Papers by Keyword: Hydrides

Abstract: The crystal structure and overall magnetic properties of RE(Fe_0.25Co_0.75)₂H_y with RE = Ho and Er, were analyzed versus y, the hydrogen content. The single phase C15 type compounds, synthesized using cold crucible HF melting, were hydrogenated and controlled using a PCI apparatus. The impact of hydrogen insertion on the cell parameter, the Curie temperature T_C and the magnetization saturation were determined. All compounds and hydrides were found to be ferrimagnets, the magnetic moment of RE and 3d elements being opposite since Ho and Er belong to the 2^nd row of rare earth elements. Depending on the formula, a typical compensation point was in evidence. The 2^nd order character of the ferri ↔ paramagnetic transition was established using the Arrott plot method. Magnetization isotherms vs. magnetic field gives quantified results for the magnetic entropy variation by application of the Maxwell relation. Control of y, the hydrogen content, shows it is interesting for improvement of the MCE of the starting compounds. However increasing y leads to decreased T_C at the lowest temperatures, suggesting potential cryogenic uses.

Abstract: The fast and spontaneous hydrogen diffusion in HCP structures leads to the hydride precipitation. It is often pointed as causing embrittlement and rupture in zirconium alloys for applications in the nuclear industry. In our previous works TEM, DSC, SEM-EBSD and XRD were used to study the hydride stability after many precipitation-dissolution thermal cycles as well as the crystallographic hydride phase nature and the hydride-substrate crystallographic orientation relationships as a function of the hydrogen content. Results showed that the evolution of the dissolution and precipitation energies is correlated to the concentration of hydrogen atoms available to reprecipitate, which is submitted to a diffusion controlled by the misfit dislocation migration. In the present work in-situ TEM thermal cycling was performed in order to locally investigate the crystallographic stability of zirconium hydrides of different structures after many dissolution-reprecipitation cycles.

Abstract: Hydrogen sorption-desorption processes by zirconium alloy Zr-1Nb was investigated in this article. Sievert apparatus was used for investigation of hydrogen interaction with material. It was shown that hydrogenation at gas atmosphere at 350, 450 and 550 °C temperatures leads to the δ-hydride formation in all material volume. Hydrogen sorption rates were calculated on the linear parts of sorption curves and equal to 0.5·10^-4 wt%/s at 350 °C, 9.5·10^-4 wt%/s at 450 °C and 23.1·10^-4 wt%/s at 550 °C. Short-wave diffraction of synchrotron radiation was used for investigation of hydrides dissociation during heating. Hydrides dissociation formed after hydrogenation at gas atmosphere occurs at temperature 522-540 °C. Further temperature increasing leads to β-phase appearance which indicates α to β phase transition at zirconium.

Abstract: The magnetic anisotropy (MA) of trihydrides of single-crystalline rare-earth (R) intermetallics R₂Fe₁₇H_3±x (R = Tb and Dy) with small deviations in the hydrogen concentration (x = 0 – 0.4) was investigated via the magnetic and magnetoelastic measurements in a wide temperature region. The easy-axis MA of Tb₂Fe₁₇H_3±x is preserved in the entire range of magnetically ordered state while an “easy-cone – easy-plane” transition takes place in Dy₂Fe₁₇H_3±x above ambient temperatures. The trihydrides demonstrate a significant decrease of magnetic anisotropy as the temperature increases. The temperature variation of thermal expansion of Dy₂Fe₁₇H₃ displays anomalies in the vicinity of a spin-reorientation transition. The longitudinal magnetostriction λ_||(T) of the same compound changes its sign at 340 K. The compounds studied can find specific applications in practice.

