Nonlocal Continuum Theory of Liquid Crystals

A nonlocal continuum theory of liquid crystals is constructed to explain and predict the physical behavior of liquid crystals under long range intermolecular forces Balance laws consist of conservation of mass and mocroinertia, balance of momenta and energy. Constitutive equations are given for the equibilibirium and non-equilibirium parts of the stress, couple strees, free energy, entropy an nonlocal body force and couple. Thermodynamic restrinctions and material frame-indifference are studied. The theory is valid for liquid crystals having arbitrary shapes (inertia), Passage is made to the thread-like molecuels and to local theory. Applications are considered to two-dimensional problmes, steady, plane shear flows and disperison of twist waves.     

Molecular Cooperation in Photo-Induced Electron Transfer in Monolayer Assemblies

Quenching of fluorescence due to electron transfer is observed in monolayer assemblies when electron donor D and electron acceptor A are either at the same interface or separated by one spacer monolayer. When the donor and acceptor monolayers are separated by two or more fatty acid interlayers no fluorescence quenching is detected.     

On the Origin of Trapping Centres in Organic Molecular Crystals

Traps for current carriers in molecular crystals may be due either to imperfections of the crystal structure (structural traps) or to chemical impurities introduced into the lattice (chemical traps). In the present paper simple hypotheses are presented explaining the formation of traps of the two groups. Perturbations of the crystal structure are not localized in one unit cell, but may extend over several farther cell. The local values of the polarization energy of the crystal lattice will change near to the centre of the defect, thus the “perturbed” molecules can act as traps for currentcarriers.Energetic distributions of traps as calculated under such conditions are foundto be in a good agreement with those determined experimentally. Trapping states may be also formed by impurity molecules having either greater electron affinity or smaller ionization energy than the host ones. The depth of the chemical traps estimated from the relationships given in the paper has been compared with the experimental data.     

Measurement of the Refractive Index and Estimate of the Order Parameter in Oriented Liquid Crystals

The refractive indices of a cholesteric liquid crystal, cholesteryl dodecyl carbonate (CDC), and a smectic liquid crystal, p-n-octoxybenzilidene-p-n-butylaniline (OOBBA), have been measured as a function of temperature in the transition temperature regions. The measurements were made on oriented liquid crystals with an Abbe refractometer. By combining the data on ordinary and extraordinary refractive indices, the relative order parameter, α_aS/α, is plotted as a function of temperature. The behavior of the order parameter near the cholesteric to isotropic transition of CDC is similar to a nematic liquid crystal, whereas, the order parameter in OOBBA shows only a slight temperature variation like many solids.     

Synthesis and characterization of methylammonium tetrachlorocuprate (II) dihydrate crystals

Single crystals of methylammonium tetrachlorocuprate (II) dihydrate were obtained by slow evaporation method at room temperature. The crystals were characterized through powder XRD, thermogravimetric (TG‐DTA), low temperature DSC studies and FTIR and far IR spectra. While the powder XRD confirms the crystallinity of the compound, the TG and DTA studies confirm the formation of the compound in the stoichiometric ratio and the presence of two water molecules of crystallization. The low temperature DSC study indicates a thermal anomaly at ‐3°C. The ordering of CH₃NH₃ group at low temperature causes phase transitions in the compound. The IR spectra characterize the various chemical bonding and water molecules in the compound. Laser Raman spectral studies under progress would help understand the mechanism of phase transitions at low temperatures. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis,thermal and IR spectral characterization of ammonium tetrabromocuprate (II) dihydrate crystals

Single crystals of a new compound, ammonium tetrabromocuprate (II) dihydrate were obtained by slow evaporation method. The crystals were characterized through thermogravimetric (TG), differential thermal analysis (DTA), low temperature differential scanning calorimetric (DSC) methods and infrared (IR) spectroscopic technique. While the TG study confirms the stoichiometry of the compound, the thermal hystereses in DSC indicate the occurrence of first order phase transitions in the compound at low temperatures. The FTIR spectrum of the compound characterizes the various chemical bond vibration frequencies. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and thermal,phase transition and FTIR spectral characterization of tetraethylammonium tetrachlorocuprate (II) crystals

