes Na+ exchange for H+ ions. This ion-exchange leads to a decrease in the interlamellar spacing and an increase in the surface area of the samples.

<p>Bismuth titanate Bi₄Ti₃O₁₂, is one of the bismuth based layered ferroelectric materials that is a candidate for replacing the lead based ferroelectric materials in for instance non-volatile ferroelectric random access memories (FRAM). This is due to the fact that the bismuth based ferroelectrics consists of pseudo perovskite units sandwiched in between bismuth oxide layers, which gives them a better fatigue nature.</p><p>In this thesis thin films of Bi₄Ti₃O₁₂have been deposited by chemical vapour deposition (CVD) using the metal iodides, BiI₃ and TiI₄ as precursors. Films grown on MgO(001) substrates were found to grow epitaxially. The electrical properties were determined for films grown on Pt-coated silicon and good properties such as a high dielectric constant (ε) of 200, low <i>tan</i> δ of 0.018, a remnant polarisation (<i>P</i>_r) of 5.3 μC/cm² and coercive field (E_c) as high as 150 kV/cm were obtained. Thin films in the Bi-Ti-O system were also deposited by atomic layer deposition (ALD) using metalorganic precursors.</p><p>In addition to the ternary bismuth titanates, films in the binary oxide systems <i>i.e.</i> bismuth oxides and titanium oxides were deposited. Epitaxial TiO₂ films were deposited both by CVD and ALD using TiI₄ as precursor. The rutile films deposited by ALD were found to grow epitaxially down to a temperature of at least 375 ¢ªC on α-A1₂O₃(0 1 2) substrates. The TiO₂ ALD process was also studied <i>in-situ</i> by QCM. Different bismuth oxides were deposited by halide-CVD using BiI₃ as precursor on MgO(0 0 1) and SrTiO₃(0 0 1) substrates and the results were summarised in an experimental CVD stability diagram. The Bi₂O_2.33 phase was found to grow epitaxially on both substrates.</p>