Nanosize icosahedral quasicrystal in Mg90Ca10 glass: an ab initio molecular dynamics study

Rapid solidification of Mg(90)Ca(10) from its liquid state is studied by means of an ab initio molecular dynamics technique, and its local structure is investigated by various analyzing methods. The liquid and amorphous states are found to have slightly different short range order even though the perfect and defective icosahedral bonding environments are major bonding elements of both liquid and amorphous states. Perfect icosahedrons with a small frequency exist in the liquid state, more develop during the cooling process and they become the leading building units in the glass state, indicating an icosahedral short range order in Mg(90)Ca(10) glass. Also the linked icosahedrons lead to an icosahedral medium range order. Furthermore, an ordered arrangement of some icosahedrons in the hexagonal symmetry is observed in the glass model, representing a nanoscale icosahedral quasicrystalline phase in Mg(90)Ca(10) glass.     

Local structure of liquid Ti: Ab initio molecular dynamics study

Local order of liquid Ti is studied by ab initio molecular dynamics to address the unique liquid structure factor in experiments reported recently. The present study reveals that the local order of liquid Ti is in the form of fragments of the distorted icosahedral short range order, where the distortion is induced by strong bond order effects. We show that the fragments in the short-bond rich region separated from the background liquid account for the pronounced feature in structure factor of liquid Ti.     

液态Al80Fe20在快速冷却中的MD模拟

通过Tight binding (TB)势的分子动力学模拟分析了Al80Fe20合金熔体的中程有序结构以及快速凝固过程中体系微观结构的演变规律，发现在倒空间，其结构因子的小角部分都出现了一个预峰.在平衡态（1450 K），模拟结果得到了X射线衍射实验的进一步印证，这被认为是体系中存在中程有序的标志.随着温度的降低，预峰的高度逐渐增大，说明体系中原子团簇尺寸越来越大.通过运用键对分析技术和键取向序参数，发现体系中存在着大量的二十面体短程有序单元.在对平衡态化学短程序参数α的计算过程中，得到了负值的α，证实了熔体中存在着较强的化学序.在FZ偏结构因子中， SAl Fe(Q)在400 K的第二峰较之SAl Al(Q)和SFe Fe(Q)发生了更为明显的劈裂，表明在非晶形成能力方面， Al、Fe元素之间的轨道杂化作用比Al元素或Fe元素单独作用要强.从BT偏结构因子中我们也发现，表征化学序的SCC(Q)在17.5 nm－1左右处出现了第一峰，而这个峰位恰是总结构因子中出现预峰的峰位.因此可推断，正是体系中的化学序导致了中程有序结构的产生.     

Redox-driven transport of copper ions in an emulsion liquid membrane system

A new redox-driven type of emulsion liquid membrane separation is described. Milligram amounts of copper(II) in 0.2 M hydrochloric acid were reduced to copper(I) in the presence of ascorbic acid (1 M≡1 mol l⁻¹). The copper solution was emulsified with a (1+4) mixture of toluene and n-heptane using Span-80 (sorbitan monooleate) as an emusifier. The resulting water-in-oil emulsion was dispersed in 0.2 M hydrochloric acid containing hydrogen peroxide and neocuproine (2,9-dimethyl-1,10-phenanthroline) by stirring for 10 min. The copper in the internal aqueous phase was selectively transported to the external one, leaving other heavy metals (e.g., Mn, Co, Ni, Cd and Pb) in the internal aqueous phase. After collecting the dispersed emulsion globules, they were demulsified by heating and the metals in the segregated aqueous phase were determined by graphite-furnace atomic absorption spectrometry (GFAAS). The selective transport of copper offered the multielement separation of trace heavy metals from a copper matrix, allowing the GFAAS determination of impurities at the 0.01% level in copper metal.     

Polytetrahedral order and chemical short-range order in metallic melts

The reasons are investigated for the prepeak and the asymmetry of the second peak in the structure factor curve that are observed in a variety of metallic melts. The prepeak is observed as an additional maximum in the left wing of the main peak of the structure factor for multicomponent melts and is attributed to their chemical short-range order (CSRO). The asymmetry of the second peak in the structure factor, which is usually explained by the “icosahedral” (polytetrahedral) order in the melt, is observed both for multicomponent systems and for pure metals. However, some aluminum alloys with transition metals exhibit the two features simultaneously, which requires an explanation. An X-ray diffraction study of the liquid ternary Al_66.6Mn_16.7Co_16.7 alloy is performed at 1393 K and that of liquid copper at 1353 K, 1403 K, and 1553 K. The reverse Monte Carlo (RMC) method is used to derive structural models of these and other melts. Structural analysis of these melts is conducted using Delaunay simplices. A theoretical simulation of CSRO is performed in the model of liquid aluminum, the structure factor of which does not have these features. It is discussed that CSRO can exist in a melt regardless of the presence of the polytetrahedral order.     

