A cluster approach to hydrogen chemisorption on the GaAs(1 0 0) surface

Chemisorption properties of atomic hydrogen on the Ga-rich GaAs(1 0 0), (2×1) and β(4×2) surfaces are investigated using ab initio self-consistent restricted open shell Hartree–Fock (ROHF) total energy calculations with Hay–Wadt (HW) effective core potentials. The effects of electron correlation have been included using many-body perturbation theory through second order with the exception of β(4×2) symmetry due to computational limitations. The semiconductor surface is modeled by finite sized hydrogen saturated clusters. The effects of surface reconstruction have been investigated in detail. We report on the energetics of chemisorption on the (1 0 0) surface layer, including adsorption beneath the surface layer at an interstitial site, and also report on the possible dimer bond breaking at the bridge site. Chemisorption energies, bond lengths, and charge population analysis are reported for all considered sites of chemisorption.     

Stiffness reduction and stress analysis in cracked [0m 0/90n 0]s laminates

Summary A new method of modelling effective properties and stress distribution in cracked laminates [12]–[17] is applied to assess the loss of the effective Young, Kirchhoff and Poisson moduli of the [0 _m ⁰/90 _n ⁰] _s laminates with transverse aligned cracks in the 90° layer and to find the stress distribution between interacting cracks and in the vicinity of isolated cracks. The decaying curve describing the loss of the effective Young's modulus in the tensile direction lies slightly over the curve found by Hashin [7] with the help of similar stress assumptions. The reduction of Kirchhoff's modulus is predicted in the same way as in Hashin [7] and Tsai and Daniel [28]. All components of the state of stress are given by closed-form relations. The stresses within periodicity cells are expressed in terms of macrodeformations. The explicit formulae for stresses derived in the present paper make it possible to interrelate the crack spacing with a magnitude of the applied stress. To this end a local stress-type criterion of the onset of cracks nucleation is used and reasonable good fit with experimental data of Highsmith and Reifsnider [9] is observed.     

Microstructure and corrosion behaviour of additively manufactured Ti–6Al–4V with various post-heat treatments

This paper presents the results of an experimental investigation into the effects of solution treatment, age and stress relief on the corrosion behaviour of direct metal laser sintered Ti–6Al–4V. The evidence of microstructural change and phase evolution, as affected by heat-treatment temperature, was characterised through scanning electron microscopy and X-ray diffraction. The corrosion behaviour was evaluated electrochemically in Ringer's solution, at 37°C. It was determined that the non-equilibrium α’ phase, with a small amount of β nuclei, formed in the as-printed sample. Enhancement in the resistance of the passive oxide layer on the alloy was observed after solution treatment and age, as well as after high-temperature stress relief. The structure of the passive layer of the surface showed a heat-treatment temperature dependency.     

Heterophase structures and their quantitative characteristics in (1 − x)Pb(Fe1/2Nb1/2)O3 − xPbTiO3 near the morphotropic phase boundary

Features of phase coexistence in solid solutions of (1 − x)Pb(Fe_1/2Nb_1/2)O₃ − xPbTiO₃ with the perovskite-type structure are studied at 0.05 ≤ x ≤ 0.08. The role of elastic matching of the tetragonal P4mm and monoclinic Cm phases of the ferroelectric nature is considered near the morphotropic phase boundary. Model concepts on the stress relief in heterophase structures are developed and applied to interpret the phase content in (1 − x)Pb(Fe_1/2Nb_1/2)O₃ − xPbTiO₃. Good agreement between the calculated and experimental dependences of the volume fraction of the tetragonal phase on x is observed. It is shown that the studied phase coexistence under conditions for the complete stress relief can take place at elastic matching of the single-domain monoclinic phase and the tetragonal phase split into two types of 90° domains.     

Structure of Pt3Co(1 1 0)c(2×4)-O surface studied by scanning tunneling microscopy

A Pt₃Co(1 1 0)c(2×4)-O surface has been investigated by scanning tunneling microscopy (STM), low-energy electron diffraction, and Auger electron spectroscopy. At a very initial oxidation stage exposed at 500°C, creation of missing and/or added row structures running to the [0 0 1] direction on clean Pt₃Co(1 1 0)2×1 surface was imaged from the steps. The surface is fully covered by a well-ordered c(2×4) structure at 2 L oxygen exposure. Atomic-resolution STM image shows the added row-type anti-phase Co-O zigzag chains along the [0 0 1] direction. Based on the images, the structure model for the c(2×4) surface was suggested as a first oxidized layer, which comes from the chemical reaction forming stoichiometric Co monoxide. Further oxygen exposure above 5 L, Co-O clusters imaged to large dots were formed on the surface with the size of about 5-7 Å.     

