Sol-gel processing, designing porosity, pore size and polarity, and shaping processes

Abstract

Solution-sol-gel or sol-gel (for short) processing starts with a solution or a sol which becomes a gel. The solution can be prepared from either inorganic salts or organic compounds which then is hydrolyzed and condensed to make a sol or a gel. One can stop at the sol stage which refers to a dispersion of particles of colloidal dimensions in a liquid, or proceed to the gel state which refers to a three-dimensionally-linked solid network with liquid filling the pores. These pores are interconnected in the wet gel state. If one stops at the sol state, the sols can be subsequently used for gel-making. Colloidal silica sols in a typical size range of 5–20 nm are readily available commercially [1] which can be used in making gels. Colloidal dried powders such as boehmite are also available commercially which can be utilized in the processing of gels. Silica and other particles of colloidal dimension can also be prepared by hydrolysis of their chlorides in an oxygen-hydrogen flame at high temperatures in the range 1000–1800 °C. When these materials are formed in the flame, they appear like smoke and thus they are referred to as fumed products (for example, fumed silica).