Showing papers on "Phase (matter) published in 2007"

An in situ X-ray diffraction study of the reaction of Li with crystalline Si

Abstract: The electrochemical reaction of lithium with a crystalline Si composite electrode at room temperature was investigated using in situ X-ray diffraction (XRD) techniques. The study confirmed that crystalline Si becomes amorphous during the first lithiation. The range of the coexistence region between crystalline Si and amorphous lithiated Si is 3350 ± 200 mAh/g. The highly lithiated amorphous Li x Si phase was found to crystallize into Li 15 Si 4 rapidly at about 60 mV (vs Li/Li + ) and this Li 15 Si 4 phase exists over a relatively narrow range of capacity. During delithiation, the Li 15 Si 4 phase coexists with amorphous Li z Si with z = 2.0 +0 -1 . Once all the Li 15 Si 4 phase disappears, the amorphous phase persists as the remaining Li is extracted. The formation of the Li 15 Si 4 phase can be avoided if the potential of the Si electrode is controlled above 70 mV during cycling. Based on the electrochemical and XRD data, a "phase diagram" was constructed to show the phase changes and regions of phase stability that occur during the lithiation and delithiation of the Si electrode.

Structure of phase III of solid hydrogen

Abstract: Hydrogen, being the first element in the periodic table, has the simplest electronic structure of any atom, and the hydrogen molecule contains the simplest covalent chemical bond. Nevertheless, the phase diagram of hydrogen is poorly understood. Determining the stable structures of solid hydrogen is a tremendous experimental challenge1,2,3, because hydrogen atoms scatter X-rays only weakly, leading to low-resolution diffraction patterns. Theoretical studies encounter major difficulties owing to the small energy differences between structures and the importance of the zero-point motion of the protons. We have systematically investigated the zero-temperature phase diagram of solid hydrogen using first-principles density functional theory (DFT) electronic-structure methods4, including the proton zero-point motion at the harmonic level. Our study leads to a radical revision of the DFT phase diagram of hydrogen up to nearly 400 GPa. That the most stable phases remain insulating to very high pressures eliminates a major discrepancy between theory5 and experiment6. One of our new phases is calculated to be stable over a wide range of pressures, and its vibrational properties agree with the available experimental data for phase III.

On the Theory of Organic Catalysis "on Water"

Abstract: A molecular origin of the striking rate increase observed in a reaction on water is studied theoretically. A key aspect of the on-water rate phenomenon is the chemistry between water and reactants that occurs at an oil-water phase boundary. In particular, the structure of water at the oil-water interface of an oil emulsion, in which approximately one in every four interfacial water molecules has a free ("dangling") OH group that protrudes into the organic phase, plays a key role in catalyzing reactions via the formation of hydrogen bonds. Catalysis is expected when these OH's form stronger hydrogen bonds with the transition state than with the reactants. In experiments more than a 5 orders of magnitude enhancement in rate constant was found in a chosen reaction. The structural arrangement at the "oil-water" interface is in contrast to the structure of water molecules around a small hydrophobic solute in homogeneous solution, where the water molecules are tangentially oriented. The latter implies that a breaking of an existing hydrogen-bond network in homogeneous solution is needed in order to permit a catalytic effect of hydrogen bonds, but not for the on-water reaction. Thereby, the reaction in homogeneous aqueous solution is intrinsically slower than the surface reaction, as observed experimentally. The proposed mechanism of rate acceleration is discussed in light of other on-water reactions that showed smaller accelerations in rates. To interpret the results in different media, a method is given for comparing the rate constants of different rate processes, homogeneous, neat and on-water, all of which have different units, by introducing models that reduce them to the same units. The observed deuterium kinetic isotope effect is discussed briefly, and some experiments are suggested that can test the present interpretation and increase our understanding of the on-water catalysis.

