Showing papers on "Shell (structure) published in 2007"

A meshfree thin shell method for non‐linear dynamic fracture

Abstract: A meshfree method for thin shells with finite strains and arbitrary evolving cracks is described. The C 1 displacement continuity requirement is met by the approximation, so no special treatments for fulfilling the Kirchhoff condition are necessary. Membrane locking is eliminated by the use of a cubic or quartic polynomial basis. The shell is tested for several elastic and elasto-plastic examples and shows good results. The shell is subsequently extended to modelling cracks. Since no discretization of the director field is needed, the incorporation of discontinuities is easy to implement and straightforward.

Type-II core/shell CdS/ZnSe nanocrystals: synthesis, electronic structures, and spectroscopic properties.

Abstract: We report a two-step synthesis of highly luminescent CdS/ZnSe core/shell nanocrystals (emission quantum yields up to 50%) that can produce efficient spatial separation of electrons and holes between the core and the shell (type-II localization regime). Our synthesis involves fabrication of cubic-singony CdS core particles that are subsequently overcoated with a layer of ZnSe in the presence of surfactant-ligands in a noncoordinating solvent. Studies of different growth regime of the ZnSe shell indicate that one approach to obtaining high emission efficiencies is through alloying the CdS/ZnSe interface with CdSe, which leads to the formation of an intermediate ZnCdSe layer with a graded composition. We perform theoretical modeling of these core/shell nanocrystals using effective mass approximation and applying first-order perturbation theory for treating both direct electron-hole coupling and the core/shell interface-polarization effects. Using this model we determine the range of geometrical parameters of the core/shell structures that result in a type-II localization regime. We further applied this model to evaluate the degree of electron-hole spatial separation (quantified in terms of the electron-hole overlap integral) based on measured emission wavelengths. We also discuss the potential applicability of these nanocrystals in lasing technologies and specifically the possibility of single-exciton optical gain in type-II nanostructures.

Universal Scaling of Plasmon Coupling in Metal Nanostructures: Extension from Particle Pairs to Nanoshells

Abstract: It has been recently shown that the strength of plasmon coupling between a pair of plasmonic metal nanoparticles falls as a function of the interparticle gap scaled by the particle size with a near-exponential decay trend that is universally independent of nanoparticle size, shape, metal type, or medium dielectric constant. In this letter, we extend this universal scaling behavior to the dielectric core-metal shell nanostructure. By using extended Mie theory simulations of silica core-metal nanoshells, we show that when the shift of the nanoshell plasmon resonance wavelength scaled by the solid nanosphere resonance wavelength is plotted against the shell thickness scaled by the core radius, nanoshells with different dimensions (radii) exhibit the same near-exponential decay. Thus, the nanoshell system becomes physically analogous to the particle-pair system, i.e., the nanoshell plasmon resonance results from the coupling of the inner shell surface (cavity) and the outer shell surface (sphere) plasmons over a separation distance essentially given by the metal shell thickness, which is consistent with the plasmon hybridization model of Prodan, Halas, and Nordlander. By using the universal scaling behavior in the nanoshell system, we propose a simple expression for predicting the dipolar plasmon resonance of a silica-gold nanoshell of given dimensions.

Three-layer core/shell structure in Au-Pd bimetallic nanoparticles.

Abstract: This paper describes the internal structure of Au-Pd nanoparticles exhibiting newly discovered three-layer core/shell morphology, which is composed of an evenly alloyed inner core, an Au-rich intermediate layer, and a Pd-rich outer shell. By exploitation of spatially resolved imaging and spectroscopic and diffraction modes of transmission electron microscopy (TEM), insights were gained on the composition of each one of the observed three layers, indicating a significant extent of intimate alloy among the monometallic elements.

The design and simulated performance of a coated nano-particle laser

Abstract: The optical properties of a concentric nanometer-sized spherical shell comprised of an (active) 3-level gain medium core and a surrounding plasmonic metal shell are investigated. Current research in optical metamaterials has demonstrated that including lossless plasmonic materials to achieve a negative permittivity in a nano-sized coated spherical particle can lead to novel optical properties such as resonant scattering as well as transparency or invisibility. However, in practice, plasmonic materials have high losses at optical frequencies. It is observed that with the introduction of active materials, the intrinsic absorption in the plasmonic shell can be overcome and new optical properties can be observed in the scattering and absorption cross-sections of these coated nano-sized spherical shell particles. In addition, a “super” resonance is observed with a magnitude that is 103 greater than that for a tuned, resonant passive nano-sized coated spherical shell. This observation suggests the possibility of realizing a highly sub-wavelength laser with dimensions more than an order of magnitude below the traditional half-wavelength cavity length criteria. The operating characteristics of this coated nano-particle (CNP) laser are obtained numerically for a variety of configurations.

