Sol Formation and Gelation

The sol-gel chemistry of metal salts is more complex than that of metal alkoxides because of the numerous molecular species that can be formed depending on the oxidation state of the metal, the pH of the reaction solution and the concentration of the reactants. Since the sol-gel polymerization of inorganic salts varies widely among the different metal ions, this section will only present a general summary of the topic. For a detailed description of the mechanism of condensation and gelation, the reader is referred to the following review on the sol-gel chemistry of transition metal oxides [7]. 

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Multicomponent sol—gel fibers have been successfuUy developed (1,52). The early stages of sol formation and gelation are cmcial for controlling the fiber microstmcture. Aluminosilicates, zirconates, and aluminates (1,18,52) can be prepared by sol—gel methods. MuUite [55964-99-3] 3AI2 O3 -2Si02, fibers... 

Silica in the form of thin films as well as oxide monoliths, fibers, and powders can be prepared from sol-gel method. In contrast with the fabrication of conventional inorganic glasses at much higher melting temperature, sol-gel processing is performed at low temperatures to produce oxide materials with desirable hardness, optical transparency, chemical durability, tailored porosity, and thermal resistance. The sol-gel method involves formation of a colloidal suspension (sol) and gelation to form a network in a continuous liquid phase (gel). One starts with an aqueous solution containing oxides or alkoxides, mutual solvent, and catalyst. Usually an external catalyst is added like mineral acids and ammonia as well as acetic acid, KOH, amines, KF, and HF for rapid and... 

The reaction mechanism during gel formation differs from that of precipitated silicas and comprises sol formation followed by the gelation step, where the liquid precursor sol has the same chemical composition as the gel formed from it. This might be described by interaction of separate micelles in the sol first by hydrogen bonding, followed by condensation. More details may be taken... 

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