Showing papers on "Phase (matter) published in 2005"

A potential model for the study of ices and amorphous water: TIP4P/Ice.

Abstract: The ability of several water models to predict the properties of ices is discussed. The emphasis is put on the results for the densities and the coexistence curves between the different ice forms. It is concluded that none of the most commonly used rigid models is satisfactory. A new model specifically designed to cope with solid-phase properties is proposed. The parameters have been obtained by fitting the equation of state and selected points of the melting lines and of the coexistence lines involving different ice forms. The phase diagram is then calculated for the new potential. The predicted melting temperature of hexagonal ice (Ih) at 1 bar is 272.2 K. This excellent value does not imply a deterioration of the rest of the properties. In fact, the predictions for both the densities and the coexistence curves are better than for TIP4P, which previously yielded the best estimations of the ice properties.

Copper oxide nanocrystals.

Abstract: It is well-known that inorganic nanocrystals are a benchmark model for nanotechnology, given that the tunability of optical properties and the stabilization of specific phases are uniquely possible at the nanoscale. Copper (I) oxide (Cu(2)O) is a metal oxide semiconductor with promising applications in solar energy conversion and catalysis. To understand the Cu/Cu(2)O/CuO system at the nanoscale, we have developed a method for preparing highly uniform monodisperse nanocrystals of Cu(2)O. The procedure also serves to demonstrate our development of a generalized method for the synthesis of transition metal oxide nanocrystals. Cu nanocrystals are initially formed and subsequently oxidized to form highly crystalline Cu(2)O. The volume change during phase transformation can induce crystal twinning. Absorption in the visible region of the spectrum gave evidence for the presence of a thin, epitaxial layer of CuO, which is blue-shifted, and appears to increase in energy as a function of decreasing particle size. XPS confirmed the thin layer of CuO, calculated to have a thickness of approximately 5 A. We note that the copper (I) oxide phase is surprisingly well-stabilized at this length scale.

Reactions in Micellar Systems

Abstract: The notion of "green chemistry " has encouraged even synthetic organic chemists to include water as a solvent. Incredible selectivities and activities can be achieved through the addition of amphiphiles with a defined structure. The morphology of supramolecular assemblies or associates formed by surfactants vary according to the temperature and concentration. As a rule, reactions are typically conducted using simple spherical aggregates, that is, micelles in the nanometer range. The strong polarity gradient present between the hydrophilic surface and the hydrophobic core of the micelle means that both nonpolar and polar reagents can be solubilized. This solubilization results in reactants becoming more concentrated within the micelle than in the surrounding water phase and leads to an acceleration of the reaction and causes selective effects. The kinetic treatment of reactions in micellar systems can be accomplished by considering them as microheterogeneous two-phase systems.

Precipitates and intermetallic phases in precipitation hardening Al–Cu–Mg–(Li) based alloys

Abstract: The present study contains a critical review of work on the formation of precipitates and intermetallic phases in dilute precipitation hardening Al–Cu–Mg based alloys with and without Li additions. Although many suggestions for the existence of pre-precipitates in Al–Cu–Mg alloys with a Cu/Mg atomic ratio close to 1 have been made, a critical review reveals that evidence exists for only two truly distinct ones. The precipitation sequence is best represented as: supersaturated solid solution→co-clusters→GPB2/S"→S where clusters are predominantly Cu–Mg co-clusters (also termed GPB or GPB I zones), GPB2/S" is an orthorhombic phase that is coherent with the matrix (probable composition Al10Cu3Mg3) for which both the term GPB2 and S" have been used, and S phase is the equilibrium Al2CuMg phase. GPB2/S" can co-exist with S phase before the completion of the formation of S phase. It is further mostly accepted that the crystal structure of S' (Al2CuMg) is identical to the equilibrium S phase (Al2CuMg). Th...

