116 research outputs found
Angular rigidity in tetrahedral network glasses
A set of oxide and chalcogenide tetrahedral glasses are investigated using
molecular dynamics simulations. It is shown that unlike stoichiometric
selenides such as GeSe and SiSe, germania and silica display large
standard deviations in the associated bond angle distributions. Within
bond-bending constraints theory, this pattern can be interpreted as a
manifestation of {\it {broken}} (i.e. ineffective) oxygen bond-bending
constraints. The same analysis reveals that the changes in the Ge composition
affects mostly bending around germanium in binary Ge-Se systems, leaving
Se-centred bending almost unchanged. In contrast, the corresponding Se twisting
(quantified by the dihedral angle) depends on the Ge composition and is reduced
when the system becomes rigid. Our results establishes the atomic-scale
foundations of the phenomelogical rigidity theory, thereby profoundly extending
its significance and impact on the structural description of network glasses.Comment: 5 pages, 4 figure
Structural, vibrational and thermal properties of densified silicates : insights from Molecular Dynamics
Structural, vibrational and thermal properties of densified sodium silicate
(NS2) are investigated with classical molecular dynamics simulations of the
glass and the liquid state. A systematic investigation of the glass structure
with respect to density was performed. We observe a repolymerization of the
network manifested by a transition from a tetrahedral to an octahedral silicon
environment, the decrease of the amount of non-bridging oxygen atoms and the
appearance of three-fold coordinated oxygen atoms (triclusters). Anomalous
changes in the medium range order are observed, the first sharp diffraction
peak showing a minimum of its full-width at half maximum according to density.
The previously reported vibrational trends in densified glasses are observed,
such as the shift of the Boson peak intensity to higher frequencies and the
decrease of its intensity. Finally, we show that the thermal behavior of the
liquid can be reproduced by the Birch-Murnaghan equation of states, thus
allowing us to compute the isothermal compressibility
Order and disorder in calcium–silicate–hydrate
Despite advances in the characterization and modeling of cement hydrates, the atomic order in Calcium–Silicate–Hydrate (C–S–H), the binding phase of cement, remains an open question. Indeed, in contrast to the former crystalline model, recent molecular models suggest that the nanoscale structure of C–S–H is amorphous. To elucidate this issue, we analyzed the structure of a realistic simulated model of C–S–H, and compared the latter to crystalline tobermorite, a natural analogue of C–S–H, and to an artificial ideal glass. The results clearly indicate that C–S–H appears as amorphous, when averaged on all atoms. However, an analysis of the order around each atomic species reveals that its structure shows an intermediate degree of order, retaining some characteristics of the crystal while acquiring an overall glass-like disorder. Thanks to a detailed quantification of order and disorder, we show that, while C–S–H retains some signatures of a tobermorite-like layered structure, hydrated species are completely amorphous.ICoME2 Labex (ANR-11-LABX-0053)A*MIDEX projects (ANR-11-IDEX-0001-02)Program “Investissements d’Avenir
Topological Origin of Fracture Toughening in Complex Solids: the Viewpoint of Rigidity Theory
In order to design tougher materials, it is crucial to understand the
relationship between their composition and their resistance to fracture. To
this end, we investigate the fracture toughness of usual sodium silicate
glasses (NS) and complex calcium--silicate--hydrates (CSH), the binding phase
of cement. Their atomistic structure is described in the framework of the
topological constraints theory, or rigidity theory. We report an analogous
rigidity transition, driven by pressure in NS and by composition in CSH.
Relying both on simulated and available experimental results, we show that
optimally constrained isostatic systems show improved fracture toughness. The
flexible to stressed--rigid transition is shown to be correlated to a
ductile-to-brittle transition, with a local minimum of the brittleness for
isostatic system. This fracture toughening arises from a reversible molecular
network, allowing optimal stress relaxation and crack blunting behaviors. This
opens the way to the discovery of high-performance materials, designed at the
molecular scale
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