Search results for "nucleation"
showing 10 items of 364 documents
The growth of charged platelets.
2014
Growth models of charged nanoplatelets are investigated with Monte Carlo simulations and simple theory. In a first model, 2-dimensional simulations in the canonical ensemble are used to demonstrate that the growth of a single weakly charged platelet could be limited by its own internal repulsion. The short range attractive interaction in the crystal is modeled with a square well potential while the electrostatic interactions are described with a screened Coulomb potential. The qualitative behavior of this case can also be described by simply balancing the attractive crystal energy with the screened Coulomb repulsion between the crystal sites. This repulsion is a free energy term dominated b…
Nucleation and cavitation in parahydrogen
2012
We have used a density functional approach to investigate thermal homogeneous nucleation and cavitation in parahydrogen. The effect of electrons as seeds of heterogeneous cavitation in liquid parahydrogen is also discussed within the capillary model. (C) 2011 Elsevier B.V. All rights reserved.
Hydration of alite containing aluminium
2011
Abstract The most important phase in Portland cement is tricalcium silicate, which leads during its hydration to the nucleation and growth of calcium silicate hydrate [referred to as C–S–H, (CaO)x–SiO2–(H2O)y]. The development of this hydrate around the cement grains is responsible for the setting and hardening of cement pastes. The general term for designating the tricalcium silicate in cements is alite. This name relates to all polymorphs containing various foreign ions inserted in their structure. These ions may influence the intrinsic reactivity, and once released during the dissolution, they may interact also with C–S–H. One of the most likely species to be inserted in the alite struct…
Two-Step Nucleation Process of Calcium Silicate Hydrate, the Nanobrick of Cement
2018
Despite a millennial history and the ubiquitous presence of cement in everyday life, the molecular processes underlying its hydration behavior, like the formation of calcium–silicate–hydrate (C–S–H), the binding phase of concrete, are mostly unexplored. Using time-resolved potentiometry and turbidimetry combined with dynamic light scattering, small-angle X-ray scattering, and cryo-TEM, we demonstrate C–S–H formation to proceed via a complex two-step pathway. In the first step, amorphous and dispersed spheroids are formed, whose composition is depleted in calcium compared to C–S–H and charge compensated with sodium. In the second step, these amorphous spheroids crystallize to tobermorite-typ…
Experimental investigation of calcium silicate hydrate (C-S-H) nucleation
1999
Due to the importance of calcium silicate hydrate (C-S-H) in cement chemistry, its nucleation mode and parameters influencing it were investigated. It has been observed that the C-S-H nucleation follows the general laws governing the nucleation. The degree of supersaturation has been found to be the main parameter controlling homogeneous nucleation rates. The lime concentration in solution, well known to be the most important parameter determining the kinetic, morphological and structural features of C-S-H, also controls the nucleation characteristics of heterogeneous nucleation, i.e. during hydration of cement. The correlation between heterogeneous nucleation of C-S-H and possible final me…
Nucleation mechanism for the direct graphite-to-diamond phase transition
2011
Graphite and diamond have comparable free energies, yet forming diamond from graphite is far from easy. In the absence of a catalyst, pressures that are significantly higher than the equilibrium coexistence pressures are required to induce the graphite-to-diamond transition. Furthermore, the formation of the metastable hexagonal polymorph of diamond instead of the more stable cubic diamond is favored at lower temperatures. The concerted mechanism suggested in previous theoretical studies cannot explain these phenomena. Using an ab initio quality neural-network potential we performed a large-scale study of the graphite-to-diamond transition assuming that it occurs via nucleation. The nucleat…
Electrocrystallization of Monolayer-Protected Gold Clusters : Opening the Door to Quality, Quantity, and New Structures
2017
Thiolate-protected metal clusters are materials of ever-growing importance in fundamental and applied research. Knowledge of their single-crystal X-ray structures has been instrumental to enable advanced molecular understanding of their intriguing properties. So far, however, a general, reliable, chemically clean approach to prepare single crystals suitable for accurate crystallographic analysis was missing. Here we show that single crystals of thiolate-protected clusters can be grown in large quantity and very high quality by electrocrystallization. This method relies on the fact that charged clusters display a higher solubility in polar solvents than their neutral counterparts. Nucleation…
Snapshots of calcium carbonate formation – a step by step analysis
2017
Abstract Recent advances in our understanding of CaCO3 nucleation from solution have provoked new and challenging questions. We have studied CaCO3 formation using precipitation by carbonate ester hydrolysis which ensures precipitation from a strictly homogeneous solution state and allows “titrating” carbonate to a solution with a given Ca2+ concentration on a timescale suited for kinetic studies. Nucleation and crystallization were traced by combining dynamic light scattering (DLS) and transmission electron microscopy (TEM). DLS served as in situ technique to identify the nucleation time, to monitor particle size evolution, to discriminate different precipitation mechanisms and to validate …
Precipitation and calcination synthesis methods forming nano-sized platinum catalytic particles for methanol and hydrogen oxidation
2013
Under varying experimental conditions of calcination and precipitation reactions, different particle sizes and levels of platinum on carbon supported (Pt/C) catalysts were obtained. Rapid precipitation following a chemical reaction ensured formation of nano-sized catalytic particles using super-saturated concentrations under controlled conditions was a significant contribution in understanding the synthesis process and how it relates to an increased number of catalytic reaction sites ultimately providing superior electrochemical (EC) activity. These conditions influenced nucleation and growth rates of the catalytic particles. The super-saturation concentrations of the reactants in the react…
Template induced crystallization of biominerals on self-assembled monolayers of alkylthiols
1998
Abstract We have studied the templated crystallization of the three polymorphs of calcium carbonate on self-assembled monolayers of ω -substituted alkylthiols on gold surfaces. Our interest has been to model biomineralization processes by examining the effect of structured organic templates on heterogeneous nucleation at the inorganic–organic interphase. The crystallization of the three polymorphs was carried out from solution at two different temperatures. The crystals that were formed on the surface were examined using scanning electron microscopy. Phase quantification was achieved through Rietveld analysis of the X-ray diffraction profiles. This study provides significant clues on factor…