Surface composition of ion bombarded nickel based alloys

\u3cp\u3e The composition of Ni \u3csub\u3ex\u3c/sub\u3e Pd \u3csub\u3ey\u3c/sub\u3e (x, y = 1, 5) and NiMoRe alloys irradiated by 3 and 4 keV Ar \u3csup\u3e+\u3c/sup\u3e ions along the normal to the sample surface was investigated in situ by low energy ion scattering spectroscopy (LEIS). The analysis was performed using a 5 keV Ne \u3csup\u3e+\u3c/sup\u3e ion beam. It was found that the composition of the topmost layer of bombarded Ni \u3csub\u3ex\u3c/sub\u3e Pd \u3csub\u3ey\u3c/sub\u3e alloys nearly corresponds to the alloy stoichiometry. In the case of irradiation of the NiMoRe alloy, a surface enrichment with the heavier components (Mo and Re) was found. The thermally activated Gibbsian segregation in NiPd and NiPd \u3csub\u3e5\u3c/sub\u3e alloys was also studied using LEIS. \u3c/p\u3

Velocity scaling of ion neutralization in low energy ion scattering

The ion fraction P+ is measured for He+ ions scattered by 129° from a Cu surface. Both the primary energy and the angles of incidence and of exit are varied. From our results we conclude the following: along the incoming and outgoing trajectories, neutralization is due to Auger processes and depends on the normal velocity component v¿ only. At higher energies, additional charge exchange is due to collision induced neutralization and reionization, both depending on the total ion energy only. Also in this regime P+ depends on v¿, but via a two-valued function of the scattering geometry at fixed energ

Comparative analysis of the low-energy He+ ions scattering on Al and Al2O3 surfaces

Using the Anderson–Muda–Newns approach, the neutralization rate and the ion survival probability have been calculated for the large angle scattering of low-energy He+ ions by Al and by Al2O3. The two-band model of the electronic energy spectra is applied for the case of alumina. The electron promotion has been shown to play an important role in the processes of the He+ ions scattering by aluminum and alumina. The experimentally observed absence of the matrix effect is discussed on the basis of the obtained results

The surface of the perovskites LiBaF3 and BaZrO3 studied by low-energy ion scattering

Low‐energy ion scattering has been performed on the normal perovskite BaZrO3 and the only known inverse perovskite LiBaF3. BaO2, ZrO2, and BaF2 were used to quantify the surface coverage of the different constituents. In BaZrO3 the surface coverage showed an enrichment of Ba. Calibration to the metaloxides gave surface coverages of 80% and 20% for Ba and Zr, respectively. For the LiBaF3, also a higher Ba coverage was detected, and, compared to BaF2, found to be 75%. The enrichment of this twelvefold coordinated cation can be explained in terms of a segregation effect due to a lower surface free energy of the bariumoxide and ‐fluoride with respect to the zirconiumoxide or lithiumfluoride. In view of our earlier experiments on spinel structures, preferential exposure of certain planes of the perovskite structure is another possibility, which has yet to be evaluated. The results indicate that a surface terminated by cations in the B‐site, in most cases transition metals, does not present a reliable model when discussing the catalytic activity of the perovskites

New model for ion neutralization at surfaces

We investigate the neutralization of low energy He+ ions in close collisions with metal surface atoms. In order to describe the neutralization process as completely as possible, we consider Auger neutralization (AN), resonant neutralization (RN) and resonant ionization (RI). Our calculation agrees well with experimental data and shows that in some metals (like Pd) AN is the dominant process, whereas in others (like Al) RN and RI contribute significantly for energies above the threshold for reionization

Silica poisoning of oxygen membranes

Perovskite oxide membranes (La0.6Sr0.4Co0.2Fe0.8O3) are used for the separation of oxygen from air. In oxygen permeation experiments these membranes showed a peculiar behavior. Besides poor performance, a characteristic coloring also appeared on the surface of the membranes. In order to understand what was happening to the surface of the membranes, they were analysed with Low-Energy Ion Spectroscopy (LEIS) and X-ray Photoelectron Spectroscopy (XPS). The analyses showed that the surface of the LSCF membrane was covered with a SiO2 layer, which obviously reduced the performance and caused the coloring. It was established that the source of the silicon was siloxane containing grease that was used in the manual valves of the setup. In a new improved permeation setup, where grease-free valves were used, the LSCF membranes showed remarkably better performance. The LEIS measurements showed also that the permeation experiment of 300 h did not affect the surface composition of the membranes. The contamination-free LSCF membranes only showed the presence of La, Sr and O in the outermost atomic layer. The observed absence of Co and Fe suggests that further improvement of the membrane performance is possible

