Lamellar Zones Research Articles

The intrinsic twinning and enigmatic twisting of aragonite crystals

It is generally accepted that aragonite crystals of biogenic origin are characterized by significantly higher twin densities compared to samples formed during geological processes. Based on our single crystal X-ray diffraction (SCXRD) and transmission electron microscopy (TEM) study of aragonite crystals from various localities, we show that in geological aragonites, the twin densities are comparable to those of the samples from crossed lamellar zones of molluscs shells. The high twin density is consistent with performed calculations, according to which the Gibbs free energy of twin-free aragonite is close to that of periodically twinned aragonite structure. In some cases, high twin densities result in the appearance of diffuse scattering in SCXRD patterns. The obtained TEM and optical micrographs show that besides the twin boundaries (TBs) of growth origin, there are also TBs and especially stacking faults that were likely formed as the result of local strain compensation. SCXRD patterns of the samples from Tazouta, in addition to diffuse scattering lines, show Debye arcs in the [Formula: see text] plane. These Debye arcs are present only on one side of the Bragg reflections and have an azimuthal extent of nearly 30°, making the whole symmetry of the diffraction pattern distinctly chiral, which has not yet been reported for aragonite. By analogy with biogenic calcite crystals, we associate these arcs with the presence of misoriented subgrains formed as a result of crystal twisting during growth.

Microstructural Evolution and Tensile Testing of a Bi–Sn (57/43) Alloy Processed by Tube High-Pressure Shearing

Tube high-pressure shearing (t-HPS) processing was performed on a eutectic Bi–Sn (57/43) alloy for 0.25, 1, 5 and 20 turns. The selected samples were stored at room temperature for up to 56 days to examine the strain weakening and self-annealing behavior of the alloy. The results showed that t-HPS processing gradually refined the microstructure and led to decreasing of microhardness, but microhardness increased slowly during the subsequent storage at room temperature. Shear localization of the eutectic structure during t-HPS processing was observed as large amounts of narrow dense lamellar zones were visible in the deformed microstructures. The Bi–Sn (57/43) alloy processed by t-HPS exhibited significantly enhanced superplastic properties with elongations up to >1800% in a sample after t-HPS processing for 20 turns. This high elongation is attributed to the breaking of the lamellar structure and the very small grain size.

Role of nanostructured modifiers in composite corundum ceramics preparation

Influence of highly disperse nanostructured modifiers of alumina – magnesia, partially stabilized zirconia – on the consolidation processes of composite ceramics of industrial corundum powders annealed at 1600–1700 °C, changes of its microstructure and physico-mechanical properties is investigated. It is established, that due to processes of self-diffusion of active modifiers there is a distribution of their nanograins on the borders of miсroparticles of corundum powder. In addition, nanostructured modifiers fill a pore space that causes sliding of particles under mechanical and thermal loads of material and transfers the mechanism of fragile destruction to pseudo-plastic. The entered nanostructured modifiers promote the process of lamellar zones formation throughout the material that also strengthens its mechanical properties. The correlation of composite ceramics structure and their physico-mechanical characteristics are developed.

Relationship between twisting phenomenon and structural discontinuity of stacked lamellae in the spherulite of poly(ethylene adipate) as studied by the synchrotron X-ray microbeam technique

