Silica or various silicate aerogels can be characterized by highly porous, open cell, low density structures. The synthesis parameters influence the three-dimensional porous structures by modifying the kinetics and mechanism of hydrolysis and condensation processes. Numerous investigations have shown that the structure of porous materials can be tailored by variations in synthesis conditions (e.g., the type of precursors, catalyst, and surfactants; the ratio of water/precursor; the concentrations; the medium pH; and the solvent). The objectives of this review are to summarize and elucidate the effects of chemical conditions on the nanoporous structure of sol-gel derived silicate aerogels.

The main aim of this study is to synthesize calcium silicate ceramics that exhibit suitable properties to be used for biomedical applications. In the present work, attention was paid to the understanding of processing-structure relationships. A particular effort was made to clarify the identification of Ca-O-Si bonds by means of spectroscopy. The calcium silicate systems were prepared via a sol-gel route, varying the chemical compositions, the catalyst concentration, and the temperature and time of aging and heat treatment. The processes and the phases evolved during the sol-gel procedure were determined. The bond systems were investigated by Fourier transform infrared (FTIR) and (29)Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and the aggregate structures by scanning electron microscopy (SEM), small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), and X-ray diffraction (XRD) measurements.