Control of the electronic phase of a manganite by mode-selective vibrational excitation

Abstract: Controlling a phase of matter by coherently manipulating specific vibrational modes has long been an attractive (yet elusive) goal for ultrafast science. Solids with strongly correlated electrons, in which even subtle crystallographic distortions can result in colossal changes of the electronic and magnetic properties, could be directed between competing phases by such selective vibrational excitation. In this way, the dynamics of the electronic ground state of the system become accessible, and new insight into the underlying physics might be gained. Here we report the ultrafast switching of the electronic phase of a magnetoresistive manganite via direct excitation of a phonon mode at 71 meV (17 THz). A prompt, five-order-of-magnitude drop in resistivity is observed, associated with a non-equilibrium transition from the stable insulating phase to a metastable metallic phase. In contrast with light-induced and current-driven phase transitions, the vibrationally driven bandgap collapse observed here is not related to hot-carrier injection and is uniquely attributed to a large-amplitude Mn-O distortion. This corresponds to a perturbation of the perovskite-structure tolerance factor, which in turn controls the electronic bandwidth via inter-site orbital overlap. Phase control by coherent manipulation of selected metal-oxygen phonons should find extensive application in other complex solids--notably in copper oxide superconductors, in which the role of Cu-O vibrations on the electronic properties is currently controversial.

Morphology and Phase Segregation of Spin-Casted Films of Polyfluorene/PCBM Blends

Abstract: In this study the morphology of spin-casted films of polymers blended with [6,6]-phenyl C61-butyric acid methyl ester (PCBM) has been studied. It was found that the lateral structure formation in the films is favored by rapid solvent evaporation and strong polymer−PCBM repulsion. The formation of homogeneous films is favored by slow evaporation and weak polymer−PCBM repulsion. The effect of solvent evaporation rate is the opposite of what is found for spin-casting polymer−polymer blends. The results can be explained by the kinetics of phase separation and the phase behavior involving limited solubility and crystallization of PCBM.

Random polymer models

Abstract: Random Polymer Models and Their Applications The Homogeneous Pinning Model Weakly Inhomogeneous Models The Free Energy of Disordered Polymer Chains Disordered Pinning Models: The Phase Diagram Disordered Copolymers and Selective Interfaces: The Phase Diagram The Localized Phase of Disordered Polymers The Delocalized Phase of Disordered Polymers Numerical Algorithms and Computations.

Integrated continuous microfluidic liquid–liquid extraction

Abstract: We describe continuous flow liquid–liquid phase separation in microfluidic devices based on capillary forces and selective wetting surfaces. Effective liquid–liquid phase separation is achieved by using a thin porous fluoropolymer membrane that selectively wets non-aqueous solvents, has average pore sizes in the 0.1–1 µm range, and has a high pore density for high separation throughput. Pressure drops throughout the microfluidic network are modelled and operating regimes for the membrane phase separator are determined based on hydrodynamic pressure drops and capillary forces. A microfluidic extraction device integrating mixing and phase separation is realized by using silicon micromachining. Modeling of the phase separator establishes the operating limits. The device is capable of completely separating several organic–aqueous and fluorous–aqueous liquid–liquid systems, even with high fractions of partially miscible compounds. In each case, extraction is equivalent to one equilibrium extraction stage.

Extraction of Proteins from Biological Fluids by Use of an Ionic Liquid/Aqueous Two-Phase System

Abstract: An ionic liquid/aqueous two-phase system based on the hydrophilic ionic liquid 1-butyl-3-methylimidazolium chloride (BmimCl) and K(2)HPO(4) has been employed for direct extraction of proteins from human body fluids for the first time. Proteins present at low levels were quantitatively extracted into the BmimCl-rich upper phase with a distribution ratio of about 10 between the upper and lower phase and an enrichment factor of 5. Addition of an appropriate amount of K(2)HPO(4) to the separated upper phase results in a further phase separation, giving rise to an improved enrichment factor of 20. FTIR and UV spectroscopy demonstrated that no chemical (bonding) interactions between the ionic liquid and the protein functional groups were identifiable, while no alterations of the natural properties of the proteins were observed. The partitioning of proteins in the two-phase system was assumed to have been facilitated by the electrostatic potential difference between the coexisting phases, as well as by salting out effects. The system could be applied successfully for the quantification of proteins in human urine after on-line phase separation in a flow system. The use of an ionic liquid, as a green solvent, offers clear advantages over traditional liquid-liquid extractions, in which the use of toxic organic solvents is unavoidable.

