An application of the optics of oil films to the study of engineering surfaces

The flow of an oil film draining down a rough surface is studied, to show that as a result of the surface tension, only the long wave components of the roughness give rise to corrugation of the film surface. The oil thus serves two purposes, to provide a reflecting surface and to provide a low-pass spatial filter for the surface roughness spectrum. Observation in out-of-focus reflected light shows the film corrugations clearly, thus giving selective information about the rough surface underneath.     

Influence of the viscosity and the substrate on the surface hydrophobicity of polyurethane coatings

Tailor-made polyurethane (PU) dispersions were synthesized as coatings for paperboard for dry food packaging. For this purpose a low moisture-vapor transmission rate and a high surface hydrophobicity are desirable characteristics, which are both met by PU. However, it was found that the surface hydrophobicity of water-borne PU dispersions depends strongly on the viscosity of the dispersion. This dependency was studied by static contact angle measurements (SCA) as well as a novel technique using digital pulsed-force mode atomic force microscopy (DPFM-AFM). Comparison of the results validated that DPFM-AFM is a valuable tool to characterize the surface hydrophilicity. Both techniques confirmed that the surface hydrophobicity varies with the viscosity and that an optimum viscosity for the PU coating with a maximum surface hydrophobicity can consequently be determined. It was found that both lower as well as higher viscosities led to a less hydrophobic surface.     

Influence of the state of the bulk of the sample on the formation and thermal stability of the surface silicide on W(100)

The formation and destruction of the surface silicide on W(100) after cleaning of the sample surface and bulk in various regimes is studied by high-resolution Auger electron spectroscopy. It is shown that the cleanness of the bulk has practically no influence on the laws governing the formation of the surface silicide when Si atoms are adsorbed on a heated W surface and that almost up to completion of its formation all the silicon atoms impinging on the surface, from the very first, remain on it and are incorporated into the surface silicide. The destruction of the surface silicide depends in a definite manner on the state of the bulk, and at T=1400 K it is apparently limited in the early stages by the passage of Si atoms from the surface to the subsurface layer and in subsequent stages by the diffusion of silicon within the substrate. The bulk silicon density that limits the destruction of the surface silicide is estimated. Zh. Tekh. Fiz. 67, 137–140 (July 1997)     

Effect of the surface-modifying macromolecules on the duration of the surface functionalization

Functionalization of polystyrene films by the preferential surface enrichment of surface-modifying macromolecules (SMM) to achieve a hydrophilic surface with long effective duration is described. The comb-like amphiphilic copolymers (PKG-g-PS) based on styrene-maleic anhydride copolynier (SMA) backbone was synthesized by esterification of SMA with poly(ethylene glycol) (PEG). When PEG-g-PS was melt blended with polystyrene, the preferential surface enrichment of PEG-g-PS was much evident resulting in the large increase of the surface polarity. The effective duration of the surface functionalizatoin was also hugely extended as SMMs were added into the blends. Furthermore, more polyether chain segments on PEG-g-PS could selectively migrate to the surface by the inducement of polar solvent. Compared with the PEG-g-PS/PS and PEG/PEG-g-PS/PS blends, the surface polarity and the effective duration of surface modification both increased greatly when PEG-g-PS was used as the compatibilizer of PEG/PS blends. It was an effective solution to balance the conflict between the duration and efficiency of the surface-modifying additives.     

Reconstruction dependence of the etching and passivation of the GaAs(001) surface

The microscopic nature of the selective interaction of iodine with an As- and Ga-stabilized GaAs(001) surface has been investigated by the photoelectron emission and ab initio calculations. The adsorption of iodine on the Ga-stabilized (4 × 2)/c(8 × 2) surface leads to the formation of the prevailing chemical bond with gallium atoms; to a significant redistribution of the electron density between the surface Ga and As atoms; and, as a result, to a decrease in their binding energy. Iodine on the As-stabilized (2 × 4)/c(2 × 8) surface forms a bond predominantly with surface arsenic atoms. Such a selective interaction of iodine with the reconstructed surfaces gives rise to the etching of the Ga-stabilized surface and the passivation of the As-stabilized surface; this explains the layer-by-layer (“digital”) etching of GaAs(001) controlled by the reconstruction transitions on this surface.     

Effect of the cathode spots of a vacuum arc on the properties of the surface layer of structural materials

The application of vacuum-arc discharge provides great opportunities for metallic-surface cleaning and the development of products with the required functional surface properties. The effect of vacuumarc treatment on the roughness of the surface, its mechanical properties, and the structure and composition of the surface layer is studied. The processing and cleaning of the surface of steel samples by cathode spots of a vacuum-arc discharge form a structurally changed layer with a thickness of up to 10 μm and a microhardness exceeding the initial one by 1.4–1.5 times. The deposition of 20% solution of KOH and NaOH alkalis (substances which reduce the electronic work function of the scale surface) on the processed steel surface reduces the specific power inputs of vacuum-arc cleaning by a factor of 1.5–2.4. The depletion of the surface layer of low-grade steels by impurity elements enhances its anticorrosion properties.     

