Surface Energy Of Metals Research Articles

Comparison of Surface Energy of Bcc Alkali Metals and Transition Metals Using Maeam

The surface energies of the low index (100), (110) and (111) planes of some bcc alkali metals (Li,Na,K,Rb,Cs) and for the bcc transition metals (Fe,W,Mo,Cr,Ta,Nb and V) have been calculated using the Modified Analytical Embedded Atom Method (MAEAM).The surface energy of each (hkl) plane in alkali metals was found to be much more lower than those of the transition metals. The experimental values of surface energies are not tied to specific surfaces and are obtained for polycrystalline materials. They do not correlate with computed values published for any of these surfaces. This is expected since experiments give direct results. For all bcc metals the order of the surface energy is such that Es(110) < Es(100) < Es(111) which is in agreement with the results obtained except for (111) surface. Our calculated result shows that Es(111) < Es(110) < Es(100). This is not in agreement with the result of MAEAM.[1]

SURFACE ENERGY ENGINEERING OF Cu SURFACE BY STRAIN: FIRST-PRINCIPLES CALCULATIONS

Surface energies of strained Cu surfaces were studied systematically using first-principles methods. Results showed that the strain-stabilization of Cu surface was anisotropic and strongly related to the strain distribution. This strain-induced approach could be used as an effective way to engineer the surface energies of metals.

In-flight gas phase growth of metal/multi layer graphene core shell nanoparticles with controllable sizes

In this report, we present a general method for a continuous gas-phase synthesis of size-selected metal/multi layer graphene (MLG) core shell nanoparticles having a narrow size distribution of metal core and MLG shell for direct deposition onto any desired substrate kept under clean vacuum conditions. Evolution of MLG signature is clearly observed as the metal-carbon agglomerates get transformed to well defined metal/MLG core shell nanoparticles during their flight through the sintering zone. The growth takes place via an intermediate state of alloy nanoparticle (Pd-carbon) or composite nanoparticle (Cu-carbon), depending upon the carbon solubility in the metal and relative surface energy values. It has been also shown that metal/MLG nanoparticles can be converted to graphene shells. This study will have a large impact on how graphene or graphene based composite nanostructures can be grown and deposited in applications requiring controllable dimensions, varied substrate choice, large area and large scale depositions.

Influence of adsorbates on the segregation properties of Au–Pd bimetallic clusters

In this work, we investigate the surface segregation phenomena of 923-atom Au–Pd bimetallic clusters upon CO and O adsorption by using Monte Carlo simulations. The metal–metal interaction was modeled by the Johnson’s embedded atom method (EAM) and the metal–adsorbate interaction potential was developed by density functional theory (DFT) calculations. It is found that Pd atoms would like to occupy the surface sites for Au–Pd clusters upon adsorption, which is different to the surface segregation of Au on the surface of free Au–Pd clusters. This phenomenon may be due to a competition between surface energies of Au and Pd metals and their interaction strength with CO and O for the clusters upon adsorption. It is expected that our results can be used to understand and explain the experimental results of Pd atoms occupying the surface layer of Au–Pd bimetallic clusters in reactive environments.

Energy profile of hydrogen evolution on metals with consideration of their surface energy

It has been established that there is a quantitative relationship between the experimental value of hydrogen overvoltage, \(a = \eta _{H_2 }\), in the Tafel equation and the surface energy, ΔUS, of metal with fcc and hcp structure, which is calculated theoretically in metal surface physics. An equation of slow discharge of hydrogen ions on metal is deduced, in which the a value is a linear function of the value where ΔUS is the minimal surface energy of the specified metal, α = 0.5 is the transition coefficient, and F is the Faraday number. A prevailing tendency toward a decrease in hydrogen overvoltage on polycrystalline surfaces of metals with fcc, hcp, and bcc lattices with an increase in the minimal surface energy related to the appropriate (111), (0001), and (110) facets of these lattices has been observed. Results of analysis of the relationships between and the surface energy of metal are interpreted within the theory of active centers for heterogeneous catalysis with involvement of a thermodynamic model of the metal surface enriched with atom vacancies.

