Basal Facets Research Articles

The dentition of Manidens condorensis (Ornithischia; Heterodontosauridae) from the Jurassic Cañadón Asfalto Formation of Patagonia: morphology, heterodonty and the use of statistical methods for identifying isolated teeth

The recently described Manidens condorensis is one of the most completely known taxa of the family Heterodontosauridae from the southern landmasses. However, some dental aspects are not well known due to preservational problems in the type material. This contribution reports new isolated teeth found in the Cañadón Asfalto Formation (Early-Middle Jurassic). These teeth are referred to Manidens condorensis based on the presence of autapomorphic characters of the unusual dentition of this taxon, such as the highly asymmetric tooth crowns and small crenulations on each denticles. The isolated crowns are well preserved and reveal the presence of undescribed and new autapomorphical features, including apical and basal wear facets on the occlusal surface of isolated crowns and a wear surface also in the caniniform tooth. We carried out statistical analyses (including morphogeometrical and discriminant analyses), using the holotype crowns as a morphological starting point, for characterising shape variation of the crowns along the toothrow and for identifying the position of isolated crowns. These analyses allow defining morphological regions within the postcaniniform toothrow and produce a metrically based discriminant function to predict the hypothetical position of future discoveries, providing a methodological framework that could be applied to other extinct heterodont dinosaurs.

Colloidal CdSe/Cu3P/CdSe Nanocrystal Heterostructures and Their Evolution upon Thermal Annealing

We report the synthesis of colloidal CdSe/Cu(3)P/CdSe nanocrystal heterostructures grown from hexagonal Cu(3)P platelets as templates. One type of heterostructure was a sort of "coral", formed by vertical pillars of CdSe grown preferentially on both basal facets of a Cu(3)P platelet and at its edges. Another type of heterostructure had a "sandwich" type of architecture, formed by two thick, epitaxial CdSe layers encasing the original Cu(3)P platelet. When the sandwiches were annealed under vacuum up to 450 °C, sublimation of P and Cd species with concomitant interdiffusion of Cu and Se species was observed by in situ HR- and EFTEM analyses. These processes transformed the starting sandwiches into Cu2Se nanoplatelets. Under the same conditions, both the pristine (uncoated) Cu(3)P platelets and a control sample made of isolated CdSe nanocrystals were stable. Therefore, the thermal instability of the sandwiches under vacuum might be explained by the diffusion of Cu species from Cu(3)P cores into CdSe domains, which triggered sublimation of Cd, as well as out-diffusion of P species and their partial sublimation, together with the overall transformation of the sandwiches into Cu(2)Se nanocrystals. A similar fate was followed by the coral-like structures. These CdSe/Cu(3)P/CdSe nanocrystals are therefore an example of a nanostructure that is thermally unstable, despite its separate components showing to be stable under the same conditions.

Why the 46° halo is seen far less often than the 22° halo?

The goal of this study is to explore some effects of surface roughness on phase functions of hexagonal ice crystals. The simulations were performed using a Monte-Carlo ray-tracing method. The peculiarity of our statistical model of surface roughness is the following: (a) the surface roughness of the prismatic facets can be anisotropic, and (b) the roughness degree of basal and prismatic facets can be different.The main result is the existence of a sufficiently large range of the roughness parameters when a phase function of hexagonal ice crystals has a pronounced 22° halo whereas the 46° halo is totally smoothed. It is speculated that the anisotropic roughness is the unique model that is able to explain the only-22°-halo feature of phase functions of thin plates. It is also proposed to use the model of rough basal-facets as a proxy for ensembles of hollow-ended columns and/or bullets.

