Showing papers on "Phase (matter) published in 1995"

Importance of phase fluctuations in superconductors with small superfluid density

Abstract: THE superconducting state of a metal is characterized by a complex order parameter with an amplitude and a phase In the BCS-Eliashberg mean-field theory1, which is a very good approximation for conventional metals, the phase of the order parameter is un-important for determining the value of the transition temperature Tc and the change of many physical properties brought about by the transition Here we argue that superconductors with low super-conducting carrier density (such as the organic and high-Tc oxide superconductors) are characterized by a relatively small phase 'stiffness9 and poor screening, both of which imply a significantly larger role for phase fluctuations As a consequence, in these mat-erials the transition to the superconducting state may not display typical mean-field behaviour, and phase fluctuations, both classical and quantum, may have a significant influence on low-temperature properties For some quasi-two-dimensional materials, notably underdoped high-temperature superconductors, the onset of long-range phase order controls the gross value of Tc as well as its systematic variation from one material to another

High-Rate, Gas-Phase Growth of MoS2 Nested Inorganic Fullerenes and Nanotubes

Abstract: The gas-phase reaction between MoO3-x and H(2)S in a reducing atmosphere at elevated temperatures (800 degrees to 950 degrees C) has been used to synthesize large quantities of an almost pure nested inorganic fullerene (IF) phase of MoS(2). A uniform IF phase with a relatively narrow size distribution was obtained. The synthesis of IFs appears to require, in addition to careful control over the growth conditions, a specific turbulent flow regime. The x-ray spectra of the different samples show that, as the average size of the IF decreases, the van der Waals gap along the c axis increases, largely because of the strain involved in folding of the lamella. Large quantities of quite uniform nanotubes were obtained under modified preparation conditions.

Liquid-crystalline phases as templates for the synthesis of mesoporous silica

Abstract: THE synthesis of inorganic mesoporous materials using ionic surfactant template molecules was first reported in 19921,2, and surfactant-mediated synthesis has since been used to form a variety of mesoporous materials3–8. Such materials could find application in catalysis, membrane and separation technology, and molecular engineering. Previous syntheses have used low surfactant concen-trations, and the templating mechanism (which is still controversial) is thought to be a cooperative process involving the interaction of inorganic ions with discrete surfactant aggregates1,2,5,9. Here we report the templating of silica mesostructures from ordered liquid-crystalline mesophases: the resulting silica phase, with pores of ∼3 nm diameter, is a cast of the organic mesophase. As the phase diagram of the surfactant/silica/water system at high surfactant concentration is similar to the (known) phase diagram of surfactant/water alone, this approach should introduce an element of predictability into the synthesis of mesoporous materials.

Polyisoprene-Polystyrene Diblock Copolymer Phase Diagram near the Order-Disorder Transition

Abstract: The phase behavior of ten polyisoprene-polystyrene (PI-PS) diblock copolymers, spanning the composition range from 0.24 to 0.82 polyisoprene volume fraction (f PI ), has been studied near the order-disorder transition (ODT). Dynamic mechanical spectroscopy, transmission electron microscopy, and neutron and X-ray scattering have been used to characterize phase transition temperatures and ordered state symmetries. Five distinct microstructures were observed for this chemical system : spheres, hexagonally packed cylinders (HEX), lamellae (LAM), hexagonally perforated layers (HPL), and a bicontinuous cubic phase having an Ia3d space group symmetry. The bicontinuous Ia3d phase only occurs in the vicinity of the ODT between the HEX and LAM states at compositions of 0.65 ≤ f pI ≤ 0.68 and 0.36 ≤ f pI ≤ 0.39 (prior report). Farther from the ODT, within these composition ranges, the HPL phase occurs. We did not find the ordered bicontinuous double diamond (OBDD) morphology at any composition or temperature studied, and the overall phase diagram is qualitatively different from those reported previously for PI-PS block copolymers.

Electronic and optical properties of three phases of titanium dioxide: Rutile, anatase, and brookite

Abstract: Using the self-consistent orthogonalized linear-combination-of-atomic-orbitals method in the local-density approximation, the electronic structure and the optical properties of three phases of titanium dioxide have been studied. For rutile, the calculated band structure, equilibrium lattice constant, and bulk modulus are in good agreement with other recent calculations and with experimental data. The results on the ground-state properties of anatase and brookite are reported. Compared with the rutile phase, anatase has similar ground-state properties except for a larger band gap, whereas brookite has relatively smaller bulk modulus. The optical properties of these three phases are also calculated using the band-structure results and compared with the available measurements. For the rutile phase, the anisotropic properties of the dielectric function are in good agreement with the reflectance spectroscopy. For the anatase phase, there are very limited experimental optical data for comparison. For the brookite phase, no experimental data are available. Our calculations show subtle differences in the optical properties of these three phases.

