Papers by Author: Yves Limoge

Abstract: Despite its importance as a material in many domains, SiO₂ is still a very badly known material from the point of view of materials science. Experimentally the silicon and oxygen diffusion has been determined in silica as well as in quartz, but several discrepancies arise between different authors. From a theoretical point of view the various possible atomic defects have mostly been studied in an electronic perspective, so even the simplest ones remained quite poorly known till recently, the silicon related ones remaining completely unknown. The great similarity between silica and quartz properties is in favour of a common model. The determination of the precise formation and migration energies of the various defects is then of paramount importance for the understanding of the kinetic properties of SiO₂. We will present in this paper the results of a study of the formation and mobility properties of oxygen and silicon defects in the view of determining the self-diffusion mechanism(s). Our work relies on up to date ab-initio methods: total energy calculations in a DFT-LDA approach, using either plane wave or pseudo-atomic basis for the wave functions and pseudopotentials.We shall discuss the role of the various parameters controlling the kinetic behaviour: chemical potential of the species, nature of the main impurities, cristallinity, and preparation mode of the sample.

Abstract: SiO2 layers were deposited by reactive d.c ion sputtering (using 1keV Ar+ ion gun) from a high purity silicon target in an oxygen ambient. The base pressure in the deposition chamber was 4.7·10-9mbar, and the substrate temperature was held at 550 °C. The argon partial pressure during ion gun operation was 1·10-3mbar. Structural characterization of the films was performed by Rutherford backscattering spectrometry (RBS analysis), electron microprobe analysis, X-ray diffraction (XRD analysis) and Raman spectroscopy. Reactive sputtering proved to be efficient for the deposition of silica at an oxygen partial pressure of 2·10-4mbar and an electrical current on the target of 5.5mA.

Abstract: In this conference we try to give a survey of the main characteristics of aging of oxides under irradiation in the perspective of the recent developments of the ab-initio modeling capabilities. After a brief recall of the relevant radiation – matter interactions, we present the main aspects of materials aging under irradiation, I) defect creation either elastically or inelastically, ii) microstructure evolution due to defect elimination, iii) radiation enhanced diffusion, iv) phase changes under irradiation.

Abstract: In this paper we propose a couple of experimental lines in order to probe specific features of oxygen and silicon diffusion in -quartz and SiO2 in general. We start from the results of atomistic first principles calculations and from their predictions concerning point defects formation free enthalpies; we suggest that measurements with variable oxygen partial pressure could confirm that, in well defined regimes, oxygen diffusion is controlled by doubly negative oxygen interstitials. Along similar lines, for silicon diffusion, one should check the real equilibrium conditions of samples, with gaseous oxygen or bulk silicon, or closed conditions. We discuss our predictions for silicon diffusion, that could clarify some discrepancies between experimental results. Another possible probe would be to perform measurements under hydrostatic pressure, in order to measure the formation volume of the migrating species and to compare it to the theoretical values.