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  • 1
    ISSN: 1573-2746
    Keywords: Surfaces ; computer simulation ; free energy ; segregation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract Atomistic simulations of segregation to (100) free surface in Ag−Au, Au−Pd, and Cu−Ni alloy systems have been performed for a wide range of temperatures and compositions within the solid solution region of these alloy phase diagrams. In addition to the surface segregation profiles, surface free energies, enthalpies, and entropies were determined. These simulations were performed within the framework of the free energy simulation method, in which an approximate free energy functional is minimized with respect to atomic coordinates and atomic site occupation. The effects of the relaxation with respect to either the atomic positions or the atomic concentrations are discussed. For all alloy bulk compositions (0.05 ≤ C ≤ 0.95) and temperatures (400 ≤ T(K) ≤ 1,100) examined, Ag, Au, and Cu segregates to the surface in the Ag−Au, Au−Pd, and Cu−Ni alloy systems, respectively. The present results are compared with several theories for segregation. The resultant segregation profiles in Au−Pd and Ag−Au alloys are shown to be in good agreement with an empirical segregation theory, while in Cu−Ni alloys the disagreement in Ni-rich alloys is substantial. The width of the segregation profile is limited to approximately three to four atomic planes. The surface thermodynamic properties depend sensitively on the magnitude of the surface segregation, and some of them are shown to vary linearly with the magnitude of the surface segregation.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1573-2746
    Keywords: Segregation ; grain boundaries ; enrichment factor
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract A high-angle grain boundary is modeled as a planar defect characterized by its thickness and atomic density. We successively examine the elastic and electronic contributions to the solute/grain boundary binding energy. We deduce the effect of the grain boundary physical parameters on its propensity for segregation. The thickness of high-angle grain boundaries is not a fundamental parameter for segregation. The atomic density in the grain boundary controls the electronic binding energy. The rate of change of elastic constants with the density is the important factor in the elastic contribution to segregation. We conclude that segregation to boundaries with small excess volumes is not precluded.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1573-2746
    Keywords: SiC ; whisker ; HRTEM ; iron ; VLS
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract SiC whiskers were grown from the reaction of silicon monoxide (SiO) with activated carbon containing iron impurities. Growth proceeds through a VLS growth mechanism with SiO and CO as reacting gases. HRTEM combined with EDS shows that the SiC whisker is topped by a Fe3Si catalyst droplet. The SiC whisker is found to be one-dimensionally disordered along the [111] growth direction of an fcc crystal structure. Although the catalyst droplet is usually larger than the top face of the whisker, we observed a number of situations where the diameter of the droplet was smaller. The study of the SiC-Fe3Si interface showed that the growth is nucleated from the edges.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Interface science 1 (1993), S. 115-124 
    ISSN: 1573-2746
    Keywords: Asymmetrical grain boundaries ; structural units ; rational and irrational boundaries ; f.c.c. lattice
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract It is shown how the decomposition scheme into structural units can be developed for rational asymmetrical tilt grain boundaries without computer simulations. The method is illustrated on the Σ9 and Σ11 grain boundaries in the f.c.c. lattice for which experimental observations of the boundary structures are available. Besides short-period boundaries, which may serve as elementary structural units, the irrational aperiodic asymmetrical boundaries are also considered. There are clear experimental indications that some of them, namely those composed of dense-packed atomic planes, and called “special” asymmetrical boundaries, behave differently from the other more general boundaries.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1573-2746
    Keywords: Segregation ; grain boundaries ; structural order
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract We introduce a model for high-angle grain boundaries in the form of a two-dimensional array of compressive forces. Segregation profiles driven by the stress field around the boundary are calculated for several levels of translational order. We find an enhancement of segregation with increasing degree of translational disorder, in agreement with experimental observations.
