Distinct Structural Types Research Articles

Liquid crystals of disc-like molecules

Liquid crystals of disc-like molecules fall into two distinct structural types, the columnar and the nematic. The columnar phase in its simplest form consists of discs stacked aperiodically in columns, the different columns constituting a two-dimensional array, whereas the nematic phase is an orientationally ordered arrangement of discs without any long-range translational order. After a brief review of the structures of the various mesophases, two topics are considered in somewhat greater detail. (i) An extension of McMillan’s mean field model of smectic A to liquid crystals of disc-like molecules. The translational order is now assumed to be periodic in two dimensions. Calculations show that when the lattice is hexagonal, or departs from it only slightly, the transition from the columnar to the isotropic phase may take place either directly or via a nematic phase, depending on the model potential parameters Interpreting a to be a measure of the chain lengths as in McMillan's model, the theoretical phase diagram is shown to be in broad agreement with the experimental trends. As the asymmetry of the lattice is increased the theory predicts the occurrence of a smectic A phase as well. The new smectic A phase is biaxial. (ii) A theoretical study of fluctuations in the columnar liquid crystal. It is shown that the Frank elasticity of the liquid-like columns stabilizes the two-dimensional order, a result that was in fact envisaged by Peierls and Landau in the 1930s. The meansquare fluctuation of the lattice as well as the Debye—Waller factor show a certain dependence on the linear dimensions of the sample.

Anglican Parish of Blenheim: strengthening to the Church of the Nativity

The Church of the Nativity in Blenheim is in two distinct structural types. The nave, narthex and tower are of brick veneered reinforced concrete and steel portal frames constructed about 1959 to replace an old timber building. Check calculations showed this to generally meet latest code requirements.
 The transept and chancel was, prior to the strengthening work, of unreinforced brick masonry built about 1906 - with some reinforced buttresses and reinforced concrete bands. The whole building is roofed with concrete tiles.
 The initial assessment of the building was made in terms of Clause 301A of the Municipal Corporation Act in 1976.
 This building is regarded as one of Blenheim's finest older buildings and the members of the Church did not wish to lose, or detract from, its character, either by obvious strengthening or rebuilding.

Metal (II) hexafluoroarsenates: Preparation and vibrational spectroscopy of MF 2·2AsF 5 (M = Mg, Ca, Sr, Ba, Mn, Co, Ni, Cd, Hg, Pb) adducts [1

Adducts of the type MF 2·2AsF 5 (M = Mg, Ca, Sr, Ba, Mn, Co, Ni, Cd, Hg, Pb) have been prepared by reaction of the metal difluorides with AsF 5 in anhydrous HF at room temperature. Vibrational spectroscopic studies and X-ray powder diffraction patterns of the solids indicate that adducts of three distinct structural types occur. The differences in structure have been rationalized in terms of the fluoride ion donor abilities of the metal difluorides.

Atomic and solid-state effects in the angle resolved photoemission cross section from an adsorbate: 4d levels of c(2 × 2)Te on Ni

The photon energy dependence of the normal emission from the Te - 4d levels of an ordered Te c(2 × 2) overlayer on Ni(100) shows two distinct types of structure. The main feature is due to an atomic effect, a delayed onset, a peak followed by a Copper minimum. Superimposed on this atomic profile are intensity modulations of 30 to 40% due to coherent scattering from the substrate. The atomic contribution is in qualitative agreement with theoretical calculations, while the modulations due to solid-state effects are not reproduced by a LEED final state type calculation.

An ultrastructural study of the neurosecretory canopy cell of the pond snail Lymnaea stagnalis (L.), with the use of the horseradish peroxidase tracer technique.

The paired, electrotonically coupled neurosecretory Canopy Cells (CC) of the pond snail Lymnaea stagnalis were microiontophoretically injected with horseradish peroxidase (HRP). Whole mount preparations and ultrathin sections of injected CC were studied to describe in detail the morphology of the CC, their axon tracts and neurohaemal areas. The CC release their secretory product at three different sites, viz. from the soma and from axon terminals in the intercerebral commissure and in the median lip nerve. Neural control over the CC occurs by few synapses found exclusively on the CC axon, not on the cell body. One type of "en passant" synapse was identified. Two morphologically distinct types of synapselike structures (SLS) are numerous. The site of electrotonic coupling between the two CC is most probably located in the cerebral commissure. Serial sectioning showed that the axons contact each other over more than 130 micrometers. The contact is very intimate and the axon membranes interdigitate in a complex manner. Gap junctions, which are often described as the sites of electrotonic coupling, were not found.

