Presence Of Convection Research Articles

Concentration profiles of As in a Ga rich solution during electroepitaxy of GaAs using a computer simulation technique

Concentration profiles of As in front of a GaAs crystal interface growing in a Ga rich solution have been determined during the electroepitaxial growth of GaAs layers using a computer simulation technique. The effect due to Peltier heating or cooling and electromigration during growth have been incorporated to simulate the concentration profiles. The growth velocity in the absence and presence of convection, due to Peltier effect and electromigration, are calculated under different conditions. It is observed that there is a transition in the movement of arsenic atoms towards the GaAs crystal interface from smooth and orderly to turbulent and wavy as the intensity of electric field increases during the electroepitaxial growth of GaAs.

Langmuir probe characteristics in a high pressure plasma in the presence of convection and ionization

The current to a Langmuir probe operating in a high pressure flowing plasma is calculated for the case where convection of electrons or ions into the sheath adjacent to the probe and thermal generation of electrons and ions within the sheath can both be significant factors in determining probe current. In addition to generating explicit relations for planar, spherical and cylindrical probes for the asymptotic case in which only generation need be considered, a solution is also obtained for the extensive transition between this state and the purely convective state, for which explicit solutions already exist. The theory, which enables the electron/ion density to be calculated from the probe current is applicable to both positive and negative probes.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Three-dimensional probabilistic modelling of equiaxed eutectic solidification in the presence of convection

A three-dimensional probabilistic model describing the solidification of a small specimen containing a metallic alloy of eutectic composition has been developed. For a uniform temperature, the heterogeneous nucleation of the grains is assumed to be randomly distributed within the domain. The growth kinetics of the grain interface may be that of either a coupled regular or irregular eutectic ( i.e. Jackson-Hunt type of model) or given by the diffusion of carbon through the austenite shell for the case of nodular cast iron. The grains are allowed to move within the liquid according to different convection models, such as no movement, random movement of the grains associated with turbulent effects, inhomogeneous fluid flow velocity or sedimentation. In order to trace the position of the grain interface, each grain surface is subdivided into a given number of elementary surfaces. The grain impingement, the volume fraction of solid, the effective solid-liquid interface, the average number of nearest neighbours, the grain size distribution and stereological relationships may all be calculated by means of such models. The effect on the cooling curve may also be deduced.

Entrainment from a bed of particles by thermal convection

Differentiation in magma chambers, in the Earth's core and in the partially molten early Earth is a competitive process between sedimentation and re-entrainment of crystals in the presence of convection. Previous studies show that the particles suspended in convective layers eventually settle and do so almost as fast as in the absence of convection. However, the nature and magnitude of the competing entrainment has remained unclear. Here we provide a simple theory and experimental evidence showing that entrainment occurs at the crests of dunes created in the particle bed at the base of a convecting fluid. In both laminar and turbulent regimes, the dune formation and entrainment are driven by viscous stresses produced by thermal plumes. At sufficiently high Rayleigh numbers the particles are probably entrained by Reynolds stresses. Entrainment in the Earth's core is hardly possible because it requires unreasonably small crystals. Entrainment of 10 −2–10 −1 cm diameter crystals is very likely in magma oceans. For magma chambers entrainment requires large viscosities (> 10 6 P) and even when it occurs, the total amount of the suspended solid fraction is very small.

Influence of convection on the composition profiles of thick GaAlAs layers grown by liquid phase electroepitaxy

Liquid phase electroepitaxy (LPEE) — the method of current controlled layer growth from a solution at constant temperature — was used to grow thick GaAlAs layers on GaAs substrates. We have shown that in the presence of convection in the solution, compositionally uniform layers can be grown despite compositional nonuniformity of the source material used. Layers grown in the absence of convection had an Al content which increased with thickness. This reflects the composition profile of the source material prepared by the cooling down of the system.

Macrosegregation during plane front directional solidification of CsI-1 wt% T1I alloy

Macrosegregation produced during vertical Bridgman directional solidification of CsI-1 wt% T1I in crucibles of varying diameter, from 0.5 to 2.0 cm, has been examined. Gravity driven convection is present in the melt even in the smallest crucible diameter of 0.5 cm. Observed solutal profiles are in agreement with the analytical boundary layer model of Favier, which describes macrosegregation in the presence of convection. The scintillation efficiency of CsI decreases along the specimen length as the thallium iodine content of the alloy increases.

