@misc{riedel_tailoring_hierarchical_2023, author={Riedel, L., Markmann, J., Weißmüller, J., Shi, S.}, title={Tailoring hierarchical nanoporous gold on dual length scales}, year={2023}, howpublished = {journal article}, doi = {https://doi.org/10.1103/PhysRevMaterials.7.116001}, abstract = {Dealloyed nanoporous metals with a hierarchical structure provide model systems for low-density structural and functional nanomaterials. It has been suggested that these materials are distinguished by particularly stringent design principles, with precisely defined characteristic length scales, and with geometrically similar structures on each hierarchy level, and that the length scales can be independently tuned on each level. Studying nanoporous gold made by two-step dealloying, we here demonstrate the tunability of the microstructure, independently for the upper and the lower hierarchy level. Small-angle (SAXS) and ultrasmall-angle x-ray scattering (USAXS) revealed sharp interference peaks corresponding to each of the two levels, confirming the stringent structural definition. Exploiting USAXS, we resolve and study upper-hierarchy-level ligament spacings of up to 600 nm. The length scales inferred from the peak positions correlate excellently with structure sizes determined by analysis of electron micrographs. This suggests a scaling factor that allows for size conversion between the two approaches. Furthermore, the analysis of the small-angle scattering enables a characterization of the volume-specific surface area, in good agreement with the estimate based on the ligament size and the leveled-wave model as an approximate description of the material's microstructure.}, note = {Online available at: \url{https://doi.org/10.1103/PhysRevMaterials.7.116001} (DOI). Riedel, L.; Markmann, J.; Weißmüller, J.; Shi, S.: Tailoring hierarchical nanoporous gold on dual length scales. Physical Review Materials. 2023. vol. 7, no. 11, 116001. DOI: 10.1103/PhysRevMaterials.7.116001}} @misc{wu_on_the_2023, author={Wu, Y., Markmann, J., Lilleodden, E.T.}, title={On the consequences of intrinsic and extrinsic size effects on the mechanical response of nanoporous Au}, year={2023}, howpublished = {journal article}, doi = {https://doi.org/10.1016/j.matdes.2023.112175}, abstract = {In this study, the consequence of intrinsic and extrinsic size effects on mechanical responses of nanoporous gold is investigated via microcompression testing. By varying the micropillar diameter (D) between 1 µm and 20 µm and the ligament size (L), 50 nm and 350 nm, a critical ratio (α = D/L = 20) was found, above which the test structure can be considered a representative volume element, resulting in identical mechanical response and uniform deformation. Below that value, both flow stress and elastic modulus decrease with decreasing pillar diameter, as evidenced for a measurement series with a fixed ligament size of 350 nm where the flow stress decreased by more than 50% (from approximately 5 to 2.5 MPa) and the elastic modulus was reduced from approximately 0.5 GPa to almost zero. Stochastic behavior along with non-uniform deformation and failure is observed for α < 10, suggesting that the size of the load-bearing units in this material is about 10 times the corresponding ligament size.}, note = {Online available at: \url{https://doi.org/10.1016/j.matdes.2023.112175} (DOI). Wu, Y.; Markmann, J.; Lilleodden, E.: On the consequences of intrinsic and extrinsic size effects on the mechanical response of nanoporous Au. Materials & Design. 2023. vol. 232, 112175. DOI: 10.1016/j.matdes.2023.112175}} @misc{ovri_mechanistic_origin_2023, author={Ovri, H., Markmann, J., Barthel, J., Kruth, M., Dieringa, H., Lilleodden, E.}, title={Mechanistic origin of the enhanced strength and ductility in Mg-rare earth alloys}, year={2023}, howpublished = {journal article}, doi = {https://doi.org/10.1016/j.actamat.2022.118550}, abstract = {Magnesium (Mg) alloys with low concentrations of rare earth additions are known to exhibit strengths and ductility that are significantly higher than those obtained in traditional Mg alloys. However, the mechanisms that underlie these improvements are still open to debate. We assessed these mechanism(s) by carrying out in-depth analysis of the deformation behavior in single crystals of pure Mg and a homogenized Mg-0.75 at.% Gd alloy oriented for twinning, pyramidal- and basal-slip. We observed a fivefold increase in basal CRSS, an eightfold increase in twinning CRSS and a fourfold decrease of the pyramidal/basal CRSS (P/B) ratio due to Gd addition. We also observed that while twinning and pyramidal slip activities were similar in the two material systems, basal slip was radically different. Specifically, basal slip was planar in the alloy but wavy in pure Mg. Our work reveals that these observations are a consequence of Gd-rich short-range ordered (SRO) clusters in the alloy. We show that interactions between dislocations and the SRO clusters would lead to significant increases in strength and slip activity, and consequently, ductility improvements in homogenized polycrystalline Mg-Gd alloys.}, note = {Online available at: \url{https://doi.org/10.1016/j.actamat.2022.118550} (DOI). Ovri, H.; Markmann, J.; Barthel, J.; Kruth, M.; Dieringa, H.; Lilleodden, E.: Mechanistic origin of the enhanced strength and ductility in Mg-rare earth alloys. Acta Materialia. 2023. vol. 244, 118550. DOI: 10.1016/j.actamat.2022.118550}} @misc{li_enhanced_electrochemical_2022, author={Li, J., Markmann, J., Mameka, N.}, title={Enhanced electrochemical actuation of nanoporous gold-polypyrrole hybrid under load}, year={2022}, howpublished = {journal article}, doi = {https://doi.org/10.1063/5.0093921}, abstract = {This work examines the actuation strain response of a nanoporous gold-polypyrrole electrochemical actuator under compression. The strain is monitored by in situ dilatometry and dynamic mechanical analysis when the material is wetted by an aqueous electrolyte and subjected to cyclic potential variation under various compressive loads ranging from −0.27 to −22.30 MPa. Contrary to previous studies that report reduced actuation amplitudes under load in the individual constituents of the material—nanoporous gold and polypyrrole, we find the strain amplitudes of the hybrid increase with increasing load and even while being deformed by plastic deformation. In this contribution, we discuss the phenomenon by taking into account the variations of the effective Young's modulus of the material that occur simultaneously with the actuation.}, note = {Online available at: \url{https://doi.org/10.1063/5.0093921} (DOI). Li, J.; Markmann, J.; Mameka, N.: Enhanced electrochemical actuation of nanoporous gold-polypyrrole hybrid under load. Applied Physics Letters. 2022. vol. 121, no. 2, 021901. DOI: 10.1063/5.0093921}} @misc{li_evolution_of_2022, author={Li, Y., Dinh-Ngo, B., Markmann, J., Weissmüller, J.}, title={Evolution of length scales and of chemical heterogeneity during primary and secondary dealloying}, year={2022}, howpublished = {journal article}, doi = {https://doi.org/10.1016/j.actamat.2021.117424}, abstract = {We study the evolution of silver-rich regions, or ‘clusters’, during the making of nanoporous gold by dealloying. The clusters, which are remnants of the master alloy that have evaded corrosion, impact the functional behavior of the material. Furthermore, they carry information on the structure size in the initial stages of dealloying. Using kinetic Monte Carlo simulations, we emulate electrochemical dealloying at various electrode potentials. Our simulations illustrate the two-stage characteristic of the process, where primary dealloying generates the initial network of nanoscale ligaments, while the subsequent secondary dealloying is characterized by coarsening and further dissolution. Silver-rich clusters, embedded in essentially pure gold, form during primary dealloying throughout the range of dealloying potentials of the study. At this point, their size scales with that of the ligaments. Both sizes decrease with increasing dealloying potential, and the trends of size versus potential agree with a Gibbs-Thompson type relation. Yet, when coarsening increases the ligament size during secondary dealloying, the size of the silver clusters remains constant. Directly accessing the initial ligament size of nanoporous gold in experiment is challenging, yet our study links this size to that of the silver-rich clusters. The clusters survive even in the later stages of dealloying and their size can be measured. This provides an experimental signature of the initial size.}, note = {Online available at: \url{https://doi.org/10.1016/j.actamat.2021.117424} (DOI). Li, Y.; Dinh-Ngo, B.; Markmann, J.; Weissmüller, J.: Evolution of length scales and of chemical heterogeneity during primary and secondary dealloying. Acta Materialia. 2022. vol. 222, 117424. DOI: 10.1016/j.actamat.2021.117424}} @misc{elder_grain_boundary_2021, author={Elder, K.L.M., Beck Andrews, W., Ziehmer, M., Mameka, N., Kirchlechner, C., Davydok, A., Micha, J.-S., Chadwick, A.F., Lilleodden, E.T., Thornton, K., Voorhees, P.W.}, title={Grain Boundary Formation Through Particle Detachment During Coarsening of Nanoporous Metals}, year={2021}, howpublished = {journal article}, doi = {https://doi.org/10.1073/pnas.2104132118}, abstract = {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.}, note = {Online available at: \url{https://doi.org/10.1073/pnas.2104132118} (DOI). Elder, K.; Beck Andrews, W.; Ziehmer, M.; Mameka, N.; Kirchlechner, C.; Davydok, A.; Micha, J.; Chadwick, A.; Lilleodden, E.; Thornton, K.; Voorhees, P.: Grain Boundary Formation Through Particle Detachment During Coarsening of Nanoporous Metals. Proceedings of the National Academy of Sciences of the United States of America: PNAS. 2021. vol. 118, no. 30, e2104132118. DOI: 10.1073/pnas.2104132118}} @misc{li_nanoporous_goldpolypyrrole_2021, author={Li, J., Markmann, J., Weissmüller, J., Mameka, N.}, title={Nanoporous gold-polypyrrole hybrid electrochemical actuators with tunable elasticity}, year={2021}, howpublished = {journal article}, doi = {https://doi.org/10.1016/j.actamat.2021.116852}, abstract = {This study explores the effective elastic response of hybrid electrochemical actuators based on nanoporous gold – decorated with a thin film of an electrosynthesized polypyrrole – during charging in an aqueous electrolyte. We found a new and yet unrevealed phenomenon in the hybrids – a reversible change of the material’s elasticity with alternating stiffening and softening behavior. Remarkably, the stiffness variations are larger and of opposite sign as compared to a non-coated nanoporous gold. The amplitude of the elastic modulus variation increases with the thickness of the polypyrrole layer, pointing to a dominant role of processes in the polymer bulk upon its doping or undoping. We propose that the reversible stiffness of the hybrid material is governed by the competition between the increased intermolecular bonding as consequence of interactions between the charged chains and dopant anions in the polymer and its plasticization due to solvent intake.}, note = {Online available at: \url{https://doi.org/10.1016/j.actamat.2021.116852} (DOI). Li, J.; Markmann, J.; Weissmüller, J.; Mameka, N.: Nanoporous gold-polypyrrole hybrid electrochemical actuators with tunable elasticity. Acta Materialia. 2021. vol. 212, 116852. DOI: 10.1016/j.actamat.2021.116852}} @misc{shi_scaling_behavior_2021, author={Shi, S., Li, Y., Ngo-Dinh, B., Markmann, J., Weissmüller, J.}, title={Scaling behavior of stiffness and strength of hierarchical network nanomaterials}, year={2021}, howpublished = {journal article}, doi = {https://doi.org/10.1126/science.abd9391}, abstract = {Structural hierarchy can enhance the mechanical behavior of materials and systems. This is exemplified by the fracture toughness of nacre or enamel in nature and by human-made architected microscale network structures. Nanoscale structuring promises further strengthening, yet macroscopic bodies built this way contain an immense number of struts, calling for scalable preparation schemes. In this work, we demonstrated macroscopic hierarchical network nanomaterials made by the self-organization processes of dealloying. Their hierarchical architecture affords enhanced strength and stiffness at a given solid fraction, and it enables reduced solid fractions by dealloying. Scaling laws for the mechanics and atomistic simulation support the observations. Because they expose the systematic benefits of hierarchical structuring in nanoscale network structures, our materials may serve as prototypes for future lightweight structural materials.}, note = {Online available at: \url{https://doi.org/10.1126/science.abd9391} (DOI). Shi, S.; Li, Y.; Ngo-Dinh, B.; Markmann, J.; Weissmüller, J.: Scaling behavior of stiffness and strength of hierarchical network nanomaterials. Science. 2021. vol. 371, no. 6533, 1026-1033. DOI: 10.1126/science.abd9391}} @misc{li_datasets_for_2020, author={Li, Y., Dinh-Ngô, B., Markmann, J., Weissmüller, J.