Immiscible Metals Research Articles

Strengthening nanocrystalline immiscible bimetallic composite by high-entropy effect

Immiscible bimetallic composites are a kind of transpiration cooling material with potential in high-temperature service. Aiming at boosting their load-bearing capacity, the refractory phase was replaced with the multi-principal refractory high-entropy phase in the present work. Bi-phase metallic nanocrystalline NbMoTaW–Cu composites were fabricated successfully by the powder metallurgy method. The average grain size of the NbMoTaW phase in the sintered composite was kept to be 15 nm. Two interfacial configurations of BCC/FCC and BCC/amorphous/FCC were found in the composite. Using atom probe tomography, notable compositional inter-diffusion between the immiscible metals was disclosed, and the thickness of the mutual diffusion layer in the NbMoTaW–Cu composite was 2.2 times that in the W–Cu composite. The mechanisms of entropy effect on the formation of amorphous configuration and interfacial mutual diffusion were explained based on thermodynamic calculations. The yield strength and Vickers hardness of the nanocrystalline NbMoTaW–Cu composite are 52% and 27% higher than those of the W–Cu counterpart, respectively. In addition, the NbMoTaW–Cu composite processes excellent resistance to high-temperature softening even at 900 °C. The improved mechanical properties were associated with solid solution strengthening of the refractory metal phase, as well as the constraint effect and strengthening of interface between the refractory metal and Cu phases. This work provides novel guidance for designing advanced immiscible metallic composites with excellent mechanical performance.

The Perfect Imperfections in Electrocatalysts.

Modern day electrochemical devices find applications in a wide range of industrial sectors, from consumer electronics, renewable energy management to pollution control by electric vehicles and reduction of greenhouse gas. There has been a surge of diverse electrochemical systems which are to be scaled up from the lab-scale to industry sectors. To achieve the targets, the electrocatalysts are continuously upgraded to meet the required device efficiency at a low cost, increased lifetime and performance. An atomic scale understanding is however important for meeting the objectives. Transitioning from the bulk to the nanoscale regime of the electrocatalysts, the existence of defects and interfaces is almost inevitable, significantly impacting (augmenting) the material properties and the catalytic performance. The intrinsic defects alter the electronic structure of the nanostructured catalysts, thereby boosting the performance of metal-ion batteries, metal-air batteries, supercapacitors, fuel cells, water electrolyzers etc. This account presents our findings on the methods to introduce measured imperfections in the nanomaterials and the impact of these atomic-scale irregularities on the activity for three major reactions, oxygen evolution reaction (OER), oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). Grain boundary (GB) modulation of the (ABO3 )n type perovskite oxide by noble metal doping is a propitious route to enhance the OER/ORR bifunctionality for zinc-air battery (ZAB). The perovskite oxides can be tuned by calcination at different temperatures to alter the oxygen vacancy, GB fraction and overall reactivity. The oxygen defects, unsaturated coordination environment and GBs can turn a relatively less active nanostructure into an efficient redox active catalyst by imbibing plenty of electrochemically active sites. Obviously, the crystalline GB interface is a prerequisite for effective electron flow, which is also applicable for the crystalline surface oxide shell on metal alloy core of the nanoparticles (NPs). The oxygen vacancy of two-dimensional (2D) perovskite oxide can be made reversible by the A-site termination of the nanosheets, facilitating the reversible entry and exit of a secondary phase during the redox processes. In several instances, the secondary phases have been observed to introduce the right proportion of structural defects and orbital occupancies for adsorption and desorption of reaction intermediates. Also, heterogeneous interfaces can be created by wrapping the perovskite oxide with negatively charged surface by layered double hydroxide (LDH) can promote the OER process. In another approach, ion intercalation at the 2D heterointerfaces steers the interlayer spacing that can influence the mass diffusion. Similar to anion vacancy, controlled formation of the cation vacancies can be achieved by exsolving the B-site cations of perovskite oxides to surface anchored catalytically active metal/alloy NPs. In case of the alloy electrocatalysts, incomplete solid solution by two or more mutually immiscible metals results in heterogeneous alloys having differently exposed facets with complementary functionalities. From the future perspective, new categories of defect structures including the 2D empty spaces or voids leading to undercoordinated sites, the multiple interfaces in heterogeneous alloys, antisite defects between anions and cations, and the defect induced inverse charge transfer should bring new dimensionalities to this riveting area of research.

