Shell Junctions Research Articles

DESIGN OF SHELL CONJUNCTION ZONES IN PRESSURE VESSELS

Separation forces occur in shell junctions of fuel tanks due to the change in its geometry. To compensate the forces, thrust rings are placed in these zones. Moreover, there is a necessity to determine a form and dimensions of the trust ring cross section with consideration of the operation conditions. Welding is used to join trust rings with a shell in case of conventional manufacturing. Nowadays, additive manufacturing technology is constantly being developed. Additive method allows to create objects with various geometry, in a layer-by-layer way of addition of the material in accordance with a computer model. These new technological capabilities make the task of determining of the geometry of the fuel tank in the shell’s junctions zone actual. 
 The following work provides a consideration of two approaches to the determination of the conversion geometry of shells in-between area instead of trust ring. The first approach is based on determination of a median surface with the use of rational cubic splines and membrane theory of shells. The second approach is based on the use of topology optimization of the initial design. The choice of the first approach relates to the fact that standard energy functional like potential energy of uniform bent rod or uniform sheet can be described with cubic splines. In the following work the use of rational cubical splines for build-up a transitive area in a junction zone of the spherical-conical vessel is considered. Spline parameters are determined based on the condition of median surface propagation of the transitive zone in its probable location. Thickness of the shell in the transitive area was evaluated due to the membrane theory of shells with the use of Huber von Mises Hencky theory of failure. Obtained solutions are tested in numerical models of spherical-conical vessel. Comparison of two approaches is carried out and practical recommendations are given.

Optimization of a GaAs/AlGaAs p-i-n heterojunction nanowire solar cell for improved optical and electrical properties

GaAs/AlGaAs based nanowires are promising candidates for photovoltaic applications due to their high absorption coefficient, low surface reflection, and efficient collection of photogenerated carriers. This study focuses on optimizing the performance of p-i-n GaAs/AlGaAs nanowire solar cell arrays having a radial junction using optoelectronic simulations. The research investigates the optimal doping for the GaAs core and AlGaAs shell, as well as the impact of shell thickness and junction positions on solar cell performance. Additionally, the study examines the effect of various surface effects, including the presence of surface traps, surface recombination velocities, and associated lifetime degradation. Our studies find that a high doping density for the shell and core region is crucial for achieving an appropriate band configuration and carrier extraction. It also highlights that having a larger doping density is more important than having a larger lifetime. Finally, the research examines the effect of different aluminum compositions on photogeneration inside the nanowire and shows that having a high aluminum composition can confine most photogeneration to inner GaAs regions, potentially allowing for thicker AlGaAs shells, which can efficiently prevent surface recombination.

Nonlinear static analysis of multi-segmented toroidal shells under external pressure

This paper investigates the nonlinear static responses of multi-segmented toroidal shells with egg-shaped cross sections. The discontinuity effect in terms of displacement and the slope at the toroidal shell edge must be constrained by the Lagrange multiplier technique. The structural shape of the multi-segmented toroidal shells can be described by the differential geometry of the shell with the six-center-combined toroidal shells. The energy functional of the multi-segmented toroidal shells can be written in the appropriate form based on the principle of virtual work. The numerical results of the displacement responses and the volume change can be obtained by the nonlinear finite element method via the fifth-order polynomial shape function and a direct iterative method. In this study, the Lagrange multiplier is the internal force at the toroidal shell junction sustaining the smooth curve of the multi-segmented toroidal shells under several loads. Finally, the results of the volume change and the displacement responses for various geometric parameters are presented and discussed.

Analysis of discontinuity stress at shell and elliptical head juncture using theory of beam on elastic foundation

A thin-walled pressure vessels under internal pressure generates higher amount of stresses at shell-head juncture compared to membrane stress. These localized stresses are generated due to change in the shape of the meridian. The change in shape of the meridian is seen at the head and shell junction. The discontinuity in shape changes the direction of resultant stress. To predict the accurate stresses around the discontinuity region, bending of beam on an elastic foundation is used as part of the analytical analysis. Charts have been plotted of various stresses generated near juncture. The method is applied to pressure vessel with cylindrical shell and elliptical head. The theoretical results are then compared to the analytical results obtained from finite element analysis (FEA). In the present work, the discontinuity stresses have been evaluated analytically and the same is also compared with the finite element analysis results. It is found that the results of FE analysis closely agrees with the analytical results maximum percentage variation of 1.19.

