The influence of crystallization conditions on the onset of dendritic growth of calcium carbonate

Understanding the crystallization of calcium carbonate is relevant in numerous fields like biomineralization, geology and industrial applications where calcium carbonate forms. In order to enhance the knowledge about the formation of calcium carbonate with focus on the vaterite polymorph, in this work calcium carbonate has been crystallized in aqueous solutions at temperatures from 5 °C to 90 °C. Special attention has been directed to higher temperatures for which the effect of the initial supersaturation on the resulting crystal morphologies and the onset of dendritic growth have been studied. In analogy to snow crystal formation, it has been found that in a certain temperature range hexagonal plate‐like crystals form at low supersaturation whereas dendritic patterns start to appear at higher supersaturation. The symmetrical branches characteristic for dendritic growth get less pronounced as the temperature is decreased. The results reported here related to the interdependence between supersaturation, crystal morphology and growth mechanisms, can be used in future works to predict particle formation and to design crystal architectures. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystallization habit of calcium carbonate in presence of sodium dodecyl sulfate and/or polypyrrolidone

The synthesis of calcium carbonate (CaCO₃) crystals from aqueous solutions containing sodium dodecyl sulfate (SDS), poly(N-vinyl-1-pyrrolidone) (PVP) or SDS/PVP complexes has been performed through a slow titration method. It was found that aragonite and calcite coexisted in the prepared crystals. The formation of aragonite in the precipitation systems without magnesium ions indicates that at ambient temperature ca. 26.0°C, initially formed amorphous CaCO₃ could also transfer into aragonite in the sedimentary phase, which indicates the controlling factor of organic additives in the nucleation and growth process of CaCO₃ crystals. The appearance of hexagonal crystals in the suspensible phase confirmed the hexagonal crystallization cell of vaterite, and revealed the colloidal-dispersion function of the SDS/PVP complex in the crystallization process of CaCO₃.     

Temperature fluctuations in a LiNbO3 melt during crystal growth

The synthesis of calcium carbonate (CaCO₃) crystals from aqueous solutions containing sodium dodecyl sulfate (SDS), poly(N-vinyl-1-pyrrolidone) (PVP), or SDS/PVP complexes has been performed through a slow titration method. It was found that aragonite and calcite co-existed in the prepared crystals. The formation of aragonite in the precipitation systems without magnesium ions indicates that at ambient temperature ca. 26.0°C, initially formed amorphous CaCO₃ could also transfer into aragonite in the sedimentary phase, which indicates the controlling factor of organic additives in the nucleation and growth process of CaCO₃ crystals. The appearance of hexagonal crystals in the suspensible phase confirmed the hexagonal crystallization cell of vaterite, and revealed the colloidal-dispersion function of the SDS/PVP complex in the crystallization process of CaCO₃.     

Mechanoluminescence of halogenate crystals

Mechanoluminescence (ML) has been studied in twenty halogenate crystals. All piezoelectric crystals show ML similar to the emission from second positive group of molecular nitrogen, and all the crystals not exhibiting ML are non-piezoelectric. Some of the non-piezoelectric crystals also exhibit nitrogen emission ML which is comparable in intensity to that of piezoelectric crystals. The ML appears only during the different steps developing in the fracture region of the force versus compression curve of the crystals. No considerable change in the ML activity is found due to the number of crystallization or due to the addition of different impurities from 100 to 10.00 ppm. The ML activity per mole of the halogenate crystals varies around four orders of magnitude. It is concluded that in addition to the piezoelectrification of the newly created surfaces there exists some other process of electrification which may cause the ML exitation in halogenate crystals.     

On the incorporation mechanism and distribution mode of impurities in potassium acid phthalate crystals grown from aqueous solutions

Mechanism of impurity structure formation in crystals grown from aqueous solutions has been studied on the example of potassium acid phtalate (abbreviated hereafter as KAP) single crystals. Gold decoration technique at an electronmicroscopic scale has been applied to the study of the distribution of uncontrolled impurities on KAP cleavage face (010) after 10, 20 and 30 days of growth, taking into consideration different growth rates in 〈001〉 and 〈001 〈 directions. A technique for visualization of impurities in water, based on the adsorption of these impurities by the surface of amorphous film of nitrocellulose (parlodion) and the vacuum decoration with gold of these impurities, has been developed. Differences in the impurity structure of KAP regions located in 〈001〉 and 〈001〉 directions from the seed have been established. In 〈001〉 direction after 20 days of growth impurity assemblies 0.1—0.4 μm in size are revealed, and in 〈001〉 direction heterogeneous impurity structure is revealed only after 30 days of growth. The real (impurity) structure of KAP outside impurity assemblies is quite homogeneous and is the same throughout the whole crystal volume, the impurities incorporating mainly into complex active centres. From comparison of the changes in time of the impurity structure of water used for crystallization solutions and the impurity structure of KAP crystals a conclusion is made that the impurity structure of crystals is “programmed” in the impurity structure of crystallization solutions which regularly changes with time, i. e. impurities from different kinds of assemblies which are selectively adsorbed by the growing crystal faces. The role of the adjacent to the growing face interfacial layers which control the growth rate and have a complex impurity structure is stressed.     