Contribution of the surface state to the observation of the surface Kondo resonance

Tunneling spectra obtained on and near Co atoms adsorbed on Ag(111) show at 5 to 6 K a Kondo resonance that appears as a characteristic dip around the Fermi energy. The feature is present up to 1.5 nm around Co atoms adsorbed on terraces with the surface state onset in the occupied region of the density of states. On a narrow terrace, where the surface state onset lies in the unoccupied region of the density of states, it is only present up to 0.5 nm. This difference demonstrates directly the importance of the surface state electrons in the observation of the surface Kondo resonance.     

Non-linear analysis of the surface profile resulting from the one-dimensional Rosensweig Instability

The instability of the free surface of a ferrofluid subject to an external magnetic field normal to the undisturbed surface is investigated in one dimension. Using a pertubation analysis up to fifth order in the surface deflection the character of the bifurcation is elucidated. Moreover, from the amplitudes of the higher harmonics in the pertubation ansatz the approximate static profile of the surface in the non-linear regime is obtained.     

The inversion of the dielectric function in the presence of an interface

For a system containing a metal-vacuum interface an exact formal solution is presented for the inverse RPA dielectric function. The solution is valid for an arbitrary shape of the electron density profile at the surface. It contains(a) the inverse dielectric Lindhard function representing the bulk screening effects and (b) surface terms which describe the modification of the screening propertles near the interface. From the pole contribution of the surface term an exact equation for the surface plasmon dispersion is derived. In order to check the applicability of the general result for ?^?1, the surface plasmon dispersion and the screening length of a homogenous field applied perpendicular to the surface are briefly discussed by adopting the infinite barrier model.     

A note on the electron density profile at the surface of metals

A two step model of electron density profile at the surface of a metal is in use for the study of surface plasmas. The same model is used here to calculate surface energy and work function. The parameters in the model are determined by minimizing surface energy. It is shown that the model is a reasonable approximation to the true profile only in low density metals.     

On the possible application of LEED to the study of surface potential barriers

The structure of surface barrier resonances in LEED is shown to be strongly dependent upon the shape and position of the potential energy barrier at the crystal surface. This suggests that systematic high-resolution measurements of LEED intensities in the energy range of surface barrier resonances may be a useful tool for studies of surface barrier structure. Such measurements should be performed at several angles of incidence and at a variety of azimuths with primary beams of very low energy (<30 eV).Computations are presented for a model crystal surface of Cu(001) to illustrate these conclusions. The results are in agreement with McRae's model for the surface barrier resonance in LEED.     

Calculation of the surface energy of bcc-metals with the empirical electron theory

We have used the dangling bond analysis method (DBAM) based on the empirical electron theory (EET) to establish a database of surface energy for low index surfaces of the bcc-metals such as V, Cr, Fe, Nb, Mo, Ta, and W. And a brief introduction of the new surface energy models will be presented in this paper. Under the first-order approximation the calculated results are in agreement with experimental and other theoretical values. And the calculated surface energy shows a strong anisotropy. As we predicted, the surface energy of the close-packed (1 1 0) is the lowest one of all index surfaces. It is also found that the dangling bond electron density and the spatial distribution of covalent bonds have a great influence on surface energy of various index surfaces. The new calculation method for the research of surface energy provides a good basis for models of surface science phenomena, and the model may be extended to the surface energy estimation of more metals, alloys, ceramics, and so on, since abundant information about the valence electronic structure (VES) can be generated from EET.     

Investigation of the 4H-SiC surface

The silicon carbide (SiC) surface is more complex than that of silicon and can be carbon-terminated or silicon-terminated, and can exist as several reconstructions. Investigations of the surface structure as a function of temperature, under ultrahigh vacuum (UHV) conditions using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED), are presented. The 4H-SiC surface can be passivated using a silicon deposition/evaporation technique to reconstruct the surface. This has a significant effect on the electrical behaviour of metal contacts to the silicon carbide surface, critical in any electronic device. Atomic resolution STM studies of the 4H-SiC surface have revealed step features and micropipe defects in unprecedented detail. STM has also been used to image clusters of metal deposited on the 4H-SiC surface. The effect of annealing on the behaviour of these nickel clusters is also presented. The surface of the silicon carbide is extremely important in the fabrication of silicon carbide electronic devices and this paper presents a discussion of the SiC surface with particular reference to its impact on SiC device applications in power electronics.     

