Mingpu Wang

The particle size dependence of cohesive energy of metallic nanoparticles

Based on the concept that the cohesive energy can be regarded as the required energy to separate the metallic crystal into individual atoms by destroying the metallic bonds, a method is developed in this letter to calculate the cohesive energy of metallic nanoparticles. The calculation predictions are consistent with the experimental values of Mo and W nanoparticles.

A simplified model to calculate the surface-to-volume atomic ratio dependent cohesive energy of nanocrystals

A simplified model for the surface-to-volume atomic ratio dependent cohesive energy of nanocrystals is obtained in this letter. On the basis of the model, we predict that the cohesive energy can not only decrease but also increase with the increasing surface-to-volume atomic ratio under different conditions. The model predictions are found to be in agreement with the experimental values of Mo and W nanoparticles. The conditions on the cohesive energy of an embedded nanocrystal increasing or decreasing with the increasing surface-to-volume atomic ratio are discussed. The relations between the cohesive energy of nanoparticles, nanowires and nanofilms, and the shape effect on nanoparticles are also discussed. Since the cohesive energy is regarded as being directly related to the nature of the thermal stability of nanocrystals, the relationship to the phenomenon of melting and superheating given by our cohesive energy model seems sensible.

Dynamics of phase transformation of Cu-Ni-Si alloy with super-high strength and high conductivity during aging

The precipitation behaviors of the Cu-Ni-Si alloys during aging were studied by analyzing the variations of electric conductivity. The Avrami-equation of phase transformation kinetics and the Avrami-equation of electric conductivity during aging were established for Cu-Ni-Si alloys, on the basis of linear relationship between the electric conductivity and the volume fraction of precipitates, and the calculation results coincide well with the experiment ones. The transformation kinetics curves were established to characterize the aging process. The characteristics of precipitates in the supersaturated solid solution alloy aged at 723 K were established, and the results show that the precipitates are β-Ni3Si and δ-Ni2Si phases.

Size‐, Shape‐ and Composition‐Dependent Alloying Ability of Bimetallic Nanoparticles

Based on the surface-area-difference model, the formation enthalpies and the formation Gibbs free energies of bimetallic nanoparticles are calculated by considering size and shape effects. Composition-critical size diagrams were graphed for bulk immiscible bimetallic nanoparticles with the developed model. The results reveal that both the formation enthalpy and formation Gibbs free energy decrease with the decrease of particle size. The effect of rising temperature is similar to the diminishing of particle size on reducing the formation Gibbs free energy. Contrary to the positive formation enthalpy of the bulk immiscible system, a negative formation enthalpy is obtained when the particles are smaller than a critical size. With the decrease of size, the alloying process first takes place in the dilute solute regions, then broadens to the dense solute regions and finally, particles with all compositions can be alloyed. The composition-critical size diagram is classified into three regions by the critical size curves with shape factors of 1 and 1.49, that is, the non-alloying region, alloying region and possible alloying region. The model predictions correspond well with experimental evidences and computer simulation results for Cu-Ag, Au-Ni, Ag-Pt and Au-Pt systems.

Structure of Unsupported Small Palladium Nanoparticles.

A tight binding molecular dynamics calculation has been conducted to study the size and coordination dependence of bond length and bond energy of Pd atomic clusters of 1.2–5.4 nm in diameter. It has been found that the bond contraction associated with bond energy increases in the outermost layer about 0.24 nm in a radial way, yet in the core interior the bond length and the bond energy remain their corresponding bulk values. This surface bond contraction is independent of the particle size.

Structure characteristic and its evolution of Cu-W films prepared by dual-target magnetron sputtering deposition

Abstract Immiscible Cu-W alloy thin films were prepared using dual-target magnetron sputtering deposition process. The structure evolution of Cu-W thin films during preparation was investigated by X-ray diffraction, transmission electron microscopy and high resolution transmission electron microscopy. In the initial stage of dual-target magnetron sputtering deposition process, an amorphous phase formed; then it crystallized and the analogy spinodal structure formed due to the bombardment of the sputtered particles during sputtering deposition process, the surface structure of the film without the bombardment of the sputtered particles was the amorphous one, the distribution of the crystalline and amorphous phase showed layer structure. The solid solubility with the analogy spinodal structure was calculated using the Vegard law. For Cu-13.7%W (mole fraction) film, its structure was composed of Cu-11%W solution, Cu-37%W solution and pure Cu; for Cu-14.3%W film, it was composed of Cu-15%W solution, Cu-38%W solution, and pure Cu; for Cu-18.1%W film, it was composed of Cu-19%W solution, Cu-36% W solution and pure Cu.