Uniform thickness and colloidal-stable CdS quantum disks with tunable thickness: Synthesis and properties

Uniform thickness and colloidal-stable CdS quantum disks have been reproducibly prepared using cadmium acetate, elemental sulfur, fatty acids and octadecene as the starting materials without any size/shape sorting. The thickness could be varied between 1.2 and 2.2 nm, i.e., 4.5, 5.5, 6.5 and 7.5 monolayers of CdS along the thickness direction. These single crystalline disks with lateral dimensions between 20 and 100 nm adopted the zinc blende crystal structure with 〈100〉 (possibly mixed with 〈111〉) as the thickness direction. The basal planes and side facets were terminated with cadmium carboxylates, which dictated the thicknesses to be half a monolayer more than an integer number. Formation of CdS quantum disks probably occurs through a “nucleation-growth” mechanism, instead of aggregation of pre-formed magic clusters. Completion of a full monolayer along the lateral direction was found to be rather fast if two-dimensional nucleation was initiated on existing disks, which helped formation of atomically flat and thickness-controlled disks. As disk thickness decreased, the crystal lattice was found to dilate gradually, which has not been observed with CdS quantum dots. Compared with CdS quantum dots and rods, the disks displayed weakened quantum confinement and their photoluminescence lifetime (tens of picoseconds) was about two orders of magnitude shorter. Open image in new window

Faceting in AlN bulk crystal growth and its impact on optical properties of the crystals

AbstractThe surface of bulk AlN single crystals grown by PVT on Al‐polar (0001) seeds is typically constructed from prismatic, pyramidal, and basal plane facets. Simultaneous growth on different facets with different surface kinetics leads to formation of a zonar structure which is visible due to local differences in coloration in the wafers. Four zones on an AlN wafer representing (1) growth on {1013} pyramidal facets, (2) growth adjacent to ridges and (0001) basal plane facets, and (3) + (4) outer and centre areas corresponding to surface depressions are investigated. They show striking differences in optical absorption (OA) spectra, cathodoluminescence (CL) spectra and chemical analysis (SIMS). In areas near ridges and (0001) facets and in centres of depressions, deep‐UV absorption as well as luminescence is significantly increased. We assign OA and CL bands observed in these zones to the presence of carbon and/or the lack of oxygen due to local segregation effects caused by faceting. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Facile synthesis of flower-like 3D ZnO superstructures via solution route

A novel flower-like 3D ZnO superstructure with high uniformity was fabricated on a large scale through a facile aqueous solution route at room temperature. The as-prepared ZnO superstructures with a diameter of about 3 μm are assembled by large amounts of interleaving nanosheets, which have a uniform thickness of about 15 nm and a well-crystalline structure with {100} planes as basal facets. The hierarchical microstructures, which were formed via a two-stage nucleation–growth process, highly depend on the concentration of NaOH and trisodium citrate dihydrate. The photoluminescence result indicates that the ZnO superstructures possess a relatively strong UV emission. The photodecomposition of methyl orange indicates that such ZnO superstructures possess excellent photocatalytic activity.

ChemInform Abstract: One‐Step Green Synthesis of Cuprous Oxide Crystals with Truncated Octahedra Shapes via a High Pressure Flux Approach.

AbstractCu2O truncated octahedral crystals exposing the active basal (110) facets are synthesized from a mixture of CuO and Cu using NaOH/KOH (1:1) as a flux (autoclave, 0.2 GPa, 300—600 °C, 6 h).

Comparative Molecular Dynamics Study of Vapor-Exposed Basal, Prismatic, and Pyramidal Surfaces of Ice

We present the results of molecular dynamics simulations in which ice I(h) slabs with free basal, prismatic, 28° pyramidal, and 14° pyramidal facets are exposed to vapor. All simulations were carried out at 250 K using a six-site intermolecular potential. Characteristics common to all facets include spontaneous development of a quasi-liquid layer (QLL) within ∼10 ns and QLL stratification into outer (ε(1)) and inner (ε(2)) sublayers having on average two and three hydrogen bonds, respectively. Vapor pressure, based on the rate of escape of molecules from the QLL to the vapor phase, is found to be greatest for the 14° pyramidal and basal facets (∼230 Pa), while significantly lower values are obtained for the prismatic and 28° pyramidal facets (∼200 Pa). The geometric thickness of the QLL also varies between facets, with the 14° pyramidal having the greatest thickness. The free prismatic and pyramidal facets exhibit significant anisotropic diffusivity, in-plane motion being faster in the trans-prismatic direction than in the basal-to-basal direction. The in-plane diffusion length is greatest for the 28° pyramidal facet and smallest for the prismatic facet. This diversity of facet-specific properties provides a rich set of possibilities for mechanisms of ice crystal growth and ablation.

