Results of crystal growth of bismuth-antimony alloys (Bi100–xSbx) in a microgravity environment

Results of two experiments are presented for growth of crystals from (Bi_100–xSb_x) alloys in a microgravity environment. In the growth experiments different variants of the Bridgman technique were used. It was shown that in crystal growth from the melt in closed ampoules under microgravity conditions convection can be prevented completely. Therefore it is possible to grow crystals from melts of some components under diffusion controlled conditions of mass transfer. In microgravity a reduced interaction between the melt and the confining walls was observed even if they have large contact with each other. The investigation of surface morphology corroborated the importance of surface effects for crystal growth from the melt under microgravity conditions. Measurements of electronic properties of crystals grown in microgravity showed a good quality in comparison to earth grown crystals. Because under microgravity conditions in closed ampoules the diffusion controlled mass transfer can be realized and the interaction between the melt and confining wall is reduced, homogeneous crystals with high perfection can be grown melts of some components.     

The Synthesis of Crystals of Chalcogenides of K,Zr, Hf,Hg, and Some Other Elements in Halide Melts under Conditions of Stationary Temperature Gradient

Crystallography Reports - The growth of crystals of metals, alloys, chalcogenides, and pnictides in melts of alkali metal halides under conditions of stationary temperature gradient has been...     

Anomalous effects in dopant distribution in Ge single crystals grown by the floating zone technique aboard spacecrafts

Specific features of Ga-dopant distribution in Ge single crystals grown by the floating zone technique from lightly and heavily Ga-doped Ge melts aboard the Photon spacecrafts have been studied. An anomalously strong concentration dependence of the effective coefficient of Ga distribution in Ge crystallized from melts with a well-developed free surface, which has no analogues under the terrestrial conditions, has been established for the first time. This anomaly is interpreted as a consequence of specific concentration-dependent processes of heat and mass transfer in the molten zone, and, first of all, the intensification of the concentration-capillary convection with an increase of the doping level. The quantitative characteristics of the crystallization parameters necessary for the construction and verification of hydrodynamic models are obtained experimentally. The effect of the observed phenomena on the electrophysical homogeneity of the grown single crystals and possible methods for the control of dopant distribution in such crystals are discussed.     

Block (mosaic) structure as an indicator for cooling of crystals grown from a melt

The specific features of the formation of the block (mosaic) structure with low-angle misorientation of the block boundaries are investigated using fluorite and germanium crystals as an example. These structural features are considered an indicator of conditions under which plastic deformation and polygonization occur during cooling of the crystals grown from melts.     

Synthesis of chalcogenide and pnictide crystals in salt melts using a steady-state temperature gradient

Some examples of growing crystals of metals, alloys, chalcogenides, and pnictides in melts of halides of alkali metals and aluminum at a steady-state temperature gradient are described. Transport media are chosen to be salt melts of eutectic composition with the participation of LiCl, NaCl, KCl, RbCl, CsCl, AlCl₃, AlBr₃, KBr, and KI in a temperature range of 850–150°C. Some crystals have been synthesized only using a conducting contour. This technique of crystal growth is similar to the electrochemical method. In some cases, to exclude mutual influence, some elements have been isolated and forced to migrate to the crystal growth region through independent channels. As a result, crystals of desired quality have been obtained using no special equipment and with sizes sufficient for study under laboratory conditions.     

On the Mechanism of Crystal Growth from Undercooled Melts

The mechanism of crystal growth from undercooled melts is studied experimentally by means of a simple capillary technique. Thymol and Na₂S₂O₃ · 5 H₂O are used as model substances. Dislocation-free crystal faces of these compounds are obtained by continuous growth in the capillary or by prolonged annealing under appropriate conditions. Two-dimensional mechanism of growth was experimentally verified on such perfect crystal planes. The conditions under which the surface nuclation mechanism operates are described accounting for the supersaturation, the influence of impurities etc. Transition from two-dimensional into spiral growth of purposedly defected crystal faces is demonstrated and investigated. The mechanism of formation of perfect crystals is discussed and further experimental evidence on the possibility of two-dimensional growth from melts is given.     

