Electrochemical cleaning of metallic surfaces

The aim of this research is to investigate the feasibility of depleting or modifying solid steel’s surface layer by electrochemically affecting the non-metallic inclusions, such as oxides and sulphides, thus providing zero calibration and modified surface properties. It has been shown that this is possible by decomposing or modifying non-metallic inclusions. This is achieved by the application of cathodic refining of the surface of solid steel, utilising molten salt as the electrolyte. The thermodynamic and electrochemical basis of the experimental results is presented, along with SEM/EDX information. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Emissvity of metallic microcontoured surfaces

In this paper, we study the diffraction of electromagnetic radiation by a periodic micro-rough surface separating vacuum from a metal with a finite conductivity. We submit the integral method to the surface impedance boundary condition. Thus the numerical implementation is greatly reduced. We compare the numerical emissivities obtained by this approach to those we have calculated through the rigorous multilayer modal method. This enables us to show that the mentioned approximate method has two regions of validity: one corresponding to fairly flat surfaces and the other to very deep surfaces. It is well known that both the Kirchhoff approximation and the constant flat boundary impedance approximation are also valid for fairly flat surfaces. Our investigation aims also to establish whether these two approximate methods lead to the same results, and whether the integral method submitted to the surface impedance condition has a larger domain of validity. Concerning deep surfaces with a period smaller than the wavelength, we introduce the homogenization process in order to study its accuracy.Finally, this work permitted to identify three different regimes depending on the surface slopes: the simple scattering regime, the homogenization regime and the intermediate regime. For the latter, if the period is in the order of the wavelength, then we will show that the emissivity can be exalted.     

Interactions between charges and metallic surfaces

We report numerical results for the potential of interaction between two charged particles on a metallic surface. The density functional formalism approach is utilized. The interaction is cast in the form of an image interaction with a correction parameter λ, which depends on the positions of the particles. We find that for molecular distances, λ for the metal mediated interaction, between two charges is approximately given by the average of λ for the self interaction of there charges with the surface. However, the derivatives of the two charges λ, with respect to the particle positions (needed for the evaluation of forces and force constants), cannot be simply expressed as a function of single charge parameters.     

The Rashba-effect at metallic surfaces

Surface states, that show a k-dependent splitting resulting from spin-orbit coupling, show wide similarities to a two-dimensional electron gas in semiconductor heterostructures, where the Rashba-effect lifts the spin-degeneracy of the bands and allows spin-manipulation by an electric field. We discuss the conditions for such a Rashba-effect at metallic surfaces by comparing ab initio calculations for Au(1 1 1), Ag(1 1 1), and Lu(0 0 0 1) and investigate the influence of electric and magnetic (exchange) fields on these surface states.     

The jellium theory of metallic surfaces

A theory for the interaction of a half-space jellium with charged particles is considered within the context of the conventional theory of quantised fields. This leads to conclusions which contradict the results of the approaches that employ the quasistatic approximation for solving the same problem. In particular it is shown that the interaction due to the scalar surface modes is far less dominant at large distances than is currently believed and the volume modes are shown to be chiefly responsible for the potential in this limit. This is in qualitative agreement with the conclusions recently obtained by Ekardt. The total potential is given by the sum of contribution from the two types of mode and is formally applicable for all Z > 0.     

Magnetostatic field noise near metallic surfaces

We develop an effective low-frequency theory of the electromagnetic field in equilibrium with thermal objects. The aim is to compute thermal magnetic noise spectra close to metallic microstructures. We focus on the limit where the material response is characterised by the electric conductivity. At the boundary between empty space and metallic microstructures, a large jump occurs in the dielectric function which leads to a partial screening of low-frequency magnetic fields generated by thermal current fluctuations. We resolve a discrepancy between two approaches used in the past to compute magnetic field noise spectra close to microstructured materials.     

Scattering of low-energy atoms by metallic surfaces

The problem of a neutral low-energy atom impinging on a well-defined metallic surface is approached from first principles. The solid and its potential energy of interaction with the incident atom is treated in the most general way, but under the following assumptions: (a) the conduction electrons interact adiabatically with the lattice ions and the gas atom; (b) no chemical reactions occur; (c) the one-phonon approximation is valid. The scattering amplitudes for surface and bulk mode excitations are obtained in terms of the dynamical properties of the metallic surface. Direct collisions of the incident atoms with the lattice ions are shown to give a negligible contribution to the scattering. The most important contribution comes from the interaction of the gas atom with the surface conduction electrons; the excitation of lattice vibrational modes occurs through the electron-phonon term of the Hamiltonian. The general expressions for the scattering amplitudes obtained show that the scattering is incoherent. With further assumptions one obtains a separation of the scattering amplitude into a coherent and incoherent part.     

