Hexagonal Texture Research Articles

Restitution conditions of the parent cubic texture from the inherited hexagonal texture in the β–α phase transformation

Different factors calculated from theCcoefficients of the inherited orientation distribution function (ODF) allow us to check the importance of the variant selection. When no variant selection or a slight variant selection occurs by the cubic-to-hexagonal phase transformation, it is possible to calculate the ODF of the cubic parent phase present at high temperature from the ODF of the inherited hexagonal phase at room temperature. When a stronger variant selection occurs, qualitative information about the parent ODF can be obtained by using a specific correlation function, which we have named R(g).

Highly Ordered Parabolic Focal Conics in Lyotropic Systems

Parabolic focal conics develop at 10−20 wt % surfactant in the two-phase lamellar−isotropic regions of various systems (SDS−hexanol−decane−water, alkylpolyglucoside APG600−fatty alcohols pentanol, hexanol, decanol, dodecanol, tetradecanol, oleyl alcohol). Extended regular quadratic two-dimensional lattices are formed in microslides (0.05−0.4 mm thickness) when only a small amount (10−15 wt %) of the isotropic phase is present. Five different focal planes can be visualized with the polarization microscope. The midplane focus gives pictures which look like primitive two-dimensional lattices of equally sized vesicle droplets. The 1:1 mixtures give irregular pattern formation. Removal of the isotropic phase by a 20 min centrifugation at 3000g leads to ordinary focal conics. Parabolic focal conics seem to be formed when the isotropic phase and the coexisting lamellar phase have comparable compositions, and this leads to the assumption that both phases should be in a near-critical state. The formation of parabolic focal conics looks more like a spinodal demixing than a crystallization process with nucleation. The very regular parabolic focal conics are transient states which change to mosaic textures, which look very similar to hexagonal fan textures, whereas polycrystalline parabolic focal conics transform to ordinary focal conics after several days or weeks.

Ultramicrotomy of industrial alumina.

Industrial materials, such as alumina, often pose difficulties in their preparation for TEM examination. Composite and particulate materials are particularly difficult to prepare using conventional thinning techniques, i.e., ion beam and chemical jet thinning. Ultramicrotomy (UM) can be used to produce TEM specimens with a uniform thickness and an unaltered composition. Some crystalline materials, i.e., alumina hydrate, were difficult to section due to conflict between the cutting direction and cleavage planes. Sectioning was successful when these two directions were mutually parallel or perpendicular. At other orientations shattering occurred. Microcrystalline particulate materials, i.e., calcined alumina, were sectioned successfully in particles < 70 microns in diameter. The phases found in industrial alumina particles were gamma, delta, theta, and alpha alumina. Gamma alumina consisted of fine-grained, equiaxed crystallites. The selected area electron diffraction (SAED) patterns indicated poor crystallinity with a distinct hexagonal texture. Delta and theta alumina appeared as an undifferentiated intermediary microstructure of elongated grains. The SAED patterns indicated poor crystallinity, but without a distinct texture. Alpha alumina was found to be a coarse-grained crystalline phase with high diffraction contrast. SAED patterns consisted of fine, randomly oriented spots. Considerable variation was observed in the distribution of phases. In some specimens, discrete particles of gamma and alpha predominated. In others, particles were a mixture of phases representative of the bulk composition. To characterise these samples, TEM of numerous whole particles was required. Ultramicrotomy was the only preparation technique capable of producing such samples.

DNA Mesophases: A Structural Analysis in Polarizing and Electron Microscopy

Abstract In aqueous solution, pure DNA forms multiple liquid crystalline phases when the polymer concentration is increased. A structural analysis of the different phases and of their transitions is presented here in polarizing and in electron microscopy. The cholesteric phase presents usually periodic patterns with series of nested arches in electron microscopy. The defects of the structure can be analyzed with the resolution of the electron microscopy and compared to the data available in polarizing microscopy. We show how the germs of the more concentrated phase nucleate preferentially along some types of defects of the cholesteric phase. The concentrated phasg is columnar hexagonal and behaves as a lamellar structure. Each layer (25 to 40 A thick) is composed of DNA molecules aligned in parallel. Numerous screw dislocations, either left or right-handed, can be seen in the structure. Besides, the evolution of the hexagonal textures, induced by a progressive dehydration of the preparation, can be follow...

The growth of highly ordered textures in planar poly(γ-benzylglutamate) liquid crystals

Abstract We have observed the growth of highly ordered textures in homogeneously aligned poly(γ-benzyl-glutamate) (PBG) liquid crystals as the polymer concentration increases. The textures are similar to the hexagonal columnar textures exhibited by recent studies on highly concentrated solutions of helical biological polymers. In a geometry of homeotropic alignment, no evidence of such texture growth was seen, which is probably due to strong binding interactions between PBG polymers and the substrates.

