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    SFB 986: Subproject Z01 - Central Task of Collaborative Research Center
    Das Teilprojekt Z1 ist für die wissenschaftliche und administrative Koordination des gesamten SFB verantwortlich und verwaltet dessen zentrale Mittel.
      60
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    Auswirkungen von Typ 2 Diabetes Mellitus auf das Frakturverhalten und die biomechanischen Eigenschaften im kortikalen Knochen
    Projektpartner: M.Sc., Eva Maria Wölfel Heisenberg Forschungsgruppe & Bioengineering Institut für Osteologie und Biomechanik UKE Prof. Dr. Björn Busse Heisenberg Forschungsgruppe & Bioengineering Institut für Osteologie und Biomechanik UKE Ausgangssituation: Bei der metabolischen Erkrankung Typ 2 Diabetes Mellitus besteht eine Unempfindlichkeit der Zellen gegenüber des Hormons Insulin. Aufgrund der alternden Gesellschaft, einem zunehmend gesetzten Lebensstil sowie erhöhtes Vorkommen von Übergewicht steigt die Prävalenz von Typ 2 Diabetes Mellitus weiter an. Im Verlauf der Krankheit kann es zu Schädigungen des kardiovaskulären Systems, der Nieren und Nerven kommen. Zusätzlich geht Typ 2 Diabetes Mellitus mit einem erhöhten Frakturrisiko einher, dessen Ursachen bislang unzureichend erforscht sind. Dieses liegt auch an der Tatsache, dass entsprechende Technologien das Frakturrisiko bei Diabetes nicht darstellen können. Patienten mit Typ 2 Diabetes Mellitus zeigen oft trotz erhöhtem Frakturrisiko eine normale oder sogar erhöhte Knochenmineraldichte auf. Da die Diagnose eines erhöhten Frakturrisikos bisher auf der Bestimmung der Knochenmineraldichte basiert, wird die Diagnose von Diabetes Patienten mit erhöhtem Frakturrisiko erschwert. Entscheidende Knochenmaterialqualitätsparameter können jedoch nicht ohne weiteres mit den aktuell vorhandener Medizintechnik und klinischen Bildgebungsmodalitäten quantifiziert werden. Zur Bestimmung der zugrundeliegenden Mechanismen der diabetischen Knochenkrankheit sind daher Analysen am Knochengewebe erforderlich, um Daten erheben zu können welche dann in die Verbesserung der medizinisch-technischen Diagnosemöglichkeiten einfließen können. Zielsetzung: Obwohl es bereits erste Studien gibt, welche die Veränderungen der Knochenmaterialqualität unter Typ 2 Diabetes Mellitus analysieren, fehlt es bislang an direkten Korrelationen zwischen Knochenmaterialqualitätsparametern und dem Frakturverhalten von diabetischen Knochen. Aus diesem Grund sollen in diesem Projekt die Kollagen- und Mineraleigenschaften, als maßgebliche Bestandteile der Knochenmatrix, mit der Bruchzähigkeit und dem Kriechverhalten der diabetischen Knochenmatrix korreliert werden, um die veränderte Knochenmatrix mit dem Frakturverhalten zu verstehen. Vorgehensweise: Um das Frakturverhalten von Knochen im Labor zu ermitteln, werden zunächst Bruchzähigkeitstests in an diabetischen und gesunden Knochen durchgeführt. Für die Festigkeit des Knochens spielt der Mineralgehalt eine entscheidende Rolle. Daher werden die Knochendichte und der Mineralgehalt der Probe mit Hilfe von hochauflösender Bildgebung bestimmt. Zur Untersuchung des Verhaltens des Knochens unter Belastung, werden die Knochenproben mit einer Indenterspitze punktiert. Aufgrund der verschiedenen Methoden kann das Frakturverhalten mit Informationen zur Knochenmaterialqualität (Mineralisation, Knochendichte, Steifigkeit) korreliert und so der Einfluss von Diabetes Mellitus auf die Eigenschaften der Knochenmatrix näher definiert werden. Ziel ist es die Haupteinflussfaktoren zu identifizieren, die mit dem Frakturrisiko bei Diabetes korrelieren. Diese Faktoren sollen dann in die Diagnoseprotokolle von aktuellen medizinisch-technischen Bildgebungssystemen implementiert werden, um die Knochengesundheit von Diabetespatienten genauer zu beobachten und langfristig zu schützen.
