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.

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.

The Importance of Transient Nucleation and Non-Equilibrium Viscosity for Glass Formation

1985 ◽  
Vol 57 ◽  
Author(s):  
Kenneth F. Kelton
Keyword(s):  
Steady State ◽  
Metallic Glasses ◽  
Glass Formation ◽  
Volume Fraction ◽  
Molecular Cluster ◽  
Transient Effects ◽  
Glass Forming ◽  
Model Glass ◽  
Transient Nucleation ◽  
Non Equilibrium

AbstractThe process of nucleation and growth in glasses and undercooled liquids is modeled by directly simulating the evolution of the molecular cluster distribution under both isothermal and non-isothermal conditions. Results of that simulation for the nucleation rate during the quench, and for the number of nuclei produced and the volume fraction transformed at the end of the quench are presented. The following three points are discussed: (1) The importance of transient, or non-steady state, nucleation rates on glass formation is assessed by considering three model glass forming systems: lithium disilicate, a relatively good glass former, and two metallic glasses, (Au85Cu15)77Si9Gd14 and Au81Si19. (2) Using experimentally determined values for the steady state nucleation rates and growth velocities for Pd40Ni40P20, it is demonstrated that, in agreement with recent experimental results, this alloy may be cycled at rates on the order of 1 K/sec between the melting and glass transition temperatures without crystallization. Transient effects are shown to be unimportant under these conditions in this system. (3) The effect on glass formation of a non-equilibrium viscosity during the quench due to configurational freezing is evaluated by assuming a phenomenological model for the changing viscosity.

scholarly journals Heterogeneous versus homogeneous crystal nucleation of hard spheres

Soft Matter ◽  
2019 ◽  
Vol 15 (47) ◽  
pp. 9625-9631 ◽  
Author(s):  
Jorge R. Espinosa ◽  
Carlos Vega ◽  
Chantal Valeriani ◽  
Daan Frenkel ◽  
Eduardo Sanz
Keyword(s):  
Nucleation Rate ◽  
Heterogeneous Nucleation ◽  
Cell Walls ◽  
Homogeneous Nucleation ◽  
Hard Spheres ◽  
Crystal Nucleation ◽  
Experimental Measurements ◽  
Homogeneous Crystal

Heterogeneous nucleation at the cell walls may at least partly explain the reported discrepancy between experimental measurements and simulation estimates of the homogeneous nucleation rate.

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.

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>

Molecular dynamics simulation of homogeneous nucleation of supersaturated potassium chloride (KCl) in aqueous solutions

CrystEngComm ◽  
2019 ◽  
Vol 21 (48) ◽  
pp. 7507-7518 ◽  
Author(s):  
Soroush Ahmadi ◽  
Yuanyi Wu ◽  
Sohrab Rohani
Keyword(s):  
Molecular Dynamics ◽  
Aqueous Solution ◽  
Molecular Dynamics Simulation ◽  
Aqueous Solutions ◽  
Potassium Chloride ◽  
Homogeneous Nucleation ◽  
Md Simulation ◽  
Crystal Nucleation ◽  
Dynamics Simulation

Molecular dynamics (MD) simulation is used to investigate the mechanism of crystal nucleation of potassium chloride (KCl) in a supersaturated aqueous solution at 293 K and 1 atm.

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.

Nucleation and Glass Formation

1985 ◽  
Vol 57 ◽  
Author(s):  
D. R. Uhlmann ◽  
M. C. Weinberg
Keyword(s):  
Glass Formation ◽  
Nucleation Theory ◽  
Crystal Nucleation ◽  
Oxide Glass ◽  
Classical Nucleation Theory ◽  
Nucleation Kinetics ◽  
Formation Behavior ◽  
Homogeneous Crystal ◽  
Exponential Factors

AbstractThe role of nucleation kinetics in affecting glass formation behavior is discussed. Also considered are measurements of homogeneous crystal nucleation in a variety of liquids. For a number of oxide glass-forming liquids, available data indicate pre-exponential factors which are larger than those predicted from classical nucleation theory by factors of 1017 to 1049. Possible sources of this discrepancy are discussed.

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.

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