Chemistry and Fundamentals of the Sol–Gel Process

Sol–gel materials are metastable solids that are formed in kinetically controlled reactions from molecular precursors, which constitute the building blocks for the later materials. This chapter discusses in detail the chemical and physical principles behind the individual steps of sol–gel processing. In alkoxide-based systems, hydrolysis reactions of Si-OR groups must precede condensation to generate the Si-OH groups, which are necessary for condensation. The chapter discusses gelation focusing on silica, but is analogous for other materials. The sol–gel transition (gel point) is reached when a continuous network is formed. The chemical reactions leading to network formation are not finished with gelation, and structural rearrangements take place in the wet gels. This phenomenon increases the stiffness of the gels and is called aging. The final step in the synthesis of many sol–gel materials is some postsynthesis treatment after or concomitant with drying, depending on the intended properties or uses.

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