      40
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    SFB 986: Subproject B10 - Funktionalisierung von hierarchischen nanoporösen Metallen durch aktive organische Filme
    This research project aims to explore approaches for surface functionalization of hierarchical nanoporous (h-np) gold by stimulus-sensitive organic layers in order to develop novel material concepts for multifunctional hard-soft hybrids as sensor and actuator materials. Targeted control of the elastic as well as plastic behavior of these materials by electrical signals and chemical stimuli is pursued using experimental and computational methods. Two types of organic coatings are employed: the conductive polymer polypyrrole (PPy) and self-assembling monolayers (SAM) of organic thiols. The component selection of the resulting functional material systems was motivated both by the variety of potential new functionalities they can impart to nanoporous (np) metals and by a significant potential to improve mechanical properties, based on our preliminary work. This is expected to lead to strategies for the development of multifunctional material systems with tailored properties, where a function is accompanied by targeted improved mechanical behavior.
      164
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    Mechanical behavior of bulk supercrystalline ceramic-organic nanocomposites
    The proposed work aims at elucidating the mechanical behavior of bulk supercrystalline ceramic-organic nanocomposites. These are materials that consist of inorganic, usually crystalline, nanoparticles, surface-functionalized with organic ligands, and assembled into periodic structures, analogous to atomic lattices. They represent a rising field of materials science and nanoengineering, which has been finding applications in optoelectronics, plasmonics, magnetic materials, battery electrodes, catalysts and sensors. Their use for structural purposes is however largely unexplored, even if promising mechanical properties have started to emerge in bulk ceramics-based supercrystalline materials.An effective way to enhance their mechanical properties has been found in inducing crosslinking among the organic chains that are anchored to the ceramic nanoparticles’ surfaces. This limits the mobility of the inorganic building blocks with respect to each other, leading not only to high values of strength, hardness and stiffness, but also to significant changes in the materials’ behavior under loading. Preliminary work points towards the concurrent effects of material compaction, plastic-like deformation and extrinsic toughening mechanisms. Defects remindful of the ones typically observed in crystalline materials – such as interstitials, stacking faults, dislocations and shear bands – have also been detected.The interconnections among mechanical behavior and the local nanostructure deformations are yet to be clarified, and this is what the present proposal aims at tackling. For materials with various degrees of crosslinking of the organic ligands, the stress-strain relationships, creep and compressibility behavior are to be derived. In parallel, the defects and deformations induced by both material processing and mechanical loading will be characterized. The methods of choice involve nanoindentation with various tips, for the derivation of the materials’ constitutive response, and atomic force and electron microscopy for the visualization of imperfections, from point-defects to dislocations and inter-supercrystalline boundaries. Together with their empirical evaluation, the connections between supercrystalline deformations and overall mechanical behavior will be modeled by applying and adapting the classic theories from the literature on slip systems activation, strain and stress fields around line defects, and dislocation glide. The correlations between material processing, supercrystalline structure and mechanical properties will then be inferred.
      203
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    Integrated Process Simulation of Powder Metallurgical Shaping and Sintering Applied to the Optimization of Porcelain Tile Manufacturing
    Sintering is a key processing step for manufacturing powder metallurgy-based products used in innumerable applications, including devices for upcoming sustainable energy scenarios as fuel cells, solar cells and electrolyser cell, and also for traditional ceramics such as cements and tiles. Sintering at high heating rates is based on a fast formation of a dense outer layer, which controls the further flux of heat to the interior of the compact. Thereby, the amount of energy available for sintering is enhanced and densification without extensive grain growth becomes easier. The economic and environmental benefit of fast sintering is related to lower quantity of demanded energy and produced emissions. Nevertheless, the influence of composition, heat transfer and sintering atmosphere is still to be fully understood. Thus, modelling and simulation of powder materials subjected to fast sintering can clarify the effects of phase evolution, sample size and geometry on the densification of ceramic bodies. Modeling and simulation of sintering generally refers to different approaches not only including distortion calculations, morphology issues or stress distribution in the sintering body but also simulating microstructural evolution, mainly densification. However, the current models for densification during sintering do not follow strictly the experimental data. Moreover, available sintering data, even for the same material, are just useful for the very specific experimental settings. Hence, the common practice is to determine experimentally the appropriate conditions to meet the requirements. Thus, there is a lack of a practical model able to predict densification of powder-based materials from sintering data of widely used ceramic materials. The focus of the project is to develop models and calculation approaches for modeling the densification step of ceramic powder sintering, particularly at high heating rates and short treatment times, predicting microstructure evolution and defects. Modeling particles rearrangement and densification will be performed using Discrete Element Model (DEM) which will be coupled to grid-based model for calculation of liquid transport. Moreover, this project aims to validate the model by designing ceramic materials by fast sintering associated with selected literature experimental data in order to verify the domain of the model in forecasting the behaviour of widely used ceramics.
