Formation Of New Nuclei Research Articles

Evolution Behavior of Shear Band in Al‐0.5Mg‐0.4Si‐0.1Cu Alloy during Hot Compression Process

To clarify the evolution behavior of shear bands in Al‐0.5Mg‐0.4Si‐0.1Cu alloy under high‐temperature conditions, isothermal compression tests are carried out under 400, 450, and 500 °C with 50% compression at a strain rate of 1 s−1. Through electron backscattered diffraction (EBSD), Schmid factors are mainly distributed between 0.395 and 0.495, making lattices easy to rotate and shear bands formed. The shear band, a local plastic deformation structure, has a significant impact on the dynamic recrystallization and preferred orientation of grains. Through the recrystallization distribution maps, subgrains absorb a large amount of dislocations, which increases the grains’ orientation difference, resulting in the formation of recrystallized grains with high‐angle grain boundaries. In addition, bulging grain boundaries promote the formation of new nuclei. The newly formed nuclei will generate and grow by absorption of dislocations, which is the feature of discontinuous dynamic recrystallization. These two phenomena are further confirmed by transmission electron microscope tests. Finally, R‐Cube textures mainly form in shear bands at 400 °C, while in the matrix, E‐textures mainly form at 400 and 450 °C, and R‐Goss textures are rotated to form at 500 °C, which is confirmed by EBSD test results.

Experimental study on visualization of TBAB hydrate formation in confined small channels

Hydrate crystals' microscopic characteristics and agglomerating morphologies play an important role in the hydrate reaction equipment's flow assurance and heat transfer. In order to study the formation process of hydrate, experimental quartz tubes with small diameters were selected, and a fully visualized flow loop system was established. The study investigated the effects of supercooling, solution concentration, tube diameter, and flow rate on hydrate blockage time and agglomeration morphology. The results show that under the same supercooling conditions, the density of single crystal hydrate formed in 10 wt%-20 wt% TBAB (Tetra-n-butylammonium bromide) hydrate solution is low, and the crystal front appears needle-like or sword-like, which is conducive to mass and heat transfer. High concentration TBAB solution(30 wt%-40 wt%) can form dense hydrate crystals, and the tip effect was significantly weakened. For low concentration TBAB solution, the increased flow rate can accelerate the formation of new nuclei and inhibit the growth of large crystals. Grout solutions with different particle sizes can be obtained by adjusting the flow rate. It was found that the initial morphology of TBAB hydrate crystals included spherical and cylindrical shapes, and the morphological evolution process of a single hydrate crystal was similar. Finally, a microscopic physical model of the TBAB hydrate growth and aggregation process was established.

Study of the Silicon Electrochemical Nucleation in LiCl-KCl-CsCl-K2SiF6 Melt

In this work, we studied the kinetics of the cathodic process and the regularities of the initial stages of silicon electrodeposition with methods of cyclic voltammetry, square-wave voltammetry, and chronoamperometry on a glassy carbon substrate from a LiCl-KCl-CsCl melt with K2SiF6 at a temperature of 545 ± 5°C. It is shown that the cathodic process of silicon reduction proceeds in one stage, and it is not electrochemically reversible. The diffusion coefficient of silicon ions found by CV and chronoamperometry was 8.44·10−11 and 1.00·10−10 m2 s−1, respectively. It was also found that the nucleation of silicon on glassy carbon is progressive; the formation of new nuclei proceeds continuously against the background of the growth of existing ones. Based on electrochemical measurements, various modes of silicon electrodeposition in the form of thin films were chosen: potentiostatic, pulse, reverse and galvanostatic with preliminary anodizing. As a result of electrolysis, silicon films were obtained, which were analyzed by SEM and XRD methods. The thickness of such deposits during electrolysis reaches several microns, and it consists of many spherical nuclei up to 0.7 microns in diameter. The content of impurities in deposits is extremely low, and the main contaminant is oxygen (0.4–1.2 wt%).

