Anomalous phase transitions in soft colloid-polymer binary mixtures

We have shown earlier [1] that these PGNPs resemble star polymers or spherical brushes in terms of their morphology in the melt. However, these particles show dynamics in melt which is quite different from other soft colloidal particles. Since most of the work on soft colloidal particles have been performed in solutions we have now explored the phase behavior of the PGNPs in good solvent using microscopic structural and dynamical measurements on binary mixtures of homopolymers and soft colloids consisting of polymer grafted nanoparticles. We observe anomalous structural and dynamical phase transitions of these binary mixtures, including appearance of spontaneous orientational alignment and logarithmic structural relaxations, as a function of added homopolymers of different molecular weights. Our experiments points to the possibility of exploiting the phase space in density and homopolymer size, of such hybrid systems, to create new materials with unique properties.

Quantum Phase Diagram of One-Dimensional Spin and Hubbard Models with Transitions to Bond Order Wave Phases

Similar quantum phase diagrams and transitions are found for three classes of one-dimensional models with equally spaced sites, singlet ground states (GS), inversion symmetry at sites and a bond order wave (BOW) phase in some sectors. The models are frustrated spin-1/2 chains with variable range exchange, half-filled Hubbard models with spin-independent interactions and modified Hubbard models with site energies for describing organic charge transfer salts. In some range of parameters, the models have a first order quantum transition at which the GS expectation value of the sublattice spin < S-A(2)> of odd or even-numbered sites is discontinuous. There is an intermediate BOW phase for other model parameters that lead to two continuous quantum transitions with continuous < S-A(2)>. Exact diagonalization of finite systems and symmetry arguments provide a unified picture of familiar 1D models that have appeared separately in widely different contexts.

Quantum Phase Diagram of One-Dimensional Spin and Hubbard Models with Transitions to Bond Order Wave Phases

Similar quantum phase diagrams and transitions are found for three classes of one-dimensional models with equally spaced sites, singlet ground states (GS), inversion symmetry at sites and a bond order wave (BOW) phase in some sectors. The models are frustrated spin-1/2 chains with variable range exchange, half-filled Hubbard models with spin-independent interactions and modified Hubbard models with site energies for describing organic charge transfer salts. In some range of parameters, the models have a first order quantum transition at which the GS expectation value of the sublattice spin < S-A(2)> of odd or even-numbered sites is discontinuous. There is an intermediate BOW phase for other model parameters that lead to two continuous quantum transitions with continuous < S-A(2)>. Exact diagonalization of finite systems and symmetry arguments provide a unified picture of familiar 1D models that have appeared separately in widely different contexts.

New 4-(2-(4-alkoxyphenyl)-6-methoxypyridin-4-yl) benzonitriles: synthesis, liquid crystalline behavior and photo physical properties

A new series of luminescent 4-(2-(4-alkoxyphenyl)-6-methoxypyridin-4-yl) benzonitriles containing three ring systems, viz. methoxy pyridine, benzonitrile and alkoxy benzene with variable alkoxy chain length, with bent-core structures were synthesized as potential mesogens and characterized by spectral techniques. Their liquid crystalline behavior was investigated by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and variable temperature powder X-ray diffraction (PXRD) measurements. The study reveals that compounds with shorter chain lengths i.e. m = 4] exclusively exhibit the nematic phase while compounds with longer chain lengths i.e. m = 6-14 (only even)] show predominantly the orthorhombic columnar phase. Single crystal X-ray analysis of 4-(2-(4-butyloxy/octyloxyphenyl)-6-methoxypyridin-4-yl) benzonitriles reveals that they possess slightly non-planar unsymmetrical bent structures and their molecular packing consists of nonconventional H-bond interactions; it also explains the observed liquid crystalline phase. An optical study indicates that the title compounds are good blue emitting materials showing absorption and emission bands in the range 335-345 nm and 415-460 nm, respectively. An electrochemical study of 4-(2-(4-octyloxyphenyl)-6-methoxypyridin-4-yl) benzonitrile shows a band gap of 1.89 eV with HOMO and LUMO energy levels of -5.06 and -3.17 eV, respectively. Also, density functional theory (DFT) calculations confirm its optimized geometry, electronic absorption and frontier molecular orbital distributions.

