Pulsed-Field-Gradient Measurements of Time-Dependent Gas Diffusion

Pulsed-field-gradient NMR techniques are demonstrated for measurements of time-dependent gas diffusion. The standard PGSE technique and variants, applied to a free gas mixture of thermally polarized xenon and O₂, are found to provide a reproducible measure of the xenon diffusion coefficient (5.71 × 10⁻⁶m²s⁻¹for 1 atm of pure xenon), in excellent agreement with previous, non-NMR measurements. The utility of pulsed-field-gradient NMR techniques is demonstrated by the first measurement of time-dependent (i.e., restricted) gas diffusion inside a porous medium (a random pack of glass beads), with results that agree well with theory. Two modified NMR pulse sequences derived from the PGSE technique (named the Pulsed Gradient Echo, or PGE, and the Pulsed Gradient Multiple Spin Echo, or PGMSE) are also applied to measurements of time dependent diffusion of laser polarized xenon gas, with results in good agreement with previous measurements on thermally polarized gas. The PGMSE technique is found to be superior to the PGE method, and to standard PGSE techniques and variants, for efficiently measuring laser polarized noble gas diffusion over a wide range of diffusion times.     

Noninvasive Measurements of Gas Exchange in a Three-Dimensional Fluidized Bed by Hyperpolarized 129Xe NMR

We present a novel nuclear magnetic resonance (NMR) technique that provides a noninvasive, direct measurement of gas exchange in a three-dimensional gas-fluidized bed of solid particles. The NMR spectrum of hyperpolarized ¹²⁹Xe gas in an Al₂O₃ particle bed displays three resolved peaks corresponding to xenon in bubbles, the interstitial spaces (emulsion), and adsorbed on particles. Modified NMR exchange and saturation recovery sequences, together with data analysis based on an exchange-coupled set of Bloch equations, yield gas exchange rate constants between the emulsion and adsorbed phases, and between the bubble and emulsion phases. The results are in approximate agreement with previously unverified predictions from well-known models of fluidized bed behavior. Incorporation of NMR imaging methodologies would straightforwardly allow similar measurements on a spatially resolved basis. Authors' address: Ross W. Mair, Harvard Smithsonian Center for Astrophysics, MS 59, 60 Garden Street, Cambridge, MA 02138, USA     

Pore-Structure Determinations of Silica Aerogels byXe NMR Spectroscopy and Imaging

Silica aerogels represent a new class of open-pore materials with pore dimensions on a scale of tens of nanometers, and are thus classified as mesoporous materials. In this work, we show that the combination of NMR spectroscopy and chemical-shift selective magnetic resonance imaging (MRI) can resolve some of the important aspects of the structure of silica aerogels. The use of xenon as a gaseous probe in combination with spatially resolved NMR techniques is demonstrated to be a powerful, new approach for characterizing the average pore structure and steady-state spatial distributions of xenon atoms in different physicochemical environments. Furthermore, dynamic NMR magnetization transfer experiments and pulsed-field gradient (PFG) measurements have been used to characterize exchange processes and diffusive motion of xenon in samples at equilibrium. In particular, this new NMR approach offers unique information and insights into the nanoscopic pore structure and microscopic morphology of aerogels and the dynamical behavior of occluded adsorbates. MRI provides spatially resolved information on the nature of the flaw regions found in these materials. Pseudo-first-order rate constants for magnetization transfer among the bulk and occluded xenon phases indicate xenon-exchange rate constants on the order of 1 s⁻¹for specimens having volumes of 0.03 cm³. PFG diffusion measurements show evidence of anisotropic diffusion for xenon occluded within aerogels, with nominal self-diffusivity coefficients on the order ofD= 10⁻³cm²/s.     

Direct observation of atomic diffusion in warm rubidium ensembles

We present a robust method for measuring diffusion coefficients of warm atoms in buffer gases. Using optical pumping, we manipulate the atomic spin in a thin cylinder inside the cell. Then, we observe the spatial spread of optically pumped atoms in time using a camera, which allows us to determine the diffusion coefficient. As an example, we demonstrate measurements of diffusion coefficients of rubidium in neon, krypton and xenon acting as buffer gases. We have determined the normalized (273 K, 760 Torr) diffusion coefficients to be 0.18 ± 0.03 cm²/s for neon, 0.07 ± 0.01 cm²/s for krypton and 0.052 ± 0.006 cm²/s for xenon.     

