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  • Quantum fluctuations and lineshape anomaly in a high- silver-coated InP-based metallic nanolaser

    Metallic nanocavity lasers provide important technological advancement toward even smaller integrable light sources. They give access to widely unexplored lasing physics in which the distinction between different operational regimes, like those of thermal or coherent light emission, becomes increasingly challenging upon approaching a device with a near-perfect spontaneous-emission coupling factor β. In fact, quantum-optical studies have to be employed to reveal a transition to coherent emission in the intensity fluctuation behavior of nanolasers when the input–output characteristic appears thresholdless for β=1 nanolasers. Here, a new indicator for lasing operation in high-β lasers is identified by showing that stimulated emission can give rise to a lineshape anomaly manifested as a transition from a Lorentzian to a Gaussian component in the emission linewidth that dominates the spectrum above the lasing threshold.
  • Characterization and Multiscale Modeling of the Mechanical Properties for FDM-Printed Copper-Reinforced PLA Composites

    Additive manufacturing is an emerging technology and provides high design flexibility to customers. Fused deposition modeling (FDM) is an economical and promising additive manufacturing method. Due to its many advantages, FDM received great attention in recent years, and comprehensive studies are being undertaken to investigate the properties of FDM-printed polymers and polymer composites. As a result of the manufacturing technology employed in FDM, inner structures are changed with different process parameters, and thus, anisotropic properties are observed. Moreover, composite filaments such as particle- or fiber-reinforced polymers already have anisotropy before FDM printing. In this study, we investigate the effect of different process parameters, namely layer thickness and raster width on FDM-printed copper-reinforced poly(lactic acid) (PLA). Mechanical characterizations with a high-resolution camera are carried out for analyzing the deformation behaviors. Optical microscopy characterizations are performed to observe the mesostructural changes with various process parameters. Scanning electron microscopy (SEM) and an energy-dispersive X-ray spectroscopy (EDS) analysis are conducted for investigating the microstructure, specifically, copper particles in the PLA matrix. A 2D digital image correlation code with a machine learning algorithm is applied to the optical characterization and SEM-EDS images. In this way, micro- and mesostructural features, as well as the porosity ratios of the specimens are investigated. We prepare the multiscale homogenization by finite element method (FEM) simulations to capture the material’s response, both on a microscale and a mesoscale. We determined that the mesostructure and, thereby, the mechanical properties are significantly changed with the aforementioned process parameters. A lower layer thickness and a greater raster width led to a higher elasticity modulus and ultimate tensile strength (UTS). The optical microscopy analysis verified this statement: Decreasing the layer thickness and increasing the raster width result in larger contact lines between adjacent layers and, hence, lower porosity on the mesoscale. Realistic CAD images were prepared regarding the mesostructural differences and porosity ratios. Ultimately, all these changes are accurately modeled with mesoscale and multiscale simulations. The simulation results are validated by laboratory experiments.
  • Digital Reset. Redirecting Technologies for the Deep Sustainability Transformation

    Time seems out of joint. The world society has experienced a centennial pandemic, the global thermometer has displayed a sequence of hottest years on record, and Russia’s war on Ukraine has shattered political order. Unsurprisingly, the economy is severely affected. Governments worldwide hope that digital technologies can provide key solutions. Yet this report shows that digitalisation, in its current and mainstream form, is rather aggravating than solving many of the pressing social and environmental crises at hand. What is needed instead is a deep sustainability transformation that fundamentally reorganises the economy and all its sectors – agriculture, mobility, energy, buildings, industry, and consumption. The report “Digital Reset” shows how digital technologies can support the quest for such a deep sustainability transformation. The report provides a blueprint for the European Union on how to reconceptualise digitalisation so that it first and foremost contributes to achieving carbon neutrality, resource autonomy and economic resilience while supporting equity and fully respecting citizen’s rights and privacy. The report is the outcome of the two-year international science-policy dialogue “Digitalization for Sustainability” (D4S), and presents an up-to-date comprehensive analysis of opportunities, risks and governance options regarding digitalization and sustainability.
  • Extrusion-based ceramic additive manufacturing with robocasting: numerical study of printability and particle orientation

