Dieter Richter

Grazing incidence neutron spin echo spectroscopy: instrumentation aspects and scientific opportunities

Grazing Incidence Neutron Spin Echo Spectroscopy (GINSES) opens new possibilities for observing the thermally driven dynamics of macromolecules close to a rigid interface. The information about the dynamics can be retrieved as a function of scattering depth of the evanescent neutron wave, on the length scale in the range of some 10-100 nm. Using a classical neutron spin echo spectrometer with a laterally collimated beam, dynamics can be measured in grazing incidence geometry. We show examples of how the interface modifies the dynamics of microemulsions, membranes and microgels. Instrumental details and possible improvements for this technique will be presented. The key issue is the low intensity for dynamics measurements with an evanescent neutron wave. Conceptual questions how a specialised instrument could improve the experimental technique will be discussed.

Tailored Polymer Additives for Wax (Paraffin) Crystal Control

Depending on the type of crude deposits and refined technology applied, this fraction can vary between 10 and 30% (Coutinho et al., 2000). Although they are energetically desirable because of their increased combustion enthalpy with respect to C5-C17 alkanes, the long chain C18-C40 n-paraffins (waxes) are technically embarrassing when (1) their concentration is too high, (2) crude oils are extracted from deep sea reservoirs and pipeline transported through cold regions or (3) diesel fuels are used during the winter time (Kern & Dassonville, 1992). In these conditions, such fluids undergo dramatic degradation of viscoelastic properties due to precipitation of waxes as a consequence of the temperature drop and reduction of their solubility.

10. Hydrogen in Metals

Publisher Summary This chapter discusses the investigation of the diffusion process. Quasi-elastic scattering has provided a good background for the qualitative understanding of the elementary diffusive step, at least for the simple bcc and fcc hydrides. On the other hand, there are no theoretical concepts for calculating, for instance, the characteristic residence times or the host-lattice relaxation times, connected with the diffusional jump, on a first-principle basis, because the corresponding many-particle problem of the lattice is very difficult to solve. It explains that diffusion in metals with dilute impurities is approximately understood on a semi-empirical basis, and also a certain understanding exists of the interesting phenomena, such as trapping modes and tunnelling states of hydrogen around traps. As a by-product of the research on diffusion, it has turned out that quasi-elastic scattering is a powerful tool for determining self-diffusion constants, in particular for metal alloy hydrides and storage materials, where other methods are either difficult to apply or to fail. The chapter also discusses that hydrogen vibrations were also used as a local probe in order to gain information on the hydrogen site symmetry in hydrogen impurity clusters and in amorphous metal hydrides. However, there exist no ab initio interpretations of these potentials in terms of the electronic properties of the hydrides. In general, there is a great variety of experimental data from neutron scattering on hydrides, but deeper theoretical interpretation is still required.