La thèse présente l'élaboration et l'étude des films minces (35 nm) cristallins et stœchiométriques de la magnétite Fe304(001)/MgO. La qualité de ces films est étudiée par différentes techniques (DEL, XPS, XMCD, Effet Kerr, Auger). Nous supposons que les films se recouvrent partiellement, même sous ultra vide, par de Fe2O3. Pour la première fois, l'étude de la structure électronique de la bande t2g de ces films est réalisée par photoémission résolue en angle (ARPES)à température ambiante et à 75 K, en utilisant des photons à basses énergies (6eV - 21 eV). Nous avons constaté que le signal de la photoémission est composé des états électroniques de deux périodicités liées à la zone de Brillouin primitive de la surface (a=4.2 Å) et à la zone de Brillouin de la reconstruction de la surface (a=8.4 Å). Nous pensons que la présence des états liés à la reconstruction sont probablement responsables de la chute de la densité d'état à EF. Une signature de Verwey est observée par ARPES à basse température. Nous estimons que la réalisation des films ultra minces de magnétite est difficilement aboutie. / With its half-metallic behavior predicted theoretically, the magnetic oxideFe3O4 (TC = 863 K) is promising for applications in spintronics as thin films.High quality films and the electronic band structure are still a challenge to faceexperimentally. We managed to develop single crystalline Fe3O4(0 0 1) films(35 nm) on MgO. Analysis by XPS, XAS and XMCD allowed to characterizethe quality of the films prepared. The magnetic study shows a perfect XMCDsignal and a form magnetic anisotropy which lays down the axis of easy magnetizationin the film plane. Our samples are stable during the photoemissionmeasurements. The photoemission measurements of the t2g band show thatthe electronic bands cannot be described by a DFT calculation. Indeed, spinpolarons due to strong electron-phonon coupling mechanism are involved inthe electronic transport. Angle-resolved photoemission shows a dispersion ofthe t2g band in the ����M direction corresponding to two periodicities [the unitcell of the surface reconstruction a = 8,4 Å (30% of the signal) and the simpleunit cell of the surface a = 4;2 Å]. At a temperature T < TV (TV = 120 K,Verwey temperature), the angle-resolved photoemission shows the opening ofa 100 meV band gap, with a rigid shift of the spectral weight of the t2g bandto the high binding energy side.

A formação de espinélio de aluminato de magnésio (\'MG\'\'AL IND.2\'\'O IND.4\'; espinélio), através da combinação de óxido de alumínio (\'AL IND.2\'\'O IND.3\'; alumina) com geradores de poros à base de óxido de Magnésio (\'MG\'\'O\'; magnésia) ou hidróxido de magnésio (\'MG\'(\'OH)IND.2\'; HM), resulta em estruturas de elevada porosidade (acima de 50%) e com baixa tendência a densificar mesmo em temperaturas elevadas (1500-1650°C). Devido a isso, esse sistema tem grande potencial de aplicação tecnológica para isolamento térmico e filtração em altas temperaturas. Considerando sua utilização em larga escala como isolante térmico verifica-se a necessidade de desenvolvimento de novas rotas de síntese mais eficazes. A reação de hidroxilação do \'MG\'\'O\' pode ser controlada, utilizando como matéria-prima tanto a magnésia sínter como com a magnésia caustica, desta forma, a expansão de peças de cerâmicas porosas feitas a base de \'MG\'\'O\' também podem ser controladas. Deste modo, neste trabalho pretende-se investigar uma nova rota para a obtenção de espinélio utilizando o \'MG\'\'O\' como ligante hidráulico e incorporar poros, in situ, utilizando-se da decomposição do HM em uma matriz de alumina. Os resultados mostraram que a área superficial específica (ASE) da magnésia influencia fortemente nas propriedades mecânicas do material moldado, comprovando que para as ASEs maiores a magnésia é viável como ligante hidráulico, sendo 0,6 MPa para o modulo de ruptura por compressão diametral, o melhor resultado deste trabalho, enquanto o sistema à base de cimento de aluminato de cálcio (CAC) teve 0,5 MPa de módulo de ruptura, e o sistema à base de \'alfa\'-Bond teve 0,3 MPa de módulo, ambos sistemas de ligantes hidráulicos conhecidos na literatura. A grande diferença no módulo de ruptura por compressão diametral mostrou que o tempo de cura e a ASE da magnésia influenciaram-no fortemente.Enquanto, as amostras com alta ASE de magnésia, em torno de ~60 \'M POT.2\'/g, tiveram 0,6 