Single-point magnetic resonance imaging study of water adsorption in pellets of zeolite 4A

The water uptake process in commercial type particles of zeolite 4A has been studied using a single-point MRI method. True proton density, T1, T2, and T*2 relaxation times were obtained with submillimetric resolution, overcoming the restrictions of short T*2 signals. The molecular mobility in nonequilibrium conditions has been characterized by relaxation time mapping. A clear reduction of the water sorption rate was observed by comparing MRI profiles of a loosely packed bed and gravimetric measurements of spread particles from the same sieved zeolite batch.     

Spatial and temporal mapping of water content across Nafion membranes under wetting and drying conditions

Water transport and water management are fundamental to polymer electrolyte membrane fuel cell operation. Accurate measurements of water content within and across the Nafion layer are required to elucidate water transport behavior and validate existing numerical models.We report here a direct measurement of water content profiles across a Nafion layer under wetting and drying conditions, using a novel magnetic resonance imaging methodology developed for this purpose. This method, multi-echo double half k-space spin echo single point imaging, based on a pure phase encode spin echo, is designed for high resolution 1D depth imaging of thin film samples. The method generates high resolution (<8 μm) depth images with an SNR greater than 20, in an image acquisition time of less than 2 min. The high temporal resolution permits water content measurements in the transient states of wetting and drying, in addition to the steady state.     

NONCOOPERATIVE MANAGEMENT OF THE NORTHEAST ATLANTIC COD FISHERY: A FIRST MOVER ADVANTAGE

The point of departure for this analysis is Bjørndal and Lindroos [2012], who developed an empirical bioeconomic model to analyze cooperative and noncooperative management of Northeast Atlantic cod. In their analysis, only constant strategies were analyzed for noncooperative games. In this paper, nonconstant strategies are considered. Moreover, the fishery in question is characterized by cooperative management. What may happen in the real world is that one nation breaks the cooperative agreement by fishing in excess of its quota. Often, it takes time for the other agent to detect this and respond. In this paper, we allow this kind of delayed response into a two‐agent noncooperative game so that, if country 2 exceeds its quota, there will be a time lag before this is detected by country 1; moreover, there may also be a delay until country 1 is able to respond. Results show that the outcome critically depends on the length of these two lags as well as initial conditions.     

Centric scan SPRITE for spin density imaging of short relaxation time porous materials

The single-point ramped imaging with T1 enhancement (SPRITE) imaging technique has proven to be a very robust and flexible method for the study of a wide range of systems with short signal lifetimes. As a pure phase encoding technique, SPRITE is largely immune to image distortions generated by susceptibility variations, chemical shift and paramagnetic impurities. In addition, it avoids the line width restrictions on resolution common to time-based sampling, frequency encoding methods. The standard SPRITE technique is however a longitudinal steady-state imaging method; the image intensity is related to the longitudinal steady state, which not only decreases the signal-to-noise ratio, but also introduces many parameters into the image signal equation. A centric scan strategy for SPRITE removes the longitudinal steady state from the image intensity equation and increases the inherent image intensity. Two centric scan SPRITE methods, that is, Spiral-SPRITE and Conical-SPRITE, with fast acquisition and greatly reduced gradient duty cycle, are outlined. Multiple free induction decay (FID) points may be acquired during SPRITE sampling for signal averaging to increase signal-to-noise ratio or for T2* and spin density mapping without an increase in acquisition time. Experimental results show that most porous sedimentary rock and concrete samples have a single exponential T2* decay due to susceptibility difference-induced field distortion. Inhomogeneous broadening thus dominates, which suggests that spin density imaging can be easily obtained by SPRITE.     

The internal magnetic field distribution, and single exponential magnetic resonance free induction decay, in rocks

When fluid saturated porous media are subjected to an applied uniform magnetic field, an internal magnetic field, inside the pore space, is induced due to magnetic susceptibility differences between the pore-filling fluid and the solid matrix. The microscopic distribution of the internal magnetic field, and its gradients, was simulated based on the thin-section pore structure of a sedimentary rock. The simulation results were verified experimentally. We show that the 'decay due to diffusion in internal field' magnetic resonance technique may be applied to measure the pore size distribution in partially saturated porous media. For the first time, we have observed that the internal magnetic field and its gradients in porous rocks have a Lorentzian distribution, with an average gradient value of zero. The Lorentzian distribution of internal magnetic field arises from the large susceptibility contrast and an intrinsic disordered pore structure in these porous media. We confirm that the single exponential magnetic resonance free induction decay commonly observed in fluid saturated porous media arises from a Lorentzian internal field distribution. A linear relationship between the magnetic resonance linewidth, and the product of the susceptibility difference in the porous media and the applied magnetic field, is observed through simulation and experiment.     

Membrane gas diffusion measurements with MRI

Gas transport across polymeric membranes is fundamental to many filtering and separation technologies. To elucidate transport mechanisms, and understand the behaviors of membrane materials, accurate measurement of transport properties is required. We report a new magnetic resonance imaging (MRI) methodology to measure membrane gas phase diffusion coefficients. The MRI challenges of low spin density and short gas phase relaxation times, especially for hydrogen gas, have been successfully overcome with a modified one-dimensional, single-point ramped imaging with T(1) enhancement, measurement. We have measured the diffusion coefficients of both hydrogen gas and sulfur-hexafluoride in a model polymeric membrane of potential interest as a gas separator in metal hydride batteries. The experimental apparatus is a modified one-dimensional diaphragm cell which permits measurement of the diffusion coefficient in experimental times of less than 1 min. The H(2) gas diffusion coefficient in the membrane was 0.54 +/- 0.01 mm(2)/s, while that of sulfur-hexafluoride was 0.14 +/- 0.01 mm(2)/s, at ambient conditions.