Résumé
Below 10 GPa, hydrostatic pressures P are usually obtained in diamond-anvil cell with soft pressure transmitting medium such as methanol-ethanol mixture. When such experiments are performed in this P range for v-SiO2, the volumetric strain of the sample agrees with that expected from its bulk modulus, . It was revealed recently that v-SiO2 submitted to high pressures under He atmosphere exhibits surprisingly small relative volume changes leading to an apparent B greater than hundred GPa at ambient P, much higher than the well-known modulus GPa of silica. Using Brillouin light scattering we evidence that the elastic moduli are only weakly affected by the presence of He. The difference between the static and the acoustic measurements reveals that silica pressurized under He behaves as an open system allowing He to penetrate into the interstitial free volume of the network. In that case, the static measurement underestimates the compressibility by a term due to the gas charging the sample. The large difference found here shows that He continues penetrating v-SiO2 up to our highest investigated P, i.e. 6 GPa. Such a strong effect of the pressurizing medium on the static compressibility is usually observed in materials with open porosity, such as zeolites. It results from pore filling by molecules of the pressurizing fluid. An estimate of the amount of He entering v-SiO2 is derived from the measured refractive index, leading to the unexpected value of more than 1 mole of He per mole of SiO2 at 6 GPa [1]. Finally we present molecular simulations relating the amount of fluid adsorbed to the volumetric strain of the network. We show that a generalized poromechanical approach can be applied to silica glass in which the free volume exists only at the sub-nanometric length scale. In that picture, the small apparent compressibility of silica under high He or Ne pressure results from an adsorption-induced expansion [2].[1] C. Weigel, A. Polian, M. Kint, B. Rufflé, M. Foret, R. Vacher,, “Vitreous Silica Distends in Helium Gas: Acoustic Versus Static Compressibilies” Phys. Rev. Lett., 109, 245504 (2012).[2] B. Coasne, C. Weigel, A. Polian, M. Kint, J. Rouquette, J. Haines, M. Foret, R. Vacher, B. Rufflé, “The Startling Poroelastic Behavior of Vitreous Silica” submitted (2014).