Comparative study of morphology and surface composition of Al–Cr–Fe alloy powders produced by water and gas atomisation technologies

The morphology and surface composition of Al–Cr–Fe alloy powders of 0–63 and 63–100 μm size fractions, produced by gas and water atomisation, have been studied by scanning electron microscopy and Auger electron spectroscopy. While gas atomised particles are of spherical shape, water atomised powders are usually irregular in shape with a complex branched relief. The morphology and composition of surface oxides have been estimated. The surface oxide film is composed of aluminium oxides/hydroxides and contains no Fe and Cr atoms. Two to five water molecules are associated with one Al₂O₃ molecule on the surface of powders. The surface oxide film has a non-uniform thickness, with thick oxide islands separated by thinner oxide film. The parameters of the surface film morphology, such as the island coverage, the oxygen content and the thin film thickness, depend on the atomisation technology used and powder size fraction. Heavily and weakly oxidised powder groups present in all powder fractions are distinguished by Auger spectra analysis. Relationships between heavily and weakly oxidised powder groups are discussed as a function of atomisation technology and size fraction.     

The effect of second neighbour repulsion on fcc binary alloy phase diagrams: A Monte Carlo study

We report the results of the Monte Carlo simulation of the phase diagram of fcc binary alloys using a 3-D Ising model with nearest and next-nearest neighbour repulsive interactions. Calculations are carried out for a ratio of second- to first-neighbour interaction energies of 0.4. The resulting phase diagram contains three superstructures A₂B₂, A₂B and A₅B, each separated by a disordered fcc phase. There was no evidence for the formation of an A₃B phase. These results are in qualitative agreement with CVM results.     

Development of cube textured Ni–5 at.%W alloy substrates for coated conductor application using a melting process

Biaxially textured Ni–5 at.%W substrates have been prepared by cold rolling, followed by three different annealing routes. In this paper, the processes of melting Ni and W metals, flat rolling, various annealing methods are described in detail. The Ni–5 at.%W tapes annealed under either high vacuum or flowing Ar (7% H₂) gas were characterized by X-ray pole figures, ODF, EBSD as well as AFM analysis. The texture analysis indicated that as fabricated tapes have a sharp cube texture formed after annealing at a wide temperature range of 800–1100 °C. The high quality of cube orientation on tapes was obtained after a two-step annealing (TSA), where the percentage of the cube texture component was as high as 93.5% within a misorientation angle smaller than 8° from EBSD analysis. Furthermore, it was also observed that the number of twin boundaries in this tape decreased with respect to that of tapes annealed both in vacuum and one-step gas annealing. From AFM on 1 μm² areas, it was concluded that the roughness (RMS) on the tape surface reached 0.98 nm.     

Structural investigation of Si0.5Ge0.5 alloy for optoelectronic applications: Ab initio study

The structural, electronic and optical properties of the binary silicon–germanium alloy have been investigated using the projector augmented-wave (PAW) calculations with a powerful VASP package (Vienna ab initio simulation package). The structural properties of Si_0.5Ge_0.5 alloy have been calculated using total energy calculations and compared with our empirical model of bulk modulus. The electronic band structure and density of state of Si_0.5Ge_0.5 alloy show that the conduction band minimum (CBM) is located at the X point and the valence band maximum (VBM) is located at the Г point, resulting in indirect (Г–X) energy band gap of 0.48 eV. The results of the refractive index and optical dielectric constant of Si_0.5Ge_0.5 alloy are also obtained. The PAW's results are in good agreement with experimental, theoretical and our model results.     

Visualization of rigorous sum rules for Franck–Condon factors: spectroscopic applications to Xe2

A fully quantum mechanical approach to the calculation and normalization of the Franck–Condon factors for diatomic species is described. The treatment is based on the fundamental demand of completeness of the energy eigenfunctions, which results in the rigorous sum rule for the Franck–Condon overlap integrals. The importance of this general rule has been discussed and thoroughly illustrated in the case of diatomic xenon molecules. Exactly solvable reference potentials for this system have been constructed and a complete basis of the actual energy eigenstates (including both bound and scattering states) has been created. Several direct spectroscopic applications to xenon excimers are presented, and their good agreement with relevant experimental data demonstrated. In particular, a kinetic model is proposed to explain the observed oscillatory structures in the fluorescence spectra of Xe₂^* [Chem. Phys. Lett. 117 (1985) 301] related to their classical left turning points. The same model gives a uniform explanation to the well-known first and second emission continua of rare gases.     

