Compression of aerogels

When an aerogel is pressurized in a mercury porosimeter, the network is compressed, but no mercury enters the pores. Therefore, porosimetry cannot be used to measure the pore size distribution in an aerogel, but it does provide a measure of the bulk modulus of the network. For silica aerogels, the network is linearly elastic under small strains, then exhibits yield followed by densification and plastic hardening. In the plastic regime it is found that the bulk modulus has a power-law dependence on density with an exponent of ≈ 3.2. For the same gels, the linear elastic modulus (before compression) also obeys a power law, but the exponent is ⋍ 3.6, as found by several other groups. If a gel is compressed to a pressure, P₁, that exceeds the yield stress, then returned to ambient pressure, the plastic deformation is irreversible; if that gel is then compressed to pressure P₂ 〉 P₁, it behaves elastically up to ⋍ P₁, then yields and follows the same power-law curve. Thus the location of the yield point of a previously compressed material indicates the maximum pressure to which the sample had been subjected; in particular, the compression curve can be used to estimate the capillary pressure exerted on a xerogel during drying.