RAMAN-STUDY OF STRUCTURAL DEFECTS IN SIO2 AEROGELS

The structure of the silica aerogels was studied by Raman spectroscopy . The spectra of the solid network resembles that of bulk silica with additional bands related to organic groups and a large amount of OH gr oups. The typical bands due to ring breathing also called defect bands D-1 and D-2 located at 490 and 610 cm(-1) are present. However, the e volution of the D-2 band compared to that of OH band (980 cm(-1)) seem s apparently, in contradiction with the results previously reported in the literature. During heat treatments between 25 and 300 degrees C t he D-2 and the OH bands increase simultaneously. Generally, in silica glass the defect band D-2 grows at the expense of the OH groups. This result is explained by the oxidation of the organic compounds which, i n this temperature range, leads to the formation of the both species ( OH) and those related to siloxane rings. Si-29 MAS NMR results are in agreement with the Raman study.