MPa de módulo de ruptura, as amostras com baixa ASE, ~1 \'M POT.2\'/g, não tiveram alteração no módulo de ruptura. Verificou-se também um aumento gradual no módulo de ruptura segundo a ASE, para um valor intermediário de ASE, ~30 \'M POT.2\'/g, o módulo foi de 0,4 MPa, valor ainda comparável a outros sistemas que se utilizam ligantes hidráulicos. Observou-se que a expansão das amostras durante a cura foi influenciada pela sua ASE, quanto maior a ASE, maior a expansão. Os resultados de PTG e do módulo de ruptura combinados com as imagens de MEV e os difratogramas, das amostras calcinadas, mostraram a influência da temperatura e da ASE na formação da cerâmica porosa por essa rota, sendo a temperatura uma variável de controle já conhecida, observada no diagrama de equilíbrio de fases. Entretanto, a influência da ASE do \'MG\'\'O\' para o controle da formação do espinélio é desconhecido da literatura / Magnesium aluminate spinel (\'MG\'\'AL IND.2\'\'O IND.4\'; spinel) formation by the combination of aluminum oxide (\'AL IND.2\'\'O IND.3\'; alumina) with magnesium oxide (\'MG\'\'O\', magnesium) or magnesium hydroxide (\'MG\'(\'OH) IND.2\'; HM), results in structures of high porosity (above 50%) and with low tendency to densify even at elevated temperatures (1500-1650°C). Due to this, this system has great potential of technological application for thermal insulation and filtration in high temperatures. Considering its large scale use as thermal insulation, it is necessary to develop new and more efficient routes of synthesis. The hydroxylation reaction of \'MG\'\'O\' can be controlled using both sinter magnesia and caustic magnesia as a feedstock, so the expansion of porous ceramic pieces made with \'MG\'\'O\'can also be controlled. Thus, in this work we intend to investigate a new route to obtain spinel using \'MG\'\'O\' as a hydraulic binder and to incorporate pores, in situ, using the decomposition of HM in an alumina matrix. The results showed that the specific surface area (ASE) of the magnesia strongly influences the mechanical properties of the molded material, proving that for the larger ASEs the magnesia is viable as a hydraulic binder, being 0.6 MPa for the diametral compression rupture modulus, the best result of this work, while the calcium aluminate cement (CAC) system had 0.5 MPa of modulus of rupture, and the \'alfa\'-Bond based system had 0.3 MPa of modulus, both systems of hydraulic binders known in the literature. The large difference in the diametral compression rupture modulus showed that the curing time and the ASE of the magnesia strongly influenced it. While high ASE magnesia samples, around ~60 \'M POT.2\'/g, had 0.6 MPa of modulus of rupture, samples with low ASE, ~1 \'M POT.2\'/g, had no change in modulus of rupture. There was also a gradual increase in the ASE burst modulus, for an ASE intermediate value ~30 \'M POT.2\'/g, the modulus was 0.4 MPa,a value still comparable to other systems using hydraulic binders. It was observed that the expansion of the samples during curing was influenced by their ASE, the higher the ASE, the greater the expansion. The results of PTG and the rupture modulus combined with SEM images and the diffractograms of the calcined samples showed the influence of temperature and ASE on the formation of the porous ceramic by this route, the temperature being a known control variable, observed in the phase equilibrium diagram. However, the influence of \'Mg\'\'O\' ASE on the control of spinel formation is unknown in the literature

O aluminato de magnésio (MgAl2O4; espinélio) apresenta propriedades mecânicas superiores quando comparado aos materiais cerâmicos tradicionais, tais como elevados módulo elástico (273 GPa) e resistência à flexão (110 MPa), associadas à baixa densidade (3,58 g/cm3), baixo índice de reflexão (1,736), índice de transmissão óptica elevado no espectro visível e espectros no infravermelho com comprimentos de onda médios (0,2 - 5,5 µm), além da ausência de anisotropia óptica, devido à sua estrutura cúbica. No entanto, MgAl2O4 é utilizado principalmente como material refratário, apesar de possuir grande potencial em aplicações que exijam blindagem transparente leve. Nanopartículas de espinélio já foram preparadas anteriormente por diferentes métodos. Contudo, o domínio de um processo industrial contínuo, escalonável e versátil para a preparação de MgAl2O4 dopado ainda permanece como