1,721,454 research outputs found
X-ray analysis of microstructure in Au-Fe melt spun alloys
No full textSupersaturated Au80Fe20and Au70Fe30solid solutions have been obtained by rapid solidification. The lattice constant a0of the as-quenched FCC phase was of 4.025 and 3.989 Å for Au80Fe20and Au70Fe30, respectively. Thermal treatments have been performed in the temperature range between 320°C and 550°C. A precipitation reaction has been observed in Au80Fe20only after prolonged annealing at low temperature (320°C/168 h). In Au70Fe30an increase in the lattice constant has been observed after thermal treatments, giving values ranging from 3.998 to 4.044 Å. Precipitation occurs at grain boundaries and into grains. Low-temperature annealings give lamellar precipitates, which grow and transform into globules at higher temperatures. A Williamson-Hall plot for the FCC phase shows a significant scatter due to occurrence of planar defects. A detailed analysis of microstructure have been obtained by Rietveld analysis of peak broadening. Precipitation reaction leaves a Fe-depleted matrix, which shows a cristallite size of about 200 Å and an r.m.s. microstrain of 6 × 10-3. Compound fault probability has been estimated from asymmetric peak profile. © 2003 Elsevier Science B.V. All rights reserved
Spectroscopic and Structural Characterization of Thermal Decomposition of γ-Mg(BH4)2: Dynamic Vacuum versus H2 Atmosphere
Full text linkMg(BH4)2 attracts a particular interest as material for hydrogen storage because of its high gravimetric capacities and being suggested as a rehydrogenable compound. Although extensively studied, besides the whole decomposition process, a large debate is still present in the literature about the temperatures leading to the different (and in many cases, unknown) products. In this paper, an ad hoc designed thermogravimetric study, together with a critical review of literature data, allowed to identify the products for low reaction rates. Two reaction environments have been considered: dynamic vacuum and hydrogen atmosphere. In order to guarantee quasi equilibrium conditions, the samples were obtained after 10 h isotherms in the RT-400 °C range. The decomposition of γ-Mg(BH4)2 has been here characterized adopting a new approach and by XRD and medium infrared spectroscopy, together with experimental techniques used for the first time for this process (Far-IR and UV-Vis-NIR spectroscopies). Density functional calculations were performed to help the identification of the amorphous products. A possible process mechanism was delineated and in particular that: (a) Mg(BH4)2 decomposition starts at 200 °C; (b) MgB4H10 is proposed, for the first time, as the phase responsible for its reversibility for T < 270 °C, which would implicitly restrict the Mg(BH4)2 reversible capacity to 3.7 mass%
SOLID-STATE NMR COUPLED WITH GIPAW AB-INITIO CALCULATIONS IN METAL HYDRIDES AND BOROHYDRIDES
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Structural study of a new B-rich phase obtained by partial hydrogenation of 2NaH + MgB2
No full textThe structure of an unknown crystalline phase observed during the hydrogen absorption reaction of the powder mixtures 2NaH þ MgB2 at high pressure has been studied by abinitio structure determination from powder diffraction. The sequence of un-overlapped peaks extracted from the X-ray powder diffraction pattern could be indexed with a primitive cubic cell with lattice parameter a ¼ 7.319 A°. The diffraction patterns of the peaks are matched with the Pa-3 space group. The stoichiometry of the hydrogen absorption reaction suggests the presence of a high-boron content phase in the compound under investigation. Assuming this phase to be composed only by boron atoms and therefore having a density similar to that found for boron polymorphs, the solution with a space group of Pa-3 leads to reasonable BeB interatomic distances.JRC.F.2 - Cleaner energ
A thermodynamic investigation of the LiBH4-NaBH4 system
No full textThe LiBH4–NaBH4 pseudo-binary system has been investigated by X-ray diffraction, temperature-programmed photographic analysis, and differential scanning calorimetry, in order to establish the phase diagram. The polymorphic orthorhombic-to-hexagonal phase transition of LiBH4 was observed at 94 °C in samples containing NaBH4, i.e. 15 °C lower than for pure LiBH4, which indicates the dissolution of sodium into LiBH4. The formation of solid solutions was confirmed by powder X-ray diffraction measurements performed as a function of temperature. A new eutectic composition between Li0.65Na0.35BH4 and Li0.70Na0.30BH4, with a melting temperature of 216 °C, is observed. Ab initio calculations have been performed to establish the relative stabilities of the pure compounds in orthorhombic, hexagonal and cubic structures. The obtained experimental and calculated data were compared with available literature values and they were used for a thermodynamic assessment of the LiBH4–NaBH4 system by the calphad method. The enthalpy of mixing for solid and liquid solutions has been estimated on the basis of experimental data
Solid State NMR spectra and GIPAW plane-wave investigation of hydrogen storage materials
Full text linkPreparation and Characterization of Master Alloys Fe48Cr15Mo14C15B6Y2 Metallic Glasses
Full text linkThe purpose of this work is the characterization of a master alloy of metal glass based on iron Fe48Cr15Mo14C15B6Y2. Two types of alloys studied B1 which has been prepared by the use of pure element and the other B2 which has been prepared by the use of raw materials. The thermal and structural properties of the samples are measured by a combination of high temperature differential scanning calorimeter (HTDSC), X-ray diffraction and scanning electron microscopy (SEM). Chemical compositions are checked by energy dispersive spectroscopy analysis.Fil: Bendjemil, Badis. University of Badji -Mokhtar; Argelia. Faculty of Sciences and Technology; ArgeliaFil: Seghairi, Nassima. Faculty of Sciences and Technology; ArgeliaFil: Lavorato, Gabriel Carlos. Dipa rtimento di Chimica Universita' di Torino; Italia. Comisión Nacional de Energía Atómica. Gerencia del Área Investigaciones y Aplicaciones no Nucleares; ArgentinaFil: Castellero, Alberto. Dipa rtimento di Chimica Universita' di Torino; ItaliaFil: Bougdira, Jamal. Université de Nancy, Faculté des Sciences et. Techniques; FranciaFil: Vinai, Franco. Instituto Nazionale di Ricerca Metrologica; ItaliaFil: Baricco, Marcello. Dipa rtimento di Chimica Universita' di Torino; Itali
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