A facile surfactant-free fabrication of single-crystalline truncated Fe3O4 cubes

Single-crystalline truncated Fe 3 O 4 cubes with active basal facets have been successfully fabricated through a facile surfactant-free hydrothermal route. The presented materials were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS), dynamic light scattering (DLS), selection area electron diffraction (SAED) and magnetic property measurement system (MPMS). The results showed that all products are Fe 3 O 4 with face-center-cubic (FCC) structure. The morphology of Fe 3 O 4 depends on the contents of hydroxide ions, hydrazine hydrate and reaction time. The well-defined truncated Fe 3 O 4 cubes with active basal facets {1 0 0} were fabricated when the pH value, the hydrazine hydrate content and reaction time are 10, 10 mL and 24 h, respectively. The as-prepared Fe 3 O 4 cubes exhibit excellent magnetic properties, which endow the materials with great potential applications in many fields.

Sintering behavior of nanostructured WC–Co composite

Sintering of nanocrystalline WC–Co, retaining the nanoscaled grain sizes is a matter of great interest. Many non traditional methods have been used to achieve this goal as the required result has not been achieved using the traditional method of liquid phase sintering. The present study is an attempt to find out the limitations of traditional sintering method and optimize it to get nanocrystalline WC–Co composites. The effect of temperature, time and composition variation has been studied to see the sintering behavior of nanostructured WC–Co composite. Detailed X-ray diffraction (XRD) analysis as well as microstructural examination showed that sintering at 1350°C for 1h with 5% binder composition (Co) is a set of critical temperature, time and composition which can provide the required results. The grain growth is through coalescence. At critical conditions the majorities of WC grains are faceted and attains the shape of prismatic and basal facets.

Effects of growth direction and polarity on bulk aluminum nitride crystal properties

Aluminum nitride (AlN) single crystals, approximately 25mm in diameter and up to 15mm thickness, were grown by the physical vapor transport (PVT) method in tungsten crucibles. To study the effect of growth direction and polarity, growth was performed using AlN seeds of very different orientations, i.e., Al-polar and N-polar basal and rhombohedral planes. For some growth runs, dual seeds consisting of two halves of switched polarity were used to compare growth under nearly identical settings. Specially roughened seed crystals were prepared to promote large voids in AlN crystals. Stable rhombohedral and (0001) c-plane facets form the Al-polar side of the void, whereas a wavy and less faceted surface occurs at the N-polar side. The growth rate on Al-polar rhombohedral facets is significantly lower compared to the one on the Al-polar basal facet. Regardless of initial seed orientation, the as-grown crystal surface always grows in Al-polar direction and appears to be composed of pyramids bounded mainly by rhombohedral (01−12) and (01−13) surfaces. Finally, the coloration of AlN crystal was found to depend on growth orientation: the material grown on (0001) facet is almost colorless, but when it is grown on rhombohedral facets it demonstrates characteristic amber color. The material grown on N-polar facets appears dark brown and opaque.

Polarity of prismatic facets delimiting WC grains in WC–Co alloys

This study reports a determination of the polarity of WC facets in WC–Co alloys sintered at the liquid state. In these alloys, WC grains are delimited by basal facets and two sets of { 1 0 1 ¯ 0 } prismatic facets, one set of facets being much more developed than the other. A variation of the shape is observed as a function of the composition of the alloy. High resolution transmission electron microscopy is used to investigate the polarity of the prismatic facets owing to the typical triangular patterns appearing on the images. The effect of the composition is investigated using two alloys with different carbon potentials and one containing VC and Cr 3C 2 as grain growth inhibitors. The interpretation of the images shows that in all cases, the same set of prismatic planes is favoured.

