The investigation of the kinking process with help of cholesteric liquid crystals

Process of kinking in CsI crystals has been studied with help of CLC (cholesteric liquid crystals) sensitive to deformation and temperature. It is shown that kinking formation is accompanied by local heat in the kink band. The velocity of deformation is the more the larger the quantity of heat in the kink band.     

Electrical instability in a semiconductor gas discharge system

Electrical instability in a SI GaAs plates of the semiconductor gas discharge gap system (SGDGS) is studied experimentally in a wide range of the gas pressures, interelectrode distances and different diameters of the cathode areas. While being driven with a stationary voltage, it generates current and discharge light emission (DLE) instabilities with different amplitudes of the oscillation. It is shown that under the experimental conditions the interelectrode distance played only a passive role and was not responsible for the appearance of the DLE instability. At the same time for different diameters D of the GaAs plate areas the expanded range of current and DLE oscillations are observed. SGDGS with an N‐shaped CVC was analyzed using both the current and DLE data showing the electrical instability in the GaAs cathode. It was found that application of high feeding voltage to this cathode give rise to non‐uniform spatial distribution of the DLE, which disturbed the operation of the system. The experiment presents also a new metod to study and visualization of the electrical instabilities in high‐resistivity GaAs plates of large diameter. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Reflectance,Transmittance and Color Gamut of Superimposed Layers of Cholesteric Liquid Crystals

The spectral reflectance curves of superimposed layers of cholesteric liquid crystals (CLC) having the same handedness add as there is little absorption. The peak reflectance of the sum cannot exceed 0.5 and the transmittance cannot be less than 0.5, which is the theoretical limit. The shape of the reflection and transmission band resembles rectangles and thus can be compared with the spectral curves of optimum colors. The chromaticity loci of CLC colors and its luminous reflectance equals 0.5 of the value of the luminous reflectance attached to the optimum colors. The color gamut thus produced can be shown to be greater than that which has been obtained with pigments.

A significant improvement can be obtained using superimposed layers of CLC having opposite handedness. Their spectral curves add and the peak reflectance is not limited by the value of 0.5 but approaches 1.0. Use of CLC's of opposite handedness is not always practical as they have different chemical and physical properties. A similar result can be obtained using two CLC layers of the same handedness and a half-wave retardation plate interposed between them. This plate converts the transmitted through the upper layer right-handed (left-handed) component into a left-handed (right-handed) one, which is then reflected from the lower layer and contributes to the reflected left-handed (right-handed) component from the upper layer. The chromaticity loci and color gamut of such superimposed CLC pairs of coatings now approaches that of optimum colors of the same luminous reflectance.     

Cholesteric liquid Crystals: Physical Properties and Molecular-Statistical Theories

It seems certain now that cholesteric liquid crystals (cholesterics, or CLC, for short) have been the first examples of mesomorphic state of matter known to humankind. Apart from the well-known discovery of the two-stage melting of cholesteryl benzoate made by Reinitzer in 1888¹ and cited in almost all the textbooks, one may recall some other papers where the authors, quite unaware of the real significance of their observations, presented undeniable evidence of the liquid crystalline state of matter. E.g., Professor Planer from the Lvov University as early as in 1861 did observe, putting it in the presentday terms, selective reflection from the planar texture of cholesteryl chloride forming a monotropic cholesteric mesophase on cooling.² One should also note that the works of Fergason^3,4 outlining the prospects of practical applications for CLC, did, in fact, precede similar endeavors by Heilmeier in the field of nematics. From the viewpoint of organic chemistry, a number of mesogenic cholesterol derivatives had been already synthesized and thoroughly studied⁵ by the time the name of MBBA (p-methoxybenzylidene-p′-n-butylaniline), to say nothing about cyanobiphenyls, was not yet known to anybody. The mere listing of CLC prospective applications^6,7. seems so impressive that, keeping in mind the above-described historical background, one should expect to find cholesterics in a key position in the field of liquid crystal science.     

