Point-defect properties in body-centered cubic transition metals with analytic EAM interatomic potentials

Analytic embedded-atom method (EAM) interatomic potentials for body-centered cubic (bcc) transition metals have been constructed. The total energy is regarded as consisting of a pair potential part, an embedding interaction part and a modification term. All these parts are analytic functions of the atomic separations only and are fitted to various bulk properties, such as cohesive energy, vacancy formation energy, lattice constant and elastic constants. The present potentials are shown to predict a more realistic pressure–volume relationship so that interactions at separations smaller than that of the first-nearest neighbors can be treated. Interstitial formation energies are calculated for various configurations, using quenched molecular dynamics. Vacancy, surface and phonon spectra have been also studied and satisfactory agreement with available experimental data has been found.     

碳纳米管负载金属氮化物催化剂的制备及其性能研究

采用等体积浸渍法结合程序升温还原技术制备出一系列负载型过渡金属氮化物催化剂。利用XRD、BET、TPR等表征手段,结合氨分解制氢反应,研究了它们的表面性质和反应性能。发现改性碳纳米管负载的氮化钴催化剂具有较大的比表面积,对氨分解制氢反应的催化活性最高。新鲜态CoNx/CNTs催化剂的比表面积可达151.85m2/g,在650℃时氨转化率为80.3%,850℃达到完全转化。     

Computational method for atomistic homogenization of nanopatterned point defect structures

The development of an approximation method that rigorously averages small‐scale atomistic physics and embeds them in large‐scale mechanics is the principal aim of this work. This paper presents a general computational procedure based on homogenization to average frozen nanoscale atomistics and couple them to the equations of continuum hyperelasticity. The proposed application is to nanopatterned systems in which complex atomic configurations are organized in a repeating periodic array. The finite element method is used to solve the equations at the large scale, but the small‐scale equation is representative of lattice‐statics. The method is predicated on a quasistatic zero‐temperature assumption and, through homogenization, leads to a coupled set of variational equations. The numerical procedure is presented in detail, and 2‐D examples of ultra thin film layers of carbon one atom thick are shown to illustrate its applicability. Homogenization naturally gives rise to an inner displacement term with which point defects are explicitly modelled and their non‐linear interactions with global states of multiaxial strain are studied. Published in 2004 by John Wiley & Sons, Ltd.     

用AEAM模型研究Ti与过渡族bcc金属二元合金的结构

运用AEAM模型理论,计算了Ti金属的总能量,并和Rose方程的结果作了比较;同时系统地研究了Ti与bcc金属组成的二元合金系统形成焓,并对计算结果进行了分析,给出了Ti与bcc金属组成的各合金结构稳定性与成分关系,为AEAM模型理论研究二元合金结构及稳定性提供了思路.     

Local modes of interstitial atoms in transition metals

Local modes of the C, N, O and H interstitial atoms were measured in many transition metal solid solutions by inelastic neutron scattering. The metal-interstitial force constants were estimated from the experimental data and are graphically presented as functions of metal-interstitial distances. A correlation of these functions with the data, predicted by the embedded atom theory, is revealed and discussed.     

Molecular dynamics study of thermal properties of noble metals

Molecular dynamics simulations have been applied to investigate thermal properties of Ag and Au. Semi-empirical potentials, based on the embedded atom method (EAM) have been employed to calculate lattice parameter, energy per atom, mean square displacements and radial distribution function for the two metals. Thermal properties like specific heat, thermal coefficient of linear expansion and melting temperature are deduced from the calculated parameters. Results are found to compare well with the experimental results.     

Observation of relaxation phenomena in thermophysical properties of metals

Studies of relaxation phenomena related to thermophysical properties of metals are reviewed. They provide important information not available from other techniques. Observation of relaxation phenomena was used in studying ordering in alloys, glass transition in Supercooled liquids, and equilibration of point defects in a crystal lattice. Opportunities to observe the equilibration phenomena in materials are provided by subsecond thermophysical measurements. A simple method is proposed for a straightforward determination of the defect contribution to the enthalpy of metals.Paper presented at the Fourth International Workshop on Subsecond Thermophysics, June 27–29, 1995, Köln, Germany.     