One-step production of polymeric microtubes by co-electrospinning

Abstract: Herein we demonstrate the ability to fabricate polymeric microtubes with an inner diameter of approximately 3 mm through co-electrospinning of core and shell polymeric solutions. The mechanism by which the core/ shell structure is transformed into hollow fibers (microtubes) is primarily based on the evaporation of the core solution through the shell and is described here in detail. Additionally, we present the filling of these microtubes, thus demonstrating their possible use in microfluidics. We also report the incorporation of a protein (green fluorescent protein) within such fibers, which is of interest for sensorics.

Sharp bounds on the critical stability radius for relativistic charged spheres

Abstract: In a recent paper by Giuliani and Rothman \cite{GR}, the problem of finding a lower bound on the radius $R$ of a charged sphere with mass M and charge Q

Telemetry method and apparatus using magnetically-driven mems resonant structure

Abstract: A telemetry method and apparatus using pressure sensing elements remotely located from associated pick-up, and processing units for the sensing and monitoring of pressure within an environment. This includes remote pressure sensing apparatus incorporating a magnetically-driven resonator being hermetically-sealed within an encapsulating shell or diaphragm and associated new method of sensing pressure. The resonant structure of the magnetically-driven resonator is suitable for measuring quantities convertible to changes in mechanical stress or mass. The resonant structure can be integrated into pressure sensors, adsorbed mass sensors, strain sensors, and the like. The apparatus and method provide information by utilizing, or listening for, the residence frequency of the oscillating resonator. The resonant structure listening frequencies of greatest interest are those at the mechanical structure's fundamental or harmonic resonant frequency. The apparatus is operable within a wide range of environments for remote one-time, random, periodic, or continuous/on-going monitoring of a particular fluid environment. Applications include biomedical applications such as measuring intraocular pressure, blood pressure, and intracranial pressure sensing.

Palladium-Coated Gold Nanoparticles with a Controlled Shell Thickness Used as Surface-Enhanced Raman Scattering Substrate

Abstract: We report the synthesis and characterization of gold core palladium shell (Au@Pd) nanoparticles with thickness-controlled shell as an improved transition-metal substrate for surface-enhanced Raman scattering (SERS). By changing the molar ratio of H2PdCl4 to Au, the Pd shell thickness can be precisely controlled from a few nanometers down to ca. one monolayer. A series of characterizations were performed using transmission electron microscopy (TEM), UV−vis, SERS, and electrochemical techniques. The results confirmed the core−shell structure and the uniform and pinhole-free nature of the Pd shell, ensuring the properties of Pd without possible interference from Au. Consistent with theoretical prediction, the core−shell setting borrows high SERS activity from the Au core through the long-range electromagnetic enhancement in addition to the enhancement from the Pd shell itself. Moreover, their SERS activity can be optimized by the tunable shell thickness and core size. The nm-Au@Pd/Pd electrodes allow us to o...

A large deformation solid-shell concept based on reduced integration with hourglass stabilization

Abstract: In this paper a new eight-node (brick) solid-shell finite element formulation based on the concept of reduced integration with hourglass stabilization is presented. The work focuses on static problems. The starting point of the derivation is the three-field variational functional upon which meanwhile established 3D enhanced strain concepts are based. Important additional assumptions are made to transfer the approach into a powerful solid-shell. First of all, a Taylor expansion of the first Piola–Kirchhoff stress tensor with respect to the normal through the centre of the element is carried out. In this way the stress becomes a linear function of the shell surface co-ordinates whereas the dependence on the thickness co-ordinate remains non-linear. Secondly, the Jacobian matrix is replaced by its value in the centre of the element. These two assumptions lead to a computationally efficient shell element which requires only two Gauss points in the thickness direction (and one Gauss point in the plane of the shell element). Additionally three internal element degrees-of-freedom have to be determined to avoid thickness locking. One important advantage of the element is the fact that a fully three-dimensional stress state can be modelled without any modification of the constitutive law. The formulation has only displacement degrees-of-freedom and the geometry in the thickness direction is correctly displayed. Copyright © 2006 John Wiley & Sons, Ltd.

