Physical-Chemistry Parameters of SiO2 Monoliths Processed Via Sol - Gel

High quality glasses may be prepared from liquid precursors, via sol - geL Starting materials and variable control determine the final characteristics of silica monoliths for optical fiber preforms, prepared via sol - gel, as we have found in previous studies. The rate of hydrolysis. and condensation reactions, as wcU as the time required for densification of a gel, is dependent on water/alkoxide ratio, pH and the presence of drying control chemical additives. The physical and optical properties of the preforms are also determined by the specific conditions of the sol - gel processing.In this study we present an additional analysis of pH effects on the kdnetics, before and during the material syneresis. The effect of CO2 as a contaminant, is also considered. Samples are prepared from hydrolysis of tetrametoxysane in methanol - water mixtures, in the presence of a - picoline. At 20°C, the initial values of pH range between 8 and 6 and show an exponential decrease as the hydrolytic polymensation proceeds, tending to acidic or more acidic values. These steps also are followed by TEM and SEM and are correlated with the sort of aggregates formed in the material along the percolation process. Optical properties of the preforms are interpreted in terms of the clusters formed from those aggregates.Details on instrumental monitoring of [H+] along the alcogelation process are discussed.