Growth of CaMoO4 and SrMoO4 crystals in silica gel media

Growth kinetics and characterization of calcium and strontium molybdate crystals grown in silica gel have been studied under a variety of parameters. The changes in nucleation characteristics, growth habit, quality of these crystals were carefully observed and are found directly related to pH of the medium. The profound influence of pH on spontaneous crystallization of CaMoO₄ crystals has been carefully investigated; and its crystallization range has been determined. The wide morphological change of SrMoO₄ with respect to pH variation has been studied. The quality of the crystals has been critically examined.     

Controlled deposition and transformation of amorphous calcium carbonate thin films

Controlled synthesis of amorphous calcium carbonate (ACC) films was realized by using the multiple templates, which are composed of a self‐assembled film (SAF, insoluble Poly (ε‐caprolactone) film) and a soluble modifier (poly allylamine), as modifiers. The formation of self‐assembled film was analyzed by monitoring the morphologies using atomic force microscopy. Even more noteworthy, using anhydrous ethanol as media, the ACC‐to‐vaterite‐to‐calcite transformation was also investigated, and the obtained products were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, and X‐ray diffraction. The results demonstrated that organic solvent has profound influence on transformation of amorphous calcium carbonate thin films. In the air of anhydrous ethanol, the controlled synthesis of calcium carbonate films with different morphologies, such as planar films with a few sporadic particles, symmetric rhombohedral crystals, novel crystals with symmetrical terraced convexity formation of calcite, was obtained by the fine‐tuning of induction time. It provides a new and simple method to prepare polymorphic CaCO₃ crystal films from the ACC films by controlling the crystallization process (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

Preparation and structure of calcium peroxide‐templated porous calcium carbonate crystals

Crystalline calcium carbonate with randomly dispersed porous structure was prepared through co‐ crystallization with calcium peroxide and the following template elimination by a post heating treatment and washing with water. The artificial CaCO₃ possess abundant macro‐mesopores structures and high surface area. This approach may open a new general route for the preparation of crystals with high porosity and structure specialty. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of calcium carbonate in polyacrylamide hydrogel: Investigation of the influence of polymer content

The growth of calcium carbonate crystals has attracted growing attention as a model system for biomineralisation processes. Organic molecules and gelatinous matrices are known to play an essential role in the formation of hard tissues. For the investigation of the function of specific influence factors, a model experiment is necessary. Several hydrogels were previously tested as growth matrices for calcium carbonate.

For laboratory experiments, a double diffusion set-up for the growth of crystals in gels was established earlier. Calcium carbonate crystals were grown in polyacrylamide hydrogels.

Here the influence of the polymer content in the hydrogels on the crystallisation behaviour is reported. Time-resolved and spatially resolved crystallisation experiments were conducted. The collected calcium carbonate precipitates were analysed by light microscopy, scanning electron microscopy and X-ray diffraction.

The morphology of the developing crystals was found to be dependent on the polymer content of the hydrogels.     

Growth of hierarchical calcium carbonate crystals templated by biomolecules of lotus root

In this paper, crystal growth of calcium carbonate (CaCO₃) in the presence of biomolecules of lotus root was investigated. Scanning electron microscopy, Fourier transform infrared spectroscopy and X‐ray powder diffractometry were used to characterize the products. The results indicate that calcite spherical particles were constructed from small rhombohedral subunits. Similar CaCO₃ crystals were also gained when crystal growth of CaCO₃ in aqueous solution containing extracts of lotus root was performed, suggesting that the soluble biomolecules of lotus root play a crucial role in directing the formation of hierarchical calcite spherical particles. The possible formation mechanism of the CaCO₃ crystals by using lotus root is also discussed, which can be interpreted by particle‐aggregation based non‐classical crystallization laws. The biomolecules of lotus root might induce and control the nucleation and growth of calcium carbonate crystals. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Modeling the distribution of Ga and Sb impurities in Ge‒Si single crystals grown by double feeding of the melt: Growth conditions for homogeneous single crystals

Mathematical modeling of the distribution of Ga and Sb impurities in homogeneous (with respect to the content of the main components) single crystals of Ge–Si alloys, grown by double feeding of the melt, has been performed in the Pfann approximation. It is shown that the axial gradient of impurity concentration in Ge–Si crystals can be controlled in wide limits by changing the ratio of crystallization rate and the rates of feeding of the melt by silicon and germanium rods. The conditions for growing alloy single crystals, homogeneous both with respect to the content of the main components and to the impurity concentration distribution, have been determined.     