}, title={Datasets for the microstructure of nanoscale metal network structures and for its evolution during coarsening}, year={2020}, howpublished = {journal article}, doi = {https://doi.org/10.1016/j.dib.2019.105030}, abstract = {The datasets in this work are files containing atom position coordinates of volume elements approximating nanoporous gold made by dealloying and annealing. The material is represented in an as-prepared state and in various stages of coarsening, as described in Phys. Rev. Mater, 3 (2019) 076001. Realistic initial structures of different solid fractions have been constructed by the leveled-wave algorithm, approximating mixtures at the end of early-stage spinodal decomposition. The microstructural evolution during coarsening by surface diffusion was approximated by on-lattice kinetic Monte-Carlo simulation. The data sets refer to solid fractions from 0.22 to 0.50, providing for different initial connectivity of the bicontinuous structures. Coarsening at two temperatures, 900 K and 1800 K, explores two different degrees of surface energy anisotropy – more faceted at 900 K and more rough at 1800 K. Each structure takes the form of a face-centred cubic lattice with approximately 32 million sites. A site can be occupied by either void or atom. 3D periodic boundary conditions are satisfied. Tables list each structure's properties, and specifically the specific surface area, two different measures for the ligament size, the net topological genus as well as the scaled genus. The atom coordinate files may serve as the basis for geometry analysis and for atomistic as well as finite element simulation studies of nanoporous as well as spinodally decomposed materials. The data sets are accessible via the TORE repository at http://hdl.handle.net/11420/3253.}, note = {Online available at: \url{https://doi.org/10.1016/j.dib.2019.105030} (DOI). Li, Y.; Dinh-Ngô, B.; Markmann, J.; Weissmüller, J.: Datasets for the microstructure of nanoscale metal network structures and for its evolution during coarsening. Data in Brief. 2020. vol. 29, 105030. DOI: 10.1016/j.dib.2019.105030}} @misc{wu_electrochemomechanical_coupling_2019, author={Wu, Y., Markmann, J., Lilleodden, E.}, title={Electro-chemo-mechanical coupling of nanoporous gold at the microscale}, year={2019}, howpublished = {journal article}, doi = {https://doi.org/10.1063/1.5128049}, abstract = {The observation of reversible strengthening and stiffening of nanoporous gold (NPG) under electrochemical potential has opened opportunities to exploit this material for multifunctional applications. Yet the complex structural geometry and length-scales involved make a definitive understanding of structural correlations to the behaviors difficult at best. Achievement of coupled electro-chemo-mechanical testing at the micrometer scale is a key step toward this goal. Here, we introduce an experimental approach to investigate the elastic and plastic behaviors of NPG under electrochemical potential at the microscale using a modified nanoindentation setup and multiple load function. The in situ experiments in electrolyte show a significant increase by 32% in strength of pillars in a positive potential regime where oxygen adsorption occurred. This response was found to be reversible, which agrees with macroscopic results, while the elastic modulus was shown to be insensitive to the applied potential—an observation inconsistent with recent bulk dynamic mechanical analysis results.}, note = {Online available at: \url{https://doi.org/10.1063/1.5128049} (DOI). Wu, Y.; Markmann, J.; Lilleodden, E.: Electro-chemo-mechanical coupling of nanoporous gold at the microscale. Applied Physics Letters. 2019. vol. 115, no. 25, 251602. DOI: 10.1063/1.5128049}} @misc{li_topology_evolution_2019, author={Li, Y., Ngo, B.-N.D., Markmann, J., Weissmueller, J.}, title={Topology evolution during coarsening of nanoscale metal network structures}, year={2019}, howpublished = {journal article}, doi = {https://doi.org/10.1103/PhysRevMaterials.3.076001}, abstract = {Many experiments exploit curvature-driven, surface-diffusion-mediated coarsening for tuning the characteristic structure size of metal network structures made by dealloying, such as nanoporous gold. Here we study this process by kinetic Monte Carlo simulation. The initial microstructures are leveled Gaussian random fields, approximating spinodally decomposed mixtures, of different solid fraction φ. Earlier work establishes these structures as valid representations of the nanoporous gold microstructure. We find that the coarsening law for the characteristic spacing between the ligaments of the network is universal, whereas the time evolution of the characteristic ligament diameter is not. The expected time exponent 1/4 is confirmed by our simulation. Contrary to what may be expected based on continuum models, the degree of surface faceting or roughness has no apparent effect on the coarsening kinetics. In the time interval of our study, the network connectivity—as measured by a scaled density of topological genus—remains sensibly invariant for networks with φ≥0.3, consistent with previous reports of a self-similar evolution of the microstructure during coarsening. Yet, networks with lesser φ lose their connectivity on coarsening and can even undergo a percolation-to-cluster transition. This process is slow for φ only little below 0.3 and it accelerates in networks with lesser φ. The dependency of the connectivity evolution on φ may explain controversial findings on the microstructure evolution of nanoporous gold in experimental studies.}, note = {Online available at: \url{https://doi.org/10.1103/PhysRevMaterials.3.076001} (DOI). Li, Y.; Ngo, B.; Markmann, J.; Weissmueller, J.: Topology evolution during coarsening of nanoscale metal network structures. Physical Review Materials. 2019. vol. 3, no. 7, 076001. DOI: 10.1103/PhysRevMaterials.3.076001}} @misc{liu_surfacedriven_actuation_2019, author={Liu, L.-Z., Mameka, N., Markmann, J., Jin, H.-J., Weissmueller, J.}, title={Surface-driven actuation: Sign reversal under load and surface load-memory effect}, year={2019}, howpublished = {journal article}, doi = {https://doi.org/10.1103/PhysRevMaterials.3.066001}, abstract = {Motivated by suggestions that hybrid nanomaterials from nanoporous metal and aqueous electrolyte can be used as actuators, we study the impact of an external load on the actuation behavior of nanoporous gold impregnated with aqueous electrolyte. At no load, we observe the well-documented trend for a more positive electrode potential prompting elongation of the nanoporous body. For purely capacitive electrode processes we confirm that the elastic response to external load is simply superimposed on the potential-induced elongation, so that the strain per electric charge is invariant with the load. The observations so far are consistent with the expectation for surface-stress driven actuation in a linear elastic materials system. Surprisingly, however, actuation in the regime of oxygen electrosorption responds strongly to loading: as the load is increased, the strain per charge gradually drops to zero and even inverts its direction. In other words, the actuator moves backward when asked to do work against a substantial external load. Furthermore, we demonstrate that the length change in response to lifting the oxygen adsorbate layer depends on the load that was present at the instant of oxysorption. This “load memory effect” has analogies to shape-memory behavior in massive alloys. Yet, contrary to shape-memory alloys, the microscopic origin is here a surface phase transition. We argue that the observation is a signature of the reorientation of local surface domains with anisotropic surface stress, and that the required atomic transport process acts only while mobile adatoms are supplied during the deposition or lifting of the oxygen adsorbate layer.}, note = {Online available at: \url{https://doi.org/10.1103/PhysRevMaterials.3.066001} (DOI). Liu, L.; Mameka, N.; Markmann, J.; Jin, H.; Weissmueller, J.: Surface-driven actuation: Sign reversal under load and surface load-memory effect. Physical Review Materials. 2019. vol. 3, no. 6, 066001. DOI: 10.1103/PhysRevMaterials.3.066001}} @misc{gnegel_numerical_investigation_2019, author={Gnegel, S., Li, J., Mameka, N., Huber, N., Düster, A.}, title={Numerical Investigation of Polymer Coated Nanoporous Gold}, year={2019}, howpublished = {journal article}, doi = {https://doi.org/10.3390/ma12132178}, abstract = {Nanoporous metals represent a fascinating class of materials. They consist of a bi-continuous three-dimensional network of randomly intersecting pores and ligaments where the ligaments form the skeleton of the structure. The open-pore structure allows for applying a thin electrolytic coating on the ligaments. In this paper, we will investigate the stiffening effect of a polymer coating numerically. Since the coating adds an additional difficulty for the discretization of the microstructure by finite elements, we apply the finite cell method. This allows for deriving a mesh in a fully automatic fashion from the high resolution 3D voxel model stemming from the 3D focused ion beam-scanning electron microscope tomography data of nanoporous gold. By manipulating the voxel model in a straightforward way, we add a thin polymer layer of homogeneous thickness numerically and study its effect on the macroscopic elastic properties systematically. In order to lower the influence of the boundary conditions on the results, the window method, which is known from homogenization procedures, is applied. In the second part of the paper, we fill the gap between numerical simulations and experimental investigations and determine real material properties of an electrolytic applied polypyrrole coating by inverse computations. The simulations provide an estimate for the mechanical properties of the ligaments and the polymeric coating and are in accordance with experimental data.}, note = {Online available at: \url{https://doi.org/10.3390/ma12132178} (DOI). Gnegel, S.; Li, J.; Mameka, N.; Huber, N.; Düster, A.: Numerical Investigation of Polymer Coated Nanoporous Gold. Materials. 2019. vol. 12, no. 13, 2178. DOI: 10.3390/ma12132178}} @misc{bargmann_generation_of_2018, author={Bargmann, S., Klusemann, B., Markmann, J., Schnabel, J.E., Schneider, K., Soyarslan, C., Wilmers, J.}, title={Generation of 3D representative volume elements for heterogeneous materials: A review}, year={2018}, howpublished = {journal article}, doi = {https://doi.org/10.1016/j.pmatsci.2018.02.003}, abstract = {This work reviews state of the art representative volume element (RVE) generation techniques for heterogeneous materials. To this end, we present a systematic classification considering a wide range of heterogeneous materials of engineering interest. Here, we divide heterogeneous solids into porous and non-porous media, with 0 < void volume fraction < 1 and void volume fraction = 0, respectively. Further subdivisions are realized based on various morphological features. The corresponding generation methods are classified into three categories: (i) experimental methods targeting reconstruction through experimental characterization of the microstructure, (ii) physics based methods targeting simulation of the physical process(es) responsible for the microstructure formation and evolution, and (iii) geometrical methods concentrating solely on mimicking the morphology (ignoring the physical basis of the microstructure formation process). These comprise of various mathematical tools such as digital image correlation, tessellation, random field generation, and differential equation solvers. For completeness, relevant up-to-date software tools, used at various stages of RVE generation – either commercial or open-source – are summarized. Considered methods are reviewed based on their efficiency and predictive performance with respect to geometrical and topological properties of the microstructures.}, note = {Online available at: \url{https://doi.org/10.1016/j.pmatsci.2018.02.003} (DOI). Bargmann, S.; Klusemann, B.; Markmann, J.; Schnabel, J.; Schneider, K.; Soyarslan, C.; Wilmers, J.: Generation of 3D representative volume elements for heterogeneous materials: A review. Progress in Materials Science. 2018. vol. 96, 322-384. DOI: 10.1016/j.pmatsci.2018.02.003}} @misc{jin_mechanical_response_2018, author={Jin, H.-J., Weissmueller, J., Farkas, D.}, title={Mechanical response of nanoporous metals: A story of size, surface stress, and severed struts}, year={2018}, howpublished = {journal article}, doi = {https://doi.org/10.1557/mrs.2017.302}, abstract = {Nanoporous metals made by dealloying are macroscopic network architectures that can contain ∼1015 nanoscale struts or ligaments per sample. Their mechanical performance is critical to their applications as functional or lightweight high-strength materials. Testing nanoporous metals at the macroscopic scale offers opportunities for unraveling the properties of nanoscale solids in general. The central questions in this area include whether the macroscopic strength and elastic modulus of nanoporous metals can be correlated with the properties of nanoscale ligaments by the classical Gibson–Ashby equations, whether the dealloying-made network structure differs from the conventional foam metals, how network connectivity influences mechanical response, and how ligament size and surface properties affect the elastic and plastic response of nanoscale solids and that of nanoporous metals, particularly the tension–compression asymmetry in strength. This article reviews the fundamental observations of the mechanical response of nanoporous metals with a focus on gold and the emerging understanding of the aforementioned issues.