Showing papers on "Critical radius published in 2008"

Growth thermodynamics of nanowires and its application to polytypism of zinc blende III-V nanowires

Abstract: Theoretical model for the growth thermodynamics of nanowires in different epitaxial techniques is presented, which enables one to determine morphological and structural configurations of the nanowire ensemble with minimum formation energy. It is demonstrated that nanowire ensembles are metastable and controlled entirely by the growth kinetics. The model is applied to studying the polytypism of zinc blende III-V nanowires. It is shown that structural transition should occur within a certain domain of radii and vapor supersaturations. Different polytypes between wurtzite and zinc blende structures with periodicity up to 18 layers are analyzed. It is demonstrated that $4H$ polytype has the lowest formation energy and the largest critical radius of transition amongst all polytypes. Numerical estimates predict the critical radius of structural phase transition of $17\char21{}25\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ for GaAs nanowires growing on the $\mathrm{Ga}\mathrm{As}(111)B$ substrate.

Volume reflection dependence of 400 GeV/c protons on the bent crystal curvature.

Abstract: The trend of volume reflection parameters (deflection angle and efficiency) in a bent (110) silicon crystal has been investigated as a function of the crystal curvature with 400 GeV/c protons on the H8 beam line at the CERN Super Proton Synchrotron. This Letter describes the analysis performed at six different curvatures showing that the optimal radius for volume reflection is approximately 10 times greater than the critical radius for channeling. A strong scattering of the beam by the planar potential is also observed for a bend radius close to the critical one.

Quasihomogeneous nucleation of amyloid beta yields numerical bounds for the critical radius, the surface tension, and the free energy barrier for nucleus formation.

Abstract: Amyloid aggregates are believed to grow through a nucleation mediated pathway, but important aggregation parameters, such as the nucleation radius, the surface tension of the aggregate, and the free energy barrier toward aggregation, have remained difficult to measure. Homogeneous nucleation theory, if applicable, can directly relate these parameters to measurable quantities. We employ fluorescence correlation spectroscopy to measure the particle size distribution in an aggregating solution of Alzheimer’s amyloid beta molecule (Aβ1–40) and analyze the data from a homogeneous nucleation theory perspective. We observe a reproducible saturation concentration and a critical dependence of various aspects of the aggregation process on this saturation concentration, which supports the applicability of the nucleation theory to Aβ aggregation. The measured size distributions show a valley between two peaks ranging from 5to50nm, which defines a boundary for the value of the nucleation radius. By carefully controlli...

Effects of turbulence on self-sustained combustion in premixed flame kernels: A Direct Numerical Simulation (DNS) study

Abstract: The effects of mean flame radius and turbulence on self-sustained combustion of turbulent premixed spherical flames in decaying turbulence have been investigated using three-dimensional direct numerical simulations (DNS) with single step Arrhenius chemistry. Several flame kernels with different initial radius or initial turbulent field have been studied for identical conditions of thermo-chemistry. It has been found that for very small kernel radius the mean displacement speed may become negative leading ultimately to extinction of the flame kernel. A mean negative displacement speed is shown to signify a physical situation where heat transfer from the kernel overcomes the heat release due to combustion. This mechanism is further enhanced by turbulent transport and, based on simulations with different initial turbulent velocity fields, it has been found that self-sustained combustion is adversely affected by higher turbulent velocity fluctuation magnitude and integral length scale. A scaling analysis is performed to estimate the critical radius for self-sustained combustion in premixed flame kernels in a turbulent environment. The scaling analysis is found to be in good agreement with the results of the simulations.

Determination of activation parameters for dislocation formation from a surface in fcc metals by atomistic simulations

Abstract: Defects in free surfaces are expected to be seeds for the nucleation of dislocations, which is the likely way nanoscale materials suffer plastic deformation. The nucleation results in the competition between the image force attracting the dislocation to the surface and the applied strain. In this work, two methods based on molecular dynamics simulations using an embedded atom method (EAM) potential are used to determine the activation energy and the critical radius for the formation of dislocations from a surface defect in a typical fcc metal.

Critical loss radius in a Penning trap subject to multipole fields

Abstract: When particles in a Penning trap are subject to a magnetic multipole field, those beyond a critical radius will be lost. The critical radius depends on the history by which the field is applied, and can be much smaller if the particles are injected into a preexisting multipole than if the particles are subject to a ramped multipole. Both cases are relevant to ongoing experiments designed to trap antihydrogen.

