Pyramidal Faces Research Articles

Investigation of SR method grown 〈0 0 1〉 directed KDP single crystal and its characterization by high-resolution X-ray diffractometry (HRXRD), laser damage threshold, dielectric, thermal analysis, optical and hardness studies

〈0 0 1〉 directed potassium dihydrogen orthophosphate (KDP) single crystal was grown by Sankaranarayanan–Ramasamy (SR) method. The 〈0 0 1〉 oriented seed crystals were mounted at the bottom of the platform and the size of the crystals were 10 mm diameter, 110 mm height. Two different growths were tried, in one the crystal diameter was the ampoule's inner diameter and in the other the crystal thickness was less than the ampoule diameter. In the first case only the top four pyramidal faces were existing whereas in the second case the top four pyramidal faces and four prismatic faces were existing through out the growth. The crystals were grown using same stoichiometric solution. The results of the two growths are discussed in this paper. The grown crystals were characterized by high-resolution X-ray diffractometry (HRXRD), laser damage threshold, dielectric, thermal analysis, UV–vis spectroscopy and microhardness studies. The HRXRD analysis indicates that the crystalline perfection is excellent without having any very low angle internal structural grain boundaries. Laser damage threshold value has been determined using Nd:glass laser operating at 1054 nm. The damage threshold for the KDP crystal is greater than 4.55 GW cm −2. The dielectric constant was higher and the dielectric loss was less in SR method grown crystal as against conventional method grown crystal. In thermal analysis, the starting of decomposition nature is similar in SR method grown KDP crystal and conventional method grown crystal. The SR method grown KDP has higher transmittance and higher hardness value compared to conventional method grown crystals.

Well-Formed One-Dimensional Hydroxyapatite Crystals Grown by an Environmentally Friendly Flux Method

Highly crystalline, well-developed hydroxyapatite (HAp) crystals were grown successfully for the first time by a flux cooling method at a temperature less than 500 °C under air pressure. Environmentally friendly growth was induced by heating a mixture of solute [Ca(NO3)2·4H2O, (NH4)2HPO4, and KOH] and flux (KNO3 and LiNO3) at 400−500 °C and holding at this temperature for 10 h. After that, the mixture was cooled at various rates of 5, 200, and >1000 °C·min−1 (water quenching). The obtained HAp crystals, which had average sizes of up to 18.0 × 2.1 μm and aspect ratios of up to 8.6, were colorless and transparent. They had hexagonal prismatic one-dimensional (1-D) forms with pyramidal or truncated pyramidal end faces. The 1-D crystal forms and sizes were obviously dependent on the growth conditions, such as the holding temperature and the cooling rate of the high-temperature solution. High-resolution transmission electron microscopy (HR-TEM) images indicated that the grown HAp crystals were of a very good c...

Synthesis and characterization of ( h0l) oriented high-silica zeolite beta membrane

The ( h0l) oriented high-silica zeolite beta membrane was prepared with improved method in fluoride media. Dealuminated zeolite beta nanocrystals used as seeds were preliminary deposited on support, and then the seeded supports were vertically placed in autoclaves for hydrothermal synthesis. After the secondary growth of seeded supports at 413 K for 2 days, zeolite membranes were characterized by X-ray diffraction and scanning electron microscopy to determine their morphology and preferred orientation. Zeolite beta membranes with a thickness of intergrown crystal layer around 2 μm were prepared and the crystals with typical truncated square bipyramidal morphology were packed tightly with their pyramidal ( h0l) face perpendicular to the plane of membrane. In addition, ICP-AES chemical analysis was utilized to confirm the composition of seeds and membrane, and the Si/Al molar ratio of zeolite beta membrane was around 5000. The ( 00l) and randomly orientated zeolite beta membranes were also prepared by changing the composition of starting synthesis gel and pH value. A competitive growth model was proposed to explain the formation of oriented membranes. The ( h0l) oriented high-silica zeolite beta membrane with few cracks exhibited higher pervaporation flux.

