Dissertations / Theses on the topic 'SrFeOx'

Non-stoichiometric strontium iron oxide is described by an abbreviated formula SrFeOx (2.5 ≤ x ≤ 3.0) exhibits a variety of interesting physical and chemical properties over a broad range of temperatures and in different gaseous environments. The oxide contains a mixture of iron in the trivalent and the rare tetravalent state. The material at elevated temperature is a mixed oxygen conductor and it, or its derivatives,can have practical applications in oxygen conducting devices such as pressure driven oxygen generators, partial oxidation reactors in electrodes for solid oxide fuel cells (SOFC). ¶ This thesis examines the behaviour of the material at ambient and elevated temperatures using a broad spectrum of solid state experimental techniques such as: x-ray and neutron powder diffraction,thermogravimetric and calorimetric methods,scanning electron microscopy and Mossbauer spectroscopy. Changes in the oxide were induced using conventional thermal treatment in various atmospheres as well as mechanical energy (ball milling). The first experimental chapter examines the formation of the ferrite from a mixture of reactants.It describes the chemical reactions and phase transitions that lead to the formation of the oxide. Ball milling of the reactants prior to annealing was found to eliminate transient phases from the reaction route and to increase the kinetics of the reaction at lower temperatures. Examination of the thermodynamics of iron oxide (hematite) used for the reactions led to a new route of synthesis of the ferrite frommagnetite and strontium carbonate.This chapter also explores the possibility of synthesis of the material at room temperature using ball milling. ¶ The ferrite strongly interacts with the gas phase so its behaviour was studied under different pressures of oxygen and in carbon dioxide.The changes in ferrite composition have an equilibrium character and depend on temperature and oxygen concentration in the atmosphere. Variations of the oxygen content x were described as a function of temperature and oxygen partial pressure, the results were used to plot an equilibrium composition diagram. The heat of oxidation was also measured as a function of temperature and oxygen partial pressure. ¶ Interaction of the ferrite with carbon dioxide below a critical temperature causes decomposition of the material to strontium carbonate and SrFe12O19 . The critical temperature depends on the partial pressure of CO2 and above the critical temperature the carbonate and SrFe12O19 are converted back into the ferrite.The resulting SrFe12O19 is very resistant towards carbonation and the thermal carbonation reaction does not lead to a complete decomposition of SrFeOx to hematite and strontium carbonate. ¶ The thermally induced oxidation and carbonation reactions cease at room temperature due to sluggish kinetics however,they can be carried out at ambient temperature using ball milling.The reaction routes for these processes are different from the thermal routes.The mechanical oxidation induces two or more concurrent reactions which lead to samples containing two or more phases. The mechanical carbonation on the other hand produces an unknown metastable iron carbonate and leads a complete decomposition of the ferrite to strontiumcarbonate and hematite. ¶ Thermally and mechanically oxidized samples were studied using Mossbauer spectroscopy. The author proposes a new interpretation of the Sr4Fe4O11 (x=2.75) and Sr8Fe8O23 (x=2.875)spectra.The interpretation is based on the chemistry of the compounds and provides a simpler explanation of the observed absorption lines.The Mossbauer results froma range of compositions revealed the roomtemperature phase behaviour of the ferrite also examined using x-ray diffraction. ¶ The high-temperature crystal structure of the ferrite was examined using neutron powder diffraction.The measurements were done at temperatures up to 1273K in argon and air atmospheres.The former atmosphere protects Sr2Fe2O5 (x=2.5) against oxidation and the measurements in air allowed variation of the composition of the oxide in the range 2.56 ≤ x ≤ 2.81. Sr2Fe2O5 is an antiferromagnet and undergoes phase transitions to the paramagnetic state at 692K and from the orthorhombic to the cubic structure around 1140K.The oxidized formof the ferrite also undergoes a transition to the high-temperature cubic form.The author proposes a new structural model for the cubic phase based on a unit cell with the Fm3c symmetry. The new model allows a description of the high-temperature cubic form of the ferrite as a solid solution of the composition end members.The results were used to draw a phase diagramfor the SrFeOx system. ¶ The last chapter summarizes the findings and suggests directions for further research.

This work consists of mainly two parts. The first part deals with the sintering behaviour and the microstructural stability of SrFe_1-xCr_xO_3-δ, and the second with transport properties of membranes of the same compositions. The most important experimental tools have been dilatometry and oxygen permeability measurements. Supplementary tools were x-ray diffraction analysis (XRD), scanning electron microscope (SEM) and particle size distribution analysis.

