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Ionic transport in electrolyte-imbibing nanopores is considered a common bottleneck in the functional applications of bi-continuous nanoporous (NP) metals, which in turn offer a unique opportunity to understand structure-transport relationships at nanoscales. By dealloying an Ag-Au alloy and isothermal coarsening, we can control the pore size of NP Au in the range of 13 nm to 2.4 µm and the porosity between 38 % and 69 %. By reduction-induced decomposition of AgCl, we can further control the structural hierarchy and the pore orientation of NP Ag. In these NP metals, we measure the effective conductivities of 1 M NaClO 4 to range from 7 % to 44 % of that of a free solution. The tortuosity of NP Au displays weak dependences on both the pore size and the porosity, consistent with the observed self-similarity in coarsening, except for those of pores narrower than 25 nm, which we consider deviating from the well-coarsened pore geometry. For NP Ag, rapid transport is observed for the hierarchical and the oriented structures; the former can be explained with the Maxwell-Garnett equation and the latter underlines random orientations as the common cause of slow transport. We then demonstrate the practical significance of the structure-transport relationship in the application of NP Ag in CO 2 reduction. [Display omitted] [ABSTRACT FROM AUTHOR]

Liquid metal dealloying is a promising technique to elaborate porous metallic materials with potential applications for catalysis and energy storage. It consists in the selective dissolution of an element out of a precursor alloy by immersing it in a liquid bath. To model this process, we introduce a one-dimensional diffusion model that relies on the thermodynamic properties of the ternary system, and on a maximum velocity criterion of the reaction front. The model is compared to experimental measurements conducted on NiCu samples dealloyed in liquid Ag. Regarding the kinetics, the model enables to reach experimental time-scales, replicate experimental trends, and reveal the influence of the ligament structure on the dealloying rate. Also, by assuming thermodynamic equilibria between solid and liquid phases in the dealloyed region, the microstructure gradient obtained numerically is in satisfactory agreement with experimental measurements. These numerical results are obtained without adjustable parameters, which opens the possibility to predict dealloyed microstructures as function of the process parameters (precursor and bath composition, dealloying time and temperature, etc.). [Display omitted] [ABSTRACT FROM AUTHOR]

The strengthening effect of interface in materials is usually studied by monitoring the change of strength in response to varying structure sizes, such as the varying grain or phase sizes. In this study, we demonstrate that the strength of a nanostructured material can be recoverably tuned by modifying interfacial structure without changing structure size. Specifically, we studied the Cu/Ag spinodoid alloys fabricated by dealloying and electrochemical deposition, which exhibit a cube-on-cube orientation relationship between two interpenetrating nanophases. This material remains stable against coarsening even at temperatures near the eutectic melting point, due to the presence of low-energy semi-coherent interfaces and the lack of defects such as grain boundaries. Using cyclic thermal annealing, we are able to alter the solid solubility of both nanophases and, consequently, the lattice mismatch between them, resulting in a recoverable tuning of hardness and strength. The amplitude of hardness modulation decreases with increasing feature size (λ), and changes sign when λ exceeds ∼ 500 nm. This is attributed to a competition between two strengthening mechanisms: the interface-induced strengthening that is more prominent at a lower λ , and the solid solution hardening that is largely λ -insensitive. The finding indicates that interface hardening prevails over solution hardening when λ is below ∼ 500 nm. Current study paves the way for the development of strong and stable nanostructured materials whose properties can be optimized by independently tailoring feature sizes and interfacial structures. [Display omitted] [ABSTRACT FROM AUTHOR]

