scholarly journals The Effect of Hydrogen on Pore Formation in Aluminum Alloy Castings: Myth Versus Reality

Metals ◽  
2020 ◽  
Vol 10 (3) ◽  
pp. 368 ◽  
Author(s):  
Murat Tiryakioğlu
Keyword(s):  
Pore Formation ◽  
Liquid Aluminum ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Fundamental Change ◽  
Aluminum Casting ◽  
Solid Aluminum ◽  
Alloy Castings ◽  
Casting Industry

The solubility of hydrogen in liquid and solid aluminum is reviewed. Based on classical nucleation theory, it is shown that pores cannot nucleate either homogeneously or heterogeneously in liquid aluminum. Results of in situ studies on pore formation show that pores appear at low hydrogen supersaturation levels, bypassing nucleation completely. The results are explained based on the bifilm theory introduced by Prof. John Campbell, as this theory is currently the most appropriate, and most likely, the only mechanism for pores to form. Examples for the effect of hydrogen on pore formation are given by using extreme data from the literature. It is concluded that a fundamental change in how hydrogen is viewed is needed in aluminum casting industry.

scholarly journals Reversible disorder-order transitions in atomic crystal nucleation

Science ◽  
2021 ◽  
Vol 371 (6528) ◽  
pp. 498-503
Author(s):  
Sungho Jeon ◽  
Taeyeong Heo ◽  
Sang-Yeon Hwang ◽  
Jim Ciston ◽  
Karen C. Bustillo ◽  
...  
Keyword(s):  
Early Stage ◽  
Nucleation Theory ◽  
Crystal Nucleation ◽  
Classical Nucleation Theory ◽  
Gold Nanocrystals ◽  
Size Dependent ◽  
In Situ Electron Microscopy ◽  
Nucleation Mechanisms ◽  
Structural Fluctuations

Nucleation in atomic crystallization remains poorly understood, despite advances in classical nucleation theory. The nucleation process has been described to involve a nonclassical mechanism that includes a spontaneous transition from disordered to crystalline states, but a detailed understanding of dynamics requires further investigation. In situ electron microscopy of heterogeneous nucleation of individual gold nanocrystals with millisecond temporal resolution shows that the early stage of atomic crystallization proceeds through dynamic structural fluctuations between disordered and crystalline states, rather than through a single irreversible transition. Our experimental and theoretical analyses support the idea that structural fluctuations originate from size-dependent thermodynamic stability of the two states in atomic clusters. These findings, based on dynamics in a real atomic system, reshape and improve our understanding of nucleation mechanisms in atomic crystallization.

scholarly journals An aerosol chamber investigation of the heterogeneous ice nucleating potential of refractory nanoparticles

2010 ◽  
Vol 10 (3) ◽  
pp. 1227-1247 ◽  
Author(s):  
R. W. Saunders ◽  
O. Möhler ◽  
M. Schnaiter ◽  
S. Benz ◽  
R. Wagner ◽  
...  
Keyword(s):  
Silicon Oxide ◽  
Cloud Model ◽  
Active Surface ◽  
Accommodation Coefficient ◽  
Contact Angles ◽  
Nucleation Theory ◽  
Cloud Formation ◽  
Classical Nucleation Theory ◽  
Ice Nuclei

Abstract. Nanoparticles of iron oxide (crystalline and amorphous), silicon oxide and magnesium oxide were investigated for their propensity to nucleate ice over the temperature range 180–250 K, using the AIDA chamber in Karlsruhe, Germany. All samples were observed to initiate ice formation via the deposition mode at threshold ice super-saturations (RHithresh) ranging from 105% to 140% for temperatures below 220 K. Approximately 10% of amorphous Fe2O3 particles (modal diameter = 30 nm) generated in situ from a photochemical aerosol reactor, led to ice nucleation at RHithresh = 140% at an initial chamber temperature of 182 K. Quantitative analysis using a singular hypothesis treatment provided a fitted function [ns(190 K)=10(3.33×sice)+8.16] for the variation in ice-active surface site density (ns:m−2) with ice saturation (sice) for Fe2O3 nanoparticles. This was implemented in an aerosol-cloud model to determine a predicted deposition (mass accommodation) coefficient for water vapour on ice of 0.1 at temperatures appropriate for the upper atmosphere. Classical nucleation theory was used to determine representative contact angles (θ) for the different particle compositions. For the in situ generated Fe2O3 particles, a slight inverse temperature dependence was observed with θ = 10.5° at 182 K, decreasing to 9.0° at 200 K (compared with 10.2° and 11.4° respectively for the SiO2 and MgO particle samples at the higher temperature). These observations indicate that such refractory nanoparticles are relatively efficient materials for the nucleation of ice under the conditions studied in the chamber which correspond to cirrus cloud formation in the upper troposphere. The results also show that Fe2O3 particles do not act as ice nuclei under conditions pertinent for tropospheric mixed phase clouds, which necessarily form above ~233 K. At the lower temperatures (<150 K) where noctilucent clouds form during summer months in the high latitude mesosphere, higher contact angles would be expected, which may reduce the effectiveness of these particles as ice nuclei in this part of the atmosphere.

