Effect of oxidation treatment on the crack propagation rate of aerogels

Evolution of the Mechanical Properties During the Gel–Glass Process

“…At low K I , V is two orders of magnitude higher for the hydrophilic gel. We note that n is identical in this K I range for the two samples, which means that the mechanism associated to crack propagation is the same; n is close to 15 and corresponds to a corrosion mechanism by OH À (Despetis et al 2001). At higher K I , n changes and becomes equal to 35, the classical n value found for vitreous silica, associated to a corrosion mechanism by water molecules (Suratwala and Steele 2003).…”

Subcritical crack growth in silica aerogel

“…If the medium is basic, corrosive species are hydroxide ions and the activation energy of the reaction is changed: n value becomes equal to 15 [2]. However, for silica aerogels in contact with water vapor we observed two slopes in the region I [3,4]. Because these two domains are associated to two different corrosion mechanisms, we proposed that at low crack propagation rates (n = 15), hydroxide ions resulting from the dissociation of highly organized water molecules adsorbed on the gel surface, react with the silica network.…”

“…The experimental details have been reported in previous papers [3,4]. The crack growth rates have been measured within the range 10 À6 -10 À3 m/s with optical devices allowing us to estimate a change in the crack tip length of about 10 lm.…”

Mechanical Behavior of Nanocomposite Aerogels

“…The literature explains that some monolithic aerogels can cracks several months later because of subcritical flaw propagation (Etienne et al 1995;Despetis et al 2001). This behavior being the results of internal stresses and progressive rupture of siloxane bonds by the action of water at the tip of the flaws.…”