Kinetics of dissolution of BaF2 crystals

Kinetics of dissolution of BaF₂ crystals in aqueous solutions of HNO₃ and HCl acids have been studied for the different acid concentrations in the temperature interval 20–100°C. It is established that this process is diffusion controlled. Activation energy is constant, but pre-exponential factor is dependent on the concentration of the acid.     

Kinetics of growth of calcite crystals in presence of ammonium ions

The growth of calcite crystals from an aqueous supersaturated solution of initial concentration 0.15 mol m⁻³ containing 10 mol m⁻³ of (NH₄)₂CO₃ has been studied at 298 K. The progress of growth has been followed through determining the composition of the solution by chemical analysis. The crystal growth is controlled by the surface reaction mechanism and satisfies a kinetic equation of the second order with the rate constant 3.7 × 10⁻⁶ m⁴ mol⁻¹ s⁻¹ that is independent on the surface area of the present solid. NH₄⁺ ions do not change the crystal growth mechanism but decrease the growth rate of calcite crystals.     

Growth and dissolution kinetics of potassium sulfate in pure solutions and in the presence of Cr3+ ions

The growth and dissolution kinetics of potassium sulfate was studied based on single crystal measurements. The growth rate is correlated to the supersaturation with power low equation. At all the temperatures studied, the growth rate order lies in the range of 1‐1.5 with the surface integration process as the controlling step. The estimated value of the activation energy of growth is 39.4 kJ/mol. The dissolution rate order decreases with increasing the temperature. The diffusion step is controlling the dissolution process. The addition of 5 ppm Cr³⁺ ions reduces the growth rate. Both growth rate dispersion and dissolution rate dispersion occur in the growth and dissolution processes of potassium sulfate. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Kinetics of dissolution and calcium sulphate growth on {111} faces of calcium fluoride crystals etched in sulphuric acid

The effect of temperature (20–100°C) and concentration of H₂SO₄ etching solutions on the etching behaviour of {111} faces of calcium fluoride crystals is investigated. Whisker growth of calcium sulphate is observed. Temperature of the etchant also has an effect on their formation. The kinetics of dissolution at the sites of dislocations and general dissolution have been studied. Studies on induction period and its dependence on the concentration of the acid and temperature are also described. It is observed that: (i) at low and high acid concentrations, the dissolution is diffusion controlled while it is predominantly reaction-rate controlled in the intermediate concentration range and (ii) the growth rate of calcium sulphate whiskers decreases with time and their induction period decreases with increase in the temperature of the solution. The implications are discussed.     

Precipitation and dissolution of KCaCl3 impurity in NaCl crystals

Impurity precipitation in NaCl crystals containing CaCl₂ and KCl in various proportions is studied by means of flotation density measurements. The paper is concerned with the determination and dissolution, the kinetics and activation energy, and the mechanism of nucleation. Two temperature regions for solid solution decomposition were revealed. The characteristic stages in density variation upon isochronal annealing are shown to be correlated with radical changes in the X-ray diffraction patterns. Evidence is given that the nucleation is heterogeneous for the stable phase and homogeneous for the metastable one. – The activation energy in the low-temperature region has been found to be 0.8 ± 0.1 eV, which suggests precipitation to be diffusion-limited. From the kinetics studies it follows that the composition of the metastable phase corresponds to KCaCl₃ and the rate-controlling stage in stoichiometric crystals is a K⁺ diffusion.     

Investigations of impurity striations in potassium bichromate crystals

Impurity striations in potassium bichromate crystals (KBC, lopezite) formed during crystal growth from aqueous solution were revealed by chemical etching and analyzed. Striations were revealed as etch grooves, as rows of dislocation etch pits and as rows of flat‐bottomed etch pits. Various types and groups of striations have been visualized. Some striations were due to lateral segregation of impurities caused by convection flow of the mother solution, other were formed during growth stoppages whereas induced striations were generated by changes in hydrodynamical conditions. Growth rates changes resulted in zonal distribution of impurities, formation of planar lattice strain, rows of clusters of point defects and rows of dislocations. Generation of striations with different intensities in various sectors is a proof of the selective capture of impurities. Ratios of growth rates of various faces of KBC crystals growing in forced and free convection regimes were determined by induced striations. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Geometrical properties of dislocations in potassium bichromate crystals

The geometry of growth dislocations present in potassium bichromate crystals grown from aqueous solution has been studied by etching. Etch topographs composed of dislocation etch grooves and grooves representing sector boundaries and growth bands have been analysed. It was found that most of the dislocation lines are straight and have well-defined directions in each sector. Refractions of dislocations at growth sector boundaries and at other defects have been observed.     

Oriented inclusions in single crystals of potassium dihydrogen phosphate

Different forms of oriented inclusions in single crystals of potassium dihydrogen phosphate (KDP) grown by gel method and from aqueous solutions are reported. Small rod like chain and channel types of liquid inclusions are illustrated. These inclusions are oriented along 〈001〉, 〈011〉 and 〈010〉 directions and are attributed to mother liquor trapped in the body of the crystal during growth.     

Magnetostimulated changes of microhardness in potassium acid phthalate crystals

A decrease in microhardness along the (010) cleavage in potassium acid phthalate single crystals by 15–18% after the application of a permanent magnetic field is revealed for the first time. It is shown that the effect revealed is of the volume character. The role of interlayer water in the processes stimulated by a magnetic field is studied. Interlayer water does not cause the observed changes; it only plays the part of an indicator of these changes in potassium acid phthalate crystals in a magnetic field. It is established that microhardness in the (100) plane of the crystal in an applied a magnetic field first increases by 12–15% and then remains constant in time within the accuracy of the experiment. The possibility of varying the crystal structure of potassium acid phthalate crystals by applying magnetic fields inducing rearrangement in the system of hydrogen bonds or in the defect structure is discussed.     

