Showing papers on "Phase (matter) published in 1987"

Fine phase mixtures as minimizers of energy

Abstract: Solid-solid phase transformations often lead to certain characteristic microstructural features involving fine mixtures of the phases. In martensitic transformations one such feature is a plane interface which separates one homogeneous phase, austenite, from a very fine mixture of twins of the other phase, martensite. In quartz crystals held in a temperature gradient near the α-β transformation temperature, the α-phase breaks up into triangular domains called Dauphine twins which become finer and finer in the direction of increasing temperature. In this paper we explore a theoretical approach to these fine phase mixtures based on the minimization of free energy.

Structure‐property predictions for new planar forms of carbon: Layered phases containing sp2 and sp atoms

Abstract: Structure, thermodynamics, and electronic properties are predicted for a new low energy phase of carbon which contains planar sheets equally occupied by sp2 and sp carbon atoms. The isolated planar sheets have the same planar symmetry as do the layers in graphite (p6m) and can be formally viewed as resulting from the replacement of one‐third of the carbon–carbon bonds in graphite by –C 3/4 C– linkages. This material, called graphyne, is predicted to have a crystalline state formation energy of 12.4 kcal/mol carbon, which appears to be much lower than for any carbon phase which contains acetylenic groups as a major structural component. Based on the major structural reorganization required for graphitization and the observed high temperature stability of known model compounds, high temperature stability is predicted for graphyne. While graphyne will have similar mechanical properties as graphite, it is predicted to be a large bandgap semiconductor (Eg=1.2 eV) rather than a metal or semimetal. Based on this...

Effective Thermal Conductivity of Composites with Interfacial Thermal Barrier Resistance

Abstract: A modification of the original theories of Rayleigh and Maxwell permitted the deriva tion of expressions for the effective thermal conductivity of composites consisting of a continuous matrix phase with dilute concentrations of dispersions with spherical, cylin drical and flat plate geometry with a thermal barrier resistance at the interface between the components.

Formation of dispersed phase in incompatible polymer blends: Interfacial and rheological effects

Abstract: The formation of dispersed phase in blends of incompatible polymers during melt extrusion with a co-rotating twin screw extruder was studied, using nylon and polyester as the matrix and ethylene-propylene rubbers as the dispersed phase. A master curve is obtained, i.e., Gηmα/γ = 4p±0.84, where G is the shear rate, γ the particle diameter, η the interfacial tension, ηm the matrix viscosity, ηd the dispersed-drop viscosity, and p = ηd/ηm. The plus (+) sign applies for p > 1, and the minus (−) sign for p < 1. Thus, the dispersed-drop size is directly proportional to the interfacial tension and the ±0.84 power of viscosity ratio. The dispersed drops are the smaller, when the interfacial tension is the lower and the viscosity ratio is the closer to unity. The interfacial tension is largely controlled by the polarities of the two phases, and can be varied over several orders of magnitude by using appropriate dispersants.

On the Equilibrium Thickness of Intergranular Glass Phases in Ceramic Materials

Abstract: The fundamental question as to whether thin intergranular films can adopt an equilibrium thickness in polycrystalline ceramics is addressed. Two continuum approaches are presented, one based on interfacial energies and the other on the force balance normal to the boundary. These indicate that there will exist a stable thickness for the intergranular film and that it will be of the order of 1 nm. The origin of an equilibrium thickness is shown to be the result of two competing interactions, an attractive van der Waals-disperson interaction between the grains on either side of the boundary acting to thin the film and a repulsive term, due to the structure of the intergranular liquid, opposing this attraction. As both of these interactions are of short range (

Sum-frequency vibrational spectroscopy of a Langmuir film: Study of molecular orientation of a two-dimensional system.

Abstract: Using infrared-visible sum-frequency generation as a probe, we have obtained the vibrational spectrum of a monolayer of pentadecanoic acid on a water surface in the CH stretch region. We have monitored the molecular orientation at different surface densities. It was found that the molecules in the liquid condensed phase have their alkane chains extended and oriented nearly normal to the surface, while in the liquid expanded phase the chains are highly disordered.

Phase equilibria of fluid interfaces and confined fluids

Abstract: Phase transitions at fluid interfaces and in fluids confined in pores have been investigated by means of a density functional approach that treats attractive forces between fluid molecules in mean-field approximation and models repulsive forces by hard-spheres. Two types of approximation were employed for the hard-sphere free energy functional: (a) the well-known local density approximation (LDA) that omits short-ranged correlations and (b) a non-local smoothed density approximation (SDA) that includes such correlations and therefore accounts for the oscillations of the density profile near walls. Three different kinds of phase transition were considered: (i) wetting transition. The transition from partial to complete wetting at a single adsorbing wall is shifted to lower temperatures and tends to become first-order when the more-realistic SDA is employed. Comparison of the results suggests that the LDA overestimates the contact angle γ in a partial wetting situation. (ii) capillary evaporation of a fluid...

