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Item type:Publication, Access status: without files , A comparison of ligament geometries in real and computer-generated nanoporous gold based on cross-section descriptors(2023-10-05); Nanoporous gold (np-Au) exhibits a bi-continuous network structure of nanoscale pores and solid ligaments, which form a random 3D network. Geometrical descriptors are developed that allow for characterization, statistical analysis and comparison of a large number of ligaments in real and computer generated np-Au structures. In addition to the voxel resolution and ligament length, the cross-sections of the ligaments are described by roundness, circularity, and compactness. The structures are analyzed in initial and coarsened conditions for various solid fractions. Although the structures do not change topologically, the results confirm significant changes in the geometry during coarsening. It is found that after coarsening the computer-generated structures compare much better to tomographic reconstructions of annealed np-Au structures, although there are still differences in the free ligament length. Therefore, in addition to the topological similarity, similarity of geometrical descriptors is found to be equally important for deriving predictive models based on computer-generated structures.Publicationtype: Journal Article25 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Simulated annealing framework for generating representative volume elements of materials with complex ligamentous microstructuresAt the microscale, various materials from biological tissues to nanoporous metals are formed by networks of ligaments. Here we propose a highly efficient simulated annealing (SA) framework for generating synthetic representative volume elements (RVE) of such materials. It can produce RVE where the microstructural characteristics both on the network level (e.g., node valency and ligament length) and on the level of individual ligaments (e.g., curvature) can be predefined by the user via probability distributions. As an application example of our framework, we generate a large variety of RVEs, analyze their mechanical properties by the finite element method, and establish through this approach links between microstructural descriptors and macromechanical properties of materials with ligamentous microstructures.Publicationtype: Journal Article49 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Data-based analysis of nanoporous metals: from microstructural features to structure-property relationships(2023); In this thesis, a comprehensive workflow for an FE beam modeling approach was developed, which allows to predict the macroscopic elastic and plastic mechanical behavior of nanoporous structures. An in-depth characterization of various geometrical and topological descriptors provided detailed insights about the comparability of experimental nanoporoug gold and computer-generated structures. The combination of the macroscopic mechanical predictions by FE simulations and the structural descriptors derived for the over one hundred computer-generated structures enabled to propose an improved scaling law.Publicationtype: Doctoral Thesis178 187 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Densification of nanoporous metals during nanoindentation: The role of structural and mechanical propertiesThe analysis of the densification behavior of nanoporous metals in nanoindentation is challenging in simulations and experiments. A deeper understanding of the densification behavior provides valuable information about the different deformation mechanisms in nanoindentation and compression experiments. The developed two-scale model allows for predicting the densification field for variable microstructure and elastic–plastic behavior. It could be shown that the penetration depth of the densification field is mainly controlled by the ratio of the macroscopic work hardening rate to yield stress. The shape as well as the value at characteristic isolines of densification depend mainly on the macroscopic plastic response of the nanoporous material. This could be confirmed by nanoindentation experiments, where the densification under the indenter was measured for ligament sizes from 35 to 150 nm. Although the depth of the densification field was underpredicted by the simulations, the experiments confirmed the predicted trends. Graphical abstract: [Figure not available: see fulltext.]Publicationtype: Journal ArticleScopus© Citations 4 69 261 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , How nanoporous silicon-polypyrrole hybrids flex their muscles in aqueous electrolytes: in operando high-resolution x-ray diffraction and electron tomography-based micromechanical computer simulations(American Physical Society (APS), 2022-11-28); ; ; ; Macroscopic strain experiments have revealed that silicon crystals traversed by parallel, channel-like nanopores functionalized with the artificial muscle polymer polypyrrole (PPy) exhibit large and reversible electrochemomechanical actuation in aqueous electrolytes. On a macroscopic scale these actuation properties are well understood. However, on the microscopical level this system still bears open questions, as to how the electrochemical expansion and contraction of PPy acts on to np-Si pore walls and how the collective motorics of the pore array emerges from the single-nanopore behavior. Here we