Outer Radius Of Shell Research Articles

Electronic states in GaAs/AlGaAs/GaAs core/shell cylindrical quantum wires

Cylindrical quantum wires (CQWRs) based on GaAs/ AlxBGa1−xBAs /GaAs core/shell semiconductors are multilayer semiconductor nanostructures that exhibit interesting electronic properties. In this study, we explore in detail the electronic band structure and electronic transmission spectrum in these core/shell CQWRs in relation to various parameters such as inner and outer shell radius and aluminum-doping rate. Using Green’s function formalism, an advanced theoretical approach to solving Schrödinger’s equation in the effective mass approximation, taking into account the confinement potential of the cylindrical geometry. The results show that as the difference between the outer and inner radii of the shell increases, the number of electronic states increases due to the weakening of geometric confinement. In addition, as the concentration of Aluminum in the shell increases, the electronic states move to higher energies, due to the increase in confinement by the height of the potential. This study aims to provide an overview of the electronic states in core/shell nanowires, due to modification in Al molar concentration and shell size; they offer advantages such as better electronic transmission. We can control the electronic band structure of this nanowire, which can be used to fabricate high-performance electronic devices such as transistors, diodes and sensors.

Fabrication and characterization of core–shell microparticles containing an aqueous core

Core–shell microparticles containing an aqueous core have demonstrated their value for microencapsulation and drug delivery systems. The most important step in generating these uniquely structured microparticles is the formation of droplets and double emulsion. The droplet generator must meet the performance and reliability requirements, including accurate size control with tunability and monodispersity. Herein, we present a facile technique to generate surfactant-free core–shell droplets with an aqueous core in a microfluidic device. We demonstrate that the geometry of the core–shell droplets can be precisely adjusted by the flow rates of the droplet components. As the shell is polymerized after the formation of the core–shell droplets, the resulting solid microparticles ensure the encapsulation of the aqueous core and prevent undesired release. We then study experimentally and theoretically the behaviour of resultant microparticles under heating and compression. The microparticles demonstrate excellent stability under both thermal and mechanical loads. We show that the rupture force can be quantitatively predicted from the shell thickness relative to the outer shell radius. Experimental results and theoretical predictions confirm that the rupture force scales directly with the shell thickness.Graphical abstract

Structure dependent third order nonlinear susceptibility in the presence of impurity and magnetic field in CdS/ZnS core/shell quantum dot

Third order nonlinear optical susceptibility due to intersubband optical transition of an electron confined in a CdS/ZnS core-shell quantum dot is investigated for various sizes of core and shell radii in the presence of magnetic field. It is computed by employing the variational approach within the effective mass approximation and the two level density matrix method is applied to obtain the transition energy between two levels. The dependence on third order susceptibility of the incident light is investigated. The results show that the third order nonlinear optical susceptibility is blueshifted with the reduction of inner core radius for a constant outer radius and the same trend is observed with the decreasing outer shell radius. The resonant wavelength and the peak of third order susceptibility will be managed by the control over the structure of core-shell materials and the external perturbation.

Thermal and mechanical stabilities of Core-shell microparticles containing a liquid core

Thorough understanding of the behaviour of core–shell microparticles with a liquid core is essential for determining their performance in applications under different operation conditions. This paper reports the behaviour of core–shell particles with a liquid core under thermal and mechanical loads. First, we formulated an analytical model for the heating process of a core–shell microparticle with a liquid core. Next, we utilised an axisymmetric model of an elastic spherical shell upon compression to describe the deformation of a core–shell microparticle. Finally, we conducted experiments to validate these models. Both thermal and mechanical models agree well with the experimental data. The maximum temperature a core–shell microparticle can withstand depends on the liquid, the geometry, and the material of the shell. The critical compression force before rupture of a core–shell microparticle depends on the Poisson’s ratio of the shell material and the shell thickness relative to the outer shell radius. The rupture force and rupture temperature increase with increasing shell thickness.

Theoretical Study of Sensitivity and Localized Surface Plasmon Resonance of Ag-Dielectric Core-Shell Multi-layered Nanosphere