The mechanism of silicon modification in aluminum-silicon alloys: Impurity induced twinning

Abstract: Modification of silicon by sodium in aluminum silicon eutectic alloy has been examined in detail by optical, SEM, and TEM methods. The aluminum phase is not significantly affected but the silicon becomes very heavily twinned. Modification by quenching does not involve an increase in twin density. Consideration of the atomic positions which attend the formation of growth twins on {111} planes suggests that adsorbed impurity atoms of suitable size, on the solid-liquid interface, could be responsible for changing the {111} stacking sequence, so promoting ‘impurity induced twinning’; the optimum hard sphere radius ratio would be ≈ 1.65. It is proposed that this condition could be the first and principal requirement for a modifying agent to be effective in this system. It is shown further, that other reputed modifiers do also induce a higher twin density. Variations in the efficiency of individual elements to promote such an effect are discussed in terms of other relevant factors which include melting points and vapor pressures, the free energies of formation of compounds — notably of oxides, and the forms of alloy phase diagrams.

Observation of a glass transition in suspensions of spherical colloidal particles.

Abstract: Concentrated suspensions of submicron colloidal spheres were studied both by dynamic light scattering and by direct observation of their phase behavior. In agreement with recent theory and computer simulation, the measured dynamic structure factor developed an essentially nondecaying component, implying ``structural arrest,'' at almost the same concentration as that at which a long-lived amorphous or glass phase was first observed.

Process for the preparation of dispersible colloidal systems of a substance in the form of nanoparticles

Abstract: The process according to the invention comprises: (1) the preparation of a liquid phase consisting essentially of a solution of a substance A in a solvent or in a mixture of solvents, and containing the substance B in solution or as a dispersion, (2) the preparation of a second liquid phase consisting essentially of a non-solvent or a mixture of non-solvents for the substances A and B, and supplemented with one or more surfactants, the solvent or mixture of solvents of the first phase being miscible in all proportions with the non-solvent or mixture of non-solvents of the second phase, (3) the addition of the first phase to the second phase with moderate agitation so as to produce a colloidal suspension of nanocapsules, the wall of which is constituted by the substance A and the core by the substance B, (4) if desired, the removal of all or part of the solvent or the mixture of solvents and of the non-solvent or the mixture of non-solvents so as to produce a colloidal suspension of nanocapsules of the desired concentration or to produce a powder of nanocapsules following the addition of stabilizing substances.

Water interaction with the superconducting YBa2Cu3O7 phase

Abstract: We show that the superconducting YBa2Cu3O7 phase is highly sensitive to water and water vapor. This is probably due to the presence of nonequilibrium Cu3+ ions in this compound. In particular, the YBa2Cu3O7 phase decomposes in water to CuO, Ba(OH)2 and Y2BaCuO5 and evolves oxygen. Samples with a reduced oxygen content, e.g., YBa2Cu3O6.0, also decompose in an aqueous ambient. The superconductivity of YBa2Cu3O7 samples is greatly degraded by the interaction with water and humid air. This effect should not preclude practical application of these materials since it should be possible to protect them with coatings of metal, glass, or plastic.

Constant thermodynamic tension Monte Carlo studies of elastic properties of a two-dimensional system of hard cyclic hexamers

Abstract: A constant thermodynamic tension Monte Carlo method is introduced and applied studies of the elastic properties of a two-dimensional system of hard cyclic hexamers. Elastic compliances and elastic constants are determined at a number of different values of the pressure. The existence of the phase transition between a tilted and a straight phase is confirmed. The results obtained strongly suggest that the Poisson modulus can be negative in the tilted phase.

Observation of a phase transition of stored laser-cooled ions.

Abstract: Clouds of two to about fifty simultaneously stored, laser-cooled ${\mathrm{Mg}}^{+}$ ions in a Paul trap were observed in two phases, which are clearly distinguishable by their excitation spectra. Transitions between these phases can be induced either by a variation of the power of the laser radiation used to cool the ions or by a change of the size of the radio-frequency voltage applied to the trap. Transitions between a "crystalline" phase and a "gaseous" phase can be repeatedly observed by variation of the appropriate parameters. The two phases and the transitions between them have also been recorded by a photon-counting image system.