Orbital phase resolved spectroscopy of GX 301-2 with MAXI

GX 301-2, a bright high-mass X-ray binary with an orbital period of 41.5 d, exhibits stable periodic orbital intensity modulations with a strong pre-periastron X-ray flare. Several models have been proposed to explain the accretion at different orbital phases, invoking accretion via stellar wind, equatorial disc, and accretion stream from the companion star. We present results from exhaustive orbital phase resolved spectroscopic measurements of GX 301-2 using data from the Gas Slit Camera onboard MAXI. Using spectroscopic analysis of the MAXI data with unprecedented orbital coverage for many orbits continuously, we have found a strong orbital dependence of the absorption column density and equivalent width of the iron emission line. A very large equivalent width of the iron line along with a small value of the column density in the orbital phase range 0.10-0.30 after the periastron passage indicates the presence of high density absorbing matter behind the neutron star in this orbital phase range. A low energy excess is also found in the spectrum at orbital phases around the pre-periastron X-ray flare. The orbital dependence of these parameters are then used to examine the various models about mode of accretion on to the neutron star in GX 301-2.

Temperature-Induced Reversible First-Order Single Crystal to Single Crystal Phase Transition in Boc-gamma(4)(R)Val-Val-OH: Interplay of Enthalpy and Entropy

Crystals of Boc-gamma y(4)(R)Val-Val-OH undergo a reversible first-order single crystal to single crystal phase transition at T-c approximate to 205 K from the orthorhombic space group P22(1)2(1) (Z' = 1) to the monoclinic space group P2(1) (Z' = 2) with a hysteresis of similar to 2.1 K. The low-temperature monoclinic form is best described as a nonmerohedral twin with similar to 50% contributions from its two components. The thermal behavior of the dipeptide crystals was characterized by differential scanning calorimetry experiments. Visual changes in birefringence of the sample during heating and cooling cycles on a hot-stage microscope with polarized light supported the phase transition. Variable-temperature unit cell check measurements from 300 to 100 K showed discontinuity in the volume and cell parameters near the transition temperature, supporting the first-order behavior. A detailed comparison of the room-temperature orthorhombic form with the low-temperature (100 K) monoclinic form revealed that the strong hydrogen-bonding motif is retained in both crystal systems, whereas the non-covalent interactions involving side chains of the dipeptide differ significantly, leading to a small change in molecular conformation in the monoclinic form as well as a small reorientation of the molecules along the ac plane. A rigid-body thermal motion analysis (translation, libration, screw; correlation of translation and libration) was performed to study the crystal entropy. The reversible nature of the phase transition is probably the result of an interplay between enthalpy and entropy: the low-temperature monoclinic form is enthalpically favored, whereas the room-temperature orthorhombic form is entropically favored.

Exact Phase Retrieval for a Class of 2-D Parametric Signals

We address the problem of two-dimensional (2-D) phase retrieval from magnitude of the Fourier spectrum. We consider 2-D signals that are characterized by first-order difference equations, which have a parametric representation in the Fourier domain. We show that, under appropriate stability conditions, such signals can be reconstructed uniquely from the Fourier transform magnitude. We formulate the phase retrieval problem as one of computing the parameters that uniquely determine the signal. We show that the problem can be solved by employing the annihilating filter method, particularly for the case when the parameters are distinct. For the more general case of the repeating parameters, the annihilating filter method is not applicable. We circumvent the problem by employing the algebraically coupled matrix pencil (ACMP) method. In the noiseless measurement setup, exact phase retrieval is possible. We also establish a link between the proposed analysis and 2-D cepstrum. In the noisy case, we derive Cramer-Rao lower bounds (CRLBs) on the estimates of the parameters and present Monte Carlo performance analysis as a function of the noise level. Comparisons with state-of-the-art techniques in terms of signal reconstruction accuracy show that the proposed technique outperforms the Fienup and relaxed averaged alternating reflections (RAAR) algorithms in the presence of noise.