Abstract: Spatial distributions of a potential established inside a crystal by an externally applied potential are investigated theoretically in Pd and PdH. The calculations were performed in the framework of the linear-response approach with full inclusion of the electron band structure obtained within a self-consistent pseudopotential scheme. Analysis shows that in clean palladium the potential amplitude is maximal at the octahedral sites of the fcc lattice where the hydrogen ions predominantly reside. On the other hand the corresponding amplitude is significantly lower at the palladium ion positions. We observe drastic modifications in the screening properties in Pd upon hydrogen absorption. Nevertheless, the potential amplitude in PdH is also small at the Pd sites, whereas that at the H sites is significantly larger. However, in PdH the H ions are located at places where a local shallow minimum in the induced potential amplitude is observed. These results only slightly depend on the magnitude of the momentum transfer.

Abstract: Magnetic properties of the hydrides of an intermetallic compound Ho₂Fe₁₄B are studied in the temperature range 78 700 K. The temperature dependencies of magnetization σ (Т) are obtained under continuous control of the hydrogen content in the samples investigated. The dependence of the Curie (T_C) and the spin-reorientation transition (T_SR) temperatures on hydrogen pressure (P) and its concentration (x) are determined. The dependence of hydrogen concentration on pressure at constant temperatures (isotherms x (P)) in the vicinity of T_C and the temperature variation of hydrogen concentration at a constant pressure (isobars x (T)) are investigated.

Abstract: Microstructures of electron beam welded joints for TA15 titanium alloy with different hydrogen contents were observed and analyzed by SEM and TEM. And the influence of hydrogen on microstructure of the joints was investigated. The results show that the microstructure of the weld metal is lamellar α+β phase after hydrogen charging. In the range of hydrogen contents discussed in this study (from 0 to 0.101 wt%), With the increase of hydrogen content, there is little change in the appearance of the microstructure of the weld metal. The presence of hydrogen can promote the growth of twins in electron beam welded joints. With the increase of hydrogen content, the number of twins is increased. When hydrogen content reaches to a certainty level, hydrides are found in TA15 electron beam welded joints.

Abstract: In this paper, the crystal structure and hydrogen storage properties of the sodium hydride at different x value (NaH_xD_1-x, NaH_xT_1-x, NaD_xT_1-x; x=0, 0.25, 0.5, 0.75, 1.0) are investigated by using density functional theory within the generalized gradient approximation (GGA). The calculated results of NaH (D, T) are in good agreement with the other theoretical results. It has been found that, densities decreased with the increase of x value, while lattice parameters stay constant. The hydrogen storage properties of sodium hydride were predicted. The density-value x (ρ-x) relationship, the variations of the hydrogen storage properties with different crystal structure were obtained systematically.

Abstract: The influence of a crystallographic symmetry break on the morphology of precipitates during the coherent precipitation of a trigonal phase in a close packed hexagonal matrix is analyzed. It is pointed out that in spite of the isotropy of the stress free strain of the precipitate in the basal plane, the existence of an extra elastic constant in the precipitate (associated to the loss of symmetry) induces a morphological evolution from a shape having a symmetry of revolution around the threefold axis to a needle-like one oriented along the compact directions in the basal plane. These general considerations are applied to the case of zêta zirconium hydrides the crystallography of which has been recently identified to be coherent with that of the alpha Zr matrix. The influence of symmetry break and elastic heterogeneity on precipitation morphology has been numerically addressed by using different approaches. An analytical approximation to the elastic energy based on Eshelby equivalent inclusion method allows obtaining a qualitative criterion to determine the occurrence of a shape bifurcation of zêta hydrides.

Abstract: Occurrence of phase transitions in TixVyCrz-H systems was analyzed versus temperature. Hydrogen evacuates from the hydrogenated compounds via two main steps. The 1st step corresponds to the destabilization of the fcc hydrogenated lattice leading to a fcc → bcc martensitic type transformation. The 2nd step occurs according to the decomposition of the resulting hydride via evacuation of hydrogen from the bcc lattice. Both the kinetics and temperature of 1st and 2nd step transformations depend on the composition of the starting TixVyCrz alloys.