Calculated vibrational and electronic properties of various sodium thiogermanate glasses

We study the vibrational and electronic properties of (x)Na$_2$S-(1-x)GeS$_2$ glasses through DFT-based molecular dynamics simulations, at different sodium concentrations ($0<x<0.5$). We compute the vibrational density of states for the different samples in order to determine the contribution of the Na$^+$ ions in the VDOS. With an in-depth analysis of the eigenvectors we determine the spatial and atomic localization of the different modes, and in particular in the zone corresponding to the Boson peak. We also calculate the electronic density of states as well as the partial EDOS in order to determine the impact of the introduction of the sodium modifiers on the electronic properties of the GeS$_2$ matrix.Comment: 9 pages, 6 figures; to appear in Chemical Physics (typo. corrected

Electronic structure of amorphous germanium disulfide via density functional molecular dynamics simulations

Using density functional molecular dynamics simulations we study the electronic properties of glassy g-GeS$_2$. We compute the electronic density of states, which compares very well with XPS measurements, as well as the partial EDOS and the inverse participation ratio. We show the electronic contour plots corresponding to different structural environments, in order to determine the nature of the covalent bonds between the atoms. We finally study the local atomic charges, and analyze the impact of the local environment on the charge transfers between the atoms. The broken chemical order inherent to amorphous systems leads to locally charged zones when integrating the atomic charges up to nearest-neighbor distances.Comment: 13 pages, 9 figures; to appear in Phys. Rev.

Vibrational signature of broken chemical order in a GeS2 glass: a molecular dynamics simulation

Using density functional molecular dynamics simulations, we analyze the broken chemical order in a GeS$_2$ glass and its impact on the dynamical properties of the glass through the in-depth study of the vibrational eigenvectors. We find homopolar bonds and the frequencies of the corresponding modes are in agreement with experimental data. Localized S-S modes and 3-fold coordinated sulfur atoms are found to be at the origin of specific Raman peaks whose origin was not previously clear. Through the ring size statistics we find, during the glass formation, a conversion of 3-membered rings into larger units but also into 2-membered rings whose vibrational signature is in agreement with experiments.Comment: 11 pages, 8 figures; to appear in Phys. Rev.

Propriétés physico-chimiques des verres chalcogénures GeS2 et xNa2S-(1-x)GeS2 (une étude par dynamique moléculaire "ab initio")

MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Experimental investigation of a dielectric liquid-vapor interface between two vertical planar electrodes: Influence of the DC electric field and temperature

International audienceThe effect of a DC electric field on a liquid-vapor interface was investigated at different temperatures using a Pellat-like test cell filled with HFE-7000 and HFE-7100 dielectric fluids. Special attention was given to the local observations of the meniscus as this information is crucial in two-phase heat transfer applications. The results confirmed first that the dielectrophoretic force controls both the mean position and the shape of the meniscus as the visualizations revealed that the interface could be totally flattened when the electric field increased. The visualizations showed secondly that free charges in the liquid could affect the interface behavior in two ways: by slightly modifying the pressure profile in the liquid bulk, and by leading the formation of electrojets close to the wall. The temperature dependence of the vapor dielectric strength led to a drastic limitation at low temperature of the maximum electric field that the fluid could withstand before breakdown. This point should be carefully considered in electrohydrodynamics enhanced two-phase cooling systems