Inter-granular glassy phases in the low-CaO-doped HIPed Si3N4ceramics: a review

AbstractThis review outlines the essence of a progressive study on the glassy inter-granular film (IGF) in a model ceramic system, the low-CaO-doped HIPed high-purity Si3N4. This was initiated from the finding of a systematic variation of equilibrium IGF thickness following the dopant chemistry, manifesting its fundamental important to ceramic processing. By employing analytical transmission electron microscopy to measure the local chemistry in IGF, however, significant discrepancy was found between trends of local IGF chemistry and thickness. A stable IGF composition was revealed in this system, while a bi-level distribution of Ca segregation establishes a correspondence between the IGF structure and the surface crystallography. The detection of similar levels of nitrogen in IGF through the whole series further supports the presence of a rather stable IGF chemistry. After the saturation of dopants in the stable IGF, extra CaO was found to re-distribute in pockets by enrichment at tips, leading to a liquid phase separation with the Ca-rich phase wetting the entrance zone contacting IGF. The perspective for establishing a comprehensive correlation between the inter-granular phases and the bi-modal microstructure induced by faster growth of basal facets is briefly discussed to pave the way for future work.

Doppler lidar measurements of oriented planar ice crystals falling from supercooled and glaciated layer clouds

AbstractThe properties of planar ice crystals settling horizontally have been investigated using a vertically pointing Doppler lidar. Strong specular reflections were observed from their oriented basal facets, identified by comparison with a second lidar pointing 4° from zenith. Analysis of 17 months of continuous high‐resolution observations reveals that these pristine crystals are frequently observed in ice falling from mid‐level mixed‐phase layer clouds (85% of the time for layers at −15 °C). Detailed analysis of a case study indicates that the crystals are nucleated and grow rapidly within the supercooled layer, then fall out, forming well‐defined layers of specular reflection. From the lidar alone the fraction of oriented crystals cannot be quantified, but polarimetric radar measurements confirmed that a substantial fraction of the crystal population was well oriented. As the crystals fall into subsaturated air, specular reflection is observed to switch off as the crystal faces become rounded and lose their faceted structure. Specular reflection in ice falling from supercooled layers colder than −22 °C was also observed, but this was much less pronounced than at warmer temperatures: we suggest that in cold clouds it is the small droplets in the distribution that freeze into plates and produce specular reflection, whilst larger droplets freeze into complex polycrystals. The lidar Doppler measurements show that typical fall speeds for the oriented crystals are ≈︁ 0.3 m s−1, with a weak temperature correlation; the corresponding Reynolds number is Re ∼ 10, in agreement with light‐pillar measurements. Coincident Doppler radar observations show no correlation between the specular enhancement and the eddy dissipation rate, indicating that turbulence does not control crystal orientation in these clouds. Copyright © 2010 Royal Meteorological Society

Evolution of the WC grain shape in WC–Co alloys during sintering: Cumulated effect of the Cr addition and of the C content

The evolution of the WC grain shape in Cr doped WC–Co alloys is studied at several stages of the sintering treatment. The common shape of WC is a prism based on a truncated triangle. The habit planes are two prismatic facets and the basal plane. In this work, the WC grain morphology is quantified using transmission electron microscopy at several temperatures. Two shape factors are used to measure the anisotropy between the two prismatic facets and between the prismatic and basal facets. The cumulated effect of the Cr addition and C content is studied. The Cr addition increases the anisotropy between prismatic facets in the W rich alloy. A significant increase of the elongation factor is recorded in the C rich alloy while the effect is more limited in the W rich alloy. The results are discussed as a function of the factors influencing the grain shape: grain growth and difference in energy between the facets.

Evolution of the WC grain shape in WC–Co alloys during sintering: Effect of C content

Abstract After liquid phase sintering in the Co binder WC grains are faceted. The common shape of WC is a prism based on a truncated triangle. The habit planes are two prismatic facets and the basal plane. In this work, the grain shape evolution is quantified at several stages of the sintering treatment using transmission electron microscopy. Two shape factors are used to measure the anisotropy between the two prismatic facets and between the prismatic and basal facets. The influence of the C content in the alloy is studied. The C/W ratio has a limited effect on the anisotropy between prismatic facets while a significant effect is recorded for the anisotropy between the basal and prismatic facets. The results are discussed as a function of the factors influencing the grain shape: grain growth, difference in energy between the facets and contacts with other WC grains.