Phase transitional behavior in K0.5Na0.5NbO3–LiTaO3 ceramics

Abstract: In addition to x-ray diffraction, Raman spectrum measurements provide direct evidence of the tetragonal and orthorhombic phases coexistence in lead-free ceramics (1−x)K0.5Na0.5NbO3 (KNN)–xLiTaO3 when x equals 5mol%. This is caused by the phase transition temperature between tetragonal and orthorhombic decreasing to around room temperature due to the Li and Ta doping in KNN, and not by constituting a region of morphotropic phase boundary as presented by most published papers. The results indicate that although this kind of ceramic displays good properties, it needs further study to verify if it is suitable to be used in a varying temperature environment.

Synthesis of NaYF4 Nanocrystals with Predictable Phase and Shape

Abstract: In this Full Paper, a water/alcohol/oleic acid system was developed to prepare NaYF 4 nanocrystals with predictable size, shape and phase. The structural and kinetic factors that govern the phase and shape evolution of NaYF 4 nanocrystals have been carefully studied, and the influence of NaF to Y 3+ ratio, reaction time and temperature on the phase and shape evolution of the as-prepared NaYF 4 samples was systematically investigated and discussed. It was found that the NaF to Y 3+ ratio was responsible for the shape evolution while temperature and reaction time was the key for the phase control of the NaYF 4 nanocrystals. This study would be suggestive for the precisely controlled growth of inorganic nanocrystals, especially for those usually crystallizing in diverse crystal structures.

Steady-state hydrodynamic instabilities of active liquid crystals: hybrid lattice Boltzmann simulations.

Abstract: We report hybrid lattice Boltzmann (HLB) simulations of the hydrodynamics of an active nematic liquid crystal sandwiched between confining walls with various anchoring conditions. We confirm the existence of a transition between a passive phase and an active phase, in which there is spontaneous flow in the steady state. This transition is attained for sufficiently "extensile" rods, in the case of flow-aligning liquid crystals, and for sufficiently "contractile" ones for flow-tumbling materials. In a quasi-one-dimensional geometry, deep in the active phase of flow-aligning materials, our simulations give evidence of hysteresis and history-dependent steady states, as well as of spontaneous banded flow. Flow-tumbling materials, in contrast, rearrange themselves so that only the two boundary layers flow in steady state. Two-dimensional simulations, with periodic boundary conditions, show additional instabilities, with the spontaneous flow appearing as patterns made up of "convection rolls." These results demonstrate a remarkable richness (including dependence on anchoring conditions) in the steady-state phase behavior of active materials, even in the absence of external forcing; they have no counterpart for passive nematics. Our HLB methodology, which combines lattice Boltzmann for momentum transport with a finite difference scheme for the order parameter dynamics, offers a robust and efficient method for probing the complex hydrodynamic behavior of active nematics.

Application of equivalent model method to dynamic rheo‐optical properties of crystalline polymer

Abstract: Assuming that the crystalline polymer consists of the crystalline and the non-crystalline phase, a mechanical model for the crystalline polymer is constructed. We introduced into this model such complicated circumstances as some parts of the crystalline phase will be largely deformed under a stress concentration and other parts not so much. The temperature dependence of complex moduli of the crystalline polymers with various degrees of crystallinity is illustrated by using the model. On the basis of the model, we have derived an equation for calculating the strain-optical coefficient of the crystalline polymer from those of the crystalline and the non-crystalline phase. The equation gives a relation among the rheo-optical quantities, the viscoelastic quantities and the parameters representing the fine structure and the crystallinity of the polymer. It is concluded from this equation that the strain optical coefficients cannot be expressed by such a simple additive relation as usually employed.

Holographic Nuclear Physics

Abstract: We analyze the phases of the Sakai-Sugimoto model at finite temperature and baryon chemical potential. Baryonic matter is represented either by 4-branes in the 8-branes or by strings stretched from the 8-branes to the horizon. We find the explicit configurations and use them to determine the phase diagram and equation of state of the model. The 4-brane configuration (nuclear matter) is always preferred to the string configuration (quark matter), and the latter is also unstable to density fluctuations. In the deconfined phase the phase diagram has three regions corresponding to the vacuum, quark-gluon plasma, and nuclear matter, with a first-order and a second-order phase transition separating the phases. We find that for a large baryon number density, and at low temperatures, the dominant phase has broken chiral symmetry. This is in qualitative agreement with studies of QCD at high density.