Localized surface plasmon resonances (LSPRs) of Ag-dielectric-Ag multi-layered nanoshell are studied by quasi-static approximation and plasmon hybridization theory. Absorption properties of multi-layered nanoshell with the silver core and nanoshell separated by a dielectric layer exhibit strong coupling between the core and nanoshell. The result shows absorption spectrum of LSPRS is influenced by the refractive index of surrounding medium, the dielectric constant of middle dielectric layer, the thickness of inner core radius and outer shell layer. LSPR shift of the longest wavelength \(\left |\omega _{-}^{-}\right >\) is red-shifted with increasing the inner core radius. It is interesting to find that longer wavelength \(\left |\omega _{-}^{+}\right >\) mode is mainly effected by the ratio constant of the surrounding medium refractive index e 4 to the middle layer dielectric constant e 2. \(\left |\omega _{-}^{+}\right >\) mode takes place a blue-shift with increasing inner core radius when e 2 > e 4, a red-shift when e 2 \) mode shows the different property as that of increasing the inner core radius. The underlying mechanisms are analyzed with the plasmon hybridization theory and the distribution of induced charge interaction between the inner core and outer shell. In addition, the influence of core radius, middle dielectric layer radius and outer shell radius to sensitivity of Ag-dielectric-Ag multi-layered nanoshell are also reported, a higher sensitivity could be gotten by adjusting geometrical parameters. Our theoretical study could give an easy way to analyze properties of the core-shell nanosphere based on plasmon hybridization theory and the induced charge interaction, and usefully broaden the applications in nano-optics.

Nucleation and growth of a core-shell composite nucleus by diffusion.

The critical radius of a core-shell-type nucleus grown by diffusion in a phase-separated solution is studied. A kinetic critical radius rather than the thermodynamic critical radius of standard classical nucleation theory can be defined from the diffusional growth equations. It is shown that there exist two kinetic critical radii for the core-shell-type nucleus, for which both the inner-core radius and the outer-shell radius will be stationary. Therefore, these two critical radii correspond to a single critical point of the nucleation path with a single energy barrier even though the nucleation looks like a two-step process. The two radii are given by formulas similar to that of classical nucleation theory if the Ostwald-Freundlich boundary condition is imposed at the surface of the inner nucleus and that of the outer shell. The subsequent growth of a core-shell-type postcritical nucleus follows the classical picture of Ostwald's step rule. Our result is consistent with some of the experimental and numerical results which suggest the core-shell-type critical nucleus.

Drag and Separation Flow Past Solid Sphere with Porous Shell at Moderate Reynolds Number

Axisymmetric viscous, two-dimensional steady and incompressible fluid flow past a solid sphere with porous shell at moderate Reynolds numbers is investigated numerically. There are two dimensionless parameters that govern the flow in this study: the Reynolds number based on the free stream fluid velocity and the diameter of the solid core, and the ratio of the porous shell thickness to the square root of its permeability. The flow in the free fluid region outside the shell is governed by the Navier–Stokes equation. The flow within the porous annulus region of the shell is governed by a Darcy model. Using a commercially available computational fluid dynamics (CFD) package, drag coefficient and separation angle have been computed for flow past a solid sphere with a porous shell for Reynolds numbers of 50, 100, and 200, and for porous parameter in the range of 0.025–2.5. In all simulation cases, the ratio of b/a was fixed at 1.5; i.e., the ratio of outer shell radius to the inner core radius. A parametric equation relating the drag coefficient and separation point with the Reynolds number and porosity parameter were obtained by multiple linear regression. In the limit of very high permeability, the computed drag coefficient as well as the separation angle approaches that for a solid sphere of radius a, as expected. In the limit of very low permeability, the computed total drag coefficient approaches that for a solid sphere of radius b, as expected. The simulation results are presented in terms of viscous drag coefficient, separation angles and total drag coefficient. It was found that the total drag coefficient around the solid sphere as well as the separation angle are strongly governed by the porous shell permeability as well as the Reynolds number. The separation point shifts toward the rear stagnation point as the shell permeability is increased. Separation angle and drag coefficient for the special case of a solid sphere of radius r = a was found to be in good agreement with previous experimental results and with the standard drag curve.

Influence of dielectric core, embedding medium and size on the optical properties of gold nanoshells

The amplitudes of the dipole and quadrupole modes for gold nanoshells have been investigated with various dielectric constants for the core and the embedding medium and with various size of the nanoshells by means of Mie theory. With the increase in the dielectric constant of the core, it is found that the strengths of dipole and quadrupole modes become weak. We also observe with the increasing dielectric constant of the embedding medium that the strength of the quadrupole mode is enhanced quickly, whereas that of dipole mode increases first and then decreases. We further find that the amplitude maximum of the dipole mode appears when the total radius of the nanoshell reaches to 50 nm even if the ratio of the inner and outer shell radii is fixed. We have ascribed the changes of the dipole and quadrupole modes to the competition among the variations of induced surface charges, conduction electrons and oscillation electrons.