Wide-angle and small-angle X-ray scattering measurements were performed simultaneously using an X-ray beam 1 μm in size to detect the inner structure of a spherulite of poly(ethylene adipate) [PEA, –(O(CH2)2OCO(CH2)4CO–)n–]. The lamellae were found to twist periodically by 180° in the area between the neighboring bright rings observed in the polarized optical microscope image of the spherulite. However, as known from the discontinuity between the repeated lamellar zones discovered by Woo et al. (Macromolecules, 45:1375–1383, 2012), the twisted lamellae must not be assumed to be continuously twisted long plates spreading out over the spherulite. Rather, these lamellae should be considered to have limited areas of ~7 μm in length along the radial direction and with disconnected end zones. The present X-ray scattering data revealed the structural relation between the repeated arrays of the disconnected lamellar blocks and the 180° twisting phenomenon, which occurs only in the finite and disconnected flat-on lamellar zones. Designer supramolecular polymers are a growing field of polymer materials. The designability and flexibility in their structures and functionality have attracted a great deal of attention in polymer science, as well as in supramolecular chemistry. These polymeric structures are formed from one or more molecular components via reversible bonds; therefore, monomeric and polymeric states are in equilibrium on the relevant experimental timescale. The dynamic nature of supramolecular polymers in terms of chain lifetime and conformational flexibility are determined by external conditions. This adaptivity can result in stimuli-responsive structures and properties. This article describes the use of our host–guest structures based on a calix[5]arene, a bisporphyrin, and a self-assembled capsule in the synthesis of supramolecular polymers.

Understanding of degradation-resistant behavior of nanostructured thermal barrier coatings with bimodal structure

Nanostructured thermal barrier coatings (TBCs) often provide high degradation resistance, as well as extended lifetime. However, the underlying mechanism has not been fully understood. In this study, the sintering characteristics of nanostructured yttria-stabilized zirconia (YSZ) coatings were investigated, and compared with those of the conventional YSZ coatings. Multiscale characterizations of the changes in microstructures and properties were performed. Results showed that the enhanced high-performance durability was mainly attributed to different sintering mechanisms of lamellar zones and nanozones. Sintering characteristics of the lamellar zones were similar to those of the conventional coatings. Stage-sensitive healing of two-dimensional (2D) pores dominated the sintering behavior of the lamellar zones. However, the differential densification rates between nanozones and lamellar zones of the nanostructured TBCs led to the formation of coarse voids. This counteractive effect, against healing of 2D pores, was the main factor contributing to the retardation of the performance degradation of bimodal TBCs during thermal exposure. Based on the understanding of the performance-degradation resistance, an outlook towards TBCs with higher performances was presented.

Stage-sensitive microstructural evolution of nanostructured TBCs during thermal exposure

Nanostructured thermal barrier coatings (TBCs) often exhibit bimodal structure comprised of both nanozones and lamellar zones, and therefore, their sintering behaviour can be different from that of conventional coatings. In this study, changes in the microstructure and properties of nanostructured TBCs were investigated. The results show that their microstructural evolution is highly time-sensitive during long thermal exposure at 1150 °C. In stage I (0–20 h), changes in mechanical properties were significant. The dominant microstructural change was faster healing of flat pores, whereas the macroscopic structure seemed less affected. In stage II (20–500 h), the changes in properties were much slighter and some large macroscopic voids appeared. In brief, the microscopic healing of pores in lamellar zones leads to a significant change in mechanical properties in stage I, whereas sintering of the nanozones leads to macroscopic voids in stage II.

A comprehensive mechanism for the sintering of plasma-sprayed nanostructured thermal barrier coatings

Abstract Nanostructured thermal barrier coatings (TBCs) are being widely researched for their superior thermal barrier effect and strain compliance. However, the sintering occurs inevitably in nanostructured TBCs that comprise both nanozones and lamellar zones, although the mechanism of sintering in such bimodal coatings is not yet clear. This study investigates the changes in microstructure and properties of nanostructured TBCs during thermal exposure with the aim to reveal the sintering mechanism operative in these coatings. Results show that the sintering process occurs in two stages. It was found that in the initial shorter stage (~0–10 h), the properties increased rapidly; moreover, this change was anisotropic. The main structural change was the significant healing of the intersplat pores through multiconnection. During the subsequent longer stage, the change in the properties was much smaller, where it was observed that the pores continued to heal, albeit at a much lower rate. Furthermore, the faster densification of the nanozones induced during sintering became significant, resulting in an opening at the interface between the nanozones and the lamellar zones. In brief, the pore healing at the lamellar zones affects the properties, especially in the initial stage. The presence of nanozones has a positive effect in that the performance degradation during the overall thermal exposure is slowed down. An understanding of this competing sintering mechanism would enable the structural tailoring of nanostructured TBCs in order to increase their thermal insulation and thermal cycling lifetime.