High temperature stress relief cavitation of an Al-bearing α-brass

High temperature stress relief intergranular cavitation and subsequent room temperature embrittlement of an aluminium-bearing-brass has been studied metallographically. The behaviour of a cast susceptible to cavitation has been compared to one which does not exhibit intergranular cavitation during stress relief, and which is subsequently more ductile at room temperature. A number of micro-analytical techniques (SIMS and EDX) failed to reveal any difference in the grain boundary chemistry between a cast susceptible to intergranular cavitation and one which was not, but it is suspected that the combined action of dissolved gases (e.g. hydrogen) and trace element impurities plays a major role in cavitation. The cavities formed during stress relief were often polyhedral in shape and it is considered that this occurs by the diffusion of matter around the cavity surface to attain a lower energy surface configuration. Second phase particles were found to play only a minor role in the nucleation of cavities. Room temperature intergranular fracture surfaces of material, in which cavities had formed during stress relief, were interpreted in terms of high temperature cavity formation and coalescence combined with low temperature plastic void growth and interlinkage.     

Coupled heat and mass transfer in mixed convection over a VHF/VMF wedge in porous media: The entire regime

Summary The development of flow and heat transfer of a viscous electrically conducting fluid in the stagnation point region of a three-dimensional body with an applied magnetic field is studied when the external stream is set into an impulsive motion from rest and at the same time the surface temperature is suddenly raised from that of the surrounding fluid. This analysis includes both short time solution (Rayleigh-type of solution) and the steady-state solution as time tends to infinity (Falkner-Skan type of solution). The unsteady three-dimensional boundary layer equations represented by a system of parabolic partial differential equations are solved numerically using an implicit finite-difference scheme. For certain particular cases analytical solutions are obtained. In the absence of the magnetic field, the reverse flow occurs in the transverse component of the velocity in a certain portion of the saddle-point region (–1c<–0.4, wherec=b/a is the ratio of the velocity gradients in they- andx-directions at the edge of the boundary layer). The magnetic field delays or prevents the reverse flow. The surface shear stresses in the principal and transverse directions and the surface heat transfer increase with the magnetic field both in nodal point (0c1) and saddle point (–1c<0) regions. For a fixed magnetic field, the surface shear stress inx-direction and the surface heat transfer increase with time in nodal and saddle point regions, but the surface shear stress in the transverse direction increases with time for 0<c1 and decreases with increasing time for –1c<0.     

Electron energy loss spectroscopy study of the effect of low-energy Ar-ion bombardment on the surface structure and composition of Pt80Co20 (1 1 1) alloy

Electron energy loss spectroscopy has been employed for investigation of the effect of 600 eV Ar⁺-ion irradiation in the dose range 7×10¹⁶-4×10¹⁷ ions/cm² on the atomic structure and surface composition of alloy Pt₈₀Co₂₀(1 1 1). A method of the layer-by-layer reconstruction of the lattice interplanar distance changes based on the analysis of the plasmon spectra excitation was proposed. The ion bombardment was shown to result in a non-monotonic variation of the lattice interplanar distance due to formation of the stable defects, with the topmost layer being in the state of compression. Using the ionization energy loss spectra, a layer-by-layer concentration profile of the alloy components was reconstructed for different doses of ion irradiation of the surface. The Ar⁺-ion bombardment of the alloy was found to result in the preferential sputtering of Co and in the enrichment of the near-surface region by Pt atoms with formation of an altered layer, which is characterized by a non-monotonic concentration profile dependent on the irradiation dose. The results obtained are discussed in the framework of the models of preferential sputtering and radiation-induced segregation.     

Oxide scale stresses in polycrystalline Ni200

An X-ray diffraction (sin² ψ) method has been successfully used to measure the oxidation stresses at room temperature in annealed and electropolished samples of polycrystalline Ni200 coupons oxidized in the temperature range 760 to 982° C for 4 h. The stresses on the free surface of the oxide (σ ₁₁ andσ ²²) were compressive and the average stress through the thickness normal to the oxide layer was found to be tensile. Surface stresses on the oxides formed at temperatures up to 927° C were found to be isotropic and both surface stresses and the average normal stress increased with increasing temperature of oxidation. At 982° C, the surface stresses were lower and this was attributed to the deformation and fracture of oxide layer resulting in stress relaxation.     

