Field-induced phase separation in one dimension

Phase separation is induced in the one-dimensional Ising chain (or lattice-gas model of a fluid) by means of an external field that changes sign in the middle of the chain. The magnetization profile (or density profile of the analogous fluid) is obtained analytically. It is found to decay exponentially rapidly to the bulk-phase magnetizations (or densities), the exponential decay parameter being the correlation length in the bulk phases in the presence of the field. This is in accord with earlier theoretical ideas. The interfacial tension is also obtained analytically. In an appropriately defined limit of large neighboring-site spin-spin interactions and small external field the interface becomes infinitely broad while the amplitude of the profile and the interfacial tension both vanish, in close imitation of the approach to a critical point in a real fluid. In this asymptotic limit the interfacial tension is related to the amplitude of the profile in the way that is predicted by earlier theories of interfaces near critical points, with critical-point exponents now those appropriate to one dimension. The exact interfacial profile and tension are used to test several approximations, including a corrected form of the barometric law and local (square-gradient) and nonlocal forms of the van der Waals theory.     

The interfacial surface tension of a quark-gluon plasma fireball in a hadronic medium

We calculate the interfacial surface tension of a QGP-fireball in a hadronic medium in the Ramanathan et al statistical model. The constancy of the ratio of the surface tension with the cube of the critical transition temperature is in overall accordance with lattice QCD findings. It is in complete agreement with a recent MIT bag model calculation of surface tension. The velocity of sound in the QGP droplet is predicted to be in the range (0.27 ± 0.02) times the velocity of light in vacuum and this value is independent of both the value of the transition temperature and the model parameters.      

Effects of pressure on structural,electronic, and mechanical properties of α,β,and γ uranium

The first-principles methods have been employed to calculate the structural, electronic, and mechanical properties of the α, β, and γ phases of uranium under pressure up to 100 GPa. The electronic structure has been viewed in forms of density of states and band structure. The mechanical stability of metal U in the α, β, and γ phases have been examined.The independent elastic constants, polycrystalline elastic moduli, as well as Poisson's ratio have been obtained. Upon compression, the elastic constants, elastic moduli, elastic wave velocities, and Debye temperature of α phase are enhanced pronouncedly. The value of B/G illustrates that α and γ phases are brittle in ground state.     

Structural,elastic, electronic and thermal properties of M2SbP (M = Ti,Zr and Hf)

The structural, elastic, electronic and thermal properties of M₂SbP (M = Ti, Zr and Hf) were studied by means of a pseudo-potential plane-wave method based on the density functional theory within both the local density approximation and the generalised gradient approximation. The optimised zero-pressure geometrical parameters, i.e. the two unit cell lengths (a, c) and the internal coordinate (z), were in good agreement with available experimental and theoretical data. The effect of high pressure, up to 20 GPa, on the lattice constants shows that the contractions along the a-axis were higher than along c-axis. The anisotropic independent elastic constants were calculated using the static finite strain technique. Numerical estimations of the bulk modulus, shear modulus, Young's modulus, Poisson's ratio, average sound velocity and Debye temperature for ideal polycrystalline M₂SbP aggregates were performed in the framework of the Voigt–Reuss–Hill approximation. The calculated band structures show that all studied materials are electrical conductors. Analysis of the atomic site projected densities showed that the bonding is of covalent–ionic nature with the presence of metallic character. The density of states at the Fermi level is dictated by the transition metal d–d bands; the Sb element has little effect. Thermal effects on some macroscopic properties of M₂SbP were predicted using the quasi-harmonic Debye model, in which the lattice vibrations are taken into account. The variations of the volume expansion coefficient, heat capacity and Debye temperature with pressure and temperature in the ranges 0–50 GPa and 0–2000 K were obtained successfully.     

Interfacial tension in water/n-decane/naphthenic acid systems predicted by a combined COSMO-RS theory and pendant drop experimental study

The interfacial tension of systems containing water, n-decane, and model naphthenic acids were investigated using a predictive model based on COSMO-RS theory and experimental pendant drop measurements. Five naphthenic acid homologues that are considered to be representative of surfactants inherent to crude oil were dissolved in n-decane at equal concentrations. The interfacial tensions of the five systems at an acid concentration of 1.66?mol% relative to n-decane were experimentally determined to be 27–30?mN/m. The interfacial tensions of the five different acid-decane phases against water were also predicted using density functional theory (DFT) calculations and COSMO-RS theory. The accuracy of the predictions was very good as confirmed through pendant drop measurements of the interfacial tension. The mean-absolute-deviation between experimental and predicted values was 2.6?mN/m thus demonstrating the high predictive power of COSMO-RS theory for calculating the interfacial tension at oil–water interfaces in the presence of surface-active compounds.     

