Study of the Growth Morphology of Ammonium Oxalate Monohydrate Crystals Obtained from Aqueous Solutions

The experimental results of the growth morphology of ammonium oxalate monohydrate [(NH₄)₂C₂O₄ · H₂O; AO] single crystals obtained from aqueous solutions at 30 and 40 °C and supersaturation up to 9% are presented. The observations are compared with the theoretical morphology predicted by PBC analysis and Braivais-Donnay-Harker law.     

Study of growth kinetics of ammonium oxalate monohydrate crystals from aqueous solutions

Experimental results of the dependence of linear growth rates of ammonium oxalate monohydrate [(NH₄)₂C₂O₄ · H₂O; AO] single crystals on solution supersaturation are presented. The AO crystals were grown by constant-temperature, constant-supersaturation method at 30 and 40 °C in the supersaturation range of 1–9%. It was observed that the supersaturation dependence of growth rates follows the parabolic growth law. Analysis of the supersaturation dependence of linear growth rates of AO crystals showed (1) that growth models involving surface diffusion and direct incorporation of growth units give kinetic parameters similar to those reported for other compounds grown from solutions, and (2) that the the BCF model of cooperating screw dislocations is also applicable. An inverse relationship between the estimated values of the length, L, of the line containing the dislocations and growth rate, R, and a direct relationship between L and interplanar distance, d_hkl, of the face {hkl} were found. Both these relationships are associated with the process of generation of screw dislocations in the growing layer.     

Ferrous Tartrate recognition of sodium chloride {111} and its effect on crystal growth

The surface morphology and growth pattern of some molecules could be altered by the recognition of ferrous tartrate (FeC₄H₄O₆, FeTA). We have explored this phenomenon in the aspect of growth rate and morphology alteration of sodium chloride crystals where few researches have been done. In this paper, the crystallization behavior of NaCl with added FeTA was studied by supersaturation test, conductivity, scanning electron microscopy (SEM), and X‐ray diffraction (XRD). The results showed that the supersaturation of NaCl solution could be increased to about 5.5 % in presence of FeTA prior to the onset of crystallization. Furthermore, SEM images and XRD results indicated that the addition of FeTA could change the morphology of NaCl crystal from cubic to octahedron by impeding the growth of {111} and {110} faces. Besides, the interaction between NaCl {111} face and FeTA was discussed and the possible spatial structure of FeTA was speculated through the lattice parameters of NaCl.     

Effect of Mn(II) ions on the growth of ammonium oxalate monohydrate crystals from aqueous solutions: I. Growth habit and surface morphology

The effect of concentration of Mn(II) ions on the growth habit and the surface micromorphology of different as‐grown faces of ammonium oxalate monohydrate (AO) single crystals grown from aqueous solutions was studied at a constant temperature of 30 °C and predefined supersaturations up to 20%. It was observed that the growth habit and the surface morphology of the crystals strongly depend on the supersaturation used for growth and the impurity concentration in the solution. The experimental results were analysed in terms of connected nets determined from different projections of the structure of AO crystals. Analysis of the observations revealed that: (1) the directions of connected nets corresponding to basic growth units composed of single (NH₄)₂C₂O₄ · H₂O molecules are in excellent agreement with the low‐index crystallographic directions of the orientations of growth layers, (2) all faces appearing in the growth morphology of AO crystals are F faces, and (3) the {001} face growing from pure aqueous solutions is essentially a kinetically rough face but the presence of Mn(II) impurity leads to their appearance in the morphology due to increase in the strength of bonds of the connected nets composing the surface graph.     

Influence of a polyacrylate antiscalant on gypsum nucleation and growth

Calcium sulphate dihydrate (gypsum) crystallization was studied under conditions, of supersaturation and temperature, simulating a brackish water desalination unit using solar energy. The effect of an commercial sodium salt of poly(acrylic acid), based compound known as RPI, on homogeneous nucleation and growth of gypsum was also examined. Gypsum was precipitated by mixing aqueous CaCl₂ and Na₂SO₄ solutions. It was found that, with increasing temperature or supersaturation, the induction time decreases and the growth rate increases. By using classical nucleation theory, the interfacial tension and the nucleation rate values were estimated. It was shown that the interfacial tension is temperature dependent. The addition of increasing quantities of RPI, in the same conditions of temperature and supersaturation, prolongs the induction time, decreases the nucleation rate and increases the interfacial tension. The addition mode of RPI (in calcium or in sulphate solution) was found as an important parameter in controlling the inhibition process of gypsum crystallization. XRD and SEM analysis showed that RPI antiscalant strongly affected the texture and the morphology of the deposit gypsum. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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)     