Determining angles of incidence and heights of quantum dot faces by analyzing X-ray diffuse and specular scattering

Scattering of X rays by structures with multilayer ensembles of quantum dots MBE-grown in the In(Ga)As-GaAs system is studied by high-resolution grazing X-ray reflectometry. The peaks of the diffuse scattering intensity are discovered for the first time in structures with both vertically uncorrelated and vertically correlated quantum dots. It is shown that the position of the peak is totally determined by angle of inclination of the quantum dot pyramidal faces (the so-called blaze condition for diffraction gratings), which was theoretically predicted earlier. Comparison with the results of scattering simulation carried out by the technique of boundary integral equations indicates that a simple geometrical condition allows one to exactly determine the value of from the position of the intensity peak, the shape of which depends on many parameters. As follows from the theory and experiment, the width and height of the peaks for samples with vertically correlated quantum dots are larger than for those with uncorrelated dots. The roughness and interdiffusion of interfaces and the height of quantum dots are found from the position and amplitude of Bragg peaks. Thus, the conventional application of high-resolution grazing X-ray reflectometry is extended in this work to determination of the quantum dot geometry.

Chemistry of Zircon in Rare Metal Granitoids and Associated Rocks, Eastern Desert, Egypt

AbstractTypological study, including paragenic, morphological, textural, and chemical characteristics of zircon from nine rare metal granitic stocks and associated greisens, was carried out in order to identify the metallogenic processes of their host granitoids. The investigated zircon‐bearing granitoids and type occurrences can be categorized into magmatically and metasomatically specialized types. The magmatic type includes: (i) peralkaline, Zr + Nb‐enriched, A1‐granite (e.g. Um Hibal); (ii) metaluminous, Nb + Zr + Y‐enriched, A2‐type alkali granite (e.g. Hawashia and Ineigi); and (iii) peraluminous, Ta ≥ Nb + Sn + Be ± W‐enriched, Li‐albite granite (e.g. Nuweibi, Igla and Abu Dabbab). The metasomatized granites are Nb>>Ta + Sn + Zr + Y + U ± Be ± W‐enriched and hydrothermally altered alkali feldspar granite (i.e. apogranite; e.g. Um Ara, Abu Rusheid, and Um Naggat). Zircon of peralkaline granite is characteristically equant with well‐developed pyramidal faces and short prisms (i.e. pseudo‐octahedral form) with length/width ratios in the range of 2:1–1:1. It is of Zr0.990Hf0.007SiO4 composition and is associated with hypersolvus assemblage consisting of alkali feldspar, quartz, aegirine and minor reibeckite. Zircon of metaluminous alkali granites is of Zr0.99Hf0.01SiO4 composition and is associated with sub‐ to transolvus assemblage of K‐feldspar, quartz, plagioclase and annite‐siderophyllite mica. It is prismatic with length/width ratios in the range of 5:1–3:1, doubly terminated with small pyramidal faces. Compositionally, zircon of Li‐albite granite ranges between Zr0.925Hf0.075SiO4 and Zr0.705Hf0.295SiO4. It is idiomorphic with a simple combination of prism and bipyramidal terminations with a length/width ratio of 3:1–2:1. This zircon commonly exhibits a normal zoning with rims consistently higher in Hf than cores. The higher Hf content, of this zircon coupled with its association with topaz, tantalite and lithian micas (e.g. zinnwaldite and Li‐white mica), indicates a higher solubility of Hf‐fluoride complexes and their more stabilized state at lower temperature in Li‐ and F‐rich sodic melts. Zircon of apogranite association ranges in composition between Zr0.967Hf0.013SiO4 in the lower unaltered alkali feldspar granite zone and Zr0.805Hf0.064(Y, U, Th, heavy rare‐earth elements) [HREE])0.125SiO4 in the apical metasomatized (i.e. microclinized, albitized, and greisenized) apogranite zones. This compositional change appears to reflect a roofward increasing in μKF, μNaF, and μHF of the exsolved fluids. Columbite, xenotime, thorite, cassiterite, beryl and fluorite are common associates of this zircon. This zircon is of bipyramidal to typical octahedral form with complete absence of prism concurrently with conspicuous development of pyramid, thus the zircon crystals have a length/width ratio of 1:1–0.5:1. The neoformed metasomatic zircon commonly exhibits either normal or reverse zoning with rims consistently different in Hf, U, Y, and HREE than cores, reflecting disequilibrium conditions (e.g. sudden change in P, T, salinity, and pH) between the growing crystals and the exsolved fluids.