Transport phenomena in crystal growth from solution

This is a review of the role of transport phenomena in crystal growth from solution. Transport phenomena are normally taken to include fluid motion (convection), heat transfer and mass transfer. These prosesses strongly influence growth rate, crystal morphology, and defect formation. Mass transfer arises primarily from concentration gradients, but can also be caused by a temperature gradient (Soret effect), an electric field, acceleration of gravity, etc. In solutions, mass transfer is strongly enhanced by convection; the motion of the solution carries material with it. Similarly, convection can be caused by a variety of driving forces; a pressure gradient, mechanical forces, buoyancy, electric fields, and even the growth of the crystal itself. Except in liquid metals, heat transfer in liquids is also strongly enhanced by convection. It is important to realize that the simple form of Fick's first law is often not valid in crystal growth from solutions, i.e. the mass transfer flux is not necessarily equal to the product of a concentration gradient and a diffusion coefficient. In multicomponent systems the flux of a component depends on the concentration gradients of all the other constituents. It is possible for a component to move from a region where its concentration is low to a region where its concentration is high. If the component is charged, its movement is also strongly influenced by an electric field, which may be generated by the diffusion process itself. The flux of a solute into the surface of the crystal equals the growth rate, and includes a contribution caused by the movement of the solution toward the crystal surface. This convective contribution has the effect of increasing the growth rate beyond that predicted by Fick's first law, especially when the solubility is large. It is also important to note that the stagnant film model for heat and mass transfer in the presence of convection is a fiction and can lead to erroneous predictions. There is no unstirred layer near the surface of the crystal. With convection, the mass transport rate is not proportional to the diffusion coefficient and the heat transfer rate is not proportional to the thermal conductivity. The operational challenge in crystal growth from solutions is to maximize the growth rate without trapping the solution as inclusions in the crystal. Constitutional supercooling is always present in solution growth, but does not prevent growth without interface breakdown. High quality crystals can be grown from solutions because step propagation is faster than step generation, as manifested by the presence of facets. Faceted growth can become unstable when step trains decelerate, which occurs when steps begin in regions of high supersaturation and move to regions of low supersaturation. In order to avoid this condition, either crystals must be grown very slowly or well-controlled vigorous convection must be used. However if one desires to avoid inclusion formation in the transition from dissolution to growth, it is advantageous to grow in the absence of convection so that the transition from dissolution to growth may be made gradual.

Experimental determination of the thermal diffusivity of molten alkali halides by the forced Rayleigh scattering method. I. Molten LiCl, NaCl, KCl, RbCl, and CsCl

As a series of experimental determinations of the thermal diffusivity of molten alkali halides, this paper describes measurements on five molten alkali metal chlorides (LiCl, NaCl, KCl, RbCl, and CsCl) in the temperature range up to 1440 K by the forced Rayleigh scattering method. K2Cr2O7 is employed as a dye substance to color the transparent molten salts. The accuracy is estimated to be ± 4 to ±11 % depending on the measured salts. In comparison with the present results converted into thermal conductivity, most of the previous experimental data obtained by steady-state methods show larger values, up to about five times, which may be due to the systematic error caused by the presence of convection and radiation. It is found that the thermal conductivity of these series of molten alkali metal chlorides decreases with increasing molecular weight, and their temperature coefficients are weakly negative.

Break-down of a planar liquid-solid interface during directional solidification; influence of convection

The influence of convection on the development of morphological instability at the liquid-solid interface during directional solidification in a positive thermal gradient has been examined in Pb-10wt%Sn and succinonitrile-1.9wt%acetone. The onset of interfacial breakdown occurs at higher growth speeds in the presence of convection. The linear stability analysis due to Favier and Rouzaud which uses the “deformable” mass flow boundary layer concept shows a good agreement with the experimentally observed behavior.

The Stefan thermodiffusion problem in the presence of convection

The Stefan thermodiffusion problem is considered, taking into account convective motions in the liquid phase. The solvability of this problem is proved in spaces of smooth functions.

Traveling Wave Solutions of the Stefan and the Ablation Problems

The problem of the stability of traveling wave solutions of the ablation and the Stefan problems is considered in the presence of convection. It is proved that under reasonable assumptions on the initial datum the solution has a traveling wave as an asymptotic limit, and the phase shift is computed.

