Helium-3 gas self-diffusion in a nematically ordered aerogel at low temperatures: enhanced role of adsorption

Abstract

We performed 3 He gas diffusion measurements for the first time in a highly porous ordered Al 2 O 3 aerogel sample at a temperature of 4.2 K using a nuclear magnetic resonance field gradient technique.A strong influence of 3 He adsorption in the aerogel on self-diffusion is observed. The classical consideration of adsorptive gas diffusion in mesopores leads to anomalously high tortuosity factors. The application of a more sophisticated model than the simple combination of empirical two-phase diffusion and the Knudsen gas diffusion models is required to explain our results. Anisotropic properties of the aerogel are not reflected in the observed gas diffusion even at low gas densities where the anisotropic Knudsen regime of diffusion is expected. The observed gas densification indicates the influence of the aerogel attractive potential on the molecular dynamics, which probably explains the reduced diffusion process. Perhaps this behavior is common for any adsorptive gases in nanopores.

Diffusion Anisotropy of Gaseous Helium-3 in Ordered Aerogels at Low Temperatures

“…Recently Kondrashova et al 15 reported strong diffusion anisotropy of liquid cyclohexane at room temperature in the specially prepared porous silicon wafers by the electrochemical anodization technique. The first diffusion measurements of strongly adsorbing gas for characterization of globally anisotropic aerogels was performed by Lee et al 9 However, the obtained results are questionable as was already discussed in Kuzmin et al 10 In this study the globally anisotropic aerogel was produced using slightly different conditions of its growth if compared to a usual growth condition of isotropic aerogels; therefore, the degree of its anisotropy was modest (15%). Because of recent technology developments, the production of aerogels with a strong global anisotropy became feasible, and tools for their characterization are in high demand.…”

“…At low temperatures 3 He forms adsorbed layers on the surface of aerogels that suppress the measured diffusivity in aerogel and lead to its anomalous pressure dependence. This behavior is typical for the two-phase system with a fast exchange between phases: 10,25,26 diffusion in the gas phase is much faster than in the adsorbed 3 He layer, and the lifetimes in each phase are much shorter than measurement times. The actually measured apparent diffusion coefficient D is usually assumed to be a fraction-weighted averaged diffusion in both phases:…”

“…The magnetic field gradient was applied at a set of angles φ against the mean aerogel fibers alignment axis within the range of 360°. Spin-echo amplitude decays were measured using a standard Hahn echo sequence, and the diffusion coefficient was obtained after a proper fit of data as described by Kuzmin et al 10 Standard saturationrecovery and CPMG techniques were used for complementary measurements of 3 He spin−lattice and spin−spin relaxation times. The NMR experiments were performed with both helium-3 and helium-4 aerogel preplating at 4.2 K. The helium-4 preplating was used to reduce the effect of adsorbed helium-3 on gas diffusion measurements.…”

“…He in the adsorbed layer revealed the same diffusivity in gas phase as in the pure 3 He experiment. 10 Diffusion in gas phase D gas is usually written as…”

“…This procedure of extracting of 3 He amounts in two phases from the adsorption isotherm was described in detail in our previous work. 10 We also computed the 3 He density in the gas phase using the obtained adsorption data, the sample porosity ϵ ,and the volume V:…”