Spontaneous crystallization of potassium chloride from aqueous and aqueous‐ethanol solutions; Part 3: Model of the crystallization process

A model of spontaneous crystallization process was proposed. The model describes kinetics of the crystallization process after the end of the induction period. To test the model the published earlier data on crystallization and aggregation kinetics of potassium chloride at its spontaneous crystallization from supersaturated aqueous and aqueous‐ethanol solutions were used. It was found excellent coincidence of the experimental and theoretical data on concentration of the salt and the total number of crystals in solution at crystallization. Somewhat change for the worse was at the theoretical calculations of crystal size distribution at the end of the crystallization process. It indicated that the ways of calculation of size of crystals and their weight fraction in deposit were very approximate. The model allows predicting with satisfactory accuracy kinetics of crystallization using such general parameters of potassium chloride as the specific surface energy and the height of the nucleus‐bridges between crystals at coalescence. It needs further test of the model for other salts. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spontaneous crystallization of sodium chloride from aqueous‐ethanol solutions; Part 3: Examination of a model of the crystallization process

In the present work the examination of the proposed earlier model of spontaneous crystallization process was done. The model describes kinetics of the crystallization process after the end of the induction period. To test the model the published data on crystallization and aggregation kinetics of sodium chloride at its spontaneous crystallization from supersaturated aqueous‐ethanol solutions were used. It was found an excellent coincidence of the experimental and theoretical data on concentration of the salt and the total number of crystals in solution at crystallization. The model allows predicting with satisfactory accuracy kinetics of crystallization using such general parameters of sodium chloride as the specific surface energy and the height of the nucleus‐bridges between crystals at coalescence. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spontaneous crystallization of potassium chloride from aqueous and aqueous‐ethanol solutions Part 1: Kinetics and mechanism of the crystallization process

Kinetics of spontaneous crystallization of potassium chloride from aqueous and aqueous‐ethanol solutions were studied. During the crystallization of the salt the electrical conductance and optical transmission of the supersaturated solutions were measured automatically. For monitoring of the total surface of growing potassium chloride crystals at the crystallization the turbidimetric method was used. The growth rate and activation energy of the crystals were determined. The crystal growth rate was proportional to supersaturation. When the volume fraction of ethanol in the solution increased from 0 to 25.76%, the activation energy of the growth process did not change and was about 60 kJ·mol^‐1. Aggregation of the crystals was found. The aggregation kinetics of the crystals may be described approximately by the famous Smoluchowski equation for coagulation of colloidal particles. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mechanism of aggregation and intergrowth of crystals during bulk crystallization from solutions

Available literature data on aggregation kinetics of crystals of a number of salts during their bulk crystallization from solutions have been analysed. The proposed earlier mechanism of aggregation and intergrowth of crystals during bulk crystallization owing to formation of nucleus‐bridges between crystals was tested and confirmed. The aggregation kinetics of crystals was described by the familiar Smoluchowski equation for coagulation of colloidal particles. However, in a bulk crystallization process, the aggregation constant in this equation decreased as supersaturation in a solution lowered. An expression for the aggregation constant in this equation was proposed. The proposed mechanism of crystal intergrowth duringt bulk crystallization allowed evaluating the specific surface energy of tested salts, which turned out to be in reasonable agreement with published literature data. It was concluded that the intergrowth of crystals during bulk crystallization from solutions proceeded via formation of nucleus‐bridges between crystals. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Part 1: Kinetics and mechanism of the crystallization process

The kinetics of spontaneous crystallization of sodium chloride from aqueous‐ethanol solutions were studied. During the crystallization the electrical conductance and optical transmission of the supersaturated solutions were measured automatically. For monitoring of the total surface of growing potassium chloride crystals at the crystallization the turbidimetric method was used. The growth rate and activation energy were determined. The crystal growth rate was proportional to supersaturation. When the volume fraction of ethanol in solution increased from 14.85 to 29.72%, the activation energy of the growth process did not change and was about 50 kJ· mol^‐1. Aggregation of the crystals was found. The aggregation kinetics of the crystals may be described approximately by the famous Smoluchowski equation for coagulation of colloidal particles. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis of nano‐crystalline zeolite β: Effects of crystallization parameters

