La structure pérovskite, largement étudiée depuis plusieurs décennies, présente un vaste champ d’application. Ces matériaux peuvent notamment être utilisés en tant que condensateurs, matériaux pour les piles à combustible, catalyseurs, analogues à la pérovskite mantellique (Bridgemanite), cellules photovoltaïques ou encore comme matériaux de confinement des déchets nucléaires de haute activité. La structure pérovskite est dite simple (ABO3) ou complexe (A’A’’BO6, AB’B’’O6, A’A’’B’B’’O6, …) en fonction de la composition chimique et de l’arrangement atomique des éléments dans la structure. L’incorporation d’éléments trivalents dans les pérovskites simples A2+B4+O3 induit des modifications structurales et des problèmes de compensation de charges. L’étude cristallochimique de ces pérovskites substituées est nécessaire à la compréhension des propriétés physiques de ces matériaux. Dans le cadre de cette thèse, l’incorporation de terres rares trivalentes, simulant potentiellement des actinides, dans des pérovskites orthorhombiques calciques a été étudiée. Pour ce faire, nous avons incorporé des terres rares trivalentes par frittage à haute température (1550 °C) dans des pérovskites CaSnO3 et CaTiO3.Dans un premier temps, l’étude de l’incorporation de néodyme dans la pérovskite CaSnO3 a été menée par microsonde électronique, MEB, diffraction des rayons X (poudres, monocristaux), spectroscopie µ-Raman, et MET. Il a été montré qu’environ 17 % at de Nd pouvait être incorporé dans une pérovskite CaSnO3 préformée. Les études par diffraction des rayons X sur monocristaux, sur les échantillons les plus concentrés (≈ 14 et 17% at Nd), ont montré que l’incorporation du Nd s’effectue en site A avec une compensation de charge par migration du calcium en site B. A notre connaissance, ce type de mécanisme n’a jamais été reporté dans la littérature. Les études par microsonde électronique, diffraction sur poudres et spectroscopie µ-Raman ont permis d’établir que l’incorporation du Nd dans la pérovskite CaSnO3 s’opérait selon le même schéma, sur toute la gamme de composition.Dans un second temps, au vu des résultats obtenus, nous avons étudié l’incorporation d’une série de terres rares trivalentes (La, Pr, Nd, Sm, Gd, Dy, Er, Yb) dans une pérovskite CaTiO3 préformée, suivant un protocole similaire. L’objectif consistait à évaluer l’effet du rayon ionique de la terre rare sur le mécanisme d’incorporation de cette dernière. Cette étude par microsonde électronique, MEB, diffraction des rayons X sur poudres et spectroscopie Raman a révélé qu’il était possible d’incorporer les différentes terres rares selon un schéma similaire à celui observé pour le système (1-x) CaSnO3 – x Nd2O3, indépendamment du rayon ionique de celles-ci.Ce travail expérimental a mis en évidence un nouveau mécanisme d’incorporation de terres rares dans deux pérovskites calciques (CaSnO3, CaTiO3) et pourrait contribuer à moduler les propriétés physico-chimiques de ces matériaux en fonction des conditions de synthèse / The perovskite structure is widely studied since several decades and a myriad of materials presenting this structure are used in numerous applications. These materials are notably used as capacitors, anodes for solid oxide fuel cells, catalysts, mantellic perovskite analogues, solar cells or matrix for the nuclear waste storage. The perovskite structure can be described as simple (ABO3) or complex (A’A’’BO6, AB’B’’O6, A’A’’B’B’’O6, …) depending on its chemical composition and its structural features. The incorporation of trivalent elements in A2+B4+O3 perovskites induces structural modifications and charge compensation problems that can be accommodated by several ways. The crystallochemistry of these materials must be understood in order to better apprehend their physical properties. During this thesis, the incorporation of trivalent rare earths (used as potential actinide analogues) in orthorhombic calcic perovskites