Showing papers on "Shell (structure) published in 2008"

Scattering Theory Derivation of a 3D Acoustic Cloaking Shell

Abstract: Through acoustic scattering theory we derive the mass density and bulk modulus of a spherical shell that can eliminate scattering from an arbitrary object in the interior of the shell—in other words, a 3D acoustic cloaking shell. Calculations confirm that the pressure and velocity fields are smoothly bent and excluded from the central region as for previously reported electromagnetic cloaking shells. The shell requires an anisotropic mass density with principal axes in the spherical coordinate directions and a radially dependent bulk modulus. The existence of this 3D cloaking shell indicates that such reflectionless solutions may also exist for other wave systems that are not isomorphic with electromagnetics.

Nonlocal shell model for elastic wave propagation in single- and double-walled carbon nanotubes

Abstract: This paper investigates the transverse and torsional wave in single- and double-walled carbon nanotubes (SWCNTs and DWCNTs), focusing on the effect of carbon nanotube microstructure on wave dispersion. The SWCNTs and DWCNTs are modeled as nonlocal single and double elastic cylindrical shells. Molecular dynamics (MD) simulations indicate that the wave dispersion predicted by the nonlocal elastic cylindrical shell theory shows good agreement with that of the MD simulations in a wide frequency range up to the terahertz region. The nonlocal elastic shell theory provides a better prediction of the dispersion relationships than the classical shell theory when the wavenumber is large enough for the carbon nanotube microstructure to have a significant influence on the wave dispersion. The nonlocal shell models are required when the wavelengths are approximately less than 2.36×10 −9 and 0.95×10 −9 m for transverse wave in armchair (15,15) SWCNT and torsional wave in armchair (10,10) SWCNT, respectively. Moreover, an MD-based estimation of the scale coefficient e 0 for the nonlocal elastic cylindrical shell model is suggested. Due to the small-scale effects of SWCNTs and the interlayer van der Waals interaction of DWCNTs, the phase difference of the transverse wave in the inner and outer tube can be observed in MD simulations in wave propagation at high frequency. However, the van der Waals interaction has little effect on the phase difference of transverse wave.

Controlled Rupture of Magnetic Polyelectrolyte Microcapsules for Drug Delivery

Abstract: In this study, a magnetic-sensitive microcapsule was prepared using Fe3O4/poly(allylamine) (Fe3O4/PAH) polyelectrolyte to construct the shell. Structural integrity, microstructural evolution, and corresponding release behaviors of fluorescence dyes and doxorubicin were systematically investigated. Experimental observations showed that the presence of the magnetic nanoparticles in the shell structure allowed the shell structure to evolve from nanocavity development to final rupture of the shell under a given magnetic stimulus of different time durations. Such a microstructural evolution of the magnetic sensitive shell structure explained a corresponding variation of the drug release profile, from relatively slow release to burst-like behavior at different stages of stimulus. It has proposed that the presence of magnetic nanoparticles produced heat, due to magnetic energy dissipation (as Brown and Neel relaxations), and mechanical vibration and motion that induced stress development in the thin shell. Both ...

Encapsulation and Growth of Gold Nanoparticles in Thermoresponsive Microgels

Abstract: Gold nanoparticles are encapsulated within thermoresponsive pNIPAM migrogels through an easy two-step protocol. The core/shell structure of the composite is characterized by TEM, AFM, PCS, and UV-vis spectroscopy. The restricted environment and the high porosity of the microgel shell are studied through the overgrowth of the gold core.

