Elastic Properties of Silica Xerogels

Abstract

To calculate elastic constants, longitudinal and tranverse acoustic wave velocities were measured for silica xerogels as a function of relative humidity (rh). The silica xerogels studied are microporous with open porosity of 53 vol%. The longitudinal wave velocity exhibits a minimum at about 35% rh. The transverse wave velocity decreases to a constant value for 35% rh. Consequently, Young's modulus is a minimum at about 35% rh, whereas the shear modulus decreases to a constant value at 35% rh. The bulk modulus and Poisson's ratio exhibit minimum values at about 15% rh. Young's modulus decreases from 4.91 to 3.42 GPa at 35% rh and then increases to 3.60 GPa at 55% rh. Poisson's ratio decreases from 0.184 to 0.164 at 15% rh and then increases to 0.272 at 55% rh. Below 35% rh, silica xerogels adsorb a monolayer of hydroxyls, whereas above 35% rh silica xerogels show pore filling.