Showing papers on "Shell (structure) published in 2013"

Upconverting Near‐Infrared Light through Energy Management in Core–Shell–Shell Nanoparticles

Abstract: Lanthanide-doped upconversion materials, capable of converting low-density (< 1000 W cm ) near-infrared (NIR) excitation to ultraviolet (UV) and visible emissions, have generated a large amount of interests in the areas of information technology, biotechnology, energy, and photonics. Significantly, recent developments in the synthetic and multicolor tuning methods have allowed easy access to upconversion nanoparticles with well-defined phase and size, core–shell structure, optical emission, and surface properties. The technological advances provide promising applications in sensitive biodetection and advanced bioimaging without many of the constraints associated with conventional optical biolabels. Despite the attractions, further progress in using upconversion processes has been largely hindered because upconversion nanoparticles are typically sensitized by Yb ions that only respond to narrowband NIR excitation centered at 980 nm. The absorption of 980 nm light by the water component in biological samples usually limits deep tissue imaging and induces potential thermal damages to cells and tissues. Excitation of conventional upconversion nanoparticles at other wavelengths has been proposed to minimize the effect of water absorption. But the use of this technique is limited mainly by the largely sacrificed excitation efficiency. Efforts have also been devoted to tuning the NIR response of photon upconversion through integration of various sensitizers such as metal ions (e.g.; Nd, V or Cr) and organic dyes. The progress has resulted in visible emission by NIR excitation in the 700–900 nm range where the transparency of biological samples is maximal. However, upconversion emission across a broad range of spectra in these systems have not been demonstrated largely owing to the uncontrollable nonradiative processes. Herein, we describe a novel design, based on nanostructural engineering to separate unwanted electronic transitions for constructing a new class of materials displaying tunable upconversion emissions spanning from UV to the visible spectral region by single wavelength excitation at 808 nm. We also show that these nanoparticles can surpass the constraints associated with conventional upconversion nanoparticles for biological studies. The nanostructure design for management of energy transitions is depicted in Figure 1. A core–shell–shell nanoparticle platform is used to host light-harvesting, upconvert-

Successive Layer-by-Layer Strategy for Multi-Shell Epitaxial Growth: Shell Thickness and Doping Position Dependence in Upconverting Optical Properties

Abstract: One pot successive layer-by-layer (SLBL) strategy is introduced to fabricate the core/shell upconversion nanoparticles (NPs) for the first time by using high boiling-point Re-OA (rare-earth chlorides dissolved in oleic acid at 140 °C) and Na-TFA-OA (sodium trifluoroacetate dissolved in oleic acid at room temperature) as shell precursor solutions. This protocol is flexible to deposit uniform multishell on both hexagonal (β) and cubic (α) phase cores by successive introducing of the shell precursor solutions. Shell thickness of the obtained NPs with narrow size distribution (σ < 10%) can be well controlled from 1 monolayer (∼0.36 nm) to more than 20 monolayers (∼8 nm) by simply tuning the amounts of the shell precursors. Furthermore, the tunable doping positions (core doping and shell doping) can also be achieved by adjusting the species and addition sequence of the shell precursors. As a result of the high quality uniform shell and advanced core/shell structures, the optical properties of the obtained core...

Lamp Having Outer Shell to Radiate Heat of Light Source

Abstract: A lamp includes an outer shell having heat conductivity, a base provided in the outer shell, and a cover provided in the outer shell. The outer shell has a light source support, and a heat radiating surface exposed to the outside of the outer shell. The light source support is formed integral with the heat radiating surface. A light source is supported on the light source support. The light source is heated during lighting, and thermally connected to the light source support. The light source is covered with the cover.