Study Of Nucleation And Growth Ev Al-Zn Alloys Using TEM

1993 ◽  
Vol 321 ◽  
Author(s):  
G. Sundar ◽  
E. A. Kenik ◽  
J. J. Hoyt ◽  
S. Spooner
Keyword(s):  
Completion Time ◽  
Nucleation And Growth ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Initial Value ◽  
Two Phase ◽  
Transmission Electron ◽  
Time Required ◽  
Zn Alloys

ABSTRACTNucleation and growth studies were conducted on Al-Zn alloys at several temperatures using transmission electron Microscopy (TEM) with an in-situ furnace. The value of the critical undercooling was established by noting the lowest temperature at which precipitates were no longer observed, following a quench into the two-phase metastable region. These results were compared with the Langer-Schwartz Model of nucleation and growth in which it is predicted that the half-completion time (i.e, the time required for the supersaturation to reach half its initial value) diverges for initial supersaturations which are higher than those predicted by the classical nucleation theory.

An in Situ Hrem Study of Crystal Nucleation in Amorphous Silicon thin Films

1990 ◽  
Vol 202 ◽  
Author(s):  
A. S. Kirtikar ◽  
J. Morgiel ◽  
R. Sinclair ◽  
I-W. Wu ◽  
A. Chiang
Keyword(s):  
Thin Films ◽  
Solid Phase ◽  
High Resolution Electron Microscopy ◽  
Nucleation Theory ◽  
Crystal Nucleation ◽  
Classical Nucleation Theory ◽  
Interface Reactions ◽  
Silicon Thin Films ◽  
Resolution Electron

ABSTRACTIn Situ high resolution electron microscopy has proved to be a valuable tool in investigations involving interface reactions in a number of thin film systems. We have applied this technique to dynamically record nucleation and growth sequences during the amorphous (a-) to crystalline (c-) phase transformation in silicon thin films. Interpretation of the recordings has yielded a wealth of information on the mechanisms and to some extent the kinetics of solid phase crystallization. In our recordings, we have been able to capture the critical nucleus at the a-Si-SiO2 interface. Incorporating this into classical nucleation theory enables us to make an estimate of the a-Si-c-Si interfacial energy.

Altitude Test Facility Humidity Control to Generate Defined Icing Conditions

2020 ◽  
Vol 142 (2) ◽  
Author(s):  
Felix M. Barth ◽  
Stephan Staudacher ◽  
Constanze Schiewe
Keyword(s):  
Optical Measurements ◽  
Nucleation Theory ◽  
Test Facility ◽  
Classical Nucleation Theory ◽  
Model Parameters ◽  
Fundamental Part ◽  
Aerosol Particle Size ◽  
The University ◽  
Ingested Nanoparticles

Abstract In altitude test facility (ATF) operation, the requirements to control humidity to generate defined icing conditions are gaining more and more importance. In this context, the ability to predict humidity and condensation becomes a fundamental part of ATF control. For this purpose, classical nucleation theory has been applied in combination with in situ measurements to derive a model suitable to predict the onset of condensation during very low temperature ATF operation. The model parameters have been acquired inside the ATF of the University of Stuttgart downstream of its air coolers. This makes the application or assumption of generalized atmospheric aerosol data unnecessary. Polydisperse nano aerosol distributions were measured and statistically evaluated, showing that a constant distribution of nano aerosol particle size can be assumed. The composition of the ingested nanoparticles was analyzed and Arizona test dust was chosen as a valid substitute material for the application in the prediction model leading to a conservative prediction. The approach has been successfully verified using optical measurements during ATF testing. Its prediction accuracy fulfills the requirements of ATF control for a variety of icing conditions in component and engine altitude testing.

Atomistic Simulation of Pore Formation in Lipid Bilayers in the Presence of Dimethylsulfoxide: Further Evidence for Entropic Driven Pore Formation

2008 ◽  
Author(s):  
Raghava Alapati ◽  
Dorel Moldovan ◽  
Ram Devireddy
Keyword(s):  
Lipid Bilayer ◽  
Lipid Bilayers ◽  
Pore Formation ◽  
Configurational Entropy ◽  
Line Tension ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Pore Shape ◽  
Dmso Concentration ◽  
Pore Nucleation

