Phonon Contribution to Thermal Boundary Conductance at Metal Interfaces Using Embedded Atom Method Simulations

The phonon contribution to the thermal boundary conductance (TBC) at metal–metal interfaces is difficult to study experimentally, and it is typically considered negligible. In this study, molecular dynamics simulations (MDS), employing an embedded atom method (EAM) potential, are performed to study the phonon contribution to thermal transport across an Al–Cu interface. The embedded atom method provides a realistic model of atomic behavior in metals, while suppressing the effect on conduction electrons. In this way, measurements on the phonon system may be observed that would otherwise be dominated by the electron contribution in experimental methods. The relative phonon contribution to the TBC is calculated by comparing EAM results to previous experimental results which include both electron and phonon contributions. It is seen from the data that the relative phonon contribution increases with decreasing temperature, possibly accounting for more than half the overall TBC at temperatures below 100 K. These results suggest that neglect of interfacial phonon transport may not be a valid assumption at low temperatures, and may have implications in the future development of TBC models for metal interfaces.     

Ni针尖/Au基体纳米压痕的分子动力学模拟

采用原子镶嵌势函数（EAM）模拟Ni针尖（约1．5mm）／Au基体纳米压痕过程．研究结果表明,当Ni针尖与Au基体间距离达到一定值时（约0．23 mm）,机械的不稳定性使得针尖与基体间发生跳跃接触,产生纳米压痕和黏附现象（Au原子包裹在Ni针尖周围）．当压头离开基体表面,Ni针尖被拔起,随后在针尖与基体间形成连续的由Au组成的细颈．同时计算得到整个系统在针尖接近基体、跳跃接触、压痕、黏附、形成缩颈和一系列分离过程中的势能变化．     

Collision cascades in pure δ -plutonium

Molecular dynamics simulations of the formation and annealing of large collision cascades in delta-phase plutonium are presented. The defect evolution is followed with time up to 2 ns. Simulations are performed with the MEAM potential at three different temperatures; at 600 K where the pure delta phase is thermodynamically stable; at 300 K where the delta phase can only be maintained in a metastable state with minor additions of gallium or aluminum; and at 180 K where plutonium should transform to the alpha phase. It is found in all three cases that the atomic structure within the cascade evolves through a glass-like state. At 600 K, this structure recovers very slowly; at 300 K it persist up to 2 ns with no discernable trend to recover eventually; and at 180 K the amorphous structure initiated by the collision cascade spreads through the entire crystal and converts it to a glass-like structure.     

Surface reconstruction and elastic behavior of silicon nanobeams: The impact of applied deformation

Using molecular dynamics calculations with a recently developed modified embedded atom method (MEAM) potential, the overall elastic behavior of silicon (100) nanobeams under externally applied strain at room temperature is investigated. As uniaxial tensile strain increases, the stability range of any relevant reconstructions changes, thus, the surface region undergoes a series of reconstructions. In nanostructures, such as nanoplates, nanobeams, and nanowires, this phenomenon is significant and changes the elastic response. The results indicate that the elastic behavior of nanostructures is not only size-dependent, but also load-dependent.     

An improved molecular dynamics potential for the Al-O system

Molecular dynamics (MD) simulations of aluminum oxide material and the aluminum oxidation process require a sufficiently sophisticated and well-calibrated potential, one that takes into account locally varying Al/O ratios and adaptive charge transfer between Al and O atoms. In this work we show that the Charge Transfer Ionic Potential (CTIP) by Zhou et al. [X.W. Zhou, H.N.G. Wadley, J.-S. Filhol, M.N. Neurock, Phys. Rev. B 69 (2004) 035402] in combination with a new, “Reference Free” version of the Modified Embedded Atom Method (RFMEAM) potential performs well for this purpose. This new potential has been parameterized by systematically fitting it to a large database of different Al_xO_y crystal energies, over a range of lattice constants and elastic deformations, using a recent method which separates the electrostatic and non-electrostatic fitting steps. The resulting potential yields more realistic atomic charges, crystal energies and lattice constants than earlier potentials. In particular, we show that the angular forces in the MEAM part are essential for α-Al₂O₃ to be the lowest-energy aluminum oxide. We compare the performance of our potential with the potential of Zhou et al., which lacks angular forces and was parameterized using a less involved fitting procedure, and show the results of a few molecular dynamics simulations. The two-step fitting method is generally applicable and can be adopted for constructing potentials for other metal-oxide systems.     

Effects of size and surface on the elasticity of silicon nanoplates: Molecular dynamics and semi-continuum approaches

In this paper, the size effects on the elastic behavior of single crystal silicon nanoplates terminated by {100} surfaces is studied by means of molecular dynamics (MD) using a modified embedded atom method. The results indicate that the {100} surfaces undergo 2 × 1-type reconstruction, which significantly influences the mechanical properties of nanoplates. The simulations are carried out at room temperature and structural relaxation is performed. The effective Young's modulus, in extensional mode, is determined for different thicknesses. The surface energy, surface stress and surface elasticity of layers near the surfaces (non-bulk layers) are obtained. These surface properties are used as inputs for a recently developed two-dimensional plane-stress semi-continuum framework. The framework can be seen as the link between the surface effects calculated by atomistic simulations and the overall elastic behavior. The surface properties of nanoplates of a few layers are shown to deviate from thicker plates, suggesting a size dependence of surface parameters and, especially, surface energy. For thicknesses below 3 nm, there is a difference between the effective Young's modulus, calculated by the semi-continuum approach and that calculated directly by MD. The difference is due to the size dependence of surface parameters.     