Atomistic insights into aqueous corrosion of copper

Corrosion is a fundamental problem in electrochemistry and represents a mode of failure of technologically important materials. Understanding the basic mechanism of aqueous corrosion of metals such as Cu in presence of halide ions is hence essential. Using molecular dynamics simulations incorporating reactive force-field (ReaxFF), the interaction of copper substrates and chlorine under aqueous conditions has been investigated. These simulations incorporate effects of proton transfer in the aqueous media and are suitable for modeling the bond formation and bond breakage phenomenon that is associated with complex aqueous corrosion phenomena. Systematic investigation of the corrosion process has been carried out by simulating different chlorine concentration and solution states. The structural and morphological differences associated with metal dissolution in the presence of chloride ions are evaluated using dynamical correlation functions. The simulated atomic trajectories are used to analyze the charged states, molecular structure and ion density distribution which are utilized to understand the atomic scale mechanism of corrosion of copper substrates under aqueous conditions. Increased concentration of chlorine and higher ambient temperature were found to expedite the corrosion of copper. In order to study the effect of solution states on the corrosion resistance of Cu, partial fractions of proton or hydroxide in water were configured, and higher corrosion rate at partial fraction hydroxide environment was observed. When the Cl(-) concentration is low, oxygen or hydroxide ion adsorption onto Cu surface has been confirmed in partial fraction hydroxide environment. Our study provides new atomic scale insights into the early stages of aqueous corrosion of metals such as copper.     

15种中成药中有害重金属铅、镉、铜含量的测定

为了解中药重金属污染情况,测定了市售常用15种中成药中重金属铅、镉、铜的含量,其中铅、镉采用石墨炉原子吸收法,铜采用火焰原子吸收法测定.结果表明,15种中成药均不同程度被这3种重金属污染,通心络中铅、镉、铜含量都已超标,桑菊感冒片镉超标,中药重金属污染问题仍然存在.     

Indirect flame atomic absorption detection for the liquid chromatographic separation of alkaline metals

Alkaline metals and ammonium ion are separated by cationic exchange liquid chromatography using a 60 microg ml(-1) aqueous copper solution as the mobile phase at 1.5 ml min(-1). Detection is carried out by measuring copper with an atomic absorption spectrometer directly interfaced to the chromatograph. A simple T-piece is used to compensate the difference between the nebulizer uptake rate and the chromatographic flow-rate with air. The method is applicable to the analysis of alkaline metals and ammonium in waters. Average recovery ( n = 16) was 100.3 +/- 4.0%.     

High energy x-ray scattering studies of the local order in liquid Al

The x-ray structure factors and densities for liquid aluminum from 1123 K to 1273 K have been measured using the beamline electrostatic levitator. Atomic structures as a function of temperature have been constructed from the diffraction data with reverse Monte Carlo simulations. An analysis of the local atomic structures in terms of the Honeycutt-Andersen indices indicates a high degree of icosahedral and distorted icosahedral order, a modest amount of body-centered cubic order, and marginal amounts of face-centered cubic and hexagonal close-packed order.     

Structural study of supercooled liquid transition metals

Local structural models for supercooled liquid transition metals (Ti, Ni, and Zr) are proposed based on a reverse Monte Carlo analysis of high-energy x-ray diffraction data taken from the electrostatically levitated liquids. The resulting structures are characterized by their bond angle distributions, Honeycutt and Andersen indices [J. Phys. Chem. 91, 4950 (1987)], and bond orientational order parameters. All analyses suggest that an icosahedral short-range order is present in these supercooled liquids, but it is distorted in liquid Ti. These results are in agreement with the observed evolution of the high-q shoulder on the second peak in the structure factor S(q) and with an earlier analysis based on a local cluster model.     

