Diffusion NMR Methods Applied to Xenon Gas for Materials Study

TL;DR

This paper explores advanced diffusion NMR techniques using xenon gas to study porous materials and flow dynamics, revealing structural information and high-velocity imaging capabilities with potential applications in material characterization.

Contribution

It introduces the application of Pulsed Gradient Spin Echo NMR to xenon gas for analyzing porous media and flow, including the first gas-phase NMR diffusive-diffraction data.

Findings

D(t) approaches long-time asymptote, revealing structural info

Velocity-sensitive imaging of high flow rates in xenon gas

First observation of diffusive-diffraction in gas-phase NMR

Abstract

We report initial NMR studies of i) xenon gas diffusion in model heterogeneous porous media, and ii) continuous flow laser-polarized xenon gas. Both areas utilize the Pulsed Gradient Spin Echo techniques in the gas-phase, with the aim of obtaining more sophisticated information than just translational self-diffusion coefficients - a brief overview of this area is provided in the introduction. The heterogeneous or multiple-length scale model porous media consisted of random packs of mixed glass beads of two different sizes. We focus on observing the approach of the time-dependent gas diffusion coefficient, D(t), (an indicator of mean squared displacement) to the long-time asymptote, with the aim of understanding the long-length scale structural information that may be derived from a heterogeneous porous system. The Pade approximation is used to interpolate D(t) data between the short and long time limits. Initial studies of continuous flow laser-polarized xenon gas demonstrate velocity-sensitive imaging of much higher flows than can generally be obtained with liquids (20 - 200 mm/s). Gas velocity imaging is, however, found to be limited to a resolution of about 1 mm/s due to the high diffusivity of gases compared to liquids. We also present the first gas-phase NMR scattering, or diffusive-diffraction, data: namely, flow-enhanced structural features in the echo attenuation data from laser-polarized xenon flowing through a 2 mm glass bead pack.