Clustering Of Cu Atoms Research Articles

Substrate‐Adatom Interface Engineering of Transition Metal Decorated Boron‐Doped Graphene Sheet for Enhanced Adsorption of Xe and Kr – A Systematic Ab initio Study

AbstractEfficient adsorption and segregation of Xe and Kr gases are of high importance in commercial and nuclear industries. Systematic ab initio calculations reveal that transition‐metal (TM) decorated boron‐doped graphene (BDG‐TM) sheet can act as an efficient substrate for adsorptive capture of Xe and Kr (adatoms). Substantial enhancement in the adsorption energy is obtained on BDG‐TM substrates, and it varies as BDG‐Cu > BDG‐Ni > BDG‐Fe > BDG‐Zn. The improvement is approximately four times that of the pristine BDG and twice that of the conventional metallic substrates. TM‐decoration alters the charge distribution at the substrate‐adatom interface, which brings a considerable change in the polarization and induced dipole moment of adatom, leading to significant improvement in the . Partial density of states analysis shows a splitting and significant interaction of Xe‐p with TM‐d orbitals near the Fermi level of Fe, Ni, and Cu decorated systems, unveiling strong adsorption. Further, the effect of clustering and dispersion of Cu atoms on are analyzed using a first‐principle‐based genetic algorithm, which reveals that clustering of Cu atoms deteriorates the of Xe and Kr. Thus, for experimental realization, a BDG sheet with uniformly dispersed fine Cu particles is proposed as a substrate.

Effects of Al on the precipitation of B2 Cu-rich particles in Fe–Cu ferritic alloy: Experimental and theoretical study

The fundamental mechanism of the precipitation kinetics and stability of nano-sized Cu-precipitates are crucial to the development of ultra-high strength low carbon ferritic steels. The effects of Al on the formation and stabilization of B2 Cu-rich precipitates in Fe–Cu based steels were studied using experimental method combining the first-principles calculations and molecular dynamics simulations. With increasing aging time, the Cu-rich particles with B2 structure precipitated in the ferritic matrix with cube-on-cube and coherent relationship, and Al addition leads to a relatively higher precipitation hardening effect. The first-principle calculations indicate that the metastable B2 FeCu phase has relatively higher mixing energy and small lattice mismatch with the ferritic matrix, and the addition of Al leads to the lower mixing energy of B2 precipitates and the B2 structure Fe(CuAl) compound possesses more smaller lattice mismatch with ferritic matrix. To better understanding the effect of Al on the evolution process of Cu-rich particles, it is essential to investigate the migration and clustering process of Cu atoms during the isothermal tempering at the atomic scale. To this end, we first developed a new interatomic potential of Fe–Cu–Al using the force-matching method based on ab initio calculations, then the formation and evolution of Cu clusters in the Fe–Cu–Al ternary were studied using molecular dynamics method with the new EAM potential. The simulation results indicate that Al addition promotes the clustering of Cu atoms due to the attractive interaction between Cu and Al atoms which enhances nucleation rate of the Cu-rich precipitates.

Temperature-dependent nucleation kinetics of Guinier-Preston zones in Al–Cu alloys: An atomistic kinetic Monte Carlo and classical nucleation theory approach

Guinier-Preston (GP) zones, which form in Al–Cu alloys, exhibit characteristic patterns of Cu-rich, disk-shaped atomic clusters along the {100} planes. GP zones have received much attention not only for their precipitation-hardening effect on the Al matrix, but also from fundamental interest in the physics and chemistry of the nanoscale organization of solute atoms. In this study, we established an atomistic kinetic Monte Carlo (kMC) modeling technique for exploring the nucleation and formation processes of GP zones in Al–Cu alloys by constructing an effective on-lattice multibody potential for a dilute Al–Cu-vacancy system by density functional theory calculations. Our model can describe the clustering of Cu atoms via successive exchanges with vacancies and reproduce the characteristic planar nanoprecipitates along the {100} planes in a manner consistent with the crystallographic nature of the GP zones in the early-stage aging of Al–Cu alloys. The time evolution and critical nucleus size of Cu clusters were characterized at various temperatures based on the kMC results. Consequently, we predicted the existence of an optimum temperature (i.e., nose temperature) for the formation of Cu clusters at which the cluster growth was maximized, which was attributable to the interplay between the critical nucleation barrier and the diffusion rate. In addition, the critical nucleus size and temperature for cluster formation were examined based on classical nucleation theory along with the developed multibody potential. These provided an insight into the competition between the enthalpic and entropic effects on the formation of GP zones in the Al–Cu system.

