Nikolaus Nestle

Spin transport in GaAs

We present a spectroscopic method for studying spin transport in semiconductors. Our time-resolved experiments have an important implication for spin electronics as they show that spin-polarized electron drift is possible in semiconductors over typical device lengths in high electric fields. We demonstrate an almost complete conservation of the orientation of the electron spin during transport in GaAs over a distance as long as 4 μm and fields up to 6 kV/cm.

Heavy metal uptake of alginate gels studied by NMR microscopy

Abstract In recent years, heavy metal uptake by biopolymer gels such as calcium alginate or chitosan has been studied by various methods. The main motivation for these studies was the possible application of such materials in environmental technology. Most of the work done on absorption phenomena in such materials was concerned with equilibrium concentrations of different heavy metal ions and their dependence on parameters such as temperature, pH and concentrations of competing ions. A few of these studies also referred to kinetic aspects. However, no experiments on the spatial distribution of heavy metals during the absorption process are known to us. Therefore, we have introduced NMR microscopy into this field. By this method, both spatially- and temporally-resolved data of the absorption process can be measured. As for the possible image contrasts, there are mainly two mechanisms: changes in relaxation behaviour of water protons in the presence of heavy metal ions; and changes in magnetic susceptibility upon loading with paramagnetic ions. In this contribution, results of both imaging methods are presented. From these data, it is obvious that the ion intrusion does not follow the ordinary diffusion equation. Instead, there are several different forms of ion intrusion behaviour. In the case of rare earth ions, the shrinking core model is found to give a satisfying description of the experiments.

NMR imaging of heavy metal absorption in alginate, immobilized cells, and kombu algal biosorbents

In this contribution, an NMR imaging study of heavy metal absorption in alginate, immobilized‐cell biosorbents, and kombu (Laminaria japonica) algal biomass is presented. This method provides the good possibility of directly monitoring the time evolution of the spatial distribution of the ions in the materials. From these results, we demonstrate that rare earth ions are absorbed with a steep reaction front that can be described very well with a modified shrinking core model, while copper ions are absorbed with a more diffuse front.

Nuclear magnetic resonance study of diffusion and relaxation in hydrating white cement pastes of different water content

While the nuclear spin relaxation time changes in hydrating cement materials have been widely studied by various groups during the last 20 years, data on the self-diffusion behavior of the pore water during hydration of a cement paste are much scarcer. Taking advantage of improved spectrometer hardware for pulsed field gradient diffusometry and a specialized pulse sequence which is designed to compensate the detrimental effects of inner magnetic field gradients in the sample we have studied the water self-diffusion behavior in pastes prepared from white cement at various water/cement ratios. For the same mixtures, studies of the transverse spin relaxation behavior were also conducted. A comparison of the results from both techniques shows that the diffusion coefficient starts to decrease only much later than the relaxation times for all pastes studied.

Oxygen determination in oxygen-supersaturated drinking waters by NMR relaxometry

In recent years, a rising number of different table waters supersaturated with oxygen have hit the market with claims of both positive health effects and an increase in athletic performance. In addition to research on the correctness of these claims and their possible physiological reasons, the appearance of oxygen supersaturation as a marketing promise also creates a need for appropriate analytical techniques allowing a rapid and reliable determination of oxygen contents in such waters. Here, we present NMR relaxometry as a possible analytical tool for such studies. NMR relaxation in oxygen supersaturated water is not only of interest in the context of monitoring the oxygen content in such drinking water products, but also might offer some interesting possibilities in medical and environmental science MRI applications. As a contact-less measuring method, NMR relaxometry avoids disturbance of the measurement due to outgassing. The method was applied in the concentration range from 10 mg l(-1) to over 100 mg l(-1) dissolved oxygen. In addition to freshly sampled drinking waters, also oxygen losses during storage of the water in open drinking vials was studied.

Transient high concentrations of chain anions in hydrating cement - indications from proton spin relaxation measurements

Studies of the dynamics of liquid water in the pores of hydrating cement materials were performed by means of various nuclear magnetic resonance techniques, such as spin-echo T2 relaxometry, echo-detected saturation-recovery T1 relaxometry and pulsed field gradient diffusometry. While the diffusion coefficients and the transverse relaxation times were found to decrease monotonically with hydration time, the longitudinal relaxation time exhibits a transient minimum during the onset of the acceleration phase. Earlier explanations of the minimum can be ruled out on the basis of our experimental data and a new one is suggested. The findings are corroborated by observations in cement pastes with added fine particles.

NMR microscopy of heavy metal absorption in calcium alginate beads

In recent years, heavy metal uptake by biopolymer gels, such as Cal-alginate or chitosan, has been studied by various methods. This is of interest because such materials might be an alternative to synthetical ion-exchange resins in the treatment of industrial waste waters. Most of the work done in this field consisted of studies of equilibirum absorption of different heavy metal ions with dependence on various experimental parameters. In some publications, the kinetics of absorption were studied, too. However, no experiments on the spatial distribution of heavy metals during the absorption process are known to us. Using Cu as an example, it is demonstrated in this article that NMR microscopy is an appropriate tool for such studies. By the method presented here, it is possible to monitor the spatial distribution of heavy metal ions with a time resolution of about 5 min and a spatial resolution of 100 µm or even better.

Robust spatially resolved pressure measurements using MRI with novel buoyant advection-free preparations of stable microbubbles in polysaccharide gels

MRI of fluids containing lipid coated microbubbles has been shown to be an effective tool for measuring the local fluid pressure. However, the intrinsically buoyant nature of these microbubbles precludes lengthy measurements due to their vertical migration under gravity and pressure-induced coalescence. A novel preparation is presented which is shown to minimize both these effects for at least 25 min. By using a 2% polysaccharide gel base with a small concentration of glycerol and 1,2-distearoyl-sn-glycero-3-phosphocholine coated gas microbubbles, MR measurements are made for pressures between 0.95 and 1.44 bar. The signal drifts due to migration and amalgamation are shown to be minimized for such an experiment whilst yielding very high NMR sensitivities up to 38% signal change per bar.

Blastfurnace slag cements: A construction material with very unusual nuclear spin relaxation behavior during hardening

In this contribution, nuclear magnetic resonance ~NMR! relaxometry results on hardening hydraulically activated blastfurnace slag pastes are presented. The results are found to be very different from the behavior of other hydraulic building materials such as cement and gypsum. A mechanism based on water diffusion in inner field gradients is suggested in order to explain the observed relaxation behavior and corroborated by additional experimental evidence from magnetic susceptibility measurements and NMR relaxation measurements on slag pastes prepared from nonwatery fluids.

PFG NMR and internal magnetic field gradients in plant-based materials

In this contribution, it is demonstrated that inner magnetic field gradients can seriously affect the results of stimulated echo PFG NMR experiments on plant-based materials even if there is no notable content of paramagnetic substances. Such effects could be observed both in experiments on water in pharmaceutical grade cellulose powder materials and on eggplant fruit tissue. In both cases, it was observed that the effects of internal magnetic field gradients led to different relative values of the diffusion coefficient compared to values obtained with a gradient-compensating pulse sequence.