Dresden 2003 – wissenschaftliches Programm
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M: Metallphysik
M 13: Wasserstoff in Metallen
M 13.3: Vortrag
Montag, 24. März 2003, 15:15–15:30, IFW D
Hydrogen diffusion in nanostructured graphite studied by NMR — •Eva Stanik1, Günter Majer1, Shin-ichi Orimo2, and Hironobu Fujii2 — 1Max-Planck-Institut für Metallforschung, Heisenbergstr. 3, 70569 Stuttgart, Germany — 2Faculty of Integrated Arts and Sciences, Hiroshima University, Japan
Nanostructured hydrogen-graphite-systems, CnanoHx, with up to 7.4 wt.% of hydrogen were prepared by mechanical milling of graphite under hydrogen atmosphere. Nuclear magnetic resonance (NMR) measurements on CnanoHx (x = 0.24, 0.31, 0.96) have been performed to study the mobility of hydrogen in these materials. The NMR spectra are well represented by the sum of a Lorentzian line (width about 5 kHz at room temperature) and a broader (about 50 kHz) Gaussian line. They can be ascribed to hydrogen in graphite interlayers and to covalent C-H bonds. At lower temperatures the hydrogen mobility is reduced and thus also a broadening of the Lorentzian line occurs.
The proton spin-lattice relaxation rate Γ1 has been measured on the same samples over wide temperature ranges at 67.7 and 32.6 MHz. Γ1 can be decomposed into three contribitions according to Γ1 = Γ1d + Γ1e + Γ1p. The contributions due to the interaction with conduction electrons Γ1e and paramagnetic impurities Γ1p dominate below about 200 K and show a rather weak temperature dependence. The dipolar relaxation Γ1d provides information on the hydrogen hopping frequencies. We find an activation enthalpy for hydrogen motion of about Ea = 0.15 eV, a value that is close to the van der Waals binding energy of hydrogen to a graphene sheet.