Inspirados por resultados que mostram grande capacidade de seleção de spins em sistemas quirais, produzimos, principalmente em forma de filmes, óxidos tipo perovskita SrFeO3− com conhecida estrutura magnética helicoidal. Foi usando método químico a partir de Fe(NO3)3.9H2O, SrCO3 e ácido cítrico como reagentes precursores. Devido à complexidade de produção, a rota de fabricação foi detalhadamente explorada e descrita. Medidas de transporte eletrônico foram feitas aplicando correntes com frequências controladas para excitar o filme depositado. Foram encontrados dois mecanismos de transporte que aparentemente dependem da espessura da amostra, com uma possível transição perto de 700 nm. O mecanismo de transporte para temperaturas acima da transição magnética é mais sensível à frequência e à temperatura. Por outro lado, duas transições atribuídas a fases magnéticas foram encontradas em temperaturas ao redor de 105 K e 135 K. As temperaturas coincidem com as transições correspondentes à transição magnética helicoidal. Mediante o uso dos modelos de Mott e de Polarons pequenos conseguimos determinar duas fases magnéticas separadas por uma transição(crossover) que possivelmente seja antiferromagnética. Conclui-se que filmes da perovskita que apressentam transições correspondentes às reportadas como helicoidais, foram fabricados com sucesso.
Inspired by results that showed great ability of chiral systems to select spins in electronic transport, we produced films of SrFeO3− perovskite-type oxides that present helical magnetic structure. We used a chemical method starting from Fe(NO3)3.9H2O, SrCO3 and citric acid as precursors. Due to the complexity of production, the manufacturing route has been extensively explored and described. Electronic transport measurements were made by applying currents with controlled frequencies to the deposited film. Two transport mechanisms were found. Apparently they depend on the thickness of the sample, with a possible transition near 700 nm. The transport mechanism for temperatures above the magnetic transition is more sensitive to both frequency and temperature. On the other hand, two transitions attributed to magnetic phases were found around the temperatures of 105 K and 135 K. The temperatures coincide with the transitions corresponding to the helical magnetic transition. Through the use of the Mott and small Polarons models we can determine two magnetic phases separated by a transition (crossover) that is possibly antiferromagnetic. It is concluded that films of the perovskite that present transactions corresponding to those reported as helical, were manufactured successfully.

