On the impact of capillarity for strength at the nanoscale

The interior of nanoscale crystals experiences stress that compensates 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. In order to clarify the issue, 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 $\textit{surface stress}$ as not forceful and instead predicts a significant contribution of the surface energy, as measured by the $\textit{surface tension}$. The predictions for the combined potential- 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 the experiment

Anomalous compliance and early yielding of nanoporous gold

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

Surface-controlled mechanical properties of bulk nanoporous gold

In dieser Arbeit wird ein intelligentes Hybridmaterial auf Basis von nanoporösen Gold in einem wässrigen Elektrolyt vorgestellt, dessen Steifigkeit und Festigkeit reversibel durch externe Signale eingestellt werden können. Die Durchstimmbarkeit dieser effektiven mechanischen Eigenschaften wird über das Anlegen von elektrischen Potentialen an der großen Gold-Elektrolyt Grenzfläche innerhalb des Werkstoffes realisiert. Auf diese Art lässt sich der Oberflächenzustand durch elektrische Polarisation und Ionensadsorption kontrollieren. Die Verformungsexperimente wurden in situ in einer elektrochemischen Zelle, eingebaut in einen dynamisch-mechanischen Analysator und in einer mechanischen Prüfmaschine, durchgeführt. Die Ergebnisse der in situ Experimente zeigen, dass die Oberflächenexzesselastizität die effektive elastischen Antwort von Nanostrukturen beeinflusst während auf die Plastizität die Oberflächenspannung wesentlich einwirkt.This study highlights a smart hybrid material based on nanoporous gold in an aqueous electrolyte whose stiffness and strength are reversibly tuned by external stimuli. The tunability of these effective mechanical properties is realized via application of electric potentials to the large gold-electrolyte interface of the material, and consequent control of the surface state through electrical polarization and adsorption. The experiments are implemented in situ under environmental control in a dynamic mechanical analyzer and a mechanical testing device. The results of the in situ experiments provide conclusive evidence for the impact of surface excess elasticity on the effective elastic response of nanostructures and support a hypothesis that surface tension is a dominant factor in controlling their plasticity

Piezoelectric gold : strong charge-load response in a metal-based hybrid nanomaterial

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.DF

Nanoporous gold : testing macro-scale samples to probe small-scale mechanical behavior

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 L−1 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

Scaling laws of nanoporous metals under uniaxial compression

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

Numerical Investigation of Polymer Coated Nanoporous Gold

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

Numerical investigation of polymer coated nanoporous gold

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.Deutsche Forschungsgemeinschaft (DFG