Abstract
Bulk mechanical alloying was successfully applied to solid-state synthesis of Mg2Ni, which is a promising hydrogen storage alloy. Through XRD, TEM and PCT measurement, the synthesized product was characterized to be a single-phase Mg2Ni, excluding co-synthesized MgNi2 or residuals of nickel or magnesium as in conventional processing. The synthesized Mg2Ni has a fine microstructure with the grain size of 10–20 nm. Thermodynamic data for hydride formation are coincident with reference data, so that nano-structuring in the solid-state synthesized, stoichiometric Mg2Ni has little role to make essential change in hydrogen storage capacity and mechanism. Physical modification by enrichment of nickel via the bulk mechanical alloying enables us to obtain the nickel-enriched, nano-structured Mg2Ni and to demonstrate the phase transformation of Mg2NiH4 takes place from high temperature phase (HT-phase) to low temperature phase (LT-phase) with decreasing the holding temperature. Negative shift of formation entropy of Mg2NiH4 from HT-phase to LT-phase is corresponding to the hydrogen ordering, where a part of octahedral vacancy sites in Mg2Ni structure is only occupied by hydrogen atoms. Formation of LT-phase is also verified by XRD analysis working together with PCT measurement. Positive shift of formation enthalpy of Mg2NiH4 from HT-phase to LT-phase drives a significant increase of plateau pressure in the PCT diagram. Non-equilibration of nano-structured Mg2Ni leads to control of hydride formation process. Use of these non-equilibrium phase materials helps us to understand various unknown properties of metal hydrides.
