On the heat of formation of solid alloys

Universal Maximum Strength of Solid Metals and Alloys

Interstitial electron density ρ_{o} is offered as a direct metric for maximum strength in metals, arising from universal properties derived from an electron gas. ρ_{o} sets the exchange-correlation parameter r_{s} in density-functional theory. It holds also for maximum shear strength τ_{max} in polycrystals [M. Chandross and N. Argibay, Phys. Rev. Lett. 124, 125501 (2020)PRLTAO0031-900710.1103/PhysRevLett.124.125501]. Elastic moduli and τ_{max} for polycrystalline (amorphous) metals are linear with ρ_{o} and melting T_{m} (glass-transition T_{g}) temperature. ρ_{o} or r_{s}, even with rule-of-mixture estimate, predicts relative strength for rapid, reliable selection of high-strength alloys with ductility, as confirmed for elements to steels to complex solid solutions, and validated experimentally.

Catalysis of Alloys: Classification, Principles, and Design for a Variety of Materials and Reactions

Alloying has long been used as a promising methodology to improve the catalytic performance of metallic materials. In recent years, the field of alloy catalysis has made remarkable progress with the emergence of a variety of novel alloy materials and their functions. Therefore, a comprehensive disciplinary framework for catalytic chemistry of alloys that provides a cross-sectional understanding of the broad research field is in high demand. In this review, we provide a comprehensive classification of various alloy materials based on metallurgy, thermodynamics, and inorganic chemistry and summarize the roles of alloying in catalysis and its principles with a brief introduction of the historical background of this research field. Furthermore, we explain how each type of alloy can be used as a catalyst material and how to design a functional catalyst for the target reaction by introducing representative case studies. This review includes two approaches, namely, from materials and reactions, to provide a better understanding of the catalytic chemistry of alloys. Our review offers a perspective on this research field and can be used encyclopedically according to the readers' individual interests.

Thermodynamic stability of Pd-Ru alloy nanoparticles: combination of density functional theory calculations, supervised learning, and Wang-Landau sampling

Solid-solution alloy nanoparticles (NPs) comprising Pd and Ru, which are immiscible in the bulk state, have been synthesised and show excellent catalytic performance. To date, most studies have evaluated the stability of alloy NPs at 0 K only. Because the thermodynamic stability of Pd-Ru alloy NPs may differ from that of the alloy in the bulk state, the stable configuration of the NPs must be evaluated under a finite temperature. Such stability evaluations are critical for developing the durable NPs as catalysts. Therefore, the thermodynamic stability of Pd-Ru alloy NPs was analysed using density functional theory (DFT), supervised learning (SL), and Wang-Landau sampling. We calculated the excess energy of Pd-Ru alloy NPs, which depends on their composition, structure, NP size, adatom type, and defects, and applied SL to all models. The excess energies of the Pd-Ru alloy NPs expressed by structural information, such as the surface-to-volume ratio, correlated with those calculated using DFT. Wang-Landau sampling based on the energy estimated by SL gave the thermodynamic stability of Pd-Ru alloy NPs with a stable configuration under a finite temperature. The solid-solution atomic configuration was subdivided into partially mixed configurations in the surface layer or in the core of the NPs, which is different from the bulk state. The partially mixed configuration was determined by the overall composition and surface properties. The findings from the combined method could contribute to a better understanding of the alloy-NP stability and their application in catalysis.

Electronegativity and chemical hardness of elements under pressure

SignificanceOver the years, many unusual chemical phenomena have been discovered at high pressures, yet our understanding of them is still very fragmentary. Our paper addresses this from the fundamental level by exploring the key chemical properties of atoms-electronegativity and chemical hardness-as a function of pressure. We have made an appropriate modification to the definition of Mulliken electronegativity to extend its applicability to high pressures. The change in atomic properties, which we observe, allows us to provide a unified framework explaining (and predicting) many chemical phenomena and the altered behavior of many elements under pressure.