Revealing the structural properties of hydrogenated black TiO2 nanocrystals

Abstract: Our recent discovery of hydrogenated black TiO2 nanocrystals has triggered intense research interests for many applications. The understanding of the properties of this new material, however, falls far from satisfactory. We present here a novel and quantitative structural analysis of hydrogenated black TiO2 nanocrystals from X-ray diffraction (XRD) based crystal morphology modeling assisted with high-resolution transmission electron microscopy measurements. This analysis method provides an in-depth understanding of the structural properties of the hydrogenated black TiO2 nanocrystals, which is otherwise hardly achievable in a quantitative way. Specifically, the percentage of various crystalline facets and the ratio of the crystalline/amorphous phases are clearly revealed, besides the surface excess pressure and stress over the crystalline phase, and the stabilizing function of the amorphous shell. This study thus sheds more light on the black TiO2 nanocrystals and may inspire more applications based on the newly revealed structural properties. We also demonstrate that the unique analysis method of XRD-derived crystal morphology modeling would be extremely useful to gain an in-depth understanding of the structural properties and behaviors of nanocrystals, especially in the case where only XRD is available and other direct measuring methods or high-end instruments are not available or are cost-prohibitive. Thus, it would promote great research interest in nanomaterials research with rich structural information, even under budget-limiting situations.

Static analysis of functionally graded doubly-curved shells and panels of revolution

Abstract: The Generalized Differential Quadrature (GDQ) Method is applied to study four parameter functionally graded and laminated composite shells and panels of revolution. The mechanical model is based on the so-called First-order Shear Deformation Theory (FSDT), in particular on the Toorani-Lakis Theory. The solution is given in terms of generalized displacement components of points lying on the middle surface of the shell. The generalized strains and stress resultants are evaluated by applying the Differential Quadrature rule to the generalized displacements. The transverse shear and normal stress profiles through the thickness are reconstructed a posteriori by using local three-dimensional elasticity equilibrium equations. In order to verify the accuracy of the present method, GDQ results are compared with the ones obtained with semi-analytical formulations and with 3D finite element method. A parametric study is performed to illustrate the influence of the parameters on the mechanical behavior of functionally graded shell structures made of a mixture of ceramics and metal.

A unified formulation for vibration analysis of functionally graded shells of revolution with arbitrary boundary conditions

Abstract: This paper describes a general formulation for free, steady-state and transient vibration analyses of functionally graded shells of revolution subjected to arbitrary boundary conditions. The formulation is derived by means of a modified variational principle in conjunction with a multi-segment partitioning procedure on the basis of the first-order shear deformation shell theory. The material properties of the shells are assumed to vary continuously in the thickness direction according to general four-parameter power-law distributions in terms of volume fractions of the constituents. Fourier series and polynomials are applied to expand the displacements and rotations of each shell segment. The versatility of the formulation is demonstrated through the application of the following polynomials: Chebyshev orthogonal polynomials, Legendre orthogonal polynomials, Hermite orthogonal polynomials and power polynomials. Numerical examples are given for the free vibrations of functionally graded cylindrical, conical and spherical shells with different combinations of free, shear-diaphragm, simply-supported, clamped and elastic-supported boundary conditions. Validity and accuracy of the present formulation are confirmed by comparing the present solutions with the existing results and those obtained from finite element analyses. As to the steady-state and transient vibration analyses, functionally graded conical shells subjected to axisymmetric line force and distributed surface pressure are investigated. The effects of the material power-law distribution, boundary condition and duration of blast loading on the transient responses of the conical shells are also examined.

