Solubility isotherms of hydrogen in epitaxial Nb(110) films

Abstract

The solubility isotherms of hydrogen in epitaxial Nb͑110͒ films have been determined via in situ x-ray lattice-parameter measurements. The Nb films were grown by molecular-beam epitaxial techniques on Al 2 O 3 (1120) substrates with thicknesses ranging from 32 to 527 nm. All isotherms can be well described by the standard expression for the solubility of a monatomic lattice gas. Aside from the metal-hydrogen interaction, which appears to be bulklike, all other properties exhibit a strong thickness dependence, including the hydrogen-hydrogen interaction, the critical temperature for the ␣-␣Ј transition, and the maximum latticeparameter change that can be reached in saturation. For the critical temperature we find a scaling with the film thickness, which most likely is due to the elastic boundary condition provided by the rigid substrate. The clamping of the epitaxial film to the substrate hinders critical fluctuations of macroscopic hydrogen density modes from developing, thereby lowering the critical temperature for the ␣-␣Ј transition.

An optical method to determine the thermodynamics of hydrogen absorption and desorption in metals

“…5 In thin films, residual clamping to the substrate is known to reduce the attractive long range H-H interaction and T c . 19,20 However, the good agreement found between buckled thin film and bulk in desorption shows that thermodynamic quantities determined on thin films are relevant for the search for hydrides if the strain induced by clamping to the substrate is reduced by the proper activation procedure. In summary, an optical method to determine the enthalpy, entropy and hysteresis of hydrogen absorption and desorption of thin metal films has been demonstrated for a metallic hydride.…”

Solid Solutions of Hydrogen in Complex Materials

“…The behavior of hydrogen in samples with reduced dimensions is one of the more recent areas of research in metal-hydrogen systems [20,158,159,160,161,162,163,164,165,166,167,168,169,170]. The properties of these systems are affected by the proximity of surfaces or interfaces providing additional sites to the DOSE.…”

Nanoskalige Metall- Wasserstoff- Systeme

“…1 Reimer et al [REI92,REI93] berichten 1992 von einem weiteren interessanten Verhalten dünner epitaktischer Nb-Schichten unter H-Beladung: Sie entdecken eine Verschmälerung der Rocking-Kurve des (110)-Peaks nach H-Beladung, das 'hydrogen annealing'. 1996 findet dieselbe Arbeitsgruppe (Song et al [SON96]) aber auch eine starke Verbreiterung der Rocking-Kurve epitaktischer Nb-Schichten nach HBeladung.…”

“…Song et al [SON96,SON97,SON99] haben den Einfluss der Schichtdicke insbesondere an epitaktischen Nb-H-Schichten untersucht. Song et al [SON96] veröffentlichten 1996 'Löslichkeitsisothermen' 2 von 32 bis 527 nm epitaktischen Nb-Schichten, aus denen sie Aussagenüber die kritische Temperatur der α − α'-Entmischung gewinnen. Sie finden eine deutliche Abhängigkeit der kritischen Temperatur von der Schichtdicke in Form eines Skalen-Gesetzes.…”

“…Song et al [SON96] finden zudem eine starke Abhängigkeit der maximalen vertikalen Gitterdehnung Hmax von der Schichtdicke: Während 20 nm Nb sich bei einem maximalen Druck von 200 mbar um 0,065 dehnt, zeigt die 520 nm Nb-Schicht nur 0,045 Gitterdehnung.…”