Sphere Walls Research Articles

Solid state transformations from spheres to polyhedrons in hollow Fe spheres

The present work analyzes the sintering conditions for ∼ 3 mm inner diameter hollow iron spheres, sintered for manufacturing composite structures. Optimal sintering variables were investigated modifying temperature from 700 to 1200◦C, while times were between 1 and 3 h. Results showed that packing of the spheres increased with time and temperature: at 700◦C sintering was not enough; at temperatures from 800◦ to 1000◦C spheres were well sintered with porosity between them; while at higher temperatures were completely packed. Densities ranged from 0.6 gcm−3 to a maximum of 1.1 gcm−3 for spheres 100% packed, where it was observed a sphere-to-polyhedron shape transformation, with maximum values of penetration (0.39 mm) and sintering neck width (1.42 mm). Complete packing of the Fe particles of the sphere walls was also observed. The use of Design of Experiments made possible to establish correlations between sintering variables and characteristics such as neck width, penetration, porosity and packing. These results could be used as a starting point for the adequate selection of the sintering conditions of hollow Fe spheres for manufacturing hollow composite structures, taking into account not only the characteristics of the sintered hollow spheres but also of the Fe particles forming their walls.

Novel template-free procedure of polyacrylonitrile-derived carbon hollow spheres preparation in the presence of palladium

A novel preparation method of hollow carbon spheres (HCSs) was developed. For the first time, polyacrylonitrile-derived HCSs were obtained under IR heating of joint precursor of the polymer and palladium compound in the presence of alkali. The simultaneous carbonization of polyacrylonitrile and its chemical activation with KOH in the presence of forming palladium nanoparticles provide the production of multiple HCSs located on the surface of the highly-porous carbon support. According to the proposed HCS preparation mechanism, the presence of potassium hydroxide during PAN pyrolysis leads to changes in both the products of pyrolysis and their concentration, which provides to the formation of a large amount of HCSs on palladium nanoparticles. The obtained HCSs are characterized by an outer diameter of 40–500 nm and a wall thickness of 3–15 nm. TEM micrographs and analysis of structural characteristics showed that HCS walls consist of oxidized graphite with an interplanar distance of 3.96 Å. It was found that HCSs are non-uniform, some of them have thickly sites built by additional graphene sheets on the graphitic sphere walls.

Convective Heat Transfer and Entropy Generation Around a Sphere Within Cuboidal Enclosure

The intricate three-dimensional natural convection and entropy generation of a flow between cuboidal enclosure and a spherical surface is numerically examined in this study. The sphere is maintained hot and centrally located in cubical enclosure with four cooled walls, whereas top and bottom walls are perfectly insulated. The effects of inner shape, Rayleigh numbers, and inclination angles on fluid flow and heat transfer characteristics are analyzed in detail for Rayleigh numbers ranging from to and a tilted angle of the enclosure varying from 0 to 90°. The working fluid is air so that the Prandtl number equates to 0.71. The developed mathematical model is governed by the coupled equations of continuity, momentum, and energy and is solved by a finite volume method. Based on numerical results, two correlation expressions for predicting average Nusselt number of lateral walls of the cubical enclosure and the inner sphere are proposed as a function of Rayleigh number and inclination angle under different wake parameters. By comparing inner spherical shape results with that of cylindrical shape, it can be inferred that using a spherical shape provides the highest thermal performance for heat transfer. Moreover, this comparison reveals that the inner spherical shape decelerates the unsteadiness for the considered inclinations. Results indicate also that the optimal average heat transfer rate is obtained for extreme Rayleigh number, , and an inclination of 90° for both cases of the inner sphere and lateral walls of the cuboidal enclosure. The Bejan number and total entropy generation are found to be increased by increasing the inclination angle for the considered Rayleigh numbers. Maximum local entropy generation due to heat transfer and that due to fluid friction decrease gradually by increasing the inclination angle. The results of this investigation find extensive array of applications in several engineering devices, which will be mentioned in the paper content.

Interplay of adsorption and surface mobility in tracer diffusion in porous media.