Ordering and segregation in isolated Au–Pd icosahedral nanoclusters and nanowires and the consequences of their encapsulation inside carbon nanotubes

Metropolis Monte Carlo sampling in the semi-grand canonical ensemble with empirical potentials is used to predict equilibrium ordered structures and segregation properties of small icosahedral Au–Pd nanoclusters and helical nanowire segments over the whole range of compositions at low temperatures. The cases of free-standing clusters and wires are compared with the same systems encapsulated in carbon nanotubes.A number of chemically ordered structures and segregation states are identified and found to be consistent with the same interplay between size mismatch, mixing enthalpy and surface energies of elemental metals which determines the thermodynamic equilibrium of binary metal alloys. Encapsulation has the effect of modifying the surface energies of nanoclusters and wires, with considerable consequences on their thermodynamic states, although the metal–graphite interaction strength is low as compared with the metal cohesive energy and the carbon–carbon binding energy.

Nonuniform Scaling Applied to Surface Energies of Transition Metals

We construct a generalized gradient approximation of the exchange-correlation energy that satisfies the nonuniform scaling in one dimension and is accurate in the whole quasi-two-dimensional (Q2D) regime. Using spatial and energetic analyses of metal (111) surfaces, we show that the Q2D behavior is important at the surface of most transition metals, and that the here proposed Q2D-generalized gradient approximation functional predicts for these metals accurate surface energies as well as bulk properties.

Monte Carlo simulation of surface segregation phenomena in extended and nanoparticle surfaces of Pt–Pd alloys

The surface segregation phenomena in the extended and nanoparticle surfaces of Pt–Pd alloys have been studied using the Monte Carlo (MC) simulation method and the modified embedded-atom method (MEAM) potentials developed for Pt–Pd alloys. The MEAM potentials were fitted to reproduce the experimental values of the lattice parameters, cohesive energies and surface energies of pure Pt and Pd metals, as well as the density functional theory calculation results of the lattice parameters and heat of formation of L12 Pt3Pd, L10 PtPd and L12 PtPd3 crystal. Using the MC method and the developed MEAM potentials, we calculated the Pt concentrations in the outermost three layers of the equilibrium (111), (100) and (110) extended surfaces as well as the outermost surfaces of the equilibrium cubo-octahedral nanoparticles of Pt–Pd alloys. Our simulation results showed that the Pd atoms would segregate into the outermost layers of the extended surfaces and the Pt concentration would increase monotonically from the extended surfaces into the bulk. The equilibrium Pt–Pd nanoparticles were found to have Pd-enriched shells and Pt-enriched cores. In the shell of the Pt–Pd nanoparticles, the Pd atoms were predicted to preferably segregate to the (100) facets rather than the (111) facets.

Surface properties of 3d transition metals

Using the projector augmented wave method within density functional theory, we present a systematic study of the layer relaxation, surface energy and surface stress of 3d transition metals. Comparing the calculated trends for the surface energy and stress with those obtained for 4d and 5d metals we find that magnetism has a significant effect on the surface properties. Enhanced surface magnetic moments decrease the size of the surface relaxation, lower the surface energy and surface stress, leading to compressive stress in Cr and Mn.

Correlation energy functional from jellium surface analysis

Using the wave-vector analysis of the jellium exchange-correlation surface energy, we show that the PBEint generalized gradient approximation (GGA) of Fabiano et al. [Phys. Rev. B 82, 113104 (2010)] is one of the most accurate density functionals for jellium surfaces, being able to describe both exchange and correlation parts of the surface energy, without error compensations. We show that the stabilized jellium model allows us to achieve a realistic description of the correlation surface energy of simple metals at any wave vector $k$. The PBEint correlation is then used to construct a meta-GGA correlation functional, modifying the one-electron self-correlation-free Tao-Perdew-Staroverov-Scuseria (TPSS) one. We find that this new functional (named JS) performs in agreement with fixed-node diffusion Monte Carlo estimates of the jellium surfaces, and is accurate for spherical atoms and ions of different spin-polarization and for Hooke's atom for any value of the spring constant.