Gas bubble associated mass transport in Bridgman-Stockbarger melt growth system

Experimental evidence has been obtained showing that gas bubbles may be responsible for melt stirring in Bridgman-Stockbarger growth system which results in Pfann type impurity distribution profiles along the crystal length. The hypothesis of the gas bubble associated mass transfer mechanism is supported by the production of Y₃Al₅O₁₂–Nd³⁺ single crystals under conditions which eliminate or limit gas bubble nucleation in the melts with Nd³⁺ distribution profiles similar to those generally observed in melt growth systems where the mass transport in the melts is limited to diffusion.     

Etching behaviour to MgO crystals in relation to chemical reactions

Experimental results of an investigation of the etching behaviour of {100} faces of MgO crystals under different etching conditions in acids, alkalies and inorganic salts are presented. It is observed that etching takes place in aqueous solutions of acids and acidic salts and in the melts of NaOH and KOH. On the basis of an analysis of the results it is suggested that the energetics of chemical reactions dictate the choice of etchants for a crystal.     

Concept of specific supersaturation (specific supercooling) and practical aspects of its application

It is demonstrated that the normal growth rate of crystals from solutions and melts depends on the area of the crystallization front. New growth parameters, namely, the specific supersaturation and the specific supercooling, are introduced, and the quantitative dependence of the normal growth rate on these parameters is determined. It is noted that the normal growth rate affects the formation of defects. The criteria and the range of the possible use of the specific supersaturation (specific supercooling) for optimizing the conditions of crystal growth from solutions and melts are revealed.     

Macroscopic Phenomena in Polymer Liquid Crystals

Rigid and semirigid backbone polymers based on either helical structures or conjugated aromatic bonding often exhibit nematic ordering in solution or as melts. Because of the large axial ratio of these molecules, the resulting nematic phases exhibit hiahly anisotropic elastic and viscous properties. The study of macroscopic phenomena in these liquid crystals serves two purposes. First is the elucidation of the relationship between macroscopic material parameters and molecular properties. Second is the understanding of the consequence of the highly anisotropic nature of these materials, namely new phenomena not observed in low molecular weight liquid crystals. Several new phenomena will be described related to the Frederiks transition and flow instabilities.     

Influence of F-Anions on the Habit and Growth Mechanism of Potassium-rich Feldspars

Orthoclase-rich alkalic feldspar crystals were grown from melts next to Orthoclase-Albite-Quartz eutectic composition at 2 kb in the presence of a surplus of a supercritical aqueous phase by isobaric cooling at a rate of approximately 10 °C/day. Some runs were made with KF-bearing fluid phase, in two runs the melt was formed during intensive thermoconvection of the fluid phase. Tabular crystals were only formed from F-bearing melts, crystals grown from F-free melt were always fan-shaped. We assume this fact is due to a more easy supply of Al to the growing crystal in melts with F content.     

Phase equilibria and prospects of crystal growth in the system LiF–GdF3–LuF3

Scheelite type LiGdF₄, LiLuF₄, and mixtures of both end members were prepared by a hydrofluorination route from the rare earth oxides and commercial LiF. The samples were purified by melting in HF/Ar mixtures, and were investigated by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and energy dispersive X‐ray spectroscopy (EDX) techniques. Both end members show unlimited miscibility in the solid phase. Mixed crystals containing at least 65 mol‐% LiLuF₄ melt under direct formation of the liquid phase. The gap between solidus and liquidus is narrow. LiGdF₄ and mixed crystals with less then 65 mol‐% LiLuF₄ decompose peritectically under formation of (Gd,Lu)F₃. Crystal growth is expected to be possible either from Lu‐rich melts with the appropriate scheelite composition or from Gd‐rich melts containing an excess of LiF. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Initial stages of nucleation of carbide phases in oxide melts

The initial stages of nucleation of Mo₂C and W₂C crystals in the tungstate-molybdate-carbonate melts of the known compositions on Ag, Au, Cu, Pt, and Ni substrates are studied by the method of electrodeposition depending on the electrocrystallization conditions—temperature, deposition time, initial current pulse, and current density.     

Ionic Conductivity of KTiOPO<Subscript>4</Subscript> Single Crystals Grown by Flux Crystallization under Different Conditions

The ionic conductivity of three KTiOPO₄ crystals grown from high-temperature solution–melts in combination with the Czochralski technique under different conditions has been investigated. The first crystal was obtained at a cooling rate V_g = 0.2–0.5 mm/day and a ratio of potassium and phosphorus concentrations in the solution–melt [K]/[P] = 2. The other two crystals were grown at a much higher velocity (V_g = 3–7 mm/day) from solution–melts with [K]/[P] = 1.5 and 1. It is shown that the crystal grown upon slow cooling at [K]/[P] = 2 has the lowest ionic conductivity: σ_||c = 1.0 × 10^–5 and 3 × 10^–11 S/cm at 573 and 293 K, respectively.     