Simple three parameter equations for correlating liquid phase compositions in subcritical and supercritical systems

Two Chrastil type expressions have been developed to model the solubility of supercritical fluids/gases in liquids. The three parameter expressions proposed correlates the solubility as a function of temperature, pressure and density. The equation can also be used to check the self-consistency of the experimental data of liquid phase compositions for supercritical fluid-liquid equilibria. Fifty three different binary systems (carbon-dioxide + liquid) with around 2700 data points encompassing a wide range of compounds like esters, alcohols, carboxylic acids and ionic liquids were successfully modeled for a wide range of temperatures and pressures. Besides the test for self-consistency, based on the data at one temperature, the model can be used to predict the solubility of supercritical fluids in liquids at different temperatures. (C) 2014 Elsevier B.V. All rights reserved.

An alluaudite Na-2+Fe-2x(2-x)(SO4)(3)(x=0.2) derivative phase as insertion host for lithium battery

A new desodiated derivative compound, Na0.89Fe1.8(SO4)(3), was prepared by the chemical oxidation of alluaudite Na2.4Fe1.8(SO4)(3) Phase using NOBF4 as oxidant. The structure and valency of Fe were characterized by X-ray diffraction (XRD) and Fe-57 Mossbauer spectroscopy. Intercalation behavior of lithium ions in the structure of Na0.89Fe1.8(SO4)(3) was gauged by electrochemical analyses and ex-situ X-ray diffraction. A high capacity of 110 mAh g(-1) at 0.1 C was obtained with a good rate kinetics within a range of 0.1-10 C(1 C = 118 mAh g-1) involving a high Fe3+/Fe2+ redox potential of 3.75 V (vs. Li/Li+). These results confirmed that the Na2.4-delta Fe1.8(SO4)(3) framework was stable even after oxidation and forms a new competitive cathode for the reversible intercalation of lithium ions. (C) 2014 Elsevier B.V. All rights reserved.

Differential response of cholesterol based pyrimidine systems with oxyethylene type spacers to gelation and mesogen formation in the presence of alkali metal ions

A new series of lipophilic cholesteryl derivatives of 2,4,6-trichloro-pyrimidine-5-carbaldehyde has been synthesized. Oxyethylene spacers of variable lengths were inserted between the hydrogen bonding promoting pyrimidine core and the cholesteryl tail in order to understand their effect on the selfassembly of these compounds. Only compound 1a with the shortest spacer formed a gel in organic solvents such as n-butanol and n-dodecane. While other members (1b and c) having longer spacers led to sol formation and precipitation in n-butanol and n-dodecane respectively. The self-assembly phenomena associated with the gelation process were investigated using temperature-dependent UVVis and CD-spectroscopy. The morphological features of the freeze-dried gels obtained from different organic solvents were examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The solid phase behaviours of these molecules and their associated alkali metal ion complexes were explored using polarized optical microscopy (POM) and differential scanning calorimetry (DSC). The molecular arrangements in the xerogel and in the solid state were further probed using a wide-angle Xray diffraction (WAXD) technique. Analysis of the wide-angle X-ray diffraction data reveals that this class of molecules adopts a hexagonal columnar organization in the gel and in the solid state. Each slice of these hexagonal columnar structures is composed of a dimeric molecular-assembly as a building block. Significant changes in the conformation of the oxyethylene chains could be triggered via the coordination of selected alkali metal ions. This led to the production of interesting metal ion promoted mesogenic behaviour.