Ti(C,N) cermets — Metallurgy and properties

Abstract: An overview of the metallurgical reactions during the vacuum sintering process of powder mixtures for the manufacture of cermets is presented. The relatively complex phase reactions in the multi-component system Ti/Mo/W/Ta/Nb/C,N-Co/Ni are discussed. The liquid binder phase reacts with titanium carbonitride by preferentially dissolving titanium carbide leaving titanium nitride undissolved. The compositions and the amounts of the gas species set free during the sintering process were monitored and led —together with differential thermal analysis — to a better understanding of the mechanisms that govern the sintering behaviour. The properties and the microstructure of cermets depend on the nature and the alloy status of the prematerials. The composition of the prematerials with respect to the carbon-nitrogen ratio, the stoichiometry of the hard phase and the amount and composition of the binder phase have a decisive influence on the properties and the cutting performances of the final products. Optimization of the properties with respect to the desired performance is possible. Examples of the cermet cutting performance in various applications are discussed.

Computer simulations of vapor-liquid phase equilibria of n-alkanes

Abstract: For petrochemical applications knowledge of the critical properties of the n‐alkanes is of interest even at temperatures where these molecules are thermally unstable. Computer simulations can determine the vapor–liquid coexistence curve of a large number of n‐alkanes ranging from pentane (C5) through octatetracontane (C48). We have compared the predicted phase diagrams of various models with experimental data. Models which give nearly identical properties of liquid alkanes at standard conditions may have critical temperatures that differ by more than 100 K. A new n‐alkane model has been developed by us that gives a good description of the phase behavior over a large temperature range. For modeling vapor–liquid coexistence a relatively simple united atom model was sufficient to obtain a very good agreement with experimental data; thus it appears not necessary to take the hydrogen atoms explicitly into account. The model developed in this work has been used to determine the critical properties of the long‐chain alkanes for which experiments turned out to be difficult and contradictory. We found that for the long‐chain alkanes (C8–C48) the critical density decreases as a function of the carbon number. These simulations were made possible by the use of a recently developed simulation technique, which is a combination of the Gibbs‐ensemble technique and the configurational‐bias Monte Carlo method. Compared with the conventional Gibbs‐ensemble technique, this method is several orders of magnitude more efficient for pentane and up to a hundred orders of magnitude for octatetracontane. This recent development makes it possible to perform routinely phase equilibrium calculations of complex molecules.

Design of electrical conductive composites : key role of the morphology on the electrical properties of carbon black filled polymer blends

Abstract: Effect of carbon black (CB) on the morphology of filled polyethylene (PE)/polystyrene (PS) blends has been investigated by image analysis of optical micrographs (2-D analysis) and by the selective extraction of one phase of the binary blends (3-D analysis). The macroscopic electrical resistivity of the filled polyblends strongly depends on the selective localization of CB in one phase or at the interface and above all on the double percolation, i.e. percolation of the polymer phases and percolation of the CB particles. The selective localization of CB in the PE phase has remarkable effects on the polyblend phase morphology. The phase cocontinuity is indeed extended over a much larger composition range, and the phase morphology is stabilized toward post-thermal treatment at 200 °C. In the case of double percolation and selective localization of CB at the polyblend interface, electrical conductivity is observed at a CB content as low as 0.4 wt %.

Phase Behavior of Block Copolymer/Homopolymer Blends

Abstract: We examine weakly segregated blends of AB diblock copolymer and A homopolymer with similar degrees of polymerization. The relative stability of numerous phases is examined and phase diagrams are constructed using self-consistent field theory. While the pure diblock system is only found to exhibit the body-centered cubic (spherical), hexagonal (cylindrical), bicontinuous Ia3d cubic (gyroid), and lamellar ordered phases, we find that the addition of homopolymer stabilizes close-packed spherical, bicontinuous Pn3m cubic (double-diamond), and hexagonally-perforated (catenoid) lamellar phases. We find that, in general, the minority-component region of a microstructure can only accommodate a limited amount of homopolymer before macrophase separation occurs. On the other hand, the majority-component regions can swell indefinitely with the addition of homopolymer, eventually resulting in an unbinding transition. We associate the region of highly-swollen microstructures with the micellar region observed in real systems. For the lamellar and hexagonal phases, we examine the distribution of homopolymer within the microstructure, and for the lamellar phase, we calculate the effect of homopolymer on the dimensions of the A- and B-rich microdomains.