Nonlinear ultrasonic response of voids and Cu precipitates in body-centered cubic Fe

Interpreting nonlinear ultrasonic signals detected in a nondestructive evaluation of radiation damage requires the knowledge of the correlation between defects and nonlinearity. In this work, molecular dynamics simulations are performed to study the effect of distributed vacancies, voids, Cu atoms, and Cu precipitates on the nonlinear ultrasonic response in body-centered cubic (bcc) Fe. The nonlinearity parameter calculated from the second harmonic amplitude in the perfect lattice is 2.73. Vacancies are found to increase the nonlinearity. However, clusters of vacancies in the form of spherical voids show an opposite effect. This finding can be used to conveniently distinguish vacancies from voids in the material. Unlike vacancies, individual Cu atoms decrease the nonlinearity. Clustering of Cu atoms into Cu precipitates further decreases the nonlinearity. Interestingly, precipitates with a diameter of 2 nm and larger exhibit a similar effect despite their different structure and coherency with the Fe matrix.

Direct bonding of copper and liquid crystal polymer

Copper (Cu) film bonded with polymer substrates offers better dimensional stability than that of deposited Cu especially in humid environment. However, poor adhesion is an issue in both cases. In this paper, we demonstrate a two-step, low temperature direct bonding technique for liquid crystal polymer (LCP) and copper (Cu) using oxygen plasma. The bonded interface showed high peel strength. This strong adhesion is due to the interdiffusion of Cu and carbon atoms into LCP and Cu film respectively. These phenomena resulted in three-dimensional metal clusters of Cu atoms and islands at the LCP/Cu interface, which gradually diffuse into the LCP. Interfacial elemental analysis reveals an intermediate oxide layer formation due to Cu2O and CO.

Atomic scale study of CU clustering and pseudo-homogeneous Fe–Si nanocrystallization in soft magnetic FeSiNbB(CU) alloys

A local electrode atom probe has been employed to trace the onset of Cu clustering followed by their coarsening and subsequent growth upon rapid (10s) annealing of an amorphous Fe73.5Si15.5Cu1Nb3B7 alloy. It has been found that the clustering of Cu atoms introduces heterogeneities in the amorphous matrix, leading to the formation of Fe rich regions which crystallizes pseudo-homogeneously into Fe–Si nanocrystals upon annealing. In this paper, we present the data treatment method that allows for the visualization of these different phases and to understand their morphology while still quantifying them in terms of their size, number density and volume fraction. The crystallite size of Fe–Si nanocrystals as estimated from the atom probe data are found to be in good agreement with other complementary techniques like XRD and TEM, emphasizing the importance of this approach towards accurate structural analysis. In addition, a composition driven data segmentation approach has been attempted to determine and distinguish nanocrystalline regions from the remaining amorphous matrix. Such an analysis introduces the possibility of retrieving crystallographic information from extremely fine (2–4nm sized) nanocrystalline regions of very low volume fraction (< 5Vol%) thereby providing crucial in-sights into the chemical heterogeneity induced crystallization process of amorphous materials.

Atom probe tomography study of ultrahigh nanocrystallization rates in FeSiNbBCu soft magnetic amorphous alloys on rapid annealing

Rapid annealing (4–10s) induced primary crystallization of soft magnetic Fe–Si nanocrystals in a Fe73.5Si15.5Cu1Nb3B7 amorphous alloy has been systematically studied by atom probe tomography in comparison with conventional annealing (30–60min). It was found that the nanostructure obtained after rapid annealing is basically the same, irrespective of the different time scales of annealing. This underlines the crucial role of Cu during structure formation. Accordingly, the clustering of Cu atoms starts at least 50°C below the onset temperature of primary crystallization. As a consequence, coarsening of Cu atomic clusters also starts prior to crystallization, resulting in a reduction of available nucleation sites during Fe–Si nanocrystallization. Furthermore, the experimental results explicitly show that these Cu clusters initially induce a local enrichment of Fe and Si in the amorphous matrix. These local chemical heterogeneities are proposed to be the actual nuclei for subsequent nanocrystallization. Nevertheless, rapid annealing in comparison with conventional annealing results in the formation of ∼30% smaller Fe–Si nanocrystals, but of identical structure, volume fraction and chemical composition, indicating the limited influence of thermal treatment on nanocrystallization, owing to the effect of Cu.

