Nucleation kinetics in deionized charged colloidal suspension

A systematic experimental study on the nucleation, crystallization and crystal-growth of one-component charged colloidal particles (122 nm diluted in pure water with densities between 0.5 μm⁻³ < n_p < 5 μm⁻³) is present by means of time resolved static light scattering spectroscopy revealing the heterogeneous and homogenous nature of the crystallization. The interactions between the charged colloidal particles are sufficiently strong to cause crystallization which described in terms of Debye-Hückel approximation. Crystallization starts always with the formation of compressed structurally heterogeneous precursor domains. The results show that the heterogeneous nucleation at the cell walls starts simultaneously with the homogeneous bulk nucleation and the rate density of the heterogeneous nucleation appears slightly higher. It has been also found that the overall crystallization consists of at least a two-step nucleation process involving formation of early stage nuclei or crystal precursor then followed by the main crystallization. The induction time, the number density of nuclei and the growth rate of crystals, is strongly dependent on particle concentration and on whether the nucleation are homogeneous in cell center or heterogeneous on cell walls.     

Study on crystallization kinetics and the crystal internal defects of HMX

In this paper, the solubility data of HMX (1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocane) in acetone from 323.15 K to 293.15 K were accurately measured by use of the laser‐monitoring observation technique. Intermittent dynamic method was utilized to study crystallization kinetics of HMX in acetone. The data of crystallization kinetics were obtained by moment analysis, and the parameters of the growth rate and nucleation rate equations were derived by using multiple linear least squares method. Subsequently, growth rate and nucleation rate at different conditions were calculated according to these equations. In addition, Optical Microscopy Images (qualitative) and Particle Apparent Density (quantitative) experiments were applied to study the crystal internal defects of HMX under different crystallization conditions. It can be found that the crystal apparent density of HMX is in the range of 1.8993 g·cm⁻³ to 1.9017 g·cm⁻³, very close to the theory density of HMX; the internal defects and the crystal size do not increase after 25 °C, from which we predict that the HMX crystal growth reaches the steady growth segment. These results suggest that the nucleation rate is a significant factor influencing the crystal internal defects, and larger nucleation kinetics can reduce crystal internal defects.     

Crystal/glass phase change in K1−xRbxSb5S8 (x = 0.25, 0.50, 0.75) studied through thermal analysis techniques

K_1−xRb_xSb₅S₈ (x = 0.25, 0.5, 0.75) is a well-defined single-phase system that undergoes a reversible phase-change. We determined the activation energy of glass transition and crystallization, respectively, for the three compositions using the Kissinger and Ozawa-Flynn-Wall equations. The results have shown that for K_0.25Rb_0.75Sb₅S₈ the crystallization mechanism could be interpreted in terms of a single-step reaction. For the other two compositions the glass-to-crystal transformation is a process of increasing mechanistic complexity with time and it involves simultaneously several different nucleation and growth events. The slope of the lines in the Avrami plots was observed to be independent of heating rate for K_0.25Rb_0.75Sb₅S₈ and the mean value of the activation energy was found to be 262 ± 6 kJ/mol. For the other two compositions, the slope varies with the heating rate. In the K_0.25Rb_0.75Sb₅S₈ glasses, bulk nucleation with three-dimensional crystal growth appears to dominate the phase-change process.     

Observation of nucleation effect on crystallization in lithium aluminosilicate glass by viscosity measurement

The crystal nucleation effect in lithium aluminosilicate glasses was investigated by the viscosity measurement with aid of the fiber elongation method. The abrupt increase of viscosity due to the crystallization of glass was observed in viscosity-temperature curve but the minimum viscosity temperature (T_η) related with crystallization showed a strong dependence on the nucleation state such as nucleation temperature, nucleation time and heating rate. The results by viscosity agreed well with those of DTA. The nucleation effect on the microstructure of glass-ceramics was also discussed. Finally, the nucleation effect on the crystallization kinetics was approached quantitatively by calculating the crystal volume from viscosity value.     

A kinetic treatment of glass formation VII: Transient nucleation in non-isothermal crystallization during cooling

