Papers by Keyword: EXAFS

Abstract: In the present study, pure elemental powders of Fe and S were mixed to give the desired compositions of Fe50S50. A SPEX 8000D high-energy ball mill was used to synthesize iron sulfide powders under an Ar-filled atmosphere. The prepared powders were examined by conventional X-ray diffractometry and synchrotron X-ray absorption spectroscopy. The experimental results revealed that mechanochemical reactions occurred during the ball milling process for all the compositions. The Fe50S50 phase was obtained after ball milling for 20 h. Extended X-ray absorption fine structure (EXAFS) results revealed that the nearest neighbor bond lengths of the radial distribution function (RDF) for iron decreased when iron sulfides formed. X-ray absorption near edge structure (XANES) of S K-edges distinguished better the structural evolution of these iron sulfides.

Abstract: The commercial material PM2000 is investigated as a representative for the material class of Oxide Dispersion Strengthened (ODS) steels. ODS steels are envisaged as substance of structural components in Very High Temperature Reactors (VHTR). The VHTR concept is considered by the Generation IV International Forum, an initiative researching the next generation of nuclear power plants. The chosen ODS is mainly used for non-nuclear applications. In order to justify the applicability of this material within a neutron irradiative environment, the evolution of radiation damage must be investigated. PM2000 samples are therefore exposed to 4He2+ irradiation at different temperatures. The potential structural change is measured as a function of the radiation parameters, using Extended X-Ray Absorption Fine Structure (EXAFS) spectroscopy on the yttrium K-edge and on the iron K-edge. A degradation of the dispersoids or the steel matrix would show the limitation of this material candidate.

Abstract: The biosorption of arsenic species by dried lettuce leaves (L.sativa) was investigated. Arsenic sorption, that is not effective on in natura biomass, was enhanced when the biomass was previously loaded with Fe(III). Analysis of X-ray Absorption Near-Edge Spectroscopy (XANES) spectra showed that iron was incorporated as Fe(III) and arsenic as As(V), regardless the contact with the lowest or highest valence species of these elements. The features of Extended X-ray Absorption Fine-Structure Spectroscopy (EXAFS) spectra suggest that the nearest neighboring atoms of iron ions are the same in all the samples, even in the As-Fe loaded ones. These results indicate the arsenic oxyanions as the sorbed species on the iron-loaded biomass.

Abstract: Ga K-edge EXAFS spectra have been analyzed to elucidate the local coordination structure of Ga in two representative selenide Ge-As-Se and Ge-Sb-Se glasses all doped with Pr. Gallium turned out to be coordinated with 4 Se atoms in its first neighboring shell. This implies that Ga does not follow the 8-N rule associated with the short-range order structures of typical covalent glasses, further indicating there being more ionic-bond nature in the Ga-Se bonds compared to other heteropolar chemical bonds in the selenide glasses. This is decisive for the Pr3+ ions to be incorporated in the selenide glasses. In this case, the GaSe4 units can be electrically neutralized by the doped Pr3+ ions that act as a charge compensator. As such, inside the selenide glasses, distributions of Pr3+ ions and the Ga tetrahedral units are closely correlated. Spectroscopic properties of rare earths embedded in these Ga-containing selenide glasses thus can be explained in connection with the proposed role of Ga.

Abstract: The effect of heat treatment on mechanical prosperities of Mg65Cu25Gd10 glass has been studied in this paper. The thermal stability and the structure of as-spun and heat treated Mg65Cu25Gd10 alloys were examined by differential scanning calorimetry (DSC) and X-ray diffractometry (XRD). The compression tests were performed using a computer controlled tensile test machine. Extended X-ray absorption fine structure (EXAFS) was applied to study the structural changes of Mg65Cu25Gd10 glass during annealing. Results showed that the decline of mechanical properties for Mg65Cu25Gd10 glass after annealing was closely related to the changes in local environment of Cu atoms and appearance of crystalline phases, and not the Gd local structure.

Abstract: XAFS measurements of the Cu, Ni and Zr K-edges for the melt-quenched Zr67Cu33 and Zr67Ni33 metallic glasses were curried out using synchrotron radiation at 20K. Fitting calculations for the EXAFS results reveal that local structure around Ni and Zr in Zr67Ni33 is well represented by those for the crystalline Zr2Ni, while local structure around Zr in Zr67Cu33 is better fitted by an icosahedron rather than crystalline Zr2Cu. Such differences of the local structure attribute to the differences of the stability of super-cooled liquid state and glass formability between Zr67Cu33 and Zr67Ni33 metallic glasses.

Abstract: The purpose of this research is to investigate the micro-mechanism of anelastic behavior of metallic glass. The changes of atomic arrangement in Cu50Zr50 metallic glass were observed directly by EXAFS method during tensile deformation. The transmission-mode EXAFS measurements around Zr K-edge and Cu K-edge of the sample in various amounts of tensile deformation conditions were done. The interatomic distance between Cu-Zr increased, and that between Zr-Zr decreased simultaneously, with the amount of tensile deformation increase of the sample at first stage of deformation. When the atomic distances reached to certain values respectively, they recovered to the initial values of no tensile deformation condition at first stage, and then the atomic distances changed as same manner as the first stage of deformation, respectively. This is the first observation on the change of atomic arrangement during tensile deformation of metallic glass.

Abstract: We present extended X-ray absorption fine structure (EXAFS) and anelastic spectroscopy measurements performed on (La0.63Ca0.37)(Mn1-yMy)O3 manganites (M = Cr or Ni and y = 0.03, 0.08). The ferromagnetic metallic phase maintains its long-range character even after Mn substitution with non- Jahn-Teller ions, but both the doping species (Ni or Cr) lower the Curie temperature and broaden the magnetic transition. EXAFS suggests the presence of distorted insulating zones in the framework of phase inhomogeneity. Moreover, a strong dependence of the real part of the complex dynamic Young’s modulus and of the elastic energy-loss coefficient on the Mn substitution is evidenced by the anelastic spectroscopy spectra. These results confirm the correlation among local lattice disorder, dynamic and magnetic degrees of freedom and also evidence the possibility to tune manganites physical properties acting on their chemical composition.

Abstract: The alloying and phase formation in Ni-Hf samples with 0.2-, 2-, and 5-at.% Hf were studied by X-ray diffraction (XRD) technique and scanning electron microscopy (SEM). Both characterization methods, XRD and SEM, reveal the presence of the HfNi5 phase (fcc structure) where the excess Ni atoms are present in the form of Ni or Ni-rich segregations in the sample containing 5-at.% Hf. The sample with 2-at.% Hf is characterized by the presence of the two phases present in the 5-at.% sample and by Hf atoms, which occupy substitutional lattice positions in the Ni lattice. Finally, in the third sample with 0.2-at.% Hf, the Hf atoms mainly substitute the Ni atoms in the lattice. This analysis is being complemented with additional information on the local structure around Hf by extended X-ray absorption fine structure spectroscopy (EXAFS).