Formation and Growth of Tetragonal Lysozyme Crystals at Some Boundary Conditions

Formation and growth of tetragonal lysozyme crystals at the interface of protein solution with glass substrate have been studied. Morphology of crystals formed and grown on a flat glass surface are given and discussed. The influence of a second flat glass plate as well as some differences in the supersaturation is shown; the four {101} pyramidal faces vanish, giving a place of a hemisphere or a “new” {001} face.

X-ray Topography of Hydrothermally Grown Emerald Single Crystals Using Laboratory and Synchrotron Sources

Hydrothermally grown emerald crystals have been investigated by X-ray topography methods using both laboratory and Synchrotron sources. Because of the disintegration of the given nonsingular seed face (556) into smaller pyramidal faces, growth sectors have been formed. These are accompanied by growth bands consisting of the isochrones of the dopants that occur because of the fluctuating hydrodynamic conditions during the growth process. Both kinds of defects have been topographically detected and are discussed referring to their appearance on the imaged topographs.

Simultaneous Control of Shape and Properties of AZ31 Magnesium Alloy Sheets by Incremental Forming

In this study, simultaneous control of shape and hardness of AZ31 magnesium alloy sheets by incremental forming process was proposed. Influences of working temperature, giving strain and heat treatment temperature on hardness were investigated. AZ31 magnesium alloy sheets were formed into pyramidal shape by incremental forming process at 100°C and 200°C. Major strains given to the pyramidal faces were 0.1,0.2 and 0.3. After forming, heat treatment was carried out at various temperatures. Combination of final shape and total equivalent plastic strain were changed by multi-step forming and hardness distribution was measured. From the experimental results, it was found that when heat treatment was done at 200°C for 10 min after forming at 200°C, hardness of the specimen became higher than that of as received one. When heat treatment was carried out at 175°C for 60 min after forming at 100°C, hardness of the specimen also increased. In the case of multi-step forming, strain was accumulated at forming temperature of 100°C and hardness increased, but not at the forming temperature of 200°C. By multi-step forming at 100°C, asymmetric hardness distribution with symmetric shape was realized and simultaneous control of shape and hardness was succeeded.

Plešovice zircon — A new natural reference material for U–Pb and Hf isotopic microanalysis

Matrix-matched calibration by natural zircon standards and analysis of natural materials as a reference are the principle methods for achieving accurate results in microbeam U–Pb dating and Hf isotopic analysis. We describe a new potential zircon reference material for laser ablation ICP-MS that was extracted from a potassic granulite facies rock collected in the southern part of the Bohemian Massif (Plešovice, Czech Republic). Data from different techniques (ID-TIMS, SIMS and LA ICP-MS) and several laboratories suggest that this zircon has a concordant U–Pb age with a weighted mean 206Pb/ 238U date of 337.13 ± 0.37 Ma (ID-TIMS, 95% confidence limits, including tracer calibration uncertainty) and U–Pb age homogeneity on the scale used in LA ICP-MS dating. Inhomogeneities in trace element composition due to primary growth zoning prevent its use as a calibration standard for trace element analysis. The content of U varies from 465 ppm in pristine parts of the grains to ~ 3000 ppm in actinide-rich sectors that correspond to pyramidal faces with a high degree of metamictization (present in ca. 30% of the grains). These domains are easily recognized from high intensities on BSE images and should be avoided during the analysis. Hf isotopic composition of the Plešovice zircon (> 0.9 wt.% Hf) is homogenous within and between the grains with a mean 176Hf/ 177Hf value of 0.282482 ± 0.000013 (2SD). The age and Hf isotopic homogeneity of the Plešovice zircon together with its relatively high U and Pb contents make it an ideal calibration and reference material for laser ablation ICP-MS measurements, especially when using low laser energies and/or small diameters of laser beam required for improved spatial resolution.

