Showing papers on "Critical radius published in 2011"
TL;DR: In this article, the formation mechanisms of GaN nanowires grown on a Si$x$N${y}$ amorphous interlayer within a self-induced approach by molecular beam epitaxy have been investigated by combining in situ reflection high-energy electron-diffraction measurements with ex situ high-resolution transmission electron microscopy imaging.
Abstract: The formation mechanisms of GaN nanowires grown on a Si${}_{x}$N${}_{y}$ amorphous interlayer within a self-induced approach by molecular beam epitaxy have been investigated by combining in situ reflection high-energy electron-diffraction measurements with ex situ high-resolution transmission electron microscopy imaging. It is found that GaN initially nucleates as spherical cap-shaped islands with a wetting angle of $42\ifmmode\pm\else\textpm\fi{}{7}^{\ifmmode^\circ\else\textdegree\fi{}}$. Subsequently, these islands coarsen and undergo a shape transition toward the nanowire morphology at an experimental critical radius of 5 nm. As the epitaxial constraint is very weak on an amorphous interlayer, the equivalent Laplace pressure due to the effects of surface stress has been taken into account. Analytical and finite-element method calculations show that the Laplace pressure results at the nanoscale dimensions in significant volume elastic strain in both spherical caps and nanowires. From thermodynamic considerations, it is revealed that the related strain energy density is slightly in favor of the shape transition toward the nanowire geometry owing to its higher ability to relieve the strain. Nevertheless, the anisotropy of surface energy is an even stronger driving force, since the nanowires are composed of $c$- and $m$-planes with very low surface energies. It is deduced that an energy barrier does exit for the shape transition and may be related to edge effects, resulting in a growth condition-dependent critical radius.
117 citations
TL;DR: In this article, a series of analytical calculations are used to study both the effect of the thermal gradients and the stability of the molten zone in the laser floating zone growth of Al 2 O 3 -Y 3 Al 5 O 12 eutectic composite.
Abstract: A series of analytical calculations is used to study both the effect of the thermal gradients and the stability of the molten zone in the laser floating zone growth of Al 2 O 3 –Y 3 Al 5 O 12 eutectic composite. The thermal gradients in the solidification interface have been calculated and the axial gradient compared with the experimental one of 4.5 × 10 5 K/m. For these calculations the coefficients of heat transfer from the molten zone to the ambient at the solid-melt interface have been previously obtained. The thermal stresses generated by the high thermal gradients can induce crack formation during the cooling depending on the rod diameter. The theory predicts that it is possible to grow rods free of cracks up to R = 1.7 mm, at low rates (10 mm/h) in close agreement with the experimental critical radius of 1.6 mm.The dependence of the zone length on the input laser power used to carry out the growth is shown. The study of the floating zone profile allows determining the maximum stable zone length, verifying the stability criterion established by some authors.
44 citations
TL;DR: In this paper, the nucleation and growth of nanoscale precipitates in a new class of high-strength, multicomponent, ferritic steels has been studied with complementary state-of-the-art microstructural characterization techniques of atom probe tomography for individual embryos and precipitates and small-angle neutron scattering for their statistical averages.
Abstract: The nucleation and growth of nanoscale precipitates in a new class of high-strength, multicomponent, ferritic steels has been studied with complementary state-of-the-art microstructural characterization techniques of atom probe tomography for individual embryos and precipitates and small-angle neutron scattering for their statistical averages. Both techniques revealed a bimodal size distribution, with subnanometer embryos, and nanoscale precipitates. The embryos, which have a radius of \ensuremath{\sim}0.4 nm, are enriched in Cu and served as preferential sites for nucleation. The critical radius for nucleation was determined to be \ensuremath{\sim}0.7 nm. Subsequent growth of the precipitates is dictated by volumetric diffusion, as predicted by the Lifshitz--Slyozov--Wagner theory.
41 citations
TL;DR: In this article, a thermodynamic model based on Gibbs free energy and availability is developed for the onset of heterogeneous nucleation on heated surfaces with different wettabilities in pool boiling, taking into consideration the temperature gradient in the superheated liquid layer adjacent to the wall as well as the contact angle between the liquid and the wall.
