Growth morphology of potassium bichromate crystals from aqueous solutions

The results of a systematic study of growth morphology of potassium bichromate single crystals on seeds and by self-nucleation at different temperatures and supersaturations are presented. The observed growth morphology is analysed in the light of theoretical predictions. An increase in both growth temperature and supersaturation leads to a symmetrical morphology, and the transition from asymmetrical to symmetrical morphology is monotonous. The predicted sequence of morphological importance agrees only in the case of crystals grown at low temperatures and supersaturations.     

The precipitation of alkaline-earth metal molybdate powders from aqueous solution. Crystal numbers,final morphology and sizes

The precipitation of barium, strontium and calcium molybdates was studied from neutral equivalent solutions of concentrations from 0.0004 to 0.4 M at 25 °C. Crystal growth started after induction periods; the precipitiations were heterogeneously nucleated at low supersaturations and homogeneously nucleated at medium to high supersaturations. Barium molybdate was precipitated as tetragonal bipyramids, strontium molybdate generally as prisms and calcium molybdate as platelets. Crystal numbers at medium to high supersaturations increased with increasing inital metal molybdate concentrations according to the relation, The final crystal lengths in this range than decreased from maximum values (at the critical concentrations) with increasing initial metal molybdate concentrations according to the relation. Generally, for precipitation from solutions at any concentration, larger crystals were obtained in the precipitates of the salt of higher solubility.     

Wachstum von Graphitkristallen in kohlenstoffhaltigen Eisenschmelzen bei höheren Überschreitungen

The rates of growth for the basic and the prismatic faces of graphite crystals are calculated in dependence of the supersaturation on the assumption that the faces at high supersaturations grow by twodimensional nucleation from an iron-carbon melt. Near the thermodynamic equilibrium prismatic faces grow more quickly than basic faces. At high supersaturations the ratio of the growth rate is reversed. The results are suitable to explain the different kinds of graphite crystals in cast iron.     

Polyhedral stability ‐ on the growth of flat crystal faces on small molecule and protein crystals

The old standing problem of face morphology is discussed. A special emphasis is put on the macroscopically flat faces, appearing on small molecule crystals mostly during calm growth, under low supersaturations. As distinct, protein crystals are growing with macroscopically flat faces even under surprisingly high supersaturations. Explanations of these facts are suggested by considering the surface micro‐profile of crystal faces which growth is driven by screw dislocations. It is shown that, due to kinetic reasons, the tips of the growth hillocks and the valley between them have to be levelled to some extent (at least on a quasi‐atomic scale) under low enough supersaturations. The amplitude of the surface roughness has to be suppressed also due to the surface energy gain, especially under quasi‐equilibrium. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The precipitation of lead sulphate from aqueous and aqueous alcohol solutions: Nucleation,final sizes and morphology

The precipitation of lead sulphate was studied from 0.0001 to 0.01 M aqueous solutions (supersaturations 3 to 600) and from 20% aqueous ethanediol, methanol and ethanol solutions, in polypropylene beakers, at ambient temperature: the experimental techniques were conductivity measurements and optical microscopy. The precipitations were heterogeneously nucleated at low supersaturations and homogeneously nucleated at intermediate to high supersaturations. New crystal morphologies generally developed at some what higher supersaturations in the aqueous alcohol systems. The final crystal lengths at first increased with increasing initial metal salt concentration and then decreased with this parameter; the largest crystals at any concentration were obtained from solutions in which lead sulphate solubility was highest. The critical supersaturations (for the onset of homogeneous nucleation) increased from 36 (in water) to 50 (in 20% aqueous ethanol): the surface energies for the formation of nuclei correspondingly increased from 90 to 110 mJ m⁻² in good agreement with the Nielsen-Söhnel relation. The nucleation and crystal growth processes are taking in an aqueous environment of similar water activity to that of the bulk solutions.     

一水甲酸锂晶体生长界面过饱和度的测量

本文采用激光全息相衬干涉显微术研究了有机非线性光学晶体一水甲酸锂晶体的生长,计算了晶体生长的界面过饱和度.我们的研究结果表明,晶体生长的界面过饱和度随体过饱和度的增加而非线性增加;不同晶面的界面过饱和度不同;当体过饱和度增加到一定程度时,不同晶面的界面过饱和度趋于相同.     

The precipitation of alkaline-earth metal and transition metal ‘oxinate’ powders from aqueous solution. Crystal numbers and final sizes

The precipitation of barium strontium, calcium, magnesium, zinc, cadmium and lead, manganese, cobalt, nickel and copper 8-quinolinolates (‘oxinates’) was studied from equivalent solutions, at pHs from 4.5 to 10, by optical microscopy: the metal cation and overall ‘oxinate’ with ‘oxine’ concentrations were varied from 0.0002 to 0.020 M (while the mean metal oxinate concentrations varied from 10⁻⁷ to 0.001 M). Crystal growth started after induction periods; the precipitations were heterogeneously nucleated at low supersaturations and homogeneously nucleated at medium to high supersaturations. The final precipitate crystal numbers depended on the number of nuclei formed during the induction periods. Crystal numbers at medium to high supersaturations increased with increasing initial metal oxinate concentration according to the relation, The final crystal lengths in this supersaturation range then decreased (from maximum values) with increasing initial mean metal oxinate concentration according to the relation, For precipitation from solutions of any concentration at any pH, smaller crystals were generally obtained in the precipitates from solutions of the metal oxinate of lower solubility.     

