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Item type:Publication, Access status: with files , A geometric model for the fracture toughness of porous materialsDifferent models for the fracture toughness, KIC, of porous materials have been proposed to describe KIC as a function of the porosity P. They have in common that beside P at least one additional parameter exists that has to be adjusted to the measured data. Based on the cubic structure, we present a geometrical 3D model without any arbitrary parameter, which predicts the KIC the toughness, GC, and the Young's modulus, E, of a porous material. The model comprises three variants, depending on the material properties like open or closed porosity. It is in good agreement with a large amount of experimental data from different research groups.Publicationtype: Journal ArticleScopus© Citations 31 606 526 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Metal nanoparticle growth within clay-polymer nacre-inspired materials for improved catalysis and plasmonic detection in complex biofluids(American Chemical Society (ACS), 2017-05-27); ; ; ; Recent studies have shown that layered silicate clays can be used to form a nacre-like bioinspired layered structure with various polymer fillers, leading to composite films with good material strength, gas-barrier properties, and high loading capacity. We go one step further by in situ growing metal nanoparticles in nacre-like layered films based on layered silicate clays, which can be used for applications in plasmonic sensing and catalysis. The degree of anisotropy of the nanoparticles grown in the film can be controlled by adjusting the ratio of clay to polymer or gold to clay and reducing agent concentration, as well as silver overgrowth, which greatly enhances the surface enhanced Raman scattering activity of the composite. We show the performance of the films for SERS detection of bacterial quorum sensing molecules in culture medium, and catalytic properties are demonstrated through the reduction of 4-nitroaniline. These films serve as the first example of seedless, in situ nanoparticle growth within nacre-mimetic materials, and open the path to basic research on the influence of different building blocks and polymeric mortars on nanoparticle morphology and distribution, as well as applications in catalysis, sensing, and antimicrobial surfaces using such materials.Publicationtype: Journal Article110 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Nanoporous-gold-based composites : toward tensile ductility(Macmillan Publishers Limited, part of Springer Nature, 2015-06-12); ; ; ; We report on mechanical tests on interpenetrating-phase nanocomposite materials made by vacuum impregnation of nanoscale metal networks with a polymer. The metal component is nanoporous gold made by dealloying, whereas two epoxy resins and polyurethane are explored as the polymer component. The composites are strong and deformable in compression. Although previous observations invariably indicate tensile brittleness for nanoporous gold, composite samples made from cm-sized nanoporous samples enable macroscopic tensile and four-point bending tests that show ductility. This implies that the high strength of individual metal objects such as nanowires can now be incorporated into a strong and ductile material from which macroscopic things can be formed. In fact, a rule-of-mixture-type analysis of the stresses carried by the metal phase suggests quantitative agreement with data reported from separate experiments on small-scale gold nanostructures.Publicationtype: Journal ArticleScopus© Citations 60 294 409 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Thickness-dependence of the breakdown strength: analysis of the dielectric and mechanical failureThe breakdown strength as well as the mechanical strength of ceramic materials decreases with increasing volume. The volume-effect of the mechanical strength can be explained by the Weibull theory. For the breakdown strength the same explanation has been often assumed. In order to validate this assumption breakdown strength and mechanical strength of alumina samples with defined porosities were compared. Differences in the Weibull moduli of breakdown and mechanical strength distributions indicate that the volume-effect cannot explain the thickness-dependence of the breakdown strength. In particular, the thickness-dependence of the breakdown strength always leads to a Weibull modulus of two which is not in agreement with the measured Weibull moduli for samples with constant thickness. It can be concluded that the thickness-dependence of the breakdown strength cannot be explained by the Weibull concept. A recently developed breakdown model which is based on space charge injection is able to explain the experimental results.Publicationtype: Journal ArticleScopus© Citations 85 645 3576 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Dielectric breakdown of alumina single crystalsThe bulk breakdown behaviour of alumina single crystals with two different crystal orientations, {11-20}-plane (single