40 citations
TL;DR: In this paper, the authors report on the properties of ZnO:F films deposited by RF sputtering on polyethylene naphthalate (PEN) substrates and compared them with films deposited on glass.
28 citations
TL;DR: Using the effective-index approximation, it is shown that touching spherical metal surfaces form a broadband omnidirectional absorber of gap surface plasmons (GSP), representing thereby a two-dimensional analogue of an optical black-hole realized without use of metamaterials.
Abstract: Using the effective-index approximation we show that touching spherical metal surfaces form a broadband omnidirectional absorber of gap surface plasmons (GSP), concentrating all GSP waves travelling within a certain radius at the point of contact (at which the field intensity tends to infinity even in the presence of metal absorption) and representing thereby a two-dimensional analogue of an optical black-hole realized without use of metamaterials. The developed wave analysis is supplemented with the geometrical optics (adiabatic) description providing explicit expressions for the critical radius (radius of the event horizon) and buildup of field enhancement along ray trajectories.
27 citations
TL;DR: In this article, the Ginzburg-type gradient energy term was found to make only partial contribution to the interfacial energy, and a similar or even larger contribution from the bulk terms in the free energy functional also has to be included.
24 citations
TL;DR: In this article, the image force and stability of a screw dislocation inside a coated cylindrical inhomogeneity with interface stresses are analyzed using a three-phase composite cylinder model.
Abstract: The image force and stability of a screw dislocation inside a coated cylindrical inhomogeneity with interface stresses are analyzed. A three-phase composite cylinder model is utilized to study the issues. The stress boundary condition at the interface of the coating layer is modified by incorporating surface/interface stress. The analytical solution of complex functions of the inhomogeneity, the coating layer, and the infinite matrix is derived by means of the complex variable method. With the aid of the obtained stress fields and the Peach–Koehler formula, the explicit expression of the image force acting on the screw dislocation is obtained. The critical radius of the inhomogeneity where the screw dislocation stays is evaluated. The influence of the interface stresses on the image force and the critical radius of the inhomogeneity is derived. The results indicate that the negative interface stresses attract the dislocation, and the positive interface stresses repel the dislocation. The critical radius of inhomogeneity will increase for considering the negative interface stresses and will decrease for considering the positive interface stresses.
23 citations
TL;DR: In this paper, the growth of highly nonlinear optical 4-N, N -dimethylamino-4′-N ′-methyl stilbazolium tosylate (DAST) crystals was carried out with the addition of activated carbon into the methanol solutions.
21 citations
TL;DR: The surface tension of a helium nanobubble is a convex function with respect to the bubble radius, and is estimated to vanish at a critical radius of approximately 1 nm.
Abstract: Interfacial properties of helium nanobubbles in water at normal conditions have been investigated using large-scale molecular dynamics simulations for systems including over one million atoms. The surface tension of a helium nanobubble is a convex function with respect to the bubble radius, and is estimated to vanish at a critical radius of approximately 1 nm.
16 citations
TL;DR: A methodology for calculating the curved surface tension in local supersaturated environments is applied, showing that the Tolman length is negligible far from the critical region, however, near the critical point, the TolMan length becomes positive and appears to diverge.
Abstract: In this work, a weighted density functional theory has been used to study the equilibrium and metastable processes for argon. In the theoretical approach, the two- and three-body interactions of the fluid molecules are considered simultaneously, and the renormalization group transformation is applied to address the long-range fluctuations inside the critical region. The global phase equilibria, planar and curvature-dependent surface tensions, critical radius, and nucleation rates of argon are investigated systematically. The results are in good agreement with the experimental data. Meanwhile, this work applies a methodology for calculating the curved surface tension in local supersaturated environments, showing that the Tolman length is negligible far from the critical region. Near the critical point, however, the Tolman length becomes positive and appears to diverge.
TL;DR: In this paper, the size distribution of active nucleation sites on single horizontal copper and steel tubes with different diameter and surface finish has been calculated from heat transfer measurements over wide ranges of heat flux and selected pressure.