Melting and freezing of spherical bismuth nanoparticles confined in a homogeneous sodium borate glass

Abstract: The melting temperature and the crystallization temperature of Bi nanoclusters confined in a sodium borate glass were experimentally determined as functions of the cluster radius. The results indicate that, on cooling, liquid Bi nanodroplets exhibit a strong undercooling effect for a wide range of radii. The difference between the melting temperature and the freezing temperature decreases for decreasing radius and vanishes for Bi nanoparticles with a critical radius $R=1.9\text{ }\text{nm}$. The magnitude of the variation in density across the melting and freezing transitions for Bi nanoparticles with $R=2\text{ }\text{nm}$ is 40% smaller than for bulk Bi. These experimental results support a basic core-shell model for the structure of Bi nanocrystals consisting of a central crystalline volume surrounded by a structurally disordered shell. The volume fraction of the crystalline core decreases for decreasing nanoparticle radius and vanishes for $R=1.9\text{ }\text{nm}$. Thus, on cooling, the liquid nanodroplets with $Rl1.9\text{ }\text{nm}$ preserve, across the liquid-to-solid transformation, their homogeneous and disordered structure without crystalline core.

Thermodynamics of vesicle growth and instability

Abstract: We describe the growth of vesicles, due to the accretion of lipid molecules to their surface, in terms of linear irreversible thermodynamics. Our treatment differs from those previously put forward by consistently including the energy of the membrane in the thermodynamic description. We calculate the critical radius at which the spherical vesicle becomes unstable to a change of shape in terms of the parameters of the model. The analysis is carried out for the case both when the increase in volume is due to the absorption of water and when a solute is also absorbed through the walls of the vesicle.

Nucleation of a liquid on aerosol nanoparticles

Abstract: Nucleation of liquid water in the Earth's atmosphere occurs via heterogeneous nucleation on aerosol particles. We consider nucleation on both water-insoluble and water-soluble aerosol particles. We find that, for particles of the same radius, nucleation on soluble particles dominates. Soluble particles dissolve in the liquid phase and form a droplet even at coexistence. The radius of this droplet essentially determines the supersaturation at which nucleation occurs: the larger the droplet the smaller the supersaturation required before it nucleates to form the bulk liquid. We find that the supersaturation is best measured by the Kelvin radius, which is the radius of a droplet of pure liquid that coexists with vapour of a given supersaturation. We show that nucleation occurs at a universal value of the ratio between the radius of the droplet at coexistence and the Kelvin radius.

Homogeneous nucleation in thermal dust-electron plasmas.

Abstract: The homogeneous nucleation in the dust-electron plasma, which is formed in the zone of metal powder combustion products in the premixed laminar two-phase flame, has been studied. The classical nucleation theory has been used to determine the free energy and the critical radius of the nucleus. The influence of nucleus charging as a result of interaction between the nucleus and the electronic component of the plasma on the free energy has been determined. The dependence of the nucleus' critical radius on the plasma temperature and number density of the plasma's dust component has been determined. The proposed theoretical model shows that nucleation in the thermal dust-electron plasma is a self-consistent process, which is opposed to changing of the plasma's disperse structure.

Effect of Oblique Magnetic Field on Release Conditions of Dust Particle from Plasma‐Facing Wall

Abstract: Effects of oblique magnetic field on release condition of a spherical dust particle from a plasma-facing vertical wall is studied analytically. The magnetic pre-sheath and the Debye sheath are coupled to obtain the plasma quantities and the electric field at the wall, which are necessary to analyze the release condition. It is clarified that for the deeper potential drop inside the Debye sheath than the floating one the critical radius for release increases as the magnetic field approaches parallel to the wall. The smaller dust than the critical one can be released from the wall. On the other hand in the case of the shallower potential drop the critical radius disappears at some angle of the oblique magnetic field. From this analysis we find that the size of the released dust particle can be controllable by adjusting the plasma parameters such as plasma density and temperature as well as biasing the wall potential. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

On the filling of a pore in a solid-liquid nanodispersed system

Abstract: Thermodynamic equations have been derived that describe the process of the liquid migration from a pore into the bulk of a sintered body. The existence of the migration pressure in a nanocomposite body has been proved and the condition for the equilibrium of a liquid in a pore has been deduced. A concept of a pore critical radius has been formulated and an expression for the radius value has been suggested. All pores, whose radii exceed the critical one, are stable in a sintered body. The pores, whose radii are below the critical radius, are filled by the liquid phase.

