Two-dimensional Growth Of Nuclei Research Articles

Non-isothermal crystallization kinetics of the heavy-group lanthanide dititanates

The systematic investigation of crystallization kinetics of heavy-group lanthanide dititanate pyrochlores, HL2Ti2O7 (HL=Lu, Tm, Er, Ho, Y, Dy, and Tb) is presented. Materials are prepared by the Pechini-type polymerized complex route, and their thermal properties were studied by differential thermal analysis under non-isothermal conditions. For this group, the crystallization temperature became larger with increasing the ionic radius of lanthanides, from 1066 K for Lu2Ti2O7 to 1125 K for Tb2Ti2O7. The linear dependence of crystallization temperature on radius of heavy lanthanide ions is found to follows Tc[K]=168.86+920.89×rion[Å] semi-empirical equation. Apparent activation energies of crystallization were obtained from calculations based on the Arrhenius equation, isoconversional method and Pérez-Maqueda method, and are ∼805 kJ mol−1 calculated by three different methods. The crystallization kinetics followed the so-called Avrami–Erofeev (A3) model, as confirmed using several criteria, namely, the Málek, master plot, and Pérez-Maqueda methods. The crystallization mechanism of all lanthanide dititanate pyrochlores is found to be the same, and the crystallization process involves a constant rate of nucleation and two-dimensional growth of nuclei.

Non-isothermal crystallization kinetics of Y2Ti2O7

The crystallization kinetics of Y2Ti2O7 powders prepared by a Pechini-type polymerized complex route was studied using differential thermal analysis under non-isothermal conditions. Apparent activation energies of crystallization of 816 and 804kJmol−1 were obtained from calculations based on the Arrhenius equation and isoconversional analysis, respectively. The crystallization kinetics followed the so-called A3 model, as confirmed using several criteria, namely, the Málek, master plot, and Pérez-Maqueda methods. The measurement results and calculated crystallization parameters (n=3, m=2) showed that the crystallization process includes a constant rate of nucleation and two-dimensional growth of nuclei, and excluded the possibility of surface crystallization.

Direct reduction of nickel catalyst with model bio-compounds

The effects of temperature and S/C on the reduction extent and kinetics of a steam reforming NiO/α-Al2O3 catalyst were systematically investigated using five bio-compounds commonly produced during the fermentation, pyrolysis and gasification processes of biomass (acetic acid, ethanol, acetone, furfural and glucose). Reduction was also performed with methane and hydrogen for comparison. Kinetic modelling was applied to the NiO conversion range of 0–50% using the Handcock and Sharp method. The two-dimensional nuclei growth model (A2) was found to fit very well except for glucose. For all the bio-compounds, the apparent activation energy of NiO reduction was between 30 and 40kJ/mol. Their pre-exponential factors decreased in this order: CH4>ethanol≈acetone>acetic acid>furfural>glucose, probably due to the different activities of reducing species they produced. Optimal molar steam to carbon ratios for reduction kinetics were found between 1 and 2.

Non-isothermal and isothermal crystallization kinetics and their effect on microstructure of sintered and crystallized TiNbZrTaSi bulk alloys

Non-isothermal and isothermal crystallization kinetics of mechanically alloyed Ti65Nb23.33Zr5Ta1.67Si5 metallic glass (MG) powder and their effect on microstructure and mechanical property of spark-plasma-sintered and crystallized bulk counterparts were investigated by using Johnson–Mehl–Avrami–Kolmogorov equation. Results show the MG powder has two distinct crystallization steps, which precipitates bcc β-Ti and hexagonal (Ti, Zr)2Si phases successively. The larger apparent activation energy of the second crystallization step than that for the first one indicates the easier occurrence of precipitation for bcc β-Ti phase. Comparative analysis of crystallization kinetics indicates that regardless of non-isothermal and isothermal crystallization types, the first crystallization step is dominated by diffusion-controlled three- and two-dimensional growth of nuclei for the early and late crystallization stage, respectively, while the second one is governed by diffusion-controlled three-dimensional growth of nuclei in the whole crystallization process. Both nucleation rates of these two steps increase firstly and then reduce in the early and late crystallization stage, respectively. The processing technologies combined with crystallization kinetics decide different grain sizes and yield strengths for the sintered and crystallized bulk alloys. The results obtained provide a new insight into tailoring microstructure and mechanical property of bulk alloys by designing and optimizing processing technologies based on crystallization kinetics.