Factors Controlling the Stability of Colloid-Stabilized Emulsions: I. An Experimental Investigation

Abstract: Experimental data are presented to show the influence of colloidal particles on the stability of oil-water emulsions. It is shown that these solids stabilize emulsions both by providing steric hindrance to drop-drop coalescence and by modifying the rheological properties of the interfacial region. Coalescence occurs as a result of the displacement of the colloids along the interface. Results are presented on the effects of pH and salt concentration in the aqueous phase, the concentration of surfactant in the oleic phase, and the properties of the solid particles on the type and the stability of emulsions formed.

Phase diagram of colloidal dispersions of anisotropic charged particles : equilibrium properties, structure, and rheology of laponite suspensions

Abstract: We discuss the phase diagram of aqueous dispersions of colloidal platelike charged particles (300 A x 10 A). Particle concentration and ionic strength are the two parameters controlling the system. The suspensions undergo a sol/gel transition without macroscopic phase separation. Shear rheology is used to monitor this transition and to locate the appearance of the mechanical gel phase. Increasing the ionic strength shifts the sol/gel transition to lower volume fraction. Direct inspection of this gel phase by cryofracture, TEM and SAXS shows correlated but well-separated particle populations. In order to check the reversibility and the equilibrium properties of this transition, the equation of state was determined by osmotic stress. At fixed ionic strength, the osmotic pressure first increases at low particle concentration, then reaches a pseudoplateau, and increases again for higher concentrations. The location of such a singularity in the equation of state of the suspension defines a thermodynamical transition coinciding with the mechanical phase transition. In order to analyze the origin of this gel or glassy phase, the role of particle anisotropy, coupled with diffuse layer repulsion, is discussed.

Properties of Metal Silicides

Abstract: Crystal structure mechanical and thermal properties growth and phase transformations thermochemical properties band structure Schottky barrier heights heterojunctions and interfaces electrical transport optical properties and functions impurity diffusion self-diffusion hyperfine interactions.

Membrane lipid domains and dynamics as detected by Laurdan fluorescence.

Abstract: 2-Dimethylamino-6-lauroylnaphthalene (Laurdan) is a membrane probe of recent characterization, which shows high sensitivity to the polarity of its environment. Steady-state Laurdan excitation and emission spectra have different maxima and shape in the two phospholipid phases, due to differences in the polarity and in the amount of dipolar relaxation. In bilayers composed of a mixture of gel and liquid-crystalline phases, the properties of Laurdan excitation and emission spectra are intermediate between those obtained in the pure phases. These spectral properties are analyzed using the generalized polarization (GP). TheGP value can be used for the quantitation of each phase. The wavelength dependence of theGP value is used to ascertain the coexistence of different phase domains in the bilayer. Moreover, by following the evolution of Laurdan emission vs. time after excitation, the kinetics of phase fluctuation in phospholipid vesicles composed of coexisting gel and liquid-crystalline phases was determined.GP measurements performed in several cell lines did not give indications of coexistence of phase domains in their membranes. In natural membranes, Laurdan parameters indicate a homogeneously fluid environment, with restricted molecular motion in comparison with the phospholipid liquid-crystalline phase. The influence of cholesterol on the phase properties of the two phospholipid phases is proposed to be the cause of the phase behavior observed in natural membranes. In bilayers composed of different phospholipids and various cholesterol concentrations, Laurdan response is very similar to that arising from cell membranes. In the absence of cholesterol, from the steady-state and time-resolved measurements of Laurdan in phospholipid vesicles, the condition for the occurrence of separate coexisting domains in the bilayer has been determined: the molecular ratio between the two phases must be in the range between 30% and 70%. Below and above this range, a single homogeneous phase is observed, with the properties of the more concentrated phase, slightly modified by the presence of the other. Moreover, in this concentration range, the calculated dimension of the domains is very small, between 20 and 50 A.

