Showing papers in "Crystal Research and Technology in 2003"

Review: Nucleation in solutions revisited

Abstract: Existing and new results in nucleation in solutions are outlined from a unified point of view. The thermodynamics of the process is considered and expressions are given for the supersaturation, the nucleation work and the size of the nucleus in homogeneous or heterogeneous nucleation. It is shown how the nucleation theorem can be used for a model-independent determination of the nucleus size from experimental data. The mechanism and kinetics of nucleation are also considered and formulae are presented for the supersaturation dependence of the monomer attachment frequency and the stationary rate of homogeneous or heterogeneous nucleation. General expressions for the induction time and the critical supersaturation ratio for crystallization are given. An approximate formula is derived for estimating the width of the metastable zone with the help of data for the solubility of the substance crystallized. Existing experimental data are used for verification of the validity of some of the presented theoretical dependences.

Growth and properties of LiGaX2 (X = S, Se, Te) single crystals for nonlinear optical applications in the mid-IR

Abstract: Single crystals LiGaX 2 (X = S, Se, Te) of optical quality were grown, with transparency ranges at 5 cm -1 absorption level of 0.32-11.6 μm, 0.37-13.2 μm and 0.54-14.2 μm, respectively. The first two, LiGaS 2 and LiGaSe 2 , have a wurtzite-type structure whereas LiGaTe 2 is tetragonal (chalcopyrite lattice). The three refractive indices were measured in the whole transparency ranges of LiGaS 2 and LiGaSe 2 and n a and n c were found to be very close (quasi-uniaxial optical anisotropy) with a crosspoint at 6.5 μm (LiGaS 2 ) and 8 μm (LiGaSe 2 ). Sellmeier equations were fitted and phase-matching conditions for second harmonic generation (SHG) were calculated: the 1.467-11.72 μm spectral range for the fundamental is covered by LiGaS 2 and LiGaSe 2 .

Non-stoichiometry related defects at the melt growth of semiconductor compound crystals: a review

Abstract: An overview on the defect knowledge in IV-VI (PbTe), II-VI (CdTe) and III-V (GaAs, InP) semiconductor compound crystals in dependence on non-stoichiometric growth conditions from the melt is given. The treatment starts with the today's informations about the phase relations, i.e. the shape of the existence regions versus stoichiometry. Non-stoichiometry related melt growth phenomena are specified. After that selected intrinsic point defects and their influence on the impurity incorporation as function of deviation from stoichiometry are discussed. The interaction processes between native point defects and dislocation formation is touched. Finally, the serious mechanisms of precipitation and inclusion incorporation are shown. Possible measures of defect minimization are mentioned.

Optical characterization of thermally evaporated thin CdO films

Abstract: Cadmium Oxide films have been prepared by vacuum evaporation method on a glass substrate at room temperature. Detailed structural, optical, and electrical properties of the films are presented at different annealing temperatures. The crystal structure of the samples was studied by X- ray diffraction. The spectral absorption coefficient of the CdO film at the fundamental absorption region (450-650nm) was determined using the spectral data of transmittance. The direct and indirect band gap energies were determined and found to be 2.33 eV and 1.95 eV respectively. The third order optical nonlinearities χ(3) of CdO films has been measured used the z-can technique. The real and imaginary parts of χ(3) have been measured at 514 nm and found to be 1.7x10-3 esu and 3.0x10-3 esu, respectively. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermal and optical properties of glasses of the system Bi2O3 – B2O3

Abstract: In the binary system Bi 2 O 3 - B 2 O 3 glasses were prepared in the composition range 57. 5 mol% - 80 mol% B 2 O 3 by defined slow cooling of large melt samples (about 75 cm 1 , each). Temperatures of crystallization, of melting and of glass transition were determined and density data of the glasses were derived using the hydrostatic weighting method. Thermal expansion coefficients and high precision refractive indices, together with their dispersion, were measured. The measured physical properties indicate subtle discontinuous structural changes of the glasses with glass composition, that match with the ranges of existence of the crystalline compounds of the binary system Bi 2 O 3 - B 2 O 3 . Thermal investigations together with X-ray powder diffraction analyses of crystallized glass samples prove the so far doubtful existence of a borate compound named BiBO 3 in the PDF within the composition range 52. 5 - 57.5 mol% B 2 O 3 .

