Increasing the lasing efficiency in cholesteric liquid-crystal photonic structures

Two types of lasing in cholesteric liquid crystals (LCs) in the range of luminescence of laser dye molecules have been investigated. The first type belongs to the Bragg modes at the photonic band edge, which propagate along the normal to the LC layer. The second type of lasing is related to the modes leaking into the substrate and propagating at small angles to the LC layer. It is shown that the Bragg lasing efficiency can be significantly increased under wide-aperture optical pumping. The method proposed for increasing the lasing efficiency is based on suppressing the excitation of leaky laser modes using partially absorbing thin films as the coatings for LC-orienting substrates. Both experimental results and the theoretical model of the effect using the numerical simulation data are discussed.     

Increasing the growth velocity of coupled eutectics in directional solidification of off-eutectic alloys

A simple and effective approach of employing an abrupt increase in growth velocity is presented to obtain entirely coupled eutectics in directional solidification of off-eutectic alloys. Using the proposed approach, the velocity required for the growth of entirely coupled eutectics in off-eutectic alloys can be increased remarkably. In directionally solidified Al–40 wt% Cu hypereutectic alloys with a thermal gradient of 250 K/cm, primary Al₂Cu dendrites appeared at constant growth velocities ranging from 5 to 100 μm/s and entirely coupled eutectics formed at 1 μm/s. Employing the abrupt increase in growth velocity, the growth velocity of purely coupled eutectics could attain 100 μm/s.     

The Hydrothermal Synthesis of Petalite Mineral Phases and their Morphological Changing by Increasing of Temperature

In this investigation the Petalite minerals were synthesized hydrothermally at different temperatures from a glass with the composition Li₂CO₃:Al₂O₃: SiO₂ and the relation 1: 1: 8. On that occasion the following mineral phases arised:

• at low temperatures (410°C) α-petalite (unit cell å = 11.737 Å/b̊ = 5.17 Å/c̊ = 7.63 Å, space group P₂/a);
• At 490 °C α-petalite was splitted to quartz and an orthorhombic β₁-petalite (unit cell å = 18.24 Å/c̊ = 10.54 Å/c̊ = 10.57 Å, space group P_mC_n). β₁-petalite could be observated until 1000 °C.
• at 560 °C tetragonal β₂-petalite (unit cell å = 7.50 Å/c̊ = 9.07 Å, space group P 4₃2₁2) in parallel with β₁-petalite;
• Behind 750 °C β₂-petalite was not observed.
• Behind 900 °C a pseudohexagonal mineral phase developed from β₁-petalite. May be it is possible to indicate this phase as hexagonal (unit cell å = 5.217 Å/c̊ = 5.464 Å, space group P 6₂22 or P 6₄22). The author suggests to denote this phase as β₃-petalite.

     

Low frequency relaxation in liquid crystals in relation to structural relaxation in glass-formers

Liquid crystals (LC) are the state of matter intermediate between isotropic liquids and the crystalline state. LC-forming molecules have strongly anisotropic shapes (rod-like in most cases). This leads to an interaction potential that consists of distance-dependent and orientation-dependent parts. Rotational dynamics of LC molecules falls into two frequency regions. Rotations about the short axes are strongly hindered by the potential barrier and thus coupled to fluctuations of the molecular centers of mass. This in turn causes these longitudinal or “flip-flop” motions, characterized by a relatively large relaxation time τ_||, to exhibit considerable temperature, pressure and volume dependences. Experimental relaxation times determined to date for various LC phases (nematic, smectic A, C, and E) for different thermodynamic conditions (isobaric, isothermal and isochoric) are discussed herein, adopting the formulae applied for characterization of the structural relaxation times of glass-formers (GF). This analysis appears fruitful; in particular, the strength parameter characterizing the steepness of the interaction potential can be determined from the relaxation times, and τ_|| is independent of temperature and pressure along the nematic-isotropic transition line, similar to the behavior of the structural relaxation time along certain transitions in GFs.     

Investigation of Boron Distribution in Solids Demonstrated in Nickel and in Silica Glass

By means solid state track autoradiography has been demonstrated that boron is enriched on grain boundaries of nickel and on texture boundaries of silica glass. This result has been proved on nickel by SIMA and on silica glass by luminescence autoradiography.     

Intermolecular Interactions in Crystalline Hydroxychloroquine Sulfate in Comparison with Those in Selected Antimalarial Drugs

Abstract The crystal structure of hydroxychloroquine sulfate (OHClQ) was determined in order to compare its conformation and intermolecular interactions to those in the crystalline chloroquine phosphate (ClQP) and quinine salicylate (QSal) monohydrate. The crystals of OHClQ are monoclinic with the space group P2₁/c and unit-cell dimensions: a = 10.4966(1) ?, b = 8.8056(1) ?, c = 21.8603(3) ?, β = 101.074(1)°. The quinoline antimalarial drugs may interact with their putative receptors by formation of characteristic hydrogen-bonded rings. The protonated nitrogen atoms and/or hydroxyl groups of the drug cation are proton donors, while the oxygen atoms of anions are proton acceptors. Water molecules may intermediate in these interactions. Graphical Abstract Hydroxychloroquine sulfate is a drug used in the treatment of malaria and rheumatic diseases. The X-ray structure analysis shows an important role of intermolecular hydrogen bonds in the crystal architecture. Comparison with chloroquine phosphate and quinine salicylate indicates that the organization of the drug cations is determined by the anions.      