The analysis of crystallization statistics has been modified to allow for time-dependent (transient) nucleation. To establish its accuracy, the numerical analysis has been applied to isothermal crystallization kinetics and shown to yield crystallization versus time curves which compare very closely with curves calculated analytically with or without the inclusion of transient nucleation.The numerical analysis including transient has been used to calculate the critical cooling rates for glass formation in anorthite and o-terphenyl considering (1) only homogeneous nucleation and (2) homogeneous nucleation + heterogeneous nucleation for 10⁷ heterogeneities cm^?3 with contact angles between 40° and 100°. It has been shown that inclusion of time-dependent nucleation in the calculations does not change the critical cooling rates for glass formation calculated assuming steady-state homogeneous nucleation in both materials. The critical cooling rate in anorthite calculated including steady-state heterogeneous nucleation was found to be decreased only slightly by the inclusion of time-dependent nucleation; while the critical cooling rates calculated for o-terphenyl were not change at all by the inclusion of time-dependent nncleation.The lack of an effect of time-dependent nucleation on the critical cooling rates calculated assuming only homogeneous nucleation is explained by the relatively small transient times on the high temperature side of the nucleation peak (a temperature range which has an overwhelming effect on the overall crystallization process because of the relatively high crystal growth rates in this range).Although the critical cooling rates associated with heterogeneous nucleation are large, the nucleation here takes place at relatively small undercooling where the transient times are relatively small. Thus, transient nucleation causes only a temporary delay in the over all crystallization, and its effect on the critical cooling rate is small.     

Effects of Li replacement on the nucleation, crystallization and microstructure of Li2O-Al2O3-SiO2 glass

The Li replacement including the Li₂O replaced by other oxides and the expensive Li₂CO₃ replaced by low-cost spodumene mineral was studied to lower the product cost of (Li₂O-Al₂O₃-SiO₂, LAS) glass ceramic, and the effects of Li replacement on the nucleation, crystallization and microstructure of LAS glass were investigated by the differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that Li₂O replacement increases the crystallization activation energy, lowers the crystal growth, and increases the nucleation and crystallization temperature by restraining the formation of crystalline phases. The Li₂CO₃ replacement decreases the crystallization activation energy, promotes the crystal growth, without affecting the nucleation, and lowers the crystallization temperature by adding some beneficial compositions with mixed alkali effect.     

Impact of amorphous Ge thin layer at the amorphous Si/Al interface on Al-induced crystallization

We investigated the impact of an amorphous Ge (a-Ge) thin layer inserted at the amorphous Si (a-Si)/Al interface on Al-induced crystallization. In situ observation of the growth process clarified that the nucleation rate is drastically reduced by insertion of a-Ge, which led to increase in the average size of crystal grains. This was interpreted as resulting from decrease in the driving force of crystallization, mainly due to the larger solubility of Ge in Al than that of Si in Al. The obtained films were SiGe alloys with lateral distribution of Ge content, and its origin is discussed based on the two-step nucleation process.     

Crystallization kinetics of Fe73.5−xMnxCu1Nb3Si13.5B9 (x = 0, 1, 3, 5, 7) amorphous alloys

In this study, we have investigated the effect of substituting Mn for Fe on the crystallization kinetics of amorphous Fe_73.5−xMn_xCu₁Nb₃Si_13.5B₉ (x = 1, 3, 5, 7) alloys. The samples were annealed at 550 °C and 600 °C for 1 h under an argon atmosphere. The X-ray diffraction analyses showed only a crystalline peak belonging to the α-Fe(Si) phase, with the grain size ranging from 12.2 nm for x = 0 to 16.7 nm for x = 7. The activation energies of the alloys were calculated using Kissinger, Ozawa and Augis-Bennett models based on differential thermal analysis data. The Avrami exponent n was calculated from the Johnson-Mehl-Avrami equation. The activation energy increased up to x = 3, then decreased with increasing Mn content. The values of the Avrami exponent showed that the crystallization is typical diffusion-controlled three-dimensional growth at a constant nucleation rate.     

Crystallization behavior of (GeS2)0.1(Sb2S3)0.9 glass

The crystal growth kinetics of antimony trisulfide in (GeS₂)_0.1(Sb₂S₃)_0.9 glass has been studied by microscopy and DSC. The linear crystal growth kinetics has been confirmed in the temperature range 492 ? T ? 515 K (E_G = 405 ± 7 kJ mol⁻¹). The applicability of standard growth models has been assessed. From the crystal growth rate corrected for viscosity plotted as a function of undercooling it has been found that the most probable mechanism is interface controlled 2D nucleated growth. The non-isothermal DSC data, corresponding to the bulk sample, can be described by the Johnson-Mehl-Avrami equation.     

Crystal Growth of Diazabicyclo[2.2.2]octane-1,4-diium dichromate

Abstract  

The crystal growth of (C₆H₁₄N₂)[Cr₂O₇], consists of a diazabicyclo[2.2.2]octane-1,4-diium cation and a discrete dichromate anion, in solution has been determined under the influence of various factors. Four factors, which influence the crystal size, crystal growth and nucleation, were chosen. These factors are concentration, temperature, type of solvent and method of crystallization. Taguchi design of experiments (DOE) method has been used for to plan a minimum number of experiments and optimization of crystallization processes. Crystallization of diazabicyclo[2.2.2]octane-1,4-diium dichromate with desirable particle size, shape and minimum time of crystal growth as performance characteristics were studied. The changes in the crystal growth habit were observed and could be explained by the differences in the morphology on the various photograph of this compound.     