Surface energies of metals in both liquid and solid states

Although during the last years one has seen a number of systematic studies of the surface energies of metals, the aim and the scientific meaning of this research is to establish a simple and a straightforward theoretical model to calculate accurately the mechanical and the thermodynamic properties of metal surfaces due to their important application in materials processes and in the understanding of a wide range of surface phenomena. Through extensive theoretical calculations of the surface tension of most of the liquid metals, we found that the fraction of broken bonds in liquid metals ( f) is constant which is equal to 0.287. Using our estimated f value, the surface tension ( γ m ), surface energy ( γ SV ), surface excess entropy (−d γ/d T), surface excess enthalpy ( H s ), coefficient of thermal expansion ( α m and α b ), sound velocity ( c m ) and its temperature coefficient (−d c/d T) have been calculated for more than sixty metals. The results of the calculated quantities agree well with available experimental data.

Bond-Energy and Surface-Energy Calculations in Metals

A simple technique appropriate for introductory materials science courses is outlined for the calculation of bond energies in metals from lattice energies. The approach is applied to body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal-closest-packed (hcp) metals. The strength of these bonds is tabulated for a variety metals and is seen to be somewhat smaller than those typically found for the covalent bonds in molecular substances or polymers. The calculated bond energies are then combined with experimental lattice constants to illustrate the estimation of the surface energy of metals with these three crystal structures. The agreement with experiment is reasonably good, given the difficulties inherent in measuring the surface tension of metals and the considerable extrapolation required for surface-energy calculation.

Coating geometries of metals on single-walled carbon nanotubes

This paper studies the coating geometries of metals on single-walled carbon nanotubes (SWNTs) on the basis of the nucleation theory and wetting theory. The metal surface energy, cohesion energy, diffusion barrier, and metal-SWNT interfacial energy are calculated using first-principles calculation. Metals including Fe, Al, Au, Pd, Ni, and Ti are considered. For Ti, Ni, and Pd, low metal-SWNT interfacial energies and high diffusion barriers are responsible for forming continuous or quasicontinuous layers on the SWNT surface. In contrast, Al and Au have small diffusion barriers and poor SWNT surface wetting, thus they tend to aggregate and form large clusters. Although the binding energy between Fe and SWNTs is large, due to the large cohesion energy and poor wetting, Fe may form isolated clusters. All results are in good agreement with experimental observations.

Survey on wetting of SiC by molten metals

Abstract Good wetting and low reactivity of metal/ceramic couples are key factors in many technological processes, in particular in metal/ceramic joining, to avoid degradation of ceramics and to achieve the desired properties during service. Silicon carbide is a covalent material of great technological interest due to its excellent overall properties. Starting from a survey of the surface energies of SiC and liquid metals, the reactivity and wettability of pure metal/SiC systems, as well as the wetting behavior and mechanisms of (liquid metal + Si)/SiC systems are reviewed for understanding the interfacial bonding and for supporting the development of application-oriented processes like non-reactive brazing. Silicon chemisorption and interactions between the molten alloys and SiC at the metal/substrate interface and the intrinsic properties of the alloys or of the pure metals are considered to play the key roles in interfacial bonding. In particular, additions of Si can limit or even suppress the substrate dissolution leaving the solid–liquid interface nearly undisturbed. At the same time, oxidation–deoxidation processes at the SiC surface are the basic mechanisms to be controlled in order to allow the liquid phases to contact a “pure” SiC surface. The need of further investigations, covering basic interfacial phenomena including both experiments and ab-initio modeling of the solid–liquid interfaces is strongly underlined.

Dependence of the surface energy of metals on pressure

A theoretical model has been developed within the method of electron-density functional to describe the effect of pressure on the surface energy of pure metals.

Calculation of energy characteristics of the metal surface taking into account the lattice relaxation effects

Within the framework of the electron-density functional theory method, a self-consistent calculation of the shift of two near-surface ionic planes in different metals has been carried out. The effect of the near-surface lattice relaxation on the surface energy of metals and on the electron work function from these metallic surfaces has been investigated. To describe the arising strong inhomogeneity of the electronic system in the near-surface region, gradient corrections have been taken into account for the kinetic and exchange-correlation energy. To calculate the effect of electron-ion interaction on the energy characteristics of the surface, the Heine-Abarenkov pseudopotential has been used.

