Wet and dry model granulates under mechanical load : a confocal microscopy study

Granular matter, also known as bulk solids, consists of discrete particles with sizes between micrometers and meters. They are present in many industrial applications as well as daily life, like in food processing, pharmaceutics or in the oil and mining industry. When handling granular matter the bulk solids are stored, mixed, conveyed or filtered. These techniques are based on observations in macroscopic experiments, i.e. rheological examinations of the bulk properties. Despite the amply investigations of bulk mechanics, the relation between single particle motion and macroscopic behavior is still not well understood. For exploring the microscopic properties on a single particle level, 3D imaging techniques are required.rnThe objective of this work was the investigation of single particle motions in a bulk system in 3D under an external mechanical load, i.e. compression and shear. During the mechanical load the structural and dynamical properties of these systems were examined with confocal microscopy. Therefor new granular model systems in the wet and dry state were designed and prepared. As the particles are solid bodies, their motion is described by six degrees of freedom. To explore their entire motion with all degrees of freedom, a technique to visualize the rotation of spherical micrometer sized particles in 3D was developed. rnOne of the foci during this dissertation was a model system for dry cohesive granular matter. In such systems the particle motion during a compression of the granular matter was investigated. In general the rotation of single particles was the more sensitive parameter compared to the translation. In regions with large structural changes the rotation had an earlier onset than the translation. In granular systems under shear, shear dilatation and shear zone formation were observed. Globally the granular sediments showed a shear behavior, which was known already from classical shear experiments, for example with Jenike cells. Locally the shear zone formation was enhanced, when near the applied load a pre-diluted region existed. In regions with constant volume fraction a mixing between the different particle layers occurred. In particular an exchange of particles between the current flowing region and the non-flowing region was observed. rnThe second focus was on model systems for wet granular matter, where an additional binding liquid is added to the particle suspension. To examine the 3D structure of the binding liquid on the micrometer scale independently from the particles, a second illumination and detection beam path was implemented. In shear and compression experiments of wet clusters and bulk systems completely different dynamics compared to dry cohesive models systems occured. In a Pickering emulsion-like system large structural changes predominantly occurred in the local environment of binding liquid droplets. These large local structural changes were due to an energy interplay between the energy stored in the binding droplet during its deformation and the binding energy of particles at the droplet interface. rnConfocal microscopy in combination with nanoindentation gave new insights into the single particle motions and dynamics of granular systems under a mechanical load. These novel experimental results can help to improve the understanding of the relationship between bulk properties of granular matter, such as volume fraction or yield stress and the dynamics on a single particle level.rnrn

Spurenanalyse von Aktiniden in der Umwelt mittels Resonanzionisations-Massenspektrometrie

In dieser Arbeit wurde die Methode der resonanten Ionisation von neutralen Atomen mittels Laserstrahlung auf die leichten Aktinide Thorium, Uran, Neptunium und Plutonium angewendet und für die Ultraspurenanalyse optimiert. Der empfindliche Nachweis dieser Aktinide stellt eine Herausforderung für die Beobachtung und Bestimmung von radioaktiven Verunreinigungen aus kerntechnischen Anlagen in der Umwelt dar. In einem für diese Untersuchungen entwickelten Quadrupolmassenspektrometer konnte durch Resonanzionisationsspektroskopie jeweils eine Reihe unbekannter Energiezustände in der Elektronenhülle des neutralen Atoms der oben genannten Aktinide identifiziert, sowie effiziente Anregungsschemata für die resonante Ionisation entwickelt und charakterisiert werden. Durch die verwendete in-source-Ionisation, die aufgrund der guten Überlagerung von Laserstrahlung und Atomstrahl eine hohe Nachweiseffizienz gewährleistet, konnten diese Untersuchungen bereits mit einem, für Radionuklide notwendigen, geringen Probeneintrag erfolgen. Die resonante Ionisation erlaubt durch die selektiven resonanten Prozesse eine Unterdrückung unerwünschter Kontaminationen und wurde auf den analytischen Nachweis von Ultraspurengehalten in Umweltproben, sowie die Bestimmung der entsprechenden Isotopenzusammensetzung optimiert. Durch die effiziente in-source-Ionisation mit leistungsstarker gepulster Laserstrahlung, konnten Nachweiseffizienzen im Bereich von bis zu 1% erreicht werden. Dabei wurden für Plutonium in synthetischen Proben, aber auch in ersten Umweltproben, Nachweisgrenzen von 10^4-10^5 Atomen erzielt. Die Verwendung spektral schmalbandiger Dauerstrichlaser und eine Ionisation transversal zum frei propagierenden Atomstrahl ermöglicht durch Auflösung der Isotopieverschiebung eine hohe Selektivität gegenüber dominanten Nachbarisotopen, wohingegen die Ionisationseffizienz deutlich abnimmt. Hiermit konnte für das Ultraspurenisotop U-236 eine Nachweisgrenze bis hinab zu 10^-9 für das Isotopenverhältnis N(U-236)/N(U-238) bestimmt werden.