The effects of variation in Si/Al ratio (25 and 100) and crystallization temperature (80 °C to 180 °C, at an interval of 20 K) on crystal size of zeolite β were studied. Products obtained at different synthesis parameters were characterized by powder X‐ray diffraction, IR spectroscopy, thermal analysis, scanning electron microscopy and nitrogen adsorption. Increase in crystal size with crystallization temperature and Si/Al molar ratio was observed. Crystal morphology at 140 °C was spherical whereas at 180 °C it was of irregular shape. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spectral and conductivity features of novel ternary Tl1–xIn1–xSnxS2 crystals

Complex studies of novel Tl_1–xIn_1–xSn_xS₂ single crystalline alloys (x = 0.1; 0.2; 0.3; 0.4; 0.5) were performed. Here we present the study of the effect of the partial cation substitution of Sn ions by In ions on their optical absorption and photoconductivity characteristics. The dependences of optical and electric properties of the crystals of the crystalline solid solutions on temperature and composition are discussed within a framework of intrinsic defect states forming then fundamental absorption. The calculations of the charged defects responsible for the experimentally determined values of Δ₀ were done. Some increase of the concentration for charged defects with the sample temperature is probably related to the thermal ionization of some of the defects that were neutral at lower temperature. The results obtained will be analyzed within the framework of the intrinsic defect's model.     

Heterogeneous Nucleation of Hen‐Egg‐White Lysozyme — Molecular Approach

The heterogeneous nucleation of hen‐egg‐white lysozyme (HEWL) crystals has been repeatedly investigated using a double‐(thermal)‐pulse technique, thus detaching nucleation from growth stage. n(t) dependencies of the nucleus number n, on templates of poly‐L‐lysine, vs time, t were plotted and the steady‐state nucleation rates I were determined. They were compared with the results obtained earlier for surfaces rendered hydrophobic (by means of hexamethyl‐disilazane) as well as for bare glass surfaces. In the present paper we determine the number of HEWL molecules in the (heterogeneously formed) critical nucleus. It turned out that it is build of 3 (to 4) HEWL molecules on glass substrate and 8 molecules for both hexamethyl‐disilazane and poly‐L‐lysine templates. The energy Ak required for heterogeneous formation of a critical nucleus on poly‐L‐lysine has been calculated, on the basis of the steady‐state nucleation rates I. Intermolecular binding energy in the HEWL crystal lattice has been estimated again (approximately 10‐9 erg/molecule). This time the basis was the adhesion of HEWL crystals to poly‐L‐lysine substrate.     

Crystallization features of YBa2Cu3O7‐δ in the Y2BaCuO5‐BaCuO2‐CuO and Y2Cu2O5‐BaCuO2 systems

Based on the data of X‐ray phase and microstructure analysis, the sample composition was optimized in order to provide maximum size of the textured macrograins of YBa₂Cu₃O_7‐δ and of the crystallites in the Y₂BaCuO₅‐BaCuO₂‐CuO, Y₂Cu₂O₅‐BaCuO₂ systems. The growth rate has been studied and the YBa₂Cu₃O_7‐δ growth activation energy has been calculated for the samples of Y₂BaCuO₅+3BaCuO₂+2.3CuO, Y₂BaCuO₅+3BaCuO₂+0.6CuO, and Y₂Cu₂O₅+3.5BaCuO₂ compounds in the temperature range of 1240‐1270K for the case of use of the Y₂Cu₂O₅ and Y₂BaCuO₅ precursors with an average grain diameter of 10 μm and 1mm. A crystallization mechanism of YBa₂Cu₃O_7‐δ in the Y₂BaCuO₅‐BaCuO₂‐CuO and Y₂Cu₂O₅‐BaCuO₂ systems in the case of different sizes of Y₂BaCuO₅ and Y₂Cu₂O₅ precursor grains was proposed and validated. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Characteristics of the Mixed Crystals of the KNO3‐NH4NO3‐H2O System at 298 K

The equilibrium studies of the KNO₃‐NH₄NO₃‐H₂O system at 298 K have been conducted. The isotherm of solubility and the curve of distribution have been mathematically described. The parameters of the crystal lattice and the enthalpy of crystallization of the [K_x(NH₄)_1‐x]NO₃ solid solutions as a function of their composition have been presented. The structure of salts KNO₃�xNH₄NO₃ with different x values have been solved and refined.     

