Dynamics of hydrogen interaction on and in uranium

Bazley, Scott Gordon (2004) Dynamics of hydrogen interaction on and in uranium. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

The adsorption of hydrogen on polycrystalline uranium surfaces, with modified oxygen overlayers, has been investigated using supersonic molecular beams, TPD and XPS techniques. An indirect channel dominated hydrogen adsorption on clean uranium with an absolute sticking probability of 8.6x10^-2. This produced a single second order desorption peak (α₁) and the TPD spectrum at 450 - 410 K that had a saturation coverage of 0.22 ML. The activation energy of desorption was calculated to be 74.58 ± 7 kJmol^-1. In contrast, an activated direct channel dominated the adsorption of hydrogen on oxidised uranium surfaces with an absolute sticking probability of 1x10^{-2 .}The TPD spectra exhibited three desorption peaks at 250 (β₂), 470 (β₁) and 650 - 610 (β₃) K. These were attributed to weakly bonded hydroxyl groups that were affected by the presence of subsurface metal regions, to desorption from the oxide and to the presence of defects or edges on the oxide that increased the strength of the hydrogen bonding to the surface respectively. The "initial" surface also exhibited three desorption peaks, with a fourth being observed at 63 meV, an additional peak (α₃) growing as a shoulder on the β₂peak at 350 K. This was attributed to either edge effects or the population of unfavourable sites near the metal/oxide interface.

Using the Time Lag method, we have investigated the role of the uranium oxide overlayer in the permeation of deuterium through 0.178 and 1 mm thick uranium metal membranes. Palladium coating the inlet surface enabled permeation through "clean" metal to be investigated. There appears to be a number of factors that control the level of influence the oxide overlayer is seen to have on the diffusion process; the thickness of the oxide overlayer, the thickness of the metal membrane, the microstructure of the oxide overlayer and the level of impurities in the gas-phase and the bulk.

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