High-Temperature-Stable Au@SnO2 Core/Shell Supported Catalyst for CO Oxidation

Abstract: High-temperature-stable Au@SnO2 core/shell supported catalyst was prepared by a simple intermetallics-based dry-oxidation approach in which the size of the core can be controlled easily by varying the size of the pre-made Au seeds. The change of their structure was investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). In the as-prepared supported catalysts, Au particles with a mean size of ca. 15 nm were highly encapsulated by the SnO2 shell. Moreover, the Au@SnO2 core/shell supported catalysts showed superior catalytic activity compared to non-encapsulated Au−SnO2. XPS spectra showed that the interactions between the Au catalyst and oxide support in the well-encapsulated Au@SnO2 core/shell nanoparticles are much stronger than those in the non-encapsulated Au−SnO2 nanoparticles, further indicating the synergetic confinement effect in such nanoscaled catalyst/support core/shell systems.

Stress-driven buckling patterns in spheroidal core/shell structures

Abstract: Many natural fruits and vegetables adopt an approximately spheroidal shape and are characterized by their distinct undulating topologies. We demonstrate that various global pattern features can be reproduced by anisotropic stress-driven buckles on spheroidal core/shell systems, which implies that the relevant mechanical forces might provide a template underpinning the topological conformation in some fruits and plants. Three dimensionless parameters, the ratio of effective size/thickness, the ratio of equatorial/polar radii, and the ratio of core/shell moduli, primarily govern the initiation and formation of the patterns. A distinct morphological feature occurs only when these parameters fall within certain ranges: In a prolate spheroid, reticular buckles take over longitudinal ridged patterns when one or more parameters become large. Our results demonstrate that some universal features of fruit/vegetable patterns (e.g., those observed in Korean melons, silk gourds, ribbed pumpkins, striped cavern tomatoes, and cantaloupes, etc.) may be related to the spontaneous buckling from mechanical perspectives, although the more complex biological or biochemical processes are involved at deep levels.

Ultrahigh stress and strain in hierarchically structured hollow nanoparticles

Abstract: Nanocrystalline materials offer very high strength but are typically limited in their strain to failure, and efforts to improve deformability in these materials are usually found to be at the expense of strength. Using a combination of quantitative in situ compression in a transmission electron microscope and finite-element analysis, we show that the mechanical properties of nanoparticles can be directly measured and interpreted on an individual basis. We find that nanocrystalline CdS synthesized into a spherical shell geometry is capable of withstanding extreme stresses (approaching the ideal shear strength of CdS). This unusual strength enables the spherical shells to exhibit considerable deformation to failure (up to 20% of the sphere’s diameter). By taking into account the structural hierarchy intrinsic to novel nanocrystalline materials such as this, we show it is possible to achieve and characterize the ultrahigh stresses and strains that exist within a single nanoparticle during deformation. Nanocrystalline materials usually exhibit high strength and their deformation caused by stress is limited. Nanocrystalline CdS with spherical and hierarchical shell geometry is shown not only to withstand extreme stresses, but also to deform considerably before failure.

Synthesis and Optical Properties of Type II CdTe/CdS Core/Shell Quantum Dots in Aqueous Solution via Successive Ion Layer Adsorption and Reaction

Abstract: 3-Mercaptopropionic acid stabilized CdTe/CdS core/shell quantum dots (QDs) were prepared in an aqueous solution following the synthetic route of successive ion layer adsorption and reaction. The photoluminescence quantum yield of the CdTe QDs could reach 40%, from 8% of the bare core, via the control of the shell thickness. The CdTe/CdS QDs exhibited also a significant red shift of emission and excitation peaks when the shell layer grew. The experiments revealed that the CdTe/CdS QDs evolved from type I to type II core/shell structures with the increase of the shell thickness, and the evolution process is affected by the core size, shell thickness, surface quality of the core and shell, as unraveled by steady-state and time-resolved spectroscopy. The lack of photoluminescence lifetime lengthening was ascribed to the surface influence of the shell.