La compréhension des aspects fondamentaux de la diffusion de l'oxygène dans les oxydes solides à des températures modérées, jusqu'à température ambiante, est un enjeu majeur pour le développement d'une variété de dispositifs technologiques dans un avenir proche. Cela concerne, par exemple, le développement de la prochaine génération des électrolytes et membranes solides d'oxygène pour les piles à combustible de type SOFC. Autrement, les réactions d'intercalation de l'oxygène réalisées à basse température présentent un outil puissant pour contrôler le dopage en oxygène ainsi que des propriétés physiques. Dans ce contexte, les oxydes ayant une structure type brownmillérite (A2BB'O5) ou type K2NiF4, ont attiré beaucoup d'attention, car ils montrent une mobilité de l'oxygène déjà à température ambiante.Dans cette thèse, nous avons étudié les mécanismes d'intercalation d'oxygène dans SrFeO2.5+x, ainsi que Pr2NiO4+x par des méthodes de diffraction in situ, réalisées sur des monocristaux dans une cellule électrochimique spécifiquement conçue, explorant principalement le rayonnement synchrotron. Ceci a permis d’explorer en 3D tout le réseau réciproque, et d'obtenir des informations précieuses sur la diffusion diffuse, sur les faibles intensités des raies de surstructure, ainsi que des informations sur la fraction volumique des différents domaines de maclage au cours de la réaction, impossibles à accéder par diffraction de poudre.Les deux systèmes montrent des changements structuraux complexes, accompagnés par une mise en ordre de l'oxygène à longue portée. Au cours de l'intercalation d'oxygène nous avons mis en évidence deux phases intermédiaires, SrFeO2.75 et SrFeO2.875, possédant des lacunes en oxygène ordonnées à longue échelle. En raison du maclage, avec jusqu'à douze possibles individus, nous avons suivi directement la formation et l'évolution des domaines de maclage ainsi que leur micro-structure apparentée. Nous avons ainsi observé un mécanisme de réaction topotactique pour SrFeO2.5 vers SrFeO2.75, tandis que l'oxydation de SrFeO2.75 conduit à des importants réarrangements de l’oxygène, associés à un changement de nombre de domaines de maclage. La réduction électrochimique de la phase orthorhombique Pr2NiO4.25 donne Pr2NiO4.0 comme produit final, ayant la même symétrie, tandis que la phase tétragonale Pr2NiO~4.12 apparaît comme phase intermédiaire. Utilisant un monocristal avec un diamètre de 50 microns, la réaction se déroule dans des conditions d'équilibre dans moins que 24 heures, ce qui implique un coefficient de diffusion de l’oxygène anormalement élevé, supérieur à 10-^11cm2*s-1 à température ambiante. Nous avons également étudié le diagramme de phase de Pr2NiO4.25 sur monocristal jusqu’à 1100°C en chauffant sous air. Une série complexe de transition de phases a été mise en évidence, la vraie symétrie de Pr2NiO4.25 s’avérée en fait monoclinique.Outre l'exploration des diagrammes de phases complexes de SrFeO2.5+x et Pr2NiO4+d, nous avons pu étudier les changements détaillés concernant la micro-structure à l'aide de diffraction sur monocristaux in situ, impossible à accéder par des méthodes de diffraction de poudre classique. Les changements de la micro-structure des domaines va bien au-delà des composés étudiés ici et porte une grande importance pour extrapoler sur la performance, la stabilité et la durée de vie par exemple des matériaux utilisés pour le stockage de l’énergie
Understanding fundamental aspects of oxygen diffusion in solid oxides at moderate temperatures, down to ambient, is an important issue for the development of a variety of technological devices in the near future. This concerns e.g. the progress and invention of next generation solid oxygen ion electrolytes and oxygen electrodes for solid oxide fuel cells (SOFC) as well as membrane based air separators, oxygen sensors and catalytic converters to transform e.g. NOx or CO from exhaust emissions into N2 and CO2. On the other hand oxygen intercalation reactions carried out at low temperatures present a powerful tool to control hole doping, i.e. the oxygen stoichiometry, in electronically correlated transition metal oxides. In this aspect oxides with Brownmillerite (A2BB’O5) and K2NiF4-type frameworks, have attracted much attention, as they surprisingly show oxygen mobility down to ambient temperature. In this thesis we investigated oxygen intercalation mechanisms in SrFeO2.5+x as well as Pr2NiO4+x by in situ diffraction methods, carried out on single crystals in especially designed electrochemical cell, mainly exploring synchrotron radiation. Following up oxygen intercalation reactions on single crystals is challenging, as it allows to scan the whole reciprocal lattice, enabling to obtain valuable information as diffuse scattering, weak superstructure reflections, as well as information of the volume fraction of different domains during the reaction, to highlight a few examples, difficult or impossible to access by powder diffraction. Both title systems are able to take up an important amount of oxygen on regular and interstitial lattice sites, inducing structural changes accompanied by long range oxygen ordering. For SrFeO2.5+x the uptake of oxygen carried out by electrochemical oxidation yields SrFeO3 as the final reaction product. The as grown SrFeO2.5 single crystals we found to show a complex defect structure, related to the stacking disorder of the octahedral and tetrahedral layers. During the oxygen intercalation we evidenced the formation of two reaction intermediates, SrFeO2.75 and SrFeO2.875, showing complex and instantly formed long range oxygen vacancies. Due to the specific twinning with up to totally twelve possible twin individuals, we directly follow up the formation and changes of the specific domain and related micro-structure. We thus observed a topotactic reaction mechanism from SrFeO2.5 to SrFeO2.75, while further oxidation lead to important rearrangements in the dimensionality of the oxygen defects in SrFeO2.75, implying the formation of an additional twin domain in course of the reaction. The electrochemical reduction of orthorhombic Pr2NiO4.25 yields stoichiometric Pr2NiO4.0 as the final reaction product with the same symmetry, while tetragonal Pr2NiO~4.12 appears as a non-stoichiometric intermediate phase. Using a single crystal with 50µm diameter, the reaction proceeded under equilibrium conditions in slightly less than 24h, implying an unusually high oxygen ion diffusion coefficient of > 10^-11cm2*s-1 at already ambient temperature. From the changes of the associated twin domain structure during the reduction reaction, the formation of macro twin domains was evidenced. Heating up Pr2NiO4.25 single crystals in air revealed a complex series of phase transition, evidencing the true symmetry of the starting phase to be in fact monoclinic. Beside exploring the complex phase diagrams of SrFeO2.5+x and Pr2NiO4+d we were able to investigate detailed changes in the micro-structure using in situ single crystal diffraction techniques, impossible to access by classical powder diffraction methods. The importance of changes in the domain structure goes far beyond the investigated title compounds and has utmost importance of the performance, stability and lifetime of e.g. battery materials

