NMR measurements of hyperpolarized 3He gas diffusion in high porosity silica aerogels

TL;DR

This study uses NMR measurements of hyperpolarized 3He gas to noninvasively analyze the microscopic structure and diffusion properties of high-porosity silica aerogels, revealing non-uniformities and differences in diffusion behavior.

Contribution

It introduces a novel application of hyperpolarized 3He NMR to characterize the microstructure and diffusion in silica aerogels, including a phenomenological model for non-uniform pore structures.

Findings

Diffusion behavior varies among aerogel samples despite similar porosities.

Pressure dependence indicates non-uniform pore structures on micrometer scales.

Numerical models align with experimental diffusion measurements.

Abstract

Hyperpolarized 3He is used to nondestructively probe by NMR the structure of custom-made and commercial silica aerogels (97% and 98.5% porous). Large spin-echo signals are obtained at room temperature and very low magnetic field (2mT) even with small mounts of gas. Attenuation induced by applied field gradients results from the combined effects of gas diffusion and confinement by the porous medium on atomic motion. Nitrogen is used as a buffer gas to reach equivalent 3He pressures ranging from 5 mbars to 3.5 bars. The observed pressure dependence suggests a non-uniform structure of the aerogels on length scales up to tens of micrometers. A description by broad phenomenological distributions of mean free paths is proposed, and quantitatively discussed by comparison to numerical calculations. The investigated aerogel samples exhibit different effective diffusion characteristics despite comparable nominal porosities.