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Nanoporous gold with a hierarchical structure has prospects as an advanced functional material with enhanced mechanical properties, but how the hierarchical structure affects its mechanical properties compared to a unimodal structure has not been revealed. Here, we investigate the mechanical behavior of hierarchically-structured nanoporous gold and unimodally-structured nanoporous gold with the same relative density by micropillar compressive tests in dry and electrolyte environment. The ligament size at the upper-level structure in hierarchically-structured nanoporous gold and the ligament size in unimodally-structured nanoporous gold are kept similar, while having hierarchically-structured samples with ligament sizes of 10 to 50 nm at lower-level structure. We find that hierarchically-structured nanoporous gold shows greater compressive strength and pronounced stress-variation by oxidization of the surface compared to unimodally-structured nanoporous gold. A ligament-size dependency on the lower-level structure in hierarchical samples is observed, with compressive strength and stress variation by surface oxidation increasing as the lower-level ligament size decreases. Three-dimensionally reconstructed structure analysis suggests that the enhanced mechanical properties of hierarchically-structured nanoporous gold are attributed to the better-connected network of ligaments originating from two separated dealloying-coarsening procedures. The influence of dislocation activities depending on characteristic sizes is also discussed to elucidate the distinguished mechanical behavior. [Display omitted] [ABSTRACT FROM AUTHOR]

Tensile and compressive tests are carried out on four nanoporous gold samples with ligament sizes of 56, 149, 402, and 868 nm, all without grain boundaries and with similar number of ligaments regardless of ligament size. We find that tensile yield strengths are greater than compressive yield strengths up to ligament size of 402 nm, while yield strengths are similar in tension and compression for d L of 868 nm. The dependence of tensile yield strength on ligament size is described well using a size-effect exponent -1.18 for four samples, whereas bulk-like behavior, i.e. no ligament-size dependency, is observed for ligament size greater than 402 nm in compression; compressive yield strength depending on ligament size is described by a size-effect exponent -1.49 for ligament sizes ranging from 56 nm to 402 nm. The asymmetries in yield strength and plasticity between tension and compression are discussed in terms of the deformation behavior of individual ligaments based on a suggested open-cell model and dislocation activities in the ligaments. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Enhancing tensile strength is crucial to increasing the applicability of nanoporous materials including nanoporous gold (np-Au) that show mechanical weakness because of their nanoporous structure despite other superior characteristics. We fabricated twinned and textured np-Au foils with an average twin spacing of 7.9 nm. The foils exhibit an ultimate tensile strength (UTS) of 87.5 MPa when the loading axis is normal to the twin boundaries. This UTS value is approximately three times greater than that for np-Au with rare twins of 27.4 MPa. The high UTS can be ascribed to the twin boundaries acting as effective barriers to dislocation slip, resulting in the strain-hardening of the load-bearing ligaments. [ABSTRACT FROM AUTHOR]

Nanoporous gold (np-Au) samples of ligament sizes 68.6, 248.6, 462.9, and 710.9 nm are prepared by heat treatment, and representative volumes are reconstructed by focused ion beam (FIB) tomography. The increase in relative density by thermal coarsening is not pronounced. We analyze the distribution of ligament size, surface-to-volume ratio, and scaled connectivity density for coarsened np-Au, revealing that np-Au coarsens in a self-similar way. The measured activation energy for thermal coarsening supports that it is accomplished by surface diffusion of Au. [ABSTRACT FROM AUTHOR]

Enhanced resistance to crack propagation in nanoporous gold (np-Au) with fine grains is observed comparing with conventional np-Au with coarse grains. Cyclic hot-rolling is adopted to prepare fine grain structure in Au-Ag precursor alloys, which evolve to fine grained np-Au with grain size of 3.69 µm after dealloying. Np-Au with fine grains exhibits stable crack growth with enhanced stress intensity for crack propagation K Jc of 0.133 MPa∙m1/2 while np-Au with coarse grains shows linear elastic fracture behavior with stress intensity factor for crack initiation K Ic of 0.042 MPa∙m1/2. Enhanced crack resistance in fine grained np-Au is achieved with intergranular fracture, which is attributed to the effective strain energy release through grain boundary plasticity. [Display omitted] [ABSTRACT FROM AUTHOR]

