Silica aerogels prepared via rapid supercritical extraction: Effect of process variables on aerogel properties

We have examined experimentally the effects of rapid supercritical extraction (RSCE) process variables and their resulting pressure and temperature characteristics on aerogel properties. We employ an RSCE process that uses a hydraulic hot press to seal and heat a contained mold until the aerogel precursors reach a supercritical state. After a short stabilization period the hot press restraining force is lowered and the supercritical fluid is allowed to escape, leaving behind an aerogel monolith. The entire process can be accomplished in fewer than 3 h. To control the process, we set the restraining force, the maximum temperature, the heating and cooling rates, the pressure release rate and the mold volume fill ratio (related to the amount of initial precursor material). To investigate the effects of these variables we made silica aerogels from a TMOS-based recipe. We varied the volume of precursor material from 10 to 15 mL (60-97% fill volume), the restraining force from 43 to 111 kN, the temperature heat rate from 0.7 to 4.2 °C/min, the maximum temperature from 288 to 371 °C and the pressure release rate from 0.23 to 0.66 MPa/min. The RSCE process is robust. We were able to make transparent, monolithic aerogels under almost all conditions with little effect on the resulting aerogel properties. Typical density measurements yielded values of approximately 0.065 g/mL (bulk) and 1.9 g/mL (skeletal). The samples were translucent and transmitted 70% of the light at 800 nm (for 5-mm thick samples). The BET surface areas ranged from 517 to 590 m²/g. Maximum temperature was the only variable found to have a significant effect on the aerogels’ properties. As the maximum temperature increased from 288 to 371 °C the surface area decreased from 560 to 395 m²/g and average pore diameter (BJH desorption) increased from 21 to 32 nm.