Anisotropy of Electric Properties in Binary Cholesteric Mixtures

Dielectric permittivity and electric conductivity anisotropies were determined for cholesterol laurate, cholesterol caprilate as well as for their mixtures 25:75%, 50:50% and 75:25% (by weight). Connection between anisotropies of pure substances and their mixtures were found. The mixtures 75:25% and 25:75% have positive conductivities anisotropies within the cholesteric range as for the initial esters; the sign of the dielectric anisotropy is the same as for the prevailing ester. Within the smectic range the dielectric anisotropy is negative for both mixtures and pure compounds, in accordance to Carr's hypothesis with reference to the conduction mechanism in smectics.

The phase transitions are revealed in most cases by maxima or minima of the dielectric anisotropies.     

Lasing Spectra of Dye Doped Cholesteric Liquid Crystals Under Reversible Phototuning of the Frequency

We investigate the lasing frequency phototuning in a distributed feedback (DFB) laser based on cholesteric liquid crystals (CLC). To improve the CLC planar texture quality, we use new materials such as nematic ZhK-654 and an analog of cholesteryl oleate as a twisting additive. The selective excitation of the cis-form molecules of the azoxy-nematic is carried out with the use of a combination of an interference filter with maximum transmission at 436 nm and a cut-off filter. Using the orienting substrates with SnO₂ allows us to reduce the threshold pump power by more than an order of magnitude. The proposed combination of filters for the selective excitation enables us to align the ranges of lasing wavelengths in both directions.     

Behaviour of Nematic Liquid Crystals in Inhomogeneous Electric Fields

The behaviour of homogeneously aligned nematic liquid crystal layers in inhomogeneous electric fields has been theoretically and experimentally investigated. The variation of the phase retardation of monochromatic light along nematic liquid crystal layer at the edges of a cell electrode has been obtained. Anisotropic character of the resolution of liquid crystal devices is shown.     

Electroluminescence in Amorphous Pyrazoline Films Under DC Fields

We have investigated the electroluminescence, EL, behavior in amorphous films of a pyrazoline derivative under dc fields. The current vs. voltage characteristics indicated a space charge limited current behavior with traps distributed exponentially within the forbidden energy gap. EL was detected in the current range of double carrier injection which ocurred after all traps in the sample were filled up. Spectral studies showed that the light emission zone was loalized around the cathode side of the pyrazoline film. The brightness, measured as a function of current, indicated the EL in the lower current density range to be dominated by delayed luminescence.     

Growth Kinetics of Adenine Sulphate Crystals

The kinetics of growth and dissolution of adenine sulphate in 1.5 mol · 1⁻¹ H₂SO₄ at different super- and undersaturations has been studied. The solubility of adenine sulphate in H₂SO₄ was determined over the temperature range 20–50 °C. The growth rate of the (100) face is by about 25% higher than that of (111) and the crystal habit is changed by a high supersaturation almost to a bipyramide. The surface diffusion was determined as a prevailing processes and the BaS model gives the best fit with experimental data.     

一水甲酸锂晶体生长动力学和生长机理的研究

本文实时测定了312.16K时一水甲酸锂晶体(LFM)(010)和(021)面的生长速率。根据Bennema修正的用于水溶液晶体生长的BCF表面扩散模型,计算了晶体生长过程的激活能以及动力学系数。结果表明LFM晶体生长主要是表面扩散控制的螺位错生长机制。     

Theoretical and Practical Studies on Effects of External Electrostatic Electric Field on Nucleation and Growth Kinetics of Protein Crystals

The crystallization technique where an electric field is applied is an extremely powerful tool to control the crystallization processes of various materials. In particular, the method with application of an external electrostatic electric field can have a significant effect on the phase equilibrium of the liquid and solid phases. This review demonstrates that the crystallization processes of proteins are significantly impacted by the application of an external electrostatic electric field: (1) Control of both the increase and decrease in the nucleation rate can be achieved by changing the applied frequency of the external electrostatic electric field. (2) The effect of the external electrostatic electric field on the nucleation rate can be controlled by regulating the thickness of the electric double layer (EDL) formed at the interface. (3) The quality of the grown crystals can be improved by an increase in the step free energy under application of an external electrostatic electric field at 1 MHz. The effect of the external electrostatic electric field on nucleation and growth kinetics during crystal growth of proteins is also discussed based on a thermodynamic perspective.     