Effect of Zn substitution on the AC induction heating properties of ZnxMn1-xFe2O4 (x = 0.1–0.9) nanoparticles prepared using microwave assisted synthesis

ZnxMn1-xFe2O4 (x = 0.1–0.9) magnetic nanoparticles (MNPs) were prepared using a microwave-assisted coprecipitation method, and the effect of Zn substitution on the AC induction heating properties of the MNPs was investigated. With increasing Zn substitution, owing to the lower solubility product of Zn2+ ions, the formation of new nuclei was preferred over grain growth, which reduced the average crystallite size. The saturation magnetization initially increased with Zn substitution, attained the maximum value at x = 0.5, and decreased beyond that due to Yafet-Kittel type triangular spin ordering. The prepared MNPs exhibited superparamagnetic behaviour at ∼ 300 K. AC induction heating studies of the MNPs indicated a specific absorption rate of ∼ 130 ± 4 W/gFe at x = 0.1. The AC induction heating efficiency did not exhibit any non-monotonic variation at x = 0.5, and progressively decreased with increasing Zn concentration. This was attributed to the reduction in the MNP size and anisotropy energy density at higher Zn concentration that caused the relaxation dynamics to be Nèel dominated with lower effective relaxation time. AC induction heating studies on the agar-immobilized samples confirmed the Brownian relaxation mediated magneto-thermal energy conversion at lower Zn concentration. The obtained results demonstrated that saturation magnetization alone does not influence the AC induction heating efficiency and relaxation dynamics play a significant role.

Membrane-Catalyzed Aggregation of Islet Amyloid Polypeptide Is Dominated by Secondary Nucleation.

Type II diabetes is characterized by the loss of pancreatic β-cells. This loss is thought to be a consequence of membrane disruption, caused by the aggregation of islet amyloid polypeptide (IAPP) into amyloid fibrils. However, the molecular mechanisms of IAPP aggregation in the presence of membranes have remained unclear. Here, we use kinetic analysis to elucidate the aggregation mechanism of IAPP in the presence of mixed zwitterionic and anionic lipid membranes. The results converge to a model in which aggregation on the membrane is strongly dominated by secondary nucleation, that is, the formation of new nuclei on the surface of existing fibrils. The critical nucleus consists of a single IAPP molecule, and anionic lipids catalyze both primary and secondary nucleation, but not elongation. The fact that anionic lipids promote secondary nucleation implies that these events take place at the interface between the membrane and existing fibrils, demonstrating that fibril growth occurs at least to some extent on the membrane surface. These new insights into the mechanism of IAPP aggregation on membranes may help to understand IAPP toxicity and will be important for the development of therapeutics to prevent β-cell death in type II diabetes.

Preparation of TiO2/Bi2S3 Composite Photo-Anode Through Ultrasound-Assisted Successive Ionic Layer Adsorption and Reaction Method for Improving the Photoelectric Performance

One important component of modern thin film solar cell designs is the photo-anode. TiO2 is the most common photo-anode material. However, TiO2 has no response over its wide band-gap to most of the visible light spectrum. Semiconductor coupling can improve the photoelectric conversion efficiency of TiO2 thin film photo-anodes. In our study, TiO2 arrays were coupled with the narrow band gap material Bi2S3 to form a TiO2/Bi2S3 composite. The Bi2S3 was deposited by successive ionic layer adsorption and reaction method (SILAR), then the effect of the number of deposition cycles on the composite film performance was studied. The as-obtained samples were characterized by x-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, UV–Vis diffuse reflectance spectroscopy and photo-electrochemical measurements. When the number of deposition cycles reached 10, the composite film showed the best optical and electrical properties. To improve the film performance, the SILAR method was modified with an ultrasound-assist. Under the same deposition cycle number (3 and 5), the current density of TiO2/Bi2S3 by ultrasound-assisted SILAR method was larger than that with the SILAR method alone. During the ultrasound-assisted SILAR process, the high temperature and high pressure provide by ultrasound accelerated the formation of new nuclei and increased the ion diffusion rate in the reaction solution, thus forming more Bi2S3 materials.

