XPS study of hydrogen and oxygen interactions on the surface of the NiZr intermetallic compound

摘要

The cathodic discharge of hydrogen on the surface of massive specimens of NiZr intermetallic compound readily forms the trihydride NiZrH_3 which exhibits the same crystallographic parameters as the trihydride obtained by gas phase charging. Moreover microstructural studies of the first stage of the hydriding process during cathodic charging of massive samples allow to follow the influence of the microstructure of NiZr on the germination of cracks and on the first stage of the decrepitation associated with H absorption. These results show that the NiZr microstructure could strongly affect both the hydriding kinetic, the hydride stability and further the amount of hydrogen stored in the intermetallic compound. The XPS analysis of the surface of NiZr after cathodic charging shows the coexistence of zirconium core levels characteristic of Zr-H and Zr-O bonds. The hydrogen absorption affects the electronic properties of Zr atoms in the intermetallic compound in a similar way of what is observed in pure Zr: an H-induced band is observed about 6 eV below the Fermi level together with a chemical shift of the Zr core levels characteristic of the hydride formation. The presence of oxygen on the surface decreases the intensity of the Ni2p core levels whereas the intensity drop of the satellite located at 5 eV (below Ni 2p) is associated with an H-induced filling of the 3d band of Ni atoms in NiZr. The cosegregation of oxygen and zirconium on the surface is clearly evidenced by XPS in the 20-380 ℃ temperature range. A preferential Zr enrichment of the surface is observed at higher temperature; the occupancy fraction of the surface sites by Zr atoms is as large as 98% at 600 ℃. This is presumably a consequence of both the oxygen dissolution in NiZr and a preferential segregation of Zr atoms on the surface.