Volker Schmidt

Universität Ulm, Ulm, Baden-Württemberg, Germany

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Publications (121)264.29 Total impact

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    ABSTRACT: Mechanical stress-strain curves are estimated by means of numerical simulation in order to analyze and compare the mechanical properties of a real Strontium-modified Al-Si alloy with virtually designed materials. The virtual materials are generated by a competitive stochastic growth model of the 3D coral-like morphology of the eutectic Si in Al-Si alloys. The experimental data for the real material was acquired using FEB/SEM tomography. The numerical simulations are based on finite element methods. The effects of coarsening the mesh size and using different degrees of the finite elements are discussed. The simulations show that there is high conformity between the mechanical properties of the real and virtual materials. Experiments are also performed to show that the mechanical behavior of the realizations of the stochastic model is sensitive to changes in the parameters that control the morphological characteristics of the Si component.
    Archive of Applied Mechanics 01/2015; · 1.44 Impact Factor
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    Lothar Heinrich, Sebastian Lück, Volker Schmidt
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    ABSTRACT: We consider spatially homogeneous marked point patterns in an unboundedly expanding convex sampling window. Our main objective is to identify the distribution of the typical mark by constructing an asymptotic $\chi^2$-goodness-of-fit test. The corresponding test statistic is based on a natural empirical version of the Palm mark distribution and a smoothed covariance estimator which turns out to be mean square consistent. Our approach does not require independent marks and allows dependences between the mark field and the point pattern. Instead we impose a suitable $\beta$-mixing condition on the underlying stationary marked point process which can be checked for a number of Poisson-based models and, in particular, in the case of geostatistical marking. In order to study test performance, our test approach is applied to detect anisotropy of specific Boolean models.
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    ABSTRACT: Focused ion beam tomography has proven to be capable of imaging porous structures on a nano-scale. However, due to shine-through artefacts, common segmentation algorithms often lead to severe dislocation of individual structures in z-direction. Recently, a number of approaches have been developed, which take into account the specific nature of focused ion beam-scanning electron microscope images for porous media. In the present study, we analyse three of these approaches by comparing their performance based on simulated focused ion beam-scanning electron microscope images. Performance is measured by determining the amount of misclassified voxels as well as the fidelity of structural characteristics. Based on this analysis we conclude that each algorithm has certain strengths and weaknesses and we determine the scenarios for which each approach might be the best choice.
    Journal of Microscopy 09/2014; · 1.63 Impact Factor
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    ABSTRACT: A new algorithmic approach to segmentation of highly porous three dimensional image data gained by focused ion beam tomography is described which extends the key-principle of local threshold backpropagation described in [1]. The technique of focused ion beam tomography has shown to be capable of imaging the microstructure of functional materials. In order to perform a quantitative analysis on the corresponding microstructure a segmentation task needs to be performed. However, algorithmic segmentation of images obtained with focused ion beam tomography is a challenging problem for highly porous materials if filling the pore phase, e.g. with epoxy resin, is difficult. The grey intensities of individual voxels are not sufficient to determine the phase represented by them and usual thresholding methods are not applicable. We thus propose a new approach to segmentation, that pays respect to the specifics of the imaging process of focused ion beam tomography. As an application of our approach, the segmentation of three dimensional images for a cathode material used in polymer electrolyte membrane fuel cells is discussed. We show that our approach preserves significantly more of the original nanostructure than an thresholding approach.
    Materials Characterization 09/2014; · 1.88 Impact Factor
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    ABSTRACT: A set of computationally generated granular packings of frictionless grains is statistically analyzed using tools from stochastic geometry. We consider both the graph of the solid phase (formed using the particle mid-points) and the pore-phase. Structural characteristics rooted in the analysis of random point processes are seen to yield valuable insights into the underlying structure of granular systems. The graph of the solid phase is analyzed using traditional measures such as edge length and coordination number, as well as more instructive measures of the overall transport properties such as geometric tortuosity, where significant differences are observed in the windedness of paths through the different particle graphs considered. In contrast, the distributions of pore-phase characteristics have a similar shape for all considered granular packings. Interestingly, it is found that prolate and oblate ellipsoid packings show a striking similarity between their solid-phase graphs as well as between their pore-phase graphs.
