Heat treatment effects on the surface chemisorption behavior of strained uranium : The H2O/U reaction

The initial interaction of H2O vapor with polycrystalline uranium surfaces was studied with samples initially strained, then strain relieved by heat treatments, performed in the temperature range up to 650K. The chemisorption characteristics of these surfaces were studied by a combination of direct recoils spectrometry and X-ray photoelectron spectroscopy. X-ray diffraction measurements were used to determine the level of strain relief induced by each of the heat treatments. For all the samples, full water dissociation on the metal surface is observed. The reactivity of the samples towards water is clearly strain dependent, with the sticking coefficient decreasing as strain is relieved. It also seems that for strained samples the initial growth of the oxide is mostly inwards, while for the more relaxed samples lateral growth is dominant. Two interesting phenomena were observed for specific samples. For the 420K relieved sample, partial dissociation process is observed on top of the forming oxide, in contrast to the full dissociation observed for the other samples. For the 650K relieved sample, clustering of the adsorbed hydrogen atoms (resulted by water dissociation) on the metal surface is observed, in contrast to the homogeneous dispersion of H, occurring on the surface of all other samples. These two phenomena will further be studied in conjunction with microscopic metallurgical observations.