      1292
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    Investigation of the deformation and fracture behavior of hierarchical dental enamel
    In the previous funding period of this DFG project we experimentally investigated the influence of the different hierarchical levels on the mechanical properties of dental enamel, in particular by using micromechanical experiments under tension and compression. We could reveal for the first time, that there exists a pronounced mechanical tension compression asymmetry especially in regard to the influence of hierarchy. This asymmetry has not been included in the existing theoretical micromechanical models yet. Notably our investigations showed that it is not clear, whether the hierarchical structure or the hard and soft combination of hydroxyapatite with its soft protein sheets is dominant for the exceptional mechanical properties of enamel. A major reason for this gap in understanding is, that there does not exist a 3 dimensional structural imaging of the structure of the hydroxyapatite nanofibers and the interfaces between the enamel prisms. The objective of this renewal proposal is therefore to understand, whether the presence of soft proteins is necessary for the damage tolerant behavior of dental enamel, or whether its complex hierarchical structure is more essential. In order to reveal that we want to compare mechanical experiments with enamel samples with and without proteins. In parallel we want to use TEM tomography to image the 3D structure of the hydroxyapatite nanofibers, their interfaces and FIB-tomography to image the interfaces of the enamel prisms. We expect, that the combination of the micro- and macromechanical experiments on different hierarchical levels together with high resolution imaging will allow answering the central scientific question of this project. A secondary objective of this renewal proposal is to understand, whether dental enamel as a non living tissue has the ability of crack healing, as indicated in one publication. It is related to the first objective of this proposal and will elucidate the role of proteins in enamel. Finally we think that the findings of this project will contribute in developing better and novel dental materials and non-metallic engineering materials.
      612
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    Analysis of the mechanisms of dielectric breakdown in ceramic materials
    Our experiments from the first project period showed that space charge limited conduction (SCLC) is the dominating conduction mechanism in 5µm-5mm thick ceramics at dielectric breakdown. This conduction mechanism is also the reason for the thickness-dependence of the breakdown strength. This means that all breakdown models which assume ohmic conduction at breakdown for 5µm-5mm thick ceramics are incorrect. In the first project period, we developed a breakdown model in analogy to the energy release rate concept known from fracture mechanics. This breakdown model is able to explain the experimentally found thickness-dependence of the breakdown strength as well as the dependence of the breakdown strength to the permittivity.In the following the scientific objectives of the renewal proposal are described:1. The developed breakdown model will be validated. The model predicts a dependence of the breakdown strength to the length of conducting filaments which initiate the breakdown process. These conducting filaments will be imitated experimentally by electrically conducting channels of different lengths which will be introduced by FIB (Focused Ion Beam) into defect-free substrates. The breakdown strength of these so prepared substrates will be correlated to the length of the FIBed channels and compared to the prediction of the breakdown model.2. The conduction mechanism of SCLC is thickness-dependent which means that the onset of SCLC in shifted to higher voltages in thicker samples. Therefore, there should be a limiting thickness above which the conduction mechanism will not be SCLC at breakdown anymore. In order to validate this assumption, experiments on thick samples are planned. The thickness is chosen that way that SCLC cannot establish before breakdown happens. In this third thickness range the breakdown strength should at least show a different thickness-dependence compared to what was already measured. The newly proposed breakdown model is not valid for this thickness range anymore and we can define an upper validity limit for the model in terms of sample thickness. 3. The breakdown strength of stacks should be evaluated. The dominating conduction mechanism at breakdown is SCLC. Therefore, it can be assumed that the breakdown of a layered sample cannot be explained by the linear ohmic model of a layered dielectric. Here, the influence of stacks with a thickness ranging in the thickness-dependent as well as in the thickness-independent breakdown strength range will be evaluated. Besides stacks of same material also stacks of different materials in terms of permittivity will be tested. We expect that the breakdown strength of a high-permittivity material can be enhanced by a thin layer of a low-permittivity material. This is of notable interest for technical application. In order to describe the breakdown strength of layered samples the linear model of a layered dielectric should be enhanced by SCLC.
