On the dependence of true hydrogen equilibrium pressure on the granular size distribution of the Zr(Fe0.5Mn0.5)2 getter alloy

The influence of grain size distribution on the achievement of the chemical equilibrium between the solid solution region and the hydrogen gas phase for the St909 alloy isotherms has been investigated for two different grain size distributions to test the possibility of using this alloy without a previous cycling treatment. The small grain size distribution gave Delta H = -33.54 +/- 1.03 kJ/mol, Delta S = -18.31 +/- 3.43 J/K mol in a H_2 concentration range 0.05 < x < 0.09, and Delta H =-42.30 +/- 1.06 kJ/mol, Delta S = -21.78 +/- 4.49 J /K mol in a D_2 concentration range 0.02<0.09. A comparison with Delta H and Delta S values in the infinite dilution range for H_2 and T_2 obtained for the conditioned St737 alloy suggested that the small grain distribution gave H_2 pressure values close to but not yet at equilibrium. A qualitative inquiry on the possible factors limiting the achievement of equilibrium is also presented and suggests surface dissociation phenomena as the most probable cause.