NMR study of hydrogen diffusion in uranium hydride

Abstract

The diffusion of hydrogen in uranium hydride is studied employing the NMR technique. From measurements of spin-spin relaxation time T 2, the activation energy for hydrogen diffusion in β- UH 3 is determined to be E a = (19.25 ± 0.4) kcal mole and the preexponential factor to be A 0 ≈ 5 × 10 14 Hz. It is shown that these results are in fair agreement with spin-lattice relaxation time T 1 data. Assuming that hydrogen diffusion proceeds via vacancies whose concentration is temperature dependent, it is concluded that E a is the sum of the energies of vacancy formation and barrier height, and that A 0 contains an entropy change factor. Using vacancy concentration data calculated by Libowitz, we estimate the barrier height energy to be E b ≈7 kcal/ mole. Using a value for the frequency of hydrogen vibration v 0 determined from inelastic neutron scattering by Rush et al., we estimate the entropy change due to vacancy formation and the hydrogen atom jump to be about S k B ≈3 . Similar measurements on samples containing less hydrogen than is needed to compose stoichiometric UH 3, show that the rate of diffusion is enhanced by the presence of excess metal in the sample. The jump frequency at 500°K in UH 3 is found to be approximately 10 6 Hz while for the two-phase samples of H/U = 2.8 and 2.5, it is larger by a factor of about 3 and 3.5, respectively.