Mechanical Properties Improvement of Silica Aerogel through Aging: Role of Solvent Type, Time and Temperature

摘要

Effective parameters that enhance mechanical properties during aging were investigated in the present study. Silica aerogels were made from tetraethyl orthosilicate (TEOS), water, methanol and NH4F in molar ratio 1:4:8:2×10~(-3) using a one-step method. Different time, temperature and aging solvents in aging stage were studied. Subsequently, solvent exchange with n-hexane, modification under TMCS solution and ambient pressure drying (APD) were performed for all samples. The aerogels had densities within the range of 0.1 and 0.6 g/cm~3. The FTIR, mechanical properties, density and BET results, porosity, pore volume, pore diameter and surface area of the samples, were discussed. The compression properties of the gel increased with the increase in the time and temperature of aging. It was observed that solvents with more polarity improved polymerization, which enhanced the mechanical properties of the related samples. However, the stresses and capillary forces of water during drying were so large that inhibited "spring-back effect" during APD, and consequently a collapsed silica network with higher density was fabricated. In other words, the specific compression strength and modulus declined drastically. For methanol, alcohols inhibit the reactions inconveniently causing more shrinkage. In aging by n-hexane, capillary pressure declined significantly and thereby shrinkage was eliminated and silica aerogels with low bulk densities (0.095 g/cm~3), high specific surface areas (600 m~2/g), and large pore volumes (2.6 cm~3/g) were synthesized.