Developments for transactinide chemistry experiments behind the gas-filled separator TASCA

Topic of this thesis is the development of experiments behind the gas-filled separator TASCA(TransActinide Separator and Chemistry Apparatus) to study the chemical properties of the transactinide elements.rnIn the first part of the thesis, the electrodepositions of short-lived isotopes of ruthenium and osmium on gold electrodes were studied as model experiments for hassium. From literature it is known that the deposition potential of single atoms differs significantly from the potential predicted by the Nernst equation. This shift of the potential depends on the adsorption enthalpy of therndeposited element on the electrode material. If the adsorption on the electrode-material is favoured over the adsorption on a surface made of the same element as the deposited atom, the electrode potential is shifted to higher potentials. This phenomenon is called underpotential deposition.rnPossibilities to automatize an electro chemistry experiment behind the gas-filled separator were explored for later studies with transactinide elements.rnThe second part of this thesis is about the in-situ synthesis of transition-metal-carbonyl complexes with nuclear reaction products. Fission products of uranium-235 and californium-249 were produced at the TRIGA Mainz reactor and thermalized in a carbon-monoxide containing atmosphere. The formed volatile metal-carbonyl complexes could be transported in a gas-stream.rnFurthermore, short-lived isotopes of tungsten, rhenium, osmium, and iridium were synthesised at the linear accelerator UNILAC at GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt. The recoiling fusion products were separated from the primary beam and the transfer products in the gas-filled separator TASCA. The fusion products were stopped in the focal plane of TASCA in a recoil transfer chamber. This chamber contained a carbon-monoxide – helium gas mixture. The formed metal-carbonyl complexes could be transported in a gas stream to various experimental setups. All synthesised carbonyl complexes were identified by nuclear decay spectroscopy. Some complexes were studied with isothermal chromatography or thermochromatography methods. The chromatograms were compared with Monte Carlo Simulations to determine the adsorption enthalpyrnon silicon dioxide and on gold. These simulations based on existing codes, that were modified for the different geometries of the chromatography channels. All observed adsorption enthalpies (on silcon oxide as well as on gold) are typical for physisorption. Additionally, the thermalstability of some of the carbonyl complexes was studied. This showed that at temperatures above 200 °C therncomplexes start to decompose.rnIt was demonstrated that carbonyl-complex chemistry is a suitable method to study rutherfordium, dubnium, seaborgium, bohrium, hassium, and meitnerium. Until now, only very simple, thermally stable compounds have been synthesized in the gas-phase chemistry of the transactindes. With the synthesis of transactinide-carbonyl complexes a new compound class would be discovered. Transactinide chemistry would reach the border between inorganic and metallorganic chemistry.rnFurthermore, the in-situ synthesised carbonyl complexes would allow nuclear spectroscopy studies under low background conditions making use of chemically prepared samples.

Massenspektrometrische Untersuchungen des Komplexierungsverhaltens von Actinid-Ionen in Lösung

Da die Langzeit-Radiotoxizität von abgebrannten Kernbrennstoffen von Plutonium und den minoren Actiniden dominiert wird, sind diese Elemente im Fokus der Untersuchungen bezüglich der Entsorgung der radioaktiven Abfälle.rnUm ein besseres Verständnis der Selektivität der Partitioning-Liganden BTP und BTBP bezüglich der Extraktion von trivalenten Actiniden zu erlangen, wurden die Komplexe, die diese mit Lanthaniden in octanolischer Lösung bilden charakterisiert. Das unterschiedliche Extraktionsverhalten der Lanthaniden untereinander konnte dabei auf unterschiedliche Präferenz zur Bildung von Ln(BTP)3-Komplexen abhängig vom Ionenradius der Lanthaniden zurückgeführt werden. Darüber hinaus konnte gezeigt werden, dass abhängig vom sterischen Anspruch der BTBP-Liganden in Eu(BTBP)2-Komplexen Nitratliganden in der ersten Koordinationssphäre gebunden werden. rnDa das Verhalten von Plutonium unter geochemischen Bedingungen von besonderem Interesse für die Risikoabschätzung von nuklearen Endlagern ist, widmet sich der zweite Teil der Arbeit dem Hydrolyse- und Kolloidbildungsverhalten von wässrigen Plutoniumlösungen in den Oxidationsstufen IV bis VI. Daher wurden die Lösungsspezies von sowohl Zirconium(IV) als Analogon für Plutonium(IV), als auch die von Uran(VI) und Plutonium(VI) direkt mittels massenspektrometrischer Methoden charakterisiert und quantifiziert. Darüber hinaus wurde die kinetische Hemmung der Reduktion von Pu(V) zu Pu(IV) und nachfolgender Kolloidbildung untersucht, welche sich durch oberflächeninduzierte Reduktion an kolloidalen Kristallisationskeimen deutlich beschleunigen lässt.rn