Dynamic properties of water in silicalite-1 powder

Self-diffusion of D₂O in partially filled silicalite-1 crystals was studied at 25°C by ²H nuclear magnetic resonance (NMR) with bipolar field gradient pulses and longitudinal Eddy-current-delay. For the first time, reliable experimental diffusion data for this system were obtained. Analysis of NMR diffusion decays revealed the presence of a continuous distribution of apparent self-diffusion coefficients (SDCs) of water, ranging from 10⁻⁷ to ∼10⁻¹⁰ m²/s, which include values much higher and lower than that of bulk water (∼10⁻⁹ m²/s) in liquid phase. The observed distribution of SDC changes with variation of the diffusion time in the range of 10–200 ms. A two-site Kärger exchange model was successfully fitted to the data. Finally, the water distribution and exchange in silicalite-1 pores were described by taking into account (a) a gas-like phase in the zeolite pores, a gas-like phase in mesopores and an intercrystalline gas-like phase and (b) intercrystalline liquid droplets with intermediate exchange rate with the other phases. The other phases experience fast exchange on the NMR diffusion time scale. Diffusion coefficients and mean residence times of water in some of these states were estimated.     

High Temperature Diffusion of 6Li Adsorbed on a Ru(001) Single Crystal Surface as Seen by Pulse NMR

Diffusion measurements on lithium atoms adsorbed on a ruthenium single crystal were performed in the high temperature regime (1100–1200 K). Pulsed NMR techniques were utilized to produce and observe the decay of magnetization patterns from which the diffusion coefficient was extracted. The observed temperature dependence could be described by D = (10 ± 7) cm²/s · exp (−(0.46 ± 0.07) eV/kT). The extremely high diffusion coefficient and prefactor are understood by a gas like adsorbate behavior. The electric field gradient has been measured with ⁷Li: V _zz = −5.0 ± 0.1 10¹⁵ V/cm² with an inhomogeneity of less then 1% as judged by the width of the satellite transitions.     

Electrochemical determination of the lithium ion diffusion coefficient in TiS2

Long-time chronoamperometry of TiS₂ electrodes immersed in saturated LiClO₄/DMF solution was employed to investigate the charge transport processes which govern the rate of Li⁺ intercalation in TiS₂. The intercalation rate and hence, the current, appears to be controlled by the rate of Li⁺ diffusion within the TiS₂. A model has been developed which predicts the current-time behavior under the control of Li⁺ solid state diffusion. The close agreement of this model with the experimental data allows the solid state diffusion coefficient and other transport parameters (such as effective electrode area) to be evaluated from the measured average grain boundary distance. Typical TiS₂ grain boundary distances in the 3–10 μm range yield a geometric mean value of 1.3 × 10^?9 cm²/s for the solid state diffusion coefficient; this is in close agreement with previously reported diffusivities as measured by NMR spin-lattice relaxation techniques.     

Production of nitrogen-free, hyperpolarized 129Xe gas

129 Xe with a nuclear polarization far above the thermal equilibrium value (hyperpolarized) is used in NMR studies to increase sensitivity. Gaseous, adsorbed, or dissolved xenon is utilized in physical, chemical, and medical applications. With the aim in mind to study single-crystal surfaces by NMR of adsorbed hyperpolarized ¹²⁹Xe, three problems have to be solved. The reliable production of ¹²⁹Xe with highest nuclear polarization possible, the separation of the xenon gas from the necessary quench gas nitrogen without polarization loss, and the dosing/delivery of small amounts of polarized xenon gas to a sample surface. Here we describe an optical pumping setup that regularly produces xenon gas with a ¹²⁹Xe nuclear polarization of 0.7(±0.07). We show that a freeze–pump–thaw separation of xenon and nitrogen is feasible without a significant loss in xenon polarization. The nitrogen partial pressure can be suppressed by a factor of 400 in a single separation cycle. Dosing is achieved by using the low vapor pressure of a frozen hyperpolarized xenon sample. Received: 12 June 1998     

