Sol gel synthesis of multiphase lanthanum oxicarbonate and langasite thin films

Precursors of lanthanum oxide and of (3La2O3, 5Ga2O3, SiO2) system were first elaborated then thermally decomposed into polycrystalline lanthanum oxide and langasite La3Ga5SiO14. The as prepared lanthanum oxide powder was then hydrated and carbonated to give hydroxicarbonates: thermal decomposition of these complex system La2O3-X(OH)y(CO3)z was studied by Fourier Transform infrared spectroscopy. Spin coating process applied to these precursors was optimized to elaborate thin films of lanthanum oxicarbonate La2O2CO3, lanthanum oxide La2O3 and langasite, on silicium substrates (Si-(001)). Thin films are partly textured. Making use of same spin coating process, multiphase thin films of langasite covered with La2O3 particles were obtained.Precursors of lanthanum oxide and of (3La2O3, 5Ga2O3, SiO2) system were first elaborated then thermally decomposed into polycrystalline lanthanum oxide and langasite La3Ga5SiO14. The as prepared lanthanum oxide powder was then hydrated and carbonated to give hydroxicarbonates: thermal decomposition of these complex system La2O3-X(OH)y(CO3)z was studied by Fourier Transform infrared spectroscopy. Spin coating process applied to these precursors was optimized to elaborate thin films of lanthanum oxicarbonate La2O2CO3, lanthanum oxide La2O3 and langasite, on silicium substrates (Si-(001)). Thin films are partly textured. Making use of same spin coating process, multiphase thin films of langasite covered with La2O3 particles were obtained

Synthesis and characterization of nanosized Ce1−xbixo2−δ solid solutions for catalytic applications

AbstractThis study consists of elaborating and characterizing some nanometric materials in basic of rare-earth oxides by the soft chemistry technique. the first step of this work consists of synthesizing nanometric pure ceria by sol-gel process. In the second one, the Bismuth doped ceria by co-precipitation method was realized in order to obtain ceria-based solid solution, to improve its catalytic property by creation of oxygen vacancies. The solubility limit of Bi2O3 in CeO2 was determined to be around 20 atom %. The effect of thermal treatment temperatures on the average crystallite sizes and lattice parameters was done for pure ceria and Ce1−xBixO2−x/2 (x = 0.15 and 0.2). The different elaborated samples are subject of structural characterization (XRD). Catalytic reactivity of these materials in presence of “air- toxic gas” mixtures is studied by Fourier Transform Infra-Red Spectroscopy (FTIR)

Hydrostatic compression on YBa2_{2}Cu3_{3}O7−δ_{7-\delta}/polymer composites

High-Tc superconducting YBa$_{2}$Cu$_{3}$O_{7\mbox{-}\delta} phase was produced by a solid state reaction method. A series of YBa$_{2}$Cu$_{3}$O_{7\mbox{-}\delta}/polymer composites were fabricated. The structure, physical property, levitation and mechanical strength of composites were carried out. The Meissner effects associated with the 91K superconducting transition were found to be roughly preserved in the composites. Elastic experiments are carried out by applying a uniaxial pressure. Young's modulus of compression and compressibility was calculated. It increases with the volume fraction $\Phi$ s of superconductor. Poisson's coefficient was determined. The role of contacts between component is discussed

Modélisation des propriétés électriques d'un composite supraconducteur

Le comportement électrique d'un composite à base de supraconducteur est simulé par un circuit comprenant divers éléments RLC montés en parallèle et/ou en série. Une justification en termes de constituants du composite est donnée à chaque composant élémentaire du modèle. L'impédance du circuit est calculée en fonction des trois variables : température, fréquence et fraction volumique du supraconducteur