was studied. Thus, trivalent rare earths were incorporated at high temperature (up to 1550°C) in CaSnO3 and CaTiO3 perovskites.In the first part of this study, the incorporation of Nd in CaSnO3 was studied by means of electronic microprobe (EMPA), scanning electron microscopy (SEM), powder X-ray diffraction, single crystal X-ray diffraction, µ-Raman spectroscopy and transmission electronic microscopy (TEM). It was shown that approximately 17 at % of Nd can be incorporated in a preformed CaSnO3 perovskite. The single crystal X-ray diffraction on the most concentrated samples (Nd ≈ 14 at % and Nd ≈17 at %) showed that Nd is replacing Ca in the A site of the perovskite with charge compensation by Ca migration into the perovskite B site (instead of Sn). This mechanism can be written as:(1-x) CaSnO3 + x Nd2O3  (Ca1-2xNd2x)(Sn1-xCax)O3To our knowledge, this mechanism was never reported in the literature. The chemical studies (EMPA), coupled with the structural studies (powder X-ray diffraction, µ-Raman spectroscopy) on the less Nd concentrated samples showed that this substitution mechanism occurs in the whole range of composition.In the second part of this study and regarding the previous substitution mechanism, we studied the incorporation of eight rare earths (La, Pr, Nd, Sm, Gd, Dy, Er, Yb), presenting decreasing ionic radii, in a preformed calcium titanate perovskite, CaTiO3, following a similar protocol. This crystallochemical study by means of EMPA, powder x-ray diffraction and µ-Raman spectroscopy strongly suggests that a substitution mechanism similar to the one observed in the (1-x) CaSnO3 – x Nd2O3 system was possible for the (1-x) CaTiO3 –x Ln2O3 (Ln = La, Pr, Nd, Sm, Gd, Dy, Er, Yb) perovskite systems.This experimental study evidenced a new substitution mechanism of trivalent rare earths element in two calcic perovskites (CaSnO3, CaTiO3). This result could be in turn used for crystal engineering purposes

Die Existenz metallreicher Perowskite der Zusammensetzung (SE3X)M (X = O, N; SE = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Er, Ho, Tm, Lu; M = Al, Ga, In, Sn) wurde untersucht. Die Charakterisierung der Proben erfolgte mit Röntgenpulverdiffraktometrie und Elementaranalysen (O und N). Oxide (SE3O)Al mit SE = La, Ce, Pr, Nd und Sm und (SE3O)In mit SE = Ce, Pr und Nd wurden erhalten. Die Reihe der Verbindungen (SE3N)Al (SE = La, Ce, Pr, Nd, Sm) wurde um die Seltenerd-Metalle SE = Gd, Tb, Dy, Ho, Er und Tm erweitert. Die metallreichen Perowskite (SE3N)Sn (SE = La, Ce, Pr, Sm) und (SE3N)Ga (SE = Ce, Pr, Sm, Gd, Tb) wurden erstmals beschrieben. Die thermische Stabilität (DSC) der Phasen (SE3X)M ist für die Nitride allgemein am höchsten. Nitride von Al und Ga zersetzen zwischen 1000 °C und 1200 °C, Stannide bleiben bis 1250 °C thermisch stabil. Messungen der magnetischen Suszeptibilität und der LIII-Absorbtionskanten sind in Einklang mit einer Elektronenkonfiguration SE3+. Die gemessenen elektrischen Widerstände sind charakteristisch für schlechte metallische Leiter. Verschiedene Gehaltschnitte SE3Al-(SE3X)Al und SE3In-(SE3X)In wurden mit Röntgenpulverdiffraktometrie und DTA untersucht. Die Oxide und Nitride (SE3X1-y)M (SE = La, Ce; X = N, O) weisen nur geringe Phasenbreiten auf. Die Carbide (Ce3C1-y)In zeigen hingegen signifikante Phasenbreiten. In den Systemen EA-In-N wurden röntgenografisch phasenreine Pulver von (Ca4N)[In]2 und (EA19N7)[In4]2 (EA = Ca, Sr) erhalten. Durch Elementaranalysen auf H, C, N, O, EA und In und Neutronenbeugung am Pulver können alternative Zusammensetzungen mit einer ausgeglichenen Ladungsbilanz im Sinne des Zintl-Konzepts für diese Phasen ausgeschlossen werden. Im System La-Al wurde die neue Phase La16Al13 beobachtet und an Einkristallen sowie an Pulvern charakterisiert. Das in der Literatur im Cu3Au-Strukturtyp beschrieben kubische Polymorph von Ce3Al wurde auf einen ternären metallreichen Perowskit (Ce3X)Al zurückgeführt.