Nanoporous metals (np-M) have emerged as promising materials owing to their high surface area-to-volume ratio and electrical/thermal conductivity. There exists a group of processing methodologies by which np-M are formed through a top-down nanostructure evolution driven by the selective removal of a sacrificial component, all of which are a variation of dealloying. Nanoporosity evolution through current dealloying methodologies, however, is governed by strict requirements including sufficient separation in "reactivity" of the participating components and a homogeneous solid solution precursor alloy. This limits the viable alloy systems that may be used and the range of np-M's that may be formed. Here, we report thermal decomposition of crystalline transition metal dichalcogenides (TMDs) as a new processing methodology for np-M formation, adding to the spectrum of dealloying protocols. We demonstrate application of this process to the formation of a broader class of np-M including W, Re, Mo, and Ta with feature sizes below 100 nm. The presented facile thermal treatment of TMDs offers a new methodology for the evolution of nanoporosity in a broad range of metals. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Dealloying is a selective corrosion process during which the more electrochemically active elements in a parent alloy dissolve while the remaining more noble atoms diffuse along the solid-electrolyte interface, resulting in the formation of a nanoporous microstructure composed of core-shell ligaments. In this work, we study an extension of the physics of dealloying in which the parent alloys (Cu Fe Mn alloys) contained an engineered composition of elements selected such that, upon dissolution of one of them, the remaining elements phase separate as they become concentrated in the undissolved ligaments. Such simultaneous dealloying and phase separation leads to the formation of rich variety of new porous structures containing nanocomposite ligaments, as well as unanticipated structure evolution such as nanowire growth that are attributed to solution side effects. Image 1 [ABSTRACT FROM AUTHOR]

