A method for producing gradient density aerogel

The Stardust Mission used gradient density aerogel as the hypervelocity particle capture medium in both the cometary and interstellar collection grids. The development and production of these materials was performed exclusively for this mission as a means of improving the efficiency of the collection process. The density of an aerogel can be dictated by controlling the ratio of the condensable silica to that of the solvent used in the aerogel precursor solution. A density gradient was established by gradually mixing the precursor solution for low density aerogel into the precursor solution for high density aerogel and continuously pumping the resultant mixture into a mold. The aerogel designed for the capture of cometary particles had a density gradient ranging from 10 mg/cm3 to 50 mg/cm3 across the 3 cm profile of each block. Whereas those designed for the capture of interstellar grains had a gradient ranging from 10 mg/cm3 to 20 mg/cm3 across a 1 cm profile. Since various physical properties, e.g., refractive index, thermal conductivity, acoustic impedance, dielectric constant, are correlated to the density, they also vary with the density. This method of establishing a density gradient in aerogel can be used to produce other material gradients, e.g., gradient oxide, gradient dopant, in any sol–gel based material.