Formation of insoluble silver-phases in an iron-manganese matrix for bioresorbable implants using varying laser beam melting strategies

The combination of immiscible metals is targeted to fulfil the requirements for certain technical and biomedical applications. The degradation behavior of iron (Fe) is adjustable by the insertion of silver (Ag) phases with high electrochemical potential causing anodic dissolution of the Fe-based matrix. To process such immiscible materials, powder-based processes like laser beam melting (LBM) can be applied. The continuous fusion process characterized by a small melt pool, strong melt flow, and rapid solidification enables the creation of well-dispersed Ag phases. To enable the adaption of insoluble phases this study aims to understand the mechanism responsible for the incorporation of insoluble phases within the matrix. Based on the characterization of FeMn with 5 wt.-% Ag processed via LBM the surface tension is determined as the essential influencing factor and a model of the melt pool including melt flow is developed. The Ag forms an almost closed layer on the top surface with an increased thickness between adjacent melt tracks. The lower surface tension of Ag compared to FeMn keeps Ag on the top and the outward-directed melt flow of underlying FeMn transports the Ag to the sides of the melt pools. During the generation of the next slice, deep parts of the Ag layer remain in the bulk material. By adaption of process conditions, e. g. the hatch distance, the size and shape of Ag phases are influenced. Moreover, the chemical composition depends on the LBM strategy, since the diffusion of alloying elements is differently pronounced.

Metallurgical joining of immiscible system: Pure Mg and pure Fe

Understanding the metallurgical joinability of pure Mg and pure Fe and their interfacial microstructure lays the foundation for deciphering the complicated mechanism of joining various magnesium alloys and steels in applications. Pure Mg and pure Fe were successfully joined to form a metallurgical bond for the first time using the friction stir assisted scribe technique. Through detailed characterization, we found that the two immiscible metals are bonded by a critical interfacial oxide layer that spans only ~40 ± 10 nm. We present the first direct experimental evidence of the presence of an Mg/Fe oxide-rich interfacial layer using high-resolution electron microscopy and discuss the contribution of oxide formation toward a successful joining mechanism.

Strategy of thermodynamic and kinetic improvements for Mg hydride nanostructured by immiscible transition metals

High hydrogen density and low material costs make Mg as one of the most promising candidates for solid-state hydrogen storage. However, the practical applications of Mg are restricted by high reaction temperature and slow kinetics of hydrogen absorption/desorption. Here we present the improvements of both thermodynamics and kinetics of the hydride/deuteride of Mg (MgH2/MgD2) by utilizing the immiscible Mg–Cr system. Nanometer-sized MgD2 domains with the average crystallite size of ~10 nm embedded in a Cr matrix are formed in deuterated Mg0.25Cr0.75. X-ray diffraction and nuclear magnetic resonance spectroscopy studies show that the MgD2 domains are heavily distorted, which leads to the thermodynamic destabilization lowering the reaction temperature. Mg0.25Cr0.75 can reversibly absorb and desorb hydrogen/deuterium at a low temperature of 473 K. The enthalpy ΔH for deuterium desorption of Mg0.25Cr0.75−D is 72.1 kJ mol−1−D2, which is lower than ~74 kJ mol−1−D2 for bulk MgD2. The apparent activation energy for hydrogen desorption of Mg0.25Cr0.75−H is decreased to 75 kJ mol−1 from ~160 kJ mol−1 for bulk Mg, in which the dehydrogenation of nanometer-sized MgH2 is controlled by one-dimensional diffusion of hydrogen. Our work demonstrates that MgH2 nanostructured by an immiscible matrix is a useful strategy to alter the thermodynamic and kinetic properties.