Shape, Size Evolution, and Nucleation Mechanisms of GaAs Nanoislands Grown on (111)Si by Low-Temperature Metal–Organic Vapor-Phase Epitaxy

The shape, size evolution, and nucleation mechanisms of GaAs nanoislands grown at 400 °C on As-stabilized (111)Si by metal–organic vapor phase epitaxy are reported for the first time. GaAs crystallizes in the zincblende phase in the very early nucleation stages until a continuous epilayer is formed. GaAs nanoislands grow (111)-oriented on Si as truncated hexagonal pyramids, bound by six equivalent {120} side facets and a (111) facet at the top. Their diameter and height appear to increase linearly with the deposition time, yielding a constant aspect ratio of ∼1/4. The nanoisland density (before coalescence) stays constant with time at ∼2 × 1010 cm–2, suggesting that the nucleation occurs at specific Si surface sites (defects) during the very early growth stages, rather than being due to the continuous formation of new nuclei. To understand the molecular-level mechanisms driving the low-temperature MOVPE growth of GaAs on Si, we applied a deposition–diffusion–aggregation (DDA) nucleation model, which predicts a linear evolution of the overall GaAs growth rate with surface coverage, in good agreement with experimental observations, under the assumption that direct impingement of trimethylgallium (Me3Ga) molecules onto the nanoisland surface dominates the material nucleation and growth rate; the contribution of Me3Ga adsorbed onto the As-stabilized (111)Si is negligible, pointing out the reduced reactivity of the Si surface (As passivation). Our DDA model allows estimation of the effective reactive sticking coefficient of Me3Ga onto GaAs, which is equal to 2.82 × 10–5: the small value is compatible with the Me3Ga large steric hindrance and the competitive role of methyl radicals in surface adsorption at low temperature.

Melt crystallization at large deviations from equilibrium

Abstract Kinetic model of bulk crystallization of supercooled melt is presented. It correctly takes the change in metastability level of parent phase in the process of formation and growth of the new phase nuclei into account. The distinctive feature of the model is the isolation of “forbidden” zones in the volume of crystallizing material where the formation of new nuclei is suppressed. The model allows one to find characteristic time of the nucleation stage, dependence of the volume of crystal phase on time, and mean size of crystal grains in solidified material.

Effects of seeding on the fast crystallization of ZSM-11 microspheres with intergrowth morphology and small particle size

A facile and efficient route toward the synthesis of submicron ZSM-11 with intergrowth morphology and controllable particle size is developed, which is characterized by adding micron ZSM-11 microspheres into a little spot of TBA-contained [(TBA)2O/SiO2 = 0.005] gel precursors. By tuning the seed contents and aging period duration at ambient temperature, well crystallized ZSM-11 zeolites with adjustable particle size from 0.2 to 1.0 μm were obtained. Compared with the micron seed particles, the prepared submicron ZSM-11 exhibited both enhanced external surface area and mesopore volumes. The existence of seed particles was found to provide surface area for the formation of new nuclei, while the aging period benefits the occurrence of nucleation behavior, both of which are responsible for the fast crystallization rate as well as the small particle size of products. Besides homogeneous seeds, heterogeneous seeds, such as ZSM-5 and Y can also do a contribution to accelerate the crystallization rate. It was found that the common building units between ZSM-5 seed and ZSM-11 product are favorable for both the nucleation and crystal growth process, while the locally arranged structure of fragments from Y seed are mainly responsible for the crystal growth.

