The Study of Undercooling of Homogeneous Nucleation for Metallic Melting

2007 ◽  
Vol 561-565 ◽  
pp. 1941-1944
Author(s):  
Cheng Huang ◽  
Bo Song ◽  
Pei Zhao
Keyword(s):  
Melting Temperature ◽  
Liquid Iron ◽  
Heterogeneous Nucleation ◽  
Homogeneous Nucleation ◽  
Mathematic Model ◽  
Wetting Angle ◽  
Calculation Results ◽  
Maximum Undercooling

In this paper, the thermodynamic and kinetic requirements of heterogeneous and homogeneous nucleation of metallic melting were suggested. Based on the kinetic requirements of nucleation, the mathematic model of wetting angle of heterogeneous nucleation was developed, Based on the wetting angel model, it was predicted that the maximum undercooling of homogeneous nucleation for melts is two thirds of melting temperature. With the wetting angel model, the wetting angles of different catalysts in liquid iron were calculated, and calculation results are in agreement with that of other researchers.

Amorphous silica coating on α-alumina particles

1995 ◽  
Vol 53 ◽  
pp. 210-211
Author(s):  
J. W. Mellowes ◽  
C. M. Chun ◽  
I. A. Aksay
Keyword(s):  
Amorphous Silica ◽  
Heterogeneous Nucleation ◽  
Homogeneous Nucleation ◽  
Silica Coating ◽  
Full Density ◽  
Optimal Conditions ◽  
Fine Grained ◽  
Alumina Particles ◽  
Complete Mullitization ◽  
Powder Compacts

Mullite (3Al2O32SiO2) can be fabricated by transient viscous sintering using composite particles which consist of inner cores of a-alumina and outer coatings of amorphous silica. Powder compacts prepared with these particles are sintered to almost full density at relatively low temperatures (~1300°C) and converted to dense, fine-grained mullite at higher temperatures (>1500°C) by reaction between the alumina core and the silica coating. In order to achieve complete mullitization, optimal conditions for coating alumina particles with amorphous silica must be achieved. Formation of amorphous silica can occur in solution (homogeneous nucleation) or on the surface of alumina (heterogeneous nucleation) depending on the degree of supersaturation of the solvent in which the particles are immersed. Successful coating of silica on alumina occurs when heterogeneous nucleation is promoted and homogeneous nucleation is suppressed. Therefore, one key to successful coating is an understanding of the factors such as pH and concentration that control silica nucleation in aqueous solutions. In the current work, we use TEM to determine the optimal conditions of this processing.

Direct Nucleation of Crystalline SiGe on Substrates by Reactive Thermal CVD with Si2H6 and GeF4

1997 ◽  
Vol 467 ◽  
Author(s):  
Fumio Yoshizawa ◽  
Kunihiro Shiota ◽  
Daisuke Inoue ◽  
Jun-ichi Hanna
Keyword(s):  
Growth Rate ◽  
Gas Phase ◽  
Heterogeneous Nucleation ◽  
Homogeneous Nucleation ◽  
Film Growth ◽  
Early Stage ◽  
Gaseous Mixture ◽  
The Other ◽  
Thermal Cvd ◽  
Initial Deposition

ABSTRACTPolycrystalline SiGe (poly-SiGe) film growth by reactive thermal CVD with a gaseous mixture of Si2H6 and GeF4 was investigated on various substrates such as Al,Cr, Pt, Si, ITO, ZnO and thermally grown SiO2.In Ge-rich film growth, SEM observation in the early stage of the film growth revealed that direct nucleation of crystallites took place on the substrates. The nucleation was governed by two different mechanisms: one was a heterogeneous nucleation on the surface and the other was a homogeneous nucleation in the gas phase. In the former case, the selective nucleation was observed at temperatures lower than 400°C on metal substrates and Si, where the activation of adsorbed GeF4 on the surface played a major role for the nuclei formation, leading to the selective film growth.On the other hand, the direct nucleation did not always take place in Si-rich film growth irrespective of the substrates and depended on the growth rate. In a growth rate of 3.6nm/min, the high crystallinity of poly-Si0.95Ge0.05in a 220nm-thick film was achieved at 450°C due to the no initial deposition of amorphous tissue on SiO2 substrates.

Numerical Simulation of Heat and Mass Transfer Enhancement in Absorption Process With Double-Side Film-Inversion

2008 ◽  
Author(s):  
Ya-Ping Chen ◽  
Chen-Jie Shi ◽  
Ming-Heng Shi ◽  
Chen-Min Ling
Keyword(s):  
Mass Transfer ◽  
Heat And Mass Transfer ◽  
Vertical Plane ◽  
Mathematic Model ◽  
Absorption Process ◽  
Transfer Coefficients ◽  
Mass Transfer Coefficients ◽  
Plane Plate ◽  
Before And After ◽  
Calculation Results