Enhanced assumed strain (EAS) and assumed natural strain (ANS) methods for one‐point quadrature solid‐shell elements

Abstract: A reduced enhanced solid-shell (RESS) finite element concept has been suggested recently by Alves de Sousa et al. (Int. J. Numer Meth. Engng 2005; 62:952; Int. J. Numer Meth. Engng 2006; 67:160; Int. J. Plasticity 2007; 23:490). Developments on the 'RESS' element were motivated by the following reasons: first, solid-shell elements automatically incorporate the normal stress along the thickness direction, which makes them more suitable for the simulations with double-sided contact than their shell counterparts; second, they have only translational degrees of freedom, which alleviates some difficulties associated with formulating complex shell formulations using nodal rotations; third, general constitutive models can be used and a reformulation for plane-stress conditions is not necessary. Furthermore, traditionally, solid elements have been developed based on fully integrated schemes with limited number of integration points per single element layers which renders them computationally expensive. On the contrary, the solid-shell element by Alves de Sousa et al. (Int. J. Numer. Meth. Engng 2005; 62:952; Int. J. Numer. Meth. Engng 2006; 67:160; Int. J. Plasticity 2007; 23:490) was developed for one single-layer shell structure with reduced in-plane and multiple integration points along the thickness direction of the shell. The formulation consists of several combinations of well-known techniques to ameliorate locking problems as is the case of the enhanced assumed strain (EAS) method. However, the proposed 'RESS' solid shell did not consider the transverse shear components of hourglass (or physical stabilization) parts. This negligence produces a slightly flexible behavior of the element, but it can also cause the appearance of hourglass modes for several non-linear applications including large rigid body rotations. Sometimes, the negligence brings a non-positive-definite status on the stiffness matrix. In order to overcome such drawbacks, a modified assumed natural strain (ANS) method considering the top and bottom surfaces of the element was incorporated for the transverse shear components. At the same time, new hourglass strains for the membrane field were constructed based on the stabilization vectors of Liu et al. (Comput. Meth. Appl. Mech. Engng 1998; 154:69). With these modifications, the improved element (called 'M-RESS') passes both the membrane and bending patch tests and performs with remarkable stability and accuracy in sheet-forming simulations. Copyright © 2007 John Wiley & Sons, Ltd.

Layer resolved structural relaxation at the surface of magnetic FePt icosahedral nanoparticles.

Abstract: The periodic shell structure and surface reconstruction of metallic FePt nanoparticles with icosahedral structure has been quantitatively studied by high-resolution transmission electron microscopy with focal series reconstruction with sub-angstrom resolution. The icosahedral FePt nanoparticles fabricated by the gas phase condensation technique in vacuum have been found to be surprisingly oxidation resistant and stable under electron beam irradiation. We find the lattice spacing of (111) planes in the surface region to be size dependent and to expand by as much as 9% with respect to the bulk value of Fe52Pt48. Controlled removal of the (111) surface layers in situ results in a similar outward relaxation of the new surface layer. This unusually large layerwise outward relaxation is discussed in terms of preferential Pt segregation to the surface forming a Pt enriched shell around a Fe-rich Fe/Pt core.

Synthesis of Fe3O4@Polyaniline Core/Shell Microspheres with Well-Defined Blackberry-Like Morphology

Abstract: Superparamagnetic Fe3O4@polyaniline core/shell microspheres with well-defined blackberry-like morphology have been synthesized via a simple in situ surface polymerization method. The thickness of the polyaniline (PANI) shell can be selectively obtained by tuning the reaction time and monomer concentration. The poly(vinylpyrroldine) (PVP) plays an important role in the coating process. The present method can be extendable to fabricate other magnetic/conductive core/shell composites, and these unique core/shell spherical materials could find applications in catalyst supports or biomedical areas.

