An unusual type of unmixing in NaCl-KCl mixed crystals at room temperature

Water-catalysed decomposition of solid solutions of NaCl–KCl has been investigated at room temperature. The “wet” decomposition proceeds at abnormally high rate as compared to the usual “dry” decomposition. The formation and movement of the decomposition boundary is characteristic of the “wet” process. The influence of divalent impurities and partial decomposition of the usual type has been studied on the rate of the decomposition boundary movement. The mechanism of the unmixing is suggested which involves two interrelated process: the phase separation and formation of a system of interlocking microcracks acting as channels for the water transport from the surface into the bulk of the crystals. The open porosity resulting from the unmixing is confirmed experimentally.     

Coherent spinodal decomposition of equimolar mixed crystals of NaCl-KCl

Bulk density changes have been observed in equimolar mixed crystals of NaCl-KCl by flotation technique at RT. The results are accounted for on the basis of the theory of coherent spinodal transformation. The interdiffusion coefficient has been estimated from the kinetics of the transformation in the crystals.     

Solutal convection during growth of organic NLO crystals

Effect of solutal and thermal convection plays very important role when a large thermal gradient is applied during crystal growth. To address this problem, we have purified m.nitroaniline (m.NA) and m. dinitrobenzene (m.DNB) and studied crystal growth and effect of growth parameters on the optical quality. Crystals of pure and binary alloy of m.dinitrobenzene and m.nitroaniline were grown by vertical directional solidification method in a two zone transparent furnace. Effect of doping and temperature gradient on the solid-liquid interface morphology and quality of crystal was determined by studying the bulk transparency and nonlinearity.     

Understanding the growth mechanism of CuI crystals during gel growth experiments

Cuprous iodide crystals have been grown with decomplexation method in silica gel. Various crystal morphologies, such as polycrystalline aggregate, skeletal, dendrite, hopper crystals and regular tetrahedron crystals, were observed in different growth regions with an optical microscope. Their surfaces were photographed using a binocular metallographic microscope and the results were explained with the crystal growth mechanism which was determined by supersaturation. These observations support the general hypothesis that the concentration of reactant affects the relative growth rate by controlling the nucleation and diffusion. The mutual influence of the crystals grown in different regions was also discussed. Additionally, the suitable condition for getting regular tetrahedron crystals or large hopper crystals was obtained by changing the concentration of CuI·HI complex in the later period of crystal growth. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Defects in quartz crystals initiated during their hydrothermal growth

A complex investigation (performed by means of X-ray topography, γ-ray diffractometry, optical methods) of the defects in as-grown and tempered quartz single crystals has been described. The results should contribute to the improvement of the quality of hydrothermally synthetized quartz.     

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.     

Dendritic growth of snow crystals

The hypothesis that allows the interpretation of dendritic growth of a snow crystal in terms of diffusion-limited aggregation is criticized. The results of simulation of growth of quasi-two-dimensional crystals in two-and three-dimensional media based on the classical two-parametric model of diffusion-limited aggregation are used as an argument in this criticism. It is established that the model dimensionality considerably influences morphology of the grown crystal. The mechanism of dendritic growth of a snow crystal in which the main part is played by the surface processes at the ice/water interface is suggested.     

Iso-epitaxial growth of n-octacosane crystals

Iso-epitaxial(epilayer)growth of n-octacosane crystals is reported. The morphology of the epilayers is in the form of triangular growth islands with or without truncature. Epilayer growth precedes spiral growth mechanism which occurs at medium and low supersaturations, respectively. The occurrence of triangular and truncated rhombic platelets is found to be a manifestation of growth at medium supersaturations.     

Molybdenum-oxygen interaction during growth of refractory oxide crystals from the melt in vacuum

The behavior of the Mo: O₂ system in a temperature range of 2350?2500 K under a pressure of 1 × 10^?5 bar has been investigated. The compositions of gas and solid phases and the main chemical reactions describing molybdenum oxidation are determined for different ratios of the basic components. The thermodynamic calculations made it possible to recommend a applying a specific atmosphere during the growth of leucosapphire and aluminum yttrium garnet crystals from the melt by the Bagdasarov method.     

Study of inclusions and causes of their formation during growth of barium metaborate crystals

The complex study of barium metaborate crystals has been undertaken with the aim to determine the conditions of their growth with the minimum density of inclusions. It is shown that almost all the inclusions are multiphase formations of the melt type, and, in addition to the mixture of the main components of the quasieutectic composition, they can also be enriched with gas-forming impurities not removed during barium metaborate synthesis. It is shown that the amount and the composition of melt-like inclusions in crystals depends on the removal of by-products of the reactions used for barium metaborate synthesis from the system, the patterns of the convective flows in the flux, the character of its flow under the interface, the axial temperature gradient in the solution, and the temperature fluctuations at the crystallization front.     

