Growth aspects of barium oxalate monohydrate single crystals in gel medium

Single crystals of barium oxalate monohydrate (BaC₂O₄.H₂O, BOM) were grown in pure form by controlled diffusion of Ba²⁺ using the gel technique at different temperatures. Starting from aqueous Ba²⁺ chloride (BaCl₂) and acetic acid (C₂H₂O₄) in gel, this method offers a low‐cost and an easiest alternative to other preparation methods for the production of barium oxalate bulky single crystals. The optimal conditions for the growth of BOM crystals in silica gel were found by investigating different growth parameters such as gel pH, gel aging and crystallization temperature. Irrespective of all such crystallization environments, growth rate of the crystals were initially less and then exhibited supersaturation effect leading to non‐linearity. Gel aging and temperature has profound effect on nucleation density that resulted less number of crystals of maximum size in the gel matrix. Perfect single crystals were grown on gels of higher pH. The macropore morphology and porosity was controlled by changing age of the gel. It has been found that temperature has a fabulous effect in controlling the nucleation density by altering the supersaturation conditions for the formation of critical nuclei. The entire growth kinetics informed that the grown crystals were derived by the one dimensional diffusion controlled process. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Haze in sapphire crystals grown by SAPMAC method

Haze defect in SAPMAC method grown sapphire crystal was studied in detail. It is shown that haze is composed by a large number of CO₂ bubbles, and haze always appears in the axis region of the crystal since the bubbles formed in front of the crystallization surface are most always draged to the convection rolls in front of the central part of the crystallizaiton surface by melt and then engulfed by the rolls. Moreover, the effects of pulling rate on the formation of haze were analyzed and means for restraining haze was suggested. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and properties of KTP crystals from a new flux

High-quality KTiOPO₄ (KTP) crystals were grown by a top seeded solution growth (TSSG) method using K₈P₆O₁₉–BaF₂ as a flux. The volatility of different solvents, such as K₈P₆O₁₉ (K₈), K₈–NaF, K₈–KF, and K₈–BaF₂, was measured. These fluoride additives in K₈ fluxes and their compositional effects on the growth of KTP crystals were studied and discussed. The transmissivity and optical homogeneity of KTP crystals were also measured.     

Control over gas bubble distribution in shaped sapphire crystals

Bubble distribution in shaped sapphire crystals grown by Stepanov technique is found to depend on the field of velocities of the melt flows between the top surface of a die and the crystallization front. A die design defining the flows in the meniscus is a tool for control over the bubble distribution.     

Growth mechanism of crystals from aqueous solutions

Microscopic processes occurring on the surface of a growing crystal or a dissolving one were observed by microcinematography. The crystals under observation were grown either in a drop of solution by evaporation or in a constant-temperature microscope stage at a chosen supersaturation. Small (approx. 0.1 mm) and large (approx. 10 mm) crystals of NaCl, Pb(NO₃)₂, NaNO₃, CdI₂, KDP and ADP were studied. It is concluded qualitatively that the layers, in general polygonal, originating in one or several active centres, are formed on the crystal face, never at the corners or edges. – The average velocity of layer motion was studied quantitatively in dependence on their thickness and supersaturation. The layer motion at constant supersaturation considerably fluctuated. – Surface patterns created by moving layers agree in most cases with predictions of the dislocation theory. Two categories of steps were found on the surface: ”︁real”︁ macrosteps and shock waves. – The velocity of layer motion for most compounds lies within (1–10) · 10⁻⁴ cm · s⁻¹.     

On the void distribution and size in shaped sapphire crystals

Ribbon and rod sapphire pulling has been performed in three different crystal growth equipments in order to study the effect of the installation, of the atmosphere, of the die shape, of the feed material and of the pulling rate on the distribution, number and diameter of the characteristic voids (micro‐bubbles) in the crystals. The location of the bubbles in the crystals depends on the die geometry; however, in most cases they are essentially located close to the crystal periphery and then can be efficiently removed by lapping. After statistical analysis of the results, it is demonstrated that the number of gas moles incorporated in the crystals, inside the voids, is totally independent of any growth parameter. It is also shown that the bubble diameter depends only on the pulling rate. Consequently, for a given pulling rate, the number of bubbles auto‐adjusts in order to satisfy the constant molar gas incorporation. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of sapphire by the kyropoulus method

Single crystals of leuko-sapphire (α-Al₂O₃) are grown from the melt by an improved Kyropoulos technique using Mo or W crucibles. The influence of the temperature field on the structural and optical quality has been studied. Best results were obtained when heat losses from crystal and melt are minimized.     

Growth of single crystals of potassium chloride from gels

The growth of single crystals of potassium chloride (KCl) in silica gels upto 3 × 3 × 3 mm³ in size is described. To crystallize KCl incorporated in the gel, hydrochloric acid has been used to crystallize the KCl in the gel media. The crystalline perfection has been studied by the chemical etch pit technique.     