FTIR spectroscopic, thermal and growth morphological studies of calcium hydrogen phosphate dihydrate crystals

Abstract: Calcium hydrogen phosphate dihydrate (CHPD or DCPD) is found quite frequently in urinary calculi (stones). The CHPD crystals were grown by the single diffusion gel growth technique in sodium metasilicate gel. The crystals were found to be having platelet and broad needle type morphologies. The crystals were analyzed by FTIR spectroscopy. The thermal properties were studied by employing the thermogravimetric analysis. Various kinetic and thermodynamic parameters for dehydration were estimated. The selected platelet was studied by SEM for the growth morphologies indicating that the crystals grew in the form of leaflets having prominent (010) faces. This was in agreement with earlier reported studies. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Garnet layers prepared by liquid phase epitaxy for microwave and magneto‐optical applications – a review

Abstract: The magnetic iron garnets possess two outstanding properties, an excellently narrow linewidth of the ferromagnetic resonance and a magneto-optical activity that is without competition when the iron garnet is substituted with bismuth. Here the growth of bismuth substituted iron garnet layers with a bismuth content of up to 1.8 formula units is reported. The influence of solution compositions and undercooling on the growth and bismuth incorporation is discussed. Morphological instabilities due to misfit stress are avoided by the use of lattice-matched substrates. Two applications of iron garnet layers, namely a electronically controllable magnetostatic delay line for the communication technology in the GHz range and magneto-optic garnet sensor layers for real-time visualisation of eddy currents with high spatial resolution for the non-destructive evaluation are presented. Furthermore the detection, measurement, and visualisation of magnetic fields by magneto-optic garnet layers are reviewed.

Thermal conductivity and refractive indices of Nd:GdVO4 crystals

Abstract: The thermal conductivities of Nd:YAG, M(Y,Gd)VO4 crystals were measured at 298 K. The value of Nd:GdVO4 crystal along direction was 11.4 W/mK, which was higher than that of YAG crystal measured to be 10.7 W/mK. The principal refractive indices of Nd:GdVO4 crystal in the temperature range from 20 °C to 170 °C were determined by auto-collimation method. Based on the measured values of refractive indices, the Sellmeier equation and expression of temperature dependence of refractive indices have been obtained. The measured results show that the birefringence Δn is 0.22007 at 20 °C and temperature coefficient of birefringence is 4.33 × 10−6/°C for 1.064 μm. These results prove that the GdVO4 crystal is a laser crystal with excellent thermal and birefringence properties. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Some physical and magnetic properties of Mg‐Zn ferrite

Abstract: Some physical properties (such as lattice parameter, density and porosity) and magnetic properties of the system Mg 1-x Zn x Fe 2 O 4 ; where x=0,0.1,0.2,0.3,0.4,0.5and 0.6 have been studied. It was found that the lattice parameter increases with increasing the zinc concentration. The composition dependence of the physical properties is divided into two regions. The first one is for x ≤ 0.3 and the second one is for x > 0.3. From the magnetization measurements, the basic composition (MgFe 2 O 4 ) shows the lowest magnetization, while the composition of x=0.4 shows the highest one. The behaviour of magnetization M versus composition shows also two regions for x 0.3. The behaviour of M versus x was discussed in the bases of cation distribution. From the B-H loops, the remanence induction B r , saturation induction B s and the coercive force H s were determined and studied with x. The Curie temperature T c was determined from the measurements of the initial permeability μ i versus temperature. It was found T c decreases with increasing Zn-content. Also paramagnetic temperature T was determined from the behaviour of M s vs. T. In general it was found T p > T c by about 7-10 K.

YVO4 crystals - puzzles and challenges

Abstract: Work on optical properties of undoped and rare-earth doped YVO 4 crystals is briefly reviewed. Fundamental spectroscopic parameters relevant to laser performance of YVO 4 containing erbium, thulium and holmium, derived from absorption and emission spectra and from excited state relaxation dynamics, have been presented. Laser performance of YVO 4 :Er has been reported and laser potential of YVO 4 doped with thulium and holmium has been assessed. Anomalous phenomenon of up-conversion of IR radiation into red emission in YVO 4 :Yb,Ho and puzzling anti Stokes emission in nominally undoped YVO 4 have been discussed.

Growth of high resistivity CdTe and (Cd,Zn)Te crystals

Abstract: The growth of high resistivity CdTe and (Cd,Zn)Te is successfully performed with different kinds of doping. In the literature intentionally undoped as well as doped crystals are presented with resistivities up to 10 10 Ωcm. In this paper we review the growth of high resistivity CdTe and (Cd,Zn)Te. The mechanism of compensation is discussed regarding the different dopants and deep levels, which seem to be responsible for the high resistivity. A common compensation model explains the high resistivity by deep levels. The doping and the influence on the compensation mechanism is compared for several elements like tin, germanium and chlorine. The material properties and the crystal quality of undoped and doped CdTe as well as (Cd,Zn)Te are shown.