L'objectif principal de ce travail est l'étude de la dynamique du laser à électron libres sur anneaux de stockage, dans le domaine spectral UV-VUV et en présence de l'instabilité micro-onde. Cette instabilité affecte tous les anneaux de stockage en fonctionnement pour le laser à électrons libres. Une étude à la fois théorique et expérimentale du comportement de ce système a été mise en place, dans laquelle les résultats théoriques ont été comparés aux mesures effectuées à Super ACO (France) et à ELETTRA (Italie). Dans un premier temps, le comportement du système en présence de l'instabilité micro­onde est décrit qualitativement par un modèle simple. Ensuite, dans le but de prédire de façon quantitative le comportement du faisceau, nous avons développé un code numérique unidimensionel de la dynamique du laser à électrons libres sur anneau de stockage en présence de l'instabilité micro-onde. Le code décrit l'évolution du champ électrique du laser dans la cavité optique, couplée à l'évolution du faisceau d'électrons dans l'anneau, et a été validé par comparaison avec les résultats expérimentaux obtenus à Super ACO et ELETTRA. L'étude expérimentale nous a amené en premier lieu à caractériser le comportement du faisceau dans l'anneau de stockage. Ceci a été effectué seulement à Super ACO. Nous avons mesuré les caractéristiques du faisceau d'électrons (distribution électronique, dispersion en énergie, etc. ) pour le fonctionnement du laser à électrons libres, et nous avons caractérisé deux effets du champs de sillage sur le faisceau d'électrons. Le premier effet, communément appelé la ''distortion du puits de potentiel", modifie les caractériques du faisceau sans générer aucune instabilité. Le deuxième effet du champ de sillage est la génération d'instabilités dans le faisceau d'électron, en particulier l'instabilité micro-onde. Cette instabilité induit en moyenne sur le temps un accroissement de la dispersion en énergie, qui s'accompagne d'un accroissement de la longueur des paquets d'électrons dans l'anneau. Ensuite nous avons caractérisé le comportement du laser, mesuré les caractéristiques de l'impulsion laser sous différentes conditions de désaccord, i. E. Synchronisation entre l'impulsion laser et les paquets d’électrons. Les paramètres du système dynamique, tels que le gain, les pertes de la cavité optique etc. , ont été déduits de ces mesures. La principale conclusion de ce travail est que l'instabilité micro-onde peut sérieusement dégrader les performances du laser à électrons libres sur anneau de stockage. Pour tout projet de construction d'un tel système, l'impédance de l'anneau de stockage, qui régit la puissance de l'instabilité micro-onde, doit être la plus petite possible. Le laser est en compétition avec l'instabilité micro-onde, et est capable de l'amortir dans une plage de courant dont la borne supérieure est déterminée par le gain, les pertes de la cavité optique, et la puissance de l’instabilité. Pour des courants supérieurs, le laser n’est pas stable, ou est tout simplement éteint par l'instabilité. Le code numérique que nous avons développé a été utilisé pour simuler avec succès le comportement de deux lasers à électrons libres. Il peut donc être utilisé comme outil pour l'étude de tout nouveau laser à électrons libres sur anneau de stockage
The purpose of the present work is to study the dynamics of the storage ring free electron laser in the UV-VUV wavelength range and in the presence of the microwave instability. In practice this instability is always present during the operation of a free electron laser in a storage ring, and it may degrade the performance of the laser. In order to investigate the behavior of this complex system, we used a dual approach, in which theoretical results have been compared with measurements performed on two storage ring free electron lasers, at Super ACO (France) and at ELETTRA (Italy). As a first theoretical step, we analyzed a simple model which describes qualitatively the behavior of the storage ring free electron laser only, including the effects due to the microwave instability. In order to get more quantitative predictions, which can not be obtained from the simple model, we developed a 1-dimensional numerical code for the evolution of the laser electric field along the optical cavity axis. The code was validated by the comparison with experimental measurements performed at Super ACO and at ELETTRA. The experimental approach has also been clone in two steps. The first step was to characterize the electron beam behavior in the storage ring without the free electron laser. This has only been clone at Super ACO. We measured the beam characteristics (electron distribution, beam energy spread, etc. ) for the free electron laser operation, and we characterized two effects of the wakefield on the electron beam. The first effect is the so-called potential well distortion, which only modifies the beam properties, leaving the beam stable. The second effect induces instabilities in the beam, in particular the microwave instability, with on average an enhancement of energy spread and a proportional bunch lengthening. Next we characterized the laser behavior, measured the laser pulse properties (pulse duration, average power, etc. ) under detuning conditions, and deduced from measurements the parameters of the dynamical system, such as the gain at start-up, the cavity loss, etc. The main conclusion of the present work is that the microwave instability can seriously degrade the performance of the storage ring free electron laser. For any design of such a system one has to make sure that the impedance of the storage ring, which scales the strength of the microwave instability, is as low as possible. The laser is in competition with the instability, and is able to damp it in a range of current determined by the gain of the free electron laser, the optical cavity Joss and the microwave instability strength. At higher current the laser is unstable, or simply, it may be switched off by the microwave instability. The numerical code we developed has been used to simulate successfully the behavior of two different storage ring free electron lasers. It can be used reliably as a tool for the design of new storage ring free electron lasers

