Shrinkage xerogel

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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