Investigations of silica alcogel aging using coherent light

Abstract

Light scattering methods have previously been used to monitor the formation of gels. In this report we present new light scattering techniques to study the properties of silica alcogels during the aging process. Monitoring one particular polarization transformation of scattered light with time reveals a clear increase in internal strain in standing alcogels with time. The stress birefringence coefficient of an acid-catalyzed SiO 2 gel was found to be 3.4 x 10 12 Brewsters. Additionally, the evolution of the stiffness of alcogels was investigated using laser speckle methods. Specifically, image analysis of specklegrams obtained during multi-frequency acoustic excitation of aging gels was used to non-destructively measure the hardening of alcogels. For an acidcatalyzed gel with a theoretical density of ~0.05 g/cm 3 SiO 2 , the rate of hardening is found to be greatest between gelation and 2X the gel time, and drops considerably thereafter. The Young's modulus of the gel can be monitored over time with this method and was found to range from 6.2 x 10 3 N/m 2 after 6 hours to 2.2 x 10 5 N/m 2 after 24 hours for acid-catalyzed silica gels.

Aerogel nanocomposites for energy applications

“…This can prove to be a major drawback for industrial applications. Various approaches have been used to strengthen gels, including reinforcement with carbon nanotubes (which additionally makes the aerogel conductive) 41 ; coating gel structure with polymer [42][43][44] to increase the stiffness; and chemically reinforcing the particle network to produce a flexible, hydrophobic material 45 . However, a simpler approach to strengthen silica (and other) aerogels is to increase the connection between particles in the network through an aging process 46 .…”

“…Aging is often carried out in the original solution, where more material can be added to the existing structure, but the process may also occur in pure solvents, where strengthening is likely due to dissolution and redeposition processes. In general, aging has been shown to be an effective method to increase the strength and stiffness of gels 41,64 .…”