TETRAHEDRALLY COORDINATED BORON IN Li-BEARING OLENITE FROM "MUSHROOM" TOURMALINE FROM MOMEIK, MYANMAR

So-called “mushroom” tourmaline, a botryoidal aggregate of pale pink tourmaline crystals, occurs in the Palelni mine, Khetchel village, Molo quarter, Momeik Township, northeast of Mogok, Mandalay Division, Shan State, Myanmar. It has been characterized by crystal-structure determination, electron-microprobe analysis (EMPA), secondary-ion mass spectrometry (SIMS), uranium extraction, and inductively coupled plasma – mass spectrometry (ICP–MS). This sample consists of sprays of radially arranged crystals up to ~9 mm in length and ~50–150 μm in diameter. Chemical data show that Al increases, whereas the Mn content decreases, from the center of the aggregate to the pyramidal faces. The composition at the center can be described as a Mn-bearing elbaite with ~1.6 wt% MnO. Near the pyramidal faces, the composition of these tourmaline crystallites is relatively Al-rich (up to 43.6 wt% Al 2 O 3 ), and Mn-poor (0.1–0.3% MnO). A crystal-structure refinement of the Li-bearing olenite (near the pyramidal faces), in combination with analytical results, gives the optimized formula X (Na 0.55 Ca 0.08 Pb 0.01 □ 0.36 ) Y (Al 2.13 Li 0.75 Mn 2+ 0.01 □ 0.11 ) Z Al 6 T (Si 5.34 B 0.66 )O 18 (BO 3 ) 3 V (OH) 3 W [(OH) 0.50 O 0.26 F 0.24 ], with a 15.7561(6), c 7.0703(5) A. The structure refinement yields a relatively high [4] B content (Si 0.87(1) B 0.13(1) ), and a T –O> distance of 1.604 A, the shortest observed in natural tourmaline to date. Together with Al-rich tourmalines from other localities, a positive correlation between Al at Y site and [4] B with r 2 ≈ 0.998 was found. Furthermore, this tourmaline aggregate shows a relatively high Pb content (1640 ppm) and a significant Bi content (153 ppm). The REE pattern (∑REE: ~7.2 ppm) exhibits a weak negative Eu anomaly, a strong positive Tm anomaly, and a positive Tb anomaly. Such a pattern cannot be derived simply from a late-stage fractionated magma of a S-type granite or from differentiated crustal melts alone.

Growth of zinc platelets on zinc single crystals under diffusion conditions

The surface morphology of zinc platelets on zinc single crystals in the presence of hydrogen or argon is investigated. Platelets appear only on the pyramidal and prismatic faces and not upon the basal face of the zinc single crystals. It is shown that the smooth basal faces of the platelets change their shape in skeletal. The results are discussed and are in conformance with Chernov's theory for the disturbance of the morphological stability of crystal forms. The kinetics of growth of platelets on the pyramidal {1 0 1¯ 1} and prismatic {1 0 1¯ 0} faces of zinc single crystals and the alteration with time t the edge length a of the {0 0 0 1} basal face of the same crystals in the presence of argon were studied. The results of measurement of the edge a with time show that at the moment when the critical size a k is reached, the transition from kinetic to diffusion regimes starts. The comparison between the time when the visible platelets appear on the zinc crystals and the a 2 vs. t curves revealed that the first platelets grow exactly at the transition between the two regimes. It has been established that during the initial stage, when the size L is small, the platelets grow at a constant rate. During further growth, as the size L increases and reaches the critical dimension L k , a change in the L 2 vs. t curves appear: the linear curve is transformed into parabolic one. In conformance with the morphological theory of Chernov in the isotropic case, it is concluded that the platelets grow in a kinetic and a diffusion regime. During the growth of the platelets, part of them after reaching a certain thickness terminate their growth. The termination of the frontal growth is probably related to the transformation of their atomically rough surfaces into flat faces. The kinetic investigations led to the conclusion that part of the platelets do not grow following a screw dislocation mechanism. The observed transformation of triangular platelets into trapezoidal suggests that their frontal faces are atomically rough. The growth of the trapezoidal platelets is discussed and explained by a normal mechanism, while the growth of triangular platelets is attributed to a screw dislocation growth pattern.

Synthesis of all-silica BEA-type material in basic media

Previous investigations have shown that an all-silica BEA-type (Si-BEA) material is difficult to be nucleated in basic media and that the fluorosilicic precursors formed in near neutral media play an important role in its formation. The present study reports on the synthesis of this microporous material under basic conditions by using TEA 2SiF 6 as an additional silica source that provides preformed fluorosilicate species. The effect of the amount of TEA 2SiF 6 on the purity of the final product, crystallization kinetics and morphological features of Si-BEA were systematically studied. The investigation showed that above a certain concentration corresponding to TEA 2SiF 6/SiO 2 = 0.07 pure Si-BEA was obtained and further increase in the concentration improved substantially the crystallization kinetics. The increase in TEA 2SiF 6 content led also to a gradual increase of the size of the pyramidal ( h 0 l) face coupled with the decrease of pinacoidal (0 0 1) one. Thus, the morphology of the zeolite crystals can be varied from well developed truncated bipyramids to plate-like crystals with dominating pinacoidal faces. A consequence of this morphological changes is the number the pore openings of the channel systems running along the a- and c-axes per total external surface. The framework defectness and water content in the synthesized materials were studied by 29Si NMR spectroscopy and TG analysis, respectively. This combined study revealed that materials with a high level of hydrophobicity, due to the specific effect of fluorine anions, have been synthesized under basic conditions.