Free vibration analysis of functionally graded cylindrical shells including thermal effects

Abstract: Free vibration analysis of simply supported FG cylindrical shells for four sets of in-plane boundary conditions is performed. The material properties are assumed to be temperature-dependant and gradually changed in the thickness direction of the shell. The effects of temperature rise are investigated by specifying arbitrary high temperature on the outer surface and the ambient temperature on the inner surface of the cylinder. Distribution of temperature across the shell thickness is found from steady state heat conduction only in the thickness direction. The equations of motion are based on Love's shell theory and the von Karman–Donnell-type of kinematic nonlinearity. The static analysis is first performed to determine the prestressed state induced by the thermal loadings, using the exact solution of the governing equations and then the equations of motion are solved by Galerkin's method. The results are obtained to indicate the effects of power law index on the natural frequencies and corresponding mode shapes in the thermal environment.

Modeling of the mass-transfer kinetics in chromatographic columns packed with shell and pellicular particles.

Abstract: The equations of the general rate model of chromatography and those of simple models (the POR, equilibrium-dispersive, and transport-dispersive models) are derived for beds packed with shell particles. Shell particles are made of a solid, nonporous core surrounded by a shell of a porous material that has properties similar to those of the fully porous materials conventionally used in HPLC. These equations have no algebraic solutions, but the moments of the peaks eluted under linear conditions can be calculated, affording the HETP equation for these columns. The discussion of the contribution to the HETP of the mass-transfer resistances shows that shell particles exhibit much lower plate heights for large molecular size compounds (e.g., peptides and proteins) than do fully porous particles, this advantage increasing with decreasing thickness of the shell. In contrast, the efficiencies of columns packed with shell particles and with fully porous particles having the same diameters are nearly the same for lo...

Review of the Synthetic Chemistry Involved in the Production of Core/Shell Semiconductor Nanocrystals

Abstract: Passivation of CdSe semiconductor nanocrystals can be achieved by overcoating the particles with a homogeneous shell of a second semiconductor. Shell layers are grown in monolayer steps to ensure homogeneous growth of the shell. The relative band edges of the two materials determine the photoreactiveity of the resultant core-shell nanocrystals. The critical role of ligands in minimizing nucleation of the shell material during the growth of the passivating layer is emphasized. The delocalization of charge carriers into the shell layers can be followed spectroscopically during the growth processes. The relative spectral shifts are directly correlated to the relative energies of the band edges.

Mass-transfer kinetics in a shell packing material for chromatography.

Abstract: A shell particle consists of a solid, nonporous core that is surrounded with a shell of a porous solid having essentially the same physicochemical properties as those of the conventional porous particles used as packing media in chromatography. The diameter of the solid core and the thickness of its shell or the external diameter of the particle characterizes the chromatographic properties of the packing material. The potential advantage of this particle structure would be the shorter average path length experienced by solute molecules during their diffusion across the particles of packing material when they are retained. Compounds having slow pore diffusion would exhibit higher efficiencies on columns packed with shell than with conventional, fully porous particles. Using columns packed with Halo, a new type of porous silica shell particles, we assess the gain achieved with this principle for peptides of moderate molecular weights and for small proteins.

Structure of the first- and second-neighbor shells of simulated water: quantitative relation to translational and orientational order.

Abstract: We perform molecular dynamics simulations of water using the five-site transferable interaction potential (TIP5P) model to quantify structural order in both the first shell (defined by four nearest neighbors) and second shell (defined by twelve next-nearest neighbors) of a central water molecule. We find that the anomalous decrease of orientational order upon compression occurs in both shells, but the anomalous decrease of translational order upon compression occurs mainly in the second shell. The decreases of translational order and orientational order upon compression (called the "structural anomaly") are thus correlated only in the second shell. Our findings quantitatively confirm the qualitative idea that the thermodynamic, structural, and hence dynamic anomalies of water are related to changes upon compression in the second shell.

Mn 2 + as a Radial Pressure Gauge in Colloidal Core/Shell Nanocrystals

Abstract: The fluorescence of Mn2+ doped in the shell of CdS/ZnS core/shell nanocrystals at radially controlled position is used as a local probe of pressure in the nanocrystal shell. The redshift of the fluorescence with increasing shell thickness indicates a pressure of more than 4 GPa for 7.5 monolayers of ZnS. The radial dependence and magnitude of the pressure derived from the Mn2+ fluorescence shift are in good agreement with the spherically symmetric elastic continuum model and the 7% misfit between the core and the epitaxial shell.

