The growth process of silicon crystals grown with the float- zone method is strongly influenced by convection in the melt zone. Crystal growth under microgravity (μg)offers the advantage to reduce the buoyancy related convection. The size restriction which we have under terrestrial conditions due to the hydrostatic pressure is also avoided. But due to the low Prandtl number of silicon (Pr=0.02) and the comparatively high temperature dependence of surface tension (δy/δT=-.28x10^-3 N/mK) even small free melt zones exhibit time-dependent thermocapillary (Marangoni) convection. Thus irregular dopant inhomogeneities occurred in doped crystals grown under 1g conditions as well as in crystals grown under (mu) g. Attempts to enhance the dopant distribution by applying static magnetic fields resulted in most cases in either a reduced radial homogeneity, or in time-dependent thermoelectromagnetic convection (TEMC). A promising alternative are transverse rotating magnetic fields: the time-dependent thermocapillary convection is not damped as in the case of static fields, but a 2D-axisymmetric, azimuthal flow is superimposed. Several experiments performed under 1g demonstrated the positive effect of rotating magnetic fields on the microscopic dopant distribution as well as on the radial dopant profile homogeneity.

The solidification of liquids to produce materials is almost invariably affected by natural convection occurring in the liquid, due to concentration or temperature gradients. Fundamental studies of solidification studies of solidification are hampered by the existence of this convection and such studies have been facilitated by experiments in the microgravity of an orbiting spacecraft, where convection can be nearly eliminated. The paper describes early results from an alternative approach where buoyancy forces can be countered by a magnetic body force that arises from the temperature dependence of the magnetic susceptibility of the liquid. Velocities in a liquid volume, across which a temperature difference was imposed, have been measured by particle image velocimetry. The velocity were found to be significantly reduced when a magnetic field was imposed on the liquid using a superconducting magnet.

The effect of applied axial and radial thermal gradients and an axial static magnetic field on the macrosegregation profiles of Bridgman-grown GeSi alloy crystals has been assessed. The axial thermal gradients were adjusted by changing the control setpoints of a seven-zone vertical Bridgman furnace. The radial thermal gradients were affected by growing samples in ampoules with different thermal conductivities, namely graphite, hot-pressed boron nitride (BN), and pyrolytic boron nitride (PBN). Axial macrosegregation profiles of these samples fell between the predictions for a completely mixed melt and one where solute transport is dominated by diffusion. All of the samples were grown on Ge seeds. This resulted in a period of free grown until the Si concentration in the solid was in equilibrium with the Si concentration in the liquid. The length of crystal gown during this period was inversely proportional to the applied axial thermal gradient. Several samples were grown in an axial 5 Tesla magnetic field. Measured macroscopic segregation profiles on these samples indicate that the magnetic field did not, in general, reduce the melt flow velocities to below the growth velocities.

A novel method for control and homogenization oxygen distribution in silicon crystals by using electromagnetic force (EMF) to rotate the melt without crucible rotation has been developed. We call it electromagnetic Czochralski method. An EMF in the azimuthal direction is generated in the melt by the interaction between an electric current through the melt in the radial direction and a vertical magnetic field. (B). The rotation rate (ω_m) of the silicon melt is continuously changed from 0 to over 105 rpm under I equals 0 to 8 A and B equals 0 to 0.1 T. Thirty-mm-diameter silicon single crystals free of dislocations could be grown under several conditions. The oxygen concentration in the crystals was continuously changed from 1 X 10¹⁷ to 1 X 10¹⁸ atoms/cm³ with increase of melt rotation by electromagnetic force. The homogeneous oxygen distributions in the radial directions were achieved. The continuous change of oxygen concentration and the homogenization of oxygen distribution along the radial direction are attributed to the control of the diffusion-boundary-layer at both the melt/crucible and crystal/melt by forced flow due to the EMF. This new method would be useful for growth of the large-diameter silicon crystals with a homogeneous distribution of oxygen.