Dynamic aging of dislocations in materials with a high crystalline relief: Competition between diffusion and impurity entrainment

A model of the dynamic interaction of dislocations with the impurity subsystem of crystals that have a high lattice potential relief (Peierls barriers) has been developed. It is shown that the microscopic structure of migration barriers for impurities near a dislocation core may cause qualitatively different behavior of the impurity atmosphere on a moving dislocation. It is justified that the impurity kinetics during atmosphere formation includes two stages. The first (initial) stage is fast and significantly nonequilibrium; it is followed by the second stage, characterized by a slower approach to equilibrium. The initial stage manifests itself at a sufficiently fast dislocation motion and may lead to an anomalous increase in the driving force (or the yield strength of the material) with an increase in the temperature in some range. Blocking of the dislocation motion by impurities may cause inverse brittle-ductile transition, which is observed in some materials with an increase (rather than the usual decrease) in temperature.     

Selective crystallization of amorphous silicon thin film by a CW green laser

A simple and effective method for selective CW laser crystallization of a-Si (CLC) without pre-patterning of a-Si has been reported. By using a metallic shadow mask instead of a photolithographic process, we can reduce the process steps and time compared with a conventional CLC process. It shows very high performance – mobility of 173 cm²/s, I_off of ∼10⁻¹³ A @ V_d = −5 V, I_on/I_off of >10⁸ – as a p-channel poly-Si TFT even without any pre-/post-treatment to improve TFT characteristics such as plasma hydrogenation.     

Compensation for the spectral dispersion of light polarization states in electro-optic modulators based on chiral liquid crystals

The effect of compensation for the spectral dispersion of light polarization states at the output of a single-domain layer of a chiral liquid crystal (CLC) is experimentally studied. It is shown that such dispersion can be decreased significantly with the aid of phase plates of two types that have different signs of the spectral dispersion of birefringence. The dispersion compensation allows one to significantly increase the operating spectral range of fast light modulators based on chiral nematic liquid crystals (NLCs).     

Studies of Specific Adsorption Sites for Sodium Ion and Water on the Interface of Mixed Micelles of Novel Type II Lyomesophases

Novel aqueous type II lyotropic mesophases that align in magnetic fields have been synthesized for binary and ternary mixtures of amphiphiles. A system in which major components of the micelle bilayer are decyltrimethylammonium and dodecanoate ions is an aligning mesophase from 100% to 35% dodecanoate, but replacement of the cationic amphiphile with hexadecyltrimethylammonium facilitates the extension to aligning mesophases up to 100% replacement of the dodecanoate ions. A study of sodium and deuterium quadrupole splittings observed in their nuclear magnetic resonance spectra, confirms and extends a three site adsorption theory for sodium ion in the mixed head group interface. The first site is associated with more strongly hydrated ions and head groups and involves adjacent dodecanoate head groups, the second is not so strongly hydrated and involves one dodecanoate group while the third is a weakly bound water at - N(CH₃)⁺ groups and a sodium remote from the interface layer in close to average isotropic motion.

Mixed detergent mesophases of type II, which spontaneously align in the field, have also been synthesized from variable mixtures of hexadecyltrimethylammonium bromide and decylammonium chloride and from mixtures of potassium dodecanoate and sodium decylsulphate. In the mixed cationic system the sodium quadrupole splittings are always low but water is tightly bound to the -NH₃ ⁺ head group. In the anionic detergent mixture sodium appears about as strongly bound to dodecanoate and decylsulphate head groups. In most of the mixed mesophases some decanol is required (maximum 25% of the bilayer) in order to render the desired type II mesophase stable over a range of mixtures of the charged detergents. Specific site adsorption for sodium and water, peculiar to different head groups renders the effect of the added decanol as primarily a convenient diluent in the bilayer to preserve mesophase integrity.     

Instability of the melt flow in VGF growth with a traveling magnetic field

The linear instability of a thermally stratified melt flow in the VGF configuration driven by a traveling magnetic field (TMF) is considered numerically and experimentally. The dependency of the instability threshold on the governing parameters is found for several cuts through the parameter space covering a wide range of possible applications. In a first approximation the linear instability occurs when the dimensionless TMF forcing parameter reaches the magnitude of the Grashof number. This is particularly true in a medium-sized crucible where the first instability is axisymmetric and sub-critical. As the Grashof number increases the flow develops self-similar boundary layers and the instability becomes three-dimensional. The instability originates in the bottom boundary layer where the convection tends to suppress the imposed temperature gradient in the central part of the melt zone. It is shown that the TMF may serve as a tool to control the phase interface shape without causing flow instationarity when the crucible diameter exceeds a certain value. This value is estimated to be around 6 cm for GaAs. The flow stays stable if the TMF is used for a reversal of the meridional flow with the aim to remove a possible dopant concentration peak on the axis.     