um desafio para expandir as aplicações deste material. Dentre as vias de síntese habituais utilizadas para produzir nano-óxidos, a Pirólise de Spray gerado por Pulverização Ultrassônica (PSPU) tem sido utilizada com sucesso para sintetizar nanopartículas esféricas, nanofios, nanofitas e nanovaretas. Neste contexto, o presente trabalho confirma o potencial da PSPU para produzir espinélio dopado em um processo contínuo. A influência dos parâmetros envolvidos na síntese do aluminato de magnésio por meio desta técnica, assim como o efeito da presença do cálcio e do fluoreto de lítio sobre a morfologia e a estrutura das partículas, foram investigadas por fluorescência de raios X, difração de raios X, espectroscopia no infravermelho, granulometria por difração laser, microscopia eletrônica de varredura e adsorção de N2. Finalmente, as propriedades mecânicas do produto final sinterizado foram avaliadas visando estabelecer uma correlação com as condições de síntese. O processo de síntese de aluminato de magnésio por meio do sistema de PSPU desenvolvido nas dependências do Departamento de Engenharia Metalúrgica e de Materiais da Escola Politécnica da Universidade de São Paulo demostrou-se eficaz para a produção de amostras de aluminato de magnésio puro após a etapa de sinterização, desde que respeitada a estequiometria do composto após a etapa de síntese. Por meio desta técnica, esferas micrométricas de MgAl2O4 dopadas com Ca(NO3)2 e LiF, apresentando tamanhos médios de cristalito na faixa de 3,5 - 7,0 nm e áreas de superfície específicas de 20 a 40 m2/g, foram produzidas como aglomerados esféricos de aproximadamente 2,5 µm. Durante o processo, as partículas permaneceram a temperaturas elevadas durante um curto período de tempo (de 35 a 70 segundos), permitindo a estabilidade de fases e aumento do tamanho de grãos limitado. Destaca-se ainda que as condições de síntese e/ ou incorporação de aditivos devem ser ajustados para a obtenção de amostras com maior área de superfície específica após a PSPU, o que acarretará em um produto final sinterizado com maior teor de densificação e dureza. Dessa forma, os melhores resultados foram obtidos a maiores temperaturas de pirólise e com incorporação do aditivo LiF, demonstrado a necessidade de futuros estudos mais aprofundados a respeito dos limites máximos destas variáveis para a obtenção de um produto final otimizado. Finalmente, as propriedades balísticas das amostras também foram analisadas através da aplicação de fórmulas empíricas para avaliação da fragilidade (B) e da habilidade do material dissipar energia balística (critério D), onde se observou que as amostras sintetizadas sem aditivos apresentaram boa concordância em relação aos valores reportados na literatura para a alumina. A amostra aditivada com LiF, no entanto, apresentou um incremento no critério D de cerca de 43% em relação à alumina com 99,7% de pureza, evidenciando o efeito deste aditivo nas propriedades balísticas do aluminato de magnésio produzido pela PSPU. / Magnesium aluminate (MgAl2O4; spinel) possesses superior mechanical properties when compared to traditional ceramic materials, such as high elastic modulus (273 GPa) and flexural strength (110 MPa), associated with low density (3.58 g/cm3), low reflection index (1.736), high optical transmission in visible and mid-wavelength infrared spectra (0.2 - 5.5 µm), and no optical anisotropy due to its cubic structure. However, MgAl2O4 is primarily used as a refractory material, despite its great potential as a transparent lightweight armor. Spinel nanoparticles have been previously prepared by different methods. Nevertheless, a continuous, scalable, and versatile process for the preparation of doped MgAl2O4 still remains a challenge for expanding applications. Among the usual synthesis routes used to produce nano-oxides, Ultrasonic Spray Pyrolysis (USP) has been successfully employed to synthesize nanoparticles as spheres, nanowires, nanoribbons and nanorods. In this context, the present work confirms the potential of USP to produce doped spinel in a continuous setup. The influence of the parameters involved in the synthesis of magnesium aluminate through this technique, as well as the effect of the presence of calcium and lithium fluoride on the morphology and structure of the particles, were investigated by X-ray fluorescence, X-ray diffraction, infrared spectroscopy, laser diffraction