Phase Diagram Boundaries Research Articles

Using dislocations to probe surface reconstruction in thick freely suspended liquid crystalline films.

Surface interactions can cause freely suspended thin liquid crystalline films to form phases different from the bulk material, but it is not known what happens at the surface of thick films. Edge dislocations can be used as a marker for the boundary between the bulk center and the reconstructed surface. We use noncontact mode atomic force microscopy to determine the depth of edge dislocations below the surface of freely suspended thick films of 4-n-heptyloxybenzylidene-4-n-heptylaniline (7O.7) in the crystalline B phase. Here, 3.0±0.1 nm high steps are found with a width that varies with temperature between 56 and 59°C. Using a strain model for the profile of liquid crystalline layers above an edge dislocation to estimate the depth of the dislocation, we find that the number of reconstructed surface layers increases from 4 to 50 layers as the temperature decreases from 59 to 56°C. This trend tracks the behavior of the phase boundary in the thickness dependent phase diagram of freely suspended films of 7O.7, suggesting that the surface may be reconstructed into a smectic F region.

Phase diagram of ternary polymeric solutions containing nonsolvent/solvent/polymer: Theoretical calculation and experimental validation

Mathematical modeling of phase inversion membrane fabrication has been focused by many researchers, as the experimental study of phenomenological events is almost impossible, mainly, due to the high rate of exchanges and the small thickness of cast film. The appropriate coupling of conservation law of mass and thermodynamic stability boundaries and their consequent kinetic mechanisms is the only way to establish a rigorous thermo-kinetic predictive model. In this work, we focused on development of a reliable method of phase diagram construction i.e. determination of thermodynamic boundaries, where a compressible regular solution theory based model (CRS) has been used to resolve the essence of experimental knowledge on interaction parameters of models such Flory–Huggins. Attempts were made to develop thermodynamic regions such as binodal, spinodal, gelation, vitrification and etc. For validation of calculation results, required experimental phase diagram and thermodynamic boundaries were retrieved from literature. Ternary systems of (1) polyethersolfune/N-methylpyrrolidone/water and (2) cellulose acetate/acetone/water were considered as the case study, here, for which the methodology can be extended then to other amorphous polymer containing systems.

High-pressure structural anomalies and electronic transitions in the topological Kondo insulator SmB6

We have scrupulously investigated the high-pressure structural behaviour of the topological Kondo insulator SmB6 through synchrotron powder x-ray diffraction. Lattice modifications are observed at a critical pressure of for three separate compression experiments at ambient and low temperatures. The initially subtle compression of the Sm-B bond length shows a sharp kink downward at the critical pressure. These anomalies coincide with the phase diagram boundaries for the pressure-induced metallization and the concurrent appearance of long-range magnetic ordering in the compound. We propose a scenario of delocalization of 4f electrons and the gradual filling of 5d conduction bands, which can also provide some clarification on the valence dependence of Sm with pressure.

Geometric phase and phase diagram for a non-Hermitian quantumXYmodel

We study the geometric phase for the ground state of a generalized one-dimensional non-Hermitian quantum XY model, which has transverse-field-dependent intrinsic rotation-time reversal symmetry. Based on the exact solution, this model is shown to have full real spectrum in multiple regions for the finite size system. The result indicates that the phase diagram or exceptional boundary, which separates the unbroken and broken symmetry regions corresponds to the divergence of the Berry curvature. The scaling behaviors of the groundstate energy and Berry curvature are obtained in an analytical manner for a concrete system.

