NMR measurements of hyperpolarized He3 gas diffusion in high porosity silica aerogels

Magnetic-field coils for metastability-exchange optical pumping, spectroscopy, and magnetic resonance of helium

Optical pumping on the 23S-23P transition (1083 nm) of metastable 3He or 4He atoms is used for science and applications. Gas is usually enclosed in elongated cells with lengths ranging from several centimeters to several meters for efficient absorption. Good magnetic-field homogeneity is needed for weak diffusion-induced relaxation and long nuclear magnetic resonance (NMR) signal lifetimes. A compact coil system is designed using a target-oriented numerical optimization method. It provides a suitably uniform field over cell volumes, with characteristics depending on the chosen optimization parameters. Additional pairs of coils can be used to generate transverse field components with contributions to the total field inhomogeneities that are discussed.

An overview of Helium-3 NMR: Recent developments and applications

The present review is focused on experimental and theoretical methods together with applications of helium NMR in chemistry and biochemistry. It comprises two main sections, the first dealing with standardization and instrumentation for 3He NMR spectroscopy and the second dealing with its practical applications, mainly those in general and organic chemistry with a special emphasis on the rapidly developing and exciting area of fullerenes encapsulating helium atoms. Several general applications of 3He NMR spectroscopy in physical chemistry and biomedicine are also briefly discussed.

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

We report on the first observation of diffusion anisotropy of gaseous helium-3 entrapped in ordered aerogels at 4.2 K. The origins of ³He diffusion anisotropy in aerogels of different porosity are discussed. The correlations between gas diffusion coefficient and basic parameters of aerogels, such as porosity, fiber diameter, and fiber's degree of alignment, are inspected using simple diffusion simulations within the framework of classical diffusion model in both oriented and chaotic aerogels under conditions of diffuse (Knudsen diffusion) and specular reflections of atoms from the walls. The failure of the two-phase and Knudsen diffusion models at low temperature in isotropic and anisotropic aerogels is observed. The effect of a wall attractive potential on the gas dynamics is suspected to play a crucial role in the gas diffusion and its anisotropy. The rough theoretical estimates of that effect at low temperatures in aerogel space confirm this assumption. The observed peculiar diffusion is universal and is expected to occur with other probe gases at higher temperatures.

Anisotropic reduced diffusion in dilute liquid 3He-4He mixture in ordered aerogel

We report on the first observation of ³He diffusion anisotropy in ³He-⁴He liquid mixture confined in ordered aerogels at 1.5-4.2 K temperatures. The used aerogels are arrays of long Al₂O₃ parallel 8 nm strands. The possible origins of diffusion anisotropy are considered and the changes of roton properties introduced by parallel aerogel strands are discussed. Among the responsible mechanisms we account for Knudsen diffusion, potential anisotropy of layer mode excitations or of bulklike excitations, and helium vortices. The observed reduced ³He diffusion in aerogels is discussed and suggested to appear due to helium excitations at strong confinement conditions. These observations pave the way for future experiments to gain insight into the crossover regime expected at lower temperatures (below 1 K) for which roton density is lower and ³He collisions with strands play significant role.

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

We performed ³He gas diffusion measurements for the first time in a highly porous ordered Al₂O₃ aerogel sample at a temperature of 4.2 K using a nuclear magnetic resonance field gradient technique. A strong influence of ³He 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.

Achieving high spatial resolution and high SNR in low-field MRI of hyperpolarised gases with Slow Low Angle SHot

MRI of hyperpolarised gases is usually performed with fast data acquisition to achieve high spatial resolutions despite rapid diffusion-induced signal attenuation. We describe a double-cross k-space sampling scheme suitable for Slow Low Angle SHot (SLASH) acquisition and yielding an increased SNR. It consists of a series of anisotropic partial acquisitions with a reduced resolution in the read direction, which alleviates signal attenuation and still provides a high isotropic resolution. The advantages of SLASH imaging over conventional FLASH imaging are evaluated analytically, using numerical lattice calculations, and experimentally in phantom cells filled with hyperpolarised (3)He-N(2) gas mixtures. Low-field MRI is performed (here 2.7 mT), a necessary condition to obtain long T(2)(∗) values in lungs for slow acquisition. Two additional benefits of the SLASH scheme over FLASH imaging have been demonstrated: it is less sensitive to the artefacts due to concomitant gradients and it allows measuring apparent diffusion coefficients for an extended range of times.

Role of urea in the preparation of highly porous nanocomposite aerogels

The preparation of highly porous CoFe2O4-SiO2 nanocomposite aerogels was successfully achieved by a novel sol-gel procedure involving urea-assisted co-gelation of the precursor phases. This method allows fast gelation, giving rise to an aerogel with 97% porosity. The formation of CoFe2O4 nanocrystals homogeneously distributed within the matrix occurs after calcination at 750 degrees C and is complete at 900 degrees C. Despite the high temperature required for the formation of the CoFe2O4 nanocrystals, the high porosity typical of aerogels is still retained.

Diffusion in binary gas mixtures studied by NMR of hyperpolarized gases and molecular dynamics simulations

The dependence of the individual mean square displacement of rare gases in binary mixtures is studied by a combined experimental and theoretical approach. We show that the diffusion constant can be varied in a considerable range by changing the molar fractions of the mixtures. On the experimental side, NMR diffusion measurements are done on hyperpolarized 3He and 129Xe, mixed with several inert buffer gases, in the presence of a magnetic field gradient. The results are compared to diffusion coefficients obtained from atomistic molecular dynamics simulations based on Lennard-Jones type potentials of the corresponding gas mixtures, and to appropriate analytical expressions, yielding very good mutual agreement. This study is the first quantitative validation of the effects of the mutual interactions between gas particles on the individual diffusion properties. It is shown that the dependency of gas phase diffusion properties on the local chemical environment may not be neglected, e.g. in diffusion-controlled chemical reactions.