New ordered MAX phase Mo_2 TiAlC_2: Elastic and electronic properties from first-principles

First-principles computation on the basis of density functional theory(DFT) is executed with the CASTEP code to explore the structural, elastic, and electronic properties along with Debye temperature and theoretical Vickers' hardness of newly discovered ordered MAX phase carbide Mo_2TiAlC_2. The computed structural parameters are very reasonable compared with the experimental results. The mechanical stability is verified by using the computed elastic constants. The brittleness of the compound is indicated by both the Poisson's and Pugh's ratios. The new MAX phase is capable of resisting the pressure and tension and also has the clear directional bonding between atoms. The compound shows significant elastic anisotropy. The Debye temperature estimated from elastic moduli(B, G) is found to be 413.6 K. The electronic structure indicates that the bonding nature of Mo_2TiAlC_2 is a mixture of covalent and metallic with few ionic characters. The electron charge density map shows a strong directional Mo–C–Mo covalent bonding associated with a relatively weak Ti–C bond.The calculated Fermi surface is due to the low-dispersive Mo 4d-like bands, which makes the compound a conductive one.The hardness of the compound is also evaluated and a high value of 9.01 GPa is an indication of its strong covalent bonding.     

Analysis of the electrical properties of p–n GaAs homojunction under dc and ac fields

In this work, the n-type GaAs films were grown on p-type GaAs single crystalline substrate by metal organic chemical vapor deposition (MOCVD). The temperature dependence of the current density–voltage (J–V) characteristics of n-GaAs/p-GaAs homojunction contacts were measured in the temperature range 293–413 K. These characteristics showed a rectifying behavior consistent with a potential barrier formed at the interface. The forward current density–voltage characteristics under low voltage biasing were explained on the basis of thermionic emission mechanism. The high values of ideality factor (n) may be ascribed to the presence of an interfacial layer. Analysis of the experimental data under the reverse voltage biasing suggests a dominant mechanism was found to be a Schottky effect. The impedance properties and the alternating current (ac) conductivity of n-GaAs/p-GaAs homojunction were investigated as a function of frequency and temperature. The ac conductivity was found to obey the universal power law. The variation of the exponent s with the temperature suggested that the conduction mechanism is an overlapping large-polaron tunneling (OLPT) model associated with correlated barrier hopping (CBH) model at the higher temperature.     

Ti,Nb, Al及其二元合金状态方程的第一原理计算及其应用

金属状态方程对于探究金属及合金原子的相互作用中起到了至关重要的作用.本文使用第一原理计算了Ti, Nb, Al及其二元合金能量与原子间距关系(E-r关系),并使用得到的E-r关系计算了金属及合金的体弹性模量,结果与实验值吻合的很好.计算结果表明,使用不同的赝势,计算不同金属表现出不同的适用性;二元合金的E-r曲线处于对应合金化元素曲线之间;并且合金的E-r曲线会更靠近合金内含量较高的元素的E-r曲线;并发现合金E-r关系可通过组成合金的纯金属的E-r关系计算获得,且用该方法计算材料的体弹性模量与实验值非常符合.     

Chain length dependencies of the bending modulus of surfactant monolayers

The effect of the surfactant chain length n on the bending modulus kappa of surfactant monolayers is simulated with a mesoscopic oil-water-surfactant model. We confirm a power law, kappa is proportional to np, as predicted by mean-field theory and found experimentally, and find p approximately 1.5 at a constant surface density and p approximately 1.0 at a constant interfacial tension. This agrees quite well with both mean-field theory (p=2-3, assuming constant surface density) and experiments (at constant surface tension). Our results suggest that the previously reported agreement between theory and experiment may be fortuitous and caused by the difference in surfactant types.     

Hydrophobic force,a Casimir-like effect due to hydrogen-bond fluctuations

Hydrophobic force, interfacial tension, and transverse density profile in a confined water system are addressed from first principles of statistical mechanics in a lattice model for water. Using the molecular mean field theory technique we deduce explicit expressions for each of the above mentioned phenomena and show that hydrophobic force is a manifestation of a Casimir-like effect due to hydrogen-bond fluctuations in confined water. It is largely influenced by the long range correlations of orientational fluctuations. Furthermore, the temperature dependence of hydrophobic force between large non-polar surfaces is suggested to be different from that between small solutes. The mechanisms contributing to characteristic behavior in each case are identified. In the case of large surfaces, the prevalence of discrete fluctuation modes in the confinement direction and their entropic contribution to the overall free energy dominate the temperature dependence. Mode discretization is also implicated in the variation of interfacial tension with separation distance between confining surfaces and characteristic density profile of the confined fluid. All the computations are parameter free and compare favorably with results of molecular dynamics simulations and experiments.     

Interfacial roughness in a near-critical binary fluid mixture: X-ray reflectivity and near-specular diffuse scattering

Measurements are presented of the X-ray specular reflectivity and near-specular diffuse scattering of the interface in a near-critical mixture of hexane and perfluorohexane. A lineshape analysis of the scattered intensity at each temperature yields values for the interfacial tension and interfacial width. The temperature variation of the tension and width so-obtained are consistent with current understanding of this interface, which holds that there is, firstly, an intrinsic width over which the fluid density varies smoothly from one coexistence composition to the other, and, secondly, that the interface acquires an additional and larger statistical interfacial width as a result of capillary fluctuations. Received 1 April 1998     

Surface tension and density of liquid Bi–Pb,Bi–Sn and Bi–Pb–Sn eutectic alloys

Surface tension and density measurement of liquid Bi₅₆Pb₄₄, Bi₄₃Sn₅₇ and Bi₄₆Pb₂₉Sn₂₅ eutectic alloys was carried out by using the large drop method over the temperature range of 380–750 K. The regular solution model has been used in conjunction with Butler's equation to calculate the surface tension of binary and ternary alloys of the Bi–Pb–Sn system, while the surface tension of ternary alloys has also been predicted by using geometric models. The new experimental results were compared with the calculated values of the surface tension as well as with the data available in the literature.     

