Morphokinetics: Growth of Mesoporous Silica Curved Shapes
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The nucleation and growth of mesoporous silica fibers and gyroids was the focus of this kinetic study. The authors obtained images of the smallest objects implicated in the creation of these shapes by tapping-mode atomic force microscopy and transmission electron microscopy. Dynamic light scattering provided temporal information on the size evolution of the growth objects. It is shown that increasing pH correlates with slower nucleation and a shape transition from fibers to gyroids. This knowledge is essential to further advancement of the synthesis of mesoporous materials with controlled morphologies.Crystal (programming language)
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Nucleation of barite (BaSO4) has broad implications in geological, environmental, and materials sciences. While impurity metals are common, our understanding of how they impact nucleation remains dim. Here, we used classical optical microscopy compared to fast X-ray nanotomography (XnT) to investigate heterogeneous nucleation of barite on silica in situ with Sr2+ as an impurity ion. The observed barite nucleation rates were consistent with classical nucleation theory (CNT), where barite crystals displayed a nonuniform size distribution, exhibiting distinct morphologies and incubation periods in Sr-free solutions. While undetectable with optical microscopy, nanotomography revealed that addition of Sr2+ enhances nucleation rates driven by the pre-factor in CNT, likely because both adsorbed Ba2+ and Sr2+ act as precursor sites on which nucleation occurs. Sr2+ simultaneously inhibits growth, however, leading to a homogeneous distribution of smaller crystals. This finding will enable an improved predictive understanding of nucleation in natural and synthetic environments.
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Nucleation plays a key role in solidification processing.The present overview focuses on a series of competitions incorporated in nucleation upon non-equilibrium solidification of bulk undercooled alloy melt.According to the classical nucleation theory and the newly developments in nucleation field,three nucleation modes,namely,continuous nucleation,site saturation and free growth are proposed for different solidification conditions.Competition of nucleation modes,which consists of continuous nucleation and athermal nucleation(site saturation and free growth),competition between one point and copious nucleation,competition between homogeneous and heterogeneous nucleation,were separately discussed in various alloy systems.At the same time,experiment evidences were given and analyzed to prove these competitions.Additionally,some criterions for determining nucleation mechanism were also proposed.
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An experimental investigation to explore the interaction between bubbles forming at adjacent nucleation sites is presented. The results obtained are consistent with the results of Calka’s and Knowles’ experimental investigations and confirm that nucleation site activation/deactivation, whereby a bubble growing at a nucleation site is able to promote/hinder the formation of a bubble at an adjacent nucleation site by depositing/displacing a vapor nucleus in the nucleation cavity, is instrumental in determining how a bubble forming at a nucleation site influences the nucleation of the subsequent bubble at an adjacent nucleation site.
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The nucleation and growth kinetics for the synthesis of ZnO nanoparticles from ZnCl2 and NaOH in ethanol were determined as a function of the reactant concentrations and the concentration of added water. It was found that the presence of water is essential for the controlled nucleation of ZnO. The nucleation rate increases with increasing reactant concentrations, which is in agreement with classical nucleation theory.
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When the nucleation of a stable crystalline phase directly in a supersaturated old phase is greatly retarded, the crystal nuclei might nucleate within faster-forming particles of an intermediate phase. Here we present a theoretical investigation of the kinetics of this two-step nucleation of crystals and derive general expressions for the time dependence of the number of crystals nucleated within the particles of the intermediate phase. The results reveal that crystal nucleation can be strongly delayed by the slow growth of the particles and/or by the slow nucleation of the crystals in them. Furthermore, the linear part of the time dependence of the number of nucleated crystals is determined by the formation rate of the intermediate particles. This is in contrast with the one-step nucleation of crystals when this linear part is determined by the rate of crystal nucleation directly in the old phase. Criteria are proposed for distinction between the one- and two-step nucleation mechanisms, based on the supersaturation dependence of the delay time for nucleation. The application of the theoretical approach to the analysis of experimental data on the nucleation of crystals and other ordered aggregates of protein and other soluble materials is discussed.
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The induction times for electrodeposition of individual Ag nanoparticles on Pt nanodisk electrodes in acetonitrile were used to determine the critical nucleus size and activation energy barrier associated with the formation of Ag nuclei. Induction times for the nucleation and growth of a single Ag nanoparticle were determined following the application of a potential step to reduce Ag+ at overpotentials, η, ranging from −130 to −70 mV. Sufficiently small Pt electrodes (5.1 × 10–10–2.6 × 10–11 cm2) were used to ensure that the detection of a single Ag nucleation event occurred during the experimental observation time (150 ms–1000 s). Multiple measurements of Ag nucleation induction times were recorded to determine nucleation rates as a function of η using cumulative probability theory. Both classical nucleation theory (CNT) and the atomistic theory of electrochemical nucleation were employed to analyze experimental nucleation rates, without a requisite knowledge of the nucleus geometry or surface free energy. Using the CNT, the number of atoms comprising the critical size nucleus, Nc, was estimated to be 1–9 atoms for η ranging from −130 to −70 mV, in good agreement with 1–5 atoms obtained using atomistic theory. The experimental nucleation rates were also used to determine the activation energy barriers for nucleation from the CNT, which varied from 3.31 ± 0.05 to 13 ± 1 kT over the same range of η.
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