Studies on optical properties of polycrystalline SnO2:Sb thin films prepared using SnCl2 precursor

Abstract: Optical properties of spray deposited antimony (Sb) doped tin oxide (SnO2) thin films, prepared from SnCl2 precursor, have been studied as a function of antimony doping concentration. The doping concentration was varied from 0-4 wt.% of Sb. All the films were deposited on microscope glass slides at the optimized substrate temperature of 400 °C. The films are polycrystalline in nature with tetragonal crystal structure. The doped films are degenerate and n-type conducting. The sheet resistance of tin oxide films was found to decrease from 38.22 Ω/□ for undoped films to 2.17 Ω/□ for antimony doped films. The lowest sheet resistance was achieved for 2 wt.% of Sb doping. To the best of our knowledge, this sheet resistance value is the lowest reported so far, for Sb doped films prepared from SnCl2 precursor. The transmittance and reflectance spectra for the as-deposited films were recorded in the wavelength range of 300 to 2500 nm. The transmittance of the films was observed to increase from 42 % to 55 % (at 800 nm) on initial addition of Sb and then it is decreased for higher level of antimony doping. This paper investigates the variation of optical and electrical properties of the as-deposited films with Sb doping. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

An improved prediction model of morphological modifications of organic crystals induced by additives

Abstract: Additives have a great effect on the crystal morphology of organic compounds. The molecular modeling techniques were applied to predict the morphological modifications induced by additives. Besides molecular modeling software additionally some simulation approaches are needed. The build-in and the surface docking approaches were applied to some host-additive-systems. It will be presented here that the suitability of these two approaches depends on the degree of anisotropy of the intermolecular bonding of the host substance. A procedure on how to choose an appropriate prediction approach for the individual crystal systems on investigation is suggested.

Optical constants of DC magnetron sputtered titanium dioxide thin films measured by spectroscopic ellipsometry

Abstract: Optical constants of DC magnetron sputtered $TiO_2$ thin film have been determined by Spectroscopic Ellipsometry in the photon energy range 1.2 to 5.5 eV at room temperature. The measured dielectric-function spectra reveal distinct structures at energies of the E1, E1 + \bigtriangleup 1 and E2 critical points are due to interband transitions. The root mean square roughness of the magnetron sputtered $TiO_2$ thin films evaluated by ex-situ atomic force microscopy is 5.8 nm. The Dielectric constant values were found to be substantially lower than those for the bulk $TiO_2$. The dielectric related Optical constants, such as the refractive index, extinction coefficient, absorption coefficient and normal incidence of reflectivity determined from the spectroscopic ellipsometry data are presented and analyzed. The optical constants of the films were also determined using the optical transmittance measurements and the results were discussed.

Optical characterization and crystal structure of the novel bronzetype CaxBa1‐xNb2O6 (x = 0.28; CBN‐28)

Abstract: Calcium barium niobate (CBN), also like strontium barium niobate (SBN), belongs to the materials family of partially filled tetragonal tungsten bronzes, which show relaxor-type ferroelectric phase transitions and large electro-optic effects. For the first time, it was possible to grow large single crystals of Ca 0.25 Ba 0.75 Nb 2 O 6 (CBN-25) and CBN-28 by using different growing techniques. The successful single crystal growth allowed to investigate several physical properties of the novel phase. Herein, we report on measurements of differential thermal analysis (DTA), wavelength dispersion of the refractive indices, temperature dependence of the birefringence and light absorption. Additionally, results of the single crystal X-ray structure analysis are presented.