for particle size analysis, scanning electron microscopy and N2 adsorption. Finally, the mechanical properties of the sintered product were evaluated in order to establish a correlation with the synthesis conditions. The magnesium aluminate synthesis process through the USP system developed at the Department of Metallurgical and Materials Engineering of the Polytechnic School of the University of São Paulo was effective for the production of pure magnesium aluminate samples after the sintering stage, if the stoichiometry of the compound after the synthesis step is observed. Through this technique, micrometric spheres of Ca(NO3)2 and LiF doped MgAl2O4 with crystallite size in the range from 3.5 - 7.0 nm and specific surface areas varying from 20 to 40 m2/g, were produced as spherical agglomerates of approximately 2.5 µm. During the process, the particles stay at high temperatures for a short period (from 35 to 70 seconds), allowing phase stability and limited coarsening. It should also be noted that the synthesis conditions and / or the incorporation of additives must be adjusted in order to obtain samples with greater specific surface area after the USP, which will result in a sintered final product with a higher densification and hardness. Therefore, the best results were obtained at higher pyrolysis temperatures and with the incorporation of LiF additive, demonstrating the need for further studies on the maximum limits of these variables to obtain an optimized final product. Finally, the ballistic properties of the samples were also analyzed by the application of empirical formulas to evaluate the brittleness (B) and the ability of the material to dissipate ballistic energy (criterion D), where it was observed that the samples synthesized without additives showed good agreement with the values reported in the literature for alumina. The sample containing LiF additive, however, showed an increase in the D criterion of about 43% in relation to alumina with 99.7% purity, evidencing the effect of this additive on the ballistic properties of magnesium aluminate produced by USP.

Metallic interconnects in intermediate temperature solid oxide fuel cells (IT-SOFC) stacks form Cr2O3 scales on their surface. Such oxide scales can be further oxidized to Cr6+ containing gaseous species that migrate and deposit at the cathode triple phase boundaries, causing significant degradation in the performance of the SOFCs. This phenomenon is termed as ‘Cr-poisoning’. A solution to this problem is the application of coatings on the interconnects that act as a diffusion barrier to Cr migration. Two different Cu/Mn spinel compositions, Cu1.3Mn1.7O4 and CuMn1.8O4, were studied as coating materials. Dense coatings were deposited on both flat plates and meshes by electrophoretic deposition (EPD) followed by subsequent thermo-mechanical or thermal densification steps. At room temperature, Cu1.3Mn1.7O4 coatings were found to have a mixture of CuO and spinel phases, while CuMn1.8O4 coatings were found to have a mixture of Mn3O4 and spinel phases. However, CuMn1.8O4 is a pure spinel phase between 750 °C and 850 °C. After densification processing and high temperature oxidation, a Cr2O3 layer was formed at the coating/alloy interface, which partially reacted with the spinel coatings to form a dense cubic spinel layer of the general composition (Cu,Mn,Cr)3-xO4. In addition, Cr-rich precipitates, formed in the dense layer close to coating/alloy interface. It is believed that these are Cr2O3 precipitates, formed when the solubility of Cr in the spinel phase is reached. Solubility experiments using powders showed that 1 mole of CuMn1.8O4 can effectively getter 1.83 moles of Cr2O3 at 800°C. Electrical conductivity of (Cu,Mn,Cr)3-xO4 was found to be at least two orders of magnitude higher than that of Cr2O3. The coatings acted as an effective Cr getter whose lifetime depends on the oxidation temperature, coating thickness, and the overall porosity in the coating. In-cell electrochemical testing showed that the CuMn1.8O4 coatings on Crofer 22 APU meshes performed significantly better than commercial Cu/Mn spinel coatings. The CuMn1.8O4 coatings gettered Cr effectively for 12 days at 800 ºC, leading to no performance loss of the cell due to Cr-poisoning. Significantly longer lifetime can be achieved at 750 ºC or lower, which is the target operational temperature regime of IT-SOFCs.