Varying the spherical shell geometry in rotating thermal convection

The effect of spherical shell geometry on rapidly-rotating thermal convection is studied in a suite of high resolution three-dimensional numerical simulations. The geometry is characterized by the radius ratio, χ = ri/ro , where ri is the inner shell radius, and ro is the outer shell radius. In this study, χ is varied over the broad range 0.10 to 0.92 in calculations of Boussinesq rotating convection subject to isothermal, rigid boundary conditions. Simulations are performed at Prandtl number Pr = 1 and for Ekman numbers E = 10−3, 3 × 10−4 and 10−4. Near the onset of convection, the flow takes the form of rolls aligned parallel to the rotation axis and situated adjacent to the inner shell equator. The dimensionless azimuthal wavelength, λ c , of the rolls is found to be independent of the shell geometry, only varying with the Ekman number. The critical wave number, mc , of the columnar rolls increases in direct proportion to the inner boundary circumference. For our simulations the critical Rayleigh number Rac at which convection first occurs varies in proportion to E −1.16; a result that is consistent with previous work on rotating convection. Furthermore, we find that Rac is a complex function of χ. We obtain the relation , which adequately fits all our results. In supercritical convection calculations the flows form quasi-geostrophic sheet-like structures that are elongated in the radial direction, stretching from the inner boundary toward the outer boundary.

Classification of two-shell nanotubes with commensurate structures of shells

A classification of two-shell carbon nanotubes with commensurate structures of shells is proposed. The classification is based on the concept of equivalence classes as a set of shells with chiral indices of the (kf, kg) type, where f and g are the chiral indices of the equivalence class and k is the index of the shell diameter. All two-shell nanotubes with commensurate shells that are characterized by the chiral indices (k1f1, k1g1) and (k2f2, k2g2), where k1 and k2 are integral numbers, make up a family of nanotubes of different radii but with equal geometric parameters (such as the intershell distance, the unit cell length of the nanotube, and the difference between the chiral angles of the shells). The geometric parameters of nanotubes are calculated for a number of families, the distribution of different types of two-shell nanotubes with commensurate shells over the outer-shell radii is determined, and the threshold forces required to induce relative motion of nanotube shells are evaluated. The possible use of two-shell nanotubes with commensurate shells in nanostructures is discussed.

Effects of an Electrically Conducting Inner Core on Planetary and Stellar Dynamos

The influence of an electrically conducting inner core on the characteristics of kinematic spherical α2 dynamos is investigated both analytically and numerically. Emphasis is placed on the magnetic coupling between the outer turbulent fluid spherical shell where magnetic fields are generated and the electrically conducting inner core, which can control the way in which the magnetic field in the shell is produced. We illustrate the subtlety of the electromagnetic boundary conditions at the core-shell interface: continuity of the radial magnetic field, continuity of the tangential electric field, and continuity of the tangential magnetic field. In the special cases of zero and infinite core electrical conductivity, the core-shell interface conditions take different explicit forms and in the latter case depend on whether the dynamo is stationary or oscillatory. Dynamo solutions depend on η, the ratio of inner to outer shell radius; β, the ratio of core to shell magnetic diffusivity; α(r), the radially dependent α function in the outer turbulent fluid spherical shell; and Rα, the magnetic Reynolds number of the shell. For small cores, η 5.3. Time-dependent dynamo behavior can arise from the α-effect alone in the presence of a sufficiently large and electrically conducting inner core. This phenomenon could be relevant to the solar dynamo for which η = 0.8 and β = 10-3. It could also be relevant to the dynamos of planets with sufficiently large inner cores; Earth's inner core is too small, however, for the oscillatory phenomenon to be directly applicable to the geodynamo.

Infrared Spectroscopy of Ultracompact HiiRegions

We present intermediate-resolution (λ/Δλ ≈ 60) spectra of 21 ultracompact H II regions in the spectral range from 3 to 13 μm. The 9.7 μm silicate feature is seen in absorption, and the 12.8 μm [Ne II] fine structure line is seen in emission toward most of the observed nebulae. The strengths of both features vary enormously from nebula to nebula, suggesting large variations in the column densities of both Ne II and silicates toward these objects. Near-IR features attributed to polycyclic aromatic hydrocarbons (PAHs) are detected in six of the sources. Spherically symmetric dust shell models were calculated to obtain the best fits to those nebulae for which distances are known, and spectral energy distributions are available in the range of 1 mm to 1 μm. The models are used to infer properties of the dust cocoon such as the distribution of density and temperature with radius, shell thickness, outer shell radius, and dust abundances. Our results are consistent with previous models that predict large dust cocoons with central cavities, sharp temperature gradients, and approximately constant density in the outer regions.