Nucleation of microcellular foam: Theory and practice

Abstract: Microcellular polymer foams exhibit greatly improved mechanical properties as compared to standard foams due to the formers' small bubble size. Microcellular foams have bubbles with diameters on the order of 10 microns, volume reductions of 30 to 40 percent, and six or seven times the impact strength of solid parts. They are produced through the use of thermodynamic instabilities without the use of foaming agents. This method leads to a very uniform cell size throughout a part's cross section. A theoretical model for the nucleation of microcellular foams in thermoplastic polymers has been developed and experimentally confirmed. This model explains the effect of various additives and processing conditions on the number of bubbles nucleated. At levels of secondary constituents below their solubility limits, an increase in the concentration of the additive or the concentration of gas in solution with the polymer increases the number of bubbles nucleated. Nucleation in this region is homogeneous. Above the solubility limit of additives, nucleation is heterogeneous and takes place at the interface between second phase inclusions and the polymer. The number of bubbles nucleated is dependent on the concentration of heterogeneous nucleation sites and their relative effect on the activation energy barrier to nucleation. In the vicinity of the solubility limit, the two mechanisms compete.

The effect of viscosity ratio on the morphology of polypropylene/polycarbonate blends during processing

Abstract: The size of the minor phase in an immiscible polymer blend can have a significant effect on properties such as the impact strength Few studies, however, have quantitatively considered the parameters controlling the size of the dispersed phase In this paper, light and scanning electron microscopy have been used to examine the size of the minor phase in polypropylene/polycarbonate blends after melt processing The size was examined as a function of both the viscosity ratio (p) and the torque ratio (TR) The viscosity ratio is studied in the regions of p > 1 and p < 1 p has a marked effect on the morphology of the dispersed phase with the phase size increasing by a factor of 3 to 4 times from p = 45 to p = 173 Reduction in the size of the minor phase was achieved below p = 1 with the minimum particle size occurring at p ≃ 015 The results for these systems indicate the presence of upper and lower limits of p beyond which deformation becomes difficult This is shown to be similar in some respects to the behavior of Newtonian fluids in shear flow, although the upper limit extends beyond that observed in the Newtonian fluid studies

A differential scanning calorimetric study of the thermotropic phase behavior of model membranes composed of phosphatidylcholines containing linear saturated fatty acyl chains.

Abstract: The thermotropic phase behavior of a series of 1,2-diacylphosphatidylcholines containing linear saturated acyl chains of 10-22 carbons was studied by differential scanning calorimetry. When fully hydrated and thoroughly equilibrated by prolonged incubation at appropriate low temperatures, all of the compounds studied form an apparently stable subgel phase (the Lc phase). The formation of the stable Lc phase is a complex process which apparently proceeds via a number of metastable intermediates after being nucleated by incubation at appropriate low temperatures. The process of Lc phase formation is subject to considerable hysteresis, and our observations indicate that the kinetic limitations become more severe as the length of the acyl chain increases. The kinetics of Lc phase formation also depend upon whether the acyl chains contain an odd or an even number of carbon atoms. The Lc phase is unstable at higher temperatures and upon heating converts to the so-called liquid-crystalline state (the L alpha phase). The conversion from the stable Lc to the L alpha phase can be a direct, albeit a multistage process, as observed with very short chain phosphatidylcholines, or one or more stable gel states may exist between the Lc and L alpha states. For the longer chain compounds, conversions from one stable gel phase to another become separated on the temperature scale, so that discrete subtransition, pretransition, and gel/liquid-crystalline phase transition events are observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Neutron diffraction study on the structural phase transition in GeTe

Abstract: The structural phase transition in GeTe has been investigated by neutron diffraction in the temperature range 295-716 K. Accurate positional and thermal parameters have been obtained as a function of temperature both in the rhombohedral hR2 phase(binary analogue of grey arsenic structure A7) and in the cubic CF8 phase (NaCl-type, B1). The temperature variation of the rhombohedral distortion and the relative shifts of the Ge and Te substructures show that the structural phase transition in GeTe is a displacive type.

Modification of supercritical fluid phase behavior using polar cosolvents

Abstract: Tres forte augmentation de solubilite dans certains cas (6 fois pour l'acide aminobenzoique dans CO 2 avec addition de methanol). Interpretation par les interactions intermoleculaires

Adsorption and capillary condensation of fluids in cylindrical pores by Monte Carlo simulation in the Gibbs ensemble

Abstract: A recently proposed technique [1] for the direct determination of phase coexistence properties of fluids by simulation was extended for the prediction of adsorption and capillary condensation of simple fluids in narrow cylindrical pores. Two variations of the technique were developed, the first for equilibrium between a bulk fluid phase and the interior of a pore, and the second for the determination of capillary condensation points. The metholology was applied for the calculation of equilibria for the pure Lennard-Jones (6, 12) fluid in a structureless cylindrical pore that interacts with the fluid via an integrated Lennard-Jones potential. The potential parameters were chosen to approximate fluid Ar in a pore of solid CO2. The phase diagram of the fluid was determined as a function of temperature and pore radius. The gas/liquid critical temperature decreases as the pore radius is reduced. The results from the proposed technique were found to be in good agreement with recent results from independent mole...