}, note = {Online available at: \url{https://doi.org/10.1557/mrs.2017.302} (DOI). Jin, H.; Weissmueller, J.; Farkas, D.: Mechanical response of nanoporous metals: A story of size, surface stress, and severed struts. MRS Bulletin. 2018. vol. 43, no. 1, 35-42. DOI: 10.1557/mrs.2017.302}} @misc{okulov_dealloyingbased_metalpolymer_2018, author={Okulov, A.V., Volegov, A.S., Weissmueller, J., Markmann, J., Okulov, I.V.}, title={Dealloying-based metal-polymer composites for biomedical applications}, year={2018}, howpublished = {journal article}, doi = {https://doi.org/10.1016/j.scriptamat.2017.12.022}, abstract = {Here, we developed interpenetrating-phase metal-polymer composites mimicking mechanical behavior of cortical bone and occupying previously unclaimed region at the Ashby diagram in the area of intermediate strength and low stiffness. The composites consist of dealloying-based open porous TixHf100 − x alloys (scaffolds) impregnated by polymer. The scaffolds significantly contribute to strength (215–266 MPa) and stiffness (15.6–20.8 GPa) of the composites while the polymer phase provides their high strain rate sensitivity (0.037–0.044). Tuning scaffolds' connectivity by preloading and/or their chemical composition allows fine optimization of composites' mechanical properties. The results suggest that the composites may provide a basis for promising future implant materials.}, note = {Online available at: \url{https://doi.org/10.1016/j.scriptamat.2017.12.022} (DOI). Okulov, A.; Volegov, A.; Weissmueller, J.; Markmann, J.; Okulov, I.: Dealloying-based metal-polymer composites for biomedical applications. Scripta Materialia. 2018. vol. 146, 290-294. DOI: 10.1016/j.scriptamat.2017.12.022}} @misc{weissmueller_mechanochemistry_breaks_2018, author={Weissmueller, J.}, title={Mechanochemistry breaks with expectations}, year={2018}, howpublished = {journal article}, doi = {https://doi.org/10.1038/s41929-018-0061-1}, abstract = {Tensile strain of a solid surface can result in either strengthening or weakening of bonds with adsorbates. Adsorption energies of different adsorbate/site combinations may be shifted in different directions — a striking violation of the Brønsted–Evans–Polanyi relation.}, note = {Online available at: \url{https://doi.org/10.1038/s41929-018-0061-1} (DOI). Weissmueller, J.: Mechanochemistry breaks with expectations. Nature Catalysis. 2018. vol. 1, no. 1, 238-239. DOI: 10.1038/s41929-018-0061-1}} @misc{okulov_open_porous_2018, author={Okulov, I.V., Okulov, A.V., Soldatov, I.V., Luthringer, B., Willumeit-Roemer, R., Wada, T., Kato, H., Weissmueller, J., Markmann, J.}, title={Open porous dealloying-based biomaterials as a novel biomaterial platform}, year={2018}, howpublished = {journal article}, doi = {https://doi.org/10.1016/j.msec.2018.03.008}, abstract = {The close match of stiffness between implant material and bone is critically important to avoid stress-shielding effect and ensure a fast healing of injured tissues. Here, we introduce liquid metal dealloying method for synthesis of robust open porous biomaterials possessing low Young's modulus. The remarkable advantage of the liquid metal dealloying method is a large flexibility in selecting chemical composition of a desired porous biomaterial together with unique tunable microstructure. To demonstrate the versatility of the method, a number of open porous TixZr100-x alloys with different chemical compositions and microstructural characteristics was developed by dealloying precursor (TixZr100-x)yCu100-y alloys in liquid magnesium. The effects of the processing conditions and the precursors' chemical composition on the microstructure of the porous TixZr100-x as well as their mechanical behavior were discussed in detail. In particular, the porous TixZr100-x distinguish themselves due to a low and tunable stiffness ranging from 3.2 to 15.1 GPa and a rather high strength reaching up to 480 MPa. This unique combination of mechanical properties of the new open porous TixZr100-x alloys becomes even more interesting in view of preliminary biological tests highlighting their excellent cytocompatibility. Overall, the liquid metal dealloying provides an opportunity for designing a new biomaterials platform with flexible tunable functionality.}, note = {Online available at: \url{https://doi.org/10.1016/j.msec.2018.03.008} (DOI). Okulov, I.; Okulov, A.; Soldatov, I.; Luthringer, B.; Willumeit-Roemer, R.; Wada, T.; Kato, H.; Weissmueller, J.; Markmann, J.: Open porous dealloying-based biomaterials as a novel biomaterial platform. Materials Science and Engineering C. 2018. vol. 88, 95-103. DOI: 10.1016/j.msec.2018.03.008}} @misc{soyarslan_3d_stochastic_2018, author={Soyarslan, C., Bargmann, S., Pradas, M., Weissmueller, J.}, title={3D stochastic bicontinuous microstructures: Generation, topology and elasticity}, year={2018}, howpublished = {journal article}, doi = {https://doi.org/10.1016/j.actamat.2018.01.005}, abstract = {Motivated by recent experimental investigations of the mechanical behavior of nanoporous metal we explore an efficient and robust method for generating 3D representative volume elements (RVEs) with strikingly similar behavior. Our approach adopts Cahn's method of generating a Gaussian random field by taking a superposition of standing sinusoidal waves of fixed wavelength but random in direction and phase. In its theory part, our study describes closed-form expressions for how the solid volume fraction affects the binarization level, mean structure size, specific surface area, averages of mean and Gaussian curvature, and the scaled topological genus. Based on numerical studies we report on criteria for achieving representative realizations of the structure by proper choice of the number of waves and element size. We also show that periodic structures are readily created. We analyze the mechanical properties considering linear and infinitesimal elasticity and evaluate the residual anisotropy (which can be made small) and the effective values of the Young's modulus and Poisson's ratio. The numerical results are in excellent agreement with experimental findings for the variation of stiffness with solid fraction of nanoporous gold made by dealloying. We propose scaling relations that achieve naturally a perfect agreement with the numerical and experimental data. The scaling relation for the stiffness accounts for a percolation-to-cluster transition in the random field microstructure at a finite solid fraction. We propose that this transition is the origin of the previously reported anomalous compliance of nanoporous gold.}, note = {Online available at: \url{https://doi.org/10.1016/j.actamat.2018.01.005} (DOI). Soyarslan, C.; Bargmann, S.; Pradas, M.; Weissmueller, J.: 3D stochastic bicontinuous microstructures: Generation, topology and elasticity. Acta Materialia. 2018. vol. 149, 326-340. DOI: 10.1016/j.actamat.2018.01.005}} @misc{wang_oxygen_adsorption_2018, author={Wang, L.C., Zhong, Y., Widmann, D., Weissmueller, J., Behm, R.J.}, title={Oxygen Adsorption and Low-Temperature CO Oxidation on a Nanoporous Au Catalyst: Reaction Mechanism and Foreign Metal Effects}, year={2018}, howpublished = {journal article}, doi = {https://doi.org/10.1007/s11244-017-0881-2}, abstract = {To further our understanding of the role of trace impurities of the second metal in the catalytic performance of unsupported, nanoporous Au (NPG) catalysts, in particular for the activation of O2, we have prepared a NPG catalyst by electrochemical leaching of Cu from a AuCu alloy and investigated its behavior in the CO oxidation reaction. The structural and chemical properties of the as-prepared catalyst as well as that after reaction for 1000 min were characterized by scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The nature of the surface oxygen species and the oxygen storage capacity were investigated and quantified by multi-pulse experiments in a temporal analysis of products (TAP) reactor. The catalytic behavior in the low-temperature CO oxidation reaction was evaluated both in a TAP reactor under dynamic vacuum conditions and in a conventional micro-reactor under atmospheric pressure. We discuss implications of these results and of similar data obtained previously on a Ag-containing NPG catalyst on the reaction mechanism and on the role of the second metal in the reaction and its impact on the reaction characteristics.}, note = {Online available at: \url{https://doi.org/10.1007/s11244-017-0881-2} (DOI). Wang, L.; Zhong, Y.; Widmann, D.; Weissmueller, J.; Behm, R.: Oxygen Adsorption and Low-Temperature CO Oxidation on a Nanoporous Au Catalyst: Reaction Mechanism and Foreign Metal Effects. Topics in Catalysis. 2018. vol. 61, no. 5-6, 446-461. DOI: 10.1007/s11244-017-0881-2}} @misc{weissmueller_dealloyed_nanoporous_2018, author={Weissmueller, J., Sieradzki, K.}, title={Dealloyed nanoporous materials with interface-controlled behavior}, year={2018}, howpublished = {journal article}, doi = {https://doi.org/10.1557/mrs.2017.299}, abstract = {Dealloying, the selective dissolution of less noble elements from an alloy, enables the preparation of monolithic macroscale bodies, which at the nanostructure level exhibit a network of “ligaments” with a well-defined characteristic size that can be tuned to between a few nanometers and several microns. These porous solids can be made with macroscale dimensions, and, prior to dealloying, can be shaped to form engineered components. Their surface-to-volume ratio is extremely large and their bicontinuous structure provides transport pathways to tune the surface state under control of an electric or chemical potential. These materials present new opportunities for exploring the impact of surfaces on material behaviors and for exploiting surface effects in novel materials design strategies. New experimental approaches unraveling surface effects involving small-scale plasticity and elasticity have been demonstrated. Approaches to new functional materials include electrochemical potential switching of strength, stiffness, fracture resistance, fluid sorption, actuation, and quasi-piezoelectric strain sensing.}, note = {Online available at: \url{https://doi.org/10.1557/mrs.2017.299} (DOI). Weissmueller, J.; Sieradzki, K.: Dealloyed nanoporous materials with interface-controlled behavior. MRS Bulletin. 2018. vol. 43, no. 1, 14-19. DOI: 10.1557/mrs.2017.299}} @misc{michl_electrocapillary_coupling_2018, author={Michl, A., Weissmueller, J., Mueller, S.}, title={Electrocapillary Coupling at Metal Surfaces from First Principles: On the Impact of Excess Charge on Surface Stress and Relaxation}, year={2018}, howpublished = {journal article}, doi = {https://doi.org/10.1021/acs.langmuir.7b04261}, abstract = {We study the response of the surface stress to excess charge via ab initio simulation of metal surfaces in an external electric field. We focus on “simple” sp-bonded metals to gain insight into the mechanisms underlying electrocapillary coupling. Both the direct effect on the surface stress via charging of the bonds and the indirect effect resulting from the charge-induced relaxation are analyzed and discussed in relation to the trends of the coupling coefficients, which—owing to a Maxwell relation—are determined in terms of the response of the work function to strain. Al(111), Mg(0001), and Na(110) are investigated as prototypical sp-bonded metal surfaces with positive, vanishing, and negative coupling parameters, respectively. Mg(0001) and Al(111) exhibit an inward relaxation of the first atomic layer upon negative charging, whereas an outward relaxation occurs for Na(110). The indirect contribution of the relaxation to the coupling coefficient has the same sign as the total response and makes up about 30% of its magnitude for Al(111) and Na(110). Our study highlights that even the response behavior of the so-called simple metals is by no means readily captured within simple models.}, note = {Online available at: \url{https://doi.org/10.1021/acs.langmuir.7b04261} (DOI). Michl, A.; Weissmueller, J.; Mueller, S.: Electrocapillary Coupling at Metal Surfaces from First Principles: On the Impact of Excess Charge on Surface Stress and Relaxation. Langmuir. 2018. vol. 34, no. 16, 4920-4928. DOI: 10.1021/acs.langmuir.7b04261}} @misc{okulov_nanoporous_magnesium_2018, author={Okulov, I.V., Lamaka, S.V., Wada, T., Yubuta, K., Zheludkevich, M.L., Weissmueller, J., Markmann, J., Kato, H.