The very initial stage of hydride formation on polycrystalline gadolinium

The initial development of hydrides on hydrogen-exposed metallic surfaces displays a complex behavior with two possible surface-associated stages [M.H. Mintz, in: K.H.J Buschow, R.W. Cahn, M.C. Flemings, B. Ilschner, K.J. Kramer, S. Mahajan (Eds.), Encyclopedia of Materials: Science and Technology, Elsevier Sci. 2002, pp 1-9; Y. Ben-Eliyahu, M. Brill, M.H. Mintz, J. Chem. Phys. 111 (1999) 6053]: (i) Rapid formation of a dense pattern of small (sub-micrometric) hydride spots, which almost instantaneously attain their near final size and practically cease any further development (resulting in a type of "induction" or "incubation" period). (ii) Development of a few "growth centers" (GC's) of hydride "patches", which spread over the surface and lead to the massive hydriding stage. In the present study, the features of the very initial stage (i.e., stage (i)) are characterized for the H₂/Gd (polycrystalline) system, using Hot Stage Microscopy (HSM), X-ray diffraction analysis and Atomic Force Microscopy (AFM) techniques. The main characteristics are:(1)Typical hydride nucleation patterns composed of dense overlapping "spots" aligned along certain orientations on given metallic grains.(2)Very rapid initial development rates (within seconds), followed by an exponential decay of the precipitate growth rates.(3)A strong dependence of reactivity on the orientation of the metallic grains, with some grains displaying a very high reactivity (i.e., formation of very dense nucleation patterns) and other grains displaying a long-term inertness.(4)A time delay ("induction" period) between stage (i) and the flowing stage (ii) (i.e., GC's formation), which predominantly depends on the hydriding pressure and temperature. The above features and their related kinetics were studied as a function of hydrogen pressure (in the range 6 × 10² to 2 × 10⁴ Pa) at 373 K. Possible mechanisms controlling these reactions are discussed.