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Item type:Publication, Access status: without files , Nanostructured MWCNT/Polypyrrole Actuators with Anisotropic Strain Response(2016-04-01); ; ; ; Conducting polymers (CP) as an active material in electrochemical actuators are often likened to "artificial muscles". To transfer the isotropic change of volume in the CP component into an anisotropic actuation as in a natural muscle, different design approaches, mostly bilayer or triple layer bending devices, have been studied. Herein, we report a novel actuator design with an almost linear strain response, consisting of nanostructured aligned multi-walled carbon nanotube arrays in which each MWCNT is individually coated with a thin layer of polypyrrole.Publicationtype: Journal Article59 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , High-temperature stable Zirconia particles doped with Yttrium, Lanthanum, and Gadolinium(Wiley-VCH, 2016); ; ; ; Zirconia microspheres synthesized by a wet-chemical sol–gel process are promising building blocks for various photonic applications considered for heat management and energy systems, including highly effi cient refl ective thermal barrier coatings and absorbers/emitters used in thermophotovoltaic systems. As previously shown, pure zirconia microparticles deteriorate at working temperatures of ≥1000 °C. While the addition of yttrium as a dopant has been shown to improve their phase stability, pronounced grain growth at temperatures of ≥1000 °C compromises the photonic structure of the assembled microspheres. Here, a new approach for the fabrication of highly stable ceramic microparticles by doping with lanthanum, gadolinium, and a combination of those with yttrium is introduced. The morphological changes of the particles are monitored by scanning electron microscopy, ex situ X-ray diffraction (XRD), and in situ high-energy XRD as a function of dopant concentration up to 1500 °C. While the addition of lanthanum or gadolinium has a strong grain growth attenuating effect, it alone is insuffi cient to avoid a destructive tetragonal-to-monoclinic phase transformation occurring after heating to >850 °C. However, combining lanthanum or gadolinium with yttrium leads to particles with both effi cient phase stabilization and attenuated grain growth. Thus, ceramic microspheres are yielded that remain extremely stable after heating to 1200 °C.Publicationtype: Journal ArticleScopus© Citations 19 399 363 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , In situ high-energy X-ray diffraction during hot-forming of a multiphase TiAl alloy(Multidisciplinary Digital Publishing Institute (MDPI), 2015-11-30); ; ; ; Intermetallic γ-TiAl based alloys exhibit excellent high-temperature strength combined with low density. This makes them ideal candidates for replacing the twice as dense Ni base super-alloys, currently used in the medium temperature range (~700 °C) of industrial and aviation gas turbines. An important step towards the serial production of TiAl parts is the development of suitable hot-forming processes. Thermo-mechanical treatments often result in mechanical anisotropy due to the formation of crystallographic textures. However, with conventional texture analysis techniques, their formation can only be studied after processing. In this study, in situ high-energy X-ray diffraction measurements with synchrotron radiation were performed during hot-forming. Thus, it was possible to record the evolution of the phase constitution as well as the formation of crystallographic texture of different phases directly during processing. Several process temperatures (1100 °C to 1300 °C) and deformation rates were investigated. Based on these experiments, a process window can be recommended which results in the formation of an optimal reduced texture.Publicationtype: Journal ArticleScopus© Citations 18 435 294 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Large-scale parallel alignment of platelet-shaped particles through gravitational sedimentation(2015-05-18); ; ; ; Parallel and concentric alignment of microscopic building blocks into several orders of magnitude larger structures is commonly observed in nature. However, if similarly aligned structures are artificially produced their thickness is generally limited to just about one or two orders of magnitude more than the dimensions of the smallest element. We show that sedimentation provides a promising approach to manufacture solid materials consisting of well-aligned platelet-shaped particles while being more than 30 000 times thicker than the individual particle. Such sediments contain up to 28 vol% of particles without any further treatment and can be densified to 67 vol% particle fraction by subsequent unidirectional pressing. The degree of orientation of the platelet-shaped particles within the sediments was tracked by high-energy X-ray diffraction measurements. The Hermans orientation parameter, a statistical measure of the quality of alignment, was determined to be 0.63 ± 0.03 already for as-sedimented samples while the standard deviation of the orientation distribution of particles, another measure of average misalignment, was found to be (21.5 ± 1.4)°. After pressing, these values further improved to (0.81 ± 0.01) and (14.6 ± 0.4)°, respectively. Such quality of alignment competes with, if not even exceeds, values reported in the literature.Publicationtype: Journal ArticleScopus© Citations 40 406 364 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Synthesis and thermal stability of zirconia and yttria-stabilized zirconia microspheres(Elsevier, 2015); ; ; ; Zirconia microparticles produced by sol-gel synthesis have great potential for photonic applications. To this end, identifying synthetic methods that yield reproducible control over size uniformity is mportant. Phase transformations during thermal cycling can disintegrate the particles. Therefore, understanding the parameters driving these transformations is essential for enabling high-temperature applications. Particle morphology is expected to influence particle processability and stability. Yttria-doping should improve the thermal stability of the particles, as it does in bulk zirconia.Publicationtype: Journal ArticleScopus© Citations 