Abstract This paper reports the critical role of microstructural length scale in dealloying for the fabrication of bimodal or monolithic functional nanoporous metal structures and the underlying mechanisms and kinetics. Two dual-phased (Al 2 Cu AlCu) precursor alloys Al65Cu35 and Al55Cu45 (at.%) were selected to demonstrate the concept. Microstructural observations revealed that the two constituent phases Al 2 Cu and AlCu in each alloy can undergo either sequential dealloying, which leads to bimodal nanoporous Cu, or simultaneous dealloying , which results in monolithic nanoporous Cu. In-situ and ex-situ synchrotron X-ray diffraction (XRD), focused ion beam scanning electron microscopy (FIB-SEM), and potentiodynamic polarization scans were used to identify the detailed phase evolution processes and kinetics. It is concluded that microstructural length scale plays a decisive role in regulating the dealloying pathways. Sequential dealloying of Al 2 Cu and AlCu occurs when both phases are micrometer-scaled, while simultaneous dealloying takes over when both phases are nanoscaled. The nanosize effect of Al 2 Cu and AlCu can override their intrinsic difference in electrochemical potential at the micro- or macro-scale, and the advantage of tetragonal Al 2 Cu over monoclinic AlCu in crystallographic transition to face-centered-cubic (FCC) Cu by dealloying. The high-resolution in-situ synchrotron XRD data revealed a two-stage kinetic process for dealloying of Al 2 Cu to Cu. The Avrami-Erofe′ev kinetic model provides an excellent description of each stage. The underlying rationales and implications are discussed. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract Dealloyed nanoporous metals are emerging as a new class of structural and functional materials for a wide range of applications. Nevertheless, the dealloying process usually leads to significant volume shrinkage, large internal stresses, surface oxidation, stress-corrosion cracking and thereby extensive fabrication flaws. This is particularly serious for nanoporous non-noble metals because of their high affinity with oxygen and resulting serious oxidation. Therefore, dealloyed nanoporous metals usually have poor mechanical performances, particularly, under tension which is highly sensitive to flaws. Consequently, high tensile strength has not been achieved from technically-important and economic nanoporous transition metals, such as Ni and Cu. In this study we report flaw-free nanoporous Ni fabricated by utilizing an ultrafine grained precursor alloy, high-temperature dealloying and post-dealloying annealing. The resulting nanoporous Ni shows an excellent tensile strength, which is one order of magnitude higher than all reported tensile strengths of dealloyed nanoporous metals. The strong and ductile nanoporous Ni developed in this study can be scaled up for large-scale structural and functional applications where tensile properties are required. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract A major challenge in the synthesis of high surface area metals via subtractive processes such as dealloying is maintaining the mechanical integrity of the resulting porous materials. This problem is especially apparent in liquid metal dealloying, in which high-temperature selective dissolution in a molten metal bath leads to bicontinuous porosity formation. In liquid metal dealloying of polycrystalline alloys, grain boundary separation leads to the detachment of individual grains. In this work, we show that addition of small amounts of silicon to Nb Ti or Ta Ti parent alloys leads to the generation of self-assembled arrays of intermetallic (niobium silicide or tantalum silicide) plates that are structurally merged with the usual bicontinuous porosity seen in dealloying. These silicide plates pass through grain boundaries and hold the niobium or tantalum network intact without strongly affecting the microstructural evolution during dealloying. Our approach yields a mechanically robust porous metal-intermetallic composite, which can be further processed to form tertiary materials via re-impregnation by a new third phase. The materials design strategy introduced here can be generalized to serve as a platform to form dense multiphase nanocomposites. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract Nanoporous gold (NPG), formed by the chemical or electrochemical dealloying of binary or ternary solid solutions, has strong prospects as a catalyst, sensor substrate, membrane, actuator, and other applications. In view of the wide range of thermochemical conditions expected, understanding the evolution of NPG during thermal coarsening is necessary to assess and improve the prospects for its functionality. The addition of Pt to NPG (NPG-Pt) was shown in recent years to have an effect of inhibition of coarsening, both during dealloying and during oxidizing post-treatment. In this study, the direct observation of the coarsening of nanoligaments by simultaneous transmitted electron (TE) and secondary electron (SE) imaging reveals the complex nature of the coarsening process. As a result of adding Pt, alterations in the volume fraction of ligaments are shown to have a strong impact on the operating coarsening mechanism and kinetics in a low-pressure hydrogen environment. Pt was also shown to increase the temperature for the onset of thermal coarsening. The important roles of surface diffusion and the possibility of coalescence-controlled coarsening kinetics are discussed in detail. Reproducible observations of ligament collapse and void annihilation serve to expand the current understanding of thermal coarsening of NPG. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Selective dissolution (dealloying) of more reactive elements from a solid solution alloy can result in the formation of a nanoporous surface layer enriched in the more noble elements. For example, in Alloy 800 (wt.% Fe-32Ni-21Cr), a remnant nanoporous Ni-rich film is formed following the selective dissolution of Fe and Cr in boiling 50 wt.% NaOH caustic solutions (135–140 °C). This surface film has been proposed to act as a precursor to stress corrosion cracking (SCC) by a cleavage mechanism. This study investigates the effect of crystallographic orientation on the characteristics of a dealloyed layer (i.e., thickness and size of ligaments/pores) formed on Alloy 800. Electron backscattered diffraction (EBSD) is used to correlate grain orientation with the geometry and morphology of the dealloyed layer formed in the boiling caustic environment. Results indicate that the dealloyed layer formed on grains with planes close to the { 111 } family on the sample surface, has a finer geometry (i.e., a thinner layer compared with other grain orientations and consisting of smaller diameter ligaments). Further SCC experiments on Alloy 800 specimens revealed a lower density of cracks initiated from surface planes that were close to { 111 } , confirming a link between dealloyed layer geometry, texture, and SCC. [Display omitted] [ABSTRACT FROM AUTHOR]

Most experimental studies on synthesis of nanoporous structures via selective dissolution of electroactive species in metal-alloy nanomaterials report the conditions and the time required to achieve nanoporosity. Depending on the dissolution conditions the timescales of nanoporosity evolution can vary over several orders from sub-second timescales to days, and as such evolution for each condition may appear unrelated. We show that the extent of dissolution (EOD), i.e., the fraction of electroactive atoms dissolved, and not time, is an appropriate descriptor for nanoporosity evolution. Using kinetic Monte Carlo simulations we demonstrate that morphological features obtained over a wide range of temperature, alloy composition, material interactions, etchant concentration and overpotential values are nearly identical for a given EOD. Timescales for different conditions are related by a scaling parameter. Dissolution behavior observed with one experiment can be used to predict results for another experiment, e.g., we relate linear sweep and potential step to constant potential experiments on the basis of EOD. This understanding may be useful for predicting morphology for slow etching/corrosion/leaching conditions from accelerated dissolution experiments as well as design of nanoporous materials. [ABSTRACT FROM AUTHOR]

