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Saddle-point energies and Monte Carlo simulation of the long-range order relaxation in CoPt

EUROPEAN PHYSICAL JOURNAL B 2005年第4期45卷 443-448页

作者： Allalen, M Bouzar, H Mehaddene, T Univ Osnabruck Fachbereich Phys D-49069 Osnabruck Germany Univ M Mammeri LPCQ Tizi Ouzou 15000 Algeria Tech Univ Munich Dept Phys D-85747 Garching Germany

We present atomic-scale computer simulations in equiatomic L1(0)-CoPt where Molecular Dynamics and Monte Carlo techniques have both been applied to study the vacancy-atom exchange and kinetics relaxation. The atomic potential is determined using a Tight-Binding formalism within the Second-Moment Approximation. It is used to evaluate the different saddle-point energies involved in a vacancy-atom exchange between nearest-neighbour sites. The potential and the saddle-point energies have been used to simulate the relaxation of the long-range order in CoPt using a Monte Carlo technique. A vacancy migration energy of 0.73 +/- 0.15 eV and an order-disorder transition temperature of 935 K have been found.

关键词： structural modeling: serial-addition models, computer simulation Order-disorder transformations, statistical mechanics of model systems Self-diffusion in metals, semimetals, and alloys

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Single-crystal structural study of the pressure-temperature-induced dimerization of C₆₀

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EUROPEAN PHYSICAL JOURNAL B 2004年第1期37卷 25-37页

作者： Moret, R Launois, P Wågberg, T Sundqvist, B Agafonov, V Davydov, VA Rakhmanina, AV Univ Paris 11 Phys Solides Lab CNRS UMR 8502 F-91405 Orsay France Umea Univ Dept Phys S-90187 Umea Sweden Univ Tours Fac Pharm Chim Phys Lab F-37200 Tours France Russian Acad Sci Inst High Pressure Phys Troitsk 142092 Moscow Region Russia

We present a study by Raman spectroscopy and X-ray diffraction/diffuse scattering of C-60 single-crystals treated at high-pressure and high-temperature. This allowed us to obtain structural information on the C-60 dimer state which can be considered as an intermediate state in the polymerization process. In the 1-6 GPa pressure range the crystals are primarily formed of dimers with additional minor fractions of monomers, 1D and 2D polymers, as shown by the analysis of the Raman spectra. The dimers are disordered within an average cubic lattice derived from that of the monomer. Single-crystal diffraction patterns reveal a characteristic diffuse scattering intensity distribution which has been simulated by calculating the diffuse scattering produced by dimer and trimer model structures. Satisfactory agreement is obtained for random positional and orientational disorder of the C-60-C-60 dimers although a small concentration of similarly disordered trimers is likely. In a first approximation the dimer/trimer disorder can be considered as random but various inter-dimer correlations are probably present, as discussed.

关键词： Fullerenes and fullerene-related materials structural modeling: serial-addition models, computer simulation Fullerenes and related materials

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Atomic scale models of Co-Ag mixed nanoclusters at thermodynamic equilibrium

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EUROPEAN PHYSICAL JOURNAL D 2004年第1期29卷 33-38页

作者： Van Hoof, T Hou, M Free Univ Brussels B-1050 Brussels Belgium

The configurations at thermodynamic equilibrium of CoxAg201-x nanoparticles are explored for 0 50, Co regroups at the centre of the cluster and intersects {111}-facets when Ag atoms are not numerous enough to form an... 详细信息

The configurations at thermodynamic equilibrium of CoxAg201-x nanoparticles are explored for 0 < x < 201 by means of Metropolis Monte Carlo simulations with a semi-empirical embedded atom potential at temperatures front 100 K to 1000 K. Remarkable configurations are predicted in this temperature range. As a consequence of a competition between strain and Co binding at low temperature, for x < 20, Co is distributed just below the cluster surface layer into groups of no more than 5 atoms, favouring well-defined positions, and the cluster central area is avoided. To increase the temperature favours the clustering of these small groups. Their dissolution is predicted at temperatures higher than the melting temperature of the cluster. For x > 50, Co regroups at the centre of the cluster and intersects {111}-facets when Ag atoms are not numerous enough to form an entire surface shell. At these stoechiometries, temperature is not sufficient to mix Ag and Co, even above the melting point. At still smaller Ag concentrations, the Ag atoms are distributed at lowest coordination sites, along the edges of the cluster, avoiding the cluster facets as well as inner sites. At intermediate stoechiometries (20 < x < 50), either oblate Co groups below the surface or a compact group at the centre of the cluster are possible.

