Polarization thermal

I. V. Koptyug, S.A. Altobelli, E. Fu-kushima, A.V. Matveev, R.Z. Sagdeev 2000, (Thermally polarized H NMR microimaging studies of liquid and gas flow in monolithic catalysts), J. Magn. Reson. 147, 36. 

S. D. Beyea, A. Caprihan, C. F. M. Clewett, S.J. Glass 2002, (Spatially resolved adsorption isotherms of thermally polarized perfluorinated gases in Y-TZP ceramic materials using NMR imaging), Appl. Magn. Reson. 22, 175-186. 

Gas-phase MR will undoubtedly find more widespread use in studies of catalysts and catalytic reactors initial studies have been done with thermally polarized gases. Clearly, it will be of interest to image gas flows in reactors in this application, the measurement strategies used to image gas and liquid flows will be similar. However, gas- and liquid-phase species diffusing within a porous catalyst will be influenced to differing extents by the physical and chemical characteristics of the catalyst. These... 

In the experiments illustrated above, a remarkable amount of information becomes available rather quickly and easily when flowing HP xenon gas is used as a probe material. Although some of the information could have been obtained with thermally polarized xenon, such experiments would have to be carried out on sealed, pressurized samples, but to obtain information on the temperature-programmable interlayer void space would be far more difficult if not impossible. 

NaY zeolite at 60.1 ppm. The image was obtained for a 3mm slice with full chemical shift imaging (note that for thermally polarised Xe this type of imaging experiment would be far more demanding in terms of experimental time even than chemical shift resolved imaging, as practiced for the Aerogel samples[30]), and was obtained in 30 min. Thus, the improvement in imaging with HP xenon over thermally polarized xenon is impressive, and indicates that there are real prospects for applications in the characterization of materials. 

DNP at very low magnetic fields is attractive for two reasons. First, the electron saturation frequency at low fields is in the radiofrequency range, where it is much easier to obtain high-power radiation sources, amplifiers and transmission equipment. In fact, the first experimental verifications of the Overhauser effect were conducted between 1 and 5 mT,2/85/86 likely due to the ease of constructing a suitable magnet and equipment to perform ESR saturation. The second reason for adding DNP to a low-field system is to help overcome the limited thermal polarization at low... 

More recent studies of gas-phase imaging with more direct relevance to chemical processing and reaction engineering have involved the examination of thermally polarized gas (and liquid) flow in monolithic catalysts. Koptyug et al. (2000a) have obtained quantitative, spatially resolved velocity maps for the flow of thermally polarized acetylene, propane, butane and water flowing through the channels of alumina monoliths with an in-plane spatial resolution of 400 pm. The monoliths had a channel cross-section of 4.0 mm2 and a wall channel... 

Fig. 9-10. Representative thermal polarized optical micrograph of the nematic schlieren texture displayed by 13 ( = 5) on cooling from the isotropic melt to 159 C.

Instead of polarized noble gases, thermally polarized NMR microimaging was used to study of liquid and gas flow in monolithic catalysts. Two-dimensional spatial maps of flow velocity distributions for acetylene, propane, and butane flowing along the transport channels of shaped monolithic alumina catalysts were obtained at 7 T by NMR, with true in-plane resolution of 400 xm and reasonable detection times. The flow maps reveal the highly nonuniform spatial distribution of shear rates within the monolith channels of square cross-section, the kind of information essential for evaluation and improvement of the efficiency of mass transfer in shaped catalysts. The water flow imaging, for comparison, demonstrates the transformation of a transient flow pattern observed closer to the inflow edge of a monolith into a fully developed one further downstream. 

Finally, ONP may be achieved with the aid of applied electromagnetic (i.e., radiofrequency, rf or microwave, mw) fields (c), and thus can most simply be considered as extensions of conventional DNP methods these approaches are referred to as radio-frequency induced ONP (RFONP) and microwave-induced ONP (MIONP), respectively. In conventional DNP, magnetization from thermally polarized electron spins (residing, for example, in permanent paramagnetic centers) is driven via hyperfine interactions to surrounding nuclei using resonant or near-resonant AC fields. More specifically, this process is governed by the conventional Overhauser effect in the case of dynamic hyperfine interactions (e.g., in metals or semiconductors),... 

Fig. 9-14. Representative thermal polarized optical micrograph of the nematic o...

Fig. 9-15. Representative thermal polarized optieal micrograph of the focal-conic texture displayed...

The technique of depolarized light intensity (DLI) microscopy was introduced by Magill (80) in 1960. Basic elements of the apparatus were a light source, polarizers, a sample holder, an analyzer, and a suitable recording system. Barrall and Johnson (74) and Miller (75, 76) have described applications of this technique to polymeric samples. Miller (75) prefers to call this technique thermal polarization analysis (TPA). 

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