A Kinetic Study of the Polymorphic Transformation of Nimodipine and Indomethacin during High Shear Granulation

The objective of the present study was to investigate the mechanism, kinetics, and factors affecting the polymorphic transformation of nimodipine (NMD) and indomethacin (IMC) during high shear granulation. Granules containing active pharmaceutical ingredient, microcrystalline cellulose, and low-substituted hydroxypropylcellulose were prepared with ethanolic hydroxypropylcellulose solution, and the effects of independent process variables including impeller speed and granulating temperature were taken into consideration. Two polymorphs of the model drugs and granules were characterized by X-ray powder diffraction analysis and quantitatively determined by differential scanning calorimetry. A theoretical kinetic method of ten kinetic models was applied to analyze the polymorphic transformation of model drugs. The results obtained revealed that both the transformation of modification I to modification II of NMD and the transformation of the α form to the γ form of IMC followed a two-dimensional nuclei growth mechanism. The activation energy of transformation was calculated to be 7.933 and 56.09 kJ·mol(-1) from Arrhenius plot, respectively. Both the granulating temperature and the impeller speed affected the transformation rate of the drugs and, in particular, the high shear stress significantly accelerated the transformation process. By analyzing the growth mechanisms of granules in high-shear mixer, it was concluded that the polymorphic transformation of NMD and IMC took place in accordance with granule growth in a high-shear mixer.

Nano- and macro-geometrical structural change of caffeine and theophylline anhydrate tablets during hydration process by using X-ray computed tomography

The effects of nano- and macro-geometrical factors on the hydration kinetics of caffeine (CA) and theophylline anhydrate (TA) tablets at high humidity were investigated using X-ray high-resolution computed tomography (CT). Hydration profiles of CA and TA tablets obtained at 25 and 50 MPa, 96% relative humidity, and 20 °C were measured by weight and X-ray CT. The total tablet volume (G-V) and average tablet density (G-D) calculated based on the volume and weight of tablets, and tablet volume (CT-V) and tablet density (CT-D) were evaluated by X-ray CT. The hydration kinetics of CA and TA tablets followed two-dimensional growth of nuclei (Avrami-Erofee) and three-dimensional phase boundary equations, respectively. The increase in the G-V of TA tablets was initially more than, but later less than, that of CA tablets. The G-D of CA tablets varied extensively and was constant initially, whereas that of TA tablets decreased significantly in the initial stage. The CT-V of CA tablets gradually increased initially, but that of TA tablets increased significantly early on. The inter-granular volume (IG-V) of both tablets decreased initially with large fluctuations, but then increased. The CT-D of CA tablets decreased significantly, but that of TA tablets did not decrease. The hydration kinetics of CA and TA tablets was affected by changes in the geometrical structure of the tablets. X-ray CT is a powerful tool for evaluating dynamic changes inside tablets.

Thickness Dependent Nano-Crystallization in Ge2Sb2Te5 Films and Its Effect on Devices

A thickness dependent nano-crystallization in ultra-thin Ge2Sb2Te5 films was studied by in-situ exothermal and isothermal electrical resistivity measurements. When the film thickness is below 20 nm, an exponential increase of the crystallization temperature with decreasing film thickness was found. Furthermore, the crystallization speed was decreased with decreasing film thickness. Based on Kissinger plot and Johnson–Mehl–Avarami (JMA) model, the crystallization kinetics including the crystallization mechanism, the corresponding activation barrier and the Avrami coefficient was investigated. The crystallization activation energy increased from 2.86 eV in 30 nm thick film to 4.66 eV in 5 nm thick film. The increasing of activation energy was explained by a thickness dependent interfacial energy model and was believed as the main reason for the increasing crystallization temperature. In the films thinner than 10 nm, the two-dimensional nucleus growth corresponding to an Armani coefficient n<1 was proposed to attribute to the decreasing crystallization speed. In addition, the effects of this nano-crystallization on line-type phase change devices were discussed.

