The effect of hydrogen stoichiometry on palladium strain and resistivity

The strain and the electrical resistivity of a Pd sample stressed by a constant tension have been investigated through a series of hydrogenation cycles in a continuous H stoichiometry [0⩽x⩽0.8] range. The isotropic lattice expansion for both “as drawn” and “annealed” Pd sample reveals a strain of only 1% from pure Pd to PdH0.8 in disagreement with literature data available; the measured effect is minimum at x=0.13 (α+β phase) and then from x=0.6 (β phase) it has an exponential increase. The contribution of the mechanical tensile stress on the total relative elongation of the wire is also investigated. An increase of the Pd sample tensile strain after each hydrogenation cycle is reported for “as drawn” samples, while for “annealed” samples the reverse behaviour is observed. Moreover, annealed samples show considerably higher value of tensile strain compared to “as drawn”. The variation of mechanical strain versus H content, for both “annealed” and “as drawn”, has a maximum at x=0.52. Strain variation and resistivity variation versus H content exhibit similar behaviour.