Adsorption of sulfur on heated (10\overline 1 0) Re

High-resolution Auger electron spectroscopy is used to study the adsorption of sulfur on a Re surface and to determine the regions of thermal stability of the resulting adsorption states. The formation of surface sulfide is observed and its stoichiometry, Re₂S, and the absolute sulfur concentration on the rhenium surface N _S =(6.0±1)×10¹⁴ atom/cm² are determined. It is shown that the surface sulfide is destroyed by thermal desorption of sulfur from the rhenium surface and the desorption activation energy is estimated as E _d =(3.3±0.2) eV for a coverage close to the surface sulfide and E _d =(4.9±0.2) eV in the limit θ→0. Pis’ma Zh. Tekh. Fiz. 25, 57–64 (July 26, 1999)     

Instability and Reconstruction of Thin Liquid Layer Under Inversed Gravitation Conditions

Evolution of shape of thin liquid layer covering a solid substrate under conditions of inversed gravitation (the van der Waals's and gravitation forces have opposite directions) with increasing the layer thickness is studied numerically. It is shown that at thickness higher than the critical value 10 ^–4 cm flat surface of neutral liquid layer becomes unstable, and the stationary solitary wave of surface deformation is formed, i.e. the reconstruction transition takes place at the surface. With increasing the layer thickness amplitude of the wave grows gradually, and at amplitudes much higher than the critical thickness shape of the wave is close to shape of single macroscopic droplet. Influence of external electric field on charged droplet's shape is evaluated, also. The obtained theoretical dependencies are used for treatment of results of the experiments with charged liquid hydrogen droplets.     

First-principles study of the (1 1 0) polar surface of cubic PbTiO3

Under GGA, the cleavage energy, surface energy, surface grand potential, surface relaxation, and surface electronic structure have been calculated for five different terminations of PbTiO₃ (1 1 0) surface by using PAW method implemented in VASP. Taking into account the results of two neutral PbTiO₃ (1 0 0) surfaces, the favorable PbTiO₃ (1 1 0) and (1 0 0) surfaces are the TiO₂-terminated (1 0 0) surface, the PbO-terminated (1 0 0) surface, and the O-terminated (1 1 0) surface successively in view of surface energy minimization. The surface grand potential calculations show that two neutral PbO- and TiO₂-terminated (1 0 0) surfaces are favored in the moderate Pb and O chemical potentials, two mutual complementary TiO- and Pb-(1 1 0) terminations are stable in Pb-poor environment and in O- and Pb-rich conditions, respectively. A non-negligible rumpling of O-terminated (1 1 0) surface is found in the third O₂ layer and large lateral displacements between Ti and O atoms on the PbTiO layer lead to the initial O-Ti-O alignment broken. Different from the Fermi levels of the three nonstoichiometric TiO-, Pb- and O-terminations which are located in the band gap, the Fermi level of the PbTiO- termination is located at the bottom of the conduction band and that of the O₂-termination is located at the top of the valence band due to increment and decrement of the occupation states for polarity compensation.     

Growth process of CuO(1 1 1) and Cu2O(0 0 1) thin films on MgO(0 0 1) substrate under metal-mode condition by reactive dc-magnetron sputtering

The growth process of CuO and Cu₂O thin films on MgO(0 0 1) substrates by reactive dc-magnetron sputtering was studied by reflection high-energy electron diffraction (RHEED) and atomic-force microscopy (AFM). The RHEED pattern and AFM image showed that (1) three-dimensional Cu(0 0 1) islands grew on MgO under the nonreactive sputtering condition, (2) CuO(1 1 1) was deposited layer by layer on MgO at 400 °C under the reactive sputtering condition, and (3) the film deposited at 600 °C in the initial growth stage was composed of three-dimensional Cu islands because O₂ gas could not be incorporated into them due to the low sticking coefficient of O₂ on MgO under the reactive sputtering condition. The layer-by-layer CuO(1 1 1) thin-film growth process is discussed from the viewpoint that Cu and oxygen species are supplied in stoichiometry onto the MgO substrate to form CuO thin-film crystals while maintaining minimum interfacial energy between CuO and MgO.     

Adsorption of Ni on Si(1 0 0) surface

The chemisorption of one monolayer Ni atoms on ideal Si(1 0 0) surface is studied by using the self-consistent tight binding linear muffin-tin orbital method. Energies of adsorption systems of Ni atoms on different sites are calculated. It is found that Ni atoms can adsorb at fourfold site above the surface and bridge site below the surface. The adsorption of Ni atoms can readily diffuse and penetrate into the subsurface. A Ni, Si mixed layer might exist at the Ni-Si(1 0 0) interface. The layer projected density states are calculated and compared with that of the clean surface. The charge transfers are also investigated.     

