Optical extinction spectroscopy of single silver nanoparticles

The extinction spectrum of single silver nanoparticles with size ranging from 20 to 80 nm is investigated with the spatial modulation spectroscopy technique using either a tunable laser or a white lamp as the broadband source. Results are in good agreement with the prediction of the Mie theory, permitting to extract the nanoparticle size from the measured absolute value of the optical extinction cross-section. In contrast, the deduced refractive index of the nanoparticle environment and the reduction of the electron mean free path show a large dependence on the precise value of the bulk silver dielectric function.     

Electronic structure of Ru impurities in iron

The molecular cluster discrete variational method has been employed to study the magnetic properties of Ru impurities and Ru monolayers in iron. With the introduction of Ru atoms in the neighborhood of an Fe atom the local moment changes appreciably and the magnetization decreases steadily. By increasing Ru concentration both ferromagnetic and antiferromagnetic Fe–Ru couplings can be expected in dilute alloys. The calculations indicate that the contact hyperfine fields of Fe atoms are strongly dependent on the chemical environment as well as on the local symmetry.     

Prediction of Γ-L and L-X crossovers in Ga<Subscript>x</Subscript>Al<Subscript>1-x</Subscript>Sb alloys by empirical pseudopotential method

Using empirical pseudopotential method Γ-L crossover is found for the Ga_0.74Al_0.26Sb. The conduction band minimum is observed to switch at the (0.87, 0, 0) point for Ga_0.51Al_0.49Sb which shifts to the X point for Ga_0.21Al_0.79Sb and remains at X leading finally to indirect band gap in AlSb. Band structure calculations for a large number of alloys are performed and bowing parameters b_X and b_L are proposed for the E_X and E_L respectively. Our findings may serve as directive to select the materials in a range of composition to examine the bowing parameters and thereby effective mass experimentally for the Ga_xAl_1-xSb alloys.     

Study of solid-state reactions in Sm(Co,Fe, Cu,Zr)z 2:17-type alloys by means of in situ electrical resistivity measurements

In the present work, solid-state reactions in Sm₂(Co, Fe, Cu, Zr)₁₇-type alloys have been investigated by means of in situ electrical resistivity measurements. Changes in the electrical resistivity of a Sm(Co_0.74Fe_0.1Cu_0.12Zr_0.04)_8.5 alloy after solid solution treatment at 1190 °C, quenching to room temperature, and during isothermal ageing at temperatures between 400 and 900 °C, have indicated microstructural/phase changes occurring at temperatures below those commonly used for the development of high coercivity in Sm(Co, Fe, Cu, Zr)_z-type materials. Subsequent crystallographic and magnetic transition measurements have shown a high degree of correlation with respect to the changes observed in the electrical resistivity during isothermal ageing.     

Dissimilar autogenous full penetration welding of superalloy K418 and 42CrMo steel by a high power CW Nd:YAG laser

Experiments of autogenous laser full penetration welding between dissimilar cast Ni-based superalloy K418 and alloy steel 42CrMo flat plates with 3.5 mm thickness were conducted using a 3 kW continuous wave (CW) Nd:YAG laser. The influences of laser welding velocity, flow rate of side-blow shielding gas, defocusing distance were investigated. Microstructure of the welded seam was characterized by optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). Mechanical properties of the welded seam were evaluated by microhardness and tensile strength testing. Results show that high quality full penetration laser-welded joint can be obtained by optimizing the welding velocity, flow rate of shielding gas and defocusing distance. The laser-welded seam have non-equilibrium solidified microstructures consisting of γ-FeCr_0.29Ni_0.16C_0.06 austenite solid solution dendrites as the dominant and very small amount of super-fine dispersed Ni₃Al γ′ phase and Laves particles as well as MC needle-like carbides distributed in the interdendritic regions. Although the microhardness of the laser-welded seam was lower than that of the base metal, the strength of the joint was equal to that of the base metal and the fracture mechanism showed fine ductility.     

Surface oxidation behavior of MgNd alloys

The oxidation kinetics of MgNd alloys oxidized in pure O₂ at high temperatures has been investigated. The results revealed two stages of the reaction: A fast initial oxidation was followed by a slow oxide growth with a parabolic kinetics. For MgNd alloys (Nd = 25 wt.%), the oxidation rate increased with the enhancement of the oxidation temperature. A sudden ignition was found for this alloys oxidized at 873 K up to about 80 min. Moreover, the increase of the Nd content would harm the oxidation resistance of the MgNd alloys. By Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis, it was found that a triplex structure of oxide film formed. The outer layer was composed of MgO, Nd₂O₃ and Nd(OH)₃, the middle layer mainly consisted of MgO and Nd₂O₃, and the inner layer was the transitional layer made of MgO, Nd₂O₃ and the content of the substrate. The protective oxidation was associated with the formation of the dense Nd₂O₃/MgO layer during isothermal oxidation process. The oxidation mechanisms for the formation of oxide film are discussed.     

