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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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    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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    SFB 986: Subproject A06 - Synthesis and characterization of hierarchical, multifunctional ceramic/metal-polymer materials systems
    The aim of this project is creating new bioinspired materials, through the design, synthesis and characterization of hierarchically structured composites. Such materials consist of hard (ceramic/metal) and soft phases (polymers and organic ligands). Pre-structured functionalized nanoparticles received from projects A1 and A2 are assembled into a material of progressively higher hierarchical levels. The processing steps involve self-assembly, hot-pressing and spouted bed spray granulation (with projects A2 and A3). The objective for the highest hierarchical level is an anisotropic brick-and-mortar structure, analogous to the one characterizing nacre and other exceptionally strong and tough natural materials. The composites, as well as their individual hierarchical levels, will be characterized both structurally (in collaboration with projects A7, Z2 and Z3) and mechanically (in collaboration with projects Z2 and Z3), from the nano- to the macro-scale. The key scientific question to be addressed concerns the role played by each building unit, at the different length scales, in the material’s macroscopic features. To do so, it is imperative to determine which properties and arrangements of the building units (polymeric, ceramic, metallic and nanocomposite particles) are necessary and sufficient to attain the desired mechanical and multifunctional characteristics in the final bulk material. Gaining this knowledge will ultimately lead to the tailored design of novel nanocomposites.
      150
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    SFB 986: Subproject C04 - Additive manufacturing by selfassembly of colloidal suspensions for tailor-made photonic properties
    TP C4 aims at the fabrication of multi-scale and multiphase photonic coatings by combining methods of additive fabrication with the self-assembly of colloidal suspensions and thus combining principles of colloidal self-organization with the scalability and form flexibility of direct writing. We expect that this will enable us to control the particle arrangement in the µm to nm scale on planar and curved cm-sized substrates and to produce ceramic photonic structures for adaptable photonic properties, structural colors and high temperature applications.
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