Zeitschriftenaufsätze

Understanding the connection of molecular structure and optical properties of rare earth doped luminescent materials is essential for fabrication of state-of-the-art active laser media. On the other hand, rare earth ions can be used as a probe ion for the molecular structure of the host material if the structure-property correlations are known. Therefore, this work combines molecular dynamics simulations, Judd-Ofelt theory and UV-vis-NIR absorption spectroscopy including the behavior of the structure-sensitive hypersensitive absorption transitions of Er3+ to expand the knowledge on the local molecular structure in the immediate vicinity of the doped rare earth ions in dependence of glass composition. For this purpose, glasses of the compositions (35-x) BaO &hahog; x MgO &hahog; 10 Al2O3 &hahog; 55 SiO2 (mol%) (x = 0, 7.5, 15, 25, 35) and (20-x) BaO &hahog; x MgO &hahog; 20 Al2O3 &hahog; 60 SiO2 (mol%) (x = 0, 10, 20), doped with 2 × 10^20 ions/cm^3 Er3+ were prepared and analyzed. Clear differences in the absorption spectra between glasses of different BaO/MgO ratios, i.e. different network modifier field strengths, and different network modifier oxide to Al2O3 ratios are found and discussed in detail. Glasses with high BaO concentrations and high network modifier oxide to Al2O3 ratios provide lower rare earth coordination numbers with oxygen in general but higher coordination probabilities with non-bridging oxygen, which results in notably increased splitting of the optical transitions of the doped rare earth ions and higher hypersensitivity / lower local site symmetry for the doped rare earth ions in the investigated compositions. Based on our results and results from other publications the local rare earth site symmetry in glasses can in general be correlated with the rare earth coordination number.

Ultrasonic metal welding (USMW) is a highly attractive joining technology due to high energy efficiency and solid-state joint formation. Various joining solutions for conductor materials can be realized with USMW. Still, a big challenge for complex industrial applications is an adequate process monitoring that allows to cope with inevitable and complex process fluctuations. In this work, the suitability of contactless temperature measurements for process monitoring of copper sheet welding is examined and compared with the suitability of vibration measurements by means of machine learning methods. Different sensor signals acquired during welding on a metrological test rig are used for predicting the tensile shear strength of the joint. Results show that quality predictions based on temperatures exceed the state-of-the-art monitoring based on the welding energy. Yet, solely temperature-based predictions are exceeded by quality predictions based on either welding machine signals or tool vibration measurements. To further explore temperature-based quality analysis, joint microstructure analyses are carried out. These reveal concurring joint formation mechanisms associated with the mechanical and thermal process domains. To finally cover both domains in quality prediction, a sensor-fusion-based regression model is set up relying on vibration and temperature measurements. This fusion model exceeds all previously considered regression models with a mean absolute percentage error of 7.4% on the test data set. These results stress the importance of both process domains and suggest the combination of temperature and vibration measurements as a good starting point for future industrial monitoring of USMW processes.

The basic idea of cryogenic machining of elastomers is to lower the process temperature under glass transition temperature, causing the transformation of the viscoelastic properties of elastomers into brittle with better machinability outcomes. However, because of the heat generated by plastic deformation and chip formation in the primary shear zone and friction between the tool and the workspace, even with cryogenic cooling, the resulting temperature in the cutting area is often higher than the glass transition temperature. As a result, it can cause a partially rubbery state of the workpiece. In this paper, the effect of cryogenic cooling on the milling of acrylonitrile-butadiene rubber was analysed. Three different cooling setups, namely, indirect, direct and flow cooling, were proposed and their influence on temperature distribution in the cutting zone was studied in conjunction with different tool geometries and parameters (depth of cut, rotational speed, and feed rate). Direct cooling provides the best-resulting temperature distribution with the lowest surface roughness (1.27 to 1.47 μm) as it acts as a lubricant between the tool and workpiece and cools the tool and workpiece simultaneously.On the other hand, increasing the depth of cut and rotational speed also increases surface roughness. The best results show samples with grooves obtained with a rotational speed of 5000 rpm, depth of cut 0.25 mm and feed rates between 75 and 300 mm/min with surface roughness between 0.86 and 1.29 μm. Those samples show clean grooves with sharp edges, minimal surface roughness and geometric deviation, with defined ductile chip formation.

Nanosponges are subject of intensive research due to their unique morphology, which leads among other effects to electrodynamic field localization generating a strongly nonlinear optical response at hot spots and thus enable a variety of applications. Accurate predictions of physical properties require detailed knowledge of the sponges’ chaotic nanometer-sized structure, posing a metrological challenge. A major goal is to obtain computer models with equivalent structural and optical properties. Here, to understand the sponges’ morphology, we present a procedure for their accurate 3D reconstruction using focused ion beam tomography. Additionally, we introduce a simulation method to create nanoporous sponge models with adjustable geometric properties. It is shown that if certain morphological parameters are similar for computer-generated and experimental sponges, their optical response, including magnitudes and hot spot locations, are also similar. Finally, we analyze the anisotropy of experimental sponges and present an easy-to-use method to reproduce arbitrary anisotropies in computer-generated sponges.

The exchange interaction of a brominated Co-porphyrin molecule with the Cooper pair condensate of Pb(111) is modified by reducing the Co-surface separation. The stepwise dehalogenation and dephenylation change the Co adsorption height by a few picometers. Only the residual Co-porphine core exhibits a Yu-Shiba-Rusinov bound state with low binding energy in the Bardeen-Cooper-Schrieffer energy gap. Accompanying density functional calculations reveal that the Co dz2 orbital carries the molecular magnetic moment and is responsible for the intragap state. The calculated spatial evolution of the Yu-Shiba-Rusinov wave function is compatible with the experimentally observed oscillatory attenuation of the electron-hole asymmetry with increasing lateral distance from the magnetic porphine center.