Sol-Gel Routes to Ceramics and Glasses I. Gels

Abstract

This work describes some significant differences between alkoxide gels and colloidal particulate gels which are important for fundamental understanding and for the manufacture of ceramic and glass products. The experimentally observed differences in rheology between the two gel types are rationalized using fundamental theories advanced by Iler. The most significant difference is the irreversible nature of the gel formed from ethanolic solution of tetraethyl orthosilicate (TEOS) and the highly reversible nature of the fumed silica gels in water and in alcohol. Two possible reasons for this difference are advanced: variation in the fundamental type of dominant network bond, viz. primary chemical bonds for the alkoxide gels versus secondary physical type bonds for the fumed silica gels, and in the amount of adsorbed liquid that could function as an internal lubricant. Other important observations relating to the fundamental structure of the alkoxide gels involve the duration of highly exothermic reactions and the formation of colloidal particles that appeared to promote gellation. These observations support the conclusion that the alkoxide gels could be formed by a network of colloidal particles or by precursors to these solid particles, i.e., by aggregates of hydrolyzed TEOS oligomers.