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A sol is a solution with particles suspended in it. When the particles in the sol form long polymers (chains) that span the entire sol, a gel is formed.
Instead of the traditional approach to creating "melt" glasses, the sol-gel method begins by dissolving the materials in a sol (see an example recipe). This process allows for a more homogenous distribution of dopant in the final product (find out more about the rare earth ions used as dopants). The silicon alkoxide (hybrid organic/inorganic molecule), TMOS, is normally insoluble in water, but the addition of an acid or base allows the silicate to mix with the water and catalyzes the formation of a network of polymers and cross-linkages that lead to the creation of a gel. Using an acid leads to longer chains and fewer cross-linkages, resulting in a gel with smaller pores, while an alkaline catalyst promotes cross-linkages and leads to larger pore-sizes. After allowing the cross-linkages sufficient time to form, the gel is heated to 110oC in order to boil off the water, leaving the solid silicon-dioxide network behind. The remaining organic compounds in the gel give it a yellowish color. This product can be annealed to create glass.