On the damping of unsteady flow by compliant boundaries

This paper concerns unsteady motion of an incompressible inviscid fluid near a flexible surface which, in responding to the surface pressure field, absorbs energy. The modification of the flow consequent on energy removal at the boundaries is examined. Energy absorption always occurs when the mechanical surface properties include an element of dissipation. But surface dissipation is not essential; surface waves have a similar property. Unsteady fluid induced forces excite surface waves which carry with them energy that must have originated in the flow. The question of how flow characteristics change as energy is gradually given up to the boundary is examined through a particular model problem from which it becomes evident that surface motion draws vorticity towards the surface. The model chosen is that of a rectilinear vortex adjacent to a weakly responding boundary. Surface motion induces a velocity perturbation which is shown to move the vortex towards the surface whenever the fluid gives energy to that surface.     

Theoretical studies of the bonding of oxygen to models of the (100) surface of nickel

Electronic wavefunctions have been obtained as a function of geometry for an O atom bonded to Ni clusters (consisting of one to five atoms) designed to model bonding to the (100) surface of Ni. Electron correlation effects were included using the generalized valence bond and configuration interaction methods. For the (100) surface, we find that the charge distribution for the full O overlay er is consistent with taking a positively charged cluster. The four surface atoms in the surface unit cell and the atom beneath the surface are important in determining the geometry, leading to a Ni⁺₅O cluster as the model for the (100) surface. The optimum oxygen position with this model is 0.96 Å above the surface (four-fold coordinate site) in good agreement with the value (0.90 ± 0.10 Å) from dynamic LEED intensity analysis. The atom beneath the surface allows important polarization effects for the positively charged cluster. The bonding to the surface involves bridging two diagonal surface Ni atoms. There is an O(2pπ) pair which overlaps the other diagonal pair of Ni atoms leading to nonbonded repulsions which increase the distance above the surface. There are two equivalent such structures, the resonance leading to a c(2 × 2) structure for the O overlayer.     

北极典型冰下声信道建模及特性

将Burke-Twersky (BT)散射模型与射线理论相结合研究北极典型冰下的水声信道特性。BT模型将极地冰水界面的冰脊视为随机分布在自由表面的半椭圆柱。首先根据BT模型分别对高频和低频情况下的冰面反射系数取近似,计算不同频率的冰面反射系数。然后结合射线理论计算冰下声场并分析冰下信道特性,并与相同条件下绝对软界面的水声信道进行对比研究。结果显示,由于冰界面的存在,冰水界面与绝对软界面相比,冰面反射系数较小,使得部分声线不会传播很远,且随频率的增加衰减越发严重,因此不利于声信号远距离传播;此外在信道结构上,由于冰层反射系数较小,冰下信道多径相较于无冰的水-空气界面其多途现象不明显。研究结果对认知极地冰下水声信道特性以及开展极地水声系统性能预报具有一定意义。      

On the effect of microscopic roughness of the metal surface on the surface plasmon dispersion relation and photoemission

The effect of microscopic surface roughness on the opical characteristics of metal is considered. The field near the rough surface has been calculated on the basis of integral equations containing only smoothly varying field components. It appears that the influence of surface plasmons can cause, as a result of drastic enhancement of the field at the surface, an increase by several orders in such observed surface effects as photoemission and Raman scattering by adsorbed molecules. The results of calculation are compared with the experimental data obtained from the study of photoemission from silver with rough surface. Expressions have been derived for the dispersion relation for surface plasmons on a rough surface.     

On the determination of Cauchy surfaces from intrinsic properties

We consider the problem of determining from intrinsic properties whether or not a given spacelike surface is a Cauchy surface. We present three results relevant to this question. First, we derive necessary and sufficient conditions for a compact surface to be a Cauchy surface in a spacetime which admits one. Second, we show that for a non-compact surface it is impossible to determine whether or not it is a Cauchy surface without imposing some restriction on the entire spacetime. Third, we derive conditions for an asymptotically flat surface to be a Cauchy surface by imposing the global condition that it be imbedded in a weakly asymptotically simple and empty spacetime.This research was supported in part by the National Science Foundation grants PHY 70-022077 and PHY 76-20029 as well as the National Aeronautics and Space Administration grant NGR 21-002-010National Science Foundation Pre-doctoral Fellow     

On the role of the surface currents in conducting targets in the formation of the diffraction and transition radiation of relativistic electrons

The surface current method and the pseudophoton method are widely used in the study of the interaction between relativistic electrons and matter. A simple analysis reveals the contradictions between these methods as to the excitation of the currents on the surface of the conducting target. To solve this contradiction, the surface currents on the downstream and upstream surfaces of the conducting target were measured in the geometry of the diffraction radiation. The surface currents were experimentally recorded on the upstream target surface, from which the backward diffraction radiation is generated. At the same time, the surface currents are absent on the downstream target surface, which is conventionally considered as a source of diffraction radiation in the direction of the motion of electrons. Analogous results were obtained in the same geometry in a beam of real photons. On the whole, these results confirm the applicability of the pseudophoton method for the analysis of the effects of the interaction between the field of relativistic photons and the thick (thicker than the skin layer) conducting targets and inapplicability of the surface current method for the radiation in the direction of motion of electrons.