凝胶体系中生物矿物生长的研究进展

近年来,生物矿化的模拟和仿生材料的研究十分引人注目.凝胶体系是模拟生物矿化的有效介质.本文综述了凝胶中羟基磷灰石、透钙磷石、磷酸钙等磷酸盐,文石、方解石、球霰石等碳酸钙,碳酸钡和草酸钙等生物矿物生长的研究进展,讨论了生长参数如凝胶种类、凝胶化学性质、吸附到凝胶表面的物质种类、介质pH、生长时间等对这些矿物晶体生长的影响,最后指出了该领域所面临的问题和将来的发展方向.     

Initial transient in Zn-doped InSb grown in microgravity

Three Zn-doped InSb crystals were directionally solidified under microgravity conditions at the International Space Station (ISS) Alpha. The distribution of the Zn was measured using SIMS. A short diffusion-controlled transient, typical for systems with k >1 was demonstrated. Static pressure of ∼4000 N/m² was imposed on the melt, to prevent bubble formation and de-wetting. Still, partial de-wetting has occurred in one experiment, and apparently has disturbed the diffusive transport of Zn in the melt.     

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.     

Convection in melts and impurity distribution in semiconductor crystals

Impurity distributions in semiconductor melts and crystals grown from these melts are experimentally and numerically studied on an example of Ga-doped Ge crystals. It is shown that inhomogeneous dopant distribution is observed in the form of striations and is caused by the convective flows in the melt and their nonstationary rearrangement in the vicinity of the crystallization front. The character of heat and mass transfer under the microgravity conditions is predicted. The necessity of precision experiments under terrestrial and, especially, space conditions is emphasized.     

Silicon induced in situ hole formation in optical fibers

Random hole optical fibers (RHOFs) represent a new class of holey fiber, which utilizes randomly sized and spaced porosity in the cladding region to confine light to the core region of the fiber. The porosity in the cladding region acts to lower the average refractive index relative to that of the core region. This paper presents studies on silicon induced in situ hole formation in optical fibers. Silicon powder was added to the optical fiber preform and during drawing of the optical fiber, the silicon powder caused the formation of bubbles which were subsequently drawn into tubes in the fiber.     

Water-network percolation transition in hydrated blue-green algae in vivo

We found that dc conductivity percolation process typical for low hydrated porous materials shows up in bulk viable blue-green algae, Arthrospira (Spirulina) platensis (strain Laporte 1963/M-132/2b) at unusually low hydrations, more than an order of magnitude lower than in, e.g., hydrated yeast [D. Sokolowska, A. Krol-Otwinowska, J.K. Moscicki, Phys. Rev. E 70 (2004) 052901]. The critical exponent is characteristic for two-dimensional network. Comparison with results for yeast and other similar materials shows that the hydration percolation threshold is a sensitive indicator of wettability of water accessible surface in porous bio- and abiotic materials.     

Bipolar-homogeneous structural phase transition in nematic droplets formed in the polymer matrix in a magnetic field

The phase transition from the bipolar structure to the homogeneous structure in droplets formed from the nematic liquid crystal 4-n-pentyl-4′-cyanobiphenyl in poly(vinyl butyral) in the presence of a magnetic field has been investigated. The phase transition is associated with the expansion of the regions with a tilted orientation of the director in the vicinity of defects (the disappearance of boojums) and with the alignment of the nematic director lines in the bulk of the droplet in the direction of the magnetic field. In the temperature range T = 24−34°C, cyclic mutual transformations between the bipolar configuration of the nematic director and the homogeneous structure, which have a period of ∼0.5–3.5 s and result from thermal fluctuations of the order parameter, are observed in droplets 3–15 μm in size after the forming field is switched off.     

Compositional trends in glass transition temperatures in inorganic network glasses

We present a simple mathematical model of glass transition based on the analysis of molecular agglomeration in overcooled liquids. With simple assumptions concerning local configurations and their and bonding energies, and with elementary combinatorics we are able to derive the dependence of the glass transition temperature on chemical composition in non-organic covalent glasses.     

Structural changes in MgO by activation in a planetary mill

Mechanical activation of MgO in a planetary mill results in structural changes comparable to those observed in a vibration mill. Under the given conditions they are, however, achieved in considerable less time. Furthermore, a so far unreported lattice dilatation could be observed. The activated MgO showed an increased reactivity with respect to the ambient atmosphere. As a result magnesium hydroxide and a basic magnesium carbonate are formed.     

Light Scattering in Nematic Liquid Crystals in Non-Equilibrium Steady States

Thermal fluctuations in nematic liquid crystals about a non-equilibrium stationary state due to an external temperature gradient or due to an external shear stress are theoretically investigated. The influence of the non-equilibrium situation on the light scattering spectrum (especially on the equilibrium Lorentzian due to transverse director fluctuations) is discussed.