Scandium in aluminium alloys

Abstract: A considerable part of the available literature on scandium in aluminium alloys is reviewed. Experimental data and assessments of the binary Al–Sc phase diagram, a wide range of ternary Al–Sc–X phase diagrams and a few higher order phase diagrams are accounted for, with emphasis on the aluminium rich part of the diagrams. The phase which is in equilibrium with Al, Al3Sc, can form by several different mechanisms, all of which are described. The precipitation kinetics of Al3Sc in binary Al–Sc alloys are discussed, and an overview of the reported influences of ternary alloying elements on the precipitation of Al3Sc is given. The Al3Sc phase particles can serve as a grain refiner in the Al melt, a dispersoid for controlling the grain structure of the alloy and a strengthening precipitate. Several examples of these three effects are mentioned, both in binary Al–Sc alloys, and in more complex alloys. The reported effects of Sc on the precipitation behaviour in Al–Cu, Al–Mg–Si, Al–Zn–Mg and Al–Li alloys ...

Kinetics of materials

Abstract: Preface. Bibliography. Acknowldegments. Notation. Symbol Table-Roman. Symbol Table-Greek. 1. Introduction. PART I: MOTION OF ATOMS AND MOLECULES BY DIFFUSION. 2. Irreversible Thermodynamics: Coupled Forces and Fluxes. 3. Driving Forces and Fluxes for Diffusion. 4. The Diffusion Equation. 5. Solutions to the Diffusion Equation. 6. Diffusion In Multi-Component Systems. 7. Atomic Models for Diffusion. 8. Diffusion in Cerystals. 9. Diffusion Along Crystal Imperfections. 10. Diffusion in Noncrystalline Materials. PART II: MOTION OF DISLOCATIONS AND INTERFACES. 11. Motion of Dislocations. 12. Motion of Crystalline Surfaces. 13. Motion of Crystalline Interfaces. PART III MORPHOLOGICAL EVOLUTION DUE TO CAPILLARY AND APPLIED MECHANICAL FORCES. 14. Surface Evolution due to Capillary Forces. 15. Coarsening due to Capillary Forces. 16. Morphological Evolution, Diffusional Creep, and Sintering. PART IV: PHASE TRANSFORMATIONS. 17. General Features of Phase Transformations. 18. Spinodal and Order-Disorder Transformations. 19. Nucleation. 20. Growth of Phases in Concentration and Thermal Fields. 21. Concurrent Nucleation and Growth. 22. Solidification. 23. Precipitation. 24. Martensitic Transformations. Appendix A: Densities, Fractions, and Atomic Volumes of Components. Appendix B: Structure of Crystalline Interfaces. Appendix C: Capillarity and Mathematics of Space Curves and Interfaces. Illustration Credits. Cited Author Index. Figure Index. Topic Index.

Prediction of TiO2 nanoparticle phase and shape transitions controlled by surface chemistry.

Abstract: The effects of surface chemistry on the morphology and phase stability of titanium dioxide nanoparticles have been investigated using a thermodynamic model based on surface free energies and surface tensions obtained from first principles calculations. It has been found that surfaces representing acidic and alkaline conditions have a significant influence on both the shape of the nanocrystals and the anatase-to-rutile transition size. The latter introduces the possibility of inducing phase transitions by changing the surface chemistry.

Liquid crystal ‘blue phases’ with a wide temperature range

Abstract: Liquid crystal 'blue phases' are highly fluid self-assembled three-dimensional cubic defect structures that exist over narrow temperature ranges in highly chiral liquid crystals. The characteristic period of these defects is of the order of the wavelength of visible light, and they give rise to vivid specular reflections that are controllable with external fields. Blue phases may be considered as examples of tuneable photonic crystals with many potential applications. The disadvantage of these materials, as predicted theoretically and proved experimentally, is that they have limited thermal stability: they exist over a small temperature range (0.5-2 degrees C) between isotropic and chiral nematic (N*) thermotropic phases, which limits their practical applicability. Here we report a generic family of liquid crystals that demonstrate an unusually broad body-centred cubic phase (BP I*) from 60 degrees C down to 16 degrees C. We prove this with optical texture analysis, selective reflection spectroscopy, Kossel diagrams and differential scanning calorimetry, and show, using a simple polarizer-free electro-optic cell, that the reflected colour is switched reversibly in applied electric fields over a wide colour range in typically 10 ms. We propose that the unusual behaviour of these blue phase materials is due to their dimeric molecular structure and their very high flexoelectric coefficients. This in turn sets out new theoretical challenges and potentially opens up new photonic applications.