Interdiffusion of Elements During Ultrasonic Additive Manufacturing

This paper reports evidence for enhanced elemental interdiffusion during ultrasonic additive manufacturing (UAM) across metal boundaries of copper-aluminum, nickel-gold, and nickel-gold-aluminum. The high solute interdiffusion measured by energy dispersive X-ray spectroscopy line scans is rationalized with calculated vacancy concentrations orders of magnitude larger than thermal equilibrium values. The above estimates are supported by existing knowledge related to defect physics and UAM thermal cycles. The observation of pronounced elemental mixing are evidence for the presence of enhanced non-equilibrium immiscible metal interdiffusion during UAM processing.

Probing Kinetics and Mechanism of Formation of Mixed Metallic Nanoparticles in a Polymer Membrane by Galvanic Replacement between Two Immiscible Metals: Case Study of Nickel/Silver Nanoparticle Synthesis.

Galvanic replacement between metals has received notable research interest for the synthesis of heterometallic nanostructures. The growth pattern of the nanostructures depends on several factors such as extent of lattice mismatch, adhesive interaction between the metals, cohesive forces of the individual metals, etc. Due to the difficulties in probing ultrafast kinetics of the galvanic replacement reaction and particle growth in solution, real-time mechanistic investigations are often limited. As a result, the growth mechanism of one metal on the surface of another metal at the nanoscale is poorly understood so far. In the present work, we could successfully probe the galvanic replacement of silver ions with nickel nanoparticles, stabilized in a polymer membrane, using two complementary methods, namely, small-angle X-ray scattering (SAXS) and radiolabeling, and the results are supported by density functional theory (DFT) computations. The silver-nickel system has been chosen for the present investigation because of the high degree of bulk immiscibility caused by the large lattice mismatch (15.9%) and the weak adhesive interaction, which makes it a perfect model system for immiscible metal pairs. Membrane, as a host medium, plays a crucial role in retarding the kinetics of atomic and particle rearrangements (nucleation and growth) due to slower mobility of the atoms (monomers) and particles within the polymer network. This allowed us to examine the real-time concentration of silver monomers during galvanic replacement of silver ions with nickel nanoparticles and evolution of Ni/Ag nanoparticles. From combined experiment and DFT computations, it has been demonstrated, for the first time to the best of our knowledge, that the majority of silver atoms, which are produced on the nickel nanoparticle surface by galvanic reactions, do not form traditional core-shell nanostructures with nickel and undergo a self-governing sequential nucleation and growth of silver nanoparticles via formation of intermediate prenucleation silver clusters, leading to the formation of mixed metallic nanoparticles in the membrane. The surface of NiNPs has a heterogeneous effect on the silver nucleation pathway, which is evident from the reduced critical free energy barrier of nucleation (ΔGcrit). The present work establishes an original mechanistic pathway based on a sequential nucleation model for formation of mixed metallic nanoparticles by the galvanic replacement route, which opens up future possibilities for size-controlled synthesis in mixed systems.

Metallurgical Joining of Immiscible System: Pure Mg and Pure Fe

Understanding the metallurgical joinability of pure Mg and pure Fe and their interfacial microstructure lays the foundation for deciphering the complicated mechanism of joining various magnesium alloys and steels in applications. Pure Mg and pure Fe were successfully joined to form a metallurgical bond for the first time using the friction stir assisted scribe technique. Through detailed characterization, we found that the two immiscible metals are bonded by a critical interfacial oxide layer that spans only ~40 ± 10 nm. We present the first direct experimental evidence of the presence of a Mg/Fe oxide rich interfacial layer using high-resolution electron microscopy and discuss the contribution of oxide formation toward a successful joining mechanism.

