Influence of adsorbed sulfur on surface reaction kinetics and surface self-diffusion on iron

The nitrogenation of iron in N₂ and the carburization of iron in CH₄H₂ mixtures were studied in a flow apparatus by a resistance method in the presence of adsorbed sulfur. From these measurements the adsorption isotherm for sulfur on iron was obtained at 850°C and the change of the surface energy in dependence on the sulfur activity could be calculated. These results and measurements of the growth of grain boundary grooves were used to determine the influence of adsorbed sulfur on the surface self-diffusion of iron, which is enhanced by a factor of 3 to 5 in the presence of a saturated layer of adsorbed sulfur.     

Al表面的"类液"结构及其自扩散通道

应用分子动力学方法,采用嵌入势模型在熔点下和熔点上对Al(001),(110)和(111)表面的原子结构和自扩散现象进行研究.发现这些表面的第一层原子在低于熔点时,Al(110)面在700±10 K,Al(001)面在 860±10 K,Al(111)面在 930±10 K呈现明显自扩散且最终转变为"类液"结构,而其余各层仍保留有序状态.对这种"类液"结构进行均方位移、结构有序参数、径向分布函数和z向粒子密度分析,发现其结构和扩散行为与熔化的Al表面不同,并能在一定温度区间稳定存在.在"类液 关键词： 表面结构 分子动力学 自扩散     

Mechanism of low-temperature ion-bombardment-activated surface self-diffusion

Elementary events of low-temperature surface erosion induced by the bombardment with accelerated helium atoms and ions were studied at the atomic level. It is established that the regular arrangement of surface atoms is disturbed due to the release of energy of formation of interstitial atoms emerging at the surface and to the expenditure of part of this energy on the formation of surface defects in excited states. The adatom excitation energy allowing the short-range diffusion processes was determined experimentally.     

Anomalous effects of hydrostatic pressure on ice surface self-diffusion

This paper presents an experimental study of the formation time of the groove that occurs at the intersection of a grain boundary and the free surface of a pure ice sample immersed in silicone oil at a pressure of approximately 10 MPa and temperature higher than ? 25 °C. Using the theoretical approach presented by Mullins (1957) [14], the surface diffusion coefficient is obtained. It is shown that, at 10 MPa, the ice surface diffusion coefficient is more than twice that observed at atmospheric pressure.     

Effect of impurities on the surface morphology of Fe(111)

The surface composition and morphology of Fe(111) have been examined through a combined analysis that includes low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and scanning tunneling microscopy (STM). The preferential segregation of sulfur has been clearly identified by AES upon annealing. The STM images exhibit numerous triangular pits of various sizes, and the LEED patterns show diffused n × 1 spots. The triangular pits reveal a Sierpinski gasket fractal. For sulfur-free Fe(111), nitrogen segregates to the surface upon annealing, forming a 4√3 × 4√3 superstructure that is identified by LEED patterns and STM images. The STM images show nanoscale triangular clusters regularly aligned in a hexagonal 4√3 × 4√3 configuration. Ultra-thin chromium film deposited on a nitrogen-segregated Fe(111) surface with post-annealing induces further nitrogen segregation, resulting in the formation of triangular pyramid-shaped CrN nanoclusters.     

Directional anisotropy of surface self-diffusion on Pt(110)

The rate of surface self-diffusion on a Pt(110) single crystal in the [1$\text{1}$0] and [001] directions was measured at 1200–1750 K by monitoring the decay of a sinusoidal surface profile. The surface diffusion rate in the [1$\text{1}$o] direction was much faster than in the [001] direction. The activation energy of surface self-diffusion was 1.70 and 3.16 eV for the [1$\text{1}$0] and [001] directions, respectively, in good agreement with theoretical estimates. For large amplitudes of the profile the decay rate for the [001] direction was also dependent on the amplitude. This behavior can be explained by the appearance of (111) facets on the profile, which cause a retardation of the profile decay.     