Stability of cyanocobalamin in sugar-coated tablets

The purpose of this study was to clarify the stability of cyanocobalamin (VB 12-CN) in sugar-coated tablets containing fursultiamine hydrochloride (TTFD-HCl), riboflavin (VB 2), and pyridoxine hydrochloride (VB 6), and to identify the factors affecting the stability of VB 12-CN in these sugar-coated tablets. The stability of VB 12-CN was investigated using high-performance liquid chromatography while decomposition was evaluated kinetically. The decomposition of VB 12-CN in sugar-coated tablets with high equilibrium relative humidity (more than 60%) under closed conditions showed complex kinetics and followed an Avrami–Erofe’ev equation, which expresses a random nucleation (two-dimensional growth of nuclei) model. We showed that equilibrium relative humidity, the incorporation of VB 2 and VB 6, and sugar coating, are the main factors influencing decomposition and that these factors cause the complex decomposition kinetics.

Epitaxial growth of celestite on barite (0 0 1) face at a molecular scale

In situ AFM experiments have been conducted in order to obtain information about kinetics of celestite epitaxial growth on barite. Growth has been promoted by passing aqueous solutions supersaturated with respect to celestite over freshly cleaved barite (0 0 1) surfaces. Solution supersaturation, β celestite, was varied from 1 to 45.7 ( β celestite = a ( Sr 2 + ) · a ( SO 4 2 - ) / K sp , cel ) . At supersaturations below 10 neither two-dimensional nucleation neither step advancement are observed on barite (0 0 1) surfaces. However, once the two-dimensional nucleation barrier is overcome ( β celestite > 10), nuclei preferentially form on cleavage steps parallel to [1 0 0], [1 1 0] and [1 2 0] directions and more scarcely on terraces. The subsequent growth of two-dimensional nuclei leads to the development of celestite “islands”. Their morphology is defined by (0 0 1) face and {2 1 0} and {1 0 0} forms and can be explained on the basis of PBC theory. The coalescence of such islands results in the formation of a homogeneous SrSO 4 layer. Growth rates along [0 0 1] direction have been measured for the whole supersaturation range. The growth rate equation for “Birth and Spread” crystal growth mechanism has been successfully fitted to our experimental data. The fitting process has provided basic growth parameters in a good agreement with theoretical ones. Both the high transitional supersaturation required for two-dimensional nucleation and the high interfacial energy value obtained from the fitting of the “Birth and Spread” equation ( σ 001 cel – bar = 0.137 J / m 2 ) indicate low affinity of SrSO 4 growth units for barite (0 0 1) faces. This is consistent with the relative high mismatch between celestite and barite structure. The behaviour of the epitaxial growth described in this work can help to interpret the oscillatory zoning frequently occurring in both natural and synthetic crystals of the Ba x Sr 1− x SO 4 solid solution.

Characterisation of Supported Chromium Oxide Catalysts by Kinetic Analysis of H2-TPR Data

The kinetics of reduction of chromium oxide catalysts was studied by applying nonlinear regression analysis to the temperature-programmed reduction (TPR) data of six different CrOx/Al2O3 samples (1.2–14 wt% Cr). Different kinetic models were tested. Three heating rates were required to discriminate between the different kinetic models. The best-fit kinetic models and the estimated parameters for each sample are reported. The two-dimensional nuclei growth model suitably described the kinetics of reduction of samples with loading close to or beyond the monolayer content (>5 wt%). The overall reduction mechanism seemed to be the same irrespective of the method of catalyst preparation. The chromium content was important, however. Reducibility increased regularly with chromium loading, supporting the idea of a topochemical reduction mechanism. The sample with low chromium content (∼1 wt%) was reduced by a different overall mechanism than were the other samples and its reduction behaviour was best described by homogeneous first-order kinetics (random nucleation). Complete line-shape analysis of TPR patterns offers a good means for quantitative characterisation of alumina-supported chromium oxide catalysts, since it provides information on dynamic redox behaviour of the active sites.