On the Determination of the Concentration of Crystal Nuclei in Glasses by DSC

An interesting technique was proposed by Ray et al. over a decade ago to determine the number density of nucleation sites, Nq, or the nucleation rate, I(T), in glasses that exhibit internal nucleation. Their approach is based on the measurement of the areas under the differential scanning calorimetry (DSC) crystallization peaks of partially crystallized glass samples. In this study, we review their method and test a modified equation recently proposed by some of us, which also takes into account the crystal morphology and impingement. We compare Nq obtained with both methods for a Li2O·2SiO2 glass. Small glass monoliths were treated at 620°C for different time periods for crystal growth, without any nucleation treatment, and subsequently analyzed by DSC up to 800°C. We thus estimated Nq from the area under the DSC crystallization peaks. The corrected approach resulted in Nq values which were not affected by the pair of growth times chosen, as expected, but the obtained values were two‐ to fivefold lower than those calculated with the Ray model. Taking into account previously reported nucleation rates and the corresponding induction periods as a function of temperature (for specimens of the same glass batch), we estimated the number of nuclei formed during the DSC heating/cooling steps, and also measured them by optical microscopy (OM). Finally, we compared the obtained values from OM with the Nq values determined by the DSC method. The Nq resulting from the original and new equations were approximately two orders of magnitude larger than those experimentally determined for the same glass using optical microscopy. This difference is attributed to the formation of new nuclei during the heating and cooling paths of the DSC runs and to surface crystallization, which are not taken into account in the DSC expressions.

Isothermal and non-isothermal crystallization of a fresnoite glass

Several analytical models have been widely applied to determine the activation energy of crystallization, E, and the Avrami index, n. In this work, the parameters E and n for the crystallization of fresnoite (2BaO·TiO2·2SiO2) glass were calculated using different approaches (Ozawa, Kissinger, and Matusita & Sakka). Monolithic glass samples (≈36mg) were heated in a DSC furnace from room temperature to 650°C at 40°C/min, and held at that temperature for 5min. To induce the formation of new nuclei mostly in the nucleation zone, these samples were heat-treated in the temperature range of 650°C to 750°C at heating rates of 10°C/min and 20°C/min. They were then reheated at different heating rates (ϕc=6, 8, 10, 15 and 20°C/min) until they became completely crystallized. The experiments were carried out in a DSC furnace under an argon atmosphere (20ml/min), using Pt crucibles. Additionally, to confirm the Avrami coefficient and the predominant crystallization mechanism, the evolution of the crystallized fraction over time was determined independently by powder X-ray diffraction of the fresnoite glass. In this case, the samples were heat-treated in a tubular furnace at 720°C for different lengths of time. The Avrami index obtained by the non-isothermal DSC and XRD experiments are in excellent agreement and demonstrate that fresnoite crystallizes predominantly in the volume and occurs by simultaneous crystal nucleation and growth. The activation energy of crystallization predicted by Kissinger and Matusita & Sakka's models was EC=(688±22)kJ/mol, and ECK=(940±70)kJ/mol, respectively.

Determining the Crystal Volume Fraction of BS2 Glass by Differential Scanning Calorimetry and Optical Microscopy

In this work, the crystal volume fraction, α(t), of a barium disilicate (BS2) glass‐ceramic was carefully investigated by optical microscopy (OM) and differential scanning calorimetry (DSC). X‐ray diffraction experiment revealed that the reflected peaks of the glass‐ceramic, which was prepared by heat treatment at 1000°C for 12 h, were indexed as the low‐temperature orthorhombic sanbornite mineral phase. This result was confirmed by the refinement of the crystal structure parameters. Bulk samples were then heat‐treated in the range of 760°C to 830°C. In each case, the α(t) values obtained by DSC were higher than those determined by OM due to surface crystallization and the formation of new nuclei during the heating/cooling steps in the DSC experiments. OM and DSC techniques were also used to estimate the number of preexisting nuclei, Nq, in a set of samples heat‐treated at 790°C directly in the DSC furnace. At this temperature, it was found that the Nq obtained directly by OM measurements were in reasonable agreement with those calculated from the combination of overall crystallization with crystal growth kinetics.