An axially traveling magnetic wave can be used to induce a meridional base flow in a cylindrical zone of an electrically conducting liquid, such as a crystal growth melt. This flow generated non-intrusively can be conveniently controlled, in magnitude and direction, to derive potential benefits for crystal growth applications. In particular, it can be used to effectively stir the melt in long cylindrical columns. It can be used to modify the species concentration field and the thermal field in the melt and to also affect the interface shape of the growing crystal. The basic theory of such an application is developed and preliminary data from a model fluids experiment, using a mercury column, are presented.

Because magnetization force is body force, it is possible to induce convection and buoyancy driven flows. A vertical magnetization force can modify the vertical acceleration and quench natural convection. The effect of horizontal and vertical magnetization forces on natural convection is studied. We present numerical simulations of the velocity and temperature distributions of a nonconducting fluid heated from below in the presence of an imposed, nonuniform magnetic field, generated with a solenoid-type magnet. The vertically placed magnet induces horizontal and vertical magnetic forces due to the gradient of magnetic strength, and the horizontal force is found to play an important to damp natural convection. When an imposed magnetic field of strength H₀ in the middle of the magnet, is less than a critical value, H_0c, the damping effect increases with increasing H₀. For H₀ > H_0c, natural convection is completely replaced by convection induced by the magnetic field. These results were discussed, comparing either the effect of vertical forces. Our results indicate a novel method to control convection of nonconducting fluids, especially in crystal formation processes.

A Computational Fluid Dynamics (CFD) code is used to determine the potential benefit of pulsed Metal-Organic Chemical Vapor Deposition (MOCVD). When AlN is grown using MOCVD over a range of pressures (30 to 270 Torr) and substrate temperatures (400°C to 900°C), gas-phase mixing of the precursor (TMA1) and ammonia hydride (NH₃)leads to adduct formation. This adduct formation may produce some undesired particulate by-products and deplete the precursors at elevated pressure and temperature. In order to reduce this gas-phase parasitic reaction, the pulsed inlet condition as proposed by Bachmann et al. is utilized to effectively separate the precursor form ammonia in gas- phase. It is predicted that for high reactor pressure (270 Torr), the growth efficiency of AlN can be enhanced by a factor of 3 through the pulsed MOCVD technique while simultaneously reducing the particle formation. The improvement by pulsed MOCVD is also demonstrated for a proposed 3D (North Carolina State University) research reactor.

Phthalocyanine films have been synthesized by vapor deposition on quartz substrates. Some substrates were coated with a very thin gold film for introducing electrical fie.d These films have been characterized by x-ray diffraction and scanning electron microscopy. The films have excellent chemical and optical stability. However, the surface of these films grown without electrical field shows whisk-like morphology. When films are deposited under an electrical field, a dense film with flat surface is obtained. A change of film in growth orientations and solid state phase is also observed for the film synthesized under electrical fields.

In this work, manganese segregation in vertical Bridgman- grown GaSb crystals has been investigated for different ampule diameters and several growth conditions. Experimental data of their impurity distribution from atomic absorption spectroscopy and Hall measurements have been obtained and compared with numerical analysis carried out with the finite element commercial program.

Magnetic force, i.e., magnetization force is body force and the function of density, and it is possible to induce buoyancy and convection similarly to the gravitational one. Magnetization force under 1D magnetic field gradient is generally shown by the product of density (p), mass magnetic susceptibility (x_g), magnetic field strength (H) and its gradient (dH/dy). Several experiments to control bubbles by using magnetic buoyancy forces were conducted under microgravity as follows: (1) Magnetic transport of bubbles: In pure water and glycerol/water mixture which are diamagnetic, the magnetic buoyancy force caused by a strong permanent magnet could transport bubbles toward a stronger magnetic field and to fix bubbles at the maximum point of magnetic strength. The transporting velocity was found to decrease with decreasing the radius of bubbles and increasing the viscosity. (2) Collision and fusion of two bubbles: It is almost impossible to observe the collision of bubbles clearly on the earth. the technique of magnetic control of bubbles nd microgravity condition made this observation possible. (3) Magnetic support of chemical reaction to produce bubbles (2H₂O₂→O₂+2H₂O on Pt catalyst). When small O₂ bubbles were removed from the surface of catalyst by magnetic buoyancy force, the decomposition reaction was observe to continue smoothly even under microgravity. On the other hand, in the absence of the magnet, the reaction was observed to stop under microgravity. Thus, the present study suggest the potential of using magnetic buoyancy forces to control bubbles in space experiments.