O espinélio MgAl2O4 é utilizado prioritariamente na indústria de refratários. Atualmente há várias técnicas de síntese, visto que não é encontrado na natureza. A técnica de sais fundidos apresenta grande potencial para otimização e redução do custo energético na produção deste óxido. Este trabalho teve como objetivo o desenvolvimento da técnica de síntese via sais fundidos para obtenção do espinélio MgAl2O4, e a caracterização do pó produzido via diferentes técnicas. A dissertação explora as variáveis da produção do espinélio a partir de Al2O3 e MgO em KCl, e também alternando o sal para MgCl2 e o eutético KCl-MgCl2. As seguintes alterações foram testadas para verificar o resultado no pó produzido: redução da temperatura para 1150°C, aumento do MgO nos precursores em 10% e 30%, redução da granulometria da Al2O3 precursores para 0,60 μm, variação da razão sal/precursor (S/P) para 1:1 e 9:1. Foi testado o sal MgCl2 nas proporções de precursores estequiométrica, -50% de MgO e -100% de MgO. Por fim, o sal KCl-MgCl2 na razão eutética foi testado. Para caracterização do pó foram utilizadas as técnica de análise cristalográfica por difração de raios X (DRX) e pela técnica de RAMAN, granulometria por dispersão a laser (GDL), microestrutura por microscopia eletrônica por varredura (MEV) e análise de cristalitos por mircroscopia eletrônica de transmissão (MET). Os pós produzidos apresentaram as fases MgAl2O4, Al2O3 e MgO. A redução de temperatura reduziu a quantidade de espinélio em pequena proporção, a alteração de estequiometria dos precursores no sal KCl reduziu a quantidade de Al2O3 em um primeiro momento e também sobrou MgO na reação para um aumento maior de MgO. A redução de granulometria da Al2O3 propiciou aumento da proporção de espinélio na reação. A proporção S/P 1:1 não mostrou alteração, contudo para 9:1 houve sedimentação e um efeito de diluição/precipitação. Para o sal MgCl2, houve precipitação de MgO a partir do sal, alterando o perfil granulométrico dos pós e as fases obtidas: MgO e MgAl2O4. A redução de MgO aumentou a conversão para MgAl2O4, reduzindo também a granulometria. O sal na proporção eutética apresentou boa conversão de espinélio e pouco aumento de granulometria. / The MgAl2O4 spinel is mainly applied at the refractory industry. There are many techniques to produce that oxide, since it is not found in the nature. The molten salts technique shows potencial energy savings and cost reduction to produce spinel. This work aimed at the development of the molten salts synthesis technique to obtain the MgAl2O4 spinel, and characterization of the powder produced via different techniques. The dissertation explores the variables of production of spinel from Al2O3 and MgO in KCl, and also alternating salt to MgCl2 and KCl-MgCl2 eutectic. The following changes were tested to verify the result of the powder produced: reducing the temperature to 1150 ° C, an increase in the MgO precursor at 10% and 30% reduction in particle size of the Al2O3 precursor to 0.60 micrometres variation ratio of salt / precursor (S / P) to 1: 1 and 9: 1. MgCl2 salt was tested in the stoichiometric proportion of precursors, -50% of MgO and -100% MgO. Finally, the KCl-MgCl2 salt eutectic ratio was tested. For characterization of the powder was used the crystallographic analysis technique of X-ray diffraction (XRD) and RAMAN technique, laser scattering for particle size analisys, scanning electron microscopy (SEM) and crystallite analysis by transmission electron mircroscopy (TEM). The powders produced presented the MgAl2O4 phase, Al2O3 and MgO. The reduction in temperature reduced the amount of spinel in small proportion, the stoichiometry change of the precursors in the KCl salt reduced the amount of Al2O3 at first and also left MgO in reaction to a greater increase of MgO. The reduction of the Al2O3 particle size resulted in an increase in the proportion of the spinel reaction. The ratio S/P 1: 1 showed no change, however to 9: 1 was sedimentation and dilution / precipitation effect. To MgCl2 salt, there was precipitation of MgO from the salt, changing the physical profile of the powder and the obtained phases: MgO and MgAl2O4. The reduction of MgO increased the conversion MgAl2O4, by reducing the grain size. The salt in the proportion eutectic showed good conversion spinel and little increase in particle size.