Growth Kinetics and Mechanism of TGS Crystals

The growth kinetics of TGS crystals was studied at high supersaturations under the Curie temperature. The kinetics data proved that the crystal growth was mainly controlled by BCF surface diffusion model. The continuous growth was fitted to the growth rate data of (110) face. Its edge energy, Jackson factor, activity energies, kinetic coefficients were calculated.     

Growth Kinetics of Barium Sulphate in Suspension

The growth rate of barium sulphate seed crystals from stirred solutions was studied conductometrically at 25°C by a stopped-flow technique. The supersaturation ranged from 3 × 10⁻⁷ to 3 × 10⁻⁸ mol BaSO₄/cm³. The seed crystals were grown in the system during the initial (steady-state) period of the experiment. Crystal size distributions were determined by optical microscopy. The growth rate of barium sulphate under the conditions of the experiments can be expressed by a quadratic function of supersaturation. The results, which suggest an interface rate-controlling mechanism, are discussed with respect to published data.     

Reorientation of Liquid Crystals by Superposed Optical and Quasistatic Electric Fields

The electric field of a CW laser beam can reorient a nematic liquid crystal. Experiments on the influence of a superposed quasistatic electric field are reported showing that this can reinforce or weaken the laser induced reorientation depending on the substance and geometry.     

Dynamic Behaviour of Ferroelectric Liquid Crystalline Thin Layers in Electric Fields

The direction of the spontaneous polarization of a thin ferroelectric liquid crystalline film can be switched up and down between two stable states by applying an electric field. This process is investigated theoretically using the elastic continuum theory of liquid crystals. There are found two reorientation mechanisms with different switching rates. The regimes of fast bulk switching and relatively slow domain wall motion are separated by a critical threshold field.     

Determination of the Cholesteric Screw Sense

If a cholesteric liquid crystal is placed between a glass plate with planar alignment and a spherical lens with concentric surface alignment, a double spiral disclination line appears. The spiral rotation sense depends on the helix screw sense of the cholesteric phase and the radial distance between the disclination lines on the pitch magnitude. A theoretical calculation of the form of the spiral disclination lines is given and compared with experimental results.     

Periodic Structure in Pblg-Dioxane System Under Electric Field

Periodic patterns were observed in PBLG-dioxane system under an electric field which was higher than the threshold field of the isotropic-nematic transition. The size of the well oriented zone became larger with the field strength, while the lattice constant of the pattern was independent of the electric field.     

Chemical CdS Thin-Film Deposition Influenced by External Electric and Magnetic Fields

Cadmium sulfide polycrystalline thin films were grown on glass substrates by the chemical-bath deposition technique (CBD) under application of both a pulsating direct electric and a dc magnetic field acting on the Cd- and S-ions during growth in an aqueous solution which contains CdCl₂ and thiourea as source material for the cadmium and sulfur components, respectively. The field orientation in each case of deposition was chosen perpendicular as well as parallel to the growing CdS-film interface. Two different values of the magnetic field strength were applied. Various properties of the films were studied, such as optical transmission and reflection, surface morphology, X-ray diffraction, and dark electrical resistivity, as a function of the applied field configurations, which reveal a clear effect of the field presence during growth.     