Superior Perovskite Oxide‐Ion Conductor; Strontium‐ and Magnesium‐Doped LaGaO3: I, Phase Relationships and Electrical Properties

Abstract: The single-phase, cubic-perovskite region of the LaO1.5-SrO-Gao1.5-MgO phase diagram was determined from room-temperature and high-temperature X-ray diffraction. Two impurity phases were identified, LaSrGaO4 and aSrGa3O7. The conductivity of the oxygen-deficient perovskite phase was shown to be essentially a purely oxide-ion conductivity sigmao over a wide range of oxygen partial pressures 10-22 lessthan equal to PO2 lessthan equal to 1 atm. The highest values of sigmao = 0.17 and 0.08 S/cm were found for La0.8Sr0.2Ga0.83Mg0.17O0.2815 at 800° and 700°C, respectively; they remain stable over a week-long test. The Arrhenius plot of sigmao is curved, dividing into a high-temperature region T > T* similar/congruent 600°C and a low-temperature region T < T*. Above T* all the oxygen vacancies appear to be mobile; below T* they progressively condense into clusters of ordered vacancies.

Positron Annihilation Spectroscopic Evidence to Demonstrate the Flux-Enhancement Mechanism in Morphology-Controlled Thin-Film-Composite (TFC) Membrane

Abstract: In this study, positron annihilation lifetime spectroscopy (PALS) is applied to explain the flux-enhancement mechanism in thin-film-composite (TFC) membranes prepared by using dimethyl sulfoxide (DMSO) as an additive in the interfacial polymerization. The TFC membranes show a large increase in water flux, up to 5-fold, compared to nonadditive membrane. Atomic force microscopy (AFM) shows that surface roughness and surface area increase when DMSO in the aqueous phase solution phase works to increase miscibility of the aqueous and the organic phase by reducing the solubility difference of two immiscible solutions. X-ray photoelectron spectroscopy (XPS) reveals the variation of the chemical compositions to the extent that there is a considerable increase in the cross-linked amide linkages of the flux-enhanced TFC membranes. The effects of these structural changes on the molecular-size free volume properties are evaluated by PALS studies. The PALS results are the first to experimentally show that the thin films of cross-linked aromatic polyamide RO membranes are composed of two types of pores having radii of about 2.1-2.4 A from tau3, network pore, and 3.5-4.5 A from tau4, aggregate pore. The increase in the size and number of network pores by means of DMSO addition during interfacial polymerization enhances the water flux notably. The size of aggregate pores also increases and may contribute to enhance water flux, although their number inevitably decreases as the number of network pores becomes increased. Details on the correlations between RO performances and o-Ps lifetime parameters are clearly described based on the pore-flow model of reverse osmosis developed by Sourirajan et al.

Processing and Piezoelectric Properties of Lead‐Free (K,Na) (Nb,Ta) O3 Ceramics

Abstract: The preparations of (K 0.5 Na 0.5 )NbO 3 (KNN)-based ceramics were studied as lead-free piezoelectric materials. The authors found that the addition of CuO greatly enhanced the sinterability of the KNN-based ceramics. The sinterability and piezoelectric properties of these ceramics were dependent upon the A/B ratio, CuO doping, and the formation of a solid solution with KTaO 3 . Perovskite (K 0.5 Na 0.5 ) x NbO 3 (x = A/B ratio) was synthesized with A/B ratios of 1.00 and 1.05 by CuO doping, while (K 0.5 Na 0.5 )NbO 3 contained K 4 CuNb 8 O 23 as a second phase with A/B ratios below 0.98. Although the A site-rich (K 0.5 Na 0.5 )NbO 3 (x = 1.00 and 1.05) ceramics exhibited deliquescence, the A site-poor (K 0.5 Na 0.5 )NbO 3 (x ≤ 0.98) ceramics with K 4 CuNb 8 O 23 had higher densities without deliquescence. K 4 CuNb 8 O 23 also improved Q m , which reached 1400 for the (K 0.5 Na 0.5 ) 1.0 NbO 3 doped with 0.5 mol% K 4 CuNb 8 O 23 . The formation of a solid solution with KTaO 3 raised the melting point of the system and also improved its sinterability. The k p and Q m of (K 0.5 Na 0.5 ) 0.97 (Nb 0.95 Ta 0.05 )O 3 with CuO were 0.41 and 1400, respectively.

