A crystallizer for single crystal growth from solution

The theoretical basis and the construction of the new type of the two-vessel WALKER -KOHMAN crystallizer adequate for the optical observations of single crystals growing from solution is described.     

Mechanism of growth of ZnO single crystals from hydrothermal alkali solutions

The results obtained in experiments on the growth of zinc oxide, ZnO, under hydrothermal conditions are generalized. The polar growth and nonstoichiometry of ZnO crystals are analyzed in terms of crystal chemistry with due regard for the physicochemical conditions of the growth medium. The effect of the Li⁺ and of di-and trivalent metal (Co²⁺, Fe²⁺, Mn²⁺, Fe³⁺, Mn³⁺, Sc³⁺, In³⁺) impurities on the kinetics and some physical characteristics of ZnO crystals are studied, and the chemical reactions occurring on the surfaces of the (0001) and \((000\bar 1)\) faces are considered.     

Hydroxyapatite synthesis from biogenic calcite single crystals into phosphate solutions at ambient conditions

Hydroxyapatite is one of the most important bone substitute biomaterials. Here, it has been successfully overgrown on biogenic seed crystals at ambient conditions. Single crystals of calcite from Atrina rigida, Paracentrotus lividus and Heterocentrotus mammillatus have been soaked in phosphate solution with different concentrations and pHs for 2 months. X-ray powder diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy have been used to characterize soaking precipitates. The results show that the conversion of calcite to hydroxyapatite occurs to an extent which depends on composition and morphology of seed crystals, and starting concentration and pH of phosphate solutions. In the same experimental conditions, synthetic calcite single crystals did not convert to hydroxyapatite. The morphological observations suggest for hydroxyapatite formation, a mechanism that involves a superficial dissolution of calcite and a subsequently overgrowth of hydroxyapatite. Moreover, the final architectural assembly of the hydroxyapatite crystals resembles the shape of the starting biogenic seed crystals.     

Efficient conditions for the growth of MoO3 single crystals under oxygen atomsphere

Single crystals of MoO₃ have been grown under an oxygen atmosphere using a horizontal tubular furnace. The temperature and pressure necessary to grow satisfactory crystals in oxygen along [100] and [010] directions are determined.     

Bulk GaN single crystals: growth conditions by flux method

Hexagonal GaN platelet crystals with a size of 1–4 mm have been grown by a Li-based flux method. The influence of growth conditions such as the molar ratio of starting materials, temperature, pressure, the position of Li₃N in the crucible on the growth of GaN single crystals was studied. The quality of GaN single crystal was checked by optical microscope and X-ray rocking curve.     

Effect of the growth rate on thermal conditions during the growth of single crystals with melt feeding

An attempt is made to establish a correlation between the radial and axial growth rates and the change in the conditions of heat transfer from a growing crystal to the atmosphere of the water-cooled vacuum furnace for the growth of large alkali halide single crystals. It is found experimentally that an increase in the growth rate leads to an increase in the automatic compensation of the melt temperature by the main heater. In this case, the thickness of the layer of melt condensate on the end face and the lateral surface of the crystal decreases. It is revealed that the possibility of growing infinitely long ingots in the presence of intense melt evaporation is restricted by the possibilities of the heat transport through the boundary between the furnace atmosphere and the cooled furnace walls, onto which melt condensate deposits.     

Hybrid technique for growing homogeneous single crystals of semiconductor solid solutions from melt

A concept and fundamentals of a hybrid technique for growing homogeneous single crystals of semiconductor solid solutions (SSs) from melt are presented. The growth consists of two stages. In the first stage, a single crystal with a specified concentration of the more refractory component is grown by an innovative method of directional constitutional supercooling of a melt in the steady-state mode. In the second stage, the melt is fed with the second component. Using an example of the classical Ge-Si system, the concentration profiles of the components along the crystal axis are calculated and the SS growth dynamics ensuring single crystallinity over the entire ingot length is determined. An analysis of the results yields the optimal technological parameters and conditions for growing homogeneous single crystals of SSs of a specified composition and size.     