Film-inversion is an effective way recently developed to enhance heat and mass transfer in absorbers. However, only one-side of round or rectangular tube i.e. half of the total heat transfer area is used to form film-inverting configuration in the published literature. The paper presents a double-side film-inverting scheme, which consists of two plate bundles and a set of comb shaped conjunction guiders between them for leading solution film from both-sides of each couple of the upper plate bundle to the opposite sides of the bottom ones. A two-scale crosswise corrugation plate bundle, which has vertical large corrugations and horizontal small ones, is suggested instead of the plane plate bundle. The horizontal small corrugation can make the film turbulent and film distribution uniform before and after inversion with surface tension effect, thus increasing the heat and mass transfer coefficients of the absorption process. A mathematic model for heat and mass transfer in absorption process with aqueous Li-Br solution falling film-inverting on two sequential vertical plane plates was established and solved numerically. The distributions of dimensionless velocity, temperature and concentration of liquid film profile before and after film-inverting were obtained. The influence of the number of inversion on heat and mass transfer characteristics was analyzed. The calculation results show that the heat and mass transfer coefficients of the once-film-inverting scheme have about 58% and 73% increment respectively over these of the none film-inverting scheme.

scholarly journals Melting Temperature and Wetting Angle of AlN/Dy2O3 and AlN/Yb2O3 Mixtures on SiC Substrates

2015 ◽  
Vol 18 (5) ◽  
pp. 957-962 ◽  
Author(s):  
Aline Corecha Santos ◽  
Ana Paula Luz ◽  
Sebastião Ribeiro
Keyword(s):  
Melting Temperature ◽  
Wetting Angle

Crystal Nucleation in Submicron Droplets of Pure Elements

1993 ◽  
Vol 321 ◽  
Author(s):  
Louis M. Holzman ◽  
Thomas F. Kelly ◽  
W. N. G. Hitchon
Keyword(s):  
Heterogeneous Nucleation ◽  
Homogeneous Nucleation ◽  
Crystal Nucleation ◽  
Droplet Radius ◽  
Electrohydrodynamic Atomization ◽  
Submicron Droplets ◽  
Diffraction Patterns

ABSTRACTLiquid-to-crystal nucleation has been studied extensively through droplet experiments to locate examples of homogeneous nucleation. However, prior to this work very few examples have been found, which implies that the experiments have not been able to isolate heterogeneous nucleants in a small percentage of the droplets as is required. In this research, electrohydrodynamic atomization (EHD) is used to produce sub-Micron droplets of pure elements that are largely free of heterogeneous nucleants.Diffraction patterns of individual EHD-produced droplets are viewed to determine the fraction of crystalline droplets produced as a function of droplet radius. These results are compared to theories for surface and volume heterogeneous nucleation and for homophase nucleation. It is found that Si and Ge nucleate through either homogeneous nucleation or nucleation by homophase impurities. Nucleation results for vanadium and iron were not conclusive.

Bubbles and element clusters in rock melts: A chicken and egg problem

2021 ◽  
Author(s):  
Renelle Dubosq ◽  
Pia Pleše ◽  
Brian Langelier ◽  
Baptiste Gault ◽  
David Schneider
Keyword(s):  
Heterogeneous Nucleation ◽  
Homogeneous Nucleation ◽  
Growth Dynamics ◽  
Gas Bubbles ◽  
Atom Probe ◽  
Bubble Nucleation ◽  
Interface Element ◽  
Rich Cluster ◽  
Heterogeneous Distribution ◽  
Rapid Decompression

<p>The nucleation and growth dynamics of gas bubbles and crystals play a vital function in determining the eruptive behaviour of a magma. Their rate and relative timing, among other factors, are controlled by the magma’s ascent rate. Investigating the kinetics of decompression-induced degassing and crystallization processes can thus give us insight into the rheology of magmas. For example, the rapid decompression of magmas inhibits microlite crystallization and bubble nucleation during ascent leading to crystallization and degassing at shallow levels. This results in a drastic increase in viscosity and an over pressured system, which can lead to violent eruptions. Although many experiments and numerical simulations of magma decompression have been carried out, nascent and initial bubble nucleation remain poorly understood. It is widely accepted that there are two ways bubbles can nucleate within a melt: heterogeneous (on a pre-existing surface) and homogeneous nucleation (within the melt), where homogeneous nucleation requires a higher volatile supersaturation. It has since been tentatively suggested that homogeneous nucleation is simply a variety of heterogeneous nucleation where nucleation occurs on the surface of submicroscopic crystals. However, evidence of these crystals is equivocal. Thus, we have combined novel 2D and 3D structural and chemical microscopy techniques including scanning transmission electron microscopy (STEM), electron energy-loss spectroscopy (EELS) mapping, and atom probe tomography (APT) to investigate the presence of sub-nanometer scale chemical heterogeneities in the vicinity of gas bubbles within an experimental andesitic melt. The combined STEM and EELS data reveal a heterogeneous distribution of bubbles within the melt ranging between 20-100 nm in diameter, some of which have Fe and/or Ca element clusters at the bubble-melt interface. Element clusters enriched in Fe, Ca, and Na are also observed heterogeneously distributed within the melt. The reconstructed APT data reveals bubbles as low ionic density regions overlain by a Na-, Ca-, and K-rich cluster and heterogeneously distributed Fe clusters within the bulk of the melt. Based on these observations, our data demonstrate the existence of nano-scale chemical heterogeneities within the melt and at the bubble-melt interface of bubbles that were previously interpreted to be nucleated homogeneously within the melt, therefore contributing to the proposed hypothesis that homogeneous nucleation could in fact be a variety of heterogeneous nucleation. These results highlight the need to redefine homogeneous nucleation and revisit whether bubbles or crystals occur first within volcanic melts. </p>