Free vibration analysis of double-bonded isotropic piezoelectric Timoshenko microbeam based on strain gradient and surface stress elasticity theories under initial stress using differential quadrature method

The size-dependent effect on free vibration of double-bonded isotropic piezoelectric Timoshenko microbeams using strain gradient and surface stress elasticity theories under initial stress is presented. This article is developed for isotropic piezoelectric material. Due to the high surface-to-volume ratio, surface stress has an important role with micro- and nanoscale materials. Thus, the Gurtin–Murdoch continuum mechanic approach is used. Governing equations of motion are derived by Hamilton's principle and solved by the differential quadrature method. The effects of pre-stress load, surface residual stress, surface mass density, surface piezoelectrics, Young's modulus of surface layers, three material length scale parameters, thickness to material length scale parameter ratios, various boundary conditions, and two elastic foundation coefficients are investigated. It is concluded that the effect of pre-stress load in greater modes is negligible for higher aspect ratios and this effect is similar to lower aspect ratios. Also, the size-dependent effect on the dimensionless natural frequency for strain gradient theory is higher than that for modified couple stress theory and classical theory, which is due to increasing stiffness of the Timoshenko microbeam model. Moreover, the results show that dimensionless natural frequency affects more by considering the material length scale parameters with respect to surface effect. The results are compared with the obtained results from the literature and show good agreement between them. It is concluded that the amplitude of the transverse displacements (w₀) for a microbeam (MB) is more than the transverse displacements (w₁) for a piezoelectric microbeam (PMB). On the other hand, using a piezoelectric layer for PMB, the amplitude of the transverse displacements (w₁) reduces considerably with respect to MB, in which this effect leads to increase the stiffness of the microbeam and stability of microstructures. With considering the piezoelectric layer, the obtained results can be used to control the amplitude and vibration of microstructures, prevent the resonance phenomenon, design smart structures, and can be employed for micro-electro-mechanical systems and nano-electro-mechanical systems.     

Treatment of singularities in cracked bodies

Three-dimensional finite-element analyses of middle-crack tension (M-T) and bend specimens subjected to mode I loadings were performed to study the stress singularity along the crack front. The specimen was modeled using 20-node isoparametric elements with collapsed, non-singular elements at the crack front. The displacements and stresses from the analysis were used to estimate the power of singularities using a log-log regression analysis along the crack front. The analyses showed that finite-sized cracked bodies have two singular stress fields. The near-field singular stress has the form =C ₀(,z)r ^-/12' +D ₀ (0,)R The first term is the cylindrical singularity with the power -1/2 and is dominant over the middle 96 percent (for Poisson's ratio = 0.3) of the crack front and becomes nearly zero at the free surface. The second singularity is a vertex singularity with the vertex point located at the intersection of the crack front and the free surface. The second singularity is dominant at the free surface and becomes nearly zero away from the boundary layer. The thickness of the boundary layer depends on Poisson's ratio of the material and is independent of the specimen type. The thickness of the boundary layer was about 0%, 2%, 4%, and 5% of the total specimen thickness for Poisson's ratio of 0.0, 0.3, 0.4, and 0.45, respectively.Because there are two singular fields near the free surface, the strain-energy-release rate (G) is an appropriate parameter to measure the severity of the crack front. The G-distribution for M-T and bend specimens were different.Nomenclature a crack length, m - E Young's modulus, GPa - G strain-energy-release rate, J/m' - G _p plane-strain, strain-energy-release rate, J/m² - H height of specimen, m - P load per unit length, N/mm - R, O, spherical coordinate system - r, O, z cylindrical coordinate system - S remote tension stress, N/mm² - t specimen thickness, m - t _ti ith layer thickness, m - u, v, w displacements inx-, y-, andz-directions, m - x, y, z Cartesian coordinates, m - W half-width of the specimen, m - power of the stress singularity - power of displacement field - v Poisson's ratio - _y normal stress in y-direction, GPa     

Studies of diamond films/crystals synthesized by oxyacetylene combustion flame technique