Application of genetic algorithm in calculation of the phase behavior of binary and ternary polymer solutions

A genetic algorithm as an optimization procedure has been developed to predict the phase behavior of polymer solutions. The phase equilibrium diagrams of binary and ternary polymer solutions have been determined using the appropriate form of Flory–Huggins free-energy function for polymer solutions. A concentration and temperature dependent form of the interaction parameter has been used to reflect the effect of temperature and polymer properties in the free-energy form. The proposed genetic algorithm is applied to compare the phase behavior results of some typical polymer solutions with the results of the classical determination methods and then applied to some conventional ternary polymer solutions as polymer–solvent–nonsolvent systems. The proposed algorithm use a set of individual states as the initial chromosomes and uses the general rules as crossover, mutation, and with use of a fractional objective function determines the binodal points or the phase diagram boundaries of polymer solutions. The properties of an industrially relevant polymer solution, a polystyrene–cyclohexane solution, have been used to emphasize on the industrial application of the proposed algorithm. The algorithm has been used to predict the phase behavior of the two polymer–solvent–nonsolvent systems as polystyrene-butanone-methanol and polystyrene-butanone-propanol at three different temperatures and results show good agreement with the experimental observations. The algorithm also has the capability to predict both the concentration-independent and concentration-dependent interaction parameters among the different components. The genetic algorithm is an easy-to-use, state-of-art, and very fast optimization tool, and has very strong capability to solve nonlinear systems in chemical and polymer engineering topics.

High Pressure Kinetics Studies of Water Solidification in Dynamic-DAC

The phase diagram of water is both unusual and complex, exhibiting a wide range of polymorphs including proton-ordered or disordered forms. In addition, a variety of stable and metastable forms are observed. The richness of H2O phases attests the versatility of hydrogen bonded network structures that include kinetically stable amorphous ices. In addition, natural gas hydrates are observed upon cooling and/or pressurization. The structures of gas hydrates are quite different from ices, yet guest molecules are enclathrated inside hydrogen bonded water cages. Hence, the presence of small impurity molecules can affect strongly the behaviours of hydrogen bonding and alter the phase diagram and phase boundaries of H2O. In this paper, we report the effect of methane impurity on solidification and melting of H2O under dynamic loading conditions. The presence of impurity mainly affects the growth rate of metastable ice VII, slowing down from 1.4 m/s in pure H2O to 0.26 m/s in impure H2O, neither the occurrence of metastable ice VII nor the melting of ice VI.

Phase structure of monolayer graphene from effective U(1) gauge theory on honeycomb lattice

Phase structure of monolayer graphene is studied on the basis of a U(1) gauge theory defined on the honeycomb lattice. Motivated by the strong coupling expansion of U(1) lattice gauge theory, we consider on-site and nearest-neighbor interactions between the fermions. When the on-site interaction is dominant, the sublattice symmetry breaking (SLSB) of the honeycomb lattice takes place. On the other hand, when the interaction between nearest-neighboring sites is relatively strong, there appears two different types of spontaneous Kekul\'e distortion (KD1 and KD2), without breaking the sublattice symmetry. The phase diagram and phase boundaries separating SLSB, KD1, and KD2 are obtained from the mean-field free energy of the effective fermion model. A finite gap in the spectrum of the electrons can be induced in any of the three phases.

Preparation of nanoparticles by solvent displacement for drug delivery: A shift in the “ouzo region” upon drug loading

As biodegradable nanoparticles meet with increasing interest for drug delivery applications, a series of investigations were carried out to understand the mechanism of the formation of drug-loaded nanoparticles using the solvent displacement method. Although previous explanations referred to Marangoni convection as the driving force for nanoprecipitation, recent publications describing the so-called “ouzo effect” sparked these current studies using a novel negatively charged polymer, poly(vinyl sulfonate- co-vinyl alcohol)- graft-poly( d, l-lactide- co-glycolide) (P(VS-VA)-g-PLGA), and a positively charged model drug, salbutamol. Interfacial tension did not influence the nanoparticle formation as would be expected if governed by Marangoni convection, but ternary phase diagrams outlined the so-called “ouzo regions” defining the polymer and solvent concentrations leading to stable nanoparticle suspensions for both this novel polymer and unmodified poly( d, l-lactide- co-glycolide) (PLGA). Physicochemical properties, morphology and drug loading of the nanoparticles were analyzed, and stable P(VS-VA)-g-PLGA nanoparticles with and without salbutamol ranged in size from 59–191 nm. The “ouzo region” phase diagram boundaries shifted considerably upon drug loading, which can be explained by the increased solubility of the polymer–drug complex. This behavior necessitated a substantial adjustment of polymer concentrations required to produce drug-loaded nanoparticles with characteristics comparable to blank nanoparticles. In conclusion, the use of “ouzo diagrams” is a beneficial tool to manufacture nanoparticles with specified physicochemical properties by the solvent displacement method.