Le CO2 est une source de carbone propre pour les réactions chimiques, nombreux chercheurs ont étudié l'utilisation du CO2. Les alcools supérieurs sont des additifs de carburant propres. La synthèse des alcools supérieurs à partir de l'hydrogénation du CO a également été étudiée par de nombreux chercheurs, mais il existe peu de littératures sur la synthèse des alcools supérieurs à partir de l'hydrogénation du CO2, qui est une réaction complexe et difficile. Les catalyseurs utilisés pour la synthèse des alcools supérieurs nécessitent au moins deux phases actives et une bonne synergie. Dans notre étude, nous avons étudié les catalyseurs spinelle basés sur Co. Cu. Fe et l'effet des supports (CNTs et TUD-1) et celui des promoteurs (K, Na, Cs) à la réaction de HAS. Nous avons trouvé que le catalyseur CuFe-précurseur-800 est favorable pour la synthèse d'hydrocarbures en C2+ et d'alcools supérieurs. Dans l'hydrogénation du CO2, Co agit comme catalyseur de méthanisation plutôt que comme catalyseur FT, en raison du mécanisme de réaction différent entre l'hydrogénation du CO et celle du CO2. Afin d'inhiber la formation d'hydrocarbures de quantités importante, il est préférable de choisir des catalyseurs sans Co dans la réaction d'hydrogénation du CO2. En comparant les fonctions des CNT et du TUD-1, nous avons constaté que le CNT est un support parfait pour la synthèse de produits à longue chaîne (alcools supérieurs et hydrocarbures C2+). Le support TUD-1 est plus adapté à la synthèse de produits à un seul carbone (méthane et méthanol) .L'addition d'alcalis en tant que promoteurs conduit non seulement à augmenter la conversion de CO2 et H2, mais augmente également la sélectivité des produits visés fortement, des alcools supérieurs. Le catalyseur 0.5K30CuFeCNTs possède une productivités les plus élevées (370.7 g ∙ kg-1 ∙ h-1) d'alcools supérieurs à 350 ° C et 50 bar. / CO2 is a clean carbon source for the chemical reactions, many researchers have studied the utilization of CO2. Higher alcohols are clean fuel additives. The synthesis of higher alcohols from CO hydrogenation has also been studied by many researchers, but there are few literatures about the synthesis of higher alcohols from CO2 hydrogenation, which is a complex and difficult reaction. The catalysts that used for higher alcohols synthesis need at least two active phases and goodcooperation. In our study, we tested the Co. Cu. Fe spinel-based catalysts and the effect of supports (CNTs and TUD-1) and promoters (K, Na, Cs) to the HAS reaction. We found that catalyst CuFe-precursor-800 is beneficial for the synthesis of C2+ hydrocarbons and higher alcohols. In the CO2 hydrogenation, Co acts as a methanation catalyst rather than acting as a FT catalyst, because of the different reaction mechanism between CO hydrogenation and CO2 hydrogenation. In order to inhibit the formation of huge amount of hydrocarbons, it is better to choose catalysts without Co in the CO2 hydrogenation reaction. Compared the functions of CNTs and TUD-1, we found that CNTs is a perfect support for the synthesis of long-chain products (higher alcohols and C2+ hydrocarbons). The TUD-1 support are more suitable for synthesis of single-carbon products (methane and methanol).The addition of alkalis as promoters does not only lead to increase the conversion of CO2 and H2, but also sharply increased the selectivity to the desired products, higher alcohols. The catalyst 0.5K30CuFeCNTs owns the highest productivities (370.7 g∙kg-1∙h-1) of higher alcohols at 350 °C and 50 bar.

