Showing papers on "Shell (structure) published in 2005"

Engineering InAs(x)P(1-x)/InP/ZnSe III-V alloyed core/shell quantum dots for the near-infrared.

Abstract: Quantum dots with a core/shell/shell structure consisting of an alloyed core of InAs(x)P(1-x), an intermediate shell of InP, and an outer shell of ZnSe were developed. The InAs(x)P(1-x) alloyed core has a graded internal composition with increasing arsenic content from the center to the edge of the dots. This compositional gradient results from two apparent effects: (1) the faster reaction kinetics of the phosphorus precursor compared to the arsenic precursor, and (2) a post-growth arsenic-phosphorus exchange reaction that increases the arsenic content. The cores have a zinc blend structure for all compositions and show tunable emission in the near-infrared (NIR) region. A first shell of InP leads to a red-shift and an increase in quantum yield. The final shell of ZnSe serves to stabilize the dots for applications in aqueous environments, including NIR biomedical fluorescence imaging. These NIR-emitting core/shell/shell InAs(x)P(1-x)/InP/ZnSe were successfully used in a sentinel lymph node mapping experiment.

Growth and structure in abalone shell

Abstract: The growth and self-assembly of aragonitic calcium carbonate found in the shell of abalone (Haliotis) is described. This was accomplished through the close examination of laboratory-grown flat pearl samples and cross-sectional slices of the nacreous shell. Further understanding of the sequenced assembly has been obtained. It has been confirmed that the growth of the aragonite component of the composite occurs by the successive nucleation of aragonite crystals and their arrest by means of a protein-mediated mechanism; it takes place in the “Christmas-tree pattern” [Nature 49 (1994) 371]. It is shown that the protein layer is virtually absent where plates on a same plane abut (along lateral surfaces of tiles). This suggests a mechanism of c-axis aragonite growth arrest by the deposition of a protein layer of approximately 20–30 nm that is periodically activated and determines the thickness of the aragonite platelets, which are remarkably constant (0.5 m). This platelet size was measured for animals with shell diameters of 10, 50, and 200 mm and was found to be constant. The overall growth process is expressed in terms of parameters incorporating the anisotropy of growth velocity in aragonite ( Vc, the velocity along c axis, and Vab, the velocity in basal plane). Comparison of laboratory-raised and naturally-grown abalone indicates growth regulated by the level of proteinaceous saturation. Naturally-grown abalone exhibits mesolayers (growth bands) ∼0.3 mm apart; it is proposed that they result from seasonal interruptions in feeding patterns, creating thicker ( ∼10–20m) layers of protein. These mesolayers play a critical role in the mechanical properties, and are powerful crack deflectors. The viscoplastic deformation of the organic inter-tile layers is responsible for the significant improvement of tensile strength over the tensile strength of monolithic aragonite. © 2004 Elsevier B.V. All rights reserved.

On structural optimization of composite shell structures using a discrete constitutive parametrization

Abstract: In this article a novel method for structural optimization of laminated composite shell structures such as wind turbine blades is presented. The outer shape of a wind turbine blade is typically determined by aerodynamic considerations and therefore not subject to change. Furthermore, the thicknesses of the shell structures are also considered fixed. The design objective is chosen to be a global quantity such as maximum stiffness or lowest eigenfrequency with a constraint on the total mass, such that the cost of material can be considered. The design optimization method is based on ideas from multiphase topology optimization where the material stiffness (or density) is computed as a weighted sum of candidate materials, and the method is easy to implement in existing finite element codes. The potential of the method to solve the combinatorial problem of proper choice of material, stacking sequence and fibre orientation simultaneously for maximum stiffness or lowest eigenfrequency design is illustrated on both small test examples and a real-life main spar from a wind turbine blade. Copyright © 2004 John Wiley & Sons, Ltd.

Non-linear analysis of shells with arbitrary evolving cracks using XFEM

Abstract: A new formulation and numerical procedures are developed for the analysis of arbitrary crack propagation in shells using the extended finite element method. The method is valid for completely non-linear problems. Through-the-thickness cracks in sandwich shells are considered. An exact shell kinematics is presented, and a new enrichment of the rotation field is proposed which satisfies the director inextensibility condition. To avoid locking, an enhanced strain formulation is proposed for the 4-node cracked shell element. A finite strain plane stress constitutive model based on the logarithmic corotational rate is employed. A cohesive zone model is introduced which embodies the special characteristics of the shell kinematics. Stress intensity factors are calculated for selected problems and crack propagation problems are solved. Copyright © 2004 John Wiley & Sons, Ltd.