The precipitation of transition metal oxalate powders from aqueous solution. Crystal numbers and final sizes

The precipitation of manganous, ferrous, cobalt, nickel and copper oxalate hydrates was studied from equivalent solutions of concentrations from 0.001 to 0.3 M at pHs 7 to 6, by optical microscopy and other methods. Crystals growth started after induction periods: the precipitations were heterogeneously nucleated at low supersaturations and homogeneously nucleated at medium to high supersaturations. The crystal numbers of the final precipitates depended on the number of nuclei (and crystallites) formed during the induction periods. At medium to high supersaturations, crystal numbers increased with increasing initial metal oxalate complex ion concentrations according to the relation. N = N₁C_mox^β, where β was 5. The N values increased in the order Mn ≪ Fe < Co < < Ni < Cu. The final crystal lengths, in this range, then decreased with increasing metal oxalate complex ion concentrations according to the relation l_fin = l₁/C_mox^γ, where γ was 1.3. For precipitations from solution of any concentration, smaller crystals were generally obtained in the precipitates of the metal oxalate of lower solubility; nickel oxalate precipitations were the exception to this.     

Influence of the supersaturation on the mode of thin film growth

The thermodynamic criterion of the mechanism of growth of thin films is reconsidered. It is shown that the supersaturation plays a significant role in determining the growth mode in addition to already discussed interrelation between the specific surface energies of the substrate, deposit and the substrate-deposit interface. The conclusions are in good agreement with experimental observations in deposition of f.c.c. metals on f.c.c. metal substrates that at positive surface energy change the island growth is favoured at supersaturations lower than some critical one while at higher supersaturations a layer growth takes place. The STRANSKI -KRASTANOV mechanism consisting in formation of threedimensional nuclei on top of previously deposited layers of the overgrowth is considered in more details. It is concluded that its occurrence depends on the sign and the value of the lattice misfit between the substrate and the overgrowth, on the interaction forces between them and on the thermodynamic stability of the deposit. The validity of the classical nucleation theory for explanation of the growth mode transition is also discussed.     

In situ AFM investigation of 2D nucleation on the (100) face of KDP

Surface morphology of the (100) face of potassium dihydrogen phosphate (KDP) crystals which were grown at different supersaturations at 25 °C was investigated by in situ atomic force microscopy (AFM). Various AFM images of 2D nucleation under different growth conditions were presented. It is found that the growth of KDP is controlled by polynuclear nucleation mechanism at the high supersaturation. With reduction of the supersaturation, the growth velocity of 2D nuclei becomes very slow and shows typical anisotropy. It is found that the process of coalescence of 2D nuclei does not lead to defect. The experiments show that the growth mechanism for KDP at 25 °C changes between step flow and 2D nucleation in the supersaturation range of 4.5‐5%. The triangular nuclei which are close to equilateral triangle are observed in the experiment at the supersaturation σ = 6% for the first time, showing typical anisotropic growth. Through observing the dissolution of 2D nuclei, the dissolving process can be regarded as the reverse process of growth. We also find that the microcrystals landing on the surface at σ = 9% would grow and coalesce with each other and there is no observable defect in the coalescence. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth Kinetics and Mechanism of TGS Crystals

The growth kinetics of TGS crystals was studied at high supersaturations under the Curie temperature. The kinetics data proved that the crystal growth was mainly controlled by BCF surface diffusion model. The continuous growth was fitted to the growth rate data of (110) face. Its edge energy, Jackson factor, activity energies, kinetic coefficients were calculated.     

Various Growth Shapes of Na2S2O3 5H2O Crystals

The relation between the growth shapes and supersaturations and/or supercoolings was investigated in the Na₂S₂O₃ 5H₂O crystals: (i) The growth shapes changed from a hexagonal prismatic crystal, aggregate of platy crystals and to a spherulite with an increase of supersaturations and/or supercooloings. (ii) The deposition rate suddenly increased at a certain supersaturation and/or supercooling, where the growth shape changed from the hexagonal prism to the aggregate of platy crystals. (iii) The deposition rates of spherulites become higher with increasing the solute concentrations. (iv) The formation process of the spherulites were in situ observed by the Schlieren and/or Mach‐zehnder interferometer methods.     

Autodeformation bending of gypsum crystals grown under the conditions of counterdiffusion

The dynamics and kinetics of growth-induced bending of gypsum crystals grown from solution have been studied. Crystallization was performed by the method of chemical reaction under the conditions of component counterdiffusion. It is established that autodeformation bending occurs in the [001] direction at the growth front and is caused by cationic impurities. The crystal curvature depends on the anisotropy of growth rate and increases at lower supersaturations. The mechanism of growth-induced crystal bending is suggested which takes into account the heterometry stresses providing the appearance of a bending moment at the growth front.     