First- principles modeling is used to predict hydrogen permeability through Palladium (Pd)-rich binary alloy membranes as a function of temperature and H2 pressure. We introduce a simplified model that incorporates only a few factors and yields quantitative prediction. This model is used to predict hydrogen permeability in a wide range of binary alloy membranes and to find promising alloys that have high hydrogen permeability. We show how our efficient Density Functional Theory (DFT)-based model predicts the chemical stability and proton conductivity of doped barium zirconate (BaZrO3), barium stannate (BaSnO3), and barium hafnate (BaHfO3). Our data is also used to explore the physical origins of the trends in chemical stability and proton conductivity among different dopants. We also study potassium tantalate (KTaO3), which is a prototype perovskite, to examine the characteristics of undoped perovskites. Specifically, we study the impacts of isotope effects, tunneling effects, and native point defects on proton mobility in KTaO3. It is important to find and develop solid-state Li-ion electrolyte materials that are chemically stable and have high ionic conductivities for high performance Li-ion batteries. We show how we predict the chemical stability of Li7La3Zr2O12, Li7La3Sn2O12, and Li7La3Hf2O12 with respect to carbonate and hydroxide formation reactions.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Nesta tese, o efeito magnetocalórico é estudado teoricamente partindo de um hamiltoniano modelo que leva em conta uma rede magnética formada por diversas sub-redes magnéticas acopladas. No hamiltoniano são consideradas as interações de troca, Zeeman e magnetoelástica. Primeiramente, o hamiltoniano é apresentado em sua forma generalizada para R sub-redes magnéticas e a influência dos parâmetros do modelo na temperatura de Nèel e na temperatura de compensação é analisada no sistema com duas sub-redes magnéticas. Encontramos que, dependendo dos parâmetros de troca, arranjos ferrimagnético, antiferromagnético e ferromagnéticos podem ser obtidos. O efeito magnetocalórico foi sistematicamente estudado para diversos arranjos possíveis, posteriormente foi estudado em compostos reais do tipo R3Fe5O12 (RIG), sistema formado por três sub-redes magnéticas. Retornando ao sistema com duas sub-redes magnéticas foi analisada a influência da interação magnetoelástica no efeito magnetocalórico nos arranjos ferrimagnéticos obtidos previamente. Aplicando este modelo para uma estrutura cúbica do tipo perovskita, estudamos o efeito magnetocalórico nos compostos EuZrO3 e EuTiO3. Uma metodologia para a obtenção da magnetização de uma amostra policristalina foi apresentada e ainda estudamos o efeito magnetocalórico anisotrópico de natureza antiferromagnética. / In this work, the magnetocaloric effect is studied theoretically considering a model Hamiltonian, which accounts for a magnetic lattice formed by several coupled magnetic sublattices. The model Hamiltonian includes the exchange, Zeeman and magnetoelastic interacrions. Firstly, R sublattices are considered to present the Hamiltonian in a generalized form, then the model parameters influence on the Nèel temperature and the compensation temperature are analised for the system considering two magnetic sublattices. Depending on the exchange parameters, ferrimagnetic, antiferromagnetic and ferromagnetic arrangements can be obtained. The magnetocaloric effect was systematically studied for several profiles and applied to the system with three magnetic sublattices, R3Fe5O12 (RIG). In sequence, the magnetoelastic interaction influence on the magnetocaloric effect of the systems with two coupled magnetic sublattices was analysed for the ferrimagnetic profiles studied previously. This model was applied to the cubic perovskite-like structure, and the magnetocaloric effect was studied on the EuZrO3 and EuTiO3 compounds. A metodology to calculate the policrystalline magnetization was presented and the anisotropic magnetocaloric effect from the antiferromagnetic nature of the system was studied .

