Statistical Thermodynamic Analysis for Isothermal Hydrogenation Performances of Mg_2-yPr_yNi₄Intermetallics (y = 0.6, 0.8, 1.0)

摘要

Isothermal hydrogenation performances of intermetallic Mg_2-yPr_yNi₄ alloys with y = 0.6, 0.8 and 1.0 reported by Terashitaet al.were analyzed on the basis of statistical thermodynamics under a simplifyinga priori assumption of constant nearest neighbourH-H interactionE(H-H) in a given phase at arbitrary T aiming at characterizing basic aspects of state of H atoms in the interstitial sites in H-storage alloy. To fulfill this a priori assumption, number θ of available interstitial sites per metal atom was chosen by preliminary search attempt at the onset of the statistical thermodynamic analysis. Primary H solution in Mg_2-yPr_yNi₄ was analyzed by the model with θ = 0.15. The chosen ? value 0.15 for the model analysis was close to be 1/6 (≈ 0.167) which was half of 1/3 (=[Mg + Pr]/[Mg + Pr + Ni])implying that about half of the (Mg + Pr)-related interstitial sites were provided as the available sites for occupation by H atoms in the primary H solution of Mg_2-yPr_yNi₄. On the other hand, hypo-stoichiometric M₄H₃ type hydride of Mg_2-yPr_yNi₄ was analyzed by the model with θ = 0.75 and θ' = 0.333 where ?' refers to the lower limiting composition of the phase. This model yielded situation with E(H-H) = 0 for any Mg_2-yPr_yNi₄examined. Chosen value of θ' = 0.333 appeared to imply that the filling of Ni-related interstitial sites by H atoms started after preferential full occupation of the (Mg + Pr)-related interstitial sites by H atoms in the two-phase equilibrium range at invariable p(H₂) plateau during H-charging.