Controlling Shell Thickness in Core−Shell Gold Nanoparticles via Surface-Templated Adsorption of Block Copolymer Surfactants

Abstract: When Au nanoparticles are encapsulated within shells of cross-linked, block copolymer amphiphiles, the structure of the shells is determined by the initial interaction between the amphiphile and the nanoparticle surface. In the case of small nanoparticles, for which particle size is comparable to the dimension of the block copolymer (ρAu/Rg ≈ 1), particles act like solutes that are dissolved within polystyrene-block-poly(acrylic acid) (PS-b-PAA) micelle cores. In the case of larger nanoparticles (ρAu/Rg > 1), PS-b-PAA adsorption is templated by the particle surface, and a concentric core−shell structure is formed. The thickness of this shell can be predicted from theoretical models of polymer adsorption onto highly curved surfaces and controlled by varying the ratio of polymer to available nanoparticle surface area. We anticipate that these rules will illustrate how cross-linked copolymer shells with predetermined thickness can be used to stabilize and functionalize a variety of nanoparticle materials.

Analysis of Thin Plate Structures Using the Absolute Nodal Coordinate Formulation

Abstract: The absolute nodal coordinate formulation can be used in multibody system applications where the rotation and deformation within the finite element are large and where there is a need to account for geometrical non-linearities. In this formulation, the gradients of the global positions are used as nodal coordinates and no rotations are interpolated over the finite element. For thin plate and shell elements, the plane stress conditions can be applied and only gradients obtained by differentiation with respect to the element mid-surface spatial parameters need to be defined. This automatically reduces the number of element degrees of freedoms, eliminates the high frequencies due to the oscillations of some gradient components along the element thickness, and as a result makes the plate element computationally more efficient. In this paper, the performance of a thin plate element based on the absolute nodal coordinate formulation is investigated. The lower dimension plate element used in this investi...

Core-shell zeolite microcomposites

Abstract: Core–shell zeolite composites possessing a core and a shell of different zeolite structure types have been synthesized. A characteristic feature of the obtained composites is the relatively large single-crystal core and the very thin polycrystalline shell. The incompatibility between the core crystals and the zeolite precursor mixture yielding the shell layer has been circumvented by the adsorption of nanoseeds on the core surface, which induced the crystallization of the shell. The pretreated core crystals are subsequently subjected to a continuous growth in a zeolite precursor mixture. The feasibility of this synthetic approach has been exemplified by the preparation of core–shell β-zeolite–silicalite-1 composites. The synthesized composites have been characterized using X-ray diffraction, high-resolution transmission electron microscopy, and scanning electron microscopy. The integrity of the shell layer has been tested via N2-adsorption measurements on materials comprising a calcined core (β-zeolite) and a non-calcined tetrapropylammonium (TPA)-containing shell, the latter being non-permeable for the N2 molecules. These measurements have shown that 86 % of the β-zeolite crystals are covered with a defect-free TPA–silicalite-1 shell after a single hydrothermal treatment, while after three consecutive crystallization steps this value reaches 99 %. The shell integrity of the calcined composite has been studied by the adsorption of butane, toluene, and 1,3,5-trimethylbenzene, which confirmed the superior performance of the triple-shell composites.

Non-linear vibrations of doubly curved shallow shells

Abstract: Large amplitude (geometrically non-linear) vibrations of doubly curved shallow shells with rectangular base, simply supported at the four edges and subjected to harmonic excitation normal to the surface in the spectral neighbourhood of the fundamental mode are investigated. Two different non-linear strain–displacement relationships, from the Donnell's and Novozhilov's shell theories, are used to calculate the elastic strain energy. In-plane inertia and geometric imperfections are taken into account. The solution is obtained by Lagrangian approach. The non-linear equations of motion are studied by using (i) a code based on arclength continuation method that allows bifurcation analysis and (ii) direct time integration. Numerical results are compared to those available in the literature and convergence of the solution is shown. Interaction of modes having integer ratio among their natural frequencies, giving rise to internal resonances, is discussed. Shell stability under static and dynamic load is also investigated by using continuation method, bifurcation diagram from direct time integration and calculation of the Lyapunov exponents and Lyapunov dimension. Interesting phenomena such as (i) snap-through instability, (ii) subharmonic response, (iii) period doubling bifurcations and (iv) chaotic behaviour have been observed.