Sodium ion conduction in single crystal vermiculite

The diffusion of sodium in single crystals of Llano vermiculite has been studied using conductivity measurements. The crystals were maintained in the bilayer water state by using aqueous contacts. The diffusion coefficient at 25°C was found to be 1.9×10⁻⁸ cm²/s, with an enthalpy of motion of 11.7 kcals/mole. These values are in good agreement with sodium tracer and proton NMR studies and indicate that the sodium ions probably diffuse with their hydration spheres. The conductivity in the range 10 to 90°C is less than that in sodium beta alumina and much less than that of either surface clay cations or of aqueous sodium chloride solutions.     

Magnetic resonance imaging of convection in laser-polarized xenon

We demonstrate nuclear magnetic resonance (NMR) imaging of the flow and diffusion of laser-polarized xenon (129Xe) gas undergoing convection above evaporating laser-polarized liquid xenon. The large xenon NMR signal provided by the laser-polarization technique allows more rapid imaging than one can achieve with thermally polarized gas-liquid systems, permitting shorter time-scale events such as rapid gas flow and gas-liquid dynamics to be observed. Two-dimensional velocity-encoded imaging shows convective gas flow above the evaporating liquid xenon, and also permits the measurement of enhanced gas diffusion near regions of large velocity variation.     

Radiation-enhanced diffusion and fission gas release from recrystallised grains in high burn-up \hbox{UO}_{2} nuclear fuel

The effective diffusion coefficient that gives a steady-state xenon concentration of 0.2-0.3wt% in the recrystallised grains of high burn-up UO 2 fuel is calculated to lie in the range 10 m 24 to 10 m 22 m 2 s m 1 . These values are one to three orders of magnitude lower than the value currently accepted for the radiation-enhanced diffusion coefficient. The time required to reach the steady-state concentration depends on the local fission rate, the grain size distribution and the precise magnitude of the radiation-enhanced diffusion coefficient, and can take from 2 to 10 years. Additional calculations reveal that substantially less than 10% of the fission gas inventory is released from the original UO 2 grains in the outer region of the fuel prior to recrystallisation. In contrast, with a diffusion coefficient of 10 m 22 m 2 s m 1 more than 80% of the fission gas is released from the recrystallised grains of the high burn-up structure in one year.     

NMR of Xe on CO/Ir(1 1 1) and on multilayer Xe/Ir(1 1 1)

Nuclear magnetic resonance (NMR) is performed on monolayer (ML) amounts of adsorbed ¹²⁹Xe on a single crystal substrate. The inherently low sensitivity of NMR is overcome by using highly nuclear spin polarized ¹²⁹Xe that has been produced by optical pumping. A polarization of 0.8 is regularly achieved which is 10⁵ times the thermal (Boltzmann) polarization. The experiments are performed with a constant flux of xenon atoms impinging on the surface, typically 4 ML/s. The chemical shift (σ) of ¹²⁹Xe is highly sensitive to the Xe local environment. We measured profoundly different shifts for the Xe bulk, for the surface of the Xe bulk, and for Xe on CO/Ir(1 1 1). The growth of the bulk is seen in a phase transition like change of σ as a function of temperature at constant Xe flux. At temperatures where no bulk forms at a flux of 4 ML/s, the xenon exchange rate was measured by a spin inversion/recovery method. The exchange time of Xe is found to be 0.24 s at 63.4 K and 64.4 K and somewhat longer at 61.2 K. An analysis is given involving the desorption out of the second layer and fast mixing of first and second layer atoms at these temperatures.     

Configuration and Performance of a Mobile 129Xe Polarizer

A stand-alone, self-contained and transportable system for the polarization of ¹²⁹Xe by spin exchange optical pumping with Rb is described. This mobile polarizer may be operated in batch or continuous flow modes with medium amounts of hyperpolarized ¹²⁹Xe for spectroscopic or small animal applications. A key element is an online nuclear magnetic resonance module which facilitates continuous monitoring of polarization generation in the pumping cell as well as the calculation of the absolute ¹²⁹Xe polarization. The performance of the polarizer with respect to the crucial parameters temperature, xenon and nitrogen partial pressures, and the total gas flow is discussed. In batch mode the highest ¹²⁹Xe polarization of P_Xe = 40 % was achieved using 0.1 mbar xenon partial pressure. For a xenon flow of 6.5 and 26 mln/min, P _Xe = 25 % and P _Xe = 13 % were reached, respectively. The mobile polarizer may be a practical and efficient means to make the applicability of hyperpolarized ¹²⁹Xe more widespread.     