Calculation of the surface tension and the surface energy of Lennard–Jones fluids from the radial distribution function in the interface zone

A detailed study is presented of the calculation of the surface tension and the surface energy of Lennard–Jones fluids from the radial distribution function and the density profile. To do so, a modification is made to Lekner and Henderson's statistical mechanics approach by introducing two simple analytical expressions for the radial distribution function of the interface zone. In these expressions the radial distribution functions of the liquid and vapour phases are weighted via step or exponential variations. The well- known exponential model for the density profile in the interface zone is considered. Finally, results are compared with values from experiment, from computer simulation and from relevant theoretical developments. It is shown that the use of the proposed radial distribution function in the interface zone represents a significant improvement in applying Lekner and Henderson's approach.     

Model of a nanoparticle in the theory of inhomogeneous medium

Basic thermodynamic characteristics (density distribution profile near the surface of a nanoparticle, adsorption, and interfacial surface tension) of the structureless nanoparticle-vapor/liquid equilibrium system are calculated using a unified approach. A joint solution of the basic equation in the Van der Waals theory of an inhomogeneous medium for the density distribution profile in the spherical system of coordinates and the Gibbs equation for the interfacial tension and absorption is obtained. The features of nucleation of nanoparticles are considered. The results generalize some familiar formulas and provide a more adequate interpretation of experimental results.     

Investigation of vapour--liquid nucleation properties for spherical and chain-like fluids by density functional theory

The excess Helmholtz free energy functional for nonpolar chain-like molecules is formulated in terms of a weighted density approximation （WDA） for short-range interactions and a Weaks Chandler Andersen （WCA） approximation and a Barker Henderson （BH） theory for long-range attraction. Within the framework of density functional theory （DFT）, vapour liquid interracial properties including density profile and surface tension, and vapour-liquid nucleation properties including density profile, work of formation and number of particles are investigated for spherical and chain- like molecules. The obtained vapour liquid surface tension and the number of particles in critical nucleus for Lennard- Jones （L J） fluids are consistent with the simulation results. The influences of supersaturation, temperature and chain length on vapour liquid nucleation properties are discussed.[第一段]     

Rayleigh relation and scaling power laws in minor hysteresis loops

Seven scaling power-law relations were discovered in the minor hysteresis loops of ferromagnetic materials several years ago. These relations include universal constants that are independent of the type of material and temperature, along with coefficients that provide information about the material and lattice defects. Four universal constants and four coefficients are explained in connection with the Rayleigh relation, which is caused by two scaling laws for l(H) and p(H), where l(H) is the pinning and unpinning length in the domain wall displacement, H is the magnetic field, and p(H) is the number of domain walls involved. The dependence of the coefficients on the dislocation density, ρ, is explained to increase in proportion to the square root of ρ. This result agrees with experimental results. The force profile of the domain walls has similar structures at every point.     

Mechanical and lattice dynamical properties of the Re2C compound

In this work, density functional theory calculations on the structural, mechanical, and lattice dynamical properties of Re₂C within ReB₂‐type structure are reported. The generalized gradient approximation has been used for modeling exchange–correlation effects. We have predicted the lattice constants, bulk modulus, bond distances, elastic constants, shear modulus, Young's modulus, Poisson's ratio, hardness, Debye temperature, and sound velocities of this compound. Furthermore, the band structure, phonon dispersion curves and corresponding density of states are computed. The obtained results are in good agreement with the available experimental and other theoretical data. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The second and third order elastic constants of alkali metals

The second and third order elastic constants of the alkali metals have been calculated on the long wave method using the Heine-Abarenkov lacal model potential with different exchange-correlation corrections. It is found that the use of an exchange correlation correction which satisfies the compressibility sum rule leads to a good agreement between the calculated and measured second order elastic constants of the alkali metals Na, K, Rb and Cs. The shear elastic constants however come out correct even if the compressibility sum rule is violated by the exchange-correlation correction. The third order elastic constants and the pressure derivatives of the second order elastic constants and the pressure derivatives of the second order elastic constants calculated on the HA local potential are lower than the experimental values at room temperature. The discrepancy is pronounced for the heavier alkali metals. Similar calculations using the Wallace potential for Li, Na and K and the Schneider-Stoll potential for Rb give the pressure derivative in good agreement with experiment. In view of the important role by the exchange correlation correction, Suzuki's results calculated without taking this correction into account can only be accepted with some reservation.