Polycrystalline Fibers Research Articles

Roles of potassium ions, acetyl and l-glyceryl groups in native gellan double helix: an X-ray study

Native gellan, the natural form of the polysaccharide excreted by the bacterium Pseudomonas elodea, has a tetrasaccharide repeating unit that contains l-glyceral and acetate ester groups, and forms only weak and elastic gels. Based on X-ray diffraction data from well oriented and polycrystalline fibers of its potassium salt, the crystals structure of native gellan, including ions and water, has been determined and refined to a final R-value of 0.17. The molecule forms a half-staggered, parallel, double helix of pitch 5.68 nm which is stabilized by hydrogen bonds involving the hydroxymethyl groups in one chain and both carboxylate and glyceryl groups in the other. Two molecules are packed in an antiparallel fashion in a trigonal unit cell of side a = 1.65 nm. Although the gross molecular morphology and packing arrangements are isomorphous with those observed in the crystal structure of potassium gellan, which is devoid of any substitutions, native gellan exhibits exceptional changes in its ion binding characteristics with respect to gellan. In particular, the l-glyceryl groups do not allow the gellan-like coordinated interactions of the ions and the carboxylate groups, within and between double helices, which are necessary for strong gelation. These results at the molecular level explain, for the first time, the differences in the behavior of the polymer with and without substitutions.

Structure evolution in polycrystalline zirconium dioxide fibers

The reported results form a part of the complex investigations carried out at the Ukrainian Scientific-Research Institute of Refractories on the structure evolution processes in the polycrystalline fibers based on zirconium dioxide.

Effects of disorder on fibre diffraction patterns

Diffraction patterns from oriented polycrystalline specimens of fibrous polymers sometimes contain features indicating that the constituent microcrystallites are disordered. Such disorder affects the relationship between the crystal structure and the diffracted intensities. Therefore, the effects of disorder must be considered when determining structures from fibre diffraction data. Theory is developed here that describes the intensities diffracted by disordered polycrystalline fibres, and is used to calculate diffraction patterns from simulated specimens with various kinds of disorder. The results show how Bragg and continuous diffraction are distributed on such diffraction patterns, and implications for structure determination are discussed.

KRS‐5 Crystal Growth in Various Ambient Gases and Polycrystalline Fiber Preparation by an Extrusion Method

AbstractKRS‐5 single crystals have been grown in three kinds of ambient gases: air, Ar, and Ar + I2. The scattering centers in crystals were inspected and the metal impurities and oxygen content of the crystals were analysed. The polycrystalline fibers with 0.5–1 mm in diameter have been prepared by an extrusion method. The stability and attenuation of the fiber are discussed.

Infrared optical fibers for surgical applications.

The basic characteristics of fibers that are appropriate for surgical use in the infrared are reviewed. New fiber materials, such as sapphire, fluorozirconate and chalcogenide glasses, and polycrystalline fibers are discussed as well as their applicability for surgical procedures.

A re-examination of the crystal structure of A-DNA using fiber diffraction data.

Classical A-DNA helices with h = 0.25 nm may represent the greatest mass per unit length attainable by polynucleotide duplexes. The X-ray diffraction pattern from polycrystalline and well-oriented fibers of calf thymus DNA in its A-form has been carefully re-examined. Indexing on the basis of a C-face-centered monoclinic unit cell of dimensions a = 2.170 nm, b = 3.990 nm, c = 2.803 nm and beta = 96.82 degrees is superior to alternatives that have been proposed. Two right-handed. Watson-Crick base-paired, helical DNA chains with 2 X 11 nucleotides per 2.803 nm pitch, each carrying C3'-endo furanose rings, pass through the unit cell. The crystallography requires the two chains in the duplex to be antiparallel and conformationally identical but the 11 nucleotides in each pitch may be distinct. However, a secondary structure with a mononucleotide asymmetric unit provides as good an X-ray agreement as one with 11 distinct nucleotides. This relative lack of variability is quite different from what is observed in fibrous B-DNAs.

