Ph.D. Theses

his dissertation aims to fill the gap in experimental evidence by first investigatingthe influence of material properties and He plasma parameters on the He-W inter-action broadly and then isolating a parameter variations directly linked to the Heinteraction mechanisms of interest.For this, recrystallised polycrystalline tungsten (polycrystalline tungsten (poly-W)) samples, undamaged and irradiation-damaged to varying doses, were first usedin several exposure series screening the influence of He flux, fluence, energy, andpre-existing defects. To optimise the surface preparation for single-crystalline tung-sten (single-crystalline tungsten (sc-W)), open-volume defect concentrations in thenear-surface region of the samples were characterised by Doppler-broadening spec-troscopy of the positron annihilation line (Doppler-broadening spectroscopy of thepositron-electron annihilation line (DBS)). [...]

The research reactors FRM II and RHF support global non-proliferation efforts by converting their highly enriched uranium fuel elements to low enriched uranium, a process requiring comprehensive neutronic and thermal-hydraulic analyses. This work presents the development of the Python code TAILLEFER, a sampling-based software that enables the efficient integration of uncertainty analysis into steady-state thermal-hydraulic (SSTH) modeling. [...]

Modern electric motors require efficient flux guidance in NOES rotors, but conventional cut‑out flux barriers reduce mechanical stability. Residual‑stress‑based barriers, as currently investigated, offer an alternative. [...]

This thesis examines ternary UMoX alloys (X = Zr, Pt, Nb, Ti, W) as alternative research reactor fuel candidates. Microstructure, solubility, γ-phase stability and Al interdiffusion were analyzed under thermal and irradiation conditions. [...]

Positron annihilation lifetime spectroscopy (PALS) is a powerful tool for defect characterisation in a wide range of materials. To investigate inhomogeneous defect distributions, for example, near fatigue cracks or in dispersive alloys, with PALS, a monochromatic pulsed positron beam of variable energy with a diameter in the range of 1 µm and a pulse width of 200 ps FWHM is required. [...]

This thesis presents a comprehensive investigation of the structural and magnetic properties of biocompatible iron oxide nanoparticles coated with three different ligand materials: sodium citrate, (3-aminopropyl)triethoxysilane (APTES), and dextran. The influence of the coating agents on the agglomeration of iron oxide nanoparticles and their oxidation stability over time was studied. [...]

This work is dedicated to the detailed investigation of the atomic and magnetic structurein a selection of germanium-based melilite-type compounds, namely Ba2TGe2O7(T = Cu, Co and Mn) and the Ba2Cu1-xMnxGe2O7 solid solution series. These materialsgained high interest in the current condensed matter research as they revealed astrong magnetoelectric coupling that could not be explained by the conventional modelsof spin-driven multiferroicity. [...]

Neutron depth profile analysis is a method for the high-precision determination of concentration profiles of certain nuclides in near-surface layers. For an extension to 3D profiling, a detector system based on double-sided silicon strip detectors with a thin and homogeneous dead layer was developed and characterised. [...]