The anomalous exhibition of GSF energy at surface of fcc metals

The generalized stacking fault (GSF) energy curves for (1 1 1) surface of fcc metals are calculated by the second nearest-neighbor modified embedded atom method (2NN-MEAM), in order to investigate the deformation mechanism of (1 1 1) surface. Except the energy reduce for all these metals, strange energy curves are found for Au, Pd and Pt, especially for Au. Combining the surface GSF energy data and the experimental results, we find that the deformation mechanism should be explained by not only the values of the stable stacking fault energy γ_sf and unstable stacking fault energy γ_usf, but the whole shape of a metal’s energy curve.     

Surface effect on the necking of hyperelastic materials

Necking is a widely observed instability phenomenon. Surface tension often plays a significant role in the deformation of soft materials, especially at the micro scale, but its influence on the necking behavior of soft structures remains unclear. In this paper, we use the energy method to explore surface effects on the necking of soft plates and cylindrical bars under uniaxial tension. Analytical solutions are derived for the critical conditions of necking instability in the two types of structural components with a power-law constitutive relation. It is found that surface energy shows different impacts on their necking behaviors – it tends to postpone the occurrence of necking in soft plates but promote that in soft cylinders. This work not only deepens our understanding of the instability of soft materials but also helps design soft devices and robots.     

Surface effect on the nonlinear forced vibration of cantilevered nanobeams

The nonlinear forced vibration behavior of a cantilevered nanobeam is investigated in this paper, essentially considering the effect due to the surface elastic layer. The governing equation of motion for the nano-cantilever is derived, with consideration of the geometrical nonlinearity and the effects of additional flexural rigidity and residual stress of the surface layer. Then, the nonlinear partial differential equation (PDE) is discretized into a set of nonlinear ordinary differential equations (ODEs) by means of the Galerkin’s technique. It is observed that surface effects on the natural frequency of the nanobeam is of significance, especially for the case when the aspect ratio of the nanobeam is large. The nonlinear resonant dynamics of the nanobeam system is evaluated by varying the excitation frequency around the fundamental resonance, showing that the nanobeam would display hardening-type behavior and hence the frequency-response curves bend to the right in the presence of positive residual surface stress. However, with the negative residual surface stress, this hardening-type behavior can be shifted to a softening-type one which becomes even more evident with increase of the aspect ratio parameter. It is also demonstrated that the combined effects of the residual stress and aspect ratio on the maximum amplitude of the nanobeam may be pronounced.     

Surface profiling using fringe projection technique based on Lau effect

In this paper, the surface profile of objects is determined by using a grating projection system. The method is based on the concept of self-imaging, namely, the Lau effect. Periodic fringe patterns are generated from the projection of a grating illuminated by multiple mutually incoherent quasi-plane wavefront. The fringe patterns are then projected on to the object surface and the deformed grating image is captured by a CCD camera for subsequent analysis. Phase shifting techniques have been employed to determine the phase value. With suitable calibration of the system and the phase value obtained, the surface profile of the objects can be determined. The phase variation is achieved by using a linear translation stage incorporated to the grating. In this study, two specimens are tested to demonstrate the validity of the method. One is a spherical cap with a height of 4 mm, and the other is a coin. The experimental results are compared with results obtained by mechanical stylus method. In the case of the spherical cap the results are also compared with fringe projection method based on Talbot effect.     

Surface energy modification by electron beam

We observe a pronounced variation of wettability properties in solid state materials induced by a low-energy electron beam. The phenomenon occurs in several stages characterized by various mechanisms. We show that for low electron doses the irradiation leads to decrease in the wetting of a dielectric surface due to induced surface electric potential. The higher electron charge leads to formation of a chemical monolayer on material’s surface. It has been found that the electron irradiation strongly modifies the surface free energy of SiO₂ by decreasing its total surface free energy value, almost twice. However, electron-induced variations of dispersive and polar components of the surface free energy are quite different and depend of incident electron charge.     

Surface free energy of a finite system without interfaces at low temperatures

We derive the specific surface free energy for a rather general system at low temperatures that can be rewritten as a gas of non-interacting contours (polymers). To this end, we use a standard cluster expansion series for the system?s partition function. A specific regime of ‘weak’ boundary conditions is assumed to ensure that no interfaces or large droplets occur in the system. We illustrate the general results, using a simple lattice–gas model.     