[Display omitted] Dealloying has emerged as a new route to the synthesis of porous or nanoporous materials for a variety of novel structural/functional applications. However, dealloyed (nano-)porous metals are notoriously brittle and often fracture catastrophically in bending or tension, which is detrimental to their applications. In this paper, we report that under tension, the irreversible fracture strain (ε i) of a macroscopic porous Fe(MnCr) alloy prepared by liquid metal dealloying can be enhanced from nearly zero to 4.1 % by introducing weak domain boundaries, while previous samples with homogeneous (nano-)porous structure usually fracture at ε i below 1.0 %. Tensile ductility is enhanced only if the mean domain size (D ¯) is not too much larger than mean ligament diameter (L ¯), evidenced by the dramatic increase in ε i as D ¯ / L ¯ decreases from ∼ 24 to ∼ 7. The "ductile" deformation of these materials is associated with the diffuse failure under tension, arising from the promoted nucleation of micro-cracks and the crack deflection or branching along weak domain boundaries at lower D ¯ / L ¯. This strategy might also apply to other (nano-)porous materials self-organized in dealloying or fabricated by other methods. [ABSTRACT FROM AUTHOR]

De-alloying, i.e. selective dissolution of alloys, is currently studied to produce nanoporous gold items suited for use in catalysis, electrochemical applications, sensors and actuators. Both crystalline and amorphous alloys can be selectively etched. In the former, less noble atoms are removed from surface terraces of grains layer by layer, while noble ones form mounds. These evolve by undercutting and electrolyte percolation to form a ligament network. The mechanism of ligament development by de-alloying amorphous alloys is unknown. Here we show that for de-alloying a Au-based glass, in this case Au 40 Cu 28 Ag 7 Pd 5 Si 20 , percolation of the electrolyte through cracks of the native surface oxide initiates the formation of protuberances which are soon undercut. An interlayer develops, where Au crystals germinate, grow to nanometer size by diffusion and impinge. This is how ligaments start to coarsen. The interlayer is found at all stages between coarsened ligaments and amorphous phase. The ligaments are defective polycrystals, as opposed to single crystals obtained from crystalline alloys. [ABSTRACT FROM AUTHOR]

Liquid metal dealloying (LMD) has recently emerged as a novel technique to fabricate bulk nanostructures using a bottom-up self-organization method, but the literature lacks fundamental studies of this kinetic process. In this work, we conduct an in-depth study of the kinetics and fundamental microstructure evolution mechanisms during LMD using Ti Ta alloys immersed in molten Cu as a model system. We develop a model of LMD kinetics based on a quantitative characterization of the effects of key parameters in our system including alloy composition, dealloying duration, and dealloying temperature. This work demonstrates that the dealloying interface is at or near equilibrium during LMD, and that the rate-limiting step is the liquid-state diffusion of dissolving atoms away from the dealloying interface (diffusion-limited kinetics). The quantitative comparison between theoretically predicted and measured dealloying rates further reveals that convective transport and rejection of the dissolving element during coarsening of the structure also influence the dealloying kinetics. [ABSTRACT FROM AUTHOR]

Nanoporous Cu foams having various pores and strut size were fabricated by chemical dealloying process with Al-Cu precursors. Microstructural evolution of three-dimensional configuration was investigated by electron microscopes aided focused ion beam (FIB) technique. The specific surface area and strength of the Cu foams increased with decreasing pore and strut sizes. The measured strength and specific surface areas were correlated with the Exner-Pohl model by modifying the parameters using the microstructural observation. The modified model demonstrated the mechanical behaviors of nanoporous Cu foams accounted for the strut size and the deformation-twinning model. [ABSTRACT FROM AUTHOR]