Growth and characterization of superconductive Nd2‐xCexCuO4‐y single crystals

Nd_2‐xCe_xCuO_4‐y single crystals with different x values have been successfully grown by Traveling Solvent Floating Zone method (TSFZ). Electronic properties of Nd_2‐xCe_xCuO_4‐y single crystals with various x have been studied in detail. The results show that superconductivity can be found in crystals with x > 0.1 (0.13‐0.18) directly grown at oxygen‐reduced atmosphere without post‐annealed, while no superconductivity appears in crystals with x < 0.1 at the same atmosphere. It is also found that, the segregation coefficient of Ce is determined to be 0.946 and transition temperature Tc (onset) reaches maximum value of 23.5 K at nominal composition x = 0.165. With further increase of Ce concentration, transition temperature of single crystals declines due to the precipitation of secondary Phases. In addition, the variation of lattice constants of Nd_2‐xCe_xCuO_4‐y single crystals with different x is also given. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystallization of silicalite‐1 from clear synthesis solutions: Effect of template concentration on crystallization rate and crystal size

Synthesis of silicalite‐1 powders and membranes from initially clear solutions with different tetrapropylammonium hydroxide or bromide concentrations was studied. While tetrapropylammonium bromide acts only as template, tetrapropylammonium hydroxide provides both the template and hydroxyl ions to the synthesis medium. The effects of template and hydroxyl ion concentration on the product yield, crystallization rate and crystal size were investigated. Pure and highly crystalline silicalite‐1 was obtained with all compositions. The nucleation time decreases from 100 h to 20 h and the crystal size decreases from 3.5 μm to 0.35 μm as the template amount x is increased from 5 to 30 moles at a batch composition of 80SiO₂.xTPAOH.1500H₂O at 95 °C. Yield of silicalite‐1 passes through a maximum at intermediate TPA concentration. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation of strontium chloride hexahydrate by batch‐cooling crystallization: Control of crystal size and morphology

The effects of initial supersaturation, cooling rate, and stirring rate on the morphology of strontium chloride hexahydrate (SrCl₂·6H₂O) crystals were investigated by batch‐cooling crystallization, providing optimal operating conditions. Uniform needle‐like crystals with a length of 1200.50 μm and a width of 100.92 μm on average were obtained. The corresponding aspect ratio of length to width was about 11.90. Moreover, the morphological modification of SrCl₂·6H₂O crystals using cetyltrimethylammonium chloride (CTAC) was studied. When 20.30 mmol·L⁻¹ of CTAC was added, the length and width of crystals were 792.71 μm and 233.25 μm, respectively, and the corresponding aspect ratio decreased to 3.40. The shape of SrCl₂·6H₂O crystal changed to granule‐like, probably because of the strong interaction of CTAC with the SrCl₂·6H₂O facets with a denser distribution of Cl⁻ ions. This study offers a simple, flexible, and highly efficient approach to regulate the morphology of SrCl₂·6H₂O crystals and opportunities for multiple applications of SrCl₂·6H₂O.     

Electronic Structure and X‐Ray Photoelectron Spectroscopy of Wurtzite GaxAl1‐xN Solid Alloy

Band energy structure of Ga_xAl_1‐xN solid alloy (x=0.15; 0.65) is calculated. All one‐electron methods of band energy calculations (self‐consistent norm‐conserving pseudo‐potential (NCPP), linear muffin tin orbital method (LMTO) and linear combination atomic orbital (LCAO)) methods give considerable disagreement with the experimental data. We demonstrate that better agreement with experiment is achieved when the NCPP wavefunctions are orthogonalised to the core‐like states. Simultaneously additional geometry optimization improve the agreement with the optical data. We have carried out theoretical and experimental investigation of the band density of states for two representatives of the investigated wurtzite Ga_xAl_1‐xN solid alloys (with x=0.15 and 0.65).     

Effect of temperature and isomorphic atom substitution on optical absorption edge of TlInS2xSe2(1‐x) mixed crystals (0.25 ≤ x ≤ 1)

The optical properties of the TlInS_2xSe_2(1‐x)mixed crystals (0.25 ≤ x ≤ 1) have been investigated through the transmission and reflection measurements in the wavelength range of 400–1100 nm. The optical indirect band gap energies were determined by means of the analysis of the absorption data. It was found that the energy band gaps decrease with the increase of selenium atoms content in the TlInS_2xSe_2(1‐x)mixed crystals. The transmission measurements carried out in the temperature range of 10–300 K revealed that the rates of change of the indirect band gaps with temperature are γ = –9.2×10^–4 eV/K, –6.1×10^–4 eV/K, –4.7×10^–4 eV/K and –5.6×10^–4 eV/K for TlInS₂, TlInS_1.5Se_0.5, TlInSSe and TlInS_0.5Se_1.5 crystals, respectively. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Interplay between phase formation mechanisms and magnetism in the Sr2FeMoO6–δ metal‐oxide compound