Evidence of an extra‐terrestrial source for the mesospheric sodium layer

Lidar observations of the mesospheric sodium layer, made at São José dos Campos (23°S, 46°W), show three distinct types of organized structure in the vertical distribution of sodium. Profiles averaged over several days show a smooth but asymmetrical distribution. A sequence of profiles for a given night normally shows a wavelike structure which descends through the layer with time. Very occasionally an extremely narrow peak is observed at a constant height for several hours. On one occasion a layer 2.5 km wide, with scale heights of 700 m on the bottomside and 900 m on the topside, was observed to persist at a constant height for three hours. It is concluded that such a layer could neither be produced by neutral density perturbations nor by photochemical processes and therefore indicates a source of sodium in the mesosphere.

Wall structure changes in low-loss magnetic bubble materials

Transitions between underdamped and overdamped radial motion in magnetic bubble domains are investigated in a low-loss rare-earth garnet material. Three distinct types of domain wall structures, which are present during underdamped motion, have been identified. Bubble walls were subjected to a bias field pulse ( H ) and tested for underdamped motion sometime (τ) later. The first type of structure follows the form H = H' \exp (\tau/\tau_{0}) , with 170 nsec nsec for the first transition, and is not statically stable. Transitions associated with the second type are characterized by a constant critical angle ( \bar{\Psi}_{c} ) between the magnetization in the middle of the wall, and the plane of the wall. For the first transition, \bar{\Psi}_{c} = 230\deg , and for the second, \bar{\Psi}_{c} = 370\deg . These structures are statically stable. The third type of structure is not statically stable, and H is independent of τ. The first and second wall structure types are associated with multiple transitions while the third only exhibits a single transition.

Grain boundary segregation phenomena observed by gold decoration of NaCl bicrystals

Examination by electron microscope of gold decorated surfaces near grain boundaries in nominally pure NaCl bicrystals reveals features that derive directly from the presence of the boundary: a central strip along the boundary, and isolated structures scattered on either side, within which nuclei density is larger than normal. Two distinct types of structure are identified as second-phase precipitates and pre-precipitation segregates of the same residual divalent impurity. The segregates can account for previously observed enhanced grain boundary diffusion.

Décomposition thermique du monoxide de carbone sur une surface de molybdène: Formation de couches superficielles de carbone et de carbure

We have investigated the decomposition of carbon monoxide on polycrystalline and (001), (110) monocrystalline molybdenum surfaces. This study was performed by massspectrometry, for thermal desorption studies, Auger electron spectrometry (AES), low energy electron diffraction (LEED) and photoelectron spectroscopy (ESCA). By heating the clean Mo surface in CO or by heating the Mo surface covered with CO, the dissociation of chemisorbed CO leads to a build-up of carbon layer which inhibits the subsequent adsorption. Two distinct types of fine structure are associated with the KLL line of carbon Auger spectra. If the Mo surface is heated at a temperature between 300 and 1500 K, the Auger peak is characteristic of a “graphite layer”. If the Mo surface is heated at a temperature up to 2000 K, the Auger peak is characteristic of a “carbure” layer. This “carbure layer” give rise to a surstructure which agrees with a Mo 2C surface layer and was also investigated by ESCA. Chemical shifts of (1s) C and (3d) Mo photoemission bands were observed and attributed to the bounding between Mo and C atoms in the Mo 2C layer.

Crustal Structure of Baffin Bay from the Earthquake-generated Lg Phase

The Lg phase generated by earthquakes is used to infer the gross crustal structure of Baffin Bay. The principle of interpretation is that the presence of the Lg phase on an earthquake record is sufficient proof of the existence of continental crustal structure along the entire propagation path, while the absence of the Lg phase is a consequence but not sufficient proof of the existence of a section of oceanic crust somewhere along the propagation path. Examples are given of earthquake records involving propagation paths through three distinct types of gross crustal structure, namely shield-type continental, intermediate-type continental, and oceanic. These examples are used to interpret the records at four seismic observatories located around Baffin Bay for eleven earthquakes, which occurred in and around the Bay in the years 1965 to 1967. The records reveal two areas of Baffin Bay that appear to have oceanic crust, one in the northern part of the Bay at the entrance to Nares Strait, and one in the deep central part of the Bay separated by a bridge of continental crust from Cape York on Greenland to Cape Macculloch on Baffin Island.