Subsurface bonding of hydrogen to metallic surfaces

It is shown that the energy of solution of a light interstitial impurity increases as it approaches the surface of the crystal. Just below the first ionic layer this increment is considerable (0.1—1 eV) and leads to a new kind of bond (subsurface bond) which may be stronger than chemisorption. Calculations for Ni, Pd, Pt and Nb show good agreement with experiment.     

Near-ionisation of neutral hydrogen outside metallic surfaces

The problem of neutral hydrogen outside a perfectly conducting plane surface is re-examined in an approximation in which the interaction potentials, derived from quantum electrodynamics, take a pure image form and the distance R of the proton from the surface is treated as a parameter rather than a dynamical variable. The numerically evaluated eigenfunctions and eigenvalues vary from the normal hydrogenic form for R → ∞ to a form in which the electron is considerably delocalised with a smaller binding energy. The results are expected to hold only for R > R_c ～ 3 Bohr radii, but the deviations from the hydrogen results are much greater than could be found by perturbation theory.     

Hurst exponent analysis of moving metallic surfaces

We report on the application of Hurst exponent analysis to digital speckle patterns for investigating moving rough surfaces in the presence of defects. Digital speckle patterns were generated by recording the scattered light from moving surfaces illuminated by a laser beam. It was found that it is possible to identify the presence of the defects by means of the variation of the Hurst exponent along the sample.     

Nanoporous surfaces via impact of molten metallic droplets

Here we describe a new pathway for the production of nanoporous surfaces, by recourse to molten droplet impact and solidification. The nanopores in this case are frozen in bubbles that nucleate in the melt due to gas supersaturation within 100 nanoseconds of impact. Initial observations and previous analysis are presented, as well as ongoing work to control or pattern porosity via process variation and substrate pre-treatment. This method is presumably not limited in material, and has potential to create large area, functional surfaces.     

Calculation of positronium emission fraction from metallic surfaces

Abstract

Theoretical calculations of positronium emission fraction are compared with experiments. The results show that positronium emission fraction is quite sensitive to screening effects. Thus, positronium formation will be a very good example for testing dynamical screening effects. An experimental angle-resolved energy spectrum of positronium is required for more detailed discussion.     

Swelling of metallic surfaces irradiated by helium ions

At doses close to threshold for exfoliation or bubble formation the surfaces have been investigated by SEM and, after isolation of surface layers, also by RBS. Swelling as a function of energy and material has been determined. The data are interpreted in the form of volume occupied by a helium atom. We find an indication for a volume decrease with increasing energy. A special example of blister formation is also discussed, where we find evidence for a non-uniform fracture plane.     

Time-evolving statistics of cavitation damage on metallic surfaces

The statistics of surface damage on polycrystalline aluminium plates caused by acoustic cavitation is studied experimentally as a function of time. Cavitation is shown to produce a uniform distribution of crater-like holes with different depth, area and eccentricity. Most notably, the size distribution of such craters evolves with time from a gamma function into a power law. By contrast, on the surface of a martensitic Cu–Ni–Al crystal cavitation damage generates ramified patterns, reminiscent of a fractal object.     

Structural changes of metallic surfaces induced by ultrasound

The effect of ultrasounds on the surface of metallic plates with cubic faced centered (Al, Ag, Cu) or hexagonal (Zn) structure was studied. Treated surfaces were strongly attacked: a micrometric roughness and a superficial oxidation (characterized by roughness measurements and scanning electron microscopy) were induced. X-ray diffraction analyses showed a preferential reorientation of the maximum density planes for both crystallographic structures.     

Footprint organization of chiral molecules on metallic surfaces

We study the behavior of chiral molecules adsorbed on clean metallic surfaces using a lattice-gas model and Monte Carlo simulation. The aim is to model and simulate the structure (footprints and organization) formed by molecules on the surface as they adsorb. The model, which is applicable to chiral species like S- and R-alanine, or similar, discloses the conditions to generate different ordered phases that have been observed in experiments by other authors.In our model, each enantiomer may adsorb in two different configurations (species) and several effects are taken into account: inhibition, blockage of neighboring adsorptive sites (steric effects) and promotion of sites representing, in some sense, modifications in the surface properties due to molecule-surface interactions. These adsorption rules are inspired by the enantiomeric character of adsorbed species. We perform a systematic study of the different phases formed in order to qualitatively understand the mechanism for the formation of adsorbate structures experimentally found by other authors.