crystal A) and {0001}-plane (single crystal C), have been studied. Therefor plan-parallel single crystal samples were electrically loaded until dielectric breakdown was achieved. For each crystal orientation, a characteristic breakdown channel direction through the sample could be defined. In C-oriented crystals the breakdown channel originated parallel to the c-axis. For A-oriented crystals however, the breakdown channel crossed the sample in an oblique direction; the angle between crystal surface and breakdown channel was 60°. Here, the breakdown channel crossed the sample along an A-plane. Although the breakdown channel paths of A and C crystals are different, the observed breakdown strength are identical within the scatter range.Publicationtype: Working Paper822 1644 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , What do we know about surface charges on cracks in ferroelectric ceramics?The present work investigates the static and time dependent electric potential distribution around cracks in a poled ferroelectric ceramic by Kelvin Probe Force Microscopy (KFM). In a first step a Vickers indentation crack in poled lead zirconate titanate (PZT) was subjected to static electric fields of up to 500 V/mm in poling direction, and the potential distribution around the crack as measured. In a second step, the polarity of the applied voltage was reversed against the poling direction during the measurement of the potential. Using a simple model, an effective dielectric constant of the crack, as well as the surface charge density on the crack face were calculated as a function of the distance from the crack tip, the applied field and the time. The results are discussed with reference to free charges on the crack surface, electrically induced domain switching at the crack tip and crack bridging.Publicationtype: Conference Paper620 844 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Influence of finite notch root radius and optically determined crack length on the measured fracture toughness of brittle materialsWhen measuring the fracture toughness of ceramics, for instance with the Single Edge V-Notched Beam (SEVNB) method, a known problem is given by the finite notch root radius. If this radius is larger than a certain small value of a few micrometers, the measured stress intensity factor is higher than the true intrinsic value. Another problem is that due to technical and physical reasons the optically measured crack length at stable crack growth is smaller than the true crack length, which might lead to an underestimation of the fracture toughness. In the given paper an experimental approach is presented to register these effects quantitatively with stable crack advance and to finally obtain the correct fracture toughness. This is achieved by applying a new computer aided automatic control system for stable crack advance and by analyzing the results in force - displacement and crack length - force diagrams. The used material is zirconia-toughened alumina.Publicationtype: Working PaperScopus© Citations 15 1148 664 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Experimenteller Aufbau zur Messung der Energiefreisetzungsrate für Risswachstum in PZT unter elektromechanischer LastFour-point-bending V-notched specimens of PZT poled parallel to the long axis are fractured under conditions of controlled crack growth in a custom-made device. In addition to the mechanical loading an electric field is applied parallel or anti parallel to the poling direction. This is a typical loading condition for the application of piezo- and ferroelectric ceramics, respectively. To determine the different contributions to the total energy release rate the mechanical and piezoelectric compliance as well as the electrical capacitance of the sample are acquired continuously with a special technique as a function of the crack length. The derivation of the data with respect to the crack surface allows calculating all parts of the total energy release rate on the basis of a single bending experiment for the first time. The main focus here is the presentation of the experimental method.Publicationtype: Conference Paper812 527 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Zur Bestimmung bruchmechanischer Kennwerte am Piezo-AktorCracks at thin electrodes are a known failure mode of piezoelectric actuators. For that reason the fracture mechanical parameters determining the stable crack grow for such interfacial cracks are of interest. The fracture mechanical parameters will be determined theoretically and experimentally. The measurement of the critical loads leading to crack growth are performed at 4-point bending specimens. Using the theoretical solution for the near tip crack field of the generalized two-dimensional problem the intensity factors will be defined. The determination of the critical intensity factors results from numerically calculated shape functions for the 4-point bending specimen. The crack closure integral gives the connection between the intensity parameters and the energy release rate.Publicationtype: Conference Paper1175 332