Effect of Microstructure on the Mirror-Like Surface Quality of FCC and BCC Metals

The microstructure of machined metals changes near the tool-affected zone. This paper presents some new results concerning mirror-like surface cutting of aluminum, copper and tungsten. The microstructure of aluminum and copper represents polycrystalline mild metals with face centered cubic (FCC) crystal structure. Examination of a mirror-like surface by optical microscopy, scanning electron microscopy (SEM), electron backscattered diffraction and atomic force microscopy revealed that grain boundaries and twin boundaries were present, which separates two domains for different crystal orientation. The Young's modulus that depends on orientation can change considerably on these boundaries and, consequently, the value of elastic deformation of the layer under the machined surface. This effect modified the roughness. Ultraprecision machining of tungsten, which is a body centered cubic (BCC) metal, proved useless using diamond and/or boron-nitride tools. Since tungsten is a very brittle metal at room temperature, its ductile to brittle transition temperature is much higher. Therefore, in contrast to normal cutting, the material that is incapable of plastic deformation will cause brittle fracture of the chip and bad surface quality.     

Volume variation of Gruneisen parameters offcc transition metals

The volume variation of the Gruneisen parameters of tenfcc transition metals, up to 40% compression, has been studied on the basis of a model approach proposed by Antonovet al. The results are reasonably good for six metals except for Rh, Ag, Au and Ni when compared with available experimental and other theoretical values. The model requires an appropriate modification for Rh, Ag, Au and Ni.     

Optical properties of oxide glasses containing transition metals: Case of titanium- and chromium-containing glasses

One of the major driving forces for the development of new glasses is the demand for high optical non-linearity with reduced cost and a higher damage resistance. Oxide glasses with large non-linear refractive index and non-linear absorption coefficient are promising materials for fiber telecommunication and for non-linear optical devices such as ultrafast optical switches, power limiters, real time holography, self-focusing, white-light continuum generation and photonic applications. To get insight into the optical absorption in amorphous materials, studies are still needed for revealing the nature of photoelectronic excitations in these materials by comparison with that in crystals which have been understood firmly based on band theory. Although the IR absorption loss in oxide glasses is larger than of fluorides, low light scattering loss is expected in these oxide glasses because they have lower glass transition temperature. In addition, small concentration of dopant such as alkaline metal and alkaline earth metal elements gives rise to the structural relaxation of the frozen-in density fluctuations even below glass transition temperature T_g, adding to the reduction of T_g as well. A review of the fundamentals and recent research advances in optical properties of oxide glasses containing chromium or titanium is presented.     

Influence of transition metals on the surface acidic properties of titania prepared by sol-gel route

A series of titania catalysts containing chromium, molybdenum as well as tungsten has been prepared by colloidal sol-gel route using metatitanic acid as the precursor. Structural characterization of the prepared catalysts was done with Energy dispersive X-ray analysis, X-ray diffraction, BET surface area and pore volume measurements. The influence of different transition metals like chromium, molybdenum and tungsten on the surface acidic properties of titania is investigated in detail. Two independent methods have been used to study the surface acidity of these catalyst systems: temperature programmed desorption of ammonia which is a measure of total acidity and thermodesorption studies using 2,6-dimethyl pyridine which is a measure of Bronsted acidity. Cumene cracking reaction is carried out over the prepared systems for further characterizing the acidity of the prepared catalysts. Remarkable enhancement in the surface acidity is observed after transition metal incorporation. The catalytic activity of the prepared catalysts was tested towards the dehydrogenation of cyclohexane.     

First-principles study of phase transition and structural properties of AlAs

We have investigated the phase transition and structural properties of AlAs in three crystallographic structures, i.e., B3 (zinc blende), B1 (rocksalt), and B8 (nickel arsenide), at high pressures using the full-potential linearized muffin-tin orbital (FP-LMTO) scheme within the generalized gradient approximation correction (GGA) in the frame of density functional theory (DFT). For B8 structure, it is found that the c/a ratios kept nearly constant (0.2% fluctuation) corresponding to V/V₀  0.7–1.05 (V is the primitive cell volume and V₀ is the experimental equilibrium volume of B3 structure), which is in full agreement with experiment, but the c/a ratios increase linearly with the values of V/V₀ decreasing corresponding to V/V₀  0.4–0.7. This indicates under low pressure the compression along c-axis and a-axis is the same, but the compression along c-axis is more difficult than along a-axis under higher pressure. Based on the condition of equal enthalpies AlAs is found to undergo a structural phase transition from B3 to B8 at 5.34 GPa, in agreement with the experimental value of 7 ± 5 GPa, and is speculated to undergo the B3–B1 transition at 6.24 GPa.     