Enthalpies of Formation of Noble Metal Binary Alloys Bearing Rh or Ir

The modified embedded atom method proposed by authors has been applied to calculating the enthalpies of formation of random alloys and the ordered intermetallic compounds for noble metal binary systems bearing Rh or Ir. The present results are in good agreement with those of Miedema theory, available experiments and the first-principles quantum mechanics calculations. The present results indicate that Cu-Rh, Cu-Ir, Ag-Rh, Ag-Ir, Au-Rh, Au-Ir, Pd-Rh and Pd-Ir systems are repulsive, however, Ni-Rh, Ni-Ir, Pt-Ir, Pt-Rh and Rh-Ir systems form solid solutions and Ni-Rh, Ni-Ir and Pt-Rh show ordering tendency.     

Modeling of hydrogen embrittlement in single crystal Ni

A large-scale molecular dynamics simulation by the embedded atom method was carried out on hydrogen embrittlement of a single crystal containing 1,021,563 nickel atoms. The details of the deformation in the specimen were identified by a new method of the deformation analysis. Plenty of slip deformation occurred around the crack tip and in the bulk of the hydrogen-free specimen. Hydrogen embrittlement was most serious in the specimen hydrogen-charged in the notched area. Serious embrittlement was also observed in the specimen hydrogen-charged in the slip planes, in which dislocation emission was localized at the crack tip and enhanced on the planes where hydrogen atoms were located. It is considered that the fracture process is due to the hydrogen-enhanced decohesion mechanism.     

Corrosion Behavior of High Energy–High Rate Consolidated Graphite/Copper Metal Matrix Composites in Chloride Media

The corrosion behavior of particulate reinforced graphite/copper (Gr_p/Cu) metal matrix composites (MMCs) was studied in 3.5 wt.% sodium chloride solution using electrochemical techniques, ionic solution analysis, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD) techniques. The materials under investigation were high energy-high rate (HEHR) consolidated Gr_p/Cu metal matrix composites. HEHR processing employs a 10 MJ homopolar generator that supplies a 100 kA pulse to rapidly heat and solidify the composite powder compact. This short time at high temperature and the preferential heating and melting at the graphite-copper interface serve to encapsulate the graphite reinforcement, thus providing a highly densified composite product. Initially the open circuit potential corrosion behavior of 1.2, 5, 15, 25, and 40 volume percent Gr_pCu composites was studied in aerated and deaerated 3.5 wt.% NaCl solution using SEM and EDX. Subsequently, the environmental stability of these composites was studied using electrochemical techniques such as polarization resistance and potentiodynamic polarization. The severity of corrosive attack increased with increasing graphite content and in aerated solutions. In addition, solutions from these tests were analyzed to determine the relative amounts of copper and carbon present in the electrolyte after polarization tests. Microscopic analysis techniques were used to characterize the corrosion morphologies and the extensive localized corrosion occurring at the graphite-copper interface. The effectiveness of benzotriazole as a corrosion inhibitor for the copper MMCs was also studied.     

Nucleation of dislocations and twins in fcc nanocrystals: Dynamics of structural transformations

This paper reports on a molecular dynamics study of structural rearrangements in a copper nanocrystal during nucleation of plastic deformation under uniaxial tension. The study shows that the resulting nucleation of partial dislocations on the free surface and their glide occurs through local fcc→bcc→hcp transformations via consistent atomic displacements. We propose an atomic model for the generation of dislocations and twins based on local reversible fcc→bcc→fcc transformations, with the reverse one proceeding through an alternative system. The model gives an insight into possible causes and mechanisms of the generation of partial dislocations and mechanical twins in two and more adjacent planes of plastically deformed nanocrystals. The obtained data allow a better understanding of how plasticity is generated in nanostructured materials.     

Role of Thick‐Lithium Fluoride Layer in Energy Level Alignment at Organic/Metal Interface: Unifying Effect on High Metallic Work Functions

The function of ≈3‐nm thick lithium fluoride (LiF) buffer layers in combination with high work function metal contacts such as coinage metals and ferromagnetic metals for use in organic electronics and spintronics is investigated. The energy level alignment at the organic/LiF/metal interface is systematically studied using photoelectron spectroscopy and the integer charge transfer model. The thick‐LiF buffer layer is found to pin the Fermi level to ≈3.8 eV, regardless of the work function of the initial metal due to energy level bending in the LiF layer caused by depletion of defect states. At 3‐nm thickness, the LiF buffer layer provides full coverage, and the organic semiconductor adlayers are found to physisorb with the consequence that the energy level alignment at the organic/LiF interface follows the integer charge transfer model's predictions.