Academic literature on the topic 'Sr2Fe2O5'

The purpose of this work is to study the effect of lanthanum (La) concentration on the phase formation, conductivity, and thermophysical properties of perovskite-like strontium ferrite ceramics. At the same time, the key difference from similar studies is the study of the possibility of obtaining two-phase composite ceramics, the presence of various phases in which will lead to a change in the structural, strength, and conductive properties. To obtain two-phase composite ceramics by mechanochemical solid-phase synthesis, the method of the component molar ratio variation was used, which, when mixed, makes it possible to obtain a different ratio of elements and, as a result, to vary the phase composition of the ceramics. Scanning electron microscopy, X-ray phase analysis, and impedance spectroscopy were used as research methods, the combination of which made it possible to comprehensively study the properties of the synthesized ceramics. Analysis of phase changes depending on lanthanum concentration change can be written as follows: (La0.3Sr0.7)2FeO4/LaSr2Fe3O8 → (La0.3Sr0.7)2FeO4/LaSr2Fe3O8/Sr2Fe2O5 → (La0.3Sr0.7)2FeO4/Sr2Fe2O5. Results of impedance spectroscopy showed that with an increase in lanthanum concentration from 0.10 to 0.25 mol in the synthesized ceramics, the value of the dielectric permittivity increases significantly from 40.72 to 231.69, the dielectric loss tangent increases from 1.07 to 1.29 at a frequency of 10,000 Hz, and electrical resistivity decreases from 1.29 × 108 to 2.37 × 107 Ω∙cm.

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.

碩士
國立成功大學
材料科學及工程學系碩博士班
91
Double Perovskite-type oxides Sr2FeMoO6 are related to half-metallic Ferromagnets (or ferrimagnets), which shows remarkable magnetoresistance at room temperature. Therefore, this property is attractive from the standpoint of both physics and engineering. The solid-state reaction of those precursors of Strontium iron oxide and Strontium molybdenum oxide were used to this study. In the purposes of this study are to evaluate the reaction kinetics and polycrystalline sample with residual SrMoO4 and the formation mechanism of SrFeMoO6. The solid-state reaction of SrFeO3-x and SrMoO4 to form the Sr2FeMoO6 at different temperature and heating rate was to investigate the formation kinetics. The non-isothermal energy kinetic empirical model was proposed to evaluate the activation energy. The reaction temperature of Sr2FeMoO6 was performed by the solid-state reaction of SrFeO3-x and SrMoO4 had been reduced about 100℃. Nevertheless, there have no much influence in electric and magnetic properties. In the study of the sample with residual SrMoO4 phase, the sample would have the high resistivity and low field magnetoresistance (LFMR). It was found that the non-magnetic and insulating SrMoO4 phase dose not reside at the boundary of granular sample but some boundaries are rich in the Sr ion. It is suggested that SrMoO4 might not play a role in enhancing LFMR. The possible mechanism of the increase of LFMR is discussed. However, in aspect of formation mechanism of non-stoichiometric Sr2Fe2-xMoxO6, the additional element of Mo could be help to stabilize the solid solution of Sr2Fe2-xMoxO6, the equation of electrical neutrality could be written as Sr2+2(Fe4+2-2xFe3+xMo5+x)O2-6, any composition will be obeyed the equation. Then, we found that the TEM micrographs of samples with composition of x=0.6 whose to appear the striped structure such as Moiré figure or stacking fault in the grain, and some dislocations could be existed in other grain. However, the composition of x=0.6 is a critical point for the phase transition. Therefore, both factors of local compositional variations and phase transition could be interrupted the existence strange defect in the solid solution at x=0.6.

碩士
國立成功大學
材料科學及工程學系碩博士班
92
In this study, different ratios of the precursor phases of SrFeO2.97 and SrMoO4 were used to prepare Sr2FeMoO6 by a solid-state reaction technique. The residual SrMoO4 was observed to exist in the samples with SrFeO2.97/ SrMoO4 ratio of 0.9 : 1 and 0.8：1. It was found that the sample with a residual SrMoO4 phase had higher resistivity, lower magnetization, but higher low field magnetoresistance (LFMR). It was found that nano-sized amorphous-like clusters of SrMoO4 phase were located inside the grains rather than at grain boundaries. Besides, anti-phase boundaries (APB) were observed in all samples of Sr2FeMoO6 with residual SrMoO4 phase. The possible mechanism for the conduction and LFMR of Sr2FeMoO6 is attributed to spin-dependent scattering at the unusual APB. 　　The study of magnetism and conductive mechanism of Sr2Fe2-xMoxO6, with doping content of Mo increase, thus leading to antiferromagnetic coupling increase, and consequentially enhance the saturation magnetization. The linkage of Fe3+-O2--Mo5+ which leads to the formation of a narrow band results in enhancing the conductivity. Furthermore, the low temperature conductivity can be explained away by Mott’s variable range hopping (VRH) mechanism while the high temperature conductivity is contributed to the thermally activated small polaron hopping (SPH) mechanism.

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). ¶ ...