Applications of the rheo-dielectric technique

An apparatus for dynamical mechanic measurements was modified in order to allow broadband dielectric spectroscopy measurements (10 mHz to 10 MHz) on systems under shear stress. Shear oscillatory deformation was applied to different systems under conditions of controlled deformation amplitude, choosing different shear frequencies and amplitudes, including the range of the non-linear viscoelastic response. The plate–plate geometry was used, with the two metallic sample holders serving both as mechanical tools and as electrodes of a capacitor. The rheo-dielectric technique was used to study the effect of oscillatory shear on two classes of systems: nematic liquid crystals and polymeric melts.     

Investigation of the variations in the crystallization front shape during growth of gadolinium gallium and terbium gallium crystals by the Czochralski method

Numerical investigation of the variations in the crystallization front shape during growth of gadolinium gallium and terbium gallium garnet crystals in the same thermal zone and comparison of the obtained results with the experimental data have been performed. It is shown that the difference in the behavior of the crystallization front during growth of the crystals is related to their different transparency in the IR region. In gadolinium gallium garnet crystals, which are transparent to thermal radiation, a crystallization front, strongly convex toward the melt, is formed in the growth stage, which extremely rapidly melts under forced convection. Numerical analysis of this process has been performed within the quasistationary and nonstationary models. At the same time, in terbium gallium garnet crystals, which are characterized by strong absorption of thermal radiation, the phase boundary shape changes fairly smoothly and with a small amplitude. In this case, as the crystal is pulled, the crystallization front tends to become convex toward the crystal bulk.     

A study of the chemical reactions involved in Li–Ca–N system

The chemical reactions and phases involved in the potential flux system of Li–Ca–N for the growth of bulk GaN crystals have been investigated under varying conditions. It is found that no preferential nitrification of Li or Ca by N₂ in Li–Ca melts at 500 °C. Only the ternary compound LiCaN is identified in the Li–Ca–N system under the present experimental conditions. Static N₂ pressures are found to enhance the formation of LiCaN compared to an N₂ stream. LiCaN forms from two possible pathways: one is a modified metathesis chemical reaction represented by Li₃N+Ca→LiCaN+2Li, and the other is a combination chemical reaction represented by Li₃N+Ca₃N₂→3LiCaN. The formation of LiCaN by the metathesis reaction is thermodynamically favored over the other pathway. In addition, the formation of LiCaN might benefit from a slightly larger initial amount of Li₃N compared with Ca or Ca₃N₂.     

A Pragmatic Model for the Simulation of Self-induced Striations in Quartz Crystals

A mechanism is investigated which might lead to fluctuations in the growth of quartz crystals due to the periodic incorporation of impurities. The model ought to be applicable also to the incorporation of stoichiometric partners during the growth of complicated crystals from non-stoichiometric melts. The processes have been simulated with help of an equivalent circuit experimentally and by computation. Some typical cases are presented in the paper.     

Formation of dislocations in Sb crystals grown by pulling from melts

Results of experimental investigations on regularities of the formation of dislocations in crystals of pure antimony during pulling from melts are reported. The mean density of dislocations is compared with the values calculated by different theories.     

Bridgman growth and properties of potassium lithium niobate single crystals

Transparent KLN crystals 10mm in diameter and 25 to 45mm in length have been grown by the modified vertical Bridgman technique from different melts in the range of 3035mol% K₂O, 1723mol% Li₂O and 4350mol% Nb₂O₅. The growth conditions are a growth rate of less than 0.25 mm/hr, temperature gradient in solid-liquid interface of 23 °C/mm and growth direction of <110>. As-grown KLN crystals have tetragonal tungsten bronze structure. Most of the as-grown crystals do not crack when cooling through the paraelectric/ferroelectric phase transition. 180° domain structures are observed after the KLN crystal was etched in boiling 2HNO₃:Hf. Dielectric properties and transmission spectrum of the as-grown KLN crystals are measured.