Growth of snow crystals from frozen water droplets

Growth of frozen water droplets, falling freely in a supercooled cloud, was investigated by using a large cloud chamber 6.5 m in height. Water droplets containing silver iodide in suspension were dropped from the top of the chamber. The size of frozen water droplets was between 20 and 100 μm in diameter. They were nucleated at about − 5 °C in most cases and froze into single-crystalline particles. Frozen water droplets grew at temperatures between − 5 and − 33 °C. Two modes were confirmed at the initial growth stage. In one mode, twenty circular crystal faces appeared during the growth. They were 2 basal, 6 prism and 12 pyramidal faces. {1120} crystal faces, whose existence has been suggested by theory, were confirmed in this experiment. In the other mode, steps or irregular patterns appeared in place of circular pyramidal and prism faces. After further growth frozen water droplets grew to short column-like ice crystals in both modes, and finally grew to snow crystals with the crystal habit as shown by Nakaya's diagram.

Morphology evolution of anatase TiO 2 nanocrystals under a hydrothermal condition (pH = 9.5) and their ultra-high photo-catalytic activity

Morphology evolution of anatase TiO 2 nanocrystals under a hydrothermal condition (pH = 9.5) was observed and a relationship between the morphology evolution and photo-catalytic activity was investigated. A titanium hydroxide nanogel was neutrally precipitated from a TiCl 4 aqueous solution and then used as the precursor in the hydrothermal process. In the hydrothermal process, the nanogel was finally crystallized and grew to a capped bipyramidal nanocrystal which has faceted {1 0 1} pyramidal faces and are capped with rounded (0 0 1) faces, passing through an elongated structure with zigzag {1 0 1} faces. The final morphology was evolved via four growth stages in series: (i) formation and growth of anatase nuclei with consuming the nanogel, (ii) rapid growth along [0 0 1] direction by oriented attachments between (0 0 1) faces to produce an elongated structure with zigzag {1 0 1} pyramidal faces, (iii) flattening of zigzag {1 0 1} pyramidal faces by solution and precipitation, and (iv) rapid growth along [0 0 1] direction by solution and precipitation (Ostwald ripening). The photo-catalytic activity of anatase nanocrystals was closely related to the morphology evolution. Especially, the capped bipyramidal nanocrystals in the last growth stage showed an excellent photo-catalysis behavior: some of them were more than three times active than a commercial photo-catalyst (P25). The ultra-high photo-catalytic activity originated in the well-developed {1 0 1} surfaces, since the 4-coordinated Ti ions in the step edge of {1 0 1} face effectively adsorbed hydroxyl ions. Therefore, the particle morphology (surface property) was important as much as the particle diameter in TiO 2 photo-catalysis.

Control of the Morphology of All-Silica BEA-type Zeolite Synthesized in Basic Media

All-silica BEA-type (Si-BEA) zeolite readily crystallizes in fluoride quasi-neutral media, while its synthesis under basic conditions is difficult. To compare the morphological features of BEA-type materials synthesized in quasi-neutral and basic media, a two-step nucleation procedure, which allows controlled synthesis of Si-BEA under basic conditions, was developed. The procedure consists of (i) aging of the initial system at a close to neutral pH, and (ii) raising the pH and a secondary aging in basic media. This procedure was further employed to study the effect of the synthesis formulation, especially its sodium and potassium content, on the crystal morphology of Si-BEA. With an increase in the pH of the system, a gradual decrease of the size of the pyramidal (h0l) face coupled with an increase of the pinacoidal (00l) face was observed. Thus, the morphology of zeolite crystals can be varied from well-developed truncated bipyramids to platelike crystals, where the pinacoidal face is dominant. A consequ...