Effect of sample topology on the critical fields of mesoscopic superconductors

Abstract: THE superconducting state of a material can be suppressed by either increasing the temperature (T) or applying a magnetic field (H). For bulk samples, the form of the H–T phase boundary is mainly determined by the material itself; sample topology can be neglected because the surface-to-volume ratio is small1. But for mesoscopic samples, this ratio becomes very large and nucleation of the superconducting state should depend strongly on the boundary conditions imposed by the sample shape, analogous to the role of the confining potential on the energy levels in the quantum-mechanical 'particle-in-a-box' problem2. Here we describe measurements of the superconducting H-T phase boundary of a range of mesoscopic aluminium structures (lines, squares and square rings) which show clearly the effect of sample topology. The H-T phase boundaries determined experimentally are in excellent agreement with theoretical calculations.

Observation, prediction and simulation of phase transitions in complex fluids

Abstract: Preface. A: Experiment. Phase Transition of Spherical Colloids W.C.K. Poon, P.N. Pusey. Liquid Crystal Phase Transitions in Dispersions of Rodlike Colloidal Particles H.N.W. Lekkerkerker, P. Buining, J. Buitenhuis, G.J. Vroege, A. Stroobants. Phase Transitions in Colloidal Suspensions of Virus Particles S. Fraden. B: Theory. Colloidal Suspensions: Density Functional Theory at Work J.-P. Hansen. Bilayer Phases M.E. Cates. Liquid Crystal Interfaces M.M. Telo Da Gama. Statistical Mechanics of Directed Polymers D.R. Nelson. C: Simulation. Introduction to Monte Carlo Simulation M.P. Allen. Numerical Techniques to Study Complex Liquids D. Frenkel. Molecular Dynamics Techniques for Complex Molecular Systems M. Sprik. Gibbs Ensemble Techniques A.Z. Panagiotopoulos. Phase Transitions in Polymeric Systems K. Binder. Simulations and Phase Behaviour of Liquid Crystals M.P. Allen. D: Seminars. Equilibrium Sedimentation Profiles of Screened Charged Colloids. A Test of the Hard-Sphere Equation of State V. Degiorgio, R. Piazza, T. Bellini. Dynamics of N-Nonadecane Chains in Urea Inclusion Compounds as Seen by Incoherent Quasielastic Neutron Scattering and Computer Simulations M. Souaille, J.C. Smith, A.-J. Dianoux, F. Guillaume. Theory of Phase Equilibria in Associating Systems: Chain and Ring Aggregates, Amphiphiles, and Liquid Crystals R.P. Sear, G. Jackson. Can a Solid be Turned into a Gas Without Passing through a First Order Phase Transition? R. Lovett. Index.

Rapid formation of intermetallic compounds interdiffusion in the CuSn and NiSn systems

Abstract: Fundamental investigations were carried out to examine a new interconnection technology which is based on the rapid formation of intermetallic compounds composed of a high melting component (e.g. Cu or Ni) and a low melting component (e.g. Sn) between two layers of the high melting component at temperature just above the melting point of Sn. This reaction is known as isothermal solidification. The growth of the intermetallic compounds of the CuSn and NiSn systems were studied in thin films at temperatures from 513 to 673 K. At the beginning of interdiffusion, the Sn-rich intermetallic compounds ν (Cu6Sn5) and Ni3Sn4 grow fastest with a non-parabolic time dependence. The Cu3Sn phase grows parabolically, however, not the Ni3Sn2 phase. The Ni3Sn phase does not nucleate below 623 K in specimens with clean Ni/Sn interfaces. For the first time, the influence of the small grain size of the Cu or Ni thin films were studied by performing similar experiments with Cu or Ni single crystals. In the CuSn system the interdiffusion coefficients for Cu3Sn obtained from the thin film experiments are twice those obtained from the single crystal experiments. In the NiSn system there are no differences between thin film and single crystal results.

Reaction-controlled morphology of phase-separating mixtures.

