Hydrogen interactions with titanium thin films

Nanocrystalline titanium films were prepared on sapphire substrates in an UHV chamber, by means of ion beam sputter deposition under Ar-atmosphere at the pressure of 1,5·10^-4 mbar, with a deposition rate of 2,1 nm/min. The crystal structure was investigated by means of X-Ray diffraction. For electrochemical hydrogen charging, the films were covered by a 30 nm thick layer of Pd in order to prevent oxidation and facilitate hydrogen absorption. The samples were step-by-step charged with hydrogen by electrochemical charging. An Ag/AgCl (sat.) and Pt wires were used as the reference and the counter electrode, respectively. XRD measurements were performed before and after hydrogenation in order to investigate the effect of hydrogen on the microstructure. The main characteristics of hydrogen's absorption behaviour, as well as the thermodynamics and phase boundaries of titanium-hydrogen thin films are discussed in detail with specific emphasis on the comparison to titanium-hydrogen bulk system. Shifted phase boundaries and narrowed miscibility gaps were observed in Ti-H nanocrystalline film system. Large compressive stresses built up in these films during hydrogen absorption. In low hydrogen concentration range, α-phase region, the films behaved linear elastically, whereas with further hydrogenation, plastic deformation processes occur. Ti-hydride phase is quite stable, indicating an insignificant hydrogen loss in Ti-films.