The Guest-Host Interaction in Liquid Crystals: 1. A New Fast Dichroic Shutter

A new fast optical shutter, based on the guest-host interaction, that uses a 2 mm optical path length, has been demonstrated. Using a suitable pleo-chroic dye as guest in a cyanobiphenyl nematic host it has been shown that the shutter can be turned on and off in a few milliseconds. The electro-optic cell uses transverse electric fields, and field strength, frequency and temperature studies have been made. It has been shown that both increases and decreases in transient light levels may be obtained by changing the polarisation state of the incident light.     

Metastable zonewidth, induction period and interfacial energy of zinc tris(thiourea) sulfate

Metastable zonewidth and induction period measurements on ZTS and ZTSP crystals are presented. The effect of pH on the solubility, metastable zonewidth and induction period has been studied. The effect of substitution of phosphate on the nucleation parameters has been presented. The interfacial energy for pure and phosphate mixed ZTS solutions at 30°C has also been estimated. There is a significant variation in the induction period and interfacial energy due to decrease in solution pH and phosphate substitution.     

Growth and characterization of Bis Thiourea Zinc Acetate (BTZA)

Single crystals of BTZA have been grown by low temperature solution growth method using slow cooling process at an optimized pH of 3.5. The grown crystals have been examined under an optical microscope to study its surface morphology. The morphological studies show that the growth takes place by spreading of growth layers. Formation of rectangular shaped etch pit on the as‐grown crystal has been explained in relation to the growth conditions. Single crystal X‐ray diffraction analysis has been carried out to confirm the monoclinic system. Transmission spectrum reveals that the crystal has a low UV cut off of 434.5 nm and has a transmittance of 100%. Dependence of micro hardness on load has been studied. Powder X‐ray Diffraction and FT‐IR have been carried out to characterize the grown crystals. BTZA forms metal – sulfur bond and has good optical transmission in the entire visible region, which is the essential requirement for a non‐linear crystal. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Light Scattering Experiments during Freezing and Melting

Anomalous light scattering has been observed at the solid-liquid interfaces of high purity systems during freezing as well as during melting. The model substances used are water, salol, cyclohexane and xenon. From light scattering experiments the existence of a layer of pre-ordered material at the solid-liquid interface is deduced. Light is scattered by long range fluctuations in this layer. The thickness of this layer is a few μM. The correlation length of the fluctuations in this layer has been determined to be ξ ≈ 260 nm. Dynamic and static light scattering experiments lead to the same correlation length. Xenon, salol and cyclohexane cover a range of a factor of 15 in viscosity at the melting temperature. Growth velocities have been varied by a factor of about 200. Thermal gradients at the solid-liquid interface have been varied in a wide range. No dependence of the correlation length on these parameters or on substance has been found in the experiments. The observations do not depend on the crystallographic orientation of the crystal surface nor on the orientation of the crystal surface relative to gravity. The interface-layer may be compared to the quasiliquid layer which has been found at crystal vapour interfaces at conditions close to the melting point. The existence of a pre-ordered layer in-between a crystal and its melt means that the crystal does not grow into the melt but into the pre-ordered layer.     

Melt Convection in a Czochralski Crucible

2d-axisymmetric of natural and forced convection in the melt of a Czochralski equipment has been performed. The influence of the flow on the shape of the interface has been studied.     

Electrocrystallization of silver: Growth kinetics of screw-dislocation-free (100) crystal faces

At given conditions, especially at higher supersaturation, the growth rate of a close packed, perfect crystal face depends on the formation rate of two-dimensional nuclei and on the propagation rate of the monoatomic layers. This multinuclear multilayer growth as well as the advancement rate of growth steps have been studied experimentally on electrocrystallization of silver. The advancement rate of mono- and polyatomic growth steps has been measured on screw dislocation-free (100) crystal faces. For low overvoltages a linear dependence of the rate on overvoltage has been found. A strong influence of the surface condition of the crystal face — “fresh” or “aged” on the step advancement rate has been established. It was also found that on a “fresh” surface mono- and polyatomic steps advance with the same rate. The average monoatomic step spacing of the polyatomic step has been determined. The kinetic constants of the step growth rate are established and a conclusion regarding the mechanism of electrolytic deposition of silver is drawn. The initial current—time curves were recorded on applying potentiostatic pulses on a perfect crystal face. The shape of these curves coincides very well with those theoretically calculated for the cases of multinuclear growth. On the basis of the theoretical dependences, one can determine from these curves the formation rate J of two-dimensional nuclei at a given overvoltage η since the rate of step advancement is known. A linear dependence of log J on 1/η has been established. The values of the pre-exponential term in Volmer's equation and the specific edge energy of the two-dimensional nucleus have been determined. The surface condition of the crystal face influences strongly also the process of two-dimensional nucleation.     