Optical and microhardness studies of semiorganic nonlinear optical material: L‐ histidine tetrafluoroborate

Abstract: L-histidine tetrafluoroborate (L-HFB) a semiorganic nonlinear optical material has been synthesized in aqueous solution at 50°C and characterized by FT-IR and FT-Raman spectroscopy studies. The solubility was determined in different solvents such as water, methanol and water mixture of methanol. The single crystals with dimensions 15x12x3 mm3 were grown by slow evaporation method within four weeks with approximate growth rate of 0.25 mm/day. The grown crystals have been subjected to single crystal X-ray diffraction studies to determine the unit cell dimensions and morphology. The Kurtz powder second harmonic generation test shows that the compound is a potential candidate for frequency conversion. The refractive index has been measured using He-Ne laser. The microhardness test was carried out and the load dependence hardness was observed. The material has a wide transparency in the entire visible region. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Particle size of powders under hydrothermal conditions

Abstract: Various non-oxide (CuI, AgI, AgCl, PbS, CuS and ZnS) and oxide (ZnO, TiO2, SnO2, CeO2 and ZrO2) powders were prepared under hydrothermal conditions to investigate the effects of temperature, pH and precursors on the particle size of powders. It was found that the particle sizes of PbS, CuS and ZnS powders were much smaller than that of CuI, AgI and AgCl powders prepared under the same conditions. The particle sizes of TiO2, SnO2, CeO2 and ZrO2 powders are much smaller than that of ZnO powders prepared under the same conditions. It is concluded that the solution conditions have a certain effect on the particle size of powders under the hydrothermal conditions. The particle size of powders increased with the rising of temperature. Additional factors affecting the particle size were uncovered through studying the nucleation mechanism. The particle size was mainly related to the Madelung constant and the electric charge number of ions. Powders with smaller particle size resulted from systems that possessed the larger Madelung constant and ionic charge number. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of crystallization conditions on the mechanism and rate of crystal growth of potassium sulphate

Abstract: Growth rate of potassium sulfate crystals was studied in a fluidized bed crystallizer. Higher growth rates were observed at larger bed heights. Larger crystals grow faster than smaller size crystals due to the changed fluid dynamics. The growth rate was found to be very sensitive to the pH value of the solution. The growth rate increases with increasing the pH value and the increase of growth in the acidic medium is faster than in the basic medium. The presence of small amounts of Cr 3+ ions reduces the growth rate of potassium sulfate dramatically. The presence of Cr 3+ ions lowers the saturation temperature and increases the width of the metastable zone, i.e. shifts the metastability of the solution to be at lower level of supersaturation.

Growth and characterization of semiconducting cadmium selenide thin films

Abstract: Semiconducting thin films of cadmium selenide have been grown by conventional thermal evaporation technique. The effect of various growth parameters like rate of deposition and deposition temperature has been studied in detail. Films deposited at room temperature are cadmium rich with segregated selenium globules. A deposition temperature of 453K has been found to yield stoichiometric, homogeneous films. The films have been analysed for optical band gap and thermal activation energies. Films of low electrical resistivity have been obtained for possible applications

Rb3Lu(PO4)2:Ce and Cs3Lu(PO4)2 :Ce - new promising scintillator materials

Abstract: Cs 3 Lu(PO 4 ) 2 :Ce and Rb 3 Lu(PO 4 ) 2 :Ce single crystals have been studied under gamma-, x-ray and UV excitation. For all types of excitation the luminescence of these materials is dominated by a broad Ce-emission band positioned at the UV/visible light boundary. Highly efficient and relatively fast scintillation of both materials identify them as promising scintillators.

Mechanical activation of precursors for nanocrystalline materials

Abstract: Nanostructured materials win big scientific interest and increasingly economic meaning through their specific exceptional properties. Precursors that were compacted by pressing and sintering are normally used preparation of materials. In present work, the influence of mechanical activation by grinding on the structure as well as on compacting and sintering behavior of oxides from magnesium, aluminium and silicon has been investigated. Starting materials for each metal oxide differ in microstructure, dispersity, and porosity. The influence of mechanical activation on the destruction of crystalline structure to nanocrystalline, as well as to the amorphous stage and the compaction of powders with nano-particles, as well as structures with nanoscale pores have been compared. The possibilities of the consolidation of nanostructured materials were investigated. The mechanical activation took place in a disc vibration mill. The mechanical activated materials as well as their pressing and their sintering products were characterized by density, particle-size distribution, specific surface, pore-structure, microstructure, and crystallite size by X-ray powder diffraction (XRD). The mechanical activation of the model-substances led, in most cases, to an improvement of the compaction properties; thus, this improvement can be achieved with subsequent sintering densities up to 98% of the theoretical density. From these experiments, generalizations transferable to other materials can be made.

