Ugo Bardi

Initial stages of oxidation of the Ni3Al alloy: structure and composition of the aluminum oxide overlayer studied by XPS, LEIS and LEED

The oxidation of the Ni3Al alloy was studied by XPS, LEED and LEIS. At pressures of oxygen under approximately 5 × 10−5 Pa and at temperatures higher than approximately 700 K, a layer of Al2O3 grows on the surface forming islands of uniform thickness of the order of 5 A. The structure of these islands is related to that of γ-alumina.

XRD and XPS study on reactive plasma sprayed titanium–titanium nitride coatings

Reactive plasma spraying of titanium in a nitrogen-containing plasma gas allows the formation of hard titanium composite coatings reinforced by in situ formed titanium nitrides (TiN and Ti2N). In the present study, the formation of titanium nitrides during the reactive spraying of titanium was studied by means of two different analysis techniques: X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The diffusion and solidification sequence predicted by the Ti-N phase diagram was consistent with the experimental results. The nitrogen retained in the coating was found mainly as titanium nitrides and not as interstitial nitrogen in the titanium lattice. It is hypothesised that the formation and direct solidification of TiN occurs during the flight of the sprayed particles. The presence of solid TiN constrains the thermal contraction of titanium splats upon solidification. The observed cell distortion of the residual α-Ti is thus attributed to tensile stresses arising from the solidification process.

The mineral economy: a model for the shape of oil production curves

Abstract The production and the depletion of mineral resources, and especially oil and fossil fuels, has been an object of extensive predictive modeling. These predictions are often derived from Hubberts model which is based on the fitting of the experimental data to a symmetric, bell-shaped curve. Although this model describes several historical cases, in particular, crude oil production in the lower 48 US states, not all theoretical models for the “mineral economy” are based on symmetric curves. Also, not much attention has been dedicated so far to the mechanisms which lead to such a behavior. In particular, scarce attention has been dedicated to the factors which may make the production curve asymmetric, e.g. a decline in production more abrupt than the growth. In the present paper, the author uses a stochastic model to examine factors affecting these phenomena. The results of the simulations indicate that the production curves of a non-renewable resource may be asymmetric in dependence on factors such as the search strategy or the presence of technological improvements. Considering worldwide oil production, the simulations indicate that the after-peak downward slope might turn out to be considerably more steep than the upward slope, something that could have unpleasant effects on the economy.

A comparative study of high velocity oxygen fuel, vacuum plasma spray, and axial plasma spray for the deposition of CoNiCrAlY bond coat alloy

In the aerospace field as well as in the stationary gas turbine field, thermally sprayed coatings are used to improve the surface properties of nickel-super-alloys materials. Coatings are commonly used as bond coat and antioxidation materials (mainly MCrAlY alloys) and as thermal barrier coatings (mainly yttria partially stabilized zirconia). The purpose of the current study was to assess the properties of thermally sprayed bond coat CoNiCrAlY alloys comparing the performance of three different techniques: vacuum plasma spray (VPS), high velocity oxygen fuel (HVOF), and axial plasma spray (AxPS). The quality of the deposited films has been assessed and compared from the point of view of microstructural (porosity, oxide concentration, unmelted particles presence) and mechanical (hardness) characteristics. The surface composition and morphology of the coatings were also determined. Specific efficiency tests were performed for the three examined technologies. The highest quality coatings are obtained by VPS, but also high velocity oxygen fuel and AxPS sprayed films have interesting properties, which can make their use interesting for some applications.

Surface alloying at the SnPt(111) interface: a study by x-ray photoelectron diffraction

Abstract The formation of surface alloys obtained by annealing ultrathin films of Sn deposited on the Pt(111) surface was investigated by low energy electron diffraction and X-ray photoelectron diffraction. The Pt(111)(2 × 2)-Sn and Pt(111)(√3 × √3)R30°-Sn form after deposition of amounts of Sn in the range from 0.2 to 1 ML and subsequent annealing at 1000 K are found by XPD to be single atomic layer surface alloys. Depositing 4–5 ML of Sn and annealing at 400–600 K produces an ordered phase exhibiting a (2 × 2) LEED pattern. This phase is a multilayer surface alloy and it has the same structure as the bulk Pt 3 Sn alloy.

