From SiO1.5CH3 to vitreous SiO2: A structural evolution study

The conversion of SiO1.5CH3 into SiO2 glass by pyrolysis in air has been studied by combining density measurements, vibrational spectroscopy, positron annihilation spectroscopy, and nuclear magnetic resonance. The results show that a critical transition takes place between 400 and 500 degrees C and at 600 degrees C the organic features are completely removed. The conversion of the hybrid structure into a fully inorganic network takes place through the formation of metastable 3-membered rings that progressively evolve into more stable 5+ rings as the pyrolysis temperature increases. The evolution of the organic moieties is coupled with the formation of Si-OH and molecular water trapped in the network which are well visible between 500 and 800 degrees C. In such temperature regions, a clear evolution of the network density can be detected, though the amount of free volume is nearly constant (due to the presence of trapped H2O). Only at 1250 degrees C, the network is comparable with a melt-derived glass.