Molecular-scale mechanisms of crystal growth in barite

Models of crystal growth have been defined by comparing macroscopic growth kinetics with theoretical predictions for various growth mechanisms1,2. The classic Burton–Cabrera–Frank (BCF) theory3 predicts that spiral growth at screw dislocations will dominate near equilibrium. Although this has often been observed2,4, such growth is sometimes inhibited4,5, which has been assumed to be due to the presence of impurities6. At higher supersaturations, growth is commonly modelled by two-dimensional nucleation on the pre-existing surface according to the ‘birth and spread’ model7. In general, the morphology of a growing crystal is determined by the rate of growth of different crystallographic faces, and periodic-bond-chain (PBC) theory8,9 relates this morphology to the existence of chains of strongly bonded ions in the structure. Here we report tests of such models for the growth of barite crystals, using a combination of in situ observations of growth mechanisms at molecular resolution with the atomic force microscope10,11 and computer simulations of the surface attachment of growth units. We observe strongly anisotropic growth of two-dimensional nuclei with morphologies controlled by the underlying crystal structure, as well as structure-induced self-inhibition of spiral growth. Our results reveal the limitations of both the BCF and PBC theories in providing a general description of crystal growth.

Determination of Heat of Hydration and Hydration Kinetics of Theophylline by Thermal Analysis.

Heat of hydration of theophylline anhydrate to the monohydrate at 298 K was evaluated by three different methods. The values determined by heat of solution with a microcalorimeter and heat of dehydration with a differential scanning calorimeter were -9.34±0.65 and -9.82±0.48 kJ/mol, respectively. The values obtained directly with a heat conduction microcalorimeter under 97.3, 95.4 and 93.7% relative humidities were -9.48±0.49, -9.28±0.63 and -9.37±0.74 kJ/mol, respectively. From thermograms obtained with a microcalorimeter at the isothermal transition of theophylline anhydrate to monohydrate, hydration rates were analyzed by a deconvolution method. Hydration rates were in good agreement with those obtained by the Karl Fischer method. Hydration mechanisms were also analyzed on the basis of solid-state kinetic models and the transition followed a mechanism of random nucleation and two-dimensional growth of nuclei.

Scanning tunneling microscopy of epitaxial YBa2Cu3O7−x films prepared by thermal plasma flash evaporation method

The surface morphology of epitaxial YBa2Cu3O7−x films prepared by thermal plasma flash evaporation was extensively investigated by scanning tunneling microscopy. Under epitaxial film growth conditions with the deposition rate up to 0.42 μm/min, two-dimensional nucleus growth and spiral growth were observed. The main deposition species in this process was found to be the cluster ranging from 0.3 to 9 nm and the size of the cluster influenced the growth mode strongly. Theoretical analysis based on the two-dimensional critical radius revealed that smaller clusters became weakly bonded nuclei resulting in spiral growth and larger clusters became stable nuclei resulting in two-dimensional nucleus growth, which we named two-dimensional cluster nucleus growth. The clusters generated in the plasma boundary layer undoubtedly involve sufficient energy necessary for crystallization and show quite different characteristics from those of the clusters generated in vacuum by adiabatic expansion process. Hence, this process must be named “hot cluster epitaxy.’’

Pseudopolymorphism and phase stability of 7-piperidino-1,2,3,5-tetrahydroimidazo-[2,1-b]quinazolin-2-one (DN-9693)

The phase stabilities of three pseudopolymorphs of DN-9693 were studied by using thermogravimetry and differential thermal analysis. The thermal dehydration of DN-9693·2HCl·H2O proceeds by the mechanism of two-dimensional growth of nuclei. The thermal dehydration of 0.5 mol of H2O per mol of DN-9693·2HCl·2H2O, and that of 2 mol of H2O per mol of DN-9693·2HCl·3H2O, proceed by the mechanisms of three-dimensional diffusion and three-dimensional phase boundary reaction, respectively, but the dehydration followed overlaps with the thermal elimination of HCl. The half-lives for the dehydration at 25°C show that DN-9693·2HCl·H2O is the most stable form.

2D nuclei evolution during epitaxial growth

Growth of two-dimensional nuclei on the isotropic surface is considered. Studies are made of all the stages of their evolution up to the monolayer completion. The dependence of the nucleus growth rate on its size is obtained. The evolution of the nuclei size distribution function is considered.