TL;DR: In this article, a method of calculating a possible stability loss by a rotating circular annular disc of variable thickness is suggested within the theory of perfect plasticity with the help of small parameter method.
Abstract: A method of calculating a possible stability loss by a rotating circular annular disc of variable thickness is suggested within the theory of perfect plasticity with the help of small parameter method. A characteristic equation for a critical radius of a plastic zone is obtained as a first approximation. The formula for the critical angular velocity, determining the stability loss of the disc according to the self-balanced form, is derived. The method using which we can take into account the disc’s geometry and loading parameters is also specified. The efficiency of the proposed method is shown in Section 5 while considering an illustrative example. The values of critical angular velocity of rotating are found numerically for different parameters of the disc.
TL;DR: In this paper, it was shown that a light-cored vortex can be unstable in spite of the "stable stratification" of density, and that a wave interaction mediated by shear is the mechanism for the instability.
Abstract: We study the stability of a vortex in an axisymmetric density distribution. It is shown that a light-cored vortex can be unstable in spite of the 'stable stratification' of density. Using a model flow consisting of step jumps in vorticity and density, we show that a wave interaction mediated by shear is the mechanism for the instability. The requirement is for the density gradient to be placed outside the vortex core but within the critical radius of the Kelvin mode. Conversely, a heavy-cored vortex, found in other studies to be unstable in the centrifugal Rayleigh―Taylor sense, is stabilized when the density jump is placed in this region. Asymptotic solutions at small Atwood number At show growth rates scaling as At 1/3 close to the critical radius, and At 1/2 further away. By considering a family of vorticity and density profiles of progressively increasing smoothness, going from a step to a Gaussian, it is shown that sharp gradients are necessary for the instability of the light-cored vortex, consistent with recent work which found Gaussian profiles to be stable. For sharp gradients, it is argued that wave interaction can be supported due to the presence of quasi-modes. Probably for the first time, a quasi-mode which decays exponentially is shown to interact with a neutral wave to give exponential growth in the combined case. We finally study the nonlinear stages using viscous direct numerical simulations. The initial exponential instability of light-cored vortices is arrested due to a restoring centrifugal buoyancy force, leading to stable non-axisymmetric structures, such as a tripolar state for an azimuthal wavenumber of 2. The study is restricted to two dimensions, and neglects gravity.
01 Jan 2011
TL;DR: In this paper, five fallacies used in the description of the particle's journey are discussed, and it is shown why the particle will never reach the critical radius and therefore cannot form a black hole.
Abstract: Einstein [1] sets out field equations that describe a matterfree field. A German military officer, Karl Schwarzschild [2], shortly before he died, derived a solution of the field equations for a static gravitational field of spherical symmetry. Schwarzschild’s solution is referred to as the Schwarzschild metric. Einstein [3] showed that matter cannot be compacted below a critical radius defined by the Schwarzschild metric. Weller [4] shows that compacting matter below the critical radius to form a black hole results in a violation of the conservation of momentum and energy. Why, then, do many believe that black holes exist in Einstein’s relativistic space time? The belief appears to have arisen based, at least partly, on an incorrect description of the journey of a particle falling radially towards a hypothetical mass compacted below the critical radius. The description is incorrect in that the particle reaches and crosses the critical radius. Herein are discussed five fallacies used in the description of the particle’s journey. Preliminary to addressing the fallacies, it is shown why the particle will never reach the critical radius.
TL;DR: In this article, a linear model of thermoelastic damping in annular microplate is considered for calculating the quality factor of this damping, and the clamped boundary condition is applied in inner and outer circular edges.
Abstract: In this paper, a linear model of thermoelastic damping in annular microplate is considered for calculating the quality factor of this damping. In this model clamped boundary condition is applied in inner and outer circular edges. In the results, there are critical radius and thickness which are calculated analytically and in these dimensions, the thermoelastic damping becomes maximal. The critical radius and thickness depend on material properties, vibration modes and frequencies, dimensions of microplate.