2-D solution for free vibrations of parabolic shells using generalized differential quadrature method

Abstract: The Generalized Differential Quadrature (GDQ) procedure is developed for the free vibration analysis of complete parabolic shells of revolution and parabolic shell panels The First-order Shear Deformation Theory (FSDT) is used to analyze the above moderately thick structural elements The treatment is conducted within the theory of linear elasticity, when the material behaviour is assumed to be homogeneous and isotropic The governing equations of motion, written in terms of internal resultants, are expressed as functions of five kinematic parameters, by using the constitutive and kinematic relationships The solution is given in terms of generalized displacement components of the points lying on the middle surface of the shell The discretization of the system by means of the Differential Quadrature (DQ) technique leads to a standard linear eigenvalue problem, where two independent variables are involved The results are obtained taking the meridional and circumferential co-ordinates into account, without using the Fourier modal expansion methodology Several examples of parabolic shell elements are presented to illustrate the validity and the accuracy of GDQ method Numerical solutions are compared with the ones obtained using commercial programs such as Abaqus, Ansys, Femap/Nastran, Straus, Pro/Mechanica Very good agreement is observed Furthermore, the convergence rate of natural frequencies is shown to be very fast and the stability of the numerical methodology is very good The accuracy of the method is sensitive to the number of sampling points used, to their distribution and to the boundary conditions Different typologies of non-uniform grid point distributions are considered The effect of the distribution choice of sampling points on the accuracy of GDQ solution is investigated New numerical results are presented

Synthesis of Ag@AgAu Metal Core/Alloy Shell Bimetallic Nanoparticles with Tunable Shell Compositions by a Galvanic Replacement Reaction

Abstract: The fabrication of metallic nanostructures with controllable shapes and sizes is important in delivering the promise of sizeand shape-tunable properties of nanomaterials. The properties of nanometals can be further modified by incorporating more than one metallic component into a common particle to form, for example, a bimetallic nanoparticle. It has been confirmed that co-operative and synergistic interactions between the metallic components could lead to an overall more useful functionality. The bimetallic nanoparticles may be fabricated as alloy nanoparticles where the two constituent metals are mixed at the atomic level or as core/shell nanoparticles where the two components are separated by distinct phase boundaries. It is known that bimetallic nanoparticles with the same overall composition but different composition distributions can exhibit different properties. The geometric distribution of the metals within a particle between the two extremes of alloy (maximallymixed) and core/ shell (minimally mixed) nanoparticles may therefore be used to increase the versatility in property tuning. In an effort to extend the envelope of possibility further, we will demonstrate the synthesis of a new type of core/shell nanostructure where the core is a pure metal and the shell is the alloy of two metals with adjustable compositions, using the Ag–Au system as an example. The novel Ag@AgAu bimetallic nanoparticles were produced by the replacement reaction between Ag nanoparticles and HAuCl4. [19–24] The replacement reaction between Ag nanoparticles and common Au precursor salts has been used by several groups to produce hollow nanostructures in the

Core/single-crystal-shell nanospheres for controlled drug release via a magnetically triggered rupturing mechanism

Abstract: Core/single-crystal-shell nanospheres are constructed from a poly-(N-vinyl- 2-pyrrolidone) (PVP)-modified silica core with an outer layer of single-crystal iron oxide shell. The nanospheres show outstanding release-and-zero-release characteristics via the addition and removal, respectively, of an external high-frequency magnetic field.

A metamaterial frequency-selective super-absorber that has absorbing cross section significantly bigger than the geometric cross section

Abstract: Using the idea of transformation optics, we propose a metamaterial device that serves as a frequency-selective super-absorber, which consists of an absorbing core material coated with a shell of isotropic double negative metamaterial. For a fixed volume, the absorption cross section of the super-absorber can be made arbitrarily large at one frequency. The double negative shell serves to amplify the evanescent tail of the high order incident cylindrical waves, which induces strong scattering and absorption. Our conclusion is supported by both analytical Mie theory and numerical finite element simulation. Interesting applications of such a device are discussed.

Thermal and mechanical instability of functionally graded truncated conical shells

Abstract: Thermal and mechanical instability of truncated conical shells made of functionally graded material (FGM) is studied in this paper. It is assumed that the shell is a mixture of metal and ceramic that its properties changes as a function of the shell thickness. The governing equations are based on the first-order shell theory and the Sanders nonlinear kinematics equations. The results are obtained for a number of thermal and mechanical loads and are validated with the known data in the literature.