Crystal fibers of BiSrCaCuO materials grown by the laser floating zone method

The technological importance of good-quality fibers of the new high- T c superconducting oxide materials warrants their investigation through a variety of preparatory techniques. In particular, the laser floating zone (LFZ) growth method is very versatile for rapidly growing fibers of specific length, diameter and orientation. This report describes a study of the preparation, morphology and electrical properties of dense, oriented, LFZ-grown polycrystalline fibers derived from ceramic precursors in the BiCaSrCuO system. The features of the resulting materials are discussed on the basis of the procedural variables and chemical history of the polycrystalline precursors.

The morphological stability of polycrystalline fibers

Zirconium acetate (yttrium nitrate) precursor fibers, produced by dry spinning, were placed upon a single crystal Al 2O 3 substrate. Upon heat treatment, the precursor fibers pryrolyzed and densified, producing polycrystalline ZrO 2 (8 mol% Y 2O 3) fibers which were sintered to the substrate. Further heat treatment to induce grain growth produced the following sequential microstructural changes: 1. (a) grain growth proceeded until single grains spanned the fiber to produce a “bamboo” configuration, 2. (b) grain boundary grooves deepened and the surfaces between the boundaries became curved, and 3. (c) if the distance between grain boundaries was greater than 1.4 times the fiber diameter, the fiber broke into a row of separate grains. Thermodynamic calculations show that when the grain size to fiber diameter ratio exceeds a critical value, the break up of the fiber into individual grains lowers the free energy. These observations and calculations can be used to predict the conditions required to retain fiber morphology within composites during high temperature processing and use.

Absorption edges of mixed silver-halide crystals and polycrystalline optical fibers

Optical absorption edges of mixed AgClxBr1−x (0≤x≤1) crystals and polycrystalline fibers were investigated as a function of composition. Both visible edge, resulting from electronic transitions, and infrared edge due to multiphonon processes behave as the one-mode (amalgamation) type of mixed crystals, shifting continuously with composition. Quantitative expressions were used to discuss the results. Polycrystalline extruded fibers preserve this spectral window of silver-halide crystals, except for slight deviations which are explained by the small-grain structure of the fibers.

Optical and mechanical properties of infrared fibers

Optical and mechanical properties have been investigated for As/sub 2/S/sub 3/ and As/sub 2/Se/sub 3/ glass fibers and a KRS-5 polycrystalline fiber. A bending effect was serious for a KRS-5 fiber but not for chalcogenide fibers. In an aging test, a KRS-5 fiber showed a deterioration of transmissivity due to adsorption of moisture as well as plastic deformation of the crystal. Optical loss spectra of chalcogenide fibers were observed to change characteristically due to environmental factors such as humidity, high temperature, and ultraviolet radiation. The potential usefulness of these fibers is discussed. >

The role of microarchitecture and microstructure in the preservation of taxonomic closely related scleractinians

The scleractiniansAcropora palmata andAcropora cervicornis are common framebuilders in the various Pleistocene fringing reefs of Barbados. Both exihit the same diagenetic fabrics, but the rate of diagenetic alteration varies considerably.A. cervicornis is dominated by dissolution with minor calcite precipitation or neomorphism. This leads to a drastic reduction of the fossil record in older terraces. In contrast,A. palmata still has relics of unaltered microstructure in the older reefs. This difference in record potential is a result of the genetically fixed diameter of the polycrystalline fibers comprising the spherulitic trabecular microstructure; these are distinctly thicker inA. palmata.

Pressure Dependence of the Optical Absorption Edge of KRS-5: An Interferometric Study

There has been much recent interest in infrared-transmitting materials because of their potential of becoming the next generation of optical communication fibers. One of these polycrystalline fibers, thallium bromo-iodide (commonly known as KRS-5), is considered to be a good candidate because it is both transparent down to 25 μm and, unlike fluoride glasses, is not hygroscopic. Moreover, it has been found that the long 10.6-μm wavelength is strongly absorbed by animal tissues and thus the combination of a CO2 laser with a KRS-5 fiber would be good choice for medical surgery. Usually, fibers of KRS-5 are extruded continuously from a high-pressure ram extrusion apparatus; thus it is important to understand the effect of pressure on the optical properties of KRS-5 fibers, particularly as pressure has been shown to induce a large red shift in the optical absorption edges of thallium halide salts. Furthermore, it is not clear whether the frequency shift is a reversible process in KRS-5 and thus whether the pressure used in the manufacturing process will have an impact on the transmitting characteristics of the final products.