Exploring the First Steps in Core–Shell Electrocatalyst Preparation: In Situ Characterization of the Underpotential Deposition of Cu on Supported Au Nanoparticles

The underpotential deposition (upd) of a Cu shell on a non-Pt nanoparticle core followed by galvanic displacement of the Cu template shell to form core–shell electrocatalyst materials is one means by which the Pt-based mass activity targets required for commercialization of PEM fuel cells may be reached. In situ EXAFS measurements were conducted at both the Au L3 and the Cu K absorption edges during deposition of Cu onto a carbon-supported Au electrocatalyst to study the initial stages of formation of such a core–shell electrocatalyst. The Au L3 EXAFS data obtained in 0.5 mol dm–3 H2SO4 show that the shape of the Au core is potential dependent, from a flattened to a round spherical shape as the Cu upd potential is approached. Following the addition of 2 mmol dm–3 Cu, the structure was also measured as a function of the applied potential. At +0.2 V vs Hg/Hg2SO4, the Cu2+ species was found to be a hydrated octahedron. As the potential was made more negative, single-crystal studies predict an ordered bilayer of sulfate anions and partially discharged Cu ions, followed by a complete/uniform layer of Cu atoms. In contrast, the model obtained by fitting the Au L3 and Cu K EXAFS data corresponds first to partially discharged Cu ions deposited at the defect sites in the outer shell of the Au nanoparticles at −0.42 V, followed by the growth of clusters of Cu atoms at −0.51 V. The absence of a uniform/complete Cu shell, even at the most negative potentials investigated, has implications for the structure, and the activity and/or stability, of the core–shell catalyst that would be subsequently formed following galvanic displacement of the Cu shell.

Structural evolution and the kinetics of Cu clustering in the amorphous phase of Fe-Cu-Nb-Si-B alloy

An attempt has been made to investigate the evolution of the structure of the amorphous phase of Fe73.9 Cu0.9 Nb3.1 Si13.2 B8.9 (finemet) alloy by a combination of wide-angle x-ray scattering, small angle x-ray scattering (SAXS), Mössbauer spectroscopy and X-ray absorption near edge spectroscopy on the supposition that they would provide complementary information. Before the onset of nanocrystallization, the amorphous phase undergoes a structural relaxation resulting in small increase in the hyperfine field and a decrease in the width of the first diffraction maxima. There is an increase in the topological ordering in the system, though chemical inhomogeneity sets-in due to the clustering of Cu atoms in the pure amorphous state of this alloy. Annealing at 400 °C (well below the crystallization temperature) for different time durations results in occurrence of Cu clusters having fcc structure. Kinetics of Cu clustering is studied using SAXS. The incubation time for the clustering at 400 °C is ∼120 min. With further annealing, the average cluster size gradually increases from the initial value of ∼0.4 nm, reaching a value of ∼0.6 nm after annealing for 720 min. Cluster size exhibits a t1/2 dependence, suggesting a diffusion controlled growth.

Interaction of He with Cu, V, and Ta in bcc Fe: A first-principles study

Precipitates often play key roles in improving the mechanical performance of structural materials. Using first-principles density functional theory method, we have calculated the geometry and energetics of small Xn and XnHe (X = Cu, V, and Ta) clusters in bcc Fe matrix to investigate the effect of He on X precipitation on the initial stage in neutron-irradiated Fe alloys. Both substitutional and interstitial He attract solute atoms. The attraction of a substitutional He and a Cu atom is as strong as 0.30 eV (nearest neighbor) or 0.25 eV (next-nearest neighbor), even stronger than the vacancy-Cu pair. Such an attraction facilitates the clustering of Cu atoms. By comparison, the attraction of He to V (0.02 eV) or Ta (0.22 eV) is weaker than that of a vacancy. We find that one He can bind up to four Ta atoms to form a tetrahedron, despite the fact that in the absence of He, Ta atoms prefer to stay away from each other. The effect of He on the solute-solute and solute-matrix interactions can be understood from the facts that He behaves both as a free-volume filler and as a bonding insulator.

(Meta-)stable phases and pre-Guinier-Preston zones in AlCu alloys constructed from ab initio relaxed atomic positions - Comparison to experimental methods

Vacancies on different sublattices in the stable and meta-stable Al2Cu phases Θ and Θ′ have been studied by ab initio calculations. We calculate their formation energies and relaxation features. These phases exist in over-aged dilute AlCu alloys and have to grow under Al-rich conditions. Our calculations show that the preferred site for structural vacancies is on the Cu sublattice. Characteristic for early stages of aging dilute AlCu alloys are pre-Guinier–Preston (GP) zones. Here, we calculated the atomic arrangements of small clusters of Cu atoms in the aluminum matrix (Cu platelets on the {100}-planes of Al). Strong – energy lowering – relaxations of Al atoms toward the habit plane of Cu atoms possibly explain the stability of these planar configurations favored over three-dimensional agglomerations in real materials. The ab initio atomic positions for Θ- and Θ′-phases as well as for pre-GP zones have been finally used to calculate both X-ray absorption fine structure spectra and positron annihilation data – namely positron lifetimes and momentum distributions. Comparisons to the few existing experimental data are discussed and suggestions for further experiments are given.

