Showing papers on "Phase (matter) published in 1988"

Enthalpy-porosity technique for modeling convection-diffusion phase change: application to the melting of a pure metal

Abstract: The melting of pure gallium in a rectangular cavity has been numerically investigated using the enthalpy-porosity approach for modeling combined convection-diffusion phase change. The major advantage of this technique is that it allows a fixed-grid solution of the coupled momentum and energy equations to be undertaken without resorting to variable transformations. In this work, a two-dimensional dynamic model is used and the influence of laminar natural-convection flow on the melting process is considered. Excellent agreement exists between the numerical predictions and experimental results available in the literature. The enthalpy-porosity approach has been found to converge rapidly, and is capable of producing accurate results for both the position and morphology of the melt front at different times with relatively modest computational requirements. These results may be taken to be a sound validation of this technique for modeling isothermal phase changes in metallurgical systems.

Phase equilibria of associating fluids : spherical molecules with multiple bonding sites

Abstract: The effect of molecular associations on the phase coexistence properties of fluids with one or two directional, attractive centres is investigated. The individual molecules are represented by hard-sphere repulsive cores with off-centre, square-well attractive sites. Such a system’s thermodynamic properties can be calculated by using expressions based on a theory recently proposed by Wertheim. Isothermal-isobaric Monte Carlo simulations of hard-sphere fluids with one or two attractive sites are shown to be in good agreement with the results of the theory. In order to study the system’s phase equilibria using the theory, a simple van der Waals mean-field term is added to account for the dispersion forces. The critical points and phase equilibria of the associating fluids are determined for various values of the strength and range of the attractive site. Furthermore, results are presented for the degree of association in the gas and liquid phases along the vapour pressure curve. The theory can treat fluids with strong hydrogen-bonding associations such as those found in the carboxylic acids, the aliphatic alcohols, hydrogen fluoride, water etc.

Pattern selection in fingered growth phenomena

Abstract: A variety of non-equilibrium growth processes are characterized by phase boundaries consisting of moving fingers, often with interesting secondary structures such as sidebranches. Familiar examples are dendrites, as seen in snowflake growth, and fluid fingers often formed in immiscible displacement. Such processes are characterized by a morphological instability which renders planar or circular shapes unstable, and by the competing stabilizing effect of surface tension. We survey recent theoretical work which elucidates how such systems arrive at their observed patterns. Emphasis is placed upon dendritic solidification, simple local models thereof, and the Saffman-Taylor finger in two-dimensional fluid flow, and relate these systems to their more complicated variants. We review the arguments that a general procedure for the analysis of such problems is to first find finger solutions of the governing equations without surface tension, then to incorporate surface tension in a non-perturbative manne...

Anderson localization and interactions in one-dimensional metals.

Abstract: A one-dimensional interacting electron gas in a random potential exhibits a localized-delocalized transition for increasingly attractive interactions. We develop here a renormalization-group approach to study this transition. Our treatment allows us to obtain the phase diagram and the exponents of the correlation functions in the delocalized regime. The boundary between the two regimes is found to depend both on disorder and the strength of the interactions. For (nearly) spin-isotropic interactions the delocalized phase is dominated by superconducting fluctuations of either singlet or triplet type. The temperature dependence of the conductivity in the delocalized phase is also obtained and a nonuniversal power-law behavior is found. A description of the crossover towards the localized phase is given and the localization length is computed. An analogous description is developed for the localized-superfluid transition of a one-dimensional boson gas. In this case the transition to the localized regime occurs for increasingly repulsive interactions. We suggest a phase diagram with two different localized phases. Finally, we discuss some possible implications of our model for real quasi-one-dimensional metals.

Bulk synthesis of the 81-K superconductor YBa2Cu4O8 at high oxygen pressure

Abstract: The superconducting phase YBa2Cu4O8 ('124') differs from YBa2Cu3O7 ('123') in having a double, instead of single, Cu–O chain running parallel to the b axis. It was first observed as a lattice defect in partly decomposed 123 powders1, and then as an ordered defect structure in inhomogeneous 123 thin films2,3, but has not up to now been synthesized in bulk. As part of a systematic study of the pressure–temperature–composition phase diagram of pseudo-binary Y–Ba–Cu–O systems at high oxygen pressure4,5, we have determined the approximate field of thermodynamic stability of the 124 phase. Here we report synthesis of the phase in bulk at 400 bar O2 and 1,040 °C. In agreement with previous measurements in thin films, the transition temperature is 81 K. Unlike the 123 compound, the oxygen content in 124 is thermally stable up to 850 °C, which may prove important for applications.