Single crystals of tetraethylammonium tetrachlorocuprate (II), [N(C₂H₅)₄]₂CuCl₄, were grown by slow evaporation method at room temperature. The crystals were characterized through powder XRD, thermogravimetric (TG‐DTA), low temperature differential scanning calorimetric (DSC) studies and FTIR spectroscopy. While the powder XRD pattern of the compound shows sharp Bragg peaks confirming the crystallinity of the compound, the TG‐DTA studies confirm formation of the compound in the stoichiometric ratio. The thermal anomalies observed in DSC curve at ‐120°C in the heating cycle and around ‐30°C in the cooling cycle indicate a first order phase transition. The phase transition was predicted to be associated with the ordering of CuCl₄^2‐ and successive long range orientation of [N(C₂H₅)₄]⁺ ions which are disordered at high temperatures. The cationic [N(C₂H₅)₄]⁺ plays a role in phase transitions at low temperatures. The sharp exothermic peak observed in high temperature DSC indicates a structural phase transition when [N(C₂H₅)₄]CuCl₃ is formed on heating the compound. The FTIR spectra of the compound characterize the various chemical bonding and water molecules adsorbed in the compound. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and characterization of (NH4)2CuBr2Cl2·2H2O crystals

Single crystals of a new compound, (NH₄)₂CuBr₂Cl₂.2H₂O, were grown from saturated aqueous solution at room temperature by slow evaporation method. The grown crystals were characterized through elemental, powder XRD, thermal and DSC analyses and FTIR and far IR spectra. The elemental analysis and the decomposition pattern formulated using the TG‐DTG studies confirm the stoichiometry of the compound. The crystallinity of the compound is confirmed from the powder XRD pattern. A preliminary single crystal X‐ray diffraction structural analysis reveals that the title compound belongs to the orthorhombic system with a = 7.7466 Å, b = 7.783 Å and c = 8.1211 Å. The low temperature DSC shows thermal anomalies at –161.1, –156.5, –152.4, –145.2, –134, –18.5, and 1.4°C during the heating run and at –4.3, –54.8, –66.1, –90.6, –109.7 and –147.2 °C during the cooling run. The thermal hysterses indicate first order phase transitions in the title compound at these temperatures. The FTIR spectra were used to assign the characteristic vibrational frequencies due to NH₄⁺, CuX₄^2– ions and other chemical bonds. The effect of substitution of two bromine atoms on the phase transitions of a closely related crystal, diammonium tetrachloro cuprate dihydrate is also discussed. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and Mesomorphic Properties of New Columnar Liquid Crystal Containing 1,3,5-triiminebenzene with Pendant 1,3,4-thiadiazole Group

A novel discotic liquid crystal series based on 1,3,5-benzenetrisazomethine derivatives with three pendant 2-amino-5-(4′-n-alkoxy)phenyl-1,3,4-thiadiazole has been synthesized, which is the first columnar molecules containing 1,3,4-thiadiazole moiety exhibiting a discotic liquid crystal. The molecular structure of compounds was confirmed by FT-IR, ¹H-NMR, and mass spectroscopy and elemental analysis. The electron excitation properties of these compounds were investigated by UV-vis absorption spectroscopy. Their liquid crystalline properties were studied by polarizing optical microscopy and differential scanning calorimetry. The formation of a columnar mesophase was found to be dependent on the number of methylene unit in alkoxy side chains.     