129Xe and131Xe NMR of xenon on silica surfaces at 77 K

Little is known about¹²⁹Xe NMR spectral features and spin-lattice relaxation behavior, and the dynamics of xenon atoms, for xenon adsorbed on solid surfaces at cryogenic temperatures (≤77 K), where exchange with gas-phase atoms is not a significant complication. We report¹²⁹Xe NMR experiments at 9,4 T that provide such information for xenon adsorbed onto the hydroxylated surface of a number of microporous silica samples at 77 K. A convenient design for these cryogenic experiments is described. Dynamics of surface-adsorbed xenon atoms on the time scale of seconds can be observed by¹²⁹Xe NMR hole-burning experiments; much slower dynamics occurring over hours and days are evidenced from changes with time of the¹²⁹Xe NMR chemical shifts. The peak maxima occur in the region ca. 180–316 ppm, considerably downfield of¹²⁹Xe shifts previously reported on surfaces at higher temperatures, and closer to the shift of xenon bulk solid (316.4±1 ppm). The¹²⁹Xe spin-lattice relaxation timesT ₁ range over five orders of magnitude; possible explanations for both nonexponential relaxation behavior and extremely shortT ₁ values (35 ms) are discussed. Preliminary¹³¹Xe and¹H NMR results are presented, as well as a method for greatly increasing the sensitivity of¹²⁹Xe NMR detection at low temperatures by using closely-spaced trains of rf pulses.     

Observation of micropores in hard-carbon using Xe NMR porosimetry

The existence of micropores and the change of surface structure in pitch-based hard-carbon in xenon atmosphere were demonstrated using ¹²⁹Xe NMR. For high-pressure (4.0 MPa) ¹²⁹Xe NMR measurements, the hard-carbon samples in Xe gas showed three peaks at 27, 34 and 210 ppm. The last was attributed to the xenon in micropores (<1 nm) in hard-carbon particles. The NMR spectrum of a sample evacuated at 773 K and exposed to 0.1 MPa Xe gas at 773 K for 24 h showed two peaks at 29 and 128 ppm, which were attributed, respectively, to the xenon atoms adsorbed in the large pores (probably mesopores) and micropores of hard-carbon. With increasing annealing time in Xe gas at 773 K, both peaks shifted and merged into one peak at 50 ppm. The diffusion of adsorbed xenon atoms is very slow, probably because the transfer of molecules or atoms among micropores in hard-carbon does not occur readily. Many micropores are isolated from the outer surface. For that reason, xenon atoms are thought to be adsorbed only by micropores near the surface, which are easily accessible from the surrounding space.     

Formation of a saturation wave front and dynamics of magma state characteristics in its zone

The influence of the diffusion zone width (parameter χ), which bounds the nucleation process in the nucleus neighborhood, on the dynamics of magma melt state at the initial stage of explosive eruption is numerically analyzed in the context of the Iordanskii-Kogarko-van Vijngaarden model of multiphase media. It is shown that in the region of 5 ÷ 50, the value of χ does not affect the time of forming the front of the zone of magma saturation with nuclei, but determines their maximum density, which decreases by three orders of magnitude, from 510¹⁰ to 810⁷ m⁻³, as the parameter χ grows by one order of magnitude. The most considerable losses of gas dissolved in magma are observed for χ = 5 and, consequently, as soon as 100 ms later after the beginning of decompression, there appear zones behind the wavefront, where the melt viscosity grows to hundreds of thousand Pa·s. Under the restriction of diffusion to bubbles, the decompression wave has a classical profile that is slightly perturbed by the front of saturation zone. However, in spite of the fact that the density of the number of nuclei in the cavitating magma upon removing the restriction of gas diffusion into bubbles remains the same as in the case of restriction, the effect of diffusion of additional mass of gas into bubbles changes radically the wavefield structure. Substantially increasing gas pressure in the bubbles due to diffusion with the melt viscosity growing by several orders of magnitude, which prevents bubble growth, changes appreciably the dynamics of the main characteristics of the cavitating magma state.     

