Globally Anisotropic High Porosity Silica Aerogels

TL;DR

This paper explores methods to engineer high porosity silica aerogels with global anisotropy through axial strain and controlled growth, characterized by SAXS and optical birefringence.

Contribution

It introduces two novel methods for inducing and characterizing global anisotropy in silica aerogels during fabrication.

Findings

Anisotropy causes optical birefringence in aerogels.

Optical cross-polarization effectively characterizes anisotropy.

Both strain and shrinkage induce measurable anisotropy.

Abstract

We discuss two methods by which high porosity silica aerogels can be engineered to exhibit global anisotropy. First, anisotropy can be introduced with axial strain. In addition, intrinsic anisotropy can result during growth and drying stages and, suitably controlled, it can be correlated with preferential radial shrinkage in cylindrical samples. We have performed small angle X-ray scattering (SAXS) to characterize these two types of anisotropy. We show that global anisotropy originating from either strain or shrinkage leads to optical birefringence and that optical cross-polarization studies are a useful characterization of the uniformity of the imposed global anisotropy.