Controllable Synthesis of Single-Crystalline Fe3O4 Polyhedra Possessing the Active Basal Facets

In the present paper, we report on the controllable preparation of single-crystalline Fe3O4 microrhombic dodecahedra possessing the active basal facets in high yield through a facile hydrothermal route. The growth mechanism of the microcrystals is discussed. From a crystallographic point of view, the shapes of the crystals are determined by the ratio of the growth rate along to directions. The pH values of the precursor solution strongly affected the morphologies of the products. When pH value was adjusted to 8–10, well-defined Fe3O4 truncated microrhombic dodecahedra exposing the active basal surfaces ({110}) with narrow size-distribution (<10%) were obtained. The sizes of the Fe3O4 truncated microrhombic dodecahedra are dependent upon the reaction time. The magnetic properties of the products are sensitive to the crystal size. The synthesized well-defined Fe3O4 microcrystals with active basal facets may have important applications in many fields.

Morphological analyses of minute crystals by using stereo‐photogrammetric scanning electron microscopy and electron back‐scattered diffraction

We present a new method for the morphological analyses of minute faceted crystals by combining stereo-photogrammetric analysis of scanning electron microscope images and electron back-scattered diffraction. Two scanning electron microscope images of the same crystal, recorded at different tilt angles of the specimen stage, are used to determine the orientations of crystal edges in a specimen-fixed coordinate system. The edge orientations are converted to the indices [uvw] in the crystal system using the crystal orientation determined by electron back-scattered diffraction analysis. The Miller indices of crystal facets are derived from the indices of the edges surrounding the facets. The method is applicable to very small crystal facets. The angular error, as derived from tests using a calcite crystal of known morphology, is a few degrees. To demonstrate the applicability of the method, the morphology of boehmite (gamma-AlOOH) precipitated from solution during the dissolution of anorthite was analyzed. The micrometre-sized boehmite crystals are surrounded by two {010} basal facets and eight equivalent side facets that can be indexed equally well as {323}, {434} or {545}. We suggest that these side facets are in fact {111}, the morphology having been modified slightly (by a few degrees) by a small extension associated with opening along (010) microcleavage planes. Tiny {140} facets are also commonly observed.

Special WC/Co orientation relationships at basal facets of WC grains in WC–Co alloys

In WC–Co alloys prepared by liquid phase sintering WC grains are partly surrounded by the Co binder and are terminated by basal and prismatic facets. This work reports transmission electron microscopy (TEM) observations on basal WC/Co interfaces. Two kinds of preferred orientation relationships are found. The one most common corresponds to the joining of (0001)WC with (111)Co planes. It leads to a large parametric misfit of 15% and is observed for only small pools of Co, mainly located in corners formed by the contact between WC grains. The second type of interface corresponds to the joining of (0001)WC with (001)Co and is more rarely found. For random WC/Co orientation relationships a thin WC1−x cubic layer about two atomic rows is revealed at the interface.

The Giant Growth of the Facets of 4He Crystal

Below the first and second roughening transitions the facets of a helium crystal demonstrate two qualitatively different regimes of growth. At small deviations from the equilibrium a crystal grows slowly via surface defects such as screw dislocations and Frank-Reed sources. The second regime is observed at high overpressurizations. A single crystal goes over into new state with the growth rates by several orders of the magnitude higher than the previous ones. The transition occurs simultaneously in both the basal and lateral facets. The time of formation of an anomalous state decreases drastically with the increase of the overpressurization and lies within the range of 0.1–200 ms. An average overpressurization. Dp0 above which the anomalous state appears increases monotonically with the growth of temperature. The measurements of the facet growth rate immediately after a transition show that it reaches magnitudes up to ∼ 5 m/s at ∼ 20 mbar. Impurities of 3He with a concentration of 180 ppm retard the formation of the fast phase, namely, a boundary Dp0(T) shifts to higher values. The features of this phenomenon (phase diagram, effect of impurity) are similar to the “burst-like growth” observed in the range of 2–250 mK at the (0001) facet without dislocations. Possibly, kinematical multiplication of steps plays a significant role in these effects, yielding a qualitatively new channel for the growth of an ideal facet.