Shrinkage—Xerogels

In 1911, igmondy (288) visualized that gel is made up of very fine grains separated by what are in effect very fine capillary tubes and showed how this view harmonizes with the phenomena observed in dehydrating and rehydrating SiO] gels. For instance, the diminution of vapor tension of the water is a direct consequence of the fineness of the capillary.  

Since the gel consists of a three-dimensional network of particles or short chains of particles linked rigidly together and since the silica particles themselves are essentially inelastic, it is apparent that, as the gel dries and shrinks owing to the surface tension of the water in the pores, the network must fold or crumple (289, 290). The cause of collapse of an aggregate during drying has been discussed in detail by Bikerman (291). The final gel structure will depend on the structure of the gel originally formed in solution, but it will be a contracted and distorted version of the latter. 

However, the shrinkage of silica gel is essentially irreversible. It is probable that, as certain bonds break, the portions of the network thus released come into contact with other members and new contacts and bonds are formed. In this manner, the packing density is increased and the pore diameters are diminished. 

It is for this reason that, in the case of very coarse gels or precipitates containing particles larger than about 20-30 nm. where the spaces or pores are also of this order of magnitude, the dried products are not strongly compressed but are light and friable.- -------------- - 

Low density gels are obtained only when shrinkage is avoided as the liquid phase is removed. This can be accomplished in five ways  

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The hydrogel has an open structure (i.e. a low particle coordination number) and is mechanically weak. The removal of the aqueous liquid phase normally leads to a drastic shrinkage of the silica framework and die formation of additional siloxane bonds (Fenelonov et al., 1983). The resulting xerogel is therefore much stronger and more compact, but inevitably has a lower pore volume. 

FIGURE 12.2 (a) Comparison of the linear shrinkage of mullite (3Al203-2Si02) micronic powder, 600°C thermally treated gel (xerogel) and pristine gel. (After Reference 87.) (b) Plot of the R.T. microhardness for mullite and Nasicon (Na3Zr2Si3POi2) as a function of thermal treatment. (After Reference 27.)... 

A last complication results from the measurement of the thickness of supported layers. This can easily be done with SEM for calcined layers. Wet lyogel, or even dry xerogel, layers cannot be measured in this way. Reproducible thickness measurements on wet lyogel films could not be obtained with other easy-to-perform methods. Consequently layer thicknesses were measured after calcination. Estimates of the shrinkage in the thickness direction were made for supported alumina (boehmite) membranes dried at 40°C and 60% RH made with a standard precursor solution of 1 mol AlOOH/1 stabilised at pH = 4 by... 

An aerogel is a special type of xerogel from which the liquid has been removed in such a way as to prevent any collapse or change in the structure as liquid is removed (8). This is done by drying a wet gel in an autoclave above the critical point of the liquid so that there is no capillary pressure and therefore relatively little shrinkage. The product is mostly air, having volume fractions of solid as low as about 0.1% (8), hence the term aerogel. 

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