Misleading fringes in TEM images and diffraction patterns of Si nanocrystallites

Abstract: High-resolution transmission electron microscopy (HRTEM) images and electron diffraction patterns of twinned Si nanocrystallites were recorded along various directions and analyzed in detail. We point out that special attention must be paid when interpreting HRTEM images and diffraction patterns of twinned Si nanocrystallites, because elongation of reciprocal lattice points could fabricate misleading fringes and patterns. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Poole-Frenkel electrical conduction in europium oxide films deposited on Si(100)

Abstract: Thin Eu2O3 films were prepared on Si (P) substrates to form MOS devices. The oxide crystal structure was determined by X-ray diffraction (XRD). The electrical transport properties of the devices with amorphous and crystalline Eu oxide were investigated. The current-voltage and current-temperature characteristics suggest a Poole-Frenkel (PF) type mechanism of carrier transport through the device when the applied field is more than 105 V/cm. A deviation from PF leakage current course was found and attributed to the current carrier trapping. We have also observed that, the dielectric spectra of MOS structure are different when the insulator is an amorphous or crystalline thin film. From which we calculate the relaxation time (τ) of the interface (insulator/semiconductor) dipoles. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Crystallographic Shear Defect in Molybdenum Oxides: Structure and TEM of Molybdenum Sub-oxides Mo18O52 and Mo8O23

Abstract: Molybdenum trioxide and molybdenum sub-oxides are of great interests in catalysis due to their utilities as model system to elucidate the correlations between the structure and the catalytic performance. The sub-oxides are usually an intermediate phase during catalytic reaction in which the lattice oxygen is involved. We show the identification of the two common molybdenum sub-oxides Mo 18 O 52 and Mo 8 O 23 , derived from MoO 3 by crystallographic shearing (CS), by means of electron diffraction and High-Resolution Transmission Electron Microscopy (HRTEM) in combination with image simulation. The coincidence of simulated electron diffraction patterns and high-resolution images with the experimental ones indicates the feasibility of CS structure determination by these techniques.

Photoelectronic and electrical properties of CuIn5S8 single crystals

Abstract: To identify the impurity levels in CuIn5S8 single crystals, the dark electrical conductivity and photoconductivity measurements were carried out in the temperature range of 50–460 K. The data reflect the intrinsic and extrinsic nature of the crystals above and below 300 K, respectively. Energy band gaps of 1.35 and 1.31 eV at 0 K and 300 K, were defined from the dark conductivity measurements and the photocurrent spectra, respectively. The dark and photoconductivity data in the extrinsic temperature region reflect the existence of two independent donor energy levels located at 130 and 16 meV. The photocurrent-illumination intensity dependence (F) follows the law IphαFγ, with γ being 1.0, 0.5 and 1.0 at low, moderate and high intensities indicating the domination of monomolecular, bimolecular and strong recombination at the surface, respectively. In the intrinsic region and in the temperature region where the shallow donor energy level 16 meV is dominant, the free electron life time, τn, is found to be constant with increasing F. In the temperature region 140 K < T < 210 K, the free electron life time increases with increasing illumination intensity showing the supralinear character. Below 140 K, τn decrease with decreasing illumination intensity. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Asymmetric twinning crystals of Zinc oxide formed in a hydrothermal process

Abstract: Well crystallized ZnO powders with various morphologies such as hour glass-shaped, shortened prismatic, flake-like and prismatic form have been synthesized by adopting the system of Zn(Ac) 2 -KOH-H 2 O via a hydrothermal process at 200°C for 2 h. It was found that the as-synthesized hour glass-shaped ZnO crystallites are asymmetric both in shape and stoichiometry: while the atomic ratio of O/(Zn+O) at the bigger end ranges from about 60% to 40% along the polar axis of the hour glass-shaped ZnO crystallites, the atomic ratio of O/(Zn+O) at the smaller end remains almost constant at approximately 50%.

Advances in modeling of wide‐bandgap bulk crystal growth

Abstract: We review key aspects of sublimation growth of wide-bandgap semiconductors like SiC, AlN and GaN, and show how modeling can help to solve a number of practical problems. As the temperature distribution in the growth chamber is the most critical factor in the sublimation technology, we discuss in detail specific features of heat transfer with the focus on the porous materials normally involved in the growth system: powder source, thermal insulation, and graphite. The species transport is simulated using advanced models accounting for a multicomponent vapor and the kinetics of chemical interaction of nitrogen-containing species with the surface of group-III nitrides. The effect of growth conditions on parasitic phase formation is analyzed. To optimize the growth system design and operating conditions, we suggest an inverse-problem approach instead of commonly used trial-and-error methods. This simulation procedure has proved quite efficient for getting design solutions, which could hardly be found by conventional straightforward methods. In particular, we demonstrate the effectiveness of the inverse modeling for finding the growth conditions, which provide crystals of desired shapes. A special analysis is made to establish a correlation between the growth conditions and defect distribution in the grown crystal, with the focus on thermoelastic stress produced by temperature gradients. The high-temperature dynamics of gliding dislocations and consequent plastic stress relaxation in the material is examined. The prospective of global modeling of wide-bandgap crystal growth by sublimation and still open questions are discussed.