Effect of electrical field on growth of gallium arsenide layers from vapour phase

The effect of electrical fields on the growth rate and morphology of gallium arsenide layers in the GaAs AsCl₃ H₂ system is investigated. The fields both parallel and perpendicular to the substrate surface are used. It is found that application of the parallel field results n increase of growth rate of (111)A face but does not change that of 2° (001). The morphology of layers grown in such a field is better than that grown without any. The application of perpendicular field results in decrease of growth rate for both crystallographic orientations, the layer morphology worthening. In both cases the effect is greater when the substrate has a negative potential. Possible mechanisms of the field influence on the growth rates of gallium arsenide layers are discussed such as: supersaturation decrease because of nucleation in vapour phase and the change in the rates of surface processes.     

Effect of pH value on the growth morphology of KH2PO4 crystal grown in defined crystallographic direction

KH₂PO₄ single crystals were grown in aqueous solution at different pH values by using “point seeds” with a defined crystallographic direction at 59 degree to the Z axis. Atomic Force Microscope (AFM) was applied to observe the surface morphology of (100) face. It was found that at the same supersaturation, the larger steps appeared at the lower pH value before appearance of 2D nucleus. We found that 2D nucleus was occurred at σ ≤ 0.04 when pH value is <2.8. The occurrence of 2D nucleus was caused by the decreasing step‐edge free energy with the decreasing of pH value in the growth solution. In this paper, we observed the morphologies of (100) faces of KDP crystals which grew in solutions with different pH values. 2D nucleuses appeared on the terrace of growth steps when pH value down to 2.8 and 3.2 at supersaturation of 0.04, while pH value down to 2.4, only 2D nucleation control the growth. Therefore, the pH value can change the growth mechanism of KDP crystals.     

Themodynamic Approach to Diffusion-Controlled Epitaxial Silicon Deposition in Flow System from SiCl4 + H2 Mixtures

Treatment of diffusion-controlled chemical vapour deposition (CDV) usually neglects supersaturation at the gas-solid interface, which is the basic condition for crystal growth. A new approach based on metastable equilibria and on the concept of activity, taking into account departure from equilibrium has been given and applied to the system resulting from SiCl₄ + H₂ mixtures. The control of activity at the gas-solid interface should provide a method of controlling the morphology and perfection of CVD films.     

Polymorphic composition and morphology of calcium carbonate as a function of ultrasonic irradiation

This paper reports on the precipitation of CaCO₃ polymorphs, having various crystal morphologies under different conditions. In particular, systems that were subject to ultrasonic irradiation were compared to the corresponding reference systems in the absence of such a treatment. The application of ultrasonic irradiation predominantly resulted in a change of particle size distribution and polymorphic composition of the precipitate, in comparison to the reference systems. Thus, it was found that the supersaturation and temperature influenced the size distribution, in both the reference and sonicated systems. A mixture of calcite, vaterite and aragonite was obtained in all reference systems, at 25 °C. At this temperature, the sonication caused the vaterite content to increase, while aragonite was not detected. In reference and sonicated systems at 80 °C, only aragonite precipitated. The results also indicate that the principle parameter responsible for the morphology of vaterite was the initial supersaturation: at higher supersaturation spherical vaterite particles precipitated, while at lower supersaturation hexagonal platelets were obtained. The morphological investigations also indicated different mechanisms of vaterite formation in the systems in which precipitation was initiated at higher supersaturation: spherulitic growth of vaterite was observed in sonicated systems, while the aggregation of primary particles was predominant in the reference systems. At lower supersaturation, the effect of c(Ca²⁺)/c(CO₃²⁻) on the morphology of hexagonal platelets of vaterite was observed as well. By varying the c(Ca²⁺)/c(CO₃²⁻), significant changes of the polymorphic composition were observed only in the sonicated systems, at 25 °C.     