Mechanical properties and microstructural change in (Cu–Fe) immiscible metal matrix composite: Effect of Mg on secondary phase separation

Phase morphology and the distribution of the strengthening phase generally have a strong influence on the movement of dislocations. The presence of minor alloying element Mg in Cu–30Fe alloy significantly affects both the morphology and distribution of the strengthening phase. Dendritic α-Fe is transformed into a spherical morphology because of changes in the chemical composition during the solidification process. The presence of Mg leads to the formation of secondary phase-separated Cu within the α-Fe phase and to increasing heterogeneity of the gradient size of the α-Fe phase. The increased volume fraction of α-Fe enhanced by secondary separated Cu, refinement of α-Fe, and the degree of distribution enhanced the yield strength of this alloy by 40% compared with that of Cu–30Fe. This result can be used as a guide to modifying the morphology of Cu–Fe alloys and improve understanding of strengthening mechanism of this system.

Equilibrium shape of core(Fe)–shell(Au) nanoparticles as a function of the metals volume ratio

The equilibrium shape of nanoparticles is investigated to elucidate the various core–shell morphologies observed in a bimetallic system associating two immiscible metals, iron and gold, that crystallize in the bcc and fcc lattices, respectively. Fe–Au core–shell nanoparticles present a crystalline Fe core embedded in a polycrystalline Au shell, with core and shell morphologies both depending on the Au/Fe volume ratio. A model is proposed to calculate the energy of these nanoparticles as a function of the Fe volume, Au/Fe volume ratio, and the core and shell shape, using the density functional theory-computed energy densities of the metal surfaces and of the two possible Au/Fe interfaces. Three driving forces leading to equilibrium shapes were identified: the strong adhesion of Au on Fe, the minimization of the Au/Fe interface energy that promotes one of the two possible interface types, and the Au surface energy minimization that promotes a 2D–3D Stranski–Krastanov-like transition of the shell. For a low Au/Fe volume ratio, the wetting is the dominant driving force and leads to the same polyhedral shape for the core and the shell, with an octagonal section. For a large Au/Fe ratio, the surface and interface energy minimizations can act independently to form an almost cube-shaped Fe core surrounded by six Au pyramids. The experimental nanoparticle shapes are well reproduced by the model, for both low and large Au/Fe volume ratios.

Nanoalloys of Metals Which Do Not Form Bulk Alloys: The Case of Ag-Co.

Ag and Co metals do not form macroscopic solid or liquid alloys. However, AgmCon clusters have been produced in dual-target dual-laser vaporization experiments, and the same occurs for other pairs of immiscible metals. We have performed density functional calculations to shed light on this phenomenon. The main result, obtained for clusters with sizes m + n not larger than 11, is that the cohesive energies justify that those clusters can be formed starting from free Ag and Co atoms, which is the case in the vaporization experiments. At the same time, mixing of Ag and Co is difficult even at the nanoscale. This is revealed by the application of several miscibility criteria. Those two features become, nevertheless, compatible in the clusters. Even if the cluster sizes considered are small, the emerging trend becomes clear: Co atoms form a core in the inner part, surrounded by a shell of Ag atoms on the surface. A consequence is that core-shell clusters can be formed from pairs of metals that do not form macroscopic alloys. The magnetic moments μ of the AgmCon clusters are due mainly to the Co atoms, and the presence of Ag induces a reduction in the magnitude of μ.

Nanostructured Gradient Material Based on the Cu–Cr–W Pseudoalloy Fabricated by High-Energy Ball Milling and Spark Plasma Sintering