}, title={Nanoporous magnesium}, year={2018}, howpublished = {journal article}, doi = {https://doi.org/10.1007/s12274-018-2167-9}, abstract = {In this study, we present freestanding nanoporous magnesium as a novel lightweight material with high potential for structural and functional applications. Thus far, the high reactivity of Mg with oxygen and aqueous media prevented the fabrication of nanoporous Mg. First, in order to synthesize nanoporous Mg, we fabricated a bicontinuous nanocomposite consisting of interpenetrating Mg and non-Mg phases by liquid metal dealloying. The non-Mg phases in the nanocomposite protect Mg against corrosion. Second, we etched the non-Mg phases from the nanocomposite, leaving nanoporous Mg, using HF solution. This process is advantageous because the nanoporous Mg was passivated by a MgF2 layer during the etching. Our approach is very flexible, and we demonstrate that versatile microstructures of the nanoporous Mg—e.g., nanoscale bicontinuous network, hierarchical, or plate-like structures—can be designed for the given needs. More importantly, these nanoporous Mg samples can readily be exposed to air without being harmed by corrosion.}, note = {Online available at: \url{https://doi.org/10.1007/s12274-018-2167-9} (DOI). Okulov, I.; Lamaka, S.; Wada, T.; Yubuta, K.; Zheludkevich, M.; Weissmueller, J.; Markmann, J.; Kato, H.: Nanoporous magnesium. Nano Research. 2018. vol. 11, no. 12, 6428-6435. DOI: 10.1007/s12274-018-2167-9}} @misc{shi_synthesis_of_2018, author={Shi, S., Markmann, J., Weissmueller, J.}, title={Synthesis of uniform bulk nanoporous palladium with tunable structure}, year={2018}, howpublished = {journal article}, doi = {https://doi.org/10.1016/j.electacta.2018.07.081}, abstract = {This work presents systematic investigations on the synthesis of hierarchical nanoporous Pd via electrochemical dealloying of CuPd alloys in sulfuric acid. The impact of electrode potential, dealloying temperature, and additional annealing on microstructure and morphology is explored. Dealloying Cu85Pd15 in 1M sulfuric acid at elevated temperature provides a facile strategy to produce bulk nanoporous Pd samples which are uniform, hierarchically nanoporous, and free of macro-scale cracks. The question “Why will one-step template-free dealloying yield a hierarchical and not unimodal nanoporous structure?” is discussed. The impact of passivation and of a percolating Cu-rich cluster on the pore structure is inspected. A structural instability concept for dealloying of dilute master alloys is preferred as the underlying mechanism. Nanoporous Pd with classical, unimodal pore structure and tunable ligament size ranging from 80 to 270 nm emerges when the as-prepared hierarchical nanoporous Pd is annealed. The material of this study may provide a model system that complements nanoporous Au for studies of bulk nanoscale metal networks as functional and structural materials.}, note = {Online available at: \url{https://doi.org/10.1016/j.electacta.2018.07.081} (DOI). Shi, S.; Markmann, J.; Weissmueller, J.: Synthesis of uniform bulk nanoporous palladium with tunable structure. Electrochimica Acta. 2018. vol. 285, 60-69. DOI: 10.1016/j.electacta.2018.07.081}} @misc{mameka_tailoring_the_2018, author={Mameka, N., Luehrs, L., Heissler, S., Gliemann, H., Woell, C.}, title={Tailoring the Strength of Nanoporous Gold by Self-Assembled Monolayers of Alkanethiols}, year={2018}, howpublished = {journal article}, doi = {https://doi.org/10.1021/acsanm.8b01368}, abstract = {Because of the large specific surface area, the properties of nanoporous metals and in particular their mechanical properties are sensitive to chemical modifications of their surfaces. Here, we exploit self-assembled monolayers (SAMs) to modify a surface of nanoporous gold and study their effect on plastic behavior. The SAMs investigated here (i) are made from alkanethiols, which consist of a sulfur headgroup that strongly binds to metal substrates, a hydrocarbon chain, and an end group, and (ii) are known to spontaneously self-organize into well-ordered, dense two-dimensional molecular films on the surface of coinage metals. Alkanethiols with various chain lengths and terminal groups were used to prepare SAMs on bulk nanoporous gold, and compression tests were performed on the SAM-modified and nonmodified macroscopic samples. Our experiments reveal a substantial, up to 50%, increase of the flow stress due to thiol adsorption. We attribute the strengthening to the adsorption locking of dislocation end points at the surface, which is mediated by the fairly strong metal–sulfur interaction.}, note = {Online available at: \url{https://doi.org/10.1021/acsanm.8b01368} (DOI). Mameka, N.; Luehrs, L.; Heissler, S.; Gliemann, H.; Woell, C.: Tailoring the Strength of Nanoporous Gold by Self-Assembled Monolayers of Alkanethiols. ACS Applied Nano Materials. 2018. vol. 1, no. 12, 6613-6621. DOI: 10.1021/acsanm.8b01368}} @misc{wang_local_flow_2017, author={Wang, K., Hartig, C., Blankenburg, M., Mueller, M., Guenther, R., Weissmueller, J.}, title={Local flow stresses in interpenetrating-phase composites based on nanoporous gold — In situ diffraction}, year={2017}, howpublished = {journal article}, doi = {https://doi.org/10.1016/j.scriptamat.2016.09.026}, abstract = {We report a synchrotron in situ diffraction experiment exploring stress evolution during compression of interpenetrating-phase nanocomposites based on nanoporous gold and polymer. While previous experiments provided indirect indication of local flow conditions based on macroscopic effective flow stress and micromechanics models such as Gibson-Ashby scaling law, the lattice parameter data of our experiment access the flow stress directly. At small structure size we find excellent agreement with previous reports, supporting the match between material and model of those studies. Yet, deviations at larger structure size suggest that coarsening generates defects in metal network structure that are ignored by standard models.}, note = {Online available at: \url{https://doi.org/10.1016/j.scriptamat.2016.09.026} (DOI). Wang, K.; Hartig, C.; Blankenburg, M.; Mueller, M.; Guenther, R.; Weissmueller, J.: Local flow stresses in interpenetrating-phase composites based on nanoporous gold — In situ diffraction. Scripta Materialia. 2017. vol. 127, 151-155. DOI: 10.1016/j.scriptamat.2016.09.026}} @misc{ngo_on_the_2017, author={Ngo, B.-N.D., Roschning, B., Albe, K., Weissmueller, J., Markmann, J.}, title={On the origin of the anomalous compliance of dealloying-derived nanoporous gold}, year={2017}, howpublished = {journal article}, doi = {https://doi.org/10.1016/j.scriptamat.2016.11.006}, abstract = {The origin of the anomalously compliant behavior of nanoporous gold is studied by comparing the elasticity obtained from molecular dynamics (MD) and finite element method (FEM) simulations. Both models yield a compliance, which is much higher than the predictions of the Gibson-Ashby scaling relation for metal foams and thus confirm the influence of other microstructural features besides the porosity. The linear elastic FEM simulation also yields a substantially stiffer response than the MD simulation, which reveals that nonlinear elastic behavior contributes decisively to the anomalous compliance of nanoporous gold at small structure size.}, note = {Online available at: \url{https://doi.org/10.1016/j.scriptamat.2016.11.006} (DOI). Ngo, B.; Roschning, B.; Albe, K.; Weissmueller, J.; Markmann, J.: On the origin of the anomalous compliance of dealloying-derived nanoporous gold. Scripta Materialia. 2017. vol. 130, 74-77. DOI: 10.1016/j.scriptamat.2016.11.006}} @misc{krekeler_silverrich_clusters_2017, author={Krekeler, T., Strasser, A.V., Graf, M., Wang, K., Hartig, C., Ritter, M., Weissmueller, J.}, title={Silver-rich clusters in nanoporous gold}, year={2017}, howpublished = {journal article}, doi = {https://doi.org/10.1080/21663831.2016.1276485}, abstract = {High-resolution elemental mapping in a transmission electron microscope shows that the residual silver in dealloying-made nanoporous gold (NPG) is aggregated in nanoscale clusters. Kinetic Monte Carlo simulation confirms that these regions are buried relics of the master alloy that have never been exposed to corrosion. The surface of as-dealloyed NPG is covered by at least one atomic monolayer of nearly pure gold. The preferential location of silver in the bulk is relevant when interfaces control the material's function, as in catalysis and sensing. Annealing in air homogenizes the alloy by surface diffusion.}, note = {Online available at: \url{https://doi.org/10.1080/21663831.2016.1276485} (DOI). Krekeler, T.; Strasser, A.; Graf, M.; Wang, K.; Hartig, C.; Ritter, M.; Weissmueller, J.: Silver-rich clusters in nanoporous gold. Materials Research Letters. 2017. vol. 5, no. 5, 314-321. DOI: 10.1080/21663831.2016.1276485}} @misc{okulov_dealloyingbased_interpenetratingphase_2017, author={Okulov, I.V., Weissmueller, J., Markmann, J.}, title={Dealloying-based interpenetrating-phase nanocomposites matching the elastic behavior of human bone}, year={2017}, howpublished = {journal article}, doi = {https://doi.org/10.1038/s41598-017-00048-4}, abstract = {The long-term performance of orthopedic implants depends crucially on a close match between the mechanical behavior of bone and of the implant material. Yet, the present man-made materials with the required biocompatibility and strength are substantially stiffer than bone. This mismatch results in stress shielding, which can lead to the loss of bone mass and may even lead to a revision surgery. Here we report a new materials design strategy towards metal-polymer composites that are based on constituents with established biocompatibility and that can be matched to bone. Ti-based nanoporous alloys, prepared by liquid-metal dealloying, are infiltrated with epoxy to form interpenetrating-phase nanocomposites. At up to 260 MPa, their yield strength is technologically interesting for a deformable light-weight material. More importantly, Young’s modulus can be adjusted between 4.4 and 24 GPa, which affords matching to bone. As another parallel to bone, the strength of the composite materials is strain-rate dependent. These findings suggest that the novel composite materials may provide the basis for promising future implant materials.}, note = {Online available at: \url{https://doi.org/10.1038/s41598-017-00048-4} (DOI). Okulov, I.; Weissmueller, J.; Markmann, J.: Dealloying-based interpenetrating-phase nanocomposites matching the elastic behavior of human bone. Scientific Reports. 2017. vol. 7, 20. DOI: 10.1038/s41598-017-00048-4}} @misc{elsner_surface_excess_2017, author={Elsner, B.A.M., Mueller, S., Bargmann, S., Weissmueller, J.}, title={Surface excess elasticity of gold: Ab initio coefficients and impact on the effective elastic response of nanowires}, year={2017}, howpublished = {journal article}, doi = {https://doi.org/10.1016/j.actamat.2016.10.066}, abstract = {Predicting the influence of the surface on the effective elastic properties of nanoscale structures and nanomaterials remains a challenge, which we here address on both levels, continuum and atomic. Density Functional Theory (DFT) computation at the atomic level yields the first reliable surface excess elastic parameters for the (111) and (001) surfaces of gold. At the continuum level, we derive closed-form expressions for the effective elastic behavior that can be combined with the DFT-derived excess elastic parameters to obtain the effective axial, torsion, and bending stiffness of circular nanowires with surface excess elasticity. The two approaches use different reference frames, and we emphasize the need for consistent stress definitions and for conversion between the separate stress measures when transferring results between the approaches. We present excess elastic parameters separately for Cauchy and 2nd Piola-Kirchhoff stresses, demonstrating that the conversion substantially modifies their numerical value and may even invert their sign. The results afford an assessment of the contribution of the surface excess elastic parameters to the effective elastic response of nanoscale beams or wires. This assessment sheds doubt on earlier suggestions relating experimental observations of an effective stiffening or softening at small size to the excess elasticity of clean surfaces.}, note = {Online available at: \url{https://doi.org/10.1016/j.actamat.2016.10.066} (DOI). Elsner, B.; Mueller, S.; Bargmann, S.; Weissmueller, J.: Surface excess elasticity of gold: Ab initio coefficients and impact on the effective elastic response of nanowires. Acta Materialia. 2017. vol. 124, 468-477. DOI: 10.1016/j.actamat.2016.10.066}} @misc{shi_actuation_by_2017, author={Shi, S., Markmann, J., Weissmueller, J.}, title={Actuation by hydrogen electrosorption in hierarchical nanoporous palladium}, year={2017}, howpublished = {journal article}, doi = {https://doi.org/10.1080/14786435.2017.1311428}, abstract = {We report a strategy for preparing macroscopic samples of nanoporous (np-) Pd by electrochemical dealloying. Starting out with the master alloy , single-step dealloying in 1 M at C provides a hierarchical network structure with two well-defined ligament sizes, 35 and 10 nm. The material is distinguished by its uniform microstructure and its excellent mechanical deformability. Thereby, it may provide an alternative to dealloying-made nanoporous gold as a model system for nanoscale functional materials. Our study exemplifies this by exploring actuation through electrochemically controlled hydrogen sorption. Hydrogen underpotential deposition, bulk sorption isotherms and the concentration strain coefficient are found to agree closely with previous studies of H adsorption on planar surfaces and of hydrogen absorption in bulk, respectively. The actuation strain reaches amplitudes up to 4.0%. Even though each strain cycle brings the np-Pd-H through the phase transformation, the strain amplitude remains stable during much more than 1000 cycles. Furthermore, in view of the macroscopic sample size in all three dimensions, the switching time for actuation is remarkably fast.