V. Nikolov

Relationship between the hydrodynamics in the melt and the shape of the crystal/melt interface during Czochralski growth of oxide single crystals: I. Determination of the critical crystal rotation rate from physical simulation data on free and forced convections

Abstract Physical simulation with model glycerol-water solutions imitating oxide melts was used to investigate free and forced convections in the liquid under the conditions of Czochralski single crystal growth. The investigations were carried out with a wide range of physical and geometrical parameter values of the crucible-liquid-crystal system. Equations were obtained for determining the rates of the down-flow of free convention and the up-flow of forced convection. A relationship between critical crystal rotation rate ensuring a flat crystal/liquid interface and the system parameters was found assuming equality of the flow rates during the transition from a convex to a flat interface.

On the conditions of formation of a flat crystal/melt interface during Czochralski growth of single crystals

The effect of some geometrical and physical parameters of the crucible-liquid-crystal system on the shape of the crystal/melt interface during the growth of single crystals by the Czochralski method is studied by physical simulation. The critical rotation rates at which a flat interface is obtained are established for different crystal diameters, model liquid viscosities and temperatures difference between the crucible walls and the crystal. The data obtained are generalized by a relationshhip of dimensionless numbers. This relationship is checked in Czochralski growth of bismuth germanium oxide (Bi12GeO20) single crystals is found to be satisfied in experiments with a high-temperature melt.

Simulation studies of the hydrodynamics in high-temperature solutions for crystal growth by the TSSG method

Abstract Investigations with model liquids of the convection flow rate under the crystal during its growth from a high-temperature solution according to the TSSG method have been carried out in the cases of: (a) forced convection, (b) free convection, and (c) simultaneous free and forced convection. Dependences between the rate of convection flow and the physicochemical properties of the liquid have been found and presented in their explicit form. For the case of simultaneous free and forced convection, which mostly occurs, intervals of convection flow rates are found for different crystal diameters, within which a flat crystal/liquid interface is formed. A dependence is established between the flow rate and the character and amplitude of temperature fluctuations on the interface. The conditions have been determined under which the formation of a flat interface in the absence of temperature fluctuations may be expected.

Physicochemical properties of high-temperature solutions of the system Na2OB2O3BaOFe2O3 suitable for the growth of BaFe12O19 single crystals

The system Na/sub 2/O-B/sub 2/O/sub 3/-BaO-Fe/sub 2/O/sub 3/ has been studied in order to find appropriate solvents and conditions of growth of barium hexaferrite from high-temperature solutions. The concentration and temperature regions of stability of the phase BaFe/sub 12/O/sub 19/ have been determined. The temperature dependences of the dynamic viscosities of solutions of this system are established and data on the density and volatility of the latter are obtained. On the basis of the experimental results, a solvent composition is proposed consisting of 70 mol % Na/sub 2/O and 30 mol % B/sub 2/O/sub 3/. The advantages of this solvent in comparison with the B/sub 2/O/sub 3/ and Na/sub 2/O based solvents used up to now are discussed.

Physicochemical properties of high-temperature solutions of the systems BaOB2O3Fe2O3 and BaOB2O3BaF2Fe2O3 suitable for the growth of BaFe12O19 single crystals

The systems BaOB2O3Fe2O3 and BaOB2O3BaF2Fe2O3 have been investigated and the concentration and temperature regions of stability of the BaFe12O19 phase in them are determined with a view to choosing suitable solvents and growth conditions of barium hexaferrite single crystals. The temperature dependences of the dynamic viscosity of solutions of these systems are studied. It is shown that solutions containing BaF2 have considerably lower viscosity values. The density and volatility of the solutions at their saturation temperatures are determined. On the basis of the data obtained, an appropriate solvent composition from the system BaOB2O3BaF2 has been proposed. Solutions of BaFe12O19 in this solvent have viscosity and saturation temperatures which are convenient for growth experiments.

Simulation studies of the hydrodynamics in high-temperature solutions for crystal growth. I. Forced convection

Abstract Investigations have been carried out on the free convection in model liquids (glycerol, glycerol-water solutions, silicone oil and liquid paraffin). All possible heating conditions and temperature profiles for growing crystals from high-temperature solutions have been considered. Dependences between the rate of the up (down) convection flow and the parameters of the liquid have been established and the explicit form of these dependences has been found.

Effect of the hydrodynamics in high-temperature solutions on the quality of pure and substituted YIG single crystals grown by the TSSG method

Abstract Single crystals of Y 3 Fe 5 O 12 , Y 3 Fe 5− x Al x O 12 and Y 3 Fe 5- x Ga x O 12 ( x = 0.5 and 1.0) have been grown from high-temperature solutions by the top-seeded solution growth (TSSG) method. The experiments were carried out under different conditions which, according to previous simulation studies on the hydrodynamics of high-temperature solutions [Peshev et al., J. Crystal Growth 65 (1983) 173], formated convex, flat or concave crystal/solution interfaces in the presence or absence of temperature fluctuations on these interfaces. The experimental results have confirmed those predicted by the simulation studies. The hydrodynamic conditions in the solution during the growth are shown to substantially affect the quality of the single crystals obtained. Crystals grown with a flat crystal/solution interface and in the absence of temperature fluctuations have demonstrated minimum levels of striations, inclusions and dislocations.

Study of the selectivity of a V2O5−ZrO2 catalyst for partial oxidation ofo-xylene at high temperatures

It has been established that at high temperatures (above 450°C) the V2O5−ZrO2 catalyst exhibits a higher selectivity in the oxidation ofo-xylene to phthalic anhydride than does the conventional V2O5−TiO2(a) catalyst. The catalyst selectivity is found to increase with respect to partial oxidation ofo-xylene, the valuable by-product maleic anhydride being obtained. Studies by different physicochemical methods have shown that V2O5−ZrO2 undergoes no significant phase and structural changes under high-temperature conditions.

Stability region and growth conditions of Al- and Ga-substituted Y3Fe5O12 single crystals in solvents from the BaO-B2O3-BaF2 system

Abstract The crystallization region of Y 3 Fe 4 AlO 12 and Y 3 Fe 4 GaO 12 was investigated in BaO-B 2 O 3 -BaF 2 solvents. The effect of each of the six components in the system on the concentration and temperature limits of the garnet phase stability region was shown. The experimental data indicated that solvents containing 0.60–0.77 moles of B 2 O 3 and 0.00–0.46 moles of BaF 2 per 1 mole of BaO are most suitable for the growth of the two garnets by slow cooling. When a gradient transport technique is used, the best sovents are those containing 0.00–0.60 moles of BaF 2 and 0.70–1.00 or 0.60–1.00 moles of B 2 O 3 per 1 mole of BaO in the cases of Y 3 Fe 4 AlO 12 and Y 3 Fe 4 GaO 12 , respectively.

Simulation studies of the hydrodynamics in high-temperature solutions for crystal growth. VI. Temperature fluctuations at the crystal/liquid interface

Abstract The temperature fluctuations at the crystal/liquid interface have been studied using model liquids. The character of the fluctuations varied depending on whether in the liquid there was only free convection or both free and forced convections were simultaneously present. It has been established that in the second case the amplitude of fluctuations is determined mainly by the rate of the resultant convection flow and the liquid viscosity. A maximum amplitude of fluctuations is observed at a resultant flow rate w fl = O when the free and forced convections proceed in opposite directions, and at w fl = min when both convection flows have the same direction. Larger amplitudes of temperature fluctuations are registered in low-viscosity liquids. The conditions under which a flat crystal/liquid interface showing no temperature fluctuations should be formed have been determined for crystal diameters of 10, 15 and 20 mm, respectively.