Fabrication and characterization of resorcinol–formaldehyde gradient aerogels by double-diffusive convection

Density gradient aerogel is a kind of porous material with non-uniform density change in a specific direction. It has some unique properties and can be used in some extreme conditions. The resorcinol–formaldehyde (RF) gradient aerogels were prepared by the method based on double diffusion convection. We studied the influences of injection velocity and diffusion before gelation on the distribution of gel density gradient, and obtained an optimized injection velocity scheme. The RF density gradient aerogels were characterized by x-ray transmission imaging, scanning electron microscopy (SEM), and nitrogen adsorption–desorption isotherm. The density distribution and microstructure of the RF density gradient aerogels were obtained. The solution obtained by the optimized multi-velocities injection method presents a gradient change in the low-density part that is greater than the high-density part. This uneven change can offset the diffusion caused by the long gel time of the low-density part, and finally form a gel whose density changes uniformly with the thickness. The aerogel obtained by the optimized method ranges from 100 to 400 mg cm⁻³, and the density of the sample varies linearly. The results indicate that this is an efficient and direct method for preparing density gradient aerogels, and aerogels with specific density trends can be obtained by adjusting the injection velocities.