Nanostructured mechanical composites of immiscible metals Cu, Cr, and 5–70 wt % W; nanostructured consolidated materials based on them; and Cu/Cu–Cr–W nanostructured gradient material with various W contents are fabricated in this work by combining short-term (up to 150 min) high-energy ball milling (HEBM) and spark plasma sintering (SPS). To fabricate Cu–Cr–W mechanical composites, HEBM of Cu + Cr + (5–70 wt %)W is performed using an Activator-2S planetary ball mill with a revolution rate of drums of 1388 rpm and a planetary disc of 694 rpm in argon for 150 min. The Cu–Cr–W mechanical composites are consolidated by SPS at temperatures of 800–1000°C and pressure of 50 MPa for 10 min. The nanostructured gradient sintered material based on Cu–Cr–W pseudoalloys is compacted layer-by-layer in the following sequence (from pure copper to pseudoalloy with an increase in the tungsten weight fraction): Cu/Cu–Cr–5% W/Cu–Cr–15% W/Cu–Cr–70% W and sintered at 800°C for 10 min. The crystal structure, microstructure, and properties of Cu–Cr–W mechanical composites and consolidated materials based on them are investigated depending on fabrication conditions. It is shown that the nanostructure formed in mechanical composites at the short-term HEBM stage (up to 150 min) is retained after SPS for all Cu–Cr–W (5–70 wt % W) compositions. The SEM and EDS data evidence that W (d ~ 20–100 nm) and Cr (d ~ 20–50 nm) refractory particles are homogeneously distributed in the material bulk (in a copper matrix). The hardness of consolidated Cu–Cr–W samples (15 wt %) formed from nanostructured powder mixtures (after 150-min HEBM) by SPS at t = 800°C exceeds the hardness of samples sintered from the mixture of initial components (without HEBM) by a factor of ~6. The hardness for the nanostructured Cu–Cr–70% W composition (tSPS = 1000°C) is higher by a factor of ~3 than for microcrystalline analogs. Samples Cu–Cr–15% W and Cu–Cr–70% W have the largest relative density up to 0.91. The resistivity of nanostructured Cu–Cr–W compositions exceeds this characteristic for microcrystalline samples approximately twofold. This can be caused by an increase in grain boundaries and the accumulation of various defects in the material at the HEBM stage. These results show the prospects of using the combination of short-term HEBM and subsequent SPS for the formation of consolidated nanocrystalline Cu–Cr–W composites and gradient materials based on them.

Nanostructured gradient material based on Cu—Cr—W pseudo alloy prepared by high energy ball milling and spark plasma sintering

This study was conducted to obtain nanostructured mechanically activated composite particles from immiscible metals Cu, Cr and 5÷70 wt.% W, nanostructured bulk materials based on them and Cu / Cu—Cr—W nanostructured gradient material with different tungsten content by combined short-term (up to 150 min) high-energy ball milling (HEBM) and spark plasma sintering (SPS). Cu— Cr—W mechanically activated composites were obtained by HEBM of Cu + Cr + (5÷70 wt.%)W powder mixtures in the Activator-2S ball planetary mill at the rotating speed of 1388 rpm for the grinding chamber and 694 rpm for the planetary disk in an argon atmosphere for 150 min. Cu—Cr—W mechanically activated composite particles were consolidated by SPS in the temperature range of 800— 1000 °C at a pressure of 50 MPa for 10 min. The nanostructured gradient sintered material based on Cu—Cr—W pseudo alloys was pressed layer by layer in the following sequence (from pure copper to pseudo alloy with increasing tungsten content): Cu / Cu—Cr—5%W / Cu—Cr—15%W / Cu—Cr—70%W and sintered at 800 °C for 10 min. The crystal structure, microstructure, and properties of Cu— Cr—W mechanically activated composites and consolidated materials based on them were studied depending on production conditions. It was shown that the nanostructure formed in mechanically activated composites at the short-term HEBM stage (up to 150 min) was preserved for all Cu—Cr—W (5÷70 wt.% W) compounds after SPS. Based on SEM and EDX, refractory particles of W (d ~ 20÷100 nm) and Cr ( d ~ 20÷50 nm) were uniformly distributed in the material volume (in the copper matrix). The hardness of Cu—Cr—W (15 wt.% W) bulk samples obtained from nanostructured powder mixtures (after 150 min HEBM) by SPS at 800 °C was approximately 6 times higher than the hardness of samples sintered from the mixture of starting components (without HEBM). For the Cu—Cr—70%W nanostructured compound ( t sps = 1000 ° С ) the hardness value was ~3 times higher than that for microcrystalline analogues. The highest re­lative density of 0.91 was achieved for Cu—Cr—15%W and Cu—Cr—70%W samples. Electrical resistivity for nanostructured Cu—Cr—W composites were 2 times higher than for microcrystalline samples. Apparently, this is due to an increase in grain boundaries and various defects accumulated in the material at the HEBM stage. The obtained results show that combined short-term HEBM and subsequent SPS is a promising way to produce nanocrystalline Cu—Cr—W composites and gradient materials based on them.