Ce travail de thèse porte sur les relations entre structure et propriétés physiques de nouveaux oxydes de métaux de transition à valence mixte de type SrFe1-x(Sc,Sn)xO3-d (M = Sc, Sn). La caractérisation structurale a été effectuée par diffraction X et par microscopie électronique en transmission. La spectrométrie Mössbauer a permis d'accéder aux degrés d'oxydation du fer et à son environnement local. Les propriétés physiques des composés ont été caractérisées par mesures de résistivité avec ou sans champ magnétique appliqué, mesures de susceptibilité magnétique, mesures d'aimantation en fonction du champ magnétique ou de la température. La substitution du fer par un ion trivalent non magnétique tel que le Sc3+ conduit, comme attendu, à une diminution de la teneur en oxygène sans affecter la structure cristalline du composé. Cependant, la substitution est limitée et le composé limite SrFe0,5Sc0,5O2,5 peut être également synthétisé dans une structure chimiquement ordonnée de type brownmillerite. Bien que les propriétés de transport et les propriétés magnétiques soient fortement affectées par une telle substitution, la magnétorésistivité n'est pas significativement modifiée. La substitution du fer par un ion tétravalent tel que le Sn4+ ne permet pas de maintenir constante la teneur en oxygène. Celle-ci augmente avec le taux de substitution. La substitution n'est pas limitée, et un changement de structure est observé pour un taux de substitution supérieur à 0,5 : la structure de la pérovskite n'est plus cubique mais orthothrombique. Toutes les propriétés physiques étudiées (transport, magnétisme, magnétorésistivité) sont significativement influencées par la substitution
In this thesis, the relationship between structure and physical properties of new transition-metal oxides of the SrFe1-x(Sc,Sn)xO3-d (M = Sc, Sn) type has been investigated. The structural characterization was performed by X-ray diffraction and transmission electron microscopy. Mössbauer spectrometry has been used to investigate both the oxidation state of iron and its local environment. The physical properties of the compounds were characterized by resistivity measurements with and without applied magnetic field, magnetic susceptibility measurements, magnetization measurements as a function of magnetic field or temperature. The substitution of iron by a nonmagnetic trivalent ion such as Sc3+ leads to a decrease in the oxygen content without affecting the crystalline structure of the compound. However, the substitution is limited and the SrFe0,5Sc0,5O2,5 limit compound can also be chemically synthesized in a brownmillerite ordered structure. Although transport and magnetic properties are strongly affected by such substitution, magnetoresistivity is not significantly altered. The substitution of iron by a tetravalent ion such as Sn4+ cannot maintain constant oxygen content, which increases with the substitution rate. Substitution is not limited, and a structural change is observed for a substitution rate higher than 0. 5. All the physical properties investigated (transport, magnetism, magnetoresistivity) are significantly influenced by the substitution

Ce mémoire concerne l'influence d'un degré d'oxydation inusuel d' un element de transition (Cu(III), Cu(IV), Fe(IV)) sur les proprietes physico-chimiques des réseaux oxygènes dérivés de structure perovskite. Les différentes structures du compose du cuivre trivalent LaCuO3 (formes basse et haute pression) ont été étudiées. Le role de plusieurs facteurs sur les propriétés physiques de ce matériau a pu etre mis en évidence, en particulier: 1) l'accroissement de la covalence moyenne de la liaison Cu-O du a la stabilisation de la valence mixte Cu(III)/Cu(IV) obtenue par substitution du Sr au La; 2) la distorsion structurale induite par substitution de l'yttrium au lanthane; 3) la pression d'oxygène de synthèse. La seconde partie de ce travail permet, au travers d' une étude Mössbauer par sonde diamagnétique locale (119Sn), d' expliquer le mécanisme de dismutation du fer tétravalent au sein d'un réseau perovskite AFeO3 (A=Ca, Sr)

La mise au point d'une méthodologie : les mesures électrochimiques sur céramiques a été réalisée dans le but d'étudier, de façon qualitative ou quantitative, le comportement électrocatalytique des perovskites non-stoechiométriques de type La_1-xSrFe_1-zCO_zO_3-y, vis à vis de la réaction de dégagement de l'oxygène. Un mécanisme réactionnel à cinq étapes élémentaires a été proposé, et les étapes limitantes ont été discutées sur la base de calculs cinétiques et thermodynamiques. On en a déduit des critères d'amélioration des performances électrocatalytiques de ce type de matériaux.

