Evaluation of sol-gel processing as a method for fabricating spherical-shell silica aerogel inertial confinement fusion targets

A feasibility study has been undertaken of a method for fabricating uniform, hollow silica aerogel spheres of controlled size, thickness, and porosity that may be required for manufacture of cryogenic inertial confinement fusion targets. The method is a combination of a droplet generation method and sol-gel processing. The parameters controlling the properties of the resulting silica aerogel spheres are the detailed chemical makeup of the reactant solution (which consists of tetraethylorthosilicate, ethyl alcohol, and water) and the gelation medium (which consists of either ammonia and nitrogen gas or ammonium hydroxide), and the dimensions and relative positions of the nozzles used for the droplet generation. To monitor the hydrolysis and polymerization reactions infrared spectroscopy and 1H and 29Si nuclear magnetic resonance were used. The characterization of the silica spheres was performed using mercury intrusion porosimetry, optical microscopy, and scanning electron microscopy. Hollow silica aerogel spheres with densities as low as 61 mg/cm3 were obtained from a reactant solution of 134 composition (namely, TEOS:EtOH:H2O=1:3:4) by using a levitation-gelation method. Thermogravimetric analysis and differential thermal analysis were used, along with the measurement of weight loss and linear shrinkage, to study the response of the aerogel spheres under heat treatment.