The key parameters controlling the interference of convection with segregation are examined. A numerical model was developed to evaluate the suitability of the Peclet number and the Segregation number for predicting segregation behavior in microgravity. The model demonstrates the drawbacks of the Peclet number. The recently proposed Segregation number includes the equilibrium segregation coefficient k and the growth rate R and therefore appears to be a more appropriate scaling parameter for predicting segregation behavior in space experiments. On earth, diffusion-controlled growth is possible only in very small melts, under special conditions. When diffusion-controlled growth is not feasible, controlled stirring is preferred to the free convection. The advantages of steady forced convection over the unsteady unpredictable natural convection are discussed.

Application of a low intensity axial magnetic field can promote significant convection during Bridgman growth of GeSi when resident thermoelectric currents at the growth interface are large due o difference of thermoelectric powers of the melt and of the crystal and the tangential temperature gradient at the interface. Thermoelectromagnetic convection (TEMC) in the GeSi melt is characterized by a meridional flow driven by the rotation of the fluid due to the azimuthal Lorentz force from currents in the radial direction, concentrated near the interface, and the axial magnetic field. A similar flow is caused by a rotating magnetic field (RMF). When the field is rotating sufficiently fast, a time-averaging azimuthal Lorentz force (almost uniform axially) causes a steady rotation of the melt, and an associated meridional convection (Ekman cells) near the interface. In this work, we developed a computational model to study convection of the GeSi melt in a microgravity environment in the presence of low intensity magnetic fields.

An in situ observation setup for the growth process based on near-IR microscopic interferometry was modified for a short- duration low-gravity experiment. Subsequently the observation in the environments were performed to reveal the influence of strongly-damped fluid flow on the growth process using the parabolic flights of an airplane and the free-fall of a drop capsule. As result, the dissolution and growth rates were successfully obtained using the setup with a high accuracy. It was also found that the rates were strongly decelerated during the low gravity conditions.

The wall free growth of CdTe by the Bridgman technique under microgravity was performed. This phenomenon called Detached or Dewetting growth was studied in the flight mission STS- 95. At this mission two CdTe crystals were grown co-doped with vanadium and zinc. Identical samples were grown with different ampoule designs. The effect of Detached or Dewetting growth was observed by analysis of crystal surface and roughness by optical and mechanical methods. All results of the crystals grown under microgravity are compared to the earth grown reference samples. Surface differences can be found between the (mu) g and the 1g samples. The surface roughness measurements demonstrate that the detaching was partially successful.

THree recent microgravity experiments have been hampered by convection caused by unwanted voids and/or bubbles which evolved in the melt. In this paper, we study steady and oscillatory thermocapillary and natural convective flows generated by a bubble in the surrounding fluid environment using a controlled model experiment with silicone oil. The dynamic characteristics of the time-dependent convection are captured using a combined numerical-experimental approach. It is shown that only below the critical Marangoni number, steady states conditions are attainable. With increasing Ma number, there is a complete transition from steady state up to a final non-periodic fluctuating flow regime through several complicated symmetric and asymmetric oscillatory states. The most prevalent oscillatory mode corresponds to a symmetric up and down fluctuation of the temperature and flow fields associated with an axially travelling wave. Careful examination of the numerical results reveals that the origin of this class of convective instability is closely related to an intricate temporal coupling between large-scale thermal structures which develop in the fluid and the temperature sensitive surface of the bubble. Gravity and natural convection play an important role in the formation of these thermal structures and the initiation of the oscillatory convection. Consequently, in low-g, the time evolution of the temperature and flow fields around the bubble are very different from their 1-g counterparts for all Ma numbers.

