Influence of gel aging and Na2SiO3/H2O molar ratio on monolithicity and physical properties of water–glass-based aerogels dried at atmospheric pressure

The experimental results on the effects of gel aging and molar ratio of Na2SiO3/H2O on monolithicity and physical properties of water–glass-based silica aerogels are presented. The aerogels were prepared by atmospheric pressure drying using a silylation process. The gel-aging period was varied from 1 to 96 h. The percentage of volume shrinkage and hence the density and optical transmission of the aerogels increased with gel aging. The molar ratio of Na2SiO3/H2O was varied by a factor of 10, from 1.77 × 10−3 to 1.77 × 10−2. The bulk density, percentage of volume shrinkage and optical transmission decreased with an increase in the initial Na2SiO3/H2O molar ratio. The Na2SiO3 was passed through a cationic exchanger before preparation of the sol, in all cases. Silica aerogels were obtained with zero percentage of volume shrinkage and density as low as 0.150 g cm−3 with a Na2SiO3/H2O molar ratio ⩾8.86 × 10−3. At this ratio, however, the percentage of optical transmission is reduced to 50% (at 750 nm for a 0.8 cm thick sample) and further decreases for higher molar ratios. The gelation time decreases with an increase in the Na2SiO3/H2O molar ratio. The aerogels were characterized by density, optical transmission and porosity measurements. The results are discussed by taking into account the hydrolysis and condensation reactions, syneresis effects and the bulk modulus of the gel network.