• Spherical nanoindentation creep tests were performed on nanoporous gold samples with ligament sizes of 30, 103, 417, and 986 nm. • Ligament-size dependent creep was observed in terms of total creep strain and creep rate for ligament sizes ranging from 103 nm to 986 nm while the greatest creep is shown for ligament size of 30 nm. • By coating a thin amorphous alumina layer on nanoporous gold, creep increased in the range of ligament sizes from 103 nm to 986 nm while creep decreased for ligament size of 30 nm. • The primary creep mechanism was regarded as the dislocation-mediated creep for all ligament sizes, but the governing dislocation source was different stochastically; dislocation was dominantly carried by dislocation nucleation at the surface for ligament size of 30 nm, while it was gradually controlled by initial dislocation sources as increasing ligament size. Spherical nanoindentation creep tests are conducted on four nanoporous gold samples with ligament sizes of 30, 103, 417, and 986 nm. We find the ligament-size dependent creep, increase of total creep strain and creep strain rate with increasing ligament size, in the range of ligament sizes from 103 nm to 986 nm while the most pronounced creep is shown for ligament size of 30 nm. By coating a 2-nm-thick alumina layer on the ligaments, creep increased for ligament sizes ranging from 103 nm to 986 nm, while it decreased for ligament size of 30 nm. The ligament-size dependent creep and effect of surface coating are discussed with dominant dislocation mechanism in the ligaments under the dislocation-mediated creep for each ligament size. [Display omitted] [ABSTRACT FROM AUTHOR]

Thermomechanical reliability remains challenging in through-silicon via (TSV) manufacture, a key technology in three-dimensional packaging of integrated circuits. A primary issue in reliability is the residual stress created during manufacture and operation by mismatch in thermal expansion coefficients of Cu, the TSV filling material, with surrounding materials. Nanoindentation is suggested as a tool to measure the distribution of residual stress in the amorphous top layer near Cu TSVs. Formation of Cu TSV generates the tensile residual stress in the vicinity of the TSVs, and the maximum residual stress in top SiO 2 insulating layer is 322 MPa. The residual stress increases as 652 MPa by post-heat treatments, and increases as 390 MPa for higer current density of TSVs electroplating. [ABSTRACT FROM AUTHOR]

The ease of Te sublimation from Bi 2 Te 3 -based alloys significantly deteriorates thermoelectric and mechanical properties via the formation of voids. We propose a novel strategy based on atomic layer deposition (ALD) to improve the thermal stability of Bi 2 Te 3 -based alloys via the encapsulation of grains with a ZnO layer. Only a few cycles of ZnO ALD over the Bi 2 Te 2.7 Se 0.3 powders resulted in significant suppression of the generation of pores in Bi 2 Te 2.7 Se 0.3 extrudates and increased the density even after post-annealing at 500 °C. This is attributed to the suppression of Te sublimation from the extrudates. The ALD coating also enhanced grain refinement in Bi 2 Te 2.7 Se 0.3 extrudates. Consequently, their mechanical properties were significantly improved by the encapsulation approach. Furthermore, the ALD approach yields a substantial improvement in the figure-of-merit after the post-annealing. Therefore, we believe the proposed approach using ALD will be useful for enhancing the mechanical properties of Bi 2 Te 3 -based alloys without sacrificing thermoelectric performance. [ABSTRACT FROM AUTHOR]