Thermal Vibration, Buckling and Dynamic Stability of Functionally Graded Cylindrical Shells Embedded in an Elastic Medium:

Abstract: This article is the result of an investigation on the effect of thermal load on vibration, buckling and dynamic stability of functionally graded cylindrical shells embedded in an elastic medium, based on the first-order shear deformation theory (FSDT) considering rotary inertia and the transverse shear strains. A heat conduction equation along the width of the shell is applied to determine the temperature distribution. Material properties are assumed to be graded with distribution along the width according to a power-law in terms of the volume fractions of the constituents. Calculations, effects of material composition, thermal loading, static axial loading, medium stiffness and shell geometry parameters on vibration, buckling and the parametric resonance are described. The new features of thermal vibration, buckling and dynamic stability of functionally graded cylindrical shells embedded in an elastic medium and some meaningful and interesting results in this paper are helpful for the application and des...

Plane symmetric thin-shell wormholes: Solutions and stability

Abstract: Using the cut-and-paste procedure, we construct static and dynamic, plane symmetric wormholes by surgically grafting together two spacetimes of plane symmetric vacuum solutions with a negative cosmological constant. These plane symmetric wormholes can be interpreted as domain walls connecting different universes, having planar topology, and upon compactification of one or two coordinates, cylindrical topology or toroidal topology, respectively. A stability analysis is carried out for the dynamic case by taking into account specific equations of state, and a linearized stability analysis around static solutions is also explored. It is found that thin-shell wormholes made of a dark energy fluid or of a cosmological constant fluid are stable, while thin-shell wormholes made of phantom energy are unstable.

Solution-based II-VI core/shell nanowire heterostructures.

Abstract: We demonstrate the solution-phase synthesis of CdS/CdSe, CdSe/CdS, and CdSe/ZnTe core/shell nanowires (NWs). On the basis of bulk band offsets, type-I and type-II heterostructures are made, contributing to the further development of low-dimensional heteroassemblies using solution-phase chemistry. Core/shell wires are prepared by slowly introducing shell precursors into a solution of premade core NWs dispersed in a noncoordinating solvent at moderate temperatures (215−250 °C). Resulting heterostructures are characterized through low- and high-resolution transmission electron microscopy, selected area electron diffraction, and energy dispersive X-ray analysis. From these experiments, initial shell growth appears to occur through either Stranski−Krastanov or Volmer−Weber island growth. However, beyond a critical shell thickness, nucleation of randomly oriented nanocrystals results in a polycrystalline coat. In cases where overcoating has been achieved, corresponding elemental analyses show spatially varying ...

Electron Mean Free Path in a Spherical Shell Geometry

Abstract: Mean free path is calculated for the shell geometry under the assumptions of (i) diffusive, (ii) isotropic, and (iii) billiard, or Lambertian, scattering. Whereas in a homogeneous sphere case the difference between different models of surface scattering is reflected merely by a different slope of the linear dependence of a mean free path Leff on the sphere radius R, qualitatively different nonlinear dependencies on the inner and outer shell radii result for different model cases in the shell geometry. A linear behavior of Leff on the shell thickness (D) can only be established for the billiard model in the thin shell limit, in which case Leff ≈ 2D, whereas, in the same limit, Leff ≈ (D/2)ln(2R/D) in the diffusive case and Leff ≈ 14(2RD)1/2/[3ln(2R/D)] in the isotropic case. The shell geometry turns out a very sensitive setup to distinguish between the different models of electron scattering, which could be performed in future experiments on single and well-controlled dielectric core-metal shell nanopartic...