Microstructure and Its Effect on the Magnetic, Magnetocaloric and Magnetostrictive Properties of Tb55Co30Fe15 Glassy Ribbons

In the present work, the microstructure and its effect on the magnetic, magnetocaloric, and magnetoelastic properties of the Tb₅₅Co₃₀Fe₁₅ melt-spun ribbon were investigated. The ribbon exhibits typical amorphous characteristics in its X-ray diffraction examination and differential scanning calorimetry measurement. However, the magnetic properties of the ribbon indicate that the ribbon is inhomogeneous in the nanoscale, as ascertained by a high-resolution electron microscope. Compared to the Tb₅₅Co₄₅ amorphous alloy, the Tb₅₅Co₃₀Fe₁₅ ribbon shows poor magnetocaloric properties but outstanding magnetostriction. A rather high value of reversible magnetostriction up to 788 ppm under 5 T was obtained. The mechanism for the formation of nanoparticles and its effect on the magnetocaloric and magnetostrictive properties were investigated.

Random alloy and intermetallic nanocatalysts in fuel cell reactions

Fuel cells that use small organic molecules or hydrogen as the anode fuel can power clean electric vehicles. From an experimental perspective, the possible fuel cells' electrocatalytic reaction mechanisms are obtained through in situ electrochemical spectroscopy techniques and density functional theory calculations, providing theoretical guidance for further development of novel nanocatalysts. As advanced nanocatalysts for fuel cells' electrochemical reactions, alloy nanomaterials have greatly improved electrocatalytic activity and stability and have attracted widespread attention. Enhanced electrocatalytic performance of alloy nanocatalysts could be closely related to the synergistic effects, such as electronic and strain effects. Depending on the arrangement of atoms, alloys can be classified into random alloy and intermetallic compounds (ordered structure). Intermetallic compounds generally have lower heats of formation and stronger heteroatomic bonding strength relative to the random alloy, resulting in high chemical and structural stability in either full pH solutions or electrochemical tests. Here, we summarize the latest advances and the structure-function relationship of noble metal alloy nanocatalysts, among which Pt-based catalysts are the main ones, as well as comprehensively understand why they significantly affect the electrocatalytic performance of fuel cells. Novel alloy nanocatalysts with a robust three-phase interface to achieve efficient charge and mass transfer can obtain desirable activity and stability in the electrochemical workstation tests, and is expected to acquire a higher power density on fuel cell test systems with harsh test conditions.

Nanomaterials by design: a review of nanoscale metallic multilayers

Nanoscale metallic multilayers have been shown to have a wide range of outstanding properties, which differ to a great extent from those observed in monolithic films. Their exceptional properties are mainly associated with the large number of interfaces and the nanoscale layer thicknesses. Many studies have investigated these materials focusing on magnetic, mechanical, optical, or radiation tolerance properties. Thus, this review provides a summary of the findings in each area, including a description of the general attributes, the adopted synthesis methods and most common characterization techniques used. This information is followed by a compendium of the material properties and a brief discussion of related experimental data, as well as existing and promising applications. Other phenomena of interest, including thermal stability studies, self-propagating reactions and the progression from nano multilayers to amorphous and/or crystalline alloys, are also covered. In general, this review highlights the use of nano multilayer architectures as viable routes to overcome the challenges of designing and implementing new engineering materials at the nanoscale.

Extracting an Empirical Intermetallic Hydride Design Principle from Limited Data via Interpretable Machine Learning

An open question in the metal hydride community is whether there are simple, physics-based design rules that dictate the thermodynamic properties of these materials across the variety of structures and chemistry they can exhibit. While black box machine learning-based algorithms can predict these properties with some success, they do not directly provide the basis on which these predictions are made, therefore complicating the a priori design of novel materials exhibiting a desired property value. In this work we demonstrate how feature importance, as identified by a gradient boosting tree regressor, uncovers the strong dependence of the metal hydride equilibrium H₂ pressure on a volume-based descriptor that can be computed from just the elemental composition of the intermetallic alloy. Elucidation of this simple structure-property relationship is valid across a range of compositions, metal substitutions, and structural classes exhibited by intermetallic hydrides. This permits rational targeting of novel intermetallics for high-pressure hydrogen storage (low-stability hydrides) by their descriptor values, and we predict a known intermetallic to form a low-stability hydride (as confirmed by density functional theory calculations) that has not yet been experimentally investigated.

