Fabrication and characterization of hollow silica aerogel spheres using sol-gel processing in the dual nozzle generation system

A system was designed for fabricating uniform, hollow silica aerogel spheres of controlled size, thickness, and porosity that may be required for manufacture of cryogenic ICF (Inertial Confinement Fusion) Targets. The method was a combination of a droplet generation method and sol-gel processing. The parameters controlling the properties of the resulting silica aerogel spheres were the detailed chemical makeup of the reactant solution (which consists of tetraethylorthosilicate, ethyl alcohol, and water) and the gelation medium (which consisted of either ammonia and nitrogen gas or ammonium hydroxide), and the dimensions and relative positions of the nozzles used for the droplet generation. A detailed study designed to understand and control the kinetics of the sol-gel processing that is responsible for the hollow silica-aerogel sphere formation is reported. Specifically, the optimum rheology and stoichiometry of the reactant solution, the make-up of the gelation/levitation gas mixture, and the characteristics of the resulting silica-aerogel spheres, such as size, thickness, porosity, pore size, and density, were investigated. Supercritical drying was performed for the elimination of residual solvents without shrinkage and cracking of hollow spheres. To monitor the hydrolysis and polymerization reactions FTIR (Fourier Transform Infrared Spectroscopy), and $\sp1$H, $\sp{29}$Si, and Solid-State NMR (Nuclear Magnetic Resonance) were used. The characterization of the silica spheres was performed using MIP (Mercury Intrusion Porosimetry), BET, optical microscopy, interferometry, and SEM (Scanning Electron Microscopy). TGA (Thermogravimetric Analysis) and DTA (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. Nitrogen adsorption-desorption isotherms were obtained to get such information as average pore size, pore size distribution, and specific surface area for the characterization of the internal structure of silica aerogel spheres. Hollow silica aerogel spheres with a low density (61 mg/cc) and a high specific surface area (1109 m$\sp2$/g) were obtained from a reactant solution of 134-composition (namely, TEOS:EtOH:H$\sb2$O = 1:3:4) by using a gas levitation gelation method.U of I OnlyETDs are only available to UIUC Users without author permissio