Abstract

In this work a thermodynamic database describing the Mg-Al-Li-Na-H system was constructed using thermodynamic modeling through the CALPHAD method. The constructed database was used to assess the hydrogen storage properties of the system at different pressures and temperatures and to understand the phase relationships and reactions mechanisms.
The liquid phase in the binary systems was reassessed using the modified quasichemical model (MQM) or taken from the literature. Since hydrogen atoms occupy the interstitial positions in the solid phases, terminal solid solution phases were described using the compound energy formalism (CEF). Two sublattices were used where Mg, Al, Li, and Na atoms mix randomly in the first sublattice to allow for their mutual solubility and H atom and vacancy mix in the second sublattice. All the gases included in this study were considered ideal. The calculated phase diagrams and thermodynamic properties were compared to experimental data from the literature and showed good agreement. The ternary systems were extrapolated from the corresponding binaries using the asymmetric Kohler-Toop technique where H is singled out as the asymmetric component. In this study, all the calculations were performed using the FactSage software where all the cited models are already coded and implemented.
The constructed database allowed the calculation of pressure-composition isotherms of MgH2-10 wt. % NaH, Mg-10 at. % Al and Mg-4 at. % Al. Phase diagrams, pressure temperature diagrams, and reaction pathways of the composites MgH2-AlH3, MgH2-NaAlH4, MgH2-Na3AlH6, and MgH2-AlLiH4/AlLi3H6 were calculated and showed good agreement with the experimental data from the literature. The results provided more details about the de/hydrogenation processes, the amount and composition of phases, and the effect of the pressure and temperature. It is found that Al and Li when added to Mg improve the hydrogen potential of the system and the most promising compositions were predicted.
The strategy used in this study will allow the experimental investigations to focus on the kinetics aspect of the de/hydrogenation reactions for these alloys.