In a recent study, Moldovan et al [1] have demonstrated that in the presence of 11.3 mol% dimethylsulfoxide (DMSO) a pore can nucleate and grow spontaneously in a lipid bilayer even in the absence of an externally applied stresses or electric field. The authors rationalized the spontaneous pore nucleation, in the absence of any external driving force, by considering the combined effect of the entropy of pore shape fluctuation and the significant decrease of the bilayer line tension in the presence of DMSO. Building on the classical nucleation theory developed three decades ago by Lister [2] the authors propose a new formulation for the bilayer free energy that incorporates the pore shape configurational entropy. According to this study, in the presence of DMSO, the pore nucleates spontaneously and grows provided the bilayer line tension decreases below a threshold value, λ. In this study we report our recent simulation results on the effect of DMSO concentration on both bilayer line tension and bilayer structural stability with respect to pore nucleation. The lipid bilayer systems investigated in this study by molecular dynamics (MD) consists of 96 molecules (48 in each leaflet) of dimyristoylphosphatidylcholine (DMPC) immersed in DMSO-water solutions at various concentrations. In all MD simulations reported the bilayer systems were followed over 100 ns. Our simulations results indicate the existence of a critical DMSO concentration below which there are no pores nucleated in the lipid bilayers. Our findings corroborate and complement the entropy-based pore nucleation model proposed earlier by Moldovan et al. [1].

Studies on nucleation phenomena occurring in aqueous solutions supersaturated with calcium sulfate

1978 ◽  
Vol 56 (6) ◽  
pp. 831-838 ◽  
Author(s):  
D. M. Keller ◽  
Ronald E. Massey ◽  
O. E. Hileman Jr.
Keyword(s):  
Aqueous Solutions ◽  
Nucleation Rate ◽  
Calcium Sulfate ◽  
Nucleation Theory ◽  
Critical Parameters ◽  
Classical Nucleation Theory ◽  
In Situ Generation ◽  
Temperature Surface

The results of a set of experiments designed to delineate the nucleation rate, supersaturation, temperature surface for the system CaSO4•2H2O/H2O are reported. The data were obtained using the droplet technique coupled with in situ generation of SO42− under the conditions that dS/dt was large during the early part of the induction period and it was zero during the actual nucleation events. The observed shape of the surface is compared with that suggested by the classical theory of nucleation and differences are commented on. The critical parameters important in classical nucleation theory are reported.

A Modified Classical Nucleation Theory for Plastic Foam Processing

2015 ◽  
Author(s):  
Anson Wong ◽  
Chul B. Park
Keyword(s):  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Stress Effect ◽  
Free Energy Barrier ◽  
Local Pressure ◽  
Cell Nucleation ◽  
Gas Clusters ◽  
Plastic Foaming ◽  
Stress Variations

This paper elucidates a modified Classical Nucleation Theory (CNT) that considers the effects of stress variations and existence of microvoids in plastic foaming processes. The stress effect has been incorporated into the CNT by a local pressure variation term that describes the dynamic conditions of industrial plastic foaming processes (e.g., extrusion foaming, injection foam molding). Also, the free energy barrier equation has been modified to include nucleation events from existing gas clusters. Both phenomena have been observed experimentally via in-situ plastic foaming visualization to confirm the modified theory. Combining the stress and preexisting gas clusters considerations, the modified CNT described in this paper improves the traditional CNT in describing cell nucleation phenomena in plastic foaming processes.

scholarly journals A lab-on-a-chip approach integrating in-situ characterization and reactive transport modelling diagnostics to unravel (Ba,Sr)SO4 oscillatory zoning

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Jenna Poonoosamy ◽  
Mohamed Mahrous ◽  
Enzo Curti ◽  
Dirk Bosbach ◽  
Guido Deissmann ◽  
...  
Keyword(s):  
Solid Solutions ◽  
Reactive Transport ◽  
Lab On A Chip ◽  
Nucleation Theory ◽  
Oscillatory Zoning ◽  
Classical Nucleation Theory ◽  
Reactive Transport Modelling ◽  
Transport Modelling ◽  
Co Precipitation

AbstractThe co-precipitation of sulphate minerals such as celestine and barite is widely studied because their formation is ubiquitous in natural and anthropogenic systems. Co-precipitation in porous media results in crystallization of solid solutions yielding characteristics such as oscillatory zoning that are rarely observed in bulk solution or in batch experiments. In the past, the precipitation of compositionally-zoned (Ba,Sr)SO4 crystals was observed post-mortem in macroscopic silica gel counter-diffusion experiments. Their formation was originally explained by the difference in the solubility products of the end-members combined with diffusion-limited transport of solutes to the mineral-fluid interface, while a later study favored the idea of kinetically controlled reactions. With recent advances combining in-operando microfluidic experiments and reactive transport modelling, it is now possible to verify hypotheses on the driving forces of transport-coupled geochemical processes. We developed a “lab on a chip” experiment that enabled the systematic study of the nucleation and growth of oscillatory-zoned (Ba,Sr)SO4 crystals in a microfluidic reactor. The compositions of the solid solutions were determined by in-situ Raman spectroscopy. Our investigation shows (1) that the composition of the nucleating phases can be approximated using classical nucleation theory, (2) that the oscillatory zoning is not solely controlled by the limited diffusional transport of solutes, and (3) that nucleation kinetics plays a major role in the switch between different stoichiometric compositions. The zoning phenomena is governed by the complex interplay between the diffusion of reactants and the crystallization kinetics as well as other factors, e.g. surface tension and lattice mismatch.

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