      76
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    FOR 2213: Subproject "Quantum mechanically based quantitative description of metal alloy surfaces as function of the environment
    The goal of this project is to predict and understand the structural properties of metal alloy surfaces containing Au as function of their environment. This includes properties as the segregation profile of the surface, the crystallographic structure and its stability. Here, "environment" may stand for temperature, bulk concentration, or the influence of adsorbates. Based on these studies, we will be able to control the influence of the individual parameter, e.g. by analyzing the electronic structure, on the surfaces properties. Starting point will be the alloy surfaces of Ag-Au and Cu-Au, because they belong to the most promising candidates for the making of nanoporous Au. They will be studied via the combination of first-principles methods based on Density Functional Theory (DFT) with methods from statistical physics, namely the so-called Cluster Expansion method (CE) and Monte-Carlo (MC) simulations.While the CE allows for scanning huge parameter spaces, MC simulations will give us access to finite temperatures. In that way, the influence of temperature on atomic ordering phenomena in the near-surface regime will be studied by implementation of the CE Hamiltonian into Monte-Carlo programs. Then, on the basis of the stabilized cluster expansion, surface phase diagrams and ordering parameters will be constructed. These diagrams will provide a look-up-table for experimental studies, because they allow us to detect at which temperature and concentration the highest enrichment/depletion of Au in the near surface layer is reached. This also includes short-range order. In parallel, the structural results will be supported by the quantitative analysis of intensity spectra received from Low Energy Electron Diffraction (LEED) measured in the project SP5. Such structure determinations may reach accuracy smaller than one percent of the atomic diameter. This will allow for a detailed understanding of the alloy surface structures.
      118
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    Mechanical-chemical coupling at surfaces: prediction and experiment
    While the partial atomic volume of foreign atoms in solid solutions or of constituents in intermetallic compounds, and hence the coupling between the stress state and the chemical potential, have been studied already for many decades by materials scientists, up to now the analogous processes at surfaces have not received much attention. The key goal of this project is the investigation of the coupling between the mechanics of surfaces and adsorption. The basic phenomenon is that a capillary force, the surface stress, is coupled to changes of state at the materials surface. Thus, the adsorption of atoms from gas or ions from solution, or the enrichment/depletion of electronic charge at the surface prompts deformation of the material. This phenomenon has recently found application in modern biosensors, whose miniaturized cantilevers deform measurably upon adsorption of atoms or molecules. Furthermore, it has been shown that the controlled reversible adsorption/desorption of atoms or ions can prompt large strain in nanoporous solids that promise application as metallic actuators with large amplitude and strain energy density. Understanding of the microscopic mechanisms behind the coupling of changes of state at materials surfaces and the stress state is still rudimentary at best. A theory that could predict the coupling strength for a given materials surface and adsorbate is still missing. Moreover, the surface stress changes upon reversible adsorption and desorption of light elements, which are particularly relevant for applications in actuation and sensing, remain poorly studied. In this context, the present proposal aims at 1.) providing for the first time quantitative values for the coupling between capillary force and adsorption from both experiment and theory and 2.) insight into the underlying microscopic mechanims. As model processes we plan to study the adsorption of H and O on Au, Pt and Ir surfaces. The theoretical method for calculation of the coupling strength is density functional theory. For efficient and exhaustive screening of the configuration space we additionally employ cluster expansion methods. In experiment, changes of the surface stress upon adsorption from gas and electrosorption are determined from cantilever bending. Complementary measurements of the variation of the electrode potential as a function of strain during different electrosorption processes are carried out with Dynamic Electro-Chemo-Mechanical Analysis, a technique developed by one of the applicants.
      87