Sterol structure determines the separation of phases and the curvature of the liquid-ordered phase in model membranes

Abstract: The existence of lipid rafts in biological membranes in vivo is still debated. In contrast, the formation of domains in model systems has been well documented. In giant unilamellar vesicles (GUVs) prepared from ternary mixtures of dioleoyl-phosphatidylcholine/sphingomyelin/cholesterol, a clear separation of liquid-disordered and sphingomyelin-enriched, liquid-ordered phases could be observed. This phase separation can lead to the fission of the liquid-ordered phase from the vesicle. Here we show that in cholesterol-containing GUVs, the phase separation can involve dynamic redistribution of lipids from one phase into another as a result of a cross-linking perturbation. We found that the molecular structure of a sterol used for the preparation of GUVs determines (i) its ability to induce phase separation and (ii) the curvature (positive or negative) of the formed liquid-ordered phase. As a consequence, the latter can pinch off to the outside or inside of the vesicle. Remarkably, some mixtures of sterols induce liquid-ordered domains exhibiting both positive and negative curvature, which can lead to a new type of budding behavior in GUVs. Our findings could have implications for the role of sterols in various cell-biological processes such as budding of secretory vesicles, endocytosis, or formation of multivesicular bodies.

The melting temperature of the most common models of water

Abstract: The melting temperature of ice Ih for several commonly used models of water sSPC, SPC/ E,TIP3P,TIP4P, TIP4P/Ew, and TIP5Pd is obtained from computer simulations at p= 1 bar. Since the melting temperature of ice I h for the TIP4P model is now known fE. Sanz, C. Vega, J. L. F. Abascal, and L. G. MacDowell, Phys. Rev. Lett. 92, 255701 s2004dg, it is possible to use the Gibbs‐Duhem methodology fD. Kofke, J. Chem. Phys. 98, 4149 s1993dg to evaluate the melting temperature of ice I h for other potential models of water. We have found that the melting temperatures of ice I h for SPC, SPC/E, TIP3P, TIP4P, TIP4P/Ew, and TIP5P models are T= 190 K, 215 K, 146 K, 232 K, 245 K, and 274 K, respectively. The relative stability of ice I h with respect to ice II for these models has also been considered. It turns out that for SPC, SPC/E, TIP3P, and TIP5P the stable phase at the normal melting point is ice II sso that ice Ih is not a thermodynamically stable phase for these modelsd. For TIP4P and TIP4P/Ew, ice Ih is the stable solid phase at the standard melting point. The location of the negative charge along the H‐O‐H bisector appears as a critical factor in the determination of the relative stability between the I h and II ice forms. The methodology proposed in this paper can be used to investigate the effect upon a coexistence line due to a change in the potential parameters. © 2005 American Institute of Physics . fDOI: 10.1063/1.1862245g

Mechanisms of room temperature metastable tetragonal phase stabilisation in zirconia

Abstract: Mechanisms of tetragonal phase stabilisation, at room temperature, in nanocrystalline ( 1 μm) are reviewed in detail. The merits, demerits and scope of each individual model are outlined. The analysis of the literature shows that, although the mechanism of tetragonal phase stabilisation in bulk ZrO2 is well understood, the room temperature tetragonal phase stabilisation mechanism in undoped, nanocrystalline ZrO2 is controversial. Various proposed models, based on surface energy (nanocrystallite size), strain energy, internal and external hydrostatic pressure, structural similarities, foreign surface oxides, anionic impurities, water vapour and lattice defects (oxygen ion vacancies), are discussed in detail. It is proposed that generation of excess oxygen ion vacancies within the nanocrystalline ZrO2 is primarily responsible for the room temperature tetragonal phase stabilisation, below a critical size. Hence, the mechanism of ...

Cloud Point Extraction as a Procedure of Separation and Pre‐Concentration for Metal Determination Using Spectroanalytical Techniques: A Review

Abstract: Cloud point extraction is a separation and preconcentration procedure that has been extensively applied for trace metal determination in several different matrices. Its major advantages are simple experimental procedures, low cost, high preconcentration factors, and environmental safety. These aspects include it in a set of analytical methods in agreement with the “green chemistry” principles. The surfactants characteristics and the process of micelle formation are outlined for a better understanding of the technique. After general considerations about the cloud point extraction basis and its extraction mechanism for metal chelates are considered, selected spectroanalytical techniques and their application for analysis of the micellar phase are discussed. The micellar extraction in metal speciation analysis, the on‐line incorporation of cloud point extraction to flow injection analysis, and coupling with capillary electrophoresis are described.

