Defects in Gel-Derived Glasses

Abstract

ABSTRACTDefects in gel-derived glasses were reviewed for the three principal stages of the sol-gel process: solution, porous gel, and consolidated glass. The most prominent defects in gel-derived SiO2 are Raman active defects characterized by sharp bands at 490 cm−1(D1) and 608 cm−1(D2) 18O enrichment, experimental estimates of heats of formation, and structural studies of cyclic model compounds were used to identify the structural origins of D1 and D2 in gels. We conclude that D1 and D2 in gel-derived SiO2 are the oxygen breathing modes of 4- and 3-fold siloxane rings, respectively, as originally proposed by Galeener to explain these Raman-active defects in conventional vitreous silica (v-SiO2). During heat treatments near Tg (defined by n = 1013.5 poises), the gel-derived SiO2 structure rapidly approaches that of conventional v-SiO2.