Made available in DSpace on 2014-10-09T12:49:51Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:03:04Z (GMT). No. of bitstreams: 1 09826.pdf: 5905522 bytes, checksum: b0354cc2f21014da9657b88019b2b992 (MD5) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP / FAPESP:99/07068-0

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Nesta tese, o efeito magnetocalórico é estudado teoricamente partindo de um hamiltoniano modelo que leva em conta uma rede magnética formada por diversas sub-redes magnéticas acopladas. No hamiltoniano são consideradas as interações de troca, Zeeman e magnetoelástica. Primeiramente, o hamiltoniano é apresentado em sua forma generalizada para R sub-redes magnéticas e a influência dos parâmetros do modelo na temperatura de Nèel e na temperatura de compensação é analisada no sistema com duas sub-redes magnéticas. Encontramos que, dependendo dos parâmetros de troca, arranjos ferrimagnético, antiferromagnético e ferromagnéticos podem ser obtidos. O efeito magnetocalórico foi sistematicamente estudado para diversos arranjos possíveis, posteriormente foi estudado em compostos reais do tipo R3Fe5O12 (RIG), sistema formado por três sub-redes magnéticas. Retornando ao sistema com duas sub-redes magnéticas foi analisada a influência da interação magnetoelástica no efeito magnetocalórico nos arranjos ferrimagnéticos obtidos previamente. Aplicando este modelo para uma estrutura cúbica do tipo perovskita, estudamos o efeito magnetocalórico nos compostos EuZrO3 e EuTiO3. Uma metodologia para a obtenção da magnetização de uma amostra policristalina foi apresentada e ainda estudamos o efeito magnetocalórico anisotrópico de natureza antiferromagnética. / In this work, the magnetocaloric effect is studied theoretically considering a model Hamiltonian, which accounts for a magnetic lattice formed by several coupled magnetic sublattices. The model Hamiltonian includes the exchange, Zeeman and magnetoelastic interacrions. Firstly, R sublattices are considered to present the Hamiltonian in a generalized form, then the model parameters influence on the Nèel temperature and the compensation temperature are analised for the system considering two magnetic sublattices. Depending on the exchange parameters, ferrimagnetic, antiferromagnetic and ferromagnetic arrangements can be obtained. The magnetocaloric effect was systematically studied for several profiles and applied to the system with three magnetic sublattices, R3Fe5O12 (RIG). In sequence, the magnetoelastic interaction influence on the magnetocaloric effect of the systems with two coupled magnetic sublattices was analysed for the ferrimagnetic profiles studied previously. This model was applied to the cubic perovskite-like structure, and the magnetocaloric effect was studied on the EuZrO3 and EuTiO3 compounds. A metodology to calculate the policrystalline magnetization was presented and the anisotropic magnetocaloric effect from the antiferromagnetic nature of the system was studied .

