Reversible changes of state in nanoscale metal-hydrides

The properties of nanostructured materials differ significantly from those of coarse grained materials. For materials of smaller grain sizes, the overall properties are strongly influenced by the interfaces. It is shown, that this is not only due to the large volume fraction of the grain boundary region, but also to a long range elastic interaction between grain boundaries and crystallites which shift the thermodynamic equilibrium in the crystallites. Nanocrystalline palladium is examined as a model system and it is shown that due to the excess hydrogen concentration in the grain boundaries an interface stress leads to a pressure in the crystal grains causing quantitative changes in the phase diagram. A theory of chemo-elastic equilibrium presented here explains the experimentally observed narrowing of the miscibility gap and a shift of the critical point. X-ray diffraction, dilatometric and volumetric measurements are performed. Adsorption isotherms of hydrogen in the grain boundaries are determined experimentally. For the first time, the hydrogen distribution on grain boundaries and the crystallites are derived. Interface stress and stretch are also determined experimentally. Calorimetric studies indicate a broadening of the energy site distribution in the grain boundaries and clustering of hydrogen at the transition to the hydride-phase. Analysis of the pair distribution function confirm hydrogen enrichment in an ordered layer at the interface. The layer thickness is estimated with several methods. The results presented here can be applied to related systems such as thin films, multilayers and metal-hydrides for hydrogen storage applications. (orig.)