The Clustering of Cu Atoms in Neutron Irradiated Reactor Pressure Vessel Steels Studied by Positron Annihilation

Defect studies of neutron-irradiated Cr-Mo-V (VVER-440) type reactor pressure vessel steels were performed in the present work. The steels were irradiated in the nuclear power plant reactor under the conditions of a regular operation. Characterization of the irradiation induced defects was performed by two complementary techniques of positron annihilation spectroscopy: (i) positron lifetime spectroscopy was used for identification of defects and determination of defect densities, (ii) coincidence Doppler broadening was employed for investigation of Cu atom aggregates. Long range diffusion of Cu atoms is assisted by the irradiation induced vacancies. The solute Cu atoms form small clusters in the irradiated steels. Subsequent isochronal annealing of the irradiated steel leads to vacancy assisted clustering of Cu atoms and formation of small precipitates. The Cu clusters exhibit maximum diameter at 400oC. Above this temperature the clusters dissolve again in the matrix.

Interaction between Cu and Sn in the Early Stages of Ageing of Al-1.7at.%Cu-0.01at.%Sn

The interaction between vacancies and Sn and Cu solute atoms in an Al-1.7at.%Cu- 0.01at.%Sn alloy was investigated by exploring the effect of incorporating natural ageing into conventional age hardening treatment. It was found that provided the artificial ageing temperature does not exceed a critical value between 160°C and 200°C, a narrow window of natural ageing (3-100 h) will result in a significant acceleration of the age hardening response and no decrease in peak hardness. Transmission electron microscopy showed that this effect reflects a large and rapid increase in number density of Cu GP(I) zones, and, to a lesser extent, of θ". The distribution and number density of θ' are essentially unaffected. Three-dimensional atom probe provided strong evidence that refinement of GP(I) zone distribution is not due to clustering of Cu atoms onto pre-existing Sn clusters. Instead it appears to be caused by a subtle interaction between vacancies, Sn and Cu atoms.

Hardening and microstructural evolution in A533B steels under high-dose electron irradiation

Commercial A533B steels (0.12, 0.16wt% Cu) irradiated at 290°C up to 22mdpa with 5MeV electrons were examined by hardness measurements, positron lifetime spectroscopy, the coincidence Doppler broadening (CDB) technique and three-dimensional atom probe microanalysis (3DAP). The radiation-induced hardening increased with electron dose and lay on the same trend of neutron-induced hardening within data scatter. CDB measurements revealed that clustering of Cu atoms occurred at doses over 1mdpa and proceeded with increasing dose. 3DAP results showed that well-defined Cu-rich precipitates with a diameter of less than 2nm were formed at doses of 10 and 22mdpa. The precipitates had a shell structure consisting of a Cu–Fe core region and a surrounding Mn–Ni–Si shell, which are similar to those formed under neutron irradiation. The size and number density of the precipitates were consistent with previous neutron data. Positron lifetime spectroscopy showed that no microvoids were formed. The electron irradiation caused almost the same hardening efficiency and evolution of Cu-rich precipitates as those under neutron irradiation on a dpa basis.

First-principles prediction of equilibrium precipitate shapes in Al-Cu alloys

A first-principles thermodynamic approach for studying coherent precipitation in size-mismatched alloys is presented and applied to the well studied (but still controversial) problem of Guinier-Preston (GP) zone formation in Al-Cu alloys. The exceptionally soft anharmonic elastic response of Cu along {100} is crucial in determining the shape of clusters of Cu atoms embedded in the Al matrix which are plate-like {100} structures. First-principles calculations involving both strain and interfacial energies yield two types of discshaped precipitate, namely GP1 and GP2, which are single Cu monolayers and two monolayers separated by three A1 layers respectively, in agreement with many (but not all) measurements. Competition between the thermodynamic driving force (favouring GP2) and interfacial energy around the perimeter of the zones (favouring GP1) leads to a size-dependent transition in the equilibrium precipitate shape from GP1 to GP2 at about 150 A. This theoretical approach (generally applicable to metals, ceramics and semiconductors) should aid in understanding the coherent precipitate shapes in a variety of alloy materials.