Surface self-diffusion studies on the (111) surface of vanadium

The temperature dependence of the surface self-diffusion coeffiecient of vanadium was investigated experimentally under ultra-high vacuum conditions by using the sinusoidal profile decay technique. The rates of decay of profiles were measured in situ by using the laser diffraction technique. These diffusion studies were carried out on the (111) plane and the diffusion direction on this plane was controlled to be 〈2&#x0304;11〉. For the temperature range 1374–1521 K, the results were found to follow the relationship: $\text{D}{}_{\mathrm{<mtext>s</mtext><mtext>(111)</mtext>}}\text{〈}\text{2}\text{?}\text{11〉 = 66}\text{exp}\text{[?48,000)RT]}\text{cm}{}^2\text{/}\text{s}$. A comparison of activation energy with a pairwise potential model shows that a significant lattice relaxation is present when an atom occupies the saddle point configuration. Saddle point energies for various body centered cubic metals, calculated from the experimental data on surface diffusion, have been shown to vary linearly with the heat of vaporization.     

Atomic mechanisms and kinetics of self-diffusion on the Pd(001) surface

The atomic mechanisms and kinetics of self-diffusion of Pd adatoms on a single-crystal Pd(001) surface are investigated using molecular dynamics simulation. It is shown that the migration of Pd adatoms on the Pd(001) surface predominantly occurs through the relay-race mechanism with the participation of substrate atoms. The activation energy for an elementary event of relay-race self-diffusion is calculated from the kinetic equation describing the change in the concentration of surface adatoms labeled at the initial instant of time.     

NMR determination of topological structure and self-diffusion of linear polymers

A general approach is proposed to calculate the attenuation of spin echo diffusion and the self-diffusion coefficients D of linear polymers with different average molecular weights M _w . It was shown that the changes in the topological structure of linear polymers resulted from the formation of a physical entanglement network at M _w > 10⁵ are responsible for the experimentally observed anomalous diffusion D(M _w ).     

Two-dimensional excitons and hydrogenlike impurities on a curved surface

The shifts and splittings are found for a two-dimensional donor and a Wannier-Mott exciton on a curved surface. The appearance of fine structure in the excitonic luminescence line as a result of the motion of the system as a whole in a potential of geometric origin is estimated. The relative contributions of the Coulomb potential and kinetic energy operator to the corrections for curvature depend strongly on the geometric form of the surface. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 2, 136–140 (25 July 1998)     

Measurement of gold surface self-diffusion by scanning tunneling microscopy

Measurement of gold surface self-diffusion by the method of surface profile decay, using a scanning tunneling microscope (STM) have been done on a polycrystalline gold film deposited on a glass substrate. The peak-to-peak surface roughness was measured as a function of annealing time after annealing at 170 °C with a special pan-cake furnace in the STM. The gold surface diffusion coefficient at 170 °C can then be extracted from these measurements.     

Effect of hydrostatic pressure on self-diffusion in metal nanoparticles

The thermodynamics approach has been developed to describe the self-diffusion in nano-sized solids. It has been established that identical homologous temperatures of metal nanoparticles with their fixed characteristic size give the identical coefficients of diffusion under different pressures. The dependence of the activation enthalpy of diffusion on pressure and on the characteristic size of nanoparticles is first obtained.     

Effect of hydrostatic pressure on self-diffusion in metal nanoparticles

The thermodynamics approach has been developed to describe the self-diffusion in nano-sized solids. It has been established that identical homologous temperatures of metal nanoparticles with their fixed characteristic size give the identical coefficients of diffusion under different pressures. The dependence of the activation enthalpy of diffusion on pressure and on the characteristic size of nanoparticles is first obtained.     