Non-stoichiometric strontium iron oxide is described by an abbreviated formula SrFeOx (2.5 ≤ x ≤ 3.0) exhibits a variety of interesting physical and chemical properties over a broad range of temperatures and in different gaseous environments. The oxide contains a mixture of iron in the trivalent and the rare tetravalent state. The material at elevated temperature is a mixed oxygen conductor and it, or its derivatives,can have practical applications in oxygen conducting devices such as pressure driven oxygen generators, partial oxidation reactors in electrodes for solid oxide fuel cells (SOFC). ¶ This thesis examines the behaviour of the material at ambient and elevated temperatures using a broad spectrum of solid state experimental techniques such as: x-ray and neutron powder diffraction,thermogravimetric and calorimetric methods,scanning electron microscopy and Mossbauer spectroscopy. Changes in the oxide were induced using conventional thermal treatment in various atmospheres as well as mechanical energy (ball milling). ¶ ...

碩士
淡江大學
物理學系碩士班
102
We report the study of charge/spin-ordering in single crystal SrFeOx using resonant soft x-ray scattering. Probing the charge and spin ordering using x-rays is difficult due mainly to the very weak diffraction intensity. However, this difficulty can be overcome by the use of resonant x-ray diffraction. In this study, we performed the resonant soft x-ray diffraction at the Fe L-edge and O K-edge, and observed a huge enhancement from the spin and charge ordering reflections. The wave vectors of charge- and spin-ordering in SrFeOx were observed to be Qco ≈ (0, 0, 0.64) and Qso ≈ (0, 0, 0.5) with transition temperatures at Tco ≈ 107 K for charge ordering and Tso ≈ 65 K for spin. These reflections were also further confirmed to originate from charges and spins, respectively, using the different polarized x-rays.

博士
國立臺灣大學
物理研究所
101
We calculate the self-energy and dispersion relation of magnons in the spiral spin state of SrFeO3. Our model consists of the superexchange (SE) interaction and the itinerant ferromagnetic (SD) interaction with or without external magnetic field. The SE interaction part comes from the electrons in the t2g orbitals, and the SD part comes from the itinerant electrons which reside mostly in the eg orbitals. For the SE part, we start with treating spins as classical vectors and then considered the quantum fluctuation with the Holstein-Primakoff transformation. Then Bogoliubov transformation is used to diagonalize the Hamiltonian. For the itinerant ferromagnetic part, we use a projection operator method to simplify the Hamiltonian. Finally the self-energy and the dispersion of magnons are calculated and they give a linear relation between frequency and wavevector.

碩士
國立臺灣大學
化學工程研究所
81
Sensors have been widely used recently in various areas such as automatical exhaust monitor, poisonous gas detector and reaction on controlling device. P-type perovskite SrFeO3-x oxide is a good sensor material because of its durability and stability properti- es. In this work, the perovskite materials were prepared by sol- gel method and coprecipitation method, and their physical proper- ties were studied by means of XRD, TPD/TPR, TEM, and TGA/DTA. It is found that the sol-gel method is superior than the coprecipit- ation method in preparation and physical properties . We former yields thematerials of smaller particle size, and higher oxygen desorption and hydrogen consumption. The TPDand TPR results show that their oxidizing and reducing abilities are better. When the SrFeO3-x madeby sol-gel method is used as CO gas sensor, The re- sistance of material decreases with increasing temperature. The- refore, tends to decrease the η(response ratio). The reason may be ascribed to thereduction of oxygen adsorption with the incre- ase in temperature. The hole numbers increase at higher tempera- ture. The sensor resistance is found toincrease with increasing CO concentration, showingthe reaction of holes in the sensor with CO is deeper. The experimental result, shows that the optimum temperature for CO detecting is in the rangeof 280-340℃. Resist- ance variance is large enough,and detceting time is short too. reproducibility of response after several oxidation and reduction cycle is better, Finally the elementary sensorial kinetics are proposed.

碩士
淡江大學
物理學系碩士班
100
We report the study of the magnetoresistance behavior and the charge/spin modulations in a high quality single crystal of SrFeO3-δ using magnetization, conductivity, and x-ray scattering. The crystal displays a sharp magnetic transition at T ≈ 60 K and a nonlinear conductivity behavior at about T ≈ 110 K, suggesting a 3-δ of about 2.81. In addition, there exists the hysteresis phenomenon with a transition width of about 10 K at T ≈ 60 K. A charge modulation was located at (0, 0, L ± q), q ≈ 0.36 to have a transition temperature of T ≈ 110 K by the use of x-ray scattering with x-ray energy of 10 keV and 12 keV. This is in accord with the nonlinear transport transition as observed by conductivity measurement. By the means of resonant soft x-ray diffraction around the Fe L-edge, we also observed the spin modulation (from paramagnetic to antiferromagnetic states) in agreement with the formation of spin ordering doubling the c-axis.