Core / shell-type catalyst particles comprising metal or ceramic core materials and methods for their preparation

Abstract: The invention is directed to core/shell type catalyst particles comprising a Mcore / Mshell structure with Mcore = inner particle core and Mshell = outer particle shell, wherein the medium diameter of the catalyst particle (dcore+shell) is in the range of 20 to 100 nm, 5 preferably in the range of 20 to 50 nm. The thickness of the outer shell (tshell) is about 5 to 20 % of the diameter of the inner particle core of said catalyst particle, preferably comprising at least 3 atomic layers. The inner particle core (Mcore ) of the particles com- prises metal or ceramic materials, whereas the material of the outer shell (Mshell) comprises precious metals and/or alloys thereof. The core/shell type catalyst particles are preferably supported on suitable support materials such as carbon black and can be used as electrocatalysts for fuel cells and for other catalytic applications.

Core/shell-type catalyst particles and methods for their preparation

Abstract: The invention discloses core/shell type catalyst particles comprising a Mcore / Mshell structure with Mcore = inner particle core and Mshell = outer particle shell, wherein the medium diameter of the catalyst particle (dcore+shell) is in the range of 20 to 100 nm, preferably in the range of 20 to 50 nm. The thickness of the outer shell (tshell) is about 5 to 20 % of the diameter of the inner particle core of said catalyst particle, preferably comprising at least 3 atomic layers. The core/shell type catalyst particles, particularly the particles comprising a Pt-based shell, reveal a high specific activity. The catalyst particles are preferably supported on suitable support materials such as carbon black and are used as electrocatalysts for fuel cells.

The Effect of Silica Coating on the Optical Response of Sub-micrometer Gold Spheres

Abstract: The effects derived from the growth of silica shells on the optical properties of sub-micrometer gold spheres have been investigated as a function of shell thickness. The dipole plasmon resonance mode of these large spheres has been found to be significantly more sensitive to changes in the medium refractive index than the quadrupolar mode, in agreement with theoretical modeling based on the boundary element method (BEM) for concentric spheres. Near-field maps of both resonance modes in such coated nanoparticles show that the areas with significant near-field enhancement get progressively located within the silica shell as its thickness increases, with basically no enhancement outside of the shell when this is thicker than the sphere radius.

Transient heat conduction in functionally graded thick hollow cylinders by analytical method

Abstract: In this article, transient heat conduction in a cylindrical shell of functionally graded material is studied by using analytical method. The shell is assumed to be in axisymmetry conditions. The material properties are considered to be nonlinear with a power law distribution through the thickness. The temperature distribution is derived analytically by using the Bessel functions. To verify the proposed method the obtained numerical results are compared with the published results. The comparisons of temperature distribution between various time and material properties are presented.

Negative axial radiation forces on solid spheres and shells in a Bessel beam.

Abstract: Prior computations predict that fluid spheres illuminated by an acoustic Bessel beam can be subjected to a radiation force directed opposite the direction of beam propagation. The prediction of negative acoustic radiation force is extended to the cases of a solid poly(methylmethacrylate) PMMA sphere in water and an empty aluminum spherical shell in water. Compared with the angular scattering patterns for plane wave illumination, the scattering into the back hemisphere is suppressed when the radiation force is negative. This investigation may be helpful in the development of acoustic tweezers and in the development of methods for manipulating objects during space flight.

Vibration of functionally graded cylindrical shells based on higher order shear deformation plate theory with ring support

Abstract: In this paper, a study on the vibration of thin cylindrical shells with ring supports made of a functionally gradient material (FGM) composed of stainless steel and nickel is presented. The cylindrical shells have ring supports which are arbitrarily placed along the shell and which impose a zero lateral deflection. The study is carried out based on third order shear deformation shell theory (T.S.D.T). The objective is to study the natural frequencies, the influence of constituent volume fractions and the effects of configurations of the constituent materials on the frequencies. The properties are graded in the thickness direction according to the volume fraction power-law distribution. The analysis is carried out with strains-displacement relations from Love's shell theory. The governing equations are obtained using an energy functional with the Rayleigh-Ritz method. Results are presented on the frequency characteristics, influence of ring support position and the influence of boundary conditions. The present analysis is validated by comparing results with those available in the literature.