We model the diffusion of a tracer that interacts with the internal surface of a porous medium formed by a packing of solid spheres. The tracer executes a lattice random walk in which hops from surface to bulk sites and hops on the surface have small probabilities compared to hops from bulk sites; those probabilities are related to bulk and surface diffusion coefficients and to a desorption rate. A scaling approach distinguishes three regimes of steady state diffusion, which are confirmed by numerical simulations. If the product of desorption rate and sphere diameter is large, dominant bulk residence is observed and the diffusion coefficient is close to the bulk value. If that product is small and the surface mobility is low, the tracers are adsorbed most of the time but most hops are executed in the bulk. However, for high surface mobility, there is a nontrivial regime of dominant surface displacement, since the connectivity of solid walls allows the tracers to migrate to long distances while they are adsorbed. In this regime, we observe rounded tracer paths on the sphere walls, which are qualitatively similar to those of a recent experiment on polystyrene particle diffusion. The calculated average residence times are proportional to the bulk and surface densities of an equilibrium ensemble of noninteracting tracers, and the relation between those densities sets the adsorption isotherm. Simulations performed with initially uniform (nonequilibrium) distribution of tracers in the pores show other nontrivial results in cases of dominant surface residence: slow increase of the mean-square displacement at short times, since the tracer has not explored a homogeneous medium, and a remarkable slowdown between the first encounter with a solid wall and the first hop from that point. Relations between our results and other models of diffusion and adsorption in porous media are discussed.

Photoluminescence of the integrating sphere walls, its influence on the absolute quantum yield measurements and correction methods

Determination of the absolute quantum yield (QY) of photoluminescence (PL) or electroluminescence is commonly performed using an integrating sphere (IS) – versatile device for radiometry applications. The key feature of IS is very high diffused reflectance of its internal surface. Two materials are commonly used: (a) the sintered high-density polytetrafluoroethylene (Spectralon) and (b) materials based on barium sulphate (Spectraflect). Using PL-micro-spectroscopy we show that both materials reveal PL from localized centers excitable by UV, blue and green light emitting broad PL spectrum extending up to the red spectral region. The main effect of PL from IS-walls is introduction of non-constant parasitic background which is mixed with PL from a tested sample during the QY measurements. We develop theoretical description of QY determination which includes effects of PL from IS walls. This allows us to propose and test a reliable and universal correction for the IS-related PL background. Finally, a method of “black sample” is proposed and applied to estimate PL QY of Spectraflect which is shown to decrease from 0.09 to 0.015% for excitation shift from 320 to 440 nm.

On the behavior of degree of polarization surfaces at the limit of Poincaré sphere walls

Recently a new class of mathematical entities—the gyrovectors—came into light in physics. The most intuitive examples of gyrovectors are the relativistic allowed velocities and the Poincare polarization vectors. Taking advantage of the (also recently elaborated) approach of degree of polarization surfaces in polarization theory, I make an analysis of the strange behavior of these entities in the interaction between the polarized light and the orthogonal dichroic polarization devices. The same approach can be applied in the theory of relativity as well as in many other branches of optics (multilayer optics, ray optics, laser cavity physics, quantum optics) where the Lorentzian character of the transformations which govern some specific problems has been recognized.

High-Force Display Capability and Wide Workspace With a Novel Haptic Interface

We present a novel haptic interface, named VirtuaPower, having a high-force display capability (FDC) and a wide workspace with a six-degree-of-freedom position measurement and FDC. The overall mechanism of VirtuaPower is designed based on a parallel pair of five-bar mechanisms supporting the end-platform. This novel mechanism enhances the high-FDC and overcomes the problem of small-size workspace, which are the pros and cons of parallel mechanisms, respectively. First, we present the conceptual design and prototype of VirtuaPower. Then, we formulate the forward kinematic and the constraint and space Jacobian. Using these formulations, we analyze two types of workspaces and FDC. In both analyses, VirtuaPower shows remarkable performances, especially in terms of its FDC, compared to other haptic interfaces having an equivalent size. In addition, we validate the static, step, and frequency responses of the force-display performance by performing experiments using the test bench of fixed end setup with a six-dimensional force/torque sensor. Finally, we evaluate the suitability of VirtuaPower as a haptic interface by realizing two types of virtual sphere walls and a virtual reality environment.

