Shell Aspect Ratio Research Articles

A geometrical model for the description of the AlN shell morphology in GaN-AlN core-shell nanowires

A geometrical model based on the one formulated by Foxon et al. [J. Cryst. Growth 311, 3423 (2009)] is developed to describe the morphology of AlN shells in GaN-AlN core-shell nanowires grown by plasma-assisted molecular beam epitaxy. The shell aspect ratio is studied as a function of the atomic beam flux incidence angles and of the ratio between Al and N species. The comparison between experimental data and the developed geometrical model suggests the diffusion of about 55% of Al atoms from the side walls to the top surface.

Tuning the number of plasmon band in silver ellipsoidal nanoshell: refractive index sensing based on plasmon blending and splitting

Because of the geometric features of both rod and shell, dielectric-silver core–shell ellipsoidal nanostructure with 12–40 nm semi-major axis may bring forth four surface plasmon resonance (SPR) absorption peaks at most. Theoretical calculations based on quasi-static approximation show that there is surrounding refractive index-dependent plasmon blending and splitting in the absorption spectra, which makes the number of plasmon band of the silver ellipsoidal nanoshell is tunable. The sensitivity of the plasmon blending and splitting to the surrounding refractive index may be improved by increasing the shell thickness, aspect ratio or core refractive index. This local refractive index dependent-plasmon blending and splitting presents a new sensing picture based on tuning the number of SPR absorption peaks.

Simple Shape-Controlled Synthesis of Carbon Hollow Structures

This study reports a simple method for the controlled synthesis of uniformly shaped carbon hollow structures by an ethanol-assisted thermolysis of zinc acetate. The experimental evidence reveals that the generated zinc oxide nanostructures act as in-situ templates to form the carbon hollow structures. The morphologies, including the shell thickness, cavity size, and aspect ratio, can be controlled by the reaction time and the heating procedure, and hollow nanospheres, nanocapsules, nanorods, and microtubes can be obtained. Experimental results show that the as-synthesized carbon hollow structures exhibit excellent thermal and structural stability to temperatures as high as 1200 °C.

Multiobjective optimization of laminated composite cylindrical shells for maximum frequency and buckling load

This paper deals with multiobjective optimization of laminated cylindrical shells to maximize a weighted sum of the frequency and buckling load under external load. The layer fibre orientation is used as the design variable and the multiobjective optimization is formulated as the weighted combinations of the frequency and buckling under external load. The first order shear deformation theory is used for the finite element formulation of the laminated shells. Five shell configurations with eight layers are considered as candidate designs. The modified feasible direction method (MFD) is used as optimization routine. For this purpose, a program based on FORTRAN is used for the optimization of the laminated shells. Finally, the effect of different weighting ratios, shell aspect ratio, shell thickness-to-radius ratios and boundary conditions on the optimal designs is investigated and the results are compared.

Beads Comprising a Hierarchical Porous Core and a Microporous Shell

Two different types of zeolitic materials were structured in core−shell composites possessing a large core (BEA-type) and a thin shell (MFI-type). The core structure comprised a hierarchical porous organization where the access to the zeolite micropores was enabled by macro- and mesopores. This structure was prepared by macroporous anion exchange resin templating, that is, zeolite β was crystallized within the pores of the resin beads. The organic macrotemplate was then removed by combustion leaving stable 300−500 μm macrospheres of zeolite β. Preliminary seeding of the calcined beads with silicalite-1 nanocrystals induced the formation of a well-intergrown silicalite-1 shell during the hydrothermal treatment. The shell thickness did not exceed 1.0 μm, thus providing a material with a very high core−shell aspect ratio. The integrity of the shell layer was tested by N2 adsorption measurements on materials comprising a calcined core and a noncalcined organic templatecontaining silicalite-1 film, thus provid...

Numerical and experimental investigations of the response of aluminum cylinders with a cutout subject to axial compression

Understanding how a cutout influences the load bearing capacity and buckling behavior of a cylindrical shell is critical in the design of structural components used in automobiles, aircrafts, and marine applications. Numerical simulation and analysis of moderately thick and thin unstiffened aluminum cylindrical shells ( D/ t=45, 450 and L/ D=2, 5, 10), having a square cutout, subjected to axial compression were systematically carried out in this paper. The investigation examined the influence of the cutout size, cutout location, and the shell aspect ratio ( L/ D) on the prebuckling, buckling, and postbuckling responses of the cylindrical shells. An experimental investigation on the moderately thick-walled shells was also carried out. A good correlation was observed between the results obtained from the finite element simulation and the experiments. Furthermore, empirical equations, in the form of a ‘buckling load reduction factor’ were developed using the least square regression method. These simple equations could be used to predict the buckling capacities of several specific types of cylindrical shells with a cutout.

Wall-Thickness Dependence of Cooling-Induced Deformation of Polystyrene Spherical Shells

Experiments on the wall-thickness dependence of the cooling-induced deformation (CID) of polystyrene (PS) spherical shells were carried out. For the experiments, the PS shells were fabricated by the density-matched emulsion method using the hand-shaken microencapsulation technique. The number-averaged and weight-averaged molecular weights of the PS were Mn = 1.1 × 105 and Mw = 4.0 × 105, respectively. The diameter of the PS shells was ~400-550 μm. To investigate the wall-thickness dependence of the CID, the wall thickness of the PS shells was varied between 5 and 60 μm. In the experiments, the PS shells were cooled by using liquid nitrogen, and their images were captured at 0 and −190°C. For the investigation of the CID, two shapes of each shell that were measured at 0 and −190°C were compared. The thinner PS shells showed larger CID. The maximum deformation was almost 1% of the outer radius when the shell aspect ratio (outer radius)/(wall thickness) was higher than 20. The repeatability of the CID was studied, and the results implied that residual stress in the PS shells had an influence on the CID.