博士
淡江大學
物理學系博士班
106
The synchrotron radiation based spectroscopic techniques provide element-specific information of electronic and atomic structures for fundamental researches, material studies and industrial applications. Techniques include X-ray diffraction (XRD) for lattice structure, X-ray absorption near edge structure (XANES) and X-ray linear dichroism (XLD) for electronic and orbital structures, X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) for information of the occupied states and inherent elementary excitations, extended X-ray absorption fine structure (EXAFS) for local atomic structure. In addition, neutron scattering related techniques provide direct evidence and texture of magnetic structure, such as elastic and inelastic neutron scattering (ENS and INS) for spatial correlations and dynamics of spin moments, small angle neutron scattering (SANS) for spatial correlations of magnetic clusters. These techniques are considerable and powerful techniques for the study of various complex couplings in novel single crystal materials with special magnetic structure. In the first part of this thesis, the local electronic and atomic structures of the high-quality single crystal of SrFeO3-δ were studied using temperature-dependent x-ray absorption (XAS) and valence-band photoemission spectroscopy (VB-PES) to investigate the origin of anisotropic resistivity in the ab-plane and along the c-axis close to the region of thermal hysteresis (near temperature for susceptibility maximum, Tm~78 K). All experiments herein were conducted during warming and cooling processes. The Fe L3,2-edge XLD results show that during cooling from room temperature to below the transition temperature, the unoccupied Fe 3d eg states remain in persistently out-of-plane 3d3z2-r2 orbitals. In contrast, in the warming process below the transition temperature, they change from 3d3z2-r2 to in-plane 3dx2-y2 orbitals. The nearest-neighbor (NN) Fe-O bond lengths also exhibit anisotropic behavior in the ab-plane and along the c-axis below Tm. The anisotropic NN Fe-O bond lengths and Debye-Waller factors stabilize the in-plane Fe 3dx2-y2 and out-of-plane 3d3z2-r2 orbitals during warming and cooling, respectively. Additionally, a VB-PES study further confirms that a relative band gap opens at low temperature in both the ab-plane and along the c-axis, providing the clear evidence of the charge-density-wave nature of SrFeO3-δ single crystal. In the second part, ENS and INS experiments were performed on a single crystal of Ni0.4Mn0.6TiO3 (NMTO) to study the spatial correlations and dynamics of spins in the XY-like spin-glass (SG) state. Magnetization measurements reveal signatures of SG behavior in NMTO with a freezing temperature of TSG ~ 9.1 K. The ENS experiments indicated that the intensity of magnetic diffuse scattering starts to increase around 12 K, which is close to TSG. Also, spin-spin correlation lengths (ξ) at 1.5 K are approximately 21.05±0.6 and 72.99±1.6 Å in the interlayer and the in-plane directions, respectively, demonstrating that magnetic correlations in NMTO exhibit quasi two-dimensional-like antiferromagnetic order. INS results show quasi-elastic neutron scattering (QENS) profiles below TSG. The life-time of dynamic correlations (τ), obtained from the QENS profiles, are approximately 16.27±0.8 and 15.88±1.9 psec at 10 K for two positions (0.00, 0.00, 1.52) and (0.01, 0.01, 1.50), respectively. Therefore, our experimental findings demonstrate that short-range-ordered antiferromagnetic clusters with short-lived spin correlations are present in the XY-like SG state of NMTO at a temperature of approximately TSG. In final part, SANS, RIXS, and XAS experiments on the single crystal of NMTO have been carried out to study the role of electronic and atomic properties in the XY-like SG state. SANS presents the texture of toroidal glass and phase diagram of NMTO. RIXS experiments provide the evidence of crystal field (d-d) excitations at Ni and Mn L3-edge. However, spin-flip excitations are only observable which are prominent at Ni L3-edge RIXS. Using temperature dependent XANES, XLD and EXAFS studies along with RIXS, it has been shown that symmetry breaking/ lattice distortion occurs at low temperature near SG phase transition due to the local disorder in Ni-O bond lengths, and further induced the orbital preference occupation charge from out-of-plane state (3d3z2-y2) to in-plane state (3dx2-y2). We therefore believe that XY-like SG state of NMTO is associated with the unoccupied in-plane (3dx2-y2) states.

