Advances in low-field NMR relaxometry

• Fortschritte in der Niederfeld-NMR Relaxometrie

Jaschtschuk, Denis; Blümich, Bernhard (Thesis advisor); Adams, Alina Reghina (Thesis advisor)

1. Auflage. - Aachen : Verlagshaus Mainz (2019)
Book, Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2019

Abstract

In the context of this work, three unilateral NMR sensors were designed, constructed and characterized. For the new sensors, the concept was chosen to vary the angle of the polarization of the magnets. This affects the size and the position of the sensitive slice. It increases the penetration depth, which is a crucial parameter for investigations with unilateral NMR. The special magnet arrangement of both sensors results in a reduction of the magnetic field and gradient strength compared to prior state of the art sensors. Consequently, the weaker diffusive attenuation leads to a decreased diffusion weighting. Furthermore, in combination with the extended lateral size of the sensitive area, it leads to an increased SNR, compared to the PM5 and Mini-MOUSE. The third sensor is the Hexarray. It is a compact unilateral sensor with two sensitive slices. It operates at 4.9 MHz at 4 mm above the sensor surface. If one coil is operated while the other is in the waiting period and vice versa it is possible to perform measurements quasi-simultaneously without any mutual interactions. If both spots measure a similar sample, this setup can be used to increase the SNR by adding up both decays. Studies of a summer, winter and sport tire have illustrated well, how spin density contrast and especially relaxation time distributions can access valuable information non-invasively. It was possible to identify the multilayered structures and furthermore, deviations between the different tires. A possible assignment of the tread cap, tread base, cap ply and potential interfaces to the observed layers was suggested. A more detailed insight in the sophisticated nature of the elastomer composite was gained by inverse Laplace transformation. Relaxation time distribution profiles revealed a diversity in the number and molecular mobility of components used for the fabrication of tires. Depth profiles of ancient frescoes of varying quality and conservation state, excavated in Ostia Antica, were presented. For the first time NMR has proven to be able to identify covered fresco layers by variations in amplitude and relaxation time. Furthermore, it was shown that wall painting located close to each other show similar characteristics in the NMR profiles. Furthermore, the affect of restoration on the information accessible via NMR was shown. These results confirm the fingerprinting capabilities of NMR depth profiles of antique frescoes. It has been demonstrated that mobile, unilateral NMR is a valuable tool for non-invasive studies of ancient wall paintings. Relaxation measurements with a compact Halbach NMR sensor were used to analyze the adsorption and desorption of CH4 in CdIF-13, a novel MOF, under various pressures and temperatures. Measurements between 0 bar and 150 bar at 0 °C and 20 °C revealed three main peaks in the relaxation spectrum. According to the transverse relaxation time, integral and FWHM of the peaks it was possible to assign them to the MOF powder, free CH4, and intra- and inter-pore CH4. Furthermore, it was possible to link the hysteresis behaviour observable for such a MOF with changes in the T2 values. Further measurements are necessary to clearly verify the presence of intra- and inter-pore CH4. Measurements with other MOFs, exhibiting a different CH4 uptake behaviour, might help identifying different trends observable via NMR relaxometry.