Is silica glass a porous material?
Résumé
Silica exists under a number of different forms, either crystalline or vitreous. The density of silica glass is about 5% lower than that of cristobalite and 20% lower than that of quartz, two crystals of the same chemical composition. Further, vitreous silica can be permanently densified under high pressure and high temperature by more than 20%, keeping an amorphous structure. What is the origin of these differences at the microscopic scale ? We will first review our knowledge of the structure of silica glass : the basic element of the structure is a SiO4 tetrahedron as in crystals, but the angles between tetrahedra are irregular. Then, results of experiments for silica glass under compression will be presented. Under pressures in the GPa range, the results depend drastically on the fluid used as a pressure-transmitting medium for the compression : while the volume of the silica sample changes dramatically in argon, only a very small decrease in size is observed under helium. This shows that helium penetrates inside silica, the open and flexible structure of the glass allowing gas atoms to distend the network, the same way a porous material deforms upon fluid adsorption. Furthermore, a fine quantitative analysis of the behavior of the Raman modes in function of pressure shows that changes in force constants between atoms and bond angles between tetrahedra in the network occur simultaneously.