Metallic particles of high cobalt content in Apollo 15 soil samples

Single phase alpha-kamacite containing more than 3.2 wt % Co and gamma-taenite containing from 30 to 60 wt % Ni from the Apollo 15 soils - 15031, 15071, 15081, 15261, and 15271 - have been examined by metallographic and electron microprobe techniques. In addition two phase alpha + gamma particles from soils 14003, 15071, 15261, and 15271 with Ni and Co contents well outside the meteoritic range have also been examined. Two distinct types of alpha-gamma structure occur, one analogous to 'clear taenite' in ordinary chondrites, and the other analogous to a 'Widmanstaetten' structure in Ni-rich ataxites. The measured Ni gradients in the two-phase particles are very similar to those meteorites having the same structure. However the Co content is much higher than the meteoritic samples, up to 12 wt % in the alpha phase. Approximate phase equilibria data for the Fe-Ni-Co system indicate equilibration of the two-phase particles during cooling to approximately 350 C. Estimates of cooling rates and second-phase growth times indicate that the maximum time necessary for the development of the high-Co two-phase structures is roughly 25 to 100 m.y. These estimates argue for the development of the two-phase structures during formation of the lunar crust, at a depth of 10 to 20 km beneath the moon's surface.

Histology and ultrastructure of the tube foot epithelium in the phanerozonian starfish, Astropecten

The pointed tube feet of the primitive phanerozonian starfish Astropecten were examined by both light and electron microscopy. The cuticle overlying the whole tube foot is composed, in part, of acidic mucopolysaccharides, and ultra structurally consists of two layers: an inner layer of loosely packed fibers and a more dense fibrous outer layer. The outer epithelium of the tube foot tip possesses at least four distinct structural cell types: ciliated cells, vacuolated cells, cells containing small, dense spherical granules (80-160 mμ ), and cells with long rows of dense oval secretory granules (500 x 800 mμ The larger secretory granules are composed of both neutral and acidic mucopolysaccharides and lack the complex substructure of similar secretory granules in the suckered tube feet of more advanced starfish. The possible functions of the cuticle and cell types as well as the relationship between the ultrastructure, chemical nature, and function of the large secretory granules are discussed.

Auger spectroscopy of carbon on nickel

The Auger spectrum of almost clean nickel is presented, in which twelve Auger and associated peaks have been resolved, for a primary electron energy of 2300 eV. Most of the observed peaks could be identified satisfactorily, but uncertainty remains about two of them. A peak arising from the L 2M 2,3V transition was not observed, although other peaks based on L 2 ionization were resolved. Two distinct types of fine structure associated with the carbon Auger peak were observed, and attributed to Ni 3C and to graphite on the surface, respectively. On heating above about 400°C, the Ni 3C decomposed, whereupon a continuous transition of one type of fine structure into the other occurred. At the same time a doublet of low-energy peaks at about 13 and 18 eV appeared and increased as the amount of graphite on the surface increased. The doublet was shown to be associated with graphite by comparison with the results of Scheibner et al on single-crystal graphite, and also by recording secondary electron energy distributions from amorphous graphite and from carbon fibres. Ni 3C was formed by quenching the nickel from about 900 °C, presumably by diffusion of carbon from just inside the foil to the surface, which in this context acts similarly to a grain boundary.

Posterior Capsule of Haller's Organ in the Lone Star Tick, Amblyomma americanum (Acari: Ixodidae)

The complex external and internal structure of the capsule of Haller's organ in the Lone Star tick, Amblyomma americanum (L.), was studied using scanning electron microscopy. Externally, the organ opening consists of a complex rectangular aperture. Internally, 2 distinct types of structures were observed: irregularly shaped elements and thin-walled setae. The variation, placement, and configuration of these structures are described.

Annual Meeting of the (American) African Studies Association

Annual meetings of one sort or another are part of the ritual cycle of academic culture. Normally, these are of two distinct structural types: the disciplinary, in which the participants are of a specific group such as anthropologists, historians, political scientists, sociologists, as well as those being enculturated; and the interdisciplinary, devoted to a specific theme or geographical area, in which members and prospective members of the specific disciplines concerned participate. In the second category we may place the annual meeting of the (American) African Studies Association.