TL;DR: The critical mass and dimensions of simple geometries containing highly enriched uraniumdioxide (UO2) and uraniummononitride (UN) encapsulated in tungsten-rhenium alloys are determined using MCNP5 criticality calculations as discussed by the authors.
01 Jan 2011
TL;DR: In this paper, positive switching impulse discharge tests were carried out using rod electrodes with different diameters, hemisphere and cone shape in their heads, and the influence of shape and diameter of rod electrode on 50% flashover voltage was concluded.
Abstract: Rod-plane air gap is the most typical gap in the research of air gap discharge,and its switching impulse characteristic is a very important factor for dimension of air gaps in the project design of transmission lines and converter stations/substations in transmission project.To study switching impulse characteristic of rod-plane air gap deeply,positive switching impulse discharge tests were carried out using rod electrodes with different diameters,hemisphere and cone shape in their heads.The influence of shape and diameter of rod electrode on 50% flashover voltage was concluded.To obtain the critical radius at different altitudes,comparison tests were carried out in Beijing,Xining and Yangbajing,and the critical radius corresponding to different gap distances were obtained through changing the radius of sphere connecting rod electrode.It is shown that when radius of rod electrode is less than critical radius,the change of radius and shape of rod electrode will not affect 50% flashover voltage of air gap;The critical radius will increase when gap distance increases,and the critical radius corresponding to a certain gap distance will increase at high altitude.
TL;DR: In this paper, the induction period of various proportion of urea-thiourea zinc chloride crystal in water has been measured experimentally using the visual observation method, which is inversely proportional to the nucleation rate.
Abstract: The induction period of various proportion of urea–thiourea zinc chloride crystal in water has been measured experimentally using the visual observation method. The induction period, which is inversely proportional to the nucleation rate, has been used to estimate the interfacial tension between the urea–thiourea zinc chloride and water; hence, the nucleation parameters like critical radius (r*), number of molecules in the radius (r*) and Gibbs free energy change for the formation of a critical nucleus (∆G*) have been calculated.
TL;DR: In this article, the expansion mechanism inside carbon dioxide expander is analyzed theoretically and the mechanism analysis of metastable equilibrium indicates that there is a potential barrier during phase change process and energy is required to overcome the potential barrier when the new interface is formed from primary phase.
Abstract: The expansion mechanism inside carbon dioxide expander is analyzed theoretically in this paper. The mechanism analysis of metastable equilibrium indicates that there is a potential barrier during phase change process. That is to say energy is required to overcome the potential barrier when the new interface is formed from primary phase. The superheat of liquid is the impetus of phase change and has an exponential decrease with the increase of the saturated temperature of fluid. The analysis also indicates that there is a critical radius during the phase change process. The bubble will grow up when its radius is larger than the critical value, otherwise, will disappear. When considering the metastable phase change, calculation of P and V during expansion process indicates that the phase-change will be delayed with the decline of pressure, which is called lag phenomenon. The phase-change delay time decreases with the increase of initial temperature. When the initial temperature is close to the critical temperature, the delay time is close to zero. The phase-change delay brings energy losses, which decrease with the increase of initial temperature and its decrease ratio also has a decrease trend. When the initial pressure is 10 MPa, the energy loss will be 1.06 W with an initial temperature of 10°C while 0.34 W with that of 20°C.
TL;DR: In this article, the metastable zone width and induction period were determined for the aqueous solution growth of l -histidine trifluoroacetate (LHTF), and the surface free energy values were calculated from the experimental data.
21 Aug 2011-Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment
TL;DR: In this paper, an improved theoretical model to describe the mechanism of bubble formation for Freon 115 as the active medium has been developed, which accurately predicts the neutron-induced bubble density measured on OMEGA with both liquid bubble chambers and gel detectors.