High-quality diamond films/crystals were synthesized using the oxyacetylene combustion flame technique at atmospheric pressure in a narrow acetylene-rich region. Three nozzle configurations, single-, tilted- and multi-nozzle, were used to explore possible ways to improve the uniformity of diamond films and to increase the deposition areas. It was found from the systematic investigation that the surface morphology and crystal structure of diamond films are strongly dependent on the processing parameters such as the gas mixture ratio, r, of acetylene to oxygen, substrate temperature, and nozzle configurations. The appearance of two-dimensional spiral steps on (1 1 0) diamond surfaces was observed, which have not previously been reported in the literature. This phenomenon is explained using the concept of surface reconstruction. The observed layered steps on (1 0 0), (1 1 0), and (1 1 1) diamond planes strongly suggest that under certain conditions the synthetic diamond crystals could grow with a layer mechanism on any major plane, at least in the case of films made using combustion flames. Experimental results from X-ray diffraction and Raman spectroscopy show the presence of compressive stress along the 1 0 0 direction in the diamond films. The films also have good optical transparency, indicating potential for optical coating applications. The hardness, growth rate, film uniformity, and deposition areas of diamond films are discussed. Advantages and limitations of these three flame-torch deposition techniques are also presented.     

Surface phase transitions upon reduction of epitaxial WO3(1 0 0) thin films

The surface structure and morphology of WO₃(1 0 0) thin films were studied using scanning tunneling microscopy (STM) and low-energy electron diffraction. The films experienced a net-reducing environment when annealed in oxygen at 800 K leading to surface phase transitions from p(2×2) to p(4×2), and from p(4×2) to a mix p(4×2) and p(3×2). Increasing the annealing temperature to 830 K in ultra-high-vacuum (UHV) led to a fully p(3×2) reconstructed surface. Continued UHV annealing above 800 K caused (1×1) islands to appear on the p(3×2) surface and the film color to darken. Eventually, prolonged UHV annealing led to a (1×1)-terminated surface with straight steps oriented in [0 0 1] or [0 1 0] directions due to crystallographic shear planes. The randomly spaced steps on the (1×1) surface indicated variations in the local stoichiometry in the film. An added row model proposed for the p(4×2) structure is also shown to be consistent with the p(3×2) structure. The formation of the p(4×2) structure from the p(2×2) structure was attributed to W⁵⁺ migration into the bulk to form the troughs between the added rows. Reduction of the p(4×2) structure caused the troughs to narrow rather than deepen, suggesting that W⁵⁺ or added row surface diffusion competes with migration of reduced W ions into the bulk when the p(3×2) structure forms. The STM images also show evidence that the (1×1) structure forms through coalescence of the added rows.     

Effect of Si on the oxidation reaction of α-Ti(0 0 0 1) surface: ab initio molecular dynamics study

Abstract

We present our ab initio molecular dynamics (MD) study of the effect of Si on the oxidation of α-Ti(0?0?0?1) surfaces. We varied the Si concentration in the first layer of the surface from 0 to 25 at.% and the oxygen coverage (θ) on the surface was varied up to 1 monolayer (ML). The MD was performed at 300, 600 and 973 K. For θ = 0.5 ML, oxygen penetration into the slab was not observed after 16 ps of MD at 973 K while for θ > 0.5 ML, oxygen penetration into the Ti slab was observed even at 300 K. From Bader charge analysis, we confirmed the formation of the oxide layer on the surface of the Ti slab. At higher temperatures, the Si atoms diffused from the first layer to the interior of the slab, while the Ti atoms moved from second layer to the first layer. The pair correlation function shows the formation of a disordered Ti-O network during the initial stage of oxidation. Si was found to have a strong influence on the penetration of oxygen in the Ti slab at high temperatures.     

An investigation of ion implantation-induced near-surface stresses and their effects in sapphire and glass

Using both cantilever bending and indentation fracture techniques, the generation of near-surface compressive stresses by ion-implantation into sapphire and glass has been monitored and characterized. In all cases, the surface stresses initially increase with ion dose until a critical dose (dependent on material and ion species/ energy) is reached. Beyond this dose, stress relief has been observed and, for sapphire implanted with both Y⁺ and Ti⁺, this has been attributed to the formation and growth of an amorphous layer as monitored by hardness testing. The stress relief has been simply modelled and values estimated for the mechanical strength of the amorphous layer produced. For sapphire, the integrated stress produced over the near-surface volume was found to increase linearly with dose; values of the integrated stress produced by the two different species were similar when considered in terms of energy deposition. Estimates of the contribution to the integrated stress of both the implantation-induced damage and the implanted species profile suggest that the implanted profile makes a minor but significant (20%) contribution. Broadly similar behaviour was observed for soda-lime-silica glass specimens implanted with both C⁺ and N⁺. While the origins of the compressive stress produced are probably similar to those in crystalline materials (i.e. defect production and ion-stuffing), no microstructural explanations for both the observed hardening with increasing dose and stress relief have been forthcoming. However, high-dose implantation of N⁺ into glass leads to blistering and concomitant softening.