Deposition of calcium carbonate films by a polymer-induced liquid-precursor (PILP) process

A polypeptide additive has been used to transform the solution crystallization of calcium carbonate to a solidification process of a liquid-phase mineral precursor. In situ observations reveal that polyaspartate induces liquid–liquid phase separation of droplets of a mineral precursor. The droplets deposit on the substrate and coalesce to form a coating, which then solidifies into calcitic tablets and films. Transition bars form during the amorphous to crystalline transition, leading to sectorization of calcite tablets, and the defect textures and crystal morphologies are atypical of solution grown crystals. The formation of nonequilibrium crystal morphologies using an acidic polypeptide may have implications in the field of biomineralization, and the environmentally friendly aspects of this polymer-induced liquid-precursor (PILP) process may offer new techniques for aqueous-based processing of ceramic films, coatings, and particulates.     

Mathematical modeling of impurity distribution in the melt meniscus in the growth of profiled sapphire crystals

The distribution of impurities in the growth of profiled sapphire crystals is simulated. The distribution of impurities was calculated with the use of the diffusion equation with convective terms. The melt flow was found by solving the Navier-Stokes equation. The distributions of impurities over the melt meniscus are obtained at different crystallization rates. The maximum concentration supersaturation in the meniscus is studied as a function of its geometric parameters.     

Impact of magnetic field on the nucleation and morphology of calcium carbonate crystals

The influence of static magnetic field of strength 0.75 T on the nucleation of calcium carbonate crystals has been investigated. Particle size analysis shows that magnetic field can cause marked difference in distribution. One of the major impacts of magnetic exposure is the increase in number of the critical nuclei formed. Also, magnetic field promotes the formation of parallelepipedic calcite crystals and the dissolution of the smaller crystals by Ostwald ripening mechanism. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Physico-chemical processes in single crystal growing of calcium aluminates by zone melting

The growing of single crystals of calcium aluminates of compositions 12 CaO · 7 Al₂O₂, CaO · Al₂O₃, and CaO · 2 Al₂O₃ by zone melting under vacuum of 10⁻⁵ mm Hg permitted to establish that some of the Al³⁺ ions in octahedral coordination have been driven back by the moving crystallization front. Energetically, this process can be represented on the basis of the viscous flow model with the activation energy of −45 kcal/mole. The possible mathematical models have been considered for the processes of preparation of single phase crystals of the aluminates mentioned above which take account of incongruent vaporization of the component oxides and refining of the melt from structural impurities by the moving crystallization front.     

Influence of stirring speed on the crystallization of calcium carbonate

Control over crystal morphology of calcium carbonate (CaCO₃) was investigated by simply changing the stirring speeds in the process of CaCO₃ formation. Scanning electron microscopy (SEM) and powder X‐ray diffraction (XRD) measurements explore the morphology evolution of CaCO₃ at varying stirring speeds. As the stirring speeds increase, rhombohedral calcite, spherical vaterite, and monoclinic crystal with coexistence of calcite phase and vaterite phase were formed, suggesting a facile control over calcium carbonate crystallization in constructing crystals with desired morphology. Moreover, almost pure vaterite spherical particles of narrow particle size distribution were formed at optimum stirring speed. Finally, also elucidated in this work is the mechanism investigation into the construction of various crystal forms via this simple route. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Properties of Undoped InP and GaP Bulk Crystals Prepared by Floating Zone Melting

Undoped GaP and InP crystals which are produced by floating zone melting without a crucible were investigated. The background concentration was 10¹⁵cm⁻³ after several passages of the molten zone. The fundamental residual impurities are carbon and silicon penetrating into the crystals during the crystallization process. After purification, the electron mobility of GaP was 200 cm²/V. s, and for InP – 4000 cm²/Vs. The compensation decreased to 20%. A very effective purification was observed with respect to the carbon atoms, but for silicon this result is observed only after several passages of the molten zone. GaP and InP are basic materials for optoelectronic devices and it is important to investigate the possibility for production of pure bulk crystals. A proper method for this aim is the floating zone melting, where the crucible effect is removed and the influence of residual impurities is observed. In this work the influence of the number of floating zone passages on the electrical and luminescence properties of InP and GAP bulk crystals has been investigated.     

The Influence of Impurities on Crystallization Kinetics of Sodium Chloride

The influence of impurities on the crystallization kinetics of NaCl was investigated in a fluidized bed crystallizer. The growth and dissolution rates were related to the supersaturation and impurity concentrations. The effect of different impurities on the growth rate of NaCl crystals can be divided into thermodynamic effects where the impurities influence the solubility and kinetic effects where the impurities will suppress the growth rate compared to the pure NaCl. A mathematical model describing crystal growth rates from aqueous solution as a function of impurity concentration is presented. The model explains impurity concentration effects on the crystal growth rate in terms of an impurity effectiveness factor and a Langmuir adsorption isotherm for the impurity.     

On the variation of the impurity composition in LaF3 crystals grown by the Bridgman-Stockbarger method

Single crystals of the composition LaF₃ with CeF₃, YF₃, and SrF₂ impurities have been grown by the Bridgman-Stockbarger method in a graphite crucible with six cells. The CeF₃ concentration has been measured in the LaF₃ charge prior to the growth experiment and in the grown LaF₃ crystals upon it. It is shown that, depending on the growth conditions, the CeF₃ concentration can change during crystallization from 25% of its initial concentration in the charge up to 550%.