    Granular Matter 08/2014; 16(4). · 1.50 Impact Factor
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    ABSTRACT: The relationship between the 3D morphology of gas-diffusion layers (GDL) of HT-PEFCs and their functionality is analyzed. A stochastic model describing the microstructure of paper-type GDL is combined with the Lattice-Boltzmann method (LBM) to simulate gas transport within the GDL microstructure. Virtual 3D microstructures representing paper-type GDL are generated by a stochastic model, where the binder morphology is systematically modified. On these structures, single phase single component gas flow is computed by the LBM. Quality criteria evaluating the spatial homogeneity of gas supply are introduced and related to the binder morphology. The spatial homogeneity of the gas supply is analyzed by a parametrized stochastic model describing the gas flow at the exit of the GDL. This approach gives insight into the spatial structure of the gas flow at the GDL exit. The quality of gas supply is quantified by characterizing size and arrangement of regions with high gas supply. This stochastic gas flow model predicts the quality of gas supply for further binder morphologies. Analyzing the quality criteria and the stochastic evaluation of the spatial structure of the gas flow field at the GDL exit, it is found that the binder morphology has an essential influence on the gas supply.
    Transport in Porous Media 07/2014; 103(3). · 1.55 Impact Factor
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    ABSTRACT: Microstructure has an important influence on the performance of SOFC electrodes. In recent years considerable progress has been achieved in describing microstructure parameters (e.g. TPB, tortuosity, particle size distributions) on a quantitative level based on high-resolution tomography. However, the performance of fuel cell electrodes is based on a complex interplay of various transport and electrochemical processes. Hence, in order to unravel the influence of microstructure (and microstructure degradation) on the electrode performance, it is not sufficient to just quantify the critical microstructure parameters, but also to incorporate these parameters into models that allow simulation of electrode reaction mechanism including the complex interplay of various physico-chemical processes. In this study we first present the recent progress in the elaboration of the relationship between effective transport properties with the transport relevant parameters (i.e. percolating phase volume fraction, tortuosity, constrictivity, size distributions of particle bulges and bottlenecks). Furthermore a model was developed to simulate the complex reaction mechanism of Ni-YSZ anodes. This model is capable to incorporate all relevant microstructure parameters that influence charge transport (ionic, electric) and charge transfer (fuel oxidation). The model allows distinguishing between different components of the ASR, which are related either to limitations of charge transport (ionic, electric) or charge transfer (electrochemistry) within the anode. In literature the influence of active reaction sites (i.e. TPB) is strongly emphasized. In the present paper we also focus on limitations in charge transport due to microstructure effects. Examples are presented which highlight the effects of grain size on the effective electric and ionic conductivity and corresponding anode performance. The data are compared with experimental data from EIS. The presented methodology gives new insight on the effects of microstructure variation, because it links critical microstructure parameters with anode performance and with the associated ASR components from different rate limiting processes.
    11th European SOFC Forum, Lucerne, Switzerland; 07/2014
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    ABSTRACT: We present a synchrotron X-ray tomographic study on the morphology of carbon fiber-based gas diffusion layer (GDL) material under compression. A dedicated compression device is used to provide well-defined compression conditions. A flat compression punch is employed to study the fiber geometry at different degrees of compression. Transport relevant geometrical parameters such as porosity, pore size and tortuosity distributions are calculated. The geometric properties notably change upon compression which has direct impact on transport conditions for gas and fluid flow. The availability of broad 3D paths, which are most important for the transport of liquid water from the catalyst layer through the GDL, is markedly reduced after compression. In a second experiment, we study the influence of the channel-land-pattern of the flow-field on shape and microstructure of the GDL. A flow-field compression punch is employed to reproduce the inhomogeneous compression conditions found during fuel cell assembly. While homogenously compressed underneath the land the GDL is much less and inhomogeneously compressed under the channel. The GDL material extends far into the channel volume where it can considerably influence gas and fluid flow. Loose fiber endings penetrate deeply into the channel and form obstacles for the discharge of liquid water droplets.