碩士
國立臺中教育大學
科學教育與應用學系科學教育碩士班
103
The mixing and heating method and the hydrothermal method are applied to synthesizing two visible-light-driven (VLD) photocatalysts, SrFeO3-x/g-C3N4 and BiOBr/PbBiO2Br, respectively. X-ray diffractometer, Field Emission-Scanning Electron Microscopy-Energy Dispersive Spectroscopy, Field Emission-Transmission Electron Microscopy-Energy Dispersive Spectroscopy, Fourier Transform Infrared Spectroscopy, UV-vis diffuse reflectance spectra, Surface Area and Porosity Analyzer, and Ultraviolet Photoelectron Spectroscopy are utilized for distinguishing the synthesized samples. SrFeO3-x/g-C3N4 and BiOBr/PbBiO2Br degraded crystal violet dye and chloramphenicol solution are used for simulating the environmental target pollutants. From the reaction rate constant of the degraded crystal violet dye solution, the 4wt.% SrFeO3-x is added g-C3N4 for the calcination at 500℃ for 2 hours. The acquired SrFeO3-x/g-C3N4 0.0997h-1 is higher than g-C3N4 0.0209h-1 and SrFeO3-x 0.0014h-1, and the degradation efficiency constant of BiOBr/PbBiO2Br appears 0.0601h-1 which apparently enhances the photocatalytic efficiency in comparison to BiOBr 0.0207h-1 and PbBiO2Br 0.0017h-1. The distinguishing with reactive species and Electron Paramagnetic Resonance shows the important roles of free radicals, ․OH and․O2— , in the photocatalytic reaction. In order to understand the pathway of the crystal violet dye and chloramphenicol degradation, the reactive intermediate in the HPLC-PDA-ESI-MS separation process is used for predicting the possible photocatalytic degradation mechanism for the research basis of crystal violet dye and chloramphenicol degradation.

O desenvolvimento de materiais nano estruturados tem interesse nas áreas da ciência, tecnologia e inovação, principalmente no que concerne aos materiais cerâmicos avançados. Materiais constituídos por óxidos do tipo perovesquite (ABO3) são desenvolvidos para vários tipos de aplicação como filmes catalíticos, fotocatalíticos e como elétrodos, quer como cátodos quer como ânodos. Os óxidos SrTiO3, SrTi0,5Fe0,5O3 e SrFeO3 foram sintetizados por via sólida através do método cerâmico, sendo a temperatura final de síntese 1200ºC. A caracterização estrutural dos compostos obtidos na forma de pós foi feita por DRX e a caracterização morfológica por SEM e TEM. Foram preparados filmes dos compostos sintetizados, em substrato de espuma de níquel, pelo método de “slurry-paint” usando como dispersante Triton X-100. A caracterização morfológica dos filmes foi feita por SEM. Prepararam-se igualmente filmes imobilizando partículas de TiO2 comercial, de modo a comparar o desempenho fotocatalítico dos pós sintetizados, com estrutura perovesquite, com o do fotocatalisador mais referido na literatura, o TiO2. Todos estes filmes foram usados como fotocatalisadores, recorrendo a radiação U.V. de 254 nm, para a degradação de soluções de corante Acid Orange 7 (AO7). A monitorização das degradações foi realizada por medidas de espectrofotometria do ultravioleta e visível. Dos compostos com estrutura tipo perovesquite sintetizados usados nos ensaios de fotocatálise, o que promoveu maior percentagem de remoção de corante ao fim dos ensaios de 6 h, foi o SrFeO3. No entanto, o melhor resultado obtido foi com o composto comercial TiO2. Na série de perovesquites estudadas, os resultados obtidos indicam um aumento de efeito fotocatalítico com o aumento do teor de Fe. Para os compostos SrTiO3, SrTi0,5Fe0,5O3, e SrFeO3 e TiO2 as percentagens de remoções de absorvância ao fim das 6 h, foram de 84,8, 90,0, 95,4 e 96,8 %, respetivamente.
The development of nano-structured materials has been of great interest in the scientific, technological and innovation areas, particularly regarding advanced ceramic materials. Materials constituted from perovskite-type oxide (ABO3) are developed for various types of applications, such as catalytic or photocatalytic films, and electrodes, as cathodes or as anodes. The SrTiO3 oxide, SrFeO3 and SrTi0,5Fe0,5O3 were synthesized by solid route with the ceramic method, and the final synthesis temperature reached 1200ºC. The structural characterization of the compounds, which were obtained in the form of powders, was prepared by XRD, and the morphological characterization by SEM and TEM. Films were prepared from the synthesized compounds in nickel foam substrate by using the slurry-paint method and the Triton X-100 was used as a dispersant. The morphological characterization of the films was made by SEM. Films were also prepared by immobilizing commercial TiO2 particles in order to compare the photocatalytic performance of the synthesized powders with the perovskite structure, with the most popular photocatalyst in scientific literature – the TiO2. All of the films were used as photocatalysts, by using 254 nm UV radiations for the degradation of the Acid Orange 7 dye solution (AO7). The degradation was monitored by the action of ultraviolet and visible spectrophotometry. SrFeO3 showed the highest absorbance removal, within the synthesized perovskite - type structure compounds, after 6 hours of assays. However, the best result was obtained with the commercial TiO2 compound. In the perovesquites series studied, the results indicate an increase of photocatalytic effect with an increase of the Fe content. The percentages of absorbance removal for the azo dye, after 6 hours of assays, for the compounds SrTiO3, SrTi0,5Fe0,5O3, SrFeO3 and TiO2, , were 84.8; 90.0; 95.4 and 96.8 % respectively.