Double Hydrophilic Block Copolymer Monolayer Protected Hybrid Gold Nanoparticles and Their Shell Cross-Linking

Abstract: This paper describes the syntheses of core/shell gold nanoparticles stabilized with a monolayer of double hydrophilic block copolymer and their stimuli responsiveness before and after shell cross-linking. The hybrid nanoparticles consist of gold core, cross-linkable poly(2-(dimethylamino)ethyl methacrylate) (PDMA) inner shell, and poly(ethylene oxide) (PEO) corona. First, diblock copolymer PEO-b-PDMA was prepared via the reversible addition-fragmentation chain transfer (RAFT) technique using a PEO-based macroRAFT agent. The dithioester end group of PEO-b-PDMA diblock copolymer was reduced to a thiol end group. The obtained PEO-b-PDMA-SH was then used to prepare diblock copolymer stabilized gold nanoparticles by the "grafting-to" approach. 1,2-Bis(2-iodoethoxy)ethane (BIEE) was utilized to selectively cross-link the PDMA residues in the inner shell. The stimuli responsiveness and colloidal stability of core/shell gold nanoparticles before and after shell cross-linking were characterized by laser light scattering (LLS), UV-vis transmittance, and transmission electron microscopy (TEM). At pH 9, the average hydrodynamic radius Rh of non-cross-linked hybrid gold nanoparticles starts to increase above 35 degrees C due to the lower critical solution temperature (LCST) phase behavior of the PDMA blocks in the inner shell. In contrast, Rh of the shell cross-linked gold nanoparticles were essentially independent of temperature. Core/shell gold nanoparticles before and after shell cross-linking exhibit reversible swelling on varying the solution pH. Compared to non-cross-linked core/shell gold nanoparticles, shell cross-linking of the hybrid gold nanoparticles leads to permanent core/shell nanostructures with much higher colloidal stability and physically isolates the gold core from the external environment.

Energy absorption of aluminium alloy circular and square tubes under an axial explosive load

Abstract: The energy absorption of circular and square aluminium alloy tubes subjected to an axial explosive load, which is transmitted to a tube by a small attached mass, is discussed. Particular attention is paid to the interaction between the inertia of the attached mass and a tube when the importance of the initial compression phase is revealed. The effect of this phase on the mean load, which is characteristic of the energy absorption capacity of structural elements, is demonstrated. The influence of the material models on the prediction of the response of aluminium alloy circular and square tubes is also discussed in relation with the temperature effects caused by the high strain rates. The analysis shows that the material properties play an important role for the formation of the buckling pattern due to the finite duration of the initial compression phase when plastic stress waves at different speeds propagate along a shell.

Structure of spherical three-dimensional Coulomb crystals.

Abstract: An analysis of the structural properties of three-dimensional Coulomb clusters confined in a spherical parabolic trap is presented. Based on extensive high-accuracy computer simulations the shell configurations and energies for particle numbers in the range 60 < or = N < or = 160 are reported. Further, the intrashell symmetry and the lowest metastable configurations are analyzed for small clusters and a different type of excited state that does not involve a change of shell configuration is identified.

Preparation and mechanical properties of thermal energy storage microcapsules

Abstract: A series of heat energy storage microcapsules was prepared using melamine-formaldehyde resin as the shell material and the mechanical properties of the shell were investigated. A phase change material whose melting point was 24 °C was used as core and the quantity of heat involved in phase transition was 225.5 J/g. Average diameter of the microcapsules varied from 5 to 10 μm, and the globular surface was smooth and compact. The mechanical properties of the shell were evaluated by observing the surface morphological structure change after application of pressure by means of scanning electron microscopy. When the mass ratio of the core and shell material is 3:1, a yield point of about 1.1×105 Pa was found and when the compression was increased beyond this point the microcapsules showed plastic behavior. This has been attributed to the cross-link density and to the high degree of reaction of the shell material. Different yield points subsequently reflected differences in the mechanical behavior. It was also found that the mechanical intensity of double-shell microcapsules was better than that of single shelled ones.

Convective-conductive transitions and sensitivity of a convecting ice shell to perturbations in heat flux and tidal-heating rate: Implications for Europa

Abstract: We investigate the response of conductive and convective ice shells on Europa to variations of heat flux and interior tidal-heating rate. We present numerical simulations of convection in Europa's ice shell with Newtonian, temperature-dependent viscosity and tidal heating. Modest variations in the heat flux supplied to the base of a convective ice shell, �F , can cause large variations of the ice-shell thickness �δ . In contrast, for a conductive ice shell, largeF involves relatively small �δ . We demonstrate that, for a fluid with temperature-dependent viscosity, the heat flux undergoes a finite-amplitude jump at the critical Rayleigh number Racr. This jump implies that, for a range of heat fluxes relevant to Europa, two equilibrium states—corresponding to a thin, conductive shell and a thick, convective shell—exist for a given heat flux. We show that, as a result, modest variations in heat flux near the critical Rayleigh number can force the ice shell to switch between the thin, conductive and thick, convective configurations over a ∼10 7 -year interval, with thickness changes of up to ∼10-30 km. Depending on the orbital and thermal history, such switches might occur repeatedly. However, existing evolution models based on parameterized- convection schemes have to date not allowed these transitions to occur. Rapid thickening of the ice shell would cause radial expansion of Europa, which could produce extensional tectonic features such as fractures or bands. Furthermore, based on interpretations for how features such as chaos and ridges are formed, several authors have suggested that Europa's ice shell has recently undergone changes in thickness. Our model provides a mechanism for such changes to occur.