A Model of agglomerate formation during bed wetting in the process of disc granulation

A Model of agglomerate formation during bed wetting in the process of disc granulation This paper presents a method for quantitative assessment of the mechanisms of nucleation and granules growth by layering in the process of bed wetting during periodic disc granulation. This study included two initial, consecutive stages of a process with defined time courses. The first phase was a time period, in which only formation of new nuclei took place, while in the second stage simultaneous nucleation and growth of granules as a result of sticking raw material grains to pre-existing nuclei occurred. Different kinds of binding liquid were used for bed wetting in each phase. In the first phase, an aqueous solution of dye was used, and pure distilled water in the second stage. The contribution of particular mechanisms to the formation of agglomerates at different time points within the second phase of the process was determined in this study. To do that the results of bed granulometric analysis, mass balance of size fractions and the analysis of contents of a marker (dye) delivered to the bed with the binding liquid during the first phase in agglomerates were used. To assess the concentration of the dye in different size fractions of the batch, spectrophotometric analysis was utilised. The study was performed using UV-VIS JASCO V-630 spectrophotometer equipped with an integrating sphere. The sieve analysis, spectrophotometric studies and mass balance were used to determine changes in the weight of the dye containing nuclei and of the nuclei containing no dye. The aforementioned analyses were also used to assess changes in the weight of formed granules and of raw material particles attached to nuclei during simultaneous nucleation and growth of granules.

ChemInform Abstract: Ferroelectricity of Polymers Based on Vinylidene Fluoride

Characteristics of the ferroelectricity in polyvinylidene fluoride (PVDF) and copolymers based on vinylidene fluoride (VDF) with different previous thermomechanical histories are analysed. Switching of polarisation and the fluctuation mechanism of formation of new nuclei upon the appearance of conformation defects such as kink bonds are considered. The role of the space charge, which stabilises the new polarisation position, is noted. It is shown that the Curie transition can be either a first- or second-order phase transition depending on the copolymer composition. The influence of the morphology of PVDF on some ferroelectric characteristics is found. In particular, the morphology of thick lamellar crystals is favourable for enhanced spontaneous polarisation. It is shown that a ferroelectric structure approaching the texture of an ideal single crystal can be created in Langmuir–Blodgett films. As the number of unimolecular layers decreases, the concentration of conformational defects in these films increases and the crystallographic order in the lattice is violated. This intensifies the low-temperature Curie transition. The bibliography includes 247 references.

Enhanced oxygen precipitation in neutron-irradiated nitrogen-doped Czochralski silicon crystal

Nitrogen and vacancies were intentionally introduced into Czochralski silicon crystal by doping and neutron irradiation, respectively, to study the effect on oxygen precipitation. A series of isochronal anneals in the temperature range of 700–1150 °C revealed that vacancies showed the prominent enhancement effect on oxygen precipitation at temperatures below 1000 °C, whereas nitrogen exhibited the evident enhancement in oxygen precipitation at temperatures above 900 °C. It is suggested that the vacancies induced by neutron irradiation form VnOm (n, m≥1) complexes acting as nuclei to facilitate oxygen precipitation in silicon. However, the VnOm complexes are not stable at higher temperatures; thus, the enhancement effect of vacancies on oxygen precipitation decreases evidently as the temperature increases above 1000 °C. In addition to the formation of new nuclei based on the VnOm complexes, the growth of grown-in oxygen precipitates is also facilitated by the incorporation of high concentration vacancies in the neutron-irradiated silicon. In nitrogen-doped silicon, the enhanced oxygen precipitation at high temperatures primarily depends on nitrogen concentration rather than vacancy concentration as a result of forming stable N2V2 complexes acting as precipitate nuclei.

Modeling the possible role of iodine oxides in atmospheric new particle formation