Kinetic analysis of TG data: Thermal dehydration of Ni(II), Co(II) and Mg(II) acetate tetrahydrates

A study has been made of the kinetic behaviour of three tetrahydrated acetates undergoing dehydration. Differential kinetic analysis was used to discriminate between models. A two-step model, namely two-dimensional nuclei growth and phase boundary reaction in cylinders, is required to describe the course of the dehydration of the three solids. If the Coats-Redfern method is used to fit the kinetic data to any of the models used, the E and A values obtained depend on the heating rate. A model with more than one step must be used.

A detailed study of the mechanism of an organic solid state reaction

The thermal cis → trans-isomerization of azobenzene was examined in various states of aggregation. Below the eutectic temperature of educt and product the reaction rate is controlled by two-dimensional nuclei growth up to a reaction extent 0.15. Around an extent between 0.15 and 0.20 a distinct change in mechanism occurs. The following region can be described by a three-dimesional diffusion law. The evaluation of the data was done by fitting master curves and by a modified Weibull distribution. Above the eutectic temperature the reaction kinetics follow the Prout-Tompkins law caused by liquid eutectic layers. In solution and in the melt the kinetics are first order. In addition the phase diagram of the pseudobinary system cis-/trans-azobenzene was determined.

Kinetics of the isothermal transformation of β-PbO 2 into α-PbO 2

The isothermal transformation of β-PbO 2 into α-PbO 2 was studied. This process takes place at 440–450 °C and is accompanied by a decrease in the ratio Pb(IV)/(Pb(II) + Pb(IV)), although it always remains higher than 0.9. The kinetic analysis of the isothermal data shows that the activation energy of the process is ca. 150 kJ mol −1. The transformation involves a two-dimensional growth of nuclei that are related to the OH content of the samples. For ground β-PbO 2, the higher Pb(II) and OH content causes a change in the kinetic law governing the reaction, which is described by a contracting area model.

Thermochemical study of theophylline and its hydrate.

Anhydrous theophylline exhibited polymorphism and two modifications, which were named form I and form II, were isolated. The melting point and the enthalpy of fusion of each polymorph determined by differential scanning calorimetry (DSC) were 273.4±1.0°C and 26.4±0.3 kJ/mol for form I and 269.1±0.4°C and 28.2±1.1 kJ/mol for form II. The higher melting form I had a smaller density. In contrast to caffeine, theophylline formed a monohydrate. The dissociation vapor pressure curves of theophylline hydrate and caffeine hydrate were obtained and the difference in their stabilities was discussed. The enthalpy of dehydration was determined by DSC under closed conditions. The dehydration under isothermal conditions appeared to proceed acoording to the mechanism of random uncleation followed by two-dimensional growth of nuclei as represented by the Avrami-Erofe'ev equation. The solubilities of theophylline and its hydrate were determined as a function of temperature. From the van't Hoff type plot, the transition temperature between the hydrate and the anhydrous form was determined.

The dehydration kinetics of theophylline monohydrate powder and tablet.

The dehydration of theophylline monohydrate in powder and tablet forms was studied by using an infrared water-content measuring instrument. Intact theophylline powder was recrystallized from distilled water, and ground powder was obtained by grinding in a ceramic mortar with a pestle. The samples were dehydrated as follows. The intact and ground powders were placed on a plate 8cm in diameter. A powder bed was formed on a plate 3cm in diameter as a model of a loosely packed tablet, and a tablet 2.0cm in diameter was obtained by compression at 1.0t/cm2. Dehydration kinetic mechanisms of various dosage forms of theophylline monohydrate were clarified by using various kinds of solid-state kinetic models. The dehydrations of intact and ground powders followed two-dimensional growth of nuclei and three-dimensional growth of nuclei equations, and the activation energies (Ea) were 20.1 and 23.0 kcal/mol, respectively. The dehydrations of powder bed and tablet followed two-dimensional phase boundary and threedimensional phase boundary equations, and the Ea values were 15.4 and 11.5 kcal/mol, respectively. It seems that the dehydration of intact powder starts randomly at each particle and proceeds two dimensionally, but that of ground crystals proceeds three-dimensionally from the cracks in particles. In the case of the powder bed the dehydration proceeds two-dimensionally from the surface particles of the powder bed into the inner particles. The dehydration of a tablet proceeds three-dimensionally from the outside of the tablet into its inside.