Kinetics of crystal growth of mirabilite in aqueous supersaturated solutions

The crystallization of sodium sulfate decahydrate (Na₂SO₄·10H₂O, mirabilite) from supersaturated solutions was investigated using stable supersaturated solutions seeded with mirabilite seed crystals. The experiments were done in batch, stirred reactors in which the supersaturated solutions were prepared either by dissolution of sodium sulfate anhydrous at 32 °C followed by cooling to 18 or 20 °C or by mixing equal volumes of equimolar ammonium sulfate and sodium hydroxide solutions at 20 °C. Inoculation of the solutions supersaturated only with respect to mirabilite with seed crystals was accompanied with temperature increase of the thermostated solution. Despite the fact that crystal growth was initiated with seed crystals, the process started past the lapse of induction times inversely proportional to the solution supersaturation. The rates of crystal growth were measured both from the temperature rise and from the concentration–time profiles, which were linearly correlated. The measured crystal growth rates showed a parabolic dependence on supersaturation at low supersaturations. For higher values this dependence changed to linear, a behavior consistent with the BCF spiral crystal growth model. The morphology of the crystals growing at 20 °C showed typical prismatic habit, while at 18 °C when crystallized from cooled sodium sulfate solutions changes in the crystal habit to a leaf like morphology were observed.     

Investigation of nucleation kinetics and crystal defects of HMX

The nucleation kinetics of HMX (cyclotetramethylene tetranitramine, C₄H₈N₈O₈) in γ‐butyrolactone was studied in cooling process by induction time method. The laser scattering method was used to measure the solubility data and metastable region of HMX in γ‐butyrolactone. The induction time was measured over a range of supersaturation at different temperatures. Then, the nucleation mechanism of HMX in γ‐butyrolactone was investigated by analysis the relationships between induction time and supersaturation. The results indicated homogeneous nucleation dominated at high supersaturation of S >1.35, while the heterogeneous nucleation dominated at low supersaturation of S < 1.35. The values of interfacial tension at different final temperatures were calculated to indicate the ability of HMX to be crystallized. The growth mechanism of HMX was investigated by the data fitting applying different growth mechanism models and identified as two‐dimensional nucleation‐mediated (2D) growth. Finally, the effects of supersaturation and temperature on the crystal defects were analyzed based on the nucleation kinetics. When the temperature is below 303.15K, homogeneous nucleation dominated the nucleation process at higher supersaturation. Fine HMX crystals with more defects were produced. On the contrary, heterogeneous nucleation mechanism dominated at lower supersaturation. large regular HMX crystals with fewer defects were formed when the temperature is above 318.15K.     

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 faces of K2Co x Ni1 – x(SO4)2 · 6H2O mixed crystals

Data on the morphology and normal growth rate of the (110) and (001) faces, velocities of step motion, and slopes of dislocation hillocks on the (001) face of K₂Co _x Ni₁– _x (SO₄)₂ · 6H₂O crystals at different supersaturations of solutions with a Co/Ni ratio equal to 1: 1 or 1: 2 have been obtained using a Michelson interferometer. The morphology of the (110) faces is found to be the same for solutions of both compositions. Powerful dislocation sources with large Burgers vectors dominate on the (001) face. The morphology of the (001) surface is rougher than that of (110), especially in a 1: 2 solution at high supersaturation. The (110) faces grow more slowly as compared with (001). The kinetic coefficients of steps on the (001) face are identical in the 1: 1 and 1: 2 solutions. The influence of the kinetic and morphological characteristics of (001) faces on the single-crystal quality is analyzed.

     

Growth Rate anisotropy of the K2(S,Cr)O4 Mixed Crystals Grown from Aqueous Solution

The ratio of growth rates of the K₂(S,Cr)O₄ crystals in the [100] and [001] directions (the anisometry coefficient) is proportional to the initial effective supersaturation of the solutions from which the crystals were grown. The values of the factors of asymmetry and excess of distribution of the crystal anisometry coefficient are affected by the composition of solution. The change in the values of these factors is caused by the change in the value of the initial supersaturation of solution according to the specific features of the solubility diagram.     

Effect of Mn(II) ions on the growth of ammonium oxalate monohydrate crystals from aqueous solutions: II. Growth kinetics

The experimental results of the effect of concentration of Mn(II) ions on the growth kinetics of different faces of ammonium oxalate monohydrate single crystals from aqueous solutions at a constant temperature and different predefined supersaturations are described and discussed. It was observed that: (1) at a given supersaturation σ, Mn(II) ions lead to a decrease in the growth rates of different faces of AO crystals, (2) the growth of a particular face of the crystals occurs above a critical supersaturation σ_d but there is also another supersaturation barrier σ* when the rate abruptly increases with σ, (3) the values of σ_d and σ* increase with increasing concentration of the impurity, and (4) the values of σ_d depend on the growth kinetics of a face but those of σ* are independent of face growth kinetics. The experimental R(σ) data for different Mn(II) concentrations c_i were analysed according to the model involving complex source of cooperating screw dislocations and concepts of instantaneous and time‐dependent impurity adsorption. It was found that: (1) for a given face the differential heat of adsorption Q_diff is higher during instantaneous impurity adsorption than that during time‐dependent adsorption, and (2) the values of Q_diff involved during instantaneous adsorption are related with face growth kinetics but those during time‐dependent adsorption are independent of face growth kinetics.     

