958 research outputs found
Surface energetics and structure of the Ge wetting layer on Si(100)
Ge deposited on Si(100) initially forms heteroepitaxial layers, which grow to a critical thickness of ~3 MLs before the appearance of three-dimensional strain relieving structures. Experimental observations reveal that the surface structure of this Ge wetting layer is a dimer vacancy line (DVL) superstructure of the unstrained Ge(100) dimer reconstruction. In the following, the results of first-principles calculations of the thickness dependence of the wetting layer surface excess energy for the c(4×2) and 4×6 DVL surface reconstructions are reported. These results predict a wetting layer critical thickness of ~3 MLs, which is largely unaffected by the presence of dimer vacancy lines. The 4×6 DVL reconstruction is found to be thermodynamically stable with respect to the c(4×2) structure for wetting layers at least 2 ML thick. A strong correlation between the fraction of total surface induced deformation present in the substrate and the thickness dependence of wetting layer surface energy is also shown
Method for Computing Short-Range Forces between Solid-Liquid Interfaces Driving Grain Boundary Premelting
We present a molecular dynamics based method for computing accurately
short-range structural forces resulting from the overlap of spatially diffuse
solid-liquid interfaces at wetted grain boundaries close to the melting point.
The method is based on monitoring the fluctuations of the liquid layer width at
different temperatures to extract the excess interfacial free-energy as a
function of this width. The method is illustrated for a high energy Sigma 9
twist boundary in pure Ni. The short-range repulsion driving premelting is
found to be dominant in comparison to long-range dispersion and entropic forces
and consistent with previous experimental findings that nanometer-scale layer
widths may only be observed very close to the melting point.Comment: 5 pages, four figure
Structural disjoining potential for grain boundary premelting and grain coalescence from molecular-dynamics simulations
We describe a molecular dynamics framework for the direct calculation of the
short-ranged structural forces underlying grain-boundary premelting and
grain-coalescence in solidification. The method is applied in a comparative
study of (i) a Sigma 9 120 degress twist and (ii) a Sigma 9 {411}
symmetric tilt boundary in a classical embedded-atom model of elemental Ni.
Although both boundaries feature highly disordered structures near the melting
point, the nature of the temperature dependence of the width of the disordered
regions in these boundaries is qualitatively different. The former boundary
displays behavior consistent with a logarithmically diverging premelted layer
thickness as the melting temperature is approached from below, while the latter
displays behavior featuring a finite grain-boundary width at the melting point.
It is demonstrated that both types of behavior can be quantitatively described
within a sharp-interface thermodynamic formalism involving a width-dependent
interfacial free energy, referred to as the disjoining potential. The
disjoining potential for boundary (i) is calculated to display a monotonic
exponential dependence on width, while that of boundary (ii) features a weak
attractive minimum. The results of this work are discussed in relation to
recent simulation and theoretical studies of the thermodynamic forces
underlying grain-boundary premelting.Comment: 24 pages, 8 figures, 1 tabl
First-principles study of the energetics of charge and cation mixing in U_{1-x} Ce_x O_2
The formalism of electronic density-functional-theory, with Hubbard-U
corrections (DFT+U), is employed in a computational study of the energetics of
U_{1-x} Ce_x O_2 mixtures. The computational approach makes use of a procedure
which facilitates convergence of the calculations to multiple self-consistent
DFT+U solutions for a given cation arrangement, corresponding to different
charge states for the U and Ce ions in several prototypical cation
arrangements. Results indicate a significant dependence of the structural and
energetic properties on the nature of both charge and cation ordering. With the
effective Hubbard-U parameters that reproduce well the measured
oxidation-reduction energies for urania and ceria, we find that charge transfer
between U(IV) and Ce(IV) ions, leading to the formation of U(V) and Ce(III),
gives rise to an increase in the mixing energy in the range of 4-14 kJ/mol of
formula unit, depending on the nature of the cation ordering. The results
suggest that although charge transfer between uranium and cerium ions is
disfavored energetically, it is likely to be entropically stabilized at the
high temperatures relevant to the processing and service of urania-based solid
solutions.Comment: 8 pages, 6 figure
