63 research outputs found
First principles study of a sodium borosilicate glass-former I: The liquid state
We use ab initio simulations to study the static and dynamic properties of a
sodium borosilicate liquid with composition 3Na_2O-B_2O_3-6SiO_2, i.e. a system
that is the basis of many glass-forming materials. In particular we focus on
the question how boron is embedded into the local structure of the silicate
network liquid. From the partial structure factors we conclude that there is a
weak nanoscale phase separation between silicon and boron and that the sodium
atoms form channel-like structures as they have been found in previous studies
of sodo-silicate glass-formers. Our results for the X-ray and neutron structure
factor show that this feature is basically unnoticeable in the former but
should be visible in the latter as a small peak at small wave-vectors. At high
temperatures we find a high concentration of three-fold coordinated boron atoms
which decreases rapidly with decreasing T, whereas the number of four-fold
coordinated boron atoms increases. Therefore we conclude that at the
experimental glass transition temperature most boron atoms will be four-fold
coordinated. We show that the transformation of [3]B into [4]B with decreasing
T is not just related to the diminution of non-bridging oxygen atoms as claimed
in previous studies, but to a restructuration of the silicate matrix. The
diffusion constants of the various elements show an Arrhenius behavior and we
find that the one for boron has the same value as the one of oxygen and is
significantly larger than the one of silicon. This shows that these two network
formers have rather different dynamical properties, a result that is also
confirmed from the time dependence of the van Hove functions. Finally we show
that the coherent intermediate scattering function for the sodium atoms is very
different from the incoherent one and that it tracks the one of the matrix
atoms.Comment: 15 pages; 14 figure
First principles study of a sodium borosilicate glass-former II: The glass state
We use ab initio simulations to investigate the properties of a sodium
borosilicate glass of composition 3Na_2O-B_2O_3-6SiO_2. We find that the
broadening of the first peak in the radial distribution functions g_BO(r) and
g_BNa(r) is due to the presence of trigonal and tetrahedral boron units as well
as to non-bridging oxygen atoms connected to BO_3 units. In agreement with
experimental results we find that the [3]B units involve a significant number
of non-bridging oxygens whereas the vast majority of [4]B have only bridging
oxygens. We determine the three dimensional distribution of the Na atoms around
the [3]B and [4]B units and use this information to explain why the sodium
atoms associated to the latter share more oxygen atoms with the central boron
atoms than the former units. From the distribution of the electrons we
calculate the total electronic density of states as well its decomposition into
angular momentum contributions. The vibrational density of states shows at high
frequencies a band that originates from the motion of the boron atoms.
Furthermore we show that the [3]B and [4]B units give rise to well defined
features in the spectrum which thus can be used to estimate the concentration
of these structural entities. The contribution of [3]B can be decomposed
further into symmetric and asymmetric parts that can also be easily identified
in the spectrum. We show that certain features in the spectrum can be used to
obtain information on the type of atom that is the second nearest neighbor of a
boron in the [4]B unit. We calculate the average Born charges on the bridging
and non-bridging oxygen atoms and show that these depend linearly on the angle
between the two bonds and the distance from the connected cation, respectively.
Finally we have calculated the frequency dependence of the dielectric function
as well as the absorption spectra.Comment: 18 pages; 16 figure
New interaction potentials for borate glasses with mixed network formers.
We adapt and apply a recently developed optimization scheme used to obtain effective potentials for 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 high temperature generated by ab initio molecular dynamics simulations, and density, coordination, and elastic modulus of glass at room temperature from experiments. The new interaction potentials are shown to reliably reproduce the structure, coordination, and mechanical properties over a wide range of compositions for binary alkali borates. Furthermore, the transferability of these new interaction parameters allows mixing to reliably reproduce the properties of various boroaluminate and borosilicate glasses
The critical role of the interaction potential and simulation protocol for the structural and mechanical properties of sodosilicate glasses
We compare the ability of various interaction potentials to predict the
structural and mechanical properties of silica and sodium silicate glasses.