Dynamic stability analysis of the solidification of binary melts in the presence of a mushy region: changeover of instability

A dynamic linear instability analysis of steady-state binary melt solidification with a mushy region has been carried out. Such an instability differs from a conventional morphological one of a planar solid–liquid interface and is connected with the perturbations in the steady-state solidification velocity. Solidification with a narrow mushy region has been revealed to be absolutely unstable with a monotonous instability of a “hard” type. An increase of the mushy region width leads to an instability changeover from the “hard” type to the oscillatory “soft” one. Both the critical changeover width and the neutral stability curves have been determined as the functions of relevant physical and operating parameters. The steady-state solidification regime with a broad mushy region is absolutely stable. Thus, the mushy region width has been shown to represent a stabilizing factor.     

Evolution of the growth interface in liquid phase diffusion growth of bulk SiGe single crystals

The article presents a study for the evolution of growth interface in crystal growth by Liquid Phase Diffusion (LPD). Specific LPD experiments were designed to grow compositionally graded, germanium‐rich Si_xGe_1‐x single crystals of 25 mm in diameter with various thicknesses. Measured interface shapes show the evolution of the growth interface. Silicon compositions were measured by the Energy Dispersive X‐ray analysis (EDX) in the growth and radial directions. The study shows the feasibility of extracting the desired seeds of uniform composition from LPD grown crystals, for subsequent use in other epitaxial growth processes. © 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim     

Growth,composition, ferroelectric and magnetic properties of new multiferroic Pb3.3Mn4.8Ni1.1Ti0.56O15.3 single crystals

Multiferroic single crystals in the novel system Pb‐Mn‐Ni‐Ti‐O have been grown by the high temperature solution growth method. At room temperature the crystals are indexed in the hexagonal space group P6₃cm. The dielectric and magnetic properties along with the temperature dependence of the c‐lattice parameter have been studied in the temperature range 2 K ‐ 500 К. The magnetic measurements reveal a paramagnetic to antiferromagnetic phase transition around 48 K. The dielectric permittivity exhibits a maximum at 430 K, indicating ferroelectric to paraelectric phase transition. The temperature dependent Raman and XRD measurements around 430 K reveal an anomaly and abrupt change of the lattice parameter along the z‐axis respectively, thus confirming the ferroelectric‐to‐paraelectric phase transition.     

Synthesis and Crystal Structure of Mono-Substituted Ferrocene-Containing Liquid Crystals

Abstract

Ten mono-substituted ferrocene-containing liquid crystals, which containing Schiff base or ester group, were prepared and their mesogenic behaviors were investigated. Two molecular structures were analyzed by X-ray. 4-ferrocenylphenyl 3′-chloro-4′-(2-methylbutyloxy)-benzoate, triclinic crystal system, P1 space group, a=12.035(2)Å, b=14.402(3)Å, c=15.137(3)Å, α =103.07(3)Å, ß = 102.01(3)Å, γ = 96.70(3)Å, Z=4, 8141 independent reflections, R=0.0528, Rw=0.1724. 4-ferrocenylphenyl 3′-chloro-4′-decanyloxybenzoate, triclinic crystal system, P1 space group, a= 11.663(2)Å, b= 11.729(2)Å, c=22.005(4)Å, α=76.10(3)Å, ß =87.56(3)Å, γ = 77.89(3)Å, Z=4, 7935 independent reflections, R=0.0784, Rw=0.2265.     

Investigation of the structure and mechanisms of thermal motion in nanostructured undecylenic acid

Abstract

We have compared the structure, phase transitions and the thermal motion mechanisms of the undecylenic acid in bulk and inside a porous matrix using powder X-ray diffraction, calorimetry and IR-spectroscopy methods. The matrix used was a porous silicon with cylindrical pores (D?=?20?nm). The interaction between the walls of the porous silicon and the nanocrystals of the undecylenic acid results in a crystalline structure, which is significantly different from the structure of bulk undecylenic acid. A phase transition in the wall-adjacent layer of the undecylenic acid nanocrystals was shown to take place. The solid-state phase transition temperatures, as well as the melting point, were shown to be lower for nanocrystals. Topological soliton motion mechanism was suggested to explain the thermal motion of the undecylenic acid molecules in the rotator phase.     