Effect of exchange reduction on the temperature dependence of the depinning field of domain walls

Recent experimental data suggest that for some 2:17 type magnets, the coercive field exhibit non-monotonic temperature dependence. As the temperature is increased, the switching field increases, attains a maximum, and then decreases. We study the effect of a reduction of the exchange for the spins in the pinning boundaries on the temperature dependence of the coercive field. In our model the pinning boundary consists of periodic arrays of faces of pyramids. Simulation results of non-monotonic temperature dependence of the depinning field is found, in agreement with experimental observation. We speculate that the pinning potential can change from repulsive to attractive as the temperature is increased. The location of the domain wall is also changed.

Scanning vector Hall probe microscopy

We have developed a scanning vector Hall probe microscope for mapping magnetic field vector over magnetic samples. The microscope is based on a micromachined Hall sensor and the cryostat with scanning system. The vector Hall sensor active area is ∼5×5 μm 2 . It is realized by patterning three Hall probes on the tilted faces of GaAs pyramids. Data from these ‘tilted’ Hall probes are used to reconstruct the full magnetic field vector. The scanning area of the microscope is 5×5 mm 2 , space resolution 2.5 μm, field resolution ∼1 μT Hz −1/2 at temperatures 10–300 K.

Cell organization and ultrastructure of a magnetotactic multicellular organism

Magnetotactic multicellular aggregates and many-celled magnetotactic prokaryotes have been described as spherical organisms composed of several Gram-negative bacteria capable to align themselves along magnetic fields and swim as a unit. Here we describe a similar organism collected in a large hypersaline lagoon in Brazil. Ultrathin sections and freeze fracture replicas showed that the cells are arranged side by side and face both the external environment and an internal acellular compartment in the center of the organism. This compartment contains a belt of filaments linking the cells, and numerous membrane vesicles. The shape of the cells approaches a pyramid, with the apex pointing to the internal compartment, and the basis facing the external environment. The contact region of two cells is flat and represents the pyramid faces, while the contacts of three or more cells contain cell projections and represent the edges. Freeze-fracture replicas showed a high concentration of intramembrane particles on the edges and also in the region of the outer membrane that faces the external environment. Dark field optical microscopy showed that the whole organism performs a coordinated movement with either straight or helicoidal trajectories. We conclude that the organisms described in this work are, in fact, highly organized prokaryotic multicellular organisms.

Atomic force microscopy studies on surface morphologies of CdHg(SCN)4 crystals grown in solutions containing excessive amount of Cd(II) cations

AbstractSurface morphologies of CdHg(SCN)4 (CMTC) crystals grown from solutions with excessive amount of Cd(II) cations (5%, 20% and 50% in molar ratio) have been investigated by atomic force microscopy (AFM). [Cd(SCN)n]2‐n (n ≤ 4) complex anions formed by addition of excessive Cd(II) cations in the solutions have been found to act either as growth units or impurities during CMTC crystal growth. On the prismatic faces, incorporation of [Cd(SCN)n]2‐n (n ≤ 4) complex anions as growth units leads to the formation of well‐oriented protuberance trains at the step fronts, named as “locally anisotropic crystal growth”. These protuberances become fewer, less distinct and nearly disappeared with the increase of excessive Cd(II) cations in the solutions. The pyramidal face, however, varies from regular 2D nucleation growth at a low concentration of Cd(II) to much rougher growth surfaces at high concentrations, exhibiting typical surface morphologies where crystal growth is completely inhibited by impurities. Observations in this experiment provide a new picture of crystal growth.

Study of the flow around surface-mounted pyramids

The turbulent flow around square-based wall-mounted pyramids in thin and thick boundary layers was experimentally investigated as a function of the pyramid apex angle, ζ, and angle of attack, α, based on mean flow surface patterns, pressure and velocity measurements. For thin boundary layers, wake periodicity is observed. For slender pyramids (15°<ζ<75°), the periodic formation and shedding of vortices is observed. The shedding frequency scales with the length scale L=w′(w′/h)–1/4, where h is the pyramid height and w′ is the frontal (projected) base width. For broad pyramids, wake periodicity exists but cannot be related to vortex shedding. Vortex shedding appears related to the existence of a double vortex structure along the side faces of the slender pyramids. The location of the separation point upstream and the attachment point downstream of the obstacle also scales with L. For thick boundary layers, no wake periodicity is observed and the mean flow structure in the wake differs from that seen for thin boundary layers. The location of the separation and attachment points scales only approximately with L.