Direct Growth of Aggregates on {110} Faces of Cadmium Mercury Thiocyanate Crystals

New growth phenomena ‐ direct incorporation of aggregates have been observed on the {110} faces of cadmium mercury thiocyanate CdHg(SCN)₄ crystals by atomic force microscopy. These aggregates grow in two forms: some directly cover up the steps and forms new growth layers; while others are just incorporated at the step edges. These aggregates, which are mostly oriented along [111] direction, are formed by small columnar structural units. The aggregates have the similar structure of CdHg(SCN)₄ crystals and greatly vary in nature with the variation of solution supersaturation σ and growth time t. With the increase of σ the aggregates become larger, consistent with the variation of growth units dimension with the supersaturation; and with the increase of growth time the aggregates become more structurally substantial. These observations have led to a new understanding about the crystal growth.     

LPE growth of DH laser structures with the double source method

For the reproducible processing of double heterojunction injection laser structures to stripe geometry laser diodes, e.g. by proton bombardment, the thickness of the p-Ga_0.7Al_0.3As confining layer and of the p-GaAs top layer are of great importance. It is shown that the thickness control of these layers grown by liquid phase epitaxy can be improved considerably by introducing two source crystals into the growth system. Both source crystals are brought into contact with the solutions consecutively and prior to the seed. If this method is combined with the use of relatively thin melts (about 2 mm), a growth scheme may be chosen such that the growth rates of the p-GaAlAs and of the p-GaAs layers have reached a practically constant value on the seed crystal, independent of the initial degree of supersaturation. This behaviour is found to be in accordance with the diffusion-limited growth model applied for thin solutions, including a temperature dependence for both the diffusion constant and the slope of the liquidus curve. The results indicate that a second solid phase may appear in the p-GaAlAs solutions at a supersaturation as small as 4°C, which reduces the growth rate on the substrates by a factor of about 2. This situation is different from that of GaAs solutions, where a second phase appears only at a much higher degree of supersaturation. For the p-GaAlAs and the p-GaAs layers a thickness control of ⁺_-0.2 μm is thus achieved with this growth method, without the necessity of very precise temperature control and weighing so as to keep the total supersaturation less than 0.2°C.     

Crystallization kinetics of MgSO4 · 7 H2O at higher ranges of the driving force

By means of batch crystallization technique the crystallization kinetics of MgSO₄ · 7 H₂O at 25 °C at higher ranges of the supersaturation from pure aqueous solutions was investigated. It was observed that the growth rate highly depends on the habit of crystals and on their ratio of length to width, respectively. This ratio is a function of the initial supersaturation of each crystal.     

Growth kinetics and adsorption trapping of impurity during crystallization of NH<Subscript>4</Subscript>Cl in the NH<Subscript>4</Subscript>Cl-CuCl<Subscript>2</Subscript>-H<Subscript>2</Subscript>O-CONH<Subscript>3</Subscript> system

The growth of NH₄Cl crystals and their trapping of copper impurity in the NH₄Cl-CuCl₂-H₂O-CONH₃ quaternary system have been experimentally studied. The epitaxial adsorption of copper complexes on (100) faces leads to a sharp decrease in the growth rate in good correspondence with the Bliznakov-Chernov equation. The copper impurity enters the crystal composition in amounts up to 6.5 mol %. The impurity distribution coefficient nonlinearly changes with the copper concentration in the solution: it is much larger than unity at low concentrations and sharply decreases with an increase in supersaturation. Such behavior is indicative of the adsorption mechanism of copper trapping by NH₄Cl crystals. Single-crystal X-ray study shows that the impurity is incorporated in NH₄Cl crystals in the form of oriented intergrowths of different complex coppercontaining compounds. The concentration and variety of impurity phases increase with an increase in the copper content in the solution and decrease with an increase in supersaturation. Heterogeneous 2D isomorphous trapping of copper impurity by NH₄Cl crystals induces high (up to 60 MPa) internal stresses in them, as a result of which anomalous birefringence and splitting of crystals occur.