Irreversible volume shrinkage of silica aerogels under isostatic pressure

An isostatic pressure provided by a Hg porosimeter is applied on monolithic silica aerogels. Due to the small pore size, the mercury does no intrude the aerogel and induces a compression. The bulk modulus is measured for an aerogel series. Above a certain pressure, the aerogel does not recover its initial dimension and, consequently, it may be densified using this method. The densification is more important for oxidized aerogel. This irreversible compaction is explained by an interpenetration of clusters constituting the aerogel. Such an interpenetration increases the contact number between reactive species, borne by the arms of the clusters. The reactive species for silica aerogels are the silanol groups. They polycondense to form siloxane bonds; hence, the aerogel network is more reticulated. The reaction is deduced from the structural change observed by infrared spectroscopy.