A valence band photoemission study of Pb adsorption on Rh(1 0 0) and Rh(1 1 0)

We have studied the adsorption of Pb on the Rh(1 0 0) and (1 1 0) surfaces by photoemission and low energy electron diffraction (LEED), and tested the chemical properties by adsorption of CO. Pb forms two distinct c(2 × 2) phases on Rh(1 0 0), according to the temperature of the substrate. The phase formed below about 570-620 K, denoted α-c(2 × 2), reduces the coverage of adsorbed CO but does not affect the valence band spectrum of the molecule. The phase formed above this temperature, denoted β-c(2 × 2), also reduces the coverage of adsorbed CO but the valence band spectrum of the adsorbed CO is strongly affected. The two phases are also characterised by a slightly different binding energy of the Pb 5d_5/2 level, 17.54 eV for the α phase and 17.70 for the β phase. The Pb/Rh(1 1 0) surface shows two ordered Pb induced phases, c(2 × 2) and p(3 × 1). CO adsorbs on the first with reduced heat of adsorption and with a valence band spectrum that is strongly altered with respect to CO adsorbed on clean Rh(1 1 0), but does not adsorb on the p(3 × 1) structure at 300 K. We compare the present results with previous results from related systems.     

A phenomenological explanation for the buckling of surface alloys

A phenomenological model, in which the interactions between the nearest-neighbor (NN) atoms are described as bondings but not hard sphere contacts, is proposed to explain the unexpected reduced buckling in surface alloy systems. In the model, the forces acting on an adsorbate atom from its NN substrate atoms in different layers may be either repulsive or attractive, depending on whether the bond between the adsorbate atom and its NN substrate atoms is compressed or stretched. It is found that the forces on the adsorbate atom from its NN substrate atoms in the sub-surface layer play a more important role for the buckling of surface alloy than those from its NN substrate atoms in the surface layer do. The bucklings expected by the model are significantly smaller than those predicted by the simple hard sphere model and are in good agreement with the experiments when the equilibrium bond length of the NN adsorbate-substrate atom pairs is taken as the sum of the corresponding metal radii.     

Characteristics of deep penetration laser welding of dissimilar metal Ni-based cast superalloy K418 and alloy steel 42CrMo

Experimental trials of autogenous deep penetration welding between dissimilar cast Ni-based superalloy K418 and alloy steel 42CrMo flat plates with 5.0 mm thickness were conducted using a 3 kW continuous wave (CW) Nd:YAG laser. The influences of laser output power, welding velocity and defocusing distance on the morphology, welding depth and width as well as quality of the welded seam were investigated. Results show that full keyhole welding is not formed on both K418 and 42CrMo side, simultaneously, due to the relatively low output power. Partial fusion is observed on the welded seam near 42CrMo side because of the large disparity of thermal–physical and high-temperature mechanical properties of these two materials. The microhardness of the laser-welded joint was also examined and analyzed. It is suggested that applying negative defocusing in the range of Raylei length can increase the welding depth and improve the coupling efficiency of the laser materials interaction.     

Structural change of liquid Si15Te85 and Si20Te80 with temperature: Ab initio molecular dynamics simulations

Using ab initio molecular dynamics simulations, the structural and electronic properties of liquid Si₁₅Te₈₅ and Si₂₀Te₈₀ at two temperatures were studied respectively. Compared with available experimental data, the calculated structure factors are acceptable. From symmetry arguments, the calculated partial bond-angle distribution functions suggest that with increasing temperature the extensive tetrahedral network structures persist longer in liquid Si₂₀Te₈₀ than those do in liquid Si₁₅Te₈₅. Our results indicate that the local tetrahedral structure around Si atoms and the Peierls-like distorted local atomic structure around Te atoms both play important roles in the structural change of liquid Si₂₀Te₈₀ and Si₁₅Te₈₅, which also suggest that the mechanisms of the structural change upon cooling in liquid Si₂₀Te₈₀ and Si₁₅Te₈₅ are of no essential difference. The results of DOS and LDOS indicate that the variation of the dip in DOS at E_F mainly results from the change of Te p orbitals.