On the kinetic of crystallization of zinc oxalate (II). Determination of the specific surface energy at the crystal solution interface by kinetic studies

An apparatus based on turbidimetry was constructed for studying the kinetics of crystallization of sparingly soluble salts. The kinetics of crystallization of zinc oxalate was investigated at various supersaturations and pHs of the medium. The specific surface energy at the crystal-solution interface was determined from the induction periods of the S-shaped curves by means of the classical theory of nucleation. The calculations were made using the slope of the logarithmic dependence of I or τ, respectively, on supersaturation (equations 1 and 2) and the critical supersaturation which was also determined on the basis of kinetic data. Various values of σ were found for the different supersaturations. Different values for this quantity were also obtained when supersaturation was presented by the concentration ratio (S = C/C₀) or by the ratio of the product of the two ions concentrations in the supersaturated solution to that in the saturated one (S = ab/L_p). These σ values were lower than those obtained using the critical supersaturations at the two different solution pHs. For the present it is impossible to give a definite explanation of the results obtained. The experiments for determining the specific surface energy at the crystal-solution interface will be continued.     

Crystallization of inorganic salts from aqueous solutions in a microwave field

The crystallization of some inorganic salts (KH₂PO₄, NaCl, Sr(NO₃)₂, KNO₂, Ca(OH)₂) by the thermal-gradient (with decreasing temperature) and solvent-evaporation methods using microwave heating of solutions is investigated. It is established that the growth rates of single crystals in a microwave field are an order of magnitude higher than obtained in other known techniques at comparable crystallization temperatures and supersaturations. For example, the growth rate of prismatic faces {100} of KH₂PO₄ crystals is as high as 11 mm/day at supersaturations of ～1.2%. The results obtained are discussed in the context of the effect of microwave radiation on the adsorption surface layers of crystals. Fine-grained phases of the salts under study are obtained by evaporation of the solvent.     

Thermodynamics and kinetics of the first monolayer adsorption of xenon on the (0001) graphite face

We show that Auger electron spectroscopy is a suitable means to study the kinetics of condensation of the first layer of xenon on (0001) graphite. The amount of rare gas condensed ? is measured at 75 K and 76 K versus time for different supersaturations. The coverage at first varies linearly with time, then asymptotically tends to ?=1. Within a broad range of coverage (? <0.9), the growth rate is determined by the incident flux only.     

Nucleation velocity of NaPO3 melts

The time τ for visible crystallization of NaPO₃ melts is measured in the whole range of undercoolings from the melting temperature T_m to the temperature of vitrification T_g. It is shown that the time τ, needed for the appearance of a visible crystallization at high supersaturations is determined by the linear growth rate, whereas the rate of heterogeneous nucleation is the main factor, determining the experimentally measured time τ at low supersaturations. The specific surface energy σ at the melt-crystal interface is calculated from the data on nucleation velocity in the vicinity of T_m and the temperature dependences of both heterogeneous and homogeneous nucleation velocity for NaPO₃ melts are constructed. It is concluded that at normal cooling rates only nucleation, initiated by active crystallized cores could be observed for NaPO₃.     

The effect of temperature on the kinetics of scale growth

The temperature dependence of the kinetics of incrustations growth on heat-exchange surfaces from solutions of NH₄NO₃ was examined. With increasing temperature the formal kinetic exponent of growth is decreasing and the kinetic constant of growth is increasing. The linear rate of growth of NH₄NO₃ incrustations is increasing with rising temperature within the whole range of measured supersaturations.     

Determining the primary nucleation and growth mechanism of cloxacillin sodium in methanol–butyl acetate system

The primary nucleation and growth mechanism of cloxacillin sodium in methanol–butyl acetate system are determined on the basis of induction time measurements. The induction time of cloxacillin sodium is experimentally determined by the laser scattering method at different supersaturations and temperatures. The measured induction times are then treated using the models of mononuclear and polynuclear mechanisms. It is discovered that the primary nucleation mechanism of cloxacillin sodium is identified as polynuclear mechanism, which relates the induction time and the supersaturation for various growth mechanisms. On the basis of these analyses, the growth mechanism of cloxacillin sodium is two-dimensional nucleation-mediated growth at 285 K, and changes into spiral growth with increasing temperature (at 291 and 297 K).     

Some observations of growth hillocks and growth layers on potassium hydrogen tartrate crystals

Some observations of growth hillocks and growth layers on the as-grown surfaces of potassium hydrogen tartrate (KHT) crystals obtained at 30 °C from aqueuos solutions of different supersaturations are reported and discussed. It is found that macrospirals, isolated growth hillocks, and growth layers are typical features observed on these crystals. Bunching of rectilinear growth layers originating from growth hillocks produced parallel macrosteps on the {010} faces. In certain cases, interaction of thick rounded layers of unequal height, emitted from neighbouring strong sources, gave rise to residual steps (pseudo-interlaced steps). Isolated growth hillocks produced by microbes were also observed.