New Type of Membrane Material for Water Desalination Based on a Cross-Linked Bicontinuous Cubic Lyotropic Liquid Crystal Assembly

Abstract: A polymeric membrane material for efficient water desalination based on a cross-linked type I bicontinuous cubic (QI) lyotropic liquid crystal (LLC) assembly is described. This ordered, nanoporous, polymer material is formed by the self-assembly of a cross-linkable gemini amphiphile in water and contains interpenetrating organic networks separated from one another by a continuous, ultrathin water layer surface (ca. 0.75 nm gap spacing) with overall cubic symmetry. Supported membranes of this material are produced by hot-pressing the initial LLC monomer gel at high pressure through a commercial microporous hydrophilic membrane support at 70 °C and then radically photocross-linking the infused QI monomer phase. In stirred dead-end water filtration tests, the resulting 40-μm thick, optically transparent, supported LLC membranes exhibit 95−99.9% rejection of dissolved salt ions, neutral molecules and macromolecules, and molecular ions in the 0.64−1.2 nm size range in a single pass. This rejection performance ...

Limitations of the Wegener–Bergeron–Findeisen Mechanism in the Evolution of Mixed-Phase Clouds

Abstract: Phase transformation and precipitation formation in mixed-phase clouds are usually associated with the Wegener–Bergeron–Findeisen (WBF) process in which ice crystals grow at the expense of liquid droplets. The evolution of mixed-phase clouds, however, is closely related to local thermodynamical conditions, and the WBF process is just one of three possible scenarios. The other two scenarios involve simultaneous growth or evaporation of liquid droplets and ice particles. Particle evolution in the other two scenarios differs significantly from that associated with the WBF process. Thus, during simultaneous growth, liquid droplets compete for the water vapor with the ice particle, which slows down the depositional growth of ice particles instead of promoting their growth at the expense of the liquid as in the WBF process. It is shown that the WBF process is expected to occur under a limited range of conditions and that ice particles and liquid droplets in mixed-phase clouds are not always processed i...

Material decomposition mechanisms in femtosecond laser interactions with metals

Abstract: A numerical hydrodynamic study of femtosecond laser ablation is presented. A detailed analysis of material decomposition is performed using a thermodynamically complete equation of state with separate stable and metastable phase states and phase boundaries. The lifetime of the metastable liquid state is estimated based on the classical theory of homogeneous nucleation. In addition, mechanical fragmentation of the target material is controlled based on available criteria. As a result, several ablation mechanisms are observed. A major fraction of the ablated material, however, is found to originate from the metastable liquid region, which is decomposed either thermally in the vicinity of the critical point into a liquid-gas-mixture or mechanically at high strain rate and negative pressure into liquid droplets and chunks. The calculation results explain available experimental findings.

Oxide materials with low thermal conductivity

Abstract: A heuristic approach to identifying candidate materials with low, temperature-independent thermal conductivity above room temperature is described. On the basis of this approach, a number of compounds with thermal conductivities lower than that of 8 mol% yttria-stabilized zirconia and fused silica have been found. Three compounds, in particular, the Zr 3 Y 4 O 12 delta phase, the tungsten bronzes, and the La 2 Mo 2 O 9 phase, exhibit potential for low thermal conductivity applications. As each can exhibit extensive substitutional solid solution with other, high atomic mass ions, there is the prospect that many more compounds with low thermal conductivity will be discovered.

The anomalous behavior of the density of water in the range 30 K < T < 373 K

Abstract: The temperature dependence of the density of water, rho(T), is obtained by means of optical scattering data, Raman and Fourier transform infrared, in a very wide temperature range, 30 < T < 373 K. This interval covers three regions: the thermodynamically stable liquid phase, the metastable supercooled phase, and the low-density amorphous solid phase, at very low T. From analyses of the profile of the OH stretching spectra, we determine the fractional weight of the two main spectral components characterized by two different local hydrogen bond structures. They are, as predicted by the liquid-liquid phase transition hypothesis of liquid water, the low- and the high-density liquid phases. We evaluate contributions to the density of these two phases and thus are able to calculate the absolute density of water as a function of T. We observe in rho(T) a complex thermal behavior characterized not only by the well known maximum in the stable liquid phase at T = 277 K, but also by a well defined minimum in the deeply supercooled region at 203 +/- 5 K, in agreement with suggestions from molecular dynamics simulations.