二维异核铜-钠配位聚合物的合成和晶体结构

合成了铜-钠混合金属配位聚合物 [(CuL)2(CH3OO)Na(H2O)4]2 (简写为1)并进行了元素分析和单晶X射线衍射分析,L为{2-(2′-酚基)- Δ 2-噻唑-4-羧酸}.晶体结构分析表明配合物形成Cu4Na2的六核混合金属大环.相邻的Cu4Na2环之间通过进一步的配位作用连结成包含更大的Cu8Na4大环的二维网状结构.该晶体属于三斜晶系Pī空间群,晶胞参数分别为:a = 1.0777(4) nm, b = 1.5221(6) nm, c = 1.6994(6) nm, α = 84.58(1)°, β = 76.31(1)°, γ = 89.97(1) °, V = 2.696 (2) nm3, Z = 2, Mr= 1447.26, dc = 1.783 g·cm-3, T = 293(2) K, μ = 1.816 mm-1, F(000) = 1472, R1 = 0.0794, wR2 = 0.1819 [I>2σ(I)].     

Primary converters for optical sensors of physical values based on polymer dispersed cholesteric liquid crystal

The investigations of system based on cholesteric liquid crystal (CLC) sensors of physical quantities with an information transfer optical channel are carried out in the paper. The results of experimental investigations of spectral characteristics of polymer dispersed cholesteric liquid crystals (PDCLC) are represented. The application such material as primary transducer of a temperature sensor is proposed. The structure of an optical fiber temperature sensor with successive allocations of primary transducers and shifted areas of selective reflection band has been designed.     

Synthesis,structural and thermal studies of tetrathioureacopper(I) chloride crystals

Tetrathioureacopper(I) chloride, hereafter abbreviated as TCC, was synthesised and single crystals were obtained from saturated aqueous solution by slow evaporation (solution growth) method at room temperature. The crystals obtained are bright, colourless and transparent having well defined external faces. The grown crystals were characterized through elemental analysis, single crystal X‐ray diffraction study, thermal analysis, electron spin resonance spectroscopy and Fourier Transform infrared spectroscopy. The elemental analysis confirms the stoichiometry of the compound. The single crystal diffraction studies indicate that TCC crystallises in the tetragonal lattice and the unit cell parameters are a = b = 13.4082 Å, c = 13.8074 Å, V = 2482.29 ų, α = β = γ = 90°. Space group and the number of molecules per unit cell (Z) are found to be P4₁2₁2 and 8 respectively. The TG curve of the sample shows a prolonged decomposition from 210 to 628.3 °C, from which the decomposition pattern has been formulated. The endothermic peaks in the DTA curve indicate melting and decomposition of the compound at 165.2 and 633.8 °C respectively. An exothermic peak in high temperature DSC indicates a phase transition in the compound at 274.8 °C. Thermal anomalies observed in the low temperature DSC at –163.3, –152.0, –141.5, –108.3, 1.0 and 12.1 °C in the heating run and –157.1 and –153.9 °C in the cooling run reveal first order phase transitions in the crystal. The peaks observed at –146.2 °C in both the heating and cooling runs suggest occurrence of a second order phase transition in this compound. The IR spectroscopic data were used to assign the characteristic vibrational frequencies of various groups present in the compound. The ESR study confirms that the copper is in the +1 oxidation state in the complex. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On splitting of dislocations at a reverse motion in germanium single crystals

The dislocation dissociation after the reverse motion along a slip plane in Ge single crystals have been investigated. It has been established that the dissociation width as well as the constriction density were independent on the motion direction. The results obtained have been used to clarify the role of the dislocation splitting in the formation of the phenomenon of the dislocation mobility asymmetry.     

Switching of SBN crystals: Comparison with the model case (DTGS)

It is shown that a number of specific features of the electric-field induced polarization reversal and transition to the single-domain state observed in ferroelectric crystals of SBN-0.61 and DTGS (the model object)— delayed or incomplete transition to the single-domain state in strong fields, hysteresis loop degradation, and time dependence and the memory effect of the residual domain density—are due to the pinning of forward motion (intergrowth) of dynamic domains with charged inclined walls in the absence of lateral motion.     