In this study, nanoporous silver with tunable porosity was fabricated by chemically dealloying Ag-based metallic glasses in HCl solution. By systematically investigating the dynamic change in pore characteristics of the nanoporous silver with dealloying condition, the correlation between ligament size and dealloying parameter was successfully established. It was also found that the evolution behavior of the ligament size can be well described by a diffusion based growth kinetic model. In addition, our study of the capacitive performance of the nanoporous silver shows that these newly developed materials are promising for applications as electrode/substrate materials in electrochemical supercapacitors. [ABSTRACT FROM AUTHOR]

The effects and mechanisms of dealloying Zn-diffused Fe–Mn alloys over a wide range of processing parameters have been investigated using a four-step surface modification technique. The dealloying process was applied to construct tailorable, nanoporous Fe–Mn alloys for the development of functional, bioresorbable implants. The results showed that nanoscale features can be deliberately designed by selectively leaching out the less noble metals from the outer surface layer to create an extensive variety of porous topographies. It was found that initial microstructure, Zn diffusion time, etching rate, annealing temperature and time, and the type of etching media all play a role in customizing the resulting dealloyed morphology. The resulting nano-sized structures have the potential to enhance initial cell attachment for future bioactive, resorbable materials. [ABSTRACT FROM AUTHOR]

[Display omitted] This study explores microstructure and phase decomposition in nanoporous Au-Pt made by dealloying. The starting alloy, Ag 75 Au 12.5 Pt 12.5 , forms a uniform solid solution. Removing the Ag by dealloying at room temperature forms nanoporous equimolar Au-Pt with a ligament size as small as 4 nm. That alloy's composition is in the Au-Pt phase diagram's regime of spinodal instability. Surprisingly, in view of the instability and of the substantial atomic rearrangements accompanying dealloying, X-ray diffraction and transmission electron microscopy reveal a homogeneous single-phase state in the bulk of the nanoporous material. This can be traced back to enrichment of Pt at the surface, which depletes the bulk in Pt. The Pt-depleted bulk is in the metastable region of the alloy phase diagram, between the binodal and the spinodal. Annealing prompts curvature-driven coarsening by diffusion, diminishing the number of sites for Pt surface segregation. The ensuing enrichment of the bulk in Pt is accompanied by the formation first of crystallographically coherent, Pt-rich regions and later of semi-coherent regions of the Pt-rich phase. The morphology of the coherent regions is compatible with spinodal decomposition. Yet, the microstructure evolution pathway is nonstandard, since decomposition here concurs with rapid coarsening of the porous microstructure. Among the prospect of the nanoporous alloy are its high kinetic stability and the opportunity to tune the surface composition through the annealing temperature. [ABSTRACT FROM AUTHOR]

The transition behavior from a Nb–Ni precursor to porous Nb and the coarsening of ligaments by a dealloying reaction in a Mg melt are investigated. Based on these results, the kinetics of the reaction are discussed. When a Nb 25 Ni 75 (at.%) disk was immersed in a Mg melt, the Ni content decreased the most at the surface of the disk and less so towards the interior of the disk. In this disk, it was found that transition layers of body-centered cubic-Nb and Ni 6 Nb 7 formed. These transition layers grew, following a parabolic law, and the activation energy of such growth was close to values reported for the diffusion of solute atoms in liquids, suggesting that diffusion of Ni in the Mg melt was the rate-controlling process of the dealloying reaction. The ligament size depended on time and temperature, following a power law with an exponent of 4, suggesting that surface diffusion was a key part of coarsening. These results agree well with those reported for conventional dealloying in an aqueous solution. Based on the kinetics analyses, the dealloying conditions were optimized to maximize the specific surface area, allowing a Nb electrolytic capacitor to be produced with a maximum mass-specific capacitance of 650,000 μFV g −1 , about three times larger than the highest value previously reported. [ABSTRACT FROM AUTHOR]