Preparation of calcium carbonate crystals in the presence of trisodium citrate

The dumbbell‐like calcium carbonate (CaCO₃) crystals were synthesized in the presence of trisodium citrate. Different morphologies were obtained by changing the reaction temperature and the trisodium citrate concentration. The obtained samples were characterized by means of X‐ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that the morphology of CaCO₃ crystals was markedly affected by the reaction temperature and citrate anion concentration. The possible growth mechanism of CaCO₃ crystals was proposed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigations on the Nucleation Kinetics of L-Arginine Phosphate single crystals

The nucleation parameters, such as radius of the critical nucleus and critical free energy change have been evaluated for LAP single crystals. The interfacial tension determined by conducting the induction period measurements has been used for the evaluation of nucleation parameters. The determined interfacial tension is found to be comparable with theoretical literature values.     

Kinetics of etching of dislocations in magnesium orthosilicate single crystals

Cleavages of undoped, doped and natural magnesium orthosilicate crystals have been simultaneously etched in concentrated hydrochloric acid solution. The lateral and normal velocities of the growth of pits were then measured at different temperatures for the varieties of crystals. The time dependence of the growth of pit dimensions is found to be linear, while the temperature dependence of the growth of pits is found to be exponential, viz. V = A exp (− E/kT). The dissolution parameters, e. g. the activation energy (E) and the pre-exponential factor (A) for dissolution along the surface and along the dislocation lines have been computed. It is observed that: (1) the activation energy (E₁) of dissolution along [001] direction is found to be greater than the activation energy (E_b) along [100] direction, irrespective of whether the crystals are doped, undoped or natural, and (2) the doping of the crystal with manganese has lowered down the activation energy (E_d) to such an extent that it is less than E₁ and E_b. The implications are discussed.     

Investigations of the energy flow of X-ray wavefields in crystals

The energy flow of the X-ray wavefields in the Laue case of diffraction and its importance for X-ray topography and interferometry are discussed.     

Growth of rhombohedral insulin crystals and in vitro modeling of their dissolution in the blood stream

Insulin is the only protein that is secreted in a crystalline form in a human healthy body. To mimic the secretion process we used NaCl salting‐out to growing tiny rhombohedral Zn‐insulin crystals. The dissolution of the insulin crystals is of special interest for the therapeutical praxis, because the human body is supplied with the physiologically active monomers of the insulin through dissolution of the crystalline granules secreted in the pancreatic β‐cells. Sets of tiny rhombohedral Zn‐insulin crystals, which resembled the granules secreted in the β‐cells, were subjected to dissolution in blood plasma and model solutions. The impacts of the solution composition, flow rate, pH and ionic strength on the insulin crystal dissolution were investigated. The effect of the blood plasma was determinant because it dissolved the rhombohedral Zn‐insulin crystals almost instantly, while the effects of solution's physicochemical characteristics were of minor importance. In addition, we found that the presence of abundant zinc ions suppressed the dissolution of the insulin crystals. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mathematical modelling of sand dissolution in a glass melting channel with controlled glass flow

This work presents the results of a mathematical modelling of sand dissolution in a horizontal glass-melting channel with circulation flows when varying the length of the channel and the time needed for the dissolution of the sand particles. The recently found optimal combination of intensive transversal circulations with forward flow, slowed down by a positive longitudinal temperature gradient near the glass level, has been set in the melting channel. The results of the modelling were projected into the values of the space utilisation for the sand-dissolution process. The high values of space utilisation were repeatedly attained under these conditions in a broad interval of channel lengths and sand-dissolution times. Both the growing length of the channel and the growing sand-dissolution time increased the ratio of the transversal to longitudinal temperature gradient necessary for attaining the maximum space utilisation. The pull rate of the channel grew linearly with the growing ratio of the channel length to the sand-dissolution time. The results provide a theoretical base for the design of glass-melting segments with controlled glass flow, characterised by lower energy consumption and higher melting performance.     

Optical absorption of Fe3+ Ion in LiF crystals

The absorption spectrum of LiF—Fe³⁺ single crystals was investigated at liquid nitrogen temperature. Six new absorption bands, unobserved earlier in other works, were found, which were located at 11 200, 16 000, 22 700, 27300, 30 500, and 41 900 cm⁻¹. The spectrum was interpreted in the cubic cristalline field approximation with D_q = 1397 cm⁻¹, B = 657 cm⁻¹, C = 3226 cm⁻¹. Above mentioned bands were assigned to the transitions from a ground state ⁶A_1g to the levels ⁴T_1g(G), ⁴T_2g(G), ⁴A_1g(G) (⁴E_g), ⁴E_g(D), ²T_1g(F), and ⁴T_1g(F), respectively.     

Decoration of growth and dissolution steps on the surfaces of L-arginine phosphate monohydrate crystals

Observations of decoration of growth and dissolution steps on the {100} surfaces of non-linear optical L-arginine phosphate monohydrate (LAP) crystals during their withdrawal through an n-hexane layer placed above its supersaturated and undersaturated solutions are described. This is the first report of decoration of growth or dissolution steps on the surfaces of crystals at or near room temperature in a liquid medium by the crystallizing or dissolving material.