Nuclear-magnetic-resonance measurements of the hydrogen dynamics in nanocrystalline graphite

Hydrogen-loaded nanocrystalline graphite samples have been prepared by mechanical milling under a hydrogen atmosphere. Milling vials and balls made of agate and Zr O2 have been used to prepare samples with hydrogen contents between 1 and 2 wt %. The proton nuclear-magnetic-resonance (H1 -NMR) spectra of these samples are well represented by the sum of a broad Gaussian and a more narrow Lorentzian line corresponding to hydrogen in C-H covalent bonds as well as to hydrogen in methyl groups. The temperature dependence of the Lorentzian line can be ascribed to a hindered rotation of the methyl groups. The corresponding activation enthalpy of about 0.12 eV has been deduced from the spin-lattice relaxation rates between 250 and 450 K. Below about 200 K the relaxation rates are temperature independent but they depend strongly on the NMR frequency and on the parameters of the sample preparation. The relaxation due to paramagnetic impurities as well as the cross relaxation of the proton spins with spins of quadrupolar impurity nuclei are proposed to contribute significantly to the measured spin-lattice relaxation rates.