Radial basis function method applied to doubly-curved laminated composite shells and panels with a General Higher-order Equivalent Single Layer formulation

Abstract: The aim of this work is the application of Radial Basis Function (RBF) method to a General Higher-order Equivalent Single Layer (GHESL) formulation for the free vibrations of doubly-curved laminated composite shells and panels. The theoretical development of the present paper is based on the well-known Carrera Unified Formulation. In particular, the fundamental nuclei of a multi-layered doubly-curved shell structure are deducted and explicitly defined. The Differential Geometry (DG) tool has been used to geometrically define each of the structures under consideration: doubly-curved, singly-curved and degenerate shells. The 2D free vibration shell problems are numerically solved using RBFs, where the shape parameters have been optimized using two different algorithms. In fact, the shape parameters of RBFs depend not only on the choice of the radial basis functions, but also on how the points are located on the given computational domain. Modifying the positions of such points, the shape parameters change too. It has been discovered that, once the shape parameters have been optimized for a given grid distribution, they can be rounded off and used for every kind of structure. This is a very important aspect because when, using a fixed parameter, the RBF method becomes a “parameter free” numerical technique. In order to demonstrate the accuracy, stability and reliability of the present methodology, many comparisons are presented with reference to literature results, that are obtained by using Generalized Differential Quadrature method. The above numerical applications of this study are also compared with finite element method solutions.

Postbuckling of nanotube-reinforced composite cylindrical shells under combined axial and radial mechanical loads in thermal environment

Abstract: A postbuckling analysis is presented for nanocomposite cylindrical shells reinforced by single-walled carbon nanotubes (SWCNTs) subjected to combined axial and radial mechanical loads in thermal environment. Two types of carbon nanotube-reinforced composite (CNTRC) shells, namely, uniformly distributed (UD) and functionally graded (FG) reinforcements, are considered. The material properties of FG-CNTRCs are assumed to be graded in the thickness direction, and are estimated through a micromechanical model. The governing equations are based on a higher order shear deformation shell theory with a von Karman-type of kinematic nonlinearity. The thermal effects are also included and the material properties of CNTRCs are assumed to be temperature-dependent. A boundary layer theory and associated singular perturbation technique are employed to determine the buckling loads and postbuckling equilibrium paths. The numerical illustrations concern the postbuckling behavior of perfect and imperfect, FG-CNTRC cylindrical shells under combined action of external pressure and axial compression for different values of load-proportional parameters. The results for UD-CNTRC shell, which is a special case in the present study, are compared with those of the FG-CNTRC shell.

FGM and Laminated Doubly-Curved and Degenerate Shells Resting on Nonlinear Elastic Foundations: A GDQ Solution for Static Analysis with a Posteriori Stress and Strain Recovery

Abstract: This work focuses on the static analysis of functionally graded (FGM) and laminated doubly-curved shells and panels resting on nonlinear and linear elastic foundations using the Generalized Differential Quadrature (GDQ) method. The First-order Shear Deformation Theory (FSDT) for the aforementioned moderately thick structural elements is considered. The solutions are given in terms of generalized displacement components of points lying on the middle surface of the shell. Several types of shell structures such as doubly-curved shells (elliptic and hyperbolic hyperboloids), singly-curved (spherical, cylindrical and conical shells), and degenerate panels (rectangular plates) are considered in this paper. The main contribution of this paper is the application of the differential geometry within GDQ method to solve doubly-curved FGM shells resting on nonlinear elastic foundations. The linear Winkler-Pasternak elastic foundation has been considered as a special case of the nonlinear elastic foundation proposed herein. The discretization of the differential system by means of the GDQ technique leads to a standard nonlinear problem, and the Newton-Raphson scheme is used to obtain the solution. Two different four-parameter power-law distributions are considered for the ceramic volume fraction of each lamina. In order to show the accuracy of this methodology, numerical comparisons between the present formulation and finite element solutions are presented. Very good agreement is observed. Finally, new results are presented to show effects of various parameters of the nonlinear elastic foundation on the behavior of functionally graded and laminated doubly-curved shells and panels.

Facile fabrication of core-shell structured Ag@carbon and mesoporous yolk-shell structured Ag@carbon@silica by an extended Stöber method

Abstract: Layer upon layer: A facile extended Stober method has been developed for the synthesis of diverse metal-carbon spheres, which includes core-shell-structured Ag@carbon, rattle-type Ag,AgBr@meso-SiO, and yolk-shell-structured Ag@carbon@meso-SiO (see figure).

