Globally anisotropic high porosity silica aerogels

Abstract

a b s t r a c tWe discuss two methods by which high porosity silica aerogels can be engineered to exhibit global anisotropy. First, anisotropy can be introduced with axial strain (i.e. axial compression). 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.

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“…One of our goals was to measure the mean-free-path and its anisotropy. Consistent with expectations based on other characterization methods 13 the RSA appears to be anisotropic and recent reports indicate that this type of anisotropy stabilizes an anisotropic superfluid state. 4,29 Additionally, a second anisotropic superfluid state emerges at even lower temperatures where the theory indicates that this is the manifestation of a biaxial state.…”

“…However, this can be ruled out in the present work from our direct measurements using optical birefringence and measurements on similar samples with small angle x-ray scattering. 3,13 Additionally, for 3 He imbibed into the aerogel structure, spatial inhomogeneity in the density of aerogels will produce a non-zero width in the transition temperature to the superfluid 3 He state, where the appropriate length scale to resolve such inhomogeneity is the superfluid coherence length 0.1 µm. However, in similar aerogels we do not find any evidence for such inhomogeneity to high precision.…”

“…They were grown with a one-step method and rapid supercritical extraction of the solvent. 13,14 The surface area of the aerogel from nitrogen adsorption (BET) 15 is ∼400 m 2 /g and the mean distance between silica strands, i.e. the aerogel correlation length, can be estimated from numerical calculations to be ∼50 nm.…”

“…16 The homogeneity of the samples was characterized with optical birefringence technique and filtered white light. 13 Inhomogeneity in the aerogel density, if present on length scales longer than the wavelength of light, will induce a preferred optic axis that would be apparent in an image of the sample placed between crossed polarizers. The resulting light intensity variation serves as a sensitive method of determining the aerogel uniformity and density anisotropy, Fig.…”

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“…However, the synthesis can be controlled such that the anisotropy is either globally axial, or completely avoided altogether. 22) In the latter case, an image with polarized light of a homogeneous and isotropic aerogel sample is black for any angle of presentation with respect to the polarizers. In the case of homogeneous axial anisotropy the image is black only for the anisotropy axis (optical axis) parallel to either of the polarizers.…”