New solvents for the growth of substituted BaFe12O19 single crystals from high-temperature solutions

The solubility of a series of hexaferrite derivatives of BaFe₁₂O₁₉ in solvents of the system Na₂O-B₂O₃ with oxide ratios of 7:3 and 3:2 has been investigated. The temperature dependences of the saturation concentration in these solvents are determined for ferrites with the nominal compositions Ba_0.8Pb_0.2Fe₁₂O₁₉, BaFe₁₀Ga₂O₁₉, BaFe₁₀Al₂O₁₉ and BaFe₈Mn₂ Ti₂O₁₉. Single crystals of BaFe₁₂O₁₉, in which part of the metal ions are replaced by various amounts of Pb²⁺, Ga³⁺, Al³⁺ and Mn²⁺ + Ti⁴⁺ ions, are g rown from the solutions by the slow cooling technique. The distribution coefficients of the substituting ions and the compositions of the crystals obtained are established by microprobe analysis (EPMA). Information on the position of substitution is obtained from the Mössbauer spectra.     

Surface morphology and quality of potassium bichromate crystals obtained from aqueous solutions in relation to growth conditions

The characteristics of macrospiral layers on growth surfaces of potassium bichromate crystals and their quality are examined in relation to growth conditions. It was observed that the layers become thick and rounded, more inclusions are trapped and the crystal quality deteriorates with an increase in supersaturation, while an opposite effect is noted with an increase in growth temperature. The observations are discussed from the standpoint of crystal growth theories.     

Preparation of Cu2ZnSnS4 single crystals from Sn solutions

We investigated the phase diagrams of the Cu₂ZnSnS₄ (CZTS)–Sn pseudobinary system in order to obtain knowledge useful for the growth of high-quality CZTS single crystals using a solution-based method. For Sn solutions saturated with less than ～60 mol% CZTS, the solutes are separated into two phases (CZTS phase+SnS_x phase+liquid phase). On the other hand, for solutions with more than 60 mol% CZTS, the solutes are single phase (CZTS phase+liquid phase). The CZTS single crystals were obtained from a 70 mol% CZTS solution (liquid temperature 850 °C) at 900 °C. The powder X-ray diffraction (XRD) pattern of the CZTS single crystal shows preferred orientations of (112), (220) and (312) planes, confirming the Kesterite structure of CZTS. The Raman spectrum shows three peaks at 287, 338, 371 cm^?1, which corresponded to CZTS peaks. The composition of the CZTS single crystal along the growth direction is found to be slightly Cu-poor, Zn-rich and S-rich. Therefore, it is assumed that the Cu vacancy is the dominant p-type conduction mechanism.     

Hydrodynamic conditions for growth of large KDP crystals

Three-dimensional mass transfer is simulated. The factors determining the hydrodynamic conditions for efficient mass transfer in growth of large KDP single crystals (cross sections up to 45 × 45 cm²) from solutions in a real 300-l-crystallizer are established. The conditions for the motion of the supersaturated solution to the surface of a growing crystal in the direction opposite to that of gravitation are attained. The growth mode developed provides growth rates that are five times higher of large high-quality optical KDP crystals.     

Small-angle X-ray scattering study of conditions for the formation of growth units of protein crystals in lysozyme solutions

The structural composition of lysozyme solutions favorable for the formation of the tetragonal form of protein crystals was studied by synchrotron-based small-angle X-ray scattering depending on the protein concentration and the temperature. Along with lysozyme monomers, dimers and octamers are found in crystallization solutions; the octamer content increases with an increase in the protein concentration.     

A novel growth method for ZnAl2O4 single crystals

ZnAl₂O₄ is a well‐known wide band gap compound semiconductor (E_g=3.8eV), ceramic, opto‐mechanical, anti‐thermal coating in aero‐space vehicles and UV optoelectronic devices. A novel method for the growth of single crystals of a ternary oxide material was developed as a fruit of a long term work. Material to be grown as metal incorporated single crystal was taken as precursor and put into a bath containing acid as reaction speed up reagent (catalyst) as well as solvent with a metal foil as cation scavenger. Using this method, ZnAl₂O₄ crystals having hexagonal facets are prepared from a single optimized bath. Structural and compositional properties of crystals were studied using Philips, Xpert ‐ MPD: X‐ray diffractometer and Philips, ESEM‐TMP + EDAX. Thus technique was found to be a new low cost and advantageous method for growth of single crystals of ternary oxide a material. We hope that these data be helpful either as a scientific or technical basis in material processing. Dedicated to Prof. P. Ramasamy © 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim     

Preferred growth of bismuth single crystals

The results of the study of the factors influencing the orientation of the ( 111 ) cleavage plane of bismuth are presented. It has been observed that at high purity the growth rates and temperature gradients do influence the orientation whereas at low purities no such influence is observed.     