Thermodynamics of Solutions of Various Nonmesomorphic Solutes in the Nematic and Isotropic Phases of 7CB

Thermodynamic results from gas-liquid chromatography, (GLC) are reported for thirty two nonmesomorphic solutes, at infinite dilution, in the nematic and isotropic phases of p-n-heptyl-p'-cyanobiphenyl (7CB). The solutes fall into five categories; normal alkanes, branched alkanes, alkenes, hexadienes and aromatics. The results are examined within the framework of statitical thermodynamic models. The observed trends are interpreted on the basis of solute structure (e.g. shape, size, flexibility, polarity and polarizability). The results conclusively show that the presence of one or more double bonds in the solute structure increases its compatibility with the nematic and isotropic phases of the liquid crystalline solvent. This effect is more drastic when the double bonds are conjugated. Other implications of this study include shape selective GLC separation of geometric isomers and an examination of the effect of solute structure on the stability of the nematic phase at finite solute concentrations.     

Diffusion induced segregation in the case of the ternary system sphalerite,chalcopyrite and cubanite

A mathematical model for describing natural and experimental diffusion induced segregation (DIS) in the case of a (Zn,Fe)S single crystal with three coexisting phases is derived. As main result, a new and quite general segregation principle for ternary systems is discovered where one phase has a flat free energy density and serves as catalyst for the segregation of the other two phases. The model includes also a stochastic noise term to represent fluctuations of the copper concentration. Numerical simulations in 2‐d underline the physical significance of the model and allow to make quantitative predictions. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

太赫兹声子极化激元在LiNbO3微结构中的衍射和干涉动态过程的时间分辨成像

利用飞秒激光脉冲,在50μm厚的铌酸锂晶体薄片中产生了太赫兹声子极化激元波.使用飞秒加工技术在铌酸锂晶体薄片上刻制了针对太赫兹声子极化激元波的衍射和干涉微结构.利用已经构建的偏振门探测成像系统实时动态地记录了太赫兹声子极化激元波的时间和空间传播过程.对不同频率的声子极化激元波在不同尺寸狭缝的动态衍射和干涉的过程进行了时间分辨的二维成像探测和分析,直观地展示了其衍射和干涉的时空特性.     

Synthesis and study of mesomorphic properties in rod-like chalconyl compounds

In order to investigate the influence of the central linking group and effect on flexibility on mesophase behavior of liquid crystal we have synthesized newly homologous series viz. 3-(4-Octyloxyphenyl)-1-(4′-n-alkoxy phenyl) prop-2-en-1-one (Series-l) consists of thirteen homologues (C₁ to C₈, C₁₀, C₁₂, C₁₄, C₁₆, C₁₈). Textures of a nematic phases are threaded or Schlieren type. The textures of smectic and nematic phase are directly judged from the heating and cooling condition. Group efficiency order for mesophases is derived on the basis of thermal stabilities from the comparative study of thermometric data determined from structurally analogous series. A transition curve as depicted in a phase diagram behaves in normal manner. Structural and conformational characterization of these new compounds had achieved by ¹H NMR, ¹³C NMR, IR, and elemental analysis.     

Intramolecular Photochemistry and Energy Transfer in Molecular Crystals of [2.2] metacyclophane-1-ene and 1,2-diphenylcyclopentene

The intramolecular photocyclisation reactions of the stilbene analogues [2.2] metacyclophane-1-ene (MCPE) and 1,2-diphenylcyclopentene (DPCP) have been investigated in the solid state. In MCPE there is little or no energy transfer through the crystal prior to photoreaction, which occurs in regions of otherwise perfect crystal. This may in large part be due to a slow energy transfer rate because excited molecules relax to a distorted state which is only weakly coupled to its neighbours. In DPCP in the solid state photocyclisation will not occur, presumably for steric reasons. Energy transfer in DPCP is measurably slower than that expected for a rigid lattice.