Drying with supercritical

Fig. 3. Effect of using either liquid or supercritical carbon dioxide on the textural properties of titania aerogels calcined at the temperatures shown. (—), dried with Hquid carbon dioxide at 6 MPa and 283 K (-------), dried with supercritical carbon dioxide at 30 MPa and 323 K. Reproduced from Ref. 36.

The capillary forces are reduced by using hydrophobic photoresists, by rinse additives, by freeze-drying or by drying with supercritical CO2 [9],... 

Thin film super-low dielectric constant silica aerogel is investigated for application in ultra-large-scale integrated circuits. It is believed that aerogels could more than double the computer speed [ 11J. Such thin films can be made by using TMOS based solution mixed with dimethyl sulfoxide, and dried with supercritical carbon dioxide after coating. A relative dielectric constant as low as 1.1 can be obtained. 

Jarzebski, A.B., Lachowski,A.I., Lorenc, J., Mashnska-Sohch, J., and Turek, W. 1992. Effect of drying with supercritical carbon dioxide on enhancement and modification of polymeric catalysts activity. Chem. Eng. ScL, 47(5) 1321-1322. 

The two clay samples were expanded using a SiOj-TiOj sol solution prepared by a procedure described by Yamanaka and coworkers (4). Tetraethylorthosilicate is first hydrofyzed with an excess of an HQ (IN) and ethanol mixture, and then reacted with titanium tetraisopropoxide (hydrolyzed with IN HQ). to form a sol with composition TiOj-lOSiO. The sol is then added to a slurry containing 1% clay. The resulting mixture is stirred for 90 minutes at 60 C. After filtration and washing, part of the clay is allowed to diy in air at 60 C. The remaining clay was washed with ethanol (to displace interlamellar water) and then dried with supercritical COj at 120 atm and 40°C A schematic of the clay catalyst s preparation is shown in Figure 5-1. 

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.

Figure 10.18. Photographs of aerocelluloses prepared from caustic soda solutions of cellulose (cellulose 7% w/w in aqueous NaOH 8% w/w) regenerated in water at 25° C, washed with acetone then dried with supercritical CO2. Heights of the samples are, respectively, 3 and 9 cm (courtesy of T. Budtova, MINES ParisTech/CEMEF, Sophia Antipolis, France).

Fig. 5. 23 Occurrence of cracks in gels dried with supercritical CO2 depending on washing parameters as defined in Fig. 5.22b (taken from van Bommel and de Haan (1994)). (The shaded region marks the supercritical state of the single component CO2).

Orlovic, A., Petrovic, S., Skala, D., 2005. Mathematical modeling and simulation of gel drying with supercritical carbon dioxide. ]. Serb. Chem. Soc. 70 125-136. 

Methods have been developed by which the porous network structure is retained upon drying, the most important being drying with supercritical fluids and the so-called ambient-pressure drying, where formation of Si—O—Si bridges is prevented by hydrophobization of the pore walls. The obtained materials are called aerogels (because the pore liquid is replaced by air) [17] and will be treated in Qiapter 18. 

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