Effect of sodium chloride on gelatinization of silicic acid and the formation of novel polysilicic acid crystals

Silicic acid high purity was produced by passing sodium silicate solution under pressure through a column packed with ion exchange resin. The effect of sodium chloride concentration on the gel formation process was studied. We proposed that, besides amorphous sol, chain-like compounds were also formed in the silicic acid gelatinization process. Polysilicic acid solution was obtained by slow polymerization of the silicic acid at lower temperature. Some organic solvents were added to this solution at room temperature, resulting in a novel material species - partial crystallized polysilicic acid powder. Characterization techniques such as XRD, IR, TEM, TGA and DTA were used to investigate the materials. The new findings in the present study supported the previously proposed mechanism for silicic acid chain polymerization.     

On the influence on tracht and habit of urea crystals (II)

It is shown that during crystallization urea forms mixed crystals, adsorption mixed crystals, and epitaxial growth from aqueous solutions with inorganic salts, the lattice parameters of which are lying within the tolerance of efficiency parameters of urea. The extent of the formation of mixed crystals is determined by the composition of the solution. The slats added to the solution affect changes of tracht and habit of urea and thus are important for improving storage and transport properties. Similar results can be found for modified melts of urea.     

Effect of different solvents on the habit of meta‐nitroaniline single crystals

The habit of the organic non‐linear optical material meta‐nitroaniline (mNA) crystallized from different organic solvents such as acetone, benzene, ethyl acetate, n‐hexane, methanol and toluene were studied. Solubility of mNA in these solvents at various temperatures in the range between 288 and 323 K was determined by gravimetric method. Crystals were grown by restricted evaporation of solvents method. Solutions with different solvents having different chemical nature and polarity yielded crystals with different habits: one‐dimensional needles, two‐dimensional rhombic platelets and three‐dimensional octahedral. In addition, the mNA crystals show unidirectional growth behaviour along its polar [001] direction irrespective of the solvents used. All the grown crystals were found to be orthorhombic system with point group mm2 and space group Pbc2₁ which was confirmed by powder X‐ray diffraction study. Optical transmittance study showed that the grown mNA single crystals have optical transparency in the wavelength range between 430 and 1550 nm. SHG efficiency of the grown mNA crystal was 3 times grater than KDP. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effects of anions on rapid growth and growth habit of KDP crystals

The synthesis of KDP from its raw material has been found exist in the growth solution. In the crystal growth experiment, significant extension of specific faces was observed at low dopant concentration. At high doping concentration, the growth rate of the whole crystal decreased with no significant habit modification. The inhibiting effects of phosphite and other H-bonding anionic ions on the growth of pyramidal faces are discussed. Rapid growth rate experiments have been carried out with purified material and an averaged growth rate of 18.6mm/day was obtained.     

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.

     

Dielectric studies on sodium fluoroantimonate single crystals

Dielectric properties of grown single crystals of Na₂SbF₅ and Na₃Sb₂F₉ have been carried out. The values of the dielectric constant of Na₂SbF₅ and Na₃Sb₂F₉ are about 24.1 and 14.3, respectively, at temperatures from 30 °C to 250 °C and 225 °C for 100 kHz.     

Influence of solvent and crystallization method on the crystal habit of metronidazole

In the present study, metronidazole was crystallized in several solvents, according to both the “cooling crystallization” and the “crystallization by non‐solvent addition”. Particle properties, such as crystal habit, elongation ratio, and mean particle size, were determined by SEM analysis. Structural changes and development of polymorphic forms were excluded by both Differential Scanning Calorimetry (DSC) and X‐Ray Powder Diffractometry (XRPD). Crystal habit (and thus elongation ratio) was typically influenced by both the solvent polarity index and the crystallization method: solvents with higher polarity index tended to promote acicular or stick‐shaped crystals with a high elongation ratio, while isodimensional crystals were promoted by decreasing the polarity index, as was particle aggregation. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the nature of growth hillocks on sodium chlorate crystals

Optical examination of as-grown {100} surfaces of sodium chlorate crystals grown from aqueous solution revealed the presence of elliptical growth hillocks. The hillocks were present on both enantiomorphous forms and originated from dislocations, inclusions, and microcrystals attached to the growing surface. The value of the surface entropy factor equal to 4.55 at 313 K suggests that crystals grow via/or with the participation of dislocation mechanism, and the hillocks are dislocation growth centers. Compound mechanism controlled growth of some crystals because edge nucleation and dislocation centers operated simultaneously on the same surfaces.     