Made available in DSpace on 2015-05-14T13:21:35Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 2485638 bytes, checksum: 60d79f7bd25e1ad9c1ac5a5e78d9dc30 (MD5) Previous issue date: 2014-02-27 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / The potassium niobate, KNbO3, of orthorhombic perovskite presenting ferroelectric, optical property, in addition to their use in heterogeneous photocatalysis. This work consists of two stages - the synthesis of potassium niobate and its evaluation in the photodegradation of the dye Remazol Yellow Gold (RNL). The synthesis of KNbO3 was performed by polymeric precursors and catalysts were characterized by X- ray diffraction ( XRD ), infrared spectroscopy (IR) spectroscopy, ultraviolet visible (UV-Vis) region and area analysis surface . XRD results showed that the perovskite had a far-reaching organization, and there was a decrease in the secondary phase (K4Nb6O17). IR spectra confirmed these data by presenting a reduction of the bands related to vibrations with short double bond character of the Nb = O. The surface area values were lower with increasing temperature. The photocatalyst was applied to the dye Remazol yellow discoloration of gold (RNL) and the parameters used were: radiation time (2 and 4 h), the amount of catalyst (5 or 20 mg) and the change in pH (3 and 6). It was observed that a higher radiation time, a smaller quantity of catalyst and lower pH favoring a better photocatalytic efficiency, obtaining a result of discoloration of 55 %. A comparative study was done with a temperature of 700 º C, as the best result obtained in the previous tests, and observed a photocatalytic efficiency of 67% discoloration. / O niobato de potássio, KNbO3, é uma perovskita ortorrômbica que apresenta propriedade ferroelétrica, óptica, além da sua utilização em fotocatálise heterogênea. O presente trabalho consiste em duas etapas a síntese de niobato de potássio e a sua avaliação na fotodegradação do corante Amarelo Remazol Ouro (RNL). A síntese do KNbO3 foi realizada pelo método dos precursores poliméricos e os catalisadores foram caracterizados por difratometria de raios-X (DRX), espectroscopia na região do infravermelho (IV), espectroscopia na região do ultravioleta visível (UV-Vis) e análise de área superficial. Os resultados de DRX mostraram que as perovskitas apresentaram uma organização a longo alcance, e houve uma diminuição da fase secundária (K4Nb6O17). Os espectros de IV confirmaram esses dados ao apresentarem uma redução das bandas referentes às vibrações curtas com caráter de dupla ligação do Nb=O. Os valores de área superficial foram menores com o aumento da temperatura. O fotocatalisador foi aplicado na descoloração do corante remazol amarelo ouro (RNL) e os parâmetros utilizados foram: o tempo de radiação (2 e 4 h), a quantidade de catalisador (5 ou 20 mg) e a variação do pH (3 e 6). Observou que um maior tempo de radiação, uma quantidade menor de catalisador e menor pH favoreceram uma melhor eficiência fotocatalítica, obtendo um resultado de 55% de descoloração. Um estudo comparativo foi feito com a temperatura de 700 ºC, como o melhor resultado obtido nos testes anteriores, e observou uma eficiência fotocatalítica de 67 % de descoloração.

Made available in DSpace on 2014-10-09T12:49:51Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:03:04Z (GMT). No. of bitstreams: 1 09826.pdf: 5905522 bytes, checksum: b0354cc2f21014da9657b88019b2b992 (MD5) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP / FAPESP:99/07068-0

Made available in DSpace on 2016-08-18T18:19:32Z (GMT). No. of bitstreams: 1 Dissertacao de Antonio Luiz Martins Junior.pdf: 2188938 bytes, checksum: 2158404e55575579b79bcf24267df24b (MD5) Previous issue date: 2013-10-30 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In recent years, researchers and scientists has been devoted more to the study of inorganic materials, in particular the oxides known as perovskites, because its technological use. The analysis of their properties have been made by various techniques, among these we can mention the Raman spectroscopy and infrared, which show very sensitive to changes in the structure of these oxides. Thus, this work presents the study of phonons at room temperature the double perovskites Ca₃Mn₂WO₉ (CMWO) and Ca₃Mn₂NbO₉ (CMNO) by means of Raman scattering techniques and transmittance Infrared. Both compounds crystallize in a monoclinic system space group P2₁/n, with