Thermal transport in au-core polymer-shell nanoparticles.

Abstract: Thermal transport in aqueous suspensions of Au-core polymer-shell nanoparticles is investigated by time-resolved measurements of optical absorption. The addition of an organic cosolvent to the suspension causes the polystyrene component of the polymer shell to swell, and this change in the microstructure of the shell increases the effective thermal conductivity of the shell by a factor of approximately 2. The corresponding time scale for the cooling of the nanoparticle decreases from 200 ps to approximately 100 ps. The threshold concentration of cosolvent that creates the changes in thermal conductivity, 5 vol % tetrahydrofuran in water or 40 vol % N,N-dimethylformamide in water, is identical to the threshold concentrations for producing small shifts in the frequency of the plasmon resonance. Because the maximum fraction of solvent in the polymer shell is less than 20 vol %, the increase in the effective thermal conductivity of the shell cannot be easily explained by contributions to heat transport by the...

Ultrafast carrier dynamics in core and core/shell CdSe quantum rods: Role of the surface and interface defects

Abstract: Femtosecond pump-probe spectroscopy, in the visible spectral range, is used to study fast processes after photoexcitation in quantum confined wurtzite CdSe core and $\mathrm{CdSe}∕\mathrm{CdS}∕\mathrm{ZnS}$ core/shell nanorods. Effective-mass theory is applied to assign the energy levels in linear absorption spectra. Sequential bleaching of the excitonic states and photoinduced absorption are observed in the transmission difference spectra. A strong difference was found between the core and core/shell samples. In fact, in the transient transmission difference spectra, the decay time of the higher energy states is faster in the core than in the core/shell samples (200 fs versus 500 fs), due to the trapping in the unpassivated surface states of the bare core. Stimulated emission (SE) has been achieved in CdSe core and core/shell quantum rods at room temperature. As a striking feature, the SE is sustained for a shorter time in the core/shell sample (25 ps) compared with the core sample (50 ps). We demonstrate that this result is due to the competition between SE and photoinduced absorption from defect states in the midgap of the core/shell sample. Such midgap states are related to the formation of extended defects at the core/shell interface, due to lattice strain relaxation.

Induction heating device

Abstract: PROBLEM TO BE SOLVED: To provide a technology for not placing an induction heating coil above a workpiece in a high-temperature state. SOLUTION: The induction heating coils 111 and 112 that are divided into two parts and have such a shape as to surround a cylindrical furnace shell 105, wherein a to-be-heated material is stored, from a side face thereof, are displaced. By moving the induction heating coils 111 and 112 horizontally with respect to the furnace shell 105, movement to a place where induction heating is conducted and receding from there are performed. Thus, location of the induction heating coils 111 and 112 above the furnace shell 105 can be prevented to avoid a situation in which the induction heating coils 111 and 112 are heated above the furnace shell 105. COPYRIGHT: (C)2011,JPO&INPIT

Effective Modeling and Nonlinear Shell Analysis of Thin Membranes Exhibiting Structural Wrinkling

Abstract: Thin solar sail membranes of very large span are being envisioned for near-term space missions. One major design issue that is inherent to these very flexible structures is the formation of wrinkling patterns. Structural wrinkles may deteriorate a solar sail's performance and, in certain cases, structural integrity. A geometrically nonlinear, updated Lagrangian shell formulation is employed using the ABAQUS finite element code to simulate the formation of wrinkled deformations in thin-film membranes. The restrictive assumptions of true membranes as defined by tension field theory are not invoked. Two effective modeling strategies are introduced to facilitate convergent solutions of wrinkled equilibrium states. They include 1) the application of small, pseudorandom, out-of-plane geometric imperfections that ensure initiation of the requisite membrane-to-bending coupling in a geometrically nonlinear analysis and 2) the truncation of corner regions, where concentrated loads are prescribed, to improve load transfer, mesh quality, and kinematics and to reduce severe concentration of membrane stresses. The corner truncation necessitates replacing the concentrated force with a statically equivalent distributed traction. Several numerical studies are carried out, and the results are compared with recent experimental data. Good agreement is observed between the numerical simulations and experimental data.