Microstructural characterization of silica aerogels using scanning electron microscopy

Abstract

In this paper the experimental results of microstructural characterization of silica aerogels using scanning electron microscopy (SEM) are reported. 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, catalysts and solvents; gel ageing periods and supercritical drying conditions. It has been observed that strong acidic catalysed gels resulted in smaller pore and particle sizes, and hence more transparent but readily cracked aerogels; whereas weak-basic catalysed gels gave larger pore and particle sizes, and hence slightly less transparent and monolithic aerogels. Microstructures of very low density (0.05 gm cm⁻³) gels 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 have been found to be necessary in order to obtain highly transparent and monolithic silica aerogels. In addition, autoclave heating and solvent evacuation rates of around 25°Ch⁻¹ and 4 cm³ min⁻¹, respectively, resulted in the best quality silica aerogels in terms of monolithicity and transparency.