The limiting factor for oxygen exchange at the surface of fuel cell electrolytes

The outermost surface layer of the yttria stabilised zirconia (YSZ) electrolyte in the solid oxide fuel cell plays an important role in the performance of the cell. By combining 16O/18O exchange experiments with quantitative surface analysis by low energy ion scattering spectroscopy (LEIS), the relation between the composition of the outermost atomic layer and the oxygen kinetics at the surface of the electrolyte can be studied. The results suggest a linear relation between an increasing amount of impurity oxides present at the surface and a decreasing oxygen exchange. Ceramics invariably contain glassy impurities, which segregate to the grain boundaries during sintering. Our results, however, show that the accumulation is restricted to the outermost atomic layer and proceeds until complete coverage is obtained. This observation underlines the importance of the used surface analysis technique. The strong accumulation of the impurities at the surface is observed even in the purest YSZ materials available. A decrease of the total bulk impurity concentration by a factor of 10–100 is necessary to ensure that the YSZ surface cannot be covered completely by impurities. The amount of exchanged oxygen at the outermost surface layer reduces from 50% at a clean yttria stabilised zirconia surface to zero at a surface completely covered by impurity oxides. Although the oxygen exchange experiments pertain to a temperature of 500 °C it is believed that the impurity oxides also have a strong influence at higher temperatures

A surface science study of model catalysts. 1. Quantitative surface analysis of wet-chemically prepared Cu/SiO\u3csub\u3e2\u3c/sub\u3e model catalysts

\u3cp\u3eCu/SiO\u3csub\u3e2\u3c/sub\u3e model catalysts containing nanometer-sized Cu particles on a flat silica model support were wetchemically prepared and characterized in detail by a variety of surface science techniques. The particle size and shape, particle number density, metal surface coverage, total metal loading, and oxidation state of the particles were determined by ultrahigh vacuum atomic force microscopy, electron microscopy, low-energy ion scattering, Rutherford backscattering spectrometry, and X-ray photoelectron spectroscopy. Deposition of a Cu precursor on a flat Si wafer with a SiO\u3csub\u3e2\u3c/sub\u3e top layer by spin-coating was followed by calcination in air at 450 °C. This preparation method produces both homogeneously distributed hemispherical CuO particles with an average height of 8 nm and highly dispersed oxidic Cu species. Subsequent reduction in hydrogen at 250 °C results in metallic and more rounded Cu particles with an average height of 8 nm.\u3c/p\u3

New insights into the nature of the active phase of VPO catalysts : a quantitative static LEIS study

The atomic composition of the outermost atomic layer of equilibrated VPO catalysts was studied with low-energy ion scattering. Using external reference samples, absolute numbers for the atomic densities of both P and V could be determined. VPO catalysts prepared in either aqueous or organic media showed large amounts of carbonaceous surface species. From catalysts prepared in an aqueous medium the carbonaceous species could be removed using a mild oxidation treatment without significantly changing the average valence of the vanadium atoms in the catalyst as evidenced from XPS. After this cleaning treatment the surface concentration of vanadium (atoms/cm2) agrees well with that expected for a vanadyl pyrophosphate structure. However, the surface phosphorus concentration is twice as high as that in vanadyl pyrophosphate, leading to a surface P/V ratio of 2.0±0.2. This shows that VPO catalysts may be terminated by a distorted vanadyl pyrophosphate structure, where the excess amount of phosphorus is positioned between the vanadyl units and the phosphate groups. The catalyst surface is also compared to VOPO4 phases. A significant contribution of a phase such as a -VOPO4 could explain the observed surface P/V ratio of 2.0