Pressure dependence of viscosity

We reanalyze the pressure dependence of viscosity of liquids of constant composition under isothermal conditions. Based exclusively on very general considerations concerning the relationship between viscosity and "free volume," we show that, at moderate values of pressure, viscosity increases, as a rule, with increasing pressure, provided the liquid is in stable or metastable (undercooled) equilibrium states. However, even if the behavior of the viscosity is governed by free volume effects, deviations from a positive pressure dependence are possible, when the liquid's thermal expansion coefficient is negative. We derive an equation that allows one to quantitatively determine the pressure dependence of viscosity, which requires, in the simplest case, only the knowledge of the temperature dependence of viscosity at constant pressure, the thermal expansion coefficient, and the isothermal compressibility of the liquid. As an example, the negative pressure dependence of water in the range of temperatures 0-4 degrees C and of several silicate liquids, such as albite, jadeite, dacite, basalts, etc., could be explained in such a way. Other glass-forming liquids initially (for moderate pressures) show a positive pressure dependence of viscosity that changes to a negative one when subjected to high (approximately GPa) isostatic pressure. A detailed analysis of water and already mentioned silicate melts at GPa pressures shows that, in addition to free volume effects, other pressure induced structural transformations may have to be accounted for in a variety of cases. By this reason, the theoretical analysis is extended (i) in order to describe the pressure dependence of viscosity for systems that are in frozen-in thermodynamic nonequilibrium states (glasses, i.e., undercooled liquids below the glass transition temperature Tg) and (ii) to systems which undergo, in addition to variations of the free volume, pressure induced changes of other structural parameters. In such cases a decrease of viscosity with increasing pressure may occur, in principle, even if the thermal expansion coefficient is positive. In this way, the present analysis grants a general tool to estimate the pressure dependence of viscosity and supposedly settles the controversy in the current literature.     

Molten and solidification properties of copper nanoparticles

The melting and solidifying processes are carried out using molecular dynamics simulations. The influencing mechanism of the simulation size and the crystal configuration after solidification on the molten and the solidification properties is explored. The results demonstrate that the crystal structure of solidified copper nanoparticle is sensitive to the size of the copper nanocubes. Polycrystalline appears in the solidified copper nanoparticle for the relatively larger copper cubes; correspondingly, the grain boundaries give rise to the increase of the average atomic energy. Whereas for the relatively small copper cubes, the solidified copper nanoparticles mainly present monocrystalline structure. Moreover, the relationship between the internal pressure of the liquid copper droplets and the droplet diameter is studied to clarify the surface tension property at nanoscale. It is found that the internal pressure of the liquid copper droplets is logarithmically linearly dependent on the diameter of the copper droplets, implying that the surface tension of the liquid copper is not sensitive to the diameter of the liquid copper droplets. The present findings will be helpful to the preparation of the copper nanoparticle‐based thin ribbon. Copyright © 2016 John Wiley & Sons, Ltd.     

Ab initio melting curve of molybdenum by the phase coexistence method

Ab initio calculations of the melting curve of molybdenum for the pressure range 0-400 GPa are reported. The calculations employ density functional theory (DFT) with the Perdew-Burke-Ernzerhof exchange-correlation functional in the projector augmented wave (PAW) implementation. Tests are presented showing that these techniques accurately reproduce experimental data on low-temperature body-centered cubic (bcc) Mo, and that PAW agrees closely with results from the full-potential linearized augmented plane-wave implementation. The work attempts to overcome the uncertainties inherent in earlier DFT calculations of the melting curve of Mo, by using the "reference coexistence" technique to determine the melting curve. In this technique, an empirical reference model (here, the embedded-atom model) is accurately fitted to DFT molecular dynamics data on the liquid and the high-temperature solid, the melting curve of the reference model is determined by simulations of coexisting solid and liquid, and the ab initio melting curve is obtained by applying free-energy corrections. The calculated melting curve agrees well with experiment at ambient pressure and is consistent with shock data at high pressure, but does not agree with the high-pressure melting curve deduced from static compression experiments. Calculated results for the radial distribution function show that the short-range atomic order of the liquid is very similar to that of the high-T solid, with a slight decrease of coordination number on passing from solid to liquid. The electronic densities of states in the two phases show only small differences. The results do not support a recent theory according to which very low dT(m)dP values are expected for bcc transition metals because of electron redistribution between s-p and d states.     

Properties of the discotic-nematic to isotropic transition: influence of short range orientational order

The present investigation concerns the analysis of the influence of short range orientational correlation on the thermodynamic properties of discotic-nematic liquid crystals. Two-site cluster approximation is applied to the orientational molecular coordinates to include the short range orientational correlation. The role of short range orientational order, dispersion interaction, molecular length-to-width ratio and pressure on the thermodynamic and orientational behaviour of discotic nematogens close to the discotic-nematic to isotropic transition are analysed. It is observed that the short range orientational order has a strong influence on the thermodynamic properties and that the transition properties of both the calamitic and discotic mesogens exhibit quite similar behaviour.     

Properties of the discotic-nematic to isotropic transition: influence of short range orientational order

The present investigation concerns the analysis of the influence of short range orientational correlation on the thermodynamic properties of discotic-nematic liquid crystals. Two-site cluster approximation is applied to the orientational molecular coordinates to include the short range orientational correlation. The role of short range orientational order, dispersion interaction, molecular length-to-width ratio and pressure on the thermodynamic and orientational behaviour of discotic nematogens close to the discotic-nematic to isotropic transition are analysed. It is observed that the short range orientational order has a strong influence on the thermodynamic properties and that the transition properties of both the calamitic and discotic mesogens exhibit quite similar behaviour.     

