Abstract:We have characterized the short-range order in the liquid and undercooled states of Au-Si alloy at the eutectic composition using molecular dynamics simulations. The interactions are described via a modified embedded-atom model refined to take into account the liquid properties. For the eutectic liquid, the local structure is characterized by a strong Au-Si affinity, namely a well-pronounced chemical short-range order which leads to the slowing down of the formation of icosahedral local motifs in the undercool… Show more
“…13 Indeed, at higher temperatures, the shear cell data exhibit an anomalous change in slope, presumably a result of a convection-enhanced diffusion profile. Jakse et al 21 have determined a mean self-diffusivity in this melt using molecular dynamics (MD) simulations, which is in good agreement with experimental values. At the T m of this alloy, we derive D Au in Au 81 Si 19 as 7.5(60.4) Â 10 À10 m 2 s À1 , and 3.7(60.1) Â 10 À9 m 2 s À1 at the T m of pure liquid gold.…”
supporting
confidence: 71%
“…Essentially, this is the result of a less densely packed local structure, 22 in which the more mobile silicon atoms reduce the effective energy barriers for atomic transport. Similar 20 (error bars are on the order of the symbol size), along with mean self-diffusion coefficients hDi from MD simulations 21 (filled diamonds). The filled square is the self-diffusion coefficient of pure liquid gold from MD simulations.…”
We use incoherent quasielastic neutron scattering to study the atomic dynamics of gold in a eutectic Au81Si19 melt. Despite the glass-forming nature of this system, the gold self-diffusivity displays an Arrhenius behavior with a low activation energy characteristic of simple liquids. At high temperatures, long-range transport of gold atoms is well described by hydrodynamic theory with a simple exponential decay of the self-correlation function. On cooling towards the melting temperature, structural relaxation crosses over to a highly stretched exponential behavior. This suggests the onset of a heterogeneous dynamics, even in the equilibrium melt, and is indicative of a very fragile liquid.
“…13 Indeed, at higher temperatures, the shear cell data exhibit an anomalous change in slope, presumably a result of a convection-enhanced diffusion profile. Jakse et al 21 have determined a mean self-diffusivity in this melt using molecular dynamics (MD) simulations, which is in good agreement with experimental values. At the T m of this alloy, we derive D Au in Au 81 Si 19 as 7.5(60.4) Â 10 À10 m 2 s À1 , and 3.7(60.1) Â 10 À9 m 2 s À1 at the T m of pure liquid gold.…”
supporting
confidence: 71%
“…Essentially, this is the result of a less densely packed local structure, 22 in which the more mobile silicon atoms reduce the effective energy barriers for atomic transport. Similar 20 (error bars are on the order of the symbol size), along with mean self-diffusion coefficients hDi from MD simulations 21 (filled diamonds). The filled square is the self-diffusion coefficient of pure liquid gold from MD simulations.…”
We use incoherent quasielastic neutron scattering to study the atomic dynamics of gold in a eutectic Au81Si19 melt. Despite the glass-forming nature of this system, the gold self-diffusivity displays an Arrhenius behavior with a low activation energy characteristic of simple liquids. At high temperatures, long-range transport of gold atoms is well described by hydrodynamic theory with a simple exponential decay of the self-correlation function. On cooling towards the melting temperature, structural relaxation crosses over to a highly stretched exponential behavior. This suggests the onset of a heterogeneous dynamics, even in the equilibrium melt, and is indicative of a very fragile liquid.
“…The procedure of aforementioned semi-empirical potentials parametrization is easy for pure metals, however its complexity grows with increasing number of alloying elements: for binary systems, it is still feasible [15,16], in comparison to being difficult for multicomponent systems [17]. As it has been recognised, amongst these potentials, the most widely used are the embedded atom model [18] and its recent version -the modified embedded atom model [13,19,20] or the angular-dependent formalism [21], which allow to sufficiently reflect the experimentally determined thermodynamic, structural and physical properties [15,22]. Indeed, first-principles calculations provide the best and most detailed atomic-scale information, but they sometimes could be insufficient to study bulk materials' properties like viscosity or diffusion due to the limit in the number of atoms used.…”
“…The diffusion constant profile in Fig. 2(a) is significantly less abrupt than that corresponding to the (001) surface displayed in Fig S8 and is characterized by a value in the bulk-like part lower than that of the bulk eutectic liquid 19 , in accordance to a higher degree of FFS as shown in Fig. 2(b) .…”
Section: Resultsmentioning
confidence: 69%
“… Figure 2 Results from the large scale classical MD simulations with the MEAM potential at T = 600 K for the Si (111)-(6 × 6)/AuSi interface determined in the liquid slab for ( a ) the self-diffusion coefficient for Au as a function of z , and ( b ) the percentage of five-fold symmetry as a function of z , The shadowed red lines in both panels represent the density profiles in arbitrary units of the crystalline substrate for visual purpose only, in order to highlight the AuSi liquid slab z -range. In each case, the green dotted line arrows mark the positions of both interfaces, and the red dot-dashed horizontal line marks the corresponding value of the bulk eutectic liquid 19 , 20 . …”
Bringing a liquid into contact with a solid is known to generally promote crystal nucleation at the freezing temperature. In contrast, it is much more difficult to conceive that a solid surface may hinder nucleation and favor large undercooling effects. Here we report on ab initio and classical molecular dynamic simulations to capture the underlying structural mechanism responsible for this striking effect. We find that the substrate/liquid interactions exert an important influence on in-plane ordering of the adjacent liquid layers in the undercooling regime. In particular, we identify that the presence of atomic arrangements with five-fold symmetry (FFS) on the substrate surface in the form of pentagonal atomic motifs allows the liquid to be undercooled well below its freezing temperature. Our findings clearly demonstrate that this pentagonal-coordinated surface enhances the presence of local arrangements with FFS in the adjacent liquid layers that prevents the crystal nucleation. Finally we suggest new technological developments to attain large undercooling effects.
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