Showing papers on "Shell (structure) published in 2011"

Effects of thickness stretching in functionally graded plates and shells

Abstract: The present work evaluates the effect of thickness stretching in plate/shell structures made by materials which are functionally graded (FGM) in the thickness directions. That is done by removing or retaining the transverse normal strain in the kinematics assumptions of various refined plate/shell theories. Variable plate/shell models are implemented according to Carrera’s Unified Formulation. Plate/shell theories with constant transverse displacement are compared with the corresponding linear to fourth order of expansion in the thickness direction ones. Single-layered and multilayered FGM structures have been analyzed. A large numerical investigation, encompassing various plate/shell geometries as well as various grading rates for FGMs, has been conducted. It is mainly concluded that a refinements of classical theories that include additional in-plane variables could results meaningless unless transverse normal strain effects are taken into account.

Plates and Shells for Smart Structures: Classical and Advanced Theories for Modeling and Analysis

Abstract: Smart structures that contain embedded piezoelectric patches are loaded by both mechanical and electrical fields. Traditional plate and shell theories were developed to analyze structures subject to mechanical loads. However, these often fail when tasked with the evaluation of both electrical and mechanical fields and loads. In recent years more advanced models have been developed that overcome these limitations. Plates and Shells for Smart Structures offers a complete guide and reference to smart structures under both mechanical and electrical loads, starting with the basic principles and working right up to the most advanced models. It provides an overview of classical plate and shell theories for piezoelectric elasticity and demonstrates their limitations in static and dynamic analysis with a number of example problems. This book also provides both analytical and finite element solutions, thus enabling the reader to compare strong and weak solutions to the problems.

Double-emulsion drops with ultra-thin shells for capsule templates

Abstract: We introduce an emulsification technique that creates monodisperse double-emulsion drops with a core–shell geometry having an ultra-thin wall as a middle layer. We create a biphasic flow in a microfluidic capillary device by forming a sheath flow consisting of a thin layer of a fluid with high affinity to the capillary wall flowing along the inner wall of the capillary, surrounding the innermost fluid. This creates double-emulsion drops, using a single-step emulsification, having a very thin fluid shell. If the shell is solidified, its thickness can be small as a hundred nanometres or even less. Despite the small thickness of this shell, these structures are nevertheless very stable, giving them great potential for encapsulation. We demonstrate this by creating biodegradable microcapsules of poly(lactic acid) with a shell thickness of a few tens of nanometres, which are potentially useful for encapsulation and delivery of drugs, cosmetics, and nutrients.

Wave Function Engineering for Ultrafast Charge Separation and Slow Charge Recombination in Type II Core/Shell Quantum Dots

Abstract: The size dependence of optical and electronic properties of semiconductor quantum dots (QDs) have been extensively studied in various applications ranging from solar energy conversion to biological imaging. Core/shell QDs allow further tuning of these properties by controlling the spatial distributions of the conduction-band electron and valence-band hole wave functions through the choice of the core/shell materials and their size/thickness. It is possible to engineer type II core/shell QDs, such as CdTe/CdSe, in which the lowest energy conduction-band electron is largely localized in the shell while the lowest energy valence-band hole is localized in the core. This spatial distribution enables ultrafast electron transfer to the surface-adsorbed electron acceptors due to enhanced electron density on the shell materials, while simultaneously retarding the charge recombination process because the shell acts as a tunneling barrier for the core localized hole. Using ultrafast transient absorption spectroscopy...

ZnO@MoO3 core/shell nanocables: facile electrochemical synthesis and enhanced supercapacitor performances

Abstract: Core/shell nanostructures often exhibit novel physical and chemical properties. Herein ZnO@MoO3 core/shell nanocables have been synthesized in large quantities by a simple electrochemical method at room temperature and the shell thickness of MoO3 can be controlled by changing the deposition time. The synthesized core/shell nanocables were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The inner ZnO nanorod shows a single-crystal structure and has preferential growth in the [0001] direction. The MoO3 shell also shows a single-crystal structure with a [0001] direction. The prepared ZnO@MoO3 core/shell nanocables have been successfully employed as supercapacitor electrodes and give a specific capacitance of 236 F g−1 at scan rate of 5 mV s−1, which is much larger than that of MoO3 nanoparticles. In addition, ZnO@ MoO3 core/shell nanocables show high electrochemical stability and can withstand over 1000 cycles with no obvious decrease in the specific capacitance.