Crystallization behavior and microstructure of (CaO·ZrO2·SiO2)–Cr2O3 based glasses

The effects of chromium oxide on the crystallization behavior of glass compositions in the calcium, zirconium and silicon oxides system were investigated by differential thermal analysis, X-ray diffraction and scanning electron microscopic. Results indicate that crystallization is predominantly controlled by a surface nucleation mechanism, even though a partial bulk nucleation has been encountered in compositions containing more than 1.0 mol% of doping oxide. The effect of heating rate on differential thermal analysis curves was studied in order to investigate nucleation mechanisms and to extract the corresponding crystal growth activation energies E_c for the different crystalline phases. Activation energy (E_c) was found to be 490 ± 5 kJ/mol for 5.0 mol% chromium oxide in glasses. The most suitable nucleation temperature was determined as 810 °C for the above mentioned glass. The results of this study have highlighted that a small percentage of chromium oxide strongly affects the crystal formation thereby reducing the time and temperature of the thermal treatment and enhancing the degree of crystallization of calcium, zirconium and silicon oxides glasses.     

Controlled crystallization and ionic conductivity of a nanostructured LiAlGePO4 glass–ceramic

A Li_1.5[Al_0.5Ge_1.5(PO₄)₃] glass composition was subjected to several crystallization treatments to obtain glass–ceramics with controlled microstructures. The glass transition (T_g), crystallization onset (T_x) and melting (T_m) temperatures of the parent glass were characterized by differential scanning calorimetry (DSC). The glass has a reduced glass transition temperature T_gr = T_g/T_m = 0.57 indicating the possibility of internal nucleation. This assumption was corroborated by the similar DSC crystallization peaks from monolithic and powder samples. The temperature of the maximum nucleation rate was estimated by DSC. Different microstructures were produced by double heat treatments, in which crystal nucleation was processed at the estimated temperature of maximum nucleation rate for different lengths of time. Crystals were subsequently grown at an intermediate temperature between T_g and T_x. Single phase glass–ceramics with Nasicon structures and grain sizes ranging from 220 nm to 8 μm were then synthesized and the influence of the microstructure on the electrical conductivity was analysed. The results showed that the larger the average grain size, the higher the electrical conductivity. Controlled glass crystallization allowed for the synthesis of glass–ceramics with fine microstructures and higher electrical conductivity than those of ceramics with the same composition obtained by the classical sintering route and reported in literature.     

Effects of Nb on the formation of icosahedral quasicrystalline phase in Ti-rich Ti-Zr-Ni-Cu-Be glassy forming alloys

As-cast (Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈)_100−xNb_x (0 ? x ? 5) (Ф3) glassy forming alloys were investigated in order to clarify the role of Nb on the formation of icosahedral quasicrystalline phase (I-phase) in Ti-rich Ti-Zr-Ni-Cu-Be glassy system. It is found that an I-phase is formed in Ti-Zr-Ni-Cu-Be glassy alloy by addition of Nb element; however, the nucleation rate of I-phase increases, whereas the grain growth rate decreases with increasing Nb content. Moreover, with increasing Nb content, the thermal stability against crystallization increases, while the temperature range of stability of the I-phase decreases.     

A study of the primary crystallization of a mixed anion silicate glass

This paper presents the results of a study of the primary crystallization of a multicomponent mixed anion silicate glass. The primary phase, leucite, and the secondary phase, diopside, were formed by surface crystallization, while the secondary phase, phlogopite, was formed by volume crystallization. The influence of the particle size of glass powder samples in the range 0-1 mm on the temperature of the DTA crystallization peak, T_p, the height of the peak, (δT)_p, and the parameter /(ΔT)_p was studied. The behaviors of the parameters (δT)_p and /(ΔT)_p depend only on the change in the surface-to-volume nuclei ratio, as is the case with polymorphic crystallization. However, the particle size ranges in which the surface, volume and mixed crystallization mechanism dominate are considerably narrower for this glass. The influence of the duration of a pre-DTA heat treatment on the parameters T_p, (δT)_p, and ΔT_p for fixed temperatures in the range T = 550-750 °C was investigated. The T_p of the samples thermally treated for different times, at fixed temperatures, decreased up to t = 5 h and then increased to a value close to that of an as-quenched sample, after which the value remained constant. The curves of (δT)_p, and ΔT_p as a function of time for fixed temperatures show a maximum. The influence of the temperature of the pre-DTA heat treatment on the parameters T_p, (δT)_p, and ΔT_p for fixed times of t = 1−5 h was also investigated. On increasing the pre-DTA heat treatment time, the curves changed significantly. The curves for 3 and 5 h in the temperature range 580-660 °C were similar to the nucleation curve, which indicated that the volume nucleation process proceeded in this temperature range. The behavior of these parameters, as a result of the simultaneous action of different nucleation mechanisms and crystal growth differ from those previously reported for the case of polymorphic crystallization.     

