Publikationen

During operation of a lithium ion cell electrode storage particles experience an inhomogeneous volume change due to local differences in the internal lithium concentration. The resulting mechanical stress can become large enough to provoke particle fracture, an aging mechanism considered to have a severe detrimental impact on the life time of lithium ion cells. In this work, we use a coupled model of mechanical stress, lithium diffusion and crack growth to study the problem of fracture in storage particles. The model was successfully applied to study crack growth during a single half cycle of lithium insertion or extraction in earlier investigations. It was demonstrated that, under specific circumstances, particle breakage may occur in a single half cycle. Here, we consider the second half cycle and examine under which conditions cracks that either remained stable or underwent growth in the first half cycle, can lead to particle fragmentation during the subsequent half cycle. From both numerical results and supportive analytic solutions, we find that growth of a through crack during Li insertion is a strong indicator for particle breakage, either in one or two half cycles. Such a relationship is not found for growth of a surface crack during Li extraction.

Fracture of storage particles is considered to be one of the major reasons for capacity fade and increasing power loss in many commercial lithium ion batteries. The appearance of fracture and cracks in the particles is commonly ascribed to mechanical stress, which evolves from inhomogeneous swelling and shrinkage of the material when lithium is inserted or extracted. Here, a coupled model of lithium diffusion, mechanical stress and crack growth using a phase field method is applied to investigate how the formation of cracks depends on the size of the particle and the presence or absence of an initial crack, as well as the applied flux at the boundary. The model shows great versatility in that it is free of constraints with respect to particle geometry, dimension or crack path and allows simultaneous observation of the evolution of lithium diffusion and crack growth. In this work, we focus on the insertion process. In particular, we demonstrate the presence of intricate fracture phenomena, such as, crack branching or complete breakage of storage particles within just a single half cycle of lithium insertion, a phenomenon that was only speculated about before.

Storage particles of lithium ion batteries undergo significant mechanical stress during charging and discharging due to the inhomogeneous volume change within the particles when lithium is inserted and extracted. This stress potentially leads to fracture of the particles resulting in detrimental effects for the capacity and internal resistance of a lithium ion battery, such as the growth of additional solid electrolyte interface, loss of contact in conductive pathways or complete disintegration of the electrode. Here, we tackle the problem of fracture in storage particles by merging a coupled model of mechanical stress and diffusion of lithium ions with a phase field description of an evolving crack. This approach allows the simultaneous study of the evolution of the lithium concentration together with the growth of a crack without restrictions to specific geometries, simulation dimensions and presuppositions regarding the crack path. The model was successfully applied to study crack growth during lithium insertion in an earlier work. It was shown that inertia effects play a crucial role with respect to a possible fragmentation of the storage particles. Here, we focus on the opposite charging condition and examine the circumstances under which unstable crack growth and particle breakage can occur during lithium extraction.

Recent research into lithium ion battery storage particles has seen the development of many models to predict lithiation stresses generated during operation, and their effects on performance. Due to computational considerations most of the particles studied have idealized geometry with smooth surfaces, such as spheres. In reality, storage particles used in battery electrodes are acicular and have sharp edges and corners. In order to study the effect of these edges and corners on the generation of lithiation stress, we perform a parameter study on the development of lithiation strain and the resulting stress in cubic-shaped particles. We use a previously developed coupled stress-diffusion model, as well as three non-dimensional parameters, to quantify the stress response of cubic-shaped particles as a function of their material properties. Our results show that a change in material properties can lead to differences in both the value of maximum stress as well as its location in the particle. Both lithium insertion into and extraction from the particle are considered. © 2016 Springer Science+Business Media Dordrecht

The microvascular endothelium of the gut barrier plays a crucial role during inflammation in inflammatory bowel disease. We have modified a commonly used intestinal cell model based on the Caco-2 cells by adding microvascular endothelial cells (ISO-HAS-1). Transwell filters were used with intestinal barrier-forming Caco-2 cells on top and the ISO-HAS-1 on the bottom of the filter. The goal was to determine whether this coculture mimics the in vivo situation more closely, and whether the model is suitable to evaluate interactions of, for example, prospective nanosized drug vehicles or contrast agents with this coculture in a physiological and inflamed state as it would occur in inflammatory bowel disease. We monitored the inflammatory responsiveness of the cells (release of IL-8, soluble intercellular adhesion molecule 1, and soluble E-selectin) after exposure to inflammatory stimuli (lipopolysaccharide, TNF-α, INF-γ, IL1-β) and a nanoparticle (Ba/Gd: coprecipitated BaSO4 and Gd(OH)3), generally used as contrast agents. The barrier integrity of the coculture was evaluated via the determination of transepithelial electrical resistance and the apparent permeability coefficient (Papp) of NaFITC. The behavior of the coculture Caco-1/ISO-HAS-1 was compared to the respective monocultures Caco-2 and ISO-HAS-1. Based on transepithelial electrical resistance, the epithelial barrier integrity of the coculture remained stable during incubation with all stimuli, whereas the Papp decreased after exposure to the cytokine mixture (TNF-α, INF-γ, IL1-β, and Ba/Gd). Both the endothelial and epithelial monocultures showed a high inflammatory response in both the upper and lower transwell-compartments. However, in the coculture, inflammatory mediators were only detected on the epithelial side and not on the endothelial side. Thus in the coculture, based on the Papp, the epithelial barrier appears to prevent a potential inflammatory overreaction in the underlying endothelial cells. In summary, this coculture model exhibits in vivo-like features, which cannot be observed in conventional monocultures, making the former more suitable to study interactions with external stimuli.

