Synthesis and physical properties of TEOS-based silica aerogels prepared by two step (acid-base) sol-gel process

The experimental results on the synthesis and physical properties of tetraethoxysilane- (TEOS) based silica aerogels produced by two step (acid-base) sol-gel process, are reported. The oxalic acid (A) and NH4OH (B) concentrations were varied from 0 to 0.1 M and from 0.4 to 3 M, respectively. Monolithic and transparent aerogels have been obtained for the values of A = 0.001 M and B = I M. The effect of time interval (t) before the base catalyst (NH4OH) addition to the acidic sol was studied from 0 to 72 h. The time interval at t = 24 h of NH4OH addition was found to be the best, in terms of low volume shrinkage, high optical transmission and monolithicity. The molar ratio of EtOH/TEOS (S) was varied from 3 to 7.5. Monolithic and transparent aerogels were obtained for an S value of 6.9. Also, the effects of molar ratio of acidic water, i.e., H2O/TEOS (W-1) and basic water, i.e., H2O/TEOS (W-2) on the physical properties of the aerogels have been studied. Highly transparent (similar to 90%) and monolithic aerogels with lower volume shrinkage (< 10%) were obtained for the molar ratio 1:6.9:3.5:2.2 of TEOS: EtOH: acidic (H2O):basic (H2O). The results are discussed by taking into consideration the hydrolysis and polycondensation reactions. The aerogels were characterized by scanning electron microscopy (SEM), optical transmission, bulk density, volume shrinkage and porosity measurements. (C) 2004 Elsevier SAS. All rights reserved.