Nucleation time-lag from nucleation and growth experiments in deeply undercooled glass-forming liquids

A new approach is proposed to explain the strong difference between the induction periods (nucleation time-lags) obtained from nucleation rate measurements and from crystal growth experiments for lithium silicate glasses; and their similar magnitude for a Na₂O · 2CaO · 3SiO₂ glass. For these two glass families, the time-lags for nucleation estimated from crystal growth kinetics were compared with those directly obtained from nucleation experiments. A theoretical analysis was performed employing analytical solutions of the Frenkel-Zeldovich equation. In such analysis, the frequently assumed condition of size-independence of the thermodynamic properties of the crystallites was used. Provided this assumption is correct, time-lag data obtained in the two above mentioned ways should coincide. Consequently the significant difference between the values of nucleation time-lag for lithium silicate glasses from nucleation and growth data gives a strong indirect evidence for the deviation of the properties of critical nuclei from the respective parameters characterizing the state of the newly evolving macrophase. For Na₂O · 2CaO · 3SiO₂ glass at intermediate stages of crystallization we show that the average composition of the growing crystals is close to that of the near-critical nuclei. The fact that the nucleation and growth rates of this soda-lime-silica glass refer to the same phase provides an explanation for the similarity of the induction periods estimated from nucleation and growth experiments.     

Nucleation and growth of needle-like fluorapatite crystals in bioactive glass–ceramics

The nucleation behaviors of glass–ceramics with different Ca–mica (Ca_0.5Mg₃AlSi₃O₁₀F₂)/fluorapatite ratios were investigated. By using differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscope with an energy dispersive spectrometer (SEM/EDS), the effect of CaO and P₂O₅ addition on the nucleation behaviors was studied. Results showed that the addition of CaO and P₂O₅ promoted nucleation process and led to the formation of different nucleation phases. After further heat treatment, Ca₅(PO₄)₃F crystals were of needle-like morphology, instead of particle-like reported in previous studies. This can be attributed to the one-dimensional rapid growth of fluorapatite along the c-axis. The values of the Avrami parameter, n, and the dimensionality of crystal growth, m, are found to be 2 and 1, respectively, which indicated that the bulk nucleation is the dominant mechanism in crystallization, and one-dimensional growth of fluorapatite is preferred. Since needle-like fluorapatite crystals are of the same morphology to hydroxyapatites in human bones, the glass–ceramics thus prepared show excellent bioactivity in vivo.     

Self organized nano crystallinity of magnetite precipitated from a 4.9Na2O · 33.3CaO · 17.1Fe2O3 · 44.7B2O3 glass

Glasses with the mol% composition 4.9Na₂O · 33.3CaO · 17.1Fe₂O₃ · 44.7B₂O₃ were melted, rapidly quenched using a twin roller technique, and subsequently tempered in the range from 550 to 620 °C. This led to the crystallization of magnetite with mean crystallite sizes in the 10-20 nm range. Using higher temperatures resulted in a larger quantity of formed crystallites and slightly larger mean crystallite sizes. Larger tempering times did not lead to substantial crystal growth. The time law of Ostwald ripening was not followed. This is explained by an increase in viscosity of the residual glassy phase during nucleation and crystal growth. Here, the smaller iron concentration near the crystals leads to higher viscosities and to the formation of a diffusional barrier around the crystals, which reduces further crystal growth. The crystallization stops, if T_g of the residual glassy phase is equal to the tempering temperature. Magnetite nano crystals with sizes in the 10-20 nm range offer a wide range of applications, such as the preparation of ferrofluids or of materials for medical diagnostics and therapy.     

Crystallization of vitamin C in a continuous DT MSMPR crystallizer – Size independent growth kinetic model approach

The experimental results concerning continuous mass crystallization process in L(+)‐ascorbic acid – water system are presented and discussed. Influence of L(+)‐ascorbic acid concentration in a feeding solution and mean residence time of suspension in laboratory DT MSMPR crystallizer on product crystal size distribution as well as nucleation and growth kinetics were determined. Kinetic parameter values were evaluated on the basis of size–independent growth (SIG) kinetic model (McCabe's ΔL law). It was observed, that within the examined range of crystallizer productivity (120–1600 kg LAA crystals m^–3h^–1) crystal product population of mean size L_m from 0.2 to 0.3 mm and CV from 66.6 to 49% is withdrawn. Linear growth rate values present decreasing trend (from ca. 7 · 10^–8 to ca. 6 · 10^–8 m s^–1) with the productivity increase (assuming constant mean residence time of suspension τ = 900 s). Occurrence of secondary nucleation within the circulated and mixed suspension, resulting from crystal attrition and breakage, was observed. The parameter values in design equation connecting linear growth rate and suspension density with nucleation rate were determined. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)