[Display omitted] Dealloyed nanoporous (NP) metals are expected to be stronger by more than one order of magnitude than conventional porous metals of same porosity. However, the strengths of random-structured NP metals in practice are limited by the low mechanical efficiency of their network structure. In this paper, we report that an anisotropic nanoporous gold (NPG) with unidirectional pore channels can also be self-organized in dealloying. This material combines a mechanically efficient topology structure with ultra-strong nanoscale solid skeleton. In consequence, its out-of-plane mean strength (461 ± 52 MPa) is more than twice that of the strongest NPGs reported in previous studies. Because of the structural anisotropy, unidirectional NPGs deform by the formation and expansion of kink bands under compression, unlike the uniform densification of most previous NPGs. The pore-channel misalignment and other defects such as the voids in pore walls may have reduced macroscopic strength, which accounts for a large scatter in the measured strength and a gap between theoretical and experimental data. We anticipate that the structure and mechanical properties of unidirectional NPGs will be further improved in the future by optimizing dealloying parameters. A combination of high strength, unidirectional pore channels, and excellent thermal and electrical conductivity of metallic pore-wall phase might enable novel applications of this material in many areas. [ABSTRACT FROM AUTHOR]

Abstract: Nanoporous metals made by dealloying possess significant geometric complexity—they are random, bicontinuous structures that also possess bubbles within ligaments, regions of very high negative, positive, and saddlepoint curvature, and significant polyfaceting. Here we introduce methods to geometrically quantify the structure of nanoporous metals employing simulated model nanoporous metals generated via large-scale kinetic Monte Carlo simulations as the basis of discussion. A method is introduced to transform these simulated structures into smooth triangulated meshes using new mesh-smoothing algorithms that hybridize mean curvature flow and signal processing approaches to mesh fairing. The technique is assessed by comparing the exact genus of high-genus meshes with the genus calculated via the Gauss–Bonnet formula, and works well to find the local curvature at all points of simulated surfaces of high topological genus. Specific geometric quantification of nanoporous metals is discussed for two quantities: (i) the relative surface area fraction of different crystal facets, which is important for catalysis; and (ii) the curvature distribution on the surface of porous metals, important for applications in which high curvature features are active (e.g. optical sensing). [Copyright &y& Elsevier]

Abstract: Nanoporous metals prepared by alloy corrosion may assume the form of monolithic, millimeter-sized bodies containing around 1015 nanoscale ligaments per cubic millimeter. Here, we report on the fabrication and mechanical behavior of macroscopic, crack-free nanoporous gold samples which exhibit excellent ductility in compression tests. Their yield stress is significantly lower than that expected based on scaling laws or on previous nanoindentation experiments. Electron backscatter diffraction imaging reveals a polycrystalline microstructure with grains larger than 10μm which acquire a subdomain structure during plastic flow, but remain otherwise intact. We highlight the action of lattice dislocations which can travel over distances much larger than the ligament size. This results in a collective deformation of the many ligaments in each grain. Remarkably, the dislocation cores are partly located in the pore channels. The results suggest a critical view of the conversion between indentation hardness and yield stress in previous work. [Copyright &y& Elsevier]

[Display omitted] 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 character 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. [ABSTRACT FROM AUTHOR]

Abstract: Freestanding nanoporous gold beams fracture during dealloying owing to volume shrinkage. This study illustrates that annealing after release, yet prior to dealloying, prevents failure during the selective dissolution step. Experiments in which annealing was performed at temperatures ⩽400°C illustrate that permanent buckling of the freestanding solid alloy beams is required to prevent failure of the nanoporous beams during dealloying. In contrast, annealing of beams prior to release, or annealing temperatures ⩽200°C (which do not cause permanent buckling deformation), do not mitigate dealloying failures. This work illustrates that the fabrication yield and the residual stresses (in the final nanoporous beams) correlate with the permanent buckling deflection of the alloy due to annealing, with the maximum yield (>95%) and the minimum residual stress (∼4–7MPa) occurring for annealing at ∼300°C. The present experimental results are used with elementary elastic buckling models to explain the critical temperature that produces permanent deflection, as well as discuss the mechanisms underlying the above phenomena. [Copyright &y& Elsevier]