    The emergence of additive manufacturing (AM) techniques enabled a new class of lightweight engineering parts with advanced functionalities. WhileAMis already established in industry for metal and polymer applications, AM of ceramics is still in its infancy. Functional ceramics, however, provide material properties that allow engineering applications in which metal and polymer materials fail. An emerging AM technique consist of various compositions; in this work, a particular paste is applied that consists of prolate platelet-like and sphere-like particles. The paste composition affects the rheology - a crucial printing parameter. A challenge with respect to RC is to consider the complex interaction between paste composition, paste rheology, filfor ceramics is robocasting (RC) – a technology to extrude continuous rod-like filaments to create layer-wise structures from a ceramic paste. This paste canament shape and settling behavior after printing, as well as orientation of the particles within the filament. This thesis addresses this interaction with the help of numerical simulations making use of two meshless Lagrangian particle methods, smoothed particle hydrodynamics (SPH) and discrete element method (DEM). SPH serves as a fluid solver to model the paste behavior and DEM models the dynamics of the suspended particles. There is one limitation in this approach with respect to the high number of suspended particles in the filament as a full consideration all of them easily exceeds the capabilities of today’s computational resources. For this reason, a multi-scale approach is applied: On a particle level, where particles are fully resolved, the influence of the paste composition onto the rheology is studied and the particle orientation after printing is predicted; on a macroscopic scale, where particles are unresolved, the rheology extracted from the microscopic scale is used to model the filament extrusion during printing. Numerical results are compared to experiments and literature where applicable. RC is successfully modeled on both scales. The microscopic simulations revealed the dominance of the platelet particles onto the rheology. The addition of platelet particles allows to make the filaments strong enough to carry multiple layers but the filling fraction should not exceed 30 vol% to allowtechnical printability. Identified as a rather free parameter is the sphere phase that is currently used to provide a filling fraction large enough to prevent filament shrinkage during drying. As the composition of both particle types affect the rheology, its influence onto the settling behavior is studied and summarized within a process map. From this map, the rheology can be extract that is required to tailor the shape of the final part ranging from filigree to dense bulk. The rheology does not only affect the final part geometry, but also the velocity profile in the paste during extrusion. Both affect the material properties; the former obviously, the latter as it affects the particle orientation within the filament. Usually, anisotropic particle orientation states are favored as they allow well-defined material properties. For this purpose, this thesis addresses applicable means to affect the particle orientation. Here, the most important parameter is identified to be the amount of deformation that has been found insufficiently small in the nozzle center to reach anisotropic orientation states. Often, particle orientation is predicted with analytical orientation prediction models.Many of these models exist but none of these is designed to be applied to multicomponent pastes. Here, the model by Folgar and Tucker (FT) is found to reliably predict the orientation within a wide variation of paste compositions. An efficient pathway for process optimization is suggested. First, using the process map as guidance, a paste composition should experimentally be identified that is suitable to yield the desired geometrical shape. This paste should then be characterized with respect to its rheology profile, based on which numerical simulations should be applied to extract the deformation acting on the particles. Then, process optimization can be applied to iteratively design a nozzle geometry that increases the deformation where found insufficiently small. If desired, this process optimization can be done in combination with the FT model to get a visualization of the particle orientation along the specimen.
  • Nukleophile Kupferhydride aus Diwasserstoff und deren Anwendungen in katalytischen 1,4- und 1,2-Reduktionen

    Die vorliegende Dissertation behandelt die kupferkatalysierte Erzeugung von nukleophilen Hydriden aus Diwasserstoff (H2), um stöchiometrisch eingesetzte abfallproduzierende Metallhydride durch einen Katalysator zu ersetzen und so eine atomökonomische Alternative zu aktuell verwendeten Verfahren zu bieten. Der erste Teil dieser Arbeit beschäftigt sich mit der Entwicklung einer kupferkatalysierten konjugierten Reduktion von α,β-ungesättigten Estern und Amiden mit H2 als Hydridquelle. Durch den Einsatz kommerziell erhältlicher Kupfer(I)/NHC-Komplexe in Verbindung mit einer sterisch anspruchsvollen Alkoxidbase konnte eine Vielzahl von α,β-ungesättigten Estern bei 10 bar H2-Druck chemoselektiv zu den entsprechenden gesättigten Estern umgesetzt werden. Es konnte gezeigt werden, dass eine asymmetrische Variante der konjugierten Reduktion unter Verwendung eines chiralen NHC-Liganden möglich ist. Im zweiten Teil dieser Arbeit konnte erstmals eine homogenkatalytische Transferhydrierung über Kupferhydridintermediate basierend auf Alkoholen als H2-Äquivalenten gezeigt werden. Im letzten Teil dieser Arbeit wurde die erste kupferkatalysierte 1,2-Reduktion von Estern mit H2 entwickelt. Der Einsatz eines bifunktionellen Katalysators, basierend auf einem Kupfer(I)/NHC-Komplex und einem Guanidin-Organokatalysator in der Seitenkette, ermöglichte die Reaktion von „harten“ Ester-Elektrophilen mit „weichen“, bisher für diese Elektrophile nicht ausreichend nukleophilen Kupfer(I)hydriden. Viele unterschiedliche Ester konnten erfolgreich in der Katalyse umgesetzt werden. Die chemoselektive 1,2-Reduktion von Estern in Anwesenheit von Alkenen zeigt die deutliche Abgrenzung zu vorherigen Hydrierkatalysatoren und die Bildung nukleophiler Hydride aus H2. Erste Beispiele zur Reduktion von Estern zu Aldehyden, sowie zur Reduktion von Amiden zu Alkoholen konnten präsentiert werden.