Kinetics and thermodynamics of near eutectic Mg-Mg2Ni composites produced by casting process

Abstract

This article reports the hydrogenation properties of several Mg–Mg2Ni composite specimens with increasing Ni content: 4.4, 11.3 (eutectic), and 16.3 (in at%). Mg–Mg2Ni composites were prepared by means of induction melting, followed by simple mechanical chipping of the casts. The hydrogenation and dehydrogenation reaction kinetics were studied, and reaction mechanisms were described by means of solid-gas reaction modeling. Absorption and desorption properties were evidenced to be diffusion controlled, with hydrogen diffusion through hydrided/dehydrided phases being the rate limiting step in most cases (the migration of metal/hydride interface at a constant velocity being rate-limiting in only few of them). The kinetics study was supported by a thorough thermal analysis to provide in-depth insights of the decomposition reaction. Hence, thermogravimetry (TG) and pressurized differential scanning calorimeter (PDSC) were combined to investigate the dehydrogenation properties such as hydrogen gravimetric density, reaction onset temperature, enthalpy and activation energy as a function of Ni content. Structural analysis included X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM, TEM) to discuss the structural stability and microstructural evolution as a function of cycles, notably during the activation procedure. Finally, cyclic performance was evaluated for 100 cycles, using a custom-made large-scale reactor to demonstrate scale-up feasibility.