We developed a calculation method of 1D crystal growth of InAs-GaAs binary semiconductors and investigated the effect of the cooling rate an the temperature gradient on the crystal growth process under zero gravity conditions. We found that crystal of uniform compositions can be grown by the 1D growth method with a nonuniform concentration gradient in the solution if the growth rate is 3-5 mm/h. Furthermore, we investigated the occurrence of constitutional supercooling and found that the degree of supercooling can be reduced if the temperature gradient ins larger than 30 K/cm. We also developed a calculation method of 2D crystal growth of InAs-GaAs binary semiconductors, in which buoyancy convection induced by both temperature and concentration differences is taken into account, and investigated the effect of residual gravity and the inclination of the crucible on the crystal growth process. We found that convection is induced even under 10^-6 g conditions and that the crystals-solution interface is deformed by the convection. Convection is reduced and diffusion conditions can almost be realized if the growth direction is opposite to residual gravity. Convection becomes quite strong when the growth direction is perpendicular to gravity. The possibility of growing uniform crystals when the growth direction is inclined against the gravitational direction is discussed.

A general 2D and 3D finite element model of non-dilute alloy solidification was used to simulate growth of HgCdTe in terrestrial and microgravity conditions. Verification of the 3D model was undertaken by comparison with previously published result on convection in an inclined cylinder. For low growth velocities, plane front solidification occurs. The location and the shape of the interface were determined using melting temperatures obtained from the HgCdTe liquids curve. The low thermal conductivity of the solid HgCdTe causes a thermal short circuit through the ampoule walls, resulting in curved isotherms in the vicinity of the interface. Double-diffusive convection in the melt is caused by radial temperature gradients and by material density inversion due to the combined effects of composition and temperature. Cooling from below and the rejection at the solid-melt interface of the heavier HgTe-rich solute each tend to reduce convection. Because of these complicating factors, dimensional rather than non-dimensional modeling was performed. The predicted interface shape is in agreement with one obtained experimentally by quenching.

In the given work the problem of force action of stationary homogeneous crossed electric and magnetic fields on conducting vapor-liquid medium hydrodynamics in microgravity conditions is considered. Magnetic hydrodynamics mathematical model of weak dispersed conducting diamagnetic vapor-liquid medium is developed. For closure of magnetohydrodynamic equations the method of spatial average of variables is used by means of local self concerted field. Experimental evaluations of crossed electric and magnetic fields force action effectiveness of gas bubbles hydrodynamics in weak conducting carrying liquids in microgravity conditions are realized. It is shown in the original, that the force on gas bubbles in conducting carrying liquids of stationary homogeneous electric and magnetic fields is more effective and predictable, that from the side of vibrating, thermal, electrostatic and magnetic force fields.

Several Cu-based alloys such as Cu-Co, Cu-Fe and Cu-Nb, which are of considerable technological importance, possess a flattened liquidus implying a thermodynamic tendency to immiscibility upon undercooling. In this paper, both container and containerless processing were used to study the undercooling behavior, metastable liquid separation and microstructural development in the Cu-based systems. For undercooling experiments in an oxide flux, the melt separation temperature could be measured and the metastable liquid miscibility gap has been directly determined; while containerless processing in the 105 m drop tube permits larger undercooling to be achieved prior to solidification. All phase-separated samples were found to exhibit droplet- shaped morphologies, however there were various droplet size distributions, dependent upon composition, undercooling, and cooling rate.

Glasses of Na₂ xTe₂(x=2,4 and 6) compositions were remelted and evaporated while supported by a platinum heater coil in the low gravity (~10^-5g) drop shaft at the Japan Microgravity Center (JAMIC). The evaporating species from all the melts, which formed a spherical cloud surrounding the melt during the few seconds low gravity time were identified to be amorphous particles of TeO₂. These particles were highly spherical, 5 to 10 μm in diameter, and were, on the average, 6 to 8 times larger than the particles grown from similar experiments at 1-g. The melt remaining after evaporation was splattered on to a glass plate positioned at about 3.5 cm directly below the melt during the high-g (~8 to 10 g) deceleration of the drop capsule and crystallized almost instantaneously. The chemical composition of the crystallized splatters was same as that of the starting glass. The crystallization tendency of these sodium tellurite platters was estimated to be at least 1000 times greater than that of an identical melt at 1-g. No suitable explanation was found for the high crystallization tendency of the drop shaft splatters, but a sudden 5 orders of magnitude increase in the gravity level is suspected to be a contributing facto for this effect.