The influence of inhibitors on the growth of barium sulfate crystals in suspension: Scale prevention (II)

The inhibiting influence of four organic bisphosphonates with different substituents on the growth rate of barium sulfate crystals, suspended in a supersaturated solution, has been investigated. The growth process has been studied conductometrically. Plots of both the relative supersaturation versus time and the mean linear rate versus the growth affinity have been given. — The results have been interpreted using a hypothesis, stating that growth inhibition occurs due to simultaneous coordination of the cations and hydrogen bonding of the anions at the active growth spots on the crystal surface.     

DC conductivity in single and mixed alkali vanadophosphate glasses

Three different series of single and mixed alkali vanadium phosphate glasses have been prepared by a melt quench method. DSC studies were carried out on few of these samples and their glass transition temperatures were determined. The glass transition temperatures were found to decrease with alkali content in single alkali systems and increase with second alkali content in mixed alkali systems. The dc electrical conductivity has been measured as a function of temperature. The data has been analyzed in the light of Mott’s small polaron hopping model and activation energies were determined. In one set of single alkali glasses activation energies were found to increase with alkali content and in another set of single alkali systems a transition from predominantly electronic to ionic conduction has been observed above 0.16 mol fraction of alkali content. The mixed alkali glasses have shown higher activation energies and lower conductivities compared to single alkali doped glasses and this has been attributed to a mixed alkali influence on the electrical conduction in these systems.     

Factors influencing the periodic precipitation of BaMoO4 crystals in silica gel

Influence of electric and magnetic fields on the periodic precipitation of BaMoO₄ crystals in Silica gel has been discussed. The retarding effect of gel density on the advancing precipitation front has been qualitatively analysed. The role of impurity on the formation of rings has been investigated.     

Phase stability and ordering of nematogen at molecular level: A computer-aided modeling

The present article deals with the phase stability and ordering of nematogen, viz., 6-octyloxy-2-naphthylyl-4-octoxybenzoate (ONOB) at molecular level. A comparative picture has been given between molecular charge distribution, and phase stability based on AM1, PM3, CNDO, and MNDO methods. The modified Rayleigh–Schrodinger perturbation method along with multicentered-multipole expansion method has been employed to evaluate the long-range intermolecular interactions, while a “6-exp” potential function has been assumed for the short-range interactions. The total interaction energy values obtained through these computations have been used to calculate the probability of each configuration at room temperature, nematic-isotropic transition temperature, and above transition temperature using the Maxwell–Boltzmann formula. Further, the entropy of each configuration has been computed during the different modes of interactions. An effort has been made to develop a computational model at molecular level based on configurational entropy, and translational rigidity parameters introduced in this article.     

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.     

Growth of CaMoO4 and SrMoO4 crystals in silica gel media

Growth kinetics and characterization of calcium and strontium molybdate crystals grown in silica gel have been studied under a variety of parameters. The changes in nucleation characteristics, growth habit, quality of these crystals were carefully observed and are found directly related to pH of the medium. The profound influence of pH on spontaneous crystallization of CaMoO₄ crystals has been carefully investigated; and its crystallization range has been determined. The wide morphological change of SrMoO₄ with respect to pH variation has been studied. The quality of the crystals has been critically examined.     

PCAM: a multi-user facility-based protein crystallization apparatus for microgravity

A facility-based protein crystallization apparatus for microgravity (PCAM) has been constructed and flown on a series of Space Shuttle Missions. The hardware development was undertaken largely because of the many important examples of quality improvements gained from crystal growth in the diffusion-limited environment in space. The concept was based on the adaptation for microgravity of a commonly available crystallization tray to increase sample density, to facilitate co-investigator participation and to improve flight logistics and handling. A co-investigator group representing scientists from industry, academia, and government laboratories has been established. Microgravity applications of the hardware have produced improvements in a number of structure-based crystallographic studies and include examples of enabling research. Additionally, the facility has been used to support fundamental research in protein crystal growth which has delineated factors contributing to the effect of microgravity on the growth and quality of protein crystals.     