Pressure-temperature phase diagram ofSrTiO3up to 53 GPa

We investigate the cubic to tetragonal phase transition in the pressure-temperature phase diagram of strontium titanate SrTiO3 (STO) by means of Raman spectroscopy and X-ray diffraction on single crystal samples. X-ray diffraction experiments are performed at room temperature, 381 and 467 K up to 53 GPa, 30 GPa and 26 GPa respectively. The observation of the superstructure reflections in the X-ray patterns provides evidence that the crystal undergoes at all investigated temperatures a pressure-induced transition from cubic to the tetragonal I4/mcm phase, identical to the low-temperature phase. No other phase transition is observed at room temperature up to 53 GPa. Together with previously published data, our results allow us to propose a new linear phase boundary in the pressure-temperature phase diagram. The data are analyzed in the framework of the Landau theory of phase transitions. With a revised value of the coupling coefficient between the order parameter and the volume spontaneous strain, the model built from pressure-independent coefficients reproduces satisfactorily the boundary in the phase diagram, but fails at reflecting the more pronounced second-order character of the pressure-induced phase transition as compared to the temperature-induced transition. We propose a new Landau potential suitable for the description of the pressure-induced phase transition. Finally, we show that particular attention has to be paid to hydrostatic conditions in the study of the high-pressure phase transition in STO.

Field-Induced Bose-Einstein Condensation of Triplons up to 8 K inSr3Cr2O8

Single crystals of the spin dimer system Sr(3)Cr(2)O(8) have been grown for the first time. Magnetization, heat capacity, and magnetocaloric effect data up to 65 T reveal magnetic order between applied fields of H(c1) approximately 30.4 T and H(c2) approximately 62 T. This field-induced order persists up to T(c)(max) approximately 8 K at H approximately 44 T, the highest observed in any quantum magnet where H(c2) is experimentally accessible. We fit the temperature-field phase diagram boundary close to H(c1) using the expression T(c) = A(H-H(c1))(nu). The exponent nu = 0.65(2), obtained at temperatures much smaller than T(c)(max), is that of the 3D Bose-Einstein condensate (BEC) universality class. This finding strongly suggests that Sr(3)Cr(2)O(8) is a new realization of a triplon BEC where the universal regimes corresponding to both H(c1) and H(c2) are accessible at (4)He temperatures.

Phase Diagram and High-Pressure Boundary of Hydrate Formation in the Carbon Dioxide−Water System

Experimental investigation of the phase diagram of the system carbon dioxide-water at pressures up to 2.7 GPa has been carried out in order to explain earlier controversial results on the decomposition curves of the hydrates formed in this system. According to X-ray diffraction data, solid and/or liquid phases of water and CO2 coexist in the system at room temperature within the pressure range from 0.8 to 2.6 GPa; no clathrate hydrates are observed. The results of neutron diffraction experiments involving the samples with different CO2/H2O molar ratios, and the data on the phase diagram of the system carbon dioxide-water show that CO2 hydrate of cubic structure I is the only clathrate phase present in this system under studied P-T conditions. We suppose that in the cubic structure I hydrate of CO2 multiple occupation of the large hydrate cavities with CO2 molecules takes place. At pressure of about 0.8 GPa this hydrate decomposes into components indicating the presence of the upper pressure boundary of the existence of clathrate hydrates in the system.