}, note = {Online available at: \url{https://doi.org/10.1080/14786435.2017.1311428} (DOI). Shi, S.; Markmann, J.; Weissmueller, J.: Actuation by hydrogen electrosorption in hierarchical nanoporous palladium. Philosophical Magazine. 2017. vol. 97, no. 19, 1571-1587. DOI: 10.1080/14786435.2017.1311428}} @misc{graf_xray_studies_2017, author={Graf, M., Ngo, B.-N.D., Weissmueller, J., Markmann, J.}, title={X-Ray Studies of Nanoporous Gold: Powder Diffraction by Large Crystals with Small Holes}, year={2017}, howpublished = {journal article}, abstract = {X-ray diffraction studies of nanoporous gold face the poorly understood diffraction scenario where large coherent crystals are riddled with nanoscale holes. Theoretical considerations derived in this study show that the ligament size of the porous network influences the scattering despite being quasi single crystalline. Virtual diffraction of artificially generated samples confirms the results but also shows a loss of long-range coherency and the appearance of microstrain due to thermal relaxation. Subsequently, a large set of laboratory X-ray investigations of nanoporous gold fabricated by different approaches and synthesis parameters reveal a clear correlation between ligament size and size of the coherent scattering domains as well as extremely high microstrains in samples with ligament sizes below 10 nm.}, note = {Graf, M.; Ngo, B.; Weissmueller, J.; Markmann, J.: X-Ray Studies of Nanoporous Gold: Powder Diffraction by Large Crystals with Small Holes. Arxiv.org, Condensed Matter, Materials Science. 2017. 1708.07789.}} @misc{wang_a_nanoporous_2017, author={Wang, K., Stenner, C., Weissmueller, J.}, title={A nanoporous gold-polypyrrole hybrid nanomaterial for actuation}, year={2017}, howpublished = {journal article}, doi = {https://doi.org/10.1016/j.snb.2017.04.025}, abstract = {We discuss actuation with a hybrid nanomaterial that is made by electro-polymerizing pyrrole on the internal surfaces of dealloying-derived nanoporous gold and then letting aqueous electrolyte be imbibed in the remaining pore space. In this way, active polypyrrole films are contacted by two separate but individually contiguous conduction paths, providing efficient transport of ions in the electrolyte channels and of electrons in the metal skeleton. The metal skeleton also serves to enhance the mechanical behavior of the actuator. Actuation exploits the dimension changes of the polymer when ions are exchanged with the electrolyte in a pseudo-capacitive way, at potentials negative of the classic oxidation/reduction of polypyrrole. Our experiments with millimeter-size bulk samples indicate fast switching and substantially larger strain amplitude than nanoporous metal actuators.}, note = {Online available at: \url{https://doi.org/10.1016/j.snb.2017.04.025} (DOI). Wang, K.; Stenner, C.; Weissmueller, J.: A nanoporous gold-polypyrrole hybrid nanomaterial for actuation. Sensors and Actuators B. 2017. vol. 248, 622-629. DOI: 10.1016/j.snb.2017.04.025}} @misc{mameka_on_the_2017, author={Mameka, N., Markmann, J., Weissmueller, J.}, title={On the impact of capillarity for strength at the nanoscale}, year={2017}, howpublished = {journal article}, doi = {https://doi.org/10.1038/s41467-017-01434-2}, abstract = {The interior of nanoscale crystals experiences stress that compensates for the capillary forces and that can be large, in the order of 1 GPa. Various studies have speculated on whether and how this surface-induced stress affects the stability and plasticity of small crystals. Yet, experiments have so far failed to discriminate between the surface contribution and other, bulk-related size effects. To clarify the issue, here we study the variation of the flow stress of a nanomaterial while distinctly different variations of the two capillary parameters, surface tension, and surface stress, are imposed under control of an applied electric potential. Our theory qualifies the suggested impact of surface stress as not forceful and instead predicts a significant contribution of the surface energy, as measured by the surface tension. The predictions for the combined potential-dependence and size-dependence of the flow stress are quantitatively supported by the experiment. Previous suggestions, favoring the surface stress as the relevant capillary parameter, are not consistent with our experiment.}, note = {Online available at: \url{https://doi.org/10.1038/s41467-017-01434-2} (DOI). Mameka, N.; Markmann, J.; Weissmueller, J.: On the impact of capillarity for strength at the nanoscale. Nature Communications. 2017. vol. 8, 1976. DOI: 10.1038/s41467-017-01434-2}} @misc{graf_nanoporous_gold_2017, author={Graf, M., Roschning, B., Weissmueller, J.}, title={Nanoporous Gold by Alloy Corrosion: Method-Structure-Property Relationships}, year={2017}, howpublished = {journal article}, doi = {https://doi.org/10.1149/2.1681704jes}, abstract = {Nanoporous gold (NPG) made by selective corrosion, or dealloying, serves as a model system for the investigation of electrochemical and mechanical properties of nanomaterials. While various dealloying protocols are in use, it is typically assumed that the structural characteristics are identical and independent of the preparation technique. Yet, reported properties such as strength, Young's modulus, or catalytic behavior can vary widely. Here, we compare the microstructure and the mechanical behavior of NPG structures prepared by three different synthesis protocols reported in the literature. We find that corrosion rates, the content of residual sacrificial metal, the average ligament size and the densification by shrinkage strongly depend on the synthesis protocol and show the consequences on the mechanical properties. We finally deduce different correlations between microstructure and composition for different dealloying routes.}, note = {Online available at: \url{https://doi.org/10.1149/2.1681704jes} (DOI). Graf, M.; Roschning, B.; Weissmueller, J.: Nanoporous Gold by Alloy Corrosion: Method-Structure-Property Relationships. Journal of the Electrochemical Society. 2017. vol. 164, no. 4, C194-C200. DOI: 10.1149/2.1681704jes}} @misc{juarez_nanoporous_metals_2017, author={Juarez, T., Biener, J., Weissmueller, J., Hodge, A.M.}, title={Nanoporous Metals with Structural Hierarchy: A Review}, year={2017}, howpublished = {journal article}, doi = {https://doi.org/10.1002/adem.201700389}, abstract = {Nanoporous (np) metals have generated much interest since they combine several desirable material characteristics, such as high surface area, mechanical size effects, and high conductivity. Most of the research has been focused on np Au due to its relatively straightforward synthesis, chemical stability, and many promising applications in the fields of catalysis and actuation. Other materials, such as np-Cu, Ag, and Pd have also been studied. This review discusses recent advances in the field of np metals, focusing on new research areas that implement and leverage structural hierarchy while using np metals as their base structural constituents. First, we focus on single-element porous metals that are made of np metals at the fundamental level, but synthesized with additional levels of porosity. Second, we discuss the fabrication of composite structures, which use auxiliary materials to enhance the properties of np metals. Important applications of these hierarchical materials, especially in the fields of catalysis and electrochemistry, are also reviewed. Finally, we conclude with a discussion about future opportunities for the advancement and application of np metals.}, note = {Online available at: \url{https://doi.org/10.1002/adem.201700389} (DOI). Juarez, T.; Biener, J.; Weissmueller, J.; Hodge, A.: Nanoporous Metals with Structural Hierarchy: A Review. Advanced Engineering Materials. 2017. vol. 19, no. 12, 1700389. DOI: 10.1002/adem.201700389}} @misc{cheng_semiordered_hierarchical_2017, author={Cheng, C., Luehrs, L., Krekeler, T., Ritter, M., Weissmueller, J.}, title={Semiordered Hierarchical Metallic Network for Fast and Large Charge-Induced Strain}, year={2017}, howpublished = {journal article}, doi = {https://doi.org/10.1021/acs.nanolett.7b01526}, abstract = {Nanoporous metallic actuators for artificial muscle applications are distinguished by combining the low operating voltage, which is otherwise reserved for polymer-based actuators with interesting values of strain amplitude, strength, and stiffness that are comparable of those of piezoceramics. We report a nanoporous metal actuator with enhanced strain amplitude and accelerated switching. Our 3D macroscopic metallic muscle has semiordered and hierarchical nanoporous structure, in which μm-sized tubes align perpendicular with the sample surface, while nm-sized ligaments consist of the tube walls. This nanoarchitecture combines channels for fast ion transportation with large surface area for charge storage and strain generation. The result is a record reversible strain amplitude of 1.59% with a strain rate of 8.83 × 10–6 s–1 in the field of metallic based actuators. A passive hydroxide layer is self-grown on the metal surface, which not only contributes a supercapacitive layer, but also stabilizes the nanoporous structure against coarsening, which guarantees sustainable actuation beyond ten-thousand cycles.}, note = {Online available at: \url{https://doi.org/10.1021/acs.nanolett.7b01526} (DOI). Cheng, C.; Luehrs, L.; Krekeler, T.; Ritter, M.; Weissmueller, J.: Semiordered Hierarchical Metallic Network for Fast and Large Charge-Induced Strain. Nano Letters. 2017. vol. 17, no. 8, 4774-4780. DOI: 10.1021/acs.nanolett.7b01526}} @misc{luehrs_plastic_poissons_2017, author={Luehrs, L., Zandersons, B., Huber, N., Weissmueller, J.}, title={Plastic Poisson’s Ratio of Nanoporous Metals: A Macroscopic Signature of Tension–Compression Asymmetry at the Nanoscale}, year={2017}, howpublished = {journal article}, doi = {https://doi.org/10.1021/acs.nanolett.7b02950}, abstract = {The suggestion, based on atomistic simulation, of a surface-induced tension−compression asymmetry of the strength and flow stress of small metal bodies so far lacks experimental confirmation. Here, we present the missing experimental evidence. We study the transverse plastic flow of nanoporous gold under uniaxial compression. Performing mechanical tests in electrolyte affords control over the surface state. Specifically, the surface tension, γ, can be varied in situ during plastic flow. We find that decreasing γ leads to an increase of the effective macroscopic plastic Poisson ratio, νP. Finite element simulations of a network with surface tension confirm the notion that νP of nanoporous gold provides a signature for a local tension–compression asymmetry of the nanoscale struts that form the network. We show that γ promotes compression while impeding tensile elongation. Because the transverse strain is partly carried by the elongation of ligaments oriented normal to the load axis, the surface-induced tension–compression asymmetry acts to reduce νP. Our experiment confirms a decisive contribution of the surface tension to small-scale plasticity.}, note = {Online available at: \url{https://doi.org/10.1021/acs.nanolett.7b02950} (DOI). Luehrs, L.; Zandersons, B.; Huber, N.; Weissmueller, J.: Plastic Poisson’s Ratio of Nanoporous Metals: A Macroscopic Signature of Tension–Compression Asymmetry at the Nanoscale. Nano Letters. 2017. vol. 17, no. 10, 6258-6266. DOI: 10.1021/acs.nanolett.7b02950}} @misc{graf_electrocatalytic_methanol_2017, author={Graf, M., Haensch, M., Carstens, J., Wittstock, G., Weissmueller, J.}, title={Electrocatalytic methanol oxidation with nanoporous gold: microstructure and selectivity}, year={2017}, howpublished = {journal article}, doi = {https://doi.org/10.1039/c7nr05124g}, abstract = {The properties of Nanoporous Gold (NPG) obtained by the selective dissolution of Ag from an Au–Ag alloy can be tuned by the details of its fabrication, and specifically the residual Ag content is correlated to the ligament size of the material. We link this correlation to methanol electro-oxidation. Specifically, two different NPG types (obtained by potentiostatic dealloying) are compared with one obtained by free corrosion. They show remarkable differences in activity. Quantitative product analysis reveals that NPG shows nearly selective oxidation of CH3OH to HCOO− when NPG is used as an active electrode in contrast to planar Au. This trend can further be enhanced when applying finer nanoporous structures that are linked to a higher Ag content. X-ray photoelectron spectroscopy (XPS) reveals changes in the nature of residual Ag from which we conclude that Ag is not a passive component in the methanol oxidation process.}, note = {Online available at: \url{https://doi.org/10.1039/c7nr05124g} (DOI). Graf, M.; Haensch, M.; Carstens, J.; Wittstock, G.; Weissmueller, J.: Electrocatalytic methanol oxidation with nanoporous gold: microstructure and selectivity. Nanoscale. 