Z = 2. The Raman and infrared spectra are interpreted by the group factor analysis in terms of the space group P2₁/n. Also the vibrational modes of stretching and folding B were reported "- O. The observed spectra are very similar to the cubic perovskite structure prototype dual Fm3m indicating that the phases of the study result of minor distortions cubic cell. It is also observed an unexpected band in the Raman spectra of both materials. We suggest that this additional bandwidth would result from occupational disorder of the sites of the cation B, which is evidenced in both phases. / Nos últimos anos, pesquisadores e cientistas tem se dedicado mais ao estudo de materiais inorgânicos, em particular aos óxidos denominados perovskitas, devido o seu aproveitamento tecnológico. A análise de suas propriedades tem sido feita por diversas técnicas, entre estas podemos citar as espectroscopias Raman e de Infravermelho, que se mostram muito sensíveis às mudanças na estrutura desses óxidos. Sendo assim, este trabalho apresenta o estudo de fônons à temperatura ambiente das perovskitas duplas Ca₃Mn₂WO₉ (CMWO) e Ca₃Mn₂NbO₉ (CMNO), por meio das técnicas de espalhamento Raman e transmitância de Infravermelho. Ambos os compostos cristalizam em um sistema monoclínico com grupo espacial P2₁/n, com Z = 2. Os espectros Raman e infravermelho são interpretados por meio da análise do grupo fator em termos do grupo espacial P2₁/n. Também foram reportados os modos vibracionais de estiramento e dobramento do B - O. Os espectros observados são muito semelhantes ao do protótipo cúbico de estrutura perovskitas dupla Fm m indicando que as fases em estudo resultam de pequenas distorções da célula cúbica. Observa-se também uma banda inesperada nos espectros Raman de ambos materiais. Nós sugerimos que essa banda adicional seria resultado da desordem ocupacional dos sítios do cátion B, que é evidenciada nas duas fases.

Oxygen Transport Membranes (OTMs) can drastically reduce the energy and cost demands of processes that require pure oxygen, as they offer the possibility to combine a separation unit with a chemical reactor. One of the most commercially viable applications of OTMs is the partial oxidation of hydrocarbons for syngas production. A typical OTM configuration is a sequential arrangement of layers, i.e. an inactive support, a fuel oxidation layer, a dense layer and an oxygen reduction layer. However, one of the limitations of the OTM system is the low catalytic activity and stability of the materials currently used for the fuel oxidation layer. Moreover, the traditional deposition techniques that are used for the catalysts preparation are difficult to perform, as the fuel oxidation layer is buried deeply in the structure of the OTM. To simplify the OTM fabrication and improve the catalysts activity and stability, this thesis explores the exsolution of Ni nanoparticles from two different host lattice compositions, as potential materials for the fuel oxidation layer of OTMs. The (La₀.₇₅Sr₀.₂₅)(Cr₀.₅Mn₀.₄₅Ni₀.₅)O₃ (LSCMNi5) perovskite was selected, as the first candidate material for the OTMs. During reduction, the exsolution of Ni nanoparticles from the perovskite lattice took place and enhanced significantly the catalytic activity of the material regarding methane conversion. However, these nanoparticles were oxidised during the first hours of the testing and slowly reincorporated into the perovskite structure, leading to drop in the performance. Thereafter, the (La₀.₇₅Sr₀.₂₅)(Cr₀.₅Mn₀.₄₅Ni₀.₅)O₃ (LSCMNi5) perovskite was selected as an alternative composition. When the oxide lattice was sufficiently reduced, the exsolution of Fe-Ni alloy nanoparticles occurred. The catalytic testing suggested that the Fe-Ni alloy nanoparticles on LSCFNi5 presented lower activity for methane conversion comparing to the Ni nanoparticles on LSCMNi5, but higher stability in oxidising conditions. By increasing the Ni doping on the B-site of LSCF to 15 mol%, the catalytic activity of the material regarding methane conversion was increased and exceeded that of LSCMNi5. A CH₄ conversion of 70% was achieved, which was 20 times higher than that of the initial LSCF perovskite. Therefore, by tailoring the perovskite composition and the exsolution of the Fe-Ni alloy nanoparticles, it was possible to synthesize a material for the fuel oxidation layer of OTMs, which combined the high catalytic activity of Ni and the good redox stability of Fe.