Ionization and phase behavior of fatty acids in water: application of the Gibbs phase rule

Abstract: The phase behavior of several medium-chain (10- and 12-carbon) and long-chain (18-carbon) fatty acids in water was examined as a function of the ionization state of the carboxyl group. Equilibrium titration curves were generated above and below fatty acid and acid-soap chain melting temperatures and critical micelle concentrations, and the phases formed were characterized by X-ray diffraction, 13C NMR spectroscopy, and phase-contrast and polarized light microscopy. The resulting titration curves were divided into five regions: (i) at pH values less than 7, a two-phase region containing oil or fatty acid crystals and an aqueous phase; (ii) at pH approximately 7, a three-phase region containing oil, lamellar, and aqueous (or fatty acid crystals, 1:1 acid-soap crystals, and aqueous) phases; (iii) between pH 7 and 9, a two-phase region containing a lamellar fatty acid/soap (or crystalline 1:1 acid-soap) phase in an aqueous phase; (iv) at pH approximately 9, a three-phase region containing lamellar fatty acid-soap (or crystalline 1:1 acid-soap), micellar, and aqueous phases; and (v) at pH values greater than 9, a two-phase region containing micellar and aqueous phases. Interpretation of the results using the Gibbs phase rule indicated that, for oleic acid/potassium oleate, the composition of the lamellar fatty acid/soap phase varied from approximately 1:1 to 1:3 un-ionized to ionized fatty acid species. In addition, constant pH regions observed in titration curves were a result of thermodynamic invariance (zero degrees of freedom) rather than buffering capacity. The results provide insights into the physical states of fatty acids in biological systems.(ABSTRACT TRUNCATED AT 250 WORDS)

Precipitation of the δ-Ni 3 Nb phase in two nickel base superalloys

Abstract: The precipitation of the equilibrium δ-Ni3Nb phase has been studied in two niobium bearing nickel base superalloys—INCONEL 718 and INCONEL* 625—both of which are hardenable by the precipitation of the metastableγ″-Ni3Nb phase. The morphology and the distribution of precipitates have been examined and the crystallographic orientation relationship between the austenite and theδ phases has been determined. The nucleation of theδ phase at stacking faults within pre-existing δ" precipitates has been discussed.

Silver halide photographic material.

Abstract: A silver halide photographic material is disclosed, comprising a support having provided thereon at least one silver halide emulsion layer, wherein the silver halide emulsion layer contains, in the dispersion medium phase of the emulsion, one or more kinds of inorganic fine particles having a refractive index the total weight of the fine particles contained in the unit volume of the dispersion medium phase is from 1.0 to 95 wt %, the dispersion medium phase containing the fine particles is substantially transparent to the photosensitive peak wavelength light of the emulsion layer, and the photographic material is exposed and processed in the development process comprising at least a developing step and a fixing step.

Nucleation of a new phase from the interaction of two adjacent phases: Some silicides

Abstract: The reactions of metal layers with their silicon substrates resulting in the formation of various silicides are considered generally not only as phenomena common to all diffusion couples where new phases are formed, but also as typical of all transitions from two to three phases. The conditions under which such transitions will display the same characteristics as encountered in the usual one-to-two phase transitions (condensation, crystallization, boiling) are analyzed by comparison to the classical theory of nucleation. Because of the lack of knowledge about the exact values of the relevant parameters, the discussion is carried out mostly in descriptive thermodynamic terms. Although nucleation effects are analyzed in general terms, the main focus of attention is a class of reactions where nucleation dominates the formation of a new phase; a salient feature of these reactions is the absence of any equilibrium temperature, although the nucleation temperatures are relatively well defined within narrow limits. Nucleation effects are correlated to such material characteristics as the stability of the nucleated phases, and to such kinetic characteristics as the sequence of phase formation. The modification of the energy levels of the different phases brought about by stress, ion bombardment, or the replacement of usual phases by metastable ones, are considered with respect to their effect on nucleation processes. The nearly total absence of literature references to nucleation in metal-metal diffusion couples is discussed with respect to some specific aspects of the metal-silicon reactions.

Preparation of the High-TcPhase of Bi-Sr-Ca-Cu-O Superconductor

Abstract: Co-decomposition of mixed nitrates of Bi, Pb, Sr, Ca and Cu around 830°C under low oxygen pressure led to the formation of a high-Tc superconducting phase of Bi(Pb)-Sr-Ca-Cu-O with Tc(zero) at 107.5 K. A sample prepared by a conventional solid state reaction method under low oxygen pressure also showed the superconducting transition at 107.5 K. X-ray powder diffraction and magnetic susceptibility measurements on these samples revealed the high-Tc phase without 80 K or semiconducting phase. The reaction under low oxygen pressure has an effect to lower the temperature with broad ranges to render the high-Tc phase of the Bi-Sr-Ca-Cu-O.

