Ultrasound-assisted Freeze-Drying Strategy to Enhance Nanoparticle Dispersion in Aerogels: A Case Study on Polyimide/silica Nanocomposite Aerogel

Thermal protection materials should possess light weight, excellent thermal insulation, flame retardant and hydrophobic properties. Aerogel are considered a promising thermal protection material due to its high porosity and low density. Among them, nano-silica/polyimide composite aerogel has been intensively studied as they can solve the problem of fragility of nano-silica aerogel while ensuring thermal insulation and flame retardant performance. However, the problems of nanoparticle agglomeration and the tedious preparation process remain unsolved. The ultrasound-assisted freeze-drying (UAFD) strategy presented in this paper avoids the tedious silica solution preparation process by hydrolyzing silica precursors to achieve good dispersion of silica. This strategy replaces the traditional approach of functionalized modification of silica nanoparticles to inhibit their agglomeration with a new method of nanoparticle dispersion by ultrasound and rapid freezing to lock the dispersed state. The UAFD strategy enables the rapid and efficient preparation of well-dispersed nano-silica/polyimide composite aerogels, and the hierarchical pore structure of the composite aerogels is imparted by this facile preparation strategy that cooperates with the co-regulation capability of the tert-butanol on the morphology of the ice template. The aerogels have substantially improved thermal insulation, flame retardant, and hydrophobic properties when compared with those prepared by conventional freeze-drying methods.