present synchrotron-based, in operando x-ray diffraction on the evolving electrostrains in epilayers of this material grown on bulk silicon. An analysis of these experiments with micromechanical finite-element simulations, that are based on a full three-dimensional reconstruction of the nanoporous medium by transmission electron microscopy (TEM) tomography, shows that the in-plane mechanical response is dominantly isotropic despite the anisotropic elasticity of the single-crystalline host matrix. However, the structural anisotropy originating from the parallel alignment of the nanopores led to significant differences between the in- and out-of-plane electromechanical response. This response is not describable by a simple two-dimensional arrangement of parallel cylindrical channels. Rather, the simulations highlight that the dendritic shape of the silicon pore walls, including pore connections between the main channels, causes complex, highly inhomogeneous stress-strain fields in the crystalline host. Time-dependent x-ray scattering experiments on the dynamics of the actuator properties hint towards the importance of diffusion limitations, plastic deformation, and creep in the nanoconfined polymer upon (counter)ion adsorption and desorption, the very pore-scale processes causing the macroscopic electroactuation. From a more general perspective, our study demonstrates that the combination of TEM tomography-based micromechanical modeling with high-resolution x-ray scattering experiments provides a powerful approach for in operando analysis of nanoporous composites from the single nanopore up to the porous-medium scale.Publicationtype: Journal ArticleScopus© Citations 5 54 55 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , On the factors impacting the mechanical response of nanoporous gold-polymer compositesNanoporous gold (NPG) is a bicontinuous network of nanoscale gold ligaments and pores, the dimensions of which can be adjusted to exploit strengthening at small ligament sizes. Nevertheless, its mechanics are limited by the lack of ductility under tensile loading. Recently, the impregnation of the open porous structure with an epoxy phase created a metal-polymer composite that combined strengthening at small structural length-scales with enhanced tensile ductility. The present work aims to clarify the effect of factors influencing the mechanical behaviour of composite materials, particularly the properties of the epoxy phase and interface – properties that are still unexplored in NPG-epoxy composites. To this end, a series of novel approaches were developed to investigate the independent contribution of each factor, focusing on the development of representative model samples, micromechanical testing, and finite element simulation. A novel method to understand the effect of the composite microstructure in the epoxy was made possible by isolating the matrix phase. Taking inspiration from mining techniques, an approach based on gold etching in iodine-iodide solutions successfully leached out the Au from NPG-polymer composites. The remaining bi-continuous open porous polymer (NPP) was investigated using nanoindentation to explore possible size effects as those found in nanoporous gold. The interface was investigated using a model system where a single planar boundary between gold and epoxy was achieved. Using both nanoindentation approaches and novel microshear experiments, the interphase regime and interfacial strength were measured. Moreover, the experimental results inspired a modelling approach of the interfaces for finite element simulations, which was employed to study the interfacial behaviour as a function of the balance between the properties of constituent phases and interfaces. The interfacial model was also employed to simulate the effect of the interfacial strength on the mechanical response and delamination evolution of NPG-epoxy composites. The findings from the investigations on the isolated factors improved the interpretation of the composite mechanical response. No evidence of a structural length-scale effect in the polymer was found, and the response of the isolated polymer phase could be well predicted by an isostrain law applied to the properties measured on the bulk polymer. Furthermore, no evidence supporting the presence of an interlayer was found between the gold and epoxy phases. The measurement of the interfacial strength showed that the interface in NPG-epoxy composite is the weakest link in the composite's deformation, which was also observed in the simulations. The isolated effect of the interfacial properties was attenuated in the composite by the interconnected structure, which in compression allows the load transfer even after interface failure.Publicationtype: Doctoral Thesis218 1483 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Grain boundary formation through particle detachment during coarsening of nanoporous metals(2021-07-27); ; ; ; Grain boundary formation during coarsening of nanoporous gold (NPG) is investigated wherein a nanocrystalline structure can form by particles detaching and reattaching to the structure. MicroLaue and electron backscatter diffraction measurements demonstrate that an in-grain orientation spread develops as NPG is coarsened. The volume fraction of the NPG sample is near the limit of bicontinuity, at which simulations predict that a bicontinuous structure begins to fragment into independent