Use of wall-less 18F-doped gelatin phantoms for improved volume delineation and quantification in PET/CT

Positron emission tomography (PET) with 18F-FDG is a valuable tool for staging, planning treatment, and evaluating the treatment response for many different types of tumours. The correct volume estimation is of utmost importance in these situations. To date, the most common types of phantoms used in volume quantification in PET utilize fillable, hollow spheres placed in a circular or elliptical cylinder made of polymethyl methacrylate. However, the presence of a non-radioactive sphere wall between the hotspot and the background activity in images of this type of phantom could cause inaccuracies. To investigate the influence of the non-active walls, we developed a phantom without non-active sphere walls for volume delineation and quantification in PET. Three sizes of gelatin hotspots were moulded and placed in a Jaszczak phantom together with hollow plastic spheres of the same sizes containing the same activity concentration. 18F PET measurements were made with zero background activity and with tumour-to-background ratios of 12.5, 10, 7.5, and 5. The background-corrected volume reproducing threshold, Tvol, was calculated for both the gelatin and the plastic spheres. It was experimentally verified that the apparent background dependence of Tvol, i.e., a decreasing Tvol with increasing background fraction, was not present for wall-less spheres; the opposite results were seen in plastic, hollow spheres in commercially-available phantoms. For the types of phantoms commonly used in activity quantification, the estimation of Tvol using fillable, hollow, plastic spheres with non-active walls would lead to an overestimate of the tumour volume, especially for small volumes in a high activity background.

Novel Concept for Pure Diffusive Capillary Membrane Oxygenators

The preeminent limitation of silicone membrane oxygenators is the poor gas permeability compared with microporous hollow fiber oxygenators (MHFO). However, the imponderability of plasma leakage, foam formation, and brittleness are all hazards that result in blood trauma formation, hereby limiting the application of MHFO during long-term oxygenation therapies. Here, we introduce a novel type of pure diffusive capillary-form silicone membrane called silicone hollow sphere. Silicone hollow sphere walls embed hollow microspheres into the core. The lodging of such microspheres promotes a higher gas exchange performance (as a result of the reduction of dense material) without altering the total thickness of capillary walls; thereby the demanded mechanical strength for handling is nevertheless conserved. Out of the same silicone material, seven SiHSp fibers with six different design specifications and a control were constructed to define experimentally the appropriate configuration for subsequent production. Each fiber was used in a miniaturized module oxygenator of a constant effective membrane surface area (Amem = 0.02 m) and length (L =183 mm) for a fair evaluation. Modules were investigated in vitro with porcine blood. O2 and CO2 transfer rates weighed 12.6 mlO2/min and 10.4 mlCO2/min, respectively, for one type of SiHSp, comparable with microporous polypropylene (OXYPHAN) exhibiting 14.1 mlO2/min and 13.2 mlCO2/min, respectively, at a maximum blood flow rate (Qmax = 200 ml/min). Silicone hollow sphere fibers show a promising competency to MHFs. They also show an evident dominancy over the conventional silicone fibers, evaluated by the control module, which emphasizes the advantage of this design.

Controllable synthesis of Ce1−xZrxO2 hollow nanospheres via supercritical anti-solvent precipitation

Nanocrystalline Ce 1-x Zr x O 2 hollow nanospheres were successfully synthesized via supercritical anti-solvent precipitation using supercritical CO 2 as the anti-solvent. It was found that the as-produced samples exhibited hollow spherical structures with uniform diameters ranging from 30 to 50 nm and the sphere walls were composed of various oriented nanocrystallites, with sizes of 3-7 nm. The results of high-resolution transmission electron microscopy showed that the formation of the hollow structures could be controlled by adjusting the solution concentration. The results of temperature-programmed reduction and oxygen storage capacity measurements showed that the hollow nanospheres had enhanced redox properties. A possible mechanism for the formation of Ce 1-x Zr x O 2 hollow nanospheres has also been proposed and experimental investigated.

