Hydrogen absorption by thin Pd/Nb films deposited on glass.

Abstract

Hydrogen absorption by 200--2000-\AA{}-thick Pd-capped Nb films, between 5 and 110 \ifmmode^\circ\else\textdegree\fi{}C, was studied by simultaneous four-probe resistivity and volumetric measurements. The resistivity as a function of hydrogen concentration was measured while charging the films with hydrogen, and was used to compute the change in hydrogen concentration in the film, during the reaction with oxygen. For the thinnest films (200 \AA{} thick), the hydrogen charging and discharging curves indicate that a first-order gas-liquid-like phase transition with a ${\mathit{T}}_{\mathit{c}}$ of 70--75 \ifmmode^\circ\else\textdegree\fi{}C takes place. The H-Nb phase diagram for the 200-\AA{} film looks like the H/bulk Nb \ensuremath{\alpha}-\ensuremath{\alpha}\ensuremath{'} phase diagram which has a higher ${\mathit{T}}_{\mathit{c}}$ (173 \ifmmode^\circ\else\textdegree\fi{}C). We attribute the substantial modifications of the film's phase diagram to the clamping of the Nb film at its interfaces with glass and Pd and to the nanostructure of the films. \textcopyright{} 1996 The American Physical Society.