    Journal of Power Sources 05/2014; 253:123–131. · 5.26 Impact Factor
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    ABSTRACT: The microstructure influence on conductive transport processes is described in terms of volume fraction ε, tortuosity τ, and constrictivity β. Virtual microstructures with different parameter constellations are produced using methods from stochastic geometry. Effective conductivities σeff are obtained from solving the diffusion equation in a finite element model. In this way a large database is generated, which is used to test expressions describing different micro-macro relationships such as Archie's law, tortuosity and constrictivity equations. It turns out that the constrictivity equation has the highest accuracy indicating that all three parameters (ε, τ, β) are necessary to capture the microstructure influence correctly. The predictive capability of the constrictivity equation is improved by introducing modifications of it and using error-minimization, which leads to the following expression: σeff = σ0 2.03 ε1.57β0.72/τ2 with intrinsic conductivity σ0. The equation is important for future studies in e.g. batteries, fuel cells and for transport processes in porous materials. © 2014 American Institute of Chemical Engineers AIChE J, 2014
    AIChE Journal 02/2014; · 2.58 Impact Factor
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    ABSTRACT: A quantitative method for the characterization of nanoscale 3D morphology is applied to the investigation of a hybrid solar cell based on a novel hierarchical nanostructured photoanode. A cross section of the solar cell device is prepared by focused ion beam milling in a micropillar geometry, which allows a detailed 3D reconstruction of the titania photoanode by electron tomography. It is found that the hierarchical titania nanostructure facilitates polymer infiltration, thus favoring intermixing of the two semiconducting phases, essential for charge separation. The 3D nanoparticle network is analyzed with tools from stochastic geometry to extract information related to the charge transport in the hierarchical solar cell. In particular, the experimental dataset allows direct visualization of the percolation pathways that contribute to the photocurrent.
    Advanced Functional Materials 02/2014; · 10.44 Impact Factor
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    ABSTRACT: We show how regional prediction of car insurance risks can be improved for finer subregions by combining explanatory modeling with phenomenological models from industrial practice. Motivated by the control-variates technique, we propose a suitable combined predictor when claims data are available for regions but not for subregions. We provide explicit conditions which imply that the mean squared error of the combined predictor is smaller than the mean squared error of the standard predictor currently used in industry and smaller than predictors from explanatory modeling. We also discuss how a non-parametric random forest approach may be used to practically compute such predictors and consider an application to German car insurance data.
    Statistics & Risk Modeling. 01/2014; 31(2).
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    ABSTRACT: A novel parametrized model that describes the 3D microstructure of compressed fiber-based materials is introduced. It allows to virtually generate the microstructure of realistically compressed gas-diffusion layers (GDL). Given the input of a 3D microstructure of some fiber-based material, the model compresses the system of fibers in a uniaxial direction for arbitrary compression rates. The basic idea is to translate the fibers in the direction of compression according to a vector field which depends on the rate of compression and on the locations of fibers within the material. In order to apply the model to experimental 3D image data of fiber-based materials given for several compression states, an optimal vector field is estimated by simulated annealing. The model is applied to 3D image data of non-woven GDL in PEMFC gained by synchrotron tomography for different compression rates. The compression model is validated by comparing structural characteristics computed for experimentally compressed and virtually compressed microstructures, where two kinds of compression – using a flat stamp and a stamp with a flow-field profile – are applied. For both stamps types, a good agreement is found. Furthermore, the compression model is combined with a stochastic 3D microstructure model for uncompressed fiber-based materials. This allows to efficiently generate compressed fiber-based microstructures in arbitrary volumes.
    Journal of Power Sources 01/2014; · 5.26 Impact Factor
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    ABSTRACT: Simulations of organic semiconducting devices using drift-diffusion equations are vital for the understanding of their functionality as well as for the optimization of their performance. Input parameters for these equations are usually determined from experiments and do not provide a direct link to the chemical structures and material morphology. Here we demonstrate how such a parametrization can be performed by using atomic-scale (microscopic) simulations. To do this, a stochastic network model, parametrized on atomistic simulations, is used to tabulate charge mobility in a wide density range. After accounting for finite-size effects at small charge densities, the data is fitted to the uncorrelated and correlated extended Gaussian disorder models. Surprisingly, the uncorrelated model reproduces the results of microscopic simulations better than the correlated one, compensating for spatial correlations present in a microscopic system by a large lattice constant. The proposed method retains the link to the material morphology and the underlying chemistry and can be used to formulate structure–property relationships or optimize devices prior to compound synthesis.
