Drying, of gel

Overview. Three approaches are used to make most sol—gel products method 1 involves gelation of a dispersion of colloidal particles method 2 employs hydrolysis and polycondensation of alkoxide or metal salts precursors followed by supercritical drying of gels and method 3 involves hydrolysis and polycondensation of alkoxide precursors followed by aging and drying under ambient atmospheres. 

Transition-metal mixed oxides active in combustion catalysis have been prepared by two main procedures i) classical coprecipitation / calcination procedures starting from metal nitrates and/ or alkoxides ii) preparation based on the supercritical drying of gels prepared from organic complexes (alkoxides, acetylacetonates or acetates), producing aerogels . Details on the second preparation can be found in Ref. 13. 

Co-solvent included to facilitate the rapid drying of gels without cracking. 

Akbar and Goer ling (A2) presented a theoretical interpretation of shrinkage and moisture flow during the drying of gel and paste-type substances. 

The drying of gels has been extensively investigated [7], and only a brief survey is given here. 

Aerogels are materials prepared by supercritical drying of gels produced by hydrolysis ofalkoxides (14). High-surface-area boehmite (75,76) or pse-udoboehmite (77) can be prepared by this method, and these materials can subsequently be transformed into high-surface-area aluminas. 

Scherer, G.W. (1990) Stress and fracture during drying of gels, J. Non-Cryst. Solids 121, 104. Model for drying gels. 

Laudise, R.A. and Johnson, J., Supercritical drying of gels. Journal of Non-Crystalline Solids, 79, 155-164, 1986. 

Figure 10.14. SEM micrographs of polyurethane aerogels coming frcm supercritical CO2 drying of gels synthesized with saccharose in different solutions of DMSO and etac with volume fraction ofDMSO <0.3 (top) and >0.3 (bottom) (microscopy conducted by M. Repoux at MINES ParisTech/CEMEF, Sophia AntipoUs, France) [39].

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