Copper as an additive in the As 0.4Se 0.3Te 0.3 glass: Mean atomic volume and Tg

Data on the mean atomic volume ( V) and the glass transition temperature ( T g) of the As 0.4Se 0.3Te 0.3 glass, modified with Cu are reported and discussed. The V and T g of the parent glass are invariant with addition of ≤1 atomic percent (at.%) of Cu up to which clusters of Cu atoms occupy interlayer positions in the layered network of the parent glass. For additions of Cu>1 at.%, a decrease in V and an increase in T g are observed. The composition dependencies of V and T g are consistent with a model according to which, with progressive increase in the Cu content, some of the original As 2Se 3 and As 2Te 3 structural units of the parent glass are replaced, respectively, by CuAsSe 2 and CuAsTe structural units. For Cu≥23.08 at.%, after the formation of CuAsSe 2 structural units, there is no free Se available for the formation of As 2Se 3 structural units; as no free Se is available, a change of slope occurs in the V–composition and T g–composition data at the Cu content of 23.08 at.%. The increase in T g with increasing Cu content is due to an increase in the average coordination number brought about by an increase in cross linking due to the formation of tetrahedral CuAsSe 2 and CuAsTe complexes in the glasses.

APFIM Studies of Nanocrystallizations of Amorphous Alloys

AbstractThis paper reports recent atom probe analysis results of Fe-Zr-B(-Cu) and Al-Ni-Ce(-Cu) amorphous alloys, in which nanocrystalline microstructures develop by primary crystallization. In these alloy systems, enrichment of slow diffusing solute was found at the interfaces between primary crystals and amorphous matrix during the nucleation and growth stage. In the case of ternary Fe-Zr-B, no evidence for compositional heterogeneities were found prior to the onset of crystallization reaction. On the other hand, clustering of Cu atoms was observed in quaternary Fe-Zr-B-Cu alloy prior to the crystallization reaction. In the ternary Fe-Zr-B alloy, nucleation sites seem to be provided by the quenched-in nuclei which were observed as medium range ordered (MRO) domains by HREM. In the as-quenched Al-Ni-Ce(-Cu) alloy, compositional fluctuations were present from the as-quenched state. These observations suggest that nuclei for primary crystallization are provided in various forms such as MRO domains, solute clusters and compositional heterogeneities.

Computer Simulations of Cluster-Solid Interactions: Energetic Clusters and Soft Landings

ABSTRACTThe interaction of small energetic clusters of Cu atoms with Cu surfaces has been investigated by molecular dynamics computer simulations. The simulations are fully dynamical and three dimensional; they employ embedded atom method potentials. The simulations described here are an extension of our past work on cluster solid interactions [1]. In this work, the effects of the angle of incidence, energy, size and temperature of the cluster are examined. In addition, soft landings of Cu clusters on Cu are simulated.

Electron states of ultrathin Pd and Cu films on (001)Ag

We present the results of a theoretical calculation of the electronic structure of a monolayer of Pd and Cu deposited on the (001) surface of Ag. The adsorbate atoms are placed in a central site with the same lateral periodicity as a parallel plane of the substrate. The method of calculation is a parametrized LCAO with hopping integrals determined through a fit to bulk data. Charge transfer effects are taken approximately into account by assuming the intraatomic parameters to vary as the valence orbital ionization potentials. Energy bands and local densities of states are given and the nature and location of the adsorbate states are discussed. The results are compared with the experimental data: good agreement is found for the Pd on Ag system. For Cu on Ag we find that our assumptions do not explain the photoemission data and suggest that some clustering of Cu atoms occurs during the deposition.

NMR study of magnetic transition in the Cu1-xNixsystem

The 63Cu NMR on CuNi alloys in the critical concentration range for the transition to the paramagnetic state (45 at.%<or=x<or=54 at.%) has been studied between 4.2K and 300K by the spin-echo technique. The magnetic properties of such alloys are known to be strongly dependent on local environment effects and chemical clustering. Two types of CuNi alloys which are characterized by a different degree of chemical homogeneity have been examined. The observed inhomogeneously broadened 63Cu line exhibits features which are characteristic of the magnetic polarization due to Ni atoms in different magnetic states (magnetic and nearly magnetic) resulting from local environment effects. From the analysis of the NMR parameters a model is proposed to interpret the magnetic polarization of the observed Cu atoms showing the fundamental role of nearly magnetic atoms in the vicinity of these Cu sites. At the same time it is shown that chemical inhomogeneities resulting from clustering of Cu atoms strongly reduce the magnetic polarization of these atoms.