Surface segregation of Al substrate metal on Zr film surface

Segregation of substrate Al on thin Zr film, Zr/Al/Al system was investigated by heating the specimen in a UHV chamber. Dual-cathode magnetron-sputtering source was used for deposition of Zr film as well as thin Al film to avoid aluminum oxide formation at Zr/Al interface. Al segregates on Zr film surface at 730 K. It was found that oxide-free interface between film and substrate is important for segregation in Al system. The diffusion coefficient calculated for surface segregation and inter-metallic compound showed that the grain boundary diffusion and bulk diffusion are very close in Zr/Al/Al system. Hence, it is important to control specimen heating to cause surface segregation by grain boundary diffusion.     

The influence of the electron structure into the stacking fault energy of noble metals

The study of the electron structure of Ag, Au and Cu transition metals by EELS shows that the greatest electron concentration in conduction band observes in Ag and decreases through Au to Cu. At the same time the fraction of free-electron in conduction band of Ag is less than 0.2 while this value is about of 0.5 for Au and more than 0.6 for Cu. Thus the electron system of these elements consists from high-energy free-electron and low-energy nearly free-electron subsystems. The SFE of Ag, Au and Cu increases with increasing free-electron fraction in electron system of these metals. The explanation of this phenomenon was discussed in the frame of electron theory of metals.     

Interaction of dopant atoms with stacking faults in silicon

The width of a stacking fault ribbon bound by a pair of partial dislocations in silicon crystals was unchanged when boron and gallium atoms of p-type dopant were agglomerated nearby the ribbon by annealing, even though the width increased when n-type dopant atoms were agglomerated as previously reported [Y. Ohno, Y. Tokumoto, I. Yonenaga, Thin Solid Films, accepted for publication]. The origin of the width-increase in n-type crystals was proposed as the reduction of the stacking fault energy, from 58±5 down to 46±5 mJ/m², due to an electronic interaction between the ribbon and the n-type dopant atoms, and the interaction energy was estimated to be 0.15±0.05 eV. On the other hand, the interaction of p-type dopant atoms with stacking faults was not detected.     

Surface structure and energy of B2 type intermetallic compound NiAl

The surface structure and energies for 22 surfaces of NiAl, an ordered intermetallic compound of B2 structure, have been studied by using embedded atom method. The results show that, for alternating Ni and Al surfaces with odd numbers of the sum of their three Miller indices, the energy difference between the Ni terminated surface and Al terminated surface increase linearly with increasing the interlayer distance. So from surface energy minimization, the Al terminated surface is favorable for each alternating Ni and Al surface. This is in agreement with experimental results. However, the energy of the (1 1 0) surface belonged to the other kind of the surface consisted of stoichiometric atomic layers and with even numbers of the sum of their three Miller indices, is the lowest in all two kinds of the surfaces. Therefore the (1 1 0) texture of NiAl appears mostly in the experiments.     

Surface extended energy loss fine structures above the K-edge of oxygen on Al

Surface Extended Energy Loss Fine Structures (SEELFS) have been detected above the K-edge of oxygen adsorbed on an Al sample. This is the first evidence that the local geometry of adsorbates on metal surfaces can be investigated with the electron energy loss technique in the reflection mode. This experimental technique is highly surface sensitive and accurate bond lengths can be obtained. The oxidation model implied by our results for the OAl system is in agreement with data in the literature.     

Determination of the effective surface region thickness and of Begrenzungs effect

The thickness, x_s, of the effective surface region, defined as the region in which an electron travelling inside a material experiences surface excitations, as well as Begrenzungs effect, i.e. the variation of the bulk inelastic cross section in the surface region are theoretically determined for aluminium and silicon using the QUEELS-ε(k,ω)-REELS software. This software allows to determine the energy-differential inelastic electron scattering cross sections for reflection-electron-energy-loss spectroscopy (REELS) within the dielectric response theory. This study has been carried out for electron energies between 300 eV and 5000 eV. We find that the bulk inelastic cross section decreases exponentially with the distance to the surface, that x_s is practically independent of electron incidence and exit angles and that x_s=[2(v_B)^0.4/ω_s]v^0.6, where ω_s is the surface plasmon frequency, v the electron velocity and v_B the Bohr velocity ().     