Ontogenic Changes in Microhabitat Distribution of Juvenile Bay Scallops, Argopecten irradians irradians (L.), in Eelgrass Beds, and Their Potential Significance to Early Recruitment.

Ontogenetic changes in the vertical distribution of a cohort of juvenile bay scallops, Argopecten irradians, on eelgrass, Zostera marina, were followed throughout the summer and early fall in two Long Island embayments (New York, USA). Despite site-specific differences in eelgrass height and density, more than 95% of post-settlement scallops remained attached above the bottom until they reached a shell height of about 11 mm. Over a 5-week period, scallops gradually relocated until, at a mean size of 31 mm, all occurred on the bottom. The decline in percent attachment coincided with a 5-fold increase (from 16 to 84 μmoles min-1 g muscle dry wt-1) in the activity of octopine dehydrogenase (proposed here as an index of the scallops' capacity for burst swimming activity), and in maximum rate of increase in the shell aspect ratio. While attached to eelgrass, scallops were nonuniformly distributed, with greatest concentration at mid-canopy. Following disturbance, they rapidly regained above-ground position, attaining asymptotic heights within 3-10 h. This and prior studies suggest that the climbing behavior of the bay scallop is an adaptive response to high predation pressure at small sizes. Enhanced scope for activity (predator avoidance) may enhance survival of scallops at intermediate sizes, when they become too heavy to maintain elevation but have not yet attained effective refuge in size.

Stationary flow model of ablatively imploded spherical shells

The stationary flow model of spherical ablation is extended to shells, solutions with a density discontinuity at the critical surface, and charged-particle-beam-driven ablation. Parameter studies of the shell solutions show the relationship between shell aspect ratio, relative ablative mass removal or burnthrough, laser power, and shell material type. The discontinuous solutions are shown to occur when the critical surface and sonic surface coalesce. The relationship of these discontinuous solutions to particular physical situations is shown to be ambiguous in a way that must be resolved by microscopic transport calculations. Charged-particle-driven ablative implosion processes are shown to resemble laser-driven ablation. However, qualitatively different ablation processes occur in different regimes of the power and particle range of the incident beam. Procedures are described by which stationary solutions can be used to predict and interpret the results of experiments and numerical simulations.

Furnace profile effects on glass shell formation

In the course of blowing glass shells in a furnace drop tower it has been observed that furnace temperature profile affects shell aspect ratio. Wall uniformity appears less sensitive to temperature profile as long as some minimum-time/temperature history is achieved for a given glass composition. Quantitative results will be reported from experiments wherein the furnace temperature profile is systematically varied and the resultant shells are statistically analyzed for aspect ratio and wall uniformity.

Study of Ablatively Imploded Spherical Shells

The implosion of spherical-shell targets driven ablatively by six 1.05-..mu..m laser beams at an irradiance of approx.10/sup 13/ W cm/sup -2/ has been studied by streak time-resolved x-ray shadowgraphy. Measurements of acceleration determined the ablation pressure. Departures from one-dimensional simulations showed increasingly degraded implosions as the shell aspect ratio r/..delta..r was increased from 10 to 110 for shells with surface perturbations of +- 0.05 ..delta..r.

Study of Ablatively Imploded Spherical Shells

The implosion of spherical-shell targets driven ablatively by six 1.05-\ensuremath{\mu}m laser beams at an irradiance of \ensuremath{\sim} ${10}^{13}$ W ${\mathrm{cm}}^{\ensuremath{-}2}$ has been studied by streak time-resolved x-ray shadowgraphy. Measurements of acceleration determined the ablation pressure. Departures from one-dimensional simulations showed increasingly degraded implosions as the shell aspect ratio $\frac{\mathcal{r}}{\ensuremath{\Delta}\mathcal{r}}$ was increased from 10 to 110 for shells with surface perturbations of $\ifmmode\pm\else\textpm\fi{}0.05\ensuremath{\Delta}\mathcal{r}$.

The hen's egg: Factors affecting shell splitting during boiling

Synopsis Experiments are described from which it is concluded that the shell will split when an egg is boiled if thermal expansion of the egg's liquid or gaseous contents, which have a higher coefficient of expansion than the shell, is insufficiently relieved by escape through the membranes and pores. The probability of splitting was enhanced if the shell had low structural strength due to a pre‐existing crack or to large size, skew shape or low effective thickness; once these factors had been taken into account there was no significant effect of the shell's aspect ratio, colour, glossiness, roughness or degree of ridging, nor of the hen's age or a strain history of selection for egg production. The incidence of splitting depended also on storage time and temperature: it was very low in newly laid eggs; in eggs stored at room temperature it rose to a maximum at about 5 d, then fell and was again low after about 21 d; in eggs stored at — 2 °C the time‐scale was increased and the incidence of splitting was still high after 14 d storage. These effects are due to two changes occurring during storage: progressive reduction of permeability of the membranes and shell to the liquid contents of the egg, which impedes escape of the contents when they expand during cooking, and progressive increase in the size of the air space, which provides expansion space. Allowing refrigerated eggs to warm to room temperature before cooking and adding salt to the cooking water did not significantly affect the incidence of splitting; putting eggs into cold rather than boiling water did not reduce the incidence of splitting except when the air space was large, following prolonged storage at room temperature. The belief, common among housewives, that shell splitting occurs more commonly now than in earlier decades may be well‐founded but, if so, the change is probably due to a combination of two practices that have been introduced to maintain interior egg quality but which coincidentally raise the probability of shell‐splitting during boiling: rapid distribution from hen to housewife and refrigerated storage. There is no evidence that shells are now thinner, that hens of modern, high‐production strains lay eggs with weaker shells or that white shells are weaker than brown ones.