particles during coarsening. Phase-field simulations of coarsening using a computationally generated structure with a volume fraction near the limit of bicontinuity are used to model particle detachment rates. This model is tested by using the measured NPG structure as an initial condition in the phase-field simulations. We predict that up to ∼5% of the NPG structure detaches as a dealloyed Ag75Au25 sample is annealed at 300 ◦C for 420 min. The quantity of volume detached is found to be highly dependent on the volume fraction and volume fraction homogeneity of the nanostructure. As the void phase in the experiments cannot support independent particles, they must fall and reattach to the structure, a process that results in the formation of new grain boundaries. This particle reattachment process, along with other classic processes, leads to the formation of grain boundaries during coarsening in nanoporous metals. The formation of grain boundaries can impact a variety of applications, including mechanical strengthening; thus, the consideration and understanding of particle detachment phenomena are essential when studying nanoporous metals.Publicationtype: Journal ArticleScopus© Citations 7 72 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Image segmentation and analysis for densification mapping of nanoporous gold after nanoindentation(Springer, 2021-07-14); ; ; ; Segmentation of scanning electron microscopy (SEM) images of focused ion beam (FIB) cross-sections through indented regions in nanoporous gold (np-Au) is carried out. A key challenge for image analysis of open porous materials is the appropriate binarization of the pore and gold ligament regions while excluding material lying below the cross-sectional plane. Here, a manual approach to thresholding is compared to global and local approaches. The global thresholding resulted in excessive deviations from the nominal solid fraction, due to a strong gray-scale gradient caused by the tilt angle during imaging and material shadowing. In contrast, the local thresholding approach delivered local solid fractions that were free of global gradients, and delivered a quality comparable to the manual segmentation. The extracted densification profiles vertically below the indenter as well as parallel to the surface showed an exponential-type decay from the indenter tip towards the nominal value of 1 far from the indenter. Graphic abstract: [Figure not available: see fulltext.]Publicationtype: Journal ArticleScopus© Citations 3 148 234 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , A strategy for dimensionality reduction and data analysis applied to microstructure–property relationships of nanoporous metalsNanoporous metals, with their complex microstructure, represent an ideal candidate for the development of methods that combine physics, data, and machine learning. The preparation of nanporous metals via dealloying allows for tuning of the microstructure and macroscopic mechanical properties within a large design space, dependent on the chosen dealloying conditions. Specifically, it is possible to define the solid fraction, ligament size, and connectivity density within a large range. These microstructural parameters have a large impact on the macroscopic mechanical behavior. This makes this class of materials an ideal science case for the development of strategies for dimensionality reduction, supporting the analysis and visualization of the underlying structure–property relationships. Efficient finite element beam modeling techniques were used to generate ~200 data sets for macroscopic compression and nanoindentation of open pore nanofoams. A strategy consisting of dimensional analysis, principal component analysis, and machine learning allowed for data mining of the microstructure–property relationships. It turned out that the scaling law of the work hardening rate has the same exponent as the Young’s modulus. Simple linear relationships are derived for the normalized work hardening rate and hardness. The hardness to yield stress ratio is not limited to 1, as commonly assumed for foams, but spreads over a large range of values from 0.5 to 3.Publicationtype: Journal ArticleScopus© Citations 10 90 133 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Comment to "Skeletonization-based beam finite element models for stochastic bicontinuous materials: application to simulations of nanoporous gold" by C. Soyarslan et al. [J. Mater. Res. 33(20), 3371 (2018)]Soyarslan et al. [J. Mater. Res. 33(20), 3371 (2018)] proposed a beam-finite element model for the computation of effective elastic properties of nanoporous materials, where the ligament diameter along the skeleton is determined with the biggest sphere algorithm. Although this algorithm is often used in the literature, it is known that it systematically overestimates the diameter in network structures. Thus, the need for further stiffening of the junction zones as proposed by the authors is in contradiction to the literature. Furthermore, the factor 40 appears to be one order of magnitude too high. We show that the 3D microstructures generated from random Gaussian fields contain features that are violating the assumption of circular cross-sections and, therefore, cannot be captured by the biggest sphere algorithm. Consequently, the authors required an unphysically high value of 40 to compensate this hidden effect.Publicationtype: Journal Article30