Jürgen Markmann

Microstructure evolution during rolling of inert-gas condensed palladium

During cold-rolling of nanocrystalline Pd consolidated from clusters we observed a strong increase in stacking fault density, conclusive evidence for lattice dislocation activity. However, the absence of texture and the retention of an equiaxed grain shape even after large deformation suggested grain boundary sliding and grain rotation as concurring processes. The rate of tensile creep at 313 K and at low stress is in agreement with predictions for Coble creep.

Deformation twinning in nanocrystalline Pd

Defect structures of plastically deformed nanocrystalline Pd investigated by high-resolution transmission electron microscopy are presented. Material with an average grain size of about 15 nm was prepared by inert-gas condensation, and this was plastically deformed by cold rolling up to a true strain of 0.32 at a strain rate of about 0.3 s−1. Abundant deformation twinning on {111} planes was found and Shockley partial dislocations identified. Remarkably, in each grain, twinning occurs only on a single set of parallel planes. This implies that only one out of the five independent slip systems required for the general deformation of a grain is active, a finding which suggests that grain rotation and grain-boundary sliding must be active together with twinning.

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.

Switchable imbibition in nanoporous gold

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.

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 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.

Low temperature processing of dense nanocrystalline yttrium-doped cerium oxide ceramics

Nanocrystalline cerium oxide ceramics of high homogeneity and nearly full density were prepared. The starting material was synthesized by the direct homogeneous precipitation method using hexamethylenetetramine (HMT). Two alternative routes for consolidation into green bodies were employed: (i) centrifugal casting of an electrostatically stabilized colloidal solution and (ii) cold isostatic pressing of dried ceria powder. The green bodies were sintered at temperatures between 700 and 1000°C. The sinter activities of both nanocrystalline materials were strongly enhanced if compared to microcrystalline ceria. Green bodies, which were generated by colloidal processing exhibited the highest sinter activities, associated with a unique pore structure. The effect of yttrium doping on the grain size after sintering at high temperatures was also investigated. The combination of yttrium doping and colloidal processing allowed for the synthesis of dense nanocrystalline cerium oxide ceramics by pressureless sintering.

On the origin of the anomalous compliance of dealloying-derived nanoporous gold

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.

Actuation by hydrogen electrosorption in hierarchical nanoporous palladium

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.

Microstrain in nanocrystalline solids under load by virtual diffraction

We propose a method for computing virtual X-ray diffractograms for nanocystalline materials under uniaxial load based on molecular-dynamics simulation data. While the increase in diffraction microstrain during deformation is generally taken as evidence for the generation of lattice dislocations, our virtual diffraction data show extra microstrain even at small load, before the onset of lattice dislocation activity. We show that the microstrain data have a natural explanation in the elastic response of a heterogeneous medium. The results imply that the conclusions of previous experimental in situ diffraction data for nanocystalline metals may deserve a critical examination.

On the impact of capillarity for strength at the nanoscale

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.Nanoscale crystals can experience lattice instability and a tension-compression asymmetry of their strength. Here, Mameka and colleagues scrutinize the proposition that these phenomena arise from surface-induced stress, finding that the surface tension contributes significantly, while the surface stress does not measurably impact the nanomechanical behaviour.