The coarsening of the two-phase, bi-continuous microstructure via surface diffusion is simulated using a three-dimensional Monte Carlo (MC) model. The coarsening exponent obtained from simulations initially decreases and then saturates at ~4 as the temperature increases. Kinetic analysis on nanoporous gold (np-Au) samples, annealed at 450 and 600 °C, confirms the prediction of the model. The coarsening probability ratio of np-Au between two annealing temperatures, calculated from the reported activation energy for coarsening of np-Au, is comparable to that of the model, signifying that the simulation successfully mimics the kinetic evolution of the thermally coarsened np-Au via surface diffusion. Unlabelled Image [ABSTRACT FROM AUTHOR]

Annealed, prestrained, and ball-milled nanoporous gold (np-Au) samples were prepared. Since the microstructures of the precursor alloys, such as the crystallographic orientation and grain size, were mostly preserved during the dealloying process, prestrained np-Au is believed to have higher initial dislocation density, and ball-milled np-Au is believed to have higher densities of initial dislocation and grain boundary comparing to annealed np-Au. The time-dependent deformation behavior of np-Au samples with various microstructures was characterized with two parameters; creep strain exponent n and activation volume V ∗ using spherical nanoindentation creep tests. We found that primary mechanism of time-dependent plasticity for annealed and prestrained np-Au samples is dislocation slip and that for ball-milled np-Au sample is grain boundary sliding. In dislocation slip-dominant time-dependent deformation in np-Au, a higher initial dislocation density lowers n and V ∗ . In grain boundary sliding-dominant time-dependent deformation in np-Au, the values of n and V ∗ are similar to those for dislocation slip-dominant time-dependent deformation; however, the creep strain rate in quasi-steady-state is higher than that for dislocation slip-dominant time-dependent deformation. [ABSTRACT FROM AUTHOR]

Stretch-flangeability as a parameter of formability is measured on dual-phase (DP) and complex-phase (CP) steels by the hole-expansion ratio (HER), and nanoindentation is introduced to assess the hardness of constituent phases before and after HER testing. Hole-expansion ratios of two dual-phase (DP1, DP2) and one complex-phase (CP1) steels are measured as 51%, 126% and 136%, respectively. The primary site of void formation is found to be the interfacial boundary for DP1, the ferrite phase close to the martensite phase where numerous geometrically necessary dislocations (GNDs) are formed for DP2, and the martensite phase for CP1. The hardness ratio of the hard to soft phase is a key indicator of formability, and this introduces the stress concentration from strain disparity at the interfacial boundaries between the hard and soft phases. Here, it is founded that strain-hardenability of constituent phases depends on the hardness of the GND layer, and the strain disparity under deformation is determined by the GND layer hardness as well as the hardness ratio of the hard to soft phase. This study suggests the GND layer as a stress-dispersion layer and the hardness of GND layers in soft phases as a critical role in formability. [ABSTRACT FROM AUTHOR]

Based on electron backscatter diffraction (EBSD) and energy dispersive spectroscopy (EDS), open-cell Ni foams fabricated by electroplating on a removable template were investigated. Hollow struts or cavities inside the Ni foams were observed, implying traces of a removable template. The crystallographic orientations and grain structures of the Ni foam struts were also examined. Individual grains of Ni foams covered the struts completely through the thickness direction. The crystallographic orientations of the Ni foam struts display a random distribution. Σ 3 twinning boundaries were also observed in the Ni matrix. During a two-hour heat treatment at 1273 K under atmospheric condition, refined NiO layers clearly formed on the surface of the Ni matrix. The overall grain sizes of the NiO are smaller than those of the Ni matrix. The obvious crystallographic relationship between the Ni matrix and the NiO layers was observed at the high misorientation angles. Ni foams with high porosity revealed a stress drop and NiO layers that formed during atmospheric annealing caused a predominant stress drop. Young's modulus and the hardness of the Ni matrix and NiO layers were measured from a Ni foam cell structure consisting of nodes, by means of nano-indentation tests. [ABSTRACT FROM AUTHOR]