Preparation and characterization of inhomogeneous RF aerogels with continuously varying densities

Inhomogeneous resorcinol-formaldehyde (RF) aerogels with continuously varying densities were prepared by a facile yet effective method. The preparation involved sol–gel technique and supercritical drying. The low-density sol was pumped into the high-density sol continuously, meanwhile the mixed sol was blended and pumped into a cylindrical container continuously for directional gelation. Then the varying-density gel was washed in hot ethanol and dried by supercritical drying. The samples were investigated by scanning electron microscope and X-ray phase contrast imaging instrument. The problems of density mutation and non-monotonicity of density in samples were studied and well solved by controlling the blending and pumping of sols. Qualified samples with thickness of ~10–28 mm and density of ~0.19–0.62 g/cm3 were obtained. Further work is focusing on the samples’ performance in the experiments of trapping high-velocity particles and impact compression.