Effect of gravity on copper phthalocyanine thin films III: Microstructure comparisons of copper phthalocyanine thin films grown in microgravity and unit gravity

This is the third paper in a series of three describing the first closed-cell physical vapor transport (PVT) experiments in which thin films of organic pigments were epitaxially deposited onto highly oriented seed films of another organic. Furthermore, the experiments were conducted in both low earth orbit (LEO) on the Space Shuttle Orbiter and in the laboratory as ground controls using the 3M-PVTOS (PVT of organic solids) apparatus. In this last paper of the series, scanning electron microscopy is used to compare the ground control CuPc samples with the LEO-grown samples. The results are notable because they show that a distinctly different microstructure exists in the centers of the LEO-grown films from that in the ground control films. A further very important observation is that the circular perimeters of the LEO-grown films have a microstructure much like that of the ground control films in both their center and edge regions. This provides a strong self-consistency check that the structural differences are attributable to the absence or presence of convection and not some other PVT parameter or ampoule artifact. The form and radial variation of the physical microstructure among the LEO and ground control samples are fully consistent with the observations and conclusions of the optical, visible spectroscopy, ellipsometry, interferometry, reflection X-ray diffraction and IR spectroscopy of the two preceding articles in this series.

Search for auroral belt E∥ fields with high‐velocity barium ion injections

Four high‐velocity shaped charge Ba+ injections were conducted from two Black Brant‐10 rockets at collision‐free altitudes (770‐975 km) over northern Alaska (L = 7.4–10.6) in April 1984 under active auroral and magnetic disturbance (Kp 4+ and 5) conditions. The motions of the Ba+ “pencil” beams from these injections were accurately triangulated to altitudes ranging from 9000 to 14000 km from multistation image observations. Scanning photometer observations from five sites provided checks on the triangulations as well as auxiliary data, including observations that illustrate the lack of any anomalous ionization that might be expected from critical velocity effects. Well‐defined initial conditions and improved software for predicting the unperturbed, E∥ = 0, trajectories in the presence of convection, E⊥, fields permitted an accurate detection of changes in the motion which could be attributed to E∥ fields. Large (&gt;1 keV) potential changes that might be anticipated from double‐layer or V‐, U‐, and S‐shaped potential structures were not encountered even though the Ba+ rays were clearly located on auroral arc flux tubes on at least several occasions and were at various times in close proximity to auroral flux tubes for many minutes. Abnormally intense E⊥ fields that might also indicate that the above potential structures were also not observed. Transient accelerations and/or decelerations involving magnetic field‐aligned energy changes ≤375 eV were, however, encountered by each of the seven principal Ba+ rays tracked to high altitudes. Acceleration events were only slightly more frequent than deceleration events. Interpretation, taking into account limits on the duration of the events and simultaneous auroral conditions, favors explanation in terms of propagating waves, soliton trains, or other pulse forms provided that the propagation is primarily field‐aligned. The similarity of energy and intermittent occurrence characteristics suggests that these E∥ perturbations are likely to be associated with suprathermal electron bursts, both upward and downward. One release, which uniquely took place in a region of intense (200 V/km) rotational shear in the E⊥ field, was followed by the appearance of a small population of suprafast Ba+ ions traveling well ahead of the main high‐velocity Ba+ rays. A second set of faint streaks, representing suprafast Ba+ ions from the low‐velocity debris cloud, simultaneously appeared above the debris rays. These suprafast ions are attributed to transverse energizations of small fractions of both the initial Ba+ jet and Ba+ debris immediately following the release. Transverse energizations for the highest velocity ions extended to 5879 eV with maximum flux near 952 eV. Tip motions for the fastest Ba+ debris ions were not obtained, but triangulation of one streak showed a maximum energization of 131 eV. Wave‐particle stochastic heating models may be relevant to explaining both the high energies and the dependence of the energy on initial velocities.

Thermal conductivity measurement of molten salts by the transient hot-wire method. (2nd report. Measurements of molten NaNO3 and KNO3 using ceramic-coated probes).

The present paper reports absolute measurements of the thermal conductivity of molten salts by the transient hot-wire method using newly developed ceramic-coated probes. Al2O3 has been employed as the insulation material and the following techniques have been utilized to produce insulation layers : ion plating for a thin platinum hot-wire and plasma-spray for thick titanium struts. The measurements have been performed on pure NaNo3 in the temperature range 311°C to 388°C and KNO3 349°C to 439°C. The experimental data have an estimated accuracy of ±3%. In comparison with the present results, most of he previous experimental data obtained by steady-state methods show higher values which may be due to the presence of convection and radiation.