Crystal and magnetic structures of Fe0.25TiSe2 and Fe0.48TiSe2

The crystal and magnetic structures of the lamellar FexTiSe 2 (x = 0.25 and 0.48) antiferromagnets have been refined from neutron diffraction powder data at several temperatures ranging from 2 to 300 K. For both compounds, Ti is found to be substituted for 8 % of Fe so that the separation of the two metals is not complete. The distortion of the Ti-containing octahedra decreases from x = 0.25 to x = 0.48 i.e. as the lower-occupied d-subband filling increases due to charge transfer upon intercalation. The origin of the previously evidenced magneto-elastic effect for x = 0.48 is shown to lie in the existence of ferromagnetic coupling between nearest-neighbouring iron moments along b (dFe-Ee = 3.59 Å), the exchange parameter of which increases with decreasing distance. The Ti conduction electron spins are shown to be greatly polarized by the Fe localized moments leading to the onset of a commensurate spin density wave below TN. The refinement of the 2 K intensities yields Ti moment values equal to 0.07(3) and 0.31(3) μB for x = 0.25 and 0.48, respectively.Les structures cristallographiques et magnétiques des composés lamellaires antiferromagnétiques FexTiSe2 (x = 0,25 et 0,48) ont été déterminées à partir de l'affinement des spectres de diffraction de neutrons réalisés sur poudres dans le domaine 2-300 K. Une permutation partielle entre les sous-réseaux du fer et du titane est mise en évidence. Pour les deux composés, elle correspond à une substitution de 8 % du fer par le titane. La distorsion de l'octaèdre TiSe6 s'atténue de x = 0,25 à x = 0,48, à savoir lorsqu'augmente le remplissage de la sous-bande d par suite du transfert de charge consécutif à l'intercalation. Pour la phase Fe0,48TiSe2 magnétiquement ordonnée, l'arrangement des spins permet d'expliquer l'effet magnéto-élastique précédemment observé. L'origine du phénomène réside dans la nature ferromagnétique de l'interaction entre premiers voisins fer le long de b (dFe-Fe = 3,59 Å), dont l'intensité augmente lorsqu'ils se rapprochent. Les spins des électrons de conduction du titane sont notablement polarisés par les moments localisés du fer, ce qui se traduit par l'établissement au-dessous de TN d'une onde de densité de spin commensurable. L'affinement des intensités des spectres à 2 K conduit à l'attribution d'un moment de 0,07(3) et 0,31(3) μ B pour x = 0,25 et x = 0,48 respectivement

Magnetic field expulsion in superconducting granular ceramics and in polymer/superconductor composites

The magnetic interaction between a permanent magnet and superconducting ceramics such as YBa2Cu3O7-δ and Bi1,6Pb0,4Sr2Ca2Cu3O10+δ depend on the superconducting state of each phase and on the junctions between grains. In the case of polymer/superconductor composites, screening effects depend on the volume fraction of superconductor. Measurements of the evolution of the levitation force (F=A/dγ) as a function of the interaction distance d are used to characterize the effective response of the ceramics or composites to the magnetic flux penetration. Some of the abnormal variations of the exponent ε and of the term "A" ( in F=A/dγ) could be reinterpreted in terms of a change in superconducting regime. Other observed variations of γ should be due to the variation of the effective field from the cylindrical magnet

The influence of synthesis way and dopant on the crystallites size of ceria

The present paper deals with the preparation of pure ceria by different methods : a conventional precipitation and a new sol-gel process. The size of crystallites was also carried out, aiming at studying the role of synthesis way on properties of pure ceria.
In order to investigate the influence of doping, samples with different molar contents of Mn$^{2+}$ were synthesized by co-precipitation. The doping level in all samples is expressed in molar percentage. A linear correlation between the lattice parameters and the Mn content confirm the incorporation of Mn in the ceria lattice. The evolution of crystallites size versus the lattice parameter was investigated

Multifunctional rare earth or bismuth oxide materials for catalytic or electrical applications

We present a review on catalytic or electrical properties of materials based on rare earth (RE) oxides (CeO2, La2O3, Lu2O3) or bismuth based composite systems CeO2-Bi2O3, susceptible to be integrated into catalytic microsystems or gas sensors. The polycrystalline solids can be used as catalysts allowing conversion of CO or CH4 traces in air-gas flows. Fourier Transform infrared spectroscopy is used to determine the conversion rate of CO or CH4 into CO2 through the variations versus time and temperature of vibrational band intensities. The time dependent reactivities are interpreted in terms of an adapted Avrami model. In these catalytic analyses the nature of surfaces of polycrystalline solids seems to play a prominent role in catalytic efficiency. Electrical impedance spectroscopy allows analyzing the variation of conductivity of the system CeO2-Bi2O3. In this system, the specific high ionic conduction of a Bi2O3 tetragonal phase might be linked to the high catalytic activity

Elaboration and characterization of europium doped ceria solid solutions

Cerium dioxides doped or substituted by europium have been prepared using low (320 $^{\circ}$C) and high temperature (1350 $^{\circ}$C) processes. The Eu substituted ceria phase obtained at high temperature is a solid solution Ce$_{1-{\rm x}}$Eu$_{\rm x}$O$_{2-\delta}\ 0\le {\rm x}\le 0.30$.The stœchiometry of the pure CeO$_{2-{\rm z}}$ ceria was controlled using an impedance spectroscopy approach permitting evaluation of sample stœchiometry (z). The various substituted samples were studied using X-ray diffraction. In initial low temperature [ 1/2 ${\rm Eu}_{2}$O$_{3}$ - CeO$_{2}$] systems, the ceria particles are nanosized (D = 4.3 nm). In sintered samples, a linear variation of cell parameters is clearly observed

Photocatalytic and photoluminescent properties of a system based on SmPO4 nanostructure phase

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