A reduced integration solid-shell finite element based on the EAS and the ANS concept—Large deformation problems

Abstract: In this paper we address the extension of a recently proposed reduced integration eight-node solid-shell finite element to large deformations. The element requires only one integration point within the shell plane and at least two integration points over the thickness. The possibility to choose arbitrarily many Gauss points over the shell thickness enables a realistic and efficient modeling of the non-linear material behavior. Only one enhanced degree-of-freedom is needed to avoid volumetric and Poisson thickness locking. One key point of the formulation is the Taylor expansion of the inverse Jacobian matrix with respect to the element center leading to a very accurate modeling of arbitrary element shapes. The transverse shear and curvature thickness locking are cured by means of the assumed natural strain concept. Further crucial points are the Taylor expansion of the compatible cartesian strain with respect to the center of the element as well as the Taylor expansion of the second Piola–Kirchhoff stress tensor with respect to the normal through the center of the element. Copyright © 2010 John Wiley & Sons, Ltd.

Magnetic properties of a cylindrical Ising nanowire (or nanotube)

Abstract: Magnetic properties (phase diagram and magnetization) of a cylindrical Ising nanowire or nanotube are investigated by the use of the effective-field theory with correlations. Particular emphasis is given to the effects of the surface and its dilution on them. Much attention is paid to the thermal variation of the magnetization when the spins at the surface are coupled antiferromagnetically to the ferromagnetic core spins by the negative shell coupling. The effects of dilution at the surface, the surface exchange interaction, and the shell coupling on the magnetization profiles are investigated. We find a number of characteristic phenomena for them.

Towards controlled synthesis and better understanding of highly luminescent PbS/CdS core/shell quantum dots

Abstract: A two-step cation exchange procedure has been developed for synthesizing PbS/CdS core/shell quantum dots (QDs) with a much thicker shell than previously reported, which expands the flexibility of the current cation exchange approach. The thick-shell QDs allow relatively easy observation of the core/shell morphology by transmission electron microscopy as well as exhibiting characteristic absorption and emission of CdS when the shell thickness reaches 1.8 nm. X-ray diffraction patterns show gradual transformation from a rock salt PbS pattern to a zinc blend CdS pattern with increasing shell thickness and the overall diffraction pattern is indeed the same as that of the CdS standard when the shell is thicker than 3.6 nm. The thick-shell QDs were further analyzed by energy dispersive X-ray spectrometry. It is found that Pb only exists in the core region and is essentially absent in the shell region. All of these results consistently suggest that the shell is made of CdS, instead of ternary PbxCd1−xS alloy in thick-shell QDs. As direct experimental identification of the shell composition in thin-shell QDs is difficult, experimental data and calculations are combined to indirectly probe this issue. The comparison of band gap versus core size plots of different compositional models indicates that it is highly likely that the thin shell is also made of CdS. Importantly, these core/shell PbS/CdS QDs not only show significantly increased quantum yield up to 67% at the optimal shell thickness of about 0.7 nm, they are also much more photostable and thermally stable than the shell-free PbS QDs.

Analysis of the Shell Thickness Distribution on NaYF4/NaGdF4 Core/Shell Nanocrystals by EELS and EDS

Abstract: The structure and chemical composition of the shell distribution on NaYF4/NaGdF4 core/shell nanocrystals have been investigated with scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS), and energy-dispersive X-ray spectroscopy (EDS). The core and shell contrast in the high-angle annular dark-field (HAADF) images combined with the EELS and EDS signals indicate that Gd is indeed on the surface, but for many of the particles, the shell growth was anisotropic.

Fabrication of γ-MnO2/α-MnO2 hollow core/shell structures and their application to water treatment

Abstract: In this work, we present a novel concept of structural design for the synthesis of a new type of core/shell structure comprising γ-MnO2 cores inside hollow α-MnO2 shells. The approach involves two main steps. First, MnCO3 microellipsoids were mixed directly with KMnO4 solution, which lead to a manganese oxide shell on the surface of the MnCO3. Then, thermal annealing of this type of core/shell structure resulted in the formation of γ-MnO2/α-MnO2 ellipsoids, accompanying with the phase transition in both the core and shell. The as-prepared γ-MnO2/α-MnO2 ellipsoids were used as adsorbent in water treatment, and showed an excellent ability to remove organic pollutants and heavy metal ions without any other additives.

Using Shape to Control Photoluminescence from CdSe/CdS Core/Shell Nanorods

Abstract: CdSe/CdS core/shell nanorods can exhibit high photoluminescence quantum yields, but it is not yet clear what processes determine the yields and how they can be controlled. Moreover, the effective band alignment between the core and the shell affects quantum yield, but its nature is still under debate. We systematically studied quantum yields when the shell is excited as a function of both core size and shell volume. Using time-resolved photoluminescence decay measurements and transient-absorption spectroscopy, we found that quantum yields are determined by a balance between radiative and nonradiative recombination rates, and not by single-carrier trapping. The radiative recombination rate decreases as the nanorod volume increases, independent of the core size. The results indicate that high quantum yields can be obtained only by limiting the size of the shell and point to an effective quasi-type-II band alignment for all of the nanorods in this study.