2017. vol. 9, no. 45, 17839-17848. DOI: 10.1039/c7nr05124g}} @misc{jalas_electrochemical_tuning_2017, author={Jalas, D., Shao, L.-H., Canchi, R., Okuma, T., Lang, S., Petrov, A., Weissmueller, J., Eich, M.}, title={Electrochemical tuning of the optical properties of nanoporous gold}, year={2017}, howpublished = {journal article}, doi = {https://doi.org/10.1038/srep44139}, abstract = {Using optical in-situ measurements in an electrochemical environment, we study the electrochemical tuning of the transmission spectrum of films from the nanoporous gold (NPG) based optical metamaterial, including the effect of the ligament size. The long wavelength part of the transmission spectrum around 800 nm can be reversibly tuned via the applied electrode potential. The NPG behaves as diluted metal with its transition from dielectric to metallic response shifted to longer wavelengths. We find that the applied potential alters the charge carrier density to a comparable extent as in experiments on gold nanoparticles. However, compared to nanoparticles, a NPG optical metamaterial, due to its connected structure, shows a much stronger and more broadband change in optical transmission for the same change in charge carrier density. We were able to tune the transmission through an only 200 nm thin sample by 30%. In combination with an electrolyte the tunable NPG based optical metamaterial, which employs a very large surface-to-volume ratio is expected to play an important role in sensor applications, for photoelectrochemical water splitting into hydrogen and oxygen and for solar water purification.}, note = {Online available at: \url{https://doi.org/10.1038/srep44139} (DOI). Jalas, D.; Shao, L.; Canchi, R.; Okuma, T.; Lang, S.; Petrov, A.; Weissmueller, J.; Eich, M.: Electrochemical tuning of the optical properties of nanoporous gold. Scientific Reports. 2017. vol. 7, 44139. DOI: 10.1038/srep44139}} @misc{steyskal_electrochemically_tunable_2016, author={Steyskal, E.-M., Qi, Z., Poelt, P., Albu, M., Weissmueller, J., Wuerschum, R.}, title={Electrochemically Tunable Resistance of Nanoporous Platinum Produced by Dealloying}, year={2016}, howpublished = {journal article}, doi = {https://doi.org/10.1021/acs.langmuir.6b01734}, abstract = {The extremely high surface-to-volume ratio of nanoporous platinum (np-Pt) produced by dealloying was applied for tuning electrical resistance by surface charging. In the as-dealloyed state, a characteristic sign-inversion of the charging-induced resistance variation occurs, which can be associated with the electronic structure of PtO. After electrochemical reduction, the relative resistance variations of np-Pt of up to 58% could be generated by electrochemically induced adsorption and desorption, which was 1 order of magnitude larger compared with that of cluster-assembled nanocrystalline Pt. Although the maximum resistance variation was also higher than that of dealloyed nanoporous gold (np-Au), the resistance variation related to the imposed charge was reduced owing to the higher bulk resistance of Pt compared with that of Au. The sign-inversion behavior of the resistance could be recovered by re-oxidation.}, note = {Online available at: \url{https://doi.org/10.1021/acs.langmuir.6b01734} (DOI). Steyskal, E.; Qi, Z.; Poelt, P.; Albu, M.; Weissmueller, J.; Wuerschum, R.: Electrochemically Tunable Resistance of Nanoporous Platinum Produced by Dealloying. Langmuir. 2016. vol. 32, no. 31, 7757-7764. DOI: 10.1021/acs.langmuir.6b01734}} @misc{stenner_piezoelectric_gold_2016, author={Stenner, C., Shao, L.-H., Mameka, N., Weissmueller, J.}, title={Piezoelectric Gold: Strong Charge-Load Response in a Metal-Based Hybrid Nanomaterial}, year={2016}, howpublished = {journal article}, doi = {https://doi.org/10.1002/adfm.201600938}, abstract = {Impregnating the pores of nanoporous gold with aqueous electrolyte yields a hybrid nanomaterial with two separate and interpenetrating charge transport paths, electronic conduction in the metal and ionic conduction in the electrolyte. As the two paths are capacitively connected, space-charge layers along the internal interfaces are coupled to electric potential differences between the paths and can be controlled or detected thereby. The present experiments show that the space charge couples to mechanical deformation of the hybrid material, so that external loading generates an electric current. The electric signal originates from charge displacement along the entire internal interface; the signal is particularly robust since the interface area is large. The charge transfer in response to load constitutes a piezoelectric response, yet the mechanism is quite different to classic piezoelectricity. The analysis in this work predicts links between electromechanical coupling parameters for strain sensing and actuation, which are in excellent agreement with the experiment.}, note = {Online available at: \url{https://doi.org/10.1002/adfm.201600938} (DOI). Stenner, C.; Shao, L.; Mameka, N.; Weissmueller, J.: Piezoelectric Gold: Strong Charge-Load Response in a Metal-Based Hybrid Nanomaterial. Advanced Functional Materials. 2016. vol. 26, no. 28, 5174-5181. DOI: 10.1002/adfm.201600938}} @misc{mameka_nanoporous_gold_2016, author={Mameka, N., Wang, K., Markmann, J., Lilleodden, E.T., Weissmueller, J.}, title={Nanoporous Gold - Testing Macro-scale Samples to Probe Small-scale Mechanical Behavior}, year={2016}, howpublished = {journal article}, doi = {https://doi.org/10.1080/21663831.2015.1094679}, abstract = {Nanoporous gold made by dealloying exemplifies how the exciting mechanical properties of nanoscale objects can be exploited in designing materials from which macroscopic things can be formed. The homogeneous microstructure and the possibility of adjusting the ligament size, L, between few and few hundred nm, along with the high deformability and reproducible mechanical behavior predestine the material for model studies of small-scale plasticity using reliable macroscopic testing schemes on mm- or cm-size samples. Such experiments tend to agree with the Gibson-Ashby scaling relation for strength versus solid fraction, while suggesting an essentially scaling of the local strength of the ligaments. By contrast, the elastic compliance is dramatically enhanced compared to the Gibson-Ashby relation for the stiffness. Contrary to intuition, the anomalously compliant behavior of the nanomaterial goes along with a trend for more stiffness at smaller L. This article discusses surface excess elasticity, nonlinear elastic behavior and specifically shear instability of the bulk, network connectivity, and the surface chemistry as relevant issues which deserve further study.}, note = {Online available at: \url{https://doi.org/10.1080/21663831.2015.1094679} (DOI). Mameka, N.; Wang, K.; Markmann, J.; Lilleodden, E.; Weissmueller, J.: Nanoporous Gold - Testing Macro-scale Samples to Probe Small-scale Mechanical Behavior. Materials Research Letters. 2016. vol. 4, no. 1, 27-36. DOI: 10.1080/21663831.2015.1094679}} @misc{luehrs_elastic_and_2016, author={Luehrs, L., Soyarslan, C., Markmann, J., Bargmann, S., Weissmueller, J.}, title={Elastic and plastic Poisson’s ratios of nanoporous gold}, year={2016}, howpublished = {journal article}, doi = {https://doi.org/10.1016/j.scriptamat.2015.08.002}, abstract = {We explore the elastic and plastic Poisson’s ratios, νEνE and νPνP, of nanoporous gold, using digital image correlation during compression experiments including load/unload segments. The two coefficients differ significantly, with νEνE independent of the ligament size, L , and with a trend for View the MathML sourceνP∝L at not too large L . Disorder in the network of ligaments may explain why νEνE is smaller than predicted by lattice-based models. Finite element simulations, based on the Deshpande–Fleck constitutive law, validate the data analysis. The constitutive law captures work-hardening and transverse flow of nanoporous gold in good agreement with the experiment.}, note = {Online available at: \url{https://doi.org/10.1016/j.scriptamat.2015.08.002} (DOI). Luehrs, L.; Soyarslan, C.; Markmann, J.; Bargmann, S.; Weissmueller, J.: Elastic and plastic Poisson’s ratios of nanoporous gold. Scripta Materialia. 2016. vol. 110, 65-69. DOI: 10.1016/j.scriptamat.2015.08.002}} @misc{qi_porous_gold_2015, author={Qi, Z., Vainio, U., Kornowski, A., Ritter, M., Weller, H., Jin, H., Weissmueller, J.}, title={Porous Gold with a Nested-Network Architecture and Ultrafine Structure}, year={2015}, howpublished = {journal article}, doi = {https://doi.org/10.1002/adfm.201404544}, abstract = {A preparation strategy is developed for monolithic samples of nanoporous gold with a hierarchical structure comprising two nested networks of solid “ligaments” on distinctly different structural length scales. The electrochemical dealloying protocol achieves a large retention of less noble element in a first corrosion step, thereby allowing an extra corrosion step which forms a separate structural hierarchy level. The beneficial impact of adding Pt to the Ag–Au master alloys that are more conventionally used in dealloying approaches to nanoporous gold is demonstrated. At ≈6 nm, the lower hierarchy level ligament size emerges extremely small. Furthermore, Pt favors the retention of Ag during the first dealloying step even when the master alloy has a high Au content. The high Au content reduces the corrosion-induced shrinkage, mitigating crack formation during preparation and favoring the formation of high-quality macroscopic (mm-sized) samples. The corrosion effectively carves out the nanoscale hierarchical ligament structure from the parent crystals tens of micrometers in size. This is revealed by X-ray as well as electron backscatter diffraction, which shows that the porous crystallites inherit the highly ordered, macroscopic crystal lattice structure of the master alloy.}, note = {Online available at: \url{https://doi.org/10.1002/adfm.201404544} (DOI). Qi, Z.; Vainio, U.; Kornowski, A.; Ritter, M.; Weller, H.; Jin, H.; Weissmueller, J.: Porous Gold with a Nested-Network Architecture and Ultrafine Structure. Advanced Functional Materials. 2015. vol. 25, no. 17, 2530-2536. DOI: 10.1002/adfm.201404544}} @misc{ngo_anomalous_compliance_2015, author={Ngo, B.-N.D., Stukowski, A., Mameka, N., Markmann, J., Albe, K., Weissmueller, J.}, title={Anomalous compliance and early yielding of nanoporous gold}, year={2015}, howpublished = {journal article}, doi = {https://doi.org/10.1016/j.actamat.2015.04.021}, abstract = {We present a study of the elastic and plastic behavior of nanoporous gold in compression, focusing on molecular dynamics simulation and inspecting experimental data for verification. Both approaches agree on an anomalously high elastic compliance in the early stages of deformation, along with a quasi immediate onset of plastic yielding even at the smallest load. Already before the first loading, the material undergoes spontaneous plastic deformation under the action of the capillary forces, requiring no external load. Plastic deformation under compressive load is accompanied by dislocation storage and dislocation interaction, along with strong strain hardening. Dislocation-starvation scenarios are not supported by our results. The stiffness increases during deformation, but never approaches the prediction by the relevant Gibson–Ashby scaling law. Microstructural disorder affects the plastic deformation behavior and surface excess elasticity might modify elastic response, yet we relate the anomalous compliance and the immediate yield onset to an atomistic origin: the large surface-induced prestress induces elastic shear that brings some regions in the material close to the shear instability of the generalized stacking fault energy curve. These regions are elastically highly compliant and plastically weak.}, note = {Online available at: \url{https://doi.org/10.1016/j.actamat.2015.04.021} (DOI). Ngo, B.; Stukowski, A.; Mameka, N.; Markmann, J.; Albe, K.; Weissmueller, J.: Anomalous compliance and early yielding of nanoporous gold. Acta Materialia. 2015. vol. 93, 144-155. DOI: 10.1016/j.actamat.2015.04.021}} @misc{deng_electrocapillary_coupling_2015, author={Deng, Q., Gosslar, D.-H., Smetanin, M., Weissmueller, J.}, title={Electrocapillary coupling at rough surfaces}, year={2015}, howpublished = {journal article}, doi = {https://doi.org/10.1039/c5cp00167f}, abstract = {We investigate the impact of the surface roughness on the experimental value of the electrocapillary coupling coefficient, ς. This quantity relates the response of electrode potential, E, to tangential elastic strain, e, and also measures the variation of the surface stress, f, with the superficial charge density, q. We combine experiments measuring the apparent coupling coefficient ςeff for gold thin film electrodes in weakly adsorbing electrolyte with data for the surface roughness determined by atomic force microscopy and by the capacitance ratio method. We find that even moderate roughness has a strong impact on the value of ςeff. Analyzing the mechanics of corrugated surfaces affords a correction scheme yielding values of ς that are invariant with roughness and that agree with expectations for the true coupling coefficient on ideal, planar surfaces. The correction is simple and readily applied to experiments measuring ςeff from surface stress changes in cantilever bending studies or from the potential variation in dynamic electro-chemo-mechanical analysis.