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Nucleation Theory (nucleation + theory)

Distribution by Scientific Domains

Polymers and Materials Science	63%

Selected Abstracts

Reconciling the Classical Nucleation Theory and Atomic Scale Observations and Modeling,

ADVANCED ENGINEERING MATERIALS, Issue 12 2006
G. Martin
Here the author recalls the classical assumptions and the resulting expressions to be used in Cluster Dynamics stricto sensu and then, he shows how to establish a precise formal link between Classical Nucleation Theory (CNT) and Cluster Dynamics (CD). [source]

Driving force in first-order phase transitions and its application to gas hydrate nucleation from a single phase

AICHE JOURNAL, Issue 9 2009
Bernardo Carreón-Calderón
Abstract Classical nucleation theories of general application are taken as starting point to analyze the driving force for multicomponent gas hydrate nucleation from a single homogeneous phase. It is shown that the ratio between the specific surface energy and the critical radius of nucleation has a single value irrespective of the analyzed driving force expression. From this result, two driving force expressions for multicomponent gas hydrate nucleation are derived in the context of the so-called generalized nucleation theory, and it is demonstrated that the driving force for gas hydrates can be estimated using the same information given for the determination of the incipient formation points of the dispersed phase from a saturated phase. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source]

Determination of nucleation parameters and the solid liquid interfacial energy of the KCl-ethanol-water system

CRYSTAL RESEARCH AND TECHNOLOGY, Issue 5 2007
Waid Omar
Abstract The kinetic parameters of homogeneous nucleation of KCl in different ethanol-water solvent mixtures were determined at 25°C from the experimental measurements of the width of the metastable zone at different cooling rates. The ethanol mass ratio in the ethanol water solvent mixture was varied from 0-0.9 and the metastable zone width for each solvent mixture was measured under the cooling rates of 10, 20 and 30 K/h. The influence of ethanol ratio on the activity coefficient was calculated. It was found that increasing the ethanol ratio in the solvent mixture leads to an increase in the mean molal activity coefficient. The experimental results obtained showed that the increase in the ethanol ratio in the solvent widens the metastable zone for the crystallization of KCl. Also it has inferred from the calculations based on the classical nucleation theory that increasing of the ethanol ratio in the solvent mixture resulted in an increase of the nucleation rate order, increase of the critical size of nuclei and increase of the solid liquid interfacial energy. It has been found that the solid liquid interfacial energy can be good correlated with inversely proportionality to the solubility. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Kinetic Monte Carlo Simulations of Precipitation,

ADVANCED ENGINEERING MATERIALS, Issue 12 2006
E. Clouet
Abstract We present some recent applications of the atomistic diffusion model and of the kinetic Monte Carlo (KMC) algorithm to systems of industrial interest, i.e. Al-Zr-Sc and Fe-Nb-C alloys, or to model systems. These applications include study of homogeneous and heterogeneous precipitation as well as of phase transformation under irradiation. The KMC simulations are also used to test the main assumptions and limitations of more simple models and classical theories used in the industry, e.g. the classical nucleation theory. [source]

Driving force in first-order phase transitions and its application to gas hydrate nucleation from a single phase

Solidification in heat packs: III.

AICHE JOURNAL, Issue 2 2003
Metallic trigger
The metallic trigger used in commercial heat packs initiates solidification by releasing minute crystals of solid sodium acetate trihydrate into the subcooled solution. These crystals are harbored in submicron cracks on the disk's surface and are released when the disk is flexed. Using scanning electron microscopy, such seed crystals are observed on the surface of a disk after flexing it. Classic nucleation theory is used to investigate the behavior of crystals residing in the cracks on the metallic disk's surface during heating and cooling. Sodium acetate trihydrate crystals are capable of surviving in a 1-nm crack or smaller at regeneration temperatures of the order of 353 K. These seed crystals grow to the mouth of the crack at temperatures below the liquidus, but can only promote solidification of the whole solution surrounding the disk at temperatures below 256 K or when the disk is flexed. [source]

Controlled Crystallization of Calcite Under Surface Electric Field Due to Polarized Hydroxyapatite Ceramics