Effect of Ni and Co impurities on the electronic structure and magnetic properties of BCC iron

A theory of disordered binary alloys A_xB_1−x (A=Ni, Co; B=Fe; x0.06) is used to determine the changes in the electronic structure and magnetic properties of body centered cubic (BCC) iron induced by doping with nickel and cobalt impurities. This approximation is an extension of the cluster-Bethe lattice method, in which we incorporate electronic correlations, itinerant and localized nature of electrons 3d, and both long-range and short-range chemical correlations. The magnetism is described by means of a Hubbard Hamiltonian that in conjunction with Green's functions techniques is used to calculate local densities of electronic states. For it we take an atom in the real lattice and it is joined to a Bethe's lattice with like coordination number. The magnetic moments on sites occupied for A and B atoms are obtained self-consistently. Nickel and cobalt impurities in BCC iron can provide crucial information on the modification of the electronic band structure and magnetic moments from pure Fe. The results obtained are compared with those of both pure Fe and binary alloys of Co–Fe and Ni–Fe, which have been obtained by other authors using methods such as: first-principles electronic structure calculations using the layer Korringa–Kohn–Rostoker (KKR), the full-potential linearized augmented plane wave method, the KKR coherent potential approximation combined with the local-density functional method and by the tight-binding linear-muffin-tin orbitals method, obtained good agree. These results and other that recently we have published indicate to us that our methodology can be a new alternative for calculations of the electronic structure and magnetic properties of impurities and alloys of ferromagnetic transition metals.     

Quenching of self-diffusion by impurities in plastic crystals as a tentative explanation of experimental discrepancies

A model has been developed which can explain observed discrepancies in the determination by different techniques of the activation energy for self-diffusion in plastic crystals. In this model, impurities play an essential role. The basic hypothesis is that an impurity stabilizes the crystal lattice by reducing the self-diffusion around it, to a range of several molecular distances. This hypothesis allows a semi-quantitative fit of published experimental N.M.R. data on perfluorocyclohexane, and the parameters of the model are found to be consistent with previous work on cyclohexane. Moreover, as observed experimentally, it leads to the conclusion that activation energies deduced from N.M.R. linewidth measurements are in general smaller than those deduced from radiotracer data. It is concluded that for infinitely pure samples, all techniques should lead to convergent results for self-diffusion activation energies.     

The effect of bulk impurities on localized surface states

The effect of the occurrence of bulk impurities in a crystal on the existence and localization of surface states is examined by investigating a simple one-dimensional model. It is found that the bulk impurity has a small effect on the existence conditions for localized states associated with the surface, but the degree of localization of the corresponding charge density can be altered appreciably by the presence of such impurities. The penetration depth associated with localized surface levels can be much greater that than estimated for a pure crystal. The charge density associated with the electrons which occupy localized surface states may vary in a real crystal over a broader region near the surface than would be deduced from models of pure crystals. This will affect physical quantities, such as surface impedance, which are dependent on this charge distribution.     

Effect of particles on the flow structure and dispersion of solid impurities in a two-phase axisymmetric jet

The Euler approach is used for studying the structure of a flow and the propagation of a disperse impurity in a submerged two-phase jet for small values of the mass concentration of particles (M _L1 = 0 to 0.5) upon a variation of the size and material of particles in a wide range. The effect of particles on the propagation of a two-phase jet, gas turbulence, and solid phase dispersion is analyzed. The addition of particles decreases the jet opening angle, increases the jet range, suppresses turbulence, and deteriorates turbulent mixing with the surrounding submerged space. It is shown that at the first stage, particle accumulation effects (pinching) in the axial region of the jet appear upon an increase in the particle size and the density of the particle material. Then, upon an increase in the inertia of particles, pinching changes to intense scattering of the disperse phase in the initial cross sections of the jet. The results are compared with the results of measurements for mono- and polydisperse two-phase jet flows.     

Enhancement of water self-diffusion at super-hydrophilic surface with ordered water

It has been well acknowledged that molecular water structures at the interface play an important role in the surface properties, such as wetting behavior or surface frictions. Using molecular dynamics simulation, we show that the water self-diffusion on the top of the first ordered water layer can be enhanced near a super-hydrophilic solid surface. This is attributed to the fewer number of hydrogen bonds between the first ordered water layer and water molecules above this layer, where the ordered water structures induce much slower relaxation behavior of water dipole and longer lifetime of hydrogen bonds formed within the first layer.