Microstructure of sol-gel processed TiO₂/SiO₂ glasses

Ultra-low expansion TiO₂/SiO₂ glasses were prepared by the sol-gel method. Their microstructures were examined using optical microscopy and transmission electron microscopy with energy dispersive spectroscopy (TEM/EDS). A variety of macro-, micro- and nano-defects were observed and are described, and these are related to the sol/gel processing parameters. However, the intrinsic microstructure of the glasses was found to be homogeneous, amorphous and, in general, comparable to glasses prepared by high temperature flame hydrolysis techniques. This observation is significant because these gels were diphasic mixtures of TiO₂ and SiO₂ colloidal species initially and, further, because their sintering was completed below the liquidus temperature.