Este estudo investigou a síntese de espinélio MgAl204 (MA) por combustão em solução (SCS), a caracterização microestrutural dos pós obtidos, a sua sinterização e a sua ação inibidora da combustão de carbono. Como precursores, foram empregados o nitrato de alumínio, nitrato de magnésio e sacarose em água. A fase cristalina somente foi obtida após um tratamento térmico a 900°C do pó como-sintetizado. Os pós obtidos foram caracterizados por análise termodiferencial (ATD) e termogravimétrica (ATG), granulometria por dispersão a laser (GDL), BET para análise de área superficial, análise cristalográfica por difração de Raios X (DRX) e por espectroscopia Raman e FTIR, e a morfologia por microscopia eletrônica de varredura (MEV) e microscopia eletrônica de transmissão (MET). Os pós de MA apresentaram-se como constituídos de cristalitos nanométricos, com tamanho médio de 21,69nm (calculado via software WinFit) e 35,32nm (estimado por MET). Por análise granulométrica, constatou-se um tamanho de aglomerado médio de 8,21μm, em uma distribuição tetramodal, com uma larga distribuição de tamanho. A área superficial dos pós MA, tratados termicamente a 900°C, alcançou 17,47m²/g. Por MEV, constatou-se que esses aglomerados apresentam uma morfologia irregular, além de vazios (poros internos). Os pós de MA assim obtidos foram prensados uniaxialmente e queimados em diferentes temperaturas (1350-1650°C). Os corpos sinterizados foram caracterizados quanto às propriedades físicas (densidade aparente, absorção de água, porosidade aparente e retração linear), mecânicas (resistência à flexão 4 pontos) e quanto à microestrutura (por MEV). A maior densidade aparente obtida foi de 2,607g/cm3, para uma temperatura de 1650°C, representando uma densificação de 51%, para uma retração linear de 10,06%. A máxima resistência à flexão foi de 51,33MPa e o maior módulo de elasticidade 26,13GPa. A avaliação da ação inibidora dos pós de MA foi realizada pela sua incorporação a diferentes teores em flocos de grafite e posterior aquecimento em termobalança. Constatou-se que os pós de MA apresentaram potencial para o uso como aditivos antioxidantes de materiais à base de carbono em altas temperaturas, pois levaram à diminuição da perda de massa do grafite durante o seu aquecimento e a um aumento da temperatura onset. / This study investigated the synthesis of MgAl204 spinel (MA) by combustion in solution, microstructural characterization of the powders, the sintering process and the inhibiting action of carbon combustion. As precursors were used aluminum nitrate, magnesium nitrate and sucrose in water. The crystalline phase was obtained only after a heat treatment at 900 ° C of the as-synthesized powder. The powders were characterized by differential thermal analysis (DTA) and thermogravimetric (TGA), laser scattering for particle size, BET for analysis of surface area, crystallographic analysis by X-ray diffraction (XRD) and by Raman and FTIR spectroscopy, and morphology by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The powders were presented as consisting of nanosized crystallites with an average crystallite size of 21,69nm (calculated via Winfit software) and 35,32nm (estimated by TEM). The sieve analysis had an average agglomerate size of 8,21μm, in a tetramodal distribution with a wide size distribution. The surface area of the powders calcined at 900°C reached 17,47m²/g. By SEM, it was verified that these clusters have an irregular morphology, and voids (internal pore). The MA powders thus obtained were uniaxially pressed and fired at different temperatures (1350-1650). The powders sintered were characterized by physical properties (bulk density, water absorption, apparent porosity and linear shrinkage), mechanical (flexural strength 4 points) and the microstructure (SEM). The higher bulk density obtained was 2,607g/cm3, to a temperature of 1650 ° C, representing a densification of 51%, for a linear shrinkage of 10.06%. These ceramic samples also had the highest flexural strength (of 51,33MPa) and higher elastic modulus (26,13GPa). The evaluation of the inhibitory action of MA was held by its incorporation into different levels of graphite flakes and subsequent heating in thermobalance. It was noted that the MA powders showed potential use as antioxidant additives of carbon based materials in high temperatures that led to decreased mass loss of graphite flakes during their heating and an increase in the onset temperature.