74 483 642 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Fracture behavior of hydroxyapatite nanofibers in dental enamel under micropillar compression(Elsevier, 2012-11-10); ; ; ; Understanding the structure-property relations of load-bearing natural materials can provide design strategies for the generation of bio-inspired materials showing enhanced mechanical properties. Dental enamel is a highly damage-tolerant biological nanocomposite having a high mineral content. Its hierarchical structure consists of hydroxyapatite nanofibers enveloped by proteins on the smallest length scale. In this work, we show the fracture behavior of nanofibers in enamel by conducting micropillar compression tests. The evaluation and the result of the mechanical properties are discussed.Publicationtype: Journal Article90 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Hierarchical flexural strength of enamel : transition from brittle to damage-tolerant behaviour(Royal Society of London, 2011-10-26); ; ; ; Hard, biological materials are generally hierarchically structured from the nano- to the macroscale in a somewhat self-similar manner consisting of mineral units surrounded by a soft protein shell. Considerable efforts are underway to mimic such materials because of their structurally optimized mechanical functionality of being hard and stiff as well as damage-tolerant. However, it is unclear how different hierarchical levels interact to achieve this performance. In this study, we consider dental enamel as a representative, biological hierarchical structure and determine its flexural strength and elastic modulus at three levels of hierarchy using focused ion beam (FIB) prepared cantilevers of micrometre size. The results are compared and analysed using a theoretical model proposed by Jäger and Fratzl and developed by Gao and co-workers. Both properties decrease with increasing hierarchical dimension along with a switch in mechanical behaviour from linear-elastic to elastic-inelastic. We found Gao's model matched the results very well. This journal is © 2011 The Royal Society.Publicationtype: Journal Article177 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Novel ceramic-polymer composites synthesized by compaction of polymer-encapsulated TiO2-nanoparticles(Elsevier, 2011-10-13); ; ; ; A novel processing route for producing composites from ceramic particles and a thermoplastic polymer with high ceramic content was developed. Via a radical emulsion polymerization reaction in an aqueous suspension, titanium dioxide is encapsulated by a thin layer of poly(methyl methacrylate). Subsequently, the coated particles are compacted by applying high pressure (∼1GPa) at a temperature above the glass transition temperature of the polymer (∼160°C). This technique enables producing dense, hard and stiff composites at low processing temperatures. Microstructural investigations of composites by scanning electron microscopy confirm successful coating of titanium dioxide particles by polymer. Compositions were estimated from thermogravimetric measurements. A maximum TiO2 volume content of almost 70% was achieved. For characterizing mechanical properties, Vickers microhardness as well as flexural strength and elastic modulus were determined. With respect to pure PMMA, composites exhibit a 10-fold increase in microhardness. Furthermore, a strong increase in elastic modulus with TiO2 contents, up to 40GPa at 66vol.% TiO2 was observed. These moduli are among the highest found in literature for ceramic polymer composites. However, bending strength of the material is still low.Publicationtype: Journal Article40 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Corrosion of magnesium in static and dynamic (bioreactor) systems(European Society for Biomaterials, 2011); ; ; ; Publicationtype: Conference Paper19 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , High-energy X-rays : a tool for advanced bulk investigations in materials science and physics(Hindawi Publishing Corporation, 2003-09); ; ; High-energy X-rays between 30 keV and 1MeV, such as provided by modern synchrotron radiation sources as the ESRF and HASYLAB, bear the advantage of high penetration into most materials. Even heavy element compositions can be accessed in their volume. The range of applications is huge and spreads from nuclear spectroscopy to the characterization of metal extrusion under industrial conditions. This article compiles an overview over the most common instrumental diffraction techniques. Modern two-dimensional detectors are used to obtain rapid overviews in reciprocal space. For example, diffuse scattering investigations benefit from the very flat Ewald sphere as compared to low energies, which allow mapping of several Brillouin zones within one single shot. Diffraction profiles from liquids or amorphous materials can be recorded easily. For materials science purposes, whole sets of Debye–Scherrer rings are registered onto the detector, their diameters and eccentricities or their intensity distribution along the rings relating to anisotropic strain or texture measurements, respectively. At this point we stress the resolution of this technique which has to be carefully taken into account when working on a second generation synchrotron source. Energy-dispersive studies of local residual strain can be studied by a dedicated three-circle diffractometer which allows accurately to adjust the scattering angle from a defined gauge volume. Triple axis diffractometry and reciprocal space mapping is introduced and can be employed for highest resolution purposes on single crystal characterization, even under heavy and dense sample environments. Thus, the perfection of single crystals can be mapped and strain fields and superstructures as introduced by the modulation from ultrasonic waves into crystals or epitaxially grown Si/Ge layers can be investigated in detail. Phase transitions as magnetic ordering can be studied directly or through its coupling to the crystal lattice. Time resolved studies are performed stroboscopically from a ub-nanosecond to a second time scale. The combination of these techniques is a strong issue for the construction and development of future instruments.Publicationtype: Journal Article426 816