Abstract
Peculiar xerogels and aerogels constituted by a silica network, made of spherical fully dense silica particles having the same size, are investigated by adsorption of nitrogen at 77.4 K. Comparison of sorption data between materials dried via different methods, gentle drying at room temperature, alcohol supercritical drying and CO2 supercritical drying, shows that the specific surface area is associated to the particle sizes and necks established between them during drying and not to the sample density. The dissolution-redeposition of silica, which occurs in the alcohol supercritical drying process, induces a decrease of specific surface area and consequently an increase in the mechanical properties comparatively to CO2 supercritical drying. Investigating pore volume measurements as a function of dwell time, which is the interval of time allowing a pressure change of 0.01%, we corroborate that for compliant materials the full volume can not be detected because of capillary stresses. So the time required to perform correct measurements of the pore volume decreases with sample bulk density increase and elastic properties increase. All these experiments qualitatively corroborate the theory proposed previously.
Highlights
Silica xerogels and aerogels are materials exhibiting a high porous volume
Comparison of sorption data between materials dried via different methods, gentle drying at room temperature, alcohol supercritical drying and CO2 supercritical drying, shows that the specific surface area is associated to the particle sizes and necks established between them during drying and not to the sample density
The bulk density of aerogels is within the range 0.19 - 0.48 g/cm3 depending on the dilution in aqueous solution of the starting silica sol and on the supercritical drying technique
Summary
Silica xerogels and aerogels are materials exhibiting a high porous volume They are characterized by pores having a size smaller than 100 nm. According to the small pore sizes, gas condensation occurs and induces capillary stresses These capillary stresses do not have any effect on porous materials with acceptable mechanical properties. During a nitrogen adsorptiondesorption isotherm experiment a length variation results from the compression of the sample which occurs as nitrogen molecules condense within the pores. This length change has been followed using an optical device [1] and more recently has been quantified using a linear variable differential transducer [2]. The porous volume measured using adsorption technique is far smaller than the true porous volume of aerogel
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