Abstract: A comprehensive study on the relationship between yield strength, relative density and ligament sizes is presented for nanoporous Au foams. Depth-sensing nanoindentation tests were performed on nanoporous foams ranging from 20% to 42% relative density with ligament sizes ranging from 10 to 900nm. The Gibson and Ashby yield strength equation for open-cell macrocellular foams is modified in order to incorporate ligament size effects. This study demonstrates that, at the nanoscale, foam strength is governed by ligament size, in addition to relative density. Furthermore, we present the ligament length scale as a new parameter to tailor foam properties and achieve high strength at low densities. [Copyright &y& Elsevier]

[Display omitted] Nanoporous gold (NPG) made by dealloying takes the form of a network of nanoscale struts or "ligaments". It is well established that the material's mechanical behavior is strongly affected by its ligament size, L and by its solid volume fraction, φ. We explore the mechanical behavior of NPG, with an emphasis on establishing a consistent data set with comparable L but covering a significant range of initial φ. Specimens are prepared from Ag-Au master alloys with their Au atom fraction, x Au 0 in the range 0.20–0.35. Since dealloying replaces Ag with voids, φ may be expected to agree with x Au 0. Yet, spontaneous plastic deformation events during dealloying can result in macroscopic shrinkage, decoupling φ from x Au 0. This raises the question, how do φ and x Au 0 separately affect the mechanical behavior? We confirm two recent suggestions, namely i) a modified Roberts-Garboczi-type scaling law for Young's modulus versus φ of the material in its as-prepared state and ii) the relevance of an apparent "load-bearing solid fraction" for Young's modulus as well as strength. Yet, remarkably, we find that stiffness and strength of the as-prepared material show a much better correlation to x Au 0 as compared to φ. This can be understood as a consequence of the microstructural changes induced by shrinkage. Studying the microstructure evolution during annealing, we also confirm the suggestion that coarsening entails an enhanced loss in stiffness for samples with lesser solid fraction. This finding confirms concerns about the notion of self-similar coarsening as a general behavior of dealloying-made network materials. [ABSTRACT FROM AUTHOR]

The fabrication of Cu 3 Sn at near room temperatures and atmospheric pressure remains out of reach. We propose a novel concept to address this challenge by exploiting both the classical nano-island model for dealloying and the nanocurvature effect on nucleation, in conjunction with bespoke precursor alloy design. In-situ synchrotron X-ray diffraction (XRD) confirmed significant formation of Cu 3 Sn at both 70°C and 55°C in 180 min via a dealloying-realloying process in the designed precursor alloy Al67Cu18Sn15, compared with much slower formation at 125°C by conventional approaches. Furthermore, Cu 3 Sn formed just 10 min after Cu 6 Sn 5 emerged at 70°C, being two orders of magnitude faster than in Cu/Cu 6 Sn 5 diffusion couples at 125°C. The energy barrier to the formation of a lenticular Cu 3 Sn nucleus on Cu 6 Sn 5 nanocaps was assessed systematically using the CALPHAD approach through the initial driving force (DF) concept and DF -plateau method. The predictions were benchmarked against diffusion couple studies, which fully supported the experimental results. In addition, the in-situ synchrotron XRD data on phase evolution was analysed in detail using the well-established Johnson-Mehl-Avrami-Kolmogorov (JMAK) model, which provided further fundamental support to our conceptual design. The concept and methodology demonstrated are expected to be applicable to the fabrication of other challenging materials. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] 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. [ABSTRACT FROM AUTHOR]