Graphene-related materials (GRM) inherit unique combinations of physicochemical properties which offer a high potential for technological as well as biomedical applications. It is not clear which physicochemical properties are the most relevant factors influencing the behavior of GRM in complex biological environments. In this study we have focused on the interaction of GRM, especially graphene oxide (GO), and Caco-2 cells in vitro. We mimiked stomach transition by acid-treatment of two representative GRM followed by analysis of their physicochemical properties. No significant changes in the material properties or cell viability of exposed Caco-2 cells in respect to untreated GRM could be detected. Furthermore, we explored the interaction of four different GO and Caco-2 cells to identify relevant physicochemical properties for the establishment of a material property-biological response relationship. Despite close interaction with the cell surface and the formation of reactive oxygen species (ROS), no acute toxicity was found for any of the applied GO (concentration range 0-80 µg/ml-1) after 24 h and 48 h exposure. Graphene nanoplatelet aggregates led to low acute toxicity at high concentrations, indicating that aggregation, the number of layers or the C/O ratio have a more pronounced effect on the cell viability than the lateral size alone.

Within the framework of the ENFSI Expert Working Group Firearms/GSR a novel proficiency test on the Forensic Determination of Shooting Distance – FSDS2015 – was implemented. This proficiency test was developed out of collaborative studies which were previously carried out by a number of pre-selected ENFSI laboratories. The aim of this test was to assess the laboratories’ performance in visualizing the lead patterns on a shot object, and compare the questioned patterns with provided test shot patterns. The participating laboratories were requested to estimate the presumed shooting distance following their individual laboratory specific methods (SOPs) for shooting distance / muzzle-to-target determination. The submitted results were compiled by means of z scores according to the IUPAC and EURACHEM guidelines, and an extended statistical evaluation was performed. This is one of the first proficiency tests in the field of qualitative forensic methods where z scores were successfully utilized. This paper summarizes the results of the study and presents the overall performance of the participating laboratories.

One-dimensional zinc oxide (ZnO) nanostructures of different sizes were synthesized on silicon wafer substrates by controlling the substrate condition using chemical etching and a vapor-liquid-solid (VLS) process. In this work, a thin layer of gold was deposited on the silicon substrate and used as catalyst. Gold is one of the most frequently-used catalysts in the chemical vapor-deposition method. By annealing the gold-coated thin films at different temperatures, the layer was transformed into gold islands, due to the dewetting effect. To investigate the effect of the dewetting process on ZnO nanostructures, samples with various thicknesses are annealed at different temperatures. The results are compared with the uncoated and chemically-etched silicon wafers. Structural and morphological properties of the samples were analyzed using X-ray diffraction and scanning-electron microscopy. Formation of nanowires and nanorods was observed, and their sizes were dependent on the sizes of the gold islands. Photoluminescence spectra of the samples at room temperature were measured and visible emissions were observed from the synthesized nanostructures at room temperature.

Nano-TiO2 filled polypropylene was compounded on a twin-screw extruder and diluted to a filler con-tent of 1 vol.-%. Nanocomposite and neat matrix material were then injection molded to plates and vibration welded. Thin films of welded specimens were investigated in micro-tensile testing under the light-microscope in order to achieve global and local tensile properties and compared to results of tensile tests for two welding pressures on neat and filled polypropylene. Based on investigations, a model of the welding area has been created with ANSYS, which then was coupled and compared with experimental results based on macroscopic test coupons. Prediction of break position based on stress distribution calculated in the model was in good agreement with experimental results.

In this study, carbon nanotubes reinforced polyoxymethylene with different filler loadings was joined by using linear vibration welding technique. The tensile properties of vibration welded polyoxymethylene nanocomposites with different carbon nanotube contents were studied as functions of filler loading and weld pressure. The results showed that the addition of carbon nanotubes into polyoxymethylene slightly improved the matrix tensile strength and pronounced decreased the ductility of pure polyoxymethylene. Interestingly, the weld strength of the nanocomposites was also higher than the polymer matrix strength even at high weld pressure of 2 MPa. Possible reasons for this high weld strength are discussed based on the morphological investigations.