Abstract: Neutron imaging is used in inertial confinement fusion (ICF) experiments to measure the core symmetry of imploded targets. Liquid bubble chambers have the potential to obtain higher resolution images of the targets for a shorter source–target distance than typical scintillator arrays. Due to the fact that nucleation models used in gel detectors research cannot always give correct estimates for the neutron-induced bubble density inside a liquid bubble chamber, an improved theoretical model to describe the mechanism of bubble formation for Freon 115 as the active medium has been developed. It shows that the size of the critical radius for the nucleation process determines the mechanism of bubble formation and the sensitivity of the active medium to the 14.1-MeV incident neutrons resulting from ICF implosions. The bubble-growth mechanism is driven by the excitation of the medium electronic levels and not by electrons ejected from the medium's atoms as happens for the bubble chambers used to detect charged particles. The model accurately predicts the neutron-induced bubble density measured on OMEGA with both liquid bubble chambers and gel detectors.
Journal Article•
TL;DR: In this paper, the evolution law of non-dendrite microstructure of Al-Si alloy slurry fabricated by near liquidus casting was investigated from the perspective of melt structure.
Abstract: By comparing different experimental schemes and using multiple polishing in situ observation and metallographic analysis,the evolution law of non-dendrite microstructure of Al-Si alloy slurry fabricated by near liquidus casting was researchedFurther,from the perspective of melt structure,the formation mechanism and stable existence of non-dendrite microstructure was discussedThe results show that,for Al-797Si,at 390-400 ℃,the process of transfer and rheoforming of semi-solid slurry will cost 15s by conventional near liquidus castingHowever,in the temperature range not high above liquidus,because of the effect of applied rotating magnetic filed,the time of transfer and slurry rheoforming is elongatedThe main reason is that the disturbance caused by rotating magnetic filed for overheating melt always influences the nucleation and growth during solidificationOn one hand,the disturbance changes the sizes,shapes and distribution of atomic clusters,which provides the conditions for increasing nucleation rate;on the other hand,it also decreases the solute concentration gradients of solid-liquid interface,which restrains composition overcooling and increases the critical radius of globular growth for crystal nucleus in stable state
TL;DR: In this article, the effects of traveling magnetic field on degassing of aluminum alloys were investigated, and the critical radius of the pores was calculated, and it was shown that critical radius decreases with increasing the magnetic density linearly when the traveling magnetic force is applied during solidification, which promotes the heterogeneous nucleation of pores.
01 Jan 2011
TL;DR: The formation of nuclei and clusters from single atomic or molecular species plays an important or even decisive role in laser-induced gas-phase synthesis of powders,Nanopowders laser-CVD, pulsed-laser deposition (PLD) etc.
Abstract: The formation of nuclei and clusters from single atomic or molecular species plays an important or even decisive role in laser-induced gas-phase synthesis of powders,Nanopowders laser-CVD, pulsed-laser deposition (PLD) etc. Of similar importance are (heterogeneous) nucleation processes in laser-annealing, recrystallization, thin-film formation, etc. Depending on the particular application, nucleation and cluster formation can be desirable or undesirable. An example is PLD, where clusters formed within the vapor plume may condense on the substrate and thereby deteriorate epitaxial film growth. However, by employing a moderate background pressure, the same technique can be used to fabricate thin nanocrystalline films with unique physical properties.
TL;DR: In this paper, the authors investigated the energy transfer from the metal cluster to the Er ions critically depends on the evolution of the Au metal clustering process, starting from dispersed metal atoms up to the formation of nanometer-sized clusters.
Abstract: We present recent results on the occurrence of an Er fluorescence sensitization induced by ultra-small Au clusters (NC) incorporated in Er-doped silica by a low fluence ion implantation; it is shown that the energy transfer from the metal cluster to the Er ions critically depends on the evolution of the Au metal clustering process, starting from dispersed metal atoms up to the formation of nanometer-sized clusters. To investigate sub-nanometric Au structures and dopant ions (Au, Er) dispersed into the matrix, extended X-ray absorption fine structure spectroscopy has been used because, by measuring the local site around the dopant (Er, Au) independently of any long-range order considerations, it can detect both few-atom clusters and the possible presence of dopant dispersed into the matrix. This investigation allowed us to directly follow the early stages of Au nucleation, where clusters nearly have a the critical radius (i.e. aggregates formed by two to three atoms), and to relate the Au NC size with the ...
