Particle aggregation in alumina aerogels

Alumina aerogels were synthesized by low temperature CO₂ supercritical drying (SCD) of gels via the Yoldas process. The aerogels have a surface area of ˜425 m²/gr, similar to that obtained under high pressure/temperature SCD. The surface area and the cluster size of the aerogels are strongly influenced by the amount of acid during gelation. Gels and aerogels were studied by small angle x-rays scattering (SAXS), and the data were analyzed using the Fisher–Burford equation. The SAXS results along with TEM observations support the existence of a hierarchical aggregation at the gelation stage, having a mass fractal dimension of D_m = 2.6−2.8. During the SCD the morphology collapses to form a structure with surface fractal dimension D_s = 2.6−2.9.