Crystal and molecular structures of several tetrakis (nitrile)copper(I) complexes

The crystal structures of several [Cu(RCN)₄]X salts (R = Me, X = SO₃CF₃ ^–; R = Ph, X = BF₄ ^–, ClO₄ ^–, and PF₆ ^–) were determined using single crystal X-ray diffraction. All of the compounds contain distorted tetrahedral Cu(I) centers and noncoordinating anions, with the acetonitrile and benzonitrile structures containing three and one unique CuL₄ ⁺ complex in their respective asymmetric units. One important distortion is observed in the benzonitrile-Cu bonds, which are bent up to 23° away from linearity. The result is a flattened complex that maximizes the – tacking of the aromatic rings and is the dominant packing interactions between the complexes.     

Effects of aromatic core and flexible terminal chain structures on the properties of luminous liquid-crystalline gold(i) complexes for functional materials

ABSTRACT

Luminescent liquid-crystalline (LC) Au complexes with various aromatic cores and flexible terminal chains were synthesized. The aromatic core aspect ratio crucially affected the LC temperature range and melting point of the complexes. Moreover, branched flexible chains reduced the melting point, thus expanding the LC temperature range. In the crystalline and LC phases of the complexes, fluorescence was quenched by the aggregation-caused quenching effect, but phosphorescence was enhanced by the crystallization-induced phosphorescence effect. However, no phosphorescence was observed in the isotropic phase. Thus, not only aggregation but also regularity of molecular aggregates is necessary for the observed crystallization-induced phosphorescence effect.     

Metastable States and CDW Conductivity in NbSe3

A study of the response of niobium triselenide to pulsed currents shows that the field-induced motion of the charge-density waves (CDWs) leaves them in long-lived metastable states in which, presumably, some distortion is stabilised by pinning. Evidence of this is provided by a reduction in Ohmic conductivity after current flows non-linearly through a specimen originally in thermal equilibrium, and by a transient increase in the non-linear conductivity on subsequent reversals of the current. A gradual increase in the non-linear conductivity during pulses applied repeatedly in the same sense is found to arise from a thermally-induced change in the strength of the pinning. The possible origin of this change is discussed.     

The Investigation of Heat,Release in Slip Bands Consisting of Edge or Screw Dislocations

The temperature rise in screw and edge dislocation slip bands of the NaCl crystals by deformation by compression has been studied with the help of cholesteric liquid crystals CLC.     

Macrosteps dynamics and the growth of crystals and epitaxial layers

Step pattern stability of the vicinal surfaces during growth was analyzed using various surface kinetics models. It was shown that standard analysis of the vicinal surfaces provides no indication on the possible step coalescence and therefore could not be used to elucidate macrostep creation during growth. A scenario of the instability, leading go macrostep creation, was based on the dynamics of the step train, i.e. the step structure consisting of the high (train) and low (inter-train) density of the steps. The critical is step motion at the rear of the train which potentially leads to the step coalescence i.e. creation of the double and multiple step. The result of the analysis shows that the decisive factor for the step coalescence is the step density ratio in and out of the train. The ratio lower than 2 prevents double step formation irrespective of the kinetics. For higher ratio the coalesce depends on step kinetics: fast incorporation from lower terrace stabilizes the single steps, fast incorporation from upper leads to step coalescence. The double step is slower than the single steps, so the single steps behind catch up creating multistep and finally macrostep structure. The final surface structure consists of the macrosteps and superterraces, i.e. relatively flat vicinal segments. The macrostep alimentation from lower superterrace leads to emission of the single steps which move forward. Thus the single step motion is dominant crystal growth mode in the presence of the macrosteps. These steps finally are absorbed by the next macrostep. The absorption and emission of single steps sustain the macrostep existence, i.e. the macrostep fate is determined the single step dynamics. The condition for single step emission was derived. In addition, the macrosteps are prone to creation of the overhangs which results from surface dynamics coupling to impingement from the mother phase. The angular preferential access of the bulk material to the macrostep edge, leads to the overhang instability and creation of inclusions and dislocations.