Drying subcritical

Only recently, the possibility of using subcritical drying techniques to produce xerogels has been explored most of them will be treated in Sections 5.5.3 and 5.5.4 on vacuum and convective drying, respectively. Here, only one method close to supercritical drying as followed by Kirkbir et al (1998a, b) will be presented. Wet silica gels were first washed (e.g., with ethanol) and then heated in a pressure 

These threshold pressures are related to the temperature at which the gel dries out and, hence, to the maximal surface tension that the gel experiences they depend on synthesis conditions which control the structural and mechanical properties of the gel, such as pore size and strength, respectively. Although process pressures can be significantly reduced (by 50%) by this approach, the previously described problems associated with the elevated temperatures remain. 

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Subcritical drying can sometimes yield porous xerogels when the gel structure is strong enough to withstand capillary pressures [11,31], although it is not possible to produce carbon xerogels with both small pores and high pore volumes... 

In contrast to supercritical drying, operating parameters for subcritical drying including temperature and pressnre are below the critical point. Wei et al. [35] nsed E-40 (multi-polysiloxane) as silicon source and isobutyl alcohol as solvent to prepare Si02 gel. The prepared gel was placed in autoclave, adding an appropriate amonnt of isobutyl and surfactant, preserved sometime under pressure of 2.3-2.6 MPa, temperature of 240°C-260°C, and then deflated slowly and cooled naturally hydrophobic SiOj aerogel was finally obtained. 

Exploiting subcritical drying or atmospheric pressure drying based on modification technology for nanomaterials due to their low operation cost and operation pressure, and mild operation temperature... 

Brandt R, Fricke J (2004) Acetic-acid-catalyzed and subcritically dried carbon aerogels with a nanometer-sized structure and a wide density range. J Non-Cryst Solids 350 131-135. 

Reuss M, Ratke L (2008) Subcritically dried RF-aerogels catalyzed by hydrochloric acid. J Sol-Gel Sci Technol 47 74-80. 

It has to be pointed out that the very same process that is observed during desorption upon nitrogen sorption analysis is active when using any other probe, for example when subcritically drying a sample [74, 75]. To use the same evaluation concept for drying shrinkage the fluid related quantities such as the surface tension and the relative pressure and the temperature have to be replaced in the above relationships. 

Subcritical drying A drying process perfomaed at a temperature T and a pressure P below the critical point temperature and the pressure P of the fluid contained in the porosity of the wet gel... 

In contrast to supercritical drying, operating parameters for subcritical drying including temperature and... 

Smitha, S., Shajesh, P., Aravind, P.R., Rajesh Kumar, S., Krishna Pillai, P., and Warrier, K.G.K. (2006) Effect of aging time and concentration of aging solution on the porosity characteristics of subcritically dried silica. Microporous Mesoporous Mater., 91, 286-292. 

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