Nathalie Olivi-Tran

Fragmentation by thermal relaxation of zirconium oxide aerogel

Zirconia aerogels are made of connected fractal clusters composed of small crystalline particles. When heated at a low temperature (i.e. 0.13 times the melting temperature), the mass transport process which predominantly occurs is surface diffusion. Due to the local character of surface diffusion, fragmentation appears in the sample during thermal annealing. This particular evolution of the structure is numerically analysed and experimentally studied by small-angle x-ray scattering.

Pure, Si and sp3-C-doped graphene nanoflakes: A numerical study of density of states

Abstract We built graphene nanoflakes doped or not with C atoms in the sp3 hybridization or with Si atoms. These nanoflakes are isolated, i.e. are not connected to any object (substrate or junction). We used a modified tight binding method to compute the π and σ density of states. The nanoflakes are semiconducting (due to the armchair geometry of their boundaries) when they are pure but they become conducting when doped because doping removes the degeneracy of the density of states levels. Moreover, we showed that the π Fermi level and the Fermi level of both π and σ electrons are not superimposed for small isolated nanoflakes.

A percolation approach to aerogel gas permeability

We performed a numerical and theoretical study of the critical behaviour of the permeability to gases in aerogels of different densities. In order to compute the critical exponent, it has been necessary to calculate the percolation critical density for densifying aerogels. As a final result, the permeability critical exponent is found to be different to that of random media. The theoretical and numerical results agree very well.

Analysis of a Lennard-Jones fcc structure melting to the corresponding frozen liquid: Differences between the bulk and the surface

Abstract We computed a Lennard Jones frozen liquid with a free surface using classical molecular dynamics. The structure factor curves on the free surface of this sample were calculated for different depths knowing that we have periodic boundary conditions on the other parts of the sample. The resulting structure factor curves show an horizontal shift of their first peak depending on how deep in the sample the curves are computed. We analyze our resulting curves in the light of spatial correlation functions during melting . The conclusion is that the differences between bulk and surface are quite small during melting and that at the end of melting, only the very surface happens to be less dense than the bulk. This result is intrinsic to the shape of the Lennard Jones potential and does not depend on any other parameter.