Solubility and diffusivity of hydrogen in complex materials

A general model based on Statistical Mechanics and Random Walk is presented which allows to desribe the behavior of hydrogen in disordered systems, i. e. metallic glasses, amorphous silicon, nanocrystalline metals, deformed me tals, disordered metallic solutions, and metallic multi layers. The various systems are specified by a lattice with an appropriate site energy disorde r and a distribution of site transitions rates. Lattice sites are filled ac cording to Fermi-Dirac Statistics because double occupancy is excluded. Thu s the model is applicable to adsorption on heterogeneous surfaces or soluti ons of small particles in oxide glasses and polymers. With a given distribu tion of site energies a relationship between chemical potential (Fermi ener gy) of hydrogen and its concentration can be derived and compared with expe rimental results. It is a unique feature of hydrogen that its chemical pote ntial and its diffusion coefficient can be determined rather easily by elec trochemical techniques or by measuring partial pressures at moderate temper atures around 300 K. With increasing H-content the sites are usually filled from lower to higher energies. As a consequence Henry's Law is not fulfill ed and the diffusion coefficient increases because at high concentrations l ow energy sites are saturated and additional H-atoms have to perform their random walk through sites of low occupancy or small time of residence, resp ectively Some results for metallic glasses, nanocrystalline metals, deforme d metals, and metallic multi layers are presented and compared with the mod el. Thus information on the interaction between defects (dislocations, grai n boundries, distorted tetrahedral sites in glasses) and hydrogen a-re obta ined. For extended defects the diffusion is strongly anisotropic, i.e. it d iffers in a Pd/Nb-multi layer by a factor of 10(5) for diffusion in plane a nd out of plane.