Influence of sol-gel processing parameters on the ultrasonic sound velocities in silica aerogels

Experimental results on ultrasonic sound velocities in silica aerogels as a function of molar ratios of tetramethoxysilane (TMOS) precursor, ammonia (NH4OH) catalyst, methanol (MeOH) solvent, and water are reported in this paper. The molar ratios of NH4OH/TMOS, MeOH/TMOS, and H2O/TMOS were varied from 3.7 x 10(-6) to 1, 1 to 80, and 2 to 24, respectively. Very low molar ratios of NH4OH/TMOS (10(-6) to 10(-4)), MeOH/TMOS (1 to 8) and H2O/TMOS (2 to 4) resulted in semi-transparent or opaque aerogels with very low longitudinal and transverse sound velocities of nearly 110 and 67 m s(-1), respectively. On the other hand, very high molar ratios of these combinations gave increased sound velocities of greater than 145 and 85 m s(-1) with all cracked but transparent aerogels. The best quality silica aerogels in terms of monolithicity, transparency, and low sound velocities of 113 and 68 m s(-1) were obtained with TMOS:MeOH:H2O:NH4OH in the molar ratio of 1:12:4:5 x 10(-3). The aerogels with lower and higher densities were subjected to various humidity conditions in air, and the resulting variations in the sound velocities were measured. The thermal treatment of low density silica aerogels (rho approximate to 50 kg m(-3)) up to 500 degrees C also changed the sound velocity by greater than 30%. The systematic and detailed measurements of sound velocities allowed us to determine the corresponding Poisson ratio for all of the aerogels. The results are discussed by considering the hydrolysis and condensation reactions and the resulting densities of silica aerogels. (C) 1997 Elsevier Science Ltd.