SOL-GEL STRATEGIES FOR CONTROLLED POROSITY INORGANIC MATERIALS

The porosity (i.e., pore volume, pore size, and surface area) of ceram ic materials prepared by sol-gel processing depends on the size and st ructure of primary particles or polymers formed by condensation reacti ons, the organization of these structures, often by aggregation, to fo rm a gel, and the collapse of the gel by drying. This paper reviews th ese ideas in the context of the formation of thin films suitable for i norganic membranes and introduces a number of specific strategies desi gned to control pore sizes in the range appropriate for gas separation : (1) aggregation of fractals; (2) management of capillary pressure, ( 3) control of condensation rate, and (4) the use of organic or micropo rous templates in composite thin film structures. These strategies are contrasted with the more traditional particle packing approach to pre paring controlled porosity materials.