In Situ Study of the Breakout Crystallization in the Poly(butadiene)-block-Poly(ε-caprolactone) Thin Film

Despite its wide occurrence in soft confined block co-polymers, breakout crystallization remains poorly understood and is difficult to control. In this work, thin films of cylinder-forming poly(butadiene)-block-poly(ε-caprolactone) (PB-b-PCL) diblock co-polymers, with PCL being the minority block, have been chosen as the study subject. We demonstrate a new route to study the breakout crystallization by obtaining the microphase separation structure within terraced lamellae first and then in situ tracking down the lamellar coalescence, resulting from the development of the crystal growth front. We find that the crystal growth front has sucked materials from the surrounding amorphous lamellae, which lead to the decrease of the lamellar zones and coalescence of the microphase separation structure. Dividing the breakout crystallization into parallel breakout and vertical breakout, we illustrate that it is the crystallization-driven molecular diffusion that make the molecules overcome the topography constraint and grow into large-scale spherulite. Moreover, the results show that the polymer microphase separation structure has a significant influence on the crystal nucleation and greatly retarded the crystal growth rate. With a well-designed microphase separation structure within terraces and an easily tunable atomic force microscopy in situ imaging technique, an intensive study of the breakout crystallization and concomitant microdomain coalescence has been offered.

Octadecylsilane hybrid silicas prepared by the sol–gel method: Morphological and textural aspects

A series of octadecylsilane-modified silicas was prepared by the sol–gel method through the hydrolysis and cocondensation of tetraethylorthosilicate (TEOS) with octadecyltriethoxysilane (ODS). The ODS:TEOS ratio was varied between 0:100 and 100:0. The resulting carbon content was between 2.5 and 53.4%. In the case of pure ODS, the resulting silica presented 68.6% of C. Hybrid silicas were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, and 29Si nuclear magnetic resonance spectroscopy. Spheres of ca. 0.5–1.0 μm were obtained in the case of hydrolysis of pure TEOS. The combination of ODS:TEOS ratio yielded systems combining spherical and lamellar patterns zones. Monitoring the particle growth, it seems that spherical particles grow around lamellar zones, these latter concentrating the organosilicon moieties. The degree of cross-linking of ODS moieties was shown to be dependent on the ODS addition time and stirring speed.

Growth and textural ageing in long bones of the American alligator Alligator mississippiensis (Crocodylia: Alligatoridae)

Growth series of femora, tibiae, and humeri of the American alligator Alligator mississippiensis were examined to assess the relationship between bone surface textures and relative skeletal maturity. Element texture types were compared with both size-based and size-independent maturity estimates. Selected elements were thin sectioned to observe the histological structures underlying various surface textures. Results suggest little to no relationship between bone textures and skeletal maturity in Alligator. Controlling for additional factors suspected to affect textural variation - sexual dimorphism, seasonally interrupted growth, wild vs. captive habitat, and geographical range - provides little resolution. Indeterminate growth is almost certainly a factor; however, this alone cannot explain all observed variability. Histological analyses reveal that highly porous surface textures are often associated with zones composed of fibrolamellar bone; smoother textures are generally underlain by lamellar zones or annuli. Textures of intermediate porosity may be associated with more than one histological pattern. Until the factors affecting bone texture changes in modern crocodylians are better understood, it is recommended that the textural ageing method be applied with caution to studies of fossil archosaurs with crocodylian-like or unknown growth regimes.

Phase Equilibria and Equation of State of a Mixed Cationic Surfactant−Glycolipid Lamellar System

We establish experimentally the equation of state relating pressure and composition of a ternary model glycolipid/synthetic cationic surfactant/water system at room temperature. It is shown that an adhesion energy induced by the sugar headgroups is necessary to explain the presence of two distinct critical points that close two biphasic lamellar−lamellar zones. Between these two critical points, a turbid single-phase region is evidenced.