Helical conformations of gellan gum

We have obtained X-ray diffraction patterns of high quality from oriented polycrystalline fibres of the extracellular microbial polysaccharide gellan gum. The diffraction from the lithium salt form indexes on a unit cell with dimensions a=b=1.56±0.03 nm, c (fibre axis)= 2.82±0.05 nm, α= β=90°, γ=120°, with strong meridional reflections observed on the 3rd, 6th and 9th layer lines. The chemical structure consists of a tetrasaccharide repeat: å3)-β- d-Glcp-(1å4)-β- d-GlcpA-(1å4)-β- d-Glcp-(1→4)-α- l-Rhap-(1→. We have examined molecular conformations consistent with this chemical structure, with the measured density of 1.47 g ml −1, the unit cell dimensions and the periodicity and symmetry deduced from the X-ray fibre diffraction patterns. The axial projection of 2.82/3=0.94 nm is unusually short for an unbranched tetrasaccharide repeat (less than half the theoretical maximum), so in addition to very contradicted single helices, a number of intertwined double helix structures capable of generating the appropriate symmetry and layer line spacing were investigated. Computer model building using a linked-atom least-squares (LALS) system yielded four stereochemically acceptable models for a single molecule: a right-handed (3 1 contracted single helix, right- and left-handed (6 1 and 6 5) contracted double helices and a left-handed (3 2) extended double helix. Examination of the packing of molecules in the unit cell eliminated both 6-fold double helix models. Even after extensive and detailed calculations, the best of each of the surviving models still yield relatively poor agreement between the measured and calculated structure factors. The reliability factors for 3 t single helices packed parallel and antiparallel, extended 3 2 double helices packed parallel and antiparallel were computed to be 0.86, 0.79, 0.71 and 0.65 respectively. This discrepancy between the measured and calculated structure factors can be related to the general distributio of diffracted intensity seen in the X-ray diffraction pattern. Most of the diffracted intensity is concentrated on or close to the equator, and remains so even when the lattice sampling is suppressed by reducing crystallinity whilst maintaining orientation. These observations suggest models with concentrations of electron density on planes closely parallel to the fibre axis, as would be expected from almost fully extended polysaccharide chains. Of all the models considered the extended 3 2 double helices, packed antiparallel, would be favored on general considerations and best fit with the measured X-ray intensities.

Infrared polycrystalline waveguides made of silver halides

Single crystals of AgCl and AgBr and of their solid solution KRS-13 were grown. The lowest absorption was exhibited by the solid solution: it amounted to 6×10−5cm−1 at 10.6μ and 1.4×10−4cm−1 at 5–6μ. An investigation of the edges of many-phonon absorption yielded estimates of the minimum intrinsic optical losses, which were 10−1–10−2 dB/km at wavelengths in the range 5–10μ. Extrusion was used in the preparation of polycrystalline fiber waveguides from AgCl, AgBr, and KRS-13. The optical losses at 10.6μ in the waveguides made of AgCl (0.5 dB/m) and in KRS-13 (1.3 kB/m) were governed by the absorption, whereas in the AgBr waveguide (3.5 dB/m) they were governed by the scattering. A polycrystalline KRS-13 waveguide was prepared for the first time and it exhibited a high breaking strength (~150 MPa and a low minimum radius of elastic bending (~5 cm).

Highly refractory heat-insulating articles based on polycrystalline oxide fibers (Review)

The authors discuss the production of polycrystalline fibers from highly refractory oxides, mainly alumina and zirconium dioxide which are the most readily available and the cheapest materials, but highly refractory fibers may be obtained also from other oxides. The fibers are obtained in the monoand polycrystalline forms and the preparation methods for polycrystalline highly refractory fibers are described in some detail. The properties for the main types are given. An addition of small amounts of alumina, polycrystalline fibers to mullite-siliceous glass fiber has a significant influence on the reduction in the shrinkage.