Low-angle boundary investigation in silicon single crystals

Low-angle boundaries in Si single crystals grown from the melt in [111], [100], [112], [110], [118] and [115] directions were investigated by chemical etching, copper decoration technique and X-ray topography. — LAB of four types were found in planes parallel to the crystal growth axes. Rearrangement of dislocations from slip bands into LAB was observed in heavily-doped Si crystals. The origin of LAB in melt grown Si crystals is discussed. It is shown that these boundaries are well interpreted in terms of dislocation alignment formation in the thermal stress field of the growing crystal.     

The growth of single crystals of Ni-W alloy under conditions of high temperature gradient

The structure of single crystals of the NV-4 nickel alloy containing 32–36 wt % W is investigated. The temperature gradient at the crystallization front and the velocity of the crystallization front are the variable parameters of directional crystallization. The degrees of structural perfection of the single crystals grown under different conditions are compared. The crystallization parameters providing growth of single crystals that have high structural perfection and can be successfully used as seeds for the growth of single-crystal blades are determined. Typical defects formed upon directional crystallization of single crystals of the Ni-W (35 wt %) alloy are examined. The studied defects are classified, and the factors responsible for the disturbance of the single-crystal structure are analyzed.     

The inhibitory growth mechanism of saccharides on the growth of ice crystals from aqueous solutions

The inhibitory growth mechanism controlling the growth of ice crystals due to the presence of saccharides, as in plant cells, was experimentally studied at low temperature. As a result, it was found that the growth rate of ice crystals in saccharide solutions depended on the type and concentration of the saccharides. The experimental facts can be explained by the concept that giant hydrated clusters of saccharides create the framework that causes a resistance to the volume diffusion of water molecules.     

Nucleation and growth kinetics for the crystallization of ice from dextrose solutions in a continuous stirred tank crystallizer with supercooled feed

The crystallization of ice in aqueous dextrose solutions is studied in an adiabatic continuous stirred tank crystallizer with a supercooled feed stream. The effective diameter of the ice crystals was determined for various values of mean crystal residence time, feed supercooling, magma density, stirring rate, and dextrose concentration. For all process conditions the supercooling was measured at 9-12 different locations in the crystallizer. These local supercoolings were averaged algebraically to yield the bulksupercooling. From the experimental results growth and nucleation rates have been calculated. By comparing the experimental growth rates to growth rates calculated by means of a mathematical model kinetics for the inbuilding of water molecules into the ice lattice have been determined. The growth rate appears to be directly proportional to the interface supercooling. The rate constant decreases exponentially with increasing weight percentage of dextrose in the solution. The nucleation rate was found to be directly proportional to total crystal surface per unit volume of suspension and proportional to the bulksupercooling to the power 2.1. Nucleation is believed to occur by breakage of dendrites from the surface of parent crystals.     

On the origin of supersaturation barriers during the growth of single crystals from aqueous solutions containing impurities – a review

A generalised treatment of the appearance of supersaturation barriers σ_d, σ* and σ** during the growth of single crystals is outlined from the standpoint of well‐defined critical values of relative step velocities on a face. The final theoretical expressions are based on the premise that: (1) there are critical values of the relative step velocities associated with different average distances between adsorbed impurity particles during instantaneous, time‐dependent and time‐independent adsorption of the impurity on the growing surface, (2) the growth rate of a face is proporptional to velocity of steps on the growing face, and (3) Freundlich and Langmuir adsorption isotherms apply for different impurities. The theoretical expressions are then used to critically analyse the experimental data on supersaturation barriers observed during the growth of ammonium oxalate monohydrate and potassium dihydrogen phosphate single crystals from aqueous solutions containing different impurities. It was found that: (1) Langmuir adsorption isotherm is more practical for the analysis of the experimental data of the dependence of supersaturation barriers σ_d, σ* and σ** on the concentration c_i of an impurity, and (2) the ratios σ_d/σ* and σ*/σ** of successive supersaturation barriers for an impurity either increases or remains constant with an increase in impurity concentration c_i, and may be explained in terms of the mechanism of adsorption of impurity particles. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)