SnO2@TiO2 double-shell nanotubes for a lithium ion battery anode with excellent high rate cyclability.

Abstract: SnO2@TiO2 double-shell nanotubes have been facilely synthesized by atomic layer deposition (ALD) using electrospun PAN nanofibers as templates. The double-shell nanotubes exhibited excellent high rate cyclability for lithium ion batteries. The retention of hollow structures during cycling was demonstrated.

A modified variational approach for vibration analysis of ring-stiffened conical–cylindrical shell combinations

Abstract: This work presents a modified variational method for dynamic analysis of ring-stiffened conical–cylindrical shells subjected to different boundary conditions. The method involves partitioning of the stiffened shell into appropriate shell segments in order to accommodate the computing requirement of high-order vibration modes and responses. All essential continuity constraints on segment interfaces are imposed by means of a modified variational principle and least-squares weighted residual method. Reissner-Naghdi's thin shell theory combined with the discrete element stiffener theory to consider the ring-stiffening effect is employed to formulate the theoretical model. Double mixed series, i.e., the Fourier series and Chebyshev orthogonal polynomials, are adopted as admissible displacement functions for each shell segment. To test the convergence, efficiency and accuracy of the present method, both free and forced vibrations of non-stiffened and stiffened shells are examined under different combinations of edge support conditions. Two types of external excitation forces are considered for the forced vibration analysis, i.e., the axisymmetric line force and concentrated point force. The numerical results obtained from the present method show good agreement with previously published results and those from the finite element program ANSYS. Effects of structural damping on the harmonic vibration responses of the stiffened conical–cylindrical–conical shell are also presented.

Nonlinear vibrations of functionally graded cylindrical shells

Abstract: In this paper, the nonlinear vibrations of functionally graded (FGM) circular cylindrical shells are analysed. The Sanders–Koiter theory is applied to model the nonlinear dynamics of the system in the case of finite amplitude of vibration. The shell deformation is described in terms of longitudinal, circumferential and radial displacement fields. Simply supported, clamped and free boundary conditions are considered. The displacement fields are expanded by means of a double mixed series based on Chebyshev orthogonal polynomials for the longitudinal variable and harmonic functions for the circumferential variable. Both driven and companion modes are considered; this allows the travelling-wave response of the shell to be modelled. The model is validated in the linear field by means of data retrieved from the pertinent literature. Numerical analyses are carried out in order to characterise the nonlinear response when the shell is subjected to a harmonic external load; a convergence analysis is carried out by considering a variety of axisymmetric and asymmetric modes. The present study is focused on determining the nonlinear character of the shell dynamics as the geometry (thickness, radius, length) and material properties (constituent volume fractions and configurations of the constituent materials) vary.

Prevalence of anisotropic shell growth in rare earth core-shell upconversion nanocrystals.

Abstract: Through a series of carefully executed experiments, we discovered the prevalence of anisotropic shell growth in many upconversion NaREF4 systems caused by a combination of factors: selective adsorption of ligands on the core surface due to the core crystal structure, ligand etching, and the lattice mismatch between core and shell components. This could lead to incomplete shell formation in core–shell nanocrystals under certain conditions. Shell growth is always faster in the a and b crystallographic directions than in the c direction. In the case of a larger lattice mismatch between the core and shell, shell growth only occurs in the a and b directions resulting in an oblong core–shell structure. These findings are useful for rationalizing shell-dependent emission properties, understanding the emission mechanisms in complex core–shell nanostructures, and for creating accurate models of core–shell designs for multifunctionality and optimal performance in applications.

An isogeometric solid-like shell element for non-linear analysis

Abstract: An isogeometric solid-like shell formulation is proposed in which B-spline basis functions are used to construct the mid-surface of the shell. In combination with a linear Lagrange shape function in the thickness direction, this yields a complete three-dimensional representation of the shell. The proposed shell element is implemented in a standard finite element code using Bezier extraction. The formulation is verified using different benchmark tests.

