Study of liquid crystals at the institute of crystallography

The history of studying liquid crystal at the Shubnikov Institute of Crystallography is briefly reviewed. The scientific results of researchers from the institute (starting from the 1960s and leading to the formation of a new field of science: physics of liquid crystals) are presented. In particular, it is shown how the physics of electro-hydrodynamic instability was developed in the 1970s. The origin of the physics of photonic crystals is considered. The discovery of the ferroelectric properties of ultrathin films of polar molecules is described. A breakthrough in the molecular theory of ferroelectricity in liquid crystals in the new century is discussed. The entire abundance of liquid-crystalline phases and processes occurring in them is estimated. An analysis of the participation of researchers from the Institute of Crystallography in the development of liquid-crystal science demonstrates their valuable contribution, which offers perspectives of new studies in this field.     

Numerical analysis of the velocity of SiC growth by the top seeding method

Velocity of crystal growth of SiC in a process of solution growth was studied on the basis of a global model of heat and mass transfer in conjunction with a phase diagram of the Si–C system. The growth rate was estimated by flux of carbon to a seed crystal. The results of calculation showed that growth velocity was increased when temperature of a seed crystal was increased. The temperature dependence of growth velocity was mainly determined by the phase diagram of the Si–C system, although the flow pattern was slightly modified by changing temperature distribution in the furnace.     

Effect of the temperature on the TMOS sono-hydrolysis and a study of the structure of the resulting aged sonogels

The effect of temperature on the oxalic acid catalyzed sono-hydrolysis of tetramethoxysilane (TMOS) was studied by means of a heat flux calorimetric method. The activation energy of the process was measured as (24.5 ± 0.8) kJ/mol in the temperature range between 10 and 50 °C. The structural characteristics of the resulting sonogels, after long period of aging in saturated conditions, were studied by means of small angle X-ray scattering. The structure can be described as formed by ∼2.7 nm mean size mass fractal-like aggregates (clusters) of primary silica particles of ∼0.3 nm mean size, all imbibed in a liquid phase. The average mass fractal dimension of the clusters was found to be 2.58. The primary particle density was estimated as 2.23 g/cm³, in good agreement with the value frequently quoted for fused silica. The volume fraction of the clusters in the saturated sonogels was estimated as about 28%. The moment in which the meniscus of the liquid phase penetrates into the clusters under rapid evaporation process has been detected by an inflection in the first derivative of the curve of weight loss in a simple thermogravimetric test.     

Levitation of metallic melt by using the simultaneous imposition of the alternating and the static magnetic fields

This study developed a new levitation method, which used the simultaneous imposition of static and alternating magnetic fields. Dynamic behavior was measured for pure Cu and pure Ni melts levitated by the proposed method. The oscillation due to surface tension and convection in levitated Cu melts were hardly observed at static magnetic fields exceeding 1 T. Only the rotation of this axis parallel to the static magnetic field was observed under high static magnetic fields. The proposed method demonstrated that metallic melt could be statically levitated like a solid sphere. It was also found that stable levitation of paramagnetic Ni melt was rather difficult at static magnetic fields exceeding 5 T, because of the magnetization force.     

Measurement of the refractive index of glass fibers by the Christiansen-Shelyubskii method

The Christiansen principle was employed to measure the refractive index of borosilicate glass fibers (13–41 μm diameter) over the visible range. The refractive index for glass fibers at 589.3 nm was measured by temperature and wavelength scan and values obtained were in close agreement. The refractive index for glass fibers as a function of wavelength was measured to an accuracy of < 10⁻⁴. The uniformity of the refractive index for a bundle of fibers of slightly different diameter was calculated using the modified Shelyubskii method and compared to experimental values. Theoretical calculations of the transmission by the present work suggest that, for high optical clarity and transmission of Christiansen cell (or transparent composite consisting of glass fiber and polymer), the refractive index must be controlled to the fifth decimal place. For example, the maximum transmission of a fiber/liquid mixture cell at 25°C can increase from 89 to 97% when the standard deviation is reduced from 13 × 10⁻⁵ to 9 × 10⁻⁵.     

A novel sol-gel method for the synthesis of γ-aluminium oxide: development of the sol-gel transformation and characterization of the xerogel

A novel method for the synthesis of aluminium oxide gel has been developed, whereby the sol-gel transformation was investigated. Aluminium tri-sec-butoxide was used as precursor while acetone was chosen as solvent. The synthesis was carried out in a special reactor, which allowed the dosing of steam. ²⁷Al NMR spectroscopy showed that during the sol-gel process the signal at δ∼3 ppm increases strongly corresponding to the formation of hexacoordinated aluminium species. Beside hydrolysis and condensation reactions, the coordination of acetone to a strong Lewis acid aluminium site occurs, which was shown by FTIR and ²⁷Al NMR spectroscopy. Viscosimetric analysis showed that at the beginning of the sol-gel process short polymers are observed while before the gelation a three-dimensional polymer network is formed. After pyrolyzing the gel a high surface area γ-aluminium oxide xerogel was formed. The effect of heating on the morphology and structure was examined by nitrogen physisorption (BET and pore size distribution), XRD and ²⁷Al MAS NMR spectroscopy.     

