Synthesis And Characterization Of Flexible Silica Aerogels

The flexible silica aerogels which exhibited high flexibility were developed from native silica aerogel by altering the raw materials and preparing technology. There are two methods to prepare flexible aerogels: chemical derivatization method and polymer cross-linking method. Nowadays, the use of aerogel monoliths has been restricted due to their inherent fragility and poor mechanical property. Therefore, it is urgent to develop flexible silica aerogels, which will greatly expand the commercial application of these materials.In this paper, the main research contents include:synthesis of the flexible silica aerogels by supercritical fluid drying (SCFD); the influence of different solvents, precursor and catalyst concentrations on the properties and microstructures; the heat treatment of the flexile aerogels.The main conclusions of this paper are as follows:The flexible aerogel monoliths with less volume shrinkage are obtained using methyltriethoxysilane (MTES) precursor by a two-step acid-base catalyzed sol-gel process followed by the supercritical fluid drying (SCFD). Methanol (MeOH) is used as the solvent while oxalic acid (C2H2O4) and ammonium hydroxide (NH4OH) are used as the acid catalyst and base catalyst, respectively. The optimum molar ratio of the MeOH/MTES (M) is 19.5. The optimum concentration of oxalic acid and ammonium hydroxide are 0.001 M and 10 M, respectively. The as-prepared aerogel monoliths are thermally stable up to a temperature of 533℃. The bulk density of the monoliths is 60 kg/m3 which shows high flexibility with the Young's modulus (Y) at 4.82×104 N/m2. The samples exhibit a nano-structure with the specific surface area, pore volume, average pore diameter at 395 m2/g,0.966 cm3/g, and 9.78 nm, respectively.The flexible aerogel monoliths with less volume shrinkage are obtained using methyltriethoxysilane (MTES) precursor. Ethanol (EtOH) is used as the solvent while oxalic acid (C2H2O4) and ammonium hydroxide (NH4OH) are used as the acid catalyst and base catalyst, respectively. The molar ratio of the EtOH/MTES (M) is fixed at 35. The 60%(v/v) TEOS/EtOH was used as the ageing solution in the reaction. The bulk density of the samples is less than 0.1 g/cm3 and the porosity is as high as 90%. The aerogel monoliths could thermally stable up to a temperature of 470℃.After annealed at different temperatures, the polymer chains of the flexible monoliths are damaged to a certain extent and led to a striking decline of the adhesion of the inter-particles, which is responsible for the deterioration in the flexibility of the flexible silica aerogels.