Amplitude and frequency stabilisation of a Tm–Ho:YAG laser for coherent lidar applications at 2.1 μm

We developed a low-noise, single-frequency Tm–Ho:YAG laser tunable in the wavelength interval between 2087 and 2099 nm. To suppress both amplitude and frequency fluctuations the laser has been stabilised by two different control loops. Intensity noise has been effectively reduced using a feedback loop acting on the pump diode current, based on a biquadratic bandpass filter, which provides up to 17 dB suppression at the relaxation oscillation peak. Absolute frequency stabilisation has been achieved by locking the oscillator to the P(12) absorption line of the HBr molecule at 2097.2 nm using the fringe side locking technique, obtaining a long-term frequency stability better than 32 kHz over an observation time of 60 min. This stabilised source is aimed to injection seeding of a coherent lidar system for high precision measurements of wind velocity.     

Morphological,Structural, and Compositional Evolution of Pt–Ni Octahedral Electrocatalysts with Pt‐Rich Edges and Ni‐Rich Core: Toward the Rational Design of Electrocatalysts for the Oxygen Reduction Reaction

The progress in colloidal synthesis of Pt–Ni octahedra has been instrumental in rising the oxygen reduction reaction catalytic activity high above the benchmark of Pt catalysts. This impressive catalytic performance is believed to result from the exposure of the most active catalytic sites after an activation process, chemical or electrochemical, which leads to a Pt surface enrichment. A foremost importance is to understand the structure and the elemental distribution of Pt–Ni octahedral, which leads to an optimal catalytic activity and stability. However, the factors governing the synthesis of the Pt–Ni octahedra are not well understood. In this study, unprecedented surface atomic segregation of Pt atoms in a Ni‐rich Pt–Ni octahedral nanoparticle structure is established by advanced electron microscopy. The Pt atoms are almost exclusively located on the edges of the Pt–Ni octahedra. This structure is formed in a pristine form, i.e., prior to any chemical or electrochemical etching. A new growth mechanism is revealed, which involves the transformation from an octahedron with a Pt‐rich core to a Ni‐rich octahedron with Pt‐rich edges. This observation may pave the way for a deeper understanding of this class of Pt–Ni octahedral nanoparticles as an electrocatalyst.     

The effect of a silica support: a density functional theory study of the C–H bond activation of ethane on a nickel oxide cluster

The support effect is an important issue in heterogeneous catalysis. A systematic density functional theory computational study is reported here to better understand the C–H bond activation steps in the reaction between C₂H₆ and a model silica‐supported Ni₃O₃ cluster, as well as that between C₂H₆ and an unsupported Ni₃O₃ cluster. Two mechanisms, namely, a radical mechanism (denoted as mechanism A) and a concerted mechanism (denoted as mechanism B) were examined. Both of these mechanisms contain two steps. For the C–H bond activation taking place via mechanism A, the involvement of the model silica support does not change the most favorable pathway significantly; however, it does result in a modest increase in the reaction barrier and the overall Gibbs energy change. For the C–H bond activation taking place via mechanism B, the involvement of the model silica support leads to an increase in the reaction barrier in the first step. The product of this step has a noticeable difference in the structures for the Ni₃O₃ moiety in the unsupported and model silica‐supported systems. The result of charge analysis shows that there is no noticeable charge transfer between the silica support and Ni₃O₃ when they are in the starting reactants, while there is an electron withdrawal from Ni₃O₃ by the silica support when they are in transition states, intermediates, or products. The results here provide deeper insights into the support effect on the C–H bond activation of lower alkanes on supported transition metal catalysts. Copyright © 2015 John Wiley & Sons, Ltd.     

Beam‐size‐related phenomena and effective normalization in energy‐dispersive EXAFS for the study of heterogeneous catalysts,powder materials and the processes they mediate: observations and (some) solutions

The effects of focal spot size and the nature of powder samples (such as heterogeneous catalysts) on the quality of data obtainable from a dispersive EXAFS experiment are characterized at ID24 of the ESRF. Using examples of supported Pd catalysts, it is shown that, for a given photon flux, massive improvements in data quality can be achieved by increasing the size of the dispersive beam in the vertical, whilst concurrently applying a methodology to account for scattering effects emanating from the samples under study. These improvements are demonstrated using progressively practical and demanding examples. Questions regarding optimal beam dimensions for the study of such materials, how to counter undesirable effects that arise from the coherence of the source, how to obtain similar results consistently across the 5–30 keV bandwidth of ID24, and whether a methodology for simultaneous normalization in dispersive EXAFS is of significant utility in such circumstances are discussed.