Gel drying

The (hying of gels is best described as a function of the solvent weight loss in the following stages [30], which are the same as those for green body drjnng. 

Constant rate period. Liquid flows to the surface to replace that lost to evaporation. This is the stage where most of the shrinkage and warping occurs, as a result of the c pillaiy pressvire differentials within the film. 

Critical point. The gel matrix reaches a point where it can no longer shrink to release the solvent necessary to reduce the capillary pressure. At this point the liquid meniscus enters the matrix and drying from within the film begins. This is when a crack is most likely to appear in the material, especially those produced with alkoxides, where the M—O—M network is stiff. Cracking occurs more with thicker films than with thin films. 

First falling rate period. The last layers of solvent are removed by flow along the pores walls to the surface. 

Second falling rate period. The volatilized solvent is removed by the diffusion of vapor to the surface. 

The classical sol-gel drying process basis relies on solvent elimination from the matrix without generating a two-phase system and the related capillary forces that cause partial or total destruction of the gel nanostructure. If the liquid phase is removed from the gel in a nondestructive manner, a porous solid should be left behind with approximately the same shape and volume as the original gel. Several techniques have been developed and improved in order to obtain the best solid material from any given sol-gel process. Because of the extremely small pore sizes and the high surfeice area of the gel structure, the drying process has a tremendous impact on the properties [147] and economic key factors of the final material [148,149]. 

In order to remove the liquid phase from any gel structure, in terms of the 

In relation to the different phase change phenomena involved, the following three processes have been developed and individually optimized for the various 

One must note that regardless of the drying protocol used, the classical definitions given in (i) and (ii) are often used interchangeably with the term aerogel. This is adequate if the materials satisfy the standard definition criteria for aerogel materials [23]. 

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Figure 6. Raman spectra of silicates silica gel dried at 50°C, heated to 600°C, consolidated at 1100°C, and conventionally prepared fused silica.

Figure 8. 29Si MASS and CPMASS NMR spectra of silica gels dried at 50°C, heated to 200 and 600°C, and consolidated at 1100°C. Hydrated samples were exposed to 100% RH at 25°C for 24 hours prior to analysis.

J.H. Harreld, T. Ebina, N. Tsubo, and G. Stocky, Manipulation of pore size distributions in silica and ormosil gels dried under ambient pressure conditions. J. Non-Cryst. Solids 298, 241—251 (2002). 

Procedure Weigh accurately about 0.5 g of potassium hydrogen phthalate in a 100 ml conical flask. Add 25 ml of glacial acetic acid and attach a reflux condenser fitted with a silica-gel drying tube. Warm until... 

Sihca gel Drying of gases, refrigerants, organic solvents, transformer oils desiccant in packings and double glazing dew point control of natural gas. 

The sol is made of a stable suspended solution of metal salts or solvated metal precursors containing solid particles of nanometer diameter. Polycondensation or polyesterification results in the appearance of particles in a new phase called the gel . Aging, drying and dehydration are steps required to achieve solid-form ultra-fine particles. Coarsening and phase transformation occur simultaneously with aging. Gel drying is associated with the... 

Flies are placed in contact with the foil electrodes using a conductive electrode gel. Two rows of gel are applied using a 3 ml syringe (without a needle) taking care to ensure that all gel residue in the central gap is removed. It is necessary to replace the gel after 2-4 pacing sessions because bubbling and gel dry out affect the conductivity and fly contact, respectively. 

Gels dried by conventional methods, known as xerogels (xero- = dry), generally have lost the very open structure of the wet gel. The reason for this is that the liquid phase, as it evaporates and shrinks, exerts powerful capillary forces on the pore walls through the surface tension at the meniscus, resulting in collapse or cracking of the large-scale structure of the gel. 

Although Schobein failed at marketing guncotton, researchers in France discovered that solvents such as diethyl ether and alcohol transformed nitrocellulose to a gel that could be molded into various shapes. Furthermore, spread thin on a flat surface, the gel dried to a tough, clear, transparent film. This workable nitrocellulose material was dubbed collodion, and its first application was as a medical dressing for cuts. 

The LPS silica is prepared by extracting the water from a low solids hydrogel with an organic solvent to avoid the compression of aqueous surface tension. This leaves a fragile catalyst of very high pore volume. Such preparations often provide dramatic examples of the effect of pore volume because the same gel dried directly in an oven will frequently be less active or even completely dead. Invariably, nitrogen sorption shows the inactive silica to contain a low pore volume mainly inside small pores, e.g., less than 60 A diameter. In contrast, the active sample dried by extraction usually has equal or greater volume inside small pores, and, in addition, considerable volume... 

Careful control of hydrolysis and condensation rales by use of acid catalysis in Method 3 results in very narrow pore si/e distributions, which minimizes stress gradients during drying by thermal evaporation under ambient pressure and low temperatures. Gels dried in this manner are... 

Deuterated gel. Dried curdlan powder was mixed into a 2% w/v suspension in heavy water. This suspension was spread on a glass plate and placed in an airtight container, shown schematically in Figure 5. The container was heated to approximately 100°C, thus gelling the polymer. The film was washed in D2O, to stop the rapid loss of D2O from the hot film, and then carefully removed and dried. (All operations except that of heating were carried out in the dry-box.)... 

Drying to remove solvent from a gel drying method (e.g. evaporative vs. supercritical vs. freeze drying) temperature and heating rate pressure and pressurization rate time... 

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