HYDROGEN SOLUBILITY IN LANI5

In a previous study (H. Uchida, A. Hisano, K. Terao, N. Sate and A. Na gashima, J. Less-Common Met., 172-174 (1991) 1018). we pointed out tha t LaNi5 powder samples exhibit higher H solubilities than block sample s at H concentrations lower than [H]/[LaNi5] = 1X10(-1). This may be a scribed to the ready stress relaxation of small alloy particles and to the limited expansibility of block samples with larger crystal grains . In this investigation, the pressure-composition-temperature (P-C-T) relations were measured for the LaNi5-H system using well annealed sto ichiometric LaNi5.0. The size of the alloy samples was extensively cha nged from fine powders with a 20 mu m median size to a cubic block sam ple with 10.5 mm in the side length to examine the dependence of H sol ubility on the sample volume and stress accumulation. The systematic m easurements revealed that the dependence of H solubility on sample siz e vanishes when the particle size becomes lower than 50-64 mu m in dia meter or when the cubic block sample size exceeds 9 mm in the side len gth. The relative partial molar enthalpy Delta (H) over bar(H), for H solution was found to be dependent on sample size. At H concentrations lower than [H]/[LaNi5] = 3.5 x 10(-2), the Delta (H) over bar(H) valu es for all samples were almost constant independent of H concentration ; however, the Delta (H) over bar(H) became more positive as the sampl e size was reduced, indicating reduction of attractive metal-H interac tions because of ready volume expansions of fine particles. With incre asing II concentration at [H]/[LaNi5] > 3.5 x 10(-2), the Delta (H) ov er bar(H) values tended to become more positive and this effect became more marked as the sample size was increased from fine particles to b locks with larger grains, which may be caused by the accumulation of h igher compressive stresses in block samples.