Ref HAL: hal-03032695_v1
DOI: 10.1038/s41586-020-2869-5
WoS: 000588830300008
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5 Citations

Ref HAL: hal-02986296_v1
DOI: 10.1063/5.0019514
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Résumé:

Using molecular dynamics simulations we investigate the dependence of the structuraland vibrational properties of the surfaces of sodo-silicate glasses on the sodium content as well as the nature of the surface. Two types of glass surfaces are considered:A melt-formed surface (MS) in which a liquid with a free surface has been cooleddown into the glass phase and a fracture surface (FS) obtained by tensile loadingof a glass sample. We find that the MS is more abundant in Na and non-bridgingoxygen atoms than the FS and the bulk glass, whereas the FS has higher concentration of structural defects such as two-membered rings and under-coordinated Si thanthe MS. We associate these structural differences to the production histories of theglasses and the mobility of the Na ions. It is also found that for Na-poor systems thefluctuations in composition and local atomic charge density decay with a power-lawas a function of distance from the surface while Na-rich systems show an exponentialdecay with a typical decay length of ≈ 2.3 Å. The vibrational density of states showsthat the presence of the surfaces leads to a decrease of the characteristic frequenciesin the system. The two-membered rings give rise to a pronounce band at ≈ 880 cm−1which is in good agreement experimental observations.

Ref HAL: hal-02904329_v1
DOI: 10.1073/pnas.2005638117
WoS: WOS:000546772500002
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Ref HAL: tel-02902601_v1
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Résumé:

La compréhension de la structure et du comportement mécanique des matériaux àl’échelle microscopique est cruciale pour la conception de nouveaux produits aux pro-priétés spécifiques. Cette thèse vise à obtenir des informations microscopiques sur lespropriétés, notamment celles de la fracture, des verres d’oxydes qui sont parmi lesmatériaux les plus utilisés au monde. À cette fin, nous utilisons des techniques desimulation atomistique de pointe pour étudier la silice et des silicates de sodium, c’est-à-dire les compositions représentatives pour de nombreux verres d’oxydes. À l’aide desimulations de dynamique moléculaire à grande échelle, la fracture dynamique des ver-res est étudiée de manière approfondie. Nous montrons que les propriétés mécaniquesdes verres sont considérablement plus sensibles au potentiel d’interaction et au pro-tocole de simulation utilisés qu’à leurs propriétés structurelles. La fracture du verrede silice est due aux ruptures de liaisons en pointe de fissure, tandis que la fracturedes verres riches en Na s’accompagne d’une croissance et d’une coalescence des cavités.Nous montrons également que l’origine microscopique du comportement transitoireprésenté par la rigidité des verres en fonction de leur composition se trouve dans ledéplacements atomiques non affines des atomes constituants. On constate que lessurfaces générées suite à la fracture sont considérablement plus rugueuses que les sur-faces formées par fusion et présentent un comportement en loi logarithmique à l’échellenanométrique (≤ 10 nm). En utilisant des simulations premiers principes, les signa-tures vibrationnelles et électroniques de certaines unités structurales, abondantes surla surface du verre, sont identifiées. De plus, l’ionicité et la force de divers types de li-aisons sont extraites à partir de ces simulations. Enfin, nous introduisons une méthodenouvelle pour caractériser la structure des liquides et des verres. Notre analyse montreque ces systèmes ont une structure tridimensionnelle étonnamment ordonnée.

Ref HAL: hal-02563236_v1
Ref Arxiv: 1910.06181
DOI: 10.1016/j.jnoncrysol.2020.119895
WoS: 000519653700018
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Ref HAL: hal-02563181_v1
DOI: 10.1063/1.5142605
WoS: 000519911600001
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Résumé:

We adapt and apply a recently developed optimization scheme used to obtain effective potentialsfor aluminosilicate glasses to include the network former boron into the interaction parameter set.As input data for the optimization, we used the radial distribution functions of the liquid at hightemperature generated by ab initio molecular dynamics simulations, and density, coordination andelastic modulus of glass at room temperature from experiments. The new interaction potentials areshown to reproduce reliably the structure, coordination and mechanical properties over a widerange of compositions for binary alkali borates. Furthermore, the transferability of these newinteraction parameters allows mixing to reliably reproduce properties of various boroaluminateand borosilicate glasses.

Ref HAL: hal-02540128_v1
Ref Arxiv: 1908.01484
DOI: 10.1021/acs.macromol.9b02166
WoS: 000526399500029
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1 Citation
Résumé:

In order to determine in polymeric systems the geometrical mesh size ξ, we simulate a solution of coarse-grained polymers with densities ranging from the dilute to the concentrated regime and for different chain lengths. We determine the monomer density fluctuation correlation length ξc from the monomer structure factor as well as the radial distribution function, showing that the identification ξ = ξc is not justified outside of the semidilute regime. In order to better characterize ξ, we compute the pore size distribution (PSD) following two different definitions, one by Torquato et al. and one by Gubbins et al. We find that the mean values of the two distributions, ⟨r⟩T and ⟨r⟩G, display the behavior predicted for ξ by scaling theory, and argue that ξ can be identified with either one of these quantities. This identification allows to interpret the PSD as the distribution of mesh sizes, a quantity which conventional methods cannot access. Finally, we show that it is possible to map a polymer solution on a system of hard or overlapping spheres, for which Torquato’s PSD can be computed analytically and reproduces accurately the PSD of the solution. We give an expression that allows ⟨r⟩T to be estimated with high accuracy in the semidilute regime by knowing only the radius of gyration and the density of the polymers.