Structural Anisotropy and Birefringence in Microporous Glasses

Abstract

Microstructural birefringence was observed by immersing microporous glass specimens in liquids with various refractive indices. The sign of the birefringence may be positive or negative depending on the base glass compositions, with the optic axis parallel to the direction of materials transport during leaching. The magnitude of the birefringence varies systematically as a function of the refractive index of the immersing liquid and depends on heat‐treatment conditions of the base glass (possibly on soluble minor‐phase compositions) as well as on leaching conditions (e.g. leaching temperatures and acid concentrations). This birefringence can be erased by soaking the specimens in a dilute alkali solution. Scanning electron micrographs before and after soaking indicate that the birefringence is created by the structural anisotropy of SiO2 gels produced as decomposed deposits from the soluble minor phase within the cavity of the silica skeleton. Copyright © 1978, Wiley Blackwell. All rights reserved