TRANSMISSION ELECTRON MICROSCOPY ANALYSIS OF NbF_5-DOPED MgH_2

摘要

The microstructure of MgH_2 with 1 mol % NbF_5, prepared by high-energy ball milling (HEBM), was studied using high resolution transmission electron microscopy (HR-TEM) with an X-ray energy dispersive spectroscope (EDS) before and after hydrogen sorption cycles. The TEM samples were prepared without any air exposure by a novel, focused ion beam (FIB) system specially designed for highly air sensitive materials. During HEBM, the doping agent, NbFs, melted and was distributed as an extremely thin, film-like, amorphous phase along the grain boundaries of the nanocrystalline MgH_2. After 10 sorption cycles, amorphous Nb-F phase was transformed into crystalline Nb hydrides. The Nb hydride layer would be responsible of the improvement of the H sorption kinetics of MgH_2. This Nb hydride layer was also believed to suppress the grain growth of nanocrystalline MgH_2 quite effectively and thus maintain its initial catalytic activity throughout repeated hydrogen sorption cycles. The MgF_2 byproduct formed by fluorine transfer reaction during ball milling and hydrogen sorption cycles was decisive for initial activation of the hydrogen desorption of MgH_2.