Studies on twin boundaries in calcite crystals

Natural single crystals of calcite have been cleaved along (100) planes and cleavage faces have been etched in 2% and 3% citric acid solutions. Etching produces twin boundaries oriented in 〈010〉 directions. The etch pits on the two sides of the twin boundary are oppositely oriented. It has been conjectured that the rows of pits might have been formed due to etching of dislocations on twin boundaries. One to one correspondence of twin boundaries has been established on matched cleavage faces. This is further confirmed by studying the induced twin regions produced on a (100) cleavage plane by indenting that plane itself. The implications are discussed.     

Thermal effects in low-temperature deformation: The response to strain rate changes

The response of a deforming crystal to an abrupt strain rate change has been considered in the framework of the thermal model for low temperature anomalies of plastic deformation. Various modes of the deformation behaviour following strain rate changes have been predicted depending on the location of the point representing the state of the specimen in the plane of control parameters. A quantitive analysis of two characteristic modes of the deformation behaviour experimentally observed by KOMNIK and DEMIRSKI has been carried out with the aid of computer calculations and a good agreement with their data has been found.     

Electrical conductivity of new zinc phosphate glass/metal composites

Zinc phosphate–glass/metal composites have been successfully prepared. Glass with composition of 45 mol% ZnO–55 mol% P₂O₅ (ZP) has been filled with metallic powders (nickel and cobalt). The glass matrix thermal stability has been assessed by differential thermal analysis technique. The morphology has been examined by scanning electronic microscopy, showing almost homogenous composites. Comparison between the measured and calculated densities as a function of metallic content exhibits a good coherence and allows the estimation of porosity inside the composites. X-ray diffraction analysis has revealed that the ZP-matrix phase is amorphous when the temperature treatment is below the glass transition temperature T_g. However, the principal peaks observed in the case of the composites have been assigned to the metallic crystals of nickel or cobalt fillers. It has been found that the phosphate glass phase is not affected by the growing of the metallic network. The electronic conductivity measurements versus filler volume fraction have been investigated for the first time on phosphate–glass/metal composites. This study has shown the occurrence of a conducting transition at around 30% filler volume fraction. The obtained result has been interpreted on the basis of the statistical percolation theory frame.     

Kinetics of dissolution of potassium alum crystals in a flow cell in the presence of Fe++ cations

Dissolution rates of the {111} face of potassium alum crystals have been measured in situ in a flow cell. Dependence of the mass dissolution fluxes on the subsaturation and on the presence of Fe⁺⁺ cations has been studied. No influence of these cations over the concentration range 0–165 ppm on the dissolution kinetics has been observed. In addition, the dissolution process has been found to be of a purely diffusive character.     

Crystal growth of potassium perchlorate in gelatin gels

The suitability of gelatin gel for the growth of good quality single crystals of potassium perchlorate has been investigated. Growth in a more rigid and simple apparatus has been accomplished and described here. The grown crystals were identified by X-ray analysis. A study of the relation of crystal morphology to concentration of feed solutions and growth temperature has also been made. Studies made on chemical etching of the cleavages have revealed that the crystals are relatively perfect. At times, large liquid inclusions which introduce stresses in the crystals are responsible for a large dislocation concentration.     

The structure of star-branched copolymers. A Monte Carlo study

A coarse-grained model of a many-arm star-branched polymer chain has been studied. The chains were flexible heteropolymers built of two kinds of united atoms (segments) – hydrophobic and hydrophilic. The positions of segments were restricted to the vertices of a [310] hybrid lattice. The force field consisted of the softened excluded volume and the long-distance potential between a pair of non-bonded segments. The properties of these macromolecular models were studied in a wide range of temperatures from random coil to dense globule states. Monte Carlo simulations were performed using an algorithm based on the chain’s local changes of conformation and employing the Replica Exchange technique. The influence of the polymer length, the sequence of segments in the chain and the temperature on the dimension and the structure of chains have been studied. It has been shown that the process of the heteropolymer chain collapse to low-temperature structures is a complicated phenomenon and a possible explanation of this behavior has been discussed.     

Heat and Momentum Transfer Numerical Analysis in a Vertical Bridgman Growth System

In this work the momentum and heat transfer on a Bridgman system for the growth of GaSb has been studied. The main objective was to obtain some information about the role of the different processes like conduction, radiation and convective effects both in the melted material and the surrounding environment. These simulations are based on a 2D axi‐symmetrical model using a finite element method based code. The simulations have been carried out both in steady and transient states. It has been demonstrated that the consideration of a moving environment is important in the distribution of temperatures. The effects of the variations of thermal conductivities and emisivities on the thermal and velocity fields have been investigated. The results show that the key parameters are the thermal conductivities of the different materials present in the system, which produce significant changes in the convective flows inside the melt.