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Tortuosity measurement and the effects of finite pulse widths on xenon gas diffusion NMR studies of porous media.

Abstract：

:We have extended the utility of NMR as a technique to probe porous media structure over length scales of approximately 100-2000 microm by using the spin 1/2 noble gas 129Xe imbibed into the system's pore space. Such length scales are much greater than can be probed with NMR diffusion studies of water-saturated porous media. We utilized Pulsed Gradient Spin Echo NMR measurements of the time-dependent diffusion coefficient, D(t), of the xenon gas filling the pore space to study further the measurements of both the pore surface-area-to-volume ratio, S/V(p), and the tortuosity (pore connectivity) of the medium. In uniform-size glass bead packs, we observed D(t) decreasing with increasing t, reaching an observed asymptote of approximately 0.62-0.65D(0), that could be measured over diffusion distances extending over multiple bead diameters. Measurements of D(t)/D(0) at differing gas pressures showed this tortuosity limit was not affected by changing the characteristic diffusion length of the spins during the diffusion encoding gradient pulse. This was not the case at the short time limit, where D(t)/D(0) was noticeably affected by the gas pressure in the sample. Increasing the gas pressure, and hence reducing D(0) and the diffusion during the gradient pulse served to reduce the previously observed deviation of D(t)/D(0) from the S/V(p) relation. The Pade approximation is used to interpolate between the long and short time limits in D(t). While the short time D(t) points lay above the interpolation line in the case of small beads, due to diffusion during the gradient pulse on the order of the pore size, it was also noted that the experimental D(t) data fell below the Pade line in the case of large beads, most likely due to finite size effects.

journal_name

Magn Reson Imaging

journal_title

Magnetic resonance imaging

authors

Mair RW,Hürlimann MD,Sen PN,Schwartz LM,Patz S,Walsworth RL

doi

10.1016/s0730-725x(01)00247-8

subject

Has Abstract

pub_date

2001-04-01 00:00:00

pages

345-51

issue

3-4

eissn

0730-725X

issn

1873-5894

pii

S0730-725X(01)00247-8

journal_volume

pub_type

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Tortuosity measurement and the effects of finite pulse widths on xenon gas diffusion NMR studies of porous media.