Thermomechanical vibration analysis of a functionally graded shell with flowing fluid

Abstract: This paper reports the results of an investigation into the vibration of functionally graded cylindrical shells with flowing fluid, embedded in an elastic medium, under mechanical and thermal loads. By considering rotary inertia, the first-order shear deformation theory (FSDT) and the fluid velocity potential, the dynamic equation of functionally graded cylindrical shells with flowing fluid is derived. Here, heat conduction equation along the thickness of the shell is applied to determine the temperature distribution and material properties are assumed to be graded distribution along the thickness direction according to a power-law in terms of the volume fractions of the constituents. The equations of eigenvalue problem are obtained by using a modal expansion method. In numerical examples, effects of material composition, thermal loading, static axial loading, flow velocity, medium stiffness and shell geometry parameters on the free vibration characteristics are described. The new features in this paper are helpful for the application and the design of functionally graded cylindrical shells containing fluid flow.

A measurable Lawson criterion and hydro-equivalent curves for inertial confinement fusion

Abstract: It is shown that the ignition condition (Lawson criterion) for inertial confinement fusion (ICF) can be cast in a form dependent on the only two parameters of the compressed fuel assembly that can be measured with existing techniques: the hot spot ion temperature (Tih) and the total areal density (ρRtot), which includes the cold shell contribution. A marginal ignition curve is derived in the ρRtot, Tih plane and current implosion experiments are compared with the ignition curve. On this plane, hydrodynamic equivalent curves show how a given implosion would perform with respect to the ignition condition when scaled up in the laser-driver energy. For 3 50keV2.6⋅g∕cm2, where ⟨ρRtot⟩n and ⟨Tih⟩n are the burn-averaged total areal density and hot spot ion temperature, respectively. Both quantities are calculated without accounting for the alpha-particle energy deposition. Such a criterion can be used t...

Morphological instability of misfit-strained core-shell nanowires

Abstract: We investigate the morphological instability of a misfit-strained cylindrical core-shell nanowire by performing a linear stability analysis. For this aim, the stress and strain distributions of a core-shell nanowire with a sinusoidally perturbed surface are calculated to first order, properly accounting for the core-shell interface. In addition, the effect of surface stress on the stress and/or strain distributions is considered. Due to the large surface-to-volume ratio of nanosized objects, this is indispensable. The outcome of the stability analysis is threefold: First, our calculation shows that surface stress strongly influences the nature of the fastest growing mode of perturbation. It turns out that the axially symmetric mode does not necessarily grow fastest. Second, we find that the system is most unstable in the initial phase of shell growth, i.e., for thin shell thicknesses. Interestingly, considering thin shells and large misfits $(\ensuremath{\gtrsim}3%)$, we find that there exists a core radius for which stability becomes maximal. Under the conditions considered this radius is in the range of about 5--10 nm. Third, there exists a parameter range for which the experimental observation that Ge-rich islands grown on thick silicon nanowires tend to be aligned in two rows on the opposite sides of the nanowire agrees with the outcome of our calculation.

Experimental study of porous silicon shell pillars under retentive conditions.

Abstract: Experimental measurements of the retention capacity and the band broadening in perfectly ordered porous shell pillar array columns are presented for a wide range of retention conditions and layer thicknesses. The porous silicon shells were obtained using electrochemical anodization of the solid silicon pillars obtained using deep reactive ion etching. Using 10-μm-wide pillars, minimal reduced plate height values of the order of hmin = 0.4−0.5 were obtained under nonretained conditions, even in cases where the outer shell made up 20% of the total diameter. Under retained conditions, minimal plate heights around hmin = 0.9 were obtained, even at retention factors up to k′ = 12. Using a model based on Giddings nonequilibrium theory, and using a newly calculated value for the stationary zone configuration factor for the case of porous shell cylinders, a plate height model describing the band broadening in porous shell pillar arrays has been established. The validity of this model is demonstrated by showing that the geometrical parameters appearing in the model and fitted using band-broadening measurements under nonretained conditions can be used to relatively accurately predict the band broadening under retained component conditions. Using this model, some speculations on the ultimate performance of porous pillar array columns could be made.