JRP CALL information
Supported By

European Commission

Short description of the work
1. Two U-based bcc alloys, U0.85Mo0.15 and U0.80Zr0.20, in the form of splats provided by CUNI, were subjected to a surface science study using photoelectron spectroscopy (XPS and UPS). Although the Mo-alloy has its surface covered, as expected, by oxidized Uranium, in the case of Zr alloy the U metal is invisible, and the surface is covered by Zr oxide. Using in situ sputtering by Ar ions, we succeeded to remove the surface layer and reveal the bulk of the alloy. Subsequent study of isochronal annealing (up to 700 oC) and/or exposure to O2 mapped conditions under which the surface segregation and oxidation proceeds under well defined UHV conditions.
2. Using sputter deposition from elemental U and Mo targets we succeeded to synthesize clean (oxygen-free) films of U-Mo alloys with variable stoichiometry. Electronic structure study using U-4f and valence-band spectra did not show any dramatic development (e.g. localization, electronic transfer) of the U-5f states in the course of Mo dilution. This is consistent with the known insensitivity of magnetic susceptibility to Mo concentration, and points to a significant hybridization of the U-5f and Mo electronic states, which prevents to 5f localization happening due to increasing U-U spacing. The 4f core-level spectra are rather similar for pure U (assumed to be in the alpha-U state) and Mo alloyed presumed gamma-U films, fine differences will be subject of detailed analysis. The spectra of corresponding films and splat mentioned above are in a good agreement.

3. As next, we undertook synthesis by sputter deposition of UH3 and UH3 alloyed with Mo by means of reactive sputtering. This is a tricky part, hydrogen in UHV equipment leads to desorption of impurities from the walls, and films tend to oxidation with increasing H pressure. On the other hand, one needs high H2 pressure in the sputter gas, as binding energies of H in the U-hydride are small. After some experimenting with gas purification and modification of sputtering conditions we started to obtain a robust state, which can be associated with UH3, although the valence –band spectra are somewhat different from those described in literature. The 4f-core level spectra are practically identical to the published ones. Several thick films were deposited, capped by Mo, and transported to the lab in Prague, so as to characterize them by XRD (GLAXRD) and magnetization study to check the presence of regular UH3. Doping U by Mo leads surprizingly to disappearance of hydrogen, spectroscopy points to pure metal. It may be that we are hitting the border of thermodynamic stability. Finally testing what is the critical Mo concentration, we succeeded to obtain similar valence band spectra as in UH3 (amount of hydrogen cannot be really quantified by XPS/UPS) for 10 and 20 at. % Mo, the question is whether Mo was really embedded into the structure. That would need to make thicker films for XRD and magnetometry studies. There was not sufficient time for that in the present run.
4. Surface science study of the bulk hydride (UH3)0.85Mo0.15 revealed a thick surface film of UO2. Its removal required a prolonged sputtering, which however clearly led to desorption of hydrogen from the surface.
5. TEM study of (UH3)0.85Mo0.15 and (UH3)0.80Zr0.20 had to be postponed, as the glove boxes at TEM were not operational due to a defect in N2 supply. It will be presumably still done in December 2015, albeit users will not be present in the lab.

Main visitor contact data
Name: Dr. Ladislav Havela
Organisation: Charles University, Prague

JRP Identification
JRP nr: TALI-C06-06
JRP title: Surface science of gamma-U alloys and their hydrides
JRP scope: Scope 3: Actinide materials

Visited Associated Pooled Facility
Visited APF during the stay: JRC-ITU - Labs & Hot-cells
Name of the APF Contact Person: Dr. Thomas Gouder

Other APF and organisation involved in the JRP
Other organisations involved:
Other APF involved in the project:

Description of the work done at the associated pooled facility
Start date of the stay: 10/29/2015
End date of the stay: 12/2/2015
Quantity of access: 25
Access Unit: Days

Other APF visitors of the JRP during the stay
Visitor 2: Mykhaylo Paukov