159 research outputs found
On the glass transition temperature in covalent glasses
We give a simple demonstration of the formula relating the glass transition
temperature, , to the molar concentration of a modifier in two types
of glasses: binary glasses, whose composition can be denoted by
, with ^ an element of III-rd or IV-th group (e.g. B, or Si,
Ge), while is an alkali oxide or chalcogenide; next, the network
glasses of the type , e.g. , , etc.
After comparison, this formula gives an exact expression of the parameter
of the modified Gibbs-Di Marzio equation.Comment: 15 pages, LateX; ([email protected]), ([email protected]
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
Rigidity transitions and constraint counting in amorphous networks: beyond the mean-field approach
Subj-class: Disordered Systems and Neural NetworksComment: 12 pages, revtex, 3 figure
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
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