Desequilibres entre phases solides et fluides mineraux dans la petrogenese

1962 ◽  
Vol S7-IV (2) ◽  
pp. 157-164 ◽  
Author(s):  
Georges Deicha
Keyword(s):  
Crystal Growth ◽  
Solid Solutions ◽  
Heterogeneous Nucleation ◽  
Homogeneous Nucleation ◽  
Fluid Solution ◽  
Secondary Intervention ◽  
The Moment

Abstract The importance of phenomena conditioned by the disequilibrium between gases, liquids, or magmas, and solid minerals is discussed. Four modes of crystal growth are capable of causing the precipitation of a crystalline species in a supersaturated or supercooled fluid solution. In homogeneous nucleation crystallization occurs at the moment when saturation is reached and only in the presence of preformed crystals of the same species. Germinal nucleation occurs when crystallization develops in conditions of disequilibrium causing the formation of minerals of the same composition but of different structure. Crystals of this mode are twinned. Metastable nucleation involves the formation of a species different in structure from the stable crystal which developed after twinning. In heterogeneous nucleation crystallization of a species has its beginning on the surface of another crystal. This introduces impurities which inhibit nucleation and crystal growth. Secondary intervention of liquid or solid solutions in the rocks also produces phenomena equally effected by disequilibrium.

A model for the simultaneous heterogeneous and homogeneous nucleation of gas bubbles

1979 ◽  
Vol 368 (1734) ◽  
pp. 331-343 ◽  
Keyword(s):  
Heterogeneous Nucleation ◽  
Homogeneous Nucleation ◽  
Bubble Nucleation ◽  
Rate Theory ◽  
Rare Gases ◽  
Bubble Density ◽  
Competitive Mechanism ◽  
The Matrix ◽  
Arrhenius Energy ◽  
First Time

Electron microscope observations of cavities in steels have shown that they are often associated with microstructural features as well as occurring randomly in the matrix. Previous theoretical studies of gas bubble nucleation have concentrated on either homogeneous or heterogeneous nucleation. In the present work we extend the homogeneous nucleation rate theory for rare gases in solids to include, for the first time, the competitive mechanism of heterogeneous nucleation. The model indicates that the temperature dependence of the bubble density varies as the binding energy between a trap and a single gas atom is changed, and provides an interpretation of the high Arrhenius energy found from observed cavity densities.

Heat Transfer Analysis of the Micron-Scale Agglomerates of TiO2 Precursor during the Detonation Process

2011 ◽  
Vol 306-307 ◽  
pp. 1138-1141
Author(s):  
Yan Dong Qu ◽  
Xiao Jie Li ◽  
Hong Hao Yan
Keyword(s):  
Heat Transfer ◽  
Mathematic Model ◽  
Heat Transfer Analysis ◽  
Temperature History ◽  
Detonation Products ◽  
Synthesis Mechanism ◽  
Temperature Distributions ◽  
Calculation Results ◽  
Detonation Method ◽  
Detonation Process

Removing water excessively could result in the formation of sphere-like agglomerates of TiO2 precursor with 1-2 μm in size, which was used to prepare TiO2 nanoparticles by detonation method. Different temperature distributions of TiO2 precursor agglomerates influenced the components of detonation products. In order to obtain the temperature distributions, a mathematic model was introduced to study the heat transfer behaviors of the TiO2 precursor agglomerates during the detonation process. The temperature history at spherical center with different spherical radiuses and time was also studied. The calculation results were in good agreement with the experimental results. The heat transfer analysis laid the foundation for the synthesis mechanism research of TiO2 nanoparticles, and it was also helpful to design and optimize experimental procedure.

Transient Nucleation in Devitrification

1993 ◽  
Vol 321 ◽  
Author(s):  
A. L. Greer
Keyword(s):  
Classical Theory ◽  
Heterogeneous Nucleation ◽  
Glass Formation ◽  
Homogeneous Nucleation ◽  
Amorphous Solids ◽  
Crystal Nucleation ◽  
Transient Effects ◽  
Interphase Interface ◽  
Transient Nucleation ◽  
Important Test

ABSTRACTA review is given of transient crystal nucleation in glassy or amorphous solids. The types of behaviour are surveyed. It is shown that the kinetics can be quantitatively modelled and that the matching of experiment and theory provides an important test of the classical theory. Examples are considered of homogeneous nucleation (affecting glass formation), heterogeneous nucleation, and nucleation at an interphase interface. While the emphasis is on transient effects of the kind implicit in the classical theory, it is shown that transients can arise for other reasons as well, thus potentially complicating the interpretation of experiments.

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