Investigation of the competitive grain growth during solidification of single crystals of nickel-based superalloys

The competitive growth of columnar grains in a single-grain selector, which is used for directional solidification of single-crystal blades from nickel-based superalloys, has been investigated by electron back-scattered diffraction and local X-ray diffraction analysis. It has been found that the competitive grain growth in a starter block is determined by the crystallographic factor: rapidly growing grains with the axial orientation close to the [001] direction dominate in this part of the casting. For the competitive grain growth in a helicoidal separator, the geometric factor (the position of a grain at the input of the separator) is also important. The results obtained suggest that an appropriate geometry of the single-grain selector was chosen. In addition, the distribution of the orientations of columnar grains obtained by electron backscattered diffraction, can be used for approximate estimation of the yield of suitable (i.e., with the deviation of the axial orientation from the [001] direction within a specified tolerance) single-crystal blades.     

On the mechanism of trace-element uptake during the hydrothermal growth of sulfide mineral crystals

The stable species of cadmium and mercury are formed in iron and lead sulfides at elevated temperatures and pressures under hydrothermal conditions. Their formation proceeds through different mechanisms. The most general mechanism involves the uptake of trace elements in the form of structural species due to isomorphous exchange. The results obtained for cadmium in pyrrhotite indicate that structural defects substantially affect the incorporation of impurities into the crystal structure. It is found that trace elements are accumulated on the surface in the absence of defects and are incorporated into the crystal bulk in the form of structural impurities in the presence of defects. Moreover, at elevated temperatures and pressures, trace elements can form their own (surface nonautonomous) phases. These phases are formed not only at trace-element concentrations close to saturation (Cd in Fe_1.0S) but also under conditions far from saturation as a result of the interaction of trace elements with the oxidized surface of the mineral (the cadmium uptake by galena with the formation of either a sulfate surface phase or a sulfate-chloride surface phase). An important mechanism of trace-element uptake by sulfides during the hydrothermal growth of crystals is associated with external and internal adsorption of impurities by defects. This adsorption manifests itself both in an increase in the content of sorbed mercury in galena at a high sulfur activity due to the interaction of mercury with lead vacancies and in the formation of dislocation cadmium species, which results in an increase in the coefficient of cadmium distribution in the crystal-solution system.     

A statistical thermodynamic model for oxygen segregation during Czochralski growth of silicon single crystals

A thermodynamic model derived from atomic scale statistics is formulated for the crystal–melt interface where oxygen segregation occurs during silicon crystal growth by the Czochralski method. The model shows that the segregation coefficient is close to but less than unity. Approaches for controlling oxygen concentration in the resulted crystal are discussed.     

Spiral growth of potassium bichromate crystals

Some observations of growth spirals and hillocks on {001} surfaces of potassium bichromate crystals grown from aqueous solutions are presented and discussed. It is shown that a spiral pattern in fact represents a macrospiral developed as a result of the cooperative action of a group of dislocations.     

Dendritic growth forms of borax crystals

A variety of dendritic forms of borax grown from solutions by the film formation method is given. The changing growth morphology is followed as a function of concentration and temperature. The initial, intermediate and final growth morphologies are described and discussed. Influence of evaporation rate and supersaturation on the mechanism of growth is assessed. It is suggested that under all crystallization conditions, borax crystals have dendritic form in the initial stages of growth.     

The growth of HgI2 crystals

The paper presents a system for αHgI₂ crystal growth by the temperature oscillation method. The system has a capability of crystal growing at an excess I₂ or Hg vapour pressure. Optimum conditions for producing crystals up to 2 cm³ by volume have been established. The crystals grown at an excess I₂ vapour pressure have higher resistivity and higher drift electron and hole mobilities — μ_e = 120 cm² V⁻¹ s⁻¹ and μ_h = 6 cm² V⁻¹ s⁻¹, respectively.     

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.     

Solid-phase growth of CuGaS2 crystals

Homogenized mixtures of copper, gallium, and sulphur were annealed for 200 hrs. under vacuum at about 1000–1050°C (i.e. below the melting point of the CuGaS₂ phase) in quartz ampoules in a vertical position, and then cooled down in a temperature gradient. The final charge was made up of three well-defined portions: a yellow polycrystalline CuGaS₂ with large blocks of CuGaS₂ (close to stoichiometry), a black portion with darkgreen aggregates of small CuGaS₂ crystals, and yellow laminae of CuGaS₂ single crystals, slightly inclined away from [112] direction. — A coupled growth mechanism — solidstate-diffusion-assisted coalescence/vapour transport — is assumed.