In situ off-axis electron holography of real-time dopant diffusion in GaAs nanowires

Off-axis electron holography was used to reveal remote doping in GaAs nanowires occurring during in situ annealing in a transmission electron microscope. Dynamic changes to the electrostatic potential caused by carbon dopant diffusion upon annealing were measured across GaAs nanowires with radial p-p+ core–shell junctions. Electrostatic potential profiles were extracted from holographic phase maps and built-in potentials (Vbi ) and depletion layer widths (DLWs) were estimated as function of temperature over 300–873 K. Simulations in absence of remote doping predict a significant increase of Vbi and DLWs with temperature. In contrast, we measured experimentally a nearly constant Vbi and a weak increase of DLWs. Moreover, we observed the appearance of a depression in the potential profile of the core upon annealing. We attribute these deviations from the predicted behavior to carbon diffusion from the shell to the core through the nanowire sidewalls, i.e. to remote doping, becoming significant at 673 K. The DLW in the p and p+ regions are in the 10–30 nm range.

Variational formalism for generic shells in general relativity

We investigate the variational principle for the gravitational field in the presence of thin shells of completely unconstrained signature (generic shells). Such variational formulations have been given before for shells of timelike and null signatures separately, but so far no unified treatment exists. We identify the shell equation as the natural boundary condition associated with a broken extremal problem along a hypersurface where the metric tensor is allowed to be nondifferentiable. Since the second order nature of the Einstein–Hilbert action makes the boundary value problem associated with the variational formulation ill-defined, regularization schemes need to be introduced. We investigate several such regularization schemes and prove their equivalence. We show that the unified shell equation derived from this variational procedure reproduce past results obtained via distribution theory by Barrabès and Israel for hypersurfaces of fixed causal type and by Mars and Senovilla for generic shells. These results are expected to provide a useful guide to formulating thin shell equations and junction conditions along generic hypersurfaces in modified theories of gravity.

A Naghdi Type Nonlinear Model for Shells with Little Regularity

In this paper a new nonlinear two-dimensional shell model of Naghdi’s type is formulated for shells which middle surface is parameterized by a $W^{1, \infty}$ function. Therefore the model inherently contains undeformed geometries with corners, so the model also includes models of junctions of nonlinear shells. Deformation of the shell is described by a pair $(\boldsymbol{\psi}, {\mathbf{R}})$ of independent unknowns, where $\boldsymbol{\psi}$ is the deformation of the middle surface and ${\mathbf{R}}$ is a function with value in rotations that describes rotation of the shell cross-section. The model is formulated as the minimization problem for the total energy functional that includes flexural, membrane, shear and drill energies differently scaled with respect to the thickness of the shell. We relate the new model for smooth enough undeformed geometry to the known shell models in two ways. First we restrict the proposed model on two particular subsets of admissible functions and obtain exactly the flexural shell model and a perturbation of the Koiter shell model. More important, we consider asymptotics, using $\Gamma$ –convergence, of the proposed model with respect to the thickness as a small parameter in the membrane and flexural regime and obtain exactly the nonlinear membrane and flexural shell model obtained as $\Gamma$ –limits starting from nonlinear three–dimensional elasticity. In that way we link rigorously the proposed model with the nonlinear three–dimensional elasticity.

Strength and stability of spherical-conical shell assemblies under external hydrostatic pressure

Abstract Thin shells are increasingly finding new applications under the sea. In this study, we consider a thin-walled shell-of-revolution assembly comprising a deep spherical shell dome (deeper than a hemisphere) axisymmetrically and tangentially joined to a steep-sided conical shell, the whole being a closed shell structure intended for stationary deployment beneath the surface of the sea in relatively shallow water. The closed shell structure, which might serve as an underwater observatory, is intended to operate at a constant depth, anchored to the seabed against flotation forces, with the thin steel shell walls being required to withstand the external hydrostatic pressure of the surrounding water. We use shell theory to investigate the discontinuity stresses that occur at the junction of the spherical shell and the conical shell, and employ FEM to explore the buckling behaviour of the thin shell. While discontinuity stresses are relatively small, they may influence the lower buckling modes of the shell, which are found to be largely confined to the region of the cone that is adjacent to the junction. Considerations are extended to a doubly-curved variant of the cone in the form of a paraboloid of revolution. As expected, double curvature enhances buckling capacity and also influences the mode shapes.