Release of a “light” thylakoid membrane fragment with high F730/ F685 fluorescence emission ratio (77 K) by digitonin disruption from “low-salt”-destacked or phosphorylated thylakoids of pea

Digitonin disruption of chloroplasts, phosphorylated in the light, results in the release of a light thylakoid fragment, sedimenting above 50 000× g and coprecipitating with the stroma lamellar subchloroplast fraction isolated by differential centrifugation; this fragment can be isolated as a zone, resolved between the grana and stroma lamellar zones, on sucrose density gradients upon ultracentrifugation; the extent of release depends on the light intensity during phosphorylation. The fragment is also released by digitonin action from isolated grana after phosphorylation, or from chloroplasts suspended in “low-salt” Tricine (0.5 mM MgCl 2). It is characterized by a high F 730/ F 685 fluorescence emission ratio at 77 K, and is enriched in the supramolecular complex of photosystem I (CPIa) and also contains light-harvesting complex II (LHC-II). The results suggest that the fragment originates in destacked, peripheral, grana thylakoids, and may contaminate the light stroma lamellar subchloroplast fraction isolated by differential centrifugation from digitonin-disrupted, phosphorylated or “low-salt” chloroplasts. Therefore, the enrichment in LHC-II components of the stroma lamellar fraction isolated from digitonin-disrupted, phosphorylated or “low-salt” chloroplasts cannot be considered as evidence for the lateral movement hypothesis. The enhancement of the F 730/ F 685 fluorescence emission ratio at 77 K, observed in chloroplasts on phosphorylation, may reflect the association of granal LHC-II with granal photosystem I (PSI).

POLLEN GRAIN DEVELOPMENT AND TAPETAL CHANGES IN STRELITZIA REGINAE (STRELITZIACEAE)

The proexine that forms within the callosic envelope before the end of the microspore tetrad period is thick (about 1 μm) and exceptionally complex. It has components equatable with tectum, columellae, and a nexine that includes lamellar zones. All these components persist in the exine although late in development they become difficult to recognize because this exine is reduced in thickness, apparently by stretching, to a maximum of 0.2 μm. Strelitzia is an example of an exine template, with receptors for sporopollenin, that is not maintained during development. The Strelitzia microspore surface changes from an exine like that on an interaperture sector to the channeled intinelike system common for the apertures of pollen grains. The exine on sterile grains gives what may be a rare view of a stabilized immature exine. The mature exine on viable pollen grains resembles this early exine only in the most impressionistic way. Tapetal cells go through at least one cycle of hyperactivity, dedifferentiation, mitosis, and then again hyperactivity before they finally decline.

Pollen Grain Development and Tapetal Changes in Strelitzia Reginae (Strelitziaceae)

The proexine that forms within the callosic envelope before the end of the microspore tetrad period is thick (about 1 μm) and exceptionally complex. It has components equatable with tectum, columellae, and a nexine that includes lamellar zones. All these components persist in the exine although late in development they become difficult to recognize because this exine is reduced in thickness, apparently by stretching, to a maximum of 0.2 μm. Strelitzia is an example of an exine template, with receptors for sporopollenin, that is not maintained during development. The Strelitzia microspore surface changes from an exine like that on an interaperture sector to the channeled intinelike system common for the apertures of pollen grains. The exine on sterile grains gives what may be a rare view of a stabilized immature exine. The mature exine on viable pollen grains resembles this early exine only in the most impressionistic way. Tapetal cells go through at least one cycle of hyperactivity, dedifferentiation, mitosis, and then again hyperactivity before they finally decline.

Cell shape and organization of cytoskeleton and surface fibronectin in non-tumorigenic and tumorigenic rat liver cultures.