Experimental Study of the Structure of Sporadic E (Summary)

Short‐term characteristics of sporadic E propagation were observed on a forward‐scatter radio link equipped with an angle‐of‐arrival measuring device. The evolution with time of the field intensity n, the fading rate N, and the width ∆α of the angular spectrum was studied in detail for several typical Es events. It reveals the coexistence of two distinct structural types of layers characterized as follows:

Nitrous Oxide Produced by the Catalytic Oxidation of Organic Nitrogen Compounds

Nitrous oxide is produced by the catalytic oxidation of organic nitrogen compounds of several distinct structural types with Hopcalite catalyst at a space velocity of 21,000 hours(-1) and at temperatures near 300 degrees C.

Sedimentary Structures in Modern Carbonate Sands of the Bahamas: ABSTRACT

Sedimentary structures of carbonate sand bottoms near Bimini, Sandy Cay, and the Berry Islands have been examined in depths ranging to 10 ft. below MLW. Box cores with a vertical face 18 in. high and 12 in. wide were impregnated in the field with epoxy resin to preserve and accentuate the structures. Four distinct types of internal structures occur in contrasting depositional environments. Type I, characterized by 3-10 cm.-thick sets of cross-strata and either horizontal or curving truncation surfaces, is found in intertidal, rippled, oolite shoals. Type II, with 20-40 cm.-thick sets of tabular cross-strata and horizontal truncation surfaces, is found in intertidal-subtidal, unrippled, grapestone bars. Type III, with even, parallel laminae inclined at low angles to the horizontal, is found in the backshore and foreshore of beaches. Type IV is mottled, lacks stratification, and is found in unrippled level bottoms. Bahamian field observations confirm pertinent hydrodynamic conclusions of J. R. L. Allen, H. W. Menard, and D. B. Simons. Type I is observed to form by small ripple trains induced at low flow intensities by tidal currents. Type II is judged to form by a large-scale ripple induced at high flow intensities by rare storms. According to M. M. Ball, deposits with similar structure were formed by Hurricane Donna in Florida Bay. Type III is observed to form from swash and backwash currents on beaches. Type IV reflects an environment in which organisms destroy stratification as fast as it is produced. These four types should be considered as end-members representing hydrodynamically pure situations. Many actual sites represent alternating conditions (beach-shoal, beach-bar, etc.), as is clear from preserved structures. End_of_Article - Last_Page 533------------

Taxonomic Significance of Spermathecal Structure in Some Species of Tribollum

RECENT examination of spermathecae in tenebrionid beetles associated with stored products has revealed that there are two distinct types of structure which are termed here the ‘tenebrio’ type and the ‘chamber’ type. Their basic plan and relationship to the rest of the female reproductive system are similar ; but there are certain differences which, in regard to the genus Tribolium, are considered to be of some phylogenetic significance. In both types, a duct of varying width and length, which terminates in the main sperm storage region, arises from the common oviduct. In Tenebrio molitor this duct is short and wide and ends in a group of convoluted, thin-walled tubes bound together by a tough membrane. Arising from their common base is a single, longer structure which may be glandular, having large-celled walls and a restricted lumen. Dissection of fertilized females has shown that the convoluted tubes carry sperm. In the chamber type, found in Gnathocerus, Alphitobius, Alphitophagus and some Tribolium species, there is a similar duct which runs to a strongly chitinized chamber in which motile spermatozoa are easily detected. This is variously shaped and arising either from the end opposite the duct entrance or from the side is a more-or-less elongate glandular region. In Palorus species this basic pattern is modified, associated with a muscular bursa.

Surface migration on tantalum crystals under influence of electric and thermal gradients

Grains in a thin tantalum ribbon with (112) surfaces exposed develop two distinct types of surface structure when heated in vacuum with direct current for a few hundred hours at 2150°K. The first type forms if the electric gradient has a positive component parallel to the [1̄11] direction, and consists of ridges about 0.0012 cm apart running transverse to the electric field. The ridges develop (121) or (211) surfaces facing toward the positive, and (001) and (101) surfaces facing the negative; surfaces of the type (103) also sometimes develop. The second type of structure forms if the electric gradient has a negative component parallel to the [1̄11] direction, and consists of the original undisturbed (112) surface, sometimes with ledges which form broad shallow steps. The steep surfaces of the steps face toward the negative and have indexes (110), and either (101) or (011). Opposite sides of the same grain have the different structures, one side of the first type and the other of the second. A thermal gradient produces the same effect, the hot end being equivalent to positive. An explanation of these phenomena is presented, based upon the assumption that the rate of drift of atoms and ions on (121) or (211) surfaces is greater than that on the other surfaces mentioned. If the initially exposed face of the grain is oriented differently from (112), structures different from those described are formed.