Effect of Relaxation Time on Spin Coating Instability

Abstract: The relaxation time affects significantly the radius of a drop relaxing before spin coating especially for the liquids of low viscosity. Even for a fixed volume of liquid and rotation speed, the initial film thickness and the corresponding rotational Reynolds number are all changed. Furthermore, the critical radius for the onset of rivulet instability is dramatically altered. In the previous study, we reported a "turning phenomenon" for the Coriolis effect: it destabilizes high Bond number flows and stabilizes low Bond number flows. However, a comparison among variations of critical radius with rotational Reynolds numbers for different relaxation times shows that it is not the Coriolis force but the effect of a disturbance in a spreading drop of a fluid of low viscosity that causes this "turning phenomenon".

Effect of an electric field applied during the quenching on hardness of carbon steel

Abstract: It is found that application of an electric field during the quenching of 0.46% carbon steel in water increases its hardness. The hardness of the quenched surface increases with the strength of the electric field. The effect of the electric field on the hardness of the quenched surface is larger than that in the interior of the material. The quenching with an electric field gives a tinier martensite microstructure than the quenching without it. The possible mechanisms of electric field increasing the quenched hardness are: (a) the field reduces the critical radius of the martensite and the nucleation energy, and increases the nucleation rate; (b) the field reduces the vacancy concentration at the charged surface, thereby retarding the vacancy influence on the rate of pearlite formation.

Growth, clustering and morphology of intermetallic alloy core‐shell nanodroplets

Abstract: Sm-Fe-Ta-N core-shell (CS) nanospheroids were fabricated from hot liquefied nanodroplets by 157 nm pulse laser deposition in nitrogen gas. The Sm 13.8 Fe 82.2 Ta 4.0 intermetallic alloy was used as the target. At low laser energy (20 mJ), spherical CS of 1-35 nm radius were fabricated on a Si/Ta substrate forming uniform films. The small nanodroplets were grown in the plume from the gas phase, and the larger ones (>50 nm radius) from the target's hydredynamic ejection. The critical radius of the droplets and their surface energy per unit area was found to be 7.5 nm and 3.8 μJ/cm 2 respectively. A number of CS solidified in the plume and consist of 2.5-5 nm radius crystalline nucleus surrounded by a <35 nm radius amorphous spherical shell. This structure prevents the oxidization of the crystalline nucleus because oxidization is confined on the surface of the CS. Furthermore, multicrystalline nanodomains (embryos) were identified in a single CS from both homogeneous and heterogeneous nucleation.

The limiting current in a one-dimensional situation: Transition from a space charge limited to magnetically limited flow

Abstract: For a nonrelativistic electron beam propagating in a cylindrical drift tube, it is shown that the limiting current density does not saturate to the electrostatic one-dimensional (1D) estimate with increasing beam radius. Fully electromagnetic particle-in-cell (PIC) simulation studies show that beyond a critical aspect ratio, the limiting current density is lower than the 1D electrostatic prediction. The lowering in the limiting current density is found to be due to the transition from the space charge limited to magnetically limited flow. An adaptation of Alfven’s single particle trajectory method is used to estimate the magnetically limited current as well as the critical radius beyond which the flow is magnetically limited in a drift tube. The predictions are found to be in close agreement with PIC simulations.

Silicon self-assembled nanodots fabricated using a radio-frequency magnetron sputtering method

Abstract: Silicon nanodot is a promising nanostructured material for future single-electron devices in nanoelectronic system. The self-assembly growth of silicon nanodots on sapphire substrate was investigated, with highlights on the very early stage of nucleation and the growth process. The scope of study covers both the theoretical approach and experimental works. A classical theory of nucleation was applied to a liquid-solid phase transition, combined with high temperature supercooling to establish the expression for the net energy change in the formation of silicon nanodots. Using a computer program, the predicted parameters, such as critical radius (r*), critical energy (?G*), surface energy (?NS), and free energy change per unit area (?Gv) were obtained and tabulated into a dome-like shape nucleus following the Volmer-Weber growth mode. Experimental works have been conducted using a radio-frequency magnetron sputtering under the varying conditions of 5-20 minutes deposition time, 100-400°C substrate temperature and 50-200 W radio-frequency power. Optimum experimental conditions for the onset of silicon nanodot were found to be at 5 minutes/400 oC/100 W setting. Characterization measurements have been done on this sample using AFM, PL, XRD and EDX. Observation from AFM indicated the presence of small islands with an average diameter of 40.81 nm. The results from PL analysis revealed the existence of a peak which corresponded to a bandgap energy of 1.78 eV. This was further confirmed by the presence of 0.48 at.% of silicon on the substrate using EDX. A further XRD analysis gave no indication of a crystallinity phase probably due to extremely small amount of silicon formed on the substrate. The results showed that the formation of dome-like silicon nanodots on sapphire substrate occurred during the first 3 minutes of deposition, ascribed by the surface energy mismatch at interface and governed by a Volmer-Weber growth mode. A further growth of silicon nanodots were found to change their properties and strongly dependent on the experimental conditions.