Crystal structure of the 80 K superconductor YBa2Cu4O8

Abstract: Recently a new Y–Ba–Cu–O superconducting oxide has been prepared as a distinct phase in thin films1–3. The new phase was first seen as a defect structure in bulk samples of YBa2Cu3O7 (refs 4–7) and found to contain an additional copper layer7. The proposed defect structure has a c-axis spacing of ∼27 A and a cation ratio of YBa2Cu4. Y–Ba–Cu–O films containing a majority phase with a 27 A unit cell have been proposed as evidence of a distinct phase with the same structure as the defects (ref. 1 and K. Char et al., unpublished results). Films of the new phase superconduct with a transition temperature of 80 K (K. Char et al., unpublished results and refs 2 and 3). We show by X-ray crystallography of a film containing 85% of the new phase that the stoichiometry can be written as YBa2Cu4O8. The new structure differs from YBa2Cu3O7 in that the single, linear Cu–O chain parallel to the b-axis has been replaced by a double Cu–O chain with edge-sharing, square-planar oxygen coordination.

Structure and mechanical properties of giant lipid (DMPC) vesicle bilayers from 20 degrees C below to 10 degrees C above the liquid crystal-crystalline phase transition at 24 degrees C.

Abstract: We have used micromechanical tests to measure the thermoelastic properties of the liquid and gel phases of dimyristoylphosphatidylcholine (DMPC). We have found that the rippled P beta' phase is only formed when a vesicle is cooled to temperatures below the main acyl chain crystallization transition, Tc, under zero or very low membrane tension. We also found that the P beta' surface ripple or superlattice can be pulled flat under high membrane tension into a planar structure. For a ripple structure formed by acyl chains perpendicular to the projected plane, the projected area change that results from a flattening process is a direct measure of the molecular crystal angle. As such, the crystal angle was found to increase from about 24 degrees just below Tc to about 33 degrees below the pretransition. It was also observed that the P beta' superlattice did not form when annealed L beta' phase vesicles were heated from 5 degrees C to Tc; likewise, ripples did not form when the membrane was held under large tension during freezing from the L alpha phase. Each of these three procedures could be used to create a metastable planar structure which we have termed L*beta' since it is lamellar and plane-crystalline with acyl chains tilted to the bilayer plane. However, we show that this structure is not as condensed as the L beta' phase below 10 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth of the 2223 Phase in Leaded Bi-Sr-Ca-Cu-O System

Abstract: Growth of the 2223 phase has been studied in the leaded Bi-Sr-Ca-Cu-O system. Differential thermal analysis showed that the enhanced formation of the 2223 phase occurs in the temperature range between 835°C and 869°C where partial melting occurs. The density of the pellet was found to increase by 30% at one hour and then to decrease by 40% from the highest density attained. The former is attributed to the growth of the 2223 phase with the aid of the partially melted liquid phase and the latter to the flaky shape of the resultant 2223 crystals. The effect of Pb addition for enhanced 2223 phase formation will be discussed.

Synthesis, structure and superconductivity in the Ba1-xKxBiO3-y system

Abstract: The recent discovery of superconductivity near 30 K in Ba1−xKxBiO3−y (x≈0.4)1,2 is remarkable for two reasons. It is the first copper-free oxide superconductor that has a transition temperature (Tc) above that for the best intermetallic superconductor; and the structure is reported to be cubic, which excludes a two-dimensional metal–oxygen sublattice analogous to the CuO2 planes believed to be responsible for superconductivity in the copper-oxide-based superconductors. Cava et al.1 described a synthesis technique which involved starting with a 100% excess of KO2. At least part of the excess potassium was found to be present in the final sample (in a form not detectable by X-ray diffraction), resulting in samples that were not suitable for resistivity measurements and making a precise determination of the potassium and oxygen content in the superconducting phase impossible. Here we describe a two-step synthesis technique starting with a stoichiometric oxide composition, which yields single-phase samples suitable for transport measurements. Neutron powder diffraction studies of samples with varying potassium concentration show that superconductivity in Ba1−xKxBiO3−y, occurs only in a cubic perovskite phase which is stable at ≲600°C and which forms only for x > 0.25. Within this cubic phase, Tc is highest for compositions near the structural phase transition (x ≈ 0.25) and decreases with increasing x.

Quasimelting and phases of small particles.