Effects of cold sphere walls in PET phantom measurements on the volume reproducing threshold

We studied quantitatively the effects of the discontinuity introduced in an otherwise homogeneous background by the cold walls of the standard spherical glass inserts commonly used in phantom measurements for calibration of threshold-based approaches to volumetric evaluation of PET investigations. We concentrated especially on the question of threshold-based volume determination. We computed analytically the convolution of an isotropic Gaussian point-spread function with the insert geometry (hot sphere + cold wall + warm background) and derived the theoretical background dependence of the volume reproducing threshold. This analysis shows a clear wall-related reduction of the optimal threshold with increasing background. The predictions of our theoretical analysis were verified in phantom measurements at background fractions between 0 and 0.29. Defining the background-corrected relative threshold (Tabs: absolute volume reproducing threshold, A: measured activity at centre, B: background), we find that for a wall-less sphere T is independent of the background level. In the presence of cold walls, T drops (for not too small spheres, where recovery at the centre approaches 100%) from about 43% at B/A = 0 to about 25% at B/A = 0.5. Applying these thresholds to wall-less spheres leads to sizeable overestimates of the true volumes (43% at B/A = 0.5 for a sphere of 6 ml volume). We conclude that phantom measurements with standard sphere inserts for calibration of optimal thresholding algorithms introduce a systematic bias if performed at finite background levels. The observed background dependence is an artefact of the measurement procedure and does not reflect the conditions present in actual patient investigations.

Performances of a [formula omitted]-delayed neutron detection array at Peking University

A β-delayed neutron detection array composed of a neutron sphere and two neutron walls was constructed in the State Key Laboratory of Nuclear Physics and Technology at Peking University. Recently the performances of this detection array were largely improved and tested with a Co60 source, cosmic rays and C16 and N17 radioactive beams. The Tyvek 1056D paper and silicone grease were chosen for the reflection and coupling materials, respectively. For the neutron sphere with large detection solid angle (30% of 4π steradian), the intrinsic efficiency is about 14.1% at a neutron energy of 1MeV and the detection threshold is about 350keV; for the neutron walls with flexible setup, these values are 36.5% and 200keV, respectively. The combined array of neutron sphere and neutron walls has successfully been applied to measure the β-delayed neutrons emitted from neutron-rich unstable nuclei.

Scroll waves in spherical shell geometries.

The evolution of scroll waves in excitable media with spherical shell geometries is studied as a function of shell thickness and outer radius. The motion of scroll wave filaments that are the locii of phaseless points in the medium and organize the wave pattern is investigated. When the inner radius is sufficiently large the filaments remain attached to both the inner and outer surfaces. The minimum size of the sphere that supports spiral waves and the maximum number of spiral waves that can be sustained on a sphere of given size are determined for both regular and random initial distributions. When the inner radius is too small to support spiral waves the filaments detach from the inner surface and form a curved filament connecting the two spiral tips in the surface. In certain parameter domains the filament is an arc of a circle that shrinks with constant shape. For parameter values close to the meandering border, the filament grows and collisions with the sphere walls lead to turbulent filament dynamics. (c) 2001 American Institute of Physics.

Hisingerite: A Ferric Kaolin Mineral with Curved Morphology

AbstractHisingerite, first described in 1810, has been variously regarded as noncrystalline, as a septechlorite, as ferric allophane, as ferric halloysite and as poorly crystalline nontronite. Hisingerite from the original localities of Gillinge and Riddarhyttan in Sweden has a composition close to Fe2O3·2SiO22H2O. X-ray diffraction (XRD) analysis of Riddarhyttan hisingerite yields very broad maxima at 7.7, 4.44, 3.57, 2.56, 2.26, 1.69 and 1.54 Å, and that from Gillinge is similar. Cation exchange capacities are 2.2 meq/100 g (Riddarhyttan) and zero (Gillinge). Transmission electron microscopy (TEM) shows a fabric of concentric spheres and part spheres, with diameters of about 140 Å and walls up to six 7-Å layers thick. High-resolution images of the sphere walls reveal a 2-layer structure similar to that of kaolinite. A calculated diffraction pattern based on a model of 4 concentric shells of ferric kaolinite structure matches the observed pattern quite closely. Some other hisingerites, notably that from Bellevue King Mine, Idaho, show 10-Å layers as well as 7-Å layers, and this hisingerite has a CEC of 32 meq/100 g and a weak 15.5-Å X-ray reflection in addition to a pattern similar to Riddarhyttan hisingerite. It is concluded that hisingerite is a curved ferric 7-Å 1:1 layer silicate analogous to halloysite, and that many of the hisingerites reported in the literature contain admixed nontronite.

