Effects of prior hydrogen dissolution and hydridingon hydrogen solubility in LaNi 5

Abstract

Pressure-composition-temperature ( P-C-T) relations for the LaNi 5−H system weremeasured using well annealed LaNi 5 block and powder samples with a stoichiometry of x=5.0 in LaNi x . The block sample exhibited lower hydrogen solubility than the powder sample for hydrogen concentrations [H]/[LaNi 5]<1×10 −1, which may be ascribed to higher internal stresses accumulated in the block sample by H 2 dissolution. Marked hydrogen trapping and hydrogen solubility enhancement in LaNi 5 took place not only in the samples activated by hydriding-dehydriding cycling, but also in the samples cycled in advance in a solid solution. The mean value of the relative partial molar enthalpy for H 2 dissolution at infinite dilution is −20.2 kJ(mol H) −1, constant up to hydrogen concentrations of [H]/[LaNi 5]=1.5×10 −2. However, the negativity of the enthalpy value becomes markedly increased by the hydrogen trapping effect induced in LaNi 5 even by cyclic H 2 dissolution in a hydrogen solid solution region.