Study of the metastable region in the growth of GaN using the Na flux method

It was revealed that the metastable region, in which liquid phase epitaxy (LPE) of GaN single crystals proceeds without the generation of polycrystals, expands with growth temperature in the Na flux method. The metastable region appears when LPE growth proceeds at temperatures above 1073 K, although generation of polycrystals inevitably occurs on a crucible at temperatures less than 1073 K. The highest growth rate of 14 μm/h in a small experimental setup was achieved at a temperature of 1163 K with a nitrogen pressure of 5.5 MPa due to complete suppression of the growth of polycrystals on a crucible, even though the supersaturation at this condition reached a fairly high level.Also, an LPE crystal with a flat surface could easily be obtained under high-temperature conditions.     

Molecular dynamics study of the influence of mobile cations on the reconstruction of an irradiated silicate glass

The effect of ballistic collisions in a simplified nuclear glass was investigated by molecular dynamics. Systematic results were obtained in the 0–16 keV energy range. Following a damage peak, reconstruction of the glass structure was observed in terms of the overall degree of polymerization. The reconstruction was facilitated by the presence of mobile cations. The dynamics of restoration of the SiO₄ tetrahedrons during displacement cascades and during the formation of a structure from a random configuration can be fitted to curves corresponding to the same analytic formula. This similarity allowed us to examine the influence of mobile cations (alkalis or alkaline earths) on the formation of SiO₄ entities in smaller systems formed from random configurations. The formation rate of SiO₄ tetrahedrons accelerates with the Na₂O or CaO percentage to reach a maximum rate above about 10 mol% Na₂O or CaO. This threshold corresponds to the disappearance in the glass structure of zones comprising a central bridging oxygen surrounded by two first-neighbor bridging oxygen rings. No mixed alkali effect was observed in systems containing both Na and Cs because the formation of SiO₄ does not require long-range diffusion of mobile cations.     

Measurement of Flexo-coefficients for Nematic LCs without the Influence of the Impurity Ions by Using the Chromatographic Isolation Method

The sum of the flexo-coefficients (e₁₁+e₃₃) was measured by the capacitance characteristic depending on the applied dc voltage in the HAN cell. The voltage for the minimum value of the capacitance was shifted by the influence of flexoelectric effect. One of the important problems for the evaluation for the flexo effect was the influence of impurity ions. Then, the chromatographic isolation phenomenon was used to separate impurity ions when the LC material was injected into the empty cell by the capillary action. The coefficient (e₁₁+e₃₃) of ZLI-4792 was measured, and a value of 26.0 pC/m was obtained.     

The effect of the translational symmetry of crystalline silicon on the structure of amorphous germanium in the interfacial region

The structure of an amorphous Ge layer near an interface with a Si(111) crystal was studied by quantitative high-resolution electron microscopy. It was found that the translational symmetry of a Si crystal leads to the crystal-like order in the positions of Ge atoms in the interfacial region, the width of which is about 1.4 nm. In this region, the average orientation of interatomic bonds tilted with respect to the interface compensates for the difference in the bond lengths in crystalline Si and amorphous Ge and is responsible for the tetragonal distortion of the most likely atomic positions.     

Dependence of the step velocity on its length on the (010) face of the orthorhombic lysozyme crystal

The length of a two-dimensional critical nucleus has been measured, and the Gibbs-Thomson formula has been experimentally verified on orthorhombic lysozyme crystals. The step velocity was found to be independent of its length. The critical length initiating the step motion can be determined by a low density of kinks on a step, and not the critical-nucleus size. The invalidity of the Gibbs-Thomson formula in this case is discussed.     

Applications of the rheo-dielectric technique

An apparatus for dynamical mechanic measurements was modified in order to allow broadband dielectric spectroscopy measurements (10 mHz to 10 MHz) on systems under shear stress. Shear oscillatory deformation was applied to different systems under conditions of controlled deformation amplitude, choosing different shear frequencies and amplitudes, including the range of the non-linear viscoelastic response. The plate–plate geometry was used, with the two metallic sample holders serving both as mechanical tools and as electrodes of a capacitor. The rheo-dielectric technique was used to study the effect of oscillatory shear on two classes of systems: nematic liquid crystals and polymeric melts.     

Application of virtual screening and molecular dynamics for the analysis of selectivity of inhibitors of HU proteins targeted to the DNA-recognition site

DNA-Binding HU proteins are essential for the maintenance of genomic DNA supercoiling and compaction in prokaryotic cells and are promising pharmacological targets for the design of new antibacterial agents. The virtual screening for low-molecular-weight compounds capable of specifically interacting with the DNA-recognition loop of the HU protein from the mycoplasma Spiroplasma melliferum was performed. The ability of the initially selected ligands to form stable complexes with the protein target was assessed by molecular dynamics simulation. One compound, which forms an unstable complex, was eliminated by means of a combination of computational methods, resulting in a decrease in the number of compounds that will pass to the experimental test phase. This approach can be used to solve a wide range of problems related to the search for and validation of low-molecular-weight inhibitors specific for a particular protein target.     