About entangled networks of worm-like micelles: a rejected hypothesis

We report new results from small-angle neutron scattering ond12-cyclohexane/lecithin/water micellar solutions performed as a function of the water content (w0), temperature (T) and dispersed phase volume fraction (ϕ). The data from dilute samples are interpretable in terms of the existence of giant cylindrical reverse micelles and are well fit with a core-shell model (that provides the micelle structure and dimensions) with values of 28 and 45 A for the inner core and the outer shell radii, almost independent on temperature and concentration. Such a result could appear consistent with the current idea that worm-like micelles are living polymers. On the contrary, the appearance of a sharp interference maximum at high concentrations (ϕ>0.15) suggests arguments against the current hypothesis of an entangled network of giant flexible cylinders. Further arguments against the current hypothesis are given by the close similarity between the above described results and those from free of water micelles (for sure not cylinders). All the data are well fitted in terms of a unique model taking into account the micellar form factor plus a hard sphere structure factor. The data analysis suggests a micellar size distribution determined by the competition between concentration and interaction effects on which temperature plays not a minor role. Following our results, the current hypothesis of a gel structure in terms of an entangled network can be assumed as wrong and some caution has to be taken in assuming wormlike micelles as living polymers.

Three-Dimensional Infinite Prandtl Number Convection in a Spherical Shell with Temperature-Dependent Viscosity.

We study the numerical simulations of 3-D inifinite Prandtl number convection with temperature-dependent viscosity in a spherical shell. Control volume method coupled with SIMPLE algorithm is used to solve the basic equations. Both constant and temperature-dependent viscosity cases are studied under the condition that the ratio of the inner to the outer shell radius is 0.55, which is appropriate for mantle convection of the Earth, with only basal heating. For the case of constant viscosity, when the Rayleigh number (Ra) is less than 105, steady state solutions (both cubic and tetrahedral types) are obtained. However, for Ra = 106, we find only a time-dependent solution. For the temperature-dependent viscosity case, we assume that the viscosity changes with temperature exponentially and we can successfully obtain solutions with the viscosity variation up to 10000. For the viscosity contrast of 1000 times and the surface Rayleigh number of Ra0 = 5 × 103, we get the flow patterns which consist of the sheet-like downwelling and small upwellings.

The infrared emission from dust surrounding newly formed O stars

The dust cocoon around a newly formed 06 ZAMS star embedded in its natal molecular cloud is modeled for a wide range of radial density distribution and grain properties. The most important result of this investigation is that the only models that satisfy all the observational constraints require thin dust shells with relatively large inner radii composed of grains with about one-half the graphite/silicate abundance ratio of the MRN-DL mixture for the diffuse interstellar medium. It is also shown that constant density models produce the best overall fit to the observations; the dust cocoons are optically thick at wavelengths shortward of approximately 7 microns, and the warm dust cocoons are large; the average dust temperatures drop very steeply just outside the inner boundary of the dust shell to less than 100 K in less than 0.1 percent of the outer shell radius; the addition of water ice mantles to refractory grain cores produces a strong 3.07 micron absorption feature and weaker 12 and 45 micron absorption features; and all models imply rather massive dust shells. 37 refs.

Transversely isotropic infinite cylindrical shell subjected to a radial axisymmetric line load

The problem of an infinite transversely isotropic circular cylindrical shell subjected to an axisymmetric radial external line load is investigated using elasticity theory, classical shell theory and shear deformation theory. The results obtained by these methods are compared for two ratios of inner to outer shell radius and for varying degrees of anisotropy. Some typical results are given here to show the effect of anisotropy and the thickness of the shell on the distribution of stresses and displacements.

Transversally Isotropic Cylinders Under Band Loads

Abstract : The problem of a transversally isotropic circular cylindrical shell subjected to periodically spaced, axisymmetric band loads is investigated. The three-dimensional elasticity solution is obtained and stresses and displacements are evaluated for values of the modulus ratio equal to two, five and ten. Also presented are expressions for the stresses and displacements obtained by use of a thin orthotropic shell theory. The results are compared to determine the effects of a change in modulus ratio on the stresses and displacements of the cylinder and the accuracy of the shell theory. A detailed analysis of the data is carried out for a ratio of inner to outer shell radii equal to 0.70, a ratio of distance between band loads to outer shell diameter equal to 0.2., and a ratio of band width to distance between band loads of 0.2. (Author)

Further comparison of elasticity and shell theory solutions.

Abstract : The problem of an infinite circular cylindrical shell subjected to periodically spaced axisymmetric band loads is investigated using a Reissner-Naghdi higher-order shell theory. The generalized equilibrium equations, which include the effects of transverse normal stress and transverse shear deformation, were applied to the specific problem at hand. These equilibrium equations were successfully uncoupled, and an expression describing the transverse displacement of the median surface was obtained. The solution of this expression was then used to calculate the stress resultants, stresses, and displacements. A comparison of the resulting stresses and displacements with the exact elasticity solution and some lower-order shell theory solutions of the same problem was carried out for ratios of inner to outer shell radius equal to 0.7, 0.8, 0.87, 0.9, 0.93, and for a ratio of distance between band loads to outer shell diameter equal to 0.2. The ratio of band width to distance between band loads was also equal to 0.2. The Poisson's ratio used is equal to 0.3. (Author)