419 research outputs found
Viscosity and viscosity anomalies of model silicates and magmas: a numerical investigation
We present results for transport properties (diffusion and viscosity) using
computer simulations. Focus is made on a densified binary sodium disilicate
2SiO-NaO (NS2) liquid and on multicomponent magmatic liquids (MORB,
basalt). In the NS2 liquid, results show that a certain number of anomalies
appear when the system is densified: the usual diffusivity maxima/minima is
found for the network-forming ions (Si,O) whereas the sodium atom displays
three distinct r\'egimes for diffusion. Some of these features can be
correlated with the obtained viscosity anomaly under pressure, the latter being
be fairly well reproduced from the simulated diffusion constant. In model
magmas (MORB liquid), we find a plateau followed by a continuous increase of
the viscosity with pressure. Finally, having computed both diffusion and
viscosity independently, we can discuss the validity of the Eyring equation for
viscosity which relates diffusion and viscosity. It is shown that it can be
considered as valid in melts with a high viscosity. On the overall, these
results highlight the difficulty of establishing a firm relationship between
dynamics, structure and thermodynamics in complex liquids.Comment: 13 pages, 8 figure
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
Fracture Toughness of Silicate Glasses: Insights from Molecular Dynamics Simulations
Understanding, predicting and eventually improving the resistance to fracture
of silicate materials is of primary importance to design new glasses that would
be tougher, while retaining their transparency. However, the atomic mechanism
of the fracture in amorphous silicate materials is still a topic of debate. In
particular, there is some controversy about the existence of ductility at the
nano-scale during the crack propagation. Here, we present simulations of the
fracture of three archetypical silicate glasses using molecular dynamics. We
show that the methodology that is used provide realistic values of fracture
energy and toughness. In addition, the simulations clearly suggest that
silicate glasses can show different degrees of ductility, depending on their
composition.Comment: arXiv admin note: text overlap with arXiv:1410.291
Creep of Bulk C--S--H: Insights from Molecular Dynamics Simulations
Understanding the physical origin of creep in calcium--silicate--hydrate
(C--S--H) is of primary importance, both for fundamental and practical
interest. Here, we present a new method, based on molecular dynamics
simulation, allowing us to simulate the long-term visco-elastic deformations of
C--S--H. Under a given shear stress, C--S--H features a gradually increasing
shear strain, which follows a logarithmic law. The computed creep modulus is
found to be independent of the shear stress applied and is in excellent
agreement with nanoindentation measurements, as extrapolated to zero porosity
Stretched Exponential Relaxation of Glasses at Low Temperature
The question of whether glass continues to relax at low temperature is of
fundamental and practical interest. Here, we report a novel atomistic
simulation method allowing us to directly access the long-term dynamics of
glass relaxation at room temperature. We find that the potential energy
relaxation follows a stretched exponential decay, with a stretching exponent
, as predicted by Phillips' diffusion-trap model. Interestingly,
volume relaxation is also found. However, it is not correlated to the energy
relaxation, but is rather a manifestation of the mixed alkali effect
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