Dynamic Contact Angle of Spreading Thin Film on a Rotating Disk

Abstract: The effect of dynamic contact angle on fingering instability during spin coating is investigated by flow visualization experiment. A modified rotational Bond number, which demonstrates the dynamic contact angle of fluid front that affects the dimensionless critical radius, is proposed in this work. For high rotational Bond number, the variation in dimensionless critical radius is not linearly proportional to the rotational Reynolds number because of the variation in critical contact angle if the relaxation time is fixed at zero. By modifying the rotational Bond number with the change in critical contact angle, the dimensionless critical radius becomes a function of the modified rotational Bond number.

Radius of energy transfer as an indicator of the interaction of aromatic amines with an olefin

Abstract: Perrin-type dependences for the efficiency of quenching of phosphorescence of aromatic amines by hex-1-ene in ethanol at 77 K were obtained and the radii of nonradiative energy transfer were estimated. The critical radius of T-T energy transfer decreases with enhancement of steric hindrances in the amine molecule all other things being equal.

Dynamics of charged dust particles in protoplanetary discs

Abstract: We study the effect of an imposed magnetic field on the motion of charged dust particles in magnetically active regions of a protoplanetary disc. Assuming a power law structure for the vertical and the toroidal components of the magnetic field for the regions beyond magnetically dead region of the disc, the radial and the vertical velocities of the charged particles, in the asymptotic case of small particles, are calculated analytically. While grains with radii smaller than a critical radius significantly are affected by the magnetic force, motion of the particles with larger radii is independent of the magnetic field. The critical radius depends on the magnetic geometry and the charge of the grains. Assuming that a grain particle has one elementary charge and the physical properties of the disc correspond to a minimum-mass solar nebula, we show that only micron-sized grains are affected by the magnetic force. Also, charge polarity determines direction of the radial velocity. For such small particles, both the radial and the vertical velocities increase due to the magnetic force.

Heterogeneous nucleation on convex and concave spherical surfaces

Abstract: Nucleation is a phenomenon of broad scientific interest and technological importance. It refers to the very early stages of the formation of a new phase, which can be solid, gaseous, and liquid, in a metastable parent phase. Most nucleation occurs heterogeneously unless the metastable parent phase from which the nuclei form is perfectly homogeneous and isolated from any catalyzing medium. This thesis project deals with heterogeneous nucleation on convex and concave spherical surfaces. In brief, the main achievements of the project are • An innovative analytical thermodynamic approach has been invented, which enabled rigorous thermodynamic formation of the energy barrier to nucleation, the critical radius and the shape factor, for nucleation on both convex and on concave surfaces. The rigorous thermodynamic analyses conducted have revealed a number of features for heterogeneous nucleation on convex and concave spherical surfaces as opposed to heterogeneous nucleation on a flat substrate surface. These are described in detail in Chapters 2 and 4. • Nucleation is the easiest on a concave spherical surface while it is the most difficult on a convex spherical surface assuming the contact angle and the critical embryo radius are the same. Nucleation on a flat substrate surface falls in between. This is determined by their shape factors. • The ratio R = 20r*, where R is the radius of the spherical substrate and r* is the critical embryo radius, (always define the symbols when they are first used. No one knows what they mean. R could be gas constant) can be regarded as a sufficiently accurate boundary that distinguishes between spherical and flat substrates for heterogeneous nucleation. • The investigation of the growth of crystal nuclei on a convex spherical surface has revealed that no growth barrier exists to the growth of a nucleus on a convex spherical substrate surface regardless of R r*. All nuclei formed on a convex spherical substrate surface are thus transformation nuclei. Turnbull’s transformation nucleus model or the recently developed free growth model does not apply to the growth of a spherical-cap nucleus on a convex spherical surface. • For heterogeneous nucleation in undercooled liquid metals, the cap thickness varies in a very narrow range by just a few angstroms and is typically about a few atomic layers thick according to Turnbull’s nucleation rate equation. The variations in the cap thickness are generally limited to less than 1A when the contact angle Ɵ is varied in the range from 0° to 45°. It is anticipated that these findings will help to better understand the classical models for heterogeneous nucleation and provide new insights into the control of heterogeneous nucleation where convex or concave spherical substrate surfaces are involved.