acase@tulane.edu

Les matériaux luminescents sensibles à la température ou l’exposition auxrayons ultra-violets sont de plus en plus recherchés. Dans ces travaux, deux famillesde composés ont été synthétisés et caractérisés.La première famille est celle des composés ZnAl2O4 dopés aux ionsmanganèse de structure spinelle. Ces composés présentent des propriétés dethermo-chromo-luminescence irréversible avec une variation d’émission du rouge auvert en fonction de l’histoire thermique subie par le matériau. Ici les variations delongueurs d’onde d’émission sont dues au changement de l’environnement des ionsluminescents manganèse en fonction de la température. Il a été démontré que lagamme de température de commutation peut être modulée par une variation de lacomposition de la matrice hôte. La seconde famille est celle des elpasolites decomposition Rb2KInF6 dopé au cérium. Ces composés présentent des propriétés dephoto-chromo-luminescence réversible par oxydo-réduction entre les ions cérium etindium. Sous irradiation UV, il est possible de créer des espèces (In+) quiluminescent dans le rouge. Nous avons montré que le retour aux propriétés initialespeut avoir lieu sous irradiation UV à plus hautes longueurs d’onde ou par chauffage.La spécificité du phénomène associé à cette structure et cette compositionparticulière peut s’expliquer par des considérations structurales, la situation spatialedes ions cérium et indium permettant des échanges électroniques par recouvrementorbitalaire de ces deux ions. / Research for luminescent materials sensitive to temperature or UV exposurevariation are increasing. Here we manage to synthesized and characterized materialswhich show such properties.First, spinel structure materials as ZnAl2O4 doped with manganesecompounds exhibit irreversible thermo-chromo-luminescence properties. Thismaterial shows a variation of emission wavelength from red to green with a variationof the thermal history of the sample. These variations are due to a modification of themanganese environment in the structure with the temperature. It was demonstratedthat the temperature range of the luminescence switch can be tuned by themodification of the matrix composition. Secondly, cerium-doped Rb2KInF6compounds with elpasolite structure exhibit reversible photo-chromo-luminescenceproperties. Under UV irradiation, new luminescent species (In+), created by redoxreaction between Ce3+ and In3+ species, and which emits in the red range, can beproduced. We demonstrate the reversibility of the phenomenon by UV irradiation orafter a thermal treatment. The specificity of these properties with this structure andthis composition can be explained by structural consideration: Ce3+ and In3+positioning in the cell allowing electronic exchanges between these ions thanks toorbital overlap.

Ab Initio calculations suggest that partially lithiated layered LixMnO₂ transforms to spinel in a two-stage process. In the first stage, a significant fraction of the Mn and Li ions rapidly occupy tetrahedral sites, forming a metastable intermediate. The second stage involves a more difficult coordinated rearrangement of Mn and Li ions to form spinel. This behavior is contrasted to LixCoO₂. The susceptibility of Mn for migration into the Li layer is found to be controlled by oxidation state which suggests various means of inhibiting the transformation. These strategies could prove useful in the creation of superior Mn based cathode materials. / Singapore-MIT Alliance (SMA)