TL;DR: In this paper, a self-consistent model of growth and structure of semiconductor nanowires is proposed and the critical radius of the transition from the hexagonal wurtzite (WZ) structure to the cubic structure of zinc blende (ZB) type is calculated as a function of parameters of the system of materials and the gaseous medium supersaturation.
Abstract: A self-consistent model of growth and structure of semiconductor nanowires is proposed. The crystal phase of group III–V semiconductor nanowires is studied. The critical radius of the transition from the hexagonal wurtzite (WZ) structure to the cubic structure of zinc blende (ZB) type is calculated as a function of parameters of the system of materials and the gaseous medium supersaturation. The model presented here is applicable to both gas-phase and molecular beam epitaxies and allows one to calculate the probability of formation of the WZ and ZB phases under various deposition conditions.
28 Mar 2011
TL;DR: This paper deduce determinate upper and lower bounds of the critical radius of the communication radius in wireless multi-hop line networks with unstable links, and validate them by extensive simulations.
Abstract: Connectivity is a crucial performance parameter for wireless multi-hop networks, which is affected by factors like the communication radius, the node density as well as the link quality. In this paper, we solve the critical communication radius problem in wireless multi-hop line networks with unstable links. Specifically, we deduce determinate upper and lower bounds of the critical radius, and validate them by extensive simulations. The range of the critical radius for a line network of n nodes with link connection probability p is ln n/n ≤ r c ≤ 2 ln n/np.
Journal Article•
TL;DR: In this article, a new method to calculate critical radius of rod-plane air gap was proposed and the reason causing the critical value was analyzed, where the spherical surface with radii from 19 mm to 475 mm was chosen as the end of the rod electrode to achieve the discharge voltage.
Abstract: During the tests of positive switching impulse discharge voltage of rod-plane air gap,the radius of the end of rod electrode would influences the discharge voltage.To obtain the regularity of discharge voltage varying with the radius of the end of rod electrode,the spherical surface with radii from 19 mm to 475 mm are chosen as the end of the rod electrode to achieve the discharge voltage of rod-plane air gaps while the air gaps are from 2 m to 5 m.Testing data show that when the radius of the end of rod electrode increases to a certain critical value,the discharge voltage starts to increase sharply,so this critical value is called the critical radius of the end of rod electrode.A new method to calculate critical radius is proposed and the reason causing the critical value is analyzed.To investigate the variation of critical radius of rod-plane air gap under different altitudes,the same tests are performed in Beijing whose altitude is 50 m and in Yangbajing located in Tibet whose altitude is 4300 m respectively,and the critical radius of the two places are obtained.Tested results show that the value of critical radius varies with both air gap distances of rod-plane and altitude where the air gap locates.
TL;DR: In this article, the effects of high density ultrasonic field coupling on the microstructures and properties of Al-12Si alloy were investigated, and it was shown that when the melt undergoes ultrasonic coupling processing prior to solidification, the nucleation rate of liquid phase can be raised to make α(Al) dendrite transform towards near equiaxed grains, the growth of Si phase is restrained and eutectic Si microstructure is refined due to acoustic streaming effect and thermal mechanism.
Abstract: The effects of high density ultrasonic field coupling on the microstructures and properties of Al-12Si alloy were investigated. It is shown that when the melt undergoes ultrasonic coupling processing prior to solidification, the nucleation rate of liquid phase can be raised to make α(Al) dendrite transform towards near equiaxed grains, the growth of Si phase is restrained and eutectic Si microstructure is refined due to acoustic streaming effect and thermal mechanism; When the melt undergoes ultrasonic coupling processing during the melt solidification, large degree of supercooling is produced in the liquid phase in the solidification interface front edge to reduce the critical radius of crystal nucleus and critical work of nucleation and break up, rupture by melting and refine the Si phase to improve obviously the strength of Al-12Si alloy due to its cavitation effect, acoustic streaming action and heat undulation; The crushing effect of ultrasonic coupling on Si phase occurs mainly during the crystallizing solidification and threshold sound intensity exists.