A growth mechanism is studied by investigating the morphology and structure of ZnO films under different growth conditions and orientations. ZnO films are deposited on )0001) sapphire and quartz substrates by off-axis sputtering deposition at various pressures and temperatures. All films reveal highly textured structures on glass substrates and epitaxial growth on sapphire substrates. X-ray diffraction, scanning probe microscopy, and electrical measurements are used to characterize these films. The full width at half maximum of theta rocking curves for epitaxial films is less than 0.5°. In textured films, it rises to several degrees. The morphology on the surface of textured films is of a granular round shape. The epitaxial films reveal flat surface but some hexagonal facets appear when the growth temperature is increased. At a pressure of 150 mTorr, a morphology transition form large flat grains to hexagonal facets occurs at 550°C. Also, the conductivity of films decreases with the increase of growth temperatures.

Containerless solidification of BiFeO₃ oxide has been carried out under microgravity with Electrostatic Levitation Furnace (ELF) aboard on the sounding rocket (TR-IA). It is a first containerless experiment using ELF under microgravity for studying the solidification of oxide insulator material. Spherical BiFeO₃ sample with diameter of 5mm was heated by two lasers in oxygen and nitrogen mixing atmosphere, and the sample position by electrostatic force under pinpoint model and free drift model. In order to compare the solidification behavior in microgravity with on ground, solidification experiments of BiFeO₃ in crucible and drop tube were carried out. In crucible experiment, it was very difficult to get single BiFeO₃ phase, because segregation of Fe₂O₃ occured very fast and easily. In drop tube experiment, fine homogeneous BiFeO₃ microstructure was obtained in a droplet about 300 μm. It implies that containerless processing can promote the phase selection in solidification. In microgravity experiment, because the heating temperature was lower than that of estimated, the sample was heated into Fe₂O₃+liquid phase region. Fe₂O₃ single crystal grew on the surface of the spherical sample, whose sample was clearly different from that observed in ground experiments.

We successfully prepared amorphous Bi-Pb-Sr-Ca-Cu-O spherical particles by a containerless melt solidification method. These particles were obtained by the melt solidification of Bi_2-xPb_xSr₂CaCu₂O_y [(Bi,Pb)-2212] powder during the free-fall using a propane gas torch. The diameter of these particles was about 100 micrometers . The crystalline (Bi,Pb)-2212 superconductors were obtained by the transformation from the amorphous state under several oxygen partial pressures. (Bi,Pb)-2212 phase was formed via (Bi,Pb)₂Sr₂CuO_y phase for all P₀₂ values. Low P₀₂ was more effective to introduce Pb into Bi₂Sr₂CaCu₂O_y lattice and only the heat treatment under P₀₂ equals 0.01 atm was successful in obtaining Pb-rich (Bi,Pb)-2212 with modulation-free structure. The critical composition separating Pb-poor modulated and Pb-rich modulation-free structures in (Bi,Pb)- 2212 was x equals 0.5. However, no lamellar structure composed of modulated and modulation-free regions was present in any particles for all P₀₂ values. Enhancement of critical current density was obtained by the Pb substitution.

The electrical conductivity and the atomic diffusion of ⁷Li in the molten lithium borates have been evaluated for studying transport phenomena in the melt. The dc electrical conductivity of melt with high viscosity shows exponential temperature dependence, σ=σ_oexp for the specimens with various compositions, x=Li₂O/(Li₂O + B₂O₃). The activation energy of dc conductivity estimated are 1.47eV for x equals 0.2, 1.10eV for x=0.25, 1.23 eV for x=0.34 and 0.77eV for x=0.5, respectively. The thermal activation energy for the atomic diffusion of ⁷Li in the melt evaluated by nuclear magnetic resonance technique, 0.37eV for x=0.07 and 0.69eV for x=0.67. Activation energy of Li diffusion is much smaller than those of the electrical conductivity in boron rich specimens. These are comparable in Li richer specimens. It may be suggested that small motive Li⁺ cations dominate the electric conduction in the molten borates in Li rich specimens. While, the anionic group, boric oxides, which are large in size and difficult to move in the melt may be responsible for the thermal activation of electrical conduction in boron rich specimens.