Structural and spectral studies on gypsum crystals under simulated conditions of phosphoric acid production with and without organic and inorganic additives

Abstract: In the dihydrate process to produce phosphoric acid, phosphate ore [Ca 10 F 2 (PO 4 ) 6 ] is leached with sulfuric and weak phosphoric acids to produce phosphoric acid and gypsum as a by-product. Crystallization of gypsum occurs as the leaching is taking place. The effect of organic and inorganic additives on the structure and spectrum of gypsum crystals under simulated conditions of phosphoric acid production is studied using x-ray diffraction and infrared spectroscopy. Structure and spectrum of formed gypsum crystals in the absence of additives are slightly different from the standard gypsum crystals (card No. 6-0046), which reflect the effect of preparation medium on the crystal structure of gypsum crystals. Presence of additives such as cetyl trimethyl ammonium bromide and 1,2-dihydroxybenzene-3,5-disulfonic acid, Al 3+ and Mg 2+ increase the crystallinity of gypsum, while presence of additives such as citric acid and sodium dodecyl sulfate decrease the crystallinity of gypsum. Presence of Al 3+ and Mg 2+ as additives lead to the formation of calcium sulfate hemihydrate beside calcium sulfate dihydrate. Presence of sodium dodecyl sulfate as an additive inhibits the crystallization of gypsum and leads to the formation of anhydrite and calcium sulfate hemihydrate.

Crystallization of potassium sulfate by cooling and salting-out using 1-propanol in a calorimetric reactor

Abstract: A study was made on a isothermal process for the crystallization of potassium sulfate as an alternative to the cooling process. The process employs addition of 1-propanol to aqueous salt solutions to achieve the salting-out of the K 2 SO 4 . This work was carried out using an automated Mettler Toledo model RC1 reactor-crystallizer with 800 ml capacity, and controlled isothermally at 25°C to test the crystallization of K 2 SO 4 by addition of the alcohol, and from 50 to 10°C for the cooling crystallization. In both systems, the line of nucleation points was shown to be approximately parallel to the saturation curve, with an average width of 13°C or 3 % mass for crystallization by cooling, compared with 0.2 to 1 % by salting-out. In experiments on crystallization by cooling, the K 2 SO 4 crystals were 0.27 mm in mean size, showed 7 % agglomeration, and contained 8.5 % moisture. Crystals obtained by salting-out had a mean size of 0.79 mm, 28 % agglomeration, and 9-10 % moisture content. A crystal shape factor of approximately of 0.7 was obtained in both systems, apart from the agglomeration.

Influence of high frequency vibrations on fluid flow and heat transfer in a floating zone

Abstract: This paper deals with the numerical simulation of fluid flow and heat transfer in a silicon floating zone (FZ) subjected to axial vibrations. Corresponding experiments were performed under microgravity conditions within the framework of the European sounding rocket Maxus-4 flight. The study concerns a liquid zone maintained between two rigid rods of equal radii. The growing crystal is subjected to vibrations in the direction of the zone axis. Numerical modelling of the convective heat transfer in the liquid zone is carried out accounting for melting and crystallization. The flows and the heat transfer in the melt are calculated on the basis of the equations of thermal vibrational convection under non-uniform vibrations. The calculations are made for zero gravity and 1-g conditions accounting for both pulsational and average deformations of the free surface. The temperature at the free surface is fixed and corresponds to the experimental one. The shape and the position of the solid-liquid interfaces are given by the isotherm of the melting point of the material. The problem is solved by a finite difference method. The boundary-fitted curvilinear coordinate system is employed. The calculations are made for the parameters corresponding to the radiation heated floating zone growth of silicon, i.e. a crystal diameter of 10 mm and an aspect ratio of the melt zone between 1 and 1.5. The vibration amplitude is varied in the range of 3 to 4.5 μm and the vibration frequency lays in the interval of 2 to 4.5 kHz. It is shown that for zero gravity and for 1-g conditions, the vibrations lead to a decrease of the thermocapillary flow intensity near the crystal-melt interface. For zero gravity conditions vibrations lead to a flattening of the crystallization front.