Electrical characteristics of funnel-shaped silicon nanowire solar cells

The electrical characteristics of funnel-shaped silicon nanowire (SiNW) solar cells are introduced and numerically analyzed. The funnel-shaped NW consists of a cylinder over a conical unit. Its aim is to maximize the optical absorption over a large wavelength range and hence the electrical efficiency by increasing the number of resonance wavelengths or by enlarging the resonance wavelength range. The conical part has different radii in the axial direction, which increases the number of resonance wavelengths. Further, the coupling between the supported modes by the upper cylinder and the lower tapered cone offers multiple optical resonances required for broadband absorption. The optical characteristics and generation rates through the studied design are obtained using 3-D finite difference time domain. However, the electrical properties are calculated using finite element via the Lumerical device software package. In this regard, radial and axial junctions are examined for the suggested design and compared with the conventional cylindrical SiNW counterpart. In this investigation, short circuit current density, open circuit voltage, fill factor, and power conversion efficiency (PCE) are simulated to quantify the optoelectronic performance of the reported design. Furthermore, the effects of the doping concentration and carrier lifetime on the performance of the funnel-shaped design are reported. The proposed SiNWs offer PCE and short circuit density of 12.7% and 27.6 mA/cm2, respectively, for the axial junction. However, the funnel design with core–shell junction shows an efficiency and short-circuit current (Jsc) of 14.13% and 31.94 mA/cm2, respectively. Therefore, the suggested design has higher efficiency than 6.4% and 9.6% of the conventional cylindrical SiNWs according to the axial and core shell junctions, respectively.

Analytical formulation and numerical modelling for multi-shell toroidal pressure vessels

Depending on the required solution, this paper presents results for the state of stress and buckling of a uniformly pressurized elastic toroidal vessel of four segments. In the first part of the paper, a closed-formed stress solution is formulated for the novel shell form, by adopting the membrane solution as the particular solution of the Reissner-Meissner general bending-theory equations, and an approximate bending solution is used to quantify discontinuity effect at the shell junctions. In the second part of the paper, linearized stability equations are formulated and simplified for the segmented toroidal vessel buckling problem. The membrane results obtained in the first part is used to predict the pre-buckling state and the stability equations are approximately solved for the segments in the middle regions of the toroidal vessel, using the Galerkin’s scheme. This leads to an expression for estimating the critical buckling pressures of pressurized isotropic toroids. Numerical results from the proposed methods are presented and compared with those from a finite element method solution.

Finite Element Analysis of Skirt to Shell Junction in a Pressure Vessel

Objectives: The objective of this research work is to find the effect of providing hotbox at skirt to shell junction of a pressure vessel on the stresses induced at this junction. Methods/Statistical Analysis: In present work, since the vessel is to be supported at some elevation from ground, a conical skirt support is used. Two models of skirt to shell junction are made, in Unigraphics 10, one without hotbox while other with hotbox. A thermo-structural analysis of models is performed using ANSYS Workbench15. Stress linearization is done and stresses are limited to code allowable to ensure protection against plastic collapse and local failure. Findings: The result of finite element analysis for the case of model without hotbox shows that thermal stresses as high as 329 MPa are induced at the y-ring, which is much higher than the allowable stress at that temperature. When a hotbox is provided at this region, the stress is found to be reduced to 35.082 MPa, which is less than the allowable stress. So, the vessel will work safely if a hotbox of minimum 480 mm length is provided at the critical junction of skirt and shell. Stress is linearized at five stress classification lines. The linearized stresses and their combinations for four load cases are compared with the code allowable limit, and are found to be less than allowable stress. This ensures the protection of vessel against plastic collapse and local failure. Application/Improvements: Such analysis is needed to be performed for long vessels supported on skirt type of support, as the total vessel loads will be transferred to skirt from this junction only.

Ultraphotostable Mesoporous Silica-Coated Gap-Enhanced Raman Tags (GERTs) for High-Speed Bioimaging.