The applicability of DTA to the study of ceramic fibres

To determination of thermal properties of different ceramic fibres the DTA method has been applied. Characteristic temperatures of the glass fibres have been determined. As a result of the study of aluminosilicate fibres temperature of mullite crystallisation has been evaluated. Alumina spinel polycrystalline fibres were obtained from organic solutions and then they were heated to get ceramic structure. DTA and TG methods can be applied to the investigation of chemical processes during decomposition of organic component and forming of fibras ceramic structure.

Digital processing of X-ray diffraction patterns from oriented fibers

Abstract Accurate measurement of intensity data from x-ray diffraction patterns of oriented fibers is one of the major difficulties in the determination of polymer structures. Accurate diffraction data are particularly important when discrimination between different, but similar, molecular models is required. Although digital processing of diffraction patterns has provided more accurate data than was obtainable by traditional measurements, many of the methods used have been partly subjective and restricted to well-resolved diffraction patterns. Background is a major source of error in intensity measurement. A general method of two-dimensional background removal has been developed. Objective methods of measuring the intensities of Bragg reflections from x-ray diffraction patterns of polycrystal-line fibers are described. A profile fitting method (which is an extension of the previously reported angular deconvolution procedure) for measuring diffraction data from noncrystalline oriented polymers is also pre...

Transmission Measurement of Polycrystalline Silver Halide Fibres in the 1-11 μm Wavelength Region

The wavelength dependence of attenuation in polycrystalline silver halide fibres in the 1–11 w m wavelength range is reported. The results indicate that the total attenuation does not obey the u -4 dependence observed in silica, but is close to the u -2 dependence reported recently for other polycrystalline fibres.

Optical losses in polycrystalline thallium halide fibres at CO- and CO 2 -laser wavelengths

The scattering and absorption loss distribution along polycrystalline fibres from KRS-5 was measured by the laser calorimetry method. The minimal optical losses at 10.6 μm were 0.35 dB/m and were mainly caused by absorption. At 5–6 μm, absorption and scattering losses were equal and their sum was 1.6 dB/m.

Dermatan sulfate: Molecular conformations and interactions in the condensed state

The molecular conformations and manner of aggregation has been determined for three allomorphs of the connective tissue polysaccharide dermatan sulfate by analysis of X-ray diffraction from oriented, polycrystalline fibers of sodium salts. One allomorph is unique among glycosaminoglycans in having right-handed (8(3)) helical chains. Two such chains pack antiparallel in a tetragonal unit cell (a = b = 1.267 nm, c = 7.353 nm) with P4(3)2(1)2 space group symmetry. The 3(2) chains of the second allomorph are organized in a trigonal unit cell (a = b = 1.460 nm, c = 2.823 nm, space group symmetry P3(2)21) containing two left-handed antiparallel polysaccharide molecules. (The chirality of this allomorph has been assumed to be the same as in other 3-fold glycosaminoglycan helices, since discrimination between 3(1) and 3(2) symmetries was found not to be possible.) The archiral 2(1) helices of the third allomorph, pack probably in an orthorhombic unit cell (a = 1.151 nm, b = 1.065 nm, c = 1.878 nm, space group symmetry P2(1)2(1)2(1)) that contains again two antiparallel polymer molecules. Each dermatan sulfate chain is stabilized intramolecularly by O3-O5 hydrogen bonds across the beta (1 leads to 4) linkage. There are two intermolecular hydrogen bonds per tetrasaccharide repeat in the tetragonal structure and two per disaccharide in the trigonal structure. Fourier difference syntheses indicated equivalents of four sodium ions per tetrasaccharide and two sodium ions per disaccharide in the tetragonal and trigonal structures, respectively. The cations are either partially or fully hydrated and link dermatan sulfate chains either intra- or intermolecularly by involving besides other polyanion oxygen atoms, carboxylate and sulfate oxygen atoms. The probable mode of packing in the orthorhombic structure indicates a pair of hydrogen bonds between adjacent antiparallel polysaccharide chains and suggests plausible cationic sites in the unit cell.