Rate-Controlling Steps for the Hydriding Reaction of the Intermetallic Compound Mg2Ni

Mg2Ni samples were prepared by sintering a pelletized mixture under an argon atmosphere in a stainless steel crucible at 823 K. The XRD pattern of the prepared Mg2Ni sample showed a well crystallized Mg2Ni phase. The hydriding and dehydriding properties of the prepared samples were examined at 518-593 K under relatively low hydrogen pressures of 3-7 bar H2. At 573 K under 7 bar H2, the activation of the Mg2Ni sample was completed at the number of cycles of six (n = 6). At n = 7, the hydrided fractions of the sample were 0.53 (1.99 wt% H) at 4.97 min, 0.72 (2.71 wt% H) at 9.52 min, 0.81 (3.05 wt% H) at 31.15 min, and 0.81 (3.05 wt% H) at 60.07 min. The particle sizes of the prepared Mg2Ni were not homogeneous and the particles had irregular shapes. We analyzed the rate-controlling steps for the hydriding reaction of the intermetallic compound Mg2Ni by examining the dependences of hydriding rates on hydrogen pressure and temperature in the same reacted fraction ranges. The analyses in the same reacted fraction ranges were done in order to eliminate the influence of the interfacial area on the hydriding rate. When the driving force, which is the difference between the applied hydrogen pressure Po and equilibrium plateau pressure Peq at a given temperature, was low, the nucleation of Mg2Ni hydride controlled the hydriding rate of Mg2Ni. After the nucleation of the Mg2Ni hydride, the rate-controlling step of the hydriding reaction of Mg2Ni was analyzed to be the forced flow of hydrogen molecules through pores, inter-particle channels, or cracks.

Structural, morphological, magnetic and hydrogen absorption properties of LaNi5 alloy: A comprehensive study

A comprehensive study of structural, morphological, magnetic and hydrogen absorption properties of LaNi 5–H system was investigated. The X-ray diffraction patterns show that as-synthesized LaNi 5 alloy is single phase with CaCu5-type structure while some weak peaks of elemental nickel also appeared after several hydrogenation/dehydrogenation (H/D) cycling. The presence of pure Ni was also followed using the room temperature magnetic measurements. After H/D cycling, the particle size decreases and particle size distribution was found nearly uniform compared to noncycled alloy. The pressure-composition isotherms (PCIs) of the hydrogen absorption reaction were determined in the temperature range 20–80°C using a homemade Sievert's type experimental apparatus, and then the enthalpy and entropy of hydride formation were calculated. The hydriding kinetic mechanism of LaNi 5 was evaluated using the different fitting models: Jander diffusion model (JDM), Johnson–Mehl–Avrami (JMA) and Chou models. All employed models confirm an increase in the hydriding reaction rate with temperature. However, the calculated results using JMA model show a better agreement with the experimental data and hence we believe that diffusion along with nucleation and growth is the rate-controlling step for the hydriding reaction. The values of activation energy for hydriding reaction were also obtained by JD and JMA models.