Tin oxide gel shrinkage during CO 2 supercritical drying

Abstract

Shrinking behavior of tin oxide gels during solvent exchange in CO 2 supercritical drying was investigated. The drying process consisted of two solvent-exchanging steps, including a first replacement of water by acetone and a second of acetone by liquid CO 2. Gels prepared from a basic sol with a ζ-potential of −75 mV exhibited extensive shrinkage during both steps, while those from an acidic hydrous precipitate of lower potential (| ζ-potential|<35 mV) shrank only during the second step. The shrinkage during the first step correlates with the ξ-potential reduction with increasing acetone concentration, while that during the second step for both gels can be attributed to osmotic compressive pressure that arises from an increase in the liquid–solid (gel skeleton) interfacial energy with increasing CO 2 concentration. Dramatically different aerogel microstructures were obtained by controlling the gel surface potential in conjunction with different combinations of solvent-evaporating and supercritical drying.