Range Of Diffusion Times Research Articles

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.

Restricted diffusion of an 19F-labelled organic acid in human erythrocytes analyzed by 19F pulsed field gradient NMR.

The diffusion coefficient of an 19F-labelled organic acid (3-trifluoromethylhippurate: TFMH) was measured in intact human red blood cells (RBCs) and sealed right-side-out ghosts at 22.5 +/- 0.05 degrees C. Diffusion coefficients were measured using stimulated echo and spin-echo pulsed field gradient sequences. The apparent diffusion coefficient (Da) of TFMH in the intracellular space was much smaller than in the extracellular medium. This was due to restricted diffusion of intracellular TFMH since i) the intracellular Da decreased when the diffusion time was increased, and ii) the smaller Da component disappeared when the membrane was permeabilized with saponin. From the intracellular Da values obtained over a range of diffusion times, from 5 ms to 1 s, the apparent radius of the diffusion barrier was estimated to be 3 +/- 1.1 micron assuming spherical geometry. Despite the high protein concentration in the intracellular space of the intact RBC, the intracellular values of Da were similar to the values obtained in the ghosts at the same diffusion times. We therefore conclude that the small diffusion coefficient of intracellular TFMH is mainly explained by the restricted diffusion due to the cell boundary.

Self-diffusion in CNS tissue by volume-selective proton NMR.

We have used a volume-selective NMR pulse sequence to measure the diffusion coefficient (D) inside a selected cuboid volume. The sequence is a variant of the stimulated echo method for volume-selective spectroscopy, and allows sequential measurement of D in any of three orthogonal directions and for a range of diffusion times. The echo attenuation from which D is calculated is free from the cross terms that can arise because of interactions between volume selection and diffusion-sensitizing gradients. Anisotropic and diffusion-time-dependent behavior is observed in brain structures known to have axon fibers aligned primarily in one direction.

Analysis of P and Sb Diffusion during Thermal Oxidation in Silicon

Using experimental results of oxidation-enhanced/retarded diffusions (OED/ORD) for P/Sb in silicon, OED and ORD equations are rewritten into differential equations for self-interstitials and vacancies (I and V). An extended relation of the local equilibrium between I and V, VIm = n, is derived from asymptotic forms of formal solutions of OED and ORD equations. The differential equations are solved using VIm=n, i.e., OED and ORD equations are simultaneously solved without any assumptions. Consequently, OED/ORD were governed by only the interstitialcy/vacancy mechanism. The local equilibrium between I and V did not occur in the present diffusion time range. From the obtained results, diffusivities of I and V, i.e., D I=2.4×10-9 cm2 s-1 and D V=2.1×10-10 cm2 s-1, and also their thermal equilibrium concentrations, C I0=3.3×1016 cm-3 and C V0=2.2×1017 cm-3, were determined at 1100°C.

Partially restricted diffusion in a permeable sandstone: Observations by stimulated echo PFG NMR

We demonstrate a variant, insensitive to eddy current effects, of an alternating pulsed field gradient technique designed to null the effect of background gradients in liquid-saturated porous media, using a 38 mm diameter sample of a natural sandstone. Measurements of the effective diffusivity confirm predictions of a decline as the square root of an effective diffusion time. A value of the ratio S V p for the dominant pores is extracted, yielding with T 1 a value for the surface relaxivity. We test also a geometry-dependent data collapse recently suggested for a range of diffusion times and wavenumbers. The data agree with a sheet-like pore model for this granular sandstone, and fail to agree with a tube-like model; a pore length scale is also extracted.

Thermal Pulse Diffusion of Zn in GaAs from an Elemental Source

We have studied Zn diffusion from an electroplated elemental source, using rapid thermal annealing techniques to investigate a range of diffusion times from 3 to 30s. Improved surface morphology is obtained by exploiting the high vapor pressure of Zn to remove excess source material, leaving only one to two monolayers of Zn tightly bound to the substrate. Resultant diffusions form shallow (0.1–0.2 μm) heavily doped regions. Junction properties are almost independent of diffusion time and linear in temperature over the range investigated.

Horizontal diffusion in the atmosphere: A Lagrangian-dynamical theory

A form of Langevin's equation is derived that is applicable to the atmospheric diffusion problem. The resulting equation for the particle displacement variance σ 2 y has limits at small and large diffusion times equal to asymptotic predictions of statistical diffusion theories but provides, in addition, estimates over the broad, middle range of diffusion, which is important in regional and larger-scale atmospheric applications. Predictions of the theory compare well with standard atmospheric diffusion data sets over a range of diffusion times, from seconds to days. When parameters of the theory are determined from short-range plume diffusion data, the theory predicts large-scale eddy diffusivity, K, in the known atmospheric range, a striking confirmation of the ability of this theory to describe atmospheric diffusion.

Diffusion of silver in gallium arsenide at 1000 degrees C

Radioactive silver was diffused into GaAs at 1000 degrees C. The diffusions were carried out over a range of diffusion times and with different amounts of extra arsenic in the ampoule. Profiles were plotted. It is proposed that the silver diffuses primarily by the movement of interstitial atoms but that most of the silver in the GaAs occupies a substitutional site. Electrical measurements were also carried out on diffused specimens, using capacitance-voltage and Hall techniques. All samples were p-type and a reasonably good correspondence was found between the silver and hole concentrations. It was established that the p-type conductivity was not due to thermal conversion and it was concluded that the silver acts as an acceptor in GaAs, occupying the gallium site.

Plasma Confinement in a Toroidal Quadrupole

The confinement of plasma in a toroidal quadrupole configuration is studied. The plasma parameters are, ion temperature = 50 eV, electron temperature = 6 eV, and plasma density 1013-107 cm−3. The confinement times in terms of Bohm diffusion time range from 20 to 40. The loss of the plasma is to the walls. The plasma fluctuations which are present in large amplitudes are the most likely cause of plasma loss. The superposition of a toroidal field reduces the plasma loss.