Crack supercritical drying

Fig. 1. Crack-free monolithic titania-silica aerogel photos, (a) aerogel prepared by hi -temperature ethanol supercritical drying, (b) aerogel prepared by low-temperature CO2 supercritical drying.

Cracking of PS because of capillary forces can be circumvented if one avoids crossing the liquid-vapor boundary in the phase diagram of the solvent. This is the case for supercritical drying [Ca4] or freeze drying [Ami], as shown in the inset of Fig. 6.12. 

Strategies to reduce or eliminate cracking are discussed including chemical additives, supercritical drying and thickness effects. 

Cracks can also appear during the pressure release in the autoclave. In the supercritical drying process, the gel is subjected to high temperature and high pressure. When the critical point is reached, the pressure of the autoclave is decreased while the temperature is kept constant. At this instant, the pressure applied to the supercritical fluid is equal to that within the pores. The supercritical fluid has a very low density and viscosity compared with that of the liquid at room temperature however, the low permeability of the gel resists the flow of the supercritical fluid out of the gel. In other words, if the supercritical fluid release is performed too fast a pressure gradient appears. In this case the supercritical fluid within the gel, which is in compression, suddenly expands and the solid part suffers tensile stress. Experiments show that cracking depends on the pressure release rate, on the nature of the gel (basic or neutral), and on its geometrical dimensions. 

Figure 2.7. Typical depressurization crack (perpendicular to the largest surface) experimented by the silica gel during supercritical drying (illustrated here on a 1 cm thick wet silica tile having a liquid permeability between 5 and 10 nm, dried with supercritical CO2 at 313 K and 90 bar, and submitted to an autoclave depressurization of 0.15 bar min ). Courtesy of Rigacci A.

Even if supercritical drying permits to obtain perfectly monolithic and transparent sihca-based thermal insulators, the process to obtain such crack-free large plates remains stUl too far from industrial large-scale commercialization. This was the initial reason why different processes have been studied to develop subcritical routes to access rapid massive commercialization. Among the various studies, aging of the gels in silica precursor containing solution has permitted to reach room temperature thermal conductivities as low as... 

Cracking of the solid part of the gel is the second drawback usually encountered during drying. Freeze drying and supercritical drying are two processes which have been investigated to circumvent these difficulties. 

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