Surface-enhanced Raman scattering (SERS) tags can be utilized as optical labeling nanoprobes similar to fluorescent dyes and quantum dots for bioimaging with additional advantages of fingerprint vibrational signals as unique optical codes and ultranarrow line widths for multiplexing. However, the development of the SERS imaging technique is much less well-established compared to the devlopment of fluorescence imaging mainly because of speed limitations. An effective strategy for improving the SERS imaging speed and simultaneously maintaining the photostability of SERS tags has not, to the best of our knowledge, been reported. In this work, mesoporous silica- (MS-) coated gap-enhanced Raman tags (GERTs) were designed with built-in Raman reporters for off-resonance near-infrared laser excitation and reduced photothermal effects, leading to ultraphotostability during laser irradiation. Additionally, they achieve large amplification of Raman signals by combining the chemical (CHEM) and electromagnetic (EM) enhancement effects due to the subnanometer core-shell junction, so SERS imaging can be performed in a dramatically reduced duration. With these unique structural and optical advantages, MS GERTs exhibit high storage, pH, serum, and photostabilities; strong Raman enhancements; and favorable biocompatibility. Therefore, MS GERTs achieve long-term cell imaging that can last for 30 min without being photobleached and also maintain decent imaging effects. Furthermore, MS GERTs enable continuous and stable imaging in living tissues for more than 30 min. With these advantages, MS GERTs might potentially have more biomedical applications.

CALCULATION OF LOCAL STRESS STATE FOR SHELLS INTERSECTION

<p>This paper presents current calculation methods in order to determine the state of stress in the nozzle – shell junction section, their peculiarities, and limitations. These can be applied only to the linear-elastic behavior of the materials components. In the case of nonlinear behavior, beyond the yield stress limit, there is proposed a calculation method based on the concept of specific energy participation.</p>

Double Detection of Mycotoxins Based on SERS Labels Embedded Ag@Au Core-Shell Nanoparticles.

A sensitive surface-enhanced Raman scattering (SERS) signal dependent double detection of mycotoxins is achieved for the first time, without the aid of nucleic acid amplification strategies. SERS labels embedded Ag@Au core-shell (CS) nanoparticles (NPs) as novel SERS tags are successfully prepared through a galvanic replacement-free deposition. SERS tags produce stable and quantitative SERS signal, emerging from the plasmonic coupling at the junction of Ag core and Au shell. SERS tags engineered Raman aptasensors are developed for the double detection of ochratoxin A (OTA) and aflatoxin B1 (AFB1) in maize meal. The limits of detection (LODs) are as low as 0.006 ng/mL for OTA and 0.03 ng/mL for AFB1. The developed protocol can be extended to a large set of different SERS tags for the sensitive detection of multiple targets that possess different lengths of aptamers.

Nanooptics of Plasmonic Nanomatryoshkas: Shrinking the Size of a Core-Shell Junction to Subnanometer.

Quantum effects in plasmonic systems play an important role in defining the optical response of structures with subnanometer gaps. Electron tunneling across the gaps can occur, altering both the far-field optical response and the near-field confinement and enhancement. In this study, we experimentally and theoretically investigate plasmon coupling in gold "nanomatryoshka" (NM) nanoparticles with different core-shell separations. Plasmon coupling effects between the core and the shell become significant when their separation decreases to 15 nm. When their separation decreases to below 1 nm, the near- and far-field properties can no longer be described by classical approaches but require the inclusion of quantum mechanical effects such as electron transport through the self-assembled monolayer of molecular junction. In addition, surface-enhanced Raman scattering measurements indicate strong electron-transport induced charge transfer across the molecular junction. Our quantum modeling provides an estimate for the AC conductances of molecules in the junction. The insights acquired from this work pave the way for the development of novel quantum plasmonic devices and substrates for surface-enhanced Raman scattering.