}, note = {Online available at: \url{https://doi.org/10.1039/c5cp00167f} (DOI). Deng, Q.; Gosslar, D.; Smetanin, M.; Weissmueller, J.: Electrocapillary coupling at rough surfaces. Physical Chemistry Chemical Physics. 2015. vol. 17, no. 17, 11725-11731. DOI: 10.1039/c5cp00167f}} @misc{wang_nanoporousgoldbased_composites_2015, author={Wang, K., Kobler, A., Kuebel, C., jelitto, H., Schneider, G., Weissmueller, J.}, title={Nanoporous-gold-based composites: toward tensile ductility}, year={2015}, howpublished = {journal article}, doi = {https://doi.org/10.1038/am.2015.58}, abstract = {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.}, note = {Online available at: \url{https://doi.org/10.1038/am.2015.58} (DOI). Wang, K.; Kobler, A.; Kuebel, C.; jelitto, H.; Schneider, G.; Weissmueller, J.: Nanoporous-gold-based composites: toward tensile ductility. NPG Asia Materials. 2015. vol. 7, e187. DOI: 10.1038/am.2015.58}} @misc{kitzler_the_electrochemomechanical_2015, author={Kitzler, T., Maawad, E., Toebbens, D.M., Ziehmer, M., Markmann, J.}, title={The Electro-Chemo-Mechanical Coupling in Lithium Alloy Electrodes and Its Origins}, year={2015}, howpublished = {journal article}, doi = {https://doi.org/10.1149/2.0461514jes}, abstract = {A method to identify and separate the influence of changes in the surface stress from the bulk stress in a model lithium-ion battery electrode during electrochemical cycling was developed. The strategy for this separation is based on the different influence of surface and bulk stresses on the coupling between electrode potential and mechanical strain as measured by dynamic electro-chemo-mechanical analysis and the coupling between the transferred electric charge and the elastic strain as determined by wide angle X-ray scattering. Using both methods, it was possible to uncover the behavior of an apparent surface stress evoked by the bulk stress due to grain boundary alloying of lithium in a gold film. Additionally, the analysis allowed for a determination of a range in surface stress due to underpotential deposition of one monolayer of lithium as the interval between −3.1 to −1.9 N/m.}, note = {Online available at: \url{https://doi.org/10.1149/2.0461514jes} (DOI). Kitzler, T.; Maawad, E.; Toebbens, D.; Ziehmer, M.; Markmann, J.: The Electro-Chemo-Mechanical Coupling in Lithium Alloy Electrodes and Its Origins. Journal of the Electrochemical Society. 2015. vol. 162, no. 14, A2684-A2691. DOI: 10.1149/2.0461514jes}} @misc{deng_less_noble_2015, author={Deng, Q., Gopal, V, Weissmueller, J.}, title={Less Noble or More Noble: How Strain Affects the Binding of Oxygen on Gold}, year={2015}, howpublished = {journal article}, doi = {https://doi.org/10.1002/ange.201504715}, abstract = {Many heterogeneous catalysts exploit strained active layers to modulate reactivity and/or selectivity. It is therefore significant that density functional theory, as well as experimental approaches, find that tensile strain makes the gold surface more binding for oxygen, in other words, less noble. We show that this behavior does not apply when re-structuring of the gold surface is allowed to occur simultaneously with the adsorption of oxygen. In situ cantilever-bending studies show the surface stress to increase when oxygen species adsorb on a (111)-textured gold surface in aqueous H2SO4. This implies a positive sign of the electrocapillary coupling parameter and, hence, a trend for weaker oxygen binding in response to tensile strain. These conflicting findings indicate that different electrosorption processes, and specifically oxygen species adsorption on the bulk-terminated surface, exhibit fundamentally different coupling between the chemistry and the mechanics of the surface.}, note = {Online available at: \url{https://doi.org/10.1002/ange.201504715} (DOI). Deng, Q.; Gopal, V.: Less Noble or More Noble: How Strain Affects the Binding of Oxygen on Gold. Angewandte Chemie. 2015. vol. 127, no. 44, 13173-13177. DOI: 10.1002/ange.201504715}} @misc{mameka_electrical_stiffness_2014, author={Mameka, N., Markmann, J., Jin, H.-J., Weissmueller, J.}, title={Electrical stiffness modulation—confirming the impact of surface excess elasticity on the mechanics of nanomaterials}, year={2014}, howpublished = {journal article}, doi = {https://doi.org/10.1016/j.actamat.2014.04.067}, abstract = {Local variations in the stiffness at surfaces may affect the elastic response of nanostructures, yet experiments disagree on the magnitude and even sign of the surface excess elastic constants. The present study reports the variation in the effective macroscopic stiffness of bulk samples of nanoporous gold when the surface state is modulated under potential control in an electrochemical environment. Using in situ experiments in a dynamic mechanical analyzer to measure the storage and loss moduli, we show that adsorption of ⩽1⩽1 atomic monolayer of oxygen species as well as a capacitively controlled excess of electrons at the surface stiffen the material, while oxygen desorption/electron depletion enhance the compliance. Relative changes in the effective stiffness of up to 8% imply the variation of a surface excess elastic constant of the order of 60 N m−1, much larger than the absolute value of that constant deduced from previous atomistic simulation studies of clean surfaces. Since the electrode potential affects exclusively the surface, our observations provide conclusive evidence for the impact of local stiffness variation at surfaces on the effective elastic response of nanostructures.}, note = {Online available at: \url{https://doi.org/10.1016/j.actamat.2014.04.067} (DOI). Mameka, N.; Markmann, J.; Jin, H.; Weissmueller, J.: Electrical stiffness modulation—confirming the impact of surface excess elasticity on the mechanics of nanomaterials. Acta Materialia. 2014. vol. 76, 272-280. DOI: 10.1016/j.actamat.2014.04.067}} @misc{zhong_crack_mitigation_2014, author={Zhong, Y., Markmann, J., Jin, H.-J., Ivanisenko, Y., Kurmanaeva, L., Weissmueller, J.}, title={Crack Mitigation during Dealloying of Au25Cu75}, year={2014}, howpublished = {journal article}, doi = {https://doi.org/10.1002/adem.201300211}, abstract = {The suggested use of nanoporous gold for functional and structural applications requires uniform and specifically crack-free monolithic bodies, ideally with a structure size in the range of 10 nm or below. Here we investigate electrochemical dealloying of two different starting alloys, Au25Ag75 and Au25Cu75, as pathways towards that goal. With an emphasis on the processes that lead to crack formation, we discuss the role of the parameters (i) lattice parameter change, (ii) dealloying potential and rate, and (iii) thermo-mechanical treatment of the master alloys. The Cu-based alloys are found to give superior homogeneity at very small structure sizes, provided that intermediate temperature treatments are avoided. A complete suppression of crack formation was achieved by application of a dealloying potential of 1.1 V versus Ag/AgCl, at a ligament size of 11 nm.}, note = {Online available at: \url{https://doi.org/10.1002/adem.201300211} (DOI). Zhong, Y.; Markmann, J.; Jin, H.; Ivanisenko, Y.; Kurmanaeva, L.; Weissmueller, J.: Crack Mitigation during Dealloying of Au25Cu75. Advanced Engineering Materials. 2014. vol. 16, no. 4, 389-398. DOI: 10.1002/adem.201300211}} @misc{deng_mechanical_modulation_2014, author={Deng, Q., Smetanin, M., Weissmueller, J.}, title={Mechanical modulation of reaction rates in electrocatalysis}, year={2014}, howpublished = {journal article}, doi = {https://doi.org/10.1016/j.jcat.2013.10.008}, abstract = {Many modern catalyst materials exploit a strained surface layer as the active component. Here, we explore how the catalytic activity is affected by changes in the lattice parameter, focusing on the hydrogen evolution reaction on Au and Pt electrodes in H2SO4 as a model process. We present a lock-in technique that allows the modulation of the reaction current to be followed in situ while a small cyclic elastic strain is imposed on the electrode material. We find that tensile strain enhances the exchange current density and the reactivity at low overpotential, ΔE, whereas the trend is inverted and the reactivity diminished at higher ΔE. We introduce kinetic rate equations for Heyrowsky and Tafel kinetics, allowing for strain dependence of the hydrogen adsorption enthalpy as well as the activation enthalpy. The results link the reactivity modulation to electrocapillary coupling coefficients that are open to investigation by experiment or ab initio computation. The inversion in sign of the coupling as the function of ΔE emerges in agreement with experiment.}, note = {Online available at: \url{https://doi.org/10.1016/j.jcat.2013.10.008} (DOI). Deng, Q.; Smetanin, M.; Weissmueller, J.: Mechanical modulation of reaction rates in electrocatalysis. Journal of Catalysis. 2014. vol. 309, 351-361. DOI: 10.1016/j.jcat.2013.10.008}} @misc{huber_scaling_laws_2014, author={Huber, N., Viswanath, R.N., Mameka, N., Markmann, J., Weissmueller, J.}, title={Scaling laws of nanoporous metals under uniaxial compression}, year={2014}, howpublished = {journal article}, doi = {https://doi.org/10.1016/j.actamat.2013.12.003}, abstract = {This study is motivated by discrepancies between recent experimental compression test data of nanoporus gold and the scaling laws for strength and elasticity by Gibson and Ashby. We present a systematic theoretical investigation of the relationship between microstructure and macroscopic behaviour of nanoporous metals. The microstructure is modelled by four-coordinated spherical nodes interconnected by cylindrical struts. The node positions are randomly displaced from the lattice points of a diamond lattice. We report scaling laws for Young’s modulus and yield strength, which depend on the extension of nodal connections between the ligaments and the solid fraction. A comparison with the scaling laws of Gibson and Ashby revealed a significant deviation for the yield stress. The model was applied for identifying a continuum constitutive law for the solid fraction. Matching the model’s predicted macroscopic stress–strain behaviour to experimental data for the flow stress at large compression strain requires the incorporation of work hardening in the constitutive law. Furthermore, the amount of disorder of the node positions is decisive in matching the model results to the experimental observations of an anomalously low stiffness and an almost complete lack of transverse plastic strain.}, note = {Online available at: \url{https://doi.org/10.1016/j.actamat.2013.12.003} (DOI). Huber, N.; Viswanath, R.; Mameka, N.; Markmann, J.; Weissmueller, J.: Scaling laws of nanoporous metals under uniaxial compression. Acta Materialia. 2014. vol. 67, 252-265. DOI: 10.1016/j.actamat.2013.12.003}} @misc{xue_switchable_imbibition_2014, author={Xue, Y., Markmann, J., Duan, H., Weissmueller, J., Huber, P.}, title={Switchable imbibition in nanoporous gold}, year={2014}, howpublished = {journal article}, doi = {https://doi.org/10.1038/ncomms5237}, abstract = {Spontaneous imbibition enables the elegant propelling of nano-flows because of the dominance of capillarity at small length scales. The imbibition kinetics are, however, solely determined by the static host geometry, the capillarity, and the fluidity of the imbibed liquid. This makes active control particularly challenging. Here we show for aqueous electrolyte imbibition in nanoporous gold that the fluid flow can be reversibly switched on and off through electric potential control of the solid–liquid interfacial tension, that is, we can accelerate the imbibition front, stop it, and have it proceed at will. Simultaneous measurements of the mass flux and the electrical current allow us to document simple scaling laws for the imbibition kinetics, and to explore the charge transport in the metallic nanopores. Our findings demonstrate that the high electric conductivity along with the pathways for fluid/ionic transport render nanoporous gold a versatile, accurately controllable electrocapillary pump and flow sensor for minute amounts of liquids with exceptionally low operating voltages.}, note = {Online available at: \url{https://doi.org/10.1038/ncomms5237} (DOI). Xue, Y.; Markmann, J.; Duan, H.; Weissmueller, J.; Huber, P.: Switchable imbibition in nanoporous gold. Nature Communications. 2014. vol. 5, 4237. DOI: 10.1038/ncomms5237}} @misc{hoppe_abinitio_modeling_2014, author={Hoppe, S., Michl, A., Weissmueller, J., Mueller, S.