Present capability of the High Temperature Electrostatic Levitator (HTESL) at JPL for the containerless materials processing is described. The capability includes the measurements of various thermophysical properties and the studies of undercooling and nucleation phenomena. The thermophysical reports that can be measured include the density, the volume expansion, and ratio between the specific heat capacity and the hemispherical total emissivity, the surface tension,the viscosity, the spectral emissivity, and the electrical resistivity. For liquids where the Wiedemann-Franz-Lorenz law applies, their thermal conductivities can be determined indirectly from the electrical resistivities. The capability of determining the statistical nature of nucleation and the Temperature-Time-Transformation (T-T-T)curves has also been demonstrated. Experimental results on pure metals, alloys, and semiconductors were used to support the argument.

Viscosity of semiconductors and metallic melts is usually measured by oscillating cup method. This method utilizes the melts contained in vacuum sealed silica ampoules, thus the problems related to volatility, contamination, and high temperature and pressure can be alleviate. In a typical design, the time required for a single measurement is of the order of one hour. In order to reduce this time to a minute range, a high resolution angular detection system is implemented in our design of the viscometer. Furthermore, an electromagnet generating a rotational magnetic field (RMF) is incorporated into the apparatus. This magnetic field can be used to remotely and nonintrusively measure the electrical conductivity of the melt. It can also be used to induce a well controlled rotational flow in the system. The transient behavior of this flow can potentially yield of the fluid. Based on RMF implementation, two novel viscometry methods are proposed in this work: a) the transient torque method, b) the resonance method. A unified theoretical approach to the three methods is presented along with the initial test result of the constructed apparatus. Advantages of each of the method are discussed.

Measurement of 3D three-component velocity fields is of profound importance in microgravity fluid experiments including crystal growth, two-phase flows, and thermocapillary phenomena. Stereoscopic imaging velocimetry (SIV) is an optical nonintrusive technique for measuring gross-field flow, which is advantageous in system simplicity for building compact hardware and in software efficiency for continual near-real-time velocity monitoring. However, the challenge is how to increase spatial resolution, that is, marker particle density while maximizing data recovery rate. In this paper, the new SIV algorithms which utilize neural networks, are presented. Preliminary results from both simulating calculation and experiment show that the neural network algorithms offer very good potential for performance enhancement and has proven to be very useful for the SIV technique.

Diamond synthesis under microgravity was successfully executed using Japanese free-flyer (SFU, Space Flyer Unit) launched in 1995. This program achieved the space-based production of diamond by plasm-assisted Chemical Vapor Deposition (CVD). The results showed the better quality of diamond than that of earth-grown diamond and the microgravity effects on gaseous chemistry of CVD processes. To obtain a detailed understanding of plasma processes under microgravity, the post flight experiments of the SFU program were executed. A diagnostic study of hydrogen-methane plasma was applied to the CVD experiments using the 4.5s drop-shift facility in Toki, Japan. We found the direct effects of microgravity on the pattern formation of plasma discharge. The plasma instabilities affected the emission characteristics of plasma. Plasma temperature was estimated based on the emission spectra as a function of position from anode. Remarkable differences of the electron temperature of plasma were confirmed between the microgravity and the normal-gravity conditions.