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 7 2009
Norio Wada
We examined effects of surface electric fields for the crystallization of calcite on polarized hydroxyapatite ceramics with and without polyacrylic acid (PAA) as soluble additive. Both on negatively and positively charged surfaces without PAA, the only precipitates were rhombohedra calcite crystals with the face of the {10.4} plane favorably oriented parallel to the surfaces. This oriented growth was explained by the nucleation theory in the presence of an external electric field. However, the addition of PAA drastically changed the situation of the calcite crystals, i.e., the crystallites were the hemispheric aggregates of calcite needles with a facetted rhombohedral {10.4} end face and flat island-shaped aggregates of ones with a rough (00.1) end face having a triangular shape. The calcite needles grew along the crystallographic [00.1] axis. This oriented growth was explained by epitaxy on the PAA,Ca2+ complexes adsorbing on the surfaces. The morphology of the PAA,Ca2+ complex assemblies adsorbing on the surfaces before the calcite nucleation was an important factor to control the structure of calcite aggregates formed following. This morphology was controlled by properties of the surface electric field and the spatial distribution of the negatively and positively charged sites in the PAA,Ca2+ complexes. [source]

Abnormal Grain Growth in Alumina with Anorthite Liquid and the Effect of MgO Addition

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2002
Chan Woo Park
Abnormal grain growth (AGG) in alumina with anorthite liquid has been observed with varying anorthite and MgO contents, at 1620°C. When only anorthite is added to form a liquid matrix, the grain,liquid interfaces have either flat or hill-and-valley shapes indicating atomically flat (singular) structures. The large grains grow at accelerated rates to produce AGG structures with large grains elongated along their basal planes. This is consistent with the slow growth at low driving forces and accelerated growth above a critical driving force predicted by the two-dimensional nucleation theory of surface steps. With increasing temperature, the AGG rate increases. The number density of the abnormally large grains increases with increasing anorthite content. The addition of MgO causes some grain,liquid interfaces to become curved and hence atomically rough. The grains also become nearly equiaxed. With increasing MgO content the number density of the abnormally large grains increases until the grain growth resembles normal growth. This result is qualitatively consistent with the decreasing surface step free energy associated with partial interface roughening transition. [source]

Effect of die temperature on the morphology of microcellular foams

POLYMER ENGINEERING & SCIENCE, Issue 6 2003
Xiangmin Han
A study on the extrusion of microcellular polystyrene foams at different foaming temperatures was carried out using CO2 as the foaming agent. The contraction flow in the extrusion die was simulated with FLUENT computational fluid dynamics code at two temperatures (150°C and 175°C) to predict pressure and temperature profiles in the die. The location of nucleation onset was determined based on the pressure profile and equilibrium solubility. The relative importance of pressure and temperature in determining the nucleation rate was compared using calculations based on classical homogeneous nucleation theory. Experimentally, the effects of die temperature (i.e., the foaming temperature) on the pressure profile in the die, cell size, cell density, and cell morphology were investigated at different screw rotation speeds (10 , 30 rpm). Experimental results were compared with simulations to gain insight into the foaming process. Although the foaming temperature was found to be less significant than the pressure drop or the pressure drop rate in deciding the cell size and cell density, it affects the cell morphology dramatically. Open and closed cell structures can be generated by changing the foaming temperature. Microcellular foams of PS (with cell sizes smaller than 10 ,m and cell densities greater than 10 cells/cm3) are created experimentally when the die temperature is 160°C, the pressure drop through the die is greater than 16 MPa, and the pressure drop rate is higher than 109 Pa/sec. [source]

The turning point on plots of log , and log t of Mo's equation

POLYMER INTERNATIONAL, Issue 7 2009
Jian-bin Song
Abstract BACKGROUND: Mo's equation based on the Avrami equation and Ozawa equation has been successfully used in non-isothermal crystallization kinetics by many researchers. However, in recent years we have found that plots of log ,,log t of Mo's equation are not straight lines, but there appears a turning point at , = 7 °C min,1 or so. The aim of this article is to analyze in detail the reason for the occurrence of this turning point by studying the non-isothermal crystallization of poly(vinylidene fluoride) (PVDF) using differential scanning calorimetry. RESULTS: A turning point at about 7 °C min,1 appeared on plots of log ,,log t. The cooling rate where the turning point occurred showed little change with increasing relative crystallinity, but the temperature decreased. It is noted that this turning-point temperature is lower than transition temperature of regime I , II reported for PVDF. CONCLUSION: We deemed that the occurrence of turning points on plots of log ,,log t of Mo's equation can be ascribed to the different crystallization regimes based on the Hoffman nucleation theory, and presumed that the turning point corresponded to transition of regime I , II. Finally, we considered that Mo's equation should be analyzed using two beelines, instead of one beeline. Copyright © 2009 Society of Chemical Industry [source]