    Journal of Chemical Theory and Computation 01/2014; 10(6):2508-2513. · 5.39 Impact Factor
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    ABSTRACT: Summary In adult articular cartilage, the pericellular matrix (PCM) mediates chondrocyte-matrix-interactions and is associated with the spatial cellular organization. Immunofluorescence microscopy, multiphoton-induced autofluorescence and second harmonic generation (SHG) imaging, as well as point pattern analyses revealed that both PCM and spatial organization were absent in fetal chondrocytes.Introduction In adult articular cartilage, the pericellular matrix mediates the biomechanical, biophysical and biomechanical interactions between the chondrocyte and the extracellular matrix. The PCM is also associated with the spatial organization of human superficial chondrocytes, which are situated in four distinct patterns of strings, clusters, pairs or single cells. However, little is known about the PCM and the spatial organization during fetal development. In this study, we asked the question whether fetal chondrocytes display a spatial organization comparable to that of adult chondrocytes, and whether a PCM is present or absent in the early stages of fetal cartilage development.Methods Articular cartilage sections (100µm thickness) were prepared from the condyles of human fetal knee joints (pregnancy weeks 7–10) and from macroscopically intact condylar areas (knee joint replacement procedures). The samples were characterised by immunofluorescence microscopy and multiphoton-induced autofluorescence imaging combined with quantitative SHG signal profiling, a technique that allows quantifying the fibrillar collagen content. The spatial organization was analyzed by point pattern analyses as we described previously. For these analyses the Cartesian coordinates of each nucleus were determined by converting the immunofluorescence images into gray-scale images and finding the local gray-scale maxima with ImageJ (NIH).Results In adult superficial cartilage, the PCM surrounded single or groups of chondrocytes and defined their spatial organization of groups of cellular strings. PCM was characterised by a strong collagen VI staining signal and high collagen intensity as measured by SHG (156.7±12.4). In fetal cartilage, the condyles were characterised by a high cellular density, no recognizable spatial organization, and little amounts of extracellular matrix. No collagen VI was detected in the matrix surrounding the fetal chondrocytes. Furthermore, SHG imaging revealed that the fibrillar collagen intensity was significantly weaker (4.7±0.8; p<0.001) when compared to the adult tissues. During maturation from the fetal to the adult stage the cell density per volume decreased significantly (p<0.001). The cell distance from each fetal chondrocyte to its nearest neighbor was 15.70±0.12µm and significantly larger for adult cartilage (35.86±1.37µm; p<0.001). The level of spatial clustering as measured by the integral of the pair correlation function was significantly higher in the adult cartilage (p<0.001). Overall, a cellular grouping comparable to that of adult chondrocytes was not present in fetal cartilage. Together these parameters suggest that the spatial organization typical for adult condylar cartilage was not present in human fetal condylar cartilage.Discussion/Conclusion In human fetal articular cartilage the spatial organization typical for adult chondrocytes was not present. Instead, the chondrocytes were situated densely and in close proximity. This study determined for the first time that the collagenous components that are typical for the adult PCM were not present in fetal cartilage. In conclusion, the PCM and also the spatial organization of superficial human chondrocytes develop with cartilage maturation and thus are not inborn but instead acquired characteristics.
    Bone & Joint Journal Orthopaedic Proceedings Supplement. 01/2014; 96-B(SUPP 11):323.
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    ABSTRACT: Distributional properties and a simulation algorithm for the Palm version of stationary iterated tessellations are considered. In particular, we study the limit behaviour of functionals related to Cox–Voronoi cells (such as typical shortest-path lengths) if either the intensity γ0 of the initial tessellation or the intensity γ1 of the component tessellation converges to 0. We develop an explicit description of the Palm version of Poisson–Delaunay tessellations (PDT), which provides a new direct simulation algorithm for the typical Cox–Voronoi cell based on PDT. It allows us to simulate the Palm version of stationary iterated tessellations, where either the initial or component tessellation is a PDT and can furthermore be used in order to show numerically that the qualitative and quantitative behaviour of certain functionals related to Cox–Voronoi cells strongly depends on the type of the underlying iterated tessellation.
    Journal of Statistical Computation and Simulation 01/2014; 84(7). · 0.63 Impact Factor
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    ABSTRACT: A methodology is proposed that is suitable for efficient simulation of continuous-time Markov chains that are nearly-completely decomposable. For such Markov chains the effort to adequately explore the state space via Crude Monte Carlo (CMC) simulation can be extremely large. The purpose of this paper is to provide a fast alternative to the standard CMC algorithm, which we call Aggregate Monte Carlo (AMC). The idea of the AMC algorithm is to reduce the jumping back and forth of the Markov chain in small subregions of the state space. We accomplish this by aggregating such problem regions into single states. We discuss two methods to identify collections of states where the Markov chain may become ‘trapped’: the stochastic watershed segmentation from image analysis, and a graph-theoretic decomposition method. As a motivating application, we consider the problem of estimating the charge carrier mobility of disordered organic semiconductors, which contain low-energy regions in which the charge carrier can quickly become stuck. It is shown that the AMC estimator for the charge carrier mobility reduces computational costs by several orders of magnitude compared to the CMC estimator.