Double perovskites with the structural formula A₂BB′O_6±δ, especially the strontium ferromolybdate Sr₂FeMoO_6–δ, have attracted a lot of attention due to their unique magnetic and electrical properties and a possible application in spintronic devices. However, a strict correlation between the functional characteristics of Sr₂FeMoO_6–δ and its synthesis technology has been lacking up to date. Thus, we have studied in the present work the crystallization kinetics of Sr₂FeMoO_6–δ using reagents with different pre‐history as well as the structural and magnetic properties of the obtained compounds. Differences in the crystallization kinetics as well as higher magnetic inhomogeneity of Sr₂FeMoO_6–δ synthesised from a mixture of MoO₃, Fe₂O₃, SrCO₃ in comparison with the compound synthesised from the SrFeO_2.5 and SrMoO_4–y precursors have been found and interpreted. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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)     

Growth of big single crystals of Sr2YRu1‐xCuxO6 from high temperature solutions

Single crystals of Sr₂YRu_1‐xCu_xO₆ (x = 0 ‐ 0.4) have been grown from PbO‐PbF₂ based solutions in the temperature range 1150 – 1350°C. A silicon carbide heating element furnace (with a recrystallized alumina tube lining) in a vertical configuration was used to grow the crystals in platinum crucibles. Conditions for the stable growth of big crystals have been investigated. The morphology of the crystals containing Cu was found to change from octahedral to cube octahedral as the growth temperature is increased from 1150 to 1350°C. Crystals measuring up to 4.5 mm across and 2.5 mm thick have been grown from 1250°C. The incorporation of Cu into the crystals was ascertained by EDS and x‐ray diffraction analysis. A diamagnetic transition which increased in magnitude and temperature with x was observed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation and investigation of (CuInSe2)x(2ZnSe)1‐x and (CuInTe2)x(2ZnTe)1‐x solid solution crystals

The (CuInSe₂)_x(2ZnSe)_1‐x and (CuInTe₂)_x(2ZnTe)_1‐x solid solution crystals prepared by Bridgman method and chemical vapor transport have been studied. The nature of the crystalline phases, the local structure homogeneity and composition of these materials have been investigated by X‐ray diffraction (XRD) and Electron Probe Microanalysis (EPMA) methods. The analysis revealed the presence of chalcopyrite‐sphalerite phase transition between 0.6 ≤ X ≤ 0.7. Lattice constants, value of σ position parameter and bond length between atoms were also calculated. It was found that the lattice parameters exhibit a linear dependence versus composition. The transmission spectra of solid solution crystals in the region of the main absorption edge were studied. It was established that the optical band gap of these materials changes non‐linearly with the X composition. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of x‐irradiation on indentation size effect and formation of cracks for [Ky(NH4)1‐y]2ZnCl4 mixed crystals

The effects of x‐beam irradiation with different doses on microhardness and its related physical constants on [K_y(NH₄)_1‐y]₂ZnCl₄ mixed crystals with concentrations, y equals 0.000, 0.232, 0.644, 0.859 or 1.000 has been studied. The tests were performed for x‐doses from 0.2 kGy up to 1.6 kGy for loads from 20 to 160 g. The variation of hardness on (010) faces of orthorhombic [K_y(NH₄)_1‐y]₂ZnCl₄ mixed crystals with load were studied. The experimental results showed that, the hardness decreased as the x‐doses increased. Variation of the microhardness follows the normal ISE trend for low x‐doses and un‐irradiated crystals, then follows the reverse ISE trend for high x‐doses. Analysis of the experimental results revealed that: the radial cracks length, indentation size and applied indentation load are mutually related, and these dependences related with fracture mechanics are the basis of Meyer's empirical law. Indentation size effect (ISE) can be explained satisfactory by Hays‐Kendall's approach and proportional specimen resistance model. Brittleness of two cracks system are applicable for characterizing cracks around indentation impression (i.e. radial cracks) and another is (lateral cracks) for [K_y(NH₄)_1‐y]₂ZnCl₄ mixed crystals, crystals in the load range 60 – 160 g. It is shown that indentation induced microhardness decreases, whereas the length of radial cracks induced on indentation increases with the increase of x‐doses. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)