Transparent silica aerogels

This paper begins with the story of monolithic, transparent silica aerogels when in 1968 a paper was published by Nicolaon and Teichner where at the end of it two lines quoted that "...in a basic medium, the aerogels so obtained were totally transparent even through a thickness of 1 cm". This remark attracted the attention of scientists working in high energy physics with charged particles and looking for the construction of a new type of a solid-state Cerenkov counter. Due to their very high thermal insulation properties and their optical transparency in the visible region, they soon were proposed for double pane windows. Transparent silica is also interesting in the production of hollow spheres for inertial confinement fusion as well as for daylight thermal insulation applications. Parameters influencing optical transparency both at the sol-gel stage (monomer precursors and partially polymerized precursors of silica, catalysts, solvents) and at the drying step: supercritical drying at high or low temperature, were described. (C) 1998 Elsevier Science B.V. All rights reserved.