Surface energy and mechanical behaviour of hardened cement paste

The Bangham equation indicates a direct proportionality between the length change of a porous body and a change of surface energy. In our experiments surface energy of hardened cement paste has been modified by desorbing or adsorbing water molecules. The internal pressure created by surface energy could be directly determined with the help of Mössbauer experiments. Based on these results it is possible to determine to what extent shrinkage or swelling is caused by a change of surface energy. Using Griffith's criterion our findings can explain quantitatively the decrease of stength as a function of increasing water content. Results are in good agreement with other methods of studying surface energy such as experiments to determine van der Waals forces.     

Surface energy and hybridization studies of amorphous carbon surfaces

Surface properties of a large number of amorphous carbon (a-C) films have been investigated using contact angle measurements and X-ray photoelectron spectroscopy (XPS). Dense a-C surfaces with variable sp³/(sp² + sp³) average hybridization were grown using sputtering or pulsed laser deposition (PLD) and were further chemically modified by thermal annealing, ion bombardment or covalent grafting of organic monolayers. The average carbon hybridization, impurity level and mass density, were deduced from XPS and photoelectron energy loss spectroscopy (PEELS). The depth sensitivity of the dispersive (Lifshitz–van der Waals) interaction, estimated at 1–2 nm from the dependence of γ^LW on the grafted perflorodecene molecule coverage, is much better than XPS which probes a 3–5 nm depth. The observation of a non-monotonic behavior in the correlation between surface hybridization and electron donor component of surface energy reveals that the average carbon hybridization alone does not describe the entire surface energy physics. The role of π bond clustering in the polar interactions is thus considered and some implications on surface reactivity and mutual interactions with molecular or biomolecular species are discussed.     

基于平面波展开法的二维光子晶体表面模式研究

采用平面波展开法研究了四种二维光子晶体结构(圆柱介质柱四方晶格、圆柱介质柱三角晶格、正方介质柱四方晶格、正方介质柱三角晶格)的带隙宽度随介质柱尺寸变化的关系.使用平面波展开法计算常规晶格和表面缺陷晶格的模式并进行结果叠加,研究了各结构的二维光子晶体在带隙宽度最大时的表面模式.结果表明,同种晶格的光子晶体带隙宽度随着介质柱的尺寸增大呈先增后减趋势,存在最大值.随着表面介质柱尺寸的增加,四种晶格表面模式曲线均呈下降趋势.四方晶格光子晶体与三角晶格相比,表面介质柱尺寸的变化范围更大,但能获取表面模式频率范围较小.     

基于平面波展开法的二维光子晶体表面模式研究

采用平面波展开法研究了四种二维光子晶体结构(圆柱介质柱四方晶格、圆柱介质柱三角晶格、正方介质柱四方晶格、正方介质柱三角晶格)的带隙宽度随介质柱尺寸变化的关系.使用平面波展开法计算常规晶格和表面缺陷晶格的模式并进行结果叠加,研究了各结构的二维光子晶体在带隙宽度最大时的表面模式.结果表明,同种晶格的光子晶体带隙宽度随着介质柱的尺寸增大呈先增后减趋势,存在最大值.随着表面介质柱尺寸的增加,四种晶格表面模式曲线均呈下降趋势.四方晶格光子晶体与三角晶格相比,表面介质柱尺寸的变化范围更大,但能获取表面模式频率范围较小.     

Surface energy of ordered NiAl and Ni3Al alloys

The surface energies of NiAl alloy with the B2 structure and Ni₃Al alloy with the L1₂ structure for the (100), (110) and (111) faces are calculated by the model electron density functional method. The anisotropy of the surface energy of the alloys is compared with the anisotropy of of the pure metals. It is shown that the value of of NiAl and Ni₃Al is determined by the thermodynamic properties of the alloy and agrees well with the values obtained using Miedema's semiempirical formula.Institute of Physics of Strength and Study of Materials, Russian Academy of Sciences, Siberian Division. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 80–86, October, 1994.     

Thickness dependent stripe structure stability of Ag films on Si(1 1 1)-(4 × 1)-In substrate

The thickness dependent stripe structure stabilization of Ag films on Si(1 1 1)-(4 × 1)-In substrate is thermodynamically considered. It is found that for the stability of the structure, there is a competition between the sum of elastic energy and stacking fault energy in the film and the film-substrate interface energy. The presence of equilibrium of them leads to a critical film thickness. Beyond it, the stripe structure will transform into a flat one. Our prediction for n_c of Ag films shows reasonable agreement with experimental data. In addition, according to the established model, it is predicted that Au could also form the above stripe structure on this substrate with a similar n_c value of Ag.