}, title={Ab-initio modeling of electromechanical coupling at Si surfaces}, year={2014}, howpublished = {journal article}, doi = {https://doi.org/10.1063/1.4893375}, abstract = {The electromechanical coupling at the silicon (100) and (111) surfaces was studied via density functional theory by calculating the response of the ionization potential and the electron affinity to different types of strain. We find a branched strain response of those two quantities with different coupling coefficients for negative and positive strain values. This can be attributed to the reduced crystal symmetry due to anisotropic strain, which partially lifts the degeneracy of the valence and conduction bands. Only the Si(111) electron affinity exhibits a monotonously linear strain response, as the conduction band valleys remain degenerate under strain. The strain response of the surface dipole is linear and seems to be dominated by volume changes. Our results may help to understand the mechanisms behind electromechanical coupling at an atomic level in greater detail and for different electronic and atomic structures.}, note = {Online available at: \url{https://doi.org/10.1063/1.4893375} (DOI). Hoppe, S.; Michl, A.; Weissmueller, J.; Mueller, S.: Ab-initio modeling of electromechanical coupling at Si surfaces. Journal of Applied Physics. 2014. vol. 116, no. 7, 073507. DOI: 10.1063/1.4893375}} @misc{roy_wrinkling_of_2014, author={Roy, A., Kundu, S., Mueller, K., Rosenauer, A., Singh, S., Pant, P., Gururajan, M.P., Kumar, P., Weissmueller, J., Singh, A.K., Ravishankar, N.}, title={Wrinkling of Atomic Planes in Ultrathin Au Nanowires}, year={2014}, howpublished = {journal article}, doi = {https://doi.org/10.1021/nl502259w}, abstract = {A detailed understanding of structure and stability of nanowires is critical for applications. Atomic resolution imaging of ultrathin single crystalline Au nanowires using aberration-corrected microscopy reveals an intriguing relaxation whereby the atoms in the close-packed atomic planes normal to the growth direction are displaced in the axial direction leading to wrinkling of the (111) atomic plane normal to the wire axis. First-principles calculations of the structure of such nanowires confirm this wrinkling phenomenon, whereby the close-packed planes relax to form saddle-like surfaces. Molecular dynamics studies of wires with varying diameters and different bounding surfaces point to the key role of surface stress on the relaxation process. Using continuum mechanics arguments, we show that the wrinkling arises due to anisotropy in the surface stresses and in the elastic response, along with the divergence of surface-induced bulk stress near the edges of a faceted structure. The observations provide new understanding on the equilibrium structure of nanoscale systems and could have important implications for applications in sensing and actuation.}, note = {Online available at: \url{https://doi.org/10.1021/nl502259w} (DOI). Roy, A.; Kundu, S.; Mueller, K.; Rosenauer, A.; Singh, S.; Pant, P.; Gururajan, M.; Kumar, P.; Weissmueller, J.; Singh, A.; Ravishankar, N.: Wrinkling of Atomic Planes in Ultrathin Au Nanowires. Nano Letters. 2014. vol. 14, no. 8, 4859-4866. DOI: 10.1021/nl502259w}} @misc{deng_electrocapillary_coupling_2014, author={Deng, Q., Weissmueller, J.}, title={Electrocapillary Coupling during Electrosorption}, year={2014}, howpublished = {journal article}, doi = {https://doi.org/10.1021/la501353g}, abstract = {The electrocapillary coupling coefficient, ς, measures the response of the electrode potential, E, to tangential elastic strain at the surface of an electrode. Using dynamic electro-chemo-mechanical analysis, we study ς(E) simultaneously with cyclic voltammetry. The study covers extended potential intervals on Au, Pt, and Pd, including the electrosorption of oxygen species and of hydrogen. The magnitude and sign of ς vary during the scans, and quite generally the graphs of ς(E) emphasize details which are less obvious or missing in the cyclic voltammograms (CVs). Capacitive processes on the clean electrode surfaces exhibit ς < 0, whereas capacitive processes on oxygen-covered surfaces are characterized by ς < 0 on Au but ς > 0 on Pt and Pd. The findings of ς < 0 during the initial stages of oxygen species adsorption and ς > 0 for hydrogen electrosorption agree with the trend that tensile strain makes surfaces more binding for adsorbates. However, the large hysteresis of oxygen electrosorption on all electrodes raises the question: is the exchange current associated with that process sufficient for its measurement by potential response during small cyclic strain?}, note = {Online available at: \url{https://doi.org/10.1021/la501353g} (DOI). Deng, Q.; Weissmueller, J.: Electrocapillary Coupling during Electrosorption. Langmuir. 2014. vol. 30, no. 34, 10522-10530. DOI: 10.1021/la501353g}} @misc{viswanath_electrocapillary_coupling_2013, author={Viswanath, R.N., Weissmueller, J.}, title={Electrocapillary coupling coefficients for hydrogen electrosorption on palladium}, year={2013}, howpublished = {journal article}, doi = {https://doi.org/10.1016/j.actamat.2013.07.013}, abstract = {The surface stress f, a capillary force at solid surfaces, has important implications for the behavior of nanomaterials. Surface stress is known to vary considerably when atoms adsorb on the surface, yet the underlying mechanisms are poorly understood. Our in situ dilatometry study of H adsorption on porous nanocrystalline Pd provides quantitative data for the response of f to changes in the adsorbate coverage. The porous body is immersed in aqueous electrolyte and H adsorption is controlled and measured electrochemically. The surface stress response is quantified by means of the electrocapillary coupling parameter, ς, defined as the derivative of f with respect to the superficial charge density. The results support previous, more indirect, findings for ς. We show that ς is precisely predicted by a model based on the continuum mechanics of superficial layers containing misfitting solute.}, note = {Online available at: \url{https://doi.org/10.1016/j.actamat.2013.07.013} (DOI). Viswanath, R.; Weissmueller, J.: Electrocapillary coupling coefficients for hydrogen electrosorption on palladium. Acta Materialia. 2013. vol. 61, no. 16, 6301-6309. DOI: 10.1016/j.actamat.2013.07.013}} @misc{michl_signinverted_response_2013, author={Michl, A., Weissmueller, J., Mueller, S.}, title={Sign-inverted response of aluminum work function to tangential strain}, year={2013}, howpublished = {journal article}, doi = {https://doi.org/10.1088/0953-8984/25/44/445012}, abstract = {We have investigated the response of the work function, W, of low-index aluminum surfaces to tangential strain by using first-principles calculations based on density functional theory. This response parameter is a central quantity in electrocapillary coupling of metal electrodes relating to the performance of porous metal actuators and surface stress based sensing devices. We find that Al surfaces exhibit a positive response for all orientations considered. By contrast, previous studies reported negative-valued response parameters for clean surfaces of several transition metals. We discuss separately the response of W to different types of strain and the impact of the strain on the Fermi energy and the surface dipole. We argue that the reason for the abnormal positive sign of the Al response parameter lies in its high valence electron density.}, note = {Online available at: \url{https://doi.org/10.1088/0953-8984/25/44/445012} (DOI). Michl, A.; Weissmueller, J.; Mueller, S.: Sign-inverted response of aluminum work function to tangential strain. Journal of Physics: Condensed Matter. 2013. vol. 25, no. 44, 445012. DOI: 10.1088/0953-8984/25/44/445012}} @misc{qi_hierarchical_nestednetwork_2013, author={Qi, Z., Weissmueller, J.}, title={Hierarchical Nested-Network Nanostructure by Dealloying}, year={2013}, howpublished = {journal article}, doi = {https://doi.org/10.1021/nn4021345}, abstract = {Applications of porous microstructures in functional materials often impose conflicting requirements on the pore size, which may be met by hierarchical structures that combine porosity on distinctly different length scales. Here we report an electrochemical dealloying strategy that yields bulk samples of porous gold with a hierarchical microstructure. A nanoscale network of solid ligaments forms the lower hierarchy level, which is nested within the geometrically similar, but much larger, network of the upper hierarchy level. Starting from a dilute solid solution of Au in Ag, controlled electrochemical corrosion yields nanoporous Ag–Au alloy as an intermediate product. Coarsening of the porous alloy creates the large ligaments of the upper hierarchy level. Those are then again dealloyed, which creates the fine ligaments of the lower hierarchy level. We show that the material exhibits enhanced charge transport kinetics while maintaining a large specific surface area.}, note = {Online available at: \url{https://doi.org/10.1021/nn4021345} (DOI). Qi, Z.; Weissmueller, J.: Hierarchical Nested-Network Nanostructure by Dealloying. ACS Nano. 2013. vol. 7, no. 7, 5948-5954. DOI: 10.1021/nn4021345}} @misc{wang_catalytic_activity_2013, author={Wang, L.-C., Zhong, Y., Jin, H., Widmann, D., Weissmueller, J., Behm, R.J.}, title={Catalytic activity of nanostructured Au: Scale effects versus bimetallic/bifunctional effects in low-temperature CO oxidation on nanoporous Au}, year={2013}, howpublished = {journal article}, doi = {https://doi.org/10.3762/bjnano.4.13}, abstract = {The catalytic properties of nanostructured Au and their physical origin were investigated by using the low-temperature CO oxidation as a test reaction. In order to distinguish between structural effects (structure–activity correlations) and bimetallic/bifunctional effects, unsupported nanoporous gold (NPG) samples prepared from different Au alloys (AuAg, AuCu) by selective leaching of a less noble metal (Ag, Cu) were employed, whose structure (surface area, ligament size) as well as their residual amount of the second metal were systematically varied by applying different potentials for dealloying. The structural and chemical properties before and after 1000 min reaction were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The catalytic behavior was evaluated by kinetic measurements in a conventional microreactor and by dynamic measurements in a temporal analysis of products (TAP) reactor. The data reveal a clear influence of the surface contents of residual Ag and Cu species on both O2 activation and catalytic activity, while correlations between activity and structural parameters such as surface area or ligament/crystallite size are less evident. Consequences for the mechanistic understanding and the role of the nanostructure in these NPG catalysts are discussed.}, note = {Online available at: \url{https://doi.org/10.3762/bjnano.4.13} (DOI). Wang, L.; Zhong, Y.; Jin, H.; Widmann, D.; Weissmueller, J.; Behm, R.: Catalytic activity of nanostructured Au: Scale effects versus bimetallic/bifunctional effects in low-temperature CO oxidation on nanoporous Au. Beilstein Journal of Nanotechnology. 2013. vol. 4, 111-128. DOI: 10.3762/bjnano.4.13}} @misc{ziehmer_examination_of_2013, author={Ziehmer, M., Tschoepe, A., Birringer, R., Markmann, J.}, title={Examination of the Energy Phase Space of Mixed Copper Grain Boundaries by Orientation Imaging Microscopy (OIM) and Sphere-on-a-Plate Method}, year={2013}, howpublished = {journal article}, doi = {https://doi.org/10.1016/j.actamat.2013.05.042}, abstract = {This article reports on an experimental study of the energy phase space of mixed copper grain boundaries by a combination of electron backscatter diffraction (EBSD) and the so-called sphere-on-a-plate method. Single crystal copper spheres with diameters of a few microns were sintered onto flat single crystal {1 1 1} copper plates, resulting in random initial grain boundary configurations. EBSD measurements together with an assumption about the grain boundary plane orientation were used for the determination of the five macroscopic degrees of freedom of the grain boundaries. The tilt and twist components of the grain boundaries were calculated making use of the interface plane scheme representation of grain boundaries. Upon annealing, the spheres rotated along gradients in the grain boundary energy phase space. Thus, points of the trajectories of single spheres could be recorded between the single annealing steps, allowing for tracing the path of single spheres towards and into energy minima regions. The results gathered from 13 spheres underline a strong complexity of the grain boundary energy phase space.}, note = {Online available at: \url{https://doi.org/10.1016/j.actamat.2013.05.042} (DOI). Ziehmer, M.; Tschoepe, A.; Birringer, R.; Markmann, J.: Examination of the Energy Phase Space of Mixed Copper Grain Boundaries by Orientation Imaging Microscopy (OIM) and Sphere-on-a-Plate Method. Acta Materialia. 2013. vol. 61, no. 15, 5586-5594. DOI: 10.1016/j.actamat.2013.05.042}} @misc{wang_composites_of_2013, author={Wang, K., Weissmueller, J.}, title={Composites of Nanoporous Gold and Polymer}, year={2013}, howpublished = {journal article}, doi = {https://doi.org/10.1002/adma.201203740}, abstract = {novel materials design strategy that exploits the trend of “smaller is stronger” in metal nanostructures by incorporating them into a bulk composite material.}, note = {Online available at: \url{https://doi.org/10.1002/adma.201203740} (DOI). Wang, K.; Weissmueller, J.: Composites of Nanoporous Gold and Polymer. Advanced Materials. 2013. vol. 25, no. 9, 1280-1284. DOI: 10.1002/adma.2012037