Asymptotic stability in the presence of convection

t) in [0,1] x [0,00). Put simply, theorem 2.1 asserts that bounds on the initial and boundary data which satisfy the appropriate differential inequalities in the domain under consideration are in fact bounds on the solution inside the domain as well. Thus this result is very much in the spirit of the more familiar maximum principle. As an illustration of the applicability of theorem 2.1, suppose that in problem (2.1) a

Growth of needle-shaped crystals in the presence of convection.

The motion of the freezing front between a needle-shaped crystal and a supercooled liquid is analyzed for situations where there is forced convection aligned with the crystal axis. It is shown that in the absence of capillary effects the shape of the crystal-melt interface is a paraboloid of revolution, similar to that found in situations where diffusion is the sole heat-transfer mechanism. A relation between the supercooling, the product of tip velocity and tip radius, and the strength of the flow is derived which reduces to the well-known Ivantsov theory in the absence of convection.

Estimation of diffusion coefficients by a capillary method in the presence of oppositely directed convection of the melt

The paper presents a theoretical analysis of linear diffusion in the presence of convection with a constant flow rate for an instantaneous plane source and for diffusion in infinite capillary. A conclusion can be drawn that diffusion in the melt moving at the ratev together with the source is described, with respect to the system of coordinates connected with the capillary, by a superposition of molecular diffusion and translational motion of the melt. Self-diffusion of51Cr in Fe-Cr-O systems of different compositions and self-diffusion of59Fe in molten iron at 1860 K have been investigated experimentally. The results of theoretical analysis have been applied in the evaluation of diffusion coefficients in the presence of convection.

Thermal Convection in 3He–4He Mixtures near the Tricritical Point

Exploratory experiments are described that investigate 3He–4He mixtures near the tricritical point as convection systems. Measurements of the temperature difference between the horizontal boundaries of a convection cell heated from above exhibit features attributable to phase separation and the bulk superfluid transition but show no feature consistent with the onset of stationary convection. The presence of convection is inferred from enhanced thermal conduction and the existence of a time-dependent state showing similarities to Busse's oscillatory instability. The effect of superfluid film flow was very noticeable and measurements of the film superfluid transition temperature are presented for four concentrations of 3He in 4He.

FREEZING OF SALT SOLUTIONS ON A VERTICAL WALL

Abstract The freezing of water-salt (sodium chloride) solution on a vertical wall of a two-dimensional cavity is investigated experimentally. The influence of natural convection on the freezing rate and on the interface shape is examined. Flow visualization revealed the presence of convection confined to a region above a stable solutally stratified fluid. The thermally and solutally driven natural convection was found to strongly affect the shape of the solidification front and the rate of freezing. Due to continuous rejection of salt at the interface, a stable solutally stratified region of solution develops at the bottom of the cavity where the convection is absent

Analytical and experimental study of transport processes during directional solidification and crystal growth

Heat and mass transfers in the melt are known to be major phenomena controlling solidification of alloys or doped semiconductors. These are both diffusion and convection dependent even in non-stirring melt because in the presence of gravity natural convection of thermal or solutal origin can perturb diffusion, leading to unsteady transfers. The methods of the order of magnitude analysis are used to define scaling laws for the solute boundary layer extent and concentration in the presence of convection. These laws can be introduced into analytical boundary layer models predicting segregation or morphological stability thresholds. Various experiments in metallic systems are described. In order to get significant data, particular attention is given to the control of the parameters governing the flow and to the measurement of properties directly influenced by convection: (a) Surface flow velocities are measured on liquid metal layers which are sufficiently thin for the buoyancy effects to be negligible. (b) Ga doped Ge crystals have been grown from the melt under different thermal conditions on the ground as well as in a microgravity environment, and segregation profiles have been compared with theoretical analyses. (c) An original method has been developed which allows an in-situ measurement of the temperature of the solid-liquid interface during solidification, by making use of the Seebeck effect. Chemical transients, morphological destabilization as well as interfacial kinetic effects can thus be detected in real time. Two direct extensions of the previous theoretical and experimental studies are also briefly presented: (1) The effects of unsteady flow regimes on the solidification process are considered for two different physical situations: on the ground in the case of high driving forces for convection and in a microgravity environment with a non-stationary residual g-level (g-jitters). (2) Natural convective effects at fully destabilized solidification fronts are studied (dendritic solidification). Simple laws can also be given for this extreme case where the solute concentration can be supposed as imposed at any point within the interdendritic zone due to the local equilibrium between the solid and liquid phase.