Growth Kinetics of Potassium-Dihydrogen Phosphate Crystals in Solution

Growth rate data of (101) faces of KDP crystals as a function of the relative supersaturation of the solution are analyzed on the basis of BURTON , CABRERA and FRANK surface-diffusion model applied to the growth from solution. The characteristic constants in the BCF function are determined for 50°C. Approximate values of the free activation energies for dehydration, desorption, surface diffusion and entering the kinks from the surface are calculated. The experimental data previously obtained by MULLIN and AMATAVIVADHANA and by BELOUET are also discussed.     

Influence of Electric and Magnetic Fields on the Molecular Alignment in EBBA

The effects of external de and af electric fields of strengths up to 25kV/cm on the nematic liquid crystal p-ehtoxybenzylidene-[pacute]-n-butyl aniline (EBBA) are investigated. The effectiveness of electric fields and of high magnetic fields in producing molecular alignment are compared. Changes in the molecular alignment are observed in terms of changes in the rf dielectric constant. These measurements are as fruitful as microwave measurements in studying the degree of ordering of molecules in bulk samples of liquid crystals. The threshold voltages for chevron formation in the dielectric regime are found to be considerably lower than the threshold voltages necessary induce changes in dielectric constant in the same regime.     

Behaviour of Ultrasonic Velocity in Cholesteric Liquid Crystals

Ultrasonic velocities (V) have been determined, employing a fixed path double crystal interferometer, in three cholesteric liquid crystals, namely cholesteryl propionate, cholesteryl laurate and cholesteryl myristate in their isotropic and anisotropic phases including the region of the phase transition. The variation of specific volume (v) is also studied in the same temperature range by a special dilatometer constructed for this purpose. Anomalous behaviour of ultrasonic velocity is observed near the isotropic-cholesteric phase transition in all three compounds. In the two polymesomorphic liquid crystals, namely cholesteryl laurate and cholesteryl myristate, contrary to the ultrasonic behaviour of cholesteryl stearate, prominent velocity dips are observed at cholesteric-smectic transition temperatures. The parameters adiabatic compressibility (β_ad) and molar sound velocity (R) are estimated and they are found to exhibit sudden jumps at cholesteric-smectic and isotropic-cholesteric transitions. The thermal expansion and temperature co-efficient of compressibility are found to show abnormal increase near the phase transition indicating the existence of large-magnitude pre-transitional effects near the phase transition. A comparative study of the ultrasonic behaviour of six aliphatic esters of cholesterol has shown that the magnitude of the velocity dip observed at the isotropic-cholesteric transition increased with increase of molecular weight and only cholesteryl acetate shows deviation.     

Infrared Spectroscopic Studies of Homologous Cholesteric Liquid Crystals

Abstract

Infrared (500–2500 cm^?1) spectra of cholesteryl alkyl carbonates (methyl, ethyl and oleyl) have been examined in the solid (cholesteryl methyl carbonate and cholesteryl ethyl carbonate) and liquid crystalline (cholesteryl oleyl carbonate) states. An attempt has been made to correlate the variation in the intensities and frequency shifts with the molecular structures. The shift in the carbonyl frequency in the case of cholesteryl oleyl carbonate may be correlated with the affect on the coplanarity with benzene ring.     

Kinetics of Scale Growth from Quiescent Solutions of Soluble Substances

The kinetics of scale growth on a cooling surface from quiescent aqueous solutions of oxalic acid, Na₂HPO₄ × 12 H₂O, KNO₃, Na₂SO₄ × 10 H₂O, and Na₃PO₄ × 12 H₂O saturated at 65, 32, 48.2, 31, and 39.2 °C, resp., was studied. The incurstations formed during 2 min of growth consist of loosely packed dendrites or needle-like crystals (the former 3 substances), or of tightly packed crystals forming a scale with a “smooth surface” (the latter 2 salts). The kinetic order of growth, g, assessed on the basis of experimental data was smaller than unity in all the studied cases; the g values of smooth incrustations were higher than those of the dendritic ones. It follows from the considerations concerning the possible mechanism of scale growth that the growth is controlled by a mononuculear mechanism. The experimental values of g compare reasonably well with the expected theoretical values.