Point defects and their clusters in bcc metals

Recently Doyama and Kogure have developed new EAM potentials of bcc metals (DK_EAM). The validity of DK_EAM potential of Fe and V is examined by calculating a thermal expansion coefficient, a phonon dispersion and properties of point defects. In Fe, a calculated thermal expansion coefficient is negative and its absolute value is much smaller than that of experimental data. The phonon dispersion relation shows fair agreements at several branch. The calculated density of state of phonon shows a lack of a peak at high frequency determined experimentally. In V, the thermal expansion coefficient is negative and the crystal volume decreases significantly during a molecular dynamics (MD) simulation at 1200 K. This means that the embedding energy and the repulsive pair potential are not calculated correctly at the position which deviates from the stable site of atoms. We calculate the property of point defects in Fe. The relaxation volume of vacancies in Fe is very small which leads to the void formation as a vacancy cluster grows to a larger cluster. A MD simulation shows that two atoms in interstitial clusters approach very closely. This suggests the repulsive pair potential in DK_EAM is weak. By combining the pair potential of DK_EAM with the classical pair potential, the DK_EAM is modified. The results are compared with the previous calculation and the experimental data.     

An analysis of the high-temperature entropy of transition metals

The high-temperature entropy at constant pressure, obtained from heat capacity data, is analyzed for Mo, Nb, Pd, Rh, Ru, Sc, Ti, V, W, Y, and Zr. The vibrational contribution to the entropy defines an entropy Debye temperature, _S(T). Due to anharmonicity, _S(T) decreases with the temperature T, typically by 10–20% between room temperature and the melting point. An unusual temperature dependence of _S(T) may indicate inadequacies in the underlying data. On this ground, the C _p data recommended by Hultgren et al. (1973) for W and Ru seem doubtful. A comparison is made between _S and the elastic-limit Debye temperature _D for the metals with a cubic lattice structure. _S of Nb is found to be almost independent of T, a feature related to the fact that the elastic constant c ₄₄ increases with T.     

A new spd tight-binding model of magnetism in transition metals

We present the extension of an efficient spd tight-binding model to the case of spin polarized materials by including electron–electron interactions, in an extended Hubbard–Hartree–Fock formalism. It is applied with success to rhodium and palladium clusters and slabs.     

Computer simulation of point defects in fcc metals using EAM potentials

The activation energies for the motion of interstitials and vacancies, vacancy clusters have been calculated using embedded atom potentials. The most exciting results obtained are that the stable interstitial configuration is a [1 0 0] split type and the saddle point was a body-centered interstitial. This is quite different from widely accepted computational results. The activation energies for the motion of vacancies, di-vacancies and tri-vacancies have been calculated. The atom above the center of a 60° tri-vacancy relaxes to the center of the tetrahedron in copper but does not relax to the center of the tetrahedron in silver. In the case of gold the energy relaxed to the center is very small and the atoms move around in the tetrahedron. The details depend upon the potentials.     

Heat capacity of transition metals at high temperatures

In recent years better data have appeared for thermophysical properties of some transition elements at high temperatures (e.g., Ta, Nb, Mo, W, Pt) using subsecond techniques and refinements in thermometry. These data enable us to deduce values for the heat capacity at a constant volume C _v and compare them with the expected lattice vibrational contribution. We attribute the differences chiefly to electronic effects and compare these with results of low-temperature measurements. While calculating the C _p-to-C _v correction, we obtain values for the Grüneisen parameter which are useful in assessing the reliability of the measured data.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

纳米金属/介孔TiO2复合体磁性

本文研究了不同纳米金属含量对溶胶-凝胶法制备的纳米Fe,Co,Ni/介孔TiO2复合体的饱和磁化强度、剩余磁化强度、矩形比和矫顽力的影响.通过振动样品磁强计对纳米复合分体磁学参量(-7.96×105 A·m-1＜H＜7.96×105 A·m-1)进行测定;结果表明:纳米金属含量对复合体的磁学参量有着复杂的影响.