Self-consistent solution of Kohn-Sham equations for infinitely extended systems with inhomogeneous electron gas

The density functional approach in the Kohn-Sham approximation is widely used to study properties of many-electron systems. Due to the nonlinearity of the Kohn-Sham equations, the general self-consistence searching method involves iterations with alternate solving of the Poisson and Schr\"{o}dinger equations. One of problems of such an approach is that the charge distribution renewed by means of the Schr\"{o}dinger equation solution does not conform to boundary conditions of Poisson equation for Coulomb potential. The resulting instability or even divergence of iterations manifests itself most appreciably in the case of infinitely extended systems. The published attempts to deal with this problem are reduced in fact to abandoning the original iterative method and replacing it with some approximate calculation scheme, which is usually semi-empirical and does not permit to evaluate the extent of deviation from the exact solution. In this work, we realize the iterative scheme of solving the Kohn-Sham equations for extended systems with inhomogeneous electron gas, which is based on eliminating the long-range character of Coulomb interaction as the cause of tight coupling between charge distribution and boundary conditions. The suggested algorithm is employed to calculate energy spectrum, self-consistent potential, and electrostatic capacitance of the semi-infinite degenerate electron gas bounded by infinitely high barrier, as well as the work function and surface energy of simple metals in the jellium model. The difference between self-consistent Hartree solutions and those taking into account the exchange-correlation interaction is analyzed. The case study of the metal-semiconductor tunnel contact shows this method being applied to an infinitely extended system where the steady-state current can flow.

Fluorescence quenching of quantum dots by gold nanorods and its application to DNA detection

Gold nanorods (GNRs) with different localized surface plasmon resonance (LSPR) peaks and different thicknesses of polymer-coating were synthesized to validate that the distinct GNR-based quenching process of quantum dot (QD) emission follows the metal surface energy transfer theory. The quenching efficiency is directly related to the spectral overlap of GNR-LSPR absorption and QD-emission, as well as the separation distance between the two nanoparticles. GNRs are advantageous for DNA conjugation, and the high efficiency of GNR-QD quenching system was utilized for sensitive DNA detection.

The negative adsorption of chromium atoms on alloy-oxide film boundaries during oxidation in air and anodic passivation of Fe-Cr and Ni-Cr alloys

Equilibrium of Cr atoms between the surface layer and bulk of a binary alloy was analyzed. The Gibbs adsorption equation was used to obtain the dependence of the adsorption activity of atoms in the surface layer on their activity in the bulk. An approximate thermodynamic method was used to calculate the adsorption of Fe (Ni) and Cr atoms in the surface layers of Fe-Cr and Ni-Cr alloys. According to calculations, there was negative adsorption, XCr ≪ 1, in the surface layer of the alloys caused by a large difference between the Gibbs surface energies of Cr and Fe (or Ni). The negative adsorption of Cr shifted chemical reaction equilibria on the alloy-oxide film boundary both in oxidation in air and in anodic passivation, 3FeO (NiO) + 2Cr = Cr2O3 + 3Fe(Ni), toward oxide film enrichment in the FeO (or NiO) oxide. A unified method for calculating the composition of oxide films on alloys was used for both processes. The method was based on the use of the initial data on the Gibbs surface energy of metals constituting alloys. The calculated oxide film compositions were close to the experimental X-ray photoelectron spectroscopy data.

Many-body effects on surface stress, surface energy and surface relaxation of fcc metals

The results of an investigation aimed at isolating the role played by pair-wise and many-body contributions in determining the surface stress, surface energy and surface relaxation for clean, low-index fcc metal surfaces are presented. General expressions for surface stress and surface energy are developed for generic embedded atom method (EAM) potentials and are contrasted with results derived from central-force pair potentials. Calculations are presented for specific forms. We find that these pair-wise and many-body contributions usually act in opposition to each other. Pair potentials tend to produce compressive surface stress contributions and favor outward surface relaxations, while the many-body component produces tensile surface stress contributions and favors inward relaxations. We also bring to light certain relationships between surface and bulk thermodynamic properties within the framework of these models. It is shown how surface stress can be expressed in an approximate form involving terms proportional to the product of an elastic modulus and the neighbor spacing characteristic of the material, and constants of proportionality that depend only on surface orientation. This form provides a good fit to a set of first-principles results as well. Finally, we demonstrate how various surface properties display a strong correlation with the ratio of bulk-to-shear moduli, B eq/ G eq.