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  • 6
    ISSN: 1573-2746
    Keywords: Silicon ; defects ; hyperfine interactions ; oxidation ; computer simulation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract We assess several models for the environment of the P b center (Si dangling bond center) at the interface of Si(111) with its oxide. The comparison of hyperfine constants observed with those predicted using large cluster models favors a local structure in which there is an Si-Si bond within the oxide close to the Si dangling bond. Such Si-Si bonds are also suggested by a number of other experiments and are consistent with the “reactive layer” model proposed to rationalize a range of oxidation studies.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1573-2746
    Keywords: Grain boundaries ; texture ; grain boundary misorientation distribution ; orientation distribution function
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract Orientation distribution functions in two recrystallized austenitic stainless steels (AISI types 304 and 316L) with known grain boundary misorientation distributions have been studied. Previously obtained data on grain boundary spectra in these steels have been re-examined and analyzed from the point of view of texture analysis. The results obtained have shown that there is no unambiguous relatonship between grain boundary misorientation distribution and grain orientation distribution (ODF) determined by the X-ray analysis in the materials under study. This ambiguity is due to the following reason. In the grain boundary misorientation statistics only nearest-neighbor grains are taken into account, but in the orientation distribution function orientations are averaged over the entire volume of the specimen independent as to whether the grains are adjacent or not. Two main results were established for the steels under study: (i) Textures of the two steels differ, though their grain boundary misorientation distributions are similar; and (ii) misorientations of the majority of grain boundaries can be described as rotations about the axes close to 〈110〉.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Springer
    Interface science 1 (1993), S. 163-174 
    ISSN: 1573-2746
    Keywords: Anisotropy ; grain boundaries ; interfaces ; segregation ; solid solubility
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract Based on thermodynamic analysis of interfacial segregation, the segregation enthalpy ΔH o of a solute I in a given matrix was found to depend linearly on two mutually independent terms reflecting the type of interface ϕ and the solid solubility limit X infI sup* at temperature T and can be written as In this equation, the structural dependence of interfacial segregation is contained in ΔH *(ϕ) which corresponds to the extrapolated segregation enthalpy of a solute with unlimited solubility in the matrix. The product [Tln(X infI sup* )] is essentially constant with temperature, and can therefore be obtained from data for maximum solid solubility, [Tln(X infI sup* )]max. The parameter v〉0 represents the relationship between the activity a infI sup* of a solute at the bulk solid solubility limit in a given matrix and X infI sup* , a infI sup* =(X infI sup* ) v , and is characteristic for the matrix. Using recent experimental data for silicon, phosphorus, and carbon segregation at well-characterized grain boundaries in oriented bicrystals of α-iron, the averaged value $$\bar \upsilon$$ was determined. Values of ΔH *(ϕ) range from -8 kJ/mol (general grain boundaries) up to +8 kJ/mol (special grain boundaries). These values are discussed and used for a more precise and generalized construction of grain boundary segregation diagrams of α-iron.
    Type of Medium: Electronic Resource
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  • 9
    ISSN: 1573-2746
    Keywords: thin films ; high temperature superconductor ; grain boundaries ; microstructure ; transport properties
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract The microstructure and transport properties of various 90° grain boundaries in (103) oriented YBa2Cu3O7(YBCO) thin films grown epitaxially in situ by 90° off-axis sputtering are compared. The (103) films grown on (101) LaAlO3 and (101) SrTiO3 substrates have specific sets of 90° grain boundaries in both principal in-plane directions: 90° [010] twist boundaries along the [101] direction, and 90° [010] symmetrical tilt boundaries and 90° [010] basal-plane-faced tilt boundaries along the (301) direction. No weak-link behavior is observed across some of these boundaries by transport critical current density and normalized magnetic field dependence of J c measurements along both those in-plane directions. High-resolution transmission electron microscopy reveals variations in the structure and microfaceting of the 90° boundaries, which may contribute to the absence of weak-link behavior. These results have important implications for understanding the behavior of step-edge Josephson junctions.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Springer
    Interface science 1 (1994), S. 243-257 
    ISSN: 1573-2746
    Keywords: Adsorption ; embedded-atom method ; Fourier representation ; surface migration ; vibration frequencies
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract This paper reports on the embedded-atom method (EAM, developed by Johnson) calculations of adsorption parameters for Cu and Ni on {110} unrelaxed substrates of Mo and W. The following are calculated: (i) the equilibrium height h c of an adatom; (ii) the optimum coefficients W 0 A hk of a truncated Fourier representation of the adatom-substrate interaction potential; (iii) the desorption energies E des ; and (iv) the activation energies Q of surface migration. It is shown that (a) the calculated values of E des agree satisfactorily with available empirical data; (b) the scale factor W 0 is approximately proportional to the bonding as measured by E des , whereas the normalized coefficients A hk are determined by the {110} substrate symmetry; (c) E des and Q are respectively dominated by the embedding energy and pair potential of the EAM; and (d) W 0 and E des for an atom in a monolayer are only about half as much as the corresponding quantities for an isolated atom.
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