Steel and Titanium Hollow Sphere Foams

ABSTRACTMetal hollow sphere foams are fabricated by bonding millimeter sized metal alloy hollow spheres at points of contact. The spheres are formed as powder shells from slurries. For stainless steel spheres, the starting powder is a mixture of iron and chromium oxide. Thermal treatment in hydrogen reduces the oxides to Fe/Cr alloys with less than 2% porosity in sphere walls. The nominal composition is close to that of 405 stainless. Carburization in CO/CO2 atmosphere followed by heat treatment produces foams of either 410 or 420 type stainless steels depending on carbon content. Compressive stress-strain behavior was measured on point contact bonded stainless foams both before and after carburization. Hardness measurements on steel sphere walls were used to estimate the yield strength. Relative strengths of the foams were positioned between open and closed cell models. This was encouraging because bonding in the foams was less than optimum and the hollow sphere walls contained defects. As processing improves, strengths should increase. To produce titanium alloy spheres, the starting powder is titanium alloy hydride. Thermal treatment in an inert atmosphere decomposes the hydride and sinters the titanium powder in the sphere walls to greater than 96% relative density. Both titanium and Ti-6V-4V spheres and foams have been produced. Oxygen contents are a concern for titanium compositions and processing is being altered to reduce oxygen levels to increase ductility

Noncrystalline Fe-Si-Al-Oxyhydroxides

AbstractHigh-resolution transmission electron microscopy of noncrystalline Fe-Si-Al-oxyhydroxide gels shows a common structure of hollow packed spheres having external diameters ranging from 50 to 1000 Å. Some sphere walls display a concentric structure, particularly if the gel composition is close to that of a crystalline clay mineral (e.g., smectite, kaolin). The spheres probably formed by expansion of void space (bubbles) as the surrounding gel contracted 5–10% because of partial ordering of the Fe-Si-Al-oxygen network. Much of the water contained in such noncrystalline minerals is incorporated within the bubbles.

Efficient averaging spheres for visible and ultraviolet wavelengths

On the basis of a theoretical analysis that identifies the design criteria for optimization of efficiency, an averaging sphere with a barium-sulfate target was constructed that has an efficiency of 80-90% in the visible. In spite of the decreased reflectance of the sphere walls in the uv, this sphere is still 30% efficient at 300 nm and, thus, can be used for near-uv work. For applications below 300 nm, a cell containing a fluorescent dye is placed inside the sphere in order to shift the incident radiation into the longer wavelength region in which the reflectance of the sphere walls is higher. The use of this fluorescent wavelength converter, which was designed so that the sphere remains usable in the visible, resulted in a sphere efficiency of about 20% in the region between 200 nm and 300 nm and 50% in the visible. The averaging properties of these spheres were found to be adequate for high-accuracy photometric measurements.

Sphere-Rod Morphogenesis in Arthrobacter crystallopoietes II. Peptides of the Cell Wall Peptidoglycan

Cell walls of Arthrobacter crystallopoietes grown as spheres and as rods were solubilized by treatment with the B enzyme from Chalaropsis, an N-acetylmuramidase. The neutral glycopeptides were then isolated by chromatography on ECTEOLA cellulose. The glycopeptides, consisting of disaccharide-peptide units interlinked by peptide cross-bridges, were fractionated by gel filtration on Sephadex columns into oligomers of various sizes. The size distribution ranged from monomers with no cross-bridges to polymers with a high degree of polymerization, but did not differ significantly between cell walls from cells grown as spheres or rods. Some small differences in the distribution of C- and N-terminal amino acids were found. Analyses revealed that all the peptide bridges in the glycopeptide fractions from rod cell walls were formed by one l-alanine residue. In sphere cell walls, l-alanine was also found, but, in addition, higher oligomers of the glycopeptide contained glycine in their cross-bridges. These results were confirmed by determinations of C- and N-terminal amino acids released after lysostaphin and AL-1 enzyme digestions and by Edman degradations. Models representing the structures of the sphere and rod cell walls are presented. These structures indicate that the sphere cell wall is probably a more loosely knit macromolecule than is the rod cell wall.