M. Bououdina

Phase stability and neutron diffraction studies of Laves phases Zr(Cr1−xMx)2 with M = (Cu0.5Ni0.5) and 0 < x < 0.2 and their hydrides

Abstract Laves phases of composition Zr(Cr 1− x M x ) 2 with M = (Cu 0.5 Ni 0.5 ) and 0 ⩽ x ⩽ 0.2 have been synthesised with the hexagonal C14- and/or cubic C15-type structures. By X-ray and neutron diffraction we have determined the structures of the alloys, and in particular the sites for the 3d metal substitution. The position of the hydrogen atoms in the structures and the occupation of the different sites of the hydrides have been determined. Purity and quality of polycrystalline alloys have been checked by using scanning electron microscopy and energy dispersive X-ray analysis techniques. The effects of the M/Cr substitution have been studied for phase stability, maximum hydrogen content and hydrogen site stability and analyzed in view of applications as electrodes in H M batteries.

Effect of nickel alloying by using ball milling on the hydrogen absorption properties of TiFe

Abstract After ball-milling, TiFe under a pure argon atmosphere, with a small amount of pure nickel, can readily absorb large amounts of hydrogen without any activation process. The H⧸M system reaches equilibrium after 8 h under 10 bars of hydrogen pressure (H⧸M = 0.62). Even after 10 days exposure to the air, the same sample reacts with hydrogen, but with slower kinetics. The impact of the ball-milling time on the hydrogen absorption kinetics and the maximum hydrogen content is discussed.

Structural studies of Laves phases Zr(Cr1−xNix)2 with 0≤x≤0.4 and their hydrides

Abstract Hexagonal C14-type Laves phases of the series Zr(Cr 1− x Ni x ) 2 with 0≤ x ≤0.4 have been synthesised. Purity and quality of polycrystalline alloys have been checked using SEM and EDX techniques. By X-ray and neutron diffraction we have analyzed the structure of the alloys, and determined the sites for the Ni substitution. The positions of hydrogen atoms in the structure and the occupation of the different interstitial sites of the hydrides have been determined. The effects of the Ni/Cr substitution have been studied with regard to the phase stability, the maximum hydrogen content and the hydrogen site stability.

Study of the hydrogenation/dehydrogenation processes of ZrCr0.7Ni1.3, a Laves phase-rich multi-component system, by in-situ neutron diffraction under hydrogen gas pressure

Abstract Powder neutron diffraction phase analyses were performed on a ZrCr 0.7 Ni 1.3 sample by using the Rietveld method. The average phase proportions are 59% C15-Laves phase, 36% Zr 7 Ni 10 and 5% elemental free chromium. In-situ neutron diffraction experiments were performed at 323 K under deuterium pressure. Cyclic refinements were carried out on the data recorded during both the absorption and desorption cycles. A non-reversible amount of absorbed hydrogen was found to be stored by the minor Zr 7 Ni 10 phase. After complete hydrogenation, the Zr 7 Ni 10 phase was found to transform into an amorphous phase, accompanying the overall reduction of reversible hydrogen capacity of the material.

The effects of heat treatments on the microstructure and electrochemical properties of the ZrCr0.7Ni1.3 multiphase alloy

Abstract Qualitative and quantitative X-ray analysis using the Rietveld method of the as cast ZrCr 0.7 Ni 1.3 alloy, shows an average phase abundance of 62.7% C15-Laves phase, 36.6% of Zr 7 Ni 10 and less than 1% of chromium. The annealed alloy at 1373 K reveals a complete disappearance of pure Cr with an important reduction of the amount of the Zr 7 Ni 10 phase based on EDS/SEM and XRD results. Pressure-Composition-Temperature (PCT) measurements show that the alloy behaves as a single phase with faster kinetics. Electrochemical measurements shows a relatively high discharge capacity of the cast alloy, c =320 mAhg −1 after 20 cycles. Contrary to that, the discharge capacity of the annealed alloy decreases by about a factor of 2 with c =156 mAhg −1 .

Magnetisation and neutron diffraction studies of HoFe12−xTaxXy (0.5≤x≤0.7, X=H, C)

Magnetisation measurements, in particular the angular dependences of the components of the magnetisation vector, parallel and perpendicular to the applied field were measured on oriented powder samples of the HoFe11.4Ta0.6 alloy and its hydride and carbide. These experiments allow a study of the magnetocrystalline anisotropy and an accurate determination of the spin reorientation process occurring in the hydride. Moreover powder neutron diffraction experiments led to a determination of the crystallographic and magnetic structures of these materials, in particular the values of the iron and holmium moments on the different sites. Analysis of the results allows an estimate of the magnetocrystalline anisotropy and of the exchange interactions. Accordingly their changes after hydrogen or carbon insertion in the starting alloy are determined.

New RFe12-xTax compounds and their related hydrides and carbides (R = Tb to Lu, 0.5≤x≤0.7)

Abstract New RFe 12− x M x alloys as well as parent hydrides and carbides have been synthesised with M=Ta and for rare earth metal R belonging to the series Tb to Lu. The range of stability of the compounds was found rather narrow, i.e. 0.5≤ x ≤0.7 making this system very similar to that recently reported for M=Nb. The crystal cell parameters and the Curie temperature of the compounds and their parent interstitial have been measured, the latter being particularly high.

Stability of Laves phases type of alloys ZrCr2(1-x)M2x and their hydrides

Abstract The semi-empirical model developed by Miedema et al. (Journal of Physics F., 1973, 3, 1558) has been used in the case of pseudo-binary alloys ZrCr2(1 − x)M2x where M = 3d metal or Mo. The stability of these alloys has been computed and then compared to the experimental diagram of the existence of the crystal structures C14 and C15. In the second part, the model has been applied to the corresponding hydrides. The impact of the nature and the stoichiometry of the substituted element M has been analysed.

Pressure–composition measurements and microstructural analysis of the Laves phases Zr(Cr0.5Mo0.2M0.3)2 with M=Fe, Co, Ni

Abstract The Laves phases Zr(Cr 0.5 Mo 0.2 M 0.3 ) 2 with M=Fe, Co, Ni have been synthesised in the hexagonal C14 type structure. Hydrogen absorption–desorption isotherms have been determined by pressure–composition isotherms (P–C–T) within the temperature range 298–423 K. We have observed that for more than 50% of chromium substitution, the compounds always have a relatively high capacity (H/M>1). Moreover, some isothermal curves reveal the existence of a sloping plateau. To understand the origin of this plateau, the phase composition and purity of polycrystalline alloys have been checked using SEM and EDX techniques.

Study of the system Zr1−xTix(Cr0.5M0.4V0.1)2 – H2 (0≤ x ≤0.2, M=Fe, Co, Ni).

Abstract Alloys of composition Zr 1− x Ti x (Cr 0.5 M 0.4 V 0.1 ) 2 have been synthesised with the hexagonal C14-type Laves phase structure. The crystal structure was studied by the neutron powder diffraction technique. The hydridation properties have been carried out from pressure–composition measurements using a thermogravimetric apparatus. The effects of Zr/Ti and Cr/M substitutions have been studied in view of potential hydrogen storage applications.