Six thermophysical properties of both the solid and liquid zirconium measured using the high-temperature electrostatic levitator at JPL are presented. These properties, are: density, thermal expansion coefficient, constant pressure heat capacity, hemispherical total emissivity, surface tension and viscosity. For the first time, we report the densities and the thermal expansion coefficients of both the solid as well as liquid Zr over wide ranges of temperatures. Over the 1700-2300 K temperature span, the liquid density can be expressed as ρ_l(T)=6.24 x 10³ - 0.29 (T - T_m) kg/m³ with T_m=2128 K, and the corresponding volume expansion coefficient as αl = 4.6 x 10^-5/K. Similarly, over the 1250-2100 K range, the measured density of the solid can be expressed as ρ_s(T)=6.34 x 10³ - 0.15(T - T_m), giving a volume expansion coefficient α_s = 2.35 x 10^-5/K. The constant pressure heat capacity of the liquid phase could be estimated as C_pl(T) = 39.72 - 7.42 x 10^-3(T - T_m) J/mol/K if the hemispherical total emissivity of the liquid phase ε_Tl remains constant at 0.3 over the 1825 - 2200 K range. Over the 1400 - 2100 K temperature span, the hemispherical total emissivity of the solid phase could be rendered as ε_Ts(T) = 0.29 - 9.91 x 10³ (T - T_m). The measured surface tension and the viscosity of the molten zirconium over the 1850 - 2200 K range can be expressed as σ(T) = 1.459 x 10³ - 0.244 (T - T_m) mN/m and as η(T) = 4.83 - 5.31 x 10^-3(T - T_m) mPa.s, respectively.

This research applies a state of the art x-ray transmission microscope, to image the solidification of metallic or semiconductor alloys in real-time. By employing a hard x-ray source with sub-micron dimensions, resolutions of up to 2 micrometers can be obtained with magnifications of over 800 X. Specimen growth conditions were optimized and the best imaging technologies applied to maintain x-ray image resolution, contrast and sensitivity. In addition, a special furnace design is required to permit controlled growth conditions and still offer maximum resolution and image contrast. We have successfully imaged in real-time: interfacial morphologies, phase growth, coalescence, incorporation of phases into the growing interface, and the solute boundary layer in the liquid at the solid-liquid interface. We have also measured true local growth rates and can evaluate segregation structures in the solid; a form of in situ metallography. Composition gradients within the specimen cause variations in absorption of the flux such that the final image represents a spatial integration of composition. During this study, the growth of secondary phase fibers and lamellae form eutectic and monotectic alloys have been imaged during solidification, in real-time, for the first time in bulk metal alloys.

Growing silicon crystals form free melt zones, the flow regime is usually dominated by time-dependent convection, resulting in temperature fluctuations in the melt and subsequently in irregular dopant distributions in the crystal. The contribution of buoyancy and thermocapillary convection can be separated and their specific characteristics determined by taking advantage of microgravity conditions. For quantification of convective temperature fluctuations, temperature measurements have been performed in liquid silicon zones. Both half zone and full zone and full zone arrangements have been investigated. In case of the latter one, temperature measurements have been performed during the growth process to analyze the relation between the temperature fluctuations and the dopant distribution. All experiments have been carried out in monoellipsoid mirror furnaces. For temperature measurements, sheathed thermocouples or graphite-coated blackbody sensors have been used. The maximum temperature fluctuations were up to 7K in the half-zone case and 0.7K in the full-zone one. In both cases the main frequencies are in the range of 0.05 to 0.5Hz but they are slightly shifted to higher values with increasing Marangoni number. For the half-zone configuration, four thermocouples and up to two black-body sensor were inserted into the melt. Between certain pairs of thermocouples and sensors, a well developed phase correlation or 180 degrees anti-phase correlation has been detected indicating a pulsating flow regime. In the case of the floating zone experiments, a very good agreement is found between the frequency characteristics of the temperature signal and the frequency distribution of dopant irregularities.

Surface oscillation due to oscillatory Marangoni flow in a liquid bridge of molten silicon was observed using phase- shift interferometry. The molten silicon surface was described by phase-distribution profiles with a sampling rate of 30 Hz. From the phase-distribution profiles, we analyzed the oscillation of the radial displacement, axial gradient, and azimuthal gradient of the molten silicon surface. The oscillation of the radial displacement, axial gradient showed an in-phase relationship. However, in-phase oscillation was not observed between the radial displacement and the azimuthal gradient. Marangoni frequencies was observed at 0.1 to 5 Hz in which the frequencies higher than 1Hz had not previously been observed by conventional methods. We also found eigenfrequencies of the liquid bridge at 8.8Hz and 11.5Hz.

