Chapter 8: Beyond the Limits of Conventional Pulsed Gradient Spin Echo (PGSE) Diffusometry: Generalization of the Magnetization-grating Principle Check Access

Standard PGSE techniques have proven to be powerful and popular tools for diffusion studies in materials. However, there are a number of challenges that might arise if the investigation is to refer to media of a more complex nature. Examples are heterogeneous, micro-textured, porous, granular, composite or high-viscosity materials. In these cases, there will be tendencies to anomalous diffusion, superimposed components, extremely small displacement rates, and displacement restrictions. Artifacts in the experiments or in interpretations can moreover be due to eddy currents, magneto-acoustic ringing, short-T₂-limitations, and internal field gradients. In this chapter, alternative NMR techniques based on (a) strong, stationary field gradients in the fringe field of magnets, (b) rotating-frame imaging with the aid of gradients of the RF field amplitude, and (c) multiple stimulated echoes arising in modulated demagnetizing fields will be described. The common principle of all these variants is to generate a so-called grating of the z magnetization, which then tends to be leveled by translational displacements of the spin-bearing nuclei. It will be shown how such techniques can help to solve or avoid the before mentioned problems. Finally, it will be proposed to combine the diverse diffusometry techniques to cover an unsurpassed time scale ranging from microseconds to seconds with options to extend the scale even down to nanoseconds and up to hours or days by additionally referring to NMR relaxometry and microimaging methods, respectively.