    Methodology And Computing In Applied Probability 01/2014; 16:465. · 0.65 Impact Factor
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    ABSTRACT: As a straightforward generalization of the well-known Voronoi construction, Laguerre tessellations have long found application in the modelling, analysis and simulation of polycrystalline microstructures. The application of Laguerre tessellations to real (as opposed to computed) microstructures—such as those obtained by modern 3D characterization techniques like X-ray microtomography or focused-ion-beam serial sectioning—is hindered by the mathematical difficulty of determining the correct seed location and weighting factor for each of the grains in the measured volume. In this paper, we propose an alternative to the Laguerre approach, representing grain ensembles with convex cells parametrized by orthogonal regression with respect to 3D image data. Applying our algorithm to artificial microstructures and to microtomographic data sets of an Al-5 wt% Cu alloy, we demonstrate that the new approach represents statistical features of the underlying data—like distributions of grain sizes and coordination numbers—as well as or better than a recently introduced approximation method based on the Laguerre tessellation; furthermore, our method reproduces the local arrangement of grains (i.e., grain shapes and connectivities) much more accurately. The additional computational cost associated with orthogonal regression is marginal.
    Journal of Statistical Physics 11/2013; · 1.40 Impact Factor
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    ABSTRACT: Compared to the cytoplasmic F-actin abundance in cells, nuclear F-actin levels are generally quite low. However, nuclear actin is present in certain cell types including oocytes and under certain cellular conditions including stress or serum stimulation. Currently, the architecture and polymerization status of nuclear actin networks has not been analyzed in great detail. In this study, we investigated the architecture and functions of such nuclear actin networks. We generated nuclear actin polymers by overexpression of actin proteins fused to a nuclear localization signal (NLS). Raising nuclear abundance of a NLS wild-type actin, we observed phalloidin- and LifeAct-positive actin bundles forming a nuclear cytoskeletal network consisting of curved F-actin. In contrast, a polymer-stabilizing actin mutant (NLS-G15S-actin) deficient in interacting with the actin-binding protein cofilin generated a nuclear actin network reminiscent of straight stress fiber-like microfilaments in the cytoplasm. We provide a first electron microscopic description of such nuclear actin polymers suggesting bundling of actin filaments. Employing different cell types from various species including neurons, we show that the morphology of and potential to generate nuclear actin are conserved. Finally, we demonstrate that nuclear actin affects cell function including morphology, serum response factor-mediated gene expression, and herpes simplex virus infection. Our data suggest that actin is able to form filamentous structures inside the nucleus, which share architectural and functional similarities with the cytoplasmic F-actin.
    Histochemie 10/2013; · 2.61 Impact Factor
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    ABSTRACT: A stochastic model is proposed for the efficient simulation of complex three-dimensional microstructures consisting of two different phases. The model is based on a hybrid approach, where in a first step a graph model is developed using ideas from stochastic geometry. Subsequently, the microstructure model is built by applying simulated annealing to the graph model. As an example of application, the model is fitted to a tomographic image describing the microstructure of electrodes in Li-ion batteries. The goodness of model fit is validated by comparing morphological characteristics of experimental and simulated data.
    Modelling and Simulation in Materials Science and Engineering 05/2013; 21(5):055004. · 1.93 Impact Factor
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    ABSTRACT: The performance of polymer solar cells critically depends on the morphology of the interface between the donor- and acceptor materials that are used to create and transport charge carriers. Solar cells based on poly(3-hexylthiophene) and ZnO were fully characterized in terms of their efficiency and three-dimensional (3D) morphology on the nanoscale. Here, we establish a quantitative link between efficiency and morphology by using the experimental 3D morphology as direct input for a 3D optoelectronic device model. This model includes the effects of exciton diffusion and quenching; space-charge; recombination, generation, drift and diffusion of charge carriers; and the injection/extraction of carriers at the contacts. The observed trend in internal quantum efficiency as a function of layer thickness is reproduced with a single set of parameters. Several morphological aspects that determine the internal quantum efficiency are discussed and compared to other organic solar cells. This first direct use of morphological data in an optoelectronic device model highlights the importance of morphology in solar cells.
    Advanced Energy Materials 05/2013; 3:615. · 14.39 Impact Factor

Publication Stats

660 Citations
264.29 Total Impact Points


  • 2005–2014
    • Universität Ulm
      • • Institute of Stochastics
      • • Workgroup of Electron Microscopy
      • • Institute of General Medicine
      Ulm, Baden-Württemberg, Germany
  • 2009
    • Orange Labs
      Rhône-Alpes, France
    • Technische Universiteit Eindhoven
      • Department of Chemical Engineering and Chemistry
      Eindhoven, North Brabant, Netherlands