Enantiomorphism in sodium chlorate crystals

Creation of enantiomorphous forms in sodium chlorate and enantiomorphous nature of surface micromorphology were studied by optical microscopy. It was shown that when the small number of crystals nucleates spontaneously in unstirred, unseeded solutions then exclusively left- (L) or right-handed (D) crystals can be formed. At the large number of nucleated crystals the probability of the creation of L and D form is nearly the same. L and D crystals can be distinguished by the shape of growth or dissolution centers.     

Growth habit of crystals of refractory compounds prepared from high temperature solutions

The growth habit of single crystal refractory compounds (borides, carbides and silicides) obtained using a solution-melt method involving various growth conditions is reviewed. The effect of external and internal factors on the growth habit is analyzed. The influence of such external factors as the cooling mode, the cooling rate, the effect of variation of solute concentration, the stoichiometry of the initial components, the presence of additives in the system, the change of the maximum soaking temperature, the nature of the initial components and the solvent metal and microgravity on the growth habit are described. Examples of various anisotrophic properties in crystals of refractory compounds as well as data on crystal michrohardness anisotropy, obtained by the authors, are given. In conclusion the comparative evaluation of the effect of external factors on growth habit is provided.     

Data on nucleation and growth of sodium trimolybdate crystals

The influence of acidification on crystallization and kinetics of growth of sodium trimolybdate has been interpreted. Furthermore the process of nucleation at the surface layer of the solution has been described.     

Study of the oriented overgrowth of sodium nitrate crystals

Formation of overgrowths of NaNO₃ on crystalline and noncrystalline substrates has been investigated employing the solvent evaporation method. Calcite, Gypsum, Sodium chloride, Mica, Glass and Polystyrene films formed on glass have been employed as substrate materials. The solvents employed are water, liquid ammonia, methyl alcohol and n-butyl alcohol. Random orientation of the overgrowths is observed on the substrates except in the case of calcite and mica. It is found that the morphology of the overgrowths manifests in the form of rhombic-, triangular- and hexagonal platelets. The occurrence of (001), (110) and (111) twins is observed. The effect of supersaturation and nature of the substrate on the formation of overgrowths is considered.     

Growth and characterization of mercurous chloride single crystals

Mercurous chloride was purified by repeated sublimation using multichamber distillation column. Single crystals were grown by Physical Vapour Transport technique. During the process of purification of the material, very interesting habit modifications like dendrites and needles were observed. Vickers hardness tests were carried out on the (110) plane of the crystal for different orientations of the indentor. The shape of the impressions for different orientations of the indentor reveals the non-cubic structure of the plane. No structural phase transition in mercurous chloride was observed in calorimetry and optical studies.     

Physical vapor transport growth of mercurous chloride crystals

We have carried out experiments to derive a quantitative understanding of the physical vapor transport (PVT) process and to identify convective effects on the crystal growth process. The experimental growth velocity was several orders of magnitude lower than the theoretically predicted value. The effusion holes were used to disturb the impurity boundary layers. We observed a change of 18% (not an order of magnitude) in growth velocities. The Arrhenius behavior of growth rate with temperature was used to derive the sticking coefficient. Experimental result on growth velocity as the aspect ratio was varied showed that with increasing convection, the growth rate increased up to a certain value and then dropped to a constant value. This indicated that a bifurcation had occured with a resulting change in transport behavior.