Morphological changes associated with neoplastic transformation of epithelial cells were studied in a series of IAR cell lines derived from rat liver. The series included three independently obtained, non-tumorigenic lines and five derived, tumorigenic lines. The morphology of cell surfaces was observed by scanning electron microscopy; the distribution of actin, tubulin and fibronectin was determined by indirect immunofluorescence. All the non-tumorigenic lines had a typical epithelioid morphology: isolated cells of these lines spread on the substratum had a discoid shape and contained circular, marginal bundles of microfilaments and microtubules. In denser areas, the cells formed monolayered sheets with characteristic marginal bundles of microfilaments near the free edges. Decreased spreading of isolated cells on the substratum was the characteristic feature that distinguished tumorigenic lines from their non-tumorigenic parent lines. In particular a decrease in the size of the ring-like, peripheral lamella and its disintegration into several discrete lamellar zones were often observed; as a result, the cell shape was altered from discoid to polygonal or elongated. The altered distribution of microfilament bundles and microtubules was characteristic in elongated cells; the pattern of the cytoskeletal elements of these cells resembled that of polarized fibroblasts. Complete disappearance of microfilament bundles was observed in cells of only one tumorigenic line. Various degrees of disorganization of monolayered cell sheets were observed in tumorigenic cultures, accompanied by an altered distribution of microfilament bundles. The alterations in the fibronectin-containing structures were more complex: there were often fewer fibronectin "spots' and fibrils at the lower surfaces of cells of tumorigenic cultures as compared with those of non-tumorigenic ones; there were more fibrils in dense cultures of certain lines but fewer in others. It is concluded that alterations in the ability to spread on the substratum and to form cell-cell contacts are common features of morphologically transformed fibroblastic and epithelial cultures. However, the actual changes in the cytoskeletal structures that accompany these alterations are different in transformed cultures of various tissue types.

Landslides and related phenomena on Suduroy, the Faeroe Islands

The formation of landslides on Suduroy, the Faeroe Islands, is greatly favoured by the geological setting and the tectonic pattern of the island. Some landslides involve lavas, the coal-bearing sequence and the luff-agglomerate zone, which served as lubricating agents for the slipped masses. The biggest landslides occur along scarps with the direction NW-SE and are believed to have a relation to the lamellar zones which are characterized by closely spaced fissures. The NW-SE trend is the main tectonic direction on Suduroy.

The relation of lipids to the mineral components in salivary calculi

Using lipid histochemical and microradiographic methods, the distribution of lipids and the inorganic component in different areas of salivary calculi was investigated. Two main groups of lipids were found, i.e. hydrophilic phospholipids and hydrophobic lipids. The hydrophilic phospholipids were shown to be composed mainly of phosphosphingosides and of the hydrophobic lipids cholesterol was demonstrated. Phosphosphingosides were localized in more or less lamellar zones as also in large homogeneous areas. In some cases the calculi were surrounded by a structureless non-mineralized zone exhibiting a strongly positive reaction for phosphosphingosides. Cholesterol was found in isolated band-formed areas. Phosphosphingosides and cholesterol were especially prevalent in areas showing a low or slight degree of mineralization. In areas showing a high degree of mineralization the corresponding lipids gave a weak or absent histochemical reaction. After EDTA-decalcification, however, an increase in the concentration of lipid was noticed in high mineralized areas, indicating a masking process. The influence of various factors on the presence and distribution of different lipids was discussed.

Mechanism of Exfoliation (Layer Corrosion) of AI-5%Zn-1%Mg

AbstractThe mechanism of exfoliation in AlZn5Mg1 alloy has been studied by making a survey of the phasesb occurring in the alloy in the naturally aged and artificially aged conditions, preparing the phases in pure state and investigating their electrochemical properties. It has beenfound that in the naturally aged alloy, the attack is confined to lamellar zones in the structure giving rise to the exfoliation. Finely dispersed α-Al(Fe, Me)Si phase particles arranged in streaks along the extrusion or rolling direction act as cathodes and the anodic areas consist of narrow zones of the Zn- and Mg-rich matrix, next to the particles. Between the streaks of α-Al(Fe,Me)Si particles, layers of matrix are left unattacked. Since the main factor determining the amount of α-Al(Fe,Me)Si phase is the Fe content, an increase in Fe content will decrease resistance to exfoliation. In the artificially aged condition, the alloy is not prone to exfoliation, but shows a type of general attack. α-Al(Fe,Me)Si particles again co...