Abstract. We studied the possible role of iodine oxides in atmospheric new particle formation with the one-dimensional marine boundary layer model MISTRA, which includes chemistry in the gas and aerosol phase as well as aerosol microphysics. The chemical reaction set focuses on halogen (Cl-Br-I) chemistry. We included a two-step nucleation parameterization, where in the first step, the "real" nucleation process is parameterized, i.e., the formation of cluster-sized nuclei via homogeneous condensation of gases. We considered both ternary sulfuric acid-ammonia-water nucleation and homomolecular homogeneous OIO nucleation. For the latter, we derived a parameterization based on combined laboratory-model studies. The second step of the nucleation parameterization treats the "apparent" nucleation rate, i.e., the growth of clusters into the model's lowest size bin by condensable vapors such as OIO. We compared different scenarios for a clean marine versus a polluted continental background atmosphere. In every scenario, we assumed the air to move, independent of its origin, first over a coastal region (where it is exposed to surface fluxes of different reactive iodine precursors) and later over the open ocean. According to these sensitivity studies, in the clean marine background atmosphere OIO can be responsible for both homogeneous nuclei formation and the subsequent growth of the clusters to detectable sizes. In contrast to this, in the continental case with its higher levels of pollutants, gas phase OIO mixing ratios, and hence related nucleation rates, are significantly lower. Compared to ternary H2SO4-NH3-H2O nucleation, homogeneous OIO nucleation can be neglected for new particle formation in this case, but OIO can contribute to early particle growth, i.e., to apparent nucleation rates. In general, we found OIO to be more important for the growth of newly formed particles than for the formation of new nuclei. According to our studies, observations of particle "bursts" can only be explained by hot spot-like, not by homogeneously distributed emissions.

Simulation of VC precipitate evolution in steels with consideration for the formation of new nuclei

A numerical method of simulation of precipitate ensemble evolution is suggested, accounting for the formation of new centres of nucleation. This method is based on the mean field approximation for the description of evolution of the particles present and on the classical theory of nucleation for calculation of the formation rate of new nuclei. It may be used for simulation of evolution of constant composition precipitates in multicomponent alloys at different stages of the process. The method is used for simulation of VC precipitate evolution in steels, and the results of these calculations are compared with the available experimental data.

Effect of mixing on nanoparticle formation in micellar route

Nanoparticle formation using reverse micellar route has been applied to a large variety of nanoparticles in the recent past. In all these preparation methods, two micellar solutions containing reactants are brought together in small volume containers and the contents are agitated. While the mixing issues may not be important for such small-scale operations, large-scale production using this method will call for mixing of large volumes of micellar solutions in either batch systems or continuous reactors. The present study brings out the effect of inefficient mixing on large-scale operations by combining an approximate population balance model for particle formation proposed in this work with an existing model for mixing in turbulent flows. The approximate population balance model has been validated by comparing results with rigorous Monte-Carlo simulations for several cases. The model for capturing the effect of mixing on final particle size distribution visualizes competition between particle growth and formation of new nuclei to be similar to that known for series–parallel reactions. The present study predicts strong effect of intensity of mixing on both the average particle size and the breadth of the size distribution of nanoparticles, indicating that scale up of this process will require considerable care.

Preparation of monodisperse silica particles with controllable size and shape

The effects of dilution of tetraethyl orthosilicate (TEOS) with ethanol on the shape and monodispersity of silica particles were investigated. The results indicated that the dilution of TEOS with ethanol can depress the formation of new nuclei and the aggregation or adhesion of particles and make the distillation of TEOS unnecessary to achieve monodispersed silica spheres. A seeded growth technique using continuous drop addition of TEOS diluted with 4× volume of ethanol was developed to improve monodispersity and spheric shape and increase the size of silica particles. The monodisperse silica particles (150 nm ± 5%−1.2 μm ± 1%) with fine spheric shape were successfully synthesized by the seeded growth technique. Using the homemade 280 nm ± 2.8% silica spheres, we prepared opals of high quality which showed periodically ordered packing and a photonic band-gap effect.

Grain coarsening of hematite in the Gushan iron deposit, Anhui Province, China

The iron ores of the Gushan mine occur in the contact zone of a Mesozoic diorite intrusion and are composed primarily of hematite microcrystallites and chalcedony. The hematite microcrystallites have undergone post-mineralization recrystallization and coarsening with resultant formation of lath-shaped hematite porphyroblasts. Microscopic investigation reveals that recrystallization and coarsening of the hematite ores of the Gushan mine took place without the formation of new nuclei, due to the coalescence of the microcrystallites. The whole process could have begun with the mutual approach of the microcrystallites, followed by grain rotation to realize parallelism and ending by the welding of these grains to form optically homogeneous porphyroblastic hematite.