The non-intrusive measurement of velocity and temperature fields inside liquid metals is very difficult owing to their opacity for light in the near-visible range and the lack of suitable tracer particles. This is one of the reasons why numerical modeling of liquid metal flow has become increasingly important, in particular in crystal growth from the melt. Apart from the desire to model technical processes as realistically as possible, simple model studies have been carried out to investigate the fundamental fluid physics. These simple models are also very valuable for the test and the development of velocity-measurement techniques using, e.g. x-rays. This paper reports on recent advancements in the numerical analysis of thermocapillary flow in the most wide-spread model for the float-zone process of silicon crystal growth. In the model, tangential free surface forces drive a toroidal vortex flow inside a cylindrical volume of liquid. On an increase of the driving shear stresses induced by thermocapillarity the flow undergoes a sequence of pattern forming instabilities. The properties of these transitions, which are quite different for transparent high- and opaque low-Prandtl-number fluids, the physical mechanisms, and the structure o the associated flow fields will be addressed. Results have been obtained by 3D linear stability analyses and full numerical simulations of the governing equations.

3D unsteady numerical simulations were conducted, by means of the finite difference method, to understand the characteristics of stationary and oscillatory 3D Marangoni flows in half-zone liquid bridges of low Prandtl number fluids (Pr=0.01 and 0.02) with different aspect ratios (As=1.0, 1.2 and 1.8). The results clearly explain the transition processes; first from an axisymmetric to a 3D steady flow and the second from a 3D steady to 3D oscillatory flow. Critical Marangoni numbers for each transition as well as the frequency were determined for various conditions. These results were compared with available results of both linear stability analysis and non- linear numerical simulations. The present results agreed well with previously reported values within 6%. In a short liquid bridge, the 3D flows indicate two-fold symmetry in azimuthal direction (m=2). In a longer liquid bridge, however, there appeared a 3D flow with m=1. These basic flows become unstable against time dependent disturbances at Ma_c2.

The floating zone technique is a promising containerless method to realize larger and high-quality crystals of semiconductors under microgravity. Thermocapillary convection, which is caused by the presence of unbalanced surface tension in floating zone, is important for mass and heat transport in crystal growth and is studied with half- zone liquid bridge model in present paper. The free surface of liquid bridge is idealized as non-deformable and adiabatic from the environmental gas. the 3D oscillatory thermocapillary flow is investigated numerically by directly solving incompressible Navier-Stokes, energy and continuity equations with finite volume method. The surface tension is added directly in Navier-Stokes equations. the relationship between Marangoni number Ma and dimensionless frequency f* equals fH²κ^-1 and also flow structures are studied.

Bulk growth of wide band gap II-VI semiconductors by physical vapor transport (PVT) has been developed and refined over the past several years at NASA Marshall Space Flight Center. Results from a modeling study of PVT crystal growth of ZnSe are reported in this paper. The PVT process is numerically investigate using a 2D formulation of the governing equations and associated boundary conditions. Both the incompressible Boussinesq approximation and a compressible model are tested to determine the influence of gravity on the process and to discern the differences between the two approaches. The influence of a residual gas in included in the models. The results show that both the incompressible and compressible approximations provide comparable results and the presence of a residual gas tends to measurably reduce the mass flux in the system. Detailed flow, thermal and concentration profiles are provided. The simulations show that the Stefan flux dominates the system flow field and the subtle gravitational effects can be gauged by subtracting this flux from the calculations. Shear flows, due to solutal buoyancy, of the order of 50 micron/s for the horizontal growth orientation and 10 micron/s for the vertical orientation are predicted. Whether these flows can fully account for the observed gravity related growth morphological effects and inhomogeneous solute and dopant distributions is a matter of conjecture. A template for future modeling efforts in this area is suggested which incorporates a mathematical approach to the tracking of the growth front based on energy of formation concepts.