Supercritical drying process

Solgel processing specifies a type of solid material synthesis method performed in a liquid and at low temperatures [152], The produced inorganic solids, mostly oxides or hydroxides, are formed by chemical transformation of chemical solutes termed precursors. The solid is formed as the result of a polymerization process that involves the establishment of M-OH-M or M-O-M bridges between the metallic atoms M of the precursor molecules [152], The drying process, after the gel formation, is carried out at a relatively low temperature to produce a xerogel, or by a supercritical drying process. 

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. 

The rM5-modified alcogel is then subjected to supercritical drying. The supercritical drying process is based on the carbon dioxide extraction method [4]. The standard process parameters pressure and temperature versus time are shown in Figure 28.3. The substitution of carbon dioxide for alcohol was performed at a supercritical condition of carbon dioxide. The temperature of substitution is 80°C and the pressure is 16 MPa. At this condition, alcohol is soluble in supercritical state carbon dioxide with every concentration. The whole process of supercritical drying takes ordinarily about 2 days. The maximum size of TMSA obtained by the above process is approximately 200 x 300 mm. ... 

Figure 28.3. Temperature and pressure patterns of CO2 extraction method of supercritical drying process.

Figure 28.16 shows the distributions of the refiractive index of P-EW s TMSA blocks, SP-30. These data were obtained using about 1,300 pieces of manufactured blocks. The refractive index is usually measured by the prism method [19]. The refractive index was in the range of 1.029-1.033 and almost all the blocks are within 1.029-1.031. This dispersion is caused by the differences between batches of manufacturing, solutions for alcogels, supercritical drying processes, measuring conditions of the refractive index, etc. 

Yokoyama M (1991) Materials prepared by supercritically drying process. Development and appUcations of porous ceramics II CMC 295-309. 

HOT supercritical drying High temperature supercritical drying process, at temperatures >260°C, in an alcohol... 

The supercritical drying process has been proposed by Kistler.(Kistler, 1932) to dry, without textural modification, very tenuous solids wetted with a solvent. 

The supercritical drying process which was first proposed by S.S. Kistler is now well controlled. It requires a good knowledge of the role played by the different process parameters. These parameters must be carefully selected as a function of the structural, textural... 

Yokogawa H., Yokoyama M. Hydrophobic silica aerogels. J. Non-Cryst. Solids 1995 186 23-29 Yokoyama M. Materials prepared by supercritically drying process. Dev. Appl. Porous Ceram. II 1991 295-309... 

Yokoyama M. Materials Prepared by Supercritically Drying Process. Development and Applications of Porous Ceramics II. CMC, 1991, pp. 295-309 Zimmermann A., Gross J., Fricke J. Constant-Q acoustic attenuation in silica aerogels. J. Non-Cryst. Solids 1992 186 238-243... 

Currently, the supercritical drying process is the best solution to avoid such capillary forces. With the supercritical fluid drying, it is possible to move from the liquid phase via the supercritical fluid directly to the gas phase without touching the two-phase area. With optimized drying parameters, capillary forces can be avoided and nanoporous structures are maintained [55-57]. 

The supercritical drying process is carried out batchwise (see Figure 8.22). The material to be dried is inserted into a high-pressure vessel equipped with a quickacting closure at the top. The samples are soaked in the organic solvent before the... 

Although many different varieties of aerogels can be made, the most common is silica aerogel. Without additional modification, silica aerogel is prone to the adsorption of water, which causes collapse of the structure. However, modification of the gel prior to the supercritical drying process with trimethylsilyl results in a hydrophobic silica aerogel [110]. This practice is very common in siUca aerogel production, especially in application as thermal insulation. 

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