Strain evolution in the single point incremental forming process: digital image correlation measurement and finite element prediction

Abstract: Incremental Sheet Forming (ISF) is a relatively new class of sheet forming processes that allow the manufacture of complex geometries based on computer-controlled forming tools in replacement (at least partially) of dedicated tooling. This paper studies the straining behaviour in the Single Point Incremental Forming (SPIF) variant (in which no dedicated tooling at all is required), both on experimental basis using Digital Image Correlation (DIC) and on numerical basis by the Finite Element (FE) method. The aim of the paper is to increase understanding of the deformation mechanisms inherent to SPIF, which is an important issue for the understanding of the high formability observed in this process and also for future strategies to improve the geometrical accuracy. Two distinct large-strain FE formulations, based on shell and first-order reduced integration brick elements, are used to model the sheet during the SPIF processing into the form of a truncated cone. The prediction of the surface strains on the outer surface of the cone is compared to experimentally obtained strains using the DIC technique. It is emphasised that the strain history as calculated from the DIC displacement field depends on the scale of the strain definition. On the modelling side, it is shown that the mesh density in the FE models plays a similar role on the surface strain predictions. A good qualitative agreement has been obtained for the surface strain components. One significant exception has however been found, which concerns the circumferential strain evolution directly under the forming tool. The qualitative discrepancy is explained through a mechanism of through-thickness shear in the experiment, which is not fully captured by the present FE modelling since it shows a bending-dominant accommodation mechanism. The effect of different material constitutive behaviours on strain prediction has also been investigated, the parameters of which were determined by inverse modelling using a specially designed sheet forming test. Isotropic and anisotropic yield criteria are considered, combined with either isotropic or kinematic hardening. The adopted constitutive law has only a limited influence on the surface strains. Finally, the experimental surface strain evolution is compared between two cones with different forming parameters. It is concluded that the way the plastic zone under the forming tool accommodates the moving tool (i.e. by through-thickness shear or rather by bending) depends on the process parameters. The identification of the most determining forming parameter that controls the relative importance of either mechanism is an interesting topic for future research.

Compression, crumpling and collapse of spherical shells and capsules

Abstract: The deformation of thin spherical shells by applying an external pressure or by reducing the volume is studied by computer simulations and scaling arguments. The shape of the deformed shells depends on the deformation rate, the reduced volume V/V0 and the Foppl–von Karman number γ. For slow deformations the shell attains its ground state, a shell with a single indentation, whereas for large deformation rates the shell appears crumpled with many indentations. The rim of the single indentation undergoes a shape transition from smooth to polygonal for γ7000(ΔV/V0)− 3/4. For the smooth rim the elastic energy scales like γ1/4 whereas for the polygonal indentation we find a much smaller exponent, even smaller than the exponent 1/6 that is predicted for stretching ridges. The relaxation of a shell with multiple indentations towards the ground state follows an Ostwald ripening type of pathway and depends on the compression rate and on the Foppl–von Karman number. The number of indentations decreases as a power law with time t following Nind~t− 0.375 for γ=8×103 and γ=8×104 whereas for γ=8×105 the relaxation time is longer than the simulation time.

Effect of CdS shell thickness on the optical properties of water-soluble, amphiphilic polymer-encapsulated PbS/CdS core/shell quantum dots

Abstract: Cation exchange, recently explored for synthesizing core/shell quantum dots (QDs), causes continuous core size change during shell formation. By carefully varying parent PbS QD size and cation exchange conditions, we have synthesized PbS/CdS core/shell QDs with a similar PbS core size of ∼4.5 nm yet a different CdS shell thickness. This enables us to study the effect of shell thickness on the properties of PbS QDs after their transfer from chloroform into watervia poly(maleic anhydride-alt-1-octadecene-co-poly(ethylene glycol)). It was found that the quantum yield (QY) of PbS cores in water firstly increases with shell thickness up to ∼0.7 nm, reaching 33%, owing to better surface passivation and then decreases to 1.7% when the shell thickness reaches 2.3 nm. Such decline is due to the formation of new defects with shell deposition. In contrast, the variation amplitude of QY during water transfer monotonically decreases and QD photostability monotonically improves with shell thickness. It is clear that although newly introduced defects play a fundamental role in the absolute QY, they do not show any overwhelmingly negative effects on the variation of QY with environments and photostability. The colloidal stability of QDs in buffers containing different salt concentrations seems to be not affected by the shell thickness, possibly due to the same steric stabilization effect of the amphiphilic polymer in all samples. Further investigation on a series of core/shell samples confirms that ∼0.7 nm is an optimal shell thickness for various core sizes investigated herein, consistently yielding the maximum QY and reasonably good photostability.