While most structural quantities show a relatively mild dependence on the
potential used, the mechanical properties such as the failure stress and strain
as well as the elastic moduli depend very strongly on the potential, once
finite size effects have been taken into account. We find that to avoid such
finite size effects, samples of at least 75,000 atoms are needed. Finally we
probe how the simulation ensemble influences the fracture properties of the
glasses and conclude that fracture simulations should be carried out in the
constant pressure ensemble.Comment: Minor corrections. An additional figure (current Fig. 3) and
correction in Tab. I (interchange of SiO and OO parameters' lines
Fracture of silicate glasses: Micro-cavities and correlations between atomic-level properties
We use large-scale simulations to investigate the dynamic fracture of silica
and sodium-silicate glasses under uniaxial tension. The stress-strain curves
demonstrate that silica glass is brittle whereas the glasses rich in Na show
pronounced ductility. A strong composition dependence is also seen in the crack
velocity which is on the order of 1800 m/s for glasses with low Na
concentration and decreases to 700 m/s if the concentration is high. We find
that during the fracture of Na-rich glasses very irregular cavities as large as
3-4 nm form ahead of the crack front, indicating the presence of nanoductility
in these glasses. Before fracture occurs, the local composition, structure, and
mechanical properties are heterogeneous in space and show a strong dependence
on the applied strain. Further analysis of the correlations between these local
properties allows to obtain a better microscopic understanding of the
deformation and fracture of glasses and how the local heating close to the
crack tip, up to several hundred degrees, permits the structure to relax
Surface properties of alkali silicate glasses: Influence of the modifiers
Using large-scale molecular dynamics simulations, we investigate the surface
properties of lithium, sodium, and potassium silicate glasses containing 25
mole % of alkali oxide. The comparison of two types of surfaces, a melt-formed
surface (MS) and a fracture surface (FS), demonstrates that the influence of
the alkali modifier on the surface properties depends strongly on the nature of
the surface. The FS exhibits a monotonic increase of modifier concentration
with increasing alkali size while the MS shows a saturation of alkali
concentration when going from Na to K glasses, indicating the presence of two
competing mechanism that influence the properties of a MS. For the FS, we find
that larger alkali ions reduce the concentration of under-coordinated Si atoms
and increase the fraction of two-membered rings, implying an enhanced chemical
reactivity of the surface. For both types of surfaces, the roughness is found
to increase with alkali size, with the effect being more pronounced for the FS
than for the MS. The height-height correlation functions of the surfaces show a
scaling behavior that is independent of the alkali species considered: The ones
for the MS are compatible with the prediction of the frozen capillary wave
theory while the ones for the FS show a logarithmic growth, i.e., on the
nanoscale these surfaces are not self-affine fractals. The influence of the
modifier on the surface properties are rationalized in terms of the interplay
between multiple factors involving the size of the ions, bond strength, and
charge balance on the surface
Density effects on the structure of irradiated sodium borosilicate glass: A molecular dynamics study
International audienceWe have carried out Molecular Dynamics simulations on a sodium borosilicate glass in order to analyze how the structure of the glass during irradiation is affected by the choice of the density in the liquid state before cooling. In a pristine form generated through the usual melt-and-quench method, both short- and medium-range structures are affected by the compressive or tensile environment under which the glass model has been generated. Furthermore, Na-rich areas are much easier to compress, producing a more homogeneous glass, in terms of density, as we increase the confinement during the quench. When the glass is subjected to displacement cascades, the structural modifications saturate at a deposited energy of approximately 8 eV/atom. Swelling appears for the glasses that were initially prepared under compression, while contraction is evident for the ones prepared under tension. We have equally prepared glass models using a fast quench method, and we have found that they present an analogous disorder as the glasses submitted to displacement cascades. Compared to the irradiated glass, we found that the magnitude of the modifications for the fast quenched glass is lower, most notably in terms of boron and sodium coordination, the percentage of non-bridging oxygens and in the ring distributions. This later result agrees with statements extracted from recent experimental works on nuclear glasses
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