Dielectric and Relaxor Studies of Ba Modified PLZT (1.2/55/45) System

The morphotrophic phase boundary compositions of Ba doped Pb 1 − 3x/2 La x Δ x (Zr y Ti 1 − y )O 3 (1.2/55/45) ceramics are synthesized for x = 0 to 6 mole% by mixed-oxide method and characterized for phase formation, microstructure, dielectric and relaxor studies. Density of ceramics is increased as Ba concentration is increased. Coexistence of rhombohedral and tetragonal phases is strong in 5 mole % Ba doped PLZT composition near morphotrophic phase boundary of PLZT phase diagram. Scanning electron micrographs indicate the grain size control as Ba concentration increases in PLZT system. ϵ rt and Tanδ are increased up to 4 mole% Ba doped PLZT system. Dielectric maximum (ϵ Tc ) is decreased to 4 mole% Ba and increased to 6 mole% Ba. Curie temperature (T c ) decreased substantially with increase of Ba concentration in PLZT system. Relaxor behavior of the system is characterized over range of 200 kHz.

Phase Diagram and High-Pressure Boundary of Hydrate Formation in the Ethane−Water System

Dissociation temperatures of gas hydrate formed in the ethane-water system were studied at pressures up to 1500 MPa. In situ neutron diffraction analysis and X-ray diffraction analysis in a diamond anvil cell showed that the gas hydrate formed in the ethane-water system at 340, 700, and 1840 MPa and room temperature belongs to the cubic structure I (CS-I). Raman spectra of C-C vibrations of ethane molecules in the hydrate phase, as well as the spectra of solid and liquid ethane under high-pressure conditions were studied at pressures up to 6900 MPa. Within 170-3600 MPa Raman shift of the C-C vibration mode of ethane in the hydrate phase did not show any discontinuities, which could be evidence of possible phase transformations. The upper pressure boundary of high-pressure hydrate existence was discovered at the pressure of 3600 MPa. This boundary corresponds to decomposition of the hydrate to solid ethane and ice VII. The type of phase diagram of ethane-water system was proposed in the pressure range of hydrate formation (0-3600 MPa).

Phase and orientational ordering of low molecular weight rod molecules in a quenched liquid crystalline polymer matrix with mobile side chains.

We study the phase diagram and orientational ordering of guest liquid crystalline (LC) rods immersed in a quenched host made of a liquid crystalline polymer (LCP) matrix with mobile side chains. The LCP matrix lies below the glass transition of the polymer backbone. The side chains are mobile and can align to the guest rod molecules in a plane normal to the local LCP chain contour. A field theoretic formulation for this system is proposed and the effects of the LCP matrix on LC ordering are determined numerically. We obtain simple analytical equations for the nematic/isotropic phase diagram boundaries. Our calculation show a nematic-nematic (N/N) first order transition from a guest stabilized to a guest-host stabilized region and the possibility of a reentrant transition from a guest stabilized nematic region to a host only stabilized regime separated by an isotropic phase. A detailed study of thermodynamic variables and interactions on orientational ordering and phases is carried out and the relevance of our predictions to experiments and computer simulations is presented.

Coexistence of antiferromagnetic ordering and high- Tc superconductivity in electron-doped superconductor Nd 2− xCe xCuO 4

Muon spin relaxation and magnetic susceptibility measurements were performed for the electron-doped superconductor Nd 2− x Ce x CuO 4 (NCCO) with a particular attention to the vicinity of the boundary in phase diagram between the antiferromagnetic (AF) and the superconducting phases. The obtained phase diagram clearly exhibits a competitive change in the volume fraction between the two phases around the phase boundary; a dramatic decrease of the AF phase and a sudden appearance of bulk superconductivity upon doping beyond x∼0.13. However, the spatial coexistence of both phases is also suggested around the boundary. In contrast to the hole-doped system, the onset of T c is approximately independent on the doping in the underdoped region of NCCO.

Morphotropic ceramic solid solutions of the Pb(B3+ ½Nb½)O3 - PbTiO3 binary system

The structure, dielectric, ferroelectric and electromechanical properties of lead lutecium niobate-titanate (PLuNT) and lead erbium niobate-titanate (PErNT) binary systems are reported. The data of phase diagram, crystallographic symmetry and morphotropic phase boundaries (MPB) are provided. High values of the electromechanical coupling coefficients kp = 0.66, kt = 0.48, k31 = 0.36 of (1−x)PLuN − xPT ceramics are attained in compositions near the MPB at x≈0.41.