Phase equilibria and solvation forces for fluids confined between parallel walls

Abstract: The thermodynamics of fluids confined between two adsorbing solid substrates (walls) is revisited. Attention is focused on the phase equilibria of an open system characterized by the variables μ (chemical potential), T (temperature), and H (wall separation). Clapeyron equations for the shape of lines of coexistence are derived and used to interpret the results of earlier calculations of two first‐order transitions, namely capillary condensation of an undersaturated ‘‘gas’’ to ‘‘liquid’’ and prewetting (thick–thin film transition) at finite H. At such transitions the adsorption Γ and the solvation force f jump discontinuously. Criticality of a confined fluid is associated with the divergence of the derivatives (∂Γ/∂μ)T,H and (∂2Γ/∂μ2)T,H or equivalently, with the divergence of (∂f/∂H)T,μ and (∂2f/∂H2)T,μ. The presence of the additional field variable H, and its conjugate density f, implies that the phase equilibria of a confined fluid can be much richer than those of a bulk fluid or of a single interface (H=∞). By extending the formalism to multicomponent systems Clapeyron equations are derived for the coexistence of phases in confined fluid mixtures. An equation for the shift in chemical potential (or concentration) of the phase separation curve of a binary liquid mixture resulting from confinement at constant pressure and temperature is presented. This equation, which becomes exact for large separations H, is the appropriate analog for mixtures of the Kelvin equation used to describe capillary condensation in pure fluids; it can also be regarded as a generalization to nonzero concentrations of the Ostwald–Freundlich formula for the dependence of solubility on particle size. Our analysis provides a framework for interpreting recent solvation force measurements on phase‐separating liquid mixtures.

Free energy perturbation method for chemical reactions in the condensed phase: a dynamic approach based on a combined quantum and molecular mechanics potential

Abstract: Etude theorique de la substitution nucleophile du chloromethane par l'ion chlorure en phase gazeuse et en solution

Phase diagram of silicon by molecular dynamics.

Abstract: Using the Stillinger-Weber potential we explored the liquid, crystal, and amorphous phase diagram of silicon by molecular dynamics. We obtain the chemical potential of the crystal by following the crystal-vapor coexistence curve from the T=0 harmonic solid up to the melting point. The liquid free energy is found by reversible expansion. The thermodynamic melting point is 1691\ifmmode\pm\else\textpm\fi{}20 K, which is very close to the experimental value of 1683 K. Contrary to experiment, the calculated supercooled liquid phase does not undergo a first-order transition to the fourfold-coordinated amorphous structure upon cooling, since the chemical potentials of these structures are almost equal over a wide range of temperatures. Diffusion coefficients, heat capacities, and expansivities are compared with experiment.

Alumina‐Silica Phase Diagram in the Mollite Region

Abstract: The alumina-silica phase relations of samples annealed in oxygen and quenched were studied by optical microscopy, image analysis, X-ray diffraction, and the electron microprobe. The solid solution boundaries of mullite changed with increasing temperature and joined at 1890°C and a composition of 77.15 wt% alumina. The melting point of mullite was 189°C with a peritectic between 76.5 and 77.0 wi% alumina.

Reentrant phase transition of N‐isopropylacrylamide gels in mixed solvents

Abstract: Reentrant volume–phase transitions are observed in N‐isopropyoacrylamide gels in the methanol–water mixtures. When the solvent composition is varied systematically, the gel undergoes two transitions: a discontinuous collapsing followed by a discontinuous swelling. The reentrant transition defines a closed‐loop instability phase boundary having both upper and lower critical points. The closed‐loop phase boundary depends on temperature and diminishes to a point at approximately 0 °C. A simple mean field theory is presented to describe the phenomenon, which reveals an alteration of free energy of alcohol–water interaction by presence of polymer network. In the case of ethanol–water mixtures, there appear two closed‐loop phase boundaries, whose physico‐chemical basis are not yet clear.

Crystallization of amorphous silicon during thin-film gold reaction

Abstract: The crystallization of a‐Si in a‐Si (50‐nm) and Au (5‐nm) thin‐film bilayers has been investigated during heat treatment in a transmission electron microscope. When crystallization of a‐Si first begins at 130 °C, the Au‐Si alloy (Au and a precursor phase) reflections observed at lower temperatures vanish, and several new reflections from metastable Au‐Si compounds occur. Dendritically growing islands of poly‐Si are observed after heating at 175 °C. If the samples are held at a constant temperature of 175 °C for 10 min, the poly‐Si islands coalesce. The formation of poly‐Si depends on the diffusion of Au into a‐Si and the formation of metastable Au‐Si compounds, which act as transport phases for both Si and Au. After crystallization Au segregates to the front and back surfaces of the poly‐Si film. The result of this work and earlier diffraction investigations are interpreted in terms of superlattices based on a sublattice. A fundamental body‐centered‐cubic structure with a=5.52 A and composition Au4Si is s...