129Xe NMR in study of tissues and plants

The high sensitivity of the¹²⁹Xe nuclear magnetic resonance (NMR) chemical shift to the environment was used for characterization of biological tissues and plants. The xenon gas was dissolved under moderate pressure by means of a special device in small pieces of human and animal tissues (heart, muscle, lung, kidney, liver, spleen, brain, sinew, cartilage and hypodermic fat) or plants (leaves, stems, grains, fruits) and the NMR parameters were measured in vitro. The observed line with the chemical shift ～ 180 ppm was attributed to the xenon located in various cellular structures such as lipid shells, intracellular formations. A xenon spectrum in the lungs obtained in vitro coincides with that in the lungs of a mouse measured in vivo by other investigators. The NMR parameters were found to reveal noticeable distinctions between normal and tumour-affected tissues. The analysis of the¹²⁹Xe NMR spectra of the sinew and the cartilage revealed the dependence of the magnetic parameters on the age of the substance. This fact could be accounted for by the changes of the absorption ability of a biological system due to age transformations. The results obtained in comparison with biochemical data reveal the promissory outlook of¹²⁹Xe NMR for the investigation of the state of biological tissues and for medical diagnostics.     

Time-of-flight remote detection MRI of thermally modified wood

We demonstrate that time-of-flight (TOF) remote detection (RD) magnetic resonance imaging (MRI) provides detailed information about physical changes in wood due to thermal modification that is not available with conventional nuclear magnetic resonance (NMR) based techniques. In the experiments, xenon gas was forced to flow through Pinus sylvestris pine wood samples, and the flow paths and dispersion of gas atoms were observed by measuring ¹²⁹Xe TOF RD MRI images from the samples. MRI sensitivity of xenon was boosted by the spin exchange optical pumping (SEOP) method. Two different samples were studied: a reference sample, dried at low temperature, and a modified sample, which was thermally modified at 240 °C after the drying. The samples were taken next to each other from the same wood plank in order to ensure the comparability of the results. The most important conclusion is that both the smaller dispersion observed in all the TOF RD experiments independent of each other and the decreased amount of flow paths shown by the time projection of z-encoded TOF RD MRI experiment imply that a large amount of pits connecting tracheid cells are closed in thermal modification. Closed pits may be one reason for reduced moisture content and improved dimensional stability of wood achieved in thermal modification. This is the first time biological samples have been investigated by TOF RD MRI.     

Superficial migration of tritium physisorbed on monocrystalline nickel (111)

The aim of this study is the measurement of superficial migration coefficient of tritium physisorbed on monocrystalline nickel without chemisorbed sublayer. The chosen crystalline orientation was (111) because it offers the greatest concentration of adsorption sites per square centimeter. A clean surface sample is obtained by mechanical polishing, chemical etching and finally ionic bombardment by high purity argon gas. The pressure in the experimental vessel is maintained below 10^?9 torr, by liquid helium cryopumping after zeolite sorption pumping.A little spot of adsorbed tritium is produced by introduction of a finite amount of tritium gas on the clean surface of the nickel sample through a stainless steel tube. Temperatures of nickel and of the gas introduction tube are respectively regulated at 5 K and 35 K. Tritium is used as a radioactive marker and its 10 keV β-radiation is measured by a channeltron type detector which permits the localization of the deposit without acting on the surface. We observed that tritium sorbed at 5 K is quite immobile (at the time scale of our experiment). After heating up to a fixed temperature T chosen between 10 K and 20 K, the deposite profile variation in function of time is observed to determine the superficial diffusion coefficient D. For the values of T from 13 K to 20 K, D varies from 10×10^?6 to 150×10^?6 cm² sec^?1. A diffusion activation energy of 200 cal mole^?1 is deduced from the exponential increase of the curve. A vibrational frequency can be evaluated to 3×10¹² sec^?1. The rate of desorption permits the evaluation of sorption energy at about 1800 cal mole^?1 in good agreement with usual results concerning physorption of H₂ on metals.