Silicate-based thin layers and glass

The BaO-B₂O₃-SiO₂ system

In collaboration with Damien Vandembroucq (PMMH)

Study of the phase separation phenomena occurring in the case of thin layer of BaO-B₂O₃-SiO₂ combining SEM and AFM observations (and in-situ SEM at different temperatures) with phase field based simulations.

The Na₂O-CaO–Al₂O₃–SiO₂ (NCAS) system

In industrial silicate glasses, chemical diffusion of elements plays an important role at all steps of the glass melting and transformation processes. Elemental migration has a complex dependency with temperature and glass composition, because of inter-species couplings, which are required for the displacement of ions within a strongly polymerized structure. No model has been proposed yet in order to predict coupled multicomponent diffusion in silicate melts, with the exception of a few narrow regions of simplified systems (with respect to complex phase diagrams of industrial interest). In addition, extrapolating beyond such narrow regions would require a deep fundamental understanding of diffusion-controlling mechanisms, which is still lacking.

Our aim is to study and model multicomponent diffusion in silicates, in the Na₂O-CaO–Al₂O₃–SiO₂ (NCAS) system, optionally doped with other relevant elements, for conditions relevant to flat glass melting and transformation. Our approach consists in studying diffusive couplings between species as well as the influence of glass structure on such couplings, in order to obtain predictive models of diffusion phenomena encountered in the glass industry, via the basic understanding of kinetic and thermodynamic microscopic mechanisms at the origin of diffusion. To this purpose, we use an original combination of experimental and numerical techniques. The concentration profiles along diffusion gradients are measured using various micro-analysis techniques, with a spatial resolution ranging from nanometers to millimeters. From such measures, we obtain the diffusion matrices of the compositions of interest. Diffusion matrices are the matrix extension of a scalar diffusivity, and are found in a multicomponent version of Fick’s law taking into account coupling between species. For a local view of diffusion, local configurations and their dynamics have been obtained thanks to structural measurements. The observed phenomena are rationalized by identifying the dominant exchange reactions between species, which can be obtained from the eigenvectors of the diffusion matrices.

We also explore the persistence of these exchange reactions throughout the phase diagrams of interest (within the Na₂O-CaO–Al₂O₃–SiO₂ system, and with the addition of Zn or  Sn as dopants), and at different temperatures (in the molten state, and for metastable liquids close to the glass transition). Finally, we strive to give a thermodynamic interpretation of our results using Onsager’s formalism for multi-diffusion.