Growth morphology of potassium bichromate crystals from aqueous solutions

The results of a systematic study of growth morphology of potassium bichromate single crystals on seeds and by self-nucleation at different temperatures and supersaturations are presented. The observed growth morphology is analysed in the light of theoretical predictions. An increase in both growth temperature and supersaturation leads to a symmetrical morphology, and the transition from asymmetrical to symmetrical morphology is monotonous. The predicted sequence of morphological importance agrees only in the case of crystals grown at low temperatures and supersaturations.     

Growth of KDP crystals from solutions with mechanical impurities

Growth of KDP crystals from aqueous solutions with SiO₂ particles whose size ranges from 10^–2 to 400 μm in the static and dynamic modes has been studied. The effect of mother-solution supersaturation and particle size and concentration on the process of particle capture by a growing crystal is considered as well as types of inhomogeneities formed in the crystal under the influence of these factors. It is shown that the larger the particle size, the higher the probability of particle capture by a crystal. The influence of supersaturation, growth rate, face morphology, and particle concentration on particle capture and defect formation in crystals is also discussed.     

Growth of new optical crystals from boron-containing fluxed melts

The reactivity of viscous melts has been analyzed as a function of the degree of anion polymerization. Precise techniques are proposed for detecting the phase relations and kinetics of high-temperature crystallization of new materials in four-to-six-component borate and tantalate systems. For rare earth polyfunctional borates RAl₃(BO₃)₄ (R = Y or lanthanides), efficient frequency conversion with parameters greatly exceeding the corresponding parameters for known media has been demonstrated for the first time. The conditions for fabrication of refractory optical single crystals having high thermal conductivity and other record characteristics (in comparison with the known diode-pumped lasers) for femtosecond integrated optics have been optimized. Bases of the laboratory technology of growth of such crystals are considered.     

Growth morphology of crystals from kinetic and geometric perspective

The growth morphology of crystals is analysed from macroscopic point of view and the recent developments in this field are surveyed. In particular, it is shown that, for specific geometry characterized by interfacial angles a given face can increase in size for a very wide range of relative growth rates and need not be the slow‐growing face. Even growing faster than the neighbouring faces, such a face can increase in its size. However, there are other faces with other specific geometry, which can decrease the size growing more slowly than one of the neighbouring faces. If the growth time is sufficiently long, such a face may disappear and not be represented in the final crystal morphology. In the present survey the concepts given earlier [5] are extended. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Basal twinning as a room temperature plastic deformation mode of chromium- and titanium-doped sapphire crystals

The degree of basal twinning caused by indentation was determined quantitatively on a series of chromium and titanium doped sapphire crystals with variours dopant contents. It depends on the dopant content significantly. The effect of titanium ions in reducing the degree of twinning is more pronounced than that of chromium dopants. This “size effect” of the dopant ions is consistent with the mechanism of basal twinning as reported by KRONBERG .     

The mechanism of formation of light-scattering centers in sapphire crystals grown in gas atmospheres

The mechanism of the formation of light-scattering centers in sapphire crystals grown by horizontal directed crystallization in gas atmospheres reducing with respect to the Al₂O₃ melt was studied. The experimental regularities here significantly differ from those observed upon formation of other conventional defects in sapphire crystals (vacancy pores, gas bubbles, and so on). It is shown that the known formation mechanisms of macroscopic ≥1 mm) inclusions in crystals are not acceptable in this case. Using the model of bulk crystallization is proposed to describe the obtained regularities.     

Modeling habit forms of sapphire crystals using the principles of periodic-bond-chain method

The potential of the periodic-bond-chain method for calculating the sequence of manifestation of faces of corundum single crystals is considered. The leading role of the faces of the pinacoid, high rhombohedron, and hexagonal prism is demonstrated. The calculation results are compared with the experimental data on faceting the lateral surface of cylindrical sapphire single crystals grown by the Stepanov method and with the faceting data for crystals grown by the flux method and natural crystals.

     

Growth of large single crystals of stearic acid from solution

The growing of large single crystals of stearic acid (CH₃(CH₂)₁₆COOH), one of the saturated fatty acids, from organic solutions is reported. There are two parts in this experimental investigation; (1) to clarify the relationship between polymorphism of stearic acid having three modifications, namely, the A-form (orthorhombic), the B- and C-forms (monoclinic) and the growth conditions in the case of solution growth, because this complicated relationship has apparently made it very difficult to obtain large single crystals, (2) to produce large single crystals of stearic acid under the optimum growth conditions according to part (1). As a result large single crystals of stearic acid more than 8 × 8 mm² in area of the B- and C-modifications were successfully obtained. The values of long spacing, the crystal habit and the cleavage plane are also reported.