Abstract: The role of externally-controlled chemical reactions in the selection of patterns in phase-separating mixtures is presented. Linearized theory and computer simulation show that the initial long-wavelength instability characteristic of spinodal decomposition is suppressed by chemical reactions, which restrict domain growth to intermediate length scales even in the late stages of phase separation. Our findings suggest that such reactions may provide a novel way to stabilize and tune the steady-state morphology of phase-separating materials. Pattern formation in reaction-diffusion systems occurs throughout nature. It is well known, for example, that spiral waves and other interesting steady-state patterns can be generated by simple chemical reactions [1]. In contrast, transient patterns are formed during phase separation by spinodal decomposition in both small molecule and polymer mixtures [2,3]. These patterns, whose characteristic length scale depends on the specificity of the components of the mixture, coarsen and disappear when macroscopic phase separation is achieved at asymptotically long times. It would be desirable to devise a mechanism by which these phase-separating morphologies could be stabilized. In this Letter, we argue that chemical reactions can be used to stabilize and tune the characteristic length scale of patterns arising in phase-separating materials. Unlike the usual scenario of spinodal decomposition, where concentration fluctuations of all length scales larger than a certain critical length scale spontaneously grow with time, we show that chemical reactions introduce two cutoff lengths, thereby

Cryo-TEM and SANS Microstructural Study of Pluronic Polymer Solutions

Abstract: Cryogenic temperature transmission electron microscopy (cryo-TEM) and small-angle neutron scattering (SANS) were combined to give complete microstructural characterization of aqueous solutions of Pluronic F127, an (EO) 99 (PO) 65 (EO) 99 triblock copolymer. The images of vitrified specimens observed by cryo-TEM provided direct information about the building blocks of these systems, i.e., spheroidal micelles. This confirmed the structural model obtained from analysis of SANS data, which, in addition, provide quantitative information about the system. Occasionally spheroidal micelles were pressed closer to each other during specimen preparation, thus giving rise locally to higher concentration. The cubic phase that was thus formed was directly imaged. The existence of this lyotropic liquid crystalline phase was confirmed by SANS of an originally higher concentration solution. When exposed to shear, the polycrystalline phase transforms into a monodomain crystal with cubic symmetry. The system was also used to demonstrate the potential of selective electron beam radiolysis to enhance contrast in radiation-sensitive, inherently low-contrast systems.

Interfacial tension in high-pressure carbon dioxide mixtures

Abstract: High-pressure interfacial- and surface-tension phenomena govern the migration and recovery of oil and gas from hydrocarbon reservoirs. The phenomena are of particular relevance to phase separation and mass transfer in light hydrocarbon fractionation plants and in propane deasphalting in lubricating oil refining. Interfacial tensions of carbon dioxide-water-alcohol mixtures were measured at temperatures in the range 5--71 C and pressures 0.1--18.6 MPa, using the capillary rise method. The alcohols were methanol (0.136 mf), ethanol (to 0.523 mf), and isopropyl alcohol (to 0.226 mf). Interfacial tension (IFT) decreased linearly with both temperature and pressure din the low-pressure range (gaseous CO{sub 2}) but was largely independent of pressure at high pressure (liquid or supercritical CO{sub 2}). There was a zone in the vicinity of the critical pressure of CO{sub 2}-as much as 20 C below and 10 C above the carbon dioxide critical temperature--where IFT became small. This is attributed to the formation of a second CO{sub 2}-rich phase. The isotherms exhibited a crossover pressure near 3 MPa for all systems examined.

Structure and properties of silicon XII: A complex tetrahedrally bonded phase

Abstract: Angle-dispersive powder diffraction using an image-plate area detector and synchrotron radiation have been used in conjuction with first-principles pseudopotential calculations to examine the structural, electronic, and vibrational properties of the recently discovered phase XII of silicon (the R8 phase). The R8 phase is synthesized by decompression of the high-pressure \ensuremath{\beta}-Sn phase and exists over a relatively wide pressure range of 2--12 GPa. Although there are structural similarities between BC8 and R8, the latter phase exhibits substantially greater local deviations from ideal tetrahedral bonding and is the most distorted crystalline structure containing fourfold-coordinated silicon. We present a detailed investigation of the pressure response of the BC8 structure, suggest plausible atomic trajectories for the \ensuremath{\beta}-Sn to R8 transition, and we investigate the energy of R8 silicon relative to those of other tetrhedral forms.

Experimental Phase Diagram of a Binary Colloidal Hard-Sphere Mixture with a Large Size Ratio

Abstract: We determined the phase diagram of a binary hard-sphere dispersion with size ratio 9.3. Phase separation into a fluid and a crystal of large spheres is observed. The fluid-solid binodal is determined by measurements of compositions of coexisting phases. The results agree qualitatively with recent theories, although the latter strongly overestimate the depletion activity of the smaller spheres. By fluorescent labeling we are able to measure the mobility of both particles separately. We found evidence for a new glassy phase in which only the small spheres are mobile.