Abstract: Calculations of the Gibbs free energies of single-crystal and multiply twinned small metal particles clearly indicate for the first time both the presence of a quasimolten phase (where the particles are continuously fluctuating between different structures) at temperatures well below the melting point and the existence of distinct phase regions for different particle shapes. These results have important implications for the understanding of epitaxial growth processes; for instance, at temperatures where the quasimolten and single-crystal phases are contiguous one would expect far better epitaxial growth than at temperatures where the multiply twinned phases are stable.

General patterns of the phase behavior of mixtures of water, nonpolar solvents, amphiphiles, and electrolytes. 2

Abstract: Determination des diagrammes de phase pour les systemes eau-NaCl-agent de surface non ionique

Multiple liquid crystal phases of DNA at high concentrations.

Abstract: DNA packaging in vivo is very tight, with volume concentrations approaching 70% w/v in sperm heads, virus capsids and bacterial nucleoids1–3. The packaging mechanisms adopted may be related to the natural tendency of semi-rigid polymers to form liquid crystalline phases in concentrated solutions4–8. We find that DNA forms at least three distinct liquid crystalline phases at concentrations comparable to those in vivo, with phase transitions occurring over relatively narrow ranges of DNA concentration. A weakly birefringent, dynamic, 'precholesteric' mesophase with microscopic textures intermediate between those of a nematic and a true cholesteric phase forms at the lowest concentrations required for phase separation. At slightly higher DNA concentrations, a second mesophase forms which is a strongly birefringent, well-ordered cholesteric phase with a concentration-dependent pitch varying from 2 to lOμm. At the highest DNA concentrations, a phase forms which is two-dimensionally ordered and resembles smectic phases of thermotropic liquid crystals observed with small molecules.

Growth of the 2223 Phase in Leaded Bi-Sr-Ca-Cu-O System : Electrical Properties of Condensed Matter

Abstract: Growth of the 2223 phase has been studied in the leaded Bi-Sr-Ca-Cu-O system. Differential thermal analysis showed that the enhanced formation of the 2223 phase occurs in the temperature range between 835°C and 869°C where partial melting occurs. The density of the pellet was found to increase by 30% at one hour and then to decrease by 40% from the highest density attained. The former is attributed to the growth of the 2223 phase with the aid of the partially melted liquid phase and the latter to the flaky shape of the resultant 2223 crystals. The effect of Pb addition for enhanced 2223 phase formation will be discussed.

Ellipsometric, electrochemical, and elemental characterization of the surface phase produced on glassy carbon electrodes by electrochemical activation

Abstract: Ellipsometry was used to monitor the In situ growth of a fllm on a glassy carbon electrode, whlch was electrochemlcally anodized at 1.8 V In 0.1 M H,SO,. The layer grew contlnuously In an uninhlblted fashion wlth constant optical constants to a thlckness of at least 925 nm. It was nearly transparent at wavelengths of 545 and 632.8 nm. X-ray and elemental analysls of bulk quantltles of the phase Indicate that It Is an amorphous form of graphlte oxlde. Comblned elllpsometrlc and electrochemical measurements show that the phase activates the surface and that deactivation occurs upon extennslve reductlon of the layer. The electrode actlvlty, as monitored by the voltammetrlc response In dilute solutlons of catechol, hydroquhne, and 2,3-dlcyanohydroqulnone, varled wlth the extent of reductlon of the layer.

Phase Separation of Liquid Crystals in Polymers

Abstract: New optoelectronic materials based on polymer dispersed liquid crystals (PDLC) show great potential for application in displays, temperature sensors, optical computing and for solar energy control. We report liquid crystal, termoset or thermoplastic materials. PDLC materials may be formed by several different processes. The liquid crystal may be dissolved in low molecular weight polymer precursors, in a thermoplastic melt or with a thermoplastic in a common solvent. Subsequent polymerization, cooling of the polymer melt or solvent evaporation lead to liquid crystal immiscibility, droplet formation and growth, and polymer gelation. The optoelectronic properties of these materials are affected by the droplet morphology. Specific examples are presented for each of these processes and it is demonstrated how the droplet morphology and density, and thus device performance, can be controlled by each method. The thermoplestics are suitable for forming films by a variety of techniques. A range of polymers...

Phase transitions in magnetic superlattices

Abstract: We investigate the magnetic-field- and temperature-dependent equilibrium structure of magnetic superlattices formed from two ferromagnetic materials which couple antiferromagnetically at the interfaces. Both a macroscopic, Landau-Ginzburg, and a microscopic approach are used. Due to competing exchange and Zeeman interactions, a variety of phases exist in the superlattice. There are aligned phases where all the spins are either parallel or antiparallel to the applied field, and there is a twisted phase where the spins in each layer lie at a different angle with respect to the applied field. We show that small changes in the layering structure can lead to dramatic changes in the phase diagram.