Calculation of orientation factors for crystalline polymers from X-ray pole figure intensities

The characterization of polymer orientation is discussed by expanding the orientation distribution function (ODF) for the crystalline phase into a series of generalized spherical harmonics (GSH) or into a series of symmetric generalized spherical harmonics (SGSH). A transformation relation is derived between the two expansions. By means of this transformation the orientation factors (moments of the ODF) can be calculated from the SGSH series expansion coefficients. As an example, this transformation is demonstrated for crystals of hexagonal symmetry and fiber texture.

The ultrastructure of the midgut and the formation of peritrophic membranes in a harvestman, Phalangium opilio (Chelicerata Phalangida)

The ultrastructure of the midgut epithelium of Phalangium opilio was examined. In the anterior part of the midgut the epithelium consists of three different types of cells, called resorption, digestion, and excretion cells according to their presumed functions. Excretion cells may represent old digestion cells. The relation between resorption and digestion cells needs further investigation. The epithelium of the posterior part of the midgut consists of two types, transport and secretion cells, which seem to serve mainly for the resorption of water and the secretion of peritrophic membranes, respectively. Peritrophic membranes are secreted by the anterior midgut epithelium mainly in a period between 2 and 4 h after feeding. Chitin or chitin precursors could be localized in vesicles and in the brush border of midgut cells, and in the peritrophic membranes, using colloidal gold labelled with wheat germ agglutinin. Two different textures of chitin-containing microfibrils were found in the peritrophic membranes, either a random or a hexagonal texture. The latter results if the microfibrils polymerize between the basal parts of the microvilli. Irregularities of the hexagonal texture can be correlated with an irregular pattern of the microvilli. In the posterior midgut peritrophic membranes with a random texture, chitin-containing microfibrils are continuously secreted in the form of patches.

Effective conductivity of regular conducting channeling textures in two dimensions

Composite materials exist, which are composed of elongated conductors and granular insulators. For the effective conductivity of such materials, the ordinary effective medium theory and the random resistor network models cannot be efficiently applied. As an approach to the problem, we compute by the finite element method the effective conductivity of triangular, square, and hexagonal channeling textures, composed of continuous conductive channels and islands of insulators. As shown by the results, the effective conductivities of the three patterns are nearly equal when the channel widths of the textures are the same. And compared with a granular conductor-insulator composite material, the three textures have a very conductive geometrical configuration, because neither island nor dead end conducting regions exist in the three patterns. If we accumulate such data for the effective conductivity of materials with a definite composite structure, we can predict the outline of the geometrical configuration of a composite material or inhomogeneous medium by measurement of the conductivity and volume components.

The saturation magnetostriction of thin polycrystalline films of iron, cobalt, and nickel

The saturation magnetostriction λs of Fe, Co, and Ni films was measured by the cantilevered substrate technique. The values of λs, −6.8×10−6 and −38×10−6, obtained for Fe and Ni, respectively, are in accord with bulk published values. A unique value of λs could not be obtained for Co because the sputtered or evaporated Co films were admixtures of face-centered-cubic and hexagonal close packed phases and the magnitude and sign of λ varied with the fcc content. Increasing the substrate temperatures during film deposition decreases the fcc content; however, this also caused the development of a C-axis hexagonal texture normal to the film plane. Annealing the films at ∼500 °C caused a similar but more pronounced texture. Then, because of the uniaxial anisotropy of Co, the films could not be magnetically saturated and the magnetostrictive behavior changed; i.e., the transverse and longitudinal λ were both negative and in some instances the transverse λ was larger than the longitudinal λ.

Crystallography of CdTe layers on CdS grown by chemical vapor transport

The crystal quality of CdTe layers grown on single-crystalline CdS by chemical vapor deposition is evaluated. Different x-ray techniques have been applied to investigate the crystal perfection and defects. Good CdTe layers grow in the sphalerite phase with strong twinning about the [111̄] growth direction. No mixture of cubic and hexagonal phases, polytypes, stacking faults, and fiber texture are observed. Imperfect layers contain polycrystalline areas.

Feinstruktur peritrophischer membranen

The peritrophic membranes, which is several groups of animals are produced by midgut epithelia, have been investigated most thoroughly in insects. The following results concern Crustacea: In the peritrophic membranes chitin containing microfibrils are embedded in the ground substance consisting of proteins and mucopolysaccharides. In addition to a felt-like texture (“Streuungstextur”), microfibrils are arranged in textures of higher order (orthogonal texture — “Gittertexture”; hexagonal texture — “Wabentexture”).

The identification of close-packed hexagonal nickel textures in nickel electrodeposits. A critique

The identification of close-packed hexagonal nickel textures in nickel electrodeposits. A critique