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Résumé:

Silica glass is a typical example of disordered material. Theoretically, molecular dynamics computer simulation is a common and effective way to investigate structural and dynamic microscopic properties of silica glass, such as pair correlation functions, bond angle distribution, vibrational density of states and so on. In this work, we have prepared several different silica models solely within the framework of first-principles approach of the order of hundred atoms (38 Si and 76 O atoms) and analyze their structural, electronic and vibrational properties. The SiO2 models have been generated by quenching liquid silica (equilibrated at 3600 K and with a density equal to 2.20 g/cm3 ) to room temperature (300 K) using different cooling rates. We have compared the microscopic properties with experimental and previous simulation results. We have found that the cooling rate affects the properties of generated silica samples and that the slowly quenched model gives better agreement with the experimental results. These models have been gradually compressed up to 2.67 g/cm3 density, which corresponds to about 7 GPa: just below the elastic to plastic transition regime (estimated around 8 to 10 GPa) of silica glass. To characterize microscopic properties of compressed silica models below the transition pressure is an important step to investigate the mechanism of the transition with further compressed silica models (above the transition pressure). We show the properties of these compressed silica models and compare them with those of the uncompressed ones. Another type of models are also generated by compressing and equilibrating the liquid silica at 3600 K to 2.67 g/cm3 and then quenched to 300 K. The microscopic properties of these samples will be also presented and discussed.

Ref HAL: hal-00533613_v1
Ref Arxiv: 0811.0223
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
Résumé:

We discuss the relaxation dynamics of a simple lattice gas model for glass-forming systems and show that with increasing density of particles this dynamics slows down very quickly. By monitoring the trajectory of tagged particles we find that their motion is very heterogeneous in space and time, leading to regions in space in which there is a fast dynamics and others in which it is slow. We determine how the geometric properties of these quickly relaxing regions depend on density and time. Motivated by this heterogeneous hopping dynamics, we use a simple model, a variant of a continuous time random walk, to characterize the relaxation dynamics. In particular we find from this model that for large displacements the self part of the van Hove function shows an exponential tail, in agreement with recent findings from experiments and simulations of glass-forming systems.

Commentaires: Paper presented at the 10th Granada Conference on Computational and Statistical physics Journal: AIP Conf. Proc. 1091, 95 (2009)

Ref HAL: hal-00533612_v1
Ref Arxiv: 0811.1447
Ref. & Cit.: NASA ADS
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We study a microscopically realistic model of a physical gel and use computer simulations to investigate its static and dynamic properties at thermal equilibrium. The phase diagram comprises a sol phase, a coexistence region ending at a critical point, a gelation line, and an equilibrium gel phase unrelated to phase separation. The global structure of the gel is homogeneous, but the stress is supported by a fractal network. Gelation results in a dramatic slowing down of the dynamics, which can be used to locate the transition, which otherwise shows no structural signatures. Moreover, the equilibrium gel dynamics is highly heterogeneous as a result of the presence of particle families with different mobilities. An analysis of gel dynamics in terms of mobile and arrested particles allows us to elucidate several differences between the dynamics of equilibrium gels and that of glass-formers.

Commentaires: 9 pages, 7 figures, paper presented at the 10th Granada Seminar on Computational and Statistical Physics Journal: AIP Conf. Proc. 1091, 166 (2009)

Ref HAL: hal-00364535_v1
Ref Arxiv: 0809.1613
DOI: 10.1016/j.commatsci.2008.03.046
WoS: 000264463800015
Ref. & Cit.: NASA ADS
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24 Citations
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We present a genetic algorithm for the atomistic design and globaloptimisation of substitutionally disordered bulk materials and surfaces.Premature convergence which hamper conventional genetic algorithms due toproblems with synchronisation is avoided using a symmetry adapted crossover.The algorithm outperforms previously reported Monte Carlo and genetic algorithmsimulations for finding low energy minima of two simple alloy models withoutthe need for any redesign.

Commentaires: 9 pages, 4 figures

Ref HAL: hal-00368331_v1
DOI: 10.1016/j.susc.2007.09.047
WoS: 000252904700018
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14 Citations
Résumé:

We present the structural properties of thin films of a calcium alummosilicate glass generated by classical molecular dynamics (MD). The films are generated by two methods: in the first, the films are created in the liquid state and quenched to 300 K; in the second, the films are generated at room temperature. Depending on the method, film thickness and surface roughness are different but the main structural characteristics of the films are similar. The atomic concentration appears to be inhomogeneous from the center of the films to the surface and new structural entities are present at the film surfaces. The surfaces are depleted in calcium atoms and are enriched in aluminum atoms. This atomic arrangement induces a de-polymerized area under the surface. At the surface, all structural properties are modified in comparison with those of the bulk: interatomic distances, angular distributions, ring size, coordinations. In order to confirm the surface properties, we relaxed a surface using ab initio molecular dynamics. Some modifications appear but they do not significantly change the results obtained by classical MD, validating the use of interatomic potentials for the study of such films. (C) 2007 Elsevier B.V. All rights reserved.