Growth velocity measurements for the PVT process as a function of orientation of the g-vector were also made. The experimental results clearly showed that the growth velocity varied with g-vector for a particular temperature profile. The effects of convection on crystal quality were studied by varying the thermal conditions (source and crystal temperatures) which affects thermal convection during PVT. The results showed that crystals grown at low Rayleight numbers had better homogeneity. While no microgravity experiments were conducted, computation of mass flux for the horizontal orientation for various gravitational levels showed two distinct regions; above 10^-3 g where the flow was convective and strong circulating cells appeared, and also below 10^-3 g, where the flow was purely diffusive and no circulating cells were predicted. Therefore it is postulated that for the conditions of growth considered, space flight experiments with acceleration less than 10^-3 g could yield crystals grown under diffusive transport.     

GaN single crystals of different habit grown from solution at near atmospheric pressure

Near atmospheric pressure solution growth is one of the many developing methods for growing bulk GaN from solution. Apart from other approaches, this method holds certain advantages, such as relatively low growth pressure and temperature, and the ability to grow high quality GaN crystals with different orientations by varying the solvent composition. GaN whiskers of millimeter scale size with exceptional mechanical and optical properties were grown from solution. Crystals of near isotropic shape were also grown from solution by manipulating additives in the basic solvent.     

Influence of emulsion types on nucleation kinetics and growth habit in the cooling crystallization

Experiments on unseeded batch cooling crystallization were made to investigate the influence of emulsion [solution in non‐solvent (S/NS) emulsion and non‐solvent in solution (NS/S) emulsion] on crystal size and growth habit of various materials such as hexahydro‐1,3,5‐trinitro‐1,3,5‐triazine, ammonium sulfate, potassium dihydrogen phosphate (KDP), and γ‐glycine. Size of crystals obtained from the S/NS emulsions was found to be significantly larger than that obtained from the NS/S emulsions. Those results were explained by broad induction time distribution of nucleation in the isolated solution droplets and subsequent seeding effect by free movement of early‐induced crystals by vigorous agitation. As a result, the population density of crystals was shown to be smaller in the S/NS emulsions than that in the NS/S emulsions. In the S/NS emulsions, desupersaturation rate should be slow and high supersaturation is subsequently generated during crystal growth stage. Therefore, it may be concluded that crystal morphology of the materials with supersaturation dependent growth habit, such as ammonium sulfate, KDP and γ‐glycine, can be controlled by selection of emulsion type.     

Influence of impurity atmosphere on the deformation of silicon crystals

The Alexander–Haasen theory, which describes the deformation kinetics of silicon crystals, has been generalized for impurity crystals. The deformation kinetics of an impurity sample is calculated in a wide range of parameters, including the cases of partial and complete entrainment of impurities by moving dislocations. The developed model, despite its simplicity, adequately describes the qualitative transformation of the stress–strain curves of impurity silicon crystals in dependence of the impurity concentration and other material parameters. The manifestation of negative velocity dependence of the yield stress, observed in natural experiments, is analyzed.     

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 and characterization of tris allylthiourea mercuric chloride crystals

Single crystals of tris allylthiourea mercury chloride (ATMC), a semi‐organic nonlinear optical material, have been grown from a low‐temperature solution growth technique by slow cooling method. In the present work, we have grown ATMC crystal which is a new nonlinear optical material (NLO) having high optical quality and its second harmonic generation (SHG) efficiency is thrice that of Urea. The grown crystals have also been subjected to various characterization studies like X‐ray diffraction, UV‐VIS, FT‐IR and TGA‐DTA. X‐ray powder diffraction study confirmed the crystal structure of ATMC, UV‐VIS /FT‐IR spectral analysis showed the crystal has good optical transmission in the entire visible region and TGA‐DTA studies showed the thermal properties and complex degradation of ATMC crystal. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of striction on soliton interaction in crystals

Striction-mediated attraction of domain walls, solitons in incommensurate phases, and Abrikosov vortices in superconductors are considered. It is shown (a) that it is this type of attraction that can be responsible for a soliton-density jump in lock-in transitions and (b) that the strain-induced vortex interaction in superconductors with a high Ginzburg-Landau parameter is higher by one or two orders of magnitude than was assumed earlier.