Elemental analysis of steel scrap metals and minerals by laser-induced breakdown spectroscopy

The atomic emission of laser-induced plasma on steel samples has been studied for quantitative elemental analysis. The plasma has been created with 8 ns wide pulses using the second-harmonic from a Q-switched Nd:YAG laser, in air at atmospheric pressure. The plasma emission is detected with temporal resolution, using an Echelle spectrometer of wide spectral range (300–900 nm) combined with an intensified charge coupled device camera. A plasma temperature of 7800 ± 400 K is determined using the Boltzmann plot method, from spectra obtained under optimized experimental conditions.As an example of an industrial application the concentration of copper in scrap metals is studied, which is an important factor to determine the quality of the samples to recycle. Cu concentrations down to 200 ppm can be detected. Another application of the laser-induced plasma spectroscopy method is the measurement of the nickel and copper concentrations in an iron-containing sample of reduced magma from the 1870s expedition to western Greenland by Adolf Erik Nordenskiöld. Different spectral lines of nickel are used for calibration, and their results are compared.     

Perturbation theory for discotic nematic liquid crystals of axially symmetric molecules: effect of dispersion interaction

We investigate the influence of dispersion interaction on a variety of thermodynamic properties of discotic nematic liquid crystals at the discotic nematic-isotropic transition. We report calculations for a hard oblate ellipsoidal system, superposed with an attractive interaction represented by dispersion interaction subjected to different external pressures ranging from 1 to 300 bar. We consider a model system (which simulates a discotic nematic liquid crystal) in which molecules are assumed to interact via a pair potential having both repulsive and attractive parts. The repulsion part is represented by a repulsion between hard oblate ellipsoids of revolution and is a short range, rapidly varying potential. The attractive potential, a function of centre of mass distance and relative orientation between two molecules, is represented by dispersion interaction. The properties of the reference system and first order perturbation term are evaluated using a decoupling approximation which decouples orientational from translational degrees of freedom. The inclusion of fourth and sixth rank orientational order parameters in the calculation slightly improves the result. The role of pressure on phase transition parameters has also been studied.     

Perturbation theory for discotic nematic liquid crystals of axially symmetric molecules: effect of dispersion interaction

We investigate the influence of dispersion interaction on a variety of thermodynamic properties of discotic nematic liquid crystals at the discotic nematic-isotropic transition. We report calculations for a hard oblate ellipsoidal system, superposed with an attractive interaction represented by dispersion interaction subjected to different external pressures ranging from 1 to 300 bar. We consider a model system (which simulates a discotic nematic liquid crystal) in which molecules are assumed to interact via a pair potential having both repulsive and attractive parts. The repulsion part is represented by a repulsion between hard oblate ellipsoids of revolution and is a short range, rapidly varying potential. The attractive potential, a function of centre of mass distance and relative orientation between two molecules, is represented by dispersion interaction. The properties of the reference system and first order perturbation term are evaluated using a decoupling approximation which decouples orientational from translational degrees of freedom. The inclusion of fourth and sixth rank orientational order parameters in the calculation slightly improves the result. The role of pressure on phase transition parameters has also been studied.     

A perturbed Yukawa chain equation of state for refractory liquid metals

The perturbed Yukawa chain equation of state (EoS) has been employed to calculate the liquid density of refractory metals over a wide range of temperatures and pressures. The model uses three independent parameters: m-segment number, σ-segment size, and ε/k-segment energy. For pure components, parameters have been obtained by fitting the models to experimental data on liquid densities. Our calculations on the liquid density of tantalum, rhenium, molybdenum, titanium, zirconium, hafnium and niobium from undercooled temperatures up to several hundred degrees above the boiling point and pressures ranging from 0 to 200?MPa reproduces very accurately the experimental pVT data.     

Crystallization of a binary Lennard-Jones mixture

Transition interface path sampling combined with straightforward molecular dynamics simulation was applied to study the mechanism and kinetics of the crystallization of an undercooled 3:1 binary Lennard-Jones mixture with diameter ratio 0.85 and equal interaction strengths. We find that this mixture freezes via the formation of crystalline clusters consisting of a fcc-rich core and a bcc-rich surface layer, with an excess of large particles and particle species distributed randomly. A detailed comparison reveals that the transition mechanism is similar to that of the pure fluid but occurs with much smaller nucleation rates even at comparable degrees of undercooling. Also, the growth of the crystalline cluster in the mixture proceeds at a pace about 1 order of magnitude slower than in the pure system. Possibly, this slow dynamics of the mixture is related to the occurrence and subsequent relaxation of icosahedral structures in the growing crystal as well as in the liquid surrounding it.