关键词： Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals structural modeling: serial-addition models, computer simulation

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Structure and stability of non-molecular nitrogen at ambient pressure

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EUROPHYSICS LETTERS 2004年第3期65卷 400-406页

作者： Nordlund, K Krasheninnikov, A Juslin, N Nord, J Albe, K Univ Helsinki Accelerator Lab FIN-00014 Helsinki Finland Tech Univ Darmstadt Inst Mat Wissensch D-64287 Darmstadt Germany

Non-molecular solid nitrogen, metastable even at zero pressure and low temperature, was recently manufactured by Eremets et al. (Nature, 411 (2001) 173) and investigated with respect to its pressure stability. In this study we present analytical potential molecular-dynamics simulations that allow reproducing all the stages of the experiment. The bonding structure and energetics of the N polymeric state obtained from the dynamical simulations are verified to be reasonable using a plane-wave ab initio method. We predict that the metastable low-pressure phase has predominantly 3-folded bonded atoms in a disordered configuration. The energy accumulated in the metastable phase is about 1 eV/atom.

关键词： New materials: theory, design, and fabrication structural modeling: serial-addition models, computer simulation

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Thermal evolution of cluster assembled Ni₃Al materials modelled at the atomic scale

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EUROPEAN PHYSICAL JOURNAL D 2003年第3期27卷 231-237页

作者： Moskovkin, P Hou, M Free Univ Brussels B-1050 Brussels Belgium RRC Kurchatov Inst Moscow Russia

Diffusion properties of Ni3Al cluster assembled nanostructured materials are investigated at the atomic scale. Two different model samples are considered, at equilibrium at 300 K. One is obtained by modelling cluster compaction under 2 GPa external pressure and the second by accumulating low energy deposited clusters on a Ni surface. They differ essentially by their density, the latter sample presenting an interconnected network of nanopores, which is not observed in the former. At elevated temperatures, cluster coalescence is observed in both, as well as an intense atomic diffusion at the internal surfaces and nanograin interfaces. A method is presented which allows, in a good approximation, to distinguish between the two phenomena and to estimate diffusion coefficients. At temperatures above 400 K, it is found for both samples, irrespective to their density, that the diffusion activation energy at the internal surfaces and interfaces is as low as in a liquid while the grain cores remain crystalline.

关键词： structural modeling: serial-addition models, computer simulation Atomic and molecular clusters Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals

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Structure character in small-carbon-cluster deposition on diamond surface

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EUROPEAN PHYSICAL JOURNAL D 2003年第3期23卷 369-373页

作者： Li, ZJ Pan, ZY Wei, Q Du, AJ Huang, Z Zhang, ZX Ye, XS Bai, T Wang, C Liu, JR Fudan Univ Inst Modern Phys Shanghai 200433 Peoples R China Chinese Acad Sci Shanghai Inst Microsyst & Informat Technol Ion Beam Lab Shanghai 200050 Peoples R China NW Inst Nucl Technol Xian 710024 Peoples R China

Experimentally, hydrogen-free diamond-like carbon (DLC) films were assembled by means of pulsed laser deposition (PLD), where energetic small-carbon-clusters were deposited on the substrate. In this paper, the chemisorption of energetic C-2 and C-10 clusters on diamond (001)-(2x1) surface was investigated by molecular dynamics simulation. The influence of cluster size and the impact energy on the structure character of the deposited clusters is mainly addressed. The impact energy was varied from a few tens eV to 100 eV. The chernisorption of C-10 was found to occur only when its incident energy is above a threshold value (E-th). While, the C-2 cluster was easily to adsorb on the surface even at much lower incident energy. With increasing the impact energy, the structures of the deposited C-2 and C-10 are different from the free clusters. Finally, the growth of films synthesized by energetic C-2 and C-10 clusters were simulated. The statistics indicate the C-2 cluster has high probability of adsorption and films assembled of C-2 present slightly higher SP3 fraction than that of C-10-films, especially at higher impact energy and lower substrate temperature. Our result supports the experimental findings. Moreover, the simulation underlines the deposition mechanism at atomic scale.