Prestressed Morphing Bistable and Neutrally Stable Shells

Abstract: This study deals with prestressed shells, which are capable of “morphing” under large deflexions between very different load-free configurations. Prestressing involves plastically curving a flat, thin shell in orthogonal directions either in the opposite or same sense, resulting in two unique types of behavior for isotropic shells. Opposite-sense prestressing produces a bistable, cylindrically curved shell provided the prestress levels are large enough and similar in size: This effect forms the basis of a child’s “flick” bracelet and is well known. On the other hand, same-sense prestressing results in a novel, neutrally stable shell provided the levels are also sufficiently large but identical: The shell has to be made precisely, otherwise, it is monostable and is demonstrated here by means of a thin, helically curved strip. The equilibrium states associated with both effects are quantified theoretically and new expressions are determined for the requisite prestress levels. Furthermore, each stability response is revealed in closed form where it is shown that the neutrally stable case occurs only for isotropic materials, otherwise, bistability follows for orthotropic materials, specifically, those, which have a shear modulus different from the isotropic value. Finally, prestressing and initial shape are considered together and, promisingly, it is predicted that some shells can be neutrally stable and bistable simultaneously.

One‐Pot Synthesis of Silica@Coordination Polymer Core–Shell Microspheres with Controlled Shell Thickness

Abstract: Coordination polymers have received much attention due to their unique applications, such as gas storage, [ 1 ] catalysis, [ 2 ] separation, [ 3 ] and optics. [ 4 ] In addition to the typical macroscaled crystalline coordination polymer materials, microand nanoscaled crystalline or amorphous coordination polymer particles (CPPs) have been recently developed by several research groups. [ 5–8 ] CPPs are now being developed into a new branch of colloidal materials with unique applications in gas storage materials, [ 6 ] heterogeneous catalysts, [ 7 ] and imaging probes. [ 8 ] Meanwhile, the merger of coordination polymers with other solid materials can extend the scope of the utilization of these materials, however, the conjunction of coordination polymers to other solid materials to produce hybrid materials is barely studied. [ 9–11 ] Seed coating and chemical modifi cation of the support surface are general useful methods for directing the initiation and growth of coordination polymers on the surface of solid materials because the heterogeneous growth of coordination polymers on the support surface is usually poor. [ 10 ] Usually a carboxylate-terminated surface induces a regular growth of coordination polymers. [ 11 , 12 ] Here we present a novel advance in the technical use of coordination polymers for the preparation of core–shell structures. We fi nd that coordination polymerization of organic building blocks and metal nodes can be initiated on carboxylate-terminated silica particles to produce unique silica@coordination polymer core–shell microspheres. Silica particles are often used in the construction of core-shell structures due to their optical transparency, easy of production with narrow size distribution, and low cost. [ 13 ] Furthermore, we also demonstrate that the shell thickness in core–shell structures can be judiciously controlled by adjusting the amount of reactants, including coordination polymer precursors and silica particles. In a typical synthesis, silica@coordination polymer core– shell microspheres were prepared by the following process ( Scheme 1 ): Carboxylate-terminated silica particles with a diameter of 0.99 μ m were added to a N,N -dimethylformamide (DMF) solution of coordination polymer precursors, which contained metal nodes, In(NO 3 ) 3 , and the organic building blocks isophthalic acid (H 2 IPA). [ 14 ] Carboxylate groups on the silica surface interact with In 3 + and initiate the growth of coordination polymers. [ 11 ] This interaction should induce a regular growth of

LED light bulb equipped with light transparent shell fastening structure

Abstract: An LED light bulb includes a power conversion board, at least one light source baseboard electrically connected to the power conversion board, a heat sink and a light transparent shell to hold the light source baseboard. The heat sink has a wedged groove with a first holding portion formed thereon. The light transparent shell includes a sphere and a neck wedged in the wedged groove. The sphere and neck have an inner wall surrounded to hold the power conversion board and an outer wall opposite to the inner wall. The neck has a longitudinal shell retaining portion on the outer wall corresponding and fastening to the first holding portion and at least one transverse shell retaining portion. The wedged groove has at least one second holding portion corresponding and fastening to the transverse shell retaining portion to restrict relative turning of the light transparent shell and heat sink.