Effect of the counter-ion of the basification agent on the pore texture of organic and carbon xerogels

Organic and carbon xerogels were prepared by polycondensation of resorcinol with formaldehyde in water, followed by evaporative drying and, eventually, pyrolysis. The pH of the precursor’s solution was fixed at 6.0 in all cases by adding various hydroxides as basification agent. Three alkali metal hydroxides (LiOH, NaOH and KOH) and three alkaline earth metals hydroxides (Ca(OH)₂, Ba(OH)₂, Sr(OH)₂) were used. It was found that the pore texture of the organic and carbon xerogels is totally independent on the cation size, but depends on the charge and concentration of the counter-cation. Indeed, the pore size of the alkaline earth metal loaded samples is larger than that of the alkali metal-doped xerogels. As a matter of fact, to reach the same initial pH, the concentration in alkali metal hydroxide must be twice that of the alkaline earth metal base. The effect of ions on the pore texture was thus attributed to electrostatic effects on the microphase separation process that occurs prior to gelation.     

Evaluation of the highly coherent surface structure of the GaAs (411)A plane using scanning tunneling microscopy

We have observed the surface structure of (411)A GaAs by atomically resolved scanning tunneling microscopy for the first time. A highly ordered structure of straight atomic rows stretching along the [01 ] direction was observed. The structure was found to be composed of two kinds of alternately arranged rows with different contrasts. From this image, a surface structure model was proposed. It was also indicated that the surface roughness of a (411)A surface must be smaller than that of a (100) surface due to the corrugated surface structure in the former case.     

Microscopic study of the H2O vapor treatment of the silicon grain boundaries

We have proposed annealing in H₂O vapor as a new effective and low-cost method for passivating polycrystalline silicon grain boundaries and for improving the performance of poly-Si based devices. The effect of H₂O vapor treatment was experimentally found to differ from analogous anneal in nitrogen and hydrogen. Mechanism of the H₂O vapor treatment was studied by Kelvin force microscopy, used to measure the potential change at individual grain boundaries and point defects. The potential change was dependent on the grain boundary character and it correlated with the crystalline disorder and internal stress observed by microscopic Raman spectroscopy. Effect of H₂O vapor passivation was experimentally connected with the potential change at the grain boundaries before and after the treatment.     

High precision measurements of the temperature dependence of the sound velocity in selected liquid metals

The temperature dependence of the sound velocity in liquid lead, tin, bismuth and antimony has been measured with high precision (errors of less than 0.35%) by the ultrasonic pulse transmission technique. The measurements were performed from the melting temperature to approximately 1000 °C. A smooth temperature dependence of the sound velocity was found in liquid lead and antimony. In liquid lead, a linear dependence with a negative temperature coefficient was observed whereas for liquid antimony the sound velocity displays a distinct maximum. The high precision of the measurements enabled uncovering localized features in the temperature dependence of the sound velocity in liquid tin and bismuth. The measurements provide some information on the temperature dependent structure of the molten state of these four elements.     

Discussion on the mechanism of photodoping

The photo-enhanced reaction between metallic silver and vitreous chalcogenides is known as “photodoping”. Based on a series of experimental results, a model for photodoping was proposed. It was assumed in this model, that a junction barrier at the silver-chalcogenide interface worked for separating photocarriers. Holes are captured by metallic silver, and electrons are trapped by active or loosely bound chalcogen atoms after travel toward the interior of a glass layer. The Coulomb attraction field between ions thus formed is large enough to overcome the kinetic barrier in the process of silver diffusion. The square root dependence for the growth of the photodoped depth with exposure time has been explained in the light of this proposed model.     

Effect of the inclusion of a water soluble high molecular weight oligomer on the surface characteristics of silica gel

The surface characteristics of pure and polyglycol 4000 containing silica gels were investigated by nitrogen adsorption at ? 195°C and water adsorption at 35°C. The addition of polyglycol 4000 during the precipitation of silica gel led to a remarkable increase in the surface area as well as the pore volume and on the other hand a sharp drop in c-BET constant.Thermal treatment led to a variation in the surface characteristics of both pure and polyglycol containing silica gels, but in different ways. Inclusion of polyglycol 4000 led to a retardation of the dehydroxylation process that occurs at elevated temperature. The decrease in surface area at 400°C was much higher in polyglycol 4000 containing silica gel than in the pure simple, an effect which correlated with shrinkage of the polymer at high temperature. Slight increase in surface area at 500°C may be attributed to the phase change from β to α form. For the high content polyglycol sample the phase change effect was counteracted by the shrinkage of the polyglycol.Water vapor adsoprtion indicated that polyglycol 4000 containing silica gel was more lyophobic in character than the pure gel at all temperatures except at 200°C.