LEED crystallographic investigation of ultrathin films formed by deposition of Sn on the Pt(111) surface

Abstract Deposition of Sn on the Pt(111) surface followed by annealing at 1000 K leads to the formation of ordered phases showing (2 × 2 and ( 3 × 3 ) R 30° LEED patterns, depending on the surface coverage of Sn. Both these phases were studied by LEED dynamical analysis. The best agreement between experimental and calculated I–V curves was obtained by means of models based on the formation of mixed Pt-Sn layers on the surface where Pt and Sn atoms are nearly coplanar with a slight upward buckling of Sn atoms. The structures of these phases are similar to those already observed for the Pt3Sn(111) surface.

An unusual common ion effect promotes dissolution of metal salts in room-temperature ionic liquids: a strategy to obtain ionic liquids having organic–inorganic mixed cations

A simple strategy has been reported to prepare new ionic liquids with binary systems of organic–inorganic cations exploiting the common ion effect, i.e. dissolving metal salts with organic or inorganic anions (bistriflylimide or nitrate) in ionic liquids bearing the same anions. The resulting concentrated solutions of metal cations in ionic environments, which may have great potential for electrochemical processes, have been characterized by X-Ray photoelectron spectroscopy (XPS) and electrospray ionization mass spectrometry (ESI-MS)

Study of the reconstructed (001) surface of the Pt80Co20 alloy

The structure of the clean, (001) oriented single crystal surface of a platinum alloy containing 20 % at of cobalt (‘Pt 80 Co 20 ’) was studied. He + Low Energy Ion Scattering Spectroscopy data indicated that the outermost surface plane of the alloy is composed of virtually pure platinum. Low Energy Electron Diffraction showed a complex pattern which can be interpreted as a c(58 × 22) coincidence unit mesh. A model of the surface can be proposed in terms of a reconstructed surface layer which forms a compact ‘quasi-hexagonal’ plane. The structure and bond lengths in the outermost plane of Pt 80 Co 20 (001) are nearly identical to those of the reconstructed Pt(001) surface.

The surface structure and composition of and oriented single crystals of the ordered alloy Pt3Sn

The surface composition and structure of 〈111〉 and 〈100〉 oriented single crystals of the ordered alloy Pt3Sn (Ll2 or Cu3Au-type) were determined using a combination of low-energy electron diffraction (LEED) and low-energy ion scattering spectroscopy (LEISS). The clean annealed surfaces displayed p(2 × 2) and c(2 × 2) LEED patterns, respectively, and the Sn/Pt LEISS intensity ratios at normal incidence were consistent with the surface compositions expected for bulk termination surface structures. In the case of the 〈100〉 crystal, preferential termination in the mixed (50% Sn) layer was indicated, which appears to be the consequence of the thermodynamic tendency for Sn to be at the surface. A strong variation in Sn/Pt LEISS intensity ratios with incidence angle was observed on both surfaces, indicating an outward displacement of tin atoms from the plane of platinum atoms.

THE SURFACE STRUCTURE AND COMPOSITION OF THE LOW INDEX SINGLE CRYSTAL FACES OF THE ORDERED ALLOY Pt3Sn

The surface composition and structure of 〈111〉, 〈100〉, and 〈110〉 oriented single crystals of the ordered alloy Pt3Sn (Ll2 or Cu3Au-type) were determined using the combination of low energy electron diffraction (LEED) and low energy ion scattering spectroscopy (LEISS). The clean annealed surfaces displayed LEED patterns and Sn/Pt LEISS intensity ratios consistent with the surface structures expected for bulk termination. In the case of the 〈100〉 and 〈110〉 crystals, preferential termination in the mixed (50% Sn) layer was indicated, suggesting this termination to be the consequence of a thermodynamic preference for tin to be at the surface.