An approach to detecting a reducing gas by radial modulation of electron-depleted shells in core–shell nanofibers

Abstract: Based on the radial modulation of electron-depleted shell layers in SnO2–ZnO core–shell nanofibers (CSNs), a novel approach is proposed for the detection of very low concentrations of reducing gases. In this work, SnO2–ZnO CSNs were synthesized by a two-step process: core SnO2 nanofibers were first prepared by electrospinning, followed by the preparation of ZnO shell layers by atomic layer deposition. The radial modulation of electron depletion in the CSN shells was accomplished by controlling the shell thickness. The sensing capabilities of CSNs were investigated with respect to CO and NO2 that represent typical reducing and oxidizing gases, respectively. In the case of CO at a critical shell thickness, the CSN-based sensors showed greatly improved sensing capabilities compared with those fabricated on the basis of either pure SnO2 or pure ZnO nanofibers. In sharp contrast, CSN sensors revealed inferior sensing capabilities for NO2. The results can be explained by a model based on the radial modulation of the electron-depleted CSN shells. The model suggests that CSNs comprising dissimilar materials having different energy-band structures represent an effective sensing platform for the detection of low concentrations of reducing gases when the shell thickness is equivalent to the Debye length.

Direct measurement of coherency limits for strain relaxation in heteroepitaxial core/shell nanowires

Abstract: The growth of heteroepitaxially strained semiconductors at the nanoscale enables tailoring of material properties for enhanced device performance. For core/shell nanowires (NWs), theoretical predictions of the coherency limits and the implications they carry remain uncertain without proper identification of the mechanisms by which strains relax. We present here for the Ge/Si core/shell NW system the first experimental measurement of critical shell thickness for strain relaxation in a semiconductor NW heterostructure and the identification of the relaxation mechanisms. Axial and tangential strain relief is initiated by the formation of periodic a/2 ⟨110⟩ perfect dislocations via nucleation and glide on {111} slip-planes. Glide of dislocation segments is directly confirmed by real-time in situ transmission electron microscope observations and by dislocation dynamics simulations. Further shell growth leads to roughening and grain formation which provides additional strain relief. As a consequence of core/she...

Two-dimensional theory of piezoelectric shells considering surface effect

Abstract: Two-dimensional (2D) general equations of piezoelectric shells with nano-thickness are presented in an orthogonal curvilinear coordinate system, in which the surface effect is considered by treat the shell as a bulk core plus two surface layers. The general 2D equations can be directly degenerated into those of particular shells such as flat plates, cylindrical shells and so on by setting the Lame coefficients and the principal radii of curvature to certain values. Using the derived 2D equations of cylindrical shells, the free vibration of a radially polarized piezoelectric cylindrical shell of finite length is analyzed. Numerical results show that the surface effect has a remarkable influence on the natural frequency of the shell at the nano-scale.

Torsional vibration and buckling of the cylindrical shell with functionally graded coatings surrounded by an elastic medium

Abstract: In this study, the torsional vibration and buckling analysis of cylindrical shell with functionally graded (FG) coatings surrounded by an elastic medium is investigated. The material properties of FG coatings are assumed to be graded in the thickness direction according to a simple power law distribution in terms of the volume fractions of the constituents. A two-parameter foundation model or Pasternak foundation model is used to describe the shell–foundation interaction. The basic equations are derived and solved by using Galerkin method and the expressions for dimensionless torsional buckling load and dimensionless torsional frequency parameter of the cylindrical shell with FG coatings resting on the Pasternak elastic foundation are obtained. Finally, the influences of geometrical parameters, volume fraction distribution of FG coatings and foundation stiffnesses on the dimensionless torsional buckling load and dimensionless torsional frequency parameter are discussed. The present results are compared with the available data for a special case.