Elastic versus Alloying Effects in Mg-Based Hydride Films

Magnesium thin films covered with a layer of Pd absorb hydrogen at much higher pressures than bulk Mg. Such an effect was originally explained as a consequence of elastic clamping on Mg by the capping Pd layer. An alternative interpretation later suggested that the pressure increase could originate from simple alloying between Mg and Pd. Here we resolve this controversy by measuring the hydrogenation and dehydrogenation isotherms of Mg-Pd thin film alloys over a wide range of compositions. Our results disentangle the effects of elastic clamping and alloying and highlight the role of plastic deformations.

Structure and properties of a novel boride: ThNi12B6

Characterization of the crystal structure and physico-chemical behaviour of ThNi₁₂B₆clearly documents the two-dimensional character of Fermi surfaces originating from 3³4²-Ni-nets.

Cohesion, Compound Formation and Phase Diagrams from First Principles

The physical picture of cohesion and compound formation provided by parameter-free, self-consistent, energy-band calculations will be described. For transition-metal elements, the calculations allow us to “see” which electrons are holding the solid together and which are holding it apart. For compounds, calculated heats of formation agree well with available measurements and are in general agreement with those given by Miedema's empirical formula. (The agreement with Miedema is paradoxical, in that Miedema's conception of the formation process differs qualitatively from ours.) Preliminary, but very encouraging, results of efforts to extend the analysis to disordered materials and to the calculation of phase diagrams are described.

A Depth Selective Mössbauer Study of ion Implanted Stainless Steel

High austenitic stainless steel of composition Fe62Ni2OCrl8 was implanted with Eu ions and analyzed with Depth Selective Conversion Electron Mössbauer Spectroscopy (DCEMS). DCEMS gives information about the depth profile of phases, in this case about implantation induced changes in phase composition as function of depth. The samples were complementary analyzed with Rutherford Backscattering Spectrometry (RBS) to get the element profile of the implanted ions. The main experimental result is a martensitic transformed depth region that coincides with the Eu depth distribution. The potential of DCEMS for application in the field of materials modification by implantation techniques is demonstrated.

The Influence of an Adsorbate Layer on Adatom Diffusion and Island Nucleation: Fe on Si(111)-√3×√3-Au

Using scanning tunneling microscopy, the influence of a thin Au layer on the diffusion of Fe adatoms and the subsequent island nucleation on a Si(111) surface is investigated. The adsorbate induces the Si(111)-√3×√3-Au structure that increases the surface mobility of subsequently deposited Fe atoms, resulting in the formation well-defined nanoclusters. Surprisingly, the domain walls-inherent to the √3×√3-Au reconstruction-do not influence the surface diffusion, which demonstrates that the passivation is of much more importance for the self-assembly than the surface corrugation. Using the decoupling of the diffusion and nucleation on the surface and the reaction with the surface and conventional nucleation theory, the activation energy for surface diffusion E(d) = 0.61 eV and the critical cluster size i = 3 are determined, which reveal the microscopic details of the diffusion and nucleation processes.

Iron silicide nanostructure formation on Au induced superstructures on Si(111)

Using scanning tunnelling microscopy we have investigated the formation of low dimensional Fe-Si structures on Au covered Si(111) surfaces. The ultrathin Au layer induces a variety of surface reconstructions, depending on the coverage and temperature: Si(111)-5 x 2, [alpha-squareroot 3 x squareroot 3,beta-squareroot 3 x squareroot 3], and 6 x 6-Au. The subsequent deposition of 0.28 ML (monolayers) of Fe at 400 degrees C results in the formation of Fe-Si nanostructures whose morphological properties critically depend on the underlying surface. All Au induced reconstructions give rise to an increase in diffusion length as compared to the bare Si(111)-7 x 7 surface, thereby allowing the growth of well-separated nanostructures at considerably lower temperatures. Ultimately, the decoupling of surface diffusion and temperature, induced by the Au layer, can be exploited to tailor the island dimensions and density. With an appropriate choice of substrate, passivating layer and deposited material, nanostructures with the desired properties can be grown in a controlled way.