Ginzburg-Landau theory of crystalline anisotropy for bcc-liquid interfaces
The weak anisotropy of the interfacial free-energy is a crucial
parameter influencing dendritic crystal growth morphologies in systems with
atomically rough solid-liquid interfaces. The physical origin and quantitative
prediction of this anisotropy are investigated for body-centered-cubic (bcc)
forming systems using a Ginzburg-Landau theory where the order parameters are
the amplitudes of density waves corresponding to principal reciprocal lattice
vectors. We find that this theory predicts the correct sign,
, and magnitude, , of this anisotropy in good agreement
with the results of MD simulations for Fe. The results show that the
directional dependence of the rate of spatial decay of solid density waves into
the liquid, imposed by the crystal structure, is a main determinant of
anisotropy. This directional dependence is validated by MD computations of
density wave profiles for different reciprocal lattice vectors for
crystal faces. Our results are contrasted with the prediction of the reverse
ordering from an earlier formulation of
Ginzburg-Landau theory [Shih \emph{et al.}, Phys. Rev. A {\bf 35}, 2611
(1987)].Comment: 9 pages, 5 figure
Capillary force-induced structural instability in liquid infiltrated elastic circular tubes
The capillary-induced structural instability of an elastic circular tube
partially filled by a liquid is studied by combining theoretical analysis and
molecular dynamics simulations. The analysis shows that, associated with the
instability, there is a well-defined length scale (elasto-capillary length),
which exhibits a scaling relationship with the characteristic length of the
tube, regardless of the interaction details. We validate this scaling
relationship for a carbon nanotube partially filled by liquid iron. The
capillary-induced structural transformation could have potential applications
for nano-devices
Heuristic generation via parameter tuning for online bin packing
Online bin packing requires immediate decisions to be made for placing an incoming item one at a time into bins of fixed capacity without causing any overflow. The goal is to maximise the average bin fullness after placement of a long stream of items. A recent work describes an approach for solving this problem based on a ‘policy matrix’ representation in which each decision option is independently given a value and the highest value option is selected. A policy matrix can also be viewed as a heuristic with many parameters and then the search for a good policy matrix can be treated as a parameter tuning process. In this study, we show that the Irace parameter tuning algorithm produces heuristics which outperform the standard human designed heuristics for various instances of the online bin packing problem
Atomic-scale structure of the SrTiO3(001)-c(6x2) reconstruction: Experiments and first-principles calculations
The c(6x2) is a reconstruction of the SrTiO3(001) surface that is formed
between 1050-1100oC in oxidizing annealing conditions. This work proposes a
model for the atomic structure for the c(6x2) obtained through a combination of
results from transmission electron diffraction, surface x-ray diffraction,
direct methods analysis, computational combinational screening, and density
functional theory. As it is formed at high temperatures, the surface is complex
and can be described as a short-range ordered phase featuring microscopic
domains composed of four main structural motifs. Additionally, non-periodic
TiO2 units are present on the surface. Simulated scanning tunneling microscopy
images based on the electronic structure calculations are consistent with
experimental images
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Direct imaging of short-range order and its impact on deformation in Ti-6Al.
Chemical short-range order (SRO) within a nominally single-phase solid solution is known to affect the mechanical properties of alloys. While SRO has been indirectly related to deformation, direct observation of the SRO domain structure, and its effects on deformation mechanisms at the nanoscale, has remained elusive. Here, we report the direct observation of SRO in relation to deformation using energy-filtered imaging in a transmission electron microscope (TEM). The diffraction contrast is enhanced by reducing the inelastically scattered electrons, revealing subnanometer SRO-enhanced domains. The destruction of these domains by dislocation planar slip is observed after ex situ and in situ TEM mechanical testing. These results confirm the impact of SRO in Ti-Al alloys on the scale of angstroms. The direct confirmation of SRO in relationship to dislocation plasticity in metals can provide insight into how the mechanical behavior of concentrated solid solutions by the material's thermal history
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