Core-shell ZnO@CuInS2 hexagonal nanopyramids with improved photo-conversion efficiency

A soft chemical route has been employed to synthesize ZnO@CuInS2 (ZnO@CIS) core–shell hexagonal nanopyramids using 2-mercaptopropionic acid as a surface-functionalized agent. The core–shell formation was confirmed by the X-ray diffraction, high resolution transmission electron microscope and X-ray photoelectron spectroscopy characterizations. UV–visible absorption spectrum of the core–shell nanopyramids shows a red shift of the band edge of pure ZnO in the visible region along with broadening of the absorption spectrum. ZnO@CIS nanopyramids can be used as an antireflection coating on Silicon solar cell, exhibit lower reflectance and better light trapping ability than the Silicon solar cell with ZnO nanopyramids. Moreover higher photocurrent is obtained for the n-ZnO/n-CIS core–shell junction due to favorable band bending. The efficiency of the p-Si/n-ZnO solar cell (6.6%) is improved to 10% for p-Si/n-ZnO@n-CIS configuration.

Influence of Electrical Design on Core–Shell GaAs Nanowire Array Solar Cells

In this paper, we perform coupled optical and electrical simulation for core–shell junction GaAs nanowire array solar cells to obtain the important physical insights of how the cell efficiency is affected by various key parameters, including core–shell doping, junction position, carrier lifetime, and surface effect. Our study reveals that junction design in core–shell nanowires in terms of doping and geometry is largely restricted and affected by the small nanowire dimension, requiring different optimization from those of conventional planar solar cells. To take advantage of the radial p-n junction, high core and shell doping are essential to achieve effective radial carrier collection. Moreover, maintaining thin nanowire shell could effectively offset the detrimental surface effect for improved efficiency.

A theoretical formulation for the stress analysis of multi-segmented spherical shells for high-volume liquid containment

A linear-elastic theoretical formulation is presented for the complete determination of the state of stress in large thin-walled liquid-filled vessels in the form of multi-segmented spherical shells. The transfer of membrane forces between adjacent shell segments is such that only vertical equilibrium of stress resultants needs to be preserved. The edge effect in the vicinity of the shell junctions is quantified on the basis of an approximate but accurate bending theory for spherical shells. The effectiveness of the developed formulation is demonstrated by consideration of a numerical example. Agreement with the results of finite-element modelling is excellent, showing that the presented theoretical formulation is a reliable, computationally efficient and accurate means of obtaining stresses in large multi-segmented spherical vessels.

Enhancement of CdSe/ZnS quantum dot-based LED by core-shell modification

In this study, we performed an analysis of quantum-dot-based light-emitting diodes (QD-based LEDs) to investigate the defects at the interface between the core and the shell of the quantum dots (QDs) using gradient-shell QDs (CdSe/Cd1-xZnxSe1-ySy/ZnS, G-QDs) and single-shell QDs (CdSe/ZnS, S-QDs). QDs of the general core-shell type have defects at the core and shell junction interface due to the different lattice constants. However, G-QDs have a low number of lattice defects in the form of a step function between the core and the shell owing to their chemical composition and can more easily confine electron-hole pairs (EHP). Therefore, we fabricated QD-based LEDs by using two emissive layer (G-QDs and S-QDs) and analyzed their characteristics, including their brightness and efficiency.

Application of RCC-MR for the structural design of tube sheet of intermediate heat exchanger for a sodium cooled fast reactor

The structural mechanics behavior of tube sheets of a sodium to sodium heat exchanger for a fast reactor, with circular tube holes pattern, less addressed subject in the literature, is investigated in detail. The tube sheet design rules recommended in the French design code RCC-MR-2007 and the associated solid mechanics basis are explained. A finite element analysis of tube sheets of intermediate heat exchanger of a typical 500MWe pool type fast reactor is presented to study the effects of some specific parameters viz., (i) small solid rim portion with connecting shell and (ii) grooves on rim area. For the analysis, the distribution of holes on the last row is assumed to be symmetric and axial stiffening of tubes on tube sheet is included toward realistic estimation of stresses in the tube sheets. The effects are studied on the primary and secondary stresses induced along the interface between solid to perforated region. The aspects covered include linearization of radial and circumferential stress components, thereby deriving primary membrane and bending stress intensities along the radial directions with particular focus at the interfaces between solid portions and perforated portions including the effect of filet radius at the junction of tube sheet and shell. These investigations thus help to optimize the design of IHX tube sheets with high confidence. The analysis has been carried out by CAST3M, a structure analysis software granted by CEA, France.