MICROSTRUCTURAL CHARACTERIZATION OF SILICA AEROGELS USING SCANNING ELECTRON-MICROSCOPY

In this paper the experimental results of microstructural characteriza tion of silica aerogels using scanning electron microscopy (SEM) are r eported. In order to understand the reasons for shrinkage, opacity and cracking of the aerogels, detailed SEM observations have been made on the aerogels prepared using various molar ratios of precursors, catal ysts and solvents; gel ageing periods and supercritical drying conditi ons. It has been observed that strong acidic catalysed gels resulted i n smaller pore and particle sizes, and hence more transparent but read ily cracked aerogels; whereas weak-basic catalysed gels gave larger po re and particle sizes, and hence slightly less transparent and monolit hic aerogels. Microstructures of very low density (0.05 gm cm(-3)) gel s indicate that the gels form a highly crosslinked polymer network and , then, the spherical particles form on the network at higher aerogel densities. Gel ageing resulted in neck growth between SiO2 particles. Precise control of pore and particle sizes using sol-gel parameters ha ve been found to be necessary in order to obtain highly transparent an d monolithic silica aerogels. In addition, autoclave heating and solve nt evacuation rates of around 25 degrees C h(-1) and 4 cm(3) min(-1), respectively, resulted in the best quality silica aerogels in terms of monolithicity and transparency.