Enhanced adhesion of Cu-W thin films by ion beam assisting bombardment implanting

Abstract Cu-W thin film with high W content was deposited by dual-target DC-magnetron co-sputtering technology. Effects of the substrates surface treating technique on the adhesive strength of Cu-W thin films were studied. It is found that the technique of ion beam assisting bombardment implanting of W particles can remarkably improve the adhesive property of Cu-W thin films. Indentation and scratching test show that, the critical load is doubled over than the sample only sputter-cleaned by ion beam. The enhancing mechanism of ion beam assisting bombardment implanting of Cu-W thin films was analyzed. With the help of mid-energy Ar+ ion beam, W atoms can diffuse into the Fe-substrate surface layer; Fe atoms in the substrate surface layer and W atoms interlace with one another; and microcosmic mechanical meshing and diffusing combination on atom-scale among the Fe and W atoms through the film/substrate interface can be formed. The wettability and thermal expansion properties of the W atoms diffusion zone containing plentiful W atoms are close to those of pure W or W-based Cu-W film.

Unique Cu@CuPt Core–Shell Concave Octahedron with Enhanced Methanol Oxidation Activity

Although tremendous efforts have been devoted to the exploration of cost-effective, active, and stable electrochemical catalysts, only few significant breakthroughs have been achieved up to now. Therefore, exploring new catalysts and improving catalyst activity and stability are still major tasks at present. Controllable synthesis of Pt-based alloy nanocrystals with a uniform high-index surface and unique architecture has been regarded as an effective strategy to optimize their catalytic efficiency toward electrochemical reactions. Accordingly, here we present a one-pot facile solvothermal process to synthesize novel unique Cu@CuPt core-shell concave octahedron nanocrystals that exhibit both outstanding activity and long durability. By regulating temperatures during the synthesis process, we were able to control the reduction rate of Cu and Pt ions, which could subsequently lead to the sequential stacking of Cu and Pt atoms. Owing to the concave structure, the as-prepared core-shell nanoparticles hold a high-index surface of {312} and {413}. Such surfaces can provide a high density of atomic steps and terraces, which is suggested to be favorable for electrochemical catalysts. Specifically, the Cu@CuPt core-shell concave octahedron presents 8.6/13.1 times enhanced specific/mass activities toward the methanol oxidation reaction in comparison to those of a commercial Pt/C catalyst, respectively. Meanwhile, the as-prepared catalyst exhibits superior durability and antiaggregation properties under harsh electrochemical conditions. The facile method used here proposes a novel idea to the fabrication of nanocrystals with desired compositional distribution, and the as-prepared product offers exciting opportunities to be applied in direct methanol fuel cells.

Effects of dimensionality on the ballistic phonon transport and thermal conductance in nanoscale structures

Using the scattering-matrix method, we investigate ballistic phonon transport and thermal conductance at low temperatures in a two-dimensional and a three-dimensional nanoscale structures modulated with a finite Cu/W superlattice. A comparative analysis for two-dimensional and three-dimensional models is made. The results show that the thermal conductance in three-dimensional model is larger than that in two-dimensional model for same structural parameters and temperature. In characteristics, however, the thermal conductance displays similar behaviors in both two-dimensional and three-dimensional models. Moreover, some interesting physical phenomena such as stop-frequency gap, resonant transmission, and universal quantized thermal conductance are observed in the structures. A brief analysis of these results is given.

Microstructure evolution and thermal stability of nanocrystalline Cu-Nb alloys during heat treatment

Abstract The microstructure evolution and high thermal stability of the mechanically-alloyed supersaturated nanocrystalline Cu-10%Nb alloy during subsequent heat treatment were investigated by X-ray diffractometry and transmission electron microscopy (TEM). The results show that no significant change of the microstructure of the solid solution can be detected after annealing at 300–400 °C. The pronounced phase separation can be detected at 700 °C. After annealing for 30 min at 900 °C, almost all the Nb atoms precipitate from the solid solution, and the average Cu grain size is about 37 nm. As the solute atoms hinder the migration of fcc phase, at Cu grain boundaries, no significant grain growth occurs before large amount of Nb atoms precipitates from Cu matrix, and the decrease of internal strain and density of dislocation is small. Furthermore, the nanosized Nb precipitates can also help to reduce the Cu grains growth through precipitating pinning effect. Therefore, the mechanically-alloyed nanocrystalline Cu-Nb alloys have a high thermal stability. And the contaminations brought into the Cu matrix by milling can influence the phase formation and the thermal stability of Cu-Nb alloys during heat treatment.