Self-assembled binary immiscible Cu-transition metal multilayers prepared by co-sputtering deposition

A series of Cu-X (X = W, Mo, Nb, Cr, V, Al) films with similar Cu content (about 50 at.%) were prepared by dual-target co-deposition technique. By using this method, we found that the multilayered structure can be detected in Cu-W, Cu-Mo, Cu-Nb, Cu-V and Cu-Cr films, but not in Cu-Al films. Except for Cu-Al system, the multilayered structures are made up of Cu-rich and X-rich sublayers rather than conventional pure Cu and X layers. Based on our previous work, we further proved that the confocal targets, rotation of sample stage and the existence of diffusion barrier elements are the essential conditions for obtaining the multilayered structure within the films. The formation mechanism of the multilayered structure by co-deposition was discussed. Initially, the multilayered structure is formed by co-sputtering deposition with the sample stage rotation. And the multilayered structure can remain stable and become more prominent with the uphill diffusion of Cu in immiscible systems. However, in miscible systems, because of the interdiffusion and forming intermetallic compounds between Cu and X (such as Al), the initial multilayered structure can be interrupted and disordered. In addition, the morphology of multilayered structure formation also depends on the diffusion barrier abilities of the elements.

Progress in Research on the Alloying of Binary Immiscible Metals

Progress in Research on the Alloying of Binary Immiscible Metals

Immiscible bi-metal single-atoms driven synthesis of electrocatalysts having superb mass-activity and durability

Properties of metal elements can be modified by alloying. Given that catalytic efficiencies are often maximized using metal single-atoms (SAs), two immiscible metals can improve the catalytic activity when they can be present as non-agglomerated dual SAs. Here, we report yet-unexplored synthesis of high-performance electrocatalysts utilizing immiscibility of Cu/Ru bimetal atoms. In the synthesized electrocatalyst (Cu/Ru@GN), both Cu-SAs and Ru-SAs are dispersed on N-doped graphitic-matrix (GN), while other Cu-SAs are bridged to Ru nanoparticles (NPs) surface via nitrogen. It shows superior mass-activity with outstanding catalytic performance and durability for hydrogen evolution reaction (HER), outperforming commercial Pt20wt%/C. Cu-SAs along with Ru-NPs enhance electric conduction or charge-transfer rate of GN-templates for fast kinetics. Owing to Cu-Ru immiscibility, the coordination of Cu-SAs by N atoms which bridge to Ru-NPs surface, introduces new active sites and brings about long-term stability (over 600 h in acidic media) by preventing aggregation of Ru-NPs.

Aerosol Synthesis of High Entropy Alloy Nanoparticles.

Homogeneously mixing multiple metal elements within a single particle may offer new material property functionalities. High entropy alloys (HEAs), nominally defined as structures containing five or more well-mixed metal elements, are being explored at the nanoscale, but the scale-up to enable their industrial application is an extremely challenging problem. Here, we report an aerosol droplet-mediated technique toward scalable synthesis of HEA nanoparticles with atomic-level mixing of immiscible metal elements. An aqueous solution of metal salts is nebulized to generate ∼1 μm aerosol droplets, which when subjected to fast heating/quenching result in decomposition of the precursors and freezing-in of the zero-valent metal atoms. Atomic-level resolution scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy analysis reveals that all metal elements in the nanoparticles are homogeneously mixed at the atomic level. We believe that this approach offers a facile and flexible aerosol droplet-mediated synthesis technique that will ultimately enable bulk processing starting from a particulate HEA.

