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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 84 (1998), S. 5261-5269 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Three-dimensional molecular dynamics simulations of ion beam assisted deposition (IBAD) are performed to determine the mechanisms of crystallographic texture selection during the IBAD of polycrystalline films. A face centered cubic bicrystal consisting of [111] and [110] oriented grains is grown while an ion beam bombards the growing film at normal incidence. As the film grows, the grain boundaries delimiting the [111] and [110] grains move towards each other, eventually pinching off the [111] grain such that the film texture changes from equal densities of [111] and [110] to purely [110]. Examination of single crystals grown in the presence of ion beams shows two important effects: ion beam induced atomic sputtering from the surface and ion beam induced damage are significantly reduced when the ion beam is oriented along channeling directions of the crystals. The first observation suggests that grains with channeling directions aligned parallel to the ion beam grow more quickly than those where they are not aligned. This leads to grain-to-grain variations in the film thickness that increase in magnitude during growth. Variations in thickness result in a shadowing effect that further slows the growth of the less thick (nonaligned) grains—eventually leading to pinch-off of these grains. The second observation suggests that the stored energies within the grains with channeling directions aligned parallel to the ion beam will be lower than that of the nonaligned grains. This difference in stored energy (in the form of crystal defects) is shown to lead to grain boundary migration—a process equivalent to primary recrystallization. Both of these effects can lead to changes in crystallographic texture during film growth and both are observed in the bicrystal simulations. Which mechanism will dominate under a prescribed set of conditions remains to be delineated. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 59 (1991), S. 2535-2537 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report observations of two distinct types of phase-separated microstructures in co-deposited Al-Ge films. In the initial stages of growth, lateral phase separation is observed, with a temperature dependence consistent with surface diffusion. As the film grows thicker, the Ge-rich phase becomes increasingly buried, and a transverse phase-separated microstructure results, consisting of an Al-rich layer covering a Ge-rich layer. This observation is explained in terms of the competition between surface and interfacial free energies. We discuss the kinetic aspects of the phase separation process, and the resulting behavior in the thick-film limit.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 60 (1986), S. 1793-1796 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Both the steady-state and transient solute (impurity) concentrations at the surface of a film during its deposition are calculated as a function of deposition rate, temperature, diffusivity, bulk film solute concentration, and segregation energy. The steady-state solute concentration at the surface is found to increase upon increasing either the solute diffusivity or the magnitude of the (negative) segregation energy, or upon decreasing the deposition rate. The transient concentration profile at the start of deposition relaxes toward the steady-state profile as 1/t. Implications of the present results for film growth mechanisms and methods to control the degree of segregation via manipulation of deposition parameters are discussed.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 6197-6197 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 5090-5097 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present results of variational calculations of the Helmholtz free energy and the thermodynamic properties of a series of metallic liquids and solids (Ag, Au, Cu, Ni, Pd, Pt) described by embedded-atom-method potentials. For the solids, we use a variational procedure based on an Einstein-model reference state. The free energies of liquids are calculated with an approximate variational method proposed by Ross. At the respective melting points, the present results for the Helmholtz free energy are within about 1% of the results of accurate Monte Carlo (MC) calculations with the same interaction potentials, both for the fluid and the solid. The average error in the melting points calculated with the present procedure relative to Monte Carlo results is about 7.5%. The internal energies and entropies are compared to MC results, and show, in general, good agreement.
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The effects of lanthanide co-dopants on oxygen diffusion in yttria-stabilized zirconia (YSZ) are studied using a combined first principles density functional theory (DFT)/kinetic Monte Carlo (kMC) modeling approach. DFT methods are used to calculate barrier energies for oxygen migration in different local cation environments, which are then input into kMC simulations to obtain long-time oxygen diffusivities and activation energies. Simulation results show a substantial increase in the maximum value of the oxygen diffusivity upon co-doping and in the dopant content at which this value is obtained for Lu-co-doped YSZ; while relatively little change is seen for Gd-co-doped YSZ. Examination of the DFT barrier energies reveals a linear scaling of barrier heights with the size of cations at the diffusion transition state. Using this strong correlation, oxygen diffusivity is examined in YSZ co-doped with several lanthanide elements. The oxygen diffusivity decreases with dopant atomic number (and decreasing dopant ion size) for co-dopants smaller than Y, and changes relatively little when Y is replaced by co-dopants larger than it. These results are broadly consistent with experiment, and are explained in terms of cation-dopant and vacancy concentration-dependent correlation effects, with the aid of a simple analytical model.
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  • 7
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: We present a multiscale modeling approach to study oxygen diffusion in cubic yttria-stabilized zirconia. In this approach, we employ density functional theory methods to calculate activation energies for oxygen migration in different cation environments. These are used in a kinetic Monte Carlo framework to calculate long-time oxygen diffusivities. Simulation results show that the oxygen diffusivity attains a maximum value at around 0.1 mole fraction yttria. This variation in the oxygen diffusivity with yttria mole fraction and the calculated values for the diffusivity agree well with experiment. The competing effects of increased oxygen vacancy concentration and increasing activation energy and correlation effects for oxygen diffusion with increasing yttria mole fraction are responsible for the observed dopant content dependence of the oxygen diffusivity. We provide a detailed analysis of cation-dopant-induced correlation effects in support of the above explanation.
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  • 8
    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.
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  • 9
    ISSN: 1573-2746
    Keywords: Grain boundaries ; 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 [001] Σ5 twist boundaries in Cu−Ni, Au−Pd, and Ag−Au 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 grain boundary segregation profiles, grain boundary 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. For all alloy bulk compositions (0.05 ≤ C ≤ 0.95) and temperatures (400 ≤ T (K) ≤ 1,100) examined, Cu and Au segregates to the boundary in the Cu−Ni and Au−Pd alloy systems, respectively; although in the Ag−Au alloys, the majority element segregates to the boundary. The width of the segregation profile is limited to approximately three to four (002) atomic planes. The classical theories for the segregation, and the effects of the relaxation with respect to either the atomic positions or the atomic concentrations, are discussed. The boundary thermodynamic properties depend sensitively on the magnitude of the boundary segregation, and some of them are shown to vary linearly with the magnitude of the grain boundary segregation.
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  • 10
    ISSN: 1573-2746
    Keywords: Adhesion ; heterophase interface ; first-principles electronic structure calculations
    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 bonding characteristics, interfacial energetics, and electronic structure associated with adhesion at the Mo-MoSi2(001) heterophase interface are investigated using the first-principles, self-consistent local orbital method. We found both the adhesive energy and peak interfacial strength for the interface to be 10%-15% smaller than the respective values for cleavage along the (001) planes in crystalline Mo and MoSi2. The equilibrium interlayer separation between Mo and MoSi2 is found to lie between the interplanar spacings of crystalline Mo and MoSi2. The interfacial adhesive bonding is attributable to the combination of a nearly uniform band of charge accumulation at the interface and directional charge accumulation between atoms across the interface. These first-principles calculations demonstrate that the universal-binding-energy relation can be extended to describe adhesion between dissimilar materials.
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