ODNP enhanced NMR relaxometry and diffusometry: Hardware development and applications to fuel cell materials

In nuclear magnetic resonance (NMR) the demand for compact, low-cost instruments that can substitute expensive superconducting magnets is growing. Although compact NMR devices based on permanent magnets that can resolve 1H chemical shift differences are commercially available, the magnetic field strength of these devices is limited, which sets boundaries to the signal intensity and quality. Hyperpolarization techniques to boost the NMR signal beyond ist thermally given polarization are well known and applied on superconducting magnets. Thus, implementing these methods on compact NMR instruments will be the next step of development to gain an increased signal quality and widen the range of applications for those magnets.

One part of this thesis deals with the development of compact magnets for an X-Band Overhauser Dynamic Nuclear Polarization (ODNP) setup and the construction of the hyperpolarization hardware itself. Two concepts of small open-access magnets are presented, simulated, and experimentally tested that allow a direct shimming of the magnetic field by displacing magnet blocks without the need of further pole shoes or shim pieces. The superior design is transferred to a bigger magnet, which can fit EPR resonators and is the center piece of the ODNP hardware. For this hardware an ODNP amplification board is constructed and a commercially purchased EPR resonator is modified to include NMR coils. All components are adjusted to each other, the communication between them is established and the basic functionality of the hardware demonstrated. Additionally to the hardware construction, a fast field mapping method is introduced to characterize the detection volume of compact NMR devices. This method facilitates the characterization and hence construction of permanent NMR magnets and saves about one order of magnitude in measurement time compared to the established procedures.

Beside the construction of compact hyperpolarization setups, new applications areas for these techniques should be explored. In this context the idea of ODNP enhanced Laplace NMR experiments is explained and experimentally demonstrated by CPMG, inversion recovery, and PFG diffusion experiments on a model sample. Furthermore, these techniques are applied to study the influence of ODNP spin probes on the dynamic properties of Nafion membranes. Since ODNP relies on the presence of paramagnetic spin probes their influence on the material properties must be studied before any conclusion on material properties can be drawn from hyperpolarization experiments. A cornucopia of established and novel methods is applied to dissect Nafion properties in the presence of spin probes ranging from Small Angle X-ray Scattering (SAXS), Thermal Gravimetric Analysis (TGA), conductivity measurements, PFG NMR diffusometry, field cycling NMR relaxometry, ODNP relaxometry to Electron Paramagnetic Resonance (EPR), and new combinations thereof.