Microstructure morphology and concentration modulation of nanocomposite thin-films during simulated physical vapor deposition

Metallic, nanocomposite, thin-films with intertwined morphologies can be synthesized via physical vapor deposition of two immiscible metals. Understanding the phase-ordering kinetics controlling the way the microstructure develops is crucial to obtaining reliable and enhanced functionalities in these nanostructured thin-films. Here, we study the complex relationship between the vapor-deposition conditions and the resulting self-assembled nanoscale morphologies by using phase-field simulations. Our phase-field model accounts for the deposition of the incident vapor phase of a binary alloy onto a substrate, surface interdiffusion, and the subsequent spinodal decomposition in the resulting elastically inhomogeneous thin-film. We systematically investigated the effects of deposition rate, dissimilar bulk and surface kinetics, phase fraction, and dissimilar elastic response on the resulting microstructure. Four distinct classes of achievable self-assembled microstructure morphologies are observed throughout: lateral, vertical, random, and nanoprecipitate concentration modulations. Through our systematic investigation of competing mechanisms, we provide insights on the complex relationships between alloy species and deposition conditions to obtain specific nanostructured morphologies of binary, nanocomposite, thin-films.

Nanoporous high-entropy alloys with low Pt loadings for high-performance electrochemical oxygen reduction

Pt-based alloys have been widely used as high-performance electrocatalysts for the oxygen reduction reaction (ORR). These catalysts contained high Pt contents, typically 60–70 at.% or higher, since further decreasing Pt contents would significantly lower the desired electro-catalytic activity. High-entropy alloys (HEAs) with built-in stability through their low free-energy phases provided a promising route to prepare Pt-based alloy catalysts with less than 50 at.% Pt while maintaining high ORR activity and stability in various environments. In this work, we reported a top-down de-alloying synthetic method to controllably incorporate five immiscible metals in one nanoscale solid phase. By predetermining four elements (Al, Cu, Ni and Pt) and alternating the fifth element including Pd, V, Co, Mn, etc., a series of nanoporous HEAs (np-HEAs) with Pt content of ~20–30 at.% were obtained. Among these quinary alloys, the Al-Cu-Ni-Pt-Mn np-HEA exhibited the best ORR catalytic activity and electrochemical cycling durability, well exceeding the commercial Pt/C catalysts. We expected these nanostructured HEA would offer a new class of alloy catalysts in many electrochemical reactions.

Effect of Cold-Sintering Parameters on Structure, Density, and Topology of Fe-Cu Nanocomposites.

The design of advanced nanostructured materials with predetermined physical properties requires knowledge of the relationship between these properties and the internal structure of the material at the nanoscale, as well as the dependence of the internal structure on the production (synthesis) parameters. This work is the first report of computer-aided analysis of high pressure consolidation (cold sintering) of bimetallic nanoparticles of two immiscible (Fe and Cu) metals using the embedded atom method (EAM). A detailed study of the effect of cold sintering parameters on the internal structure and properties of bulk Fe–Cu nanocomposites was conducted within the limitations of the numerical model. The variation of estimated density and bulk porosity as a function of Fe-to-Cu ratio and consolidation pressure was found in good agreement with the experimental data. For the first time, topological analysis using Minkowski functionals was applied to characterize the internal structure of a bimetallic nanocomposite. The dependence of topological invariants on input processing parameters was described for various components and structural phases. The model presented allows formalizing the relationship between the internal structure and properties of the studied nanocomposites. Based on the obtained topological invariants and Hadwiger’s theorem we propose a new tool for computer-aided design of bimetallic Fe–Cu nanocomposites.