Why is the Partial Molar Volume of CO2 So Small When Dissolved in a Room Temperature Ionic Liquid? Structure and Dynamics of CO2 Dissolved in [Bmim+][PF6-]

Abstract: When supercritical CO2 is dissolved in an ionic liquid, its partial molar volume is much smaller than that observed in most other solvents. In this article we explore in atomistic detail and explain in an intuitive way the peculiar volumetric behavior experimentally observed when supercritical CO2 is dissolved in 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim+] [PF6-]). We also provide physical insight into the structure and dynamics occurring across the boundary of the CO2 ionic liquid interface. We find that the liquid structure of [Bmim+] [PF6-] in the presence of CO2 is nearly identical to that in the neat ionic liquid (IL) even at fairly large mole fractions of CO2. Our simulations indicate, in agreement with experiments, that partial miscibilities of one fluid into the other are very unsymmetrical, CO2 being highly soluble in the ionic liquid phase while the ionic liquid is highly insoluble in the CO2 phase. We interpret our results in terms of the size and shape of spontaneously forming cav...

Nanostructuring, compositional fluctuations, and atomic ordering in the thermoelectric materials AgPbmSbTe2+m. The myth of solid solutions

Abstract: The nature of the thermoelectric materials Ag1-xPbmSbTem+2 or LAST-m materials (LAST for Lead Antimony Silver Tellurium) with different m values at the atomic as well as nanoscale was studied with powder/single-crystal X-ray diffraction, electron diffraction, and high-resolution transmission electron microscopy. Powder diffraction patterns of different members (m = 0, 6, 12, 18, ∞) are consistent with pure phases crystallizing in the NaCl-structure-type (Fm3m) and the proposition that the LAST family behaved as solid solutions between the PbTe and AgSbTe2 compounds. However, electron diffraction and high resolution transmission electron microscopy studies suggest the LAST phases are inhomogeneous at the nanoscale with at least two coexisting sets of well-defined phases. The minority phase which is richer in Ag and Sb is on the nanosized length scale, and it is endotaxially embedded in the majority phase which is poorer in Ag and Sb. Moreover, within each nanodomain we observe extensive long range orderin...

Controlled synthesis of CeO2 nanorods by a solvothermal method

Abstract: Pure phase CeO2 nanorods (about 40?50?nm in diameter and 03?2??m in length) were synthesized through a solvothermal synthesis method The addition of ethylenediamine is critical to obtain CeO2 nanorods Other experimental conditions, such as the solvent composition, surfactant and the cerium source precursor were of importance in the final product morphology The reaction temperature and reaction time also had significant influence on the yield of CeO2 nanorods A possible formation mechanism of CeO2 nanorods was discussed mainly based on the dependences of controlling parameters on the final morphologies In addition, the optical properties of CeO2 nanorods were investigated The UV?visible adsorption spectrum and photoluminescence spectrum of the CeO2 nanorods showed unusual red-shift and enhanced light emission, respectively

Carbon monoxide clathrate hydrates: Equilibrium data and thermodynamic modeling

Abstract: Carbon monoxide occurs in abundance throughout the cosmos, potentially in clathrate form, whereas on Earth, it forms a notable constituent of industrial flue gases. It has been proposed that hydrate technology could be used in CO2 separation from flue gases, and in subsea flue gas CO2 disposal. This—and the likely widespread occurrence of CO clathrates in the cosmos—means it is important that the phase behavior of CO hydrates is known. Here, we present experimental H-L-V (hydrate–liquid–vapor) equilibrium data for CO, COCO2, and COC3H8 (propane) clathrate hydrates. Data were generated by a reliable step-heating technique validated using measured data for CO2 and CH4 hydrates. Data for CO and COC3H8 clathrates have been used in the optimization of Kihara potential parameters for CO, reported here, facilitating the extension of a thermodynamic model to the prediction of CO hydrate equilibria. Model predictions are validated against independent experimental data for COCO2 (structure I) systems, with good agreement being observed. © 2005 American Institute of Chemical Engineers AIChE J, 2005