Dielectric and Raman investigations of structural phase transitions in (C2H5NH3)(2)CdCl4

Temperature-dependent Raman and dielectric measurements have been carried out on (C2H5NH3)(2)CdCl4 single crystals. Raman studies reveal the presence of two structural phase transitions below room temperature at 216 K and 114 K. The phase transitions are marked by anomalies in temperature dependence of wave-number and full width half maximum (FWHM) of several vibrational modes. The transitions are also accompanied by anomalies in dielectric measurements. Raman and dielectric data indicate that the transition at 216 K is order-disorder in nature and is driven by re-orientation of organic ions, while the transition at 114 K is due to coupling between the CdCl6 octahedron and the organic chain. Further high temperature dielectric measurements reveal the presence of one more structural phase transition around 473 K across which dispersion in dielectric parameters is observed. The activation energies and relaxation time obtained for high temperature dielectric phases are characteristic of combined reorientation motions of alkyl ammonium cations.

Active Power Decoupling with Reduced Converter Stress for Single-Phase Power Conversion and Interfacing

Single-phase DC/AC power electronic converters suffer from pulsating power at double the line frequency. The commonest practice to handle the issue is to provide a huge electrolytic capacitor for smoothening out the ripple. But, the electrolytic capacitors having short end of lifetimes limit the overall lifetime of the converter. Another way of handling the ripple power is by active power decoupling (APD) using the storage devices and a set of semiconductor switches. Here, a novel topology has been proposed implementing APD. The topology claims the benefit of 1) reduced stress on converter switches 2) using smaller capacitance value thus alleviating use of electrolytic capacitor in turn improving the lifetime of the converter. The circuit consists of a third leg, a storage capacitor and a storage inductor. The analysis and the simulation results are shown to prove the effectiveness of the topology.

Active Power Decoupling with Reduced Converter Stress for Single-Phase Power Conversion and Interfacing

Single-phase DC/AC power electronic converters suffer from pulsating power at double the line frequency. The commonest practice to handle the issue is to provide a huge electrolytic capacitor for smoothening out the ripple. But, the electrolytic capacitors having short end of lifetimes limit the overall lifetime of the converter. Another way of handling the ripple power is by active power decoupling (APD) using the storage devices and a set of semiconductor switches. Here, a novel topology has been proposed implementing APD. The topology claims the benefit of 1) reduced stress on converter switches 2) using smaller capacitance value thus alleviating use of electrolytic capacitor in turn improving the lifetime of the converter. The circuit consists of a third leg, a storage capacitor and a storage inductor. The analysis and the simulation results are shown to prove the effectiveness of the topology.

Order-disorder phase transition and multiferroic behaviour in a metal organic framework compound (CH3)(2)NH2Co(HCOO)(3)

We have investigated the multiferroic and glassy behaviour of metal-organic framework (MOF) material (CH3)(2)NH2Co(CHOO)(3). The compound has perovskite-like architecture in which the metal-formate forms a framework. The organic cation (CH3)(2)NH2+ occupies the cavities in the formate framework in the framework via N-H center dot center dot center dot O hydrogen bonds. At room temperature, the organic cation is disordered and occupies three crystallographically equivalent positions. Upon cooling, the organic cation is ordered which leads to a structural phase transition at 155 K. The structural phase transition is associated with a para-ferroelectric phase transition and is revealed by dielectric and pyroelectric measurements. Further, a PE hysteresis loop below 155 K confirms the ferroelectric behaviour of the material. Analysis of dielectric data reveal large frequency dispersion in the values of dielectric constant and tan delta which signifies the presence of glassy dielectric behaviour. The material displays a antiferromagnetic ordering below 15 K which is attributed to the super-exchange interaction between Co2+ ions mediated via formate linkers. Interestingly, another magnetic transition is also found around 11 K. The peak of the transition shifts to lower temperature with increasing frequency, suggesting glassy magnetism in the sample. (C) 2016 AIP Publishing LLC.