关键词： structural modeling: serial-addition models, computer simulation Atomic, molecular, and ion beam impact and interactions with surfaces Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals

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Elastic heterogeneity and phase stability under hydrostatic pressure

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EUROPHYSICS LETTERS 2003年第2期64卷 211-217页

作者： Holliday, K Lammert, PE Crespi, VH Penn State Univ Dept Phys University Pk PA 16802 USA Penn State Univ Mat Res Inst University Pk PA 16802 USA

structural phase transitions typically arise from differences in the average stiffness, nominal bond length and/or lattice topology between competing structures. We show how heterogeneity in the nominal bond lengths and stiffnesses can shrink and soften a material, thereby favoring a heterogeneous state, even when the average bond length and stiffness are unchanged. We quantify the relationship between lattice coordination, the variance of the local bond stifnesses and the macroscopic rigidity. We also demonstrate that long-range order in three dimensions can survive perturbative disorder in the nominal bond length.

关键词： structural modeling: serial-addition models, computer simulation Order-isorder transformations, statistical mechanics of model systems Solid-solid transitions

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Model of silica glass from combined classical and ab initio molecular-dynamics simulations

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EUROPEAN PHYSICAL JOURNAL B 2000年第4期13卷 631-636页

作者： Benoit, M Ispas, S Jund, P Jullien, R Univ Montpellier 2 Lab Verres F-34095 Montpellier France

A Car-Parrinello (CP) molecular dynamics simulation of vitreous silica, combined with classical molecular dynamics. is presented. The equilibration of the liquid, quench and relaxation of the glass are performed classically using the van Beest, Kramer and van Santen (BKS) potential [12] and the resulting configuration (coordinates and velocities) is used as input for the CP simulation. A remarkable stability of the CP dynamics is observed justifying this procedure and validating the BKS potential. Several structural and electronic properties are calculated and compared with experiments.

关键词： Glasses Molecular dynamics calculations (Car-Parrinello) andother numerical simulations structural modeling: serial-addition models, computer simulation

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Comparison of two representations of a random cut of identical sphere packing

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EUROPEAN PHYSICAL JOURNAL B 2000年第3期14卷 403-406页

作者： Oger, L Richard, P Troadec, JP Gervois, A Univ Rennes 1 Grp Mat Condensee & Mat CNRS UMR 6626 F-35042 Rennes France CEA Saclay DSM Serv Phys Theor F-91191 Gif Sur Yvette France

We compare two-dimensional froths obtained by radical tessellation of random planar cuts made through disordered assemblies of monosize spheres at different packing fractions C from C = 0 to C = 0.64 with two-dimensional stereological cuts obtained through the three-dimensional froths made with the same packing. We have built numerically the packings using different algorithms. The study of both topological and metric properties shows significant differences between the two representations.

关键词： structural modeling: serial-addition models, computer simulation computer simulation of liquid structure

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Tunneling states in tetrahedrally bonded amorphous semiconductors

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EUROPHYSICS LETTERS 1999年第4期48卷 403-409页

作者： Dyrting, S Fang, HP Szeto, KY Sheng, P Hong Kong Univ Sci & Technol Dept Phys Hong Kong Peoples R China

Using molecular dynamics, are construct a structural model of amorphous silicon which produces well-relaxed and long-lasting structures with radial distribution functions that agree well with experimental results. The model produces a sufficient number of double-well potentials with low asymmetries, enabling a structurally-explicit microscopic portrayal of tunneling states in covalently bonded amorphous systems. A new structural characterization method is used to identify the possible origin of the tunneling states in amorphous silicon. Based on the study of the short-range order, we suggest that the tunneling states in amorphous silicon are predominantly associated with dangling bonds.

关键词： Amorphous semiconductors, metals, and alloys structural modeling: serial-addition models, computer simulation Molecular dynamics and particle methods

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