Adsorption in a model of interacting branched polymers on a fractal lattice

We have studied a model of self-interacting branched polymers on the three-dimensional Sierpinski gasket lattice, in the presence of an attractive impenetrable fractal boundary. Using an exact renoramlization group approach, we have determined the phase diagram boundaries of this model, and its critical properties in different regions of the phase diagram.

Two Variant Modeling of Shape Memory Materials: Unfolding a Phase Diagram Triple Point

For shape memory materials, the combined effects of hysteresis and phase mixing are described in the absence of stress by the four transformation temperatures: Mf, Ms, As, Af. These temperatures can be viewed as the four-way splitting of a single transformation temperature that applies when both effects are absent. Treating the combined effect of hysteresis and phase mixing in the presence of stress have led to internal variable models in which transformation zones replace abrupt transition boundaries in traditional stress vs. temperature thermodynamic phase diagrams. We discuss how these descriptions emerge from a hysteresis and phase mixing mediated unfolding of an underlying phase diagram for a three species system involving austenite and two variants of martensite. The unfolding concept dictates certain general features of the resulting phase diagram zonal geometry, correlates this geometry with particular transformations and transformation combinations, and provides insight into the evolutionary kinetics that governs these transformations. A complete evolutionary model for arbitrary temperature and single stress component histories can be extracted from knowledge of the purely thermal behavior coupled with material data specification in terms of: transformation strain, austenite and martensite elastic moduli, transformation latent heat, and the start and finish stresses for martensite detwinning. The accompanying evolutionary kinetics is given an incremental formulation that captures shape memory and superelasticity, and also allows for the identification of attracting states associated with periodic stressing. The kinetics consistently permits an algebraic description of transformation progression for phase species undergoing depletion, but not for those phase species that are in flux or are increasing under multiple concurrent transformation. This latter result is established by an analysis of the path independent integrability of the evolution kinetics which serves to clarify the resulting description with respect to other models.

Lattice dynamical study of Zr0.85Ca0.15O1.85 and CeO2

Inelastic neutron scattering studies of Zr0.85Ca0.15O1.85 at 300, 1200 and 1500 K show LA and TA phonons rapidly broadening with phonon wavevector q in all observed directions ((00 xi ),( xi xi 0),( xi xi xi ), (- xi /2, - xi /2, xi )). Optical phonon branches could not be observed. Both results can be related to the underlying static disorder. Comparative studies of well ordered CeO2 show that the influence of anharmonicities is negligible in both these oxides up to temperatures of 1500 K. The phonon linewidths of Zr0.85Ca0.15O1.85 are exceptionally large at 1200 K. This can be related to phase diagram boundaries occurring in this temperature regime. The temperature dependence of the linewidths is associated with the hindered cubic:tetragonal phase transition. The line broadening is markedly anisotropic. Elastic constants c11, c12 and c44 were derived from the data at 1200 K.

Real time spectroellipsometry study of the evolution of bonding in diamond thin films during nucleation and growth.

Real time spectroellipsometry (RTSE) has been performed during the nucleation and growth of nanocrystalline thins films prepared by plasma-enhanced chemical vapor deposition onto Si substrates. RTSE shows that a significant volume fraction of nondiamond (or ${\mathrm{sp}}^{2}$-bonded) carbon forms during thin film coalescence and is trapped near the substrate interface between $\ensuremath{\sim}300\AA{}$ diamond nuclei. Although this behavior is observed over a range of C${\mathrm{H}}_{4}$/${\mathrm{H}}_{2}$/${\mathrm{O}}_{2}$ gas compositions, the interfacial ${\mathrm{sp}}^{2}$-bonded carbon volume can be minimized by operating just within the growth-etch boundary of the diamond growth phase diagram.